Throttle

The term throttle has come to refer, informally, to any mechanism by which the power or speed of an engine is regulated, such as a car's accelerator pedal.

What is often termed a throttle (in an aviation context) is also called a thrust lever, particularly for jet engine powered aircraft.

In a motor vehicle the control used by the driver to regulate power is sometimes called the throttle, accelerator, or gas pedal.

An exception to this generalization is newer diesel engines meeting stricter emissions standards, where such a valve is used to generate intake manifold vacuum, thereby allowing the introduction of exhaust gas (see EGR) to lower combustion temperatures and thereby minimize NOx production.

It controls the engine power output, which may or may not reflect in a change of RPM, depending on the propeller installation (fixed-pitch or constant speed).

[1] Some modern internal combustion engines do not use a traditional throttle, instead relying on their variable intake valve timing system to regulate the airflow into the cylinders, although the result is the same, albeit with less pumping losses.

The throttle body is usually located between the air filter box and the intake manifold, and it is usually attached to, or near, the mass airflow sensor.

The ECU then increases the amount of fuel injected by the injectors in order to obtain the required air-fuel ratio.

The additional height afforded by the dome helps to avoid any liquid (e.g. from bubbles on the surface of the boiler water) being drawn into the throttle valve, which could damage it, or lead to priming.

It is used in conjunction with the reversing lever to start, stop and to control the locomotive's power although, during steady-state running of most locomotives, it is preferable to leave the throttle wide open and to control the power by varying the steam cut-off point (which is done with the reversing lever), as this is more efficient.

This is not always a requirement; in fact, the thrust of a solid-fuel rocket is not controllable after ignition, and is instead pre-planned by varying the shape of the void down the center of the booster when the fuel is molded.

However, liquid-propellant rockets can be throttled by means of valves which regulate the flow of fuel and oxidizer to the combustion chamber.

They are also useful in situations where the airspeed of the vehicle must be limited due to aerodynamic stress in the denser atmosphere at lower levels (e.g. the Space Shuttle).

Rockets characteristically become lighter the longer they burn, with the changing ratio of thrust:weight resulting in increasing acceleration, so engines are often throttled (or switched off) to limit acceleration forces towards the end of a stage's burn time if it is carrying sensitive cargo (e.g. humans).

Vehicles not equipped with the EPC warning light indicate issues with the throttle by illuminated check engine symbol.