916 Fuel Flow

Richard,

This issue has been around a while, and was first brought up on this site about four years ago regarding the 912iS engine.  Other folks have noted a 9-11% error, with the indication less than actual. I believe the fuel flow calculation is based on the ECU totalizing injector pulses, factored for the pulse width at any given time.  Apparently it’s not perfect. There are a lot of variables, and I would think even a small change in fuel pressure at the fuel rail can affect the accuracy of the reported fuel flow. I’ve seen this discussed regarding automotive engines, which over time have gotten much more accurate so maybe eventually Rotax’s will also. 

Here is a link to an earlier thread, which includes a response from Garmin saying it’s a Rotax issue. And you are correct that Garmin offers no calibration once “FADEC” is chosen for fuel flow. That makes sense, they would not want to accept the liability for altering what is being reported by Rotax.   I’m sure Rotax knows about this by now, but they probably have limited control since they buy the injectors and ECU from a third party.  

https://www.rotax-owner.com/en/912is-technical-questions/8427-inaccurate-fuel-flow-on-garmin-g3x-touch

Thanks for the additional information.  I should have suspected there were previous discussions on this issue.  The Garmin experimental folks alway attend the annual Rans fly-in, which happens to be this coming weekend.  I’ll see what they have to say but won’t have high expectations.

This issue was discovered some time ago with the Rotax 912iS, and we’ve brought it up with Rotax. The fuel flow displayed by the ECU is a "calculated" value and often reads about 11% lower than the actual fuel consumption, depending on the throttle setting (Power vs. Eco mode). Rotax provides fuel flow and consumption charts, and they’ve included this information in the installation and operator's manual.

Fuel Flow and ECU Operation:

• Fuel Flow [kg/h]: The fuel flow is based on specific ambient and engine conditions. The tolerance range under stationary flight conditions is 10% for 5500-5800 rpm (power output > 55 kW) and 4000-5000 rpm (power output > 35 kW). The actual flight range must be evaluated by the aircraft manufacturer for real-world applications.

• Throttle Modes:

  • Below approximately 97% throttle position, the ECU operates the engine in Eco Mode, optimizing injection and ignition systems for the best economy and fuel efficiency.
  • Above 97% throttle, the ECU switches to Power Mode, enriching fuel for maximum performance, particularly for takeoff and climb.

Actual Fuel Flow Accuracy:

The true power output and fuel consumption are highly dependent on the engine installation, adherence to maintenance protocols, and operational usage. These values must be determined and provided by the aircraft manufacturer.

For the 916iS/915iS and 912iS engines, the only way to obtain an accurate fuel flow reading is by installing a fuel flow transducer on both the inlet and return sides of the fuel system. The fuel flow value displayed by the ECU is only a "calculated" estimate and is approximately 11% inaccurate (on the low side). This means the engine is consuming more fuel than what is shown on the display.

Recommendation:

To achieve accurate fuel flow data, a transducer must be installed on both the inlet and return sides of the fuel system. Relying on the ECU's fuel flow data is not advised, as it is an estimated value.