Poppet valve

A poppet valve is the type of valve system used in most piston engines, used to seal the intake and exhaust ports. The valve is usually a flat disk of metal with a long rod known as the valve stem out one end. The stem is used to push down on the valve and open it, with a spring used to close it when the stem is not being pushed on.

The engine normally operates the valves by pushing on the stems with cams. The shape of the cam can be used to control how quickly (or slowly) the valve is opened. The cams are normally placed on the camshaft which is then geared to the crankshaft. In earlier designs the camshaft was located near the crankshaft, and pushed on the valves through a system of rods and rockers. This led to significant energy losses in the engine. More modern designs have the cam on top of the engine pushing directly on the valve stem, a system known as overhead cam. Often there are two cams, one for the intake and one for exhaust, creating the dual overhead cam, or DOHC which is found in almost all modern engines.

In the early days of engine building, the poppet valve was a major problem. Metallurgy was not what it is today, the rapid opening and closing of the valves against the cylinder heads led to rapid wear. They would need to be re-ground every two years or so, in an expensive and time consuming process known as a valve job. Adding tetra-ethyl lead to the petrol reduced this problem to some degree, but improved alloys have generally made this problem disappear completely.

See also:

sleeve valve