Mixture control
 

Not discussing about how to lean mixture, but on something else:

1. As per POH (eg. C172), leaning is done when power is less than 75%. Let's say cruising at 5,000ft, with throttle full open, power is 70%. Can you lean? Throttle is full open! With throttle full open, isn't there a "fuel enrichment" function which adds slightly extra amount of fuel with throttle full open for cooling? Won't you be leaning, but fuel enrichment is "riching" it again...?

2. While set up (properly leaned, fix pitch prop) cruising at say 5,000ft, you fly into an area of slightly different ambient conditions causing RPM change & also airspeed change. Is it ok to leave the mixture lever as it is and adjust throttle to regain your required airspeed?

The fuel enrichment function works mechanically based on throttle position if the engine is equipped with such carburettor/injection system. Basically, instead of increasing fuel flow linearly with moving the throttle forward, the fuel flow starts to increase "exponentially". Try to imagine you have two valves for setting the fuel flow (I know this isn't the case for carburetted engines, just trying to explain). The first one is controlled by throttle and the second is controlled by mixture. For example: you set the throttle to desired position and the first valve gives you 10 USgal/h fuel flow. If you leave the mixture full rich, the second valve is fully open and the fuel flow remains unchanged, but if you pull the mixture towards the lean, the second valve starts to close and reduces the fuel flow coming from the first valve, so the enrichment function on the first (throttle-controlled) valve doesn't have influence on the second (mixture-controlled) valve. For example in Arrow II, the full throttle fuel flow at MSL (ISA) is 16 USgal/h and using mixture you can achieve fuel flow to the engine anywhere in range from 16 (full rich) and 0 USgal/h (idle cut-off).

In a normal C172, equipped with Lycoming O-320 you will get much more than 70% at 5000ft pressure altitude (ISA conditions)! As per my tables for C172N (O-320-H2AD), the engine produces 75% power at 8000ft pressure altitude, ISA conditions and 2650 rpm (fixed pitch prop). All this is at "Recommended Lean Mixture", which is about peak EGT (manual says 25-50 RPM below peak RPM on the lean side). Nobody is saying you can't lean above 75% power - you have to do it in the climb if you want achieve satisfactory performance, but you can't lean it to peak RPM (best power), since the CHTs would head for the sky.

While most instructors will tell you that every power adjustment (increase as well as decrease), requires full rich mixture, this just isn't true. If you have to increase power a bit (not full throttle of course) for let's say 10 seconds, just leave the mixture where it is and reduce the power to cruise RPM when you don't need the extra power anymore. As far as reducing power goes, there's absolutely nothing wrong with reducing power while leaving mixture intact - and if the engine starts running rough (and it probably won't until you go near idle), just enrich the mixture until it's smooth again.

A very good point. Unfortunately most instructors treat the mixture control as an engine on off switch with only two positions, full rich and ICO, and don't teach there students how to lean. The fact is the engine should be leaned whenever you are below 75 % power regardless of the altitude. I also leave the mixture leaned on descents, maybe enrichening slightly if it is a prolonged descent. The mixture only goes full rich on the prelanding check and not even there, if I am operating from a high elevation airport.

Confusion exists from folk lore spread by 'teach by numbers' instructors who do not keep themselves up to date. Leaning should always be carried out with reference to the aircraft or engine manufacturers instructions. This, below, is for normally aspirated Lycoming engines

Various Lycoming Flyer articles have emphasized proper leaning at the manufacturer’s recommended cruise power. Before delving into the savings to be obtained by leaning, it may be appropriate to again review those factors that affect leaning at cruise.
First, we must know that cruise power for Lycoming normally aspirated engines is generally considered to be 55% to 75% of the maximum power for which the engine is rated. At these power settings, the engine may be leaned at any altitude. There has been confusion about the reference to not leaning below 5000-feet density altitude. Remember that this reference only applies to those power settings above the cruise range — those normally used for takeoff and climb. Once cruise power has been set, leaning to best economy should be standard procedure as damage to the engine will not occur from leaning at cruise power settings.

FS - You should be referring to density altitude not pressure altitude for leaning

I was trundling home the other night in an AA5, about FL50, decided that I needed to climb to FL90, mixture rich, full throttle - 300fpm.

I then remembered that when I did a high altitude flying course in Arizona, we leaned for best power. Leaning a little, leaving the speed alone, I got 550fpm and was at FL90 in a reasonably sensible time.

One forgets these things, and if I'd not remembered that on this occasion I'd never have made FL90 and been forced to return home at about 2000ft, dodging cloud and ATZs.

But leaning isn't just for efficient cruising, it can be for best power at elevated density altitudes as well, even when it's about ISA-10, as it was that night.

I admit however, that I usually find it most convenient when changing flight condition to go fully rich, then lean again to the condition I want.

G

While this comment may (or may not) be correct for the C172, it is certainly not true for the general case. At the very least, it will depend (for normally aspirated engines) on the carburettor / engine combination.

For example, the Teledyne Continental Manual for the C75, C85, C90 & O-200 series of engines reads as follows:

7-7. LEVEL FLIGHT
...
During flight, observe the following procedures and precautions:

a. If a Stromberg carburetor is installed leave mixture control in the "FULL RICH" position at altitudes less than 5000 ft above sea level. If a Marvel-Schebler carburetor is installed the mixture control may be adjusted to obtain best rich power at any cruising altitude.
...

Without wishing to be controversial ;), I would suggest the safe course is to follow the procedure in the POH or Engine Manual.

JD
:)

Thanks, but if you look at my post thoroughly, you will see I added ISA conditions where pressure altitudes are mentioned. I did it just to make it more understandable to OP in case he goes looking in Cessna's manual which is in form of a table with pressure altitudes and ISA, ISA+20 and ISA-20 columns.

Otherwise I agree, air-to-fuel ratio is mass of the air in the cylinder divided by mass of the fuel in the cylinder, which is actually air mass flow divided by fuel mass flow. And air mass flow depends on air temperature and pressure, so it is in fact the density altitude which is important - and most manufacturers use density altitude in performance tables/charts.

First off my statement to "always" lean the engine is 100 % correct as a response to the original post who was asking about leaning a C 172. There are no C 172's that should not be leaned in cruise.

Your comment about the Stromberg carb is interesting. Personally I have never seen a Stromberg carb as every C 65-75-85 engine I have ever flown has had a Marvel-Schebler carb installed. Any aircraft with a stromberg carb is going to be so old it will not have a POH as we currently understand the term to mean.

However many, if not most, pilots and owners do not seem to know that both Lycoming and Continental publish their own operating manuals for pretty much every engine they have ever made. The engine information in the POH will have been derived from these manuals but engine manufacturers manual will go into much more detail than the POH. Every aircraft owner IMO should have a copy of the engine manual of their aircraft in their library. For renter pilots most 4 place rental machines will have a version of the ubiquitous Lycoming 0 320, so that manual is a good investment

Correct, unless its a Beagle Pup!

I operated a C90 with a Stromberg for a few years and was left somewhat disappointed by the 'information' generally available on the net.

A Stromberg leans perfectly well - once you understand what it is doing and how that works. It is nothing like so immediate as a Marvel - you need to let the new float chamber level/pressure 'settle', but with a simple single probe EGT and CHT it was very obvious what was happening.

Once I had that sorted, I leaned it all the time - just like a Marvel, but you have to be much slower and smaller making the adjustments and waiting for their effects.

But with all these engines once at any decent altitude leaning is essential. That aircraft went to 12,000 a few times and without leaning there is no way it would have got there!

What about a change in airspeed while cruising at the same altitude?

Example:

Cruising at 5,000ft, properly leaned (LOR), 1800RPM, 100kts.
Now, at same altitude, you want to increase speed to 130kts, which would require about 2200RPM (this is just an example!).
What do you do with the mixture?

a. Mixture full rich, power increase to 2200RPM or so as required to maintain 130kts, then lean (LOR).
b. Power increase to 2200RPM or so as required to maintain 130kts, then mixture full rich, and then lean again to LOR.
c. None of the above (then pls give an anwer...)

In the real world, c. None of the above.

Just rich it up a little before you increase the power. It's nice to avoid going too lean, even for brief periods, so a bit of rich assures that. Going full rich at altitude is not necessary, or ideal. If you're way up there, full rich will make it run rough, until you lean it again.

Though a well set up fuel injected engine can be leaned with precision, a carburetted engine really cannot. Any leaning will be a compromise. Temperature scanners help. With one, you will probably see that only certain power settings give you anything close to equal fuel distribution across all of the cylinders.

Leaning is nice, and appropriate at higher altitudes, but otherwise, the cost saving and "good" it does to lean an engine, can be quickly turned into costly bad, if you go too lean for too long. Perhaps for reasons of perceived economy, coupled with control and opportunity to fiddle with knobs, fixed wing pilots tend to lean in flight. The same piston engine series are generally not leaned in flight when they are installed in a helicopter. Yes, helicopters don't often fly as high as fixed wing regularly do, but the training, and flight manual usually say to leave it full rich.

Don't get me wrong, I do lean all the time in fixed wing aircraft, but I don't get carried away about that final precision of lean, unless it's a very long flight leg, and optimizing range becomes a factor.If you're running 100LL, and your exhaust is whitish grey, you did fairly well.

PilotDAR,

1. "Just rich it up a little before you increase the power",
but wouldn't it be better if after this new power setting is done, to "reset" the "system" - that is, with this new power setting, you don't know if you're still at eg. 50RPM lean of peak.
So "reset" by mixture full rich, then lean until 50RPM lean of peak, just to be sure...

2. Basically what I'm concerned with is having set up at eg. 50RPM lean of peak, then playing around with the throttle at the same altitude (& ambient conditions), will it be always 50RPM lean of peak at every other throttle setting?

3. Another concern, eg. I am cruising at 5,000ft, 2200RPM, full rich, 100kts. Then I lean, get a peak of 2300RPM, so lean another 50RPM. Now I am flying at 2250RPM, properly leaned, but due to higher RPM now (due to proper leaning) airspeed is 110kts. I want 100kts. Is it ok to just adjust the power as required to get 100kts?

This site is quite an eye opener: Pelican's Perch - More Stories

Following on from that...

What do you mean by 'too lean'? that doesn't appear to make much sense without some form of context.

Yes; that is basically correct.

If you set the mixture to around say peak EGT, then you can vary the MP quite a lot and the engine will remain around peak EGT.

The exception is when there is a big altitude change e.g. in a descent from FL150 to 3000ft you will need to enrich the mixture, because if you were at peak EGT at FL150, and then descend, the engine will go leaner and leaner and long before you are at 3000ft it will be way LOP and will probably start running rough.

I have a writeup on non-turbo fuel injected engine management here. It is quite long but you will get the general idea.

I don't bother with LOP; it does not produce any more MPG, except as a side effect of delivering less power and flying slower as a result of that ;) And even with GAMI injectors my engine does not sound that smooth when LOP.

Regarding the original Q, it is correct that one should not lean a Lyco motor when the power setting is above 75%. However, this is a crude guideline because, following takeoff, the power drops continuously as one climbs, and it is obvious that there is nothing inside the engine which will suddenly wake up at 75% and blow it up :) The constant-EGT climb technique is generally accepted as totally safe, and it delivers an easy method for climbing all the way to one's operating ceiling without touching anything but the mixture lever.

What is really important is setting a reasonably high IAS ASAP after takeoff, to avoid exceeding ~400F on any cylinder. For me (TB20) that means trimming for 120kt pretty soon. And sometimes it cannot be done; for example I departed from some place in Turkey at +35C and could not keep it below 430F, but that is still OK. Some SID profiles also make it hard - look up some of the ones in the Alps, for amusement :) If a SID has a 10% climb profile then it is probably a no-no for piston ops.

True. If you have proper instrumentation and an injected engine (preferably with matched injectors like GAMI) you can do a lot more, including leaning above 75%.

Some friends of mine fly a Sportscruiser with full engine instruments and an injected engine. They lean as soon as they're off the ground, based on a target CHT of 375F I believe. (Of course leaning for any CHT requires patience - a few turns of the vernier mixture control at a time, and then 60 seconds wait for the temperatures to settle.)

Only when in the cruise do they properly lean for best economy using the EGT.

But for those of us that fly a non-turbo, carbureted engine, a fixed pitch prop, and no more instruments than the RPM, a very crude single CHT and the Mk. 1 eardrum, leaning above 75% power is not worth the risk of cooking the cylinders. Below 75% lean for best RPM (best power) or slightly richer than the point where the engine starts running rough (best economy).

I agree.

Should also mention that the constant-EGT method involves hardly any leaning at low levels. The mixture lever comes back just a little bit initially.

I would not use a constant-CHT method. The CHT will depend a lot on the airflow, and will obviously be directly related to the ambient air temperature. And anybody doing that probably has just a single CHT gauge so other cylinders could be running too hot.

I see a lot of exacting numbers in this discussion, and description of very specific techniques. Other than lean of peak, I'm not suggesting that any of them are not worthy of consideration, but I am suggesting that for leaning Lycomings, and Continentals, particularly those with carburetors, one technique is not universally appropriate. Indeed, a technique found to be excellent on one engine, could verge on being harmful on another of the same model. There are too many variables for a one size fits all leaning technique.

Certainly, if you have a well instrumented engine, and a lot of experience with operating it (peterh337 for example), you can have the technique down to a fine art, and get excellent results. If, however, you're going from one to the next in a rental fleet, and the instrumentation is modest, it would be wise to allow for some variation.

Think of the forest instead of the trees for a minute. Those engines were designed, built and even repaired the first few times, long before we really cared about the cost of fuel, or optimizing it's use. Back in the day, leaning was more an exercise in treating the engine well, in not fouling it up with lead deposits, rather than saving every penny in avgas. In the originally approved configurations of these planes, the ability to lean precisely was not designed in.

Add to that, repairs done to the engine along the way. Cylinders are weld repaired, and the precise aerodynamics of the inside of the combustion chamber might not have been maintained, the induction system parts might have been exchanged, the venturi in the carb might have been changed for a different type, or slightly damaged in a small undetected carb fire, the baffles could be imperfect, leading to more condensation of the fuel in the induction of one cylinder than the other. Add this all together, and the effect of leaning of one engine found to be ideal, will likely not be ideal on the next engine.

If leaning at less than 75% power is over done, the immediate effect is negligible. Over a slightly longer term though (many minutes) combustion chambers can get hot spots, valve guides, seats, and the valves themselves can be affected. The cost of repairing a stuck exhaust valve will far exceed the saving in fuel getting you to that point.

If you're renting the plane, the operator has already budgeted the fuel consumption at greater than that obtained by precise leaning. The operator has also budgeted the occasional cylinder repair, but would rather not. Unless the operator directs you otherwise, and the flight manual instructs, yes, lean, and understand why you are doing it, but err to the rich side as much as you can. Fuel is the cheapest coolant available to that engine.

If you are using the same very well instrumented, large engine aircraft over and over, and the owner encourages aggressive leaning, within the limits of the flight and engine manuals, learn to do it. The big engines burn a lot, and there are certainly savings to be had.

I personally do not agree with lean of peak operation, and I have never seen a flight manual, or engine operating manual for a GA aircraft which presented that type of operation as being acceptable. I have certainly seen some Lycoming engine operating manuals which prohibited it. If the owner of the aircraft you are flying, is also the qualified engine maintainer, and they are instructing you to run LOP, and the engine manufacturer does not prohibit it, consider it. Otherwise, if you're running LOP, and you stick a valve, are you willing to pay for the repair?

At 5000' going to full rich would be okay, but getting higher, it's not a good idea. At full rich, the engine is likely to stumble, and startle you and your passengers un-necessarily. I once had a 180 HP Cardinal just about stop when I went to full rich changing tanks at 11,500 feet. If I lean and aircraft with no EGT, I will lean until the first sign of any drop in engine power I can perceive, and enrichen it about 10% mixture control position from there. In 35 years of flying, I am not aware that this has ever created a problem.

Just wondering, why would a valve stick from LOP operation?

LOP itself is a non-issue because by definition the engine is cooler (EGT and CHT) than at peak EGT, and peak EGT is authorised by Lyco at/below 75% of max rated power.

The problem is a general one of excessive CHT combined with high cylinder pressures. There is no real data on what values are damaging in the short term but they are higher than most people think.

GAMI's own tests suggest that detonation needs a CHT of about 500F, which absolutely nobody with half decent instrumentation should ever get anywhere near, but a pilot without instrumentation very well could do so simply by flying at Vx or Vy for 10+ minutes.

For long engine life, the general consensus, such as there is :) is that one should cruise at 65% of max rated power, and peak EGT (or LOP if you want to fly slower, yet maintain full throttle) are perfectly fine at that power setting.

During climb, one cannot get more than 65% above about 12000ft (chart).

My nice-weather IFR cruise setting is FL100, 2400rpm, peak EGT, full throttle, which is about 65% power. For better MPG I go to FL120 and drop down to 2200rpm and 9.5GPH which according to this is about 45% power.

This type of operation produces a very long engine life, with clean cylinders, valves, etc.

Peter337's point about managing CHT is a very good one. I bet more light aircraft engines have been cooked by overheating than have ever been damaged by over leaning. Long Vy climbs on a hot day are a good way to overheat and permanently damage the cylinders. Unfortunately your average rental trainer/tourer is unlikely to have a CHT gauge, and if it does it probably doesn't work. The only way to know the engine is getting too hot is to watch the oil temp. Unfortunately this is an indirect measurement of CHT and so you could do damage before the oil temp indicates an excessively high value. In any case immediate action should be taken if the oil temp is observed approaching the redline ( ie full rich mixture and increase airspeed).

In the absence of engine instrumentation the easiest way to avoid burn't cylinders is, once clear of the aerodrome, do a shallow cruise climb at full power, and an airspeed of 15 % or so below the normal level flight cruise value.

Peter337 also gives very precise leaning instructions which work well if you are flying the same fully instrumented, fuel injected, CS prop equipped, tourer on a frequent basis; but are of no practical use when flying a simple rental airplane.

For those aircraft I echo what pilot DAR said, slowly pull the mixture knob out until you get a reduction in RPM and the onset of rough running. Enrichen until smooth and you get maximum RPM and then a "little" bit more. The magnitude of the "little" varies depending on how far the mixture control travel is. The point of the exercise is to get the engine firmly in the "best power" mixture range.

Remember leaning is authorized whenever the power is below 75 % and for Cessna's in particular, in the climb above 5000 feet. If you are going cross country you must lean to get anywhere near book figures and for any high altitude operation leaning is a must to get adequate power.

Yes. Though I agree with this, by the time your got the engine to this point either you have damaged it, or you are about to.

Enrichen mixture, yes, increase airspeed , yes, but not to the point of a dive with the power off, that will shock cool the engine, which is the last thing you want to do with an engine that hot.

The important thing is to not get yourself there in the first time. Parachuting pilots are usually thoroughly briefed about treating engines gently in this regard.

If the aircraft you are flying has a CHT (if it has cowl flaps, it is required to), watch it as you change power. If you can see the pointer move at all, you're changing power too quickly. If the aircraft you're flying has an "aftermarket" CHT, and not a probe on every cylinder, be aware that the probe might not be installed on the hottest (or coolest) cylinder. Use that reading with caution...

How would you damage an engine from 'over leaning'?

Full power climbs at lower airspeeds with the engine leaned will do it. The CHT's could be OK at full rich, but will be so high with the engine leaned that you toast the cylinders.

I wonder how many planes will have the engine below say 400F when doing a Vy climb, even with mixture fully rich.

In my limited experience of the higher performance types, probably none of those.

Yet, during training (PA38, C152, PA28-140 -161 -180) we were doing Vy climbs all the way to altitude. None of those planes had any EGT or CHT gauges, however. In the FAA IR (Arizona, ISA+10 or so) we were doing Vy climbs to 11,000ft and the only gauge was the oil temp.

And when people do have a CHT gauge it tends to be on #5 or #6 (on a 6-cyl) which common sense tells you should be the hottest but common sense is usually wrong in this case because it is the middle ones (#3 or #4 - usually #3 if the oil cooler is mounted on that side, robbing some air for itself) which run the hottest. These are typical full-rich deg-F climb CHT figures for mine, #1 to #6

380 359 398 374 386 373 194
where the last one is the oil temp. And that was climbing out of St Gallen LSZR where the OAT was +14C so not exactly warm. Looking at the EGTs, that part of the climb was full-rich, too, trimmed for 120kt to get plenty of cooling air.

You can see #3 is 25F hotter than #6.

So it would not suprise me if a lot of un-instrumented engines out there are being run very hot, because Vy climbs are common on the training scene.

Pure touring aircraft tend to be more tightly cowled then trainers in order to increase cruise speeds. They will also usually have cowl flaps. I think the standard trainers are deliberately designed to have extra cooling flow so that people can get away with flight school foolishness like unnecessary Vy climbs. Also most trainers are powered by the Lycoming 0 320 , possibly the most bullet proof engine ever made.

The reality is your PPL training will not give you very much useful preparation for operating a more sophisticated aircraft and so a check out with somebody who has actually real world experience operating the same or similar type of aircraft is highly desirable.

Eh? If you lean past peak, CHTs will get cooler not hotter, so that statement requires a qualification for it to make sense.

As a general rule it is impossible to get a carburated engine like you find on the vast majority is trainers/simple tourers, to run smoothly at lean of peak mixture settings because of the inherent uneven fuel distribution between cylinders.

But full rich mixture will deliver more fuel than can be burned, the excess will carry away heat and thus reduce CHT's. Leaning will eliminate most of the "extra" fuel thus loosing the cooling effect and causing a rise, and sometimes a dangerous rise, in CHT's.

I want to emphasize that I am taking about aircraft in common use. That is single engine fixed gear Piper and Cessna aircraft manufactured in the 1970's and early 1980's. I am sure someone can find some obscure aircraft with exactly 13.5 airframes in existence where the above comment does not apply but for aircraft that average reader of this forum has even seen let alone flown, I believe my statement in post number 28 is accurate.

I can not prove that damage would happen, but I am so alert to the possibility, that I have no interest to the possibility of doing damage. Fuel distribution as quite variable, and positional of EGT probes can be quite imprecise as well, leading to erroneous readings. If I could see every cylinder indicating near identical EGT, which a belief that the probes were perfectly positioned, and the engine manufacturer recommended LOP operations, I might try it. Otherwise, the savings don't warrant the risk in my estimation.
If I'm flying an extreme long over water, I'll be looking for best range, but sure don' want to cough an engine!

I really do agree with the numerous cautions that Big Pistons has presented here (so won't repeat them). When you change the combustion in the cylinder, you can be repositioning the flame front. That can expose internal areas of the piston, cylinder head, and exhaust valve to temperatures for which they were not designed. By way of a minor (an perhaps only somewhat relevant example) what the EGT when you're doing a mag check - Does the EGT increase when one mag is grounded? Less power, why the higher EGT!

I sure don't have all the answers, and if I thought I did, the next model engine, with different combustion chamber characteristics would require a restart in my thinking anyway. During inflight and ground based detonation testing I have undertaken in Continental 470's and 520's, I have seen rapid and unexpected temperature change trends with leaning, and not a full power. These have caused me to err on the side of caution when leaning. Leaning is necessary for good engine health, and I always lean my O-200 at all altitudes (with reference to a bar graph EGT), but I still keep it 100 or so rich of peak. Fuel, no matter how costly, is still cheaper than engine maintenance....

A good explanation Silvaire, thanks. It does serve to add to my thought that as combustion is now happening downstream of where it is intended to, although the combustion chamber is not feeling that fire, the open exhaust valve is, and that could be a beginning or aggravating point for sticking the exhaust valves(s) open. The exposed valve stem is potentially receiving deposits of hotter that expected, products of combustion, which had they occurred where they were supposed to, would pass by harmlessly.

I cannot draw a conclusive link between valve sticking, and unconventional mixture settings, but during years of work at the engine shop I saw a number of cases of stuck exhaust valves, and these were often linked to the pilot's report of the use of 100LL, and extreme leaning. I too, lean harder when I occasionally must run 100LL, and stuck two exhaust valves last summer for the fist time in years.

For those not familiar with stuck exhaust valves, the immediate affect is that you are now developing one fewer cylinder worth of power, which on a four cylinder engine is significant.While this is happening, and depending upon which position the stuck valve has taken, worst case engine damage can include broken rockers bosses (expensive cylinder repair or replacement), bent push rods, and cam and follower damage (may fill your engine with metal, and require replacement cam$haft).

To save a bit of fuel, it's just not worth it! If I'm really worried about making the coast of Iceland in higher than forecast winds, it's a chance I might take. For day to day running around, I'd rather waste a gallon an hour....

Which is why the EGTs rise when doing the mag checks.

Only to a very small degree. An LOP mixture is a relatively weak burn. The gas pressure is low and as a result the whole combustion chamber pressure is low.

I think it was GAMI who showed that valve stem temperature closely tracks the CHT, first and foremost. This is why CHT management is so important.

As an aside, people who fly "deep LOP" (say 50F LOP) find a useful MPG improvement by using low RPM. I doubt anybody knows exactly why but it is most likely because the fixed spark timing on these engines produces a more favourably timed spark. I certainly find 2200 produces a ~5% (IIRC) better MPG than 2500 (at the same IAS etc) but I don't fly LOP (I fly around peak EGT).

You waste a lot more than that, potentially.

80-100F ROP ("best power") is about 10% worse in MPG than peak EGT.
150F ROP (mixture full rich) is about 25-30% worse in MPG than peak EGT.

LOP alone does not give you measurably better MPG if you fly at the same IAS.

Though I am not certain, It's not so much heat or not at the exposed ('cause open) exhaust valve stem which concerns me, it's the only partly burned products of combustion getting and sticking there which worries. me. I do agree that if LOP, probably everything got burned, but it's still flowing past there in a way that the engine manufacturer might not have accounted for in the design.

As for the waste gallon per hour... One of the rewards of enduring a quarter century of ridicule for owning a 150 spam can, is that the difference between wasteful and efficient really is only about a gallon per hour. Now, when I fly the Navajo, things are different!

Everything should get burnt at peak EGT, or anywhere LOP.

I can offer one data point: my engine got rebuilt (Lyco SB569 crank swap) at ~700hrs, after ~650hrs of peak EGT operation 100% of the time (except climbs), and the only items which were outside new limits were the exhaust valve stems (they were about 0.002" outside and still within overhaul limits), and the cam followers. The engine was otherwise spotless.

Now, some 400hrs on, the compressions are all ~78/80.

I am not aware of anybody who has operated similarly (which is quite common among pilots of similar aircraft, though some operate nearer to 75% power whereas I fly low level at ~65%) and who has found anything different.

Turbo engines are a different thing; they seem to crack cylinders well before TBO no matter what one does :)

Simplified: at peak EGT you have stoichiometric mixture, meaning: every fuel molecule has the potential to find one matching oxygen molecule and get burned perfectly.
In a 'rich' mixture, there are more fuel molecules than oxygen molecules, in a 'lean' mixture there are more oxygen molecules than fuel molecules.

The reason both a richer and leaner mixture produce less heat is because they burn less quick and therefore act like a retarded ignition: the peak power pulse happens later in the cycle when the cylinder head has moved downwards more and there is less pressure (and heat) in the cylinder.

Same with single magneto operation: the flame-front needs more time to burn the whole air/fuel mixture and this also acts like a retarded ignition thus EGT rises.

Hottest CHT happens about 40°F before the hottest EGT, so peak EGT is better for the engine than 'enriching slightly for the grandma and kids'

About the savings: in my aircraft, at TBO time, the LOP savings in fuel consumption more than pay for the engine overhaul. The EDM700 has more than paid for itself in reduced engine and ignition system troubleshooting time.

One more reason: at peak or LOP, there is NO carbonmonoxide produced (deadly in case of exhaust/heater leaks)

Back to one of my original question:

What if you are properly leaned (eg. ROP), cruising at eg. 5,000ft, then due to atmospheric condition changes, your RPM went up by 100 and thus causing your airspeed to increase by 10kts, you want to maintain your original airspeed so you adjust your throttle.
BUT, what about your mixture now? You have entered an area of significant atmospheric condition, shouldn't you "reset" your mixture, that is, put it full rich, then lean again to eg. ROP?

Shumway,

In trying to consider your original question, I'm trying to think of a situation where, at a constant altitude, a change in atmospheric conditions could cause a change in RPM of 100, and an increase in speed of 10 kts. Could you present a more defined example of this situation?

No; it's not that critical - by a long way.

Unless you are operating in the LOP region, the power v. mixture curve has quite a flat top.

PilotDAR,

The 100RPM & 10kts increase is just an example.

The situation is cruising at the same altitude over long distance, I enter an area of significant atmospheric condition, maybe an area where the temperature drops significantly & density increases due to whatever phenomenon (cold front or something like that, just as an example).
That would cause the engine RPM & thus airspeed to change.

Now the leaning was done earlier, before this atmospheric condition change occured.
The question is, other than adjusting the throttle to maintain the original intended airspeed (reduce the RPM to lose the extra 10kts gained), shouldn't the mixture be readjusted again due to this "new" atmospheric condition?

well, I suppose that I do not have an answer of that scenario, because I cannot imagine it occurring. in a standard GA aircraft, you are not going to fly far enough, or for long enough to encounter enough atmospheric change to cause such an engine performance change. indeed, I don't think that a 172 could operate in conditions varied enough to produce such a performance change!

I doubt that in the real world of GA flying, you're going to encounter an operating change enough to affect engine operation at one power setting detectably. Probably the change in weight resulting from fuel burn will be noticeable first.

As I have previously suggested, unless you're flying a perfectly set up fuel injected engine, you're never going to optimize fuel flow/mixture to the extent you propose. Bear in mind that the C 172 S Flight Manual instructs that "lean" is 50F cool of peak, and operation lean of peak is prohibited. This is not the first engine for which I have seen this prohibition.

As I have said, understanding the concepts is good, understanding the limitations, and working within them is vital and required. Saving that last drop of fuel is not worth the effort, or the risk of operating the engine outside it's approved parameters. I would expect that a good wash and wax will have more noticeable affect on the cruise performance and economy of the flight, than attempting to optimize the mixture to the "enth" degree....

Correct, especially for simple carburated engines