A throttle valve is a device installed in the fuel systems of engines which regulates the pre-determined amounts of fuel and air which reach the combustion areas of the engines. The amount of fuel and air that goes into the combustion chamber regulates the engine speed and, therefore, engine power. In a fuel-injection engine, it is part of the metering device that feeds fuel to the fuel injectors. In a turboprop, turbojet or fanjet engine, the throttle valve is a metering valve in the system that feeds fuel to the combustion chambers. 
The throttle valve is connected to the accelerator pedal on a car or truck, or to the throttle knob on a piston engine airplane. When power is needed, the operator opens the throttle/accelerator which in turn increases the supply of fuel/air mixture thereby increasing the speed of the engine. And vice versa, when the throttle valve closes it reduces the flow of air/fuel mixture. 
A throttle valve has a control member which is angularly positioned by two stepper motors which drive the control member through differential gearing. The drives to the control member are arranged so that a single step displacement of the respective motors in the same direction drives the control member by different amounts in opposite directions, providing a high resolution drive. The control member may be driven at a high speed by reversing one of the motors. 
The throttle valve is a butterfly valve that usually consists of a disc mounted on a spindle. The disc is roughly circular, and it has the same diameter as the main air passage in the throat or "venturi".
In a carburetor, the throttle valve is usually located at the bottom of the carburetor, between the jet nozzle and the intake manifold. The throttle spindle is connected to the accelerator in such a manner that when the pedal is depressed, the valve opens. When the pedal is released, the valve closes. Fuel injected engines use throttle valves to regulate engine power, even though the fuel is also regulated through the injectors.