Soot particles in piston-top pool fires and exhaust at 5 and 15 MPa injection pressure in a gasoline direct-injection engine

The effect of fuel injection pressure on in-flame and exhaust soot structure was investigated in an optical spark-ignition direct-injection engine at fixed early injection timing of 320°C crank angle before top dead center to simulate significant wall wet ting and pool fire conditions. Soot particles were sampled on TEM grids via thermophoretic force directly from piston-top pool fires and at the entrance of the exhaust port. Two selected injection pressures of 5 and 15 MPa were investigated based on the sampled soot images and key statistical parameters of soot structures. Results showed that the 5-MPa injection produces smaller soot aggregates composed of smaller soot primary particles within the pool fire than those of the 15-MPa injection. The observed trend was related to a concentrated spray plume and taller pool fires formed on the piston top, which limited the soot growth within the fuel-rich flame. In the exhaust, however, relatively larger soot particles are observed for the lower injection pressure due to enhanced soot oxidation occurred at higher injection pressure. This was supported by the in-flame soot internal structure with less defined core-shell boundaries and higher fringe separation for the 15-MPa injection, indicating higher reactivity for the increased chance of soot oxidation. This more reactive status was maintained in the exhaust soot, resulting in smaller soot aggregates and primary particles despite larger piston-top pool fire soot.