A comprehensive RSM approach for modeling and optimization of performance and emission characteristics of a diesel engine fueled with waste tire pyrolysis oil and transformer oil blends

In this study, waste transformer oil (WTRO) and waste tire pyrolysis oil (WTPO) were mixed with standard diesel fuel in different volumetric ratios to prepare four fuel samples (D100, WTRO25WTPO25, WTPO50, WTRO50). In the first stage, the fuels were characterized physicochemically thru density (832.3–862.3 kg/m3), kinematic viscosity, cloud/flow point, lower heating value, and FT-IR spectroscopy; all mixtures except for WTPO50 (1.77 mm2/s) meet the ASTM D975 viscosity limits (1.9–6.0 mm2/s). In the second phase, performance and exhaust emission tests were conducted under full load conditions in a single-cylinder, four-stroke, air-cooled compression ignition (CI) engine within the 1200–2400 rpm range. Experimental data were analyzed using second-order multi-term Response Surface Methodology (RSM) models; high statistical consistency was achieved for the response variables BTE, BSFC, CO, and NOx with R2 = 0.964–0.978 (p < 0.001). ANOVA results revealed the dominant determining position of engine speed in performance parameters, while Pareto analyzes showed that the Speed·WTRO interaction term was statistically significant in emission parameters. In terms of engine performance, the WTRO50 blend increased torque and brake power by 8.9% and 8.7% respectively at 1200 rpm compared to D100, while reducing BSFC by 4.3%. WTPO50, due to its low lower heating value (40.74 MJ/kg) and high aromatic content, reached the highest BSFC value (486.01 g/kWh) at 2400 rpm. The WTRO25WTPO25 triple mixture exhibited a balanced emission profile while producing performance values close to D100. Using the Derringer–Suich suitability function and the Differential Evolution algorithm, scenario-based weighted multi-criteria optimization was performed under four priority scenarios (Base, Performance, Economy, Emission-Focused). Overall fitness values remained within a narrow band (D = 0.346–0.535; ΔD = 0.189), however, the optimal engine speed exhibited a statistically significant shift of 832.6 rpm between scenarios (Performance Scenario S-2: 1542.9 rpm; Emission-Focused Scenario S-4: 2375.5 rpm). In all optimization scenarios, the optimal WTRO ratio was determined to be 50% and WTPO = 0%; this finding statistically proves the inadequacy of raw and unprocessed WTPO compared to WTRO in terms of performance-emission balance. The study presents three original engineering contributions: (i) the first RSM-based study evaluating WTRO and WTPO as simultaneous dual diesel replacements; (ii) the first quantitative demonstration of an optimal speed shift of 832.6 rpm between performance-prioritized and emission-prioritized scenarios; (iii) the statistical proof that raw WTPO requires pre-processing under all four scenarios. The findings show that WTRO offers a technically viable waste valorization approach as a diesel substitute up to 50% and provides measurable CO and NOx emission benefits. HC, PM, and soot emissions could not be measured as they were beyond the measurement capacity of the Testo 350 analyzer used in this study; a comprehensive environmental assessment (HC/PM/soot measurement and life cycle analysis) has been left for future studies.