What takes place during each stroke of a four stroke engine?

Stroke 1: Intake, as piston moves downwards from top dead center (TDC) to bottom dead center (BDC), it draws in the air fuel mixture through the intake valve.

Stroke 2: Compression, with both valves closed, the piston moves upwards from BDC to TDC, compressing the air fuel mixture.

Stroke 3: Combustion, with the piston at TDC, the spark plug ignites the air fuel mixture, forcing the piston down to BDC, this is also called the power stroke.

Stroke 4: Exhaust, the exhaust valve opens and the piston moves from BDC to TDC, forcing the exhaust gases out of the cylinder.

Summary: Suck, Squeeze, Bang, Blow