Spark Ignition - Fan Motor Brushes

Nearly every underhood fluid has a low enough required energy of ignition to be ignited by sparks (arcs) from brushes in normally operating electric motors. To test the possibility of motor sparks igniting fluid vapors, measurements were made of the spark energy of a cooling fan from a 1991 Plymouth Acclaim. For comparison, measurements were also made of minimum ignition energy of ethylene glycol and water Results of the authors' tests [1] showed:

Spark energy available from normal operation:

Minimum ignition energy :

Published values for minimum ignition energy of various underhood fluids are:

Testing as of 2005 has demonstrated that the minimum energy required to ignite underhood fluids is well below the energy available from the motor spark produced by least one representative motor. Many questions remain; the minimum ignition energy in vehicles may be quite different than laboratory values due to factors such as air flow, temperature and droplet size. Furthermore, propagation from inside motors may be unlikely or impossible.

Until further questions are answered, investigators should maintain an awareness of the potential for motor spark ignition, but should also understand there are reasons to see it as a low probability in most vehicle fires. For more information, see reference [1].

 

References

  1. Shields, L., Scheibe, R., El Sharkawi, M., “Spark Ignition of Underhood Fluids,” SAE 2005-01-1559, 2005.

  2. Pereira, A., “Safe Handling Procedures for Insulating Oil with High Concentration of Combustible Gases,” Bonneville Power Administration, 1996.

  3. Blanc, M. V., Guest, P. G., Von Elbe, G., Lewis, B., “Ignition of Explosive Gas Mixtures by Electric Sparks – III Minimum Ignition Energies and Quenching Distances of Mixtures of Hydrocarbons and Ether with Oxygen and Inert Gases,” Third Symposium on Combustion and Flame and Explosion Phenomena, 1949.

  4. Fitzgerald and Kingsley, Electric Machinery, McGraw-Hill, 1972.