Exhaust purification system

An exhaust purification system comprising:

• a NOx-occlusion-reduction-type catalyst that is provided in an exhaust passage of an internal combustion engine and occludes NOx in exhaust when the exhaust is in a lean state and reduces and purifies the occluded NOx when the exhaust is in a rich state;
• an electronic control unit (ECU) for reducing and purifying NOx occluded in the NOx-occlusion-reduction-type catalyst by putting the exhaust into the rich state by repetitively performing fuel injection control of at least one of post injection and exhaust pipe injection at a predetermined interval;
• and
• a processor which prohibits execution of NOx purge in a case where a fuel injection amount resulting from an accelerator operation is equal to or smaller than a lower limit threshold value,
• wherein the processor does not prohibit the execution of the NOx purge during the execution of the NOx purge,
• wherein the ECU is configured to determine a plurality of start conditions for starting NOx purge including:
• a second start condition, that an estimated NOx occlusion amount value of the NOx-occlusion-reduction-type catalyst increases to a value equal to or greater than a predetermined occlusion amount threshold value,
• a third start condition, that a NOx conversion efficiency of the NOx-occlusion-reduction-type catalyst is lowered to a value equal to or smaller than a predetermined conversion efficiency threshold value,
• a seventh start condition, that a low-temperature state where the NOx-occlusion-reduction-type catalyst is at a temperature lower than a predetermined catalyst temperature threshold value continues during a predetermined time period, and
• an identification flag is set for identifying based on which of the start conditions that the NOx purge is executed,
• wherein the ECU is configured to determine a fourth prohibition condition where an in cylinder fuel injection amount is smaller than a predetermined lower limit injection amount threshold value, and
• the ECU is further configured to identify the type of the NOx purge on the basis of the identification flag, and to not apply the fourth prohibition condition during the execution of the NOx purge in the cases of the second start condition, the third start condition, and the seventh start condition.