Accurate modeling of evaporating sprays is critical for diesel engine simulations. The standard spray and evaporation models in KIVA-3V tend to under-predict the vapor penetration, especially at high ambient pressure conditions. A sharp decrease of vapor penetration gradient is observed soon after the liquid spray is completely evaporated due to the lack of momentum sources beyond the liquid spray region. In this study, a gas particle model is implemented in KIVA-3V which tracks the momentum sources resulting from the evaporated spray. Lagrangian tracking of imaginary gas particles is considered until the velocity of the gas particle is comparable to that of the gas phase velocity. The gas particle continuously exchanges momentum with the gas phase and as a result the vapor penetrations are improved. The results using the present gas particle model is compared with experimental data over a wide range of ambient conditions and good levels of agreement are observed in vapor penetration. The present model also provides good levels of grid independency in vapor penetration. The gas particle model is further integrated with detailed chemistry models to simulate diesel spray combustion in a constant-volume chamber, and improved lift-off lengths are obtained. Diesel engine simulations are also performed using the present gas particle model, detailed chemistry, and two-step soot model to predict the soot emissions at different exhaust gas recirculation (EGR) levels.