Poppet valve

A poppet valve (also sometimes called mushroom valve) is a valve typically used to control the timing and quantity of gas or vapor flow into or out of an engine, but with many other applications. It consists of a hole or open-ended chamber, usually round or oval in cross-section, and a plug, usually a disk shape on the end of a shaft known as a valve stem. The working end of this plug, the valve face, is typically ground at a 45° bevel to seal against a corresponding valve seat ground into the rim of the chamber being sealed. The shaft travels through a valve guide to maintain its alignment. A pressure differential on either side of the valve can assist or impair its performance. In exhaust applications higher pressure against the valve helps to seal it, and in intake applications lower pressure helps open it. The poppet valve was invented in 1833 by American E.A.G. Young of the Newcastle and Frenchtown Railroad. Young had patented his idea, but the Patent Office fire of 1836 destroyed all records of it. The word poppet shares etymology with "puppet": it is from the Middle English popet ("youth" or "doll"), from Middle French poupette, which is a diminutive of poupée. The use of the word poppet to describe a valve comes from the same word applied to marionettes, which, like the poppet valve, move bodily in response to remote motion transmitted linearly. In the past, "puppet valve" was a synonym for poppet valve; however, this usage of "puppet" is now obsolete. The poppet valve is fundamentally different from slide and oscillating valves; instead of sliding or rocking over a seat to uncover a port, the poppet valve lifts from the seat with a movement perpendicular to the plane of the port. The main advantage of the poppet valve is that it has no movement on the seat, thus requiring no lubrication. In most cases it is beneficial to have a "balanced poppet" in a direct-acting valve. Less force is needed to move the poppet because all forces on the poppet are nullified by equal and opposite forces.

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