Pollutant emissions from D.I. Diesel engines strongly depend on injection system characteristics and mainly on injection pressure and timing. In the latest years some solutions have been proposed based on very high fuel pressure values (up to 150 MPa). Among them, the so called “Common rail” system configuration, being able to electronically control needle lift and injection pressure, seems to be particularly promising. Much experimental and theoretical work has been done to improve system performance for automotive applications. With the aim of investigating the influence of some details of geometrical configuration on the injector operating mode, a mathematical model able to describe the pressure-time history in any section of the delivery pipe and the fuel injection rate through the nozzle has been developed, based on a semi-implicit finite volumes approach. The computed results have been compared with experimental data provided by the Institut Français du Pétrole. After calibrating the model, the influence of timing and of some geometrical characteristics on injector performance has been analysed in more detail.