Engine Cycle (Stroke)

The engine cycle is the complete sequence an internal combustion engine goes through to produce power: intake of the air–fuel mixture, compression, combustion (the power stroke) and exhaust. A stroke is a single movement of the piston from top to bottom or bottom to top.

In a four-stroke engine the cycle takes four strokes, or two crankshaft revolutions, and valves control the gas flow. In a two-stroke engine the same events are combined into two strokes, or one revolution, using ports in the cylinder wall and the crankcase to move the mixture. That is why people speak of two-stroke and four-stroke engines; in everyday speech "cycle" and "stroke" are often used loosely for the same thing.

Two-strokes give more power for their size and are simpler, while four-strokes are more economical, cleaner and have a broader spread of torque.