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

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.

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.

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.

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.

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 H2S 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.