How Far LOP is Too Far LOP?

[RoundTwo]

Yesterday I rescued the plane from its AOG state after the left mag, presumably the condenser, went dead during a 30 minute flight. Thank goodness we were within Uber range of home. 
Yesterday was also the first time I tried to run the engine LOP. Even without GAMI injectors, I was very surprised how far LOP I could get it to run. Since it was only a very short flight, I had only climbed to 4,500’, but the engine ran fine, down into the 7’s before any roughness was detected. What got my attention though, were the CHT’s. I was bouncing right around about 300°. I started thinking about lead scavenging issues, so I richened things up to get some more heat in the heads and continued on towards home.

This got me thinking: How far LOP is too far LOP and how cold is too cold for CHT temps?

[RoundTwo]

6 minutes ago, Evan said:

Most tend to land around 20-30 LOP. That is where you can maximize BSFC. However, down low you might want to be further lean to avoid the “red box.” Up high, closer to peak EGT. 

I was showing 60% power, so as far as I am aware, the red box had all but disappeared. Looks like a great setting for loitering and sight seeing as long as it’s warm enough to enable lead scavenging.

[mike_elliott]

From what I have been told, When LOP, HP is best determined by FFx14.9 for higher compression non turbo engines and 13.5*FF for Turbo engines. Keep CHTs at or above 280 for lead scavenging. 

[A64Pilot]

39 minutes ago, RoundTwo said:

I was showing 60% power, so as far as I am aware, the red box had all but disappeared. Looks like a great setting for loitering and sight seeing as long as it’s warm enough to enable lead scavenging.

You were fine there. I’m usually down low due to short flights and I run usually about 50C LOP but will enrichen in cooler Wx to keep cyl head temps up.

I don’t have an analyzer just the factory instruments, so I set at power setting low enough and lean enough so that I’m sure I can’t hurt anything. Not being able to watch each cyl independently I just run conservatively. Normal cruise for me is 8GPH, which calculates out to 60% power.

If you go high enough it’s likely you may decide to run richer as you just lose so much power LOP, but at that point power is down so much you can’t hurt anything with any mixture.

People have run LOP for decades to save fuel before engine analyzers, so you can do so safely so long as your conservative.

Most efficient is richer than that, but in truth at that point we are splitting hairs.

On edit these IO-360’s are the poster children for LOP, I’ve never seen any other engine run so well LOP, my IO-540 and IO-520 sure wouldn’t.

[Shadrach]

On 4/29/2023 at 8:12 AM, RoundTwo said:

Yesterday I rescued the plane from its AOG state after the left mag, presumably the condenser, went dead during a 30 minute flight. Thank goodness we were within Uber range of home. 
Yesterday was also the first time I tried to run the engine LOP. Even without GAMI injectors, I was very surprised how far LOP I could get it to run. Since it was only a very short flight, I had only climbed to 4,500’, but the engine ran fine, down into the 7’s before any roughness was detected. What got my attention though, were the CHT’s. I was bouncing right around about 300°. I started thinking about lead scavenging issues, so I richened things up to get some more heat in the heads and continued on towards home.

This got me thinking: How far LOP is too far LOP and how cold is too cold for CHT temps?

The lowest and flattest part of the BSFC curve (read most efficient) starts right around peak EGT and continues to about 25° before efficiency starts to decrease again (Lycoming graph below). I have been using the lean side of the mixture spectrum for nearly 20year. It’s come to be  my opinion that most Lycoming IO360s will run quite a bit leaner than needed. I normally try to run what I will call “Best LOP power”. That means that I run my RICHEST cylinder as close to peak EGT as prudent with consideration to CHTs and OAT/DA. The other three cylinders will be somewhat further LOP. Consideration to OAT/DA means a CHT that’s acceptable on a 90° summer day might not be acceptable on a 15° winter day.  I run my engine leanest (maybe 40°) at high power settings when at lower altitudes on cold, low DA days. In winter, DAs can be around SL at 3500msl where I live. As my operating DA goes up, the mixture gets richer. At DAs of ~6k and above, peak is usually sufficient to keep CHT’s in the desired range.  I try to keep CHTS between 300-350° In level flight, 300° is challenging in the winter time for #1 and #4. Under 70% power, there almost no operational benefit to running leaner than peak unless the engine is running warmer than desired. Ignition timing does have some effect on LOP ops. IO360s can be timed at 20° or 25°. This makes for a small but noticeable difference in how the engine operates throughout the mixture spectrum.

So after all of the above, the answer is, it depends. However, I think many run leaner than necessary out of fear or misunderstanding.

 

[Shadrach]

46 minutes ago, mike_elliott said:

From what I have been told, When LOP, HP is best determined by FFx14.9 for higher compression non turbo engines and 13.5*FF for Turbo engines. Keep CHTs at or above 280 for lead scavenging. 

14.9 for 8.5 C/R. 

15.1 for 8.7 C/R (Lyc IO360)

the difference is splitting hairs but the higher compression engine is a touch more thermally efficient.

[jlunseth]

Turbo is 13.7 for the Mooney engines as far as I know. As I understand it, there is a different compression ratio for some turbo engines that would result in a different fuel flow constant, but I am pretty sure the constant is 13.7 for the various Mooney turbos. Talking true turbos now, not NA's with an aftermarket charger.

Let me put it this way. The constant for the TSIO360's is 13.7.

I have quite a bit of experience running with cold CHT's. Around here in the winter my #6 will be at or below 240 dF, which is the bottom end of the normal operating range in the POH. Not much I can do to get the temp higher in the low altitudes (i.e. single digit MSL) when the temps are super cold. I do try running ROP or even right at peak sometimes, but there will always be a couple of cold cylinders. I have had little to no lead scavenging. "Little to no" means that about once a year a mag will fail the run up test and I need to run it very lean to clean it, then test it again and it works fine. Other than that, the engine does not seem to care about lower temps.

If I want to run at a lower power while LOP though, I reduce the power setting. That is, my normal cruise setting is 34" and 11.1 GPH at 2450. Without touching the fuel knob I will lower the MP to 24 point something. The fuel flow will automatically come down to around 8.5-8.8 in my engine. That gives me about 120 kts. clean. I suppose I could try just reducing the fuel flow and with the reduced power the MP would come down also, but reducing MP seems to be the better way to do it.

[Ibra]

Usually 20F-30F LOP otherwise power and speed decrease: while FF goes down when one go 75LOP, I don't think the IAS stays where it is such that MPG gets any better than 25LOP

Where I am not sure is CHT, my understanding one need to keep their cylindres warm even if going "deep LOP", I don't fly 45% at 30F LOP as CHT is way too low under 300F...I recall a rule of thumb stating that CHT should be higher than 300+OAT/2 (in F), say 60F OAT outside one would like to see CHT > 330F (and of course less than 380F) otherwise it is not healthy for pistons & rings 

Edited April 29, 2023 by Ibra

[ArtVandelay]

It’s the lead scavenging that requires higher temperatures, EGTs above 1100°F and cylinder temperatures above 300°.

[Shadrach]

33 minutes ago, ArtVandelay said:

It’s the lead scavenging that requires higher temperatures, EGTs above 1100°F and cylinder temperatures above 300°.

I run CHTs under 300 all winter, no issues with lead scavenging nor deposits. Comps last annual were 80/80, 80/80, 80/80, 79/80.

[ArtVandelay]

I run CHTs under 300 all winter, no issues with lead scavenging nor deposits. Comps last annual were 80/80, 80/80, 80/80, 79/80.

It’s not a piston issue, it’s lead deposits on the valves (stuck valves) that would result if cylinders are run too cool. I assume your EGTs are well above 1100, preferably 1300s.

[Shadrach]

5 hours ago, ArtVandelay said:

It’s not a piston issue, it’s lead deposits on the valves (stuck valves) that would result if cylinders are run too cool. I assume your EGTs are well above 1100, preferably 1300s.

Yes my point was that CHTs are not necessarily a good indicator. As for EGTs, my full rich takeoff EGTs are high 1100s to low 1200s.  Cruise in the mid 1400 to high 1300s.  That sometime yields CHTs in the 270 range on #1 and #4. Borescope shows clean and happy cylinders with minimal deposits. No morning sickness issues in 1000s of hours of operation.

[Pinecone]

Everyone has gone into the weeds.

How far is too far LOP?

Easy, either it runs so rough you can read the instruments from the vibration 

OR

The engine stops running.

 

[kortopates]

Easy peasy, it’s to far LOP when power drops off such that you can’t sustain level flight!
Or maybe you just don’t want to go that slow.
Now if your a turbo pilot you have much more air to work with and can run deeper at higher power settings unlike the NA engines.


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[PT20J]

Lean it until it runs rough and then richen until it’s just smooth. That will put you at the lowest spot on the BSFC curve. If the airspeed is too slow, richen. If you are below 75% (Lycoming) or 65% (Continental) you are outside the red box at any power setting. If you are below 400 F you are outside the red box. Keep it simple.

 

 

[Shadrach]

8 minutes ago, PT20J said:

Lean it until it runs rough and then richen until it’s just smooth. That will put you at the lowest spot on the BSFC curve. If the airspeed is too slow, richen. If you are below 75% (Lycoming) or 65% (Continental) you are outside the red box at any power setting. If you are below 400 F you are outside the red box. Keep it simple.

 

 

Roughness is entirely dependent on available MP. There are many scenarios where enrichening to smooth would leave my engine well lean of best BSFC.

[PT20J]

9 minutes ago, Shadrach said:

Roughness is entirely dependent on available MP. There are many scenarios where enrichening to smooth would leave my engine well lean of best BSFC.

It is interesting to ponder why the BSFC curve has a U shape. Starting at stoichiometric mixture (peak EGT) as we lean the mixture the BSFC decreases approximately linearly. Why? It is because leaner mixtures burn at lower temperatures causing less chemical dissociation resulting in a greater percentage of the energy from combustion being converted to work. However, because  there is less fuel available, the power decreases even though the efficiency increases. Theoretically, the BSFC would continue to decrease until the mixture was so weak that the engine stopped running. But, in a real engine, it doesn't; it levels off and then starts to rise. The reason is that very lean mixtures burn slower and also increase cycle to cycle cylinder pressure variations. Both effects decrease efficiency. The cycle-to-cycle variations between cylinders, and within individual cylinders, are what the pilot senses as "roughness."

From, Heywood, John B., Internal Combustion Engine Fundamentals: