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. Often, an engine coolant line also runs through it in order for the engine to draw intake air at a certain temperature (the engine's current coolant temperature, which the ECU senses through the
relevant sensor) and therefore with a known density. 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, which is mechanically connected to the throttle linkages, which, in turn, rotate the throttle plate. In cars with
electronic throttle control (also known as "drive-by-wire"), an
electric actuator controls the throttle linkages and the accelerator pedal connects not to the throttle body, but to a sensor, which outputs a signal proportional to the current pedal position and sends it to the
ECU. The ECU then determines the throttle opening based on the accelerator pedal's position and inputs from other engine sensors such as the engine coolant temperature 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 (closed) 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, immediately drawn inside by its vacuum. Usually a mass airflow sensor measures this change and communicates it to the ECU. The ECU then increases the amount of fuel injected by the injectors in order to obtain the required
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
solenoid 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 to allow the engine to idle when the throttle is closed. from the E92
BMW M3 showing eight individual throttle bodies A throttle
body is somewhat analogous to the
carburetor in a non-injected engine, although it is important to remember that a
throttle body is not the same thing as a
throttle, and that carbureted engines have throttles as well. A throttle body simply supplies a convenient place to mount a throttle in the absence of a carburetor venturi. Carburetors are an older technology, which mechanically modulate the amount of air flow (with an internal throttle plate) and combine air and fuel together (
venturi). Cars with fuel injection don't need a mechanical device to meter the fuel flow, since that duty is taken over by injectors in the intake pathways (for
multipoint fuel injection systems) or cylinders (for
direct injection systems) coupled with electronic sensors and computers which precisely calculate how long should a certain injector stay open and therefore how much fuel should be injected by each injection pulse. However, they
do still need a throttle to control the airflow into the engine, together with a sensor that detects its current opening angle, so that the correct air/fuel ratio can be met at any RPM and engine load combination. The simplest way to do this is to simply remove the carburetor unit, and bolt a simple unit containing a throttle body and fuel injectors on instead. This is known as
single-port injection, also known by different marketing names (such as "throttle-body injection" by
General Motors and "central fuel injection" by
Ford, among others), and it allows an older engine design to be converted from carburetor to fuel injection without significantly altering the
intake manifold design. More complex later designs use intake manifolds, and even
cylinder heads, specially designed for the inclusion of injectors.
Multiple throttle bodies Most fuel injected cars have a single throttle, contained in a
throttle body. Vehicles can sometimes employ more than one throttle body, connected by linkages to operate simultaneously, which improves
throttle response and allows a straighter path for the airflow to the cylinder head, as well as for equal-distance intake runners of short length, difficult to achieve when all the runners have to travel to certain location to connect to a single throttle body, at the cost of greater complexity and packaging issues. At the extreme, higher-performance cars like the E92
BMW M3 and
Ferraris, and high-performance motorcycles like the
Yamaha R6, can use a separate throttle body for each cylinder, often called "
individual throttle bodies" or ITBs. Although rare in production vehicles, these are common equipment on many racing cars and modified street vehicles. This practice harks back to the days when many high performance cars were given one, small, single-venturi carburettor for each cylinder or pair of cylinders (i.e. Weber, SU carburettors), each one with their own small throttle plate inside. In a carburettor, the smaller throttle opening also allowed for more precise and fast carburettor response, as well as better atomization of the fuel when running at low engine speeds. == Other engines ==