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It’s commonly assumed that a diesel engine works, in principle, the same as a gasoline engine, but when you hear of a diesel engine runaway, a lot of people do not know how to stop a diesel. While the basic function is nearly identical, there are some unique variables in the operation of a diesel engine that set it apart from its gasoline counterparts, and also force it to exhibit some of its own issues.

To compare and contrast, let’s first look at the function of a gasoline engine. Most gasoline engines are four-stroke in design, which means that there are four unique strokes to the reciprocation of a piston: Intake, compression, combustion and exhaust, or more commonly known as suck-squish-bang-blow. The amount of air being drawn into the engine for the perfect 14.7:1 air/fuel ratio is determined by the throttle body, which is a butterfly valve on top of the engine. The ignition source of a gasoline engine is the spark plug, so when the engine is instructed to shut off, spark is removed and the engine comes to a stop.

Now let’s move onto the diesel. A diesel engine is either two- or four-stroke in design. But for the sake of the conversation, we will only discuss four-stroke as it is now the most common design in commercial and consumer application. The four-stroke diesel uses the same suck-squish-bang-blow method as a gasoline engine, but it totally lacks spark plugs and a throttle body. The perfect 14.7:1 air/fuel mixture of a diesel is determined by the fueling system. Diesels ignite their fuel by taking advantage of the laws of thermodynamics: compressing a gas creates heat, and diesels compress their air charge so tightly that when the fuel is injected directly into the cylinder, it immediately combusts. Diesel’s fueling is controlled by a governor, which in principle is basically designed to maintain engine speed as inputted by the operator. Injectors themselves are mechanically actuated by their own lobe on the cam shaft because of their incredibly high injection pressure of 30,000PSI+. So, because there is no air control or electric ignition source, a diesel’s engine speed is controlled by varying the amount of fuel supplied to the cylinders.

Now we will pose the question: What happens when you can’t shut the fuel off to a diesel? We now have a runaway engine that can’t be controlled.

The runaway diesel is relatively rare, but it can still affect diesels in these circumstances. The runaway is when the engine draws extra fuel from unintended sources, and starts over-speeding past the redline to higher and higher speeds, sometimes getting up to 10 times the engines rated speed, until it is destroyed by mechanical failure or seizure due to lack of lubrication. There are two ways diesels can runaway.

 1. Excess Fuel Supply

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As covered, the cylinders in a diesel engine are fed by an injector which is fed by an injector pump, that takes the standard fuel supply and compresses it to north of 15,000psi. Engine speed requested by the operator adjusts the amount of fuel entering the cylinder via the injectors, while the governor regulates the fuel pressure in the lines and limits the maximum engine speed. Any failure of any part of the engine can result in uncontrolled levels of fuel entering the cylinder and causing a runaway. Mechanical diesels of the past, such as the 12-valve Cummins, are susceptible to this because there is no electronic control of the injection system, however modern diesels are not immune to this either.

2. Oil Intake:

Because diesel engines operate by sheer heat of compression, the kinds of fuel they can use are exclusively limited to their combustibility. There are many military diesel engines that are designed as multi-fuel engines and can quite literally run on anything that burns. Since motor oil has similar energy and combustion properties as diesel fuel, any feedback into the engine will result in this runaway scenario. The most common points of entry are past the cylinder rings, where the oil becomes mist and burns. The extra fuel causes the engine revolutions to increase, causing still more oil mist to be forced out of the crankcase 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 normal fuel supply cut off entirely, and will run faster and faster until it is destroyed.

Another common entry point is from a failed oil seal on a turbocharger, overfilling the crankcase with oil or other mechanical issues.

 

There are multiple ways of stopping a runaway. The most common method used is to starve the engine of air. Some industrial engines have a mechanical shutter system that will seal the intake system. Consumer engines generally do not have this feature, and covering the air intake or smothering the engine with a CO2 fire extinguisher will stop the engine. Engines in operation that are fitted to a manual transmission can sometimes be stopped by shifting to a high gear and using the braking system in an attempt to bring the engine below its minimum sustainable RPM speed and causing it to stall.

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