Differences in thermo-physical parameters of fuels have high impact on the ignition, combustion and emission. Pure rapeseed FAME and diesel fuel with a cetane number of 60 have been compared to reference fuel. In an optical accessible vessel the fuels have been injected in order to investigate the spray, the ignition and soot formation.The high cetane number fuel showed similar behavior in spray phase to the reference fuel but the FAME fuel is more present at all operating points due to low volatile fuel components.The ignition and combustion process was investigated via chemical luminescence (CL) and laser induced incandescence (LII). In engine investigations a reduced ignition delay is detected in case of high cetane-number. The more sensitive optical techniques show differences in the combustion process. The ignition behavior of the reference fuel and the increased cetane number fuel were similar until the cetane increaser of the high cetane fuel came into effect. The FAME showed a shortened ignition delay but was found to be slower in combustion propagation.For the LII investigation a Nd:YAG pulsed laser with a wavelength of 532 nm was used. The light sheet excited the soot particles up to temperatures where the black body radiation is shifted towards the UV-spectra. The soot formation of the reference fuel is equal to the soot formation of the high cetane number fuel until the cetane increaser affected the flame and increased soot production. The soot production of the FAME is advanced in time but overall significantly lower.