# Nozzle Design of Plug-and-Play Passive Pre-Chamber Ignition Systems for Natural Gas Engines
## 3 Results and Discussion
### Influence of Nozzle Design on the Main Combustion  
After the combustion of the PC, hot jets eject and ignite the MC. Chemical, thermal, and turbulent effects enhance the flame in the MC. The three effects are derived from the improvement of the chemical reaction kinetics by active radicals, high temperature, and  
Figure 10: Mass fractions of cold and hot jets for 7 cases during jet ejection. sufficient mixing of air and fuel. Compared with the single-point ignition of the traditional spark plug, turbulent jet ignition can significantly increase the ignition energy by about two orders of magnitude. Figure 11 shows the MC and PC pressure and heat release rate curves of the cases during combustion. After ignition, the PC pressure increases, accompanied by cold and hot jets entering the main combustion chamber. Then, the pressure of the MC decreases slightly before burning, due to the downward movement of the piston. After jet ignition, the MC pressure rises rapidly and exceeds that of the PC. At this time, part of the gas mixture flows back into the PC. After the backflow finishes, the pressures of the MC and the PC are gradually balanced. The PC nozzle's lateral angle, vertical angle, and orifice diameter significantly impact the in-cylinder pressure, which is mainly reflected in the pressure peak and change rate. Figure 11 compares the in-cylinder pressure and heat release rate curves for lateral angles of 0\({}^{\circ}\), 10\({}^{\circ}\), and 20\({}^{\circ}\). Appropriately increasing the lateral angle can improve the combustion phase and pressure peak. Still, as the angle increases and exceeds the critical value, the ignition energy decreases significantly, accompanied by an increase in the ignition delay. On the one hand, the center of the stable swirl is prone to retain the residual gas. On the other hand, the inherent pressure gradient of the swirling flow is opposite to the pressure wave generated by combustion, which is unfavorable for jet injection. Figure 11 compares the in-cylinder pressure and heat release rate curves with vertical angles of 120\({}^{\circ}\), 130\({}^{\circ}\), and 140\({}^{\circ}\). It can be seen that the parameter sensitivity of the vertical  
Figure 11: Cylinder pressure and heat release rate (HHR) of PC and MC for all cases during combustion. angle is much higher than that of the lateral angle and orifice diameter. Increasing or decreasing the vertical angle from the optimal value will significantly decrease the ignition energy, but the mechanisms are different. The 120\({}^{\circ}\) small vertical angle design of Case 4 makes the jets of different nozzles interfere with each other, reduces the jet velocity, and then impacts the turbulence effect. The 140\({}^{\circ}\) large vertical angle design of Case 5 results in uneven flame distribution, with more jet flowing into the squish area. In addition, Case 5 presents a higher fraction of cold jets. Although the combustion phase is advanced, the ignition energy is decreased accordingly. Figure 11c compares the in-cylinder pressure and heat release rate curves for orifice diameters of 1.0 mm, 1.3 mm, and 1.6 mm. The scavenging effect of the PC with a large orifice diameter is the best, but the jet penetration distance and the ignition energy decline, resulting in reduced combustion efficiency. After simulation ranking, four schemes are selected. The maximum pressure, knock frequency, EGR rate, and fuel consumption of the schemes and the original engine are tested on the bench, as shown in Figure 12. After the PC retrofit, the maximum pressure of the engine increases, while the knock remains near the original level. The EGR rate of Case 1 is significantly improved under operating Condition 2, i.e., under normal power. However, under the rated power of operating Condition 4, due to the high PC temperature affecting the scavenging charge, each scheme's EGR rate is challenging to increase compared to the original engine.