RE: octane meathods

[Bob Davis <rkd@sover.net>]

Brett Dikeman said on Thursday, December 11, 1997 at  11:57 PM:
>I would be interested in
>seeing how the various methods are calculated/tested.

This had me running for my copy of Taylor (1).  Basically, octane ratings
compare the detonation tendency of the subject fuel to standards such
as isooctane (less detonation tendency than average fuel) and normal 
heptane (more detonation tendency than average).

Basically, you take the fuel to be tested and run it through a test motor
at standard conditions.  You set the compression ratio of the motor
(obviously not your common street car motor, the CRF test motor is a
special single cylinder rig with variable CR) so that the knock intensity
matches the standard knock intensity as measured on the knock indicator.
Then, using this set up, you run various mixtures of the above two fuels
until you get a mixture that matches the knock intensity of the fuel under
test.  The percentage of isooctane required for the match is the octane
number.  The CFR (Cooperative Fuel Research) Committee motor method
and research methods differ as follows:

Factor                   Motor Method          Research Method
------------------          --------------------          --------------------------
Inlet temp.               300 deg. F                   125 deg. F
Jacket temp.            212 deg. F                   212 deg. F
Speed                     900 rpm                       600 rpm
Humidity                0.0036-0.0072          0.0036-0.0072
  (mass/mass dry air)

(From Taylor, Vol. 2, p. 144)

Taylor claims that since 1940, the research method numbers have shown 
greater correlation with passenger car road-test octane numbers then the
Motor method.

Given the above, how does one get octane rating above 100?  Well, for fuels
with less detonation tendency than isooctane, there are some test
procedures involving either the addition of lead to isooctane or the use of
performance numbers using the rich and lean mixtures and calculations using
the imep of isootane and of the fuel under test.  For a detailed discussion, I 
refer you to Taylor.

(1) Charles Fayette Taylor, "The Internal-Combustion Engine in Theory and
     Practice, Volume 2: Combustion, Fuels, Materials, Design, Revised
     Edition", The M.I.T. Press, Cambridge, Mass. and London, England,
     1968 and 1985.