4-Stroke Engine

Intake: Piston descends into the cylinder and sucks in the gases which pass through the intake valve. The exhaust valve is closed.
Compression: Two valves are closed. Piston rises and compresses the mixture ignited by the spark plugs.
Explosion/Expansion: Piston propelled downwards, movement transmitted to the propeller by the crankshaft.
Exhaust: Intake closed, exhaust open. Piston rises and ejects burnt gases. Then back to the beginning of the cycle.