The four-stroke engine cycle
A four-stroke engine completes one operating cycle in four piston strokes and two crankshaft revolutions: 720 degrees. A stroke is one journey between the top and bottom of the cylinder. The following sequence describes a simplified spark-ignition gasoline engine.
1. Intake: the piston moves down
The intake valve opens as the piston moves from top dead center toward bottom dead center. The resulting pressure difference draws a fresh charge into the cylinder. In port-injected engines, fuel mixes with air before entering; in direct-injected engines, fuel is injected into the cylinder. The exhaust valve is treated as closed in this simplified sequence.
2. Compression: the piston moves up
Both valves are closed while the piston rises. The same charge now occupies a smaller volume, raising its pressure and temperature. The spark normally occurs before top dead center so combustion has time to develop. Exact ignition timing varies with speed, load and engine design.
3. Power: combustion pushes the piston down
Burning fuel raises cylinder pressure. Expanding gases push the piston downward, transmitting force through the connecting rod to the crankshaft. Both valves remain closed for most of this stroke. This is the stroke that supplies work to sustain rotation and drive the load.
4. Exhaust: the piston moves up again
The exhaust valve opens and the rising piston pushes burned gas out toward the exhaust manifold. The cycle begins again as the intake valve opens. Real engines open and close valves around the stroke boundaries, and may briefly have both valves open; the four neat stages are a teaching simplification.
Why 720 degrees, rather than 360?
A single crankshaft revolution contains two piston strokes: one down and one up. Completing all four strokes therefore takes two revolutions. Each cylinder has one power stroke per cycle. In a conventional four-stroke valvetrain, the camshaft rotates at half crankshaft speed.
How a two-stroke cycle differs
A two-stroke engine completes its cycle in one crankshaft revolution and uses a different gas-exchange arrangement. It is not simply a four-stroke engine with two steps removed. Intake and exhaust processes must overlap with other parts of the cycle.
Connect the steps
- Intake: down
- Compression: up
- Power: down
- Exhaust: up
Count movements, not just rotations
Start a cylinder at the beginning of intake. One down-and-up pair completes intake and compression: one crankshaft revolution. A second down-and-up pair completes power and exhaust: another revolution. The total is four strokes and 720 degrees. These are idealized boundaries; real valve events overlap or shift around them.
Try it in 3D
Use the engine cycle controls to compare intake and power. Both move the piston down; check how the valves and the purpose of the movement differ.
Open the related explorer →Why does the camshaft turn at half crankshaft speed?
The valves need to repeat their sequence once per four-stroke cycle. That cycle lasts two crankshaft revolutions, so one camshaft revolution covers the same sequence.