## 2 Materials and Methods  
All experiments were performed in Vilnius Gediminas Technical University, Laboratory of Internal Combustion Engines (J. Basanaviciaus str. 28, Vilnius, Lithuania). NISSAN's HR16 DE in-line four-cylinder spark ignition engine combined with engine load stand AMX200/100 (0.9 Nm precision) was used for the experiments. Engine displacement was 1.598 dm\({}^{3}\) (cylinder bore-78.0 mm, piston stroke 86.6 mm), engine power was 84 kW at 6000 rpm and torque was 156 Nm at 4400 rpm. The engine compression ratio was 10.7. A plate heat exchanger was added to maintain a stable engine operating temperature during all measurements. A standard oxygen sensor was replaced with a wideband oxygen sensor BOSCH LSU 4.9 for air fuel (A/F) ratio monitoring. The intake air mass was measured with a BOSCH hot film air mass meter, type HFM 5 (accuracy 2%). Additional injectors for CH\({}_{4}\) were added to the engine's intake manifold. The engine's spark timing (ST) and fuel injection quantities were adjusted via a reprogrammable MOTEC M800 electronic control unit (ECU), which has the ability to control additional injectors; in this case, the injectors for CH\({}_{4}\) were controled. Figure 2 shows a schematic diagram of an engine with load stand and measurement equipment. The in-cylinder pressure was measured for every selected biogas composition and analyzed using the AVL BOOST software utility tool BURN. The in-cylinder pressure data were obtained using a spark plug with integrated pressure sensor AVL ZI31_Y7S and signal amplifier AVL DiTEST DPM 800; detailed information is given in Table 2. The signal of the sensor was processed and recorded using the LABVIEW REAL TIME module. During data collection, two samples were collected, each containing 100 combustion engine cycles. The exhaust gas temperature was measured in the exhaust manifold, while the exhaust gas composition was measured from offshoot pipe (for reducing the exhaust gas flow and temperature) connected to the exhaust pipe before the catalytic converter. Exhaust gas analyzer AVL DICOM 4000 was calibrated for the biogas engine. The exhaust gas analyzer parameters are listed in Table 3. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Property** & **Pressure Sensor** & **Property** & **Signal Amplifier** \\  & **Z131\_Y7S** & **Property** & **DiTEST DPM 800** \\ \hline Measuring range & 0-200 bar & Input range & 6000 pC \\ Sensitivity & 12 pC/bar & Signal output & 1 mV/pC \\ Capacitance & 5 pF & Zero offset & 0.5 V \\ Acceleration & 0.001 bar/g & Signal amplitude & 0.5–4.5 V \\ sensitivity & 40–350 \({}^{\circ}\)C & Temperature range & \(-\)10–120 \({}^{\circ}\)C \\ Linearity & \(\pm\)0.5\% & Power supply & 8 V–32 V \\ Natural frequency & 130 kHz & Dimensions & \(l\) = 131 mm, \(d\) = 13.8 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: In-cylinder pressure measuring equipment. Figure 2: Experiment setup. 1—HR16 DE SI engine; 2—connecting shaft; 3—engine load stand; 4—load stand ECU and display; 5—crankshaft position sensor; 6—spark plug with pressure sensor; 7—in-cylinder pressure recording equipment; 8—exhaust gas temperature sensor; 9—exhaust gas temperature display; 10—wideband oxygen sensor; 11—reprogrammable engine ECU; 12—exhaust gas analyzer; 13—air mass flow meter; 14—throttle control servo motor; 15—CH4 injector; 16—low pressure regulator; 17—high pressure regulator; 18—fuel mass flow meter for CH4; 19—fuel mass flow meter display; 20—CH4 cylinder; 21—high pressure regulator; 22—fuel mass flow meter for CO2; 23—fuel mass flow meter display; 24—CO2 cylinder. For the experiment, biogas was reprocessed as pure biogas containing only methane and carbon dioxide [41]. Two separated gas tanks of CO\({}_{2}\) and CNG (as substitutes for methane) were used for imitating biogas. Recreated were four different biogas compositions containing 50%, 40%, 20% and 0% of CO\({}_{2}\) by volume.