## 4 Results
### Emission Performance Analysis  
Figure 9a shows the change curve of the average NO mass fraction in the cylinder under supercritical fuel injection and subcritical conditions. The calculation results for the initial fuel temperature of 650 and 400 K shows that the mass fraction of NO at a fuel temperature of 650 K is 2.3 times the fuel temperature of 400 K in the steady state. As shown in Figure 9b, compared with the experimental data of [PERSON] [21], the NO emissions of the engine are 0.34 g/kW-h and 0.71 g/h at the initial fuel temperature of 650 and 400 K, respectively. The numerical simulation results are close to the experimental results. Thus, the computational model is reliable under subcritical fuel injection conditions, and the emission performance under subcritical conditions can be compared with that under supercritical conditions. The mass fraction of NO under the supercritical fuel injection condition is higher than that under the subcritical condition. At 720-735\({}^{\circ}\) CA, the combustion is premixed. The average temperature of the fluid under the supercritical condition is higher than that under the subcritical condition. Therefore, the generation rate of NO is rapid under the supercritical condition. In Figure 10a, the NO generation rate under the supercritical injection condition is faster than that under the subcritical condition. Furthermore, the region where NO is generated corresponds to the high-temperature region of the temperature distribution cloud diagram at 730\({}^{\circ}\) CA in Figure 10b,  
Figure 8: Flow velocity distribution in the cylinder under five operating conditions. Figure 9: (**a**) Change curve of NO; (**b**) NO emission comparison average mass fraction. indicating that the formation of NO is closely related to the temperature distribution in the cylinder. Similar to thermal efficiency, NO plays a dominant role in NOx products. Figure 11 shows the changes in the average mass fraction of soot in the cylinder with CA under supercritical fuel injection and subcritical conditions. According to the results of [PERSON] (2018), when the fuel temperature is 400 and 650 K, the quality of soot emissions is 56.08 and 34.4 mg/KW\(\cdot\)h, respectively. The quality of soot emissions obtained from the calculated results is shown in Figure 10b. The results are close to the experimental ones. Thus, the numerical simulation of soot emission is reliable. As shown in Figure 11a, under subcritical and supercritical conditions, the fuel temperature exerts no significant effect on the average mass fraction of soot. When CA increases, the average weight of soot increases to the maximum then gradually decreases because soot formation is the result of the interaction between the formation and oxidation of inert products ([PERSON] et al., 2018). Figure 11: (**a**) Change curve of the mass fraction of soot; (**b**) comparison of soot emission. Figure 10: NO production rate and temperature at 730\({}^{\circ}\) CA. (**a**) Comparison of diffusion rate at 730\({}^{\circ}\) CA; (**b**) Temperature distribution at 730\({}^{\circ}\) CA. Figure 11a shows that the average mass fraction of soot before 750\({}^{\circ}\) CA is gradually increased. After 750\({}^{\circ}\) CA, the average mass fraction of soot begins to decrease. However, during the increase in the mass fraction of soot, the mass fraction change ratio of soot under the subcritical fuel injection condition is faster than that under the supercritical condition in the early stages of combustion. Under the subcritical state fuel injection condition, fuel and air mixing is not sufficient to form a number of high-temperature and lean oxygen areas. Therefore, part of the fuel cracking produces hydrocarbons or gas-phase precursors of carbon particles in the combustion process. In addition, soot is also formed by further condensation and polymerization of hydrocarbons or gas-phase precursors of carbon particles.