When a diesel engine is exposed to an external fuel source, such as an airborne combustible hydrocarbon in the surrounding environment, it naturally ingests the mixture into the air intake system. Since diesel engines control fuel and not air, the engine can no longer maintain speed control.
On January 13, 2003 in Rosharon, Texas, two tank trucks unloaded waste liquids into an open collection pit at a disposal facility. Unknown to either the driver or to disposal facility personnel, the waste material was highly volatile, and a flammable vapor cloud formed in the unloading area. Vapor was drawn into the air intakes of the trucks’ running diesel engines, causing one of them to over-speed, backfire, and ignite the flammable cloud.
This incident allowed the unique opportunity to compare two identical engines operating side-by-side, exposed to the same external fuel source where one ran away and the other did not. This side-by-side comparison of mechanical fingerprints found on the engine’s components showed proof of fuel ingestion and engine over-speed followed by a “destructive backfire.”
What is the solution?
Controlling combustion air is the key to absolute engine control during an emergency. The first and most direct method for eliminating a runaway is to simply shut off the engine at the loading/unloading destination. If there is a need to keep the engine running, an air intake shut-off valve should be installed.