Optimization of Spirulina biodiesel-ammonium hydroxide blends with exhaust gas recirculation for enhanced diesel engine performance and emission reduction

• Optimized Spirulina biodiesel-ammonium hydroxide blends for cleaner diesel engines. • Box–Behnken design identified 15.2 % MES, 14.9 % AH, and 5.2 % EGR as optimal blend. • Ammonium hydroxide significantly improves engine performance metrics. • Spirulina biodiesel reduces emissions, supported by ANOVA and validation tests. • Carbon nanotubes enhance ammonia decomposition, improving fuel efficiency. This study optimized the volume concentrations of methyl ester of Spirulina microalgae (MES), ammonium hydroxide (AH), and exhaust gas recirculation (EGR) percentage in diesel fuel to achieve sustainable, efficient, and clean energy. MES was produced through the transesterification of algae biomass derived from wastewater treatment, and all fuel samples were enriched with 100 ppm of carbon nanotubes (CNT) to reduce ammonia decomposition activation energy. A Box-Behnken design was employed to formulate the fuel matrix, and each sample was tested in a single-cylinder diesel engine under maximum load conditions. Analysis of variance (ANOVA) revealed that AH concentration had the most significant influence on performance (contributing ∼73 % to the model), while MES concentration had the greatest impact on emissions (∼49 %). The optimal blend, 15.2 % MES, 14.9 % AH, and 5.2 % EGR, achieved a balanced improvement in engine performance and emission reduction. Confirmation experiments validated the model predictions, showing a 95 % confidence interval alignment with experimental data and high R 2 values, confirming statistical reliability. These findings demonstrate that optimized MES-AH-EGR blends can significantly enhance engine efficiency while reducing environmental impact, offering a viable pathway for sustainable diesel engine operation.