SZ…KELY ENGINEERING
Tom Szťkely, P.E., LEED AP

EXPLANATIONS & EXAMPLES - Vol. 7, No. 7
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July 31, 2007

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Of Fuels and Explosives, or, Dynamite and the Octane Effect

Two issues ago , I wrote about how hard things hit is much more dependent upon how fast theyíre going than on how big they are, with the impetus for the piece being a newspaper story implying a Cessna could be a terrorist threat.

One of my readers then e-mailed me asking about a Cessna loaded with explosives. I replied that the explosives had to get deep within a building to have any effect on its structure, and that a Cessna simply couldnít do the job. It seems as though the issue started people thinking because another reader asked what I thought about the official explanation for the crash of TWA Flight 800, to which I replied that I agreed with it.

The two questions have made it clear to me that while that piece provided answers which would allow one to internalize the gestalt behind the slogan "speed kills", it raised other questions about explosives and explosions.

Thus, I have been led to research the issue, so as to obtain the detailed knowledge needed to write this piece, a serendipitously satisfying salutary side effect of my predilection to pontificate in print.

Iíve always known (well Iíve known for as long as I can remember knowing anything) that gasoline is highly explosive, and that liquid fuels in general contain potential energy in extremely concentrated form. The interesting things Iíve learned in my research have to do with the differences between fuels, explosives, and fire.

My tendency towards being sesquipedalian in my writing would often lead me to use "conflagration" in lieu of "fire", but I discovered a fuel-air explosion was not actually an explosion, but a "deflagration".

Huh? Well this was a case where something I always knew, just wasnít so. That is, I had at one time learned that an explosion was defined as fast moving fire.

Gasoline explosion , yes. Dynamite, TNT, Plastique, Semtex, RDX, etc., no.

That is, the explosion of a fuel-air mixture actually (usually) deflagrates, while a true [high] explosive detonates.

The difference between the two is that the latter propagates supersonically , and is why the "knock" effect of using low-octane fuel in a high compression gasoline engine is referred to as detonation.

Before I explain the implications of the differences between detonation and deflagration, a digression into an explanation of octane is in order.

Octane rating , the definition of a gasolineís detonation resistance, is so named because Octane, like Butane, Methane, Propane, etc., is a flammable hydrocarbon, and in the early days of high-compression gasoline engines, the octane rating referred to the detonation resistance of a gasoline-antiknock agent mix as a percentage compared to that of a pure Octane-Gasoline mix. Thus 87-octane gasoline is 87% as resistant to detonation as a pure 100% gasoline-octane mixture.

Diesel engines  utilize such high compression that the heat of compression detonates the fuel-air mixture, while gasoline engines rely on a spark plug to ignite the mixture at just before the point of maximum compression. The gasoline deflagration then proceeds, in a small fraction of a second, throughout the mixture from the spark plug to the top of the piston. If the engine compresses the mixture to a heat above the auto-ignition temperature of the fuel-air mixture, it detonates rather than burns, and is why one way of curing knock in older engines (whose combustion chambers may have become smaller because of carbon buildup) is via switching to a higher-octane fuel.

Itís also why using higher than recommended octane gasoline, especially in a new engine, is a waste of money.

Pre-ignition on the other hand, is the setting off of the mixture in a gasoline engine too early in the cycle, usually by a glowing piece of carbon left in the combustion chamber. The mixture still deflagrates rather than detonates, but does so at the wrong time.

OK, back to the implications of the differences between detonation and deflagration.

Objects (including a wave of combustion products) which move in air (at sea level) at velocities in excess of 1090 feet per second or 761 miles per hour are supersonic; moving faster than sound.

The crack of a bullwhip and the thunderclap following a lightning strike are sonic booms; pressure waves piling up and crashing into each other. This overpressure is what makes a high explosive detonation such as that of a stick of dynamite so much more destructive than that of a low explosive deflagration, such as that of a gasoline-air mixture.

This, however, is not to dismiss the destructive power of a fuel-air deflagration. That is, while a stick of dynamite (a little less than a half pound of the stuff) has an energy content of about 2000 BTUs, a like amount of jet fuel has an energy content of 10,000 BTUs.

With jet fuel weighing about 6.7 pounds per gallon, a gallon of it sloshing around in TWA 800ís center fuel tank had the energy equivalent of about 67 sticks of dynamite.

Before you go nuts worrying about the gasoline in your car, consider that a Big Mac, at 2,200 BTUs contains more energy than a stick of dynamite, and a pound of wood contains just a bit less energy than two sticks of dynamite.

Whatís going on here?

Well neither a Big Mac in your body nor the wood in a fireplace or stove burns at a rate anything approaching deflagration.

A gasoline-air mixture deflagrates at about one foot per second. A pound of wood might take from minutes to hours to be consumed. You could take half a day to burn a Big Mac.

Power is how fast energy is released, and is why the supersonic detonation of a stick of dynamite is more powerful than the subsonic deflagration of a half-pound of jet fuel or gasoline in air.

I mean, think about it, even a "tame" dynamite, detonating at a velocity of about Mach 5, in traveling about 5000 times as fast as a low-explosive gasoline-air deflagration, has its shock wave hit objects about 25 million times as hard as the shock wave from the gasoline-air deflagration. (Remember? All that stuff from two issues ago about a faster object hitting harder than a slower one in proportion to the square of the velocity difference?) You go five thousand times as fast and you hit 25 million times as hard.

Earlier on in this piece I said fuel-air mixtures usually deflagrate rather than detonate. Pentane-air mixtures detonate with about as high a velocity as a "tame" dynamite ("tame", because there are dynamites with three times the detonation velocity of a "tame" dynamite), and whaddaya know, JPL conducted experiments in 1980 which had gasoline-air mixtures detonating nearly 6000 times as fast as a gasoline-air deflagration, or 20% faster than my "tame" dynamite baseline.

Ouch! No wonder detonation in internal combustion engines has been known to trash pistons.

Keep in mind that while detonation is supersonic combustion and deflagration is subsonic combustion, NFPA 69 defines an explosion as a situation where such combustion ruptures the container in which it occurs, due to the overpressure caused by the constrained combustion. That is, that even "mere" deflagration in the center tank of TWA 800 could raise the pressure in the tank to the point of blowing it apart.

Think about this. If you can blow the top off a pot which takes 5 to 10 minutes to come to a boil, or pop the safety of a steam boiler, by an overpressure of a few pounds, what happens to the temperature (and thus the pressure) in a sealed tank where even a deflagration can release the energy of tens to hundreds of gallons of vaporized jet fuel in a few seconds?

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