The fuel injection in internal combustion engines plays a crucial role in the mixture formation, combustion process and pollutants' emission. Its correct modeling is fundamental to the prediction of an engine performance through a computational fluid dynamics simulation. In the first part of this work a tridimensional numerical simulation of a multi-hole’s injector, using ethanol as fuel, is presented. The numerical simulation results were compared to experimental data from a fuel spray injection bench test in a quiescent vessel. The break up model applied to the simulation was the combined Kelvin-Helmholtz Rayleigh-Taylor, and a sensitivity analysis of the liquid fuel penetration curve, as well on the overall spray shape was performed according to the model constants. Experimental spray images were used to aid the model tuning. The final configuration of the KH-RT model constants that showed best agreement with the measured spray was C3 equal to 0.5, B1, 7 and Cb, 0. In the second part of this work, a complete simulation of a single-cylinder research engine, with ethanol direct injection, was performed with the characterized fuel injector operating at homogeneous mode, 2000 rpm engine speed and 8 bar of engine BMEP. The engine performance parameters results were compared to experimental data and curves for in-cylinder pressures and heat release rate are plotted. It is also presented a numerical fuel evaporation curve. Based on 3D simulated results, the piston and valves impingements were detected.