CHT again

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astaib
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Re: CHT again

Post by astaib »

Hello,

I tried to make also this test, but I wasn't able to record the results (no automatic pilot and always someone to bother me in the plane).
But out of my memory the spread was also less than 0.5 gph.
One question: how do you do to be very accurate at 0.1 gph? The mixture lever is not very smooth and unfortunately not as good as what we can find in other plane like cessna or piper with the mixture lever that you can turn?

I will try to make other test and record this summer during our holidays trips to Sardegna, Alps and Brittany.

Arnaud.
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perossichi
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Re: CHT again

Post by perossichi »

My cyl 1 would routinely run very hot (low 400s) when run ROP. Only way I could control it was to run LOP. At 40-50 LOP, I would still get 385-395. I tried everything to diagnose and fix the problem. GAMI spread is less than .3 gph.

After hearing about the Arizona baffle on this forum, I installed it and all problems with cyl 1 went away. All cylinders are now running below 380 even at full power cruise at low altitudes. Only very slow climbs can get temps high - such as repeated Chandelles.

Now cyl #4 is the hottest but I see no reason to be concerned with differences in cyl temps as long as all cylinders are running cool below 380.

I routinely run GAMI spread tests to check for clogged injector nozzles rather than having the nozzles cleaned at each annual— following the advice of Mike Busch. I get very similar results in each spread test in terms of measured GAMI spread. But find similar variation to Rich’s experience in the actual peak EGTs.
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astaib
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Re: CHT again

Post by astaib »

Hi Peter,

Thanks for that feedback. It's really encouraging.

I think that I will have to do install it too.

Arnaud.
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Re: CHT again

Post by BlueYonder »

This whole conversation is...surprising. I just flew from Seattle to Austin and back -- 11 days -- in a variety of conditions. Most cruise was at 22/2200 or 22/2300; fuel burn ran to 7.5- 8.0. But the CHTs were, as always, in the low 400s the whole way, with #3 coming in the hottest at 430-440 most of the time (and up to 470 if I bumped up to 2500 on climb).

This is how this plane (2007 XL) has run for the 2.5 years I've owned it. I can't imagine how y'all are getting such low CHTs; we seem to be doing the same things, but I'm consistently getting a much hotter result. Thoughts on what might be different?
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perossichi
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Re: CHT again

Post by perossichi »

I don’t mean to be alarmist but those are extremely high CHTs. You didn’t say what alt that was. You might try LOP if you are not running LOP already. At least 40 degrees LOP.
If I saw a CHT at even 410, I would immediately go very rich, put the fuel pump on, and throttle back and reduce angle of climb to increase climb speed. I try to climb out at no less than 90.

Redline is 500 but I’d be very worried about runaway temps at 420+. Detonation which could lead to Preignition.

I suppose it is possible that this is an indication problem not a real problem. But the fact that all cylinders have high temps would argue against that.

You should check baffle seals and the metal inter cylinder baffles. eSometimes baffle seals get folded over the wrong way or twisted. You can see black marks on the inside of top cowling from the black rubber baffle seals. It is also possible that one of the baffle seals is missing. The baffle seals create a pressure differential between top and bottom of the engine which creates airflow around the cylinder fins. They must have a snug fit around the whole top of the engine in order to work properly. cyl 3 is right side closest to firewall. Check inter cylinder baffle between 3 and 1.

I’d also check the timing. It is supposed to be 25 degrees before top dead center. It is possible your timing is advanced which can raise CHTs. Just a few degrees can make a big difference.
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Re: CHT again

Post by BlueYonder »

This is the third time I've taken the plane more than halfway across the country and back, and this is how it's run since I got it. I've run the problem by a variety of A&Ps, and nobody seems to have regarded this as a problem, let alone addressed it in a way that seems to change anything. (I had the baffle seals replaced two years ago.) So I finally just decided that this is how DA40s are -- you fly them with one eye on the CHT gauge all the time.

I'm running fairly leaned out, with the EGTs around 1400. Was I mis-trained on this? If I run it richer, of course, it cools down, but then I'm also up to a fuel burn of 9+gpm.

It's in a Diamond service center right now getting another issue looked at. I'll bring this to the mechanic's attention while they've got it. Open to anybody's guesses as to what might be going on.
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perossichi
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Re: CHT again

Post by perossichi »

To run LOP, you lean past max EGT.

ROP if you lean CHT and EGT will increase

LOP if you lean further both will decline.

Once you figure out where you want to run you lean on the basis of fuel flow.

It sounds like you are running LOP, but absolute EGTs are meaningless so I can’t tell for sure.

Since you have eliminated the baffle seal, that leaves intercylinder baffles and timing. Advanced timing could yield high CHTs and relatively low EGT. Retarded timing would yield low CHTs and high EGTs.
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perossichi
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Re: CHT again

Post by perossichi »

Also, Diamonds have efficient and very tight cowling so they should run cool as opposed to older aircraft.

CHTs are measured using thermocouples so if there is something wrong with connections etc you would get very erratic or low temp readings not high.
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Re: CHT again

Post by waynemcc999 »

Sara, I agree with Peter. Cruise CHTs of 430 are way too high. I've put 1600 hours (in 4 years) on my 2008 DA40 almost always cruising LOP with highest CHT below 375. I would think the issue is more one of cooling, than on how you're running the engine. On the other hand, if you're intending to run LOP, you may be running too close to peak and should lean even more to get the CHTs down well below 400.
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Re: CHT again

Post by astaib »

It is recommended to either:
-enrich after peak in order to be 100 degrees F less than peak
-lean from peak in order to be 20 to 50 degrees F less than peak.

Then and only then you read the fuel flow and consequently the CHT and see what happen.

If you have to enrich more than the values above to have CHT below 400 or lean more than the values above to also have CHT below 400, it can show a cooling, timing... issue or also that the outside temperature is too high.
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