Internal combustion engine air-fuel ratio feedback control method functioning to compensate for aging change in output characteristic of exhaust gas concentration sensor

Claim 1

In a method of feedback-controlling an air-fuel ratio of a mixture supplied to an internal combustion engine having an exhaust system and a sensor arranged in said exhaust system for sensing a concentration of an exhaust gas, including comparing an output value from said exhaust gas concentration sensor and a predetermined reference value, and feedback-controlling said air-fuel ratio of the mixture to a desired value by at least one of proportional control and integral control depending upon results of the comparison, said proportional control including correcting said air-fuel ratio by a first correction value when said output value from the exhaust gas concentration sensor changes from a rich side to a lean side or vice versa with respect to said predetermined reference value, and said integral control including correcting said air-fuel ratio by a second correction value whenever a predetermined time period elapses, as long as said output value from the exhaust gas concentration sensor is on the lean side or rich side with respect to said predetermined reference value, the improvement comprising the steps of: (a) calculating a first time period required for said output value from the exhaust gas concentration sensor to make a transition from a peak value on the rich side to said predetermined reference value with respect to said predetermined reference value; (b) calculating a second time period required for said output value to make a transition from a peak value on the lean side to said predetermined reference value with respect to said predetermined reference value; (c) obtaining a ratio of said calculated first time period to said calculated second time period; and (d) altering, in dependence upon said ratio obtained in said step (c), a value of at least one predetermined control factor applied to control of said air-fuel ratio.