EGT Mystery

[skippydiesel]

I have two EGT probes on my Rotax 912ULS (just went 90 hrs) read by a Dynon Skyview.

 

About 10 hrs ago, one of them started to peak at over temp. This coincide with a remodeling of the exhaust pipe that the probe is in. 

 

Checked and double checked everything, including carby balance - no apparent cause.

 

Swapped EGT probe from left to right exhaust - problem followed EGT - problem must be defective EGT (In my limited experince defective EGT's under read or fail).

 

Replaced probe (today) with new Dynon supplied part. Most of the time the EGT's read one a little higher than other, Unfortunatly still have problem.

 

Took note of when it overreads - Happens when Airmaster CS, switched from Cruise to Climb  and rpm reduced for a gradual descent (wiered) EGT went into the red (again). Increase rpm -  EGT temp drops. Reduce rpm further, to say 3500. EGT drops

 

Any suggestions???

[turboplanner]

14 hours ago, skippydiesel said:

I have two EGT probes on my Rotax 912ULS (just went 90 hrs) read by a Dynon Skyview.

 

About 10 hrs ago, one of them started to peak at over temp. This coincide with a remodeling of the exhaust pipe that the probe is in. 

 

Checked and double checked everything, including carby balance - no apparent cause.

 

Swapped EGT probe from left to right exhaust - problem followed EGT - problem must be defective EGT (In my limited experince defective EGT's under read or fail).

 

Replaced probe (today) with new Dynon supplied part. Most of the time the EGT's read one a little higher than other, Unfortunatly still have problem.

 

Took note of when it overreads - Happens when Airmaster CS, switched from Cruise to Climb  and rpm reduced for a gradual descent (wiered) EGT went into the red (again). Increase rpm -  EGT temp drops. Reduce rpm further, to say 3500. EGT drops

 

Any suggestions???

Summarising what you've said, you remodelled the pipe and since then it has been running hotter than the other one with an EGT probe?

[sfGnome]

If the problem followed the probe swap, then it shouldn’t be related to the exhaust remodelling. Is it fair to assume that when you swapped the probes, you didn’t swap the wiring at the same time? That precludes noise spikes being picked up by the wires as they didn’t change while the reading did. Hmmm… I join you in your confusion.
 

Out of interest, are the probes simple thermocouples that require correct thermocouple wire all the way back to the meter, or do they have inbuilt electronics so they can just be connected with ordinary wire?

[skippydiesel]

16 minutes ago, sfGnome said:

If the problem followed the probe swap, then it shouldn’t be related to the exhaust remodelling. Is it fair to assume that when you swapped the probes, you didn’t swap the wiring at the same time? That precludes noise spikes being picked up by the wires as they didn’t change while the reading did. Hmmm… I join you in your confusion. Correct
 

Out of interest, are the probes simple thermocouples that require correct thermocouple wire all the way back to the meter, or do they have inbuilt electronics so they can just be connected with ordinary wire? The probes & wiring has been  supplied by Dynon (compatible with their Dynon Skyview)

 

[IBob]

You said the problem 'followed the EGT' when you moved the probe. Did you mean the problem remained at the same pipe.....or it moved when you moved the probe?

Some thoughts:
1.The EGT probes are a standard K type thermocouple, two dissimilar metals welded together at the tip, produce a small voltage when heated. So not hard to check with a multimeter set to mV. There will be lookup tables for temp vs voltage.

2. It is my understanding that those same metals must be must be used all the way back to the Dynon. Red and yellow I think they are. Asking the stupid question: you don't have a join where the red/yellow have been crossed up?

3. The exhaust gases will not pass down the pipe in a uniform manner, especially close to the heads. Are the probes installed as recommended, and fully into the pipe? Could there a be leak there resulting in a hot spot?
 

4. If all that checks out, it does seem likely that it is associated with the remodelled pipe and/or anything that was disturbed at that time.

[skippydiesel]

1 hour ago, IBob said:

You said the problem 'followed the EGT' when you moved the probe. Did you mean the problem remained at the same pipe.....or it moved when you moved the probe?

 

Moved with the probe/thermocouple

Some thoughts:
1.The EGT probes are a standard K type thermocouple, two dissimilar metals welded together at the tip, produce a small voltage when heated. So not hard to check with a multimeter set to mV. There will be lookup tables for temp vs voltage.

2. It is my understanding that those same metals must be must be used all the way back to the Dynon. Red and yellow I think they are. Asking the stupid question: you don't have a join where the red/yellow have been crossed up?

 

No system worked as expected, until I refitted the modified exhaust pipe. Blamed the pipe initially but found problem moved with probe - so not the pipe.

3. The exhaust gases will not pass down the pipe in a uniform manner, especially close to the heads. Are the probes installed as recommended, and fully into the pipe? Could there a be leak there resulting in a hot spot?

 

Probes installed as per Rorax manual (both exactly same distance from flange). How would a leak result in a hot spot?
 

4. If all that checks out, it does seem likely that it is associated with the remodelled pipe and/or anything that was disturbed at that time.

 

Problem moved from left to right when I swapped probes so unlikly to be exhaust pipe

 

Food for thought from correspondent in Rotax Owner Forum:

 

If we assume your probes are OK, and the high EGT is actually occurring, then the conditions you describe make perfect sense.  EGT represents a measure of energy wasted through the exhaust, and CHT represents energy used within the engine to produce thrust.  When you quickly reduce the prop pitch, this unloads the engine and you would expect to see an increase in EGT and decrease in CHT.  Once you increase RPM to fully load the engine at the reduced prop setting, the energy is once again directed to thrust, and EGT would decrease while CHT increases.  Think of it this way, the combustion heat has to go one place or another.  Unload the engine and the heat goes out the exhaust, load the engine and it goes into the heads and out to the radiator.  

Because we don’t have control of mixture, we tend not to think about these principles as much.  With conventional aircraft that have mixture control you see these effects more clearly.  It’s a common mistake for a pilot to unload the engine to attempt to reduce EGT, only to experience the opposite effect.

 

My response:

 

Thanks for that excellent reasoning .

There is one problem - the high readings followed the swapping of the probe from left to right.

Following your logic the low reading (assumed to be correct) probe must be the faulty one ??????

[skippydiesel]

Update:

 

After much discussion, it seems at least possible that, on reducing power (high inlet vacuum) a small air leak could be causing a momentary lean condition = higher temperature on that side.

 

I closely inspected the left (offending) side carburettor & inlet manifold - found, what may be, fuel residue around the balance tube fitting

 

Removed fitting, cleaned thread and reinstalled with HT thread sealant.

 

Unfortunatly persistent rains have rendered the airfield unusable - a situaton that is likly to continue for at least 10 days after last rain (forecast for this Wednesday). Assuming no more rain on already waterlogged soil, I will be able to test fly at that time - living in hope.😈