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Maybe this is a bit too technical for an aviation forum, but it specifically concerns a flight engine.(Rotax 912 iS) Road vehicle engines will probably have different behavior. As you may know, the 912iS is a fuel injected engine where you can't manually control the mixture.

I have logged some data on a dyno bench, and i can clearly see a relation between RPM and Air Fuel Ratio. The higher the rpm, the leaner the mixture. Strangely enough, throttle position or MAP value has no clear relation with the AFR, much unlike a car's engine. I thought i'd ask here since you guys may know what variables a flight engine uses to determine the AFR it wants to maintain.

My guess is that the following variables influence the AFR, but i don't know how and how much:
-Throttle (with exception of WOT or near WOT, which causes rich mixture)
-MAP (i'd guess higher MAP makes for a richer mixture, but not)
-Temperature of intake air, and exhaust gas etc.
-Temperature of oil, coolant etc. (i'd guess a richer AFR is maintained during startup=low temps)
-Known volumetric efficiency at certain circumstances

Finally, are there other factors affecting the AFR, like angular motion of the throttle valve? With carbs used in cars you always had an acceleration pump or a damper in case of a constant depression carb. This is for the engine to attain the desired rpm/speed more rapidly.

Thanks for the help, cheers. i'd like to hear if there's other forums more focused on technical aspects of flight.

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  • $\begingroup$ welcome to aviation.se. please note that we are not a discussion forum, we are a Q&A site. it makes quite a difference. Please consider editing your post to contain only one clear major question, currently I have some difficulties understanding what exactly you are interested in. $\endgroup$ – Federico Sep 5 '16 at 13:53
  • $\begingroup$ As the title says, i'm interested in what circumstances an engine bases it's AFR/mixture on. ie. what circumstances affect the AFR/mixture a flight engine wants to maintain, given the engine regulates the AFR/mixture fully automatically. $\endgroup$ – Bart Sep 5 '16 at 13:57

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