Combustion, Performance, and Emission Analysis of Oleic Acid Diethanolamide-Stabilized Water-in-Diesel Emulsions

Abstract Water contamination in diesel fuel storage and transportation systems remains a persistent challenge, leading to microbial induced corrosion and potential engine reliability issues. Rather than removing the water, the present study explores its utilization within the fuel through the formation of stable water-in-diesel emulsions (WiDE). A novel surfactant, oleic acid diethanolamide (OADEA), synthesized at an optimal ratio of 70:30, was employed to stabilize the emulsions. Although OADEA contains nitrogen (∼4.1% by mass), the fuel-bound nitrogen contribution is only 0.25–0.41% of total fuel mass, and the thermal NOx suppression from water microexplosion dominates, as evidenced by up to 77% NOx reduction. The formulated fuels contained 5% (wt./wt.) water and 6–10% (wt./wt.) surfactant, producing transparent emulsions that remained stable for more than 15 months without phase separation. Engine testing demonstrated that the emulsion fuels exhibited performance comparable to neat diesel, with similar brake thermal efficiency, fuel economy, and power output across all operating loads. Most of the prepared emulsions showed favorable emission characteristics. Under idling conditions, emissions of unburnt hydrocarbons (HC) and carbon monoxide (CO) increased with increasing surfactant concentration due to reduced combustion temperatures. However, as torque increased to 10–20 N m, both CO and HC emissions decreased sharply owing to enhanced combustion efficiency from water-induced microexplosion. A substantial reduction in NOx emission was observed, with a maximum reduction of 77.23% achieved for the O6 formulation under idling conditions (0 N m torque). The results demonstrate that OADEA-stabilized water-in-diesel emulsions can effectively incorporate free water during storage while functioning as a drop-in fuel, offering a dual-functional approach to enhance operational reliability and reduce primary pollutant emissions from diesel engines.