A real-time, on board, diesel engine emissions estimation with an empirical, table-based approach that accounts for up to eight (8) input parameters, for optimum emissions estimation under steady state or transient engine operation. The method considers a steady state NOx model, steady state Particulate Matter model, transient NOx model and transient Particulate Matter models to populate a table in memory. The switch between steady state and transient models, real time emissions estimations is based on the rate of change of engine speed (RPM). If the rate of change of RPM exceeds a predetermined threshold, transient models for NOx and Particulate Matter are used to operate the engine and reduce emissions of NOx and Particulate Matter.