## 4 Discussion  
The combustion of B5 and biogas is disadvantageous to the rise in the cylinder pressure due to the poor quality of the biogas used. Besides, the burning of the fuels was found beneficial to the rate of heat released at low to medium engine loads due to higher in-cylinder temperature compared to the use of diesel under injection at the valve approach. The reactivity of the fuel was also found highly beneficial in reducing PPRR. The use of B5 and biogas injected at the valve demonstrated slow combustion as the engine load increased, but the combustion was more advanced with the use of premixed biogas while the trend was found similar to the outcome of diesel and biogas, which was also more beneficial to advanced combustion phase. Besides, the burning of B5 and biogas injected at the valve gave the highest indicated thermal efficiency of 36.33% at a load of 5.5 bar IMEP. In general, the results showed that the in-cylinder mixing, as a consequence of biogas injection at the valve, significantly influenced slow combustion with the rise in the indicated thermal efficiency. The reactivity of B5 and low-quality biogas did not considerably benefit the combustion phase but indicated thermal efficiency. The use of low-quality biogas also significantly affected the in-cylinder pressure development resulting in very poor combustion.  
The CO\({}_{2}\) emission increased as the engine load increased during combustion of B5 and biogas injected at the valve. However, the burning of these fuels demonstrated the capability in reducing the CO emission, though increased from 4.5 to 5.5 bar IMEP, then decreased as the load increased. The UHC emission also significantly reduced as the load increases while NOx emission was found less at 4.5-5.5 bar IMEP but increased drastically as the engine load proceeds to full 6.5 bar IMEP. The trends of the emissions were found comparable to that of the conventional diesel RCCI combustion for both premixed and biogas injected at the valve. Besides, the patterns of the emissions in response to the change in the engine speed, for B5 and biogas injected at the valve, signify that the high engine speed, 2000 rpm, selected might also contribute to high emissions of both premixed and biogas injection at the valve; thus, demanding for more investigation of the workability of the approach of injection at the valve for low engine speed. Inferentially, the outcome of the emissions revealed that using B5 and biogas with high CO\({}_{2}\) concentration, in RCCI mode, was disadvantageous in elevating CO\({}_{2}\) emission. The use of B5 and biogas injected at the valve was quite beneficial in rising the in-cylinder temperature and reducing the stream of fuels entering the crevices that aid in cutting down the CO and UHC emissions but rose the NOx emission. The fuel reactivity reduced the CO and UHC emissions, averagely, by 20.33% and 10% respectively.  
The approach facilitated a reduction in the persistent challenge of the CO and UHC emissions, compared to the premixed method. However, the research recommended that the fuel delivery hose should be constructed as an integral part of the air intake manifold and its outlet to be part of the valve seat for proper control of the gaseous fuel delivery with the aid of valve closing. The concept should be investigated numerically for better evaluation and understanding.