In-Cylinder Soot Oxidation in an Optically Accessible Automotive Diesel Engine

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Copyright: Rao, Lingzhe
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Abstract
The problematic soot emission of diesel engines is the net result of the competing in-cylinder formation and oxidation processes. Previous studies primarily focused on the soot formation, whereas the oxidation of soot particles remains an unexplored and least studied topic. To bridge this gap, this thesis aims to improve the fundamental knowledge about the in-cylinder soot oxidation and its significant role in lowering late-cycle soot levels. Of particular interest is how fuel injection conditions as well as jet-swirl and jet-jet interactions impact soot oxidation process. In this regard, various optical/laser-based imaging diagnostics were performed to unveil the development of high-temperature reaction and soot in small-bore optical diesel engines, which include OH* chemiluminescence, planar laser-induced fluorescence of hydroxyl (OH-PLIF) and planar laser-induced incandescence (PLII) imaging. Additionally, thermophoresis-based particle sampling techniques were implemented to obtain transmission electron microscope (TEM) images of soot aggregates and primary particles for detailed morphology analysis. It was observed that either increased fuel injection pressure or advanced injection timing enhances the formation of soot particles because of higher combustion temperature caused by enhanced premixed combustion or advanced combustion phasing. However, the increased high-temperature reaction also leads to enhanced soot oxidation, which outperforms the increased soot formation and thereby resulting in overall reduction of late-cycle soot. It was also found the implementation of after-injection enhances soot oxidation through additional temperature rise. For this approach, the close-coupled after-injection was found to add more soot but also induces higher soot oxidation as compared to that of its long-dwell counterpart. The air-fuel mixing was also found to affect the soot oxidation significantly. From innovative multi-location in-bowl soot sampling experiments, it was shown that the oxidation of soot particles is higher on the up-swirl side of the wall-interacting jet due to enhanced mixing. The decreased jet-jet interaction could suppress the initial soot growth while stimulating soot oxidation as a result of enhanced mixing. The outcome of this thesis provides a guideline for diesel engine developers to design fuel injection strategies achieving lower remaining soot through enhanced soot oxidation even if the soot formation is higher.
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Author(s)
Rao, Lingzhe
Supervisor(s)
Kook, Shawn
Chan, Shaun
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Publication Year
2020
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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