This work examines how methanol addition influences the performance and exhaust emissions of a diesel engineoperating on B10 containing Jatropha biodiesel over 1200–2200 rpm. Methanol was added to the B10 fuel at concentrations of 3–15% by volume. Increasing the methanol fraction caused engine power to decline byapproximately 2–9%, whereas brake specific fuel consumption increased by about 3–10%. These changes weremainly associated with the lower energy content and ignition quality of methanol. In contrast, progressively higher methanol contents produced marked reductions in exhaust emissions, with nitrogen oxides (NOx) decreasing by2–38%, carbon monoxide (CO) by 15–48%, and particulate matter (PM) by 25–62%. The emission improvement resulted from the combined effects of fuel-bound oxygen, enhanced evaporation, and better fuel–air preparation, which favored oxidation while limiting pollutant formation. Considering both engine performance and emissionbehavior, methanol concentrations below 10% provided the most suitable compromise under the investigatedoperating conditions. These findings provide a practical basis for selecting methanol blending levels in diesel–biodiesel fuels and contribute to the development of lower-emission fuel strategies for compression-ignition engines, supporting cleaner and more sustainable engine operation