The main engine modes of loading are engine start, idle and light loads, medium loads, full loads and sudden transitions from small to large loads.
By the starting engine is needed a very rich mixture (a=0,2-0,6) since the crankshaft rotational speed is low, and so the fuel evaporates poorly and part of it condenses on the cold walls of the cylinders.
Engine operation at idle and light load is possible with (a=0,7-0,8). The combustible mixture entering to the engine cylinders mixes with residual gases, and so enrichment of the mixture improves its flammability and provides the stable operation of the engine.
The car engine operates at medium load most of the time , that is, not fully open throttle. For this mode, a lean mixture with an excess air ratio a=1,05-1,15 (economical mixture) is needed to ensure economical operation of the engine. With a sharp opening of the throttle, depletion of the combustible mixture is possible, as the amount of incoming air increases. The carburetor must have a device to prevent this depletion.
The engine works with a full load when accelerating the car, driving at maximum speed and overcoming steep climbs and heavy sections of the road. In this case, to obtain the highest engine power, the carburetor must prepare an enriched mixture with a coefficient (a=0,85-0,95).