Exhaust gas purification system for internal combustion engine

Abstract

Concerning an internal combustion engine equipped with an NSR catalyst and an SCR, this invention is intended to provide an exhaust gas purification system for an internal combustion engine, the system being used to suppress degradation of emissions due to blow-through of NOx. The exhaust gas purification system, used for the internal combustion engine susceptible of lean-burn operation, includes an NSR catalyst disposed in an exhaust passageway of the internal combustion engine, an SCR disposed on a downstream side relative to the NSR catalyst, and an element for executing rich-spike control at a predetermined interval during lean-burn operation. On the basis of a first rule as a relationship between the time interval and the amount of NH 3 generated in the NSR catalyst during rich-spike control, and a second rule as a relationship between the time interval and the amount of NOx blown through the NSR catalyst to the downstream side thereof during rich-spike control, the time interval is set to stay in a period during which a total of the amount of NOx blown through will be reduced and purified in the SCR. Preferably, the time interval is set so that a ratio of the amount of NH 3 generation to that of NOx blow-through is greater than a predetermined ratio.

1 . An exhaust gas purification system for an internal combustion engine which operates in lean-burn mode, the system comprising:
 an NOx storage reduction catalyst (hereinafter referred to as an NSR catalyst) disposed in an exhaust passageway of the internal combustion engine;   an NOx selective catalytic reduction catalyst (hereinafter referred to as an SCR) disposed on a downstream side relative to the NSR catalyst; and   rich-spike means for executing rich-spike control at a predetermined time interval during lean-burn operation;   wherein the rich-spike means includes:
 first-rule acquisition means for acquiring, as a first rule, a relationship between the time interval and an amount of NH 3  generated in the NSR catalyst during rich-spike control; 
 second-rule acquisition means for acquiring, as a second rule, a relationship between the time interval and an amount of NOx blown through the NSR catalyst to the downstream side thereof during rich-spike control; and 
 time interval setting means for setting the time interval on the basis of the first rule and the second rule so that a total of the amount of NOx blown through will be reduced and purified in the SCR. 
   
     
     
         2 . The exhaust gas purification system for the internal combustion engine according to  claim 1 , wherein the time interval setting means sets the time interval so that a ratio of the amount of NH 3  generation to the amount of NOx blow-through is greater than a predetermined ratio. 
     
     
         3 . The exhaust gas purification system for the internal combustion engine according to  claim 1 , wherein the time interval setting means sets the time interval in a range that an increase rate of the amount of NH 3  generation in the first rule becomes larger than a predetermined rate. 
     
     
         4 . The exhaust gas purification system for the internal combustion engine according to  claim 1 , wherein the time interval setting means sets the time interval in a range that the amount of NH 3  generation in the first rule becomes larger than a predetermined amount. 
     
     
         5 . An exhaust gas purification system for an internal combustion engine which operates in lean-burn mode, the system comprising:
 an NOx storage reduction catalyst (hereinafter referred to as an NSR catalyst) disposed in an exhaust passageway of the internal combustion engine;   an NOx selective catalytic reduction catalyst (hereinafter referred to as an SCR) disposed on a downstream side relative to the NSR catalyst; and   a rich-spike unit that executes rich-spike control at a predetermined time interval during lean-burn operation;   wherein the rich-spike unit includes:
 a first-rule acquisition device that acquires, as a first rule, a relationship between the time interval and an amount of NH 3  generated in the NSR catalyst during rich-spike control; 
 a second-rule acquisition device that acquires, as a second rule, a relationship between the time interval and an amount of NOx blown through the NSR catalyst to the downstream side thereof during rich-spike control; and 
 a time interval setting device that sets the time interval on the basis of the first rule and the second rule so that a total of the amount of NOx blown through will be reduced and purified in the SCR.