Air-fuel ratio control of engine

Abstract

A catalytic converter ( ) having a three-way catalyst which stores oxygen is disposed in the exhaust passage ( ) of an engine ( ). An oxygen storage amount of the catalyst is estimated based on the output of a universal exhaust gas oxygen sensor ( ) provided upstream of the catalytic converter ( ). A control unit ( ) controls an air-fuel ratio of the fuel mixture supplied to the engine ( ) through a fuel injector ( ) so that the oxygen storage amount coincides with a target value. An excess/deficiency oxygen amount in the exhaust gas is accumulated when the output of an oxygen sensor ( ) which detects the oxygen concentration downstream of the catalytic converter ( ) is in an excess oxygen region which is higher than a stoichiometric oxygen concentration region. An average oxygen excess ratio is calculated by dividing the accumulated value by an accumulated intake air amount. The output of the universal exhaust gas oxygen sensor ( ) is corrected based on the average oxygen excess ratio. In the same manner, the output of the universal exhaust gas oxygen sensor ( ) is corrected based on the average oxygen excess ratio when the output of the oxygen sensor ( ) is in an deficiency oxygen region which is lower than the stoichiometric oxygen concentration region. These corrections compensate for fluctuations in the output resulting from deterioration of the universal exhaust gas oxygen sensor ( ) or due to manufacturing errors and the calculation accuracy of the oxygen storage amount of the catalyst is thereby increased.