Cars bursting into flames after just minor contact is the ultimate action movie cliche. I was wondering, how often do cars actually get engulfed in fire due to an accident? What specifically must happen in an accident to cause a major explosion?
I do remember from Mythbusters, for what it’s worth, that shooting the gas tank certainly does not cause any cool explosions, even when the gas tank is out of the car. It’s a TV show, though, so that evidence should be taken with the appropriate dosage of salt
.50 cal tracers will do it, if you hit the tank
Or so I’ve heard
If the right part of the car has been on fire for a while it can begin boiling the gasoline in the tank. This doesn’t cause an explosion, but can make a particularly impressive column of fire.
If the venting mechanism of the tank was sealed, by melting plastic for example, pressure could build up. If the fire department put water on it the combination of cooling/embrittlement and contraction/pressure spike could cause a big vapor flash in the case of sudden catastrophic failure of the tank. Which still technically wouldn’t be an explosion, but would probably be described as such by most people.
If you’ll notice the trend, neither of these happen until after the car’s has a chance to burn for a bit. An explosion is possible if the gas tank is ruptured and the car is not on fire and a source of ignition is subsequently encountered after the fuel vapor has had a chance to mix with the air for a bit. The movie crash-boom isn’t possible.
Edit: I’m talking about gasoline powered internal combustion engines. Hybrids with certain battery types are prone to getting very angry very quickly after container failure.
They used tracer rounds on the above mentioned episode of Mythbusters, but they weren’t .50 cal.
Nonetheless, even a .50 cal won’t do it. Gasoline requires a precise fuel-air mixture to explode. Dropping something hot into a container of liquid gasoline won’t do it.
Yeah - “burn” not “explode”. Sorry for the exaggeration
I’ve witnessed several car accidents, two of which would have exploded if they had been in Hollywood. In one, a pickup truck tried to make a light before it turned red. He didn’t make it. Instead, he ran over the hood of a little white car that couldn’t see him coming. The pickup truck flipped over completely in the air (I thought that only happened in Hollywood too, but I guess not), landed on its side, and skidded to a halt about 3 feet away from my door. No explosion, but it sure made me want to change my undies afterwards.
The second time a woman had fallen asleep and drifted over into the tires of an 18 wheeler. At this point she woke up, and being rather startled (you know, because she just smacked the tires of an 18 wheeler) she yanked on the steering wheel to get her away from the truck, thereby launching herself completely off of the other side of the road. The car literally went end over and a surprisingly large number of times, but didn’t explode. Amazingly, the driver and her passenger both walked away from the crash.
My father also died in a car crash that reduced our family station wagon into a lump of twisted metal barely recognizable as a car. It didn’t explode either.
The only time I have seen a car go up Hollywood style was when we were all driving home from college one day. A car up ahead of us pulled off of the road and everyone jumped out. There was some sort of electrical fire inside the car, and they had all smelled smoke. The car went from a small bit of smoke inside the passenger compartment to being completely engulfed in flames in a matter of minutes, not quite the big WHOOOSH of a Hollywood flame, but rather impressive just the same. Once the flames really got going, I was surprised at how little time it took before the car become completely engulfed. The gas tank did eventually go up, but at that point it was kind of explosion that 1010011010 described above.
I think if you really want to get an explosion, you have to get the gas tank ripped open in such a way that the gasoline splatters into the air. Once you get a nice mixture of gasoline and air, you’ve got all of the ingredients for a fuel-air bomb. All you need to do is add a spark and you’ve got your Hollywood style kaboom. I think this was exactly the problem with the old 70’s Ford Pintos. In a rear end accident the gas tank would come apart and the car would explode. I could be wrong, though. I think that ever since the Pinto, cars have been intentionally designed in such a way as to make those kinds of explosions almost impossible.
Can’t find the cite, but once upon a time I heard there was a fairly consistent 2% of all motor vehicle accidents that resulted in one or more vehicles catching on fire. This was many years ago and has probably become a little lower today.
Mind you this is everything from a little oil or battery fire through totally cooked, 2%.
The only explosion I heard of in my EMT days was a motor vehicle collision involved a bobtail propane tanker. The tank was breached by one of the impacts (multi vehicle collision). Found an arc or small fire on one of the involved vehicles and BOOM!
What about when people stick a kerosene or gasoline soaked rag in the gas tank and light it, does this result in the Hollywood type explosion, or just a persistent fire.
Check this out! :eek:
In order to make this effective, you can’t simple “splatter” the gasoline; you literally have to vaporize it. This isn’t hard to do–a fuel air bomb does this with a very small “cold” charge before a thermal charge ignites the fuel, and the explosions set off by the F/X guys for your archtypical Hollywood explosion work the same way–but I sincerely doubt that this would incidentially occur from a car accident.
Gasoline is surprisingly nonvolatile; in fact (while I don’t encourage you to try this at home) dropping a lighted cigarette into an open container of gasoline is likely to result in nothing more than a dead cigarette, particularly if it is a cold day. Given that the stoichiometric mass ratio of air to gasoline is 14.7:1 (that is, for complete combustion you need 14.7 times as much mass of air as gasoline) you’re going to have a hard time getting any significant combustion in a tank of gasoline before it chokes itself out unless it’s almost empty. Gasoline leaking into the open air will ignite if it sees the autoignition temperature of ~260 Celcius, but it won’t explode, though it may burn very energetically, especially if there is something for it to wick to.
Bascially, the only way gasoline will “explode” (that is, deflagrate energetically) is if you vaporize it and then rapidly ignite it. It won’t do this in a car accident, particularly if the car–having been forced off of a cliff–hasn’t even impacted yet. The exploding car thing is pure Hollywoodism.
Stranger
Yipes! That car was doing an impressive speed up to the impact.
And still, it wasn’t really an explosion (pretty good fire, though).
By the way - there is a substantial difference between dropping something hot (such as a lit cigarette) into gasoline, and dropping an open flame into it. A cig won’t light gas under normal circumstances, but a match will almost every time. A tracer round will also do it, if the tank is ruptured, enough gas leaks out, the fumes stay around and reach LEL but don’t exceed UEL.
If the Mythbusters couldn’t start a fire with gas and tracers they must not have been trying very hard. You get a couple dozen guys firing a bunch of full-auto’s with tracers at a fuel tank, and there’s gonna be fire. “Engulfed in fire” as the OP put it, would be a decent description. “Explosion”? Not so much.
ETA - MythBusters Revisited II successfully started fire with .223 tracers and gas tank. Says “confirmed will explode” but also says “one ignites tank” so I don’t know if it ignited or exploded.
But what exactly makes an explosion?
An explosion requires both a fuel and an oxidizer. Explosives like gunpowder contain both. The trouble with gasoline is that it contains fuel, but does not contain oxidizer.
Therefore, gasoline cannot explode. Only a mixture of gasoline and oxygen can explode. And this requires some pretty unusual situations that are highly unlikely to occur in a car crash.
Now, will a crashed car catch on fire and start burning briskly? It could happen. Could that fire spread really quickly? I could happen. Car crashes, fuel tank ruptures, gasoline catches on fire, pretty soon all flammable parts of the car are burning. But that’s not an explosion unless you get vaporized gasoline mixed with air and ignited, remember the fire triangle. This happens regularly on Hollywood movie lots, hardly ever in real car crashes.
IIRC, if a car is on fire, the real threat of explosion comes from the tires and the bumpers.
I’ve seen gas tanks ignite due to lead poisoning from a .50 cal. Granted, it was a boat, and it was twice on the same boat, but it happened.
While I was serving aboard a patrol boat in Florida, we happened upon a Cuban migrant boat, as we often did. This boat was home made from sheets of steel welded together in the shape of a hull, an old car engine driving a propeller, 2 gas tanks, and a back up mast with sail. The whole thing was scratch built. When we found them, they were relying on the sail as the engine had died. Once we took the Cubans off the boat, we sank it as a hazard to navigation. We shot it up with the .50 which ignited the gas tanks. It certainly wasn’t a destructive explosion as seen on TV, but the tanks went up in an impressive fireball.
Bumpers?
Maybe the styrofoam? Ok I don’t know.
A fireman friend of mine said he has been to a hundred car fires. They lift up the hood and put them out. He says he has no fear of explosion.
Modern cars have gas(nitrogen) charged impact absorbers that may explode if heated enough. Most of the time a seal fails and the N just WOOSHES out.
Spread the fire out but did not extinquish it. Experence is what you get when you don’t get what you wanted.