Patent ID: 7779619
Filing Date: 2010-08-24
Classification: F01N,F02D,Y02T

Abstract:
1. An air fuel ratio control apparatus for an internal combustion engine, 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 of a mixture in the exhaust gas upstream of said catalyst; a variety of kinds of sensors that detect operating conditions of said internal combustion engine; a first air fuel ratio feedback control section that adjusts the air fuel ratio of the mixture supplied to said internal combustion engine in accordance with an output value of said upstream air fuel ratio sensor and a predetermined control constant thereby to make said air fuel ratio oscillate in rich and lean directions in a periodic manner; a downstream air fuel ratio sensor that is arranged at a location downstream of said catalyst for detecting an air fuel ratio in the exhaust gas downstream of said catalyst; an average air fuel ratio oscillation section that operates said control constant based on an amount of oxygen occlusion of said catalyst so as to make an average air fuel ratio, which is obtained by averaging said periodically oscillating air fuel ratio, oscillate in the rich and lean directions; and a catalyst degradation diagnosis section; wherein said catalyst degradation diagnosis section calculates a correlation between the oscillation of said average air fuel ratio and an output value of said downstream air fuel ratio sensor, and diagnoses the degradation of said catalyst based on the correlation thus calculated, wherein said catalyst degradation diagnosis section calculates a first inversion period based on an oscillation width to the rich direction and to the lean direction of said average air fuel ratio, and a second inversion period based on an oscillation width of the output value of said downstream air fuel ratio sensor; and wherein said catalyst degradation diagnosis section also calculates an oscillation width ratio from a ratio of said first oscillation width and said second oscillation width, and diagnoses the degradation of said catalyst based on said oscillation width ratio thus calculated.