Method and apparatus for measuring lean-burn engine emissions

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 (36) capable of alternatively storing and releasing NOx when exposed to exhaust gases that are lean and rich of stoichiometry, respectively, optimises vehicle fuel economy while complying with emissions standards by prohibiting lean engine operation only when tailpipe emissions, calculated in terms of grams of an exhaust gas constituent per vehicle mile travelled, exceeds a permissible threshold value. The threshold value is preferably itself periodically determined based upon a detected or determined level of vehicle activity, such that vehicle activity characterised by greater transient engine operation prescribes a relatively lower level of permissible vehicle emissions.