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A single-shaft exhaust gas-driven turbocharger includes two parallel-flow first-stage centrifugal compressors in series with a single second-stage centrifugal compressor, and a one-stage turbine arranged to drive both the first- and second-stage centrifugal compressors via a single shaft on which th

A single-shaft exhaust gas-driven turbocharger includes two parallel-flow first-stage centrifugal compressors in series with a single second-stage centrifugal compressor, and a one-stage turbine arranged to drive both the first- and second-stage centrifugal compressors via a single shaft on which the compressors and turbine are fixedly mounted. The compressor housing defines from one to a plurality of circumferentially spaced inlet ducts for the second wheel of the first stage, and from one to a plurality of circumferentially spaced interstage ducts leading from a vaneless diffuser of the first stage into the inlet of the second stage.

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1. An exhaust gas-driven turbocharger, comprising: a shaft having a first end and a second end;a one-stage turbine wheel mounted on the shaft proximate the first end;a turbine housing surrounding the turbine wheel and defining ducting for directing exhaust gas into and out of the turbine wheel;a fir

1. An exhaust gas-driven turbocharger, comprising: a shaft having a first end and a second end;a one-stage turbine wheel mounted on the shaft proximate the first end;a turbine housing surrounding the turbine wheel and defining ducting for directing exhaust gas into and out of the turbine wheel;a first-stage compressor wheel mounted on the shaft proximate the second end, the first-stage compressor wheel comprising first and second impellers disposed back-to-back;a second-stage compressor wheel mounted on the shaft at a location intermediate the first-stage compressor wheel and the turbine wheel; anda compressor housing assembly surrounding the first- and second-stage compressor wheels and defining: inlet ducting configured to direct two separate portions of air in generally opposite axial directions into respective inlets of the first and second impellers of the first-stage compressor wheel, wherein the inlet ducting comprises a common duct configured to conduct air for both the first and second impellers of the first-stage compressor wheel, a first duct configured to conduct one portion of the air from the common duct into the inlet of the first impeller, and a plurality of separate second ducts configured to conduct a remaining portion of the air from the common duct into the inlet of the second impeller, wherein the second ducts are circumferentially spaced apart from one another about a rotational axis defined by the shaft;interstage ducting configured to direct pressurized air from an exit of each of the impellers of the first-stage compressor wheel into an inlet of the second-stage compressor wheel; anddischarge ducting configured to collect pressurized air discharged from the second-stage compressor wheel for supply to an intake of an internal combustion engine;wherein the first- and second-stage compressor wheels and the turbine wheel are all fixedly mounted on the shaft and therefore rotate at the same angular rate of rotation. 2. The exhaust gas-driven turbocharger of claim 1, wherein the interstage ducting comprises a plurality of separate third ducts circumferentially spaced apart from one another about the rotational axis. 3. The exhaust gas-driven turbocharger of claim 2, wherein the second and third ducts alternate with one another in the circumferential direction about the rotational axis. 4. The exhaust gas-driven turbocharger of claim 3, wherein there are three second ducts and three third ducts. 5. The exhaust gas-driven turbocharger of claim 2, wherein there is space between each second duct and the third ducts adjacent thereto, such that there is thermal isolation between the second and third ducts. 6. The exhaust gas-driven turbocharger of claim 2, wherein each of the third ducts has a substantially constant flow area between the exit of the first-stage compressor wheel and the inlet of the second-stage compressor wheel. 7. The exhaust gas-driven turbocharger of claim 2, wherein there are three third ducts respectively connected to three partial volutes that received pressurized air from the first-stage compressor wheel, each of the partial volutes occupying substantially one-third of a 360° circumference about the rotational axis. 8. The exhaust gas-driven turbocharger of claim 7, wherein a vaneless diffuser connects each of the first and second impellers of the first-stage compressor wheel to the partial volutes. 9. The exhaust gas-driven turbocharger of claim 1, further comprising a first bearing assembly rotatably supporting the shaft at a location between the turbine wheel and the second-stage compressor wheel, and a center housing formed separately from the compressor housing assembly, the first bearing assembly being contained in the center housing. 10. The exhaust gas-driven turbocharger of claim 9, wherein the compressor housing assembly includes a first-stage compressor housing that contains the first-stage compressor wheel and defines the inlet ducting and interstage ducting, and a second-stage compressor housing that contains the second-stage compressor wheel and defines the discharge ducting in the form of a discharge volute for the second-stage compressor wheel, the second-stage compressor housing being formed separately from the first-stage compressor housing. 11. The exhaust gas-driven turbocharger of claim 10, wherein the center housing defines a generally annular flange that extends generally radially outwardly, the first-stage compressor housing is affixed to the flange of the center housing, and the second-stage compressor housing is affixed to the flange of the center housing independently of the affixation of the first-stage compressor housing thereto. 12. The exhaust gas-driven turbocharger of claim 11, wherein the first-stage compressor housing defines a plurality of separate legs that are circumferentially spaced apart from one another and that extend generally axially and are affixed to the flange of the center housing. 13. The exhaust gas-driven turbocharger of claim 12, wherein each of the legs is formed integrally with a respective one of the second ducts. 14. The exhaust gas-driven turbocharger of claim 10, further comprising a second bearing assembly rotatably supporting the shaft at a location between the first- and second-stage compressor wheels, the second bearing assembly comprising a foil air bearing. 15. The exhaust gas-driven turbocharger of claim 14, wherein the foil air bearing includes a foil bearing journal formed as an integral part of the first-stage compressor wheel. 16. The exhaust gas-driven turbocharger of claim 14, wherein the foil air bearing also serves as a seal between the first- and second-stage compressor wheels, and is arranged such that air that leaks through the seal cools the foil air bearing. 17. The exhaust gas-driven turbocharger of claim 14, wherein the second-stage compressor housing is connected by a flexible connection to the first-stage compressor housing in the region of the interstage ducting. 18. The exhaust gas-driven turbocharger of claim 17, wherein the flexible connection comprises a generally annular portion of one of the compressor housings received within a generally annular portion of the other of the compressor housings, with a radial clearance existing between the annular portions, a flexible compressible element being disposed between the annular portions to seal the flexible connection. 19. An exhaust gas-driven turbocharger, comprising: a shaft having a first end and a second end;a one-stage turbine wheel mounted on the shaft proximate the first end;a turbine housing surrounding the turbine wheel and defining ducting for directing exhaust gas into and out of the turbine wheel;a first-stage compressor wheel mounted on the shaft proximate the second end, the first-stage compressor wheel comprising first and second impellers disposed back-to-back;a second-stage compressor wheel mounted on the shaft at a location intermediate the first-stage compressor wheel and the turbine wheel; anda compressor housing assembly surrounding the first- and second-stage compressor wheels and defining: inlet ducting configured to direct two separate portions of air in generally opposite axial directions into respective inlets of the first and second impellers of the first-stage compressor wheel;interstage ducting configured to direct pressurized air from an exit of each of the impellers of the first-stage compressor wheel into an inlet of the second-stage compressor wheel; anddischarge ducting configured to collect pressurized air discharged from the second-stage compressor wheel for supply to an intake of an internal combustion engine;wherein the first- and second-stage compressor wheels and the turbine wheel are all fixedly mounted on the shaft and therefore rotate at the same angular rate of rotation,wherein the first-stage compressor wheel is affixed to the shaft by a boreless joint. 20. The exhaust gas-driven turbocharger of claim 19, wherein the interstage ducting comprises a plurality of separate third ducts circumferentially spaced apart from one another about the rotational axis. 21. The exhaust gas-driven turbocharger of claim 20, wherein the second and third ducts alternate with one another in the circumferential direction about the rotational axis. 22. The exhaust gas-driven turbocharger of claim 19, wherein there is space between each second duct and the third ducts adjacent thereto, such that there is thermal isolation between the second and third ducts. 23. The exhaust gas-driven turbocharger of claim 19, further comprising a first bearing assembly rotatably supporting the shaft at a location between the turbine wheel and the second-stage compressor wheel, and a center housing formed separately from the compressor housing assembly, the first bearing assembly being contained in the center housing. 24. The exhaust gas-driven turbocharger of claim 23, wherein the compressor housing assembly includes a first-stage compressor housing that contains the first-stage compressor wheel and defines the inlet ducting and interstage ducting, and a second-stage compressor housing that contains the second-stage compressor wheel and defines the discharge ducting in the form of a discharge volute for the second-stage compressor wheel, the second-stage compressor housing being formed separately from the first-stage compressor housing. 25. The exhaust gas-driven turbocharger of claim 24, wherein the center housing defines a generally annular flange that extends generally radially outwardly, the first-stage compressor housing is affixed to the flange of the center housing, and the second-stage compressor housing is affixed to the flange of the center housing independently of the affixation of the first-stage compressor housing thereto. 26. The exhaust gas-driven turbocharger of claim 25, wherein the first-stage compressor housing defines a plurality of separate legs that are circumferentially spaced apart from one another and that extend generally axially and are affixed to the flange of the center housing. 27. The exhaust gas-driven turbocharger of claim 26, wherein each of the legs is formed integrally with a respective one of the second ducts. 28. The exhaust gas-driven turbocharger of claim 24, further comprising a second bearing assembly rotatably supporting the shaft at a location between the first- and second-stage compressor wheels, the second bearing assembly comprising a foil air bearing. 29. The exhaust gas-driven turbocharger of claim 28, wherein the foil air bearing includes a foil bearing journal formed as an integral part of the first-stage compressor wheel.