Deterioration diagnostic system of exhaust gas purifying catalyst

A deterioration diagnostic system of an exhaust gas purifying catalyst provided in an exhaust passageway of an internal combustion engine, which is operable to diagnose deterioration of the exhaust gas purifying catalyst based on output signals of upstream and downstream air-fuel ratio sensors which are provided on an upstream side and a downstream side of the exhaust gas purifying catalyst, the deterioration diagnostic system comprising:

• an upstream side switching frequency calculator, operable to calculate the number of times of switching on the upstream side from a rich side to a lean side or from the lean side to the rich side based on the output signals of the upstream air-fuel ratio sensor and an upstream side rich/lean determination level;
• a reference value calculator, operable to calculate a downstream side rich/lean determination level having a first width which is used for performing a downstream side rich/lean determination;
• a downstream side rich/lean determination level adjuster, operable to set the first width of the downstream side rich/lean determination level to a second width that is larger than the first width when the number of times of switching on the upstream side is smaller than a predetermined value;
• a downstream side switching frequency calculator, operable to calculate the number of times of switching on the downstream side from the rich side to the lean side or from the lean side to the rich side based on the output signals of the downstream side air-fuel ratio sensor and the downstream side rich/lean determination level having the second width;
• and
• a deterioration determiner, operable to determine the deterioration of the exhaust gas purifying catalyst based on the number of times of switching on the downstream side and the number of times of switching on the upstream side.

A method for diagnosing deterioration of an exhaust gas purifying catalyst provided in an exhaust passageway of an internal combustion engine based on output signals of upstream and downstream air-fuel ratio sensors which are provided on an upstream side and a downstream side of the exhaust gas purifying catalyst, comprising:

• calculating the number of times of switching on the upstream side from a rich side to a lean side or from the lean side to the rich side based on the output signals of the upstream air-fuel ratio sensor and an upstream side rich/lean determination level;
• calculating a downstream side rich/lean determination level having a first width which is used for performing a downstream side rich/lean determination;
• setting the first width of the downstream side rich/lean determination level to a second width that is larger than the first width when the number of times of switching on the upstream side is smaller than a first predetermined value;
• calculating the number of times of switching on the downstream side from the rich side to the lean side or from the lean side to the rich side based on the output signals of the downstream side air-fuel ratio sensor and the downstream side rich/lean determination level having the second width;
• calculating an intake air volume into the internal combustion engine;
• calculating a frequency ratio that is a ratio between the number of times of switching on the downstream side and the number of times of switching on the upstream side when the intake air volume is no less than a predetermined lower limit value and is no more than a predetermined upper limit value;
• and
• determining the deterioration of the exhaust gas purifying catalyst when the frequency ratio is larger than a second predetermined value.