Exhaust emission purification system for diesel engine

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

• a first exhaust emission purification device, arranged in an exhaust passage of an engine, having a first continuously regenerating type diesel particulate filter and a first SCR catalyst;
• a bypass exhaust passage placed side by side with an exhaust passage upstream of said first exhaust emission purification device;
• a second exhaust emission purification device, arranged in said bypass exhaust passage, having a second continuously regenerating type diesel particulate filter and a second SCR catalyst;
• reducing agent supply means for NOx cleanup arranged upstream of a diverging portion of said bypass exhaust passage;
• exhaust gas temperature region detection means for detecting an exhaust gas temperature region of the engine;
• exhaust gas temperature rise means for raising exhaust temperature;
• channel switching means for controlling the flow of exhaust gas to said bypass exhaust passage;
• and
• control means for controlling said exhaust gas temperature rise means and said channel switching means correspondingly to an exhaust gas temperature region obtained by detecting by said exhaust gas temperature region detection means, wherein
• said control means is constructed such that when said exhaust gas temperature region detection means detects that said exhaust gas temperature is within a predetermined low-temperature region,
• said exhaust gas temperature rise means raises said exhaust gas temperature, and said channel switching means switches the channel in such a manner that the exhaust gas flows through said bypass exhaust passage, and after causing the exhaust gas to pass through said second exhaust emission purification device, causes the exhaust gas to pass through said first exhaust emission purification device,
• when it is detected by the gas temperature region detection means that the exhaust gas temperature is not in the prescribed low temperature region, the control means causes the channel switching means to switch the flow path so as not to permit a major portion of the exhaust gas to flow through the bypass exhaust passage, thereby causing the major portion of the exhaust gas not to flow through the second exhaust emission purification device but to flow through the first exhaust emission purification device alone.