How dangerous is a flaming car?

June 30, 2012 8:56 PM

If a car becomes engulfed in flames, will it eventually explode like it does in the movies?

I drove by a car fire the other day and now I'm wondering how much danger I was in. I saw it go from "flames licking around the tires" to "fully engulfed" in about ten seconds. Was the next step "explode like a bomb"? That's usually what happens on TV.

For what it's worth, I've been following the story in the local news and it appears that no one was seriously hurt.

posted by gentian to Travel & Transportation (20 answers total)

Best answer: No.

I worked in accident reconstruction for years, specializing in car fires. I still do I guess, but more infrequently. Cars do not explode like in movies. Gas tanks do not blow up. The part of gasoline that burns is the vapors, the liquid itself does not burn.

What generally happens when a car is allowed to burn to completion is the tank itself will melt through (almost all are plastic tanks now), allowing the gas to escape, then ignite. It will look like a large grease fire flare up, not a boom or blow up.

This is rarish though. The tank itself contains a fair amount of liquid that makes it take a lot to heat up to the melting point of the plastic tank. Generally by the time this would happen the car is already put out. If you let a car burn in the middle of nowhere it would possibly happen. I'd put the chances at 30%ish. There is a steel plate of the car floor between the fire and the tank, and the gas may boil, but it won't normally escape.

What people see when they say OMG IT BLEW UP is the car tires. They will get hot and explode, this adds instant increase to any fire by blowing a shitload of air into the fire at pressure, and blowing the flames themselves out violently.
posted by sanka at 9:10 PM on June 30, 2012 [110 favorites]

It is extremely unlikely. Unless the vehicle was carrying some sort of volatile explosives, nothing in your average gasoline or diesel vehicle will be explode. At most if the fuel tank was down to fumes with the perfect ratio of oxygen to fuel, there might be a slight possibility of a small explosion, but not the sort of hood-flinging, tire-frisbee, mirror-dice-tossing explosion you see in movies.

Of course the story might be different if it had a couple of propane tanks in the trunk.
posted by thebestsophist at 9:11 PM on June 30, 2012

The current issue of Entertainment Weekly addresses just this, and they say no. That actual car explosions aren't that interesting. They lay out all the techniques that movie car explosions use.
posted by FlyByDay at 9:20 PM on June 30, 2012

I have to agree with the above. As a result of a misspent youth, I saw a bunch of cars intentionally set ablaze in the middle of the desert where they eventually burnt til there was nothing left to burn. The closest they came to exploding was the initial moment where someone (usually the dumbest of the group)threw a lit road flare into the car right after the upholstery had been soaked with a gallon or two of gasoline. This always resulted in a giant mushroom of a fireball that would die down fairly quickly. The rest of the fire was actually kind of boring to watch after you'd seen a couple of cars burn to the ground.
posted by buggzzee23 at 9:24 PM on June 30, 2012 [3 favorites]

I was in this car when it caught on fire and it burned for quite a while before a fire truck showed up. It didn't explode.
posted by Confess, Fletch at 9:47 PM on June 30, 2012 [4 favorites]

Ditto here - I've driven by a few cars-on-fire on the side of the road, and basically everyone just stands back and watches it burn (until the fire dept arrives). No explosions, just a lot of smoke.

The Mythbusters have also tried to blow up cars a few times, and it's very hard (impossible?) to do accidentally.

This is great opportunity to remind everyone to keep a fire extinguisher in your car for exactly this reason - as the OP mentioned, a car goes from "on fire" to "engulfed" pretty dang quickly. Having a fire extinguisher on hand can make the difference between a good story and a smoldering pile of metal.
posted by jpeacock at 9:48 PM on June 30, 2012

You need an air fuel mixture, and even these will only explode in a very narrow range of concentrations. So no. If you let the right amount of petrol evaporate inside a sealed car and lit a spark, that might give you an impressive bang. The explosions you see in movies are the result of explosive charges.
posted by epo at 3:00 AM on July 1, 2012

Here is a video that lets you see what might happen to a car if you set in on fire.
posted by Sinadoxa at 4:39 AM on July 1, 2012 [1 favorite]

Propane tank explosions (aka BLEVE Boiling Liquid Expanding Vapor Explosion) are also very rare- there was a railway car full of the stuff in Cali on fire, and even then the firefighters were able to drain off the tank. Even if you do get one hot enough for long enough for the device to fail, it likely won't be a catastrophic failure of the container, but a valve failure. This would result in venting- big exciting shooting flame, but a regular fire wouldn't exceed the blowoff rate of the safety valve, so explosions are very unlikely. Think about all idiots with bbq's out there - right there you've got all the ingredients for disasters and explosions- and it remains exceptionally rare event.
posted by zenon at 6:50 AM on July 1, 2012

I've seen a number of cars burn all the way down, and none of them ever exploded. One had a plastic petrol cap and that eventually gave out and caused a bit of a flame thrower effect. Boiling petrol venting through a narrowish pipe will do that.

Tires can pop though. Which is somewhat impressive.
posted by aychedee at 8:50 AM on July 1, 2012

It probably doesn't need to be mentioned, but just in case it does...while cars will not explode like they so dramatically do in film/tv, this shouldn't encourage anyone to underestimate the danger of vehicle fires — they burn fast and hot, putting a lot of volatile and poisonous gases into the air. You want to be well away from one and you want to get out of any vehicle that's on fire as quickly as possible.

This includes being cautious with engine fires. Someone with much more knowledge than myself ought to clarify, but I'd advise much caution when/if you detect an engine fire and decide to use an extinguisher — especially, say, when opening the hood.
posted by Ivan Fyodorovich at 1:42 PM on July 1, 2012

When someone set fire to my mother's car, even though there was actual gasoline poured on it, it didn't explode, it just melted weirdly after being fully engulfed in flames. (The driveway also melted. But the fire was so contained that the car sitting next to it, and the wooden garage door, neither more than a few feet away, were entirely untouched.)
posted by jeather at 3:12 PM on July 1, 2012 [1 favorite]

Engine fires are very dangerous because the the engine compartment contains rubber hoses full of high-pressure gasoline. If you open the hood, you might get sprayed with burning gas.
posted by ryanrs at 5:37 PM on July 1, 2012

Someone with much more knowledge than myself ought to clarify, but I'd advise much caution when/if you detect an engine fire and decide to use an extinguisher — especially, say, when opening the hood.

Never, ever open the enclosing panel to a fire to try and put it out. Only ever open anything when the fire is well and truly out. The air to the fire is being restricted and by opening the bonnet you'll allow a sudden influx of fresh oxygen which will cause the fire to rapidly expand into the area you are standing in. Shove the extinguisher into an access hole like directly underneath the car and to one side - about a foot from the wheel - and point upwards (assuming it is safe enough to get that close). You need to send the extinguishant through the same hole the air is getting in through, not create a great big new hole above the fire.

The temptation is to wave the extinguisher nozzle all over the flames in a light saber style, but the key is to put the heart of the fire out, not the end result of the flames you can see. Emptying an extinguisher into a hole that leads to the fire can be surprisingly effective as the sheer volume in a concentrated area and the air flow the fire itself creates (ie direct to the heart of itself) will help. Even better would be one extinguisher load in each side of the engine bay from underneath. If that doesn't do it, likely you need MUCH more extinguishers than you had.
posted by Brockles at 8:02 AM on July 2, 2012 [3 favorites]

"Never, ever open the enclosing panel to a fire to try and put it out. Only ever open anything when the fire is well and truly out. The air to the fire is being restricted and by opening the bonnet you'll allow a sudden influx of fresh oxygen which will cause the fire to rapidly expand into the area you are standing in."

Yeah, that was my thinking, but I wanted someone with more authority to say so.

About fifteen years ago I was for some reason standing outside my apartment when my neighbor pulled into the parking lot and jumped out of the car. She ran over to me and exclaimed that she thought her car was on fire. I'd thought I'd seen a flame under the engine compartment as she pulled in, but there was no flame or smoke visible as it was sitting there.

She asked if I had an extinguisher, and I remembered that there was one in the shared laundry room. We ran together to it — at which point I hesitated about breaking the glass to retrieve it. She didn't, she broke the glass and we ran back to the car. At that point, she reached to open the hood and I stopped her for the very reason you describe. I suggested that we ought to try to get to it without opening it. Again she plunged ahead, opened it up, and sprayed the engine compartment. If there'd been a fire, at that point it was extinguished.

I've always been a bit disappointed in myself for my hesitation at breaking the glass to retrieve a fire extinguisher, because that was stupid. But I think it also arguably undermined my credibility for her when I counseled against opening the hood. That could have gone differently and she could have been badly injured because I didn't prevent her from doing it.

In general it's a bad idea to open an enclosed space where there's fire — engine compartment or bedroom or wherever, for precisely the reason that a fire that's consumed most of the oxygen (but not the fuel) burns at one (relatively low) level and a fire that's fed a bunch of oxygen and has plenty of fuel burns at a (much higher) level.
posted by Ivan Fyodorovich at 8:45 AM on July 2, 2012 [1 favorite]

Gasoline lines should not be under high pressure, as there's no fuel pump running. The radiator hoses may still be under pressure though, assuming the car was running recently and the engine is hot.

You don't have to worry about backdraft w/a car engine because the hood is not an airtight seal - the entire underside of the engine compartment is wide open, the fire is getting plenty of oxygen already. You have more to worry about burning your hand on the hood/release lever trying to open it.

Of course, this can all vary by car make/model, this is my experience with cars that I have owned.
posted by jpeacock at 7:09 PM on July 2, 2012

Cars that you have owned that caught fire?

The bonnet (hood) is a fairly air tight seal, and air is restricted (not prevented) through the underside. There will be an increase in air flow, and hence flame-up, if you lift the bonnet. I didn't say backdraft, I said it will flare up.

Fuel lines will remain under pressure even with the engine off, as the regulator will hold pressure at the fuel rail and bleed back through a modern high pressure pump can be very slow. Assume every feed line (or every line if you don't know feed from return) is under pressure in a car engine at all times.
posted by Brockles at 7:38 PM on July 2, 2012 [1 favorite]

there's no fuel pump running

Bad assumption. In their hurry to exit the burning car, the driver may not have turned off the key.
posted by ryanrs at 9:38 AM on July 4, 2012

What about magnesium used in some components and blocks? A quick search of youtube reveals many videos where car fires involving magnesium undergo a flare or "poof". Here's one here: http://www.youtube.com/watch?v=zLoF99LYdts
posted by Pliskie at 6:52 PM on July 30, 2012

Those flare ups are usually the result of large quantities of water being dumped on very hot metals as a direct result of the fire fighters actions, so it is unlikely to be much of an issue for an engine fire danger to the person on the street.
posted by Brockles at 6:05 AM on July 31, 2012