| 摘要 |
PROBLEM TO BE SOLVED: To efficiently control exhaust emission by efficiently using a plurality of catalysts disposed in series in the exhaust pipe of an engine. SOLUTION: First to third sensors 24 to 26 for detecting the air/fuel ratio or rich/lean of exhaust emission are installed on the upstream and downstream sides of an upstream side catalyst 22 and on the downstream side of a downstream side catalyst 23, a target air/fuel ratio is set based on the outputs of the second sensor 25 and/or third sensor 26, and the air/fuel ratio is feedback- controlled based on a deviation between the target air/fuel ratio and the output of the first sensor 24. In addition, the control range (rich side/lean side guard value) of the target air/fuel ratio is switched according to the combination of the rich and lean of the outputs from the second sensor 24 and the third sensor 26. Thus the control range of the target air/fuel ratio can be switched so that the exhaust emission control ratio of an entire system can be increased to efficiently use both the upstream side catalyst 24 and the downstream side catalyst 25 so as to control the exhaust emission. |
