Valve Timing

In a piston engine, the valve timing is the precise timing of the opening and closing of the valves.

In four-stroke cycle engines and some two-stroke cycle engines, the valve timing is controlled by the camshaft. It can be varied by modifying the camshaft, or it can be varied during engine operation by the relatively new technology of variable valve timing. It is also affected by the adjustment of the valve mechanism, and particularly by the tappet clearance. However, this variation is normally unwanted.

Many two-stroke cycle and all wankel engines do not have a camshaft or valves, and the port timing can only be varied by machining the ports, and/or modifying the piston skirt (two stroke applications). Some supercharged two-stroke diesel engines do however have a cylinder head and camshaft similar to a four-stroke cycle engine.

In a steam engine, the control of the valve timing is an important part of the operation of the engine.