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Severe Jittering Oil Pressure

in 912 / 914 Technical Questions 9 years ago

  • Re: Severe Jittering Oil Pressure

    by » 8 years ago

    I am going to jump into this discussion based on my 40 years experience as a professional hydraulics engineer. Although my experience has been mostly with higher pressure systems (2000-5000 psi) I believe the same principles still may apply. The symptoms I am reading about here indicate to me a high likelihood of very aerated oil entering the oil pump, the bubbles then get compressed, then violently decompressed as they pass thru the regulator valve, causing havoc at the regulator as Bill already suggested. It hasn't been said in this discussion, but we do know that highly aerated oil continually enters the oil tank whenever the engine is running. This is because the engine crankcase pressure is way more than enough to continually scavenge/push ALL the oil out of the crankcase (the 912 being a "dry sump" engine). That is why the oil tank is such a complicated device with screens and baffles to try to de-aerate the oil before it gets sucked back into the oil pump. Any air leaks in the oil line plumbing and fittings will obviously increase this inherent problem, as has already been suggested. But there are several other areas to check which could be worsening the problem:

    1- Are you using the Rotax recommended oil? Oil anti-foam additives are very important in this kind of system.
    2- Is your engine compression good? Poor compression will give more blowby, increasing the amount of "air" in the scavenged oil.
    3- Is your oil tank vent line free and clear, allowing the separated air to get out of the tank?
    4- Have you completely disassembled the oil tank to check that every piece is in proper position? Is the all-important screen sealing properly at the top and bottom caps? Some folks put the bottom cap in upside down-very bad. Is the spot welded vertical seam on the screen tight all the way? Is the center baffle in place?
    5- This tank has a nearly impossible job to do and everything must be working properly for it to have a chance.
    6- One last thought-are your engine oil operating temperatures up in the recommended area? Cool, thick oil is much harder for the air to separate from as it passes thru screens and baffles. If not, maybe you need to block off part of your oil cooler.

    My 2 cents worth. Hope it helps.
    Thank you said by: Bill Hertzel, David HEAL
      
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  • Re: Severe Jittering Oil Pressure

    by » 8 years ago

    Hi everyone,sorry i haven't posted lately,work has gotten in the way,we did disassemble the oil tank,and checked it,but that,s not to say that it is functioning properly,or next step is to by pass the oil cooler,hoping it may be sucking in air through a micro crack that opens up when hot??,if no success then try another oil tank. will keep you posted
    John
      
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  • Re: Severe Jittering Oil Pressure

    by » 8 years ago

    John, Alex, Or Anyone Else who would like to try an Experiment,

    I have been studying the problem and noticed a few interesting items.



    Notice that the Valve Seat is an insert and is conceivably removable.
    Also, notice that the valve seat appears to be aligned with and blocking most of the return flow channel to the Left of it.

    If anyone has a surplus, but otherwise serviceable, Oil Pump housing and is willing to part with it for a short period of time for the sake of experimentation, I would be willing to remove the original Valve Seat and custom machine a new flat style seat and matching valve spring cap of the type used in the 912iS engine.
    I would then return the pump housing to you, (all at no cost) so that you could test it to see if this cures the problem.

    Any Takers?

    EDIT: Later examination showed that the Valve seat is NOT blocking the Return Flow channel. The opening in the photo turns out to be a casting recess on the back side that just happened to break through in the cutaway operation. The actual channel is about 10mm below the valve seat and not at all visible.
    OilPumpValve1.jpg (You do not have access to download this file.)

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

      
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  • Re: Severe Jittering Oil Pressure

    by » 8 years ago

    Hi Bill,
    I might be able to get my hands on a serviceable pump body from a friend. Please send me an email at "alex at fpcounsel dot com".
    Alex
    Thank you said by: Bill Hertzel
      
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  • Re: Severe Jittering Oil Pressure

    by » 8 years ago

    First, a special "Thank You" to Alex who has supplied a ULS Oil Pump assembly for close examination.

    The first item of Note is that the apparent problem with the Valve seat blocking the Return flow channel is not a problem at all.
    The opening to the left of the Valve seat in the previous photo turns out to be a casting recess in the back side that just happened to break through in the cutout process.
    The actual return channel is about 10mm lower and not visible at all in the photo.

    The first thing that was looked at was the differences in the pump rotor parts.
    Besides the obvious 4 lobes to 6 lobe change, there is a big difference, with BIG being the operative word.

    If you consider that with a warm engine at idle; if the oil pressure drops below ~40psi, that would indicate that the pressure relief valve has closed completely and that none of the pump output is being bypassed.
    And that even with 100% of the pump output going to the engine, the engine would still like a little more oil to maintain the nominal pressure. This is not a terrible thing as this only happens low RPM's.

    When Rotax redesigned the pump for the 912iS to 6 lobes they also increased the diameter 42% producing a pump that conceivably is capable of delivering at least twice the oil volume to the engine.

    So the idea of swapping out the rotor parts can not even be considered.

    Stay Tuned... More Comparisons to come.
    OilPump1.jpg (You do not have access to download this file.)

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

      
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