The actual trends in development and series application of mixture formation techniques for SI engines converge irrevocably to a process after scavenging, by direct injection, the reason being the higher thermal efficiency in a wide operation range of the engine, leading to substantially lower bsfc and pollutant emission. After numerous and successful research projects of direct injection for two-stroke engines, the most of series applications are being introduced for four-stroke automotive engines. A main reason for this profitable way consists in the better fluid dynamic conditions and in the longer time for mixture formation in the case of the four-stroke process. The aim of the fluid dynamic analysis presented in the paper was to find a configuration of the injection system and its parameters able to achieve optimum conditions for mixture formation as a controlled spray distribution in the air for a wide engine operation range and with special regard to the two-stroke cycle respectively to very small swept volume as extreme requirements. The analysis consists in a process simulation and a fuel jet optimization based on the CFD code FLUENT, followed by the experimental determination of the jet form. The comparison of computational results and the jet visualization, presented for three different injector needle forms, confirms the combined method of analysis and optimization as very effective.