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This study investigates the effects of waste cooking oil (WCO) biodiesel–diesel blends on engine performance, combustion efficiency, and exhaust emissions under varying operating conditions. Experimental investigations were conducted using three fuel types—pure diesel, Diesel WCO20, and Diesel WCO40—by systematically evaluating the effects of load, engine speed (RPM), and air–fuel ratio (k). The results demonstrated that increasing the biodiesel fraction significantly improved combustion quality and reduces harmful emissions. Compared with conventional diesel, WCO20 and WCO40 blends decreased carbon monoxide (CO) emissions by up to 71%, carbon dioxide (CO2) by 11%, and particulate number (PN) by nearly six times. A moderate increase in NOx emissions was observed for WCO20 due to higher in-cylinder temperatures; however, NOx levels decreased again for WCO40 because of slower combustion and lower temperature peaks. Brake thermal efficiency (BTE) increased by 7.4%, while brake-specific fuel consumption (BSFC) exhibited a nonlinear
dependence on biodiesel concentration—initially increasing for WCO20 and later decreasing for WCO40, reaching 254 g kWh-1. Statistical tests (Kruskal–Wallis, p \ 0.05) confirmed significant differences among fuel types, especially for CO, NO, and PN emissions. Statistical analysis and multivariate regression modeling were applied to evaluate the influence of
engine load, speed, and excess air ratio (k) on combustion efficiency and emission characteristics. The developed models demonstrated high predictive capability, with coefficients of determination (R2) reaching up to 0.99. Overall, the WCO40 blend provided a favorable balance between engine efficiency and emission reduction, although moderate increases in NOx and slight variations in HC emissions were observed under certain operating conditions between engine efficiency and emission reduction, proving its potential as a sustainable and environmentally friendly diesel substitute without requiring engine modifications.
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