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Oil pressure fluctuations

in 912 / 914 Technical Questions 6 years ago

  • Re: Oil pressure fluctuations

    by » 6 years ago

    Short update:
    Fluctuations are best and clearly seen with the mechanical gauge. The different properties of sensors and displays were misleading:

    The mechanical gauge is the fastest
    The VDO-Sensor is slower
    The new Rotor and the Kavlico-sensors are both way faster than the VDO
    The analog VDO-instrument is damped in a way that it masks fluctuations completely, its useless for our purpose
    The Dynon Display is way faster than the VDO analog, but with guessed 3 to 4 Hz barely fast enough to show the fluctuations. You see erratic changes instead of harmonic oscillations as with the mechanical gauge
    The Bluetooth-sensor with external display is also too slow.

    We checked the fluctuations: they appear in two (of two) Rotax 912S with 400 and 600h resp. tested when the oil is hotter than 65°. They are influenced, but not cured, by valve ball, valve mushroom, spring, even the bottom nut (long sleeve vs short sleeve). Oil pump housing new, Rotors old. Those didn't't seem to have much influence.

    Thus: don't look at your engine with a mechanical instrument, you won't like what you see. Bill is right.
      
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  • Re: Oil pressure fluctuations

    by » 6 years ago

    There are two pipistrel virus aircrafts here locally in Alberta with 912iS engines and dynon skyviews that have the same fluctuations. We both have good psi at low altitude but the higher we climb the lower the oil pressure becomes. The other guy replaced his oil pump and spring with no change so we know it’s not the pump or spring causing this. Possibly a dynon sensor or wiring issue? Only way we can be sure is to install an analog gauge and confirm what the oil pressure is at high altitudes. I’m seeing 16 psi at 14,000’. Still within limits but bairly, discomforting when crossing over Rocky Mountains here in Canada to have such low psi. Anyone else ever have this issue?
      
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  • Re: Oil pressure fluctuations

    by » 6 years ago

    @dzubot: you may do a ground run with as much throttle as possible waiting for the oil temp rising. I would challenge air pressure dependency, maybe oil temperature or rpm related. Dynon setup correctly for sensor type?
    16psi =1,1 bar (if real) are not airworthy. What are the values @ 2200 engine rpm? It is very easy to set the oil pressure higher.

    Rotas Franz, the distributor in Germany and IIRC West Europe tests engines 1. with mechanical sensor and damped mechanical gauge, 2. with actual rotax sensor and Rotax flydat 3. if in doubt with equipment to test the electrical sensor (like when you were using an oscilloscope).
      
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  • Re: Oil pressure fluctuations

    by » 6 years ago

    ...
    Rotas Franz, the distributor in Germany and IIRC West Europe tests engines 1. with a mechanical sensor and Damped Mechanical Gauge ...


    A Rhetorical Question...
    Why would he need to Dampen a Mechanical Gauge??? ;)

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

      
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  • Re: Oil pressure fluctuations

    by » 6 years ago

    @Bill: I agree. Now with the solution below, pressure in flight is ok at 3.9 bar


    To sum this topic up: we systematically examined most of the factors that could affect oil pressure indication in the cockpit.

    Factors may be:

    0. div
    Mass/sensor cable connections
    Vibrations, heat affecting the sensor

    1. Oil pump
    4- and five-star rotors of the oil pump
    Oil pump housing
    oil pump front cover

    2. Oil pressure regulation
    valve ball
    valve mushroom
    valve spring
    valve bottom screw (low/high sleeve)

    3. Pressure sensing
    a. analog purely mechanical non damped
    b. Old pre 12/07 VDO Rotax supplied sensor (still available from VDO)
    c. New Kavlico Rotax-supplied sensor
    d. Bluetooth-pressure sensor

    4. Pressure indication
    a. mechanical gauge for 3a
    b. analog electrical instruments for 3b and c
    c. Dynon D180 Display for 3 b and c
    d. mobile phone for 3d

    Results:
    I. Cabling and heat, vibrations could be excluded by external voltage, external cabling, synchronous readings of built in and externalized heated sensors.
    Pump rotors did not affect the readings
    II. Every other item listed above influences the pressure read in the cockpit.
    III. the Rotax engine has indeed marked oil pressure fluctuations, amplitude 0.3 to 0.6 bar. This is factory normal. Cause is most probably the hydrodynamics of the oil pressure valve, hence the influence of every part of the valve.
    IV. This can only be seen with a sensor-instrument combination fast enough to display the fluctuations.
    Fastest sensor is the Rotax/Kavlico, Rotax VDO is way slower, mechanical non damped gauge is fast, but gets in resonance quickly-useless.
    Fastest display is the Dynon, analog electrical instrument is way slower, Bluetooth was the slowest.

    Conclusion
    Thus, the disturbing values displayed with the Dynon/Kavlico-new Rotax combination are real but not seen before as the old sensor was to slow to show the fluctuations. Semi-official statement from Rotax confirmed these fluctuations to be seen on the bench regularly.

    Solutions
    Our several-thousand-dollar solution :( : old Rotax VDO sensor type installed at the firewall for protection against vibration, rock stable values with the Dynon-Display. I dislike the high pressure oil lines outside of the engine, but the alternative with new sensors would be looking at constantly changing oil pressure values while inflight.

    What Dynon might contemplate: add some averaging/damping at the display setup for the new sensors.

    Redesign of the pressure valve regulation. The low/high sleeve spring screw and the mushroom-like ball replacement show that Rotor tried to adress this in the past (to no avail).
    There are some promising ideas third party ideas which appear to work, but their user base is limited
      
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