CHT Oscillation

[OR75]

Anyone have an explanation of have seen similar behavior on CHT ? engine is an IO-360-A3B6D.

#4 cylinder

EGT is a straight line

 

 

[carusoam]

There are two parts to this question...

1) heat generation.

2) heat dissipation.

Heat generation is usually shown with the EGT charts.  If they are all the same, we go to the CHT charts to find why the cooling of two cylinders is different than the others.  

Share the EGT chart!

If the EGTs are close, it is an airflow/cooling challenge.  If the EGTs are equally wacky, cleaning FIs could be a solution.

Then there is the follow-up.  Is the data real, or is it altered by the probes due to age or cleanliness. Temporarily swapping sensors can shed some light.

Typical PP knowledge, not a mechanic...

Best regards,

-a-

[KSMooniac]

CHT typically cannot change very quickly, must less oscillate, due to the thermal mass.  I can't tell what the time scale is on your chart, but I'm assuming the oscillation is fairly rapid.  I suspect you have an instrumentation issue, and if you have a JPI system I would start by disconnecting the probe from the harness which has a small ring terminal on each wire and a screw, star washer, and nut.  The star washer must be BETWEEN the ring terminals, and of course tight and free from corrosion.  If you have a different system, then I'm not sure where the weak point is.

[N601RX]

Electrical noise or probe/connection intermittent.  Also be aware that the compression algorithm JPI uses for their data rounds most of the numbers up and down by multiples of 6 which may make things like this worse than they really are. Look at the raw data for the CHT and you can see this.

 

[carusoam]

Two engineers looking at a piece of art....

One sees sinusoidal oscillations on a single CHT, that is odd.

The other sees a 50°F temperature spread between CHTs, that is odd.

What is the oddity that the OP is seeing? (Could be a third thing altogether)

Best regards,

-a-

[PaulM]

Check the wiring to the thermocouple.  I had a TIT probe (Type K) bounce about 30-40 degrees.  It was a loose screw+nut on the wire where the probe leads are connected to the wire loom. 

for example : http://www.aircraftspruce.com/pages/in/probesandsenders_cht/gem3.php 

(as previously mentioned above). 

My mechanic pointed to probe X and said, all tight.  I pointed out that was an EGT probe on the exhaust riser and not just before the turbine. We pulled the lower cowl and found the TIT probe and tracked down the loose connection. 

As for the differential.. well that would need to know the probe types, location (some are washers, others are probes. )
 

[OR75]

so here is the CHT and EGT tracks. Time scale so the oscillations on CHT 4 have ~ 1 mn period

Insight G2 engine monitor

Cylinder 3 is the hottest and 1 the coolest which makes sense

In one of his article , Mike Bush links an EGT oscillation to valve rotation and possible hot spot / warning

 

 

[carusoam]

The saw tooth nature of the green CHT4 has been discussed before.

EGT3(red) stands out a bit.  But EGT numbers are not so critical, how is the GAMI spread?

Anyone remember the saw tooth graph situation?

This thread was a sawtooth EGT or TIT, I believe.  See if it is similar...   https://www.savvyanalysis.com/flight/487165/3ee05526-c621-4950-b2ec-e39ee45c4002

Best regards,

-a-

 

[Stephan Kablitz]

HI, OR75!

We recently have dicsovered a similar pattern om our IO-360-A3B6D. Did you ever find out the cause of your oscillating CHT#4?

Brgds, Stephan

[Fly Boomer]

On 11/28/2022 at 10:41 AM, Stephan Kablitz said:

HI, OR75!

We recently have dicsovered a similar pattern om our IO-360-A3B6D. Did you ever find out the cause of your oscillating CHT#4?

Brgds, Stephan

You should include a tag for @OR75 like this.  Also, be patient -- This topic is 6 years old.

[carusoam]

Welcome aboard Stephan!

OR75 was around here earlier today…. 

If you want to…

You can discuss your observations here…

Often, people download their data, send it to Savvy… then share the link here…

or post some pics….

Best regards,

-a-

[Stephan Kablitz]

The frequency of the oscillation suggests, it has something to do with the exhaust valve. And we do have a slight leakage at the exhaust valve at TDC, however no leakage at all at BDC. And since the leakage occurs only when the engine is warm, we believe the root cause to be the hydraulic tappet.

I am still puzzled however, that we see the oscillating signal at the CHT#4 probe but not at all at the EGT#4 probe.

 

Edited December 23, 2022 by Stephan Kablitz

[Guest]

7 minutes ago, Stephan Kablitz said:

The frequency of the oscillation suggests, it has something to do with the exhaust valve. And we do have a slight leakage at the exhaust valve at TDC, however no leakage at all at BDC. And since the leakage occurs only when the engine is warm, we believe the root cause to be the hydraulic tappet.

I am still puzzled however, that we see the oscillating signal at the CHT#4 probe but not at all at the EGT#4 probe.

 

Look for loose connections on the CHT wiring, and loose baffles.

[Fly Boomer]

@Stephan Kablitz I am not as knowledgeable as the others here, but I did run across this topic:

https://mooneyspace.com/topic/43031-bad-probe-or-something-more-sinister/

I believe both @M20Doc and @kortopates have commented in this topic.

Also, this article by Mike Busch discusses oscillation, but I believe he is observing varying EGT:

https://resources.savvyaviation.com/wp-content/uploads/articles_aopa/AOPA_2017-02_actionable-intelligence-from-big-data.pdf

And this one on baffling:

https://resources.savvyaviation.com/wp-content/uploads/articles_aopa/AOPA_2020-12_its-baffling.pdf

 

[skykrawler]

2 hours ago, Stephan Kablitz said:

The frequency of the oscillation suggests, it has something to do with the exhaust valve. And we do have a slight leakage at the exhaust valve at TDC, however no leakage at all at BDC. And since the leakage occurs only when the engine is warm, we believe the root cause to be the hydraulic tappet.

 

Easy to swap the 2 and 4 CHT probes and see if the problem moves or stays on the cylinder.

If you have evidence of an exhaust valve leak it should be verified with a bore scope and resolved.  Flying with a leaking exhaust valve is asking for trouble.  It can lead to an off field landing and exposure to high percentages for bad results.

[Marc_B]

1 hour ago, Fly Boomer said:

And this one on baffling:

https://resources.savvyaviation.com/wp-content/uploads/articles_aopa/AOPA_2020-12_its-baffling.pdf

 

My bet is it's baffle related.  Been discussed multiple times in the past.  Sawtooth EGT is perhaps valve problem, but sawtooth CHT is likely probe or baffle related.

 

[N201MKTurbo]

If the thermocouple wires are shorted together somewhere other than the tip of the sensor, it will measure at the short. This can account for the CHT changing, but it is hard to explain why it would be so periodic.

[skykrawler]

7 hours ago, N201MKTurbo said:

If the thermocouple wires are shorted together somewhere other than the tip of the sensor, it will measure at the short. This can account for the CHT changing, but it is hard to explain why it would be so periodic.

Is it not true that EGT is thermocouple (produces voltage) and CHT is thermistor (changes resistance)?

[N201MKTurbo]

52 minutes ago, skykrawler said:

Is it not true that EGT is thermocouple (produces voltage) and CHT is thermistor (changes resistance)?

All the scanners I’ve worked on use type K thermocouples for everything.

The factory CHT sender was a thermistor and the factory EGT was a thermocouple.