4thyear (2012-2013) Sheet No.

(2)
1) In a four stroke cycle S.I. engine the cam shaft runs
(a) at the same speed as crank shaft (b) at half the speed of crank shaft
(c) at twice the speed of crank shaft (d) at any speed irrespective of crank shaft speed

2) Explain with suitable sketches the working of a four stroke Otto engine.

Intake stroke: This starts with the piston at TC and ends with the piston at BC, which draws fresh mixture
into the cylinder.
Compression stroke: when both valves are closed and the mixture inside the cylinder is compressed to a
small fraction of its initial volume. Toward the end of the compression stroke, combustion is initiated and
the cylinder pressure rises more rapidly.
Expansion stroke: This starts with the piston at TC and ends at BC as the high-temperature, high-pressure,
gases push the piston down and force the crank to rotate.
Exhaust stroke: where the remaining burned gases exit the cylinder: first, because the cylinder pressure
may be substantially higher than the exhaust pressure: then as they are swept out by the piston as it moves
toward TC.

3) Explain the following terms as applied to I.C. engine:

Bore: Diameter of the cylinder or diameter of the piston face, which is the same minus a very small clearance.

Stroke: Movement distance of the piston from one extreme position to the other: tdc to bdc or bdc to tdc.

T.D.C.: is Position of the piston when it stops at the furthest point away from the crankshaft.
Top because this position is at the top of most engines (not always),
Dead because the piston stops at this point.

B.D.C.: is Position of the piston when it stops at the point closest to the crankshaft. It is not always at the
bottom of the engine.

Clearance volume: the volume of the compressed charge which is also the volume contained in the
cylinder above the top or crown of the piston when the piston is at TDC.
 Swept volume: the displacement volume swept by the piston in one stroke and is equal to the product of
the inner cross-sectional area of the cylinder and the stroke length of the piston.

Compression ratio: the ratio between the total volume to the clearance volume.

2L N
Piston speed: mean piston speed =
60

4) Discuss the difference between ideal and actual valve timing diagrams of petrol engine

For ideal valve time:

The intake valve open at TDC and closed at BDC.
The exhaust valve open at BDC and closed at TDC.

For actual valve time:

The intake valve open before TDC and closed after BDC.
The exhaust valve open before BDC and closed after TDC.
This difference to overcome the mechanical motion of the valve.

5) Define compression ratio. What is its range for S.I. and C.I. engines? What factors limit the
compression ratio in each type of these engines?
The compression ratio is the ratio between the total volumes of the cylinder to the minimum volume of
the cylinder.

For S.I.E. it is between 6 to 10.

It is not less than 6 because the efficiency is very low and not exceeds above 10 because the pressure
and temperature of the charge be high and will reach the self ignition point for petrol fuel.

For C.I.E. it is between 14 to 20.

It is not less than 14 because the pressure and temperature of the charge is still low and they not reach
the ignition point of the diesel fuel and not exceeds above 20 because the engine be heavy weight and
the increase in efficiency be low according to the amount of fuel used.

6) Show by suitable sketches the following types of cylinder arrangements indicating their
principal applications
a) In-line engines b) “V” engines c) Opposed piston engines d) Radial engines

7) Classify the internal combustion engine with respect to:

a) Cycle of operation:
a. Otto Cycle
b. Diesel Cycle
c. Dual Cycle
b) Type of ignition:
(a) Spark Ignition.
(b)Compression Ignition
c) Cylinder arrangements:
a. In cylinder
b. V-type engine
c. Radial engine
d. Opposite engine
d) Type of fuels used:
(a) Gasoline.
(b) Diesel Oil or Fuel Oil.
(c) Gas, Natural Gas, Methane.
(d) LPG.
(e) Alcohol-Ethyl, Methyl.
 (f) Dual Fuel. There are a number of engines that use a combination of two or more fuels.
Some, usually large, CI engines use a combination of methane and diesel fuel.
(g) Gasohol. Common fuel consisting of 90% gasoline and 10% alcohol.
e) Method of charging the cylinder:
For Fuel: a- Carbureted.
b- Multipoint Port Fuel Injection. One or more injectors at each cylinder intake.
C- Throttle Body Fuel Injection. Injectors upstream in intake manifold.

For Air: a- Naturally Aspirated. No intake air pressure boost system.

b- Supercharged. Intake air pressure increased with the compressor driven off of
the engine crankshaft
c- Turbocharged. Intake air pressure increased with the turbine-compressor
driven by the engine exhaust
d- Crankcase Compressed. Two-stroke cycle engine which uses the crankcase
as the intake air compressor. Limited development work has also been done on
design and construction of four-stroke cycle engines with crankcase
compression.
f) Type of cooling:
(a) Air Cooled.
(b) Liquid Cooled, Water Cooled.

8) Compare S.I. and C.I. engines with respect to:

a) basic cycle b) fuel used c) introduction of fuel d) ignition
e) compression ratio f) speed g) efficiency h) weight
Spark Ignition Engine Compression Ignition Engine
Basic Cycle Operated on Otto cycle. Operated on Diesel cycle.
Fuel Used Gasoline should have higher self Diesel fuel should have lower
ignition temperate. self ignition temperate.
Method of Introducing Fuel is evaporated, mixed with Fuel is injected inside the engine
of Fuel air and then supplied to engine cylinder inform of a fine spray.
cylinder.
By carburetor or injector By injector
Ignition Fuel is ignited by means of Fuel is ignited due to high
electric spark generated inside temperature at end of
the cylinder by spark plug. compression stroke.
Compression Ratio Operating at range of 7-10 Operating at range of 14-20
Speed Higher, because of lighter Lower, because of heavier
engine. engine.
Efficiency Lower because of lower Higher because of higher
compression ratio. compression ratio.
Weight Lighter than Diesel Heavier than Petrol

9) Define the following efficiencies:

a) indicated thermal efficiency
 b) brake thermal efficiency
c) mechanical efficiency
d) relative efficiency
e) volumetric efficiency

10) Choose the appropriate answer

1-Engines of different cylinder dimensions, power and speed can be compared on the basis of
a) maximum pressure b) fuel consumption c)mean effective pressure d)unit power

2-Thermal efficiency of C.I. engine is higher than that of S.I engine due to
a) fuel used b) higher compression ratio
c) constant pressure heat addition d) none of the above

3- S.I. engines are of

a) light weight b) high speed c) homogeneous change of fuel and air d) all of the above

4-Compressin ratio in diesel engine is of the order of

a) 5-7 b) 7-10 c) 10-12 d) 14-20

5- The volumetric efficiency of the S.I. engine is comparatively

a) lower than C.I. engine b) higher than C.I. engine
c) will the same as C.I. engine d) none of the above

11) The brake thermal efficiency for S.I. engine varies from:
a)40-50% b)45-60% c)25-30% d)15-25%

12) In a 4- stroke S.I. engine

a) The exhaust valve opens at 50o before bdc and closes 15o after tdc
b) The inlet valve opens about 25o before tdc and closes 15o after bdc
c) The exhaust valve opens 10o after bdc and closes 25o after tdc
d) The inlet valve opens 50o before tdc and closes 15o after bdc

13) The indicated thermal efficiency of 4-stroke S.I. engine producing 500 kW, mechanical
efficiency 0.8 and fuel consumption 300kg/hr (HHV=40MJ/kg)is
a) 0.12 b) 0.1875 c) 0.2 d) 0.2175

14) The volumetric efficiency of a 4-stroke engine (stroke volume 30 liters) running at 4000 rpm.
producing 50 kW and consuming 3000kg/hr of air at 1 bar and 300 K is
a) 60% b) 71.8% c) 75% d) 80%

15) The b.mep of a passenger car running on diesel fuel in comparison with the b.mep of a petrol
engine is
a) more b) equal c) less d) dependent on compression ratio

16) The volumetric efficiency of an internal combustion engine will depend upon
1-period of valve overlap 2-density of fresh charge
3-pressure of residual gas 4-design of intake manifold
Of these statements:
a) 1,2,3 are correct b)2,3,4 are correct c)all are correct d)1,3,4 are only correct

17) Engine cylinders arranged in the form of the letter” V” are

1-perfectly balanced 2-produce more power than in-line arrangement
 3-easy to manufacture 4-having smaller crankshaft than in-line ones
Of these statements:
a) all are correct b) only 2 is wrong c) only 4 is correct d) only 1 and 2 are correct

18) Select the suitable engine for the following purposes indicating the reasons for your selection.
1-engine power 70kW and its speed 200 rpm used for driving pump to raise crude oil from oil wells
2-engine drive passenger car with variable speed its power 30kW at 4000 rpm
3-Trucks engine, variable speed its power 90kW at 2500 rpm
4-marine engine its power 10000kW at 150 rpm
5-stationary engine used for electrical generation its power 8000kW at 7000 rpm
1 &3 &4 Diesel Engine 2 Petrol Engine 5 Gas turbine
19) What is meant by “quantity control” and “quality control” engines? Compare between these
engines.
The quantity control: it means that we can control of the quantity of mixture which enter to the cylinder of
the engine by control of the amount of throttle valve open.

The quality control: it means that we can control of the quality of mixture which enter to the cylinder of
the engine by control of the amount of fuel inject to the combustion chamber.

1) In petrol engine the pressure in the cylinders at 30% and 70% of the compression
strokes are 1.3 and 2.6 bar respectively. If the compression follows the law PV 1.33
= constant. Find the compression ratio?

Given:
Pa = 1.3 bar Pb = 2.6 bar

The solution:
Here: take V1 is total volume and V2 is the clearance volume be unit =1
V
The compression ratio r = 1 = V1
V2
The stroke volume = V1 - V2 = r - 1
Va = 1 + 0.7 (r − 1) = 0.3 + 0.7r
Vb = 1 + 0.3(r − 1) = 0.7 + 0.3r

Pa Va1.33 = Pb Vb1.33
1 1
Va  Pb  1.33  2.6  1.33
=  =  = 1.684
Vb  Pa   1.3 
also :
Va 0.3 + 0.7 r
= 1.684 =
Vb 0.7 + 0.3r
1.18 + 0.505 r = 0.3 + 0.7 r
 r = 4.51

2) The ideal Otto engine working on the air standard has a temperature and pressure
at the beginning of 25ºC and 1 bar respectively, and a thermal efficiency of 48%.
Determine the pressure and temperature at the end of compression.
 Given:
P1 = 100 kPa T1= 298 K η = 0.48

The solution:
Point 1: P1 = 100 kPa & T1= 298 K
k −1
1
where : th = 1 −   = 0.48
r
= (1 − th ) k −1 r =
1 1 1 1
= = 5.13
r (1 −th ) 1k −1 (1 − 0.48) 10.4
P2 = P1  (r ) P2 = 100  (5.13) = 986.7 kPa
k 1.4
Point 2:
T2 = T1  (r ) = 298 (5.13)
k −1
= 573.14 K
0.4

3) A petrol engine works on Otto cycle. the pressure, temperature and volume of air
at the beginning of compression are 0.93 bar, 38 oC and 0.028 m3 and the pressure
at the beginning and end of heat addition are 7.87 bar and 22 bar. If there are 250
cycles per minute, find power developed by the engine and the cycle efficiency.

Given:
P1 = 93 kPa T1= 311 K V1 = 0.028 m3 P2 = 7.87 bar
P3 = 22 bar N = 250 cycle per minute

The solution:
Point 1: P1 = 93 kPa & T1= 311 K & V1= 0.028 m3
Point 2: P2 = P1  (r )k 787 = 93  (r )1.4 r = 4.6
T2 = T1  (r )k −1 = 311 (4.6)0.4 = 572.6 K
V1 0.028
V2 = = = 0.0061 m 3
r 4.6
P   2200 
Point 3: P3 = Pmax = 2200 kPa & T3 = T2   3  = 572.6    = 1600.7 K
 2
P  787 
k 1.4
1  1 
Point 4: P4 = P3    = 2200    = 259.8 kPa
r  4.6 
k −1 0.4
1  1 
T4 = T3    = 1600.7    = 869.4 K
r  4.6 
 P1 (V1 − V2 ) 93  (0.028 − 0.0061)
where : m= = = 0.0228 kg
R  T1 0.287  311
Wnet = Qadd − Qrej = m  Cv  (T3 − T2 ) − m  Cv  (T4 − T1 )
Wnet = 0.0228  0.718  (1600.7 − 572.6) − 0.0228  0.718  (869.4 − 311) = 7.7 kJ
250
Power = Wnet  No. of cycles per sec. = 7.7  = 32.1 kW
60
Wnet 7.7
th = = = 0.46
Qadd 16.83
 4) The rotational speed of 4-strock engine working on an ideal Otto cycle is 4000 rpm.
The initial pressure and temperature are 100 kPa and 300 k respectively. If the
maximum volume to the minimum volume ratio is 11,heat add to system 2000 kJ/kg
air and stork volume is 2.2 lit, find the following:
a) The theoretical thermal efficiency
b) The engine's power and m.e.p.
Given:
P1 = 100 kPa T1= 300 K r = 11 Qadd = 2000 kJ/kg
Vs = 2.2 lit

The solution:
Point 1: P1 = 100 kPa & T1= 300 K
Point 2: P2 = P1  (r ) = 100  (11)1.4 = 2870.1 kPa
k

T2 = T1  (r )k −1 = 300  (11)0.4 = 782.8 K

Point 3: Qadd = Cv  (T3 − T2 )  2800 = 0.718  (T3 − 782 .8)  T3 = 4682 .5 K
T   4682.5 
P3 = P2   3  = 2870.1   = 17168.2 kPa
 2
T  782.8 
k 1.4
1 1
Point 4: P4 = P3    = 17168.2    = 598.1 kPa
r  11 
k −1 0.4
1 1
T4 = T3    = 4682.5    = 1794.4 K
r  11 

Wnet = Qadd − Qrej = Cv  (T3 − T2 ) − Cv  (T4 − T1 )

Wnet = 0.718  (4682.5 − 782.8) − 0.718  (1794.4 − 300) = 1727 kJ/kg air
P1 100
Wnet = 1727   Vs = 1727   0.0022 = 4.41kJ
R  T1 0.287  300

Wnet 1727
 th = = = 0.864
Qadd 2000
Wnet 4.41
mep = = = 2004.5 kPa
Vs 0.0022

4000 1
Power = Wnet  No. of cycles per sec. = 4.41  = 147 kW
60 2
 5) An engine working on a theoretical diesel cycle a suction pressure of 100 kPa, air
temperature of 300k, and a compression ratio of 16.if the heat add to the cycle is
1500 kJ/kg of air . Find the pressure and temperature at the key points in the cycle.

Given:
P1 = 100 kPa T1= 300 K r = 16 Qadd = 1500 kJ/kg

The solution:
Point 1: P1 = 100 kPa & T1= 300 K
P2 = P1  (r ) = 100  (16) = 4850.3 kPa
k 1.4
Point 2:
T2 = T1  (r ) = 300  (16) = 909.4 K
k −1 0.4

Point 3: Qadd = Cp  (T3 − T2 ) 1500 = 1.005  (T3 − 909 .4)  T3 = 2401 .94 K
P3 = P2 = 4850 .3 kPa
T 2401.94
rcut = 3 = = 2.64
Point 4: T2 909.4
k 1.4
r   2.64 
P4 = P3   cut  = 4850.3    = 389.3 kPa
 r   16 
k −1 0.4
r   2.64 
T4 = T3   cut  = 2401.94    = 1168.3 K
 r   16 