The two stroke engine requires one revolution of crankshaft or two strokes of the piston to complete the cycle of events (ie, auction, compression, power and exhaust shows the working principle of two stroke petrol engine

First Stroke Assume that the piston is at its B.D.C position. During this stroke, the piston moves upwards from bottom dead centre to top dead centre. It closes the transfer port and the exhaust port. The charged air petrol mixture which is already there in the cylinder is compressed. Due to upward movement of the piston, a partial vaccum is created in the crank case and a fresh charge is drawn into the crank case through the uncovered inlet port. 

At the end of this stroke, the piston reaches the T.D.C position. 

Two Stroke Petrol Engine

Second stroke The compressed charge is ignited in the combustion chamber by means. of an electric spark produced by the spark plug, slightly before the completion of the compression stroke. Due to combustion piston is moved downward direction. During this stroke, the inlet port is covered by the piston and the fresh charge is compressed in the crank case. Further downward movement of the piston uncovers the exhaust port and then the transfer port. 

The expanded gases start escaping through the exhaust port and at the same time fresh charge which is already compressed in the crank case, is forced into the cylinder through the transfer port. The charge strikes the deflector on the piston crown, rises to the top of the cylinder and pushes out most of the exhaust gases. The piston is now at the bottom dead center position. The cylinder is completely filled with the charge. The cycle of events is then repeated.

See More: Petrol Engine-Working Principle

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