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.
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.
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 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 at top dead centre (TDC) of the compression stroke.
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.
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. In some instances, a "throttle" is known as a "thrust lever" (as in most Boeing aircraft.) This is chiefly due to the fact that a standard "throttle" is associated with internal combustion engnes.
CPUs manufactured by Intel feature a self throttle mechanism to prevent CPU from overheating while still maintain functionality. This serves as a fail-safe system. CPU consumes more electricity under workload, but use significantly less power when idle. The more power it consumes the more heat it produces. When its heatsink has failed or inadequate to dissipate the heat generated by the CPU, the CPU temperature increase past a predefined threshold, then the throttle function interrupts the running processes forcing the CPU turn into idle state and/or by other means limit the workload, therefore decrease its power consumption allowing it to cool down. The computer system may experience reduced performance as a result, but may still be fully functional. Overheating ultimately force the computer to hang, shutdown or causing damage to the hardware. The self throttle mechanism is further developed, combined with SpeedStep serving more purposes.