Flying over square

[Jabawocky]

Jas, keep reading mate....you almost have this sussed out.

Fuel cooling, sure you can generate some evaporative cooling effect with fuel but not in your engine, pour some on your arm and wave it about, it will feel a fraction cooler than the rest of your arm but bugger all really.

Now for the facts. Going richer does NOT mean fuel is cooling. It means a less violent bang thru a more lethargic mixture.

It has the effect of retarding timing, not the spark timing just the flame front. Thus lower value of PPP and further from TDC. Hence lower CHT, and a bit less power for all that extra fuel.

Remember in our fixed spark timing engines the way we can control the combustion event is by mixture control. In your car, especially with EFI lots of things are done differently. Even the old Holden engines (old for me) had vac advance.


Trent..".no that fuel is burnt. And the reason for that flame spitting you see on many race cars is not relevant to this discussion, as good as I am at thread drift I think we should leave this tread with an educational trail for pilots not rev heads. Even though some of us are both:}

[Trent 972]

Jaba, I know you're a GAMI fan, so perhaps a quote from Bill Cunningham (Powermaster Engines) on the GAMI (http://www.gami.com/articles/frugalflyer.php) website might interest you. CAUTION article contains the words 'unburned fuel', continue at your own risk.

Bill Cunningham, founder of Powermaster engines in Tulsa, Oklahoma, said the key to keeping cylinders healthy is keeping them cool. That can be accomplished with a rich mixture setting that pours unburned fuel through them, or a very lean setting that restricts fuel and creates less heat in combustion.
;)
ps. evaporation only happens with a difference in humidities. Pour petrol on your skin while you're standing in the rain and you will be wet and have a higher calorific value, but not cooler. :E
Unburned fuel molecules (denser than air molecules) absorb heat from the combustion and transport that heat through the exhaust process.
At least that was what I was taught in my OWT classes.

[Wallsofchina]

Right Clinton. When the engine is leaned, there is less unburnt fuel and the combustion chamber is hotter. Check a spark plug and you'll see it is white ash under those conditions. Lean produces maximum power but it is a very fine balance between power and damage. Rich produces less power but gives more reliability due to the fuel cooling the combustion area. Check a spark plug and you'll see a film of unburnt black carbon under those conditions.

[Jabawocky]

Clinton you are a funny dude:ok: I did not think you folk down there had a sense of humor, but I was wrong.

Trouble is someone is going to believe that little gem :}


Trent
You are almost king of the wind ups, now being a clever chap, how about you explain to the viewers why that "play on words" almost says but does not quite an OWT.

Think about it this way, is that black soot unburnt? No, it's black and sootie, it is burnt, but maybe not fully combusted. So what is it Mr Cunningham is trying to tell people, especially those who lived a diet of OWT's and believe fuel cools;)

I am sure you will wrk it out.

PS this iPad is great.....just not for debates on pprune:ugh:

[jas24zzk]

Jas, I agree with everything except
Quote:
ROP is a hotter temp than PEAK
Think EGT guage and richening the mixture from peak EGT, the EGT will decrease. That is the unburnt fuel removing heat from the cylinder.

:ugh::ugh::ugh::ugh::ugh:

Freudian slip. WTF was i thinking....lord only knows :{

Unburnt Fuel/not completely burnt fuel (charge) is really just a play/choice of words, meaning pretty much the same thing :)

Rare thing to see todays car's with that lovely fawn coloured deposits in the exhaust. Maybe thats a lead thing.

[Wallsofchina]

Fawn is the Holy Grail on petrol. Thank goodness for computers

[Trent 972]

Jaba, I can see us getting closer (not in a biblical sense though :E)
Black and sooty and sticky, that's why it sticks to the inside of the exhaust. As opposed to light grey (ok, fawn then) and not sooty like fully combusted petrol.
Don't have an iPAD :(, bloody kids think it's too techo for me and won't let me use it.
In my very best 'Pauline' PLEASE EXPLAIN Mr. Cunninghams use of 'unburned fuel'. You tell me that's not possible, remembering GAMI have used the article on their website.

[jas24zzk]

Lean produces maximum power but it is a very fine balance between power and damage. Rich produces less power but gives more reliability due to the fuel cooling the combustion area.

That might need some re-thinking. If LOP is best power, why does you POH show a higher TAS for ROP than it does for LOP??

[WannaBeBiggles]

I must say that I'm really enjoying this thread and some of the associated articles, but thought I'd just post the below quote from the GAMI article for those who have a tendency to just take other peoples advice on face value and then consider it fact.


Not all GA engines can run safely LOP. Carbureted engines, for example, lack precise fuel/air metering systems and typically run rough and lose power LOP. And electronic engine monitors that show cylinder head and exhaust gas temperatures for every cylinder are necessary for safe LOP operations.


I'm by no means arguing the merit of LOP operations, but just wanted to ensure some of the uninitiated don't go running an engine LOP when it is not set up to do so.

As for "flying over square" - the R985 (supercharged radial) I operate does 36.5/2300 @ Max and climbs out at 30/2000, 'nuff said :ok:

[43Inches]

As what Trent said regarding fuels. Fuel air mixture will determine how much of the charge is burnt in the time available during the power stroke. "Peak" EGT is just that, the peak exhaust temperature. If you richen the mixture from that position you gradually flood the cylinder with fuel until you eventually end up with a rich cut, this will result in progressively cooler EGT.


ROP. You are rich, so if fuel is having a cooling effect, it certainly isn't doing it at the point of measurement. ROP is a hotter temp than PEAK. What is happening? the unburnt fuel is being burnt in the exhaust, causing a rise in what the probe is seeing.

Best Power allows a slightly higher density charge to enter the cylinder and therefore produce more power. The excess fuel above peak mixture allows better cooling reducing damage caused by excessive combustion heat at higher power settings.


LOP. Your fuel is lower, so the fire is smaller, so less heat can be generated. You are also in a zone that the oxygen levels permit total burn of the fuel.

Mixture burns slower lean so engine may run slightly hotter, EGT is cooler however. EGT is really a fuel management tool, whats going on in the engine is more reflected in CHT and oil temps.


Peak. You are burning everything max, but you are opening the door to detonation and things like that.

Detonation and preignition can happen at any operating mixture setting due to a number of reasons. At high power and running peak you just dramatically increase the chance of it in a healthy engine. Lycoming actually recomends operating at peak during cruise as long as proper cruise powers are adhered to (less than 75% normally aspirated and less than 65% for super/turbo charged).

[Back Seat Driver]

Last edited by Trent 972; 19th Oct 2011 at 07:49. Reason: will someone let me know if Clinton says something intelligent, because he's still 'ignored' here! :D :D
Not Yet.

[Tankengine]

43 inches,
From post #261:

Shockcooling not a problem within allowed temp ranges?:confused:

While I agree operations outside those ranges are wrong, don't assume no damage can be done inside them!

Glider tugs for instance can crack cylinders very easily if throttle reduced too quickly on descent, a steady reduction of power is needed.[along with help from modern CHT warning systems and cowl flaps can help]:hmm:

[Chimbu chuckles]

When trying to get your head around rich mixtures 'cooling' the engine it might be more productive to think about oxy acetylene torches. Pure acetylene produces a lazy, dirty (sooty) 'cool' flame. As you squeeze the oxygen trigger on the torch you're leaning the mixture and it burns hotter and cleaner. Keep adding oxygen and you get that hot blue flame - peak EGT if you.

That is exactly how a rich mixture 'cools' your engine - it's more correct to say 'produces less heat' than cools.

[Wallsofchina]

jas24zzk

That might need some re-thinking. If LOP is best power, why does you POH show a higher TAS for ROP than it does for LOP??

leaner = hotter, richer = cooler is for the combustion chamber.

ROP rich of peak is for peak exhaust temperature
LOP lean of peak is also for peak exhaust temperature

This might be where some of the disagreements are coming from.

[Aerozepplin]

leaner = hotter, richer = cooler is for the combustion chamber.

Wouldn't that would make LOP operations impossible, because the engine would get too hot?

http://www.warmkessel.com/jr/flying/td/jd/images/pelperch_aps_leaning_chart.jpg (http://www.warmkessel.com/jr/flying/td/jd/images/pelperch_aps_leaning_chart.jpg)

This chart is the similar to those published by Lycoming, Continental, and various regulators.

What damages an engine? Heat.

Quite simply, the data shows that around peak EGT, the cylinder head temperatures, and so the internal pressure (pressure and temperature being directly proportional) are at their maximum. The reason that people keep making a big deal out of all this LOP/ROP thing is that it shocks us that the engine manufacturers give advice that is EXACTLY opposite to what the data suggests.

On other matters: While I'm no expert, I am willing to debate the concept of LOP in a carburetted engine, or without an engine monitor.

I think either is perfectly acceptable.

If you operate where the POH suggests you are already operating at the worst possible place. If you're flying at 50ROP you are producing the highest CHTs, and coupled with a high-ish power setting these may be too high. If you fly at 50ROP at high altitude or with a low setting, then that of course may be fine.

My point is, if you have no idea what your EGTs/CHTs are, and you lean as per the flying school "till the engine is rough, then richen till smooth", you likely have cylinders that are LOP, and some that are ROP. The roughness was the leanest "falling off" the BHP curve as shown in the graph. So if you mixture distribution is good you may have part of your engine LOP anyway! This being without intending to operate beyond peak, because as everyone know, engines explode there.

So you have two options:
Lean as far as possible for fuel economy and to keep your engine clean, and at a sensibly low power setting to ensure that the cylinders close to peak are happy.

Or, fly conservatively rich at whatever setting you want. Noting of course that mixture alters the power produced, and most tables include the condition that best power is set.

These are only my opinions, largely collected from reviewing the data available, so feel free to disagree.

[Wallsofchina]

Aero

Wouldn't that would make LOP operations impossible, because the engine would get too hot?

No, different parts of the engine, different times.

[mcgrath50]

The way I have always thought of it is fire needs fuel, air and heat (ie; spark). At a certain level these give the biggest flame (therefore hottest and most powerful). Any change to fuel or air (you can't change spark in flight) from this certain level will result in a cooler, less powerful and more 'wasteful' flame.

By wasteful I mean something won't be totally burnt at ROP this is fuel and therefore 'dirty' and LOP this is air therefore 'clean'.

Is this a fairly accurate simple summary?

[Jabawocky]

Walls of China and others.....do we need to go through this all again? The hard part is explaining every little detail here in a post on pprune.

I am seriously thinking of start an APS type seminar process like they have in the USA, and I have thought about colaboration with one particular engine shop that has the gear. Other option is bring the boys from APS out, but that is not as easy as it sounds. I have talked to them about it.

So lets get a few things straight, and best you all learn to love this chart, this is not some GAMI produced slant on things (take note Trent) it is universally accepted and has been from day dot. It appears in some of your engine manuals and other engine manufacturer documents.

This one I believe is coutesy of TCM.
http://www.avweb.com/newspics/tcm_mixture_sweep.jpg

And study this one hard too, coutesy of Advanced Pilot Seminars
http://www.beechtalk.com/forums/download/file.php?id=21519&t=1

Some of you guys are really well off the track. A bit of your version of OWT's.

Walls Of China, can I suggest you contact me via PM, I am happy as always to help educate folk but it requires several things not possible on here I have found. Namely a lot of documentation, a long phone call or two, and some desire to go out and prove it to yourself. If you want to see why your leaner = hotter comments above raise my eyebrows I suggest you spend a few bucks and come up here so I can show you.....heck I might even let you do it yourself and watch with your own eyes. ;)

In summary, looking at the TCM graph, a richer mixture is a lethagic mixture (hard to get going bang) and is containing more available BTU's than a leaner one. As you lean you get a mixture more volatile and it goes bang better and quicker giving you a bigger PPP and one closer to TDC, this is when you get more force and x crank distance torque and at a given RPM > Power. You also get a rising CHT. Going richer again just reverses all that and the CHT goes down again. As Chimbu chuck said, its not cooling, its just creating less heat to begin with.

As you lean past peak EGT you start getting a more lethagic mixture again, less of them BTU or calorie thingo's, less PPP and at a crank position further from TDC again, and guess what....dropping CHT's.

If anyone does not get this, I am serious at a cost sharing basis of flight and pay me for my time I will arrange a ad hoc seminar at YCAB, we can all go play....and no I do not think for one minute you will kill my IO540, and you can learn for yourselves. I have all the gear and it may just save your life one day, by knowing what your engine monitor tells you. If we get a small bunch of keen folk together it will be a fun day, or two, and will not cost everyone a fortune. I doubt I will retire on the proceeds, but it will be fun and educational for all of us.

Where is Warbo?......he was kind enough to let me fly his beautiful 185 while we did exactly this. Was a great exercise.:ok:

PM me if you are interested.......and bring your CFI/Gr 3 instructors along too http://www.beechtalk.com/forums/images/smilies/icon_box.gif :}

[Tankengine]

Jaba,
My experience with these engines was a long time ago, [Jets or gliders mostly last 20+ years:E], the only independent cylinder EGT/CHT indicators showed a fair difference between cylinders at some times. Most EGT indicators were only on one cylinder from memory.
Do you lean on the hottest, coolest, richest, leanest, average?:confused:

[43Inches]

On other matters: While I'm no expert, I am willing to debate the concept of LOP in a carburetted engine, or without an engine monitor.



There is no problem with heat at all LOP, if there is you must of exceeded your peak EGT limit on the way there. The problem is uneven distribution of the lean mixture to all cyllinders causing engine roughness and excessive vibration. A float type carby would almost be impossible to get even fuel distribution to all pots, which is why the engine manufacturers advise against it in these aircraft. This has all been covered before.

There is another factor affecting effective leaning of an engine and that is vibration. The smoother you can get the engine to run the longer the engine componants and accesories will last. Too rich there is above normal vibration, and too lean in most engines it will run rough before it cuts out.

Quite simply, the data shows that around peak EGT, the cylinder head temperatures, and so the internal pressure (pressure and temperature being directly proportional) are at their maximum. The reason that people keep making a big deal out of all this LOP/ROP thing is that it shocks us that the engine manufacturers give advice that is EXACTLY opposite to what the data suggests.


The engine manufacturers are quite specific about when an engine should be leaned, what limits apply and how to go about it. Lycoming has some good notes on the whole thing. Lycoming specifies anything other than peak during cruise at appropriate power settings is throwing money away and the engine is designed to run that way. They give you the leaning procedure for peak and best power as well as lean of peak but the latter they discourage in any smaller engine models. The main point they make continuously is that non of their engines should be leaned at high power (above 75% non turbo or abv 65% turbo) unless rich limit roughness is encountered and then only leaned just enough to ensure smooth operation.

[Trent 972]

Lycoming has some good notes on the whole thing.
Quoting 43 inches. Is that pen!s size?
Lycoming Flyer Operations (http://www.lycoming.textron.com/support/tips-advice/key-reprints/pdfs/Key%20Operations.pdf) (4.5MB pdf)

[43Inches]

Two entries in the flyer answer the original question;

page 43; An Explanation Of Power Settings
page 65; A Review of Old Wives Tales

43inches MAP + 2575 RPM = 350hp (ISA, SL) = Happy Chieftain at take-off

[Jabawocky]

Jaba,
My experience with these engines was a long time ago, [Jets or gliders mostly last 20+ yearshttp://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/evil.gif], the only independent cylinder EGT/CHT indicators showed a fair difference between cylinders at some times. Most EGT indicators were only on one cylinder from memory.
Do you lean on the hottest, coolest, richest, leanest, average?:confused:

Most have a CHT on one cylinder and an EGT either on one or at the joint of 3 or 2.

So your question do you lean to???? How would you ever really know? You have no way of telling just from those instruments.

For a friend who recently aquired an RV6 with 0-320 and fixed pitch prop, I was able to calculate by looking at graphs and knowing what fuel flows should be, a suggested engine setting for cruising at 5500-9500. This worked rather well. He reported excellent speeds (faster than 2 x 7's and a 6 on the trip and less fuel burn.) and his CHT was a bit lower than running ROP. So you can assume I guessometrically calculated that spot on :ok:.

The other RV6 with a slower speed and way higher fuel burn must not know what the mixture knob is for.

So you can do it..........but I am not about to start trying to explain it on here, we have enough trouble getting the basics through :hmm:.

43inches
There is no problem with heat at all LOP, if there is you must of exceeded your peak EGT limit on the way there.
What is your peak EGT limit :confused: Except for the TIT in your chieftan, there really is no such thing. Apart from anything else it will vary depending on probe location.

A float type carby would almost be impossible to get even fuel distribution to all pots, yep that is true, however I have managed to get several carby engines LOP, go bac a page or two and take a look at the picture I posted. :ok: Some will, and some won't and some will only if you experiment a heap to get it there.

The main point they make continuously is that non of their engines should be leaned at high power (above 75% non turbo or abv 65% turbo)
And if you had the right equipment installed you could start at 75% and by the time you leaned to say 20LOP you would end up with your EMS saying around 65% power. I have several photos which I have posted in the past showing this.

And from the Lycoming Flyer, I love the way TCM and Lycoming have tip toed backwards on LOP operations over the years, you can see it in the way they write stuff :}, but what is rather disturbing is this little gem. Seems old habbits die hard
In  the 
climb configuration, we recommended full throttle throughout the climb for internal fuel cooling with RPM reductions initially to 3000 RPM and then 2750 RPM for prolonged climb.  :ugh:

[43Inches]

What is your peak EGT limit http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/confused.gif Except for the TIT in your chieftan, there really is no such thing. Apart from anything else it will vary depending on probe location.

Sorry wrote that post far too fast to see that slip. The point was that LOP Heat is not an issue if the engine is operated correctly. LOP will go through peak EGT on the way and if there was any problem with heat it would be at that point. As mixture is progressively leaned from peak all temps are reduced as the graph clearly shows. The limit of leaning beyond peak has nothing to do with heat, its about stable engine operation and effective power output.

The manufacturers write their books based on the lowest common denominator. They stay away from LOP because if they started even giving the slightest hint its ok we all know some pilots who would be sitting 50 LOP full power in the climb with the engine shaking itself off its mounts "But I'm saving fuel!". To their credit they do have a disclaimer there effectively saying that if you do have the right equipment to monitor the engine and know how to use it you may lean to your hearts desire....

[Chimbu chuckles]

I have climbed my Bonanza LOP-was smooth as a baby's bum. The engine doesn't know what phase of flight it's in. As long as you keep your speed up it's fine-of course in a NA aircraft your ROC will be unacceptable by the time you're at 5000', but in a turbo you can climb LOP as high as you like.

[Jabawocky]

Agreed Chimbu Chuckles :ok:

Mind you the retard vehicle will climb at a few hundred feet per minute at 9-11k feet while LOP. :ok:

I am gong to "borrow" a phrase from George Brayly, and folks this is GOLD.

It's not how hard you run your engine, It's how you run your engine hard!

[jas24zzk]

Thinking about Jaba's quote...

The engineering in these things has always amazed me. You get talking to people 'that know' and they scoff at air cooling. (seems to work to me) They are then amazed that we tell them we run em at 75% power till the fuel runs out. (well ok almost)

They squat on the belief that no factory (unmodified) engine can do that, and insist its been modified. :ugh:

You watch them shake their heads in disbelief when you tell em a 540 only makes 300hp or so. Then they really fall over when you tell em the operator (pilot) has control over mixture, normally answered with 'why the **** would they let you do that' :ugh: Some (most) mechanics really should stick to changing oil.

TBH I really think few pilots really understand this stuff, and the attitude of most CPL's doesn't help, as most are too busy studying for the next rating to go take a sidestep for this stuff (and no not trying to wind anyone up), but surely they could ensure the student knows where to get it. If it was taught properly at PPL level there wouldn't be any OWT's floating about. Maybe the schools should look to appoint someone to teach this stuff as a specialist...from the get go.

If it's not addressed, and it has to be, as The info I see and hear too often, says that what is being taught is getting worse. Then engines are going to suffer, and owners will decide to either opt out, or decide that its cheaper to fit a FADEC, and the knowledge will become just another victory for the IT nerds.

Jas

[Jabawocky]

Good point about the FADEC there jas, the problem is the old mixture cable is more reliable in a mechanical sense. I am an automation person, heck it is what I do for a living, but automation relies on good inputs either digital or analog.

If I have an egt probe fail in some way, or if in fact they all do, I can still operate LOP just as the FADEC does however if it starts to lose a few sensory inputs it has no idea what to do and has to default to some preset performance.

Give me a properly tuned IO-XXX with manual controls and an EMS any day ov a FADEC.

I started to read that Lycoming Flyer closer last night, I read one paragraph out to Mrs Jaba and even she was shocked:eek:, so my task apart from work today is go through it and highlight those little GEM's that are lurking in there as a result of a re-write that missed a lot of OWT's . They went to great trouble to dispell the myth of over square, but left so much behind.

PS jas that was not my quote.....it was George Brayly. :ok:

[43Inches]

Interesting to note that the Lycoming recomended procedure for economy leaning a normally aspirated engine will result in LOP operation in a good engine. The procedure requires mixture to be leaned until rough or significant loss of power occurs and then enrichen only sufficiently to regain smooth reasonable power operation. If the engine is able to run smoothly LOP then this procedure will result in LOP operation.

[Wallsofchina]

Sit still Jabba, you're a moving target. Let's just stay with FADEC for the exercise. I decided to do some reading and found good support for LOP and long engine life in a number of reports. Didn't find any recommendation from Continental, and didn't find any empirical evidence for it. The Gami site firmly supported it, and although they are selling injectors, made sense in what they said. They gave an example of a Cirrus SR22 with Continental O-550N engine and FADEC (Full Authority Digital Engine Control) of 2200 hours against the only stated TBO I could find of 1700 hours.
That's one single case, but I read a lot of similar claims where the consensus was that with fuel injection, telemetry and control like FADEC the engine could be managed for a satisfactory life.

Bear in mind, with the pressure of reducing emissions such as cancer causing Nox and Particulates, engine manufacturers are using digital technology to do different things at different times of the stroke including multiple injector shots at different times with the intent of burning non combusted items etc. and also to allow operation at leaner mixtures to substantially reduce fuel consumption because of the ever growing cost of fuel.

Add all this together and LOP makes a lot of sense.

What DIDN'T have much support was running LOP on a carburetted engine because there is a lot less control accuracy, and the power output from each cylinder will be different, so you don't really know what you are doing to the engine with the settings you are using. Flow balancing, where the inlet side of the engine is modified so all cylinders produce the same power, is a way to get closer, but if you really want the reduced fuel consumption maybe the solution is to run the engine out then upgrade to the newer technology.

Maybe I shouldn't have jumped in with basic engine principles because this thread was all about operating an engine at reduced power and lower rpm compared to the full power output situation.

However, some of the comments strayed away from peak exhaust gas temperature and into the combustion chamber itself.

The Chart which Jabba attributed to TCP shows up all over the place, and is part of a powerpoint presentation called gregs_show, and shows a Cylinder Head Temperature curve related to Exhaust Gas Temperature curve in a typical less-than-full power manipulation of air/fuel ratio.

I agree, if I came up Jabba, you could show me how you could replicate this, but then I could show you how I could melt your pistons, because I've melted a few.

The relatively even cylinder head temperature you see on a gauge is the net result of a number of temperature changes dancing around the combustion chamber at various points in the cycle.

My original comments about lean being hotter and rich being cooler relates to gas temperature in the chamber in response to Clint's comment, and your #262 and #264.

The chemical and physical properties of air don't change. When it's compressed it heats, when it expands it cools.

On the compression stroke, intake air is compressed and heats to a peak at top dead centre. The flame front then kicks in and there is a momentary second rise followed by a rapid fall as the gas expands.

If the mixture is too lean, it's all burnt and there is nothing left to cool the second rise, so it doesn't take long for any protrusions such as spark plug electrodes, piston edges and valves to start to deteriorate or melt.

If the mixture is rich, there is a flow of unburnt fuel over these items, cooling them.

Since gasolene has a very narrow band of tolerance for combustion, and doesn't have to be very rich for the engine to run rough, Jabba as you've found you don't see it dripping out the exhaust.

To make this easier to understand, let's look at methanol where the engine will make full power with an extremely rich mixture. Only a fraction of this is burnt, and the methanol is virtually like a garden hose turned on your engine.

I'm able to make an aircooled engine pump out maximum power reliably in an enclosed compartment with no vents, and if you look at a Top fueler dragster you'll see methanol streaming out at low rpm (it's turned into a gas at full power)

So in discussing the subject of this thread, the theory needs to stay out in the exhaust temp, cruise power, and mixture control area.

[Jabawocky]

Interesting to note that the Lycoming recomended procedure for economy leaning a normally aspirated engine will result in LOP operation in a good engine. The procedure requires mixture to be leaned until rough or significant loss of power occurs and then enrichen only sufficiently to regain smooth reasonable power operation. If the engine is able to run smoothly LOP then this procedure will result in LOP operation.

43inches

There is a broard sweeping statement in all of this. One first of all has to assume that the injectors are well balanced. I flew an IO540 recently, straight out of the box, genuine Lycoming, and bugger me if the peak egt from 1-6 all happened in under 0.2GPH, it went LOP no trouble at all sorts of power settings.

On the other hand.....mine was a PIG, I could not get LOP without it being rough. We had to do no end of tuning to get it right and the fuel flows per injector vary a lot.

So if you had my engine out of the box and followed the Lycoming suggestion.....where exactly do you think you would be? :ooh: Yep.....

The more we discuss this the more there is a lack of education out there. And it is no wonder when you read these articles and they say"giving pilots too much information is not always a good thing" well not giving enough has been the whole problem from the start. :ugh:

[Jabawocky]

Walls of China,

Seems you have had a quick lesson in this stuff, but the comments here
That's one single case, but I read a lot of similar claims where the consensus was that with fuel injection, telemetry and control like FADEC the engine could be managed for a satisfactory life.
....tells me you have many many more hours of research to go.

Add all this together and LOP makes a lot of sense. Really now.......again I suggest keep doing the homework, you have only just scratched the surface.

What DIDN'T have much support was running LOP on a carburetted engine because there is a lot less control accuracy, and the power output from each cylinder will be different, so you don't really know what you are doing to the engine with the settings you are using.
Really..... I knew EXACTLY what I was doing. The reason there is not much support for it is its pretty damned hard to get a carby engine LOP....this is true, but armed with the CORRECT knowledge and instrumentation and if you are lucky enough to have an engine that will comply, it can be done. You just can not expect average Joe out there to do it. Unfortunately.

I agree, if I came up Jabba, you could show me how you could replicate this, but then I could show you how I could melt your pistons, because I've melted a few.
Ohhh really, well I am very capable of showing myself that too. detonation is rather easy to induce at takeoff power, due to a mixture cable pulling back due to thrust, we had the mixture arm cracking back a bit on takeoff, if we did not have an EMS we would have flown like this for the last 500 hours, because a random single CHT probe would have shown nothing. We would have problems any time now.

So for everyones sake what engine and airframe and under what conditions did you melt those pistons? Would make for interesting reading. :ok:

If the mixture is too lean, it's all burnt and there is nothing left to cool the second rise, so it doesn't take long for any protrusions such as spark plug electrodes, piston edges and valves to start to deteriorate or melt.

If the mixture is rich, there is a flow of unburnt fuel over these items, cooling them.

I think you are misguided in the belief fuel inside a cylinder is doing any such cooling of any signifigance. Maybe a bit more study at the GAMI engine test cell would help here.

[jas24zzk]

Quote:
If the mixture is too lean, it's all burnt and there is nothing left to cool the second rise, so it doesn't take long for any protrusions such as spark plug electrodes, piston edges and valves to start to deteriorate or melt.

If the mixture is rich, there is a flow of unburnt fuel over these items, cooling them.

I think you are misguided in the belief fuel inside a cylinder is doing any such cooling of any signifigance. Maybe a bit more study at the GAMI engine test cell would help here.


From all the readings linked here, and few extra's I have found on my own, i came to the conclusion, that if you have enough fuel coming in to affect any sort of cooling, you are going to have so much fuel coming in, you are going to foul spark plugs.....if the engine will run at all.

For an injected engine, even tho the the run into the combustion chamber is a short one, the fuel temp won't be too far behind the air temperature. It has already absorbed the latent heat coming through the FCU, Lines and injectors.

Cheers
Jas

[Trent 972]

You may be doing a disservice to your argument by discounting the effect of unburnt fuel.
I googled "unburnt fuel effect on cylinder temperature" and found some of the hits to be interesting, (This thread returned the #1 hit :ok:) certainly enough info to provoke thought.
The University of Wisconsin, Engine Research Center (http://www.erc.wisc.edu/) paper entitled
International Multidimensional Engine Modeling User’s Group Meeting 2004, Detroit, MI (http://www.erc.wisc.edu/documents/8-Zhang_Kung_Haworth.pdf) says in part

Unburned fuel is a non-negligible contribution to UHC (Unburned Hydro Carbons) only in cases with high swirl or where a TRLC (Top-Ring-Land Crevice) has been considered.

Anything that passes through the ignition sequence inside the cylinder, that has a lower temperature of the burnt fuel (approx 1400 deg C) will absorb some of that heat and transport it through the exhaust cycle.

ps. This is not a reply to Clinton, because I can't see what he wrote. still :E

[Wallsofchina]

Jabba, I'm not enthusiastic about studying more on this subject because I'm only a little Cherokee driver who was taught to warn passengers not to touch the red knob.

I was more interested in trying to explain base engine technology.

Source: Wikipedia, Top Fuellers

The block is machined from a piece of forged aluminium. It has press-fitted, ductile iron liners. There are no water passages in the block, which adds considerable strength and stiffness. The engine is cooled by the incoming air/fuel mixture. Like the original Hemi, the racing cylinder block has a long skirt (to reduce piston "rocking" at the lower limit of piston travel). There are five main bearing caps, which are fastened with aircraft-standard-rated steel studs, with additional reinforcing main studs and side bolts. There are three approved suppliers of these custom blocks.
The cylinder heads are machined from aluminum billets. As such, they, too, lack water jackets and rely entirely on the incoming air/fuel mixture for their cooling. The original Chrysler design of two large valves per cylinder is used.


Jas, on petrol engines fuel doesn't squirt out the exhaust pipe or flood the apron, but it's coming out as a rapidly expanding rapidly evaporating gas.

If yo want to discuss the thread subject, staying on the exhaust temperature is good - by that stage the gas has expanded, and it has evaporated so you don't have to get all excited or teary about cooling/not cooling. Its a different set of numbers in the exhaust.

[Jabawocky]

Fine, then keep out of the productive discussion then. :ugh:

Strange how you do not answer questions real well. I'll assume those pistons were in a drag engine, and while I have little to none first hand experience with them I have enough to know they are way different to what we are playing with here.

So enough of the distracting antics, and if Trent wants to push the little bit of unburnt fuel cools theory, fine, but let mea sk this question. If you move from beyond 50ROP the CHT will drop, so between 50 and 125ROP the CHT is dropping but how much fuel is unburnt, and how much heat is that tiny amount taking with it?

Seriously its two tenths of three sixteenths of farkhall. The only way it could really make a big difference is if it was so rich it was about to quit, and then the CHT would be down due to lack of power. If it made 1 degree F that would be a surprise.

So regardless of the micro arguments, its seems that the bulk of piston operations are done with lack of knowledge and instruments. :ooh:

[Aerozepplin]

nobody knows

Exactly!

That's why in my opinion the "don't LOP with a carb or without an engine monitor" idea is false. If you lean till rough, then richen a little, you're already running a cylinder(s) lean of peak. If you enrichen a little bit more, you've now moved the LOP cylinder(s) closer to ROP, and so hotter. Of course, the cylinder(s) that were already at peak or just ROP will now be even more ROP, so slightly cooler.

So, if you have a carb and/or no monitor, you don't know where your engine is running. So my advice to students is lean as far as possible for fuel economy and a cleaner engine, and fly at a sensible power setting for your density altitude, so that the engine stays cool. That does mean that some of your engine is at the worst place, ie 50ROP, but what options do you have?

The only other option is to fly with a very conservatively rich mixture, and whatever power setting you feel like.

[Trent 972]

Jaba, So enough of the distracting antics, and if Trent wants to push the little bit of unburnt fuel cools theory, fine, but let mea sk this question. If you move from beyond 50ROP the CHT will drop, so between 50 and 125ROP the CHT is dropping but how much fuel is unburnt, and how much heat is that tiny amount taking with it?

I can't answer that question.
Jaba asks- How much heat is that tiny amount taking with it? 75 degress worth!
I don't know the percentage of fuel not burnt, but if in stable cruise and the only change that is made is to enrichen the mixture enough to decrease the EGT's by 75C degrees (from 50C ROP to 125C ROP(your figures)), then as a consequence the CHT's will slowly decrease, as a function of external cooling and due to less heat being applied from the inside at a rate that would differ for each individual installation and circumstance.
Nothing else has changed therefore the addition of the extra fuel has the ability to drop the EGT by 75 degrees or even more.
Now you tell me that a lower EGT will do anything other than lower the CHT in a constant cruise config.
You stated earlier that fuel cannot pass through the ignition process and remain unburned. Well that's wrong. I'm not a thermodynamist but I know enough to know that.

[Jabawocky]

So my advice to students is lean as far as possible for fuel economy and a cleaner engine, and fly at a sensible power setting for your density altitude, so that the engine stays cool. That does mean that some of your engine is at the worst place, ie 50ROP, but what options do you have?

Aerozep, for the bulk of teh O-320 or O-360 type fleet, once at a suitable power setting around 65-70% lean until you get either the fuel flow or the speed you want. You will not harm anything. By the time you get to peak or just past it you will be fine anyway. Of course doing this at full combat power down low is not such a smart idea :ooh:.

Here is a sample of numbers that work for an O-320 and fixed pitch prop RV6.---Yes a proper plane with the little wheel on the back :p
This is not what I recomend you all try to use....I know this worked in this machine. But you might start to see how the theory works, even with a carby.

Basically if you set 75% power and lean LOP you will have 65%.

What is 65%, well it should give you about 156KTAS.

So here is some rule of thumb calculations I have done for you.

Take off and lean to target EGT in the climb.

Under 3000 feet set 2100RPM or less for econ cruise. Lean to LOP.

If cruising at 4000 or higher continue the climb as per target EGT method.

To achieve 65% power LOP use these numbers,

4000/5000 feet = 2100 RPM
6000’ = 2250 RPM
7000’ = 2350 RPM
8000’ and above….probably full throttle and lean for best power or economy, use your TAS and make small changes. If your TAS is around 156-160 you are doing it right.

For fuel planning purposes, take off assuming you get full RPM would be 60LPH, but you do not, so I reckon its no more than 54LPH maximum.

65% LOP fuel flow will be around 25-26 LPH.

If you run way rich…….and full bore……anywhere up to 45+ LPH

[Trent 972]

Clinton, the sight of your last two postings, picqued my interest enough to have a look, and you offer this unattributed quote, but none the less, I don't disagree.
The truth is that a mixture just slightly richer (maybe 25-50F ROP) than a chemically-correct mixture burns the fastest, while the flame front slows down if the flame front is EITHER richer.....
However, would you like to explain to me what becomes of the fuel/ air mixture that is not reached by the 'slowed down flame front', during the fraction of a second it is present in the cylinder.
I'll give you a hint.
If the flame front does not reach that portion of unburnt fuel, then it does not reach its ignition temperature and is ejected into the exhuast manifold in gaseous form as unburnt hydro carbon. Not as liquid petrol falling from the exhaust pipe, as some wits on here seem to think.
I'll look forward to your next post, as I was quite surprised your last one was a lot better than your previous. We're all learning.

[Jabawocky]

Trent, one of us is not reading the others posts properly, it may be me but lets look at what I said and what you just said. I think we said the same thing, except I must point out I am talking deg F not C as F seems to be the norm.

I don't know the percentage of fuel not burnt, but if in stable cruise and the only change that is made is to enrichen the mixture enough to decrease the EGT's by 75F degrees (from 50F ROP to 125F ROP(your figures)), then as a consequence the CHT's will slowly decrease, as a function of external cooling (air through the fins) and due to less heat being applied from the inside Yep thats what I have said all along) at a rate that would differ for each individual installation and circumstance.

So what is your point of difference? This??


Nothing else has changed therefore the addition of the extra fuel has the ability to drop the EGT by 75 degrees or even more. Yep and it does it by changing the combustion event, not by some latent heat effect.

I think Chimbu Chuckles summed this up just recently....here are his words
When trying to get your head around rich mixtures 'cooling' the engine it might be more productive to think about oxy acetylene torches. Pure acetylene produces a lazy, dirty (sooty) 'cool' flame. As you squeeze the oxygen trigger on the torch you're leaning the mixture and it burns hotter and cleaner. Keep adding oxygen and you get that hot blue flame - peak EGT if you.

That is exactly how a rich mixture 'cools' your engine - it's more correct to say 'produces less heat' than cools.


And what were you trying to say here, my apologies but I do not follow you here.Now you tell me that a lower EGT will do anything other than lower the CHT in a constant cruise config.

And thisone maybe needs a better explanation, but again we are aguing over nothing:ugh: it makes no difference whatsoever to the reality of things.You stated earlier that fuel cannot pass through the ignition process and remain unburned. Well that's wrong. I'm not a thermodynamist but I know enough to know that.
NOT FULLY COMBUSTED, is that better? The fuel molecules that are not fully combusted. Just like throw a log on the fire, pull it out after 30 minutes its burned, but it is far from fully combusted. Either way whatevber is left is doing bugger all cooling. Unless someone can show me signifigant cooling from not fully combusted fuel, we can therfore assume that it is irrelevant............does this feel like mythbusters ;)

[Trent 972]

Degrees F or Degrees C doesn't matter really. I fly Euro Trash and you fly stuff Noah bought over from America with his ark. :ENOT FULLY COMBUSTED, is that better? The fuel molecules that are not fully combusted.
There is no such thing as a 'not fully combusted fuel molecule'.
A burnt hydro carbon (fuel molecule) has an altered chemical state and presents as CO2 and H2O.
If you disagree, don't tell these people (http://www.dynesystems.com/gas-analyzer.htm) because they spent a lot of money designing and building an instrument to measure it. (Hydro Carbon)
Wouldn't it be dumb if they built a machine to measure something that doesn't exist.
Definition for HC
A hydrocarbon is any molecule composed of ONLY Hydrogen and Carbon, such as Methane (CH4), ethane (CշH6), ethene (CշH4), ethylene (commonly known as Acetylene (CշHշ)),or benzene (C6H6).

Either way whatevber is left is doing bugger all cooling. Unless someone can show me signifigant cooling from not fully combusted fuel, we can therfore assume that it is irrelevant
It's not irrelevant, it's just that you don't believe in it.

[Jabawocky]

You’re just winding us up, right?

Yes he is and in the process I am going round in circles not making sense either. trent is right, a molecule is burnt and that is it, but a whole F:mad: bunch of them swimming along together then, is that better.

Either way it is irrelevant.

It's not irrelevant, it's just that you don't believe in it.

Ohh yes it is Trent. Go find me proof there is a signifigant Cooling effect, no not a less heat generated, a signifigant cooling from those random bunches of unburnt hydro carbons, and show me how this really is providing serious CHT cooling and I will believe in it.

I can give you lots of proof the Santa came to visit, the cake was missing bar a few crumbs and the Bundy bottle half empty. Proof indeed! :ok:

[Trent 972]

If the flame front does not reach that portion of unburnt fuel, then it does not reach it's ignition temperature and is ejected into the exhuast manifold in gaseous form as unburnt hydro carbon.You cannot seriously believe that a fuel/air mixture that is burning will suddenly stop doing so, just because an exhaust valve opens.Are you asking me or telling me? I didn't say anything about exhaust valves.

Clinton, I provided a link earlier to
The University of Wisconsin, Engine Research Center (http://www.erc.wisc.edu/) paper entitled
International Multidimensional Engine ModelingUser’s Group Meeting 2004, Detroit, MI (http://www.erc.wisc.edu/documents/8-Zhang_Kung_Haworth.pdf) Quote:
Unburned fuel is a non-negligible contribution to UHC (Unburned Hydro Carbons) only in cases with high swirl or where a TRLC (Top-Ring-Land Crevice) has been considered.
Perhaps you can explain to them what they have published has errors of fact.
(That HC's cannot remain unburned on their trip from fuel tank to exhaust pipe exit).What were those fools thinking, anyway.

Jaba, That fat B@stard drinks your rum too?
I don't know how he does the job.

[On eyre]

EGT is basically irrelevant unless you have a turbocharged engine when TIT (turbine inlet temperature) limits may apply.
CHT is the killer and for most carburetted engines is variable between cylinders which is a problem.
In my RV-9A fixed pitch prop and IO-360 with multipoint CHT and EGT indicators I normally just select a power setting I am comfortable with (high RPM if I want to go fast or lower for long range cruise ie 2200 RPM) then lean to give a max CHT of 380 degrees on the hottest cylinder (not much spread between cylinders anyway) and for long range cruise this gives a TAS of 140 kts and fuel flow of 26-27 L/hr - don't worry about EGT at all.

[Trent 972]

Clinton, I'm actually enjoying the discussion. I must be a bit odd.
The NSW RTA (Arseholes) use this Summary of Emission guideline (http://www.infrastructure.gov.au/roads/environment/impact/emission.aspx) for measuring engine exhaust, and according to them HC (which by earlier supplied definition can only be unburnt fuel), is possible and allowed from a petrol engine exhaust, to the specified amount . (Obviously not applicable to aviation ops. Rather just to prove a point).

But let’s assume I’m wrong and a ‘portion of unburnt fuel … is ejected into the exhuast[sic] manifold in gaseous form’

Therefore as per your earlier offer, I will assume you are wrong.

Are you wrong about anything else?
---
Not wishing to hog the debate, I'll add my reply to Jaba and Clints following posts, here.
Talking about some experimental tests on a DIESEL engine using n-heptane as a fuelNot so, it is an international standard (sans spark plug ignition) for data consistancy.HCCI engines (http://en.wikipedia.org/wiki/Homogeneous_charge_compression_ignition) can operate on gasoline, diesel fuel, and most alternative fuels.
Clint, it's all very well to accuse me of being disingenious, but you won't accept proof that petrol can remain unburnt in an engine. BTW I notice you quote the most recent RTA requirements for emission levels. Those would be after the gasses have passed through the catalytic converter, and yet the HC's can still be present even after that. Ho Hum that's all from me folks. Still one of the best threads running ATM IMHO.
ps. I'm not really Santa Claus, I just pay a lot toward his wages.

[jas24zzk]

I don't get it :ugh:

[Jabawocky]

Trent 972 is Santa Claus :eek:

Now I know where all the rum goes :ok:

What are you doing man :ugh:
4.2 Effect of Wall Temperature
The effect of a 50° C reduction in wall
temperatures can be seen by comparing cases
PHI_25 and TWAL_50 (Table 2 and Fig. 2).
Again, the anticipated effects are found with
decreasing wall temperature: a delay in
ignition, lower peak temperature (not shown),
and higher CO and nonfuel UHC emissions.
The influence of TCI is similar to that
discussed in Section 4.1: a small effect on
global thermodynamics, and significant effects
on emissions.


Unburned fuel is a non-negligible
contribution to UnburnedHydroCarbons only in cases with
high swirl or where a TRLC has been
considered.


And what are they doing in this International Multidimensional Engine Modeling User’s Group Meeting ??

Talking about some experimental tests on a DIESEL engine using n-heptane as a fuel.

Come off the grass trent, get back on the straight and narrow. I am over playing games, stick to the real world here please or some Moderator will crack the Sierra Hotel 1 Tango's and lock the thread. Some folk are learning a heap and that would be a shame.

If you want to denate it more, put your money where you keyboard is and go to ADA, next March I think :ok:. I could show you stuff on my airborne dyno rig ;) but I doubt I could convince you there.

Hooroo! :ok:

[Jabawocky]

For those with a mind for education, here are some links I should have put up before.

Discount the fact GAMI sells injectors, these guys know more about the engines we fly behind than anyone on the planet.

General Aviation Modifications, Inc. (http://www.gami.com/articles/bttfpart1.php)

General Aviation Modifications, Inc. (http://www.gami.com/articles/bttfpart2.php)

General Aviation Modifications, Inc. (http://www.gami.com/articles/bttfpart3.php)

General Aviation Modifications, Inc. (http://www.gami.com/articles/frugalflyer.php)

Enjoy!

[Wally Mk2]

I can't believe the amount of coverage this subject has gotten , makes the QF debacle seem like childs play!:) Entertaining I guess the wild theories/stories that abound in here.



Wmk2

[Trent 972]

Solid info Jaba.
That 4th link was posted 51 posts back (#269) by an uneducated fella, user name Trent 972. :ok:

[Jabawocky]

he does do some good work at times.....just have to keep him on topic. :ok:

Jaba has that problem too of course:=

[osmosis]

So, do any of the round engine old pharts know how the auto lean works?

(Sorry folks, I wrote that late last night after consuming two bottles of NZ's finest and shouldn't have been anywhere near a computer.)

[werbil]

Even the non geared R985 Pratt is same, don't let the prop drive the engine, the counter weights get really unhappy.


Unless the counter weights are psychic there is no way they can tell whether the engine is being driven by the prop or by the fuel...

[T28D]

The Auto lean is a balance of pressure input fuel to metered fuel across a modulated diaphragm that is fed sense static pressure as well as the dynamic pressure in the carby throat. 2 diaphragms with a balance beam that ultimately modulates fuel flow to the "injector"

Auto lean only works on those round engines that have pressure carburettors ( also known as throttle bodies) which are actually something between a float carby and inlet injection as practiced by Continental and Lycoming. Older float carburettor engines are a lot triclier to lean.

The series of balanced pressure diaphragms are set up before the supercharger which is important, and the system is really sensitive to backfires which can tear the dynamic air pressure diaphragm and then the mixture control becomes U/S so no overfuelling on start.

[T28D]

Any radial if the prop drives the engine the whole engine gets unhappy, factors such as Keystone rings, even in non geared engines are an issue, not to understate the need to maintain positive pressure on the master bearing, the engines were never designed to be wind turbines.

[T28D]

Theory of operation
http://upload.wikimedia.org/wikipedia/commons/thumb/e/ef/Bendix_A-B.jpg/220px-Bendix_A-B.jpg (http://en.wikipedia.org/wiki/File:Bendix_A-B.jpg) http://bits.wikimedia.org/skins-1.18/common/images/magnify-clip.png (http://en.wikipedia.org/wiki/File:Bendix_A-B.jpg)
Fuel regulator air diaphragm dividing chambers A and B


There are four chambers in the fuel regulator portion of the carburetor. They are referred to by letters A, B, C, and D, with the A chamber closest to the throttle body. The fuel metering servo valve responds to pressure differentials across the diaphragms. The resulting diaphragm movement controls fuel flow into the engine under all flight conditions.[9] (http://en.wikipedia.org/wiki/Bendix-Stromberg_pressure_carburetor#cite_note-Flight-8)
Chamber A The diaphragm located closest the carburetor body is the air metering diaphragm. It measures the difference in air pressure taken from two locations within the carburetor. Chambers A and B are on opposite sides of the air metering diaphragm. The velocity of the air flow entering the carburetor is measured by placing one or more venturi directly in the airflow. The venturi creates a lower than atmospheric pressure that changes with the velocity of the air. The negative air pressure from the venturi is connected to "Chamber A" on the side of the air metering diaphragm closest to the carburetor body. As the air pressure in chamber A is decreased, the diaphragm is pulled toward the carburetor body.[9] (http://en.wikipedia.org/wiki/Bendix-Stromberg_pressure_carburetor#cite_note-Flight-8) Chamber B The mass of the air entering the carburetor was measured by placing a number of impact tubes directly in the airflow, generating a pressure higher than atmospheric pressure that represents the real-time air density. The impact tube pressure is connected to "Chamber B" on the side of the air metering diaphragm farthest from the carburetor body. As the air pressure in chamber A is increased, the diaphragm is moved toward the carburetor body toward the fuel metering valve. Chamber A also contains a spring that creates a force toward the fuel metering valve when the air flow is absent.[9] (http://en.wikipedia.org/wiki/Bendix-Stromberg_pressure_carburetor#cite_note-Flight-8) The difference in pressure between the two air chambers creates what is known as the air metering force, which moves the fuel metering valve open when it is greater than the opposing force or closed when it is less than the opposing force.[9] (http://en.wikipedia.org/wiki/Bendix-Stromberg_pressure_carburetor#cite_note-Flight-8)
http://upload.wikimedia.org/wikipedia/commons/thumb/5/5f/Bendix_C-D.jpg/220px-Bendix_C-D.jpg (http://en.wikipedia.org/wiki/File:Bendix_C-D.jpg) http://bits.wikimedia.org/skins-1.18/common/images/magnify-clip.png (http://en.wikipedia.org/wiki/File:Bendix_C-D.jpg)
Fuel regulator fuel diaphragm dividing chambers C and D


The second diaphragm is the fuel metering portion of the regulator, and is located farthest from the carburetor body. It measures the difference in fuel pressure taken from two locations within the regulator itself. Chambers C and D are on opposite sides of the fuel metering diaphragm.[9] (http://en.wikipedia.org/wiki/Bendix-Stromberg_pressure_carburetor#cite_note-Flight-8)
Chamber C Chamber C contains unmetered fuel, that is the pressure of the fuel as it enters the carburetor. The pressure in this chamber moves the metering valve outward when the fuel pressure is higher than the pressure in chamber D, on the opposite side of the diaphragm.[9] (http://en.wikipedia.org/wiki/Bendix-Stromberg_pressure_carburetor#cite_note-Flight-8) Chamber D Chamber D contains metered fuel, that is fuel that has already passed through the metering valve, but not yet injected into the air stream. The pressure in this chamber moves the metering valve inward when the fuel pressure is higher than the pressure in chamber C, on the opposite side of the diaphragm.[9] (http://en.wikipedia.org/wiki/Bendix-Stromberg_pressure_carburetor#cite_note-Flight-8) The difference in pressure between the two fuel chambers creates the fuel metering force, which acts to close the servo valve.
The air metering force from chambers A and B apply a force to open the servo valve, and is opposed by the fuel metering force from chambers C and D which apply a force to close the servo valve. These two forces combine into movement of the servo valve to adjust the fuel flow to the precise amount required for the needs of the engine, and the needs of the pilot.[9] (http://en.wikipedia.org/wiki/Bendix-Stromberg_pressure_carburetor#cite_note-Flight-8)

[osmosis]

Thank you, 28. So the auto lean is driven soley by the pressure differentials and does not take into account any engine temperatures. Is it in-flight adjustable at all? I'm trying to link it back to this thread's main (but not original) topic of various engine operating temperatures.

[Avgas172]

It would appear that I may once again have to refer to the discombobulator ....

[T28D]

No temp inputs to Auto Mixture it is set in one position by detent, in the case of a Wright 1820 as an example it is good to 51.5 inches before you must go to full rich.