<p>This article focuses on understanding the transient effects of the gasoline direct injection (GDi) process, which plays an important role in mixture formation and combustion efficiency in this type of engine. Owing to the growing interest of the scientific community in better understanding the performance of engines with other alternative fuels, this work includes not only iso-octane as a substitute fuel for gasoline but also surrogate fuels such as hexane, pentane, heptane, ethanol, and multiple components. In GDi injections, the injector plays a major role in defining the air–fuel mixture quality. The study of these phenomena has become challenging because of the reduced size of the orifices, high flow velocities, and multiphasic flows. Computational fluid dynamics (CFD) tools allow insight and understanding into complex flow physics, which are considered in this research. Unsteady Reynolds-Averaged Navier Stokes (URANS) is selected to account for the turbulent effects. The volume-of-fluid (VOF) approach is employed to analyze the flow inside the nozzle and the first 2–5 mm of the spray. A homogeneous relaxation model (HRM) accounts for the mass exchange between flow phases. Validation against experimental data proves how the variation in discharge pressure minimally influences the macroscopic parameters of internal flow. The density and volatility of different fuels directly affect injection. This effect must be considered, especially in flash boiling conditions, where fuels with a greater tendency toward phase change produce strong flash boiling, resulting in a higher probability of spray collapse and lower engine performance.</p>