Date: Tue, 02 Mar 1999 11:54:45 -0800
From: RED <>
Subject: Re: Can someone explain OCTANE?

Mike Smith wrote: > > Justin VanAbrahams wrote in message <>... > >GK wrote: > >> > >> Can someone please explain exactly what octane is and how it effects an > >> engine? > > > >Let me first establish I am not a scientist. I might not know > >what I'm talking about. If that's the case, anyone out there is > >free to correct me... :) > > Uh... OK, then. ;-) > > >Octane is a chemical present in fuel which resists ignition. > >In gasoline, it's primary function is the prevent the mixture from > >burning abnormally, which can happen when the gas reaches a > >temperature (or a pressure) when it actually ignites itself. > > Bzzt. Octane *might* exist in various amounts in gasoline as a by-product > of the manufacturing process, but it is not a primary ingredient. Octane, > C8H18, is a hydrocarbon that exhibits properties similar to gasoline, > particularly that is is combustible (duh) and that it exhibits a flash point > (temperature at which spontaneous combustion takes place, which is what > causes knock) of around 800 degrees. Because it has properties that are > similar to those of gasoline, and it is a repeatably reproducible substance > of an exact, known formula, it makes a good "benchmark" against which to > compare real gasoline, which is actually a mix of many different > hydrocarbons. (Plus other substances like detergents and oxygenators like > MTBE). > > -- > Mike Smith. No, the other one. Well, you get only a B- in organic chemistry. The "octane" of octane rating isn't really the straight-chain compound, octane. It is actually what was once known as "isooctane" which was shortened to just "octane" for convenience. The compound is actually 2,2,4-trimethylpentane. It has the formula C8H18, but is much more branched than octane, thus having greater volatility. Its boiling point is 99C vs ~125C for octane. Originally, gasolines were tested against a mixture of straight-chain heptane, C7H16, and isooctane. The percentage isooctane in the mixture that gave the same knocking properties as the gasoline being tested was the "octane number." At least that's how it was originally done. How it's done now, I'm not sure. Mike is right in saying that any given gasoline mixture may or may not actually contain any isooctane (or heptane for that matter). Bob

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