Octane Ratings

[ricksummon]

On gas pumps, there's a sticker that has the octane rating of the gasoline. It also usually says that the octane rating is determined by the (R + M) / 2 method. What the heck are R and M? For that matter, what exactly is an octane rating? I know that "octane" is a specific kind of hydrocarbon, but does an "octane rating" of 85 mean that the gasoline is exactly 85% pure octane?

[bibliophage]

The octane number measures nothing more or less than the anti-knock property of the fuel. Heptane is defined to have an octane number of zero. 2,2,4-trimethylpentane (an isomer of octane) is given a rating of 100. An 87-octane gasoline has the same anti-knock properties as a mixture of 87% iso-octane and 13% heptane. A gasoline need not have any octane in it to get a rating of 100. (In practice I'm sure all gasoline does contain octane). I believe it's possible to to have an octane rating higher than 100. High octane ratings do not necessarily mean higher energy content.

[bibliophage]

By the way, R and M refer to 2 different ways of calculating the octane rating. See Fuel Ratings (http://www.earlham.edu/~chem/pages/gasoline/grades.html) which also explains how they calculate octane ratings over 100.