<p>This study presents a numerical investigation into the influence of first, second, and third-generation biodiesel fuels on combustion behavior, thermal performance, and emission characteristics of a single-cylinder diesel engine. Biodiesel samples representative of blends (B20) of soybean, castor, beef tallow, and algae along with a waste plastic oil blend (WPO30) were technically assessed by Diesel-RK (v4.3.0.189) software. The results highlight the fact that fuel physico-chemical properties especially viscosity, oxygen content, and cetane number determine intimately the interaction of spray atomization, combustion efficiency, and formation of emissions. Although biodiesel blends showed larger droplet sizes than diesel, they had a comparable or even better combustion performance because their oxygen content allowed improved oxidation in the cylinder. This in turn caused a decrease in combustion products resulting from incomplete burning while at the same time working on the level of fuel consumption depending on the differences of energy density and volatility among the feedstocks. Overwhelmingly, biodiesel blends produced a significant rise in NO₂ emissions, which was explained by the presence of higher temperatures and more oxygen in locations, while WPO30 oil went opposite presumably due to its difference in chemical composition and combustion character. Differences in CO₂ emissions in part showed differences in carbon in fuels and how thoroughly they burned. The findings revealed the BSFC of 20% SME is 0.2578 kg kWh⁻¹, while it is 0.2321, 0.2563, 0.2392, and 0.2468&#xa0;kg kWh⁻¹ for 20% CME, 20% BOME, 20% AME, and 30% WPO, correspondingly. While a 20% blending ratio (B20) was consistently selected for all biodiesel fuels based on its well-established balance between performance, combustion stability, and emission benefits reported in the literature, waste plastic oil (WPO) was evaluated at a higher blending ratio (30%) due to its distinct physicochemical properties and fuel behavior. The NO_x was raised by 2.3599%, 47.3198%, 7.4023%, &amp; 37.1706% for 20% SME, 20% CME, 20% BOME, &amp; 20% AME, respectively, except in WPO, decreasing by 8.61% for 30% WPO as compared to neat diesel. The CO₂ was enhanced for 20% (SME, BOME, AME) and 30% WPO by 10.82314%, 10.48159%, 2.38268% and 5.1062%, respectively, while it was decreased for 20% CME by 0.4085%. Overall, the study demonstrates that emission and performance outcomes are governed by a balance between spray characteristics and fuel-bound oxygen effects, with chemical properties often outweighing atomization limitations. The findings provide deeper insight into the spray–combustion–emission relationship and support the suitability of selected biodiesel blends as partial diesel substitutes under optimized conditions.</p>