JP-A-2005-207405 (U.S. Pat. No. 7,051,725 B2) shows a cylinder air-fuel ratio control system including the controller. In this system, air-fuel ratios of a plurality of cylinders are estimated for the respective cylinders on the basis of the output of one air-fuel ratio sensor disposed in an exhaust confluent portion where exhaust gas from the plurality of cylinders joins, air-fuel ratio correction quantities of the respective cylinders (cylinder air-fuel ratio correction quantities) for correcting variations in an air-fuel ratio between the cylinders are computed for the respective cylinders, the cylinder air-fuel ratio correction quantities are learned by smoothing processing or the like, and cylinder air-fuel ratio control for controlling the air-fuel ratios (fuel injection quantities) of the plurality of cylinders for the respective cylinders is performed on the basis of the cylinder air-fuel ratio correction quantities and their learning values. Further, in this cylinder air-fuel ratio control system, on operating condition that the estimation of the cylinder air-fuel ratio is difficult (the computation of the cylinder air-fuel ratio is difficult), the cylinder air-fuel ratio control is performed by the use of the learning values of the cylinder air-fuel ratio correction quantities.
In this cylinder air-fuel ratio control system, for example, when the estimation accuracy of the cylinder air-fuel ratio is decreased by a deviation in sampling timing of the output of the air-fuel sensor or a variation in the fuel injection quantity is increased by individual difference of the fuel injection system of each cylinder (variation in manufacture or deterioration with time of a fuel injection valve), variations in the air-fuel ratio between the cylinders may be increased to increase the cylinder air-fuel ratio correction quantity and its learning value. However, it can be thought that as the cylinder air-fuel ratio correction quantity and its learning value are increased, the accuracies thereof are degraded. Hence, when the learning value of the cylinder air-fuel ratio correction quantity is updated without limitation, there is a possibility that the cylinder air-fuel ratio correction quantity is erroneously learned to perform the cylinder air-fuel ratio control in an erroneous direction. This contrarily raises the possibility of impairing the accuracy of the cylinder air-fuel ratio control.
Moreover, this engine control system is mounted with a self-diagnosis function of performing various abnormality diagnoses such as deterioration diagnosis of a catalyst, abnormality diagnosis of a fuel system, abnormality diagnosis of an air-fuel ratio sensor, diagnosis of state of combustion (detection of misfire), and diagnosis of fuel property.
These abnormality diagnoses are performed by the use of the output of the air-fuel ratio sensor and parameters affected and changed by the air-fuel ratio (for example, engine revolution speed). Hence, when the state of control of the cylinder air-fuel ratio control is not stabilized, there is a possibility that the abnormality diagnoses will be erroneously performed because of the effect of the unstable state of control.