Abstract:
A more efficient multi-stage turbocharging system and method for internal combustion engine systems is set forth. The present invention recovers the loss of a portion of exhaust energy that conventionally occurs in bypassing exhaust flow from one stage to another in a multi-stage turbocharging system. The preferred method of preserving such exhaust energy is through converting a portion of the exhaust energy of the bypassed flow from pressure to kinetic energy (velocity) by passing the bypassed flow through a VGT vane outlet or other variable geometry valve/nozzle, and then not allowing the accelerated flow to dissipate energy before reaching the subsequent stage&#39;s turbine wheel, where the accelerated flow may then be converted to a mechanical rotational force by the lower pressure turbine&#39;s wheel. Preferred hardware for achieving the object of the invention is also set forth, including a preferred two-volute low pressure turbocharging system with a VGT mechanism in one turbine volute only, or an alternative low pressure turbocharger with two low pressure turbines on a common shaft (again, preferably, with a VGT mechanism in one turbine only).

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the priority benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application No. 60/605,898, filed Aug. 31, 2004, which application is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to internal combustion engine systems with multi-stage turbocharging systems, and the use of multi-stage turbines in general.  
         [0004]     2. Description of the Related Art  
         [0005]     Turbocharging systems, such as for use with internal combustion engines, are well-known in the art. A turbocharger comprises an exhaust gas turbine coupled to a gas intake charge compressor. The turbine operates by receiving exhaust gas from an internal combustion engine and converting a portion of the energy in that exhaust gas stream into mechanical energy by passing the exhaust stream over blades of a turbine wheel, and thereby causing the turbine wheel to rotate. This rotational force is then utilized by a compressor, coupled by a shaft to the turbine wheel, to compress a quantity of air to a pressure higher than the surrounding atmosphere, which then provides an increased amount of air available to be drawn into the internal combustion engine cylinders during the engine&#39;s intake stroke. The additional compressed air (boost) taken into the cylinders can allow more fuel to be burned within the cylinder, and thereby offers the opportunity to increase the engine&#39;s power output.  
         [0006]     In a turbocharged internal combustion engine system, the wide range of speed and power output levels at which the internal combustion engine may operate presents challenges for designing an appropriately matched turbocharging system with good mechanical efficiency for working with the engine. For example, while smaller turbochargers provide boost quickly and more efficiently at lower engine speeds, larger turbochargers provide boost more effectively at higher engine speeds. Because of the relatively narrow flow range over which a turbocharger operates efficiently, relative to the broader flow range generated by internal combustion engines, it is known in the prior art (e.g., in cases of high boost need), to provide a multi-stage turbocharging system, involving both a smaller (i.e. “high pressure”) turbocharger and a larger (i.e. “low pressure”) turbocharger, wherein the smaller high pressure turbocharger operates at lower engine speeds and the larger low pressure turbocharger takes over at higher engine speeds. It has been found valuable to switch between the two turbocharging stages through use of a bypass system to divert exhaust gas flow around the higher pressure turbocharger to the lower pressure turbocharger as needed.  
         [0007]     As a result, bypassing exhaust flow around a turbine gas expander is also well-known in the art. Typically, turbine bypass systems are used in the prior art primarily to regulate system pressure across the higher stage turbine wheel, and can be operated by selectively bleeding off a portion of the upstream exhaust gas over a pressure drop through a bypass channel when backpressure caused by the turbine&#39;s operation causes the system pressure upstream of the turbine to exceed desired levels. Bleeding of the exhaust gas through the bypass channel is generally controlled by a small regulating valve (called a “wastegate”) in the exhaust piping channel around the turbine. A typical wastegate valve operates somewhat like a trap door, opening a port from the higher pressure turbine inlet to a lower pressure area by diverting a portion of the exhaust flow through a bypass channel around the turbine, with the bypassed exhaust flow naturally expanding over the pressure drop in the bypass channel and then reuniting with the remaining exhaust flow downstream of the bypassed turbine.  
       OBJECT OF THE INVENTION  
       [0008]     An object of the present invention is to provide a more efficient multi-stage (i.e., with two or more stages) turbocharging system for internal combustion engine systems.  
         [0009]     In furtherance of the object of this invention, it has been recognized that prior art wastegate and bypass mechanisms are a source of unnecessary loss of useful energy in prior art multi-stage turbocharging systems. Therefore, a further object of the present invention is to provide an efficient means for preserving, capturing, utilizing, and/or reducing the amount of energy otherwise lost in bypassing between stages in multi-stage turbocharging systems, in order to further improve the efficiency of internal combustion engine systems utilizing multi-stage turbocharging systems.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention reduces the unrecovered loss of exhaust gas energy that otherwise occurs in bypassing exhaust flow from one stage to another in a conventional multi-stage turbocharging system. The preferred method of preserving such exhaust energy is through converting a portion of the exhaust energy of the bypassed flow from pressure to velocity by passing the bypassed flow, while substantially still at the higher exhaust energy level present upstream of the bypassed turbine, through a variable geometry valve/nozzle, turbine VGT vanes, or other reduced cross-sectional area nozzle, and then not allowing the accelerated flow to substantially lose that increased recoverable kinetic energy before reaching the subsequent stage&#39;s turbine wheel. This may be done, for example, through placing the variable geometry valve or VGT vane outlet adjacent to the lower pressure turbine wheel&#39;s blades (or sufficiently nearby such blades to avoid substantial dissipation of the increased acceleration/momentum effect), and at an appropriate incidence angle to the lower pressure turbine wheel&#39;s blades. The increased momentum resulting from accelerating the flow may then be imparted to the lower pressure turbine&#39;s wheel, and thereby allow converting the energy to a mechanical rotational force as is known in the art. Alternative means and preferred turbocharging hardware embodiments for efficiently preserving or capturing energy lost between stages in a multi-stage turbocharging system are also discussed. This system may be utilized between stages with internal combustion engine or other multi-stage turbine systems encompassing three or four (or more) stage systems as well. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale.  
         [0012]      FIG. 1  is a schematic diagram of an internal combustion engine system with a prior art multi-stage turbocharging system.  
         [0013]      FIG. 2  is a schematic diagram of an internal combustion engine system with a first turbocharging and bypass arrangement of the present invention.  
         [0014]      FIG. 3  is a schematic diagram of an internal combustion engine system with a second, alternative bypass arrangement of the present invention.  
         [0015]      FIG. 4  is a more detailed view of the turbocharging and bypass arrangement of the system shown in  FIG. 3 .  
         [0016]      FIG. 5  is a schematic diagram of an internal combustion engine system with another alternative turbocharging and bypass arrangement of the present invention.  
         [0017]      FIG. 6A  presents a preferred variable geometry valve/nozzle means for use with a two-volute low pressure turbine, with the valve/nozzle means being VGT vanes, shown in an open position, in a second volute of a two-volute low pressure turbine.  
         [0018]      FIG. 6B  provides another view of the  FIG. 6A  preferred variable geometry valve/nozzle means, but shown with the valve/nozzle means in a closed position.  
         [0019]      FIG. 6C  presents a conventional rotating VGT vane, for use in a turbine.  
         [0020]      FIG. 6D  presents an alternative VGT vane with an articulating trailing edge.  
         [0021]      FIGS. 7A and 7B  are sectional views of a two-volute turbine, showing an alternative variable geometry valve/nozzle means embodiment of the present invention for use in a second volute of a two-volute low pressure turbine.  
         [0022]      FIG. 8  is a sectional view of a two-volute turbine, showing a second alternative variable geometry valve/nozzle means embodiment of the present invention for use in a second volute of a two-volute low pressure turbine.  
         [0023]      FIG. 9A  illustrates the preferred internal combustion engine multi-stage turbocharging and bypass arrangement of the present invention, with a partially cut-away view of volute  53 ′ in the invention.  
         [0024]      FIG. 9B  illustrates the two volute turbine of the preferred embodiment of an internal combustion engine multi-stage turbocharging and bypass arrangement of the present invention.  
         [0025]      FIG. 10  presents a cut-away view of a two-volute turbine in a side-by-side orientation, such as for use in the preferred embodiment of  FIGS. 9A and 9B  of the invention.  
         [0026]      FIG. 11  is a schematic view of another alternative embodiment of an internal combustion engine system of the present invention, with two turbines on a common shaft.  
         [0027]      FIG. 12  is a more detailed view of the  FIG. 11  two turbines on a common shaft turbocharging and bypass arrangement of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]      FIG. 1  shows an internal combustion engine system with a multi-stage turbocharging and bypass system from the prior art. Referring to  FIG. 1 , ambient air enters the system through intake line  11 . The intake air may optionally be mixed with recirculated exhaust gas (EGR) to form a charge-air mixture. The ambient air or EGR/ambient air mixture (“charge-air”) mixture flows through and is compressed by a first-stage low pressure air compressor  12 .  
         [0029]     After compression in compressor  12 , the intake air may flow through a second-stage high pressure air compressor  16  for further compression. Alternatively, the intake air may be diverted at port  13  to optional bypass channel  14  and return to the intake line at port  17 , as regulated by the opening or closing of optional bypass valve  15 .  
         [0030]     Intake air then enters the intake manifold  18  and into combustion chambers  20  of engine  19  through conventional valves (not shown) in a conventional manner. Following combustion in the combustion chambers  20 , the warm, pressurized exhaust gases leave the combustion chambers  20 , at a first, higher, exhaust gas energy level, through conventional valves (not shown) in a conventional manner, and flow from engine  19  through exhaust manifold  21  to exhaust line  28 .  
         [0031]     After leaving the exhaust manifold  21 , exhaust gas in exhaust line  28  may flow through a high pressure turbine gas expander  25 . High pressure turbine gas expander  25  in exhaust line  28  is coupled to the high pressure air compressor  16  in the intake line  11  through shaft  29 ′, and together the combined expander and compressor device forms a high pressure turbocharger  30 . Alternatively to flowing through high pressure turbine  25 , a portion of the exhaust gas may be selectively diverted at port  22  to bypass channel  23  and return to the exhaust line at port  26 , as regulated by opening or closing of port  22  through operation of wastegate valve  24 , which is operated (actively or passively) to open in response to system pressure buildup upstream of turbine  25 . Wastegate valve  24  may be located anywhere within bypass channel  23 . It should be noted that even though the wastegated exhaust gas does not pass through turbine expander  25 , the pressure difference between the bypassed exhaust gas flow and the exhaust gas that has passed through turbine expander  25  is lost to natural expansion and dissipation of any increased velocity of the bypassed exhaust gas, either in bypass channel  23  or upon reuniting with the lower pressure exhaust flow in exhaust line  28  at port  26 .  
         [0032]     Downstream of turbine gas expander  25 , the exhaust gas at this second, lower, exhaust gas energy level may then flow through low pressure turbine gas expander  27  for further expansion, and then exit the system through exhaust line  28 . It should also be noted for  FIG. 1  that turbine gas expander  27  in exhaust line  28  is coupled to low pressure air compressor  12  in intake line  11  through shaft  29 , and together the expander  27  and compressor  12  integrated device form a low pressure turbocharger  31 .  
         [0033]      FIG. 2  presents a first improvement on the prior art multi-stage turbocharged internal combustion engine system of  FIG. 1 , as one embodiment operating in accordance with principles of the present invention. For ease of discussion in highlighting aspects of this embodiment of the invention over the prior art, the embodiment of  FIG. 2  is presented herein as identical to  FIG. 1  of the prior art in all respects (i.e., with identical components, numeration, system configuration and operation), except as hereafter described.  
         [0034]     Referring to  FIG. 2  in comparison to the  FIG. 1  prior art, it will be noted that certain changes from the prior art have been made with relation to the bypass system around high pressure turbine  25 . Like valve  24  of  FIG. 1 , valve  34  of  FIG. 2  regulates (e.g. through a pressure differential) the quantity of exhaust gas diverted from exhaust line  28  through bypass means channel  33  to port  36 . However, in  FIG. 2 , valve  34  and return port  36  are geometrically configured closer to, and at a more complementary angle for direction of the bypass flow toward the inlet of turbine  27 .  
         [0035]     These changes in  FIG. 2  are made in recognition that a portion of the energy in the bypassed exhaust gas that is diverted through bypass channel  33  is converted from pressure to kinetic energy (velocity) at valve  34  by passing the bypassed flow through valve  34 , with valve  34  acting as a reduced cross-sectional area nozzle. Valve/nozzle  34  therefore acts in  FIG. 2  as a nozzle when in an open position, by providing a reduced cross-sectional area flow path for the bypassed exhaust gas. As an example, valve/nozzle  34  may open to form a flow path in the shape of a short tube with a taper or constriction (reduced cross-section) designed to speed up (and preferably also direct) the flow of exhaust gas. As will be known in the art, there are many known alternative structures that may also perform this similar “nozzle” function of speeding up the flow of a gas or fluid, which are also intended to be encompassed within this patent&#39;s use of the terms “nozzle” or “nozzle means” herein.  
         [0036]     The accelerated flow exiting valve/nozzle  34  is there reunited at intersection point  36  with the flow in exhaust line  28  (or directly at an inlet to turbine  27 ), preferably in an orientation resulting in an optimal combined direction for the exhaust flows just prior to, and at an appropriate incidence angle to, the turbine wheel blades of turbine  27 , as will be known in the art. The accelerated flow is there converted, combined with the exhaust flow in exhaust line  28 , to a mechanical rotational force by turbine  27 . By locating port  36  sufficiently near the turbine wheel blades of turbine  27 , the accelerated flow is not allowed to substantially dissipate energy before reaching the turbine wheel of turbine  27  for work extraction. Regarding selection of acceptable distances between valve/nozzle  34  and the turbine wheel, it will be understood that the closer the distance will result in greater recovery of energy, and that through experimentation the distance can be increased until such point that the increase in recovery of energy from the bypass acceleration is no longer measurable with normal state of the art sensors and thus would no longer fall within the scope of this invention.  
         [0037]     Thus, in  FIG. 2 , bypass means  33  and valve/nozzle  34  provide bypassing of pressurized exhaust gas from the engine around the high pressure turbine to an inlet of the lower pressure stage turbine in this embodiment, by leaving the bypassed flow in a complementary flowing direction with the main exhaust flow just prior to the turbine wheel blades of turbine  27 , regardless of (and thus it is irrelevant for this particular embodiment) whether port  36  lies as a direct inlet to turbine  27  or as a substantially equivalent return port to exhaust line  28  just prior to turbine  27 .  
         [0038]      FIG. 3  presents the same embodiment as  FIG. 2 , but illustrating that the length of the bypass route  33  is irrelevant and may be substantially eliminated, if desired. In addition, for either  FIG. 2  or  3 , the bypass route may optionally begin directly from exhaust manifold  21  instead of exhaust line  28 , if desired, such as is illustrated in  FIG. 5  (discussed below).  
         [0039]      FIG. 4  illustrates in more detail one embodiment of reuniting of the accelerated bypass flow with the main exhaust flow prior to and at an appropriate incidence angle to the turbine wheel blades of turbine  27  as discussed for  FIGS. 2 and 3  above. As shown in  FIG. 4 , bypass exhaust flow  49  in bypass route  33  passes through valve  34  in a reduced cross section (nozzle) area of bypass route  33  and/or port  36 , which produces an accelerated bypass exhaust flow  51 . Other “nozzle means” for accelerating the bypass exhaust flow may alternatively be used, as is known in the art. Accelerated bypass exhaust flow  51  then combines with the lower velocity main exhaust flow  50  in exhaust line  28  (or, alternatively, within the turbine  27  itself), forming combined exhaust flow  52 . Combined exhaust flow  52  preferably shortly thereafter hits the turbine blades  48  at a desired angle to cause turbine wheel  47  to spin, as is known in the art. Note, however, that it is not necessary for the bypass flow to reunite with the main exhaust flow prior to impact with the lower pressure turbine&#39;s wheel blades for the energy to be recovered.  
         [0040]      FIG. 5  presents an alternative embodiment, with bypass route  43  connected directly to exhaust manifold  21 , instead of to exhaust line  28 . In this manner, for each of these embodiments, it will be understood that bypass means  43  may be shortened to be no more than a direct fluid connection between exhaust manifold  21  and an inlet to low pressure turbine  27 . In addition, returning to  FIG. 5 , in  FIG. 5 a  two-volute turbine  27 ′ (e.g.  FIG. 10 ) replaces turbine  27 , with one volute  53  of turbine  27 ′ receiving lower velocity and energy exhaust from exhaust line  28  downstream of high pressure turbine  25 , and the other volute  53 ′ of turbine  27 ′ receiving the higher energy and velocity (accelerated) bypassed exhaust directly from exhaust manifold  21  (without being reunited with other exhaust gas prior to impact with the lower pressure turbine&#39;s wheel blades). Volutes  53  and  53 ′ of turbine  27 ′ need not be of the same size. Two-volute turbines such as turbine  27 ′ are known in the art, although more commonly with volutes of the same size, such as for use with divided exhaust manifolds.  FIG. 10  presents a cut-away view of a sample two-volute turbine  27 ′. It will also be understood that the flows from the two volutes of turbine  27 ′ may be coordinated in various ways with regard to the targeting of the respective flows toward the blades of the turbine wheel, if desired.  
         [0041]     For  FIGS. 2 through 5  above, it has already been discussed that valve  34  in the bypass route may function in the present invention as both (i) a regulating valve to control bypass flow, and (ii) as a nozzle that converts a portion of the exhaust energy of the bypassed flow from pressure to kinetic energy (in the form of increased velocity of the bypassed exhaust flow). Given the wide range of exhaust flows generated in internal combustion engines that operate under wide ranges of engine speed and load conditions, it is preferable with the present invention to utilize a valve/nozzle means with variable geometry capability in accelerating the bypassed exhaust flow, to extend the system&#39;s benefits and effectiveness over a wider range of engine operation.  
         [0042]     There are various structures that may be utilized to serve the functions of valve/nozzle means  34  with the preferred variable geometry capability. In  FIGS. 6A and 6B , as a preferred embodiment for use with a two-volute turbine  27 ′ (or also for single volute turbine  56  of the two turbine arrangement in  FIG. 12 , discussed below), VGT vanes  54  surrounding the turbine wheel  47  function as the valve/nozzle means.  FIG. 6C  presents a larger view of a conventional VGT vane  54 .  
         [0043]      FIG. 6A  illustrates the vanes  54  in an open orientation, allowing and guiding passage of bypassed exhaust flow  49  to the turbine blades  48 , and additionally acting as variable geometry nozzles in accelerating the exhaust flow  49  into the turbine blades  48 . In contrast,  FIG. 6B  shows the vanes  54  of  FIG. 6A  in a completely closed orientation (i.e. here, lined up “tail to nose” around the turbine wheel  47 ), thereby sealing and blocking any bypass exhaust flow through the volute  53 ′ to the turbine blades  48 . In this manner, the position of the VGT vanes  54  can operate fully as a regulating valve to open or shut off flow, and dictates the back pressure applied to the exhaust line  28  and/or exhaust manifold  21  in the system, and thus also controls the pressure drop allowed for the main exhaust flow through the high pressure turbine  25 . This consequently provides flow control through the alternative exhaust paths, including proportional flow control, to extend the system&#39;s benefits and effectiveness over a wider range of engine speed and load operating conditions.  
         [0044]     As an alternative to VGT vanes  54  for two-volute turbine  27 ′,  FIGS. 7A and 7B  utilize a sliding plate mechanism  54 ′ in volute  53 ′ to perform the valve/nozzle function in regulating and accelerating the bypass flow in volute  53 ′ to turbine blades  48 . Likewise,  FIG. 8  utilizes a sliding member/mechanism  54 ″, as shown for a two-volute double flow turbine housing (wherein for this second example the two volutes are concentrically disposed with respect to the circumference of the turbine wheel  47 , as opposed to being side-by-side with respect to the circumference of the turbine wheel  47 ).  
         [0045]      FIGS. 9A and 9B  now present the preferred embodiment of the multi-stage turbocharging system of the present invention.  FIG. 9A  is similar to the embodiment of  FIG. 5 , except as noted below. In the  FIG. 9A  preferred embodiment, valve/nozzle  34  is replaced by Variable Geometry Turbine (VGT) mechanism  54  in one volute, volute  53 ′, of two-volute turbine  27 ′. The two volutes  53  and  53 ′ are configured in a side-by-side orientation to each other with respect to their orientation around the circumference of the turbine wheel  47 , as shown by the partial cut-away view in  FIG. 9A , and as also more clearly shown in  FIG. 9B  and in  FIG. 10 . VGT mechanism  54  is presented herein in  FIGS. 9A and 9B  as conventional rotating adjustable vanes  54  (as also shown in  FIGS. 6A-6C ), but it will be understood that other VGT and/or other nozzle mechanisms may also be equivalently employed (e.g. a sliding nozzle mechanism as used by Cummins or in  FIGS. 7A-7B  and  8 , or a vane with an articulating trailing edge ( FIG. 6D ), as a few examples) without departing from the scope of the invention.  
         [0046]     As is known in the art, adjustable VGT vanes  54  act as nozzles to throttle exhaust gas and use the resulting restriction to create an accelerated, high velocity exhaust gas stream, and also to guide and direct that exhaust gas stream into the turbine wheel blades  48  (e.g., as is represented in  FIG. 6A ). Thus, in one embodiment, VGT mechanism  54  comprises conventional VGT vanes, which are rotating vanes arranged in a circle in the turbine volute  53 ′, with the vanes able to rotate uniformly to form wider or narrower paths for the exhaust gas to the turbine blades  48 . VGT mechanism(s)  54  are preferably placed closely adjacent the turbine blades  48  such that the kinetic energy of the bypassed exhaust flow passing by such vanes is fully preserved and not lost prior to the bypassed exhaust flow hitting turbine blades  48  at the optimal angle, as will be understood in the art. In contrast, second volute  53  of turbine  27 ′ is preferably a fixed volute without a VGT mechanism  54 , but may optionally use VGT as well, if desired. The flow from both volutes  53  and  53 ′ target portion(s) of the turbine wheel blades  48  as desired, as for example shown in the sample embodiment of  FIG. 9B .  
         [0047]     Further referring to  FIG. 9A , high pressure turbine  25  (presented simply in block form) is fluidly connected to exhaust manifold  21  by exhaust line  28 . High pressure turbine  25  may optionally contain a VGT mechanism, if desired. Exhaust gas enters and leaves high pressure turbine  25  through an inlet and outlet in conventional manner (not shown), to continue in exhaust line  28  to volute  53  of low pressure turbine  27 ′, where it is further expanded. The further expanded exhaust gas then leaves low pressure turbine  27 ′ through an outlet in conventional manner (not shown), to continue in exhaust line  28  for exhaust gas recirculation or for release from the exhaust system. Bypass means/turbine inlet  43  of low pressure turbine  27 ′ is also fluidly connected to exhaust manifold  21 , allowing high pressure exhaust gas to bypass high pressure turbine  25  to volute  53 ′ of low pressure turbine  27 ′. VGT mechanism  54 , as discussed above, here shown as adjustable rotating VGT vanes as one embodiment, acts in volute  53 ′ in place of a valve to regulate flow of bypassed exhaust gas flow  49  through volute  53 ′, and also acts as a nozzle means to convert the exhaust energy in bypassed exhaust flow  49  to kinetic energy (velocity) to create accelerated bypass exhaust flow  51 , and to guide or direct the accelerated bypass exhaust flow  51  to hit turbine blades  48  at an appropriate incidence angle (e.g., as shown in  FIG. 6A ) for spinning of turbine wheel  47 . The placement of the VGT mechanism  54  near the turbine blades  48  allows the kinetic energy and increase in momentum of the bypassed exhaust flow to be preserved (by not allowing deceleration and expansion) for conversion to mechanical force at the turbine blades  48 . The expanded exhaust gas from volute  53 ′ then leaves low pressure turbine  27 ′ through an outlet in conventional manner (not shown), to continue in exhaust line  28  for exhaust gas recirculation or for release from the exhaust system, as discussed above.  
         [0048]      FIG. 11  presents an alternative preferred embodiment of the engine system and turbocharging system of the present invention, similar to  FIG. 5  and to  FIGS. 9A and 9B , but comprising two low pressure turbines  56  and  57  on a common shaft  29 ″ instead of a two-volute low pressure turbine  27 ′. Turbine  56  utilizes a VGT mechanism  54  in a configuration and manner similar to volute  53 ′ from  FIGS. 5 and 9 A- 9 B, and receives bypass exhaust flow from bypass route  43  in one of the manners as previously described above. Turbine  57 , on the other hand, preferably utilizes a fixed geometry, and receives exhaust gas from exhaust line  28  that has already passed through high pressure turbine  25 , as also previously described above. Each low pressure turbine  56  and  57  includes a separate turbine wheel arrangement (identified as turbine wheels  47  and  47 ′, and blades  48  and  48 ′, as shown in  FIG. 12 ), with the rotating wheels  47  and  47 ′ connected by common rotating shaft  29 ″, which is also part of shaft  29 , which connects the two turbines  56  and  57  to compressor  12  (as shown in  FIG. 11 ). The compressor  12 , shaft  29  and  29 ″, and two turbine arrangement  56  and  57  comprise turbocharger  31 ′ in this embodiment.  
         [0049]     After expansion in turbines  56  and  57  of  FIG. 9 , the exhaust gas flows that leave turbines  56  and  57  are thereafter combined downstream in exhaust line  28  (or within the two turbine turbocharger arrangement itself, in the alternative).  
         [0050]     It will be understood from the foregoing that there are various other embodiments that could also be formed to achieve the novel objectives and methods of the inventions herein, and that such variations with equivalent functions and goals are also intended to fall within the scope of this patent. For example, the objectives of the inventions herein may apply to multi-stage turbines for gas or fluid flows in other applications than in conjunction with internal combustion engine turbocharging systems. This patent is therefore intended to be limited solely by the claims, in the manner allowed by law.