Basics of a car

SOME READERS of this column have `accused' me of trying to get the general reader of The Hindu to run an automobile race before learning to walk, so this and a few more articles will attempt to cover some basic concepts. I beg the temporary indulgence of `advanced' readers.

I owe an enormous debt of gratitude to Mr. Robert Ireson for teaching me these concepts while I was learning to drive as a callow youth.

Main requirements

The motor car is essentially an individual means of transport; for many users its principal appeal lies in the sense of independence that it affords. To achieve this independence a number of essential requirements must be met and although over a century of evolution has produced a remarkable number of variations in the general layout it is common knowledge that all cars share the following features:

A carriage unit which, since it cannot be confined to rail tracks, must be steerable from within itself and must be able to travel in safety and comfort over roads of varying surface finish and flatness.

An entirely self-contained power unit, that is, one which does not need to draw energy continuously from an outside source.

Arrangements for making use of the power output to propel the vehicle, by rotating its road wheels.

Means for stopping the vehicle.

This first article is mainly concerned with the choice of power unit.

Choice of power unit

If the car is to be entirely independent it must utilise a prime mover. This is a general title for any device that can produce mechanical energy directly from a fuel, and almost invariably this is done by extracting the fuel's heat energy and using this to bring a gas to high temperature and pressure. The gas is then permitted to expand in a suitable mechanism and produce motion.

A simple form of prime mover is the rocket, in which the gas produced directly by combustion of a fuel expands through a nozzle and creates a propulsive force by jet reaction. Rocket powered motoring, however, has a place only in science fiction.

Prime movers may be divided into two classes, namely, those in which the gas is obtained by external combustion separately from the engine (for example, steam generated within a boiler), and those in which the high temperature gas is derived from the burning of fuel within the engine itself (internal combustion).

For practical purposes the classes may be further sub-divided between piston-type engines and turbines.

Briefly, in a piston engine, the gas is used to force a `plug' along the interior of a smooth bored cylinder, the plug or piston being coupled mechanically (normally by a crank mechanism) so that its motion can be utilised as the rotation of a shaft. In a turbine, the gas blows through a type of 'fan' which rotates at high speed.

These sub-divisions present four basic types of power unit:

- External combustion piston engine, example, steam engine complete with boiler

- External combustion turbine, example, steam turbine with boiler

- Internal combustion (IC) piston engine, example, petrol and diesel engines. These are essentially 'hot air' devices, that is, the expanding gas is air which is drawn into the engine and heated by the burning of fuel. An incidental function of the air is that it provides the oxygen essential for combustion.

- Internal combustion or gas turbine. Here also air is used.

Despite its high degree of complication and essentially crude operating principle, the IC piston engine currently reigns supreme in automobile applications. However, successful steam powered and electric cars have been built in the past and the future is likely to see the increasing use of hybrid (petrol- electric or diesel-electric) vehicles as well as of fuel cell powered automobiles.

Main parts of an engine

The cylinder: In its simplest form this is a circular tube closed at one end.

The piston fits closely inside the cylinder. Ideally, it would be perfectly gas-tight yet perfectly free to move up and down inside the cylinder.

The connecting rod connects the piston to the crankshaft. At the piston end is a pin called the gudgeon pin which is fitted into holes in the piston and the connecting rod to connect them together.

The crankshaft is the main shaft of the engine and is carried in bearings in the crankcase. Offset from the main part of the shaft is the crank pin on which the connecting rod is fitted and is free to turn.

The arrangement is such that rotation of the crankshaft causes the piston to move up and down inside the cylinder: As the piston moves upwards, the space between its top surface and closed end of the cylinder is reduced, that is, the gas trapped in this space is compressed. As the piston moves downwards the space above it is increased, that is, the gas in this space expands.

Pushing the piston up and down in the cylinder can rotate the crankshaft. Starting with the position shown in Fig. 1, the crankshaft rotates clockwise as the piston is pushed downwards until the piston reaches the lowest point of its travel. At this point, the crank pin will be directly under the centre of the crankshaft, and the centres of the gudgeon pin, crank pin and crankshaft will all lie in a straight line. In this position pressure on the piston will have no turning effect on the crankshaft, and this position is therefore called a dead centre. Another dead centre is when the piston is at the extreme top of its travel. These two dead centres, known as the bottom dead centre (BDC) and top dead centre (TDC) respectively, mark the extreme limits of the piston's travel.

In its movement upwards from the bottom dead centre the piston traverses a space which is known as the swept volume. This volume (multiplied by the number of cylinders in the case of a multi- cylinder engine) is termed the capacity or displacement of the engine. It is commonly measured in cubic centimetres (cc); it is usual to refer to the 'size' of an engine in ccs or litres (1 litre = 1,000 cc).

The cylinder has dimensions such that with the piston at its topmost position or top dead centre, there is a space remaining above it in which the combustion occurs. In comparing the volume of the space left above the piston at bottom and top dead centres we use the term `compression ratio' which indicates the degree to which the gas is squeezed on the up-stroke. Current engines have compression ratios ranging from about 7:1 to about 10:1.

C. Manmohan Reddy

(Continued)

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