What does the term "four-stroke" in a four-stroke engine refer to?

The term "four-stroke" in a four-stroke engine specifically refers to the number of distinct movements made by the piston during one complete cycle of operation. In a four-stroke engine, there are four key strokes: intake, compression, power, and exhaust. Each of these strokes corresponds to a different phase in the engine's operation.

During the intake stroke, the piston moves down to draw in the air-fuel mixture. In the compression stroke, the piston moves back up to compress this mixture. The power stroke occurs when the mixture is ignited, forcing the piston down again, and finally, the exhaust stroke expels the burnt gases as the piston moves back up. This sequential movement of the piston through these four strokes is what characterizes a four-stroke engine.

Understanding this concept is fundamental as it differentiates four-stroke engines from other types of engines, such as two-stroke engines, which complete their cycle with only two strokes of the piston.