## 3 Results and Discussions  
### Combustion Characteristics  
#### 3.1.1 In-Cylinder Pressure  
To evaluate the effect of various engine loads on the combustion characteristics, in-cylinder pressures for different fuels are compared under different engine loads and at a constant engine speed of 1500 rpm. Figure 2 shows the variations of in-cylinder pressures for four different fuels at engine loads of 30 Nm, 60 Nm and 90 Nm. The physicochemical properties of fuel directly affect the combustion characteristics of diesel engine, and the combustion effect of mixture in cylinder can be analyzed by cylinder pressure [16]. As clearly shown in Figure 2, the peak in-cylinder pressure of the blend fuels (BP10, BP30 and BP50) is lower than that of pure diesel (BP0). The pattern of variation of in-cylinder pressure with a crank angle for BP10, BP30 and BP50 blends is similar under all engine loads. It may be related to the disadvantages of crude palm oil (CPO), such as high viscosity, high density and low cetane index. Especially for high viscosity, as shown in Table 1, the viscosity of CPO is nearly 16 times higher than that of pure diesel, which directly deteriorates the atomization effect and further affects the formation of a homogeneous mixture. Similar results were also found by [PERSON] et al. [16] and [PERSON] et al. [17]. [PERSON] et al. [16] also pointed out that the low volatility of fuel also leads to poor mixture preparation and atomization, and then reduction of the peak in-cylinder pressure. In addition, as the engine load increases from 30 Nm to 90 Nm, the in-cylinder pressure of the blend fuels gradually approaches that of pure diesel. Moreover, it seems that the initial in-cylinder pressure of blend fuels begins to rise earlier than that of pure diesel. These are the consequence of the oxygen-containing characteristics of CPO, which accelerate the combustion rate. Figure 3 shows the comparison of peak in-cylinder pressure of all test fuels according to engine load. The in-cylinder pressure peaks of pure diesel are 69.00 bar, 75.40 bar and 83.10 bar at 30 Nm, 60 Nm and 90 Nm, respectively. From Figure 3, it is obvious that the peak in-cylinder pressures of the blend fuels are significantly lower than that of the pure diesel. As mentioned earlier, this is related to the fuel characteristics of CPO, such as low cetane index, high density and high viscosity (see Table 1). With the addition of 10%, 30% and 50% by volume of CPO to pure diesel, the peak in-cylinder pressure at 30 Nm is reduced by 2.90%, 3.62% and 3.33%, respectively; at 60 Nm it is reduced by 2.12%, 1.46% and 3.32%, respectively; at 90 Nm it is reduced by 1.20%, 2.65% and 2.41%, respectively. On the other hand, with the increase of engine load from 30 Nm to 90 Nm, the increase of peak in-cylinder pressure is mainly related to the increase of injected fuel in the cylinder. This result is consistent with [18]. [PERSON] [18] also reported that the peak in-cylinder pressure is obtained later with increase of engine load due to more fuel molecules tending to combust. Compared with 30 Nm, the peak in-cylinder pressures \begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Accuracy** \\ \hline CO (ppm) & \(\pm\)0.62\% \\ NOx (ppm) & \(\pm\)0.25\% \\ HC (ppm) & \(\pm\)5\% \\ Smoke opacity (\%) & \(\pm\)1\% \\ Load monitoring (Nm) & \(\pm\)0.2\% \\ Speed measuring (rpm) & \(\pm\)5 \\ Fuel consumption (g) & \(\pm\)2 \\ Fuel injection pressure (bar) & \(\pm\)1 \\ Intake air temperature (\({}^{\circ}\)C) & \(\pm\)3 \\ Cooling water temperature (\({}^{\circ}\)C) & \(\pm\)3 \\ \hline \end{tabular}
\end{table} 
Table 4: Specifications of exhaust emission device and measurement systems. of blend fuel under medium (60 Nm) and high load (90 Nm) are closer to those of pure diesel. This is because under a low load, the temperature in the cylinder, including wall temperature and residual gas temperature, is relatively lower compared with that under high load, which leads to delayed ignition.