M20K - Leaning and Engine Temp Practices

[DaveAnderson]

Hi Everyone, this is an old topic, but I wanted to ask a question. 

 

As I just got my 231 a month ago, it didn't have an engine monitor in it, so I stuck in the jpi edm900.  The first couple of flights, I was cruising 31/2400 and getting around 13.5 gph.  I just flew back from Vegas this afternoon and was playing w/ the LOP function and got down to 8.8-9gph.   After messing around, i stuck it back up to 13.3, cause I was concerned about the TIT.  At 8.9gph, my TIT was 1550, running at 30"/2400 rpm, and my CHT's were 360 at the hottest on C1, 350/350/341/317 and then  291 on Cylinder #6.

 

Does that sound like a reasonable setting?  I feel like the TIT should be down around or below 1500 to be comfortable... What do you guys think?

 

Thanks,

David

[DonMuncy]

I'm no expert, but I wouldn't worry about the TIT as long as it stays below redline. My information is that TIT is just to protect the turbo. If the CHTs are below 380 and TIT below redline, I would be happy.

[fluffysheap]

If the turbo isn't dripping out the exhaust, it's fine

 

In all seriousness, continuous redline is 1650.  Anything under that is fine.  It will be hotter LOP, and at higher altitude.  With the settings you listed, you actually are being a bit conservative.  You should be able to increase to something like 33"/10.5GPH/1600TIT at FL200 or lower..  Or you can keep it where you have it for a little better fuel economy (in still air).

[DaveAnderson]

Cool.  I'm pouring over the info now on the EDM data (I'm a total nerd when it comes to looking @ this stuff), and it looks like 9gph at 1550 w/ all the cht's below 360....30/2400.  I'll push the MP up a bit next time and see what we get, but with that setting, I was cruising at 195kts GS with a nice little tail wind, so I didn't see any reason to push it.

 

Thanks! 

 

-David

[Jrob]

This is kinda a topic resurrection. I have been working with LOP ops in my 231 (intercooler, Merlyn, and gamis) and still have problems keeping the TIT below 1630. The FF will get down to under 9 and still run very smooth. What impact does MP have on the TIT? Do most of you guys do the big pull starting with MP at 33" and then add MP back? And please educate the misguided, if HP when LOP is determined by FF how does MP play into that?

[schule]

Our 231 LB has a Merlyn and GAMIs, but no intercooler. I start the big pull at 28" and then add back in to around 30-31" with 2500-2550 RPM. FF between 9.5-10.8 gph. RPM, MP, and FF can be a limiting factor based upon altitude, TAS, and OAT. MP quickly affects TIT and CDT, RPM quickly affects CHT, and FF can affect all three simultaneously. It usually takes a bit of experimentation each flight to tweak MP and RPM to allow a LOP FF equal to good HP/TAS while keeping CHTs around 380, TIT between 1580-1630, and CDT below 280.

MP and RPM determine a HP possibility based on FF relative to temp limits. More MP may help cool the TITs a bit, but in my LB it quickly causes CDT issues which should not be a problem with intercooler. I also find the airflow across the engine can make a 20 deg change in CDT and TIT. I can make small tweaks with cowl flaps or IAS.

Warning. Your results may vary.

[DaveAnderson]

Jrob, congrats on the 231, you're going to love it!

 

My 231 LB doesn't have the Intercooler, Merlyn, or Gami's, but it runs really well LOP.  After I roll out from the climb to cruise, I give it about two minutes to get up to speed before messing around with anything other than closing the cowl flaps and pulling it back to 31"/2500, which is what I want my settings to be for normal cruise.  After that, I slowly dial the mixture back to ~9.5gph and depending on the altitude, may have to increase the MP since i don't have the auto waste gate.  My TIT is rarely above 1600 and CHT's are @ or below 380 when running LOP.  

 

Do you have a JPI EDM or other digital readout on the TIT gauge?

 

David

[Jrob]

Thanks for all the input. I'll report back next weekend after I can get out and fly a bit. Tell me if this sounds ok for a procedure.

1. I'll climb at 33"/2600 to probably 14500ish

2. I'll leave the cowls open and pull to 10 gph or even lower if needed.

3. I assume I will have to add some mp back to bet back to 30-31"

4. If all temps are good, I'll start closing up and enriching to get CHTs (#2) just under 380dF but still worry that TIT will be the limiting factor (from past attempts)

[kortopates]

Thanks for all the input. I'll report back next weekend after I can get out and fly a bit. Tell me if this sounds ok for a procedure.

1. I'll climb at 33"/2600 to probably 14500ish

2. I'll leave the cowls open and pull to 10 gph or even lower if needed.

3. I assume I will have to add some mp back to bet back to 30-31"

4. If all temps are good, I'll start closing up and enriching to get CHTs (#2) just under 380dF but still worry that TIT will be the limiting factor (from past attempts)

 

I'd suggest the following:

0) Before flight, pick 2 or 3 power setting you want to collect data for, begin conservatively with 55% power and go up from there consecutively as long as TIT allows, such as adding another 5-10% so that you go to 60% or 65% then another 5% or 10% for a 3rd power level etc.. I would add 5% at a time. Ultimately you're looking to learn how much power % you can operate within temperature constraints you are comfortable with as well as what the specific numbers are for that power setting. Not just CHT and TIT but how many degrees LOP you are are for EACH cylinder; thus while you're collecting data you'll be looking at EGTs too. CHT is not an issue, but EGT & TIT will be in 2 possible ways. Eventually with enough power you will likely find that either that i) your TIT exceeds your  comfort threshold (that could be redline or it could be a more conservative 1550-1580F) or ii) you are not able to get EACH cylinder sufficiently LOP before the engine begins to run rough (that could be 10-20F LOP for each cylinder at  lower power settings to 60-80F LOP at higher power settings approaching max cruise power. Don't pull these numbers out of hat, but read Deakin's Pelican Perch (see avweb) articles and allow your self to make an informed decisions. Or better yet, take the APS online (or in person) class when you can.

But I personally would not recommend following Mike Busch's simplified approach - at least not initially - to forget about EGT and just look at TIT. Instead first educate yourself about the process and your engine and then with proven experience in both you can elect to simplify just using TIT as your guide. But this should be only AFTER you have collected the data to prove to yourself that all your cylinders are sufficiently LOP for the given power setting configuration you intend to use. If you don't want to do this, then just stick to lower power settings such as around 60% to keep your engine out of trouble. This is only a real concern for us turbo guys since the NA aircraft can't operate at high power settings LOP unless they're flying real low to begin with. With a turbo we can get into trouble real quick.

 

So you are ready to begin your flight once you have i) calculated fuel flows and ii) pre-planned temperature limits for TIT (single limit for all pwr settings) and a minimum target degrees LOP  for each cylinder based on power setting. Additionally for optimum data collection results, I would recommend i) changing the data sampling rate to 1 sec from the default of 6 (e.g. JPI) if your unit allows, and ii) know how to put your engine analyzer into LOP mode rather than ROP mode when you press the lean button. 

 

In flight, the following is just one way to do this that allow accurate data collection. But if possible take a safety pilot up with you to help with data collection and/or traffic watching.

1) climb at max power to altitude typically ~36" with an intercooled 231 or 252 (but whatever your intercooler documentation suggest), max rpm (2700 GB/LB/MB or 2600 SB), full rich. You should find that it runs cooler (CHT and TIT) at max power with full rich than it does using the POH cruise climb power setting in your POH. 

2) level off and set power to your first power setting, suggest 55% ROP per your POH, using the prop RPM you customarily use for cruise flight.  Configure cowl flaps as required (hopefully closed)

3) allow plane to stabilize in temps and speed. If desired, also write down Indicated air speed and fly 3 GPS tracks at 120 degrees apart to calculate True air speed later at both ROP and LOP for each power setting to allow for comparison later.

4) Begin the lean test, set your engine analyzer into LOP mode. Then slowly lean the mixture to your pre-calculated fuel flow for the desired power level (e.g., 55% = 9.5 gph). Once at target fuel flow, start raising MP slowly to continue leaning the mixture. While doing this, you are watching lean find operation in LOP mode on your engine analyzer. It will inform you when the first cylinder peaks, and as you continue leaning it will indicate each cylinder as it peaks. When the last cylinder peaks it will then indicate how many degrees LOP the last cylinder is as you continue to add air (i.e. continue to lean by adding air). Stop leaning when either you reach your target number of degrees LOP or get roughness that would make a back seat pax nervous (that's not very much). At 55%, even running at peak is safe, but you want to ensure all cylinders are running LOP, including the last to peak with at least your targeted number of degrees LOP before going to a higher power setting. After familiarizing yourself with the literature (e.g., Deakin's articles) you'll see at 55% you are fine just running at 10 LOP.

 

At 55% power this should be an easy exercise and useful learning experience. In performing this, you will have learned you may or may not have a large variety of variation in degrees LOP between your cylinders. For example, your last to peak may be only 10F LOP while your first to peak may now be 60-80F or even more LOP. This variation is what leads to the engine roughness we often get with continued leaning and stems in part from how well balanced our injectors are but can also vary significantly due to potential issues in your induction system or ignition system. I am only making these points to underline the importance of collecting the data at the cylinder level before you begin to lean solely based on TIT; especially at high power levels. But once you've convinced yourself every cylinder is running sufficiently LOP for any given documented power level settings then you should be comfortable moving on to quickly leaning to your documented power setting (FF, MP & RPM) and just using TIT to tell you your engine is running as expected. Many skip this important data collection stage, but given the cost of our engine's I personally think that's unwise.

 

If one doesn't have a modern engine analyzer with the data collection capabilities to do this then your next safest bet is to use the TIT only method while sticking to lower power settings (<65%) till you have the proper data to support higher power settings. Without data recording capabilities you're really operating blind.  

 

 

Note when leaning, for the purpose of collecting this data, don't be concerned about exceeding your target TIT or even redline TIT momentarily for upto approx 30 seconds as you go back down the LOP side, but avoid a transient deviation of more than 1700F as your POH advises. 

Should you find you encounter roughness before you achieve your desired degree LOP for your last cylinder to peak, then there are few things you can do to help. You can first try again with a different RPM (such as going from 2400 to 2500 cruise rpm), and you can also try going from massive plus to fine wire plugs. All of this assumes you already are free of induction leaks and your ignition system including magnetos, wires and plugs are all in good shape and timing is correct but these are also reasons that can lead to engine roughness that manifest itself when LOP. So if you are encountering roughness LOP, a good diagnostic is to perform a magneto test in the air while LOP, but preferably with a low enough power setting that you have a smooth running engine and be below 12K when you perform the test. (if the engine quits on one mag, before you go back to both, pull the mixture to idle to avoid backfire and then slowly push the mixture in with both mags on till it re-lites - an unlikely event but be prepared)  

 

After the above you should feel comfortable operating LOP with a high degree of confidence based on the data you collected - not blind faith. But its a very good practice to periodically re-check your data as well as periodically perform the airborne ignition test while LOP to check the health of your ignition system.

 

Way more than I intended to write, but hopes that helps a few folks get started with LOP ops.

[FlyWalt]

And I thought Jets were complex :-)

[Jrob]

Wow! Thanks for the explanation and procedural insight, kortopates. I have gone thru the avweb readings but your explanation put that information into a digestible and easy to follow format. Can't say thanks enough and look forward to getting some Data points next week.

[jlunseth]

Sigh.

 

I have tried to stay out of this, but,,,,

 

In the 231 you will either have a fixed wastegate or the Merlyn differential pressure controller.  Neither of them is an "automatic" wastegate, or absolute pressure controller.  With an absolute pressure controller, you set an MP, let's say 30", and you can fiddle with the fuel flow pretty much all you want and the controller will maintain the MP at 30".  The Merlyn is an "all things being equal" controller.  In other words, if you set a cruise power level and leave it alone, the Merlyn will maintain the MP you set, overcoming any tendency to bootstrap.  But it will only do so if all other things, including fuel flow, remain equal.  Fuel flow and MP in the 231 are interrelated.  If you change one, the other will also change.  So it is not possible to set an MP of, say, 30" and change the mixture without having the MP also change.  And likewise, it is not possible to set a fuel flow of, say, 9.5 GPH and change only the MP without having the fuel flow change.  The 231 does not work that way.  The only way I have found to maintain, say, a fixed fuel flow while varying MP, is to fiddle with both knobs at once.  In other words, to do that task, you will need to adjust the fuel flow as you change MP, in order to get the fuel flow to stay at a given setting.  It does not work the way a 252 works, or an Acclaim, which have truly automatic wastegates (absolute pressure controllers).

 

One also cannot start with an MP, let's say 34", and "big pull" the mixture to a particular setting, and simply because the fuel flow one pulls to is 11.5, know that the engine is now LOP.  One cannot know that because the MP has now changed, probably by about 5" if you started at full rich.  And one may be operating the engine not LOP, but right at peak, or ROP, or with some cylinders on one side and some on another.  That is the case because being LOP does not depend on having just a specific fuel flow, such as 11.5 GPH.  It depends on having a specific fuel/air ratio, and in doing the "big pull" in the 231, you have changed both numbers and therefore have changed the ratio.

 

One can certainly set a power setting, such as 11.5 and 28, or 11.5 and 33, and so long as the temps (EGT and TIT) stay more or less normal and under the TIT redline, one may think one is operating the engine LOP, and one may get away with it for a single flight, or several flights.  But if anyone went to the APS seminar, or understands what is really going on in these operations, temps are only a secondary indication of where the cylinders are operating.  The whole idea is to stay away from the Internal Cylinder Pressures that come with operating at peak.  Or that happen to a single cylinder if you are operating five of them LOP and one at peak or ROP.  Long experience in the fleet, of operating turbos in "fire breathing dragon" mode with the TIT right up close to the redline just because the book says you can, is exactly what causes top overhauls and turbo overhauls to be required at about half of TBO - or less.

 

If anyone can come up with a way of actually measuring how far lean of peak you are operating each cylinder in your 231 (and not in a 252 or Bravo or Acclaim, which are different), let me know how.  I have a 930 with alot of above standard capability, and I cannot, simply because so many things are changing at the same time in the 231 controls.  You can certainly find a peak, and you can lean back from that peak, but that peak was at a specific MP, and in further leaning you have now changed both, so the peak that the engine analyzer found is no longer a valid reference in determining degrees lean of peak, or even if you are lean of peak.

 

About the only way I have found to do it, is to put the analyzer in LOP mode and find a peak with a setting of about 11.5 GPH, then add back MP (while fiddling with the mixture to maintain 11.5) so long as safe temps are not exceeded.  If you add back MP while manually maintaining a more or less constant fuel flow, you should be going leaner than you were at the peak.  How much leaner you will never know, and that is the problem. 

[231LV]

hmmm....I must be simple and dumb....I pull back to 75% power from full power climb (after leveling off), close the cowl flaps, dial the prop rpm to 2500 and pull the red knob....set the JPI for LOP and start twisting the knob til all 6 jugs are registering on the lean side...add back MP to 30 inches and set the A/P...enjoy the scenery and monitor the temps

[jlunseth]

That is about the safest way to do it.

But let me ask, when you twist the knob til all 6 jugs are LOP and then add MP to 30", where does the fuel flow end up? Cause when you twisted the knob until LOP and then added back MP you also added more fuel unless you also manually adjusted the fuel flow to keep it at a set number. You may not have meant to, but that is what the control system did. So now that you have added MP and fuel, how do you know you are LOP, or how far LOP you are? The JPI found peak at a specific air fuel ratio, and you have changed both.

[schule]

Jlunseth has a very good point. It all depends on everything else, but you can get real close using the big pull to a known FF. However you must test you LOP setting once there and see that MP, RPM, FF, CHT, CDT, EGTs, TIT, and Oil Temps are within safe limits and the FA mixture is outside the RED BOX.

I don't trust or use the JPI lean find because the actual EGTs lag by 10-15 seconds and it usually results in exceeding my comfort level/POH limits to get through to LOP unless I am low altitude and very low power settings. This mandates a big pull to avoid cooking everything.

Once LOP you can test if you are safely outside the RED BOX by adding in FF toward peak. I typically can not find peak due to TIT, but I am able to ensure I am at least 70-80 deg LOP (60-70% HP) and sometimes more to keep all temps safe.

An unexpected added benefit of my 231 setup is a reduction from my cruise settings to approach setting of 2100 RPM, 25" MP is the FF drops to 6.5 without adjusting mixture and I arrive at a nice approach speed (120 KTAS) still LOP. At this power setting I am now 20-50 LOP and can easily get to 150 ROP going to 7.5 GPH if heavy weight or need a little more power to climb/fly faster. Drop the gear & 1/2 flaps approaching the FAF and reduce MP to 17-19 makes for a nice stabilized 3 deg 90 KIAS approach with minimal engine tinkering during a high workload critical phase of flight.

Much like on a NA engine where many variables can cause MP changes the 231 is easy to fly and make changes if you keep the turbo happily up to speed.

[jlunseth]

The issue with the 231 is that if you change fuel flow the MP changes and vice versa.  This is different from many other systems.  "Peak" is determined by picking a specific power setting, and finding that point at which the EGT for the last cylinder to peak (LOP ops) is highest.  Degrees lean of peak is then how many degrees cooler the EGT is from that temp.  The issue is that for every power setting that peak temp. will be different.  So if, in the 231, you add back MP but leave fuel flow untouched thinking you are going more lean, the engine control system is changing both.  You are now at a different power setting than you were when you found initially found peak, you are at a higher MP and a higher fuel flow.  And the peak you found is no longer valid at all.  So you can't use that peak to evaluate how far you are LOP (or ROP) because that peak is no longer valid.

 

what you can do is find a peak for a specific fuel flow, go a little LOP from that, and then add back MP while manually adjusting the fuel flow to stay at "x".  

 

One thing that I have found in actual experience with my 231 though, that really bothers me, is that GAMI/APS teaching that power on the LOP side is dictated solely by fuel flow.  If you set 11.5 GPH and let's say 30" of MP, you should be at 75% HP.  If you increase the MP to let's say 34" and hold fuel flow at 11.5, you should be going more degrees LOP, but horsepower should not change.  That is APS's thesis.  The problem is that in my 231, it does.  If you set a fuel flow of 11.5 and an MP of 30", and then increase MP to a higher number like 34" (assuming temps are ok), airspeed increases.  It increases significantly.  And there is only one reason airspeed increases, and that is that horsepower increases, which should not happen according to APS. 

 

So something is going on in the 231 engine that does not fit their model.  I have written them about it, and their response was that they do not know the Merlyn system and therefore can't help. 

 

The bottom line is that regardless of your engine analyzer, it is virtually impossible to get an accurate reading of how far degrees LOP you are, which you need to know to stay out of the red box.  All you have to manage with, is temps, and I personally am uncomfortable with TIT's in the 1600's.  You are in fire breathing dragon territory up there. 

[schule]

When you, for example, set 11.5 gph and 30 MP at a particular LOP setting and then increase MP 34" does the FF increase? If so then HP has increased. Or does the FF stay the same and airspeed goes up? If FF is increasing faster than air mass the LOP setting is changing and HP may have increased. There are a plethora of variables affecting LOP operations.

In a 100LL SI engine peak is right around 12.7 FA ratio. More air and/or less fuel from there will move you more LOP. Less air and/or more fuel will move you ROP. A turbo recovers energy from the exhaust air mass to generate air pressure used to generate exhaust air mass. ICP greatly effects the energy of the exhaust pressure wave. LOP in a turbo makes this a very delicate proposition to keep the turbo up to speed and thereby generating enough boost.

FF can be used LOP to estimate HP, but only at an all is equal point LOP because Fuel it the limiting component. Because, as you move more and more LOP the actual flame front characteristic changes. Too LOP and the flame cannot ignite.....e.g idle cut-off. As you move from Peak EGT=fastest possible flame front, to LOP the flame front slows and peak ICP moves beyond its optimum design point and reducing HP even if MP and FF appear to be the same. Aircraft SI engines are designed to create peak ICP at a specific point beyond TDC. Mag timing is set unless you have an electronic ignition. Very little change in that ICP timing occurs at any given FF ROP or Peak, but LOP it rapidly slows to a point where HP is quickly reduced even if FF looks the same. FF is only one indicator of FA ratios.

FA ratio matters. You can estimate air mass with MP and RPM, and the FA ratio by seeing how FF effects EGT. Also don't forget OAT, IAS (Ram Pressure), and Altimeter settings also effect air mass approaching the air filter.

[carusoam]

Too simple(the procedure).....not dumb.

JL always makes good points. Great Mooney experience beyond the N/A level...

Even in the N/A world set MP then mixture...

Changing MP always requires a refinement on the mixture to know where you are in the LOP spectrum.

In the TC world, you use the big pull to get in the ball park without over-heating things.

You can go or maintain or reset LOP from the cold side using the JPI. Set it up properly to make it work right.

There are also fast working TCs...? From JPI. The lag is somewhere in the 1 or 2 seconds time frame.

These would be beneficial over the heavier 15 second version.

Have you seen those?

Since the health of the turbo depends on quick response sensors it may make sense to swap them.

How does that sound?

Best regards,

-a-

[jlunseth]

schule-

 

On your question about whether fuel flow increases if I increase MP from 11.5 GPH and 30", to 34", the answer is absolutely.  In the 231, if you do nothing to manually control fuel flow, and you change (in this case increase) MP, then fuel flow changes in the same direction.  Moreover the change is not linear.  The 231 control system is set to rapidly increase fuel flow as MP is increased near full power.  The purpose is to create a very rich flow to cool the cylinders. 

 

However, if you are wondering about my statement that, if I increase MP at, say, 11.5 GPH and LOP, airspeed (and therefore horsepower)  increase, in my experiments where I found that result I was manually controlling fuel flow by dialing it out as I increased MP, so that fuel flow remained a constant 11.5 GPH, or at least as constant as I was able to maintain with my fingers.  Added MP did not simply increase the degrees LOP at which the engine was operating, it increased horsepower.  I should also mention that from experience with the aircraft, I am aware that airspeed will slowly increase for quite awhile at a cruise power setting if one simply levels off from a climb and makes a cruise setting without letting the aircraft speed up at full power first.  I was careful to avoid that.  But if I go from 11.5 and 30 or thereabouts to 34, the airspeed increases 5 - 10 kts.

 

That should not happen of course.  Once on the LOP side of a stoichiometric mixture adding more air should cool the cylinders, raise the EGT's and TIT, but not affect horsepower at all.  And yet, in the 231, it does.  Something happens that is not the norm, and I don't know what it is. 

[231LV]

That is about the safest way to do it.

But let me ask, when you twist the knob til all 6 jugs are LOP and then add MP to 30", where does the fuel flow end up? Cause when you twisted the knob until LOP and then added back MP you also added more fuel unless you also manually adjusted the fuel flow to keep it at a set number. You may not have meant to, but that is what the control system did. So now that you have added MP and fuel, how do you know you are LOP, or how far LOP you are? The JPI found peak at a specific air fuel ratio, and you have changed both.

 

 

 

right about 10 gph.....but I always twist until I show at least 3 bars on the last jug to peak pushing me well into the lean side of peak...this takes me down to about 26 MP...add back to 30 and the FF runs up to about 10 GPH...Jugs running cool....same set up running ROP is closer to 13 GPH...I can verify using the formula for power on the lean side...turns out I am getting  65% power running lean....once established. I can re-evaluate where I am using the JPI.....having already done this numerous times, I know that 10 gph puts my engine right where it is happiest...

[carusoam]

JL,

I think I missed something...in your last statement.

You are LOP and add more air...

You stated CHTs cool, EGTs and TIT and increase?

Is my NA engine measuring EGTs differently than the TC engine?

Leaning past peak in my engine lowers both EGTs and CHTs. My TIT hasn't been installed yet....

Are you measuring peak and LOP based on TIT? In this case, the cylinders can be ROP or near peak as the TIT has time to burn the last of the gas. This would explain EGTs increasing while going LOP using TIT...

I'll be studying my bravo simulator later...

Thanks,

-a-

[kortopates]

The issue with the 231 is that if you change fuel flow the MP changes and vice versa.  This is different from many other systems. 

 

The first statement is absolutely correct - but not the second. Any increase in MP or RPM will increase Fuel flow, and decrease in either will be accompanied by a decrease in fuel flow as Schule made the point in the first paragraph. This has nothing to do with the type of wastegate. Its controlled by the fuel pump and the throttle body. The fuel pump is providing fuel pressure primarily proportionate to RPM, while the throttle body is metering the fuel based on throttle position. So increasing throttle to increase MP gets a corresponding increase in fuel flow. Increasing Prop RPM increases fuel pressure put out by the pump also increasing fuel flow. The only thing different among the various Continental installations is that our turbo charged motors have a altitude compensating aneroid in the fuel pump to provide an auto-leaning feature - it senses ambient pressure. All in all the Continental fuel injection system is the simplest. In contrast the Lycoming RSA system uses venturi to schedule fuel flow proportionate to mass air flow; compensating for air pressure and temperature unlike our simple Continental Continuous Flow system that operate on RPM and throttle body position only. A bit verbose to make the point that fuel flow will change in response to a MP/Throttle or RPM change in any installation but hopefully the explanation makes it clear why.

 

All of the wategates, either air pressure based Merlin or the TCM hydraulically controlled unit in the TSIO-360-MB and SB engines are merely trying to maintain the same MP. The primary difference between the Merlin and TCM hydraulically controlled wastegate is that TCM will do a much better job of maintaining constant MP as you climb and descend, but even the TCM wastegate will vary a bit  in setting below max with altitude changes and is only a truly "set and forget" device at max power while climbing. 

 

So there is nothing unusual about needing to continually tweak fuel flow as you increase throttle to lean the mixture going further LOP. Assuming your engine analyzer has MP in addition to fuel flow (and preferably RPM) this is very easy. There is no need to twist both throttle and Mixture knobs simultaneously. As you increase throttle by about 1/2" MP, you do need to lean the fuel flow by the couple tenths it went up. But this hardly invalidates the process. As Anthony pointed out, the JPI probes are fast acting and you see an immediate temp change. This tweaking doesn't require a lot of time, but its also why I suggested starting at a low power setting of 50-55% power because one does need to learn the process at lower power levels so they can move through it faster at higher power levels when higher temperatures are more critical and need to be avoided. The fact that the EGT's will vary a bit up and down as the mixture fluctuates is not going to invalidate your engine analyzers ability to continue to track for peak EGT and then show you the delta from peak. Its not that hard and we're only talking about 10-15F temperature variations.

 

Of course power and speed will not go up with further leaning past peak (on  the LOP side) - just the opposite. But power and speed will go up by just increasing MP or throttle without adjusting back the change in fuel flow, since the increase in MP also accompanied by increased fuel flow. That's why it's so critical that your engine analyzer includes fuel flow. If you are also correcting to maintain constant fuel flow to increase leaning with increased throttle, I am very puzzled since if you keep increasing throttle to further lean the engine is just going to stop or get very rough.  

[jlunseth]

Well, the difference between the 252/Bravo/Acclaim controller and the 231 controller is more than that.  An absolute pressure controller tries to maintain the MP that is set.  Set 30" and it maintains 30" regardless of what you do to fuel flow.  Yes, probably it varies as to success and smoothness.  The 231 differential controller does not try to maintain the set MP.  It attempts to maintain a difference - the difference between MP and upper deck pressure - and the purpose is to eliminate bootstrapping, not to automate MP control in the turbo.  The 231 controller will not even try to restore MP if you do something that changes MP, and that something in the case of leaning the engine, is reducing fuel flow, which results in power reduction. 

 

I am trying to get that across to the 231 drivers here, because many of the statements made about the leaning process assume that MP, once set, will remain stable (relatively) at the set level.  In the 231 it does not remain at the set level at all if you reduce fuel flow.  A reduction in fuel flow, without manually increasing MP, will result in a significant drop in MP.  Depending on where you start and where you end up, it will be several inches, and you are now at a completely different power setting than when you found peak with your analyzer. 

 

I agree that many systems do interlink MP and fuel flow, change MP and fuel flow will follow.  Usually the opposite is not the case though, in other words change fuel flow once the MP is set and the MP does not follow.   

 

On your last point Paul, about power and speed not increasing with further leaning past peak (on the LOP side), that is exactly the problem in the 231 that puzzles and concerns me.  Power does actually increase if MP is increased while fuel flow remains constant on the LOP side.  The difference is not small.  It should not change, but it does.   The question is why?  I don't know the answer.  It could be instrument error (11.5 GPH at one MP is not the same fuel flow as 11.5 GPH at another MP because of some error?), it could be because the engine at 28"-30" and 11.5 GPH was not LOP to begin with, even though the engine analyzer said it was.  Which begs the question, if you thought you were "x" degrees LOP and you were actually ROP, where are you now?   It could be some greater efficiency in fuel delivery in the very much not tuned intake system of the 231.  I don't know what it is, I only know that there is a difference.  Add MP while keeping the fuel flow at 11.5 GPH and power increases.  That is what I see.

[carusoam]

Got to love the complexity of aviation's super-models!

To get a better idea of how MP is effected would require knowing the speed of the turbine or at least the volume of exhaust going through it...

The hotter the exhaust is an indication of increased volume. Going LOP, the exhaust volume rapidly decreases with the EGT.

Huge balancing act to go LOP without knowledge of turbo speed.

Suttle benefit to having a super charger. The compressor speed is tied, belted or geared to the engine rpm.

If you go too far LOP, do you risk stopping the turbo? Without the turbo spinning, the engine dies and becomes difficult to restart at altitude...

Too many questions?

Best regards,

-a-

[jlunseth]

Going LOP results in the EGT's increasing rather than decreasing.  I suppose if you got to the point where you were starving the engine and it was close to stopping you would see a reversal in EGT's.  But in the range of what is standard for cruise, EGT's increase when operating LOP, compared to an equivalent ROP setting.  Someone else on our board had the explanation.  As I recall, there is a secondary combustion effect that occurs in the exhaust.  EGT's also "go up" because it is not necessary to go as far LOP as it is ROP.  For example, 60 LOP may be about as far out of the red box as 150 ROP.  Both are subtractions from peak EGT at that power setting, so it is simply a matter of subtracting less for LOP than for equivalent ROP.  Exhaust volume may decrease, but again, unless you get to a point where you are starving the engine you don't risk stopping the turbo.  Yes, you could pull the mixture full out and stop the engine, but not likely.  I know from experience that you can pull it quite far back, to a point where the engine is barely operating, and the engine will not quit.  Since you are moving through the air, the prop helps the engine and it does not stop, in fact, it is pretty hard and takes some maneuvering to get the prop to actually stop.