As farfetched as it sounds, this author investigated a Recreational Vehicle (RV) fire where the cause was tire failure. Consider these facts:

• RV being operated on highway when right-front tire blew out.
• Driver pulled over to right side of highway.
• Witnesses photographed active fire at right-front corner of RV.
• Fire damage to RV included near-complete melting of right-front aluminum wheel hub, with zero melting of the other five aluminum hubs.
• Burn patterns on RV exterior indicated a low point of burning at right-front wheel well, and V-like patterns rising upward and spreading outward from there.

While the facts above are fully consistent with a fire origin at or near the right front wheel well, and no other origin location would be equally consistent, the evidence above fails to provide direct evidence of any fire causation mechanism. To discover the cause of the fire, our team found and inspected an exemplar RV of the same make and model and discovered an entirely new set of facts about the RV design – none of which survived the fire.  Consider the inset photo below and the following additional facts:

• Several sources of combustible plastic and rubber were present inside the wheel well.
• A set of four conductors (two of which were 10 AWG solid copper wire) inside a plastic wire loom was run through the wheel well.
• The heavy-gauge wires provided power to the front passenger seat adjustment motor and were energized whenever the ignition key was in run or accessory mode.
• The wire loom and wires were a few inches above the tire’s upper surface and a few inches inward from the tire’s inward edge.
• When a rotating tire fails, elements of steel belting can partially disengage and whip around repeatedly at high speed, impacting softer materials within their reach.

Thus, after inspecting the exemplar, our team was able to supplement the burn pattern information with design information that confirmed a source of ignition (energized conductors with contemporaneously-damaged insulation) with several sources of fuel (plastic, rubber and plywood) in the area of origin.  Our causation scenario was the only hypothesis under consideration that was fully consistent with all of the facts – tire failure, followed by steel-belt whipping and damaging energized conductors, followed by ignition of nearby combustible plastics, followed by fire spread to right-front corner of RV structure.

The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations.  We hope you are instructed by this content, and we encourage you to contact us if you seek additional information.

The author of this blog is pleased to announce that a new Mobile Investigation Workshop has been added to the Martin Thermal Engineering collection of tools.

The workshop is equipped with many types of hand and power tools for disassembling products and extracting evidence from a fire scene, as well as instruments that are vital to the conduct of appliance tests and exemplar examinations, including thermocouples, pressure gauges, and flow meters. Multiple video cameras with tripods can be deployed to monitor and record mechanical meters (e.g. gas volume).

We can also extract samples of automotive fluids and combustion gases for submission to an analytical lab for chemical characterization. Evidence chain of custody paperwork is maintained in a folder on board. Marking and labeling tags and signs can be employed when multiple pieces of evidence require identification.

A battery charging station, complete with inverter is soon to be installed, which will make the mobile workshop self-sustaining for multi-day inspections.

The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations. We hope you are instructed by this content, and we encourage you to contact us if you seek additional information.

Over the past 25 years, the National Institute of Standards and Technology (NIST) has developed, augmented and improved an important computational tool for fire investigators – the Fire Dynamics Simulator (FDS) code.

FDS is a flow, heat, and chemistry modeling application that is utilizes Computational Fluid Dynamics (CFD) to model fires and other flows that are important to fire safety engineers and fire investigators. While other commercial CFD programs exist, their cost to license and their computational cost (hours of runtime) tend to make them inaccessible for small enterprises, including many fire investigation firms.

Fortuitously, FDS offered breakthroughs on several fronts that helped bring the power of CFD modeling to smaller practitioners. (Recently, a user-friendly, front-end package was developed by Thunderhead Engineering http://www.thunderheadeng.com/ and can be licensed for a reasonable annual fee.) Because the underlying code was developed by the U.S. Government, it is downloadable and usable by the general public with no license fees. Secondly, the program can be run on Windows-based desktop and laptop computers, so there is no need to purchase expensive hardware. Perhaps most importantly, the program uses a computational simplification called “Large Eddy Simulation” (LES) to enhance the speed at which complex flows can be solved numerically.

While LES does employ a computational shortcut (where only large eddies are directly solved and the dissipative energy generation of the small eddies is modeled as a byproduct of the large eddies) as compared to CFD programs that utilize Direct Numerical Simulation (where all the equations are solved for all sizes of turbulent eddies), the LES technique nevertheless produces fully-validated results for many fire problems.

The video posted below is a FDS simulation of a gasoline leak under a car inside a garage. The model incorporates a source of heptane vapor being released from a square “puddle” under the vehicle’s engine compartment. Contours of heptane concentration versus time are shown in the simulation output – red is flammable, orange is possibly flammable, yellow is probably not flammable, and green/blue/purple are not flammable. It is noteworthy that the red contours do cover the entire floor surface, but because gasoline vapors are heavier than air, the flammable concentrations don’t rise up very high above the floor level.

Also posted below is a plot of hydrocarbon concentration measured by four sensors located in the corners of the garage, at an elevation of approximately 18 inches above the floor. The Consumer Product Safety Commission performed tests of the safety of water heaters in garages approximately 25 years ago and found that flash fires involving gasoline spills ignited by the pilot flame of a water heater were largely preventable if the water heaters were installed on pedestals so that the flames were at least 18 inches up in the air.

The plot below validates the CPSC’s finding. The maximum concentration at the four sensors during the 10 minute duration of the simulation run was 7 parts per million, by volume. This is more than 99.9% smaller (three orders of magnitude smaller) than the Lower Flammable Limit of heptane (1.0 percent by volume). In other words, the mixture is too lean to produce a deflagration at the elevation of the water heater’s burner, if the heater is installed on pedestal, in adherence with the requirements of the National Fuel Gas Code.

The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations. We hope you are instructed by this content, and we encourage you to contact us if you seek additional information.

News reports about fires involving Tesla Motors’ “Model S” electric vehicle have played a role in the company’s recent stock price decline.  However, these fires should be viewed in perspective because they share a common root cause.  Two of the three fires occurred right after the driver accidentally ran over a dangerous road hazard, and the third involved a high speed, front-end collision.

A recent report[1] published by the National Fire Protection Association (NFPA) asserted that of all automobile fires occurring after collisions, 69% originated in the engine or wheel area, whereas only 9 percent originated at the fuel tank or along the fuel lines.  This fact indicates that whatever substance happened to be the first one ignited after a collision (e.g., combustible fluids or plastics), the energy for ignition (e.g., electric arc or hot surface) was most often located in the engine area.

It is obvious that a large number of “fuels” and “ignition sources” co-exist in vehicles during normal use with very low likelihood they will come together in a way that causes a hostile fire.  But collisions and road hazards can puncture reservoirs and deform insulating materials in ways that expose these elements to each other and create conditions for fires to start.  The greater the vehicle’s speed at the time of collision, the more likely it will sustain damage that compromises the protective barriers keeping fuels away from ignition sources.

So the question regarding Tesla’s Model S seems to be whether its barriers are adequate to protect its battery cells. At present,Tesla seems to believe their “armor” is adequate.  Their website even states “…you are 5 times more likely to experience a fire in a conventional gasoline car than a Tesla!”[2]

Unfortunately, their comparison misses two important points:

(1) All three of the Tesla fires involved collisions, whereas the baseline rate they used to compare conventional car fires included many additional (unrelated) causation factors;

(2) By far, the largest percentage of all vehicle fires occurs in vehicles that are more than 5 years old.  Most of these fires have causes rooted in inadequate maintenance, flawed service, or operator carelessness.

Since all of the Tesla fires involved collision damage, fires involving improper maintenance or operation of gasoline-fueled cars should be excluded from any contrasting evaluation.  The more salient electric versus gasoline vehicle comparison would be “fires per collision exceeding a certain speed differential”.  Although we don’t have any actual data, we expect a comparison of collision-related fires would yield incident rates more similar for the two types of vehicles than the Tesla blog suggests.

However, it remains to be seen whether older all-electric cars are in fact safer overall than older gasoline cars.  Since reservoirs for gasoline, engine oil, and transmission fluid are simply not present in electric cars, it may still turn out that fleets of electric cars experience fewer total fires per mile driven than fleets of gasoline or diesel cars, over their respective lifetimes.

One additional noteworthy statistic in the NFPA report is that there was a steady decline in both annual automobile fires and fires per mile driven over the 20-year period from 1988 to 2007. Today, significantly less than 190,000 car fires occur annually (down by about half from 1988), and fewer than 75 fires per billion miles driven occur nationwide (down by about two-thirds).

These statistics are good news for both car drivers and car manufacturers.

When I was a teenager, I remember my Dad occasionally becoming frustrated with my reluctance to rake leaves and pull weeds.  Thinking myself a modern-day Tom Sawyer, I once suggested to him that my interest in weeding and raking would go up tremendously if I could have a couple of friends over to assist.  In response, he recited an old proverb he had been told in his youth, “As my Dad always said, ‘Two boys is half a boy, and three boys is no boy at all.’”

While I wasn’t exactly thrilled with my Dad’s cold water pronouncement, the adage stuck with me.  Years later, it popped back into my brain in a most unusual context – while I was performing an automobile fire investigation.

According to NFPA 921 “Guide for Fire and Explosion Investigations” there are a wide variety of substances that may serve as the material first ignited in a motor vehicle fire, including gasoline or diesel fuel; lubricating oil; transmission, power steering, brake, or windshield wiper fluids; coolant; battery vapors; plastic wire insulation, casings, trays, or trim; rubber hoses or belts; natural or synthetic fabrics or upholstery; cargo; and other contents.  Sources of ignition are only slightly less plentiful – hot surfaces; friction; mechanical sparks; electric arcs; overloaded wiring; open flames; and smoking materials.

A relatively common cause of vehicle fires is the unsafe installation of aftermarket electrical appliances, and this author has investigated many such incidents.  While many third-party audio/video systems are installed with kits that are engineered to work flawlessly in conjunction with the vehicle’s original wiring, some are not meant for automobile installation at all.  The biggest problems are (a) devices that consume too much power, (b) use of wiring that is improperly sized or inadequately safeguarded against excess current, and (c) installation of wiring in locations that are insufficiently protected and susceptible to insulation damage.

In at least three vehicle fire investigations, I have determined the cause to be improper wire installation that led to (a) chafing, (b) insulation failure, (c) short-circuit current flow from battery to ground, and (d) melting and ignition of the surrounding plastic wire insulation.

Not surprisingly, the other common feature in each of these fire incidents was inadequate overcurrent protection.  In one case, there was no fuse installed anywhere in the circuit.  In the second case, the fuse was installed on the wrong leg (neutral instead of hot).  And in the third case, a fuse was present, but its rating was too high to protect the conductor from overheating.

Which brings us back full circle to the adage my father told me (with minor adaptation) – “A fuse on the wrong leg is like half a fuse, and an overrated fuse is like no fuse at all.”