Date: Wed, 03 Mar 1999 21:29:19 -0500
From: "Bart L. Grossman" <grossmannopsam.net>
Subject: Re: Can someone explain OCTANE?


Your a softy. I would have marked it a C- for content, then gave it a D when attitude is added in. RED wrote: > Mike Smith wrote: > > > > Justin VanAbrahams wrote in message <36DA82F3.407nopsam.com>... > > >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 -- Bart L. Grossman 1 Nickerson RD Peabody, MA 01960

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