Soot formation of gasoline surrogate components in laminar and turbulent flames

Stringent emission regulations aim to restrict soot emissions by particle mass and particle number in the transport sector. Whilst the concept of direct injection gasoline (GDI) engines is very promising with respect to efficiency, complying with the legislative soot emission limits is challenging for GDIengines. A sufficient reduction of soot emissions in GDI engines requires a detailed understanding and accurate modeling of soot formation processes of gasoline fuels in engines. This study provides fundamental investigations of the soot formation process of common gasoline surrogate components. First, soot volume fraction measurements in ethylene, n-heptane, iso-octane, and toluene counterflow flames were performed by means of laser-induced incandescence (LII). This study demonstrates that large gradients in temperature and local gas composition, which are inherently present in counterflow flames, induce substantial beam steering effects. An approach is developed to ensure accurate LII measurements based on tailoring the mean laser fluence. The collected LII signal in the counterflow flames is thus nearly independent of beam steering effects. The soot volume fraction profiles in the counterflow flames were then determined for a wide range of strain rates and stoichiometric mixture fractions. Sensitivities of soot formation on strain rate and stoichiometric mixture fraction were determined for each fuel, which are found to be strongly fuel dependent. Simulations of the flames reveal that applied models are capable of predicting the soot volume fraction with remarkable accuracy for ethylene. For the gasoline surrogate components, however, the overall soot volume fractions are overpredicted. A reaction pathway analysis suggests that, in these flames, more soot precursors are formed via the reaction pathways involving fuel pyrolysis products. Finally, flame structure, local gas temperature, local soot volume fraction, and primary soot particle diameter were simultaneously detected by means of optical diagnostics in turbulent toluene flames. Joint statistics of flame and soot properties indicate that, due to differential diffusion of soot, high soot concentrations are present at conditions of low temperatures and low OH concentrations. In the soot oxidation region, the presence of large particles suggests that oxidation is not sufficiently fast to burn soot completely