An accelerator pedal is a device, used in many types of vehicles, that allows an operator to modulate engine power remotely. It is generally paired with a brake pedal, and sometimes a clutch, enabling a driver to control the speed of the vehicle almost exclusively with his feet. An accelerator pedal is typically connected to a throttle directly, either by cables or, electronically, to a computer that mechanically adjusts the throttle based on pedal input.
Beginning with the first engined vehicles, there has always been a need for the driver to adjust engine output in order to control speed. In gasoline-powered internal combustion engines, the accelerator pedal adjusts the amount of air allowed into the combustion chamber, with the corresponding supply of fuel being regulated by a carburetor or fuel injection. In early designs, the pedal itself was directly tied to a butterfly valve, located either in the carburetor itself or the throttle body, that could let more or less air in.
Many modern engines use a drive by wire system, in which there is no direct physical connection between the pedal and the throttle. Rather pedal pressure is translated by a computer, which regulates air intake in response to driver input, while maximizing efficiency. Detractors of this design claim the driver loses a degree of control when a computer is introduced into the equation, but car manufacturers contend technology has reached the point where there is no loss of what some people refer to as driver feel.
The accelerator pedal of a diesel engine engine functions differently. Instead of controlling the flow of air, it adjusts the amount of fuel entering the combustion chamber. In a diesel engine, it is the compression of the fuel that causes it to ignite, as opposed to the introduction of air. Therefore there is no actual throttle. For the driver, however, the effect of pushing the pedal down is the same.
Accelerator pedal designs are almost always developed so that the pedal itself can be pushed down to the floor with the user's toe or upper portion of the foot, while the heel remains on the floor. This allows a much greater degree of control, as the ankle joint can modulate pressure rather than the hip joint, which would effectively be the case if the entire foot rested on the pedal. Designs are also usually much narrower compared to brake pedals, with the idea that the brake should be easier to find in case of an emergency.