What is a runaway diesel engine?

Diesel engine runaway is when a diesel engine accelerates uncontrollably without any action on the accelerator pedal. This is caused by a sudden increase of fuel in the engine cylinder coming from unwanted sources. The increased energy available for combustion leads to a surge in revolutions per minute (rpm) of the engine and, consequently, the car's speed.

Why does it happen?

Diesel engines are usually designed to allow unlimited air intake into the cylinders, unlike gasoline (petrol) engines with carburetors controlled by throttles that meters the air into the cylinders. Further, diesel engines (also known as compression-ignition engines) do not have spark plugs. Instead, they ignite the air-fuel mixture in the cylinders through compression. The ignition of the fuel that has been injected into the combustion chamber is initiated by the high temperature which a gas achieves when compressed (so-called adiabatic compression). In fact, the combustion in the diesel engine's cylinders operates by compression of any sufficient fuel, even vegetable fuel or coal dust. Finally, diesel engines can be prone to drawing fuel from unintended sources (see more below). Therefore, given that a diesel engine has unlimited air intake, does not need spark plugs, and can draw fuel from unintended sources, any unwanted fuel supply can lead to infinite rotation, i.e., a runaway diesel engine situation.

What are the effects of runaway diesel?

A runaway diesel engine situation can be heard due to the surge in revolutions (rpm) and can be visible as the burning fuel will create a smokescreen from the exhaust and also around the engine. In addition, there is a high risk of mechanical failure of the engine or bearing seizure when the engine overspeeds at higher revolutions per minute (rpm).

How can diesel engines draw fuel from unintended sources?

Diesel engine runaway in cars can be caused by various reasons. One common cause is associated with turbo engines. Many diesel engines are turbocharged and are lubricated by oil. In case of a leakage of oil due to worn turbos and failure of turbo seals, high-pressure oil may be sent into the engine or the air intake, causing the runaway. Further, excess thrust movements and pressure on the machine, together with worn bearings and seals, can result in oil leaks and a runaway diesel engine. Broken internal pipes, overfilling the crankcase with oil, or a worn throttle linkage can also be the underlying cause of a runaway.

Another cause is due to mist caused by flammable gases. On the one hand, hydrocarbon vapor intended to be exhausted into the atmosphere can leak through the sides of the worn-out piston. Over time, oil mist can collect and enter the cylinder through the engine crankcase. This can accelerate the engine speed, which then leads to engine runaway. On the other hand, when an engine is highly worn, excess gasses can make their way into the crankcase, then into the cylinders via the so-called crankcase breather pipe. A diesel engine is equipped with such a breather pipe to vent the crankcase. The tube, which is connected to the engine's air intake, eliminates the release of raw hydrocarbon vapors into the atmosphere.

This results in an excess of unwanted hydrocarbon vapor that diesel engines easily burn as fuel since this mist and the ordinary diesel fuel have similar properties. This will cause the engine's revolutions to increase and create even more smoke forced out of the crankcase and into the engine. A diesel engine will make such an excessive amount of extra fuel that it will run even without its regular fuel supply. This eventually results in the machine running faster and faster until it is covered in black or blue smoke when this excess fuel oil is burned up, and ultimately the engine is destroyed.

Preventative measures

Avoid a case of engine runaway: Regular checks of the turbo seals need to be done to avoid any occurrence of a lubrication oil leak. Moreover, regular evaluations of the status of the engine's piston need to be carried out to ensure its efficiency. Also, care has to be exercised to avoid overfilling the cylinder with fuel. This ensures that the fuel seeps only through the piston rings. Also, make sure you buy a vehicle with a diesel engine from a well-established diesel engine manufacturer.