Exergetic and Environmental Sustainability Analysis of Second-Generation Hybrid Biodiesel Blends with α-Al₂O₃ Nano-additive

Abstract The increasing demand for sustainable energy has accelerated research on alternative fuels for diesel engines. In this study, the combustion, performance, emission, and sustainability characteristics of biodiesel blends derived from chicken fat (CF20), waste cooking oil (WC20), and their combined blend (CF10WC10), each enhanced with 50 ppm α-Al₂O₃ nanoparticles, were experimentally investigated in a variable compression ratio (VCR) diesel engine. The novelty of this work lies in the combined utilization of dual waste-derived biodiesel feedstocks with catalytic α-Al₂O₃ nanoparticles under varying compression ratios, along with a comprehensive evaluation of combustion, thermodynamic (energy–exergy), emission, and sustainability characteristics, which has not been extensively reported in the literature. The combustion analysis revealed that pure diesel exhibited the highest peak cylinder pressure of 68.74 bar, while WC20 showed a comparable value of 66.88 bar, indicating efficient combustion behaviour among the tested blends. Performance results demonstrated that the WC20 blend achieved a maximum energy efficiency of 27.7% and exergy efficiency of 26.1%, showing performance close to conventional diesel fuel. Emission analysis indicated that NOx emissions increased by approximately 18.7% for biodiesel blends compared with diesel under higher load conditions, primarily due to enhanced oxygen availability and elevated combustion temperatures, while other emissions remained within acceptable limits. The sustainability assessment further indicated that WC20 exhibited an improved sustainability index compared with other biodiesel blends. Overall, the results demonstrate that waste-derived biodiesel blends enhanced with α-Al₂O₃ nanoparticles can provide stable combustion characteristics and competitive thermodynamic performance, highlighting their potential as viable alternative fuels for compression ignition engines.