Patent ID: 6763657
Filing Date: 2004-07-20
Classification: F01N,F02D,Y02T

Abstract:
A method of purifying exhaust gas of an internal combustion engine comprising a NOx absorber catalyst that absorbs NOx when an oxygen concentration of the exhaust gas is excessively increased and releases the absorbed NOx and reduces the NOx at the same time when the oxygen concentration of the exhaust gas is lowered, wherein an air-fuel ratio of the exhaust gas flowing into the said NOx absorber catalyst is temporarily changed from a lean air-fuel ratio to a rich air-fuel ratio in order to release the NOx absorbed by the NOx absorber catalyst and reduce the NOx;wherein the NOx absorber catalyst can absorb NOx but does not release the absorbed NOx when a temperature of the NOx absorber catalyst is within a first temperature range from a predetermined temperature T0 to a first predetermined temperature T1, and the NOx absorber catalyst can sufficiently absorb NOx but does not sufficiently release the absorbed NOx when a temperature of the NOx absorber catalyst is within a second temperature range from the first predetermined temperature T1 to a second predetermined temperature T2, the method comprising: preventing change of air-fuel ratio of the exhaust gas to the rich operation state for the purpose of releasing and reducing NOx, if temperature of said NOx absorber catalyst is lower than the first predetermined temperature T1, and setting the air-fuel ratio to be less rich at first and then grow gradually richer at the time of temporarily changing the air-fuel ratio of the exhaust gas to the rich air-fuel ratio for the purpose of releasing and reducing NOx, if temperature of said NOx absorber catalyst is higher than the first predetermined temperature T1 and lower than the second predetermined temperature T2.