Abstract
By means of EGR, it is possible to reduce NOx emissions, but smoke occurs to form at the same time. For suppressing NOx and smoke simultaneously, it is recognized that another combustion strategy is required to avoid local over-rich regions in the mixture at EGR conditions. In the present study, a variable valve timing (VVT) mechanism is applied at medium engine speed and load in a light duty HSDI diesel engine. By means of late intake valve closing (LIVC), compressed gas temperatures near the top dead center (TDC) are lowered, thereby preventing too early ignition and increasing ignition delay to enhance fuelair 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. Simultaneous reduction in both NOx and smoke is made possible by the combustion concept applied in this study, which is called Miller-PCI (Miller-Premixed Compression Ignition) combustion. NOx and smoke suppression mechanism in the Miller- PCI combustion is analyzed using a 3D-CFD code combined with detailed chemistry. LIVC can achieve a significant NOx and smoke reduction due to lowering combustion temperatures (LTC: Low Temperature Combustion) and avoiding local over-rich regions in the mixtures, respectively.
Keywords:Premixed Diesel Combustion, Miller-PCI Combustion, VVT, Effective Compression Ratio, Late Intake Valve Closing (LIVC), -T map, Emissions