Patent Grant
8671677
(Not Applicable)
(Not Applicable)
This invention is in the field of free piston Stirling machines and more particularly is directed to an improved free piston Stirling machine of the gamma class which minimizes the dead volume normally associated with the gamma configuration.
In a Stirling machine, a working gas is confined in a working space comprised of an expansion space and a compression space. The working gas is alternately expanded and compressed in order to either do work or to pump heat. Each Stirling machine has at least two pistons, one referred to as a displacer and the other referred to as a power piston and often just as a piston. The reciprocating displacer cyclically shuttles a working gas between the compression space and the expansion space which are connected in fluid communication through a heat accepter, a regenerator and a heat rejecter. The shuttling cyclically changes the relative proportion of working gas in each space. Gas that is in the expansion space, and gas that is flowing into the expansion space through a heat exchanger (the accepter) between the regenerator and the expansion space, accepts heat from surrounding surfaces. Gas that is in the compression space, and gas that is flowing into the compression space through a heat exchanger (the rejecter) between the regenerator and the compression space, rejects heat to surrounding surfaces. The gas pressure is essentially the same in the entire work space at any instant of time because the expansion and compression spaces are interconnected through a path having a relatively low flow resistance. However, the pressure of the working gas in the work space as a whole varies cyclically and periodically. When most of the working gas is in the compression space, heat is rejected from the gas. When most of the working gas is in the expansion space, the gas accepts heat. This is true whether the machine is working as a heat pump or as an engine. The only requirement to differentiate between work produced or heat pumped, is the temperature at which the expansion process is carried out. If this expansion process temperature is higher than the temperature of the compression space, then the machine is inclined to produce work so it can function as an engine and if this expansion process temperature is lower than the compression space temperature, then the machine will pump heat from a cold source to a warm heat sink.
Stirling machines can therefore be designed to use the above principles to provide either: (1) an engine having a piston and displacer driven by applying an external source of heat energy to the expansion space and transferring heat away from the compression space and therefore capable of being a prime mover for a mechanical load, or (2) a heat pump having the power piston (and sometimes the displacer) cyclically driven by a prime mover for pumping heat from the expansion space to the compression space and therefore capable of pumping heat energy from a cooler mass to a warmer mass. The heat pump mode permits Stirling machines to be used for cooling an object in thermal connection to its expansion space, including to cryogenic temperatures, or heating an object, such as a home heating heat exchanger, in thermal connection to its compression space. Therefore, the term Stirling “machine” is used to generically include both Stirling engines and Stirling heat pumps, the latter sometimes being referred to a coolers.
Until about 1965, Stirling machines were constructed as kinematically driven machines meaning that the piston and displacer are connected to each other by a mechanical linkage, typically connecting rods and crankshafts. The free piston Stirling machine was then invented by William Beale. In the free piston Stirling machine, the pistons are not connected to a mechanical drive linkage. A free-piston Stirling machine is a thermo-mechanical oscillator and one of its pistons, the displacer, is driven by the working gas pressure variations and differences in spaces or chambers in the machine. The power piston, is either driven by a reciprocating prime mover when the Stirling machine is operated in its heat pumping mode or drives a reciprocating mechanical load when the Stirling machine is operated as an engine.
As well known in the art, there are three principal configurations of Stirling machines. The alpha configuration has at least two pistons in separate cylinders and the expansion space bounded by each piston is connected to a compression space bounded by another piston in another cylinder. These connections are arranged in a series loop connecting the expansion and compression spaces of multiple cylinders. The beta Stirling has a single power piston arranged within the same cylinder as a displacer piston. A gamma Stirling is similar to a beta Stirling but has the power piston mounted in a separate cylinder alongside the displacer piston cylinder.
As is well known, in free-piston Stirling engines and coolers, the displacer and the piston both must be able to freely operate with minimum friction. Since oil or similar lubricants are impractical for use in Stirling machines, non-contact bearings of various types have come to be generally applied. Some researchers use radially stiff flat springs to support the moving parts so as to avoid contact during operation while others have used static gas bearings. All these methods require extremely close tolerances in order to avoid excessive leakage losses and mechanical contact between the moving parts. In the standard displacer-piston beta arrangement, the precision requirements of the displacer and piston compound each other since the displacer rod penetrates the piston. The co-axial alignment of the displacer rod within the piston places additional demands on precision in both displacer and piston and is therefore a strong cost driver.
These problems can be seen in the prior art beta type free piston Stirling machine illustrated in
In the beta arrangement of
The well-known gamma configuration overcomes this alignment problem by arranging the displacer and piston in separate cylinders so that their individual requirements for precision do not interfere with each other as in the case of the beta configuration. However, a disadvantage of the gamma arrangement is that it has a higher dead volume than the beta configured machine. Further, in most prior art gamma machines, the placement of the piston and displacer in separate cylinders results in both an oscillating torque and a force on the casing that is more difficult to balance than the single oscillating axial force on the casing in the beta machine. This latter problem has been identified in at least one design published in the open literature where two opposing pistons are used to remove the oscillating torque component on the casing.
A second problem associated with beta free-piston machines is that the dynamic balancing technique that is universally used relegates these machines to operation at a single frequency. Arranging single frequency operation for engines is difficult and requires that the machine be frequency stabilized by, for example, direct electrical grid connection. On coolers, single frequency operation is easily established since the machines are electrically driven. However, even on these machines, there is sometimes a thermodynamic advantage in changing the operating frequency, which is not possible if a dynamic balancer is used. An ideal configuration for a free-piston Stirling machine would have:
a. No more precision than required for good thermodynamic operation.
b. A minimum dead volume.
c. Balancing under all operating conditions including different operating frequencies.
It is therefore an object and feature of the invention to provide a free piston Stirling machine in a gamma configuration that has power pistons with masses and orientations for balancing the vibration forces of the pistons and, most importantly, minimizes the dead (unswept) volume of the work space in order to reduce the size and mass of the machine and improve its efficiency.
The invention is an improved free piston Stirling machine having a gamma configuration. The machine includes a displacer having an inner end and is reciprocatable within a displacer cylinder along a displacer axis. Two or more power pistons are arranged in a balanced configuration for canceling their momentum vectors to minimize vibration. Each piston has an inner end and is reciprocatable within a cylinder having an inner end. Each cylinder has an unobstructed opening at its inner end that opens into a common volume of the workspace. The common volume is defined by the intersection of inward projections of the displacer cylinder and the piston cylinders. The displacer and the pistons each have a range of reciprocation that extends into the common volume. A displacer drive rod functioning like a piston is reciprocatable in a drive rod cylinder. The displacer drive rod and its cylinder are positioned outside the common volume and on the opposite side of the common volume from the displacer. The displacer is connected to the displacer drive rod by a displacer connecting rod. The displacer and pistons have complementary interfacing surface contours formed on their inner ends.
In describing the preferred embodiment of the invention which is illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific term so selected and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.
The invention utilizes the gamma configuration in the free-piston mode with two or more pistons and a single displacer. The pistons are preferably arranged at right angles to the displacer motion. In order to minimize dead volume, the displacer drive area is provided on the displacer spring, which is mounted below the pistons so that the pistons do not have to engage or contact and therefore accommodate the displacer drive rod as in conventional beta machines. This allows the pistons to approach each other to a minimum distance. The displacer and piston motions may be designed to intersect each other for even greater dead volume reduction. The pistons are sized, positioned and reciprocate so as to balance their net forces that are applied to the casing of the machine and cause vibration. This achieves substantial although incomplete balancing. The displacer remains unbalanced but is generally of low mass compared to the overall mass of the machine so that the residual motion is actually quite small and in many cases, acceptable. The displacer amplitude (around 5 to 10 mm) divided by the mass ratio of the overall machine to the displacer (around 20 to 50) gives the residual vibration amplitude. If additional balancing is required, a conventional dynamic balancer could be used but it would be of much smaller mass and size since only the force from the displacer motions would need to be balanced. The pistons are separated assemblies that do not mechanically interact with each other or with the displacer. In fact, the displacer assembly can be made completely separate from the pistons.
Two power pistons 52 and 54 are arranged in a balanced configuration for canceling their momentum vectors. In this embodiment, the balanced arrangement is that both pistons 52 and 54 reciprocate along an axis 56 within their respective cylinders 58 and 60. The pistons 52 and 54 reciprocate in opposed relation so that they operate in phase in the sense that both move inwardly and both move outwardly at the same time. In other words during operation they have the same angle of their periodic, approximately sinusoidal, motion with respect to a point between them. Each piston 52 and 54 has an inner end 62, 64. The term “inner” is used to indicate generally the central region of the machine between the pistons and the displacer. The piston cylinders 58 and 60 and the displacer cylinder 44 all have an unobstructed opening at their inner ends into a common volume of the workspace.
The term “common volume” is used to describe a part of the inner volume of the work space. “Common volume” as used in this specification and the claims is the volume within the intersection of inward projections of the displacer cylinder and the piston cylinders as further defined in this paragraph. The inward projection of the displacer cylinder is illustrated in
The terms “dead” volume or space and “unswept” volume or space are also used. In all gamma configured Stirling machines, the inner end of the displacer and the inner end of each piston bound (form a boundary of) a portion of the work space. The displacer and each piston reciprocate in their respective cylinders along a range of reciprocation which varies as a function of working conditions. There is, however, always an inner space or volume that is unswept because it is never entered by the displacer or a piston. That unswept space is referred to as a dead or unswept space or volume. A prior art beta free piston Stirling machine can be configured so there is no dead space because the displacer and piston can move into (occupy) the same cylindrical volume at different times and phases of the cycle. However, in a gamma free piston Stirling machine there is always a dead space and, in prior art machines, it is relatively large. As far as known, because it is necessary to avoid collisions between the pistons or between the displacer and one or more pistons, the range of reciprocation of the pistons and the displacer in prior art gamma machines are maintained far apart and never even come close to the common volume. The invention minimizes the dead space by configuring the components of the gamma free piston Stirling machine so that they are able to enter the common volume and by shaping the reciprocating displacer and pistons so that they can approach each other within the common volume with a minimum of volume between the inner ends of the displacer and pistons. Some small dead volume remains necessary to assure avoidance of collisions.
Returning to a description of the embodiment of
Although known to those skilled in this art, it is believed desirable to explain the function of the displacer drive rod 66. In a free piston Stirling machine, the gas pressure in the work space varies cyclically and approximately sinusoidally. The gas pressure in the work space is applied to a cross sectional area of the pistons 52 and 54 and the displacer 40 to provide the drive forces that move them. Because the work space gas pressure varies cyclically, the gas pressure variations drive the pistons 52 and 54 and displacer 40 in their cyclic motion, although the displacer 40 is out of phase with the pistons 52 and 54. The drive force on each piston 52 and 54 is easily seen as the cross sectional area of the piston in a plane perpendicular to its axis of motion multiplied by the working space pressure.
In the prior art, a rod of the same diameter along its length extends all the way between the displacer and either a gas spring or a bounce or back space. For example, in the beta configured machine of
In the invention, the displacer 40 is driven in reciprocation in the same manner. However, in the invention, the displacer drive rod 66 and the displacer drive rod cylinder 68 are positioned outside the common volume and on the opposite side of the common volume from the displacer 40. That is done so that the displacer drive rod 66 and the displacer drive rod cylinder 68 are outside the common space and therefore are located where the pistons 52 and 54 can not collide with them. Consequently, the term “displacer drive rod” is adopted to designate the piston upon which working space pressure variations apply the force that drives the displacer in reciprocation. The term “displacer connecting rod” is adopted to designate the mechanical link that connects the displacer drive rod to the displacer. In the invention, the displacer connecting rod 70 can be made to have a small diameter or thickness, considerably smaller than the displacer drive rod 66, and this is done to allow maximum excursion of the pistons into the common volume. The wide diameter rod does not need to extend all the way through the common volume.
Another important feature of the invention is that the displacer 40 and pistons 52 and 54 have complementary interfacing surface contours formed on their inner ends. The term “complementary interfacing surface contours” means that the end surfaces of the pistons and displacer have shapes and locations so that they can approach each other with a small or minimum volume between the interfacing surfaces. In this manner, these reciprocating components can move significantly far into the common volume so that most of the common volume is no longer a dead or unswept space.
Referring again to
The inner end 42 of the displacer 40 is shaped conically in order to intersect the motion of the pistons 52 and 54, which are themselves shaped to accept the displacer motions without collision. The degree of intersection is a designer's choice. Zero intersection results in maximum unswept volume while maximum intersection results in minimum unswept volume. The displacer drive rod 66 is placed beyond the reach of the pistons 52 and 54.
Referring to
As known in the art, the displacer's cyclical motion leads the pistons' cyclical motion. So, not only are the displacer and pistons shaped to avoid collisions, the pistons can occupy some of the same space/volume as the displacer at different times, as in the beta machine because the displacer is moving outwardly when the pistons are still moving inwardly. The degree that each piston and the displacer travel into the common volume is a designers engineering choice. The closer the machine is designed to have them approach each other and approach the connecting rod the more reduction in dead volume but the greater the risk that operation could go outside of the designed range of reciprocation and result in a collision.
Returning to
Mechanical planar springs 78 are attached to the displacer drive rod 66. The displacer 40 and pistons 52 and 54 travel in a cylinder assembly that may simply be one piece with intersecting axes for the displacer and piston cylinders 44, 58 and 60. The pistons 52 and 54 may be connected to linear alternators, gas compressors and/or other mechanical loads or to motors which drive the pistons 52 and 54 depending on whether the machine is an engine or a cooler (heat pump).
Synchronicity of the piston motions is achieved by a common workspace, a common bounce space and a common alternator/motor connection.
The inner ends of the pistons 52 and 54 and the displacer 40 can alternatively have other complementary interfacing surface contours. For example, they could have stair-stepped contours. As another alternative, the displacer 40 could be a simple cylindrical shape with, for example a planar end perpendicular to its axis, and each piston 52 and 54 could have a complementary semi-cylindrical cut-out aligned along a radial of the cylindrical piston. If there are more than two pistons, as subsequently discussed, the pistons can also have relief (cut outs) for the other pistons as well as cavities or cut outs that are complementary with the displacer connecting rod. Migrating rotation of the pistons 52 and 54 during operation that would cause a misalignment of the complementary interfacing surface contours is prevented by a planar spring 78 or a linear alternator.
As illustrated in
There are other balanced arrangements for three or more pistons. Any number of pistons can be arranged with axes of reciprocation that are equi-angularly spaced including a three dimensional arrangement. Additionally, pistons can be arranged to reciprocate along axes with still other relative orientations. Pistons having different masses may also be used with the only requirement for balancing the vibrations being that their momentum vectors sum to zero.
Even without any vibration balancer, the only residual vibration of a machine embodying the invention is the vibration resulting from the momentum of the displacer and the consequent reaction momentum of the casing. Therefore, it is desirable to reduce the mass of the displacer as much as practical because the displacer is the only component causing vibration. Because amplitude of the casing vibration is proportional to the mass of the displacer multiplied by the amplitude of the displacer divided by the total mass of the remainder of the machine multiplied by the amplitude of the casing, vibration amplitude is proportional to the ratio of the displacer mass to the mass of the remainder of the machine. Therefore, there an incentive to make the mass of the displacer as small as possible, relative to the entire mass of the machine.
From the above, it can be seen that, although a typical prior art gamma configured free piston Stirling machine has a large and therefore undesirable dead volume, embodiments of the invention greatly reduce and nearly eliminate the dead volume while retaining the other benefits of the gamma configuration. This reduction in the dead volume gives a higher capacity per unit of machine volume (i.e. the size of the entire machine). The reduction improves the specific capacity of the machine where specific capacity is defined as the work or power per unit of volume of the machine, whether operated as an engine or a cooler/heat pump.
A visual comparison of the drawings of
This detailed description in connection with the drawings is intended principally as a description of the presently preferred embodiments of the invention, and is not intended to represent the only form in which the present invention may be constructed or utilized. The description sets forth the designs, functions, means, and methods of implementing the invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and features may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention and that various modifications may be adopted without departing from the invention or scope of the following claims.
This application claims the benefit of U.S. Provisional Application No. 61/223,449 filed Jul. 7, 2009. The above prior application is hereby incorporated by reference.
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