Diesel engine runaway
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Diesel engine runaway is a rare condition affecting diesel engines, in which the engine draws extra fuel from an unintended source and overspeeds at higher and higher RPM and producing up to 10 times the engine's rated output until destroyed by mechanical failure or bearing seizure through lack of lubrication.
A gasoline engine is fed by either a carburetor with a butterfly valve or a throttle body with a throttle valve and fuel injectors. In both cases, the operated valves (in case of a car, the gas pedal) control the volume of air-fuel mixture taken into the engine and control engine speed. Finally, the combustion cycle requires an ignition system in form of an electric spark, that is circuited to a switch (in case of a car, the ignition key). Thus in the rare case of throttle linkage failure, it is very easy to bring a runaway petrol engine to a stop by switching off the ignition.
A diesel engine's speed is controlled by varying the amount of fuel supplied to the cylinders in the unit injector and since no air restriction is present (and is moreover accelerated by the exhaust driven turbocharger or compressor mechanism) an unlimited supply of air is thus possible. There are two distinct mechanisms that cause runaways.
Excess fuel supply
As stated above, the cylinders in a diesel engine are fed by a unit injector, which in turn is fed by a crankshaft driven injection pump. Maximum pump RPM (and thus the pressure of fuel supplied to the injectors) is regulated by a centrifugal governor. The operator regulates the nozzle clearance of the injector and the governor thus adjusts the fuel pressure to keep stable RPM. Failure of any part of the mechanism, particularly the governor can cause an uncontrolled amount of fuel to enter the nozzles creating a positive feedback loop.
Oil supply into the intake
In many vehicles, a crankcase breather pipe feeds into the air intake to vent the crankcase without releasing raw hydrocarbon vapors into the atmosphere. In a heavily worn engine, hot gases, including unburnt fuel can blow past the piston rings and into the crankcase. This creates an excess oil mist, which is then drawn from the crankcase into the air intake via the breather. A diesel engine will readily burn this oil mist as fuel, since engine oil has similar energy content and combustion properties as diesel fuel. The extra fuel causes the engine revolutions to increase, causing still more oil mist to be forced out of the crankcase and into the engine, and a positive feedback loop is created. The engine quickly reaches a point where it is generating so much fuel from its own crankcase oil that it can sustain operation even with the normal fuel supply shut off, 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 a worn or incorrectly assembled throttle linkage. In vehicles or installations that use both diesel engines and bottled gas (e.g. propane, natural gas, acetylene) or operate in an area where vapors may accumulate, a gas leak drawn back into the engine air intake can supply unintended fuel.
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.
Stopping a runaway engine
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 CO
2 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 and parking brake fully applied, and letting out the clutch quickly to slow the engine RPM to a stop, without moving the vehicle.
Notable incidents involving diesel engine runaway
- In the Texas City Refinery explosion, an instance of diesel engine runaway is thought to have provided the ignition source that triggered the massive explosion. After the refinery's blowdown stack failed and started releasing raffinate into the air, a pickup truck that had been parked near the blowdown stack with its engine idling was engulfed by the vapor cloud released and the engine began to race. As staff at the refinery attempted to stop the truck's now-overheating engine, it backfired, igniting the vapor cloud and triggering the disaster.
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