The unquenchable EGT

17 posts in this topic

Posted (edited)

Since day one of my little planes new life I have been unable to get my cruise EGT temps to acceptable levels.

Here is the things I have done or tried:

1. Checked probe location and found that someone had placed the probes holes 3.5 inches out on the Exhaust Manifold. Welded up the holes and re-drilled them 38MM (1.49 inches) out from the face of the port. Installed new probes.

2. Rebuilt cards with new jetting and leaned the Idler circuit

3. Needle clip to the richest setting. Anything leaner that the richest position made the EGT temp rise over 1200 in cruise. 

4. Added pitch to the prop. This was the only thing that seemed to help but it took so much pitch that the performance was not acceptable for normal operation.  

5. Purchased a longer propeller to load the engine. Again high pitch setting was the only noticeable difference.

6. Purchased and installed one size bigger Jet needle. Didn't see much of a change to the mid range as reported by others. In fact I seen a slight raise in the temps.

7. Read every article I could find. CPS, Ultralight news, several independent articles, this form, Kitfox form, and the Rotax form. Most articles I copied and saved in my builders reference manual.

8. Monitored Plug color per Jimchuk. The plugs said I was OK so I continued to keep searching.

 

I was talking to a friend the other day and was explaining to him what I'd been dealing with. He asked if I had tried a larger main jet. I told him that my temps were good at WOT and I didn't believe this was the problem. He suggested I install the next larger main jet, run it at the current needle setting, lean the needle if needed and see if that had any effect on the my mid-range problem. At this point I was willing to try anything. I threw in the next bigger set of main jets and took it for a flight around the pattern. What did I see......hotter EGT temps than before. You have to be kidding me I thought!!! I thought adding fuel cools the circuit?? Why was it that every time I added more fuel with a jet temps showed hotter??? 

I went home, got my articles out and started reading through them again. I found the answer and it had to do with the installation of the EGT probes. If you use the 1" EGT probe (which I did)you have to shim the probe out with 7 AN-3 washers (which I didnt) so the tip of the probe is placed inside the manifold no deeper than half the diameter of the pipe. If you insert the tip of the probe all the way to the other side you are outside the cone of the flame. The cone of the flame (cooler part of the flame) is where the temperature is supposed to be taken. Anything outside of that area will produce hotter readings. As the article explained.... you will know this by raising EGT temperatures if you enrichen the circuit. When you add more fuel to the system the excess fuel gets pushed out of the cylinder and is burned inside the manifold. The more flame the hotter temp 

Now you can purchase probes with 1/2 inch long sensors right from Westach. The bayonet probes can be adjusted. Unfortunately I cut the excess clamp off which made one of them to short to add the washer between the probe and the pipe. I was able to install washers under one of my probes. So far it looks like this was the problem all the time. 

A lot of the headaches I've ran into with buying someone else's project was because things were installed incorrectly some of which I took apart and reinstalled back the same way. If you buy someone else's project it never hurts to double check the installation instructions on everything. 

I hope this will help someone who may have the same issue or as Joey called it is" Chasing a Mermaid".

Edited by NorthIdahoAvidflyer
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Posted

I thought the EGT probes were supposed to be 10cm out from the flange!  

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Posted

I was in the same boat when I got my plane. I got it in a April and didn't fly until October because of the mess the previous owners made. It definitely sucks but you'll get a better understanding of your plane if you rip into everything and inspect it yourself. 

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Posted

I thought the EGT probes were supposed to be 10cm out from the flange!  

Not sure where you got those numbers. 38mm or 1.49 inches from the flange face.

exhaustb.jpg

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Posted

I was mistaken.  the 1990 version of the 582 manual states, on page 15: "Cylinder head temperatures are measured at the spark plug seat.  Exhaust gas temperatures are measured at 100 mm (3.94 in.) from the cylinder sleeve."   It would seem logical to use the flange as the reference for these probes, but no!  I had now better check the locations of my EGT probes. Thanks for the correction.

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Posted (edited)

Just to muddy the waters even more, the 1990 build manual says to drill the holes for the EGT probes 4" away from the lower edge of each flange, measured along the bottom centerline of each branch of the "Y" (this is indicated in a sketch).  It also says, in a footnote: "Absolutely precise location of EGT probes is not critical.  The attaching bands will 'effect' {affect} exact location.".  Oh Boy!  It appears likely that new things have been learned since 1990!   My probes are certainly not 38mm out from the flange center, more like 50mm.  So Vance, where did that drawing come from?  Does anybody else want to chime in here?  I've been believing that my EGTs were significant.  I was even contemplating a T-compensated digital gauge to replace my analog ones.  If location of those probes is more critical than it says in my 1990 build manual, I at very least will be more careful about keeping my EGTs away from the 1200F limit.   My probes are about 3-1/8" out from flange bottom, measured along the bottom of the Y branches on centerline.  Any further out, and the bands wouldn't fit.  . 

Edited by Turbo

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Posted

Find a newer engine to look at. The newer engines have prewelded bosses on the manifold in the correct location.

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Posted (edited)

Wow! Another clear-as-mud discourse on where to drill the damn hole!  It'd be tempting to just get a new Y-manifold with the holes pre-drilled, but for the doubtless exhorbitant price!  Isn't EGT really only about keeping the plugs clean without melting a piston?  If mine read high due to the probes being too far downstream, i reckon it's conservatve if the probe tips are centered in the ducts.  I think I'll validate the latter, then call it good enough!   Thanks for digging into this, Vance!

Edited by Turbo

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Posted

I found two other places where it said to place the probe out 100 mm from the piston skirt on a 582.  I believe you are right that the probes could be close to the specified location and you may not see a lot of temperature difference. When my probes were out at 3.5 inches the EGT's showed over 1300 every time I ran it. Mind you the tips of my probes were still stuck all the way through to the outer edge of the pipe so I could not really tell you what actual placement difference makes. It makes sense to me that if you put the probe in the hotter part of the flame that you will get a higher reading. From gas welding you will learn that the tip of the cone is the hottest part of the flame and the inner cone is the cooler part of the flame. It makes sense that the probe should be placed inside the center of the cone. The goal is to monitor the relationship between the mixture and the heat being exerted on the piston. I will be able to tell you more once I install the new probes how much of a difference tip placement really makes.  

 

 

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Posted

THe main problem is that we all post what we believe, or what we were told, or what we remember, so it becomes clear as mud. Then we say stuff like "wow it's confusing." No it is not confusing, it is as clear as can be, if we bother to just read what the manufacturer said to do. This is from the installation manual, written 19 years ago:

 

 

 

egtlocation.jpg

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Posted

Thanks, Nick.  That's nice and clear, unlike some other sources. 

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Posted

I received and installed the new probes yesterday afternoon. I took it around the pattern a couple times and it appears the new probes have decreased my EGT's by 100 degrees. This puts me in the 1100 range instead of the 1200 range. If anything else it makes it nicer to look at. 

Nick, Thank you for posting the picture. 100 MM from the piston skirt is what all the articles I read say. Not sure why the Avid Construction manual said to put the holes out so much further. I have two Rotax 582 manuals downloaded on my phone and either of them mention EGT probe placement. They are newer manuals. It does talk about EGT limits. They must be service manuals and installation manuals. I will find the specific installation instructions and download them also. 

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Posted

pretty sure the install and service manuals are on this site :)  

:BC:

 

 

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Posted

nice link!  Look under historical documents for the repair manual.

:BC:

 

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Posted (edited)

Update: I flew several hours this last weekend and I could not be happier with the EGT temps. I'm running a steady 1125/1125. Changing the probes to the right location made all the difference. I actually found myself watching the EGT's less and less over the weekend. 

After several months its so nice to find and fix the problem. Of all the things I read there was little mention of probe placement. The Devils in the Details. 

Thank you for all the advice.

Edited by NorthIdahoAvidflyer
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