Fuel additives for biodiesel: Impacts on fuel properties, combustion behavior, engine performance, and emissions – A systematic review

Biodiesel is a key low-carbon fuel in the global energy transition; however, its widespread application is limited by low oxidative stability, poor cold-flow properties, and slight penalties in engine performance and fuel economy. In recent years, chemical additives, natural additives, and nanomaterials have been increasingly investigated to overcome these challenges. This study presents a systematic review of peer-reviewed literature published between 2020 and 2025, analyzing the effects of various additives on biodiesel fuel properties (viscosity, density, cloud point, oxidative stability, and lower heating value), combustion behavior, diesel engine performance, and exhaust emissions. The results indicate that chemical antioxidants such as TBHQ and BHT significantly enhance oxidative stability, while cetane improvers (e.g., 2-EHN) reduce ignition delay and improve combustion efficiency. Metal oxide nano-additives, including CeO 2 , TiO 2 , and Al 2 O 3 , enhance fuel atomization, catalytic combustion activity, and thermal efficiency, leading to notable reductions in HC, CO, and particulate emissions. In parallel, natural additives derived from plant-based waste streams demonstrate effective antioxidant activity, improving storage stability and which may contribute to lower environmental impact due to their renewable origin and biodegradable nature. Despite these benefits, challenges remain, including high nanomaterial production costs, dispersion stability issues, limited long-term engine data, and insufficient assessment of environmental impacts. Overall, this review highlights the potential of multifunctional additives, hybrid nano–natural formulations, and the integration of life cycle assessment and techno-economic analysis as key pathways toward the sustainable industrial deployment of diesel–biodiesel fuels.