Deterioration detecting apparatus and method for engine exhaust gas purifying device

A deterioration detecting apparatus for an engine having an exhaust purifying catalyst and oxygen responsive upstream and downstream sensors disposed upstream and downstream the catalyst, respectively, the apparatus comprising:

• air/fuel ratio feedback control means for effecting an air/fuel ratio feedback control based on an output of the upstream sensor;
• sub-feedback control means for effecting a sub-feedback control based on an output of the downstream sensor thereby to reflect the output of the downstream sensor in the air/fuel ratio feedback control;
• catalyst deterioration detecting means for detecting deterioration of the catalyst based on the outputs of the downstream sensor and the upstream sensor;
• wherein the catalyst deterioration detecting means includes gain changing means for changing a feedback gain of the sub-feedback control during a catalyst deterioration detecting operation of the catalyst deterioration detecting means;
• the air/fuel ratio feedback control means effects the feedback control to a target air/fuel ratio;
• the sub-feedback control means effects the sub-feedback control on the target air/fuel ratio by using a correction value determined based on the output of the downstream sensor;
• the gain changing means changes a magnitude of the correction value in the sud-feedback control between the catalyst deterioration detecting operation and a normal operation other than the catalyst deterioration detecting operation;
• the correction value includes at least one of a proportional term and an integral term;
• and
• the correction value is increased from a value used in the normal operation when the catalyst deterioration detecting operation is started.

A deterioration detecting method for an engine having an exhaust purifying catalyst and oxygen responsive upstream and downstream sensors disposed upstream and downstream the catalyst, respectively, the method comprising the steps of:

• effecting an air/fuel ratio feedback control to control an air/fuel ratio to a target based on an output of the upstream sensor;
• effecting a sub-feedback control to change the target of the feedback control based on an output of the downstream sensor;
• increasing a feedback gain of the sub-feedback control when the engine is in a deterioration detecting condition;
• calculating a catalyst deterioration parameter using the outputs of the upstream sensor and the downstream sensor in the deterioration detecting condition;
• detecting a catalyst deterioration when the calculated parameter reaches a reference value;
• wherein the feedback gain of the sub-feedback control is increased using a correction value determined based on the output of the downstream sensor, a magnitude of the correction value being changed in the sub-feedback control between the deterioration detecting operation and a non-deterioration detecting operation;
• the correction value includes at least one of a proportional term and an integral term;
• and
• the correction value is increased from a value used in the non-deterioration detecting operation when the deterioration detecting operation is started.