The principle of two stroke spark ignition engine is shown in the figure. Its two strokes are as follows:
During upward stroke, the piston moves upward from the bottom dead centre to top dead centre. By compressing the charge air petrol mixture in the combustion chamber of the cylinder. Due to upward movement of the piston, a partial vacuum is created in the crankcase. And a new charge is drawn into the crankcase through the uncovered inlet port. The exhaust port and transfer port are covered when the piston is at the top dead center position. The compressed charge is ignited in the combustion chamber by a spark given by the spark plug.
As soon as the charge is ignited the hot gases compress the piston which moves downward, rotating the crankshaft thus doing the useful work. During this stroke, the inlet port is covered by the piston and the new charge is compressed in the crankcase. Further downward movement of the piston uncovers first the exhaust port and then the transfer port. and hence the exhaust starts through the exhaust port.
As soon as transfer port is open, the charge through it is forced into the cylinder. 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 a fresh charge, although it is somewhat with the exhaust gases. The cycle of events is then repeated, the piston making two strokes for each revolution of the crankshaft.