Posted by David Sherman on Thursday, April 20, 2006 at 11:58AM :
In Reply to: Re: Lead Substitute? posted by PitMan on Wednesday, April 19, 2006 at 12:50PM :
Casinghead or "natural" gasoline was the only kind there was until the "catalytic cracker" was developed, which is device that breaks long hydrocarbon chains into short ones. "Volatility" isn't necessarily related to anti-knock properties. Propane, for example, is much more volatile (evaporates rapidly) than gasoline, but has good anti-knock properties. "Octane" is actually a specific hydrocarbon. The longer the hydrocarbon, the less volatile it is. Methane, propane, butane, pentane, hexane, heptane, octane, and so on are progresively larger hydrocarbon molecules.
The "octane rating" was originally a way to compare the anti-knock capability of a gasoline to a mixture containing nothing but heptane and octane. Pure heptane detonates very easily under compression, whereas pure octane does not. The "octane rating" of gasoline refers to what mixture of pure octane and heptane would have the same anti-knock capability as the gas being tested. Anti-knock additives decrease detonation without actually changing the hydrocarbon mixture. TEL was a big deal because it drastically lowered the cost of high-octane gasoline. One of my dad's old 1940-vintage college engineering handbooks lists ordinary automobile gas at 65 octane and "fighting grade aviation gas" (the best made) at 87 octane, which is now considered low-end.