Numerical Research on the Effects of Pre-Chamber Orifice Scheme on the Performance in a Large-Bore Natural Gas Engine

Pre-chamber ignition is one of the advanced technologies to improve the combustion performance for lean combustion natural gas engine, which could achieve low NOx, simultaneously. The designing scheme of the orifices, which connects the pre-chamber and the main chamber, is the main challenge limiting the further improvement. In this work, the three-dimensional computational fluid dynamics calculation based on a four-stroke engine with 320 mm cylinder bore was conducted to investigate the effects of orifice structure on the combustion and NOx performance. The results show that the schemes with 7 and 9 orifices lead to the delayed high-temperature jets formation due to the asymmetrical airflow in the pre-chamber, which retards the ignition timing but enhances the combustion in the main chamber. The 6 orifices scheme leads to the insufficient distribution of the high-temperature jets, and the 10 orifices result in the serious interference between the adjacent high-temperature jets. The increased orifice draft angle has strengthened the formation of the high-temperature jets because of the throttling effects, resulting in the higher indicated power. Although the central orifice strategy has delayed the high-temperature jets, the main chamber combustion process has been enhanced significantly due to the ignition below the pre-chamber. Compared to the baseline, the output indicated power with 2.0 mm central diameter has been increased by 3.2%, and the NOx with 2.5 mm central diameter has been reduced by 34.6%. The layered orifice strategy can effectively improve the interference and enlarge the high-temperature jets distribution in the main chamber. However, the advanced combustion and more high-temperature regions produce more NOx. According to the sensitivity analysis, the orifice number and the central orifice diameter present more significant effects on the output indicated power and the NOx.