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Fuel Orifice

Category: 912iS Technical Questions 3 years ago
Should an orifice be installed? (Chapter 73 in installation manual) My understanding is that it is primarily used to depressurize the system after shut down. we found that out early on when we must have had air in the system and undoing the hose we got sprayed. Now lets talk about when we are flying(more important issue here) If we run a tank dry it takes a while after switching to get full power restored again.probably air in the system? also after an airborne restart we may have vapor lock. Now, the question is: Will installing the orifice cure these problems and SHOULD it be installed?? I notice that Rotax says it should be mentioned in the manuals that one is not installed (obviously it is not compulsory) Is this to alert the ground engineers or the pilot? I think we spoke to Rob at Oshkosh this year and he mentioned that it should be installed. Who else has not installed it and why? anyone ever have a vapor lock issue< So many questions lol. Thanks guys for any comments. Phill
  • Re: Fuel Orifice

    by » 3 years ago


    912 Is Header tank not possible

  • Re: Fuel Orifice

    by » 3 years ago


    The orifice is used to bleed off fuel pressure at shut down and to allow a much quicker re-start if you run out of fuel.
    It absolutely should be used, it is not an optional in the latest revision of the Installation Manual.

    Shown is the Andair restrictor fitting: http://www.andair.co.uk/product/rotax-912is-nozzle/
    AndairRestrictorFitting1.JPG (You do not have access to download this file.)
    AndairRestrictorFitting2.JPG (You do not have access to download this file.)

    Thank you said by: Phill Hooker, Sefa Tak

  • Re: Fuel Orifice

    by » 3 years ago


    The fuel bypass Orifice is not "Needed" for normal operation of the engine.
    It is when "Abnormal" operations arise that the fuel bypass becomes significant.

    Aside from bleeding off pressure from the fuel lines after shutdown to avoid spraying fuel during maintenance and just generally relieving unnecessary stress to the fuel system, let's consider the temporary fuel starvation event from emptying a fuel tank before switching over to a second tank.

    The fuel pumps normally pump about 30 gallons per hour to the engine without regard to how much fuel is being used, if any.

    Once the pressure in the fuel rail reaches ~40+ psi or so, the fuel pressure regulator bypass whatever amount of fuel that is required to maintain the proper pressure.
    If the engine is only using 4 gallons per hour, the other 26 gallons get dumped back to the fuel tank to be used again.
    This means the entire contents of a 20-gallon tank go through the fuel filters, the engine, and back to the fuel tank every 40 minutes. Portions of the final 5 gallons of fuel may have passed through the fuel system a dozen times by the end of a flight.

    Once the tanks go empty and air enters the fuel lines, the fuel pump loses its wet prime and pressure drops dramatically.
    The fuel injectors loose pressure and the engine leans out and stalls.
    Once the second fuel tank has been selected, The air in the fuel lines needs to be purged for the fuel pumps to restore full flow and pressure.

    Without the fuel bypass, the pumps will maintain a marginal pressure in the fuel lines, they are able to pump air, just not very efficiently or with full pressure. They will self-prime if given a fighting chance.
    In order to get fuel to the intake side of the pumps, you need to get the air out of the pressure side of the pumps.
    There is not enough pressure to open the pressure regulator to purge the air so the only escape path will be the fuel injectors as the engine is being cranked for the restart.
    This can seem like a very long time as you start considering your engine-out options and the drama and adrenaline build.
    The air eventually gets purged and the engine comes back to life.

    With the Fuel Bypass orifice installed, the fuel pressure again drops and the engine quits.
    Once the tanks are switched and in the seconds before the engine start attempt, the fuel pumps are still running at full speed and quickly purge the intake line of the air as the pressure side orifice is able to bypass the air back to the fuel tank.
    The engine restart happens with minimal drama as the fuel rail is already up the nominal pressures and any remaining air has been purged by the pressure regulator at the 30 gph rate.

    The orifice is sized to bypass just a few gallons of fuel per hour.
    Under normal operation the fuel pumps deliver 30 gph, the engine uses 4 gph the orifice bypasses 3gph, and the Fuel Regulator returns the remaining 23 gallons.

    Air, being 600 times less dense than fuel, will vent out of the orifice hundreds of time faster than the fuel does.

    Should the orifice be installed? YES! And be sure to install it pointing UPWARDS and ideally at the top of a rise in the fuel line to facilitate the air purge.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    whertzel1@yahoo.com
    Clicking the "Thank You" is appreciated by all.

    Thank you said by: Phill Hooker

  • Re: Fuel Orifice

    by » 3 years ago


    Thanks Bill, what I thought. You were able to explain it well. Merry Xmas to you and family. Phill

  • Re: Fuel Orifice

    by » 3 years ago




    912 Is Header tank not possible


    There is nothing preventing the use of a header tank with a 912iS.
    Watch the free Builder Series Video #8 to see a CH750 built with a header tank installed in the door post.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    whertzel1@yahoo.com
    Clicking the "Thank You" is appreciated by all.

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