## 3 Results and Discussion  
### Engine Performance  
#### 3.1.1 Combustion Characteristics  
Figure 4a,b shows the combustion pressure and heat release rate for engine loads and test fuels. Figure 4c,d show the rate of combustion pressure according to the crank angle for engine loads. An analysis of combustion data is shown in Table 5. In this study, both a pilot and main injection were applied, and tests were performed at the lowest engine speed. Pilot injection quantities were the same for all test conditions with an injection duration of 0.35 ms as shown in Figure 3. For all engine loads and all blend ratios, the SOC of the pilot injection for all conditions was 16 \({}^{\circ}\)CA BTDC. Under all conditions, the heat release rate, and the rate of combustion pressure by pilot injection were different according to engine load, even though the ignition delays were all the same. At an engine load of 0 Nm, the heat release rate and the rate of combustion pressure of the pilot injection appeared \begin{table}
\begin{tabular}{c c c} \hline \hline
**Table** & **Unit** & **Condition** \\ \hline Engine Speed & rpm & 750 \(\pm\) 10 (idle speed) \\ Engine Load & Nm & 0 \& 40 \\ Total injection & 0 Nm & mcc & 7 \\ Quantity & 40 Nm & mcc & 13 \\ Cooling Water Temperature & \({}^{\circ}\)C & 85 \(\pm\) 5 \\ Intake Air Temperature & \({}^{\circ}\)C & 25 \(\pm\) 5 \\ Fuel Injection Pressure & bar & 280 \\ Injection Timing & \({}^{\circ}\)CA & Main BTDC 2/Pilot BTDC 20 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Test conditions. Figure 3: Injection timings and durations at engine loads: (a) 0 Nm, and (b) 40 Nm. almost the same for the blend ratios, but PD100 showed the lowest values. At an engine load of 40 Nm, the heat release rate and the rate of combustion pressure were reduced and were the lowest in PD100. This is because the deterioration of the injection atomization had a greater effect on combustion than the oxygen content, slowing the combustion speed of the PD100. A higher heat release rate and rate of combustion pressure of the pilot injection were observed at an engine load of 40 Nm because of the rapid pre-mixed combustion. Specifically, higher pressure and temperature were observed in the cylinder as the engine load increased. The heat release rate and the rate of combustion pressure by the pilot injection on PD100 with engine loads 0 Nm and 40 Nm were similar. This result suggests that the main factor which affects the combustion of pilot injection is the poor atomization by palm oil biodiesel blending under idle conditions. The combustion of the main injection is strongly affected by the combustion of the pilot injection [48; 49; 50]. Moreover, the maximum combustion pressure depends on the burned fuel during the pre-mixed phase depending on the properties of biodiesel, such as high viscosity, high cetane number, and low volatility [51; 52]. At an engine load of 0 Nm, the maximum combustion pressure of PD100 was the highest at 5149 kPa. Similarly, the highest pressure observed at an engine load of 40 Nm was for PD100 at 6138 kPa. In PD100, the deteriorated combustion by the poor atomization of pilot injection led to an increase in the ignition delay so that the maximum combustion pressure increased. [PERSON] et al. [52] reported the biodiesel blends showed higher peak combustion pressure than diesel fuel. And [PERSON] et al. [53] and [PERSON] et al. [54] also showed that the peak combustion pressure of biodiesel was higher than diesel. This phenomenon was more clearly visible on the graph of the rate of combustion pressure in Figure 4d at an engine load of 40 Nm. The rate of combustion pressure by the pilot injection of PD100 was slower, and the point at which combustion increased in the main injection was retarded, and the rising phase was slower compared to other blends. The points of maximum combustion pressure were 8 \({}^{\circ}\)CA ATDC for all blends at an engine load of 0 Nm and 12 \({}^{\circ}\)CA ATDC at PD100 only at an engine load of 40 Nm, and the others were 11 \({}^{\circ}\)CA ATDC.