As a substitute for conventional fossil fuels, biodiesel may be a potential future choice to deal with the scarcity of fossil fuels while lowering the emissions of pollutants from internal combustion engines. The aim of this research is to assess, through experiments and statistical analysis, the effects of using a diesel-biofuel blend on the performance and emissions of a single-cylinder direct injection engine. Utilizing used oil derived from trees of olive in the Tunisian region of Sfax was presented in this scientific study. As for the producer of biofuels from recycled olive oil, a chemical esterification procedure was carried out. Six mixes with varying amounts of diesel and biofuel were created and tested at the test facility for single-cylinder engines. The experimental results were compared to a one-dimensional engine model in terms of torque and exhaust emissions, and the results showed a good agreement between the test and the numerical data. For a better understanding of the elements that affect the engine’s response to changes in fuel composition, the thermodynamic parameters of the engine for each formulation have also been presented. According to the investigation, a blend of 80% pure diesel and 20% methyl ester-based oleic acid-based biofuel would be a workable fuel option for cleaner exhaust emissions while providing essentially the same performance.