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

Tachometer

A tachometer is an instrument measuring 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 analogue dial, but digital displays are increasingly common...

 in revolutions per minute

Revolutions per minute

Revolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...

 (RPM).

Examples of overspeed

• In aircraft an engine overspeed will occur if the propeller
  Propeller (aircraft)
  Aircraft propellers or airscrews convert rotary motion from piston engines or turboprops to provide propulsive force. They may be fixed or variable pitch. Early aircraft propellers were carved by hand from solid or laminated wood with later propellers being constructed from metal...
  
   - 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
  Axial compressor
  Axial compressors are rotating, airfoil-based compressors in which the working fluid principally flows parallel to the axis of rotation. This is in contrast with other rotating compressors such as centrifugal, axi-centrifugal and mixed-flow compressors where the air may enter axially but will have...
  
   exceeds its maximum operating RPM - this often leads to the mechanical failure of turbine blades, flameout
  Flameout
  A flameout refers to the failure of a jet engine caused by the extinction of the flame in the combustion chamber. It can be caused by a number of factors, including fuel exhaustion; compressor stall; insufficient oxygen supply; foreign object damage ; severe inclement weather; and mechanical...
  
   and complete destruction of the engine.

• In vehicles an engine can be forced to turn too quickly 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. The measurable property of a device is managed closely by specified conditions or an advance set...
  
   engines will overspeed should there be no or little load
  Mechanical work
  In physics, work is a scalar quantity that can be described as the product of a force times the distance through which it acts, and it is called the work of the force. Only the component of a force in the direction of the movement of its point of application does work...
  
   while power is applied.

Overspeed protection

Sometimes a regulator or governor

Governor (device)

A governor, or speed limiter, is a device 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 Watt or fly-ball governor, which uses a rotating assembly of weights mounted on arms to determine how fast the engine...

 is fitted to make engine overspeed impossible or less likely. For example:

• Many steam engine
  Steam engine
  A steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...
  
  s use a centrifugal governor
  Centrifugal governor
  A centrifugal governor is a specific type of governor that controls the speed of an engine by regulating the amount of fuel admitted, so as to maintain a near constant speed whatever the load or fuel supply conditions...
  
   which centrifugally
  Centrifugal force
  Centrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...
  
   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 , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...
  
   to restrict steam flow as engine speed increases.
• In motor vehicles automatic gearboxes will change gear to prevent the engine from turning too quickly.
• 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 that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

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.