CHTs too cold?

[jlunseth]

On 12/14/2020 at 8:26 AM, aviatoreb said:

What happens at 250 degrees that makes that the threshold number?

My POH has a table labeled Power Plant Instrument Markings. The lower end of the normal operating range is 250 dF.

I did some searching and found this quote from Anthony from another thread like this, same basic subject. He said, "here is a lower limit for CHT, based on the lead content of the 100LL... but I don't remember that number off the top of my head..."

@carusoam, do you remember the why of the lower limit?

I don't know, I have not encountered particular difficulty down to 220, which happens in cold winter temps on one or two of my cylinders. In a turbo the serious lower limit is the Oil Temp which cannot be below 100 dF for the protection of the turbo, oil becomes too thick to provide reliable thin film lube in the turbo bearings. OT is only indirectly related to CHT though.

My guess at it is that there might be coking or excess sooting if the CHT gets too low, but I really don't know. The fact that it is marked on the engine instruments is a clue. The design provisions of the FAR's, or for most of our aircraft the predecessor CAR's, were based on what was generally known about design, and on actual testing. Instrument markings are approved in the certification process. So it would stand to reason that something happens under 250 that is not best for the engine, such as incomplete combustion, or too much change in the wrong direction of the actual dimensions of the piston, cylinder, and components. If it were critical to safety it would be a redline, but it is not a redline and there are certainly circumstances in normal operation where temps would be cold, say, immediately after start-up or at idle or at low power, on a cold day. The instrument markings are telling you it is best not to operate there, but it is not critical. My read of it.....

[carusoam]

Let’s see if @Blue on Top has as much detailed knowledge of engine ops as he has in airframe design...   

Question about lowest CHTs for continuous operation...

The green arc is for ops... below that is acceptable for warm up... the red line, is do not pass....

what causes the green arc to have the lower limit set at 250°F?

- Interference fit between the cylinder and piston/rings?

- Lead scavenging temperature?

 

My POH is similar to JL’s M20K’s limitation section.

PP question only, not an engine designer...

Best regards,

-a-

[takair]

5 hours ago, carusoam said:

Let’s see if @Blue on Top has as much detailed knowledge of engine ops as he has in airframe design...   

Question about lowest CHTs for continuous operation...

The green arc is for ops... below that is acceptable for warm up... the red line, is do not pass....

what causes the green arc to have the lower limit set at 250°F?

- Interference fit between the cylinder and piston/rings?

- Lead scavenging temperature?

 

My POH is similar to JL’s M20K’s limitation section.

PP question only, not an engine designer...

Best regards,

-a-

I think it is a few things. The cylinders start off with a choked shape, such that the top of the cylinder is a smaller diameter. With heating they take on a purely cylindrical size. The choking causes the rings to be tighter at the top. Rings are sized such that they have a small gap when cold, but they probably tend to cause more wear when the cylinder is not up to temp. 
 

I think lead scavenging is another reason. I don’t recall the numbers, but there are optimal temps in both combustion and surface for lead scavenging. This is one reason for variable temp spark plugs. Low temps leave lead globs. These can then, ironically, cause local hot spots at sudden high power demand. So, it is good to keep the lead happy. 
 

Not an engine engineer....

[aviatoreb]

14 minutes ago, takair said:

I think it is a few things. The cylinders start off with a choked shape, such that the top of the cylinder is a smaller diameter. With heating they take on a purely cylindrical size. The choking causes the rings to be tighter at the top. Rings are sized such that they have a small gap when cold, but they probably tend to cause more wear when the cylinder is not up to temp. 
 

I think lead scavenging is another reason. I don’t recall the numbers, but there are optimal temps in both combustion and surface for lead scavenging. This is one reason for variable temp spark plugs. Low temps leave lead globs. These can then, ironically, cause local hot spots at sudden high power demand. So, it is good to keep the lead happy. 
 

Not an engine engineer....

Certainly the word engineer must originally come to describe those that design or operate engines.  Probably dating to stream engines.

I remember there was an engine overhauler guy who claimed that the choke was a bad idea and he was overhauling cylinders with zero choke, and claiming his engines made less heat, more power, and more efficiency and lasted longer.  So goes his claims.   Chuck Neyes was his name, and he eventually passed away and I am not sure if anyone is carrying on with idea.

[carusoam]

The Chuck Ney...  known for the Ney Nozzles for lubricating the cam...

http://www.aviatorshotline.com/content/chuck-ney

Best regards,

-a-

[Blue on Top]

@jlunseth  Great question!

All of you are correct in your statements above.  There are a lot of different things happening with the cylinders, pistons, rings, combustion cycles, lubrication, etc. The problem is that the gage can only mark so many things (very limited), but here's a few point to think about ...

1) We (the OEM and/or STC holders) are required by law to pass along all limitations.  IOW, if Lycoming, Continental or Porsche has a redline or green arc as a part of their certification (14CFR33) it must be shown to the pilot ... and stated as such.

2) In this case an OEM and/or STC holder cannot put a redline on the low end because every start would be in the "red", which by FAA definition is unacceptable.  (no operations in the red are acceptable).  The gage could have been marked yellow below the green, but this gets into other complications.

3) Electronic gages (actually their computing and display capabilities) could remedy this situation by allowing normal operation until the temperature gets up into the green and then give a warning when the temperature goes below the green when it shouldn't. 

4) We are having similar marking issues with electronic airspeed indicators.  The round dials are too simple to indicate everything, but the electronic indicators have way too many possibilities ... with way, way too many failure modes.

Bottom Line:  Yes, CHTs in flight should be kept in the green arc for multiple reasons.

Blue on Top, Ron

[exM20K]

For CMI power plants, the goal is, I believe, trying to extend cylinder life rather than preventing a “kaboom!”  There are two sets of lean operating guidance that are in conflict sometimes: APS’s focus on internal cylinder pressure with degrees LOP as a proxy, and Mike Busch’s focus on CHT and metallurgy. 
 

so which is more likely to lead to more cylinder or valve wear requiring expensive work: High ICP, or CHT’s out of the ideal range?  Because if I don’t care about high ICP, I can baffle or cowl flap or cold weather my way to desired CHT.

I appreciate all the effort Mike has put into his books and Savvy, but I’m more convinced by the data and history as gathered by APS, and choose to operate targeting 50 LOP, accepting whatever CHT results from that, so long as it’s in the green.

I also am making a sincere effort to fly at least weekly, which will probably be much more influential than ICP or CHT  in making TBO and beyond on these jugs.

-dan

Edited December 19, 2020 by exM20K

[jlunseth]

1 hour ago, Blue on Top said:

Bottom Line:  Yes, CHTs in flight should be kept in the green arc for multiple reasons.

That confirms the instrument marking issue, but it would be helpful to know what the factors are that make low CHT operation an issue. Near the beginning of this thread someone stated the issue that cylinders are tapered in anticipation of dimensional changes during operation, which is true, but the pistons do not contact the cylinder walls directly, they are buffered from the wall by the compressible rings, so there is some room for dimensional changes. I know in my turbo, oil temp is an issue with respect to lubing the turbo bearings, but that is an OT issue, OT must be at least 100dF for sustained operation in my TSIO360. But that would not apply to an NA. 

Just speculating, but there are control systems that likely expect warm (and therefore less viscous) oil, such as the prop governor, an oil controlled turbo wastegate, some others depending on engine. I suppose the oil pump expect less viscous oil. The engine even depends to a degree on splashing and misting in the crankcase to lubricate the rings. Most of these, however, are OT issues, as long as the OT is above a certain level it does not matter what the CHT’s are, they affect OT only indirectly.

@carusoam - Do you remember what the issue is with 100LL, Could be a number of things, insufficient vaporization of the fuel in the mix, coking or sooting, could be some concern about the chemistry and whether combustion will occur properly. Just guessing.

For most of you this is perhaps not an issue, but when we try to operate under 20dF, or even in below zero conditions up here in the frozen north, we will see cruise CHTs at or under the 250 normal operating limit. So what is the risk?

PS this is more critical for LOP ops because of the lower CHTs.

[aviatoreb]

38 minutes ago, jlunseth said:

 

For most of you this is perhaps not an issue, but when we try to operate under 20dF, or even in below zero conditions up here in the frozen north, we will see cruise CHTs at or under the 250 normal operating limit. So what is the risk?

PS this is more critical for LOP ops because of the lower CHTs.

Exactly - so I take it to mean that in the depth of winter - I simply cannot and do not operate LOP because the engine gets just too cool.

[Blue on Top]

This is a great question for Lycoming and Continental.  They set the limits on their engine.

I would say to look at the TCDS for the engine, but I am (educatedly) guessing that the "why" is not listed.

[jlunseth]

3 hours ago, aviatoreb said:

Exactly - so I take it to mean that in the depth of winter - I simply cannot and do not operate LOP because the engine gets just too cool.

I do, but only when I can get away with it. In extreme conditions I have even been known to operate closer to peak than normal, need the heat.

[N201MKTurbo]

One thing to keep in mind is the limitation only applies to the factory gauge in the factory location. Where you put the thermocouple makes a big difference. You could make a case that as long as cylinder 3 is above 250 F, you are within the certification limits.

I doubt the piston/cylinder clearance is an issue, or you would be harming your engine every time you started it. 

Edited December 19, 2020 by N201MKTurbo

[carusoam]

I don’t have numbers for the dimensions of the pistons compared to the choke of the cylinder... the space is pretty big...

There could be a limit to how compressible rings are... but start up temps on cold days can add a few variables that are hard to describe...

We wait for oilT to get in the yellow zone... CHTs seem to come up really quickly...

Flying LOP at altitude was where I would see the temps at the lower end of the green arc... the colder the day, the lower they would go... typically cruise @65%bhp a few °F LOP...

My biggest concern was more in line with wear... and getting my six cylinders to the 2k hr Mark... I originally acquired them with about 1400hrs... and was adding 100hrs each year...

I haven’t run into any signs of anything not operating properly... but being tech driven for all things machinery... I wanted to know what the limits were and what caused them to be there...

Sure, JL, evaporation rate of 100LL could get funky at some temp... as the rate of evaporation is temp dependent... 

So... if things get really cold... and the JPI starts alarming with EGTs, and the ignition system experiencing skips...    that would be alarming...

The fuel needs to evaporate... to blend with air, for complete ignition...

PP thoughts only...

Best regards,

-a-

[N201MKTurbo]

I’m pretty sure the lower limit has to do with lead scavenging. Here is a good article about:

https://www.shell.com/business-customers/aviation/aeroshell/knowledge-centre/technical-talk/techart-18-30071600.html
 

So the problems will be more long term and somewhat reversible. 

[aviatoreb]

3 hours ago, N201MKTurbo said:

I’m pretty sure the lower limit has to do with lead scavenging. Here is a good article about:

https://www.shell.com/business-customers/aviation/aeroshell/knowledge-centre/technical-talk/techart-18-30071600.html
 

So the problems will be more long term and somewhat reversible. 

I'm pretty sure you are right.

I was just checking my savvy record.  My cylinders get much colder than 250 during the last minutes of flight and taxi, as the power comes way down.  My coolest cylinder was like 230 for 5 min.  I don't think this is damaging anything.  Its just the scavvaging isn't happening, but it resumes during the next flight, and not a lot of depositing is happening I bet during that short time.  It would be a chronic problem if long flights were extremely cool I bet.

[jlunseth]

@N201MKTurbo That article was a great find! Explains the problem. I wonder how many of us are rotating spark plugs by magneto sets at every oil change.

Now, where is that fellow from a couple of months ago who was unhappy because he couldn’t idle his TSIO360LB at 750 rpm’s?

[jaylw314]

7 hours ago, aviatoreb said:

I'm pretty sure you are right.

I was just checking my savvy record.  My cylinders get much colder than 250 during the last minutes of flight and taxi, as the power comes way down.  My coolest cylinder was like 230 for 5 min.  I don't think this is damaging anything.  Its just the scavvaging isn't happening, but it resumes during the next flight, and not a lot of depositing is happening I bet during that short time.  It would be a chronic problem if long flights were extremely cool I bet.

IIRC there was a Lycoming document that recommended running the motor at 1600 RPM on the ground for 20 seconds right before shutting down to "burn off" or scavenge any lead deposits.  However, neither the O-360 operating handbook or the M20J POH specifies doing so.

When I'm worried about it, I just run up the motor to 1600 RPM, lean out the motor until the EGT's are at peak or just LOP, which reduces the RPM to about 1500 (so I'm not sitting there blasting the hangar and picking up rocks), for about 20 seconds.

[aviatoreb]

26 minutes ago, jaylw314 said:

IIRC there was a Lycoming document that recommended running the motor at 1600 RPM on the ground for 20 seconds right before shutting down to "burn off" or scavenge any lead deposits.  However, neither the O-360 operating handbook or the M20J POH specifies doing so.

When I'm worried about it, I just run up the motor to 1600 RPM, lean out the motor until the EGT's are at peak or just LOP, which reduces the RPM to about 1500 (so I'm not sitting there blasting the hangar and picking up rocks), for about 20 seconds.

Huh - there is some logic to that.

Does anyone else around here do that?

E

[ArtVandelay]

IIRC there was a Lycoming document that recommended running the motor at 1600 RPM on the ground for 20 seconds right before shutting down to "burn off" or scavenge any lead deposits.  However, neither the O-360 operating handbook or the M20J POH specifies doing so.
When I'm worried about it, I just run up the motor to 1600 RPM, lean out the motor until the EGT's are at peak or just LOP, which reduces the RPM to about 1500 (so I'm not sitting there blasting the hangar and picking up rocks), for about 20 seconds.

I do this, it’s in the SB about avoiding stuck valves. I use 1400 because I think that’s the number quoted.


Tom

[PT20J]

On 12/19/2020 at 2:09 PM, Blue on Top said:

This is a great question for Lycoming and Continental.  They set the limits on their engine.

I would say to look at the TCDS for the engine, but I am (educatedly) guessing that the "why" is not listed.

1E10 TCDS doesn’t list a minimum. The Operator’s Manual says 150F.

Skip

[jaylw314]

2 hours ago, ArtVandelay said:

I do this, it’s in the SB about avoiding stuck valves. I use 1400 because I think that’s the number quoted.


Tom

Do you happen to know the SB# or have a link?  I've been wracking my brain trying to remember where I saw it.

 

2 hours ago, aviatoreb said:

Huh - there is some logic to that.

Does anyone else around here do that?

E

I can't remember the last time I had problems starting due to lead fouling, leaning while taxiing seems to be more important, so I don't do it most of the time.  But, yeah, if it was a long, cold descent to the airport, I'll do it afterwards

[aviatoreb]

1 hour ago, jaylw314 said:

Do you happen to know the SB# or have a link?  I've been wracking my brain trying to remember where I saw it.

 

I can't remember the last time I had problems starting due to lead fouling, leaning while taxiing seems to be more important, so I don't do it most of the time.  But, yeah, if it was a long, cold descent to the airport, I'll do it afterwards

I’ve never specifically had a problem but winter flying up here in the north so I do try and pay attention.

[PT20J]

1 hour ago, jaylw314 said:

Do you happen to know the SB# or have a link?  I've been wracking my brain trying to remember where I saw it

Spark Plug Fouling.pdf

[ArtVandelay]

I found this...


https://www.lycoming.com/sites/default/files/Recommendations%20to%20Avoid%20Valve%20Sticking.pdf

[carusoam]

Interesting read Tom!

They didn’t quite give the temps...

But it was clear about one thing...   idling above 1200 rpm is enough to keep things warm enough to avoid lead shot from collecting...

-a-

 

“Prior to engine shut-down, the engine speed should be maintained between 1000 and 1200 RPM until the operating temperatures have stabilized. At this time the engine speed should be increased to approximately 1800 RPM for 15 to 20 seconds, then reduced to 1000-1200 RPM and shut-down immediately using the mix- ture control.

The engine should be operated at engine speeds between 1000 and 1200 RPM after starting and during the initial warm-up period. Avoid prolonged closed throttle idle engine speed operation (when possible). At engine speeds from Prior to engine shut-down, the engine speed should be maintained between 1000 and 1200 RPM until the operating temperatures have stabilized. At this time the engine speed should be increased to approximately 1800 RPM for 15 to 20 seconds, then reduced to 1000-1200 RPM and shut-down immediately using the mix- ture control.
The engine should be operated at engine speeds between 1000 and 1200 RPM after starting and during the initial warm-up period. Avoid prolonged closed throttle idle engine speed operation (when possible). At engine speeds from 1000 to 1200 RPM, the spark plug core temperatures are hot enough to activate the lead scaveng- ing agents contained in the fuel which retards the formation of the lead salt deposits on the spark plugs and exhaust valve stems. Avoid rapid engine speed changes after start-up and use only the power setting required to taxi.
NOTE
For information which retards the formation of the lead salt deposits on the spark plugs and exhaust valve stems. Avoid rapid engine speed changes after start-up and use only the power setting required to taxi. “