For the development of new HD Diesel engines which meet the demands of future international emission regulations and, at the same time, ensure economic operation modern development tools need to be used, especially for an optimisation of the combustion principle. To find the optimal engine set up, the variation of a large number of engine and injection system parameters, i.e. injection system, number of nozzle holes and sizes, injection rate profiles, etc. is required. To speed up the design process, modern optical engine diagnostics and 3D-numerical simulation can help to analyse the highly transient in-cylinder processes in detail. These methods provide essential insight to understand the complicated physical and chemical interactions and acting mechanisms during mixture formation, combustion and pollutant formation as well as the function of components of the system.In this paper most useful state-of-the-art optical diagnostic tools, i.e. transparent engine with high speed combustion photography, 0D and 2D soot -temperature and - relative concentration analysis, as well as 3D-numerical combustion simulation are presented and discussed. The application of these modern techniques in addition to conventional engine bench tests is demonstrated in a comparison of the performances of a Pump Line Nozzle (PLN) and a Common Rail (CR) Injection system. This analysis shows that a CR system with the current design does not provide advantages for HD truck engines when compared with a modern PLN system.