Exhaust gas purification system for internal combustion engine

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

• an NOX storage reduction catalyst provided in an exhaust passage of the internal combustion engine, which stores NOX in exhaust gas when the exhaust gas has a lean air-fuel ratio higher than a theoretical air-fuel ratio and desorbs NOX stored in it to let the desorbed NOX be reduced by unburned fuel in the exhaust gas when the exhaust gas has a rich air-fuel ratio lower than the theoretical air-fuel ratio;
• a fuel supply device for supplying fuel to the exhaust gas upstream of the NOX storage reduction catalyst;
• and
• a controller comprising at least one processor,
• wherein the controller configured to:
• determine, after fuel is supplied to a fuel tank of the internal combustion engine, a cetane number of the fuel used in the internal combustion engine after the supply of fuel;
• and
• perform an NOX reduction process by supplying fuel to the exhaust gas through the fuel supply device when an amount of NOX stored in the NOX storage reduction catalyst is larger than or equal to a specific NOX storage amount and a temperature of the exhaust gas flowing into the NOX storage reduction catalyst exceeds a specific threshold temperature that is determined on the basis of the cetane number of the fuel, the threshold temperature being set lower when the cetane number of the fuel is low than when the cetane number of the fuel is high,
• wherein when a quantity of heat generated in the NOX storage reduction catalyst per unit time is smaller than a specific value while the NOX reduction process is being performed, the controller suspends the NOX reduction process in progress, and performs the NOX reduction process later on when the temperature of the exhaust gas flowing into the NOX storage reduction catalyst exceeds an updated threshold temperature higher than the specific threshold temperature.