A piston moves down, up, down and up again inside a cylinder. A connecting rod and crankshaft turn that back-and-forth movement into rotation. In the simplified four-stroke petrol engine shown here, the stages are intake, compression, power and exhaust.

One complete cycle takes two crankshaft revolutions. Only the power stroke delivers the cycle’s main expansion work; the crankshaft, flywheel and other cylinders carry the mechanism through the remaining strokes. Real valve and ignition timing extend beyond this simplified picture.