Patent ID: 7165392

Claim:
A system for purifying exhaust gas passing through an exhaust passage in an engine, the system comprising: a HC trap catalyst disposed in the exhaust passage, the HC trap catalyst being operative to trap HC contained in the exhaust gas; and a control unit being programmed to: calculate a HC quantity which is trapped by the HC trap catalyst; calculate a temperature of the HC trap catalyst; judge whether or not the HC trap catalyst is activated; control a target excess air ratio to a preset value that is smaller than a normal value and determined based on the calculated HC quantity which is trapped by the HC trap catalyst and the calculated temperature of the HC trap catalyst, during a time period from starting the engine to judging that the HC trap catalyst is activated, to thereby rise an exhaust gas temperature; and when the calculated temperature of the HC trap catalyst is lower than a first temperature and the calculated HC quantity reaches an upper limit, the target excess air ratio is set to a value leaner than a value to which the target excess air ratio has been set before the calculated HC quantity reaches the upper limit.