Commonly, the spray process in Direct Injection (DI) diesel engines is modeled with the Euler Lagrangian discrete droplet approach which has limited validity in the dense spray region, close to the injector nozzle hole exit. In the presented research, a new reactive spray modelling method has been developed and used within the 3D RANS CFD framework. The spray process was modelled with the Euler Eulerian multiphase approach, extended to the size-of-classes approach which ensures reliable interphase momentum transfer description. In this approach, both the gas and the discrete phase are considered as continuum, and divided into classes according to the ascending droplet diameter. The combustion process was modelled by taking into account chemical kinetics and by solving general gas phase reaction equations. The newly developed method was incorporated into the commercial computational fluid dynamics code AVL FIRE™, and it was used in combination with the previously validated spray sub-models. The method takes into account the liquid jet disintegration, droplet atomization, droplet collision, droplet evaporation, and subsequent vapor combustion.The computed results are validated against available experimental data, and a good agreement of the mean pressure, temperature and rate of heat release was achieved.