REFLECTOR: The definitive answer on octane and av-gas

Jim Sower canarder at frontiernet.net
Sun Feb 6 00:08:31 CST 2005


Jim,
Boy!  Are we ever coming up with different stuff!  What are your 
sources?  Mine (mostly on the EAA web site and conversations with the 
EAA STC and government liaison guys) say waaaay different stuff from 
yours.  Here's my reality:

Like mine say research octane produces a LOT higher number than motor 
octane, so (R+M)/2 ends up being about 5 points higher than M.  87 Mogas 
is about 80 motor octane and 93 mogas is about 88 motor.

Also, I'm told that the 80/87, 100/110, 110/130 and 115/145 octane 
ratings of avgas in the old days (up through the 60s) reflected  
/lean/rich/ Motor octane - that the octane rating of the fuel dropped 
off as you leaned the engine.  I trained in T-28s, so our fuel had 145 
octane when we took off at 60" or so MAP and full rich mixture, but 
dropped to 115 as we leaned the engine so we had to back off on the 
boost for climb.  I'm told they dropped the rich number for reasons that 
are unclear, so 100LL and 80 Octane are lean numbers.  I'm not clear on 
this detail at all.

As for lead and octane, I'm told that Octane is a long string molecule, 
Heptane not so long, Hextane shorter, etc. and that "octane" rating 
reflected the percentage of octane in the fuel - 100 octane was defined 
as just that - 100% octane, none of the shorter molecules in the mix.  
The molecules "burn" from both ends toward the middle of the molecule, 
so the longer octane molecule takes longer to "burn" than the shorter 
molecules so the shorter ones are consumed much faster.  Octane 
concentration results in slower burning fuel, a slower flame front.

Lead raises "Octane" number because the Lead in Tetraethel-Lead has four 
receptors that the fuel molecules bind to.  Since they're bound to the 
lead molecule at one end, they can't be consumed as fast, burning from 
"free" end toward the end that's bound to the lead so burning a "bound" 
fuel molecule takes longer than burning "free" one from both ends toward 
the middle. 

All fuel molecules bound to lead sort of "double" their burn time and 
thus raise the octane of the fuel.  Octane molecules can bind to the 
lead too, and like the others take longer to burn, and that's how you 
get numbers over 100 octane.

Unleaded Avgas is pretty much right around the corner.  I'm told it will 
be nothing much but the regular gas you buy at the local Exxon (or 
whatever) station except that it will contain NONE of the additives that 
are added at the terminal at tank-wagon load time (MTBE, ETBE, 
Proprietary additives (Tom Cat Piss and the like) and Alcohol).  When 
all of this comes down, basically all manufacturers' restrictions and 
prohibitions around Mogas will simply vanish, and we'll all pretend that 
they were never there :o)  Sadly, the Old Wives Tales and Urban Legends 
around Mogas and Lead will be more difficult to purge.

In any event, if I'm misinformed I need to get the straight skinny ... 
Jim S.

Jim Agnew wrote:

>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
>>>_______________________________________________
>>>To change your email address, visit
>>>      
>>>
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>>
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>>>      
>>>
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>>
>
>
>=====
>James F. Agnew
>Jim_Agnew_2 at Yahoo.Com
>Tampa, FL
>Velocity 173 Elite Aircraft Completed & Flying
>_______________________________________________
>To change your email address, visit http://www.tvbf.org/mailman/listinfo/reflector
>
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>
>  
>
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