Throttle

In an engine, the throttle is the mechanism by which the engine's power is increased or decreased. Throttle may refer to both the part inside the engine which directly regulates the fuel flow, or the human controls (pedal, lever, electronic) that the operator uses to indirectly control an engine's power.

Internal combustion engines

In an internal combustion engine, the throttle is a valve that directly regulates the amount of air entering the engine, and indirectly controls the amount of fuel, and thus regulates the engine speed. In a vehicle, the control that human operators use may be referred to as a 'gas pedal' or 'accelerator'.

The throttle is typically a butterfly valve. In a fuel-injected engine, the throttle valve is housed in the throttle body. In a carbureted engine, it is found in the carburetor. The butterfly valve in a fuel-injected throttle body has one or more holes in it to allow the engine air when idling.

When a throttle is wide open, the intake manifold is usually at ambient atmospheric pressure. When the throttle is partially closed, a manifold vacuum develops as the intake drops below ambient pressure.

Usually the throttle valve is mechanically linked with the throttle pedal or lever. In vehicles with electronic throttle control, the throttle valve is electronically controlled, which allows the ECU greater possibilties in reducing air emissions.