• Re: odd dips in fuel pressure, pump problem?

    by » 9 months ago

    I don't  think you can always blame fuel vaporization all the time. You have to look at all possibilities. A hugh number of low wings have fuel fluctuations and high wings don't  seem to have issues near as often. If this was true about vaporization  always being the cause tens of thousands would be having issues. Especially since some countries and users use ethanol fuel far above 10% without issues.. For instance in Arizona where I live most Rotax users use 10% ethanol and our daytime temps are 105F - 115F. I have low wing owners from all over the country call about fuel fluctuations on low wings. Things like hose routing, electric fuel pump too high above fuel fuel level in the wing (draft height), old fuel  pumps, wrong return fuel jet size, poor wiring connections, bad fuel pressure sender. 

    When you're  flying in cruise your moving fuel through the hose to the carbs and recirculation hose. You have cooling air moving through the cowl. If vaporization was always the issue why aren't  thousands of cars stranded along side the road with vaporization. Yes they have a pressurized system, but hundred of  millions of different  engines using ethanol don't  have any issues.

    Not to mention even though they state up to 10% - 15% or up to 20% plus that is rarely the true number at the pump. For instance they post 10% in Arizona, but after 10+ years of testing almost every batch of fuel owners only find 6%-7% with a once in a blue moon 8%. 

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

  • Re: odd dips in fuel pressure, pump problem?

    by » 9 months ago

    I also don't understand why it takes about 40 min of cruise flight to occur.  The knee jerk answer to that is that it take time for everything to heat soak, but I refuse to believe that takes more than 5-10 min, and certainly not 40.  By the time I reach climb altitude, the oil, coolant, CHT, and cowl temps are higher than they will be in cruise.  

  • Re: odd dips in fuel pressure, pump problem?

    by » 9 months ago


    I believe there is a serious misunderstanding of the issue of heat soak and even what RVP is.  

    First, just forget ethanol for a min, it has little to nothing to do with the subject of fuel vapor.  The issue is fuel temperature and the boil off of fuel from liquid to a gas (vapor) at higher temperatures.  Ethanol has nothing to do with this.  In actual fact ethanol has a very high octaine and the RVP of just ethanol is very low.  

    Here are the numbers: 

    Ethanol is an organic compound with the chemical formula CH₃CH₂OH. It is an alcohol, with its formula also written as C₂H₅OH, C₂H₆O or EtOH, where Et stands for ethyl. Ethanol is a volatile, flammable, colorless liquid with a characteristic wine-like odor and pungent taste. " 
    Boiling point78.37 °C
    Molar mass46.068 g/mol
    IUPAC IDethanol
    Density789 kg/m³
    Vapor pressure5.95 kPa
    Classificationorganic compound, alcohol
    So for purposes of the discussion please leave ethanol out of it.  Ethanol n itself is not the problem here and has nothing to do with the pressure problem originally put forth.  
    It takes time for fuel to heat, it is not immediate and can be worse after flights when you sit on the ground and "heat soak" the system from radiant heat of the engine block, exhaust and other parts under a cowl.  Circulation of heated fuel can and most often is a major issue.
    I am old enough to remember big block engines in the 1970s that had great HP and performance and they had no return lines.  These same engines now must have reworks to run todays fuel and the issue is not octane it is RVP.  (some put fuel pumps in the fuel tanks with rework kits)  If you can't get fuel under 11 RVP then you are doomed to have major issues in warm and hot weather.  Please contact your local fuel supplier and find out what they are selling you.  At the very least swap out your fuel to AVGAS, ASTM D910 100LL or Swift 94 for example,  for a tank and see what difference that makes before you speculate as to the cause and start buying fuel pumps you may very well not require.
    The automotive world has literally hundreds of pages of articles and videos on old cars and trucks with how to solve the "new" vapor lock problems with today's fuels.  I am rather surprised we still have this conversation about a return line today.  Back in the 1970s high performance big block engines were fitted with special fuel filters that had ...guess what...a return line on them to solve boiling fuel issues.  
    just my 10 cents worth.

  • Re: odd dips in fuel pressure, pump problem?

    by » 9 months ago

    I understand vapor lock, and I know that auto fuels typically have a much higher RVP.  I was hoping to have an oil analysis this year that wasn't full of lead, so I've avoided 100LL completely.  I have been thinking of trying a tank of it as a test. 

    I don't agree with the whole heat soak fuel argument though.  My plane sits in the hangar at over 100F, so everything is soaked to that temp before I even crank the engine.  In flight the temps are probably 20F lower, so the fuel in the tank is certainly getting cooler, not hotter.  At cruise power, the fuel flow far exceeds the fuel return though that tiny orifice, so I can't see that small amount of heated fuel being much of a problem looping back through the gascolator.  My gascolator, and fuel lines aren't anywhere close to exhaust except by Rotax design at the fuel pump itself. They are fire-sleeved, and have lots of direct cool airflow.  

    I'd worry more about the boiling at the carbs themselves, but you'd think you'd get roughness if those were boiling.  Maybe I'll investigate some cool air directly to the carbs.  

    Fortunately, there seems to be nothing life threatening about the current behavior, since the boost pump will resolve it, and the Dynon will warn me.  I'll continue careful testing.  




  • Re: odd dips in fuel pressure, pump problem?

    by » 9 months ago

    I noticed something that grabbed my attention in your statement: "The only change since last summer is that I now have direct airflow to the pump and more flow through the cowl than I ever did before."

    In the Rotax installation manual, in section 73-00-13, on Page 13, under subsection "DRAINAGE LINE," there's an important caution: "Observe correct routing of the fuel pump drainage line. The fuel pump drainage line must be routed into a ram-air and vacuum-free zone, according to the requirements and release of BRP-Rotax. The fuel pump drainage line must not be routed into the slipstream. Ram pressure or vacuum impairs the fuel pressure."

    For further details, please refer to the Rotax installation manual available here: https://www.rotax-owner.com/pdf/IM_912_Series_Ed3_R0.pdf.

    We conducted tests on this matter and discovered that every inch of mercury (Hg) of vacuum introduced into this fuel pump drainage line led to a direct reduction in fuel pressure. The same effect was observed when pressure was introduced into this fuel pump drainage line. 

    Additionally, you might find it beneficial to read a Blog article we released on the Rotax-Owner website, authored by Rainbow Aviation. You can find the article here: https://www.rotax-owner.com/en/rotax-blog/item/77-rotax-912-fuel-pumps.

    The new-style BDC fuel pumps are highly reliable and robust pumps that rarely cause any issues. Re-routing the drainage line for this fuel pump is a straightforward task and is worth attempting to determine whether it resolves the issue of fluctuating fuel pressure. Please report back and let us know if this resolved the issue or not. 

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