Main limiting factor in the application of 3D-CFD simulations within an engine development is the very high time demand, which is predominantly influenced by the number of cells within the computational mesh. Arbitrary cell coarsening, however, results in a distinct distortion of the simulation outcome. It is rather necessary to adapt the calculation models to the new mesh structure in order to ensure reliability and predictability of the 3D-CFD engine simulation. In the last decade, a fast response 3D-CFD tool was developed at FKFS in Stuttgart. It aims for a harmonized interaction between computational mesh, implemented calculation models and defined boundary conditions in order to enable fast running simulations for engine development tasks. Their susceptibility to errors is significantly minimized by various measures, e.g. extension of the simulation domain (full engine) and multi-cycle simulations. In this way, a predictive analysis of influencing parameters on the engine flow field allows a thorough definition of the engine design and operating strategy. Comparable measures were taken for the numerical description of injection processes. The fuel injection, which essentially influences the combustion performance, is highly sensitive to a variety of parameters. These include the fuel properties, injector geometry and injection conditions. However, their numerical description has no general validity. It is rather reasonable to follow application specific procedures in order to meet the demand for injection simulations in accordance to the “fast response” methodology. The paper addresses recent analyses and findings on the numerical characterization of fuel injection processes with a focus on spatial and temporal discretization as well as the fuel modeling and their respective influence on macroscopic spray properties.