Widely used diesel engines significantly emit harmful NOx, CO₂ and particulate matter to the environment. Biodiesel blends, although it can reduce emissions, but face challenges in viscosity, flash point, pour point, cold weather performance, etc. This study investigated the performance and emission behaviour of biodiesel-diesel blends with an aim to redue emissions and improve performance. Multiscale entropy analysis was performed to understand combustion instability and complexity, emphasizing the impact of fuel composition and engine speed at full load on engine performance and emission. Tomato-papaya bodieslel-diesel blends and tomato biodiesel-diesel blends were used for experimental investigation. At 2200 rpm, the results indicate consistent and regular combustion, enhancing engine efficiency however increasing CO₂ and NOx emissions compared to commercial diesel. Fuels with lower complexity, such as tomato-papaya biodiesel and diesel blends, yields higher torque output, while diesel fuel with higher complexity generates lower torque. The results revealed that pure diesel exhibits the highest combustion complexity, resulting in the lower useful work and exergy, approximately 5% less than the papaya-tomato biodiesel-diesel blends.