# Nozzle Design of Plug-and-Play Passive Pre-Chamber Ignition Systems for Natural Gas Engines
## 4 Conclusions  
The plug-and-play passive pre-chamber (PC) is suitable for quick retrofit and improving fuel economy in realistic engines. In this paper, the influence of three key design parameters on ignition processes was numerically studied, including the lateral angle, the orifice diameter, and the vertical angle design. Seven PC schemes were numerically evaluated, and four were selected for the bench test. The characteristics of the scavenging process, turbulent jet ignition, and main-chamber combustion were investigated and analyzed.  
Figure 12: Experimental data of the selected schemes and original engine.  
Considering the trade-off of the ignition energy and scavenging efficiency, the ratio of the PC volume to the clearance volume is recommended to be 0.2\(\sim\)0.7%, and the area-volume of the PC is recommended to be 0.003\(-\)0.006 mm\({}^{-1}\). After the PC retrofit, the combustion duration can be shortened by 30-50%. The early combustion stage (mass fraction burned 10-50%) is significantly shortened, but the later combustion stage (mass fraction burned 50-90%) presents no noticeable improvement. Under normal power, the fuel consumption can be saved by 13.2%; however, the benefit is relatively reduced under low load or rated power conditions. In addition, the maximum pressure of the engine increases, while the knocking remains near the original level. The EGR rate is also improved, but the advantages of the PC become weakened at rated power due to the high temperature affecting the scavenging charge.  
During the scavenging stage, the lateral angle determines the swirl intensity in the PC. The angle should be appropriately reduced if a stable swirl is formed in the PC. Otherwise, if the insufficient swirl intensity causes the airflows to collide, the lateral angle should be appropriately increased. The vertical angle is another crucial factor affecting scavenging. If the swirl height cannot reach near the spark plug, the vertical angle should be appropriately reduced. If the reduction in the swirl radius causes a significant dissipation, the vertical angle should be increased appropriately. The orifice diameter influences the jet penetration distance. The diameter adjustment should also consider the trade-off between scavenging and jet injection.  
During the jet ignition stage, the lateral angle affects the cold jet fraction. The angle should be increased if the cold jet causes insufficient ignition energy. In addition, the lateral angle affects the jet symmetry, but the effect seems less critical. The vertical angle affects the jet's direction. If the jet separation at the piston throat is unbalanced, e.g., the flow in the bowl area is more significant than that in the squish area, the vertical angle should be increased, and vice versa. If the cold jet fraction rises, it may be due to an excessive vertical angle. Moreover, the vertical angle presents the most significant influence during the main chamber (MC) combustion stage. If the jet flames approach each other to interfere, the vertical angle should be increased. If the flame distribution appears to be out of balance, and the flame fraction in the squish area is too high, the vertical angle should be reduced.  
Writing--review and editing, W.L.; methodology and software, J.M.; validation and formal analysis, T.Z.; supervision, H.W.; project administration, J.P. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Key R&D Program of China (2022 YFE0100100) and National Natural Science Foundation of China (52222604).  
Not applicable.  
The authors declare no conflict of interest.