2 Stroke Engine Operation
At an exact point just before the piston reaches the top of the cylinder the spark plug fires. The spark ignites the mixture which rapidly expands propelling the piston downwards. Once the top edge of the piston uncovers the exhaust port the exhaust gasses start to vent out through the exhaust. Meanwhile the fuel/air mixture in the crankcases below the piston is being compressed as the piston descends and the reed valve closes. Once the top of the piston uncovers the transfer ports the compressed fuel/air in the crankcase passes up through the ports to start charging the cylinder with fresh mixture which also serves to expel the remaining exhaust gasses. The fuel/air mixture from the crankcase continues to pass through the transfer ports until the piston reaches the bottom.
As the piston rises it covers the transfer ports and starts to create a vacuum beneath it. Fuel/air mixture is drawn in from the carburetor, through the reed valve, and into the crankcase. In the cylinder the rising piston expels the remaining exhaust gasses out of the exhaust port. Once the top of the piston has closed the exhaust port the mixture trapped in the cylinder is compressed up into the head and the cycle repeats.