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  #1  
Old 02-28-2021, 09:44 AM
Lt Dan Lt Dan is offline
 
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Default Question for the Engineers: Why Leaner of Peak at Higher RPM?

In all that I have read on LOP operations, I have never found an answer as to why the onset of roughness seems to occur later (deeper LOP) when you are running at a higher RPM for any given power setting.

For example: 65% power, 2300 RPM: I can only run 30° LOP, but at 2500 RPM I can run 75° LOP.

Logic tells me this should be the opposite, since leaner mixtures burn slower, and you should be able to extract more energy from the burning fuel air mixture in the cylinder if it has a longer time to spend inside the cylinder pushing against the piston. Can anyone speak authoritatively on the subject?
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  #2  
Old 02-28-2021, 10:20 AM
BobTurner BobTurner is offline
 
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Onset of roughness is usually due to the mixture distribution in one cylinder being noticeably different than the others. If you re-do your ‘adjust injector aperture’ for zero difference at 2300 rpm you’ll probably see your results flip. Our injection systems are crude enough that they can only be optimized for one set of conditions.
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Old 02-28-2021, 10:27 AM
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rv6ejguy rv6ejguy is offline
 
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Quote:
Originally Posted by Lt Dan View Post
In all that I have read on LOP operations, I have never found an answer as to why the onset of roughness seems to occur later (deeper LOP) when you are running at a higher RPM for any given power setting.

For example: 65% power, 2300 RPM: I can only run 30° LOP, but at 2500 RPM I can run 75° LOP.

Logic tells me this should be the opposite, since leaner mixtures burn slower, and you should be able to extract more energy from the burning fuel air mixture in the cylinder if it has a longer time to spend inside the cylinder pushing against the piston. Can anyone speak authoritatively on the subject?
There are several possibilities:

Mixture distribution between cylinders may change with rpm due to intake runner length and angle differences, primary exhaust tube length differences and throttle plate position (if not WOT).

Most likely that cylinder to cylinder air/fuel (AFR) ratios are different at higher rpms and at least 1 cylinder is much leaner than the others. This one encounters lean misfire first, giving you roughness.

Higher RPM also results in higher chamber turbulence which may result in less charge stratification (more homogeneous mixture) near the spark plugs.

With EFI where we can easily equalize the AFR between cylinders, we can run 100 LOP easily and even 200 on most engines at any rpm.

What is your GAMI spread at 2300 and 2500?
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Old 02-28-2021, 04:26 PM
BillL BillL is offline
 
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Quote:
Originally Posted by Lt Dan View Post
Logic tells me this should be the opposite, since leaner mixtures burn slower, and you should be able to extract more energy from the burning fuel air mixture in the cylinder if it has a longer time to spend inside the cylinder pushing against the piston. Can anyone speak authoritatively on the subject?
Just some fundamental theory to work into your thinking. If combustion (heat release) takes place faster, then there is more stroke to generate work (expansion ratio).

Now- Ross has way more hands on experience here, so if there is a conflict, I defer.

I find just the opposite of your comments- I can not get much LOP (deg F) at 2700, and at each 100 RPM lower I can lean more. That is with mags, so varying the best timing, that may change. M1B std Cr. I thought it might be mixture variance or time in the intake port for evaporation. Just a data point.
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Old 02-28-2021, 10:27 PM
Lt Dan Lt Dan is offline
 
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Quote:
Originally Posted by rv6ejguy View Post

What is your GAMI spread at 2300 and 2500?
Just got done running some tests. I found it to be 0.0 to 0.1 GPH at both 2500 and 2300. I’ll try to run some more tests at additional power settings and RPMs.
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Old 03-01-2021, 05:38 AM
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rv6ejguy rv6ejguy is offline
 
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Quote:
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Just got done running some tests. I found it to be 0.0 to 0.1 GPH at both 2500 and 2300. I’ll try to run some more tests at additional power settings and RPMs.
It doesn't get much better than that! It would appear than one lean cylinder isn't creating the roughness. That would seem to leave something with ignition causing it. Strange indeed.
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Turbo Subaru EJ22, SDS EFI, Marcotte M-300, IVO, Shorai- RV6A C-GVZX flying from CYBW since 2003- 445.9 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
https://www.youtube.com/channel/UCiy...g2GvQfelECCGoQ



Last edited by rv6ejguy : 03-01-2021 at 05:44 AM.
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  #7  
Old 03-01-2021, 07:26 AM
lr172 lr172 is online now
 
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I suspect something is not right somewhere in your engine, but can't really venture a guess as to what. I run my IO-320 at 2750 in cruise (balanced injectors /1 mag and 1 EI). While I typically cruise around 40 LOP, I can easily get to 100 at that RPM. Many with FP props cruise at around 2700 RPM and Many easily get to 100 LOP with FI.

I would start by confirming your timing and examine your plugs and wires with a VOM.

Larry
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Old 03-01-2021, 08:11 AM
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DanH DanH is offline
 
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BTW, what engine/prop/fuel system/ignition are we talking about here?
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  #9  
Old 03-01-2021, 09:54 AM
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vlittle vlittle is offline
 
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Did I miss something here? Changing RPM will also change ignition timing advance in most EI systems. So you are changing two variables at once.

Combustion speed changes with mixture. For a given mixture, we can make the assumption that this is constant with RPM (not totally true, but bear with me).

Piston speed changes with RPM.

Therefore, for a given mixture, the peak cylinder pressure occurs at different piston positions at different RPMs.

Therefore, if you change RPM, you should re-lean the mixture. The measured result will change based on RPM, but the actual result, inside the piston, will still be optimal.

The problem is not with the numbers, but just a limitation of our instrumentation. At a given altitude, set your MAP and RPM, then lean your mixture to your desired point. If you change altitude, MAP or RPM, re-lean.

Ross probably has all of this magically incorporated into his software, but for the rest of us, we just have to use the stone-age method.

V
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  #10  
Old 03-01-2021, 10:33 AM
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Vern brings up a good point. If you have EI, rpm and MAP advance could be different and this would skew the EGT numbers with advance generally cooling EGTs.

The observation would depend on how all this is done. Both at WOT? Both starting from ROP, reaching peak, then LOP, and continue leaning until roughness?

Do you get any MAP change when pulling the prop back from 2500 to 2300?

When EI is adjusting advance while you lean, the true amount LOP may not be what you observe, which is a limitation of using EGTs to determine the amount LOP.
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Turbo Subaru EJ22, SDS EFI, Marcotte M-300, IVO, Shorai- RV6A C-GVZX flying from CYBW since 2003- 445.9 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
https://www.youtube.com/channel/UCiy...g2GvQfelECCGoQ


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