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I recently took delivery of an AutoGyro Cavalon equipped with an Rotax 915 iS 3A engine. My previous aircraft engine experience has been with Continental and Lycoming engines where cylinder head temps are the norm for monitoring the engine. And, hence,  I had ordered from AutoGyro the capability to sense/display cylinder head temperatures. I'm equipped with temperature sensors on all four cylinders but only, at the present, able to display cylinders 1, 2, & 3 cyl. head temps. The 4th sensor, on cyl. 4, is connected and displayed as water temperature. (All sensor readings are being displayed on a Garmin G3x Touch).

Are the reading on cylinders 1-3 really the cylinder head temperature? Is it possible that all the cylinder head temperatures are really a reading of the water temperature? Does it make sense to be looking at cylinder head temperatures on a water cooled engine? Could I expect to see a difference in temperatures from cylinder to cylinder?

Thanks in advance,

Hal

  • Re: Cylinder Head temperatures

    by » 6 months ago


    Unlike Continental and Lycoming engines, Rotax cylinder heads are liquid cooled, so you only use coolant temperature, no CHT. So, I’m betting they’re all showing coolant temp. Are they all reading the same? 


    Thank you said by: Woodruff Hal

  • Re: Cylinder Head temperatures

    by » 6 months ago


    CHT in a Rotax, at least the later versions is really Coolant temp. This does NOT have the same meaning as on an air cooled engine. I have done experiments on a four cylinder water cooled engine with temp sensors in each head and they are all within a couple of degrees of each other.

    Being water (well glycol/water) cooled the changes in temp are much slower than in an air cooled engine and are not good for signalling an issue immediately.

    EGT is a quicker and more diagnostic tool

    Now that you are in the Rotax world you will have to "unlearn" some of the tenets you have been using in the "Lyconental" world.

    The worries about shock cooling, temp stabilising before shut down, mixture leaning etc all go away.

     


    Thank you said by: Woodruff Hal

  • Re: Cylinder Head temperatures

    by » 6 months ago


    It depends on the age of your engine, or at least the age of your heads.
    The early heads had the temperature sensor mounted on the BOTTOM of the heads and measured the temperature of the head's metal.
    About 5 years ago, the newer, Rev.1 heads changed to measuring the coolant within the head and the sensor mounting location was moved to the TOP of the head.

    If your temp sensors are on the TOP of the cylinders, you are measuring Coolant Temps. Bottom = CHT, Top = COOLANT

    While it makes sense to measure all 4 EGTs,  The coolant circulation is common to all 4 heads and the temps will be nearly the same in each.
    If One cylinder went 'Cold', the hot coolant from the other 3 heads would heat up the Dead Head to match.
    One(1) COOLANT Temp Sensor is Enough.  #4 usually runs 'Slightly' hotter than the other 3.

    There is no need to worry about shock cooling either.
    It will take a while for the gallon or so of coolant to lose its heat; keeping the heads from cooling too quickly.
    In addition, AutoGyro commonly installs thermostats for both the Coolant and the Oil systems.
    As the Coolant Temp falls below ~185°F the thermostat bypasses the Radiator to maintain head temps.


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.


    Thank you said by: Woodruff Hal

  • Re: Cylinder Head temperatures

    by » 6 months ago


    Thanks to all for the responses. Indeed the sensors are on the top of each cylinder. As such it would appear that I don’t have temps of the “metal” that I was expecting but 4 readings of the coolant temperature. All readings are within a degree of each other. It’s good to know about the lack of shock cooling concerns. 

    Again, thanks. 


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