## 4 Conclusions  
Having conducted the experimental and numerical modelling research of a CI engine (\(V_{H}\) = 1986 cm\({}^{3}\), \(n\) = 2000 rpm, \(M_{B}\) = 45 Nm, and SOI = 6 CAD BTDC) and after replacing conventional diesel (D) with a dual fuel (D, HVO-M, or BM) where gas energy amounts for 60% of fuel mixture, the following conclusions can be made:  
1. When using dual fuel D40M60, BSFC increased by 6.8% and BTE decreased by 14.1%, while when using HVO40 BM60, BSFC grew 1.2 times and BTE decreased by 14.3%. The use of biomethane gas significantly increased fuel consumption, because CO\({}_{2}\) comprised 40% of BM gas volume, while LHV of biogas was ~2.7 times lower than that of methane. The use of both methane and biomethane gas reduced BTE because of the changed combustion process. 2. With the engine running on D40M60, CO\({}_{2}\) emissions declined by 17.1%, because methane has a lower C/H ratio. When using HVO40 BM60, CO\({}_{2}\) emissions increased by 10.2%, even though the C/H ratio of HVO biodiesel and methane was lower compared to that of the conventional diesel fuel. CO\({}_{2}\) emission was increased due to the fact that CO\({}_{2}\) comprised 40% of BM volume. However, biomethane is a renewable fuel and produces lower CO\({}_{2}\) emissions throughout its life cycle. 3. CO volumetric fraction of dual fuel (D40M60 and HVO40 BM60) increased from 0.01% to ~0.24% (~24 times), while HC volumetric fraction grew from 9.5 ppm to 257-280 ppm (~28 times). Emissions of incomplete combustion products increased due to the reduction of excess air ratio \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Combustion Indicators** & **Puel** & **D40/M60** & **D40/BM60** & **HVO100** & **HVO40/M60** & **HVO40/BM60** \\ \hline Combustion duration \(\Delta q_{CD}\), CAD & 50.7 & 53 & 55.8 & 51.6 & 54.6 & 55.7 \\ Combustion shape parameter \(m_{v}\) & 0.684 & 1.08 & 1.06 & 0.848 & 1.11 & 1.09 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Combustion indicators of fuel mixtures. and lower combustion temperatures. HC emissions also increased due to the overlap of intake and exhaust valves. 4. With the engine running on D40M60, NO\({}_{x}\) emission decreased by 67%, and when using HVO40 BM60, it decreased by 82%. The use of dual fuel reduced the amount of liquid fuels, which create an intense combustion and heat release in the kinetic (premixed) combustion phase. Cetane number is higher in HVO fuels, which reduces the ignition delay phase and ROHR. CO\({}_{2}\) gases contained in biomethane also suppress ROHR. 5. The use of dual fuel D40M60 reduced smoke by 23%, and using HVO40 BM60, smoke declined by 39%. HVO fuels have a lower C/H ratio and a more intensive combustion in the diffusion phase. Methane has a simpler molecular structure, a lower C/H ratio and a longer combustion time. All this helped to burn soot having formed in the course of the combustion of liquid fuels. 6. A lower ROHR in the premixed combustion phase (D40M60 and HVO40 BM60) decreased a temperature rise and NO\({}_{x}\) formation. A lower pressure rise results in a lesser load on the engine's crankshaft mechanism parts, and the engine produces less noise. When using HVO biogases, ROHR is more intense in the diffusion phase, methane and CO\({}_{2}\) gases also affect a delayed heat release and a longer combustion process, which in turn reduces smoke, also reducing BTE. In pursuit of the maximum BTE for dual fuels, SOI must be advanced. This will reduce CO and HC emissions, leaving smoke levels nearly the same, but significantly increasing NO\({}_{x}\) emissions. Conceptualization, A.R. and S.S.; methodology, A.R., S.S., and J.M. ; software, S.S. and A.R. ; formal analysis, A.R. and J.M. ; validation, A.R. and S.S.; writing--original draft preparation, A.R. and J.M. ; writing--review and editing, A.R. and J.M. ; supervision, A.R., and J.M. ; project administration, J.M. All authors have read and agreed to the published version of the manuscript. This research received no external funding.