This gasoline piston starts
at the top, the intake valve opens, and the piston moves down to let the engine
take in a cylinder full of air and gasoline. This is the intake stroke. Only
the tiniest drop of gasoline needs to be mixed into the air for this to work.
Then the piston moves back up to compress this fuel/air mixture. Compression
makes the explosion more powerful. When the piston reaches the top of its stroke,
the spark plug emits a spark to ignite the gasoline. The gasoline charge in
the cylinder explodes, driving the piston down. Once the piston hits the bottom
of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to
go out the tail pipe.