Valve stem

A valve stem is a self-contained valve which opens to admit gas to a chamber (such as air to inflate a tire), and is then automatically closed and kept sealed by the pressure in the chamber, or a spring, or both, to prevent the gas from escaping. They are most commonly used on automobile and bicycle wheels, but also for many other applications.

Schrader

Schrader valves consist of a valve stem into which a poppet valve is threaded with a spring attached. They are used on virtually all automobile and motorbike tires, and on wider rimmed bicycle tires. In addition to tires, Schrader valves of varying diameters are used in refrigeration and air conditioning systems, plumbing, engine fuel injection, suspension systems, and SCUBA regulators, allowing the user to remove and attach a hose while in use. The Schrader valves in the fuel injector rail of many automobiles is used as a quick and easy point to check fuel pressure or connect an injector cleaner cartridge.

Presta

Presta valves (also called Sclaverand valves or French valves) are normally only used on bicycles. The stem has a narrower diameter (nominally 6 mm) than the thinnest (nominally 8 mm) Schrader type, and so the bore of the hole in the rim through which the stem passes can be smaller.

Valve

A valve is a device that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings, but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure. The word is derived from the Latin valva, the moving part of a door, in turn from volvere, to turn, roll.

The simplest, and very ancient, valve is simply a freely hinged flap which drops to obstruct fluid (gas or liquid) flow in one direction, but is pushed open by flow in the opposite direction. This is called a check valve, as it prevents or "checks" the flow in one direction. Modern control valves may regulate pressure or flow downstream and operate on sophisticated automation systems.

Valves have many uses, including controlling water for irrigation, industrial uses for controlling processes, residential uses such as on / off and pressure control to dish and clothes washers and taps in the home. Even aerosols have a tiny valve built in. Valves are also used in the military and transport sectors.

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 portion of the hole where the plug meets with it is referred to as the 'seat' or 'valve seat'. The shaft guides the plug portion by sliding through a valve guide. In exhaust applications a pressure differential helps to seal the valve and in intake valves a pressure differential helps open it. Poppet valves date from at least the 1770s, when James Watt used them on his steam engines.

Etymology

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 poupe. 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.