(91octane + water) Vs. (100octane + water)

[hotrod]

Why then is a higher octane fuel more resistant to detonation?

Keep in mind that detonation is a very very complex process that even after 100 years of investigation is not fully understood. Detonation appears to be caused by uncontrolled combustion of the partially decomposed fuel. Gasoline is a mixture of several hundred different chemicals composed of very long chain hydrocarbon molecules. They burn by breaking up those chains into smaller and smaller pieces (like cracking in an oil refinery), as temperature rises (this is a form of pyrolysis). The actual burning occurs when free oxygen begins to combine with the most accessable hydrogen and carbon atoms of these fuel molecule fragments.

What happens is that at very high temperatures and pressures, fuel begins to decompose even before the main flame front reaches it. This breaks up the fuel molecules into smaller "pieces". Think of this sort of like a piece of paper beginning to char before it bursts into flame, under a magnifying glass.

Also when the main flame front passes through a volume of gas it does not all burn at once. It takes time and the more easily burned "pieces" burn off first, leaving behind some other more difficult to ignite fuel fragments. This is much like what you see in a camp fire where the smaller pieces of wood will burn to ash before the larger pieces are fully ignited. In the case of the larger pieces of wood you will see smoke and vapors cooking out of them and igniting when it reaches free oxygen in the air. The same sort of progressive burning takes place in a gasoline air mixture. If you ground the wood up into a very fine dust and blew it into a fire it would burn just like gasoline.

The effect of these two processes (pre-combustion decomposition) and (post combustion afterburning of partially burned fuel), is what controls detonation. In high speed photographs they have recorded flame fronts sweeping across the combustion chamber several times through the same fuel air mix as these successive combustion processes occur.

Some fuel components tend to break up into fragments that are prone to uncontrolled explosive combustion at high temperatures, and other fuel components tend to break up into molecule fragments that tend to burn smoothly and progressively under high temperature and pressure.

A high octane fuel simply has a higher proportion of the "well behaved" fuel components like Toluene. Toluene is a ring shaped molecule and like the other members of the Aromatic fuels (Benzene, and Xylene) tends to burn in a well behaved way. Straight chain hydrocarbons tend to produce partial combustion products that do not play well at high temperatures and pressures. They tend to decompose into mixtures that burn explosively when they reach certain critical temperature and pressure combinations.

Most octane control additives like TEL (lead) and MMT work by changing the way these decomposition products form, and changing the chemistry of the partially burned fuel.

Water injection appears to control detonation by two different mechanisms. It cools the peak combustion temperatures which changes the type of decompositon the fuel undergoes prior to normal ignition, and it changes peak cylinder pressures. Peak pressures are lower but they tend to persist longer giving more useful mechanical work out of the lower pressures.

Water vapor also has a strong effect on the actual chemistry of combustion as it appears to assist with full combustion of carbon to carbon dioxide. Absolutely dry carbon monoxide mixed with oxygen is very difficult to burn but once water is present it will ignite and burn to CO2 easily.

Larry