4 Stroke Engine Operation
At the start of the induction stroke the inlet valve opens and as the piston lowers fresh fuel/air mixture is drawn into the cylinder from the carburetor.
Once the piston reaches the bottom the inlet valve closes and the compression stroke begins. The fresh mixture is compressed by the rising piston up towards the cylinder head and spark plug.
At an exact point just before the piston reaches the top the spark plug sparks to ignite the compressed mixture. This is the start of the power stroke. The ignited mixture expands rapidly propelling the piston downwards.
As the piston reaches the bottom the exhaust valve opens. This allows the spent gas from the power stroke to escape. During this exhaust stroke the upward motion of the piston serves to pump the exhaust gases out through the valve. Once the piston has reached the top the exhaust valve closes and the cycle begins again.