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Which kind of crash does it need for a car to catch fire?

I'm asking because I was discussing about an accident where a car crashed into a full-grown brown bear. Is it realistic under normal circumstances that the car could catch fire?

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up vote 16 down vote accepted

Three things are required, fuel, oxygen, and a source of ignition. Since around 1978 when the burning Pinto issue appeared, manufacturers have tried to design vehicles that limit the three from occuring together. Cars bursting into flames after an accident is a pretty rare event. According to these stats I got from the NFPA website, only 3% of vehicle fires were the result of a collision or rollover. Almost half (49%) were the result of mechanical failure, leaks, or worn out parts. Electrical issues caused 23% of the fires. So looking at all the data in a well maintained vehicle it is possible but not likely.

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Any accident that causes the fuel system to rupture could potentially cause a fire. Petrol/Gasoline vapour is highly flammable, and sparks are frequently present in a crash situation (e.g. metal scraping along the road surface).

The tank on most passenger cars is underneath the rear of the vehicle, with a combination of metal and rubber hoses to get the fuel to the engine - it is easy for one or more of these to become dislodged in a heavy impact, or for the tank itself to be ruptured.

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5  
But it isn't like the movies. No gigantic explosions. Rarely do fuel tanks explode, and if they do they do not take out the whole car. – Nick Jul 28 '12 at 15:15
    
Even more fake in the movies is tractor trailers exploding. Diesel is not explosive. :-) – R.. Jan 26 '13 at 16:45

If the question is "what can cause a car to catch on fire" rather than "what can cause a spectacular movie-style explosion", then things are a little more wide open!

Remember that the car runs by burning gasoline inside of a metal box (the engine) - things therefore get quite hot in the normal course of operation, and a good deal of engineering has been devoted to making sure that that heat is dissipated safely. Lots of things can go wrong and cause the simple heat of running an engine to start a fire or at least make things very smoky and scary for a while.

  • Dirty air filter - I'll be honest, I have no idea why a dirty air filter would be more dangerous than a clean one. Bad for your engine, yes, but more likely to catch fire? Dunno. That being said, I still remember a visit from the fire department to our grammar school and the emphasis they laid on replacing your filters.

  • Foreign objects - This has happened to me twice: once, a sheet of paper flew up from the road and lodged under the exhaust manifold: no flames, but the cabin filled with smoke. Years later - different car - a walnut fell off our tree into the engine compartment (the grill between the edge of the hood and the windshield was broken) and nestled in between the valve covers. Again, no flames but a choking cloud of smoke and a stink that took months to go away.

  • Fluid leaks - Oil, transmission fluid, brake/steering fluid - they're all flammable (at high temperatures). Wipe up any spills, and be careful of leaking fluid lines.

  • Catalytic converter - underneath the car, runs your exhaust gas through a catalyst to burn off any unburned hydrocarbons. Gets VERY hot! I attended high school up in the mountains, and the father of one of my classmates drove over a tree branch while collecting wood in his pickup. A chunk got wedged between the catalytic converter and the truckbed (I don't know how he managed that!) and as he was driving down the hill a few days later it caught fire. He was OK, and they got the fire put out before it reached the gas tank, but the back end of the truck was a total loss.

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It is possible, though very unlikely, to create an explosion multiple factors are required.

A. A nearly empty gas tank. (Gasoline by itself is not explosive, the vapors are)

B. A failure of the venting system, allows the vapors to build.

C. A rupture of the fuel tank, allowing the fuel vapors and oxygen to mix to the right ratio to allow combustion.

D. Some sort of spark or flame that exists at the same time that the ratio is correct.

Thats off the top of my head.

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