## 2 Materials and Methods
### Engine Test  
The experiments were conducted on a 4-cylinder, common rail direct injection (CRDI) turbocharged diesel engine with a displacement of 1991 cc. Figure 1 shows the schematic of the test engine setup, and Table 2 gives the engine specifications. The CRDI diesel engine was coupled with an eddy current (EC) type water-cooled dynamometer (DYTEK230, Hwanwoong Mechatronics Co., Ltd., Gyeongnam, Korea) with a maximum load of 230 kW. The engine speed and engine load were controlled by the dynamometer through a controller. The experiments were conducted at a constant engine load of 30 Nm with three idling speeds of 750 rpm, 1500 rpm, and 2250 rpm. A precise electronic balance with 1 g precision (GP-100K, A&D Co., Ltd., Tokyo, Japan) was used to measure fuel consumption.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Properties (Units)** & **Diesel Fuel** & **Crude Palm Oil** \\ \hline Density (kg/m\({}^{3}\) at 15 \({}^{\circ}\)C) & 836.8 & 903.8 \\ Viscosity (mm\({}^{2}\)/s at 40 \({}^{\circ}\)C) & 2.719 & 42.21 \\ Calorific value (MJ/kg) & 43.96 & 39.34 \\ Cetane index & 55.8 & 49 \\ Flash point (\({}^{\circ}\)C) & 55 & 260 \\ Oxygen content (\%) & 0 & 11.4 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Properties of diesel fuel and crude palm oil.  
A piezoelectric pressure sensor (type 6056A, Kistler Korea Co., Ltd., Seongnam-si, Korea) was employed to measure the in-cylinder pressure. A charge amplifier (type 5011B, Kistler Korea Co., Ltd., Seongnam-si, Korea) was utilized to amplify the recorded in-cylinder pressure signal. The in-cylinder pressure value with a crank angle was recorded from 200 engine cycles to calculate the heat release rate (HRR) for analyzing combustion characteristics. All combustion data were acquired using a National Instruments PCI-6040E (National Instruments, Austin, TX, USA) data acquisition (DAQ) board and stored on a desktop. The CO, HC, and NOx emissions were measured by an MK2 (GreenLine MK2, Eurotron (Korea) Ltd., Seoul, Korea) and HPC-501 (Nantong Huapeng Electronics Co., Ltd., Jiangsu, China) multi-gas analyzer. The smoke opacity was measured with an OPA-102 (QROTECH Co., Ltd., Bucheon-si, Korea) smoke meter. The specifications of the exhaust emission device and measurement systems are given in Table 3. Three engine speeds, 750 rpm, 1500 rpm, and 2250 rpm, were selected as the main engine operating variables to test the blended fuels. The engine load and the pilot and main injection timings were fixed at 30 Nm, 22\({}^{\circ}\) before top dead center (BTDC), and 7\({}^{\circ}\) BTDC, respectively. Detailed operating conditions are listed in Table 4. Before recording data, the engine was first warmed up with diesel under an idling speed of 750 rpm without an engine load for 30 min until the cooling water temperature reached 85 \({}^{\circ}\)C. The experiments were conducted firstly for neat diesel fuel (CPO), and thereafter for the diesel-CPO blended fuels, CPO10, CPO30, and CPO50. After the testing of one kind of fuel, the fuel return valve was opened, all the fuel inside the fuel line, injection pump, and injector were discharged by using a fuel pump, and finally, the fuel supply and return lines were washed with the next test fuel 3 times. After the fuel change was completed, the engine was allowed to run for about 10 min to attain a steady-state condition for each new test fuel. After all tests were finished, the engine was switched back to diesel fuel until the blended fuel was removed from the fuel line, injection pump, and injector, and then the engine was stopped.  
Figure 1: Schematic diagram of the engine test bed.