Rich of peak or Lean of Peak

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awash2002
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Rich of peak or Lean of Peak

Post by awash2002 »

I'm trying to wrap my head around ROP mixture or LOP mixture settings in cruise. Any expert advice would greatly help all I know LOP slows you down and saves fuel and ROP is normal cruise setting to me anyway what is the difference between them?
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Hook
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Re: Rich of peak or Lean of Peak

Post by Hook »

If you have installed GAMIjectors and an engine analyzer it's ok to run lean of peak (according to the company marketing claims). :D Not everyone agrees.

I'll let the experts hash it out, but let's just say it is a controversial subject. As far as I'm personally concerned, it's marketing for the GAMIjector company. What do the engine manufacturers say about it?

Hook

awash2002
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Re: Rich of peak or Lean of Peak

Post by awash2002 »

On some engines like the one in the Piper Mirage or M350 now it is recommended to run LOP
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Hook
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Re: Rich of peak or Lean of Peak

Post by Hook »

Here's the manual for the M350 engine:

https://www.lycoming.com/sites/default/ ... 297-27.pdf

Search for "peak" to get the pertinent information.

I'll let the reader decide what is marketing and what is technical and decide which of these to use.

Hook

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AKar
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Re: Rich of peak or Lean of Peak

Post by AKar »

This topic has been beaten to death in several occasions, including by myself and others. While I've since somewhat adjusted my opinions in what comes to real life operations (something I do not want to play advising on in any forum, outside sharing stories, anecdotes and colorful opinions, of course!), the essence is almost fully contained in the very graph Lycoming includes in all of their manuals, I think.

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The picture is far from perfect, however: if one cannot deduce what is the role of improved injectors, engine monitors and whatnot (if any), and the benefits of running lean and rich respectively, by staring that image for a good long evening, I'd tend to tell one to stick with what the manual says. It may not always be the smartest way to do things overall, but in that case one is unlikely to outsmart the manual either, unless things shown by this simplistic graph ring many bells.

-Esa

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ratty
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Re: Rich of peak or Lean of Peak

Post by ratty »

AKar wrote: 11 Mar 2020, 11:51 . . . the essence is almost fully contained in the very graph Lycoming includes in all of their manuals, I think.


-Esa
Yes! Thank you, thank you, thank you! I had not seen a graphic "explanation" before. There's a wealth of information there.
I think I'll just run ROP in future. Operating LOP without the fancy monitoring and metering, it's too easy to be just a bit too rich and in max CHT territory.
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AKar
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Re: Rich of peak or Lean of Peak

Post by AKar »

Even if I risk some repetition, I could go through few points as the original post came our somewhat more blunt than I intended.

First, however, I don't intent to present myself as an 'expert'. What I write mostly comes from the experience of others and seeing their results, some first hand, and most via a network of contacts. Combining their experiences as 'data points' with those of mine and what various literature sources, theoretical knowledge and available numerical data from 'real world' sources (yes, I have gone through several sets of engine monitor output files over the years) tell us, can form a fairly coherent picture that is generally very well applicable. However, not every engine operating in every possible scenario, operated in every way it can possibly be operated is accounted for, and hence nothing that I say should be applied directly into real life flying without thorough consideration of all applicable circumstances and still, the judgement remains solely on the operator.

Further, what follows is simplified on purpose, and several points strictly applicable to real engines are not considered. The scope is intentionally retained as 'flight sim applicable'.

That being said,


1. The role of the injectors (GAMI or otherwise).

See the graph, and in particular, how steeply the power starts to drop after passing the peak EGT mixture towards further lean ("best economy range"). Now, consider that this graph only applies to a single cylinder. So, your engine is made up of four or six such graphs.

In fuel-injected engines, the fuel is typically pumped continuously into the induction manifold via injectors near the intake valves of each cylinder. The metered fuel flow comes as one from the Bendix servo or the fuel control unit. It is divided for each cylinder by appropriately named flow divider. And the flow should be evenly divided. The injectors themselves play huge part in this. Any mismatch in between them results in unbalanced fuel distribution in between the cylinders.

Note, that if the intake system is not perfect, also the air distribution may be unbalanced.

Both of the above mentioned cases result in uneven air-fuel mixtures in between the cylinders.

When you run lean of peak, where the power produced is a steep function of the mixture (slight difference in mixture results in proportionally large difference in power produced), if the mixture distribution is not even, the cylinders produce uneven power, and the engine tends to vibrate very noticeably. It will not start to run rough or sputter, however, until one or more of the cylinders run lean enough to fail to fire properly. This generally happens only with engines that have very poor mixture distribution, which unfortunately can include many 'stock engines'.

To enable an engine to run smoothly at lean mixtures, you need even mixture distribution. For this, you need good, even set of fuel injector nozzles. "Marketing" or not, this is where aftermarket products often come in.


2. The role of engine monitors.

The engine monitor is not a strict requirement for the lean of peak running. (Though generally recommended for any running...) One could argue it is equally or even more beneficial when running at 'handbook settings', just slightly rich of peak EGT!

With a proper engine monitor, one can detect a cylinder running abnormally lean or rich - something that is not immediately noticeable with stock gauges. One can monitor individual cylinders for CHT, and do other things. Perhaps the coolest feature is the logging, like a QAR for the engine.

A test question: with a classic, 'stock' fuel flow gauge, what will be the indication of a blocked fuel nozzle, and why?

However, to apply lean of peak, engine monitor is not required.


3. What is not included in the graph?

For one thing, the speed of combustion. Remember, we are limited to a fixed ignition timing here.

Generally speaking, in an internal combustion piston engine, we want to have as fast combustion event we can have in a controlled fashion. This fast combustion event occurs at around peak EGT, and this is approximately where your car generally adjusts the mixture unless the engine under moderate-to-heavy load. (Although it does this by sensing the remaining oxygen in the exhaust, not by finding the EGT peak reference.) However, in our aircraft piston engine, the fixed ignition timing necessitates a compromise. We must be able to operate at fairly wide ranges of RPM. So, at the maximum rated RPM, it follows that the ignition timing is just somewhat 'too advanced' (at very slow RPMs it is significantly too retarded, hence, these engines tend to idle poorly). Thus, the quick-burning, near-peak-EGT mixture results in us somewhat "hitting against a rising piston", resulting in high wasted work, directly correlating in high CHTs.


I may add a part two, consisting of topics:
- Classic leaning - pulling until roughness and then adding some;
- Why LOP?
- Why not LOP?


But I'll see if I can spare a moment - I am not sure at all if the current aviation crisis we are facing is giving me more time or less.

-Esa

SaxTeacher
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Re: Rich of peak or Lean of Peak

Post by SaxTeacher »

The Lycoming graph shown above is the key to all of this. It is a complicated subject.

The best and most comprehensive explanation I have found is the discussion given by Mike Busch in two webinars he put on. He has given these lectures at Oshkosh several times. If you want to understand it completely, watch and enjoy!

https://www.youtube.com/watch?v=_VfiPuheeGw
and
https://www.youtube.com/watch?v=X-tKyiUZ3ts

best,
SaxTeacher
N9284P (PA-24-260B)

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AKar
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Re: Rich of peak or Lean of Peak

Post by AKar »

I'd consider the graph a clue to a good half of the story.

The other half would include ignition timing and RPM.

-Esa

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