Thermal patterns (burn patterns) are helpful to the fire cause and origin analyst because they give indications of where the fire may have started. Vehicles with steel sheet metal bodies usually survive a fire with damage to the exterior paint indicating high temperatures. As the paint is burned away in a local area, corrosion of the sheet steel occurs quickly, indicating where the fire may have started. Aluminum body parts also give an indication of fire origin and heat transfer direction from the degree of melting and deformation.
Unlike steel body parts, aluminum panels can be completely consumed in a fire, yielding little information on the fire origin. Resin based fiberglass reinforced body panels are often consumed readily and may not yield much information through burn pattern recognition.
Heat transfers by three recognized modes: conduction, convection, and radiation. Conduction is the transfer of heat from one molecule to another from direct contact. Convection is the process of transferring heat via a fluid, such as convective cooling of an automobile radiator where air flows over hot radiator tubes, cooling the internal radiator coolant and heating the air. Radiation is heat transfer through electromagnetic-wave means such as infrared radiation. Most fires involve all three modes of heat transfer.
Heat also naturally transfers from high temperatures to lower temperatures. The heat from a fuel fed fire in an engine compartment transfers upward by convection and radiation causing a burn pattern on the hood. The heat may also transfer downward as a result of thermal radiation or laterally along body structures as a result of thermal conduction.
Basic Exterior Thermal Patterns on Steel Sheet Metal Bodies
Engine Compartment Patterns
Figure 2.01 depicts a narrow burn pattern over the middle of the hood area suggesting that either a malfunction of the fuel injection system has occurred, or backfiring ignited combustible materials over the engine. Heat has transferred upward from the engine by convection and radiation.
Figure 2.01
Heat is also transferred laterally by conduction across the hood, but the temperature is decreasing to a point where the paint is not damaged. Since heat transfers in the direction of decreasing temperature, tracing surface temperature
Figure 2.02
evidence toward the higher values of temperature results in a “hot” spot, which is frequently where the fire started.
Figure 2.02 shows a similar pattern that is displaced toward the bulkhead on the right side of the vehicle. Look for an electrical malfunction, such as hood courtesy lighting and the many wiring harnesses that are installed in this area.
Figure 2.03
Figure 2.03 shows a wider hood thermal pattern which encompasses several additional engine components such as the fuel system, alternator, electrical wiring, oil line, and oil seals.
Figure 2.04
In Figure 2.04, a burn pattern on the right front of the hood is noted, indicating possible electrical malfunction, battery problem, brake frictional heating or coolant leakage through the radiator cap. The burn pattern in Figure 2.05 is more widespread and covers the entire engine compartment area. A summation of all the possible causes of the fire in Figures
Figure 2.05
2.01 through 2.04 may be consistent with a fire in this case. Note that these hypothetical examples deal only with exterior burn patterns. Opening the hood may yield additional burn or melted alloy patterns that can further refine opinions as to the fire origin and movement.
Occupant Compartment Patterns
Figure 2.06
Figure 2.06 shows burn patterns around doors and sooting inside the vehicle windows. This is a classical occupant compartment fire. Possible causes are deliberate acts (arson), misuse of
Figure 2.07
smoking materials, electrical malfunctions, and exhaust system problems.
Exhaust System Patterns
Figure 2.07 shows a burn pattern under the rear of a vehicle consistent with tail pipe or muffler problems, electrical malfunctions or fuel leakage near the fuel tank.
Figure 2.08
Figure 2.08 is a burn pattern that nearly encompasses the whole vehicle. This often makes it difficult to find the fire origin based on external patterns. Vehicles in garage fires often look like this as well as vehicles involved in collision and arson related fires.
Examples
of Burn Patterns on Vehicles
Figure 2.09 is a view of one side of a motor home showing a burned roof area. The vehicle was stationary with power wiring attached for charging batteries and operating appliances.
Figure 2.09
Figure 2.10
Figure 2.10 shows the other side of the vehicle with the arrow pointing at a low burn. Since hot gases and smoke associated with a fire tend to rise (convection) and spread out, a V shaped burn pattern often results, as shown in Figure 2.10. It is logical to conclude that the fire origin is at the base of the V as indicated by the arrow. In many cases this is a valid premise, but in situations where there are heavy fuel loads nearby that are ignited, distortion of the burn pattern can occur and indicate a false origin.
Figure 2.11
Figures 2.11 through 2.14 are classical engine compartment fires as indicated by the corroded sheet metal. Once the paint is burned away, rusting of the steel sheet occurs quite rapidly, often within 24 hours. Bent hood sheet metal may be a result of fire suppression activity by the fire department as illustrated by the deformed hood frame in Figure 2.12 and is not a thermally induced deformation.
Figure 2.12
Figure 2.13
Figure 2.14
Figure 2.14 shows an engine compartment fire on an older vehicle consistent with carburetor malfunction. The burn pattern is symmetrical and tends to be centered over the carburetor. The symmetrical hood burn pattern in the center of the hood on the newer automobile
Figure 2.15
Figure 2.16
shown in Figure 2.15 is consistent with fuel injection malfunction.
Figure 2.16 shows an engine compartment fire with random high temperature areas indicated by the corroded sheet metal on the hood. The “spotty” burn pattern can result from ignition of other volatile materials in the engine compartment as well as localized venting of the fire.
Figure 2.17
Figure 2.17 shows a typical van engine compartment fire which often readily spreads into the occupant compartment.
In Figure 2.18, the engine compartment
Figure 2.18
is below the occupant compartment (cab). A fire originating there often burns up through the cab.
Figure 2.19
The vehicle in Figure 2.19 sustained an engine compartment fire that burned into the occupant compartment. The bulkhead between the engine and occupant compartment can readily transfer heat from a fire. Commonly, an engine compartment fire burns directly through wiring harness access openings or may transfer sufficient heat to ignite combustibles in the occupant compartment.
Figure 2.20
Figure 2.20 shows a badly damaged engine compartment with evidence of melted alloys and consumed aluminum fins and tubes on the radiator. The radiator is badly damaged on the right side, suggesting a fire origin to the right side of the engine compartment. (Right and left is in reference to the vehicle driver facing forward. Right is the passenger side, left is the driver’s side for vehicles with the steering wheel on the left.)
Figure 2.21
Figure 2.21 is a view of an air filter housing showing an air filter in relatively good condition. It is not burned away and is covered with soot, indicating that a fire likely originated outside the engine compartment. Engine compartment fires causing severe damage to the engine compartment usually burn away the paper filter element.
Figure 2.22
Figure 2.22 is a view of an air filter housing with little remaining of the paper air filter element after a carburetor related fire.
Figure 2.23
Figure 2.23 through 2.27 show partially burned air filter elements that may yield clues as to fire origin and movement.
Figure 2.24
Figure 2.25
Figures 2.26 and 2.27 show a partially consumed air filter
element. The left side of the element is undamaged while the right side is
nearly consumed (which indicates heat transfer from the right side of the engine compartment.)
Figure 2.26
Figure 2.27
Figure 2.28
Figures 2.28 through 2.31 show views of four different vehicles from different fire losses. Burn patterns in each case pointed to a fire origin near the catalytic converter under the rear mounted engine compartment.
Figure 2.29
Figure 2.30
These cases suggest that a design defect caused the fires, since the burn patterns are very similar, pointing to the same fire origin area: the catalytic converter.
Figure 2.31
Figure 2.32
Figure 2.33
Figure 2.32 depicts a burn pattern consistent with a fire in the instrument panel or dash area. Thermal stress from the heat below has cracked the windshield. Figure 2.33 shows the fire origin area as indicated by the arrow. Instrument panel fires tend to be electrical in nature because of the wide variety of wiring and electronic equipment mounted in that area.
Figure 2.34
Figures 2.34 and 2.35 show close-up views of the fire origin area in the instrument panel. Excavating in this area for clues can be a challenge because the solidified polymers are hard and stiff, and encapsulate material that may yield information as to the fire cause. Sometimes X-Ray analysis of the polymer residue helps identify evidence of malfunctions.
Figure 2.35
Figure 2.36 shows a badly burned right door area suggesting that a fire of electrical origin occurred in the wiring near the door lock switch.
Figure 2.36
Figure 2.37
Figure 2.37 depicts a vehicle with significant thermal damage all over the vehicle. Figure 2.38 is the rear of this vehicle. This near total burn out can be attributed to a fire that was not suppressed early, because of a long fire department response time. Arson fires also exhibit this burn pattern since the vehicle is usually taken to a remote area, set on fire and left to burn.
Figure 2.38
Figure 2.39
Figure 2.39 shows a view of a totally oxidized vehicle that was removed from a garage with no paint remaining. Burn pattern analysis was virtually impossible in this case, with little evidence pointing toward a fire origin.
Vehicles are often suspected as a cause of a fire in a garage, but in many cases the garage fire engulfs the vehicle, destroying burn patterns that may exonerate the involvement of the vehicle as a cause of the fire. Inspecting a garage fire related vehicle while it remains in the garage is desirable in order to rule in or out other causes such as a garage door opener.
Figure 2.40
Figures 2.40 and 2.41 show two separate vehicles remaining in their garages - the preferred configuration for analysis after a garage fire.
Figure 2.41
Figure 2.42 shows the engine compartment of the vehicle in
Figure 2.41, which was in relatively good condition, suggesting that the fire
most likely did not start in the engine compartment.
Figure 2.42
Figure 2.43
In Figure 2.43, a vehicle involved in a garage fire has been removed. Figure 2.44 shows the engine compartment to be in relatively good condition compared to the rest of the vehicle, suggesting that an engine compartment fire may not be the cause.
Figure 2.44
Figures 2.45 and 2.46 show views of a badly burned vehicle
that was involved in an accident. Deformations to the rear and front frame and fenders are
accident related. The fuel system was damaged during the accident causing a
fuel spill and fire.
Figure 2.45
Figure 2.46
Figure 2.47
Figure 2.47 is a view of the box of a badly burned pickup truck found in a field. The striation patterns on the box as indicated by the arrow suggest that the fire was set by dumping a flammable liquid in the box and igniting it.
Figure 2.48 shows a badly burned truck found in a field. An arson? Probably not. The vehicle became stuck and as the driver tried to get it out, the transmission overheated and ejected hot transmission fluid onto the exhaust manifold causing the fire.
Figure 2.48
Figure 2.49
Figure 2.49 is an infrared thermogram of the left front wheel of a vehicle. The hot area (arrow)
points to a defective brake, causing overheating and a fire near brake
hydraulic components.
Figure 2.57