For studying the effects of injection system properties and combustion chamber conditions on the penetration lengths of both the liquid and the vapor phase of fuel injectors in Diesel engines, a holistic injection model was developed, combining hydraulic and spray modeling into one integrated simulation tool.The hydraulic system is modeled by using ISIS (Interactive Simulation of Interdisciplinary Systems), a one dimensional in–house code simulating the fuel flow through hydraulic systems. The computed outflow conditions at the nozzle exit, e.g. the dynamic flow rate and the corresponding fuel pressure, are used to link the hydraulic model to a quasi–dimensional spray model. The quasi–dimensional spray model uses semi–empirical 1D correlation functions to calculate spray angle, droplet history and droplet motion as well as penetration lengths of the liquid and the vapor phases. For incorporating droplet vaporization, a single droplet approach has been used. In the paper the details of the modeling approaches are discussed and simulation results for conditions typical for HD DI Diesel engines are presented.The model results are compared to experimental results obtained from measuring liquid and vapor penetration of fuel sprays of a research type CR injector in a high pressure, high temperature combustion bomb. The data obtained by simultaneously applying Schlieren and Mie–Scattering diagnostics are in very good agreement with the data predicted by the holistic injection model. It is concluded therefore that the proposed model is a suitable tool for supporting R&D efforts to optimize DI diesel sprays for future engines.