Exploding cars/motorcycles/etc

Factual Questions
1

If the 80’s have taught me anything it’s that cars will always explode when they fall off a cliff. We’ve all seen real examples of this happening with airplanes, on 9/11 if nowhere else. My question is what exactly causes vehicles to explode when they hit something at high speed, and what causes some of them not to explode?

As a lowly high school physics teacher I suspect it has something to do with the massive kinetic energy of a very big car or plane travelling at high speed coming to a sudden stop. That kinetic energy needs to go somewhere, so maybe it goes into heating the fuel stored in the vehicle. Some of it goes into destroying or deforming the thing that it hit, sure. If the fuel stays in a liquid state (the fuel compartment isn’t breached) then it doesn’t do anything other than get warm. If the fuel compartment is breached and the volatile fuel becomes a mist then it may reach ignition temperature which will cause a quick chain reaction, aka an explosion.

I’m curious if this is the actual mechanism of the explosion, though. When I was younger I figured it was the electronics that activated the explosion, like in Terminator 2 when the spark plug in the T-1000’s rig set off the explosion after he crashed (never mind that diesel trucks don’t actually have spark plugs).

I’m interested in the actual step-by-step mechanism of the explosion. Is it:

  1. Fuel molecules are accelerated to a high speed.
  2. Fuel molecules are brought to a stop by a great force, giving them tremendous thermal energy.
  3. Some fuel particles disperse in a cloud, giving them free access to oxygen.
  4. The fuel particles ignite, causing a minor explosion which gives energy to the rest of the particles to ignite, causing a bigger explosion.
  5. The explosion blasts the liquid fuel into a mist which continues the chain.
  6. Big boom.

Maybe I’ve answered my own question, or maybe I’m wrong. Even if I’m right I think it would be an interesting column for Cecil to tackle.

2

In the 80s, stunt coordinators, not real life, taught you that cars off a cliff and crashing planes explode. In movies and TV, the director wants an explosion, so stunt people give him one. In real life, crashing cars and planes seldom even catch fire, let alone explode. Car fires mostly happen when the car is not moving at all.

I’m not going to touch 9/11. I’ll let somebody else dance with the conspiracy hobbyists.

3

Vehicles usually don’t explode. They can often burn very impressively.
To get an explosion due to a fuel tank blowing up, requires a specific set of circumstance. A fire adding heat to a sealed fuel tank, long enough to cause it to rupture due expansion. Then a pretty good fireball might happen. But most fuel tanks are vented. In a crash with fire, there is likely another fuel leak, that will vent pressure too.

A partially full tank with a large volume of flammable vapor in it is more likely to explode. But it is a very haphazard thing. Also difficult to achieve.

In real life, explosions on impact are very rare. Explosions during an ensuing fire are more likely. Intermittent large flare ups and jets of flame during the fire are more likely.

I got to watch a car burn. Arsonist lit it up in the parking lot across the street from my house at night. It was interesting. As the wiring melted and randomly shorted, the lights flashed, horn tooted. Don’t know if the radio turned on and played a fitting tune. But I like to imagine it did. Fire department put it out before the gas tank got involved. But I really think it would have burned out and not exploded. Not enough flammable material that close to the gas tank. Nor a good path into the engine compartment.

4

Cars will explode when the gas tank ruptures and sprays fuel about, which then ignites due to an available spark.

I saw a police dash cam video where a pickup overran a downed motorcycle. As the truck overran the bike the undercarriage/motorcycle interaction lifted the truck in the air while tearing the bike’s fuel tank apart, which then fireballed nicely as the truck continued over the motorcycle in its short airborne path. Most Hollywood-esqe vehicle crash I’ve ever seen.

5

that’s not really an “explosion,” though, just the gas vapors igniting. It’s like the “WHOOF” you get if you light a barbecue when someone used gas as the lighter fluid (NOT RECOMMENDED.)

for gasoline to cause an explosion, it needs to be in a more or less closed container, mixed with air at roughly between 12:1 and 18:1 air:gasoline, then ignited to burst the container.

n.b. this is why there’s no danger with running wiring and electric fuel pumps inside car gas tanks. The tank is full of liquid gasoline, and the space above the liquid is almost entirely gasoline vapor. there’s practically no oxygen inside the tank to support combustion.

that would practically never happen in the real world anyway, diesel fuel is not flammable at ambient temperature. you need to get it over 140 degrees F before it starts to emit flammable vapors. below that, it’s nigh impossible to ignite with a mere spark.

6

There’s nothing mysterious about gasoline igniting. No decelerating molecules required.
All that’s needed is a mist of gasoline, and metal hitting metal, or metal hitting pavement. So, if the fuel tank ruptures, and sprays gasoline around, the collapsing car frame is pretty likely to generate a spark which will ignite the vapors.
Have you ever seen a chain being dragged along the ground? I makes lots of sparks. In a wreck, if the fuel tank is ruptured, there are sure to be spark sources around.

Note that fuel tanks are designed not to rupture easily, which is why most wrecks don’t involve fireballs. But, if a car gets crushed by a semi, or sandwiched between two big vehicles… woosh!

7

Cars explode in movies because they are packed full of explosives and gasoline containers. Real world cars very rarely explode.

My father was killed when his car hydroplaned, rolled up the underside of a bridge overpass and rolled back down. No explosion.

A friend of mine in high school was killed when he lost control at a high rate of speed and slammed into a concrete bridge support at roughly 90 mph. No explosion.

Another friend of mine lost control of his car at a high rate of speed and went sailing off a mountain-side. Amazingly, no one was seriously hurt (except for the trees - he made a nice car-width path for a good distance down the mountain-side). No explosion.

I was driving into work when a woman fell asleep at the wheel, drifted over into the rear wheels of a semi-truck (which woke her up), panicked and cut the wheels hard to the other side, went flying off of the highway and flipped the car end over end several times. No explosion. The woman and her passenger both lived. Looked like her passenger had a broken nose, but otherwise no major injuries.

I was driving into work on another occasion and we stopped at a red light. The light turned green, and a car coming in the opposite direction to me immediately did a hard acceleration into the intersection. At the same time, a pickup truck tried to beat the light and failed. He ended up driving over the car’s hood and flipped the truck over in a true Hollywood style crash. The truck did a 360 and then some barrel roll through the air then slammed down onto the ground on its side and slid to a halt just inches away from my driver’s side door. No explosion. The guy in the passenger’s seat of the truck had his arm all cut up from where he landed on the window and ground as the truck door window shattered. Other than that, there were no injuries.

The only time I’ve seen a car explode was when I was driving home with my cousin from a college class many years ago. The car in front of us swerved off of the road and came to a screeching halt, and the guys that were in it immediately jumped out of the car. We stopped to see what was wrong, and they said that there was smoke coming out from the dashboard. The car went from “gee that’s a little bit of smoke” to “holy crap the entire car is engulfed in fire!” in a surprisingly short amount of time. It took a while, but eventually the gas tank exploded. No one was hurt but needless to say, the car was totaled.

The Ford Pinto was famous for potentially exploding in an accident, due to a design flaw. If the car was rear-ended, there were bolts protruding out of the rear differential that would be pushed into the gas tank, causing the tank to rip open and spray gasoline everywhere. If you throw a match into a bucket of gasoline, this may surprise most people, but most likely all that will happen is the match goes out. You need a mixture of gasoline and air along with your ignition source in order to get a big kaboom. If the car’s tank rips open and sprays gas everywhere, it’s a bit easier to end up with an explodable mix of fuel and air. Then all you need is a source of ignition.

This is why planes often explode. The wings are full of fuel, and when they rip open they spray fuel everywhere. If some of this fuel/air mixture comes in contact with something like the hot engine exhaust, it can easily ignite.

This is the key, really. You need fuel and, oxygen in the right amounts, and a source of ignition. Too much fuel and not enough oxygen and no kaboom. Too much oxygen and not enough fuel and no kaboom. No ignition source, no kaboom.

Grain elevators, coal bunkers, and similar things can also explode due to the dust and air making a fairly combustible mixture. Then all it needs is a spark.

8

Just wanted to pop in to say that directors were making cars explode when crashing way before the 1980’s. I seem to remember seeing movies made in the 1940’s that used the exploding car effect.

9

not gasoline. That doesn’t add anything much to the explosion… as it burns almost as quickly as the explosive does, and its gone in a flash. gasoline vapour is itself explosive enough… fast burning… But it doesn’t become vapour so easily, so the fuel tank won’t explode without help…basically if the car is on fire then the gasoline tank just makes jets of flame as the fuel inside evaporates and burns on exit…

As explained by mythbusters, in the movies they use explosives and a slow burning oil, like a heavy oil (ship’s fuel like bunker oil?) or naptha … to make the long lasting big orange glowing fireball effect.
(I am not saying all I learnt was from mythbusters, that would be an irony… )

10

Exactly. You don’t need to examine adiabatic compression, just:

  1. Leak,
    2, Spark,
  2. Boom.

Dennis

11

I was watching a TV show about stunts and special effects some time ago (I don’t remember the name of the show) and they said that they used containers of gasoline because that gave nice big Hollywood style fireballs.

Not my area of expertise, and that one show is all I’m basing it on. And it was a while ago so my memory could be off. But I’m fairly certain they said they use gasoline.

Your recollection of mythbusters may be a better cite.

12

A petrol car engine isn’t normally hot enough to auto-ignite petrol (by definition) and is a long way away from the fuel tank. Fuel can leak on hot parts which can vaporise it and make it easier to ignite from sparks from the electrical system or scraping metal. Catalytic Converters in the exhaust systems can be hot enough, but again are usually distant from the fuel.

Jet engines are much hotter and more exposed. Internal temperatures reach 2000 Celsius. Jet A-1 has an autoignition temperature of 210 Celsius. Plus the wings are full of fuel and just over the hot engines. Aircraft also tend to crash at higher velocity and the fuel will become an aerosol. This is why jet aircraft are much more likely to explode in flames than a car.

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13

The only way a car will truly ‘explode’ in the real world is if you put a bomb in it and set it off. Even if you put a model rocket igniter in a mostly empty gas tank (i.e. full of fumes) it’s going to be a sub-sonic, extremely ‘low’ explosive more akin to a black powder ‘flash’.

14

Diesel fuel has a much higher flash point than gasoline, so a ruptured diesel fuel tank won’t typically go up in a massive/sudden fireball. That scene in Terminator II where Patrick Cameron’s stolen wrecker crashes into a bridge support and then explodes? Nope, ain’t happening. At worst, a hot engine exhaust manifold might ignite something flammable that it gets mashed against by the impact, and the fire might slowly spread to the rest of the vehicle. If the fuel tank wasn’t ruptured by the impact, it might heat up and build pressure to the point of a BLEVE event. But it’s not going to go up in a massive blast within seconds of impact.

15

Well, I think you mean Robert Patrick (the actor who played the T-1000) in James Cameron’s film. And actually Cameron went out of his way to specifically show boththe fuel pouring out of a ruptured tank and a swinging battery cable sparking and igniting it. Only problem is diesel fuel will not even ignite that way, let alone explode (gasoline will). A for effort at least (and still an overall awesome and 99% accurate scene).

16

Heck, in Steve Martin’s “The Jerk”, IIRC the car goes flying off the cliff and explodes in midair before impact. All because Meathead was going cross-eyed thanks to Martin’s invention.

I recall a stuntman/special effects documentary from years ago, and one Hollywood trick was to use dynamite to loft some barrels of gasoline in the air, where it burst and went up in a fireball to get that big flaming mushroom cloud effect. If I recall, they used rubber cement or something and lit it first - because it was flammable and stuck to the barrels, to ensure the contents of the airborne barrels caught fire properly when they burst.

there’s a “flash point” vapour concentration necessary for an explosion at a given temperature and pressure. Too much or too little vapour for the amount of air and the explosion never happens even with a spark. Different flammable gases and vapours have different flash point requirements (hence diesel being incredibly safe - it has fewer short-chain carbon molecules, less vapour, harder to ignite at atmospheric pressure). Gas tanks won’t typically explode because even mostly empty, the vapour is too much for flash point… until you set a fire around it. But if it’s the only thing in the car that burns, really, that won’t happen unless the gas tank leaks and that leaking stuff catches fire and cooks the gas tank -or it’s such a spectacular crash, the tank bursts and creates a huge vapour cloud that encounters a spark. Since gas tanks are typically protected by some decent amount of frame, especially in the front, this rarely happens.

There was a case where Crown Vics were poorly engineered and a heavy rear collision would drive a bolt on the rear axle into the tank, bursting it as the collision crumpled it, and creating a spark. More often this was not a Hollywood explosion, so much as a rush of flaming liquid pouring into the passenger compartment. Since police used these cars and were more likely to be parked in unexpected locations (during traffic stops) they were at much greater risk of being rear-ended and a number of police were killed or badly burned in these accidents.

17

Thanks for clarifying. And yes, even an exposed pool of diesel fuel is challenging to light. A friend once poured a quantity of diesel fuel on a pile of logs, and then had to blast a diesel-soaked log with a propane torch for a minute or so before it became a self-sustaining campfire.

18

middle distillates like diesel and kerosene need to be wicked and/or heated before they’ll easily ignite.