Surface Temperatures: Underhood
Underhood surface temperature is one of the many factors upon which autoignition is dependent. There is limited published information available on underhood surface temperatures. Adding to this difficulty is the fact that exact operating conditions at the time of fire that affect underhood temperatures are also difficult to determine. The following information is based on available published tests, unpublished tests conducted or seen by the authors and general principles of vehicle mechanics. [1-18]
Typically the exhaust system or the catalytic converter surfaces are the hottest on a vehicle
In a normally operating vehicle, only exhaust system components are hot enough to autoignite fuels in the motor vehicle. Surfaces for potential ignition include exhaust manifolds, exhaust pipes nearest the manifold, catalytic converters and turbochargers.
Exhaust manifold temperatures vary for different vehicles and operating conditions
Manifolds and/or exhaust pipes on some vehicles can reach 1200 degrees F. It is rare to find temperatures this high in normal operation. The hottest locations tend to be those for which there is some constriction or impingement of exhaust gas. For example, a bend in an exhaust manifold tube immediately outside the cylinder will have a surface upon which the exhaust gas has a greater capacity to transfer heat. Exhaust system temperatures will be reduced at any point of contact with cooler components of large thermal mass, such as the cylinder head.
Turbocharger system components in contact with exhaust gas are frequently as hot or hotter than other exhaust system components.
A published study  of a 1996 Ford F150 V8 pickup truck showed temperature of the exhaust pipe entering and exiting the first catalytic converter on a level road and upgrade to be the following:
Exhaust System Temperatures at 60 mph (Degrees F.)
Catalytic Converter Inlet Catalytic Converter Outlet Level road 757 770 7% grade 979 972
A published study  of four vehicles showed manifold temperatures in the ranges provided in the following table:
Exhaust Manifold Temperatures (Degrees F.)
30 mph 70 mph Level road 250 825 7% grade 360 1020
Catalytic converter temperatures also vary for different vehicles and operating conditions
In a normally operating vehicle, the catalytic converter may be 750 degrees F, or more. If one or more cylinders are not functioning and unburned gasoline (or a richer fuel/air mixture) flows into a catalytic converter, then temperatures can increase precipitously. The source cited above  also contained the following catalytic converter temperatures for the same four vehicles.
Catalytic Converter Temperatures (Degrees F.)
30 mph 70 mph Level road 310 430 7% grade 640 735
Modeling and dynamometer data show higher catalytic converter temperatures, but these sources did not incorporate representative airflow [8,9].
Peak temperatures occur at road load, not idle
Peak temperature is related to engine speed (RPM), engine load, and the volume of exhaust gas flowing through the exhaust system.
Higher temperatures occur in extreme and improper operating conditions
When an engine is operated at high RPM or load (upgrade, towing, high wind, acceleration), exhaust system temperatures can increase dramatically. Temperatures may also increase if the engine is not functioning properly resulting in improper combustion and unburned gasoline exhausting into the catalytic converter.
Cooling occurs within minutes of shutdown
Tests have shown hot surfaces can cool down as much as 400 degrees F within 3 minutes of the vehicle being brought to a stop and the engine being shut off . The hotter the surface, the faster the expected rate of cool down. The majority of data available to the authors have shown that exhaust manifold and pipe temperatures fall immediately upon shut down, and drop below fluid autoignition temperatures within 3 minutes .
Catalytic converter temperatures may rise after the engine is shut down due to continued chemical activity in the substrate material or heat transfer from hotter internal components.
Further discussion of these data is available in the detail section for this page.
Shielding may increase the likelihood of autoignition
Even when exhaust system shielding doesn't significantly increase the temperature of a surface, the presence of a shield may increase the chance of autoignition in at least two ways. First, by reducing air flow, there is a greater opportunity to achieve air/fuel mixtures that support combustion. Second, in stagnant air, the fuel may remain in the presence of the heated surface longer and allow enough energy to be transmitted to achieve combustion (higher "residence time"). A shield may also allow an initial fire to burn and propagate when it would otherwise be blown out. [13, 15]
Engine cylinder head and block temperatures are cooler than exhaust system temperatures
The greater thermal mass of the engine and block, and the presence of the cooling system, prevent the engine block and cylinder head from achieving temperatures in the autoignition range for normally operating vehicles. Engine surface temperatures increase for a period of time after shutdown, but not to the threshold of autoignition.
A surface can be heated by dragging against the pavement
In addition to heating by exhaust gases, metal components dragging against the pavement may become hot enough to support autoignition of fluids. Steels may demonstrate their temperature history through colors that remain after they are cooled. Such dragging may occur as a result of collision or a component becoming loose enough to contact the road.
Dragging metal also provides mechanical sparks as an ignition source. You can click on the link below to review an upcoming section on mechanical spark ignition now, or continue studying autoignition.
Study Mechanical Sparks? Click here.
These are simple guidelines regarding the temperature of vehicle surfaces to support ignition; if more definitive information is needed, tests should be performed. With respect to exhaust systems, there are tests of surface temperature in published literature related to a few vehicles and specific operating conditions. The variations based on operating conditions (speed, grade) and vehicle model can be substantial. Some conclusions can be drawn from this information, however. For example, it is fairly certain that no surface in a normally operating vehicle will be of sufficient temperature after 5 minutes of idle operation for autoignition of coolant on an unshielded exhaust manifold [3, 5, 13].
For references and more detailed temperature test data, click here.