## 2 Materials and Methods  
Experimental research of energy and environmental indicators of a compression ignition engine running on diesel and gas (dual fuel) was conducted using the 1.9 TDI (Turbocharged Direct Injection) (1Z) engine loaded using the engine load bench. The CI engine power system comprises the electronically controlled axial-piston distributor injection pump BOSCH VP37 and a hole-type two-springs nozzle (opening pressure 190-200 bar). The engine was equipped with a turbocharger and a wastage as a boost-controlling device. Table 2 presents the key indicators of the CI 1.9 TDI engine, also using the additional gas supply system Dual fuel (Elpigaz-Degamix) for supplying natural gas (NG). CO\({}_{2}\) was supplied separately controlling its amount using a pressure reducer. NG and CO\({}_{2}\) gas were supplied to the intake manifold ahead of the turbocharger (Figure 1). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Properties** & **Diesel** & **HVO** & **Natural Gas** & **Biogas** \\ \hline Lower heating value \(LHV\), MJ/kg & 42.5 & 44.1 & 50.0 & 19.1 \\ Cetane number & 52.1 & \textgreater{}70.0 & - & - \\ Octane number & - & - & \textgreater{}120 & 130 \\ Auto-ignition temperature, \({}^{\circ}\)C & 180–220 & - & 650 & 600–650 \\ Stoichiometric air-fuel ratio, kg/kg & 14.3 & - & 17.2 & 6.17 \\ Carbon content, \% & 87 & 84.36 & 75 & - \\ Flammability limits, vol. \% in air & 0.6–7.5 & - & 5–15 & 7.5-14 \\ \hline \hline \end{tabular} HVO—Hydrogenated Vegetable Oil.  
\end{table} 
Table 1: Main characteristics of the fuels. Figure 1: Engine test equipment. The load bench KI-5543 was used to determine the engine's brake torque \(M_{B}\) (Nm) and the shaft speed \(n\) (rpm). \(M_{B}\) measurement error = \(\pm\) 1.23 Nm. Electronic scale SK-5000 and a stopwatch was used to determine hourly consumption of diesel \(B_{f}\) (kg/h). Measurement error of fuel consumption-0.5%. Consumption of NG and CO\({}_{2}\) was measured using the gas mass flow meter Type RHM 015 with the accuracy of \(\pm\) 0.1%. The intake air mass was measured using the meter BOSCH HFM 5 with the accuracy of 2%. VAG-Com is communicating via OBD II-ECU and displays SOI information. The pressure in the cylinder was captured using a piezoelectric sensor mounted on a spark plug _AVL_ GH13P with a sensitivity of 15.84 \(\pm\) 0.09 pC/bar. Photoelectric encoder A58M-F (Precizika Metrology, Zirmunu st. 139, Vilnius 09120, Lithuania) with a signal repeatability of 0.35156 CAD was used to determine the position of the crankshaft. The value of the gas pressure in the cylinder was recorded using the AVL DiTEST DPM 800 oscilloscope (input range 6000 pC, signal ratio 1 mV/pC) and the LabView Real software. 100 cycles in the pressure cylinder were measured, also calculating the average pressure values at each measuring point. The pressure was measured in the engine intake manifold using the pressure sensor (PS) Delta OHM HD 2304.0 m with a measurement error of \(\pm\)0.0002 MPa. The temperature was measured using type K thermocouples Temperature sensors (TS) the accuracy of \(\pm\)1.5 \({}^{\circ}\)C. Engine's exhaust gas composition was determined using the exhaust gas analyzer and the opacimeter AVL DiCom 4000. Its parameters are listed in Table 3. During the experiment, constant ambient parameters were maintained: intake air temperature--17 \({}^{\circ}\)C, air pressure--1000.6 hPa, relative humidity--52%. The number of experiments for repetitions was five, averaging the results. During the test, the engine speed was \(n\) = 2000 rpm and the engine brake torque-\(M_{B}\) = 45 Nm. These are the operating conditions of the engine in urban conditions. Tests were performed with a disconnected EGR valve, because excess air ratio decreases significantly when using biogas containing 40% of CO\({}_{2}\). Liquid fuels and dual fuels were used in the experimental research. Conventional diesel fuel (D) and the second generation biodiesel hydrotreated vegetable oil (HVO) were used as liquid fuels.