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Fuel Pump - Pressure variation

in 912 / 914 Technical Questions 10 years ago
I just replace my 15 year old Pierburg mechanical fuel pump. I usually have a 0,33KG/cm2 pressure with no variations in pressure in that pump.
These weekend i fly about 2.5 hours with the new pump and i notice a great variation on the pressure. It vary from 0,22 to 0,38 kg/cm2.
I have a 912S engine with about 1100 hours of operation.
I would like to know if this varition on pressure is commom for the new model or this is a sympton of future problem.
Thanks,
Arnaldo

  • Re: Fuel Pump - Pressure variation

    by » 10 years ago

    It is possible you still have air in the fuel lines. Give it a another flight or two and see if it levels out some.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: Fuel Pump - Pressure variation

    by » 10 years ago

    Roger, the Rotax manual states max fuel pressure of 5.8 PSI.
    The following line below that it states max of 7.26 PSI but with an asterisk...* applicable only for fuel pump from S/N 11.0036. This probably falls within the "DUH" category, but if my serial number is 12.2071 my max fuel pressure would be 7.26. Correct?
      
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  • Re: Fuel Pump - Pressure variation

    by » 10 years ago

    The old fuel pressure max for the Pierberg and A/C pumps was 5.8 psi. The new Corona pump (aluminum body with bolts around the outside perimeter) max pressure is 7.2. If you have an instrument with the old 5.8 psi max pressure set with an alarm and you just installed a newer style Corona pump you'll need to re-set the max psi limit to 7.2 and maybe re-set the yellow alert set point too.
    Seems the average Corona pump pressure is approximately 5.5 psi. Some are higher and some are lower. That said a huge majority of the readings on the gauges are wrong by anywhere from .5 to 2 psi. If your gauge seems to read excessively high or low I would test the gauge accuracy. An electric fuel pump that is also in the system and properly plumbed raises the fuel pressure approximately .5 to 1 psi (again depending on the accuracy of your gauge).

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
    Report

  • Re: Fuel Pump - Pressure variation

    by » 10 years ago

    I have made a very quick sketch to explain the mechanical fuel pump function. Its not very good, but hopefully it will help explain how a spring in the pump controls the pressure. It is much easier to see how it works with the bits in front of you.


    The cam in the gearbox drives the pump shaft in, the shaft is held against the cam by the return spring.
    When the shaft goes inwards towards the diaphragm it moves inside the "cup" on the diaphragm.
    This in turn "releases" the diaphragm and allows it to moved by the pressure spring.
    This results in fuel being pushed out of the pump.
    As the cam continues to rotate the shaft is pulled back by the return spring.
    This pulls the diaphragm back which draws fuel into the pump.
    There are two one way valves that control the direction of fuel flow.


    In the 912 installation the shaft cannot push on the back of the diaphragm and hence the only thing driving the diaphragm is the pressure spring.

    The whole process is then repeated about 25 times a second.


    The pressure spring force is 50N (approx 5kg) fully compressed in the pump. The internal diameter of the pump body is 46mm, but the effective diameter of the diaphragm is more like 40mm. (the smaller the diaphragm the higher the pressure).

    Assuming 40mm = 4cm. The area is 12.56cm2.
    Pressure = Force /Area = 5kg/12.56 = 0.39Kg/cm2. (1 kgcm2 = 1.02bar) hence Approx 0.4 Bar

    If an electric pump is connected in series the fuel pressure should remain at about 0.4bar unless the electric pump pressure is greater than 0.4 bar.

    The positioning of the pump drain needs to go to an atmospheric pressure region. If the drain pipe goes to an area of higher or lower than atmospheric pressure area the fuel pressure will increase or reduce accordingly.
    BCPCoronaSketchsm.jpg (You do not have access to download this file.)
    BCPCorona5sm.jpg (You do not have access to download this file.)
    BCPCorona6sm.jpg (You do not have access to download this file.)
    Thank you said by: David HEAL, Arnaldo Pessina
      
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  • Re: Fuel Pump - Pressure variation

    by » 10 years ago

    Hi Conrad. Excelent sketch and explanation on how that particular system works.
    I can´t understand why my pump have so much variation on pressure, like i said before betwen 0,22 to 0,38 kg/cm2.
    If everything happen as you describe, why there is variation on fuel pressure delivered by the pump?
    I suppose that the Pierburg pump worked in a similar way and the pressure was always 0,33 kg/cm2.
    Thanks ,
    Arnaldo
      
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