overspeed

(or racing), an abrupt spontaneous increase in the rate of shaft rotation of an internal-combustion engine that can lead to destruction of the engine. It is caused by a substantial increase in the supply of fuel-air mixture or fuel to the engine’s cylinders with light external loading, which occurs upon failure of the throttle valve in carburetor engines or of the high-pressure fuel injection pump in diesel engines. In two-cycle supercharged diesel engines, overspeed can also occur when the amount of oil entering the cylinders from the air filter is increased.

At the first signs of overspeed, the fuel supply is cut off and a load is applied to the engine; some diesel engines are equipped with an emergency-stop device, which shuts off the air supply to the cylinders.