Method and apparatus for enabling lean engine operation upon engine start-up

A method and system for controlling the operation of "lean-burn" internal combustion engines (12), wherein initial values for measures representing current levels of NOx and SOx stored in an emissions control device (36), such as a lean NOx trap, upon vehicle start-up are determined based at least in part upon values for the measures immediately preceding engine shut-off. Such initial values obviate the need for performing a purge event immediately upon engine start-up in appropriate circumstances, such as a brief engine shut-down, whereby the fuel economy penalty associated with such an initial purge event is avoided.