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:

Table 1: Automatic Transmission Fluid

Autoignition Temperature oF Notes Source
410-417 Laboratory - ASTM  
590-855 Full-scale vehicle tests [1]
580-610 Inside heated crucible [2]
590-600 Used – inside heated crucible
900 Heated catalytic converter [3]
<1000 Heated exhaust pipe [10]
930-1120 Recessed stainless steel plate [4]


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.

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Table 2: Brake Fluid

Material Autoignition Temperature oF Notes Source
DOT 3 540-675 Laboratory – ASTM  
DOT 3 520-635 Inside heated crucible [2]
Used DOT 3 525-645
DOT 3 570-590 On top of inverted hemisphere
DOT 3 700 Heated catalytic converter [3]
DOT 3 <1000 Heated exhaust pipe [10]
DOT 3 1010-1065 Recessed stainless steel plate [4]
DOT 3 A.P. and Unipart 735-750 Block of steel [6]
DOT 4 Castrol Girling Crimson 1110-1200

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Table 3: Coolant

Material Autoignition Temperature oF Notes Source
Ethylene Glycol (100%) 725-775 Laboratory – ASTM  
Propylene Glycol (100%) 700 Laboratory - ASTM  
Ethylene Glycol (50%) 775-1100 Full-scale vehicle tests [1]
Ethylene Glycol (100%) 1130-1245 Inside heated crucible [2]
Ethylene Glycol (50%) 1200-1230
Used ethylene Glycol (50%) 1130-1155
Propylene Glycol (100%) 960-1020
Propylene Glycol (50%) 975-1065
Ethylene Glycol (100%) 950 Heated catalytic converter [3]
Ethylene Glycol (50%) >1200 Heated catalytic converter, No ignition, test stopped at 1200 deg F
Ethylene Glycol (100%) <1000 Heated pipe [10]
Ethylene Glycol (50%) <1000
Ethylene Glycol (100%) 1020-1105 Recessed stainless steel plate [4]

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Table 4: Diesel, Biodiesel and Diesel Blend

Material Autoignition Temperature oF Notes Source
Diesel 350-625 Laboratory - ASTM  
Diesel >1200 Heated catalytic converter. No ignition, test stopped at 1200 degrees F [3]
Diesel 950-1000 Heated pipe [10]
Diesel 1010-1125 Recessed stainless steel plate [4]
B100 (Biodiesel) 705-840 Recessed stainless steel plate and fluid spray [15]
B20 (Biodiesel) 980-1300
E-diesel (Ethanol blend) 1265-1400

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Table 5: Ethanol

Autoignition Temperature oF Notes Source
685 Laboratory - ASTM  
125-1330 Recessed stainless steel plate and fluid spray [15]


Table 6: Gasoline

Material Autoignition Temperature oF Notes Source
50-100 Octane 495-853 Laboratory - ASTM  
Unleaded 1350-1550 Full-scale vehicle tests [1]
87 Octane >1200 Heated catalytic converter. No ignition, test stopped at 1200 degrees F [3]
89 Octane >1200
91 Octane >1200
Unleaded, 89 Octane 1100 Heated pipe [10]
Unleaded, 87 Octane 1135-1225 Recessed stainless steel plate [4]
87 Octane and 92 Octane 1460-1520 Nodular iron exhaust manifold [5]
1395 Stainless steel heat shield
1325-1350 Stainless steel exhaust manifold
89 Octane 1240-1445 Recessed stainless steel plate and fluid spray [15]
89 Octane 1425-1485
E85 1300-1325

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Table 7: Lubricating Oil

Material Autoignition Temperature oF Notes Source
  500-700 Laboratory - ASTM  
Engine Oil 760-870 Full-scale vehicle tests [1]
Conventional 580-635 Inside heated crucible [2]
Used Conventional 600-635
Synthetic 590-680
Used Synthetic 615-690
Conventional 660-670 On top of inverted hemisphere
Synthetic 670-790
Conventional 950 Heated catalytic converter [3]
C-V Joint Grease 1100
Conventional <1000 Heated pipe [10]
Synthetic <1000
Conventional 950-1130 Recessed stainless steel plate [4]
Conventional 1110 Block of steel [6]
Synthetic 1110

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Table 8: Power Steering Fluid

Autoignition Temperature oF Notes Source
500-700 Laboratory – ASTM  
610-800 Full-scale vehicle tests [1]
590-625 Inside heated crucible [2]
600-655 Used - Inside heated crucible
655-690 On top of inverted hemisphere
815-870 On top of inverted hemisphere 3.6 ft/s air flow
>1200 On top of inverted hemisphere 7.2 ft/s air flow. No ignition, test stopped at 1200 degrees F
850 Heated catalytic converter [3]
<1000 Heated pipe [10]
990-1145 Recessed stainless steel plate [4]

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Table 9: Refrigerants and refrigerant system lubricants

Material Autoignition Temperature oF Notes Source
R134a (atmospheric pressure) 1370 Laboratory - ASTM  
HFC-152a 849 Laboratory - ASTM    
HFO 1234yf 761 Laboratory - ASTM    
R134a (liquid and gas) >1100 No ignition on pipe heated to 1100 degrees F [10]
PAG 46 800 Heated pipe
PAG 100 & 150 900
Ester oil 1100
Hydrocarbon refrigerants Varied Flammable replacements used [7,8]


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.

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Table 10: Windshield Washer Fluid

Autoignition Temperature oF Notes Source
725-878 Laboratory - ASTM  
>1200 Heated catalytic converter. No ignition, test stopped at 1200  degrees F [3]
>1320 Recessed stainless steel plate. No ignition, test stopped at 1320 degrees F [4]


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

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