Nearly all aviation reciprocating engines are four stroke engines. We can think of a stroke as a step that the engine takes, and in a four stroke engine those steps are intake, compression, power (or ignition), and exhaust. - a good way to remember this is "*Suck Squeeze Bang Blow*"  # Intake The intake stroke begins as the piston starts its downward travel. When this happens, the intake valve opens and the fuel-air mixture is drawn into the cylinder. # Compression The compression stroke begins when the intake valve closes, and the piston starts moving back to the top of the cylinder. This phase of the cycle is used to obtain a much greater power output from the fuel-air mixture once it is ignited.  # Power The power stroke begins when the fuel-air mixture is ignited. This causes a tremendous pressure increase in the cylinder and forces the piston downward away from the cylinder head, creating the power that turns the crankshaft.  # Exhaust The exhaust stroke is used to purge the cylinder of burned gases. It begins when the exhaust valve opens, and the piston starts to move toward the cylinder head once again.