## 5 Conclusions  
In this study, experiments and three-dimensional simulations using a novel cavitation model are conducted to investigate the influence of the blending ratios of diesel and PODE, the injection strategy, and EGR rate on the combustion and emissions characteristics. The spray model used in this paper considers the influence of fuel properties on the spray characteristics. The numerical results show that the numerical model can predict the combustion process and emissions with acceptable accuracy. The numerical model created in this paper can accurately compute the change in in-cylinder pressure when burning various PODE/diesel blends, and can properly predict NO\({}_{\text{x}}\) emission by comparison with actual test data.  
(1) The self-restoring oxygen properties of PODE can efficiently minimize soot emissions while improving combustion. However, it can also raise in-cylinder temperatures and increase NO\({}_{\text{x}}\) emissions.  
(2) With the increase in blending ratio, soot emissions do not vary much at low loads, but decrease significantly at high loads. Under high-load conditions, with the increase in blending ratio, the property of self-restoring oxygen, the increase in oxidation activity of PODE/diesel blends, and the improvement of atomization effect help to reduce soot emissions.  
(3) With the advance of injection timing, the peak in-cylinder pressure and peak heat release rate increase. This is mainly because an earlier injection timing causes a longer ignition delay, which can improve fuel evaporation, and the increased volume of the mixture results in more rapid combustion. Meanwhile, with the delay of injection timing, the soot emissions increase, but the NO\({}_{\text{x}}\) emissions decrease initially and then increase when the injection timing is delayed to 710 \({}^{\circ}\)CA. The numerical simulation shows  
Figure 19: Variation in the average mole fraction of NO and soot in the cylinder with the crank angle. (**a**) NO mole fraction. (**b**) Soot mole fraction.  
Figure 20: Emission characteristics of NO\({}_{\text{x}}\) and soot under different EGR rates.  
that when the blending ratio of PODE is around 15% and the injection timing is between 710 \({}^{\circ}\)CA and 715 \({}^{\circ}\)CA, the emissions performances are optimal.  
(4) The in-cylinder combustion and emission characteristics are significantly influenced by the EGR rate. When the EGR rate increases, the maximum in-cylinder pressure and temperature decrease significantly, which helps to reduce NO\({}_{\rm x}\) emissions. However, an excessive EGR rate will result in higher soot emissions and a deteriorated combustion process. Therefore, an EGR rate in the range of 0.0 to 0.2 has a better comprehensive emission performance.  
The optimized injection time and EGR rate provides an important reference value for developing and improving the PODE engine. This numerical model can be used in the future to optimize the key parameters of the PODE engine under different usage scenarios and operating conditions.  
Conceptualization, Y.W. and S.L.; Formal analysis, Y.W.; Investigation, C.Z. and S.L.; Data curation, C.Z., Y.Z. and D.H.; Writing--original draft, Y.W.; Writing--review & editing, Z.Z.; Funding acquisition, Y.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (Grant No. 52176128) and the Shaanxi Provincial Key R&D Program (Grant No. 2019 ZDLGY15-10).  
Not applicable.  
The authors declare no conflict of interest.