A theoretical model has been developed for calculations of the evolution of fuel injected into internal combustion engine chambers. Fuel injection in the form of a gaseous jet and in the form of a liquid droplet spray are considered. The model uses the basic equations of conservation of mass, momentum, and energy in both the gaseous and liquid phases. Applications in two dimensional symmetry of the gaseous jet form of the numerical model are described in stagnant and swirling air flows. The liquid droplet spray model, including coupling between the droplets and the gas phase medium, is described. Applications of the two phase model are described for the case of axially symmetric injection. Finally, the liquid spray model and the gas jet model are applied to the same conditions, leading to a general assessment of the ranges of validity of the gas jet model.