AIR-FUEL RATIO CONTROL DEVICE FOR MULTIPLE CYLINDER INTERNAL COMBUSTION ENGINE

PROBLEM TO BE SOLVED: To enlarge a lean operation area so as to improve fuel consumption and to reduce NOx by individually controlling the air-fuel ratio of mixture cylinder by cylinder according to an operating state, and setting the air-fuel ratio of all cylinders to a lean air-fuel ratio in a first operating state, and the air-fuel ratio of partial cylinders to a rich air-fuel ratio in a second operating state. SOLUTION: In case of providing NOx purifying catalysts 7a-7d for every exhaust passage 6a-6d, the injected fuel quantity from a fuel injection valve 5 is controlled cylinder by cylinder by an ECU 10 on the basis of the output of various sensors to control an air-fuel ratio. At this time, torque T required at that time is obtained from the air quantity per one intake stroke, and according to this torque T, one of the action modes of all cylinder lean control, 1/2 cylinder lean control and all cylinder rich control is selected. Switching control to a state of operating all cylinders at a lean air-fuel ratio and a state of operating only partial cylinders at a rich air-fuel ratio or a stoichiometric air-fuel ratio is then performed to suppress torque fluctuation at the time of switching the air-fuel ratio while improving fuel consumption.