Cycle and Strokes

Engines can be either two-stroke cycle or four-stroke cycle. The complete cycle requires four piston strokes (intake, compression, power and exhaust).

Study the illustration below. In the two-stroke cycle or two-cycle engine, the intake and compression strokes and the power and exhaust strokes are combined. This condition permits the engine to produce a power stroke every two piston strokes, or every crankshaft rotation.

The actions in a cylinder can be divided into four stages, or "strokes." A stroke is the movement of the piston from the top position to the bottom position or freom the bottom position to the top position. The top position, or upper limit of piston movement, is called top dead center (TDC). The bottom position, or lower limit of piston movement, is called bottom dead center (BDC). A piston stroke or piston movement is either TDC to BDC or BDC to TDC.

The four strokes are called the intake stroke, the compression stroke, the power intake stroke and exhaust stroke. These four strokes make up the complete cycle of events in the cylinder. An engine that runs on this four-stroke cycle principle is called a four-stroke cycle engine, often shortened to four-cycle engine. The term Ott cycle is also applied to this type of engine.