## 2 Experimental Setup and Methodology  
The experiments were conducted in a six-cylinder inline engine with direct injection and a common rail injection system, the main specifications of which are shown in Table 2. The experiments were conducted under 1400 rpm and a BMEP of 0.15 MPa to 1.5 MPa. The PODE used in this paper was provided by Shandong Yuhuang Group; the mass ratio of PODE2, PODE3, and PODE4 was 2.553:88.9:8.48, respectively; the measured low calorific value was 19.6 MJ/kg. Properties of the three test fuels, P0 (PODE volume fraction: 0%), P10 (PODE volume fraction: 10%), and P15 (PODE volume fraction: 15%), are presented in Table 3. It can be observed that as the blending ratio increased, the density, surface tension, and cetane number of blends increased, while the viscosity and calorific value of blends decreased. The experimental setup is shown in Figure 1. During the tests, the intake temperature after the intercooler and coolant temperatures was controlled at 50 \(\pm\) 1 \({}^{\circ}\)C and 85 \(\pm\) 2 \({}^{\circ}\)C, respectively. The fuel injection parameters were controlled by an electronic control unit (ECU) made by Bosch. The in-cylinder pressure was measured by a Piezoelectric pressure sensor (Kistler 6052A: Kistler, Winterthur, Switzerland) coupled with a charge amplifier (Kistler 5019: Kistler, Winterthur, Switzerland). The optical crank angle encoder (Kistler 2619: Kistler, Winterthur, Switzerland) was used to trigger the cylinder pressure data acquisition with a 0.5 degree crank angle (\({}^{\circ}\)CA) increment. The gaseous exhaust, including CO, CO\({}_{2}\), HC, and NO\({}_{\rm x}\), was measured by an emission analyzer (HORIBA MEXA 7100 DEGR: Horiba, Kyoto, Japan), and the torque was measured by an eddy current dynamometer (GW400: Hunan Xiangyi Laboratory Instrument Development Co., LTD, Changsha, China). \begin{table}
\begin{tabular}{c c} \hline \hline
**Items** & **Specification** \\ \hline Model & YC6G270-30 \\ Type & six-cylinder, w type \\ Bore \(\times\) Stroke (mm \(\times\) mm) & 112 \(\times\) 132 \\ Compression ratio & 17.5 \\ Displacement (L) & 7.8 \\ Fuel injector model & CRIN-138 Eight-hole fuel injector \\ Connection Rod (mm) & 210.82 \\ IVO/IVC (\({}^{\circ}\)CA) & 13.5 BTDC/38.5 ABDC \\ EVO/EVC (\({}^{\circ}\)CA) & 56.5 BBDC/11.5 ATDC \\ Rated power/kW @ rpm & 199 @ 2200 \\ Maximum torque/Nm @ rpm & 1080 @ (1400–1600) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Engine specification. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Fuel** & **P0** & **P10** & **P15** \\ \hline Density @ 293 K (kg/m\({}^{3}\)) & 850 & 872 & 883 \\ Viscosity @ 293 K (MPa s) & 4.73 & 3.37 & 3.18 \\ Surface tension (MN/m) & 23.85 & 24.62 & 24.79 \\ Cetane number & 50.1 & 52.2 & 53.3 \\ Calorific value (k/kg) & 42,900 & 41,020 & 39,950 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Properties of diesel and diesel/PODE blends. Furthermore, the AVL DiSmoke 4000 opaque smoke meter was utilized in the test to measure the emission of particulate matter. According to the Beer-Lambert law [24], the numerical transformation between the opacity O and the light absorption coefficient K can be calculated by the following formula (Equation (1)):  
\[K=-\frac{1}{L}\ln(1-\frac{O}{100}) \tag{1}\]  
where \(L\) represents the effective length of the optical channel, 0.430 mm; \(O\) is the impermeability (%). In addition, there is a relationship between diesel \(PM\) emissions and opacity according to Equation (2). \[PM=0.180^{2}+5.57O \tag{2}\]  
where \(PM\) represents the mass concentration of particles (mg/m\({}^{3}\)). The correlation coefficient of this formula is 0.98, which is relatively accurate. Therefore, the particle emission equation can be calculated by the following formula (Equation (3)). \[PM=1800\Big{(}1-e^{-KL}\Big{)}^{2}+557\Big{(}1-e^{-KL}\Big{)} \tag{3}\]  
As can be observed, there is a positive correlation between the particle mass concentration \(PM\) and the experimentally determined light absorption coefficient.