To meet the stringent emission regulations on diesel engines, engine-out emissions have been lowered by adapting new combustion concepts such as low-temperature combustion and after-treatment systems have also been used to reduce tailpipe emissions. To optimize the control of both in-cylinder combustion and the efficiency of an after treatment system to reduce NOx, the amount of real-time NOx emissions should be determined.Therefore, in previous studies, the authors developed a real-time NO estimation model based on the in-cylinder pressure and the data available from the ECU during engine operation. The model was evaluated by comparing its results with a CFD model, which agreed well. Then, the model was implemented on an embedded system which allows real-time applications, and was verified on a 2.2-liter diesel engine. The model showed good agreement with the experimental results at various steady-state conditions and simple transient conditions.In this paper, to verify the performance and to investigate the characteristics of the real-time estimation of the model, the engine-out NO emissions measured by a fast NOx analyzer and the estimated NO emissions were compared during ECE-15 and EUDC cycles. Furthermore, to extend the NO model to a complete NOx prediction model, an empirical NO₂ prediction model is proposed based on the experiments under steady-state conditions. The in-house EGR prediction model was also applied in the NOx prediction model for accurate cycle-by-cycle prediction and used as an input during transient engine operations.