Hi all, I'm new to this forum... all good stuff. I am what you call a "turbine nut" in that my R/C skills are only good enough to fly an electric foamie about the park, but I have machined an MW54 and a turboprop, just to enjoy running them on the bench. I also developed an ECU, which is successful and which I use on my own turbines. Some of you may have seen my hobby web site at 5bears.com. Nothing commercial, just someone who loves jets.

Anyway, over the years, I have struggled with thermocouple placement and the theory behind the EGT's we so carefully track. Obviously, EGT is critical. Those of you like myself who have done a LOT of bench testing know that minute variations in T/C placement can cause MAJOR changes in displayed EGT. For example, the extension of a 1/16" type K probe through a cone into the exhaust stream, if varied by as little as a millimeter or two, may cause an EGT swing of 50 to 100 degrees. Likewise, radial position can have an equal effect. Manufacturers often give rough ideas of location, but the result isn't satisfactory. It is too easy to deceive ourselves with a bit of fiddling to measure the lower EGT.

Initially, I would locate the hottest portion of the exhaust and locate the probe there. This is conservative but limiting in that it is possible that in doing so you are essentially "derating" the turbine. Likewise, finding the coolest isn't smart. One could, in an extreme example, locate the probe 8" downstream of the turbine wheel and find a nice cool EGT but the critical components are certainly seeing higher.

My final theory is this... given that the turbine wheel is rotating, if the "hot" spot is at 7:00 looking up the pipe, each blade will pass through this hot zone, and likewise rotate through a cooler zone, and thus see an average temp. Probe tip extension in the stream should be tested, finding the hottest, then the coolest, and basically locating it in the middle of this range. Since the wheel is a rotating metallic mass, it will ultimately absorb an average amount of "work" from the hot gasses, and I think a probe placed in this manner more closely measures the true thermal load of the wheel.

Has anyone else given this thought after noticing how sensitive probe placement actually is?

When I assembed my JG 100 some years ago, I found the hottest place at idle was at the 4 O`clock position. But at full throttle, it was at 8. I thought this was because the higher case pressure, meaning the higher pressure straightened up the exhaust stream.
My experience is limited in this topic, so add plenty of room for my explanation.
Anyhow, I placed the probe at the 8 O`clock position, because 80% of the fuel is burned between half and full throttle. This worked fine for me, but my engine is not critical in temps at all.
I could be wrong again, but I think a turboprop has a hotter run, because of the resistance the exhaust meets to turn the drive and prop..?

Regards

Gudmund

Good topic

After a repaired or a built engine.
I always set my temp probe at the coolest spot on the engine. But my overheating test is simply, By looking at the turbine blades when the engine runing at idle tell me alot about the operating temp. If the blade shows low orange glow or none at all then my low end temp is ok

I really cant see the critical point in this, as long as it doesn`t exeed the max temp setting on the ECU?
If the probe is placed somewhere you get a very high temperature, I wouldn`t be worried as long as is never exeeds the max setting.
Temps close to the max is rather rare in the first place. High idle temp may slow the acceleration, but then again the settings would not be optimized.

But can you really measure a higher temp on the probe, than the combustion is inside the chamber? ( Due to a badly placed probe)

Regards

Gudmund

Gudmund,

The work taken out of the exhaust stream to turn the the turbine will result in a cooler temp exhausting the turbine.

Larry

Thats right. But is there a chance for measuring higher temps than the temp of the exhaust? By too much of the probe in the stream?

Gudmund

What is the ideal depth in average on most production engines, 1/16th, 1/8th,??? Russ

I have them 2 mm out in the flow.
Gaspar may say some words?

Regards

Gudmund

I have found that if you do not have at least 1/4 inch ( 6mm) in the flow, you are not getting a true reading. I can run my BMT with the probe about 1/8 in (3mm) in the exhaust flow and lower the egt by 200 deg. I checked the EGT with a seperate Digital pyrometer and the true temp was measured with the probe at least 1/4 inch in. At least this is true with the BMT.
I ran Ram engines a long time ago and I know that the EGT was much higher than the ECU displayed. Once again I checked the EGT with the pyrometer. In my opinion, you are "fooling" the ECU into seeing a lower EGT if you have only a small length of the probe inserted. BTW, I am in no way bashing the RAM or anyones engines or electronics.

Maybe Andre at BMT will chime in


Mickey
A-1 Hobby

Gudmund,

The location of the highest temp in the exhaust stream will probably vary for each engine. The only way to locate it is by trying different locations of the thermocouple or put multiple thermoucouples in the engine. The important reason for monitoring the temp is to note any changes in the temp at the same location which might indicate a problem somewhere in the engine. Myself I would like to see thermocouples at various locations in the engine, eg the turbine inlet, the compressor inlet and outlet, the combustion chambers and the ambient freestream total temp. I am just the type that would like more instrumentation for more info

Larry

It would certainly be safe to locate the probe so as to measure the absolute hottest stream that you can find, but I guess what bugs me is that if you do this, you are limting throttle response and possibly derating the engine. ECU's generate fuel rampings based upon EGT and RPM. During acceleration, if the fuel is introduced briskly, the ECU will have to cut the fuel increase rate as the EGT begins to bump up against the programmed maximum. That is why I think a more "average" exhaust stream placement is more indicative of the heat/work that the turbine wheel sees due to it's rotation.

It wouldn't bother me so much if the variation was, say, +/- 25 degrees, but it is more like +/- 150 with even modest repositionings. I've often wondered how it is done on the big turbines. The ideal solution is multiple averaged probes, but that is impractical.

5bears,

On the big turbines they do use multiple thermocouples, generally three, but sometimes more and they are averaged. They are located to give an average EGT for the pilot's info, I realize that is not much help for what you want.

Larry

I agree to to that one. And therefore my probe is just the 2mm in the stream.
The average is near, and since the temp is ok by wiewing the chamber after the service, I`m not worried at all. Works for me, and thats it.
If you find the average spot, then go for this one.
Homemade turbines are mostly copyes on chambers, but even small changes are displayed in temps. Experiment with it if you are not satisfied, finally in the end you will have the average.

Regards

Gudmund