The mixture formation and the combustion process of small direct-injection (DI) Diesel engines, equipped with high pressure common-rail fuel injection systems, was investigated by means of modern optical measurement techniques, such as digital high-speed cinematography and phase doppler particle analyzing (PDPA). The measurements were carried out in a high pressure chamber (HPC) and in a rapid compression machine (RCM). Several engine operating conditions with different boost pressures, injection pressures, and fuel quantities were examined. The influence of the injection and the swirl mode on the ignition delay and the flame propagation was analyzed. Different nozzle types (sac hole and VCO nozzles) with a variable number of holes and different injector types (electromagnetic/piezoelectric) were investigated. In order to observe the fuel injection, the ignition, and the combustion process simultaneously, a combined high-speed shadow graph and flame visualization technique was applied. The experimental analysis of the combustion process in the rapid compression machine yielded informations about the spray penetration and dispersing angle, the velocity, the distribution and the evaporation of the fuel droplets inside the piston bowl, the delay and the location of ignition as well as the progression of the whole combustion process including informations about the timedependent air flow during the compression and expansion stroke. The applied swirl did not show a significant influence on the spray penetration, but had a strong influence on the ignition and combustion process. Also a reducing effect on the swirl speed was observed, caused by the injected amount of high pressured fuel. PDPA measurements in the center of the spray show a characteristic and reproducible structure in the time series of droplet size and droplet velocity. The turbulence of the droplet velocity increases from the center to the edge of the spray while the mean velocity rapidly decreases.