RE: Octane vs. MPG

["Nick Pashley" <nickpashley@xxxxxxxxxxxxxxxx>]

> Date: Thu, 19 Sep 2002 11:57:19 -0700 (PDT)
> From: John Bolhuis <bolhuijo@domain.elided>
> Subject: RE: Octane vs. MPG
>
> ===
>  There is less energy (BTU's per pound) in higher octane fuels than in
> lower octane fuels. Higher octane fuels (like octane itself) have a
> benzene type of structure (ring structure), while lower octane fuels
> have a parafine type of structure (linear chain structure).

Octane is not a Benzene type structure, it is a structure of 8 Carbon atoms
connected to 18 Hydrogen atoms.  The Octane we use to rate fuels is
iso-Octane which is also called 2, 2, 4 tri-methyl pentane, and looks like
this (Hydrogen ommitted for clarity):
    C      C
     |        |
C-C-C-C-C
     |
    C

n-Octane, which looks like this:

C-C-C-C-C-C-C-C

and has an Octane number of less than zero.

As a rule, Octane number increases with disorder, but decreases with number
of Carbon atoms.  Octane rating is not a measure of flame speed, it is a
measure of autoignition temperature.  Flame speed is a strong function of
temperature, which is in itself a function of calorific value.

On the subject, the difference in calorific value, there appears to be (from
the links posted by Tyler) a 0.5% difference between n-Heptane and
iso-Octane (higher Octane number, lower CV), this, to a first order will
give you a 0.5% difference in mpg.  However, Toluene has a calorific value
10% lower than a typical alkane, and this is the type of  compound which oil
companies will use to push up Octane ratings, however, not in very large
quantities, because it is expensive, therefore the change in calorific value
between high and low octane fuels is not expected to be this high.  If we
hazard a guess at 3% difference, this would change a 20mpg figure to 20.6mpg
and I doubt if you can measure this.

Rock on!

Nick Pashley

------------------------------