Combustion and emissions performance of high-compression-ratio methanol pre-chamber Engines: Effect of pre-chamber jet hole configurations

Pre-chamber jet ignition (PCJI) shows great potential for improving combustion efficiency and reducing emissions in methanol-fueled engines. However, experimental research remains limited, particularly regarding the combined influence of pre-chamber hole number, angle, and diameter. To address this gap, seven pre-chambers with distinct geometries were evaluated on a single-cylinder methanol engine under varying excess air coefficient (λ). Results show that a six-hole design optimally balances scavenging and flow resistance, sustaining pre-chamber combustion and enhancing energy transfer. Increasing hole angle from 0° to 30° reduces peak pre-chamber combustion parameters but prolongs jet duration; 15° hole angle offers the optimal trade-off by strengthening turbulence without excessive flow disruption. Smaller hole diameters preserve higher pre-chamber temperature and pressure, improving combustion stability under ultra-lean conditions, while intermediate diameters minimize combustion duration at λ < 1.6. The configuration of six holes, a 15° hole angle, and a 1.25 mm hole diameter achieves the lowest energy losses, with a peak gross indicated thermal efficiency (GITE) of 48.60 % and a minimum specific fuel consumption (GISFC) of 377.90 g/kWh at λ = 2.0. Emissions analysis shows HC and CO rise with λ and hole size, whereas NOx emissions remain low above λ = 1.6 and are largely unaffected by hole configurations.