| abstract |
A first catalyst 21 which stores or releases oxygen according to an air-fuel ratio, and a second catalyst 22 which traps or releases NOx according to the air-fuel ratio, are provided in an exhaust passage 9 of an engine 1 . A controller 6 temporarily shifts the air-fuel ratio of the engine 1 to rich when it is determined that the NOx trap catalyst second catalyst 22 is released. At this time, the controller 6 increases a reducing agent supply amount due to the rich shift to be larger, the larger the oxygen amount stored by the two catalysts 21, 22. In this way, the air-fuel ratio in the second catalyst 22 is maintained at a target stoichiometric or rich air-fuel ratio. |