A research on diesel spray dynamic injected into stagnant ambient air in a chamber is reported in this paper. As a result of a theoretical reasoning based on momentum flux conservation in the axial direction of the diesel spray, a mathematical model which relates the momentum flux with profiles of velocity and concentration is obtained. The main contribution of the present investigation, which makes it different from previous work in the literature, is the consideration of local density variations and the deduction of the model for a generic Schmidt number. A PDPA system and additional measurements of spray momentum, mass flow rate and spray cone angle were used in order to validate the model in high density environment and real injection pressure conditions.