What fuel does the Octane Rating?

The fuel property the octane ratings measure is the ability of the unburntend 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 atvarious 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-hydrogenbonds depends on what the carbon is connected to.

Straight chain HCs such as normal heptane have secondary C-H bonds that are significantly weaker thanthe primary C-H bonds present in branched chain HCs like iso-octane.

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 compression 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.

Antiknock additives work by interfering at different points in the pre-flame reactions, with the oxygenates retarding undesirable low temperature reactions, and thealkyl lead compounds react in the intermediate temperature region to deactivate the major undesirable chain branching sequence. The antiknock ability is related to the "autoignition temperature" of the hydrocarbons.