## 2 Materials and Methods
### Experimental Procedure  
The test was conducted to investigate the engine characteristics at high speed and low load (4-7 bar IMEP) conditions in a modified approach of port injection at the valve. The manufacturer recommended operation of the engine at a speed of 2000 rpm and above. A consistent engine speed of 2000 rpm was initially considered based on the manufacturer's recommendation and varied the load from the no-load indicated mean effective pressure (4.5 bar IMEP) to maximum capacity (6.5 bar IMEP) at an interval of 0.5 bar IMEP, which correspond to an increase by 25% engine load. Subsequently, 1600 rpm, 1800 rpm, and 2000 rpm were further investigated in a premixed port injection approach to ascertain the contribution of the high speed selected in elevating some of the emissions. The research reported the engine capacity based on the actual indicated mean effective pressures (IMEP) to reflect the low load range, unlike the usual load percentages. A test was carried out to determine the appropriate proportions of the dynamometer brake applied and throttle position to achieve the average IMEP corresponding to the percentage increase in the load for each of the HRFs and speeds selected. Table 2 shows the detailed engine loading with the corresponding percentages of the dynamometer brake applied and throttle position for the regular diesel and B5 fuels at a speed of 2000 rpm. A B5 diesel was injected directly as test fuel while the regular diesel was considered a reference. The amount of direct-injected fuel for the conventional diesel combustion was reduced in such a way that the remaining amount would serve as a source of ignition and provide some heat to enable an explosion of the port injected biogas during RCCI combustion. The percentage reduced at each load capacity is offered in Figure 3a, while Figure 3b shows the analysis of the fuel quantities used for the RCCI combustion compared to the fractions reduced. The amount of fuel directly injected at every load level was preliminarily estimated, by running the engine five times under each of the conditions shown in Table 2 after which the average was taken and considered a direct-injected quantity. This quantity was maintained as a fraction of 50% for each of the directly injected fuel used, as presented in Table 2. Initially, the engine has an injection timing of 13\({}^{\circ}\) CA BTDC, then a shim of 0.8 mm was removed, which advanced the injection timing by 8 degrees, as each of the 0.1 mm advanced the injection timing by 1 deg. Therefore, an advanced injection timing of 21\({}^{\circ}\) CA BTDC was considered because early direct injection timing improves the in-cylinder mixing and fuel combustion at low load [10; 24]. The biogas was premixed port-injected at a full mixing distance of 110 mm from the injector to the air intake valve position, as premixed distance. A nearly zero mixing distance from the valve position was considered for the approach of injection at the valve. The biogas fuel was delivered at a constant pressure of 2 bar while its flow rate was varied with the engine load to ensure a 50% biogas energy fraction for both fully premixed and port injection at the valve approaches.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & **Engine Load** & & \multicolumn{2}{c}{**Diesel at 2000 rpm**} & \multicolumn{2}{c}{**B5 at**} & \multicolumn{2}{c}{**HRF**} & \multicolumn{2}{c}{**LRF**} \\  & **IMEP** & **SD** & **Brake** & **Throtle** & **Brake** & **Throtle** & **(\%)** & **Mass** & **Mass** \\ \hline
0 & 4.495 & \(\pm\) 0.083 & 22 & 53 & 23 & 53 & 0.0181 & \(\pm\)0.0066 & 0.0181 \\
25 & 4.998 & \(\pm\) 0.092 & 35 & 55 & 36 & 55 & 0.0256 & \(\pm\