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#82
May 17th 05, 01:09 PM
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On Mon, 16 May 2005 00:44:02 GMT, "Jay Honeck"
wrote: Obviously everything did run hotter, but EGTs never topped 1550 degrees, and CHTs never topped 360 degrees. This is a tad hotter than I've been running, but the engine sounded fine, and all cylinders stabilized within 100 degrees of each other -- which is as good as it gets on my normally aspirated O-540. When you think about it for a minute Jay, it makes sense that the cylinderheads cannot overheat when running lean of peak: There isn't enough fuel in the mixture to overheat them. Running LOP has it's consequences (less power developed), but overheating and detonation aren't a worry. Have you read Deakin's "Mixture Magic" column? It's EXTREMELY comprehensive. Corky Scott 
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#83
May 17th 05, 02:15 PM
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When you think about it for a minute Jay, it makes sense that the
cylinderheads cannot overheat when running lean of peak: There isn't enough fuel in the mixture to overheat them. ??? I can easily get my EGTs over 1600 degrees, and my CHTs near (or over) 400 degrees by over-leaning. At some temperature (and I don't know what it is) doesn't aluminum start to weaken? -- Jay Honeck Iowa City, IA Pathfinder N56993 www.AlexisParkInn.com "Your Aviation Destination"
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#84
May 17th 05, 02:27 PM
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In rec.aviation.owning Jay Honeck wrote:
: I can easily get my EGTs over 1600 degrees, and my CHTs near (or over) 400 : degrees by over-leaning. At some temperature (and I don't know what it is) : doesn't aluminum start to weaken? : -- Yeah... about 250-300 degrees. Comforting, no? Remember that EGTs are useful for *relative* measurements... not absolute. What one engine/airframe/power setting combination produces for EGTs has very little to do with another engine/airfram/power setting EGT. It's best used to find the peak, and to see how even individual cylinders are by dithering around the peaks. Saying one is at 1550 and another is at 1590 doesn't mean much... one may have a slightly different position in the EG stream, more resistance inthe probe wiring, etc, etc... run the mixture above and below to see where *each* peaks (below 75% of course). Again... absolute EGT number is nothing to be concerned about (within reason, but it's generally impossible to exceed reason with a certified engine). The EGT is much more important, and running at or below 400 in cruise is considered acceptable. -Cory -- ************************************************** *********************** * Cory Papenfuss * * Electrical Engineering candidate Ph.D. graduate student * * Virginia Polytechnic Institute and State University * ************************************************** ***********************
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#85
May 17th 05, 05:33 PM
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#86
May 17th 05, 05:58 PM
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On Tue, 17 May 2005 13:15:46 GMT, "Jay Honeck"
wrote: When you think about it for a minute Jay, it makes sense that the cylinderheads cannot overheat when running lean of peak: There isn't enough fuel in the mixture to overheat them. ??? I can easily get my EGTs over 1600 degrees, and my CHTs near (or over) 400 degrees by over-leaning. At some temperature (and I don't know what it is) doesn't aluminum start to weaken? Jay, have you read Deakin's article entitled "Mixture Magic"? If not, find it here and read it through and through. http://www.avweb.com/news/columns/182084-1.html He also wrote four follow up columns entitled: "Where should I run my engine" parts 1 through 4 in which he spells out in minute detail how to adjust your prop and mixture for most efficient/safe flight on the ground, in climb, at cruise and descending. But if you look at the temperature chart (produced by Continental) and what happens to the cylinderhead temps along with the EGT's as you lean from full rich, you'll see two peaked curves BUT, and this is a big but, **they don't peak together**. The curves starts as low as they're going to be when the mixture is full rich, and begin rising immediatly as you begin leaning out. As the mixture reaches the stoichiometricly correct ratio, the engine is producing the absolute maximum power it's capable of making (at that altitude). Guess what? The maximum power and the hottest cylinderhead temperatures occur at exactly the same time. But at that point, the EGT's are still rising and will continue to rise as you continue to lean, but the cylinderhead temps now start DOWN as you lean below the peak temperature. Power also starts dropping off as you continue to lean. You really need to read the above article to get the proper understanding because it takes pictures and charts to really explain it and he has them all in this article. But the point is, the charts show that as you continue to lean (in the cruise condition), once you get the mixture to the point where it is PAST PEAK, the temperatures begin to fall off. This is literally because the mixture isn't good enough to produce the kind of heat and pressure it can when the mixture is ideal. Less heat in the combustion chamber, less heat to the cylinderheads. It's actually more complicated than that but I'll let Mr. Deakin's words explain, read the article. When you say "but my temps go up when I lean", that's true ONLY while the mixture is RICH OF PEAK. When you lean past that point, continued leaning reduces engine temps for several reasons. Look at the chart, it's Continental's own chart and it does not lie: Continued leaning past the peak cylinderhead temperatures produces lower temperatures. As Deakin points out, this chart works for all spark ignition four stroke cycle engines regardless power output. Lean out the mixture and that's what the engine temps will do. It doesn't appear that you have leaned past peak in your adjustment of the mixture, till now. You've approached peak, from the rich side of the chart, but not below it. Your description of the temps you've seen and your fuel burn tell us that. It's important to understand that this technique produces the fastest cruise for the least amount of fuel burned. You can set your engine for a faster cruise but you'll burn more fuel. You can also set your engine for longest range, but you'll be going very slowly. Please note, for all who are reading this, Deakin does not believe it's advisable to use this technique (LOP operation) without having the necessary guages to monitor ALL the cylinder temps at once. But Jay, you have the JPI guage that should allow you to at least attempt to run the engine lean of peak, should you so desire. Corky Scott
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#87
May 19th 05, 02:18 PM
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#88
May 19th 05, 02:49 PM
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In rec.aviation.owning Corky Scott wrote:
: It doesn't appear that you have leaned past peak in your adjustment of : the mixture, till now. You've approached peak, from the rich side of : the chart, but not below it. Your description of the temps you've : seen and your fuel burn tell us that. : It's important to understand that this technique produces the fastest : cruise for the least amount of fuel burned. You can set your engine : for a faster cruise but you'll burn more fuel. You can also set your : engine for longest range, but you'll be going very slowly. : Please note, for all who are reading this, Deakin does not believe : it's advisable to use this technique (LOP operation) without having : the necessary guages to monitor ALL the cylinder temps at once. : But Jay, you have the JPI guage that should allow you to at least : attempt to run the engine lean of peak, should you so desire. Instrumentation is the important key. If you look through the Lycoming FUD, they say LOP is bad not because it's inherently flawed, just that most pilots are too stupid to be able to do it safely and their planes aren't equipped. Remembering that each cylinder is really its own entity as far as operation goes, individual measurements are definately necessary. So long as each one is within the safe zone of CHT (400 in cruise), you should be fine at *ANY* mixture setting provided you're below 75% power. Unfortunately, you may be screwed before you even get going if the fuel distribution is really bad on the O-540. I would set up a nice 60-65% cruise power and lean the bejeepers out of it until it definately runs rough... maybe even more. Watch where each jug peaks and see how mismatched they are. Not where they all get to the same temp... just where each one starts to go back down again. On my O-360, I can lean it to where the leanest cylinder is LOP, and the richest cylinder is at peak. Since they're all running at a cooler CHT than running 50 ROP, it's still safe so long as I'm at 75% or less. For a bit of "buffer zone" (especially since I run cargas), I'll generally limit the power to 65-70%. My PA28 airframe is really starting to hit the wall at 115 KTS or so anyway, so the additional 10% only buys 5 kts or so. -Cory -- ************************************************** *********************** * Cory Papenfuss * * Electrical Engineering candidate Ph.D. graduate student * * Virginia Polytechnic Institute and State University * ************************************************** ***********************
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