VEHICLE FIRES CAUSED BY A
Charles C. Roberts, Ph. D., P.E.
At the beginning of an investigation of a vehicle fire, the following scene presents itself. The vehicle is located in a remote field and badly burned. There is virtually no unburned paint left on the vehicle body (Figure 1).† Without further investigation, one might conclude that this is a fire of incendiary cause. After all, how many times have vehicles been observed totally destroyed, located in a remote area, where the probable cause of the fire is an incendiary act.† Further analysis may shed light on this conclusion.
According to the insured, he was driving the vehicle on his farm checking out the condition of the crop when his vehicle began to bog down in moist soil. He shifted to four wheel drive but eventually got stuck at the edge of the field. ††He tried several times to extricate the vehicle from the mud but to no avail. Suddenly a fire developed in the engine compartment and spread quickly. Without a fire extinguisher available, the insured was unable stop the fire which developed into a total loss to the vehicle. The rural fire department was notified and arrived approximately Ĺ hour later finding the vehicle totally engulfed (the vehicle fuel tank was nearly full).†
Depending on the skill of the driver and how mired the vehicle is, there is a relatively heavy load on the vehicle transmission, without adequate cooling. Mud around the transmission housing reduces heat transfer. A vehicle mired in the mud will no have the benefit of air flow through the radiator, over the engine and out through the bottom of the engine compartment.† Consequently, overheating of the transmission and transmission oil can occur. The heating of the transmission oil causes it to expand and in some cases be expelled through the transmission dipstick tube onto hot surfaces such as exhaust manifolds or turbochargers. Transmission oil readily ignites on such hot surfaces, causing a fire, typically in the engine compartment. Some dipsticks have a cam lock sealing mechanism that tends to reduce the chance of transmission fluid leakage, while other vehicles have minimal sealing of the dipstick tube. In Figure 2, the right side of the hood and engine compartment are most severely burned in the vicinity of the transmission dipstick tube. Figure 3 is a close-up of the dipstick tube where oil was likely expelled from the transmission. The transmission oil was found to be a few quarts low, which is consistent with oil having been expelled.
Figure 4 shows ruts made by the insuredís vehicle while in four wheel drive, consistent with the explanation offered by the insured. Being mired to the axles, as in this case, reduces heat transfer from the transmission and cooling system and makes the vehicle a candidate for transmission overheating and a fire.
††††††††† Figure 5 shows that transmission overheating can occur in the winter. In Figure 5, the insuredís vehicle got stuck in a snow drift. After several attempts to free the vehicle, a fire developed in the engine compartment as a result of transmission oil expulsion onto hot exhaust components. As can be seen in Figure 5, the snow drift has severely limited air flow over the radiator, causing overheating and the eventual fire. In other instances, the cooling system overheats, causing release of engine coolant and a possible fire.
An important aspect of the above case studies is that a totally destroyed vehicle in a remote area may not be a result of an incendiary act. A reduction of heat transfer from vehicle components because of the mired condition, plus overheating due to attempts to extricate the vehicle, can be a cause of the fire.