Two different types of fuel injectors - a plate-type two-jet injector and a full-cone single-jet injector - have been applied to a fired four-valve SI engine with optical access. While the two-jet injector is optimized for the employed engine, the single jet injector leads to fuel wall film deposition inside the intake. As a third variation, an already vaporized and ideally premixed fuel / air mixture was fed to the engine. The three different types of mixture formation initially generate different local air / fuel ratios inside the cylinder, but 30° CA before TDC the distributions seem to be equal and nearly homogeneous. Nevertheless, the combustion process is different and the exhaust gas composition indicates that differences must be present, which will be discussed.Laser induced fluorescence (LIF) was used to compare the fuel vapor distributions and the fluctuations of the fuel concentration during intake and compression inside the cylinder. Flame propagation was studied and is different for each of the two injector types as well as for the prevaporized mixture. The fuel used was isooctane. Triethylamine (TEA) was taken as fluorescent marker of the fuel. Excitation of TEA was achieved by a KrF excimer laser at 248 nm. An intensified CCD camera was used for detection of the UV-signals.Complementary to the optical investigations conventional exhaust gas analysis of unburnt hydrocarbons (UHC) and NOx emission was performed for the two injectors, and was correlated to the results of the optical measurements.