Engines that are maintained in proper operating condition seldom reach speeds above those for which they are desired. However, conditions may occur to cause excessively high operating speeds, such as when a ships propeller comes out of the water in rough seas. Operation of a diesel engine at excessive speeds is extremely dangerous because of the relatively heavy construction of the engines rotating parts. A high-speeding engine develops inertia1 and centrifugal forces that may seriously damage parts or even cause them to fly apart. Therefore, you must know why an engine may reach a dangerously high speed and how to bring it under control when excessive speed occurs. In some two-stroke cycle engines, lubricating oil may leak into the cylinders as a result of leaky blower seals or broken piping. Even though the fuel is shut off, the engine may continue to operate, or even run away, as a result of the combustible material coming from the un-controlled source. Engines in which lubricating oil may accumulate in the cylinders generally have an automatic mechanism that shuts off the intake air at the inlet passage to the blower. If there is no air shutoff mechanism and if shutting off the fuel will not stop an engine that is over-speeding, a cloth article such as a blanket or a pair of dungarees should be placed over the engines intake to stop airflow. This action will subsequently stop the engine.

Excessive engine speeds are more commonly found where there is an improperly functioning regulating governor than where lubricating oil accumulates in the cylinders. To stop an engine that is overspeeding because of lubricating oil in the cylinders, stop the flow of intake air. To accomplish an emergency shutdown or reduction of engine speed when the regulating governor fails to function properly, shut off or decrease the fuel supply to the cylinders.

You can shut off the fuel supply to the cylinders of an engine in several ways, either manually or automatically:

1. Force the fuel control mechanism to the NO FUEL position.

2. Block the fuel line by closing a valve.

3. Prevent the mechanical movement of the injection pump.

Overspeed safety devices automatically operate the fuel and air control mechanisms. As emergency controls, these safety devices operate only in case the regular speed governor fails to maintain engine speed within the maximum design limit. Devices that bring an overspeeding engine to a full stop by completely shutting off the fuel or air supply are generally called OVERSPEED TRIPS. Devices that reduce the excessive speed of an engine, but allow the engine to operate at safe speeds, are more commonly called OVERSPEED GOVERNORS.