DPM,
I really don't care about the term as long as we both understand just how this valve is used on our cars. The following will set the record straight in that you are correct about the term's use in this situation. As long as we agree that the "throttle" is not used to regulate engine power I think we'll be OK.
Humor me if you will...
Throttle
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This article is about the control of engine power. For other uses, see
Throttle (disambiguation).
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), 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.
Contents
Internal combustion engines
A cross-section view of a throttle valve
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).
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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.
Throttle body
The components of a typical throttle body
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.
Throttle body showing
throttle position sensor. The throttle cable attaches to the curved, black portion on the left. The copper-coloured coil visible next to this returns the throttle to its idle position when the pedal is released.
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.
Image of
BMW S65 from the e92
BMW M3 showing eight individual throttle bodies
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.
Other engines
Most
engines have some kind of throttle control, though the particular way that power is regulated is often different.
Liquid rockets are throttled by controlling the pumps that send liquid fuel and oxidizer to the combustion chamber.
Solid rockets are not controllable once their solid fuel is ignited.
Hybrid rockets, like the one used in
Space Ship One, use solid fuel and liquid oxidizer; thus, throttling is possible through the oxygen pumps, as in a full liquid rocket motor.
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 Airbus and Boeing
aircraft). This is chiefly due to the fact that "throttle" is associated with traditional gasoline engines.
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