Enhancement of CI Engine Performance and Emission Characteristics Using SiO₂ Nanoparticle-Enriched Biofuel Derived from Waste Paint and Prosopis Juliflora

This study presents an integrated approach to converting hazardous industrial paint waste and Prosopis juliflora biomass into renewable fuel via catalytic hydrothermal liquefaction (HTL) and enhancing its combustion performance using silicon dioxide (SiO₂) nanoparticles. The HTL process, optimized at 420°C with 4 wt% acid-activated bentonite catalyst and a 2:1 biomass-to-waste paint ratio, achieved a maximum bio-oil yield of 49.26 wt%. The resulting bio-oil exhibited favorable physicochemical properties, including a high heating value (45.22 MJ/kg), low viscosity (33.29 cSt at 40°C), and minimal oxygen content (1.84 wt%), confirming its suitability for compression ignition (CI) engine applications. Engine trials were conducted using a single-cylinder, four-stroke CI engine operating on diesel-biodiesel blends (DB20 and DB50), with and without the addition of 100 ppm SiO₂ nanoparticles. The SiO₂-enhanced DB20 blend demonstrated superior combustion behavior, marked by increased cylinder pressure, higher net heat release, and a sharper rate of pressure rise. Performance metrics improved significantly, with Brake Thermal Efficiency (BTE) increasing by 7% and Brake Specific Fuel Consumption (BSFC) decreasing by 8% compared to the baseline DB20 blend. Emission analysis revealed notable reductions in carbon monoxide (CO), hydrocarbons (HC), and smoke opacity by 30–45%, attributable to the improved oxidation kinetics and thermal conductivity imparted by the SiO₂ nanoparticles. A marginal increase in nitrogen oxides (NOₓ) was observed due to elevated in-cylinder temperatures. Overall, the study confirms that HTL-derived bio-oil from industrial waste paints and P. juliflora, when blended with diesel and enhanced with SiO₂ nanoparticles, can serve as a high-performance, low-emission alternative fuel for CI engines. This approach offers a sustainable solution for waste valorization, emission reduction, and energy diversification in line with circular economy and green energy goals.