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914 fuel/manifold differential pressure.

in 912 / 914 Technical Questions 13 years ago

  • Re: 914 fuel/manifold differential pressure.

    by » 6 years ago

    Yes, go with the 963143
    Thank you said by: Edward Harris
      
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  • Re: 914 fuel/manifold differential pressure.

    by » 6 years ago

    The 914's Fuel pressure is regulated relative to the AIRBOX Pressure, Not the MAP Pressure.

    From the 914 Operators Manual...
    [quote]The fuel pressure control ensures that the fuel pressure is always maintained approx. 0.25 bar (3.63 psi) above the variable boost pressure in the “Airbox“.[/quote]
    Similar verbiage also appears at multiple places in the 914 Installation Manual.

    The Hard (Metal) crossover tube connecting the Intake Manifolds after the carburetors and used for Carb Sync will have a lower pressure than the Airbox and will produce a false High-Pressure reading that will vary with throttle position.

    The Soft (Rubber) Crossover Tube connecting the intake sides of the Carburetors is connected to the Airbox and would be a more appropriate place to measure the differential pressure.

    The Soft Hose from the Fuel pressure regulator to the Airbox is the de facto reference for fuel pressure Regulation.
    This would be the preferred pressure reference source.

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

    Thank you said by: Edward Harris
      
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  • Re: 914 fuel/manifold differential pressure.

    by » 6 years ago

    Personally, I would trust Rob's experience over the "verbiage" of the Rotax documentation (considering how little effort Rotax has historically put into clear and accurate English translations). I had my 914 (which I have not started yet) plumbed with the manifold pressure sender going to the crossover tube and the differential fuel pressure going to the airbox. On Rob's advice, I am changing that. I will be putting in a tee fitting on the crossover tube and running both senders to that tee.
      
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  • Re: 914 fuel/manifold differential pressure.

    by » 6 years ago

    Bill - If I'm understanding what you're recommending, you're saying to tee off of the pressure connecting lines mentioned in 73-00-00 section 2.7 that vent the float bowls to the airbox via the solenoid? Isn't that just going to read ambient pressure unless the solenoid is open? I don't see another crossover tube on the intake side.

    Or are you suggesting to tee off of the line that connects the 'air side' (bottom) of the fuel pressure regulator to the airbox?

    Neither my A&P or I have installed a differential pressure regulator.

    More context: I am installing the fuel pressure gauge to troubleshoot an intermittent stutter at wide-open-throttle in a climb that thus far I haven't been able to reproduce on the ground. It's been very vexing. It seems that, intermittently, the 914UL will begin to stutter a bit. Reducing the throttle a bit makes the stutter go away - so far. After the last set of troubleshooting work I flew 8 touch and gos - no problem. Next day, on #5 it stuttered again. So that's 1 for 13. For a while I thought it correlated with fuel quantity and/or pitch attitude but other than being in a climb and at wide-open-throttle that doesn't seem to be 100% the case.

    I don't want to bank on the fact that it will *continue* to recover when the throttle comes back, and to be honest flying a 400-foot pattern once it happens is getting old and harshing the fun of flying off my 40 hours. So I'm planning on installing a fuel-pressure sensor and a couple of EGTs and doing additional flight testing with a laptop connected to the TCU to try and get better telemetry once and for all. Total engine time < 10 hours. We've already inspected (and rebuilt) the fuel system and done multiple inspections of the floats and carb bowls, suspecting debris.

    Any advice appreciated.
      
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  • Re: 914 fuel/manifold differential pressure.

    by » 6 years ago

    Bill - If I'm understanding what you're recommending, you're saying to tee off of the pressure connecting lines mentioned in (Heavy Maintenance Manual) 73-00-00 section 2.7 that vent the float bowls to the airbox via the solenoid? Isn't that just going to read ambient pressure unless the solenoid is open? I don't see another crossover tube on the intake side.

    Or are you suggesting to tee off of the line that connects the 'air side' (bottom) of the fuel pressure regulator to the airbox? ...


    Yes! and Yes! Either point would be appropriate. Option #2 would be my personal choice.

    The float Bowl pressure must always be the same as the mouth of the carburetor.
    This pressure will be the local ambient pressure unless the Turbo is boosting.
    The fuel pressure must always be 3-4 psi above the float bowl pressure to be able to refill the float bowl.
    The fuel pressure might be 3-4 psi at idle or 13-14 psi at boost.
    In all conditions, it will be 3-4 psi ABOVE the Float bowl pressure which in turn is the same as the Airbox pressure.

    The Fuel Pressure Regulator is also referenced to the Airbox pressure for the same reason.
    It only makes sense that if you are going to monitor the fuel pressure that you measure it from the same reference pressure that the fuel pressure regulator is using.

    Heavy Maintence Manual Chapter 73-00-00, Section 3.4.2, Page 39 ...

    [quote]CAUTION: The fuel pressure must be approx. 250 hPa above
    the constantly varying boost pressure in the “airbox”.
    Otherwise the perfect functioning of the carburetors is not assured. See 73-00-00 sec. 2. [/quote]

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

      
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