Influence of wall-wetting conditions on in-flame and exhaust soot structures in a spark ignition direct injection petrol engine

Abstract

Great attention to the efficiency benefits of spark ignition direct injection engine has been averted due to its problematic particulate emissions. In the present study, the fundamental knowledge of wall-wetting-induced spark ignition direct injection soot particles is enhanced through direct particle sampling from pool fire on the piston top surface and cylinder liner as well as from the exhaust stream. The sampled soot particles are imaged using transmission electron microscope, and the image post-processing for statistical morphology and internal structure analysis is performed to better understand the soot formation and oxidation processes. The experiments were performed in a single-cylinder optical spark ignition direct injection engine where diffusion flame luminosity was recorded using a high-speed camera through the cylinder liner window, with which the overall sooting level was understood, and the pool fire location was identified. Given the in-flame soot sampling experiments in the spark ignition direct injection engine were new, error analysis was conducted in terms of the number of fuel injections and engine run-to-run variations. This sampling technique then was applied for various injection timings in the intake stroke. The data analysis and physical interpretation was focused on a piston-wetting condition at the most advanced injection timing of 320 °CA bTDC and a liner-wetting condition at the most retarded injection timing of 180 °CA bTDC in the present study. Between these two different wall-wetting conditions, it was found that the piston-wetting condition has larger soot primary particles and soot aggregates. The internal carbon-layer fringe shows longer length, less tortuosity and smaller gap, indicating more mature and carbonised soot. This was consistent with more significant and wider distributed pool fire and thus longer soot residence time within the flames. When the exhaust soot particles were analysed, however, it was found that the reduction in soot aggregate size was much higher and the carbonisation was more progressed for the piston-wetting condition than those of the liner-wetting condition. This suggested higher soot oxidation later in the expansion/exhaust stroke for the piston-wetting condition, which potentially can be better utilised for engine applications.