A mathematical model was developed for calculating the increase in the cetane number of diesel fuels in the presence of the ignition initiator 2-ethylhexyl nitrate (EHN) as a function of the initial cetane number of the fuel and the concentration of EHN. The accuracy of the results of the calculations with the model is as good as the accuracy obtained on a bench unit. A mechanism of the reaction of EHN with the hydrocarbons in diesel fuel is proposed based on calculations with the PM6 semiempirical quantum-chemical method.