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Engine management / leaning / peak / lean of peak (merged)

I just finished the APS engine course I mentioned on here a few weeks ago. It was very, very good and I have to say I am sold on running my engine in that way (LOP) going forward. It really helps you understand what is going on in the engine at different power, mixture and timing settings. And you also get a good explanation of how to interpret CHT, EGT and TIT numbers.

Anyone who has read the famous John Deakin Pelican's Perch articles on Avweb has an inkling of the content.

It is not cheap but well worth it in my view.

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EGTK Oxford

I am not familiar with the contents of this course, so it would not be fair for me to comment either way. However, in general, lean of peak operation is not encouraged by engine manufacturers. A number of engine operations manuals only approve of operation to peak EGT, under some conditions.

Caution should be used when considering operation of an engine or an aircraft which is not in accordance with the approved methods. It's not to say that it won't work, but there may be reduced margins, or another effect which has not been considered.

Obviously more knowledge is good, but it must be applied in the context of the approved operating methods. If a "new" operating method is not approved, why not? STC's are available to anyone who would like to go through the process of demonstrating design compliance....

I used to work in an engine overhaul shop. We'd see engines come in, and sometimes we could determine a trend of non approved or careless operation, based on the condition of the engine. Fuel is a very cheap coolant for an engine, when used as intended. I learned this early, and applied it to the operation of my O-200. I overhauled it at 3600 hours SMOH, and once apart, it really did not need it. Now it had been through some cylinders during that period, but they are something of a consumable on an O-200 anyway.

In short, yes, you can economize fuel, but I'd rather waste some money on an extra gallon of fuel in an hour, than waste money on unscheduled maintenance. Perhaps my opinion would change having taken that course, but engine operation has worked really well for me all these decades, just doing what the manual says...

Home runway, in central Ontario, Canada, Canada

Yes and no. I think GAMI know something about engines too. Done properly LOP can't hurt an engine as it runs cooler with lower pressures. Running at Peak EGT is a potentially much more dangerous place to be with any higher power setting.

EGTK Oxford

I haven't been on that course but have read probably almost everything John Deakin has written, and been following the debate online for about 10 years.

In simple terms, peak EGT is authorised by Lyco at 75% of max rated power or below.

Peak EGT is also the "best MPG" point. LOP cannot deliver better MPG (other than due to second order effects, which are very small) because one is not burning the fuel any more efficiently than at peak EGT.

That's why I don't use LOP. I've done loads of flight tests and for a given IAS and prop RPM I cannot see any MPG improvement.

There is a small gain in using lower RPM, of the order of a few %, and this may be related to LOP in that, with our fixed ignition timing, a lower RPM works better w.r.t. crank angle when the mixture is lean and thus burning more slowly.

One of the things that Deakin/Braly are trying to get across is that flying "a little rich" is a bad idea because the maximum CHT point is actually slightly rich of peak EGT. The first diagram here shows it. So "giving the mixture lever a little nudge" is not doing the engine any favours. One needs to be at peak EGT, or one needs to enrich rather more than just a little bit, and about 120F ROP gives you the "best power" point. Most engines are set up for about 120F-150R ROP when all 3 levers are fully forward.

Obviously none of this is of any use unless you have EGT and CHT indication...

I don't think there is any great science behind this stuff, and when I fly with somebody and show how to set up for cruise at the top of the climb, they are usually suprised at how simple it is. At low levels, set 23", 2400RPM, lean to peak EGT (11.5USG/hr) and that's it, done, finished.

Managing the CHT is a separate thing, which needs plenty of airflow. In the TB20 (in which airflow is poor) I transition to a climb at 120kt ASAP. I never "cruise climb"; always climb with all 3 fully forward except when going high (say FL100+) in which case I use the constant-EGT lean method all the way through the climb. I have done flight tests and found that any form of "cruise climb" is a waste of time and any fuel saving is negligible.

Administrator
Shoreham EGKA, United Kingdom

I agree it isn't religion and is just about understanding what is going on in your engine. I found it very helpful to understand the relationship between the levers and cylinder temps and pressures. Peak doesn't work in the Mirage due to it being impossible to control temps there. But LOP can get you 10-15% fuel savings for the same TAS, or a higher saving for a lower TAS. But most importantly, I have learnt that ROP is probably not the best way to run my engine as far as keeping ports clean and for longevity. It is my view, others may have a different one.

As Peter says though the area just rich of peak at higher power settings is a bad place to be.

EGTK Oxford

I don't disagree but I think it's important to realise that

But LOP can get you 10-15% fuel savings for the same TAS

is relative to flying ROP, not relative to flying peak-EGT

I would certainly expect a 10%-15% MPG gain at peak EGT (or any LOP setting) relative to 100-120F ROP.

And that is why flying near one's operating ceiling is not all that efficient, because you do need "best power" (which includes max RPM) to get there. On nice days I don't fly above FL160.

There is unfortunately a "religious sect" out there which is claiming that LOP is more economical than peak EGT, and that simply defies physics, which says that once you are at stochiometric (which is as close to peak EGT as is possible to configure an engine) there is no possible further efficiency gain.

Peak doesn't work in the Mirage due to it being impossible to control temps there

What kind of CHTs would you see, in climb at say 120kt IAS?

What kind of CHTs would you see, in cruise, at say 65% power?

Administrator
Shoreham EGKA, United Kingdom

Yes clearly the most chemically efficient combustion happens at peak although the best BSFC (brake specific fuel consumption) is just lean of peak.

At peak CHTs rise well into the 400s. In cruise at high altitude I haven't tried it but 500 can be seen.

The Mirage tends to be TIT limited (which is clearly closely related to EGT) which you want to keep below 1650. At peak this will rise into the 1700s.

The key to most of the course and engines generally seems to be to understand the chart below.

EGTK Oxford

clearly the most chemically efficient combustion happens at peak

Both at Peak EGT and LOP all of the fuel gets burned and produces combustion energy so Peak EGT is not more efficient than LOP. Flying ROP is a waste of fuel and should only be done when it is the only way to prevent your engine from exploding/melting. With modern instrumentation you can control power settings so much better than what the authors of the POH's had at the time.

The Cirrus POH provides all endurance charts in LOP operation. Cessna typically uses Peak EGT for the charts. ROP was a valid technique back when fuel was cheap and our consciences were less green but nowadays there are plenty of reasons to avoid it whenever possible.

Personally I fly Peak EGT and at the lowest RPM possible/permitted for the desired power output (typically 2100 RPM).

the best BSFC (brake specific fuel consumption) is just lean of peak.

It's about 25F LOP, but one cannot set 25F LOP on any practical engine. It's too fine, and individual cylinders will usually vary by more than that.

I would bet the MPG difference between peak and 25F LOP is well under 1%, as the SFC curve is almost flat at that point.

At peak CHTs rise well into the 400s. In cruise at high altitude I haven't tried it but 500 can be seen.

That's way too high.

400F in cruise at 65% power? How the hell did the PA46 ever get certified?

Even if one goes strictly by the book (most people think 500F is way way too high, and 400F is a reasonable limit to work to) 500F is above Lyco's max official CHT, so that engine installation should have never been certified because it breaches the engine specs. Unless they certified it for LOP-only operation in cruise.

ROP was a valid technique back when fuel was cheap and our consciences were less green but nowadays there are plenty of reasons to avoid it whenever possible.

Also there is range. 15% extra range is a huge gain, in Europe, in terms of alternates with Customs and Avgas etc.

Administrator
Shoreham EGKA, United Kingdom

500F is above Lyco's max official CHT

My POH and the builtin CHT gauge show 500°F as the limit. Insane but sadly the truth. Luckily the TR182 has great airflow with a large cowl flap. I try to never go beyond 380°F.

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