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 GDI-engines. 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.
Soot volume fraction profiles in laminar counterflow flames burning ethylene, n-heptane, iso-octane, and toluene were determined experimentally for a wide range of flame conditions. Simulations of these 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. Reaction pathway analysis suggests that, in these flames, more soot precursors are formed via the reaction pathways involving fuel pyrolysis products.
Furthermore, 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 suggest that the oxidation is not sufficiently fast to burn soot completely.