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Thread: Rotax 912 IS Fine filter sub assembly

  1. #1

    Default Rotax 912 IS Fine filter sub assembly

    Dear All

    I have tried to make sense of this but not knowing what is going on inside the supplied manifold I cant figure out how this works.

    Regards,
    Anton.


    fine filter sub assembly.JPG

  2. #2
    Senior Member efwd's Avatar
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    Default Re: Rotax 912 IS Fine filter sub assembly

    There is nothing going on inside the manifold except passage ways for fuel to flow. The 180 degree turn after the filter is confusing. Fuel comes from the pump, enters the manifold. From there you see it pressurizes the manifold for which there is a fuel pressure sensor that mounts on top of the manifold. Fuel will flow across that sensor port and through the filter due to less resistance and exit the T-fitting identified by "XG" that feeds the engine. If the fuel filter becomes obstructed, the fuel will bypass the filter through the second route that parallels the fine filter. The fuel turns that 180 turn and out the "XG" fitting to the engine.
    Eddie Forward
    Flying
    SS7, 912iS, Garmin G3X

  3. #3

    Default Re: Rotax 912 IS Fine filter sub assembly

    Dear Eddie
    Thank you but it doesn't help. There is a check valve named XD. Surely the valve will stop the flow straight to the engine. This means if the filter gets blocked the whole system is blocked. If the check valve allows flow to the engine, nothing will go through the filter unless the chech valve is limited in flow rate and therefore causes a fuel pressure increase. Am I missing something here?
    Last edited by anton.willemse@gmail.com; 05-18-2022 at 01:03 PM.

  4. #4
    Senior Member Eric Page's Avatar
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    Default Re: Rotax 912 IS Fine filter sub assembly

    Quote Originally Posted by anton.willemse@gmail.com View Post
    Am I missing something here?
    Just one detail: the check valve doesn't see full fuel system pressure. Since it's connected to both sides of the filter (one side via the manifold and "XC" fittings, and the other side via the "XG" tee and "XH" tubing), it only has to hold against the normal differential pressure drop across the filter. As the filter gets blocked, that differential pressure rises closer to full system pressure, which is more than enough to overcome the valve spring, unseating the valve and allowing the engine to draw unfiltered fuel.
    Eric Page
    Building: Kitfox 5 Safari | Rotax 912iS | Dynon HDX
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  5. #5
    Senior Member jiott's Avatar
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    Default Re: Rotax 912 IS Fine filter sub assembly

    Anton, that check valve XD has a spring in it that allows it to open in the "free flow" direction only if the pressure drop across it exceeds 11.8-17.6 psi. If the fine filter clogs enough to exceed this pressure drop, the check will open and keep feeding unfiltered fuel to the engine.

    Sorry Eric, didn't see your spot on explanation.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  6. #6

    Default Re: Rotax 912 IS Fine filter sub assembly

    Thank you all. I've seen the light!

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