In-flame soot structure of a jet fuel with 24% aromatics in a small-bore optical compression-ignition engine

Abstract

This study applies a multi-location soot sampling method in a small-bore optical compression ignition engine to show in-flame particle evolution of a surrogate jet fuel with 24% aromatics (AR24) and a conventional diesel fuel. Four soot sampling probes are installed on the piston-bowl wall with 60° spacing angles for simultaneous sampling from the same firing cycle. These sampling locations represent a jet-wall impingement point (JW), an up-swirl point (US), a down-swirl point 1 (DS1) and a down-swirl point 2 (DS2) along the sooting flame path identified by high-speed soot luminosity images and planar laser induced incandescence (PLII) images. The sampled soot particles are examined with transmission electron microscope (TEM) and the images are post processed for the analysis of morphology. The results showed both fuels produce a large amount of small soot aggregates at JW point due to fuel-rich mixtures while AR24 soot aggregate number count is much less than diesel, indicating less soot formation for AR24. They grow as the flame travels along the bowl wall through aggregation, which is more significant at DS1 point than that of US point. Simultaneously, some small soot aggregates disappear due to oxidation, which is more significant at US point. As the sooting flame reaches DS2 point, only large aggregates with compact structures survived intense oxidation. The soot sampling performed for high resolution TEM and internal structure analysis of AR24 showed that the core-shell particle structure becomes clearer and the carbon-layer fringe-to-fringe gap decreases during the flame penetration, indicating the soot oxidation continued. From the results, the in-flame soot evolution is characterised by high formation of small aggregates at JW point, simultaneous aggregation and oxidation at US and DS1 point with the latter more prone to aggregation, and significant oxidation at DS2 point.