# Nozzle Design of Plug-and-Play Passive Pre-Chamber Ignition Systems for Natural Gas Engines
## 3 Results and Discussion  
### Influence of Nozzle Design on Savenging  
Scavenging refers to the removal and remixing process of residual gas in the PC; however, its effect on heat transfer is equally noteworthy. Ideal scavenging can provide a fresh homogeneous mixture to the spark plug and increase the turbulence intensity to accelerate flame propagation. The pressure difference between the PC and the MC drives scavenging. During the intake stroke, part of the residual gas is removed, and less than 15% of the fresh gas is filled. The subsequent filling is completed in the compression stroke, and the compression ratio and operating conditions determine the final residual gas fraction. This study selects low-speed and high-torque operating conditions, where the residual gas fraction reaches the maximum, for scavenging research. For research convenience, the top dead center of the compression stroke is chosen as the reference point \(0^{\circ}\)CA. Figure 4 shows a velocity distribution diagram of each PC after ignition, i.e., \(-9^{\circ}\)CA. During the compression stroke, the mixture in the MC is pressed into the PC to form a scavenging flow field. The lateral angle of the nozzle determines the swirl intensity in the PC. In Figure 4b, the lateral angle is \(0^{\circ}\), and the different airflows impact each other in the PC. A large amount of kinetic energy is dissipated near the nozzle, which affects the scavenging near the spark plug. When increasing the lateral angle to \(20^{\circ}\) in Figure 4c, a stable swirl is formed in the PC. However, in this case, the spark plug is just in the low-speed area of the swirl center, which reduces the scavenging efficiency. The \(10^{\circ}\)lateral angle in Figure 4a produces an unstable swirl with a gyro-like precession feature, enhancing the scavenging near the spark plugs. On the other hand, the vertical angle of the nozzle is another factor that affects the swirl intensity. In Figure 4d, the vertical angle is 120\({}^{\circ}\), decreasing the swirl radius. Much kinetic energy is dissipated near the nozzle, similar to that of Figure 4b, which is unfavorable for ignition. When the vertical angle increases to 140\({}^{\circ}\), as shown in Figure 4e, the swirl intensity increases but the swirl height decreases, resulting in a decline in the airflow near the spark plug. The influence of the orifice diameter is more intuitive than the nozzle angles. Figure 4f, with a diameter of 1.0 mm, has the highest swirl strength among all of the schemes. Although the flow in Figure 4g, with a diameter of 1.6 mm, is the smoothest, the exhaust gas residue is the lowest, thanks to the highest area-volume ratio. Of course, this is at the cost of reducing the jet penetration distance. To further study the influence of the nozzle design on the residual gas fraction, Figure 5 shows the mass fraction of carbon dioxide in each PC at \(-9^{\circ}\)CA. It can be seen that the carbon dioxide distribution near the nozzle is highly correlated with the velocity distribution. Near the spark plug, however, there is always a considerable amount of carbon dioxide remaining, regardless of the local velocity. Therefore, the residual carbon dioxide concentration at the spark plug and the distribution uniformity in the PC are critical indicators for evaluating the scavenging efficiency. The former can determine the ignition energy, and the latter can determine the stability of flame propagation. Among all of these schemes, Figure 5g has the lowest residual gas and the most uniform distribution, with an orifice diameter of 1.6 mm. However, due to the increase in the area-volume ratio, the subsequent jet penetration distance decreases, which is unfavorable for the ignition of the MC. In this study, the recommended area-volume ratio was 0.003-0.006 mm\({}^{-1}\). The lateral angles in Figure 5a,b are 10\({}^{\circ}\) and 0\({}^{\circ}\), respectively. Their residual gas is comparable, but the unstable swirl flow in Figure 5a makes the residual gas distribution more uniform and increases the ignition stability.