Patent Publication Number: US-2012023936-A1

Title: Nozzled turbocharger turbine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS FIELD 
     This patent application claims the benefit of U.S. Provisional Patent Application No. 61/369,147, filed Jul. 30, 2010, which is incorporated herein in its entirety by this reference. 
    
    
     TECHNICAL FIELD 
     This patent disclosure relates generally to turbochargers for use with internal combustion engines and, more particularly, to turbochargers used on internal combustion engines. 
     BACKGROUND 
     Internal combustion engines are supplied with a mixture of air and fuel for combustion within the engine that generates mechanical power. To maximize the power generated by this combustion process, the engine is often equipped with a turbocharged air induction system. 
     A turbocharged air induction system includes a turbocharger that uses exhaust from the engine to compress air flowing into the engine, thereby forcing more air into a combustion chamber of the engine than the engine could otherwise draw into the combustion chamber. This increased supply of air allows for increased fuelling, resulting in an increased engine power output. 
     The fuel energy conversion efficiency of an engine depends on many factors, including the efficiency of the engine&#39;s turbocharger. Previously proposed turbocharger designs include turbines having separate gas passages formed in their housings. In such turbines, two or more gas passages may be formed in the turbine housing and extend in parallel to one another such that exhaust pulse energy fluctuations from individual engine cylinders firing at different times are preserved as the exhaust gas passes through the turbine. These exhaust pulses can be used to improve the driving function of the turbine and increase its efficiency. 
     Internal combustion engines also use various systems to reduce certain compounds and substances that are byproducts of the engine&#39;s combustion. One such system, which is commonly known as exhaust gas recirculation (EGR), is configured to recirculate metered and often cooled exhaust gas into the intake system of the engine. The combustion gases recirculated in this fashion have considerably lower oxygen concentration than the fresh incoming air. The introduction of recirculated gas in the intake system of an engine and its subsequent introduction in the engine cylinders results in lower combustion temperatures being generated in the engine, which in turn reduces the creation of certain combustion byproducts, such as compounds containing oxygen and nitrogen. 
     One known configuration for an EGR system used on turbocharged engines is commonly referred to as a high pressure EGR system. The high pressure designation is based on the locations in the engine intake and exhaust systems between which exhaust gas is recirculated. In a high pressure EGR system (HP-EGR), exhaust gas is removed from the exhaust system from a location upstream of a turbine and is delivered to the intake system at a location downstream of a compressor. When entering the intake system, the recirculated exhaust gas mixes with fuel and fresh air from the compressor and enters the engine&#39;s cylinders for combustion. 
     In engines lacking specialized components, such as pumps, that promote the flow of EGR gas between the exhaust and intake systems of the engine, the maximum possible flow rate of EGR gas through the EGR system will depend on the pressure difference between the exhaust and intake systems of the engine. This pressure difference is commonly referred to as the EGR driving pressure. It is often the case that engines require a higher flow of EGR gas than what is possible based on the EGR driving pressure present during engine operation. 
     In the past, various solutions have been proposed to selectively adjust the EGR driving pressure in turbocharged engines. One such solution has been the use of variable nozzle or variable geometry turbines. A variable nozzle turbine includes moveable blades disposed around the turbine wheel. Motion of the vanes changes the effective flow rate of the turbine and thus, in one aspect, creates a restriction that increases the pressure of the engine&#39;s exhaust system during operation. The increased exhaust gas pressure of the engine results in an increased EGR driving pressure, which in turn facilitates the increased flow capability of EGR gas in the engine. 
     Although such and other known solutions to increase the EGR gas flow capability of an engine have been successful and have been widely used in the past, they require use of a variable geometry turbine, which is a relatively expensive device that includes moving parts operating in a harsh environment. Moreover, variable geometry turbines typically destroy or mute the pulse energy of the exhaust gas stream of the engine, which results in lower turbine efficiency and higher fuel consumption. 
     SUMMARY 
     The disclosure describes, in one aspect, a turbine. The turbine includes a turbine housing having at least two gas passages having substantially the same flow area and disposed on either side of at least one divider wall, and a turbine wheel having a plurality of blades. A nozzle ring is connected to the turbine housing and disposed around the turbine wheel. The nozzle ring includes an inner ring disposed adjacent the divider wall and at least one outer ring. A plurality of vanes is fixedly disposed between the inner and outer rings. The vanes define a plurality of inlet openings therebetween that are in fluid communication with a slot formed in the ring that surrounds the turbine wheel. The inner ring is disposed to block a portion of the inlet openings that fluidly communicate with one of the at least two gas passages in the turbine housing. 
     In another aspect, the disclosure describes an internal combustion engine. The engine includes a divided turbine having first and second inlets. A first plurality of is cylinders connected to a first exhaust conduit, which is connected to the first inlet of the divided turbine. A second plurality of cylinders is connected to a second exhaust conduit, which is connected to the second inlet of the divided turbine. A balance valve is disposed to selectively route exhaust gas from the first exhaust conduit to the second exhaust conduit. An exhaust gas recirculation (EGR) system includes a valve that selectively fluidly connects the first exhaust conduit with an intake system of the engine. The divided turbine includes a turbine housing having at least two gas passages having substantially the same flow area and disposed on either side of at least one divider wall, and a turbine wheel having a plurality of blades. A nozzle ring is connected to the turbine housing and disposed around the turbine wheel. The nozzle ring includes an inner ring disposed adjacent the divider wall and at least one outer ring. A plurality of vanes is fixedly disposed between the inner and outer rings. The vanes define a plurality of inlet openings therebetween that are in fluid communication with a slot formed in the ring that surrounds the turbine wheel. The inner ring is disposed to block a portion of the inlet openings that fluidly communicate with one of the at least two gas passages in the turbine housing. 
     In yet another aspect, the disclosure describes a nozzle ring for a turbine. The nozzle ring includes an inner ring disposed adjacent a divider wall of a turbine housing when the nozzle ring is disposed within the turbine housing. The inner ring defines a divider wall extension having a generally trapezoidal shape that includes a substantially flat base, which is adapted to be disposed adjacent the divider wall. The nozzle ring further defines a rounded base and two generally straight edges connected to the flat base and tangentially meeting the ends of the rounded base when viewed in section taken along a diameter of the nozzle ring. At least one outer ring has a radial thickness and is disposed at an axial distance relative to the inner ring. A plurality of vanes is fixedly disposed between the inner and outer rings. The vanes define a plurality of inlet openings therebetween that are adapted to be in fluid communication with one or more gas passages defined in the turbine housing. The divider wall extension portion has a radial thickness of about 40% of the total radial thickness of the at least one outer ring, and is slanted by about 60 degrees relative to the flat base such that one of the two straight edges is about 60% of the length of the other of the two straight edges. A radius of the rounded base is about 16% of the length of the longer of the two straight edges and about 25% of the length of the shorter of the two straight edges. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an internal combustion engine having a high pressure EGR system in accordance with the disclosure. 
         FIG. 2  is a section of a turbocharger assembly in accordance with the disclosure. 
         FIG. 3  is a detail section of a turbine assembly in accordance with the disclosure. 
         FIG. 4  is an outline view of a radial nozzle ring in accordance with the disclosure. 
         FIG. 5  is a section of a nozzle ring in accordance with the disclosure 
         FIGS. 6-8  are detail sections of different embodiments of nozzle ring configurations in accordance with the disclosure. 
         FIGS. 9 and 10  are sections of a turbine housing in accordance with the disclosure. 
         FIG. 11  is an outline view of a turbine wheel in accordance with the disclosure. 
         FIG. 12  is a cross section of a radial nozzle turbine in accordance with the disclosure. 
         FIG. 13  is a cross section of a mixed flow nozzle turbine in accordance with the disclosure. 
         FIGS. 14 and 15  are diagrammatic nozzle flow profile charts in accordance with the disclosure. 
         FIG. 16  is an alternative embodiment for a turbine housing in accordance with the disclosure. 
         FIG. 17  a chart comparing turbine efficiency data in accordance with the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure relates to an improved turbine configuration used in conjunction with a turbocharger in an internal combustion engine to promote the engine&#39;s efficiency and ability to drive sufficient amounts of EGR gas. A simplified block diagram of an engine  100  having a high pressure EGR system  102  is shown in  FIG. 1 . The engine  100  includes a crankcase  104  that houses a plurality of combustion cylinders  106 . In the illustrated embodiment, six combustion cylinders are shown in an inline configuration, but any other number of cylinders arranged in a different configuration such as a “V” configuration may be used. The plurality of cylinders  106  is fluidly connected via exhaust valves (not shown) to first and second exhaust conduits  108  and  110 . Each of the first and second exhaust conduits  108  and  110  is connected to a respective exhaust pipe  112  and  114 , which are in turn connected to a turbine  120  of a turbocharger  119 . A balance valve  116  is fluidly interconnected between the two exhaust pipes  112  and  114  and is arranged to route exhaust gas from the first exhaust pipe  112  to the second exhaust pipe  114  as necessary during operation. It is noted that the balance valve  116  is optional and may be omitted. 
     In the illustrated embodiment, the turbine  120  has a separated housing, which includes a first inlet  122  fluidly connected to the first exhaust pipe  112 , and a second inlet  124  connected to the second exhaust pipe  114 . Each inlet  122  and  124  is disposed to receive exhaust gas from one of the first and second exhaust conduits  108  and  110  during engine operation. The exhaust gas operates to cause a turbine wheel (not shown here) connected to a shaft  126  to rotate before exiting the turbine  120  through an outlet  128 . The exhaust gas at the outlet  128  is optionally passed though other exhaust components, such as an after-treatment device  130  that mechanically and chemically removes combustion byproducts from the exhaust gas stream, and/or a muffler  132  that dampens engine noise, before being expelled to the environment through a stack or tail pipe  134 . 
     Rotation of the shaft  126  causes the wheel (not shown here) of a compressor  136  to rotate. As shown, the compressor  136  is a radial compressor configured to receive a flow of fresh, filtered air from an air filter  138  through a compressor inlet  140 . Pressurized air at an outlet  142  of the compressor  136  is routed via a charge air conduit  144  to a charge air cooler  146  before being provided to an intake manifold  148  of the engine  100 . In the illustrated embodiment, air from the intake manifold  148  is routed to the individual cylinders  106  where it is mixed with fuel and combusted to produce engine power. 
     The EGR system  102  includes an optional EGR cooler  150  that is fluidly connected to an EGR gas supply port  152  of the first exhaust conduit  108 . A flow of exhaust gas from the first exhaust conduit  108  can pass through the EGR cooler  150  where it is cooled before being supplied to an EGR valve  154  via an EGR conduit  156 . The EGR valve  154  may be electronically controlled and configured to meter or control the flow rate of the gas passing through the EGR conduit  156 . An outlet of the EGR valve  154  is fluidly connected to the intake manifold  148  such that exhaust gas from the EGR conduit  156  may mix with compressed air from the charge air cooler  146  within the intake manifold  148  of the engine  100 . 
     The pressure of exhaust gas at the first exhaust conduit  108 , which is commonly referred to as back pressure, is higher than ambient pressure because of the flow restriction presented by the turbine  120 . For the same reason, a positive backpressure is present in the second exhaust conduit  110 . The pressure of the air or the air/EGR gas mixture in the intake manifold  148 , which is commonly referred to as boost pressure, is also higher than ambient because of the compression provided by the compressor  136 . In large part, the pressure difference between back pressure and boost pressure, coupled with the flow restriction of the components of the EGR system  102 , determine the maximum flow rate of EGR gas that may be achieved at various engine operating conditions. 
     For this reason, the backpressure at the first exhaust conduit  108  is maintained at a higher level than the back pressure at the second exhaust conduit  110  at times during engine operation when additional EGR driving pressure is desired. To accomplish this pressure increase, the turbine  120  is configured to have different exhaust gas flow restriction characteristics, with the flow entering through the first inlet  122  being subject to a higher flow restriction than the flow entering through the second inlet  124 . This different or asymmetrical flow restriction characteristic of the turbine  120  provides an increased pressure difference to drive EGR gas without increasing the backpressure of substantially all cylinders  106  of the engine  100 . At times when no back pressure increase is desired in the first exhaust conduit  108  to drive EGR gas flow, the optional balance valve  116  may be used to balance out the exhaust flow through each of the two inlets  122  and  124  of the turbine  120 . 
     In the description that follows, structures and features that are the same or similar to corresponding structures and features already described are denoted by the same reference numerals as previously used for simplicity. Accordingly, a cross section of one embodiment of the turbocharger  119  is shown in  FIG. 2 . The turbocharger  119  includes the turbine  120  and compressor  136  that are connected to one another via a center housing  202 . As shown, the center housing  202  surrounds a portion of the shaft  126  and includes a bearing (not shown) disposed within a lubrication cavity  206 . The lubrication cavity  206  includes lubricant inlet and outlet openings  208  and  210  that provide lubrication to the bearing as the shaft  126  rotates during operation. 
     The shaft  126  is connected to a turbine wheel  212  at one end and to a compressor wheel  213  at another end. The turbine wheel  212  is configured to rotate within a turbine housing  215  that is connected to the center housing  202 . The compressor wheel  213  is disposed to rotate within a compressor housing  217 . The turbine wheel  212  includes a plurality of blades  214  radially arranged around a hub  216 . The hub  216  is connected to an end of the shaft  126  by a fastener  218  and is configured to rotate the shaft  126  during operation. A detailed outline view of the turbine wheel  212  is shown in  FIG. 11 . The turbine wheel  212  is rotatably disposed between an exhaust gas inlet slot  230  defined within the turbine housing  215 . The slot  230  provides exhaust gas to the turbine wheel  212  in a radial direction along the leading edges  222  of the blades  214 . Exhaust gas exiting the turbine wheel  212  is provided to a turbine outlet bore  234  that is fluidly connected to the turbine outlet  128 . The gas inlet slot  230  is fluidly connected to inlet gas passages  236  formed in the turbine housing  215  and configured to fluidly interconnect the gas inlet slot  230  with the turbine inlets  122  and  124  ( FIG. 1 ). 
     Each of the two turbine inlets  122  and  124  is connected to one of two inlet gas passages  236 . Each gas passage  236  has a generally scroll shape that is wrapped around the area of the turbine wheel  212  and bore  234  and is open to the slot  230  around the entire periphery of the turbine wheel  212 . The cross sectional flow area of each passage  236  decreases along a flow path of gas entering the turbine  120  via the inlets  122  and  124  and exiting the housing through the slot  230 , as is generally shown in  FIG. 9  that follows. As shown, the two passages  236  have substantially the same cross sectional flow area at any given radial location around the wheel  212 . Although two passages  236  are shown, a single or more than two passages may be used. 
     A radial nozzle ring  238  is disposed substantially around the entire periphery of the turbine wheel  212 . As will be discussed in more detail in the paragraphs that follow, the radial nozzle ring  238  is disposed in fluid communication with both passages  236  and defines the slot  230  around the wheel  212 . As shown in  FIG. 2  and in the detailed view of  FIG. 3 , a divider wall  240  is defined in the housing  215  between the two passages  236 . The divider wall  240  is disposed radially outwardly relative to the slot  230  such that gas flow from the two passages  236  may be combined before entering the slot  230  and reaching the wheel. 
     In further reference to  FIGS. 4 and 5 , the nozzle ring  238  includes an inner ring  242  disposed between two outer rings, namely a first outer ring  243  and a second outer ring  244 . The inner ring  242  is positioned adjacent the divider wall  240  and forms an extension thereof, as shown in  FIG. 3 , to form a divider wall extension portion  245 . In the illustrated embodiment, the inner ring  242  has an asymmetrical shape that provides different flow areas between each of the first and second outer rings  243  and  244  and the inner ring  242  for gas passing through each of the two passages  236  into the slot  230 . A plurality of vanes  246  is symmetrically disposed between the first and second outer rings  243  and  244  and intersect the inner ring  242  as they extend axially along the rotation axis of the turbine wheel  212 . 
     The shape and configuration of the vanes  246  can be best seen in the cross section of  FIG. 5 . As shown, the vanes  246  are arranged symmetrically around a central opening  248  of the ring  238  such that inclined flow channels  250  are defined between adjacent vanes  246 . The flow momentum of gas passing through the channels  250  is directed generally tangentially and radially inward towards an inner diameter of the wheel  212  (shown in  FIG. 2 ) such that wheel rotation may be augmented. Although the vanes  246  further have a generally curved airfoil shape to minimize flow losses of gas passing over and between the vanes  246 , thus providing uniform inflow conditions to the turbine wheel, they also provide structural support to the inner ring  242 . In the illustrated embodiment there are thirteen vanes connected to the ring  238 , but any other number of rings may be used. In a preferred embodiment, the number of vanes  246  is different than the number of blades  214  such that resonance conditions are avoided during operation. 
     Returning now to  FIG. 2 , the nozzle ring  238  is disposed within a bore formed in the turbine housing  215 . A retainer  252  is disposed to retain the ring  238  within the housing  215 . The retainer  252  extends peripherally around the ring  238  and is retained to the housing by one or more fasteners  254 . Further, one or more pins  255  disposed in corresponding cavities formed in the housing and in the ring  238  may be used to properly orient the nozzle ring  238  relative to the housing  215  during assembly. The nozzle ring  238  may have a clearance fit with the bore of the housing  215  such that sufficient clearance is provided for thermal growth of each component during operation to minimize thermal stresses. 
     As best shown in  FIG. 3 , the second outer ring  244  of the nozzle ring  238  defines a contact pad  256  that abuts the retainer  252 . The contact pad  256  is disposed to provide axial engagement of the nozzle ring  238  with the housing  215 . The illustrated configuration of the nozzle ring  238  includes two pluralities of inlet openings  258  and  260 , each of which is defined between adjacent vanes  246 , the inner ring  242 , and the corresponding first or second outer rings  243  or  244 . Accordingly, a first plurality of inlet openings  258  is defined between the first outer ring  243  and the inner ring  242 , and a second plurality of inlet openings  260  is defined between the inner ring  242  and the second outer ring  244 . 
     As shown, each of the first plurality of inlet openings  258  is in fluid communication with the gas passage  236  shown on the left side of the illustration of  FIG. 3 . The inlet openings  258  permit the substantially unobstructed flow of gas therethrough. However, the inclination of the divider wall extension portion  245  of the inner ring  242 , which is towards the right in the illustration of  FIG. 3 , reduces or obstructs a portion of the flow area of each of the second plurality of inlet openings  260 . 
     The reduced flow opening of the second plurality of inlet openings  260  as compared to the first plurality of inlet openings  258  provides an asymmetrical flow restriction to gas present in one of the gas passages  236  over the other. In the embodiment shown and in further reference to  FIG. 1 , the turbine inlet  122  that is fluidly connected to the first exhaust conduit  108  is configured to be in fluid communication with the second plurality of inlet openings  260 . The turbine inlet  124  that is fluidly connected to the second exhaust conduit  110  is correspondingly in fluid communication with the first plurality of inlet openings  258 . Notwithstanding any flow diversion that may be selectively provided by the balance valve  116  ( FIG. 1 ) between the two turbine inlets  122  and  124  during operation, the reduced flow area corresponding to the second plurality of inlet openings  260  in the turbine will provide an increased gas pressure in the first exhaust conduit  108  such that the flow of EGR gas may be augmented, as previously described. 
     Therefore, the unique flow characteristics of the turbine  120  may be determined by the size, shape, and configuration of the nozzle ring  238  while other portions of the turbine may advantageously remain unaffected or, in the context of designing for multiple engine platforms, the remaining portions of the turbine may remain substantially common for various engines and engine applications. Accordingly, the specific symmetrical or asymmetrical flow characteristics of a turbine that is suited for a particular engine system may be determined by combining a turbine, which otherwise may be common for more than one engine, with a particular nozzle ring having a configuration that is specifically suited for that particular engine system. 
     The customization capability provided by a specialized nozzle ring in an otherwise common turbocharger assembly presents numerous advantages over known turbochargers. First, an engine or parts manufacturer may streamline its production by reducing the number of different turbochargers that are manufactured. In this way, waste, inventory, and costs may be reduced in the market for original and service parts. Moreover, parts may remain common even when other surrounding components and systems, such as the EGR system, undergo changes to keep up with changing performance demands. Even further, low production number engine applications, which may otherwise not have a specialized turbocharger manufactured to optimally suit them because of cost considerations, may now be more easily customized at a lower cost by simply incorporating a unique nozzle ring in an otherwise common turbocharger. These and other advantages may be realized by use of interchangeable rings for turbines as set forth herein. 
     Based on the foregoing, it should be clear that the nozzle rings may be tailored in numerous configurations to provide a desired flow restriction and flow characteristics for the turbocharger in which they are installed. Accordingly, three different embodiments of nozzle rings are shown in partial cross section in  FIGS. 6-8 . In the description that follows, structures or features that are the same or similar as corresponding structures or features previously described are denoted by the same reference numerals as previously used for simplicity. The nozzle ring shown in  FIG. 7  is the same as the nozzle ring  238  previously described, which is shown here to illustrate certain differences in the structures of the nozzle rings shown in  FIGS. 6 and 8  both to one another as well as to the nozzle ring  238  ( FIG. 7 ). 
     Accordingly, a first alternative embodiment of a nozzle ring  300  is shown in  FIG. 6 . The nozzle ring  300  includes first and second outer rings  243  and  244  disposed on either side of an inner ring  302 . Unlike the asymmetrical flow characteristics between the two pluralities of inlet openings  258  and  260  discussed previously relative to the nozzle ring  238  ( FIG. 7 ), the nozzle ring  300  ( FIG. 6 ) has a substantially balanced flow characteristic. More specifically, the nozzle ring  300  includes two pluralities of inlet openings  304  and  306 , which are defined between the vanes  246 , the sides of the inner ring  302  and the corresponding side of the first and second outer rings  243  and  244 . Given the symmetrical shape of the divider wall extension provided by the inner ring  302 , the flow area of each of the two pluralities of inlet openings is substantially equal. When coupled with the substantially equal flow areas between the two gas passages  236  ( FIG. 2 ), use of the nozzle ring  300  will provide a substantially equal flow restriction between the two inlets  122  and  124  of the turbine  120  ( FIG. 1 ). 
     The nozzle ring  300  further includes an outer ring extension  308  that is connected to the second outer ring  244  and that extends radially inward alongside the corresponding side of the slot  230 , as shown in  FIG. 6 . In certain embodiments, the extension  308  may provide additional direction to gases passing through the slot  230  towards the blades  214  of the turbine wheel  212  ( FIG. 2 ) to increase the efficiency of the turbine. 
     A second alternative embodiment of a nozzle ring  400  is shown in  FIG. 8 . In this embodiment, the divider wall extension provided by the inner ring  401  is asymmetrical to create a difference in flow area between first and second pluralities of inlet openings  402  and  404  that is roughly inverse to that of the ring  238  ( FIG. 7 ). In other words, where the ratio of flow areas in the ring  238  is about 70/30, expressed as a percentage, the corresponding ratio of flow areas in the ring  400  is about 30/70. It is contemplated that any desired ratio may be accomplished in the flow areas of inlet openings disposed on either side of the inner ring. For instance, the ratio may not only be anywhere between 30/70 and 70/30 as shown here, but any other ratio may be used by appropriately positioning and shaping the inner ring relative to the outer rings. Moreover, although the asymmetry in the illustrated embodiments is accomplished by providing a slanted inner ring relative to the first or second outer rings  243  or  244 , other methods may be used such as differently shaped inner ring cross sections and others. 
     As shown, for example, in  FIG. 7 , the divider wall extension portion  245  of the inner ring  242  has a generally oblique trapezoidal shape having a flat base  751 , along which it contacts the divider wall, and a generally rounded opposite base  753 . In the illustrated embodiment, the radial thickness of the divider wall extension portion  245  is about 40% of the total radial thickness of each of the first and second outer rings  243  and  244 . The bases  751  and  753  are connected by two straight edges  755  and  756  that extend tangentially relative to the rounded base  753 . As shown, the length of the shorter edge  755 , is about 60% of the length of the longer of the two edges  756 , while the radius of the rounded base  753  is about 16% of the length of the longer of the two edges  756  and about 25% of the length of the shorter of the two edges  755 . In the embodiment shown in  FIG. 7 , the divider wall extension portion  245  is disposed at an angle of about 60 degrees relative to the flat base  751  but other angles to yield different flow asymmetry may be used. Moreover, shapes other than oblique trapezoids having a rounded base may also be used. 
     A cross section of the turbine  120  is shown in  FIG. 9 , with an enlarged detail view thereof shown in  FIG. 10 . In these views, the general shape of one of the gas passages  236  can be seen wrapped around a center opening  502  of the housing  215 . As can also be seen, one of the turbine inlets  124  is fluidly connected to the passage  236 . The turbine inlet  124  is formed as an opening in a flange  504  that is defined on the housing  215  and used to mount the turbine  120  to the engine  100  ( FIG. 1 ). 
     As is best shown in  FIG. 10 , the gas passage  236  includes an inlet portion  506  and a turbine wheel supply portion  508 . The turbine wheel supply portion  508  has a decreasing flow area as it extends peripherally around the opening  502  beginning from a transition area  510 , which is disposed at the transition between the inlet and supply portions  506  and  508 , and ending adjacent a tip  512  of a tongue feature  514  of the housing  215 . The tongue feature  514  of the housing  215  is a wall separating the inlet portion  506  from the supply portion  508  of the housing  215 . The tongue  514  is a feature generally found in all radial turbine housings and is also an area of the turbine housing that is prone to cracking and failure due to thermal stresses and high cycle fatigue. During operation, exhaust gas entering the turbine  120  via the inlet  124  passes through the inlet portion  506  and enters the turbine wheel supply portion  508  of the passage  236 . 
     Exhaust gas entering the supply portion  508  of the passage  236  passes through the inlet openings in the ring  238 , such as the openings  260 , to radially and tangentially impinge onto the turbine wheel (not shown here), causing it to rotate. As gas passes through the openings  260  its pressure along the length of the passage will tend to decrease, which is avoided by the decreasing volume of the passage  236  as it extends around the opening  502 . 
     An additional novel feature of the turbine  120  is the tip  512  of the tongue  514  has been shortened to a greater extent than what would have been necessary to merely provide clearance for installation of the ring  238  around the opening  502 . As is best shown in  FIG. 10 , the tip  512  is disposed at a radial distance that forms a radial gap  516  with the outer diameter  518  of the ring  238 . In this way, the performance and efficiency of the turbine may be improved because, in part, the static pressure of exhaust gas reaching the end of the supply portion  508  of the passage  236  is augmented by gas in the inlet portion  506  through the gap  516 . Moreover, the reliability of the turbine  120  is improved because the shortened tongue  514  will be less prone to failure, such as from cracking or thermal stresses. In the illustrated embodiment, the tip  512  has a generally rounded shape that tangentially meets two curved sidewalls  520  and  522 . The sidewall  520  is part of the inlet portion  506  and has a generally curved shape as it follows the passage  236 . The sidewall  522  is part of the supply portion  508 . In the illustrated embodiment, the tongue  514  extends radially around the opening  502  over a radial distance of about 70 degrees. A radius of the tip  512  is about 13% of the largest thickness of the tongue  514  and about 20% of the radius of the opening  124 . The chord length of the sidewall  520  along the tongue  514  is about twice the diameter of the inlet opening  124 . 
     In reference now to  FIG. 11 , the turbine wheel  212  is generally described. Each blade  214  of the wheel  212  is spaced at an equal radial distance from its adjacent blades  214  around the hub  216 . In the illustrated embodiment, the turbine wheel  212  includes eleven blades  214  but any other number of blades may be used. Each blade  214  includes a body section  220  having a generally curved shape. The body section  220  is connected to the hub  216  along one side. A leading edge  222  is disposed at a radially outermost portion of the wheel  212  and is configured to admit a portion of a flow that operates to turn the wheel  212 . As flow enters into radial channels  224  defined between the blades  214 , the flow momentum pushes against the body sections of the blades  214 , thus imparting a moment that turns the wheel  212 . In the illustrated embodiment, for example, the wheel  212  is configured to rotate in a counterclockwise direction when viewed from the perspective of the fastener  218 . 
     The hub  216  has a generally curved conical shape such that flow entering into the channels  224  from a radial direction is turned by about 90 degrees and exits the wheel in an axial direction. The rotation of the wheel  212  is augmented as it pushes against a discharge portion  226  of each blade  214 . The discharge portion  226  has a generally curved shape that is disposed at a discharge angle  228  relative to an opposite portion of each blade  214  adjacent the inlet of the channel  224  as shown. In the illustrated embodiment, the discharge angle  228  is about 60 degrees, which is an angle that is steeper than corresponding angles used on typical turbines by about 4 to 5 degrees. 
     The wheel  212  shown in  FIG. 11  is generally configured to receive gas provided in a radial direction relative to the wheel  212 . The direction of gas flow  602 , which is denoted by arrows, can be determined by the general shape of the nozzle ring  238 , as is more particularly shown and described relative to  FIG. 12 .  FIG. 12  is a cross section of the turbocharger  119  ( FIG. 2 ). As shown in this illustration, a plane  604  extending radially relative to the axis of rotation of the wheel  212  is shown at a location substantially bifurcating the divider wall  240 . An outer, generally frusto-conical internal contour surface  605  of the inner ring  242  extends at an angle, α, relative to the plane  604  along a line denoted as  606  in the cross section of  FIG. 12 . Similarly, an inner contour surface  607  of the inner ring  242  extends at an angle, β, relative to the plane  604  along a line  608 . In embodiments using a radial flow wheel, such as wheel  212 , the angles α and β are substantially equal such that a symmetrical gas momentum velocity condition is created around the divider wall  240 . This symmetrical gas velocity around the divider wall  240  provides gas travelling generally in a radial direction relative to and toward the wheel  212 . 
     Depending on the design of the wheel, however, a mixed-flow gas velocity may alternatively be provided, which includes an axial-flow component in addition to the radial-flow discussed above. In general, turbines can be configured for radial-flow, axial-flow (for example, such as those used in jet engines), or a hybrid type of flow that includes radial and axial components, which will hereinafter be referred to as “mixed” flow to denote that the flow includes radial and axial flow characteristics. 
       FIG. 13  is a cross section of a mixed flow turbine  610 . The turbine  610  includes a mixed-flow turbine wheel  612  that receives exhaust gas passing through a nozzle ring  614 . In this embodiment, although the nozzle ring  614  includes many of the same features as previously described, it is also configured to impart an axial component to the gas momentum velocity of gas provided to the wheel  612 . More particularly, a plane  616  extending radially relative to the axis of rotation of the wheel  612  is shown at a location substantially bifurcating the divider wall  240 . In this embodiment, the outer, generally frusto-conical internal contour surface  617  of the nozzle ring  614  extends at an angle, α, relative to the plane  616  along a line denoted as  618  in the cross section of  FIG. 13 . Similarly, the inner contour surface  619  of the nozzle ring  614  extends at an angle, β, relative to the plane  616  along a line  620 . Unlike the embodiment shown in  FIG. 12 , the angles α and β in this embodiment are different, with angle β being larger than angle α such that an asymmetrical gas momentum velocity condition is created around the divider wall  240 . This asymmetrical gas velocity around the divider wall  240  provides exhaust gas having momentum components provided both radially and axially relative to and toward the wheel  212 . In the illustration of  FIG. 13 , solid line arrows  622  denote the general direction of travel for gas provided to the turbine wheel  612 . 
     When the gas flows  602  ( FIGS. 12) and 622  ( FIG. 13 ) are qualitatively compared as illustrated, it can be seen that the mixed-flow turbine  610  can operate using a turbine wheel configured to operate with gas having axial flow characteristics. This type of operation can be beneficial in part because the pressure drop of gas passing through the turbine  610  is lower than that of turbine  119 . Two qualitative charts showing certain geometrical characteristics of two different mixed flow turbines in accordance with the disclosure are shown in  FIGS. 14 and 15  for illustration. The illustrations in these figures can be considered as representing detail cross section views of the gas flow transition between a first turbine housing  702  ( FIG. 14 ) or second turbine housing  704  ( FIG. 15 ) into a turbine wheel  706 . 
     In each illustration, a radial plane  708  is defined to coincide with an inner portion of the turbine wheel  706  as shown. An axis  710  is defined along a centerline and rotation axis of the wheel  706 . The radial plane  708  and axis  710  intersect the plane of the cross sections illustrated in  FIGS. 14 and 15  to define an orthogonal coordinate system having an origin at  712  for the purpose of the following discussion. 
     In reference now to  FIG. 14 , a gas flow path  714 , which is generally denoted by a solid lined arrow, passes through an annular passage  716  formed in the turbine housing  712 , which may further include a nozzle ring having vanes  246  (see, for example,  FIG. 4 ) disposed within the annular passage  716  as previously described. The annular passage  716  is defined between an outboard inlet surface  718  and an inboard inlet surface  720 , which are shown in cross section. Exhaust gas from the annular passage  716  is provided to operate the wheel  706  as it passes through areas  722  defined between the blades  724  of the turbine wheel  706 . Gas exiting the wheel  706  passes through a generally cylindrical turbine outlet volume  726  that is defined between an outer outlet bore  728  and, at least partially, an outer portion  730  of the turbine shaft  732 . 
     As can be seen in the cross section of  FIG. 14 , an inner angle  734  has an inner centerpoint  735  and is defined between the inboard inlet surface  720  and the outer portion  730  of the turbine shaft  732  to be about 90 degrees. An outer angle  736  has an outer centerpoint  737  and is defined between the outboard inlet surface  718  and the outlet bore  728 . The outer angle  736  is a more acute and in the embodiment illustrated in  FIG. 14  is about 75 degrees. As can be seen in  FIG. 14 , the inner centerpoint  735  is located close to the origin  712 . 
     In reference now to  FIG. 15 , the gas flow path  738 , which is generally denoted by a solid lined arrow, is generally less curved than the flow path  714  of the embodiment shown in  FIG. 14  because of the general shape of the surrounding geometry. More particularly and in reference to  FIG. 15 , where features and elements that are the same or similar corresponding features and elements previously described are denoted by the same reference numerals as previously used for simplicity, the annular passage  740  is defined between outboard and inboard inlet surfaces  742  and  744 . In this embodiment, an inner angle  746  defined between the inboard inlet surface  744  and the outer portion  730  of the turbine shaft  732  has a centerpoint  747  that is close to the axis  710  but away from the origin  712  in a direction toward the generally cylindrical turbine outlet volume  726 . When compared to the inner angle  734  ( FIG. 14 ), the inner angle  746  of the embodiment shown in  FIG. 15  is larger and, as shown, measures about 115 degrees. The outer angle  748  defined between the outboard inlet surface  742  and the bore  728  in this embodiment is also larger than the outer angle  736  ( FIG. 14 ) and measures about 90 degrees. 
     With respect to the two alternative embodiments for the mixed flow turbines  702  and  704 , as shown in  FIGS. 14 and 15 , respectively, it can be seen that the general shape and inlet angles of the annular openings in the turbine housings  702  and  704  providing gas flow to the turbine wheel  706  can be adjusted to provide steeper or shallower angles of incidence of exhaust gas flowing toward the turbine wheel  706 . In this way, steeper entry angles such as those shown in  FIG. 14  have a more pronounced radial flow characteristic, while shallower angles such as those shown in  FIG. 15  have a more pronounced axial flow characteristic. These and other design parameters, coupled with the particular design of a turbine wheel, can be adjusted and selected to provide optimal turbocharger performance for each particular engine configuration. Selection of the appropriate parameters that correspond to a particular engine can involve the consideration of various turbocharger operating conditions, such as exhaust gas temperature, pressure and flow rate, desired pressure difference to drive the turbine, turbine size, and others, desired turbine A/R ratio, and others. 
     Apart from the performance characteristics of the various turbines described thus far, the structure of any of the embodiments described thus far can be modified for ease of manufacture. For example, one embodiment for a turbine  800  is shown in  FIG. 16 . The turbine  800  is, in many ways, similar to the mixed flow turbine  610  shown in cross section in  FIG. 13 , except that in this embodiment the nozzle ring  614  ( FIG. 13 ) is integrated with the turbine housing  610  ( FIG. 13 ) to provide a mixed-flow, nozzled turbine housing  802 . The nozzled turbine housing  802  retains all the desired functional characteristics previously described, but by virtue of the integration of the nozzle ring into the housing of the turbine provides a lower part count, which can reduce assembly complexity and cost as well as reduce the time required to service or remanufacture the turbine  802 . 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is applicable to radial and mixed-flow turbines, especially those turbines used on turbocharged internal combustion engines. Although an engine  100  having a single turbocharger is shown ( FIG. 1 ), any engine configuration having more than one turbocharger in series or in parallel arrangement is contemplated. 
     As is known, turbine performance depends in part on the available energy content or enthalpy per unit of gas driving the turbine. The ratio of an ideal or maximum turbine wheel velocity, which depends on the energy available to drive the turbine wheel, over the actual tangential velocity of the turbine wheel blades is commonly used to quantify turbine efficiency in a non-dimensional fashion. Accordingly, the ratio of the actual tangential velocity of a blade, U, over an ideal velocity, C, can be experimentally determined for any given pressure ratio or difference applied to a turbine, for example, on an engine or on a gas stand. The ratio of U/C is thus a non-dimensional indication of a turbine&#39;s operating state at which the efficiency of the turbine may be determined to empirically characterize the available energy and blade tangential velocity with respect to turbine efficiency. Alternatively, the U/C ratio may also be defined as the ratio of circumferential speed and the jet velocity corresponding to an ideal expansion from an inlet to an outlet condition of the turbine. 
     In  FIG. 17 , a qualitative chart  900  showing turbine efficiency  902  is plotted against the U/C ratio  904  for various embodiments of turbines the turbine, for example, a radial turbine  120 , a mixed-flow turbine  610 , a mixed-flow, nozzled turbine  802 , and a baseline turbine for comparison. The data illustrated in graphical form in the chart  900  was acquired on a gas stand and/or estimated using simulation models for all turbine types having the same or comparable frame sizes and similar pressure ratios. 
     In reference now to the chart  900 , a first efficiency curve  906  (shown in solid line) was acquired from successive runs of the baseline turbine. As can be seen from the chart  900 , the turbine efficiency was maximum at about 76% while the baseline turbine was operating at a U/C ratio of about 66%. A second efficiency curve  910  represents the performance of a radial, nozzled turbine operating at substantially the same operating conditions as those used when acquiring the plotted information from the baseline turbine. As can be seen from the second efficiency curve  910 , the radial, nozzled turbine had an efficiency of about 80% at a U/C of about 70%. In other words, the addition of the features consistent with the nozzle ring added to a baseline radial turbine resulted in an efficiency increase of about 4% points. The extent of increase in turbine efficiency, which in different turbine configurations was observed to be as much as 7.5% points between the baseline and nozzled radial turbines was unexpected. 
     A third efficiency curve  910  represents the performance of a mixed flow turbine. The third efficiency curve shows a peak efficiency of about 78% at a U/C of about 63%. In other words, the modification of the baseline turbine for mixed flow operation provided a performance increase of about 2% points at a lower U/C. A fourth efficiency curve  912  represents an estimation based on simulation of the performance of a mixed-flow nozzled turbine. Here, the peak efficiency is expected to be about 81% at a U/C of about 60%. In other words, the addition of nozzle vanes and of features that provide for mixed flow is expected to improve the efficiency of a turbine about 5% points over the baseline design, which is a considerable improvement. Additional empirical data on this and other testing is available but not presented herein for brevity. 
     It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.