Air-fuel ratio control system and air-fuel ratio control method of internal combustion engine

An air-fuel ratio control system of an internal combustion engine comprising:

• a running state detector for detecting a running state of the internal combustion engine;
• an air-fuel ratio sensor which changes an output characteristic thereof when an air-fuel ratio of exhaust gas becomes closer to a theoretical air-fuel ratio;
• a fuel feed device for feeding fuel to the internal combustion engine such that a fuel feed amount is adjustable;
• a fuel amount determiner for determining a fuel command value indicating the fuel feed amount of the fuel to be fed by the fuel feed device;
• and
• a memory for storing a first correspondence between running state reference coefficients and running states of the internal combustion engine, and a second correspondence between running state compensation coefficients and the running states, the running state reference coefficients indicating reference fuel feed amounts required to obtain target air-fuel ratios set as corresponding to the running states, respectively such that the target air-fuel ratios are different from each other and include values which are other than the theoretical air-fuel ratio;
• the running state compensation coefficients indicating values obtained by dividing the target air-fuel ratios corresponding to the reference fuel feed amounts by the theoretical air-fuel ratio;
• wherein the fuel amount determiner has a feedback control mode in which the fuel amount determiner determines the fuel command value based on an output of the air-fuel ratio sensor, and a non-feedback control mode in which the fuel amount determiner determines the fuel command value irrespective of the output of the air-fuel ratio sensor,
• in the feedback control mode, the fuel amount determiner determines a running state reference coefficient corresponding to the running state detected by the running state detector based on the first correspondence stored in the memory, determines a running state compensation coefficient corresponding to the running state detected by the running state detector based on the second correspondence stored in the memory, determines a feedback compensation coefficient used to cause the air-fuel ratio to reach a value closer to the theoretical air-fuel ratio based on an output of the air-fuel ratio sensor, and determines the fuel command value using a first formula including the determined running state reference coefficient, the determined running state compensation coefficient, and the determined feedback compensation coefficient, the air-fuel ratio control system further comprising:
• a learning value calculator for calculating a degradation learning compensation coefficient used to suppress a negative effect of degradation of the internal combustion engine which has progressed over time, based on the feedback compensation coefficient determined in the feedback control mode;
• wherein the memory updates and stores the degradation learning compensation coefficient, and
• in the non-feedback control mode, the fuel amount determiner determines the running state reference coefficient corresponding to the running state detected by the running state detector based on the first correspondence stored in the memory, reads the degradation learning compensation coefficient stored in the memory, and determines the fuel command value using a second formula including the determined running state reference coefficient and the read degradation learning compensation coefficient.