## 2 Materials and Methods
### Experimental Engine Test Bench Equipment  
In the first stage of this study, the energy and environmental indicators of a naturally aspirated spark-ignition engine (SIE), the Nissan HR16 DE (Nissan Motor Co., Ltd., Yokohama, Japan), were investigated using experimental equipment (Figure 1). According to the engine specifications, it delivers a maximum power output of 84 kW at 6000 rpm. The naturally aspirated engine was equipped with a multi-point fuel injection (MPFI) system operating at 3.5 bar fuel pressure, along with a Continuously Variable Valve Timing Control System (CVTCS). The engine was loaded using the AMX200/100 (Automex, Gdansk, Poland) eddy current engine load stand, which has a torque measurement accuracy of \(\pm\)0.9 Nm and is capable of applying a maximum torque load of 250 Nm (Table 2).  
Gasoline consumption was measured using AMX240 (Automex, Gdansk, Poland) gravimetric scales with an accuracy of \(\pm\)0.10%. Hydrogen consumption was measured using a high-pressure Coriolis mass flow metre RHEONIK RHM 015 (RHEONIK, Odelzhausen, Germany) with an accuracy of \(\pm\)0.10%. Air mass flow was measured at the intake system using a BOSCH HFM 5 (BOSCH, Stuttgart, Germany) air flow metre, which provides an accuracy of 3%. The composition of the exhaust gasses was analyzed using the AVL DiCOM 4000 (AVL DiTEST, Graz, Austria) gas analyser with the following specifications:  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **Gasoline (G100)** & **Hydrogen (H\({}_{2}\)100)** & **D-F** & **Difference Between** \\  & & **(H\({}_{2}\)100)** & **(G95H5)** & **D-F G95H5 and** \\ \hline Density at 15 \({}^{\circ}\)C, kg/m\({}^{3}\) & 736.0 & 0.09 & - & - \\ \hline Lower heating value \(LHV\), MJ/kg & 43.53 & 120.00 & 47.35 & +6.8 \\ \hline Elemental C & 86.42 & 0.00 & 82.10 & \(-\)5.0 \\ composition, \% H & 13.58 & 100.00 & 17.90 & +31.8 \\ \hline C/H & 6.36 & - & 4.59 & \(-\)27.9 \\ \hline Stoichiometric air–fuel ratio A/F, kg air/1 kg fuel & 14.7 & 35.0 & 15.7 & +8.8 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Key fuel properties [46; 47; 48].  
Figure 1: The schematic diagram of the test bench for an SIE under load.  
* CO: Measuring range, 0-10% (vol); resolution \(\pm\)0.01%.
* CO\({}_{2}\): Measuring range, 0-20% (vol); resolution \(\pm\)0.10%.
* HC: Measuring range, 0-20,000 ppm (vol); resolution \(\pm\)1 ppm.
* NO\({}_{\lambda}\): Measuring range, 0-4000 ppm (vol); resolution \(\pm\)1 ppm.
* Excess air ratio (\(\lambda\)): Measuring range, 0-9.999; resolution \(\pm\)0.001.  
In the experimental setup, liquid (G100) and gaseous (H\({}_{2}\)) fuels were injected through separate injectors installed in the intake manifold. G100 was injected using the original multi-point injection system of the HR16 DE HR16 DE (Nissan Motor Co., Ltd., Yokohama, Japan) engine. For injecting hydrogen, a separate multi-point injection system was installed in the intake manifold. To maintain the desired gasoline/hydrogen ratio, the MoTeC M800 (MoTeC, Melbourne, Australia) system was used to adjust the operating ratio of the primary and secondary injectors. The ratio was recalculated based on the mass fuel consumption of G100 and H\({}_{2}\)100. Engine operating parameters--including throttle position, engine speed, excess air ratio (\(\lambda\)), and ignition timing--were controlled using a programmable electronic engine control unit (ECU), MoTeC M800. A lambda control system maintained a stoichiometric mixture (\(\lambda\) = 1.0) over a wide range of operating conditions. Exceptions occurred at maximum throttle opening, where the mixture was enriched to an excess air ratio of \(\lambda\) = 0.9 to achieve maximum torque. At each measurement point, data were collected five times, and the standard deviation was calculated. If any measurement point showed significant variation, the measurement was repeated.