Method and system for reducing NOx tailpipe emissions of a lean-burn internal combustion engine

A method for controlling the operation of a lean-burn internal combustion engine, the exhaust gas from the engine being directed through an exhaust purification system including a catalyst, the method comprising:

• determining, during a lean engine operating condition, an incremental amount of NOx generated by the engine;
• determining an incremental amount of NOx being instantaneously stored in the catalyst;
• calculating an estimate of NOx exiting the exhaust purification system based on a difference between said incremental NOx generated by the engine and said incremental amount of NOx being stored in the catalyst;
• determining a distance traveled by the vehicle;
• calculating an average tailpipe NOx emissions per distance based on said estimate of NOx exiting and said determined distance traveled by the vehicle;
• discontinuing the lean engine operating condition based on said average tailpipe NOx emissions per distance;
• wherein said distance is determined based on vehicle speed;
• and
• wherein said average tailpipe NOx emissions per distance is calculated even during stopped vehicle conditions.