## 3 Results and Discussion
### Effects of EGR Rate  
#### 3.2.1 Combustion Characteristics  
The CP and HRR curves of D100 and P50D50 introducing different EGR rates is shown in Figure 9 and the data are listed in Table 6. Regardless of the injection pressure, the peak CP and HRR of the fuels decreased regularly with the growth of the EGR rate. When the injection pressure was 300 bar, 10% and 20% EGR rate reduced the CPmax of the D100 by 3.1% and 5.2%, respectively, while at 450 bar, the reduction was of 4.7% and 5.9%. For P50D50, the drops under the two injection pressures were 3.5% and 6.1% and 2.7% and 4.0%, respectively. The gas reintroduced by the EGR system increased the specific heat of the gas in the cylinder and reduced the oxygen content, which was not conducive to the full combustion of the fuel, thereby reducing the peak pressure [34; 35]. It was also found that the EGR weakened the peak pressure of D100, so that P50D50 showed a higher CP after the introduction of EGR. This could be caused by the oxygen contained in palm oil, which increased the oxygen concentration in the cylinder. Furthermore, since the oxygen atoms contained in palm oil could effectively increase the oxygen concentration of the oil-gas mixture, the peak attenuation of P50D50 by EGR was less than that of D100. Therefore, under high injection pressure, P50D50 suffered from a smaller image with a high EGR rate (suffered less from a high EGR rate).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Fuel**} & \multirow{2}{*}{\begin{tabular}{c} **Injection** \\ **Pressure (bar)** \\ \end{tabular} } & \multirow{2}{*}{**EGR Rate (\%)**} & \multirow{2}{*}{**CPmax (bar)**} & \multirow{2}{*}{\begin{tabular}{c} **CPmax Crank** \\ **Angle (CA)** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **HRRmax** \\ **(J/deg)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **HRRmax Crank** \\ **Angle (CA)** \\ \end{tabular} } \\ \hline \multirow{4}{*}{D100} & \multirow{4}{*}{300} & 0 & 62 & 11 & 28.58 & 12 \\  & & 10 & 60.1 & 11 & 27.98 & 12 \\  & & 20 & 58.8 & 11 & 27.41 & 12 \\  & & 0 & 70.4 & 10 & 32.47 & 10 \\  & 450 & 10 & 67.1 & 10 & 30.86 & 10 \\  & & 20 & 66.3 & 11 & 32.12 & 10 \\ \hline \multirow{4}{*}{P50D50} & \multirow{4}{*}{450} & 0 & 61.7 & 12 & 29.28 & 12 \\  & & 10 & 60.2 & 12 & 28.25 & 13 \\ \cline{1-1}  & & 20 & 59 & 12 & 27.49 & 13 \\ \cline{1-1}  & & 0 & 69.5 & 10 & 32.24 & 11 \\ \cline{1-1}  & & 10 & 67.6 & 11 & 32.66 & 11 \\ \cline{1-1}  & & 2 The influence of EGR on the ID of D100 and P50D50 is shown in Figure 10. The injection pressure in this work was the main factor that affected the ID, and a high injection pressure was conducive to a shorter ID. Because the exhaust gas introduced influenced the pre-oxidation process of the fuel, and the low heat and dilution effect of EGR delayed the progress of the chemical reaction, the ID variation was positively correlated with the EGR rate [36; 37]. Moreover, the reduced oxygen concentration in the intake due to EGR was well compensated by the specific oxygen in palm oil, which resulted in a lower ID for P50D50 than for D100 at all EGR rates.  
Figure 10: ID under different EGR rates at 300 and 450 bar.  
Figure 9: CP and HRR under different EGR rates at 300 and 450 bar.