REFLECTOR: High temps

Sun Feb 6 12:43:22 CST 2005

Some motorcycle engines run 12:1 compression ratios, some normally aspirated
racing engines run 16:1, and even higher.  Detonation is the limiting factor
with compression ratio for a spark ignition engine (no diesels).
Detonation, or the propensity for detonation is caused by compression ratio,
combustion chamber geometry, engine RPM, octane of the fuel, mixture
temperature, ignition timing and a handfull of other factors that relate to
the ones I just mentioned, like localized hot spots, endgasses (exhaust to
intake mixture ratio, EGR), etc.

If you want to prevent detonation, you want the following characteristics:
Low compression ratio.
Small piston swept volume.
High engine RPM.
High fuel octane.
Low mixture temperature.
Retarded ignition timing.

Can a Franklin run on automotive gas?  Yes.  Without a doubt.  Retard the
timing, and limit intake manifold pressure to keep the peak pressures down,
then it will run just fine.  It will be soggy, and get poor
performance/economy at sea level.  You wouldn't like it.

SAE, NACA and also GAMI have run aircraft engines with fuels at very low
octanes.  GAMI ran an engine with about half jet-a and half 100LL to
demonstrate their new ignition system could prevent detonation by monitoring
for peak pressure pulse and detonation.  At the onset of detonation, the
system retarded the ignition timing.  If you are running on the jagged edge
of performance, once you get severe detonation things like spark plugs begin
to get very hot.  The only way to stop the detonation is to reduce power,
richen the mixture, and let the cylinders cool down.

Automotive engines have had this for years.  Sure would be nice to get this
into airplanes.
 Brian Michalk  <http://www.michalk.com>
Life is what you make of it ... never wish you had done something.
Aviator, experimental aircraft builder, motorcyclist, SCUBA diver
musician, home-brewer, entrepreneur and barely single

  -----Original Message-----
  From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org]On
Behalf Of Jim Sower
  Sent: Saturday, February 05, 2005 9:51 PM
  To: Jim_Agnew_2 at yahoo.com; Velocity Aircraft Owners and Builders list
  Subject: Re: REFLECTOR: High temps

  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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