Experimental assessment of hydrogen enrichment and ZnO nanoparticle additives on the performance and emissions of a UTO biodiesel-fuelled CRDI diesel engine

The depletion of fossil fuels and stricter emission norms have intensified the search for renewable and waste-derived CI engine fuels. Used transformer oil (UTO) biodiesel offers a low-cost alternative but suffers from poor atomization and combustion owing to its high viscosity and relatively low calorific value. This study experimentally evaluates the combined effects of hydrogen enrichment (5–15 LPM) and ZnO nano-additives (50, 100 ppm) on the performance and emissions of a CRDI engine fuelled with 20 % UTO biodiesel (UTO20). Results reveal that hydrogen and ZnO addition significantly enhance combustion efficiency: the optimum blend (UTO20 + 15 LPM H₂ + 100 ppm ZnO) achieved a 33.46 % BTE (+14.7 % vs diesel) and a 0.32 kg/kWh BSFC (−28.9 % vs UTO20). CO, HC, and smoke emissions declined by 45–54 %, though NO_x rose by ∼42 %. The study uniquely demonstrates the synergistic effects of catalytic ZnO and hydrogen enrichment on waste-derived biodiesel in a CRDI engine, achieving diesel-surpassing efficiency and reduced carbon emissions, thereby advancing pathways for sustainable CI engine operation.