Exhaust emission control system of engine

An exhaust emission control system of an engine, including a NOx catalyst disposed in an exhaust passage of the engine and for storing NOx within exhaust gas when an air-fuel ratio of the exhaust gas is lean, and reducing the stored NOx when the air-fuel ratio is stoichiometric or rich, the NOx catalyst also functioning as an oxidation catalyst for oxidizing HC, the system comprising:

• a SCR catalyst disposed in the exhaust passage downstream of the NOx catalyst and for purifying NOx within exhaust gas by causing a reaction with NH3;
• a urea injector for supplying urea to the SCR catalyst by injecting urea to the exhaust passage;
• and
• a processor configured to execute:
• a fuel injection controlling module for controlling a fuel injector;
• and
• a NOx reduction controlling module for performing a NOx reduction control in which the air-fuel ratio is enriched to reach a target air-fuel ratio so that the stored NOx is reduced, the target air-fuel ratio being a ratio at which the stored NOx is reducible,
• wherein when the urea injection by the urea injector is determined to be abnormal, the NOx reduction controlling module performs an NH3-supplied NOx reduction control in which the NOx catalyst supplies NH3 to the SCR catalyst, by performing the NOx reduction control, a lean air-fuel ratio operation control, and then the NOx reduction control again, the lean air-fuel ratio operation control being a control in which the air-fuel ratio becomes leaner than the target air-fuel ratio.