## 3 Results and Discussion
### Combustion Characteristics
#### 3.1.2 Heat Release Rate  
Figure 4 demonstrates the heat release rate (HRR) of all test fuels according to various engine speeds. The energy released by the fuel at each crankshaft angle can be obtained by analyzing HRR, which is one of the important combustion parameters [26]. It can be observed from Figure 4 that the maximum value of HRR (HRRmax) for each test fuel gradually decreases with the increase of engine speed, and the location of the HRRmax occurs away from the top dead center (TDC). As the engine speed increases from 750 rpm to 1500 rpm and then to 2250 rpm, for CP00, the location of HRRmax occurs at 4 crank angle degree (\({}^{\circ}\)/CAD) after top dead center (ATDC), 10\({}^{\circ}\) ATDC, and 14\({}^{\circ}\) ATDC, respectively; for CP010, the location of HRRmax occurs at 3\({}^{\circ}\) ATDC, 9\({}^{\circ}\) ATDC, and 14\({}^{\circ}\) ATDC, respectively; for CP030, the location of HRRmax occurs at 4\({}^{\circ}\) ATDC, 10\({}^{\circ}\) ATDC, and 15\({}^{\circ}\) ATDC, respectively; for CP050, the location of HRRmax occurs at 4\({}^{\circ}\) ATDC, 9\({}^{\circ}\) ATDC, and 16\({}^{\circ}\) ATDC, respectively. In addition, for each test fuel, the amplitude of the crankshaft angle corresponding to HRR increases gradually with the increase of engine speed. Moreover, as seen in Figure 4, the start of combustion (SOC) timing for all test fuels is advanced near 8\({}^{\circ}\) BTDC with increased engine speed. As mentioned above, for analyzing the change of incylinder pressure, the combustion environment of the mixture in the cylinder is improved, and the ignition delay is shortened with the increase of engine speed.  
Figure 4: _Cont._Figure 5 shows the HRRmax of all test fuels according to various engine speeds. As shown in Figure 5, it is obvious that HRRmax shows a decreasing trend with the increase of engine speed from 750 rpm to 2250 rpm. Compared with the HRRmax at 750 rpm, the HRRmax of CP00 is reduced by 2.54% at 1500 rpm and 22.13% at 2250 rpm, respectively; the HRRmax of CP010 is reduced by 4.36% at 1500 rpm and 14.87% at 2250 rpm, respectively; the HRRmax of CP030 is reduced by 6.72% at 1500 rpm and 9.04% at 2250 rpm, respectively; the HRRmax of CP050 is reduced by 12.99% at 1500 rpm and 15.36% at 2250 rpm, respectively. On the other hand, adding CPO to diesel fuel resulted in a decrease in HRRmax for all test fuels at 750 rpm and 1500 rpm; at 750 rpm, the HRRmax of CP010, CP030, and CPO50 are respectively reduced by 4.71%, 3.11%, and 0.78% compared with that of CPO; at 1500 rpm, the HRRmax of CP010, CP030 and CPO50 are respectively reduced by 6.49%, 7.26%, and 11.43% compared with that of CPO. However, at 2250 rpm, the HRRmax of CP010, CP030, and CPO50 increased by 4.18%, 13.18%, and 7.85%, respectively, compared with that of CPO. The reason may be that the high viscosity and density of CPO lead to poor fuel atomization and curb the release of heat at low idle speed. At high speed, the temperature and mixture in the cylinder are improved, and the negative impact of the high viscosity of CPO is weakened [27].  
Figure 4: Heat release rate of (**a**) CPO0, (**b**) CPO10, (**c**) CPO30 and (**d**) CPO50 according to various engine speeds.