VEHICLE CONSTRUCTION

VEHICLE CONSTRUCTION

The automobile consists of two main parts

     1. Chassis

     2. Body

A vehicle arrangement without body is called chassis.

Various components and systems of the chassis are 

1. The power system,

Power units are clutch, gearbox transmission, differential, Rear axle shaft, universal joint &propeller shaft etc …

2. Running system  

Running systems consists brakes, wheels, frame, suspension and steering system

3. Electrical system. 

Electrical system consists of starting ciruit, charging circuit, ignition circuit lighting and horn circuit.  

2.Body

The purpose of the body is to provide accommodation to the driver and the passenger and to protect them against adverse conditions

1.4.1 Frame

It is rigid structure that forms a Skelton to hold all the major units together. The engine is mounted in the front of the frame and is connected to the clutch and transmission to form a compact power assembly. The unit is fastened to the frame through rubber-cushioned motor mounts to lessen the transfer of engine vibration. The transmission is connected to the final-drive gears through the propeller shaft and universal joints. The final-drive gears and differential to the rear wheels revolve inside the rear-axle housing.

1.4.2 Wheels and Tyres

The wheels and tyres assemblies support the frame and the units mounted on it through front and rear suspension systems.

1.4.3 Suspension System

The suspension system is made up of springs, shock absorbers and linkage so as to allow the wheels and tyres to follow the road roughness without excessively raising the frame and transmitting shock.

1.4.4 Steering System

Some parts of the steering system (steering gear) are bolted to the frame and some (steering column) connected to the body.

1.4.5 Fuel System

The fuel tank is fastened to the rear of the frame and is connected to the fuel pump through metal tubes. The exhaust pipe and muffler conducts the poisonous gases from the engine to the rear of the vehicle.

1.4.6 Electrical System

It consist of a battery, starting motor, generator and controls alongwith engine ignition system (ignition coil, distributor, spark plugs and wires).

1.4.7 Brake System

The complete brake system is incorporated in the chassis. The brake pedal controls a master cylinder which is connected to brakes on each wheel by oil lines.

1.4.8 Cooling System

A radiator is mounted on the chassis which is connected to the engine by rubber hoses to allow cooling water or coolant to circulate between radiator and engine. Fan, water pump and fan belt are other parts of the cooling system.

1.4.9 Auxiliaries

Lights, instruments, accessories, etc. are connected to the chassis and are furnished with electrical energy from battery and generator.

1.5 COMPONENTS OF AN AUTOMOBILE

The main components of an automobile are :

1. The basic structure.

2. The engine.

3. The transmission system.

4. The auxiliaries.

5. The controls.

6. The superstructure.

The items, 1 to 5 constitute automobile chassis.

The main components of an automobile are shown in Fig. 1.1.

1.5.1 The Basic Structure

It consists of the frame, the suspension system, axles, wheels and tyres. It provides support for other items of automobile.

1. Frame

There are two distinct forms of construction in common use:

1. The conventional pressed steel frame to which all the mechanical units are attached and on

which the body is superimposed.

2. The integral or frameless construction, in which the body structure combines the functions of body and frame.

The frameless construction is possible only in the case of closed car, since the  roof, screen pillars and rear panel are essentially load-taking parts of the structure.

The frameless construction has the following advantages over the conventional framed construction:

(i)         Reduced weight and saving in fuel consumption.

(ii)        Low manufacturing cost.

(iii) During collision, the body absorbs the shock due to impact and providing safety to the passengers.

(iv)       Lower body position and increased stability of the automobile.       

            These advantages are accompanied by the following disadvantages also:

(i)         Reduction of strength and durability.

(ii)        Economical only if frameless construction is adopted in mass production.

(iii)       Increased cost of repairs in case of damage to body during accidents.

(iv)       Topless cars are difficult to design with the frameless construction.

2. Suspension System

The suspension system connects the body of the vehicle with the wheels and restricts direct impact of wheels to the chassis and body.

3. Axles

The front and rear axles are beams supported at the ends and subject to the following loads :

1. The vertical load at the spring centres dur to the weight of the vehicle.

2.A fore and aft load at the wheel centre due to driving or banking effort.

3. The torque reactions due to drive or brakes.

4.A side thrust at the radius of the tyre due to centrifugal force when rounding a curve.

There are three types of live rear axles.

(a)  fully floating type.

(b)  three-quarter floating type.

(c)  semi-floating type

4. Wheels

Wire-spoked wheels have been used mainly on sports cars on account of their light weight and quickness in changing the wheel. However, the pressed steel wheel has displaced these for all ordinary purposes. Such a wheel consists of a central flanged disc pressed into a rolled section rim and retained in position by welding. Light alloy wheels are currently used in case of luxury and sports cars and are called ‘formula wheels’.

1.5.2 The Power Plant

The power plant (engine) provides power for propulsion of the vehicle. The power plant generally consists of an internal combustion engine which may be either a spart-ignition, or compression-ignition type.

The gas-turbines have also been used successfully in certain cars, though the cost has been a disadvantage in spite of their better performance compared to I.C. engines.

1.5.3 The Transmission System

            The transmission system consists of a clutch, a gear box (also called transmission) giving three, four or even five different ratios of torque output to torque input, a propeller shaft to transmit the torque output from the gear box to the rear axle and a differential gear to distribute the final torque equally between the driving wheels. The various components are:

Clutch: Its purpose is to enable the driver to disconnect the drive from the road wheels instantaneously and to engage to the road wheels gradually while moving the vehicle from rest.

Gear box (transmission): The gear box or the transmission provides the necessary leverage variation between the engine and road wheels.

Bevel pinion and crown wheel: They turn the drive round through 90^o  and also provide a permanent reduction in speed. The permanent reduction is necessitated because of the fact that speed of the engine has to be maintained at optimum level at all times, yet a minimum value of torque has to be made available at the road wheels.

Universal joints: They provide for the relative movement between the engine and the driving wheels due to flexing of road springs.

Differential: While taking turns, the driving wheels must turn at different speeds. This is done with the help of differential.

1.5.4 The Auxiliaries

The main auxiliary is the electrical equipment. This can be subdivided into four systems:

1.Supply System-Battery and Dynamo

2. The starter.

3. The ignition system-battery and magneto-ignition.

4. Ancillary devices including:

(a) Driving lights: head-lights, side-lights, tail-lights, number-plate illumination, etc.

(b) Signalling : horn, direction indicators and the brake light.

(c) Other lights: interior roof lights, panel light and reverse light if fitted (this is switched on automatically when reverse gear is engaged).

(d) Miscellaneous: radio, heater, fans, electric fuel pump, electric windscreen wipers.

1.5.5 The controls

The controls consist of:

1. Steering System.

2. Brakes.

These will be studied later in detail

1.5.6 The Superstructure

In those cases, where frameless construction is not adopted, there is a separate superstructure, i.e. the body attached to the frame. The body performs the functions of both.

The shape of the body depends upon the ultimate use for which the vehicle is meant.

1. Car Body Styles

(i) Closed Cars

            (a)  Saloon : Has two or four door, a single compartment with two rows of seats, one at the front  and the other at the rear. There is a separate luggage space at the rear of at the front.

(b)        Hatchback: A saloon with a door at the back is called hatchback.

(c)        Coupe : In this type there is only one row of seats for two persons. Only two doors are provided. An ordinary 'saloon' with roof-line at decreased height is also called coupe sometimes. The term G.T. (grand touring) added to 'coupe' means a car of performance which is better than the average.

(d)       Limousine: Driving compartment is separated from the rear compartment by a sliding glass division.

(ii)        Open cars

(a)        Sports: Has two doors or less, is provided with a collapsible hood, a fold flat windscreen and removable side-screens.

(b)        Convertible: The roof is of soft folding type and windows of special wind-up design. With these provisions the car may be either open or enclosed.

(iii)       special styles

(a) Estate Car : A saloon car with the passenger roof extended right up to the rear end. Rear door for loading with rear seats usually collapsible.

(b)        Station Wagon: Wood-panelled body in earlier days but now only with a resembling look over the steel structure. Rear doors and seats for four or five persons.

2.         Transport Vehicles

(i)         Van : These are light transport vehicles. There are seats at the front and the luggage space at the rear. The side doors are usually of sliding type. The rear door is used for loading or unloading.

(ii)        Truck: These are heavy goods commercial vehicles with all the axles attached to a single frame. Generally twin wheels are fitted on the non-steered axles. Depending upon the load capacity, there are two or more axles.

(iii)       Articulated vehicle : This is also heavy goods vehicle consisting of a tractor and a semi-trailer. The tractor provides the motive power while the trailer carries the goods. The two units are connected by means of a fifth-weel coupling. This type of vehicle has smaller turning circle than the rigid truck. Moreover, the trailer being detachable, the tractor can directly move off after reaching with some other trailer without having to wait for unloading and re-loading. However, in theis there is less traction available. Moreover, steering  is more difficult as the trailer wheels do not follow the same path as the traction wheels.

(iv)       Bus: These are used for carrying large number of people over short distances often in dense traffic. Not much space for luggage is provided, but there is adequate visibility for the passengers to know where they have to get down. Usually, there are two doors, the one at the rear for mounting and one at the front for getting down. Sometimes, the buses have two floors. Such buses are called double decker buses.

(v)        Coach: these are used for transporting passengers over long distances and are, therefore, provided with luxurious interior with comfortable adjustable seats and amenities like video.

References

1. Automobile Engineering (Vol. 1 & 2)            -           K.M.Guptha
2. Automotive Mechanics                                   -           Joseph Heitner
3. Automobile Engineering                                 -           Harbans Singh Reyd
4. Automotive Mechanics                                   -           William H. Course