Operation of the engine
In most engines a single cycle of operation (intake, compression, power, and exhaust) takes place over four strokes of a piston, made in two engine revolutions. When an engine has more than one cylinder the cycles are evenly staggered for smooth operation, but each cylinder will go through a full cycle in any two engine revolutions. When the piston is at the top of the cylinder at the beginning of the intake stroke, the intake valve opens and the descending piston draws in the air-fuel mixture