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Worldwide diesel fuels differ in their composition and therefore in thermo-physical properties. Some of these properties are known to have little effect on the combustion process. Others, like the cetane number, have dramatic influence on the combustion formation and thus on the heat release rate and more important the formation of soot and NOx. In an experiment series various commercially available fuel types, like EN 590 [1], ASTM D975 [2] and JIS K 2204 [3], have been compared to alternative diesel fuels such as FAME, GtL and premium diesel fuel with increased cetane number. A specially designed research injector was used in order to provide full optical access to one single fuel jet injected and combusted in a constant volume vessel. First, the liquid fuel phase propagation has been investigated by means of Mie-scattering and the liquid penetration depth and the spray cone angle have been evaluated.

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.

Spray- and flame-wall interactions were investigated in a combustion chamber with diesel engine conditions. Several techniques were used to perform time and spatially resolved measurements of the liquid fuel phase, the premixed and diffusion-controlled combustion close to a wall. Different wall and gas temperature variations were investigated. It was found that low temperature variations of 25K have a significant impact on the combustion process: The lower the gas temperatures, the more liquid fuel and larger vortex structures arise. Also, the ignition delay is elongated. Consequently, the premixing period is longer, which can lead to the complete disappearance of sooty combustion. The colder the wall, larger cooling of the spray and larger vortex structures of liquid fuel on the wall develop. The ignition delays again are noticeable longer at the colder wall. Therefore, the premixing period is longer and there is also much less sooty combustion when the wall temperature is lower.

The interaction between a combusting diesel spray and a wall at temperatures of 700K and 735K was investigated in a combustion chamber using optical measurement techniques. The temperatures were chosen as they appear in the range of the maximum piston surface temperatures of the latest production engines. Combustion was investigated with a dual camera setup, which is designed to take simultaneous pictures of the UV flame luminosity (FL_UV) and the visible flame luminosity (FL_VIS). The FL_UV is used to measure lean or stoichiometric combustion. The FL_VIS is capable of detecting the thermally excited soot. Mie scattering is used to study the liquid fuel phase. It was found that there is almost no FL_VIS signal visual in the 700K case, but a very strong signal in the 735K case. In general, one might expect that higher wall temperatures lead to an improved mixture formation and, consequently, lower soot production. However, the opposite was detected.