Diesel injection systems have a significant impact on the performance as well as emission and pollutant formation of modern diesel engines. Even though the geometry of atomizers became more and more complex over the last years, injection systems still have a large potential for improving the overall diesel engine combustion process. Due to the complexity of the atomization process, reliable models are not available, yet these are highly desired for supporting the design process. They have to be developed using detailed numerical simulations.In this work, the “Spray A” reference case defined by the Engine Combustion Network is simulated under realistic operation conditions using a recently developed numerical framework for multiphase flows. A Large-Eddy Simulation of the nozzle internal flow is coupled with a Direct Numerical Simulation of the interfacial outside flow and the resulting primary breakup is analyzed. Additionally, the impact of the injector geometry on primary breakup is studied. For this purpose, the effect of the taper ratio of the injector nozzle and the turbulent kinetic energy at the injector exit on the primary breakup is discussed and a parametric study is performed.It is shown that the taper ratio of the nozzle of modern injection systems has a large impact on the primary breakup because it directly influences the turbulent kinetic energy as well as the radial velocity component at the nozzle exit, which in turn affect the primary breakup.