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An exhaust-emission purifying apparatus for an internal combustion engine, includes an NOx purifying catalyst disposed in an exhaust system of the engine to purify NOx in an exhaust emission of the engine, and hold a specific substance (in addition to NOx) in the exhaust emission by adsorption or ab

An exhaust-emission purifying apparatus for an internal combustion engine, includes an NOx purifying catalyst disposed in an exhaust system of the engine to purify NOx in an exhaust emission of the engine, and hold a specific substance (in addition to NOx) in the exhaust emission by adsorption or absorption of the specific substance. A controller is operable to detect an NOx purifying ability of the NOx purifying catalyst which is lowered with an increase in the amount of the specific substance held by the catalyst. The controller compares the detected NOx purifying ability with a predetermined threshold value to determine whether the catalyst has been poisoned with the specific substance. When the controller determines that the catalyst has been poisoned with the specific substance, the controller commands at least the engine to effect a poisoning removing treatment of the catalyst, for releasing the specific substance from the catalyst. The threshold value is varied, for example, based upon the operating condition of the engine.

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1. An exhaust-emission purifying method comprising the steps of:determining a threshold value used to determine whether an NOx purifying catalyst has been poisoned with a specific substance contained in an exhaust emission from the internal combustion engine, the specific substance being a substance

1. An exhaust-emission purifying method comprising the steps of:determining a threshold value used to determine whether an NOx purifying catalyst has been poisoned with a specific substance contained in an exhaust emission from the internal combustion engine, the specific substance being a substance other than NOx, the threshold value being selected from a plurality of pre-stored threshold values; comparing an NOx purifying ability of the NOx purifying catalyst to purify NOx contained in the exhaust emission with the threshold value, to determine whether the NOx purifying catalyst has been poisoned with the specific substance; and releasing the specific substance from the NOx purifying catalyst when it is determined that the NOx purifying catalyst has been poisoned with the specific substance, wherein the threshold value decreases with an increase in load on the internal combustion engine or an increase in an operating speed of the internal combustion engine, and increases with a decrease in the load on the internal combustion engine or a decrease in the operating speed of the internal combustion engine. 2. An exhaust-emission purifying method according to claim 1, wherein the step of releasing the specific substance from the NOx purifying catalyst comprises selecting at least one of a plurality of different poisoning removing operations depending upon an operating condition of the internal combustion engine, and commanding at least the internal combustion engine to effect the poisoning removing treatment of the NOx purifying catalyst by the selected at least one of the plurality of different poisoning removing operations.3. An exhaust-emission purifying method according to claim 1, wherein an operating condition of the internal combustion engine is divided into a plurality of different operating states, and the threshold value of the NOx purifying ability is set for each of the plurality of different operating states of the internal combustion engine.4. An exhaust-emission purifying method according to claim 1, wherein the specific substance is SOx.5. An exhaust-emission purifying method comprising the steps of:detecting an operating condition of the internal combustion engine, on the basis of an amount of fuel injection into the internal combustion engine and an operating speed of the internal combustion engine; determining a threshold value used to determine whether an NOx purifying catalyst has been poisoned with a specific substance contained in an exhaust emission from the internal combustion engine, the specific substance being a substance other than NOx, such that the threshold value is changed with the detected operating condition of the internal combustion engine; comparing an NOx purifying ability of the NOx purifying catalyst to purify NOx contained in the exhaust emission with the threshold value, to determine whether the NOx purifying catalyst has been poisoned with the specific substance; and releasing the specific substance from the NOx purifying catalyst when it is determined that the NOx purifying catalyst has been poisoned with the specific substance, wherein the threshold value decreases with an increase in load on the internal combustion engine or an increase in an operating speed of the internal combustion engine, and increases with a decrease in the load on the internal combustion engine or a decrease in the operating speed of the internal combustion engine. 6. An exhaust-emission purifying method according to claim 5, wherein the operating condition of the internal combustion engine is divided into a plurality of different operating states, and the threshold value of the NOx purifying ability is set for each of the plurality of different operating states of the internal combustion engine.7. An exhaust-emission purifying method according to claim 5, wherein the specific substance is SOx.8. An exhaust-emission purifying method according to claim 5, wherein the step of releasing the specific substance from the NOx purifying catalyst comprises selecting at least one of a plurality of different poisoning removing operations depending upon the operating condition of the internal combustion engine, and commanding at least the internal combustion engine to effect the poisoning removing treatment of the NOx purifying catalyst by the selected at least one of the plurality of different poisoning removing operations.9. An exhaust-emission purifying method according to claim 8, wherein the at least one of the plurality of different poisoning removing operations includes at least one of: (1) an operation of the internal combustion engine at a stoichiometric air/fuel ratio; (2) an operation to change a path of flow of the exhaust emission through the exhaust system such that a temperature of the exhaust emission to which the NOx purifying catalyst is exposed is raised; (3) a cylinder-bank control operation of the internal combustion engine wherein at least one of a plurality of groups of cylinders of the internal combustion engine is operated at a fuel-rich air/fuel ratio while the other of the plurality of groups is operated at a fuel-lean air/fuel ratio; and (4) a post-fuel-injection control operation of the internal combustion engine wherein a fuel is injected into cylinders of the internal combustion engine after an intake stroke of the cylinders.10. An exhaust-emission purifying method according to claim 5, wherein the step of releasing the specific substance from the NOx purifying catalyst comprises commanding at least the internal combustion engine to initiate a poisoning removing treatment of the NOx purifying catalyst to release the specific substance from the NOx purifying catalyst when it is determined that the NOx purifying catalyst has been poisoned with the specific substance, and when a predetermined operating condition of the internal combustion engine is established, the method further comprising a step of commanding the internal combustion engine to be operated at a stoichiometric air/fuel ratio when the predetermined operating condition is not established while it is determined that the NOx purifying catalyst has been poisoned with the specific substance.11. An exhaust-emission purifying method according to claim 10, wherein the predetermined operating condition is established when a temperature of the exhaust emission is higher than a predetermined lower limit.12. An exhaust-emission purifying method according to claim 5, wherein the operating condition of the internal combustion engine is divided into a plurality of different operating states defined by respective combinations of an amount of fuel injection into the internal combustion engine and the operating speed of the internal combustion engine, and the threshold value of the NOx purifying ability is set for each of the plurality of different operating states of the internal combustion engine.13. An exhaust-emission purifying method according to claim 12, wherein the NOx purifying ability is detected on the basis of a concentration of NOx in the exhaust emission, and a determination as to whether the NOx purifying catalyst has been poisoned with the specific substance is effected by comparing the detected concentration of NOx with the threshold value.14. An exhaust-emission purifying method according to claim 13, further comprising detecting a change in the concentration of NOx, and wherein the step of releasing the specific substance from the NOx purifying catalyst is implemented when a minimum value of the detected concentration of NOx is higher than a predetermined first threshold value and when a maximum value of the detected concentration of NOx is higher than a predetermined second threshold value.15. An exhaust-emission purifying method according to claim 12, wherein the NOx purifying ability is detected on the basis of a concentration of NOx in the exhaust emission, and a determination as to whether the NOx purifying catalyst has been poisoned with the specific substance is effected by comparing a rich-spike time interval between adjacent rich-spike operations of the internal combustion engine, each effected when the concentration of NOx has reached a predetermined upper limit, with the predetermined threshold value.16. An exhaust-emission purifying method according to claim 15, further comprising counting a number of consecutive occurrences of the adjacent rich-spike operations, the rich-spike time interval of which is shorter than the predetermined threshold value, and wherein the step of comparing an NOx purifying ability of the NOx purifying catalyst with the threshold value to determine whether the NOx purifying catalyst has been poisoned with the specific substance comprises determining that the NOx purifying catalyst has been poisoned with the specific substance, when the counted number of the consecutive occurrences has increased to a predetermined value.17. An exhaust-emission purifying method, comprising:detecting an operating condition of the internal combustion engine, on the basis of an amount of fuel injection into the internal combustion engine and an operating speed of the internal combustion engine; determining a threshold value used to determine whether an NOx purifying catalyst has been poisoned with a specific substance contained in an exhaust emission from the internal combustion engine, the specific substance being a substance other than NOx, such that the threshold value is changed with the detected operating condition of the internal combustion engine; comparing an NOx purifying ability of the NOx purifying catalyst to purify NOx contained in the exhaust emission with the threshold value, to determine whether the NOx purifying catalyst has been poisoned with the specific substance; detecting a change in the concentration of NOx; and releasing the specific substance from the NOx purifying catalyst when a minimum value of the detected concentration of NOx is higher than a predetermined first threshold value and when a maximum value of the detected concentration of NOx is higher than a predetermined second threshold value. 18. An exhaust-emission purifying method, comprising:detecting an operating condition of the internal combustion engine, on the basis of an amount of fuel injection into the internal combustion engine and an operating speed of the internal combustion engine; determining a threshold value used to determine whether an NOx purifying catalyst has been poisoned with a specific substance contained in an exhaust emission from the internal combustion engine, the specific substance being a substance other than NOx, such that the threshold value is changed with the detected operating condition of the internal combustion engine; counting a number of consecutive occurrences of adjacent rich-spike operations of the internal combustion engine, a rich-spike time interval of which is shorter than a predetermined threshold value; comparing an NOx purifying ability of the NOx purifying catalyst with the threshold value to determine whether the NOx purifying catalyst has been poisoned with the specific substance; and releasing the specific substance from the NOx purifying catalyst when the counted number of the consecutive occurrences has increased to a predetermined value. 19. An exhaust-emission purifying apparatus for an internal combustion engine, comprising:an NOx purifying catalyst disposed in an exhaust system of the internal combustion engine, and operable to purify NOx contained in an exhaust emission from the internal combustion engine; and a controller operable to detect an NOx purifying ability of the NOx purifying catalyst, (i) the controller comparing the detected NOx purifying ability with a predetermined threshold value to determine the NOx purifying ability of the NOx purifying catalyst is below the threshold, (ii) the predetermined threshold value being changed with an operating condition of the internal combustion engine, (iii) the controller commanding at least the internal combustion engine to effect a treatment of the NOx purifying catalyst when the controller determines that the NOx purifying ability of the NOx purifying catalyst is below the threshold, wherein the threshold value decreases with an increase in load on the internal combustion engine or an increase in an operating speed of the internal combustion engine, and increases with a decrease in the load on the internal combustion engine or a decrease in the operating speed of the internal combustion engine. 20. An exhaust-emission purifying apparatus for an internal combustion engine, comprising:an NOx purifying catalyst disposed in an exhaust system of the internal combustion engine, and operable to purify NOx contained in an exhaust emission from the internal combustion engine and to hold a specific substance in the exhaust emission, in addition to NOx, by adsorption or absorption of the specific substance, an NOx purifying ability of the NOx purifying catalyst to purify the NOx being lowered with an increase in an amount of the specific substance held by the catalyst; and a controller operable to detect the NOx purifying ability of the NOx purifying catalyst, (i) the controller comparing the detected NOx purifying ability with a predetermined threshold value to determine whether the NOx purifying catalyst has been poisoned with the specific substance, (ii) the predetermined threshold value being changed with an operating condition of the internal combustion engine, (iii) the controller commanding at least the internal combustion engine to effect a poisoning removing treatment of the NOx purifying catalyst, for releasing the specific substance from the NOx purifying catalyst, when the controller determines that the NOx purifying catalyst has been poisoned with the specific substance, wherein the threshold value decreases with an increase in load on the internal combustion engine or an increase in an operating speed of the internal combustion engine, and increases with a decrease in the load on the internal combustion engine or a decrease in the operating speed of the internal combustion engine. 21. An exhaust-emission purifying apparatus according to claim 20, wherein the operating condition of the internal combustion engine is divided into a plurality of different operating states, and the threshold value of the NOx purifying ability is set for each of the plurality of different operating states of the internal combustion engine.22. An exhaust-emission purifying apparatus according to claim 20, wherein the specific substance is SOx.23. An exhaust-emission purifying apparatus according to claim 20, wherein the controller selects at least one of a plurality of different poisoning removing operations depending upon the operating condition of the internal combustion engine, and commands at least the internal combustion engine to perform the selected at least one of the plurality of different poisoning removing operations, to effect the poisoning removing treatment of the NOx purifying catalyst.24. An exhaust-emission purifying apparatus according to claim 23, wherein the at least one of the plurality of different poisoning removing operations includes at least one of: (1) an operation of the internal combustion engine at a stoichiometric air/fuel ratio; (2) an operation to change a path of flow of the exhaust emission through the exhaust system such that a temperature of the exhaust emission to which the NOx purifying catalyst is exposed is raised; (3) a cylinder-bank control operation of the internal combustion engine wherein at least one of a plurality of groups of cylinders of the internal combustion engine is operated at a fuel-rich air/fuel ratio while the other of the plurality of groups is operated at a fuel-lean air/fuel ratio; and (4) a post-fuel-injection control operation of the internal combustion engine wherein a fuel is injected into cylinders of the internal combustion engine after an intake stroke of the cylinders.25. An exhaust-emission purifying apparatus according to claim 20, wherein the controller commands at least the internal combustion engine to initiate the poisoning removing treatment of the NOx purifying catalyst when the controller determines that the NOx purifying catalyst has been poisoned with the specific substance, and when a predetermined operating condition of the internal combustion engine is established, the controller commanding the internal combustion engine to be operated at a stoichiometric air/fuel ratio when the predetermined operating condition is not established while the controller determines that the NOx purifying catalyst has been poisoned with the specific substance.26. An exhaust-emission purifying apparatus according to claim 25, wherein the predetermined operating condition is established when a temperature of the exhaust emission is higher than a predetermined lower limit.27. An exhaust-emission purifying apparatus according to claim 20, wherein the operating condition of the internal combustion engine is divided into a plurality of different operating states defined by respective combinations of an amount of fuel injection into the internal combustion engine and the operating speed of the internal combustion engine, and the threshold value of the NOx purifying ability is set for each of the plurality of different operating states of the internal combustion engine.28. An exhaust-emission purifying apparatus according to claim 27, wherein the controller detects the NOx purifying ability on the basis of a concentration of NOx in the exhaust emission, and determines whether the NOx purifying catalyst has been poisoned with the specific substance, by comparing the detected concentration of NOx with the predetermined threshold value.29. An exhaust-emission purifying apparatus according to claim 27, wherein the controller detects the NOx purifying ability on the basis of a concentration of NOx in the exhaust emission, and determines whether the NOx purifying catalyst has been poisoned with the specific substance, by comparing a rich-spike time interval between adjacent rich-spike operations of the internal combustion engine, each effected when the concentration of NOx has reached a predetermined limit, with a predetermined interval used as the threshold value.30. An exhaust-emission purifying apparatus for an internal combustion engine, comprising:an NOx purifying catalyst disposed in an exhaust system of the internal combustion engine, and operable to purify NOx contained in an exhaust emission from the internal combustion engine and to hold a specific substance in the exhaust emission, the specific substance being a substance in addition to NOx, by adsorption or absorption of the specific substance, an NOx purifying ability of the NOx purifying catalyst to purify the NOx being lowered with an increase in an amount of the specific substance held by the catalyst; and a controller operable to detect the NOx purifying ability of the NOx purifying catalyst, (i) the controller comparing the detected NOx purifying ability with a predetermined threshold value to determine whether the NOx purifying catalyst has been poisoned with the specific substance, (ii) the predetermined threshold value being selected from a plurality of available threshold values, (iii) the controller commanding at least the internal combustion engine to effect a poisoning removing treatment of the NOx purifying catalyst, for releasing the specific substance from the NOx purifying catalyst, when the controller determines that the NOx purifying catalyst has been poisoned with the specific substance, wherein the threshold value decreases with an increase in load on the internal combustion engine or an increase in an operating speed of the internal combustion engine, and increases with a decrease in the load on the internal combustion load or a decrease in the operating speed of the internal combustion engine. 31. An exhaust-emission purifying apparatus according to claim 30, wherein the controller selects at least one of a plurality of different poisoning removing operations depending upon an operating condition of the internal combustion engine, and commands at least the internal combustion engine to perform the selected at least one of the plurality of different poisoning removing operations, to effect the poisoning removing treatment of the NOx purifying catalyst.32. An exhaust-emission purifying apparatus according to claim 30, wherein an operating condition of the internal combustion engine is divided into a plurality of different operating states, and the threshold value of the NOx purifying ability is set for each of the plurality of different operating states of the internal combustion engine.33. An exhaust-emission purifying apparatus according to claim 30, wherein the specific substance is SOx.