## 4 Results and Discussion
### Effects of the Blending Ratios on Emissions
#### 4.1.2. Soot Emission Characteristics  
Figure 9 shows the variation in the average in-cylinder particle mass concentration obtained from numerical simulation for different blending ratios; the soot concentration in the figure is the in-cylinder concentration variation, based on the concentration at the time of exhaust valve opening, which is not consistent with the sampled gas volume of the concentration obtained from the actual test, so the results are only used for trend prediction. In the experimental results, under the condition that the engine speed is 1400 rpm and the BMEP is 1.1 MPa, the light absorption coefficients of particulate emission for P0, P10, and P15 are 0.1127, 0.0510, and 0.0117, respectively, and the corresponding mass fractions can be obtained by conversion to 0.0270 (mg/m\({}^{3}\)), 0.0122 (mg/m\({}^{3}\)), and 0.0028 (mg/m\({}^{3}\)), respectively. It is obvious that there is a positive correlation between the simulated and experimental results of particle mass concentration. The pattern is consistent with the experimental results. In general, the particle mass concentrations obtained from both experiments and simulations decrease significantly with the increase in the PODE blending ratio. Figure 10 shows the trend of soot emission of different fuels at 1400 rpm and under different loads. Under the low engine load, the soot emission is low and the influence of the PODE blending ratio on the soot emission is slight. However, as the engine load increases, the PODE blending ratio on the soot emission becomes significant. For example, when the BMEP is 1.5 MPa, the soot emission of P0, P10, and P15 is 0.0935 (mg/m\({}^{3}\)), 0.0384 (mg/m\({}^{3}\)), and 0.0216 (mg/m\({}^{3}\)), respectively. Compared with the P0, the particle mass concentration of P10 decreases by 58.9%, while the particle mass concentration of P15 decreases by 76.9%. The main reasons for this trend are: (1) Under low load conditions, the fuel injection volume is less; whether it is diesel or PODE/diesel blends, the oxygen in the cylinder is sufficient during combustion, so the soot emissions are lower and very close. (2) Under medium- and high-load conditions, the in-cylinder temperature is higher, the increase in fuel injection volume is likely to cause local oxygen deficiency, and diesel soot emissions increase, but the self-restoring oxygen properties of PODE can improve the local  
Figure 8: Variation in temperature field in the cylinder of fuel with different PODE blending ratio. Figure 9: Variation in the average mass concentration of particles in the cylinder with different PODE blending ratios. oxygen deficiency environment. As a result, a high ratio of PODE blending can sharply reduce the soot emissions. (3) With the use of PODE as an additive to diesel fuel, the mass fraction and oxidative activity of the volatiles increase [34,35], so particle oxidation becomes easier, and the distribution shifts toward smaller particles. As a result, particle concentration decreases. Figure 11 shows the development of the in-cylinder soot mass fraction distribution with crank angle when burning P15. It can be seen from the figure that the soot is formed at the early stage of fuel injection. In the process afterward, the soot increases sharply inside the edge of the fuel bundle as the diffusion flame develops, while the soot oxidizes and disappears rapidly at the edge of the fuel bundle. Overall, the soot mass fraction in the cylinder initially increases and then decreases rapidly along with the development of the fuel bundle. The variation in soot mass fraction in the cylinder with time shows that the effect of spraying has a great influence on the production and emission of soot. Figure 11: _Cont._  
Figure 10: Soot emission characteristics of different PODE/diesel blends under different loads. As the blending ratio increases, the change in the physical properties of the blended fuel affects the spray characteristics, especially the cavitation phenomenon, which, in turn, has an impact on the soot emission characteristics.