An engine's power can be increased or decreased by the restriction of inlet gases (i.e., by the use of a throttle), but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which the power or speed of an engine is regulated. What is often termed a throttle (in an aviation context) is more correctly called a thrust lever. For a steam engine, the steam valve that sets the engine speed/power is often known as a regulator.
In a gasoline internal combustion engine, the throttle is a valve that directly regulates the amount of air entering the engine, indirectly controlling the charge (fuel + air) 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 placed on the entrance of the intake manifold, or 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 controlled with a throttle pedal or lever via a direct mechanical linkage. In vehicles with electronic throttle control, the manual throttle control sends a signal to the Engine Control Unit (ECU), which then directly controls the position of the throttle valve. This means that the operator does not have direct control over the throttle valve; the ECU can finely control the valve in order to reduce emissions or maximize performance.
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).
The power output of a diesel engine is controlled by regulating the quantity of fuel that is injected into the cylinder. Because the engines do not need to control air volumes, they lack a butterfly valve in the intake tract. An exception to this generalization is newer diesel engines meeting stricter emissions standards, where a throttle 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.
In fuel injected engines, the throttle body is the part of the air intake system that controls the amount of air flowing into the engine, in response to driver accelerator pedal input in the main. 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 largest piece inside the throttle body is the throttle plate, which is a butterfly valve that regulates the airflow.
On many cars, the accelerator pedal motion is communicated via the throttle cable, to activate the throttle linkages, which move the throttle plate. In cars with electronic throttle control (also known as "drive-by-wire"), an electric motor controls the throttle linkages and the accelerator pedal connects not to the throttle body, but to a sensor, which sends the pedal position to the Engine Control Unit (ECU). The ECU determines the throttle opening based on accelerator pedal position and inputs from other engine sensors.
When the driver presses on the accelerator pedal, the throttle plate rotates within the throttle body, opening the throttle passage to allow more air into the intake manifold. Usually an airflow sensor measures this change and communicates with the ECU. The ECU then increases the amount of fuel being sent to the fuel injectors in order to obtain the desired air-fuel ratio. Often a throttle position sensor (TPS) is connected to the shaft of the throttle plate to provide the ECU with information on whether the throttle is in the idle position, wide-open throttle (WOT) position, or somewhere in between these extremes.
Throttle bodies may also contain valves and adjustments to control the minimum airflow during idle. Even in those units that are not "drive-by-wire", there will often be a small electric motor driven valve, the Idle Air Control Valve (IACV), that the ECU uses to control the amount of air that can bypass the main throttle opening.
Many cars have a single throttle body. Others employ more than one, chained together by linkages to improve throttle response. At the extreme, high performance cars like the E92 BMW M3 and high performance motorcycles like the Yamaha R6 use a separate throttle body for each cylinder, often called "individual throttle bodies" or ITBs.
A throttle body is somewhat analogous to the carburetor in a non-injected engine. Carburetors combine the functionality of the throttle body and fuel injectors into one in order to modulate the amount of air flow and to combine air and fuel together. Cars with throttle body injection (called TBI by General Motors and CFI by Ford) locate the fuel injectors in the throttle body, thereby allowing an older engine to be converted from carburetor to fuel injection without significantly altering the engine design.
Steam locomotives normally have the throttle or "regulator" in a characteristic dome at the top of the boiler. 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. The throttle 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. A steam locomotive throttle valve poses a difficult design challenge as it must be opened and closed using hand effort against the considerable pressure (typically 250psi) of boiler steam. Examples include the balanced "double beat" type used on Gresley A3 Pacifics.
Throttling of a rocket engine means varying the thrust level in-flight. This is not always a requirement; in fact, the thrust of a solid-fuel rocket is not controllable after ignition. However, Liquid-propellant rockets can be throttled by means of valves which regulate the flow of fuel and oxidizer to the combustion chamber. Hybrid rocket engines, such as the one used in Space Ship One, use solid fuel with a liquid oxidizer, and therefore can be throttled. Throttling tends to be required more for powered landings, and launch into space using a single main stage (such as the Space Shuttle), than for launch with multistage rockets.
In a jet engine, thrust is controlled by changing the amount of fuel flowing into the combustion chamber. In some instances, a "throttle" is known as a "thrust lever" (as in most Airbus and Boeing aircraft). This is chiefly due to the fact that "throttle" is associated with traditional gasoline engines.
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