Patent ID: 7073325

Claim:
An exhaust emission control method for treating exhaust gas emitted from an internal combustion engine, comprising the steps of: locating an emission control device in an exhaust gas passage of the internal combustion engine, the emission control device storing SOx when an air/fuel ratio of exhaust gas flowing through the emission control device is lean, and releasing the stored SOx when a temperature of the emission control device is raised to a desulfurization temperature or higher and the air/fuel ratio of the exhaust gas flowing through the emission control device becomes substantially equal to a stoichiometric air/fuel ratio or rich; performing a temperature control process to control the temperature of the emission control device to be within a predetermined temperature range whose lower limit is substantially equal to or higher than a desulfurization temperature and to raise the temperature of the emission control device only when the air/fuel ratio of exhaust gas is leaner than or equal to the stoichiometric air/fuel ratio; and performing a desulfurization process to release sulfur from the emission control device by controlling the air/fuel ratio of the exhaust gas flowing through the emission control device to be substantially equal to the stoichiometric air/fuel ratio or rich when the temperature of the emission control device is within the predetermined temperature range, wherein the temperature control process and the desulfurization process are repeated when sulfur is to be released from the emission control device.