Autoignition Temperatures in Vehicle Environments

Detailed Results

The autoignition temperature of a fluid varies considerably with circumstances. The tables below list results of published tests of autoignition temperatures under conditions likely to be found in vehicle environments.

The tables show a wide variation in the measured temperatures. For example, different researchers have found DOT 3 brake fluid ignites on hot metal at temperatures ranging from 520 to 752 degrees F (Table 2). The ignition of brake fluid at 520 degrees F was below that of the fluids for which ASTM test measurements were available; 540 degrees F was the lowest measurement documented on a brake fluid Material Safety Data Sheet. It is expected that the autoignition temperature determined by ASTM procedures would not be higher than that found in vehicle environments. The discrepancy in this case is not significant and likely due to variation in the fluids tested or experimental error.

Tests of power steering fluid (Table 8) showed ignition inside a heated crucible without imposed airflow at 590 degrees F. When the same researchers applied power steering fluid to the top of a heated and inverted hemisphere with airflow at 7.2 feet/second, no ignition was observed.

As conditions support increased residence time, reduced air flow, ideal mixture and other ideal conditions, autoignition temperatures can approach the values measured with laboratory ASTM procedures. For example, the lowest field autoignition measurements for coolant (Table 3) were recorded in tests of a vehicle with a tightly shielded exhaust manifold [1].

In an investigation, hot surface ignition of a material can be ruled in or out by comparing the autoignition temperature of the material (under similar conditions) with the expected temperature of a surface in the vehicle. Surface temperature data are summarized in the Ignition section

You can click on the link below to review surface temperature (in the Ignition Section) information now, or continue with studying Fuels.

 Review characteristics of surface temperature? click here

For the following data (Tables 1-10), the materials tested were new (not used in an operating vehicle) unless they were designated “used.” All values have been rounded to the nearest 5 degrees. For comparison, each table also starts with the minimum autoignition temperature measured in the laboratory using ASTM standard procedures (without rounding to the nearest 5 degrees).

Fluids tabulated include:

• Automatic Transmission fluid
• Brake fluid
• Coolant
• Diesel, Biodiesel and Diesel Blend
• Ethanol
• Gasoline
• Lubricating oil
• Power steering fluid
• Refrigerants and refrigerant lubricants
• Windshield washer fluid

Table 1: Automatic Transmission Fluid

In tables with “<1000” (degrees F) from source [ 4] indicated, ignition was observed when the test was run at 1000 degrees F. Tests were not run at lower temperatures to determine the lower limit of ignition. Therefore, these values were not used in the summary table.

In values from source [2], the temperatures listed are the surface temperatures at the time the fluid was introduced. Ignition sometimes occurred after a delay of several minutes. Due to the application of fluid and delay, surface temperatures sometimes fell more than 50 degrees F prior to ignition.

As part of their research Colwell and Reza [ 4] ran numerous tests; the resulting ignition temperatures were presented probabilistically rather than as singular values of ignition/no ignition. The ranges of ignition temperatures from their work that are included in the tables herein are based on their estimates of 10% and 90% probability of ignition.

(Return to list)

 

Table 2: Brake Fluid

(Return to list)

Table 3: Coolant

(Return to list)

Table 4: Diesel, Biodiesel and Diesel Blend

 (Return to list)

Table 5: Ethanol

Table 6: Gasoline

(Return to list)

Table 7: Lubricating Oil

  (Return to list)

Table 8: Power Steering Fluid

(Return to list) 

Table 9: Refrigerants and refrigerant system lubricants

Refrigerant CFC R-12 (Table 9) is being phased out and has not been legal for use in new production since 1996. HCFC R-22 is also being phased out, but may be used in new production until 2010. Neither HCFC-22 nor R134a are flammable at ambient temperature and atmospheric pressure and as such are not likely to be involved in starting a fire [11].

The European Union regulations require manufacturers to begin using refrigerants with low global warming potential (GWP) by 2011, effectively requiring R134a to be phased out. To facilitate global manufacturing, vehicle manufacturers are studying alternative refrigerants to use in production world wide [11]. Refrigerants being considered include CO2, HFC-152a and HFO 1234yf; of these both HFC152a and HFO1234yf are flammable. HFC-152a has a flammability range of 3.9-16.9% by volume and a minimum ignition energy of 0.38 mJ [12]. HFO 1234yf has a flammability range of  6.2-12.3% by volume and minimum ignition energy of more than 1000 mJ [13, 14]. Of these 2, HFO 1234yf appears to have lower flammability potential.

Refrigerant leaks also involve leakage of the compressor lubricants, which are flammable. There have been reported fires from refrigerant leakage resulting in the ignition of the system lubricant against hot exhaust system components.

There are also replacement refrigerants containing flammable hydrocarbons that are being produced, marketed, and used in vehicles. These refrigerants, some containing propane and butane, are being used in older vehicles as a replacement for the phased-out R-12, and for newer vehicles needing service to existing R-134a systems. The Environmental Protection Agency (EPA) has more information regarding specific refrigerant product flammability [9]. Systems with flammable refrigerants have caused or contributed to fast propagating fires [ 7, 8].

To view more information about replacement refrigerants,  click here.

(Return to list)

Table 10: Windshield Washer Fluid

Insufficient tests of windshield washer fluid (Table 10) were available to draw conclusions as to the range of expected autoignition temperatures.

(Return to list)

To view references for this page before continuing, click here.