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Thread: CHT/EGT selector switch?

  1. #11
    Birdseyeview's Avatar
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    Default Re: CHT/EGT selector switch?

    Jim, Thanks for pointing me in the right direction. I found the location of the EGT probe on page 10.1 of the Rotax 912S manual and the distance from the flange is stated as 2.76". It doesn't seem to say anything there about which cylinders to monitor. On page 21.1 of the installation manual it does mention that CHT should be monitored at cylinders 2 and 3 (one in front and one in back), in which both ports have a temp sensor installed. However, there's a CHT sensor port location on every cylinder head so this gets back to the basic question on what everyone is doing with CHT and EGT monitoring - Is the most common practice and/or recommendation to monitor CHT and EGT on all cylinders or just specific ones (#2 and #3)?
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  2. #12
    Senior Member Rodney's Avatar
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    Default Re: CHT/EGT selector switch?

    The EGT probe is a type K thermocouple

    Vktech 2M EGT Thermocouple K Type Temperature Probe Sensors Exhaust Screw Threads New
    by Vktech

    These are available on Amazon for about $18.00. I welded a 1/8" boss to the exhaust pipe and installed two on my engine. Plenty of cable to reach inside to the back of the instrument panel.

    Regards
    Rodney

  3. #13
    Senior Member jiott's Avatar
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    Default Re: CHT/EGT selector switch?

    Larry, CHT in cylinders 2&3 where the stock sensors are installed is where most of us leave them. EGT in the two rear exhaust headers is also standard for most folks. In my opinion, this is minimum but adequate. I do know of several that have installed EGT sensors in all 4 locations. I think this is more common for those people who have adjustable props and ignition/fuel systems where leaning and MP must be managed.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  4. #14
    Birdseyeview's Avatar
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    Default Re: CHT/EGT selector switch?

    Jim, Thank for the clarification. It is very helpful for me to know what works for others before I get into it.
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  5. #15
    Senior Member jrevens's Avatar
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    Default Re: CHT/EGT selector switch?

    Quote Originally Posted by jiott View Post
    Larry, CHT in cylinders 2&3 where the stock sensors are installed is where most of us leave them. EGT in the two rear exhaust headers is also standard for most folks. In my opinion, this is minimum but adequate. I do know of several that have installed EGT sensors in all 4 locations. I think this is more common for those people who have adjustable props and ignition/fuel systems where leaning and MP must be managed.
    I know this is going to raise a few eyebrows, but I have a couple of thoughts about this. First of all, the use of EGT measurement is a relatively recent development in the history of aircraft engine instrumentation (I’m showing my age here). Remember the history of Alcor? There’ll probably be disagreement with this, but I think that EGT is probably one of the least useful of engine monitoring instruments for a 4 cycle Rotax engine that has no manual mixture control. In case someone thinks I’m against technology somehow, I designed and built a simple comparator circuit that I used with an automotive oxygen sensor to compliment my EGT gauge on the Lycoming in my first homebuilt. That was well over 20 years ago before anyone I knew had done it. Then, consider that for over 30 years most Rotax 912 engines have somehow run just fine with only one CHT, or water temp probe. There are exceptions to every rule, and I’m not against having access to all the engine monitoring data you can, but saying that having a “minimum” of 2 & 2 is a necessity strikes me as possibly overkill. We live in an age where displayed data is over-abundant. I’m curious about how useful (and used) EGT is for the vast majority of 912 owners. I’m measuring EGT on one of my cylinders of my 912, but frankly I’ve thought about other things I could’ve done in place of that on my “old-fashioned” panel that would be more useful. For me it’s kind of like the uselessness of having an ammeter in an airplane, when a voltmeter is much more useful and can tell you everything you need to know in flight. Again, as an old guy, there is just way too much to look at on a modern glass panel for me. That’s my rant for the week.
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
    Chap. 43 honorary Lifetime

  6. #16
    Senior Member jiott's Avatar
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    Default Re: CHT/EGT selector switch?

    I agree with you John, the standard carbureted 912 has nothing you can do in flight to adjust EGT, so it is kind of useless. However, I like to look at it once in a while to make me feel good. As I see it, its only value in flight might be to give you some early warning if that cylinder is getting colder due to fouled plugs, ignition problem, fuel supply problem, etc.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  7. #17
    Birdseyeview's Avatar
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    Default Re: CHT/EGT selector switch?

    John, Thanks for the historical perspective. This is a good reminder to me of what is necessary vs what is nice. In my attempt to keep things as simple as possible in the cockpit it is often easy to get swept up in the idea that more data is always better. Although I do appreciate hearing what others are doing with their instrumentation, I'm finding it way more enlightening on why they are monitoring certain parameters and the associated logic behind their decisions. This logic is ultimately what drives the implementation of how to gather data and what to do with it once you have it. From what I've been gathering so far in my investigation EGT seems to be a lagging indicator of what's going on inside a 912 with a fixed pitch prop and may be most useful typically in troubleshooting engine issues after they occur. CHT seems to be more important. That being said, I haven't heard a compelling reason yet to monitor either CHT or EGT in every cylinder so I'm leaning toward the minimalist approach. From another historical perspective: my 912 is a 2001 vintage which apparently had coolant temp sensing that was treated as separate from the CHT, which was sensing the head casting temp. In some later engine versions it sounds like a head change was made so that CHT was sensed in the coolant passage. From stories I've heard it sounds like CHT and coolant temp are usually very close either way so I'm wondering if I could treat them as equivalent and only monitor CHT and not have the separate coolant temp sensor that my build manual calls for.
    Last edited by Birdseyeview; 11-27-2018 at 01:16 PM.
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

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