is a condition in which an engine is allowed or forced to turn beyond its design limit. The consequences of running an engine too fast varies by engine type and model and depends upon several factors, chief amongst them the duration of the overspeed and by the speed attained. With some engines even a momentary overspeed can result in greatly reduced engine life or even catastrophic engine failure. The speed of an engine is ordinarily measured
in revolutions per minute
Examples of overspeed
- In aircraft an engine overspeed will occur should the propeller - ordinarily connected directly to the engine - be forced to turn too fast by high speed airflow while the aircraft is in a dive.
- In vehicles an engine can be forced to turn too fast by changing to an inappropriately low gear.
- Most unregulated engines will overspeed should there be no or little load while power is applied.
Sometimes a regulator or governor
is fitted to make engine overspeed impossible or less likely. For example:
Large diesel engines are sometimes fitted with a secondary protection device which operates if the governor fails. This consists of a flap valve in the air intake. If the engine overspeeds, the air flow through the intake will rise to an abnormal level. This causes the flap valve to snap shut, starving the engine of air and shutting it down.