Patent ID: 6289673
Filing Date: 2001-09-18
Classification: F02D

Abstract:
An air-fuel ratio control device for an engine, comprising:a catalyst disposed in an exhaust passage of the engine, the catalyst absorbing oxygen when an inflowing exhaust gas mixture flowing into the catalyst is excessive in oxygen as compared with a stoichiometric exhaust gas mixture of a stoichiometric air-fuel ratio, and releasing oxygen stored in the catalyst when the inflowing exhaust gas mixture is deficient in oxygen as compared with the stoichiometric exhaust gas mixture;a memory storing an oxygen storage capacity corresponding to an amount of oxygen stored in the catalyst when the air-fuel ratio of an outflowing exhaust gas mixture flowing out of the catalyst changes from a ratio substantially equal to the stoichiometric ratio to a lean air-fuel ratio; anda microprocessor programmed to:calculate a current oxygen storage amount based on an oxygen absorbing rate of the catalyst which is lower when the air-fuel ratio of the outflowing exhaust gas mixture is lean than when the air-fuel ratio of the outflowing exhaust gas mixture is substantially stoichiometric;control the air-fuel ratio of the inflowing exhaust gas mixture flowing into the catalyst, based on the current oxygen storage amount so as to make the current oxygen storage amount smaller than the oxygen storage capacity when a predetermined air-fuel ratio control condition is satisfied;calculate an oxygen absorbing amount per predetermined time period, based on a product of a rate constant representing the oxygen absorbing rate and an excess oxygen amount in the inflowing exhaust gas mixture per predetermined time period when the inflowing exhaust gas mixture is excess in oxygen as compared with the stoichiometric air fuel mixture, the rate constant representing the oxygen absorbing rate being smaller when the air-fuel ratio of the outflowing exhaust gas mixture is lean than when the air-fuel ratio of the outflowing exhaust gas mixture is substantially stoichiometric;calculate an oxygen releasing amount per predetermined time period, based on a deficient oxygen amount in the inflowing exhaust gas mixture per predetermined time period when the inflowing exhaust gas mixture is deficient in oxygen as compared with the stoichiometric air fuel mixture; andcalculate the current oxygen storage amount based on the oxygen absorbing amount per predetermined time period and the oxygen releasing amount per predetermined time period.