Getting to Lean of Peak

[Devin]

I have a 67 Mooney F and have a question regarding Lean of Peak.  My mechanic at the time who flies a Mooney said just keep your CHT's under 380 and your EGT's under 1500 as a good rule of thumb.  When I'm climbing I shoot for around 115mph speed.  My #2 is always the hottest so I put more focus on that one and Lean while climbing in accordance with the mechanics advice 380/1500.  Once I'm in cruise going cross country I would like to get LOP.  I've watched a few videos on it and from what I am seeing basically you keep leaning till you get a certain temp and then the temp should start decreasing and now your on the Lean of peak side.  That's where it's a little confusing because when I Lean the temps do start rising and when I get above 380 or 1500 I will enrichen the mixture to cool engine back down under 380/1500.  It seems odd if I keep leaning the mixture the engine temps will start cooling on its own.  In my mind if I keep leaning the engine will just keep getting hotter.  I asked my mechanic and he said again just keep it 380/1500.  Any advice or explanation is greatly appreciated.

[201er]

10 minutes ago, Devin said:

That's where it's a little confusing because when I Lean the temps do start rising and when I get above 380 or 1500 I will enrichen the mixture to cool engine back down under 380/1500.  It seems odd if I keep leaning the mixture the engine temps will start cooling on its own.  In my mind if I keep leaning the engine will just keep getting hotter.  I asked my mechanic and he said again just keep it 380/1500.  Any advice or explanation is greatly appreciated.

Ever wonder why it’s called peak? EGT goes up until you reach the peak and then it starts going down. You have to be less lean of peak than you have to be rich of peak to achieve the same CHT.  

 

[Guest]

You won’t get to lean of peak that way. Your method will keep you rich of peak and is great for climb. Once you get to cruise you need a quick big pull of the mixture to get onto the back side of the curve before your chts get to hot. You can use a fuel flow or even engine roughness for an initial target. Just make sure you are less than 75% power when you play with lop and you won’t hurt anything. 

[N201MKTurbo]

I just reduce MP to 25, pull the mixture till I feel the power drop off, then increase MP to 28 and then increase the mixture till I hit my target TIT, which is 1550. This gives a FF of about 9.3 GPH and CHTs of about 380. 

[Fly Boomer]

53 minutes ago, Devin said:

Any advice or explanation is greatly appreciated.

Good advice above.  I would add that, as you have discovered, it's a bit confusing at first.  The purpose of the BMP (big mixture pull) is to get on the lean side of peak quickly (maybe a second or two).  Unless you are down around 60% power, you don't want to hang out near peak.  The trouble with that explanation is, until you have done it, you don't have any idea how far to pull the mixture.  The answer is pull it as far as you can without killing the engine, allow things to settle down, and then enrichen the mixture.  If all cylinders were on the lean side of peak, when you enrichen, all your EGTs should rise.  It's useful to know what peak is for your engine, and the only way to know is to spend a little time around peak EGT at a very low power setting.  Once you find peak, you know you need to have all cylinders on the lean side of that temperature, AND you know about where the mixture control needs to be to get there.  Once you have all cylinders on the lean side of peak (they all peak at different mixtures) then you can begin to increase manifold pressure as you monitor your CHTs.  The number I often read for Lycoming, is 400 as a max.

[redbaron1982]

1 hour ago, Devin said:

I have a 67 Mooney F and have a question regarding Lean of Peak.  My mechanic at the time who flies a Mooney said just keep your CHT's under 380 and your EGT's under 1500 as a good rule of thumb.  When I'm climbing I shoot for around 115mph speed.  My #2 is always the hottest so I put more focus on that one and Lean while climbing in accordance with the mechanics advice 380/1500.  Once I'm in cruise going cross country I would like to get LOP.  I've watched a few videos on it and from what I am seeing basically you keep leaning till you get a certain temp and then the temp should start decreasing and now your on the Lean of peak side.  That's where it's a little confusing because when I Lean the temps do start rising and when I get above 380 or 1500 I will enrichen the mixture to cool engine back down under 380/1500.  It seems odd if I keep leaning the mixture the engine temps will start cooling on its own.  In my mind if I keep leaning the engine will just keep getting hotter.  I asked my mechanic and he said again just keep it 380/1500.  Any advice or explanation is greatly appreciated.

The "why" the temps will eventually go down when leaning is because at some point you're going to remove enough fuel that the output power from the engine will start to go down, and that will bring down the CHT as well.

Think it this way: if you are ROP you are throwing more fuel than what the engine needs to produce power, the combustion will run out of oxygen before it runs out of fuel. So the extra, unburnt fuel, will cool the engine. As you lean, you are still burning the same amount of fuel in the combustion, but you're reducing how much fuel is left unburnt, so you're not cooling less, then CHT rises.

If you continue leaning, you're going to get to the point where all the fuel gets burnt during the combustion and nothing is left unburnt to cool the engine, but, and here comes the magic of LOP, if you continue leaning, now what you're going to have in excess is air, so all the fuel will burn, but you're not going to use all the air that entered the cylinder, so now your extra cooling is coming from the unused air.

One more comment, be careful with the "big quick mixture pull", it is correct, but the problem is that people understand different things by "big" and "quick", lol. So quick doesn't mean to do it in 1 second, and big doesn´t mean to pull it to almost close. If you're under 75% in a non-turbo like yours, you can take 10s (or more) to bring the mixture to the lean side without adding extra stress to your engine. For sure, always monitor CHTs so they don´t get above 400F during the process.

[Shadrach]

A 1500° EGT limit is a meaningless number  for setting power in your application. Some cylinders may not produce EGT readings that high and if they do, it may or may not be a good place to set mixture depending on which side of peak. If peak is 1500°, 1450° can be either 50° ROP or 50° LOP.

As an aside, the only time raw EGT numbers have much use is at full power, full rich. You want Take off EGTs ~1250° or less. 

I’ve been operating a 67F for over 20 years utilizing about every mixture setting on the spectrum. I’m pretty familiar with how IO360s perform in this application. In terms of CHT, #3 is almost always the hottest due to baffle design. However, this tendency can be masked if a bayonet probe is being used as it will read ~30° cooler than a standard threaded CHT probe. Conversely, a spark plug gasket thermocouple will read around 30° higher than a conventional probe. Either way, I would be surprised if number three was not your hottest cylinder.

To effectively use any mixture setting one must determine which cylinder is leanest and which cylinder is richest. For almost every IO360 A1A I have encountered #2 is richest and #3 is leanest (which contributes to it being the hottest under certain conditions). You still need to test for yourself.
 

Take a good look at the chart that @201er posted to to get a clear picture of how CHTs track EGT on each side of peak.

[Shadrach]

You should read the articles I’ve linked below if you want to maximize your understanding of the engine’s operation. 
Not only will they make you a better operator, they will make you a better troubleshooter as well.  We often speak of staying ahead of the airplane when flying. A pilot with a fundamental understanding of combustion science and an engine monitor is able to stay ahead of the engine and often recognize symptoms of trouble before it manifests catastrophically. Additionally one can learn to recognize the cause of an immediate failure without wasting time on rote, restart, checklist items.

These columns are well organized and easy to understand.  They will provide a solid foundation. 

https://www.avweb.com/features/pelicans-perch-63where-should-i-run-my-engine-part-1/

https://www.avweb.com/features/pelicans-perch-64where-should-i-run-my-enginepart-2-the-climb/

https://www.avweb.com/features/pelicans-perch-65where-should-i-run-my-enginepart-3-cruise/

https://www.avweb.com/features/pelicans-perch-66where-should-i-run-my-enginepart-4-descent/

 

 

[Devin]

Thank you all for the information.  Definitely helps me wrap my head around the concept.  A little more detailed Info.  I take off at 1500' elevation full throttle, full rich and max rpm, two positive rates of climb and gears up another positive rate and flaps up, after 600' AGL fuel pump off and I bring the RPM back to 2600 as I climb.  I climb to 11,500' and during the climb I lean the mixture as I go as but not let the temps get hotter than 380/1500.  Once I'm at 11,500' I close the cowl flaps and open the power boost.  At full throttle I'm about 19 or 20 manifold pressure at 11,500' and the boost opened gives me about 1 more inch.  I reduce the RPM to 2350 and keep leaning mixture till I get as close to 380/1500 without going over.  When it's all said and done I'm 11,500', 20-21" MP and 2350 RPM burning about 8.5 gph.  I thought I was running LOP per my maintenance guy but now I dont think I ever was.  Will I see a significantly better burn rate than 8.5 by trying to get over to the LOP side or is the way I do it now just fine?  I'm all about improving my skill and taking care of my engine but if it's a tricky process of getting over to the lean side of peak without messing my engine up in the process or killing it because I did the bump to hard will I get that much better economy or is 8.5 gph good and the engine just fine where it's at?

[Fly Boomer]

12 minutes ago, Devin said:

Will I see a significantly better burn rate than 8.5 by trying to get over to the LOP side or is the way I do it now just fine?

If your engine is happy will all cylinders on the lean side of peak, it burns cleaner, and uses less fuel.  How much less?  Not sure, but by definition, rich of peak is puking out extra unburned fuel.  Once you get to the lean side, you can start going richer and possibly adding back some manifold pressure to restore some of the power you lost by going well lean of peak, and watch your CHTs go back up.  The 380 you are using is a nice, conservative number.  Your cylinders thank you.

[Shadrach]

I’m still confused by the 1500° number. An EGT of 1500° is not even reliably attainable by a Lycoming IO360. 
 

Another question, why do you wish to run LOP at 11,500? Why not just run peak? At that altitude it gives you all of the benefits of LOP and none of the downside. For all intents and purposes, it’s just as clean and just as efficient without much power loss. 

[PT20J]

A few comments:

1. There is no need to reduce rpm to 2600 during climb. It costs you about 10 hp and lengthens your climb. 

2. The easiest technique for leaning in a climb is to note the EGT at takeoff and then lean during the climb to maintain that EGT.

3. There is no reason to worry about the exact value of EGT. It will never get hot enough in a normally aspirated engine to hurt anything. It's not even a real temperature, but it is instead the average of the pulses of much hotter exhaust gases that pass over the probe every time the exhaust valve opens. What is important is the relative value compared to the peak value.

4. At 65% power and peak EGT, an IO-360 burns about 9 gph. At 65% and best power mixture, it burns about 10 gph. 

5. Lean of peak will produce lower power than you would get with the same MAP and rpm at richer mixtures. If you are operating at 11,500 feet LOP you will certainly burn less gas, but the airplane will be slow because the power will be low.

[Rick Junkin]

@Devin These presentations will give you the knowledge you need to get started.

Cheers,
Junkman

[Ragsf15e]

3 hours ago, Devin said:

Thank you all for the information.  Definitely helps me wrap my head around the concept.  A little more detailed Info.  I take off at 1500' elevation full throttle, full rich and max rpm, two positive rates of climb and gears up another positive rate and flaps up, after 600' AGL fuel pump off and I bring the RPM back to 2600 as I climb.  I climb to 11,500' and during the climb I lean the mixture as I go as but not let the temps get hotter than 380/1500.  Once I'm at 11,500' I close the cowl flaps and open the power boost.  At full throttle I'm about 19 or 20 manifold pressure at 11,500' and the boost opened gives me about 1 more inch.  I reduce the RPM to 2350 and keep leaning mixture till I get as close to 380/1500 without going over.  When it's all said and done I'm 11,500', 20-21" MP and 2350 RPM burning about 8.5 gph.  I thought I was running LOP per my maintenance guy but now I dont think I ever was.  Will I see a significantly better burn rate than 8.5 by trying to get over to the LOP side or is the way I do it now just fine?  I'm all about improving my skill and taking care of my engine but if it's a tricky process of getting over to the lean side of peak without messing my engine up in the process or killing it because I did the bump to hard will I get that much better economy or is 8.5 gph good and the engine just fine where it's at?

Devin, @PT20J gave you the most concise answer to a few of your issues.  Don’t pull back rpm, just lean to takeoff egt in the climb (which should be ~1200-1250 as @Shadrach noted), disregard the 1500 egt as that’s pretty meaningless, the 380 cht is a nice high limit (but is only a technique) but you will definitely get above that if you let the egts creep up above ~1250 takeoff egt during climb, 2500 rpm is a nice cruise rpm unless you really want to go slow trying to burn the least amount of fuel.  Finally, why not open your ram air boost much earlier?  I wait about 1000’ up and make sure I’m in clear air, but no reason to wait until 11,500. It lets the engine breathe better and gives about .75 more mp.

Finally, at 11,500, your engine is making so little power that you really can’t hurt it.  Play with it to validate what I’m about to say… level off, set 2500 rpm, slowly lean just watching your egt.  They will all rise as you lean.  Keep leaning right through whatever numbers you previously looked for.  The egts will all “peak” at some value and start back down.  The chts will lag but also rise until the egts peak and then they will start down.  Keep leaning and the engine will eventually get rough but chts will be very low.  That’s too lean.  At 11500, you should be right around peak and still have good chts.  If you want to cool them more, go leaner and they will cool, richer will increase chts until you get richer than about 50rop then they will decrease again.

I flew my F at 11,500 last weekend for ~475nm at peak egt.  Cht was 360 for the hottest one.  Egt value is meaningless but it was around 1400.  Fuel flow was ~8.6ish.  It’s not super fast like that, but at ~140kts, it’s not bad either.