Multiple researches have focused on reducing the NO x emissions and the greatest results have been achieved when lowering the combustion temperature by employing massive exhaust gas recirculation rates (LTC). Despite this benefit, a substantial increase in the NO 2 contribution to the NO x emissions has also been observed, which is the most harmful specie and is important for the design and positioning of the after-treatment devices. To understand how NO 2 behaves and how it contributes to the total NO x (NO 2 /NO x ), not only under LTC but also for CDC conditions, a stepwise computational research study was performed with Chemkin Pro software, due to the complexity of isolating the different phenomena studied, to analyze: (1) general equilibrium conditions and (2) the influence of typical diesel engine phenomena (combustion and cooling effects) under non-equilibrium conditions.The results obtained under equilibrium state confirm the theoretical guidelines established for the NO 2 formation process. When considering a combustion process (HCCI-like mode), the previous results were corroborated as well as the fact that only poor or slow combustion processes are responsible for the NO 2 formation. Additionally, it reflected a cyclic process between NO and NO 2 , or in other words, it is suffice to just concentrate on NO to be able to predict NO 2 . Finally, the results yield after analyzing some cooling effects, inherent to how diesel engines work (the expansion stroke, dilution of combustion products with the rest of in-cylinder charge and the one caused by wall impingement), reflect that: (1) the dilution effect explains the 10% of the NO 2 /NO x ratio under CDC conditions and (2) the coupling of the dilution with the expansion stroke cooling effects can explain the NO 2 increase typical of LTC conditions. These results were also supported by some experiments performed in a single-cylinder diesel engine. Consequently, the cooling effect caused by dilution should be considered when modeling the NO 2 formation just like the expansion stroke.