"Webre, Steven P." wrote:

Martin wrote:

Ok !

But the octane rating is of the fuel, not the mixture.
You cant change the octane, once its in the tank of the aircraft. ( unless

you

put in a additive)

/Martin

Careful there..

Recall that octane "rating" of a fuel is a comparative number, not like a
definitive compositional analysis, or other "hard" measurements.

A 100 octane fuel has the same anti-knock tendencies/characteristics that a
specific mixture of iso-octane and heptane shows in a test engine under a
specific procedure in an "ASTM-CFR" knock testing engine.  Change the test
conditions and the octane "rating" of the fuel will change.

There are four octane rating methods:  Research, Motor, Aviation (lean
mixture) and Supercharge (rich mixture).  Auto fuel is the average of
Research and Motor ratings (Ever notice the small print at the auto gas pump
stating something like "minimum octane = (RON + MON)/2"?)

Aviation fuel is shown as two numbers: 80/87, 100/130, etc.  First number is
knock tendencies under normally aspirated/lean conditions.  Second is under
supercharged/rich conditions.

Smooth Air,

Steve Webre
Broussard, LA

 
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