Hi NG,

now it's my time to say something again, after mainly lurking here now
and then :-)

Knock resistance is a pretty complicated matter. The basic idea of
measuring the knock-related quality of a given fuel is to compare it to
a mixture of octane (known to be particularly knock-resistant) and
n-heptane, which will cause knocking already at very low compression
ratios. If your fuel behaves the same with regard to knock as a mixture
of 90% octane and 10% n-heptane, your octane number is 90.

Unfortunately it doesn't work quite as simple as that in the real
world. One fuel can be superior in knock resistance to another in one
engine and worse in another, or even depending on operating points in
an identical engine. A given fuel can behave similar to a 90/10%
micture of octane/n-heptane in one operating point, and like 80/20% in
another. Therefore, a defined octane number can only be attributed to a
fuel in conjunction with a defined procedure of measurement. For
automotive fuels, two measurement procedures are widely used, giving
the "motor octane number" MON and the "research octane number" RON. For
automotive fuels, these numbers typically differ by 8-10 points. In
Europe, the RON is quoted at the pump, while in north america it's the
mean value of the two, MON+RON/2, also called the "anti knock index",
AKI.

Aviation uses yet two more definitions, the "lean" and "rich" octane
number. The measurement procedure for the lean octane number is very
similar to the motor octane number and can in fact be derived from it
using a conversion table. The rich octane number is obtained in a very
different way. Instead of increasing the (n/a) test engine's
compression ratio up to the onset of knock, the test engine is boosted
and the boost pressure is increased until knock occurs. The so obtained
rich octane number is typically 30 points higher for high grade fuels
than the lean octane number. It is supposed to describe the fuel
properties particularly under high power, boosted operation.

Avgas 100LL has a lean octane number of 100, at auto pumps it would
therefore be labeled with an AKI of 104 - 105.

Yet another point: As octane is not the most knock resistant fuel in
the world, numbers for knock resistance greater than 100 are required
to describe fuels superior to pure octane. For obvious reasons a direct
comparison to a mixture from octane and n-heptane with equal property
is not possible. Ratings above 100 are derived by extrapolation. In
case of the rich octane number, values above 100 are given by comparing
the power obtained with the fuel to be rated by that obtained with 100%
octane. Fuel with 145 rich octane number allows a 45% higher power
output (obtained by boost increase) than running the same engine on
100% octane. Hence "octane" numbers above 100 should more precisely be
called "power ratings".

Two very good articles about knock and knock resistance can be found
he

http://www.prime-mover.org/Engines/G...es/octane.html
http://www.generalaviationnews.com/e...l.lasso?-token
..key=11311&-token.src=column&-nothing

Hope that helps clarifying,

regards,
Friedrich

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