Abstract:
A apparatus and method for throttling a heat engine uses a plurality of cylinder ports through a side portion of the cylinder and a sleeve that selectively opens or closes them to provide fluid communication between an interior portion of the cylinder and a reservoir area when a piston in the cylinder is below the cylinder ports. The sleeve has a plurality of throttle ports through it and moves to communicate a number of the throttle ports with a number of the cylinder ports, thereby opening the cylinder ports. Cylinder ports and throttle ports may be arranged so that as the sleeve is rotated, an increasing number of cylinder ports are opened higher up the cylinder. The cylinder ports may also be arranged so that rotating the sleeve varies the amount ports are opened. The sleeve is preferably worm-gear driven for accurate position control. Upon closing the throttle, normal operating pressures are restored via use of a check valve.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit, under 35 U.S.C. 120 of U.S. patent application Ser. No. 09/500,185, filed Feb. 7, 2000, pending. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    This application is a continuation-in-part application of pending U.S. patent application Ser. No. 09/500,185 filed on Feb. 7, 2000 (Now U.S. Pat. No. 6,263,671), which is a continuation-in-part of U.S. patent application Ser. No. 08/971,235 filed on Nov. 15, 1997 (Now U.S. Pat. No. 6,041,598). The above referenced patent applications are hereby incorporated herein by reference.  
           [0003]    1. Field of the Invention  
           [0004]    The present invention relates, generally, to heat engines. More particularly, the invention relates to Stirling cycle heat engines with a cylinder containing a working fluid and a piston moving therein.  
           [0005]    2. Background Information  
           [0006]    The maximum Stirling engine efficiency is related to the Carnot efficiency which is governed by the ratio of maximum working fluid temperature relative to the minimum fluid temperature. Improvements in technologies which increase the margin between the two temperature extremes is beneficial in terms of total cycle efficiency. The lower working fluid temperature is typically governed by the surrounding air or water temperature; which is used as a cooling source. The main area of improvements result from an increase in the maximum working temperature. The maximum temperature is governed by the materials which are used for typical Stirling engines. The materials, typically high strength Stainless Steel alloys, are exposed to both high temperature and high pressure. The high pressure is due to the Stirling engines requirement of obtaining useful power output for a given engine size. Stirling engines can operate between 50 to 200 atmospheres internal pressure; for high performance engines.  
           [0007]    Since Stirling engines are closed cycle engines, heat must travel through the container materials to get into the working fluid. These materials typically are made as thin as possible to maximize the heat transfer rates. The combination of high pressures and temperatures has limited Stirling engine maximum temperatures to around 800° C. Ceramic materials have been investigated as a technique to allow higher temperatures, however their brittleness and high cost have made them difficult to implement.  
           [0008]    U.S. Pat. No. 5,611,201, to Houtman, shows an advanced Stirling engine based on Stainless Steel technology. This engine has the high temperature components exposed to the large pressure differential which limits the maximum temperature to the 800° C. range. U.S. Pat. No. 5,388,410, to Momose et al., shows a series of tubes, labeled part number 22 a  through  d,  exposed to the high temperatures and pressures. The maximum temperature is limited by the combined effects of the temperature and pressure on the heating tubes. U.S. Pat. No. 5,383,334 to Kaminishizono et al, again shows heater tubes, labeled part number 18, which are exposed to the large temperature and pressure differentials. U.S Pat. No. 5,433,078, to Shin, also shows the heater tubes, labeled part number 1, exposed to the large temperature and pressure differentials. U.S Pat. No. 5,555,729, to Momose et al., uses a flattened tube geometry for the heater tubes, labeled part number 15, but is still exposed to the large temperature and pressure differential. The flat sides of the tube add additional stresses to the tubing walls. U.S Pat. No. 5,074,114, to Meijer et al., also shows the heater pipes exposed to high temperatures and pressures.  
           [0009]    The Stirling engine disclosed in the inventor&#39;s U.S. Pat. No. 6,041,598 overcomes the limitations and shortcomings of the above prior art by providing a dual shell pressure chamber. An inner shell surrounds the heat transfer tubing and the regenerator. The portion surrounding the heat transfer tubing contains a thermally conductive liquid metal to facilitate heat transfer from a heat source to the heat transfer tubing and also to transmit external pressure to the heat transfer tubing. An outer shell that acts as a pressure vessel surrounds the inner shell and contains a thermally insulating liquid between the inner and outer shells. Pressure of the working fluid as it flows through the regenerator is transmitted through the inner shell to the insulating liquid and back across the inner shell to the liquid metal surrounding the heat transfer tubing. This system tends to balance the pressure across the heat transfer tubing and the inner shell, thereby allowing the engine to operate with the working fluid at a high pressure to generate significant power while keeping the wall of the heat transfer tubing thin to facilitate heat transfer through it.  
           [0010]    An anticipated use of the inventor&#39;s dual shell Stirling engine is to run a 25 KW electrical generator. For that use, and others, the required power output of the engine may not be constant. Throttling of the engine is, therefore, probably necessary.  
           [0011]    Throttling of Stirling engines is typically accomplished by varying the amount of working fluid inside the engine. With this technique a significant amount of pumping and valving hardware is required to move the working fluid. This is complicated by the high working pressures which increases the size of the pumping hardware. A second technique to throttle the Stirling engine involves opening ports within the engine which are connected to dead (non-working) volumes or reservoirs. That technique increases the total system volume which lowers the power but also results in a significant reduction in efficiency due the larger dead volume which the engine is exposed to for the entire piston stroke. Houtman and Meijer et al. disclose another throttling technique that uses a variable angle plate connected directly to each piston. Reducing the plate angle results in reduced movement of the piston, resulting in reduced power levels. That throttling technique has the disadvantage of a higher system weight due to the large loads generated when converting the wobble motion of the plate to torque.  
           [0012]    The present invention provides a throttle for a Stirling engine which overcomes the limitations and shortcomings of the prior art.  
         SUMMARY OF INVENTION  
         [0013]    The present invention provides an apparatus and method for throttling a heat engine having a cylinder containing a working fluid with a piston moving therein. A plurality of cylinder ports through a side portion of the cylinder provide fluid communication between an interior portion of the cylinder and a reservoir area when the piston is below the cylinder ports. A throttle control device selectively opens or closes a number of the cylinder ports to allow a portion of the working fluid contained in the cylinder to move between the cylinder and the reservoir area through the cylinder ports and vary the pressure in the portion of the cylinder above the piston.  
           [0014]    The throttle control device includes a sleeve disposed around a portion of the cylinder. The sleeve has a plurality of throttle ports through it and moves relative to the cylinder, preferably rotationally, to selectively communicate a number of the throttle ports with a number of the cylinder ports to thereby open the cylinder ports so communicated.  
           [0015]    In one embodiment the cylinder ports are arranged in groups of vertically aligned ports and the throttle ports are arranged in groups of a stepped series of ports spaced to match the cylinder ports so that as the sleeve is rotated, an increasing number of cylinder ports are opened higher up the cylinder.  
           [0016]    In another embodiment, with the cylinder ports also arranged in groups of vertically aligned ports, the throttle ports are arranged in groups diagonally such that as the sleeve is rotated, a single cylinder port per group of cylinder ports is opened higher up the cylinder.  
           [0017]    In yet another embodiment, the cylinder ports are arranged in a single circumferential row around the cylinder and the throttle ports are arranged in a single circumferential row on the sleeve such that each throttle port aligns with each corresponding cylinder port. The sleeve rotates between a position that allows the cylinder ports to be fully open and a position that allows the cylinder ports to be completely closed, with variable positioning therebetween to thereby vary the amount the cylinder ports open.  
           [0018]    The preferred mechanism for the throttle control device includes a throttle collar attached to the cylinder that supports the throttle sleeve. A throttle worm gear is attached to the throttle sleeve and is driven by a throttle control worm that engages the throttle worm gear to rotationally position the throttle sleeve about the cylinder to selectively communicate a number of the throttle ports with a number of the cylinder ports to thereby open the cylinder ports so communicated.  
           [0019]    There is preferably a throttle fairing that surrounds the throttle control device and provides a pressure fairing to contain the working fluid passing through the cylinder ports. The throttle fairing has a series of throttle vents that provide fluid communication between the reservoir area and an area inside of the throttle fairing.  
           [0020]    Preferably there is also a check valve in the piston which allows working fluid in the reservoir area to move through it into the interior area of the cylinder when pressure in the reservoir area exceeds that of the interior area of the cylinder.  
           [0021]    To throttle the heat engine, the ports in the cylinder are selectively opened to allow communication between the reservoir area and the interior portion of the cylinder above the piston when the piston is below the ports. Working fluid vents from the interior portion of the cylinder through the open ports to the reservoir area as the piston moves up in the cylinder toward the open ports to prevent significant compression of the working fluid in the cylinder. The venting is stopped by blocking the open ports with the piston as the piston moves up past the ports to thereby resume compression of the working fluid in the cylinder. The pressure produced during compression of the working fluid is therefore reduced from that produced when the ports in the cylinder are closed, thereby effectively throttling the engine.  
           [0022]    Pressure is increased again by closing the open ports and moving working fluid from the reservoir area to the interior area of the cylinder above the piston, preferably through a check valve in the piston, to restore the amount of working fluid in the interior area of the cylinder above the piston, thereby allowing higher pressures to be produced during compression of the working fluid by the piston.  
           [0023]    The features, benefits and objects of this invention will become clear to those skilled in the art by reference to the following description, claims and drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0024]    [0024]FIG. 1 is a longitudinal vertical cross sectional view showing the overall arrangement for a complete dual shell Stirling engine.  
         [0025]    [0025]FIG. 2 is a side elevational view of a section of the cylinder in the region of the throttle showing the ports through the cylinder.  
         [0026]    [0026]FIG. 3 is a side elevational view of the throttle sleeve, throttle worm gear and throttle control worm.  
         [0027]    [0027]FIG. 4 is the view of FIG. 2 showing another embodiment for the configuration of the ports.  
         [0028]    [0028]FIG. 5 is the view of FIG. 3 showing another embodiment of the throttle sleeve corresponding to the port configuration showing FIG. 4.  
         [0029]    [0029]FIG. 6 is a cross sectional view of a portion of the cylinder and throttle sleeve at a port when the throttle sleeve is positioned such that the port is open. 
     
    
     DETAILED DESCRIPTION  
       [0030]    In the following description of the invention, the components are illustrated and described in a vertical orientation with the cylinder located above the lower housing. Terms such as upper, lower, above and below are used to describe the relative positions of components and are not intended to indicate a quality or locational requirement since the cylinder can be oriented in any position relative to the housing and crankshaft.  
         [0031]    Referring to FIG. 7, components of a dual shell Stirling engine having a power piston  12  that drives an output crankshaft  46  are illustrated. A working fluid, such as Helium, is contained in cylinder  20  above power piston  12  and is shuttled through heat transfer tubing  14 , regenerator  16 , and cooling pipes  18  by the action of a displacer piston  10 . An inner shell  30  surrounds the heat transfer tubing  14  and regenerator  16 . The upper portion  32  of inner shell  30  contains a liquid metal region  34  filled with a thermally conductive liquid metal, such as silver, which surrounds the heat transfer tubing  14 . The regenerator  16  is preferably a coiled annulus of thin material disposed between cylinder  20  and inner shell  30 . Outer shell  60  surrounds inner shell  30  and acts as a pressure vessel. The inner shell  30 , outer shell  60  and flange  38  bound a pressure backup region  42 . The pressure backup region is filled preferably with an insulating liquid material to provide pressure backup against inner shell  30  and, consequently through liquid metal region  34 , to heat transfer tubing  14 .  
         [0032]    Lower housing  22  has a reservoir area  24  between a pair of crankshaft end plates  50  which acts as a reservoir for the working fluid and is in fluid communication with the working fluid in cylinder  20  through throttle ports  40  in cylinder  20 . Low pressure seals and bearings  31  prevent the working fluid in reservoir area  24  from escaping into the space  52  outside of crankshaft end plates  50 , which is preferably pressurized with ambient air to approximately the same pressure as that in reservoir area  24 . Throttling is accomplished by controlling the openings of throttle ports  40  in cylinder  20 .  
         [0033]    Referring also to FIG. 2, a portion of cylinder  20  adjacent to throttle sleeve  28  has a series of cylinder ports  40  drilled into its side so that when the power piston  12  is at bottom dead center, the cylinder ports  40  are completely above the power piston  12  and allow fluid communication between the area inside cylinder  20  above power piston  12  and the reservoir area  24  in lower housing  22 . Open cylinder ports  40  allow the working fluid in cylinder  20  to vent to reservoir area  24  as the power piston  12  rises, thus preventing compression in the region above the power piston  12 . As the power piston  12  moves up cylinder  20  beyond cylinder ports  40 , the region above the power piston  12  is sealed and compressed. The start of the sealing is dependent on the throttle port sequence determined by the throttle control device as follows.  
         [0034]    Cylinder ports  40  are arranged in groups circumferentially around cylinder  20 . Each group has a plurality of vertically oriented ports  40 , preferably three ports per group. Referring also to FIG. 3, throttle sleeve  28  has groupings of throttle ports  41  arranged so as to provide a stepped series of ports spaced vertically to match the cylinder ports  40  in cylinder  20 . A blank portion  45  separates each grouping of throttle ports  41  around the throttle sleeve  28 . The throttle sleeve  28  fits around the cylinder  20  with a snug fit so as to provide a seal between the throttle sleeve  28  and the cylinder  20 , but loose enough that the throttle sleeve  28  can move relative to cylinder  20 . Sealing around ports  40  is accomplished by washers  47 , preferably made of material such as Teflon, which are installed in a countersunk area around ports  40  such that the tops of the washer extend slightly beyond the outer surface of cylinder  20 . FIG. 4 illustrates the relationship between washer  47 , cylinder  20  and sleeve  28 . Compressibility of washer  47  is preferably provided by a resilient O-ring  56  behind washer  47 .  
         [0035]    The throttle control device functions preferably by rotating throttle sleeve  28  around the cylinder  20  through the distance of each grouping of throttle ports  41 . There may be other configurations for sleeve  28  and throttle ports  41  that may allow sleeve  28  to move axially, or a combination of axially and rotationally, rather than rotate to accomplish a similar result, but the preferred motion of sleeve  28  is simple rotation around cylinder  20 . When the blank portion  45  covers cylinder ports  40 , throttle sleeve  28  provides a complete seal and a full-throttle condition. As the throttle sleeve  28  is rotated, an increasing number of throttle ports  41  communicate with cylinder ports  40  higher up cylinder  20  which allow the working fluid to vent from the area above the power piston  12  into the throttle housing  48  and to reservoir area  24 . The higher the cylinder ports  40 , the more power piston  12  has to travel without significantly compressing the working fluid in the cylinder  20 . Once the power piston  12  moves past the open cylinder ports  40 , the compression continues in the cylinder  20 ; but since there is less working fluid in cylinder  20 , the pressure produced by the compression is reduced. This reduction in pressure reduces the total power produced, and effectively throttles the engine. It is also possible that only one cylinder port  40  per group need be opened at a time to allow adequate venting. In that case, the throttle ports  41  for each grouping on throttle sleeve  28  could be arranged diagonally such as is illustrated in FIG. 5.  
         [0036]    Referring again to FIG. 7, once the throttle sleeve  28  is rotated to a higher throttle position, thereby covering more cylinder ports  40 , at the bottom of the stroke of power piston  12 , the pressure in cylinder  20  above power piston  12  would be less than that in reservoir area  24  and some of the working fluid in reservoir area  24  flows back into the cylinder through a check valve  54 , preferably in the top of power piston  12 , to re-pressurize cylinder  20  until the average pressures are equalized. In steady-state operation, when the power piston  12  is at the bottom of its stroke, the pressure in cylinder  20  above power piston  12  equals that in reservoir area  24  and there is no significant movement of the working fluid through check valve  54 .  
         [0037]    A throttle fairing  48  and throttle fairing blister  49  provide a pressure fairing for the throttle sleeve  28  to contain the working fluid. The throttle fairing  48  has a series of throttle vents  44  located at the lower side of the throttle fairing  48  on the surface of the lower housing  22 . The throttle vents  44  provide a means for the working fluid, preferably Helium, to move from the cylinder  20  into the reservoir area  24  of lower housing  22 .  
         [0038]    As throttle sleeve  28  rotates about cylinder  20 , it is supported by a throttle collar  42  attached to the outside of cylinder  20 . The throttle control device includes a throttle worm gear  43  attached to the throttle sleeve  28  and a throttle control worm  36  that engages the throttle worm gear  43  and drives it to rotationally position the throttle sleeve  28 . The combination of the throttle control worm  36  and the throttle worm gear  43  provide a means to reduce the gearing to improve the positioning accuracy of the throttle sleeve  28 . It is possible to control motion of throttle sleeve  28  such that only portions of cylinder ports  40  are opened, thereby providing even finer throttle control.  
         [0039]    Referring to FIGS. 6 and 7, another embodiment for the throttle has only a single cylinder port  140  at each circumferential location rather than a group of vertically oriented ports. The single circumferential row of ports  140  is preferably at approximately the same location as the uppermost port of the groups of ports  40  illustrated in FIG. 2. Throttle sleeve  128  has a corresponding series of single throttle ports  141  circumferentially arranged around it that match the locations of the cylinder ports  140 . The fine positional control of throttle sleeve  128  allows the sleeve to rotate between a position that allows the cylinder ports  140  to be fully open and a position that allows the cylinder ports  140  to be completely closed, with variable positioning therebetween to thereby vary the amount the cylinder ports  140  open. This effectively creates a variable orifice at each port. The amount of working fluid that vents through ports  140  is dependent on how open ports  140  are and the speed of the engine. Higher rpm as well as smaller openings reduce the amount of working fluid that vents.  
         [0040]    A feature of the throttling system of the present invention is the complete sealing of the upper cylinder region after the power piston  12  has passed the cylinder ports  40 . The advantage of this is that the engine will operate at a much higher efficiency at partial power than with a dead-volume throttling system which maintains the increased dead volume over the complete stroke. The reason for this improvement is tied into the Stirling cycle and its working fluid movement. The working fluid above the power piston  12  gets shuttled between the area above and the area below the displacer piston  10  during each cycle. During the power stroke the majority of the working fluid is heated and located above the displacer piston  10 . As the power piston  12  gets pushed downward, an increase in volume occurs between the displacer piston  10  and the power piston  12 . This results in movement of the working fluid from the region above the displacer piston  10  to the region below it. With the old dead-volume system, a reservoir is connected to the flow path of the working fluid as it shuttles between those locations. The total amount of working fluid in the area above the power piston and in the dead volume does not change. Therefore, when the working fluid moves during the power stroke, part of the fluid remains in the region above the power piston and does useful work and part of the fluid expands into the dead volume chamber and does useless work. This extra quantity of wasted work reduces the total engine efficiency.  
         [0041]    Rather than providing a dead volume on the flow path of the working fluid above the power piston, the present invention reduces the amount of working fluid present in the cylinder above the power piston  12 . On the compression stroke, until the ports are closed by the power piston the extra reservoir area volume reduces the compression and, thus, the amount of working fluid above the power piston. On the power stroke, all of the working fluid (though reduced in amount) moves to the region below the displacer piston  10  and expands against the power piston  12  doing useful work until the throttle ports open up again. A small amount of work is wasted when the throttle ports are opened by the power piston and the remaining compression is released into the reservoir area. The present invention thus reduces the amount of wasted work, thereby improving the throttle efficiency.  
         [0042]    The descriptions above and the accompanying drawings should be interpreted in the illustrative and not the limited sense. While the invention has been disclosed in connection with the preferred embodiment or embodiments thereof, it should be understood that there may be other embodiments which fall within the scope of the invention as defined by the following claims.