1. Method for estimating NOx creation in a combustion process of a four-stroke internal combustion engine including a variable volume combustion chamber defined by a piston reciprocating within a cylinder between top-dead center and bottom-dead center points, intake and exhaust passages, and intake and exhaust valves controlled during repetitive, sequential exhaust, intake, compression and expansion strokes of said piston, comprising: monitoring engine sensor inputs; modeling parameters descriptive of said combustion process based upon said engine sensor inputs; and estimating NOx creation with an artificial neural network based upon said parameters.