Multiple researches have focused on reducing the NOx 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 NO2 contribution to the NOx, 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 NO2 behaves and how it contributes to the total NOx, (NO2/NOx), 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 NO2 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 NO2 formation. Additionally, it reflected a cyclic process between NO and NO2, or in other words, it is suffice to just concentrate on NO to be able to predict NO2. 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 NO2/NOx ratio under CDC conditions and (2) the coupling of the dilution with the expansion stroke cooling effects can explain the NO2 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 NO2 formation just like the expansion stroke. (C) 2014 Elsevier Ltd. All rights reserved.