Y Clove (CEO) and thyme (TEO) essential oils were added to a colza biodiesel–diesel blend (B20) as natural antioxidants, and butylated hydroxytoluene (BHT) was added as a synthetic antioxidant, at 3000 ppm concentration. Fuel properties, instrumental characterizations, engine performance, combustion characteristics, and emissions of the blends were analyzed. Fourier Transform Infrared Spectroscopy (FT-IR) spectra showed that the transmission intensities of samples increased with antioxidant addition. B20 and B20–antioxidant blends were subjected to engine tests under various loads and a fixed engine speed of 1500 rpm to determine the combustion, performance, and emissions. Engine tests revealed that antioxidants caused an increase in specific fuel consumption by 0.9–2.4% and a drop of 1.1–2.2% in thermal efficiency compared to B20. Antioxidants increased in hydrocarbons HC emission due to their oxidation-inhibitive nature, while they provided substantial improvements in smoke opacity and CO2 emission. BHT reduced NO emissions by 21% compared to B20 fuel without antioxidants. BHT was more successful in curbing NO emissions but caused an average increase in CO emissions of 3.68%. CEO-blended fuel offers better engine performance and emissions improvement than TEO-blended fuel.

Biodiesel has several drawbacks, such as being prone to oxidation, having reduced stability, and having limited storage time. Antioxidants compatible with biodiesel are being used to address its drawbacks. Utilizing antioxidants effectively improves the quality of biodiesel. Enhancing the quality of biodiesel for use as a clean energy source benefits both the global economy and ecology. Therefore, we believe that our work will contribute to the advancement of the biodiesel industry worldwide. This study used blends consisting of 20% biodiesel and 80% diesel fuel. Isatin-thiosemicarbazones were tested as additives in blends at a concentration of 3000 parts per million (ppm) using an oxifast device and were compared with the chemical antioxidant Trolox. FT-IR, DSC, and TGA were used to characterize these samples. DSC measured sample crystallization temperatures (Tc). Samples with antioxidants showed decreased values compared to the non-antioxidant diesel sample D100. Several DSC tests were conducted to determine the antioxidant strengths of various samples. The results show that the FT-IR spectrum’s antioxidant effect regions grow clearer with antioxidants. The extra antioxidant is effective. Biodiesel’s oxidative stability improves with isatin-thiosemicarbazones at varying concentrations. The kinetics of thermal decomposition of isatin-thiosemicarbazones under non-isothermal conditions were determined using the Kissinger, Ozawa, and Boswell techniques. The activation energies of compounds 1 and 2 were calculated as 137–147 kJ mol⁻¹ and 173–183 kJ mol⁻¹, respectively.

The extraction of Hypericum perforatum L. (HP) was performed using the Soxhlet extraction method to evaluate its potential as an organic free-radical scavenger in biodiesel-diesel blends. Experimental blends—B100, B20D80, B20D80BHT, and B20D80HP—were prepared, incorporating Hypericum perforatum L. extract at a concentration of 3000 ppm, and compared with butylhydroxytoluene (BHT). The antioxidant properties were assessed using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FT-IR), high-performance liquid chromatography (HPLC), and the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. DSC analysis ranked the antioxidant efficiency as D100 < B20D80 < B20D80BHT < B20D80HP, demonstrating the superior stabilization effect of Hypericum perforatum L. extract. TGA and FT-IR results confirmed enhanced thermal stability, while HPLC identified key phenolic compounds such as rutin, ellagic acid, and kaempferol, which contribute to antioxidant activity. DPPH assays further confirmed the extract’s superior free-radical scavenging efficiency compared to BHT. These findings highlight Hypericum perforatum L. as a promising natural antioxidant for improving biodiesel oxidative stability.