## 3 Research Results and Discussion
### Environmental Indicators  
When using dual fuel comprising methane, CO\({}_{2}\) concentration declined compared to conventional diesel fuel, with SOI being 6 CAD BTDC: D40M60 by 17.1% and HVO40M60 by 20.4% (Figure 5). This was due to a lower methane C/H ratio. When using biogas, CO\({}_{2}\) volumetric fraction increased (D40 BM60 by 14.8% and HVO40 BM60 by 10.2%), because CO\({}_{2}\), which does not take part in combustion, comprised 40% of the biomethane gas. However, biomethane is a renewable fuel, and taking into account its complete life cycle, this increase in CO\({}_{2}\) emissions is insignificant. Having replaced conventional diesel fuel with renewable HVO reduces CO\({}_{2}\) concentration by 3.4% as HVO has a lower C/H ratio. When advancing the injection moment from SOI 0 CAD to 6-12 CAD BTDC, CO\({}_{2}\) volumetric fraction decreases due to increasing engine efficiency and decreasing fuel consumption. A change in the volumetric fraction of HC and carbon monoxide in unburned hydrocarbons is clearly correlated with engine running on fuel of different composition and changing SOI (Figures 6 and 7). With the engine running on all tested dual fuels and the SOI being 6 CAD BTDC, the CO volumetric fraction increased from 0.01% to \(\sim\)0.24% (\(\sim\)24 times) compared to conventional diesel fuel (Figure 6), while HC volumetric fraction increased from 9.5 ppm to 257-280 ppm (-28 times) (Figure 7). The combustion process deteriorates because of lean gas concentration in the air (air-gas fuel ratio ~7), when gas does not burn sufficiently. The comparison of methane and biomethane as dual fuel revealed that CO and HC emissions changed slightly, even though biogas contains 40% CO\({}_{2}\), which suppresses combustion. However, in the event of excessive air excess, CO\({}_{2}\) gas has little effect on combustion. The HC volumetric fraction also increases due to the fact that injection and exhaust valves, which are used to blow methane gas to the exhaust manifold, overlap at the end of the exhaust stroke and at the start of the intake stroke. Using dual fuel and having advanced the SOI timing to 12-15 CAD BTDC, CO volumetric fraction decreased to 0.10-0.15% and HC volumetric fraction decreased to 210-230 ppm as the combustion temperature increased and the combustion process improved. When using dual fuel, which contains methane, NO\({}_{x}\) volumetric fraction compared to conventional diesel fuel, when SOI is 6 CAD BTDC, decreased: D40M60 -by 67% and HVO40M60 by 74% (Figure 5). When using biomethane gas, NO\({}_{x}\) volumetric fraction decreased even more (D40 BM60 by 76% and HVO40 BM60 by 82%). Dual fuel reduced the amount of liquid fuels, which in turn reduced the combustion intensity and heat release in the kinetic (premixed) combustion phase. An even more intense heat release occurs in the diffusion phase with gaseous fuel combustion. The CO\({}_{2}\) gas inbiomethane additionally suppresses the combustion process. HVO fuels also reduce NO\({}_{\rm x}\) volumetric fraction (6-12%) as these biofuels have a higher cetane number and, thus, shorten the ignition delay phase, while heat release declines in the premixed phase. By advancing SOI timing, the NO\({}_{\rm x}\) volumetric fraction in the exhaust gas increases significantly operating on all tested fuel blends, because combustion occurs at a lower volume and higher temperatures. When using dual fuel, ensuring the SOI timing 6 CAD BTDC provided for by the engine control unit (ECU) software algorithm and having replaced conventional diesel with HVO fuels, smoke decreased by 23% (Figure 9). This comes as a result of a lower C/H ratio of HVO fuels and the combustion process: ROHR of HVO is more intense at the diffusion phase, which allows for a better combustion of soot. The use of dual fuel containing methane allowed reducing smoke compared to pure conventional diesel: D40M60 by 23% and HVO40M60 by 39% (Figure 5). The use of dual fuel containing biomethane allowed reducing smoke: D40M60 by 24% and HVO40M60 by 37%.