PURPOSE: To perform rapid and easy discrimination of instability of feedback and perform rapid convergence of oscillation by a method wherein a calculated feedback correction factor is compared with a detecting exhaust air-fuel ratio and based on the comparing result, stability of a feedback group is discriminated.
CONSTITUTION: During operation of an engine, a control unit 34 retrieves a map from the number of revolutions of an engine and an intake pressure to calculate a fundamental injection amount, and in a feedback control region, a feedback correction factor is computed based on the output of the LAF sensor 54. In this case, it is decided whether a difference between a target air-fuel ratio and a detecting air-fuel ratio exceeds a given value. When the difference exceeds the given value, it is decided whether the difference is positive and a difference between 1.0 and an adaptation proper factor is also positive. In which case, when the deciding result is NO, it is decided that the difference is negative and besides a difference between 1.0 and an adaptive correction factor is also negative. When one of them is YES, increment of the value of a counter is effected and when a result exceeds a reference value, it is decided that a control system is stably operated.
AKASAKI NAOSUKE