915iS High Manifold Temperature

Ken Ryan wrote:

Pilot Joe wrote:

Does that mean it won't run as efficiently as a carb engine where the pilot manually adjusts mixture to lean in flight? If you lean mixture until engine runs rough then enrich slowly until it smooths out, keeping an eye on EGT, will that result in a leaner and more efficient running engine than a Rotax EFI controlling mixture that blindly uses a default ECO map and takes into account no feedback from the running engine?

Even though the 915 may not have all of the bells and whistles to maximize efficiency that a modern automobile has, it does need to. Unlike an automobile, aircraft operate mostly in a relatively narrow power band, so the engine can just be optimized for that. The proof is in the pudding -- much higher efficiencies in terms of pounds of fuel per brake horsepower at cruise power settings.

Do you know where to find a graph that shows BSFC for the 915iS plotted against throttle position? I checked the manual.

Pilot Joe Stated:
... That is in theory but it'd need a closed loop system with an oxygen sensor to keep it accurately at 1.05. Do Rotax engines just not need closed loop because the way they are run lets it stay at close to 1.05 without feedback?

My first thought would be how long the Oxygen Sensors would last after the first tank of 100LL was loaded.
There are other ways to control the mixture without an Oxygen sensor.

Close loop implies that the input is modified based on the Output.
A lookup table could give nearly the same results if the Input is accurately known.
The Injected engines have sensors that monitor Ambient Temperature and Pressure(Altitude), along with Airbox Temp and Pressure.
Other than Humidity, that pretty well covers everything you need to know.
That data can be used to compute to a high degree of precision just how much fuel to add to the mix without looking at the exhaust to see if you were correct. (Closing the Loop!)

Just because there isn't any feedback dosen't mean the results are inaccurate.
Most CNC machines operate without any feedack.
They blindly move the tooling and assume it moved to where it was supposed to be.
As long as the initial calibration is accurate, Everything that follows is just as accurate.

Bill Hertzel wrote:

Pilot Joe Stated:
... That is in theory but it'd need a closed loop system with an oxygen sensor to keep it accurately at 1.05. Do Rotax engines just not need closed loop because the way they are run lets it stay at close to 1.05 without feedback?

My first thought would be how long the Oxygen Sensors would last after the first tank of 100LL was loaded.
There are other ways to control the mixture without an Oxygen sensor.

Close loop implies that the input is modified based on the Output.
A lookup table could give nearly the same results if the Input is accurately known.
The Injected engines have sensors that monitor Ambient Temperature and Pressure(Altitude), along with Airbox Temp and Pressure.
Other than Humidity, that pretty well covers everything you need to know.
That data can be used to compute to a high degree of precision just how much fuel to add to the mix without looking at the exhaust to see if you were correct. (Closing the Loop!)

Just because there isn't any feedback dosen't mean the results are inaccurate.
Most CNC machines operate without any feedack.
They blindly move the tooling and assume it moved to where it was supposed to be.
As long as the initial calibration is accurate, Everything that follows is just as accurate.

Why do automobile engines use those sensors and not Rotax aircraft engines? Is it because aircraft engine rpm and load doesn't change very often? Can I assume that the iS ECU keeps lambda 1.05 in eco mode even without oxygen sensors because of this?

I will guess that the lack of Oxygen sensors is because Auto engines never have to deal with leaded fuels and while Rotax could add  the restriction of Non-Leaded fuel only, that would make the engine nearly useless for anything but local flights where the operator buys the fuel off airport. 
The first time you made a flight that required refueling at an airport that did Not have Un-Leaded Fuel, (99% of them) you would be replacing the Oxygen sensors and draing and flushing the remaing Leaded fuel in order to restore operation.

100LL in your Auto will damage the Oxygen sensors and the Catalytic Converter at a minimum.   $$$

It will be a happy day when Leaded Aviation fuels are finally phased out.

Just to add a data point, the Aeromomentum uses an O2 sensor and Mark, the developer, says they last 50-100 hours using 100LL and cost about $15 on ebay. Because of the ECU mapping, failure of the O2 sensor is not a big deal.