In this study, port injection single-cylinder HCCI engines were used to conduct an experimental investigation into the impact of diesel-Eucalyptus fuel blends on HCCI engine performance and exhaust pollutants. In the tests, eucalyptus and diesel were blended in a variety of volume ratios as test fuels, including 20% eucalyptus to 80% diesel (Eu20/D80), 40% eucalyptus to 60% diesel (Eu40/D60), 60% eucalyptus to 40% diesel (Eu60/D40), and 100% diesel.