Spark Ignition (S.I.) Engines: Four stroke petrol engine and Two stroke petrol engine

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. 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.


Four stroke otto cycle engine.
Four stroke otto cycle engine.



Two stroke petrol engine

In 1878, Dugald-clerk, a British engineer introduced a cycle which could be completed in two strokes of piston rather than four strokes as is the case with the four stroke cycle engines. The engines using this cycle were called two stroke cycle engines. In this engine suction and exhaust strokes are eliminated. Here instead of valves, ports are used. The exhaust gases are driven out from engine cylinder by the fresh change of fuel entering the cylinder nearly at the end of the working stroke.

Fig.( below) shows a two stroke petrol engine (used in scooters, motor cycles etc.). The cylinder L is connected to a closed crank chamber C.C. During the upward stroke of the piston M, the gases in L are compressed and at the same time fresh air and fuel (petrol) mixture enters the crank chamber through the valve V. When the piston moves downwards, V closes and the mixture in the crank chamber is compressed. Refer Fig (i) the piston is moving upwards and is compressing an explosive change which has previously been supplied to L. Ignition takes place at the end of the stroke. The piston then travels downwards due to expansion of the gases [Fig. (ii)] and near the end of this stroke the piston uncovers the exhaust port (E.P.) and the burnt exhaust gases escape through this port [Fig. (iii)]. The transfer port (T.P.) then is uncovered immediately, and the compressed charge from the crank chamber flows into the cylinder and is deflected upwards by the hump provided on the head of the piston. It may be noted that the incoming air petrol mixture helps the removal of gases from the engine-cylinder ; if, in case these exhaust gases do not leave the cylinder, the fresh charge gets diluted and efficiency of the engine will decrease. The piston then again starts moving from bottom dead centre (B.D.C.) to top dead centre (T.D.C.) and the charge gets compressed when E.P. (exhaust port) and T.P. are covered by the piston ; thus the cycle is repeated.


Two-stroke petrol engine.
Two-stroke petrol engine.



The power obtained from a two-stroke cycle engine is theoretically twice the power obtainable from a four-stroke cycle engine.

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