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ME 2354 AUTOMOBILE ENGINEERINGL T P C

3 0 0 3

http://www.the-crankshaft.info/

UNIT I VEHICLE STRUCTURE AND ENGINES 9

Types of automobiles, vehicle construction and different layouts, chassis, frame and body, resistances to

vehicle motion and need for a gearbox, components of engine-their forms, functions and materials

UNIT II ENGINE AUXILIARY SYSTEMS 9Electronically controlled gasoline injection system for SI engines, Electronically controlled diesel injection

system (Unit injector system, Rotary distributor type and common rail direct injection system), Electronic

ignition system, Turbo chargers, Engine emission control by three way catalytic converter system.

UNIT III TRANSMISSION SYSYTEMS 9

Clutch-types and construction, gear boxes- manual and automatic, gear shift mechanisms, Over drive,transfer box, fluid flywheel torque converter, propeller shaft, slip joints, universal joints, Differential, and

rear axle, Hotchkiss Drive and Torque Tube Drive.

UNIT IV STEERING, BRAKES AND SUSPENSION SYSTEMS 9

Steering geometry and types of steering gear box-Power Steering, Types of Front Axle, Types ofSuspension Systems, Pneumatic and Hydraulic Braking Systems, Antilock Braking System and Traction

Control.

UNIT V ALTERNATIVE ENERGY SOURCES 9

Use of Natural Gas, Liquefied Petroleum Gas. Bio-diesel, Bio-ethanol, Gasohol and Hydrogen inAutomobiles- Engine modifications required Performance, Combustion and Emission Characteristics of SI

and CI engines with these alternate fuels Electric and Hybrid Vehicles, Fuel Cell.

Note: A Practical Training in dismantling and assembling of engine parts and transmission systems may be

given to the students.

TOTAL: 45 PERIODSTEXT BOOKS:

1. Kirpal Singh, Automobile Engineering Vols 1 & 2 , Standard Publishers, Seventh Edition ,1997, New

Delhi

2. Jain,K.K.,and Asthana .R.B, Automobile Engineering Tata McGraw Hill Publishers, New Delhi, 2002

REFERENCES:

1. Newton ,Steeds and Garet, Motor Vehicles , Butterworth Publishers,19892. Joseph Heitner, Automotive Mechanics,, Second Edition ,East-West Press ,1999

3. Martin W. Stockel and Martin T Stockle , Automotive Mechanics Fundamentals, The Goodheart

Will Cox Company Inc, USA ,1978

4. Heinz Heisler , Advanced Engine Technology, SAE International Publications USA,19985. Ganesan V.. Internal Combustion Engines , Third Edition, Tata Mcgraw-Hill ,2007

PEC DoME ME1353 Automobile Engineering 1


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Development of the Automobile

The progress of means for transportation has been intimately associated with theprogress of civilization.

Transportation on land has evolved from the slow moving oxcart to the high-speedautomobile.

A self-propelled vehicle used for transportation of goods and passengers on land iscalled an automobile or automotive or motor vehicle.

In general, modern automobile is a complex piece of machinery performing in asafe, economical and efficient manner.

It is comprised of a chassis and a body.

The chassis is made up of a frame supporting body, power unit, clutch or fluidcoupling, transmission system and control systems.

Wheels and tyres through suspensionsystem and axles, support the frame.

The power delivered by the power unit (engine) is transmitted through the clutchor fluid coupling, transmission system, and axles to the wheels.

The automobile is propelled on road due to friction between the tyre and roadsurface.

The various sub-systems are properly designed and held together for efficientfunctioning individually aswell as whole unit.

The protection and comfort is provided by the body and the suspension system.

The automobile has its limitations in regard to the load it can carry and the speedas well as the distance it can carry the load.

Types of Automobiles:

The different types of automobiles found on roads are presented in Chart in a comprehensive manner.

There are in general three main classifications of the various types of vehicle.

Based on the Purpose : 1) Passenger Vehicles Car, Jeep, Bus 2) Goods Vehicles Truck

Based on the Capacity: 1) Light Motor vehicles Car, Motor cycle, scooter 2) Heavy Motor vehicles Bus, coach, and tractor.

Based on the Fuel Used: 1) Petrol vehicles, 2) Diesel vehicles, 3) Alternate fuel

Based on the No. of wheels: 1) Two wheelers 2) Three wheelers 3) Four wheelers 4) Six wheelers 5)

Ten wheelers etc.

Based on the Drive of the vehicles: 1) Single wheel drive 2) Two wheel drive 3) four wheel drive. AlsoFront wheel drive, rear wheel drive and all wheel drive.

Based on the body style: closed cars, open cars and special styles

Based on the transmission: Conventional, semi-automatic, fully automatic

They are: (i) The single-unit vehicles or load carriers. (ii) The articulated vehicles. {iii)The heavy tractor

vehicles.

Classification of vehicles:



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Construction of an Automobile:

Basic structure, The Power plant, The transmission system, The auxiliaries, The controls, The

superstructure

Basic Structure:

This is the unit on which are to be built the remainder of the units required to turn it into a power

operated vehicle.

It consists of the frame, the suspension systems, axles, wheels and tyres

Power Plant: It provides the motive power for all the various functions which the vehicle or any part of it, may be

called upon to perform.

It generally consists of an IC engine which may be either of spark-ignition, or of compression ignition

type.

The Transmission system:

It consists of a clutch, a gear box, a transfer case, a propeller shaft, universal joints, final drive, and

differential gear.

The auxiliaries:

It consists of

supply system (Battery and generator), the starter, the ignition system, and ancillary devices ( Drivinglights, signaling, other lights, Miscellaneous items like radio, heater, fans, electric fuel pump,windscreen wipers, etc.)

The Controls

It consists of steering system and brakes.

The superstructure

It may be body attached with frame, frameless construction.

Layouts of Automobile:



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Main Parts of the Automobile:

The modern automobile can be categorized into two distinct sub-assemblies, the body and the chassis.

The body:

The main function of the body is to provide comfort and protection to the passengers besides giving a

good look.

The body includes the passenger compartment, the trunk, the bumpers, the fenders, the radiator grill, thehood, interior trim, glass and paint.

A wide variety of body styles, like two doors or four doors, sedans or hardtop, convertible or stationwagons are available for each chassis model.

A car body. A car chassis.

The chassis:

The chassis forms the complete operating unit and is capable of running with its own power.

It is an assembly of a vehicle without body.

The chassis includes frame, wheels, axles, springs, shock absorbers, engine, clutch, gearbox, propeller

shaft and universal joints, differential and half shafts, steering, brakes and accelerator, fuel tank, storagebattery, radiator, and silencer.

The engine is generally located at the front of the vehicle, followed by clutch, gear box, propeller shaft,

universal joint, differential, rear axle etc. The drive from the gearbox is transmitted through a short shaft to the front universal joint of the

propeller shaft.

From the propeller shaft it is conveyed to the rear wheels through a sliding splined type of universaljoint.

The bevel gear of the short shaft is driven by the rear universal joint.

This bevel gear meshes with a large bevel gear, which drives the two rear axle shafts through the

differential gear.

There are two methods of body and chassis construction, the separate body and chassis construction and

the integral construction.

In the separate body and chassis construction, the body is fitted to the chassis frame by means of anumber of body bolts, passing through the base of the body and the frame.

Pads of anti-quake or vibration materials such as rubbers are placed between the body and the frame atthe bolts to prevent quakes and rattles.



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Integral body construction

In the integral construction, the body and the chassis frame are combined as one eliminating the

mountings.

The integral construction is also called as chassis-less or unibody construction. Unlike commercial vehicles, which have a separate cab attached to a chassis, car bodies are now mostly

of integral construction, which is frameless mono-box construction.

These body shells are made up from pillars, rails, sills, and panels all welded together, and a reinforcingchannel-section under-frame with an extended sub-frame at the front is provided to replace the chassis.

Vehicle Assemblies

The main components of an automobile can be sub-grouped in the following assemblies: (i) Engine or

power plant, (ii) Running gear or basic structure, (iii) Driving system, (iv) Basic Control system, (v)

Electrical system, (vi) Accessories

Vehicle Assemblies

Engine:

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The prime movers used in almost all vehicles are either gasoline (petrol) or diesel engines.

Some specialty automobiles use a different type of engine.

The diesel engine consumes considerably less fuel than the gasoline engine, when operated at low

speeds.

An automobile engine with clutch and gearbox. The rotating combustion chamber engine is gaining popularity in small cars, and its use will probably

increase.

Turbine engines show promise, especially in commercial vehicles.

They are powerful, light weight and produce less hydrocarbons and carbon monoxides.

They are ideally suited to replace diesel engines in over-the-road load carrying vehicles.

Battery or electric vehicles are also being introduced to conserve fossil fuels and to minimise pollution.

The engine is located either in the front, mid-ship or rear.

Front mounted engines are more common in automobiles.

The engine contains mechanical parts, fuel system, cooling system, lubricating system and exhaust

system.

Figure shows an automobile engine with its clutch and gearbox.

The radiator is located at the front of the engine.

Running Gear:

The running gear comprises of the frame, suspension, springs, shock absorbers, wheels, rims and tyres.

The tyres are the only place where the automobile touches the road.

All of the engine power, steering and braking forces must operate through these tyre-to road contactareas.

Control of the vehicle is reduced or lost when the tyre does not contact the road or when skiddingbegins.

The suspension keeps the tyre in contact with the road as much as possible in all road conditions.

The suspension system must be strong enough to resist axle twisting from high engine power and frombrake reaction.

The suspension system consists of springs, shock absorbers and linkages or arms.

The frame is a rigid structure that forms a skeleton to hold all the major units together.

The wheels and tyre assemblies support the frame and the units are attached to it, through front and rearsuspension systems so as to follow the road irregularities.

Driving System:

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The driving system comprises of the clutch, transmission, driveline, differential and rear axle.

The driving system carries power to the driving wheels from the engine.

A clutch or torque converter is connected to the engine crankshaft to effectively disconnect or connect

the engine with the driveline.

The function of the transmission is to provide gear reduction, which produces high torque to start the

automobile from rest and drive it up the steep grades.

It also provides a reverse gear for backing the automobile.

A propeller shaft is required to transmit the engine power to the rear axle.

It has universal joints on each end to provide flexibility as the suspension position changes.

A differential incorporated with a rear axle, splits the incoming power to each drive wheel.

This also allows the drive wheels to turn at different speeds as they go over bumps and round corners.

Control System:

The steering and braking systems form the basic control system.

The steering gear controls the direction in which the front wheels are pointed.

The steering systems have some parts (i.e. the steering gear) bolted to the frame, some parts (i.e. thesteering column) bolted to the body and some parts closely integrated with the front suspension system.

The brake system slows down the speed of the vehicle or stops it at the driver's will. The entire brake system is located in the chassis.

The brakes are mounted inside the wheels.

The brake designs are either drum type or disc type.

Four-wheel disc brakes are more common in use.

Electrical System:

The electrical system is a part of both chassis and body.

The system includes the starting, charging, ignition, lighting and horn circuit.

Some electrical circuits are for engine operation, some for power transmission and others for lightingand operation of protective devices and accessories.

Accessories: Accessories are used to make driving more pleasant.

They include car heater, air-conditioner, radio, windscreen wiper, indicators etc.

Engine Position:

Front Engine:

There are a number of reasons for locating the engine at the front of a car as shown in Fig.

The large mass of an engine at the front of the car provides the driver protection in the event of a head-on collision, and engine-cooling system becomes simpler.

Also the cornering ability of a vehicle becomes better due to concentration of weight at the front.

Front-engine car

Rear Engine:

With the engines mounted at the rear of the vehicle the components like the clutch, gearbox and final

drive assembly can be installed as a single unit.

This arrangement requires the use of some form of independent rear suspension.

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Rear-engine layout is mostly confined to small cars, as this adversely effects on the handling of the

car.

Also it takes up a larger space in comparison to the front-engine car for carrying luggage.

However, a rear-engine layout increases the load on the rear driving wheels, providing better grip on theroad.

Figure presents one of the rear-engine cars.

The front seats are close to the front wheels than a front-engine car, and the floor is quite flat.

Rear-engine car Mid-engine car

Central and Mid-engine:

This engine location is generally confined to sports cars because this provides both good handling and

maximum traction from the driving wheels. This arrangement, however, is not convenient for everyday cars as the engine takes up space that is

normally occupied by passengers.

The mid-engine layout, shown in Fig., combines the engine and transmission components in one unit.

Drive Arrangements

Rear-wheel Drive. In this layout (Fig.) the rear wheels act as the driving wheels and the front wheels

swivel for steering of the vehicle.

The location of the main components in this arrangement makes each unit accessible.

A major drawback is the protrusion of the transmission components into the passenger compartment due

to which a larger bulge is produced in the region of the gearbox and a raised long tunnel down to the

centre of the car floor is formed to accommodate the propeller shaft. In this driving arrangement, the load transfer takes place from the front to rear of the vehicle during hill

climbing or acceleration providing good traction.

However, if the wheels lose adhesion, the driving wheels move the rear of the car sideways causing thecar to 'snake'.

Rear-wheel drive. Front-wheel drive Four-wheel drive

Front-wheel Drive:

This layout (Fig) is compact as the engine is mounted transversely and hence very popular for use on

cars.

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From space considerations the length of the engine is the critical, but the use of V-type engines for

larger power units has enabled to place the engine transversely.

Consequently, the placement of all the main components under the bonnet (hood), and the removal of

floor bulges and tunnel provide maximum space for the rear passengers.

Transverse mounting of the engine also simplifies the transmission.

The use of bevel-type final drive is eliminated; instead a simple reduction gear along with a differential

transmits the power through short drive shafts to the road wheels.

Each drive shaft is fitted with an inner and outer universal joint.

The outer joint accommodates the steering action and is specially designed to transmit the drive through

a large angle.

When the front wheels are used for steering, the driving force acts in the same direction as the wheel is

pointing.

Also the vehicle is being 'dragged' behind the front driving wheels. These features improve vehiclehandling especially in slippery conditions.

Mounting the main units in one-assembly some-times makes it difficult to gain access to

some parts, but this problem has largely been overcome now a days.

One disadvantage is that the driving wheels have fewer grips on the road when the vehicle is

accelerating and negotiating a gradient. This problem can be partly rectified by placing the engine well forward to increase

the load on the driving wheels, but the car is then liable to become 'nose-heavy' causing the

steering more arduous.

In cases where the driver's steering effort becomes excessive, the car is

often fitted with power-assisted steering.

Four-wheel Drive:

This arrangement (Fig.) is safer because of distribution of the driveto all four wheels.

The sharing of the load between the four wheels during acceleration reduces

the risks of wheel spin specifically on slippery surfaces like snow and mud. In addition the positive drive to each wheel during braking minimizes the possibility of wheel lock- up.

On an icy road or across off-highway a two-wheel-drive vehicle soon becomes non-drivable due to the

loss of grip of one of the driving wheels which causes the wheel to spin.

Specifying an Automobile

For describing an automobile, the various factors taken into consideration are:

(a) Type: Whether scooter, motor cycle, car, lorry, truck etc.

(b) Carriage capacity: Whether 1/4 tonne, 1 tonne, 3 tones, etc. or 2 seater, 4 seater, 6 seater, 30 seater,

40 seater etc.

(c) Make. The name allotted by the manufacturer. It is generally the name of the power unit indicatingkW or number of cylinders or shape of the engine block.

(d) Model: The year of manufacture or a specific code number allotted by the manufacturer.

(e) Drive, (i) Whether left hand or right hand drive, i.e. the steering is fitted on the left hand side or righthand side. (ii) Two wheel drive, four wheel drive, or six wheel drive.

As an example for specifying a truck, the typical specifications are given below: (i) Type : Truck 312 L

(ii) Capacity : 17,025 kg (iii) Drive: Right hand, 6x4 wheels, (iv) Make: Tata Mercedes-Benz(v) Model: OM 312

The Single-unit Vehicles or Load Carriers:



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These vehicles are conventional four-wheel types with two-axle design in which the front axle is a

steering non-driving axle and the rear axle is the driving axle.

With the advancement, many changes have been incorporated in the number of axles as well as the

driving system.

The Articulated Vehicles

A larger powered three-wheeler with single steering wheel in front and a conventional rear-driving axle

falls in this category.

It can be turned about its own tail due to the three-wheel construction and has a greater handling ability

in unusual places.

The coupling mechanism between semi-trailer and tractor in most of these vehicles is designed for

automatic connection and coupling up.

A lever is provided within the driver's approach for coupling operation.

A pair of retractable wheels in front can be raised or lowered automatically along with the coupling and

uncoupling operation.

The Heavy-tractor Vehicles

To move heavy loads tractor or independent tractor vehicles are used. They commonly operate in pair either in tendon or as puller or pusher.

The latter arrangement provides stability while descending appreciable gradients.

The digital figures like 4x2, 4x4, 6x4 etc. are commonly used in the classification of vehicles, where the

first figure represents the total number of wheels and the second figure the number of driving wheels.

By increasing the number of axles, the load per axle can be reduced, which protects the tyres fromoverloading and the road surface from damage.

Wheel axles are called "live" if drive and called "dead" if non-drive.

A live axle supports the payload and provides driving tractive effort, whereas a dead axle just supportsthe load.

The Motor Car

The motorcar carries passengers in the sitting position and also accommodates their luggage.

Space is also provided for the engine, the transmission system, the steering, the suspension layout, andthe braking system.

Finally, consideration is given to the styling of the body to meet various aesthetics and application

requirements.

The light motor vehicles designed to carry passengers and sometimes goods are broadly classified asfollows: (i) Saloon car (ii) Coupe (iii) Convertible (iv) Estate car (v) Pick-up.

Saloon Car:

Saloon cars have an enclosed compartment to accommodate a row of front and row of rear seats without

any partition between the driver and rear-passenger seats.

Saloon car. Hatchback car.



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A separate luggage space is made either at the front or the rear, based on the location of the engine.

One or two doors are provided on each side of the car, but if the car is a hatchback, a door replaces the

luggage space.

Coupe:

The couple is the outcome of changes is saloon-car design and has two doors, two front seats, and a hard

roof.

When two additional small seats are provided at the rear, the layout is known as "two-plus-two".

Coupe car. Convertible car.

Convertible :

Normally cars of this type have two doors and two seats, but sometimes two extra seats are alsoprovided.

Generally these have a soft folding roof and wind-up windows to make the compartment either open or

closed.

Estate Car:

In this type, the passenger roof of saloon is completely extended to the back end so that rear space isincreased.

For access a rear door is provided and sometimes the rear seats are designed to collapse to provide

additional space for carrying goods.

Estate car. Pick-up.

Pick-up:

This type of vehicle is generally classified as a two-door front-seating van with an open back (with or

without canvas roof) to carry mixed collection of goods.

Vans

Vans are light goods vehicles used for long distances or door-to-door delivery. They have seats in the front for the driver and for only one or two passengers.

The engine is usually located over or just in front of the front axle.

Medium-sized van.

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Hinged or sliding type doors are located on each side opposite the seats.

There are double doors at the rear of the van, which open outwards for easy loading.

Small vans combine the cab and the body with integral or nomo-box construction.

Large vans sometimes have separate cab and body, mounted on an independent chassis frame.

The rear axle may have twin road- wheels to have higher load carrying capacity.

Coaches

Coaches carry passengers traveling on long distance, and hence the interior is designed to provide the

best possible comfort and to minimize fatigue.

Seats are located facing the front to provide passengers the benefit of looking ahead.

For better visibility of passengers large paneled windows are provided on either side extending the full

length of the vehicle and across the back seats.

There is a door adjacent to the driver.

The passenger's doors are located opposite side of the driver's seat one towards the front and the other

towards the rear.

An emergency door is usually provided towards the centre on the opposite side of passenger's doors.

Most coaches have the two-axle arrangement, but sometimes an extra axle is also used.

As shown in the figure, engines may be mounted longitudinally in the front (Position 1), or in the mid-position horizontally (Position 2) or at the rear transversely (Position 3).

The location of the engine and transmission depends much on the length of the coach, the number of

passenger seats, the luggage space, and high or low floorboard and seat-mounting requirements.

Coach Double-decker bus

Double-decker Bus

These buses are used to transport large numbers of people having little luggage for short distances,usually in high-density traffic.

The double-decker bus occupies the minimum amount of road space.

These vehicles require a stair space for people to climb up to the upper deck (first floor).

The ground floor of the bus is arranged for seating and standing provision of the passengers.

The size and quality of seats are normally minimal due to short journeys.

Visibility for passengers inside the bus is provided sufficiently so that they can see where they are and

where to get off.

Most modern buses have two sets of doors. Passengers can enter through the front side door and pay

their fare, and can disembark by the rear side door. The engine is normally located transversely across the back of the bus or sometimes longitudinally to

one side at the back.

Lorries

Commercial vehicles used for the transportation of heavy goods are generally referred to as lorries.

These vehicles are grouped into two categories such as rigid trucks and articulated vehicles.

Rigid Trucks



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These vehicles unlike articulated vehicles are constructed to have all the axles attached to a single

chassis frame.

A simple truck has two axles and four wheels.

More number of axles and wheels are added to increase load-carrying capacity.

Classification of a Rigid Truck.

The number of wheel hubs and the number of drive axle hubs classify the rigid trucks as follows :1. A four-wheeler (4 x 2) truck with two driving wheels

2. A six-wheeler (6 x 4) truck with four driving wheels

3. A six-wheeler (6 x 2) truck with two driving wheels.4. An eight-wheeler (8 x 4) truck with four driving wheels

Rigid4x2 truck.

Rigid 6x4 truck. Rigid 8x4 truck.

Articulated Tractor and Semi-trailer: Articulated vehicles use a tractor unit for providing the propulsive power and a semi-trailer for carrying

the payload.

The tractor uses a short rigid chassis and two or three axles.

The front axle carries the steered road-wheels, and the rear axle is the driving (live) one.

The middle axle may either function as an additional drive axle or for dual steering.

The semi-trailer has a long rigid chassis with a single-axle, tandem-axle, or tri-axle layout at the rear

end.

All the trailer axles are dead axles.

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The front end of the trailer chassis is supported on the rear of the tractor chassis.

At this point it is free to swivel about a pivot known as the fifth wheel coupling.

The fifth-wheel coupling is the swivel mechanism used to attach the trailer to the tractor unit.

It contains a turntable, fixed to the rear of the tractor unit, to support the underside front end of the trailerwith a kingpin, which pivots between two half jaws.

For hitching and unhitching of the trailer and the tractor, the half jaws are moved either together to

secure the kingpin or apart to release it.

Rigid 4x2 tractor and single-axle 2 articulated Rigid 6x4 tractor and tandem-axle 4 articulated

trailers. trailers.

Rigid 6x2 tractor and tri-axle 6 articulated trailers.

Classification of Articulated Vehicle:

Different sizes of articulated tractor and trailer are available which can be classified as follows.

1. Four-wheeler and two-wheel trailer (rigid 4x2 tractor and single-axle 2 articulated trailer)

2. Six-wheeler tandem-drive-axle tractor and four-wheel trailer (rigid 6x4 tractor and tandem-axle 4articulated trailer)

3. Six-wheeler dual-steer-axle tractor and six wheel trailer (rigid 6x2 tractor and tri-axle 6 articulated

trailer)

Articulated Vehicles Compared with Rigid Trucks:

Advantages: The trailer and tractor units are interchangeable and a tractor can be immediately coupled to another

loaded trailer unit.

Articulated vehicles have much smaller turning circles than rigid trucks of the same length.

Disadvantages:

Less traction is available, because only the front end of the semi-trailer is supported by the tractor.

Tractor-and-trailer assembly have a tendency to jack about the fifth wheel under certain steered and

barking conditions.



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The tractor and trailer have only a small degree of pivoting in the vertical plane due to which they

behave unstable over rough ground.

Articulated-trailer wheels do not follow the same path as the tractor wheels, and tend to cut in or across

the road while turning a corner.

Body and Chassis

Automobile chassis and frame (structure) support various components and body of the vehicle in

addition to loads it is supposed to carry.

There are two principal types of auto body construction. The unibody construction, and the body and

chassis frame construction.

In the unibody or integral construction, individual metal parts are welded together to make up the body

assembly and provide overall body rigidity through an integral all steel welded construction.

The attachment provisions for the power train and suspension systems are provided by the under body

area, which also contributes to the strength of the vehicle. The floor plan and related sections become anintegral part of the chassis frame.

Although a separate frame is used on commercial vehicles, the majority of modern cars use integralconstruction, which produces a stronger and lighter vehicle and is cheaper on mass production.

Whether it is a car or a truck, the automobile structure has to withstand various static and dynamic loads. To appreciate the design and construction of a vehicles chassis, an understanding of the operating

environment is necessary.

The chapter presents the automobile structure as a whole.

The Chassis

The chassis frame supports the various components and the body, and keeps them in correct positions.

The frame must be light, but sufficiently strong to withstand the weight and rated load of the vehicle

without having appreciable distortion.

It must also be rigid enough to safeguard the components against the action of different forces.

The chassis design includes the selection of suitable shapes and cross-section of chassis-members.

Moreover the design looks into the reinforcement of the chassis side- and cross-member joints, and thevarious methods of fastening them together.

The materials most commonly used for frame construction is cold-rolled open-hearth steel, but

sometimes heat treated alloy steel that has equivalent strength with less weight is also used.

The steel is usually cold-pressed into channel sections so that the frame becomes strong and light.

Heavy-duty trucks sometimes use frames made from I-sections and other structural forms.

Figure illustrates the two views of a typical frame.

A typical vehicle frame

Chassis Operating Conditions



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The design of an automobile chassis requires prior understanding of the kind of conditions the chassis is

likely to face on the road.

The chassis generally experiences four major loading situations that include,

(i) vertical bending, (ii) longitudinal torsion, (iii) lateral bending, and (iv) horizontal lozenging.

Vertical Bending:

Considering a chassis frame is supported at its ends by the wheel axles and a weight equivalent to the

vehicle's equipment, passengers and luggage is concentratedaround the middle of its wheelbase, then the side-members are subjected to vertical bending causing

them to sag in the central region.

Longitudinal Torsion:

When diagonally opposite front and rear road-wheels roll over bumps simultaneously, the two ends of

the chassis are twisted in opposite directions so that both the side and the cross-members are subjectedto longitudinal torsion (Fig.), which distorts the chassis.

Longitudinal torsion Lateral bending Lozenging.

Lateral Bending.

The chassis is exposed to lateral (side) force that may be due to the camber of the road, side wind,

centrifugal force while turning a corner, or collision with some object.

The adhesion reaction of the road-wheel tyres opposes these lateral forces.

As a net result a bending moment (Fig.) acts on the chassis side members so that the chassis frame tends

to bow in the direction of the force.

Horizontal Lozenging:

A chassis frame if driven forward or backwards is continuously subjected to wheel impact with road

obstacles such as pot-holes, road joints, surface humps, and curbs while other wheels produce thepropelling thrust.

These conditions cause the rectangular chassis frame to distort to a parallelogram shape, known as

'lozenging' (Fig.).

Chassis Frame Sections

During movement of a vehicle over normal road surfaces, the chassis frame, is subjected to both bending

and torsional distortion as discussed in the previous section. Under such running conditions, the various chassis-member cross-section shapes, which find

application, include. (i) Solid round or rectangular cross-sections, (ii) Enclosed thin-wall hollow roundor rectangular box-sections, (iii) Open thin-wall rectangular channeling such as 'C, T, or 'top-hat'

sections.



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Chassis-member sections.(A) Square solid bar (B) Round solid bar (C) Circular tube with longitudinal slit (D) Circular closed tube (E) C-section

(F) Rectangular box section (G) Top-hat-section (H) I-section (I) Channel flitch plate.

Side-member Bending Resistance:

The chassis side-members, which span the wheelbase between the front and rear axles must be able totake the maximum of the sprung weight.

The sprung weight is the weight of the part of the vehicle supported by the suspension system.

The binding stiffness of these members must resist their natural tendency to sag.

The use of either pressed-out open-channel sections or enclosed thin-wall hollow round or rectangular

box-sections can provide the maximum possible bending stiffness of chassis members relative to their

weight. A comparison of the bending stiffness of different cross-sections having the same cross-sectional area

and wall thickness is presented in Fig.

Considering a stiffness of 1 for the solid square section, the relative bending stiffness for other sections

are,

Square bar 1.0

Round bar 0.95

Round hollow tube 4.3

Rectangular C-channel 6.5

Square hollow section 7.2

Practically, a 4 mm thick C-section channel having a ratio of channel web depth to flange width of about3:1 are used as chassis side-members.

This provides a bending resistance of 15 times greater than that for a solid square section with the same

cross sectional area.

For heavy-duty applications, two C-section channels may be placed back to back to form a rigid load-

supporting member of I-section (Fig. H).

To provide additional strength and support for an existing chassis over a highly loaded region (for

example, part of the side-member spanning a rear tandem-axle suspension), the side-members may havea double-section channel.

This second skin is known as a flitch frame or plate (Fig. I).



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Side-and Cross-member Torsional Resistance:

The open-channel sections exhibit excellent resistance to bending, but have very little resistance to twist.

Therefore, both side and cross-members of the chassis must be designed to resist torsional distortionalong their length.

Figure C to F illustrates the relative torsional stiffness between open-channel sections and closed thin-

wall box-sections.

Comparisons firstly between the open and closed circular sections and secondly between the rectangular

sections are made, considering the open section has a resistance of 1 in each case.

Longitudinal split tube = 1.0

Enclosed hollow tube = 62.0

Open rectangular C-channel = 1.0

Closed rectangular box-section = 105.0

This clearly explains the advantages of using channel sections over the hollow tube due to high torsional

stiffness.

The chassis frame, however, is not designed for complete rigidity, but for the combination of both

strength and flexibility to some degree.

Chassis Frame Design

A frame suitable for a light truck or minibus is shown in Fig.

The frame uses a non-independent suspension system and is consisted of two channel-shaped side-members, which are joined together with the help of a series of cross-members.

These cross-members are placed at points of high stress and are cold-riveted to the side-members.

The channel section must be chosen to minimize deflection.

Most frames of light vehicles are made of low-carbon steel having the carbon content of 0.15 - 0.25

percent.

Since the load varies at each point of the frame, so to reduce its weight either the depth of channel is to

be decreased, or a series of holes are to be drilled along the neutral axis in the regions where the load is

relatively less.

Frame for light truck.

To safeguard the frame against lozenging, gusset plates are fitted to reinforce the joins between the

side- and cross-members, or an *X' type bracing is placed between two or more of the cross members.

The frame shown in Fig. does not have sufficient rigidity against torsion, so the body has to meet this

requirement.

If the body is not designed to resist these stresses, the problems like movement between doors and

pillars, broken windscreens and cracking of the body panels may occur.

Since body jigs for pressing the integral bodies are generally very expensive, it is usual to use a separate

chassis frame when the production of a given model is not large in number.

Most of the cars have independent suspension, so the frame must be extremely rigid at the points ofjoining the main components with the body.

To achieve this, box-section members are welded together and suitably reinforced in the regions of high

stress (Fig.)

Figure presents a backbone frame, an alternative construction to the conventional rectangular frame.



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In this construction two longitudinal box section members are welded together at the centre and

separated at the front and rear to accommodate the main components.

A series of out-rigger frame members are welded to the spine to support the floor of the body.

Box section frame. Backbone-type frame.

Energy-absorbing Frame:

The chassis frames in older designs were made very stiff in order to improve safety for the occupants of

a car when involved in a collision.

This is not truly correct because on impact the structure provides the occupants an extremely high

deceleration and the force acting on the human body as it dashes against a hard surface is likely to causeserious injury or death.

Energy-absorbing frame

This problem has been overcome in most modern frames by constructing the front- and rear-end of theframe in a manner so that it crumples in a concertina manner during collision and absorb the main shock

of the impact.

Actually the body panel in the vicinity of these crumple zones are generally damaged beyond repair, butthis is a small price to pay to minimize the injury to the occupants.

Figure 21.9 illustrates the principle of designing a frame to absorb the energy of front-end and rear-endimpacts.

Chassis Side- and Cross-member Joints

Cross- and side-members are joined together to form a rectangular one-piece frame.

Open-channel sections are commonly used for cross members, but for special applications some times

tube sections are also used. The individual channel members do not have adequate stiffness against twist, but when joined together

they form a relatively rigid structure capable of withstanding both bending the torsional loading.

The attachment of the cross-members to the side channels needs special attention, because the junction

points are subjected to maximum bending as well as torsional stresses.

Commercial-vehicle side-members are generally made from flat strip pressed into C-channel of

appropriate section.



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Chassis side- and cross-member reinforcement joints(A) Top-hat-section cross-member joined to side-member flanges and web

(B) C-section cross-member with extended flanges joined to side-member flanges

(C) C-section cross-member with reinforcement gussets joined to side-member web

(D) 'Top-hat'-section cross-member with alligator-jawed enforcement joined to both flanges and web

(E) Tubular-section cross-member with reinforcement flat bracket joined to side-member web.

The web section of C-channel resists any vertical bending and the top and bottom flanges prevent the

web from buckling along its length and provide additional resistance to both bending and torsionalstresses.

Since the flanges or the outer regions of the web are the maximum stressed parts of the channel, any

attachment should, therefore, preferably be in the web section. In actual practice, joints are madebetween flanges or a combination of both web and flange joints for convenience.

Figure A shows a cross-member of 'top-hat' section joined between the web and both flanges.

Sometimes just the web alone is joined, or alternatively the upper and lower flanges from the

attachments.

These joints are mostly used for light and medium-duty work.

Figure B shows a pure channel-section flange joint and the cross-member flanges have been widened to

provide reinforcement to the joint. This joint is used only for medium duty work.

Figure C shows side- and cross-member joint where the cross-member has a lap-welded end gusset

(triangular) bracket, joined to the side channel web only.

This method of joint reinforcement allows the flange to be out of holes, which generally serve as a pointfor stress concentration. These joints are widely used for heavy-duty trucks.

Figure D shows a pressed-out two-piece cross-member that opens up at the end to form an alligator-jaw

flange-and-web reinforced joint.

This form of cross-member and attachment is often used when it is necessary to have an under slung

bridging member to clear the engine's sump-pan.

Figure E shows a round tube-section cross-member with a fillet-welded rectangular end bracket joineddirectly to the side-member web.

Tubular-section cross-members are specifically suitable for withstanding both bending and torsional

stresses at concentrated points, such as spring shackle-hangers and tandem-axle suspension pivotingsupports.

Chassis Side- and Cross-member fastening:



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The service life of a chassis structure also depends on the type of joints in which the various members

are fixed together.

Riveting, bolting, and lap welding are the three different methods of joining available.

Chassis-member joint

(A) Riveted joint (B) Bolted joint (C) Lap-welded joint.

Riveted Joints:

Cold-riveted joints (Fig. A) are most commonly used to join two chassis members.

The unformed rivet has a shank and a set head. In the process of cold forging the second head, the shankspreads out in a pair of holes in the members to be joined, and occupies any clearance existing in the

hole.

These joints provide a moderately large compressive force between the plates so that relative movement

is prevented.

Bolted Joints:

For heavy-duty applications, the bolted joints (Fig. B) are preferred, specifically if additionalcomponents are to be fastened.

The tightening of nuts and bolts sets up compressive forces between the plates, so that the corresponding

friction forces generated prevent relative movement.

If the nuts are not adequately tightened, or if they become loose due to continuous flexing and vibration

of the chassis, relative movement between the joined plates due to any clearance may cause to fretting,

noise, corrosion and finally fatigue failure.

Welded Joints:

Generally chassis side- and cross-members are not welded together. However subsections are frequently joined by lap welding.

The problem with welded joints (Fig. C) is that they produce thermal distortion and, in case of the rigid

frame, high stress concentration develops at the joints, which may eventually crack.

Additionally, welding destroys any previous heat treatment around the joint thereby weakening the

structure.

Although precautions are available to prevent these problems but they are expensive to apply.

Body work and Integral Construction:

Some Terminology Pertaining to Body:

Cab: It is the driver's cabin, which may be a closed region separated from the rest of the body (as in

truck) or may be an open region being a part of the body (as in car). Fascia. It is the frontage of the vehicle visible to the driver. It includes the dash board (instrument

board), tape recorder housing, globe box etc.

Dash board. It houses various indicators such as fuel level indicator, engine temperature indicator,

speedometer, voltmeter, ammeter, odometer, air-conditioner's control panel, ignition switch, light

switches, side indicator switch, various controls switches, automatic operation switches, etc.

Legroom. It is the space provided for the movement of legs of the driver and passengers. Sufficient

legroom is essential for a comfortable driving, riding and traveling.



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Headroom. It is the vertical distance inside the body between the floor to ceiling. This dimension is

based on the stability consideration of the vehicle, as position of CG from the ground level depends on

this height.

Shoulder Room. It is the clear horizontal distance available inside the body.

Boot Space. This is the storing space available below the rear hood.

Body Work:

Requirements:

The body work has to be structurally strong, easily accessible and of good finish.

Some of the important considerations for a good body work include the following :1. Attractive body styling.

2. Upholstery work should be well trimmed and comfortable.

3. Body structure should be rust preventing.4. Paint work and other finishing should be appealing.

5. Body should be structurally strong and light. Therefore, construction material should be of light

weight, strong and cheap.

6. Doors and windows should be conveniently located, and easier to operate.7. Controls should be located at convenient positions and should be easily approachable.

8. Arrangement of hand controls and foot pedals should be fool proof and untiring.

9. Provision of sufficient space for accommodating accessories, instruments and controls.10. Driver's and passengers seats should be comfortable and adjustable, and should be conveniently

located.

11. Interior cabin should be dust proof and sound proof.12. Body should be equipped with sufficient safety provisions.

Main Parts:

The body work includes the following main parts.1. Body safety,

2. Bonnet,

3. Side pillars,4. Rear hood,

5. Front side panel,

6. Rear side panel,7. Door pillars,

8. Windshield pillar,

9. Rear quarter pillar,

10. Body sill,11. Roof,

12. Door Panels,

13. Front bumper,14. Rear bumper

Integral Construction:

Around 1934, the all-steel body construction was introduced so that a separate frame could be

eliminated.

This frameless or integral construction provides a stiff, light construction, which is specifically suitablefor mass-produced vehicles.

Since 1945 light cars have used integral construction.

When suitably designed the body shell is capable of withstanding the various frame stresses.



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Integral body construction.

Figure illustrates some of the forces that act on a car body and the arrangement of the various bodypanels to form a unitary structure of sufficient strength to resist these forces.

The floor and roof panels resist the sagging effect caused by the weight of the occupants.

Since these two members are widely spaced, thin sheet metal is used to form a strong and lightweightbox like structure.

To increase torsional stiffness of the body the scuttle at the front is strengthened and behind the rear seat

squab cross ties are used or a ribbed metal panel is fitted.

The thickness of the sheet metal depends on the stress to be taken by the panel.

Structural members such as sills, rails and pillars are often about 1.1 mm thick, whereas panels such as

the roof are 0.9 mm thick.

Component attachment points are reinforced with thicker section. Some cases use a separate sub-frame to mount engine and other members.

Sometimes this sub-frame is connected to the body by rubber insulation mountings.

Very low (0.1 percent) carbon steel is used to provide extremely good ductility required for the pressingof the panels.

The low strength, 278 MPa, of this steel requires stiffening of the structural members, which is achieved

by spot welding into position of intricate sections, formed out of thin steel sheet.

A modified construction is necessary in case the roof cannot be fully utilized as a compression member.

This situation occurs on drop-head coupe models and where a sunshine roof, or very thin door pillars are

used.

To achieve the required strength in these cases a strong underbody frame is used.

In addition, the body-shell parts, which are subjected to torsion, are provided with extra stiffness.

A body-shell is normally fabricated either by spot-welding the panels, pillars and pressings together toform a strong box, or by buildings a skeleton or space frame (Fig.), which provides a high structural

strength.

To this frame is attached the shell, aluminium or glass-reinforced plastic (GRP) body panels, doors,roof, etc.

Steel is the most common material used for manufacturing of vehicle in high volume, because

production costs become lower once the initial investment on body jigs and other facilities has been

recovered.



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The vibration of the panels, which produces an unwanted noise called drumming, is avoided by fixing a

sound-damping material on the inside of the panels.

The driver and passengers are enclosed in a rigid cell for their safety.

Space frame Crumple zones

The front and rear of this rigid compartment are fixed with sub-frames, which are designed to concertina

on impact (Fig.).

The crumple zones of the body absorb the shock of a collision so that the rate of decelerationexperienced by the occupants is reduced.

Nowadays, it is mandatory to obtain an impact test certificate from an approved centre before vehicles

can be sold.

The vehicle can pass this severe destructive test, provided a required standard on the level of safety of

the occupants is witnessed.

The doors must remain closed during impact and must open after the test.

The inclusion of this test feature justifies the common use of special anti-burst locks at present.

The vehicles safety belts, or some other approved body restraint system, must be provided for the driver

and all passengers.

These belts must be securely anchored to suitable strengthened parts of the body.

Internal body trim, fittings and controls must all conform to safety standards. The improvements made during recent years in the design of parts such as steering wheels and control

knobs have considerably increased the safety of the occupants.

Body Shape

Body shape depends on a number of factors; these include appealing shape to the buyer, providing

comfort, and a good performance during its movement through the air.

A car body with the aerodynamic shape passes with least resistance through the air; as a consequencethe fuel economy is improved.

For a vehicle without aerodynamic shape of the body, a lot of engine power is required to drive through

the air.

This expression shows that the air resistance increases very fast as the velocity of the vehicle relative to

the air becomes high (Fig.).



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The air resistance of a vehicle is measured through wind tunnel tests.

Knowing the cross-sectional area of the vehicle and its velocity relative to the air, aerodynamic drag

coefficient (Cd) can be determined.

Values of Cd for different types of vehicle are given in Table.

Force required for overcoming air resistance

A streamlined body has a low Cd so that it provides minimum resistance when passes through the air.

Since most of the resistance is caused by the low-pressure region at the rear of the vehicle, the body

shape returns the air to this region with the minimum of turbulence after the air has flowed over thebody.

Since resistance is directly proportional to the cross-sectional area, a low and sleek sports-type car canprovide good performance.

Table: Aerodynamic drag coefficient for different types of vehicles.

Types of VehiclesCd

(dimensionless)

Racing Car 0.25 - 0.30Passenger Car 0.30 - 0.60

Convertible 0.40 - 0.65

Bus 0.60 - 0.70Truck 0.80 -1.00

Tractor and Trailers 1.25-1.35

Motor Cycle 1.75-1.85 Separation of flow at the downstream side of the vehicle, and the difference in pressure on the up stream

and down stream side of the vehicle give rise to the phenomenon called wake.

As wake is undesirable, it should be avoided or minimized by proper profiling of the body.

The contour of body should be such that in addition to minimizing drag coefficient, the separation of

flow on any part of the body should not occur and the above pressure difference should be minimum.

Wake depends on the body shape and drag coefficient depends on wake. To minimize wake rear spoiler

is added to aerodynamic styling of the body.

Several improvements are incorporated in the body to reduce air drag.

Air dam and spoiler.

(A) Air dam. (B) Rear spoiler.



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These include the recessing of protruding items such as door handles and the shaping of the body below

the front bumper to form an air dam (Fig. A).

Airflow control devices are sometimes fitted to the rear of the vehicle.

These devices, depending on their shape and location, smooth out the air flow to reduce the disturbance,or act as a spoiler to deflect the air upwards so that the adhesive force acting on the rear wheels is

increased (Fig. B).

Although these arrangements are beneficial on racing cars, their usage on domestic cars may be regardedas 'image creation' embellishments.

Vehicle Components Attachment and Location

The automobile has the following essential components and it is important to have knowledge of their

mountings and locations: (a) engine (b) gearbox (c) clutch (d) propeller shaft and universal joints, (e)

drive shafts, (f) final drive, (g) steering, and (h) brakes.

Engine, Clutch, Gearbox, and Final-drive Support Mountings

The engine, clutch, and gearbox combination and (with front-wheel drive layouts) the final drives are

usually supported on a three-point mounting system.

This configuration permits the best possible freedom of movement about an imaginary roll-centre axis. However this arrangement provides certain rigidity for withstanding the torque reaction when the engine

is developing power.

To mount various components, rubber blocks bonded on to steel plates are generally used.

Sometimes these mounts are positioned at 45 degrees to the horizontal so that the rubber is subjected to

a combination of both compression and shear elastic distortion.

This method of loading the rubber provides a flexible mount, whose stiffness increases with increase in

driving torque.

Mountings are used to (a) absorb the torque reaction during transmission of power, (b) cushion therocking movement created by the out-of balance forces of the engine, (c) prevent transmission of

vibrations from engine and transmission systems to the body structure, and (d) accommodate any

misalignment of the engine or transmission units relative to the body frame.

Location and Mounting with Front-mounted Engine and Rear-wheel Drive:

In this layout of the vehicle, the engine, clutch and gearbox are bolted together in series.

The complete assembly is then supported between both the front-wheel suspensions by a three pointmounting system.

The gear box is connected to the final drive by the propeller shaft and universal joint.

In case the final drive is unsprung, as with the rigid rear axle (Fig. A), the drive is transferred within theaxle-casing to the road-wheels.

On the other hand, if the final drive is sprung and mounted underneath the body structure by a three-

point rubber mounting (Fig. B), the drive is transferred through each drive shaft and universal joint to

the independently suspended road wheel.



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Front-mounted engine and rear-wheel drive

(A) Unsprung rigid rear axle

(B) Independent rear suspensions and rear-wheel drive and rear-wheel drive.

Advantages:

1. A front-mounted engine provides a forward centre of gravity, which tends to stabilize

the car handling at speed.2. Certain amount of safety is provided against crash to the driver and passengers with

the engine mounted in front.

3. The radiator can be best located in the front to utilize the air-stream ramming effect

as the car moves forward4. A very small slope of the propeller shaft between the gearbox and the final drive permits the use of

simple Hooke's universal joints.5. While climbing a steep slope, a partial weight transfer to the rear wheels takes place improving tyre-

to-road grip.

6. The control linkages from the clutch, gearbox, and engine to the driver's cabin can be simple and

direct.7. The longitudinal-mounted engines at the front are easily accessible for routine maintenance.

8. Tyre wear on wheels, which only steer, is marginally less than on wheels, which both steer and

drive.

Disadvantages:1. A single or split propeller shaft with universal joints and supporting bearings between the front-

mounted gearbox and the rear axle may generate vibration, drumming, howl, and other noises under

certain operating conditions.

2. The floor tunnel, necessary to provide clearance for operation of the propeller-shaft system, may

interfere with passenger leg-room.3. In case of a rigid casing for the axle and final drive, more weight is not supported by the suspension

system so that the quality of the suspension ride may be reduced.

4. Additional universal joints and drive shafts are required for independent rear suspension.5. A rear-wheel-drive vehicle, when stuck in mud, tends to plough further into the ground when

attempts are made to drive away.



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Fig. Front-mounted engine and front-wheel drive.A. Transverse front-mounted engine and front-wheel drive.

B. Longitudinal front-mounted engine and front-wheel drive.

Location and Mounting with Front-mounted Engine and Front-wheel Drive:

This kind of layouts has the engine, clutch, gearbox, and final drive built together to form a singleintegral assembly.

In the transverse engine arrangement (Fig. A), the engine, clutch, and gearbox are bolted together in

series.

The final drive forms part of the clutch bell housing and gearbox casing.

The drive shafts and their respective universal joints are placed on each side of the final-drive housing,

to transfer the propelling power to each front drive stub-axle and road-wheel.

The longitudinal-mounted engine (Fig. 21.18B) has the engine, clutch, final drive, and gearbox bolted

together in that order.

The power from the final drive is transferred to each drive shaft and then to the road wheels through its

universal joint.

In both layouts, the complete power and transmission-unit assembly is supported by a three point rubber

mounting arrangement.

Advantages:

(a) The road adhesion and acceleration are improved due to the concentration of engine, transmission,

and final-drive components on the front driving wheels.

(b) The elimination of the propeller shaft permits the use of a low floor profile and also in some casesthe centre of gravity of vehicle is lowered.

(c) The engine, gearbox, clutch, and final drive form a compact single assembly, which can be handled

easily.(d) Front-wheel-drive steered wheels are capable of driving out of pot-holes, ditches, loose soil, and

boggy ground.

(e) Simplified rigid or independent rear suspension can be used requiring minimum service.

(f) A transverse-mounted engine provides relatively more passenger room.(g) Fixing the final drive to the power and transmission unit reduces the unsprung weight, so that the

quality of ride is improved.

(h) Since steering and driving road-wheels are combined, the wheel traction and road holding on bendsare improved.

(i) Due to a forward centre of gravity, handling characteristics such as oversteer and understeer have a

tendency towards more desirable understeer response.(j) The actuating linkages for engine, gearbox, and clutch become simple.

Disadvantages:

(a) To drive the live stub-axles, constant-velocity universal joints are required to be built

into the front suspension and steering system.(b) Initial costs of the total arrangement are generally higher and also the maintenance, because of the

replacement of components, is usually faster compared with the conventional rear-wheel-drive cars.

(c) During hill-climbing the centre of gravity of the car is moved slightly backwards causing less weightto act on the front driving wheels so that reduction of tyre traction results.



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(d) Because of all the power and transmission components positioned in front, the driver and passengersmay be subjected to more noise, heat, and fumes.

(e) The concentration of weight at the front gives rise too slightly heavier steering.

Location and Mounting with Rear-mounted Engine and Rear-wheel Drive:

In this case also the power and transmission assembly is supported on the three-point mounting.

In one layout of this type of location and mounting, a horizontally opposed four-cylinder engine is

connected in series with the clutch, final drive, and gearbox (Fig. 21.19A).

At the final drive housing the power is split and transferred by the drive shaft and coupling joints to the

rear road-wheels.

Fig. Rear-mounted engine and rear-wheel drive.

A. Longitudinal rear-mounted engine and rear-wheel drive.

B. Transverse rear-mounted engine and rear-wheel drive. In an alternative layout, a transverse in-line four-cylinder engine is connected in series with the clutch,

gearbox, the final drive, where the power flow is divided and transferred to each rear driving road-wheel

through the respective couplings and drive shafts (Fig. 21.19B).

Advantages:1. Due to concentration of weight on the rear wheels, driving traction during climbing hills is improved.

2. The increased weight distribution at the rear end allows the rear wheels to be designed to take a larger

proportion of braking.3. Passengers do not experience excessive noise, heat, and fumes, as these are left behind during movement

of vehicle in the forward direction.

4. With rear-wheel drive, the steering and suspension at front-wheels can be simplified and steering

interference does not exist due to worn transmission components.5. The weight on the front wheels being relatively less, steering is somewhat lighter than for other

arrangements.6. The exhaust pipe and silencer system can be short, direct, and compact.

Disadvantages:

1. The control linkages for the engine, gearbox, and clutch are required to be extended

to the driver's position.2. To provide sufficient side-clearance for the front steered wheels the luggage-boot width

reduces.

The relatively large weight at the rear tends to make the car unstable at speed.3. The relatively lighter front end tends to make the car over-steer and very sensitive to crosswinds.

4. Installation of the cooling-system radiator and arrangement of its effective air supply are difficult.5. Provision of interior heating for the driver and front passenger may be more complicated.

Servicing and repairs of the power and transmission units are more difficult, and time

taking as they are not easily accessible.

6. In this layout the most convenient location for the petrol tank is in the front, which

may tend to become a safety hazard in a collision.

Performance Parameters:



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In order to carry out effective performance calculations of the automobiles techniques have to be developedto cater for a number of performance parameters.

The power delivered by the engine is finally made available at the drive wheels as propulsive force.

The motion of a vehicle on a level road is resisted by air and rolling resistances.When tractive effort, the force available at the contact between driving wheels and road is more than the

total resistance on level road, the surplus tractive effort contributes for acceleration, climbing

gradients and draw-bar pull.

Calculation of equivalent weight, transmission efficiency, the position of centre of gravity, stability of avehicle on a gradient and dynamics of a vehicle moving on banked track are also equally important for

the evaluation of vehicle performance.

Vehicle Drag:

Vehicle drag is a force, which resists motion and is due to the deformation of the wheel and the ground (the

later being negligible for vehicles on normal road) and the aerodynamic effects of air flow over thevehicle.

The motion of vehicle for the straight-ahead position is considered, ignoring the effect of cornering for

simplicity.

Deformation of the Wheel: The pneumatic tyre is particularly suitable for use in road vehicles because of its contribution to

comfort, its excellent adhesion properties and because it does not break up the road surface to the extent

of a more rigid wheel.

However, the vehicle load and tractive effect are not carried without deformation.

In the case of a pneumatic tyre on the hard surface of a modern road, the deformation of the tyre

accounts for 90 - 95% of the rolling resistance of a vehicle.

The term rolling resistance is the drag force of the vehicle excluding that caused by aerodynamic effects.

Windage and slippage losses are small in comparison.

The distortion, of the tyre tread as it passes through the contact area results in a hysteresis loss, whichmanifests itself as heat and a rise in the temperature of the tyre.

The rolling resistance due to the hysteresis loss from the deformed tyre is primarily a function of tyre

deflection, caused by the load carried by the tyre.

Other parameters affecting the rolling resistance of a pneumatic tyre on a hard surface are tyre

temperature, inflation pressure, vehicle speed, tread thickness, the number of plies, the mix of the rubber

and the level of torque transmitted.

The rolling resistance increases with vehicle speed if all other parameters are maintained constant.

However, in practice, an increase in vehicle speed results in an increase in tyre temperature and

pressure.

The net result is, for a given tyre, a near constant rolling resistance with vehicle speed until such a speed

is reached (Fig.) that a discernible standing wave sets in the tread in the wake of the contact area.

The resistance to motion of the tyre increases very rapidly in this condition and the energy dissipated inthe deformation caused by the standing wave is capable of destroying the tread in a very short time.

It is usual, therefore, to specify a safe maximum speed for the particular tyre that is well below the speed

at which the standing wave sets in.



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Fig. Vehicle speed vs. rolling resistance.

Since there is direct relationship between load on a tyre, deflection and the hysteresis loss and since

vehicle weight equates load on all the wheels, the rolling resistance, Rr, is expressed in terms of the non-dimensional rolling coefficient, a, as,

Rr = aW.

Since the rolling resistance cannot be taken as constant throughout the speed range of the vehicle, it is

usual to add another coefficient, b, such that, Rr = (a + bV)W, where V is the vehicle speed and W is the

weight of the vehicle.

Vehicle performance calculations are usually conducted at the full throttle condition with high and fairlyconstant torque level through out the lower vehicle speed range where the rolling resistance is important.

The rolling resistance coefficients used, therefore, should be at the appropriate torque level.

Air Flow over the Vehicle:

The moving vehicle, in displacing the surrounding air, has a resultant resisting force, called the

aerodynamics drag (simply air resistance), and is imposed upon it.

It is usual to express this drag non-dimensionally using the aerodynamic drag coefficient, Cd

where p is air density, usually taken as 1.23 kg/m3 and V is vehicle speed (m/s) relative to the air.

In order to nominate a suitable characteristic area, detailed study of the composition of the aerodynamic

drag is necessary, which is due to three separate types of aerodynamic effects.

(i) The air flow in the boundary layer resulting in the loss of momentum of the mainstream. This effect produces 'skin friction' drag.

(ii) A component from the downstream of the trailing vortices behind the vehicle, resulting in the

induced drag.

(iii) The 'normal pressure' drag, which may be found by the integration of the product (normal pressure x

area) around the vehicle. This produces a net force opposing the motion of the vehicle because the

separation of flow at the rear of the vehicle results in a lowering of the pressure on the rearward facingsurfaces.

The skin friction drag and the induced drag are usually small in comparison to the normal pressure drag.

However, the skin friction drag can reach significant proportions in the case of a long vehicle, such as a

coach. Since the major contributor to the aerodynamic drag is the normal pressure drag, the relevant

characteristic area is the 'projected frontal area', A, of the vehicle.

A = 0.8 (vehicle height above ground level X body width).

However, such an approximate expression is no real substitute for a precise measurement and its use

should be avoided.

The aerodynamic drag coefficient, Cd, for a particular vehicle can be considered as constant if side wind

effects are ignored.

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Power for Propulsion:

The motion of a vehicle moving on a road is resisted by aerodynamic forces, known as wind or air

resistance, and road resistance which is generally termed as rolling resistance.

In addition to these two types of resistances, the vehicle has to overcome grade resistance when it movesup on a gradient, because the weight of the vehicle is to be lifted through a vertical distance.

Hence, the power required to propel a vehicle is proportional to the total resistance to its motion and the

speed.

The calculation of engine power takes into account the losses in transmission.

Hence required engine power,

Air Resistance

This is the resistance offered by air to the movement of a vehicle.

The air resistance has an influence on the performance, ride and stability of the vehicle and depends

upon the size and shape of the body of the vehicle, its speed and the wind velocity.

The last term should be taken into account when indicated, otherwise it can be neglected. Hence in

general, air resistance,

Rolling Resistance:

The magnitude of rolling resistance depends mainly on

(a) the nature of road surface,(b) the types of tyre viz. pneumatic or solid rubber type,

(c) the weight of the vehicle, and

(d) the speed of the vehicle.

The rolling resistance is expressed as , where W = total weight of the vehicle, N and K =

constant of rolling resistance and depends on the nature of road surface and types of tyres = 0.0059 for

good roads = 0.18 for loose sand roads = 0.015, a representative value.

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A more widely accepted expression for the rolling resistance is given by

where V = speed of the vehicle, km/hr and Mean values of a and b are 0.015 and 0.00016 respectively.

Grade Resistance:

The component of the weight of the vehicle parallel to the gradient or the slope on which it moves istermed as 'grade resistance'.

Thus it depends upon the steepness of the grade. If the gradient is expressed as 1 in 5, it means that for

every 5 metres the vehicle moves, it is lifted up by 1 metre.

Hence, grade resistance is expressed as

Traction and Tractive Effort:

The force available at the contact between the drive wheel tyres and road is known as 'tractive effort'.

The ability of the drive wheels to transmit this effort without slipping is known as 'traction'. Hence usable tractive effort never exceeds traction. The tractive effort relate to engine power as follows.

When the tractive effort F>R, the total resistance on level road, the surplus tractive effort is utilized foracceleration, hill climbing and draw-bar pull.

Relation Between Engine Revolutions (N) and Vehicle Speed (V)

Thus, N/V ratio depends upon the overall gear ratio and wheel diameter.

A vehicle with four different gears has four different values of N/V ratio.

The N/V ratio increases as the wheel diameter increases, the overall gear ratio remaining constant.

Acceleration

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When the vehicle is accelerated, its rotating parts are also accelerated depending upon their moments of

inertia and the gear ratio in the drive line.

Due to this, weight of vehicle is increased from W to We.

This increased weight, We, is called the 'effective weight' of the vehicle.

When surplus power, i.e. surplus tractive effort is fully utilized to acceleration, then

Gradability:

The maximum percentage grade, which a vehicle can negotiate with full rated condition, is known as

'gradability'.

Hence,

Drawbar Pull:

When the excess power is fully utilized for pulling extra load attached to vehicle then, Maximum

drawbar pull = Tractive effortRoad resistance = (F R).

Road resistance in this section is made up of rolling resistance and air resistance.

Need for a Gearing System and Determination of Gear Ratios (Automobile):

Transmission

The internal combustion engine used on a vehicle operates over a limited effective speed range of 1500-

5000 rpm.

At low engine speed, a reciprocating-piston engine does not develop sufficient turning-effort or torque

to propel a vehicle forward from standstill. Even the greater torque produced at higher engine speed would be insufficient to accelerate the vehicle

at a reasonable rate.

The gearbox provides a way of varying the engine's output torque and speed to match the vehicle's speedand load.

In order to achieve a high maximum vehicle speed, combined with good acceleration and economy over

the whole speed range, a gearing system is required, which permits the engine to operate at the speedscorresponding to its best performance.

Maximum engine power, torque and economy all occur at different engine speeds.

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As a result it becomes difficult to match the gear ratio for best performance, especially when variable

operating conditions and driver demands are also to be considered.

The engine requirement to suit a given operating condition is as follows.

Operating condition Engine requirementMaximum traction Maximum vehicle

speed Maximum acceleration

Maximum economy

Maximum engine torque Maximum engine power

Maximum engine torque

Engine at mid-range speed and under light load

with a small throttle opening.

The type of engine fitted nowadays to a light vehicle generally requires a gearbox capable of providing

four forward speeds and a reverse.

This provides a reasonable performance to suit all the driving conditions except economy, which

normally needs an extra ratio, a fifth gear, that is higher than the conventional top gear.

A high gear ratio means the lower is the reduction between the engine and road wheels. Conversely thelower the gear ratio means the greater is the reduction between the engine and road wheels.

Maximum Vehicle Speed.

Maximum vehicle speed is attained when the gear is set in top and the throttle is held fully open.

A ratio of 1: 1 (direct drive) is chosen for top gear to keep the friction losses to minimum value. Consequently, the setting of top gear becomes the choice of a final drive ratio to suit the diameter of

road wheel and engine characteristic.

Fig. Power balance. A. Power required for driving the vehicle. B. Power available to drive the vehicle.C. Balance between power available and power required.

Figure illustrates the balance between the power required and the power available.

Data for the power required are obtained from the brake power curve of the engine, and for the poweravailable are based on the calculation of the power needed to overcome the tractive resistance of the

vehicle when it is moving along a level road.

The tractive resistance, sometimes called total resistance, includes :1. Air resistance which is due to movement of the vehicle through the air.

2. Rolling resistance which is due to friction between the tyre and road, and largely influenced by the

type of road surface.3. Gradient resistance occurs when the weight of the vehicle acts against the vehicle motion during

movement up a hill.

The power needed to propel a vehicle (Fig. A) increases with the cube of the speed.

In this example, a power of 150 kW is needed to drive the vehicle at 200 km/h.

The power output curve of the engine installed in this vehicle (Fig. B) indicates that the engine produces

a peak brake power of 150 kW at 5000 rpm.

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To