 The relationship between the rotation of the camshaft and the rotation of the crankshaft is of critical importance.Since the valves control the flow of the air/fuel mixture intake and exhaust gases, they must be opened and closed at the appropriate time during the stroke of the piston.For this reason, the camshaft is connected to the crankshaft either directly, via a gear mechanism, or indirectly via a belt or chain called a "timing belt" or "timing chain".