The environmental impacts and limited reserves of fossil fuels have increasingly highlighted the importance of alternative fuel sources. In this study, the effects of a sunflower oil biodiesel–diesel blend on the performance, fuel consumption, and exhaust emissions of a single-cylinder diesel engine were experimentally investigated. During the tests, pure diesel fuel (B0) and a blend containing 90% diesel and 10% sunflower biodiesel (B10) were evaluated under various load conditions. After the engine reached steady-state operating temperature, power output, fuel consumption, and exhaust emissions were measured. The findings indicate that the biodiesel blend does not adversely affect engine power and provides performance comparable to pure diesel at all load levels. While fuel consumption was similar at low and medium loads, a measurable reduction was observed at high loads when using the biodiesel blend, which is attributed to the improved combustion associated with the fuel’s higher oxygen content. Regarding exhaust emissions, the addition of biodiesel resulted in reductions in CO and HC emissions, whereas a slight increase in NOₓ emissions was observed. This increase is attributed to higher combustion temperatures and is considered manageable through appropriate emission-control strategies. In conclusion, the sunflower biodiesel blend enhances fuel efficiency while maintaining engine performance and offering environmental benefits; however, additional measures may be required to mitigate elevated NOₓ emissions.