Commercial Pilots who don't know about piston engines
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The position that you and Walter Atkinson have taken (and as far as I can tell it is an APS thing) is flat out wrong. Let me repeat your position "EGT and exhaust valve temperature are not in any way related". Wrong.
One point that is interesting from the graph in figure 8 though is that for a given EGT when ROP the valve temperature is hotter than LOP.
i.e. at say 1400 degrees LOP results in valve temp of 1200 degrees and 1400 degrees ROP results in a valve temp 1300 degrees. i.e about 100 degrees hotter.
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So how many Rastafarians can dance on the head of a pin?
Are the Pirates in the South China Sea, included in the paradigm?
A serious question, why is the exhaust valve exposed to the highest temperature during the combustion process, when exhaust is at the end of the Otto (modified) cycle?
Are the Pirates in the South China Sea, included in the paradigm?
A serious question, why is the exhaust valve exposed to the highest temperature during the combustion process, when exhaust is at the end of the Otto (modified) cycle?
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oggers, let me make it really simple.
YOUR hypothesis is true….until.
The valve temperature "tracks" the EGT, because as the mixture at any given mass airflow is leaned, the EGT goes up AND the CHT goes up. But why does the CHT go up? Because the peak cylinder pressures go up. Just like your air compressor in the garage.
Now at the moment your hypothesis is true right up until the moment they stop tracking each other and diverge. Hypothesis BUSTED. end of argument.
Now when the CHT tracks the valve temp hypothesis is tested in exactly the same way, and it does not diverge, it follows the valve temperature…..what can you make from that? Because when Lycoming did their test with far newer equipment they plotted even better curves showing this in detail. I do not have them at hand, but in several weeks I will be right there in the dyno room and will surely be able to get them, if I remember.
Now just so you know I am not making this stuff up, lets examine what else the NACA had to say about valve temperature. Remember they were not measuring ICP in those days, we can today. "The effect of spark advance on valve temperature is shown in Fig.9 This figure shows that at a F/A 0.08 the exhaust valve temperature is practically unaffected by variation of the spark advance in a range from 5-25 DBTDC. As the spark was advanced beyond 25DBTDC the valve temperature increased until at 45 DBTDC it was the highest observed in the cylinder".
Hmmmm
If we are intellectually honest with ourselves, we would ask ourselves what also tracks the same by advancing the spark timing. We know today because we can measure what they could not 8o years ago, that the ICP will rise considerably, and funny enough the CHT goes with it. Does that help prove what we are saying??? Thought not.
But wait, there is more…..we know you want more, like this set of steak knives!
In the above while advancing the timing and noting exactly the same results as the NACA were getting we not only got higher ICP tracking with CHT we had a constant decline in EGT. Yep thats right a decline in EGT!!!!
Bugger….the timing went up, the ICP went up, the CHT went up, the VALVE temperature went up and the EGT went down! 
Walter…..over to you!
YOUR hypothesis is true….until.
The valve temperature "tracks" the EGT, because as the mixture at any given mass airflow is leaned, the EGT goes up AND the CHT goes up. But why does the CHT go up? Because the peak cylinder pressures go up. Just like your air compressor in the garage.
Now at the moment your hypothesis is true right up until the moment they stop tracking each other and diverge. Hypothesis BUSTED. end of argument.
Now when the CHT tracks the valve temp hypothesis is tested in exactly the same way, and it does not diverge, it follows the valve temperature…..what can you make from that? Because when Lycoming did their test with far newer equipment they plotted even better curves showing this in detail. I do not have them at hand, but in several weeks I will be right there in the dyno room and will surely be able to get them, if I remember.
Now just so you know I am not making this stuff up, lets examine what else the NACA had to say about valve temperature. Remember they were not measuring ICP in those days, we can today. "The effect of spark advance on valve temperature is shown in Fig.9 This figure shows that at a F/A 0.08 the exhaust valve temperature is practically unaffected by variation of the spark advance in a range from 5-25 DBTDC. As the spark was advanced beyond 25DBTDC the valve temperature increased until at 45 DBTDC it was the highest observed in the cylinder".
Hmmmm
If we are intellectually honest with ourselves, we would ask ourselves what also tracks the same by advancing the spark timing. We know today because we can measure what they could not 8o years ago, that the ICP will rise considerably, and funny enough the CHT goes with it. Does that help prove what we are saying??? Thought not.But wait, there is more…..we know you want more, like this set of steak knives!
In the above while advancing the timing and noting exactly the same results as the NACA were getting we not only got higher ICP tracking with CHT we had a constant decline in EGT. Yep thats right a decline in EGT!!!!
Bugger….the timing went up, the ICP went up, the CHT went up, the VALVE temperature went up and the EGT went down! Walter…..over to you!
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Remember the purpose of the NACA report. Some US engineer during WWII decided that we needed to run the B-24 Liberator LOP to get the range AND increase the bomb load. Some General said, "That's ridiculous, everyone knows that leaner mixtures result in higher exhaust valve temperatures." The Top Secret study was undertaken to prove or disprove the adamant General's position. The general lost. The physics won. Along the way, it was discovered that EGT and exhaust valve temperature were not directly related as initially assumed. It looks like it might, but the red herring was chewed by researchers until we realized that valve temperature is most directly affected by CHT. Period. We also learned that there are actually mixtures where the exhaust gasses blasting past the exhaust valve were actually COOLING the valve. Depending on other factors, if one sets the mixture at 40dF ROP, the exhaust gasses can actually cool the exhaust valve! Try that fact on for size.
It's also worthwhile to note that science has the habit of slaying a beautiful hypothesis with an ugly fact.
(FWIW, the B-24 was the only airplane of WWII that was run LOP in cruise. It seemed to work quite well as it carried the highest bomb load the greatest distance.)
It's also worthwhile to note that science has the habit of slaying a beautiful hypothesis with an ugly fact.
(FWIW, the B-24 was the only airplane of WWII that was run LOP in cruise. It seemed to work quite well as it carried the highest bomb load the greatest distance.)
I'm not sure on what, if any, scientific issue/s we differ any more, oggers and Eddie, so I'll frame the discussion a different way.
Which of these mixture settings will, in your view, be better for a piston engine's longevity:
- 30 degrees F Rich of Peak
- 30 degrees F Lean of Peak.
Which of these mixture settings will, in your view, be better for a piston engine's longevity:
- 30 degrees F Rich of Peak
- 30 degrees F Lean of Peak.
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During a preignition event, the valve temperature is rising at the alarming rate of as much as 2 degrees per second. While this is happening, the EGT is going down significantly.
During detonation, EGT moves little or goes down slightly while exhaust valve temp goes steadily up.
How can that be if EGT affects valve temperature?
The correct observation that EGT and valve temp go up as the mixture is leaned from full rich to about 25dF ROP is the red herring. It's a correct observation with a misapplied causality.
We can hold the EGT steady on the test stand and reduce CHT and the valve temp goes down. We can raise the CHT and while holding the EGT steady and the valve temp goes up. If we hold the CHT steady and alter the EGT, the valve temp remains steady. Explain that and continue to insist that valve temp is related to EGT.
One other thing. As I have spent the last 15+ years studying this stuff, I have had to unlearn what I knew to be true, but wasn't. It was not a comfortable feeling.
During detonation, EGT moves little or goes down slightly while exhaust valve temp goes steadily up.
How can that be if EGT affects valve temperature?
The correct observation that EGT and valve temp go up as the mixture is leaned from full rich to about 25dF ROP is the red herring. It's a correct observation with a misapplied causality.
We can hold the EGT steady on the test stand and reduce CHT and the valve temp goes down. We can raise the CHT and while holding the EGT steady and the valve temp goes up. If we hold the CHT steady and alter the EGT, the valve temp remains steady. Explain that and continue to insist that valve temp is related to EGT.
One other thing. As I have spent the last 15+ years studying this stuff, I have had to unlearn what I knew to be true, but wasn't. It was not a comfortable feeling.
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(FWIW, the B-24 was the only airplane of WWII that was run LOP in cruise. It seemed to work quite well as it carried the highest bomb load the greatest distance.)
ttp://www.475thfghf.org/Lindbergh.htm
The second and critical passage made by the group concerned fuel consumption. With additional fuel cells in the J model P-38, Satan's Angels had been making six and one-half and seven-hour flights. On I July Lindbergh flew a third mission with the group, an armed reconnaissance to enemy strips at Nabire, Sagan One and Two, Otawiri, and Ransiki, all on the western shore of Geelvink Bay. Already Lindbergh's technical eye noticed something. After six and one-half hours flying time, he landed with 210 gallons of fuel remaining in his Lightning's tanks.
Two missions later, on 3 July, the group covered sixteen heavies on a strike against Jefman Island. Lindbergh led Hades Squadron's White Flight as they wove back and forth above the lumbering B-25s. After the attack the Lightning's went barge hunting.
First one, then two pilots reported dwindling fuel and broke off for home. MacDonald ordered the squadron back but because Lindbergh had nursed his fuel, he asked for and received permission to continue the hunt with his wingman. After a few more strafing runs, Lindbergh noticed the other Lightning circling overhead. Nervously the pilot told Lindbergh that he had only 175 gallons of fuel left. The civilian told him to reduce engine R.P.M.'s, lean out his fuel mixture, and throttle back. When they landed, the 431st driver had seventy gallons left, Lindbergh had 260. They had started the mission with equal amounts of gas.
Lindbergh talked with MacDonald. The colonel then asked the group's pilots to assemble at the recreation hall that evening. The hall was that in name only, packed dirt floors staring up at a palm thatched roof, one ping pong table and some decks of cards completing the decor. Under the glare of unshaded bulbs, MacDonald got down to business. "Mr. Lindbergh" wanted to explain how to gain more range from the P-38s. In a pleasant manner Lindbergh explained cruise control techniques he had worked out for the Lightning's: reduce the standard 2,200 rpm to 1,600, set fuel mixtures to "auto-lean," and slightly increase manifold pressures. This, Lindbergh predicted, would stretch the Lightning's radius by 400 hundred miles, a nine-hour flight. When he concluded his talk half an hour later, the room was silent.
The men mulled over several thoughts in the wake of their guest's presentation.
The notion of a nine-hour flight literally did not sit well with them, "bum-busters" thought some. Seven hours in a cramped Lightning cockpit, sitting on a parachute, an emergency raft, and an oar was bad, nine hours was inconceivable. They were right. Later, on 14 October 1944, a 432nd pilot celebrated his twenty-fourth birthday with an eight-hour escort to Balikpapan, Borneo. On touching down, he was so cramped his crew chief had to climb up and help him get out of the cockpit.
The group’s chief concern surfaced quickly, that such procedures would foul sparkplugs and scorch cylinders. Lindbergh methodically gave the answer. The Lightning's technical manual provided all the figures necessary to prove his point; they had been there all along. Nonetheless the 475th remained skeptical. A single factor scotched their reticence.
During their brief encounter, MacDonald had come to respect Lindbergh. Both men pushed hard and had achieved. Both were perfectionists never leaving things half done. And both had inquisitive minds. John Loisel, commanding officer the 432nd, remembered the two men talking for long periods over a multitude of topics beyond aviation. If, as MacDonald had informed his pilots, better aircraft performance meant a shorter war, then increasing the Lightning's range was worth investigating. Lindbergh provided the idea, but it was MacDonald's endorsement, backed by the enormous respect accorded him by the group, that saw the experiment to fruition. The next day, the Fourth of July, Lindbergh accompanied the 433rd on a six-hour, forty-minute flight led by Captain "Parky" Parkansky. Upon landing, the lowest fuel level recorded was 160 gallons. In his journal entry Lindbergh felt ". . . that the talk last night was worthwhile. " The 475th had lengthened its stride
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Which of these mixture settings will be better for a piston engine's longevity:
- 30 degrees F Rich of Peak
- 30 degrees F Lean of Peak.
- 30 degrees F Rich of Peak
- 30 degrees F Lean of Peak.
An excellent question, and now we're getting to the real science. How would you test that?
If only that had happened around 50 or 60 years ago.
Oh, wait....
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vh-foobar
Yes, but that set of runs was at 93% of the audible knock limit for each mixture setting and so the power was being varied. 1400LOP was an imep of about 170psi which is only 75% (!) of the 226psi they achieved at 1400ROP. Apologies but I cropped the imep curve out when I uploaded the image as I was only trying to show the EGT/valve temp curves.
One point that is interesting from the graph in figure 8 though is that for a given EGT when ROP the valve temperature is hotter than LOP.
i.e. at say 1400 degrees LOP results in valve temp of 1200 degrees and 1400 degrees ROP results in a valve temp 1300 degrees. i.e about 100 degrees hotter.
i.e. at say 1400 degrees LOP results in valve temp of 1200 degrees and 1400 degrees ROP results in a valve temp 1300 degrees. i.e about 100 degrees hotter.
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By running some engines for millions of hours at around 30 degrees rich of peak and running other engines for millions of hours at around 30 degrees lean of peak, then comparing the reliability of each group.
If only that had happened around 50 or 60 years ago.
Oh, wait....
If only that had happened around 50 or 60 years ago.
Oh, wait....
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So….are you saying that pressure is actually the driver?
Surely not?
Surely not?
The report is pretty clear the variables that effect exhaust valve temp are Mixture, IMEP, speed, spark advance, and cooling.
Last edited by vh-foobar; 9th Apr 2016 at 21:21. Reason: grammar
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While Lindy did use a LOP setting to fly the Ryan across the Atlantic in 1927, the Lindberg myth about LOP in the P-38 persists even though it is contradicted by those who were there. According to M.F. Kirby, P-38 ACE from the Pacific who was present for two of Lindy's talks about extending range, he claimed that he never talked about mixture management at all. That's consistent with my own knowledge of the P-38 Allison set-up which could NOT be manually leaned. It had only four mixture positions: Emergency rich, Auto rich, Auto Lean and Idle cutoff. Auto lean like all other WWII aircraft was set for Best Power, 80dF ROP. One P-38 pilot who actually flew with Lindberg on a combat mission said that Lindy seemed to change his mind about slow-flying when he saw his first Zero up close and personal.
According to Kirby, Paul Murray and others, Lindy ONLY talked about using high MP with low RPM to extend range. After attending a Juvat Reunion (80th FG) with Kirby, I had that confirmed by at least a dozen other P-38 pilots from the time. ALL of them claimed that Lindy's method, while useful in peacetime was a disaster in combat. The reason was that it took too long to come up to dog-fight speed from the extended range speed when you saw the enemy. Not one of them said they knew anyone who used Lindy's recommendations. They kept speed up and S-turned behind the bombers they were covering, always ready to engage at a moments notice.
"I never was interested in saving gas. I was too busy trying to save save my arse."
M.F. Kirby
According to Kirby, Paul Murray and others, Lindy ONLY talked about using high MP with low RPM to extend range. After attending a Juvat Reunion (80th FG) with Kirby, I had that confirmed by at least a dozen other P-38 pilots from the time. ALL of them claimed that Lindy's method, while useful in peacetime was a disaster in combat. The reason was that it took too long to come up to dog-fight speed from the extended range speed when you saw the enemy. Not one of them said they knew anyone who used Lindy's recommendations. They kept speed up and S-turned behind the bombers they were covering, always ready to engage at a moments notice.
"I never was interested in saving gas. I was too busy trying to save save my arse."
M.F. Kirby
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Jaba
Haha, you wish. I have explained why the valve temp peaks just before the EGT twice already. It is because: "the spread between where CHT and EGT peak as a function of mixture means the valve temp will not peak precisely at either but between them." The valve is heated by the working fluid and cooled by the head -and ultimatley the air. Therefore one would expect the valve temp to peak between peak EGT and peak CHT.
...but it does "diverge" as we can see:
..that you really really want the report to show no "divergence" between CHT and valve temp even though it doesn't!
We shall see. However, John Deakin already tried to make this point with Lycoming 'data' so maybe we already have it:
"It should be intuitively obvious then, that the valve temperature will correspond most closely with the cylinder head temperature (not the EGT), and indeed, old data from Lycoming (1966) confrirm this" he wrote, and here's the 'data':
...I added the pecked lines. There isn't actually a curve on there for the valve, but anyway he says:
"Note that CHT, valve guide, and valve head temperature all increase together, all peak at roughly the same point on the mixture curve (just rich of peak EGT), and all fall together."
...and they do sort of peak in the same place but there is a trend; The valve seat peaks leaner and hotter than CHT, and the guide leaner and hotter still and very much closer to peak EGT than peak CHT. And if the valve was on there it would definitely be hotter than the guide. And yet the APS opinion is that has nothing to do with the exhaust gas temperature.
I think most people can understand that if the guide - which isn't even in the combustion chamber - is running hotter than the valve seat, and the cylinder head, it is because it is being heated by the hot exhaust flow. Or bearing in mind that only 25% of the valve heat goes through the guide while 75% goes through the seat, why isn't the seat hotter than the guide? Because of the exhaust gas.
I find it very strange that you persist in arguing against this ground truth.
oggers, let me make it really simple.
YOUR hypothesis is true….until.
The valve temperature "tracks" the EGT, because as the mixture at any given mass airflow is leaned, the EGT goes up AND the CHT goes up. But why does the CHT go up? Because the peak cylinder pressures go up. Just like your air compressor in the garage.
Now at the moment your hypothesis is true right up until the moment they stop tracking each other and diverge. Hypothesis BUSTED. end of argument
YOUR hypothesis is true….until.
The valve temperature "tracks" the EGT, because as the mixture at any given mass airflow is leaned, the EGT goes up AND the CHT goes up. But why does the CHT go up? Because the peak cylinder pressures go up. Just like your air compressor in the garage.
Now at the moment your hypothesis is true right up until the moment they stop tracking each other and diverge. Hypothesis BUSTED. end of argument
Now when the CHT tracks the valve temp hypothesis is tested in exactly the same way, and it does not diverge, it follows the valve temperature…..
...what can you make from that?
Because when Lycoming did their test with far newer equipment they plotted even better curves showing this in detail. I do not have them at hand, but in several weeks I will be right there in the dyno room and will surely be able to get them, if I remember.
"It should be intuitively obvious then, that the valve temperature will correspond most closely with the cylinder head temperature (not the EGT), and indeed, old data from Lycoming (1966) confrirm this" he wrote, and here's the 'data':
...I added the pecked lines. There isn't actually a curve on there for the valve, but anyway he says:
"Note that CHT, valve guide, and valve head temperature all increase together, all peak at roughly the same point on the mixture curve (just rich of peak EGT), and all fall together."
...and they do sort of peak in the same place but there is a trend; The valve seat peaks leaner and hotter than CHT, and the guide leaner and hotter still and very much closer to peak EGT than peak CHT. And if the valve was on there it would definitely be hotter than the guide. And yet the APS opinion is that has nothing to do with the exhaust gas temperature.
I think most people can understand that if the guide - which isn't even in the combustion chamber - is running hotter than the valve seat, and the cylinder head, it is because it is being heated by the hot exhaust flow. Or bearing in mind that only 25% of the valve heat goes through the guide while 75% goes through the seat, why isn't the seat hotter than the guide? Because of the exhaust gas.
I find it very strange that you persist in arguing against this ground truth.
I'm not sure on what, if any, scientific issue/s we differ any more, oggers and Eddie, so I'll frame the discussion a different way.
Which of these mixture settings will, in your view, be better for a piston engine's longevity:
- 30 degrees F Rich of Peak
- 30 degrees F Lean of Peak.
Which of these mixture settings will, in your view, be better for a piston engine's longevity:
- 30 degrees F Rich of Peak
- 30 degrees F Lean of Peak.
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oggers,
You seem to be missing the point and confusing some things.
1. The centre of valve crown thermocouple is tracking the EGT more closely because it is basically an EGT probe. Look at where they positioned it
You are not looking at the relevant trace for a start.
2. The valve temperature that matters is the valve face/seat and the one represented by the back of the valve body. Stick to the important ones and don't confuse the two.
Please refer to my previous post about what happens when you advance the spark. ICP goes up, CHT goes up, valve temperature goes up and EGT goes down.
Next and for those who say APS never give anything (data) for free here is a slide from the APS class. Look closely at the Cylinder 6 trace in blue. Note that the CHT is going through the roof. The ICP was definitely going through the roof (not shown but we know) and correspondingly the EGT fell considerably. What do you think the valve temperature was doing at the seat/face and the bulk of the valve body? Please answer this.
Courtesy of Advanced Pilot Seminars
Your position has been that EGT is driving valve temperature, and that seems to be based on CORRELATION not CAUSATION. There is correlation for sure on the curve you keep showing above. However this is the wrong curve being observed in terms of an exhaust valves overall temperature and health. The appropriate one is the valve face/seat and the body represented by the back of the valve.
So Correlation some of the time does not support the facts. Causation is not from EGT, as has been shown in the NACA report on page 44 where EGT did not drive the valve temperature.
It is worth repeating, ICP is what drives valve temperature, and when you have high ICP you get high CHT, always. This is the best proxy you have for valve temperature in your plane. As is shown in the picture above.
Walter……anything to add? John Deakin perhaps?
You seem to be missing the point and confusing some things.
1. The centre of valve crown thermocouple is tracking the EGT more closely because it is basically an EGT probe. Look at where they positioned it
You are not looking at the relevant trace for a start.2. The valve temperature that matters is the valve face/seat and the one represented by the back of the valve body. Stick to the important ones and don't confuse the two.
Please refer to my previous post about what happens when you advance the spark. ICP goes up, CHT goes up, valve temperature goes up and EGT goes down.
Next and for those who say APS never give anything (data) for free here is a slide from the APS class. Look closely at the Cylinder 6 trace in blue. Note that the CHT is going through the roof. The ICP was definitely going through the roof (not shown but we know) and correspondingly the EGT fell considerably. What do you think the valve temperature was doing at the seat/face and the bulk of the valve body? Please answer this.
Courtesy of Advanced Pilot SeminarsYour position has been that EGT is driving valve temperature, and that seems to be based on CORRELATION not CAUSATION. There is correlation for sure on the curve you keep showing above. However this is the wrong curve being observed in terms of an exhaust valves overall temperature and health. The appropriate one is the valve face/seat and the body represented by the back of the valve.
So Correlation some of the time does not support the facts. Causation is not from EGT, as has been shown in the NACA report on page 44 where EGT did not drive the valve temperature.
It is worth repeating, ICP is what drives valve temperature, and when you have high ICP you get high CHT, always. This is the best proxy you have for valve temperature in your plane. As is shown in the picture above.
Walter……anything to add? John Deakin perhaps?
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Jaba
Nope. I have never taken that position. I have been very careful to never say that EGT drives valve temperature. I have pointed out that there is a correlation, between EGT and valve temp, and that CHT also correlates.
I have said that exhaust gas heats the valve. Obviously combustion heats the valve too. The two are not mutually exclusive. Walter Atkinson's position was:
Walter Atkinson claims, quote:
NACA:
Your position has been that EGT is driving valve temperature, and that seems to be based on CORRELATION not CAUSATION.
I have said that exhaust gas heats the valve. Obviously combustion heats the valve too. The two are not mutually exclusive. Walter Atkinson's position was:
Walter Atkinson claims, quote:
EGT does NOT affect valve temperature....
those who think that valve temp is in any way related to EGT must reconcile the fact [blah blah]...This is definitive evidence that the two are NOT related....valve temperature does NOT track EGT. This is 1943 data, not ours. Lycoming verified it in 1966.
those who think that valve temp is in any way related to EGT must reconcile the fact [blah blah]...This is definitive evidence that the two are NOT related....valve temperature does NOT track EGT. This is 1943 data, not ours. Lycoming verified it in 1966.
I'm not sure on what, if any, scientific issue/s we differ any more, oggers and Eddie, so I'll frame the discussion a different way.
Which of these mixture settings will, in your view, be better for a piston engine's longevity:
- 30 degrees F Rich of Peak
- 30 degrees F Lean of Peak.
Which of these mixture settings will, in your view, be better for a piston engine's longevity:
- 30 degrees F Rich of Peak
- 30 degrees F Lean of Peak.
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oggers,
You seem to be missing the point and confusing some things.
1. The centre of valve crown thermocouple is tracking the EGT more closely because it is basically an EGT probe. Look at where they positioned it You are not looking at the relevant trace for a start.
You seem to be missing the point and confusing some things.
1. The centre of valve crown thermocouple is tracking the EGT more closely because it is basically an EGT probe. Look at where they positioned it
You are not looking at the relevant trace for a start.
And on that basis you reject the report data showing that EGT and valve temp correlated well. Bearing in mind that a few posts back APS cited this report as "definitive".
2. The valve temperature that matters is the valve face/seat
Why not simply accept what the report shows loud and clear - that valve temp for the most part correlated with EGT?