Device for purifying exhaust gas of diesel engines

A device for purifying exhaust gas of diesel engines, in which an oxidizing catalyst, a diesel particulate filter for trapping particulate matters contained in the exhaust gas, and an NOx occluding/reducing catalyst comprising an NOx occluding substance and a catalytically active component, are successively arranged in an exhaust gas passage of the diesel engine from the upstream side thereof, said device for purifying exhaust gas of diesel engines comprising:

• a bypass that connects the upstream side of said oxidizing catalyst with the downstream side thereof in said exhaust gas passage to bypass said oxidizing catalyst;
• an exhaust gas passage change-over means for changing-over the exhaust gas flow into either said oxidizing catalyst or said bypass;
• an air-fuel ratio control means for controlling the air-fuel ratio of the exhaust gas;
• an NOx occlusion amount detector means for detecting the amount of NOx occluded by said NOx occluding substance;
• and
• a controller for controlling said air-fuel ratio control means and said exhaust gas passage change-over means based on a detection signal from said NOx occlusion amount detector means;
• wherein said controller controls said air-fuel ratio control means so that the exhaust gas will have a lean air-fuel ratio and controls said exhaust gas passage change-over means so that the exhaust gas will flow through said oxidizing catalyst when the amount of NOx occluded by said NOx occluding substance does not reach a predetermined value, and controls said air-fuel ratio control means so that the exhaust gas will have a rich air-fuel ratio and controls said exhaust gas passage change-over means so that the exhaust gas will flow through said bypass when the amount of NOx occluded by said NOx occluding substance exceeds the predetermined value.