Diesel engine runaway is a condition affecting diesel engines, in which the engine draws extra fuel from an unintended source other than the vehicle’s dedicated fueling system. When this happens, the engine’s speed (RPM) increases dramatically and uncontrollably, producing up to ten times the engine’s rated output, and it only stops when either the extraneous fuel flow is eliminated or the powerplant suffers severe damage (seized bearings, broken rods, crank, etc.).
Remember, oil-burners create combustion by using fuel and air only. There’s no spark. So, the more fuel, the faster that engine is going to run. On diesel engine blow-by, the combination of unburned fuel and engine oil can lead to a runaway condition if blow-by is excessive enough for that mixture to reach the combustion chamber. And, with turbodiesels, oil can sometimes cause an engine to run away.
It’s all about fuel. Most turbochargers’ internals are lubricated by engine oil, which can be the catalyst for a runaway condition if or when it breaches a turbo’s seals. Like blow-by, the lubricant enters the combustion chamber and promotes an rpm increase. In instance like this, oil is the fuel source, and the result is usually dramatic (and expensive).
In a diesel engine, the torque and the rotational speed are controlled by means of quality torque manipulation. This means that, with each intake stroke, the engine draws in air which is not mixed with fuel; the fuel is injected into the cylinder after its contents have been compressed during the compression stroke. The high air temperature near the end of the compression stroke causes spontaneous combustion of the mixture as the fuel is injected.
The output torque is controlled by adjusting the mass of injected fuel; the more fuel injected, the higher the torque produced. Adjusting the amount of fuel received per stroke alters the quality of the air-fuel-mixture, and adjusting the amount of the mixture itself is not required which is why diesel engines do not have throttle valves.
Diesel engines can combust a large variety of fuels, including many sorts of oil, petrol, and combustible gasses. This means that if there is any type of leak or malfunction that increases the amount of oil or fuel unintentionally entering the combustion chamber, the quality of the air-fuel-mixture will increase, causing torque and rotational speed to increase. Fuel and oil leaks causing engine runaways can have both internal and external causes.
Broken seals or a broken turbocharger may cause large amounts of oil mist to enter the inlet manifold, whereas defective injection pumps may cause an unintentionally large amount of fuel to be injected directly into the combustion chamber. If a diesel engine is operated in an environment where combustible gasses are used, a gas leak may result in an engine runaway if the gas can enter the engine’s inlet manifold.
How to prevent a runaway engine
Diesel engine runaway can be prevented by installing inline devices, such as Pacbrake’s PH2 PowerHalt Shut-Off Valve, a tube that contains a butterfly-style valve plate plumbed into the intake airstream. When activated – manually, or electronically/automatically – at a prescribed RPM, the internal blade closes and cuts off airflow, which in turn chokes the engine.
Installing a shut-off valve is a good idea, especially for older rigs with high mileage and engines with excessive blow-by, or trucks with big horsepower that are used for dyno and sled-pull competition. Not only can a shut-off ward off catastrophic engine failure, it also could save a truck’s occupants from being seriously or fatally wounded by shrapnel from an engine explosion.
How to stop a runaway engine
While shutting the engine off by turning the key back from the run position is an understandable knee-jerk thought, doing this won’t work for compression/ignition engines. Unlike gas engines, ignition for a diesel is not the same thing as a gasser’s spark, which also is called ignition.
The key-turn tactic is futile. A runaway oil burner has to be denied fuel – completely and immediately – or choked to death, literally, by squelching all of its air supply, by any means possible. The latter method is more widely used and has saved many engines from total destruction. Here’s what to do:
- Pop the hood and block the air intake, either physically using a cover (cloth) or plug – Air intake shut off systems are the only proven, reliable method to stop a diesel engine once it has begun to run away. Don’t wait until it’s too late to protect your people and equipment from this preventable disaster
- Use CO2 fire extinguisher – The safest method to stop a runaway diesel engine is by directing a CO2 fire extinguisher into the air intake to smother the engine.
- Cut of fuel supply – If your vehicle has a switch that blocks the fuel supply from the tank to the engine, use it. You may also manually disconnect the fuel pump electric connections usually under the rear seat on passenger cars
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 quickly letting out the clutch to slow the engine RPM to a stop, without moving the vehicle. This should be the last option because it can result in catastrophic damage to the whole transmission, mainly the gearbox, but this operation can save the engine.