Increasingly stringent emission legislation as well as increased demand on fuel efficiency calls for further research and development in the diesel engine field. Spray formation, evaporation and ignition delay are important factors that influence the combustion and emission formation processes in a diesel engine. Increased understanding of the mixture formation process is valuable in the development of low emission, high efficiency diesel engines. In this paper spray formation and ignition under real engine conditions have been studied in an optical engine capable of running close to full load for a real HD diesel engine. Powerful external lights were used to provide the required light intensity for high speed camera images in the combustion chamber prior to ignition. A specially developed software was used for spray edge detection and tracking. The software provides crank angle resolved spray penetration data. The images also provide data of ignition delay, ignition location and premixed flame propagation. The evaluation was made for an array of engine operation points with variations in fuel rail pressure, injection timing, boost pressure and charge air temperature. The influence of using pilot injections has also been investigated. This set of experiments makes it possible to analyze the impact of the various engine parameters on the spray formation and ignition processes. Some of the results are compared with the exhaust emission measurements in order to provide an insight into how the emission formation process is influenced by the spray formation and ignition processes.