1 - Piston moves down the cylinder from (TDC) top dead center to bottom dead center (BDC).Intake valve is open and the exhaust valve is shut.Downward motion of the piston creates a vacuum, which draws the air/fuel mixture into the engine through the open intake valve. 2 - Piston moves up the cylinder bore from BDC to TDC.Intake and exhaust valves are both closed.Upward motion of the piston compresses the air/fuel mixture in the combustion chamber. 3 - Spark plug fires and ignites the compressed