I recently got into a discussion with a friend about...

Dear Car Talk

Dear Car Talk | Jul 01, 2002

Dear Tom and Ray,

I recently got into a discussion with a friend about the volatility of fuels. He claimed that diesel fuel is highly volatile but gasoline is not. I told him it's the other way around. He said he knows that gasoline isn't volatile because his cousin Bubba puts out his cigarettes in a can of gasoline. I responded that what Bubba does sounds dangerous and highly unlikely. Now I'm afraid that my friend will try to duplicate Bubba's stunt. What should I advise him? -- Ken

RAY: Advise him to take out a nice, large life-insurance policy on Bubba, naming you and him as co-beneficiaries, Ken.

TOM: Here's why: The burning tip of a cigarette is somewhere between 750 and 1,100 degrees Fahrenheit, depending on whether you measure it on the side or in the middle, where it's hottest. The ignition point of gasoline is only 500 to 860 degrees F. So you do the math, Ken. Cigarette + Gasoline = Kaboom!

RAY: The truth is, it is possible to throw a lit cigarette into a bucket of liquid gasoline and have the cigarette go out. If the gasoline and/or ambient temperature is cool, and the flame is extinguished quickly, you might luck out. But here's why you're taking an awfully big risk ...

TOM: Since gasoline is highly volatile (and you are correct, Ken; it's more volatile than diesel), it's continually giving off gasoline vapor. And gasoline vapor has a lower ignition point than liquid gasoline. So if your lit cigarette spends an extra half a second in the gasoline vapor before getting into the liquid, or if it floats for a moment on top of the liquid before being extinguished, it could much more easily ignite the vapors, which would then ignite the whole thing. And that could lead to tragic -- or in Bubba's case, Darwinian -- consequences.

RAY: Diesel fuel is far less volatile, and it won't vaporize at room temperature. In fact, diesel is the same as home heating oil -- and it was chosen as a home heating fuel because of its relative safety. So you won't see diesel vapors igniting in the same way as gasoline vapors.

TOM: But still, I would strongly -- make that very strongly -- suggest to your friend that the surgeon general has determined that smoking, in and of itself, is a bad idea. And extinguishing your cigarettes in any kind of flammable liquid is a pretty clear demonstration of world-class stupidity.

RAY: And just to be on the safe side, Ken, since your friend might share some genetic material with his cousin Bubba, if you ever hear your friend utter the words "Hey guys, watch this!" run like hell.

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