Patent ID: 6865885

Claim:
A method for purifying exhaust gas passing through a particulate filter disposed in an exhaust passage in an internal combustion engine, the method comprising: determining an amount of particulates in the exhaust gas which is trapped and accumulated in the particulate filter; determining whether the particulate filter is at an adequate state for regeneration; and executing regeneration of the particulate filter by controlling oxygen concentration in the exhaust gas depending on the amount of the accumulated particulates when it is determined that the particulate filter is at the adequate state and the amount of the accumulated particulates exceeds a first preset value wherein when the amount of the trapped particulates exceeds a second preset value larger than the first preset value, the executing operation comprises reducing the oxygen concentration in the exhaust gas to a first value smaller than a second value obtained upon the regeneration being unexecuted, wherein when the amount of the accumulated particulates exceeds a third preset value larger than the second preset value, the executing operation comprises reducing the oxygen concentration in the exhaust gas to a third value smaller than a fourth value obtained when the amount of the accumulated particulates is between the second preset value and the third preset value, regardless of whether the particulate filter is at the adequate state for regeneration.