# Nozzle Design of Plug-and-Play Passive Pre-Chamber Ignition Systems for Natural Gas Engines
## 3 Results and Discussion
### Influence of Nozzle Design on Jet Ignition  
When the flame front propagates in the PC, the pressure increases rapidly, squeezing the unreacted/reacted gas through the nozzle as jet ejection. The jet ejection process can be divided into three stages, of which the dominant components are unburned gas, reaction intermediates, and reaction products, respectively. Hot high-speed jets with active radicals enhance the MC ignition through chemical, thermal, and turbulent effects. Numerous high-speed schlieren and chemiluminescence imaging studies have provided fundamental insights into jet ignition and concomitant quenching phenomena. Heat loss at the wall results in thermal quenching when the jet rushes through the nozzle. After entering the MC, the flow mixes with the unburned charge, and hydrodynamic quenching occurs. When the orifice diameter increases, the probability of the quenching phenomenon decreases, and the ignition mechanism gradually varies from turbulent jet ignition to flame ignition. The lateral angle of the PC nozzle determines the scavenging efficiency, affecting the subsequent jet ignition process. However, the direct effect of the lateral angle on the jet orientation and penetration distance is relatively insignificant. Figure 7 shows the flame front propagation process for different lateral angle schemes. When the lateral angle is set  
Figure 6: TKE in the PC of 7 cases at \(-9^{\circ}\)CA during scavenging. to 10\({}^{\circ}\), Case 1 presents the highest scavenging efficiency and the fastest flame propagation. However, because the swirl in the PC of Case 1 is not stable enough, the initial flame kernel develops asymmetrically. The flame does not spread evenly throughout the upper left side of the PC with the highest residual gas fraction. Although the asymmetric development of the initial flame kernel causes the jet to be uneven at 6\({}^{\circ}\)CA, as the combustion continues, the jet ignition becomes gradually uniform at 10\({}^{\circ}\)CA. Case 2, with a lateral angle of 0\({}^{\circ}\), exhibited similar properties. The flame propagation of Case 2 is slightly lower than that of Case 1, but the jet symmetry is better due to the kinetic energy loss caused by the impact of the airflows. In contrast, the swirl of Case 3, with a lateral angle of 20\({}^{\circ}\), is stable, and the initial flame kernel develops symmetrically, as shown in Figure 7. However, since the area with the highest residual gas fraction coincides with the spark plug, the flame propagation speed is minimal. In a word, the previous scavenging effect dramatically influences the subsequent flame propagation speed. In addition, the importance of jet symmetry does not appear to be high. Even if a significant asymmetry occurs, it will gradually disappear during combustion. Similarly to the lateral angle, the PC vertical angle can also affect jet ignition by determining the scavenging process. However, the vertical angle also has a more profound effect. The angle can affect the flow distribution in the squish and bowl areas by changing the jet orientation to control the flow separation at the piston throat. Figure 8 shows the flame front propagation process of different vertical angle schemes. The small-radius swirl in Case 4 and the low-height swirl in Case 5 reduce the scavenging efficiency and slow the flame propagation. When excluding the combustion phase difference in the three schemes, a direct comparison of Case 4 (12\({}^{\circ}\)CA), Case 1 (8\({}^{\circ}\)CA), and Case 5 (10\({}^{\circ}\)CA) shows that there are significant differences in the flow separation at the piston throat of each scheme. Case 4 has a minimum vertical angle of 120\({}^{\circ}\), the jet orientation is biased toward the bottom of the piston, and the flame front fills the bowl area first. As the pressure in the bowl area increases, the jet deforms and is squeezed into the squish area.