Overspeed 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 vary by engine type and model and depend 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 failure. The speed of an engine is ordinarily measured in revolutions per minute (RPM).
Examples of overspeed
- In aircraft an engine overspeed will occur if the propeller - ordinarily connected directly to the engine - is forced to turn too fast by high speed airflow while the aircraft is in a dive.
- In jet aircraft an overspeed results when the axial compressor exceeds its maximum operating RPM - this often leads to the mechanical failure of turbine blades, flameout and complete destruction of the engine.
- In vehicles an engine can be forced to turn too quickly by changing to an inappropriately low gear.
- In the event of runaway, a Diesel engine will overspeed due to continually increasing RPM if the condition is not quickly rectified.
Sometimes a regulator or governor is fitted to make engine overspeed impossible or less likely. For example:
- Many steam engines use a centrifugal governor which centrifugally close a throttle to restrict steam flow as engine speed increases.
- In motor vehicles automatic gearboxes will change gear to prevent the engine from turning too quickly. Additionally, almost all modern vehicles are fitted with an electronic Rev limiter device which will cut fuel supply or sparks to the engine to prevent overspeed.
- Some aircraft have constant speed units which automatically change propeller pitch to keep the engine running at the optimum speed.
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.
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