Intake - On the intake stroke the piston moves doward, sucking in carburized fuel through the open intake valve.
Exhaust valve is closed.
With both the intake and exhaust valves closed, the pistons upward stroke compresses or squeezes the fuel into the combustion chamber. Firing - Ignited by the spark, the compressed fuel explodes and forces the piston downward on its power stroke. Intake and exhaust valves are closed. Exhaust valves and seats are exposed momentarily to temperature as high as 3800 degrees F. Exhaust - The piston moves upward on its exhaust stroke and forces the burned fuel through the open exhaust valve. Intake valve is closed.