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Overspeed (engine)

 

 
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Overspeed (engine)
 
 
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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 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.






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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 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
Tachometer

A tachometer is an instrument that measures the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute on a calibrated analog dial, but digital displays are increasingly common....
 in revolutions per minute
Revolutions per minute

Revolutions per minute is a units of measurement of frequency: the number of Turn completed in one minute around a rotation around a fixed axis....
 (RPM).

Examples of overspeed


  • In aircraft an engine overspeed will occur should the propeller
    Propeller

    A propeller is a type of fan which transmits power by converting rotational motion into thrust. It can be used to drive an fixed-wing aircraft, ship, or the fluid within a pump....
     - 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
    Regulator (automatic control)

    In automatic control, a regulator is a device which has the function of maintaining a designated characteristic. It performs the activity of managing or maintaining a range of values in a machine....
     engines will overspeed should there be no or little load
    Mechanical work

    In physics, mechanical work is the amount of energy transferred by a force acting through a distance. Like energy, it is a scalar quantity, with SI of joules....
     while power is applied.


Overspeed protection

Sometimes a regulator or governor
Governor (device)

A governor, or speed limiter, is a machine used to measure and regulate the speed of a machine, such as an engine. A classic example is the centrifugal governor, also known as the James Watt or fly-ball governor, which uses weights mounted on spring-loaded arms to determine how fast a shaft is spinning, and then uses proportional contr...
 is fitted to make engine overspeed impossible or less likely. For example:

  • Many steam engine
    Steam engine

    File:Steam-powered fire engine.jpgA steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines have a long history, going back at least 2000 years....
    s rotate weights which centrifugally
    Centrifugal force

    In classical mechanics, centrifugal force is an outward force associated with rotation. Centrifugal force is one of several so-called pseudo-forces , so named because, unlike Fundamental interaction, they do not originate in interactions with other bodies situated in the environment of the particle upon which they act....
     close a throttle
    Throttle

    A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases ....
     to restrict steam flow as engine speed increases.
  • In motor vehicles automatic gearboxes will change gear to prevent the engine turning too fast.
  • Some aircraft have constant speed units which automatically change propeller pitch to keep the engine running at the optimum speed.


Large diesel engine
Diesel engine

A diesel engine is an internal combustion engine which operates using the diesel cycle . Diesel engines have the highest thermal efficiency compared to any internal combustion or external combustion engine....
s 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.

See also

  • Rev limiter
    Rev limiter

    A rev limiter is a device fitted to an internal combustion engine to restrict its maximum Revolutions per minute. This is usually carried out to prevent damage to the engine, however sometimes these devices are fitted to prevent an engine reaching the point at which it develops maximum power....