The transient characteristics of the internal flow dominate all the ensuing processes: spray, fuel-air mixture formation as well as combustion and pollutants formation. Therefore, it is crucial to understand the dynamics of the injectors' internal flow. The objective of this work is to study all transient effects that may impact the internal flow of a single hole injector under different conditions. Since the numerical investigation of such a complex flow is hampered by several factors for the real operating conditions-namely the turbulence, the cavitation and the needle motion-this work is divided into two parts.In the first part, only the effects of turbulence and cavitation are considered through the study of the effects of the fuel properties as well as the injection conditions at the fully open needle position. The impact of these effects is studied by means of the Reynolds and the cavitation number. To achieve this, simulations have been performed using an Eulerian single-fluid model based on the Rayleigh-Plesset equation for bubble growth. Both the Reynolds and the cavitation numbers affect the in-nozzle flow and it has been confirmed that they are the only dimensionless numbers that govern the flow.In the second part, the main objective is to explain the hysteresis of turbulence between needle opening and closing. Authors who identified this behavior observed that the turbulence level at the nozzle exit is mainly due to the needle motion regardless of the nozzle geometry and of the presence of cavitation. Hence, only the effects of the needle motion and the turbulence associated are studied in non-cavitating conditions in the present work. To achieve this, a reliable moving mesh strategy has been developed and implemented. Using a URANS approach, the internal flow structure and its dynamics are finely analyzed by emphasizing the transient effects of the needle motion. Especially, the hysteresis of turbulence has been explained.