Exhaust emission control apparatus of internal combustion engine and method thereof

An exhaust emission control apparatus of an internal combustion engine in which combustion is continuously performed at a lean air/fuel ratio, the exhaust emission control apparatus comprising:

• a NOX catalyst provided in a looped exhaust passage of the internal combustion engine for storing NOX contained in an exhaust gas at a lean air/fuel ratio flowing into the exhaust passage, and reducing the stored NOX in the presence of a reducing agent in the exhaust gas when the air/fuel ratio of the exhaust gas is lowered, a flow direction of the exhaust gas being reversed within the exhaust passage under predetermined conditions,
• a reducing agent supply valve that is provided in the exhaust passage upstream of the NOX catalyst, through which the reducing agent is supplied to the NOX catalyst,
• an exhaust state detector that detects a state of the exhaust gas flowing through the NOX catalyst, and
• a controller that executes (1) a reducing agent supply control by temporarily decreasing the flow rate of the exhaust gas and supplying the reducing agent from the reducing agent supply valve and (2) a correction control to correct a control parameter used in the reducing agent supply control in accordance with an exhaust state value that is obtained from an output of the exhaust state detector after the reducing agent has been supplied from the reducing agent supply valve, wherein, during the correction control, the controller determines a time period elapsing from a predetermined reference timing until the exhaust state value reaches a peak after the supply of the reducing agent from the reducing agent supply valve, and corrects the control parameter such that the time period equals a target time period.