REFLECTOR: The definitive answer on octane and av-gas
Jim Agnew
jim_agnew_2 at yahoo.com
Sun Feb 6 07:54:04 CST 2005
Jim,
You can find the entire article at
http://www.prime-mover.org/Engines/GArticles/octane.html
Its source is several other sites from what I can see and it starts with a
discussion of auto applications.
Jim
--- Jim Sower <canarder at frontiernet.net> wrote:
> 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.
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>_______________________________________________
> >>>>To change your email address, visit
> >>>>http://www.tvbf.org/mailman/listinfo/reflector
> >>>>
> >>>>Visit the gallery! www.tvbf.org/gallery
> >>>>user:pw = tvbf:jamaicangoose
> >>>>Check new archives: www.tvbf.org/pipermail
> >>>>Check old archives: http://www.tvbf.org/archives/velocity/maillist.html
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>=====
> >>>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
> >>
> >>
> >>>Visit the gallery! www.tvbf.org/gallery
> >>>user:pw = tvbf:jamaicangoose
> >>>Check new archives: www.tvbf.org/pipermail
> >>>Check old archives: http://www.tvbf.org/archives/velocity/maillist.html
> >>>
> >>>
> >>>
> >>>
> >>>_______________________________________________
> >>>
> >>>
> >>To change your email address, visit
> >>http://www.tvbf.org/mailman/listinfo/reflector
> >>
> >>Visit the gallery! www.tvbf.org/gallery
> >>user:pw = tvbf:jamaicangoose
> >>Check new archives: www.tvbf.org/pipermail
> >>Check old archives: http://www.tvbf.org/archives/velocity/maillist.html
> >>
> >>
> >
> >
> >=====
> >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
> >
> >Visit the gallery! www.tvbf.org/gallery
> >user:pw = tvbf:jamaicangoose
> >Check new archives: www.tvbf.org/pipermail
> >Check old archives: http://www.tvbf.org/archives/velocity/maillist.html
> >
> >
> >
> >
> > _______________________________________________
> To change your email address, visit
> http://www.tvbf.org/mailman/listinfo/reflector
>
> Visit the gallery! www.tvbf.org/gallery
> user:pw = tvbf:jamaicangoose
> Check new archives: www.tvbf.org/pipermail
> Check old archives: http://www.tvbf.org/archives/velocity/maillist.html
=====
James F. Agnew
Jim_Agnew_2 at Yahoo.Com
Tampa, FL
Velocity 173 Elite Aircraft Completed & Flying
More information about the Reflector
mailing list