M20K - Leaning and Engine Temp Practices

[aaronk25]

Reducing RPMs also reduces TIT due to longer duration of burn. This results in less wasted heat going out the exhaust.

My economy setting of preference is same as Mike Busch's. Turn the RPM down as low as practical such as 2100-2200rpm and run the boost up to 30-32" and about 30lop. That should put you just under the 1650tit.

Some believe the higher cylinder pressures will cause early failure. I don't. I believe heat causes failure. Just me your mileage may vary.

[carusoam]

Thank you for the reply.

Best regards,

-a-

[kortopates]

JL, I don't think you mean to say going LOP results in increasing EGT's (I assume in response to Anthony's comment about his reducing),  Perhaps what you meant, as suggested by your later comment, is that a typical LOP setting for the same (high cruise) power level will be less lean of peak (~60F) than the rich setting is ROP (~150F) and thus higher? That's certainly true. Also I trust when you say power increased you are referring to airspeed and not the power level indicated from the JPI as this is a algorithmic calculation assuming ROP operation.

 

I should clarify I didn't mean to suggest a change in fuel flow will create a significant change in MP, just that a significant change in MP (or RPM) will create a significant change in fuel flow. However, i understand you're saying with the Merlin, it does.

 

To get to a given LOP power level such as 55%, the challenge of course though is to keep fuel flow the same while increasing MP if you want to watch and collect data to see cylinders lean to peak and then track degrees of EGT below peak. (Even if you started at a higher fuel flow, once you leaned to the desired fuel flow with mixture, you'll likely need to continue leaning with more air at some point.) So have you collected any data on your JPI which illustrates the difficulty you are having trying this that you can dump and share? (e.g. savvy analysis perhaps)

[jlunseth]

Yes, you are exactly right.  If I did not make it clear, I meant to say that a LOP setting results in higher EGT's than an equivalent ROP setting.  Very roughly 100 degrees.  By "equivalent" I mean how far the setting is out of the "red box."  I did not mean that going LOP raises the EGT's, in fact it lowers EGT's just as going ROP lowers EGT's.  But once at a particular LOP setting, adding MP does raise EGT's and TIT.  So if you are operating at 11.5 GPH and 30", and you manually maintain the 11.5 GPH while increasing the MP to 34", EGT's and TIT will be higher at 34 than they were at 30.

 

Yes, I am aware that the JPI displayed horsepower is an algorithm.  On the ROP side it appears that a proprietary formula is used that considers a number of factors, just as the power tables in a POH would.  When you ask the JPI to lean LOP, the algorithm appears to switch to a straight up formula driven solely by fuel flow.  Probably it is 13.7 x GPH divided by 210 (which is 100% HP).  However, I am not relying on that in saying that an increase in MP at a fixed LOP fuel flow (say, 11.5) results, in my aircraft, in an increase in horsepower output.  I am relying on the fact that the aircraft is going significantly faster at, say, 11.5 and 34", then at 11.5 and a lower MP, say 28 or 30".  That should not be the case, but it is.

 

I might add that my engine is quite comfortable LOP.  I can dial it down quite a bit without roughness.  What I don't like, are TIT's in the 1600's and EGT's in the high 1500's, which seem to come with a relatively high MP, say 34 - 36", and a LOP fuel flow.  I use 11.5 in my examples because that is 75% HP according to APS's formula.

 

Your statement about fuel flow and MP being interrelated is exactly right.  If you find peak at a specific fuel flow, say 11.5 GPH and 28", and then simply add MP, which is what I have read in some of the posts, you are in a bad place in the 231.  You can no longer rely on the fact that you found peak and went "x" degrees lean of peak, and that is the case for two reasons.  First, peak EGT at 28" is completely different from peak at 34".  The difference will probably be around 100 degrees or more.  So simply because the algorithm in your engine analyzer is saying you are "x" degrees LOP does not mean you are.  You in effect tricked your analyzer into using the peak it found at 28, instead of the peak that would apply at 34.  Second, because the interlink is nonlinear, it likely has raised fuel flow faster than you have raised MP.  So if you started at, say, 20 degrees lean of peak at 11.5 and 28", and you simply raised the MP to a higher number, you have also enriched the mixture and you are probably operating dead in the red box.  In the 231, if the pilot is going to raised MP in an effort to go further LOP from a specific LOP fuel setting, the pilot must manually adjust fuel flow to maintain it at a constant level. 

 

It is a "two knob job."  No way around it.

[chrisk]

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...

What RPM are you at?

[CaptnJ]

So, I have a 231, I've been running LOP, the way I do it because of all of the things talked about here, moving one knob changes a coresponding setting

Start at an arbitrarily high MP, 34", BMP, feel the airspeed slow, then I can enrich the mixture, identifying the first cyl to peak, so closest to my intended power setting, then lean to ~30/40 LOP of that cyl. Every other cyl is leaner than that, and as long as the TIT is below 1620 or so I'm happy. If I note fuel flow and add MP as is suggested by the article on the GAMI site, "Back to the future" I should go faster right? I thought that was why owning a turbo is so beneficial for LOP OPS.

The thing that I'm unclear on is should I hold the FF to my noted FF or should I allow it to rise. It seems holding it would be the safer option but on Beechtalk I've seen Deakin state that it will rise on every engine when adding MP, blowing my LOP setting.

[231LV]

What RPM are you at?

2500

[carusoam]

Again, thank you for sharing your wisdom!

Best regards,

-a-

[jlunseth]

CaptnJ that is exactly my point.  If you make a LOP setting and then increase the MP without manually holding the fuel flow to a fixed number, you will have no idea where you are when you finish, LOP or ROP or some cylinders on one side and some on the other.  You are doing just fine up to the point where you add in MP and let the fuel flow go up with it.  But from what I see in my engine, the control interlink between the MP and the fuel flow is not linear.  In fact, if you add MP the interlink adds fuel at an increasing rate as MP goes up.  So in all likelihood, your final setting has you dead in the red box and right about at peak, which is exactly where you do not want to be.  If your MP is in the 30's and your fuel flow is 12 something, your engine is in the red box.  Not good. 

 

However, if you hold fuel flow to a fixed number as you add MP (that means moving the black knob in and the red knob out at the same time),  then if you were 30 LOP, you know that you are now further LOP than that.  You just have to really watch your EGT's and TIT when you do that, because they will go up quite a bit.  The CHT's will be nice and cool, and that is a good thing.  But you are exposing the valves to hotter exhaust gases, as well as the turbo.

 

Also however, doing that - simply adding MP with a fixed fuel flow - should not increase HP nor airspeed.  A stoichiometric mix is that fuel/air mix where there are enough oxygen molecules to burn the available fuel, and enough fuel that the available oxygen is consumed.  I can't tell you if that is exactly peak, but that is what the process of finding peak is all about.  Finding the stoichiometric mix.  If you add more fuel, then you are rich and not all the fuel will be consumed.  If you add more air but not more fuel, then all of the fuel will be consumed but there will be air left over, and that is a lean mixture.  So if you set a specific fuel flow, let's say 11.5 GPH which should be 75% HP in our 231 engines, adding more air will put you further lean of peak but will not produce more power because all of the available fuel has already been consumed.

 

And that is the conundrum I have seen with my engine.  If I go lean of peak at 11.5 GPH and say -10 LOP, and then hold fuel flow at 11.5 while increasing MP from, say 28 to 34 or so, I should simply be further LOP and with some reduction in HP because the less than optimal mixture results in a slower burn.  But that is not what I see.  What I see is an increase in airspeed, which means power has increased, and that can only happen a couple of ways, one being that I was not LOP to start with, which should not be the case.  The other is that some optimization of the fuel delivery or burn is going on that I do not understand.  But that increase is not a good thing, what it means, is that I really have no clue whether I am LOP or not.

 

The whole idea of running LOP or ROP is to slow the combustion rate, which reduces the peak ICP.  If you are ROP, there is excess fuel and that has a damping effect on the combustion event.  If you are LOP, there is excess air, and the combustion rate also goes down.  Either one is where you want to be, but you do not want to be at peak.  That produces a high, sharp spike in pressure from rapid combustion.  If under 65% power, the APS/GAMI people say the pressure is not enough to hurt the engine.  If over 65%, it can, and in fact at higher power settings it can result in detonation or near detonation.

 

carusoam-  thank you for the kind words.  I don't know that I am wise, just a searcher like everyone here.

[CaptnJ]

I agree, you do go faster and it is very strange that there does not seem to be any real good answer for why. But that back to the future article does speak to the boost increasing your power, but does not definitively state if the FF goes up with the added MP. I Have a G3 analyzer and I'm adding a TAS air data computer to my aircraft this winter, it allows data tracking of TAS, ALT, ect. plus all the engine data, so I'm going to be able to graph all the data and see if it helps to find a conclusion. If not it should still be a neat chart, haha. Anyway thanks for you insight.

Jeff

[jlunseth]

In the 231, if you add MP fuel flow always goes up, unless at the same time as you add MP you manually dial the fuel flow down. 

 

That happens on most aircraft engine systems, but I don't know enough about all of them to say it happens with all. 

[FBCK]

Jl I have the same setup you have on my 231 and agree with everything you said here.  The same thing happens in my 231 and the only way to see the speed increase is an increase in the horse-power ( be it a change in the combustion event or something else). If you ever get an answer I would be interested in hearing it.

[jlunseth]

When I first saw this issue (of the aircraft speeding up) I emailed John Deakin at APS.  Normally they are pretty good about answering questions, at least for people who attended their seminar (which I did).  However, their response to this one was that they are not familiar with the Merlyn so cannot be of help (I am paraphrasing).  The bottom line is that I don't think I will get an answer, sorry.  I can tell you only that according to them, and to all the engine people I have talked to about the issue, it should not happen as long as you (1) were LOP to begin with and (2) held fuel flow steady while increasing MP. 

 

carusoam you ask if I have engine data.  when I first did these experiments last spring I did, of course.  But did not save it.  I will probably play around with it some more this spring and if I do I will keep and post data. 

[FBCK]

Prehaps the folks at Merlyn who produce the wastegate have some of the answers. I'm concerned as you are that I maybe pushing too hard by not really understanding what is going on.

[jlunseth]

I don't think the issue relates to the wastegate controller.  The wastegate controller only has to do with MP, and for more horsepower to be produced when the engine is LOP there must either be more fuel or some change in the combustion event, for example the higher MP might result in better fuel delivery to the cylinders. With all respect to Mr. Deakin, I think he was just trying to avoid answering a question he did not know the answer to.

[CaptnJ]

This kind of bothered me so I tryed posting a similar question on Beechtalk, I know Deakin hangs out there. He never really responded but another guy did, I thought it was a good point, he said maybe it's not making more power maybe we're opening the throttle plate more and there for we have lower pressure delta across the compressor, there by making the compressor easier to spin and reducing back pressure in the exhaust. Because we have a small compressor it's even more pronounced. We're not making more power, we're just wasting less.

[carusoam]

So I fired up my MS FS 2004...

With a Bravo...

Ready to duplicate parts of the discussion here... Just to see how well MicroSoft handled real life...

There is one ship's gauge for CHT...

And one for TIT...

Unfortunately, there is no JPI...

Where is your TIT mounted?

The EGT gauge in the O is mounted in the confluence of three exhaust streams on one side of the engine.

The JPI sensors are mounted inches from the exhaust valves further upstream.

The exhaust gasses are cooling and expanding by the time they reach the ship's EGT gauge...

This may be similar to the 231’s TIT and JPI...?

Best regards,

-a-

[jlunseth]

Captnj-  I suppose that is possible.  The turbo is a draw on the power generated by the engine, and if you can somehow reduce that draw there is more power available to drive the prop.  However, it is hypothesis.  It would take more sensors and gauges than any of us have in our engines to understand what is happening.  Further, any such change in the combustion event would mean changes in the ICP and all of your LOP parameters.  Did you just go more LOP, less LOP, not even LOP but somewhere else?  Don't know myself. 

 

carusoam-  I have not noted where all the probes are located.  I will check, I can probably see the TIT probe, we have a door for inspection of the turbo.

 

EGT probe placement from one cylinder to the other can be different, and that is why absolute EGTs are not particularly important.

 

The only thing I noticed in your list is the item about exhaust gasses cooling and expanding by the time they reach the EGT sensor.  I believe they are expanding, yes.  But they may or may not be cooler, they may actually be getting hotter.  I am told there is a secondary event that occurs in the exhaust, causing the exhaust gas temps to actually rise.  Therefore, the TIT (the temp of the gases by the time they get to the turbo) is higher than the EGT's (the temp of the gases a short distance outside the exhaust valve).

[jlunseth]

Well, I did a little research and here is a hypothesis that fits.  There is a moment in time when both the intake and exhaust valves are open at the end of the exhaust cycle and beginning of the intake cycle.  This helps the engine by using fresh intake mixture to flush out the last of the exhaust, but in theory it can also hurt the engine a little because some amount of fresh intake mixture escapes out the exhaust valve.  The mixture would then burn in the exhaust system, which is one reason TIT is higher than EGT.  Another reason TIT is higher is that an EGT probe is exposed only to a periodic burst of exhaust gas when the exhaust valve is open, and the rest of the time when the valve is closed and the probe is cooling, so it will register a cooler temperature than the exhaust gas actually is, it is registering the probe's own average temperature given the heating by the exhaust gases when present and the cooling when not present.  The TIT probe, on the other hand, experiences a much more constant gas temperature because it receives the exhaust gases of all the cylinders.

 

Back to the moment in time when the exhaust and intake valves are open, it appears that there can be some loss of fresh fuel/air mixture out the exhaust port.  The term I learned for this was the ejection fraction, but I understand that others use a different term for it.  If MP is raised, that would cause the turbo to work harder to produce the increase MP, and that in turn would exert higher back pressure in the exhaust system.  The higher backpressure would in turn reduce or eliminate any leakage of fresh fuel/air out the exhaust port, possibly increasing the power output of the engine.  Since fuel flow is measured by a flow meter before the intake cycle, the flow meter would not register a higher fuel flow - it does not know whether some of the fuel is passing out of the exhaust port before combustion, or not. 

 

That is my own hypothesis, and I must stress that I am not an engine expert.   

 

The problem with it is that at the higher power output we still do not know what the effect has been on how far LOP the engine is running.  It also calls into question whether it is strictly true that horsepower when operating LOP is dictated by fuel flow.  It is indeed dictated by fuel flow, but there is variability in measuring how much of the fuel flow is actually used in combustion, and therefore not a direct relationship between the number from the fuel flow meter, and horsepower.  But at least the fuel/air ratio would not be changing, there would simply be more fuel/air mixture at the same ratio involved in the combustion process.

[carusoam]

Real vs. Ideal...?

If you ever get to watch an engine run at night with short exhaust pipes on it...

It makes it easier to visualize. The flames are really coming past the exhaust valve...ROP...

During LOP the flames would be greatly reduced...but probably still visual.

Valve overlap...? The period of time (degrees) when both valves are open.

LOP HP = f(FF in - FF out). FF out (LOP) of the exhaust valve is minimal to compared to FF out (ROP). In discussion, it is assumed to be zero, even though it can't be...

From mass and energy balances we could better define what's going on. But we would need to know pressure drops that are unmeasured. Estimating what's going on in a non continuous process becomes really challenging...

From last night's FS experiment I was able to measure going LOP solely using TIT...

One of the short comings with simulation... The Bravo would have vibrated off my desk with the method I was using...

Moving the throttle always moves the FF.

Best regards,

-a-

[CaptnJ]

So maybe looking at the FF LOP is not as accurate a method as we hoped in determine HP.  We could still determine our HP very accurately with true airspeed.  It takes a known amount of power to make the aircraft go a specific speed.  our performance charts would tell us, then using ROP settings we could find at 65% power we go a certain speed.  Then just pull back to LOP and fly that speed.  As long as you were below 65% power you're not supposed to hurt anything.

 

My question are...

Are the performance carts that accurate?

How does the factory determine engine HP? Dyno or extrapolation?

 

One of the things I always wondered about was the fact that LOP, you were going to be a few kts slower, well if that is true you are really not making the same HP, aerodynamically the aircraft has no idea if it is ROP or LOP.

The other thing,  if we can't really determine a true temp peak, then it does not matter if we are LOP or ROP, we don't know on either side. The only safe thing to do is fly at low power, but thats not really what i want to do.  I still feel safer LOP, even if I'm not 100% sure of the exact amount LOP.  It seems easier to control the CHT LOP.  I can always lean a little, and decent power settings are easier, there just seems to be less messing with engine controls LOP, less to worry about when changing power settings, but thats just me, opinions may vary.

[CaptnJ]

"The problem with it is that at the higher power output we still do not know what the effect has been on how far LOP the engine is running. It also calls into question whether it is strictly true that horsepower when operating LOP is dictated by fuel flow. It is indeed dictated by fuel flow, but there is variability in measuring how much of the fuel flow is actually used in combustion, and therefore not a direct relationship between the number from the fuel flow meter, and horsepower. But at least the fuel/air ratio would not be changing, there would simply be more fuel/air mixture at the same ratio involved in the combustion process." jlunseth

I'm sure you have seen the analogy of combustion as a dance party, ROP mixture- all the air finds a partner, some fuel goes home alone, stoichiometric-everyone find a partner, LOP- fuel finds a partner, some air goes home alone. But in reality there is no way that LOP every single air molecule finds a partner, the mixture distribution is just not that good. What if by adding MP, you're really just putting more air on the dance floor, there by increasing the odds fuel finds a partner. More complete fuel use = more HP ~up to a point of too lean, adding too much MP would eventually result in power loss

"Back to the moment in time when the exhaust and intake valves are open, it appears that there can be some loss of fresh fuel/air mixture out the exhaust port. The term I learned for this was the ejection fraction, but I understand that others use a different term for it. If MP is raised, that would cause the turbo to work harder to produce the increase MP, and that in turn would exert higher back pressure in the exhaust system. The higher backpressure would in turn reduce or eliminate any leakage of fresh fuel/air out the exhaust port, possibly increasing the power output of the engine. Since fuel flow is measured by a flow meter before the intake cycle, the flow meter would not register a higher fuel flow - it does not know whether some of the fuel is passing out of the exhaust port before combustion, or not." jlunseth

also with a higher MP the % of fuel loss vs total mass of the intake air charge, would be less... meaning less fuel loss during valve overlap.

did i say that right?

[carusoam]

The challenge in this part of the conversation is...

1) The throttle control is more than an MP knob...

2) Throttle adjusts both volumetric (/mass) air flow (MP) and ff at the same time.

3) in some cases FF is not linear near max throttle. Additional fuel may be allowed for cooling at high power settings.

4) The mixture knob CAN adjust just the FF.

This is the reason why the MP is set first then the mixture is adjusted to match expectations afterwards...

On the other subject...

Measuring the degree of LOP by using the planes's performance instruments...?

- yes you can

- but, the number of variables and time delay that are added make it unrealistic compared to what a modern JPI can do.

- precision of the method will be off as well...

Things that help this method of operation usable...

- vernier knobs.

- quick response T/Cs.

- engine monitor with continuous display of all EGTs.

- FF monitor.

On my JPI, the T/Cs are a bit slower than I can dial the mixture knob. If a large adjustment needs to be made, allow the JPI to catch up.

If you blow through the peak target to quickly, put a few rotations back in to identify the peak. The peak won't move...

With practice, this takes parts of a minute. With previous MP and FF targets for your specific plane it takes seconds...?

If it takes more than Parts of a minute to set LOP targets at constant MP, there is room for hardware improvement...

The reason for the big pull is it's ability to go LOP quickly by passing through the red box without wasting time...

Yes, you will be in the red box...don't stay in the red box..

APS, the big name in LOP Ops, recommends 65% HP or less to stay on the safe side while experimenting...

Know that the Red Box is larger at higher %HP and nonexistent (See APS for details) at this target.

Keep your eyes on the real applied value of LOP. There will always be theoretical challenges at the molecular level...even OAT will effect Mass Air Flow compared to Maniflod Pressure.

Identify what will work for you...

- All out speed, go ROP...

- Mooney efficiency, go LOP at 65% HP. Operating at peak, is close to the side of the red box.

- TC'd power requires knowledge of how big the red box is at the power setting you are using. How far LOP or ROP you need to be to stay out of the box.

Being able to lean properly is equally effective for LOP as it is for ROP ops...

Hope that helps...

Let me know if I missed anything or messed up.

Best regards,

-a-

The dance floor analogy is good, but

- a bit too simple to handle the cooling effect supplied by having too many boys(ROP) vs too many girls(LOP)...?

- a bit too complex because some people have better 'dance skills' and are more effective than others... This is where LL catalysts come into the discussion...?

[schule]

Some good info for experimentation on our trip this weekend.

[FBCK]

Jl I was talking to the senior engineer at Merlyn a while ago and he told me there was no relationship with MP and turbo speeds, I was a little bit surprised too. Also if you look at there website they make claims about the waste gate; it increases airspeed and reduces fuelflow, all of which we are talking about here.

 

http://www.merlynproducts.com/blackmagicm.html