Automobile - Full Notes - 6TH PDF
Automobile - Full Notes - 6TH PDF
Automobile - Full Notes - 6TH PDF
AUTOMOBILE ENGINEERING
Assessment Exam
Course Code Credits L-T-P
SEE CIA duration
Automobile 15ME655 3 3-0-0 80 20 3 Hrs
MODULE 1
ENGINE COMPONENTS AND IT’S PRINCIPLE PARTS: Spark Ignition (SI) & Compression Ignition
(CI) engines, cylinder – arrangements and their relatives merits, Liners, Piston, connecting rod, crankshaft,
valves, valve actuating mechanisms, valve and port timing diagrams, Types of combustion chambers for
S.I.Engine and C.I.Engines, methods of a Swirl generation, choice of materials for different engine components,
engine positioning. Concept of HCCI engines, hybrid engines, twin spark engine, electric car.
COOLING AND LUBRICATION: cooling requirements, types of cooling- thermo siphon system,
forced circulation water cooling system, water pump, Radiator, thermostat valves. Significance of
lubrication, splash and forced feed system. 10 Hours
MODULE 2
TRANSMISSION SYSTEMS: Clutch-types and construction, gear boxes- manual and automatic, gear
shift mechanisms, Over drive, transfer box, fluid flywheel, torque converter, propeller shaft, slip joints,
universal joints ,Differential and rear axle, Hotchkiss Drive and Torque Tube Drive.
BRAKES: Types of brakes, mechanical compressed air, vacuum and hydraulic braking systems,
construction and working of master and wheel cylinder, brake shoe arrangements, Disk brakes, drum
brakes, Antilock –Braking systems, purpose and operation of antilock-braking system, ABS Hydraulic
Unit, Rear-wheel antilock & Numerical. 08 Hours
MODULE 3
STEERING AND SUSPENSION SYSTEMS: Steering geometry and types of steering gear box-
Power Steering, Types of Front Axle, Suspension, Torsion bar suspension systems, leaf spring, coil
spring, independent suspension for front wheel and rear wheel, Air suspension system.
IGNITION SYSTEM: Battery Ignition system, Magneto Ignition system, electronic Ignition system.
08 Hours
MODULE 4
FUELS, FUEL SUPPLY SYSTEMS FOR SI AND CI ENGINES: Conventional fuels, alternative fuels,
normal and abnormal combustion, cetane and octane numbers, Fuel mixture requirements for SI engines, types of
carburetors, C.D.& C.C. carburetors, multi point and single point fuel injection systems, fuel transfer pumps, Fuel
filters, fuel injection pumps and injectors. Electronic Injection system, Common Rail Direct Injection System.
08 Hours
MODULE 5
AUTOMOTIVE EMISSION CONTROL SYSTEMS: Different air pollutants, formation of
photochemical smog and causes. Automotive emission controls, Controlling crankcase emissions,
Controlling evaporative emissions, Cleaning the exhaust gas, Controlling the air-fuel mixture,
Controlling the combustion process, Exhaust gas recirculation, Treating the exhaust gas, Air-injection
system, Air-aspirator system, Catalytic converter.
EMISSION STANDARDS: Euro I, II, III and IV norms, Bharat Stage II, III, IV norms. Motor Vehicle Act
08 Hours
Course Outcomes: Student will be able
To identify the different parts of an automobile and it’s working
To understand the working of transmission and braking systems
To comprehend the working of steering and suspension systems
To learn various types of fuels and injection systems
To know the cause of automobile emissions, its effects on environment and methods to reduce the
emissions.
TEXT BOOKS:
1. Automobile engineering, Kirpal Singh, Vol I and II (12th Edition) Standard Publishers 2011
2. Automotive Mechanics, S. Srinivasan, (2nd Edition) Tata McGraw Hill 2003.
REFERENCE BOOKS:
1. Automotive mechanics, William H Crouse & Donald L Anglin (10 th Edition) Tata McGraw Hill
Publishing Company Ltd., 2007
2. Automotive mechanics: Principles and Practices, Joseph Heitner, D Van Nostrand Company, Inc
3. Fundamentals of Automobile Engineering, K.K.Ramalingam, Scitech Publications (India) Pvt. Ltd.
4. Automobile Engineering, R. B. Gupta, Satya Prakashan, (4th Edition) 1984.
UNIT – 1
INTRODUCTION
I.C. Engines are used, in order to obtain motive power of the vehicle. In recent years, a huge
changes are made in the design of automobiles to provide safety, ease of operation, reliability,
comfort, less fuel consumption etc.
1. The power plant: It is nothing but the source of power or engine which provides motive power
to perform various functions in the vehicle. The power plant generally consists of an internal
combustion engine (I.C. Engine) which may be either of spark ignition (S.I), or of compression
ignition type. Sometimes gas turbines are also used in certain cars.
2. The basic structure: This includes frame and wheel assembly, suspension system, axles, etc.
3. The power train (transmission system): The power train carries the power from the engine to
road wheels. It consists of clutch, (for non-automatic transmissions) gear box, propeller shaft,
differential.
4. The super structure or car body.
5. The accessories which include electrical system, radio, wind shield wiper, air conditioner etc.
6. The controls: It consists of steering system, Brakes, etc.
Either S.I. or C.I. engines are used to obtain motive power to perform various functions in the
automobile. Modern automotive engines vary greatly in size and design, but the operating principles
are essentially the same as those of first models developed early.
In S.I. engine, a spark plug is an essential component which initiates combustion of fuel. The
spark plug produces an electric spark of high energy, initiates ignition of fuel. In C.I. engine, the high
temperature (greater than ignition temperature of the fuel) of compressed air ignites the fuel and this
is called self or auto ignition. The fuel pump and fuel injectors are the essential components of C.I.
engine.
Top dead centre (TDC): When the piston is at its top most position i.e., the position closest to
cylinder head, it is called top dead centre.
Bottom dead centre (BDC): When the piston is at its lowest position i.e., the position farthest from
the cylinder head, it is called bottom dead centre.
Bore: The 'Bore' is referred to the diameter of engine cylinder. It is denoted by 'D'.
Stroke length or stroke: The distance travelled by the piston between TDC and BDC is called
stroke of the piston and is denoted by 'L'.
Clearance volume: When the piston is in TDC position the cylinder volume above it, is called
Vs = L
Compression ratio: It is the ratio of volume above the piston at BDC to the volume above the piston
at TDe. It is the ratio of total volume of the cylinder (Vs + Vc), to the clearance volume. It is denoted
by ‘r’ Vs +V
Vc
For Petrol engines, it ranges from 8 to 12.
For diesel engines, it ranges from 15 to 24.
Mean effective pressure: As piston performs power stroke, cylinder pressure decreases. Thus it is
required to refer an average effective pressure throughout the whole power stroke. It is expressed in
bars.
Power: It is the work done in a given period of time. More power is required to do the same amount
of work in a lesser time.
Indicated Power (I.P.): The power developed within the engine cylinders is called indicated power.
It is expressed in kilowatts (kW). It is given by area under engine indicator diagram.
Brake Power (B.P.): This is the actual power available at the crank shaft. The indicated power
minus various power losses in the engine like friction and pumping losses in the engine, gives Brake
power. It is measured by using a Dynamometer and is expressed in kilowatts (kW).
Engine torque
It is the force of rotation acting about the crank shaft axis at any given instant of time.
It is given by T = F.r., where T = engine torque, Nm
F = force applied to the crank, N
r = effective crank radius, m
1. Cylinders
2. Piston
3. Connecting rod
4. Crank shaft
5. Valves and valve actuating mechanisms
CYLINDERS
The cylinder is the main body of an engine in which piston reciprocates to develop power. It
has to with stand very high pressure and temperature (around 2800°C). A cylinder block is one
which houses the engine cylinders. If cylinder block and crank case are made integral, then the
construction is called 'Mono block'. The cylinder material should be such that it should retain
strength at higher temperatures, should be good conductor of heat and should resist rapid wear and
tear due to reciprocating action of the piston. Generally cast iron is used. For heavy duty engines
alloy steels are used.
For cooling water circulation, passages are provided around the cylinders. Cylinder block
also carries lubrication oil to various components through drilled passages.
At the lower end of cylinder block, crank case is made integral with the block. At the top,
cylinder block is attached with the cylinder head. It houses inlet and exhaust valves. Besides, other
parts like timing gear, water pump, ignition distributor, fly wheel, fuel pump, etc., are also attached
to it. The materials used for cylinder block are grey cast Iron and aluminium alloys.
The cast iron material has the following advantages.
1. It is relatively cheap and posses good foundry properties.
2. The co-efficient of thermal expansion of cast iron is low.
3. It has high machinability and does not wear too much.
load. Further, in case of any loss of coolant, it cannot with stand high temperature and damage may
occur. It wears more than cast iron.
The grey cast iron for cylinder block has the composition; carbon - 3.5 %, silicon - 2.5%, manganese
- 0.65 %.
The Aluminium alloy cylinder blocks have the composition. Silicon - 11%, Manganese 0.5%,
Magnesium 0.4%
CYLINDER ARRANGEMENTS
Multi cylinder engines are preferred over single cylinder engines due to reasons like
(i) Giving smooth torque output (ii) Lighter fly wheel (iii) Engine compactness (iv) Easy balancing.
In multi cylinder engines, the arrangement of cylinders is very important. The following cylinder
arrangements are used to give better performance of the engine. They are,
l. In line arrangement 2.Opposed cylinders type 3.V – engine 4.Radial engine
1. In line arrangement
In this type, a number of cylinders are arranged in a line i.e., placed side by side vertically with a
common crank shaft. In this type reciprocating forces are nearly balanced.
The two cylinders are arranged horizontally opposite to each other i.e., they are placed 1800
apart facing each other with a common crank shaft. In this type, the reciprocating parts are perfectly
balanced. As two cylinders are not in line, the force in connecting rod produces a rocking couple.
3. V - engine
In this type, two cylinders are placed with their axes at 60°. The cylinders are arranged on
two arms of letter “V” with a common crank case and crank shaft it is more compact and rigid and
hence runs more smoothly at high speeds.
Fig: V-engine
4. Radial engine
In this type, a number of cylinders are arranged in radial fashion with a common crank shaft
which is placed at the centre as in figure. The number of cylinders generally used is 5, 7, 9 etc., to
obtain uniform firing intervals. This type is compact in size and gives higher Brake power per weight
ratio. This is mainly used in air craft engines.
LINERS (SLEEVES)
Engines make use of removable liners which are pressed into cylinder holes. The cylinder
liners are in the form of barrels and used to reduce the cylinder wear and hence to increase cylinder
bore life. The cylinder wear is more when cylinder block is made up of aluminium alloy. The liners
can be inserted in the cylinder bore to reduce this wear. Whenever the liners worn-out, they can be
replaced easily. Whenever a cylinder block is re-bored beyond allowable limits, liners are used to
restore its original size. These are cast centrifugally and made up of special alloy iron containing
silicon, manganese, nickel and chromium.
The liners may be further hardened by nitriding or chromium plating. In nitriding process, liners are
exposed to ammonia vapour at 5000 0C and then quenched. Chromium plating improves their
resistance to wear and corrosion. There are two types of liners (1) Dry liners and (2) Wet liners.
1. Dry liners
Dry liners
1. They may be provided either in the original design or even after wards.
2. No leak proof joint is required.
3. Construction of cylinder block is not simple.
4. As dry liners does not make direct contact with cooling water, cylinder cooling is
ineffective
5. Accurate machining of both block and outer liner surface is required, for perfect contact
between them.
Wet liners
1. These have to be included in the original cylinder design.
2. A leak proof joint between the cylinder casting & liner is required.
3. Construction of cylinder block is simple.
4. As cooling water is in direct contact with liner, better cylinder cooling is possible.
5. Accurate machining on the outer liner surface is not necessary
PISTON
The piston is a reciprocating part of the engine and converts the combustion pressure in the
cylinder to a force on the crank shaft. Pistons are slightly smaller in diameter than the cylinder bore.
The space is provided between piston and cylinder wall and is called "clearance". This 'clearance' is
necessary to provide space for a film of lubricant. Pistons are made of aluminium alloys, cast steel,
cast iron or chrome nickel. Aluminium alloy pistons are used in modern automobiles.
Functions
1. It forms a seal within the cylinder to avoid entry of high pressure gases from combustion
chamber into crank case.
2. It transmits the force of explosion to the crank shaft.
3. 3.It acts as a bearing for the gudgeon pin.
A typical I.C. engine piston is as shown in figure. The piston almost has the shape of an
Inverted bucket. The top portion of the piston is called head or crown. In some engines, pistons may
be specially designed to form desired shape of the combustion chamber. At the piston top, few
grooves are cut to accommodate the piston rings and the bands left between the grooves are known
as "Lands". They support the rings against gas pressure. The portion below rings is called piston
skirt. The skirt is provided with bosses on the inside to support the piston pin.
The Aluminium alloy pistons have the following advantages over cast iron pistons.
1. Lighter in weight, allowing higher rpm. [It is 3 times lighter than C.I. piston which is
desirable from inertia point of view].
2. It has higher thermal conductivity allowing the use of higher compression ratio.
Fig: (a) Piston with horizontal slot Fig: (b) Heat dam construction
2. Heat dam: By making heat dam i.e., by cutting a groove near the top of the piston, the heat
flow to lower part of piston can be reduced. Hence the skirt runs cooler and does not expand
too much.
3. Vertical or T slot: In this type, the top of T tends to retard the heat transfer from head to the
piston skirt. The vertical slot allows the skirt of the piston to close when heated i.e., it allows
piston skirt to expand without increase in diameter. However mechanical strength is
decreased on account of slot. Due to presence of this slot, the diameter reduces permanently
which increases engine slap. Hence fully split skirts are not used.
4. Split skirt: In a split skirt piston, skirt is either partially or completely split. When the piston
warms and begins to expand, it cannot find in the cylinder since the skirt merely closes the
split.
5. Tapered pistons: Sometimes the pistons are turned taper, the crown side being smaller in
diameter than the skirt end. As crown portion is exposed to higher tempt than skirt, that side
expands more than skirt and piston diameter becomes uniform under operating conditions.
6. Special alloy pistons: Special alloy having coefficient of expansion nearly equal to that of
cast iron (or low value) have been used in the manufacture of pistons. One such alloy is
"LOEX" alloy It is an alloy having 12-15% silicon, 1.5-3% nickel and 1 % of each of
magnesium and copper Such pistons are costlier.
7. Wire wound pistons: A band of steel wire is wound between the piston pin and oil
controlling, thus restricting the expansion of skirt.
8. Bimetal pistons: The pistons are made from both steel and aluminium. Steel is used to
manufacture skirt portion and aluminium alloy cast inside to form piston head and piston pin
bosses. For steel, coefficient of thermal expansion is quite small, piston will not expand much
and hence smaller cold clearances can be maintained.
PISTON RINGS
Piston rings are located towards the top of the piston. The top two piston rings are called
compression rings and are designed to maintain cylinder pressure. The bottom ring is called oil ring,
(may be 1 or 2 in number) they scrape the excess oil from the cylinder walls and return it through
slots to the piston ring grooves. A properly constructed and fitted ring will rub against the cylinder
wall with good contact all around the cylinder. The ring will ride in grooves that are cut into the
piston head.
The material generally used for piston rings is fine grained alloy cast iron containing silicon
and manganese. It has good heat and WCi.1rresisting qualities. Rings with molybdenum filled face
have also been introduced recently. Alloy steels are also used. The number of rings vary depending
on the engine design. It varies from two to four.
Generally the ring is cast and machined and put in position in the ring grooves. It exerts
uniform pressure against the cylinder walls. A gap is to be cut at the ends so that while inserting the
ring, it can be expanded, slipped over the piston head and released in to the ring groove. The gap is
almost closed when the piston is inside the cylinder.
Functions:
1. It forms a seal so that high pressure gases from the combustion chamber will not escape into
the crank case.
2. It provides easy passage for heat flow from piston crown to the cylinder walls.
3. It maintains enough lubrication oil cylinder walls throughout the stroke length. This reduces
ring and cylinder wear. The thickness of oil film is to be controlled and the oil should not go
up into the combustion chamber where it would burn and produces carbon deposits.
PISTON PIN
Piston pin is also known as wrist pin or gudgeon pin, used to connect Piston and connecting
rod. It transfers combustion chamber pressure and piston forces to the connecting rod. It is in tubular
shape to provide adequate strength with minimum weight. It passes through the piston bosses and
small end of the connecting rod. It is made of low carbon case hardened steel (carbon - 15%, silicon -
0.3%, manganese - 0.5%).
Piston pins are installed and secured to provide a bearing action in the following three ways.
1. The pin is fastened to the piston by set screws through the piston boss and has a bearing in the
connecting rod small end. This permits the connecting rod to swivel as required by the combined
reciprocal and rotary motion of piston and crank shaft.
2. The piston pin is fastened to the connecting rod by means of a bolt and uses the piston bosses for
bearings. Nowadays, bolt has been replaced by interference fit.
3. A floating pin is used which is free in both the connecting rod and piston. This arrangement is
most commonly used. Circlips are used to prevent end movements.
CONNECTING ROD
The connecting rods are used to connect pistons to the crank shaft. The upper end of rod
oscillates (swing back and forth) while the lower and or big end rotates (turns). It converts
reciprocating motion of the piston in to rotary motion of the crank shaft. The upper end of the rod has
a hole through it for the piston pin. The lower end must be split type. A combination of axial and
bending stresses act on the rod in operation. The axial stresses are due to gas pressure in the cylinder
and inertia force caused by reciprocating motion. Bending stresses are caused due to centrifugal
effects. Connecting rods are manufactured by casting and forging processes. The rod has an I-beam
cross section to provide maximum rigidity with minimum weight. Generally rods are made by drop
forging of steel or duralumin and also cast from malleable cast iron.
CRANK SHAFT
The crank shaft provides a constant turning force to the wheels. It receives the power from
connecting rods and subsequently transmits to the wheels. Crank shafts are made of alloy steel or
cast iron.
The crank shaft is held in position by a number of main bearings and they form axis for the
rotation of crank shaft. Their number is always one more or one less than the number of cylinders.
The crank pins are the journals for the connecting rod big end bearings and are supported by the
crank webs. The distance between the axis of the main journal and the crank pin centre lines is called
'crank through'. Oil holes are drilled from main journals to the crank pins through 'crank webs for
lubricating big end bearings.
When the engine is running, due to rotation of both crank shaft and connecting rod big end,
each crank pin will be subjected to centrifugal forces. This will tend to bend the crank shaft. To
avoid this counter weights are used. The counter weights are formed as integral part of the crank web
or may be attached separately as in fig.
On the front of the crank shaft, it is mounted with
i. Timing gear or sprocket which drives the crank shaft.
ii. Vibration damper
iii. Pulley for driving the water pump, fan and the generator. On the rear end, it is mounted with
a fly wheel.
On the main bearing journals, thrust bearing is located so as to support the loads in the direction of
shaft axis. Such loads may arise due to clutch release forces etc.
The valves located in .he cylinder head are operated by an eccentric projection called cam
which is driven at half the crank shaft speed. Different valve operating mechanisms are used and are
classified into
a) Side valve mechanism
b) Over head valve mechanism
c) Over head inlet and side exhaust valve mechanism.
a) Side Valve Mechanism: This mechanism is used for L-head engines. In this type, inlet and
exhaust valves are mounted in a single row and operated from the same crank shaft.
Nowadays, this mechanism is obsolete due to complicated shape of the combustion chamber
which leads to detonation.
b) Over Head Valve Mechanism: This mechanism is suitable for I and F head designs. The
cam operates the valve lifter which in turn actuates the push rod. This action rotates the
rocker arm about a shaft or a ball joint in some designs, to cause one end to push down on the
valve stem to open the valve.
Advantages
a) Higher volumetric efficiency.
b) Leaner air-fuel mixtures can be burnt.
c) Higher compressions can be used.
Fig: (a) Side valve mechanism. Fig: (b) Over head valve mechanism
Fig: (c) Overhead inlet and side exhaust Fig: (d) Cam shaft valve mechanism
c) Push Rod: This is placed between valve tappet and rocker arm and transmits reciprocating
motion of valve tappet to the rocker arm. Push rods are made of steel and may be either solid
or hollow. Hollow push rod is lighter and results in reduced inertia forces. It provides a
passage for the oil to lubricate the valve actuating mechanism.
d) Rocker Arm: It may be solid or hollow and changes (reverse) the upward motion of the push
rod to down ward motion of the valve and vice versa. It is made of steel (forged or stamped)
or iron (cast).
The exact number of degrees that a valve will open or close before top or bottom dead centre varies
widely, depending on engine design. This diagram shows the crank position when various operation
(suction, compression etc.) in an engine begin and end.
Theoretically, we know that inlet valve should open when piston is at TOC before suction
and close when piston is at BOC after performing suction stroke. The exhaust valve should open
when piston is at BOC before exhaust stroke and should close at the end of exhaust, when the piston
is at TDC to complete a cycle. But the valves require a finite period of time to open and close
without abruptness. Therefore, a slight lead time is necessary for proper operation of the engine.
The actual valve timing diagrams for a 4-stroke Spark-Ignition engine and diesel engines are as
shown in figures.
a) Inlet Valve: The inlet valve should open few degrees prior to the arrival of the piston at TOC
during exhaust stroke of previous cycle. This ensures full open of the valve and entry of fresh charge
in to the cylinder as soon as the piston begins to descend. If the inlet valve closes at BOC, the
cylinder would receive less charge. To avoid this inlet valve is kept open for few degrees of rotation
of the crank after suction stroke i.e., the inlet valve closing is delayed. As engine speed increases, the
inlet valve closing is delayed longer.
b) Exhaust Valve: It is necessary to open the exhaust valve before the piston reaches end of
expansion stroke. Even though this wastes some of the force of expansion, it removes greater part of
burned gases, reducing the amount of work to be done by the piston on its-return stroke.
It is seen from the valve timing diagram that both the valves (inlet and exhaust) overlap for
13 degrees of crank rotation. In petrol engine, more overlapping is not advisable, because air and fuel
mixture may pass out with the exhaust gases and is uneconomical. But in diesel engine, only air is
drawn during suction stroke and hence such problem will not arise.
This overlapping helps in scavenging, resulting in an increased output.
1. Ignition: There is always a time lag between the spark and ignition of the charge. The charge
takes some time to burn after giving the spark. Therefore, it is necessary to produce the spark early to
obtain proper combustion without losses. The angle through which the spark is given earlier is
'Ignition advance' or 'angle of advance'. In diesel engines, the opening of fuel valve before TDC is
necessary for better evaporation and mixing of the fuel. There is always lag between ignition and
supply of fuel results in early supply of fuel.
Fig: (a) Port timing diagram for 2Spetrol engine Fig: (b) Port timing diagram for 2S diesel
engine
IG Ignition
EPO - Exhaust port opens EPC - Exhaust port closes
IPO (TPO) -Inlet or transfer port opens IPC (TPC) -Inlet or transfer port closes FVO - Fuel
valve opens FVC Fuel valve closes.
The port timing diagrams for two stroke petrol and diesel engines are as shown in figures (a) and (b).
The main difference between these two is, the charging and scavenging period in the diesel
engine is (90°) greater than that off or petrol engine (70°). This is because there is no danger of loss
of fuel during scavenging of diesel engine.
L-head types
I-head T-head
Fig: Design of combustion chambers in S1 engines
Side valve engine was introduced in petrol engines, in 1910 - 30. In this type valves are placed side
by side. It is easy to lubricate the valve mechanism. It had the defects like lack of turbulence,
extremely prone to detonation, slow combustion process etc.
(b) In C.I. Engines
There are many types of combustion chambers used in C.I. Engines. Anyone of these
combustion chambers may produce good results in one field of application, but poor results or less
desirable results in another application
The turbulent chamber, pre combustion chamber and energy cell are variations of turbulent
type of chamber. All these types tend to exhibit the same general characteristics.
This type depends on turbulence to produce the required mixing of fuel and air. This does not require
as much excess air as non turbulent type. These are suitable for variable speed operation and also
produce smoother operating engines.
ENGINE RATING
All engines are rated in Power - the measure of rate at which they can do work. There are two ways
of measuring engine power - (1) The power developed by expansion of gases in the cylinder can be
determined by using indicator cards (indicated power) ; (2) By means of measuring instruments like
a prony brake or a dynamometer, the actual power which an engine delivers can be determined
(brake power).
The general methods used to define rated power of an automobile engine are
1) Maximum load carried by the engine continuously. This load is indicated on the basis of mean
effective pressure kpa. for petrol engines M.E.P varies from 640 kpa.
2) Maximum power developed by the engine. In this case the engines are rated in terms of their
maximum capacity. i.e, maximum B.P. that can be developed.
3) Using conventional formula (RAC Ring). For taxation purposes, the Royal Automobile club
made certain assumption for finding out B.P. for a 4S automobile engines. This B.P. is much less
than obtained in case (2) represents the RAC rating of engine.
The assumptions are
Piston Speed:- 1000 ft/min mep - 90 psi
Mechanical efficiency – 75%
Bp – (d2n)/2.5 Where d = diameter of the cylinder, inches n = number of cylinders.
Engine Components
1) Cylinders
2) Oil Pan
3) Inlet and exhaust manifold
4) Cylinder liners
5) Piston
6) Piston ring
7) Piston pin
8) Connecting rod
9) Crank shaft
Materials & their composition
1) Grey cast iron (carbon present in the form of flakes of graphite which makes it more Wear and
corrosions resistant) carbon - 3.5%, silicon -2.5%, manganese - 0.65%. Carbon serves to provide
graphite which improves lubrication; silicon provides wear resistance while manganese increases
the strength and toughness.
2) Aluminium alloys-silicon - II %, manganese - 0.5%, magnesium - 0.4%. Silicon reduces
expansion and increases strength and wear resistance, manganese and magnesium improves
strength of aluminium structure. Pressed steel sheet Cast Iron Special alloy iron containing
silicon, manganese, nickel and chromium Cast-iron, aluminium alloy containing silicon.
ENGINE POSITION
The engine may be conveniently placed on the chassis in different positions as given below
(a) Front Position
In most of the lighter vehicles (both private and commercial), the engine is placed at the front
and conventionally rear wheel drives are used. In some of the vehicles drive is also given to front
wheels only. The engine position remains 'the same in heavy commercial vehicles, but the cab is
brought forward over the engine to increase the pay load. The engine position at the front with rear
wheel drive system needs greater length of propeller shafts, as it has to run from front (engine side)
to the rear (road wheels) of the vehicle. Also, in this system, the number of universal joints required
are more.
(b) Rear Position
In this system, the engine is mounted close to the back axle, thereby reducing the length of
drive from engine to the axle. In this position, length of propeller shaft required is reduced and is
suitable for small cars. This position provides more space to the passengers, results in economy of
drive parts and also better engine service is possible. The fixing of gear shift lever, oil gauge and fuel
gauges, accelerator linkage is very complicated due to missing of natural draft of air during forward
motion of vehicle to the radiator.
The major portion of total weight of the vehicle lies on the rear wheels and hence helping in
traction up the hill. With rear position of the engine, the luggage has to be accommodated at front,
near the driver seat, which is a problem as wheel arches are already occupied a large place there.
3. At lower temperature, viscosity of lubricant increases and results in more frictional losses. This
reduces overall efficiency.
Fig (a): Coolant flows through an engine Fig (b) : Coolant flow path in a system using a
down - flow radiator.
Fig (c): Typical radiator. Water enters the top hose connection - I, then passes into top tank 2.
From there it flows down through core tubes 3. When it reaches bottom tank 4, it has cooled. 5
- Lower hose connections. 6 - Drain petcock.
NEED FOR ENGINE COOLING
METHODS OF COOLING
1. Air cooling
2. Water cooling
1) Air Cooling: Here, the air stream flows continuously over the heated metal surface and the rate of
heat dissipation depends on surface area of metal, air mass flow rate, thermal conductivity of metal,
temperature difference between metal surface and air.
To increase the effectiveness, the metal surface area which is in contact with air should be
increased. This is done by providing fins over cylinder barrels. The fins may be cast integral with the
cylinder or may be attached separately.
Advantages:
1. Absence of radiator cooling jackets and coolant reduces weight of the system.
2. Air cooled engines are useful in extreme climates, where water may freeze.
3. These engines warm up earlier than water cooled engines.
4. Easy maintenance as there is no leakage problem.
Disadvantages:
1. These are noisier, because of absence of cooling water which acts as sound insulator.
2. Heat transfer co-efficient for air is less. Hence less efficient cooling and results in decrease of
highest useful compression ratio.
3. Distortion of cylinder may occur due to uneven cooling all around the cylinder.
2) Water Cooling: In these systems, the water jackets surrounds engine cylinders ana cooling water
flows through these jackets. Heat is conducted through the cylinder walls to the water in the jackets
which removes the excess heat as it circulates through the radiator.
(a) Thermosyphon system: In this system the engine is connected to radiator through flexible
hoses. The difference in densities of hot and cold regions of cooling water causes water circulation
between engine and radiator. The water in circulation absorbs heat from engine cylinder and hence
cool it. The heat from the water is then dissipated into atmosphere through the radiator by conduction
and convection. This cools the water which is required for further circulation. Sometimes fans are
used behind the radiator to increase the air mass flow rate and- hence to increase cooling efficiency.
(b) Pump circulation system: This system is similar to thermosyphon system explained above.
The only difference is cooling water circulation is affected by means of a pump and a thermostat
valve controls the temperature of water.
THERMOSTAT VALVES
It is to be noted that the cooling beyond optimum limits is not desirable as it decreases the
overall efficiency of the engine. A thermostat is used to regulate the rate of cooling. It keeps the
cooling water temperature at a predetermined value.
Two types of thermostats are used in automobiles.
1. Bellows or aueroid type
2. Wax or hydrostatic type
Bellows type thermostat:
This thermostat consists of metallic bellows filled with some volatile liquids like alcohol,
acetone, ether etc., whose boiling temperature ranges between 70-85°C. One end of bellows contains
a valve and to the other end a frame is attached' which fits in to the cooling passage. The thermostat
is fitted in the water hose pipe at the engine outlet. After the engine has started, cooling system
should not operate during warming up duration~ that engine warms up early. During this warming up
period, the liquid inside the bellows has not yet changed its state and hence does not exert any
pressure on the valve. Therefore the valve remains in closed position.
If the temperature of the cooling water exceeds a pre-determined as 80°C the liquid inside the
bellows. Vaporizes and exerts a pressure on the valve. The valve opens and allows water circulation
through the radiator, As water temperature rises, valve opens gradually, thus controls the flow of
water through the radiator according to engine cooling requirement.
Wax thermostat:
It is also known as Dole thermostat. This thermostat is more reliable to operate within the
specified temperature range and is not sensitive to pressure variations. The heat carried by the
coolant is transmitted to the copper loaded wax having high thermal expansion coefficient. The
expansion of copper loaded wax makes the rubber plug to contract against the plunger and hence
exerts a force on it in upward direction. This makes the plunger to move upward and opens a valve in
the thermostat (Not shown). This allows the coolant to flow through the radiator.
ENGINE LUBRICATION
Lubrication is the most important phase of vehicle maintenance. Without lubrication, engine
cannot run smoothly even a few minutes. Inadequate lubrication results in engine troubles like scored
cylinders, burned out bearings, misfiring cylinders, dirty spark plugs, stuck piston rings, engine
deposits and sludge and more fuel consumption.
Dry or solid friction is a result of direct contact between two metallic surfaces or due to inter
locking of irregularities on metal surfaces, produces lot of heat and causes wear of the metal surface.
Hydrodynamic lubrication means, introduction of lubricating oil between two surfaces. There
is no physical contact between them and only resistance to motion is resistance offered by the oil
itself.
In boundary lubrication, the introduction of lubricant between surfaces will not cause complete
separation between them. The surfaces touch at their high spots. Boundary lubrication exist in piston
rings and valve train.
OBJECTS OF LUBRICATION
The main objects of lubrication are
(a) It reduces power loss by minimizing friction between moving parts.
(b) Decreases wear and tear of the moving components.
The lubrication also serves other purposes like.
1. Cooling effect: The lubricant absorbs heat from hot moving parts and dissipates it to the
1. Petro-oil System: In this method some amount of lubricating oil is directly mixed with the petrol.
i.e., about 25 to 30ml. of oil mixes with one litre of petrol. If oil is less, it causes damage to the
engine. If addition of oil is more, there may be excessive carbon deposits in the cylinder head and
produces poor emissions. This method is used in scooter and motor cycles [two-stroke engines].
2. Wet Sump System: In this system, the crank case contains an oil pan or sump that serves as the
oil supply or reservoir tank. It also serves as the oil cooler. Oil from the cylinders and bearings
flows by gravity back into the wet sump from where it is pumped and recirculated to the engine
lubricating system. The wet sump system is again classified into a) Splash lubrication system.
a) Pressure feed system.
b) Semi pressure feed system.
(a) Splash Lubrication System: It is the cheapest method of lubrication and was used in early
motor cycles. The lower end of the connecting rod consists of a scoop like structure as in the
figure. The oil is stored in the oil trough (being delivered from the crank case oil sump).
When the engine runs, the connecting rod oscillates and the scoop takes the oil from oil trough
and splashes on to the cylinder walls each time when it passes through BDC position. This
lubricates engine walls, gudgeon pin, main crank shaft bearings, big end bearings etc. The oil
dripping from the cylinder walls, collects in the tank where it is cooled by air flow.
(b) Pressure Feed System: This system is most commonly used in modern car engines. In this
system, the oil forces oil under pressure to the main bearings, connecting rod and cam shaft bearings
and also to the timing gears. Drilled assuages in the crank shaft carry oil from the main bearings to
the connecting rod bearings. The cylinder walls, piston pin, piston and piston rings are lubricated by
oil spray from the connecting rod and crank shaft. For the cam shaft and timing gears, there is a
separate oil line from the main oil gallery. The basic components of the wet lubricating system are
pump, strainer, pressure regulator, filter etc.
3. Dry Sump Lubricating System: In this system, two pumps are used. The ump 'A' is called
scavenging pump and is located in the crank case portion as in figure. The oil from this pump is
carne to an external tank i.e., reservoir. The pressure urn '8' urn s the oil through filter to the
cylinder and bearings. Oil dripping from cylinder and bearings in to the sump is again removed by
scavenging pump (sump pump), which supplies oil to the reservoir. As the capacity of sump pump
is greater than oil pump, oil will not be accumulated in the engine base. The oil pump draws oil
from the supply tank and delivers it under pressure to the engine bearings and oil pressure of 400-
500 kpa is maintained in main and big end bearings. A pressure of about 50-100 kpa is maintained
in timing gears and cam shaft bearings etc. This system is suitable for lubricating sport cars, jeeps
etc.
UNIT – 2
INTRODUCTION
In an engine, the combustion of fuel with oxygen in the combustion chamber provides the
energy necessary to drive the piston. In a SI engine, the liquid fuel and the air are generally mixed
prior to their arrival in the combustion chamber i.e., outside the engine cylinder. The process of
preparing this mixture is called carburetion. The basic fuel supply system in a petrol engine consists
of a fuel tank, furl lines, fuel pump, fuel filters, air cleaner, carburetor and inlet manifold. The system
responsible for preparing the correct mixture of air and fuel, and directing this mixture to each of the
cylinders is known as "Induction System". The Intake manifold is the ducting or piping through
which the fuel and air mixture travels from the carburetor to the cylinder. The throttle in the
carburetor regulates the quantity of mixture entering the cylinder. The carburetor is a device which
atomizes the fuel and mixes it with air.
CONVENTIONAL FUELS:
Traditional energy sources or fossil fuels (petroleum, oil, coal, propane, and natural gas). In
some cases nuclear materials such as uranium are also included. Some conventional sources typically
used are fossil fuels, nuclear power, hydropower, and geothermal energy.
(a) Fossil Fuels
Clean coal technologies imply much greater processing to reduce final emissions. The resources
deplete with use, so the prices will increase when demand chases supply.
(b) Hydropower
The larger hydropower dams are in place. Some want them removed, claiming that the electricity can
be offset by improved efficiency and conservation. Smaller dams are being removed, yet they may be
ALTERNATIVE FUELS:
(g) Carbon neutral fuel: Carbon neutral fuel is synthetic fuel—such as methane, gasoline, diesel
fuel or jet fuel produced from renewable or nuclear energy used to hydrogenate waste carbon dioxide
recycled from power plant flue exhaust gas or derived from carbonic acid in seawater.
(h) Hydrogen: Hydrogen is an emission less fuel. The byproduct of hydrogen burning is water,
although some mono-nitrogen oxides NOx are produced when hydrogen is burned with air.
(i) Liquid nitrogen: is another type of emission less fuel.
(j) Compressed air : The air engine is an emission-free piston engine using compressed air as fuel.
Unlike hydrogen, compressed air is about one-tenth as expensive as fossil oil, making it an
economically attractive alternative fuel.
(k) CNG fuel: CNG vehicles can use both renewable CNG and non-renewable CNG. Conventional
CNG is produced from the many underground natural gas reserves are in widespread production
worldwide today. New technologies such as horizontal drilling and hydraulic fracturing to
economically access unconventional gas resources, appear to have increased the supply of natural
gas in a fundamental way.
Renewable natural gas or biogas is a methane based gas with similar properties to natural
gas that can be used as transportation fuel. Present sources of biogas are mainly landfills, sewage,
and animal/agri waste. Based on the process type, biogas can be divided into the following: Biogas
produced by anaerobic digestion, Landfill gas collected from landfills, treated to remove trace
contaminants, and Synthetic Natural Gas (SNG)
(l) HCNG: HCNG (or H2CNG) is a mixture of compressed natural gas and 4-9 percent hydrogen by
energy.
ABNORMAL COMBUSTION:
When unburned fuel/air mixture beyond the boundary of the flame front is subjected to a
combination of heat and pressure for certain duration (beyond the delay period of the fuel used),
detonation may occur. Detonation is characterized by an instantaneous, explosive ignition of at least
one pocket of fuel/air mixture outside of the flame front. A local shockwave is created around each
pocket and the cylinder pressure may rise sharply beyond its design limits.
If detonation is allowed to persist under extreme conditions or over many engine cycles,
engine parts can be damaged or destroyed. The simplest deleterious effects are typically particle wear
caused by moderate knocking, which may further ensue through the engine's oil system and cause
wear on other parts before being trapped by the oil filter. Severe knocking can lead to catastrophic
failure in the form of physical holes punched through the piston or cylinder head (i.e., rupture of the
combustion chamber), either of which depressurizes the affected cylinder and introduces large metal
fragments, fuel, and combustion products into the oil system. Hypereutectic pistons are known to
break easily from such shock waves.
The use of a fuel with high octane rating, which increases the combustion temperature of the
fuel and reduces the proclivity to detonate;
Enriching the air–fuel ratio which alters the chemical reactions during combustion, reduces
the combustion temperature and increases the margin above detonation;
Reducing peak cylinder pressure;
Decreasing the manifold pressure by reducing the throttle opening, boost pressure or reducing
the load on the engine.
Cetane numbers: In diesel engines cetane number is a measure of ignition lag. Cetane is
straight chain paraffin assigned with a rating of 100 cetane numbers (CN) and it has good ignition
quality. It is mixed with al ha-methylnaphthalene a hydrocarbon with poor ignition quality i.e., with
zero cetane number. A CFR engine running under prescribed conditions test the fuel with this
mixture. Thus the cetane number of the fuel is defined as the percent by volume of cetane in a
mixture of cetane a I ha-methyl that produces same ignition lag as the fuel being tested, in the same
engine and under the same operating conditions.
For a diesel fuel, cetane rating is a measure of its ability to auto ignite readily when it is
injected in to the compressed air in the engine. The ignition delay is influenced by several engine
design parameters such as compression ratio, injection rate, injection time inlet air temperature etc.
The hydrocarbon composition of the fuel and its volatility characteristics also affects the ignition
delay. The cetane rating of diesel fuels ranges from 40 to 60. The octane fuels (gasoline) have cetane
numbers ranging from 10 to 20 showing their poor suitability as a diesel fuel. High cetane number
results in pre-ignition in diesel engine.
Octane numbers: The composition of fuel affects detonation. In SI engines, for a particular
fuel, the rating is done by comparing its performance with that of a standard reference fuel which is a
combination of ISO octane and n-heptane. ISO octane offers great resistance to detonations and is
assigned a rating of 100 octane number. On the other hand, n-heptane is a straight chain paraffin and
is as sign e with a rating of '0' octane number. The percentage of ISO-octane by volume in a mixture
of ISO octane and n-heptane, which exactly matches the knocking intensity of a given fuel, in a
standard engine under prescribed operating conditions is termed as "octane number" of the el. f
octane number of a fuel is 80, it means that it has a same knocking tendency of a mixture with 80%
ISO octane and 20% n-heptane by volume. The engine used to conduct test is CFR 0- operative fuel
research) variable compression ratio engine. The fuel is to be tested in the CFR engine until the
condition of detonation is reached in the engine. Then a mixture of ISO-octane and n-heptane is
prepared to produce detonation under the same conditions as the fuel under test. The percentage by
volume of ISO-octane in the mixture is nothing but the octane number of the fuel.
MIXTURE REQUIREMENTS FOR SI ENGINE
In stationary engines the desired air fuel ratio means that gives the maximum economy.
Actual air fuel mixture requirements in an operating engine vary under variable speed and load
conditions. The A/F ratios must change based on maximum over is required. Also required A/F ratio
must be provided for transient conditions like, starting a warm-u and acceleration. In all these
conditions, exhaust emission should be minimum.
In steady state operation (It means continuous operation at a given speed and over out with
normal engine temperature) of automotive engines, there are three main areas which require'
different air-fuel ratios. In each of these, the engine requirements differ. As a result the carburetor
has to modify A/F. rati9 to satisfy these demands. These ranges are
1. Idling (mixture must be enriched)
Carburetor:
A carburetor is a device that blends air and fuel for an internal combustion engine. The carburetor
works on Bernoulli's principle: the faster air moves, the lower its static pressure, and the higher its
dynamic pressure. The throttle (accelerator) linkage does not directly control the flow of liquid fuel.
Instead, it actuates carburetor mechanisms which meter the flow of air being pulled into the engine.
The speed of this flow, and therefore its pressure, determines the amount of fuel drawn into the
airstream.
When carburetors are used in aircraft with piston engines, special designs and features are
needed to prevent fuel starvation during inverted flight. Later engines used an early form of fuel
injection known as a pressure carburetor
Under all engine operating conditions, the carburetor must:
Measure the airflow of the engine
Deliver the correct amount of fuel to keep the fuel/air mixture in the proper range (adjusting for
factors such as temperature)
Mix the two finely and evenly
A carburetor basically consists of an open pipe through which the air passes into the inlet manifold
of the engine. The pipe is in the form of a Venturi it narrows in section and then widens again,
causing the airflow to increase in speed in the narrowest part. Below the Venturi is a butterfly valve
called the throttle valve a rotating disc that can be turned end-on to the airflow, so as to hardly
restrict the flow at all, or can be rotated so that it (almost) completely blocks the flow of air. This
valve controls the flow of air through the carburetor throat and thus the quantity of air/fuel mixture
the system will deliver, thereby regulating engine power and speed. The throttle is connected, usually
through a cable or a mechanical linkage of rods and joints or rarely by pneumatic link, to the
accelerator pedal on a car or the equivalent control on other vehicles or equipment.
Fuel is introduced into the air stream through small holes at the narrowest part of the
Venturi and at other places where pressure will be lowered when not running on full throttle. Fuel
flow is adjusted by means of precisely calibrated orifices, referred to as jets, in the fuel path.
CARBURETOR TYPES
Carburetors used in SI engines are classified in to up draft, down draft and horizontal (side)
draft types, according to the direction in which the air and fuel mixture is supplied by them. In down
draft type, gravity assists the flow of mixture and hence engine pulls better at lower speeds under
load. Carburetors are also classified as constant choke carburetor and constant draft or vacuum
carburetor. In constant choke type the air and fuel flow areas are always maintained constant and
depression (pressure difference) which causes fuel flow is being varied according to engine
condition. Ex.: Simple, Solex, Zenith and Carter carburetors. In constant vacuum type, the air and
fuel flow areas are being varied as per demand on the engine while depression or vacuum is always
maintained constant. It is also called as variable choke carburetor. Ex.: S.U. type carburetor.
(a) Solex Carburetor
The Solex carburetor is a down draft type and have been manufactured in India by Mis.
Carburetor Limited, Madras. It is famous for ease of starting, good performance and reliability. It is
available in various models and used in Fiat and Standard Cars and Willis Jeep. The unique feature
of this carburetor is the Bi-Starter for cold starting.
The various circuits are explained as follows:
1. Normal Running: The carburetor has a conventional float in a float chamber. For normal running,
throttle is held partly open and fuel is supplied by the main jet and air by the choke tube or venturi.
The air directly enters through the venturi and fuel passes into the well of air bleed emulsion
system. It is a tube having lateral holes and nozzles are drilled horizontally in the middle as shown in
figure. This system provides metered emulsion of fuel and air through the nozzles for the normal
working of the engine.
2. Cold Starting and Warming: This carburetor has provision of Bi-starter or progressive starter.
The starter valve is a flat disc having holes of different sizes. These holes connect starter petrol jet
and starter air jet to the passage which opens just below the throttle valve. The starter lever position
can be adjusted on the dash board and this connects air and petrol jet to the passage through holes of
different sizes. Rich mixture is used for starting and after the engine has started, richness required
decreases. This means bigger holes are the connecting holes for starting and throttle valve is in the
closed position. The whole engine suction is acted at starting passage. This suction effect draws fuel
from float chamber and the fuel passes through starter petrol jet and mixes with air entering through
the air jet. This mixture is rich enough for starting.
The starter lever is to be brought to intermediate position after starting the engine. This
connects the smaller holes into the circuit and this reduces the amount of petrol. In this condition the
main jet also delivers fuel as the throttle valve is partly opened. The starter system delivers less
mixture but it is sufficient to keep the engine in running condition, till it reaches normal running
temperature.
& 'B' are resting on a conventional float. As engine speed increases, fuel consumption will be more
and fuel level in the float chamber decreases. As a result, the float and balls' A' & 'B' comes down
and force 2F acts on the collar in upward direction. This force lifts the spindle up and provides more
opening for the fuel flow into float chamber. This increases fuel level and thus the level is
maintained constant. If fuel level increases, the float and balls' A' & 'B' moves up causing force 2F to
act in the down ward direction. The spindle partly closes the opening and reduces the fuel flow. Thus
the fuel level is maintained constant in the float chamber.
2. Starting Jet: During starting, the compensatory well is completely filled with fuel. The throttle
valve is slightly open and whole of engine suction acts near the throttle valve at point 'C'. This
suction causes flow of fuel from compensating well through starting jet line. Due to high suction,
enough quantity of fuel is supplied and thus a rich mixture is supplied to the engine. For higher
speeds the throttle valve is opened wide and suction at point 'C' is destroyed, thus stops flow of fuel
from the starting jet line.
3. Compensating Jet: The compensating jet or double jet delivers lean mixture and this compensates
for the rich mixture supplied by the main jet under increased speed of the engine and overall NF ratio
is maintained constant. The area of main jet and compensating jet is equivalent to a single main jet
which is designed to give required NF ratio for a particular speed. As fuel flow from float chamber to
the compensating well through the restricted orifice is less than that of through the compensating jet,
the fuel level in it decreases with increasing engine speed. The atmospheric air is passed into the
compensating well as it is open to atmosphere and less fuel is supplied with increase in speed. The
main jet provides richer mixture and compensating jet provides leaner mixture with increase in
engine speed and thus AIF ratio is maintained constant.
4. Acceleration: In zenith carburetor, no separate device is used for acceleration. Sudden acceleration
of the engine is not possible when throttle is fully open at higher speeds, because the compensating
well is normally dry. It is full of fuel only during idling or slow running and supplies fuel required
for sudden acceleration. So as soon as the throttle is open, the sudden depression due to inflow of the
air near the venturi draws in whole fuel from compensating well through compensating jet and
provides rich mixture for acceleration, thus only momentarily acceleration is possible.
5. Choking: A manually operated choke valve is used for starting the engine from cold weather
conditions.
lower end of the needle is inside the main jet and the needle moves up and down as the piston moves
up arid down. This changes annular area for the fuel flow. When the needle moves up area increases
and vice versa.
A damper plunger is used to regulate the rate of lift of the piston, but allows the same to fall
freely when throttle valve is closed. For acceleration, if the throttle valve is opened suddenly, the
piston lifting speed is retarded by the damper plunger and provides additional depression over the
fuel jet. This causes flow of more fuel and hence no separate acceleration pump is required.
Jet adjusting nut is used to adjust mixture strength. Tightening the nut will raise the jet and
this reduces the annular area for fuel flow. Similarly-loosening the nut lowers the jet and thus
increases fuel supply.
The unique feature of S.U. Carburetor is that it has only one jet. A constant high air velocity
across the jet is maintained even under idling condition and the necessity for a separate idling jet is
obviated. -
For cold starting a rich mixture is required. This is done by lowering the jet tube away from
the needle by means of the jet lever, there by enlarging the jet orifice. The lever is operated from the
dash board in the car.
(d) Carter Carburetor: This carburetor is an American make and used in jeep. It is a down draft
type and has three venturi (triple venturi diffusing type of choke). The small venturi is kept above the
float chamber level, other two below the petrol level, one below other.
The carburetor consists of following circuits.
1. Float Chamber Circuit.
2. Starting Circuit.
3. Idle and Low Speed Circuit.
4. Part and Full Throttle Circuit
5. Acceleration Circuit.
1. Float Chamber Circuit: It consists of a conventional float and a float chamber. Fuel enters the
float chamber from main supply. A needle valve maintains fuel level in the float chamber. When
the fuel level falls, the needle valve opens the inlet to admit more fuel. Air enters the carburetor
from the top. The choke valve in the passage remains open during normal running.
2. Starting Circuit: For starting a choke valve is provided in the air circuit. It is mounted
eccentrically. When the engine is fully choked (choke valve is closed), whole of engine suction is
applied at the main nozzle, which then delivers fuel. As the air flow is quite small, the mixture
supplied is very rich. Once the engine starts, the spring controlled choke valve opens to provide
correct amount of air during warming up period. '
3. Idle and Low Speed Circuits: In this carburetor separate idling passage is provided with low
speed port and idle port. For Idling rich mixture is required in small quantity and throttle valve is
almost closed. The full engine suction is now applied at the idle port through which the air and
fuel are drawn thus provides rich mixture. In low speed operation the throttle valve is opened
further. The main nozzle also starts supplying the fuel. In this stage fuel is delivered both by
main venturi and low speed port through idle passage.
4. Part and Full Throttle Circuit: In part throttling, fuel is delivered by the main nozzle only.
During full throttling, maximum air is passing through the venturi. To compensate this higher
rate of fuel flow is desired. This is obtained by mechanical metering method which uses a
metering rod having a number of steps of diameter sizes at its bottom. It is connected with the
accelerator pedal through Linkage. The area of opening between the metering rod jet and
metering rod governs the amount of fuel drawn into the engine. When the accelerator pedal is
pressed, the throttle is held wide open and simultaneously the metering rod is lifted up. In this
condition, the smallest diameter of the rod is inside the fuel hole (jet), providing larger flow area,
thus delivering more fuel.
5. Acceleration Circuit: The accelerating pump will not provide continuous fuel supply for
acceleration but only provides extra spurt of fuel to avoid flat spot [popping of engine]. When
accelerator pedal is pressed, pump actuates giving an extra spurt of fuel for acceleration. When
the pedal is released the pump piston moves up there by sucking fuel from float chamber for next
operation.
Fuel injection
Fuel injection is a system for admitting fuel into an internal combustion engine. It has become
the primary fuel delivery system used in automotive engines, having replaced carburetors during the
1980s and 1990s. A variety of injection systems have existed since the earliest usage of the internal
combustion engine. The primary difference between carburetors and fuel injection is that fuel
injection atomizes the fuel by forcibly pumping it through a small nozzle under high pressure, while
a carburetor relies on suction created by intake air accelerated through a Venturi-tube to draw the
fuel into the airstream.
Modern fuel injection systems are designed specifically for the type of fuel being used. Some
systems are designed for multiple grades of fuel (using sensors to adapt the tuning for the fuel
currently used). Most fuel injection systems are for gasoline or diesel applications.
Different methods of fuel injection in a 4 stroke and 2 stroke engines are as shown in fig. (a),
(b) & (c). In the manifold injection and port injection arrangements, the injector is moved farther
from the combustion chamber. This provides a longer period for mixing and warming the charge.
Fig: (a) Direct injection system Fig: (b) Port injection system
The manifold injection system may be of two types. Single point and multipoint injection. In
the first type one or two injectors are mounted inside the throttle body assembly. Fuel is sprayed at
one point or location at the center inlet of the engine intake manifold. Hence this method is also
called throttle body injection. The later type has one injector for each engine cylinder and fuel is
sprayed in more than one location. Port injection employs individual injectors delivering locally to
each port.
In SI engine continuous injection, or timed injection system is used. The later type consists
of a fuel supply pump to supply fuel at low pressure (2 bars). A fuel metering or injection pump and
nozzle are present. The nozzle injects the fuel in the manifold or cylinder head port. In some design,
the fuel is injected directly into the combustion chamber.
Timed fuel injection system injects fuel usually during the first half of the suction stroke.
Injection begins after closure of the exhaust valve. This eliminates fuel loss during scavenging.
Injection ends usually not later than 1200 after TDC, for maximum power output.
Advantages:
1. Improves fuel distribution in multi cylinder engine.
2. Increases volumetric efficiency.
3. Reduces loss of fuel during scavenging.
4. Eliminates detonation.
The functions of the fuel feed system are to store fuel for the automobile engines, to supply it
to the carburetor in the required amounts and in proper condition. It also provides an indication to the
driver of the amount of fuel in the tank. In a S.I. Engine, the fuel supply system consists of a fuel
tank, fuel lines, fuel pump, fuel filter, air cleaner, carburetor and inlet manifold.
Fuel injection pumps:
An Injection Pump is the device that pumps fuel into the cylinders of a diesel engine.
Traditionally, the injection pump is driven indirectly from the crankshaft by gears, chains or a
toothed belt (often the timing belt) that also drives the camshaft. It rotates at half crankshaft speed in
a conventional four-stroke engine. Its timing is such that the fuel is injected only very slightly before
top dead centre of that cylinder's compression stroke. It is also common for the pump belt on
gasoline engines to be driven directly from the camshaft. In some systems injection pressures can be
as high as 200 MPa (30,000 PSI).
Fuel Pumps
Many types of fuel pumps are used in the modern car fuel feed systems, all of which operate
on the same principle. A fuel pump transfers petrol from the tank to carburetor [fuel injection
system] through a fine grain filter. It must deliver petrol in sufficient volume at desired pressure to
keep the carburetor (float chamber) full of petrol, irrespective of engine speed.
There are two types of pumps which are most commonly used
1. Mechanical type fuel transfer pump [A.C. Mechanical pump].
2. Electrical fuel pump [So U. Electrical pump]
(a) Mechanical Fuel Pump: A mechanically operated diaphragm type fuel pump is shown in figure.
It is mounted on the engine and is operated by an eccentric mounted on the cam shaft of the engine.
The pump consists of a spring loaded flexible diaphragm actuated by a rocker arm which in turn
operated by an eccentric. Inlet and outlet (spring loaded) valves are provided to ensure fuel flow in
the proper direction. As rocker arm is moved by the eccentric, the diaphragm is pulled down, causes
a partial vacuum in the chamber. This causes the inlet valve to open and admits fuel into the pump
chamber through strainer. Further rotation of the eccentric will release the rocker arm and diaphragm
moves upward, causes inlet valve to close while the outlet valve opens and hence the pump delivers
fuel to the carburetor (float chamber).
When the float chamber is full of petrol, pumping of more fuel is not needed till some of it is
consumed. If the engine runs continuously at light loads, the earn shaft will be running all the time
and there is excessive pressure in the pump. This may damage the pump itself. To avoid this the
rocker arm and pull rod connection is made flexible and when the float chamber is full, the
diaphragm is not operated though the cam shaft is running.
(b) Electrical Fuel Pump: This pump contains a flexible diaphragm which is operated by electrical
means [Electro magnet]. The middle of the diaphragm is fixed to an armature. A rod extends from
middle of diaphragm and passes through a center hole in the electro magnet (solenoid). The other
end of the rod carries electrical contact points. Return springs are used to keep the diaphragm in
position. Closing the ignition switch, energies the electromagnetic winding. Thus magnetic flux is
generated which pulls the armature compressing the return spring and there by moves the diaphragm
up. This causes suction in the pump chamber and fuel is drawn into the chamber through inlet valve.
But as the armature moves, the rod disconnects the breaker points and thus interrupts the electric
supply. The electro magnet is de-energized and the armature falls back due to spring action. This
causes the diaphragm to move down creating pressure in the chamber to open outlet valve. Thus fuel
is delivered to the float chamber. The cycle repeats and fuel continues to be pumped.
These pumps need not be located close to the engine. These electrical pumps are located near
the fuel tank and are not subjected to engine heat. These pumps start operating immediately as the
ignition is switched on.
Fuel Injectors
Depending on the method of fuel control the injectors are classified into (1) Mechanical and
(2) Electronic type. Mechanical method is obsolete now. A governor was used to control fuel supply
and a fuel distributor was used to send the fuel to correct injector.
In this system an electrically driven fuel pump delivers the fuel at a specified pressure (700
kpa) into a metering distributor. The relief valve returns excess fuel to the tank and thus maintains
the metering distributor at constant pressure. The metering distributor supplies fuel to each injector
in turn. The quantity of fuel delivered is also controlled in the distributor by engine manifold
pressure. The injector is held closed until the fuel pressure opens it to deliver atomized spray of fuel.
(2) Electronic Fuel Injection:
An electric fuel pump draws the fuel from the tank through a filter and supplies the same to
the injectors at a pressure which is held constant by means of a fuel pressure regulator which returns
excess fuel to the tank. This prevents vapour lock in the fuel lines. The injectors are held closed by
spring and are opened by solenoids energized by ECU (electronic control unit). The strength of the
ECU control signal, which determines the open time of the injector to control the amount of fuel
injected depends upon the engine requirements which are determined by the ECU from the sensor
signals from critical locations.
The common sensors used are
1. Manifold absolute pressure (MAP) sensor.
2. Barometric pressure (BARO) sensor.
3. Throttle position sensor (TPS)
4. Coolant temperature sensor (CTS).
5. Vehicle speed sensor (VSS) etc.
Fuel gauges
All automotives are equipped with an electrically operated fuel gauge for indicating level of
fuel in the tank. Two types of fuel gauges are used on automobile bodies. They are - Thermostatic
type and Electromagnetic type. Both incorporate sending unit and a receiving unit.
(a) Sending Unit: It consists of a float controlled rheostat or variable resistor. The unit is mounted
on the fuel tank with float and float arm extending into the tank. The float always follows the level of
the fuel in the tank. The float position determines the amount of electrical resistance within the
variable resistor which controls the amount of electricity sent to the receiving unit on the instrument
panel.
(b) Receiving Unit: It is mounted on the instrument panel and by the amount of electricity received
from the sending unit indicates, on a calibrated gauge the amount of fuel in the tank.
The fuel injection pump delivers accurately, metered quantity of fuel under high pressure, at
the correct instant and in the correct sequence, to the injector fitted on each engine cylinder. In most
of the engines the injection pressure ranges from 7 to 30 MPa and in some cases it may be as high as
200 MPa. The timing gears drive the injection pumps and its output is controlled by drives through
accelerator pedal. The injection system has to deliver very small volume of fuel; hence the volume of
fuel to be metered is very small for each injection. The frequency of injection is quite high. For
example, in a 4 stroke, 4 cylinder diesel engine, at maximum speed of 6000 rpm, about 150 rpm of
fuel is to be metered and injected 20 times in a second. In a two stroke engine the numbers of
injections per second are twice this valve. Generally the fuel injection pumps are classified in to
jerk pump type and distributor type;
A single cylinder jerk pump type fuel injection pump is as shown in figure. It consists of a
spring loaded delivery valve, plunger, control sleeve and control rack. The fuel quality to be injected
is controlled by the plunger which contains a helix at its top end. The plunger in turn is operated by
using a cam and tappet.
In this pump, the plunger stroke remains constant, but the effective stroke is reduced by
changing the position of helix on the plunger with respect to fuel inlet port. The cam produces
forward or delivery stroke and the action of spring returns the plunger. As the plunger performs
down ward stroke, it uncovers the inlet port present in the barrel at atmospheric pressure and fills the
space above the plunger and also vertical groove and space below the helix. When the plunger raises
up, it covers the ports and compresses the fuel. The compressed fuel lifts the delivery valve and it is
supplied to the injector through the delivery valve. As the plunger moves up wards, the spill port will
be uncovered by the plunger helix and the helical groove on the plunger connects the space above the
plunger with the suction line. The oil at high pressure in the space above the plunger is by passed
back in to the pump and there by decreases pressure near the delivery valve. This closes the delivery
valve due to action of spring. The fuel quantity delivered through the delivery valve depends upon
the opening position of the spill port with respect to helical groove. Depending on the load on the
engine, the position of helical groove with respect to spill port can be changed by rotating plunger
with control rack. The quantity of fuel can be varied from zero to that required at full load by
changing the positions of the rack.
FUEL INJECTOR
The fuel injector is used
i) To atomize the fuel to the required degree of fineness.
ii) To distribute the fuel for proper mixing of fuel and air.
iii) To prevent fuel injection on cylinder walls and top of the piston.
iv) The fuel injection must start stop instantaneously.
A spring loaded fuel injector is as shown in figure. The fuel pump supplies fuel to the
injector and high pressure fuel lifts the spring loaded valve. The fuel is then injected into the
combustion chamber of the engine cylinder. As the pressure decreases, the valve is automatically
closed by the spring force. The duration of open period of the valve controls the amount of fuel
injected in to the combustion chamber.
varied i.e. A:F ratio is changed depending on the engine load. At high loads rich mixture is
supplied and lean mixture is supplied at low loads. This method is used for diesel engines.
3. Quantity Governing: In this method, the quantity of air fuel mixture supplied is varied
according to engine load. The A/F ratio of the mixture supplied to the engine at all loads
remain nearly constant. This is used for Petrol engines.
UNIT - 3
INTRODUCTION
By increasing engine speed or by increasing air density at the inlet, it is possible to increase
the amount of air inducted in to engine cylinder per unit time. As engine speed increases, inertia load
increases and this calls for rigid and robust engine to with stand stresses. Also higher engine speed
causes decrease in volumetric efficiency, higher friction and increased bearing loads. The method of
increasing the inlet air density is called super charging. This increases power output of the engine.
The super charging is achieved by supply in air or air-fuel mixture at a pressure higher than the
pressure at which the engine naturally aspirates air. This increases air density and hence mass of air
or air fuel mixture inducted for the same swept volume and there by increases power output of the
engine. A device called super charger is used to increase the pressure of air.
The power output of the engine can also be increased by increasing compression ratio. The
high compression ratio results in increase of Brake mean effective pressure and the maximum
cylinder pressure. For a given maximum cylinder pressure more power can be obtained by super
charging compared to that obtained by raising compression ratio. The increase in compression ratio
also increases exhaust temperature and results in higher thermal loads. Turbo charging uses energy
of exhaust gases to drive a turbine that increases inlet air density.
Forced induction
Forced induction is the process of delivering compressed air to the intake of an internal
combustion engine. A forced induction engine uses a gas compressor to increase the pressure,
temperature and density of the air. An engine without forced induction is considered a naturally
aspirated engine.
Forced induction is used in the automotive and aviation industry to increase engine power
and efficiency. A forced induction engine is essentially two compressors in series. The compression
stroke of the engine is the main compression that every engine has. An additional compressor
feeding into the intake of the engine makes it a forced induction. A compressor feeding pressure into
another greatly increases the total compression ratio of the entire system. This intake pressure is
called boost. This particularly helps aviation engines, as they need to operate at high altitude.
Higher compression engines have the benefit of maximizing the amount of useful energy
extracted per unit of fuel. Therefore, the thermal efficiency of the engine is increased in accordance
with the vapor power cycle analysis of the second law of thermodynamics. The reason all engines are
not higher compression is because for any given octane, the fuel will prematurely detonate with a
higher than normal compression ratio. This is called pre-ignition, detonation or knock and can cause
severe engine damage. High compression on a naturally aspirated engine can reach the detonation
threshold fairly easily. However, a forced induction engine can have a higher total compression
without detonation because the air charge can be cooled after the first stage of compression, using an
intercooler.
One of the primary concerns in internal combustion emissions is a factor called the NOx
fraction, or the amount of nitrogen/oxygen compounds the engine produces. This level is government
regulated for emissions as commonly seen at inspection stations. High compression causes high
combustion temperatures. High combustion temperatures lead to higher NOx emissions, thus forced
induction can give higher NOx fractions.
Supercharger
A supercharger is an air compressor that increases the pressure or density of air supplied to an
internal combustion engine. This gives each intake cycle of the engine more oxygen, letting it burn
more fuel and do more work, thus increasing power.
Power for the supercharger can be provided mechanically by means of a belt, gear, shaft, or
chain connected to the engine's crankshaft. When power is provided by a turbine powered by exhaust
gas, a supercharger is known as a turbo supercharger– typically referred to simply as a turbocharger
or just turbo. Common usage restricts the term supercharger to mechanically driven units.
General Overview of Superchargers
Superchargers are an external mechanism driven off the engine's auxiliary drive belt. The
mechanism can work in many fashions, but all have the same basic effect: to increase the force on
the incoming air to the engine. Since superchargers are belt-driven, they do create small amounts of
parasitic drag on the engine, however the effects of the supercharger greatly outweigh the drag.
Generally, superchargers work with gear ratios to create the desired speed of the impeller (or
other air-moving mechanism). If less boost is desired, a larger drive-pulley can be interchanged onto
the supercharger. If greater boost is desired, a smaller pulley is used.
However, boost levels can be controlled in other ways too. A waste gate or blow-off-valve can be
used in conjunction with a correctly sized pulley to have great control over boost levels.
The points to be noted during super charging are
will be more than power output without super charging for the same capacity.
4. The higher pressure and temperature may lead to detonation. Therefore fuel with better antiknock
characteristics is required.
Advantages
1. Power output of the engine can be increased
2. More quantity of charge can be inducted in to engine cylinder
3. Better atomization of fuel is possible
4. Better mixing of air and fuel can be obtained
5. Better scavenging of exhaust gases is possible
6. Torque is improved for whole speed range and better torque at low speeds
7. Faster acceleration of the engine is possible
8. The specific fuel consumption is lowered slightly
9. A better mechanical efficiency and efficient combustion is possible 10 In CI engines, exhaust
smoke is reduced
Disadvantages
1. Detonation tendency increases in SI engines
2. Heat losses due to turbulence and thermal stresses are more
3. The valve overlap period increases up to 60° of crank angle
4. Better lubrication is required
5. Better cooling of piston and valves is required
6. It increases cost of the engine
As discussed earlier, super charger is a pressure boosting device which supplies air (in case of
diesel engine) or air-fuel mixture (in case of SI engine) to the engine cylinder at higher pressure.
Different methods are used to run a super charger. The following figures show some of the
arrangements used to run super charger. In the first arrangement, the engine drives a compressor
which is coupled to it by using step up gearing. A part of the power developed by the engine is used
to run compressor and compressor super charges the engine.
In another method, a turbine coupled to the compressor is driven by engine exhaust. The
turbine used in this arrangement is free from engine except that of the exhaust pipe and air inlet pipe.
The power output of the engine is not used to run compressor. This is called Turbo charging.
In the third method, all the components i.e., engine, turbine and compressor are coupled
together with gearing. At part load, turbine develops less power which is insufficient to run the
compressor. In this case, engine supplies additional power to compensate this less power of the
turbine. If turbine is developing more power to run compressor, it can be supplied to engine.
Fig: Super charging arrangement in which engine, turbine and compressor are coupled.
In the fourth arrangement, the total power of the engine is used to run compressor and
exhaust gases from engine drives a turbine to give power output. Such arrangement is also called
"free piston engine". Sometimes, an electric motor drives compressor independently.
Fig: Super charging method in which engine runs compressor and turbine develops power
This type of super charger is most commonly used in automotive engines. It consists of an
impeller made of alloy steels and rotates at high speeds (about 80,000 rpm) inside a closely fitted
casting. If the air enters axially at the centre of impeller and radial vanes deflect air flow by 90°. Due
to centrifugal action, high velocity air from tip of radial vanes is passed to a diffuser or volute where
air pressure increased and then high pressure air is supplied to the engine. This super charger is
driven by engine through V-belt. Due to entry of high pressure air, 30% more air fuel mixture can be
forced in to combustion chamber.
The supercharger consists of an inlet port, an impeller, a scroll, and a discharge port. The air
comes in the inlet port, and is hit by an impeller. The impeller must spin at speeds of 40,000 - 60,000
rotations per minute in order to create boost. At idle speeds, the impeller does not have enough
rotational speed to produce any boost. The impeller utilizes centrifugal forces in order to produce
boost.
The impeller is the integral part of the centrifugal supercharger (depicted as black fins). As
the air comes in at the center of the compressor blades, the impeller grabs the incoming air from the
inlet port (1). Since the impeller is turning at tens-of-thousands of revolutions per minute, the air is
naturally thrown back and towards the outskirts of the fins due to centrifugal forces created by its
rotational inertia (2 &3). "At the outside of the blades, a "scroll" is waiting to catch the air
molecules. Just before entering the scroll, the air molecules are forced to travel through a venturi
(depicted as the larger grey circle), which creates the internal compression. As the air travels around
the scroll (4), the diameter of the scroll increases, this slows the velocity of the air, but further
increases its pressure (5)."
While a centrifugal supercharger is capable of very high levels of boost and high levels of
horsepower increase, the boost doesn't occur until high RPMs are reached (normally the supercharger
starts creating boost around 3000 RPMs).
The roots supercharger is among the oldest designs for pumping air. First implemented in early
1900, it was used as an industrial air-moving device. In the past thirty years however, it has been
used on many vehicles as a supercharger.
"The roots type supercharger is two counter-rotating meshed lobed rotors. The two rotors trap air
in the gaps between rotors and push it against the compressor housing as they rotate towards the
outlet/discharge port. During each rotation, a specific fixed amount of air is trapped and moved to the
outlet port where it is compressed, which is why the roots type supercharger falls under the broader
category of fixed-displacement superchargers (like the twin screw supercharger). As with all positive
displacement blowers, boost is directly related to the speed of the lobes.
The roots supercharger is known for its high levels of low-rpm boost. Used often in high torque
applications such as towing, the roots blower has also seen much use in top-fuel dragsters. The
simplicity and low-rpm of the design make it a very reliable compressor
Vane Blowers
It is a positive displacement rotary type super charger. This consists of a cylindrical casing, a
rotor with four slots; remain in contact with casing at least at one point all the time. The rotor is
eccentrically mounted and vanes slides in and out of the rotor slots in radial direction. The air is
induced in the space between the blades due to outward movement of vanes, which increases the
space between the blades. When the space reduces near the outlet of super charger, it discharges air.
The space between inner surface of body and drum decreases from inlet to outer side. The air which
enters at inlet, decreases in volume and hence pressure increases as air reaches outlet. The movement
of vanes causes flow pulsating and noisy.
The parameters such as engine knock, thermal and mechanical loads limits the power output
of the engine. Usually in SI engines, knock limit are reached first, where as in diesel engines thermal
and mechanical loads limits are reached first. If supercharging is to be done in an existing engine, it
is necessary to analyze the factors that limit the extent of super charging. This in turn depends up on
engine's ability to with stand gas loading, thermal stresses, durability, reliability, fuel economy etc.
In SI engines, the extent of super charging is mainly limited by knock. The super charging
reduces ignition delay and this result in engine knock at these high pressures.
Therefore increase in super charging pressure increases the tendency to detonate. Generally
for SI engines, super charging is employed only for air craft and racing car engines. The super
charger pressure is in the range of 1.3 to-T.S bar, corresponds to 30 to 50% super charging.
In CI engines, super charging limits are not due to combustion. The engine runs better,
smoother and quieter due to decrease in ignition delay at high super charging pressure and
temperature. But the degree of super charging is limited by thermal and mechanical load on the
engine and mainly depends on the type of super charger used and engine design. Also the engine
reliability decreases at maximum cylinder pressure, this increases heat release rate and hence thermal
load on the engine. For intake pressures less than 1.5 atm, the cost of super charging is not justified.
TURBO CHARGING
Turbochargers are a type of superchargers. It effectively 'charges' the incoming air, which is
the definition of supercharging. The turbo differs from a supercharger in that it derives its power
from a different source than previously described designs. The previous designs received power from
the driveshaft of the engine. Turbochargers derive their power from exhaust gasses.
Turbochargers use the power of the exhaust, much like a hydroelectric dam converts power
from the water into mechanical energy. A hydroelectric dam sends water through a hydroelectric
turbine. The turbine design redirects the flow of the water into a circle which is caught by
fins/blades. The water turns these blades, which turns a driveshaft. When the water has released
most all of its energy to the fins, the water then exits the turbine through a port at the center.
Turbo’s in cars act nearly the same way except the water is replaced with exhaust from the
engine. The drive shaft, in-turn, powers a centrifugal supercharger. Turbochargers are very efficient,
because they do not leech off of the engine's power. The turbo has some downsides however. Boost
cannot be controlled by simply changing a pulley. Boost must be controlled by a wastage or blow off
valve.
Another downside to a turbocharger , is the superheating of the intake air. Since the turbine
must be run by hot exhaust gasses, the heat transfers via conduction to the compressor. The
compressor becomes superheated, and therefore heats the incoming air to the engine. This can be
counteracted by implementing an intercooler.
The other main con of a turbocharger is something called turbo-lag. Turbo-lag is the time it takes
for the turbo to spool up and produce power. Since an engine does not create large amounts of
exhaust in low RPMs, the turbo creates small amounts of boost, and must have time to gain
rotational inertia from the exhaust.
Despite the added downsides, turbochargers can create very large amounts of horsepower and is
able to deliver added torque that a regular centrifugal supercharger lacks.
WORKING
The turbine uses energy from the exhaust gases to convert heat energy into rotational motion.
This rotational motion of turbine drives the compressor, which draws in ambient air from the
surrounding and pumps compressed air with high density and pressure into the intake manifold.
The exhaust gas enters the turbine inlet side of the turbocharger through a pressurized
chamber and a series of filters. The nozzle blade rings concentrates the exhaust gas on to the turbine
wheel. The movement of the turbine wheel rotates the shaft which in turn rotates the impellor of the
compressor. A part of this air goes to the labyrinths seal from the outlet side of the turbine.
As the impeller rotates, air is sucked in through the center of the impeller and due to the
heavy rotational movement, experiences circumferential velocity which pushes it outwards. A radial
velocity is gained which pushes the air further outwards on to the inducer. An additional resultant
velocity is gained due to the accurately designed inducer inlet angle which gives maximum
compressor efficiency.
Excessive pressure leads to spoiling or fouling of the impeller and inducer surfaces. These
results in change in angle of incidence and thus drop in efficiency.
All heavy fuel engines are subjected to heavy load variations which results in fluctuation of
exhaust gas pressure. A prolonged fluctuation in pressure leads to detrimental effects on the internal
parts of the compressor. For this reason, constant pressure chambers are provided in most of the
engines. The exhaust gas, instead of directly entering from the engine, first goes to the pressure
chamber and from there it is circulated to the turbine at constant pressure. This reduces the excessive
stress that gets created on the shaft bearing and sealing.
2. Larger pumping elements or nozzles are required to inject more fuel per unit time. This over
loads cams and other components.
Turbo Charging
1. The energy of exhaust gases is used to run super charger
2. It needs a waste gate control
3. It requires special exhaust manifolds
4. In CI engine it reduces smoke
5. Blade erosion takes place due to entry of dust particles
6. Larger pumping elements or nozzles are needed. This over loads cams
7. Pressure ratio is high
8. It is bulky and heavy
9. Easy scavenging
10. Poor response to load change
INTERCOOLER
An intercooler is an integral part of most blown setups. The power that a non-intercooled
turbocharger created could be maximized by using an intercooler. An intercooler can be compared to
a radiator, yet for intake air, and not coolant fluid.
The intercooler fits on the intake tract to the engine from the supercharger. Generally only
centrifugal superchargers (turbochargers included) can be intercooled, due to their mounting options.
The intercooler dramatically cools the compressed air, and in effect, packs the air closer together.
V=Volume T=Temperature
V1/T2 =V2 /T1
(Given a 2 liters or air at 200°F and an intercooler that cools to 60°F)
What volume of air at can be fit into the same 2 liters that the 200°F air held? 200°F = 366K
60°F=288K
(2L)/ (288K) = (x)/ (366K)
2.54L = x
The intercooler obviously boosts the power of the engine by stuffing more oxygen into the
cylinders. The cylinders can therefore create a larger and more vigorous explosion, and therefore
produce more power.
Intercoolers can come in two types: air-air, or air-water. Air-air systems use ambient air to
directly cool the pressurized air. Air-water systems first cool water with the ambient air around the
car, and is then filtered into an internal coolant system, where the cooled water cools the charge-air.
Turbocharger lag
A turbo works by using exhaust air to spin a turbine, which is attached to the same shaft as a
compressor. Compressed air created as the turbine spins the compressor is, in turn, fed into the
engine. This allows more horsepower to be generated by improving the engine's volumetric
efficiency, a trait based in part on the fundamental precept that the more oxygen in a given volume of
air, the more potential energy that volume has.
of an engine, turbo charging is an attractive option. This is because the proportion of horsepower a
turbo creates, as compared to the weight of its parts — a characteristic known as power to weight
ratio is favorable compared to these other options. Turbo are thus relatively common in gasoline
engines, and almost standard in mass-produced diesel engines, which are known as turbo diesels.
Turbo engines have been particularly embraced by several automobile manufacturers, including
Saab®, Mercedes Benz®, and Volkswagen®.
The basic design of a turbocharger consists of a metal — usually aluminum — center housing
and hub rotating assembly (CHRA), a turbine, a compressor, and a central shaft. The size of the
CHRA, the turbine, and the compressor dictate how much extra horsepower they can generate, and
generally also how much turbo lag is going to be created. The larger the parts, the longer the turbo
typically takes to spool, and the more turbo lag there will be.
The most common way engineers get around turbo lag is simply to use the lightest
components possible, as less inertia means less lag. A more complex way is to pair a large turbo with
a smaller one, or with a supercharger. The instant or near-instant spooling of these secondary units
helps compensate for the lag, while the larger one builds pressure, minimizing or eliminating it
completely.
UNIT - 4
IGNITION SYSTEMS
INTRODUCTION
We know that in case of Internal Combustion (IC) engines, combustion of air and fuel takes
place inside the engine cylinder and the products of combustion expand to produce reciprocating
motion of the piston. This reciprocating motion of the piston is in turn converted into rotary motion
of the crank shaft through connecting rod and crank. This rotary motion of the crank shaft is in turn
used to drive the generators for generating power. We also know that there are 4-cycles of operations
viz.: suction; compression; power generation and exhaust.
These operations are performed either during the 2-strokes of piston or during 4-strokes of
the piston and accordingly they are called as 2-stroke cycle engines and 4-stroke cycle engines.
In case of petrol engines during suction operation, charge of air and petrol fuel will be taken
in. During compression this charge is compressed by the upward moving piston. And just before the
end of compression, the charge of air and petrol fuel will be ignited by means of the spark produced
by means of for spark plug. And the ignition system does the function of producing the spark in case
of spark ignition engines.
Figure shows atypical spark plug used with petrol engines. It mainly consists of a central
electrode and metal tongue. Central electrode is covered by means of porcelain insulating material.
Through the metal screw the spark plug is fitted in the cylinder head plug. When the high tension
voltage of the order of 30000 volts is applied across the spark electrodes, current jumps from one
electrode to another producing a spark.
Whereas in case of diesel (Compression Ignition-CI) engines only air is taken in during
suction operation and in compressed during compression operation and just before the end of
compression, when diesel fuel is injected, it gets ignited due to heat of compression of air.
Once the charge is ignited, combustion starts and products of combustion expand, i.e. they
force the piston to move downwards i.e. they produce power and after producing the power the gases
are exhausted during exhaust operation.
Objectives
After studying this unit, you should be able to
Explain the different types of ignition systems,
Differentiate between battery and magneto ignition system
Know the drawbacks of conventional ignition system, and
Appreciate the importance of ignition timing and ignition advance.
Both these conventional, ignition systems work on mutual electromagnetic induction principle.
Battery ignition system was generally used in 4-wheelers, but now-a-days it is more
commonly used in 2-wheelers also (i.e. Button start, 2-wheelers like Pulsar, Kinetic Honda; Honda-
Activa, Scooty, Fiero, etc.). In this case 6 V or 12 V batteries will supply necessary current in the
primary winding. Magneto ignition system is mainly used in 2-wheelers, kick start engines.
(Example, Bajaj Scooters, Boxer, Victor, Splendor, Passion, etc.).
In this case magneto will produce and supply current to the primary winding. So in magneto ignition
system magneto replaces the battery.
Figure shows line diagram of battery ignition system for a 4-cylinder petrol engine. It mainly
consists of a 6 or 12 volt battery, ammeter, ignition switch, auto-transformer (step up transformer),
contact breaker, capacitor, distributor rotor, distributor contact points, spark plugs, etc. Note that the
Figure 4.1 shows the ignition system for 4-cylinder petrol engine, here there are 4-spark plugs and
contact breaker cam has 4-corners. (If it is for 6cylinder engine it will have 6-spark plugs and contact
breaker cam will be a perfect hexagon).
The ignition system is divided into 2-circuits:
1. Primary Circuit : It consists of 6 or 12 V battery, ammeter, ignition switch, primary winding it
has 200-300 turns of 20 SWG (Sharps Wire Gauge) gauge wire, contact breaker, capacitor.
2. Secondary Circuit: It consists of secondary winding. Secondary winding consists of about 21000
turns of 40 (S WG) gauge wire. Bottom end of which is connected to bottom end of primary and
top end of secondary winding is connected to centre of distributor rotor. Distributor rotors rotate
and make contacts with contact points and are connected to spark plugs which are fitted in
cylinder heads (engine earth).
WORKING
When the ignition switch is closed and engine in cranked, as soon as the contact breaker
closes, a low voltage current will flow through the primary winding. It is also to be noted that the
contact beaker cam opens and closes the circuit 4-times (for 4 cylinders) in one revolution. When the
contact breaker opens the contact, the magnetic field begins to collapse. Because of this collapsing
magnetic field, current will be induced in the secondary winding. And because of more turns (@
21000 turns) of secondary, voltage goes unto 28000-30000 volts
This high voltage current is brought to centre of the distributor rotor. Distributor rotor rotates
and supplies this high voltage current to proper stark plug depending upon the engine firing order.
When the high voltage current jumps the spark plug gap, it produces the spark and the charge is
ignited-combustion starts-products of combustion expand and produce power.
Note:
The Function of the capacitor is to reduce arcing at the contact breaker (CB) points. Also
when the CB opens the magnetic field in the primary winding begins to collapse. When the magnetic
field is collapsing capacitor gets fully charged and then it starts discharging and helps in building up
of voltage in secondary winding. Contact breaker cam and distributor rotor are mounted on the same
shaft. In 2-stroke cycle engines these are motored at the same engine speed. And in 4-stroke cycle
engines they are motored at half the engine speed.
3. At very high engine speed, performance is poor because of inertia effects of the moving parts in
the system.
4. Sometimes it is not possible to produce spark properly in fouled spark plugs.
In order to overcome these drawbacks Electronic Ignition system is used.
ADVANTAGES OF ELECTRONIC IGNITION SYSTEM
Ignition timing is very important, since the charge is to be ignited just before (few degrees
before TDC) the end of compression, since when the charge is ignited, it will take some time to come
to the required rate of burning.
Ignition Advance
The purpose of spark advance mechanism is to assure that under every condition of engine
operation, ignition takes place at the most favorable instant in time i.e. most favorable from a
standpoint of engine power, fuel economy and minimum exhaust dilution. By means of these
mechanisms the advance angle is accurately set so that ignition occurs before TDC point of the
piston. The engine speed and the engine load are the control quantities required for the automatic
adjustment of the ignition timing. Most of the engines are fitted with mechanisms which are integral
with the distributor and automatically regulate the optimum spark advance to account for change of
speed and load. The two mechanisms used are :
(a) Centrifugal advance mechanism, and
(b) Vacuum advance mechanism.
The centrifugal advance mechanism controls the ignition timing for full- load operation. The
adjustment mechanism is designed so that its operation results in the desired advance of the spark.
The cam is mounted, movably, on the distributor shaft so that as the speed increases, the flyweights
which are swung farther and farther outward, shaft the cam in the direction of shaft rotation. As a
result, the cam lobes make contact with the breaker lever rubbing block somewhat earlier, thus
shifting the ignition point in the early or advance direction. Depending on the speed of the engine,
The beginning of the timing adjustment in the range of low engine speeds and the continuous
adjustment based on the full load curve are determined by the size of the weights by the shape of the
contact mechanisms (rolling or sliding contact type), and by the retaining springs, all of which can be
widely differing designs. The centrifugal force controlled cam is fitted with a lower limit stop for
purposes of setting the beginning of the adjustment, and also with an upper limit stop to restrict the
greatest possible full load adjustment. A typical sliding contact type centrifugal advance mechanism
is shown in Figures 4.6(a) and (b).
Vacuum advance mechanism shifts the ignition point under partial load operation. The
adjustment system is designed so that its operation results in the prescribed partial load advance
curve. In this mechanism the adjustment control quantity is the static vacuum prevailing in the
carburetor, a pressure which depends on the position of the throttle valve at any given time and
which is at a maximum when this valve is about half open. This explains the vacuum maximum. The
diaphragm of a vacuum unit is moved by changes in gas pressure. The position of this diaphragm is
determined by the pressure differential at any given moment between the prevailing vacuum and
atmospheric pressure. The beginning of adjustment is set by the pre-established tension on a
compression spring. The diaphragm area, the spring force, and the spring rigidity are all selected in
accordance with the partial – load advance curve which is to be followed and are all balanced with
respect to each other. The diaphragm movement is transmitted through a vacuum advance arm
connected to the movable breaker plate, and this movement shifts the breaker plate an additional
UNIT - 5
POWER TRAINS
INTRODUCTION
A Transmission system uses a clutch, gear box, propeller shaft and a differential gear to
transmit power from engine to the road wheels. The power may be transmitted to rear or front wheels
or all the four wheels, depending on the type of drive used in automotive. The clutch and gear box
varies the leverage i.e. ratio of torque output to torque input. The propeller shaft transmits final
torque to the rear axle from gear box, and a differential gear equally distributes the final torque
between the road wheels (driving wheels).
A Transmission system has to perform following functions.
1. It disconnects engine from driving wheels when required.
2. The engine is connected to driving wheels without jerk.
3. It changes ratio of torque output to torque input, as desired.
4. It turns the drive through a right angle.
CLUTCH
Clutch is a device used in the transmission system of a motor vehicle to engage and
disengage the engine to the transmission. Thus the clutch is located between the engine and the
transmission. Typically a clutch consists of clutch fork, thrust bearing, diaphragm, cover, pressure
plate, clutch plate, and a flywheel
Functions of a clutch are as follows,
When the clutch is engaged, the power flows from the engine to the rear wheels through the
transmission system and the vehicle moves.
When the clutch is disengaged, the power is not transmitted to the rear wheels and the vehicles
stops while the engine is still running.
The clutch is disengaged when starting the engine, when shifting the gears, when stopping the
vehicle and when idling the engine. The clutch is kept engaged when the vehicle is moving.
The clutch also permits the gradual taking up of the load. When properly operated, it prevents
jerky motion of the vehicle.
1. Torque Transmission: The clutch should be able to transmit maximum torque of the engine.
2. Gradual Engagement: The clutch should engage gradually to avoid sudden jerks.
3. Heat Dissipation: The clutch should be able to dissipate large amount of heat which is
Dog Clutches
A dog clutch is a type of clutch that couples two rotating shafts or other rotating components
not by friction but by interference. The two parts of the clutch are designed such that one will push
the other, causing both to rotate at the same speed and will never slip.
Dog clutches are used where slip is undesirable and/or the clutch is not used to control
torque. Without slippage, dog clutches are not affected by wear in the same way that friction clutches
are. Dog clutches are used inside manual automotive transmissions to lock different gears to the
rotating input and output shafts. A synchromesh arrangement ensures smooth engagement by
matching the shaft speeds before the dog clutch is allowed to engage.
A good example of a simple dog clutch can be found in a Sturmey-Archer bicycle hub gear,
where a sliding cross-shaped clutch is used to lock the driver assembly to different parts of the
planetary gear train.
Advantages
1. When compared to single plate clutch, the normal force on friction surfaces is greater than axial
force.
Disadvantages ,
a. It is difficult to disengage the clutch if cone angle is less than 20°, as one cone tends to bind in
the other.
b. Even a small amount of wear on friction surface results in more axial movement of the cones
Centrifugal clutch
Centrifugal clutch is an automatic clutch which is controlled by the engine speed through the
accelerator. When the engine speed increases above the certain limit, the clutch engages and when
the engine speed decreases, the clutch disengages automatically.
This type of clutch design essentially consists of two members. One is the driving member
which is fitted on the driving shaft. The other one is a driven member, which is just a drum and
encloses the driving member. The driving member consists of two curved shoes or flyweights having
frictional linings on them. The shoes are anchored at one end to the back plate and are kept in
position be means of coil springs.
Fig.Centrifugal Clutch
Operation of centrifugal clutch: The driving member rotates along with the engine shaft. When the
engine speed increases, the centrifugal force also increases. At certain engine speed the shoes fly
outwards due to increased centrifugal force and they come in contact with the driven member. Now
both driving and driven members rotate together and the clutch is said to be engaged.
When the engine speed decreases, the centrifugal force also decreases. Now, the shoes return
back to their original position due to spring force, which results in a disengagement of clutch.
This type of clutch is used in mopeds.
A single disc or plate clutch consists of a clutch plate whose both sides are faced with a
frictional material. It is mounted on the hub which is free to move axially along he splines of the driven shaft.
The pressure plate is mounted inside the clutch body which is bolted to the flywheel. Both the pressure plate
and the flywheel rotate with the engine crank shaft or the driving shaft. The pressure plate pushes the clutch
plate towards the flywheel by a set of strong springs which are arranged radially inside the body. The three
levers (also known as release levers or fingers) are carried on pivots suspended from the case of the body.
These are arranged in such a manner so that the pressure plate moves away from the flywheel by the inward
movement of a trust bearing. The bearing is mounted upon a forked shaft and moves forward when the clutch
pedal is pressed
When the clutch pedal is pressed down, its linkage forces the thrust release bearing to move
in towards the flywheel and pressing the longer ends of the levers inward.
The axial pressure exerted by the spring provides a frictional force in the circumferential
direction when the relative motion between the driving and driven members tends to take place. If
the torque due to this frictional force exceeds the torque to be transmitted, then no slipping takes
place and the power is transmitted from the driving shaft to the driven shaft.
Advantages
1. Pedal movement is less and hence gear changing is easier.
2. It is more reliable. [No problem of cone binding etc.].
Disadvantages
1. The spring stiffness required is more, hence greater force is required for disengaging the clutch.
For a particular type of road, the variation of total resistance on different gradients is as shown in fig.
(b).
The fig (c) shows tractive effort (torques) available in different gears. It is clear that high torque is
available in the low gear.
The fig. (d) Shows superimposition of total resistance and tractive effort curves for different gears.
Let us consider the vehicle is moving on a gradient for which total resistance curve is I. Let
the vehicle be in the top gear and the curve 3 shows the available torque in the top gear. The inter
section of curves I and 3 gives the stabilizing speed 'OA'. At any instant, if the vehicle speed
decreases to say '08', the excess of tractive effort will accelerate it to speed 'OA'. Similarly if the
speed increases to 'OC', the excess of resistance will decelerate the speed to 'OA'. So when the
tractive effort over comes the total resistance, the vehicle speed increases such that the tractive effort
becomes equal to the total resistance.
Suppose if the vehicles move on a steeper gradient than curve I, the stabilizing speed
decreases to '00'.
Suppose, if we consider curve III, no where it crosses curve 3 (top gear). This means vehicle
will not move on the gradient with the top gear. We have to pass on to second gear to get stabilizing
speed '00'. Similarly on gradient IV, we have to shift to first gear.
During starting, more acceleration is required to gain speed quickly. In the first gear, torque
available is maximum and hence during starting, we have to shift to first gear to gain speed quickly.
Once the necessary speed has attained, the vehicle speed has to be simply maintained and
acceleration is not required. We may shift into higher gears where less torque is available.
To obtain direct gear ring member (D) and hence sliding hub (C) is slid towards left till
friction surfaces FI and F2 rob against each other and friction makes their speed equal. Further
pushing of 'D', towards left causes the ring member to override the balls and gets engaged with dog
'H' and provides direct drive from gear B via 'C' and the splines. Similarly second gear is obtained by
sliding the sliding hub 'C' to the right.
OVER DRIVE
When the standard transmission has been shifted into high gear, the ratio between the clutch
shaft and the transmission main shaft is 1:1. The unit over drive is located on the back of
transmission, between the transmission and the propeller shaft, provides a speed ratio over that of
direct or high speed ratio. The over drive allows the engine to operate at only about 70% of the
propeller shaft speed, when the vehicle is running at higher speeds.
Over drive mechanism is special equipment, causes the main shaft to over drive or turn more
rapidly than the clutch shaft. When the over drive is put into operation, it drops (decreases) the
engine speed by about 30 percent without changing vehicle speed. Suppose if a vehicle runs 40
kmph. in direct gear with an engine rpm of 2000 rpm, the use of over drive would decrease the
engine speed to 1400 rpm without changing the vehicle speed. (It maintains 40 kmph. vehicle
speeds). Essentially, the over drive consists of a planetary gear system and a freewheeling
mechanism.
Overdrive
Overdrive is the highest gear in the transmission. Overdrive allows the engine to operate at a
lower RPM for a given road speed. This allows the vehicle to achieve better fuel efficiency, and
often quieter operation on the highway. When it is switched on, an automatic transmission can shift
into overdrive mode after a certain speed is reached (usually 70+ km/h depending on the load). When
Fig: Over running clutch with rollers in engaged position (solid drive or coupling)
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2. Speed Reduction: Assume that the sun gear is stationary; turning of ring gear will cause the
planet pinion cage to turn slowly, than ring gear.
This is exactly opposite to previous condition and the system functions as speed reduction
mechanism. The driving member (ring gear) drives the driven member (cage) slowly.
3. Speed Increase: Hold the ping gear stationary and turn the planet pinion cage to drive sun gear.
The sun gear is forced to rotate faster than the 'cage, i.e. driven member (sun gear) turns faster
than driving member (planet pinion cage) and the system becomes speed increasing mechanism.
(a) Simplified view of planetary gear system (b) Planetary gear system
4. Speed Reduction: If we turn the sun gear to drive planet pinions on their shafts, by keeping ring
gear stationary, the pinions walk around the ring gear, as they are in mesh with it. The planet
pinion cage is also carried around and hence rotates at lesser speed than the sun gear speed. The
sun gear becomes driving member and turns the planet pinion cage (driven member) slowly and a
speed reduction is obtained.
5. Reverse: Keep the planet pinion cage stationary and turn the ring gear. The planet pinions
becomes idlers-and turns the sun gear in the opposite direction to the rotation of ring gear. We
get a reverse rotation system and sun gear is turning faster than the ring gear. Ring gear is the
driving member and sun gear takes the drive and turns in opposite direction to the ring gear.
6. Reverse: Hold the cage stationary and turn the sun gear to drive the ring gear in the reverse
direction, but slower than the sun gear.
7. Direct drive: The input and output shafts turns at the same speed by locking any two or three
members in the planetary gear system. The whole is locked and there is no speed change through
the reduction gear system, a direct ratio of 1:1 is obtained. However, if no member is held
stationary and no two members are locked together, then the system will not transmit power. The
input shaft will turn, without driving out put shaft.
Conditions I 2 3 4 5 6
Ring gear Dn TJ)( S S T l>Y D~
Cage TD~ DI'\ TDi Dn S S
Sun gear S S Df) TD, DII) T
Speed Ine. dee. Inc. dee. Inc. Rev dec. Rev
The above table shows various possible conditions in a planetary gear system.
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Fig: Overdrive
Fluid coupling (fluid flywheel)
It is a device for transmitting rotation between shafts by means of the acceleration and
deceleration of a hydraulic fluid. Structurally, a fluid coupling consists of an impeller on the input or
driving shaft and a runner on the output or driven shaft. The two contain the fluid (see illustration).
Impeller and runner are bladed rotors, the impeller acting as a pump and the runner reacting as a
turbine. Basically, the impeller accelerates the fluid from near its axis, at which the tangential
component of absolute velocity is low, to near its periphery, at which the tangential component of
absolute velocity is high. This increase in velocity represents an increase in kinetic energy. The fluid
mass emerges at high velocity from the impeller, impinges on the runner blades, gives up its energy,
and leaves the runner at low velocity.
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When the crankshaft turns, the driving member or impeller also rotates. The fluid flows
outwards due to the centrifugal force and circulates from the flywheel to the driven member. Now,
the fluid tends to rotate the driven member because the fluid is also carried out round by the driving
member. The fluid is also carried out round by the driving member. The fluid is also carried out
round by the driven member so; the fluid tends to rotate the driven member. Thus the torque is
transmitted from the crankshaft to the gear box shaft. The liquid coupling is not suited for use with
an ordinary gear box. It is generally used in conjunction with epicyclical gears to provide a semi or
fully automatic gear box.
Torque converter
A torque converter is generally a type of fluid coupling (but also being able to multiply
torque) that is used to transfer rotating power from a prime mover, such as an internal combustion
engine or electric motor, to a rotating driven load. The torque converter normally takes the place of a
mechanical clutch in a vehicle with an automatic transmission, allowing the load to be separated
from the power source. It is usually located between the engine's flex plate and the transmission.
The key characteristic of a torque converter is its ability to multiply torque when there is a
substantial difference between input and output rotational speed, thus providing the equivalent of a
reduction gear. Some of these devices are also equipped with a temporary locking mechanism which
rigidly binds the engine to the transmission when their speeds are nearly equal, to avoid slippage and
a resulting loss of efficiency.
Torque converters are sealed units; their innards rarely see the light of day, and when they do,
they're still pretty hard to figure out
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Imagine you have two fans facing each other. Turn one fan on, and it will blow air over the
blades of the second fan, causing it to spin. But if you hold the second fan still, the first fan will keep
right on spinning.
That's exactly how a torque converter works. One "fan," called the impeller, is connected to
the engine (together with the front cover, it forms the outer shell of the converter). The other fan, the
turbine, is connected to the transmission input shaft. Unless the transmission is in neutral or park,
any motion of the turbine will move the vehicle.
Instead of using air, the torque converter uses a liquid medium, which cannot be compressed
oil, otherwise known as transmission fluid. Automatic transmission cars use a torque converter. This
article will discuss why automatic transmission cars need a torque converter and how a torque
converter works.
The torque converter in an automatic transmission serves the same purpose as the clutch in a
manual transmission.
The engine needs to be connected to the rear wheels so the vehicle will move, and
disconnected so the engine can continue to run when the vehicle is stopped. One way to do this is to
use a device that physically connects and disconnects the engine and the transmission – a clutch.
Another method is to use some type of fluid coupling, such as a torque converter, which is located
between the engine and the transmission.
There are three components inside the very strong housing of the torque converter which
work together to transmit power to the transmission:
Pump
Turbine
Stator
The pump inside a torque converter is a type of centrifugal pump. As it spins, fluid is flung
to the outside, much as the spin cycle of a washing machine flings water and clothes to the outside of
the wash tub. As fluid is flung to the outside, a vacuum is created that draws more fluid in at the
center.
The fluid then enters the blades of the turbine, which is connected to the transmission (the
spline in the middle is where it connects to the transmission.) The turbine causes the transmission to
spin, which basically moves your car. The blades of the turbine are curved so that the fluid, which
enters the turbine from the outside, has to change direction before it exits the center of the turbine. It
is this directional change that causes the turbine to spin.
As the turbine causes the fluid to change direction, the fluid causes the turbine to spin.
The fluid exits the turbine at the center, moving in a different direction than when it entered.
The fluid exits the turbine moving opposite the direction that the pump (and engine) are turning. If
the fluid were allowed to hit the pump, it would slow the engine down, wasting power. This is why a
torque converter has a stator
The stator resides in the very center of the torque converter. Its job is to redirect the fluid
returning from the turbine before it hits the pump again. This dramatically increases the efficiency of
the torque converter.
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Operation
A torque converter has three stages of operation
Stall: The prime mover is applying power to the impeller but the turbine cannot rotate. For
example, in an automobile, this stage of operation would occur when the driver has placed the
transmission in gear but is preventing the vehicle from moving by continuing to apply the brakes. At
stall, the torque converter can produce maximum torque multiplication if sufficient input power is
applied (the resulting multiplication is called the stall ratio). The stall phase actually lasts for a brief
period when the load (e.g., vehicle) initially starts to move, as there will be a very large difference
between pump and turbine speed.
Acceleration: The load is accelerating but there still is a relatively large difference between
impeller and turbine speed. Under this condition, the converter will produce torque multiplication
that is less than what could be achieved under stall conditions. The amount of multiplication will
depend upon the actual difference between pump and turbine speed, as well as various other design
factors.
Coupling: The turbine has reached approximately 90 percent of the speed of the impeller. Torque
multiplication has essentially ceased and the torque converter is behaving in a manner similar to a
simple fluid coupling. In modern automotive applications, it is usually at this stage of operation
where the lock-up clutch is applied, a procedure that tends to improve fuel efficiency.
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The key to the torque converters ability to multiply torque lies in the stator. In the classic
fluid coupling design, periods of high slippage cause the fluid flow returning from the turbine to the
impeller to oppose the direction of impeller rotation, leading to a significant loss of efficiency and
the generation of considerable waste heat. Under the same condition in a torque converter, the
returning fluid will be redirected by the stator so that it aids the rotation of the impeller, instead of
impeding it. The result is that much of the energy in the returning fluid is recovered and added to the
energy being applied to the impeller by the prime mover. This action causes a substantial increase in
the mass of fluid being directed to the turbine, producing an increase in output torque. Since the
returning fluid is initially traveling in a direction opposite to impeller rotation, the stator will likewise
attempt to counter-rotate as it forces the fluid to change direction, an effect that is prevented by the
one-way stator clutch.
Unlike the radially straight blades used in a plain fluid coupling, a torque converters turbine
and stator use angled and curved blades. The blade shape of the stator is what alters the path of the
fluid, forcing it to coincide with the impeller rotation. The matching curve of the turbine blades helps
to correctly direct the returning fluid to the stator so the latter can do its job. The shape of the blades
is important as minor variations can result in significant changes to the converter's performance.
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During the stall and acceleration phases, in which torque multiplication occurs, the stator
remains stationary due to the action of its one-way clutch. However, as the torque converter
approaches the coupling phase, the energy and volume of the fluid returning from the turbine will
gradually decrease, causing pressure on the stator to likewise decrease. Once in the coupling phase,
the returning fluid will reverse direction and now rotate in the direction of the impeller and turbine,
an effect which will attempt to forward-rotate the stator. At this point, the stator clutch will release
and the impeller, turbine and stator will all (more or less) turn as a unit.
Unavoidably, some of the fluid's kinetic energy will be lost due to friction and turbulence,
causing the converter to generate waste heat (dissipated in many applications by water cooling). This
effect, often referred to as pumping loss, will be most pronounced at or near stall conditions. In
modern designs, the blade geometry minimizes oil velocity at low impeller speeds, which allows the
turbine to be stalled for long periods with little danger of overheating.
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The figure shows layout of Borg Warner automatic transmission. This consists of a single
stage (three members) torque converter coupled to an epicyclic gear box which gives three forward
and one reverse speed ratio. The turbine of the impeller is integral with the drums 'D)' and' 'D/ of the
clutches 'C)' and 'C/. When 'C)' is engaged, the drive goes to the sun gear 'S/ and if brake 'B)' is
applied, it gives low gear. The second gear is obtained by applying brake 'B/ instead of 'B)'. The one
way clutch 'F' prevents planet carries 'R' from rotating back ward, but allows it to rotate forwards. If
'C)' and 'C/ are engaged simultaneously, the gear is locked solid and a direct drive is obtained. For
reverse gear, C2 is engaged and brake B2 is applied. The drive then goes from S) to P, and hence to
the annulus A, the planet carrier being fixed.
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UNIT - 6
DRIVE TO WHEELS
INTRODUCTION
This chapter describes the purpose, construction and operation of drive lines which includes
driving connection between the transmission and driving wheels. In an automobile, the drive line
consists of a propeller shaft, one or two universal joints depending on the type of drive and a slip
joint. All these are used to transmit engine power from transmission to the differential which is
provided at the rear wheel axle. In some automobiles [front engine - front wheel drive], the
transmission is directly coupled to the front wheel axle by axle drive shafts, and no propeller shaft is
used. Even in rear engine rear wheel drive automobiles, the transmission may be directly coupled to
the rear wheel axle.
PROPELLER SHAFT
It is also called drive shaft. The front engine rear wheel drive automobiles require a propeller
shaft to connect transmission output shaft to the differential at the rear axle. The propeller shaft
transmits rotary motion of the transmission output shaft to the differential, causing the rear wheels to
rotate. At high speeds, whirling of propeller shaft causes bending stresses in the material which i to
be reduced. Also it has to withstand torsional loads. To reduce this, propeller shaft should be made
tubular and should be perfectly balanced. One or two universal joints are used to permit variations in
the angle of drive. The engine and transmission are rigidly attached to the frame of the vehicle while
rear axle housing with wheels and differential is attached to the frame by springs. The springs
compress or expand due to road irregularities which changes angle of drive between the propeller
and transmission shafts. This also changes distance between the transmission and the differential.
The universal joint accounts for variations in the angle of drive. The slip joint or sliding joint serves
to adjust the effective length of the propeller shaft when demanded by the rear axle movements.
The figure shows a propeller shaft with two universal joints and a sliding joint. Sliding joint is
formed by the internal splines on the sleeve attached to the left universal joint and external splines on
the propeller shaft.
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UNIVERSAL JOINTS
It is known that a solid drive line would be bent and finally broken as the angle of drive
changes. So a flexible joint called universal joint is used to move the drive line or propeller shaft
without breaking. The universal joint transmits the torque even though the rear axle is moving up and
down.
Universal joints are used as flexible connection between two rigid shafts at an angle with
each other. These joints allow angular movement between two shafts and transmit power at an angle.
In automobiles universal joints are used not only to permit power transmission from horizontal
transmission output shaft to the propeller shaft which is at an angle to the horizontal (as rear axle is
usually lower than transmission shaft), but also allow the change of this angle due to movement of
rear axle up and down on surface irregularities. Without this device power transmission under these
conditions would be impossible.
The simplest type of universal joint is Hooke's joint. It is simple and compact in construction
and is efficient for small angular movements of the propeller shaft say up to 20°. It consists of two -
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'Y' shaped yokes, one on driving (A) and another on the driven (B) shafts, whose axes are inclined to
each other. The spider or cross consists of four arms, two of which are supported in the bushes in the
yoke of shaft 'A', while the other two are supported in the yoke of shaft 'B'. Thus shaft' A' can have
angular rotation about x-x and shaft' B' about y-y. The driving shaft' A', drives the spider (cross),
causes the rotation of shaft' B' through the arms of cross 'C'.
Fig: Hooke's joint. Shaft 'A' is transmitting torque in horizontal plane; shaft 'B' is transmitting
the same at different angle.
Fig: Universal joints in action. As rear axle moves up and down, universals allow drive angles
to change without bending propeller shaft.
In the universal joint explained above, the driven shaft speed does not remain constant or
uniform. Depending on the shaft inclinations, the driven shaft speed fluctuates. It is zero for zero
angle, the magnitude increases when the angle is large. To achieve uniform driven shaft speed, two
universal joints have to be used at two ends of propeller shaft.
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The functions of live rear or back axle are
1. It acts as a beam, up on the ends of which the road wheels can revolve and through which the
weight of body and load can be transmitted, via the springs and road wheels, to the ground.
2. It acts as a housing to support the final drive, the differential and the shafts that transmit the drive
to the road wheels.
The figure shows simplified view of a live rear axle for a front engine, rear driven
automobile. The pinion shaft is supported in the bearings in axle casing and takes the drive from
propeller shaft. The pinion shaft drives a crown wheel which is in mesh with it and mounted on shaft.
The end caps are used to restrict the wheels in the axial direction. The wheels are mounted on the
ends of the axle shaft. In actual case, two half shafts are used instead of one shown here.
i. Torque Reaction: In fig, the propeller shaft applies a torque to the pinion shaft 'P', which in
turn transmitted to the axle shaft 'N. Assume that if the road wheels are fixed, then on turning
the shaft' P', the pinion will have to roll round the crown wheel 'C'. Thus, if the road wheels
are fixed with propeller shaft in the turning condition, the pinion will tend to climb around
the crown wheel. As axle casing supports pinion, it will be subjected to a force which causes
it to rotate. The torque producing this action is the equal and opposite reaction to the driving
torque which is applied to the road wheels. This phenomenon is called torque re .n, due to
which axle casing tends to turn opposite to the direction of road wheels rotation. This has
to be opposed otherwise propeller shaft would be subjected to heavy bending. This can be avoided
by attaching an arm or a member to the casing and the other end of which is secured to the frame. The
braking torque on the axle casing is opposite in direction to the torque reaction.
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ii. Driving Thrust: Driving torque produced in the engine causes the thrust to be produced in
the road wheels, which has to be transmitted from axle casing to the chassis frame and the
body of the vehicle. To do this, thrust members or radius rods are used. These members are
attached to the axle casing and chassis frame in the longitudinal direction.
iii. Weight of the Body: If we consider rear axle as a beam which is supported at the ends and
loaded at two points as shown in figure. The rear axle supports, body weight OW' through
two suspension springs and R1 and R2 are the reaction forces from the road wheels. This
weight causes shear force and bending moment in the axle shaft.
iv. The sideways forces [Side thrust] : The rear axle also experiences side thrust or pull due to
any side load on the wheel. (Ex: Cornering force). Pan hard rod may be used to hold the axle
in position against the side thrust.
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In all these arrangements, springs takes weight of the body. Many drives are used, out of which the
two important rear axle drives are (i) Hotchkiss drive & (ii) Torque tube drive.
The Hotch kiss drive is most simple and widely used system. In this arrangement, the springs
besides taking weight of the body, also take the torque reaction, driving thrust and side forces. It
consists of an open propeller shaft secured to the transmission output shaft and differential pinion
gear shaft (Bevel pinion shaft). The propeller shaft is provided with two universal joints and a sliding
joint as shown in figure. The springs are bolted to the axle casing. The front end of the spring is
rigidly fixed to the frame, while the rear end is connected to the frame by swinging [inks or shackle.
The front half of the spring will transmit the driving thrust to the frame.
It is seen that, the axle casing cannot turn under the torque reaction without causing the
springs to flex which is shown in figure (b). The springs offer considerable resistance to this
deformation, thus torque reaction is overcome. The spring deflects as it experiences torque reaction
and the bevel pinion shaft changes its position. Under this condition the axis of bevel pinion shaft
will not pass through the centre of the front universal joint. Therefore, if there is no universal joint at
the rear end, the propeller shaft will bend. To overcome this effect, two universal joints, one at the
front and other at the rear end are used.
Again the rear axle moves up and down in a circle with the front spring support at the frame
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(0) as centre. But the propeller shaft moves in a circle about '0,' as centre i.e., the centre of front
universal joint. Since the two centers do not coincide, the distance between the front universal joint
and pinion shaft of the axle [length of propeller shaft] will alter during the up and down movement of
the axle and to accommodate this, a sliding 'joint is to be used in the propeller shaft.
In both drives leaf springs take tile side thrust. When coil springs are used, they are not able
to take-side loads on therefore a separate member is used which is called "Panhard rod". The panhard
rod is in the-form of transverse radius rod fixed parallel to wheel axis with one end attached to axle
casing and the other end .to the chassis frame.
REAR AXLE SHAFT SUPPORTING
The rear live axle half shaft experiences following loads,
1. Vehicle weight causes sharing force
2. Driving torque.
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3. Bending moment due to end thrust and its reaction from the tyres on ground.
4. End thrust resulting from cornering side wind etc.
There are three types of live axle - Semi floating, three quarter floating and full floating.
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removing the wheel or disturbing the differential. In this case, the wheels remain in position even if
the axle breaks which i not possible with other arrangements. It is more expensive, bulkier and
heavier than any of the other types.
DIFFERENTIAL
When an Automobile negotiates a turn, the distance travelled by outside wheels is greater
than that travelled by inside wheels in the same time. If the wheels are mounted on dead axles, so
that the} turn independent to each other (like front wheels of an ordinary passenger vehicle), the
wheels will turn at different speeds to com pen ate for the difference in travel. But if the engine
drives the wheels, some device is necessary which will allow the wheels to revolve at different
speeds. To do this a device called differential is provided in the rear axle. This will increase tile end
of outside wheels and reduce the speed of inside wheels, when the vehicle travels around a corner, in
the mean time keep the speed of all the wheels same when the vehicle is going straight head. This
avoids skidding when the vehicle is taking turn.
Operation of differential
It consists of a drive pinion or bevel pinion, attached to the shaft which is coupled to the
propeller shaft. A crown wheel or ring gear which is bolted to the differential cage is in mesh with
the bevel pinion. The cage carries a cross pin or spider (cross pin is used when two pinion gears are
employed and spider is used when four pinion gears are used) to support two differential pinion gears
which are in mesh with the two differential side gears (sun gears) which are splined to the axle half
shafts. The ring gear (crown - wheel) is free to rotate on the half shaft as shown.
When the propeller shaft turns the bevel pinion, the pinion will turn the crown wheel. The
crown wheel in turn will revolve the differential cage and cross pin. The axle side gears will still not
turn. By adding two differential pinion gears (The cross pin will pass through these gears) that mesh
with the side gears, the revolving cage will turn the axle side gears with it.
When the vehicle is going straight ahead, the crown wheel turns the cage. The differential
pinion gears and axle side gears are moving around with the cage and pinion gears do not rotate on
the cross pin or about its own axis (no relative movement between teeth of pinion gears and axle side
gears), but apply equal torque to the two side gears. This drives both the rear wheels (half shafts) at
the same speed and crown wheel, differential cage, cross pin, pinion gears and side gears all turn as a
solid unit. Both half shafts rotate at the same speed and there is no relative movement among various
differential gears.
When the vehicle is taking a turn, the cage continuous to rotate and pull both the pinion gears
around on the cross pin. The outer wheel must turn faster than the inner wheel and for this to happen
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the outer axle side gear has to rotate faster than the inner axle side gear. To permit this, the two
differential pinion gears rotate on their axes (on cross pin). This allows them to pull on both axle side
gears, while at the same time, compensating for the difference in speed by rotating around their shaft.
The figure (c) shows the differential gears, when the vehicle is moving straight. The pinion
gear is pulling both gears, but it is not turning. In figure (d), the right side axle gear is moving faster
than the left axle gear. The pinion gear is still moving at the same speed and pulling on both gears,
but started to turn on the cross pin. This turning action added to the forward rotational speed of the
shaft and causes the right side gear to speed up and left side gear to slow down.
To understand how all this happens, assume that the cage is stationary. If we turn one side
gear, it drives the other side gear in opposite direction. Suppose if left side gear rotates 'n ' times in a
particular time, this causes right side gear to rotate 'n' times in the same period, but of course in the
opposite direction. This rotation is super imposed on the normal wheel speed when the car travels a
curve.
As an example assume vehicle speed as say 'N' rpm, when it is going straight. When it turns
right, there is a resistance to the motion of right wheel due to differential action and right side wheel
rotates back by 'n' rpm, left side wheel rotates forward by 'n' rpm. Thus the resultant speed at the
right wheel is (N-n) rpm, and this slow down the right wheel. The resultant speed at the left wheel is
(N + n) rpm, causes the left to move faster.
As the pinion gears are free to rotate on the cross pin, they act as balance and divide the
torque equally between the two wheels.
Fig (a): Distance travelled by inner and outer wheels when a car taking turn
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Fig (c): Axle side gears, Pinion gear and shaft revolve as a unit, Straight line driving
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Fig (d): Axle side gear move forward at different speeds. Pinion gear (3) is turning
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STEERING
It is necessary to run the automobile in a desired direction; this can be accomplished by
providing steering system. Besides brakes and accelerator, steering is very much essential to control
the vehicle, without which an automobile will never exist.
The main purpose of a steering system is to provide angular motion to the front wheels, when
the vehicle is taking a turn. Different steering gears and linkages are used to steer the front wheels.
The purpose of a steering system is to convert rotary motion of the steering wheel in the
driver's hands into angular motion of the front road wheels, and to multiply the driver's effort by
leverage or mechanical advantage so as to make it fairly easy to turn the wheels. The steering system
also absorbs large part of the road shocks, thus preventing them being transmitted to the driver.
Apart from the above object, the steering system also serves other purposes like,
1. It gives perfect steering condition. It means perfect rolling motion of road wheels under all
conditions.
2. When the car is moving in a straight line, it gives directional stability.
3. To reduce tyre wear.
4. To facilitate straight ahead recovery after completing the turn.
The steering system has to fulfill the following requirements.
1. The system used should be very accurate and should be easy to handle.
Till recently front wheel steered vehicles were designed. In these vehicles front wheels were
steered with rear wheels followed them. However, lately all-wheel steering or four- wheel steering
has been designed and used in some selected vehicles.
Steering Linkage used in the Vehicle with rigid axle front suspension
It consists of a drop arm or pitman arm connected between steering gear and link rod. The
link rod in turn connects steering arm through 1 a ball joint and the stub axle mounted with road
wheel is rigidly attached to the steering arm as shown in figure O. Each stub axle has a forged track
rod arm rigidly fixed to the wheel axis. To the ends of track rod arms, a track rod is attached by using
'0’ ball joints as in figure. An adjuster is also used in the track rod and it changes length of the track
rod for adjusting wheel alignment.
The steering gear provides the required leverage (mechanical advantage), so that driver's
effort required is less at the steering wheel to apply much larger force to the steering linkage. It also
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gives the desired velocity ratio so that larger angular movement of the steering wheel gives much
smaller movement of the stub axle.
By turning the steering wheel, drop arm swings and imparts a linear movement to the link
rod. The steering arm transmits this movement to the stub axle, and turns it about pivot (may be a
king pin or ball joints). The other wheel is steered through the track rod; hence only one wheel is
positively steered.
In the previous type, the main axle beam allows the stub axle to move in the horizontal plane
only. The effective track rod length does not change as there is no vertical deflection of the
suspension.
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In the independent suspension, the two stub axles can move up and down independent of each
other and causes the length of the track rod to change. For this reason, a single track rod is not
suitable.
The arrangement consists of a three piece track rod, the centre being called relay rod, one end
of which is connected to idler arm supported on body structure. The other end of the relay rod is
connected to the drop arm of the steering gear through ball joints. The relay rod is confined to move
in horizontal plane only. Movement in vertical plane is provided by the tie rods about the end ball
joints.
STUB AXLE
Stub axle is one on which the front road wheels are mounted. The king pin connects main
axle beam to the stub axle. Stub axles are made up of Nickel steels and alloy steels containing
chromium and molybdenum. Usually front axle is a dead axle and is manufactured by drop forging
of steel. As it has to withstand bending loads due to vehicle weight and torque loads due to braking
of wheels the central portions is made 'I' section and the ends of the beam are made either circular or
elliptical. This dead front axle is used in heavier vehicles.
The figure shows the arrangement of the stub axle in which king pin has been replaced by
ball joints.
STEERING GEOMETRY
It is not enough to simply place the front wheels on hubs, stand them up straight and device a
mechanism to swivel them left or right. The car could be driven but it would steer very poorly and at
higher speeds it would become dangerous to handle, and also tire life would be short.
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Wheel alignment refers to the positioning of the front wheels and steering mechanism that
promotes easy of steering, provides directional stability, reduces tire wear to a minimum. To secure
easy steering, smooth operation and several front wheel alignment factors must be considered such
as camber, king-pin or steering axis inclination, caster, toe in and toe out and turning circles (angles)
etc.
(a) Camber
Camber angle is the inclination between the centre line of the tyre and the vertical. If the
wheels are inclined or tilted outward at the top, it is called "positive camber", and if the wheels are
inclined inward at the top, it is called "negative camber". t is also called as 'wheel rake angle:.
Front wheels are not mounted parallel to each other; instead they are tilted slightly outward at
the top. This is done to prevent the top of the wheels from tilting inward too much due to excessive
loads or play in the king pins and wheel bearings.
Effect: It is noted that, to make the tyre wear more uniform, tyre should roll vertically on the
ground. Tyre will wear more on one side than the other side, when it is tilted inward or outward. The
positive camber causes the tyre to roll like a truncated cone. The positive camber makes the wheel to
toe out and tyre will wear more on the outer side. Similarly the negative camber makes the wheels to
and tyre will wear more on the inner side. Initially the wheels are provided with positive camber,
after loading automatically they come to vertical position.
It is clear that, when the vehicle is running with average load, zero camber angle gives
maximum tyre life. If the two front wheels are not provided with equal camber, the vehicle will try to
pull towards the side where the camber is higher. In the same way, if the wheels are provided with
equal camber, the crowned road has a tendency to pull away the vehicle to the side of the road. To
obviate this, usually slight higher camber is provided on the right wheel in case of right drive
vehicles which have to move on the left side. For left hand drive vehicles, left wheel is sided with
higher camber.
Camber angle is usually less than 2° and exact amount depends upon king pin inclination.
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Fig: Camber and king pin angle (steering axis inclination) on exaggerated scale.
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A zero scrub radius keeps the wheel in straight position without any tendency to toe in or toe
out as in figure (b). In this case, wheel centre line and king pin or ball joint centre line exactly meets
on the ground. This condition is called "centre point steering".
By experience, it has been proved that, if the ball joint centre line and wheel centre line meets
below the ground, it gives best results.
If both the wheels are not provided with equal combined angle, the vehicle will pull towards
the side where scrub radius is high.
Combined angle varies from 90 to 100 and scrub radius ranges up to 12 mm.
Fig: The wheel and king pin centre line meet (a) above the ground (negative scrub radius) -
wheels toe in (b) Exactly on the ground (zero scrub radius - 0 effect and (c) below the ground
(positive scrub radius) - wheels toe out
(d) Castor
The kingpins are tilted slightly from the vertical as shown in figure 6.15 (a) and (b). The
angle between the kingpin centre line and vertical, obtained in the plane of wheel is called the castor
angle. If the kingpin centre line contacts the ground at a point in front of the wheel centre, it is called
Positive Castor and if it meets behind the wheel centre line it is called Negative Castor. The castor
angles should not exceed 30. In modern vehicles negative castor ranges from 20 to 80.
Effect: Castor produces a trailing effect and hence gives directional stability by making the
wheels to lead or follow in the same direction as the vehicle moves. Incorrect castor angle results in-
hard steering, when brakes are applied vehicle pulls to one side, tendency to wander due to lack of
directional stability.
Example: Castor angle provided on the furniture rollers and on the front wheels of the bicycles, the
positive castor provided in both these cases causes the wheels to be pulled in any direction.
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Fig: Toe - in
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STEERING GEARS
Steering gear is the heart of steering system and the driver controls the direction of front
wheels with the steering gear. It converts rotational motion of the steering wheel into to and fro
motion (arc motion) of the link rod of the steering linkage which in turn swivel the front wheels.
Steering gear also provides torque multiplication. It multiplies the drives steering effort to
provide adequate force for steering column. For cars, the steering ratio or torque multiplication factor
ranges between 10 : 1 to 22 : 1 and for trucks it ranges between 24 : 1 to 32 : 1.
There are many types of steering gears used in automobiles. The important steering gears are
a) Worm and Wheel steering gear
It consists of a worm and worm wheel. In place of worm wheel, only a sector may also be
used. As the steering wheel turns, the rotation of the worm drives the worm wheel. A drop arm is
rigidly attached to the wheel spindle. So rotation of worm wheel through steering wheel causes the
drop arm to move to and fro, thereby, actuates the link rod connected to it and swivels the front
wheels.
b) Worm and Nut steering gear
It consists of a worm and a ball nut and these are arranged as shown in figure. The rotation of
steering wheel turns the worm and hence the nut moves along its length. This movement of the nut
actuates the drop arm end to move linearly and thus actuates the link rod and swivels the wheels.
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The figure shows a recirculating ball type steering gear. It consists of a worm and nut
arrangement as in figure. The steering shaft carries the worm and a nut rides on the worm with two
sets of balls in the grooves in between nut and worm. These balls reduce friction during movement of
the nut on the worm. The drop arm is rigidly attached to the wheel sector and the teeth of wheel
sector meshes with teeth of the nut. The drop arm in turn connected to the link rod, through which it
swivels the road wheels.
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The condition for true rolling is obtained when all four wheels are rolling perfectly under all
conditions of running. If the vehicle takes a turn, this fundamental condition of correct steering is
satisfied when all four wheels rotate about a common centre called as Instantaneous centre. The axes
of front wheels when produced meet the rear wheel axis at this point 'I'. It is also seen that, the inside
wheel turns through a greater angle than the outer wheel. The larger the steering angle, the smaller is
the turning circle. However, there is maximum limit to the steering angle, and is limited to 440. The
extreme positions on both sides are called 'Lock' positions.
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The turning circle is defined as the diameter of the smallest circle which the outer front wheel
of an automobile can traverse and obtained when the wheels are at lock positions.
STEERING MECHANISM
We know that for perfect steering all four wheels must rotate about Instantaneous centre. To
achieve this, inner wheel has to turn more than outer wheel. Several mechanisms are used, among
which two are important.
1. Ackermann Mechanism
2. Davis Mechanism.
The Ackermann mechanism is based on a four- bar chain mechanism, which has two longer
links RS and AB of unequal length and other two shorter links 'RA' and 'SB' of equal lengths. By
using track rod 'AB' shorter than RS (distance C or distance between kingpins), the inner wheel is
forced to turn a greater angle, when the car is taking a turn. When the car is going straight ahead, all
four wheels are parallel, but while turning, the inner and outer wheel angles become different.
The figure (a) and (b) shows the Ackermann's mechanism. It is seen that shorter links are
made integral with stub axles and are connected together through track rod. In the straight ahead
position, the shorter links makes equal inclination 'a' with the centre line of the vehicle. The dotted
line shows position of links when the car is taking left turn.
Let, I = length of the track rod 'AB' and r = Length of shorter links RA and SB.
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From figure (b), after neglecting obliquity of link AB in the turned position, the pivots A and
B moves through same distance 'X' in horizontal direction.
POWER STEERING
In heavy duty trucks and tractors, driver has to apply inadequate effort to turn the wheels. The
use of booster arrangement in steering system overcomes this drawback. The booster is put in to
operation when the steering wheel is turned. It does most of the work for steering. The power
steering system uses compressed air, electrical mechanisms, and hydraulic pressure.
The figure shows a simplified diagram of hydraulic booster. The arrangement consists of a
worm and worm-wheel, distributor slide valve, booster cylinder etc. When the steering wheel is
turned, the worm turns the sector of worm wheel and hence actuates the arm. The arm in turn
actuates the road wheels through drag link. If the resistance offered to turn the wheels is too high and
driver's effort to the steering wheel is too weak, then the worm, like a screw in a nut will be displaced
axially together with the distributor slide valve. This axial movement will admit compressed air or
oil in to booster cylinder through the pipeline. The piston in the booster cylinder will turn the road
wheels via the gear rack, the toothed worm sector, arm and drag link. In the mean time, the worm
sector will actuate the worm and will shift it along with distribution slide valve to its initial position.
This movement of slide valve will stop the piston travel in the booster cylinder. Here the system uses
power assistance in proportion to the effort needed to turn the wheels.
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CHASSIS FRAME
In an automobile, frame is the foundation part which supports the power plant and body.
The frame itself is supported on the road wheels through axles and springs.
The engine, wheels, power train, brakes and steering systems when installed on the frame, the
assembly is called chassis. The chassis can be driven by the driver on the road safely.
The main functions of the frame are
1. If with stand the engine and transmission thrust.
2. It with stands the torque stresses.
3. To support body weight, passengers and goods weight.
4. It provides base for mounting engine and transmission systems.
5. It provides the space for spring system.
Frames are made of channel or U shaped sections welded or riveted together. It withstands
the road shocks, twists and vibrations. Generally it consists of two side rails (longitudinal members)
a variable number of cross members and also an X member. The side rail has a channel section, with
the open side turned inward. These have maximum depth at their centers and tapering towards their
ends. Straight longitudinal members are used in truck and bus frames. In motor cars, the side rails are
given an arc shape both in front and rear and are referred as the double drop type. This makes the
floor of the car body to come closer to the ground.
The chassis frame with box section side members (a typical car frame), box section type and
X member type is as shown in figure.
At the front end, the frame is narrow to facilitate short turning radius of the front wheels. The
frame widens out at the rear so as to provide ample space in the body for passenger accommodation.
The side members are curved upward at the rear forming a kick up. The rear leaf springs connect
each end of this curve. This type of frame is designed for independent front wheel suspension and is
heavy in construction. The light weight cars have frame without diagonal (X) cross members. In
others types, frames uses a single X frame with no side members.
The cross members used in vehicle frame increases the rigidity and to with stand shocks,
blows, twists and vibrations, when the vehicle moves on a rough road and also during acceleration,
braking and steering to one side.
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UNIT - 7
INTRODUCTION
The ability of vehicles to negotiate rough roads and handle well at high speeds is mainly due
to proper design of suspension system. If the axles are bolted directly to the body, any uneven spot in
the road would transmit adjoining force to the vehicle which in turn results in discomfort for riding.
Hence the automobile chassis is mounted on the axle through springs. This is done to isolate shocks
on the vehicle body from road. The parts which perform the function of isolating the vehicle from
road shocks are called suspension system.
1. To prevent the road shocks from being transmitted to the vehicle parts, thereby providing suitable
riding and cushioning effect to the occupants.
2. Reduces wear on the tyre.
3. To keep the vehicle stable while in motion by providing good road holding during driving,
cornering and braking.
4. Provides safe vehicle control and free of irritating vibrations.
REQUIREMENTS
1. Vertical vibrations and pitching: The damper present in suspension system eliminates the
vibrations caused due to striking of front wheel to a bump. However, rear wheel also experiences
similar vibrations as it reaches the bump after some time and this depends on wheel base and vehicle
speed. There are three possible relations of front and rear suspension frequencies.
(i) Front frequency higher than the rear - After the initial vibration i.e., after one or two
vibrations the maximum amplitude occurs.
(ii) Front frequency equal to rear - The amplitude collapses throughout, though pitching tendency
still exists
(iii) Front frequency lesser than the rear - Practically there is no pitching tendency.
So, it is-clear that in order to reduce pitching tendency of the vehicle, the (iii) condition is suitable.
2. Rolling: The centre of gravity of the vehicle will be at certain height above the ground level. A
turning couple about the longitudinal axis of the vehicle will be induced during cornering because of
the centrifugal force acting at C.G. and forces at tyre - road contact surface. This result in a motion
called rolling. The manner in which the vehicle is sprung determines the axis about which the vehicle
will roll.
3. Brake dip: When the brakes are applied, the vehicle nose has a tendency to be lowered or to dip.
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This in turn depends up on C.G position relative to the ground, wheel base, and other suspension
characteristics
4. Unsprung weight: When the wheels hit a bump, they vibrate along with the unsprung parts which
store the vibration energy and transmit it to the sprung parts through the springs. When the weight of
unsprung parts if greater, it increases energy stored due to vibrations and thus transmits greater
shocks to the sprung parts. Therefore it is necessary to keep the unsprung weight as low as possible.
Torsion bars
It is a simple rod which is acting in torsion and takes stresses only. It nearly stores the same
amount of energy per unit weight as that of coil spring. Torsion bar is often used with independent
suspensions.
When compared with other systems, it is lighter and occupies less space. Torsion tubes may
also be used instead of torsion bars. One end of torsion bar is fixed to the frame, while the other end
is fixed to the end of the wheel arm the supported in bearing. The wheel arm is connected to the
wheel hub when the wheel hits a bump; it starts vibrating up and down and produces a torque on
torsion bar, which acts as a spring.
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b) Rear Suspension
In rear suspension systems the coil springs and leaf springs are extensively used. Fig.
illustrates a typical suspension system utilizing coil springs. The rear axle housing is mounted on
springs and is attached to a set of upper and lower control arms. Universal Coupling shown
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(represented in fig. by A & B, keep the wheel vertical and the sliding coupling "C" maintains the
wheel track constant. This method is used in Dedoin type of axle, the control arm pivot points are
rubber bushed. One end of the arm is connected to housing and the other to the frame. The arm
arrangement allows the rear axle housing to move up and down, but prevents excessive Fore and Aft
and side-to-side movement.
The main disadvantages are
1. Ignition lost is high.
2. As there is large number of parts, maintenance required is more.
3. The steering geometry is misaligned with the wear of component.
The centre of gravity of the vehicle will be at certain height above the ground level. A turning
couple about the longitudinal axis of the vehicle will be induced during cornering because of the
centrifugal force acting at C.G and forces at tyre-road contact surface. This result in a motion called
rolling. This causes the left hand suspension move out of phase with the right hand suspension. The
tendency of the front portion (Nose) of the vehicle to dip due to braking is known as Brake Dip.
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CONFERT CURVES
SPRINGS
The following are the commonly used suspension springs in Cars and Trucks.
1. Leaf or Laminated Springs.
2. Helical or Coil springs.
3. Torsion Bar
4. Rubber or elastic springs.
5. Hydro elastic springs.
6. Air springs.
Fig: Long, wide and quite thin. Entire spring is made up of one leaf.
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2. Coil Springs
Nowadays coil springs have become very popular because of the limitations involved in leaf
spring. Fig shows a front suspension system using coil springs. In the system shown, the coil spring
is held between a spring seat in the car frame and a lower control arm. The inner ends of control arms
are pivoted on the car frame, the outer ends are connected to the steering knuckle. This in turn is
attached to the control arms. The ball joints used to allow the steering knuckles to swing to the left or
right for steering. In the assembled car, the wheels are mounted from left to right pivots the front
wheels, so that the car can be steered.
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If a wheel of car hits a bump, the spring compresses. Then the spring expands after the wheel
passes bump, the expansion. of the spring causes the car to be thrown upward. Now, having over-
expanded, the spring shortens again. This action causes the wheel to momentarily leave the road and
the car drops down. The action is repeated until the oscillation gradually dies out.
Such spring action on a car would produce a very bumpy and uncomfortable ride. It could
also be dangerous because a bouncing wheel would make the car impossible to control. This would
be especially dangerous on a curve. It is obvious, therefore, that a device is needed to control the
oscillating action of the spring. This device is known as the Shock absorber.
Out of so many types of shock absorbers available such as Vane type, opposed piston etc, telescopic
shock absorber is most commonly used.
The telescopic shock absorber consists of an outer cylinder, inner cylinder, piston and piston
rod and in some cases an outer dust and rock shield. At the bottom of the inner cylinder and in the
piston a series of valves controls the movements of the hydraulic fluid within the shock absorber.
Fig illustrates the working of Telescopic shock absorber. In this, piston rod is attached to the
two way valve' 1', while valve '2' which is also a two way valve is attached between cylinder and
tube as shown in fig. The inner and outer cylinders are filled with oil. When the vehicle comes across
a bump, the eye connected to axle will move up. With this the oil below valve' I' will moves up. Due
to the resistance to the flow of oil through valve '1’ it exerts pressure on valve '2'. This allows oil to
flow through valve '2' also.
The flow of oil through valves 1 & 2 will be slow because of damping effect. In the similar way, for
the down ward movement of the eye connected to axle, because of road irregularities, the oil will
move from the upper side of valve '1 ' to the lower side and vice - versa.
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In air suspension system, four air springs (air bags) which may be of bellow type or piston
type are used instead of coil springs. The atmospheric air passes through a filter and compressor
raises its pressure to about 24kg / mm- and air at this pressure is accumulated in an accumulator. The
relief valve in the accumulator tank acts as a safety valve. This high pressure air then enters to the air
springs through lift control valve and leveling valves.
BRAKES
Brakes are used as a stopping medium to stop or slow down the vehicle or to prevent the
vehicle movement when it is parked (parking brakes). During braking the kinetic energy of the
vehicle is dissipated as heat and is the reverse of accelerating a vehicle. When driving a vehicle
engine torque produces a tractive effort at the driving wheels, and during braking, the braking torque
at the brake drums produces a negative tractive effort or retarding force at the braking wheels.
Similar to acceleration, the retarding force and rate of deceleration are also limited by adhesion
available between tyre and the ground.
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The brakes must be capable of decreasing vehicle speed faster than the engine accelerates it.
While moving down a steep gradient, the brakes are used to control the vehicle and brakes remain in
action for a longer period. This needs efficient cooling of the braking system.
BRAKING REQUIREMENTS
The function of the brakes is to develop suitable retarding force to stop the vehicle within
minimum possible distance and converts kinetic energy of the vehicle in to heat which is being
dissipated to atmosphere.
To perform the above function, the brake system has to satisfy the following requirements.
1. Irrespective of vehicle speed, load conditions, type of road, the brakes must produce maximum
possible retarding force and deceleration.
2. Irrespective of road condition and load, the pedal effort required should be same.
3. The response time of the braking system should be minimum possible.
4. The brakes must have good anti fade characteristics. The brake effectiveness should not decrease
due to prolonged application (While descending hills). This needs efficient cooling of the brake
system.
5. In an emergency, the brakes must be strong enough to stop the vehicle and in the mean time,
driver must have proper control over the vehicle. The vehicle should not skid and should be
consistent with safety.
6. The brake system should not be affected by water, dust, road grit etc.
7. The braking system should be as light as possible, easy to maintain and should give long,
economical life.
8. The braking system should produce less noise and vibrations.
9. The system should facilitate the use of independent secondary brake and parking brake.
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due to high deceleration, it might injure the passengers or would cause the load to slide and breaks
the cabin. Highly efficient brakes also causes rapid wear of tyres, (thus reduces tyre life), and brake
linings and it is difficult to control the vehicle during the application of brakes.
Thus, in general brakes with efficiencies of 50% to 80% are used to stop the vehicle within
reasonable distance. For any vehicle, the minimum brake efficiency is said to be 50% for foot brakes
and 30% for hand brakes.
The following table shows the approximate stopping distances at different speeds for various
brake efficiencies.
Table: Shortest stopping distance.
The values given in the table vary depending on type of road surface, condition of tyre treads etc.
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TYPES OF BRAKES
The automobile brakes are classified by considering several factors.
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available for heat dissipation and thus provides efficient cooling of brake drums. (Also the brake
drums are exposed to atmosphere).
The automobiles are usually provided with wheel brakes.
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DRUM BRAKES
These brakes are most commonly used and brake shoes are actuated by mechanical means am
or toggle lever) and makes contact with the inside of the brake drum. The rods and levers decreases
the pedal effort required by the driver through mechanical advantage or lever.
This consists of a brake drum which is fixed to the hub of the road wheel and the back plate is
mounted on the axle casing. On the front a .le side, the back plate is fixed to the steering knuckle
through bolts. The expander, anchor and brake shoes all are supported on the back plate which is
made from pressed steel sheet. It protects the drum and shoe assembly from mud and dust. As it
absorbs the complete torque reaction of the shoes, it is also called as "torque plate". Two brake shoes
which are semi circular in shape are anchored on the back plate. Friction linings are attached on the
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outer periphery of the brake shoes, through which it makes contact with the drum. The brake shoe
rubs against wheel rim through friction lining and locks the wheel. One or two retractor springs are
used which keeps the brake shoes away from the drum, when brakes are not applied. The brake shoes
are anchored at one end and on the other ends force is applied by using some brake actuating
mechanism, may be an expander or wheel cylinder. The expander is operated by using a link rod
which is connected to the brake pedal. The expander forces the brake shoe to rub against revolving
wheel drum from inside, there by applying the brakes. An adjuster serves to adjust the wear of
friction lining with use.
µ B = Coefficient of friction between brake lining and drum. F = Normal force applied on brake
shoes.
From the above discussion, it is clear that, the retarding force' F' is dependent on dimensions or sizes
of brake drum and wheel.
a) The braking effect can be increased by area of brake lining and pressure applied at the brake
lining.
b) The braking effect also depends upon the coefficient of friction between braking surfaces and
between tyre and road, but too higher friction coefficients may lock the wheels.
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Let P Land P T are the normal forces acting between shoe and the drum for leading and
trailing shoes respectively. This produces friction forces on the shoes which act perpendicular to the
forces P Land P T" To make the calculation simple, the following assumptions are made.
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(i) The brake consists of symmetrical linings.
(ii) The normal and friction forces act at the lining contact centre point of leading or trailing shoe.
(iii) The normal forces on leading and trailing shoes are equal.
(iv) The value of' ~B' is independent of pressure and velocity.
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DISC BRAKES
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and hence friction pads to move away from the disc, there by releasing it.
This requires higher operating force than drum type. More than one caliper may be used, but
this reduces the cooling rate.
The torque output of the disc brake is given by T = 11. W. R n,
Where, W - Force applied to each of friction pads.
n - Number of friction pads.
R- Mean radius of friction pad (r1+r2)
This torque output is not affected by the direction of disc rotation.
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MECHANICAL BRAKES
All modern cars have been using hydraulic brakes as service brakes since 1940, but
mechanical brakes are still used in parking and emergency brakes. In the mechanical brakes, the
pressure from the brake pedal is transmitted to the wheel brakes through rods and shafts or cables
and shafts.
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Fig: Expander
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Fig: Adjuster
The expander consists of a shaft, the end portion of which is made conical shape as in fig. (a).
The rollers are- used between arc portion of the shaft and brake shoes to reduce the friction. When
the shaft is pulled, conical portion moves up, moves the shoes apart through rollers. Thus the brake
shoes tend to drag on the inner surface of the brake drum, there by applying the brakes.
The adjuster is used to compensate the lining wear on the brake shoes. By screwing the
conical portion' A', brake shoes moves apart, thus taking the wear of lining.
HYDRAULIC BRAKES
Basically, the car hydraulic braking system consists of a master cylinder, steel tubing to form
connecting lines and one or two wheel cylinders for each wheel. In this type, the pedal force is
transmitted to the brake shoes through brake fluid. The force applied to the pedal is multiplied and is
transmitted to all the brake shoes. The brake fluid is incompressible and it exerts equal pressure in all
directions. The brake pedal force is equally applied on all the wheel cylinders and produces equal
braking effect on all the wheels. This force transmission is based on pascal's law which states that
"when pressure is exerted on a confined liquid, it transmits pressure without loss, equally in all
directions".
When the driver operates the brake pedal, it exerts a force on the piston of master cylinder
which is being transmitted to each wheel cylinder. The piston in the wheel cylinder transfer this force
[increased or decreased, depending on piston area, (mechanical advantage)] to the brake shoes.
The movement of piston in master cylinder causes the pistons in wheel cylinders to move
until the brake shoes engage the revolving brake drum. If an attempt is made to depress the master
cylinder piston beyond this point will transmit only pressure, but not motion.
The fig. shows schematically the hydraulic system of a car having drum brakes on all four
wheels. On the front wheels disk brakes may be used, instead of drum brakes. In Hindustan
Ambassador car, on front wheels, a separate wheel cylinder is used to operate each brake shoe (both
shoes leading) and on the rear wheels only one wheel cylinder is used to operate both the shoes (one
leading - other trailing). Here all the shoes are of floating anchor type.
A small pressure of about 50 kpa is maintained in the steel piping to keep the wheel
cylinder pistons in the expanded position, when brakes are not applied. This avoids entry of air in to
wheel cylinders when the brakes are released.
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Master Cylinder
Master cylinder is the heart of the hydraulic braking system in which hydraulic pressure is
developed. Its working is similar to a pump and converts the mechanical force on the brake pedal in
to hydraulic pressure. It is rigidly fastened to the car frame and linked by means of a pedal rod to the
service brake foot pedal. Pressure of the driver's foot on the brake pedal is transmitted through
various linkage arrangements, to a piston in the master cylinder. The forward motion of the piston in
the cylinder pushes the brake fluid. Since the brake lines and wheel cylinders are filled with brake
fluid, the piston acts on a solid column of fluid, thus forcing the wheel cylinder pistons. When the
wheel cylinder pistons have pressed the brake shoes against the drums, fluid movement ceases and
pressure increases depending on force on the piston of master cylinder.
Construction
The fig. illustrates the construction of master cylinder. Essentially it consists of a supply tank
or a reservoir and compression chamber in which the piston operates. The reservoir will supply
additional fluid, when needed, to compensate for loss of fluid in the pipe lines due to temperature
variations and due to minute leakage. The air vent provided in the cap will keep the brake fluid
always at atmospheric pressure and allows expansion and contraction of the fluid without forming
pressure or vacuum.
The compression chamber consists of an aluminums piston which is covered with rubber
seals on both the ends to prevent leakage of brake fluid. The inner face of the piston presses against a
rubber primary seal and this prevents leakage past the piston. The outer piston end has a rubber
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secondary seal to prevent fluid from leaving the master cylinder. The inner piston head has several
small bleeder ports (piston holes) that pass through head to the base of the rubber primary seal. The
piston is prevented from coming out by using stop washer and circlip as shown in figure. A push rod
is used to apply the pressure and it connects the piston to the brake pedal linkage. A fluid check
valve with a rubber cup inside is held against a rubber seat by a coil spring. The spring presses
against the check valve, while the other end is against the piston primary seal. This serves to retain
the residual pressure in the brake lines, even when the brakes are released.
On the primary seal side, a number of holes are located in the piston head. The bypass port
(compensation or relief port) and intake (recuperation or filler port) port are used to connect the fluid
reservoir to the compression chamber.
Working
When the brake pedal is pressed, push rod moves the piston inward (left) against the spring
force, till it covers the bypass port. With bypass port closed, the further movement of the piston
builds up the pressure in the compression chamber. This pressure forces the check valve inner rubber
cup to open and pass fluid in to the lines. This fluid enters the wheel cylinder and causes the pistons
in it to move out ward and force the shoes tightly against the rotating drum, thereby applying the
brakes.
When the brake pedal is released, pressure from the brake shoe return springs forces fluid
back against the check valve and the master cylinder piston moves outward (right) due to spring
action in the master cylinder. The fluid under pressure will lift the check valve off its seat, allowing
fluid to return to the cylinder.
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The spring force in the master cylinder keeps the fluid check valve pressed on its seat for
some time and hence delays the return of fluid from lines to the compression chamber. Some delay is
also caused by fluid inertia in the lines. This creates a vacuum in the compression chamber and
would result in air leakage in to the system and makes the brake useless. This problem is avoided by
having an intake port as shown. As vacuum is created, the fluid reservoir in which the fluid is at
atmospheric pressure, forces the fluid through intake port and holes in the piston which deflects the
rubber cup and enters the compression chamber, thereby destroying the vacuum. In this way a
complete column of liquid is always maintained between master cylinder piston and wheel cylinder
pistons, ready for the next brake application.
But, by the time fluid from reservoir fills the vacuum; the fluid from the lines comes back
and lifts the check valve of its seat. But compression chamber is already full and this extra fluid has
to be accommodated somehow. Otherwise, this would cause the brakes to jam, as pressure in the
lines has not been relieved fully. This problem is solved by means of a bypass port. The extra fluid
from the lines passes through the bypass port to the reservoir, where atmospheric pressure is
maintained.
Wheel cylinder
In the brake system, wheel cylinder is used to transmit the pressure of the fluid in master
cylinder to the brake shoes and force them against the revolving drum. One wheel cylinder (in some
system, two) is used to each wheel to operate the brake shoes.
The figure shows the construction of wheel cylinder and the figure shows the simplified view
of the wheel cylinder which is forcing the brake shoes outward against the drum. It consists of cast
iron housing, two aluminum pistons (in some cases sintered iron pistons are used), rubber seals
(cups), cup spreaders, coil spring and rubber boot (dust cover). The brake line from the master
cylinder is connected to the inlet port. The cylinder is drilled to provide a bleeder screw, to bleed the
air from the system, whenever required. The wheel cylinder is usually bolted to the brake backing
plate.
When brakes are applied, master cylinder forces fluid in to wheel cylinder through inlet port
and forces the pistons to move apart. This outward movement of the pistons pushes the brake shoes
against the drum.
When the brakes are released, the piston move inward due to spring force and forces the
brake fluid out of wheel cylinder.
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Disadvantages
1. It is suitable for intermittent brake applications. Mechanical linkage has to be provided for
parking purposes.
2. Even a small leakage of air in to the system makes the brake useless.
Fig: Simplified wheel cylinder action, arrow marks shows fluid pressing on rubber cup
POWER BRAKES
When the vehicle weight is more, driver cannot apply the brakes comfortably without fatigue,
some external source of energy is used-to supplement his effort which makes the brake application
easier. For this reason, many of the vehicles are equipped with power brakes. The power brakes are
used to reduce the amount of pedal pressure necessary to stop the vehicle. If energy for these brakes
are taken from the transmission of the vehicle itself, which partly helps the driver i.e., driver still has
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to put some effort while applying the brakes, then such brakes are 'called "Servo brakes or Power
assisted brakes". Practically when none of the braking effort is applied by the driver, then the brakes
'are termed as "Power brakes or Power operated brakes".
The mechanism which supplements the driver's effort in applying the brakes is called a
"servo mechanism". This servo action or self energization of brakes helps the driver to apply the
brakes without fatigue.
Mechanical servos were used initially, but these have become obsolete after the introduction
of vacuum operated servos. In vacuum servo brakes, the brakes are applied by utilizing engine
suction from inlet manifold, A small vacuum reservoir may be provided to have enough vacuum for
several brake applications even after engine has stopped. Vacuum servo brakes are of two types, both
types consists of a piston or a diaphragm operating in a cylinder and are incorporated with suitable
linkage for brake application. In the first type, on both the sides, piston is exposed to atmosphere,
when the brakes are not applied. When the brakes are applied, engine vacuum will act on one side of
piston and the differential pressure on both sides of the piston causes the linkage to operate the
brakes. In the second type, both sides of the piston are subjected to vacuum when the brakes are in
the released position. When brakes are applied, one side of piston is exposed to atmospheric pressure
and the differential pressure on both sides of the piston, causes the linkage to operate. This system is
more rapid in operation and hence preferred over the first type. The second type is called "Suspended
Vacuum" system.
Fig: Second type; suspended vacuum type Brakes in released position & when brakes are
applied.
The figure shows one type of servo vacuum or power brake. It consists of a piston and
cylinder arrangement as in figure. Master cylinder piston is connected to the one side of the piston
and the other side of it is connected to the brake pedal. A vacuum control valve is placed between the
brake and the piston. This valve admits vacuum to one side of the piston, while the other side is kept
at atmospheric pressure. This valve can also allow atmospheric pressure to reach both sides of the
piston.
When brakes are applied, the control valve closes off the atmospheric pressure to the master
cylinder side of the piston (i.e. on the right side of the piston). Further movement of the brake pedal
opens a vacuum inlet passage to this same side, and thus vacuum acts on this side of the piston. So,
on the left side atmospheric pressure is acting and a partial vacuum is acting on the right side of the
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piston. This differential pressure forces the piston to move to the vacuum side. 'As the master
cylinder piston is connected to this piston (say P), it moves toward right and thus apply pressure to
the brake system.
The power brakes have three stages of operation.
1. Brakes released
2. Applying brakes
3. Holding constant apply pressure.
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AIR BRAKES
As vehicle weight increases (like heavy buses and trucks), heavier braking effort is required
to stop the vehicle. Compressed air powered brakes are suitable for these heavy vehicles. These
brakes consist of flexible diaphragms in brake chambers and are connected to the brake rods and
controlled by hand or foot operated valve. The compressed air pressure acts on flexible diaphragm.
The brake rods connect to brake operating cams on the wheel brakes. The braking operation is
controlled by a brake valve which directs the air flow from reservoir against the diaphragms in the
brake chambers during brake application and from brake chambers to the atmosphere, when brake is
released. When the pressure in the reservoir falls below certain value, air compressor which is driven
by engine, supplies the compressed air.
The layout of an air brake system is as shown in figure. The filtered air from the compressor
passes to the reservoir through the unloaded valve which sends the air at a predetermined reservoir
pressure (about 900 kpa). The reservoir supplies air to various accessories and diaphragm units
(brake chambers) at each wheel through the brake valve.
In the air brake system, dust and other matter present in air is removed by passing it through
an air filter. An air compressor driven by the engine, raise the pressure of air to the required level and
supplies high pressure air to brake chambers at wheels through un loader valve. This valve mainly
consists of a governor valve, plunger and non-return valve and regulates the brake line pressure in
the system. A reservoir or air tank made from steel sheet stores the compressed air at the specified
pressure and is used for brake application. The reservoir is also provided with a safety valve to
control the air pressure in it. The brake valve (application valve) is used to regulate the braking
intensity in an air pressure system. Tile brake valve supplies air to the various brake chambers at the
required pressure. One brake chamber is installed on each wheel. In the brake chambers the pressure
energy of compressed air is converted in to useful mechanical work (piston movements) and is used
for brake application.
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The piston is lifted up when the air force is greater than the mechanical force. In this position
exhaust valve opens and air escapes through the exhaust port till the differential pressure on two
sides of the piston is balanced.
For increasing braking intensity, driver has to apply more mechanical force on the piston
through brake pedal. Due to force on the piston, it moves down, thus opening inlet valve more and
admitting more air under pressure. The piston reaches a new position of balance.
Whenever the driver wants to decrease the braking intensity, he will release the brake pedal.
This causes the piston to move up slightly, thus opens the exhaust valve for air to escape through
exhaust port, till a balance of forces is again established on the piston.
If the brake pedal is fully released, there is no mechanical force acting on the piston. The
piston moves up due to air pressure underneath of it, thus opens exhaust valve and closes inlet valve.
The whole pressure is released through exhaust valve and exhaust port. As the inlet valve is closed,
there is no entry of air from the reservoir and hence brakes at the wheels are released.
Brake Chamber
In brake chamber, pressure energy of compressed air is converted into useful
mechanical energy for applying the brakes. Front wheel brake chambers are usually
provided with push rods while sliding forks are fitted in rear brakes.
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UNIT - 8
INTRODUCTION
The purpose of emission control is to reduce amount of pollutants and environmentally
damaging substances released by the vehicles. If not controlled, the automobile can emit pollutants
from fuel tank, carburetor crank case and exhaust system in the atmosphere. The fuel tank and the-
carburetor emit gasoline vapours, crank case releases partly burned air-fuel mixture blown off by
piston rings and pollutants from exhaust system consists of partly burned hydrocarbons, carbon
monoxide, nitrogen oxides and sulphur oxide. The smoke may be formed due to incomplete burning
of fuel [Smoke: particles of unburned fuel and soot called particulates, mixed with air]. It took many
years for the public and the automotive industry to address the problem of these pollutants.
It is estimated that in USA alone 200 million tons of manmade pollutants adds to the air.
Therefore-these pollutants, if not controlled, adversely affect our health. Automobile manufacturers
have been working towards reduction of auto motive air pollutants when auto emissions were found
to be part of the cause of smog. The emission of pollutants can be decreased by improving
combustion efficiency which in turn needs redesigning of fuel tank, carburetor combustion chamber,
cooling system run on and exhaust system. The other way of controlling atmospheric pollution is,
destroy the pollutants after they have been formed.
The emission of pollutants in Auto motives can be reduced by
1. Closed crank case ventilation
2. Fuel tank and carburetor ventilation
3. Redesigning the engine
(i) Combustion chamber,
(ii) Cooling system,
(iii) Fuel supply system and
(iv) Ignition system
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from the engine-Crankcase, before it condenses and reacts with oil to form sludge, which may
corrodes and accelerates wear of pistons, piston rings, valves, bearings, etc. Sludge can also clog oil
lines and starve the lubricating system. As the engine oil circulates, it also carries blow by and
some unburned fuel particles which are formed due to incomplete combustion of air-fuel mixture in
to the crank case. If not removed, this dilutes the engine's oil and hence the oil does not lubricate
the engine properly resulting in excessive wear. Filtered air from the carburettor air clearer must be
circulated through the crank case to remove blow by gases and gasoline vapours from the crank
case. To prevent atmospheric pollution modern engines have a closed system called PCV system.
The flow by gases and gasoline vapours are picked up by filtered air to the engine inlet manifold
through a special PCV valve and from there enters into engine combustion chamber with fresh
charge and are burnt there.
The PCV valve consists of a spring loaded tapered valve. The valve is in closed position
under the action of crank case pressure and manifold vacuum where as the spring pressure keeps the
valve open there by regulate the flow of blow by gases. During idle or deceleration (low speed)
amount of blow by gas is less due to lesser engine load and a small PCV valve opening is needed to
move blow by gases out of crank case. The high intake manifold vacuum moves the tapered valve
against spring pressure, thus provides small opening in the valve for the flow of blow by gases.
During part throttling (or normal speed), engine load is higher than at Idle, blow by increases and
manifold vacuum decreases. The spring moves the tapered valve to increase the opening. The larger
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opening allows the entire blow by gases to enter in to the intake manifold. At high speeds or when
the engine is operating under heavy load, the throttle valve opens widely and decreases intake
manifold vacuum. The spring moves the tapered valve further down ward to provide a larger
opening through the valve. The amount of blow by gases is more, when engine load is high, hence
larger PCV valve opening is essential to allow these gases to flow through the valve in to the intake
manifold.
Fixed orifice tube PCV System
Some engines are not fitted with PCV valve. The blow by gases is routed in to the intake
manifold through a fixed orifice tube. This system works similar to PCV valve, except that the
system is regulated only by the vacuum on the orifice. The amount of blow by gas, flows in to the
intake manifold is limited by the size.
CONTROLLING EVAPORATIVE EMISSIONS [Evaporative emission control systems]:
The fuel evaporative control system capture the gasoline vapours from the fuel tank and
carburetor float bowl and prevents them from escaping in to the atmosphere. This system is called by
various names such as active emission control (EEC), evaporative control systems (ECS), cycle
vapour recovery and vapour saver (recovery) system (VVS / NRS):Since fuel' injection systems do
not have a float bowl, the ECS controls escape of fuel vapours from the fuel tank only.
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solenoid valve may be used to control flow of vapours from the fuel tank. The mechanical valves
operated by the throttle linkage. During idling, it is open and causes the vapour to flow from float
chamber to the canister. The opening of throttle closes the vent valve; likewise, the electrical vent
valve is open when ignition is off. When the ignition is on, the vent valve is closed by the
energisation of solenoid.
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Faster engine warm up and quicker choke open in reassess exhaust emissions during warm-
up. If the carburetor [fuel supply system] supplies cold air-fuel mixture, only a part of fuel will
vaporize. This makes the air-fuel mixture lean and extra rich mixture is required. Therefore, when
the engine is cold, a thermostatically controlled air cleaner is used to supply heated air quickly to the
carburetor. During cold running, air entering carburetor is heated up by thermostatic air cleaner,
which allows engine to run on a leaner air-fuel mixture during warm up.
The thermostatic air cleaner consists of a temperature sensing spring which senses
temperature of air entering the air cleaner. The air bleeds when air is cold and this applies intake
manifold vacuum to the vacuum motor. The diaphragm and hence to control am per assembly moves
up due to atmospheric pressure and thus blocks the snorkel tube. This allows all the air to enter
through the hot air pipe which is laid near to the exhaust manifold. When the engine starts, the
exhaust manifold heats up quickly, and hence allows heated air to enter into the air cleaner. This
heated air helps to vaporize the fuel delivered by carburetor or fuel injectors, which in turn improves
cold engine performance
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mixture a longer time to burn. Under certain operating conditions, this also results in formation of
more Nitrogen Oxide. The devices used to prevent vacuum advance are;
(a) Transmission Controlled Spark TCS or Transmission Regulated Spark (TRS) system: It
delays vacuum advance when the transmission is in neutral reverse and forward gears.
(b) Spark Delay Valve (SDV): It prevents vacuum advance during certain conditions of vehicle
acceleration.
4) The carbon deposits present in the combustion chamber absorb air-fuel mixture and during
exhaust releases air-fuel mixture. The HC in the exhaust gas, add pollutants to the atmosphere.
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The Air injection system consists of air pump, air switching and control valves and the one way
check valves. When the engine is cold, the air pump pushes air through nozzles to the exhaust
manifold. The nozzles are located opposite to exhaust ports and hence 02 in the air helps to burn any
HC and CO in the exhaust gas in the exhaust manifold.
When the engine warms up, ECM causes the air to pass through catalytic converter, where
HC and CO are converted into Hp and CO. The check valve avoids back flow of exhaust gases to the
air pump incase of back fire. During deceleration, the bypass valve momentarily diverts air from air
pump to the air cleaner, instead of to the exhaust manifold. This avoids back firing in the exhaust
system.
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CATALYTIC CONVERTER
The function of catalytic converter is to treat the exhaust gas to convert harmful pollutants
into harmless. All exhaust gases must pass through catalytic converter which is located in exhaust
system. The catalytic converter consists of a material called catalyst which causes a chemical change
without entering into chemical reaction. It makes two chemicals to react with each other and hence
reduces amount of HC, CO and NOx in the exhaust gases.
It consists o(two different catalysts, one to treat HC and CO and other to treat NOx' The first
catalyst promotes HC to unite with °2 to produce Hp and CO2, The second catalyst promotes CO to
react with 02 and hence to release CO2, As this converter oxidises HC and CO, it is known as
oxidising converter. The platinum and palladium are listed as oxidising catalysts.
The catalyst used for NO, splits 02 and N2 and hence NOx becomes harmless N2 and 02, the converter
is known as reducing converter and metal rhodium is used for this purpose. A large surface area of
catalytic converter is coated with catalyst. The coated surface a area or substrate is in the form of a
.bed of small beads or pellets or a ceramic honey comb. Usually honeycomb converter is round and
pellet type converter is flat.
The vehicles fitted with catalytic converter in the exhaust system must use unleaded gasoline
otherwise lead in the gasoline coats the catalyst and makes the converter ineffective. The air fuel
ratio for the mixture must be stoichiometric ratio for effective working of the catalytic converter.
EMISSION STANDARDS
As vehicle populations grow and cities become more congested the allowable emissions from
engines have been lowered to maintain air quality in major cities. The pollutants from vehicles cause
several health problems, leads to formation of smog and affects environment. Many countries are
aiming at achieving safe concentrations of these pollutants by regulating their level of emissions.
Emission standards are requirements that set specific limits to the amount of pollutants that can be
released into the environment. These emission standards regulate pollutants released by automobiles,
industry, power plants and diesel generators etc. Generally these standards regulate the emissions of
nitrogen oxides, sulphur oxides, particulate matter (PM) or soot, carbon monoxide and volatile
hydrocarbons. These emission standards puts limits for conventional pollutants and regulate green
house gases particularly carbon dioxide. In USA, emission standards are managed by the
Environmental Protection Agency. In the state of California, California's emission standards are set
to influence emission requirements that major automakers must meet. The European Union has set
its own emission standards for all road vehicles, trains, barges etc. No standards apply to seagoing
ships or aero planes. The European Union has introduced Euro 4 from 1-1-2008, introducing Euro 5
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from 1-1-20 l0 and Euro 6 from 1-1-2014. Many of the other countries also confirm to the euro 4
standards from Jan 2009. In 198~ India introduced first Indian emission regulations to limit idle
emissions. From 2000 India started adopting European emission and fuel regulations for four
wheeled light duty and for heavy duty vehicles. Indian owned emission regulations still apply to two
and three wheeled vehicles. All transport vehicles must have a fitness certificate that is renewed each
year after the first five years of new vehicle registration.
On October 6, 2003, the National Auto Fuel Policy has been announced which envisages a
phase I-program for introducing Euro 2-4 emission and fuel regulations by 20 I O.
For Two and Three wheelers, Bharat Stage II (Euro 2) was recommended from April 1, 2005 and
Bharat Stage III (Euro 3) was applied from April 1, 2008.
Emission Standards for Diesel Truck and Bus Engines, g/kWh
Year Reference CO HC NO PM
-- 17.3 - 32.6 2.7 - 3.7 x - --
1992 -
1996 -- 11.20 2.40 14.4 --
2000 Euro I 4.5 1.1 8.0 0.36
2005 Euro II 4.0 1.1 7.0 0.15
2010 Euro III 2.1 0.66 5.0 0.10
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Year Reference CO HC NO PM
Year CO HC HC+NO
x
1991 12 - 30 8 - 12
1996 6.75 - 5.40
2000 4.00 - 2.00
2005 (BSII) 2.25 - 2.00
Catalytic converters have been instrumental in reducing emissions of harmful gases from
vehicles since their inception in response to the US Clean Air Act of 1970. Regulated emissions
have been reduced approximately 1/3 while the number of cars on the road have more than doubled.
Platinum, palladium and rhodium are essential components in automobile catalytic converters
reducing engine-out emissions by well over 90%, and in some cases by over 99%.
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1) 1991 - Idle 'CO' Limits for Gasoline Vehicles and Free Acceleration Smoke for Diesel Vehicles,
Mass Emission Norms for Gasoline Vehicles.
3) 1996 - Revision of Mass Emission Norms for Gasoline and Diesel Vehicles, mandatory fitment
of Catalytic Converter for Cars in Metros on Unleaded Gasoline.
5) 2000 - India 2000 (Eq. to Euro I) Norms, Modified IDC (Indian Driving Cycle), Bharat Stage II
Norms for Delhi.
6) 2001 - Bharat Stage II (Eq. to Euro II) Norms for all Metros, Emission Norms for CNG & LGP
Vehicles.
7) 2003 - Bharat Stage II (Eq. to Euro II) Norms for 11 major cities.
8) 2005 - From 1st April Bharat Stage III (Eq. to Euro III) Norms for 11 major cities.
9) 2010 - Bharat Stage III Emission Norms for 4-wheelers for entire country whereas Bharat Stage -
IV (Eq. to Euro IV) for 11major cities. Bharat Stage IV also has norms on OBD (similar to Euro
III but diluted).
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