Patent Publication Number: US-8534994-B2

Title: Turbocharger with divided turbine housing and annular rotary bypass valve for the turbine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to U.S. patent application Ser. No. 12/611,816 filed on Nov. 3, 2009, and U.S. patent application Ser. No. 12/711,434 filed on Apr. 30, 2010, the entire contents of both said applications being hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present disclosure relates to exhaust gas-driven turbochargers having a divided turbine housing for two separate exhaust gas streams fed to the turbine. The present disclosure relates more particularly to bypass arrangements that allow exhaust gas to bypass the turbine under certain engine operating conditions. 
     In a conventional turbocharger, the turbine housing defines a bypass conduit located generally to one side of the main bore through the housing, and the bypass conduit is connected to the exhaust gas inlet or the volute of the housing via a bypass valve. The bypass valve typically is a swing or poppet style valve comprising a circular valve member that is urged against a flat valve seat surrounding the bypass passage opening. The valve usually is arranged such that the exhaust gas pressure acts on the valve member in a direction tending to open the valve. One drawback associated with such an arrangement is that it is difficult to completely seal the valve in the closed position, since gas pressure tends to open the valve. Leakage past the closed bypass valve is a cause of performance degradation of the turbine and, hence, the turbocharger and its associated engine. The typical solution to the leakage issue is to preload the bypass valve member against the valve seat, but often this does not fully eliminate leakage, and in any event it causes additional problems such as an increase in the required actuation force for opening the valve. 
     Furthermore, swing or poppet valves tend to be poor in terms of controllability, especially at the crack-open point, and it is common for the bypass flow rate to be highly nonlinear with valve position, which makes it very difficult to properly regulate the bypass flow rate. This leads to problems such as poor transient response of the turbocharger and engine system. 
     The aforementioned application Ser. Nos. 12/611,816 and 12/711,434 describe previous developments by the assignee of the present application, directed to improved bypass valve arrangements in turbocharger turbine components. The present application represents a still further development adapted particularly to turbochargers with divided turbine housing/volute configurations. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     The present disclosure describes various embodiments of turbochargers with divided turbine housings and having a novel bypass arrangement that seeks to address issues such as the ones noted above. In one embodiment, a turbocharger comprises a compressor wheel mounted within a compressor housing, and a turbine wheel mounted within a turbine housing and connected to the compressor wheel by a shaft. The turbine housing defines an exhaust gas inlet connected to a volute that surrounds the turbine wheel. The volute is a divided volute that defines two (i.e., first and second) separate spaces that respectively receive two separate streams of exhaust gas from an internal combustion engine. The turbine housing further defines an axial bore through which exhaust gas that has passed through the turbine wheel is discharged from the turbine housing. 
     The turbine housing additionally defines a pair of (i.e., first and second, or outer and inner) concentric annular bypass passages surrounding the bore and arranged to allow exhaust gas within each of the first and second separate spaces of the divided volute to bypass the turbine wheel as separate bypass streams. The first or outer bypass passage is radially outward of and concentrically surrounds the second or inner bypass passage. An annular bypass valve is disposed in communication with the bypass passages. The bypass valve comprises a fixed valve seat of annular form and a rotary valve member of annular form arranged coaxially with the valve seat relative to an axis. The valve member is disposed against the valve seat and is rotatable about the axis for selectively varying a degree of alignment between respective orifices in the valve seat and valve member, ranging from no alignment defining a closed condition of the bypass valve, to at least partial alignment defining an open condition of the bypass valve. 
     An advantage of this bypass arrangement is that exhaust gas pressure acts on the valve in a direction tending to improve, rather than hinder, sealing, in contrast to the aforementioned swing and poppet style bypass valve arrangements. 
     A further advantage is that the valve can achieve better controllability than is typically possible with swing or poppet valves, particularly at the crack-open point. 
     In a particular embodiment, the valve member defines a plurality of first or outer valve member orifices and a plurality of second or inner valve member orifices. The outer valve member orifices are circumferentially spaced apart about the axis and located at a relatively greater radial distance from the axis, while the inner valve member orifices are circumferentially spaced apart about the axis and located at a relatively smaller radial distance from the axis. That is, the outer valve member orifices are arranged in a ring that is radially outward of and concentrically surrounds the ring of the inner valve member orifices. Correspondingly, the valve seat defines a plurality of outer valve seat orifices and a plurality of inner valve seat orifices. The outer and inner valve seat orifices are arranged in concentric rings substantially as for the orifices of the valve member, so that the outer valve member orifices and the outer valve seat orifices lie at substantially the same radial distance from the axis, and the inner valve member orifices and the inner valve seat orifices lie at substantially the same radial distance (smaller than that of the outer orifices) from the axis. 
     The valve member in one embodiment is a generally flat annular disk and the outer and inner valve member orifices are circumferentially spaced apart about a circumference of the valve member. Similarly, the valve seat is a generally flat annular disk having the outer and inner valve seat orifices circumferentially spaced apart in correspondence with the outer and inner valve member orifices, respectively. There is sufficient circumferential distance between adjacent valve seat orifices to accommodate the respective valve member orifices therebetween in the closed condition of the bypass valve. In other words, in the closed condition there is no overlap between the valve member orifices and the corresponding valve seat orifices. 
     The valve can include features facilitating sealing between the valve member and valve seat. In one embodiment, either the valve member or the valve seat includes raised pads surrounding the respective orifices therein, and the pads are in contact with a substantially planar surface of the other of the valve member and the valve seat so as to provide sealing between the valve member and the valve seat. The valve seat can also include a ring-shaped raised ridge or pad located at a radius intermediate the outer valve seat orifices and the inner valve seat orifices for sealing against a surface of the valve member in order to minimize or eliminate leakage flow between the outer and inner bypass passages. Alternatively, the ring-shaped ridge or pad can be provided on the valve member for sealing against a surface of the valve seat. 
     The orifices can have various shapes, including circular or non-circular. The orifices can be circumferentially spaced uniformly or non-uniformly. 
     The turbocharger also includes a drive system for effecting the needed rotational movement of the valve member. In one embodiment, the drive system includes a rotary drive member penetrating through the turbine housing in a direction generally transverse to the axis about which the valve member rotates, and a drive arm attached to a distal end of the rotary drive member. A distal end of the drive arm engages the valve member such that rotation of the rotary drive member causes the drive arm to rotate the valve member about the axis. The drive system can further comprise a link attached to a proximal end of the rotary drive member, and a linear actuator having an actuator rod, the actuator being operable to extend and retract the actuator rod. A distal end of the actuator rod is connected to the link such that extension of the actuator rod causes the link to rotate the rotary drive member in one direction and retraction of the actuator rod causes the link to rotate the rotary drive member in the opposite direction. 
     If desired or necessary in a particular application, surfaces of components that relatively slide over one another can be coated with a friction/wear coating that reduces wear of the surfaces. For example, the raised pads and the surface they contact can be coated. The wear coating can be formed of various materials, including an aluminum diffusion coating, a nickel boron coating, or a ceramic coating (e.g., silicon nitride). In order to avoid material transfer from one surface to the abutting surface, preferably the two surfaces are coated with different materials. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       Having thus described the disclosure in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1  is an axial cross-sectional view of a turbocharger in accordance with one embodiment of the present invention; 
         FIG. 2  is a side view of the turbine assembly for the turbocharger of  FIG. 1 ; 
         FIG. 3  is a perspective view of the turbine assembly of  FIG. 2 , wherein the turbine housing is shown partly cut away to reveal internal details; 
         FIG. 4  is an axial cross-sectional view of the turbine assembly of  FIG. 1  on a first axial plane; 
         FIG. 5  is an axial cross-sectional view of the turbine assembly of  FIG. 1  on a second axial plane; 
         FIG. 6  is an axially sectioned perspective view of the turbine housing for the turbocharger of  FIG. 1 ; 
         FIG. 7  is a perspective view of the valve seat for the annular bypass valve of the turbocharger of  FIG. 1 ; 
         FIG. 8  is an axial cross-sectional view of the valve seat of  FIG. 7 ; 
         FIG. 9  is a perspective view of the rotary valve member for the annular bypass valve of the turbocharger of  FIG. 1 ; 
         FIG. 10  is an axial view of the annular bypass valve, as viewed from a position downstream of the valve looking axially upstream, showing the valve in a closed position; 
         FIG. 11  is a view similar to  FIG. 10 , showing the valve when it has just been cracked open by rotation of the rotary valve member; 
         FIG. 12  is a view similar to  FIG. 10 , showing the valve in a partially open position after further rotation of the valve member; 
         FIG. 13  is a view similar to  FIG. 10 , showing the valve in a fully open position after further rotation of the valve member; 
         FIG. 14  is an axial cross-sectional view of a turbocharger in accordance with another embodiment of the present invention; 
         FIG. 15  is a perspective view of the turbine assembly for the turbocharger of  FIG. 14 , wherein the turbine housing is shown partly cut away to reveal internal details; 
         FIG. 16  is an axial cross-sectional view of the turbine housing for the turbocharger of  FIG. 14 ; and 
         FIG. 17  is a perspective view of the turbine housing of  FIG. 16 , shown partly in section. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The present disclosure now will be described more fully hereinafter with reference to the accompanying drawings in which some but not all embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. 
     A turbocharger  20  in accordance with one embodiment of the present invention is shown in axial cross-sectional view in  FIG. 1 , and various views of a turbine assembly for the turbocharger and components of the turbine assembly are depicted in  FIGS. 2-13 . As shown in  FIG. 1 , major sub-assemblies of the turbocharger  20  include a compressor assembly  30 , a center housing assembly  40 , and a turbine assembly  50 . The compressor assembly  30  includes a compressor housing  32  and a compressor wheel  34  mounted therein and attached to one end of a rotary shaft  36 . The center housing assembly  40  includes a center housing  42  that is affixed to the compressor housing  32  and that contains bearings  44  for the rotary shaft  36 . The turbine assembly  50  includes a turbine housing  52  and a turbine wheel  54  mounted therein and attached to the opposite end of the rotary shaft  36 . 
     The turbine housing  52  defines an exhaust gas inlet  56  through which exhaust gas from an internal combustion engine is received, and a merdionally divided volute  58  that receives the exhaust gas from the inlet  56  and distributes the gas around the 360° volute for feeding into the turbine wheel  54 . In particular, the merdionally divided volute  58  defines a first volute  58   a  and a separate second volute  58   b  each of which extends 360° about the longitudinal axis of the turbine. The volutes  58   a  and  58   b  respectively receive two separate streams of exhaust gas from the engine (e.g., one stream from one sub-group of engine cylinders and the other stream from another sub-group of engine cylinders). Correspondingly, the exhaust gas inlet  56  is a divided inlet that defines two separate inlets  56   a  and  56   b  that are connected respectively to the two volutes  58   a  and  58   b.    
     The two gas inlets  56   a  and  56   b  are also open to two separate generally annular bypass passages  60   a  and  60   b  defined in the turbine housing  52 . The bypass passages  60   a  and  60   b  are concentrically arranged (outer passage  60   a  being radially outward of inner passage  60   b ) and both surround an axial bore  62  defined in the turbine housing. Exhaust gas that has passed through the turbine wheel  54  is exhausted from the turbine housing through the bore  62 . The bypass passages  60   a  and  60   b  provide an alternative pathway for exhaust gas to flow without first having to pass through the turbine wheel  54 . 
     An annular bypass valve  70  is installed in the bypass passage  60  for regulating flow through the bypass passage. The bypass valve  70  is a divided bypass valve that preserves the separation of the two exhaust gas streams in the two bypass passages  60   a  and  60   b . With primary reference to  FIGS. 3-9 , the major components of the annular bypass valve  70  include a stationary valve seat  72  and a rotary valve member  74  in abutting engagement with the valve seat. The valve seat  72  and valve member  74  are arranged between an annular outer portion  52   a  of the turbine housing  52  and an annular inner member  52   b . As shown, the inner member  52   b  is formed separately from the turbine housing  52  and is connected with an integral portion of the turbine housing, but in other embodiments the inner member can be an integral part of the turbine housing. The outer portion  52   a  and inner member  52   b  together define an annular space for receiving the valve member  74  and the valve seat  72 . The valve member  74  is prevented from moving axially upstream by a shoulder defined by the outer portion  52   a  of the turbine housing, although during operation pressure of the exhaust gas urges the valve member  74  in the downstream direction. The valve member  74  is not constrained by the turbine housing but is free to rotate about its axis and to move axially against the valve seat  72 . The valve seat  72  is prevented from moving axially, radially, or rotationally. A radially outer edge portion of the upstream face of the valve seat  72  (i.e., the left-hand face in  FIG. 5 ) abuts a shoulder defined by the outer portion  52   a  of the turbine housing, and the radially inner edge portion of the upstream face abuts a shoulder defined by the inner member  52   b , thereby putting the valve seat in a precise axial location as dictated by these shoulders. 
     The valve seat  72  (best illustrated in  FIGS. 7 and 8 ) is a generally flat ring-shaped or annular member having a plurality of orifices  73   a  and  73   b  circumferentially spaced apart about a circumference of the valve seat, the orifices extending generally axially between the upstream and downstream faces of the valve seat. The orifices  73   a  are located at a relatively larger radius from the center of the valve seat  72 , and the orifices  73   b  are at a relatively smaller radius from the center of the valve seat. The orifices  73   a  in the illustrated embodiment are uniformly spaced about the circumference of the valve seat, and likewise the orifices  73   b  are uniformly spaced (but with a different spacing from that of the orifices  73   a ), but as further described below, non-uniform spacing of the orifices is also possible and can be advantageous in some circumstances. 
     The rotary valve member  74  (best illustrated in  FIG. 9 ) is a generally flat ring-shaped or annular member having a plurality of orifices  75   a  and  75   b  circumferentially spaced apart about a circumference of the valve seat, the orifices extending generally axially between the upstream and downstream faces of the valve member. The orifices  75   a  are located at a relatively larger radius from the center of the valve member  74 , and the orifices  75   b  are at a relatively smaller radius from the center of the valve member. The orifices  75   a  in the illustrated embodiment are uniformly spaced about the circumference of the valve member, and the number and spacing of the orifices  75   a  in the valve member are the same as the number and spacing of the orifices  73   a  in the valve seat. However, as further described below, non-uniform spacing of the orifices  75   a  is also possible and can be advantageous in some circumstances; furthermore, the spacings of the orifices  73   a  and  75   a  do not have to be the same, and in some cases it can be advantageous for the spacings to be different. Similarly, the orifices  75   b  in the illustrated embodiment are uniformly spaced about the circumference of the valve member, and the number and spacing of the orifices  75   b  in the valve member are the same as the number and spacing of the orifices  73   b  in the valve seat. The valve member  74  has a substantially circular cylindrical outer edge  76  and a substantially circular cylindrical inner edge  78 , the outer and inner edges  76 ,  78  being coaxial with respect to a central longitudinal axis of the valve member, which axis is also substantially coincident with a central longitudinal axis of the valve seat  72 . The outer portion  52   a  of the turbine housing and the inner member  52   b  both define substantially circular bearing surfaces for the outer and inner edges  76 ,  78  of the rotary valve member  74  and there are clearances therebetween, so that the valve member can be rotated in one direction or the opposite direction about its central longitudinal axis in order to vary a degree of alignment between the valve member orifices  75  and the valve seat orifices  73 , as further described below. 
     The valve member  74  further defines a fork or yoke comprising a pair of projections  80  that project axially from the upstream face of the valve member. The projections  80  are circumferentially spaced apart by a small distance sufficient to accommodate the distal end  92  of an L-shaped drive arm  90  that is rigidly affixed to a distal (radially inner) end of a rotary drive member  100 . The rotary drive member  100  penetrates substantially radially through the turbine housing  52  via a bore  53  (see  FIGS. 5 and 6 ) that connects with the generally annular bypass passage  60 . The proximal (radially outer) end of the rotary drive member  100  is located outside the turbine housing  52  and is rigidly affixed to a link  110 . The link  110  has a connecting member  112  ( FIGS. 2 and 3 ) that is offset from the rotation axis of the rotary drive member  100  and that can be coupled to an actuator rod of an actuator (not shown) such that extension of the actuator rod causes the link  110  to rotate the rotary drive member  100  in one direction and retraction of the actuator rod causes the link to rotate the rotary drive member in the opposite direction. As a result, the drive arm  90  affixed to the distal end of the rotary drive member  100  in turn causes the valve member  74  to be rotated in one direction or the opposite direction about its axis. 
     With reference particularly to  FIGS. 4 ,  7 , and  8 , each of the orifices  73   a  and  73   b  in the valve seat  72  has a raised pad  82  surrounding it. The pads  82  abut the planar face of the valve member  74  ( FIG. 4 ) and serve as seals to help seal the interface between the valve member and valve seat. The valve seat  72  also defines a ring-shaped raised pad  83  located radially intermediate the orifices  73   a  and the orifices  73   b  for sealing against a surface of the valve member so as to minimize or eliminate leakage flow between the outer and inner bypass streams in the bypass passages  60   a  and  60   b . The provision of the raised pads  82 ,  82  reduces the total surface area of the valve seat  72  in frictional contact with the rotary valve member  74 , thereby reducing the total friction forces that the actuation system must overcome to rotate the valve member. 
       FIGS. 10-13  illustrate the various positions of the bypass valve  70  for regulating bypass flow. In  FIG. 10 , the valve member  74  is positioned such that each of its orifices  75   a  is located between two adjacent orifices  73   a  in the valve seat  72 , and each orifice  75   b  is located between two adjacent orifices  73   b , and there is sufficient circumferential distance between adjacent orifices  73   a, b  to accommodate an orifice  75   a, b  with no overlap therebetween. Thus, the position of  FIG. 10  represents a closed position of the bypass valve in which essentially no exhaust gas can pass through the bypass passages  60   a  and  60   b  (except perhaps for a very small leakage flow of no appreciable consequence). 
       FIG. 11  shows the valve in a “crack-open” position in which the valve member  74  has been rotated a small amount clockwise relative to  FIG. 10  such that there just begins to be some overlap between the orifices  75   a  and the orifices  73   a  and between the orifices  75   b  and the orifices  73   b.    
     With further clockwise rotation of the valve member  74  from the position of  FIG. 11 , a greater degree of overlap exists between the orifices  75   a  and  73   a  and between the orifices  75   b  and the orifices  73   b , as shown in the “partially open” position of  FIG. 12 . 
       FIG. 13  shows a fully open position of the valve, in which the maximum possible overlap exists between the orifices  75   a  and  73   a  and between the orifices  75   b  and the orifices  73   b.    
     With the described annular bypass valve  70 , exhaust gas pressure acts on the valve member  74  in a direction toward the fixed valve seat  72 , thereby tending to improve sealing between the valve member and valve seat. Furthermore, the gas pressure does not tend to open the valve, in contrast to the aforementioned swing and poppet style bypass valve arrangements in which gas pressure acts in a direction tending to open the valve and cause leakage. The improved sealing made possible by the valve  70  is thought to be significant because it can improve the transient response time of the turbocharger, by making better use of instantaneous engine pulses in the exhaust gas stream, especially at low engine speeds and gas flow rates where the pulse impact is most significant in regard to turbine efficiency. 
     A further advantage is that the valve  70  can achieve better controllability than is typically possible with swing or poppet valves, particularly at the crack-open point. In particular, the evolution of the shape and size of the flow passages through the valve as the valve member  74  is rotated can be tailored to the needs of a particular application simply by suitably configuring the sizes, angular locations (e.g., whether uniformly or non-uniformly spaced apart), and shapes of the orifices in the valve member and valve seat. Thus, while the orifices  73 ,  75  are shown as being circular in the drawings, alternatively they can be made non-circular as a way of altering the evolution of the flow passages as the valve opens. For example, the orifices could be made generally rectangular with their edges extending generally radially (possibly with a larger dimension in the radial direction than in the circumferential direction), which would result in a greater change in flow passage size per degree of valve member rotation, in comparison with the circular orifice shape. 
     As another example of the fine-tuning of the evolution of the valve flow passages made possible by the invention, the valve seat orifices  73  could have a first circumferential spacing (e.g., uniform) about the circumference, and the valve member orifices  75  could have a second circumferential spacing (e.g., non-uniform) different from the first circumferential spacing. It is further possible (though not essential) in such an embodiment for the orifices  73 ,  75  to be of different sizes and/or shapes. This could result in, for example, one flow passage (or some other subset of the total number of flow passages) beginning to open before any of the other flow passages begin to open, thereby achieving a very gradual cracking open of the bypass valve. Further rotation of the valve member  74  would then cause the other flow passages to open (perhaps in a sequential or staged fashion, e.g., one flow passage opening at a time until finally all flow passages are open). These are merely some examples of the many different ways the orifices can be configured so as to achieve a desired flow passage evolution as a function of valve member rotation. 
     As noted, if desired or necessary in a particular application, surfaces of components that relatively slide over one another can be coated with a friction/wear coating that reduces wear of the surfaces. For example, the raised pads  82 ,  83  and/or the surface of the valve member  74  that the pads contact can be coated. Other surfaces that can be coated include the end of the L-shaped drive arm  90  and/or the surfaces of the valve member  74  contacted by the end of the drive arm. The radially inner and/or outer surfaces of the valve member  74  and/or the confronting surfaces of the turbine housing portions  52   a ,  52   b  can be coated with the wear coating. The wear coating can be formed of various materials, including an aluminum diffusion coating, a nickel boron coating, a ceramic coating (e.g., silicon nitride). In order to avoid material transfer from one surface to an abutting surface, preferably the two surfaces are coated with different materials. 
     The present invention is not limited to turbochargers with meridionally divided turbine housings such as the embodiment illustrated in  FIGS. 1-6  and described above. The invention can also be applied to turbochargers having a sector-divided turbine housing such as shown in  FIGS. 14-17 . As shown in  FIG. 14 , major sub-assemblies of the turbocharger  20 ′ include a compressor assembly  30 ′, a center housing assembly  40 ′, and a turbine assembly  50 ′. The compressor assembly  30 ′ includes a compressor housing  32 ′ and a compressor wheel  34 ′ mounted therein and attached to one end of a rotary shaft  36 ′. The center housing assembly  40  includes a center housing  42 ′ that is affixed to the compressor housing  32 ′ and that contains bearings  44 ′ for the rotary shaft  36 ′. The turbine assembly  50 ′ includes a turbine housing  52 ′ and a turbine wheel  54 ′ mounted therein and attached to the opposite end of the rotary shaft  36 ′. 
     The turbine housing  52 ′ defines an exhaust gas inlet  56 ′ through which exhaust gas from an internal combustion engine is received, and a sector-divided volute  58 ′ that receives the exhaust gas from the inlet  56 ′ and distributes the gas around the volute for feeding into the turbine wheel  54 ′. In particular, the sector-divided volute  58 ′ defines a first volute sector  58   a ′ and a separate second volute sector  58   b ′ each of which extends approximately 180° about the longitudinal axis of the turbine. The volute sectors  58   a ′ and  58   b ′ (hereinafter referred to simply as “volutes  58   a ′ and  58   b ′) collectively extend about 360° about the axis. The volutes  58   a ′ and  58   b ′ respectively receive two separate streams of exhaust gas from the engine (e.g., one stream from one sub-group of engine cylinders and the other stream from another sub-group of engine cylinders). Correspondingly, the exhaust gas inlet  56 ′ is a divided inlet that defines two separate inlets  56   a ′ and  56   b ′ ( FIG. 17 ) that are connected respectively to the two volutes  58   a ′ and  58   b′.    
     The two gas inlets of the divided inlet  56 ′ are also open to two separate generally annular bypass passages  60   a ′ and  60   b ′ defined in the turbine housing  52 ′. The bypass passages  60   a ′ and  60   b ′ are concentrically arranged (outer passage  60   a ′ being radially outward of inner passage  60   b ′) and both surround an axial bore  62 ′ defined in the turbine housing. Exhaust gas that has passed through the turbine wheel  54 ′ is exhausted from the turbine housing through the bore  62 ′. The bypass passages  60   a ′ and  60   b ′ provide an alternative pathway for exhaust gas to flow without first having to pass through the turbine wheel  54 ′. 
     An annular bypass valve  70 ′ is installed in the bypass passage  60 ′ for regulating flow through the bypass passage. The bypass valve  70 ′ is a divided bypass valve identical or substantially identical to the bypass valve  70  described previously, such that its description will not be repeated here. 
     Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.