# 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
Case 5 has a maximum vertical angle of 140\({}^{\circ}\), the jet orientation is biased towards the top of the piston, and the flame front first fills the squish area and then gradually develops towards the bowl area. Case 1 is a compromise between the first two schemes. The flame front reaches the bowl and squish areas almost simultaneously, finally filling the entire MC. Therefore, the PC with an optimized vertical angle has a short combustion duration and high stability. Figure 7: Flame propagation in the PC of Case 2, Case 1, and Case 3 during jet ignition. The orifice diameter determines the scavenging efficiency and the jet velocity. In Figure 9, Case 6 has the lowest orifice diameter of 1.0 mm. The cross-sectional area of Case 6 is also the smallest, which significantly affects the scavenging efficiency. The ignition process is delayed, due to a large amount of residual gas in the PC. In the subsequent jet ignition process, driven by the considerable pressure difference, although the small orifice diameter increases the jet velocity, the benefits have approached the limit, which cannot offset the combustion phase delay that occurs at the initial stage of ignition. Case 7 has the largest orifice diameter of 1.6 mm. This case has the highest scavenging efficiency, but the unburned gas is more likely to escape as a cold jet, resulting in a decrease in the pressure and a short jet penetration distance. A short jet penetration distance increases the combustion duration and reduces flame uniformity. There is an optimal orifice diameter, considering the scavenging efficiency and the jet velocity, so that the combustion phase and jet penetration distance can meet the requirements. In the research of [PERSON] et al., the passive PC, with an orifice diameter of 1.2 mm, presents the fastest natural gas combustion speed, significantly higher than that of 1.0 mm and 1.5 mm, which is approximately consistent with the results of this paper. Figure 8: Flame propagation in the MC of Case 4, Case 1, and Case 5 during jet ignition. Figure 9: Flame propagation in the MC of Case 6, Case 1, and Case 7 during jet ignition. Monitoring the heat release rate of the MC can confirm the jet ignition timing, which can be used as a boundary to distinguish between a cold jet and a hot jet, as shown in Figure 10. Among the factors, the flame speed and the orifice diameter present the most significant influence on the cold jet fraction. Case 1 has the best scavenging effect, and the flame speed in the PC is the fastest. More unburned gas near the nozzle is ignited before escaping, therefore, the cold jet fraction is the lowest. The effect of the orifice diameter is equally essential. Although the largest orifice diameter of 1.6 mm in Case 7 improves the scavenging and flame speed in the PC, the escape of unburned gas also increases. The results show that the cold jet fraction in Case 7, with the largest orifice diameter, is higher than that in Case 6, with the smallest orifice diameter, indicating that the performance loss caused by the unburned gas escape is greater than the enhancement of the flame speed. In addition to flame speed and orifice diameter, there are other interesting effects. Compared with the 0\({}^{\circ}\) lateral angle of Case 2, the 20\({}^{\circ}\) lateral angle of Case 3 presents a stable swirl and low flame speed in the PC. However, contrary to expectations, the cold jet fractions of Case 2 and Case 3 are comparable, suggesting that the inherent pressure gradient of the swirling flow played a role. A swirling flow is characterized by a low center pressure, as opposed to the characteristics of a high flame center pressure. The phenomenon retards the propagation of the pressure wave, which helps to restrain the cold jet and increases the jet ignition energy. The vertical angle of Case 5 is 140\({}^{\circ}\), the flame speed in the PC is not high, and the mutual interference between the jets from the different orifices is minimal.