Methods and systems for treating vehicle emissions

A method comprising:

• adjusting an air/fuel ratio of a combustion engine, the engine comprising an exhaust tract connected fluidly to the combustion engine, two or more exhaust gas aftertreatment devices arranged in the exhaust tract comprising at least one nitrogen oxide storage catalyst, at least one catalyst for selective catalytic reduction arranged downstream of the nitrogen oxide storage catalyst, at least one lambda probe, at least one temperature sensor, and a control unit;
• operating a combustion engine at low load during a first condition where a torque request is less than a threshold torque request;
• changing to an operating state with a high load during a second condition where the torque request is greater than the threshold torque request;
• adjusting the air/fuel ratio from more rich to less rich during the second condition in response to an ammonia load stored in the selective catalytic reduction reaching an upper threshold ammonia load;
• adjusting the air/fuel ratio from less rich to more rich during the second condition in response to the ammonia load stored in the selective catalytic reduction reaching a lower threshold ammonia load;
• and
• adjusting the air/fuel ratio from rich to lean in when switching from the second condition to the first condition in response to the torque request being less than the threshold request.