Reference
Vehicle Fluid Auto-ignition Temperatures - Redux
By Mark C. Hughes, P.Eng.
© 2005 Sintra Engineering Inc.
My last column discussed auto-ignition temperatures for flammable liquids. Since that time, I have been involved in the revisions related to the vehicle fire section of NFPA 921 through the Society of Automotive Engineers (SAE). As part of the discussions some interesting further developments have surfaced.
Since the summer, the October 2005 edition of Fire and Arson Investigation from the IAAI presents the results of Ford Testing as it relates to flammable liquid autoignition temperatures. As before the autoignition temperatures presented were higher than those that have been previously published from ASTM International standard tests for autoignition temperatures (ASTM E659-78(2000)). There was a notable difference between the data presented in Fire Technology (through NFPA) and the results from Ford presented in the IAAI magazine. In the Ford results, the suggested auto-ignition temperatures are significantly higher than those reported in the NFPA journal article written by engineers from Exponent Failure Analysis Associates.
In particular, there are some problems with the article written by the Ford researchers in the recent article in the IAAI magazine. In the Ford article there is a general lack of referenced scientific research (only one reference citation of empirical research published in 1974) and limited test data. The Exponent article contains more scientific research data and 37 citations of different empirical research. It is also interesting that although the Fordarticle was published several months after the Exponent article, the Ford article failed to reference it or cite any of the numerous sources on prior research cited in the Exponent article. The Ford document was based upon 300 test points using a single fluid and the conclusions drawn were based upon a spreadsheet analysis.The Exponent Article was based on over 2500 test points for 14 different fluids to support their positions. The Exponent article also lists at least some ignitions at temperatures at around 1070 degrees F, whereas the Ford tests indicate they could not get ignition until the lowest point on their data was around 1325 degrees F. That difference, of approximately 250 degrees F in temperature is significant when dealing with engine compartment real world situations and scenarios. In addition, the Ford tests were based upon 1 drip every 2 seconds onto the exhaust manifold, where the Exponent testing including both the drip test and a liquid spray/stream test.
The final sentences of the Exponent article states that "Unlike the flash points and the minimum auto ignition temperature, which are all well defined combustion properties that can be measured with accepted ASTM standards, the temperature at which hot surface ignition occurs is not a fundamental fluid property and is strongly coupled to numerous factors, including the properties of the surface, the liquid spray or stream and the local airflow. Because of this coupling, the temperature required for ignition can vary. Hot surface ignition data can not be easily extrapolated to different conditions ..."
So, what does it all mean? The bottom line is that be very careful referencing autoignition temperatures for your specific application without considering the methods testers used to develop their non-standard autoignition temperatures.
Mark Hughes, P. Eng. is a principal with Sintra Engineering, a Forensic Engineering firm.
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