CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is being filed on March 3, 2017 as a PCT

International Patent Application and claims the benefit of Indian Patent Application

No. 20161 1007770, filed on March 5, 2016 and claims the benefit of Indian Patent

Application No. 20161 1008326, filed on March 10, 2016, the disclosures of which are incorporated herein by reference in their entireties.

GOVERNMENT LICENSE RIGHTS

[0002] This invention was made with government support under Contract No. DE-EE0006844 awarded by the National Energy Technology Laboratory funded by the Office of Energy Efficiency & Renewable Energy of the United States Department of Energy. The government has certain rights in the invention.

TECHNICAL FIELD

[0003] This application relates to sealing methods and structures for positive displacement devices, such as superchargers and expanders having Roots-type rotors.

BACKGROUND

[0004] Turbo rings are commonly utilized within turbochargers as a sealing mechanism. The use of turbo rings is also known in Roots-type supercharger and expander applications. However, greater sealing performance is desired.

SUMMARY

[0005] Due to the application temperature associated with expanders that utilize exhaust as a working fluid, conventional dynamic lip seals are not a potential solution to eliminate gear case-to-air cavity cross talk or communication. Turbo rings are commonly utilized within turbochargers as a sealing mechanism, but due to the different aerodynamics within a turbo compared to a Roots device this is not a standalone solution. For applications where direct recovery with a Roots-type expander is planned, cross talk needs to be eliminated to avoid oil from entering exhaust stream which will negatively impact tailpipe emissions and exhaust from entering the oil cavity which will breakdown the oil.

[0006] The proposed design involves making the leakage paths as treacherous and long as possible by incorporating labyrinth seals in addition to turbo rings which will significantly reduce the pressure subjected to the turbo rings and reduce potential for cross talk. Moreover, the disclosed designs subject a back pressure onto the cavities which will help with sealing by providing flow path resistance.

[0007] The teachings presented herein are also directed to a positive displacement or volumetric device, such as a Roots-type expander or compressor, that utilizes journal bearings or bushings to support each end of the shafts carrying the internal rotors.

[0008] 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

[0009] Figure 1 is a schematic cross-sectional view of a positive

displacement device, which is an example in accordance with aspects of the invention.

[0010] Figure 2 is an enlarged schematic cross-sectional view of the positive displacement device shown in Figure 1 .

[0011] Figure 3 is an enlarged schematic cross-sectional view of the positive displacement device shown in Figure 1 , as indicated in Figure 2. [0012] Figure 4 is a perspective view of a main body of a first seal structure of the positive displacement device of Figure 1 .

[0013] Figure 5 is a perspective view of a ring member of a first seal structure of the positive displacement device of Figure 1

[0014] Figure 6 is a side view of the seal structure main body shown in Figure 4.

[0015] Figure 7 is a side cross-sectional view of the main body shown in Figure 4, taken along the line 7-7 in Figure 6.

[0016] Figure 8 is a perspective view of a second seal structure of the positive displacement device of Figure 1 .

[0017] Figure 9 is a front view of the second seal structure of Figure 8.

[0018] Figure 10 is a side cross-sectional view of the second seal structure of Figure 8, taken along the line 10-10 in Figure 9.

[0019] Figure 1 1 is a perspective view of a third seal structure of the positive displacement device of Figure 1 .

[0020] Figure 12 is a front view of the third seal structure of Figure 1 1 .

[0021] Figure 13 is a side cross-sectional view of the third seal structure of Figure 1 1 , taken along the line 13-13 in Figure 12.

[0022] Figure 14 is a schematic cross-sectional view of a variation of the positive displacement device shown in Figure 1 , which is an example in accordance with aspects of the invention.

[0023] Figure 15 is a schematic cross-sectional view of a variation of the positive displacement device shown in Figure 1 , which is an example in accordance with aspects of the invention.

[0024] Figure 16 is a schematic cross-sectional view of a positive displacement device, which is an example in accordance with aspects of the invention. [0025] Figure 17 is a schematic perspective view of a journal bearing or bushing associated with the positive displacement device shown in Figure 16.

[0026] Figure 18 is a front view of the journal bearing or bushing of Figure 17.

[0027] Figure 19 is a side view of the journal bearing or bushing of Figure 17.

[0028] Figure 20 is a front cross-sectional view of the journal bearing or bushing of Figure 17, taken along the line 20-20 in Figure 19.

[0029] Figure 21 is a side cross-sectional view of the journal bearing or bushing of Figure 17, taken along the line 21 -21 in Figure 18.

[0030] Figure 22 shows an arrangement usable in the positive

displacement device shown in Figure 6 to control assembly location of the rotor shaft assembly.

[0031] Figure 23 further shows the arrangement of Figure 22.

[0032] Figure 24 is a schematic view of a vehicle having a fluid expander and a compressor in which the disclosed positive displacement devices may be included.

[0033] Figure 25 is a schematic side view of a positive displacement device presented to illustrate general design concepts.

[0034] Figure 26 is a schematic perspective view of the positive

displacement device shown in Figure 25.

[0035] Figure 27 is a schematic flowchart showing a process for controlling activation of the disclosed positive displacement devices.

DETAILED DESCRIPTION

[0036] 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.

Positive Displacement Device Configurations of Figures 1 -15

[0037] Referring to Figure 1 , a positive displacement device 100 is shown. As described in the section entitled "Positive Displacement Device Applications" herein, the positive displacement device 100 is a Roots-type device that can be used as an energy recovery device or as a compressor for a power plant. As shown, the device 100 includes a housing 22 with a first internal cavity 22a within which a pair of rotors 30, 32 is disposed. When used as an expander, a working fluid flows through the first internal cavity 22a to rotate the rotors 30, 32 to generate useful work. The rotors 30, 32 are respectively mounted to shafts 38, 40 which extend out of the first internal cavity 22a and into second and third internal cavities 22b, 22c. The second internal cavity 22b houses a pair of bearings 46, 48 and a set of timing gears 42, 44 respectively mounted to shafts 38, 40. A lubricant is circulated through the second internal cavity 22b to ensure that the bearings 46, 48 and timing gears 42, 44 are adequately lubricated and cooled. Bearings 50, 52 are respectively mounted to shafts 38, 40 and are located within the third internal cavity 22c. A lubricant is circulated through the third internal cavity 22c to ensure that the bearings 50, 52 are adequately lubricated and cooled. As shown, the housing 22 and internal cavities 22a, 22b, 22c are formed from an assembly of multiple parts. However, the housing 22 can be configured as a single, unitary device that defines the internal cavities 22a, 22b, 22c.

[0038] As stated previously, it is undesirable for there to be communication or "cross-talk" or fluid communication between the first internal cavity 22a and the second and third internal cavities 22b, 22c. That is, it is undesirable for the working fluid from the first internal cavity to migrate into the second and third internal cavities 22b, 22c. Likewise, it is undesirable for lubricant to migrate from the second and third internal cavities 22b, 22c into the first internal cavity. To minimize such cross-communication, a plurality of seal structures 102/104, 106/108, 1 10/1 12 are provided that create a tortuous path to lengthen the distance that any leaked fluid must travel in order to reach the adjacent internal cavity. The seal structures 102/104, 106/108, 1 10/1 12 also cause a back pressure to be formed onto the cavities which aids with sealing by providing resistance in the tortuous flow path. A seal structure 1 14 can also be provided to minimize leakage from the first internal cavity 22a to outside of the housing 22, wherein the seal structure 1 14 is creates a high pressure loss torturous path defined by an axial projection 1 14a on the end part enclosing the first internal cavity 22a and by an annular-shaped recess 22d extending in an axial direction in the housing 22, or vice versa.

[0039] As shown, the seal structures 102, 104 are provided between the first internal cavity 22a and the second internal cavity 22b. As most easily seen at Figures 1 1 -13, the seal structures 102, 104 are respectively press fit onto shafts 38, 40 and extend radially outward from the shafts 38, 40 from a base portion 102a, 104a to an axial extension 102b, 104b that is received in a recess 22e, 22f of the housing 22. Accordingly, the seal structures 102, 104 prevent the type of fluid communication between cavities that exists in designs in which the shaft simply extends through an aperture between the first and second internal cavities 22a, 22b. The seal structures 102, 104 also greatly lengthen the pathway that the working fluid and/or lubricant must traverse in order to reach the adjacent internal cavity, as the fluid/lubricant must travel along the shaft 38, 40, then along the radially extending portion 102a, 104a of the seal structures 102, 104, then along both sides of the axially extending portion 102b, 104b of the seal structures 102, 104, and then back along the opposite radial side of the seal structures 102, 104. As the clearance between the seal structures 102, 104 is very tight (but not touching), this pathway is associated with a high level of pressure drop as well, thereby further minimizing flow. This construction also reduces pressure on seals 60, 62 that are also provided to minimize leakage between the internal passages 22a, 22b.

[0040] As shown, the seal structures 106, 108 are provided between the first internal cavity 22a and the third internal cavity 22c. As most easily seen at Figures 8-10, the seal structures 104, 106 are respectively press fit onto shafts 38, 40 and extend radially outward from the shafts 38, 40 from a base portion 106a, 108a to an axial extension 106b, 108b that is received in a recess 22g, 22h of the housing 22. Accordingly, the seal structures 104, 106 prevent the type of fluid communication between cavities that exists in designs in which the shaft simply extends through an aperture between the first and third internal cavities 22a, 22c. The seal structures 106, 108 also greatly lengthen the pathway that the working fluid and/or lubricant must traverse in order to reach the adjacent internal cavity, as the fluid/lubricant must travel along the shaft 38, 40, then along the radially extending portion 106a, 108a of the seal structures 106, 108, then along both sides of the axially extending portion 106b, 108b of the seal structures 106, 108, and then back along the opposite radial side 106a, 108a of the seal structures 106, 108. As the clearance between the seal structures 106, 108 is very tight (but not touching), this pathway is associated with a high level of pressure drop as well, thereby further minimizing flow. This construction also reduces pressure on the seals 60, 62 that are also provided to minimize leakage between the internal passages 22a, 22b.

[0041] Seal structures 1 10, 1 12 are also provided as a further means of minimizing leakage between the first and third internal cavities 22a, 22c. The seal structures 1 10, 1 12, as most easily seen at Figures 4-7, are configured with a main body having a base portion 1 10a, 1 12a and an outwardly flared axially extending sleeve extension portion 1 10b, 1 12b. The flared extension portions 1 10b, 1 12b are respectively received into cavities 22i, 22j of the internal structure (e.g. an intermediate plate that forms a portion of the housing 22) between the first and third internal cavities 22a, 22c. The seal structures 1 10, 1 12 further include grooves 1 10d, 1 12d and 1 10e, 1 12e into which "turbo ring" seals 1 10c, 1 12c are received. Each seal 1 10c, 1 12c, shown in isolation at Figure 5, is configured as a metal split-ring that can be expanded over the base portion 1 10a, 1 12a and snapped into one of the main body grooves.

[0042] As with the other seal structures, the seal structures 1 10, 1 12 are press fit onto the shafts 38, 40. For fluid that might leak from the third internal cavity 22c to the first internal cavity 22a, the lubricant must first leak past the turbo ring seals 1 10c, 1 12c and then around the passageway formed between the axial sleeve extension 1 10b, 1 12b and the cavity 22i, 22j into which it extends and then through the seal structures 106, 108 (if present). For the embodiment shown at Figures 1 -3, fluid that might leak from the first internal cavity 22a to the third internal cavity 22c must first leak past the seal structures 106, 108, and then then around the passageway formed between the axial sleeve extension 1 10b, 1 12b and the cavity 22i, 22j into which it extends and then must further leak past the turbo ring seals 1 10c, 1 12c. Such a high resistance and tortuous path created by these seal structures greatly minimized the type of straight through flow that exists when only a conventional single turbo ring seal structure is provided to protect against leakage. This construction also reduces pressure on the turbo ring seals themselves. An enlarged view of the seal structures 1 10, 1 12 and the tortuous path cratered by them can be found at Figure 3 in the application.

[0043] Various combinations of utilization of the seal structures 102/104, 106/108, 1 10/1 12, 1 14 may be utilized within any given positive displacement device 100. For example, the construction shown at Figure 1 utilizes all three seal structures 102/104, 106/108, and 1 10/1 12. An enlarged view of this configuration can be found at Figures 2 and 3. Figure 14 shows an example in which the seal structure 106/108 is not utilized, but where the seal structures 102/104 and

1 10/1 12 are utilized. Thus, leakage between the first and third internal cavities 22a, 22c is controlled entirely by the seal structures 1 10/1 12. Figure 15 shows an example in which a standard turbo ring seal 1 16/1 18 is utilized and the seal structure 1 10/1 12 is not utilized. However, seal structures 102/104 and 106/108 are utilized. Thus, leakage between the first and third internal cavities 22a, 22c is controlled entirely by the seal structures 106/108 in combination with the standard turbo ring seals. Figure 16 shows a configuration in which standard seals are utilized. However, any or all of the seal structures 102/104, 106/108, and 1 10/1 12 can be utilized with the configuration shown in Figure 16.

Positive Displacement Device Configurations of Figures 16-23

[0044] Journal bearings tested on the air stand and engine with a roots device, have shown significant improvement in efficiency and power generation over the ball bearings. Journal bearings made of different materials (e.g. tin bronze, oil lite, oil lite with graphite, and 360 free cutting brass) are evaluated for performance and durability. Extensive design iterations and testing have been done with different journal bearings design parameters such as oil slots, rifling grooves and clearance with the shaft to understand the effect of these parameters on performance and functionality of the journal bearings. The engine oil forced lubrication flow, temperature, and feed pressure are critical for proper functioning of the journal bearings. Several design iterations and tests have been conducted on the roots device to understand this relationship and identify optimal flow rates and feed pressure values for different applications.

[0045] Referring to Figure 16, a positive displacement device 100 is shown of the same general type shown in Figure 1 and 14-15. Accordingly, the previous provided descriptions are fully applicable to the embodiment shown at Figure 16. In contrast to the embodiment shown at Figure 1 , the positive displacement device 100 of Figure 16 includes the use of journal bearings or bushing. As such, the second internal cavity 22b houses a pair of journal bearings or bushings 46, 48 and a set of timing gears 42, 44 respectively mounted to shafts 38, 40. A lubricant is circulated through the second internal cavity 22b to ensure that the journal bearings or bushings 46, 48 and timing gears 42, 44 are adequately lubricated and cooled. Journal bearing or bushings 50, 52 are also respectively mounted to shafts 38, 40 and are located within the third internal cavity 22c. It is noted that the features shown in Figures 16-23 and described in this section can be fully integrated into the designs shown at Figures 1 and 14-15 such that any or all of the features described and shown in this disclosure can be incorporated into a single embodiment. For example, a positive displacement device 100 could be provided with the seal structures 102/104, 106/108, 1 10/1 12, 1 14 shown in Figure 1 and the bearings 46, 48, 50, 52 shown in Figure 16, as previously noted. The combination of the disclosed journal bearings and sealing arrangements advantageously provides a minimal parasitic loss configuration that maximizes device efficiency, simplifies the assembly process and improves overall device durability.

[0046] As noted above, the shafts 38, 40 are supported by bearings 46, 48 at one end and by bearings 50, 52 at the opposite end. In the example shown at Figure 16, the bearings 46, 48, 50, 52 are plain bearings, wherein the bearings are non-rotating components that are press fit into an OD in the housing casting or otherwise secured within the housing 22. For example, the bearings can be installed with a slip fit with rotation eliminate through a pin indentation in or through a part of the housing casting.

[0047] Plain bearings of this type may be referred to as journal bearings or bushings. Referring to Figures 17-21 , an example journal bearing or bushing 200 is shown that is suitable for use as bearings 46, 48, 50, 52 in the positive displacement device 100 is shown. As shown, the bearing or bushing 200 includes an outer surface 202 which engages against a portion of the housing 22 and an inner bearing surface 204 through which the shaft 38, 40 extends. The bearing or bushing 200 also includes a plurality of ovular or racetrack shaped connecting slots 206 that extend between the surfaces 202 and 204. The shape, size, number, and location of the slots are optimized to allow oil to be delivered to the interface between the bearing surface 204 and the shaft 38, 40 such that an oil film can be formed between the two surfaces. An outside oil slot or

circumferential groove 208, which is wider than the slots 206, is provided about the outer surface 202 and allows for oil to collect and be supplied into the slots 206. Similarly, an inside oil slot or circumferential groove 210 is provided at the opposite end of the slots 206. As configured, the inlet oil port defined by the circumferential groove is 1.5 to 3 times the outlet port defined by the cumulative area of the slots 206. The housing 22 includes an oil passageway that feeds the circumferential groove 208. Oil flow rate ranges from 0 to 1500 CCM at a pressure of 1 to 3.5 bar.

[0048] For a clutched or electrically driven Roots device controls need to be in place to ensure oil has flowed into the cavity prior to device starting to spin. For example, a control system can initiate a process 1000, as shown at Figure 27, in which it is verified that the pump associated with the lubrication circuit for the expander is activated to ensure oil is sufficiently pressurized and flowing at the lubrication chambers before allowing the clutch and/or motor/generator associated with the expander to be deactivated or activated. In a step 1002, an input is received for activating the expander at a controller and in a step 1004, the rotors are prevented from rotating, for example by ensuring that a clutch is engaged or by activating a motor/generator. In a step, 1006, the pump that delivers oil to the oil chambers is activated. In a stet 1008, the rotors are allowed to rotate after an oil verification protocol has been completed. The oil verification protocol can be completed by verifying a proper oil flow/pressure condition exists by delaying deactivation or activation of the clutch or motor/generator for a period of time after the pump has been activated, by measuring the parasitic loss, or in another term monitoring the drive power to access the presence of oil. Forced lubrication ensures proper lubrication and cooling of the gear case components, including the journal bearings or bushings during all phases of operation. Ensuring that oil conditions are proper at the expander before allowing the rotors to rotate reduces potential noise issues and increases the overall durability of the device.

[0049] In the example shown, the bearing or bushing 200 is a bronze bushing sized to allow for a nominal diametrical clearance between the bushing and shaft that is about 0.010 to 0.05 mm. This low clearance helps enable tighter clearance in the working fluid or air cavity which leads to a higher efficiency unit. The small clearances associated with the use of journal bearings or bushings at each end of the rotor shafts are measurably smaller than what is generally obtainable when using ball bearings. This reduced clearance allows the rotors 30, 32 to be meshed into a tighter arrangement with less clearance between them.

[0050] This tighter clearance thus results in less rotor leakage, thereby by increasing performance of the expander. Testing or modelling over a range of industry standard operating conditions (e.g. A25, B25, C25, A50, B50, C50) has shown that the disclosed expander using only journal bearings or bushings has increased power, rotational speed, output torque, and isentropic efficiency over similarly configured expanders utilizing ball bearings.

[0051] Ball bearings in expanders have typically been used to additionally accomplish fixing the axial position of the rotors 30, 32 with respect to the interior cavity 22a of the housing 22. Thus, it is counterintuitive to use only journal bearings or bushings in such an application, as the loss of this significant positioning feature occurs. Referring to Figures 22-23, an approach is shown that allows for the shafts and rotors to be properly axially positioned and retained through the use of snap rings 250 during the assembly process. With such an approach, the timing gears 42, 44 are used to fix the axial position of the shafts and rotors. Once the timing gears 42, 44 are installed during the assembly process, the snap rings 250 can be removed. With this approach, it is possible to use only journal bearings or bushings in the expander while ensuring the proper axial positioning of the rotors 30, 32. The use of such snap rings 250 is fully applicable to all of the disclosed embodiments. For example, the snap rings 250 are also shown in a mounted position in the embodiment shown at Figure 2.

[0052] The use of only journal bearings and/or bushings to support the shafts 30, 32 has additional benefits beyond increased performance. For example, journal bearings in place of ball bearings reduce assembly complexity. A common failure mode for ball bearings in supercharger is brinelling from not supporting the components properly during the assembly process. This is not a concern when using journal bearings and/or bushings. Journal bearings and bushings also eliminate stick/slip NVH issues previously experience with greased filled needle or ball bearings.

[0053] Figures 1 and 14 to 15 show a design in which journal bearings or bushings and provided at one end of the expander and ball bearings at the opposite end while Figure 16 shows journal bearings or bushings provided at both ends. For both types of configurations and bearings, oil slinger cavities 260 are located adjacent the sides of the bearings/bushings and are defined partially by the housing 22, the shaft 38, 40, and the adjacent bushings or bearings. As most easily seen at Figure 3, the oil slinger cavities 260 include a radially extending cavity portion 260a extending away from the shaft 38, 40, and an axially extending cavity portion 260b which extends axially away from the bushing or bearing. The oil slinger cavities 260 prevent or reduce the presence of oil on the turbo rings and eliminate potential for oil to enter the air cavity 22a which would negatively impact emissions. For example, the oil slinger cavities 260 provide a place for oil to go in the event of an over flooding or seal failure issue. The oil slinger configuration and placement, in addition to the oil inlet and outlet and related internal passageway configurations may be utilized in conjunction with all of the designs presented in the application.

Positive Displacement Device Applications

[0054] The above described positive displacement devices can be used as a fluid expander 20 and a compression device 21 (e.g. a supercharger), as shown in Figure 24. 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. Figure 24 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.

[0055] As shown in Figure 24, 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.

[0056] 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.

[0057] 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.

[0058] Still referring to Figure 24, 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 US Patent 7,488,164 entitled OPTIMIZED HELIX ANGLE ROTORS FOR ROOTS-STYLE SUPERCHARGER. US Patent 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.

[0059] Referring to Figures 25 and 26, 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.

[0060] 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.

[0061] With continued reference to Figures 25 and 26, 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" or "positive displacement device" as the sealed or partially sealed working fluid volume does not change.

[0062] The expander 20 includes a housing 22. As shown in Figure 25, 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.

[0063] As additionally shown in Figure 26, 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.

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 .

[0064] 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 about an axis X1 , X2, respectively. It is noted that axes X1 and X2 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 Figure 25, 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.

[0065] 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 Figures 25 and 26, 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.

[0066] 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.