Improved understanding of the active combustion chain “injection - vaporisation - mixture formation - ignition - combustion - exhaust gas emissions” is important for the further development of IC engines with respect to fuel consumption and pollutant emissions. By means of multidimensional optical, mostly laser-based measurement techniques, a modern passenger car common rail system, applied to an optically accessible engine, was investigated. The utilisation of a new optical detection system allowed a simultaneous detection of the liquid phase by Mie scattering, the flame propagation from flame luminosity and the soot formation by laser-induced incandescence inside the combustion bowl of the engine. By such simultaneous measurements, direct dependencies of single combustion phenomena on fuel injection parameters can be resolved, and in particular soot formation and oxidation can be correlated to the actual combustion situation. Two different five-hole nozzles (VCO and mini-sac-hole) have been investigated. For both nozzles soot has not been found homogeneously distributed all over the combustion bowl but concentrated at particular locations near the walls. A longer appearance duration of high soot concentrations has been detected for the VCO nozzle, and in general the mini-sac-hole nozzle indicates reduced soot formation. A successive application of the different measurement techniques was no more necessary, resulting in a shorter operating time of the transparent engine which leads to less fatigue of the sensitive glass components. Thus, the used measurement procedure provides in combination with near-production transparent engines a very useful diagnostic tool for future engine developments.