<p>The aim of this study is to examine the synergistic effect of a ternary blend of waste cooking oil biodiesel, pentanol, and diesel, combined with the catalytic activity of Al₂O₃ nanoparticles in a direct injection (DI) diesel engine. While extensive research has been conducted on binary fuel blends (e.g., biodiesel-diesel), studies exploring the combined effect of three fuels i.e. biodiesel (from waste cooking oil), a higher alcohol (pentanol), and diesel is less common. This study investigates the potential benefits of using this specific combination in a diesel engine, with the goal of improving combustion and reducing emissions. A key focus of the study is identifying the optimal concentration of Al₂O₃ nanoparticles to achieve maximum performance and emission reduction in the ternary blend. All experimental procedures were carried out in accordance with ASTM standards. Biodiesel was produced through the transesterification process, and the sample blends were prepared accordingly. The fuel properties of the blends were found to be within the recommended limits. The samples were evaluated in a 4-stroke, constant-speed, DI diesel engine. An optimization analysis of the experimental results was performed using Taguchi/Grey relational analysis to determine the best combination of blend ratios and operating conditions. The most effective combination for butanol-based ternary blends was Bu10B30D60 with 50&#xa0;ppm of nano-additive at 50% engine load. Similarly, for pentanol-based ternary blends (Pe10B30D60), the optimal condition was with 50&#xa0;ppm of nano-additive at 100% load, while for propanol-based blends (Pr10B30D60), the optimal condition was at 0% load with the same nanoparticle concentration. According to the findings, ternary blends containing pentanol and waste cooking oil biodiesel demonstrated significantly better performance and lower emissions compared to the other blends.</p>