An exhaust purification system is provided with a NOx-occlusion-reduction-type catalyst and a NOx purge rich control unit that executes NOx purge in a case where a catalyst temperature of the NOx-occlusion-reduction-type catalyst is equal to or higher than a catalyst temperature threshold value and a NOx occlusion amount of the NOx-occlusion-reduction-type catalyst is equal to or higher than an NOx occlusion amount threshold value and executes the NOx purge even when the catalyst temperature is lower than a catalyst temperature threshold value in a case where the NOx occlusion amount is equal to or greater than the occlusion amount threshold value, a fuel injection amount resulting from an accelerator operation is equal to or greater than an injection amount threshold value, and an exhaust temperature detected by a first exhaust temperature sensor is equal to or higher than an exhaust temperature threshold value.

At a time of an abnormality diagnosis, for a NOx storage reduction catalyst, a predetermined fuel addition process is started when the temperature of the NOx storage reduction catalyst is lower than a predetermined temperature and a NOx storage amount in the NOx storage reduction catalyst is equal to or more than a predetermined storage amount. Then, the abnormality diagnosis for the NOx storage reduction catalyst is performed based on a comparison between an integrated value of a NOx outflow amount after the predetermined fuel addition process is started and before the temperature of the NOx storage reduction catalyst reaches the predetermined temperature and an integrated value of the NOx outflow amount after the temperature of the NOx storage reduction catalyst reaches the predetermined temperature during the execution of the predetermined fuel addition process.

An exhaust emission control system is provided, which includes a NO.sub.x storage catalyst provided in an exhaust passage of an engine and configured to store NO.sub.x, a urea SCR catalyst provided in the exhaust passage, downstream of the NO.sub.x storage catalyst, and a NO.sub.x reduction controlling module configured to set a NO.sub.x reduction condition and, when the NO.sub.x reduction condition is satisfied, performs NO.sub.x reduction processing in which an air-fuel ratio of exhaust gas is set to be one of near a stoichiometric air-fuel ratio and rich, and the NO.sub.x stored in the NO.sub.x storage catalyst is reduced, the NO.sub.x reduction condition being set looser for the NO.sub.x reduction processing performed the first time after the engine is started than the NO.sub.x reduction processing performed the second and subsequent times after the engine is started.

A control apparatus is applicable to an internal combustion engine having an exhaust passage arranged with an NSR catalyst and an SCR catalyst, wherein when it is necessary to decrease NH.sub.3 adsorbed to the SCR catalyst, then an air-fuel ratio of an air-fuel mixture to be com busted in the internal combustion engine is controlled to a predetermined lean air-fuel ratio which is higher than a theoretical air-fuel ratio if a temperature of the SCR catalyst is not less than a lower limit temperature at which NH.sub.3 can be oxidized, while the air-fuel ratio of the air-fuel mixture to be com busted in the internal combustion engine is controlled to a predetermined weak lean air-fuel ratio which is lower than the predetermined lean air-fuel ratio and which is higher than the theoretical air-fuel ratio if the temperature of the SCR catalyst is less than the lower limit temperature.

An exhaust purification system is provided with a NOx-occlusion-reduction-type catalyst and a NOx purge rich control unit that executes NOx purge in a case where a catalyst temperature of the NOx-occlusion-reduction-type catalyst is equal to or higher than a catalyst temperature threshold value and a NOx occlusion amount of the NOx-occlusion-reduction-type catalyst is equal to or higher than an NOx occlusion amount threshold value and executes the NOx purge even when the catalyst temperature is lower than a catalyst temperature threshold value in a case where the NOx occlusion amount is equal to or greater than the occlusion amount threshold value, a fuel injection amount resulting from an accelerator operation is equal to or greater than an injection amount threshold value, and an exhaust temperature detected by a first exhaust temperature sensor is equal to or higher than an exhaust temperature threshold value.

A three-way catalyst, an NSR catalyst, and an SCR catalyst are provided in this order for an exhaust gas passage, wherein the air-fuel ratio (AFR) is set to a first AFR which is a rich AFR before the AFR is switched from a theoretical AFR to a lean AFR, and then the AFR is set to a second AFR which is higher than the first AFR and lower than the theoretical AFR if a NOx occlusion amount is less than a threshold value during a period until an NH.sub.3 adsorption amount of the SCR catalyst becomes a predetermined adsorption amount, while the AFR is set to a third AFR which is higher than the first AFR and lower than the second AFR if the NOx occlusion amount is not less than the threshold value.

An exhaust purification system includes a NOx-occlusion-reduction-type catalyst that occludes NOx in exhaust in a lean state and reduces and purifies the occluded NOx in exhaust in a rich state, and a NOx purge rich control unit that executes NOx purge of reducing and purifying the occluded NOx by putting the exhaust into the rich state by fuel injection control, where a catalyst temperature of the NOx-occlusion-reduction-type catalyst is equal to or higher than a catalyst temperature threshold value and a NOx occlusion amount of the NOx-occlusion-reduction-type catalyst is equal to or higher than an NOx occlusion amount threshold value, and executes the NOx purge when the catalyst temperature is lower than a catalyst temperature threshold value.

Provided is an exhaust purification system including: an NOx occlusion reduction type catalyst (32), a catalyst temperature estimating unit (115), an NOx occlusion amount estimating unit (113), a regeneration control unit (100) that performs catalyst regeneration of bringing an exhaust gas into a rich state to recover NOx occlusion capacity of the NOx occlusion reduction type catalyst (32), an interval setting unit (118) that sets a target interval from the termination of the catalyst regeneration to a start of next catalyst regeneration, a catalyst regeneration start processing unit (110) that starts the next catalyst regeneration when the NOx occlusion amount is equal to or greater than a threshold and an elapsed time from the termination of the catalyst regeneration reaches the target interval, and an interval target value correcting unit (119) that extends and corrects the target interval based on the NOx occlusion amount when the catalyst temperature is lower than a predetermined catalyst activation temperature.

An exhaust purification system is provided with a NOx-occlusion-reduction-type catalyst and a NOx purge rich control unit that executes NOx purge of reducing and purifying the occluded NOx by putting the exhaust into a rich state by fuel injection control, in a case where a catalyst temperature of the NOx-occlusion-reduction-type catalyst is equal to or higher than a catalyst temperature threshold value and a NOx occlusion amount of the NOx-occlusion-reduction-type catalyst is equal to or higher than an NOx occlusion amount threshold value, and executes the NOx purge even when the NOx occlusion amount is less than the NOx occlusion amount threshold value, in a case where the catalyst temperature is equal to or higher than a catalyst temperature threshold value which is greater than the catalyst temperature threshold value.

An exhaust emission control system of an engine including a NO.sub.x catalyst disposed in an exhaust passage and for storing NO.sub.x within exhaust gas when an air-fuel ratio of the exhaust gas is lean, and reducing the stored NO.sub.x when the air-fuel ratio is approximately stoichiometric or rich, is provided. The system includes a SCR catalyst disposed downstream of the NO.sub.x catalyst and for purifying NO.sub.x by causing a reaction with ammonia, and a processor configured to execute a NO.sub.x reduction controlling module for controlling the air-fuel ratio to a target ratio so that the stored NO.sub.x is reduced. The controlling module limits the performance of the NO.sub.x reduction control when a temperature of the SCR catalyst is above a given temperature and loosens the limitation in a given engine operating state in which an exhaust gas flow rate is above a given rate despite the SCR catalyst temperature.