On engines with a chain drive, a timing chain connects the crank to the cam
via two gears called a cam gear and crank gear. On engines with a timing belt, a
cogged rubber belt connects the crank and cam. This rubber belt meshes with
two cogged gears, one for the crank, one for the cam. The cam and crankshafts
are perfectly timed mechanically. Thus, when the engine is operating, the valves
and pistons are exactly where they are supposed to be during the four-stroke
cycle. The crank rotates, causing the pistons to stroke (go up and down). The
camshaft simultaneously turns with the crank via the mechanical drive.
Camshaft
When the camshaft is timed properly, it opens and closes the valves at the
precise time necessary for the intake, compression, power, and exhaust strokes of
the four-stroke cycle. There are lobes (egg-shaped bumps) machined into the
camshaft so that, when the shaft turns, it opens and closes valves by virtue of
mechanical lift.