## 3 Results
### Effects of Injection at the Valve and Reactivity Fuels on the Combustion Attributes
#### 3.1.1 Analysis of The In-Cylinder Pressure Traces
This phenomenon implies that an explosion does not occur until 4-5 CA BTDC. Subsequently, an abrupt rise in the temperature was shown due to the explosion of the B5 at some concentrated areas in the cylinder, though not sufficient enough to erupt the whole mixture and cause a quick rise in the pressure. Presence of the low-quality biogas injected from the beginning of the intake stroke was another factor that suppressed the significant impact of the initial explosion. The explosive characteristics manifested up to around 195\({}^{\circ}\) CA irrespective of the load capacity. From 195\({}^{\circ}\) onward the autoignition temperature of methane was reached leading to more burning of the biogas and subsequently exploded. Poor quality of the biogas still affected the quick explosion of the whole mixture until it reached around 195-200\({}^{\circ}\) CA. This phenomenon significantly affected the rate of pressure development, as shown in Figure 4a above. Figure 5 indicated that the maximum in-cylinder temperature correlates with the pattern in Figure 6c, as discussed below. Amidst the power stroke, the cylinder temperature increased with the increase in the engine load because the combustion at the stage was mainly due to the burning of the biogas, which varied with the engine load. According to Figure 6c, the reactivity of B5 and biogas injected at the valve generated high maximum in-cylinder temperature, at all the load conditions except 6.5 bar IMEP. The high in-cylinder temperature could relate to the oxygenated nature of biodiesel content in the blend and reduced port mixing, which enabled more burning of the B5 at the early stages of the combustion, unlike the low-quality biogas that might affect the temperature development. However, the maximum in-cylinder temperature decreased as the engine load increased from no-load to mid-load and rose to the maximum at 6 bar IMEP; hence, the least maximum temperature manifested at mid-load. The decrease in the cylinder temperature from no-load to mid-load could be due to the ratio of the low-quality biogas, because of the high CO\({}_{2}\) concentration, that affects the rate of the heat released to contribute in raising the cylinder temperature. At the mid 5.5 bar IMEP, the energy supplied by the reactivity fuels reached equilibrium position, hence generating least heat and developed a least maximum in-cylinder temperature. As the engine load rose to a higher capacity, the heat generated due to the burning of B5 outweighed that of the biogas, causing a rise in the maximum in-cylinder temperature. The pattern of the heat release rate shown in Figure 7a could be a justification to the said argument. Besides, the fall in the maximum temperature at 6.5 bar IMEP for the burning B5 relative to diesel combustion could also relate to the less heat generated due to some heat losses subjecting the engine to be less efficient with the use of B5 (see Figure 8a and Section 3.2). Figure 6c further explained that the burning of B5 along with the biogas in a premixed mode lowers the maximum in-cylinder temperature, at all the load capacities, compared to the biogas injection at the valve. Although the trend of the maximum in-cylinder temperature generated looked closely similar to that of the biogas injected at the valve, the margin of the temperature difference was more pronounced at the higher loads. The rise in the in-cylinder temperature reaffirmed the benefits of biogas injection at the valve, which may help reduce the CO and UHC emissions at a light engine load. However, the use of B5 as an oxygenated HRF and biogas injection at the valve served more significant advantage over the regular diesel and biogas in the same mode for the light engine load operation, as evidenced in Figure 6c. The benefit of biogas injection at the valve is further shown in Figure 6d though 6 bar IMEP indicated otherwise likely because other factors such as longer ignition delay might have affected the maximum cylinder pressure developed. Figure 6: Variation of (**a**) maximum in-cylinder pressure (PMax.)