In this study, a fully optically accessible single-cylinder research engine is the basis for the visualization and generation of extensive knowledge about the in-cylinder processes of mixture formation, ignition and combustion of oxygenated synthetic fuels. Previous measurements in an all-metal engine showed promising results by using a mixture of dimethyl carbonate and methyl formate as a fuel substitute in a DISI-engine. Lower THC and NOx emissions were observed along with a low PN-value, implying low-soot combustion. The flame luminosity transmitted via an optical piston was split in the optical path to simultaneously record the natural flame luminosity with an RGB high-speed camera. The second channel consisted of OH*-chemiluminescence recording, isolated by a bandpass filter via an intensified monochrome high-speed camera. To investigate the combustion process spectrally, spatially and temporally resolved in more detail, selected operating points were recorded again via a high-speed imaging spectrograph.
Regular gasoline fuel acts as a reference and is compared to the oxygenated mixture. Since all oxygenated fuels show a heating value lower than gasoline, the injected mass increases for constant engine load. For 65 vol-% DMC 35 vol-% MeFo, the gasoline equivalent, a product of the lower heating value and the density, results in a factor of two. Accordingly, elevated cooling effects of the mixture are expected. Because of these unfavorable conditions for mixture formation, spots of diffusion flames could be detected when using oxygenated fuels.
A lambda sweep showed that the mixture did not produce significant soot even in slightly lower than stoichiometric conditions. In addition, a very late SOI of around 90 CAD bFTDC showed reduced burning duration and lower diffusion flame intensity.