Catalyst deterioration-detecting device for internal combustion engine

Claim 1

In a catalyst deterioration-detecting device for an internal combustion engine, having first and second exhaust sensors disposed in an exhaust passage of said internal combustion engine respectively on upstream and downstream sides of a catalyst body, said catalyst body being provided generally midway along said exhaust passage, which catalyst deterioration-detecting device provides first feedback control of an air-fuel ratio toward a target value in accordance with the first voltage signal which is output by said first exhaust sensor, while executing a second feedback control of said air-fuel ratio so as to correct said first feedback control in accordance with a second voltage signal which is output by said second exhaust sensor, whereby the deterioration of said catalyst body is determined when predetermined catalyst deterioration-determining conditions are fulfilled, the improvement comprising: a control means having a catalyst deterioration-determining section which determines a degraded state of said catalyst body, when said predetermined catalyst deterioration-determining conditions are established, by taking successive steps of: respectively measuring first and second voltage signal-inverted states within a predetermined arithmetic operating time in accordance with inverted states of first and second voltage signals, thereby calculating an inverted state ratio; respectively measuring first and second voltage signal-surrounding areas within said predetermined arithmetic operating time in accordance with respective areas which are surrounded by loci of periods of time during which said first and second voltage signals are inverted, thereby calculating an area ratio; respectively measuring first and second voltage signal states within said predetermined arithmetic operating time in accordance with said first and second voltage signals, thereby calculating a voltage ratio; calculating a catalyst deterioration-measured value on the basis of said inverted state ratio, said area ratio, and said voltage ratio; and, comparing said catalyst deterioration-measured value and a catalyst deterioration-determining value, the latter value being set for each engine load.