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-   -   Turbocharged engines question (https://www.pprune.org/tech-log/526982-turbocharged-engines-question.html)

crazy_duck 3rd Nov 2013 23:51

Turbocharged engines question

I have a question that i have been struggling to get an answer to.

Is it possible that a Fuel Flow Meter gives increasing indications when reducing mixture above the max power altitude (ex. Piper Seneca Turbo 12000ft), reaching peak and then starting to decrese like an EGT?

Thank you all!

Mach E Avelli 6th Nov 2013 07:32

None of the usual experts seem to want to answer this.

In THEORY, when you lean the mixture without any change in throttle setting, the fuel flow will always decrease. In practice, if when leaning you cause power to increase and there is a momentary increase in RPM as a result, I suppose it is possible for the engine to draw in more fuel which could show as an increase in fuel flow. But I would expect any such increase to be momentary, because unless the propeller is on the fine pitch stops it will govern back to the selected RPM.

A fixed pitch propeller installation (unlikely in a turbo) could possibly behave somewhat as you describe - I dunno because I have never flown a fixed pitch prop turbo charged engine, and never tried to run a turbo engine above max power altitude in full fine pitch.

Over to the experts now......

Lightning Mate 6th Nov 2013 08:16

It depends on what he means by max power altitude.

I think he means critical altitude ie the wastegate has just fully closed.

Oktas8 6th Nov 2013 09:31

Thinking this one through... (Bear in mind that all Seneca turbo charged engines are fitted with CSU: assume constant RPM.)

Retarding mixture -> reducing fuel flow -> better fuel/air mixture -> higher energy exhaust -> faster turbine spinning -> higher turbo compression -> higher intake air density -> more fuel -> higher fuel flow.

But I think that chain of logic fails at the second last step. You get more power as you retard the mixture, certainly. But I'm thinking that if you reduce fuel flow by (e.g.) 2% in step 2, it might increase again by (e.g.) 1% in the second last step.

Overall and as far as I know, FF reduces when one retards the mixture under all circumstances (barring a broken component). It's the power output and EGT that behave in a more complex way.

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