## 3 Result and Discussion
### Characterization and Properties of \"Soheil\" Biodiesel  
The properties of the produced biodiesel are among the most important topics to consider in the study of new fuels. The newly produced fuel must be able to meet existing standards in terms of physical and chemical properties. In this study, many properties of the biodiesel produced from CSO were determined. Table 8 shows the properties of biodiesel produced from CSO.  
Density has a direct impact on the injection performance of the fuel and influences fluid movement inside the pipelines, injector nozzles and orifices [61; 62; 63]. The density of \"Soheil\" methyl ester (AAMEs) is 883 kg/m\({}^{3}\). The density of AAMEs is within the range of the ASTM D4052 standard. The density of AAMEs is higher than the density of some conventional biodiesel, including soybean (0.883 g/cm\({}^{3}\)) [64], canola (0.882 g/cm\({}^{3}\)) [65], sunflower (0.880 g/cm\({}^{3}\)) [65]. One of the important features of biodiesel is its kinematic viscosity. Viscosity determines the level of the fuel fluidity and plays a vital role in combustion and in the formation of deposits and soot. If the viscosity is high, complete combustion will not take place, and atomization capability of the fuel will be weakened [66]. The results of the study showed that the viscosity of the AAMEs is 4.01 mm\({}^{2}\)/s. The viscosity of AAMEs is consistent with the requirements of the ASTMD445 standard. The viscosity of AAMEs is lower compared to conventional biodiesel such as palm (5.7 mm\({}^{2}\)/s) [65] and castor (15.25 mm\({}^{2}\)/s) [65]. A low viscosity is an advantage for AAMEs since it facilitates the fuel flow inside the engine.  
Another important characteristic of the fuel is cloud point. Cloud point is the temperature at which a cloud or a halo of crystals of paraffinic compounds appears inside the fuel (at this temperature, the sample does not lose its fluidity and is usable). The cloud point is an important fuel characteristic for low-temperature applications, and lower values of cloud point are more desirable. According to Table 8, it is evident that the cloud point of AAMEs is \(-2\)\({}^{\circ}\)C, which meets the requirements of ASTM D6751. Pour point is another important characteristic of biodiesel fuel. Pour point is the lowest temperature at which an oil specimen can flow. The pour point temperature is lower than the cloud point temperature. According to the results, the pour point of AAMEs is \(-6\)\({}^{\circ}\)C, which is consistent with ASTM D6751 standard. Another parameter with significant impact on combustion is the flash point. Flash point is the lowest temperature at which the vapor of the fuel starts to burn. Fuels with a high flash point reduce the risk of fire [67]. The flash point of AAMEs is 154 \({}^{\circ}\)C, which meets the ASTM D93 standard. One of the advantages of biodiesel in comparison to diesel is the high value of the flash point in biodiesel fuel [56]. Cetane number is an important parameter in fuel, which plays a significant role in combustion inside diesel engines. The cetane number of each fuel indicates whether combustion is carried out simply or vice versa. The cetane number has a significant effect on the parameters of diesel engine relating to combustion, performance and pollution [68; 69]. According to Table 8, it is found that the AAMEs' cetane number is 46, which meets the requirements of the ASTM D613 standard. In general, the cetane number of biodiesel fuel is greater than diesel; therefore, the efficiency of biodiesel combustion is higher than diesel. The results of measurement of other characteristics of biodiesel produced from \"Soheil\" oil are shown in Table 8. All of the characteristics of the produced biodiesel are consistent with the requirements of the ASTM D6751 standard.