Mixture control
Peter337's point about managing CHT is a very good one. I bet more light aircraft engines have been cooked by overheating than have ever been damaged by over leaning. Long Vy climbs on a hot day are a good way to overheat and permanently damage the cylinders. Unfortunately your average rental trainer/tourer is unlikely to have a CHT gauge, and if it does it probably doesn't work. The only way to know the engine is getting too hot is to watch the oil temp. Unfortunately this is an indirect measurement of CHT and so you could do damage before the oil temp indicates an excessively high value. In any case immediate action should be taken if the oil temp is observed approaching the redline ( ie full rich mixture and increase airspeed).
In the absence of engine instrumentation the easiest way to avoid burn't cylinders is, once clear of the aerodrome, do a shallow cruise climb at full power, and an airspeed of 15 % or so below the normal level flight cruise value.
Peter337 also gives very precise leaning instructions which work well if you are flying the same fully instrumented, fuel injected, CS prop equipped, tourer on a frequent basis; but are of no practical use when flying a simple rental airplane.
For those aircraft I echo what pilot DAR said, slowly pull the mixture knob out until you get a reduction in RPM and the onset of rough running. Enrichen until smooth and you get maximum RPM and then a "little" bit more. The magnitude of the "little" varies depending on how far the mixture control travel is. The point of the exercise is to get the engine firmly in the "best power" mixture range.
Remember leaning is authorized whenever the power is below 75 % and for Cessna's in particular, in the climb above 5000 feet. If you are going cross country you must lean to get anywhere near book figures and for any high altitude operation leaning is a must to get adequate power.
In the absence of engine instrumentation the easiest way to avoid burn't cylinders is, once clear of the aerodrome, do a shallow cruise climb at full power, and an airspeed of 15 % or so below the normal level flight cruise value.
Peter337 also gives very precise leaning instructions which work well if you are flying the same fully instrumented, fuel injected, CS prop equipped, tourer on a frequent basis; but are of no practical use when flying a simple rental airplane.
For those aircraft I echo what pilot DAR said, slowly pull the mixture knob out until you get a reduction in RPM and the onset of rough running. Enrichen until smooth and you get maximum RPM and then a "little" bit more. The magnitude of the "little" varies depending on how far the mixture control travel is. The point of the exercise is to get the engine firmly in the "best power" mixture range.
Remember leaning is authorized whenever the power is below 75 % and for Cessna's in particular, in the climb above 5000 feet. If you are going cross country you must lean to get anywhere near book figures and for any high altitude operation leaning is a must to get adequate power.
Last edited by Big Pistons Forever; 12th Dec 2011 at 02:36.
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before the oil temp indicates an excessively high value. In any case immediate action should be taken if the oil temp is observed approaching the redline ( ie full rich mixture and increase airspeed).
Enrichen mixture, yes, increase airspeed , yes, but not to the point of a dive with the power off, that will shock cool the engine, which is the last thing you want to do with an engine that hot.
The important thing is to not get yourself there in the first time. Parachuting pilots are usually thoroughly briefed about treating engines gently in this regard.
If the aircraft you are flying has a CHT (if it has cowl flaps, it is required to), watch it as you change power. If you can see the pointer move at all, you're changing power too quickly. If the aircraft you're flying has an "aftermarket" CHT, and not a probe on every cylinder, be aware that the probe might not be installed on the hottest (or coolest) cylinder. Use that reading with caution...
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I wonder how many planes will have the engine below say 400F when doing a Vy climb, even with mixture fully rich.
In my limited experience of the higher performance types, probably none of those.
Yet, during training (PA38, C152, PA28-140 -161 -180) we were doing Vy climbs all the way to altitude. None of those planes had any EGT or CHT gauges, however. In the FAA IR (Arizona, ISA+10 or so) we were doing Vy climbs to 11,000ft and the only gauge was the oil temp.
And when people do have a CHT gauge it tends to be on #5 or #6 (on a 6-cyl) which common sense tells you should be the hottest but common sense is usually wrong in this case because it is the middle ones (#3 or #4 - usually #3 if the oil cooler is mounted on that side, robbing some air for itself) which run the hottest. These are typical full-rich deg-F climb CHT figures for mine, #1 to #6
380 359 398 374 386 373 194
where the last one is the oil temp. And that was climbing out of St Gallen LSZR where the OAT was +14C so not exactly warm. Looking at the EGTs, that part of the climb was full-rich, too, trimmed for 120kt to get plenty of cooling air.
You can see #3 is 25F hotter than #6.
So it would not suprise me if a lot of un-instrumented engines out there are being run very hot, because Vy climbs are common on the training scene.
In my limited experience of the higher performance types, probably none of those.
Yet, during training (PA38, C152, PA28-140 -161 -180) we were doing Vy climbs all the way to altitude. None of those planes had any EGT or CHT gauges, however. In the FAA IR (Arizona, ISA+10 or so) we were doing Vy climbs to 11,000ft and the only gauge was the oil temp.
And when people do have a CHT gauge it tends to be on #5 or #6 (on a 6-cyl) which common sense tells you should be the hottest but common sense is usually wrong in this case because it is the middle ones (#3 or #4 - usually #3 if the oil cooler is mounted on that side, robbing some air for itself) which run the hottest. These are typical full-rich deg-F climb CHT figures for mine, #1 to #6
380 359 398 374 386 373 194
where the last one is the oil temp. And that was climbing out of St Gallen LSZR where the OAT was +14C so not exactly warm. Looking at the EGTs, that part of the climb was full-rich, too, trimmed for 120kt to get plenty of cooling air.
You can see #3 is 25F hotter than #6.
So it would not suprise me if a lot of un-instrumented engines out there are being run very hot, because Vy climbs are common on the training scene.
The reality is your PPL training will not give you very much useful preparation for operating a more sophisticated aircraft and so a check out with somebody who has actually real world experience operating the same or similar type of aircraft is highly desirable.
Last edited by Big Pistons Forever; 14th Dec 2011 at 04:15.
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Full power climbs at lower airspeeds with the engine leaned will do it. The CHT's could be OK at full rich, but will be so high with the engine leaned that you toast the cylinders.
But full rich mixture will deliver more fuel than can be burned, the excess will carry away heat and thus reduce CHT's. Leaning will eliminate most of the "extra" fuel thus loosing the cooling effect and causing a rise, and sometimes a dangerous rise, in CHT's.
I want to emphasize that I am taking about aircraft in common use. That is single engine fixed gear Piper and Cessna aircraft manufactured in the 1970's and early 1980's. I am sure someone can find some obscure aircraft with exactly 13.5 airframes in existence where the above comment does not apply but for aircraft that average reader of this forum has even seen let alone flown, I believe my statement in post number 28 is accurate.
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How would you damage an engine from 'over leaning'?
If I'm flying an extreme long over water, I'll be looking for best range, but sure don' want to cough an engine!
I really do agree with the numerous cautions that Big Pistons has presented here (so won't repeat them). When you change the combustion in the cylinder, you can be repositioning the flame front. That can expose internal areas of the piston, cylinder head, and exhaust valve to temperatures for which they were not designed. By way of a minor (an perhaps only somewhat relevant example) what the EGT when you're doing a mag check - Does the EGT increase when one mag is grounded? Less power, why the higher EGT!
I sure don't have all the answers, and if I thought I did, the next model engine, with different combustion chamber characteristics would require a restart in my thinking anyway. During inflight and ground based detonation testing I have undertaken in Continental 470's and 520's, I have seen rapid and unexpected temperature change trends with leaning, and not a full power. These have caused me to err on the side of caution when leaning. Leaning is necessary for good engine health, and I always lean my O-200 at all altitudes (with reference to a bar graph EGT), but I still keep it 100 or so rich of peak. Fuel, no matter how costly, is still cheaper than engine maintenance....
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A good explanation Silvaire, thanks. It does serve to add to my thought that as combustion is now happening downstream of where it is intended to, although the combustion chamber is not feeling that fire, the open exhaust valve is, and that could be a beginning or aggravating point for sticking the exhaust valves(s) open. The exposed valve stem is potentially receiving deposits of hotter that expected, products of combustion, which had they occurred where they were supposed to, would pass by harmlessly.
I cannot draw a conclusive link between valve sticking, and unconventional mixture settings, but during years of work at the engine shop I saw a number of cases of stuck exhaust valves, and these were often linked to the pilot's report of the use of 100LL, and extreme leaning. I too, lean harder when I occasionally must run 100LL, and stuck two exhaust valves last summer for the fist time in years.
For those not familiar with stuck exhaust valves, the immediate affect is that you are now developing one fewer cylinder worth of power, which on a four cylinder engine is significant.While this is happening, and depending upon which position the stuck valve has taken, worst case engine damage can include broken rockers bosses (expensive cylinder repair or replacement), bent push rods, and cam and follower damage (may fill your engine with metal, and require replacement cam$haft).
To save a bit of fuel, it's just not worth it! If I'm really worried about making the coast of Iceland in higher than forecast winds, it's a chance I might take. For day to day running around, I'd rather waste a gallon an hour....
I cannot draw a conclusive link between valve sticking, and unconventional mixture settings, but during years of work at the engine shop I saw a number of cases of stuck exhaust valves, and these were often linked to the pilot's report of the use of 100LL, and extreme leaning. I too, lean harder when I occasionally must run 100LL, and stuck two exhaust valves last summer for the fist time in years.
For those not familiar with stuck exhaust valves, the immediate affect is that you are now developing one fewer cylinder worth of power, which on a four cylinder engine is significant.While this is happening, and depending upon which position the stuck valve has taken, worst case engine damage can include broken rockers bosses (expensive cylinder repair or replacement), bent push rods, and cam and follower damage (may fill your engine with metal, and require replacement cam$haft).
To save a bit of fuel, it's just not worth it! If I'm really worried about making the coast of Iceland in higher than forecast winds, it's a chance I might take. For day to day running around, I'd rather waste a gallon an hour....
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Some of the burning mixture is going out the exhaust with one mag inactive
combustion is now happening downstream of where it is intended to
I think it was GAMI who showed that valve stem temperature closely tracks the CHT, first and foremost. This is why CHT management is so important.
As an aside, people who fly "deep LOP" (say 50F LOP) find a useful MPG improvement by using low RPM. I doubt anybody knows exactly why but it is most likely because the fixed spark timing on these engines produces a more favourably timed spark. I certainly find 2200 produces a ~5% (IIRC) better MPG than 2500 (at the same IAS etc) but I don't fly LOP (I fly around peak EGT).
I'd rather waste a gallon an hour....
80-100F ROP ("best power") is about 10% worse in MPG than peak EGT.
150F ROP (mixture full rich) is about 25-30% worse in MPG than peak EGT.
LOP alone does not give you measurably better MPG if you fly at the same IAS.
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Only to a very small degree. An LOP mixture is a relatively weak burn.
As for the waste gallon per hour... One of the rewards of enduring a quarter century of ridicule for owning a 150 spam can, is that the difference between wasteful and efficient really is only about a gallon per hour. Now, when I fly the Navajo, things are different!
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Everything should get burnt at peak EGT, or anywhere LOP.
I can offer one data point: my engine got rebuilt (Lyco SB569 crank swap) at ~700hrs, after ~650hrs of peak EGT operation 100% of the time (except climbs), and the only items which were outside new limits were the exhaust valve stems (they were about 0.002" outside and still within overhaul limits), and the cam followers. The engine was otherwise spotless.
Now, some 400hrs on, the compressions are all ~78/80.
I am not aware of anybody who has operated similarly (which is quite common among pilots of similar aircraft, though some operate nearer to 75% power whereas I fly low level at ~65%) and who has found anything different.
Turbo engines are a different thing; they seem to crack cylinders well before TBO no matter what one does
I can offer one data point: my engine got rebuilt (Lyco SB569 crank swap) at ~700hrs, after ~650hrs of peak EGT operation 100% of the time (except climbs), and the only items which were outside new limits were the exhaust valve stems (they were about 0.002" outside and still within overhaul limits), and the cam followers. The engine was otherwise spotless.
Now, some 400hrs on, the compressions are all ~78/80.
I am not aware of anybody who has operated similarly (which is quite common among pilots of similar aircraft, though some operate nearer to 75% power whereas I fly low level at ~65%) and who has found anything different.
Turbo engines are a different thing; they seem to crack cylinders well before TBO no matter what one does
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Simplified: at peak EGT you have stoichiometric mixture, meaning: every fuel molecule has the potential to find one matching oxygen molecule and get burned perfectly.
In a 'rich' mixture, there are more fuel molecules than oxygen molecules, in a 'lean' mixture there are more oxygen molecules than fuel molecules.
The reason both a richer and leaner mixture produce less heat is because they burn less quick and therefore act like a retarded ignition: the peak power pulse happens later in the cycle when the cylinder head has moved downwards more and there is less pressure (and heat) in the cylinder.
Same with single magneto operation: the flame-front needs more time to burn the whole air/fuel mixture and this also acts like a retarded ignition thus EGT rises.
Hottest CHT happens about 40°F before the hottest EGT, so peak EGT is better for the engine than 'enriching slightly for the grandma and kids'
About the savings: in my aircraft, at TBO time, the LOP savings in fuel consumption more than pay for the engine overhaul. The EDM700 has more than paid for itself in reduced engine and ignition system troubleshooting time.
One more reason: at peak or LOP, there is NO carbonmonoxide produced (deadly in case of exhaust/heater leaks)
In a 'rich' mixture, there are more fuel molecules than oxygen molecules, in a 'lean' mixture there are more oxygen molecules than fuel molecules.
The reason both a richer and leaner mixture produce less heat is because they burn less quick and therefore act like a retarded ignition: the peak power pulse happens later in the cycle when the cylinder head has moved downwards more and there is less pressure (and heat) in the cylinder.
Same with single magneto operation: the flame-front needs more time to burn the whole air/fuel mixture and this also acts like a retarded ignition thus EGT rises.
Hottest CHT happens about 40°F before the hottest EGT, so peak EGT is better for the engine than 'enriching slightly for the grandma and kids'
About the savings: in my aircraft, at TBO time, the LOP savings in fuel consumption more than pay for the engine overhaul. The EDM700 has more than paid for itself in reduced engine and ignition system troubleshooting time.
One more reason: at peak or LOP, there is NO carbonmonoxide produced (deadly in case of exhaust/heater leaks)
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Back to one of my original question:
What if you are properly leaned (eg. ROP), cruising at eg. 5,000ft, then due to atmospheric condition changes, your RPM went up by 100 and thus causing your airspeed to increase by 10kts, you want to maintain your original airspeed so you adjust your throttle.
BUT, what about your mixture now? You have entered an area of significant atmospheric condition, shouldn't you "reset" your mixture, that is, put it full rich, then lean again to eg. ROP?
What if you are properly leaned (eg. ROP), cruising at eg. 5,000ft, then due to atmospheric condition changes, your RPM went up by 100 and thus causing your airspeed to increase by 10kts, you want to maintain your original airspeed so you adjust your throttle.
BUT, what about your mixture now? You have entered an area of significant atmospheric condition, shouldn't you "reset" your mixture, that is, put it full rich, then lean again to eg. ROP?
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Shumway,
In trying to consider your original question, I'm trying to think of a situation where, at a constant altitude, a change in atmospheric conditions could cause a change in RPM of 100, and an increase in speed of 10 kts. Could you present a more defined example of this situation?
In trying to consider your original question, I'm trying to think of a situation where, at a constant altitude, a change in atmospheric conditions could cause a change in RPM of 100, and an increase in speed of 10 kts. Could you present a more defined example of this situation?
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PilotDAR,
The 100RPM & 10kts increase is just an example.
The situation is cruising at the same altitude over long distance, I enter an area of significant atmospheric condition, maybe an area where the temperature drops significantly & density increases due to whatever phenomenon (cold front or something like that, just as an example).
That would cause the engine RPM & thus airspeed to change.
Now the leaning was done earlier, before this atmospheric condition change occured.
The question is, other than adjusting the throttle to maintain the original intended airspeed (reduce the RPM to lose the extra 10kts gained), shouldn't the mixture be readjusted again due to this "new" atmospheric condition?
The 100RPM & 10kts increase is just an example.
The situation is cruising at the same altitude over long distance, I enter an area of significant atmospheric condition, maybe an area where the temperature drops significantly & density increases due to whatever phenomenon (cold front or something like that, just as an example).
That would cause the engine RPM & thus airspeed to change.
Now the leaning was done earlier, before this atmospheric condition change occured.
The question is, other than adjusting the throttle to maintain the original intended airspeed (reduce the RPM to lose the extra 10kts gained), shouldn't the mixture be readjusted again due to this "new" atmospheric condition?
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well, I suppose that I do not have an answer of that scenario, because I cannot imagine it occurring. in a standard GA aircraft, you are not going to fly far enough, or for long enough to encounter enough atmospheric change to cause such an engine performance change. indeed, I don't think that a 172 could operate in conditions varied enough to produce such a performance change!
I doubt that in the real world of GA flying, you're going to encounter an operating change enough to affect engine operation at one power setting detectably. Probably the change in weight resulting from fuel burn will be noticeable first.
As I have previously suggested, unless you're flying a perfectly set up fuel injected engine, you're never going to optimize fuel flow/mixture to the extent you propose. Bear in mind that the C 172 S Flight Manual instructs that "lean" is 50F cool of peak, and operation lean of peak is prohibited. This is not the first engine for which I have seen this prohibition.
As I have said, understanding the concepts is good, understanding the limitations, and working within them is vital and required. Saving that last drop of fuel is not worth the effort, or the risk of operating the engine outside it's approved parameters. I would expect that a good wash and wax will have more noticeable affect on the cruise performance and economy of the flight, than attempting to optimize the mixture to the "enth" degree....
I doubt that in the real world of GA flying, you're going to encounter an operating change enough to affect engine operation at one power setting detectably. Probably the change in weight resulting from fuel burn will be noticeable first.
As I have previously suggested, unless you're flying a perfectly set up fuel injected engine, you're never going to optimize fuel flow/mixture to the extent you propose. Bear in mind that the C 172 S Flight Manual instructs that "lean" is 50F cool of peak, and operation lean of peak is prohibited. This is not the first engine for which I have seen this prohibition.
As I have said, understanding the concepts is good, understanding the limitations, and working within them is vital and required. Saving that last drop of fuel is not worth the effort, or the risk of operating the engine outside it's approved parameters. I would expect that a good wash and wax will have more noticeable affect on the cruise performance and economy of the flight, than attempting to optimize the mixture to the "enth" degree....