## 3 Results and Discussions
### Combustion Characteristics
#### 3.1.3. Engine Performance  
Figure 6 compares the coefficient of variation of the indicated mean effective pressure (COVimep) for all test fuels according to various engine loads. Cyclic variations occur during combustion due to mixture composition, thermodynamic conditions, initial temperature and pressure alteration for each cycle [18]. The higher the COVimep, the more unstable the combustion, which even leads to vehicle drivability problems. As shown in Figure 6, the variation of COVimep of this CRDI diesel engine fueled with all test fuels under low (30 Nm) load and high load (90 Nm) is slightly larger than that under medium load (60 Nm) and does not show a stable change with the increase of the palm oil blend  
Figure 5: Maximum heat release rate for all test fuels according to various engine loads.  
ratio. However, at 60 Nm, the COVimep decrease gradually with the addition of palm oil to diesel. That is, the COVimep of BP10, BP30 and BP 50 decrease by 9.32%, 17.31% and 21.43% compared with that of BP0, respectively. In addition, the average value of COVimep for all test fuels at 60 Nm and 90 Nm are lower than that at 30 Nm due to the increase of the fuel extent in the mixture [24]. Generally, COVimep less than 2.5% is the upper limit of stable combustion [24], while more than 10% being the upper limit of vehicle drivability problems [25]. On the whole, all test fuels obtained relatively stable combustion without large cycle-to-cycle variations under all engine operating conditions. This may be attributed to the improved combustion quality of palm oil as an oxygenated fuel.  
Figure 7 shows the brake specific fuel consumption (BSFC) for all test fuels according to various engine loads. BSFC is an important indicator to evaluate the fuel economy of an engine. The smaller the BSFC is, the more economical the fuel is. As shown in Figure 7, the BSFCs of most blend fuels are higher than that of pure diesel. At 30 Nm, the BSFC of BP 10, BP30 and BP 50 increase by 9.16%, 8.38% and 14.23% compared with that of BP 0, respectively. At 60 Nm, the BSFC of BP 10, BP30 and BP 50 increase by 1.99%, 5.31% and 9.02% compared with that of BP 0, respectively. However, at 90 Nm, the BSFC of BP 10, BP30 and BP 50 decrease by 5.71%, 0% and 1.34% compared with that of BP 0, respectively. This means that palm oil blended fuels consume more fuel per energy extracted under medium and low load. This is because the calorific value of palm oil is lower than that of diesel (see Table 1), thereby more fuel needs to be consumed to achieve the same output. Moreover, the disadvantages of high density and high viscosity of palm oil cannot be improved under medium and low load, which further leads to incomplete combustion and consumes a lot of fuel. A similar result was reported in Ref. [26]. Generally, the lower the calorific value of the fuel, the more fuel needs to be consumed. However, in this study, the addition of palm oil reduced BSFC at 90 Nm. This shows that the variation of BSFC is not only related to physical properties of palm oil such as calorific value, density and viscosity, but also related to whether it contains oxygen. Under high load, the blend fuels get rid of the negative effects of high density and high viscosity of palm oil, and the oxygen-containing characteristics play a major role in promoting the full combustion of the fuel. In addition, it is clearly seen that the BSFCs for all test fuels are reduced with an increase of the engine load. This may be mainly attributed to the increase of temperature and pressure in the cylinder due to the increase of the engine load, which improves the combustion quality and reduces the combustion loss. This is consistent with the results of other researchers [17; 27].  
Figure 6: COVimep for all test fuels according to various engine loads.