A new design criterion for narrow throat pre-chamber to achieve high jet and ignition performance

•A dimensionless number, throat diameter ratio, was proposed for the first time to evaluate PC designs.•The numerical and experimental results present excellent matching in the flame jet dynamics.•Criteria for ultra-narrow, narrow, and wide throats were proposed for the first time.•Orifice length-to-diameter ratio of 1.036 and throat diameter ratio of 0.333 were optimal.•Narrow-throat pre-chamber is favored for high jet and ignition performance. 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 (ΔP) 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