CHTs too cold?

[Cruiser]

I was reviewing my recent flights and see that my CHTs have been running around 300°F and even lower in cruise. About 60% power and LOP of course.

 

On a flight at 9000' cruise OAT of -2°F the temperatures were

C1   C2   C3  C4  C5   C6

287 290 294 313 318 300  Avg

335 332 339 357 366 356 Max (in climb)

 

Is this too cold for extended cruise time say of two hour or so?

[BorealOne]

I'm running in that range as well - a bit warmer, but not much. I'm usually between 320-330 on C5 in those temps, 295 on the coldest.

[Txbyker]

I ran a test the other day full prop, full rpm, 8000 ft, 6C all 320-330 ish except #5.  No pic of LOP but it was low 300's.  Perhaps below zero would drop in the 200's.  What was your oil temp?

 

Russ

[David Brown]

You are in a happy place! 

 

If you get down to below 175, then think about it as a problem.

[201er]

I've had them as low as 250 on the coolest and 300 on the hottest. I too wonder if that's too cool. I was trying to run hotter on purpose but with little success. The sucky thing about exceedingly cold CHTs is that the heater doesn't give enough then and you're freezing your butt off.

[orionflt]

You are in a happy place! 

 

If you get down to below 175, then think about it as a problem.

good, because since i redid my baffles mine are in the same area

[harrispa]

A little off subject--What are you guys with the 280hp 2500 rpm using for fuel flow on takeoff (sea level)? 

Paul

[M20S Driver]

A little off subject--What are you guys with the 280hp 2500 rpm using for fuel flow on takeoff (sea level)? 

Paul

My Eagle with 280hp STC burns 23 to 23.4 gallons on sea level take offs. My CHT is around 380 at 700-800 feet climbing at 100knots. It drops to 360s at 110 knots with 24 inches of MP AT 2500rpm.

Masoud

[PTK]

The sucky thing about exceedingly cold CHTs is that the heater doesn't give enough then and you're freezing your butt off.

Cabin heat is more a function of EGT's (as opposed to CHT's) and the muffler shroud.

Always a good idea to remove shroud and carefully check muffler for cracks which would allow CO and CO2 to enter cabin!

[aaronk25]

My Eagle with 280hp STC burns 23 to 23.4 gallons on sea level take offs. My CHT is around 380 at 700-800 feet climbing at 100knots. It drops to 360s at 110 knots with 24 inches of MP AT 2500rpm.

Masoud

What are your CHTs if you stay full throttle and climb at 110-120kts? You shouldn't have to reduce MP on a naturally aspirated engine after take off.

[M20S Driver]

What are your CHTs if you stay full throttle and climb at 110-120kts? You shouldn't have to reduce MP on a naturally aspirated engine after take off.

Full power take off and climb at 120--130 knots gives me high 370s deg f in a standard day. Full power take off in a hot summer day will push CHT over 380deg f.

Masoud

[aaronk25]

Full power take off and climb at 120--130 knots gives me high 370s deg f in a standard day. Full power take off in a hot summer day will push CHT over 380deg f.

Masoud

Interesting saw the same in my friends ovation....now he has a acclaim and actually it's even cooler on climb and cruise LOP the heads were 290-300

[MooneyCFII]

What are your CHTs if you stay full throttle and climb at 110-120kts? You shouldn't have to reduce MP on a naturally aspirated engine after take off.

Not having to reduce power after T/O is usually true for normally-aspirated Lycoming engines but some normally-aspirated Continental engines have a maximum continuous MAP and RPM that is lower than takeoff MAP and RPM. (Take off power is limited to 5min typically in these cases.) The upper end of the green arc on the MAP and tach are a clue to this. 

[harrispa]

The IO550 engine in our mooney Ovations and Eagles is rated for full continuous power, there is no reduction needed.

[MooneyCFII]

The final arbiter of CHTs is, of course, the manufacturer's recommendations. The values you find in the POH supersede everything. If something is not specified in the POH then refer to the engine manufacturer's manual for your engine. If you don't have a copy of the Lycoming or Continental manual for your engine, you should get one. There is useful information there that Mooney didn't put into the POH. 

 

Even though red-line is 460F for the TSIO-360-LB1 in my '231', Continental recommends 380F-420F maximum for cruise. I know that Lycoming recommends keeping CHTs in the vicinity of 370F and not over 400F for greatest longevity. So, taking these two pieces of information together, I strive to maintain CHTs in the 340F-380F range for all flight regimes.

 

In climb for me this means that the cowl flaps must be fully open. In cruise I run with the cowl flaps in trail which usually results in 320F-360F. Closing the cowl flaps causes CHTs to rise above 400F on two cylinders so I don't feel I can operate with cowl flaps closed in cruise unless I operate at reduced power. (I normally cruise at 65% power running 25F lean-of-peak for a fuel-flow of 10GPH.) During descent with the cowl flaps closed my CHTs run around 280F-300F.

 

As for what is the minimum CHT, the green arc for CHT on my airplane is 250F-460F. That means you do not want to run with CHTs lower than 250F in flight. I think I saw someone suggest that lower is better and 175F was just fine. Based on the data in my POH, I would recommend against that.

 

This is how I have been operating aircraft for 45 years. I have successfully taken several engines beyond TBO operating in this manner. (I have only been doing lean-of-peak operation for the last 15 years and have only taken two engines to TBO using LoP operation.)

 

 

[David Brown]

Brian, TCM do not publish a minimum, but Lycoming do at 175dF.

 

Interesting the TCM water cooled cylinders had the thermostat set to 225, and the cylinder underneath is the same as all the 520/550 otherwise.

 

So it is safe to assume that even if the POH says nothing, this is OK.

 

Max temps is another matter. And this gets some folks in a spin.

 

By the way, having complete and blind faith in POH's is a fatally flawed position. I assume that was not what you were trying to say, so just for clarification for anyone reading consider this. When one critically examines many POH's and engine manuals, you find data that is contradictory to each other, to themselves one page to the next, or at complete odds with the laws of physics.

 

We know of one OEM employee who when he saw his work being displayed in the APS class as something needy of critical thinking, he even put his hand up and declared he was embarrassed to admit he wrote it and it was wrong and should not be in circulation.

 

So using critical thinking is prudent. Faith without careful consideration is sub optimal.

 

Hope that helps.

[MooneyCFII]

Huh. Yes, I had my mind in Continental-space, not Lycoming-space (which is odd because I have owned WAY more Lycoming engines than Continental engines in my life). 

 

Yes, I agree: one should try to make sense of the information and be aware of the possibility of error in the POH. If concerned, a phone call or an email to either Lycoming, Continental, or the manufacturer can set things straight. I know I have done that on several occasions. I have certainly called into question the oil temperature limits on the Russian and Chinese aircraft. That involved spending a fair bit of time talking with the engineers at Shell to determine what the proper temperatures should be for the oil to work most effectively. 

 

The problem with low CHTs has to do with differential expansion and cylinder geometry. The cylinder is made of steel while the piston is made of aluminum. The coefficient of expansion of these two materials is different. Aluminum expands more with an increase in temperature than does steel. The result is that clearances decrease as CHT rises. Optimum temperature results in optimum running clearances. Also, cylinders are not manufactured with parallel cylinder walls. They are manufactured with "choke", i.e. the cylinder is narrower near the head than at the base. This is to accommodate the difference in temperature from top to bottom of the cylinder. The idea is that, when the cylinder head reaches operating temperature, the greater temperature near the head will cause the top of the cylinder to expand more than the bottom, making the cylinder walls parallel at operating temperature. I presume this is why Lycoming says that the optimum CHT for long-life is 370F as that is where the cylinder walls will be parallel.

 

The most obvious result of this is that engines that spend more time at lower CHTs tend to have higher oil consumption due to the greater clearances. The clearest example of this was demonstrated to me when I flew my Comanche to the Paris Airshow. (Yes, Paris, France.) My leg from Gander, Newfoundland, to Shannon, Ireland, was 11 hours. Although the normal oil consumption of the Lycoming O-540 was one quart per 7 hours, the total oil consumed on that leg was 1/2 quart in 11 hours. 

 

So, I hold that cooler is not necessarily better. I think we all agree that CHTs should never be allowed to rise above 400F. But I also hold (and teach my students) that CHTs also should not be allowed to remain too cold. If your aircraft has cowl flaps or some other way to control CHT, use them. Remember, the mixture control and power settings have a great deal of effect on CHT as well. Long life for the engine depends on using ALL the engine controls properly and effectively. Don't be afraid to lean the mixture in descent to help keep the engine warm. Below 60% power there is nothing you can do with the mixture control that will hurt the engine so don't be afraid to use it.

 

BTW, the other part of this discussion gets into the care and feeding of the engine baffles. Making sure that the baffles are in good repair and that the baffle seals are not dried out (I prefer silicone seals myself) is important to long engine life too.

 

I guess we are about ready for the Lean-of-Peak and Marvel Mystery Oil arguments to start. (Do you have those here? I'm new to the Mooney list but other lists I have been on seem to have flame-fests on these two topics every couple of years. :-) FWIW, I don't think MMO hurts when used in the oil but probably shouldn't be used in the fuel as it lowers the octane rating of the fuel. I am also a proponent of LoP operation ... when done properly, of course.

 

Let the flames begin. :-)

[MooneyCFII]

Oh, with regard to water cooling, the actual cylinder temperature may be higher than the liquid coolant temperature. The most effective way to cool something with a liquid is to encourage nucleated boiling. You actually allow the coolant to boil in contact with the thing being cooled. That pulls away the most heat. What happens is that the cooler coolant then causes the steam from the nucleated boiling to immediately condense. If you have ever watched water boil in a pan you have seen this just before the water comes to a full rolling boil. You will see bubbles of steam form and then disappear as soon as they break loose from the bottom of the pan. The bubbles appear to flicker. 

 

So even with a coolant temperature set at 225F, the actual temperature near the cylinder head will be higher.

 

But even with that I would expect the manufacturer to change the choke, piston, and ring clearances to accommodate. So even if the parts appear the same and might appear to fit the air-cooled version, you might find a difference in the part number and a micrometer might tell the story.

[orionflt]

 

The problem with low CHTs has to do with differential expansion and cylinder geometry. The cylinder is made of steel while the piston is made of aluminum. The coefficient of expansion of these two materials is different. Aluminum expands more with an increase in temperature than does steel. The result is that clearances decrease as CHT rises. Optimum temperature results in optimum running clearances. Also, cylinders are not manufactured with parallel cylinder walls. They are manufactured with "choke", i.e. the cylinder is narrower near the head than at the base. This is to accommodate the difference in temperature from top to bottom of the cylinder. The idea is that, when the cylinder head reaches operating temperature, the greater temperature near the head will cause the top of the cylinder to expand more than the bottom, making the cylinder walls parallel at operating temperature. I presume this is why Lycoming says that the optimum CHT for long-life is 370F as that is where the cylinder walls will be parallel.

 

 

I'm not an engineer, but this statement doesn't make any sense to me. I agree that aluminum and steel expand at different rates and that aluminum will expand more then steel, that being said the aluminum should expand more and faster then the steel meaning there should be less tolerance with lower CHT's since the piston will be receiving more of the thermal energy then the cylinder walls thus expanding more. one of the reasons we run the engines before we do a compression check, or if we have air bypassing the piston during a cold compression check. as for the cylinders being designed fo provide a nominal clearance at normal operating temps, I agree , and that operating too hot or cold will change those clearances.  

[MooneyCFII]

I'm not an engineer, but this statement doesn't make any sense to me. I agree that aluminum and steel expand at different rates and that aluminum will expand more then steel, that being said the aluminum should expand more and faster then the steel meaning there should be less tolerance with lower CHT's since the piston will be receiving more of the thermal energy then the cylinder walls thus expanding more. one of the reasons we run the engines before we do a compression check, or if we have air bypassing the piston during a cold compression check. as for the cylinders being designed fo provide a nominal clearance at normal operating temps, I agree , and that operating too hot or cold will change those clearances.  

I think you may have misunderstood what I was saying. The clearances are larger when the cylinder is cold. Rather than thinking about expansion, think in terms of contraction. When the piston and cylinder cool off, the piston contracts more than the cylinder making the gap between piston and cylinder wall larger when colder. This means poorer oil control and more blow-by. That is why a hot cylinder has a better compression test than a cold one. So I agree with you. 

 

As for the dynamic case when temperatures are rapidly changing, such as when you first apply power during take-off, yes, more heat is being transferred to the face of the piston than to the cylinder walls. But this situation doesn't remain this way for very long. Very quickly, probably within a minute or two of applying full power for take-off, the engine reaches a point of thermal equilibrium where the temperature of the piston, cylinder, and head remain relatively constant. The clearances have settled into their normal, running values. In cruise when you reduce from full power to cruise power, we close the cowl flaps to keep the temperatures at the same point so the clearances don't change (much). 

 

The point of this whole discussion was started by, "Are my cylinder head temps too cold?" I am trying to say that both Lycoming and Continental designed their cylinders to run at a temperature "sweet spot" where the clearances are what they are supposed to be and the initial "choke" has expanded to where the cylinder walls are parallel. Lycoming says that this number is 370F. Continental mentions a maximum cruise CHT range of 380F-420F. So, to me, the Goldilocks answer is a CHT of 370F-380F -- not hotter and not colder.

 

But I do think that, if you are going to err, it is safer to err on the colder side. So in operating my airplanes, I will accept my coldest cylinder running at 330F in order to keep my hottest cylinder below 380F rather than brining the coldest cylinder up to 350F and having my hottest cylinder run at 400F.

 

So this is what I do, this is what I teach, and my engines seem to have no trouble going to TBO and beyond. You should do whatever works for you.

[carusoam]

Probably not going to get to MMO while we are stuck in...

Thermodynamics

Heat transfer

Energy conversion

Material Science

Phase change

Nucleation

Mechanical design with the use of various metals under various conditions

You GO CFII !!!

Best regards,

-a-

[MooneyCFII]

An airplane is a whole lot of physics in one can. :-)

 

I do kinda like to know how it all works. 

 

And now I'm going flying. 

[orionflt]

These types of discussions are best done over a couple of cold drinks, too bad this group is spread out over the country. I do love the knowledge base we have.

[r0ckst4r]

Reviving this post from the dead but good information here.  Based on all this information I have been recently adjusting my leaning technique and fine tuning it by CHTs.  I always fly LOP but I have started to not fly at -50 LOP and have been flying closer to peak EGT.  My hottest CHT, which is almost always #5.  Usually ends up around 350 or 360 which seems to be in range. I’m assuming that the basis of the -50 peak recommendation was to produce adequate cylinder head temps in the absence of my fancy JPI engine monitor, but if I can directly monitor my cylinder head temps shouldn’t this supersede that recommendation?   Just want to confirm if my thinking is sound.

[M20S Driver]

I did something similar after listening to Mike Busch's webinar.  I am targeting 330+ in Cruise.  This works fine in ROP but LOP is a challenge for a couple of cylinders that stay below 330.

I am a bit uncomfortable going to 360.  I am sure we will hear from guru's on this topic