Lean of peak reply.
In your article on wasting avgas, your statement under Idea 1: "As far as we're concerned, the data that indicates that correctly executed lean-of -peak operation both saves fuel and lowers EGT and CHTs is inarguable and people who claim otherwise don't know what they are talking about," requires a response.
The key temperature relating to exhaust valve and seat life is exhaust valve face temperature, not cylinder head temperature, specifically, valve edge and seat temperature. These are only indirectly related to cylinder head temperature. They are directly related to the heat transfer capability of the gases passing over them. The cooling of the exhaust valve face and seats and their resulting temperature is primarily influenced by the temperature and state of the exhaust stream.
At rich of peak, we have some unburned fuel that will cool the valves by changing state, a latent heat exchange. This powerful cooling mechanism is lost lean of peak and we must rely on the direct temperature (sensible) exchange. This is why no one, not even GAMI, will recommend takeoff and climb lean of peak. Think about it, if lean of peak insures lower cylinder head temperatures, why not take off lean of peak? They know why.
No one accurately knows or can measure valve face temperatures. What is the best way to insure cool exhaust valve faces? With excess fuel. Wright engineers, when they developed lean-of-peak operation on the R3350 turbo-compound, used sodium-cooled exhaust valves.
I will continue to fly rich of peak because with the valve technology I can afford, a gallon or two an hour is much cheaper than the alternative. Engine shops will continue to reap the benefits of lean of peak, not because pilots are ham-fisted with the mixture or ignorant, but because they have given away the major method of valve cooling with out advanced valve technology.
David E. "Crankshaft" Leue
GAMI's George Braly tells us the technology to measure valve temperatures was well established by the 1940s. This data shows that exhaust stream temperatures between 1300 and 1600 degrees F are common, with valve temperatures at 1150 to 1300 degrees. The reality is that the valve cools the exhaust stream, not the other way around.
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|Author:||Leue, David E.|
|Publication:||The Aviation Consumer|
|Article Type:||Letter to the editor|
|Date:||Jan 1, 2006|
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