## 3 Results and Discussions
### Combustion Characteristics
#### 3.1.2 Heat Release Rate  
Figure 4 illustrates the variation of heat release rate (HRR) with the crank angle of four different blend fuels under different engine loads. In Figure 4, three peaks of different sizes can be obviously observed. The leftmost one is a small trough (negative HRR), which is a slight decrease caused by the absorption of ambient temperature after pilot fuel injected in the cylinder. The middle peak and the rightmost peak are the HRR rise caused by pilot and main injection fuel combustion, respectively. Similar results have been reported by [PERSON] et al. (2018) and [PERSON] et al. (2019). It is widely known that the combustion process of diesel engines can be divided into two major phases: premixed combustion phase (PCH) and diffusion combustion phase (DCH). The PCH begins after the start of injection (SOI) and reacts rapidly due to the presence of fuel-rich combustible areas formed by mixing fuel and air during the ignition delay period. The PCH leads to a rapid increase in the pressure and heat release in the cylinder. After the air around the fuel is consumed, PCH immediately enters the DCH. Generally, PCH is much shorter than DCH due to the burning being controlled with the air-fuel mixture ([PERSON] et al., 2019; [PERSON] et al., 2019). From the combustion process analysis in Figure 4, the combustion processes of all test fuels are similar. However, on the whole, the HRR curves of the blend fuels are slightly shifted to the right which can be attributed to the ignition delay (ID). ID is defined as the time between the SOI and the start of combustion (SOC). ID can be divided into physical delay and chemical delay, which are mainly determined by the properties of the fuel itself, such as viscosity, surface tension, volatility and cetane number ([PERSON] et al., 2019). Therefore, the high viscosity and density of CPO are the direct factors affecting fuel atomization, resulting in longer ID. On the other hand, with the increase of engine load, the maximum values of HRR of blend fuels are closer to those of diesel. Especially under high load (90 Nm), the maximum HRR of BP10 (46.67 J/CA) and BP30 (47.06 J/CA) is slightly higher than that of diesel (46.46 J/CA). These phenomena are mainly because the increase in load increases the temperature and pressure in the cylinder, and reduces the negative effects of the high viscosity and high density of palm oil. Moreover, the oxygen-containing characteristics of palm oil itself positively improve combustion characteristics. Similar results were also reported by [PERSON] et al. (2019). Figure 3: Maximum in-cylinder pressure for all test fuels according to various engine loads. Figure 4: Heat release rates for all test fuels at (**a**) 30 Nm, (**b**) 60 Nm and (**c**) 90 Nm. Figure 5 plots the maximum values of HRR (HRRmax) profiles for four fuel blends under different engine loads. As clearly shown in Figure 5, the HRRmax of blend fuels are lower than that of pure diesel under medium and low load. At 30 Nm, the HRRmax of BP10, BP30 and BP50 is 30.63 J/CA, 30.38 J/CA and 29.01 J/CA, which decreases by 6.49%, 7.26% and 11.43% compared with that of BP0, respectively. At 60 Nm, the HRRmax of BP10, BP30 and BP50 is 37.11 J/CA, 37.78 J/CA and 38.19 J/CA, which decreases by 4.78%, 3.08% and 2.03% compared with that of BP0, respectively. However, at 90 Nm, the HRRmax of BP10 and BP30 increases by 0.45% and 1.29% compared with that of BP0, respectively. Moreover, only a large amount of palm oil (50% by volume) added to diesel leads to the slightly decline of HRRmax compared with pure diesel. The above results show that HRR is mainly affected by engine operating conditions and fuel properties. With the increase of engine load, a large amount of fuel and air enter the cylinder for combustion, which greatly increases the temperature and pressure in the cylinder, especially the temperature of the cylinder wall, and reduces the negative effects caused by the disadvantages of high viscosity, high density and low cetane number of palm oil.