Ask the Expert: What is Diesel Engine Runaway? 

We explain the dangerous condition called diesel engine runaway and how it occurs. *Subscribe* to our channel to see each video as it's released.
To understand runaway, you must first understand how a diesel engine works. Unlike gasoline engines which ignite the fuel using spark plugs, a diesel engine draws clean air into a combustion chamber and compresses that air until the intense heat ignites the fuel.
Diesel engines are often used in places where flammable vapors are found, such as around oil and gas wells and chemical plants. If hydrocarbon vapors are accidentally released in the air, they can be ingested into the engine along with the clean air. These vapors act as extra fuel and cause the engine speed to increase. As the engine draws in more vapors, the governor releases less diesel fuel until, eventually, the engine is running off the uncontrolled vapors as its only fuel source. If this situation is not stopped immediately, it can cause the engine to overspeed, the valves to bounce, and flames to pass through the manifold and ignite the vapors.
Once an engine starts to run away, turning off the ignition key won’t stop it. At this point, your only option is to block the air supply and cut off the vapors.