Commercial Pilots who don't know about piston engines
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Andrewr
I demonstrated this just recently, and these numbers are from memory as I did not video it, but here we go;
We were at something like 2500' and maybe 2450-2500 RPM and around 75% power LOP at 49LPH (about 40dF LOP). I then went full rich, and adjusted the MP to get me the same %age power which would have been around 24" and set appropriately 180 or so ROP, which ironically on a well set up Bendix fuel servo was on the full rich stopper or a bee's whisker off it.
Hey presto, same power, same speed, and a whopping (from memory OK) 70-75 LPH, and if I am wrong it was not far from it.
Now that is massive you say. Yep sure is and the higher the power the bigger the fuel spread because you need more so much more fuel to achieve the same peak pressures or at least keep them down.
You need to see this on the Carl Goulet Memorial Dyno test stand to fully appreciate this. No other way to describe it.
Now if we get to say higher altitudes and compare 65% and we are 100-125dF ROP and say 10dF LOP, the percentage of extra fuel is not so massive, for example on a IO520 that might be 47LPH compared to 58LPH. On my IO540 that is 41-42LPH to 53-54LPH (off the top of my head).
I think this is worth repeating, and Walter will no doubt agree……….You need to see this on the Carl Goulet Memorial Dyno test stand to fully appreciate this. No other way to describe it.
I demonstrated this just recently, and these numbers are from memory as I did not video it, but here we go;
We were at something like 2500' and maybe 2450-2500 RPM and around 75% power LOP at 49LPH (about 40dF LOP). I then went full rich, and adjusted the MP to get me the same %age power which would have been around 24" and set appropriately 180 or so ROP, which ironically on a well set up Bendix fuel servo was on the full rich stopper or a bee's whisker off it.
Hey presto, same power, same speed, and a whopping (from memory OK) 70-75 LPH, and if I am wrong it was not far from it.
Now that is massive you say. Yep sure is and the higher the power the bigger the fuel spread because you need more so much more fuel to achieve the same peak pressures or at least keep them down.
You need to see this on the Carl Goulet Memorial Dyno test stand to fully appreciate this. No other way to describe it.
Now if we get to say higher altitudes and compare 65% and we are 100-125dF ROP and say 10dF LOP, the percentage of extra fuel is not so massive, for example on a IO520 that might be 47LPH compared to 58LPH. On my IO540 that is 41-42LPH to 53-54LPH (off the top of my head).
I think this is worth repeating, and Walter will no doubt agree……….You need to see this on the Carl Goulet Memorial Dyno test stand to fully appreciate this. No other way to describe it.
So Andrew, your view is that provided the higher CHTs are under redline, it's OK. You are of course free to choose to operate engines that way (but you'll never get to touch mine).
For my part, I choose to operate my engine where it is put under the least stress to achieve the performance I need. I don't need scales on those curves to know where those settings are, because the data out of my engine monitor over many hundreds of hours (and many thousands of other monitors over millions of hours) and actual TAS and fuel consumed are objective facts.
I still reflect on that poor kid PIC and his pax on the Whyalla Airlines Chieftain. I still wonder whether the engine failures might have been avoided if the mixture had been left sufficiently rich of peak rather than leaning it to a mixture rich of peak that meant the engines were given around about the hardest pounding they could be given. (CHTs still under redline, mind you Andrew. Just before they got very cold.)
For my part, I choose to operate my engine where it is put under the least stress to achieve the performance I need. I don't need scales on those curves to know where those settings are, because the data out of my engine monitor over many hundreds of hours (and many thousands of other monitors over millions of hours) and actual TAS and fuel consumed are objective facts.
I still reflect on that poor kid PIC and his pax on the Whyalla Airlines Chieftain. I still wonder whether the engine failures might have been avoided if the mixture had been left sufficiently rich of peak rather than leaning it to a mixture rich of peak that meant the engines were given around about the hardest pounding they could be given. (CHTs still under redline, mind you Andrew. Just before they got very cold.)
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There's been a lot of discussion on here about the merits of running LOP and ROP to varying degrees but I'm interested to know what you guys think about doing neither ie running at peak EGT? I notice in the POH for the PA-32R-300 which has an IO-540 engine, the performance charts for economy cruise, range and endurance, are all based on 'mixture leaned to peak egt'. The performance cruise chart says to use 'mixture leaned to 100deg rich of peak EGT at 75% power and below'.
These are based on standard issue ex factory Lance with single probe EGT.
Any comments on the merits of these performance charts..? All I've ever heard of is leaning to peak EGT, then either leaning further to LOP or leaning less to ROP but never staying at peak EGT.
These are based on standard issue ex factory Lance with single probe EGT.
Any comments on the merits of these performance charts..? All I've ever heard of is leaning to peak EGT, then either leaning further to LOP or leaning less to ROP but never staying at peak EGT.
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This thread is full of misinformation.
**Ah, but is it? Remember what EGT is measuring - the temperature of a probe inserted at some point down the exhaust pipe. It is at best measuring the average EGT at that point.***
It is NOT measuring the average EGT. It is measuring the EGT minus the refractory period cooling between pulses of hot air.
***I would expect that if damage to valve sealing surfaces occurs, it would be most influenced by the gas temperature at the instant the valve begins to open and the temperature of the first perhaps <1% of gas to exit. ***
That is proven to be false. EGT does NOT affect valve temperature. We have hard data to prove that. The data came from a 1943 top secret NACA report done during WWII.
***Measurements I would be interested to see:
- Instantaneous EGT in the exhaust port through the combustion cycle vs measured EGT as the mixture changes
- Temperature of the valve head vs EGT as the mixture changes***
Both of those are available. The first answer is that the two temperatures track rather closely--as in immeasurably different in delta
The second is that valve temperature does NOT track EGT. This is 1943 data, not ours. Lycoming verified it in 1966. We ran the test confirming this about 15 years ago. Funny how the physics is everywhere the same. The hottest valve temperature is found with a mixture of 40dF ROP. As the EGT is going up toward peak, the valve temperature is getting cooler. Anyone who suggests that EGT affects valve temperature must reconcile this fact. In addition, there are times when the exhaust gasses blasting past the valve are actually cooling the valve. These were surprises we learned while MEASURING these things.
BTW, EGT is not a measure of the temperature of combustion. It is the result of the expansion of the 3800dF combustion temperature. That's why low compression engines have higher EGTs and higher compression engines have lower EGTs.
**Ah, but is it? Remember what EGT is measuring - the temperature of a probe inserted at some point down the exhaust pipe. It is at best measuring the average EGT at that point.***
It is NOT measuring the average EGT. It is measuring the EGT minus the refractory period cooling between pulses of hot air.
***I would expect that if damage to valve sealing surfaces occurs, it would be most influenced by the gas temperature at the instant the valve begins to open and the temperature of the first perhaps <1% of gas to exit. ***
That is proven to be false. EGT does NOT affect valve temperature. We have hard data to prove that. The data came from a 1943 top secret NACA report done during WWII.
***Measurements I would be interested to see:
- Instantaneous EGT in the exhaust port through the combustion cycle vs measured EGT as the mixture changes
- Temperature of the valve head vs EGT as the mixture changes***
Both of those are available. The first answer is that the two temperatures track rather closely--as in immeasurably different in delta
The second is that valve temperature does NOT track EGT. This is 1943 data, not ours. Lycoming verified it in 1966. We ran the test confirming this about 15 years ago. Funny how the physics is everywhere the same. The hottest valve temperature is found with a mixture of 40dF ROP. As the EGT is going up toward peak, the valve temperature is getting cooler. Anyone who suggests that EGT affects valve temperature must reconcile this fact. In addition, there are times when the exhaust gasses blasting past the valve are actually cooling the valve. These were surprises we learned while MEASURING these things.
BTW, EGT is not a measure of the temperature of combustion. It is the result of the expansion of the 3800dF combustion temperature. That's why low compression engines have higher EGTs and higher compression engines have lower EGTs.
Last edited by Walter Atkinson; 23rd Feb 2016 at 19:25.
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***Also, ignition timing advance maintained peak pressure at the most effective point as the mixture was leaned. Why then do APS say that it is a good thing to delay the peak pressure at cruise power (which is the same effect as retarded ignition timing)?***
Because the fixed timing is "averaged" in our flat engines to be "acceptable" across the mixture spectrum. The R-4350, along with some other radials had the timing retarded at max power to control ICPs and CHTs, and more importantly to stay barely outside the detonation margin. This retarded takeoff timing at max power was unacceptably retarded at cruise powers, so they had a way to alter timing to make it work efficiently in both high-power and cruise-power configurations.
One must be careful in comparing these engines and their engineering requirements to our GA engines.
The effective timing on our engines is too far forward at high powers ROP and slightly retarded when in cruise. If we retarded the timing during takeoff, the engine would produce more power and the CHTs would run lower, but if we did that, the timing would be so far retarded in cruise as to be very inefficient. It's a trade-off. We do not advocate delaying the peak pressure beyond 16dATDC. That is optimal.
'Tis amazing how often one gets misquoted by internet experts.
Because the fixed timing is "averaged" in our flat engines to be "acceptable" across the mixture spectrum. The R-4350, along with some other radials had the timing retarded at max power to control ICPs and CHTs, and more importantly to stay barely outside the detonation margin. This retarded takeoff timing at max power was unacceptably retarded at cruise powers, so they had a way to alter timing to make it work efficiently in both high-power and cruise-power configurations.
One must be careful in comparing these engines and their engineering requirements to our GA engines.
The effective timing on our engines is too far forward at high powers ROP and slightly retarded when in cruise. If we retarded the timing during takeoff, the engine would produce more power and the CHTs would run lower, but if we did that, the timing would be so far retarded in cruise as to be very inefficient. It's a trade-off. We do not advocate delaying the peak pressure beyond 16dATDC. That is optimal.
'Tis amazing how often one gets misquoted by internet experts.
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***Temperature and pressure is what turns the prop around***
That is an over-simplified and basically incorrect statement. I can demonstrate many instances where a lower combustion temperature and a lower peak pressure produces more power than higher temps and pressures.
There are two enemies of metal: heat and pressure. It's good to control them.
Mean cylinder pressure relates to HP.
Peak cylinder pressure relates to longevity.
It is quite possible to have extremely high pressures with very little to zero power being produced. ??pre-ignition??
That is an over-simplified and basically incorrect statement. I can demonstrate many instances where a lower combustion temperature and a lower peak pressure produces more power than higher temps and pressures.
There are two enemies of metal: heat and pressure. It's good to control them.
Mean cylinder pressure relates to HP.
Peak cylinder pressure relates to longevity.
It is quite possible to have extremely high pressures with very little to zero power being produced. ??pre-ignition??
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The Lycoming recommendation to be 100dF ROP at powers above 75% is suboptimal. One should be richer than that if longevity and engine health are your goals.
The Lycoming recommendation to run AT peak EGT for economy cruise is based on two facts: 1) that their engines' F:A balance is not good enough to run smoothly LOP, and 2) the difference in efficiency between peak and BSFC(min) is relatively, but measurably small. This recommendation was made for one purpose... to keep the customer service phone from ringing off the hook when their engine ran rough LOP.
Gentlemen:
There is a lot of misunderstanding and misinformation being posted in this thread. While there are certainly others who understand this well, the posts by Lead Ballon and Jabber are the posts with accurate information that can be supported by documented, repeatable, hard data.
DO NOT believe me, Lead Ballon, Jabber or any one else. Believe the Data. We have seen the data and have drawn conclusions based on that, not what we or anyone else might "think" to be right.
'Tis important to consider that without data, all you are is another person with an unsupported opinion.
The Lycoming recommendation to run AT peak EGT for economy cruise is based on two facts: 1) that their engines' F:A balance is not good enough to run smoothly LOP, and 2) the difference in efficiency between peak and BSFC(min) is relatively, but measurably small. This recommendation was made for one purpose... to keep the customer service phone from ringing off the hook when their engine ran rough LOP.
Gentlemen:
There is a lot of misunderstanding and misinformation being posted in this thread. While there are certainly others who understand this well, the posts by Lead Ballon and Jabber are the posts with accurate information that can be supported by documented, repeatable, hard data.
DO NOT believe me, Lead Ballon, Jabber or any one else. Believe the Data. We have seen the data and have drawn conclusions based on that, not what we or anyone else might "think" to be right.
'Tis important to consider that without data, all you are is another person with an unsupported opinion.
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I have seen and understand the charts, but it's hard to estimate the significance when there is no scale. I see the theoretical increase in efficiency but without a scale you can't tell whether it's 10%, 1% or 0.1%.
All I know is that people keep posting their "fuel savings" from running LOP, and when you do the calculation it's no better than what you would expect from the speed reduction.
Here's the test we need to see:
Set your LOP 150 knot cruise and note the fuel flow.
Then set the recommended best economy setting and adjust the throttle for the same 150 knots (same rpm). How much does the fuel flow differ?
All I know is that people keep posting their "fuel savings" from running LOP, and when you do the calculation it's no better than what you would expect from the speed reduction.
Here's the test we need to see:
Set your LOP 150 knot cruise and note the fuel flow.
Then set the recommended best economy setting and adjust the throttle for the same 150 knots (same rpm). How much does the fuel flow differ?
Look, it's not a difficult concept. Start with the understanding that all else being equal, power varies with mixture, and that there exists some optimum mixture where power is at a maximum. It seems a pretty simple concept that for a given desired power output there must also exist two points, on the rich and lean side of the peak, where power is equal to that desired output. and for those two mixture stetting which yield equal power output, the cruise airspeed will be the same. And if the airspeed is the same, and one setting has lower fuel flow, then the fuel burned per mile is going to be less. I'm not sure which part of this you can't get your mind wrapped around.
Avoid imitations
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From those notes, when leaned at cruise power the ignition timing was advanced from 20 to 30 degrees which favored longer valve life.
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Shy Torque
Probably? Well it might seem to make sense, but it does not work as described. The best suggestion I have is read carefully what Walter wrote above.
Probably? Well it might seem to make sense, but it does not work as described. The best suggestion I have is read carefully what Walter wrote above.
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Lead Balloon
Scammed? A little defensive there I think LB. I have asked for clarity and data. This is a public forum and so it is completely fair for advice being offered here to be scrutinised here. Of course if you don't like that, too bad. You can always take your own advice to "move on". But I am here to stay
This is why there is a 2.5 day face-to-face course.
Reacting to ad hoc questions on a blog is not an efficient or effective way to teach people engine management. It is also unreasonable to expect someone to divert their personal time to answering every supplementary question that will almost invariably arise from a narrow explanation of the issues relevant to a specific set of circumstances.
If you think you're being scammed, oggers, just move on. Leave the credulous to the snake oil salesmen.
If you don't think you're being scammed, perhaps you should ask yourself whether it's reasonable to demand that you be given, free of charge, a complete, comprehensive, written answer, covering all the 'ifs' and 'buts' and exceptions, to every question you're inclined to ask.
If you think you're being scammed, oggers, just move on. Leave the credulous to the snake oil salesmen.
Reacting to ad hoc questions on a blog is not an efficient or effective way to teach people engine management. It is also unreasonable to expect someone to divert their personal time to answering every supplementary question that will almost invariably arise from a narrow explanation of the issues relevant to a specific set of circumstances.
If you think you're being scammed, oggers, just move on. Leave the credulous to the snake oil salesmen.
If you don't think you're being scammed, perhaps you should ask yourself whether it's reasonable to demand that you be given, free of charge, a complete, comprehensive, written answer, covering all the 'ifs' and 'buts' and exceptions, to every question you're inclined to ask.
If you think you're being scammed, oggers, just move on. Leave the credulous to the snake oil salesmen.
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And, I have responded in an attempt to clarify and have provided some data. An internet forum precludes the presentation of very much data because of the bandwidth alone. I have presented over 16 hours of data dozens of times over 16 years and not one of the thousands who have looked at that data have questioned the accuracy of it. Anyone may chose whether or not they wish to take advantage of the opportunity to see and study that data, but to demand that it be provided free and at their whim seems a bit unreasonable.
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Little Donald has done very well for someone brought up in poverty in a poor family without a billion $ to their name. How many of you could be where he is today if you were only left a lousy $44million to start your investment career?
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Can the ppruners tell me if this man speaks truths?
He also tells us to go to APS.
Can the ppruners tell me if this man speaks truths?
He also tells us to go to APS.
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When ROP, power is determined by mass airflow. FF has little effect in the normally used ROP mixtures. Once they get overly rich, yes, power decreases.
When LOP, power is directly related to FF. Mass airflow plays no part.
The reason there is a Best Power mixture has nothing to do with FF. It is the mixture where the mass airflow is adequate to burn all of the fuel. That does NOT occur at stoichiometric (AKA, Peak EGT). It occurs at approximately 75-80dF ROP.
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Can the ppruners tell me if this man speaks truths?
He also tells us to go to APS.
Can the ppruners tell me if this man speaks truths?
He also tells us to go to APS.
Where we disagree with Mike is on his recommendation to lean based on CHT. This is a very flawed concept. In addition, we do not think his approach to maintenance is optimal where dispatchability is concerned. Other than those two issues and few minor peccadilloes, he's got a lot of good information.
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Hm? That's not quite correct.
When ROP, power is determined by mass airflow. FF has little effect in the normally used ROP mixtures. Once they get overly rich, yes, power decreases.
When LOP, power is directly related to FF. Mass airflow plays no part.
The reason there is a Best Power mixture has nothing to do with FF. It is the mixture where the mass airflow is adequate to burn all of the fuel. That does NOT occur at stoichiometric (AKA, Peak EGT). It occurs at approximately 75-80dF ROP.
When ROP, power is determined by mass airflow. FF has little effect in the normally used ROP mixtures. Once they get overly rich, yes, power decreases.
When LOP, power is directly related to FF. Mass airflow plays no part.
The reason there is a Best Power mixture has nothing to do with FF. It is the mixture where the mass airflow is adequate to burn all of the fuel. That does NOT occur at stoichiometric (AKA, Peak EGT). It occurs at approximately 75-80dF ROP.
Here's what *I* meant: If you had a power meter on your engine*, and you started with the throttle set and the mixture at full rich, then you started gradually pulling the mixture toward lean, you would see the indication on your power meter rise, peak, then fall as the mixture control moved toward Idle Cut Off.
I'm not sure what you mean about power not being a function of fuel flow exactly, and I'm not saying that you're wrong, but I think you may be making a semantic distinction that's not relevant to my statement.
* I flew for a number of years in an airplane which had power meters on the engines. For a constant RPM, a torque meter is a power meter. I have watched the needles rise then fall many, many times.