MOTOR VEHICLE FIRES
Unlike dwelling fires, the major damage in motor vehicle fires occurs in a short period of time, and the fire become all-encompassing. Major destruction normally happens within 15 minutes and in many cases, evidence indicating the cause of the blaze is quickly destroyed.
The fire usually develops in three states: stage 1 - beginning or incipient; stage 2 - free burning; stage 3 - smoldering.
In the incipient stage of an accidental fire, smoldering combustion occurs with temperatures often reaching 400°F. Depending on the vintage of the vehicle, this stage can last up to 15 minutes. A fire accelerated by gasoline or other flammable liquid will progress through this stage much faster. The best opportunity to determine the fire's origin and cause, assuming the fire can be stopped, is at this point.
The free burning stage sees a rapid rise in temperature to 1700°F or more. Modern vehicles contain hundreds of pounds of plastic material, the majority of which are thermoplastics containing flame retardant. Thermoplastic melting temperatures are approximately 340°F and up. It is not until these plastics begin to melt that the fire retardant gets driven out. At this point, the material becomes virgin plastic, and will burn intensely. Within 45 to 60 minutes, most cars will burn completely, without any accelerants added to assist. With combustible plastics, the temperatures reached can readily melt glass, sag roofs and crystallize seat springs, putting to rest the myth that these signs are evidence of the use of accelerants.
Smoldering is the final stage, occurring after most combustibles have been consumed. Smoldering combustion may continue for upwards of 90 minutes.
Fuel system leaks are the most common causes of automobile fires. Gasoline fires typically arise from faults in fuel line connectors, carburetor or fuel injection systems, resulting in leakage. The flash point of gasoline is 45°F. At this point, gasoline will produce a vapour that can sustain continuous flame. Above this temperature, gasoline can easily be ignited with a spark. If its temperature is raised above 495°, gasoline will auto-ignite.
Electrical system failures are the second most common cause of vehicle fires. The engine's 12-volt battery, when charging, may produce hydrogen gas, creating an explosion hazard. Primary ignition circuits in the engine compartment consist of battery and starter cables. These circuits generate sufficient current to ignite combustibles in the event of a fault condition. Typically, these circuits are not fused. The secondary ignition circuit is high-voltage (25-30kV). This voltage generates the spark in the spark ignition engine. A stray spark from this current is sufficient to ignite vapours from a fuel leak.
Vehicle arson can be difficult to prove, particularly if the fire has advanced past the first two stages. However, an intense fire in the passenger compartment will burn like a large container, burning the top of the seats and working down towards the floor. There is no oxygen at floor level, but radiation damage, resulting in the melting of synthetic material, will produce more flammable gasses. If an accelerant was on the floor, if would not burn due to lack of oxygen. This is why, even after an extensive burn, accelerant residue can still be detected in the debris of burned vehicles.
A vehicle's engine compartment contains many flammable liquids - engine oil, hydraulic brake fluid, automatic transmission fluid and engine coolant among them. Ethylene glycol, found in anti-freeze, has a flash point of 241°F, and an auto-ignition temperature of 775°F. The exhaust manifold in a running engine can reach in excess of 1000°F - high enough to auto-ignite even the heaviest hydrocarbons present in the engine compartment.
Catalytic converters can also generate heat sufficient, when combined with inadequate ventilation, to ignite floorboard carpeting.
Surprisingly, there is little published information relating to vehicle fires, and most of it deals with crash-related fires which are rare, despite media exposure to the contrary. A clever arsonist can often simulate accidental fires, particularly engine fires, making vehicle fires a challenge for any investigator.
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