Air-fuel ratio control apparatus for an internal combustion engine

An air-fuel ratio control apparatus for an internal combustion engine, applied to said engine having a catalyst disposed in an exhaust passage of said engine, comprising:

• a downstream air-fuel ratio sensor disposed in said exhaust passage and at a position downstream of said catalyst, said downstream air-fuel ratio sensor being an oxygen concentration cell type oxygen concentration sensor, which outputs a maximum output value when an amount of oxygen included in a catalyst outflow gas which is a gas flowing out from said catalyst is smaller than an amount necessary to oxidize unburnt substances included in said catalyst outflow gas, and which outputs a minimum output value when said amount of oxygen included in said catalyst outflow gas is larger than said amount necessary to oxidize said unburnt substances included in said catalyst outflow gas;
• and
• air-fuel ratio control means for controlling, based on an output value of said downstream air-fuel ratio sensor, an air-fuel ratio of a mixture supplied to said engine so as to change an air-fuel ratio of a catalyst inflow gas which is a gas flowing into said catalyst;
• wherein,
• said air-fuel ratio control means is configured so as to perform a normal air-fuel ratio feedback control to control said air-fuel ratio of said mixture supplied to said engine so that:
• a) said air-fuel ratio of said catalyst inflow gas becomes an air-fuel ratio richer than a stoichiometric air-fuel ratio when:
• i) said output value of said downstream air-fuel ratio sensor decreases, and
• ii) a magnitude of a change rate of said output value of said downstream air-fuel ratio sensor is larger than or equal to a first change rate threshold, even when the output value is larger than a middle value which is a mid-value of said maximum output value and said minimum output value, and
• b) said air-fuel ratio of said catalyst inflow gas becomes an air-fuel ratio leaner than the stoichiometric air-fuel ratio when:
• i) said output value of said downstream air-fuel ratio sensor increases, and
• ii) said magnitude of said change rate of said output value of said downstream air-fuel ratio sensor is larger than or equal to a second change rate threshold, even when the output value is smaller than said middle value.