Commercial Pilots who don't know about piston engines
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Here's a thought about the dangers of screwing up the mixture.
1) There is nothing one can do when LOP that hurts the engine. Period.
2) There are any number of things one can do to harm the engine when ROP.
So, the question is: "Why are pilots comfortable with ROP mixtures, and seem to be resistant to/uncomfortable with LOP ops?
Maybe because they "have always done it that way" with little to no understanding of what they are doing? Maybe the fact that engine manufacturers were intellectually dishonest on the subject for a long time? (We've finally gotten CMI and Lycoming on board to agree with their own data.)
As the King of Siam noted, "'Tiz a puzzlement."
1) There is nothing one can do when LOP that hurts the engine. Period.
2) There are any number of things one can do to harm the engine when ROP.
So, the question is: "Why are pilots comfortable with ROP mixtures, and seem to be resistant to/uncomfortable with LOP ops?
Maybe because they "have always done it that way" with little to no understanding of what they are doing? Maybe the fact that engine manufacturers were intellectually dishonest on the subject for a long time? (We've finally gotten CMI and Lycoming on board to agree with their own data.)
As the King of Siam noted, "'Tiz a puzzlement."
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Perhaps if you do it once it won't hurt, perhaps it doesn't matter the detonation margin is big enough, or perhaps they are both equally bad.
It's as much an illustration that other factors can influence the outcome, or its not always as straight forward as it might seem.
Last edited by vh-foobar; 14th Apr 2016 at 23:40. Reason: grammar
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Depends on the fuel control unit, but generally no it won't. How can it? The typical Lycoming is a mass air flow device so it will advance the fuel flow with it roughly (they are not perfect for the nit pickers) and maintain the F/A ratio.
Typical TCM's the RPM controls fuel pressure which is delivery so it actually gets leaner.
What is a BIG problem is when you are ROP on say a Chieftan but not anywhere near rich enough, and you get away with it in the cruise but you push it full forward on the throttle, and forget the red knob
That has been done before
Typical TCM's the RPM controls fuel pressure which is delivery so it actually gets leaner.
What is a BIG problem is when you are ROP on say a Chieftan but not anywhere near rich enough, and you get away with it in the cruise but you push it full forward on the throttle, and forget the red knob
That has been done before
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As I said previously perhaps this doesn't make much difference, or maybe I am flat out wrong about the engine. Its as much an example of how other factors can have a less than obvious effect.
Last edited by vh-foobar; 15th Apr 2016 at 12:06. Reason: grammar
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***
The example that I had in mind was advancing the throttles to full, with the mixture still set at a sensible LOP setting
***
Lycoming got it right on their full controller. When one advances the throttle when LOP, the mixture gets LEANER--exactly what we want to happen. Since no one set LOP could effectively advance the throttle to WOT as on takeoff, any enrichment feature is moot. The engine as set up will not run LOP above about 33" MP, so fuel enrichment is not an operational issue. We have tried to define why most of the engines will not run smoothly above about 32-33"MP LOP. (The exception being the GTSIO engines which runs smoothly LOP up to about 36" MP.) We have come up with some pretty good theories, all of which when tested, were wrong. (I hate when that happens)
Lycoming said the opposite--that the mixture got richer when the throttle was advanced. When we told them that's not how their how their fuel controller worked, they insisted that it did the opposite. We sent them the DATA on the TIO-540-J2BD from the test stand and they now agree that when the throttle is advanced from a LOP setting, that the mixture gets leaner--as it should.
Some fuel enrichment features are accomplished with an extra jet, others with a cam arrangement. As Jabba said, "it depends on the controller."
The example that I had in mind was advancing the throttles to full, with the mixture still set at a sensible LOP setting
***
Lycoming got it right on their full controller. When one advances the throttle when LOP, the mixture gets LEANER--exactly what we want to happen. Since no one set LOP could effectively advance the throttle to WOT as on takeoff, any enrichment feature is moot. The engine as set up will not run LOP above about 33" MP, so fuel enrichment is not an operational issue. We have tried to define why most of the engines will not run smoothly above about 32-33"MP LOP. (The exception being the GTSIO engines which runs smoothly LOP up to about 36" MP.) We have come up with some pretty good theories, all of which when tested, were wrong. (I hate when that happens)
Lycoming said the opposite--that the mixture got richer when the throttle was advanced. When we told them that's not how their how their fuel controller worked, they insisted that it did the opposite. We sent them the DATA on the TIO-540-J2BD from the test stand and they now agree that when the throttle is advanced from a LOP setting, that the mixture gets leaner--as it should.
Some fuel enrichment features are accomplished with an extra jet, others with a cam arrangement. As Jabba said, "it depends on the controller."
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***
Lycoming got it right on their full controller. When one advances the throttle when LOP, the mixture gets LEANER--exactly what we want to happen. Since no one set LOP could effectively advance the throttle to WOT as on takeoff, any enrichment feature is moot. The engine as set up will not run LOP above about 33" MP, so fuel enrichment is not an operational issue. We have tried to define why most of the engines will not run smoothly above about 32-33"MP LOP. (The exception being the GTSIO engines which runs smoothly LOP up to about 36" MP.) We have come up with some pretty good theories, all of which when tested, were wrong. (I hate when that happens)
Lycoming said the opposite--that the mixture got richer when the throttle was advanced. When we told them that's not how their how their fuel controller worked, they insisted that it did the opposite. We sent them the DATA on the TIO-540-J2BD from the test stand and they now agree that when the throttle is advanced from a LOP setting, that the mixture gets leaner--as it should.
Lycoming got it right on their full controller. When one advances the throttle when LOP, the mixture gets LEANER--exactly what we want to happen. Since no one set LOP could effectively advance the throttle to WOT as on takeoff, any enrichment feature is moot. The engine as set up will not run LOP above about 33" MP, so fuel enrichment is not an operational issue. We have tried to define why most of the engines will not run smoothly above about 32-33"MP LOP. (The exception being the GTSIO engines which runs smoothly LOP up to about 36" MP.) We have come up with some pretty good theories, all of which when tested, were wrong. (I hate when that happens)
Lycoming said the opposite--that the mixture got richer when the throttle was advanced. When we told them that's not how their how their fuel controller worked, they insisted that it did the opposite. We sent them the DATA on the TIO-540-J2BD from the test stand and they now agree that when the throttle is advanced from a LOP setting, that the mixture gets leaner--as it should.
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***
That is interesting, I wonder if it was be design or by luck... I would suggest for at least one Lycoming engineer is was luck
***
I have a very high level of confidence that it was not luck, but by design. Sadly, the corporate memory seemed to have been lost and the current understanding was flawed.
That is interesting, I wonder if it was be design or by luck... I would suggest for at least one Lycoming engineer is was luck
***
I have a very high level of confidence that it was not luck, but by design. Sadly, the corporate memory seemed to have been lost and the current understanding was flawed.
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While studying for something else, I found these if some are interested.
http://ntrs.nasa.gov/archive/nasa/ca...9930081891.pdf
and
http://ntrs.nasa.gov/archive/nasa/ca...9930091891.pdf
you can find everything from NACA on exhaust valves here
The former has a chart showing CHT, exhaust valve temperature, while mixture is varied and power held constant, Figure 8. It also has EGT (though not enough data points to see the peak properly) the significance of the graph is that’s its at a constant power. The engine is watercooled.
Its reinforced that while some of the OWT may be tales, some of the POH leaning procedures for these old aircraft is still appropriate, YMMV.
I still agree for updated engine i.e. appropriate instrumentation and fuel delivery, different leaning procedures would be appropriate.
The later report has a method of calculating the valve temperatures, but its not a simple read...
http://ntrs.nasa.gov/archive/nasa/ca...9930081891.pdf
and
http://ntrs.nasa.gov/archive/nasa/ca...9930091891.pdf
you can find everything from NACA on exhaust valves here
The former has a chart showing CHT, exhaust valve temperature, while mixture is varied and power held constant, Figure 8. It also has EGT (though not enough data points to see the peak properly) the significance of the graph is that’s its at a constant power. The engine is watercooled.
Its reinforced that while some of the OWT may be tales, some of the POH leaning procedures for these old aircraft is still appropriate, YMMV.
I still agree for updated engine i.e. appropriate instrumentation and fuel delivery, different leaning procedures would be appropriate.
The later report has a method of calculating the valve temperatures, but its not a simple read...
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Thanks for posting those links. I'll comment as I get a chance to review them.
http://ntrs.nasa.gov/archive/nasa/ca...9930081891.pdf
I have taken a cursory look at the issues on the ink above and found several interesting issues.
1) The "assumption" by the researchers that the observed increase in EGT with respect to RPM is flawed. The measured change in combustion temperature with increasing RPM is actually lower due to the delayed thetaPP during the higher RPMs, therefore so is actual EGT. The effects are actually very small but the reading changes are significant. Why, then are they (and we in instrumented aircraft) seeing higher EGT READINGS with increases in RPM? The reason this is the case is that the probe has less refractory time to cool between pulses of hot air.--it will read the same (or even a lower) temperature as higher. This has only recently become appreciated (the last 20 years or so).
2) If one looks at the change in exhaust valve temperature as CHT is altered while holding everything else consent, one quickly sees the most direct correlation of CHT-EVT as compared the other factors.
3) The unusually high EGTs in this study are due to the low compression (6.5:1) of the engine. As expected.
4) Research and analysis of hundreds of events of pre-ignition have debunked the "assumption" during this test that pre-ignition is caused by high exhaust valve temperature. This is simply not supported by reams of data from pre-ignition events where engine monitors have been present to record the events.
5) Some of the "assumptions" from the 1930s and 40s are taking a long time to dispel (recently a CMI representative giving a talk to 300 LAMEs in the US assigned effects of pre-ignition to having been, incorrectly, detonation--and that's not an uncommon error).
There is, however, some very good data in this study which has been confirmed in the decades following.
Thank you for posting this 1947 study.
http://ntrs.nasa.gov/archive/nasa/ca...9930081891.pdf
I have taken a cursory look at the issues on the ink above and found several interesting issues.
1) The "assumption" by the researchers that the observed increase in EGT with respect to RPM is flawed. The measured change in combustion temperature with increasing RPM is actually lower due to the delayed thetaPP during the higher RPMs, therefore so is actual EGT. The effects are actually very small but the reading changes are significant. Why, then are they (and we in instrumented aircraft) seeing higher EGT READINGS with increases in RPM? The reason this is the case is that the probe has less refractory time to cool between pulses of hot air.--it will read the same (or even a lower) temperature as higher. This has only recently become appreciated (the last 20 years or so).
2) If one looks at the change in exhaust valve temperature as CHT is altered while holding everything else consent, one quickly sees the most direct correlation of CHT-EVT as compared the other factors.
3) The unusually high EGTs in this study are due to the low compression (6.5:1) of the engine. As expected.
4) Research and analysis of hundreds of events of pre-ignition have debunked the "assumption" during this test that pre-ignition is caused by high exhaust valve temperature. This is simply not supported by reams of data from pre-ignition events where engine monitors have been present to record the events.
5) Some of the "assumptions" from the 1930s and 40s are taking a long time to dispel (recently a CMI representative giving a talk to 300 LAMEs in the US assigned effects of pre-ignition to having been, incorrectly, detonation--and that's not an uncommon error).
There is, however, some very good data in this study which has been confirmed in the decades following.
Thank you for posting this 1947 study.
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An interesting video, that mentions something i have not heard mentioned before in the discussions on engine ops, combustion etc,
the thing that actually influences your CHT temps. and why higher combustion pressures are the real killer, not mixture.
The boundary layer.. or sidewall cooling
the thing that actually influences your CHT temps. and why higher combustion pressures are the real killer, not mixture.
The boundary layer.. or sidewall cooling
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Mixture affects thetaPP and ICPs, which affect the thermal boundary layer, which affects the rate of heat transfer from the combustion gasses into the wall of the combustion chamber.
This is discussed in detail during the APS class
This is discussed in detail during the APS class
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Walter Atkinson:
You have overlooked where they specifically state that the EGT is "not the true exhaust gas temperature but merely indicated the temperature that may be attained by an engine part subjected to the flow of exhaust gases at the same position"; which is more relevant than instantaneous gas temperature. Nonetheless it stands to reason that EGT would increase with RPM, like it does with retarded timing.
It does not follow that EGT is lower. In the test they maintained imep as they varied RPM. In the diagram below the red pecked line represents the pressure trace with retarded PP. For the EGT to be cooler the red pecked line would have to drop into the brown area. That cannot happen whilst maintaining imep.
They did not do a test to alter CHT and EGT while holding everything else constant. I would be very interested to know how you think one would go about such a test...
The CR was 6.65 which was normal for that engine and seemingly typical for blown engines of that time, eg the Wright TCs (CR 6.7). Therefore the EGTs in the test were representative under the conditions stated. The mixture temp was 200°F which would be modest based on NACA estimates:
In the test of variable CR the difference between CR of 6.65 and 7.5 (typical GA turbo) was 55°F:
...not huge and the point of the test was to investigate how all the temps respond, not establish absolute figures for any specific CR.
A proper reading of the report reveals that the valve temperature was actually lower when they experienced preignition, due to the insulating effect of the valve deposits. It is those deposits that is the problem they identified rather than the valve temperature per se; "The foregoing results indicate that the scale and deposits probably reached a temperature sufficiently high to cause preignition whereas the temperature of the valve steel as measured by thermocouple remained substantially below this temperature". All backed up by the engineering literature:
Interesting anecdote but it has nothing to do with this test. On the two occasions when they encountered preignition, had that actually been detonation they would have recognised it due to audible knock. Your opinion that these experienced NACA researchers mistook preignition for detonation is not supported by studying the series of tests undertaken at the Cleveland lab to explore those very issues.
I have taken a cursory look at the issues on the ink above and found several interesting issues.
1) The "assumption" by the researchers that the observed increase in EGT with respect to RPM is flawed. The measured change in combustion temperature with increasing RPM is actually lower due to the delayed thetaPP during the higher RPMs, therefore so is actual EGT. The effects are actually very small but the reading changes are significant. Why, then are they (and we in instrumented aircraft) seeing higher EGT READINGS with increases in RPM? The reason this is the case is that the probe has less refractory time to cool between pulses of hot air.--it will read the same (or even a lower) temperature as higher. This has only recently become appreciated (the last 20 years or so).
"Mass-averaged exhaust temperatures are higher than time-averaged or thermocouple determined temperatures. All these temperatures increase with increasing speed, load, and spark retard, with speed being the variable with the greatest impact". Heywood JB, Internal Combustion Engine Fundamentals, chap 6, p234, 1984.
What you describe as "it will read the same (or even a lower) temperature as higher" is actually thermocouple equilibrium temperature approaching the temperature of the "pulses of hot air" as RPM increases.
The measured change in combustion temperature with increasing RPM is actually lower due to the delayed thetaPP during the higher RPMs, therefore so is actual EGT.
2) If one looks at the change in exhaust valve temperature as CHT is altered while holding everything else consent, one quickly sees the most direct correlation of CHT-EVT as compared the other factors.
3) The unusually high EGTs in this study are due to the low compression (6.5:1) of the engine. As expected.
In the test of variable CR the difference between CR of 6.65 and 7.5 (typical GA turbo) was 55°F:
...not huge and the point of the test was to investigate how all the temps respond, not establish absolute figures for any specific CR.
4) Research and analysis of hundreds of events of pre-ignition have debunked the "assumption" during this test that pre-ignition is caused by high exhaust valve temperature. This is simply not supported by reams of data from pre-ignition events where engine monitors have been present to record the events
A proper reading of the report reveals that the valve temperature was actually lower when they experienced preignition, due to the insulating effect of the valve deposits. It is those deposits that is the problem they identified rather than the valve temperature per se; "The foregoing results indicate that the scale and deposits probably reached a temperature sufficiently high to cause preignition whereas the temperature of the valve steel as measured by thermocouple remained substantially below this temperature". All backed up by the engineering literature:
"The source of preignition is usually the exhaust valve covered with mineral deposits coming from fuel and lubricant additives" Guibet JC, Duval A, New Aspects of Preignition in European Automotive Engines, SAE Technical Paper, 1972.
"Preignition is caused by the mixture igniting as a result of contact with a hot surface such as an exhaust valve." Stone R, Introduction to Internal Combustion Engines, chap 3, p74, 1992.
"The parts which can cause preignition are those least well cooled and where deposits build up and provide additional thermal insulation: primary examples are spark plugs, exhaust valves....Under normal conditions using suitable heat range spark plugs, preignition is usually initiated by an exhaust valve covered in deposits". Heywood JB, Internal Combustion Engine Fundamentals, chap 9, p453, 1984.
...but perhaps no longer the same problem as with the fuels of 1947.
5) Some of the "assumptions" from the 1930s and 40s are taking a long time to dispel (recently a CMI representative giving a talk to 300 LAMEs in the US assigned effects of pre-ignition to having been, incorrectly, detonation--and that's not an uncommon error)
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Damn! I'd like to thank everyone for providing us all with excellent reading, trolls included.
I have a question relating to the fuel enrichment on the J2B, in the piper manual it calls for a maximum normal operating power (can't remember exact acronym) of something like 40" MAP 2400RPM (for climbs etc). This is still quite a lot of power % and would require very rich mixture, however by pulling the throttles back to 40' do we loose the fuel enrichment function? Would it be better to leave everything on the stops?
I'm aware of how the Continental enrichment set up works but didn't know the fuel metering in the J2B had a similar function (I take it most Lycomings do not)
I have a question relating to the fuel enrichment on the J2B, in the piper manual it calls for a maximum normal operating power (can't remember exact acronym) of something like 40" MAP 2400RPM (for climbs etc). This is still quite a lot of power % and would require very rich mixture, however by pulling the throttles back to 40' do we loose the fuel enrichment function? Would it be better to leave everything on the stops?
I'm aware of how the Continental enrichment set up works but didn't know the fuel metering in the J2B had a similar function (I take it most Lycomings do not)
The J2B (or is it a J2BD? I forget) in the 1980 PA31-325Navajo** I operate specifies 43" nominal MAP/2575/full rich for max power, then set 38"/2400/ US 28-32GPH for climb power. I never use less than US 32GPH for climb, erring on richer than that to ensure never leaner. Even so, I'm now tending towards full rich even for climb power.
For the last 7 or 8 years I've run it LOP for cruise. Nearly always 33"/2200 RPM /27 US GPH which gives a TAS of 170kts +- 2 kts at A120.
**It's a Mike Jones 'Lock & Key' Colemill Panther Navajo, with 350HP engines. Nicest Navajo I've ever flown.
For the last 7 or 8 years I've run it LOP for cruise. Nearly always 33"/2200 RPM /27 US GPH which gives a TAS of 170kts +- 2 kts at A120.
**It's a Mike Jones 'Lock & Key' Colemill Panther Navajo, with 350HP engines. Nicest Navajo I've ever flown.
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Tins,
You are correct, full rich is the only place to be in the climb. I have just run the dyno data and you should have about 34GPH at your 38/2400 setting. Leave it full rich and that is where it should stay for takeoff, climb and any time you take the whip out.
Some tips for the cruise, try 32-33" /2400 and no more than 17GPH. At 30" about 15.5GPH. This will be around 80dF LOP.
Some folk stress out too much about TIT, so if you would rather set up 30"/2400 and then 80dF LOP then add back 2-3" then do it that way. The FCU will look after the rest.
If you want to see an hour or so of various stuff on the dyno, and Georges famous words with a nasty climb power setting where he says frankly this is not good for the engine, shortly followed by but it is consistent with the POH recommendations all the while you are watching the pressure traces just drop me a line.
The banter machine is not a TKR machine?
You are correct, full rich is the only place to be in the climb. I have just run the dyno data and you should have about 34GPH at your 38/2400 setting. Leave it full rich and that is where it should stay for takeoff, climb and any time you take the whip out.
Some tips for the cruise, try 32-33" /2400 and no more than 17GPH. At 30" about 15.5GPH. This will be around 80dF LOP.
Some folk stress out too much about TIT, so if you would rather set up 30"/2400 and then 80dF LOP then add back 2-3" then do it that way. The FCU will look after the rest.
If you want to see an hour or so of various stuff on the dyno, and Georges famous words with a nasty climb power setting where he says frankly this is not good for the engine, shortly followed by but it is consistent with the POH recommendations
all the while you are watching the pressure traces just drop me a line.The banter machine is not a TKR machine?
Hiya Jabba. I should clarify that FF is 27 GPH total ie 13.5 GPH /side, and done without GAMIs. Can't get it leaner without roughness. Owner isn't inclined to spend more just now (see below re autopilot).
I'd love to see dyno data on this engine. This plane is an early-ish Mike Jones 'Lock & Key', so apart from the Colemill mods, it had an expensive restoration from Mike Jones. It was done before glass panels became common, so still steam driven gauges (except it has a nice King EHSI that I rather like). It has 530 & 430 WAAS GPS, Avidyne MFD, XM weather, radar & stormscope so nav. is a breeze. Digital FF + an EDM760 so fuel & engine management is catered for too.
Two biggest hassles are the autopilot & Cyl 3 & 4 run hotter than the others.
Autopilot is a KFC200. Usually a good unit, but 1980 and later PA31s were required to include an accelerometer in the installation. Can't find a replacement for a reasonable price. Honeywell will build one as a special order - for only US$20,000! Used serviceable ones are rare as hen's teeth. We have our eye on one for US$7000...
I'd love to see dyno data on this engine. This plane is an early-ish Mike Jones 'Lock & Key', so apart from the Colemill mods, it had an expensive restoration from Mike Jones. It was done before glass panels became common, so still steam driven gauges (except it has a nice King EHSI that I rather like). It has 530 & 430 WAAS GPS, Avidyne MFD, XM weather, radar & stormscope so nav. is a breeze. Digital FF + an EDM760 so fuel & engine management is catered for too.
Two biggest hassles are the autopilot & Cyl 3 & 4 run hotter than the others.
Autopilot is a KFC200. Usually a good unit, but 1980 and later PA31s were required to include an accelerometer in the installation. Can't find a replacement for a reasonable price. Honeywell will build one as a special order - for only US$20,000! Used serviceable ones are rare as hen's teeth. We have our eye on one for US$7000...
When I took over managing it, I recommended to the owner that he have an EDM fitted so that LOP could be done (the POH has it using the factory single point EGT but apart from a one experiment to see how it would run on an empty leg, and a single demo to the owner to show the fuel savings, I've never run LOP without the EDM). At the time I said GAMIs might be needed post-EDM but we should have a trial period to see. That was 7 or 8 years ago
Probably a bit late to start recommending GAMIs now. Anyway the engine is only a couple of hundred hours to TBO. Maybe post overhaul I'll revisit the topic.
Probably a bit late to start recommending GAMIs now. Anyway the engine is only a couple of hundred hours to TBO. Maybe post overhaul I'll revisit the topic.