A poppet valve

A poppet valve (also called mushroom valve) is a valve typically used to control the timing and quantity of gas or vapour flow into an engine. It consists of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide . In most applications a pressure differential helps to
seal the valve and in some applications also open it. Poppet valves date from at least the 1770s, when James Watt used them on his beam engines.