## 2 Materials and Methods  
Experimental tests were carried out using specialized measuring equipment and an SI engine adapted for gaseous fuels. An overview of the engine test bench and measuring equipment is presented in Figure 2. The main components of the experimental set up include the following:  
* A naturally aspirated spark engine with three cylinders, a capacity of 796 cm3, and a compression ratio equal to 9.3. The engine control and power supply systems were modified for the use of biogas and hydrogen mixtures. The engine was tested at 1500 rpm. * An asynchronous electric motor with a power take-off system, capable of operating as electric motor (starter) and generator (load). The main purpose of this motor is to apply load on the selected point of the operating cycle. All experiments were conducted at Brake Mean Effective Pressure (_BMEP_) of 0.42 MPa. * High-pressure cylinders with methane, carbon dioxide, and hydrogen equipped with dual-stage gas regulators. * A measuring devices for flow rate, temperature, and pressure evaluation, including rotameters, Coriolis- and conductivity-type flow meters, manometers, and thermocouples. Figure 2: The experimental test rig [56]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{\(\mathbf{CH_{4}}\) vol\%} & \multirow{2}{*}{\(\mathbf{CO_{2}}\), vol\%} & \multirow{2}{*}{\(\mathbf{H_{2}}\), vol\%} & \multirow{2}{*}{\(\mathbf{LHV}\)1, MJ/kg} & \multirow{2}{*}{Marking} & \multicolumn{2}{c}{**ST 2, CAD BTDC 3**} \\ \cline{5-6}  & & & & & **Optimal for CNG 4** & **Optimal for Mixture** \\ \hline
61.88 & 38.12 & 0.00 & 17.51 & 100 BC0H2 & 28 & 36 \\
54.06 & 31.86 & 14.08 & 19.31 & 86 BGI4H2 & 28 & 36 \\
47.51 & 28.82 & 23.66 & 20.09 & 76 BG24H2 & 28 & 34 \\
41.47 & 25.37 & 33.15 & 21.39 & 67 BG33H2 & 28 & 30 \\
35.66 & 21.15 & 43.19 & 23.68 & 57 BG43H2 & 28 & 24 \\ \hline \hline \end{tabular}  
* LHV—Lower Heat Value. 2 ST—Spark Timing. 3 CAD BTDC—crank angle degrees (CAD) before top-dead-center (BTDC). 4 CNG— Compressed Natural Gas.  
\end{table} 
Table 3: Fuel composition, marking of mixtures and spark timing. To maintain stable temperature of the engine during all experiments, an additional cooling system with two plate heat exchangers was added to the engine test bench. The control thermostat, regulating the flow of the coolant, was located in the engine cooling system's primary circuit. The secondary side of the heat exchanger was equipped with a control valve, which regulates amount of heat transferred to the local central heating system. In this way, the engine temperature was maintained in the range of \(80\pm 5\)\({}^{\circ}\)C.  
During the experiment, the operation of the engine was controlled using a programmable engine electronic control unit (ECU). The used device has the capability to program and monitor engine operational parameters, i.e., preprogrammed spark timing, maps, and mixture compositions. Two types of spark timing (ST) were used for experiments with the combustion of biogas and hydrogen mixtures: the first with values corresponding to the optimal combustion of CNG (CNG ST), and the second with values adapted to the optimal combustion of a mixture of biogas and hydrogen (mixture ST) (Table 3). ST values for the mixture were chosen according to the optimal peak in-cylinder pressure position. The composition of the mixture was controlled using the lambda sensor signal and by adjusting the flow of the gas mixture to the engine inlet. During the tests, the replacement of the biogas mixture was stored in high-pressure bottles. The gas fuel was supplied to the engine through the gas mixer. The flow of each gaseous component (CH\({}_{4}\), CO\({}_{2}\), and H\({}_{2}\)) was manually adjusted with a pressure regulator to obtain the required biogas mixture composition and value of the fuel-air ratio during the combustion process. The value of the fuel-air ratio was controlled by a wideband lambda sensor. The volumetric flows of the gases were measured by rotameters equipped to the temperature and pressure sensor.