While this has been discussed over and over during the last few years of Accusim, things like 'optimal mixture', EGT references, absolute and relative temperatures still seem to confuse many. This is very understandable, because we often seek to follow written procedures to the letter (in many cases even creating a sort of procedure dependency), but the real-life written procedures that concern the leaning are often ambiguous, conflicting themselves, and offering little to none background and help in understanding the topic.
I've been doing some preliminary work to write some clean articles based on the discussions we've had on these forums over the years, but as I'm yet to find a peaceful moment to finish even a one of them, I write this quick and easy post to help with very basics should one find those confusing. What I write here applies to any properly simulated non-supercharged, fixed ignition timing airplane piston engine that burns avgas. That is, in practice, any Lycoming or Continental engine of O or IO series. While much of it applies to (turbo-)supercharged engines too (TIO and TSIO series), I don't consider the few important differences in this post, so let us just leave them out of discussion for time being!
1. Those Confusing Procedures!
So far in the fleet of Accusim, we've got four different GA engines, which all are slight variations of two basic models. What are the differences in between them that we must understand when leaning them? Would you check their respective manuals to be sure? Don't bother, I can save you some time: essentially, there are no differences at all. While the manuals kind of cherry-pick some bits from engine manufacturer's recommendations, they often don't help that much in understanding the process of leaning, instead throwing in some confusing advices to lean 50°F lean of peak EGT for economy cruise while in some other page mentioning the peak EGT for maximum economy....what? Which one is it? The answer is neither. It is easy to fool oneself with these lines, thinking that they contain some guarantee of best practice which is, in reality, not present.
In all the manuals out there, for most casual purposes there is just one graph, guideline, procedure, that one truly wants to know by heart. It can be found in several forms, below you can see it how Lycoming publishes it (the graph can easily be found from the internet so I take the liberty to post Lycoming's graph here):
The simplified picture here is really almost everything you need to know! It shows the relationships between the different engine parameters when any of our GA engines is ran at constant MP and RPM, while varying the mixture. Take a good look at it, and make sure you understand all the parameters. All of them are of special interest to us.
2. Leaning For Best Power
If for some reason (such as high altitude climb) you want to extract as much power from the engine as possible, then you'd had to lean the engine some, because the overly rich mixture when at full-rich is not optimal for our case. When having a fixed-pitch propeller, or a constant speed propeller at underspeed like when on the ground at partial power, this mixture for best power can be found by leaning until the engine peaks in RPM. There is no need to fiddle with EGT gauge in that case.
However, if you are in climb already for instance in an airplane with a constant speed prop, and therefore unable to find the peak RPM, then you can note how the EGT at that specific mixture compares to the peak value achievable. It is some 150°F rich from peak EGT. So, to lean for best power by using EGT reference, you'd find how high you'll get the EGT (find the peak), and then set the reference needle to that position (or in Comanche you'd note the figure). After that, you'd enrich the mixture so that the EGT needle comes down 150°F short of that value. In Cessnas, that's 6 tick marks below the peak value, each tick representing approximately 25°F.
Note that what the absolute EGT value happens to be is meaningless! In Cessnas you don't even know it except by cheating with tooltips, but it doesn't matter in the Comanche either. All that you care is the peak position of the needle, and how far you set the needle after finding that position!
Of note is the CHT graph. The CHT is the best indicator of the engine temperature we've got here, and high CHTs are one of our 'worst enemies' from the point of view of the cylinder and valve service life. Note that the mixture setting that results in best power is actually giving us cooler engine than some of the settings that are leaner than that! The best power mixture setting is, from that point of view, not that hard on the engine, while the economy (indicated by the specific fuel consumption graph) lacks some when compared to further leaning.
3. Leaning For Best Economy
If we want the best possible fuel economy, then instead of where the power graph peaks, we would find where the specific fuel consumption graph is at its lowest. That area is immediately on the lean side of the peak EGT. So to get there, we in principle would find where the EGT needle peaks, and lean just a bit more after that. How beautifully simple!
Of note is that the produced power drops quite sharply, so we wouldn't want to get too far to the lean side, for we'd be sacrificing our performance very quickly. Likewise, it is nice to see how the engine temps start to drop quickly as we lean further! While the EGTs stay relatively high, the CHTs that actually matter drop much faster. Flying with lean enough mixture results in nicely cool CHTs!
4. Why It Runs Roughly?
In practice however, getting the best possible economy from our engines is not that simple. Typically, if we attempt to lean them to the peak EGT and beyond, we'll encounter rough running. Why is that? The answer lies in the graph too! Again, note how the power drops sharply on the lean side of the peak EGT when compared to the rich side. What we must realize is that our imperfect fuel distribution results in that the actual mixture we achieve varies between the cylinders. Some run leaner than others, and that's why when the leanest cylinders go beyond the peak EGT, they start to loose power when compared to the cylinders that are still running somewhat richer. That uneven power production among the cylinders is what results in rough running! If we had a perfect mixture distribution, then the engine would just loose power while running smooth when we lean it further, however, in most engines just some of the cylinders start to loose power at first, resulting in roughness.
In practice this limits how much we can lean, and for maximum economy, especially in carburetted engines where the fuel mixture distribution is much worse than in fuel-injected ones, the valid method is to lean the engine until it runs rough and then enrich as little as is required for smooth, comfortable running engine. What kind of mixture would that leaning by ear produce? We can take an educated guess, again, by just looking the graph we've got. Likely the leanest cylinders would be somewhere where the produced power starts to fall quickly. This is just at the borderline of smooth running. If our EGT gauge indicates the leanest cylinder, then it would indicate slightly under the peak EGT, and on the lean side we'd be. How lean we can get depends on how the other cylinders follow - in A2A engines the mixture distribution is relatively good, so they likely follow quite closely, and their individual EGTs would likely group up at around peak EGT or so.
5. Leaning For The Worst Possible Engine Temperatures
You can by now easily check that the highest CHTs, or engine temps, occur at around 25-50°F rich of peak EGT. Does that sound familiar? By following that weird recommendation in many manuals you'd make it certain that you've got the hottest cylinders possible at given MP and RPM! Still many of us take a great deal of trouble to precisely set their engines at that very setting! Temps wise it would help to go either way from there, either by leaning for better economy or by enriching some for performance. Think about it.
But there is more to the point than blaming procedures. Remember what I wrote about the cylinders running at different mixtures in practice? This actually hides the reason why leaning for best economy is a bit difficult subject when running at very high cruise powers especially. If we had a perfect mixture distribution, we could indeed fly at very high cruise power and lean a good deal to the lean side of the peak EGT, resulting in nice and cool engine and great economy. However, because of our less than perfect mixture distribution, the smooth running criteria often encountered forces us to remain quite close to the peak EGT when we are seeking for the best economy. This results sometimes unavoidably in that at least some of the cylinders are running at around the hottest possible settings, some 25-50°F rich of peak while the leaner cylinders operate comfortably lean of peak. So, some possible overheating issues of individual cylinders may be expected, and the reason is not that we run too lean - but our inability to get those cylinders lean enough!
Again, it is important to note that we've only got information from a single cylinder in a form of EGT and from the same or another cylinder as a CHT reading. We don't know how the other cylinders are doing, except by guessing and feeling for and listening for other minute signs.
-Esa
Òûðôøüøр Ûõòúþò / Vladimir Levkov
