<p>This study investigates the influence of ethanol and n-dodecane fuel blends on the performance and emissions of HCCI engines, with particular attention to how different ethanol concentrations impact piston work, in-cylinder pressure, temperature, and emissions such as CO₂, CO, NOx, and soot. The findings demonstrate that increasing the ethanol content reduces CO and soot emissions due to the fuel's higher oxygen content and cleaner combustion profile. However, NOx emissions increase, likely resulting from leaner air–fuel mixtures that raise combustion temperatures. The 80% n-dodecane blend exhibits higher peak in-cylinder pressures and temperatures, indicating enhanced combustion efficiency. Ethanol’s higher latent heat of vaporization introduces a cooling effect that can delay its vaporization, promoting better fuel–air mixing but also potentially leading to incomplete combustion and lower in-cylinder pressure. Although ethanol-enriched blends can improve combustion efficiency and reduce certain emissions, the increase in NOx emissions underscores the complexity of balancing engine performance, combustion stability, and emissions control. This demonstrates the trade-off between improved combustion properties and environmental concerns when using fuel blends in HCCI engines.</p>