Process of Investigation

Evaluation of Autoignition

To assess the potential for autoignition as an ignition source, the investigator must obtain information about vehicle service condition prior to fire and vehicle operation at the time of fire. With this information, the investigator can compare the potential surface temperatures, conditions, and available fuels in order to evaluate the likelihood of autoignition. To do so, consideration should include:

  1. Vehicle speed at the time of the fire and for several minutes before the fire: If the vehicle had been at idle and operating normally (at idle rpm, engine operating in nominal condition), it is unlikely that surfaces are hot enough to ignite any fluid. [1,2,3]
  2. Vehicle operation: If the vehicle had been operating on a grade, fully loaded, pulling a trailer, or passing, then the probability of high surface temperatures increases. High load operation or high speed in a reduced gear translates to higher surface temperatures.
  3. Fluid vaporization: A stationary vehicle may have hot enough exhaust surfaces to vaporize leaking fluids, but insufficient heat to ignite them. In collisions, hot surface ignition may occur in the engine compartment of a striking vehicle and then flash to the struck vehicle. Ignition of the flammable vapors may also be caused by sources other than hot surfaces.
  4. Air flow conditions: Non-collision fires frequently occur immediately after vehicles come to a stop or slow from high speeds. Air flow at road speeds may prevent the ignition of leaking fluids or the propagation beyond a sheltered area. This is because the air flow may cool the hot surface, dilute the mixture and or reduce the residence time.
  5. Delays in identification of fire: Fires may initiate slowly, unobserved by witnesses for several minutes. Lubricants in particular are able to sustain a small flame (like an oil lamp). Such small fires may take time to become apparent to witnesses who are often concerned with other things - such as injury (in collisions) or their intended activities (in non-collision events). In such cases, autoignition may occur while surfaces are still hot after operation, even though witnesses did not observe it for some time. The appearance of vapor after a collision may be only vapor - or the early evidence of fire (smoke). If witnesses describe the sound of fire initiation (i.e., “I heard a big 'poof' sound”) or distinctive odors, the investigator may be able to isolate the time of fire initiation more closely and therefore be able to evaluate the potential for autoignition. Small flames may also be easier to observe at night and in instances in which hood deformation allows greater visibility to all areas of the engine compartment.
  6. Vehicle maintenance condition: Obtaining a history of vehicle condition from the operator may reveal engine or vehicle conditions related to surface temperatures. Identification of a fouled spark plug may indicate improper combustion and unburned gasoline released into the exhaust system with resultant elevated catalytic converter temperatures.
  7. Shielded manifolds, pockets on hot surfaces: Pockets in which combustible vapors are held in proximity to hot surfaces lower the temperature at which autoignition is possible (increasing the probability of autoignition) [3,4]. With higher residence time, temperatures at which autoignition occurs in a motor vehicle environment may approach (but can never be less than) ASTM E 659-78 autoignition temperature measurements.
  8. Coolant and gasoline: Both need unusually high surface temperatures (as compared to other underhood fluids) for ignition on open (unshielded) manifolds. Unless there is reason to expect temperatures of 900 degrees F or more it is unlikely that coolant will ignite on an open hot surface. Similarly, unless surface temperatures are greater than 1100 degrees F, it is unlikely that gasoline will ignite on an open hot surface [5].
  9. Time: All surface temperatures are likely to be insufficient for autoignition after 3-5 minutes with the engine off.

References

  1. Santrock, J., "Full Scale Vehicle Fire Tests of a Control Vehicle and a Test Vehicle Containing an HVAC Module Made from Polymers Containing Flame Retardant Chemicals," NHTSA docket number 98-3588-190, 1998.
  2. Fournier, E., "Under Hood Temperature Measurements of Four Vehicles," MVFRI , 2004.
  3. Authors rely upon proprietary data from surface temperature tests they conducted which are not available for publication.
  4. Colwell, J. D., et al., “Hot Surface Ignition of Automotive and Aviation Fluids,” Fire Technology, 41, pages 105-123, 2005.
  5. Shields, L, Scheibe, R., “Computer-Based Training in Vehicle Fire Investigation-Part 2: Fuel Sources and Burn Patterns,” SAE 2006-01-0548, 2006.