Novel surfactants for stable biodiesel-water emulsions to improve performance and reduce exhaust emissions of a light-duty diesel engine

[Display omitted] •Stable biodiesel-water emulsions are prepared with 6, 12 and 18% water concentrations.•Novel surfactants PGPR and RKO are used to prepare stable biodiesel-water emulsions.•Low surfactant concentration of 2% by weight used for preparing emulsion with 18% water.•Span-Tween, PGPR and Karanja oil as surfactants compared in terms of emulsion stability and engine characteristics.•Karanja oil is the most suitable surfactant considering cost, stability, sustainability and lower exhaust emissions. Emulsified biodiesel is a viable alternative to fossil diesel to reduce carbon footprint, oxides of nitrogen (NOx) and soot emissions. However, widespread commercial utilization of biodiesel-water emulsions requires cost-effective novel surfactants for producing stable emulsions. Studies on neat biodiesel-water emulsions are scarce, and the available studies are done with surfactants developed for diesel-water emulsions. The present work explores the efficacy of different surfactants in producing stable biodiesel-water emulsions and their effects on engine characteristics. The surfactants investigated include a commercial nonionic surfactant, viz. Span80-Tween80 and two novel surfactants, viz. polyglycerol poly ricinoleate (PGPR) and raw Karanja oil (RKO). The water concentrations in biodiesel-water emulsions are fixed at 6, 12 and 18% (by mass) with 2% surfactant. The emulsions prepared using a hot-plate magnetic stirrer were stable for over five months. The experiments were carried out on a light-duty diesel engine at rated speed and varying loads with the prepared emulsions. The results obtained with stable biodiesel-water emulsions with different water concentrations and surfactant types are compared with neat biodiesel as a reference fuel. At low water concentrations, the cylinder pressure increased with PGPR and RKO, whereas it was nearly the same as biodiesel with the Span-Tween combination. However, the trends between Span-Tween and RKO were reversed at higher water concentrations. The diffusion phase heat release rate increased and the combustion phasing advanced in general, with all the surfactants at all water concentrations. The engine brake thermal efficiency (BTE) increased, the brake specific fuel consumption (BSFC) reduced with emulsions, and the benefits increased at higher water concentrations with a maximum variation of 8% and 20% in BTE and BSFC, respectively. From the emission reduction perspective, biodiesel-water emulsions with RK