Double air-fuel ratio sensor system carrying out learning control operation

摘要

In a double air-fuel sensor system including two air-fuel ratio sensors upstream and downstream of a catalyst converter provided in an exhaust gas passage, an actual air-fuel ratio is adjusted in accordance with the outputs of the upstream-side and downstream-side air-fuel ratio sensors including an air-fuel ratio correction amount. When a change of an air-fuel ratio correction amount or a change of an air-fuel ratio feedback control parameter calculated in accordance with the output of the downstream-side air-fuel ratio sensor is small, a learning correction amount is calculated so that a mean value of the air-fuel ratio correction amount is brought close to a reference value. The actual air-fuel ratio is further adjusted in accordance with the learning correction amount.