Blowing up a car.

General Questions
#1

[disclaimer]
now before a mod closes this thread, let me tell everyone that i’m not looking for tips on how to destroy a toyota or stories about tying cherry bombs to matchbox cars. [/disclaimer]

i just finished watching “gone in 60 seconds” and i’ve got a question about the scene towards the end where that big silent guy blows up one of the antagonist’s car by putting a rag into his gas tank and then lighting it. i’ve seen this done in a few movies now, and i’m wondering if it really works. can this be done or is it just a movie “thing” that keeps the pyrotechnician’s union happy?

#2

It’d be sorta like a super-sized Molotov cocktail, wouldn’t it? I expect you could get similar results just dropping a match in, but that wouldn’t give you much time to get away from the fwoosh. (That’s the technical term.)

I expect the best way to find out would be to try it. You first. I’ll just stand over here and watch. :smiley:

#3

I have always thought that a molatov cocktail didn’t explode but rather the bottle broke spreading fuel all over the place which the burning rag ignited.

I think that the tube to the gas tank is to long for the buring rag to really be stuffed into it all the way to the fuel. I supose the vapors in the tank could ignite rupturing the gas tank spilling fuel everywhere thus blowing up the car.

#4

Here’s how a non-mechanic sees it:
The fuel tank is in the rear, right? I’m guessing here, but I think I’m right. Well, igniting an explosion in the rear near the gas tank would act as a spark to ignite whatever was in the gas tank, which I think would cause a huge explosion, more than what you would expect from the bomb itself.

#5

I’m sorry, I joined this one by mistake. I thought it said “blowing up a cat”.

I’ll be going now.

#6

It’s not clear how much of an explosion you would get.

Certainly, the large ball o’fire that you see in movies is the result of pyro magic; watch almost any of the “behind the scenes” shows on AMC or TLC or whatever for details on how difficult it is to make a car blow up in a spectacular fashion.

As for the rest – I’ve seen a number of car fires over the years. Even when the car is fully engulfed, there doesn’t seem to be much of an explosion.

I suspect that the petrol-soaked rag would act as a wick, and would simply burn up the fuel. Of course, when the fuel was mostly gone, and you got somewhere around a 7% gas/air mixture, you might get a bang.

Maybe you could get a similar effect (on a small, experimental scale) by filling a small oil lamp with petrol (gasoline) and lighting the wick. I recommend
(a) using a very long match
(b) doing this outdoors
© having a fire extinguisher handy.
(d) having a grown-up present :slight_smile:
(e) telling us what happened, or pointing us at the page from your local newspaper/TV station which ran the story!

#7

Want an explosion? You’ve got to mix the gasoline (preferably vapor or at least atomized) with air at an 11 to 1 ratio, then ignite it.

I suppose if you had a 12 gallon tank with one gallon of gas in it you might get a good whoomp.

To really get a good blow-the-bed-off-of-a-pickup-truck type bang you need something more uhhh… drastic than a rag and a lighter.

Not that I’ve ever been involved in blowing the bed off of a pickup truck. Oh no, not me. :wink:

#8

The most important thing to remember is…

don’t burn your lips on the exhaust pipe.

(Okay, I’m sorry, but SOMEBODY had to say it.)

Ugly

#9
  • Yes.
  • It’s even higher than that. I never saw/attempted to blow up a car, but childhood friends and myself did dispose of a few old gas cans this way. You have to pour out all the liquid gasoline, completely, just leaving what clings to the insides of the can.{{{It also has to be lit a very specific way, that I will decline to describe here. I will say that a rag stuffed into the opening won’t do it. Even a gas-soaked rag won’t do it.}}} If successful, this sorta thing makes a good “whump!” (it almost doesn’t sound like an explosion) but doesn’t do much damage, unless you’re stupid and you are holding it in your hand or something.
  • Hollywood I: almost everything you see in movies and TV is fake: you cannot explode a car by lighting the gasoline inside the tank by any simple means. If there’s liquid fuel in the tank, the air/fuel mixture will be way too rich. (-I guess they show it that way on purpose, in case somebody actually tries it)
  • Hollywood II: when we used to make high explosives, the explosions were very small very brief bright flashes that produced almost no smoke at all. The sound wasn’t a booming noise (like every movie explosion) but was a sharp CRACK that made yer ears ring a loooong time, like firing a rifle without earplugs does. Movie/TV explosions often use lots of gasoline not so much because it’s cheap, but specifically because it creates impressive-looking explosions. The sets also have to be made to fall apart, because it doesn’t really do much damage. - HTH - MC
#10

Gas burns, fumes explode. More fumes - bigger boom.

#11

This paragraph, in a chapter on how to start and manage fires in the bush, from Robert Pelton’s “Come Back Alive”, has kept me giggling for hours on end…(I know, I’m nuts).

"Chicki-Chick-Boom!

We all know that the best way to start a fire is to have a road flare, a 45 gallon drum of flammable liquid, an old wooden hut and three months of trash as kindling. I know this from personal experience from the time I wondered if a road flare could bob in a 50 gallon pool of AV gas and then woke up on my back watching bits of burning trash float down toward the ground. Yep, a road flare and highly explosive gas works mighty fine."

#12

Don’t tell me, you work for Dateline NBC, right?

Anyways, from my understanding, Molotov cocktails do actually explode. I’ve never tried on my own, or anything, but that was the impression I got from the Anarchist’s Cookbook many years ago. Incidentally, am I the only one who always thinks of Molotov cocktails whenever the Molotov-Ribbentrop Pact is mentioned?

#13
      • Molotov cocktails don’t explode, except in movies. Gasoline is not an explosive, vaporized or not. It’s just not that powerful, or sensitive. -And as to the OP, the flame won’t propagate through the gas tank opening, and even if it did, the fumes in a closed fuel tank -that has liquid fuel inside- will be way too rich to ignite. - MC
#14

Deimos, yes it does work. But only if there is just a little gas in the tank. A full tank wont do much.

#15

And even if there is a little gas in the tank, there still has to be air. Gas tends to vaporize and fill the rest of the gas tank with gas vapor - thus, very hard to set off. If you mix gasoline with air in that 11:1 to 16:1 ratio, you will get a pretty damn good explosion. How much? Well, let’s compare here:

n-Octane:

2C[sub]8[/sub]H[sub]18[/sub] + 25O[sub]2[/sub] --> 16CO[sub]2[/sub] + 18H[sub]2[/sub]O

Heat of Combustion = [18(-53.46) + 16(-85.08)] - [25(0.00) + 2(-51.07)] = -2221.46/2 = -1110.73 kcal/mol

Molecular Weight of n-octane = 114 g/mol

Heat of Combustion/gram = (-1110.73 kcal/mol)*(1/114 g/mol) = -9.74 kcal/g

trinitrotoluene (TNT)

4C[sub]7[/sub]H[sub]5/sub[sub]3[/sub] + 21O[sub]2[/sub] --> 28CO[sub]2[/sub] + 10H[sub]2[/sub]O + 6N[sub]2[/sub]

Heat of Combustion = [10(-53.46) + 28(-85.08) + 6(0.00)] - [21(0.00) + (123.63)] = -3411.45/4 = -852.86 kcal/mol

Molecular Weight of TNT = 227 g/mol

Heat of Combustion/gram = (-852.86 kcal/mol)*(1/227 g/mol) = -3.76 kcal/g

So, according to these reactions, properly oxidized gasoline should be nearly three times as powerful as TNT per mass. Scary.

(thanks to my Fierra for finding this info for me! :slight_smile: )

#16

The key to generating a large explosion is to cause a reaction that releases large quantities of energy very rapidly. This is why burning liquid gasoline does not generate an explosion; lots of heat will be released but the release of energy will occur over a relatively long period of time.

Burning gasoline vapors, however, will cause the energy to be released much more rapidly. For example, using Anthracite’s figures, if we assume a 12 gallon gas tank, we’d need about 12 parts of oxygen to completely burn one part octane. Air is ~20% oxygen, so, if we take the gas tank to contain only air and gasoline vapors in the optimal proportions, the tank would contain approximately 0.2 gallons worth of gas vapors, and 11.8 gallons of air (of which 20% of this would be oxygen). So we’d be burning ~0.033 moles of octane for about 40 kcals of heat energy. This likely wouldn’t be enough to blow up a car, although the resulting explosion would probably yield a pretty good kick. This is especially true when you take into account the rate at which the reaction would occur; gaseous octane burns fairly rapidly, but not as rapidly as, say, methane or acetylene.

#17

Anyway, the bottom line is that cars only explode under highly unusual circumstances. Cars do catch on fire, but rarely explode when they do. And simply igniting the gasoline in the tank will not make the car explode, usually it will simply cause the gasoline to burn.

Supposedly it is even possible to put out lit cigarettes in containers of gasoline. I wouldn’t do it myself, but I’ll be happy to watch from a safe distance while YOU do it…

Anthracite, isn’t TNT explosive because it already has its oxidizer built in? Gasoline is more difficult because you have to add air. So gasoline burns because it ususally uses up its local O2 and has to wait for more to move in, while TNT explodes because it has all it needs already. However, it’s been years since my last chemistry class…

#18

Most of you all forgot a little detail. After the rag burns down to the top of the gas tank filler hole, a little door clamps shut, which would probably keep it from going boom.

Next time you get gas, look at that hole, it has an automatic closing lid on the top.

#19

Well…good question. I don’t believe that is has to do directly with the containment of the oxidizer. There are other fuels that contain their own oxidizer that are not “explosive” per se. And if you look at the reaction I posted, it shows an addition of 21 moles of O[sub]2[/sub] required for every 4 moles of TNT - thus, while the TNT has some oxygen in it, it still needs 42 extra O’s to combust. So the TNT does not have the oxygen it needs contained with it.

Sorry to be terse here, I am in a rush to get out of the office today. For some further info on explosions and detonations, I posted in another thread about the difference between burning and an explosion. I would post the link, but the Board is extraordinarily slow. :frowning: If you search for “Anthracite” and “deflagration” you can find it.

#20

handy - it depends on the make & age of the car…I can assure you that my car does not have an automatic closing door to keep gas vapour in…