REFLECTOR: The definitive answer on octane and av-gas

Sat Feb 5 23:02:27 CST 2005

HOW DO YOU DETERMINE AVIATION GASOLINE OCTANE?
The octane of aviation fuel is not measured in exactly the same was as is
automobile fuel. 

Once again, you start with your trusty ASTM-CFR engine. First you set up the
ASTM-CFR for the motor method and use that method to determine the motor rating
of your fuel. You then correct that rating to the "Aviation Lean" rating using
a conversion table. Below about 110 motor octane (a performance number of 110),
the aviation lean and motor octane numbers will differ by only about 1 or 2
points. Above 110 motor octane the differences can be significant. Next you
pull out another version of the ASTM-CFR engine. This one has a fixed
compression ratio but allows you to supercharge the intake manifold. You
pressurize the intake to higher and higher values until the onset of knock.
Other than that, the parameters are the same as for the motor method used for
automobiles. The supercharge method is then used to assign the Aviation Rich
value of the fuel. Supposedly the pressurization method (as opposed to changing
compression ratios) is a throwback to the 1950s and 60s when supercharging was
common in aircraft engines. The engineers were particularly concerned with the
fuel's behavior under boost. 

Because of the different ways in which automotive and aviation gasoline octane
is measured one must be very careful when comparing absolute numbers. 100
octane avgas is not equal to 100 octane autogas (but it's close!). Note that
the lean number rating of an aviation engine will correspond very closely to
its autogas (mogas) motor rating requirement (see the above paragraph). Thus
when shopping for autogas for your 91/96 O-360, you should look for a filling
station at which the motor octane rating of the fuel is at least 91. 

You should also be careful leaning the engine as this may cause its octane
requirements to go above what the autofuel can provide. Look for an autofuel
with an octane number as far above the lower aviation octane number as you can.
If you can get one which is at or above the rich octane requirement (the higher
number) then you should be a-OK. 

Thus an engine rated for 80/87 aviation should have no trouble whatsoever
running on 89 octane (or higher) unleaded. Engines rated for 91/96 should run
on at least 91 (motor) octane unleaded but note that this is lower than the
rich limit requirements (96) of the engine. Therefore it is especially critical
to limit leaning with such an engine/fuel combo when running at high power
settings. 

HOW DO THEY GET OCTANE NUMBERS ABOVE 100?
Often it's done by pure extrapolation. A more reliable method, however, is
through the use of so-called performance numbers. Briefly, these are arrived at
by determining the instantaneous mean effective cylinder pressure (IMEP), using
the fuel under test, at the highest boost that does not cause knocking. This
number is then multiplied by 100 and the resultant is divided by the IMEP at
the highest boost that does not cause knocking on the 100 octane equivalent
fuel. 

Note that, technically, there is no such thing as an octane number above 100.
If you're at a party, avoid saying things like "110 octane gasoline" because
people will get up and walk away from you. You should say, instead, "a gasoline
with a performance number of 110." That will bring the help scurrying over with
more champagne. 

HOW DOES LEAD AFFECT OCTANE?
Tetraethyl lead raises the octane rating of a fuel not because it adds more
"octanes" to the fuel but because it makes the fuel knock at a higher
compression ratio in the ASTM-CFR. According to the latest research, octane
ratings go down with fuels comprised of long, straight, hydrocarbon chains
(paraffinic fuels). Fuels with branching hydrocarbon fuels, and aromatic fuels,
have a higher octane ratings. 

Oxygenates and alkyl lead affect the pre-flame reaction pathways by retarding
branching sequences. Lead was previously believed (by many, including myself)
to work by slowing the flame front, thus leading to a slower pressure rise in
the cylinder. While general flame-front propagation speed does affect octane
ratings, lead does not significantly affect it. 

WHAT OTHER FACTORS AFFECT OCTANE REQUIREMENTS?
Combustion chamber design, localized hot spots, piston speed, and a host of
other factors can all contribute to a engine's propensity to ping. 

Additionally, in the aviation world, altitude extremes and super/turbo charging
affect octane requirements. Increased induction pressures (such as would be
encountered in a turbo/supercharged engine) cause more rapid flame-front
propagation. Likewise, decreased exhaust pressure (as would occur at altitude)
also tends to increase flame-front propagation speed. Both of these effects can
combine to raise octane requirements - especially at altitude. 

Note that the latter effect also affects the proper fuel/air ratio for BEST
ECONOMY operation. The reason is left as an exercise for the reader. 

Jim
--- Jim Sower <canarder at frontiernet.net> wrote:

> Jim,
> Jim Agnew wrote:
> 
> >John, 
> >
> >Octane is measured at least two ways and I think that you will find that 93
> >Octane auto gas is very close to 100LL in octane.
> >
> Actually, to get "motor" octane (which is how avgas is rated) you want 
> to *subtract* about five points from the number you see at your local 
> Speedway station.  Like 93 in town is equivalent to 87 in your engine, 
> 87 in town is like 82 to the plane.  About 60% of aircraft recips (maybe 
> 80% of flat fours) were designed for 80 octane, so 87 "octane" mogas 
> works well.
> 
> >  As far as 100LL running
> >hotter I seriously doubt it.  As far as the Franklin goes, if I'm not
> mistaken
> >that it has a compression ratio about 10 to 1 so if you are running regular
> gas
> >I would think that you are going to have detonation damage if you run it for
> >long especially under high MP.  
> >
> 10:1 pistons would  DEFINITELY put Franklin in the 20% that can't use 
> regular mogas.  Hi-test might be another matter, but it would take some 
> careful research.  Electronic ignition with a tuned down advance curve 
> might help, but that would cost you a little power.  You'd make that up, 
> of course, with substantial savings in maintenance and engine life 
> compared to 100LL.
> 
> >The Cessnas are probably about 8-8.5 or less
> >compression ratio so they can probably get away with it.
> >
> >Jim
> >
> >--- John Dibble <aminetech at bluefrog.com> wrote:
> >
> >  
> >
> >>I have a Franklin, not a Lycoming and no temp problems.  Has anyone with a
> >>Lycoming
> >>tried a lower octane fuel?  My experience with Rotax engines is that using
> a
> >>higher
> >>octane fuel than what the engine requires results in high CHTs (50 F
> higher,
> >>enough to
> >>sieze the engine).  I think 100 LL is higher octane than any normally
> >>aspirated engine
> >>should need.  Several Cessnas in my flying club use 50/50 avgas/regular. 
> In
> >>a pinch,
> >>I have topped off my tanks with regular and couldn't see any difference in
> >>performance
> >>or other problems.
> >>
> >>John
> >>
> >>Jim Sower wrote:
> >>
> >>    
> >>
> >>>I am running a LIO-360-C1E and even after a lot of work on the baffling
> >>>have CHTs around 375 - 400 with occasional excursions to 425 which I try
> >>>to correct for.  From the beginning, my oil temp never ever budged off
> >>>of 180, but more recently I have had oil temps up to 210 - 220 on
> >>>occasion.  I think part of this might have to do with the firewall oil
> >>>cooler which exhausts right into the plenum - the high pressure air for
> >>>cooling the cylinders.  There can't possibly be much air flow through
> >>>that cooler, and I regard it as a significant design defect.  At some
> >>>juncture (after I get the engine running again and the plane annualed) I
> >>>am going to exhaust it to outside air under the cowl.
> >>>
> >>>Glad I'm not the only one with high CHTs ... Jim S.
> >>>
> >>>      
> >>>
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> >>    
> >>
> >
> >
> >=====
> >James F. Agnew
> >Jim_Agnew_2 at Yahoo.Com
> >Tampa, FL
> >Velocity 173 Elite Aircraft Completed & Flying
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=====
James F. Agnew
Jim_Agnew_2 at Yahoo.Com
Tampa, FL
Velocity 173 Elite Aircraft Completed & Flying