초록 ▼

A first catalyst 21 which stores or releases oxygen according to an air-fuel ratio, and a second catalyst 22 which traps or releases NOx according to the air-fuel ratio, are provided in an exhaust passage 9 of an engine 1 . A controller 6 temporarily shifts the air-fuel ratio of the engine 1 to rich

A first catalyst 21 which stores or releases oxygen according to an air-fuel ratio, and a second catalyst 22 which traps or releases NOx according to the air-fuel ratio, are provided in an exhaust passage 9 of an engine 1 . A controller 6 temporarily shifts the air-fuel ratio of the engine 1 to rich when it is determined that the NOx trap catalyst second catalyst 22 is released. At this time, the controller 6 increases a reducing agent supply amount due to the rich shift to be larger, the larger the oxygen amount stored by the two catalysts 21, 22 . In this way, the air-fuel ratio in the second catalyst 22 is maintained at a target stoichiometric or rich air-fuel ratio.

대표청구항 ▼

1. An exhaust gas purification device for an engine, comprising:an exhaust passage of the engine;a first catalyst, installed in the exhaust passage, that stores and releases oxygen in exhaust gas in accordance with an air-fuel ratio of exhaust gas;a second catalyst, installed in the exhaust passage

1. An exhaust gas purification device for an engine, comprising:an exhaust passage of the engine;a first catalyst, installed in the exhaust passage, that stores and releases oxygen in exhaust gas in accordance with an air-fuel ratio of exhaust gas;a second catalyst, installed in the exhaust passage downstream of the first catalyst, that traps and releases NOx in exhaust gas in accordance with the air-fuel ratio of exhaust gas; anda microprocessor programmed to:compute a total oxygen amount stored in the first catalyst and the second catalyst,compute a required reducing agent amount based on the total oxygen amount, wherein the required reducing agent amount is increased with increase in the total oxygen amount, andperform a rich operation that controls the air-fuel ratio of exhaust gas to rich when trapped NOx in the second catalyst is to be released until an amount of supplied reducing agent due to the rich operation reaches the required reducing agent amount, andwherein the microprocessor is further programmed to compute a high speed component stored by a noble metal of the catalysts and a low speed component stored by an oxygen storage material of the catalysts separately, and to compute the total oxygen amount from the high speed component and the low speed component. 2. The exhaust gas purification device as defined in claim 1, wherein the microprocessor is further programmed to compute the total oxygen amount stored in the first catalyst and the second catalyst based on an engine operation condition, and wherein the rich operation is performed so that the amount of reducing agent in exhaust gas corresponds to the total oxygen amount. 3. The exhaust gas purification device as defined in claim 1, wherein the rich operation is performed so that the amount of reducing agent in exhaust gas is increased with increase in an amount of trapped NOx in the second catalyst. 4. The exhaust gas purification device as defined in claim 1, wherein the microprocessor is further programmed to compute the total oxygen amount stored in the first catalyst and the second catalyst based on an engine operation condition, and to compute an amount of trapped NOx in the second catalyst based on the engine operation condition, and wherein the rich operation is performed so that the amount of reducing agent in exhaust gas corresponds to the total oxygen amount and the amount of trapped NOx. 5. The exhaust gas purification device as defined in claim 1, wherein the microprocessor is further programmed to compute an oxygen amount in exhaust gas discharged from the engine, and to compute the total oxygen amount stored in the first catalyst and the second catalyst based on an oxygen storage rate and the oxygen amount in exhaust gas discharged from the engine, wherein the oxygen storage rate varies in accordance with the total oxygen amount stored in the first catalyst and the second catalyst. 6. The exhaust gas purification device as defined in claim 5, wherein the oxygen storage rate when the total oxygen amount is smaller than or equal to a predetermined value is larger than the oxygen storage rate when the total oxygen amount is greater than the predetermined value. 7. The exhaust gas purification device as defined in claim 1, wherein the microprocessor is further programmed to compute an excess oxygen amount in exhaust gas discharged from the engine, to compute a high speed component stored by a noble metal of the catalysts and a low speed component stored by an oxygen storage material of the catalysts, and to compute the total oxygen amount by adding the high speed component and the low speed component. 8. The exhaust gas purification device as defined in claim 1, wherein the high speed component has a precedence over the low speed component when the catalysts store oxygen in exhaust gas, and oxygen in exhaust gas is stored as the low speed component when the high speed component is saturated. 9. An exhaust gas purification device for an engine, com prising:first catalyst means, installed in an exhaust passage, for storing and releasing oxygen in exhaust gas in accordance with an air-fuel ratio of exhaust gas;second catalyst means, installed in the exhaust passage downstream of the first catalyst means, for trapping and releasing NOx in exhaust gas in accordance with the air-fuel ratio of exhaust gas; andrich operation means for:computing a total oxygen amount stored in the first catalyst means and the second catalyst means,computing a required reducing agent amount based on the total oxygen amount, wherein the required reducing agent amount is increased with increase in the total oxygen amount, andcontrolling the air-fuel ratio of exhaust gas to rich when trapped NOx in the second catalyst means is to be released until an amount of supplied reducing agent due to the rich operation reaches the required reducing agent amount, andwherein the rich operation means is further for computing a high speed component stored by a noble metal of the catalyst means and a low speed component stored by an oxygen storage material of the catalyst means separately, and for computing the total oxygen amount from the high speed component and the low speed component. 10. The exhaust gas purification device as defined in claim 9, wherein the total oxygen amount stored in the first catalyst means and the second catalyst means is calculated based on an engine operation condition, and wherein the air-fuel ratio of exhaust gas is controlled so that the amount of reducing agent in exhaust gas corresponds to the total oxygen amount. 11. The exhaust gas purification device as defined in claim 10, wherein the rich operation means controls the air-fuel ratio of exhaust gas so that the amount of reducing agent in exhaust gas is increased with increase in an amount of trapped NOx in the second catalyst means. 12. The exhaust gas purification device as defined in claim 11, further comprising, means for computing an amount of trapped NOx in the second catalyst means based on an engine operation condition, and wherein the total oxygen amount stored in the first catalyst means and the second catalyst means is calculated based on the engine operation condition, and the rich operation means controls the air-fuel ratio of exhaust gas so that the amount of reducing agent in exhaust gas corresponds to the total oxygen amount and the amount of trapped NOx. 13. The exhaust gas purification device as defined in claim 12, further comprising, means for computing an oxygen amount in exhaust gas discharged from the engine, and wherein the total oxygen amount stored in the first catalyst means and the second catalyst means is calculated based on an oxygen storage rate and the oxygen amount in exhaust gas discharged from the engine, the oxygen storage rate varies in accordance with the total oxygen amount. 14. The exhaust gas purification device as defined in claim 13, wherein the oxygen storage rate when the total oxygen amount is smaller than or equal to a predetermined value is larger than the oxygen storage rate when the total oxygen amount is greater than the predetermined value. 15. A method for purifying exhaust gas discharged from an engine, comprising:computing a total oxygen amount stored in a first catalyst and a second catalyst based on an engine operation condition, wherein the second catalyst is disposed downstream of the first catalyst;computing a required reducing agent amount based on the total oxygen amount, wherein the required reducing agent amount is increased with increase in the total oxygen amount;performing a rich operation that controls the air-fuel ratio of exhaust gas to rich when trapped NOx in the second catalyst is to be released until an amount of supplied reducing agent due to the rich operation reaches the required reducing agent amount; andcomputing a high speed component stored by a noble metal of the catalysts and a low speed component stored by an oxygen storage material of the cataly sts separately, and computing the total oxygen amount from the high speed component and the low speed component. 16. The method for purifying exhaust gas as defined in claim 15, wherein the rich operation is performed so that the amount of reducing agent in exhaust gas corresponds to the total oxygen amount. 17. The method for purifying exhaust gas as defined in claim 15, further computing an amount of trapped NOx in the second catalyst based on the engine operation condition, and wherein the rich operation is performed so that the amount of reducing agent in exhaust gas corresponds to a sum of the total oxygen amount and the amount of trapped NOx. 18. The method for purifying exhaust gas as defined in claim 15, further computing an oxygen amount in exhaust gas discharged from the engine, and computing the total oxygen amount stored in the first catalyst and the second catalyst based on an oxygen storage rate and the oxygen amount in exhaust gas discharged from the engine, wherein the oxygen storage rate varies in accordance with the total oxygen amount stored in the first catalyst and the second catalyst. 19. The method for purifying exhaust gas as defined in claim 18, wherein the oxygen storage rate when the total oxygen amount is smaller than or equal to a predetermined value is larger than the oxygen storage rate when the total oxygen amount is greater than the predetermined value. 20. An exhaust gas purification device for an engine, comprising:an exhaust passage of the engine;a first catalyst, installed in the exhaust passage, that stores and releases oxygen in exhaust gas in accordance with an air-fuel ratio of exhaust gas;a second catalyst, installed in the exhaust passage downstream of the first catalyst, that traps and releases NOx in exhaust gas in accordance with the air-fuel ratio of exhaust gas; anda microprocessor programmed to:compute a total oxygen amount stored in the first catalyst and the second catalyst,compute a required reducing agent amount based on the total oxygen amount, wherein the required reducing agent amount is increased with increase in the total oxygen amount, andperform a rich operation that controls the air-fuel ratio of exhaust gas to rich when trapped NOx in the second catalyst is to be released until an amount of supplied reducing agent due to the rich operation reaches the required reducing agent amount,wherein the microprocessor is further programmed to compute an oxygen amount in exhaust gas discharged from the engine, and to compute the total oxygen amount stored in the first catalyst and the second catalyst based on an oxygen storage rate and the oxygen amount in exhaust gas discharged from the engine,wherein the oxygen storage rate varies in accordance with the total oxygen amount stored in the first catalyst and the second catalyst, wherein the oxygen storage rate when the total oxygen amount is smaller than or equal to a predetermined value is larger than the oxygen storage rate when the total oxygen amount is greater than the predetermined value. 21. An exhaust gas purification device for an engine, comprising:an exhaust passage of the engine;a first catalyst, installed in the exhaust passage, that stores and releases oxygen in exhaust gas in accordance with an air-fuel ratio of exhaust gas;a second catalyst, installed in the exhaust passage downstream of the first catalyst, that traps and releases NOx in exhaust gas in accordance with the air-fuel ratio of exhaust gas; anda microprocessor programmed to:compute a total oxygen amount stored in the first catalyst and the second catalyst,compute a required reducing agent amount based on the total oxygen amount, wherein the required reducing agent amount is increased with increase in the total oxygen amount, andperform a rich operation that controls the air-fuel ratio of exhaust gas to rich when trapped NOx in the second catalyst is to be released until an amount of supplied reducing agent due to the rich operation reaches the requir ed reducing agent amount,wherein the microprocessor is further programmed to compute an excess oxygen amount in exhaust gas discharged from the engine, to compute a high speed component stored by a noble metal of the catalysts and a low speed component stored by an oxygen storage material of the catalysts, and to compute the total oxygen amount by adding the high speed component and the low speed component. 22. An exhaust gas purification device for an engine, comprising:first catalyst means, installed in an exhaust passage, for storing and releasing oxygen in exhaust gas in accordance with an air-fuel ratio of exhaust gas;second catalyst means, installed in the exhaust passage downstream of the first catalyst means, for trapping and releasing NOx in exhaust gas in accordance with the air-fuel ratio of exhaust gas; endrich operation means for:computing a total oxygen amount stored in the first catalyst means and the second catalyst means,computing a required reducing agent amount based on the total oxygen amount, wherein the required reducing agent amount is increased with increase in the total oxygen amount, andcontrolling the air-fuel ratio of exhaust gas to rich when trapped NOx in the second catalyst means is to be released until an amount of supplied reducing agent due to the rich operation reaches the required reducing agent amount,wherein the total oxygen amount stored in the first catalyst means and the second catalyst means is calculated based on an engine operation condition, and wherein the air-fuel ratio of exhaust gas is controlled so that the amount of reducing agent in exhaust gas corresponds to the total oxygen amount,wherein the rich operation means controls the air-fuel ratio of exhaust gas so that the amount of reducing agent in exhaust gas is increased with increase in an amount of trapped NOx in the second catalyst means;means for computing an amount of trapped NOx in the second catalyst means based on an engine operation condition, and wherein the total oxygen amount stored in the first catalyst means and the second catalyst means is calculated based on the engine operation condition, and the rich operation means controls the air-fuel ratio of exhaust gas so that the amount of reducing agent in exhaust gas corresponds to the total oxygen amount and the amount of trapped NOx; andmeans for computing an oxygen amount in exhaust gas discharged from the engine, and wherein the total oxygen amount stored in the first catalyst means and the second catalyst means is calculated based on an oxygen storage rate and the oxygen amount in exhaust gas discharged from the engine, the oxygen storage rate varies in accordance with the total oxygen amount,wherein the oxygen storage rate when the total oxygen amount is smaller than or equal to a predetermined value is larger than the oxygen storage rate when the total oxygen amount is greater than the predetermined value. 23. A method for purifying exhaust gas discharged from an engine, comprising:computing a total oxygen amount stored in a first catalyst and a second catalyst based on an engine operation condition, wherein the second catalyst is disposed downstream of the first catalyst;computing a required reducing agent amount based on the total oxygen amount, wherein the required reducing agent amount is increased with increase in the total oxygen amount; andperforming a rich operation that controls the air-fuel ratio of exhaust gas to rich when trapped NOx in the second catalyst is to be released until an amount of supplied reducing agent due to the rich operation reaches the required reducing agent amount,further computing an oxygen amount in exhaust gas discharged from the engine, and computing the total oxygen amount stored in the first catalyst and the second catalyst based on an oxygen storage rate and the oxygen amount in exhaust gas discharged from the engine, wherein the oxygen storage rate varies in accordance with the total oxygen amount stored in the first ca talyst and the second catalyst,wherein the oxygen storage rate when the total oxygen amount is smaller than or equal to a predetermined value is larger than the oxygen storage rate when the total oxygen amount is greater than the predetermined value.