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A variable valve timing (VVT) mechanism was applied to achieve premixed diesel combustion at higher load for low emissions and high thermal efficiency in a light duty diesel engine. By means of late intake valve closing (LIVC), compressed gas temperatures near the top dead center are lowered, thereby preventing too early ignition and increasing ignition delay to enhance fuel-air mixing. The variability of effective compression ratio has significant potential for ignition timing control of conventional diesel fuel mixtures. At the same time, the expansion ratio is kept constant to ensure thermal efficiency. Combining the control of LIVC, EGR, supercharging systems and high-pressure fuel injection equipment can simultaneously reduce NOx and smoke. The NOx and smoke suppression mechanism in the premixed diesel combustion was analyzed using the 3D-CFD code combined with detailed chemistry.