Rich of Peak, or Lean of Peak?
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Rich of Peak, or Lean of Peak?
I am a well-trained automotive/Heavy truck technician, spent years in tech schools in the 60’s and early 70’s, worked in the industry all my life. I have attended many many continuing education classes to keep me up to date on new technology and developments, so I thought I understood all aspects of internal combustion engines, until I started flying…
Then I met mixture controls and all that I'd learned, or thought I’d learned about mixture control in petrol engines seemed to go out the window. In specific, in an automotive engine the common maxim is “A lean engine will run hot” I seemed to have just absorbed that piece of information, but when examined logically it does not make sense.
If the adiabatic correct air fuel ratio is 14.7-1, then at that ratio by weight, all the oxygen and all the fuel is consumed during combustion, then you must have the maximum temp rise, and maximum pressure rise. (P1=V1=T1).
In General Aviation circles there is always discussion about running an aircraft engine rich or lean of peak exhaust temp, say 50 deg f either side. (Please lets ignore cylinder head temps for now, too many variables) One statement I read suggested that 50 deg rich of peak puts the engine in prime detonation condition, so it’s best to avoid that area as detonation cannot be heard, as it is in automotive engines, because of the noisy environment. Running lean of peak will only reduce power slightly, reduce the risk of detonation, reduce fuel flow significantly, and keep plugs clean etc. Then there is the position that running lean of peak will cause the engine to run hot.
Now for my logic problem:
It follows if one mixes fuel and air perfectly (14.7-1) then all the fuel and all the air are consumed during combustion, producing the maximum temp/pressure rise. How then, when you run lean, can less fuel make more heat?
Is there a real world cooling effect in that small difference in the amount of fuel, say between 14.7-1 and 16.0-1?
I am interested in theory here, so please don’t tell me to read the user handbook. I have done so, and it seems to me Lycoming are in many ways still in the dark ages when it comes to engine fuel management.
Have I forgotten something I used to know? What am I missing?
Regards,
White Bear.
Then I met mixture controls and all that I'd learned, or thought I’d learned about mixture control in petrol engines seemed to go out the window. In specific, in an automotive engine the common maxim is “A lean engine will run hot” I seemed to have just absorbed that piece of information, but when examined logically it does not make sense.
If the adiabatic correct air fuel ratio is 14.7-1, then at that ratio by weight, all the oxygen and all the fuel is consumed during combustion, then you must have the maximum temp rise, and maximum pressure rise. (P1=V1=T1).
In General Aviation circles there is always discussion about running an aircraft engine rich or lean of peak exhaust temp, say 50 deg f either side. (Please lets ignore cylinder head temps for now, too many variables) One statement I read suggested that 50 deg rich of peak puts the engine in prime detonation condition, so it’s best to avoid that area as detonation cannot be heard, as it is in automotive engines, because of the noisy environment. Running lean of peak will only reduce power slightly, reduce the risk of detonation, reduce fuel flow significantly, and keep plugs clean etc. Then there is the position that running lean of peak will cause the engine to run hot.
Now for my logic problem:
It follows if one mixes fuel and air perfectly (14.7-1) then all the fuel and all the air are consumed during combustion, producing the maximum temp/pressure rise. How then, when you run lean, can less fuel make more heat?
Is there a real world cooling effect in that small difference in the amount of fuel, say between 14.7-1 and 16.0-1?
I am interested in theory here, so please don’t tell me to read the user handbook. I have done so, and it seems to me Lycoming are in many ways still in the dark ages when it comes to engine fuel management.
Have I forgotten something I used to know? What am I missing?
Regards,
White Bear.
The crux of the matter lies in the fact that most aero engines are designed to run richer than the chemically correct 14.7:1. The excess fuel pumped into the cylinders which doesnt get burnt because there isn't enough oxygen for complete combustion, is converted into it's basic elements such as carbon - hence the black smoke seen on rich running engines. This catalysation takes quite a lot of heat and leads to useful cooling of a fairly inefficient air cooled engine which relies on running rich for cooling in certain circumstances - i.e. a climb. So if you lean off, you are taking the mixture closer to 14.7:1 and taking away some of the cooling.
That's the way I understand it. Simple, but I like it that way!
That's the way I understand it. Simple, but I like it that way!
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Time for some OWT killing!
YOUR ENGINE WILL BLOW UP IF YOU ...
lots of uninformed old wives tales floating around.
Google for "John Deakin Mixture Magic" and read this as a starter.
Some misconceptions:
"the engine and the exhaust valves run hottest at peak EGT."
"The lead in Avgas acts as a lubricant to the valves."
"the engine will run hot when leaned too far"
Best to forget everything told and read the entire engine series by John Deakin. Nobody has explained it better so far.
YOUR ENGINE WILL BLOW UP IF YOU ...
lots of uninformed old wives tales floating around.
Google for "John Deakin Mixture Magic" and read this as a starter.
Some misconceptions:
"the engine and the exhaust valves run hottest at peak EGT."
"The lead in Avgas acts as a lubricant to the valves."
"the engine will run hot when leaned too far"
Best to forget everything told and read the entire engine series by John Deakin. Nobody has explained it better so far.
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I found this interesting document on the Lycoming site.
http://www.lycoming.textron.com/main...ips/index.html
http://www.lycoming.textron.com/main...ips/index.html
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White Bear
The reason for your confusion is that few people know anything about this subject. No instructor I've ever met (or got anywhere near) does. Plenty of owners of planes with engines whose management is regarded as important do know it but they aren't participating here
A good starting point is a read of articles by John Deakin at avweb.com. They do move around there a little so if you can't find them, PM me and I will dig them out.
Back to your questions:
How then, when you run lean, can less fuel make more heat
It doesn't, of course.
The highest CHT occurs in the vicinity (not quite "at") peak EGT. Once you lean past the peak, i.e. in the lean-of-peak (LOP) region, the temps all fall. Since all the fuel is getting burnt, you are extracting all the chemical energy out of it that can be extracted.
So, why not run LOP all the time (like modern car engines tend to)?
The answer, I think, is simply inadequate cooling
There is always a design compromise between getting rid of heat, engine weight, and cost. Liquid cooling makes this a lot easier but we don't have that option in those engines.
A design compromise has been chosen such that to reach their handbook max rated power, a typical Lycoming or Continental engine cannot get rid of the heat (into the exhaust, the oil and the airflow) and needs to have the combustion cooled by something. One could squirt water into it (like has been done elsewhere) but they just squirt fuel in there - in America fuel costs about the same as water
The fuel injection unit is configured so that with the mixture lever fully forward you are about 150F ROP and you can leave it there all the way through climb, wasting unburnt fuel happily out of the exhaust. Until you get to an altitude where the engine won't run because it's too rich, say 5000ft+, so you HAVE to lean, but by then it should be safe because it isn't doing anywhere near 100% power, more like 60-70%.
When the power setting is well below 100% (75% is Lyco's official position on certain engines), and the airflow (airspeed) is sufficient, one can lean to LOP. The only limit is that the engine design does not properly distribute air (and therefore fuel) equally to all cylinders, so the engine start to run rough because some cylinders develop more power than others.
The reason this doesn't matter when ROP is that if you have surplus (unburnt) fuel, uneven fuel distribution doesn't result in correspondingly uneven power distribution. Whereas at LOP (where fuel=power) it does.
So to operate LOP one needs an engine whose power distribution is balanced. A firm called GAMI has been selling matched injectors for this purpose but some engine owners don't need them as their stock injectors are well enough matched to start with. On carb engines the situation is usually hopeless.
I don't know if one can design an air cooled engine like a big Lyco which can run LOP at 100% power under all operating modes (slow speed climb and the rest). Possibly, it can't be done. These engines do deliver quite a lot of HP for their weight, considering the low RPM which is dictated by lack of a gearbox and the prop tips having to be sub-mach1.
Lycoming's position is complicated because they've been selling engines essentially unchanged for decades, and if they change any operating recommendations they open themselves to legal liability. This is especially hard if you sell an engine which is relatively easy to mismanage - if you fly mine (IO540) at 100% power 1000ft above the sea, leaned to 50F ROP, for half an hour, doing slow climbs and such, it will be scrap, 25 grand just like that. And it happens. People crack cylinders, lose camshafts, you name it. And they often try to go after Lyco, especially during the warranty. So anything Lyco do has to be ultra defensive, but providing they don't change their manuals they are OK. This makes a mockery of anything whatsoever they say on the subject.
Their quality control is certainly crap. Recently they had loads of crankshaft failures because the subcontractor who makes their cranks didn't heat treat them properly. I have long tables of engine and crank serial numbers which were recalled.
I am not saying that a big Lyco is inefficient in converting fuel into power, BTW, once set up in the operating point for which it was designed: level cruise at some 60-70% of rated max power.
Anyway there is no alternative. Some people in Europe are playing around with diesels.
The reason for your confusion is that few people know anything about this subject. No instructor I've ever met (or got anywhere near) does. Plenty of owners of planes with engines whose management is regarded as important do know it but they aren't participating here
A good starting point is a read of articles by John Deakin at avweb.com. They do move around there a little so if you can't find them, PM me and I will dig them out.
Back to your questions:
How then, when you run lean, can less fuel make more heat
It doesn't, of course.
The highest CHT occurs in the vicinity (not quite "at") peak EGT. Once you lean past the peak, i.e. in the lean-of-peak (LOP) region, the temps all fall. Since all the fuel is getting burnt, you are extracting all the chemical energy out of it that can be extracted.
So, why not run LOP all the time (like modern car engines tend to)?
The answer, I think, is simply inadequate cooling
There is always a design compromise between getting rid of heat, engine weight, and cost. Liquid cooling makes this a lot easier but we don't have that option in those engines.
A design compromise has been chosen such that to reach their handbook max rated power, a typical Lycoming or Continental engine cannot get rid of the heat (into the exhaust, the oil and the airflow) and needs to have the combustion cooled by something. One could squirt water into it (like has been done elsewhere) but they just squirt fuel in there - in America fuel costs about the same as water
The fuel injection unit is configured so that with the mixture lever fully forward you are about 150F ROP and you can leave it there all the way through climb, wasting unburnt fuel happily out of the exhaust. Until you get to an altitude where the engine won't run because it's too rich, say 5000ft+, so you HAVE to lean, but by then it should be safe because it isn't doing anywhere near 100% power, more like 60-70%.
When the power setting is well below 100% (75% is Lyco's official position on certain engines), and the airflow (airspeed) is sufficient, one can lean to LOP. The only limit is that the engine design does not properly distribute air (and therefore fuel) equally to all cylinders, so the engine start to run rough because some cylinders develop more power than others.
The reason this doesn't matter when ROP is that if you have surplus (unburnt) fuel, uneven fuel distribution doesn't result in correspondingly uneven power distribution. Whereas at LOP (where fuel=power) it does.
So to operate LOP one needs an engine whose power distribution is balanced. A firm called GAMI has been selling matched injectors for this purpose but some engine owners don't need them as their stock injectors are well enough matched to start with. On carb engines the situation is usually hopeless.
I don't know if one can design an air cooled engine like a big Lyco which can run LOP at 100% power under all operating modes (slow speed climb and the rest). Possibly, it can't be done. These engines do deliver quite a lot of HP for their weight, considering the low RPM which is dictated by lack of a gearbox and the prop tips having to be sub-mach1.
Lycoming's position is complicated because they've been selling engines essentially unchanged for decades, and if they change any operating recommendations they open themselves to legal liability. This is especially hard if you sell an engine which is relatively easy to mismanage - if you fly mine (IO540) at 100% power 1000ft above the sea, leaned to 50F ROP, for half an hour, doing slow climbs and such, it will be scrap, 25 grand just like that. And it happens. People crack cylinders, lose camshafts, you name it. And they often try to go after Lyco, especially during the warranty. So anything Lyco do has to be ultra defensive, but providing they don't change their manuals they are OK. This makes a mockery of anything whatsoever they say on the subject.
Their quality control is certainly crap. Recently they had loads of crankshaft failures because the subcontractor who makes their cranks didn't heat treat them properly. I have long tables of engine and crank serial numbers which were recalled.
I am not saying that a big Lyco is inefficient in converting fuel into power, BTW, once set up in the operating point for which it was designed: level cruise at some 60-70% of rated max power.
Anyway there is no alternative. Some people in Europe are playing around with diesels.
Last edited by IO540; 11th Feb 2005 at 08:49.
Well said that man! These ancient combine harvester engines are an absolute scandal in the 21st century!
A decent modern car engine with its FADEC, drive-by-wire throttle and high construction quality is light years ahead of the IO-360. But Lycoming has us all by the nuts.....
It should be very simple to control power, mixture, rpm, ignition and every other parameter of a typical v-p prop equipped aero engine without suffering from inefficient combustion, overheating, carburettor icing, magneto faults etc with a decent dual-redundant high speed digital computer.
A decent modern car engine with its FADEC, drive-by-wire throttle and high construction quality is light years ahead of the IO-360. But Lycoming has us all by the nuts.....
It should be very simple to control power, mixture, rpm, ignition and every other parameter of a typical v-p prop equipped aero engine without suffering from inefficient combustion, overheating, carburettor icing, magneto faults etc with a decent dual-redundant high speed digital computer.
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Beagle,
I bet you that a FADEC controller on a big Lyco would set the mixture to the same point that you set it to
There is no way to run an IO540 for example at the rated max 250HP while climbing at Vy, at peak EGT. No matter how you play with the mixture, the ign timing, whatever, the thing will still overheat because there isn't enough airflow through the engine and the oil cooler, and the oil flow rate wouldn't be enough to do it even if the oil cooler was huge.
In low level cruise, the FADEC would set the power to 65%, the RPM to 2300, the MP to 23", the flow to 11-12GPH - just like you would do manually if you knew how to do it. It could play with the ign timing; that's about it. How much difference would it make to economy? It certainly would make flying simpler.
These engines have their legacy in wartime design practices: low weight, and service intervals don't matter much. How long does a RR Merlin run for? AIUI they only just made it from one battle to the next, or (nowadays) from one airshow to the next.
I bet you that a FADEC controller on a big Lyco would set the mixture to the same point that you set it to
There is no way to run an IO540 for example at the rated max 250HP while climbing at Vy, at peak EGT. No matter how you play with the mixture, the ign timing, whatever, the thing will still overheat because there isn't enough airflow through the engine and the oil cooler, and the oil flow rate wouldn't be enough to do it even if the oil cooler was huge.
In low level cruise, the FADEC would set the power to 65%, the RPM to 2300, the MP to 23", the flow to 11-12GPH - just like you would do manually if you knew how to do it. It could play with the ign timing; that's about it. How much difference would it make to economy? It certainly would make flying simpler.
These engines have their legacy in wartime design practices: low weight, and service intervals don't matter much. How long does a RR Merlin run for? AIUI they only just made it from one battle to the next, or (nowadays) from one airshow to the next.
John Deakin's Columns are available here. Number 18 tells you a lot about mixture, but to get the whole picture you'll need to do some serious reading there.
IO540 - probably dashed unpatriotic of me, but surely 'their' DB601 with its variable blower and fuel injection was rather a superior design to the RR Merlin?
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IO540,
the highest CHT occurs at about 50-40°F ROP. This is also the highest exhaust valve temperature point. Your exhaust valves will be cooler at peak EGT than at 50° ROP.
EGT is a completely phony number only to be used in a relative to peak mode. The absolute EGT value is meaningless, the gases coming out of the exhaust valve are much hotter than that, but only one cycle in four is measured, to EGT is a 1-in-4 average of some meaningless temperature.
Most uninformed pilots/instructors/mechanics will talk about 'too lean' when they mean 'not rich enough or not lean enough' since they have never seen the back side of the EGT curve.
Most important for long engine life is CHT, a direct reflection of what is going on inside the cylinder.
Too rich at too high power level means: highest internal combustion pressure closer to the top dead center (very lean or very rich mixtures burn slower and thus fall farther from TDC where the piston is going down more and compression is less).
John Deakin's texts are unsurpassed on this subject.
the highest CHT occurs at about 50-40°F ROP. This is also the highest exhaust valve temperature point. Your exhaust valves will be cooler at peak EGT than at 50° ROP.
EGT is a completely phony number only to be used in a relative to peak mode. The absolute EGT value is meaningless, the gases coming out of the exhaust valve are much hotter than that, but only one cycle in four is measured, to EGT is a 1-in-4 average of some meaningless temperature.
Most uninformed pilots/instructors/mechanics will talk about 'too lean' when they mean 'not rich enough or not lean enough' since they have never seen the back side of the EGT curve.
Most important for long engine life is CHT, a direct reflection of what is going on inside the cylinder.
Too rich at too high power level means: highest internal combustion pressure closer to the top dead center (very lean or very rich mixtures burn slower and thus fall farther from TDC where the piston is going down more and compression is less).
John Deakin's texts are unsurpassed on this subject.
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Dear God (the aviation god I mean) I'm going to have some fun with John Deakin's column - there's 12 hours of reading in there!
Thanks for the link.
PS: I've heard that it's easily possible to run LOP on a Lyc or Cont at low power - say 45% in cruise (i.e Cirrus or similiar) - is this true?
Thanks for the link.
PS: I've heard that it's easily possible to run LOP on a Lyc or Cont at low power - say 45% in cruise (i.e Cirrus or similiar) - is this true?
Last edited by Confabulous; 11th Feb 2005 at 18:50.
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Confabulous,
If the engine will run smoothly LOP depends on the fuel/air ratio between the cylinders.
It will not hurt anything to try to run LOP if you are at 65% power or below. Find out if the engine will operate smoothly, it may even run so smoothly that it will stop running if very far LOP (this is normal, engines require some fuel to run).
Gamijectors correct a known F/A imbalance between the rear, middle and front cylinder rows in a six cylinder flat engine.
Savings of 15% or more are easily obtained LOP with minimal loss of airspeed.
Having a fuel injected engine with an all-cylinder engine monitor is a good starting point in the world of LOP.
Even some carbureted engines may run smoothly LOP, some require careful use of carb heat to better vaporize fuel droplets.
If the engine will run smoothly LOP depends on the fuel/air ratio between the cylinders.
It will not hurt anything to try to run LOP if you are at 65% power or below. Find out if the engine will operate smoothly, it may even run so smoothly that it will stop running if very far LOP (this is normal, engines require some fuel to run).
Gamijectors correct a known F/A imbalance between the rear, middle and front cylinder rows in a six cylinder flat engine.
Savings of 15% or more are easily obtained LOP with minimal loss of airspeed.
Having a fuel injected engine with an all-cylinder engine monitor is a good starting point in the world of LOP.
Even some carbureted engines may run smoothly LOP, some require careful use of carb heat to better vaporize fuel droplets.
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dirkdj
I agree 100%. I would however add
- you will save a load more than 15% going LOP, compared with mixture fully forward like every good ole PPL instructor has always told ya More like 30%.
- The fuel saving at LOP relative to peak EGT is less than 15%; probably ~ 5%, perhaps a few % more if you are able to fully open the throttle as a result of the reduced fuel flow rate (less pumping losses)
- not sure about the carb heat helping here... slick engines would have a carb per cylinder and one tweaked them individually.
- everybody should get a multicylinder engine monitor
- GAMIs correct the power imbalance regardless of how it is caused. You do a test flight and you send the data to GAMI and they send you the injectors matched for the engine, so they correct for all causes in one go.
I agree 100%. I would however add
- you will save a load more than 15% going LOP, compared with mixture fully forward like every good ole PPL instructor has always told ya
More like 30%.- The fuel saving at LOP relative to peak EGT is less than 15%; probably ~ 5%, perhaps a few % more if you are able to fully open the throttle as a result of the reduced fuel flow rate (less pumping losses)
- not sure about the carb heat helping here... slick engines would have a carb per cylinder and one tweaked them individually.
- everybody should get a multicylinder engine monitor
- GAMIs correct the power imbalance regardless of how it is caused. You do a test flight and you send the data to GAMI and they send you the injectors matched for the engine, so they correct for all causes in one go.
Avoid imitations
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An exhaust valve gets cooled by contact with the valve seat. A slow burning mixture (weak mixture) may still be in the process of combustion when the exhaust valve begins to open, therefore causing power loss and valve / seat burning.
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Confabulous is right; it will take a while to read all those articles!
Thanks for the replies guys, especially IO540 and dirkdj, what a wealth of knowledge out there.
Just for the record I have Lycoming IO 360, turning a 3 bladed McCauley prop in a Cardinal.
Regards,
W.B.
Thanks for the replies guys, especially IO540 and dirkdj, what a wealth of knowledge out there.
Just for the record I have Lycoming IO 360, turning a 3 bladed McCauley prop in a Cardinal.
Regards,
W.B.
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ShyTorque,
Only the first part of your statement is true.
I have run a TSIO360 so far lean of peak that a secondary EGT rise was visible. This was due to the gases still burning with the exhaust valve already open. It was completely harmless, the engine was so far lean of peak that it was hardly developing any power. Lowest BFSC occurs at 40°F LOP and I was probably at 90° or more so there was no advantage on range. It just proved that this engine was particulary well balanced and that each cylinder was developing exactly the same amount of horsepower.
This is by the way the reason most engines, as they come from the factory, will not run smoothly LOP unless some 'work' is done (usually balancing the injectors). The power curve falls rapidly when LOP and if some cylinders are more LOP than others, then this will be the cause of the vibration.
Valve burning occurs when there is a valve leak during the time the valve is supposed to be closed, not in the case noted above. When internal cylinder pressures are highest then any ill fitting valve will start to leak hot gases and will eventually burn. No pilot operating technique can save this valve once the process starts. Blame the manufacturer not the pilot.
Only the first part of your statement is true.
I have run a TSIO360 so far lean of peak that a secondary EGT rise was visible. This was due to the gases still burning with the exhaust valve already open. It was completely harmless, the engine was so far lean of peak that it was hardly developing any power. Lowest BFSC occurs at 40°F LOP and I was probably at 90° or more so there was no advantage on range. It just proved that this engine was particulary well balanced and that each cylinder was developing exactly the same amount of horsepower.
This is by the way the reason most engines, as they come from the factory, will not run smoothly LOP unless some 'work' is done (usually balancing the injectors). The power curve falls rapidly when LOP and if some cylinders are more LOP than others, then this will be the cause of the vibration.
Valve burning occurs when there is a valve leak during the time the valve is supposed to be closed, not in the case noted above. When internal cylinder pressures are highest then any ill fitting valve will start to leak hot gases and will eventually burn. No pilot operating technique can save this valve once the process starts. Blame the manufacturer not the pilot.
Avoid imitations
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"Valve burning occurs when there is a valve leak during the time the valve is supposed to be closed, not in the case noted above. When internal cylinder pressures are highest then any ill fitting valve will start to leak hot gases and will eventually burn. No pilot operating technique can save this valve once the process starts. Blame the manufacturer not the pilot."
How does the valve begin to leak? Uness there was an initial manufacturing fault, such as inefficient valve to seat seal or over-loose guide, which would be picked up by a compression check, it must be caused by erosion of either the valve edge itself or the valve seat. Over leaning can contribute to this in a major way as the more heat goes into the valve which can actually cause guide wear so there is a viscious circle. I think the best advice is that the engine manufacturer's advice must be followed for long engine life. Experiment outside that at your own risk.
How does the valve begin to leak? Uness there was an initial manufacturing fault, such as inefficient valve to seat seal or over-loose guide, which would be picked up by a compression check, it must be caused by erosion of either the valve edge itself or the valve seat. Over leaning can contribute to this in a major way as the more heat goes into the valve which can actually cause guide wear so there is a viscious circle. I think the best advice is that the engine manufacturer's advice must be followed for long engine life. Experiment outside that at your own risk.
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A valve begins to leak because heat is not carried away properly to the valve guide and to the valve seat. This is purely a manufacturing issue.
Old style (pre 1991) TCM cylinders had post-reamed valve guides: the icecold guides were first inserted in the hot cylinder heads. After cooling they took a certain 'set' and were then post-reamed to the exact size and concentricity. This concentricity is of utmost importance to achieve a good valve to valveseat fit. This fit is responsible for a large amount of valve cooling.
Also important is the width of this valve seat.
In order to speed up production and save a few beans, in 1991 someone at TCM changed manufacturing methods and pre-reamed the valve guides (much more efficient manufacturing) before crimping them in the cylinder heads. Concentricity would then be a matter of luck and not of design.
Result: very few cylinders made it past 600 hours before needing some kind of work, where older cylinders would run to TBO mostly without any problem. I personally ran 2 IO520's to TBO with only one burned exhaust valve. The cylinders on my friends Mooney 231 lasted no more than about 200 hours since new. Date of manufacture of the cylinder is more important than anything else.
Of course, if you call the manufacturer and tell them you have problems with their cylinders after only 200 hours, they will try to blame the pilots flying techniques. This may have worked 20 years ago, but thanks to the internet todays pilots and consumers are better informed than before.
Engine analysers with data recording show exactly what happened and what happens.
Old style (pre 1991) TCM cylinders had post-reamed valve guides: the icecold guides were first inserted in the hot cylinder heads. After cooling they took a certain 'set' and were then post-reamed to the exact size and concentricity. This concentricity is of utmost importance to achieve a good valve to valveseat fit. This fit is responsible for a large amount of valve cooling.
Also important is the width of this valve seat.
In order to speed up production and save a few beans, in 1991 someone at TCM changed manufacturing methods and pre-reamed the valve guides (much more efficient manufacturing) before crimping them in the cylinder heads. Concentricity would then be a matter of luck and not of design.
Result: very few cylinders made it past 600 hours before needing some kind of work, where older cylinders would run to TBO mostly without any problem. I personally ran 2 IO520's to TBO with only one burned exhaust valve. The cylinders on my friends Mooney 231 lasted no more than about 200 hours since new. Date of manufacture of the cylinder is more important than anything else.
Of course, if you call the manufacturer and tell them you have problems with their cylinders after only 200 hours, they will try to blame the pilots flying techniques. This may have worked 20 years ago, but thanks to the internet todays pilots and consumers are better informed than before.
Engine analysers with data recording show exactly what happened and what happens.
Avoid imitations
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So, bearing in mind that the valves and seats aren't perhaps as hardy as in years gone by (I'm not saying you're wrong, there are similar issues with older automotive engines and modern fuels) the pilot ought to be prepared to moderate his leaning technique to preserve the engine?
AVGAS is expensive, but not as expensive as an engine overhaul.
AVGAS is expensive, but not as expensive as an engine overhaul.
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I believe that dirkdj was referring to Mr Deakins article which can be read here :
http://www.avweb.com/news/columns/182155-1.html
He specifically points out that leaning technique is irrelevant as far as frying valves is concerned. The important factors are
1. Poor manufacturing methods
2. CHT's
imho it makes sense
http://www.avweb.com/news/columns/182155-1.html
He specifically points out that leaning technique is irrelevant as far as frying valves is concerned. The important factors are
1. Poor manufacturing methods
2. CHT's
imho it makes sense