## 3 Results
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Start of Combustion,**} & \multicolumn{2}{c}{**10\% MBF 2**} & \multicolumn{2}{c}{**50\% MBF,**} & \multicolumn{2}{c}{**90\% MBF,**} \\
**Fuel** & \multicolumn{2}{c}{**CAD BTDC 1**} & \multicolumn{2}{c}{**CAD BTDC**} & \multicolumn{2}{c}{**CAD ATDC 3**} & \multicolumn{2}{c}{**CAD ATDC**} \\ \cline{2-9}
**Mixture** & **Optimal** & **Optimal** & **Optimal** & **Optimal** & **Optimal** & **Optimal** & **Optimal** \\
**CNG** & **Mixture** & **CNG** & **Mixture** & **CNG** & **Mixture** & **CNG** & **Mixture** \\ \hline
100 BGoH2 & 18.1 & 20.4 & \(-\)2.0 & 1.8 & 19.4 & 15.0 & 50.1 & 48.9 \\
86 BG14H2 & 19.0 & 21.2 & \(-\)0.1 & 5.6 & 15.6 & 9.4 & 45.8 & 46.1 \\
76 BG24H2 & 19.8 & 20.1 & 1.0 & 5.7 & 13.3 & 9.0 & 41.9 & 45.0 \\
67 BG3H2 & 20.5 & 19.6 & 2.8 & 3.8 & 11.0 & 10.1 & 46.9 & 43.9 \\
57 BG43H2 & 20.6 & 17.2 & 3.9 & 0.9 & 8.9 & 12.2 & 43.9 & 44.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Combustion parameters. (Figure 6A) because of the advanced ST and more efficient combustion. The pressure increased by 16.1, 23.2, 20.7, and 19.0%, while ST advanced to 36, 36, 34, and 30 CAD BTDC. A higher hydrogen concentration accelerates combustion and reduces the need to advance ST. With the highest H\({}_{2}\) concentration, optimal ST was the same as that for CNG. Therefore, retarding ST to 24 CAD BTDC reduced the in-cylinder pressure by 8.2%, and the maximum value of the in-cylinder pressure was measured 3 CAD later. Changes in the rate of heat release when the engine was running on biogas and hydrogen mixtures are shown in Figure 7. When ST was set to 28 CAD BTDC, _ROHR_ increased with the addition of hydrogen and reached a maximum value relatively early. Comparing the pure biogas with the mixtures of biogas and hydrogen, _ROHR_ increased by 21.3, 31.1, 40.4, and 50.9% and reached its maximum value 1, 4, 7, and 9 CAD earlier due to the increased _LHV_ of the fuel mixture and speed of combustion [66]. Adjusting ST moves the maximum value of _ROHR_ closer to TDC, but the change in _ROHR_ is minimum (3.3, 3.0, 1.9, 3.3, and 0.1%) with all of the measured fuel mixtures. More detailed combustion parameters are shown in Table 5. It can be observed that the addition of hydrogen shortens the ignition lag stage, and when setting ST to 28 CAD BTDC, the ignition lag duration shortens by 4.6, 8.6, 12.0, and 12.5% when adding 14, 24, 33, and 43 vol% hydrogen, respectively. When adjusting the spark timing, the ignition lag duration reduced by 3.6, 3.9, 13.4, and 22.3% when adding 14, 24, 33, and 43 vol% hydrogen, respectively. Reduced ignition lag is achieved due to the low energy required to ignite the hydrogen and the higher flame velocity when burning the hydrogen mixture. The same tendency is visible in the flame propagation stage. When ST was 28 CAD BTDC, 50% of the _MBF_ combusted 8.6% faster with the addition of just 14 vol% hydrogen. Figure 6: Pressure in the cylinder (p\({}_{\text{cyl}}\)) depending on hydrogen concentration. (**A**)—with optimal ST for the mixture; (**B**)—with optimal CNG ST.  
An increase in temperature in the cylinder is shown in Figure 8. When ST was set to 28 CAD BTDC and the engine was running on pure biogas, a maximum temperature increase of 42.5 K/CAD was observed at 10 CAD after top dead centre (TDC). By increasing the hydrogen concentration in the fuel mixture, the temperature also increased to 53.3 K/CAD at 7 CAD after TDC, 59.23 K/CAD at 7 CAD after TDC, 65.23 K/CAD at 4 CAD after TDC, and 73.4 K/CAD at 3 CAD after TDC at 14, 24, 33, and 43 vol% hydrogen, respectively. Advanced spark timing led to an earlier and more intense temperature increase. The increase when the engine was running on pure biogas was 8.5%, while that with 14 vol and 24 vol% hydrogen was 4.1% and 6.4%, respectively. In the other cases, the maximum value of the temperature increase decreased by 0.8% and 1.9% at 33 and 43 vol% hydrogen, respectively. Figure 8: Temperature rise in the cylinder depending on hydrogen concentration.