Water-in-diesel emulsion fuels: A comprehensive review of formulation, combustion performance, emission characteristics, and technological advances

The need for cleaner and more efficient combustion in compression ignition engines has driven the search for alternative fuels; in this respect, water-in-diesel (W/D) emulsions have been identified as a promising, ready-to-use option. This review presents the influence of W/D emulsion fuel technology on engine efficiency and emissions, along with recent developments aimed at overcoming its limitations, drawing conclusions from over 120 research studies conducted over the last three decades. The underlying principle of W/D emulsions is the micro-explosion effect, in which dispersed water droplets undergo explosive vaporization, enhancing fuel atomization and air–fuel mixing, thereby improving combustion. There is a consistent and considerable decrease in two major emissions: nitrogen oxides (NOx reduction by 15–50%), primarily due to heat absorption by water vapor, which lowers peak flame temperatures, and particulate matter (PM reduction by 20–60%), resulting from enhanced combustion efficiency. However, a trade-off exists in the form of a slight increase in brake-specific fuel consumption (BSFC), which is 5–20% higher on a mass basis (and comparable on an energy basis) and, under low-load operating conditions, potentially elevated emissions of carbon monoxide (CO by 10–45%) and unburnt hydrocarbons (HC by 8–52%). The importance of surfactants is highlighted through an overview of conventional ionic and non-ionic combinations, as well as innovative biodegradable and nanoparticle-stabilized formulations that improve emulsion stability. This review also examines the synergistic effects of nanoparticles when introduced as combustion catalysts and their role in the synthesis of surfactant-free emulsions. In addition, innovative dual-fuel concepts involving emulsions and gaseous fuels such as Hythane are discussed, which provide up to 58% NOx reduction and up to 62% PM reduction in the dual-fuel mode. Through a time-sequenced analysis of research emphasis, covering formulation stability, combustion kinetics, and practical lifecycle considerations, this work serves as an important reference for researchers and engineers, elucidating the role of W/D emulsions in advancing sustainable energy systems.