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Fuel pressure

in 912iS Technical Questions 8 years ago

  • Re: Fuel pressure

    by » 8 years ago

    On my side, the pressure drops are way steeper and more worrysome, and we have checked the actual pressures as registered in our UMA sender and they check.
    The airplane is a Bristell barely 25 hours old. We originally had a Rotax fuel pressure sensor that was indicating ridiculously high pressures on my Challenger MGL iEFIS, but got a UMA one (100 psi) that is compatible with my EFIS (I have no dedicated engine monitor), and they stabilized somewhat in normal ranges. We don't have analogical pressure gauge, but have checked that after the pumps the pressure is the one that is showing in the iEFIS.

    I got 23-26 HG" readings from the manifold pressure in the MGL in an ill-advised test flight up to 7000 feet where the fuel pressure dropped to the 0.7-1.0 ranges, when I decided that discretion was the better part of valor and descended immediately.
    BTW, we exchanged the mechanical fuel pressure regulator with one cannibalized from another engine that our Rotax dealer had, and no dice. Same results.

    If anyone have any idea to diagnose this problem, I will be very grateful.
      
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  • Re: Fuel pressure

    by » 8 years ago

    This is just my two cents worth, but…..

    The fuel pressure regulator sets pressure by reference to the manifold pressure in the air box (see heavy maintenance manual).

    The fuel pressure we measure is supposed according to Dynon be measured between atmospheric and fuel ( a differential fuel pressure sender, not absolute).

    So the fuel pressure indicated is going to vary depending on the different between manifold pressure and atmospheric. If you use an absolute pressure sender instead of a differential, then the fuel pressure indicated is also going to vary with weather and altitude.


    This last bit - varying with altitude, might explain Carlos issue with flying at 7000 ft.

    If a differential sensor is used, and the pressure drops below limits when the engine is running, and the sensor is in the right place - before the final fuel filter, then I would want to check that the system can flow the required flow rate (90 l/h?) and that there is no blockage in the gascolator, fuel valve, fuel tank or a kinked hose. I would do this because a partial blockage might turn into a full blockage with negative results.
      
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  • Re: Fuel pressure

    by » 8 years ago

    Dear All,

    I have the same problem on my Sting S4 with Rotax912is, even with both fuel pumps On.
    When I am turning on base and then to final with engine in Idle, the pressure drops to
    2.45/2,5 bar. The engine becames a little bit rough. In the next days my mechanic will be in
    Rotax and will discuss about this issue and I let you know.

    Regards

    Stefano
      
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  • Re: Fuel pressure

    by » 8 years ago

    Hi Stefano

    Looking forward hearing about your findings.

    Here is what I got from the airplane factory. It fits very well the explanation with the differential pressure.
    I have Garmin G3X Touch displays in the plane.

    Kind regards, Claus


    Concerning the fuel pressure:
    The fuel pressure indicated on Garmin is low, because the Garmin does not know to figure out differencial value of the fuel pressure towards airbox. It is displaying the pressure in absolute value.

    The reduction valve, which maintains the fuel pressure on 3 bar, is connected to airbox. It maintains exactly 3 bar pressure towards manifold pressure. This is important for correct function of the injection.

    E.g. the Rotax-Integra device figures out the differential value of the fuel pressure before it is displayed on the screen. The methodics of such re-calculation was submitted to Integra´s producer directly by Rotax as the background for Integra´s development. The Garmin does not know such re-calculation and so the displayed fuel pressure value will be always significantly inexact and will depend on surrounding air pressure, air temperature and flight altitude.
      
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  • Re: Fuel pressure

    by » 8 years ago

    Dear Claus,

    thank you for your quickly answer. I have also G3x touch in my airplane.
    In my airfield there is another sting S4 (so I am pretty sure that the fuel system
    is the same like mine) with Dynon avionics, and the pressure is always 3,0 (+/- 0,1) bar
    even at low rpm or in idle.
    Do you know if Dynon avionics has inside algorithms that take into the account the differential pressure?

    thank you

    Kind regards.

    Stefano
      
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