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Different Peak EGT when leaning vs richening.

RhinoDrvr

Well Known Member
I’m still getting used to running LOP during cruise, and so far I’m loving the increased efficiency, lower CHT’s etc, and of course, fuel savings and range.

I’ve noticed a little peculiarity however that I was wondering if the group could shed some light on.

If I set cruise power (WOT/2400) and slowly lean the mixture, I typically get a fuel flow around 7.5gph running 50 LOP. When I’m closing in on his value I’ll lean the mixture .2 GPH at a time letting the temps stabilize each time until I get to 50LOP. The engine runs okay this way, but there is a very occasional stumble (maybe one in a 2 hour flight) that clears up if I push the mixture up. However EGT’s show that I truly am 50 LOP.

If instead I do a big pull to 7 GPH or so, and let the temps stabilize, then richen the mixture, find peak, then lean from that peak back to 50 LOP, I typically end up around 8.0 GPH. Again, showing 50 LOP, and the engine runs considerable smoother.

So…what gives? Shouldn’t peak EGT, and therefore 50 LOP be the same fuel flow regardless of which direction it’s approached from? I’m inclined to run the second way, since the engine runs smoother this way, but I’d like to ensure that I’m actually 50LOP using both methods, and not inadvertently ending up in the “red fin” of areas to avoid.
 
You’re right, in principle it shouldn’t make any difference. In practice, many lean just a little too fast to let egt probes stabilize, so they see a peak a little later than they would if the probes responded instantly. Going rich to lean, they think peak is a bit on the actual lean side, and vice versa going lean to rich. This is verified by the reported fuel flows.
 
Peak EGT must be done by observing manually and slowly at that. You can't count on the EMS to accurately observe a true peak, especially if moving too quickly and using .2 GPH increments. When you need a TRUE Peak, you use .1 increments and wait 60 seconds after each change. This is how we do it when balancing injectors, as we need to be precise. You don't really need a precise peak for setting cruise mixture, just set by FF and TAS. I also know exactly how my hottest EGT behaves and just lean to a constant number. It varies slightly with ambient temps, but pretty consistent. If you have EI with a moving advance, that must be factored in if using a constant #, as timing changes EGT temps. What you are seeing is pretty much expected.

Larry
 
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Not sure if you have an angle valve or parallel valve engine. If the latter, the red zone is hard to find, so not to worry about that.

No clear answer, but maybe some variables that you might think about. It takes quite a while for all temps to stabilize in the engine, oil, CHT, under cowl temps, engine case etc. They all have tiny effects. Even fuel temps, and not what is in the tank, actually getting to the injectors, as that fuel heats as soon as it departs the tank.

Also when approaching the lean misfire, consider that there are two plugs, and the mags nor plugs are perfectly equal, and combustion has an unseen random variance too. So, as approaching misfire, one plug can (and will) misfire before the other. A misfire plug seems to be equal to ~3 deg timing retard, thus pushing up that EGT. It took about 30 hrs of flying and experimenting to realize this caused a double EGT peak in the leaning sweep. The FF spread of this peak may be what you are seeing as you approach from different directions.

An example: I have noticed if doing some approaches and ending up 50 miles from home and wanted to do a speed run back. I tweak the mix for max speed as the only number important. Then about 4-6 min it slows down?? No amount of tweaking got the 2.x kts back. Still don't know exactly why, but would like too :D

You may have to experiment a quite a bit to understand what you are seeing. But regardless, the hysteresis you are reporting is affected by some of the operational variables. I am a numbers and data guy, and have measured lots of temperatures and pressures for various parameters as mentioned, but now just set my RPM, dial back the fuel to a number and tweak to a comfortable airspeed in cruise.

Even the linkage for mixture has some vagaries, I turn maybe 10 deg at a time close to the desired settings and wait 3-5 min to see a stable effect.
 
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