Narrow-throat pre-chamber (PC) possesses characteristics like high pressure buildup, high jet velocity, and thus superior ignition performance for lean combustion engines. However, its design criteria have not been proposed and the impacts of pre-chamber throat configurations are not fully elucidated. This study proposed a new design criterion by defining a dimensionless number, throat diameter ratio (Rt), for the high-performance PC engine fueled with methane. A numerical passive PC engine model was established and extensively validated utilizing experimental data of pressure, heat release rate, and natural flame luminosity images acquired from an optical PC engine. The experimental PC flame jets and simulated temperature iso-faces show excellent agreement in flame jet dynamics. The impacts of nozzle length-to-diameter ratio (Rn) and throat diameter ratio (Rt) on jet characteristics and combustion performance were investigated numerically. Results show that increasing Rn enhances the reorganization of inflow direction at the orifice, reducing pressure difference (Delta P) and jet velocity. However, decreasing Rn deflects jets prematurely, deteriorating ignition. An optimal Rn of 1.036 is identified considering both combustion performances and jet deflection angle. Foremost, three types of jet velocity peaks are uncovered and distributed within different Rt ranges: the cold jet velocity peak (0 < Rt < 0.225), the hot jet velocity peak (0 < Rt < 0.5), and the last velocity peak (0 < Rt < 1). Variation mechanisms of these peaks are elucidated. Throats are categorized as ultra-narrow (0 < Rt < 0.225), narrow (0.225 < Rt < 0.5), and wide (0.5 < Rt < 1), considering variations of Delta P and velocity peaks. Among them, narrow throats (optimal at Rt of 0.333) exhibit the best flame propagation in the main chamber, leading to fast combustion, owing to the high average mass flow discharged during the hot jet stage. Ultra-narrow and wide throats present longer combustion duration due to slender jets and low hot jet discharged mass, respectively.

A New Design Criterion for Narrow Throat Pre-Chamber to Achieve High Jet and Ignition Performance