Throttle Butterfly
The throttle butterfly is a disc-shaped valve placed in the intake tract of a petrol engine that restricts air flow as its angle changes. Because fuel is reduced proportionally as air is reduced, the butterfly angle directly determines engine power. On a diesel there is no such restricting element; there, power is set by the fuel quantity.
On carburetted or injected petrol engines the butterfly sits on a shaft and carries a lever on the outside. The electronic throttle kit is attached to this lever with a cable; as the butterfly angle increases, the speed rises.
- Even in the closed position the butterfly does not seal completely; idle air passes either around the edge of the butterfly or through a separate idle circuit.
- Play that develops in the butterfly shaft over time leads to different speeds being obtained at the same cable position.
- If the return spring has weakened, the butterfly does not close fully when the cable is released.
The application difference matters: on a diesel the cable is attached to the throttle lever of the fuel pump, on a petrol engine to the butterfly lever. The GS740 kit is declared for diesel and petrol engines with a mechanical throttle lever; in both cases what is attached to is the original mechanical lever, and the lever is not removed. On modern engines with electronic throttle, that is with a sensor pedal, there is no mechanical lever to attach to.
Context in which this term is used: Electronic speed and throttle control on a diesel engine