Diesel engine runaway
Diesel engine runaway is a rare condition affecting diesel engines, where the engine goes out of control, 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.
A gasoline engine equipped with a carburetor employs a butterfly valve, controlled by the throttle mechanism, to control the volume of air (and, thus, the amount of fuel) taken into the engine to control engine speed. A diesel engine's speed is controlled by varying the amount of fuel supplied to the cylinders.
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 feedback loop is created. The engine reaches a point where it is generating so much 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.
A runaway condition can also result from oil supplied by 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 or worn or incorrectly assembled throttle linkage. In vehicles or installations that use both diesel engines and bottled gas (e.g. propane, natural gas, acetylene) a gas leak into the engine room could also provide fuel for a runaway, via the engine air intake.
Diesel engines being used in industrial environments are subject to external hydrocarbons being introduced into the atmosphere and then being sucked into the air intake systems. This dangerous situation occurs at chemical plants, refineries, drilling sites or any environment where hydrocarbons are being produced. The BP Texas City facility was destroyed when this occurred in 2005. Federal law mandates the use of air shut off valves or esd valves on diesel engines used on offshore drilling rigs.
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. Engines fitted with a decompressor can also be stopped by operating the decompressor, and in a vehicle with a manual transmission it is sometimes possible to stop the engine by engaging a high gear (i.e. 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.
- Wellington, B.F.; Alan F. Asmus (1995). Diesel Engines and Fuel Systems. Longman Australia. ISBN 0-582-90987-2.
- "FIE system; diesel fuel system; boat fuel system". Tb-training.co.uk. Retrieved 2012-06-22.
- Launer, Donald; William G. Seifert, Daniel Spurr (2007). Lessons from My Good Old Boat. Sheridan House, Inc. pp. 161–162. ISBN 1-57409-250-2.
- Overspeed (engine)
- A General Electric ES44AC locomotive in the process of a runaway
- Video of diesel truck engine runaway
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