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UBC Theses and Dissertations

Characterizing regimes of stratified pilot-ignited direct-injection natural gas combustion in an optically-accessible engine Rochussen, Jeremy


Heavy-duty road transport is a significant contributor of greenhouse gases (GHG) and airborne pollutants, and remains challenging to decarbonize. Application of natural gas (NG) in pilot-ignited direct-injection NG (PIDING) engines has been proven to reduce emissions of pollutants and GHGs relative to conventional diesel engines. Recently, stratified-premixed NG combustion has been identified as a viable approach to further reduce PIDING pollutant emissions. However, there is insufficient experimental data on stratified-premixed PIDING combustion to guide its effective implementation. The objective of this work is to present a systematic evaluation of stratified-premixed PIDING combustion modes that span from fully-premixed to non-premixed conditions in terms of ignition, main combustion, and emissions behavior. To address these objectives, a single-cylinder research engine facility was operated in conventional all-metal and optically-accessible configurations. The facility was upgraded with a custom-designed cylinder head and high-pressure diesel/NG fuel system to investigate PIDING combustion and apply multiple simultaneous in-cylinder diagnostics to supplement existing in-cylinder OH*-chemiluminescence and 700nm imaging. Quantitative feature extraction from single-cycle combustion images was enhanced by developing a novel image segmentation algorithm to improve characterization of cyclic variability of combustion processes. Combined thermodynamic and optical analyses of injection timing, duration, and pressure effects for non-premixed PIDING combustion condition identified five distinct combustion processes, which were incorporated into an updated conceptual description of non-premixed PIDING combustion. Six regimes of stratified PIDING combustion distinguished by NG premixing time were identified and characterized for a wide range of injection pressures (14-22MPa) and equivalence ratios (0.47-0.71). Consistency of combustion regime characteristics with respect to injection pressure, equivalence ratio, and with the available literature provide confidence in the broad applicability of the identified combustion regimes (i.e. not engine-specific). NG mixture development, pilot-NG interactions, and reaction zone structure and growth rates were characterized using simultaneous in-cylinder imaging and local high-speed in-cylinder fuel concentration measurement. The conclusions of this work are summarized as a conceptual framework that parameterizes the spectrum of stratified PIDING combustion and highlights conditions where PIDING combustion performance may be improved. The key findings and novel analysis and experimental methods are broadly relevant to all pilot-ignited gaseous direct-injection combustion technologies and fuels.

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