The main objective of the present paper was to provide the necessary background knowledge to identify the main parameters, which control the in-cylinder mixture formation of a GDI-layout. As the preferential production-ready automotive system approach is that of a highpressure liquid fuel injection the examples given are related to this system. The fuel injector is a key element in the mixture preparation process. However it is impossible to impose the manufacturing requirements on this element, without a detailed knowledge of the interaction between the spray and the combustion chamber layout chosen for the application. Furthermorethe optimized compromises can be reached by use of a full 3D CFD numerical simulation tool, the virtual engine model. Finally a basic knowledge is necessary of the specific component performance in terms of the instantaneous produced spray momentum from each fuel injector atomizer. The three main layouts, swirl-, multi-hole- and CJ-atomizers provide a high flexibility in the atomizer choice. An example was given to show the procedure for the setup of a virtual engine. A small single cylinder 50 cc two-stroke engine was chosen for the example due to the high number of implicit constraints offered by this type small-bore fast rotating engine. For this two-stroke engine it was proven that the liquid highpressure Dl-technology, with a fuel injector located in counter-flow position, works perfectly well without any significant transfer of unburned fuel into the exhaust system even at engine speeds in the area of 10000 rpm. It was possible to find a cost effective constant rail-pressure compromise for this small displacement two-stroke application. It was demonstrated that the use of the virtual engine provides access to the instantaneous behavior of the parameters, which control the following three physical phenomena: The instantaneous ratio between the gas and spray momentums, the in-chamber mixture distribution prior to ignition, the thermal conditions at the piston top and in the vicinity of the injector tip. Both for two- and four-stroke applications the information about momentum and mixture distribution makes it possible to determine the compromise between the atomizer type, the injector position and the preferred rail-pressure range to apply. The information about the instantaneous thermal constraints in the vicinity of the piston top and the injector tip will provide indications about eventual cooling requirements for the piston and the risk of injector tip coking.