Most light duty vehicle engines have a timing belt, though some have timing chains. The purpose of the timing belt is to time the movement of the valves with the pistons. For instance; on the intake stroke the intake valve opens while the piston moves down, sucking in an air/fuel mixture. Next the compression stroke as the piston moves up with all valves on that cylinder closed, compressing the air fuel mixture to about 1/10 of its original volume. Next the spark plug ignites the mixture creating the power stroke and pushing the piston downward. On its next trip back up, the exhaust valve opens, letting the burned gas be pushed into the exhaust system. In a 6 cylinder engine, this happens 6 times for every RPM.
