It’s hot and CHTs are a problem! Solution??

[Utah20Gflyer]

Hello fellow Mooney types.  It’s now summer and under certain conditions it’s becoming a problem trying to produce reasonable amounts of power and also keep CHTs under control.   Full rich is no longer an option as it produces much less power and sometimes when the density altitude is high enough a rough running engine (8k DA).

If I keep it really rich my climb rate sucks!

If I lean to get better power my CHTs, specifically cylinder 4 really spikes.  
 

Climbing at a higher speed helps some but isn’t really a solution. I’m still trading a lower climb rate for cooler CHTs.  
 

I live the Mountains, sometimes I need to climb fast to clear terrain.   This last weekend I had to circle in Cache valley before I could proceed en route. 
 

So here is my question.  I’ve had an idea for a while to potentially fix this issue.  If you had a small water tank, pump, and tubing routed to the front of the cowling you could spray water into the airstream.  The evaporating water going past the cylinders would create a lot of extra cooling for the engine.  You would use distilled water to prevent mineral deposits.  The purpose of the system would be to provide 5 minutes of cooling during the initial climb out.  This would allow you to lean for more power.   
 

What about detonation?   At 7500-8500 Ft density altitude I am already below 75 percent power.  The possibility exists above 65 percent power, so some limits would have to be established to prevent abuse of the engine.  
 

I am not an engineer, just a simple man who likes flying Mooneys, sometimes when it’s hot outside.  What am I missing with this idea?  Please show me what I missed or I may just build this system and try to get an STC.  
 

Go ahead and be brutal,  I want the truth.  
 

Thanks! 

[Ragsf15e]

Truth is that you have less climb rate to work with in the summer.  Cooling and density altitude both play a part.  Fly early in the day.

There are times where cooling is ok at lower altitudes but becomes tougher as you get higher and the air is thinner. The most obvious thing to look at is your baffling and/or oil cooler.

[M20F]

You have made a very long post about CHT without saying what your CHT’s are and how you measure them.  Might want to start there. 

[A64Pilot]

Assuming your baffling is in good shape, the only real viable answer is climbing at a higher airspeed, yes that reduces your climb rate.

The water injection isn’t viable, the amount of airflow through an engine is enormous, the amount of water to achieve significant cooling through evaporation would be quite large and heavy.

 

Edited July 10, 2023 by A64Pilot

[OR75]

I would be worried about oxidation of the engine down the road and cracks developing as the temperature gradients could be very large

[Yetti]

point the nose down a little.

[ilovecornfields]

Sometimes in my Ovation taking off in the desert on a hot summer day the only thing I can do to keep the CHTs down is climb at reduced power (2400 RPM, WOT). I’ve tried other things but this works best.

[N201MKTurbo]

For the last couple of weeks, the CHTs are in the red before you start the engine...

[Utah20Gflyer]

3 hours ago, M20F said:

You have made a very long post about CHT without saying what your CHT’s are and how you measure them.  Might want to start there. 

On Saturday I was struggling to keep number 4 cylinder 420 or under while getting a very unsatisfying 200 to 300 fpm climb at 100lbs under gross.  I tried a bunch of configurations including VY very rich and 120 mph leaned a little more.  Density altitude was 7500 ft.   I tried 2700, 2600 and 2500 rpm’s, ultimately 2700 and VY produced the best climb even though it required being excessively rich.  

If I could have leaned for better power I probably could have been doing 400-500 ft per min.  Unfortunately I had to dump lots of gas in the cylinders to keep them relatively cool which really killed power production.  
 

Under normal conditions I can keep my Number 4 cylinder at 400 with excellent climb performance.   

[Utah20Gflyer]

23 minutes ago, ilovecornfields said:

Sometimes in my Ovation taking off in the desert on a hot summer day the only thing I can do to keep the CHTs down is climb at reduced power (2400 RPM, WOT). I’ve tried other things but this works best.

I haven’t tried 2400, although I did try 2500, 2600 and 2700.  

[Utah20Gflyer]

1 hour ago, OR75 said:

I would be worried about oxidation of the engine down the road and cracks developing as the temperature gradients could be very large

So your concern is shock cooling?  I can see that being an issue depending on several variables.  Obviously if you dumped liquid water on a hot cylinder that is going to be a big problem.  On the other hand a minute amount of water that has already evaporated before it gets to the cylinder would likely be ok.  Where the line between helpful and damaging is an important question in particular in relation to what benefit if any is created.  

[Utah20Gflyer]

2 hours ago, A64Pilot said:

Assuming your baffling is in good shape, the only real viable answer is climbing at a higher airspeed, yes that reduces your climb rate.

The water injection isn’t viable, the amount of airflow through an engine is enormous, the amount of water to achieve significant cooling through evaporation would be quite large and heavy.

 

Baffling (doghouse) is in good shape, under normal conditions I can easily get good climb rate and keep CHTs under 400.  
 

Your concern of adequate water to air ratio to even make a difference is a good one.   Part of the reason I was thinking the system would only be used for a short amount of time is you would want to keep the total amount of water down.  I was thinking half a gallon dispensed in 5 minutes, so that would work out to 1/10 a gallon per minute.   How much cooling does that quantity of water create when evaporated?  I’ll have to do some research.  Maybe that will help, maybe it’s not enough.  

[A64Pilot]

I think the bigger issue is the amount of airflow, it’s likely a lot larger than you realize, that huge amount of air has a lot of mass and that much mass just isn’t going to be cooled much by 4 lbs of water in 5 min, or even 8 lbs.

I believe what your looking for us called the heat of vaporization of water, but as I am no math guy I won’t touch it, it’s way over my head.

I do know large amounts of heat can be absorbed in the phase change of a liquid, it’s how vapor cycle refrigeration works.

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/heat-of-vaporization

Water has an incredible ability to absorb heat, higher I believe than anything else, but what I believe what your underestimating is the tremendous mass of the airflow through the engine in 5 min time.

Not being a math guy I’d have to try it to see, a windshield washer tank and spray nozzle off a junk car would I think be close to perfect to try, self contained tank with included 12V pump with a correct sized nozzle to spray at the pumps pressure, and cheap too.

Just need an experimental aircraft to try it on, but I don’t think it will do anything, because if it did then cyl head temp would drop out of the green in the rain and while I haven’t looked to see but I don’t think the drop is large

Edited July 11, 2023 by A64Pilot

[A64Pilot]

Oh, of course check that your cowl flaps are fully open and check the book, when fully closed I believe mine are supposed to be .25 to .50 inch open, so make sure your aren’t rigged to be more closed, sometimes people will do that in cold weather

[GeeBee]

For starters it would be an unapproved modification without approval.

I used to play with SuperCubs and they are enormously hard to keep cool with their O-320. You need to verify all your baffling is correct and tight. First you need to compare the baffling to the parts manual and verify everything is there. Then you need to place the airplane in a dark hanger and use a portable light on the back side of the baffles. Look in and check for light shining through. What you may think is tight, is not really and if you are below 120 mph you may have insufficient upper deck pressure.

The Lycoming installation manual requires 4 inches of H2O (manometer reading) of upper deck pressure. I would check to see if you have that. You can rig up some small tubing, secure an exit point out of the cowling and into the cockpit. You can get a digital manometer cheap from Amazon or you use an old airspeed indicator. 100 mph is about 4 inches. On SuperCubs, if the upper cowling was not bulging, there was not enough pressure.

Check also you oil cooler installation. Make sure it does not leak where it attaches to the baffling. Use Black RTV to seal the leaks where it fits up against the baffling. I've also seen oil cooler that seem good but when replaced with new, have good results. While it may seem to be flowing good, small internal restrictions are sometimes present and have remarkable effects when removed. 

Make sure also you are not pressurizing the lower deck with leaks in the lower cowl, such as where air cleaners, coolers and landing light fitment are located. You want maximum pressure on the upper cowl, low pressure below the engine.

For those small places, corners and crevices, RTV works great. On SuperCubs I've used Sugru to conform a baffle seal to the cowling. Just make sure your you place Saran-Wrap against what you don't want it to adhere against. 

[Utah20Gflyer]

1 hour ago, A64Pilot said:

I think the bigger issue is the amount of airflow, it’s likely a lot larger than you realize, that huge amount of air has a lot of mass and that much mass just isn’t going to be cooled much by 4 lbs of water in 5 min, or even 8 lbs.

I believe what your looking for us called the heat of vaporization of water, but as I am no math guy I won’t touch it, it’s way over my head.

I do know large amounts of heat can be absorbed in the phase change of a liquid, it’s how vapor cycle refrigeration works.

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/heat-of-vaporization

Water has an incredible ability to absorb heat, higher I believe than anything else, but what I believe what your underestimating is the tremendous mass of the airflow through the engine in 5 min time.

Not being a math guy I’d have to try it to see, a windshield washer tank and spray nozzle off a junk car would I think be close to perfect to try, self contained tank with included 12V pump with a correct sized nozzle to spray at the pumps pressure, and cheap too.

Just need an experimental aircraft to try it on, but I don’t think it will do anything, because if it did then cyl head temp would drop out of the green in the rain and while I haven’t looked to see but I don’t think the drop is large

These are good points, Thanks

Anyone have an idea of how much air actually moves through a vintage Mooney engine at 100 mph indicated?   It’s clear I need this figure to determine if the evaporation of the amount of water I am thinking of can even make a dent in the heat created by the engine.  

[Utah20Gflyer]

58 minutes ago, GeeBee said:

For starters it would be an unapproved modification without approval.

I used to play with SuperCubs and they are enormously hard to keep cool with their O-320. You need to verify all your baffling is correct and tight. First you need to compare the baffling to the parts manual and verify everything is there. Then you need to place the airplane in a dark hanger and use a portable light on the back side of the baffles. Look in and check for light shining through. What you may think is tight, is not really and if you are below 120 mph you may have insufficient upper deck pressure.

The Lycoming installation manual requires 4 inches of H2O (manometer reading) of upper deck pressure. I would check to see if you have that. You can rig up some small tubing, secure an exit point out of the cowling and into the cockpit. You can get a digital manometer cheap from Amazon or you use an old airspeed indicator. 100 mph is about 4 inches. On SuperCubs, if the upper cowling was not bulging, there was not enough pressure.

Check also you oil cooler installation. Make sure it does not leak where it attaches to the baffling. Use Black RTV to seal the leaks where it fits up against the baffling. I've also seen oil cooler that seem good but when replaced with new, have good results. While it may seem to be flowing good, small internal restrictions are sometimes present and have remarkable effects when removed. 

Make sure also you are not pressurizing the lower deck with leaks in the lower cowl, such as where air cleaners, coolers and landing light fitment are located. You want maximum pressure on the upper cowl, low pressure below the engine.

For those small places, corners and crevices, RTV works great. On SuperCubs I've used Sugru to conform a baffle seal to the cowling. Just make sure your you place Saran-Wrap against what you don't want it to adhere against. 

This looks like good information.  
 

First, If I were to proceed after determining it was a plausible design it would be to pursue an STC following all applicable rules.  
 

Second, My plane generally cools very well under all but the most extreme scenarios.  But having said that it could still have defects that could be addressed.   You have pointed out some areas to look at and I appreciate that.  I’m going to spend some time looking at these items.  I think it’s unlikely to solve the issue completely but could make a positive difference. 

Thanks! 

[Utah20Gflyer]

1 hour ago, A64Pilot said:

Oh, of course check that your cowl flaps are fully open and check the book, when fully closed I believe mine are supposed to be .25 to .50 inch open, so make sure your aren’t rigged to be more closed, sometimes people will do that in cold weather

I have fixed cowl flaps unfortunately.  Adjustment looks somewhat labor intensive.  From what I remember there is a single hole in the flap and cowl that a bolt goes through to hold it in position.   So a new position would require drilling new holes.  Not a huge deal but would require the attention of my mechanic to supervise the process.   Once new positions were created I could adjust unsupervised. 
 

If you also have fixed flaps how do yours adjust?  
 

Has anyone every done a cabin adjustable cowl flap retrofit?   If so how much is involved?

 

Thanks! 

[bcg]

In other discussions, it's been mentioned that adding just a touch of carb heat will sometimes even up fuel distribution and get the hottest cylinder a little cooler. Might be worth a try.

Sent from my Pixel 6a using Tapatalk

[Utah20Gflyer]

42 minutes ago, bcg said:

In other discussions, it's been mentioned that adding just a touch of carb heat will sometimes even up fuel distribution and get the hottest cylinder a little cooler. Might be worth a try.

Sent from my Pixel 6a using Tapatalk
 

I hadn’t thought of this, I’ll give it a try and see what affect it has.  Thanks! 

[Bolter]

@Utah20Gflyer Do you have the guppy mouth enclosure?  I believe that improves cooling flow efficiency as well as drag.

[Hank]

1 hour ago, bcg said:

In other discussions, it's been mentioned that adding just a touch of carb heat will sometimes even up fuel distribution and get the hottest cylinder a little cooler.

I've heard of this, and tried it myself when experimenting with LOP in my C. Don't recall discussion that this will help with CHT. But maybe it will. Looking forward to data!

[Pinecone]

Latent heat of vaporization of water is 1044 BTU per pound.  8 pounds per gallon.  So a bit over 8000 BTU per gallon.

One BTU is the heat to raise 1 pound of water 1 degree F.  

A typical small window air conditioner is about 5000 BTU per hour cooling capacity. 

[Utah20Gflyer]

19 hours ago, Bolter said:

@Utah20Gflyer Do you have the guppy mouth enclosure?  I believe that improves cooling flow efficiency as well as drag.

Yes, I have the closure.  

[Utah20Gflyer]

11 hours ago, Pinecone said:

Latent heat of vaporization of water is 1044 BTU per pound.  8 pounds per gallon.  So a bit over 8000 BTU per gallon.

One BTU is the heat to raise 1 pound of water 1 degree F.  

A typical small window air conditioner is about 5000 BTU per hour cooling capacity. 

Thank you, this is helpful.  
 

According to google a gallon of gasoline produces 120,000 BTUs.  My plane burns about 16 gallons per hour at full throttle prop at 2700 rpm.   That’s 1,920,000 btus per hour or 32,000 btus per minute.   If you applied one quart of water per minute that would provide 2000 btus of cooling.  That is a 6.25 percent reduction.  
 

A 6.25 percent reduction of a 420 degree CHT is 26.25. I know napkin math isn’t going to be accurate but theoretically that might reduce a 420 degree CHT to 393.75 degrees.  Maybe?

I have thought about this quite a bit since yesterday and my biggest concern is unevenly applying the water to the cylinders causing uneven thermal expansion and subsequently cylinder cracks.  Vintage Mooneys are known for their terrible cowling airflow including air coming back out after it went in.  The question of where and how to introduce the water to ensure proper air/water mixing and even cooling is the most difficult question.