In order to reduce engine out CO_2 emissions it is a main subject to find new alternative fuels out of renewable sources. For this paper, several fuels were selected which can be produced out of biomass or with hydrogen which is generated directly via electrolysis with electricity from renewable sources. All fuels are compared to conventional diesel fuel and two diesel surrogates. It is well known that there can be a large effect of fuel properties on mixture formation and combustion, which may result in a completely different engine performance compared to the operation with conventional diesel fuels. Mixture formation and ignition behavior can also largely affect the pollutant formation. The knowledge of the combustion behavior is also important to design new engine geometries or implement new calibrations for an existing engine. The fuel properties of the investigated fuels comprise a large range, for example in case of the derived cetane number, from below 30 up to 100. In the study described here, different optical diagnostic methods are used simultaneously to monitor the behavior of these fuels. Measurements have been taken in a high pressure vessel at in-cylinder conditions representative for modern diesel engines. To investigate ignition and lift-off length of the fuel jets, high-speed detection of OH*-radicals was applied. In case of mixture formation analysis, Mie scattering measurement technique was used to detect the liquid penetration length and for the gas-phase, a shadowgraphy method was used. For both measurement techniques, a high speed camera was set up. By combining these optical measurement techniques, different amounts of premixed combustion have been observed for the different fuels. This can affect strongly the pollutant formation. Adding oxygen to a fuel increases the lift-off-length significantly at the same ignition delay time. Especially, the investigated fuel OME 3,4,5 seems to be a good alternative candidate regarding its sooting tendency.
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