In the context of motor vehicles, a device that vents fuel vapours into the inlet tract of the vehicle's engine during running.
While parked, vapours released by the contents of the vehicle's fuel tank are collected, usually in a charcoal canister of some sort. During engine running these gases are released into the combustion system by the fuel tank vent valve.
To ensure the engine runs correctly when it is cold or idling, and to protect the catalytic converter (if fitted) from an over-rich fuel mixture, this may not occur until the engine has reached a particular temperature, or until it is operating under a certain load. This factor is determined by the vehicle's engine management system. After this initial opening of the valve it is modulated open and closed while the engine is running.
On older vehicles without computer control of the fuel, exhaust and ignition system the same venting system may be controlled by a vacuum pipe to the inlet manifold.
The valve, when present in a vehicle, can also be used to double as a pressure release valve for the fuel tank; without such a valve the fuel tank could eventually implode from atmospheric pressure as fuel usage causes its internal pressure to drop.