Patent ID: 7581394

Claim:
A method for operating an internal combustion engine, the method comprising the steps of: providing a first turbocharger and a second turbocharger arranged in parallel with respect to the engine, wherein the second turbocharger comprises a turbine comprising a turbine wheel disposed within a turbine housing, the turbine housing receiving exhaust gas from an engine, the turbine having an annular nozzle defined between an inboard wall and an outboard wall for guiding the exhaust gas to the turbine wheel, wherein the annular nozzle comprises a variable nozzle device comprising: a plurality of vanes mounted on the inboard wall and extending into the annular nozzle, the inboard wall and the vanes being fixed; and an axially movable, tube-shaped piston disposed within the turbine housing and having a piston end that forms the outboard wall of the annular nozzle such that the outboard wall is axially movable for varying an axial width of the annular nozzle, the piston end being stepped such that an annular first portion of the piston end extends axially farther toward the inboard wall than does a second portion of the piston end, the piston being axially movable in one direction into a fully closed position in which the first portion of the piston end passes along the radial outside or inside of the vanes and contacts the inboard wall so as to completely close the annular nozzle, and being axially movable in an opposite direction into partially open and fully open positions in which the first portion of the piston end is spaced from the inboard wall; completely closing the variable nozzle device of the second turbocharger when the engine is operating below a certain rotational speed, such that only the first turbocharger works to supercharge the engine; and opening the variable nozzle device of the second turbocharger when the engine is operating above said certain rotational speed.