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A turbocharger, having an axial compressor wheel and an axial turbine wheel mounted on a first shaft supported by a housing, and a radial compressor wheel and a radial turbine wheel mounted on a second shaft, the second shaft concentrically extending around the first shaft and being supported by the

A turbocharger, having an axial compressor wheel and an axial turbine wheel mounted on a first shaft supported by a housing, and a radial compressor wheel and a radial turbine wheel mounted on a second shaft, the second shaft concentrically extending around the first shaft and being supported by the housing. The housing defines a first duct extending axially from the exducer of the axial compressor to the inducer of the radial compressor, and a second duct extending axially from the exducer of the radial turbine to the inducer of the axial turbine. A plurality of controllable compressor guide vanes extend through the first duct, and a plurality of controllable turbine stator vanes extend through the second duct. The housing is provided with variable diffuser vanes at the exducer of the radial compressor, and with variable turbine vanes at the inducer of the radial turbine. The variable turbine vanes and the turbine stator vanes are controlled to accurately control the rotation rate of the radial and axial turbines. The compressor guide vanes are controlled to minimize surge and maximize choke flow rate.

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What is claimed is: 1. A turbocharger, comprising: a housing; a first compressor wheel and a first turbine wheel mounted on a first shaft supported by the housing; a second compressor wheel and a second turbine wheel mounted on a second shaft supported by the housing, wherein the first and the seco

What is claimed is: 1. A turbocharger, comprising: a housing; a first compressor wheel and a first turbine wheel mounted on a first shaft supported by the housing; a second compressor wheel and a second turbine wheel mounted on a second shaft supported by the housing, wherein the first and the second shafts extend concentrically to define an axial direction, and wherein the housing defines a fully enclosed axial duct, through which all air flows from the first compressor wheel to the second compressor wheel, extending axially between the first and second compressor wheels; and a plurality of compressor guide vanes extending through the axial duct intermediate the first and second compressor wheels; wherein the first compressor wheel is an axial compressor wheel configured to direct airflow axially through the axial duct toward the second compressor wheel. 2. The turbocharger of claim 1, wherein one of the first and the second shafts concentrically extends through the other. 3. The turbocharger of claim 2, wherein the second compressor wheel is a radial compressor wheel. 4. The turbocharger of claim 3, wherein the compressor guide vanes are configured to be controllably positioned at a plurality of angles, and further comprising a control system configured to control the angle of the compressor guide vanes such that at low speeds the compressor guide vanes direct airflow into the second compressor wheel in a swirling direction compatible with a blade angle of the second stage compressor blades, and at high speeds the compressor guide vanes direct airflow into the second compressor wheel in an axial or near-axial direction. 5. The turbocharger of claim 1, wherein the compressor guide vanes are configured to be controllably positioned at a plurality of angles, and further comprising a control system configured to control the angle of the compressor guide vanes such that at low speeds the compressor guide vanes direct airflow into the second compressor wheel in a swirling direction compatible with a blade angle of the second stage compressor blades, and at high speeds the compressor guide vanes direct airflow into the second compressor wheel in an axial or near-axial direction. 6. The turbocharger of claim 1, wherein: wherein the housing defines a second axial duct extending between the first and second turbine wheels; and the second turbine wheel is a radial turbine wheel configured to direct airflow axially through the second axial duct toward the first turbine wheel. 7. The turbocharger of claim 6, wherein the first turbine wheel is an axial turbine wheel. 8. The turbocharger of claim 6, and further comprising a plurality of turbine stator vanes extending through the second axial duct intermediate the first and second turbine wheels. 9. The turbocharger of claim 8, wherein the plurality of compressor guide vanes and the plurality of turbine stator vanes are each configured to be controllably positioned at a plurality of angles, and further comprising a control system configured to control the angles of the plurality of compressor guide vanes such that at low speeds the compressor guide vanes direct airflow into the second compressor wheel in a swirling direction compatible with a blade angle of the second stage compressor blades, and at high speeds the compressor guide vanes direct airflow into the second compressor wheel in an axial or near-axial direction, and configured to control the speeds of the turbine wheels by controlling the angles of the plurality of turbine stator vanes. 10. The turbocharger of claim 1, and further comprising: a plurality of compressor guide vanes; a plurality of turbine stator vanes that are configured to be controllably positioned at a plurality of angles; a plurality of turbine nozzle vanes upstream of the second turbine wheel, and a control system; wherein the first turbine wheel is an axial turbine wheel; wherein the second compressor wheel is a radial compressor wheel; wherein the second turbine wheel is a radial turbine wheel; wherein the first shaft concentrically extends through the second shaft; wherein the housing defines a second axial duct extending between the first and second turbine wheels; wherein the second turbine wheel is configured to direct airflow axially through the second axial duct toward the first turbine wheel; wherein the plurality of turbine stator vanes extending through the second axial duct intermediate the first and second turbine wheels; wherein the plurality of compressor guide vanes and the plurality of turbine stator vanes are each configured to be controllably positioned at a plurality of angles; wherein the control system configured to control the angles of the plurality of compressor guide vanes such that at low speeds the compressor guide vanes direct airflow into the second compressor wheel in a swirling direction compatible with a blade angle of the second stage compressor blades, and at high speeds the compressor guide vanes direct airflow into the second compressor wheel in an axial or near-axial direction, and configured to control the speeds of the turbine wheels by controlling the angles of the plurality of turbine stator vanes; and wherein the turbine nozzle vanes are configured to be controllably positioned at a plurality of angles, wherein the control system is configured to independently control the speed of each turbine wheel by controlling the angles of the turbine stator vanes and the turbine nozzle vanes. 11. A turbocharger, comprising: a housing; a first compressor wheel and a first turbine wheel mounted on a first shaft supported by the housing; a second compressor wheel and a second turbine wheel mounted on a second shaft supported by the housing, wherein the first and the second shafts extend concentrically to define an axial direction, and wherein the housing defines a duct extending axially between the first and second turbine wheels; a control system; a plurality of turbine stator vanes extending through the axial duct intermediate the first and second turbine wheels, the turbine stator vanes being configured to be controllably positioned at a plurality of angles; and a plurality of turbine nozzle vanes upstream of the second turbine wheel, wherein the turbine nozzle vanes are configured to be controllably positioned at a plurality of angles, wherein the control system is configured to independently control the speed of each turbine wheel by controlling the angles of the turbine stator vanes and the turbine nozzle vanes; wherein the second turbine wheel is a radial turbine wheel configured to direct airflow axially through the axial duct toward the first turbine wheel; and; wherein one of the first and the second shafts concentrically extends through the other. 12. The turbocharger of claim 11, wherein the first turbine wheel is an axial turbine wheel. 13. The turbocharger of claim 12, and further comprising a control system configured to control the relative speeds of the turbine wheels by controlling the angle of the turbine stator vanes. 14. The turbocharger of claim 11, and further comprising a control system configured to control the relative speeds of the turbine wheels by controlling the angle of the turbine stator vanes. 15. A method for controlling a turbocharger that includes a first compressor wheel and a first turbine wheel mounted on a first shaft supported by a housing, a second compressor wheel and a second turbine wheel mounted on a second shaft supported by the housing, and a plurality of compressor guide vanes, wherein the housing defines a fully enclosed duct, through which all air flows from the first compressor wheel to the second compressor wheel, extending between the first and second compressor wheels, and wherein the compressor guide vanes extend through the duct intermediate the first and second compressor wheels, comprising: actuating the plurality of compressor guide vanes through a plurality of positions, the positions being configured such that at low speeds the compressor guide vanes direct airflow into the second compressor wheel in a swirling direction compatible with a blade angle of the second stage compressor blades, and at high speeds the compressor guide vanes direct airflow into the second compressor wheel in an axial or near-axial direction. 16. A control system for controlling a turbocharger that includes a first compressor wheel and a first turbine wheel mounted on a first shaft supported by a housing, a second compressor wheel and a second turbine wheel mounted on a second shaft supported by the housing, a plurality of compressor guide vanes, and an actuator, wherein the housing defines a fully enclosed duct, through which all air flows from the first compressor wheel to the second compressor wheel, extending between the first and second compressor wheels, wherein the compressor guide vanes extend through the duct intermediate the first and second compressor wheels, and wherein the actuator is configured to actuate the plurality of guide vanes through a plurality of positions, comprising: a processing unit configured to transmit signals to the actuator such that the actuator actuates the plurality of compressor guide vanes through a selection of the plurality of positions, the selection of the plurality of positions being configured such that at low speeds the compressor guide vanes direct airflow into the second compressor wheel in a swirling direction compatible with a blade angle of the second stage compressor blades, and at high speeds the compressor guide vanes direct airflow into the second compressor wheel in an axial or near-axial direction. 17. The method of claim 15, wherein the turbocharger further includes a plurality of turbine stator vanes, wherein the housing defines a second duct extending between the first and second turbine wheels, and wherein the turbine stator vanes extend through the second duct intermediate the first and second turbine wheels, and further comprising: controlling the relative speeds of the turbine wheels by controlling the angles of the plurality of turbine stator vanes. 18. The control system of claim 16, wherein the turbocharger further includes a plurality of turbine stator vanes and a second actuator configured to actuate the plurality of turbine stator vanes through a plurality of positions, wherein the housing defines a second duct extending between the first and second turbine wheels, and wherein the turbine stator vanes extend through the second duct intermediate the first and second turbine wheels, wherein: the processing unit is further configured to control the relative speeds of the turbine wheels to desired speeds by transmitting signals to the second actuator such that the angles of the plurality of turbine stator vanes cause the turbine wheels to rotate at the desired speeds. 19. A control system for controlling a turbocharger for an engine, the turbocharger including a housing, a first rotor including a first compressor wheel and a first turbine wheel mounted on a first shaft, a second rotor including a second compressor wheel and a second turbine wheel mounted on a second shaft, a plurality of turbine stator vanes, and an actuator, wherein the first rotor has a significantly lower inertia than the second rotor, wherein the first and second shafts are supported by and concentrically coextend within the housing, wherein the housing defines a duct extending between the first and second turbine wheels, wherein the turbine stator vanes extend through the duct intermediate the first and second turbine wheels, and wherein the actuator is configured to actuate the plurality of stator vanes through a plurality of positions, comprising: a processing unit configured to transmit signals to the actuator such that the actuator actuates the plurality of turbine stator vanes through the plurality of positions to control the relative rotation rates of the turbines according to operating conditions of the engine; wherein the processing unit is configured to transmit signals to initially increase the rotation rate of the first turbine wheel relative to the second turbine wheel upon engine acceleration. 20. The control system of claim 19, wherein the processing unit is configured to transmit signals to increase the rotation rate of the second turbine wheel relative to the first turbine wheel upon operating conditions reaching high speeds. 21. The control system of claim 20, wherein the processing unit is configured to transmit signals that modulate the engine back-pressure relative to the engine inlet boost level such that the required level of Exhaust Gas Recirculation is driven at all engine operating conditions. 22. The turbocharger of claim 1, and further comprising: a plurality of turbine stator vanes extending through an axial duct intermediate the first and second turbine wheels, the turbine stator vanes being configured to be controllably positioned at a plurality of angles; and a plurality of turbine nozzle vanes upstream of the second turbine wheel, the turbine nozzle vanes being configured to be controllably positioned at a plurality of angles; wherein the control system is configured to independently control the speed of each turbine wheel by controlling the angles of the turbine stator vanes and the turbine nozzle vanes. 23. The turbocharger of claim 1, and further comprising: a plurality of turbine stator vanes extending through an axial duct intermediate the first and second turbine wheels, the turbine stator vanes being configured to be controllably positioned at a plurality of angles; a plurality of turbine nozzle vanes upstream of the second turbine wheel, the turbine nozzle vanes being configured to be controllably positioned at a plurality of angles; and a processing unit configured to transmit signals to the actuator such that the actuator actuates the plurality of turbine stator vanes through the plurality of positions to control the relative rotation rates of the turbines according to operating conditions of the engine; wherein the first rotor has a significantly lower inertia than the second rotor; and wherein the processing unit is configured to transmit signals to initially increase the rotation rate of the first turbine wheel relative to the second turbine wheel upon engine acceleration. 24. The turbocharger of claim 23, wherein the control system is configured to independently control the speed of each turbine wheel by controlling the angles of the turbine stator vanes and the turbine nozzle vanes. 25. The method of claim 15, wherein the turbocharger further includes a plurality of turbine stator vanes extending through an axial duct intermediate the first and second turbine wheels, the turbine stator vanes being configured to be controllably positioned at a plurality of angles, and a plurality of turbine nozzle vanes upstream of the second turbine wheel, the turbine nozzle vanes being configured to be controllably positioned at a plurality of angles, and further comprising: actuating the plurality of turbine stator vanes and the plurality of turbine nozzle vanes through a plurality of positions, the positions being configured to independently control the speed of each turbine wheel. 26. The method of claim 15, wherein the turbocharger further includes a plurality of turbine stator vanes extending through an axial duct intermediate the first and second turbine wheels, the turbine stator vanes being configured to be controllably positioned at a plurality of angles, a plurality of turbine nozzle vanes upstream of the second turbine wheel, the turbine nozzle vanes being configured to be controllably positioned at a plurality of angles; and wherein the first rotor has a significantly lower inertia than the second rotor, and further comprising: actuating the plurality of turbine stator vanes and the plurality of turbine nozzle vanes through a plurality of positions, the positions being configured to transmit signals to initially increase the rotation rate of the first turbine wheel relative to the second turbine wheel upon engine acceleration. 27. The turbocharger of claim 26, sending control signals to actuators configured to actuate the plurality of turbine stator vanes and the plurality of turbine nozzle vanes through a plurality of positions, the positions being selected to independently control the speed of each turbine wheel. 28. The control system of claim 16, wherein the turbocharger further includes a plurality of turbine stator vanes extending through an axial duct intermediate the first and second turbine wheels, the turbine stator vanes being configured to be controllably positioned at a plurality of angles, and a plurality of turbine nozzle vanes upstream of the second turbine wheel, the turbine nozzle vanes being configured to be controllably positioned at a plurality of angles, wherein: the processing unit is further configured to actuate the plurality of turbine stator vanes and the plurality of turbine nozzle vanes through a plurality of positions, the positions being configured to independently control the speed of each turbine wheel. 29. The control system of claim 16, wherein the turbocharger further includes a plurality of turbine stator vanes extending through an axial duct intermediate the first and second turbine wheels, the turbine stator vanes being configured to be controllably positioned at a plurality of angles, a plurality of turbine nozzle vanes upstream of the second turbine wheel, the turbine nozzle vanes being configured to be controllably positioned at a plurality of angles; and wherein the first rotor has a significantly lower inertia than the second rotor, wherein: the processing unit is further configured to actuate the plurality of turbine stator vanes and the plurality of turbine nozzle vanes through a plurality of positions, the positions being configured to initially increase the rotation rate of the first turbine wheel relative to the second turbine wheel upon engine acceleration. 30. The control system of claim 29, the processing unit is further configured to actuate the plurality of turbine stator vanes and the plurality of turbine nozzle vanes through a plurality of positions, the positions being configured to independently control the speed of each turbine wheel. 31. The turbocharger of claim 11, wherein: the control system is configured to control the angles of the turbine stator vanes and the turbine nozzle vanes through the plurality of positions to control the rotation rates of the turbines according to operating conditions of the engine; the first rotor has a significantly lower inertia than the second rotor; and the control system is configured to control the angles of the turbine stator vanes and the turbine nozzle vanes such that they initially increase the rotation rate of the first turbine wheel relative to the second turbine wheel upon engine acceleration. 32. A control system for controlling a turbocharger that includes a first compressor wheel and a first turbine wheel mounted on a first shaft supported by a housing, a second compressor wheel and a second turbine wheel mounted on a second shaft supported by the housing, a plurality of turbine stator vanes extending through an axial duct intermediate the first and second turbine wheels, the turbine stator vanes being configured to be controllably positioned at a plurality of angles, and a plurality of turbine nozzle vanes upstream of the second turbine wheel, the turbine nozzle vanes being configured to be controllably positioned at a plurality of angles, comprising: a processing unit configured to actuate the plurality of turbine stator vanes and the plurality of turbine nozzle vanes through a plurality of positions, the positions being configured to independently control the speed of each turbine wheel. 33. The control system of claim 32, wherein the first rotor has a significantly lower inertia than the second rotor, wherein: the processing unit is further configured to actuate the plurality of turbine stator vanes and the plurality of turbine nozzle vanes such that they initially increase the rotation rate of the first turbine wheel relative to the second turbine wheel upon engine acceleration. 34. A turbocharger, comprising: a housing; a first compressor wheel and a first turbine wheel mounted on a first shaft supported by the housing; a second compressor wheel and a second turbine wheel mounted on a second shaft supported by the housing, wherein the first and the second shafts extend concentrically through one another to define an axial direction, and wherein the housing defines a duct extending axially between the first and second turbine wheels; a plurality of turbine stator vanes extending through the axial duct intermediate the first and second turbine wheels, the turbine stator vanes being configured to be controllably positioned at a plurality of angles; a plurality of turbine nozzle vanes upstream of the second turbine wheel, wherein the turbine nozzle vanes are configured to be controllably positioned at a plurality of angles; and a control system configured to controllably position the plurality of turbine stator vanes and the plurality of turbine nozzle vanes through their pluralities of positions to control the relative rotation rates of the turbines according to operating conditions of the engine; wherein the first rotor has a significantly lower inertia than the second rotor; and wherein the control system is further configured to controllably position the plurality of turbine stator vanes and the plurality of turbine nozzle vanes to initially increase the rotation rate of the first turbine wheel relative to the second turbine wheel upon engine acceleration.