TURBOCHARGER BYPASS VALVING

[0001] This application claims the benefit of United States Provisional

Application serial number 60/943,338, filed June 12, 2007.

TECHNICAL FIELD

[0002] The field to which the disclosure generally relates includes turbochargers and turbocharging systems for combustion engine systems.

BACKGROUND

[0003] Combustion engine systems may be equipped with turbochargers for efficiently increasing engine output. Turbocharged engine systems include engines having combustion chambers in which air and fuel is combusted for conversion into mechanical rotational power. Such engine systems also have air induction systems upstream of the engine for conveying induction gases to the combustion chambers, and engine exhaust systems downstream of the engine for carrying exhaust gases away from the combustion chambers. A turbocharger basically includes a compressor in the induction system for generating induction boost pressure, a turbine disposed in and powered by the exhaust system for driving the compressor, and a common shaft rotatably connecting the turbine to the compressor.

[0004] Pressurized exhaust gases from the engine impinge on a bladed rotor of the turbine to pneumatically spin the rotor. The spinning rotor and shaft mechanically spin a bladed impeller of the compressor. The spinning impeller pressurizes induction gases to increase the mass of induction gases

supplied to the engine, thereby allowing more fuel to be burned for increased combustion so as to increase engine power output for a given engine displacement and speed. Such single-stage turbocharging has been implemented for many years.

[0005] More recently, multi-stage turbocharging systems replace a single turbocharger with two or more turbochargers, which collectively operate more efficiently over a wider range of engine speeds and power output levels. For example, a first turbocharger operates efficiently at lower engine speeds, and a second turbocharger operates efficiently at higher engine speeds, and both turbochargers are partially operated at medium engine speeds for a good balance of efficiency. To control exhaust gas flow through the first turbocharger, a first bypass valve is placed in parallel with the first turbocharger upstream of the second turbocharger and can open to bypass some or all of the exhaust gases around the first turbocharger. Thus, the first bypass valve modulates exhaust gas flow between the turbochargers to control transitions in operation between the turbochargers. Similarly, to control exhaust gas flow through the second turbocharger, a second bypass valve is placed in parallel with the second turbocharger and can open to bypass some or all of the exhaust gases around the second turbocharger.

[0006] But the transitions in operation between the turbochargers can be abrupt and noticeable to some vehicle drivers, and the two separate valves can be too bulky for efficient packaging in some vehicles. Abrupt transitions are typically caused when the first bypass valve is suddenly fully opened, thereby causing the first turbocharger to immediately decelerate before the second turbocharger has a chance to accelerate. This lag in operation is

caused by a momentary loss or drop in pressure of exhaust gases flowing between the first bypass valve and the second turbocharger inlet. Finally, it can be difficult to package a turbocharging system having two bypass valves, their actuators, brackets, and intermediate linkages, within the tight confines of some vehicle engine compartments.

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION One exemplary embodiment of the invention includes a turbocharger including a mixed inflow turbine rotor, and a mixed inflow turbine housing at least partially surrounding the turbine rotor. The turbine rotor includes a radial periphery and an oblique periphery adjacent the radial periphery. The turbine housing includes a first volute to convey upstream turbocharger outlet gases to the oblique periphery of the turbine rotor, a second volute to convey engine exhaust gases to the radial periphery of the turbine rotor, and a volute partition disposed between the first and second volutes and including a radially inward portion defining a first valve seat. A second valve seat is carried by the housing, and a valve is disposed on an axially downstream side of the volute partition. The valve has a first valve head disposed radially inward of the second volute and a second valve head disposed axially downstream of the first valve head. The valve is movable to engage the first valve head with the first valve seat to block flow of the engine exhaust gases from the second volute to the turbine rotor to operate the system in a series mode. Also, the valve is movable to disengage the first valve head from the first valve seat to modulate flow of the gases from both volutes to the turbine rotor to operate the system in a modulated mode.

Further, the valve is movable to fully open flow of the engine exhaust gases from the second volute to the turbine rotor to operate the system in a first bypass mode. Finally, the valve is even further movable to disengage the second valve head from the second valve seat to open a bypass flow path to bypass flow of the engine exhaust gases from the second volute around the turbine rotor and operate the system in a second bypass mode.

[0008] An additional exemplary embodiment of the invention includes a turbocharger including a turbine rotor, and a turbine housing to introduce engine exhaust gases and upstream turbocharger outlet gases to the turbine rotor. The turbocharger also includes a valve disposed between the turbine rotor and the turbine housing. The valve is movable to be advanced to a fully advanced position to block flow of the engine exhaust gases from the turbine housing to the turbine rotor to operate the system in a series mode. Also, the valve is movable to be retracted away from and back toward the fully advanced position to modulate flow of the engine exhaust gases and upstream turbocharger outlet gases to the turbine rotor to operate the system in a modulated mode. Further, the valve is movable to be retracted to a first bypass position to permit full flow of the engine exhaust gases to the turbine rotor to operate the system in a first bypass mode. Finally, the valve is movable to be further retracted beyond the first bypass position to a second bypass position to bypass the engine exhaust gases around the turbine rotor to operate the system in a second bypass mode.

[0009] Another exemplary embodiment of the invention includes a turbocharger including a turbine rotor, and a turbine housing. The turbine housing has a volute partition dividing the turbine housing into an engine

exhaust gas volute to convey engine exhaust gases to the turbine rotor and an upstream turbocharger outlet gas volute to convey upstream turbocharger outlet gases to the turbine rotor. A valve is generally radially disposed between the turbine rotor and the turbine housing and is generally axially disposed on a downstream side of the volute partition, wherein the valve is movable to block flow of the engine exhaust gases from the engine exhaust gas volute to the turbine rotor.

[0010] A further exemplary embodiment of the invention includes a turbocharger of a multi-stage turbocharging system including first and second stages. The turbocharger includes a single valve to regulate both a first stage bypass flow path and a second stage bypass flow path.

[0011] Other exemplary embodiments of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the exemplary embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS [0012] Exemplary embodiments of the present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: [0013] FIG. 1 is a schematic view of an exemplary embodiment of a combustion engine system including an engine and a turbocharging system for the engine;

[0014] FIG. 2 is a quarter-sectional perspective view of an exemplary turbocharger of the turbocharging system of FIG. 1 ; [0015] FIG. 3 is a half-sectional view of the exemplary turbocharger shown in FIG. 2, illustrating a bypass valve in a first bypass position; [0016] FIG. 4 is an enlarged view of a portion of the exemplary turbocharger shown in FIG. 3, illustrating the bypass valve in a closed position; [0017] FIG. 5 is an enlarged view of a portion of the exemplary turbocharger shown in FIG. 3, illustrating the bypass valve in an intermediate or modulated position; and [0018] FIG. 6 is an enlarged view of a portion of the exemplary turbocharger shown in FIG. 3, illustrating the bypass valve in a second bypass position.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS [0019] The following description of the embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

[0020] Referring to FIG. 1 , a combustion engine system 10 may include an engine 12 including a block assembly 14 for converting fuel and induction gases into mechanical rotational energy and exhaust gases, an induction manifold 16 to distribute the induction gases to the block assembly, and an exhaust manifold 18 to collect the exhaust gases from the block assembly 14. The engine system 10 may also include an induction system 20 that may compress and cool induction gases and convey them to the engine induction

manifold 16, and an exhaust system 22 that may extract energy from exhaust gases and carry them away from the engine exhaust manifold 18. The engine system 10 may further include a multi-stage turbocharging system 24 between the induction and exhaust systems 20, 22 for increasing engine performance.

[0021] The multi-stage turbocharging system 24 may include a turbine side 26 in the exhaust system 22 and a compressor side 28 in the induction system 20. Multi-stage turbocharging may allow for continuously variable adaptation of the turbine and compressor sides 26, 28 of the system 24 over all engine operating points. The multi-stage turbocharging system 24 may include two or more turbochargers of any size and type, that may be connected in series, parallel, or both, and that may use bypass regulation. One exemplary multi-stage turbocharging system is disclosed in U.S. Patent Application Publication 2006/0137343, which is assigned to the assignee hereof and is hereby incorporated by reference herein in its entirety.

[0022] As shown in FIG. 1 , the exemplary multi-stage turbocharging system 24 includes a first turbocharger 30 and a second turbocharger 32 according to first and second stages. For example, the first turbocharger 30 may be a relatively small high-pressure (HP) turbocharger, and the second turbocharger 32 may be a relatively large low-pressure (LP) turbocharger downstream of the first turbocharger 30. The first turbocharger 30 may include a first turbine 34 having an inlet 33 and an outlet 35, and a first compressor 36. The second turbocharger 32 may include a second turbine 38 having an inlet 37 and an outlet 39, and a second compressor 40. An exhaust gas mass flow first may be expanded by the first turbine 34 and then

by the second turbine 38, and an induction gas mass flow first may be compressed by the second compressor 40 and then by the first compressor 36 to provide boost pressure for the mass flow before it is cooled by a cooler 42 and then fed into the engine 12. The system 10 may also include a compressor bypass valve 44 in parallel with the first compressor 36, and a turbine bypass valve 46. The turbine bypass valve 46 may be any suitable type of valve, such as an adjustable three way valve or the like. As shown, and as will be described in greater detail herein below, the turbine bypass valve 46 may be integrated into the second turbine 38, and may include a first turbine bypass portion 46a and a second turbine bypass portion 46b, which may be in series with the first turbine bypass portion 46a. Exemplary first and second turbine bypass portions of the valve 46 are discussed in more detail below with reference to FIGS. 2 through 6.

[0023] The bypass valves 44, 46 may be actively or passively controlled, such as with any suitable actuators (not shown) controlled pneumatically, electrically, electronically, or in any other suitable manner. For example, the turbine bypass valve 46 may be operated by a spring-loaded diaphragm (not shown) in response to boost pressure. The diaphragm may be referenced to boost pressure downstream of the first compressor 36, so that the diaphragm is increasingly displaced with increases in boost pressure, such that the diaphragm increasingly opens the first turbine bypass portion 46a of the turbine bypass valve 46.

[0024] In this arrangement, the turbochargers 30, 32 may be tuned in such a manner that one or both of them are active at all engine operating points. For example, at relatively low engine loads and speeds, i.e. when

exhaust mass flow rate is low, much of the exhaust gas mass flow may be expanded by the first turbine 34. This results in a very quick and high rise in boost pressure in the induction system 20. But as engine load and speed increases, exhaust gas expansion may be continuously shifted to the second turbine 38 by increasing the opening of the first turbine bypass portion 46a of the turbine bypass valve 46 over a period of time. This is an example of regulated two-stage series turbocharging, which allows for continuous adaptation of the turbine and compressor sides 26, 28 to the actual requirements of the operating engine 12.

[0025] In a first mode, for example, at relatively low engine speeds and loads, the first turbocharger 30 may perform most and perhaps all of the turbocharging while the second turbocharger 32 may perform little to none of the turbocharging. Exemplary low engine loads and speeds may include, for example, 0-50% of maximum loads and speeds and, more specifically, 0-25% of maximum loads and speeds. In this mode, both of the bypass valves 44, 46 may be closed. For example, the turbine bypass valve 46 may be completely closed so that most if not all of the energy from the exhaust gas flowing from the engine 12 is used to run the first turbine 34 and, thus, compress air in the first compressor 36.

[0026] In a second mode, for example, at relatively medium engine speeds and loads, the turbocharging may be modulated between the first and second turbochargers 30, 32. Exemplary medium engine loads and speeds may include, for example, 10-80% of maximum loads and speeds and, more particularly, 25-50% of maximum loads and speeds. In this mode, the first turbine bypass portion 46a of the turbine bypass valve 46 may begin to open

so that part of the engine exhaust gas is diverted around the first turbine 34 through the turbine bypass valve 46 to the second turbine 38. As engine speed rises, the first turbine bypass portion 46a of the turbine bypass valve 46 may be increasingly opened so that more and more of the exhaust energy bypasses the first turbine 34 and is fed directly to the second turbine 38 from the exhaust manifold 18.

[0027] In a third mode, for example, at relatively high engine speeds and loads, the second turbocharger 32 may perform most and perhaps all of the turbocharging while the first turbocharger 30 may perform little to none of the turbocharging. Exemplary high engine loads and speeds may include, for example, 25-95% of maximum loads and speeds and, more specifically, 50- 80% of maximum loads and speeds. In this mode, the first turbine bypass portion 46a of the turbine bypass valve 46 may continue to open, for example, to its fully open position, while the compressor bypass valve 44 may open, for example, to its fully open position. As engine speed continues to rise, the first turbine bypass portion 46a of the turbine bypass valve 46 may be opened so that most or all of the exhaust energy may bypass the first turbine 34 and may be fed directly to the second turbine 38 from the exhaust manifold 18. Accordingly, most of the air compression may be carried out by the second compressor 40 and the compressed air may flow around the first compressor 36 through the compressor bypass valve 44.

[0028] In a fourth mode, for example, at relatively higher engine speeds and maximum load, the second turbocharger 32 may perform most and perhaps all of the turbocharging while the first turbocharger 30 may perform little to none of the turbocharging. In this mode, the compressor bypass valve

44 and both portions 46a, 46b of the bypass valve 46 may be fully open. As engine load reaches its maximum, the second turbine bypass portion 46b of the turbine bypass valve 46 opens so that most or all of the exhaust energy may bypass both the first and second turbines 34, 38 and may be fed directly to a location downstream of the second turbine 38 from the exhaust manifold 18. Accordingly, little to no compression of induction gases may be carried out in the induction system 20, to avoid overloading an already fully loaded engine 12.

[0029] Referring now to FIGS. 2 and 3, the second turbocharger 32 may be of any suitable type and construction. In one example, the second turbocharger 32 may include a central assembly 48 including a central housing 50 carrying a bearing assembly 52 for a common shaft 54 on which is mounted a compressor impeller 56 and a turbine rotor 58 at opposite ends thereof. As shown, suitable sealing devices may separate the central housing 50 from both the turbine rotor 58 and compressor impeller 56. The bearing assembly 52 may include any suitable lubrication and/or cooling components and passages. For example, the bearing assembly 52 may be in fluid communication with an engine oil lubrication system (not shown) via inlet and outlet ports 60, 62, and/or with an engine water cooling system (not shown).

[0030] The second turbocharger 32 may also include the compressor

40, which may include the compressor impeller 56 and a compressor housing 64 that may at least partially cover the impeller 56 and that may be carried by the central housing 50, such as by being fastened thereto in any suitable manner. The compressor housing 64 may define a compressor air inlet 66, and an outlet volute 68 generally circumferentially positioned about the

compressor impeller 56. Accordingly, the compressor 40 may be a centrifugal or radial-outflow device, wherein the rotating compressor impeller 56 may draw induction gases into the compressor 40 through the inlet 66, compress the gases, and expel them out of the compressor 40 through the outlet volute 68 and out an outlet 70. More specifically, rotating blades of the impeller 56 may accelerate, compress, and expel gases into the volute 68 of the compressor housing 64 where it is directed out of the outlet 70 and through ducting (not shown) toward the engine intake manifold 16.

[0031] The turbocharger turbine 38 may be a dual entry type of turbine.

The turbine 38 may include the turbine rotor 58, and a turbine housing 72 that may at least partially cover the rotor 58 and that may be carried by the central housing 50 so such as by being fastened thereto in any suitable manner. The rotor 58 may include a hub 74, which may be contoured, and a plurality of blades 76, which may extend from the hub 74 and include a radial inlet periphery 78 (FIG. 3) and an oblique inlet periphery 80 (FIG. 3) adjacent the radial inlet periphery 78. The radial inlet periphery 78 may be oriented substantially parallel to the rotational axis of the turbine rotor 58, whereas the oblique inlet periphery 80 may be oriented at an angle with respect to the rotational axis of the turbine rotor 58, such as between about 20 and about 60 degrees with respect thereto. The rotor blades 76 may also include a reduced diameter portion 82 (FIG. 3) that may extend downstream of the inlet peripheries and may be incurvately contoured.

[0032] The turbine housing 72 may include a volute portion 90 having dual volutes 84, 86 extending generally circumferentially out from the turbine rotor 58, and may also include a turbine outlet 88. In one embodiment, the

volute portion 90 defines a centripetal or radial inflow volute 84, an oblique inflow volute 86, and a volute partition 85 therebetween. The radial inflow volute 84 may be oriented to convey the flow of engine exhaust gases in a generally radially inward direction that is substantially normal with respect to the radial inflow periphery 78 of the turbine rotor blades 76. Similarly, the oblique inflow volute 86 may be oriented to convey a flow of outlet gases from the first turbine 30 in an obliquely inward direction that is substantially normal with respect to the oblique inflow periphery 80 of the turbine rotor blades 76. The volutes 84, 86 may be of any suitable shape, size, and configuration. As will be discussed further herein below, the volute partition 85 may include a radially inward portion 92 that may define a first valve seat. Accordingly, the housing 72 carries the first valve seat 92. Thus, the turbine 38 may be a mixed inflow device. For example, exhaust gas flowing directly from the engine 12, and/or indirectly from the engine 12 via the first turbine 34, may flow radially and/or obliquely against and past the rotor blades 76, and may exit substantially centrally of the housing 72 through the outlet 88. The terminology "mixed inflow" may include a radial inflow and an oblique inflow path, or may include multiple oblique inflow paths, or the like. Engine exhaust gas may be directed through the engine exhaust manifold 18 into the turbine housing 72, through the radial inflow volute 84, against and past the rotor blades 76 and out the outlet 88. Similarly, the outlet gases from the first turbine 34 may be directed into the turbine housing 72, through the oblique inflow volute 86, against and past the rotor blades 76, and out the outlet 88. The exhaust gas pressure and the heat

energy extracted from the gas causes the turbine rotor 58 and attached shaft 54 to rotate, which rotation drives the compressor impeller 56.

[0034] Referring to FIG. 3, the turbine 38 may also include an integrated bypass valving apparatus 100 that may include an integrated bypass valve 102, and a bypass valve portion 104 of the housing 72 that may be integral with the volute portion 90 of the housing 72 and may partially define a bypass outlet 106. The bypass valve 102 may be disposed axially downstream of the volute partition 85, for example, to provide dual functionality as will be described herein below. As discussed above with respect to FIG. 1 , the integrated bypass valve 102 may include the functionality of a first turbine bypass valve and a second turbine bypass valve, and may be an alternative for the valve 46 of FIG. 1.

[0035] According to the valving apparatus 100, the turbine 38 may also include a center support 108 carried by the housing 72 and radially supporting the valve 102. The center support 108 may include a flange 110, which may be attached to the housing 72 in any suitable manner, such as via a ring 112 such as a snap ring, circlip, or the like. Opposite the flange 110, the support 108 may include a turbine end 114 that may be contoured to correspond to the contour of the reduced diameter portion 82 of the turbine blades 76. The center support 108 may be axially positioned such that the reduced diameter portion 82 of the turbine blades 76 extends a desired distance into the turbine end 114 of the center support 108.

[0036] According to the valving apparatus 100, the turbine 38 may additionally include a second valve seat 116 carried by the housing 72. The second valve seat 116 may be a sealing ring that may be carried radially

between the valve 102 and the housing 72 and may be retained to the housing 72 in any suitable manner such as using a seal backup ring 118 and any suitable retainer ring 120 such as a snap ring, circlip, or the like. The second valve seat 116 may be composed of any suitable material, for example, a polymeric material of any kind. The valve 102 may be a substantially cylindrical component, such as a sleeve, slidably disposed between the center support 108 and the housing 72. An exemplary valve is disclosed in U.S. Patent 6,715,288, which is assigned to the assignee hereof and is hereby incorporated by reference herein in its entirety. At a downstream end, the valve 102 may include an annular groove 122 and, at an upstream end, the valve 102 may include a first turbine bypass portion or valve head 124 radially disposed between the center support 108 and a portion of the housing 72 axially adjacent the volute partition 85 for engaging the first valve seat 92 defined by the volute partition 85. More specifically, the first valve head 124 may include an angled outer peripheral surface 126 for sealing engagement with the angled inner peripheral surface of the volute partition valve seat 92. As shown, the angled outer peripheral surface 126 may also correspond to an angled wall of the radial volute 84. As is also shown, the valve head 124 may also include an angled inner peripheral surface 128 to correspond with an angled surface of the volute partition 85 that defines the oblique volute 86. The valve 102 may further include a second turbine bypass portion or valve head 130 axially disposed downstream of the first valve head 124, between the first valve head 124 and the groove 122, and adapted to engage the second valve seat 116. The valve 102 may also include an annular bypass groove 132 and may be

defined between the first valve head 124 and an angled surface of the second valve head 130. The valve heads 124, 130 may at least partially define the first and second bypass portions of the valve 46 discussed above with respect to FIG. 1.

[0038] According to the valving apparatus, the turbocharger 32 may further include a valve actuation apparatus 134 including a valve fork 136 for interengaging the groove 122 of the valve 102, a linkage 138 connected to the valve fork 136, and an actuator 140 mounted, for example, to the compressor housing 64 by a bracket 142 to actuate the valve 102 via the linkage 138 and fork 136. The actuator 140 may include any suitable type of actuator, for example, a pneumatic or diaphragm type of actuator, an electrically operated solenoid actuator, a fluid actuator, or the like.

[0039] In operation, and referring generally to FIGS. 3 through 6, the valve 102 may be movable among and between several positions including a fully closed position, to a partially open or modulated position, to a first bypass position, and to a second bypass position.

[0040] First, and referring to FIG. 4, the valve 102 may be advanced to a fully advanced position to block flow of engine exhaust gases from the turbine housing 72 to the turbine rotor 58 to operate the system 10 in an HP/LP series mode at low engine speeds and loads. For example, the valve 102 may be advanced so that the first valve head 124 engages the first valve seat 92. Accordingly, as exemplified by arrow T, outlet gases from the upstream first turbine flow out of the oblique volute 86, impinge on the oblique periphery 80 of the rotor 58, flow past the rotor 58 and out of the turbine 38 through the turbine outlet 88.

[0041] Second, and referring to FIG. 5, the valve 102 may be retracted away from the advanced position of FIG. 4 to any of a number of intermediate positions (and back toward the advanced position) to modulate flow of engine exhaust gases and outlet gases from an upstream turbine to the turbine rotor 58 to operate the system 10 in an HP/LP modulated mode at medium engine speeds and load. For example, the valve 102 may be retracted so that the first valve head 124 disengages the first valve seat 92 and permits a combined flow of some engine exhaust gas and some outlet gases from the upstream turbine to flow out of the radial and oblique volutes 84, 86, impinge on the radial and oblique peripheries 78, 80 of the rotor 58, flow past the rotor 58, and out of the turbine 38 through the turbine outlet 88, as exemplified by arrow C.

[0042] Third, and referring again to FIG. 3, the valve 102 may be retracted to a first bypass position to permit flow primarily of engine exhaust gases to the turbine rotor 58 to operate the system 10 in an HP bypass mode at high engine speeds and load. For example, the valve 102 may be retracted so that the angled outer peripheral surface 126 of the first valve head 124 may be substantially flush with an adjacent wall of the radial inflow volute 84 and the angled inner peripheral surface 128 of the first valve head 124 may be substantially flush with the contoured turbine end 114 of the center support 108. In this valve position, on the order of less than 10% of the fluid flow through the second turbine 38 typically flows from the first turbine 34 via the oblique inflow volute 86. Thus, in this position, the valve 102 at least partially defines a first stage bypass flow path as exemplified by arrow B-i.

[0043] Fourth, and referring to FIG. 6, the valve 102 may be further retracted beyond the first bypass position of FIG. 3 to the second bypass position to bypass engine exhaust gases around the turbine rotor 58 to operate the system 10 in an HP/LP bypass mode at high engine speeds and maximum load. In FIGS. 3 through 5, the second valve head 130 of the valve 102 is in continuous engagement with the second valve seat 116. In contrast, as shown in FIG. 6, the valve 102 may be retracted so that the second valve head 130 disengages the second valve seat 116 and permits engine exhaust gas to follow a second stage bypass flow path as exemplified by arrow B ₂ . More specifically, the engine exhaust gases bypass the turbine rotor 58 and flow through bypass passages 150 in the housing 72, past the second valve seat 116, and out of the turbine 38 through the turbine bypass outlet 106. The outlet 106 may include apertures in the flange 110 of the center support 108. The bypass passages 150 may be produced in any suitable manner, including as-cast, formed, machined, or the like. The quantity and size(s) of the bypass passages 150 may be selected on a case-by-case basis for the particular turbine and system application.

[0044] The valve 102 is packaged within the turbine housing 72 of the second turbocharger 32 and as close as possible to the turbine rotor 58. The close location of the valve 102 to the turbine rotor 58 ensures that the rotor 58 is exposed to maximal velocity of the inflowing gases for maximal expansion of, or work extraction from, the gases by the turbine 38. Prior multi-stage turbocharging systems include bypass valves located so far upstream of turbine rotors that the velocity of engine exhaust gases significantly slows

before reaching the rotors. Accordingly, the present invention provides a more efficient valving arrangement. The above description of embodiments of the invention is merely exemplary in nature and, thus, variations thereof are not to be regarded as a departure from the spirit and scope of the invention.