The transport of goods is mainly realised by the use of heavy-duty vehicles equipped with diesel engines as a drive assembly. Considering the high flexibility and reliability as well as the growing interest in saving environmental resources, a further optimisation of DI-diesel engines regarding fuel consumption and exhaust emissions is necessary. Current discussions on the application of different injection systems for passenger cars (distributor pump, common-rail, …) are also of great significance with regard to heavy-duty vehicles.Optical measurement techniques are a valuable tool to evaluate the quality and the potential of modern DI-diesel injection systems. In this work a conventional heavy-duty engine (MAN) was modified to carry out optical investigations inside the combustion bowl, concerning spray propagation and flame luminosity for different injection nozzles. With respect to the current discussions, it was equipped with a modern common-rail system. By using an external intake air charging, it is guaranteed that the engine is driven under realistic cylinder pressures and ignition delays.One aim of the investigation was the separation of flame and liquid fuel phase signals which was achieved by the simultaneous two-dimensional detection of the flame emissions and the liquid phase by the 2D-Mie-scattering-technique. For testing the reliability of the measurement technique, first tests were performed in an optically accessible high-pressure high-temperature combustion chamber which can be adjusted to engine-realistic conditions. In the engine complementary investigations concerning intake air charging and load twist were possible.The investigations in the chamber show that the mini-sac-hole nozzles, equipped with a new type of seat geometry, have a reproducible symmetrical spray propagation. This is important for a homogeneous intake of the fuel into the combustion bowl to achieve an optimum of the mixture formation between fuel and air in order to obtain low exhaust emissions and low fuel consumption. The simultaneous detection of flame luminosity and liquid phase clearly shows that, in contrast to the liquid phase, the vapour is transported further into the area of reaction where the combustion finally takes place. During the close of the needle, a typical behaviour of mini-sac-hole nozzles can be observed when droplets leave the nozzle exit. These droplets are responsible for an increase of HC emissions in the exhaust gas.Optical investigations within the transparent engine additionally showed that the application of intake air charging reduces the tendency of the liquid phase to develop a wall film which decreases the soot emissions. Furthermore, a more complete coverage of the piston bowl by the flame is observed when using intake air charging. The coverage is additionally optimised by the twist of the intake air, as one can see by the vapour being transported in direction of the twist relative to the liquid phase.