Patent ID: 8197195

Claim:
A turbine assembly for a turbocharger, comprising: a turbine housing defining a bore and defining a chamber surrounding the bore for receiving exhaust gas to be directed into a turbine wheel disposed in the bore, a radially inner side of the chamber having an axial length; and a variable nozzle having stepped two-stage vanes, the variable nozzle comprising: a tubular piston disposed in the bore of the turbine housing such that the piston is axially slidable relative to the turbine housing along the radially inner side of the chamber such that the piston blocks a variable portion of the axial length of the chamber depending on axial position of the piston, the piston having an upstream end and a downstream end with respect to a flow direction of exhaust gas along the bore of the turbine housing; and a generally annular wall that extends generally radially inwardly adjacent an upstream end of the axial length of the chamber, and an array of circumferentially spaced vanes having fixed ends joined to the generally annular wall and opposite free ends, the vanes extending across the axial length of the chamber, the vanes each having an outer surface that faces generally radially outwardly and an opposite inner surface that faces generally radially inwardly, wherein each vane defines a first vane stage proximate the fixed end of the vane and a second vane stage proximate the free end of the vane, wherein the first vane stage is contoured to guide the exhaust gas to exit the first vane stage with a first flow angle, and the second vane stage is contoured to guide the exhaust gas to exit the second vane stage with a second flow angle different from the first flow angle, each vane comprising a step transitioning from the first vane stage to the second vane stage, and wherein the piston in a closed position closes the second vane stage so that exhaust gas flows only through the first vane stage, the second vane stage being progressively opened as the piston is axially slid toward an open position.