Patent ID: 7762067

Claim:
A method for regulating exhaust gas flow to a turbocharger comprising: providing a turbine housing defining a bore extending along a downstream axial direction and defining a chamber surrounding the bore for receiving exhaust gas, and a nozzle leading from the chamber generally radially inwardly into the bore; providing a turbine wheel rotatably mounted in the bore and comprising a hub and a plurality of turbine blades extending generally radially outwardly from the hub and terminating in blade tips; providing a generally ring-shaped divider in the nozzle that divides the nozzle into a first-stage nozzle and a second-stage nozzle, the first-stage nozzle being delimited between a fixed member of the turbine assembly and the divider, the divider having an upstream surface guiding exhaust gas flowing through the first-stage nozzle and an opposite downstream surface guiding exhaust gas flowing through the second-stage nozzle, the divider having a radially innermost edge, the first-stage nozzle and second-stage nozzle each receiving exhaust gas from the chamber and each directing the exhaust gas radially inwardly onto the blade tips of the turbine wheel; providing a tubular piston disposed in the bore of the turbine housing and being axially slidable therein, the piston having a radially inner surface extending along the axial direction and having a non-flat upstream end surface, with a first radius, that includes a contact region for contacting a portion of the divider with the piston in a closed position so as to substantially close the second-stage nozzle, and movement of the piston in the downstream axial direction opening the second-stage nozzle; and configuring the piston to include a curved flow-guiding surface with a second radius proceeding from the contact region to the inner surface, the flow-guiding surface turning from a generally radially inward direction to the downstream axial direction so as to assist in causing the exhaust gas to execute said change in direction, wherein the blade tips define a contour, in a radial-axial projection, having a portion generally aligned with the second-stage nozzle that is concave in a radially outward direction, and the curved flow-guiding surface of the piston is generally complementary and parallel to said portion of the contour of the blade tips.