## 3 Results and Discussion
### Experimental Biogas Diesel Engine--Tests  
The measurements performed were preliminary tests, the aim of which was to ensure no interference in the systems regulating the operation of the engine driving the generator. It should be emphasized that in the studies of many actors [32; 33; 34; 35; 36], the total amount of nitrogen oxides (NO\({}_{\text{x}}\)) was examined regardless of the fuels and combustion systems tested. However, the work carried out measurements of the two most dangerous nitrogen oxides, which often occur together, i.e., NO and NO\({}_{2}\), of which nitrogen dioxide (NO\({}_{2}\)) is considered the most dangerous due to its impact on the health of humans and other living organisms. Raw biogas (BG) was used as the reference fuel for the tests. The assessment of the impact of the amount of biogas on the level of CO, NO, NO\({}_{2}\) and particulate matter (PM) emissions was carried out at a constant engine speed for various load levels; the percentage of biogas was changed from 40 to approximately 70-80%, i.e., until significant knocking combustion was detected in the tested engines. The biogas for testing was taken directly from the production installation and injected using a compressor into a cylinder under a pressure of 100 bar, from which a sample was taken for analysis. The composition of BG was determined using a GA2000 gas composition analyzer (Table 2), and the density and calorific value were calculated based on the biogas composition, the molar mass of the methane and carbon dioxide mixture and the calorific value of methane (Table 3). The test stand consisted of a two-cylinder diesel engine with a pre-chamber, 9 kW, liquid-cooled (Figure 4). The engine is equipped with a fuel supply system with indirect injection with a sectional injection pump. The engine is permanently connected via a PTO (power take-off) shaft to an asynchronous motor controlled by an automatic control and measurement system. The electricity generated by the asynchronous motor was directed directly to the power grid. The experimental station constitutes a compact whole and is used for simulation tests [31]. \begin{table}
\begin{tabular}{l c} \hline
**Parameter** & **Engine** \\ \hline Engine type/model & Vertical in-line diesel engine \\ Application & Energy generator \\ No. of cylinders & 2 \\ Rated speed (rpm) & 3600 \\ Rated power (kW) & 9.76 @ 3600 rpm \\ Rated torque (Nm) & 30 @ 2600 rpm \\ Displacement (cm\({}^{3}\)) & 570 \\ Aspiration system & Natural \\ Combustion system & Ball-type swirl chamber \\ Injection system & IDI Engine \\ \hline \end{tabular}
\end{table} 
Table 1: Research engine specifications [31]. The tested combustion engine operated at a constant rotational speed of 2000 rpm \(\pm\) 30 rpm, which was regulated by the injection pump settings and adjusted using the injection pump regulator depending on the engine load generated by the asynchronous engine control and measurement system. The tests began after warming up the engine so that the engine oil temperature exceeded 70 \({}^{\circ}\)C. The measurement was performed at six measurement points for which the torque load value was 0, 4.5, 8, 12, 16 and 20 Nm. After each change of parameter configuration, the engine operated for 0.5 h to stabilize the operating parameters. The operating parameters of the measurement system that were recorded automatically were gas fuel consumption and the composition, temperature and flow level of exhaust gases. The tested gas fuel was introduced into the engine at \begin{table}
\begin{tabular}{l c c} \hline \hline
**Component** & **Content** & **Uncertainty Level** \\ \hline Hydrogen sulfide & H\({}_{2}\)S & 28 ppm & \(\pm\)10\% \\ Methane & CH\({}_{4}\) & 59.9\% (v/v) & \(\pm\)3\% \\ Carbon dioxide & CO\({}_{2}\) & 41.7\% (v/v) & \(\pm\)3\% \\ Oxygen & O\({}_{2}\) & 0.7\% (v/v) & \(\pm\)1\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The chemical composition of biogas (BG) [31]. Figure 4: The schematic diagram of the experimental setup for biogas diesel engine [31]. 1—Diesel fuel tank.