Exhaust gas purifying apparatus for internal combustion engine

An exhaust gas purifying apparatus for purifying exhaust gases generated by an internal combustion engine, said exhaust gas purifying apparatus comprising:

• a NOx trap material disposed in an exhaust gas passage to absorb NOx while an air-fuel ratio of an exhaust gas is leaner than a stoichiometric air-fuel ratio and release NOx while an air-fuel ratio of an exhaust gas is richer than said stoichiometric air-fuel ratio;
• and
• control means for estimating an amount of NOx absorbed by said NOx trap material, for making an air-fuel ratio richer so as thereby to cause said NOx trap material to release NOx when said NOx trap material is judged on the basis of said amount of NOx absorbed by said NOx trap material to be lower in NOx absorbing capacity than a specified value, for estimating an amount of sulfur absorbed by said NOx trap material, and for making an air-fuel ratio richer and rising an exhaust gas temperature so as thereby to cause said NOx trap material to perform sulfur releasing when said amount of sulfur absorbed by said NOx trap material is greater than a first specified amount;
• wherein said control means conditionally restricts said sulfur releasing of said NOx trap material and restrains said air-fuel ratio from being made richer when said amount of sulfur absorbed by said NOx trap material is larger than a second specified amount that is larger than said first specified amount.

An exhaust gas purifying apparatus having a NOx trap material disposed in an exhaust gas passage for absorbing NOx while an air-fuel ratio of an exhaust gas is leaner than a stoichiometric air-fuel ratio and release NOx while an air-fuel ratio of an exhaust gas is richer than said stoichiometric air-fuel ratio so as thereby to purify exhaust gases generated by an internal combustion engine, said exhaust gas purifying apparatus comprising:

• a speed sensor operative to detect an engine speed;
• an airflow sensor operative to detect an amount of intake air introduced into said engine;
• an oxygen sensor disposed in said exhaust gas passage upstream from said NOx trap material so as to detect a concentration of oxygen in exhaust gas;
• and
• a control unit operative to control a fuel injector and an ignition plug according to said engine speed, said amount of intake air and said concentration of oxygen in exhaust gas;
• wherein said control unit has a target air-fuel ratio control map that defines a first engine operation region for engine operation in an air-fuel ratio at least leaner than a stoichiometric air-fuel ratio, a second engine operation region for engine operation in said stoichiometric air-fuel ratio and a third engine operation region for engine operation in an air-fuel ratio richer than said stoichiometric air-fuel ratio, using engine speed and engine load as parameters so as to control said fuel injector using said target air-fuel ratio map according to said engine speed and said amount of intake air;
• and
• wherein said control unit estimates an amount of NOx absorbed by said NOx trap material while said engine operates in said first engine operation region, estimates an amount of sulfur absorbed by said NOx trap material on the basis of a period of time for which said engine operates in said first engine operation region, retarding an ignition timing of said ignition plug so as thereby to rise a temperature of said exhaust gas when the condition for releasing sulfur that said engine speed and said amount of intake air are lower than specified values, respectively while said amount of sulfur is greater than a first specified amount is not satisfied or when an amount of hard-to-desorb sulfur is less than a specified amount while said amount of sulfur is greater than said first specified amount, and controls said fuel injector so as to bring an air-fuel ratio to said stoichiometric air-fuel ratio on condition that said amount of sulfur is less than a second specified amount greater than said first specified amount while said amount of NOx is greater than a specified amount of NOx.