Heavy Trucks
Heavy trucks (designated as Class 7 and 8 by the United States Department of Transportation) present some very special and unique challenges for a fire investigator. Many of these vehicles are equipped with the amenities of a modern recreational vehicle. These accessories, along with the capability to attain gross vehicle weights of near 100,000 lbs., can result in many potential modes of failure that could lead to a vehicle fire.
When a vehicle is carrying a heavy load, one of the main forces that is used and must be overcome is friction or resistance. To put an 80,000-pound vehicle in motion, a large amount of horsepower and torque must be developed. For the development to occur, a major by-product, heat, is produced. This heat is produced by the combustion of diesel fuel in the engine. Exhaust system temperatures on diesel engines can reach and exceed 800 degrees F for extended periods of time. This is a sufficient temperature to ignite many combustible components in the engine compartment of heavy trucks if they come in proximity to the exhaust system.
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The diesel fuel injection system of a heavy truck has the potential to become a fuel source for a fire. Two basic types of diesel fuel systems are used in today’s vehicles: high- and low-pressure fuel injection systems. Many high-pressure systems develop pressures well above 1,000 pounds per square inch (psi). If a diesel fuel leak occurs under this amount of pressure, the fuel will be very finely divided and atomized, which heightens its susceptibility to ignition. Even in a low-pressure system of 70 to 150 psi can cause diesel fuel to be expelled long distances and easily atomized. If this condition occurs in the presence of the heat produced by the exhaust system, a fire is likely.
One important consideration in the evaluation of heavy vehicle origin and cause investigations is the amount of combustible fuel available. Many heavy trucks carry from 200 to 300 gallons of diesel fuel. If a fire progresses and compromises the fuel tank, the resulting fire can completely consume and/or melt the vehicle. When this occurs, recognizable oxidation and burn patterns are obliterated.
Many accessories can be present in today’s commercial vehicles, including electric inverters or generators. The consideration of damage to electrical components in the vehicle due subjection to constant vibration must also be evaluated. Sometimes vehicle owners will install or modify an existing 120-volt or 12-volt system of the unit. Just as in passenger vehicles, wiring that is added or modified must be properly protected by fuses or circuit breakers. Physical damage to electrical conductors is greatly elevated in commercial vehicles due to elevated levels of vibration. Some of the same considerations for structural fires must be addressed during an investigation of the 120-volt system in a commercial vehicle; the potential for fire in a commercial vehicle must be viewed from both a vehicular and a structural standpoint.
Other components of heavy trucks can be potential ignition sources. Friction or resistance from the application of brakes on an 80,000-pound vehicle creates an enormous amount of heat. If one of the 10 brake shoe and drum, or disc brake combinations on a commercial vehicle fails to completely release, the resulting friction can quickly produce heat sufficient enough to ignite nearby oils or tires. Improperly maintained brake systems are a leading cause of commercial vehicle fires. Donan Engineering’s investigators and engineers have many years of experience in the repair, investigation, and design of heavy trucks and their components.