Method and apparatus for controlling lean operation of an internal combustion engine

A method and system for controlling the operation of "lean-burn" internal combustion engines determines a current rate of vehicle NOx emissions, and determines a threshold value for permissible vehicle NOx emissions based on at least one current value for the intake air-fuel ratio, engine speed, engine load (e.g., brake torque, manifold air pressure, or throttle position), and/or vehicle speed. A differential NOx emissions rate is calculated as the difference between the current rate and the threshold rate, and the differential rate is accumulated over time to obtain a differential measure representing the amount by which cumulative NOx emissions have fallen below permissible levels therefor. Lean engine operation is disabled when the differential NOx emissions measure exceeds a predetermined excess vehicle NOx emission value. In this manner, vehicle NOx emissions are favourably controlled even when the engine is operated "off-cycle," i.e., under operating conditions falling outside of the FTP driving cycles.