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Tuesday, March 22, 2011

Car Explosions - the Science Behind the Boom!

Today's Science in Fiction post is from guest writer Ben Garrido. Ben lives in Daejeon, South Korea where he works as a teacher and takes post-graduate courses at Pai Chai University. He has published over 40 articles in the Reno News and Review and won the Nevada Press Association's First Place Award for sports features in 2008. He is currently working on his fourth novel.

If you have any questions for Ben please email me and I will forward them on to him.

It would not be a foolish man who declared Hollywood and physics natural enemies.

Whether Brendan Fraisier tunnels to the center of the earth with little more for protection than his billowing hair or Arnold Schwarzenegger throws a steam pipe through his enemy, movies take more liberties with Newton than the Sheens take with pharmaceuticals.

In this installment of Science in Fiction, I seek to preserve you from the distortions common in Tinsel Town’s portrayal of automobiles.

1. Cars do not explode on impact. Or, let me put it
this way, cars explode only in the sense that comets strike the earth – very, very rarely. There are two major problems with blowing up a car during a wreck. The first is that automotive engineers are generally clever fellows. They have advanced degrees, lots of computer simulations and large brains and they all would prefer it if your car doesn’t explode.

Because of this, most cars have their fuel tanks carefully located towards the center of the chassis, shielded by lots of very stout metal. Even those cars with fuel tanks located in the rear have very strong crash structures between the tank and bumper skins.

This means that only a very severe accident is likely to rupture the tank and even when the tank is ruptured, the leakage will be glug-glug-glug or trickle-trickle. That matters because of problem number two.

Gasoline isn’t a very good explosive outside of an engine. If you put a lit match into a cup of gasoline, you’re going to get a small flare up and then gentle burning.

If the car runs on diesel, the open burning fire will be even less impressive - as in diesel will put a match out every bit as effectively as water. In order to actually blow up, gasoline requires fine aeration near the stoiciometric ratio of 14.7:1 air to fuel, compression (gasoline will auto ignite at a compression ration of about 13:1 depending on octane rating, ambient temperature and a number of other factors), and an ignition source.

The only people who can to duplicate those conditions in a car crash are the ones who’ve offended the Gods, walked under ladders and purchased a black cat nursery in the last hour.

Free burning fires do happen though. If you ignite spilt fuel with a hot exhaust pipe it will burn slowly and spread where ever there's more fuel. Getting a car fire out of a crash is relatively common, getting a car explosion is a one in several million type freak accident.

If you absolutely must have a car explosion, here’s an example of a plausible scenario. Some highly questionable hot rodders, for reasons known only to themselves, remove the front bumper and mount an external fuel tank it its place. This sort of rationally unjustifiable modification is most common on very old cars, like the ever popular 32 Ford.

So, if you were to run a car like this into a flat surface you could conceivably get a nice, even spread of aerated gasoline. Now, we still need an ignition source. So, call it a Malotov cocktail that hit the hood and caused the wreck. To summarize all you need is a poorly modified hot rod crashing straight into a wall after a Molotov Cocktail attack. Or you could just use a car bomb.
Image of exploding car found HERE - used under Fair Use Law.

2. Cars don’t roll over during cornering in the way that you see in the movies. There is not a single production car or truck in the western world that will do what is called a traction roll. Let me explain traction rolls. If you are driving at 100 miles per hour (or 200, or whatever) on a flat, paved area and turn the steering wheel as hard as you can, the car or truck will not roll over. Instead, you will exceed the tires' ability to generate traction and simply start skidding.

Most cars and trucks will do what is called understeer. This means the front tires start skidding before the rear tires do. If this happens, the driver will feel that the car won't change direction no matter how he/she turns the wheel. A few cars, most notably the old Volkswagen Beetles, will do what is called oversteer. This means the rear tires skid before the front tires. This makes the car spin out or fishtail. Here are some videos showing oversteer and understeer.

This is understeer and oversteer.

This is understeer .

These are professional racers intentionally oversteering. ^\

Now you are probably wondering how to flip a car/truck. The most common way people accidentally flip a truck or SUV is to go off the road and then violently correct. If the truck is in soft dirt or sand, the wheels can sink into the soft dirt. This can flip the vehicle.

Also, turning abruptly from dirt onto pavement can cause enough shock to flip some trucks and SUVs. Lastly, swerving back and forth violently can build up momentum in some trucks or SUVs and cause a rollover. In order to flip a sedan, minivan or sports car, you generally have to hit something or slide into soft dirt at very high speeds.

For an example of how hard it is to flip a sports car, consider the Aston Martin in the recent James Bond flick “Casino Royale.” It would not flip when cornered at maximum grip and run over an 18 inch ramp at 50 mph. It would not flip when cornered at maximum grip and run over a 3 foot ramp at 60 mph. In order to get that “traction roll” you see in the film, the producers had to equip 007’s car with an air powered cannon to shoot a piston into the ground that launched the car skyward.
Image for the upturned car found HERE - used under Fair Use Laws.

3. Sliding a car does not make you go faster. In movie car chases the drivers almost always spend a lot of time sliding the vehicles. This looks cool, dramatic and a lot of fun but it also slows you down. The fastest way to drive is to come to the absolute edge of traction without sliding. The only exception to this I know is when you drive on gravel or dirt roads. In this case, mild, controlled oversteer (see above) is sometimes desirable.

The Lethal Weapon movies are prime examples of this movie myth. When Danny Glover and Mel Gibson decide to chase the movie villains, they do so in clouds of smoke and from the center of epic screeching slides. If they did that on a racetrack, they could expect to increase laptimes by ten or fifteen percent. Not that I blame the producers, how else are you supposed to make a chase between an Oldsmobile station wagon and a BMW M6 competitive?
Image of Lethal Weapon 1 poster found HERE and used under Fair Use laws.

4. A sticking throttle is not a death sentence. This happens in horror flicks a lot and might creep up in your stories post Toyota's throttle sticking scandal. There are three ways to survive this in every car ever built.

Number one, turn off the ignition. This will kill the engine and you will slow down gradually.

Number two, step hard on the brakes. Every car built in the last 50 years, if not longer, has stronger brakes than engines.

Number three, take the transmission out of gear. If the car is an automatic, select "N," or if you want the same effect with lots of crunching noises and a huge repair bill, put it in park. If the car is a stick shift, put it in neutral. Doing this will probably destroy your engine, but it will for sure allow you to stop the car.

You can also choose any combination of those three ways. The smartest thing to do to avoid damage and come to a stop safely is to first step on the brakes and then turn off the ignition one click so that the engine turns off and you don’t engage the steering lock.

Stuff that won't work: the emergency brake, pumping the brakes, shifting into a lower gear.


  1. Great stuff! Exploding cars are so silly. I did see a car crash once when the radiator turned into a cloud of atomized coolant.

    I've also seen a car caught on fire by parking it on a bush. Even with all the alcohol in the back seat, it never exploded.

    The other popular one that really bugs is shooting gas cans with firearms. Even with tracer rounds, they are very difficult to ignite.

  2. I love anything with explosions! I caught Ben discussing this on CC and begged him to come to the blog for a reference.

    What I really need now is an excuse to blow up a car in my current WIP.

  3. Very cool. I watched a Mythbusters episode with my kiddoes not too long ago where they tried to blow up a car as it launched it off a cliff...and it was hard work to make that happen.

    Angela @ The Bookshelf Muse