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With 250 hours on the aircraft, I had my first experience with pump cavitation, likely from fuel vaporization.  After climbing to 4500', I turned off the aux pump and the engine immediately lost power.  Turning the aux pump back on kept the engine running, and within about 3 minutes the engine would again run on the main pump only.  The plane had been sitting for about an hour in hot sun just before takeoff so the tanks were warm. However I was using an ethanol free fuel with an advertised RVP of 7. If the RVP of 7 is accurate, I should have had a lot of headroom against this happening. 

Since this had never happened before, even in hotter weather at higher altitudes, I decided to do some fuel system tests.  The aircraft is low wing, and the fuel pump is mounted low in a compartment below the pilot footwell. This is a cool space with aluminum 3/8”  rigid fuel line from the pump to the selector valve and on to the fuel tanks.  There is moderate gravity flow to the pumps and the inlet pressure at the pump is very close to zero gauge pressure when one pump is running.  With both running the inlet pressure is approximately -.25 PSI (.5 Hg), a very slight vacuum that should not be enough contribute to vaporization.  

I also did a return flow test, which showed showed 24 GPH with one pump, and 34 GPH with both pumps.  While doing this test I ran the output line into a clear container and could see a steady stream of bubbles in the return flow stream. What I don't know is if this is the normal result of the fuel moving from a pressurized state to ambient pressure? Or, do I need to be looking for an air leak on the suction side?

 

 

  • Re: Bubbles in Return Fuel Stream

    by » 5 days ago


    In the Rotax owners video section is a group of videos detailing the construction of an aircraft with a 912iS engine. The one about the fuel systems addresses the issue of return air bubbles. The way I understand it is that bubbles are normal due to the action of the fuel pressure regulator. Mine has bubbles when the engine is running. Perhaps Bill will enter the thread conversation with his knowledge of the engine.


    Thank you said by: Jeff Blakeslee

  • Re: Bubbles in Return Fuel Stream

    by » 5 days ago


    I have a question about turning off the aux pump.  Wouldn't that make the main pump work harder to carry the entire load, thereby shortening its life?


  • Re: Bubbles in Return Fuel Stream

    by » 5 days ago


    Richard Boslaugh wrote:

    In the Rotax owners video section is a group of videos detailing the construction of an aircraft with a 912iS engine. The one about the fuel systems addresses the issue of return air bubbles. The way I understand it is that bubbles are normal due to the action of the fuel pressure regulator. Mine has bubbles when the engine is running. Perhaps Bill will enter the thread conversation with his knowledge of the engine.

    Check this 912iS Builder Series video, right about the 5 minute mark...

    https://www.rotax-owner.com/en/videos-topmenu/builder/439-912is-ch750-8

     


  • Re: Bubbles in Return Fuel Stream

    by » 5 days ago


    An RVP of 7 is very conservitive. About the same as 100LL.
    But this is California and they are known to be conservative.
    At 7, the fuel will vaporize above 7000' Pressure Attitude if it were heated above 130°F.
    You were not above 7000" and it was unlikely the fuel was above 130°F with 20+ GPM being circulated.
    That would make the fuel tanks too hot to rest a hand on them.

    The Gas Bubbles in the fuel lines are completely normal.
    They give the appearance of a foam immediately after they leave the Pressure Regulator when the pressure suddenly drops 40psi but consolidate into propper bubbles within a foot or so downstream.
    They are Not "Air" Bubbles, they are Fuel Bubbles, Mostly Butane, and will go back into solution if given the chance.

    I would check the Coarse Fuel filter located Before the Fuel pumps.
    If the filter is getting restricted, the suction of the pumps can lower the pressure in the line causing Vapor formation.
    - - -

    Turning OFF the Pump.
    Yes turning OFF a pump will stress the operating pump.
    It will also relieve the non-operating pump.
    So, overall it is a wash.
    The idea is for there to be a backup pump available if one of them should fail.
    Not to have both of them die in unison.
    If you leave Both pumps ON all the time, you will not know if one pump failed until the second one fails also.  Ouch!!!
    The MAIN and AUX Pumps are identical and the labels are arbitrary.
    They could just as well be labeled 1, 2 ... A, B ... or LEFT, Right.
    It is not uncommon to alternate the pumps in flight just to confirm that both are operational and either one can handle the full load.
    - - -

    RVP (Reid Vapor Pressure)
    This is a measure of the Vapor Pressure at 100°F Measured in PSI.
    When the Vapor Pressure exceeds the Ambient pressure, The Fuel will Boil (Vaporize).
    This can be accomplished by raising the temperature (Heating) or lowering the pressure (Altitude).
    You can actually measure RVP pressure by sealing a quantity of gasoline in a container with a pressure gauge attached and heating it uniformly to exactly 100°F.
    The pressure you measure in PSI will be the RVP.

    This is why your plastic gas cans bulge under pressure if left out in the sun.
    Do NOT Relieve the pressure on your bulging gas cans!!!  You will be wasting gas!!!
    The Gas Cans are designed to handle the pressure. Leave them Pressure sealed!!!
    Gas only goes "BAD" in Unsealed, Improperly Stored, Containers.


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Bill.Hertzel@Yahoo.com
    Clicking the "Thank You" is Always Appreciated.



  • Re: Bubbles in Return Fuel Stream

    by » 4 days ago


    Bill,

    I appreciate all of the information, it verifies what I was thinking.  The CA summer blend has an RVP of 7, but the winter blend is higher. It took a lot of looking to find it but I finally located a white paper on the Chevron site indicating the winter blend has an RVP of 13.  With further consideration I believe I was likely running the winter blend when this happened.  This makes a lot more sense mathematically.  Starting out with a fuel temp of 100, and allowing for the fuel cooling to 90 during climb out, vaporization at the pump inlet would be right around 4500'.  

    I'm working on a spread sheet to predict vapor lock conditions for my particular aircraft, taking into account the affects of temperature and pressure on the RVP, to get TVP.   I measured the fuel pump inlet pressure at my airport, which is just 125' above sea level, and the result was -.3 PSI with both pumps running.  So, with a fuel TVP of 7, I start with a Net Positive Suction Head (NPSH) of 6.7 PSI.  I'm not 100% sure of the affect of altitude on the pump inlet pressure, but I believe that as atmospheric pressure falls, the pump inlet pressure will be reduced by the same.  Do you agree with this? 


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