Patent ID: 7895826

Claim:
An air fuel ratio control apparatus for an internal combustion engine characterized by comprising: a catalyst that is arranged in an exhaust system of an internal combustion engine for purifying an exhaust gas from said internal combustion engine; an upstream air fuel ratio sensor that is arranged at a location upstream of said catalyst for detecting an air fuel ratio in an upstream exhaust gas upstream of said catalyst; a downstream air fuel ratio sensor that is arranged at a location downstream of said catalyst for detecting an air fuel ratio in a downstream exhaust gas downstream of said catalyst; a first air fuel ratio feedback control section that adjusts an amount of fuel supplied to said internal combustion engine in accordance with the air fuel ratio detected by said upstream air fuel ratio sensor and an upstream target air fuel ratio so as to make said air fuel ratio in said upstream exhaust gas and said upstream target air fuel ratio coincide with each other; and a second air fuel ratio feedback control section that operates, by using at least proportional calculation and integral calculation, said upstream target air fuel ratio in accordance with an air fuel ratio deviation between the air fuel ratio detected by said downstream air fuel ratio sensor and a downstream target air fuel ratio so as to make the detected air fuel ratio of said downstream air fuel ratio sensor and said downstream target air fuel ratio coincide with each other; wherein said second air fuel ratio feedback control section sets an integral gain of said integral calculation to be larger or an update period of said integral calculation to be smaller in accordance with an increasing flow rate of said exhaust gas, so that a change rate of said integral calculation with respect to said air fuel ratio deviation is increased; and said second air fuel ratio feedback control section also sets a proportional gain of said proportional calculation so as not to be changed with respect to a change in the flow rate of said exhaust gas.