Exhaust gas purification control of diesel engine

An exhaust gas purification device for a diesel engine, comprising:

• a diesel particulate filter which traps particulate matter contained in an exhaust gas of the diesel engine;
• an adjusting device which adjusts an oxygen concentration of the exhaust gas flowing into the filter;
• a nitrogen oxide trap catalyst which, when the oxygen concentration of the exhaust gas of the diesel engine is higher than a value corresponding to a stoichiometric air-fuel ratio, traps nitrogen oxides contained in the exhaust gas, and when the oxygen concentration of the exhaust gas is less than a value corresponding to the stoichiometric air-fuel ratio, reduces trapped nitrogen oxides;
• and
• a controller programmed to:
• calculate a particulate matter deposition amount in the filter;
• control the adjusting device, when the particulate matter deposition amount exceeds a first predetermined amount, so that the oxygen concentration of the exhaust gas does not become a value corresponding to a rich air-fuel ratio;
• control the adjusting device, when the particulate matter deposition amount exceeds a second predetermined amount smaller than the first predetermined amount, to perform a filter regeneration operation, wherein the filter regeneration operation includes an operation to maintain the oxygen concentration of the exhaust gas at a value corresponding to a lean air-fuel ratio while maintaining a temperature of the filter at a predetermined regeneration temperature;
• calculate a nitrogen oxide deposition amount on the nitrogen oxide trap catalyst, and control the adjusting device, when the nitrogen oxide deposition amount exceeds a predetermined nitrogen oxide deposition amount, to perform a nitrogen oxide trap catalyst regeneration operation including an operation to maintain the oxygen concentration of the exhaust gas at a value corresponding to a rich air-fuel ratio;
• and
• control the adjusting device, when the particulate matter deposition amount exceeds the first predetermined amount, so as not to perform the nitrogen oxide trap catalyst regeneration operation even when the nitrogen oxide deposition amount exceeds the predetermined nitrogen oxide deposition amount.