Engine exhaust gas NOx Model

Methods are provided for estimating the NO x content of exhaust gas produced by an internal combustion engine (10). The method includes determining one or more operating parameters. The method further includes applying the determined operating parameter(s) to a global NO x model. The global NO x model may be derived using a plurality of steady state data points relating to local NO x emission models. The steady state data points may be derived using a plurality of set and variable operating parameters (52, 54, 58). The global NO x model allows for prediction of the engine-out NO x content of exhaust gas in real time and without a NO x sensor, even if engine operating parameters change.