Method and apparatus for controlling the purge of an NOx trap in a lean-burn engine

A method and apparatus for controlling the operation of a "lean-burn" internal combustion engine (12) in co-operation with an exhaust gas purification system having an emissions control device (34,36) capable of alternatively storing and releasing NOx when exposed to exhaust gases that are lean and rich of stoichiometry, respectively, determines a performance impact, such as a fuel-economy benefit, of operating the engine (12) at a selected lean or rich operating condition. The method and apparatus then enable the selected operating condition as long as such enabled operation provides further performance benefits.