Patent Publication Number: US-2022234106-A1

Title: Additively manufactured rotors for superchargers and expanders

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation of U.S. patent application Ser. No. 16/079,763, filed on Aug. 24, 2018, which is a National Stage Application of PCT/US2017/019461, filed on Feb. 24, 2017, which claims the benefit of U.S. Patent Application Ser. No. 62/299,844, filed on Feb. 25, 2016, the disclosures of which are incorporated herein by reference in their entireties. To the extent appropriate, a claim of priority is made to each of the above disclosed applications. 
    
    
     TECHNICAL FIELD 
     This application relates to additively manufactured rotary components, such as Roots-type rotors for superchargers and expanders. 
     BACKGROUND 
     Various examples of Roots-type rotors for superchargers and expanders exist. In some applications, fluid temperatures necessitate the use of materials for the rotors that can withstand high temperatures. However, the use of such materials generally leads to rotors with a relatively high rotational inertia, as compared to rotors manufactured from materials that do not need to withstand high temperatures. 
     SUMMARY 
     Due to new applications of devices with rotary components (e.g. volumetric rotors or Roots-type rotors), such as WHR (waste heat recovery), ORC (organic Rankine cycle) and compound boosting with turbochargers feeding the supercharger inlet, higher temperature materials are required, lower inertia is desired to improve the transient response and different coefficients of thermal expansion are being investigated to maximize device efficiency. The materials that are able to perform at higher temperatures are typically higher density therefore we need a solution to lightweight rotors which is not cost prohibitive. 
     The teachings detailed in this disclosure are centered on the process of metal additive manufacturing for creating rotary components, such as rotors. Through the use of additive technology, formerly solid rotors can be developed in high temperature materials and hollowed out to reduce inertia. In addition to hollow rotors, the additive process allows for a lattice structure to be developed, which may allow an even thinner walled structure, thereby further reducing inertia. This invention also covers hollow or solid rotors that are printed directly onto a shaft. Another possibility covered in this invention is printing in multiple materials and printing on a coating, thereby avoiding any post processing of the rotor. The coating can also be the same material as the rotor, wherein the coating is applied at relatively higher porosity than the application for the rotor, thereby resulting in a lower density outer coating. Various portions of the rotor can be provided with differing material densities to account for different stresses at specific locations, thereby further reducing the inertia of the rotor. 
     Additional objects and advantages will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the teachings presented herein. The objects and advantages will also be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the claimed invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view of a hollow steel rotor formed by an additive manufacturing process that is mounted to a shaft, which is an example in accordance with aspects of the invention. 
         FIG. 2  is a schematic perspective view of the rotor shown in  FIG. 1  with an outer coating of the rotor not shown. 
         FIG. 3  is a schematic perspective view of the rotor shown in  FIG. 1  removed from the shaft. 
         FIG. 4  is a schematic end view of the rotor shown in  FIG. 1 . 
         FIG. 5  is a schematic end view of the rotor shown in  FIG. 1 . 
         FIG. 6  is a schematic side view of the rotor shown in  FIG. 1 . 
         FIG. 7  is a schematic cross-sectional side view of the rotor shown in  FIG. 6 . 
         FIG. 8  is a schematic perspective view of the rotor shown in  FIG. 1  in a pre-finished condition and illustrating a preferred manufacturing approach. 
         FIG. 9  is a schematic side view of the pre-finished rotor shown in  FIG. 8 . 
         FIG. 10  is a cross-sectional view of an end portion of the pre-finished rotor shown in  FIG. 8 . 
         FIG. 11  is a schematic cross-sectional side view of a lobe of the rotor shown in  FIG. 10 . 
         FIG. 12  is a schematic perspective view of a thin-walled hollow steel rotor formed by an additive manufacturing process, which is an example in accordance with aspects of the invention. 
         FIG. 13  is a schematic perspective view of a thin-walled rotor formed by an additive manufacturing process having an internal lattice structure, which is an example in accordance with aspects of the invention. 
         FIG. 14  is a schematic perspective view of a thin-walled rotor formed by an additive manufacturing process, wherein the rotor has been printed directly onto a shaft, which is an example in accordance with aspects of the invention. 
         FIG. 15  is a schematic perspective view of a printed lattice structure usable with the rotors shown in  FIGS. 12 to 14 . 
         FIG. 16  is a schematic cross-sectional view of an example thin-walled rotor showing a maximum rotor dimension. 
         FIG. 17  is a schematic cross-sectional view of the thin-walled rotor shown in  FIG. 19  showing a wall thickness. 
         FIG. 18  is a schematic view of a vehicle having a fluid expander and a compressor in which rotor assemblies of the types shown in  FIGS. 1 and 12 to 14  may be included. 
         FIG. 19  is a schematic side view of an expander within which any of the rotors shown in  FIGS. 1 and 12 to 14  can be used. 
         FIG. 20  is a schematic perspective view of the expander shown in  FIG. 19 . 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the examples which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. Directional references such as “left” and “right” are for ease of reference to the figures. 
     Rotor Design 
     Referring to  FIGS. 1-9 , a rotor  100  is shown that is produced by a 3D printing or additive manufacturing process such that a net shape or near net shape rotor is produced. The particular additive manufacturing approach utilized to produce the rotor  100  may be any of a variety of processes known in the art, such as selective laser sintering, selective laser melting, stereolithography, coaxial powder feeding, and wire feeding and fused deposition modeling. Powder-based deposition methods (powder bed or powder-fed) using laser or electron beam melting are useful. A variety of materials can be utilized to form the rotor, for example steel (e.g. 316L stainless steel) or aluminum (e.g. 1060 series aluminum). In one approach, the rotor  100  is formed in layers starting from a base end and continuing until the rotor is fully formed at an opposite end. An advantage of forming the rotor  100  with an additive manufacturing process is that the rotor  100  can be formed with internal cavities or voids such that the rotor is hollow with reduced mass and rotational inertia. Such a construction allows for a rotor  100  to be manufactured to the same size and shape but with reduced material, thus increasing performance over a solid rotor made from the same material. Such a construction also allows for the creation of a hollow rotor  100  formed from stainless steel that has the same or less rotational mass than a similarly sized solid aluminum rotor. Thus, a rotor  100  can be formed via an additive manufacturing process that has improved performance characteristics and that is also suitable for use in higher temperature applications, in comparison to a solid aluminum rotor. In some variations, the cavities or voids are filled with a second, different material, such as epoxy, after the rotor  100  has been fully formed from the first material. Where the fill material has a lower density than the material from which the rotor  100  is formed, a solid rotor  100  can be achieved that has reduced mass and rotational inertia in comparison to a solid rotor formed from only the first material. 
     As shown, the rotor  100  extends from a first end  108  to a second end  110 . A first end face  112  is provided at the first end  108  while a second end face  114  is provided at the second end  110 . A plurality of twisted or helically arranged lobes  102  extend between the first and second end faces  112 ,  114 . The lobes  102  can be alternatively provided as straight lobes and can be provided in numbers other than three lobes, for example, four lobes may be provided. As most easily viewed at  FIGS. 3, 6, and 7 , each of the lobes  102  is provided with an interior cavity  120  extending the majority of the length between the first and second end faces  112 ,  114 . As shown, the interior cavity extends between a first end  120   a  and a second end  120   b . The interior cavity  120  of each lobe  102  results in the lobe  102  having a wall  102   a  with a resulting wall thickness T 102   a  which can either be constant or varying. In one example, the wall thickness T 102   a  of the lobes  102  is greater near the inner root portion of the lobes  120  than at the outer tip portion of the lobes  120 . 
     In one aspect, the rotor lobes  102  are arranged around a central aperture  104  through which a shaft  38 ,  40  can extend.  FIGS. 1 and 2  show the rotor  100  mounted to the shaft  38 ,  40  while  FIGS. 3 to 8  show view of the rotor  100  removed from the shaft  38 ,  40 . The central aperture  104  is defined by a tubular section  106  that is also hollow. In one example, the tubular section  106  is provided with a minimum wall thickness of about 3 mm at an interior wall  102   b . In the embodiment shown, the tubular section  106  includes a stepped portion  106   a  for receiving a shoulder portion  38   a ,  40   a  of the shaft  38 ,  40 . The tubular section  106  can also be provided with second and third stepped portions  106   b ,  106   c , as shown at  FIG. 7 , for further engaging additional shoulder portions of the shaft  38 ,  40 . The tubular section  106  can be entirely formed by the additive manufacturing process or can be initially formed with the additive manufacturing process and then finally formed with a machining step, such as with a boring step. The tubular section  106  can be pressed onto the shaft  38 ,  40  or can be utilized as the shaft itself when formed as a hollow tube with additional material extending beyond the first and second faces  112 ,  114 . Alternatively, the tubular section  106  can instead be formed as a solid shaft such that shaft  38 ,  40  is an integrally formed portion of the rotor  100 . The integrally formed shaft  38 ,  40  can extend through the entire length of the rotor or could be formed such that only stubs extending from the ends of the rotor are present. 
     When producing the rotor  100 , the additive manufacturing process begins at the second end  110  and continues to completion at the first end  108 . In one example, a base  122  is first created, as shown at  FIGS. 8 and 9 . In one example, the base  122  is cylindrical and is formed at a thickness of about 10 mm (millimeters). The base  122  is used to hold the printed rotor  100  while machining and/or coating the rotor after the rotor  100  has been fully formed. After these steps are completed, the base  122  can be cut off or machined from the rest of the rotor to form the final end face  110  of the rotor. The end of the cylindrical base  122  facing the first end face  108  will form the second end face  110  of the rotor  100 . In one example, the base  122  and the end face  110  may be provided with ream holes  124  to allow powder to be removed that is entrapped within or falls into the cavities  120  during the additive manufacturing process. In one example, the ream holes  124  are about 5 mm in diameter. The ream holes can be filled with material matching that of the rotor  100  or the surface coating C 1  after the rotor  100  has been completely formed. The ream holes  124  can also be left open. The ream holes  124  can be used as location or mounting features during subsequent machining of the rotor  100 . In one alternative approach, the rotor  100  is printed onto a pre-formed shaft  38 ,  40 . 
     As material is added to build the rotor  110  up from the base  122  and towards the end face  108 , the end face  108  itself can be formed as an integral, self-supported cap. This can be accomplished by gradually building material, which may be referred to as a bridge portion  102   c , into each cavity  120  from the outer wall  102   a  or the root/inner portion  102   b  of the rotor  100 . Material can be built up from both the inner and outer walls  102   a ,  102   b  as well. Referring to  FIGS. 10 and 11 , it can be seen that the bridge portion  102   c  can be built up at an angle a 1  with respect to the wall(s)  120   a ,  120   b  (and longitudinal axis of the rotor  100 ) with added material until the cavity  120  is completely closed off and such that an end cap is formed. The angle a 1  can be between 30 to 60 degrees, and is preferably about 45 degrees. At this point, each cavity  120  has a tapered end proximate the end face  108  and is entirely enclosed with the exception of the ream holes  124 . In the example shown, the end faces  108  and  110  have a wall thickness of about 3 mm beyond the enclosed cavity  120 . Once the process is complete, the rotor  100  is formed as a monolithic structure having internal cavities within the lobes. In an alternative design, the rotor  100  is formed without the end caps defining the end faces  108 ,  110  and separate end caps are attached to the rotor  100  after the rotor has been printed. 
     In one example, the rotor  100  can be formed from different materials at different sections of the rotor  100 . Referring to  FIG. 9 , the rotor can be printed by adding the first and second materials in a printing order, wherein the first material is printed to form a first end section  102   d  of the rotor, a second, different material is printed to form a middle section  102   e , and the first material is again printed to form a second end section  102   f . The sections  102   d ,  102   e ,  102   f  are schematically shown as being separated by dashed lines on  FIG. 9 . The first and second materials can be can be any materials capable of being bonded together via an additive manufacturing process. In other examples, the portion of the rotor  100  nearest the central aperture  104  (e.g. the root portions of the lobes) is printed with the first material and the outer portions of the rotor  100  (e.g. the tip portions of the lobes) are printed with the second material. 
     The rotor  100  shown at  FIG. 1  is provided with an outer coating or layer C 1 , such as an abradable coating formed from an epoxy and graphite mixture. This coating can be applied via printing/additive manufacturing or can be applied via another process, such as electrostatic spraying. Printing the coating onto the rotor would eliminate the need for a conventional coating process, thereby reducing at least two steps in the entire production process: the finish machining operation and the coating operation. Such a step can represent a significant reduction in required capital machines to produce a rotor. In one example, the coating can be printed using the same material as the base rotor material. This would be done in a similar fashion to porous coated hips, after the rotor base is printed the exterior coating would be printed in a sparse matrix leaving a porous coating. There is concern that similar materials could lead to galling which leads to the next option of multi-material. The coating could potentially be printed with a softer or more abradable material. This would be accomplished by using a multi-material printing method. Powder bed technology or coaxial powder feed to print different materials together in a single block may be utilized. 
       FIG. 2  shows the rotor  100  without the coating C 1  where it can be seen that a pin P 1  is provided. Pin P 1  extends through the rotor  100  and into the shaft  38 ,  40  and ensures that the rotor  100  cannot rotate relative to the shaft  38 ,  40 . During assembly, the rotor  100  can be press fit onto the shaft  38 ,  40  until the stepped portions  106   a ,  106   b , and/or  106   c  fully seat on the shoulder portions (e.g.  38   a ,  40   a ) of the shaft  38 ,  40 . Once seated, a hole can be drilled through the rotor  100  and shaft  38 ,  40  and the pin can be press fit into the hole. In one example, the rotor  100  is provided with surface machining at the end faces  112 ,  114  and the lobes  102  either before or after being mounted onto the shaft  38 ,  40 , although it is preferred to conduct such machining before the pin P 1  is inserted. In one example, this machining step is performed with a 3 mm (millimeter) machining allowance with an additional about 5 mm added for coating purposes. After insertion of the pin P 1 , the layer or coating C 1  can be applied which can then be followed by a final machining step. As noted above, the coating C 1  can be printed onto the rotor outer surface instead of performing machining and coating steps. 
     Referring to  FIG. 12 , an alternative design rotor  200  is shown that is produced by a 3D printing or additive manufacturing process. As shown, the rotor  200  includes a plurality of lobes  202  that are hollow and define an interior cavity  220 . The lobes  202  are arranged around a central aperture  204 . The central aperture  204  is defined by a tubular section  206  that is also hollow. The tubular section  206  can be pressed onto a shaft or can be utilized as the shaft itself. Alternatively, the tubular section  206  can be a solid shaft. Separate end caps, formed from a metal sheet or from an additive manufacturing process, can be attached to the ends of the rotor  200 . As with rotor  100 , rotor  200  has hollow lobes  102  and thus less rotational inertia compared to a solid rotor formed from the same material. At a given wall thickness, the hollow 316L Stainless Steel rotor  200  will have less inertia than a solid 1060 Aluminum solid rotor.  FIGS. 16 and 17  show an example of a thin-walled hollow rotor lobe  202 , wherein the wall thickness t 202   a  is less than 15 percent of the maximum width w 202  of the rotor (i.e. 3 mm/20.55 mm=14.5%). A mass property analysis shows that a stainless steel hollow rotor  200  can be provided with less mass than a similarly shaped solid aluminum rotor (e.g. −252 grams for rotor  200  vs. −285 grams for a solid aluminum rotor). The same analysis shows that a stainless steel hollow rotor  200  can have lower principle moments of inertia than a solid aluminum rotor. In one example, the hollow portions of the lobes can be filled with a second material, such as a resin and/or foam material. These characteristics are fully applicable to the rotor  100  as well. 
     Referring to  FIG. 13 , a rotor  300  is shown that is produced by a 3D printing or additive manufacturing process. The rotor  300  is similar to the rotors  100  and  200 , but is also provided with a 3D printed or additively manufactured supporting lattice structure  308 . This lattice structure can be integrated into any of the disclosed rotor designs (e.g. rotors  100 ,  200 ,  400 ). The internal lattice structure  308  can allow for an even thinner walled rotor to be utilized. An internal lattice structure  308  may also provide a matrix by which one could potentially fill the rotor with a foam or polymer to reduce noise. Considerations when designing an appropriate lattice structure are: print orientation, ability of certain lattice structure designs to be printed, and minimum thickness of the lattice itself to ensure it is printable. One suitable lattice structure design for printing of the lattice structure  308  is shown at  FIG. 15 . The lattice structure is beneficial in providing structural integrity to the rotor while providing for an internal structure that has a lower density than a solid material. In one aspect, the lattice structure can be referred to as a low density structure, meaning that the density of the structure is lower than that of an entirely solid filled structure. 
     Referring to  FIG. 14 , a rotor  400  is shown that is also produced by a 3D printing or additive manufacturing process. The rotor  400  of  FIG. 14  includes a shaft  406  that is also printed concurrently with the rotor lobes  402 . In the case of either a hollow or lattice structure rotor printing directly onto the shaft  406 , total time to reach a final part can be reduced. Printing directly onto the shaft  406  would eliminate the need to finish machine the bore and press onto a shaft. If the shaft  406  is integrated into the rotor it is possible to hollow out the shaft, or only print onto partial shafts at the ends of the rotor, as is shown in  FIG. 4 . This features of this design can be fully integrated into the other disclose rotor designs (e.g. rotors  100 ,  200 ,  300 ). 
     Rotary Assembly Applications 
     The above described rotors  100 ,  200 ,  300 ,  400  (collectively rotor assembly  30 ) may be used in a variety of applications involving rotary devices. Two such applications can be for use in a fluid expander  20  and a compression device  21  (e.g. a supercharger), as shown in  FIG. 18 . In one example, the fluid expander  20  and compression device  21  are volumetric devices in which the fluid within the expander  20  and compression device  21  is transported across the rotors  30  without a change in volume.  FIG. 18  shows the expander  20  and supercharger  21  being provided in a vehicle  10  having wheels  12  for movement along an appropriate road surface. The vehicle  10  includes a power plant  16  that receives intake air  17  and generates waste heat in the form of a high-temperature exhaust gas in exhaust  15 . In one example, the power plant  16  is a fuel cell. The rotor assembly  30 ,  430  may also be used as a straight or helical gear (i.e. a rotary component) in a gear train, as a transmission gear, as a rotor in other types of expansion and compression devices, as an impeller in pumps, and as a rotor in mixing devices. 
     As shown in  FIG. 18 , the expander  20  can receive heat from the power plant exhaust  15  and can convert the heat into useful work which can be delivered back to the power plant  16  (electrically and/or mechanically) to increase the overall operating efficiency of the power plant. As configured, the expander  20  can include housing  22  within which a pair of rotor assemblies  30 ,  32  is disposed. The expander  20  having rotor assemblies  30 ,  32  can be configured to receive heat from the power plant  16  directly or indirectly from the exhaust. 
     One example of a fluid expander  20  that directly receives exhaust gases from the power plant  16  is disclosed in Patent Cooperation Treaty (PCT) International Application Number PCT/US2013/078037 entitled EXHAUST GAS ENERGY RECOVERY SYSTEM. PCT/US2013/078037 is herein incorporated by reference in its entirety. 
     One example of a fluid expander  20  that indirectly receives heat from the power plant exhaust via an organic Rankine cycle is disclosed in Patent Cooperation Treaty (PCT) International Application Publication Number WO 2013/130774 entitled VOLUMETRIC ENERGY RECOVERY DEVICE AND SYSTEMS. WO 2013/130774 is incorporated herein by reference in its entirety. 
     Still referring to  FIG. 18 , the compression device  21  can be shown provided with housing  25  within which a pair of rotor assemblies  30 ,  32  is disposed. As configured, the compression device can be driven by the power plant  16 . As configured, the compression device  21  can increase the amount of intake air  17  delivered to the power plant  16 . In one example, compression device  21  can be a Roots-type blower or supercharger of the type shown and described in U.S. Pat. No. 7,488,164 entitled OPTIMIZED HELIX ANGLE ROTORS FOR ROOTS-STYLE SUPERCHARGER. U.S. Pat. No. 7,488,164 is hereby incorporated by reference in its entirety. An additional example is provided at Patent Cooperation Treaty (PCT) International Publication Number WO 2013/148205, the entirety of which is incorporated herein by reference. 
     Referring to  FIGS. 19 and 20 , further aspects of the waste heat recovery device or expander  20  are shown. While some details of the expander  20  are discussed in this subsection and above, additional structural and operational aspects can be found in Patent Cooperation Treaty (PCT) International Publication Number WO 2014/144701 and in United States Patent Application Publication US 2014/0260245, the entireties of which are incorporated herein by reference. 
     In general, the volumetric energy recovery device or expander  20  relies upon the kinetic energy and static pressure of a working fluid to rotate an output shaft  38 . The expander  20  may be an energy recovery device  20  wherein the working fluid  12 - 1  is the direct engine exhaust from the engine. In such instances, device  20  may be referred to as an expander or expander, as so presented in the following paragraphs. 
     With continued reference to  FIGS. 19 and 20 , it can be seen that the expander  20  has a housing  22  with a fluid inlet  24  and a fluid outlet  26  through which the working fluid  12 - 1  undergoes a pressure drop to transfer energy to the output shaft  38 . The output shaft  38  is driven by synchronously connected first and second interleaved counter-rotating rotors  30 ,  32  which are disposed in a cavity  28  of the housing  22 . Each of the rotors  30 ,  32  has lobes that are twisted or helically disposed along the length of the rotors  30 ,  32 . Upon rotation of the rotors  30 ,  32 , the lobes at least partially seal the working fluid  12 - 1  against an interior side of the housing at which point expansion of the working fluid  12 - 1  only occurs to the extent allowed by leakage which represents and inefficiency in the system. In contrast to some expanders that change the volume of the working fluid when the fluid is sealed, the volume defined between the lobes and the interior side of the housing  22  of device  20  is constant as the working fluid  12 - 1  traverses the length of the rotors  30 ,  32 . Accordingly, the expander  20  may be referred to as a “volumetric device” as the sealed or partially sealed working fluid volume does not change. 
     The expander  20  includes a housing  22 . As shown in  FIG. 19 , the housing  22  includes an inlet port  24  configured to admit relatively high-pressure working fluid  12 - 1 . The housing  22  also includes an outlet port  26 . 
     As additionally shown in  FIG. 20 , each rotor  30 ,  32  has four lobes,  30 - 1 ,  30 - 2 ,  30 - 3 , and  30 - 4  in the case of the rotor  30 , and  32 - 1 ,  32 - 2 ,  32 - 3 , and  32 - 4  in the case of the rotor  32 . Although four lobes are shown for each rotor  30  and  32 , each of the two rotors may have any number of lobes that is equal to or greater than two, as long as the number of lobes is the same for both rotors. For example, the rotors shown at  FIGS. 1 and 12  each have three lobes. When one lobe of the rotor  30 , such as the lobe  30 - 1  is leading with respect to the inlet port  24 , a lobe of the rotor  32 , such as the lobe  30 - 2 , is trailing with respect to the inlet port  24 , and, therefore with respect to a stream of the high-pressure working fluid  12 - 1 . 
     As shown, the first and second rotors  30  and  32  are fixed to respective rotor shafts, the first rotor being fixed to an output shaft  38  and the second rotor being fixed to a shaft  40 . Each of the rotor shafts  38 ,  40  is mounted for rotation on a set of bearings (not shown) about an axis X 1 , X 2 , respectively. It is noted that axes X 1  and X 2  are generally parallel to each other. The first and second rotors  30  and  32  are interleaved and continuously meshed for unitary rotation with each other. With renewed reference to  FIG. 19 , the expander  20  also includes meshed timing gears  42  and  44 , wherein the timing gear  42  is fixed for rotation with the rotor  30 , while the timing gear  44  is fixed for rotation with the rotor  32 . The timing gears  42 ,  44  are configured to retain specified position of the rotors  30 ,  32  and prevent contact between the rotors during operation of the expander  20 . 
     The output shaft  38  is rotated by the working fluid  12  as the working fluid undergoes expansion from the relatively high-pressure working fluid  12 - 1  to the relatively low-pressure working fluid  12 - 2 . As may additionally be seen in both  FIGS. 19 and 20 , the output shaft  38  extends beyond the boundary of the housing  22 . Accordingly, the output shaft  38  is configured to capture the work or power generated by the expander  20  during the expansion of the working fluid  12  that takes place in the rotor cavity  28  between the inlet port  24  and the outlet port  26  and transfer such work as output torque from the expander  20 . Although the output shaft  38  is shown as being operatively connected to the first rotor  30 , in the alternative the output shaft  38  may be operatively connected to the second rotor  32 . In one aspect, the expander  20  can also be operated as a high volumetric efficiency positive displacement pump when driven by the motor/generator  70 . 
     Other implementations will be apparent to those skilled in the art from consideration of the specification and practice of the examples and teachings presented herein. It is intended that the specification and examples be considered as exemplary only, with the true scope of the invention being indicated by the following claims.