Recent development in internal combustion engines is aiming at efficient use of various fuels. Typical automotive fuels consist of many species that can affect engine combustion characteristics. Developing suitable chemical reaction mechanisms for the components of the fuel is one of the challenges in numerical simulations of engine combustion. In the present study, the reactivity adjustment model (ReAd model) is employed. In the model, a reduced reaction mechanism for a single chemical surrogate (n-heptane) is used to represent the oxidation process of a multi-component fuel by incorporating on-the-fly adjustment of reactivity of the reaction mechanism based on the reactivity difference of the components of the fuel and the composition of local mixtures. The model was implemented to KIVA-MTU, a 3-dimensional in-house CFD code and the performance of the model was validated against experimental data for 2L VW diesel engine operated under conventional diesel and natural gas-diesel RCCI combustion regimes. The results show that the ReAd model successfully captures the reactivity of multi-component fuels and performs reliably in a wide range of operating conditions of both combustion regimes.