ROP

[Fly Boomer]

1 minute ago, Little Dipper said:

What ever happened to the old days when we leaned the mixture till the engine started to run rough then gave the mixture control a quarter turn in then forget about it?

No problem as long as your temperatures are okay.

[A64Pilot]

9 minutes ago, Little Dipper said:

 What ever happened to the old days when we leaned the mixture till the engine started to run rough then gave the mixture control a quarter turn in then forget about it?   

  

Nothing, but it does assume you have a normal carbureted motor, it actually puts you about the best mix for the average carb motor.

My FI engine runs so well LOP that should I do that I’m be doing good to make 120 KIAS I’d be so lean. But as I’d be so lean it’s certainly safe. Altitude changes things, up high she isn’t as tolerant of ultra lean mixtures.

22 squared for me down low peak is 9 ish GPH, maybe 8.5 or so, but my engine will run smooth at 6 GPH.

Going to lunch yesterday I ran 22 squared at 6.5 GPH because destination was 15 min or so away, why hurry? 

[Shadrach]

32 minutes ago, Little Dipper said:

Gez.....  A weeks worth of reading on a single thread.    Sounds like every one is playing with their JPIs and worrying about their engine instead of looking out the window.   What ever happened to the old days when we leaned the mixture till the engine started to run rough then gave the mixture control a quarter turn in then forget about it?   

  

What happened? A bunch of things…most important is probably the invention of Fuel injection which made these things capable of running smoothly quite a bit leaner than optimal.

i’m so obsessed with my engine monitor  that I’m thinking of having a heads up display installed. The 6 to 10 seconds that I spend setting and verifying my power setting just isn’t enough for me to get my fix…

[GaryP1007]

I certainly got my monies worth from asking this question.  Thanks all.

[jaylw314]

On 5/28/2022 at 5:52 PM, A64Pilot said:

I can easily get deep enough LOP so that the EGT actually starts increasing again and this confuses me somewhat. If the flame front propagation is so slow that the burn isn’t complete before exhaust stroke begins, you shouldn’t be able to ignite a charge that lean, not without a stratified charge combustion chamber which we certainly are not.

My understanding is that happens due to intermittent failure of the weaker spark plug to ignite the lean mixture.  Because it's only intermittent, it's not as big as the rise on the EGT when you turn off a mag, but it increases as the misfire becomes more frequent.  It also means it's not really a substantial loss of power or anything.

I had the dangdest time trying to get LOP due to this phenomena when I had a weak left mag, because it would happen just at peak.  After getting it re-timed, it's no longer a factor, it only happens at fuel flows far below peak that I never use

Edited June 3, 2022 by jaylw314

[Shadrach]

1 hour ago, jaylw314 said:

My understanding is that happens due to intermittent failure of the weaker spark plug to ignite the lean mixture.  Because it's only intermittent, it's not as big as the rise on the EGT when you turn off a mag, but it increases as the misfire becomes more frequent.  It also means it's not really a substantial loss of power or anything.

I had the dangdest time trying to get LOP due to this phenomena when I had a weak left mag, because it would happen just at peak.  After getting it re-timed, it's no longer a factor, it only happens at fuel flows far below peak that I never use

It’s not complicated, it’s just a flame from propagating so slowly that it is still occurring during the exhaust stroke. It does represent substantial  power loss. and occurs well above the flat part of the BSFC curve. 

[PT20J]

28 minutes ago, Shadrach said:

It’s not complicated, it’s just a flame from propagating so slowly that it is still occurring during the exhaust stroke. It does represent substantial  power loss. and occurs well above the flat part of the BSFC curve. 

Radials have about as perfect mixture distribution as you can achieve in a recip. The intake pipes are all the same length and fuel first goes through the supercharger impeller so it’s well mixed before it gets to the cylinders.

I once experimented with a R-985 and could get it so lean that the mixture was still burning at the end of the exhaust stroke when the intake valve opened causing backfires. The engine wasn’t producing a lot of power but was running smoothly just before it got lean enough to backfire. 

Walter Atkinson used to own a twin Beech with R-985s and told me that if you lean very slowly you could feel a very slight roughness and then enriching slightly would be about 50F LOP. But I could never find any roughness. 

Skip

[Shadrach]

18 minutes ago, PT20J said:

Radials have about as perfect mixture distribution as you can achieve in a recip. The intake pipes are all the same length and fuel first goes through the supercharger impeller so it’s well mixed before it gets to the cylinders.

I once experimented with a R-985 and could get it so lean that the mixture was still burning at the end of the exhaust stroke when the intake valve opened causing backfires. The engine wasn’t producing a lot of power but was running smoothly just before it got lean enough to backfire. 

Walter Atkinson used to own a twin Beech with R-985s and told me that if you lean very slowly you could feel a very slight roughness and then enriching slightly would be about 50F LOP. But I could never find any roughness. 

Skip

Yes, Walt was the reason I tried it in my plane. I can get a second EGT rise at high power levels but there is a perceptible  roughness and it happens closer to 100ROP IIRC.  It was an academic exercise as I rarely need much more than 25-30LOP to keep CHTs in check even at high power.

[A64Pilot]

13 hours ago, jaylw314 said:

My understanding is that happens due to intermittent failure of the weaker spark plug to ignite the lean mixture.  Because it's only intermittent, it's not as big as the rise on the EGT when you turn off a mag, but it increases as the misfire becomes more frequent.  It also means it's not really a substantial loss of power or anything.

I had the dangdest time trying to get LOP due to this phenomena when I had a weak left mag, because it would happen just at peak.  After getting it re-timed, it's no longer a factor, it only happens at fuel flows far below peak that I never use

That makes more sense to me, never though you get similar results on one mag, regardless of mixture.

[PT20J]

15 hours ago, jaylw314 said:

My understanding is that happens due to intermittent failure of the weaker spark plug to ignite the lean mixture.  Because it's only intermittent, it's not as big as the rise on the EGT when you turn off a mag, but it increases as the misfire becomes more frequent.  It also means it's not really a substantial loss of power or anything.

I had the dangdest time trying to get LOP due to this phenomena when I had a weak left mag, because it would happen just at peak.  After getting it re-timed, it's no longer a factor, it only happens at fuel flows far below peak that I never use

If that's the cause, the engine will be running very rough at that point since it is essentially missing (even if only one spark plug is misfiring it will be very rough -- ask anyone who had the 28v SureFly voltage spike issue). Normally, the roughness at LOP mixtures is due to increased cycle-to-cycle combustion differences in each cylinder which causes power variation and roughness. But the engine isn't truly missing. At least according to Braly and Heywood,

[jaylw314]

Just now, PT20J said:

If that's the cause, the engine will be running very rough at that point since it is essentially missing (even if only one spark plug is misfiring it will be very rough -- ask anyone who had the 28v SureFly voltage spike issue). Normally, the roughness at LOP mixtures is due to increased cycle-to-cycle combustion differences in each cylinder which causes power variation and roughness. But the engine isn't truly missing. At least according to Braly and Heywood,

Why would it be running very rough?  It would not be any more rough than doing a mag check, which turns off one spark plug for every cylinder continuously.  This is only one spark plug on one cylinder intermittently.  

 

13 hours ago, Shadrach said:

It’s not complicated, it’s just a flame from propagating so slowly that it is still occurring during the exhaust stroke. It does represent substantial  power loss. and occurs well above the flat part of the BSFC curve. 

If that were simply the case, it should be happening to all cylinders simultaneously at VERY low fuel mixtures, which didn't seem to be the case for me at the time

[PT20J]

1 hour ago, jaylw314 said:

Why would it be running very rough?  It would not be any more rough than doing a mag check, which turns off one spark plug for every cylinder continuously.  This is only one spark plug on one cylinder intermittently.  

Exactly. It is smooth during a mag check because the power output if all cylinders is reduced. If one plug in one cylinder is failing to ignite the mixture, then that will cause a power imbalance between cylinders which will increase roughness.

[Shadrach]

1 hour ago, jaylw314 said:

Why would it be running very rough?  It would not be any more rough than doing a mag check, which turns off one spark plug for every cylinder continuously.  This is only one spark plug on one cylinder intermittently.  

 

If that were simply the case, it should be happening to all cylinders simultaneously at VERY low fuel mixtures, which didn't seem to be the case for me at the time

Are we talking about the same thing? A second EGT rise refers well after you’ve leaned past peak when the combustion event gets so slow that it can’t be completed during normal power stroke and continues through the exhaust stroke and is still burning when it enters the exhaust stack. Mixtures that lean are difficult to ignite and The flame front propagates very slowly (relatively speaking). Under such a scenario, the normal cycle to cycle power variations become very apparent and manifest themselves as roughness because the mixture is at the very edge of combustibility. Can we agree that there’s a mixture in which your ignition system is no longer capable of igniting the intake charge? Your engine may be so well balanced that it runs smoothly on the ragged edge of combustibility but that is not the norm. Furthermore the Fuel air mixture’s in question are so far lean has to be beyond the flat part of BSFC curve on the lien side peak. Which means less horsepower per unit of fuel. Which makes perfect sense given how much of the intake charge is being combusted after the power stroke. That portion of the combustion event is being converted to heat rather than mechanical energy and therefore represents less horsepower. 
 

I understand why you are drawing similarities between very lean mixtures and single point of  ignition combustion events, as both delay the amount of time it takes for the intake charge to fully burn. However, they’re not delayed for the same reason.  One is slower because it’s at the ragged edge of even being ignitable, The other is due simply to only having a single point of propagation.

[jaylw314]

4 hours ago, PT20J said:

Exactly. It is smooth during a mag check because the power output if all cylinders is reduced. If one plug in one cylinder is failing to ignite the mixture, then that will cause a power imbalance between cylinders which will increase roughness.

 

4 hours ago, Shadrach said:

Are we talking about the same thing? A second EGT rise refers well after you’ve leaned past peak when the combustion event gets so slow that it can’t be completed during normal power stroke and continues through the exhaust stroke and is still burning when it enters the exhaust stack. Mixtures that lean are difficult to ignite and The flame front propagates very slowly (relatively speaking). Under such a scenario, the normal cycle to cycle power variations become very apparent and manifest themselves as roughness because the mixture is at the very edge of combustibility. Can we agree that there’s a mixture in which your ignition system is no longer capable of igniting the intake charge? Your engine may be so well balanced that it runs smoothly on the ragged edge of combustibility but that is not the norm. Furthermore the Fuel air mixture’s in question are so far lean has to be beyond the flat part of BSFC curve on the lien side peak. Which means less horsepower per unit of fuel. Which makes perfect sense given how much of the intake charge is being combusted after the power stroke. That portion of the combustion event is being converted to heat rather than mechanical energy and therefore represents less horsepower. 
 

I understand why you are drawing similarities between very lean mixtures and single point of  ignition combustion events, as both delay the amount of time it takes for the intake charge to fully burn. However, they’re not delayed for the same reason.  One is slower because it’s at the ragged edge of even being ignitable, The other is due simply to only having a single point of propagation.

OK, then we may be talking about different things.  The second peak I was seeing occurred just LOP, maybe 10-20F.  As I leaned, the EGT in all cylinders went up to peak, started coming down again, then started creeping up (not at quite the same time) by about 25F before the engine got noticeably rough.  I didn't have any gross power loss, but the eventual roughness limited me to about 10F LOP

Of course, it's kind of hard to do any testing since now it doesn't happen until about 50-75 LOP, and at that point the EGT rise and roughness kick in pretty quickly 

Edited June 3, 2022 by jaylw314