In internal combustion engines, the throttle is the mechanical device that directly regulates and controls the amount of air entering the engine. This indirectly controls the amount of air/fuel mixture that is burned during each engine cycle, as the carburetor or fuel injectors maintain a relatively consistent level fuel/air ratio. The throttle is usually a butterfly valve, which consists of a flat disc that rotates on a metal rod. In the fuel-injected gasoline engines, the throttle is usually located in the throttle body.
Throttle body basics
The throttle body is a piece of mechanical equipment that controls the input of air into the engine in response to the accelerator pedal input of the driver. It is usually located between the air filter box and the intake manifold of the engine. The mass air flow sensor will either be attached to the throttle body or be located nearby. It sends a signal to the Electronic Control Unit, which then determines the proper amount of fuel that is required and sends the fuel that is needed to maintain the proper fuel/air mixture. Frequently, there is a throttle position sensor (TPS) that is attached to the shaft of the throttle that tells the ECU whether the throttle is in the closed position, the idle position, the wide open position, or any position in between.
Fuel delivery evolution
Fuel-injected engines with throttle bodies were the successor (in terms of fuel delivery) to carbureted engines. Where the carbureted engines combined fuel delivery with air intake to maintain the proper fuel to air ratio, fuel injectors handle the fuel delivery, and throttle bodies handle the air delivery. The intermediate step between the two was the vehicle with throttle body injection. This is where the fuel injectors were actually inside the throttle body itself.
Benefits of a throttle body
As the fuel delivery and air delivery functions are separated and monitored through many different channels of electronic feedback systems, throttle bodies with fuel injection make for a much more well-regulated and finely controlled fuel consumption process. This not only results in a superior gasoline miles-per-gallon rating, but significantly reduces the amount of automotive exhaust emissions from the internal combustion process.
Usage of throttle bodies
The vast majority of vehicles have one throttle body as part of their air intake system. Some vehicles have more than one, which are connected together functionally (via mechanical linkages or electronic sensors) that serve to improve throttle response. Extremely high-end/high-performance vehicles may have a throttle body for every cylinder in their engine. These are called individual throttle bodies or ITB's.
On the vast majority of vehicles that have throttle bodies, there is also a secondary electronic circuit that sends feedback information to the ECU called the idle air control (IAC). The IAC helps to control air flow through the throttle body when the engine is idling. The IAC uses a computer-controlled valve to help regulate air flow.