Exhaust emission control system of engine

An exhaust emission control system of an engine, including an NOx catalyst disposed in an exhaust passage of the engine 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 approximately stoichiometric or rich, the NOx catalyst also functioning as an oxidation catalyst for oxidizing HC, the system comprising:

• an SCR catalyst disposed in an exhaust passage downstream of the NOx catalyst and configured to purify NOx within the exhaust gas by causing a reaction with NH3;
• a urea injector configured to supply urea to the SCR catalyst by injecting urea into the exhaust passage;
• and
• a processor configured to execute:
• a fuel injection controlling module for controlling a fuel injector;
• and
• an NOx reduction controlling module for executing a first NOx reduction control in which the air-fuel ratio is enriched to reach a first target air-fuel ratio so that the stored NOx is reduced, the first target air-fuel ratio being a ratio at which the stored NOx is reducible,
• wherein when the urea injection of the urea injector is determined to be abnormal, the NOx reduction controlling module executes an NH3-supplied NOx reduction control in which the NOx catalyst supplies NH3 to the SCR catalyst, by executing an air-fuel-ratio-enriched NOx reduction control in which the air-fuel ratio is enriched to a second target air-fuel ratio that is richer than the first target air-fuel ratio,
• wherein when the air-fuel-ratio-enriched NOx reduction control is executed after executing a lean air-fuel ratio operation control in which the air-fuel ratio becomes leaner than the first target air-fuel ratio, the NOx reduction controlling module sets a post injection timing in the air-fuel-ratio-enriched NOx reduction control so that fuel injected in a post injection is combusted inside a cylinder of the engine.

An exhaust emission control system of an engine, including a NOx catalyst disposed in an exhaust passage of the engine 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 approximately stoichiometric or rich, the NOx catalyst also functioning as an oxidation catalyst for oxidizing HC, the system comprising:

• a selective catalytic reduction (SCR) catalyst disposed in the exhaust passage downstream of the NOx catalyst and configured to purify NOx within exhaust gas by causing a reaction with NH3;
• a urea injector configured to supply urea to the SCR catalyst by injecting urea into 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 of the exhaust gas 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 of the urea injector is determined to be abnormal, the NOx reduction controlling module performs a NH3-supplied NOx reduction control in a state where a larger amount of unburned fuel than an amount of unburned fuel in the exhaust passage in the NOx reduction control is supplied to the exhaust passage,
• wherein the NOx reduction controlling module determines a retarded amount of a post injection timing in the NH3-supplied NOx reduction control to be larger when the engine speed is low than when the engine speed is high.