Method and apparatus for measuring the performance of an emissions control device

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 an exhaust gas constituent, such as NOx, when exposed to exhaust gases that are lean and rich of stoichiometry, respectively, wherein a measure of the efficiency of the device (34,36) to remove the exhaust gas constituent from engine exhaust gas is determined, and a purge event for releasing previously-stored NOx is initiated when the efficiency measure falls below a threshold value.