What is the purpose of mixture control?

Question

What's the purpose of mixture control? Is it to control the amount of fuel provided to the engine with which it is associated? If that's the case, what's the purpose of throttle lever then?

Answer 1

The throttle directly controls the air admitted to the engine. In a carburetor (and most gasoline engines with fuel injection as well) this is done with a "butterfly valve" in the intake that rotates from a full throttle position parallel to the airflow, to an idle position that nearly blocks the intake (permitting only a small fraction of the full throttle flow).

Mixture control then adjusts how much fuel is added to that airflow, as a ratio (that is, the mixture compensates for throttle condition, either via manifold pressure as in a carburetor or via mass flow sensor in at least some fuel injection systems). This is important because most gasoline aircraft engines are air cooled -- which means they're partially "charge cooled", with excess heat carried away to some extent by unburned fuel in the exhaust, so adjusting mixture not only adjusts power produced at a given manifold pressure (set by the throttle), but also controls heating (richer mixture will usually give cooler exhaust gas temperature, EGT, which is a proxy for combustion temperature) -- therefore you'll set mixture rich (more fuel for a given airflow) for takeoff and climb, and then "lean out" by manifold pressure and rpm (and/or EGT) for cruise economy.

In older carburetor systems, this was done with a needle valve in the main carburetor jet, which adjusted the flow area where the gasoline was metered into the airstream (you can see this in detail by examining a model airplane engine's carburetor, where the needle valve is adjusted by a thumbscrew, only on the ground). With fuel injection systems, there's either an analog computer that meters the amount of fuel according to the airflow, or a digital computer controlling the injector(s) that does the same job with more ability to compensate for things like sudden throttle changes, temperature, outside humidity, and even fuel quality.

Almost all modern automobile engines have the latter, which is combined with the computer that manages many other functions in the car; aircraft engines are usually more resistant innovation (due to the huge cost of certification), so injections systems there are often 1970s technology, if the airplane they're in is new enough even to have that.