CHT QUESTION / IO360

[BRBENNETT]

I own an 1967 M20F / Lycoming IO360-A1A.  What do you guys see as your normal operating CHT's?  Depending on ambient temps, it varies a bit.  I have a flight logged at an OAT of 28.4F and CHT's range from 265F to 280F depending on their location under the cowling.  This summer, with an OAT 52F and CHT's 280F to 330F, again depending on location.  Cowl flaps in the closed position and in the cruise phase of flight.  Data was acquired from JPI930.  engine tach is about 300 SMOH

Any feedback is appreciated.  I am mostly concerned about minimum temps.  Lycoming publishes a max of 400 deg F, but, nowhere do they publish a minimum.  A few puffs of white smoke have recently observed and the concern is cylinder wall glazing.  I have noted no change in oil consumption & having kept a detailed log since I had the engine rebuilt.

I am going to have a borescope done very soon to have a look inside.  Could be a valve seat.

Anyway, any thoughts, personal experience, & data appreciated  from you folks!

 

 

[Guitarmaster]

When I run ROP, my 2 and 3 run about 340 and 1 and 4 run about 320.  When I run LOP, 2 and 3 run about 310 and 1 and 4 run about 280.

[Guitarmaster]

Is it possible to run too cold??  In a diesel I would say yes, but I don't think so in a gas engine.

[carusoam]

There is a thread around here about the issues of running too cold.  One was the temperature of the lead scavenger (TEL?) operation, and the other was concern for piston and cylinder interference.

I don't remember a cruise CHT Going much bellow 280°F.

Hope that helps,

-a-

[Andy95W]

4 minutes ago, Guitarmaster said:

Is it possible to run too cold??  In a diesel I would say yes, but I don't think so in a gas engine.

Trying to remember my Thermodynamic Engineering classes from 30 years ago...

Since our engines are basically heat engines, using the formulas that show the direct relationship between heat and pressure, there is an optimal temperature for our engines, below which we begin losing efficiency.  But we know as airplane owners that as you approach 400° F, the strength of the aluminum alloys in our cylinders decrease rapidly.  So we're looking for the best trade-off between longevity of our engines and efficiency of work production.

My gut feeling is that the efficiency loss between 300° CHTs and 400° CHTs is probably minuscule. 150°-200° F might be a different story, however.

[Marauder]

I own an 1967 M20F / Lycoming IO360-A1A.  What do you guys see as your normal operating CHT's?  Depending on ambient temps, it varies a bit.  I have a flight logged at an OAT of 28.4F and CHT's range from 265F to 280F depending on their location under the cowling.  This summer, with an OAT 52F and CHT's 280F to 330F, again depending on location.  Cowl flaps in the closed position and in the cruise phase of flight.  Data was acquired from JPI930.  engine tach is about 300 SMOH
Any feedback is appreciated.  I am mostly concerned about minimum temps.  Lycoming publishes a max of 400 deg F, but, nowhere do they publish a minimum.  A few puffs of white smoke have recently observed and the concern is cylinder wall glazing.  I have noted no change in oil consumption & having kept a detailed log since I had the engine rebuilt.
I am going to have a borescope done very soon to have a look inside.  Could be a valve seat.
Anyway, any thoughts, personal experience, & data appreciated  from you folks!
 
 

I've seen those temps for the 26 years I have owned my F. Here is a picture of my JPI showing it leaned out to roughly 100° ROP.



This one is running richer (prior to leaning) but at 54°. On a really hot summer day, climbing at 100 KIAS, I may come close to 380°. But after leveling and setting mixture, I will be in the 310° to 330° on that hot day.




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[TheTurtle]

If my CHT on #3 (the one that runs 30-40deg hotter than the rest) is less than 380 I keep leaning.  Im cheap as hell. 

[BRBENNETT]

Thanks for the all the responses.  Nothing seems out of order.

Much appreciated!

 

[pinerunner]

On 9/5/2017 at 10:41 PM, Guitarmaster said:

Is it possible to run too cold??  In a diesel I would say yes, but I don't think so in a gas engine.

Yes you can run too cold. Mike Busch has a talk about it as I recall ( though it might have been John Deakins). There's an additive in the fuel that helps the tetraethyl lead to go out the exhaust and not end up as deposits on valve stems and spark plugs. It works better at higher temperatures. So while you want CHT's lower than 400 degrees for sure, you don't want them as low as possible.

It's the reason I think twice about loitering mode and consider lower RPM's to keep the CHT's up when I go to lower power.

Edited September 7, 2017 by pinerunner

[Guitarmaster]

Yes you can run too cold. Mike Busch has a talk about it as I recall ( though it might have been John Deakins). There's an additive in the fuel that helps the tetraethyl lead to go out the exhaust and not end up as deposits on valve stems and spark plugs. It works better at higher temperatures. So while you want CHT's lower than 400 degrees for sure, you don't want them as low as possible.
It's the reason I think twice about loitering mode and consider lower RPM's to keep the CHT's up when I go to lower power.

Interesting. I found lower rpm drastically lowers my CHT.

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[Tommy]

On 9/7/2017 at 1:03 AM, pinerunner said:

Yes you can run too cold. Mike Busch has a talk about it as I recall ( though it might have been John Deakins). There's an additive in the fuel that helps the tetraethyl lead to go out the exhaust and not end up as deposits on valve stems and spark plugs. It works better at higher temperatures. So while you want CHT's lower than 400 degrees for sure, you don't want them as low as possible.

It's the reason I think twice about loitering mode and consider lower RPM's to keep the CHT's up when I go to lower power.

Mike Busch also suggested that lower RPM will lower your CHT. That's why he runs reverse square ie. RPM lower than Manifold. 

[Bob - S50]

8 hours ago, Tommy said:

Mike Busch also suggested that lower RPM will lower your CHT. That's why he runs reverse square ie. RPM lower than Manifold. 

That makes no sense to me.  Lower POWER should result in lower CHT.  So if you reduce the RPM but keep the MP the same, you are making less power.  However, if you decrease RPM and increase MP to make the same power, the peak pressure should move closer to TDC which should make the CHT higher.

What article did you read where he made that point?

[Bob - S50]

I found the article and I have a few comments.

1.  The engine monitor plot does in fact show a lower EGT and CHT for the lower RPM.

2.  I'm going to assume that Beech power settings work similarly to my J.  That is, according to 'flying by the numbers' and confirmed by my POH, 65% power occurs when MP + RPM equals 47.  That is, 2200+25", or 2300+24", or 2400+23", etc.  The actual number is probably different than 47 but I'll assume the relationship is similar.

3.  He uses two power settings, 2100 + 27" = 48 and 2500 + 21" = 46.  48 and 46 are not equal.  However, both settings are using the same fuel flow and both are labeled LOP.

4.  However, he does not say if they are the same amount LOP.  Since the lower RPM setting adds up to 48 vs only 46 for the higher RPM, if they were both LOP to the same degree, I would expect the first setting to have a higher fuel flow.  But they don't.  Why not?  I'm guessing because he is considerably deeper LOP.  Say maybe 100F vs only 10F for the higher RPM setting.  And looking at a standard EGT/CHT/Power/BSFC curve, further LOP means lower EGT and CHT.  That's because it slows the burn rate causing peak pressure (PP) to occur later than it would at peak.  This helps offset the earlier PP caused by the lower RPM.  The lower RPM could actually peak later than the higher RPM and the EGT could still be lower because the lower RPM also provides more time for combustion to complete before the exhaust valve opens.

Just my two cents worth.

Bob

[Tommy]

And to complicate things a further, this article calls for the complete opposite practice of leaving RPM at 2500rpm

http://www.mooneypilots.com/mapalog/cruisepower.html

 

[pinerunner]

OK I did the experiment. Setting up at 20 " of manifold pressure and 2500 RPM and slightly LOP (10 LOP on the richest and about 50 LOP on the leanest cylinder) I pulled back the prop until I had 1700 RPM. I did get a decrease in CHT of about 30 degrees (from memory; notebook was misplaced). The MP went up, as expected, to 23 " since I was pulling air through a lot slower.  Not that I didn't believe what others were saying but I like to see things for myself (except for how Mooneys behave in a spin; that I don't want to experience).

I still totally don't believe that low RPMs on takeoff are a good idea. At high power settings I stick with the old rule "don't let MP exceed RPM/100". At lower power settings I give myself permission to experiment and watch the CHT's to stay out of trouble. 

This is an interesting subject. Deakins reccomends low RPM's for descent. I want to understand this better.

[Bob - S50]

40 minutes ago, pinerunner said:

OK I did the experiment. Setting up at 20 " of manifold pressure and 2500 RPM and slightly LOP (10 LOP on the richest and about 50 LOP on the leanest cylinder) I pulled back the prop until I had 1700 RPM. I did get a decrease in CHT of about 30 degrees (from memory; notebook was misplaced). The MP went up, as expected, to 23 " since I was pulling air through a lot slower.  Not that I didn't believe what others were saying but I like to see things for myself (except for how Mooneys behave in a spin; that I don't want to experience).

I still totally don't believe that low RPMs on takeoff are a good idea. At high power settings I stick with the old rule "don't let MP exceed RPM/100". At lower power settings I give myself permission to experiment and watch the CHT's to stay out of trouble. 

This is an interesting subject. Deakins reccomends low RPM's for descent. I want to understand this better.

Well, you kind of did the experiment.  Your initial power setting was 2500/20 which adds up to 45.  Your second power setting was 1700/23 which adds up to 40.  That's a big power reduction.  I would expect the CHT to decrease because you are making a lot less power.  I'm guessing your fuel flow also went down about 2 gph.

This would be a true test:

Go up on a calm cool day so there will be few if any thermals and no turbulence.  Pick and altitude that you like to cruise at, say 5500 or 6500.

Set 2600/21 and lean to just barely LOP.  Wait a couple minutes and let everything stabilize.  Note the Speed, CHT's, and FF.

Adjust the power to 2500/22 and lean again to the same number of degrees LOP.  Note the FF.  If the FF is not the same as the first run, adjust the MP until it is.  If you really want to be particular about it, you could redo the leaning to make sure you are still the same number of degrees LOP.  But when you are done adjusting things, you want 2500 RPM, the same FF as before, and the same number of degrees LOP.  Wait a couple minutes and note the Speed and CHT's.

Adjust the power again to 2400/23 and repeat the previous process.

Do it again at 2300/24.  If you want to continue, and your POH allows those power settings, you may choose to do so.

Then compare speeds and CHT's.  CHT's will tell you which power setting is coolest and speed will tell you which combination of engine and prop efficiency gives you the most bang for the buck.

[pinerunner]

1 hour ago, Bob - S50 said:

Well, you kind of did the experiment.  Your initial power setting was 2500/20 which adds up to 45.  Your second power setting was 1700/23 which adds up to 40.  That's a big power reduction.  I would expect the CHT to decrease because you are making a lot less power.  I'm guessing your fuel flow also went down about 2 gph.

This would be a true test:

Go up on a calm cool day so there will be few if any thermals and no turbulence.  Pick and altitude that you like to cruise at, say 5500 or 6500.

Set 2600/21 and lean to just barely LOP.  Wait a couple minutes and let everything stabilize.  Note the Speed, CHT's, and FF.

Adjust the power to 2500/22 and lean again to the same number of degrees LOP.  Note the FF.  If the FF is not the same as the first run, adjust the MP until it is.  If you really want to be particular about it, you could redo the leaning to make sure you are still the same number of degrees LOP.  But when you are done adjusting things, you want 2500 RPM, the same FF as before, and the same number of degrees LOP.  Wait a couple minutes and note the Speed and CHT's.

Adjust the power again to 2400/23 and repeat the previous process.

Do it again at 2300/24.  If you want to continue, and your POH allows those power settings, you may choose to do so.

Then compare speeds and CHT's.  CHT's will tell you which power setting is coolest and speed will tell you which combination of engine and prop efficiency gives you the most bang for the buck.

Well to correct the correction..I was LOP.  Manifold pressure together with RPM do not give you power when you're LOP. The tables that are published with the typical owners manual assume leaning to best power and are not valid otherwise. To do the best job I would have to do a bit more but using the MP RPM table or MP RPM rules of thumb when running LOP would not lead to a valid answer.

I've read 14.9 HP/GPH gives a reasonable estimate of power when CHT and that was close for me a couple years ago when I compared LOP vs ROP adjusting to get the same IAS. The assumption was same airspeed must be same power but updrafts and downdrafts made it trickier; I had to average it a bit over time. I didn't do this over a bunch of settings; once was enough to satisfy me given the simple logic; LOP side fuel is the limiting reagent; ROP side air is the limiting reagent, thus the Tables (temperature matters too but effect smaller).

[Bob - S50]

3 hours ago, pinerunner said:

Well to correct the correction..I was LOP.  Manifold pressure together with RPM do not give you power when you're LOP. The tables that are published with the typical owners manual assume leaning to best power and are not valid otherwise. To do the best job I would have to do a bit more but using the MP RPM table or MP RPM rules of thumb when running LOP would not lead to a valid answer.

I've read 14.9 HP/GPH gives a reasonable estimate of power when CHT and that was close for me a couple years ago when I compared LOP vs ROP adjusting to get the same IAS. The assumption was same airspeed must be same power but updrafts and downdrafts made it trickier; I had to average it a bit over time. I didn't do this over a bunch of settings; once was enough to satisfy me given the simple logic; LOP side fuel is the limiting reagent; ROP side air is the limiting reagent, thus the Tables (temperature matters too but effect smaller).

That's true.  LOP power is determined by fuel burn and ROP it is RPM+MP.  I always set power using ROP setting and then lean to just barely LOP.  However, not all LOP settings are equal.  100F LOP is a lot less efficient (and lower CHT) than 30F LOP.  To compare apples to apples you have to keep everything equal except the variable(s) being considered.  That means FF and number of degrees LOP need to be kept constant when comparing RPM and MP settings.  Weather permitting, I hope to do the experiment myself tomorrow afternoon.  I'll let you know what I find.  I may be wrong.  If I am, I'll apologize publicly and beg forgiveness.

[jetdriven]

I haven't found an economic reason to lean past about 30f LOP. Even at around 50f LOP the fuel economy actually starts to get worse. The plane takes a major speed hit for a .2gph fuel savings. 

[Piloto]

CHT temperatures vary substantially with the cowl flap gap when in the closed position. At 1 inch gap CHT temps will not go over 300F on the climb, but fully closed temps will go over 400F on the climb. You can balance the CHT temps on each side by individual gap adjustment to each side cowl flap. There is no speed improvement by having no gap. The gap is adjusted by the turnbuckle actuating the cowl flaps.

José

[pinerunner]

1 hour ago, jetdriven said:

I haven't found an economic reason to lean past about 30f LOP. Even at around 50f LOP the fuel economy actually starts to get worse. The plane takes a major speed hit for a .2gph fuel savings. 

I agree. When I first started trying LOP I found that the EGT's started to rise again if I leaned it too much. The only explanation I can come up with was that the the fuel/air mixture was still burning when it went out the exhaust. Totally wasted calories so lower fuel efficiency expected. My GAMI spread isn't very good; number 1 is the last t go lean about 0.5 GPH as I recall. That's why I have my spread of degrees LOP so low. I never try the Wide Open Throttle LOP thing where you just use the mixture to get your LOP power. It runs too rough.

At very low power settings I set it close to peak. It basically amounts to the old "lean to roughness then enrich until it smooths out. I like to think about this stuff but when I'm flying I want things as simple as possible.

[Guitarmaster]

WOT and... Well... You can see the LOP figures.

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