Exhaust gas purifying apparatus and controller for internal combustion engine

摘要

When deterioration of a NOx purifying device due to sulfur contamination is detected, a specific operating state flag FHL is set to "1," and, at intervals equal to or shorter than, for example, three seconds, the air-fuel ratio of an air-fuel mixture to be supplied to an engine is varied so that it is alternately leaner and richer than the stoichiometric air-fuel ratio. When the temperature TCAT of the NOx purifying device exceeds a deterioration recovery temperature STCAT2 while the air-fuel ratio variation control is being executed, the SOx removing flag FHLSOx is set to "1," and for a period equivalent to the deterioration recovery time TRSOx, the air-fuel ratio is maintained richer than the stoichiometric air-fuel ratio.