Morphology, fractal dimension, size and nanostructure of exhaust particles from a spark-ignition direct-injection engine operating at different air-fuel ratios

This work studied the physicochemical characteristics of exhaust particles from a 1.48 L SIDI engine operating at different air-fuel ratios (AFRs). The morphology, fractal dimension, size and nanostructure were characterized using high-resolution transmission electron microscopy (HRTEM) in conjunction with electron energy-loss spectroscopy. The results indicate that SIDI aggregate particles produced at varying AFRs exhibit different morphologies. TEM images show that the aggregates obtained at an AFR of 14.7 are more compactly clustered than those generated under fuel-rich and fuel-lean conditions. The fractal dimension of SIDI aggregates at an AFR of 14.7 is found to be 2.21, a value that is larger than those from other AFRs. The primary particle sizes are distributed over a wide range of 5-55 nm at all AFRs, although the largest average primary particle diameter is found at an AFR of 14.7. Similar to diesel soot particles, SIDI soot particles also show characteristic shell-core and turbostratic structures at the nanoscale level. At an AFR of 14.7, the soot particles have a relatively short fringe length, a small fringe separation distance and high fringe tortuosity. As the AFR is increased, the sp2/sp3 hybridization ratios first gradually decrease and then increase, with a minimum value of 0.156 at AFR = 14.7. SIDI soot particles are likely more reactive than diesel soot particles because they possess the relatively short fringe length, large separation distance and high tortuosity.