Patent ID: 6901750

Claim:
An exhaust emission control apparatus for an internal combustion engine, comprising: an exhaust passage; a NOx retainer agent that is provided in the exhaust passage, and that takes up and retains NOx from an incoming exhaust gas if an air-fuel ratio of the incoming exhaust gas is lean of stoichiometry, and that releases NOx retained by the NOx retainer agent if the air-fuel ratio of the incoming exhaust gas is substantially stoichiometric or rich of stoichiometry; a sulfur component retainer agent that is provided in the exhaust passage upstream of the NOx retainer agent, and that takes up and retains a sulfur component contained in the incoming exhaust gas; and when the air-fuel ratio of the exhaust gas flowing into the sulfur component retainer agent is substantially stoichiometric or rich of stoichiometry, a controller performs a NOx releasing process of adjusting the air-fuel ratio of an exhaust gas flowing into the sulfur component retainer agent so that the air-fuel ratio of an exhaust gas flowing into the NOx retainer agent becomes substantially stoichiometric or rich of stoichiometry if NOx retained by the NOx retainer agent is to be released, and prohibits the air-fuel ratio of the exhaust gas flowing into the sulfur component retainer agent from becoming substantially stoichiometric or rich of stoichiometry if an amount of the sulfur component retained by the sulfur component retainer agent is at least a first predetermined amount.