Computational analysis of the influence of active pre-chamber throat structures on combustion in a large-bore natural gas engine

Efficient lean combustion in large-bore natural gas engines is vital for decarbonization in the heavy-duty power generation sector due to their exceptional thermal efficiency and low emissions. While hydrogen-assisted active pre-chamber ignition effectively extends the lean limit and enhances flame propagation by discharging high-energy turbulent reactive jets, the complex fluid-thermodynamic interactions make the throat geometry critical for jet efficacy and main-chamber ignition stability. To investigate these geometric impacts, this study evaluates nozzle design utilizing two non-dimensional parameters: the throat diameter ratio and the throat length ratio. A three-dimensional computational fluid dynamics (CFD) model was established and validated against experimental cylinder pressure, heat release rate, and emissions data, demonstrating robust predictive capability. Results show that increasing the throat length ratio generally enhances jet stability; an insufficient ratio reduces ignition energy and degrades ignition uniformity across the orifices, whereas an excessive ratio significantly increases flow resistance. Crucially, the throat diameter ratio governs the crucial trade-off between pressure differential and jet penetration. Both excessive throttling and over-sizing severely limit peak jet velocity and ignition energy transfer. Among the investigated designs, the optimal configuration (7 mm throat diameter and 20 mm length, corresponding to a diameter ratio of 0.25 and length ratio of 0.52) achieves an indicated thermal efficiency of 43.83%, representing a 1.94% improvement over the baseline.