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The throttle body is the part of a cars air intake system that controls the amount of air flowing into a fuel injection engine, in response to driver input. The throttle body is usually located between the air filter box and the intake manifold, and 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, accelerator pedal motion is communicated through the throttle cable, which activates the throttle linkages, which then moves the throttle plate. In cars with electronic throttle control (also known as "drive-by-wire"), an electric motor controls throttle linkages and the accelerator cable connects not to the throttle body but to a sensor instead. The Engine Control Unit (ECU) determines the throttle opening. On some modern cars, the accelerator cable is replaced by a pedal position sensor, which sends the pedal position to the ECU.
When the driver presses on the accelerator, the throttle plate opens up, allowing more air into the intake manifold. Either a throttle position sensor or 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 maintain the desired air-fuel ratio.
Throttle bodies may also contain valves and adjustments to control the minimum airflow during times when the engine is idle. Even in those units that are not "drive-by-wire" there may be a small electric motor that the ECU uses to change the throttle opening.
Many cars have a single throttle body, however more than one may be used, chained together by linkages, to improve throttle response. At the extreme end, cars such as the BMW M1 have a separate throttle body for each cylinder. This is often referred to as "individual throttle bodies", or ITBs.
A throttle body can be considered the equivelent of a carburetor in a non-injected engine. Carburetors combine the jobs of the throttle body and fuel injectors. e.g. to modulate air flow, and to combine air and gas. Cars with throttle body injection (known as CFI by Ford, TBI by General Motors) locate the fuel injectors in the throttle body, which allows older engines to be converted from carburetor to fuel injection without significantly altering the engine design.