Throttle

This refers to the control of engine power. For other uses, see throttle (disambiguation).

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.

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.

Other engines

Most engines have some kind of throttle control, though the particular way that power is regulated is often different.

Liquid rockets are throttled by controlling the pumps which send liquid fuel and oxidizer to the combustion chamber. Solid rockets are more difficult to throttle, but some may have mechanisms for this.

In a jet engine, engine output is also directly controlled by changing the amount of fuel flowing into the combustion chamber, usually with an autothrottle.