## 2 Materials and Methods
### Characterization of the Rheological and Physicochemical Properties of the biofuel Mixtures  
The essential physicochemical properties of the biofuel blend to be utilized in diesel engines were identified and evaluated. These properties include kinematic viscosity, density, cloud point, and pour point, all of which were measured through experimental methods. Additionally, the calorific value and cetane index were determined using predictive models. Each data point represents the median of three independent measurements, with experimental errors calculated as standard deviations to ensure accuracy and reliability. Determination of Calorific Value of Biofuel Samples  
Theoretical calorific values (CV) were calculated using the Kay Mixing Rule, based on the volumetric concentration of each component in the blend, as shown in Equation (1):  
\[\text{CV}=\Sigma\text{i}\text{ {CViXi}} \tag{1}\]  
where _CVi_ represents the calorific value of each component in megajoules per liter (MJ/L), and _Xi_ is the volumetric fraction of each component [45]. Experimental Procedure for Testing the Mechanical and Environmental Performance of Biofuel Blends in a Diesel Engine-Electrogenerator Set  
A C.I. engine (Model: AYERBE AY-4000 D, Vitoria, Spain), designed for electricity generation, was used to evaluate the energy performance and emissions of the blends, following a previously described methodology [23; 26]. The main technical specifications of the engine are listed in Table S1 in Supplementary Materials. This engine operates with a 4-stroke, single-cylinder system, featuring a bore of 78 mm and a stroke of 67 mm. It utilizes a forced air-cooling system with a flywheel fan to maintain optimal operating temperatures. The engine was operated at a constant crankshaft rotation speed, while the electrical energy demand was varied by connecting different numbers of 1000-watt heating plates. Since the engine maintained a fixed rotation speed and crankshaft torque, the variations in electrical power generated directly reflected the mechanical power produced from the combustion of the biofuels being tested. The electrical power generated was calculated by \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Property** & **Diesel (D)** & **Sunflower Oil (SO)** & **Castor Oil (CO)** &
\begin{tabular}{c} **2-Ethlyhexyl Nitrate** \\ **(EHN)** \\ \end{tabular} \\ \hline Density at 15 \({}^{\circ}\)C (kg/m\({}^{3}\)) & 825–835 & 920 & 962 & 890–963 \\ Kinematic viscosity at 40 \({}^{\circ}\)C (cSt) \({}^{1}\) & 3.20 \(\pm\) 0.01 & 37.80 \(\pm\) 0.05 & 226.20 \(\pm\) 0.05 & 1.57\(\pm\) 0.01 \\ Calorific value (MJ/kg) & 42.8 & 39.5 & 37.2 & 33.0–41.6 \\ Flash point (\({}^{\circ}\)C) & 66.0–67.5 & 220 & 228 & 76.0–77.0 \\ Auto-ignition temperature (\({}^{\circ}\)C) & 250–301 & 316 & 448 & 215–333 \\ Cetane number & 51–61.5 & 37 & 40 & * \\ \hline \hline \end{tabular}  
\({}^{1}\) Viscosity value errors were obtained from the average of 3 measurements. * The cetane number value of EHN cannot be found in the cetane number scale.  
\end{table} 
Table 1: Physicochemical properties of the different compounds used as biofuels. Kinematic viscosity values were experimentally obtained in this research. multiplying the voltage and current, as described in Equation (2). Both voltage and current were measured using a voltmeter-ammeter. Electrical Power Generated (Watts) = voltage (Volts) \(\times\) amperage (Amps) (2)  
Mechanical power was derived from electrical power measurements, and this methodology was applied to evaluate all double and triple blends with viscosity values suitable for use as biofuels. Fuel consumption was determined by measuring the volume consumed over time while operating the engine under different levels of electrical demand: low (1 kW), medium (3 kW), and high (5 kW). For each measurement, 600 mL of biofuel was used. This allowed us to calculate the fuel consumption under varying electrical loads, determining the engine consumption rate at each demand level.