Rich of Peak, or Lean of Peak?

[White Bear]

I am a well-trained automotive/Heavy truck technician, spent years in tech schools in the 60’s and early 70’s, worked in the industry all my life. I have attended many many continuing education classes to keep me up to date on new technology and developments, so I thought I understood all aspects of internal combustion engines, until I started flying…

Then I met mixture controls and all that I'd learned, or thought I’d learned about mixture control in petrol engines seemed to go out the window. In specific, in an automotive engine the common maxim is “A lean engine will run hot” I seemed to have just absorbed that piece of information, but when examined logically it does not make sense.

If the adiabatic correct air fuel ratio is 14.7-1, then at that ratio by weight, all the oxygen and all the fuel is consumed during combustion, then you must have the maximum temp rise, and maximum pressure rise. (P1=V1=T1).

In General Aviation circles there is always discussion about running an aircraft engine rich or lean of peak exhaust temp, say 50 deg f either side. (Please lets ignore cylinder head temps for now, too many variables) One statement I read suggested that 50 deg rich of peak puts the engine in prime detonation condition, so it’s best to avoid that area as detonation cannot be heard, as it is in automotive engines, because of the noisy environment. Running lean of peak will only reduce power slightly, reduce the risk of detonation, reduce fuel flow significantly, and keep plugs clean etc. Then there is the position that running lean of peak will cause the engine to run hot.

Now for my logic problem:
It follows if one mixes fuel and air perfectly (14.7-1) then all the fuel and all the air are consumed during combustion, producing the maximum temp/pressure rise. How then, when you run lean, can less fuel make more heat?

Is there a real world cooling effect in that small difference in the amount of fuel, say between 14.7-1 and 16.0-1?

I am interested in theory here, so please don’t tell me to read the user handbook. I have done so, and it seems to me Lycoming are in many ways still in the dark ages when it comes to engine fuel management.

Have I forgotten something I used to know? What am I missing?

Regards,
White Bear.

[Dan Winterland]

The crux of the matter lies in the fact that most aero engines are designed to run richer than the chemically correct 14.7:1. The excess fuel pumped into the cylinders which doesnt get burnt because there isn't enough oxygen for complete combustion, is converted into it's basic elements such as carbon - hence the black smoke seen on rich running engines. This catalysation takes quite a lot of heat and leads to useful cooling of a fairly inefficient air cooled engine which relies on running rich for cooling in certain circumstances - i.e. a climb. So if you lean off, you are taking the mixture closer to 14.7:1 and taking away some of the cooling.

That's the way I understand it. Simple, but I like it that way!

[dirkdj]

Time for some OWT killing!

YOUR ENGINE WILL BLOW UP IF YOU ...

lots of uninformed old wives tales floating around.


Google for "John Deakin Mixture Magic" and read this as a starter.



Some misconceptions:

"the engine and the exhaust valves run hottest at peak EGT."

"The lead in Avgas acts as a lubricant to the valves."

"the engine will run hot when leaned too far"



Best to forget everything told and read the entire engine series by John Deakin. Nobody has explained it better so far.

[High Wing Drifter]

I found this interesting document on the Lycoming site.

http://www.lycoming.textron.com/main.jsp?bodyPage=support/engineOperationTips/index.html

[IO540]

White Bear

The reason for your confusion is that few people know anything about this subject. No instructor I've ever met (or got anywhere near) does. Plenty of owners of planes with engines whose management is regarded as important do know it but they aren't participating here :O

A good starting point is a read of articles by John Deakin at avweb.com. They do move around there a little so if you can't find them, PM me and I will dig them out.

Back to your questions:

How then, when you run lean, can less fuel make more heat

It doesn't, of course.

The highest CHT occurs in the vicinity (not quite "at") peak EGT. Once you lean past the peak, i.e. in the lean-of-peak (LOP) region, the temps all fall. Since all the fuel is getting burnt, you are extracting all the chemical energy out of it that can be extracted.

So, why not run LOP all the time (like modern car engines tend to)?

The answer, I think, is simply inadequate cooling

There is always a design compromise between getting rid of heat, engine weight, and cost. Liquid cooling makes this a lot easier but we don't have that option in those engines.

A design compromise has been chosen such that to reach their handbook max rated power, a typical Lycoming or Continental engine cannot get rid of the heat (into the exhaust, the oil and the airflow) and needs to have the combustion cooled by something. One could squirt water into it (like has been done elsewhere) but they just squirt fuel in there - in America fuel costs about the same as water :O

The fuel injection unit is configured so that with the mixture lever fully forward you are about 150F ROP and you can leave it there all the way through climb, wasting unburnt fuel happily out of the exhaust. Until you get to an altitude where the engine won't run because it's too rich, say 5000ft+, so you HAVE to lean, but by then it should be safe because it isn't doing anywhere near 100% power, more like 60-70%.

When the power setting is well below 100% (75% is Lyco's official position on certain engines), and the airflow (airspeed) is sufficient, one can lean to LOP. The only limit is that the engine design does not properly distribute air (and therefore fuel) equally to all cylinders, so the engine start to run rough because some cylinders develop more power than others.

The reason this doesn't matter when ROP is that if you have surplus (unburnt) fuel, uneven fuel distribution doesn't result in correspondingly uneven power distribution. Whereas at LOP (where fuel=power) it does.

So to operate LOP one needs an engine whose power distribution is balanced. A firm called GAMI has been selling matched injectors for this purpose but some engine owners don't need them as their stock injectors are well enough matched to start with. On carb engines the situation is usually hopeless.

I don't know if one can design an air cooled engine like a big Lyco which can run LOP at 100% power under all operating modes (slow speed climb and the rest). Possibly, it can't be done. These engines do deliver quite a lot of HP for their weight, considering the low RPM which is dictated by lack of a gearbox and the prop tips having to be sub-mach1.

Lycoming's position is complicated because they've been selling engines essentially unchanged for decades, and if they change any operating recommendations they open themselves to legal liability. This is especially hard if you sell an engine which is relatively easy to mismanage - if you fly mine (IO540) at 100% power 1000ft above the sea, leaned to 50F ROP, for half an hour, doing slow climbs and such, it will be scrap, 25 grand just like that. And it happens. People crack cylinders, lose camshafts, you name it. And they often try to go after Lyco, especially during the warranty. So anything Lyco do has to be ultra defensive, but providing they don't change their manuals they are OK. This makes a mockery of anything whatsoever they say on the subject.

Their quality control is certainly crap. Recently they had loads of crankshaft failures because the subcontractor who makes their cranks didn't heat treat them properly. I have long tables of engine and crank serial numbers which were recalled.

I am not saying that a big Lyco is inefficient in converting fuel into power, BTW, once set up in the operating point for which it was designed: level cruise at some 60-70% of rated max power.

Anyway there is no alternative. Some people in Europe are playing around with diesels.

[BEagle]

Well said that man! These ancient combine harvester engines are an absolute scandal in the 21st century!

A decent modern car engine with its FADEC, drive-by-wire throttle and high construction quality is light years ahead of the IO-360. But Lycoming has us all by the nuts.....

It should be very simple to control power, mixture, rpm, ignition and every other parameter of a typical v-p prop equipped aero engine without suffering from inefficient combustion, overheating, carburettor icing, magneto faults etc with a decent dual-redundant high speed digital computer.

[IO540]

Beagle,

I bet you that a FADEC controller on a big Lyco would set the mixture to the same point that you set it to :O

There is no way to run an IO540 for example at the rated max 250HP while climbing at Vy, at peak EGT. No matter how you play with the mixture, the ign timing, whatever, the thing will still overheat because there isn't enough airflow through the engine and the oil cooler, and the oil flow rate wouldn't be enough to do it even if the oil cooler was huge.

In low level cruise, the FADEC would set the power to 65%, the RPM to 2300, the MP to 23", the flow to 11-12GPH - just like you would do manually if you knew how to do it. It could play with the ign timing; that's about it. How much difference would it make to economy? It certainly would make flying simpler.

These engines have their legacy in wartime design practices: low weight, and service intervals don't matter much. How long does a RR Merlin run for? AIUI they only just made it from one battle to the next, or (nowadays) from one airshow to the next.

[Jhieminga]

John Deakin's Columns (http://www.avweb.com/news/columns/182146-1.html) are available here. Number 18 tells you a lot about mixture, but to get the whole picture you'll need to do some serious reading there.

[BEagle]

IO540 - probably dashed unpatriotic of me, but surely 'their' DB601 with its variable blower and fuel injection was rather a superior design to the RR Merlin?

[dirkdj]

IO540,

the highest CHT occurs at about 50-40°F ROP. This is also the highest exhaust valve temperature point. Your exhaust valves will be cooler at peak EGT than at 50° ROP.

EGT is a completely phony number only to be used in a relative to peak mode. The absolute EGT value is meaningless, the gases coming out of the exhaust valve are much hotter than that, but only one cycle in four is measured, to EGT is a 1-in-4 average of some meaningless temperature.

Most uninformed pilots/instructors/mechanics will talk about 'too lean' when they mean 'not rich enough or not lean enough' since they have never seen the back side of the EGT curve.

Most important for long engine life is CHT, a direct reflection of what is going on inside the cylinder.

Too rich at too high power level means: highest internal combustion pressure closer to the top dead center (very lean or very rich mixtures burn slower and thus fall farther from TDC where the piston is going down more and compression is less).

John Deakin's texts are unsurpassed on this subject.

[Confabulous]

Dear God (the aviation god I mean) I'm going to have some fun with John Deakin's column - there's 12 hours of reading in there!

Thanks for the link.

PS: I've heard that it's easily possible to run LOP on a Lyc or Cont at low power - say 45% in cruise (i.e Cirrus or similiar) - is this true?

[dirkdj]

Confabulous,

If the engine will run smoothly LOP depends on the fuel/air ratio between the cylinders.

It will not hurt anything to try to run LOP if you are at 65% power or below. Find out if the engine will operate smoothly, it may even run so smoothly that it will stop running if very far LOP (this is normal, engines require some fuel to run).

Gamijectors correct a known F/A imbalance between the rear, middle and front cylinder rows in a six cylinder flat engine.

Savings of 15% or more are easily obtained LOP with minimal loss of airspeed.

Having a fuel injected engine with an all-cylinder engine monitor is a good starting point in the world of LOP.

Even some carbureted engines may run smoothly LOP, some require careful use of carb heat to better vaporize fuel droplets.

[IO540]

dirkdj

I agree 100%. I would however add

- you will save a load more than 15% going LOP, compared with mixture fully forward like every good ole PPL instructor has always told ya :O More like 30%.

- The fuel saving at LOP relative to peak EGT is less than 15%; probably ~ 5%, perhaps a few % more if you are able to fully open the throttle as a result of the reduced fuel flow rate (less pumping losses)

- not sure about the carb heat helping here... slick engines would have a carb per cylinder and one tweaked them individually.

- everybody should get a multicylinder engine monitor :O

- GAMIs correct the power imbalance regardless of how it is caused. You do a test flight and you send the data to GAMI and they send you the injectors matched for the engine, so they correct for all causes in one go.

[ShyTorque]

An exhaust valve gets cooled by contact with the valve seat. A slow burning mixture (weak mixture) may still be in the process of combustion when the exhaust valve begins to open, therefore causing power loss and valve / seat burning.

[White Bear]

Confabulous is right; it will take a while to read all those articles!
Thanks for the replies guys, especially IO540 and dirkdj, what a wealth of knowledge out there.
Just for the record I have Lycoming IO 360, turning a 3 bladed McCauley prop in a Cardinal.
Regards,
W.B.

[dirkdj]

ShyTorque,

Only the first part of your statement is true.

I have run a TSIO360 so far lean of peak that a secondary EGT rise was visible. This was due to the gases still burning with the exhaust valve already open. It was completely harmless, the engine was so far lean of peak that it was hardly developing any power. Lowest BFSC occurs at 40°F LOP and I was probably at 90° or more so there was no advantage on range. It just proved that this engine was particulary well balanced and that each cylinder was developing exactly the same amount of horsepower.

This is by the way the reason most engines, as they come from the factory, will not run smoothly LOP unless some 'work' is done (usually balancing the injectors). The power curve falls rapidly when LOP and if some cylinders are more LOP than others, then this will be the cause of the vibration.

Valve burning occurs when there is a valve leak during the time the valve is supposed to be closed, not in the case noted above. When internal cylinder pressures are highest then any ill fitting valve will start to leak hot gases and will eventually burn. No pilot operating technique can save this valve once the process starts. Blame the manufacturer not the pilot.

[ShyTorque]

"Valve burning occurs when there is a valve leak during the time the valve is supposed to be closed, not in the case noted above. When internal cylinder pressures are highest then any ill fitting valve will start to leak hot gases and will eventually burn. No pilot operating technique can save this valve once the process starts. Blame the manufacturer not the pilot."

How does the valve begin to leak? Uness there was an initial manufacturing fault, such as inefficient valve to seat seal or over-loose guide, which would be picked up by a compression check, it must be caused by erosion of either the valve edge itself or the valve seat. Over leaning can contribute to this in a major way as the more heat goes into the valve which can actually cause guide wear so there is a viscious circle. I think the best advice is that the engine manufacturer's advice must be followed for long engine life. Experiment outside that at your own risk.

[dirkdj]

A valve begins to leak because heat is not carried away properly to the valve guide and to the valve seat. This is purely a manufacturing issue.

Old style (pre 1991) TCM cylinders had post-reamed valve guides: the icecold guides were first inserted in the hot cylinder heads. After cooling they took a certain 'set' and were then post-reamed to the exact size and concentricity. This concentricity is of utmost importance to achieve a good valve to valveseat fit. This fit is responsible for a large amount of valve cooling.

Also important is the width of this valve seat.

In order to speed up production and save a few beans, in 1991 someone at TCM changed manufacturing methods and pre-reamed the valve guides (much more efficient manufacturing) before crimping them in the cylinder heads. Concentricity would then be a matter of luck and not of design.

Result: very few cylinders made it past 600 hours before needing some kind of work, where older cylinders would run to TBO mostly without any problem. I personally ran 2 IO520's to TBO with only one burned exhaust valve. The cylinders on my friends Mooney 231 lasted no more than about 200 hours since new. Date of manufacture of the cylinder is more important than anything else.

Of course, if you call the manufacturer and tell them you have problems with their cylinders after only 200 hours, they will try to blame the pilots flying techniques. This may have worked 20 years ago, but thanks to the internet todays pilots and consumers are better informed than before.

Engine analysers with data recording show exactly what happened and what happens.

[ShyTorque]

So, bearing in mind that the valves and seats aren't perhaps as hardy as in years gone by (I'm not saying you're wrong, there are similar issues with older automotive engines and modern fuels) the pilot ought to be prepared to moderate his leaning technique to preserve the engine?

AVGAS is expensive, but not as expensive as an engine overhaul.

[OVC002]

I believe that dirkdj was referring to Mr Deakins article which can be read here :

http://www.avweb.com/news/columns/182155-1.html

He specifically points out that leaning technique is irrelevant as far as frying valves is concerned. The important factors are

1. Poor manufacturing methods

2. CHT's

imho it makes sense

[dirkdj]

ShyTorque:

"So, bearing in mind that the valves and seats aren't perhaps as hardy as in years gone by (I'm not saying you're wrong, there are similar issues with older automotive engines and modern fuels) the pilot ought to be prepared to moderate his leaning technique to preserve the engine?

AVGAS is expensive, but not as expensive as an engine overhaul."

Aha, were getting there.

I don't think the metallurgy of valves has changed, nor has the fuel. Lead has been reduced from 100-130 to 100LL Avgas levels. lead has no other function than to delay detonation by slowing down the combustion process.

The best favour a pilot can do to his engine is learn how it works (same thing can be said about men or women, depending on whose side you're on).

The one thing an engine hates (if it could speak) is high internal combustion pressures. These can come when high power is used and the mixture is not rich enough or not lean enough.

A lean mixture (meaning lean of peak EGT) burns slower and the peak pressure pulse will fall later in the downgoing stroke of the piston where compression (and heat) decrease rapidly. At high power a very rich mixture also will work and requires less attention from the pilot since he just shoves the mixture full forward. The worst case is a mixture about 50° ROP at high power, this is the fastest burning mixture giving the highest CHT and highest ICP. Think of striking the top of the piston with a sledgehammer when ROP, versus a rubber hammer when LOP.

Trouble is most manufacturers POH's are first written by engineers, then rewritten by the marketing people and then rewritten by the legal department. Dont' look there for correct information (with some exceptions).

For example, the latest greatest from Wichita could run nicely LOP but that would cost a few knots to the marketing dept. Running properly rich of peak at high power takes far too much fuel thus cutting the range too much. So the marketing dept 'invents' a setting of 50° ROP to get maximum range on highest speed with complete disregard to engine health.

50° ROP is exactly the setting where ICP is highest. If it runs smoother at that setting versus LOP that only means the F/A ratios are not matched. This can be fixed on most engines.

As for Avgas being cheaper than engines, I have calculated that running LOP on most flights (except take-off and climb) I save enough money to practically pay for the overhaul of my IO520BB just with the fuel savings. My oils stays very clean, never have a fouled spark plug, my CHTs run around 300°F with a self imposed limit of 380°F on my JPI alarm (never reached even in summer). I loose about 3-4 knots of potential airspeed, probably less than 2 minutes on most flights.

I had Gamijectors set number 23 (in 1997 or so) and have flown LOP ever since, about 800 hours on my aircraft alone.

[ShyTorque]

"A lean mixture (meaning lean of peak EGT) burns slower and the peak pressure pulse will fall later in the downgoing stroke of the piston where compression (and heat) decrease rapidly."

I agree with the first part of that, but the last part doesn't actually support your argument.

The working gas temperature cools by expansion. If, for whatever reason, such as occurs with a slow-burning mixture, the peak temperature (therefore pressure) occurs later in the power stroke, less gas expansion is possible before the piston reaches BDC and the exhaust valve is open. The "spent" gas temperature therefore remains higher and the exhaust valve gets gets exposed to greater heat as the gas leaves the cylinder on the exhaust stroke.

For the same reason, a high output engine with an increased compression ratio can actually have a lower EGT than a standard engine of the same type. The engine working gases have a higher expansion ratio (and hence more fuel efficiency because more work has been absorbed by the piston for the same fuel input).

The downside of a HC engine is that fuel quality needs to be higher to prevent detonation, especially in the relatively large cylinders of a typical slow revving aero engine. Detonation prevention is why the lead content remains in AVGAS, as you say, it's not to do with the valve seats.

Here is another article, based on experimental data, which found that leaner mixtures resulted in higher exhaust valve temperatures. It also supports your points about manufacturing tolerances, valve seat widths etc, btw. It found that CHT (read rear spark plug bushing in this case) was not a good indication of exhaust valve temperature.

http://naca.larc.nasa.gov/reports/1943/naca-report-754/

[Confabulous]

Shytorque,

You make some very good points, however you seem to be content to repeat the old wive's tales that have been debunked by GAMI and Deakin - the old 'AVGAS is cheaper then enigne overhaul' stuff - just take a look at what dirk said
As for Avgas being cheaper than engines, I have calculated that running LOP on most flights (except take-off and climb) I save enough money to practically pay for the overhaul of my IO520BB just with the fuel savings.

A free engine overhaul, just by being attentive to engine data.

With regard to the engine report, it was written in 1943, and methinks things may have changed slightly in that time... actually manufacturing tolerences may have worsened since then. Either way I doubt the data is entirely relevant to today, especially when the measuring equipment might not have been as sophisticated as, say, GAMI's.

Besides, higher EGT would of course be experienced LOP - up to a point. After an arbitrary point, EGT would begin to come back down - because you're beginning to essentially starve the engine of fuel - which is really the way we want it. Everything in a Lyc or Cont is set up to give max power - hence the extra-rich linkage for cooling at max throttle. That's fantastic for takeoff, but for cruise it's unbelievably wasteful - people talk about shock cooling, but it's heat that really damages engines - consider the 450F max CHT's on most engines - alloy strength rapidly decreases above 400F, and while a standard CHT guage gives either an average or one cylinder reading, the other cylinders could be melting or detonating without your consent.

Let's get one thing straight - the more AVGAS we waste, the less will be left in a few year's time - the AVGAS additive (TEB I think??) factory is closing down in 2006... and then where will we be? What's wrong with being environmentally friendly and saving money at the same time?

For a really good, efficient AVGAS engine we need the following:

1. Liquid (not fuel) cooling.
2. 0.05% or less moving part tolerances.
3. Variable electronic ignition.
4. FADEC mixture control - 'real time', not just precalculated.
5. A BSFC of 0.385 or better.

[IO540]

ShyTorque

When you refer to moderating one's leaning, I think you are falling into the same trap as everybody else. At typical cruise power settings, say 65% or below, one cannot over-lean an engine. If one leans too far, it won't develop the desired power, or it will run too rough. But it won't be running too hot.

The danger is in leaning just a little.

But to be fair, without a multicylinder engine monitor I wouldn't know where to start anyway, so I am not in a position to criticise people who fly with the mixture fully rich all the time. The context of this entire thread is for owner pilots who have control over how the plane gets flown, and isn't relevant to typical GA (self fly hire) operations.

At say 100hrs/year, at 40 litres/hour, that's about 4000 quid a year. A decent monitor will pay for itself in two years.

[dirkdj]

ShyTorque,

Some of these things are easier to show on slides.

I am looking at an output from an engine test stand that measures the internal combustion pressure inside the cylinder versus crank angle at different fuel/air ratios (expressed in °F ROP/LOP).

Conditions: full power, wide open throttle, Sea level, Max RPM.
Spark ignition at 20° BTDC,

Situation 1: full rich (250° ROP): from about 10° BTDC sharp rise of the trace, peak at 12° ATDC, maximum of about 750 PSI, drop to 180 PSI level by 70° ATDC.

Situation 2: 75-100°ROP : from about 5° BTDC less sharp rise , peak at 18° ATDC, maximum of about 630 PSI, drop to 180 PSI by 70° ATDC.

Situation 3: Peak EGT: from about 5° BTDC even less sharp rise, peak at 20° ATDC, maximum of 600 PSI, drop to 180 PSI by 70° ATDC.

Situation 4: 50° LOP, from about 5° BTDC still less sharp rise, peak at 21° ATDC, maximum of 540 PSI, drop to 180 PSI by 70° ATDC.

Situation 5: 100° LOP, from about 5° BTDC much flatter rise, peak at 30° ATDC, maximum of 400 PSI, drop to 180 PSI by 70° ATDC.

If you plot this you will see that all traces come together by 70°ATDC while the full rich peak PSI is nearly twice that of 100° LOP, and that richer mixtures have a larger, undesirable area BTDC where the piston is still going up. This negative effect only increases with reduction of RPM since most of us don't fly at 2700 RPM all day I hope.


You are absolutely right that an engine with higher compression ratio will have higher efficiency and lower EGT than a lower compression engine of the same type. I also fly a turbonormalized IO550, it has 8.5 compression and is much more efficient and cooler than the factory TSIO550 with 7.5 compression.


As noted before, sole purpose of lead is to prevent detonation by reducing flame front speed.

The holy grail of non-fixed spark ignition technology is to have the pressure peak at 17° ATDC regardless of mixture or RPM or fuel quality. Then lead can be omitted from the fuel even at high power levels.

Confabulous:

"A free engine overhaul, just by being attentive to engine data."

Here is the data for my engine: flying normal cruise ROP consumes 60 LPH, flying normal LOP is 46 LPH. Couple knots difference in speed.

Engine life with extension: 2040 hours. Current fuel price say 1 GBP per litre.

2040 hours saving 14 litres at 1 GBP is: 28560 GBP.

A good boutique overhaul for my engine would cost probably 33-35000 US$ so there is money to spare. Of course, this calculation doesn\'t allow for take-off and climb but that would be ROP in both cases.

Confabulous:

"Besides, higher EGT would of course be experienced LOP - up to a point. After an arbitrary point, EGT would begin to come back down - because you\'re beginning to essentially starve the engine of fuel - which is really the way we want it"

The EGT curve is like a hill, the front side is the ROP portion, the top is the Peak EGT position, and the backside is the LOP portion. Any leaner mixture than Peak will make the EGT drop.

The CHT peaks about 50°F before the EGT peak.
EGT is a phony temperature, because is it averaged over 4 cycles of the engine, all that counts it the relative position to peak EGT.

[Confabulous]

Dirk,
EGT is a phony temperature, because is it averaged over 4 cycles of the engine, all that counts it the relative position to peak EGT.

True, although EGT is the perfect tool to give accurate LOP leaning - as Deakin said, pilots of big radials used to lean by the colour of the exhaust stack flame - too yellow and it was rich, blue and it was perfect, and if there was no flame you'd just lost an engine! Of course this only worked at night.

I wonder - would it be possible to put a heat-resistant optical sensor in the EG stream, note the colour/brightness of the flame and use that to help with the leaning?

[ShyTorque]

""When you refer to moderating one's leaning, I think you are falling into the same trap as everybody else. At typical cruise power settings, say 65% or below, one cannot over-lean an engine. If one leans too far, it won't develop the desired power, or it will run too rough. But it won't be running too hot.

Not necessarily true. The relevant point is that the "engine" temperature is NOT a reliable guide to the temperature of the exhaust valves. If the mixture is still burning when the exhaust valve opens, then yes, obviously there will be a drop in power. Read the 1943 article again?

[dirkdj]

Confabulous,

Remember what I said about the holy grail etc..

It is becoming true. In certification stage now.

GAMI is developing PRISM, this device will sense the exact peak pressure point via pressure transducers in the special spark plugs made by Champion. The device will then automatically adjust timing of the ignition to make the peak pressure pulse fall in the optimum range.

Pilot can adjust MP, RPM, mixture at will , the system will keep him out of trouble.

I have a recording of a Navajo engine, most detonation sensitive in the GA fleet, running at full power while the fuel is being switched to unleaded! PRISM retards the ignition automatically and no detonation anywhere in sight.

ST,

Explain to me how leaning too far can damage the engine? Leaning not far enough can damage engines for sure.

In the 1943 text I read that spark plug gasket thermocouples are not reliable indicators of exhaust valve temperatures, they had to mount thermocouples on the valve itself. To the best of my limited knowledge exhaust valve temperatures track CHT curves very closely that means hottest at about 40°F ROP .

I also know that a JPI with spark plug gasket thermocouples gives much higher CHT readings than the thermocouples that fit in the bayonet hole in the cylinder.

[ShyTorque]

Explain to me how leaning too far can damage the engine?

The situation I refer to is where a weak mixture causes delayed / extended charge burning time (not necessarily a higher combustion temperature per se, but a delayed rise of peak temperature of the burning charge on the power stroke). The gas is too hot when the exhaust valve opens.

[dirkdj]

ShyTorque,

maybe you are referring to the secondary EGT peak.

I discovered this about 5 years ago on the Mooney 231 that had an exceptionally well balanced engine. I could lean past peak EGT and see the temps fall and fall until they started to rise again. This was in fact the charge still burning with the exhaust valve opening already.

This mixture setting is far beyond anything you would use in practice, very far on the back side of the curve. Power was just enough to sustain flight.

I doubt that the absolute temperature was higher than at peak EGT, I certainly wouldn't worry about this being a factor in engine longevity since it is not a practical setting and probably produces very little heat, it's just that the EGT probe is being mislead by having continous 'fire' in the exhaust pipe rather than only one stroke in four.

EGT probes have an immediate response and are useful 'navigation' instruments on the mixture curve. What really counts is the CHT, it may take minutes to settle but you want ot keep it below 380° on the hottest cylinder. CHT is also a direct reflection of the temperatures of the exhaust valves.

[bookworm]

The paper's interesting, SkyTorque. My conclusions would be:

1) (Fig 8) The exhaust valve temperature (EVT) is peak at about 25 degF ROP EGT. Given that the CHT peaks at 50 degF ROP and that the exhaust gas is by definition peak at peak EGT, I would have thought this is the expected result: The EVT is influenced by both EGT and CHT.

2) (Fig 8) Operating at peak EGT gives the same EVT as 50 degF ROP. Operating at 50 degF LOP gives the same EVT as 150 degF ROP.

3) (Fig 7) The EVT is about 50 degF cooler at 2100 RPM than at 2400 RPM.

4) (Fig 7c) The difference in gradients of the EVT curves shows that the EVT is lower at higher MP and lower RPM than vice versa for the same power.

5) The authors suggest that the valves would probably give more satisfactory service if operated with a leaner mixture [than peak EGT] -- it's implicit I think that they'd also give more satisfactory service if operated with a vastly richer mixture too. But the worst place seems to be 25 degF ROP.

[ShyTorque]

dirkdj,

You have the benefit of a fuel injected engine that has been "balanced" as you put it, regarding providing constant mixture strength across the cylinders, plus you have a method of continuously monitoring the engine operating conditions.

Please bear in mind that balanced mixture strength is not so easy with most carburettor equipped engines - as in most light aircraft, which are still struggling on with ancient technology.

Common advice is that it is easier to tune an engine fitted with a single carburettor than with multiple ones, but from experience I have found this isn't always the case. Gasflow conditions inside the inlet manifold may have a marked effect on mixture distribution, so with a single carb fitted, mixture strength CAN markedly vary between cylinders. There is a simple device called a Colourtune - a transparent spark plug - which is fitted temporarily and allows the actual colour of the burning gases to be used as a guide to setting the correct mixture. It has always surprised me to see how the colour can vary across the cylinders of an inline four.

I know of one case where an identical engine to one of mine (inline four, single carb fitted) was bluing the exhaust pipes on cylinders 1 & 3 on a dyno run - a sure sign of higher EGT on those cylinders. After some thought, relatively minor internal mods were made to the inlet manifold to alter gasflow which cured the pipe bluing and unexpectedly released a measured 25% more power with no other changes.

This is perhaps why aero engine manufacturers are so cautious regarding leaning. It's possible that in some aircraft installations all or some of the cylinders are running at different A/F ratios. The manufacturer will give engineering and operating advice that errs on the safe side, for obvious reasons and this will probably be richer rather than weaker of stoichimetric (detonation being the other major consideration). Adjusting the mixure control outside of this advice might put one or more cylinders in the critical band. Engine instrumentation such as a single CHT or EGT gauge may be insufficient to indicate a distressed component.

It seems very logical to me that if an exhaust valve with an engineered in heat transfer problem is put in the crtitical EGT band, then damage will occur very quickly and a single event of incorrect leaning may cause it.

[dirkdj]

ShyTorque,

I fully agree that carbureted engines can be a nightmare to get balanced. Balanced in this context means that each cylinder reaches peak EGT at the same fuel flow.

It may help some to use a little carb heat in order to ease the vaporization of fuel droplets but carb. engines have atrocious fuel distribution most of the time.

Lazy manufacturing techniques meant that practically no fuel injected engines (as they come from the factory) could run smoothly lean of peak EGT so the advice was given to run ROP.

Now that the technology exists to see data per cylinder and balanced fuel injectors are available, a pilot can have a choice to run LOP or ROP.

What happens is that a lot of uninformed pilots see the EGT needle peak and then enrichen slightly for peace of mind putting them unknowingly in the danger zone slightly ROP up to about 50 °ROP when they should run much richer or much leaner to be safe.

Having an engine analyser will also show if the full rich mixture on take-off is set rich enough: there should be no rise on the CHT with a properly set full rich mixture. More fuel than specified in the TCM manual is required to keep the CHT from rising after take-off. Major contributor to short engine life.

[IO540]

bookwork

Having read the NACA report I agree with your suggestions.

However it conflicts with Deakin who says the EVT correlates much more strongly with the CHT than it does with the EGT. Deakin's knowledge of this comes, I believe, from info he got from GAMI who have done a lot of tests on heavily instrumented engines of the type we are flying with rather than on what appears to be an engine of a very different construction.

A lot must depend on the length and thickness of the valve stem and how much of its length remains in contact with the valve guide, and how well the valve guide insert (which is a different metal to the rest of the cylinder head) fits into the cylinder head. Lycoming have a particularly poor manufacturing technique here, which can result in one value getting a lot better heat transfer than another one in the same engine.

[cblinton@blueyonder.]

A good roundup!!

operating LOP is an option. The engine (with GAMIjectors installed) likes it. It runs efficiently, sees cooler CHTs, and produces less harmful combustion byproducts. ROP is also a viable option where speed is paramount and fuel burn is not a concern. Even so, keep a close eye on CHTs.
I'll be the first to admit that these engine-operating concepts are not as simple as just setting controls at fixed positions and letting the good times roll. I said earlier that the options of ROP and LOP partly depend on one's pocketbook. If engine wear and higher fuel costs are financially acceptable tradeoffs, then LOP engine management may not be for you. If, on the other hand, you would like to have the option of operating LOP, but you feel your lack of knowledge may produce more harm than good – welcome. You're not alone. Lots of people feel that way.
Just as with anything worthwhile in life, you may have to expend a little effort to learn more about your engine to enjoy the benefits. You may also have to contend with the naysayers who will try to convince you you're crazy.
In the end, remember the saying, "Knowledge is power." With a reasonable amount of effort, you will become comfortable with how your actions are affecting your engine internally. You'll actually see the result in your instruments in the form of lower CHTs and lower fuel burns. You'll enjoy favorable oil analysis. And finally, with greater understanding of the pros and cons, you'll be able to determine which option – ROP or LOP – is appropriate for your mission profile today.