Propagation Time

Knowledge of expected propagation times may assist the fire investigator in evaluating hypotheses as to the initiating fuel and ignition source of a fire. For example, in one series of tests of non-gasoline fed engine compartment fires, propagation to the passenger compartment took from approximately 5 minutes to almost 25 minutes. In contrast, gasoline pool fires propagated to the passenger compartment in approximately 1-4 minutes [1].

The next pages will show video clips of crash tests that resulted in fire and examples of propagation times measured in full-scale vehicle fire tests. Such testing is helpful in establishing sample propagation times. It should also be understood that test results are highly dependent on the conditions of the tests and may vary substantially. Some factors of importance that may be difficult to quantify in field fire investigations are wind speed and direction, leak rates (in fluid fires) and electrical resistance (in fires resultant from damaged and shorted wires).

In two unusual frontal crash tests, vehicles caught fire spontaneously (with no external ignition source or artificial fuel application). The tests were performed with all the fluid reservoirs filled, operating electrical systems and engines running. Instrumentation recorded temperatures, the presence of vapors in different regions of the engine compartment, and the fire was captured by high-speed film coverage. 

In a frontal crash test of a 1998 Honda Accord, it was struck by a 3,613 lb. deformable moving barrier in a frontal impact at 65 mph; the impact was offset 50% to the driver's side and at a 21 degree angle such that the principal direction of force was through the center of gravity of the Accord [2]. The engine of the Accord was operated at high idle for 55 minutes prior to impact to simulate on-road exhaust system operating temperatures.

The high-speed film of the crash tests shows a flame in the engine compartment from approximately 40 milliseconds to 400 milliseconds after impact. Visible flames were observed again in the region of the windshield washer fluid reservoir approximately 5 minutes after impact until the fire was extinguished after an additional 5 minutes. Instrumentation and testing indicated that the first flames were likely due to power steering fluid ignition on the hot exhaust manifold. It should be noted that mechanical and electrical sparks may also have been present and other fluids were released in the crash.

Further testing indicated that the methanol vapors in the crash-compromised windshield washer fluid reservoir could sustain a non-luminous flame for 5 minutes before spreading to other materials and becoming luminous. These results were consistent with the initial flame causing ignition of the windshield washer fluid vapor in the reservoir, an incubation period, and then fire propagation.

References

1. Tewarson, A., et al., "Post Collision Motor Vehicle Fires," Prepared for Motor Vehicle Fire Research Institute, October 2005.

2. Jensen, J., et al., "Evaluation of Motor Vehicle Fire Initiation and Propagation Part 11: Crash Tests on a Front-Wheel Drive Passenger Vehicle." NHTSA docket number 98-3588-179, 1998.