Waste tire oil in quaternary fuel blends: Optimizing performance and emissions in a light-duty diesel engine using response surface methodology

• Quaternary fuel blend optimization for diesel engines. • Response surface methodology for multi-response optimization. • WTO increased emissions, especially at higher blend ratios. • Optimized blend cut BSFC by 14% and CO by 22.7% vs. baseline. • WTO offers economic and environmental gains by reducing waste and fossil usage. Valorizing waste tire oil helps reduce landfill and carbon emissions. This study introduces an optimized quaternary blend of diesel, palm oil biodiesel (POB), diethyl ether (DEE), and waste tire oil (WTO) to enhance combustion and lower emissions in a light-duty diesel engine. The inclusion of WTO and DEE offers both opportunities and challenges in achieving a balanced formulation. Response surface methodology (RSM) with a central composite design (CCD) was employed to investigate and optimize the effects of three key input variables, DEE volume fraction, WTO volume fraction, and engine load, on brake specific fuel consumption (BSFC), and emissions of carbon monoxide (CO), nitrogen oxides (NO x ), and particulate matter (PM). The experimental results revealed that increasing engine load improved combustion efficiency and reduced CO and PM emissions. However, excessive WTO increased BSFC and pollutant emissions due to its inferior combustion characteristics. DEE showed a non-linear influence, reducing BSFC and CO at low concentrations but raising NO x and PM at higher levels. The optimized blend, 2.091 vol% DEE, 5.000 vol% WTO, at 36.367% load, achieved a 14% reduction in BSFC and a 22.7% reduction in CO compared to the baseline B20 blend, although NO x and PM slightly increased. This work offers key insights into the interactions of bio-derived, oxygenated, and waste-based fuel components. The effective use of RSM highlights its value in multi-variable optimization. Future research should explore engine durability, unregulated emissions, and techno-environmental viability to advance quaternary blends for sustainable diesel combustion.