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BobsV35B(at)aol.com
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 Posted: Mon Jun 18, 2007 3:00 am Post subject: Lean Of Peak EGT?? Was: Commander to Hawaii |
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Good Morning Al,
It is nice to hear such a knowledgeable response about the young man's trip. I am sure all will be fine.
One thing bothers me though. You have stated a suggestion that running at peak EGT will give a total burn of 24 GPH. I am not familiar with the engine involved, but that does seem like a reasonable burn for the power involved. Since it is a non supercharged engine, chances are that it would be developing well under sixty-five percent power at that altitude. Still, all is well.
However, you follow that with an ancient Old Wives Tale by implying that operating leaner than that will burn valves.
Since the highest temperature in the cylinder head will be obtained about forty or fifty degrees Fahrenheit richer than peak EGT, and, by definition, the Exhaust Gas Temperature will be lower when running more lean than Peak Exhaust Gas Temperature, why would any valve burning occur?
Twenty gallons per hour, ten gallons per engine, will definitely provide less power to the airframe and there is no advantage in economy by operating more than about twenty lean of peak EGT at the cylinder pressures involved, but there is absolutely nothing involved that would cause a valve to be burned.
Valves get burned by encountering gasses that are above their design limits. That can only happen when high temperatures and insufficient cooling is present. The most likely time for valve burning to occur is at high power and mixtures that are rich of peak EGT but not rich enough to provide proper cooling of the engine. The more likely culprit is a high power climb with a mixture that is not rich enough. Fifty to one hundred degrees rich of peak is about the worst place any engine can be operated at any power above sixty-five percent.
Lycoming does present the number of seventy-five percent as being critical while Continental likes to use the number sixty-five as the magic number, but both manufacturers will agree that cruise mixtures leaner than peak EGT at sixty-five percent power are unlikely to hurt any engine that is in conformance with it's type certificate. If it is full of crud from running too rich, something may stop a valve from closing. That could cause a valve to be burned.
However, running any engine at sixty-five percent power and a mixture that is leaner than that which will provide peak EGT can do nothing other than make a valve less likely to be burned.
Comments encouraged!
Happy Skies,
Old Bob
AKA
Bob Siegfried
Ancient Aviator
Stearman N3977A
Brookeridge Air Park LL22
Downers Grove, IL 60516
630 985-8503
In a message dated 6/18/2007 4:49:35 A.M. Central Daylight Time, alh1(at)juno.com writes:
| Quote: | fuel burn for a straight 500 at about 7500 feet at peak egt would be
about 24 gph. i know i will hear from guys that can get 20 gph, but
they burn valves an it is cheaper to burn fuel. |
See what's free at AOL.com.
[quote][b]
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28bravo(at)comcast.net
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| Posted: Mon Jun 18, 2007 7:46 am Post subject: Lean Of Peak EGT?? Was: Commander to Hawaii |
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Well put Old Bob!
- In all of the many hours of operating my 500B the red knobs were always agressively moved aftward from the moment of startup to shutdown. There is no way that sufficient heat can be produced at a 65% power setting to damage anything. I consistently see 24 GPH block fuel burns. The sweet spot is 2300 rpm/22inches (at) 7-10K. 175 KTS TAS all day long. The exhaust color is a light dove gray. Always run full rich in the climb and your engines will be happy.
Dennis N6273X
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nico(at)cybersuperstore.c
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| Posted: Mon Jun 18, 2007 8:47 am Post subject: Lean Of Peak EGT?? Was: Commander to Hawaii |
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I know the LOP debate, and some folks’ experience with engines holding to TBO while running LOP, has changed many, including my, perception. I ran my 500 ROP for years and have never had any problems with the engines. I burned 22 gph, exactly twice as much as my Twin Comanche, so I wonder whether my EGT was not calibrated correctly.
If I remember correctly, the EGT operated only off the #1 cylinder? I don’t know.
Perhaps the other cylinders were actually running LOP and held up quite fine.
Nico
From: owner-commander-list-server(at)matronics.com [mailto:owner-commander-list-server(at)matronics.com] On Behalf Of DP
Sent: Monday, June 18, 2007 8:45 AM
To: commander-list(at)matronics.com
Subject: Re: Lean Of Peak EGT?? Was: Commander to Hawaii
Well put Old Bob!
- In all of the many hours of operating my 500B the red knobs were always agressively moved aftward from the moment of startup to shutdown. There is no way that sufficient heat can be produced at a 65% power setting to damage anything. I consistently see 24 GPH block fuel burns. The sweet spot is 2300 rpm/22inches (at) 7-10K. 175 KTS TAS all day long. The exhaust color is a light dove gray. Always run full rich in the climb and your engines will be happy.
Dennis N6273X
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