Phase-boundary-guided formulation of rice bran oil-ethanol microemulsions for diesel engines with cetane number enhancement

This study investigates diesel engine performance, combustion, and emissions using rice bran oil-ethanol microemulsions with 2-ethylhexyl nitrate (2-EHN) as a cetane improver. Microemulsions were formulated with sorbitan monooleate and fatty alcohol ethoxylate as mixed surfactants and 1-octanol as co-surfactant; homogeneous phase boundaries were mapped at 40 °C across surfactant ratios to identify single-phase domains, and a base composition of 40:30:30 by volume was selected, after which 2-EHN was dosed at 0.2%, 1%, and 3% by volume for engine trials at 1500 rpm and 25–75% load. Intermediate surfactant ratios provided the widest single-phase domain. All microemulsion cetane numbers (38.86–39.89) fell below the ASTM minimum of 50, and 2-EHN addition raised cetane number dose-dependently to 47.31 at 3% by volume. Increasing 2-EHN concentration shortened ignition delay, raised peak in-cylinder pressure, and increased heat-release rates across all loads. The base microemulsion outperformed diesel in diesel-equivalent brake specific fuel consumption and brake thermal efficiency, attributable to ethanol's oxygen content and micro-explosion-enhanced atomization. The addition of 0.2% 2-EHN achieved the highest brake thermal efficiency of 19.99% at 75% load through moderate free radical generation that advanced combustion phasing without severely curtailing pre-ignition mixing time; higher dosages degraded performance non-monotonically. Specific nitrogen oxides emissions were lowest at 0.2% and 1% 2-EHN (3.61 and 3.62 g/kWh at 75% load) by suppressing the premixed combustion spike. Conversely, specific unburned hydrocarbon and carbon monoxide rose sharply at 3% (1.57 and 47.42 g/kWh at 75% load) due to wall quenching and oxygen-deficit zones. Smoke opacity remained lower than diesel across all microemulsion blends. Overall, 0.2% 2-EHN represents the practical optimum for balancing ignition quality, thermal efficiency, and emissions in rice bran oil-ethanol microemulsion fuels. • Phase behavior of rice bran oil-ethanol microemulsions mapped using mixed surfactants. • Physicochemical properties of microemulsions evaluated relative to diesel reference fuel. • Cetane improver chemistry enhances autoignition via nitrate ester radical pathways. • 2-EHN concentration governs combustion kinetics and heat release characteristics. • Additive-fuel chemistry trade-offs control NOx reduction and incomplete oxidation products.