In this paper, a novel approach for NOx generation modeling in heavy-duty (HD) Diesel engines is proposed. Only the indicated pressure and the speed measurements are used, standing on the assumption that all combustion phenomenona are reflected by the crank angle resolved pressure trajectory. A principal component analysis is performed to describe the information included in the pressure by means of a limited set of variables; this enables the use of simple identification techniques to derive a simple and reliable predictor, also suited for on-line estimation. The proposed strategy is implemented on a off-road HD Diesel engine and validated on a standard test cycle.