| abstract |
When switching an air-fuel ratio of an internal combustion engine from a stoichiometric air-fuel ratio to a lean air-fuel ratio, NH3 is rapidly supplied to an SCR catalyst. A three-way catalyst, an NSR catalyst, and an SCR catalyst are sequentially provided in an exhaust passage of an internal combustion engine, and before the air-fuel ratio in the internal-combustion engine is switched from a stoichiometric air-fuel ratio to a lean air-fuel ratio, The period corresponding to the oxygen storage amount of the NSR catalyst is set to the first air-fuel ratio which is a rich air-fuel ratio, and then the NOx occlusion amount is less than the occlusion amount threshold until the NH3 adsorption amount of the SCR catalyst reaches the predetermined adsorption amount. In this case, the second air-fuel ratio is higher than the first air-fuel ratio and lower than the stoichiometric air-fuel ratio. When the NOx storage amount is equal to or greater than the storage amount threshold, the third air-fuel ratio is higher than the first air-fuel ratio and lower than the second air-fuel ratio. Let the air-fuel ratio. [Selection] Figure 3 |