Re: 'combustion' ...my mistake, I was using it as a layman's term.....the proper term is 'ignition'.
There is a considerable differance between the two. I have seen no place in the links you posted which says high octane fuel burns slower or has less volitility..
BTW these quotes are form the last link you posted.
"6.12 How do other fuel properties affect octane?
Several other properties affect knock. The most significant determinant of octane is the chemical structure of the hydrocarbons and their response to the addition of octane enhancing additives. Other factors include:-
Front End Volatility - Paraffins are the major component in gasoline, and the octane number decreases with increasing chain length or ring size, but increases with chain branching.
Overall, the effect is a significant reduction in octane if front end volatility is lost, as can happen with improper or long term storage. Fuel economy on short trips can be improved by using a more volatile fuel, at the risk of carburettor icing and increased evaporative emissions.
Final Boiling Point.-
Decreases in the final boiling point increase fuel octane. Aviation gasolines have much lower final boiling points than automotive gasolines. Note that final boiling points are being reduced because the higher boiling fractions are responsible for disproportionate quantities of pollutants and toxins. Preignition tendency - both knock and preignition can induce each other.
"
and
"6.3 What fuel property does the Octane Rating measure?
The fuel property the octane ratings measure is the ability of the unburnt end gases to spontaneously ignite under the specified test conditions. Within the chemical structure of the fuel is the ability to withstand pre-flame conditions without decomposing into species that will autoignite before the flame-front arrives. Different reaction mechanisms, occurring at various stages of the pre-flame compression stroke, are responsible for the undesirable, easily-autoignitable, end gases.
During the oxidation of a hydrocarbon fuel, the hydrogen atoms are removed one at a time from the molecule by reactions with small radical species (such as OH and HO2), and O and H atoms. The strength of carbon-hydrogen bonds depends on what the carbon is connected to. Straight chain HCs such as normal heptane have secondary C-H bonds that are significantly weaker than the primary C-H bonds present in branched chain HCs like iso-octane [13,14].
The octane rating of hydrocarbons is determined by the structure of the molecule, with long, straight hydrocarbon chains producing large amounts of easily-autoignitable pre-flame decomposition species, while branched and aromatic hydrocarbons are more resistant. This also explains why the octane ratings of paraffins consistently decrease with carbon number. In real life, the unburnt "end gases" ahead of the flame front encounter temperatures up to about 700C due to piston motion and radiant and conductive heating, and commence a series of pre-flame reactions. These reactions occur at different thermal stages, with the initial stage ( below 400C ) commencing with the addition of molecular oxygen to alkyl radicals, followed by the internal transfer of hydrogen atoms within the new radical to form an unsaturated, oxygen-containing species. These new species are susceptible to chain branching involving the HO2 radical during the intermediate temperature stage (400-600C), mainly through the production of OH radicals. Above 600C, the most important reaction that produces chain branching is the reaction of one hydrogen atom radical with molecular oxygen to form O and OH radicals.
The addition of additives such as alkyl lead and oxygenates can significantly affect the pre-flame reaction pathways. Anti-knock additives work by interfering at different points in the pre-flame reactions, with the oxygenates retarding undesirable low temperature reactions, and the alkyl lead compounds react in the intermediate temperature region to deactivate the major undesirable chain branching sequence [13,14].
The antiknock ability is related to the "autoignition temperature" of the hydrocarbons.
Antiknock ability is _not_ substantially related to:-
The energy content of fuel, this should be obvious, as oxygenates have lower energy contents, but high octanes.
The flame speed of the conventionally ignited mixture, this should be evident from the similarities of the two reference hydrocarbons. Although flame speed does play a minor part, there are many other factors that are far more important. ( such as compression ratio, stoichiometry, combustion chamber shape, chemical structure of the fuel, presence of antiknock additives, number and position of spark plugs, turbulence etc.) Flame speed does not correlate with octane. "
