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Q ID 210516 Piston Engine

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  • Q ID 210516 Piston Engine

    Would someone please explain why this answer is correct? To me it is the most correct of the answers given. But is the question states "The mixture is not leaned" how can the fuel flow reduce? Im not sure if I'm missing something but if the mixture leaver is not moved aft. Wouldn't the fuel flow remain the same I.e the mass of fuel entering the cylinder. But in a climb with increasing altitude, the SFC would decrease slightly with decreasing air density?

    Many thanks 👍

  • #2
    Our engines expert is away for a couple of days so I will answer as best I can.

    I think you are happy with the idea that as altitude increases the density reduces so the fuel flow needs to decrease.

    In practice we find that the venturi in the carburettor doesn't work that well. Although the fuel flow reduces, it doesn't reduce in proportion to the reduced air flow. The result is that the fuel mixture progressively becomes rich. I suspect this is to do with the influence of V2 in the (1/2 rho V2).

    To compensate for the enriching we need to lean the mixture. This can be achieved either by the pilot or by an altitude capsule in the carburettor.

    To come back to the question, the fuel flow decreases with altitude but it doesn't decrease enough so we lean it further.

    I think to introduce SFC at this stage is a distraction.