Throttle

A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases (i.e., by the use of a throttle). The term throttle has come to refer, informally and incorrectly, to any mechanism by which the power or speed of an engine is regulated.

Internal Combustion Engines

In a petrol internal combustion engine, the throttle is a valve that directly regulates the amount of air entering the engine, indirectly controlling the fuel burned on each cycle due to the fuel-injector or carburetor maintaining a relatively constant fuel/air ratio. In a motor vehicle the control used by the driver to regulate power is sometimes called the throttle 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 possibilities in reducing air emissions.

Because Diesel internal combustion engines use compression ignition, they do not need to control air volumes or mixture (indeed this would be undesirable). Thus they lack a butterfly valve in the intake tract, and do not have a throttle. They instead regulate engine power by directly controlling the quantity of fuel injected into the cylinder during the power stroke.