TITLE.PM5

INTRODUCTION—OUTLINE OF SOME DESCRIPTIVE SYSTEMS 5

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Exhaust valve

Rocker arm Petrol

tank

Engine

throttle

Petrol

supply pipe

Piston

Carburettor

Connecting rod

Crank

Roller

Intercam

Crankshaft

Gear exhaust Crankcase

cam

Magnet

High tension

cable

Piston ring

Exhaust

Cooling

fins

Spark

plug

Silencer

Air inlet

Jet

Push rod

Inlet

manifold

Inlet

valve

Oil pump

Fig. 1.3. An air-cooled four-stroke petrol engine.

1.3.4. Spark ignition (S.I.) engines

These engines may work on either four stroke cycle or two stroke cycle, majority of them, of

course, operate on four stroke cycle.

Four stroke petrol engine :

Fig. 1.4 illustrates the various strokes/series of operations which take place in a four stroke

petrol (Otto cycle) engine.

Suction stroke. During suction stroke a mixture of air and fuel (petrol) is sucked through

the inlet valve (I.V.). The exhaust valve remains closed during this operation.

Compression stroke. During compression stroke, both the valves remain closed, and the

pressure and temperature of the mixture increase. Near the end of compression stroke, the fuel is

ignited by means of an electric spark in the spark plug, causing combustion of fuel at the instant

of ignition.

Working stroke. Next is the working (also called power or expansion) stroke. During this

stroke, both the valves remain closed. Near the end of the expansion stroke, only the exhaust valve

opens and the pressure in the cylinder at this stage forces most of the gases to leave the cylinder.

Exhaust stroke. Next follows the exhaust stroke, when all the remaining gases are driven

away from the cylinder, while the inlet valve remains closed and the piston returns to the top dead

centre. The cycle is then repeated.