Development of a soot particle model with PAHs as precursors through simulations and experiments

Abstract

In the present study, the polycyclic aromatic hydrocarbons (PAHs) and soot particulates emitted from a gasoline direct injection (GDI) engine were sampled and analyzed. The results show that the vapor-phase PAHs and particulate-bound PAHs exist in GDI engine exhaust emissions. Soot particles are formed by the agglomeration of the quasi-spherical primary carbon particles, and the size of the PAH cluster is close to that of the core of the primary carbon particles. To predict soot particulates evolution in the engine cylinder, a reduced toluene reference fuel (TRF)-PAH chemical mechanism consisting of iso-octane, n-heptane and toluene as gasoline surrogate fuels for the GDI engine combustion simulation was developed. The reduced mechanism contains 232 reactions and 85 species, including 17 species and 40 elementary reactions, related to the PAH formation, which could well capture the oxidation characteristics of ignition delays and laminar flame speeds, as well as PAH emissions from the GDI engine. Then, a mathematical soot growth model coupled with the reduced TRF-PAH mechanism was developed based on the method of moment. The obtained experimental soot emissions were well predicted by this soot model. The distribution of particle number density was consistent with the distribution of A4 and C2H2, and the growth and distribution of soot mass was determined by the particle surface growth due to C2H2, the nuclear reaction of A4 and the particle surface oxidation reaction.