What is the octane rating of petrol?

Answer 1

Octane rating is a measure of how resistant the fuel is to causing 'knocking' or pre-detonation in petrol(gasoline) engines. It is measured relative to a hydrocarbon which is rated at 100, and determined by testing in a test engine. There are two methods of testing though, which give results called RON or MON. In Europe RON is quoted, and regular petrol is about 92, though usually pumps give 95 RON fuel. In the US a mean of RON and MON is used and for the same fuel the figure is about 5 points lower, so 90 would be the same as 95 in Europe.

Answer 2

Pure (200 proof) Ethanol (non-denatured) will typically have a RON value of between 109 and 112 Octane. It will also have a typical MON value of between 89 and 92 Octane. Using the Road Octane Number (R+M)/2 and the average RON and MON ratings this equates to a value of around (110.5+90.5)/2 = 100.5 for what would be published at the pump on the yellow sticker provided that you could actually buy 200 proof at the pump. However you cannot do this.

Since Fuel Grade Ethanol is denatured, and the source of the denaturant varies from natural gasoline (pentanes, hexanes, and heptanes) to commercial grade gasoline, this is even slightly more variability in the Octane Value of "denatured ethanol). Although the aforementioned 100.5 value should be a reasonable target answer.

The source for these measurements are test measurements run in accordance with ASTM D2699 and D2700 (RON and MOn respectively).

The actual "Octane Number" boost you will get by adding ethanol to a fuel will vary substantially depending on what the base fuel was made of. High concentrations of parrafins such as iso-octane (2,2,4 tri-methyl pentane) will dramatically increase the boost effect.

Some literature points out that Ethanol may actually have an even higher added "combustion enhancing" effect due to the latent heat of vaporization differences when you consider ethanol.