Effect of passive pre-chamber geometry on performance and emissions of a single-cylinder natural gas large bore engine.

The maritime industry is one of the most energy-intensive sectors, characterized by high fuel consumption and significant environmental impact. As global trade mainly relies on shipping, the challenge of reducing pollutant and greenhouse gas emissions becomes ever more pressing. Natural gas (NG) is regarded as a transitional fuel, capable of lowering CO₂ emissions by 20–30% compared to conventional marine fuels. However, to fully harness this potential, significant advances in combustion technology are necessary, particularly when ultra-lean combustion strategy is adopted. One of the most promising pathways is pre-chamber combustion, a solution that can simultaneously improve the efficiency and sustainability of NG marine engines. In this scenario, the passive pre-chamber geometry plays an important role, as it directly influences ignition behavior, combustion stability and exhaust emissions. This work presents an experimental study carried out on a single-cylinder marine engine prototype, retrofitted from a diesel baseline and equipped alternatively with four passive pre-chambers featuring different geometrical configurations. The tests were carried out at an engine speed of 1500 rpm and different loads to evaluate the influence of pre-chamber geometry on engine performance and exhaust emissions. Key parameters such as combustion phasing, efficiency, and pollutant formation were analyzed and compared between the four setups. Results showed that pre-chamber design affects the interaction between the turbulent jets and the main chamber mixture, leading to significant variations in both combustion efficiency and emission trends. These findings provide new insights into the role of passive pre-chamber geometry in optimizing large-bore NG marine engines, offering a valuable contribution to the development of cleaner and more efficient propulsion systems for the maritime sector.