Motor Short Answers
Motor Short Answers
Motor Short Answers
1 The precision engineering bearing inserts are manufactured with small portion of bearing ends extending
beyond the bearing housing or caps. The installation process of these bearings requires sufficient
Ans. (b) crush.
The bearing shell itself is in two halves. The bearing shell halves are extended beyond the bearing housing by
approximately 0.3mm. This is applicable in modern thin-shelled bearings. As these bearings are thin-shelled,
they cannot support shaft on their own and hence need backing-up support of the bearing housing. Hence it is
necessary to have full contact with the back of the bearing and the bearing keep. Hence crush is provided to
achieve this. Also, it prevents the rotation of the shelf in the housing due to the rotation of the shaft.
Q.2 The method of piston cooling in which oil is delivered through the connecting rod to a compartment
within the piston, then distributed by the motion of the piston and allowed to drain to the crankcase via
one or more pipes or holes is termed as
Ans. shaker method of cooling of the piston.
In shaker method of piston cooling, it is actual movement of the piston which causes the movement of oil in the
piston. The oil outlet from the piston crown cooling space is kept at higher level than that of inlet. Hence some
oil is always retained in cooling spaces. This oil is splashed against the underside of the hot crown as the piston
moves up and down so as to have better cooling of the piston crown. This spraying action is done due to the
movement of the piston. This is very efficient piston cooling method. Nowadays, new Sulzer engine is
employing the combination of shaker and nozzle cooling method and has reported to have reduction in piston
crown temperature.
Q.3 The piston rod scrapper box incorporated in two-strokes cycle crosshead diesel engine serves to
Ans. (c) prevent the sludge and dirty oil from entering the crankcase.
In two-stroke crosshead engines, under-piston scavenge space are separated from crankcase by use of diaphragm.
Under-piston scavenge space gets accumulated with unburnt carbon, unburnt fuel and mixture oil in the form of
sludge. If this finds its way into crankcase, then crankcase oil will get deteriorated. Hence stuffing box with
scrapper and sealing rings is mounted on the diaphragm through which reciprocating piston rod passes thus
scrapping off and preventing the entry of sludge into the crankcase. Also it prevents the crankcase oil into the
under-piston space hence preventing the loss of crankcase oil.
Q.4 In large slow speed main propulsion diesel engines which of the parts listed is under tension when the
engine is running?
Ans. Tierods.
The job of the tierods is to hold bedplate column, entablature under compression so as to prevent the relative
movement between these parts. When the engine is running hence these parts are tied together and are kept under
compression by use of tierods. While doing so, the tierods itself gets into tension. Hence the firing load is
transferred to the bedplate and ship structure.
Q.5 The main advantage of unit injector over other fuel injection system is
Ans. lack of high pressure fuel lines.
In case of unit injectors, which is a combination of fuel pump and fuel valve, as the fuel valve is not separately
mounted away from the fuel pump, there is no need to provide a connecting high pressure oil carrying fuel
between the fuel pump and fuel injector. This combination improves safety and chances of high-pressure fuel
leakages also reduces. Also the possibility of fire due to leaky high-pressure pipes is reduced.
Q.6 The passages are drilled in the crankshaft of diesel engines to provide lubricating oil to
Ans. the main bearings, connecting rod bearings and piston pin bushings.
Crankshaft moves in the main bearings. At the same time the connecting rod big end bearings moves over the
crankpin. Also the connecting rod top end bearing moves over the piston pin bushing. As there is relative
movement between these parts, friction always exists between them. In order to reduce the friction and remove
the heat generated due to small amounts of film friction that may exist, it is necessary to supply sufficient
quantity of lubricating oil to all these three parts so as to have a film lubrication which will separate the moving
parts from the stationary bearings. At the same time, it will provide sufficient amount of lubricating oil for the
purpose of cooling. This arrangement is utilized only in case of small engines where the drilling of the crankshaft
is permitted. In case of big engines, there is possibility of stress concentration due to the drilling in the crankshaft
and there is a possibility of crankshaft breaking. Therefore this type of system is not utilized in big engines. In
big engines, main bearing lubricating oil is supplied separately through the crosshead bearing which will be
branched to the piston cooling as well as crosshead lubrication and through the connecting rods drilling it will go
to the bottom end bearing lubrication.
Q.7 When can the main crankcase ventilation pipes or oil drain pipes of two or more engines be connected?
Ans. As far as possible no connection should be provided between the crankcase ventilation of different
engines.
Crankcase ventilation pipes or drain pipes of two or more engines should not be connected as in case of
crankcase explosion, the heat wave may enter other healthy engine crankcase causing explosion in that engine
also. Also if one engine crankcase oil is contaminated, other engine oil may also get contaminated if the drains
are connected. Also this may occur due to the oil vapour getting mixed up.
Q.8 The main function of tierod in the construction of a large low speed diesel engine is to
Ans. absorb most of the tensile load that results from the firing forces during the operation.
The main function of the tierods is to keep the cylinder block, A-frame and bedplate tied together under
compression. At the same time it itself goes into tension. Also tierods accept most of the tensile loading that
results from the firing forces developed during operation and relieve the bedplate, entablature and columns off
these firing forces. Hence it can be seen that the tierods are always under tension so as to maintain all other parts
of the engine structure under compression and to prevent the relative movement between the parts when the
engine is in operation.
Q.9 On most modern diesel engines, the main and connecting rod bearings receive their lubricating oil by
Ans. (d) pressure feed.
On most modern diesel engines, where the load is comparatively high, it is necessary that the bearings should be
lubricated by lubricating oil. This lubricating oil will form a wedge type film between the stationary bearing and
the rotating shaft so as to prevent the metallic contact and the friction between the bearings and the journal. For
this purpose, it is necessary to supply large quantity of oil so as to have a continuous and uninterrupted supply of
lubricating oil which is required to form the film as well as cooling purpose. In order to achieve this, it is
necessary to continuously supply lubricating oil under pressure. This will overcome the piping friction and also it
will maintain uninterrupted and sufficiently large quantity of lubricating oil to the bearings.
Q.10 Fuel injection pump using the port and helix metering principle requires the use of a
Ans. (b) lapped plunger and barrel.
Due to lapped plunger and barrel, there is a good contact between the plunger and barrel. As the pump develops
very high pressure, there will be least amount of leakage between the plunger and barrel. Also due to the cutting
of helix on the sliding surface of the plunger, the sealing area between the plunger and barrel reduces. As the area
reduces, the possibilities of leakages increases. Hence in order to reduce the leakages at high pressure between
the plunger and barrel, it is necessary to lap plunger and barrel together so as to have a perfect bearing between
the plunger and barrel and to reduce the leakages. The plunger and barrel are to be replaced together as a set
because they are lapped individually with each other and they should not be replaced individually as single unit.
Q.11 Lubricating oil is supplied to the crankshaft main bearing in a marine diesel engine by
Ans. internal crankshaft passages.
In small diesel engines where the crankshaft size is small and the engine pistons are trunk-type, oil is supplied to
the main bearing by a pipe. From there through the passage in the crankshaft it is fed to the crankpin bearing thus
avoiding complicated piping to supply lubricating oil to crankpin bearing. From there, oil is led upward through
the connecting rod holes.
Q.11 The lubricating oil supplied to the crankpin bearing in a marine diesel engine Ans. by internal
crankshaft passages.
The internal passages in the crankshaft are made in such a manner, in case of small diesel engines where
crankshaft size is comparatively small, that oil is supplied to the main bearing. From there to the passage in the
crankshaft it is feed to the crank pin bearing. Thus avoiding the complicated piping to supply oil to the crankpin
bearing and gudgeon pin bearing. This is applicable only in case of small crankshafts. Bigger crankshafts may
fail under fatigue if the holes are drilled out for the passage of the oil. Hence in case of the bigger crankshafts, no
drilling of the holes is done on the crankshaft and hence this method of lubricating the bottom end bearing and
gudgeon pin bearing is not applicable. In that case, there is a separate supply of lubricating oil to main bearing
and as well as to the crosshead bearing, which will be further bifurcated into two: the crosshead bearing and
piston cooling, and the bottom end bearing, for lubricating.
Q.12 Gudgeon pin bearing are difficult to lubricate because of their oscillating motion and
As the gudgeon pin is fitted between the piston and connecting rod, it is difficult to convey oil to it. Also by the
time oil reaches this bearing after passing through the main bearing and bottom end bearing, there is a
large amount of pressure drop as well as loss of quantity of the lubricating oil. Thus this bearing is
starved of the oil. Also load on this bearing is acting downwards. Also the area of the bearing is less. All
these factors contributes to the difficulty in the lubricating the gudgeon pin bearing.
Q.13 The camshaft drive is designed to maintain the proper camshaft speed relative to the crankshaft
speed. In maintaining this relationship the camshaft drive causes camshaft to rotate at
Ans. (b) crankshaft speed in a two-stroke diesel engine i.e. camshaft and crankshaft speeds for two stroke
engines are same.
In a two stroke engines for every revolution there is one power stroke. For every power stroke, there is one fuel
injection movement and one exhaust valve opening movement. This fuel injection and exhaust valve
opening is controlled by the camshaft rotation. Hence it is necessary for camshaft to rotate at the same
speed as the crankshaft, so as to have fuel injection and exhaust to be carried out for every revolution of
the crankshaft.
Q.14 Engine having bore exceeding 250 mm, but not exceeding 300 mm, are to have at least
Ans. (c) one explosion relief door or one explosion relief valve in the way of each alternate crank throw
with a minimum of two valves.
This is a minimum requirement under the classification society rules. When arriving to this requirement figures,
the calculation of the crankcase volume and the rate at which the explosive gases are released, in order to
avoid the building of pressure and explosion of the crankcase, atleast two safety valves are needed; in
case one valve fails to open atleast the second one will respond.
Q.15 The opening of an exhaust valve on a modern large two-stroke slow-speed main propulsion diesel
engine may be actuated by
Q.16 Shaker/circulation and spray are the three general methods used in
Ans. (d) piston cooling.
As these three methods are the most effective and easier to be applied to the rapidly reciprocating pistons
without providing any external methods and this the most convenient method of supplying lubricating oil or any
other cooling medium to the piston cooling. At the same time, movement of the piston is utilized for the splash-
type shaker method of cooling for the under side of the piston crown. This method is highly effective method.
Q.17 Speed of the camshaft of a two-stroke cycle diesel engine running at 950 rpm is
Ans. 950 rpm.
In two-stroke engine, it is necessary to run the camshaft at the same speed as that of the crankshaft. As for every
revolution there is one power stroke and for every power stroke fuel has to be injected. Hence the speed of the
camshaft and the crankshaft should be equal.
Q.18 Explosion relief valve on diesel engine crankcase should relieve the pressure at not more than
Ans. 0.2 bar.
This rule is governed by the classification society and SOLAS regulations. When arriving to this figure, all
necessary calculations are done to reduce the severity of explosion. If the pressure is allowed to reach more than
this value, the crankcase temperature will be higher and the chances of explosion and severity of explosion is on
higher side. Hence it is necessary to relieve the pressure from the crankcase if it exceeds more than 0.2 bar
Q.20 The sealing surface of a diesel engine piston rings which faces are considered to be in contact with
cylinder walls in addition to the ring grooves?
Ans. (a) Bottom.
The high pressure combustion gases goes behind the piston rings to push them hard against the liner to have a
good sealing effect between the liner and the piston rings. But the gas pressure behind the piston rings should
always be maintained and there should not be any leakage from the landing faces of the rings on the grooves of
the piston; otherwise blowpast may take place. Hence the sealing, between the contact faces of the rings and the
bottom surface of the grooves on which the ring rests, is important to maintain gas pressure behind the piston
ring to give better sealing effect between the piston rings and the liner.
Q.21 Where is the fuel delivery check valve located in jerk type fuel injection system?
Ans. (c) In the injection pump housing discharge side.
As the valve job is to prevent the backflow of the fuel from the high-pressure piping and fuel valves into the
pump during the suction stroke of the pump and also at the end of delivery stroke when suction and discharge
gets short-circuited to terminate the delivery stroke. Otherwise the backflow of the oil from the fuel piping will
take place. In that case, every time at the end of delivery the high-pressure side of the piping will get partially
empty and this will cause late beginning of injection for the next stroke. Hence it is necessary to maintain the
high-pressure pipe between the fuel pump and fuel valve to be always filled with the oil. At the same time, they
should be maintained at certain high pressure.
Q.22 How many crankcase relief valves are required for a 325mm bore, 8-cylinder in-line engine?
Ans. (d) 8
As the bore is 325mm as per classification society rules, each crankcase should have atleast one relief valve
fitted. Hence in case of 8-cylinder engine, it is necessary to have 8 crankcase relief valves.
Q.23 Which of the air-intake system listed will result in lowest specific fuel oil consumption?
Ans. (b) Turbocharged.
The turbocharger will provide the extra air under pressure so that more quantity of air can be admitted into the
cylinder so that more fuel can be burnt efficiently. For this, turbochargers are provided. As these turbochargers
are driven by waste exhaust gas without any power from the engine, it is the most fuel efficient method of
improving the performance of the engine. In all other methods of supercharging, the power for driving the
supercharger is taken from the engine crankshaft. Hence there is a loss of power and from the point of view of
available power at the engine output, the specific fuel oil consumption will be on the higher side as compared to
the turbocharged engine.
Q.24 In which of the area of the crosshead engine is a permanently connected fire extinguishing system
required?
Ans. (d) Scavenging space
In open connection to the cylinders i.e. it is required for the under piston scavenge space.
Under piston scavenge spaces are connected on the cylinder underside. As under side of the piston is having fuel
unburnt loops and carbon mixture. In case of blowpast in presence of plenty of availability of air, this sludge may
catch fire. This type of fire is called crankcase fire. In order to extinguish such type of fire, permanently
connected fire extinguishing system is required. This is possible by providing a permanent connection of steam
smothering or connection to extinguish scavenge fire because there are circumstance under which most of the
other fire fighting processes will fail. In such circumstances, there is no other alternative but to flood the under
piston scavenge space weight smothering element such as carbondioxide or steam.
The rate of opening and closing of the valve will depend upon the cam-slope. As more steeper the cam
profile, faster the rate of opening and smarter the closing. This is required for the valves in order to avoid
the throttling effect when the valve is opening very very slowly or when it is closing very slowly. If the
valve opens or closes slowly, in that case, there is always a possibility of improper scavenging taking place.
In such circumstances, the efficiency of the combustion is poor. This is mostly applicable to the high-speed
engines where the time available for the process of exhaustion and suction of air is very very limited.
Q.27 Arrangement and shape of the cams on diesel engine camshaft directly controls which of the listed group of
operating condition?
Ans. (b) Firing order, valve timing and valve lift.
Justification: The position of the individual cam on the camshaft will decide the firing order of the engine.
At the same time exhaust and intake valve opening will be also decided by the positioning of the respective
cams on the camshaft. The position of the cams on the camshaft with respective crankshaft will decide the
opening and closing timing for the fuel valve, exhaust valve and intake valve. The profile of the cam will
also decide the rate of opening and the rate of closing. Also the valve lift will depends upon the roller lift
which will be decide by the total cam lift.
Q.28 Carbon deposits building up in and around injection nozzles are least likely to occur when using which of
the listed type of the fuel injection nozzles?
Ans. (c) Pintle type of nozzles.
Justification: Pintle type of nozzles are mostly used in the low powered engines where the temperature inside the
cylinder is much lower. At the same time, because of its single whole structure, there is least amount of leakages
or dripping of the nozzle. Also it has very very low sac volume and hence its dripping possibility reduces. All this
will contribute to the minimum amount of carbon deposit around the fuel nozzle.
Q.29 A port and helix fuel injection pump having upper and lower plunger helix is designed to
Ans. (b) vary the beginning and end of injection timing.
Justification: When there are two helixes cut on the plunger one helix controls the beginning of injection i.e.
lower helix controls beginning of injection and upper helix controls the end of injection. Thus it is a good old
method of variable control which were fitted on the previous generation of engines.
Q.30 Which of the following statements describes the fuel injection pump mark timed for port closing.
Ans. (a)
Injection has a constant beginning and variable ending. The helix cuts on the plunger surface and inlet port on the
barrel. The time at which the upward moving pump covers the suction port on the barrel is the beginning of
injection as the fuel oil gives compressed and it lift opens the fuel injection valve.
9nppffTill that time there is overflow of oil to the suction side. By moving upwards, depending upon the load of
engine, there is relative position of plunger helix with respect to the suction. This will decide as to when the
discharge side of the plunger will be in connection with the suction side, hence ending the delivery. Also this will
decide quantity of fuel injected depending upon the load. This system has constant beginning but the end and
quantity of injection is controlled by the helix position.
Q.32 An individual pump is designed for variable beginning and constant end of injection. For diesel
engine operating at constant speed, the start of injection will
Ans. advance as the load increases.
As the load increases, engine requires more fuel to inject and burn. As the speed is constant, more fuel is to be
injected in same time frame available, it is necessary to begin the injection earlier, i.e. to advance the beginning
of injection, so that the fuel can be completely burnt, effectively and efficiently, within the time frame available
thus giving better thermal efficiency of the engine.
Q.33 Crankshaft whose center of gravity coincides with the centerline is said to be
Ans. (b) statically balanced.
B012
Ans. (b) increase the distance of heat travel from the crown to top ring groove.
As the first ring or the top ring is subjected to maximum amount of temperature and pressure, it is difficult to
maintain the film of lubrication between the top ring and liner. Also there is every possibility that due to heat and
high pressure, the top ring will get broken or seized in the piston groove. Due to lack of lubricating oil film, there
is maximum weardown for the top ring and top end of the liner. In order to avoid this, it is necessary that the
piston ring should be maintained as far as away for the firing temperature so that a good lubricating oil film can
be maintained between the liner and piston rings. In order to achieve this, a seat damp is utilized in the engine.
Q.40 Diesel engines are classifieds as internal reciprocating engines because they
Ans. (c) burn the fuel in chamber and their energy moves the piston back and forth.
As the fuel is burning in confined space which is sealed where fuel burning takes place because of high
temperature of compressed air. All the burning process is carried out inside the cylinder liner between the top of
the piston and bottom of the cylinder head. The heat released is converted into mechanical energy by the
movement of the piston which is transmitted to the crankshaft. As the combustion takes place inside the cylinder,
it is called internal combustion engine.
B015
B017
Q.55 Cavitation and corrosion in cooling water system of diesel engine usually occurs at the pump impeller
and the waterside of the
Ans. (c) cylinder liner.
When the water enters into the cylinder liner, it gets heated up. At the same time when water passes through the
cylinder liner, the velocity of water increases due to restricted passages. As the velocity increases, as per
Bernoullis theorem, pressure drop takes place. As the pressure drop takes place, water tends to boil off at lower
temperature due to which steam bubbles are formed. These bubbles get bursted creating cavitation. Hence if the
liner is opened for inspection any time, especially at the places of restricted entries and passages, cavitation is
easily visible.
B018
Q.56 The main operating characteristics that distinguishes it from other internal combustion engines is
Ans. (c) method of igniting the fuel.
In petrol engines, the ignition is carried out with the help of igniter or spark plug. Diesel engine fully depends on
compression ignition for the purpose of igniting the fuel. In diesel engine the air is compressed. As the air is
compressed its temperature increases to a temperature which is more than the self-ignition temperature of the
fuel. When fuel is injected at this temperature, it vapourizes and mixes with air to form combustible mixture. As
the temperature is above the self-ignition temperature this mixture ignites and produces a high pressure wave.
This pressure wave and the heat is transmitted to the crankshaft by the downcoming piston.
B019
Q.57 Which term describes piston pin having bearing surface in both piston bosses and connecting rod
eye?
Ans. (b) Full floating.
If the gudgeon pin is having a press-fit in the bosses of the piston and is free to move over the top end bearing of
the connecting rod, there is a possibility that misalignment cannot be taken care off. Hence the pin is made to
float or have bearing surface in the piston boss and the connecting rod eye so that it can take care of
misalignment. At the same time the pin can have a rotating effect. When the pin has rotating effect, it will not
have continuous loading on one surface and the weardown of the bearing will be uniform thus preventing the
ovality of the bearing due to wear in only one place. In order to prevent this pin from coming out when the piston
is moving up and down, at the end of bosses, circlips are provided which prevents the gudgeon pin from coming
out and touching the liner.
B0021
Q.58 Compared to other fuel injection systems, unit injection system operates with virtually no
Ans. (a) injection lag.
In unit injection system, the fuel pump and the fuel valve consists of a single block. Hence there is no fuel piping
the valve and the pump. As there is no piping, the time required for injection from the time the fuel pump plunger
starts moving up is reduced resulting in reduction of injection lag. In case of other types of fuel pumps, where the
fuel pump is connected to the injector by long high pressure pipes, some amount of pressure drop takes place
inside the fuel pump piping. Also due to the pressure wave created, the fuel piping gets slightly expanded which
can cause fuel injection delay. Hence in order to avoid fuel injection delay, the best possible way is to fit the fuel
pump very near to the injector. In unit fuel injection system, this delay is nearly reduced to zero.
B0022
B0023
B0025
Q.61 Besides the use of piston cooling, fins provides extra strength for the
Ans. (b) piston crowns.
As the fins are provided on the underside of the piston crown, the top portion of the piston crown which is
subjected to higher temperature and pressure is supported by the fins from underside. As the support is provided
by the fins, they give extra strength to the piston crown. Hence it is possible to use the piston crown of thinner
cross-section without causing it to buckle under the influence of high temperature and pressure.
B0026
Q.62 Minimum fuel oil delivery pressure required for efficient fuel injection depends primarily on
Ans. (b) maximum pressure in the engine cylinder during injection.
When the piston is approaching TDC on compression stroke, the fuel oil injected into the cylinder. The fuel oil
injected has to overcome the resistance or backpressure offered by the high pressure compressed air. Hence the
pressure of the fuel oil injected has to be more than the pressure inside the cylinder head; otherwise it will not be
possible for the fuel to penetrate the mass of dense air available. This will result in improper combustion of the
fuel. Hence it is necessary to maintain minimum injection pressure higher than the compression pressure existing
at the time of injection.
B0027
B0028
Q.64 The two strokes of the two-stroke cycle diesel engines are
Ans. (d) compression and power.
As any diesel cycle, the two-stroke cycle will also consist of all four major processes i.e. suction, compression,
power and exhaustion. In case of two-stroke engines, the process of compression and power are distinct but air
intake and exhaustion are combined together and they form a part of the distinct compression and power stroke.
Hence there is no separate distinct stroke for intake and exhaustion as exhausting is carried out by intake of air.
B0030
Q.65 Which of the following statements pertains to propulsion engines with bore exceeding 200mm?
Ans. (c) The units will be fitted with means to display the exhaust temperature of each cylinder.
When the piston diameter exceeds 200mm, the engines are considered as highly rated engines. In such engines,
the exhaust temperature of individual units will be on higher side and sufficient amount of power is developed by
the units. In order to have proper running of the engine, it is necessary to have a balanced engine. The balanced
engine can be roughly described by comparing the individual unit temperature. If the exhaust temperatures of the
units are within prescribed range, one can consider that the engine is properly balanced. It should also be noted
that exhaust temperature is not the only indication of power balance. For this purpose, the indicator card is taken
and the power developed by the individual unit is calculated the best method. It can be a rough estimation of
power balance.
B0031
Q.66 When engine bore exceed 230mm, a berthing disc or flame arrestor is fitted
Ans. (a) to the engine air supply inlet to the starting air supply manifold for non-reversing engines.
When the bore of the engine exceeds 230mm it is considered as highly rated engine as far as power is concerned.
These engines are mostly started with compressed air. If air inlet valve or starting air valve on the cylinder leaks,
high pressure exhaust gas will enter the air manifold. As the pressure and the temperature is high, it will cause a
air starting line explosion. This will result in heavy damage to the engine and the personnel working. Also the
engine will be completely immobilized.
B0033
Q.70 Diesel engine piston ring gaps can be straight-cut or angle-cut. In compression rings the angle cutting
Ans. (d) decreases the combustion gas leakages.
Due to the angle-cut rings and if they are left and right cut, if they are fitted alternatively, the resistance offered
for the gas to flow through the gap between piston rings end i.e. butt clearance, is more. Hence less leakage takes
place from the top of the piston rings to the bottom of the piston rings through the gap between the piston rings.
Hence angle cutting is more preferred than straight cut. But in angle-cut rings, the joint at the cut position will be
slightly weaker than the straight-cut rings.
Q.71 Modern marine diesel engines equipped with mechanical fuel injection operates on combustion space
within the cycle which is
Ans. (c) combination of constant volume and constant pressure.
In the mechanical fuel injection system, when fuel is injected the piston is approaching TDC, but the fuel is not
injected exactly at TDC but much earlier than TDC. During this time, the volume is changing. At the same time,
when the piston is at TDC when firing takes place though the pressure is increasing but as the piston is moving
downwards the volume increases and the pressure remains constant. Thus there is a combination of constant
pressure and constant volume which helps to have a better thermal efficiency as compared to other fuel injection
systems.
Q.72 Which of listed design is successfully used to provide the turbulence necessary for proper combustion
in the diesel engine cylinder?
Ans. (c) Precombustion chamber.
The precombustion chamber is a part of the cylinder head where fuel is injected and combustion takes place in
this chamber. This chamber is made up of spherical shape and because of the geometry of the chamber better
turbulence is achieved. If the fuel is injected in this condition, it is burnt effectively and thus causing better
combustion and thermal efficiency increases. These engines were used in earlier days but at presently they are
not utilized because the geometry of the combustion space is made in such a manner including the piston top so
as to have a turbulence created inside the main cylinder head rather than having precombustion chamber.
Q.73 The purpose of counter-boring the top of the cylinder liner extending to the top of point of travel of
the top compression ring is to
Ans. (c) prevent wear of the liner to form ridge at the upper level of the ring crown.
Counter-boring is the word utilized to give the tapered shape to the top of the cylinder liner. Because of the
tapered boring the formation of ridge is reduced or it will not take place as there is gradual change in the
diameter of the liner. Hence if there is any ridge formation it will be removed and any metal will not be allowed
to form a ridge. If ridge formation takes place there is a possibility that the piston rings will hit the ridge which
may cause the rings to crack and the liner to break.
Q.74 The most rapid period of fuel combustion in a diesel engine cycle should begin just before the piston
reaches the TDC and
Ans. (d) should be completed just after the TDC.
The maximum possible efficiency of any diesel can be achieved if the most of fuel is burnt near the TDC. In
order to burn it near TDC, it is necessary that it should be injected before the TDC so that combustion mixture
can be formed by the TDC. At the same time, complete combustion should be carried out before the expansion
stroke begins so that no power is lost. If the combustion continues in the expansion stroke, it is called
afterburning, in which there is loss of power and high exhaust temperature which gives lot of problems to the
engine and hence result in loss of thermal efficiency. Taking into consideration all these things, it is necessary
that the completion of the fuel burning should be shortly after the piston crosses the TDC position and should not
be continued longer than this.
Q.80 What is commonly used to create turbulence in a diesel engine combustion system?
Ans. (a) Shape of the piston crown.
As the air inlet valve opens, the piston is near TDC and exhaust valve also opens during the overlap period. The
air entering from the air manifold through the air inlet valve hits over the crown and drives the exhaust gas from
the cylinder through the already opened exhaust valve. As the incoming air is hitting the piston crown, the shape
of the piston crown very important to create the flow patterns of the air which will be ultimately useful in
creating turbulence for the better combustion of the fuel. Thus the shape of the piston crown is of prime
importance. This shape will form a spherical combustion space which have better combustion efficiency and
better turbulence. Hence thermal efficiency will increase. In case of two-stroke engines, the liner scavenge port
configuration and angle at which the scavenge ports are cut is also of importance to create the turbulence.
Q.80 How many power strokes per crankshaft revolution are there in an 8-cylinder two-stroke cycle diesel
engine?
Ans. (d) 8.
As it is two-stroke engine, for every revolution there is a power stroke per cylinder. As there are 8 cylinders there
will be 8 power strokes per revolution. Hence there will 8 strokes of power.
Q.81 To shutdown a diesel engine after it exceeds the set maximum speed which type of device listed below
is used?
Ans. (c) Overspeed trip.
It is necessary for the engine to trip if the speed exceeds beyond the set speed as the inertia and the centrifugal
forces created because of high speed of the engine will give large amount of stresses onto the moving parts which
may cause the failure of the moving parts and bottom end bearing bolts. Hence to avoid these damages, it is
necessary that as soon as the speed increases beyond the preset limit the fuel should be cut-off and the engine
should be made to stop. This is achieved either mechanically or electrically depending upon the engine and the
type of injection system fitted. Mostly it is at 10% above the normal running rpm. If the engine trips because of
overspeed, the restarting of the engine is possible only after resetting the fuel lever to the start position and then
starting the engine. This is provided so that continuous stopping and starting of engine is prevented in case if the
engine trips and overspeeds.
Q.83 To function properly oil control rings used on diesel engine must distribute sufficient oil to all
cylinder parts and also
Ans. (b) prevent excessive lubrication from reaching the combustion space.
The oil control rings or scrapper rings are used in the trunk type of diesel engines so that excessive lubricating oil
which is splashed on the liner walls should be removed and drained back into the crankcase so that excessive
lubrication will not get carried away by the exhaust gas into the exhaust manifold and turbocharger which may
otherwise get fouled because of the lubricating oil burning in the exhaust piping and the combustion space. As
this oil cannot withstand high temperature, it will get burnt and form large amount of carbon deposits. These
carbon deposits will cause the piston rings to seize or turbocharger efficiency to drop.
Q.84 An important design characteristics of an explosion relief valve of a diesel engine is its ability to
Ans. (a) close quickly inorder to prevent inrush of air.
Explosion relief valves or crankcase relief valves are non-return flap valves which opens against the spring
pressure due to the formation of excess pressure inside the crankcase. When the pressure reaches in the region of
about maximum upto 0.2bars, these valves must open and let the crankcase gases to get relieved into the engine
room atmosphere. At the same time as soon as this release is carried out, the spring should close this relief valve
onto the seat. In case of crankcase explosion, viz. the primary explosion and secondary explosion, if the primary
pressure wave created is not allowed to relieve, it will damage the crankcase and the personnel. This primary
explosion leads to the secondary explosion which takes place due to ingress of fresh air into the crankcase from
the atmosphere. If the ingress of air in the crankcase is prevented after the primary explosion in such condition a
major secondary explosion can be avoided. Hence inorder to relieve the excess pressure generated into the
crankcase and to prevent the ingress of fresh air into the crankcase to avoid secondary explosion, the crankcase
relief doors are made up of non-return type.
Q.85 In modern diesel engines which of the following is used as a support for main bearings?
Ans. (a) Bed plate.
In most of the modern engines, in case of conventional crankshaft, the crankshaft rests on the bottom half of the
main bearing. The bottom half of the main bearing is resting over the saddle created in the bedplate. Thus the
complete load of the engine will be transmitted via the main bearings to the bedplate and from there to the ship
structure. But in case of undershrunk crankshafts, the crankshaft will not be resting over the bedplate but it is
overhanged or undershrunked from the bedplate. This is useful for the easy removal of the main bearing for
inspection and replacement.
Q.86 Turbulence in the cylinder of two-stroke main propulsion diesel engine is mainly created by
Ans. (d) intake port design.
In case of two stroke engines, having the scavenge ports cut in the cylinder liner, the air enters through the
scavenge ports. This air is responsible for sweeping clean the cylinder liner of any remaining exhaust gases and
creating turbulence in the liner and combustion chamber. The entry of air through the scavenge port is made at
some angle with respect to the circumference of the cylinder liner. In such condition, turbulence can be created in
the cylinder liner and combustion space. Hence to have a tangential entry into the cylinder and to have the proper
turbulence created by the scavenge air entering the cylinder liner, it is necessary to cut the cylinder liner intake
ports at a specific design which will help in promoting the turbulence in the cylinder of two-stroke engines. As
sweeping action by the scavenge air is very important in case of two-stroke engine, the exhaust removal is
carried out by the incoming air rather than the upward movement of the piston as in case of four-stroke engines.
Q.87 In internal combustion engine, which of the device listed will force the compression rings to seal the
compression gases in the space above the piston?
Ans. The gas pressure acting against the back of the rings
The gas pressure due to combustion of fuel and compression of gas will find its way through the piston ring
groove clearance i.e. the vertical clearance between the piston ring and the groove. This high pressure gases will
act on the piston rings from behind because of which the piston rings will be forced radially outwards and it will
exert a pressure onto the liner thus forming a perfect seal between ring rubbing surface and the liner. At the same
time, this piston ring will be forced downwards against the piston ring groove by the gas pressure thus preventing
the leakage of gas between the landing face of the piston ring and the groove. This clearance is of very
importance and for this the piston ring should be free in the groove so that gas can find its way behind the piston
rings to form a best possible seal. If this seal is not maintained due to any reason, the piston ring will not seal
effectively against the liner which will cause the blowpast and ultimately lead to scuffing due to the wiping out
of lubricating oil film between the liner and piston ring. The gap between the groove and the piston ring is
maximum for the top ring and it reduces as the piston rings are fitted away from the combustion space i.e. the
topmost ring will have maximum clearance and the lowermost ring will have the least clearance.
Q.90 Provision is to be made for ventilation of an enclosed diesel engine crankcase by means of a
Ans. (d) small breather or slight suction not exceeding 25mm of water.
By ventilating the enclosed crankcase of a diesel engine, the pressure inside the crankcase can be reduced. Thus
preventing the excess pressure developed which may cause the leakage in the crankcase doors and crankcase oil
may finds its way out. At the same time, it will prevent the development of high concentration of oil mist in the
crankcase. Thus reducing the possibility of crankcase explosion. In modern engines mostly, the crankcase
external fans are not provided but crankcase weld is provided which is led to a remote plate which prevents the
excessive built up of pressure inside the crankcase. This pressure should not be above 25mm of water gauge
otherwise the leakage from the crankcase and the loss of oil will be very much.
Q.91 Which of the general advantages listed does electrical pyrometers have over mechanical pyrometers?
Ans. (d) It can indicate temperature at a distant point from the source of heat.
In case of electrical pyrometer which works on the principle of thermocouple and the hot and cold junction, due
to the temperature difference between the hot and cold junctions, an electric current is produced in the circuit.
This current is directly proportional to the temperature difference between the cold and hot junction. This electric
current which is produced due to the temperature difference can be amplified by use of amplifier and can be
relayed through the wire to a remote place from where it can be indicated in the form of temperature. Hence with
electric pyrometer, it is possible to have a remote indication rather than in case of ordinary pyrometer. In ordinary
pyrometer, it is not possible to have remote indication of temperature.
Q.92 In large slow speed diesel engines the force applied to the crosshead is
Ans. (c) against the crosshead during the power and compression stroke.
In a crosshead engine, there is a crosshead shoe with two rubbing surfaces. At the same time, there are two
crosshead guides one each at port side and starboard side. During the half revolution of the engine one face of the
crosshead shoe rests against one side of the crosshead guides. This will continue upto BDC when the crank turns
around the dead center and the piston starts moving upwards. In such condition, the directional forces of the
crosshead changes and other face of the crosshead shoe will start resting against the other crosshead guide.
Hence for half the revolution one face of shoe and guides are coming in contact and for the remaining half
revolution other face of the shoe and guide are coming in contact. Hence both crosshead guides and shoes are
required. Sometimes they are called ahead or astern shoes and guides. Hence even the engine may be
unidirectional, it is necessary to have guides and shoes at both, port and starboard sides.
Q.93 A diesel engine is driving an alternator required to run at 1800rpm. The overspeed governor is
required to set within the range of
Q.94 A crankshaft whose center of gravity coincides with its centerline is said to be
(a) statically balanced
(b) dynamically balanced
Ans. (a) statically balanced only.
In case of statically balanced crankshaft the center of gravity through which all the load will pass will coincide
with the center line of the crankshaft. Hence it will not come to the standstill position at the same point again and
again. In such condition, the entire weight of the crankshaft is evenly distributed but this does not mean that
when it starts rotating, the forces which are created at different planes are balanced. The vectors of forces created
because of rotation of crankshaft may be in different planes and they may not balanced though the crankshaft is
statically balanced. For dynamic balance we have to carry out the special procedure of dynamic balancing so that
the vectors created because of the rotation of masses will be balanced with respect to each other and hence there
is no unbalanced vector which is affecting the crankshaft.
Q.101 Between the periods of injection and ignition of a fuel a diesel engine crankshaft rotates through the
Ans. c delay period.
The time between the injection and ignition is called delay period. This delay period is caused because as the fuel
enters the combustion space, it takes some time for it to mix up with the hot compressed air and evaporate and to
form a combustible mixture. Hence it is not possible to ignite the fuel at the instant it enters the cylinder, but it
takes some time and by the time this delay period is lapsed, the combustion space is filled with thorough mixture
of air and fuel so that the combustion can take place efficiently. This delay period depends on the quality of the
fuel. If the fuel is of inferior quality the ignition delay increases. If the fuel is of superior quality the ignition
delay is reduced. If the delay period is excessively high there is a possibility of knocking as well as afterburning
and loss of power.
Q.102 The cylinder liner forming the cylinder wall and the inside of the water jacket is called a
Q.139 The bearing used to support the crankshaft are generally called
Ans. (c) main bearings.
Main bearings are the bearings which support the crankshaft. The full load or the full power developed by the
engine is transmitted to the crankshaft through various moving parts such as pistons, connecting bearing,
crosshead bearing, bottom end bearing and ultimately to the crankshaft. From crankshaft, it is transmitted to the
propeller shaft. At the same time, the downward forces acting on the crankshaft are taken care of by the bearings
which supports the crankshaft. Hence they are called the main bearings as they are the most important bearing of
the system. If they are not of proper strength or are worn out, the crankshaft may get deflected and there is a
possibility that the crankshaft may crack. Also, the complete alignment of the engine is fully dependent on these
bearings; hence they are called main bearings.
Q.140 Injection pressure in a common rail fuel system is controlled
Ans. (d) by a bypass valve.
In common rail system of fuel injection, common piping supplies fuel to the individual injectors by a branch pipe
which is the feature of the common rail system. This pipe and the full system is under high pressure i.e. the
injection pressure. To control this, a bypass valve is utilized which is fitted at the end of the common pipe. By
closing or opening this valve, more or less, the fuel is bypassed from the system back to the suction of the pump,
thus reducing the pressure.
Q.141 In internal combustion engines, the crankcase vent outlets must be equipped with
Ans. (b) corrosion resistant flame screens.
Corrosion resistant flame screens are required on the vent outlets of the crankcase, in internal combustion
engines, in order to prevent spark from outside to enter the crankcase. As the crankcase atmosphere is full of oil
mist, it is only waiting for the spark to come in contact to have an explosion. Under any circumstances, the spark
must be prevented from entering the crankcase. For this purpose, it is necessary to have the flame arrester fitted
on the crankcase breather. At the same time, this flame arrester should be maintained in clean condition so that it
should have better heat transfer property so that it can extinguish the flame before entering the crankcase. It is
therefore necessary that there should be not any rust formation on the screen because of which the heat
transfering capacity and the efficiency of the arrester would be reduced. Taking into consideration the above
conditions, it is necessary to provide a rustproof flame arrester screen on the crankcase vent outlet.
Q.142 What is to be installed on an internal combustion engine if a cylinder bore exceeds 200mm?
Ans. (d) Explosion relief valves.
The engines having cylinder bore that exceeds 200mm are generally highly rated engines with high peak
pressures. At the same time, most of these engines are compressed air started engines. Taking into consideration
these conditions, it is necessary to prevent the excessive build-up of pressure inside the cylinder as this excessive
pressure may put the parts of the engine under heavy stresses, which may cause mechanical failure of these parts.
Hence, in order to avoid this, it is necessary to provide a cylinder relief valve on these engines.
Q.143 Cylinder lubricating oil for a low-speed main propulsion diesel engine is admitted to each cylinder
during
Ans. (b) the compression stroke.
Cylinder lubricating oil, for lubricating the piston rings and the liner, has to be admitted when the piston, piston
rings and the liner are in cool condition and the piston is moving upward so that oil can be retained on the piston
rings and sprayed by the piston rings on the liner walls. This is only possible during the compression stroke.
Otherwise, the piston is hot and if the lubricating oil is sprayed on it, it will evaporate very fast and will not carry
out any work of lubrication. At the same time, if lubricating oil is injected during the expansion stroke, i.e. when
the piston is moving downwards, it will have a scrapping effect rather than lubrication.
Q.144 What is the purpose of the window installed in the housing of an individual jerk-type fuel pump?
Ans. (a) To allow the pump to be timed with the engine.
Through the window of the fuel pump housing, it is possible to see the movement of the sleeve up and down. The
sleeve has one marking only. At the same time, the sides of the window also have one reference mark. When the
reference mark on the sleeve starts moving upward, it indicates that the plunger is moving upward. When the
mark on the upward moving sleeve coincides with the mark on the pump body, it indicates that the upward
moving plunger has closed the suction port of the fuel pump. This is the beginning of the injection. This can be
checked on the flywheel and, if required, can be adjusted. The end of the injection will be dependent on the
amount of fuel to be injected.
Q.145 Which of the following listed construction details of the internal combustion engines is required?
Ans. (a) A warning notice of caution against the opening of a hot crankcase for a specified period of time
after shutdown.
On most of the modern engines, the crankcase doors has been put up with a warning or notice as not to open
before particular time has lapsed. The crankcase is full of oil spray. This oil spray will be spread over the hot
surfaces of the crankcase and will get evaporated into oil mist. This oil mist, when comes in contact with the
hotspots, causes the explosion. Most of the times these crankcase explosions do not occur as the atmosphere in
there is too rich to cause an explosion. If the crankcase is hot due to some reason and oil mist has been already
formed, under such circumstances, if the crankcase door is opened before allowing sufficient amount of cooling,
there will be ingress of fresh air. This will try to dilute the crankcase atmosphere and bring it into explosive range
and an explosion might occur. Thus it is necessary to wait for specified amount of time before opening a
crankcase door.
Q.146 Which statement regarding the arrangement and location of explosion relief valve used on internal
combustion engines is true?
Ans. (b) Minimising the danger from emission of flames is a key consideration.
When a crankcase explosion takes place, the crankcase relief door opens and allows the hot gases to get relieved
through the relief door. Along with this, there is a possibility of a flame emerging from the crankcase relief door.
An arrangement should be made on the crankcase relief door so that this flame and hot gases will not emerge
directly and create danger for the personnel working in the engine room. Hence it has to be deflected to a safer
place. This is carried out by a deflector plate on the outside of the crankcase relief doors. At the same time, inside
of the crankcase relief door is provided with a flame arrester valve which also cools the flame before it emerges
in the engine room atmosphere. Thus it prevents the spreading of fire in the engine room.
Q.147 The term proper metering as applied to a diesel engine injection system can be best defined as
Ans. (a) delivering the same quantity of fuel to each cylinder for each power stroke depending upon the
engine load.
It is necessary to inject particular quantity of fuel oil, for a specified load, in order to have proper combustion of
the fuel and the best thermal efficiency of the engine. Also it is necessary that this fuel oil should be injected at
the right time. In addition, it is necessary that all the units get equal quantity of fuel so that the engine will be
mechanically and thermally balanced. For this purpose metering is done. The process of injecting the right
quantity of oil at the right time and to distribute equally for all the cylinders so as to have the full power
balancing of the engine is called metering. Actually, metering is the word used for measuring the fuel equally for
all units.
Q.149 The rate of fuel injection in a diesel engine cylinder depends primarily on
Ans. (a) the size of holes in the fuel nozzle.
Taking into consideration the size of the holes, the volume of the fuel injected through each hole will depend
upon the diameter of the hole because the length of the hole is constant for a particular fuel injector nozzle. Also,
as the hole size of the injector increases, the droplet size increases. As the droplet size increases, it has more
penetration into the cylinder volume. Hence it can overcome the compression pressure of the cylinder. Taking
into consideration both these aspects, the diameter of the hole of nozzle will be the main governing factor for the
rate of fuel injection.
Q.153 Cylinder diameters greater than 230mm require additional safety devices when the scavenging
spaces are openly connected to the cylinders. Which of the following devices will be used to protect such
spaces?
Ans. (b) Explosion relief valves.
When the under piston scavenge spaces are directly connected to the cylinder, in such conditions if there is
blowpast due to the any reasons (such as broken piston rings, excessive liner wear, seized piston rings in the
groove), it will cause scavenge fire. When scavenge fire takes place, there is a possibility of explosion occurring
inside the scavenge space because of the availability of combustible material, hot gases or sparks blowing past
and plenty air available in the scavenge space. In order to avoid such explosion, it is necessary to relieve the
pressure if it gets generated. This is carried out by the explosion relief valves.
Q.154 A loop or cross scavenge engines utilizes the motion of its piston and turbocharger to provide scavenge air.
Which of the listed mechanical design prevents the air under the piston from being pumped back through the
scavenge ports during the piston travel?
Ans. (b) length of the piston skirt.
Q.159 The duration of fuel injection developed by the individual port and helix fuel injection pump is
determined by
Ans. (b) effective pump stroke.
Effective pump stroke is the stroke when fuel is delivered by the pump. It will not take into consideration the idle
stroke before the up-moving fuel pump plunger covers the suction port or the movement of the fuel pump
plunger after the completion of the delivery stroke. Hence the position the fuel pump will be decided by the time
through which the fuel is actually delivered. When the fuel pump plunger is moving upwards, as soon as it covers
the ports, fuel injection is started. This is the beginning of the effective delivery stroke. As the plunger moves
upwards, depending upon the load on the engine, the rack position of fuel pump will rotate the fuel pump
plunger. Thus, for every load, there will be a definite disruptive position between the helix cut on the fuel pump
plunger surface and the suction port. When the plunger is moving upwards, a point will come when the top of the
plunger, i.e. the high pressure side, will get connected to the suction side thus relieving the high pressure and
causing the end of injection. This is the end of effective delivery stroke. The quantity of the fuel injected will
vary as per the effective delivery stroke which is controlled by the governor through fuel pump rack.
Q.160 The plunger in a jerk pump is rotated until the release port is uncovered. If the port remains
uncovered all the time which of the listed operation will occur?
Ans. (a) no fuel.
When the plunger is moving upwards, the fuel pump as soon as the top of plunger covers the suction port, the oil
on top of the plunger will get compressed and pressure will be developed and injection will commence. As soon
as the top of the plunger will get connected through the helix to the suction port, the suction and the delivery side
will get short-circuited and hence no pressure will be developed on top of the plunger which will cause the end of
the delivery stroke. If the rack position is nearly zero, even though the plunger is moving upwards and is
covering the suction port, the helix position is such that it will always connect the discharge side of the pump
with the suction side of the pump thus causing the short-circuiting and no pressure will be developed throughout
the stroke of the plunger. If no pressure is developed throughout the stroke of the plunger in that case there will
be no delivery of the fuel throughout the stroke of the plunger. This is called as a zero delivery stroke.
Q.161 When the lower edge of the spiral begins to uncover the release port, in a jerk pump,
Ans. (b) effective pumping stroke of the plunger ends.
As the plunger is moving upwards and the fuel is getting compressed, the pressure is developed and it will cause
the fuel injector nozzle to get lifted and the fuel is injected. Depending on the position of the rack which governs
the position of the helix with respect to the suction port, a point will come when the helix bottom portion will
come in contact with the suction port. This will cause the short-circuiting of the fuel oil between the high
pressure and the low-pressure side i.e. between the top of the plunger and the suction side. This will cause a
sudden drop in the delivery pressure due to which the fuel valve will close under the spring pressure thus causing
the end of fuel injection and hence the end of effective delivery stroke.
Q.162 The amount of fuel delivered by a helical plunger fuel injection pump is controlled by
Ans. (c) the rotation of the pump plunger.
The surface of the pump plunger has a helix cut on it. These helical grooves on the surface will be responsible for
changing the effective delivery stroke of the plunger. The moment at which the helical groove connects the
delivery side of the plunger with the suction side will be the end of injection. The position of the helical groove
can be changed with respect to the fuel pump barrel suction hole by means of rotating the fuel pump plunger
while it is moving up and down in vertical position. This movement of the plunger with respect to the pump
barrel is due the fuel pump rack which is connected to the governor. Hence depending upon the fuel load, the
governor will change the position of the rack. Due to this, the fuel pump plunger will rotate and change the
position of the helix with respect to the suction holes. Hence the quantity of fuel delivered will vary as this
causes change in the end of injection timing.
Q.163 Which of the following will occur when lower edge of the spiral on the plunger of a jerk pump
uncovers the spill ports?
Ans. The pressure drops and the fuel delivery stops.
When the lower spiral edge cut of the helix cut on the fuel pump plunger surface uncovers the suction port or the
spill port, it will have a short-circuiting effect between the top of the plunger and the suction side. As the top of
the plunger is high-pressure side, when it is connected to suction low-pressure side the pressure will drop
suddenly on the delivery side. As a result, the fuel pump will have the end of effective delivery stroke and the
fuel valve will close smartly.
Q.165 Which of the component listed is not found in a single acting engine?
Ans. (b) Double crankshaft.
In single acting engine, the action of the fuel burning and load transmission is carried out only on the top of the
piston and during the downward movement of the piston Hence it requires only one crankshaft to transfer the
power.
Q.166 In diesel engines designed with a crosshead the motion of the engine can be described as
Ans. (c) straight reciprocating.
In an engine with crosshead, the piston has a piston rod which is rigidly connected to the bottom of the piston
and the other end is connected to the crosshead. Hence when the piston moves up and down, the piston rod also
moves in a vertical direction thus causing the reciprocating movement which follows the movement of the piston.
At the same time it is passes through a stuffing box which separates the scavenge space from the crankcase
space. This vertical movement of the piston rod is converted into the rotary movement by the use of crosshead
and connecting rod combination.
Q.167 Cast iron piston used in large propulsion diesel engines are constructed with
Ans. (d) the crown being tapered and smaller than the skirt.
The crown of a large engine is always subjected to more heat rather than the skirt and hence the expansion of the
crown is higher than the skirt. Clearance is provided for the expansion. Otherwise, the crown will expand to such
an extent that it may start to touch the liner and it may cause a damage. In order to avoid such things, clearance is
kept at higher valve at the top of the piston. This is achieved by using a tapered crown so that the clearance is
maximized at the top of the crown where the temperature is maximum and reduced at the bottom where the
temperature is minimum.
Q.168 Which of the manufacturing method is recommended diesel engine fuel injection line piping?
Ans. (c) Seamless drawn.
The diesel engine fuel injection piping is subjected to very high pressure, in the range of about 1200bar. Also,
there is sudden increase and decrease of pressure which causes the fatigue in the material. Taking into
consideration these two things it is necessary that the material and the manufacturing process of the piping
should be such that it should be able to withstand the high pressure and fatigue. It is also of prime importance,
from the point of view of safety of the ship and fire hazard, that the fuel pipe should not get cracked under these
high pressures. In order to achieve these requirements, it is necessary to use fuel pump pipes made up of
seamless, solid drawn tubes.
Q.170 Which of the combustion parameters listed is use in diesel engine but not related to the injection
system?
Ans. (c) Effective stroke.
Effective stroke is not directly related with the injection system. It will control only the quantity of fuel which is
injected. It will not be related to any of the parameters required for efficient combustion such as atomization,
metering or penetration.
Q.172 Proper atomization of fuel in the diesel engine combustion chamber will
Ans. (b) improve the combustion.
Proper atomization means breaking the fuel oil droplets into fine particles or to inject the fuel under the influence
of high pressure which will convert the fuel oil into fine spray. This will cause more surface area of the fuel
available for the contact with air. As the surface area increases, the fuel and air mixing will be properly done. At
the same time, because of availability of more area, the fuel will evaporate fast and will form the combustible
mixture which is the main important point from the point of view of greater combustion of the fuel and
increasing the thermal efficiency of the engine.
Q.173 What is required for crosshead type engines that have a scavenging space in open connection to the
cylinder?
Ans. (c) The scavenge space to be permanently connected through an approved fire extinguishing system,
entirely separated from the extinguishing system of the engine room.
When the scavenge space is connected to the under side of the piston, there is every possibility of scavenge fire if
the blowpast of the hot exhaust gas takes place due to worn out cylinder liner or broken piston rings or piston
ring cut off in the groove. In such circumstances, the hot gases or the spark, entering the scavenge space, will
start burning the sludge and accumulated oil inside the scavenge space. As plenty amount of air is available this
scavenge fire may continue for a longer period and it may also go beyond control. Hence it is necessary that a
permanent arrangement must be provided so that if all other means, such as stopping of the engine and covering
the exhaust and the turbocharger suction filter with a cloth, fail. In such circumstances, it should be possible to
extinguish the fire by the use of a permanent extinguishing medium which is connected to the underside of the
piston scavenge space mostly CO2 or steam is utilized. The requirement for this is very small and it should be
independent of the engine room system because if it is connected to the engine room fire fighting system, it may
possible that you may discharge the complete safe battery of the cylinder which are actually kept for the engine
room fire. In such condition, engine room fire fighting equipment capacity will be reduced. Hence the scavenge
space fire fighting installation should be entirely independent of the engine room fire fighting installation.
Q. 174 Which of the following statement is an accurate description of the fuel injection piping used in a
diesel engine with a cylinder bore of 250mm and above?
Ans. (b) The piping is to be effectively shielded and secured to prevent fuel or the fuel mist from reaching
the source of ignition on the engine or the surrounding.
In case of engines of high capacity or those utilized in UMS ships, it is necessary that if the high-pressure pipe do
burst, the oil or the oil mist should not come in contact with hot surfaces such as exhaust manifold so as to
prevent the fire. For this purpose, it is necessary to double-walled the high pressure injection piping between the
fuel pump and the fuel valve. Hence in case of any rupture in the pipe, the outcoming high-pressure oil will not
get injected directly over the hot surfaces and will not cause fire in the engine room. Also in the double-walled
pipes, the outer portion of the pipes is connected to a leak-off tank whereby if any high pressure pipe does crack,
the leak-off will led to the leak-off tank where it will give an alarm which indicates that there is a crack in the
fuel piping
Q. 223 The rate of pressure rise during the period following fuel ignition in a diesel engine is influenced by
the length of ignition delay period due to the
Q.224 Lower water seal on a diesel engine wet cylinder must allow for liner axial movement. This seal is
most commonly a
Ans. (a) neoprene o-ring.
Neoprene o-ring is fitted at the bottom of the cylinder jacket which seal the cooling spaces from the crankcase.
Hence prevents the cooling water which is circulating around the liner from entering the crankcase. At the same
time, because of the heat generated due to running of the engine there is always a linear expansion of the liner; so
this should also be taken careof. So neoprene is the right rubber material which will allow the rings to expand in
the axial direction. At the same time, it will do the sealing job so as to prevent the water from the jacket enter the
crankcase. Hence it forms a flexible and movable seal between the jacket and the liner.
Q.225 Which of the device listed is installed in a diesel engine to isolate some of the crankshaft vibration
created by rotational and reciprocating process?
Ans. (b) Torsional vibration dampener.
Torsional vibration dampener is fitted either on the engine crankshaft and driven by it or it separately fitted in the
engine room platform or in the steering room are controlled by electric motors. These torsional vibration
dampeners consists of rotating masses. The frequency of the vibration caused by these masses will be out-of-
phase with the original frequency of vibration. Hence it will nullify the frequency caused by the rotation of the
crankshaft thus total frequency of vibration is reduced.
Q.226 Differential type fuel oil nozzle in diesel engine are closed directly by
(a) spring pressure
(b) fuel oil pressure
Ans. (a) spring pressure only
In a fuel injection nozzle, which operates on hydraulic pressure of the fuel oil, the opening of the nozzle is
carried out by fuel pressure which is developed by the fuel pump. This pressure will lift the nozzle against the
spring pressure and thus injection will take place. As soon as injection pressure drops in order to have the
efficient and positive sealing between the needle and landing face. it is necessary to close the needle by use of
spring force against the seat so it will prevent the dripping. At the same time, the hot gases will be prevented
from entering the fuel valve nozzle from the combustion space.
Q.227 Modern marine diesel engines equipped with mechanical injection operates on a combustion cycle
which is
Ans. (c) a combination of constant volume and constant pressure.
In diesel engines when the fuel is injected near to the TDC the piston is nearly in a standstill condition and
change of volume is very low. During that time, when the fuel is injected it burns under the constant volume
condition. This results in sudden increase of pressure near the TDC. Hence the volume is kept constant but the
pressure increases. When the fuel starts burning and piston starts to move towards the BDC position, during this
period the fuel burns and the pressure rises. Even though the volume increases due to the movement of the
piston, the pressure remains constant though the volume changes. So the fuel injection and burning process is
divided in constant volume and constant pressure phase.
Q.228 An efficient seal between cylinder block and cylinder head on many diesel engines is obtained with
Ans. (d) gasket.
It is necessary to form a gas-tight combustion space between the top side of the piston and the underside of the
cylinder head. This space has to seal high-pressure gases and this is possible only if the seal is perfect and
without any leakage. For that purpose, it is necessary to have some sealing arrangement between the cylinder
head and liner. In earlier days this used to be a metal-to-metal contact but to maintain a high-pressure seal with
this is a difficult process. Also, there is a possibility that some leakage may take place and the liner and the
cylinder head landing face may get damaged. In order to avoid this, a soft metal gasket is utilized which is fitted
between the cylinder head and liner. When the cylinder head is tightened against the liner flange, the softness of
the gasket will allow it to flow into the small deformities which will be formed between the cylinder head and
liner, thus forming a perfect seal.
Q.229 Oil for piston cooling is delivered through the connecting rod to a compartment in the piston head.
Then, it is distributed as a result of piston motion and finally drained to the crankcase through one or
more holes or pipe. This process is known as
Ans. (c) shaker method.
In the piston cooling, the cooling medium, the oil or water, is conveyed to the underside of the piston crown by
the pipes. There are two pipes for inlet and outlet. The outlet pipe is kept slightly higher so that the piston cooling
face will always be having some amount of cooling medium places. This cooling medium gathers the momentum
of the piston when the piston moves up and down. Due to this, the medium gets the agitating movement and
because of this it splashes against the hot piston crown underside and gives better cooling effect rather than only
jet or any other method. At present, most of the engines are using a combination of jet and agitation method, also
called the shaker method, for piston cooling. This combination gives better cooling efficiency and better running.
Q.232 The breaking up of fuel oil as it enters the diesel engine cylinder is known as
Ans. (c) atomization.
When the fuel enters the cylinder, it is at high pressure and it passes through the small holes of the nozzle tip.
Due to this, when it is injected through the nozzle tip, it gets broken up into tiny particles which are useful in
efficient burning of the fuel. This is known as atomization.
Q.233 The exhaust ports of a diesel engine using the crossflow scavenging method are closed by
Ans. (c) the reciprocating motion of the piston.
Scavenge and exhaust ports, on a cross flow scavenging system, are cut on the wall of the liner at different level.
When they are cut on the liner wall at different level, the opening and closing of these ports are carried out with
the piston movement. As the piston moves downward towards BDC, the piston top will uncover the exhaust ports
first and exhausting takes place. When it moves further it will open the scavenge ports and scavenging will take
place and scavenge air will enter the units. This is called unit injection.
Q.234 Which of the bearing listed below is most widely used for main and connecting rod bearing of a
modern high-speed diesel engine?
Ans. (b) Precision insert.
The precision insert bearings are factory made bearing which are just to be inserted into the bearing keep and no
scrapping or * of the bearing is required. These bearings are mostly thin shelled white metal bearing and they
have got better fatigue resistance and high load carrying capacity. Hence they are called precision insert bearings.
Most of the engines and bearings, where there is fluctuation of load and heavy loading such as crosshead
bearings and the bottom end bearing, the thin shelled aluminium bearings are utilized. Even on many engines,
main bearings are also made up of thin shelled bearings.
Q.235 Valve rotors are commonly used on which of the listed diesel engine cylinder head valves?
Ans. (c) Exhaust valve.
When the exhaust gas pass through the exhaust valves it is having some unburnt carbon particles. This particles
will get lodged or get in between. So when the exhaust valve closes there is a possibility that the carbon particles
may get lodged between the valve and the valve seat. If the carbon particles remains continuously in that same
position, there is a possibility that carbon will burn on the valve seat and valve. Due to this, the valve and the
valve seat will get damaged and dents will be formed on the valve seat and life of the valve will be reduced. In
order to avoid this, it is necessary that the valve should be rotated so that it will have a self cleaning effect and
whatever carbon is deposited between the valve and the valve seat, it will get removed due to the rotation of the
valve. This will increase the life of the valve. Hence it is necessary to fit a valve rotor on the exhaust valve so as
to keep the valve rotating while it is opening and closing.
Q.236 Exhaust valve opening in a diesel engine cylinder head are made as large as practical to
Ans. (c) reduce the pumping loses associated with scavenging.
When the exhaust valve is opened and the piston is moving in upwards on exhaustion stroke, there should not be
any backpressure on the piston. When there is backpressure on the piston, it will consume the energy from the
engine for exhausting the gas from the engine through the exhaust valve. Hence in order to reduce these pumping
losses, it is necessary that there should be no restriction for the exhaust gas to flow through and there should not
be any backpressure on the piston because of the exhaust gases. This is possible only by providing sufficiently
large exhaust valve area so as to avoid backpressure and loss of power because of pumping effect.
Q.239 In the cylinder head of two-stroke diesel engine valves are used for
Ans. (d) exhausting combustion gas.
The two-stroke engine mostly works on uniflow scavenging system. In uniflow scavenging system, the exhaust
valve is fitted on the cylinder head and the inlet ports are cut on the cylinder liner at much lower position. The
cylinder head valves are only used to take out the exhaust gas. The inlet of air is from the bottom position so that
there will be complete sweeping action of cylinder liner will take place and there is a less possibility of exhaust
gas mixing with the incoming air. Hence unidirectional scavenging will give better efficiency. Therefore exhaust
valves are fitted on the cylinder head and the inlet ports are cut in the liner walls.
Q.241 The intake ports of two-stroke diesel engines are opened and closed by the action of the
Ans. (b) piston movement.
On a two-stroke cycle diesel engine, the exhaust and inlet or only inlet ports are cut in the walls of the cylinder
liner at much lower height from the cylinder top. Hence the opening and closing of these ports are controlled by
the vertical movement of the piston. When the piston moves down and uncovers the ports, scavenge air enters the
cylinder. At the same time, when the piston is moving upwards and closes the scavenge ports, the scavenging
process stops. If the exhaust valve is also closed, compression stroke starts.
Q.242 Compared to four-stroke diesel engines two-stroke diesel engines have the disadvantage of
Ans. (b) high cylinder head temperature.
In two-stroke engine, for every revolution there is one power stroke i.e. for every revolution there is one fuel
firing. This means that the total power developed in individual cylinder head is more. This contributes to more
heat being developed by the engine as compared to the four-stroke engine. So the cylinder head temperature in
two-stroke engine is more than that of four-stroke engine. In reality, the exhaust temperature of a two-stroke
engine is lower than the four-stroke engine because during the scavenging process the air is driving the exhaust
gas. Along with exhaust gas, some amount of cold air is expelled from the cylinder due to which the mean
temperature recorded by the exhaust gas thermometer will always be lower, in case of two stroke engines as
compared with four stroke engines. Theoretically the exhaust temperature will be on the higher side in two-stroke
engines than four-stroke engines.
Q.243 Which of the listed conditions will affect the mean effective pressure the most in a diesel engine
cylinder?
Ans. (c) Completeness in the mixing of fuel and air.
When the fuel and air is completely mixed and fuel is fully vaporized, the combustion efficiency will always be
better because the fuel will burn efficiently in the allocated time and resulting in high peak pressure. When the
peak pressure increases automatically, the mean indicated pressure also increases and more power will be
developed by the engine.
Q.244 In comparing engines of equal horse power higher exhaust gas temperature occurs in a
Ans. (d) four-stroke cycle diesel engine.
In four-stroke cycle diesel engines, the mixing of the incoming air and outgoing exhaust gas through the exhaust
valve is less because the expelling of the exhaust gas is carried out by the movement of the piston and not by the
incoming air in case of two-stroke cycle engine. In two-stroke cycle engine, the expelling of the exhaust gas is
carried out by the incoming air. Hence, there is mixing of fresh air with the exhaust gas and some amount of
incoming air is being exhausted along with the exhaust gas through the opened exhaust valve. Therefore exhaust
temperature registered by the thermometer always drops. But, in case of four stroke engines, the exhausting is
carried out by the upward movement of the piston and it is not much assisted by the incoming air. So in the
exhaust manifold less amount of incoming air is exhausted. Hence the exhaust temperature of four-stroke cycle
diesel engine is more than two-stroke cycle diesel engine for the same amount of power developed.
Q.246 Which of the bearing listed are most widely used for the connecting rod and main bearing of a diesel
engine?
Ans. (c) Precision insert.
Q.251 Which of the bearing listed is widely used as connecting rod and main bearing on a modern diesel
engine?
Ans. (d) Precision bearing.
Precision bearing are thin shelled bearings having more load bearing capacity and resistance to fatigue. These are
factory manufactured with precision clearance maintained having thin layer of bearing materials.
Q.255 Telescopic pipes to the piston of a large slow speed main propulsion diesel engine are designed to
prevent
Ans. (c) contamination of lubricating oil by water.
When the piston is water-cooled and water is conveyed to the underside of the piston crown, it has to be
conveyed through the telescopic pipes because there is a continuous vertical movement of the piston. To achieve
this, telescopic pipes are utilized which will give the sliding sealing between the stationary pipes and
reciprocating piston. For this, there is telescopic and gland packing is utilized in order to prevent water from
leaking into the crankcase; otherwise crankcase contamination will take place in crankcase. Telescopic pipes,
arrangement along with the carbon seal provided, will prevent water from contaminating the crankcase oil.
Q.257 Large slow speed main propulsion diesel engine uses sea water to directly cool the
Ans. (c) scavenging air.
Scavenging air temperature before cooling is in the range of more than 100C which is to be cooled to about
40C. At the same time, the quantity of air available is also very large and air is having a low thermal
conductivity. Hence to cool such large quantity of air through such a large temperature difference it is necessary
that cooling medium should have a good cooling efficiency. Otherwise if it is cooled by any other medium in
such cases the cooler has to be very big. Also, seawater being a convenient medium can be thrown overboard
after cooling and new abundant cooling medium is available. Hence cooling is carried out by seawater directly
rather than indirect cooling or by using any other medium. Seawater cooling is most efficient method of cooling
scavenge air.
Q.258 The purpose of mist detector in a main propulsion diesel engine is to warn of
Ans. (a) excessive mist density in the crankcase.
Oil mist detector continuously samples the atmosphere of the crankcase and compares it with the average
atmosphere or the atmosphere in the engine room. Hence any change in the density of oil mist will be
immediately revealed or brought to notice by means of audio and visual indication. The high density in the
crankcase is a direct indication that there is hotspot present in the crankcase because of which the oil is getting
evaporated forming oil mist which may lead to crankcase explosion if ignored. Hence high oil mist density alarm
which is given by oil mist detector is a safety alarm and immediate action should be taken inorder to avoid future
destruction to the engine as well as the danger to personnel working in the engine room.
Q.259 One method of constructing large marine diesel engine and reducing the frame weight is through
Ans. (b) welding plates to form the section for assembly.
In earlier days the frame and the bed plate assembly were made up of casting. Compared to the casting the plates
which are rolled has got more strength and by assembling it properly and proper welding, it is possible to
construct a structure which has got more strength for the same weight. Hence it is possible to use a welded
structure so that the weight of the structure is reduced and its strength is increased. Hence in present generation
of engines, the longitudinal girders are made up of steel plates. They are welded in between with the transverse
girders which are made from the casting because their intrinsic structure is difficult to fabricate. The total
welding of these two different parts of bedplate has been generated into a box type bedplate which has high
strength and low weight.
Q.260 Which of the fuel injection system listed below uses a spring-loaded differential spray needle valve
and an individual pump for each cylinder?
Ans. (c) Jerk pump fuel injection.
In the jerk pump fuel injection, a sudden pressure rise of fuel is achieved. This rise is conveyed to the needle
valve where the high-pressure wave acts under the conical surface of the needle because of which the needle gets
lifted against the spring compression. Hence the oil is conveyed through the spray holes and it is injected inside
the cylinder. As soon as the suction and delivery of the pump are short-circuited the sudden drop in pressure of
fuel in the pump takes place and hence the pressure in the fuel valve drops. Under such circumstances the spring
pressure is more than the oil pressure and because of the spring pressure the valve snap closes over the seat and
prevents further injection of fuel or leakage of the fuel. This is one of the best methods of injecting the fuel at
right time and without unnecessary dripping of the fuel from the engine.
Q.261 Concerning diesel propelled vessel the astern power is provided for continuous operation astern
Ans. (b) at 70% of ahead rpm at the rated speed.
Most of the diesel engines are designed from the point of view of running ahead. The fuel pump timing, the
exhaust valve timing and the distribution of the weight of the rotating parts is made in such a way that there is a
balance in ahead direction. Hence the best possible efficiency of the engine as far as fuel is concerned is in the
ahead direction due to proper timing for the exhaust and fuel valve. At the same time, there is minimum torsional
vibration because of dynamic balancing which is taken care off at the design stage from the point of view of
ahead running. If the engine is runned at astern direction, the combustion efficiency reduces. At the same time,
because of unbalanced forces, there will be more amount of vibration. Due to profile of the ship and the
efficiency of the propeller, for the same rpm, the power required for running astern is always more than that of
what is required for running ahead. If the engine is runned at 100% rated rpm in the astern direction it may be
excessively overloaded. Hence it is always provided to give astern power at 70% of ahead rpm at the rated speed
Q.262 The crankcase of many diesel engine are kept at slightly vacuum by
Ans. (b) crankcase exhaust fans.
(In the olden days, this was very practice but not utilized presently.) By keeping the crankcase ventilated by use
of exhaust fans fitted on the crankcase, the atmosphere in the crankcase is kept slightly under vacuum. By this,
we are depriving the oil mist of air. If even little oil mist is present it will not cause any major explosion as it may
be excessively rich and it will lack of air and hence possibility of explosion will be reduced because of
excessively rich mixture which will be above high explosive limit. But this method is no more used as if there is
any leakage there is every possibility that air may enter through the leaky crankcase doors. At the same time,
because of the vacuum in the crankcase, more amount of air may leak through the under-piston scavenge through
the stuffing box into the crankcase or excessive blowpast may take place in case of four stroke engines.
Q.263 Which of the fuel injection system combines the fuel pump and nozzle in one housing?
Ans. (b) unit injector.
Unit injector is a combination of fuel pump and injector without any high-pressure pipe connected between fuel
injector and fuel valve. Hence some advantages such as elimination of pipes and reducing injection delay are
achieved.
Q.264 In unit injector upper and lower helix are machined in the lower part of plunger for
Ans. (c) accurate metering of fuel oil.
By having the helix cut on the plunger and suction hole on the barrel, and positioning the suction hole with
respect to helix by means of rotating the plunger with the help of fuel pump rack controlled by the governor, it is
possible to control the fuel injection timings. Due to the double helix cut on the plunger surface, it is possible to
control the beginning of injection and end of injection. Thus both the beginning and end of injection is controlled
thus controlling the quantity of fuel injected. Thus proper metering of quantity of fuel depending upon the load
and changing the fuel injection timing with respect to load is achieved by the double helix cut over each other in
the fuel pump plunger surface of unit injector.
Q.265 Regarding the diesel engine crankcase the general arrangement and installation should preclude the
possibility of
Ans. (a) free entry of air to the crankcase.
The crankcase atmosphere of a diesel engine is full of large amount of small oil droplets floating in the
atmosphere. At the same time, if any hotspot is created in the crankcase oil mist may also be formed which is
susceptible to burning, if sufficient amount of air is available. As hotspot and oil mist is present, only air is
required so that explosion can occur. If the entire atmosphere of crankcase is entirely filled and because of oil
mist, formed due to evaporation of oil, is rich in concentration and is above the upper explosion limit and the
chances of explosion are less. But if air is allowed to enter the crankcase, through the crankcase doors or by any
other means, then dilution of the oil mist will take place and it passes through the upper and lower explosive
limit. When it passes through this range, in presence of hotspot, this oil mist may explode and chances of
explosion are very high. This is the main reason that notices are put on the crankcase doors that the door should
not be opened for atleast half hour after the engine has been stopped.
Q.267 What function is provided by the crankcase ventilation system on some diesel engines?
Ans. To reduce the pressure in the crankcase.
Crankcase ventilation system, whether it is forced or ordinary ventilator with a flame arrestor fitted, its job is to
produce pressure inside the crankcase because in the crankcase oil in the form of droplets and numerous hot
places, such as bearings and pistons, are always present. Due to this, continuous evaporation of oil is taking
place. If these vapors are not removed from the crankcase, then the crankcase starts getting pressurised. In that
case, oil starts leaking from the crankcase doors. At the same time, consumption of crankcase oil will increase. If
oil mist increases continuously there is a possibility that it may pass through the explosive range and cause
explosion. Hence to avoid this things it is necessary that the crankcase of the internal combustion engine should
always be ventilated so that whatever gases or oil mist are accumulated will be reduced to safe limit.
Q.268 For diesel engine piston cooling lubricating oil can be supplied to the piston by
Ans. (a) oil spray.
For cooling of the piston, lubricating oil is the best and convenient medium. Oil is conveyed to the underside of
the piston by different methods: In case of four stroke engines, it passes from the main bearing to the bottom end
bearing; from bottom end bearing through the drilled passages in the connecting rod, it will be supplied to the top
end bearing where the nozzle is fitted which will inject the oil to the underside of the piston. In case of two stroke
engines, the oil may be supplied through the crosshead and the telescopic pipe, which are fitted inside the piston
rods, the oil is supplied to the underside of the piston where it will be injected through the drill passages or
nozzle so that efficient cooling can be done rather than any other method. In present engines, combination of
nozzle and shaker method of cooling rather than normal single nozzle or shaker method is used which gives
efficient cooling of the piston. (At present *RTI engine is using the combination of spray and shaker method.)
Q.269 The purpose of delivery check valve used in a diesel fuel injection jerk pump is
Ans. to prevent the backflow of oil from the pipe to the pump and also to prevent the emptying of the
delivery.
This pipe has to be filled every time so that pressure is built up resulting in delayed fuel injection and the
injection timing will not be proper. This valve shuts at the end of injection stroke thus preventing the emptying of
injection pipe and backflow of fuel to the pump. At the same time, the valve is designed in such a manner that the
closing action of the valve under the spring and returning oil is delayed slightly. Due to sudden closing of the
valve, backpressure and the pressure wave created is allowed to get released and prevents the secondary
injection. Also this valve prevents the fuel from entering the fuel valve till sufficient pressure is developed. Thus
in general the valve job is to avoid the backflow and secondary injection and delivery of fuel to the fuel injector
before the pressure is sufficiently high.
Q.272 For any piston ring to operate smoothly without scuffing, the ring must be
Ans. (b) properly lubricated.
In a diesel engine, when in operation the piston moves up and down vertically. During this motion the piston
rings are forced radially outward to have very high contact pressure between liner and piston rings thus forming a
sealed combustion space. Under this extreme pressure between the piston rings and liner, it is necessary to have a
thin film of lubricating oil so as to reduce the coefficient of friction between them. If the lubricating oil film is
not present the coefficient of friction will become excessively high because of which, under extreme pressure,
excessive heat will be generated and due to this heat, instantaneous welding between the liner and piston will
take place. At the same time, due to the vertical movement of the piston ring, this weld will get worn out thus
causing the scuffing. In order to avoid this scuffing, it is necessary that the coefficient of friction should be
reduced. This is only possible if sufficient amount of lubricating oil is spread over the liner.
Q.273 The upper piston ring in large two stroke cycle main propulsion diesel engine are most efficiently
lubricated by oil
Ans. (a) fed from a mechanical lubricator.
In a large slow speed engine, the stroke of the piston is large. At the same time the piston is separated from the
crankcase by a diaphragm and a stuffing box. Also the oil required for the cylinder lubrication is of different
characteristics, i.e. high TBN, as compared to crankcase lubricating oil. Hence taking into consideration all these
aspect it is not possible to have the flash lubrication as it is carried out in the small (four stroke) diesel engines.
Also as it is slow speed, it is difficult to splash oil to such a height. In case of two stroke engines, the oil has to be
injected into the cylinder at appropriate time so that it can be spread over the liner wall and it will be burnt and
go out as exhaust. For this process the necessary timed lubrication is possible only with the help of mechanical
lubricator which are controlled by the camshaft and crankshaft. Thus it is best method of lubricating the piston
ring and in latest engines, like Sulzer RTA, the mechanical lubricator are in conjunction with the accumulator
principle. Some of the modern engines have electronically controlled cylinder lubrication to give a minimum
quantity of lubricating oil with best possible effect and prevent excessive burning of oil. At the same time, it
avoids weardown because of lack of lubrication. All this is possible because of mechanical or electronic
controlled cylinder lubricating system.
Q.274 Fuel injector nozzles are usually of the multi-orifice type with the number and placement of the
holes arranged according to the
Ans. (d) design of the combustion chamber.
Design and geometry of the combustion chamber and the placement of the fuel injector in the combustion
chamber is so that oil can be sprayed evenly in the combustion chamber and it will be injected deep into the
combustion chamber so that it can completely mix with the amount of air available in the combustion chamber.
At the same time, it should not be injected so deep that it will not burn over the wall of the liner or on the piston
top. This can be achieved only be achieved by taking into consideration the geometry of the combustion chamber
and the number of holes in the fuel injector and the angle at which oil is injected so that the best thermal and fuel
combustion efficiency is achieved without harming the engine. For this, it is necessary to take into consideration
the geometry of the combustion chamber and design before deciding the number and diameter of holes and the
angle at which the holes are drilled in the fuel injector nozzles.
Q.275 The delivery valve installed in the port and helix fuel injection pump is designed to
Ans. (b) maintain the column of fuel in the line.
The delivery valve is fitted at the delivery side of the fuel pump and it opens against the spring force when the
plunger is moving upwards in the delivery stroke. As soon as the delivery stroke ends i.e. the short-circuiting
takes place between the suction and delivery side of the fuel pump, sudden pressure drop takes place on the top
of the plunger. As a result, the fuel from the pipe connecting the pump to the injector will return back to the
suction side. Hence it may be possible that this pipe becomes empty and everytime, for the next stroke again, you
will have to fill this pipe before the fuel injection starts which will cause the excessive delay in fuel injection.
Hence inorder to avoid this, it is necessary to maintain a column of oil at a slight pressure inside the pipe
connecting the fuel pump to the injector. This is carried out by the shutting of the delivery valve fitted on the
pump so as to avoid the back-flow of fuel from the injector to the pump. Also, this valve is made in such a
manner that there is slight delay in closing time that it will prevent the excessive development of the secondary
pressure wave and secondary injection is also avoided.
Q.277 Additional explosion relief valve are fitted in separate spaces in the crankcase such as gear or chain case
for camshaft or similar drives when the
Ans. the gross volume of such place exceeds 0.6d x .340m3
Q.278 If all the conditions such as bore, stroke, speed and mean effective pressures are equal, a two-stroke
cycle diesel engine will develop approximately
Ans. (c) twice the indicated horsepower as developed by four-stroke engine.
This is because in two-stroke diesel engine, for every revolution of the engine there will be a power stroke. The
power stroke is responsible for the development of the horsepower. Hence if all the parameters are equal then it
is the number of power strokes which governs the power developed by the engine. For every power stroke of a
four-stroke engine, it will have two revolutions of the crankshaft whereas a two-stroke engine produces power
strokes for each of its revolution. Hence two-stroke engine with all equal parameters will develop twice the
power developed by four-stroke engine.
Q.279 In order to reverse the rotation of the two-stroke cycle loop scavenging direct reversing propulsion
diesel engine the cam position must be changed for
Ans. (a) starting air fuel pumps.
For reversing the engines, it is necessary that it must be started in reverse direction. For that purpose it is
necessary to admit the starting air on the piston before TDC rather than after TDC. Once the air is admitted
before TDC, the engine will run in the reverse direction. This is possible by changing the air starting distributor
timing so that the air starting valve will open much early TDC and hence the engine will start in reverse
direction. In order to keep the engine running in reverse direction, it is necessary to alter the fuel injection timing
also because fuel pump injection timing is not symmetrical over the dead center position. It is also necessary to
adjust the timing of inlet and exhaust valve but being a loop scavenging or cross scavenging engine, where the
inlet and exhaust valves are absent and the opening and closing of the ports is controlled by the movement of the
piston, this alteration is not required.
Q.280 Viscous damper as used on a marine diesel engine is a sealed precision built device which dampens
the torsional vibration in the
Ans. (c) crankshaft.
By providing the viscous fluid inside the sealed chamber, the vibration which are created by the firing frequency
of the units on the crankshaft is dampened by providing an equal and opposite dampening frequency so that total
torsional vibrations are reduced. In order to reduce the torsional vibration, the fluid gives an opposite torque to
the vibrations, which has the dampening effect on the vibrations.
Q.281 The rotary motion of the crankshaft is obtained from the up and down motion of the piston via the
Ans. connecting rod or crosshead and piston rod assembly.
The vertical movement of the piston has to be converted into rotational movement of crankshaft. This is achieved
by providing a connecting rod which is connected at one end to the gudgeon or crosshead pin and at the other end
to the crank pin which is eccentric with respect to the arrangement of the crankshaft. Because of this arrangement
whenever the piston moves up and down the crankpin makes a complete rotation and thus converts the vertical
movement of the piston to the rotary movement of the crankshaft.
Q.282 Indirect cooling of a fuel injector nozzle holder of a diesel engine is accomplished primarily by
Ans. (b) heat conducted into the water jacket wall.
Most of the system cylinder nowadays are made to cool by bore cooling systems. In the bore cooling system, the
water for cooling is brought very near to the combustion space. Due to this water comes very near to the bore in
the cylinder where the injector is fitted and it is circulated very near the to the seat in the cylinder head where the
injector is fitted. Thus this water is brought so close to the tip of the nozzle and with sufficient amount of water
circulation, heat can be taken out indirectly from the injector through the cylinder cooling water and thus cooling
of the injector is carried out. Also, the circulating fuel oil helps in carrying out the cooling of injector.
Q.283 Diesel engine main and connecting rod precision bearing are made in two half. Each half exceeds
one-half bearing circumference by a small amount. The small amount is termed
Ans. (b) the crush.
This crush is highly essential so that when two halves of the bearing are fitted over each other and has been
tightened by bolts the bearing halves, they will have easy flow into the bearing housing and will have a good first
contact with the back side of the bearing housing thus preventing rotation of bearing in the bearing housing. At
the same time, the load of the shaft is transmitted on the bearing because thin shelled bearing due to its thinness
they are unable to carry out load on their own and so they require some backing and this backing is possible
through the crush. Also due to this crush, there is a perfect contact between the back of the bearing and the
bearing housing and this provides thermal conductivity between the bearing and the housing. Thus it keeps the
bearing cool. This crush provides the interference fit between the bearing and that of the housing.
Q.284 The port and helix metering pump used in diesel injection systems are usually designed to produce a
constant beginning and variable ending of injection. These pumps are usually
Ans. (b) timed for port closing and are controlled by the rack movement.
In helix controlled pump, when the plunger is moving upwards and the plunger top closes the suction port in the
barrel, the further movement of plunger upwards will create a hydraulic pressure on top of the plunger which will
cause the fuel valve to open and fuel injection starts. Thus the fuel injection starts when the plunger closes the
ports. At the end of injection even the port is covered by the plunger which is connected with the discharge side
of plunger by means of helix and that terminates the delivery. The quantity of the fuel depending upon the load of
the engine will depend upon the variable end of the delivery. The later the end of delivery, more is the fuel
injected. This is controlled by the rack which controls the position of the plunger helix with respect to the suction
port on the barrel thus it controls the quantity of the fuel injected.
Q.285 The end of injection in a port and helix metering pump is controlled by
Ans. short cycling or connecting the delivery ports with the suction ports in the barrel.
When the piston is upwards and it has already covered the suction port the fuel is being injected. As it is moving
upward, a time comes because of the helix cut on the plunger surface which short-circuits the discharge side of
the pump with that of suction side and it will cause the end of the delivery. Hence the combination of helix,
which is cut, and the suction port will decide quantity and point of termination of fuel injection. The position of
helix with the port cut in the barrel will be decided by the rack position which will rotate the fuel pump plunger
with that of the barrel.
Q.287 For a given size, the two-stroke diesel engine will deliver more power than a four-stroke diesel
engine because
Ans. (c) it develops more power stroke at the same speed.
It is wellknown fact that in case of two-stroke engines for every revolution there is one power stroke whereas in
four stroke engines for every two revolution there is one power stroke. Hence it is natural that, with same size of
the engine and same parameters, it is only the number of strokes which will be responsible for the quantity of the
power developed. As two-stroke engines have more power strokes compared to the four-stroke engines, it will
atleast theoretically produce more power than a four-stroke engine.
Q.288 In a jerk pump, the amount of fuel that is forced through the spray nozzle on each upward stroke
depends on
Ans. (c) how the plunger is rotated.
In a jerk type fuel injection pump, the quantity of fuel delivered by the injector into the combustion chamber will
depend on the effective delivery stroke i.e. the beginning of the delivery stroke is mostly constant and the end of
the delivery stroke is responsible for the quantity of the fuel which is delivered. The quantity of fuel delivered
will depend on the load of the engine. Hence more the load more fuel is required and vice-versa. Under such
circumstances, it is the plunger which is rotated by means of rack connected to the governor. Due to the position
of governor, the rack position changes depending upon the load on the engine. As the rack position changes the
rack will make the plunger to rotate with respect to the barrel and under such circumstances the helix which is
cut on the plunger surface will change its position with respect to the suction port on the barrel. This relative
position of the helix to the plunger surface with respect the suction hole on the barrel will decide quantity of fuel
injected which decides by the termination of the delivery stroke of plunger of fuel pump.
Q.289 Modern marine engines using mechanical fuel injectors operates on a combustion cycle which is
Ans. (a) combination of constant volume and constant pressure.
When mechanical fuel injectors are fitted on diesel engines, as soon as the fuel injection starts, i.e. when the main
piston is near to TDC, the volume is nearly constant as there is no much vertical movement of the piston. When
the fuel is ignited, it catches fire and as the volume constant it develops high pressure. In this condition, the
pressure is kept constant as the volume increases. So part of fuel ignition takes place at constant volume. As the
piston turns over the TDC and start moving downwards, some amount of fuel is still injected. During this time,
the fuel is injected and is burnt, the pressure rises which pushes the piston downwards. As the piston moves from
TDC to BDC, though the volume increases, the pressure is kept constant. Hence the ignition of fuel is carried out
partly at constant volume and partly at constant pressure. This combination method gives better thermal
efficiency.
Q.290 The amount of fuel oil delivered by the fuel injector is controlled by the
Ans. (c) rack position.
Unit type of fuel injector pump is nothing but a jerk type of fuel pump and a fuel injector together. Hence
whatever is applicable to the jerk type of fuel pump is applicable to the unit type of system. In unit injector, it is
the rack position which will control the position of helix on the plunger with respect to the suction hole on the
barrel. Hence the quantity of the fuel decided by the position of the rack which will decide the position of helix
on plunger surface with that of suction port on the barrel.
Q.291 The diesel engine valve which is subjected to most severe condition of service is
Ans. (a) the cylinder exhaust valve.
It is wellknown fact that the exhaust valve is subjected to very high temperature and pressure of the exhaust gas.
At the same time these exhaust gas which passes over the exhaust valve will consist of some unburnt particles of
carbon, ash, silica, or alumina from the fuel. Under such circumstances, when these gases pass over the exhaust
valve, they try to settle over the valve because of which it will cause the valve to burn or severe dent may be
formed on the valve seat. At the same time, the heat to which this valve is subjected the strength of the material
will reduce and hence there is a possibility of deformation of valve and valve seat. Hence taking into
consideration all these aspects, it is necessary that proper care of the valve should be taken. If the engine is
burning heavy fuel oil where the concentration of contamination such as ash, alumina, silica, unburnt carbon is
bound to be more, for self cleaning purpose and uniform thermal distribution on the seat of the valve, it is
necessary to make the valve rotate on the seat.
Q.292 For a given fuel, a change in compression ratio will affect the ignition lag by which of the listed
means?
Ans. (b) An increase in compression ratio will decrease the ignition lag.
As the compression ration increases, the quantity of air available for combustion also increases. At the same
time, the temperature of air at the end of compression stroke will also increase. Due to this the fuel which is
injected will vaporize faster and mix properly with the air and thus it will ignite instantaneously and thus reduce
the ignition lag.
Q.293 The lower section of the piston is called the skirt of the piston. This skirt of the piston is utilized in
case of four stroke engine to transfer the
Ans. athwart ship thrust to the liner wall and also guides the piston in the liner.
In case of two-stroke engine, the skirt is very small and it is only in the initial stages that this skirt is utilized as a
rubbing guide which are used only when the piston and liner are new.
Q.295 Fuel oil discharged to the diesel engine cylinder is atomized at the
Ans. (b) fuel injector nozzle tips.
As the high-pressure oil passes through the fuel injector nozzle tips, which are having the very small holes, they
are broken up into tiny particles. This process is called atomization. This is possible by passing the fuel through
the holes of the injector under high pressure. Thus converting the pressure energy of the fuel into kinetic energy
partly and thus breaking the fuel into tiny particles which is responsible for rapid evaporation and mixing with air
in the cylinder and better thermal efficiency of the engine.
Q.298 Why is it necessary to compress the air charge in the cylinder of a diesel engine?
Ans. (a) To ignite the fuel.
When the air is compressed in the cylinder of a diesel engine, it is compressed adiabetically. Due to this the
temperature at the end of compression increases to the self-ignition temperature of the fuel. Under such
circumstances, if the fuel is injected in a highly atomized condition inside the combustion space, the fuel will
ignite immediately and convert the chemical energy of fuel into mechanical energy of the engine.
Q.299 Which of the following statement concerning a closed type fuel injection nozzle is true?
Ans. (c) The most closed nozzle open inwards under the pressure acting on the differential area of the
needle valve.
In the fuel injector, the needle sits over the seat by the spring pressure. When high-pressure fuel oil enters the
fuel nozzle, it acts over the annular space and the differential area of the needle valve. As this fuel pressure acts
over a greater area, an upward force is created which overcomes the downward force of the spring thus lifting the
needle upwards. As the needle is lifted, the fuel will pass between the needle and the seat. It will further act over
the needle area thus increasing the area of pressure acting and lifting it further up and because of this, the full lift
of the needle is possible. Under such circumstances, the fuel under high pressure will pass between the needle
and seat and inject through the nozzle holes. As soon as at the end of injection when the pressure drops, the
spring force will be greater than the hydraulic pressure acting on the differential area of the needle and the needle
will snap close because of the action of the spring.
Q.300 In many diesel engines exhaust valves are being constructed with hollow stem filled with sodium in
order to
Ans. (c) assist in dissipating heat due to the extreme operating temperature.
Sodium is a good conductor of heat and as it does not evaporates easily there is less possibility of gas or vapour
pocket formation taking place and obstructing the cooling process. Thus when the stem is filled up with sodium
the liquid sodium will transfer the heat in a better way and keep the valve cool. Hence in the older engine this
method was utilized but at presently it is not utilized due to the development taken place in the material of the
exhaust valve and the cooling of the exhaust valve seat with water.
Q.301 Turbulence of the compressed air charge in the diesel engine cylinder increases
Ans. (c) the efficiency of the engine.
When the turbulence of the compressed air increases, it increases the movement of the air in the combustion. Due
to this, the air sweeps the complete volume of the cylinder with that of the exhaust gas and hence all the exhaust
gas from the cylinder will be removed and there will not be any gas in the cylinder. Thus the cylinder will be
charged with pure quality of air. Secondly, this motion of air will aide in thorough mixing of air and fuel. Thus
creating a homogenous mixture throughout the combustion space. Hence having a better combustion of the fuel
and increase in the thermal efficiency of the engine.
Q.302 Thin brass rings are inserted in the face of some chromium-plated piston rings to
Ans. (a) promote the piston ring sealing in the cylinder.
Piston rings are coated in order to prevent corrosion. At the same time, because of chromium coating the surface
of the piston rings become hard and it takes long time to weardown and form a proper sealing between the
cylinder wall and piston ring sliding surface. Hence there is a possibility of improper sealing between the piston
and liner surface and because of this blowpast of hot gases will take place. Also the chromium coating will
increase the time for breaking-in and running-in period. Hence in order to have better sealing between piston
rings and liner and give corrosion resistance to the piston rings, they are coated with chromium and an insert of
brass is put inside the sliding surface so that the brass insert will wear down fast and have a proper bearing with
the liner and provide a good sealing.
Q.303 In the Otto cycle the oil fuel is ignited at what time and by what means?
Ans. (c) Just before TDC by spark ignition.
In Otto cycle, the compression pressure is very low and hence it is not possible to achieve the air temperature to
that of the self-ignition temperature of the fuel. Hence it is necessary to ignite the fuel by means of spark ignition
just before TDC.
Q.304 The critical speed in diesel engine occurs when engine torque pulsation becomes
Ans. (b) resonant with the crankshaft natural frequency.
Engine torque pulsation takes place due to the units firing at different positions of the crankshaft. Lesser the
number of units, less pulsation will take place and higher the number of units higher the pulsation will take place
because the angular difference between the two firing strokes will be quite large. Also it will depend on the size
of the flywheel. Bigger the flywheel, lesser will be the pulsation on the engine torque. In addition to this, there is
a natural frequency of vibration in the crankshaft. If the natural frequency of vibration of crankshaft and torsional
vibration coincides, these two frequencies will resonate and total frequency will be excessively high causing
excessive vibration and damage to the engine. Hence the speed at which this happens is called critical speed.
Q.305 A diesel engine piston ring pitting face is in direct contact with
Ans. (b) cylinder liner oil film and bottom of ring groove.
The sliding face of the piston ring will be in direct contact with the vertical face of the cylinder liner wall. In
order to reduce the coefficient of friction between these surfaces and to avoid the scuffing and blowpast, it is
necessary to maintain the lubrication between the surfaces which is carried out by cylinder lubrication or splash
lubrication. Also when the cylinder is moving downwards, the pressure acts over the piston ring and it forces the
piston ring radially outward against the liner walls. At the same time, it forces the bottom of the piston ring
against the bottom portion of the ring groove. This sealing is also important to maintain the pressure behind the
piston rings. If this sealing is not maintained, the piston ring will not seal properly and blowpast will take place.
Q.308 Barrel faced, tapper faced grooved and chromium platted are all types of diesel engine
Ans. (b) piston rings
These are different shape and coatings of piston rings. They are provided to spread the lubricating oil over the
liner walls, to have a scrapping effect of excess lubricating oil or to reduce the corrosion effect of fuel on the
piston rings. They provide sealing between the piston and liner and accelerate the process of sealing between the
liner and piston rings when they in new condition i.e. to reduce the breaking and running in time.
Q.309 Injectors that use heavy fuel oil must be cooled by water or light oil to
Ans. (d) avoid carbonization of nozzle tips.
When engine is burning heavy fuel oil it is necessary to cool the piston and nozzle tips. As the nozzle tips
becomes hot the seat and needle looses its strength and because of this the proper sealing between seat and
nozzle is not achieved and the fuel injector will start leaking. Under such circumstances even after the
completion of injection, when the needle is sitted on its seat, some amount of minute drops of fuel will leak
through nozzle holes and it will get accumulated on the nozzle tips. This accumulated oil will burn at high
temperature and, due the local burning, form a carbon formation in the form of like a flower near the tip of the
injector thus damaging the injector and fuel injection pattern. Hence it is necessary to maintain the temperature
of the fuel injection nozzle tips within certain limit so as to avoid loss of strength and leakage of fuel through the
tips. Hence cooling is an important factor of fuel injector nozzle burning heavy fuel oil. Also as the fuel oil itself
is heated to high temperature and hence the cooling effect of the fuel oil is comparatively low than in case of
diesel engine fuel injector burning diesel oil where the fuel oil injected during every stroke does the cooling job.
Q.310 Which of the equal power diesel engine listed running at the same speed is least affected by the
exhaust gas backpressure?
Ans. (a) Four-stroke cycle V-type of engine.
In case of four-stroke engine, for every two revolution there is a power stroke so that the time between two
power stroke is more so that the exhaust gas, which is discharged into the exhaust manifold, will get dissipated
through the turbocharger before the next unit exhaust valve opens. Hence there is a less possibility of
backpressure. In addition, if we are using V-type of engines, where there are two banks of cylinders, they are
exhausting alternatively into the two banks of exhaust manifold. Hence there is a further delay in the exhausting
of two cylinders in the same exhaust manifold. Hence less amount of backpressure is created in four-stroke V-
type engines as compared to two-stroke engine with any configuration (in-line, or V-type).
Q.311 In a diesel engine, the time taken to heat the fuel particles turned into vapors and bring about
combustion is called
Ans. ignition delay.
The time from which the first particle of the fuel enters the combustion space through the fuel injector till the
time the first ignition starts is called the ignition delay. During this process, the fuel is atomized, vaporized,
mixed with the available air so as to forms a combustible mixture is called ignition delay. It will depend on the
compression pressure and the quality of the fuel. If the compression pressure is low or the fuel quantity is bad,
the ignition delay will be more. Higher the ignition delay there will be more amount of power loss due to
afterburning effect.
Q.312 In a diesel engine the contact surface of a piston compression ring are those in contact with
(a) back of the ring groove.
(b) bottom of the ring groove.
Ans. (b) bottom of the ring groove.
For sealing between piston ring and liner, the high pressure gases has to find its way behind the piston rings and
has to force the piston rings against the cylinder wall. This pressure behind the piston rings is to be maintained in
order to get the proper sealing between them. This is possible if there is proper sealing between the bottom of
piston ring and bottom of the ring groove. If the sealing between them is not properly maintained, the gas which
is acting from behind the piston ring will get leaked off. As this pressure is lost, there will not be sufficient
pressure between the piston ring and the liner wall and the high pressure gasses will force the rings inwards will
cause the blowpass and ring to collapse and break. Hence sealing between the between the bottom of groove and
bottom of piston rings is also of prime importance.
Q.313 The thermal energy produced by internal combustion engine is transformed into
Ans. (d) mechanical energy.
When the fuel burns on top of the piston inside the cylinder, it is the chemical energy of the fuel which is
converted into heat energy. This heat energy and the pressure energy together is acting over the piston thus
forcing the piston downwards, thus converting this energy into mechanical energy to the crankshaft. Thus it is the
thermal energy which is converted into mechanical energy by the engine.
Q.314 The reason that some two-stroke diesel engine piston rings are pinned to prevent rotation is
Ans. (c) to keep the ring ends from catching in the scavenge ports.
When the rings are pinned in the ring grooves, they are prevented from rotation. The rings can be initially set up
in such a manner so that the ends of piston rings will not come in way of scavenge and exhaust ports and thus it
will be prevented from being damaged because of getting entangled with the scavenge and exhaust ports and thus
avoid their breaking of the piston rings. Hence in older engines the pinning was done to prevent the rotation of
the rings and to keep the rings in their original position (at which they are fitted) when they are removed for
overhauling and the rings are in new condition. In modern engines, the ends of the rings are turned slightly
inwards so that when they expand due to heat, they will not get entangled with the scavenge and exhaust ports
and will not break. At the same time, the rings are allowed to rotate in the groove to give the self-cleaning effect
of the groove and to have the even distribution of heat over the entire circumference of the groove. Hence the
pinning arrangement is no longer in use.
Q.317 The primary purpose of oil control rings on a diesel engine piston is to
Ans. (c) prevent excessive lubricating oil consumption.
In a four stroke trunk piston diesel engine the liner lubrication is carried out with the help of splash lubrication
and large quantities of lubricating oil is splashed by the waves of the crankshaft onto the liner walls. This oil is
necessary to spread properly on the cylinder liner and farther lubrication between the piston rings and liner wall.
At the same time, excess oil is returned back to the crankcase otherwise it will get burnt along with the exhaust
and lubricating oil consumption will increase. At the same time, fouling of turbocharger and manifold will also
take place. Inorder to avoid this things, oil control rings are provided the job of which is to scrap the oil and send
it back to the crankcase and to avoid excess consumption of the crankcase oil.
Q.318 The compression ratio of a diesel engine refers to the ratio between the
Ans. (b) cylinder volume at top dead center and that of the bottom dead center.
At the bottom dead center, there is large amount of volume available in the cylinder and air is charged into the
cylinder at this volume. If the piston moves upward, due to the proper sealing of the compression space, the air
will get compressed. At near TDC, the volume of air will be reduced but its pressure will increase. The ratio
between the cylinder volume at TDC and BDC is called compression ratio.
Q.319 In describing basic diesel engine operation the term events refers to
(a) production of high pressure gases .
(b) removal of expanded combustion gases.
(c) admission of air to the cylinder.
(d) All of the above.
Ans. (d) All of the above.
In any diesel cycle, there are four events: fuel firing and high pressure gas is produced which will drive the piston
downwards thus producing the power stroke. At the end of power stroke, the exhaust valve will open and
exhausting will take place by which the exhaust gas will be removed which is the exhaust stroke. After
completion of exhaust stroke, when the piston is moving downwards the cylinder will be filled with fresh air
charge which is called the suction or induction stroke. This continues till the piston reaches TDC. When the
piston is moving upwards all the valves are in closed condition the air which is available on top of the piston will
get compressed to produce high pressure and high temperature air in which the fuel will be burnt. This is the
fourth event or the compression stroke. All these events together will form one cycle. This cycle can be
completed in one revolution (two-stroke) or two revolutions (four-stroke) of the engine.
Q.321 The slots are provided in the diesel engine piston oil scrapper rings to remove excess oil from the
cylinder liner and to scrap and send it back to the crankcase.317
Q.323 How are the temperature and pressure affected in a diesel engine cylinder during compression?
Ans. In the diesel engine adiabatic compression takes place because of which the pressure of air inside the
cylinder increases. Theoretically as there is no heat transfer the temperature also increases to the surrounding
hence they both increase during compression.
Q.324 Modern diesel engine using mechanical fuel injection operates on combustion cycle which is
Ans. a combination of constant volume and constant pressure.
Q.327 At bottom dead center the centerline of the connecting rod usually coincides with the
Ans. (c) centerline of the cylinder.
At the bottom dead center position the connecting rod will be exactly vertical and the piston will be at the lowest
position of the cylinder liner. Under this position, the centerline of the connecting rod and the cylinder must
coincide to get the proper alignment between the liner and the piston. If this does not take place, there is a
possibility that the liner will weardown on one side and the piston rings will weardown on one side and the
piston may get seized in the liner. Hence the most important part is to have coincide the liner and connecting
centerline when at TDC of BDC.
Q.331 A 7-cylinder two-stroke cycle single acting diesel engine with cylinder indicated horsepower
calculated to 1350kW and the brake horse power measured at 7466kW has a mechanical efficiency of
Q.332
Q.335 Vibration from the diesel engine and engine driven equipments are isolated from the hull structure
by
Ans. (d) flexible engine mountings.
Flexible engine mountings consist of shock absorbing material, such as rubberized cork, so that it will absorb the
vibration created by the machines and those vibrations will not be transferred to the ship structures or vice-versa.
They are known as elastomount and they have got the property of dampening the vibrations. Only care should be
taken while tightening them that they should not be overtightened; otherwise they will burst. Hence the
tightening of the bolts should be carried out with the help of torque
Q.336 Which of the following statements in a fuel atomization in a diesel engine cylinder is correct?
Ans. (b) Greater the atomization lesser the penetration.
For greater atomization, it is necessary to break the fuel droplets into microscopic particles. When they are
reduced in such manner their weight reduces and hence their inertia also reduces thus reducing the force at which
it will drill through the dense atmosphere of air reduces. Thus the potential energy of the particle reduces and it
cannot penetrate the dense atmosphere of the air in the combustion chamber. Hence as the atomization increases
the penetration decreases and vice-versa.
Q337 The most rapid period of fuel injection occurring in a diesel engine cylinder should begin just before
the piston reaches the TDC position
Ans. (d) should be completed after the TDC.
In diesel engine, the best possible efficiency is obtained if the fuel is burnt partially at constant volume and
constant pressure. For achieving this, the fuel injection and ignition must start just before TDC and it should
completed after the TDC. During this time, the quantity required as per the load should be injected and burnt
efficiently. If the complete combustion takes place before or after TDC, the thermal efficiency will reduce.
Q.338 When comparing different fuel for different engines the ignition quality of diesel fuel oil becomes a
less critical consideration as
Ans. (d) the engine speed decreases.
As the engine speed decrease the time for the fuel to atomize evaporate and mix with air to form a combustible
mixture is ample. When ample time is available, it is possible to burn lower grade of the fuel with low speed
engine. Hence when low grade of fuel is utilized, less critical consideration is given to the speed of the engine
because lesser the speed of the engine inferior is the quality of fuel which can be burnt efficiently.
Q. 359 The power developed by slow speed main propulsion diesel engine depends upon the:
(a) quantity of air takes in and retains in the cylinder during a given time period.
(b) proportion of trapped air that is utilized in the combustion process.
(c) thermodynamic efficiency of the engine cycle.
(d) All of the above.
Ans. (d) All of the above.
The power developed by diesel engine depends upon the how efficiently the fuel is burnt. The quantity of the fuel
that burns will depend upon the quantity of the air which is available at the end of operation stroke. This depends
upon the how much quantity of air taken inside during the suction stroke. At the same time, it will depend upon
the efficiency. Secondly when air is compressed whatever the quantity of air that is retained at the end of the
operation is also important factor. Otherwise if the piston rings are leaking or broken or stuck up, the quantity of
air is taken during the induction stroke will be higher. The retention of the air at the end of compression stroke
will be low. Under these circumstances, the power developed will be reduced. Hence the power develop will
depend upon the quantity of air taken during the induction stroke and quantity of air which is available at the end
of compression stroke in the combustion chamber. Lastly, it depends upon the thermodynamic efficiency of the
engine because higher the thermodynamic efficiency of the engine, the more efficient burning of the fuel and
more power develops for the same quantity of fuel. Hence it can very well be said that all three are most
important factors which will decide the power development.
Q.341 The cylinder lining constructed as an integral part of a block are characterized by which of the
following disadvantages.
Q.342 One end of diesel engine cylinder is sealed with the hot air and the other end by the
Ans. (b) piston.
Air is present on top of the piston and this air is compressed between the cylinder head and top of the piston thus
forming a combustion space holding high pressure and high temperature air where the fuel injected will burn and
produce the power. Hence the sealing for the compressed air is possible because of the heat which is existing
between the cylinder head and the piston.
Q.346 During fuel injection period the fuel pressure must exceed cylinder gas pressure to
Ans. (a) insure penetration and distribution of fuel in the combustion chamber.
When the fuel is injected by opening the nozzle of the fuel injector, the flow of the fuel is into the high-pressure
air. This high pressure air will try to oppose the entry of the fuel. Hence the fuel, which is entering, has to drill
through the dense atmosphere to move in further away from the tip and mix up with the complete quantity of the
air available in the combustion space. For that purpose, the pressure at which it is injected should be always more
than the compression pressure in the cylinder so that penetration deep into the dense atmosphere is possible.
Q.348 Telescopic pipe through the piston of a large of a slow speed main propulsion diesel engine are
designed to prevent
Ans. (c) contamination of the lubricating oil by water.
As the telescopic pipe has to convey the water to the piston, it has to pass through the crankcase or scavenge
space. By having the telescopic pipe, it is possible to convey between the stationery part of the pipe to the
reciprocating part of the compressor. This is possible with the help of telescopic pipe and the gland.
Q.349 The exhaust system for a turbocharged two-stroke diesel engine functions to
(a) discharge exhaust gases and smoke.
(b) permit energy to turbocharger.
(c) reduce the engine room noise.
(d) All the above.
Ans. (d) All of the above.
As the exhaust valve opens, the high pressure exhaust gas, which is slightly higher than the atmospheric pressure,
will get release from the exhaust from the cylinder. It will pass through the exhaust valve towards the
turbocharger. Thus it does the function of removing the exhaust gas and the smoke from the unit. Secondly, this
energy which is available in the exhaust gas can be utilized which may be combination of pressure and heat
energy can be utilized for driving the turbocharger so as to supply the pressure as per the engine. Thirdly, when
the exhaust gas is released from the exhaust of the engine unit it enters the exhaust manifold, which is also
bigger size, dampening of the exhaust gas will take place in the manifold and because of this it will reduce the
noise created by the engine.
Q. 350 Which of the diesel engine exhaust muffler listed is usually equipped with spark arrester?
Ans. (c) A dry-type exhaust muffler.
Dry-type exhaust muffler has been fitted with a flame or spark arrestor in the form of a copper. This will cool
down the exhaust gas coming out and prevent any flame or spark from the engine to get distributed on the desk.
Q.351 When monitoring diesel engine performance the most useful instrument to be used is
Ans. (b) exhaust gas pyrometer.
Exhaust gas pyrometer gives rough estimation of the power developed by the engine. Higher the power
developed by the engine more the exhaust temperature. At the same time, it can also monitor any discrepancies
or problems encountered by the engine. If the fuel valve is dripping or the fuel nozzle is not spraying properly or
the fuel ignition timing is not proper, even then large variation in temperature will take place. If the pistol rings
are leaking, immediately compression pressure and the peak pressure will reduce but the exhaust temperature
will be high. The exhaust temperature can be considered as a most important tool to determine the performance
of the engine and diagnose the possible cause of the malfunctioning of the engine or dropping of the performance
of the engine.
Q.352 The exhaust system of the diesel engine usually designed to remove exhaust gases and to
Ans. (d) muffer the exhaust gas noise.
The pressure waves when the exhaust gas is released from the cylinder has been dampened by the use of
muffler where in the direction of the exhaust gas changes. This causes the dampening of the noise wave
and causing the total dampening of the exhaust gas noise.
Q.353 The water jacket is placed around the exhaust manifold of propulsion diesel engine to
Ans. (a) reduce the radiation of the engine room.
When the exhaust gas manufold is water cooled, heat is carried out by water and less amount of heat is radiated
from the exhaust manifold into the engine room. Thus keeping the engine room cool. At the same time it protects
the engine from exhaust manifold fire because the temperature of the exhaust manifold here is less than 240 C
thus the chances of fire is reduced and hence it acts like a safety device.
Q.354 In a two-stroke cycle diesel engine the exhaust gases are expelled from the cylinder by
Ans. (c) pressure of the fresh air charge.
In two stroke cycle engine the exhausting is carried out with the help of incoming air and not by the movement of
the piston as in case of four stroke engine. When the piston is moving down and when the exhaust valve opens,
after certain lapse of time the scavenge ports open. By time the scavenge ports open, the gas pressure inside the
cylinder has dropped sufficiently. When the scavenge air under the pressure higher than the exhaust enters the
cylinder through the scavenge ports, at the same time, the exhaust valve is also opened. This incoming air from
the exhaust manifold will drive the exhaust gas out through the already opened up exhaust valve.
Q.355 Diesel engine muffler or silencers reduce the engine exhaust noise by
Ans. (d) reducing the exhaust gas velocity.
When the exhaust gas is passed through the muffler it is made to pass in the zigzag manner. When passing in this
manner, everytime it changes the direction and its velocity drops. As a result, vibration caused by the exhaust
gas also reduces and hence the noise is dampened and the muffler effect is carried out.
Q.357 Exhaust gases in two-stroke cycle diesel engine are discharged through
Ans. (c) exhaust ports or valve.
When the piston is moving downward and the exhaust ports are cut on the liner. As soon as the downward
moving piston uncovers the exhaust ports, the exhausting takes place. This will continue till the time the piston
when it is moving up again closes the exhaust ports. This process is the combination of exhausting and
scavenging. In case of the engines in which valves are fitted in the cylinder head, the exhausting is carried out by
opening the valve when the piston is moving down. As soon as the valve opens the pressure in the cylinder drops
and further movement of the piston downward opens the scavenge ports through which the scavenge air enters
the cylinder liner and forces the exhaust gas upward through the already opened up exhaust valve.
Q.360 In a single acting diesel engine, the cylinder liner area that is most difficulty to lubricate is
Q.362 Which of the following design features will reduce the possibility of overheating the top compression
ring of cast iron piston?
Ans. (d) Heat damp design is sometimes used in the piston head.
Due to the heat damp design which is incorporated in the piston, the piston rings are not directly subjected to heat
from combustion. As a result, the life of the piston rings increases and lubrication between the liner and piston
rings increases and the weardown of the piston ring and the liner reduces. This is used to prolong the life and
overheating of the top piston ring.
Q.363 Which of the following statements concerning the factors affecting ignition delay is correct?
Ans. (b) An increase in coolant temperature will decrease the ignition delay.
The coolant surrounds the liner and it takes out all the heat from the compressed air at the end of compression
stroke through the liner wall. If the jacket cooling water temperature is maintained at higher level, the heat
transfer from the compressed air to the cooling medium reduces automatically. When this happens, the
temperature of the air at the end of compression stroke remains at higher level. If the air temperature remains at
higher level, it helps in fast evaporation of fuel and good mixing of fuel and air instantaneously and ignite it.
Hence the delay period is reduced.
Q.365 In a large slow speed propulsion diesel engine the force applied to the engine piston is
Ans. (c) against the crosshead during the power and compression stroke.
During two-stroke cycle engine compression for every revolution there is one compression and power stroke.
During the power stroke the piston is moving downwards under the force of high-pressure gases produced on the
top of the piston due to the combustion of fuel. Hence the force acts downwards and it is taken by the bottom half
of the crosshead bearing. When the piston is moving upwards, during compression stroke, there is another half
revolution of the engine and the piston is moving towards TDC. During this time, the exhaust valve is closed
scavenge ports are closed and the air on top of the piston gets compressed so that next power stroke can
commence. Hence during the compression stroke, because of compression of the air, the force acts downwards
on the crosshead bearings. As both, during compression as well as power stroke, the force acts downwards, it is
difficult to lubricate the crosshead bearing
Q.366 One of the advantage in the use of dry liner over wet liner
Ans. (c) there is likely leakage of water into the combustion space,
In case of dry liner, there is no cooling medium involved around the liner or in between the liner and cylinder
block. As there is no cooling medium involved, there is no possibility of water leaking into crankcase and
contaminating the lubricating oil, as in case of wet liner because in wet liner there are O-rings which seals the
water space around the liner with that of the crankcase.
Q.367 Which of the construction technique listed is used on new piston rings to facilitate running or
seating?
Ans. (a) Special ring facing such as thin bearing surface of anti-friction metal.
When the piston rings are new, their sliding surfaces are not even which may cause long time to get properly
worn out and having a proper bedding between the liner and the piston rings. Hence there is a possibility of
blowpast and scuffing may take place. For this purpose, it is necessary to increase the cylinder lubrication when
new piston rings are used and running in is carried out. Inorder to accelerate the running-in process and to have
better sealing between the piston rings and liner, the piston ring sliding surface is coated with anti-friction
material such as tin-aluminium, aluminium-bronze and nickel-graphite. These materials being soft will
weardown faster and will form a good seal between the piston ring sliding surface and liner surface. This coating
will get worn out over the number of hours slowly, and in the meantime, there will be proper bedding of liner and
piston ring because the liner will worn out to form a good surface sealing between the liner and the piston ring.
Hence by providing the anti-friction coating on the sliding surface, the running in time will be reduced. At the
same time the requirement of cylinder lubrication is reduced.
Q.368 In diesel engines the four basic events: suction, compression, power and exhaust are formed once in
Ans. (a) one complete revolution in two-stroke engines and two revolutions in four-stroke engines.
In two-stroke cycle engine, the complete cycle is completed in two strokes or one revolution of the engine. For
this purpose, the four basic events are completed in one revolution or two stroke so that, for every revolution
there will be one power stroke.
Q.369 The intake air ducting in diesel engine should not drop from an area on deck where flammable
vapours released from tanks are present because
Ans. (b) the engine may overspeed and normal governor or overspeed trips will not be able to stop or
secure the engine.
When engine is drawing air rich in hydrocarbons, it acts like additional fuel to the engine. Thus whatever the
governor has allowed the fuel pump to admit, in addition to the fuel vapours present in air suction, will also act
as additional fuel. This fuel will cause the overspeeding of the engine. Under such circumstances, even though
the governor will try to reduce the engine speed by reducing the fuel, the fuel coming along with air intake, on
which we dont have any control, will continue to burn and the engine will rotate at the overspeed irrespective of
the governor cutting off the fuel supply. Hence the governor will not be able to control the speed. Even if the
overspeed trip operates and cuts off the fuel supply, as the engine is still rotating, it will draw air from the
atmosphere which is rich in hydrocarbons and continue burning inside the cylinder and cause the continuation of
running of engine irrespective of overspeed trip.
Q.371 The purpose of delivery check valve used in the diesel engine fuel injection jerk pump is to
(a) assist in quick cut-off of fuel injection .
(b) prevent fuel oil backflow from the injection pump.
Ans. (b) prevent fuel oil flow from the injection pump.
Quick cutting off of the fuel injection is carried out with the help of seal valve or the helix cut on the side of the
fuel injector pump. Hence the delivery valve will not come into picture. Delivery valve will only prevent the
backflow of the fuel from the fuel pump and fuel valve into the suction side of the fuel injection pump.
Q.372 In a diesel engine, the advantage of precombustion chamber over the open-type combustion
chamber is that
(a) it permits coarser fuel atomization
(b) it allows low fuel injection pressure
Ans. Both (a) and (b) possible.
As the precombustion chamber is a small chamber hence the atomization is not proper. The quantity of air
entered with which the amount of fuel has to mix up is also very small and proper mixing is not done. When
precombustion takes place, the ignition wave, formed because of precombustion, will form a turbulence effect in
the combustion chamber that will allow the proper mixing of air and fuel. Hence even though the atomization in
the initial stage is not proper or it is coarse, it will not have much affect on the performance of the engine. As the
precombustion chamber is a small chamber, there is no necessity of penetrating the air deep into the chamber and
hence requires low injection pressure as compared to open type combustion chamber as the depth of penetration
is low.
Q.373 A large slow speed main propulsion diesel engine must be operated with one cylinder secured when
the engine stops the affected cylinder stranded in the position preventing the engine from being started.
Which of the following actions must be taken?
Ans. (c) Admit the starting air in the position opposite to the desired direction.
There is a possibility that when one unit is cut-off the maneuverability of the engine reduces and when the engine
stops it stops in such a position that the affected unit may come under the starting position. Under such
circumstances, air will not be admitting to that unit and it is not possible to start the engine. Under such
circumstances, it is better to reverse the engine i.e. to start the engine in reverse direction, some other unit may
come under starting position and air will be admitted to the unit and the engine can be started. When the engine
starts, it must be stopped and restarted in the desired direction. There is a possibility that the engine might be
stopped in such a position that air will not be admitted to the affected unit but air can be admitted to other units
and the engine can be started in the desired direction.
Q.374 The average exhaust temperature of a two-stroke cycle diesel engine with a turbine-driven
turbocharger is lower than a similar four-stroke diesel engine running at equal load because
Ans. (d) the opening of two stroke cycle diesel exhaust ports or the valves occurs much later than in case of
four stroke cycle diesel engines.
In two stroke cycle diesel engines, the exhaust opens much later than the four-stroke cycle engine. Hence the
speed of the engine is low, and hence to get more amount of power, more amount of heat energy is released from
the exhaust gas on top of the piston to convert into power. Hence when the exhaust valve opens late, as in case of
long and superlong stroke engines, the temperature has been sufficiently dropped down so as to have lower
temperature at the exhaust valve.
Q.375 Oil control rings used in two-stroke diesel cycle engines are located near the bottom of the piston
skirt inorder to
Ans. (c) keep the excess oil away from the intake and exhaust ports.
In two-stroke engines, the oil rings or scrapper rings are provided so as to distribute the oil over the surface of the
liner rather than scrapping effect which is required in case of four-stroke engines. Hence the oil should be spread
upwards over the liner walls and it should not get washed away into the exhaust or scavenge ports. Hence the oil
control ring is provided. They increase the liner area covered by the oil film because the lower scrapper rings,
which are provided, will spread the oil over the entire length of the liner and hence it will cover the entire length
of liner. These rings were provided in the older MAN engines where there was long skirt was present and
because of proper spreading of lubricating oil over the liner, it was possible to have some amount of lubricating
oil between skirt and liner. Thus when the skirt and liner have a mechanical contact the friction is reduced
because of the presence of the oil film but in modern engines these rings are no more utilized.
Q.376 Which of the following statement represent the function of the compression rings installed in the top
of the diesel engine piston?
Ans. (a) Transmit the heat from the top of the piston to the cylinder liner.
The piston is always running hotter than the cylinder liner and the cooling of piston is a difficult task. Hence
when the piston rings are keeping sliding contact over the liner surface, some amount of heat is also transferred
from the piston rings to the liner. While doing so, the piston gets itself cooled and is prevented from overheating
and this reduces the piston ring weardown.
Q.377 In a diesel engine the time period between the fuel injection and fuel ignition is usually defined as
Ans. (d) ignition delay.
It is the time difference from the time the fuel is injected into the combustion space till the time the actual fuel
firing takes place. This will depend upon quality of fuel, compression pressure and the atomization and
penetration of the fuel.
MOTOR OPERATION
Q.378 A loose crosshead plunger assembly in a metering or proportionating pump will cause
Ans. (c) rapid wear on the plunger packing.
If there is excessive clearance between the plunger and barrel, there will be radial movement of the plunger in the
barrel due to which the packing between the plunger and barrel will get worn out on one side, thus causing the
damage and rapid weardown of the packing. Hence this plunger clearance has to be kept as low as possible and
the plunger and barrel assembly should be replaced when the clearance exceeds the limit.
Q.379 In a naturally aspirated diesel engine the volume of air intake is directly related to the engine
Ans. (b) valve size.
Valve size will govern the cross sectional area of the intake port. If the valve assembly is of bigger size more area
will be available for the air to enter and less amount of resistance will be available for air to enter because of
which the quantity of air drawn will be more. Hence volumetric efficiency of the engine will increase.
Q.380 A naturally aspirated diesel engine running at full throttle will have an intake manifold pressure
Ans. (a) slightly less than the atmospheric pressure.
When the piston is moving downwards in a naturally aspirated engine and the air intake valve is open, air enters
with a velocity inside the cylinder. As a result, there will be movement of air which will increase the velocity of
air which passes through the air intake manifold to the cylinder. When the velocity increases, as per Bernoullis
theorem, there is a pressure drop. As the pressure in the intake manifold is lower than the atmospheric pressure,
the atmospheric pressure will drive the outside air into the intake manifold and hence to the piston top through
the opened inlet valve.
Q.381 An increase in air inlet manifold pressure of a diesel engine will result in a
Ans. (c) decrease in fuel consumption per kW.
When large amount of air is admitted into the cylinder of a engine under pressure the large quantity of air can
burn large quantity of fuel in more efficient manner. Hence complete and efficient combustion of fuel is possible
because of the availability of larger quantity of air. Due to this, the thermal efficiency of the engine increases and
thus quantity of fuel consumed per kW per hour reduces i.e. the specific fuel oil consumption of oil reduces.
Q.382 On a large diesel engine installation, crankshaft axial alignment is maintained by the
Ans. (b) engine thrust bearings.
In a large diesel engines, which is used for propulsion purpose, because of the axial thrust created by the
propeller the entire engine shafting will get pushed inwards inside the engine. Due to this there will be
misalignment with the piston, piston rod, connecting rod and bottom-end bearing positions. At the same time,
there is a possibility that due to the axial movement of the crankshaft, the crankshaft web will move and touch
the transverse girder of the bedplate thus causing damage to the bedplate.
Q.383 The crankcase of many diesel engines are kept under slight vacuum
Ans. (c) reduce the risk the explosion.
If the crankcase is kept slightly under vacuum the quantity of air in the crankcase reduces. Under such
circumstances, if the fuel mist is formed it will be rich and it will not fall within the lower or higher explosive
limit. On the contrary, it will be on the higher side of the explosive limit and hence there is less possibility of it
getting ignited.
Q.384 Maintaining a lowest scavenge air temperature at all times is not recommended due to the
possibility of
Ans. (c) the formation of excessive quantity of condensate.
If the air temperature is dropped too much then excessive condensate will be formed which will interfere with
proper burning of the fuel. At the same time, it will wash out the cylinder lubrication. This condensate, along
with the sulphur present, will burn and form sulphuric acid. Hence it will give more amount of wear because of
corrosion. At the same time, the scavenge spaces and the lower part of the piston may get rusted.
Q.385 On a turbocharged medium speed diesel engine which of the following problems is an indication of
restricted air inlet passage?
Ans. (a) Engine is hard to start.
In case of turbocharged engine, if the inlet passage is choked or the turbocharger air suction filter is choked, at
the time of starting or during running, sufficient amount of air will not reach the engine cylinder and a point may
reach that the combustion will not be possible and thus the engine will have difficulty in starting on fuel. It will
turn on air, but as soon as it changes on to fuel, because of lack of availability of air it will stop.
Q.389 The air supplied to the diesel engine is compressed to provide heat for ignition of the fuel or to
decrease injection delay
Ans. (a) ignition of the fuel.
The air supplied to the engine during the induction or suction stroke is compressed by piston moving upwards
with the inlet and exhaust valves closed so as to compress the air and increase the temperature. This compression
is done adiabetically so that at the end of the compression the pressure and the temperature will be high so that
when the fuel is injected, because of the air temperature being higher than the self-ignition temperature of the
fuel, the fuel will ignite instantaneously and convert the chemical energy of fuel into pressure and heat energy to
be converted into mechanical energy.
Q.390 Fuel droplets injected into a diesel engine cylinder must have adequate penetration to
Ans. (c) thoroughly utilize the air charge.
The fuel injected should mix up properly with the available air in the combustion space. For this purpose, it is
necessary that the fuel should be atomized properly. At the same time, it should penetrate deep into the dense
atmosphere of the compressed air in the combustion space. Otherwise, if the penetration is not sufficient, local
burning will be caused and the high density air which is available away from the nozzle tip will not be utilized
efficiently. Hence under such circumstances fuel will not burn properly which will result in afterburning, loss of
power and high exhaust temperature.
Q.391 Loss of lubricating oil pressure to a main propulsion diesel engine will actuate a audible/visual
alarm.
Loss of lubricating oil pressure is a most dangerous thing which can happen to the engine as it may damage the
bearings and cause severe damage to the engine and the crankshaft. Inorder to avoid this thing, a prewarning
should be given as the drop of the pressure takes place and action has to initiated immediately by getting
audible/visual alarm. If the pressure drops still further, there is immediate danger to the engine. In addition to the
audible visual alarm, there will be tripping of the engine and the engine will stop.
Q.392 The amount of fuel injected into cylinder by unit injector is controlled by
Ans. (b) metering helix.
Metering helix will control the end of fuel injection and thus it will determine the effective delivery stroke of the
plunger. This is possible by having a proper angular relation between the helix on the plunger surface and the
suction ports. This is possible by rotating the plunger with respect to the barrel by use of rack and pinion
arrangement. The position of rack is controlled by the governor.
Q.393 The dripping of fuel from an injector nozzle after injection terminates often results in
Ans. (c) choking and blocking of the injection nozzles.
Once the injection terminates the fuel injection pump pressure drops. At the same time the fuel injector needle
valve closes. If the fuel injector needle valve still leaks, some amount of oil ooze out between the nozzle and seat
this oil will get accumulated over the tip of the nozzle. As it in not in the atomized condition or it has not
penetrated inside the combustion space, this oil as a droplet will burn on the hot nozzle during the combustion
process. Due to improper combustion of the dripping oil it will form a * and this * is mostly because of asphalt
present in the fuel because asphalt being a heavier hydrocarbon requires more time for burning and it requires
more quantity and thorough mixing of fuel with the air. All this is not possible when the dripping droplets
accumulate on the nozzle tip thus causing the * on the nozzle tip.
Q.394 The amount of fuel delivered in each cycle must be in accordance with engine load and the same
quantity of fuel must be delivered to each cylinder for each power stroke as the load. Which of the
following statements describes the requirements?
Ans. (b) Accurate metering
The complete fuel delivered to the engine will depend on the load. Hence for particular load particular amount of
fuel has to be burnt in the engine. At the same time, the total fuel given to the engine must be distributed equally
to the units so that all cylinders will produce equal power for the balancing of the engine. This two process
requires a proper quantity of fuel to be measured and delivered equally. Hence this process is called metering.
Q.395 The knock occurring when a cold diesel engine is started and continues while running at low speed
but stops when the engine reaches the normal operating speed and temperature is
Ans. (d) normal for this condition.
When the engine is started from cold, the temperature at the end of compression stroke is not sufficiently high.
When the fuel is injected in combustion space, it takes some time for it to ignite i.e. the ignition delay is more.
When the ignition delay is more, the total quantity of fuel accumulated inside the cylinder before the ignition
actually starts will go on increasing. When the accumulated fuel gets ignited, it produces a knocking sound and
knock wave. As the temperature goes on increasing, the temperature of the air at the end of compression stroke
also goes on increasing. When this happens, the fuel injected will reach the self-ignition temperature much faster
and hence there will not be much accumulation of fuel before it starts to ignite. Automatically the knocking
sound and the wave will reduce as the temperature reaches the normal operating temperature.
Q.396 The most critical time for any bearing with regards to lubrication is
Ans. (c) during starting.
Most of the bearings used on the engines work on the principle of hydrodynamic lubrication. During
hydrodynamic lubrication, there is no metal-to-metal contact between the bearing and the shaft. The bearing and
shaft is separated by lubricating oil film. This oil film is formed due to the rotation of the shaft inside the bearing.
As the speed of the shaft increases, more oil is drawn between the shaft and the bearing and thicker lubricating
oil film is formed. During starting of the engine, the speed is very low. Hence it is difficult to form a
hydrodynamic lubrication between the shaft and the bearing. During this time, for a short period, there is metal-
to-metal contact between the shaft and the bearing surface. Hence this is the most crucial period as the
characteristics of the lubricating oil (oiliness) comes into use and this is time when more wiping out of the
bearing takes place. Hence it is necessary to coat the bearings with anti-friction material to reduce the friction
between the bearing and the shaft at the time of starting because once the engine is started and the engine has
achieved the speed, the white metal does not play an important role because there is no metal-to-metal contact
and it is pure hydrodynamic lubrication. Hence white soft coating is useful for the boundary lubrication which is
existing at the time of starting the engine.
Q.397 Which of the listed diesel engine operating conditions should be checked immediately after any
diesel engine is started?
Ans. (c) Lubricating oil pressure.
Lubricating oil system is one of the most important system and if malfunctioning of lubricating oil takes place
the failure of engine is immediate and severe. Hence it is necessary to check the lubricating oil pressure and the
lubricating oil sump as soon as the engine is started. Otherwise, it will be too late and there will be irrecoverable
and irreparable damage to the engine.
Nozzle tip will get damaged because of sulphuric acid corrosion. That is the main reason most of the engines
cooling water temperature of the nozzle cooling is kept at 85C and in modern engines the cooling of the nozzle
by separate cooling system has been abandoned.
Q.399 What occurs in the combustion space of a diesel engine cylinder shortly after ignition and before
piston reaches TDC?
Ans. (c) Rapid increase in the pressure and temperature.
When the piston is nearing the TDC and the fuel is injected, the temperature inside the combustion space is
sufficiently high near the self-ignition temperature of the fuel. Hence the fuel catches fire and because of this,
sudden increase in the pressure and temperature takes place. The initial stage of ignition takes place under
constant volume therefore the temperature and the pressure increases. In the later stage, the combustion takes
place under constant pressure because the piston is moving downwards and the volume is increasing.
Q.400 The effective pump stroke of a port and helix fuel injection pump is determined by
Ans. (c) plunger control rack position.
The rack position will determine the position of the plunger helix with respect to the suction port of the barrel.
This angular position will determine the quantity and time for which the fuel is injected. Hence more the quantity
of fuel more the load and lesser the fuel lower the load. The time for which the fuel is actually injected into the
cylinder is called effective delivery stroke of the plunger which is decided by the helix position and the position
of plunger with respect to the barrel.
Q.401 The amount of fuel delivered by helical plunger type fuel injection pump is controlled by
Ans. (a) rotation of the pump plunger.
Rotation of pump plunger by means of rack and pinion arrangement will cause variation of angular position of
helix with respect to the suction port on the fuel pump barrel. Due to this, the effective delivery stroke of the fuel
pump will change which means that it will change the time for which the fuel is injected. Hence it will control
the quantity of fuel delivered in the unit because the position of plunger helix whether the termination of the fuel
injection is carried out earlier or later part of the injection stroke.
Q.402 Which of the following conditions will develop if the flow of raw cooling water to diesel engine is
obstructed?
Ans. (c) Jacket water temperature will increase.
The jacket water-cooling is a closed circuit and it uses the fresh water. The jacket water is cooled by raw water
by the use of heat exchanger. If the raw cooling water flow is obstructed, the quantity of heat removed from the
jacket cooling water is reduced. At the same time, the jacket cooling water is receiving heat from the engine
which is not being removed from the cooling water. Hence the jacket cooling water will go on rising
continuously.
Q.403 The effect stroke of a constant stroke individual fuel injection pump is varied by the
Ans. (a) controlled rack.
The controlled rack is engaged is with the pinion which in turn is engaged with the controlled sleeve of the fuel
pump. The controlled sleeve, when it rotates, will rotate the fuel pump plunger with respect to the barrel. This
rotation will change the angular position of helix with that of the suction port on the barrel. Hence ultimately the
effective delivery stroke of the pump will change and hence it will change the quantity of fuel injected.
Q.404 Which of the listed conditions occur if a diesel engine cylinder exhaust valve is leaking?
(a) loss of compression for the cylinder.
(b) misfiring or rough engine running.
(c) damaged valve.
(d) All of the above.
Ans. (d) All of the above.
When the exhaust valve is leaking automatically, the compression pressure drops. When the compression
pressure drops the fuel will not ignite properly. Hence there is a possibility that misfiring takes place. At the same
time, due to low compression pressure the fuel will not burn properly which may cause high exhaust temperature
and loss of the peak pressure. When the valve starts to leak, the high-pressure gas at high temperature will start to
pass through the leaky portion of the valve. This will result in wear or wire-drawing effect on the valve and valve
seat. Thus causing excessive damage to the valve and valve seat and excessive high exhaust temperature which
increases fast.
Q.405 High firing pressure and low exhaust temperature in a diesel engine may result from
Ans. (c) early fuel injection timing.
In case of v.i.t. controlled injection, the early injection cause the fuel to mix up better with the quantity of air
available it will have more time to mix up and form combustible mixture. Due to this, fuel will burn efficiently
and very near to the TDC which will cause high peak pressure and low exhaust temperature.
Q.407 A change in the degree of fuel atomization in a diesel engine cylinder has the greatest effect on the
Ans. (d) combustion of fuel in the cylinder.
The more degree of atomization, with proper penetration, will cause better fuel efficiency because the fuel will
be divided into very minute particles which will have more surface area available and the evaporation of the fuel
will be faster so as to form a combustible mixture. Hence the mixture will burn efficiently and promptly due to
which the combustion inside the cylinder will be efficient.
Q.408 Diesel engine oil having low cetane number can result in
Ans. (c) combustion knock.
Lower cetane number of the fuel low is the grade of the fuel and because of that ignition delay will increase.
Hence more fuel will get accumulated inside the cylinder before it catches fire. When it catches fire it will give
knocking effect because of sudden formation of pressure wave. This knocking effect is called diesel knock which
directly depends on the quality of the fuel.
Q.409 To successfully reduce an excessively high diesel engine exhaust gas temperature
Ans. (d) you should reduce the load on the engine.
As a general rule more the load on the engine more will be the exhaust temperature. Hence the first action to be
taken if high exhaust temperature is noted is to reduce the quantity of fuel which is burnt. This is possible by
reducing the load on the engine which reduces the quantity of fuel burnt and the exhaust temperature reduces
immediately.
Q.410 High exhaust temperature and black smoke exhausting from an engine can be caused by
Ans. (a) engine overload.
When engine overload takes place, the amount of fuel burnt is always higher than the quantity of the air supplied.
At the same time, high quantity of fuel is injected. This high quantity of fuel injected cannot burn in the available
time. Under such circumstances, afterburning takes and along with lack of proper quantity of air it will have
higher exhaust temperature and black smoke.
Q.411 Combustion gas venting through the expansion tank can be caused by a
Ans. (b) cracked cylinder head.
When the cylinder head is cracked, during compression stroke when high peak pressure and high exhaust
temperature takes place, these high pressure combustion gas will enter through the cracked cylinder head valve
into the water space. This gas being lighter it will try to rise upwards and get accumulated inside the expansion
tank and it will come out through the expansion tank weld. The gas coming out through the expansion tank weld
will have exhaust gas smell and water in the expansion tank will get discoloured. Also if the crack is big, more
quantity of exhaust gas will find its way into the expansion tank, the expansion tank may get bubbled out and
may get empty due to trapped exhaust gas. Also, the jacket cooling outlet temperature of that particular unit will
also be high.
Q.412 Which of the following problems can cause fluctuating pressures in the closed cooling system of a
main propulsion diesel engine?
Ans. (b) Cavitation in the cooling water pump.
If the inlet in the cooling water pump is restricted or the water is having high temperature, cavitation will take
place in the form of bubble at the suction of pump. Due to this, there will be partial loss of suction. This will
cause the variation in discharge pressure which will result in fluctuations in the pressures of closed cooling water
system.
Q.413 A fuel leak occurs in a high pressure piping between the fuel pump and fuel nozzle. This requires
immediate repairs because of the
Ans. (b) serious fire hazard.
The high pressure pipe leakage will cause the high pressure fuel to leak through the cracked portion of the fuel
pipe. Due to this, the oil coming out through the cracked portion will be in the form of a fine mist. This mist
when it comes in contact with the exhaust manifold or hot surface of the engine, as there is sufficient amount of
air in the engine, it will result in fire in the engine room. Hence immediate repair is necessary to avoid this fire.
Q.414 Individual cylinder performance in the diesel engine is routinely determined by the exhaust gas
Ans. (c) pyrometer readings.
As a general rule, the exhaust temperature will give a fairly good idea of power developed by the individual
units. The exhaust gas temperature from individual unit is taken into consideration as a measure to determine the
power developed by the individual unit. Hence as long as all unitss temperatures are equal or they are within a
prescribed limit, this indicates that all units are developing nearly same power and the engine is properly
balanced.
Q.415 Excessive diesel engine cylinder exhaust backpressure will be caused by
Ans. (c) an obstruction in the exhaust silencer.
When the exhaust valve opens and exhausting takes place through the exhaust valve, if the exhaust gas silencer
valve is blocked or choked, there will not be easy flow of exhaust gas and complete emptying of the exhaust gas
from the cylinder will not possible. Under such circumstances, when the piston is on the upward stroke it will
give a backpressure over the piston and there will be loss of power in overcoming this backpressure.
Q.416 The injection pressure of a hydraulic fuel injection nozzle can be increased by
Ans. (d) increasing compression on the pressure spring.
The fuel is injected when the pressure developed by the fuel injection pump will act on the annular space around
the fuel injection nozzle will overcome the spring compression from top on the fuel injection nozzle. As the
compression pressure increases, more pressure will be required by the fuel injector nozzle to lift up against the
spring compression. Hence more the compression pressure more will be pressure required for the fuel injector to
open. Hence the injection pressure will increase as the compression pressure of the spring increases.
Q.418 Which of the following problems could develop due to the accumulation of oil vapours into the
crankcase of diesel engine?
Ans. Crankcase explosion.
Accumulation of oil vapours in the crankcase and presence of hot spot will cause the vapour to convert into oil
mist. When the oil mist comes in contact with the hot spot in presence of large quantity of air, which is available
in the crankcase, it will form a lethal combustible mixture and it will cause a crankcase explosion.
Q.419 One problem resulting in a diesel engine fuel injector opening pressure being lower than specified
by the engine manufacturer is
Ans. (d) quantity of fuel injected tends to increase.
As the fuel injection pressure is lower than recommended by the engine manufacturer, the fuel injector valve will
open much earlier but the closing will not alter. Due to this, the quantity of fuel injected will be more than what
is required.
Q.420 Which of the listed reasons is the most likely cause of a sudden drop in compression pressure in one
diesel engine cylinder?
Ans. (b) Malfunctioning of the valves.
Malfunctioning of the valves may be because of improper adjustment of the tappet clearance or the valves may
be sticky or leaky. Under all these conditions, during compression, the compressed air will leak through the leaky
valve and automatically the compression pressure will drop.
Q.422 Failure to open the diesel engine test cock after a long period of shutdown prior to starting may
result in
Ans. (b) damage to cylinder head and piston.
If the test cock or the indicator cock is not opened and the engine is started and due to any reason there is
accumulation of fluid on top of the piston, this fluid will get compressed. As the fluid is incompressible,
excessive pressure will be developed between the piston top and cylinder head bottom portion. This will cause
the cracking of the cylinder head, damage to the piston, bending of the connecting rod or in worse case it may
cause damage to the crankshaft in the way of slippage.
Q.423 Which of the listed conditions is most likely to cause a crankcase explosion?
Ans. (b) Fuel dilation of lubricating oil.
When fuel dilation of lubricating oil takes place, automatically the flash point of the lubricating oil reduces.
Under such circumstances, if there is any hot spot present in the crankcase and the oil mist is formed along with
the oil mist there will mist of the fuel present. This mist of the fuel will be more prone to explosion and hence
explosion will be caused much early.
Q.424 repeated
Q.425 A diesel engine is operating with excessively high exhaust temperature in all cylinders. In order to
correct this condition, you should first
Ans. (a) reduce the engine load.
As a general rule if the load is more on the entire engine, the entire engine will be using more quantity of fuel.
When more quantity of fuel is burnt, automatically more heat will be generated. This will give higher exhaust
temperature for entire engine. Hence to reduce this exhaust temperature the quantity of fuel injected is reduced
and to reduce to quantity of fuel injected, you have to reduce the load on the engine.
Q.426 The overspeed trip installed on most diesel engines will stop the engine by shutting off the
Ans. (b) fuel oil supply.
When overspeed takes place in order to prevent the damage to the engine it is necessary to stop the engine. For
instantaneous stopping of the engine. the best possible way is to shut off the fuel immediately. This is done by the
overspeed trip which will bring the position of the fuel rack of all the fuel pump to zero position and no fuel will
be injected and the engine will be immediately stopped.
Q.427 White smoke exhausting from a operating diesel engine may be due to
Ans. (a) cracked liner.
Especially if the liner is cracked at the lower position, the water will enter into the combustion space. This water
when it gets evaporated, during the combustion stroke because of combustion of fuel, will form steam. This
steam will come along with the exhaust gas into the atmosphere through the funnel of the ship. When it comes
out it will get cooled down by the atmospheric temperature. This steam will be visible due to water present in it
and it will form white mist when it comes out of the funnel.
Q.428 Black smoke exhausting from an operating diesel engine can be caused by
Ans. (a) fuel dribbling from leaky fuel injector.
When the fuel is dribbling from leaky fuel injector it is actually coming into the combustion space when it is not
required. At the same time, it is not in atomized condition. Under such conditions, afterburning will take place
and fuel which is dribbling will not mix properly with the air. Hence the improper combustion of this fuel will
take place during the exhausting stroke of the engine and hence black smoke and higher exhaust temperature will
be indicated.
Q.429 A device which functions to bring a diesel engine to fullstop to protect it from damage is known as
Ans. (b) overspeed trip.
If the due to any reason the engine gets into overspeed, due to the centrifugal force caused by rotation, there will
be excessive damage may take place to the engine to the extend that the piston may come out from the cylinder
or from the crankcase or the bottom end bearings may break. In order to prevent such heavy damage, it is
necessary that the engine should stop immediately. This is possible by installing the overspeed trip which will
sense the overspeed and immediately shut off the fuel.
Q.430 Which of the following conditions can cause oil to accumulate in the cooling system of a diesel
engine?
Ans. (b) Defective oil cooler core.
If the lubricating oil cooler, in which the cooling is carried out by jacket cooling water especially in case of small
engines, is leaking the lubricating oil will find its way into the jacket cooling water. This system is utilized in the
engines working in the cold weather where the lubricating oil is cooled by jacket cooling water in a cooler.
Q.431 Lubricating oil accumulating in the cooling water system of a diesel engine will result in
Ans. (b) poor heat transfer.
When the lubricating oil find its way into the jacket cooling system it will coat over the heat transfer surface. As
lubricating oil is poor conductor of heat, it will effect the efficient heat transfer and will cause the overheating of
the liner and the cylinder head due to the overheat transfer.
Q.432 When the opening pressure of a diesel engine fuel injector is greater than that specified by the
engine manufacturer
Ans. (a) the quantity of the fuel injected decreases.
If the opening pressure of fuel injector is more than that specified by the engine manufacturer, the fuel injector
will open late and it will close at particular time depending upon the load. Hence the automatically the quantity
of the fuel injected will be reduced because of the opening of the fuel injector is late.
Q.433 Black smoke exhausting from an operating diesel engine is an indication of poor combustion which
may be caused by
Ans. (c) clogged air intake passage.
If the air intake passage is clogged, automatically the engine will be starved off the air. The quantity of fuel
injected will not burn efficiently because of lack of sufficient amount of air. If the fuel does not burn efficiently,
it will be indicated by black smoke and high exhaust temperature.
Q.435 Diesel engine lubricating oil dilation with fuel oil is indicated by
Ans. (a) decreased viscosity.
If the lubricating oil is getting contaminated due to fuel oil, the viscosity of the lubricating oil will drop down
because the viscosity of the fuel oil is much lesser than the viscosity of lubricating oil. Hence when they get
mixed together, the viscosity of lubricating oil will drop down depending upon the quantity of fuel which is
contaminating the lubricating oil. This is easily checked by the flow stick which are supplied for the on-board
testing of the lubricating oil. Also, it will drop the flash point of the lubricating oil. This is the most dangerous
situation and action should be taken immediately otherwise bearing damage will also take place.
Q.438 A classical way of checking for excessive fuel injection in one cylinder of an operating diesel engine
is
Ans. (c) frequently checking the cylinder exhaust temperature.
As a general rule, the cylinder exhaust temperature is an indication of the quantity of fuel burnt in that unit.
Hence higher the fuel burnt higher is the exhaust temperature. Hence by checking the exhaust temperature we
can get a rough idea as how the distribution of fuel is taking place for individual cylinder. If the temperature is at
higher level by rough estimation we can assume that the quantity of fuel delivered to that unit is higher than as
compared to any other units.
Q. 443 The highest load applied to the diesel engine crankshaft main bearings are
Ans. (c) firing loads.
During the firing of the fuel, the piston is nearly at TDC position and the piston rod, connecting rod and the
connecting rod bearings are nearly in the same vertical plane because of which full load of firing is transmitted to
the crankshaft. When the load is transmitted to the crankshaft, the crankshaft is supported by the main bearings.
Thus the load is transmitted to the main bearings and ultimately to the bed plate and transverse girder.
Q.444 If the diesel engine fuel injection timing is changed to delay the start of injection until the pistons
are at TDC, the engine will
Ans. (b) develop less power under load.
If the fuel is injected when the piston is at TDC, by the time it mixes up with air and forms a combustible
mixture, the piston has already passed TDC and is moving downwards. Under such circumstances, the firing of
the engine will take place when the piston is moving towards BDC. In this condition, the firing will take place
under increasing volume instead of constant volume so the pressure cannot be developed onto the piston. As the
pressure on the piston top is most important from the point of view of power developed considering the power
developed is given by
Power developed = PLAN
In this condition, power developed by the engine will reduce and whatever fuel is burnt will be wasted due to
afterburning resulting in high exhaust temperature but low power.
Q.445 Increase in compression ratio of a diesel engine while maintaining designated rate of fuel flow will
result in
Ans. (a) increase in the horsepower.
If the compression ratio increases, the air in the cylinder will be more dense and the quantity of air increases. At
the same time, the temperature of the air at the end of compression increases. As a result, the fuel injected will
burn efficiently and promptly. All this will result in more power being developed by the engine because more
heat can be released from the fuel which is burnt.
Q.446 If the jacket water temperature in a diesel engine rises rapidly above normal, you should
Ans. (b) first reduce engine load.
Jacket cooling water temperature will also roughly estimate the engine load. Higher the engine load, higher will
be the heat absorbed by the jacket water and hence the temperature will increase. Hence jacket water temperature
is a rough estimation of power or load on the engine. If the jacket water temperature is excessively high and it is
not because of any other reason such as malfunctioning of jacket water cooler, then it is necessary to reduce the
load and carry out the further investigation and rectify the fault.
Q.447 During which of the listed piston stroke of a four-stroke cycle diesel engine is the piston moving
downwards?
Ans. (a) Intake stroke.
In a four-stroke cycle diesel engines, the piston will move downwards two times: one during the expansion
stroke and one during which air will be drawn inside the cylinder called the intake stroke. During the intake
stroke, the intake valve is open and the piston is moving downwards so whatever increase in volume is there, due
to downward movement of the piston, will be filled up by the incoming air under intake stroke.
Q.449 Significant retardation of diesel engine fuel injection timing will result in
Ans. (d) reduced engine power.
Retardation of fuel injection timing will cause the fuel injection to occur late during the compression stroke. As
the fuel injection is late, by the time it forms a combustible mixture with the compressed air, the piston will be
near to TDC or it might have already passed the TDC. If it has already passed the TDC and if fuel ignition takes
place, the pressure is increasing; also the volume is increasing as the piston is moving downwards, hence the
maximum peak pressure reduces which will cause loss of power. Also, afterburning will take place and the
exhaust temperature will be high but the power developed by the engine will be low.
Q.453 When one cylinder has low compression pressure and high exhaust temperature than any of the
engine cylinders which of the conditions listed will be indicated?
Ans. (c) Leaky exhaust valve.
If the exhaust valve is leaking, the compression pressure will drop because the compressed air will get leaked out
from the leaky exhaust valve. At the same time, because of low compression pressure, the fuel which is injected
will not burn properly which will give high exhaust temperature. Also when compression is taking place and the
fuel is injected and as the fuel starts burning, as the fuel valve is leaky, high temperature and high pressure gas
will leak through the leaky exhaust valve and the exhaust temperature will rise. Hence low compression pressure,
high exhaust temperature, low peak pressure and loss of power all of them will be indicated by leaky exhaust
valve.
Q.454 A distorted spray pattern from a fuel injector can cause a diesel engine to have
Ans. (d) less power output.
The fuel should be distributed equally and evenly around the entire combustion space available. If the spray
pattern of the injector has been distorted due to any reason, the fuel will not mix properly with the air and the
complete use of the air available inside the combustion space will not be possible by the fuel. At the same time,
concentration of fuel at one particular point in the combustion space will be more than the other. Due to this, the
availability of air will not be uniform for the fuel to burn in the entire combustion space and because of this there
is a possibility that the exhaust temperature will be high and power loss will take place.
Q.455 If a few injector spray holes becomes plugged the reason could be
(a) excessive surging under governor speed.
(b)combustion knock under load.
(c) poor fuel combustion.
(d) All of the above.
Ans. (d) All of the above.
When few holes of the injector are plugged, the fuel distribution for the units in which the holes are plugged will
get disturbed. Hence the power balance of engine will change. As there is change in power balance, the governor
will try to continuously try to maintain the speed and hence it will give surging effect to the engine speed. So
engine speed will continuously fluctuating. Secondly, as the distribution of fuel is not proper and penetration and
atomization will also get affected the fuel will not burn properly and hence it may cause the fuel knock and poor
fuel combustion resulting in knocking sound, black smoke, and loss of power.
Q.456 A diesel engine fails to start because of water in the fuel. Inorder to start the engine we should
Ans. (d) drain the filter and stainer and bled off the water at each of the injection pump.
All methods should be taken to remove the water content from the entire fuel injection system upto the nozzle
tip. This is carried out by bleeding off the water content at different position including the fuel pump and injector
so that water will not be injected during the injection stroke, resulting in stopping of the engine or incomplete
combustion or fluctuation in the rpm. In such cases, if water is present engine may fail to start also.
Q.459 Air in the fuel line through a fuel injection nozzle of a diesel engine will cause the engine to
Ans. (c) operate with reduce power or stop.
If there is air in the fuel injection system, there is a possibility that the quantity of fuel injected will get reduced
or there is intermittent firing. If the air ingress in the system is excessively high, air being a compressible
medium, the fuel injection nozzle will not open and no fuel is injected into the cylinder and hence the engine may
stop.
Q.460 Which of the following problems will occur if needle valve of an injector stick in open position?
Ans. (a) Fuel injection timing may change.
If the fuel injection valve get stuck up in open position, during the suction stroke of the engine, the pressure
inside the cylinder drops. At that time, the fuel will drip through the open needle valve into the combustion space
and some part of fuel will be emptied off from the system. Hence for the next injection stroke it is necessary to
fill up the space and because of that the fuel injection timing will change. At the same time, because the valve is
staying in the open condition, the fuel will be injected as soon as the plunger covers the suction port. Hence too
much fuel will be injected which will not be under sufficient pressure resulting in the dripping of the fuel and not
spraying properly. All this will cause excessively high exhaust temperature or the relief valve will lift resulting in
loss of power and black smoke.
Q.461 A dry type exhaust muffler clogged with soot will cause
Ans. (c) loss of engine power.
When the exhaust gas muffler gets clogged, it will cause excessive backpressure on the exhausting system. In
case of turbocharged engine, the turbocharger will not attain sufficient speed and it will not supply sufficient air
for the combustion. Even in case of naturally aspirated engine because of the backpressure when the piston is
moving upwards on the induction stroke it is to force the exhaust gas out of the unit. This will cause the loss of
power developed by the engine and hence the power developed will be less.
Q 464 Which of the terms listed below represent the operational speed at which excessive engine vibration
is created?
Ans. Critical speed.
Critical speed is that speed at which the natural frequency of vibration of the shafting will coincide with the
frequency of vibration caused because of the firing of the fuel in the cylinder i.e. firing frequency. If they
coincide, there will be resonance frequency and excessive vibration of engine will take place. The condition
when this occurs is called critical condition. If this persists for longer period, there is a possibility that the
crankshaft and the connecting rod may get cracked. Also the tierods may get cracked or the foundation bolts may
get loose.
Q.466 Which of the condition listed below would cause simultaneous high cylinder pressure and low exhaust
temperature?
Q.468 If fuel injection occurs too early the diesel engine will lose power because of
Q.471 If you are suspecting misfiring due to air leakage into the air system,
Ans. (d) you should be looking for a leak on the suction side of the fuel transfer pump or booster pump.
As the fuel in the suction side of booster pump will always be lower as compared to its delivery side, it is
necessary to check at the suction side of pump for any ingress of air. In order to avoid this, there should be
sufficient height of fuel in the service tank so that possibility of ingress of air through the suction glands is
minimized.
Q.472 An increase in the load of a turbocharged diesel engine operating at constant speed will result in an
increase in
(a) exhaust temperature
(b) air box pressure
(c) brake mean effective pressure
(d) All of the above
Ans. (d) All of the above.
If the load on the turbocharger is increased the speed of the turbocharger increases. As the load on the
turbocharger increases more amount of fuel burnt increases. Because more quantity of fuel is burnt the exhaust
temperature increases. At the same time, more amount of exhaust gas is produced. This high quantity and high
temperature gas will drive the turbocharger at higher speed due to which more amount of air will enter the inlet
manifold through the turbocharger and hence scavenge air pressure will increase. As the scavenge air pressure
increases, more amount of air will enter the cylinder. As a result, compression pressure will increase and peak
pressure will increase because of firing of more quantity of fuel and burning the fuel efficiently.
Q.473 When a diesel engine is operated at partial load as compared to full load, a decrease will occur
average
(a) in air quantity aspirated.
(b) fuel injection pressure
(c) combustion pressure on power stroke
(d) compression pressure on the compression stroke
Ans. (a) in air quantity aspirated.
If it is naturally aspirated engines the quantity of air aspirated will reduce. In case of turbocharged engines, the
turbocharger at partial load will supply less quantity of air. In this respect the quantity of aspirated air will
reduce. As the quantity of air admitted into the cylinder reduces, the compression pressure during the
compression stroke will also reduce. When the compression pressure reduces at part load, the quantity of fuel
burnt will also reduce ultimately the firing pressure and mean indicated pressure, which is acting over the piston
when it is moving downwards on expansion stroke, will also reduce.
Q.476 Which of the following operating procedures should be carried out immediately before any diesel
engine is started?
Ans. (c) Verify proper lubricating oil pressure.
Verification of proper lubricating oil pressure is most important part when the engine is started because if there is
insufficient lubricating oil pressure and if the engine is allowed to run even for smallest possible time, severe
damage will occur to the crankshaft and the complete engine will be damaged beyond repair.
Q.477 The most common diesel engine fuel injection system problems are caused by
Ans. (b) dirty fuel.
If the fuel is dirty the fuel filter will get choked. The fuel injector nozzle will get choked. There will be excessive
weardown of fuel pump barrel and plunger. All this will lead to deterioration of fuel injection system.
Q.478 If sludge accumulation on the underside of the diesel engine piston it will
Ans. affect the heat transfer.
If there is sludge accumulation on the underside of the diesel engine piston in the cooling space it will affect the
heat transfer. When the heat transfer is affected ,there will less amount of heat transfer which will cause the
piston crown to overheated and ultimately it may crack.
Q.479 Faulty operation of diesel engine fuel injection nozzle can be a direct cause of
Ans. sedimentation in the fuel supply.
If the fuel supply is having sedimentation, sludge or hard particles the injection nozzle will get seized or there
will excessive wear of injection nozzle and needle and guide and because of this the spray pattern will change
and there will be faulty operation of diesel engine. Also the sludge may enter the combustion space and cause the
weardown of the liner and piston rings.
Q.480 Low cylinder compression pressure and high exhaust temperature may indicate
Ans. (b) leaky valve.
When the valves are leaking during compression the air will leak out and because of this the compression
pressure will reduce. When this happens sufficient amount of air will not be available for the fuel to burn and
under such circumstances there will loss of power and high exhaust temperature and black smoke. All this is
result of leaky valve.
Q.481 The loss of diesel engine cylinder air charge through leaky valves, piston rings, worned or scorned
liner will be indicated by which of the following sets of conditions?
Q.482 During diesel engine warm-up which type of tappet clearance adjuster listed below for a change in
length of exhaust valve?
Ans. (c) Hydraulic tappet clearance adjuster.
In hydraulic tappet clearance adjuster, there is no metal-to-metal contact between the pushrod and the stem of the
valve. The opening of the valve is carried out by the hydraulic piston and hence it replaces the pushrod and
rocker-arm combination. Hence whatever expansion of the valve and spindle takes place because of temperature,
it will be taken care off by the hydraulic oil and hydraulic cylinder that is on the top of the valve. This cylinder is
connected to the lubricating oil system through a throttle valve which will also allow for the excess oil to be
drained off and the valve will close positively.
Q.484 Worn cylinder head valve seat in a diesel engine will cause
Ans. (a) less cold tappet clearance.
As the valve will sit over the worn valve seat there will be more amount of vertical displacement of valve
spindle. As a result, the clearance between the spindle and pushrod or rocker-arm will reduce and therefore the
cold tappet clearance will reduce.
Q.485 Which operating condition of a diesel engine is indicated by excessive firing pressure?
Ans. (b) Overload.
If the engine is overloaded and if the fuel pump timings are proper, more fuel will be burnt in the engine to
accommodate for the increased load. When more fuel is burnt, automatically the firing pressure will be on the
higher side.
Q.486 If the jacket water temperature rises rapidly above normal in a diesel engine you should first
Ans. (b) reduce the engine load.
As a thumb rule, it is essential that for any rise in temperature of jacket cooling or exhaust the main reason may
be either overloading of the engine or lack of cooling. Before diagnosing or rectifying the cause it is better to
reduce the load and then carry out the necessary investigation and rectify the fault. Otherwise, time will be
wasted in diagnosing and rectifying the cause and during that time the engine may damaged. As soon as we
reduce the load, the heat generated inside the engine is released and this will help in reducing the jacket cooling
temperature or atleast maintaining it and not allowing it to reach more dangerous limit. Also if the temperature is
excessively high and if immediate action is taken to reduce the heat by increasing the flow of water, there is a
possibility that the liner may crack due to sudden change in temperature. Inorder to avoid this, it is advisable to
reduce the load and then carry out investigation.
Q.488 Which of the following procedures should be carried out when large slow speed diesel engine is
operated with one cylinder secured i.e. one cylinder is cut-off from the fuel?
Ans. (d) The engine speed should be reduced.
When the engine is operated with one unit cut-off the other unit may get over loaded and because of this the
other units may get damaged and there will be severe damage to the entire engine. At the same time, there will be
excessive vibration. In order to avoid all these things, it is necessary that when one unit is cut-off the load on
engine be reduced so that the other units will not get overloaded and the engine will work moderately over a
period of time.
Q.489 The most practical way of detecting the overload in one cylinder of an operating large low speed
main propulsion diesel engine is to
Ans. (d) check the cylinder exhaust frequently.
The exhaust temperature of the cylinder is a rough indication of power developed by the units. If the exhaust
temperature is excessively high for a particular unit and other operating conditions are normal, it can be easily
determined that the unit is overloaded. Hence the continuous observation of exhaust temperature of the units will
indicate whether the units are properly loaded or they are overloaded and whether they are equally sharing the
power or not
Q.492 If you notice smoke coming out from crankcase exhaust fans outlet of an operating diesel engines
you should suspect
Ans. broken piston rings.
In case of trunk-type piston if the piston rings are broken, high temperature and high pressure combustion gases
will enter into the crankcase because of blowpast. Due to this, the crankcase atmosphere will be filled with high
temperature, high pressure exhaust gases. This can be seen from the exhaust of the crankcase when gas comes
out of the exhaust fan outlet.
Q.493 Which of the following conditions may need to be reduced when operating a large low speed main
propulsion diesel engine at low load?
Ans. (c) Cooling water flow through aftercooler.
The cooling water flow through the aftercooler has to be reduced in order to maintain the scavenge air
temperature above the dew point; otherwise condensation of moisture will take place which will damage
the lubricating oil film which will cause the excessive weardown of the liner and the piston rings.
Q.494 A restricted diesel engine exhaust manifold operating under a normal load is indicated by
Ans. (b) low firing pressure and high exhaust temperature.
Due to the restriction in the exhaust manifold, the turbocharger rpm will be reduced and because of that less
amount of air will be pumped by the turbocharger into the engine and hence it will cause lower compression
pressure. As the complete removal of exhaust gases from the cylinder does not takes place, there will be mixing
of the exhaust gas with the incoming fresh air thus diluting it which will cause the improper combustion of the
fuel and hence low firing pressure and high exhaust temperature.
Q.496 What colour exhaust will be exhibited when a low speed two stroke main propulsion diesel engine
designated to operate on light and heavy fuel oil is operated on insufficiently preheated heavy fuel oil?
Q.497 Immediately after starting a diesel engine normal raw water and jacket water pressures are
indicated, however jacket water temperature continues to rise. If there is no change in the seawater
temperature, you should suspect
Ans. jammed up three-way thermostatic valve.
If the engine load and sea-water temperature is not changing still if the jacket water temperature is rising, it
definitely indicates that the three-way valve which controls the flow of water through the coolers so as to
maintain the jacket water temperature may be stuck up in such a manner that the jacket cooling water is getting
by-passed and is not passing through the cooler. Thus the temperature will continuously go on increasing.
Q.498 Before starting a diesel engine which has an engine-driven lubricating oil pump the engine should
Ans. (c) pressurize the lubricating oil system.
When the lubricating oil for the engine is supplied by an engine-driven pump, there will be some time delay till
the attached lubricating oil pump will take the suction and develop sufficient pressure to provide the lubricating
oil. During this transient time there is a possibility that engine bearings may get damaged. Hence it is necessary
that an externally electrically driven lubricating oil priming pump has to be started earlier and proper lubrication
of the bearings should be carried out and to keep the lubricating system under pressure. When the engine starts,
the attached lubricating oil pump will have achieved high enough pressure and the electrically driven lubricating
oil pump can be cut-off safely without causing any damage to the engine.
Q.500 Prolonged operation of a diesel engine with a closed cooling water system at low operating
temperature than designed will result in
Ans. (d) sulphuric acid formation.
If the engine is operated with the fuel having high sulphur content and low jacket cooling water temperature, the
sulphuric acid formed because of burning of sulphur will reach its dew point. As a result the sulphuric acid will
coat over the liner wall and this will cause corrosive wear to the liner and hence total liner weardown will
increase. Hence it is necessary to maintain a sufficiently high temperature if the fuel is expected to have more
amount of sulphur content.
Q.501 A large slow speed main propulsion diesel engine may become overloaded by
(a) heavily fouled hull.
(b) strong head wind and heavy seas.
Ans. (c) both of the above.
When the hull is fouled, the resistance offered by the ship for the movement increases. Hence for the same rpm,
the engine will be overloaded as it will have to produce more power to overcome the resistance. Similarly, when
there is strong head wind, the ship will have to work against the head wind and overcome the force created by the
headwind. In this condition also, the engine will be overloaded to maintain the same revolution on the engine
shaft.
Q.502 Immediately after any diesel engine is started, the engineer should check the
Ans. (b) lubricating oil pressure.
Lubricating oil is the most important component of the engine. If the lubricating oil pressure is lost, even for a
fraction of a second, there will be metal-to-metal contact between the bearing and the shaft, and they will get
damaged. Hence the lubricating oil pressure should be checked as soon as the engine is started.
Q.503 In a diesel engine lubricating oil system, which of the following part should be lubricated first?
Ans. (b) Main bearings.
In case of the four stroke engines, the oil is first supplied to the main bearings. From the main bearings, through
the holes of the crankshaft, it comes to the bottom end bearings. From the bottom end bearings, through an oil
passage, it passes to the top end bearing. This passage shows that the main bearing is lubricated first. In case of
large two-stroke engines, where separate lubrication is given to the crosshead bearing, both the main bearing and
the crosshead bearing get the lubrication together.
Q.505 Diesel engine exhaust gas temperature can be used to determine individual cylinder
Ans. (a) performance.
The exhaust gas temperature gives the approximate power developed by the engine. Hence if the exhaust
temperature is higher, we can consider that the power developed by that unit is more than the other units. This
may not true always because if there is any discrepancy developed during the fuel injection, the exhaust
temperature will be more but the power will be less. Hence the combination of exhaust temperature and the
indicator diagram will indicate the exact problem. So exhaust temperature is a rough indication of the
performance of the unit.
Q.506 Which of the following problem may occur if the opening pressure of a fuel injection nozzle is greater
than specified by the fuel injection manufacturer?
If the nozzle opening pressure is higher than specified, the nozzle will open late because the fuel pump has to
develop higher pressure to lift the nozzle and open the fuel injection valve. In such circumstances, the injection
will be delayed.
Q.507 All the diesel engine firing cylinder pressures are normal; yet all exhaust temperatures are low.
Which of the following situation is responsible for this condition?
Ans. (d) Light load.
When the temperature of all the units are low and the peak pressures are equal, it means that all units are sharing
equal power. If the exhaust temperature is low, it also indicates that all units are producing less power. When all
units are producing less power, then the total load on the engine is reduced. Hence low exhaust temperature is a
cause of low loading of the engine.
Q.508 Insufficient piston cooling of a large low-speed main propulsion diesel engine burning heavy fuel
may result in
Ans. (c) high temperature corrosion and burning of piston crown metal.
If insufficient cooling takes place, the piston crown temperature will increase. When the crown temperature
increases above 470C, the vanadium present in the fuel, along with calcium, will be melted. This molten
vanadium will form vanadium pentoxide, which is a highly corrosive compound. This will get coated over the
piston crown and cause excessive high temperature corrosion of the crown.
Q.509 Which of the following factors tends to increase scale formation on the salt-water side of a heat exchanger
used in a diesel engine cooling water system?
Ans. (c) Operating the engine while maintaining a high sea-water outlet temperature.
The sea water outlet temperature of a cooler can be high because of restricted sea-water inlet or reduced quantity
of the sea water inlet due to some reason. Under such circumstances, the flow of water through the heat
exchanger will get reduced and will get overheated resulting in high sea-water outlet temperature. This high
outlet temperature will cause the calcium and magnesium salts to get deposited inside the tube of the heat
exchanger causing the more amount scale formation.
Q.511 Any increase in exhaust gas backpressure in a four-stroke cycle diesel engine will
Ans. (a) reduce diesel engine power output.
High backpressure will cause the piston to push the exhaust gas out of the cylinder with the backpressure acting
over it from the exhaust manifold. This will cause loss of power because the power required to drive out the
exhaust gas from the cylinder will be taken from other units which are firing. Thus the total power output of the
engine will be reduced because of the pumping effect of the trapped exhaust gas by the use of the piston.
Q.518 Persistent knocking of one cylinder of a diesel engine ceases when the fuel supply of that cylinder is
secured. This problem may be a result of
Ans. (b) faulty combustion in that cylinder.
When the engine cylinder is knocking, and after shutting of the fuel if the knocking stops, it is a clear indication
that the knocking was a result of problem related to fuel injection or quality. If the knocking persisted even after
the fuel injection was cut-off, it indicates that the knocking is a mechanical knocking which is a serious problem.
Q.519 Which of the condition will occur when in an operating diesel engine if air pockets are formed
within a cylinder head circulating water passage?
Ans. Hotspots will be developed.
If the air pocket is formed, as air is a low conductor of heat, the heat will not be conducted form the hot surface
and it will not be carried away by water. Hence that particular spot will get overheated and hotspot will be
developed.
Q.520 If a diesel engine exhaust temperature is abnormally higher, the cause could be
Ans. overload of the engine or the compression pressure would be too low.
If the engine is overloaded, the exhaust temperature of all units will be higher. If the temperature of only one unit
goes up, it is an indication that there is some thing wrong with that particular unit i.e. either incomplete fuel
combustion or improper timing adjustment of that unit or too low compression pressure of that unit due to worn
out cylinder liner or leaky piston ring.
Q.521 Heat damage to the fuel injection nozzle can be prevented by avoiding
(a) excessive fuel oil temperature.
(b) long period of engine overload.
(c) hard carbon deposits and varnish on the nozzles.
(d) All of the above
Ans. (d) All of the above.
If excessive fuel oil temperature is permitted and if there is no separate cooling is given to the injector and the
circulating fuel itself is part of cooling system, in such conditions the hot fuel oil will keep the nozzle at high
temperature. This will cause damage to the fuel injector because at higher temperature the fuel injector nozzle
and the valve seats will lose its strength and it will fail. Secondly, if the engine is working at high load for long
time, the fuel injector nozzle will be subjected to high temperature from the combustion space due to which the
fuel injector nozzle may get damaged. Thirdly, if the quality of the fuel is bad, there is a possibility that hard
carbon deposits may be formed. Also the needle guide may get coated with hard varnish and because of this the
fuel oil nozzle operation may get affected and it would ultimately cause to seep in place or have excessive
weardown and it will get damaged.
Q.524 White smoke from the exhaust of diesel engine can result from
Ans. (b) low cooling water temperature.
Due to low cooling water temperature, the entire atmosphere inside the cylinder liner will be at lower
temperature because of which the exhaust gas temperature will drop down. When the exhaust gas temperature
will drop down, the water vapor in the exhaust gas will get condensed to form white smoke.
Q526 By comparing the exhaust gas outlet of each cylinder in a diesel engine, one method of determining if
the engine load is balanced is by the use of
Ans. (d) pyrometer.
Pyrometer will show the exhaust gas temperature of each unit. Generally and roughly, it is accepted that the
exhaust gas temperature is the measure of power developed by individual unit. Hence higher the exhaust
temperature, higher will be the power developed and lower the exhaust temperature lower is the power
developed. This is a rough estimation and cannot be utilized unless it is backed up by other parameters such as *
compression and * maximum.
Q.527 One cylinder of a diesel engine is persistently knocking and does not ceases when the fuel oil
supplied to the cylinder is secured. What is the following problem which can cause this?
Ans. (d) A mechanical defect in working part.
If the diesel engine one cylinder is knocking the knocking may fuel related or mechanical In order to know the
exact cause of knocking first the fuel is cut-off. If after cutting of the fuel the knocking stops, it indicates that the
knocking is due to the fuel timing or the fuel quality. But if the knocking continues even after the fuel is cut-off,
it is a clear indication that somewhere metal-to-metal contact is taking place and there is mechanical defect in the
working parts.
Q.528 A diesel engine is warmed up and white vapor is noted in the exhaust. What does it indicate?
Ans. leaky cylinder liner.
If engine is properly warmed up and is working at proper load and under such circumstances if there white
vapour is indicated in the exhaust, it is an indication that at some point water is finding its way into the
combustion space. This may be due to a liner or cylinder head crack.
Q.530 Oil accumulating in exhaust gas piping or manifold of a diesel engine can be caused by
Ans. (b) worn valve guide.
If the valve guide is worn out, the clearance between the valve guide and valve stem will increase and the valve
guide will be lubricated by the use of rocker lubrication oil. If the clearance will be more, then the finer particles
of oil will, along the stem of the valve, enter into the exhaust manifold.
Q.531 Diesel engine operating conditions are indicated by color of exhaust smoke. Blue smoke can indicate
Ans. (c) clogged drain holes in the oil control ring.
If the drain holes in the oil control ring get clogged, the oil which is trapped in the oil control link will not get
drained into the crankcase but it will remain on the cylinder liner and it will get burnt with the exhaust gas. This
burnt oil vapor will find its way along with the exhaust gas and hence the exhaust gas will have a blue tinge.
Q.532 If the diesel engine is smoking excessively under load, the cause could be
Ans. (d) plugged injection holes or low compression pressure.
If the compression pressure in one of the cylinder is low, the fuel will not burn efficiently and black smoke will
be emitted by the engine. Also if the injector holes are plugged or are choked, there will not be proper penetration
and the engine will have late ignition and because of this, there will be excessive black smoke.
Q.534 Which of the following condition can cause excessive lubricating oil consumption in a diesel engine?
Ans. (d) High lubricating oil temperature.
If the lubricating oil temperature is high, excessive vaporization of the lubricating oil will take place and
ultimately it will cause excessive lubricating oil consumption.
Q.536 Which of following condition is likely to develop if the thermocouple element of a pyrometer
becomes coated with excessive amount of combustion byproducts?
Ans. (b) Pyrometer response will be retarded.
The carbon deposits around the pyrometer sensor will retard the heat transfer because that coating will form an
insulating material and hence heat transfer will be affected. Also the heat transfer will take more time. Due to
this, the response in change in temperature will be retarded and the change in temperature will be registered not
immediately but comparatively late.
Q.537 When the opening pressure injector is greater than specified by the engine manufacturer which of
the following problem can be expected?
Ans. (a) quantity of fuel to be injected tends to decrease.
The effective delivery stroke of the pump plunger will reduce because it will take more time for the plunger to
travel before the sufficient pressure is developed. Hence as the effective delivery stroke is reduced, it will
automatically reduce the quantity of the fuel injected.
Q.538 Late fuel injection occurring, at or after TDC, is indicated by excessive exhaust smoke and
Ans. low firing pressure.
When the injection is late, the fuel will get ignited late when the piston is already on its downward travel and
because of this the firing pressure will reduce and afterburning will take place.
Q.539 In a diesel engine, late fuel injection is indicated by black or gray exhaust smoke with
Ans. (a) low firing pressure.
As the firing is late, the combustion of the fuel is late when the piston is moving downwards and because of that
the volume will increase. Hence the firing pressure will reduce.
Q.540 If a crankcase explosion occurs in a diesel engine you should stop the engine and
Ans. (b) allow the engine to cool naturally .
The crankcase doors should not be opened immediately as if they are opened there is a possibility that outside air
may enter the crankcase and result in secondary explosion. In order to avoid the secondary explosion, which is of
higher intensity and it will cause large scale destruction, it is necessary to allow the engine to cool down
naturally and not by forced ventilation.
Q.541A diesel engine will lose power if the injection occurs too late in a cycle because of
Ans. (c) maximum expansion of burnt fuel cannot take place in the cylinder.
If the fuel injection occurs too late then in such conditions the maximum pressure will be reduced. At the same
time, when the actual burning of the fuel takes place, the piston will be on its downward movement under which
the volume will increase and the pressure cannot be increased. Ultimately, the power developed by the engine
will reduce because the power developed by the engine will be given by
Power developed = PLAN
In the above formula as P reduces the power developed is reduced
Q.542 Which of the following condition will be developed when unburnt fuel contaminates the crankcase
of a diesel engine?
Ans.
When the unburnt fuel oil finds its way into the crankcase, it will mix with the lubricating oil from the crankcase
and bring down the viscosity of the lubricating oil. Also it will bring down the flash point and fire point of the
lubricating oil
Q.545 Higher than normal temperature air passing through the intake of a diesel engine will result in
Ans. (c) low horse power .
If the air entering the cylinder is at high temperature, the volumetric efficiency of the cylinder will reduce
because the quantity of the air which enters the cylinder reduces. When this happens, automatically the fuel
injected will not burn properly. Hence the thermal efficiency will drop down and will cause black smoke and
higher exhaust temperature and low power is developed.
Q.546 When the fuel injector nozzle overheats which of the following problems may developed?
Ans. (a) The fuel metering will vary.
When the nozzle gets overheated, the clearance between nozzle and needle guide will change due to which there
will excessive leakage pass the needle. Also the needle will not seat properly on the seat and there will leakage
pass the seat. Due to all this the total quantity of fuel which is injected will vary.
Q.548 If fuel injection in a diesel engine begins earlier than designed start of injection, ignition may be
delayed because of
Ans.
If there is excessive early fuel injection, as the compression pressure in the cylinder has not sufficiently reached
to cause the ignition of the fuel and because of this the fuel will get accumulated and burn with a heavy knock.
This is the reason for not advising early injection of fuel.
Q.549 Prior to starting, the purpose of turning over the main propulsion diesel engine with a cylinder test
cock open is to
Ans. (b) remove the condensation from the cylinder.
When condensation takes place of the moisture in the air on the top of the piston or if there is any water or fuel
accumulation and an attempt is made to start the engine, as the liquid is incompressible, there will be hydraulic
pressure developed which will act over the piston, cylinder head, connecting rod and crankshaft thus causing
them to bend, break or fracture. Hence it is advisable everytime to blow through the engine before starting after a
long stoppage.
Q.550 A fuel injection valve opening at pressure below than normal will result in
Ans. (b) early fuel injection.
If the fuel injection valve opening pressure is low, the pressure required to lift open the needle valve will be less.
This low pressure can be reached with a very little upward movement of the plunger and because of this fuel may
enter the chamber too early and this may also cause knocking.
Q.551 Faulty operation of diesel engine fuel injection nozzle can be directly caused by
Ans. water in the fuel oil supply or excessive fuel nozzle holder cooling or distorted fuel pattern or leakage
past the plunger into the oil drain.
(All the above noted things are possible for the faulty operation of diesel engine fuel nozzle.) Water will cause
sticking of the needle in the guide and excessive weardown of between nozzle and guide and missing of the
firing in between and so it will cause the improper combustion and surging of the engine. Excessive cooling of
the nozzle will cause the formation of sulfuric acid and it will get corroded. Due to the corrosion of nozzle, the
spray pattern of fuel which is injected will change. If the plunger and barrel clearance is more then also there will
be excessive leakage between the plunger and barrel and this will also cause the late opening of the fuel injector.
At the same time, because of the drop in pressure, the spray will not be proper and atomization and penetration
will not be good.
Q.552 Water in the fuel can prevent the engine from starting, preventing it to develop full power or run at
full speed.
Ans.
If there is water in the fuel the power developed by the engine will be reduced and it will miss firing. When
missing of firing takes place, the rpm will go on fluctuating continuously and smooth running of the engine will
get hampered.
Q.557 Operating a propulsion diesel engine at less than 30% of designed normal load for prolonged
periods will result in
Ans. (d) carbon formation on the combustion chamber surfaces.
When an engine is operated at such low loads, the turbocharger efficiency drops. When the turbocharger
efficiency drops, less quantity of air is supplied to the cylinder. Hence the quantity of air available in the cylinder
for combustion is reduced. This will automatically reduce the quantity of fuel which is burnt. At the same time,
the fuel will not burn efficiently. Thus unburnt fuel will lead to carbon formation and the combustion chamber
surfaces will get coated with the carbon deposits.
Q.559 Operating a diesel engine under light load and at lower temperatures for extended period can result
in
Ans. (a) formation of carbon on the inlet and exhaust valves.
When the engine is operated at low load and low temperature for a longer time, the turbocharger efficiency
drops. When the turbocharger efficiency drops, the quantity of air supplied to the engine reduces. This results in
improper combustion and hence large amount of carbon formation takes place in the combustion chamber. When
running at low load, the exhaust gas pressure and exhaust gas energy are not sufficient. Hence there will be
backpressure exerted on the exhaust. If the same overlap is maintained between exhaust and inlet valve is
maintained during that time, then when the inlet valve opens the exhaust pressure is not sufficiently dropped.
Under such circumstances, momentarily there will be back-flow of exhaust gas from the cylinder into the air inlet
valve before the actual flow of the inlet air to the unit starts. During this time, the formation of carbon takes place
in the air inlet valve.
Q.560 Which of the conditions listed occurring in the diesel engine would cause carbon deposits to develop in
the piston ring belt.
Ans. (d) all above i.e. faulty combustion, excessive ring temperature, over-lubrication.
The carbon deposits develops in the piston ring belt because of
1. Improper combustion: The improper combustion of fuel results in carbon being deposited.
2. Excessive ring temperature: If ring temperatures are excessive, then lubricating oil will get burnt which
also leads to the formation of carbon.
3. Over lubrication: The excessive lubricating oil that is supplied to the piston ring will get burnt and convert
into ash or carbon thus forming the excessive coating of the carbon on the ring.
Q.561 If a crankcase explosion occurs in a diesel engine which of the listed action should be taken.
Ans. (c) Allow the engine to cool before opening the crankcase.
There will be two kinds of crankcase explosion, one a primary explosion and other the secondary explosion.
When the primary explosion takes place because of the oil mist and the appearance of hot spot in the crankcase,
there is a high-pressure rise in the crankcase which is released into the engine room atmosphere with the help of
crankcase relief valves. At the same time, the non-return valve will shut and not allow the new air to enter into
the crankcase. The crankcase atmosphere will be highly rich and hence there is a less possibility of exposure.
Under such circumstances if by mistake the crankcase door is opened, fresh air will enter the crankcase and bring
the crankcase atmosphere in the range of explosive limit. When this happens, there will be heavy explosion and a
secondary explosion may take place. So the best thing to do is to allow the engine to cool down before opening
the crankcase doors.
Q.562 The burning of fuel oil in a diesel engine having a high sodium content will cause
Ans. corrosion and grooving of exhaust valves.
In general, fuels leaving the refinery have sodium content well below 50mg/kg. If the sodium content increases,
which is normally caused due to seawater contamination. A 1% seawater contamination represents potentially a
100mg/kg increase. Vanadium is also present in the fuel oil, which combines with oxygen to form V 2O5
(vanadium pentoxide), which combines with sodium to form sodium/vanadium complexes. It is well known that
there are low melting temperatures of sodium/vanadium complexes of certain critical ratios. The most critical
sodium/vanadium ratio is about 1:3. This will form a sodium/vanadium complex with a low melting point which
will flow with the exhaust gases. It will get deposited as a hard and brittle layer on the cold surfaces such as
exhaust valve spindles, turbocharger nozzles and turbine blades. This layer is highly corrosive and corrodes the
metal. It is also brittle and breaks away exposing the metal for fresh attack especially when they get deposited on
exhaust valve seats. The hard layer breaks and gives a cutting effect on the seat. Preventive measures can be
taken such as keeping the temperature of the exhaust below the melting point of V 2O5 and removal of sodium by
proper purification and proper draining of the settling tanks. Therefore high sodium content in the fuel oil will
result in corrosion and grooving of exhaust valves.
Q.563 The longer the ignition delay period resulting from improper use of low cetane fuel, the
Ans. more rapid the rise in the combustion pressure.
Longer the period of ignition delay, more quantity of fuel will accumulate before the actual firing will takes
place. As a result, there will be sudden increase in the combustion pressure. When the large quantity of
accumulated fuel ignites, there is a sudden development of the high-pressure wave. At the same time if the
ignition delay is too much, there will afterburning. When afterburning takes place, there will be high enforced
temperature.
Q.564 If the relief valve on the diesel engine cylinder lifts, the cause could be
Ans. (a) liquid in the cylinder.
When there is a liquid in the cylinder, the piston is moves upward. Hydraulic pressure will develop because of
the presence of liquid. As liquid is incompressible, large amount of hydraulic pressure is developed. In order to
save the engine components from damage, the relief valve will lift and release the pressure.
Q.567 The dry-type exhaust silencer clogged with soot will cause
Ans. (b) loss of engine power.
When the dry-type exhaust gas silencer gets clogged with soot, there will be large amount of exhaust gas to flow
and because of that the exhaust gas pressure will be higher. Hence the complete removal of exhaust gas will not
be possible which will cause the mixing of the exhaust gas with the incoming air. Thus it dilutes the incoming air
with the exhaust gas. This will cause improper combustion because of the starvation of the oxygen for the
combustion of the fuel. The exhaust temperatures will increase and consequently there will be loss of power in
the engine.
Q.570 The color of the exhaust from the diesel engine should be
Ans. (a) clear.
The clear color of the exhaust gas from the engine exhaust will indicate the best possible combustion without
any carry over of the fuel, lubricating oil, water in the fuel or improper combustion. So it should be noted that the
colour of the exhaust indicates the health of the engine or any defects in the combustion. If the colour of the
exhaust is black, it indicates improper combustion that may be due to some fuel system malfunctions or an
overloaded engine. If the colour of the exhaust is white, it is consists of water. If the colour of the exhaust is
bluish, then it indicates the burning of lubricating oil in the engine. So clear exhaust is an indication that the
combustion is perfect.
Q.571 Engine operating conditions maybe indicated by the colour of the exhaust smoke. Black smoke could
indicate.
Ans. (b) an overloaded engine.
If the engine is overloaded beyond particular limit then, the quantity of the fuel will be more but comparatively
the quantity of the air will be less. Hence larger quantity of fuel, which is injected into the unit, will be difficult
to burn in allotted time and so
after-burning will occur. This will increase the exhaust temperature and black smoke will be formed.
Q.574 With regards to diesel engine crankcase explosion, the most violent is
(i) secondary explosion.
(ii) primary explosion.
Ans. (ii) only.
Q.576 Some diesel engines are fitted with thermometer in cooling water outlet of each cylinder. The cooling water
temperature from all the cylinder being risen above the normal, you should suspect
Ans. overloading of all the cylinders i.e. overloading of the engine.
If overloading of engine takes place, then the quantity of heat taken out by the cooling water will be very high and in that
case jacket cooling water temperature will increase. Also, it may increase due to the malfunctioning of the control valve
fitted on the water cooling system.
Q.577 Some diesel engines are fitted with thermometers in the cylinder. If the cooling water temperature
from an individual cylinder begins to rise you should suspect
Ans. (b) overloading of that cylinder.
If the heat generated in that cylinder is excessively high, heat carried out by water from that unit will be very high and
hence the jacket cooling temperature of that unit will increase. It is an indication that there is a overloading of that particular
unit. This may be because of leaky fuel valve, or fuel pump rack stuck up in the high fuel condition.
Q.578 A diesel engine using lubricating oil having too high viscosity will exhibit
Ans. increase in starting difficulty in cold weather.
If we are using high viscosity oil and having low viscosity index, because of the excessively high viscosity, there will be too much of
drag and excessive friction between the bearing. The loss of power to overcome this drag will be so high that it will be
difficult to start the engine from the cold and most of the power, which is used for starting and turning the engine, will be
wasted to overcome this friction caused by the high viscosity.
Q.579 One result of operating a diesel engine at light load with excessively low cooling water temperature is
Ans. (b) increase in the cylinder misfiring.
If the engine is operating at low load and with low cooling water temperature, the compression temperature and
compression pressure will be very low. If the fuel is injected, then it may stop igniting and may cause misfiring of the
engine.
Q.580 Which of the following condition may tend to increase the ignition delay period of combustion in a
compression ignition engine?
Ans. (b) Decreasing the charge air temperature.
If the charge air temperature is decreased, automatically the temperature at the end of compression stroke will also
decrease. When the temperature at the end of compression decreases beyond certain limit, it will be difficult for the fuel
which is injected to convert into combustible mixture and large amount of time will be required to achieve this process. In
this situation, the time delay will increase, especially in compression ignition engines
Q.583 The longer the ignition period occur in a diesel engine the more rapid rise in the combustion pressure.
More delay will give more accumulation of the fuel in the combustion space before the actual firing of the fuel takes place.
As a result, the accumulated fuel will get ignited. When the accumulated fuel will get ignited suddenly or instantaneously, it
will cause excessive amount of pressure rise in the cylinder.
Q.586 Combustion knock can occur in the cylinder of a diesel engine under any condition permitting
Ans. (c) excess fuel in the combustion chamber.
Excessive fuel in the combustion chamber may be due to ignition delay and cause sudden increase in the pressure. This
excessive pressure will cause excessive knocking effect in the engine.
Q.587 Persistent knocking in one cylinder of a 8-cylinder diesel engine is most likely to be caused by
Whether the knocking in the cylinder is by fuel oil or not, it can be checked by cutting of the fuel oil and checking whether
the knock persists or not. Even if the knock persist after cutting the fuel oil it indicates that there is some excessive
clearance in some of the running parts. Most possible place is badly worn piston pin and bush. Hence every time when the
piston is changing its direction from upwards to downwards it will give the knocking effect.
Q.590 When oil vapors, oxygen and hotspots are present at the same time with in a crankcase which of the following
hazards could develop?
Ans. (a) Explosion.
When oil vapor, hotspot and oxygen is present the triangle is completed for fire resulting in explosion. This is called
primary explosion which will be followed by secondary explosion.
Q.591A possible cause for an individual piston to knock when at TDC on a slow speed two-stroke diesel engine could
be due to
(a) early fuel injection.
(b) excessive bearing play between the gears.
(c) overloading of the cylinder.
(d) All of the above.
Ans. (d) all of the above.
The main reason is that if there is excessive early fuel injection, fuel will get accumulated and there will be sudden firing of
the cylinder and cause the knock. Excessive bearing clearance will cause the mechanical knocking. Also excessive bearing
clearance between the running gears will cause low compression pressure. Due to low compression pressure automatically
the ignition will get delayed and knocking will take place. Also because of overloading of engine, excessive fuel will be
loaded in the engine and the time available to burn will be less. Hence more fuel will accumulate and that accumulated fuel
will ignite violently to give knocking. In other words, knocking will generally be because of increase in the ignition lag.
Q.594 One remedy for a high firing pressure, in addition to high exhaust temperature, in one cylinder of a diesel
engine is to
Ans. (c) adjust the fuel rack.
If both the exhaust temperature and peak pressure is high, it indicates that the quantity of fuel injected is more. Hence it is
necessary to reduce the quantity of fuel in the cylinder. This is done by adjusting the position of fuel rack. Due to this the
quantity of fuel injected will automatically get reduced and knocking will be stopped.
Q.595 If a diesel engine is difficult to start and operates at engine temperature lower than normal you should suspect
that
Ans. (a) compression pressures are below normal.
If the compression pressure is below normal, it will be difficult to start the engine immediately as the fuel does not burn
immediately. As the engine is cold and compression pressure is low, it will be difficult to start the engine.
Q.597 White smoke issuing from exhaust of an auxilary diesel engine would mean
Ans. (b) engine is cold.
If the engine is cold, the exhaust will have condensation of the moisture. At the same time, the exhaust gas generated will
cool down and the moisture in the exhaust gas will also get condensed. This will result in white smoke from the exhaust gas
outlet.
Same as 597
Q.599 Bluish smoke in the exhaust of an operating diesel engine can be caused by
Ans. (b) cold cylinder liner
When the cylinder liner is cold, lubricating oil will not get scrapped out of the liner into the crankcase but it will get burnt
along with the fuel into exhaust gas. This will result in blue smoke emitting from the exhaust manifold
Q.600 One of the factors limiting the amount of load which can be put on a modern diesel engine is
Ans. (b) exhaust temperature.
Exhaust temperature is a criteria for loading the engine because higher the exhaust temperature, the higher the temperature
to which the components of machinery in combustion chamber are subjected. Higher the temperature the components are
subjected to, they lose their strength much earlier and hence their load carrying capacity decreases. Hence the components
fail under the influence of higher temperature and pressure.
Q.602 If a diesel engine runs roughly, which one of the system listed is likely to be at fault?
Ans. (a) Fuel system.
If the fuel is of bad quality, it may have misfiring. When misfiring takes place, there will be fluctuation in the load and rpm
of the engine.
Q.604 Which of the listed prestart procedures should be carried before starting a crosshead-type diesel engine after
an overhaul?
(a) pre-lubrication of the cylinder with hand crank.
(b) open all air space drain cock.
(c) open all indicator drain cocks.
(d) All of the above.
Ans. (d) All of the above.
Before starting engine, it should be necessary that there should not be any accumulation of water or fuel on top of the
piston. Otherwise, there will be hydraulic pressure developed and cause damage to the engine. It is therefore necessary to
open the indicator cock and turn the engine on turning gear. Also, the indicator cock should be kept open before starting and
do blow-through with air. Secondly, when the engine is in standstill condition, the cylinder liner and the valve seats are dry.
Hence it is necessary to supplement it with the lubricating oil so that initially during few strokes, when the cylinder
lubricator is not injecting, sufficient amount of lubrication oil is available on the cylinder liner wall. This is carried out by
the lubricator turning by hand. Also, it is necessary in order to avoid the entry of water through the air which may be
because of condensation of moisture in the air, it is necessary to drain the air manifold.
Q.606 Late fuel injection in a diesel engine is indicated by low firing pressure with
Ans. (c) black or grey exhaust smoke.
When the fuel injection firing is late there will be afterburning. When afterburning takes place, the exhaust temperature will
be high and as the burning takes place under increase in volume condition, the pressure will drop. At the same time, there
will be large amount of black smoke.
Q.608 An incorrect spray pattern of a diesel injector nozzle can be directly caused by
Ans. overheating or overcooling of the nozzle or carbon blockage of spray holes.
Under all the three condition, there will be distortion of fuel pattern and loss of power and high exhaust temperature. If
there is overheating, the needle will not sit properly on the seat. At the same time, the clearance between the needle and
guide will change which will cause the leaking and because of which the spray pattern will change because of drop in
pressure and leaking of the needle valve. In case of overcooling, the nozzle will get distorted because of corrosion and
because of which the spray pattern will change. Thirdly, carbon blockage of spray holes will cause the distortion of the
spray as the fuel will hit the carbon deposits and it will be distorted.
Q.609 High cylinder pressure accompanied by low exhaust temperature can result from
Ans. (d) excessively injection timing.
If the fuel is injected early, there will high firing pressures but low exhaust temperature because enough amount of time will
be available for the fuel to mix properly and form a combustible mixture. Hence the combustion will be perfect. When
combustion is perfect, it will produce more amount of power and there will not be any afterburning. Hence the exhaust
temperature will be low but the firing pressure will be high.
Q.612 A diesel engine will lose power if the fuel injection occurs too early because
Ans. (b) the ignition will be delayed due to low ignition temperature.
If the injection is too early, the compression temperature and compression pressure will not be sufficient to ignite the fuel.
Hence fuel will get accumulated and the fuel will burn with explosive pressure that will give the knocking effect. At the
same time, there will be afterburning and this would cause excessive loss of power.
Q.613 When the fuel is injected in a diesel engine cylinder too early
Same as 612.
The ignition will be delayed because the temperature of air will not rich the
self-ignition temperature of the fuel. Also, there will be excess of fuel in the cylinder resulting in too
rich mixture and hence improper combustion will take place and hence there will be loss of power.
Q.614 Which of the following should always be checked prior to starting a diesel engine?
Ans. (c) crankcase oil level.
Q.615 When high firing pressure and low exhaust temperature occurs simultaneously in a diesel engine it may be
because of
Ans. early timing of the fuel injection.
If the injection is not too early, there will be sufficient time for fuel and air to mix properly. At the same time, there will be
sufficient ignition temperature so that the fuel will burn completely and efficiently, and produce more power. As there is no
afterburning, the exhaust temperature will drop and the firing pressure will be high.
Q.616 If the firing pressure in a diesel engine is high, although the exhaust temperature is normal, the cause may be
Ans. (b) high compression pressure and high scavenge air pressure.
If the scavenge air pressure and compression pressure is high, the firing pressure will be high. As good quantity of the air is
available, at slightly low temperature, it will not effect any rise in the temperature but the firing pressure will be definitely
high.
Q.618 Which of the following statement represent a rolling of a diesel engine with a cylinder indicator cock open
prior to starting?
Ans. (b) To ensure foreign material, such as water or lubricating oil or fuel, is not present in the
cylinder.
If liquid, such as water, lubricating oil, or fuel, is present on top of the piston due to any reason, as these liquids are
incompressible, they may produce large amount of pressure which will cause breaking of the piston rings, liner, cylinder
head and in worst case it may damage the crankcase. In order to avoid this, the engine must be turned by turning gear with
the indicator cock open so that if there is any pressure developed or liquid on the top of the piston, it would be released
from the cylinder and would not be able to develop any excessive pressure in the cylinder.
Q.619 In order to start a large slow speed diesel engine, on high viscosity fuel oil after an extended shutdown, the
Ans. the fuel must be preheated.
Fuel has to be preheated so as to bring its viscosity down so that it can be injected. This is done by preheating of fuel. When
the viscosity is excessively high, there will be excessive drag and the atomization will suffer. When atomization suffers, the
combustion will not be proper and sometimes, the fuel may not get fired at all. Under such circumstances, it is necessary to
preheat the fuel before starting the engine after prolonged shutdown
Q.620 After starting a diesel engine which of the listed operating condition should be checked first?
Ans. lubricating oil temperature.
Q.621 What harmful condition could result in if a diesel engine is operated at very light load for a long time?
Ans. (a) increase carbon built-up.
If a diesel engine is operated at very light load for a long period of time, the combustion is inefficient because of low
compression temperature and pressure. Also the quantity of air will be less and under such circumstances, there will be
improper combustion of fuel and improper combustion of fuel will cause the formation of carbon deposits in the
combustion chamber.
Q.622 A diesel engine should not be operated at low load for a long period of time because
(a) heavy carbon deposit will be built up on the exhaust valve and exhaust manifold.
(b) fuel dilation is increased at low load.
(c) exhaust valve may get damaged.
(d) All of the above.
Ans. (iv) All of the above.
Due to lack of air available, the combustion will be affected and this will cause the improper combustion and carbon
deposition will take place. Secondly, as sufficient amount of air is not available for combustion, some amount of fuel will
remain unburnt which will get coated over the cylinder walls and will be scrapped downwards by the oil scrapper rings and
it may finds its way to the crankcase. The excessive fuel along the cylinder wall may get scrapped along with the excessive
lubricator oil and finds its way in the crankcase and the crankcase oil will get contaminated. At the same time, the carbon
deposits formed due to improper burning may get logged on the exhaust valve seat and because of that local burning of the
seat and the valve will take place which may damage the valve. At the same time, when the exhaust valve does not close
properly, there will be wiredrawing effect and the cutting effect on the exhaust valve seat and this will cause the exhaust
temperature to rise.
Q.623 The possibility of diesel engine crankcase explosion will increase by operating the engine
Ans. (b) by a leaking crankcase inspection cover gasket.
If the crankcase inspection cover gasket is leaking, there is a possibility of air entering into the crankcase thus causing more
amount of air available for the explosion to take place. This will be primary explosion. When primary explosion takes place,
the crankcase relief door will be opened and it will release the excess pressure. As soon as the pressure is relieved, due to
the leaky gasket, fresh air will enter into it and cause the secondary explosion. Hence these chances of severe explosions are
more in case of leaky crankcase inspection cover gasket.
Q.624 Main propulsion diesel engine continuous running after shutdown, you should now attempt to
Ans. (a) stop the air supply.
There is may be a possibility that if the fuel pump racks are stuck-up in one position, then even if the stop signals are given
by the governor to the engine, some of the fuel pump rack may not be brought back to the stop condition and the engine
will continue running. For this purpose, puncture valves are provided to avoid this situation. Under such condition, if the
engine keeps running and it is necessary to bring it to stop condition, the blanketing of the suction filter of the turbocharger
is the best method. By this method, the blanketing will deprive the engine of the air. When the engine is deprived of the air
the fuel will not ignite and the engine will stop.
Q.626 If the diesel engine has been stopped due to piston seizure due to severe overheating, the
crankcase inspection cover should not be opened
Ans. until the engine is cooled
Under such circumstances as the piston is hot, there will be hotspot present inside the crankcase and there is already
possibility of high amount of fuel mist. If at this moment the crankcase door is opened for cooling purpose, the fresh air will
rush inside the crankcase. When sufficient amount of air is available along with the mist and the hotspot is present, there
will be severe explosion and severe damage.
Q.627 Which of the following condition indicate dilution of diesel engine lubricating oil by fuel?
Ans. Drop of viscosity.
The dilution of diesel engine lubricating oil by fuel can be indicated by the drop in viscosity because fuel acts like solvent.
When the fuel acts like solvent, then automatically the lubricating will be become thinner and hence its viscosity will drop.
Also, its flash point will drop.
Q.629 A large slow speed diesel marine engine is operating at rated load and speed while the vessel is in calm seas. As
the intensity of the sea increases, the engine speed governor indicates same rpm, although the load indicator
indicates an
Ans. decrease in the load limit setting.
If the governor is rpm-dependent governor, it will try to maintain constant rpm irrespective of the load change. In order to
avoid the excessive loading of the engine and to keep the rpm constant, it is necessary to set low load limit. Under such
circumstances, the fuel will be given only to maintain the rpm and the load will be reduced. (To keep the rpm constant at
higher load we have to give more fuel. Hence main engines which are directly coupled to the propeller, the governors are
mainly constant load governors where they try to keep the load constant and avoid loading resulting in drop of rpm.)
Q.631 Which of condition listed could cause the cylinder relief valve on a large low speed propulsion diesel engine to
lift?
Ans. (b) Excessive fuel injection.
When there is excessive fuel injection, there will be excessive rise in the firing pressure. When the firing pressure reaches
above the setting of the relief valve, the relief valve lifts.
Q.633 An increase in diesel engine crankcase pressure indicates generally the excessive
Ans. d excessive piston ring blowby. (Already explained)
Q.634 If the diesel engine was running at 20% overload with a smoky exhaust, you should
Ans. (c) slow the engine, allowing it to cool gradually.
You have to reduce the load on the engine so that it will be brought slowly into the normal working temperature and hence
there will no increase in the temperature. Hence sudden stoppage should not be done; otherwise, there is a possibility that
the liner or the piston may crack due to sudden cooling.
Q.635 The starting air rotates a diesel engine at a proper speed but the engine fails to start, you should check
(a) overspeed trip
(b) for an obstruction in an air filter
(c) air in the fuel line
(d) All of the above.
Ans. (d) All of the above.
If the overspeed trip has taken previously, fuel lever has gone in blocking condition and hence it will not be released unless
you reset it. In such condition, even if the engine starts up on air, it will not pick up on fuel. Secondly, if there is air in the
fuel system, the fuel pump will not inject the fuel into the cylinder and hence the engine will not start on fuel. If there is
obstruction in the air filter, engine will not change over from air to fuel because the fuel will not burn due to lack of air.
Q.636 If the diesel engine turns over at normal crank * speed but fail to start the cause could be late fuel injection or
Ans.(c) inadequate fuel injection.
If the fuel injection is inadequate, during starting the engine has to overcome the inertia of rotating parts and from standstill
it has to speedup the engine to the full speed. For this purpose, you require excessive amount of power. Hence it is
necessary that initially while starting the amount of fuel injected should be more than the normal running fuel. If the fuel
injected is less, the engine will not pickup on the fuel.
Q.637 An increase in power output of a turbocharged diesel engine operating at constant engine speed results in
(a) high exhaust temperature.
(b) increased turbocharged speed.
(c) high air box pressure.
(d) All of the above.
Ans. (d) All of the above.
As the power output of the engine increases, more amount of thermal energy is available in the exhaust gas which can be
utilized by the turbocharger. Hence the exhaust temperature will increase and more heat is available at the turbocharger
hence the turbocharger speed will increase. When the turbocharger speed increases, it will increase provide more quantity
of air and hence the scavenge air pressure will increase.
Q.638 An accumulation of carbon on one of the thermocouple of an exhaust gas pyrometer will
Ans. read low temperature due to the insulation effect of the carbon deposits.
The carbon deposited on the thermocouple will act as an insulator and the actual temperature will not be read. At the same
time, the response to the temperature will also be low.
Q.639 Which of the listed state of condition indicate early fuel injection timing?
Ans. (b) High firing pressure and low exhaust temperature.
Early injection will induce a high firing pressure but low exhaust temperature (DISCUSSED
EARLIER)
Q.641 Clogged or partially obstructed exhaust port on a diesel engine can cause
Ans. high exhaust temperature.
As the exhaust ports are clogged or partially obstructed, the exhausting will be affected and hence complete removal of
exhaust gas from the cylinder will not take place. Hence the sweeping complete action of the exhaust gas by the incoming
scavenge air will not take place. The incoming scavenge will get mixed with the exhaust gas and there will be dilution of
the air charge with the remaining exhaust gas of the cylinder liner. Due to this, improper combustion will take place which
will cause the black smoke and loss of power will take place and high exhaust temperature because of the afterburning
effect.
Q.644 If two-stroke diesel engine is overspeeding due to lubricating oil leakage of the cylinder what should you do to
stop the engine?
Ans. (c) Shut of the fuel injector and block the flow of intake air.
By shutting of the supply of intake air the speed will be reduced. But if the lubricating oil is igniting, which is causing the
running of engine even after shutting the air supply, then under such circumstances, the only possible way of stopping the
engine is to stuck off the air supply by covering the suction filter with blanket.
Q.645 Two stroke diesel engine operates electrically, overspeeds and fails to restart when the crankcase at normal
speeds Which of the following problems is likely the cause for the engine failing to restart?
Ans. (c) Failure to reset overspeed trip.
Overspeed trip is a type of switch. This is provided for a safety purpose so that the engine speed will reduce after the
overspeed trip operates. It should not restart unless it is resetted manually. To reset the overspeed trip manually, we have to
bring the fuel lever to stop condition and we have to restart. If this is done properly, then even if we try to start the engine
on air, it will turn but it will not come on fuel because the fuel is blocked because the overspeed is already operated. Hence
it is necessary to reset the overspeed trip.
Q.646 When running a large slow speed main propulsion diesel engine on heavy fuel which of the following
precautions should be observed when switching back to diesel?
Ans. (b) the temperature of fuel from the preheater should be gradually retarded after switching over
to the three way valve.
Whenever the changing from diesel oil to heavy oil or vice-versa is to be taken place, it is necessary that the temperature of
oil heated or lowered should be carried out gradually so that there is no sudden shock thermal loading of the components of
the fuel injection system. At the same, it will allow proper expansion and contraction of different parts thus maintaining the
proper clearance between the running parts of the system. Otherwise there is a possibility that one part will cool at a faster
rate than other. In that case, there is seizure of the plunger in the fuel pump or needle in the guide and there will be heavy
damage.
Q.647 If it becomes necessary to cut-out an individual cylinder of a large low speed main propulsion diesel engine
Ans. the fuel oil to the cylinder should be secured and its fuel oil rate should be reduced.
As the unit is not firing, there is no need of cylinder lubrication. Otherwise, the excessive lubrication will go along with the
exhaust and damage the turbocharger and it may catch fire in the exhaust manifold causing the overspeeding of the
turbocharger.
Ans. (b) lower normal cylinder pressure and high exhaust temperature
This will happen because of afterburning not due to burning the fuel at right time. Due to burning during the expansion
stroke there will loss of pressure and increase of temperature
MOTOR MAINTAINENCE
Q.649 Bearing wear on sleeve type bearing is measured with
(a) ball anvil micrometer.
(b) pair of outside caliper and dial indicator.
(c) pair of inside caliper.
(d) depth micrometer.
Ans. (a) ball anvil micrometer.
The ball anvil micrometer has got a ball end, instead of flat, end so that it can measure the curved surface
thickness. Bearing being curved surface, ball anvil micrometer can be used for measuring the thickness and after
measuring the thickness of the bearing it is possible to measure the weardown of the bearing.
Q.650 Wear occurring at the tip of the reduction gear teeth is usually the result of
Ans. (d) gear misalignment.
The tip of the gear teeth gets worn out entirely because of gear misalignment. When there is misalignment
between gear, the tip of the gear will rub against the bottom of the gear teeth and cause excessive weardown.
This also gives the excessive axial thrust and radial thrust which will cause the weardown of the gear teeth.
Q.651 While inspecting the main bearings on a diesel engines you find impregnated dirt and scratches. You
would therefore suspect that
Ans. (c) the lubricating oil was not properly filtered.
If the lubricating oil is not properly filtered, dirt and hard particles will find its way between the bearing and the
shafting. Due to this, there will be scratching marks on the white metal of the bearing. At the same time, the hard
dirt will get impregnated inside the soft surface of the bearing.
Q.652 When nozzle tester is being used to test a closed type fuel injection nozzle a clogged nozzle orifice
will be indicated by a
Ans. (a) distorted spray pattern.
When the nozzle hole is clogged the spray pattern will be distorted because that particular hole will not have any
injection but same quantity of fuel will be injected by remaining holes. Hence complete pattern of fuel will be
distorted and that can be checked by the use of a plate which is held under the nozzle and the nozzle is tested.
Q.653 The spray holes in the diesel engine fuel injection valve should be cleaned using carbon solvent and
Ans. (b) a special cleaning wire.
The carbon solvent will dissolve only carbon but hard particles such as silica or alumina cannot be dissolved in
the solvent. Hence it is necessary to remove them with special cleaning wire which is provided. This cleaning
wire will clean the hole but it will not damage the holes. Hence cleaning should be carried out by special
cleaning wires.
Q.654 When dissembling or assembling an injection pump plunger and barrel, you must
(a) keep the parts immersed in diesel oil.
(b) always keep the plunger and barrel together.
(c) work over a linoleum type surface.
(d) All of the above.
Ans. (d) All of the above.
When dismantling pump plunger and barrel, if we keep them in the diesel oil there is less possibility that of any
rusting taking place on the surface and thus surface finish will be maintained. Secondly, they should always be
kept together because they are lapped surface so that no dirt will enter in between them. At the same time,
because of their better sealing effect, they should always be replaced as a pair and should always assembled as a
pair. Also, it should be worked over a linoleum type surface as it is smooth and dirt-free surface. Due to this,
there is a less possibility this surface will attract any foreign dirt or any hard particles in the surface. So linoleum
type surface can be kept clean by just wiping it out and the fuel injector can be kept clean without getting the
scratching due to the hard particles.
Q.655 Visual inspection of fuel injector valve removed during overhaul shows heat discoloration of lower
end of the valve. This is indicative of
Ans. (a) valve leakage.
Fuel injector valve is a non-return valve and it seals the nozzle against the high temperature and high-pressure
gases. When the fuel injector needle valve is leaking, high temperature gases from the cylinder will enter the
nozzle because of which the nozzle will get overheated and it will get discolored.
Q.656 A visual examination of chrome plated piston compression ring at the liner port reveals vertical
brown strip on the face of the rings. This condition indicates
Ans. (c) ring blowby.
When ring blowby takes place the lubrication oil film between the piston and the cylinder liner gets distorted
resulting in the piston rings being coated with the brown tinge of the exhaust gas. At the same time, when this
happens because of lack of lubrication between the piston ring and the liner the excessive heat is generated. This
will also give discoloration of the liner and the piston rings, and vertical lining on the surface of the piston ring
called scuffing
Q.657 Uneven bolt tightening during the installation of fuel injection pump can result in
Ans. variation of clearance between the plunger and the barrel.
Due to the uneven tightening of the bolts during the installation, it will result in variation of clearance between
the plunger and the barrel. Hence the clearance between them will not be uniform all around the circumference.
There is a possibility that seizure may take place between the moving parts of the pump where there is less
clearance; therefore it is necessary to have uniform tightening.
Q.659 If the plunger or barrel of a fuel injection jerk pump gets damaged
Ans. (a) the entire pump must be replaced.
As the pump plunger and barrel is manufactured as an assembly, there is good sealing between the plunger and
barrel and both surfaces are lapped together to form air-metallic seal. Hence it should be replaced as an assembly
and because good bearing between the two surfaces is possible if they are replaced together and not replaced
individually.
Q.660 Which of the listed devices could be used as a substitute for a ring-grooving tool?
Ans. (c) Section of removed compression ring.
Q.662 It is easier to replace a dry cylinder liner than wet liner because
Ans. (c) water seals are not required.
In case of dry cylinder liner as there are no water seals or o-rings to seal the water spaces and hence replacement
of the liner becomes very easy. In case of water cooled liner the sealing rings has to be properly fitted. At the
same time, the surface on which the sealing rings are to rest should be properly cleaned; otherwise the leakage
possibilities are more which is not applicable in case of dry liner.
Q.663 To facilitate early ring seating of newly installed piston rings while still providing extended rings
wear
Ans. (c) a tapered faced ring can be used.
A tapered face ring can be utilized for faster running in of the cylinder liner and the rings due to which sealing
between the cylinder liner surface and the piston can be obtained in a shortest possible time. Due to tapered face,
the surface contact area is reduced and because of this the pressure between the ring and cylinder liner increases
which causes the faster weardown of the piston rings and cylinder liner, which ultimately results in better sealing
between them in a shortest possible time.
Q.654 Insufficient end clearance on newly fitted piston rings on a diesel engine will cause the rings to
Ans. (a) jam in the least worn part of the cylinder liner when the ring expands.
The end clearance, also called the butt clearance, is less when the piston ring expands, the two ends of the piston
rings will butt against each other. As there is no further expansion possible in the circumferential direction, they
will have to expand in the radial direction. In doing so, it will give excessive pressure over the liner wall and
cause the seizure of the piston rings into the cylinder liner.
Q.665 Heavy fuel oil will generally have an upper average ash content of .1% by weight. Which of the
following condition will occur if the ash content exceeds the above amount?
Ans. (b) Increase valve wear.
The ash which is present in the fuel oil is mostly in the form of heavy metals. Hence when the percentage of ash
increases, this ash will get deposited on the valve seat. Due to this, the valve will not get properly seated on the
valve seat and there will be increase in the weardown. At the same time, there will be burning of valve and valve
slip may occur.
Q.666 The face surface appearance of shallow groove stainless steel chrome plated compression ring
should exhibit through its operating life
Ans. (a) smooth shallow groove chrome surface.
Q.667 To determine the main bearing clearance of a propulsion diesel engine, you should measure the shell
by using ball anvil outside micrometer and measure the crankshaft journal using the
Ans. (a) outside micrometer.
The weardown of the bearing can be measured by using a ball anvil outside micrometer so that it can the
thickness of curved surface of the bearings. By determining this, the total weardown of the bearing can be found
out which can be utilized to find the inner diameter of the bearing. By measuring the pin diameter, by using an
outside micrometer; it is possible to measure the clearance of the bearing by knowing the inner diameter of the
bearing and the pin diameter.
Q.668 Visual inspection of chrome plated piston compression rings reveals a black ring face at the position
of the cylinder ports. This condition indicates
Ans. the ring which has excessive blowby.
When excessive blowby takes place the ring surface which is in contact with the liner will get discolored. It will
become blackish and dry as the lubricating oil will get burnt. This is especially present in case of first piston ring
because first ring is subjected to high temperature and pressure and so blowpast is maximum on the first piston
ring which will make the ring surface into black.
Q.669 The consistent burning of fuel oil in a diesel engine with high sulphur content will result in
Ans. (d) increased cylinder liner wear.
The excessive sulphur in the fuel which is burnt will form sulphuric acid in the cylinder. When this acid gets
condensed on the liner wall excessive corrosion of the liner will take place. In order to avoid this, it is necessary
to use cylinder oil with a high TBN value or to keep the liner wall temperature high by using the jacket cooling
water temperature at a higher range.
Q.670 Burning fuel with high sulphur content in a diesel engine will
Ans. (d) produce corrosion in the cylinder and the exhaust system at low load.
Burning fuel with high sulphur content will produce sulphuric acid and at low load the temperature of the
exhaust gas and inside the cylinder is low. During this low load operation the sulphuric acid will get condensed
over the liner wall. Hence excessive corrosion wear will take place when fuel with high sulphur content is used at
low load operation.
Q.671 Which of the following statement concerning cylinder liner wear is true?
Ans. (b) Excessive liner wear causes the wear the between the piston ring and the grooves.
Due of excessive and non-uniform liner wear, the piston rings will try to maintain the contact with the liner wall.
As a result, there is continuous radial movement of the piston rings over the groove in the piston because of
which there will be friction between the piston rings and the groove. Hence the bottom of the groove will get
worn out faster if the cylinder liner is worn out excessively and non-uniformly.
Q.672 Generally where you should find the greatest amount of wear on the cylinder liner?
Ans. (c) Opposite the top ring shortly after the piston travel has ended the compression stroke.
The liner wear is maximum when the piston is at TDC. It is time when the piston is subjected to high temperature
and low lubrication. As a result, the cylinder liner is worn-out excessively along with the piston rings because of
the excessive pressure between the liner and the piston rings and lack of lubrication due to high temperature.
Q.673 Worn main bearing will cause the compression ratio of an auxillary diesel engine
Ans. (b) to decrease.
When the main bearings are worn out, automatically the crankshaft inorder to keep contact with the bottom half
of the main bearing will try to sink downwards. Due to this, the clearance volume between the piston and
cylinder head, when the piston is at TDC, will increase. This will cause the loss of compression pressure.
Q.674 Insertion shims in the main marine diesel engine between the connecting and the bearing box would
result in
Ans. (a) increase in compression ratio.
The marine type of connecting rod has a * and if that * is fitted over the bottom end bearing housing and by
inserting the shims between the palm and the bearing housing automatically the piston is raised up and when the
piston is raised up the compression ratio increases
Q.675 Diesel engine crankshaft deflections are taken at four crankshaft positions. Good engineering
practice require the deflection gauges are indicated to be
Ans. left in place for all four readings.
When the crankshaft deflection is taken it is done at four position of the piston. As a matter of fact actually
five positions of piston. During this, we are checking the distance between the two webs. Inorder to check
the distance between the two webs, it is necessary that the dial gauge fitted between the two webs should
be continuously kept in position to get the correct reading all around the cycle.
Q.676 An acceptable method of measuring the correct rotational force applied to the connecting rod and
main bearing bush is to use a
Ans. torque range.
The tightening of the bottom end bearing bush can be done safely and properly by use of torque range. It has got
silicon clutch and hence it will never get overtightened. When a particular torque is reached it will give a sound
which will indicate proper tightening without over stressing or over tightening of the bolts. Hence torque range is
the better method of tightening the bottom end bearing bolts. At present, there are other methods such as angle
and hydraulic methods which are much better than the torque method.
Q.677 Small cracks in the surface of diesel engine crankshaft bearings are an indication of
Ans. fatigue failure.
When the crankshaft bearing is subjected to fatigue, the strength at which it will crack will lower down and
hence the cracks appear. These cracks are quiet significant and are not because of corrosion or insufficient
lubrication or abnormal wear. This will not give the cracking. Cracking is due to the material which is under
stress due to fatigue.
Q.678 If water is found in the crankcase of a diesel engine the cause may be
Ans. due to cracked cylinder head, cracked cylinder liner and leaky cylinder gasket.
In case of four-stroke engine if the cylinder head is cracked, there is a possibility that water may enter the
crankcase when the engine is in stopped condition. If the cylinder liner is cracked water may enter from
the cylinder head through the gap between piston rings and it will enter into the crankcase.
Q.681 The wear is usually greatest at the top of the cylinder in a diesel engine due to
Ans. the highest pressure being exerted.
At the TDC, when the fuel is injected, high pressure is developed. This high pressure finds its way behind the
piston rings and they are forced against the liner walls with great contact pressure and thus more amount of
weardown will take place. At the same time, near TDC because of the high temperature, the lubricating effect is
reduced and hence there is more amount of wear of the liner and the piston rings.
Q.682 Which of the following operation will have a direct impact on the rate of wear of cylinder liner?
Ans. (b) Quality of the fuel injected.
If the quality of the fuel injected is bad i.e. it has more amount of sulphur or vanadium or sodium or abrasive
particles, excessive wear down of the liner will take place due to corrosive effect. At the same time, the wear
down may also take place due to abrasive effect.
Q.683 Failure to remove the carbon ridge from the top of the cylinder when replacing the piston ring will
result in
Ans. damaged upper piston ring and the ring landing surface.
If the carbon ridge is not removed and new piston rings are replaced, the top most ring will hit at the ridge
and may get damaged. At the same time, it will damage the ring landing surface. Hence it is necessary to
grind off the ridge when the piston is removed and the rings are replaced (repressed).
Q.684 Scuffed cylinder liner bearing surface in a diesel engine can result from
Ans. (c) operating the engine on overheating.
When the engine is operating on overheating condition (which can exist due to blowpast), the lubrication
between the piston and the liner is not effective because the lubricating oil will evaporate faster. Under such
circumstances, because of loss of lubrication oil, the sliding friction between the piston and liner walls will cause
great amount of heat which will ultimately cause the scuffing of the liner walls.
Q.687 The amount of wear on a speed precision main bearing can be accurately determined by comparing
the data from previous reading to the present reading taken with ball anvil outside micrometer.
Ball anvil outside micrometer can be utilized to measure the thickness of curved surface. So by comparing the
thickness (previous and present) of the shell bearing, we will be able to find out the degree of wear of the
bearing.
Q.688 On diesel engine equipped with individual jerk type fuel pumps, adjustments should be made to the
tappet push rods of the pump to
Ans. equalize effective delivery stroke.
The effective delivery stroke of all units can be equalized when the upward moving plunger will close the suction
ports of all the fuel pumps at the same crank angle. This can be achieved by adjusting the tappet of the plunger
under the jerk pump so that the effective delivery stroke can be adjusted equally.
Q.689 The amount of wear on a split fixed sleeve type main diesel bearing can be accurately determined
with a
Ans. (c) ball anvil outside micrometer. (ALREADY DISCUSSED)
Q.690 Friction, engine wear, and oil consuming in a diesel engine are directly related to the
Ans. viscosity of the oil.
In case of lubricating oil, viscosity of the oil place an important role in reducing the friction wear and wear down
of different components of the engine because the formation of film lubrication depends on the viscosity of the
oil. The viscosity of oil has to maintained in a preset condition so as to have proper film thickness and to reduce
the friction between the rotating parts and to take out the heat generated due to friction. Hence viscosity plays an
important part in determining the engine wear and friction. At the same time, if the viscosity is too low there is
possibility that excessive consumption will take place because of leakage of oil and more amount of evaporation
will take place.
Q.691 Improper cooling of diesel engine cylinder liner due to the accumulation of scale deposits may cause
Ans. increased piston ring wear.
When the liner wall temperature is high, the lubrication between the liner and piston will get evaporated. Hence
less amount of lubrication oil film will be there between the liner and piston rings thus causing excessive friction
between liner and piston rings causing excessive weardown of the piston rings.
Q.692 The nuts of main bearing, connecting rod bolts and all other moving parts are to be secured by
Ans. split pins or other effective means.
All the nuts of these parts after tightening should be locked by means of locking washers, star washers or split
pins so that they will not tend to loosen out. This is a safety measure in addition to hydraulic tightening.
Q.693 The easiest way to locate a defective diesel engine exhaust valve is
Ans. by comparing the exhaust pyrometer readings.
By comparing the exhaust pyrometer readings, leaky exhaust valves can be easily detected because exhaust gas
temperature will be high. It will be on the continuous rise and it will indicate that the exhaust valve is leaking.
This could be found out by taking a compression and peak pressure curves where we find that both the
compression and peak pressure curves for that particular unit is reduced.
Q.694 Engine coolant accumulating in diesel engine lubricating oil can result from a
Ans. cracked cylinder head or liner.
The cooling water finding its way into the engine lubricating oil in the crankcase is only possible because of the
cylinder or liner crack or the O-ring which seals the space between the liner and cylinder block are leaking and
hence water is finding its way into the crankcase.
Q.695 If it becomes necessary to clean the spray holes in a diesel engine fuel injector
Ans. you should use a suitable sized * wire and carbon solvent.
The carbon solvent will dissolve the hard particles around the holes. Hence the hard particles because of which
the holes are clogged get dissolved and it would be easy to remove the dissolved carbon particles by use of wire
and hence carbon solvent is necessary to clean the clogged spray holes.
Q.696 While over-oiling a jerk type fuel pump it is necessary to replace pump plunger which of the part
listed below must also be replaced?
Q.697 An acceptable means of tightening connecting rods and main bearing cap bolts is
(a) to measure the torque applied to each nut and bolt assembly
(b) stretch up each nut before and after tightening
Ans. (a) to measure the torque applied to each nut and bolt assembly.
The torque should be measured so that they are tightened to the required torque and they will not be
overtightened. Hence it is necessary to measure the torque applied to each bolt at the time of tightening. At the
same time, the elongation can also be checked to countermeasure to confirm the proper tightening.
Q.699 Problems with the diesel engine fuel injection pumps are usually caused by
Ans. (b) contaminated fuel.
When the fuel is contaminated with water or insoluble hard particles under, the fuel pump plunger and barrel will
get excessively wornout because of the abrasive wear and the corrosive wear and all the problems related to the
jerk type pump will start from here onwards.
Q.700 Dirt lodged on the nozzle valve seat of a fuel injector nozzle will cause
Ans. (c) fuel leakage after and before injection.
If the dirt will be lodged at the seat, the seat will not be leak-proof. Hence whenever there is low pressure inside
the cylinder liner or in combustion space, oil will leak from the seat and it will drip in the combustion space thus
causing the high exhaust temperature and loss of power.
Q.701 When a diesel engine is operated having a piston ring cold gap clearance less than that
recommended by that manufacturer, the ring will
Ans. seize and buckle.
If the ring gap clearance is less, the ring will expand. Thus ends of the rings will butt against each other hard. As
there is no way to expand, the rings will expand radially and ultimately the rings will crack and it will collapse
and break. At the same time, due to the excessive heat, which is generated due to pressure between the piston and
the liner, the piston ring will get seized inside the liner.
Q.702 Due to excessive water in the fuel diesel engine fails to start. Before engine is started the water must
be removed from the fuel pump fuel stainer.
If excessive water is developed in the fuel in that case the engine will not start. In that case, it is necessary to
remove the water from the fuel oil at settling tanks, service tanks through the filter so that when fuel injection
starts the engine will pick up on fuel immediately.
Q.703 Failure to remove the carbon ridge at the top of the piston while removing the piston may result in
Ans. seizure of piston and rings inside the liner.
It will be difficult to remove the liner due to the carbon ridge at the top which results in the seizure of the piston
and rings. Hence it is necessary to grind the carbon ridge in order to facilitate the removal of the liner. Otherwise,
the top most ring will get jammed up between the ridge and hence the liner may come out with the liner or the
piston may get jammed up in the liner. Hence it will not be possible to push it either upwards or downwards and
this is the most critical condition we can reach to. Hence it is necessary to grind it before attempting removal of
piston.
Q.704 If the piston ring gap remains insufficient when installing new rings, you should
Ans. file the ends of the rings.
Ends of the rings are to filed to have a proper butt clearance if required. As far as possible this should not be done
because it may damage the profile of the piston rings or it may cause the excessive weardown of the liner or
vertical scratching of the liner may take place because of the alteration of the profile of the piston ring. Hence as
far as possible when new rings are to fitted the original methods must be utilized where the end gap clearance is
always maintained.
Q.705 When inspecting piston rings through the ports of two stroke diesel engines black areas on the
sealing surface are
Ans. result of blowpast
Black areas are because of the blowpast where the lubricating oil get washed away. Due to blowpast and heat and
the carbon deposit this particular portion of the ring as well as liner wall there will be a new patch of black
surface. If this is happening, cylinder lubricating oil has to be increased or checked whether the piston rings are
free in the grooves.
Q.706 An examination of the piston compression rings of an efficiently operated diesel engine should
appear with a
Ans. (a) shiny face end bottom and black end at the top.
If the engine is efficiently operated when the piston rings are in very good condition, then the sliding surface of
the piston rings over the liner should be shining without any black markings or scratching. At the same time the
piston ring is rubbing at the bottom end of the groove so that the bottom end of the groove should also be
shinning. As the gas goes behind the piston rings into the backside some carbon get deposited on the top and
hence the top end of the piston ring may become black.
Q.707 Excessive side clearance between the side and the groove will cause
Ans. the ring to hammer the piston land below the ring.
Excessive groove clearance will cause the ring to hammer on the piston ring groove bottom surface. This will
especially happen when the piston is changing its direction, i.e. when the piston is at TDC and BDC, and the
piston ring may break due to this. At the same time, the piston ring groove will wear faster.
Q.708 In certain cases the cylinder liner can be refinished rather than replaced when the properly honned
Ans. liner surface should have a crosshatched pattern.
When the liner surface have excessive scuffing or polishing effect and is not possible to retain the lubricating oil
film on the liner surface then in that case it is necessary artificially to make it roughened out under the controlled
condition. This is done by honing which is a method of cross pattern which will have the roughening carried out
in two direction so that there will be tiny cavities on the liner surface which will retain the lubricating oil on the
liner surface thus reducing the liner and the ring wear.
Q.710 Which of the following statements concerning the cylinder liner wear in a single acting diesel engine
is correct?
Ans. (c) Excessive liner wear causes wear between the piston rings and the groove.
Excessive liner wear will cause the radial movement of the piston rings inorder to maintain the contact between
the liner wall and piston ring surface. This continuous radial movement of the piston ring over the entire stroke
of the piston will cause the piston ring to slide on the bottom of the piston groove. Hence there will be excessive
weardown of the piston ring groove as well as the piston ring landing surface if the liner wornout is excessive.
Q.711 On large slow speed main propulsion diesel engine lower main bearing wear is usually measured by
using
Ans. bridge gauge.
Considering that the crankshaft is sufficiently flexible and heavy enough, it will always keep the contact with the
bottom half of the main bearing. If the bottom half main bearing wears down, the centerline of the crankshaft
moves downward inorder to keep the contact with the bearing. Hence automatically the bridge gauge reading
which measures the position of the crankshaft centerline with the fixed machine surface transverse girder. If this
increases it indicates that weardown has taken place. This reading has to be compared with the previous reading
so as to measure the weardown of the main bearing bottom half.
Q.713 Abnormal crankpin bearing and piston skirt surface wear indicates
Ans. incorrect connecting rod alignment.
The connecting rod is to be aligned perfectly with the center of the liner and piston. If this alignment is not
carried out properly, the centerline of the piston with respect to the liner will get altered. Under such
circumstances, the piston will start rubbing against the liner wall thus giving excessive weardown of piston skirt.
At the same time, it will cause alteration in the lubricating oil film produced in the bottom end bearing and hence
bottom end bearing weardown will be non-uniform and excessive.
Q.714 To determine the main bearing clearances in a diesel engine, you should use
Ans. (c) plastic gauge.
Plastic gauges are like lead wire in the form of stripes. These stripes are to flexed between the top half of the
main bearing and the journal and the main bearing is to be tightened. When this is done the plastic will flow and
the thickness of the plastic will get reduced and the width of the plastic will increase. This increase in the width
of plastic is directly calibrated with the clearance in the bearing and hence the bearing clearance can be checked
with the help of plastic gauges. These are more accurate and safe than any other method of checking.
Q.715 The ring land on a large low speed main propulsion diesel engine piston may crack due to
Ans. insufficient ring groove clearance.
If the ring groove clearance is insufficient in that case the ring will expand and get jammed up into the ring
groove. As there is no further expansion possible it will exert excessive pressure over the land of the piston ring
groove and because of this the stress concentration on the piston ring groove will exceed and cracks will
originate on piston ring groove.
Q.718 A condition contributing to diesel engine piston ring sticking into the ring grooves is Ans.
insufficient ring clearance at the ring sides i.e. the groove clearance.
If the groove clearance is insufficient, the piston ring will expand because of heat and it will occupy the complete
clearance which is available. As there is no clearance left, it will get jammed up in the piston ring grooves and
hence it will get seized up in the piston ring grooves.
Q.722 Which of the listed problems can be caused by low compression pressure in diesel engine?
Ans. Combustion will not be proper.
Actually due to low compression pressure, combustion will not be proper and when combustion is not proper
there is a possibility that more carbon will be formed on the exhaust valve. The exhaust valve may start leaking,
black smoke and turbocharger may get fouled and because of that the scavenge pressure may drop. All this things
may happen because of the low compression pressure.
Q.724 When a nozzle tester is used to check the spray pattern of an injector nozzle
Ans. (d) the oil spray could cause blood poisoning if the spray penetrates the skin of the operator.
It is well-known fact that with the high-pressure injection of the fuel, the fuel in highly atomized condition will
penetrate the skin and the diesel will go into the blood and it will cause the blood poisoning. Hence during
checking of a fuel injector valve on a test bed, it is not advisable to put your palm under the nozzle.
Q.725 When the piston is removed from a diesel engine for maintenance the piston should be examined for
Ans. scouring, crack, burned spots, gummy deposits and sticky rings.
The piston should be checked for whether the rings are free. If the rings are free then the ring grooves should be
checked for formation of carbon and ash. At the same time, the piston rings and the landing site of the piston
rings should be checked for cracks and any gummy deposits which are there on the piston rings and the grooves.
This all will indicate if the lubrication is proper and quantity and quality of the oil is in good indication.
Q.726 To reduce the load during jacking operations which of the listed device should be operated?
Ans. Indicator valve or cylinder test valve.
During jacking operations i.e. turning the engine with turning gear it is necessary to open the indicator valve.
Q.727 After removing the old set of rings which of the following conditions is indicated if a bright spot is
found on each end of broken rings?
Ans. Butt clearance was not sufficient.
If broken piston rings are to be examined and the broken ends are found out to be polished and shining, it
indicates that the butt clearance was not sufficient. The main cause of piston ring breakage is the lack of butt
clearance. Once the crack takes place, some amount of material will get removed and the clearance will
automatically come into existence. This will cause the two ends of the rings to move over each other and hence
the broken ends will get polished and automatically it will become shining.
Q.728 After an engine has been started using a * drive unit the drive gear or pinion disengage from the
flywheel due to
Ans. the high rotational speed of the flywheel.
In the * drive when the engine starts because of high speed of the flywheel, the pinion speed becomes
excessively high. As a result, the disengagement of the pinion from the flywheel takes place and which avoids
the damage caused to the pinion.
Q.729 Bearing crush as applied to diesel engine main bearing will result in
Ans. (a) positive sitting of the bearing in the housing.
Due to the crush there is interference fit between the housing and the bearing. The back side of the bearing will
have frictional contact or interference contact or interference fit with the housing which will prevent the rotation
of the bearing in the housing when the shaft is rotating. Thus it will give positive sitting of the bearing in the
housing.
Q.731 When accumulated carbon at the air inlet port of two-stroke cycle diesel engine is removed you
should take care
Ans. to avoid carbon entering the cylinder.
If carbon enters the cylinder, it may get lodged between the piston and the cylinder and cause the sticking
of the piston rings. Ultimately it will cause the breaking of the piston ring and blowpast. Hence it is
necessary that carbon should not enter the cylinder especially between the cylinder and the piston.
Q.732 Replacement piping for the diesel engine high pressure fuel system must be of same length and diameter
as the original piping to
Ans. avoid excessive amount of ignition delay.
Q.736 Exhaust valve grooving and corrosion is caused by certain component of the residual fuel oil. These
components are
Ans. sodium, sulphur and vanadium.
Vanadium, in presence of sodium, will give the hot corrosion because in presence of sodium, vanadium
pentoxide which is formed because of burning of the vanadium will melt at much lower temperature in the range
of about 500C. This molten vanadium pentoxide is highly corrosive and it will give the corrosive effect. Hence
the combination of sodium and vanadium in the ratio of 1:3 is the most dangerous combination which will cause
the grooving. Both sodium and vanadium are present in the residual fuel.
Q.737 Which of the listed cylinder liner surface condition indicates proper lubrication?
Ans. Bright appearance.
The liner surface should be dull white without excessive brightness or black spots or bright spots. Black spots
indicate the blowpast and excessive bright spots indicates scuffing. Hence well runned and properly lubricated
liner will have a dull silvery finish on the liner surface.
Q.741 You are inspecting a lower main precision bearing on a diesel engine You observe that about half the
thin babbit lining are of milky white color The condition is caused by
Ans. wiping out.
If wiping out takes place, the top layer of tin i.e. overlay is wiped out. When overlay is wiped out there is bright
nickel barrier will be seen as shiny spots. Hence half the babbit is already visible in a milky white color which
indicates that wiping has taken place because of lack of lubrication or lubricating oil having water content or
excessive loading of the bearing.
Q.742 You are testing a non-popping closed fuel injector diesel nozzle using a nozzle tester. A pressure less
than the designed valve opening pressure is applied. If no fuel appear in the spray tip this indicates
Ans. the needle valve is sitting properly and is not leaking.
The needle valve must only allow the fuel to inject when the injection pressure is reached and not below it. If the
pressure is raised to less than injection pressure and if there is no leakage, it means that the injector is in very
good condition and it is leak proof.
Q.743 Because of closed tolerance used in diesel engines fuel oil pump a worn plunger requires
Ans. (b) replacement of pump plunger and barrel.
The plunger and barrel are lapped to form a best sealing surface between them and hence it manufactured and
lapped as a pair and it should be replaced as a pair. Replacing a single unit will cause excessive leakage or may
cause seizure of plunger in the barrel. Hence it should be avoided and complete unit should be replaced, i.e. both
plunger and barrel, should be replaced.
Q.745 When inspecting pistons, liner and rings of a large two-stroke diesel engine through cylinder ports a
wet piston crown could indicate
Ans. (a) leaky fuel injector.
If the oil is accumulated on the top of the piston crown there is no other way that the oil can come on top of the
piston other than a leaky fuel injector. When engine is in stopped condition, because of leaky fuel injector, the oil
is accumulated on top of the piston. Under such circumstances, immediate action of replacing the fuel injector is
most appreciated.
Q.746 Which of the listed devices could be used as a substitute for a ring grooving tools used to remove
hard carbon deposits from the groove in preparation for installing new rings?
Ans. (a) Section of removed compression ring.
A section of a removed compression ring can be wrapped all around by a thin Emery paper and the ring section
to be moved in the groove so that all around cleaning of the groove can be possible with the help of Emery paper
which is wrapped around the section of the compression ring just removed.
Q.747 If the plunger or barrel of a fuel injection jerk pump becomes damaged
Ans. The pump and barrel of the pump must be replaced as a unit.
Q.748 Surface irregularities such as erosion and pitting on the injection pump plunger will
Ans. effect the fuel oil metering.
If there is excessive erosion or pitting on the plunger then the integrity of the sealing between the plunger and
barrel will be affected. As a result, the high-pressure oil will leak past the plunger. Hence the quantity of oil
injected into the engine will reduce and which will affect the metering of fuel to the engine.
Q.749 Side clearance (groove clearance) of the compression rings on a diesel engine piston is necessary to
Ans. for the gas to have easy access to the back of the piston rings.
Due to the easy access, it will force the piston rings against the liner walls. Hence this clearance is very
important. If this clearance is less or the space between the ring and groove gets clogged or deposited with
carbon, sufficient amount of gas will not find its way behind the piston rings. Hence the contact pressure between
the piston rings and liner will reduce which will cause the blowpast and weardown of liner and breakage of liner
and piston rings.
Q.750 If lost motion is present in a individual fuel injection pump which of the following problem will
occur?
Ans. The fuel injection will occur late.
Due to the lost motion, the exact motion of cam will not be transferred to the plunger movement and hence there
will be late injection because the position of plunger with respect to the cam will be retarded.
Q.751 In a large low speed propulsion diesel engine if the injectors have formed carbon around the nozzle
holes which of the following would help avoid further building up?
Ans. Decrease the injection cooling water temperature.
This may be happening because of the excessively hot fuel injector nozzle due to which the sealing between the
nozzle and the seat is affected and nozzle will start leaking. If the nozzle starts leaking, formation of carbon
deposit takes place.
Q.752 A condition contributing to diesel engine piston ring sticking in the ring grooves is
Ans. insufficient ring clearance at the ring gap or side clearance.
If the groove clearance of the piston ring i.e. axial clearance between the piston rings and the ring groove if it is
less then in that case the piston ring will expand. At the same time some carbon deposit may take place between
the groove and the piston ring will cause the piston ring to get seized or stick up in the groove. Hence sufficient
clearance has to be maintained between the groove and the ring.
Q.753 In diesel engine when installing new piston rings it is important to check
(a) ring gap clearance.
(b) side clearance .
Ans. Both (a) and (b).
It is necessary to check both the butt clearance and groove clearance. If any of the clearance is less, it will cause
the breaking of the piston ring or seizing of the piston in the groove which will ultimately cause the
malfunctioning or improper combustion of the fuel and which may cause liner breakage, scuffing, piston ring
breakage or loss of power.
Q.755 When accumulated carbon at air inlet port of a two stroke diesel engine is being removed you
should take care to avoid carbon particles
Ans. entering the cylinder.
(ALREADY DISCUSSED)
Q.756 You are installing new piston rings on a single acting diesel engine piston. To check the ring gap
clearance, the ring should be placed
Ans. at the point of minimum cylinder wear.
At the point of minimum cylinder wear you will have the minimum gap clearance which will be your yardstick
for the proper operation of the engine. If the clearance is satisfactory at the position of minimum weardown then
it can be surely assured that when the piston is at top position where the weardown of the liner is maximum there
will be sufficient clearance between the ends of the piston. Hence during entire stroke of piston there will not be
a single position where the piston ring ends will butt against each other and cause cracking of the piston rings,
seizure of the piston ring or cracking of the liner. Hence as a safety measure, butt clearance is always measured
where there is minimum weardown.
Q.758 Which of the following problems is the main source of fuel pump and injection system
malfunctioning?
Ans. Improper adjustment.
Improper timing adjustment of fuel pump will ultimately lead to improper functioning of the fuel injector and
fuel pump which will ultimately lead to loss of power, excessive weardown of the piston rings, black smoke,
excessive carbon deposits, turbocharger surging. All these problems will be because of improper adjustment of
the fuel pump which will ultimately cause the improper adjustment or improper spraying of nozzle holes.
Q.760 If you have determined that entrained air in fuel nozzle system is causing a diesel engine to stall you
should first check for
Ans. a loose fitting in the booster fuel pump suction piping.
If there is loose fitting on the fuel booster suction piping, the fuel booster pump will draw the air. This air
will cause the problem of air traps in the fuel injection system because booster pump suction side is the
only low pressure side in the entire fuel injection system and hence possibility of air leakage is more.
Q.761 In a large low speed diesel engine excessive piston clearance can be restored by
Ans. inserting shims between crankpin bearing box and connecting rod foot.
By inserting shims between the bearing box and connecting rod foot the piston position can be raised in the liner.
When the piston position is raised in the liner automatically the clearance volume reduces and the compression
pressure increases. It can be restored back to the original.
Q.762 Which of the following procedures should be carried out to permit the use of crosshead engines with
an inoperable aftercooler?
Ans. Run the engine at reduced speed until the cooler can be repaired or renewed If the cooler is not
working then the temperature of the air which is admitted to the engine will be very high. When the admitted air
is high the density reduces and hence the quantity of air admitted will be less and the quantity of fuel that can be
burnt will have to be reduced. At the same time, as the air temperature increases the exhaust temperature will also
increases. Hence complete thermal loading of the engine will take place and at the same time because of higher
temperature in the cylinder lubrication will also get affected. Hence in order to avoid all these things it is better to
run the engine at lower load.
Q.763 A faulty injector in one cylinder of an operating diesel engine can be located by
Ans. (a) cutting out the individual injector and noting the engine performance.
Faulty injector in general can be detected by exhaust temperature of the unit but it is not a reliable method. Hence
one has to take the draw card. By draw card you can find out the compression pressure and peak pressure. If the
compression pressure is satisfactory and peak pressure is varying, it clearly indicates that the injector is bad.
Q.764 Which of the following conditions will cause carbon deposit to form in the piston ring belt of a diesel
engine?
Carbon deposit on the piston ring belt can be caused because of faulty combustion, excessive ring temperature
or over lubrication. All this will lead to carbon formation. If the combustion is not proper, more amount of
carbon deposits will be formed on the piston rings. If excessive lubrication is given the excess oil will get burnt,
it will form carbon and ash and they will also lead to carbon deposits on the piston ring. Also if the ring
temperature is excessively high i.e. the cooling of the piston is not properly done in that case the lubricating will
burn which will form carbon deposits on the piston rings.
Q.768 Which of the following problems may occur if the clearance between the piston and the cylinder is
insufficient?
Ans. Excessive wear, scuffing of the liner and piston seizure.
There will be excessive wear of the liner as well as the piston as excessive pressure is developed by the rings
over the liner walls. At the same time, the skirt of the piston will come in contact with the liner wall and wiping
out of the lubrication oil will take place which will also cause excessive wear of the liner. At the same time, as
the expanded piston will come in contact of the liner wall it will wipe out the lubricating oil film. When the
lubricating film is wiped out because of metal-to-metal contact between the piston and liner wall there will be
scuffing of the liner and blowpast. If the clearance is excessively low and the piston expands beyond that
clearance, the piston will get seized in the liner.
Q.769 Black areas on the sealing surface of the piston rings indicates
Ans. passage of hot gases or blowpast.
Black areas is an indication of blowpast and the lubricating oil is wiped out between the ring and liner and the
areas get coated with carbon deposits from the exhaust gas or high pressure gas.
Q.771 The best tool to remove the carbon ridge at the top of engine cylinder prior to removing the piston is
to use
Ans. metal scrapper.
Metal scrapper is used to remove to ridge formed because of carbon. If the ridge is formed because of the
weardown of liner and it is a metal ridge, it is to be removed by use of hand grinder as it cannot be removed by
metal scrapper.
Q.772 Which of the following devices is commonly used to measure the clearance between the main engine
bearing and crankshaft?
Ans. (a) Plastic gauge.
Plastic gauge are utilized like lead wire and they are made to compress between the bearing and the journal and
because of the compression the thickness will reduce and the width will be increase and the increase in width can
be directly calibrated in terms of clearance.
Q.773 Which of the following condition can result in cracking of the piston land?
Ans. Insufficient ring groove clearance.
If the ring groove clearance is insufficient, the ring will expand and it will occupy the complete space in the ring
groove. Further expansion of the ring in the groove will cause excessive stresses induced on the piston ring
groove which will cause cracking of piston land.
Q.774 Which of the following conditions may occur while using fuel with high sulphur content?
Q.776 Which of the following conditions could contribute to the cracking of diesel engine cylinder head?
Ans. Blocked cooling water passages to the head.
If the cooling water passages to the head are blocked or scale formation takes place in the cooling water spaces,
the cooling of the cylinder head is reduced and it is subjected to very high temperature and pressure. Because of
this high temperature and pressure, in order to maintain the strength of the cylinder head, it is necessary to cool
the cylinder head and maintain the temperature of cylinder head. If the cooling is not carried properly, the
strength of cylinder head will reduce which will ultimately cause the cracking of the cylinder head.
Q.779 Which of the following operating conditions can occur when shims are removed from the joint
between marine type diesel engine gearing box and the foot of connecting rod?
Ans. The piston position will move downwards with respect to the cylinder head.
When the shims are removed, the piston will move downwards with respect to the cylinder head. Hence the
compression pressure and clearance volume will increase and compression ratio will reduce.
Q.780 After long period of operation a wear ridge caused by piston ring action will developed near the top
of the cylinder liner This ridge must be removed when piston rings are removed inorder to
Q.781 Crank lead can be adjusted to change which of the listed operating conditions?
Ans. Longer combustion event.
When the crank lead is adjusted the injection timing gets adjusted. When the injection timing gets adjusted, the
combustion can be delayed or advanced.
Q.782 When hydraulic valve lifter is on the base circle of the cam, zero tappet clearance is maintained by
Ans. plunger spring.
When the hydraulic valve lifter is on the base circle the plunger springs will lift it against the base circle and
hence the tappet clearance is maintained. During this time, due to the spring action on the valve plunger, the
plunger will be pushed downwards and the roller will keep contact with the base circle of the cam. Hence zero
tappet clearance will be maintained.
Q.784 Which of the listed condition can cause lacquer to be deposited on the piston skirt?
Ans. High lubricating oil temperature.
Because of high lubricating oil temperature, the lubricating oil will burn over the piston skirt. When the
lubricating oil burns a lacquer like formation will take place and deposition will take place on the skirt.
Q.785 Scuffed diesel engine cylinder liner wearing surface in a diesel engine can result from
Ans. applying load to cold diesel engine.
When the engine is cold it will take some time to form a good sealing between the piston rings and the liner.
Hence the load should be kept low initially and increased gradually. If the load is suddenly increased, blowpast
will take place between the piston ring and the cylinder liner wall. When blowpast takes place it will wipe out the
lubricating oil between the liner and piston ring and which will cause the excessive friction between the liner and
piston ring which will ultimately cause excessive scuffing of the liner.
Q.786 If a diesel engine driving a generator turns over freely but fails to fire properly, the cause could be
Ans. air in the fuel.
If the engine can accelerate sufficiently to the firing speed and the fuel is released by the governor but the engine
does not pick up on fuel, this indicates that the fuel is of bad quality and one of the reason is that the fuel contains
air or water.
Q.787 When diesel engine compression pressure is checked
Ans. the indicator cork is kept open.
When the indicator cork is connected to the cylinder head, the indicator cork is kept open so whatever the change
of pressure is taking place inside the cylinder at different positions or movements of the crank or the main piston
will be directly indicated by the stylus movement of the indicator piston.
Q.788 In a diesel engine exhaust system the cooling of the exhaust gases below their dew point will result in
Ans. sulphuric acid corrosion.
If the fuel contains sulphur it will form sulphuric acid and if this sulphuric acid is in the form of vapour but if this
vapours are allowed to cool below the dew point then it will convert into liquid form and it will cause excessive
corrosion of the liner.
Q.789 Which of the following statements is true regarding the installation of piston rings on two-stroke
diesel engine as compared to four stroke diesel engines?
Ans. Some provisions must be made in two stroke diesel engines to prevent the rings from bending in the
ports.
In two-stroke engines, there will be ports in the liner walls over which the piston rings will slide. Hence it is
necessary to make sure that the ends of the piston rings will not get entangled with the ports and the piston rings
will not break. In order to avoid such things the ends of the piston rings, in two-stroke engines, are slightly
curved inside so that during the operating temperature they will not get entangled with the ports and get broken.
Q.790 In a large low speed diesel engine the clearance between the piston crown and cylinder head is
found to be excessive. Inorder to correct this
Ans. shims should be inserted between the crankpin bearing box and connecting rod foot.
In case of marine type of connecting rod, where connecting rod is fitted on top of bottom end bearing, it is
possible to insert the shims between the crankpin bearing box and connecting rod foot so that the position of the
piston in the cylinder liner with respect to the cylinder head can be altered. Hence the compression pressure and
clearance volume can be changed.