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Lycoming's views on best economy settings
I was searching via Google for informational schematics, or even just
a written description, on Lycoming's fuel injection system because it's different from Continental's. I came across Lycoming's website in which a number of technical and informational articles reside. One of them http://www.lycoming.textron.com/main.jsp?bodyPage=support/publications/keyReprints/operation/leaningEngines.html discusses various mixture settings for their fuel injected engines. Here is item no. 5 from that article, it's a description of how to achieve "best power" and "best economy" when an EGT guage is available: 5. The exhaust gas temperature (EGT) offers little improvement in leaning the float-type carburetor over the procedures outlined above because of imperfect mixture distribution. However, if the EGT probe is installed, lean the mixture to 100oF on the rich side of peak EGT for best power operation. For best economy cruise, operate at peak EGT. If roughness is encountered, enrich the mixture slightly for smooth engine operation. ***editorial note*** If you do this (richen slightly from peak), the only place for the cylinderhead temperatures to go is up. Slightly to the rich side of peak on the graph described below is where the cylinderhead temps peak. So it would pay to be careful how much you richen from the peak setting, if the engine is running rough at that point. The Cessna 172 I rent has the EGT guage, but no CHT guage. So you can richen from peak and drone happily along not knowing that you are cooking the cylinderheads. This particular section of the website includes a nice graph, which, if you are familiar with John Deakin's "Mixture Magic" article, will look very familiar. It should because all fixed spark engines will show the exact same performance curves for things like EGT, CHT, percent power and BSFC. What was interesting to me is if you look at where peak EGT is occuring and then go down to the BSFC curve is, you will see that you have not yet reached the lowest BSFC. That doesn't occur until you've leaned a bit further. But then you would be **LOP**. Notice it does not take much additional leaning to get the lowest BSFC the engine is capable of producing. Here's the interesting part: At peak EGT, the cylinderhead temps are already starting down from their peak, and the downward curve is pretty steep once you get to peak EGT. Just a tiny bit more leaning and the cylinderhead temps dive down another full ten degrees, while the EGT has hardly gone down at all. While this is going on, the percentage of power is dropping off too, which is why economy cruise is slower than best power, of course. Lycoming finishes the graph with the following statement: "Textron Lycoming does not recommend operating on the lean side of peak EGT." Yet as their own graph shows, best economy is ONLY achieved lean of peak. What extremely interesting to me is that the difference between peak and lean of peak where best economy occurs is only a matter of a very few degrees EGT. Remember, when you are cruising at 60% power, you cannot hurt the engine no matter where you set the mixture control. You can't burn valves or cook the cylinderheads or cause detonation, it just isn't producing enough power to do that. Lycoming themselves recommends that for maximum engine life, cruise power should be limited to 65% and CHT's kept below 400 F. But since the instrument panel doesn't include a CHT guage, the only way to avoid high temps is to be way rich, or at peak EGT or below. Why Lycoming recommends against LOP operation is a mystery. MUCH cooler CHT's and less fuel being burned... what am I missing here? Does the engine run roughly at this setting? Only those who try LOP will know. Corky Scott |
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