What Is Runaway?

 

 

Diesel “runaway” is a condition that happens when fuel is unintentionally introduced to a turbocharged engine, from a source other than the vehicle’s dedicated feeling system. When this happens, the engine’s speed (rpm) increases dramatically and uncontrollably, 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.

Are you wondering, “how is this possible?” 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).

Stopping A Runaway Engine

Usually, after defining runaway, “how do you stop it?” is the question that immediately follows. Remember, we’re talking about compression/ignition engines. 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. 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:

Method 1: Pop the hood and block the air intake with a cloth – 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.

Method 2: Use CO2 fire extinguisher – The safest method to stop a runaway diesel engine is by directing a CO2 fire extinguisher into the air intake. 

Method 3: 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.

Can Runaway Be Prevented?

The good news is, yes, 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.

Source: Car Throttle

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