Multifunctional Additives for Sludge Mitigation and Fuel System Reliability in Biodiesel Blends: A Systematic Review

Abstract

This systematic review examines the mechanisms of sludge formation in biodiesel blends and evaluates the role of multifunctional additives in improving fuel system reliability. Biodiesel, particularly fatty acid methyl ester (FAME)-based blends, is susceptible to oxidation, water absorption, microbial growth, and deposit formation, which may lead to filter plugging, injector fouling, corrosion, and unstable combustion. A systematic literature review was conducted using the PRISMA framework, focusing on studies related to biodiesel degradation, sludge or deposit formation, and fuel additives. The review identifies sludge formation as a multi-stage process involving FAME oxidation, hydroperoxide decomposition, polymerization, acid formation, emulsion instability, and microbiologically influenced degradation. The findings indicate that single-function additives are insufficient to manage these interconnected pathways. Dispersant–demulsifier additives can reduce water-related sludge and particle agglomeration, antioxidant–corrosion inhibitor systems can suppress oxidative degradation and material damage, while cetane improvers can reduce combustion-related deposits. This review highlights the need for integrated multifunctional additive formulations capable of simultaneously controlling chemical degradation, water–fuel interactions, corrosion, and combustion residues. Future research should prioritize system-level additive design, long-term validation, and adaptive fuel treatment strategies for biodiesel applications.