## 4 Conclusions  
The aim of this article was to demonstrate the possibilities of using a combination of biogas and hydrogen as an alternative fuel for internal combustion engines, and solutions were provided for the problems associated with optimizing the combustion process of this mixture while achieving favorable emissions and fuel consumption. This combination of fuels could be used in stable engines, as well as in municipal or agricultural vehicles, which are close to the source of biogas. In this study, the effects of two different cases of spark timing and hydrogen addition on a spark ignition engine running on biogas were investigated. The following conclusions were drawn from the collected and analyzed experimental data:  
1. The maximum \(BTE\) value of 0.314 is reached with the advanced ST and the engine running on a mixture of biogas and 14 vol% hydrogen. Reducing ST to 28 CAD BTDC reduces \(BTE\) due to the low combustion speed and biogas combustion delay period. Further increasing the hydrogen concentration reduces the volumetric efficiency of the engine and at the same time reduces the \(BTE\). 2. In all cases, adding hydrogen to biogas increases NOx emissions. This is because of the increase in cylinder pressure and the temperature of burning fuel mixture with higher \(LHV\). The advanced ST increases NOx emissions further, because the combustion process forms peak pressure earlier and the in-cylinder temperature rises. This is also confirmed by the drop in exhaust gas temperature as a result of less burning fuel passing through the exhaust valve when ST is advanced. Figure 9: Energy distribution. (**A**)—with optimal ST for the mixture; (**B**)—with optimal CNG ST.  
3. Increased concentration of hydrogen improves the combustion process and decreases the amount of unburn fuel in the exhaust gases. At a maximum hydrogen concentration of 43 vol% and ST of 28 CAD BTDC emissions of HC fall by 40%; this is because of the lower ratio of carbon molecules inside the fuel mixture and the increased quality of combustion. In the cases of advanced ST, HC emissions rise because of the lower exhaust temperatures, which reduce the oxidation reaction of unburned HC speed. 4. Reduction of GHG is also noticeable when adding H\({}_{2}\), which reduces fuel consumption and carbon molecules in fuel mixture. In the case of 43 vol% hydrogen, CO\({}_{2}\) emissions dropped by 8.36% when ST was set to 28 CAD BTDC and by 6.96% when ST was 24 CAD BTDC. 5. The peak of the in-cylinder pressure increases with the addition of hydrogen, and it occurs closer to TDC due to the higher flame speed of added hydrogen. In the case of the highest hydrogen concentration, the value of ST 28 CAD BTDC is optimal for CNG and for the biogas-hydrogen mixture. Further reduction of ST to 24 CAD BTDC is not necessary, as the peak value of the in-cylinder pressure shifts further from TDC, and the maximum value decreases. 6. _ROHR_ increases with every portion of hydrogen, and the maximum value is reached earlier because of the increased fuel mixture's _LHV_ and the intensity of combustion. This is also confirmed by the increase in the in-cylinder temperature. In the maximum _BTE_ case (ST 28 CAD BTDC, 14 vol% H\({}_{2}\)), the increase in peak temperature reached 53.3 K/CAD at 7 CAD, and the advanced ST increased it further to 55.4 K/CAD at 1 CAD. This change also influences the increase in NO\({}_{x}\) emission. Conceptualization, D.K. and G.P. ; methodology, D.K. and G.P. ; software, D.K. ; validation, A.R. ; formal analysis, S.P. ; writing--original draft preparation, D.K. ; writing--review and editing, S.P. and D.B. ; visualization, D.B. ; supervision, A.R. and P.D. ; project administration, P.D. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. Data Availability StatementData is contained within the article.