The operation of the two-stroke and four-stroke engines are based on combustion of a fuel-air mixture. In the two-stroke engine, the piston serves as the combustion chamber. In the four-stroke engine, combustion takes place in the cylinder outside the piston.
The piston in a two-stroke engine is pushed down from the energy of the combustion. The motion of the piston and the compression of the exhaust gases delivers power to the crankshaft. The four-stroke engine uses valves at the top of the cylinder to introduce fuel-air mixes and allow exhaust gases to escape.
The fuel mixture is ignited by the spark plug when both valves are closed in a four-stroke engine. The four-stroke engine generally has a distributor that supplies a spark when the piston is at the position known as top dead center (TDC). Two-stroke engines are generally simpler and used in such applications as lawn mowers and smaller motorcycles.