Enhancing biodiesel performance: A comparative study of metal oxide nanoparticles on combustion efficiency and emission reduction

Biodiesel had been promoted as a renewable compression-ignition fuel, yet limitations in brake thermal efficiency (BTE) and elevated NOx emissions had constrained its widespread deployment. A lack of comparative, load-resolved evidence across multiple metal-oxide additives and the absence of a formal optimization framework had persisted. This study therefore quantified and optimized the effects of TiO₂, Fe₂O₃, CuO, and Al₂O₃ nanoparticles on soybean biodiesel (sustainable) combustion, efficiency, and emissions. A single-cylinder, water-cooled engine at 1500 rpm was used with engine loads of 25–100 % and nanoparticle dosages of 25, 50, and 100 ppm; measurements were repeated, and uncertainties were evaluated. A central composite design within response surface methodology (36 runs) modeled the joint influence of concentration and load on BTE and NOx. The optimized condition (100 ppm, full load) achieved a BTE of 52.94 % with NOx near 63.93 ppm; the response-surface analysis showed improvements of up to 37 % in BTE and reductions of 36–43 % in NOx relative to the low-concentration, low-load corner. At the experimental mid-dose of 50 ppm, additional quantitative outcomes were confirmed: exhaust-gas temperature fell by 1.8–2.4 % depending on oxide, CO dropped most strongly with CuO (12 %), and smoke opacity decreased by 6 % with TiO₂ at full load. Combustion indicators supported these trends: peak cylinder pressure rose to 83.7 bar with TiO₂ (4.9 % above neat biodiesel), the premixed heat-release peak increased to ∼67 J/deg. (17 %), and ignition delay shortened to 5.4 ms (10 % reduction). TiO₂ delivered the strongest NOx suppression through oxygen-storage/redox activity, stabilizing equivalence ratios and limiting temperature hot-spots, while CuO and Al₂O₃ primarily enhanced atomization and mixing, yielding consistent CO and HC co-benefits with small EGT decreases. The study established a load-concentration window at high load and 100 ppm that simultaneously raised efficiency and curtailed NOx, providing a practical dosing map for biodiesel use.