CHT - What's too hot?

[MikeOH]

The "#3 runs very hot.." thread has me wondering...

There was a reference in that thread to Mike Busch saying 450 was actually okay during break-in!

My understanding was that anything much over 400 needs to be dealt with ASAP as some large percentage of the aluminum's strength is gone.  The needing to deal with it ASAP suggests this very damaging to the cylinder; like it's going to take hundreds of hours off the life of the cylinder.  Seems there is evidence that this is true: cylinders that are run hot don't last.

So, how is it that high temps during break-in are okay?  Seems to me that if it doesn't break-in over only a few hours, you are taking a TON of life off your brand new cylinder.

Okay, expert metallurgists, explain this to me, please.

 

[carusoam]

How about a non-expert, PP?

1) Sharp edges on all things are natural in the manufacturing process....

2) getting them worn down a bit is what break in actually is...

3) the oil we use, and the temps that occur make the break in happen a bit quicker...

4) So when the process is over, get back to the regular oil....

5) Also the hot temps are a mere by-product of the break-in...

6) Most proper break-in procedures include using as much cylinder cooling that is available... mixed with max power available... and alternating the RPM... to avoid wearing any steps into the surface...

7) Flying around at low altitudes, with FT, and changing rpm every 15 minutes sounds familiar...

8) Break-in is a controlled wearing process... so not something you want to continue beyond what is required....

9) Temps above 380°F is an agreed upon temp to avoid... for improved longevity... under redline to avoid failure...

10) Above redline, the oil is at risk near the exhaust valves... coked oil in the guides disrupts the oil flow... stuck valves become a challenge... degradation of oil starts to occur at temps over 500°F, oxidation may occur in lower temps... roughly speaking these numbers can be looked up for your specific oil...

11) Compare the oil damage temps to CHT redlines.... and then think about EGTs and Lubricating the exhaust valves... 

12) Things we know... well taken care of cylinders can go to TBO... that includes the break-in.  Cylinders that live in Flaming dragon mode all the time go half the distance...  Cylinders without an engine monitor are often unsure how close to flaming dragon mode they are actually operating in...

13) unlucky thirteen... some cylinders have two CHT sensors on them... one of them is either in a piggyback position, or under a spark plug... not in the well like three other three... know where your ship’s CHT gauge sensor is... and what type of sensor it is/where is it actually measuring... this often causes confusion as the temps can be 25-50°F hot or a previous owner may have switched a location for easier monitoring on a non-primary JPI.... 5 sensors four holes... one of them is not in an ideal location...

As mentioned before... PP thoughts only, not a mechanic...

Best regards,

-a-

[MikeOH]

Familiar with 1 through 13...doesn't really address my confusion, I'm afraid.  Not worried about 'wearing a step' due to not changing RPM.

Trying to understand how much time at elevated temps is going to appreciably affect cylinder life.

Put another way: How many hours at elevated (400s kind of temp) does it take before a cylinder is broken in?  How are those hours any different, as far as affecting longevity, than operating at elevated temperatures after break-in?

I've fought moderately high temps since I bought my M20F over two years ago.  #2 would run up to 425 on climb out on a hot day.  With new baffles that number is down to just over 400 when it's hot out.  I'll pull power to keep the temps down, but someday I may need full-power and backing off won't be an option.  The way everyone talks, it sounds like running at 425 for even a few minutes is HORRIFIC  I'm NOT talking about cruising for hours at high CHTs.

Now, I'm beginning to wonder how true that really is...  if running for HOURS at 450 during break-in is okay and, presumably, doesn't destroy cylinder life, why is running 400-410 for a 15 minute climb on the occasional hot day such a problem?

[carusoam]

I don’t think you are missing anything...

The wear rate is pretty much not linear... hotter is worse...

380°F and lower is great cruise forever temps...

As you go over 400°F it will wear more, but what can you do?

Climb times and power are a bit tougher, that’s why we use deep ROP to help maintain CHTs and climb out at 120ias when able...

Climb time is pretty short compared to cruise time... hence the focus on performance, then longevity...

There is plenty around here discussing ROP using the blue or white box on modern EGTs... for climb.

That box is most likely 2-300°F ROP during the climb...

If you are familiar with setting power for T/O at higher DA airports... this is a similar EGT method being used...

For the one engine I used that went nearly to the end with one set cylinders... it was the method I used...

Deep ROP climbs, shallow climbs based on IAS, don’t spend forever in the climb... high power climbs are often used... modern cowls are magic for cooling... dog houses, not as nice... all need good sealing....

If you find a better resource than that... point it out! 

You might have success searching around where people discuss the temps they can or have achieved in the climbs...

They can be hard to remember since FF, OAT and speed make a big difference...

The one engine I mentioned didn’t run into any cylinder issues... when it came to OH time somewhere over 1600hrs...
I had lots of fear of needing cylinders at the halfway point... just expensive...

Use your engine monitor to close watch for the a step change in CHTs... this is the hint that break in is past the important part... possibly the first 10 hours ...  the rest can technically occur over the next 90 hours...

Break in occurs at full rich, low altitudes, high power settings... the temperature you get Is only related to OAT after that... 

Yes, wear is occurring. Because break-in is essentially a wearing process that can’t be avoided... if it doesn’t break-in properly you don’t get to wear the cylinder out...

While doing the break-in, the other three cylinders are along for the ride... there isn’t much you can do about that... that I know of...

PP thoughts only not a mechanic or CFI...

Best regards,

-a-

[jaylw314]

I suspect the concern about running hot for a few minutes during climb is the danger of preignition, not decreased cylinder longevity.  Bear in mind during break-in, you're supposed to run at high power at cruise speed, not climb speed, so there should be plenty of cooling air during the break-in process.  On top of that, I suspect the idea of 450F during break in does not apply to O-360 powered Mooneys, which seem to have significantly lower temps than your typical Skyhawk.  IIRC, CHT's run about 20F hotter during break in, so that's more like 360-380F in our Mooney IO-360's.

[N131MA]

When I broke in my IO550 steel cylinders by the book (Anthony's  #7) I saw high "break in" temperatures for less than one hour. After that, temps continued decreasing and settled at current state after 5-7 hours or so. If one cylinder still runs hot after five hours, I recommend

1. swapping the CHT sensor,
2. make sure this jug runs doesn't run lean (engine monitor) and
3. carefully checking airflow / engine and cylinder baffling

My old trustworthy M20E was running with 350F-360F CHT in cruise (WOT, LOP).

[G Miller]

From a Lycoming Publication

Minimum in-flight CHT should be 150 F (65 C), and maximum in most direct drive normally aspirated Lycoming engines is 500 F (260 C). Some of our higher powered more complex engines have a maximum limit of 4750 F (2450 C). Although these are minimum and maximum limits, the pilot should operate his or her engine at more 50 reasonable temperatures in order to achieve the expected overhaul life of the powerplant. In our many years of building engines, the engines have benefited during continuous operation by keeping CHT below 400 F in order to achieve best life and wear of the powerplant. In general, it would be normal during all year operations, in climb and cruise to see head temperatures in the range of 350 F to 435 F

[MikeOH]

15 hours ago, N131MA said:

snip...

My old trustworthy M20E was running with 350F-360F CHT in cruise (WOT, LOP).

That's exactly what I see, as well; and, run WOT, LOP.

It's climb out on a hot day that is my issue.

[MikeOH]

1 hour ago, G Miller said:

From a Lycoming Publication.

....

In our many years of building engines, the engines have benefited during continuous operation by keeping CHT below 400 F in order to achieve best life and wear of the powerplant. In general, it would be normal during all year operations, in climb and cruise to see head temperatures in the range of 350 F to 435 F.

Thank you!

Now, THAT is an answer.

And, it is a big relief; I've never seen 435, even in climb out on a hot day.

[0TreeLemur]

Mike Busch in "Engines" p. 85 wrote:

"I'm convinced that if every piston-powered aircraft was equipped with a modern probe-per-cylinder digital engine monitor that set off alarms anytime any CHT rose above 390F for Continentals or 410F for Lycomings, cylinder head-to-barrel separation fatalities would become largely a thing of the past."

[Cyril Gibb]

This is for T6061 aluminum.  I can't find the graph I saw before for aluminum cylinder heads. Anyone?  It's reasonably alarming, particularly because Lycoming sets the redline temp at 500F.  My personal redline is 400F, but even then only momentarily before taking dramatic action.

Temp (degF) Tensile Strength (ksi) Yield Strength (ksi)
75        45         40
212     42          38
300     34          31
400     19         15
500     7.5         5

[PT20J]

I guess we have to have something to stress out over, or flying would be all fun . It used to be shock cooling. Then it was lean mixtures frying valves. Now it seems to be CHTs. 

The origin of the curve for aluminum cylinder head yield strength is an old P&W manual (The Aircraft Engine and its Operation -- a good read, BTW) that John Deakin used in the APS courses. 

Note that the vertical axis is not scaled. The bottom could be zero. It could just as well be 90% of yield strength. The point was just to show the shape of the curve to illustrate the origin of CHT limits. As pilots, we don't really need to know the exact values -- the engineers at Lycoming, Continental and Pratt & Whitney figured that out for us.

Skip

 

 

[MikeOH]

5 hours ago, 0TreeLemur said:

Mike Busch in "Engines" p. 85 wrote:

"I'm convinced that if every piston-powered aircraft was equipped with a modern probe-per-cylinder digital engine monitor that set off alarms anytime any CHT rose above 390F for Continentals or 410F for Lycomings, cylinder head-to-barrel separation fatalities would become largely a thing of the past."

Hmm, yet, supposedly, Mike Busch is the one who said 450 during break-in is okay?!?  Which is full circle back to my original question: How is 450 degrees during break-in any less damaging than 450 during climb out?

Yes, I get it that higher temps should be avoided...but, by that logic and the strength of aluminum vs. temp graphs, we should be operating below 300 F (approx. 25% loss of strength).

At least, with my new baffling, I don't see over 410F on climb-out anymore.

Not that it applies to our Lycomings, but Skip's reference to P&W R1820 shows 450F in climb out is ok.

Edited February 29, 2020 by MikeOH

[carusoam]

Break-in is intended wearing of parts...

So if one guys says things wear faster at higher temps, good for break-in...

He isn’t wrong... But...

Modern Break-in procedures get the engine to that point with the fewest side effects...

Call it black magic if it helps...

Follow the leader is what I used... pick a good knowledgable leader with the same engine you have...

My leader was Screamin’ Eagle @Cris. His guidance was talking directly to the Continental factory... Continental has step by step guidelines for engine break-in... what oil to use, what procedure to follow...

When doing the break-in flights... Cris was unable to join me... so another Mooney CFII was brought along to help me follow the discreet steps specified by Continental...

Everything worked out well... as expected.

What would be disappointing... trying to follow the best guidance... with a single CHT gauge, high DAs, and high OATs....

Do the best you can, with what you have... or the cylinders will rust before they wear...   

Take a look at Skip’s graph above... find where 380°F is on it... the slope of the line is curving... indicating the strength of materials is declining... continuously.

So...  there are no hard rules to follow... no exact temp to avoid... just shades of gray... better if kept cooler... not as good if allowed to get hotter...

There is no question about redline... that will wear things out very fast... as the oil is getting destroyed faster at the same time...

PP thoughts only... thinking there has got to be a way to eliminate wear even better...  TBO4k...

Best regards,

-a-

[MikeOH]

2 hours ago, PT20J said:

I guess we have to have something to stress out over, or flying would be all fun . It used to be shock cooling. Then it was lean mixtures frying valves. Now it seems to be CHTs. 

Skip

 

 

LOL!

Boy, ain't that the truth!  I've been stressing over my above 400F CHTs since I've owned my plane.

Thanks to this thread I'm going to relax a bit and accept my 410F climb CHTs and mid-300s cruise CHTs.  It was really bugging me to pull back power on hot day climbs just to keep CHTs under 400.

[carusoam]

The power of MS is amazing...   

-a-

[Ragsf15e]

1 hour ago, MikeOH said:

LOL!

Boy, ain't that the truth!  I've been stressing over my above 400F CHTs since I've owned my plane.

Thanks to this thread I'm going to relax a bit and accept my 410F climb CHTs and mid-300s cruise CHTs.  It was really bugging me to pull back power on hot day climbs just to keep CHTs under 400.

Mike, you’re still that hot with ~120mph climb, very rich, cowl flaps open climb?  

I use ~1275 as a target egt for mixture in climb and my chts are usually more like 360 in climb.  Possibly my cooler oat helps in NE Washington?

Is it a stock inlet or a mod?  Mine is lasar cowl closure but not sure that helps...

[MikeOH]

@Ragsf15e

Yes; that hot with 120 mph climb, cowl flaps full open, WOT, full rich (KPOC is 1000'), 2600-2650 rpm.

My EGTs run about 1330, and that's what I lean for at high DA.

Not stock inlet; I'm not certain which mod mine is.

OAT definitely makes a difference.  This time of year, 60-75 OAT, no problem (hotter than yours, but not over 390).  Summer time when OAT is 85-100, is where I have an issue.

[HRM]

1 hour ago, carusoam said:

The power of MS is amazing...   

-a-

So is BS.

[Ragsf15e]

Just now, MikeOH said:

@Ragsf15e

Yes; that hot with 120 mph climb, cowl flaps full open, WOT, full rich (KPOC is 1000'), 2600-2650 rpm.

My EGTs run about 1330, and that's what I lean for at high DA.

Not stock inlet; I'm not certain which mod mine is.

OAT definitely makes a difference.  This time of year, 60-75 OAT, no problem (hotter than yours, but not over 390).  Summer time when OAT is 85-100, is where I have an issue.

Yeah, I can see that with the higher oat.  Interesting that your target egt is higher.  I’m sure you’ve probably checked, but what’s your sea level full rich fuel flow on takeoff?  If you’re running even a tiny bit too lean, it will cause significant increase in climb cht.

[MikeOH]

Just now, Ragsf15e said:

Yeah, I can see that with the higher oat.  Interesting that your target egt is higher.  I’m sure you’ve probably checked, but what’s your sea level full rich fuel flow on takeoff?  If you’re running even a tiny bit too lean, it will cause significant increase in climb cht.

@Ragsf15e

As far as EGTs go, I've been a believer (perhaps incorrectly?), that the absolute number is kind of irrelevant plane-to-plane due to installation differences.

Yes, I have looked at sea-level full rich fuel flow; mine runs about 16.5 gph.  Some have told me that is low, but I specifically had my mechanic look into it at my recent (Jan 2020) annual and he was not concerned.  It is my understanding that the RSA fuel servo needs to be sent in to change that setting; it's NOT field adjustable.

[Ragsf15e]

10 minutes ago, MikeOH said:

@Ragsf15e

As far as EGTs go, I've been a believer (perhaps incorrectly?), that the absolute number is kind of irrelevant plane-to-plane due to installation differences.

Yes, I have looked at sea-level full rich fuel flow; mine runs about 16.5 gph.  Some have told me that is low, but I specifically had my mechanic look into it at my recent (Jan 2020) annual and he was not concerned.  It is my understanding that the RSA fuel servo needs to be sent in to change that setting; it's NOT field adjustable.

Yeah I don’t think the absolute egt is that important either, it just made me think about the sea level fuel flow because that is pretty important.

For the record, my fuel flow is around 18gph and then I use that as a basis for my target egt during climb.  If yours is really 16.5, there’s a good chance that’s at least part of your issue.

[MikeOH]

Just now, Ragsf15e said:

Yeah I don’t think the absolute egt is that important either, it just made me think about the sea level fuel flow because that is pretty important.

For the record, my fuel flow is around 18gph and then I use that as a basis for my target egt during climb.  If yours is really 16.5, there’s a good chance that’s at least part of your issue.

@Ragsf15e

Thanks for the reminder about fuel flow.  I do need to pursue it.

What I think that really means is pulling the fuel servo and sending it in...not wild about that...$$$$

[Ragsf15e]

15 minutes ago, MikeOH said:

@Ragsf15e

Thanks for the reminder about fuel flow.  I do need to pursue it.

What I think that really means is pulling the fuel servo and sending it in...not wild about that...$$$$

Yeah I agree, that doesn’t sound good.

[0TreeLemur]

On 2/29/2020 at 2:03 PM, MikeOH said:

Hmm, yet, supposedly, Mike Busch is the one who said 450 during break-in is okay?!?  Which is full circle back to my original question: How is 450 degrees during break-in any less damaging than 450 during climb out?

Yes, I get it that higher temps should be avoided...but, by that logic and the strength of aluminum vs. temp graphs, we should be operating below 300 F (approx. 25% loss of strength).

At least, with my new baffling, I don't see over 410F on climb-out anymore.

Not that it applies to our Lycomings, but Skip's reference to P&W R1820 shows 450F in climb out is ok.

The barometric pressure in the southeast US has been really high lately.   Today it was 30.40"  with low density altitude, I was seeing 27.6" MP WOT on takeoff, the CHT on Cyl. #4 up to 411, with cowl flaps open and maintaining 100+ kts on climb out, despite the 13C air temperature.    I reduced power to 26.5" MP, which brought the CHT on that jug back to 403 or so.   The FF on the JPI900 indicates about 16 gph at takeoff power for the O-360 (carbureted) in our C model.   The Mike Busch book "Engines" has saved me multi thousand $$ already- pretty good ROI.   I'll follow what he says.