## 2 Materials and Methods  
### Experimental Test Bench  
The experiments were performed in a 4T, naturally aspirated spark-ignition engine (model MZ175, YAMAHA(r)). The engine has a volumetric capacity of 171 cm\({}^{3}\) and a compression ratio of 8.5:1. It is worth discussing the relevance of the volumetric capacity in ICEs since it provides a clear perspective of the context of the present application. In essence, this matter is essential, considering that international and governmental regulations concerning greenhouse emissions in the transportation sector are based on the volumetric capacity. The typical taxation margin is classified as low (<1000 cc), middle (1200-1500 cc), and high (>1500 cc) capacity [24]. Note that the engine used in this study is intended for power generation applications. However, its operational characteristics resemble those in commercial vehicles, extending the impact spectrum of the proposed dual-fuel technology. Table 1 lists the main features of the SI engine. A picture of the test bench is shown in Figure 1. The experimental setup consists of the SI engine, hydroxy generation system, and DAQ system, as shown in Figure 2. Firstly, the SI engine is integrated into a dynamometer to control the load condition. A crankshaft angle sensor (Beck Arnley 180-042) allows measuring engine speed. The in-cylinder pressure is measured with a piezoelectric transducer (KISTLER type 7063-A) placed in the cylinder head. The engine fuel consumption rate is obtained via a scale (OHAUS PA313) and a chronometer. On the other hand, intake airflow is measured employing a hot-wire type mass sensor (BOSCH 22,680 7f600). Additionally, the temperatures of the exhaust gases were measured using K-type thermocouples. Lastly, the measurement of CO, NOx, and HC emissions was carried out using two different gas analyzers, namely BrainBee AGS-688 and PCA(r) 400. An additional gas analyzer (BrainBee OPA-100) measured the opacity levels of the exhaust gases. The measuring instruments were integrated into a data acquisition system that processes the output data. Table 2 lists the main features of the measuring instruments of the test bench. The hydroxy gas addition was carried out by means of an HHO gas generator installed in the engine intake system (see Figure 2). Hydroxy gas is obtained through a dry cell made of stainless steel, with the ability to withstand high temperatures and currents. Additionally, this type of material does not cause a chemical reaction with the electrolytic substance. To improve cell performance, KOH was used as a catalyst at a concentration of 20% (gram of solute/volume of solution). This allows improving the conductivity of the dry cell. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Instrument** & **Manufacturer** & **Range** \\ \hline \multirow{2}{*}{Cylinder pressure} & Piezoelectric & \multirow{2}{*}{KISTLER type 7063-A} & \multirow{2}{*}{0–250 bar} \\  & transducer & & \\ \hline \multirow{2}{*}{Airflow} & \multirow{2}{*}{Air mass sensor} & BOSCH OE-22680 & \multirow{2}{*}{0–125 g/s} \\  & & 7J600 & \\ \hline Angle & Crankshaft angle & [PERSON] 180–0420 & 5–9999 RPM \\ \hline Fuel measuring & Gravimetric meter & OHAUS-PA313 & 0–310 g \\ \hline HHO gas flow & HHO flow rate & GT-556-MTR-ICV & 0–3 LPM \\ \hline Temperature & Temperature sensor & Type K & \(-\)200–1370 \({}^{\circ}\)C \\ \hline CO & & \multirow{2}{*}{BrainBee AGS-688} & 0–9.99\% \\ \cline{1-1} \cline{3-4} HC & & & 0–9999 ppm \\ \hline NOx & & PCA-400 & 0–3000 ppm \\ \hline Smoke opacity & & BrainBee OPA-100 & 0–99.9\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Specifications of measuring instruments. Figure 2: Schematic representation of the experimental test bench.