Airborne_Again wrote:
He was strongly of the opinion that you should not lean to peak EGT as that would cause burnt exhaust valves. He conceded that it would be all right if you had CHT gauges and were careful with the temperatures, but if you didn’t have that, you should not lean to peak EGT even on relatively low-powered engines like a Lyc. (I)O-360
The 2016 version of conventional thinking is that the exhaust valve temperature is a function of both the EGT and CHT, given that the valve is heated by EGT and periodically cooled by contact with the valve seat, which is at CHT.
I tend to believe that for smaller Lycomings installed on slower planes (with draggy but effective cowlings, unlike most faster planes) CHT is limited by cooling design to a temperature that adequately cools the exhaust valve regardless of EGT. That of course depends on the plane and cowling!
Just my observation.
Whenever this discussion comes up, I would start by showing this graph from the engine manufacturer (TCM in this case):
If you want the “whole story”, it comes from this classic article by John Deakin which I can highly recommend to read.
As we can see, maximum CHT is rich of peak EGT (around 50 degrees F as you correctly stated). But if you run the engine at low power (something like 60 to 65 % on a “big” IO-550-N), you could set the mixture anywhere and not run it too hot. I suppose for smaller engines this is even a higher % value. Since at my club we’re supposed to fly at such lower power settings, you could argue that it “doesn’t matter”. Though I would still think that burning less fuel, having lower CHTs and producing less carbon monoxide would be good things.
But your mechanic must know mine, and the advice you’re given resembles what you can hear at my club. I have given up discussing this as I don’t want to be accused of running the engines outside of the procedures. But there are people who outright refuse that you could run an engine safely at peak EGT, or lean of peak, and I won’t change their mind. Those are some of the worst OWTs, impossible to overcome with facts in my view. One good reason to get my own plane so I can install a good engine monitor and run it the way I want.
I think the real Q, as with all these “club / rental / instructing” scenarios, is:
There are almost no mechanics who understand how an engine works in any detail (and they don’t need to – they merely need to know how to install / repair / overhaul an engine IAW the MM) let alone understand physics, so this issue goes on an on.
If he has seen burnt valves that’s probably because somebody tried to lean it at a high power setting, possibly when climbing at Vx or Vy (high CHT) as they were trained to do in the PPL So, in a club environment, I have sympathy with his views, which in the UK are usually expressed as: don’t touch the red lever! But if you are an owner then you have a “duty” to get clued-up on this stuff, IMHO. And install an EDM700 or similar which enables you to do it properly.
Thanks everyone. What you’ve written so far supports the opinion I’ve formed over the years by reading both EuroGA, engine manufacturer recommendations and lots of material on the web.
The problem from my point of view is that very few people in my club (and probably in any club) bother to do this and they are more likely to listen to a mechanic with a 40 year relationship with the club than to me even if I think that what he says is OWT. I would be happy if everyone would even read the POH.
Oh, well. It is unlikely that you damage an (I)O-360 by running it 50° rich of peak at 65% (Cessna even recommends this!) and the important thing is that our members lean the engines at all.
Rwy20 wrote:
Whenever this discussion comes up, I would start by showing this graph from the engine manufacturer (TCM in this case):
I showed essentially the same diagram only a few minutes before the mechanic made his comments and I had just pointed out that you would get maximum CHT between peak EGT and 100°F rich of peak. But of course if you have formed an opinion from your practical experience over many years, you are not likely to be moved by analytical reasoning.
In renting, there is even less incentive for leaning correctly as it does not affect your cost if you rent wet. You pay the same if you use 10-20% more fuel and actually the way some schools are psychotic about leaning, planes are probably more expensive to rent because of it.
John Deakin has gotten it very right, LOP will save you a lot of money and maintenance too. Almost all airplanes can do it one way or the other. John told me once how to even do it with a carburetted engine, pull back till rough, increase carb heat until smooth again and lean again to rough, then add a bit until smooth again. Temps are indeed lower and peak before, so it is lean of peak and the fuel savings are quite noticable (down to 31 lph from 37 in my case) . But methods like this will never catch on in clubs because a) the old guard will see it as blasphemy and b) it takes care to do it. And if they don´t have to pay for their fuel, why save.
If CHT is low enough due to effective cooling it doesn’t actually matter whether you’re off peak CHT, on peak, or anywhere else. I think that is true for many ‘club’ types. The height of CHT curve is airframe specific.
What I don’t like about a statement like “lean to 50° rich of peak” is the figure 50. That’s a totally arbitrary number and as an engineer (the theoretician variant, not the one with oily fingers) I can’t work with arbitrary numbers at all. Why not 40 or 60, 10 or 100? Because the instrument is divided in intervals of 50? Most probably. If it would be a metric instrument, then your mechanic (the variant with greasy fingers) would probably find it optimal to lean 50° Celsius rich of peak…
When the manufacturer allows to lean for peak EGT then I see no reason for not doing that. Peak EGT can be read from the instrument, “50°F rich of peak” can not.
I am interested how accurately this can really be done without monitoring each cylinder. Mooney_Driver the comments on technique are well put, but how much is added to smooth out the engine and how certain can you be that one cylinder isnt running a lot hotter than the others? I have reservations that there is more risk associated with pilots attemtping to run LOP without being able to monitor the temperature of each cylinder, than running a little richer.
Airborne_Again – what monitors do you have in the Club aircraft for cylinder and exhaust gas temepratures?
Fuji_Abound wrote:
…and how certain can you be that one cylinder isnt running a lot hotter than the others?
If it is running a lot hotter you will hear that. The resulting engine roughness can not be missed! “Peak EGT” means “hottest possible”, doesn’t it? It is sort of an integrated value over all cylinders. One of them will be hotter than the others, no doubt, but since there is no “hotter than hottest” it will still be OK.
Except for the potential roughness, it isn’t that critical for all cylinders to reach peak EGT at the same point, since the power delivered curve is pretty flat around that region.
Even with matched injectors (like the ones you can buy from GAMI) it is rarely perfect.
The CHT is a separate “target” from the EGT, and you manage it separately, by not climbing at Vx or Vy for longer than absolutely necessary (only to clear obstacles). I then climb at 110-120kt.
A while ago I wrote some notes on this here but really it does assume a monitor like an EDM700. Without it, you can do it only approximately. Plus, on most rental planes I have seen, the instruments are shagged anyway. When I was doing the FAA IR in Arizona, in a PA28-161, the EGT gauge was buggered so the instructor leaned for peak CHT!! Well, it sort of very roughly worked and got us to 11000ft 
I wasn’t going to argue…
