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The octane rating of petrol (gasoline) is a measure of the how much compression a fuel air mixture can have before detonating (which in petrol engines is a bad thing as you want the mix to combust only when the spark plug arcs).

What are the key chemical characteristics that determine this rating? What is the molecular-level explanation? And how do the various non-hydrocarbon additives work to modify the result?

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Combustion is (usually) chain radical reaction with heavy brunching. More brunching means faster combustion and less branching is slower combustion. And detonation is wave of super-fast combustion.

So, if you can suppress branching or deaden some branches, you can increase durability of air-fuel mixture. Actually, this means, that some radical trap is needed. The oldest trap was tetraethyllead, but it is currently dropped (lead oxides as fine dust is not a health friendly substance). Non-saturated hydrocarbons of different types also can trap active radicals, transforming them into more stable allyl radicals, so fuel companies tries to keep as much of them in fuel as possible. Some other compounds, like ferrocene, also can be used to increase octane rating.

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What do you mean by branching and suppressing branching? –  Ben Norris Oct 27 '12 at 14:43
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@BenNorris: When people talk about radical-based reactions, often three types of step are referred to: initiation, a reaction which makes a radical; propagation, a reaction which preserves the number of radical species, and termination, a reaction which consumes a radical. But, if you have a reaction which produces more radicals than it consumes, you get 'branching', which accelerates the rate of the overall reaction since now more propagation and termination steps can happen. –  Aesin Oct 27 '12 at 17:39
    
Ah, okay. Since the question was about octane rating, I, as an organic chemist, was trying to relate this answer to branched molecules, for example isooctane (defined octane rating of 100) is a more branched structure than heptane (which is defined as octane rating of 0). –  Ben Norris Oct 27 '12 at 18:28
    
@BenNorris Branched hydrocarbons usually have higher octane rating as they can product tertiary radical: most stable of alkyl radicals. –  permeakra Oct 27 '12 at 20:04

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