Diesel engine runaway is a rare condition affecting diesel engines, where the engine goes out of control, consuming its own lubrication oil and running at higher and higher RPM until it overspeeds to a point where it destroys itself either due to mechanical failure or engine seizure through lack of lubrication. For instance, a 1800 rpm engine can run to 4000 or 5000 rpm or beyond.[1]
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Unlike a gasoline engine, which has a butterfly valve controlled by the throttle mechanism to control engine speed, a diesel engine's speed is controlled by varying the supply of fuel.
In many vehicles, a crankcase breather pipe feeds into the air intake to vent the crankcase; on a highly worn engine, gases can blow past the sides of the pistons and into the crankcase, then carry oil mist from the crankcase into the air intake via the breather. A diesel engine will run on this oil mist, since engine oil has the same energy content as diesel fuel, and so the engine revolutions increase as this extra "fuel" is taken in. As a result of increased revolutions, more oil mist is forced out of the crankcase and into the engine, and a vicious cycle is created. The engine reaches a point where it is generating enough oil mist from its own crankcase oil that shutting off the fuel supply will not stop it and it will run faster and faster until it is destroyed.
The unwanted oil can also come from failure of the oil seals in a turbocharged diesel engine, from overfilling the crankcase with oil, or certain other mechanical problems such as a broken internal fuel pipe. In vehicles or installations that use both diesel engines and bottled gas, a gas leak into the engine room could also provide fuel for a runaway, via the engine air intake.[2]
Several ways to stop a runaway diesel engine are to block off the air intake, either physically using a cover or plug, or alternatively by directing a CO2 fire extinguisher into the air intake to smother the engine.[3] Engines fitted with a decompressor can also be stopped by operating the decompressor, and in a vehicle with a manual transmission it is possible to stop the engine by engaging a high gear (ie 4th, 5th, 6th etc), with foot brake & parking brake fully applied, and letting out the clutch quickly to slow the engine RPM to a stop, without moving the vehicle.