912is Fuel Consumption

Category: 912iS Technical Questions 5 months ago

Greetings everyone,

I was hoping to pick the brain of those of you that take interest in my topic and take the time to resond. I thank all of you that give this the time of day. 

The company I work for uses the engine listed in the subject line on a UAS, during our first 200 hours of flight we had consistent burn rates of about 2.6 gallons per hour on 91 octane. When we switched to 100LL we saw a sgnificant jump in consumption, 3.7 gph. This has significantly reduced our time to stay on station. When we switched back to 91 octane we have yet to see a decent drop in our fuel consumption, we now average 3.5gph. 

My questions are the following:

How does the EMU recognize the fuel type? I know the engine takes note from key input variables: throttle valve position, engine speed signal, intake air temperature, ambient pressure, manifold pressure and exhaust temperature and ultimately, the required fuel quantity or injection period is determined on the basis of the calculated air density in the airbox. But I guess I am curious about the characteristics between the two fuels and how the engine performs comparatively. 

What are the expected CHT ranges for 100LL and 91octane? We see a split of about 200*F between the two fuel types. I recognize that there is a higher BTU output from gasoline than 100LL, but would this have such an impact on consumption?

Has the community of 912is users seen a difference in fuel consumption when switching fuel types?

How can I recognize when the engine is operating in ECO mode? and If there are no immediate visual indicators via the gauges, how do I see when it kicks in using the logs and the analyzer tool?


Thanks for the help, 



  • Re: 912is Fuel Consumption

    by » 5 months ago


    Just a question why do you use  100LL instead of 91? This is a very bad idea, first because this engine is build to use MOGAS or similar without lead, it's able to use 100LL but as you have see with a poor yeld, so more cosumption and less power (dificult to feel) an other problem if you use 100LL is that you must reduce the time between maintenance (/2) for oilchange but for any other things like gear box revision that come at 500h instead of 1000...

    For the EGT don't look at them because you don't have any possible adjustment for mixture, it's only for the first adjusting when the engine is installed for the first flight and if you change of leaving altitude (from sea side to montain) not for a trip, for that the calculator do it by himself.

    Good flight

  • Re: 912is Fuel Consumption

    by » 5 months ago

    The 912 iSc/iS engine has two different modes, the POWER and ECO mode which differ significantly in fuel consumption. The switchover between ECO and RICH mode is about 97 % of the throttle position.

    We suspect you were operating mostly in power mode and it skewed the data slightly?

    AV-Gas does not burn as sufficient as auto fuel, but it does have excellent anti-knock abilities and is very stable fuel that can be stored for long periods of time. Our recommendation is if you are going to continue with Av-gas more then 30% of the time, is to follow the maintenance instructions set out in the Rotax maintenance manual as the intervals change. We also recommend running a fuel additive of TCP or Declan to help scavenge the lead build-up. TCP and Declan does amazing things and does an excellent job of getting rid of that lead!

  • Re: 912is Fuel Consumption

    by » 5 months ago

    Can we use 91 on Rotax914F2?

    As last week I loose a engine after 180hrs due to 100LL used.one cylinder got damaged and engine will go for major overhaul.


  • Re: 912is Fuel Consumption

    by » 5 months ago

    100LL shouldn't have damaged the engine in 180 hrs.

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

  • Re: 912is Fuel Consumption

    by » 5 months ago

    I am wonder. 

    Propeller has no power , free rotation.Gear box checked independently Found 💯 okay.

    On Cold we have checked the compression, it was very low in cylinder no 2 i.e 25%loss after 85 psi to 60 psi. Whereas other all cylinder remains at 82, 81,80.psi

    Possible causes?

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