Key: Green Arrows - Fuel/Air Mixture Blue Arrows - Fuel Free Air Grey Arrows - Burnt Charge
The fuel air mixture is taken into the crankcase, whilst the fuel free air is stored in the transfer ports. As the charge in the combustion chamber is ignited, the piston is forced down, allowing the fuel free air to rise out of the ports. This fuel free air then flushes the burned charge out of the combustion chamber and through the exhaust port, before the fresh fuel air mixture is in place.