Effects of ultra-lean combustion coupled with high-hydrogen blending on combustion, thermodynamics and emissions in a high-compression-ratio liquid methane engine

Addressing the dual challenges of efficiency improvement and emission control in internal combustion engines, this study integrates experimental and numerical approaches to investigate the effects of high-hydrogen blending coupled with ultra-lean combustion at different rotational speeds. Results indicate that hydrogen enrichment notably intensifies in-cylinder combustion, improving indicated thermal efficiency and lean-burn capabilities. Concurrently, increasing both the excess air ratio (λ) and rotational speed (N) can synergistically suppress knock tendency and reduce NOx emissions to a minimum of 246 ppm. In addition, the findings demonstrate a near-linear quantitative relationship between EEE (effective expansion efficiency) and EER (effective expansion ratio) at all excess air ratios. Operational optimization reveals that a 50 % hydrogen energy fraction (HEF) combined with 1.5 λ at 1400 r/min achieves 37.5 % indicated thermal efficiency alongside a 5.7 % elevation in average IMEP, balancing performance and emissions efficiently. Furthermore, the performance under the condition with 40 % HEF and 2.1 λ at 1400 r/min improved instead of deteriorating during the regulation of lean burn. This research establishes viable strategies for the clean and efficient operation of natural gas engines, while promoting the implementation of lean burn technology in low-carbon power systems.