## 1 Introduction
Conversely, sunflower oil serves as a standard reference to study waste oils, primarily because it minimizes variability when compared to using waste oils of different origins. This makes it an excellent benchmark for research. Additionally, waste cooking oils, derived from over 190 million metric tons of globally generated waste vegetable oil annually [22], represent a highly sustainable and cost-effective option for biofuel production. As a result, exploring the potential of this abundant raw material as a biofuel is of great interest. The investigation of mixtures of these non-edible oils with various LVLC solvents presents a promising and cost-effective methodology for replacing fossil diesel, since based on raw material costs, castor oil ($1600/MT) and 2-EHN ($1700/MT), the costs are competitive with fossil diesel, making this an interesting approach to consider. The results so far indicate that a high percentage of fossil fuel can be substituted in diesel engines without sacrificing engine performance while achieving a significant reduction in pollutant emissions [23; 24; 25; 26; 27; 28]. Some studies have explored blends with gasoline due to their lower viscosity [24] or with hydrocarbons derived from the pyrolysis of plastic waste [26; 28]. Nevertheless, most research has focused on renewable LVLC solvents obtained from various alcohols [23; 27; 29]. However, to achieve a significant reduction in fossil diesel consumption using these mixtures, it is essential to identify a wide range of commercially available solvents. This is necessary to support the large-scale production of SVO/LVLC mixtures, given the vast quantity of fossil diesel currently in use. Moreover, identifying renewable, economically viable solvents with favorable cetane numbers is crucial. Ideally, solvents with low viscosity and high cetane numbers (LVHC) should be prioritized, as their use in blends with SVOs could further enhance results compared to those achieved with LVLC solvents investigated so far [30; 31]. This strategy could lead to improved fuel properties, higher diesel substitution rates, and greater environmental benefits. The physicochemical properties of biodiesel play a crucial role in engine emissions, significantly influencing NOx, CO\({}_{2}\), and CO outputs. Studies have identified oxygen content, aromatics, and lower heating value (LHV) as key predictors of NOx emissions. In contrast, viscosity is the most influential factor for CO emissions, while hydrogen content is critical for predicting CO2 emissions. Further analysis reveals notable correlations between viscosity, flash point, density, and hydrogen content with CO and CO2 emissions, although these properties have minimal impact on NOx emissions. Oxygen content demonstrates a strong negative correlation with NOx emissions, whereas properties such as boiling range, LHV, aromatics, and surface tension exhibit positive correlations [32]. These findings highlight the intricate relationship between fuel properties and emission behavior, underscoring the complexity of identifying optimal biodiesel blends to achieve enhanced performance with reduced pollutant emissions. To address the limitations of LVLC solvents, particularly their potential to reduce the cetane number and negatively affect combustion performance, solvents with both low viscosity and high cetane numbers (LVHC) present a promising alternative. LVHC solvents can enhance the performance of blends with SVOs, improving combustion characteristics compared to LVLC solvents. Cetane improvers are widely used to enhance the cetane number of fuels, thereby reducing engine exhaust emissions and improving overall efficiency. For example, 2-ethylhexyl nitrate (EHN) is a well-established cetane improver capable of increasing the cetane number by 12-16 points while simultaneously reducing nitrogen oxides (NOx), carbon monoxide (CO), and unburned hydrocarbon emissions [33].