Construction of an automobile

The primary components of a car are the power plant, the power transmission, the running gear, and the con­trol system. These constitute the chassis, on which the body is mounted.

The power plant includes the engine and its fuel, the carburettor, ignition, lubrication, and cooling systems, and the starter motor.

The Engine

The greatest number of cars use piston engines.The four-cycle piston engine requires four strokes of the pis­ton per cycle. The first downstroke draws in the petrol mixture. The first upstroke compresses it. The second downstroke—the power stroke—following the combus­tion of the fuel, supplies the power, and the second upstroke evacuates the burned gases. Intake and exhaust valves in the cylinder control the intake of fuel and the release of burned gases. At the end of the power stroke the pressure of the burned gases in the cylinder is 2.8 to 3.5 kg/sq cm. These gases escape with the sudden open­ing of the exhaust valve. They rush to a silencer (muf­fler), an enlarged section of piping containing expand­ing ducts and perforated plates through which the gases expand and are released into the atmosphere.

Greater smoothness of operation of the four-cycle en­gine were provided by the development of the four-cyl­inder engine, which supplies power from one or another

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of the cylinders on each stroke of the cycle. A furth increase in power and smoothness is obtained in engin of 6, 8, 12, and 16 cylinders, which are arranged in eith a straight line or two banks assembled in the form of a'

Carbu ration

Air is mixed with the vapour of the petrol in the ca burettor. To prevent the air and the carburettor fro becoming too cold for successful evaporation of the fue the air for the carburettor is usually taken from a poii close to a heated part of the engine. Modern carbure tors are fitted with a so-called float-feed chamber and mixing or spraying chamber. The first is a small chan ber in which a small supply of petrol is maintained at constant level. The petrol is pumped from the main tan to this chamber, the float rising as the petrol flows i until the desired level is reached, when the inlet closes The carburettor is equipped with such devices as accei erating pumps and economizer valves, which automati cally control the mixture ratio for efficient operatio under varying conditions. Level-road driving at constan speed requires a lower ratio of petrol to air than tha needed for climbing hills, for acceleration, or for start ing the engine in cold weather. When a mixture ex tremely rich in petrol is necessary, a valve known as th choke cuts down the air intake, permitting large quanti ties of unvaporized fuel to enter the cylinder.

Ignition

The mixture of air and petrol vapour delivered to th* cylinder from the carburettor is compressed by the firsi upstroke of the piston. This heats the gas, and the higher

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temperature and pressure facilitate ignition and quick combustion. The next operation is that of igniting the charge by a spark plug. One electrode is insulated by por­celain or mica; the other is grounded through the metal of the plug, and both form part of the secondary circuit of an induction system.

The principal type of ignition now commonly used is the battery-and-coil system. The current from the bat­tery flows through the coil and magnetizes the iron core. When this circuit is interrupted at the distributor points by the interrupter cam, a current is produced in the pri­mary coil with the assistance of the condenser. This in­duces a high-voltage current in the secondary winding. This secondary high voltage is needed to cause the spark to jump the gap in the spark plug. The spark is directed to the proper cylinder by the distributor, which connects the secondary coil to the spark plugs in the several cylin­ders in their proper firing sequence. The interrupter cam and distributor are driven from the same shaft, the number of breaking points on the interrupter cam being the same as the number of cylinders.

The electrical equipment controls the starting of the engine, its ignition system, and the lighting of the car. It consists of the battery, a generator for charging it when the engine is running, a starter and the necessary wiring. Electricity also operates various automatic de­vices and accessories, including windscreen wipers, di­rectional signals, heating and air conditioning, cigarette lighters, powered windows and audio equipment.