## 3 Results
### Engine Performance  
#### 3.1.1 Combustion Characteristics  
Typical properties of biodiesel (oxygen content in the fuel itself, high cetane number, a low calorific value, and the high viscosity) have a great influence on the injection and combustion in the cylinder. The mixture of air and fuel in the cylinder is the most unfavorable at low idle speeds, and the injection pressure is very low (fixed at 280 bar in this research). The development of fuel atomization in the cylinder deteriorated and the combustion efficiency decreased due to the high viscosity of the palm oil biodiesel. In addition, it is known that the surface tension is increased with increasing blend ratio. The high surface tension of the fuel as well as the influence of the high viscosity, also exacerbates the atomization [39; 40]. In addition, the pilot injection time, 0.3 ms, is very short. Thus, the spread of the injection spray is not large, which affects the combustion. Figure 3a shows the combustion pressure and heat release rate at an engine load of 0 Nm and a pilot injection timing of 20 \({}^{\circ}\)CA. The maximum combustion pressures are similar, but the heat release rate produced varied based on the combustion of pilot injections. The combustion starts at approximately the same point from the PD0 to the PD30 because of poor spray atomization by the high viscosity of palm oil biodiesel even though it has a higher cetane number. However, the PD100 starts the combustion slightly earlier, and the combustion becomes gentle. This suggests that the effects of oxygen content and high cetane number are more influential than the effect of high viscosity in PD100. Figure 3b reflects an engine load of 40 Nm under for a pilot injection timing of 20 \({}^{\circ}\)CA. The combustion pressure in the cylinder is higher, combustion starts at almost the same time, and the heat release rate rapidly increased. However, the peak heat release rate by the combustion of the pilot injection decreased as the blending ratio of palm oil biodiesel increased. The amount of intake air is the same, but the injected amount of the fuel increased, and the composition of the air-fuel ratio in the cylinder is not sufficient. The fuel injected at the pilot that has not been burned during the separation between the pilot and main injection is burned together with the main injection, and the heat release rate then becomes larger when the blend ratio of biodiesel is high. Figure 3c,d are the combustion pressures and the heat release rates for the engine loads of 0 Nm and 40 Nm under for a pilot injection timing of 15 \({}^{\circ}\)CA. Pilot injection is retarded 5 \({}^{\circ}\)CA. The temperature in the combustion chamber at the time of pilot injection, at 15 \({}^{\circ}\)CA, is higher than at 20 \({}^{\circ}\)CA, so the ignition delay period is shorter than for the previous pilot at 20 \({}^{\circ}\)CA. However, the mixing duration of the injected fuel and air becomes shorter. This poor mixture of fuel and air makes the heat release rate by the combustion of pilot injection lower. A higher blend ratio has a much lower heat release rate. At this duration of pilot combustion, the fuel of the pilot injection is not burned sufficiently and is burned with the main injection, resulting in a higher maximum heat release rate after the main injection. Figure 3: Combustion pressure and heat release rate. (**a**) Engine load 0 Nm and pilot BTDC 20 \({}^{\circ}\)CA, (**b**) engine load 40 Nm and pilot BTDC 20 \({}^{\circ}\)CA, (**c**) engine load 0 Nm and pilot BTDC 15 \({}^{\circ}\)CA, and (**d**) engine load 40 Nm and pilot BTDC 15 \({}^{\circ}\)CA. As the blending ratio of palm oil biodiesel increases, the effect of maximum combustion pressure and maximum heat release rate due to pilot injection timing is reduced. Figure 4a,c,e show the plots of combustion pressure and heat release rate of PD0 (pure diesel), PD20, and PD100 (pure palm oil biodiesel) under an engine load of 0 Nm at pilot injection timings 15 \({}^{\circ}\)CA and 20 \({}^{\circ}\)CA.