I have a fairly good understanding of how engines work, both carbureted or diesel. But I cannot see how, in an injected petrol engine, there is still mention of a mixture setting. In a diesel, one sends an ample quantity of fuel to the injector, and a return line takes care of the excess, simple and efficient.
Diesel engines use QUALITATIVE regulation, petrol engines use QUANTITATIVE regulation.
Qualitative regulation in diesels regulates the power output by amount of fuel injected. The amount of air is always the same. There is nothing like incombustible mixture like with the petrol engines. Mixture knob is not needed at all. The return line is for a different reason here compared to return line in petrol engines.
Quantitative regulation in petrol engines regulates the power by reducing the amount of induced air. The amount of gas has to be reduced too to keep the mixture combustible. With reduction of air mass the amount of gas is reduced automatically in carburetors by their construction (venturi - less air, lower speed, less suction, less fuel). The imperfection of this automatism and the wish of pilot to run richer/leaner can/shall be set by mixture knob.
The injected engine replaced the one "injector" in carburetor by one injector per cylinder just before the induction valve. The compensation for air mass is made differently. The mixture knob is still here for the pilot to decide how rich the mixture should be.
Must I understand that, in an injected petrol engine, the quantity of fuel injected is metered in function of both the throttle setting, and the mixture setting compensated for air pressure?
Throttle setting gives the amount of air allowed to enter the cylinders. The technical realisation is by changing the pressure (rather suction) of induced air. To this mass of air (number of oxygen molecules) the amount of gas is adjusted by some automatic mechanism. Otherwise a simple increase of power would be a very complex task for the pilot to keep the mixture in combustible region.
The mixture knob compensates the imperfection of this mechanism and allows us to adjust the mixture more precisely.
Miroc