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.
Valve is mechanical devices that control the flow of fluids, such as gases or liquids, and are used in various systems on a vehicle, including the engine, transmission, and exhaust system. In the engine, valves control the flow of air and fuel into the cylinders and the flow of exhaust gases out of the cylinders.
In the transmission, valves control the flow of hydraulic fluid used to operate the gears. In the exhaust system, valves control the flow of exhaust gases out of the engine. Valves can be controlled mechanically, through the use of a camshaft or other mechanical linkage, or electronically, through the use of a computer-controlled actuator.
Valves are technically valve 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 from volvere to turn, roll. The most straightforward and old valve is simply a freely hinged flap that drops to obstruct fluid (gas or liquid) flow in one direction but is pushed open by flow in the opposite direction.
More complex control systems using valves requiring automatic control based on an external input (i.e., regulating flow through a pipe to a changing set point) require an actuator. An actuator will stroke the valve depending on its information and set-up, allowing the valve to be positioned accurately and controlling various requirements.