Exhaust gas purification system for an internal combustion engine

An exhaust gas purification system for an internal combustion engine comprising:

• an internal combustion engine capable of fuel combustion at lean air-fuel ratios and having an exhaust conduit;
• an NOx catalyst installed in the exhaust conduit of the engine, the NOx catalyst being constructed of zeolite and carrying a metal selected from the group consisting of transition metals and noble metals;
• HC amount increasing means capable of increasing an amount of HC included in exhaust gas flowing to the NOx catalyst;
• catalyst temperature determining means for determining one of a temperature of the NOx catalyst and a temperature representing the NOx catalyst temperature and determining whether the NOx catalyst temperature is below a predetermined temperature;
• and
• HC amount increasing means operating means for operating the HC amount increasing means intermittently when the catalyst temperature determining means determines that the NOx catalyst temperature is below said predetermined temperature, wherein the NOx catalyst reduces NOx included in the exhaust gas from the engine in the presence of radicals generated through partial oxidization of HC, a relatively large amount of said radicals being produced in said NOx catalyst, said produced radicals being held within cells of the NOx catalyst at NOx catalyst temperatures which are below the predetermined NOx catalyst temperature and wherein the HC amount increasing means operating means operates to maintain the HC amount increasing means in an ON state for a first predetermined time period and, subsequently, operates to maintain the HC amount increasing means in an OFF state for a second predetermine time.