Exhaust purification device of an internal combustion engine

An exhaust purification device of an internal combustion engine comprising:

• a pair of NOx storing catalysts arranged in an exhaust passage,
• the pair consisting of an upstream catalyst and a downstream catalyst, the downstream catalyst being located downstream of the upstream catalyst relative to an inflow of exhaust gas to the pair,
• each of the upstream and downstream catalysts store NOx contained in the exhaust gas when the inflow of exhaust gas is a lean air-fuel ratio, and at least one catalyst releases the stored NOx when an oxygen concentration of the exhaust gas drops at least a threshold amount, and
• a controller programmed to judge necessity to move NOx stored in the upstream catalyst to the downstream catalyst, and to send a command to reduce the oxygen concentration of the exhaust gas by at least the threshold amount while keeping the lean air-fuel ratio when the controller judges the necessity,
• wherein when an NOx amount stored in the upstream catalyst exceeds an allowable amount and if a temperature representing temperatures of the pair of NOx storing catalysts is lower than a predetermined set temperature, the controller sends a command to temporarily lower the oxygen concentration in the exhaust gas by at least the threshold amount while maintaining the exhaust gas at the lean air-fuel ratio to cause the upstream NOx storing catalyst to release the NOx and store the released NOx in the downstream NOx storing catalyst, and
• when the NOx amount stored in the upstream catalyst exceeds the allowable amount and if the temperature representing the temperatures of the pair of NOx storing catalysts is higher than the predetermined set temperature, the controller sends a command to temporarily switch the air-fuel ratio of the exhaust gas from lean to rich to cause the upstream NOx storing catalyst and the downstream NOx storing catalyst to release the NOx and reduce the NOx.