Abstract
Spray-wall impingement caused by early fuel injection for gasoline partially premixed combustion (PPC) can lead to low combustion efficiency and a significant rise of UHC emissions. But the influence of spray wall impingement on the in-cylinder combustion process is not well understood. In this study, multiple optical diagnostics were applied to investigate the ignition, flame development and UHC formation of gasoline PPC with early single fuel-injection in a light-duty optical engine under low engine load. Natural combustion luminosity images and emission spectra were obtained. Planar laser-induced fluorescence (PLIF) of the fuel-tracer and formaldehyde were used to explore the fuel/air mixing and UHC formation in PPC, respectively. The results indicated that there was a fuel-injection time window (about -30 degrees to 60 degrees ATDC in the present study), during which the spray-impingement led to a decrease in combustion efficiency. The fuel-trapping effect in the squish region and piston crevice was shown to be the main reason because it prevented the fuel/air mixture from entering the combustion chamber. Two typical fuel injection timings of -35 degrees (PPC-35 degrees) and -60 degrees (PPC-60 degrees) were chosen for further study. For both cases, ignition sites first emerged in the fuel-rich regions and then the flames developed to the fuel lean regions. The formaldehyde PLIF images revealed distinct flame front in the flame development process. For the PPC-35 case, residual formaldehyde persisted in the fuel-lean regions late during the power stroke and might become a source of UHC emissions. When misfire happened, the combustion chamber was filled with formaldehyde. For the PPC-60 case, the flame development was composed of initial flame front propagation and following sequential auto-ignition, and the flame expansion speed of the initial flame front propagation was much higher than that in SI (spark ignition) or SACI (spark assisted compression ignition) combustion. When the injection timing was further advanced (earlier than 600), the impact of spray-wall impingement on PPC was reduced because of more time being available for fuel premixing. (C) 2016 Elsevier Ltd. All rights reserved.