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UBC Theses and Dissertations

Jet squish motion in a homogeneous-charge spark-ignition engine fueled by natural gas Kastanis, Eric James


Squish is one of the fundamental types of in-cylinder charge motion, and has been shown to be important in the generation of turbulence during the compression stroke. Turbulence intensity positively affects the mixture burning speed, which is important with inherently slow combustion systems, such as lean-burn. A previously proposed novel chamber design called ‘squish jet’ has been reported to further increase turbulence intensity compared with conventional combustion chamber designs. One bowl-in-piston (BIP) and two squish jet pistons (SJA and SJB), each with differing piston bowl geometry, were designed and built, and the operational characteristics of each were extensively investigated in an instrumented homogeneous-charge spark-ignition engine fueled by natural gas. The entire operating map of each chamber design was explored at 1500, 2000, and 2500 rpm and under naturally aspirated and supercharged inlet conditions (1.75 bar MAP). Test results showed that neither the SJA nor the SJB chamber conclusively exhibited superior brake thermal efficiency compared to the baseline BIP chamber. The observed differences were within the experimental uncertainty on the brake specific fuel consumption (BSFC) and air-fuel ratio parameters. Supplemental analysis of combustion heat release and exhaust hydrocarbon emission indicated that the SJA chamber exhibited the poorest BSFC performance. Because of uncertainty on BSFC and air-fuel ratio, NOx emission differences could neither be conclusively defined. The SJB chamber was found to result in the lowest exhaust hydrocarbon emission. Performance differences were similar under naturally aspirated and supercharged conditions. The main combustion duration of the three chambers was comparable, while the squish jet chambers exhibited longer ignition delay than the BIP chamber, and therefore favoured advanced ignition timing. Limit diagram analysis of the three chambers showed that the SJB chamber exhibited a limit space of comparable size to that of the BIP chamber, but was shifted toward advanced ignition timing. The SJA chamber limit space was similarly shifted, but was contracted in area. The results indicated that the difference in piston bowl geometry between the two squish jet chambers was as important as the addition of the jet-producing piston crown features in influencing combustion and engine performance.

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