## 3 Results and Discussion  
### Combustion Characteristics  
#### 3.1.1 In-Cylinder Pressure  
Figure 2 indicates the variation in the in-cylinder pressure of all test fuels with the crank angle according to different engine speeds. It can be clearly observed from Figure 2 that all crude palm oil (CPO)-diesel blends have no trace of pressure waves, and the in-cylinder pressure varies smoothly at the idling modes. The influence of the difference in engine speeds on the variation in in-cylinder pressure is much greater than that of the test fuels. For each test fuel, the maximum in-cylinder pressure is the highest at 750 rpm, followed by 1500 rpm and 2250 rpm. In addition, the start of combustion (SOC) gradually advanced with the increase of engine speed. The main reason may be that the increase in engine speed improves the combustion conditions of the mixture in the cylinder, such as increasing the temperature in the cylinder, especially the cylinder wall temperature. In addition, it may also be related to the increase of residual gas temperature and wall temperature, which leads to an increased charging temperature during injection and the shortening of ignition delay. Moreover, increasing the engine speed improves the air-fuel ratio and turbulence intensity [21]. The combustion process of all test fuels is almost the same, which includes two combustion phases: the premixed combustion phase followed by the diffusion combustion phase. Through the span of crankshaft angle, it can be seen that the total combustion duration gradually increases with the increase of engine speed, \begin{table}
\begin{tabular}{c c} \hline \hline
**Item** & **Conditions** \\ \hline Test fuels & CPO0, CPO10, CPO30, CPO50 \\ Engine load & 30 Nm \\ Engine speed & 750, 1500, 2250 rpm \\ Fuel injection pressure & 45 MPa \\ Pilot injection timing & \(22^{\circ}\) BTDC \\ Main injection timing & \(7^{\circ}\) BTDC \\ Intake air temperature & \(25\pm 3\)\({}^{\circ}\)C \\ Cooling water temperature & \(85\pm 3\)\({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Experimental and operating conditions. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Engine Parameter** & **Units** & **Specifications** \\ \hline Type & - & turbocharged CRDI diesel engine \\ Number of cylinders & - & 4 \\ Bore \(\times\) stroke & mm & \(83\times 92\) \\ Injector hole diameter & mm & 0.17 \\ Compression ratio & - & 17.7:1 \\ Max. power & kW/rpm & 82/4000 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Specifications of the test engine. \begin{table}
\begin{tabular}{c c} \hline \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 \hline \end{tabular}
\end{table} 
Table 3: Specifications of exhaust emission device and measurement systems. which is due to the increase of engine power output caused by the increase of engine speed, resulting in more fuel injected into the cylinder. Similar results can be found in other studies [22,23]. Figure 2: _Cont._  
Figure 3 shows the variation in maximum in-cylinder pressure for all test fuels according to various engine loads at an engine load of 30 Nm. It can be seen from Figure 3 that the maximum in-cylinder pressure gradually decreases as the engine speed increases from 750 rpm to 2250 rpm. Overall, the maximum in-cylinder pressure of all test fuels is reduced by 6.40% and 8.97% at 1500 rpm and 2250 rpm compared with 750 rpm, respectively. The maximum in-cylinder pressure generated by the engine at the lowest speed (750 rpm) is significantly higher than at the engine speed of 1500 rpm and 2250 rpm.