Method and arrangement for purifying exhaust gas in an internal combustion engine

1. A method for purifying exhaust gas in an internal combustion engine having an LNT coupled to an exhaust gas conduit downstream of the engine, and a particulate matter filter coupled to the exhaust gas conduit downstream of the LNT, comprising:

in a single process regenerating the particulate matter filter and de-sulfurizing the LNT via fluctuation of exhaust air/fuel ratio between a lean operation period and a rich operation period; and

shortening a period of lean operation as the lean air/fuel ratio increases, where the rich operation de-sulphates the LNT and the lean operation period regenerates the particulate matter filter.

2. The method of claim 1 , wherein the adjusting includes shortening the lean operation period as the level of the lean exhaust air/fuel ratio increases.

3. The method of claim 2 further comprising adjusting the level of the lean exhaust air/fuel ratio in response to a requested state of operation of the engine.

4. The method of claim 2 further comprising adjusting the level of the lean exhaust air/fuel ratio to decrease as engine load increases.

5. The method of claim 2 further comprising adjusting the rich operation period.

6. A method for purifying exhaust gas in an internal combustion engine having an LNT coupled to an exhaust gas conduit downstream of the engine, and a particulate matter filter coupled to the exhaust gas conduit downstream of the LNT, comprising:

increasing engine exhaust temperature sufficient to de-sulphate the LNT and to regenerate the particulate matter filter in a single process when a predetermined operating condition is met;

operating the engine with a rich air/fuel ratio for a first predetermined time period to de-sulphate the LNT;

subsequently operating the engine with a lean air/fuel ratio for a second predetermined time period to regenerate the particulate matter filter, where the second predetermined time period becomes shorter as the lean air/fuel ratio increases; and

repeating said de-sulphasion and regeneration in a periodic manner at the increased engine exhaust temperature.

7. The method according to claim 6 , wherein said first predetermined time period is adjusted based on engine operating conditions so as to avoid formation and emissions of H 2 S.

8. The method according to claim 7 , wherein said second predetermined time period is adjusted based on operating conditions to avoid thermal damage to the particulate matter filter.

9. The method according to any claim 8 , wherein said predetermined operating condition is met when differential pressure over the particulate matter filter exceeds a first predetermined threshold value.

10. The method according to claim 9 , wherein said repeated de-sulphasion and regeneration is terminated if said differential pressure is lower than a second predetermined threshold value.

11. A method for purifying exhaust gas in an internal combustion engine having an LNT coupled downstream of the engine, and a particulate matter filter coupled downstream of the LNT, comprising:

de-sulphating the LNT and regenerating the particulate matter filter in a single process and at an increased engine exhaust temperature;

during the de-sulphasion and regeneration, repeatedly operating the engine exhaust rich for a first phase and lean for a second phase, where the lean exhaust air/fuel ratio is generated by operating the engine with a lean air/fuel ratio;

increasing the lean air/fuel ratio as engine load decreases; and

shortening a period of the second phase as the lean air/fuel ratio increases.

12. The method of claim 11 wherein during the de-sulphasion and regeneration, operation switches between the first interval of lean exhaust and the second interval of rich exhaust, the method further comprising adjusting a period of the rich air/fuel ratio.

13. The method of claim 12 wherein the de-sulphasion and regeneration is ended with a final interval of limited oxygen concentration.

14. A method for purifying exhaust gas in an internal combustion engine having an LNT coupled downstream of the engine, and a particulate matter filter coupled downstream of the LNT, comprising:

de-sulphating the LNT and regenerating the particulate matter filter in a single process and at an increased engine exhaust temperature;

during the de-sulphasion and regeneration, repeatedly operating the engine exhaust rich for a first phase and lean for a second phase, and adjusting a period of the rich air/fuel ratio, where the lean exhaust air/fuel ratio is generated by operating the engine with a lean air/fuel ratio;

increasing the lean air/fuel ratio as engine load decreases;

shortening a period of the second phase as the lean air/fuel ratio increases;

accumulating periods of the first phases; and

terminating the de-sulphasion and regeneration based on whether the accumulated first phase period has reached a threshold value;

wherein the de-sulphasion and regeneration is ended with a final interval of limited oxygen concentration.

15. A method for purifying exhaust gas in an internal combustion engine having an LNT coupled downstream of the engine, and a particulate matter filter coupled downstream of the LNT, comprising:

de-sulphating the LNT and regenerating the particulate matter filter in a single process and at an increased engine exhaust temperature;

during the de-sulphasion and regeneration, repeatedly operating the engine exhaust rich for a first phase and lean for a second phase, and adjusting a period of the rich air/fuel ratio, where the lean exhaust air/fuel ratio is generated by operating the engine with a lean air/fuel ratio;

increasing the lean air/fuel ratio as engine load decreases;

shortening a period of the second phase as the lean air/fuel ratio increases;

accumulating periods of the second phases; and

terminating the de-sulphasion and regeneration based on whether the accumulated second phase period has reached a threshold value;

wherein the de-sulphasion and regeneration is ended with a final interval of limited oxygen concentration.

16. A system for purifying exhaust gas in an internal combustion engine, comprising:

an LNT and a particulate matter filter arranged in series in an exhaust gas conduit of the engine; and

a control unit for controlling the operation of the engine, wherein said control unit is arranged for initiating a heating phase wherein the engine is set to generate heat sufficient for said de-sulphasion of said LNT and regeneration of said particulate matter filter, said control unit initiating an LNT de-sulphasion phase during a first time period, in which the engine is set to generate a rich exhaust gas in said exhaust gas conduit, wherein said first time period is kept below a first maximum value so as to avoid release of emissions of H 2 S from said engine, said control unit subsequently initiating a particulate matter filter regeneration phase during a second time period, in which the engine is set to generate a lean exhaust gas in said exhaust gas conduit, wherein said second time period is kept below a second maximum value to avoid thermal damage to said particulate matter filter; and repeating de-sulphasion phase and said regeneration phase in a periodical manner, where the control unit shortens the second time period when increasing an air/fuel ratio of the lean exhaust gas.