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

A method and apparatus for operating a lean-burn internal combustion engine (12) in co-operation with an exhaust gas purification system having a three-way catalyst (34) and a NOx trap (36) located downstream of the three-way catalyst (34) includes a controller (14) which calculates current levels of tailpipe NOx during lean engine operating conditions based upon the difference between a determined instantaneous feedgas NOx concentration and a determined instantaneous trap efficiency. The controller (14) discontinues lean engine operation when the tailpipe NOx, expressed in terms of either grams-per-mile or grams-per-hour, exceeds a predetermined threshold level, either instantaneously or as averaged over the course of a trap purge-fill cycle.