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 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 system comprising:

• a selective catalytic reduction (SCR) catalyst disposed in the exhaust passage for purifying NOx within the exhaust gas by causing a reaction with ammonia;
• and
• a processor configured to execute:
• a NOx reduction controlling module for performing, when a NOx stored amount in the NOx catalyst exceeds a given determination amount, a first NOx reduction control in which a fuel injector performs a post injection of fuel to continuously control the air-fuel ratio to a target air-fuel ratio so that the stored NOx is reduced and the NOx stored amount falls below a given amount, the target air-fuel ratio being a ratio at which the stored NOx is reducible, the post injection causing the injected fuel to combust inside a cylinder, the performance of the first NOx reduction control permitted when an engine load is within a predetermined medium load range equal to or higher than a first predetermined engine load but lower than a second predetermined engine load,
• wherein the SCR catalyst purifies NOx within an engine operating range where the NOx reduction controlling module does not perform the post injection by the fuel injector to reduce the stored NOx.