Controller and control method for internal combustion engine

A control method for an internal combustion engine, wherein the internal combustion engine includes an exhaust purifying device, which purifies exhaust gas discharged from a plurality of cylinders, and fuel injection valves provided for the respective cylinders, the control method comprising:

• a dither control process of operating the fuel injection valves in such a manner that at least one of the cylinders is designated as a rich combustion cylinder and at least another one of the cylinders is designated as a lean combustion cylinder, an air-fuel ratio of the rich combustion cylinder being richer than a stoichiometric air-fuel ratio, and an air-fuel ratio of the lean combustion cylinder being leaner than a stoichiometric air-fuel ratio;
• a first limit requesting process of, in a case in which the fuel injection valves are operated to control air-fuel ratios of the respective cylinders to be equal to each other, requesting to limit the dither control process in such a manner that an absolute value of a difference between the air-fuel ratios of the cylinders is when a degree of variation of the injection amounts of the fuel injection valves of the respective cylinders is great than when the degree of variation is small;
• a second limit requesting process of requesting to limit the dither control process in such a manner that the absolute value is smaller when a torque fluctuation amount of the internal combustion engine is great than when the torque fluctuation amount is small;
• and
• a limiting process of limiting the dither control process in such a manner that the absolute value decreases in accordance with one of requests generated by the first limit requesting process and the second limit requesting process that causes the absolute value to be smaller than the other.

A non-transitory computer readable medium that stores a program for causing a processor to execute a control process for an internal combustion engine, wherein the internal combustion engine includes an exhaust purifying device, which purifies exhaust gas discharged from a plurality of cylinders, and fuel injection valves provided for the respective cylinders, the control process includes

• a dither control process of operating the fuel injection valves in such a manner that at least one of the cylinders is designated as a rich combustion cylinder and at least another one of the cylinders is designated as a lean combustion cylinder, an air-fuel ratio of the rich combustion cylinder being richer than a stoichiometric air-fuel ratio, and an air-fuel ratio of the lean combustion cylinder being leaner than a stoichiometric air-fuel ratio,
• a first limit requesting process of, in a case in which the fuel injection valves are operated to control air-fuel ratios of the respective cylinders to be equal to each other, requesting to limit the dither control process in such a manner that an absolute value of a difference between the air-fuel ratios of the cylinders is when a degree of variation of the injection amounts of the fuel injection valves of the respective cylinders is great than when the degree of variation is small,
• a second limit requesting process of requesting to limit the dither control process in such a manner that the absolute value is smaller when a torque fluctuation amount of the internal combustion engine is great than when the torque fluctuation amount is small, and
• a limiting process of limiting the dither control process in such a manner that the absolute value decreases in accordance with one of requests generated by the first limit requesting process and the second limit requesting process that causes the absolute value to be smaller than the other.