In this research, the performance and emissions of a Diesel engine were investigated under three fuel configurations: Diesel, a B-20 biodiesel blend with an hafnium carbide (HfC)-coated piston, and a B-20 blend with an uncoated piston. The results illustrated Diesel’s superior performance, achieving a brake specific fuel consumption (BSFC) of 0.2 kg/kWh at 7 kW brake power, coupled with the lowest emissions: CO at 30 ppm, HC at 14 ppm, NOx at 105 ppm, and smoke at 55 ppm. The HfC-coated piston running on B-20 biodiesel blend showed promising results, registering a BSFC of 0.25 kg/kWh and emissions of CO at 34 ppm, HC at 18 ppm, NOx at 110 ppm, and smoke at 60 ppm at the same brake power. In contrast, the B-20 blend in the uncoated piston recorded a BSFC of 0.28 kg/kWh with emissions of CO at 37 ppm, HC at 20 ppm, NOx at 114 ppm, and smoke at 65 ppm. The study underscores Diesel’s inherent efficiency but also highlights the potential of engine modifications, like the HfC-coating, to substantially optimize the combustion efficiency of biodiesel blends, bridging the performance and emissions gap with conventional diesel.