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The post by Bill here: https://www.rotax-owner.com/en/912is-technical-questions/6775-buds-dongle-ecu-download-error-code-messages-reference?start=45#p22924 says that the 912iS while it has dual spark plugs per cylinder it only uses one at a time and alternates between them unlike the 912ULS which uses them both practically simultaneously. I guess this lets the ECU tell if one of the spark plugs is fouled because it will detect a misfire and disable the lane with the faulty plug.

 

On Wikipedia I'm seeing that dual ignition improves efficiency and power by 5 to 10% because it creates two flame fronts to burn the gasoline faster. On the 912ULS during the mag check we force it to use only one of the two spark plugs and the RPM drops by up to 300. Does this mean that the 912iS has less power output than if it used both plugs per cylinder at the same time? Or how does it compensate from not having two flame fronts?

 

Thank you for satisfying my curiosity as I try to learn more about these excellent engines!

  • Re: Does the 912iS have any drawbacks by not using "real" dual ignition?

    by » one month ago


    In the ULS engine the two plugs are not fired simultaneously.  The B ignition is fired 4° After the A ignition.

    And the 26° and 22° ignition points are fixed by the trigger coils relative to the Crankshaft.

    Having two flame fronts is the equivalent of having one flame front started a few degrees earlier.

    When performing a ULS Mag-Check, you are also changing the virtual engine timing.

    The 26/22° timing is optimized for max power at WOT.

    Meaning that at reduced power the timing is too early and efficiency suffers.

     

    The timing in the iS Engine is not published and is variable and continuously optimised based on the current conditions.

    A review of the performance charts in the Operator's Manuals shows that eventhough the engines are the same displacement and have the same Max 100hp Rating,  the iS Engines outperform the USL engines at all other rpm values by 2-7 hp.

    The iS Engine generally make about 5% more power while consuming about 20% less fuel.

    The timimg of the alternating plug iS ignition has been optimized in the design and is in no way a shortcoming.

    - - -

    My crystal ball predicts that in a few years Rotax will announce the Sunset period for the Carbureted engines and focus production solely on the Fuel enjected engines much as the auto industry did 40 years ago. 

     


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


    Thank you said by: Pilot Joe, RotaxOwner Admin

  • Re: Does the 912iS have any drawbacks by not using "real" dual ignition?

    by » one month ago


    I'm not so sure Rotax will abandon carbureted engines any time soon.  When autos did it, they were having to comply with government mandates for better mileage, pollution control, and on-board diagnostics.  I don't see the government mandating anything similar for aircraft anytime soon.  Heck, they haven't even managed to get rid of leaded fuel yet.

    As far as market forces, there is something to be said for the KISS principle in amateur installations, at least.  With regard fuel savings (20%), I figure you need to fly about 1000 hrs (assuming $3/gal) before that will pay for the extra cost of the 912 iS vs the 912 ULS.

     


  • Re: Does the 912iS have any drawbacks by not using "real" dual ignition?

    by » one month ago


    Thanks Bill! Your answers are great as always. :)

     

    And by timings you mean the ECU's map? Has anyone ever tested it and reconstructed the map by feeding synthetic inputs and recording the output?


  • Re: Does the 912iS have any drawbacks by not using "real" dual ignition?

    by » one month ago


    Tyler Hathaway wrote:

    I'm not so sure Rotax will abandon carbureted engines any time soon.  When autos did it, they were having to comply with government mandates for better mileage, pollution control, and on-board diagnostics.  I don't see the government mandating anything similar for aircraft anytime soon.  Heck, they haven't even managed to get rid of leaded fuel yet.

    As far as market forces, there is something to be said for the KISS principle in amateur installations, at least.  With regard fuel savings (20%), I figure you need to fly about 1000 hrs (assuming $3/gal) before that will pay for the extra cost of the 912 iS vs the 912 ULS.

     

    I don't think Rotax will sunset carbureted engines just because so many people in aviation are used to them from older certified engines. Also the maintenance differs and most people are comfortable with the failure modes in a carbureted engine and don't trust a complex computer. They will probably focus on fuel injection more and more and might not make carbureted engines for the new models (just like there's no 915ULS only 915iS). But since manufacturers like Jabiru and Continental and Lycoming are still doing these engines it shows the market still wants them.


  • Re: Does the 912iS have any drawbacks by not using "real" dual ignition?

    by » one month ago


    Tyler, with respect, you are right about the extra cost versus fuel saving. However that is not the issue for some of us; my issue is 20% more range compared to the ULS and I am willing pay for that. The same argument applies to my constant speed propeller. In Australia we have some wide open gaps where there is not much of anything. My limiting mission that I planned for is a desert trip involving two two hour flights with no refueling.


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