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For example, in a C172S fuel injected type, what does the mixture control lever control exactly in the fuel system?

They have a fuel air control unit which regulates fuel flow based on air passing through butterfly valve. Then what exactly does the mixture lever control?

I guess the fuel air control unit works to maintain e.g. 15:1 fuel air ratio and the mixture control lever selects the ratio? Like if I lean the mixture lever, would the fuel air control unit now try to make a 16:1 ratio?

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Basically two reasons:

  1. You want to use richer mixture for climb as the engine would otherwise easily overheat during slow flight at high power without the excess fuel providing extra cooling. And leaner mixture for cruise for better economy.

  2. The fuel control unit is fairly dumb—to keep it simple—so it does not compensate for air density perfectly and some fine-tuning is still needed.

A FADEC (full-authority digital engine computer) equipped engine could automate both functions, and automotive engines do, but this is an old design without electronic control that they didn't want to update much, because it would be very expensive to ensure the reliability required of aviation engines for the new components.

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The throttle butterfly varies the volume of air that enters the engine, but fuel-air mixture is a function of weight of air (and fuel). The control unit doesn't sense weight of incoming air.

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