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What is the real difference in fuel economy between the iS and the ULS

in 912iS Technical Questions 7 years ago
Hi, I have heard 7%. I have heard 20-30%. Can someone tell me in terms of GPH or litres per hour? I intend to fly long distances with no fuel at the destination, so range is a real issue. Thanks!

  • Re: What is the real difference in fuel economy between the iS and the ULS

    by » 7 years ago

    At WOT the difference is small as the 912is goes full rich to keep the cylinders from overheating.
    Below 5500rpm the 912is goes into ECO Mode and leans the engine out.
    The difference at Cruise settings is significant.
    Typically 5gph vs 4gph. ~25%.
    This could yield another hour of range out of a 20gal tank.

    Example:
    https://generalaviationnews.com/2013/06/27/rotax-912-is-better-than-predicted/

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

    Thank you said by: Andrew Nielsen
      
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  • Re: What is the real difference in fuel economy between the iS and the ULS

    by » 7 years ago

    I have never operated a ULS but there are several in our club. My iS consistently gets better fuel economy but can’t scientifically quantify a number. Full throttle climb normally shows about a 22 l/hr value and cruise power setting is typically around 15 l/hr. When I am just loafing around I often see fuel flow values of 13.2 l/hr. When I am heavy and trying to keep up (as close as I can) to a faster airplane I will sometimes see 17.2 l/hr but it is still showing ECO mode. I fly a Zenith CH750 STOL and have a field elevation of 3000 ft MSL.
      
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  • Re: What is the real difference in fuel economy between the iS and the ULS

    by » 7 years ago

    I have 2 iS engines at my field that fly with the same planes with ULS engines. These planes usually fly together a lot and in formation. Fuel economy can differ. If an iS engine is propped more coarse and the ULS propped properly the ULS will do better. If a ULS is running 5200 rpm and the iS has to run 5300 or higher to keep up the ULS does better. If the same planes are propped the same .i.e. around 5600-5650 then the iS will run better fuel economy at lower altitudes. When the two engines climb up to let's say 9K in altitude the iS engine will do a much better job in fuel economy than the ULS at higher altitudes. The ULS will run rich and the iS will meter it's fuel better. Again depending on prop pitch it can be anywhere from 20%-25%. I have heard of a bigger spread from some, but rarely are these numbers scientifically gathered to rule out other factors or exact fuel use.
    The iS engines at my field go out of their fuel economy at 5300+ rpm. Bottom line in flight rpm and prop pitch are big factors.

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

      
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  • Re: What is the real difference in fuel economy between the iS and the ULS

    by » 7 years ago

    Reality check!

    This reminds me of the guy who drives 30 miles save $5 on a tank of gas but burns $6 in gas getting there and back.
    Assume that you can save a gallon an hour in fuel burn and you are burning $3 a gallon MoGas;
    It will take 1000 hours to break even on the ~$3000 added expense of the engine installation assuming you do not include the instrumentation costs.
    If you include the instrumentation cost it might take over 4000 hours to break even. You would be on your third engine by then.

    In that 1000 hours if you burned 4 gals per hour you would have burned $12,000 in fuel vs $15,000 in your $100,000 aircraft.
    Is this $3000 difference significant to you??? Your call!

    On the other hand, getting into trouble in flight and having to divert, and landing with an hour and 10 minutes of fuel remaining vs 10 minutes remaining...
    PRICELESS!

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

    Thank you said by: Daniel Jones
      
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  • Re: What is the real difference in fuel economy between the iS and the ULS

    by » 7 years ago

    I have 2 iS engines at my field that fly with the same planes with ULS engines... depending on prop pitch it can be anywhere from 20%-25%.


    It's nice to see a direct comparison. The Rotax data sheets aren't consistent. An older (2013) presentation indicated you'd do no better than 15%. More recent stuff says maybe 10% better. Surely this is dynamometer data near 75% with ideally configured carbs.

    The engines are fundamentally the same, so the difference is simply the limited ability of the Bing carbs to deliver an ideal (and identical) fuel mix to each cylinder. The lack of a mixture control hurts here. Both Cessna and Lycoming data show the O-235 to be as fuel efficient as the 912iS, but I'd bet very few pilots ever fly it so. The iS will automatically be running near ideal mixture all the time (until the electronics check out).

    Beyond bsfc, the Rotax spins it prop slower than an O-235, so the net effect likely favors the 912iS. The entire 912 series is a Rube Goldberg contraption, but it seems to work very nicely. I flew behind a ULS for the first time two weeks ago and was quite favorably impressed with the thrust and smoothness. It's only a 92 hp engine but seemed like more.

    Karl
      
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