## 3 Results and Discussion
### Effects of Injection Pressure
#### 3.1.2 Engine Performance  
Figure 4 shows the variation of the brake-specific fuel consumption (BSFC) with different fuels and injection pressures. The results show that the BSFC of the fuels at the injection pressure of 450 bar was higher than that at 300 bar. It is worth noting that the BSFC of D100 first fell and then rose with the gradual increase of injection pressure. This could be caused by the fact that the large injection pressure created a better atomization atmosphere in the early stages, and the fuel could easily evaporate and mix, which was conducive to better combustion. However, as the injection pressure continued to increase, the spray permeability worsened. The local lack of oxygen and uneven mixing of oil and gas caused by this led to the rise of BSFC [23]. It can also be observed that, due to the difference in fuel  
Figure 3: ID under various injection pressures.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Fuel**} & \multirow{2}{*}{**Injection Pressure (bar)**} & \multirow{2}{*}{**CPmax (bar)**} & \multirow{2}{*}{**CPmax Crank Angle**} & \multirow{2}{*}{**HRRmax (J/deg)**} & **HRRmax Crank** \\  & & & (\({}^{\circ}\)**CA**) & & **Angle (\({}^{\circ}\)**CA**) \\ \hline \multirow{4}{*}{D100} & 300 & 62 & 12 & 28.57 & 12 \\  & 350 & 65.1 & 11 & 31.00 & 11 \\  & 400 & 67.3 & 10 & 30.56 & 11 \\  & 450 & 70.4 & 10 & 33.16 & 10 \\ \hline \multirow{4}{*}{P50D50} & 300 & 61.7 & 12 & 29.28 & 12 \\  & 350 & 64 & 11 & 30.21 & 12 \\ \cline{1-1}  & 400 & 67.3 & 10 & 30.39 & 11 \\ \cline{1-1}  & 450 & 69.5 & 10 & 32.24 & 11 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Maximum values of CP and HRR and their corresponding crank angles (without EGR).  
heating values, the BSFC under the same injection pressure was negatively correlated with the fuel heating value.  
Brake thermal efficiency (BTE) is the reciprocal of the product of BSFC and the calorific value of the fuel, so the trends of BTE and BSFC are often opposite, as shown in previous studies [27; 28]. Figure 5 depicts the BTE of the test fuel under four injection pressures. It can be directly observed that the BTE of the same fuel was negatively correlated with the variation of BSFC. However, interestingly, the fuels with the best BSFC performance did not show the best BTE. Under the same operating conditions, D100 always exhibited the lowest BSFC, but at 350 and 400 bar, P50D50 had the highest efficiency. It can be inferred that, compared to D100, the growing rate in BSFC of P50D50 was slightly lower than the decrease in heating value.  
Figure 6 compares the exhaust gas temperature (EGT) of the two test fuels. It can be seen that the EGT of P50D50 was significantly higher than that of D100 at 300 bar. However, when the injection pressure was higher than 300 bar, the EGT of P50D50 was lower than that of D100. This shows that the fuel could not be fully atomized under low injection pressure, and the viscosity of palm oil in the mixed fuel was high, which further worsened the atomization effect and led to incomplete combustion, thus consuming a large amount of fuel and increasing EGT. In general, a lower EGT indicates that the fuel is fully burned and the engine efficiency is high [29]. In this study, a higher injection pressure improved the atomization effect. In addition, palm oil has high oxygen content and high cetane number,  
Figure 4: BSFC under various injection pressures.  
Figure 5: BTE under various injection pressures.  
which promote combustion. Therefore, the addition of palm oil to diesel resulted in lower EGT at high injection pressures.