Automobile Theory and Explanation

5

Introduction to

Automobile Engineering UNIT 1 INTRODUCTION TO AUTOMOBILE

ENGINEERING

Structure

1.1 Introduction

Objectives

1.2 Definition

1.3 Classification of Vehicles

1.4 Layout of an Automobile Chassis

1.5 Components of the Automobile

1.6 Functions of Major Components of an Automobile

1.7 Summary

1.8 Key Words

1.9 Answers to SAQs

1.1 INTRODUCTION

Automobile engineering is the one of the stream of mechanical engineering. It deals with

the various types of automobiles, their mechanism of transmission systems and its

applications. Automobiles are the different types of vehicles used for transportation of

passengers, goods, etc. Basically all the types of vehicles works on the principle of

internal combustion processes or some times the engines are called as internal

combustion engines. Different types of fuels are burnt inside the cylinder at higher

temperature to get the transmission motion in the vehicles. Most of the automobiles are

internal combustion engines vehicles only. Therefore, every mechanical and automobile

engineers should have the knowledge of automobile engineering its mechanism and its

various applications.

Objectives

After studying this unit, you should be able to

define automobile engineering,

classify the vehicles,

list the various components of automobile, and

describes the function of components of automobile.

1.2 DEFINITION

Automobile engineering is a branch of engineering which deals with everything about

automobiles and practices to propel them.

Automobile is a vehicle driven by an internal combustion engine and it is used for

transportation of passengers and goods on the ground. Automobile can also be defined as

a vehicle which can move by itself.

Examples : Car, jeep, bus, truck, scooter, etc.

6

Automobile Engineering

1.3 CLASSIFICATION OF VEHICLES

Automobiles or vehicles can be classified on different bases as given below :

On the Basis of Load

(a) Heavy transport vehicle (HTV) or heavy motor vehicle (HMV), e.g. trucks,

buses, etc.

(b) Light transport vehicle (LTV), e.g. pickup, station wagon, etc.

(c) Light motor vehicle (LMV), e.g. cars, jeeps, etc.

Wheels

(a) Two wheeler vehicle, for example : Scooter, motorcycle, scooty, etc.

(b) Three wheeler vehicle, for example : Autorickshaw, three wheeler scooter

for handicaps and tempo, etc.

(c) Four wheeler vehicle, for example : Car, jeep, trucks, buses, etc.

(d) Six wheeler vehicle, for example : Big trucks with two gear axles each

having four wheels.

Fuel Used

(a) Petrol vehicle, e.g. motorcycle, scooter, cars, etc.

(b) Diesel vehicle, e.g. trucks, buses, etc.

(c) Electric vehicle which use battery to drive.

(d) Steam vehicle, e.g. an engine which uses steam engine. These engines are

now obsolete.

(e) Gas vehicle, e.g. LPG and CNG vehicles, where LPG is liquefied petroleum

gas and CNG is compressed natural gas.

Body

On the basis of body, the vehicles are classified as :

(a) Sedan with two doors

(b) Sedan with four doors

(c) Station wagon

(d) Convertible, e.g. jeep, etc.

(e) Van

(f) Special purpose vehicle, e.g. ambulance, milk van, etc.

Transmission

(a) Conventional vehicles with manual transmission, e.g. car with 5 gears.

(b) Semi-automatic

(c) Automatic : In automatic transmission, gears are not required to be changed

manually. It is automatically changes as per speed of the automobile.

Position of Engine

Engine in Front

Most of the vehicles have engine in the front. Example : most of the cars,

buses, trucks in India.

Engine in the Rear Side

Very few vehicles have engine located in the rear. Example : Nano car.

7

Introduction to

Automobile Engineering 1.4 LAYOUT OF AN AUTOMOBILE CHASIS

Layout of an engine chasis is shown in the Figure 1.1 given below :

Figure 1.1 : Chasis of a Passenger Car

It contains the source of power, i.e. engine, the frame, which supports the engine,

wheels, body, transmission, the braking system and the steering. It also gives support to

suspension system and springs. Besides these parts

1.5 COMPONENTS OF THE AUTOMOBILE

The automobile can be considered to consist of five basic components :

(a) The Engine or Power Plant : It is source of power.

(b) The Frame and Chasis : It supports the engine, wheels, body, braking

system, steering, etc.

(c) The transmission which transmits power from the engine to the car wheels. It

consists of clutch, transmission, shaft, axles and differential.

(d) The body.

(e) Accessories including light, air conditioner/hearer, stereo, wiper, etc.

1.6 FUNCTIONS OF MAJOR COMPONENTS OF AN

AUTOMOBILE

Chasis and Frame

The chasis is formed by the frame with the frame side members and cross

members. The frame is usually made of box, tubular and channel members that are

welded or riveted together. In addition to this, it comprises of the springs with the

axles and wheels, the steering system and the brakes, the fuel tank, the exhaust

system, the radiator, the battery and other accessories. Along with this the frame

supports the body.

8


Engine or Power Plant

The engine is the power plant of the vehicle. In general, internal combustion

engine with petrol or diesel fuel is used to run a vehicle. An engine may be either

a two-stroke engine or a four-stroke engine.

An engine consists of a cylinder, piston, valves, valve operating mechanism,

carburetor (or MPFI in modern cars), fan, fuel feed pump and oil pump, etc.

Besides this, an engine requires ignition system for burning fuel in the engine

cylinder.

Transmission System (Clutch and Gear Box)

The power developed by the engine is transferred to the wheels by transmission

system. Transmission system must do three jobs :

(a) It must provide varying gear ratios. Number of gear ratios are equal to

number of gears in a vehicle.

(b) It must provide a reverse gear for moving vehicle in reverse direction.

(c) It must provide a neutral or disconnecting arrangement so that the

engine can be uncoupled from the wheels of the vehicle. In a

conventional transmission system, there is a clutch, a manually

operated transmission (gear box), a propeller shaft and a differential

or final drive.

Clutch

The purpose of the clutch is to allow the driver to couple or decouple the engine

and transmission. When clutch is in engaged position, the engine power flows to

the transmission through it (clutch). When gears are to be changed while vehicle is

running, the clutch permits temporary decoupling of engine and wheels so that

gears can be shifted. In a scooter, the clutch is operated by hand where as in a car

the clutch is operated by foot. It is necessary to interrupt the flow of power before

gears are changed. Without a clutch, it will by very difficult.

Final Drive

Final drive is the last stage in transferring power from engine to wheels. It reduces

the speed of the propeller shaft (drive shaft) to that of wheels. It also turns the

drive of the propeller shaft by an angle of 90o to drive the wheels.

Figure 1.2 : Final Drive

The propeller shaft has a small bevel pinion which meshes with crown wheel. The

crown wheel gives rotary motion to rear axles. The size of crown wheel in bigger

than that of bevel pinion, therefore, the speed of rear axles (or crown wheel) in

lower than the speed of pinion. Final drive is of two types, i.e. chain type and gear

type.

Braking System

Brakes are used to slow down or stop the vehicle. Hydraulic brakes are generally

used in automobiles, where brakes are applied by pressure on a fluid. Mechanical

brakes are also used in some vehicles. These brakes are operated by means of

9

Introduction to

Automobile Engineering leavers, linkages, pedals, cams, etc. Hand brake or parking brake is usually a

mechanical brake. These are used for parking the vehicles on sloppy surfaces and

also in case of emergency.

Gear Box

Gear box contain gearing arrangement to get different speeds. Gears are used to

get more than one speed ratios. When both mating gears have same number of

teeth, both will rotate at same number speed. But when one gear has less teeth

than other, the gear with less number of teeth will rotate faster than larger gear. In

a typical car, there may be six gears including one reverse gear. First gear gives

low speed but high torque. Higher gears give progressively increasing speeds.

Gears are engaged and disengaged by a shift lever.

Steering System

In front wheels can be turned to left and right by steering system so that the

vehicle can be steered. The steering wheel is placed in front of driver. It is

mechanically linked to the wheels to provide the steering control. The primary

function of the steering system is to provide angular motion to front wheels so that

vehicle can negotiate a turn. It also provides directional stability to vehicle when

the vehicle moves ahead in straight line.

Figure 1.3 : Simple Driving of a Steering System

Now-a-days, many vehicles are equipped with power steering which uses pressure

of a fluid to reduce steering effort. When driver turns the steering wheel, a

hydraulic mechanism comes into play to provide most of the effort needed to turn

the wheel.

Front Axle

Front axles are mounted at the end of front axle. A part of the weight of vehicle is

transmitted to the wheels through this axle. The front axle performs several

functions.

It carries the weight of the front of the vehicle and also takes horizontal and

vertical loads when vehicle moves on bumpy roads. When brakes are provided on

front wheels, it endures bending stresses and torsional stresses. It is generally

made from steel drop forging. It is robust in construction.

Suspension System

Suspension system of an automobile separates the wheel and axle assembly of the

automobile from its body. Main function of the suspension system is to isolate the

body of the vehicle from shocks and vibrations generated due to irregularities on

the surface of roads. Shock absorbers are provided in the vehicles for this purpose.

It is in the form of spring and damper. The suspension system is provided both on

front end and rear end of the vehicle.

A suspension system also maintains the stability of the vehicle in pitching or

rolling when vehicle is in motion.

10


SAQ 1

(a) Define automobile engineering.

(b) Classify the vehicles on the basis of different aspects.

(c) What are the various components of automobile?

(d) Describe the functions of various components of automobile.

(e) Describe the working of steering system mechanism

1.7 SUMMARY

1.8 KEY WORDS

1.9 ANSWERS TO SAQs

Refer the preceding text for all the Answers to SAQs.

11

Power Plants

UNIT 2 POWER PLANTS

Structure

2.1 Introduction

Objectives

2.2 Classification of IC Engines

2.3 Four Stroke Engines versus Two Stroke Engines

2.4 Working of Four Stroke Petrol Engine

2.5 Working of Four Stroke Diesel Engines

2.6 Working of Two Stroke Petrol Engines

2.7 Working of Two Stroke Diesel Engines

2.8 Main Differences between Two and Four Stroke Engines

2.9 Application of Two Stroke and Four Stroke Engines

2.10 Important Terms

2.11 Specifications of Automobile Engine

2.12 Summary

2.13 Key Words

2.14 Answers to SAQs

2.1 INTRODUCTION

Power plant or power unit of an automobile is that component or part which produces

power to drive the automobile. It is generally in the form of an internal combustion

engine running on petrol or diesel. In some cases, it can be a gas turbine or steam engine.

These are called external combustion engines. However, steam engines are now obsolete

and therefore not used for driving any vehicle.

This unit mainly covers various aspects of internal combustion engines from concept to

their principle of working. In case of an internal combustion engine (IC engines)

combustion (burning) of fuel with air takes place inside the engine cylinder.

Objectives


,

,

, and

.

2.2 CLASSIFICATION OF IC ENGINES

IC engines may be classified on different bases. Some of the main classifications are

given below :

According to Fuel Used

(a) Petrol engine

(b) Diesel engine

12


(c) LPG engine

(d) CNG (Compressed Natural Gas) engine

According to Cycle of Operation

(a) Two stroke engines

(b) Four stroke engines

According to Cycle of Combustion

(a) Otto cycle engines which work on Otto cycle.

(b) Diesel cycle engine which work on diesel cycle.

(c) Dual cycle engines which work on dual cycle.

According to Method of Ignition

Spark Ignition (SI) Engines

These engines are petrol engines in which a spark plug is used to ignite the

fuel-air mixture.

Compression Ignition (CI) Engines

Diesel engines are CI engines in which air is compressed to such a high

pressure and temperature so that burning of fuel takes place as soon as it is

injected into the cylinder due to high temperature.

2.3 FOUR STROKE ENGINES VERSUS TWO

STROKE ENGINES

Four stroke engines are those engines in which one engine cycle is completed in two

revolutions of crank shaft or four piston strokes. Various piston strokes are : suction,

compression, power and exhaust.

In two stroke engines, the entire cycle is completed in one revolution of crank shaft or

two piston strokes.

2.4 WORKING OF FOUR STROKE PETROL ENGINE

In four stroke engines, one cycle is completed with completion of four strokes. Main

features of all the strokes are discussed below and their sketch is given in Figure 2.1.

Suction or Intake Stroke

Initially the piston remains n top dead centre (TDC) position, suction valve is open

and exhaust valve remains closed. The piston now moves downward and the petrol

and air mixture (charge) enters into the cylinder. When piston reaches bottom

dead centre (BDC). The cylinder fills with the petrol air mixture. At this moment,

suction valve closes. This completes one stroke. Crank turns by 180o, i.e. it

completes half revolution.

Figure 2.1(a) : Suction or Charging Stroke

13

Power Plants Compression Stroke

Both the valves (suction and exhaust) are closed. The piston moves upwards from

BDC to TDC position. The charge is compressed inside the cylinder, i.e. its

pressure increases and volume decreases. Along with pressure temperature also

increases. The crank completes next half of revolution.

Figure 2.1(b) : Compression Stroke

Working or Expansion or Power Stroke

When the piston reaches the TDC position spark plug generates spark and the

charge is ignited and combustion of mixture takes place. Because of burning of

fuel temperature and pressure of gases increases tremendously., both the valves

remain closed. The gases expand in the cylinder and push the piston downward

and therefore, work is done by the gases on the piston. The crank revolves and

completes next half revolution. The reciprocating motion of the piston is

converted into rotary motion of crank-shaft by piston rod and crank. During

expansion, volume of gases increases. All the power for running the engine is

obtained during this stroke.

Figure 2.1(c) : Expansion or Working Stroke

Exhaust Stroke

The suction valve remains closed but exhaust valve opens. The piston moves from

BDC to TDC. The burnt gases are pushed out of the cylinder due to movement of

piston. The cylinder pressure falls down to little above atmospheric pressure. This

completes the next half revolution of the crank. By this time, crank shaft

completes two revolution and one engine cycle is completed with the completion

of four strokes. After this the same process is repeated again and again.

(d) Exhaust Stroke

Figure 2.1 : Four Stroke Cycle Petrol Engine

14


2.5 WORKING OF FOUR STROKE DIESEL ENGINES

The main features of all the four strokes in diesel engines are given below :

Suction or Intake Stroke

Initially piston is at top dead centre (TDC), exhaust valve is closed but suction

valve opens. Piston moves downwards towards bottom dead centre (BDC). As

suction valve is open, air enters into the cylinder. It is important to note that only

air enters the cylinder during suction in case of diesel engines. Cylinder is full of

air when piston reaches BDC and suction stroke in completed. Crank shaft or

crank rotates by 180o, i.e. it completes half revolution.

Figure 2.2(a) : Suction or Charging Stroke

Compression Stroke

Both the valves (suction and exhaust) are closed, piston moves from BDC to TDC.

Volume of air decreases and pressure and temperature increases. When the piston

reaches TDC, this stroke is completed and the crank completes next half

revolution. By this time crank has rotated by 360o.

Figure 2.2(b) : Compression Stroke

Expansion or Power Stroke

At the end of compression stroke, both the valve remains closed. The injector

fitted in the cylinder head injects diesel fuel in the high temperature air. The

temperature is so high that the fuel, i.e. diesel starts burning at constant pressure.

The pressure and temperature increases further due to combustion of fuel. The

gases in the cylinder push the piston downwards from TDC to BDC and expansion

process takes place. The volume of gases increases and work is obtained in this

process. The reciprocating motion of piston is converted into rotary motion of

crank shaft through piston rod and crank.

Expansion process is completed when piston reaches BDC. The crank rotates by

next half revolution. This stroke is called power stroke because power of work is

obtained in this stroke.

15

Power Plants

(c) Expansion or Working Stroke

Exhaust Stroke

After completion of expansion stroke, the piston starts moving upwards from BDC

to TDC. Suction valve is close, exhaust valve is open. As the piston moves, it

pushes the burnt gases through the exhaust vale. Thus, exhaust takes place. The

cylinder becomes empty as the piston reaches TDC. The exhaust stroke is

completed. Crank has now completed two revolutions and all the four strokes are

now completed. This completes one engine cycle. These cycles are repeated as

engine continues to run.

(d) Exhaust Stroke

Figure 2.2 : Four Stroke Cycle Diesel Engine

2.6 WORKING OF TWO STROKE PETROL

ENGINES

Two stroke and four stroke engines are different in the method of filling the cylinder

with fresh charge and also in the removal of burnt gases from the cylinder. In a four

stroke engine these processes are performed by the movement of piston during suction

and exhaust stroke. In four stroke engines these are suction and exhaust valves where as

suction (inlet) and exhaust (outlet) ports are cut in the walls of cylinder.

Whole process of working of two stroke petrol engine has been shown in Figure 2.3. The

Figure 2.3 shows a three channel system in which the fresh charge is compressed in the

crank case of the engine. This is also called crank are compression system. Figure 2.3

shows all working of two stroke petrol engine in three stages.

Exhaust and Transference

Figure 2.3(a) shows the exhaust and transfer process. When the piston moves from

TDC to BDC, i.e. downwards after expansion of gases, the piston uncovers the

exhaust port. The burnt gases start going out of the cylinder. Simultaneously the

slightly compressed charge in the crank case is forced into the cylinder through

transfer port. The deflector on the piston crown deflects this charge and the fresh

charge moves in the upward direction. This fresh charge pushes the burnt gases

16


out of cylinder. During this process, some fresh charge may also leave the cylinder

through exhaust port.

The process of cleaning of cylinder, by pushing burnt gases by fresh charge, is

known as scavenging.

Figure 2.3(a) : Exhaust and Transference

Compression

When the piston moves upwards from BDC to TDC, transfer port and exhaust

ports are closed. Compression of charge, present in the cylinder takes place.

During this motion the inlet valve open and fresh charge enters the crank case.

When the piston reaches TDC, compression process is completed.

Figure 2.3(b) : Compression and Suction

Ignition and Expansion

After compression, spark plug generates spark and ignition of fuel takes place.

Rapid rise in pressure and temperature takes place at constant volume. At this

stage both transfer port and exhaust port are closed. Expansion of burnt gases

takes place at the piston moves downward from TDC to BDC. The gases push the

piston with great force and power is obtained during this process. Simultaneously,

slight compression of fresh charge, present in crank case takes place.

(c) : Ignition and Expansion

Figure 2.3 : Two Stroke Cycle Petrol Engine

17

Power Plants After this process shown in Figure 2.3(a), i.e. exhaust and transfer of charge takes

place and cycle is repeated again. Thus, the cycle is completed in two strokes of

piston and one revolution of crank shaft. In case of petrol engines, fresh charge

consists of air petrol mixture which comes from carburetor after mixing.

2.6 WORKING OF TWO STROKE DIESEL ENGINES

Working of two stroke diesel engines is similar to that of petrol engines except the

following differences :

(a) Fuel injector is provided in the cylinder head in place of spark plug.

(b) Only air enters the crank case. After slight compression, it is passed to

cylinder and compressed in the cylinder.

(c) At the end of compression strokes fuel injector injects diesel into

compressed air. Due to high temperature of air, diesel starts burning.

Figure 2.3 can be referred to understand the working of two stroke diesel engines also.

2.7 MAIN DIFFERENCES BETWEEN TWO AND

FOUR STROKE ENGINES

(a) For the same power output the design of two stroke engine is simple where

as a four stroke engine is complex in design for manufacturer.

(b) A two stroke engine gives on working stroke for each revolution of the

crank shaft whereas a four stroke engine gives one power stroke per two

revolutions of crank shaft.

(c) Two stroke engines have suction and exhaust ports whereas four stroke

engines have suction and exhaust valves and valve mechanism.

(d) Two stroke engines lighter in weight but four stroke engines are heavier.

(e) The initial cost of two stroke engines is less than that of four stroke engines.

(f) Thermal efficiency of two stroke engines is less than that of four stroke

engines.

(g) Four stroke engines are used where efficiency is important, e.g. in cars,

busses, etc. Whereas two stroke engines are used where lower cost is

required in two wheelers, e.g. scooters and motorcycles.

2.8 APPLICATION OF TWO STROKE AND FOUR

STROKE ENGINES

Two stroke petrol engines are preferred in applications where low cost, compactness and

light-weightness are important considerations.

Example : Scooter, mopeds and motorcycle, etc.

Two stroke diesel engines are used in marine applications such as in ships where engine

space is small.

Four stroke petrol engines are now-a-days popular in motorcycle also due to their high

fuel efficiency.

Four stroke petrol engines are generally used in light vehicles such as car and jeep, etc.

where fuel efficiency is an important criteria and cost is not a limiting factor.

Four stroke diesel engines are used in heavy motor vehicles such as bus, truck and big

size carries and tractors, etc.

18


2.9 IMPORTANT TERMS

Figure 2.4 shows the cross-section of a single cylinder spark ignition internal combustion

engine. Description of different components of this engine is given below.

Figure 2.4 : Different Parts of an Internal Combustion Engine

Cylinder

The cylinder is that part in which air-fuel mixture is sucked, compressed, ignited

and expanded.

Cylinder Block

Cylinder block is made by casting and is used to support the cylinder in position.

Piston

Piston reciprocates inside the cylinder.

Combustion Chamber

The space enclosed between cylinder and upper part of the cylinder forms the

combustion chamber where fuel-air mixture burns.

Piston Rings

Piston rings are provided on the piston. These are used to seal the high pressure

side (cylinder) and low pressure side (crank case), i.e. to prevent leakage of gases.

There is one oil ring also which is used to scrap the lubricating oil at the cylinder

surface so that it returns to crank case.

Spark Plug

A spark plug is put near the top of the cylinder or in the cylinder head. It is used to

ignite the fuel-air mixture by generating a spark in petrol engines.

Fuel Injector

Fuel injector is used in diesel engines in place of spark plug.

Piston Rod

Piston rod or connecting rod connects the piston and crank.

19

Power Plants Gudgeon Pin

It is provided on the piston. It joins the piston and connecting rod.

Crank Pin

Crank pin joins the crank and piston rod.

Crank

Crank and the piston rod convert the reciprocating motion of piston into rotary

motion of the crank shaft.

Crank Shaft

It is supported on bearings attached to the crank case.

Crank Case

It is the main body of the engine to which cylinder is connected.

Valve Mechanism

A mechanism to open and close the suction and exhaust valves is also provided in

four stroke engines. This is not shown in Figure 2.4.

Top Dead Centre (TDC)

Top dead center is the upper most position upto which piston moves.

Bottom Dead Centre (BDC)

Bottom dead centre is the lower most position upto which piston comes down.

Bore (D)

Bore is the diameter of piston on cylinder.

Stroke (L)

The nominal distance through which the piston moves from one extreme position

(say TDC) to other extreme position (say BDC).

Suction Manifold

Suction or intake manifold is the pipe through which air and petrol mixture enters

the cylinder (through suction valve).

Exhaust Manifold

Exhaust manifold is the pipe through which burnt gases pass from cylinder

(through exhaust valve) to the silencer of the engine.

Stroke Volume

The volume of the cylinder between TDC and BDC is known as stroke volume.

Clearance Volume

It is the volume of cylinder left above TDC, i.e. between TDC and top of cylinder.

2.10 SPECIFICATIONS OF AUTOMOBILE ENGINE

Engine specifications may include following details :

(a) Model Designation : Model designation as specified by manufacturer.

(b) Engine Configuration : Number of cylinders and their arrangement.

(c) Fuel System : Fuel system with carburetor or with multi-point fuel

injection (MPFI).

20


(d) Displacement Volume : Stroke volume of all cylinders.

(e) Ignition System

(f) Maximum Horse Power

(g) Maximum Torque

Example : Engine Specification of Santro Car.

Model Designation : Hydraulic epsilon engine.

Configuration : In-line-4 cylinder.

Fuel System : Multi-point fuel injection (MPFI).

Displacement : 1086 cc.

Ignition System : Distributorless.

Maximum Horse Power (BHP/rpm) : 63 at 5500 rpm.

Maximum Torque (kgm/rpm) : 9.8 kg at 3000 rpm.

SAQ 1

(a) What do you understand about power plant? Explain.

(b) How do you classify internal combustion (IC) engines? Explain.

(c) Describe the working of two stroke petrol engine with neat diagrams.

(d) Describe the working of four stroke petrol engine with net diagram.

(e) Describe the working of two stroke diesel engine with neat diagram.

2.11 SUMMARY

In this unit, we have learnt the concept of power plant, its applications and types. It is

also called as power unit, used in the automobiles to develop the power. It is also called

as IC engine. This unit also explains the various types of IC engines. The construction

and working of petrol and diesel engines also explained very well. Finally, the unit

concluded with explaining the various terms used n the spark ignition petrol engine.

1.8 KEY WORDS

1.9 ANSWERS TO SAQs


21

Automobile Electrical

Systems UNIT 3 AUTOMOBILE ELECTRICAL

SYSTEMS

Structure

3.1 Introduction

Objectives

3.2 Ignition System

3.3 Requirement of an Ignition System

3.4 Types of Ignition Systems

3.4.1 Battery or Coil Ignition System

3.4.2 Magneto-ignition System

3.4.3 Electronic Ignition System

3.5 Charging System

3.6 Starting System

3.7 Functions of Components used in Circuits

3.7.1 Battery

3.7.2 Ignition or Induction Coil

3.7.3 Contact Breakers

3.7.4 Condenser

3.7.5 Distributor

3.7.6 Ignition Switch

3.7.7 Spark Plugs

3.7.8 Magneto

3.8 Functions and Working Principles of Main Components of Electrical

Systems

3.8.1 Starter

3.8.2 Dynamo or Generator

3.8.3 Alternator or AC Generator

3.8.4 Regulators for Alternator

3.8.5 Regulators for Dynamo

3.8.6 Cutout Relay

3.9 Ignition Timings

3.10 Effect of Ignition Advance and Ignition Retard

3.11 Need of Spark Advance/Retard Mechanisms

3.12 Types of Spark Advance/Retard Mechanisms

3.13 Centrifugal Spark Advance Mechanism

3.14 Vacuum Advance Mechanism

3.15 Summary

3.16 Key Words


3.1 INTRODUCTION

Automobile electrical system includes starting system, charging system, ignition system

and lighting system and some accessories. The accessories include cigarette lighter horn

and mobile charging system, etc.

22


Major components of a typical electrical systems are given below :

Ignition System

(a) Spark plugs (for petrol vehicle)

(b) Distributor

(c) Ignition coil

(d) Ignition switch, etc.

Charging System

(a) Alternator

(b) Regulator, etc.

Starting System

(a) Battery

(b) Starting motor

(c) Wiring,

(d) Switches, etc.

Objectives


define electrical system,

understand about major components of the electrical system,

describe the types of ignition systems,

know the starting system of an automobile, and

explain the functions of components used in electrical system circuits.

3.2 IGNITION SYSTEM

In spark ignition engines, a device is required to ignite the compressed air-fuel mixture at

the end of compression stroke. Ignition system fulfills this requirement. It is a part of

electrical system which carries the electric current at required voltage to the spark plug

which generates spark at correct time. It consists of a battery, switch, distributor ignition

coil, spark plugs and necessary wiring.

A compression ignition engine, i.e. a diesel engine does not require any ignition system.

Because, self ignition of fuel air mixture takes place when diesel is injected in the

compressed air at high temperature at the end of compression stroke.

3.3 REQUIREMENTS OF AN IGNITION SYSTEM

(a) The ignition system should be capable of producing high voltage current, as

high as 25000 volts, so that spark plug can produce spark across its

electrode gap.

(b) It should produce spark for sufficient duration so that mixture can be ignited

at all operating speeds of automobile.

(c) Ignition system should function satisfactory at all engine speeds.

(d) Longer life of contact points and spark plug.

23


Systems (e) Spark must generate at correct time at the end of compression stroke in

every cycle of engine operation.

(f) The system must be easy to maintain, light in weight and compact in size.

(g) There should be provision of spark advance with speed and load.

(h) It should be able to function smoothly even when the spark plug electrodes

are deposited with carbon lead or oil.

3.4 TYPES OF IGNITION SYSTEMS

There are three types of ignition systems which are used in petrol engines.

(a) Battery ignition system or coil ignition system.

(b) Magneto ignition system.

(c) Electronic ignition system.

In battery ignition system, the current in the primary winding is supplied by a battery

whereas it is supplied by a magneto in magneto ignition system.

Battery ignition system is used in cars and light truck. Magneto ignition system is used in

some scooters.

Both the systems work on the principle of mutual electromagnetic induction.

Electronic ignition systems use solid state devices such as transistors and capacitors.

3.4.1 Battery or Coil Ignition System

Battery ignition system consists of a battery of 6 or 12 volts, ignition switch, induction

coil, contact breaker, condenser, distributor and spark plugs. A typical battery ignition

system for four cylinder SI engine has been shown in Figure 3.1.

Figure 3.1 : Battery or Coil Ignition System

The primary circuit consists of battery, switch, primary winding and contact breaker

point which is grounded. A condenser is also connected in parallel to the contact breaker

points. One end of the condenser is grounded and other connected to the contact breaker

arm. It is provided to avoid sparking at contact breaker points so as to increase their life.

The secondary ignition circuit consists of secondary winding distributors and spark

plugs. All spark plugs are grounded.

The ignition coil steps up 12 volts (or 6 volt) supply to a very high voltage which may

range from 20,000 to 30,000 volts. A high voltage is required for the spark to jump

across the spark plug gas. This spark ignites the air-fuel mixture as the end of

compression stroke. The rotor of the distributor revolves and distributors the current to

24


the four segments which send the current to different spark plugs. For a 4-cylinder

engine the cam of the contact breaker has four lobes. Therefore, it makes and breaks the

contact of the primary circuit four times in every revolution of cam. Because of which

current is distributed to all the spark plugs in some definite sequence.

The primary winding of ignition coil has less number of turns (e.g. 200 turns) of thick

wire. The secondary winding has relatively large number of turns (e.g. 20,000 turns) of

thin wire.

When ignition switch in turned on, the current flows from battery to the primary

winding. This produces magnetic field in the coil. When the contact point is open, the

magnetic field collapses and the movement of the magnetic field induces current in the

secondary winding of ignition coil. As the number of turns in secondary winding are

more, a very high voltage is produced across the terminals of secondary.

The distributor sends this high voltage to the proper spark plug which generates spark

for ignition of fuel-air mixture. In this way, high voltage current is passed to all spark in

a definite order so that combustion of fuel-air mixture takes place in all cylinders of the

engine.

A ballast register is connected in series in primary circuit to regulate the current. At the

time of starting this register is bypassed so that more current can flow in this circuit.

The breaker points are held by a spring except when they are forced apart by lobes of the

cam.

Advantages

(a) Low initial cost.

(b) Better spark at low speeds and better starting than magneto system.

(c) Reliable system.

(d) No problems due to adjustment of spark timings.

(e) Simpler than magneto system.

Disadvantages

(a) Battery requires periodical maintenance.

(b) In case of battery malfunction, engine cannot be started.

3.4.2 Magneto-ignition System

This system consists of a magneto in place of a battery. So, the magneto produces and

supplies current in primary winding. Rest of the system is same as that in battery ignition

system. A magneto ignition system for a four cylinder SI engine has been shown in

Figure 3.2.

The magneto consists of a fixed armature having primary and secondary windings and a

rotating magnetic assembly. This rotating assembly is driven by the engine.

Rotation of magneto generates current in primary winding having small number of turns.

Secondary winding having large number of turns generates high voltage current which is

supplied to distributor. The distributor sends this current to respective spark plugs. The

magneto may be of rotating armature type or rotating magnet type. In rotating armature

type magneto, the armature having primary and secondary windings and the condenser

rotates between the poles of a stationary horse shoe magnet. In magneto, the magnetic

field is produced by permanent magnets.

Advantages

(a) Better reliability due to absence of battery and low maintenance.

(b) Better suited for medium and high speed engines.

(c) Modern magneto systems are more compact, therefore require less space.

25


Systems Disadvantages

(a) Adjustment of spark timings adversely affects the voltage.

(b) Burning of electrodes is possible at high engine speeds due to high voltage.

(c) Cost is more than that of magneto ignition systems.

Figure 3.2 : Magneto Ignition System

3.4.3 Electronic Ignition Systems

Electronic ignition systems use some solid state devices like transistor and capacitors,

etc. to generate right sparking voltage at right time. These systems have overcome the

limitations of conventional (battery ignition and magneto-ignition) ignition systems.

Modern automobiles make use of these systems. Two systems, common in use, are :

(a) Capacitive discharge ignition, and

(b) Transistorized coil ignition.

These systems are more reliable and require less maintenance. Wear and tear of

components is reduced and life of spark plugs is increased with the use of electronic

ignition.

3.5 CHARGING SYSTEM

Charging system is required to recharge the battery which is an important component of

electrical system of an automobile. Charging is required as the capacity of a battery to

supply current is limited to the energy stored in it in the form of chemical energy.

Battery supplies the current to run the starting motor, various lights and horn, etc.

The charging system generates electricity to recharge the battery and run other electrical

components.

3.5.1 Components of a Charging System

Charging system consists of :

Generator or Dynamo

It converts mechanical energy into electrical energy.

Regulator

It controls the generator output according to the need. It controls the current or

voltage.

Relay

It is used to control the flow of current between generator and battery. It acts as

circuit breaker.

26


3.6 STARTING SYSTEM

The starting system of an automobile is used to start the internal combustion engine.

Both SI and CI engines cannot start by itself. These engines need to be cranked by a

starting motor. This motor is also called a starter or cranking motor. Cranking of any

engine means rotating its crank shaft. Rotation of crank shaft causes the piston to

reciprocate. When piston reciprocates, suction, compression, expansion and exhaust

strokes of engine are completed. Thus, engine completes its working cycle and it starts

running.

Starting motor produces necessary torque to rotate the engine wheel (crank shaft)

through a suitable gear (one pinion on motor and other ring gear around engine wheel).

3.6.1 Components of Starting System

Starting system consists of the following :

(a) Starting Motor : Starting motor to produce rotation of crank shaft.

(b) Drive Mechanism : Drive mechanism to transfer rotary motion of starter to

the crank shaft of the engine.

(c) The ignition switch to start motor.

3.7 FUNCTIONS OF COMPONENTS USED IN

CIRCUITS

Functions of various components used in battery (coil) ignition and magneto-ignition

systems are discussed here in brief.

3.7.1 Battery

It is an important component of electrical system. The battery supplies the necessary

current to the primary winding of ignition coil which is converted into high voltage

current to produce spark. It also supplied current to run the starting motor when engine is

cranked for starting. A battery stores energy in the form of chemical energy and supplies

it for running lights and other accessories of an automobile. Lead-acid battery is

commonly used in most of the automobiles.

3.7.2 Ignition or Induction Coil

The ignition coil is step up transformer to increase the voltage form 12 volt or 6 volt to

20000-30000 volts. It consists of a primary winding and a secondary winding wound on

a laminated soft iron core. Primary winding contains about 300 turns made of thick wire.

Secondary consists of about 20000 turns of thin wire. In a can type coil, secondary is

wound on the soft core over which primary is wound. This assembly is housed in a steel

casing fitted with a cap. The cap is made of insulating material. The terminals for

electrical connections are provided in cap. This type of coil is shown in Figure 3.3. To

save the windings from moisture and to improve insulation, windings are dipped in oil.

Figure 3.3 : Cross-sectional Sketch of a Can type Ignition Coil

27


Systems One primary terminal is connected to ignition switch and other to the contact breaker.

Secondary terminal is connected to the distributor. The working of ignition coil has been

explained in Section 3.4.1.

3.7.3 Contact Breakers

Contact breaker is required to make contact and break contact of the primary circuit of

ignition system. It consists of two contact breaker points as shown in Figures 3.1 and 3.2.

One point remains fixed while the other can move. A cam is sued to move the movable

point. As cam moves, the contact is made and broken alternately. Primary circuit breaks

when the breaker points open. Magnetic field collapses due to this. This produces high

voltage current in the secondary winding which is supplied to the distributor. This

current is distributor to proper spark plug where it produces spark for ignition of fuel-air

mixture.

3.7.4 Condenser

The function of the condenser in the ignition system is to absorb and store the inductive

current generated in the coil. If condenser is not provided, the induced current will cause

arcing at the breaker points. This will cause burning of the breaker points.

3.7.5 Distributor

The distributor sends the high voltage current, generated in the secondary winding, to the

proper spark plug at proper time. If the automobile is having a four cylinder engine, it

will have four spark plugs.

The cap of the distributor is connected to the secondary winding of coil. It has a rotor

which rotates and comes in contact with the terminals (4 in number for 4 spark plugs)

placed around the rotor. As the rotor comes in contact with the terminals (numbered 1, 2,

3 and 4 in Figures 3.1 and 3.2), the current is passed to the respective spark plug at

proper time when spark is needed.

3.7.6 Ignition Switch

The function of the ignition switch is to connect the battery and starting motor in the

automobiles having self starting system.

Example : In car, jeep, etc.

Its function is to connect battery to induction coil in the battery ignition system.

3.7.7 Spark Plugs

The function of the spark plug is to produce spark between its electrodes. This spark is

used to ignite the fuel-air mixture in the spark ignition (SI) engines.

3.7.8 Magneto

Magneto is used in magneto ignition system. Magneto is a kind of generator to provide

electrical energy to run the ignition system. It is replacement of battery for ignition.

When it is rotated by the engine, it produces high voltage current to be supplied to spark

plugs through the distributor.

3.8 FUNCTIONS AND WORKING PRINCIPLES OF

MAIN COMPONENTS OF ELECTRICAL

SYSTEMS

Functions and principles of starter, dynamo, alternators and regulators have been given

in this section.

28


3.8.1 Starter

It is also known as starting motor or cranking motor. It is used to start heavy engines

which cannot be started by hand cranking.

Function of Starter

IC engines are required to be rotated at some minimum speed after which the

engines starts running by fuel supply. This initial rotation is given by the starting

motor and this is the function of a starter.

Working Principle

A motor converts electrical energy into mechanical energy. Mechanical energy is

obtained in the form of rotation of a wheel. This rotation of a wheel is used to start

the IC engine.

The motor works on the principle that “when a current carrying conductor is put in

a magnetic field, it experiences a mechanical force”. The direction of force is

determined by the Flemming‟s left hand rule.

Flemming’s Left Hand Rule

If we stretch the thumb, forefinger and middle finger such that they are

mutually perpendicular, then according to this rule :

“If the first finger points in the direction of magnetic field and the second

(middle) finger in the direction of current then the thumb will give the

direction of force acting on conductor or the direction of its motion”.

Working of Starter

When the starter switch is put on „on‟ position, the current from battery flows to

starting motor, the motor starts rotating. The motor is connected to the drive unit,

which is used to rotate the engine crank shaft. A small pinion (small gear) is fitted

on the armature shaft of the starting motor. This pinion meshes with the ring gear

when starter rotates. Thus, the fly wheel which is attached to ring gear also starts

revolving. Thus, engine crank shaft starts revolving. With the revolution of crank

shaft, the engine strokes viz. suction, compression, power and exhaust are

completed. Therefore, engine starts running. The starter is engaged to the engine

ring gear (attached to fly wheel) till the engine starts running. As soon as engine

starts running, the starter is disengaged. The starting motor is a low voltage DC

series wounded motor.

3.8.2 Dynamo or Generator

A dynamo is a machine used to convert mechanical used to convert mechanical energy

into electrical energy. When it is driven by the engine it produces electricity for running

all the electrical circuits of the automobile and keeps the battery in charged condition.

This is the function of dynamo.

Principle of Dynamo

“When a conductor moves in a magnetic field, current is produced in it. The

direction of current is determined by Flemming‟s right hand rule”.

Flemming’s Right Hand Rule

If thumb, fore finger and middle finger of right hand are stretched so that

they are mutually perpendicular then the direction of induced current in the

conductor can be found out by this rule.

“If the fore finger indicates the direction of magnetic field and the thumb

shows the direction of motion of the conductor, then middle finger will

indicate the direction induced current”. This is called Flemming‟s right

hand rule.

29


Systems A magnetic field acts between north and south poles of magnets. There are lines of

forces between two poles. When the conductor moves such that lines of force are

cut, current is induced in the conductor. This current can be used to run any

electrical components, e.g. lights and charging system, etc.

The current induced in the conductor depends upon the rate at which force lines

are cut and strength of magnetic field, etc.

The principle of dynamo has been shown in Figure 3.4.

Figure 3.4 : Principle of Dynamo

When the conductor (armature of dynamo) is rotated (by engine) in the magnetic

field, a current is induced in the conductor. The direction of flow of current in the

two legs of conductor is opposite because their direction of motion is also

opposite. The two ends of conductor connected to the commutator (two split

copper rings) and these are connected to external circuit through carbon brushes.

Thus, rotation of the armature generates current which can be used for running

electrical systems of an automobile. The magnets used are electromagnets which

are supplied energy from the generator itself. The armature consists of a core,

windings and an armature shaft.

3.8.3 Alternator or AC Generator Function

“An alternator generates alternating current (AC) unlike a dynamo which generates

direct current (DC)”.

Modern automobiles which require more electric loads are fitted with alternators instead

of dynamos. These vehicles require more electrical power because they have power

steering, power windows, electrical system for automobile transmission, etc.

A rectifier is required to convert AC to DC as all electrical equipments use DC.

Principle

The principle of working of alternator differs from that of dynamo in the manner

in which the conductor and magnetic field move relative to each other. In an

alternator the conductor remains stationary but the magnetic field is rotated.

However, conductor rotates and magnetic field remains stationary in case of a

dynamo.

In an alternator, a rotating bar magnet produces magnetic field which is cut by a

stationary conductor. Figure 3.5 shows the working principle of an alternator. The

north pole of rotating magnet is shown at top and south pole at the bottom in

Figure 3.5(a). If this magnet is rotated by half revolution such that north pole

comes down and south pole takes upper position. During this the current in the

upper leg of conductor flows in one direction. Figure 3.5(b) shows the north pole

of magnet at bottom and south pole at top. When the magnet is now rotated by

another half revolution, the direction of current in the wire is reversed. Therefore,

30


with the revolution of magnet, the current reverses its direction after each half

revolution. Thus, an alternating current flows. This is the principle of working of

an alternator.

Figure 3.5 : Principle of Alternator

3.8.4 Regulators for Alternator

A regulator controls the current and voltage produced by the alternator. It is provided to

prevent alternator to generate excessively high voltage. The battery is charged by the

current generated by the alternator. For this, the battery is connected to the stator of the

alternator through a diode. Diode allows flow of current from stator to battery but it

prevents the flow of current from battery to stator when alternator is not working. Thus,

it prevents discharging of battery back. Therefore, diode acts as regulator. It is put inside

the alternator.

Transistorized Regulators

Some regulators, e.g. transistorized regulators are placed outside the designed.

These regulators are designed to prevent the problem of damage of contact breaker

points. As contact breaker points used I other regulators, are required to open

several times in a second, they have reduced working life. To get rid off this

problem transistors are used as switches which can be actuated by very small

currents. Thus, the life of contact points is increased due to reduced arcing on

account of reduced current. If whole circuit A is based on transistorized

regulators, the system has no moving parts. This type of regulators provide a very

accurate voltage control.

3.8.5 Regulators for Dynamo

The voltage and current of a dynamo are controlled by providing an external resistance.

The regulation is required to prevent generator to generate excessive voltage and current.

In one method, a resistance is connected in the field circuit. It is connected between the

field windings and insulated brush. The field circuit is grounded through the brush inside

the generator. This is shown in Figure 3.6. The switch, shown in the Figure 3.6, remains

closed till the voltage output is not excessive. The switch connects the outer end of the

field circuit to the ground. In case, voltage increases beyond a given limit, the switch

opens. This brings the resistance in the field circuit. Because of this, the current flowing

in the field windings decreases. The voltage is also reduced.

Figure 3.6 : Regulation of Dynamo

31


Systems 3.8.6 Cutout Relay

Cutout relay acts as circuit breaker between generator and battery when dynamo is not

generating any current. It prevents the discharging of battery in case generator is not

working or running at very low speeds.

This relay is nothing but a magnetic switch which closes to connect battery and

generator when generator is running. When generator does not running, a spring breaks

the circuit between the battery and generator.

3.9 IGNITION TIMING

Ignition timing is the correct instant of generating spark just before the completion of

compression stroke. Correct, ignition timing is necessary to maximize power output of

an engine.

3.10 EFFECT OF IGNITION ADVANCE AND

IGNITION RETARD

Ignition Advance

Ignition advance is the condition when ignition of fuel occurs earlier than the

correct ignition timing. Ignition of mixture takes place near the end of

compression stroke. If the ignition is advanced it means fuel-air mixture will burn

too early before the end of compression stroke. In this case, the crank and

connecting rod will have to push the piston in order to compress the gases (for

completing the compression stroke). In this situation, the force applied on piston

by the connecting rod in upward direction may not be able to overcome the

downward force acting on piston. This downward force acting on the piston is due

to enormous pressure generated by the combustion of fuel. Under this condition,

the engine may stop or stall. Spark advance may also cause the fuel to explode

suddenly under certain operating conditions.

Ignition Retard

Ignition retard means the condition when ignition occurs after the correct ignition

timing. It is known that after ignition burning (combustion) of fuel takes place. If

ignition is retarded too much then the combustion of fuel-air mixture (charge) will

continue during power stroke (expansion stroke). Therefore, peak pressures will

not be developed. Consequently work output of the engine will decrease. In this

case, burnt gases will leave the engine cylinder at higher temperature which will

overheat the exhaust valve. It results in loss of power, overheating and sometimes

burning of exhaust valve, and excessive carbon deposits.

3.11 NEED OF SPARK ADVANCE/RETARD

MECHANISM

It is clear from the previous section that correct ignition timing is necessary to maximize

the performance of the engine. Correct ignition timing depends upon several factors.

These are compression ratio, diameter (bore) of cylinder, composition of mixture, engine

speed and load, engine temperature and quality of fuel used. Except first two factors

other factors keep on changing. Therefore, there must be an automatic mechanism of

adjust the ignition timing of engine. Sometimes the spark is to be advanced and

sometimes it is required to be retarded.

32


3.12 TYPES OF SPARK ADVANCE/RETARD

MECHANISMS

Two automatic advance mechanisms are used for spark advance and retard in engines

depending on engine speed and other operating conditions :

(a) Centrifugal spark advance mechanism.

(b) Vacuum spark advance mechanism.

Ignition timing is first set manually. After this these mechanisms are used to modify it

suitably.

3.13 CENTRIFUGAL SPARK ADVANCE

MECHANISM

This mechanism consists of two fly weights, a base plate, cam and a spring. Fly weights

are also called advance weights. The base plate is fixed to the drive shaft. The fly

weights are rotated by distributor drive shaft through the base plate. The weights are

pivoted on the base plate and also attached to the cam with the help of springs. The cam

is also joined with the distributor shaft through springs, flywheel and plate. If engine

speed increases, the fly weights are displaced out radially due to centrifugal force acting

on it. Movement of weights causes the ignition advance (spark advance). At low speeds

there is no advance while it is full advance of very high speeds. (Kindly refer to figure is

standard text book).

3.14 VACUUM ADVANCE MECHANISM

Vacuum advance mechanism consists of a diaphragm whose movement automatically

advances and retards the ignition depending upon engine speed and other operating

conditions. On side of diaphragm is connected to the induction manifold and other side

is connected to atmosphere. (Induction manifold is at lower pressure than atmospheric

and this pressure depends upon engine speed). The diaphragm is connected to the

distributor through a linkage. As engine speed increases the pressure on one side of

diaphragm decreases. This change in pressure controls the movement of diaphragm

which ultimately controls the ignition timings. At normal position of diaphragm the

ignition timing is set at fully retarded position. As engine speed increases the ignition

timings are advanced. Vacuum advance mechanism takes more care of engine load and

less of speed where as centrifugal advance mechanism takes more care of engine speed

and less of load. The scheme of vacuum advance mechanism is shown in Figure 3.7.

Figure 3.7 : Block Diagram of Vacuum Advance Mechanism

33


Systems SAQ 1

(a) Describe the requirements of an ignition system of a SI engine.

(b) List different type of ignition system. Describe the working of battery

ignition system with the help of a suitable diagram.

(c) Draw a neat sketch of magneto ignition system and explain its working.

(d) List various advantages and disadvantages of battery ignition system.

(e) Give a brief description and functions of different components of a charging

system of an automobile.

SAQ 2

(a) Describe the function and working of ignition coil.

(b) Describe in brief the function of a distributor.

(c) Describe in brief the functions of the following components of an ignition

system :

(i) Condenser

(ii) Spark plugs

(iii) Magneto

(iv) Ignition switch

(d) Describe the working principle of starter of an automobile.

(e) Write the function of a dynamo of an automobile and explain its working

principle with the help of a neat sketch.

(f) How does an alternator works? Explain.

SAQ 3

(a) What are the functions of a regulator for an alternator? How does it work?

(b) How does a regulator for dynamo works?

(c) What are different effects of ignition retard on the performance of an

automobile?

(d) What happens to the performance of a vehicle due to ignition advance?

(e) How does centrifugal advance mechanism works?

(f) What is the working principle of vacuum advance mechanism?

34


3.15 SUMMARY

Every student, who is studying the course automobile engineering, must have the

knowledge of transmission system of an automobile. Transmission system is nothing but

transmitting the power from engine to the wheels transfer clutch and gear mechanisms.

So, in this unit, we have studied about the transmission system of automobile. The

transmission system mainly comprises of clutch and gear mechanisms. We have learnt

about the functions and types of clutches and gear boxes. Clutch is mainly used to

yougase or disagause the engine to the transmission or gear box. Gear box is used to

varying the speeds of automobile according to the required conditions or according to the

need of the persons, who are driving the automobile.

3.16 KEY WORDS

3.17 ANSWERS TO SAQs


35

Transmission

UNIT 4 TRANSMISSION

Structure

4.1 Introduction

Objectives

4.2 Clutch

4.3 Principles of Clutch

4.4 Main Parts of a Clutch

4.5 Types of Clutch

4.6 Single Plate Clutch

4.7 Multiple Clutch

4.8 Clutch Pedal Free-play Adjustment

4.9 Function of Gear Box

4.10 Types of Gear Box

4.11 Sliding Mesh Gear Box

4.12 Constant Mesh Gear Box

4.13 Gear Trains

4.14 Types of Gear Trains

4.15 Summary

4.16 Key Words


4.1 INTRODUCTION

Transmission is the mechanism which is used to transfer the power developed by engine

to the wheels of an automobile.

The transmission system of an automobile includes clutch, gear box, propeller shaft axle

and wheels, etc.

Description of various types of clutches and gear boxes has been given in the following

sections of this unit. The term ‘Transmission’ is used for a device which is located

between clutch and propeller shaft. It may be a gear box, an over drive or a torque

converter, etc.

Objectives


understand the transmission system of automobiles,

list out the components of the transmission system,

describe the various functions and types of clutches and gear boxes, and

explain the advantages of clutches and gear box.

4.2 CLUTCH

Clutch is used to engage or disengage the engine to the transmission or gear box. When

the clutch is in engaged position, the engine power or rotary motion of engine crankshaft

36


is transmitted to gear box and then to wheels. When clutch is disengaged, the engine

power does not reach to gear box (and to wheels) although engine is running.

Clutch is also used to allow shifting or changing of gears when vehicle is running. For

shifting gears, clutch is first disengaged then gear is shifted and then clutch is engaged.

Clutch has to be disengaged to stop the vehicle and also at the time of idling.

4.3 PRINCIPLE OF CLUTCH

It operates on the principle of friction. When two surfaces are brought in contact and are

held against each other due to friction between them, they can be used to transmit power.

If one is rotated, then other also rotates. One surface is connected to engine and other to

the transmission system of automobile. Thus, clutch is nothing but a combination of two

friction surfaces.

4.4 MAIN PARTS OF A CLUTCH

It consists of

(a) a driving member,

(b) a driven member, and

(c) an operating member.

Driving member has a flywheel which is mounted on the engine crankshaft. A disc is

bolted to flywheel which is known as pressure plate or driving disc.

The driven member is a disc called clutch plate. This plate can slide freely to and fro on

the clutch shaft.

The operating member consists of a pedal or lever which can be pressed to disengaged

the driving and driven plate.

4.5 TYPES OF CLUTCH

Some types of clutches used in vehicles are given below :

(a) Friction Clutch : It may be (i) single plate clutch, (ii) multi-plate clutch, or

(iii) cone clutch. Multi-plate clutch can be either wet or dry. A wet clutch is

operated in an oil batch whereas a dry clutch does not use oil.

(b) Centrifugal clutch.

(c) Semi-centrifugal clutch.

(d) Hydraulic clutch.

(e) Positive clutch.

(f) Vacuum clutch.

(g) Electromagnetic clutch.

4.6 SINGLE PLATE CLUTCH

A single plate is commonly used in cars and light vehicles. It has only one clutch plate

which is mounted on the splines of the clutch shaft. A flywheel is mounted on the

crankshaft of the engine. A pressure plate is connected to the flywheel through the bolts

and clutch springs. It is free to slide on the clutch shaft with the movement of clutch

pedal. When clutch is in engaged position, the clutch plate remains gripped between

flywheel and pressure plate. Friction linings are provided on both the sides of clutch

plate. On one side clutch plate is in touch with flywheel and on other side with pressure

37

Transmission plate. Due to friction on both sides, the clutch plate revolves with engine flywheel.

Therefore, clutch transmits engine power to clutch shaft. Clutch shaft is connected to

transmission (or gear box) of automobile. Thus, clutch transmits power from engine to

transmission system which inturn rotates wheels of engine.

When the clutch plate is to be disengaged, the clutch pedal is pressed. Because of this

pressure plate moves back and clutch plate is disengaged from flywheel. Thus, clutch

shaft stops rotating even if engine flywheel is rotating. In this position, power does not

reach the wheels and vehicle also stops running. Single plate clutch is shown in

Figure 4.1.

Figure 4.1 : Single Plate Clutch

4.6 MULTIPLATE CLUTCH

Multi-plate clutch consists of more than one clutch plates contrary to single plate clutch

which consists of only one plate. Friction surfaces are made in case of multi-plate clutch.

Due to increased number of friction surfaces, a multi-plate clutch can transmit large

torque. Therefore, it is used in racing cars and heavy motor vehicles witch have high

engine power. The clutch plates are alternatively fitted with engine shaft and the shaft of

gear box. He plates are firmly held by the force of coil springs and they assembled in a

drum. One plate slides in the grooves on the flywheel and the next plate slides on spines

provided on pressure plate. Thus, each alternate plate slides in grooves on the flywheel

and the other on splines of pressure plate. If we take two consecutive plates, then one has

inner and other has outer splines.

When the clutch pedal is pressed, the pressure plate moves back against the force of coil

spring, hen the clutch plates are disengaged and engine flywheel and gear box are

decoupled. However, when clutch pedal is not pressed the clutch remain in engaged

position and the power can be transmitted from engine flywheel to the gear box. This

type of clutch has been shown in Figure 4.2.

Figure 4.2 : Multi-plate Clutch

38


4.7 CLUTCH PEDAL FREE-PLAY ADJUSTMENT

Clutch remains in engaged position when clutch pedal is not pressed. Free play

adjustment is required to maintain a given free play of the pedal after the clutch is

engaged. Before making this adjustment, correct floorboard clearance or clutch pedal

travel must be adjusted.

Floorboard clearance adjustment is made to prevent touching of floor by pedal when

clutch is engaged.

Clutch pedal travel adjustment is done to ensure total clutch disengagement when the

clutch pedal is pressed.

SAQ 1

(a) Describe the function of a clutch in a transmission system of an automobile.

(b) List various types of clutches and explain the working of a single plate

clutch.

(c) How a multi-plate clutch is able to transmit more power in comparison to a

single plate clutch.

4.8 FUNCTION OF GEAR BOX

An automobile is able to provide varying speed and torque through its gear box. Various

functions of a gear box are listed below :

(a) To provide high torque at the time of starting, vehicle acceleration, climbing

up a hill.

(b) To provide more than forward speeds by providing more than one gear

ratios. In modern cars, five forward gears and reverse gear is provided. For

given engine speed, higher speed can be obtained by running in higher

(4th and 5

th) gears.

(c) Gear box provides a reverse gear for driving the vehicle in reverse direction.

4.9 TYPES OF GEAR BOXES

(a) Selective type gear boxes :

(i) Sliding mesh gear box

(ii) Constant mesh gear box

(iii) Synchromesh gear box

(b) Progressive type gear box

(c) Epicyclic type gear box.

4.10 SLIDING MESH GEAR BOX

It is simplest type of gear box out of the available gear boxes. In this type of gear box,

gears are changed by sliding one gear on the other. This gear box consists of three shafts;

main shaft, clutch shaft and a counter shaft. In a four speed gear box (which includes one

reverse gear), the counter shaft has four gears which are rigidly connected to it. Clutch

39

Transmission shaft has one gear and main shaft has two gears. The two gears on the main shaft can

slide in the horizontal direction along the splines of the main shaft. However, the gears

on the counter shaft cannot slide. The clutch gear is rigidly fixed to the clutch shaft. It is

always connected to the countershaft drive gear.

The two gears on the main shaft can be slided by the shifter yoke by operating the shift

lever (not shown in Figures). These two gears are second gear and low/reverse gear

respectively. These gears can be meshed with corresponding gears on the countershaft

with the help of shifter yoke and shift lever. Shift lever is operated by hand in four

wheelers for changing the gears. A reverse idler gear is mounted on another (third) shaft

and is always in mesh with reverse gear on countershaft.

Neutral Position

Figure 4.3 shows sliding mesh gear box in neutral position. In this position, the

engine is in running condition, clutch remains engaged and clutch gear drives the

countershaft drive gear. The direction of rotation of countershaft is opposite to

that of clutch shaft. In this position Ist, IInd and IIIrd and reverse gears are free.

Thus, main (transmission) shaft does not rotate and automobile wheels do not

rotate. So vehicle remains stationary.

Figure 4.3 : Sliding Mesh Gear Box showing Neutral Position

First Gear

When first gear position is selected by the shift lever, first gear (large gear)

on the main shaft slides and is connected to first gear on the countershaft.

The direction of rotation of main shaft is same as that of clutch shaft. In first

gear, small gear of countershaft meshes with larger gear on main shaft,

speed reduction in the ratio 3 : 1 (approximate) is obtained.

Second Gear

When second gear is selected by the shift lever, second gear on countershaft

meshes with second gear (small gear on main shaft) on the main shaft. The

direction of main shaft is same as that of clutch shaft. Speed reduction of

the order of 2 : 1 is obtained in second gear.

Third Gear

In third gear, the main shaft is slided axially towards the clutch shaft so that

main shaft is directly connected to the clutch shaft. In this position, the main

shaft rotates at the speed of clutch shaft. Thus, a speed ratio of 1 : 1 is

obtained.

It can be noted that the clutch gear is directly connected to engine

crankshaft and main shaft is connected to the wheels through propeller

shaft.

Reverse Gear

When the shift lever is operated to engage the reverse gear, the larger

(reverse) gear of the main shaft meshes with the reverse idler gear. Reverse

40


idler gear is always connected to reverse gear on countershaft. The reverse

idler gear between countershaft reverse gear and main shaft larger gear

changes the direction of rotation of main shaft. Thus, the direction of main

shaft becomes opposite to that of clutch shaft. Therefore, wheels of the

automobile start moving in backward direction.

(Note : Countershaft is also known as lay shaft.)

In modern cars, there are five forward gears and reverse gear. Hence, they

provide five speed ratios for forward racing and one for backward

movement.

4.11 CONSTANT MESH GEAR BOX

A simplified diagram of constant mesh box has been shown in Figure 14.4. In this gear

box, all gears on the main transmission shaft are constantly connected to corresponding

gears on countershaft or lay shaft. In addition, two dog clutches are provided on the main

shaft. One dog clutch is between the second gear and cutch gear and another is between

the first gear and reverse gear. Splines are out on main shaft so that all the gears are feed

on it.

Figure 4.4 : Constant Mesh Gear Box

Dog clutches can also slide on main shaft and rotate with it. However, all the gears on

countershaft are giddily fixed to it. Different gear ratios (speed ratios) are obtained as

follows :

For Three Forward and One Reverse Gear

Top or 3rd

speed gar is obtained when the left dog clutch is slided to left to mesh

with clutch gear by using the gear shift lever. In this case, main shaft rotates at the

same speed as that of clutch gear or engine crankshaft speed which is the

maximum speed. Speed ratio obtained is 1 : 1.

Second gear is obtained when dog cutch (left side) meshes with second gear. In

this condition clutch gear rotates the drive gear on countershaft and countershaft

drives the second gear on the main shaft. All other gears on main shaft are free, so

they do not move.

In the same manner, first gear is obtained when right hand side dog clutch meshes

with first gear. Reverse gear is obtained when right side dog clutch meshes with

reverse gear on main shaft.

Advantage of Constant Mesh Gear Box

Since all the gears are in constant mesh, wear and tear of gears and any possible

damage of gears do not occur in engaging and disengaging gears. Also, any sound

are not generated in engaged/disengaged.

41

Transmission SAQ 2

(a) What do you mean by transmission in an automobile? Describe its purpose.

(b) List different type of gear boxes used in automobiles. Explain the working

of constant mesh gear box with the help of a simple diagram.

(c) Write any three differences between a sliding mesh and constant mesh gear

box.

(d) Enumerate the advantages of a constant mesh gear box over sliding mesh

gear box.

(e) How do you obtain reverse gear in a sliding mesh gear box?

4.12 GEAR TRAINS

A combination of two or more gears, which mesh in such a way that power is transmitted

from driving shaft to driven shaft, is known as gear train.

4.13 TYPES OF GEAR TRAINS

There are three types of gear trains :

(a) Simple gear train,

(b) Compound gear train, and

(c) Epicyclic gear train.

Simple Gear Train

If the axes of all the gears remain fixed relative to each other, the gear train is

known as simple gear train. A simple gear train is shown in Figure 4.5.

Figure 4.5 : Simple Gear Train

Compound Gear Train

There are more than gear on the shaft (generally intermediate shaft) in a

compound gear train. Two gears are moved on intermediate shaft, therefore, both

the gear s have same speed. A compound gear train is shown in Figure 4.6. Gears

2 and 3 will rotate at same speed as they are mounted on same shaft.

Driven gear is also known as follower.

42


Figure 4.6 : Compound Gear Train

Epicyclic Gear Train

If the axe of the shafts, on which gears are attached, move relative to a fixed axis,

then the gear train is known as epicyclic gear train.

Velocity Ratio of Gear Trains

Velocity ratio (or speed ratio) ratio of speed of driver to the speed of driven.

Speed of driver

Velocity ratio =Speed of driven

Train Value

It is the reciprocal of speed ratio.

1

Train value =Velocity Ratio

Velocity Ratio of Simple Gear Train

Case-I

When number of gears are only two. Consider Figure 4.5 which shows a

simple gear train. Gear 1 is driver and Gear 2 is driven or follower.

Let N1 is speed of driver

N2 is speed of driven

T1 is number of teeth on gear 1

T2 is number of teeth on gear 2.

Speed of driver

Speed ratio =Speed of driven

1

2

N

N

Speed ratio of any pair of gears in terms of number of teeth is given by

following relation.

1 2

2 1

Speed ratio = N T

N T

1

Train value =Speed Ratio

2 1

1 2

= N T

N T

43

Transmission Case-II

When there is an intermediate shaft in a simple gear train Figure 4.7 shows

a simple gear train with an intermediate gear (2).

Figure 4.7 : Simple Gear Train with an Intermediate Gear

Gear 1 is driver which rotates in clockwise direction. Gear 2 placed on

intermediate shaft will rotate in anticlockwise direction and driven gear

(gear 3) will rotate in clockwise direction.

Let T1, T2 and T3 are number of teeth on Gears 1, 2 and 3 respectively.

N1, N2 and are speeds of Gears 1, 2 and 3 respectively.

Considering driver (Gear 1) and intermediate gear (Gear 2) in mesh, we can

write

2 1

1 2

Train value = N T

N T . . . (i)

when intermediate and follower are considered to be in mesh.

3 2

2 3

N T

N T . . . (ii)

Multiplying Eqs. (i) and (ii)

32 1 2

1 2 2 3

NN T T

N N T T

1 2

2 1

Speed ratio = N T

N T

Thus, ratio of speed of follower and speed of driver is equal to the ratio of

number of teeth of driver and number of teeth of follower.

1

2

Speed of driverSpeed ratio =

Speed of follower

N

N

3

1

=T

T . . . (iii)

This equation shows that speed ratio is independent of the number of teeth

on the intermediate gear.

Example 4.1

A simple gear train has two gears which are mounted on two different shafts.

1 which is driver runs at 2000 rpm. The number of teeth on gears 1 and 2 are 30

and 60 respectively. Determine :

(a) Speed ratio of gear train,

(b) Train value of gear train,

44


(c) Speed of second gear, and

(d) Direction of rotation of driven if driver (gear 1) rotates in

anticlockwise direction.

Solution

Given N1 = 2000 rpm, T1 = 30 and T2 = 60

Figure 4.8

(a) Speed ratio 1 2

2 1

= N T

N T

60

=30

= 2

(b) Train value 1 1

= 0.5speed ratio 2

(c) 1

2

= speed ratioN

N

2

2000= 2

N

2

2000

2N

N2 = 1000 rpm

(d) In a simple gear train, the two gears always rotate in opposite

direction. Therefore, the direction of rotation of driver (gear 2) is

clockwise.

Example 4.2

A simple gear train consists of three gears, each mounted on separate shaft. All the

three shat are parallel. Gear 1 is driver which has 30 teeth and a speed of 600 rpm.

The number of teeth of gears 2 and 3 are 60 and 90 respectively. Determine :

(a) The speed ratio of gear train, and

(b) Direction of rotation and speed of follower if driver rotates in

clockwise direction.

Solution

Refer Figure 4.7.

Given N1 = 600 rpm, T1 = 30, T2 = 60 and T3 = 90

(a) Speed ratio speed of driver

=speed of follower

or 31

3 1

90Speed ratio = 3

30

TN

N T

Thus, speed ratio = 3.

45

Transmission

(b) Speed ratio 1

3

=N

N

3

6003

N

3

600

3N

N3 = 200 rpm

The direction of rotation of follower is same as that of driver if numbers of

intermediate gears are odd. In the present case this number is 1 (only one

intermediate gear), hence the direction of rotation of follower is clockwise.

Velocity Ration of a Compound Gear Train

Refer to Figure 4.6 which shows a compound gear train. There is one gear (gear 1)

on driving shaft. It is called driver. There are two gears (Gears 2 and 3) on

intermediate shaft. Gears 2 and 3 rotate at same speed as they are mounted on

same shaft. Gear 2 meshes with driver and gear 3 meshes with the follower or

driven gear.

Let T1, T2, T3 and T4 are number of teeth on gears 1, 2, 3 and 4 respectively.

Let N1 is speed of driver (gear 1) N4 is speed of follower and N2 and N3 are speeds

of gears 2 and 3 respectively.

N2 = N3

Consider gears 1 and 2 where gear 1 drives gear 2

2 1

1 2

N T

T T . . . (iv)

Gear 3 drives gear 4, hence, we can write

34

3 4

TN

N T . . . (v)

Multiplying Eqs. (iv) and (v), we get

32 4 1

1 3 2 4

TN N T

N N T T

34 1

1 2 4

TN T

N T T . . . (vi)

( N2 = N3)

Speed ratio 1 2 4

4 1 3

N T T

N T T . . . (vii)

i.e. speed of driver Produt of teeth on driven gears

Speed ratio = =speed of driven Product of teeth on drivers

Example 4.3

A compound gear train is used to transmit power from motor shaft to output shaft.

The motor shaft is connected to gear 1 and the output shaft is connected to gear 4.

Gears 2 and 3 are mounted on the same shaft. Motor shaft rotates at 1250 rpm in

the clockwise direction. Determine the speed and direction of output shaft and the

number of teeth on gears 1, 2, 3 and 4 are 30, 75, 20 and 50 respectively. The gear

train is shown in Figure 4.9.

46


Figure 4.9 : Compound Gear Train

Solution

Given T1 = 30, T2 = 75, T3 = 29 and T4 = 50

N1 = 1250 rpm

From Figure 4.9, it is evident that gears 1 and 3 are driving gears and gears 2 and 4

are driven gears or followers. Since, gears 2 and 3 are mounted on same shaft,

N2 = N3 and their direction of rotation will be same.

Let N4 is the speed of output shaft. It is same as the speed of gear 4.

Using formula :

Speed of first driver Produt of no. of teeth on followers

=speed of last follower Product of no. of teeth on drivers

i.e. 1 2 4

4 1 3

N T T

N T T

4

1250 75 50

30 20

N

or 4

12506.25

N

4

1250

6.25N

N4 = 200 rpm

Directional of Rotation of Output Shaft (or Gear 4)

The gear 1 rotates in clockwise direction. So, gear 2 will rotate in anticlockwise

direction because it is in mesh with gear 1. Gear 3 is on the same shaft as gear 2,

so it will also rotate in anticlockwise direction. Since, gear 4 is in mesh with gear

3, it will rotate in opposite direction, i.e. in clockwise direction.

Hence, direction of rotation of output shaft is clockwise.

SAQ 1

(a) What do you mean by gear train? List different types of gear trains.

(b) Differentiate between simple gear train and compound gear train.

(c) What do you mean by train value? How is it related to velocity ratio?

(d) Define the term, velocity ratio. What is the formula for calculating the

velocity ratio of simple gear train and compound gear train.

(e) What is epicyclic gear train?

47

Transmission SAQ 2

(a) A simple gear train consists of two gears which are mounted on two

different shafts. The two shafts are parallel. Gear 1 is driver and gear 2 is

follower. The speed of gear 1 is 600 rpm. The number of teeth on gears 1

and 2 are 20 and 60 respectively. Determine :

(i) Speed or velocity ratio of gear train,

(ii) Train value,

(ii) Speed of second gear, and

(iv) Direction of rotation of second gear if first gear rotates in clockwise

direction.

(b) A simple gear train consists of three gears each of which mounted on a

separate shaft. All the three shafts are parallel. Gear 1 is driver and rotates

at 1000 rpm. Gear 1 drives gear 2 and gear 2 drives gear 3. The number of

teeth on gears 1, 2 and 3 are 20, 30 and 50 respectively. Find :

(i) Speed ration of gear train,

(ii) Speed of follower (i.e. gear 3), and

(iii) Direction of rotation of follower if gear 1 rotates in clockwise

direction.

(c) Refer to Figure 4.9 which shows a compound gear train. It is used to

transmit power from motor shaft to output shaft. The gear 1 is mounted on

motor shaft, gears 2 and 3 are mounted on intermediate shaft, and gear 4 is

mounted on output shaft. Gear 1 drives gear 2 and gear 3 drives gear 4.

Motor shaft rotates at 1200 rpm in clockwise direction. Number of teeth on

gears 1, 2, 3 and 4 are 25, 50, 30 and 60 respectively. Determine :

(i) Speed ratio,

(ii) Direction and speed of the follower, and

(iii) Train value.

4.13 SUMMARY

Every student, who is studying the course automobile engineering, must have the

knowledge of transmission system of an automobile. Transmission system is nothing but

transmitting the power from engine to the wheels transfer clutch and gear mechanisms.

So, in this unit, we have studied about the transmission system of automobile. The

transmission system mainly comprises of clutch and gear mechanisms. We have learnt

about the functions and types of clutches and gear boxes. Clutch is mainly used to

yougase or disagause the engine to the transmission or gear box. Gear box is used to

varying the speeds of automobile according to the required conditions or according to the

need of the persons, who are driving the automobile.

48


4.14 KEY WORDS



49

Final Drive

UNIT 5 FINAL DRIVE

Structure

5.1 Introduction

Objectives

5.2 Universal Joints

5.3 Types of Universal Joints

5.4 Propeller Shaft

5.5 Final Drive or Final Reduction

5.6 Differential

5.7 Types of Differential

5.8 Rear Axles

5.9 Types of Rear Axles

5.10 Summary

5.11 Key Words


5.1 INTRODUCTION

The power developed by the engine is transferred to the wheels through clutch, gear box,

universal joints, propeller shaft, final drive, differential and rear axles. Description of

universal joints, propeller shaft, final drive, differential and rear axles has been given in

this unit.

Objectives


,

,

, and

.

5.2 UNIVERSAL JOINTS

Universal joint is used to connect two shafts at an angle for transmitting torque. In the

transmission shaft of an automobile, two universal joints are used – one between main

transmission shaft and propeller shaft and another between other end of propeller shaft

and the differential. Therefore, the universal joints make the joints flexible so that power

can be transmitted at an angle.

A universal joint takes care of rising and falling motion of the rear end of the propeller

shaft which is connected to differential. Two universal joints are shown in Figure 5.1

along with the propeller shaft.

50


Figure 5.1 : Two Universal Joints and Propeller Shaft

5.3 TYPES OF UNIVERSAL JOINTS

Three types of universal joints are commonly used. These are listed below :

(a) Cross or spider joint (variable velocity joint).

(b) Ball and trunnion joint (variable velocity joint).

(c) Constant velocity joints.

Cross Type Universal Joint

It consists of two Y-shaped yokes and a cross piece (spider). One yoke is

connected to driving shaft and other is connected to driven shaft. The cross-piece

has four-arms which are known as trunnions and are attached to the ends of yokes.

Four needle bearings are provided – one for each arm of cross-piece. These

bearings allow the yoke to swing around the trunnion when driving and driven

shaft remove together at an angle. A simple cross-type universal joint is shown in

Figure 5.2.

Figure 5.2 : Cross-type Universal Joint

This is a variable velocity joint, i.e. the driving and driven shaft do not rotate at

the same speed throughout a revolution. However, their rpm is same. This happens

because both shafts are not in straight line. Ring and trunion type and cross ball

type designs also come in this category of universal joints.

Ball and Trunnion Joint

This type of joint consists of a ball type head which is fastened to one end of the

propeller shaft. A pin is also pressed through this end of shaft. Two steel balls are

fitted at the end of this pin. The joint facilitates rotary motion through ball and

pin. The balls can also move axially.

Ball and trunnion joint is also a variable velocity joint.

Constant Velocity Universal Joint

This type of joint permits movement of both driving and driven shafts at constant

velocity. Because, two joints in this case operate at same angles. These joints are

generally used when the automobile in a front wheel (axle) drive. Because speed

variation between driving and driven shaft will introduce difficulty in steering and

excessive tyre wear.

51

Final Drive 5.4 PROPELLER SHAFT

The propeller shaft is a shaft that transmits power from transmission (gear box) to the

differential. On one end, propeller shaft in connected to main transmission shaft by

universal joint. On the other hand, it is connected to differential pinion shaft by another

universal joint. Propeller shaft transmits the rotary motion of main transmission shaft

(coming from gear box) to the differential so that rear wheels can be rotated. A sliding

(slip) joint, is also fitted between universal joint and propeller shaft on transmission side

which takes care of axial motion of propeller shaft. Propeller shaft is made of a steel tube

which can withstand torsional stresses and vibrations at high speeds.

It is important to note that the differential pinion shaft and transmission main shaft are

not in single horizontal level. The rear axle and differential is attached to automobile

frame via springs. Therefore, distance between differential and gear box keeps on

changing as vehicle moves along irregular road surface. Angle of propeller shaft also

changes due to this fact. Universal joints provided at two ends takes care of these two

changes. The propeller shaft along with universal joints has been shown in Figure 5.3.

Figure 5.3 : Propeller Shaft

A slip joint is provided between universal joint and propeller shaft to adjust for any

change in length.

5.5 FINAL DRIVE OR FINAL REDUCTION

Final drive is the last stage of power transfer from propeller shaft to rear (or front if –

automobile is front wheel driven) axles and then to wheels. It turns the propeller shaft

motion at right angle to drive the rear axle.

The final drive is composed of a bevel gear (or pinion) and crown wheel. The level

pinion is connected to propeller shaft. The pinion is in mesh with the crown wheel.

Crown wheel is part of differential. Final drive provides fixed speed reduction. Because

the crown wheel has more number of teeth and it is connected to rear axles and level

pinion has less number of teeth. Schematic diagram of final drive has been given in

Unit 1.

For final reduction in speed two types of gears can be used. One of them may be use of

level gears and another may be worm and worm wheel. Worm and worm wheel

combination provides large reduction without employing larger gears. It is strong also.

Slip Joint

The rear axle housing with wheel and differential is attached to the frame of

automobile through springs. As the vehicle moves over uneven surface, this whose

assembly moves up and down due to expansion and compression of springs. This

changes the length of propeller shaft because it is connected to differential and

gear box. Slip joint (Figure 5.3) allows for the change in length of propeller shaft.

When spring is compressed propeller shaft shortens and when spring is expanded,

propeller shaft returns to original length.

52


5.6 DIFFERENTIAL

When a four wheeler (car) takes a turn, the outer wheel turns faster than inner wheel.

Thus, there is relative movement between inner and outer wheel. The function of the

differential is to permit the relative movement between inner and outer wheels when

vehicle negotiates (takes) a turn. The torque transmitted to each rear wheel is equal in

this case, although their speed is different.

The differential is made up of a system of gears which connect the propeller shaft and

rear axles. It is a part of inner axle housing assembly. The assembly consists of

differential, rear axles, wheels and bearings.

Construction and Working

The construction of a simple differential is shown in Figure 5.5. It consists of sun

gears, planet pinion, a cage, a crown wheel and a bevel pinion. A sun gear is

attached to inner end of each rear axle (half shaft). A cage is attached on left axle.

A crown gear is attached to the cage and the cage rotates with the crown gear. The

crown gear is rotated by the bevel pinion. Crown gear and cage remain free on the

left rear axle. Two planet pinions are on a shaft which is supported by the cage.

The planet pinions mesh with the sun gears. The rear wheels are attached to outer

ends of two rear axles. When the cage rotates, sun gears rotate. Thus, the wheels

also rotate. In case one inner wheel runs slower than other when the vehicle takes

a turn, the planet gears spin on their shaft, transmit more rotary motion to outer

wheel. When vehicle runs in straight line, the crown gear, cage, planet pinions and

sun gears turn together as a unit. Thus there is no relative motion.

(a) Arrangement of Gears in a Simple Differential

(b) 3-Dimensional View of Differential

Figure 5.5

5.7 TYPES OF DIFFERENTIAL

There are three types of differential :

(a) Conventional type,

(b) Non-slip or self locking type, and

(c) Double reduction type.

53

Final Drive Conventional Type

Conventional type differential described in Section 5.6 delivers same torque to

each rear wheel. If any of the wheels slips due to any reason the wheel does not

rotate and vehicle does not move.

Non-slip or Self Locking Type

Non-slip or self locking type differential overcomes this drawback. It construction

is similar to that of conventional type differential. But, two sets of clutch plates

are provided additionally. Also, the ends of planet shafts are left loose in notches

provided on the differential cage.

Double Reduction Type

Double reduction type differential provides further speed reduction by additional

gear. This type of differential is used in heavy duty automobiles which require

larger gear reduction between engine and wheels.

5.8 REAR AXLES

Rear axle transmits power from differential to the wheels so that vehicle may move. Rear

axle is not a single piece but it is in two parts which are connected by the differential.

This is shown in Figure 5.5. Each part of rear axle is called the half shaft. Outer end of

the rear axle carries the wheel while inner end is connected to sun gear of the

differential. In vehicles which employ rear wheel drive, rear wheels are driving wheels.

However, in front wheel drive vehicles, front wheels are driving wheels. Rear axles and

differential are completely enclosed in a housing to protect them from dust, dirt, water

and any possible damage.

Functions of Rear Axle

(a) To transmit power from differential to the wheels. This is main function.

(b) To carry weight of automobile.

5.9 TYPES OF REAR AXLES

Rear axles differ on the basis of method of supporting them and mounting of rear

wheels. On this basis, these axles can be classified into three types :

(a) Half floating axle shown in Figure 5.6.

(b) Three-quarter floating axle shown in Figure 5.7.

(c) Fully floating rear axle shown in Figure 5.8.

Half Floating Axle

In a half floating rear axle, the axle is at the centre of the axle casing and the

bearings are inside the axle casing. The weight of vehicle is transmitted first to

suspension spring, then to axle casing, then to axle and finally to ground.

Figure 5.6 : Half Floating Rear Axle

54


Three-quarter Floating Axle

In three-quarter floating rear axle, bearings are on the outer side of axle casing, i.e.

between casing and wheel. In this case, major part of vehicle weight is taken by

axle casing and not by axle. This is the main advantage of three-quarter floating

type over half floating type. Thus, axle breakdown is less in this case compared to

the previous type.

Figure 5.7 : Three-quarter Floating Rear Axle

Fully Floating Rear Axle

In fully floating rear axle, the bearings are provided between axle casing and the

wheel. In this case, all the vehicle weight is transmitted to ground through axle

case and wheel. The axle is not supported by bearings but it is supported at both

ends. This type of axle is very strong and therefore, it is used for heavy duty

vehicles. In the event of breakdown of axle, wheel cannot come out. This, it is

safer but costly.

Figure 5.8 : Full Floating Rear Axle

SAQ 1

(a) What is the function of an universal joint? Where it is used in the

transmission system of an automobile?

(b) List different types of universal joints and describe the construction and

features of cross type joint.

(c) What is the function of propeller shaft? How it is connected in the

transmission system?

(d) Why is slip joint used with the propeller shaft?

(e) Write the function of final drive.

55

Final Drive SAQ 2

(a) What is the purpose of using a differential in an automobile?

(b) Describe the working of differential.

(c) Why rear axle is in two halves?

(d) List various type of rear axles and describe the working of half or

semi-floating rear axle.

(e) Which type of rear axle is used for heavy duty (load) vehicle?

5.10 SUMMARY

5.11 KEY WORDS



57

Braking System

UNIT 6 BRAKING SYSTEM

Structure

6.1 Introduction

Objectives

6.2 Functions of Brakes

6.3 Principle of Vehicle Braking

6.4 Classification of Brakes

6.5 Short Notes on Miscellaneous Braking Systems

6.5.1 Air Brakes

6.5.2 Vacuum Brakes

6.5.3 Electric Brakes

6.6 Hydraulic Brakes

6.7 Advantages and Disadvantages of Hydraulic Brakes

6.8 Construction and Working of Mechanical Brakes

6.9 Disc Brakes

6.10 Parking Brake or Emergency Brake

6.11 Bleeding of Brakes

6.12 Adjustment of Brakes

6.13 Summary

6.14 Key Words


6.1 INTRODUCTION

Baking system is necessary in an automobile for stopping the vehicle. Brakes are applied

on the wheels to stop or to slow down the vehicle.

Objectives


,

,

, and

.

6.2 FUNCTIONS OF VEHICLE BRAKING

There are two main functions of brakes :

(a) To slow down or stop the vehicle in the shortest possible time at the time of

need.

(b) To control the speed of vehicle at turns and also at the time of driving down

on a hill slope.

58


6.3 PRINCIPLE OF VEHICLE BRAKING

Braking of a vehicle depends upon the static function that acts between tyres and road

surface. Brakes work on the following principle to stop the vehicle :

“The kinetic energy due to motion of the vehicle is dissipated in the form of heat energy

due to friction between moving parts (wheel or wheel drum) and stationary parts of

vehicle (brake shoes)”.

The heat energy so generate4d due to application of brakes is dissipated into air.

Brakes operate most effectively when they are applied in a manner so that wheels do not

lock completely but continue to roll without slipping on the surface of road.

6.4 CLASSIFICATION OF BRAKES

On the Basis of Method of Actuation

(a) Foot brake (also called service brake) operated by foot pedal.

(b) Hand brake – it is also called parking brake operated by hand.

On the Basis of Mode of Operation

(a) Mechanical brakes

(b) Hydraulic brakes

(c) Air brakes

(d) Vacuum brakes

(e) Electric brakes.

On the Basis of Action on Front or Rear Wheels

(a) Front-wheel brakes

(b) Rear-wheel brakes.

On the Basis of Method of Application of Braking Contact

(a) Internally – expanding brakes

(b) Externally – contracting brakes.

6.5 SHORT NOTES ON MISCELLANEOUS

BRAING SYSTEMS

6.5.1 Air Brakes

Air brakes are applied by the pressure of compressed air. Air pressure applies force on

brakes shoes through suitable linkages to operate brakes. An air compressor is used to

compress air. This compressor is run by engine power.

6.5.2 Vacuum Brakes

Vacuum brakes are a piston or a diaphragm operating in a cylinder. For application of

brakes one side of piston is subjected to atmospheric pressure while the other is applied

vacuum by exhausting air from this side. A force acts on the piston due to difference of

pressure. This force is used to operate brake through suitable linkages.

6.4.3 Electric Brakes

In electrical brakes an electromagnet is used to actuate a cam to expand the brake shoes.

The electromagnet is energized by the current flowing from the battery. When flow of

current is stopped the cam and brake shoes return to their original position and brakes

are disengaged. Electric brakes are not used in automobiles as service brakes.

59

Braking System 6.6 HYDRAULIC BRAKES

The brakes which are actuated by the hydraulic pressure (pressure of a fluid) are called

hydraulic brakes. Hydraulic brakes are commonly used in the automobiles.

Principle

Hydraulic brakes work on the principle of Pascal’s law which states that “pressure

at a point in a fluid is equal in all directions in space”. According to this law when

pressure is applied on a fluid it travels equally in all directions so that uniform

braking action is applied on all four wheels.

Construction and Working of Hydraulic Brakes

When brake pedal in pressed, the force is transmitted to the brake shoes through a

liquid (link). The pedal force is multiplied and transmitted to all brake shoes by a

force transmission system. Figure 6.1 shows the system of hydraulic brake of a

four wheeler automobile. It consists of a master cylinder, four wheel cylinders and

pipes carrying a brake fluid from master cylinder to wheel cylinder.

Figure 6.1 : Hydraulic Brake

The master cylinder is connected to all the four-wheel cylinders by tubing or

piping. All cylinders and tubes are fitted with a fluid which acts as a link to

transmit pedal force from master cylinder to wheel cylinders.

Brake Fluid

The fluid filled in the hydraulic brake system is known as brake fluid. It is a

mixture of glycerine and alcohol or caster oil and some additives.

Master cylinder consists of a piston which is connected to peal through connecting

rod. The wheel cylinder consists of two pistons between which fluid is filled.

Each wheel brake consists of a cylinder brake drum. This drum is mounted on the

inner side of wheel. The drum revolves with the wheel. Two brake shoes which

are mounted inside the drum remain stationary. Heat and wear resistant brake

linings are fitted on the surface of the brake shoes.

Application of Brakes

When brake pedal is pressed to apply the brakes, the piston in the master cylinder

forces the brake fluid. This increases the pressure of fluid. This pressure is

transmitted in all the pipes and upto all wheel cylinders according to Pascal’s law.

This increased pressure forces out the two pistons in the wheel cylinders. These

pistons are connected to brake shoes. So, the brake shoes expand out against brake

drums. Due to friction between brake linings and drum, wheels slow down and

brakes are applied.

60


As shown in Figure 6.2, two pipes carrying braked fluid are connected to front

wheel cylinders which may be same as rear wheel cylinders. The front wheels may

also have same type of brakes (drum brakes) as shown in the rear wheels. But, in

modern cars, there are disc brakes in the front wheels and drum brakes in the rear

wheels.

Figure 6.2 : Mechanical Brake (Internal Expanding Type)

Release of Brakes

When pedal is released, the piston of master cylinder returns to its original

position due to retractor spring provided in master cylinder. Thus, fluid pressure

drops to original value. The retractor spring provided in the wheel cylinders pulls

the brake shoes and contact between drum and brake linings is broken. Therefore,

brakes are released.

6.7 ADVANTAGES AND DISADVANTAGES OF

HYDRAULIC BRAKES

Advantages

(a) Equal braking action on all wheels.

(b) Increased braking force.

(c) Simple in construction.

(d) Low wear rate of brake linings.

(e) Flexibility of brake linings.

(f) Increased mechanical advantage.

Disadvantages

(a) Whole braking system fails due to leakage of fluid from brake linings.

(b) Presence of air inside the tubings ruins the whole system.

6.8 CONSTRUCTION AND WORKING OF

MECHANICAL BRAKES

Internal expanding shoe brakes are most commonly used in automobiles. In an

automobile, the wheel is fitted on a wheel drum. The brake shoes come in contact with

inner surface of this drum to apply brakes.

The construction of internal expanding mechanical brake is shown in Figure 6.2. The

whole assembly consists of a pair of brake shoes along with brake linings, a retractor

spring two anchor pins a cam and a brake drum. Brake linings are fitted on outer surface

of each brake shoe. The brake shoes are hinged at one end by anchor pins. Other end of

brake shoe is operated by a cam to expand it out against brake drum. A retracting spring

61

Braking System brings back shoes in their original position when brakes are not applied. The brake drum

closes inside it the whole mechanism to protect it from dust and first. A plate holds

whole assembly and fits to car axle. It acts as a base to fasten the brake shoes and other

operating mechanism.

How Brakes are Applied and Released

When brake pedal is pressed, the cam turns through brake linkages. Brake shoes

expand towards brake drum due to turning of cam. The brake linings, rub against

brake drum and therefore motion of wheels is stopped. The pedal force is

transmitted to the brake shoes through a mechanical linage. This mechanism also

multiplies the force to apply the brakes effectively.

When force on brake pedal is removed, the retractor spring brings back shoes in

original position and brakes are released.

6.9 DISC BRAKES

Modern motor cars are fitted with disc brakes instead of conventional drum type brakes.

In Santro car and Maruti-800, front wheels are provided with disc brakes whereas rear

wheel are provided with drum brakes. A disc brake consists of a rotating disc and two

friction pads which are actuated by hydraulic braking system as described earlier. The

friction pads remain free on each side of disc when brakes are no applied. They rub

against disc when brakes are applied to stop the vehicle. These brakes are applied in the

same manner as that of hydraulic brakes. But mechanism of stopping vehicle is different

than that of drum brakes.

Advantage of Disc Brakes

(a) Main advantage of disc brakes is their resistance to wear as the discs remain

cool even after repeated brake applications.

(b) Brake pads are easily replaceable.

(c) The condition of brake pads can be checked without much dismantling of

brake system.

Disadvantage of Disc Brakes

(a) More force is needed be applied as the brakes are not self emerging.

(b) Pad wear is more.

(c) Hand brakes are not effective if disc brakes are used in rear wheels also.

(Hand brakes are better with mechanical brakes).

6.10 PARKIG BRAKE OR EMERGENCY BRAKE

Parking brakes or emergency brakes are essentially mechanical brakes operated by hand.

These are used to prevent the motion of vehicle when parked at a place or when parked

on slopes. In cars, these brakes are generally attached to rear wheels. In this type, a cable

connects the hand lever to the brake. Brakes are applied by pulling the lever and released

by pushing a button (provided on lever) and pressing the lever down.

6.11 BLEEDING OF BRAKES

When air enters, into the brake system and any brake line is disconnected, bleeding of

brakes has to be done. Since air is compressible so any presence of air inside brake

lining does not allow to transmit brake force to apply brakes. Therefore, the system must

be free from presence of air. Bleeding is the process of removal of air from the braking

system.

62


Bleeding Procedure

Following steps are followed for bleeding of brakes :

(a) Remove all dirt from the master cylinder filler plug. Then fill the

master cylinder upto lower edge of the filler neck by removing the

filler plug.

(b) Clean all the bleeding connections provided on all wheel cylinders.

(c) After this bleeder hose and fixture is connected to that wheel cylinder

which has longest brake line. The other rend of bleeder hose is placed

in a glass jar, and submerge this end in the brake fluid.

(d) How bleeder valve is opened by half to three quarter turn.

(e) Then press the foot pedal and allow it to return back slowly.

(f) This pumping action must be continued till all the air along with

some brake fluid comes out through bleeding hose.

(g) After this bleeding operation is carried out on all wheel cylinders.

This completes the bleeding operation. At the end master cylinder is

filled with brake fluid to required level.

6.12 ADJUSTMENT OF BRAKES

When pedal is pressed to apply brake, there should be atleast 1/2 inch free pedal

movement before breaking action starts. This may vary from company to company.

The brakes are adjusted as per the above mentioned recommendation before they are

ready to use. This is done by following a definite procedure.

(a) List the wheels by screw jack.

(b) Loosen the lock nut for the forward brake shoe and keep it in this position.

(c) Turn the eccentric with other wrench towards the front of automobile till

the brake shoe touches the drum.

(d) Release the eccentric while turning the wheel with one hand, till wheel

turns freely.

(e) Hold the eccentric in this position and tighter the lock nut.

(f) Repeat the same operation to adjust other shoe, but turn the eccentric in the

backward direction of the vehicle.

(g) Above procedure is repeated for all the four wheels.

SAQ 1

(a) Write the functions of brakes in an automobile.

(b) Explain the construction and working of mechanical brakes.

(c) Describe in brief the construction and working of hydraulic brakes.

(d) Write the advantages and disadvantages of hydraulic brakes.

(e) Write short notes on :

(i) Vacuum brakes,

(ii) Electrical brakes, and

(iii) Air brakes.

63

Braking System SAQ 2

(a) What are the functions of parking or emergency brakes?

(b) What do you mean by bleeding and adjustment of brakes?

(c) Describe the procedure of bleeding of brakes.

(d) Describe the procedure of adjustment of brakes.

(e) Why disc brakes are better than drum type brakes?

6.13 SUMMARY

6.14 KEY WORDS



65

Front Axle and Steering

UNIT 7 FRONT AXLE AND STEERING

Structure

7.1 Introduction

Objectives

7.2 Front Axle

7.3 Types of Front Axle

7.4 Stub Axle

7.5 Steering

7.6 Ackerman’s Principle of Steering

7.7 Steering Geometry

7.8 Camber Angle

7.9 King Pin Inclination

7.10 Caster Angle

7.11 Toe-in and Toe-out

7.12 Steering Gear Box

7.13 Steering Linkages

7.14 Layout of a Steering System

7.15 Summary

7.16 Key Words


7.1 INTRODUCTION

In any motor cars and other four and six wheeler vehicles, steering is main component.

Properly designed steering, works well and guides the vehicle to move in correct

direction. Mainly steering is linked to the front axel with gear train mechanism. On the

front axle, wheels are mounted, and with the help of steering wheel, the driver can turn

the vehicle in right, left or straight directions.

The function of steering mechanism is clearly explained in this unit. In this unit, we also

elaborated on the front axel and its types.

Principle of steering, steering geometry, steering gearbox and working of steering

systems have been clearly explained.

Objectives


understand about front axel and steering,

define function of front axle and steering,

explain the principle of steering mechanism,

describe the linking mechanism of front axle and steering wheel, and

understand the steering geometry.

66


7.2 FRONT AXLE

Front wheels of the vehicle are mounted on front axles. Functions of front axle are listed

below :

(a) It supports the weight of front part of the vehicle.

(b) It facilitates steering.

(c) It absorbs shocks which are transmitted due to road surface irregularities.

(d) It absorbs torque applied on it due to braking of vehicle.

Construction and Operation

Front axle is made of I-section in the middle portion and circular or elliptical

section at the ends. The special x-section of the axle makes it able to withstand

bending loads due to weight of the vehicle and torque applied due to braking. On

kind of front axle is shown in Figure 7.1 which consists of main beam, stub axle,

and swivel pin, etc. The wheels are mounted on stub axles.

Figure 7.1 : Front Axle

7.3 TYPES OF FRONT AXLES

There is two types of front axles :

(a) Dead front axle, and

(b) Line front axle.

Dead Front Axle

Dead axles are those axles, which donet rotate. These axles have sufficient rigidity

and strength to take the weight. The ends of front axle are suitably designed to

accommodate stub axles.

Line Front Axle

Line axles are used to transmit power from gear box to front wheels. Line front

axles although, front wheels. Line front axles although resemble rear axles but

they are different at the ends where wheels are mounted. Maruti-800 has line front

axle.

7.4 STUB AXLE

Stub axles are connected to the front axle by king pins. Front wheels are mounted on

stub axles arrangement for steering is connected to stub axles. Stub axle turns on kind

pins. King pins is fitted in the front axle beam eye and is located and locked there by a

taper cotter pin. Stub axles are of four types :

(a) Elliot

(b) Reversed elliot

67

Front Axle and Steering (c) Lamoine

(d) Reversed lamoine

All are differ from each other in the manner in which they are connected to the front

axle. Elliot type stub axle is shown in Figure 7.1.

7.5 STEERING

A good steering mechanism is must for a vehicle’s stability at the time of turning.

Steering of four wheeler is designed in a manner so that it will not permit lateral slip of

front wheels during steering. There must be true rolling of wheels at the time of steering.

The front wheels are mounted on front axles to allow their left and right swing for

steering the vehicle. Steering is done by providing a suitable gearing and linkage

between front wheels and steering wheel. A simplified diagram of a steering system has

been shown in the Figure 7.2.

Figure 7.2 : Simple Driving of a Steering System

7.6 ACKERMAN’S PRINCIPLE OF STEERING

Ackerman’s steering gear mechanism is based on Ackerman’s principle of steering. The

mechanism consists of a cross link BC connected to short axles AL and DM of front

wheels through short arms AB and CD. These form the bell crank levers LAB and MDC.

In case of straight motion of automobile the cross-link BC remains parallel to AD and

short links AB and CD both make angle from the horizontal axis of chasis.

(a) For Straight Motion

(b) For Right Turn

Figure 7.3 : Ackerman’s Steering Mechanism (Principle)

68


Fundamental equation of steering is satisfied when the links AB and BC are

proportioned suitably and angle is selected suitably. The condition for correct steering

is :

cot θ cot a

l

The angles and are shown in Figure 7.3(b) and distances ‘a’ and ‘l’ are shown in

Figure 7.3(a). The value of a

l lies between 0.4 and 0.5. It is generally taken near to

average of two values, i.e. 0.455. The mechanism used for automatically adjusting the

values of and for correct steering is known as Ackerman’s steering gear mechanism.

There are three values of angle for correct steering corresponding to three cases :

(a) when vehicle is running straight,

(b) when vehicle is turning to right, and

(c) when vehicle is turning to left.

7.7 STEERING GEOMETRY

When a four wheeler (car) takes a turn, all its four wheels should roll without slipping

laterally. This is possible only when the axes of four wheels intersect at one point. This

point is the centre about which the vehicle turns at that instant. At this instant, rear rotate

along two circles, where the centre of two circles is at ‘O’. The front wheels have their

different axes. These wheels also rotate along two other circles with same centre ‘O’.

Figure 7.4 shows the steering geometry of all the four wheels of the vehicle. For correct

steering, the centre of the wheels of the rear axles and centre of front wheels must

coincide.

Figure 7.4 : Steering Geometry of Four Wheels

7.8 CAMBER ANGLE

Camber angle is the angle between the vertical line and centre line of the tyre when

viewed from the front of the vehicle. Camber angle is positive when this is outward. This

happens when wheels are further apart at top than at bottom. On the contrary, camber

angle is negative when angle is inward. This happens when wheels are further apart at

bottom than at top. The camber, should not be more than 2o, because this causes uneven

or more tyre wear on one side than on other side.

The front wheels are usually fitted with positive camber angle. This is done to prevent

tilting of top of wheels inward due to excessive load or play in the king pin and wheel

bearing. The load brings the wheels to vertical position.

69

Front Axle and Steering Excessive camber is not good because it prevents proper wheel contact with the road.

Unequal camber causes the vehicle in that direction in which camber is more. This

disturbs the directional stability. Camber angle is shown in Figure 7.5.

Figure 7.5 : Camber Angle (Positive) and King Pin Inclination

7.9 KING PIN INCLINATION

It is the angle between king pin centre line and vertical line when seen from the front of

the vehicle. It is also called steering axle inclination. King pin inclination and caster are

used to improve directional stability in cars. Because of these provisions wheels tend to

return to the straight ahead position after the vehicle completes any turn (due to steering

left or right). This is also used to reduce steering effort when steering a stationary

vehicle. In addition to this, it reduces tyre wear. This inclination varies from 4 to 8o in

modern cars. The king pin inclination is shown in Figure 7.5. It should be equal on both

sides, i.e. on both front wheels.

7.10 CASTER ANGLE

Caster angle is the tilt of king pin centre line towards front of back from the vertical line.

It is the angle between the vertical line and king pin centre line in the p wheel plane

when looked from side. It is shown in Figure 7.6.

Figure 7.6 : Caster Angle (Positive)

Caster angle is positive when top of the king pin is backward and negative when it is

forward. The value of this angle in vehicles ranges from 2 to 8o. The caster angle

provides directional stability to vehicle by making wheels to follow in the direction of

movement of vehicle. The vehicle tends to roll out on turns when caster angle of both

front wheels is positive. But it tends to back or lean in on turns when caster angles are

negative. Positive caster angle increases the steering effort and tends to keep the wheels

straight. Negative caster is provided in heavy duty vehicles to reduce steering effort.

70


7.11 TOE-IN AND TOE-OUT

The front wheels are slightly turned in at front side such that the distance between

wheels at front (A) is little less than the distance at back (B), when seen from top. This

difference in distance is called to-in. It is shown in Figure 7.7. The distance B is greater

than A by 3 to 5 mm.

Figure 7.7 : Toe-in (A < B)

Purpose of Toe-in

(a) To ensure that wheels are rolling parallel.

(b) To stabilize steering and prevent slipping towards sides.

(c) To prevent excessive tyre wear.

(d) To offset the effect of small deflections in the wheel support system.

The wheels are set with to-in but they move parallel when car moves forward.

The difference in the angles between the two front wheels and frame of the car during

turns is called toe-out. While taking the turn, the inside wheel makes larger angle than

outer wheel to satisfy the condition of correct steering. The toe-out is shown in

Figure 7.8.

Figure 7.8 : Toe-out at the Time of Turning of Vehicle

At turns, inner wheels makes an angle which is more than angle of outer wheel.

Toe-out is set by maintaining proper relation between the steering knuckle arm, tie rods

and pitman arm.

7.12 STEERING GEAR BOX

Steering gears are used to reduce the steering effort and convert rotary motion of steering

wheel into straight line motion of linkage. Thus, steering gear provides mechanical

advantage also to make steering easy. Steering gears are put inside the steering gear box.

Steering gear box connects steering shaft and steering linkages.

Various types of steering gears used in different automobiles are listed below :

(a) Worm and sector type,

(b) Worm and worm wheel type,

(c) Worm and roller type,

71

Front Axle and Steering (d) Rack and pinion type, and

(e) Cam and roller type.

Worm and Sector Type Steering Gear

In a worm and sector type steering gear a worm is provided at the end of steering

shaft which meshes with a sector provided on a sector shaft. When the worm is

rotated, the sector turns which moves the linkages for steering the vehicle. The

sector shaft is also called pitman arm shaft, roller shaft or cross shaft. This is

shown in Figure 7.9.

Figure 7.9 : Worm and Sector Steering Gear

Worm and Worm Wheel Type Steering Gear

In worm and work wheel system, square threads are provided on the worm on the

steering shaft. The worm meshes with the worm wheel which is mounted on a

shaft. A drop arm is also mounted on the same shaft as shown in Figure 7.10. The

rotation of steering shaft rotates the worm and worm wheel. This rotates drop arm

by 60o to 90

o. This moves the steering linkages. This type of gear box is used in

tractors.

Figure 7.10 : Worm and Worm Wheel Steering Gear

A square shaft is generally used on which worm wheel is mounted.

Worm and Roller Type Steering Gear

In the worm and roller steering gear, a roller with two teeth is meshes with the

teeth on roller. This type of system was popular in American passenger cars.

Rack and Pinion Steering Gear

A pinion is attached at the end of the steering shaft. A rack mashes with the

pinion. The rotary movement of the steering moves the pinion which gives motion

to the rack. The movement of the rack is responsible for turning the wheels

through steering linkages.

7.13 STEERING LINKAGES

Steering linkages is connection of different links between steering gear box and front

wheels. The rotation of steering wheel is transmitted to the steering gear from which it is

transferred to the front wheels for turning them to left or right.

72


7.13.1 Steering Linkage for Conventional Rigid Axle Suspension

Steering linkage for a conventional rigid axle suspension has been shown in Figure 7.11.

It is generally used in cars which have rigid front axle.

Figure 7.11 : Steering Linkage for a Rigid Axle Suspension

The steering knuckle arm is connected to pitman arm through a drag link (link rod).

The right hand track rod arm is connected to left hand track rod arm through a track rod

(or tie rod).

Working of Steering System

When steering wheel is rotated, the motion is transmitted to pitman arm through

gear box. This motion is transmitted to drag link. Drag link transfers this

movement to stub axle which rotates about king pin. This turns the right wheel.

The left wheel is turned through the track rod and left hand track and arm.

7.13.2 Steering Linkage for Independent Front Suspension

If automobile is fitted with independent front suspensions then different type of steering

linkages are used. In these linkages, the ball joints are fitted between steering linkage

and steering arm which facilitates independent movement of the wheels. A simplified

linkage is shown in Figure 7.12.

Figure 7.12 : Steering Linkage for Independent Front Suspension

7.14 LAYOUT OF A STEERING SYSTEM

Figure 7.13 shows a simplified layout of a steering system. A typical steering system

consists of

(a) Steering wheel,

(b) Steering shaft,

(c) Steering gear box,

(d) Pitman arm,

(e) Drag link,

73

Front Axle and Steering (f) Steering knuckle arm,

(g) Tie rod, and

(h) Track rod arm, etc.

Figure 7.13 : Layout of a Steering System

SAQ 1

(a) Write the functions of steering in an automobile.

(b) Name different types of steering gear boxes.

(c) Describes worm and sector type steering gear box.

(d) What is the function of steering linkage? Describe the working of steering

linkage for rigid axle suspension.

(e) Draw a line diagram of a steering linkage for independent front suspension

type vehicle.

SAQ 2

(a) Sketch a line diagram showing the layout of a steering system. List the main

parts of which it consists.

(b) Write the Ackerman’s principle of steering. Show with the help of a

diagram when vehicle takes a right turn.

(c) Explain tow-in and toe-out with the help of suitable diagrams.

(d) Explain the following terms with the help of diagrams :

(i) Caster,

(ii) Camber and

(iii) King pin inclination.

SAQ 3

(a) What do you mean by steering geometry? Explain.

(b) Write the functions of front axle.

(c) What do you mean by :

(i) Line axle, and

(ii) Dead axle.

74


(d) What is the function of stub axles? Describe their use and list different

types.

(e) Why the following provisions made in the vehicle :

(i) Toe-in,

(ii) Toe-out,

(iii) King pin inclination,

(iv) Camber angle, and

(v) Caster angle.

7.15 SUMMARY

7.16 KEY WORDS



75

Frame and Chassis

UNIT 8 FRAME AND CHASSIS

Structure

8.1 Introduction

Objectives

8.2 Chassis

8.3 Frame

8.4 Types of Frame

8.4.1 Conventional Frame

8.4.2 Semi-integral Frame

8.4.3 Integral Frame or Frame-less Construction

8.5 Types of Sections used in Frames

8.6 Suspension System

8.7 Functions of Suspension System

8.8 Springs

8.9 Leaf Springs

8.10 Coil Springs

8.11 Torsion Bars

8.12 Shock Absorbers

8.13 Tyres

8.14 Types of Tyres

8.15 Tyre Specification

8.16 Causes of Tyre Wear

8.17 Remedies for Reducing Tyre Wear

8.18 Summary

8.19 Key Words


8.1 INTRODUCTION

The automobiles such as cars, buses and trucks, etc. are generally considered to be

consisting of two major assemblies, chassis and body.

Objectives


define chassis, frame, springs, shock absorbers,

explain the various types of frames, springs, and

describe the advantages and disadvantages of tyres, springs and shock

absorbers.

8.2 CHASSIS

Chassis is a French term which is now denotes the whole vehicle except body in case of

heavy vehicles. In case of light vehicles of mono construction, it denotes the whole

vehicle except additional fittings in the body.

“Chassis consists of engine, power train, brakes, steering system and wheels mounted on

a frame”.

76


8.3 FRAME

The frame is the main part of the chassis on which remaining parts of chassis are

mounted. The frame should be extremely rigid and strong so that it can withstand shocks,

twists, stresses and vibrations to which it is subjected while vehicle is moving on road. It

is also called underbody.

The frame is supported on the wheels and tyre assemblies. The frame is narrow in the

front for providing short turning radius to front wheels. It widens out at the rear side to

provide larger space in the body.

8.4 TYPES OF FRAME

There are three types of frames :

(a) Conventional frame,

(b) Semi-integral frame, and

(c) Integral frame (or unit frme).

8.4.1 Conventional Frame

It is non-load carrying frame. The loads of the vehicle are transferred to the suspensions

by the frame. This suspension in the main skeleton of the vehicle which is supported on

the axles through springs. The body is made of flexible material like wood and isolated

frame by inserting rubber mountings in between. The frame is made of channel section

or tubular section of box section.

Example : This type of frame is used for trucks.

8.4.2 Semi-integral Frame

In this case the rubber mountings used in conventional frame between frame and

suspension are replaced by more stiff mountings. Because of this some of the vehicle

load is shared by the frame also. This type of frame is heavier in construction.

Example : Popular in European and American car.

8.4.3 Integral Frame or Frame-less Construction

In this type of construction, there is no frame. It is also called unitized frame-body

construction. In this case, the body shell and underbody are welded into single unit. The

underbody is made of floor plates and channel and box sections welded into single unit.

This assembly replaces the frame. This decreases the overall weight compared to

conventional separate frame and body construction.

8.5 TYPES OF SECTIONS USED IN FRAMES

Three types of steel sections are most commonly used for making frames :

(a) Channel section,

(b) Tubular section, and

(c) Box section.

The cross-section of all the three types of section is shown in Figure 8.1.

(a) Channel (b) Box (c) Tubular

Figure 8.1 : Different Steel Sections used for Making Frames

77

Frame and Chassis The channel section is best suited for bending loads. Box section is good for both

bending and torsion and tubular section is good for torsion.

8.6 SUSPENSION SYSTEM

The frame and body of an automobile are mounted on front and rear axles through

springs and shock absorbers. If it is mounted directly on axles, all the socks and

vibrations will be transmitted to body causing discomfort to the passengers. The springs

and shock absorbers are used to damp the shocks and vibrations. The suspensions system

includes all those parts which are used to perform the damping action. Besides, springs

and shock absorbers, a suspension system includes other mountings also. The suspension

system of a vehicle is divided into front suspension and rear suspension.

8.7 FUNCTIONS OF SUSPENSION SYSTEM

(a) The main function of a suspension system is to prevent the socks to transmit

to car or vehicle body so that passengers may ride comfortably.

(b) To maintain the stability of vehicle during pitching and rolling actions while

the vehicle is in motion.

(c) To provide better road holding at the time of driving, braking and cornering.

(d) To allow proper steering geometry.

8.8 SPRINGS

Different types of springs are used in the suspension system of an automobile. Springs

absorb the energy which is generated due to force which comes when vehicle moves

over bumps and trenches. Springs are required to absorb the energy of shocks very

quickly and release it slowly and slowly. For this a absorber is also used. Coil springs

and leaf springs are used in the automobiles. Besides this some other devices are also

used such as torsion bars and shock absorbers. Description of these devices is given in

the following sections.

8.9 LEAF SPRINGS

These springs are made by placing several flat strips one over the other. These are made

of steel plates. One flat strip is called a leaf. Lowest leaf is of smallest length and the

length of other leaves placed above this keeps on increasing progressively. In this way,

the length of top most leaf (main leaf) largest. Main leaf has eyes at the ends. All the

leaves are clamped together at centre and sides by the centre bolt and side clamps

respectively. The centre portion of the leaf springs is connected to the axle with the help

of U-bolt. A simple sketch of leaf springs is shown in Figure 8.2.

Figure 8.2 : Leaf Spring

78


Spring eye is used to attach spring to the body frame by passing a bolt through one eye.

Other end of leaf spring is attached to a shackle through its eye. Shackle is in turn

attached to chassis. The shackle is used to accommodate any change in length of spring

due to its expansion and contraction. The contraction and expansion takes place when

the vehicle passes over road surface irregularities. Semi-elliptical springs are generally

used in all the vehicles particularly n trucks. In case, leaf springs were used in rear

suspension and independent suspension in the front. But, leaf springs are not used n cars

also.

8.10 COIL SPRINGS

Coil springs are in the form of helix. These are made from special steel. It is made from

steel wire in the form of a coil. The coil springs absorb energy when this spring is

compressed while vehicle moves over road bump. The coil springs are mainly used in

independent suspension. However, these can also be used in the conventional rigid axle

suspension. Coil springs are capable of resisting shear and bending stresses but not

torsion and side thrust.

When coil springs are used in the suspension system, other arrangements are made to

bear torsion and side thrust.

Advantages of Coil Springs

(a) Coil springs are better than leaf springs as hey can absorb almost double

energy per unit volume as compared to leaf springs.

(b) They also require less space than leaf springs and can be used in very

restricted spaces.

(c) Coil springs are lighter in weight for the same load.

(d) Compact in size.

8.11 TORSION BARS

Torsion bar is a steel rod which an take torsional and shear stresses. Torsion bar acts as

spring and keeps the lower and upper control arm parallel. The torsion bar is shown in

Figure 8.3. One end of the rod is made of hexagonal x-section which fits into lower

control arm. Other end is also hexagonal x-section which fits into an anchor attached to

an anchor. When any force acts on the wheel assembly, the torsion bar gets twisted. The

wheel axle is supported by lower control arm. The torsion bar is connected to lower

control arm. The torsion bar is used to keep the lower arm at a given height. This

suspension (torsional bar) provides cushion to road shocks by allowing the lower arm to

twist the torsion bar. The torsion bar occupies normal condition when the wheels are not

under any stress. When the wheels move up and down the torsion bar is twisted and it

absorbs the vibrations so generated.

Figure 8.3 : Simplified Figure of Torsion Bar

79

Frame and Chassis 8.12 SHOCK ABSORBERS

If only springs are used to absorb shocks, the oscillations of springs continue even after

the vehicle has passed over a bump. The oscillations cause the wheels the jumps up and

fall down till the oscillations die out. Thus, dampers or shock absorbers are used to arrest

the oscillation of springs after the vehicle passes over irregular road surface.

Shock absorbers are necessary used with coil springs. In case of leaf springs, the friction

between leaves provides some dampening effect. However, this is not sufficient

sometimes, depending upon friction between leaves. Hence, shock absorbers are

necessarily used as additional damping devices.

Function of Shock Absorbers

As explained earlier, the function of the shock absorber is to dampen the

vibrations of coil and leaf springs used in the suspension system. These vibrations

are generated when vehicle passes over a road bump.

Working of Telescopic type Shock Absorber

In modern cars, hydraulic shock absorbers are used. These absorbers use a piston

and a cylinder where cylinder is filled with a suitable oil. The oil is used to

dampen the oscillations of piston by a suitable arrangement. The construction of a

telescopic type shock absorber has been shown in Figure 8.4.

(a) Shock Absorber under Compression (b) Shock Absorber under Expansion

Figure 8.4 : Sectional View of Telescopic Type Shock Absorber

The telescopic shock absorber, mainly consists of a piston a cylinder tube and a

reservoir tube. The piston has been provided with through orifices or opening so

that fluid can pass from top to bottom or from bottom to top reservoir.

Figure 8.4(a) shows the condition when absorber is compressed. This happens

when vehicle passes over a bump. Under this condition the shock absorber

becomes short in length. The piston rod forces the piston down into cylinder tube.

Therefore, fluid under the piston is compressed to high pressure. The fluid passes

forcefully through small orifices (in piston) into the moves ahead of bump or

drops into a depression in the road, the shock absorber expands. Under this

condition the piston moves up in the cylinder tube. Because of this, fluid is forced

from upper part of cylinder tube to the lower part through the orifices provided in

the piston.

In both the cases, i.e. expansion and compression, fluid is forced through orifices.

Because of this the motion of the piston is slowed down. This puts restriction over

the spring action and vibrations of the frame are arrested in shortest time. In this

way, shocks (of bump and depression of road) are absorbed by the shock absorber.

It also prevents excessive oscillations of wheel when it passes over a bump and

depression on the road.

Shock absorbers are always provided along with springs in the suspension system

of automobiles to prevent oscillations of springs.

80


8.13 TYRES

Tyres are mounted on the rims of wheels. They enclose a tube between rim and itself.

Air is filled at a designated pressure inside the tube. The tyre remains inflated due to air

pressure inside tube. The tyre carries the vehicle load and provides cushioning effect. It

absorbs some of the vibrations generated due to vehicle’s movement on uneven surfaces.

It also resists the vehicle’s tendency to over steer or turn which cornering. Tyre must

generate minimum noise when vehicle takes turn on the road. It should provide good grip

with the road surface under all conditions.

8.14 TYPES OF TYRES

Two types of tyres are used in vehicles :

(a) Tube tyres, and

(b) Tubeless tyres.

Both these tyres are called pneumatic tyres because air is filled in them.

Tube Tyres

Tube tyres encloses a tube which is wrapped on the wheel rim. Air is forced into

tube which inflates the tube and tyre. The outer side of tyre which comes in

contact of road is made from rubber. It is called tread. Tread provides resistance to

slipping. It is very thick at the outer periphery. Beads are made at the inner bide by

reinforcing it with steel wires. Beads are very strong which have good resistance

to wearing against the wheel rim. Rayon cords are also formed into a number of

piles. Beads are cords provide good strength to tyres.

Tubeless Tyres

These tyres do not require any tube. The air at pressure is filled into the tyre itself.

The construction of tyre is same as that of tube tyre. For filling the air, a

non-return valve is filled in the tyre itself.

Advantages of Tubeless Tyres

(a) Tubeless tyres are lighter in weight.

(b) They remain cooler compared to tube tyres.

(c) The main advantage of tubeless tyre is that they remain inflated for

long time even if these are punctured by a nail if the nail remains

inside the tyre.

(d) Any hole in the tyre, due to puncture, can be repaired by rubber

plugging.

(e) A simple puncture can be repaired without removing tyre from wheel.

8.15 TYRE SPECIFICATION

Every tyre is specified by its size. Its specification is given as follows :

8.25 30 6 PR

Meaning of these Numbers

(a) 8.25 : It mean that thickness of tyre from shoulder to shoulder is

8.25 inches.

(b) 20 : It means that diameter of bead circle is 20 inches.

(c) 6 PR : It means that six ply rating. It means that tyre cosists of 6 plies.

Different type of tyres have different plies. Number of plies increase as load

increases, e.g. a ca tyre has 4 to 6 plies and a light truck may have 6 to

10 plies.

81

Frame and Chassis 8.16 CAUSES OF TYRE WEAR

Excessive tyre wear is caused due to reasons discussed below :

(a) Lower or Higher Tyre Pressure : It is recommended by manufacturer’s to

maintain correct tyre pressure. If the tyre pressure is perfect, there will be

full tread contact with the road. If tyre pressure is lower than required,

severe flexing of tyre piles, and side walls take place. In this case, excessive

heat is generated which causes excessive wear. Tyres wear out more on

both sides of tread and less of centre.

If tyre pressure is higher, the tyres wear out more at centre and les son sides.

(b) Tyres wear out more one on side than the reason is incorrect caber setting.

(c) Toe-out causes remarkable wear on tread inner end of both front wheels.

(d) High speed of vehicle is also the cause of more tyre wear and failure.

8.17 REMEDIES FOR REDUCING TYRE WEAR

(a) Maintaining correct tyre pressure.

(b) Correct camber.

(c) Proper wheel alignment.

(d) If vehicle is to be run at very high speeds tyre must be little over inflated to

reduce wear.

(e) Steering must be properly adjusted.

(f) Avoid habit of turning at higher speeds.

SAQ 1

(a) What do you mean by Chassis? Describe.

(b) What are the functions of frame?

(c) List various types of frame and describe in brief the conventional frame.

(d) What do you mean by frameless construction? Describe in brief.

(e) Differentiate between integral and semi-integral frame.

SAQ 2

(a) What are functions of suspension system of an automobile?

(b) What is the function of torsion bar suspension? How does it work? Explain

in brief.

(c) Describe a leaf spring suspension system.

(d) Describe a coil spring suspension system.

(e) What is the function of a shock absorber? How does a hydraulic shock

absorber work?

82


SAQ 3

(a) Describe in brief various types of tyres?

(b) List the advantages of tubeless of tyres?

(c) How the tyres are specified? Explain with the help of an example.

(d) What are different causes of tyre wear? Describe.

(f) How can you reduce type wear of your vehicle?

8.18 SUMMARY

In this unit, you must have gain the knowledge about the chassis and frame. Chassis and

frames are the two main parts of the automobiles. Any automobile engineering should

have the knowledge about the automobile parts and its main functions. You must have

understood about the functions and uses of chasses, frame, springs, tyres and shock

absorbers. This unit given you the knowledge on different types of spring, tyres, shock

absorbers and its functions as a automobile parts and components.

8.19 KEY WORDS


efer the preceding text for all the Answers to SAQs.

83

Frame and Chassis

FURTHER READING

William H. Crouse and Donald L. Anglin, Automotive Engines, McGraw Hill.

K. K. Jain and R. B. Asthan, Automobile Engineering, Tata McGraw Hill.

R. K. Rajput, Automobile Engineering, Laxmi Publications Pvt. Ltd.

Siegfried Herrmann, Automotive Engineering, Asia publishing House.

P. L. Ballaney, Theory of Machines, (For Numerical on Gear Trains).

84


85

Frame and Chassis

AUTOMOBILE ENGINEERING

Documents

Automobile Theory and Explanation