How does a four-stroke engine work?
A four-stroke gasoline engine repeats intake, compression, power, and exhaust to turn fuel energy into mechanical motion.
Intake
The piston moves down while the intake valve is open. Air enters; fuel is introduced according to the engine’s injection system.
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Intake
The piston moves down while the intake valve is open. Air enters; fuel is introduced according to the engine’s injection system.
Compression
With the valves closed, the piston moves up and compresses the charge. A smaller volume raises its pressure and temperature.
Power
The spark ignites the charge. Expanding gases push the piston down, transferring force through the connecting rod to the crankshaft.
Exhaust
The exhaust valve opens and the piston moves up, pushing spent gases out. The four strokes take two crankshaft revolutions.
About this visual model +
A cutaway single-cylinder engine exposes the piston, connecting rod and crank. The near cylinder wall is removed so the mechanism is visible; the narrated cycle uses highlights rather than a physical simulation.
Simplified spark-ignition cycle. Valve timing, fuel delivery, cooling, and real engine geometry vary.
- Intake valve: Opens to admit the incoming charge during intake.
- Cylinder and piston: The piston moves inside a cylinder, changing the volume of the combustion chamber.
- Spark plug: A spark initiates combustion near the end of compression.
- Crankshaft: The crank mechanism converts the piston’s reciprocating movement into rotation.
- Exhaust valve: Opens to let exhaust gases leave the cylinder.
- Fuel supply: Fuel combines with air to provide chemical energy for combustion.
Further reading: Internal combustion engine basics ↗, Four-stroke cycle ↗