HCCI/PCCI combustion concepts have been demonstrated for both high brake thermal efficiency and low engine-out emissions. However, these advanced combustion concepts still could not be fully utilized partially due to the limitations of conventional fixed spray angle nozzle designs for issues related to wall wetting for early injections.The micro-variable circular orifice (MVCO) fuel injector provides variable spray angles, variable orifice areas, and variable spray patterns. The MVCO provides optimized spray patterns to minimize combustion chamber surface-wetting, oil dilution and emissions. Designed with a concise structure, MVCO can significantly extend the operation maps of high efficiency early HCCI/PCCI combustion, and enable optimization of a dual-mode HCCI/PCCI and Accelerated Diffusion Combustion (ADC) over full engine operating maps. The MVCO variable spray pattern characteristics are analyzed with high speed photographing. Spray atomization profiles, such as droplet size distributions and velocity profiles are measured using PDA. The measurement results of using both diesel and gasoline fuels are presented and compared. Potential merits of the MVCO sprays for highly down-sized internal combustion engines are discussed based on spray and atomization characteristics. The combustion characteristics of using MVCO are studied with KIVA simulation.The impacts of the new spray patterns and characteristics on engine performance and emissions are discussed. The new MVCO can significantly reduce emissions and enhance engine efficiency through following means: (1) Providing flexible spray angles and spray patterns to match moving combustion chamber geometry and in-cylinder conditions at different injection timings, simultaneously reduce soot, NOx, HC and CO; (2) Provide variable orifice exit areas for different engine loads and injection timings; (3) Enabling low temperature premixed combustion with homogeneous atomization; (4) Enabling dual-mode early PCCI-ADC combustion through adaptive spray patterns.