Nox trap efficiency

The present invention relates to a system and a method for controlling a lean-burn internal combustion engine, and more particularly, to estimating instantaneous NOx conversion efficiency of the LNT. A more accurate method for determining a NOx storage efficiency of an exhaust gas aftertreatment device is presented. The method teaches determining NOx storage efficiency as a function of available LNT NOx storage capacity, which is calculated based on a ratio of an instantaneous value of an amount of NOx stored in the device and total device storage capacity at present operating conditions. Using this method prevents overfilling of the device and inefficient purging, thus improving emission control and fuel economy.