| 摘要 |
PROBLEM TO BE SOLVED: To obtain a method and an apparatus in which the functional state of an NOx occlusion catalyst by an NOx sensor can be detected with good accuracy. SOLUTION: When an operation control procedure in a lean airfuel ratio is started (S120), a timer is started. After a waiting time Tw elapses, a second pumping current IP2 which expresses the concentration of NOx in a gas, to be measured, on the downstream of an NOx occlusion catalyst, a sensor temperature TH, an intake-tube negative pressure Pb and an engine speed of rotation Ne are detected repeatedly (S130 to S160). Then the detected value Pb and the detected value Ne are within an allowable change width, the relative value ΔIP2(=IP2-IP2o) of the second pumping current IP2 corrected on the basis of the sensor temperature TH to a second pumping current IP2o detected first after the start of the timer is computed. When the relative value ΔIP2 is larger than a fixed value Ic, the timer is stopped (S170 to S240). When a timer value TO is smaller than a time threshold value Tth, a refresh request is output, and, when the timer value TO is larger than the time threshold value, a catalyst burning-off request is output (S250 to S270, S200). |
