Date of Award
Summer 2024
Document Type
Thesis
Degree Name
Master of Science (MS)
Department
Mechanical Engineering
First Advisor
Adam Dempsey
Second Advisor
Simcha Singer
Third Advisor
Anthony Bowman
Abstract
Increasingly stringent emissions regulations have motivated the exploration of ways to decarbonize the internal combustion engine (ICE). Alternative fuels are of interest as they can easily be implemented with existing infrastructure. Among these alternative fuels, natural gas stands out due to its large abundance and high hydrogen-to-carbon ratio, which can significantly reduce CO2 emissions. Due to its low reactivity, most natural gas engines today are premixed lean burn spark ignited (SI) engines. These engines produce criteria pollutants, such as nitrogen oxides (NOx) and carbon monoxide (CO), but due to their premixed nature, also produce relatively large amounts of unburned methane (CH4) emissions. This study explores advanced combustion strategies for natural gas engines, particularly focusing on prechamber enabled mixing controlled combustion (PCMCC), aiming to enhance performance and reduce emissions, especially methane slip, in large-bore natural gas engines. To evaluate the PCMCC concept, experimentation of a passive prechamber spark ignited (PCSI) concept was performed to provide a baseline that is representative of current premixed lean burn natural gas engine technology. Single cylinder engine (SCE) experiments were conducted to evaluate both PCSI and PCMCC combustion strategies on a Caterpillar C9.3B platform. Specialized modifications to engine hardware to adjust compression ratios and accommodate prechamber hardware were completed for both experiments. The subsystems in the engine test cell allow for flexible control of engine operation and data acquisition to analyze performance and emissions. Details of the high pressure fuel system were investigated included an understanding of hydrocarbon dew points and their role in the PCMCC concept. Additionally, apparent heat release rate (AHRR) models were developed to analyze the experimental burn rates within the prechamber and main chamber. Comparative analysis of PCSI and PCMCC concepts indicates that PCMCC of natural gas can achieve a 90% reduction in methane slip at equal NOx emissions. With use of EGR, NOx emissions can be lowered to 1 g/kW-hr or lower while maintaining low methane slip. Both concepts deliver similar gross indicated thermal efficiencies at similar compression ratios. However, without fear of knock, PCMCC can operate at higher compression ratios potentially providing an advantage in thermal efficiency compared to its premixed counterpart. Due to its non-premixed nature, the PCMCC concept shows higher combustion efficiencies, allowing for more complete oxidation of fuel to reduce methane slip. This study underscores the PCMCC concepts potential for improved emissions control and efficiency in lean burn natural gas engines.