## 3 Results
### Effects of Injection at the Valve and Reactivity Fuels on the Combustion Attributes
#### 3.1.4 The Dynamism of the Combustion Phase  
To have more understanding of the effects of these fuels, with the in-cylinder mixing of biogas as a result of injection at the valve, on the magnitude and direction of the combustion; combustion phase (CA50) was examined as the period when 50% of the fuel burned. The use of B5 and biogas injected at the valve demonstrated slow combustion as the engine load increased from 5 to full 6.5 bar IMEP, as shown in Figure 10a. Figure 10a indicated that the combustion was fastest at a charge of 5 bar IMEP, unlike the no-load 4.5 bar IMEP, though both capacities resulted in the same burning period of 34 degrees. It implies that the fastness of the combustion at a load of 5 bar IMEP might be due to the better atomization and mixture reactivity of the direct-injected fuel. The fastness of the burning at a 5 bar IMEP has also manifested in the trend of the fuel burned in Figure 11a, as magnified in the inset graph. Similarly, the use of B5 and biogas in a premixed mode also resulted in slow-burning as the engine capacity increased from no-load to full-load state, but the combustion was found more advanced across the loads compared to the biogas injection at the valve. This trend was found contrary to the outcome of diesel and biogas in a premixed mode, which advanced as the load increased from 5 to 5.5 bar IMEP, as shown in Figure 10b. The trend implies that the combustion of diesel and premixed biogas also retarded as the load increased from 5 to 6.5 bar IMEP but found more advanced compared to the result of B5 and biogas injected at the valve. The use of diesel as pilot fuel also displayed the fastness of the combustion at a charge of 5 bar IMEP similar to the attribute of B5 as discussed above. However, the variability of the combustion phase could be due to the variation in the burning period of the reactivity fuels or combustion duration; thus, longer combustion duration slows the combustion center [40]. According to Figure 10b, when the biogas was premixed, the trend of the combustion phase was relatively similar to that of the condition when it was injected at the valve, though tends to be more closely identical to the pattern of B5, which was found less retarded by 1 degree at the full 6.5 bar IMEP. In the reactivity with the conventional diesel fuel, Figure 10b further indicated that biogas injection at the valve was more influential to the retarded combustion phase at various engine loads. Analytically, burning of B5 and biogas injected at the slowed the combustion approximately by 1.4 degrees (0.486 ms) compared to the burning of conventional diesel and biogas in a premixed mode. In contrast, the use of B5 and premixed biogas advanced the combustion by 1.3 degrees (0.451 ms). It indicated that the use of B5 and biogas at the valve retarded the combustion by almost 3 degrees (0.938 ms) compared to premixed mode. The burning of diesel and biogas injected at the valve retarded approximately by 0.4 degrees roughly equivalent to 0.139 ms compared to the premixed way. [PERSON] et al. [17] observed  
Figure 9: Variation of peak pressure rise rate for **(a)** B5/biogas RCCI in a premixed and injection at the valve and **(b)** diesel/biogas in a premixed and port injection at the valve. of the fuel. Besides, there was a drop in the cylinder temperature at low to medium loads; hence more of the less reactive mixture of B5 and biogas injected at the valve could be trapped and extended the burning duration likewise the combustion period as evident in Figure 11b. As analyzed from Figure 11b, the combustion period for the burning of B5 and biogas injected at the valve was extended approximately by 5 degrees compared to the burning of diesel in the same fueling approach, unlike the use of premixed biogas that extends the period by about 1 degree. Figure 11a shows more detailed dynamism of the burning duration at various loads. Compared to the usual premixed approach, as depicted in Figure 12b, a significant reduction in the burning duration was observed with the injection at the valve.