Engine Stress, Power and Fuel Mixture Management

[N201MKTurbo]

34 minutes ago, Hank said:

Or do like Clarence and get more cylinders!   

You mean that inline twin he brags about?

[tmo]

I'm pretty sure it was some American that coined the phrase "there's no replacement for displacement"...

[exM20K]

1 hour ago, Hank said:

What has always bothered me about all of this is that they never delve into ICP, they just say "look, running here is high ICP, but running here is lower ICP, and that has to be better." Calculate theoretical ICP, juxtapose it against material stress-strain curves, then take some measurements to prove your math is correct. How much ICP is bad? How much is it reduced going from 50ROP to 100ROP or 50LOP? What forces on the pistons are our engines designed to handle? Surely there's a minimum piston force that's part of the design criteria too, where does it appear on the tables? "Reducing ICP is a good thing. Here it's higher than over here, so you want to run where it's lower" is not a universal truth.

I probably could have knocked the math out in a hour back in my senior year of college, but I haven't done much of anything with thermodynamics, heat transfer, combustion chemistry, etc., since I finished my BSME lo these many years ago . . . . 

I agree that fixating on ICP may be focusing on a spurious cause of engine wear.  However, lower ICP = lower temps generally, maybe less crankcase pressure etc.  

I don’t have the APS materials in front of me, but my recollection is that when the big fleet operators of round engines adopted SOP/LOP, they saw quantifiable, documented benefits in engine life.

there are lots of failure causes to fret. Corrosion from disuse, maintenance induced failures like improperly torqued case fasteners, metal failure in cranks or rods or pins, and so on.  LOP or ROP operation will not have much influence on these. These are catastrophic failures.  I believe the operational savings identified in the APS materials had more to do with top end life,  so while one may not be able to draw a line connecting ICP to top end wear, it’s not unreasonable to conclude that cooler cylinders pushing out cleaner exhaust with less blow by into the crank case will last longer.

if someone here has the APS materials handy and could post a synopsis of the piston airline experience, that would be great.

-Dan

[PT20J]

All the engine manufacturers caution against high CHTs and I think that is the primary operational constraint on mixture strength and power. George has tried to draw a direct correlation between ICP and CHT vs mixture strength curves. But, he never showed a complete set of data to demonstrate such a correlation when I took the APS course in Ada in 2007 and I have never found another source for this supposed direct correlation. But it doesn’t really matter because we can only measure CHT anyway.

As for the airlines, LOP was primarily a fuel saving measure. The part about reliability that conveniently seems to get left out is that the airlines set cruise power at 50% or less.

Skip

 

 

 

[aviatoreb]

2 hours ago, N201MKTurbo said:

It seems like everybody wants more power. The only way to increase your power is to increase cylinder pressure.

Not entirely true.  The peak pressure could be higher in a higher impulse event, vs peak pressure during a longer duration but lower peak impulse.  This is supposed to be the main advantage of running LOP vs ROP when comparing the same exact HP.  Less stress on the cylinder at same HP. 

[Hank]

1 hour ago, tmo said:

I'm pretty sure it was some American that coined the phrase "there's no replacement for displacement"...

Big Daddy Don Garlits, back in the 70s. Used to watch him tearing down the dragstrip, stupid fast! Not in person, I was still a kid . . . .

[PT20J]

6 minutes ago, aviatoreb said:

Not entirely true.  The peak pressure could be higher in a higher impulse event, vs peak pressure during a longer duration but lower peak impulse.  This is supposed to be the main advantage of running LOP vs ROP when comparing the same exact HP.  Less stress on the cylinder at same HP. 

I think @N201MKTurbo was referring to BMEP. It’s really the area under the ICP curve between TDC and the exhaust valve opening minus the area from ignition to TDC that represents work done on the piston. The position and magnitude of the peak has some effect, but not as much as might be implied in the APS course. That’s why MBT (maximum brake torque) timing has such a flat curve over +/- 5 degrees or so from peak. I always thought it interesting that the APS course made such a point of the peak position when discussing the effect of mixture on burn rate, but in a separate section included a chart showing that ignition timing (which of course affects the peak position) has relatively little effect.

Skip

[N201MKTurbo]

2 hours ago, PT20J said:

I think @N201MKTurbo was referring to BMEP. It’s really the area under the ICP curve between TDC and the exhaust valve opening minus the area from ignition to TDC that represents work done on the piston. The position and magnitude of the peak has some effect, but not as much as might be implied in the APS course. That’s why MBT (maximum brake torque) timing has such a flat curve over +/- 5 degrees or so from peak. I always thought it interesting that the APS course made such a point of the peak position when discussing the effect of mixture on burn rate, but in a separate section included a chart showing that ignition timing (which of course affects the peak position) has relatively little effect.

Skip

It is such a complex system. I have a book at home written in 1941 that does an exhaustive analysis of all of this. It has more calculus than the Big Bang Theory white board. This book tells you how to calculate the power output of an engine. Just the motion of the piston is way more complex than most people imagine (It is the sum of two sine functions) It goes into how to calculate the optimal max pressure angle (usually about 15 deg ATDC) The effects of mixture, cylinder pressure, temperature and timing. It is amazing what they knew back then. Everything being talked about here is in that book.

https://www.ebay.com/itm/223859902162?chn=ps&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&itemid=223859902162&targetid=1263104805526&device=c&mktype=pla&googleloc=9030049&poi=&campaignid=13918299270&mkgroupid=124965440116&rlsatarget=pla-1263104805526&abcId=9300614&merchantid=6296724&gclid=EAIaIQobChMIybrslubF9AIViBXUAR1x2Q06EAQYAyABEgIjg_D_BwE

Edited December 2, 2021 by N201MKTurbo

[jaylw314]

1 minute ago, N201MKTurbo said:

It is such a complex system. I have a book at home written in 1941 that does an exhaustive analysis of all of this. It has more calculus than the Big Bang Theory white board. This book tells you how to calculate the power output of an engine. Just the motion of the piston is way more complex than most people imagine (It is the sum of two sine functions) It goes into how to calculate the optimal max pressure angle (usually about 15 deg ATDC) The effects of mixture, cylinder pressure, temperature and timing. It is amazing what they knew back then. Everything being talked about here is in that book.

My friend, who's an IA, gave me a (very) old copy of "Aircraft Powerplants".  I was flipping through it thinking how helpful and applicable it was, and then realized it was published in the early 1960's 

[201er]

5 hours ago, Hank said:

What has always bothered me about all of this is that they never delve into ICP, they just say "look, running here is high ICP, but running here is lower ICP, and that has to be better." Calculate theoretical ICP, juxtapose it against material stress-strain curves, then take some measurements to prove your math is correct. How much ICP is bad? How much is it reduced going from 50ROP to 100ROP or 50LOP? What forces on the pistons are our engines designed to handle? Surely there's a minimum piston force that's part of the design criteria too, where does it appear on the tables? "Reducing ICP is a good thing. Here it's higher than over here, so you want to run where it's lower" is not a universal truth.

I probably could have knocked the math out in a hour back in my senior year of college, but I haven't done much of anything with thermodynamics, heat transfer, combustion chemistry, etc., since I finished my BSME lo these many years ago . . . . 

I think the point is like “we don’t really know and we don’t really have solid proof, but lower pressure/temperature has gotta be better than higher so it makes sense to keep it as low as practical for whatever increase in longevity it’s worth.”

There isn’t enough real world operational information to truly know the difference. There’s folks out there who ran LOP who had premature engine failure just like folks who ran ROP. We would need statistics across hundreds if not thousands of complete TBO cycles running predominantly ROP and predominantly LOP to know what difference it really makes.

[EricJ]

46 minutes ago, jaylw314 said:

My friend, who's an IA, gave me a (very) old copy of "Aircraft Powerplants".  I was flipping through it thinking how helpful and applicable it was, and then realized it was published in the early 1960's 

If it was recent it'd all be turbines.    Even the most recent turbines differ quite a bit from those made thirty years ago.   It used to be normal to run everything off bleed air, but the newest engines don't do that because the efficiencies are much higher and bleeding off compressor stage air screws up all the tuning.    So the newest airplanes tend to be all electric or something like that.

Everything is a moving target. 

Even with reciprocating engines the whole picture changes when you change the optimization metric.   Want max power?   Okay...this is what the cycle should look like.   Want max fuel efficiency?   Okay...now it looks like this.  etc., etc.    One of the beauties of EEC/FADEC is being able to exploit sensors to change the optimization mapping on the fly based on performance and demand.   The whole LOP/ROP/box/fin is a meager attempt at managing that by hand in an approximated steady-state condition.   It is fairly imperfect, imho. 

 

[GeeBee]

True that! A self generating FADEC unit is what is needed to end all this nonsense.

 

 

[Hank]

1 hour ago, EricJ said:

If it was recent it'd all be turbines.    Even the most recent turbines differ quite a bit from those made thirty years ago.

Yep, went from (skinny) turbines to (fat) turbofans to (less round, fatter) high bypass turbofans. Each step was a gain in fuel efficiency and engine complexity.

[EricJ]

1 hour ago, Petehdgs said:

Here is the last revision of UESP&FMM Rev7 Dec 2021.  All previous versions should be discarded. 

I realize I am not perfect and I appreciate your help and support.

One Miracle at a time!

UESP&FMM Rev7 Dec2021.pdf 1.13 MB · 6 downloads

The figures are still a mess.   There are no Figure numbers or captions, on many the axes are unlabelled and without units, so it is unclear what they are actually conveying.   With no figure numbers or captions or titles it is sometimes unclear which figure you are referring to in the text.   The sources of the charts used from elsewhere should be cited at the display of the chart.   In typical practice this is often done in the caption, and often again in a reference list.   Since a lot of the material is from elsewhere, a list of references of sources would seem appropriate. 

It is not necessary to put a copyright mark on individual pages or figures.   It really isn't necesarry to put a mark on original work at all, since original work is always automatically copyrighted to the creator.

I spent a few decades writing and reviewing research publications.   The criticisms might seem pedantic, but it makes a huge difference in readability, usability, and traceability.

[ilovecornfields]

1 hour ago, EricJ said:

The figures are still a mess.   There are no Figure numbers or captions, on many the axes are unlabelled and without units, so it is unclear what they are actually conveying.   With no figure numbers or captions or titles it is sometimes unclear which figure you are referring to in the text.   The sources of the charts used from elsewhere should be cited at the display of the chart.   In typical practice this is often done in the caption, and often again in a reference list.   Since a lot of the material is from elsewhere, a list of references of sources would seem appropriate. 

It is not necessary to put a copyright mark on individual pages or figures.   It really isn't necesarry to put a mark on original work at all, since original work is always automatically copyrighted to the creator.

I spent a few decades writing and reviewing research publications.   The criticisms might seem pedantic, but it makes a huge difference in readability, usability, and traceability.

Do you prefer PayPal or Venmo?

[Petehdgs]

10 hours ago, EricJ said:

The figures are still a mess.   There are no Figure numbers or captions, on many the axes are unlabelled and without units, so it is unclear what they are actually conveying.   With no figure numbers or captions or titles it is sometimes unclear which figure you are referring to in the text.   The sources of the charts used from elsewhere should be cited at the display of the chart.   In typical practice this is often done in the caption, and often again in a reference list.   Since a lot of the material is from elsewhere, a list of references of sources would seem appropriate. 

It is not necessary to put a copyright mark on individual pages or figures.   It really isn't necesarry to put a mark on original work at all, since original work is always automatically copyrighted to the creator.

I spent a few decades writing and reviewing research publications.   The criticisms might seem pedantic, but it makes a huge difference in readability, usability, and traceability.

Eric, I apologize for my tardiness in this reply.  There are reasons I have for doing things the way I have. 

I am assuming you have printed this and have the 10 pages there to look at and scroll through. 

The figures introduced with paragraphs 10, 15, 24, 26, and 39 are not precise.  They are to look at not to measure from. They are just pictures to illustrate a point.  All you need to know about 10 (75%) and 15 (65%) is that they illustrate the RED BOX, the bottom of the box is 400F CHT, and the width of the box is less at 65% than at 75% load.  All of this information is in the text. ROP is on the left LOP is on the right and the dotted line is Peak EGT.  In paragraph 15 I tell you to look at the 75% load box.  In the next paragraph 16 I tell you to look at 65% load, which is right there to look at.  Paragraphs 9 and 10 are the first discussions of the RED BOX, and there is a the 75% load RED BOX on the page to look at. The RED FIN is first discussed in Paragraph 23, and the first picture of it is at paragraph 24, then a different one at paragraph 26. 

The figures introduced in paragraphs 4, 18, 21, 34, 39, and after 45 are precision figures.  You can print them, measure them with an architect's scale and apply a little calculation and you will have precise measurements that are repeatable.  That's what I did.  The figure in paragraph 39 is a re-work of the RED FIN shown in Figure 26, corrected to follow the fuel and air curves more precisely.  Everything you need to know is in the text, but you can print it, measure it, and get precise readings. 

I am not a scholar.  I am a diesel mechanic and a private pilot.  This paper is not for scholars, it is for internet distribution for people who are curious about this subject and want to learn more without spending any money.  In other words airplane pilots.  Airplane pilots who are notoriously CHEAP.  Like me. 

Not all internet people are willing to take this at face value.  Some of them want to make it their own.  Some internet people are likely to pick and choose what they want and discard the rest, and then re-post it somewhere else.  I'm ok with that to a point.  The problem is when someone starts re-wording the document and then posting it in its altered form.  For some reason people love to re-edit everything.  You, yourself, are livid with me because I have not re-written or re-organized the document to meet your expectations.  Within this document I have made specific recommendations for engine operations that put the reader at a point of understanding by using the paragraphs that came before as building blocks of knowledge.  What do I want?  I want the reader to fully understand paragraph 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, and 44.  For most of the audience this means reading and understanding paragraphs 1-32 in sequential order. 

Every paragraph is important! Every paragraph builds on the previous paragraph! And changing anything, in any of the previous paragraphs, could change the reader's understanding of some key element that could affect engine operation in a negative way.  That means possibly, as a result of tampering with this document, engine damage or engine destruction could occur.  Which if it did during critical flight operations could lead to loss of life.  I take that responsibility seriously, and you should too. 

Weather you agree with what I am doing or not, you have to understand the potential liability if a loss of life should occur.  To that end, every page has a copyright notice and the name of the document, and my email address.  Anyone can email me about this document and ask any question they want to ask.  Anyone can search the internet for the key words LOP, ROP, RED BOX, RED FIN, BIG MIXTURE PULL and pull up most of the documents I used as research when creating this paper.  They can't pull up any information about the RFFA chart without emailing me directly.  That is because recreation of this chart is a difficult process that exceeds the confines of the principles presented here. 

I will say that I puzzled and puzzled over the first version of the RFFA chart trying to figure out how to make it readily adaptable to any engine, as requested last year by a poster in POA.  I have finally done that! So here is the paper to give everyone the same level of understanding that I had when I was willing to start experimenting with the operations of my own engine.  So, you could say this entire paper is about paragraphs 34 and 35.  If you don't get them, you haven't learned anything new. 

Everything they say about me is TRUE.  Good or BAD. 

One Miracle at a time. 

Edited December 5, 2021 by Petehdgs

[201er]

1 hour ago, Petehdgs said:

That means possibly, as a result of tampering with this document, engine damage or engine destruction could occur.  Which if it did during critical flight operations could lead to loss of life.  I take that responsibility seriously, and you should too. 

Weather you agree with what I am doing or not, you have to understand the potential liability if a loss of life should occur. 

You are that confident that engine damage or loss of life won’t occur by following your unaltered doc? You have done enough testing?

[Petehdgs]

13 minutes ago, 201er said:

You are that confident that engine damage or loss of life won’t occur by following your unaltered doc? You have done enough testing?

Never say never.  Engines can fail at any time.  But the RED BOX disappears below 60% load, all my initial recommendations are below that, and I recommend monitoring CHT at all times during operations.  So, yes.  Following instructions presented here, based on the work of the giants who came before me, should not harm your engine in any way.  

Edited December 5, 2021 by Petehdgs

[201er]

9 hours ago, Petehdgs said:

Following instructions presented here, based on the work of the giants who came before me, should not harm your engine in any way.  

Even when it contradicts manufacturers recommendations? You take responsibility for that?

Edited December 5, 2021 by 201er

[ilovecornfields]

7 hours ago, Petehdgs said:

Eric, I apologize for my tardiness in this reply.  There are reasons I have for doing things the way I have. 

I am assuming you have printed this and have the 10 pages there to look at and scroll through. 

The figures introduced with paragraphs 10, 15, 24, 26, and 39 are not precise.  They are to look at not to measure from. They are just pictures to illustrate a point.  All you need to know about 10 (75%) and 15 (65%) is that they illustrate the RED BOX, the bottom of the box is 400F CHT, and the width of the box is less at 65% than at 75% load.  All of this information is in the text. ROP is on the left LOP is on the right and the dotted line is Peak EGT.  In paragraph 15 I tell you to look at the 75% load box.  In the next paragraph 16 I tell you to look at 65% load, which is right there to look at.  Paragraphs 9 and 10 are the first discussions of the RED BOX, and there is a the 75% load RED BOX on the page to look at. The RED FIN is first discussed in Paragraph 23, and the first picture of it is at paragraph 24, then a different one at paragraph 26. 

The figures introduced in paragraphs 4, 18, 21, 34, 39, and after 45 are precision figures.  You can print them, measure them with an architect's scale and apply a little calculation and you will have precise measurements that are repeatable.  That's what I did.  The figure in paragraph 39 is a re-work of the RED FIN shown in Figure 26, corrected to follow the fuel and air curves more precisely.  Everything you need to know is in the text, but you can print it, measure it, and get precise readings. 

I am not a scholar.  I am a diesel mechanic and a private pilot.  This paper is not for scholars, it is for internet distribution for people who are curious about this subject and want to learn more without spending any money.  In other words airplane pilots.  Airplane pilots who are notoriously CHEAP.  Like me. 

Not all internet people are willing to take this at face value.  Some of them want to make it their own.  Some internet people are likely to pick and choose what they want and discard the rest, and then re-post it somewhere else.  I'm ok with that to a point.  The problem is when someone starts re-wording the document and then posting it in its altered form.  For some reason people love to re-edit everything.  You, yourself, are livid with me because I have not re-written or re-organized the document to meet your expectations.  Within this document I have made specific recommendations for engine operations that put the reader at a point of understanding by using the paragraphs that came before as building blocks of knowledge.  What do I want?  I want the reader to fully understand paragraph 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, and 44.  For most of the audience this means reading and understanding paragraphs 1-32 in sequential order. 

Every paragraph is important! Every paragraph builds on the previous paragraph! And changing anything, in any of the previous paragraphs, could change the reader's understanding of some key element that could affect engine operation in a negative way.  That means possibly, as a result of tampering with this document, engine damage or engine destruction could occur.  Which if it did during critical flight operations could lead to loss of life.  I take that responsibility seriously, and you should too. 

Weather you agree with what I am doing or not, you have to understand the potential liability if a loss of life should occur.  To that end, every page has a copyright notice and the name of the document, and my email address.  Anyone can email me about this document and ask any question they want to ask.  Anyone can search the internet for the key words LOP, ROP, RED BOX, RED FIN, BIG MIXTURE PULL and pull up most of the documents I used as research when creating this paper.  They can't pull up any information about the RFFA chart without emailing me directly.  That is because recreation of this chart is a difficult process that exceeds the confines of the principles presented here. 

I will say that I puzzled and puzzled over the first version of the RFFA chart trying to figure out how to make it readily adaptable to any engine, as requested last year by a poster in POA.  I have finally done that! So here is the paper to give everyone the same level of understanding that I had when I was willing to start experimenting with the operations of my own engine.  So, you could say this entire paper is about paragraphs 34 and 35.  If you don't get them, you haven't learned anything new. 

Everything they say about me is TRUE.  Good or BAD. 

One Miracle at a time. 

Wow. Just wow.

I think @EricJ may understand the life and death consequences of making the wrong decisions better then you give him credit for. I don’t think he’s the one whose response comes of as livid…

A highly intelligent and skilled professional read your entire lengthy treatise (at least twice!) and gave you constructive feedback (which you specifically asked for) in a subject in which he is an expert. He did this FOR FREE. And you’re complaining about him and criticizing him. Seriously?!

I’m sorry if not everyone thinks this is The Final Word in engine management. This subject has been discussed EXTENSIVELY here and in other places by people who are experts in the field. Many of us have taken the APS course. I apologize if you’re not getting the reception you expected but maybe you can take a few deep breaths, take yourself a little less seriously and thank the people who are actually helping you out. For free. On their own time. 

Edited December 5, 2021 by ilovecornfields

[Petehdgs]

4 hours ago, ilovecornfields said:

Wow. Just wow.

I think @EricJ may understand the life and death consequences of making the wrong decisions better then you give him credit for. I don’t think he’s the one whose response comes of as livid…

A highly intelligent and skilled professional read your entire lengthy treatise (at least twice!) and gave you constructive feedback (which you specifically asked for) in a subject in which he is an expert. He did this FOR FREE. And you’re complaining about him and criticizing him. Seriously?!

I’m sorry if not everyone thinks this is The Final Word in engine management. This subject has been discussed EXTENSIVELY here and in other places by people who are experts in the field. Many of us have taken the APS course. I apologize if you’re not getting the reception you expected but maybe you can take a few deep breaths, take yourself a little less seriously and thank the people who are actually helping you out. For free. On their own time. 

Ok.  Consider me properly admonished.  Perhaps the word livid was too strong a word.  I apologize for that.  Perhaps I really don't know what I am doing when it comes to writing.  "The figures are a mess"  doesn't really give me very much to go on.  They look a lot better than a hand drawn sketch on the back of a cardboard box.  That's what most of my working drawings look like.  It is clear that I don't understand what it is that he is trying to impart.  So I think we should discuss this in real time.  Email me and we can set it up.   

[201er]

On 11/29/2021 at 12:09 PM, Petehdgs said:

Replying to a recent thread on POA made me realize there is an interest for more understanding in the area of fuel mixture management. I think I have a good enough understanding of this subject to teach a class on it, which I intend to do for the local EAA chapter in January. In preparation for the class I wrote 45 paragraphs with illustrations, including a new RFFA chart of my own design. The big improvement is this new RFFA chart can be easily adapted to any piston engine at any power level.

UESP&FMM Rev7 Dec2021.pdf 1.13 MB · 14 downloads

Are you suggesting that you can't cruise at 75% power unless over 175ROP? That's way too rich to make best power. Lycoming says you can be at peak at 75%. Mooney says 25ROP or 100ROP.

[N201MKTurbo]

I worked for 10 years in a large software development environment where criticism was an integral part of the process. All of your work had to be scrutinized by two other developers before it became a part of the project. The criticism can sometimes be brutal. You learn to have a thick skin about it. The young bucks fresh out of school were the worst, they thought they were all geniuses and it was their way or the highway. The process didn’t require you to do anything with the criticism, but their was often very helpful advice in there that made everybody’s work better.

[GeeBee]

I think I'll switch to turbines.

 

[Petehdgs]

19 minutes ago, 201er said:

Are you suggesting that you can't cruise at 75% power unless over 175ROP? That's way too rich to make best power. Lycoming says you can be at peak at 75%. Mooney says 25ROP or 100ROP.

The RFFA chart does suggest that, yes, based on the parameters of the RED BOX and RED FIN.  That is a guideline as a starting point.  As long as CHT are good you can lean more.  The Lycoming FA Ratio chart recommends running 100ROP to 150ROP for best power.  I don't know about 25ROP.  I haven't read that in my manual.