Biodiesel has received a lot of attention in recent years as a renewable and environmentally friendly fuel. Biodiesel manufacturing, on the other hand, is still more expensive than commercial diesel, and it takes a lot of time and work. The use of waste cooking oils (WCO) in diesel engines without converting to biodiesel was examined in this study. We used 25 percent waste oil +75 percent diesel fuel and 50 percent waste oil +50 percent diesel fuel ratios to combine WCO with diesel fuel. In addition, to improve the fuel qualities of the waste cooking oil-diesel fuel blends, 5%, 10%, and 15% toluene were added. For engine testing, a single-cylinder, direct injection, air-cooled diesel engine was used.