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For diesel engines to meet current and future emissions levels, the amount of EGR required to reach these levels has increased dramatically. This increased EGR has posed big challenges for conventional turbocharger technology to meet the higher emissions requirements while maintaining or improving other vehicle attributes, to the extent that some OEMs resort to multiple turbocharger configurations. These configurations can include parallel, series sequential, or parallel - series turbocharger systems, which would inevitably run into other issues, such as cost, packaging, and thermal loss, etc. This study, as part of a U.S. Department of Energy (USDoE) sponsored research program, is focused on the experimental evaluation of the emission and performance of a modern diesel engine with an advanced single stage turbocharger.

Variable nozzle turbine (VNT) technology has become a popular technology for diesel engine application. To pivot the nozzle vane and adjust the turbine operating condition, nozzle clearances are inevitable on both the hub and shroud side of turbine housing. Leakage flow formed inside the nozzle clearance leads to extra flow loss and makes the nozzle exit flow less uniform, thus further affects downstream aerodynamic performance of the rotor. As the leakage mixing with nozzle wake flow, the process is highly unsteady, which increases the fluctuation amplitude of transient load on the rotating turbine wheels. In present paper, firstly steady CFD analysis of a turbocharger turbine was performed at different nozzle openings. Then unsteady simulation of the turbine was carried out to investigate the interaction between the leakage flow through nozzle clearance and the main flow. Nozzle clearance's effect on turbine performance was investigated.