Carb heat on Warrior (LYCOMING O-320-D3G)?
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In the Lyco/Conti context, I don't see how carb heat could lead to detonation, unless the following are also true
1) the engine is running at high power - over 80%
2) the CHT is very high - around 500F which is right at the top end
3) the mixture was leaned, enough to lead to 2) above
4) the airflow was poor, also leading to 2) above
and then the carb heat leans the mixture just enough to bring it on.
But not otherwise. Most engines are set up so that during climb (all 3 fully forward) the operating point is about 150F ROP and it would take a lot more than a bit of carb heat to bring on detonation in that condition.
Incidentally, LOP is negligibly more efficient than peak EGT. I have done some very careful tests and on the IO-540 I find that, relative to peak EGT, 75F ROP is about 10% worse (on MPG), while 25F LOP (the theoretical most efficient point for any petrol engine) is so close to the peak EGT efficiency that any gain is below the measurement resolution of about 1%. I plan to do additional tests using a different method but I am not expecting to see a better gain than 1% by using LOP.
The crucial thing in this kind of measurement is to keep the IAS and RPM constant. This ensures identical conditions of airframe drag, thrust, and prop efficiency. If you don't do that, it is dead easy to find LOP being a lot more efficient than peak-EGT because the power output is lower, the IAS is lower, and of course the plane is more efficient when flying slower - the most efficient IAS for any airframe is equal to Vbg.
1) the engine is running at high power - over 80%
2) the CHT is very high - around 500F which is right at the top end
3) the mixture was leaned, enough to lead to 2) above
4) the airflow was poor, also leading to 2) above
and then the carb heat leans the mixture just enough to bring it on.
But not otherwise. Most engines are set up so that during climb (all 3 fully forward) the operating point is about 150F ROP and it would take a lot more than a bit of carb heat to bring on detonation in that condition.
Incidentally, LOP is negligibly more efficient than peak EGT. I have done some very careful tests and on the IO-540 I find that, relative to peak EGT, 75F ROP is about 10% worse (on MPG), while 25F LOP (the theoretical most efficient point for any petrol engine) is so close to the peak EGT efficiency that any gain is below the measurement resolution of about 1%. I plan to do additional tests using a different method but I am not expecting to see a better gain than 1% by using LOP.
The crucial thing in this kind of measurement is to keep the IAS and RPM constant. This ensures identical conditions of airframe drag, thrust, and prop efficiency. If you don't do that, it is dead easy to find LOP being a lot more efficient than peak-EGT because the power output is lower, the IAS is lower, and of course the plane is more efficient when flying slower - the most efficient IAS for any airframe is equal to Vbg.
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That should have said "rich of peak operation is..."
A typo.
But then lean of peak operation has been taught since long before you began flying. It's seldom taught today...probably something you wouldn't know about or understand.
You're still unable to provide an answer, then. Like I said...no credibility for you.
A typo.
But then lean of peak operation has been taught since long before you began flying. It's seldom taught today...probably something you wouldn't know about or understand.
You're still unable to provide an answer, then. Like I said...no credibility for you.
I'm well aware of how to operate LOP in fuel injected birds with gami's, and know it's
impractical with carburetors unless you use heat.
Both LYC and TCM say to follow the POH for how to operate their engines, every POH for every carbed engine I've flown says:
Full power with heat MAY cause detonation.
A pilot is landing, the usual ISA SL conditions in his warrior.
mixture full rich, carb heat on, power 1500,
now he goes around, full power, forgets to remove heat!
a dramatic rise in cylinder pressure occurs at the moment of throttle application,
the carbs enrichment feature at full throttle increases the 100ll going into the combustion chamber, that is now getting pumped full of hot air, but the extra gas serves to cool the head, which is a good thing!
The problem arises, when the throttle is advanced to full power from near idle, and the spark timing is static at 25 BTDC, the overly rich mixture causes a more rapid burn, which produces a "peak pressure" before the nominal 14 deg ATDC, which, in turn increases the heat in the cylinder,
which causes the next charge of hot air and gas to increase the operating temps even more, which advances the "effective timing" which causes a massive pressure rise, which leads to detonation, because of carb heat at full power.
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Oh dear, it's difficult to know where to start here! Now let's see, SNS3Guppy said:
Absolutely correct ... and absolutely irrelevant! I am left with the strong impression that you've never, ever tried to operate a carburetted Lycoming or Continental LOP! They won't, because the fuel/air distribution on these engines is so appallingly variable that, in practice, each cylinder is a different engine. By the time you've leaned sufficiently far to get the second or third 'engine' LOP, the first 'engine' has experienced a lean cut - result is such an interesting amount of vibration that you most definitely aren't going to fly with it.
Those people that have doggedly pursued LOP operation of carburetted engines have established two interesting facts that can make it possible in some engine installations: a) heating the air through use of carb heat improves fuel atomisation, helping to even out the mixture distribution; and b) 'cracking' the throttle a little closed from the wide-open position creates air turbulence around the butterfly valve that also improves the mixing of the fuel and air.
There you go, SNS3Guppy, seems you don't know it all, after all. Get out there and talk to a few pilots that actually fly LOP in carburetted engines!
SNS3Guppy, that's total bo**ocks, it makes you come across as a troll whose only knowledge has come from textbooks! I presume that's not the case, so please go out and fly a carburetted Lycoming or Continental that's fitted with a multi-cylinder engine analyser and try to get all cylinders LOP without using the above techniques. Chances are you won't.
And that, SNS3Guppy makes you look like a troll that's not even very good with the textbooks! It's simply so far from the truth to be laughable. With the exception of a quite small number of specific engines, the manufacturers (Lycoming in particular, because their fuel/air distribution tends to be worse) expressly recommend against LOP operation. Try taking a look at Lycoming's 'enlightening' document SSP700, for example.
And barrow, this is terrific stuff:
Eloquently put and absolutely true - but unfortunately irrelevant because every bit of it is just as true when advancing the throttle without carb heat applied! The end difference, however, is that with carb heat applied you are generating around 15% less power - with all that that means for lower temperatures and pressures. So your conclusion:
simply doesn't follow!
Hmmm...no; carburetor heat doesn't contribute to moving a rich mixture toward a lean of peak setting, and reducing throttle doesn't either.
Those people that have doggedly pursued LOP operation of carburetted engines have established two interesting facts that can make it possible in some engine installations: a) heating the air through use of carb heat improves fuel atomisation, helping to even out the mixture distribution; and b) 'cracking' the throttle a little closed from the wide-open position creates air turbulence around the butterfly valve that also improves the mixing of the fuel and air.
There you go, SNS3Guppy, seems you don't know it all, after all. Get out there and talk to a few pilots that actually fly LOP in carburetted engines!
Any recip engine can easily be run lean of peak...simply by adjusting the mixture to a lean of peak condition.
Lean of peak operation is generally recommended by light airplane engine manufacturers
And barrow, this is terrific stuff:
A pilot is landing, the usual ISA SL conditions in his warrior.
mixture full rich, carb heat on, power 1500,
now he goes around, full power, forgets to remove heat!
a dramatic rise in cylinder pressure occurs at the moment of throttle application,
the carbs enrichment feature at full throttle increases the 100ll going into the combustion chamber, that is now getting pumped full of hot air, but the extra gas serves to cool the head, which is a good thing!
The problem arises, when the throttle is advanced to full power from near idle, and the spark timing is static at 25 BTDC, the overly rich mixture causes a more rapid burn, which produces a "peak pressure" before the nominal 14 deg ATDC, which, in turn increases the heat in the cylinder
which causes the next charge of hot air and gas to increase the operating temps even more, which advances the "effective timing" which causes a massive pressure rise .......
mixture full rich, carb heat on, power 1500,
now he goes around, full power, forgets to remove heat!
a dramatic rise in cylinder pressure occurs at the moment of throttle application,
the carbs enrichment feature at full throttle increases the 100ll going into the combustion chamber, that is now getting pumped full of hot air, but the extra gas serves to cool the head, which is a good thing!
The problem arises, when the throttle is advanced to full power from near idle, and the spark timing is static at 25 BTDC, the overly rich mixture causes a more rapid burn, which produces a "peak pressure" before the nominal 14 deg ATDC, which, in turn increases the heat in the cylinder
which causes the next charge of hot air and gas to increase the operating temps even more, which advances the "effective timing" which causes a massive pressure rise .......
....... which leads to detonation, because of carb heat at full power.
Last edited by Islander2; 27th Sep 2008 at 21:30.
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I'm well aware of how to operate LOP in fuel injected birds with gami's, and know it's
impractical with carburetors unless you use heat.
impractical with carburetors unless you use heat.
How does the use of carburetor heat make operating lean of peak practical? Have you thought that through, at all? You understand that when operating lean of peak, operation of carb heat causes the engine to run hotter and closer to peak, right?
You state that lean of peak operations are practical without carburetor heat. This is absolutely nonsensical.
a dramatic rise in cylinder pressure occurs at the moment of throttle application,
the carbs enrichment feature at full throttle increases the 100ll going into the combustion chamber, that is now getting pumped full of hot air, but the extra gas serves to cool the head, which is a good thing!
the carbs enrichment feature at full throttle increases the 100ll going into the combustion chamber, that is now getting pumped full of hot air, but the extra gas serves to cool the head, which is a good thing!
Some carburetors use enrichment valves or economizer valves, but not all.
You understand that in a normally aspirated engine, the cylinder acts as a vacum pump, do you not? When the throttle is closed, it works very much like putting your hand over the end of the hose on a vacum cleaner; manifold pressure drops because the engine is producing suction, just like the pressure drops in the vacum hose. Opening the throttle is nothing more than taking away the obstruction in the line; it's allowing ambient air pressure to return once again...hardly a dramatic rise, hardly a mind bending change in the manifold or cylinder pressure, and hardly a danger...carb heat or no.
The problem arises, when the throttle is advanced to full power from near idle, and the spark timing is static at 25 BTDC, the overly rich mixture causes a more rapid burn, which produces a "peak pressure" before the nominal 14 deg ATDC, which, in turn increases the heat in the cylinder,
which causes the next charge of hot air and gas to increase the operating temps even more, which advances the "effective timing" which causes a massive pressure rise, which leads to detonation, because of carb heat at full power.
which causes the next charge of hot air and gas to increase the operating temps even more, which advances the "effective timing" which causes a massive pressure rise, which leads to detonation, because of carb heat at full power.
Not unless you're already operating very close to peak at high power settings and low altitudes in normally aspirated power plants...on a cold day.
Particularly in the case of engines equipped with an economizer valve (enrichment feature), the use of carburetor heat moves the engine operating environment FARTHER from a detonation regime, and does NOT increase CHT or EGT. Further, there is no dramatic pressure or temperature rise, particularly in a normally aspirated powerplant, owing to a fixed timing, or to an enrichened mixture.
Detonation occurs as uncontrolled, explosive burning of the fuel air mixture in the combustion chamber. If the engine is properly timed (even in a fixed magneto installation), detonation may occur only in a narrow mixture range close to stochiometric, or peak...and doesn't occur during carb heat application in normal operation.
Now from your comments, it might occur when you're operating the engine...but that sounds more the result of someone who has no earthly idea how to operate their engine, than the result of error on the part of the average pilot.
If the engine is being operated at any significant density altitude, it's difficult, if not nearly impossible, to cause detonation damage in a normally aspirated light engine powerplant...which is part of the reason that manufacturers recommend not leaning until several thousand feet...it's nothing more than a way of idiot proofing the engine. From your description, something they haven't yet accomplished, apparently.
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Originally Posted by SNS3Guppy
Lean of peak operation is generally recommended by light airplane engine manufacturers
Originally Posted by Islander2
And that, SNS3Guppy makes you look like a troll that's not even very good with the textbooks! It's simply so far from the truth to be laughable. With the exception of a quite small number of specific engines, the manufacturers (Lycoming in particular, because their fuel/air distribution tends to be worse) expressly recommend against LOP operation. Try taking a look at Lycoming's 'enlightening' document SSP700, for example.
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SNS3Guppy, that's total bo**ocks, it makes you come across as a troll whose only knowledge has come from textbooks! I presume that's not the case, so please go out and fly a carburetted Lycoming or Continental that's fitted with a multi-cylinder engine analyser and try to get all cylinders LOP without using the above techniques. Chances are you won't.
From my perspective as a pilot, mechanic, and instructor with ample experience operating, maintaining, servicing, building, rebuilding, and caring for powerplants...you might need to get out a little more and fly.
I've operated recip engines lean of peak from small displacement engines to large radials...each with carburetors, sometimes fully outfitted with individual cylinder instrumentation, sometimes not. And yes, it really can be done. Just not by you, it would seem. By thousands of others...yes. But not by you. And that's okay...because nobody is forcing you to operate that way, and you don't need to do so.
That's really not relevant to the discussion, however, nor to the turn taken in the thread by Mr. or Mrs barrow....however misguided his or her insight may be.
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To be fair to Guppy if you look back to an earlier post of his he makes it clear that he didn't mean to write that and in fact meant the opposite
This is what SNS3 wrote:
Lean of peak operation is generally recommended by light airplane engine manufacturers, not because it's harmful to the engine if done properly but because pilots don't tend to do it properly...and there's more potential for pilot error. It's actually a much more efficient way to run an engine, when properly done, under the proper circumstances.
---for the benefit of Barrow, edited to say the above should read "Rich of peak operation is..." A typo.
---for the benefit of Barrow, edited to say the above should read "Rich of peak operation is..." A typo.
If instead you substitute 'ROP operation' for 'LOP operation' in the first line, as SNS3 would now have us believe he meant, can you tell me how, in respect of ROP operation, that pilots don't tend to do it properly and why there's more potential for pilot error (than with LOP) and on what basis you believe it's actually a much more efficient way (than LOP) to run an engine, when properly done, under the proper circumstances?
Ho, ho.
Last edited by Islander2; 27th Sep 2008 at 23:20.
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Oh please, you're being a touch gullible, aren't you?
But we seem to have concluded that it is actually very difficult indeed to damage the engine I originally started the thread about (the ever ubiquitous O-320) by using the carb heat.
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I've operated recip engines lean of peak from small displacement engines to large radials...each with carburetors, sometimes fully outfitted with individual cylinder instrumentation, sometimes not. And yes, it really can be done. Just not by you, it would seem. By thousands of others...yes. But not by you. And that's okay...because nobody is forcing you to operate that way, and you don't need to do so.
My knowledge on operating the carburetted Lycoming and Continental flat fours and sixes LOP stems largely from the feedback from a very large number of graduates of the Advanced Pilot Seminars on Engine Management that are run by Walter Atkinson, George Braly and John Deakin. If you've no objection, I'll take that very large sample size any day over your sample of one!
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But we seem to have concluded that it is actually very difficult indeed to damage the engine I originally started the thread about (the ever ubiquitous O-320) by using the carb heat.
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Originally Posted by barrow
A pilot is landing, the usual ISA SL conditions in his warrior.
mixture full rich, carb heat on, power 1500,
now he goes around, full power, forgets to remove heat!
a dramatic rise in cylinder pressure occurs at the moment of throttle application,
the carbs enrichment feature at full throttle increases the 100ll going into the combustion chamber, that is now getting pumped full of hot air, but the extra gas serves to cool the head, which is a good thing!
The problem arises, when the throttle is advanced to full power from near idle, and the spark timing is static at 25 BTDC, the overly rich mixture causes a more rapid burn, which produces a "peak pressure" before the nominal 14 deg ATDC, which, in turn increases the heat in the cylinder,
which causes the next charge of hot air and gas to increase the operating temps even more, which advances the "effective timing" which causes a massive pressure rise, which leads to detonation, because of carb heat at full power.
mixture full rich, carb heat on, power 1500,
now he goes around, full power, forgets to remove heat!
a dramatic rise in cylinder pressure occurs at the moment of throttle application,
the carbs enrichment feature at full throttle increases the 100ll going into the combustion chamber, that is now getting pumped full of hot air, but the extra gas serves to cool the head, which is a good thing!
The problem arises, when the throttle is advanced to full power from near idle, and the spark timing is static at 25 BTDC, the overly rich mixture causes a more rapid burn, which produces a "peak pressure" before the nominal 14 deg ATDC, which, in turn increases the heat in the cylinder,
which causes the next charge of hot air and gas to increase the operating temps even more, which advances the "effective timing" which causes a massive pressure rise, which leads to detonation, because of carb heat at full power.
The 'extra gas' 'cools' the head not directly, but by altering the combustion characteristics.
carb heat will enrich the mixture further, due to reduced density of air, moving you further away from detonation margins. Don't be fooled into thinking the hot air makes things hotter - the increased heat energy of the incomming air is insignificant compared to the combustion temperatures.
Also remember that while the spark is 'timed' at (let's say) 25 BTDC, the period it takes from that point to get the combustion going is measured in time (i.e. hundredths of seconds), not degrees - if the prop is spinning rapidly, the timing is effectively later - the piston will have travelled further in the interval of time required for the spark to get going. That's why running CSU engines wide open throttle with low RPM *can* be a bad thing.
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I support what Mark1234 has written, though I'm not knocking the other posts, Mark's is just the easiest to understand correctly.
I have purposefully dentonated a carburetted Continental 520 engine (we took the injection system off), and a 470 as well. For all of the factors I had to very carefully control to create detonation, carb heat really was not one of them. I'm sure it has an effect, but a minor one, compared to the other critical factors. Carb heat hot = richer mixture = greater detonation margin.
The factor which I used to most precisely cause and termnate detonation was RPM, realtive to manifold pressure (with temps carefully controlled as well) It was also necessary to run 80 octane gasoline in the 8.5:1 TCM 520 engine to get detonation. We did consistently create detonation, and measured it, and then ran our test, and proved that we had the required margins while operating in accordance with our instructions. This mod was STC approved based upon this, and other testing I did.
It is important to know, that when detonation begins, the engine will take on a life of it's own, and may continue to detonate after the adverse conditions which caused the detonation, are removed. The engine will get hotter, and continue to detonate, even after the extreme damands for power are removed. We did not cause damage to the engine doing this testing (teardown inspection) but the effects of detonation were visible on the pistons. None of the detonation we created and measured, could be detected in the cabin in any way other than the special equipment I had installed for the test. If you are detonating your engine, you will not know this until you have really damaged it - stay within the operating limits! The damage will be a hole in a piston, and all of that metal spread through your engine.
Also, the mixture distribution of carburetted engines can be affected by the RPM. Where there may be a few rich cylinders at full power, and leaning it will not affect thise relationship, a power change may effect the distribution. With an all cylinder EGT, it should be possible to find a power setting which provides more equal mixture distribution. On the big Continental engines, there is a "balance tube" which connects the left and right induction manifolds. There are three different sizes of balance tubes, and the correct size is an important factor in mixture distribution.
I do not operate engines lean of peak. We did not operated lean of peak when we flew pistons transatlantic. I would rather waste a little fuel, and have to flight plan more carefully, then to cause engine stress for a very minor benefit, and actualy have an engine problem (particularly there). Fuel is cheaper than a broken engine!
Pilot DAR
I have purposefully dentonated a carburetted Continental 520 engine (we took the injection system off), and a 470 as well. For all of the factors I had to very carefully control to create detonation, carb heat really was not one of them. I'm sure it has an effect, but a minor one, compared to the other critical factors. Carb heat hot = richer mixture = greater detonation margin.
The factor which I used to most precisely cause and termnate detonation was RPM, realtive to manifold pressure (with temps carefully controlled as well) It was also necessary to run 80 octane gasoline in the 8.5:1 TCM 520 engine to get detonation. We did consistently create detonation, and measured it, and then ran our test, and proved that we had the required margins while operating in accordance with our instructions. This mod was STC approved based upon this, and other testing I did.
It is important to know, that when detonation begins, the engine will take on a life of it's own, and may continue to detonate after the adverse conditions which caused the detonation, are removed. The engine will get hotter, and continue to detonate, even after the extreme damands for power are removed. We did not cause damage to the engine doing this testing (teardown inspection) but the effects of detonation were visible on the pistons. None of the detonation we created and measured, could be detected in the cabin in any way other than the special equipment I had installed for the test. If you are detonating your engine, you will not know this until you have really damaged it - stay within the operating limits! The damage will be a hole in a piston, and all of that metal spread through your engine.
Also, the mixture distribution of carburetted engines can be affected by the RPM. Where there may be a few rich cylinders at full power, and leaning it will not affect thise relationship, a power change may effect the distribution. With an all cylinder EGT, it should be possible to find a power setting which provides more equal mixture distribution. On the big Continental engines, there is a "balance tube" which connects the left and right induction manifolds. There are three different sizes of balance tubes, and the correct size is an important factor in mixture distribution.
I do not operate engines lean of peak. We did not operated lean of peak when we flew pistons transatlantic. I would rather waste a little fuel, and have to flight plan more carefully, then to cause engine stress for a very minor benefit, and actualy have an engine problem (particularly there). Fuel is cheaper than a broken engine!
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That's an interesting post, Pilot DAR, but I would take you to task on two important points.
That's too simplistic and takes the squabble with barrow way too far. Of the thermodynamic variables that come into play when carb heat is selected, mixture strength is but one. What actually happens is:
1. an increase in temperature of the incoming charge increases CHTs;
2. a change to the combustion event from enrichening the mixture (due to the reduced air density) reduces CHTs; and
3. a reduction in power (due both to the reduced mass flow and to the over-richness) reduces CHTs.
It's a complex interaction but allow me, too, to fall foul of massive oversimplification by hanging some numbers on these factors following selection of carb heat to full when the engine is developing, say, 85% rated BHP:
1. temperature of incoming charge air will increase by circa 100degF ... because of the complex interaction with other variables, I don't believe this will be fully reflected in increased CHTs, but a significant increase will occur;
2. mixture will enrichen by circa 15% which, assuming the cylinders were already operating rich of peak CHT, will reduce CHTs by circa 15degF;
3. power output will reduce by circa 13% (according to Lycoming), which will reduce CHT's by circa 15 degF
Simplistic? Of course, but nonetheless I would expect CHTs to rise slightly following the application of carb heat ... although certainly not by a sufficient amount to cause detonation problems on a 4-cylinder, naturally-aspirated Lycoming or Continental. It is certainly the case that I see higher CHTs on take-off in Southern Europe compared with the UK, where differences in charge temperature could be in the order of 50degF. I think the admonition not to use carb heat at high power due to detonation risk is a hangover from the days of the big radials, where the combination of charge-temperature rise from both the supercharger and carb heat very definitely could take you into detonation territory.
But hey, there's no need to speculate on this. Would someone out there with a carburetted Lycoming or Continental, in an aeroplane fitted with a multi-cylinder engine analyser, please go out, experiment and report back!
And the other thing I must take you to task over is:
It's not clear if you are talking there about specific transatlantic operations that you've been involved in, or are referring to the piston airliner heydays of DC-7s, Constellations, etc. If the latter, then you are simply wrong! The reason we know so much about lean of peak operation and, with certain safeguards, can undertake it quite comfortably (in the process, reducing fuel consumption and increasing cylinder life) is because literally millions of hours were accumulated operating Wright Cyclones, Pratt & Whitney Wasps, etc lean of peak. It's your choice not to operate LOP, but quite wrong to suggest that in, former times, it wasn't the norm.
Carb heat hot = richer mixture = greater detonation margin.
1. an increase in temperature of the incoming charge increases CHTs;
2. a change to the combustion event from enrichening the mixture (due to the reduced air density) reduces CHTs; and
3. a reduction in power (due both to the reduced mass flow and to the over-richness) reduces CHTs.
It's a complex interaction but allow me, too, to fall foul of massive oversimplification by hanging some numbers on these factors following selection of carb heat to full when the engine is developing, say, 85% rated BHP:
1. temperature of incoming charge air will increase by circa 100degF ... because of the complex interaction with other variables, I don't believe this will be fully reflected in increased CHTs, but a significant increase will occur;
2. mixture will enrichen by circa 15% which, assuming the cylinders were already operating rich of peak CHT, will reduce CHTs by circa 15degF;
3. power output will reduce by circa 13% (according to Lycoming), which will reduce CHT's by circa 15 degF
Simplistic? Of course, but nonetheless I would expect CHTs to rise slightly following the application of carb heat ... although certainly not by a sufficient amount to cause detonation problems on a 4-cylinder, naturally-aspirated Lycoming or Continental. It is certainly the case that I see higher CHTs on take-off in Southern Europe compared with the UK, where differences in charge temperature could be in the order of 50degF. I think the admonition not to use carb heat at high power due to detonation risk is a hangover from the days of the big radials, where the combination of charge-temperature rise from both the supercharger and carb heat very definitely could take you into detonation territory.
But hey, there's no need to speculate on this. Would someone out there with a carburetted Lycoming or Continental, in an aeroplane fitted with a multi-cylinder engine analyser, please go out, experiment and report back!
And the other thing I must take you to task over is:
I do not operate engines lean of peak. We did not operated lean of peak when we flew pistons transatlantic.
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Hi Islander 2,
Sorry for the confusion, I should have clarified that the piston aircraft to which I refer were all flat engined GA aircraft. I do agree that the round engines of old were regularly operated lean of peak. It is my opinion that they are outside the scope of this discussion, as they are different enough that some operating characteristics may not be comparible.
Having reviewed your numbers, I cannot present direct evidence to the contrary, though I hold the opinion that there are too many other variables at play to allow your rationale to be considered valid in isolation. Perhaps in a test cell they would be closer to reality. I can say with confidence that in practical application, the application of carb heat to an engine which is otherwise running within the recommended operating range, will not increase the chances of detonation in any meaningful way.
If I have time, I'll fly the Teal today, (O-360) and watch it's scanner. It does not lean with great precision, but I should be able to form an opinion.
Pilot DAR
Sorry for the confusion, I should have clarified that the piston aircraft to which I refer were all flat engined GA aircraft. I do agree that the round engines of old were regularly operated lean of peak. It is my opinion that they are outside the scope of this discussion, as they are different enough that some operating characteristics may not be comparible.
Having reviewed your numbers, I cannot present direct evidence to the contrary, though I hold the opinion that there are too many other variables at play to allow your rationale to be considered valid in isolation. Perhaps in a test cell they would be closer to reality. I can say with confidence that in practical application, the application of carb heat to an engine which is otherwise running within the recommended operating range, will not increase the chances of detonation in any meaningful way.
If I have time, I'll fly the Teal today, (O-360) and watch it's scanner. It does not lean with great precision, but I should be able to form an opinion.
Pilot DAR
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This is all very interesting, but isn't the reason for setting the carb heat to cold at the decision height to give full power in the event of a go-around? Nothing to do with damaging the engine..... (apart from possibly ingesting undesirable stuff when on the ground).
Join Date: Oct 2005
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I do not operate engines lean of peak. We did not operated lean of peak when we flew pistons transatlantic. I would rather waste a little fuel, and have to flight plan more carefully, then to cause engine stress for a very minor benefit, and actualy have an engine problem (particularly there). Fuel is cheaper than a broken engine!
Studies in the past determined that the ideal range for detonation occurs within about 11% of stoichiometric, or peak mixture...that's 11% rich, not lean. This is only at high power settings. It's nearly impossible to cause detonation uner normal operating cruise conditions...and both Continental and Lycoming have pernitted and even recommended peak operation at cruise power settings between 65% and 75% power for a long time...again, a big part of the reason that you'll find recommendations against leaning below three or four thousand feet...above those altitudes in a normally aspirated engine you aren't going to achieve adequate power to cause detonation. It's simply idiot-proofing the operating practices against ham-fisted pilots who don't know how to operate their engines.
I think the admonition not to use carb heat at high power due to detonation risk is a hangover from the days of the big radials, where the combination of charge-temperature rise from both the supercharger and carb heat very definitely could take you into detonation territory.
The carburetion on a large radial is different; it's a pressure carburetor rather than a float carburetor, but that doesn't make a difference in the way the carb heat is used, either...so yes, the carb heat techniques on a radial engine, being a piston engine, apply to a small flat horizontally opposed engine, too. Mixture techniques, too. A big difference is that the big pressure strom carburetors utilized autorich and autolean settings...which were routinely overridden for manual leaning in flight. I often set mixture in cruise at night by the color of the flame out the exhaust first, and then final adjustments by instrumentation.
While you can't do that on a light single or twin (see exhaust flame), you can safely operate at peak at reduced power, and no, carburetor heat isn't going to tear the engine apart. The ideal would be a carb air temp gauge, but for cost and simplicity most light airplanes are equipped with a very crude carb air box (which typically leaks on most airplanes after a while anyway). It's either full on or full off...and if you need carb heat during takeoff, during climb, during cruise, during descent, then use it.
You can think of it this way; while the greatest possibility of detonation occurs at higher power settings, and one may reasonably guess that the greatest harm might occur with at high power settings, think about why you're applying the carb heat in the first place. If you have carb ice, then you need carb heat, period. When is the greatest potential for icing? When the greatest temperature drop occurs...which occurs at high power settings when the greatest airflow takes place through the venturi. Does carb ice form at high power settings? You bet, and carb heat is needed to get rid of it.
Carb ice is common during ground operations, too, but forms a little differently, for the same reason that carb fuction at idle is different than with an open throttle. Idle fuel is obtained from a different fuel jet, with a different, independent fixed-mixture setting, by utilizing calibrated airflow leakage past the throttle plate in the carburetor. Rather than using high airflow to create a pressure drop through a venturi to draw fuel into the induction airstream, the low manifold pressure and a calibrated leak past the throttle plate combine to draw fuel in a different place than with the open throttle...and ice forms in a different place and in a different manner. Carburetor icing and heat useage therefore, is a different subject when considering throttle open and throttle closed operation.
It becomes a little more complex, however. I see far too many instructors that teach their students to reduce power to idle abeam the numbers and make a prolonged idle descent. This is hard on engines. Often the drill is to reduce to idle power by closing the throttle, apply carburetor heat, and leave it on through the descent. This is poor airmanship and poor practice, and really should be reserved only for training for engine failures. It's a time when backlash issues come into play in the engine (timing can change as much as 20 degrees or more over time with a worn woodruff key in a mag driveshaft, for example, strictly due to backlash), piston ring flutter occurs, and though few realize it, idle mixture changes substantially (resulting in a leaner idle mixture than on the ground at idle). Prolonged operation during a descent at idle is a bad idea and poor practice.
Likewise, operation with the carb heat simply left on, mindlessly, while doing prolonged idle descents is also bad idea. Use of carb heat of it's own accord isn't a bad idea, but for heaven's sake, know why it's being used and what it's doing. Simply applying the heat because the instructor said to apply it there is never wise...especially since the instructor is probably only doing what he does because he saw someone else do it...the heritage of inexperience. It's that same heritage that teaches us that peak or lean of peak operations are dangerous, and a host of other myths that pervade the hangar talk that goes on today. (Along with destructive practices like pulling the propeller through to "limber up the oil" as part of a preflight, etc).
This is all very interesting, but isn't the reason for setting the carb heat to cold at the decision height to give full power in the event of a go-around? Nothing to do with damaging the engine..... (apart from possibly ingesting undesirable stuff when on the ground).
Use of carburetor heat is there for two reasons; to help remove the ice that may have formed and to prevent it's formation by operating the carburetor in the ideal temperature range. That range isn't very big, and what manufacturers do when giving a simple butterfly carb air box with no carb air temperature gauge, is take a shotgun approach. Rather than provide you with the means to properly set carb temp, they shotgun it with extra heat in a form of overkill...apply enough to melt everything.
During a power transition, however, you pass through the ideal ranges and the less than ideal ranges, as airflow through the carb is changing, and the temperature in the cowl area (from whence the heated carb air comes) is changing...and you not only pass through the ideal carb air temp range for avoiding ice...but quite possibly both sides of that range where the ideal temperature and conditions for FORMING ice occur...you can cause ice to form.
I've seen this occur during a go-around during the right humid conditions following a rain, while doing flight training with student's, before. Application of power to go around produced a smooth transition to the climb, followed shortly thereafter by an engine failure with a very rapid buildup of carb ice. Reapplication of heat as we set up for the forced landing on the remaining runway, resulted in a restoration of power. This isn't common in my experience, but I've certainly experienced it. Use carburetor heat when and where you need it, but don't use it indiscriminately.
Having said all this, if you have instructions for your aircraft specifying a carb heat-on descent for landing, then do that, but take the time to get to know your system and the why's of what you're doing. Tools such as mixture and carb heat are NOT absolutes; just like the ideal application of carb heat is partial heat (which can only be done in conjunction with a carburetor air temperature gauge), the same applies to mixture. It's there to be adjusted, to be used as needed, and to be used with care and judgement. It's there to give you options...not simply there to be thrown on or off with abandon, or by rote.
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When is the greatest potential for icing? When the greatest temperature drop occurs...which occurs at high power settings when the greatest airflow takes place through the venturi.
The venturi effect is more pronounced when the throttle butterfly is restricting the flow of the air, as opposed to when the throttle is wide open.
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Not true; that's a common misconception. If you'll re-read, I addressed why.
Idle plate icing takes place in a different location, and for a different reason than icing through and past the venturi. It's affects the airflow differently, affects the fuel flow differently, and is not the same. Completely different icing scenarios.
A common misconception is that the high airflow through the carburetor venturi or throat will prevent icing at high power settings. This is when the greatest temperature drop and the greatest pressure drop occurs...the faster the airflow occurs through the venturi, the greater the pressure drop. Some will argue that it's the volume that makes the difference, but this isn't the case. Icing at high power settings is very possible.
Icing at idle is an entirely different mechanism, when the throttle plate is closed, and has been previously addressed.
Idle plate icing takes place in a different location, and for a different reason than icing through and past the venturi. It's affects the airflow differently, affects the fuel flow differently, and is not the same. Completely different icing scenarios.
A common misconception is that the high airflow through the carburetor venturi or throat will prevent icing at high power settings. This is when the greatest temperature drop and the greatest pressure drop occurs...the faster the airflow occurs through the venturi, the greater the pressure drop. Some will argue that it's the volume that makes the difference, but this isn't the case. Icing at high power settings is very possible.
Icing at idle is an entirely different mechanism, when the throttle plate is closed, and has been previously addressed.
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I took the Teal for a spin today. It is equipped with an O-360 Lycoming. It has a bar graph engine monitor. When operating about 50 degrees rich of peak lean at 2500' feet, the application of carb heat caused an indicated EGT rise of one to three degrees, and did not change the relationship from hot to cool cylinder. It caused a CHT rise of 2 degrees once stabilized. There was about a 1 inch MP decrease, so it did result in a power reduction.
At peak lean, the results seemed to be about the same - nothing dramatic.
I tried to operate lean of peak, but the engine just would not run smoothly. It stumbled badly, so I stayed over the water to be safe. No matter how precisely I leaned past peak, it would stumble right away, and the EGT would drop right off. Richen up, and it ran like a top. The temperature range of leaning seemed to provide about 150 degree EGT rise from rich to peak, All of the bars moved about the same about, with the hottest and coolest always remaining the same.
All of this was done at 2000 feet, 16C, with no vizible moisture, and an engine with 33 hours since overhaul.
I therefore suggest that on this engine, carb heat has little to do with EGT or CHT, affects power as expected, and is unlikely to be a cause for detonation.
I hope that this informal test helps a little.
Pilot DAR
At peak lean, the results seemed to be about the same - nothing dramatic.
I tried to operate lean of peak, but the engine just would not run smoothly. It stumbled badly, so I stayed over the water to be safe. No matter how precisely I leaned past peak, it would stumble right away, and the EGT would drop right off. Richen up, and it ran like a top. The temperature range of leaning seemed to provide about 150 degree EGT rise from rich to peak, All of the bars moved about the same about, with the hottest and coolest always remaining the same.
All of this was done at 2000 feet, 16C, with no vizible moisture, and an engine with 33 hours since overhaul.
I therefore suggest that on this engine, carb heat has little to do with EGT or CHT, affects power as expected, and is unlikely to be a cause for detonation.
I hope that this informal test helps a little.
Pilot DAR