Waste cooking oil valorization through aqueous‐ammonia emulsification and exhaust gas recirculation: Multi‐response optimization of diesel engine efficiency and emissions

Waste cooking oil (WCO) biodiesel offers a circular alternative to diesel, although its higher blending levels can introduce performance–emission trade‐offs. The present work deals with the multi‐response optimization of a compression ignition engine using a combined approach of biodiesel from waste cooking oil (WCO), aqueous ammonia (AH), and exhaust gas recirculation (EGR). The experimental design selected was a three‐factor central composite design containing 19 runs. The factors were AH concentration, WCO biodiesel content and EGR rate and were modified in the range of 1.59–18.41 vol.%, 3.18–36.82 vol.% and 1.59%–18.41%, respectively. All tests were performed at 1500 rpm and full load (BMEP of 4.14 bar). Response surface methods were used to predict brake specific fuel consumption (BSFC), brake thermal efficiency (BTE), HC, CO, NO x and CO 2 . Multi‐response optimization yielded 9.11 vol.% of AH, 15.14 vol.% of WCO biodiesel, and 5.74% of EGR as the optimal condition. The associated predicted responses were 0.293 kg/kWh BSFC, 31.03% BTE, 29.88 ppm HC, 0.33 vol% CO, 1133 ppm NO x and 11.44 vol% CO 2 . Confirmation experiments demonstrated satisfactory agreement with these predictions with uncertainties of 0.29%–3.03%. The optimized condition maintained comparable BTE with diesel while reducing HC, CO and NO x by 8.75%, 5.88% and 5.20%, respectively; however, BSFC and CO 2 increased. Results indicate that coordinated AH‐WCO‐EGR optimization can partially replace diesel while maintaining a balance between engine efficiency and critical engine‐out emissions.