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 a petrol (gasoline) 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.

In a reciprocating-engine aircraft, the throttle control is usually a hand-operated lever or knob. It controls the engine power, which may or may not reflect in a change of RPM, depending on the propeller installation (fixed-pitch or constant speed).

Because diesel 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 (although recent developments in Exhaust Gas Recirculation have introduced throttle-style designs ). They instead regulate engine power by directly controlling the quantity of fuel injected into the cylinder just before top dead centre (TDC) of the compression stroke.

Environmental aspects
Regulation of the throttle is also a mechanism for controlling engine exhaust emissions. In many modern internal combustion engines an electronic throttle is used, eliminating the older accelerator cable.

Throttle application via the accelerator pedal also results in increased sound emission from the engine. At lower operating speeds this component of vehicle noise is prominent, contrasted with higher operating speeds, for which aerodynamic and tire noise are more significant.