HK1167441A - Stirling engine - Google Patents

Description

Stirling engine

Technical Field

The present invention relates to a heat engine, and in particular to an improved stirling cycle engine incorporating a number of improvements and design features for improving the performance, manufacturability and reliability of the engine.

Background

The basic concept of the stirling engine dates back to the patent registered by robert stirling in 1817. Since then, this engine has been the subject of intense attention and evaluation. Various stirling engine systems have been prototyped and put into limited operation worldwide. One potential application area for stirling engines is as a prime mover for automobiles or as an engine power unit for hybrid electric applications. Other areas of potential use for stirling engines are for example stationary auxiliary power units, marine applications and solar energy conversion.

Stirling engines have a reversible thermodynamic cycle and can thus be used as a device to send mechanical output energy from a heat source, or as a heat pump by applying mechanical input energy. Mechanical energy can be delivered by an engine using various heat sources such as burning fossil fuel or biogas, or concentrated solar energy. This energy can be used to generate electricity or can be directly mechanically coupled to a load.

For many years, the assignee of the present application, Stirling Biopower and its predecessors have made significant advances in Stirling machine technology. While the assignee has made significant advances in stirling machine design, there is a constant need for further improvements to the machine, particularly where the desired application is mass production.

The stirling engine of the present invention shares many similarities with those previously developed by the assignee and its predecessors, including those described in U.S. patents nos. 4,439,169, 4,481,771, 4,532,855, 4,579,046, 4,615,261, 4,669,736, 4,836,094, 4,855,980, 4,707,990, 4,996,841, 4,977,742, 4,994,004, and 5,074,114, which are incorporated herein by reference. The basic features of the various stirling machines described in the above referenced patents are also implemented in connection with the present invention.

Disclosure of Invention

The stirling engine according to the invention has a so-called "modular" construction. The major components of the engine, including the drive case and the cylinder block, are bolted together along mating surfaces. A piston rod seal for the piston is transverse to the mating surface. A sliding rod seal mounted to the drive case or cylinder block can be used. The rod seal controls leakage of high pressure engine working gas to atmosphere at one end of the piston connecting rod.

In many past designs of stirling engines, the large volume of the engine housing was exposed to the high working pressure of the working gas. According to the engine of the present invention, the high pressure working fluid is confined to the extent that it can reach the opposite ends of the cylinder bore and the associated heat transfer devices and passages. Thus, the high pressure gas section of the stirling engine of the present invention is analogous to that encountered in internal combustion engines and, thus, the stirling engine can be similarly conceived in terms of considerations of high pressure component failure. This benefit is achieved in the present invention by: the drive case is maintained at a relatively low pressure, which may be close to ambient pressure, while the high pressure working fluid is confined within the cylinder block and associated components including the cylinder extension, regenerator housing and heater head.

The piston of the engine is connected to the crosshead by a piston rod. The crosshead of the engine surrounds the swashplate (swashplate) and translates the reciprocating motion of the piston connecting rods and pistons into rotational motion of the swashplate. The stirling engine of the present invention utilizes a pair of parallel guide rods mounted within the drive housing for each crosshead. The crosshead has a pair of journals that receive guide rods.

The combustion exhaust gases also contain useful heat after passing through the heater head of the engine. It is well known to use this additional heat to heat the incoming combustion air using an air preheater as a means of increasing the thermal efficiency. In accordance with the present invention, an air preheater is described that provides a compact configuration and has high thermal efficiency.

In a stirling engine of the type according to the invention employing four double acting cylinders, there are four separate working gas volumes, which are isolated from each other (except for leakage past the piston). In order to enable smooth engine operation with minimal force imbalance, the average pressure of each of the four volumes needs to be equalized. According to the invention, this is achieved in part by connecting the four volumes together via small orifices. In addition, a system is provided for determining that the average pressure in each cycle falls within a predetermined range. When a failure of the component causing the leakage occurs, a significant imbalance may cause it to have a damaging effect on the engine. The stirling engine according to the present invention has a pressure control system that unloads the engine in the event of such a failure.

The stirling engine according to the present invention has a control valve assembly which provides, in part, the aforementioned unloading feature. The control valve also provides one of the expected working gas leakage paths that form part of the pressure equalization system according to the present invention.

One of the key components in stirling engines of the aforementioned type relates to providing a highly reliable seal between the high pressure scavenging piston and the low pressure drive case of the machine. A piston rod seal assembly separates the two volumes. Each piston rod reciprocates through a piston rod seal that needs to reliably seal the piston rod to maintain a low loss rate of working gas to atmosphere. An absolute seal of gas leakage through this region is unlikely to be achieved. However, the piston rod seal assembly according to the present invention provides a low level of leakage and reduces contamination of the working gas by "pumping" of the lubricating oil located in the drive box area.

Another key design feature for improving the efficiency of a stirling engine comes from the design of the piston assembly. The scavenging piston separates the hot fluid space from the cold fluid space of the engine and reacts to the gas pressure in these areas to deliver mechanical power. It is desirable to minimize heat conduction losses between the hot and cold spaces through the piston to improve efficiency. Further, a highly reliable sliding gas seal is required between the piston ring and the cylinder bore. In addition to constituting heat losses, such leakage through the piston seal also results in a net mass exchange of working gas between the individual circulation volumes of the stirling engine. Significant differences in leakage past the piston seal can result in rapidly changing gas volumes in the cycle volume. Although there is provision according to the invention for reducing such imbalances, it is desirable to reduce the rate at which such imbalances occur.

Further benefits and advantages of the present invention will become apparent to those skilled in the art to which the invention relates from the subsequent description of preferred embodiments and the appended claims with reference to the accompanying drawings.

Drawings

FIG. 1 is a longitudinal cross-sectional view through a Stirling engine according to the present invention;

FIG. 2 is an enlarged cross-sectional view of the drive box assembly shown in FIG. 1;

FIG. 3 is an enlarged cross-sectional view of the cylinder block portion shown in the drawings;

FIG. 4 is an enlarged cross-sectional view taken from FIG. 1 showing the heater assembly of the engine of the present invention in greater detail;

FIG. 5 is an end view of a cylinder block component taken from the end of a heater assembly of the engine, with the heater assembly omitted;

FIG. 6 is a cross-sectional view of a piston assembly according to the present invention;

FIG. 7 is an enlarged view of the piston seal portion of FIG. 6;

FIG. 7a is a front view of the seal shown in FIG. 7;

FIG. 8 is a cross-sectional view of a rod seal assembly according to the present invention;

FIG. 9 is an enlarged cross-sectional view of the lid seal assembly also shown in FIG. 8;

FIG. 9a is a front view of the seal shown in FIG. 9;

FIG. 10 is an enlarged cross-sectional view of the base seal assembly of FIG. 8;

FIG. 11 is a diagrammatic view showing a pressure balancing circuit of the Stirling engine according to the present invention; and

fig. 12 is a cross-sectional view of a solenoid operated control valve according to the present invention.

Detailed Description

The stirling engine according to the invention is shown assembled in fig. 1 and designated in its entirety by reference numeral 10. The Stirling engine 10 includes a plurality or main components and assemblies, including a drive case assembly 12, a cylinder block assembly 14, and a heater assembly 16 (best shown in FIG. 4).

General configuration

The drive box assembly 12 includes a housing 18 having a pair of generally planar opposing mating surfaces 20 and 22 at opposite ends of the housing 18. The mating surface 22 is adapted to be mounted to the cylinder block assembly 14. The drive case housing 18 has a hollow interior and includes a journal 24 for mounting a drive shaft bearing. A series of crosshead guides 26 are provided around the inner periphery of the drive case housing 18. A pair of adjacent guides 26 are provided for each of four crosshead assemblies 56 of the engine (which are described below). As will be apparent from further description of the stirling engine 10, it is important that adjacent guides 26 have running surfaces that are parallel to each other within a very small tolerance.

At one end of the drive shaft 40 is a journal bearing 24. The drive case housing 18 is also provided with a central cavity in which an oil pump 44 is disposed. The oil pump 44 can be of a different type, such as a gerotor. High pressure lubricant is forced through the drilled passage 45 into a nozzle that sprays a film of lubricant onto the piston rod (described below). Additionally, lubricant is forced through the internal passages 41 in the drive shaft 40 to provide lubrication to the swash plate 52.

A sump port 50 is provided at a lower portion of the drive case 18. The lubrication system of engine 10 can be characterized as an oil sump type, whereby oil collected in the interior cavity of drive case 12 is directed by suction to oil pump 44, then pumped to a different location at oil pump 44 and sprayed as previously described.

The drive shaft 40 supports a swash plate 52, which swash plate 52 is generally circular and planar, but oriented to tilt at an angle relative to the axis of rotation of the drive shaft. Rotation of the drive shaft 40 causes the swash plate 52 to rotate about the tilt axis. This basic swash plate configuration is a well known design implemented in existing stirling engine configurations by the assignee and its predecessors. Attached to one end of drive shaft 40 is an output coupling 54, which output coupling 54 is capable of connecting to mechanical loads, which may be of different types, as previously described. The flywheel 53 rotates together with the drive shaft 40. An inductive sensor 55 is disposed near the outer diameter of the flywheel 53 and provides an electrical signal related to the rotation of the flywheel in response to teeth or gaps in the outer diameter.

The cylinder block assembly 14 defines a series of four oppositely drilled rod seal bores 48, the rod seal bores 48 having passages 37 and 35 (shown in FIG. 5) connected therebetween. A number of components, including regenerator housing 122 and cylinder extension 116, described in more detail in the following paragraphs, are attached to mounting surface 49. The cylinder block assembly 14 also defines four cylinder bores 94 aligned with the rod seal bore 48. Additional components are attached to the cylinder block surface 90. A cylinder extension 116 bolted to the cylinder block and a regenerator housing 122 are attached to the cylinder block surface 90.

Cross head assembly

With continued reference to fig. 1 and 2, the crosshead body 58 is formed as a caliper having a pair of legs 60 and 62 connected by a center bridge 64. Each of the legs 60 and 62 defines a surface for running along the guide 26. Crosshead leg 62 is also formed with a slider cup bore 72. A slider cup 74 formed with a semi-spherical surface 75 is disposed within the bore 72. The crosshead leg 60 has a surface machined with a hemispherical surface 76. The slider elements 78 and 80 also define spherical outer surfaces that are embedded into the mating slider cup surfaces 75 and 76. The opposed flat surfaces 82 and 84 are formed by slider elements and engage the swash plate 52. The swash plate surface may be formed in a conical or crown shape to create line contact with the slider element.

Cylinder block

The cylinder block assembly 14, best shown in fig. 1 and 3, includes a block casting 86 having a pair of opposed parallel generally planar mating surfaces 88 and 90. The mating surface 88 enables mounting of the cylinder block assembly 86 to the drive case housing mating surface 22. Bolts 92 hold the two components together. The stirling engine 10 according to the present invention is a four cylinder engine. Thus, the block casting 86 defines four cylinder bores 94 that are parallel to one another. As shown in FIG. 1, cylinder bore 94 defines a larger diameter section through which piston assembly 96 reciprocates, and a reduced diameter clearance bore portion for rod seal assembly 98. Four cooler bores 102 are also formed in the block casting 86 and are parallel to each other and to the cylinder bores 94. The cylinder bores 94 are arranged in square groups, but on the outer circumference of the group of cooler bores 102, as best shown in FIG. 5. In the case where the stirling engine 10 is of the double-acting type, the cylinder block casting 86 includes a working gas passage (not shown) connecting the bottom end of the regenerator bore 102 to the bottom end of the adjacent cylinder bore 94. The block casting 86 is also formed with coolant passages through which liquid coolant flows through the cylinder block 14. The pressure sensor port 34 enables the installation of a pressure sensor (not shown) for measuring the cyclic pressure at the bottom of each piston assembly 96.

Cooler assembly

A cooler assembly (not shown) is disposed within the block cooler bore. The cooler assembly may include an outer shell and a tube heat exchanger having a plurality of tubes arranged to extend between ends of the shell. The stirling cycle working gas moves back and forth between the ends of the cooler housing and through the interior of the tubes. A coolant, preferably a liquid, is pumped in a cross-flow manner through the block coolant passage 107 and through the cooler assembly to remove heat from the working gas.

Cylinder extension

The cylinder block assembly 14 also mounts a tubular top or extension 116, and these extensions 116 form a continuation of the cylinder block bore 94. At their open ends, the tubular cylinder extensions 116 are formed with skirts 117, which skirts 117 allow accurate alignment of these tubular extensions 116 with the cylinder bores 94 by guidance. The seal 118 provides a fluid seal between the cylinder block bore 94 and the tubular cylinder extension 116. The cylinder extension 116 forms heater tube manifolds 120 at opposite ends thereof.

Regenerator casing

Cup-shaped regenerator housing 122 is disposed in coaxial alignment with cooler bore 102. Regenerator housing 122 defines an open end and a closed top 126, the closed top 126 having a manifold 128 for communicating with heater assembly 16. Disposed within regenerator housing 122 is a regenerator (not shown) constructed of a material having a high gas flow rate and high thermal conductivity and heat absorption characteristics in accordance with known regenerator technology for stirling engines.

Heater assembly

The heater assembly 16 provides a means for inputting thermal energy into the stirling cycle working gas and is shown in fig. 4. Burners (not shown) are used to burn fossil fuels and other combustible materials. Alternatively, heat can be input from another source, such as concentrated solar energy, or other sources. In the stirling engine 10, the combustion gases flow axially toward the center of the engine, turning at the center to flow outwardly in a radial direction. An array of heater tubes 136 is provided to conduct heat away from the hot gases flowing radially out of the engine. The heater tubes 136 are arranged to form an inner band and an outer band with heat absorbing fins 140 located therebetween. The heater tube manifolds 120 and 128 have an inner channel that connects the inner band to the outer band of the heater tubes 136.

Air preheater

The combustion gases passing through the heater tubes 136 are still at a high temperature and have useful thermal energy that can be recovered to increase the thermal efficiency of the engine 10. This is accomplished by using an air preheater (not shown) having a circular ring-like configuration and surrounding the outer edges of the heater tubes 136. The air preheater transfers waste heat from the flue gas.

Piston assembly

The piston assembly 96 is shown in an assembled condition in fig. 6. As shown, piston assembly 96 generally includes a piston dome 146, a piston base 148, and a ring assembly 150. The piston assembly 96 slides within a cylinder liner 144 mounted within the cylinder block bore 94.

Piston dome 146 is formed with a hollow dome top portion 152 and a machined base portion 154. The arched top 152 has a hollow interior 156. Because the piston assembly 96 separates the hot and cold spaces formed by the working gas, it is desirable to minimize heat exchange between the top and bottom ends (top end on the right side and bottom end on the left side of fig. 6) of the piston assembly 96. The heat transfer through the piston assembly 96 between its ends represents a loss in thermal efficiency of the stirling engine 10. Thus, the arched top 152 has a thin-walled configuration to minimize heat transfer, while the interior 156 is hollow to minimize heat conduction paths. The piston base portion 154 has a machined stepped outer diameter. The end of the base portion 154 is formed with a threaded portion 158 for assembly with the piston base 148.

Piston base 148 has a machined bore portion formed with an internal threaded portion 162, which internal threaded portion 162 is meshed with external threaded portion 158 to enable piston dome 146 and base portion 154 to be threadably joined together and assembled as shown in fig. 6. A seal 164 located in the seal groove 160 provides a sealed connection between these components.

Referring also to fig. 7, the outer surface of the piston base 148 has a bearing groove 166 that receives a resilient piston support 168. The support 168 has a thickness sufficient to directly support against the inner surface (or bore) 145 of the cylinder liner, thereby keeping the metal components of the piston assembly 96 from directly contacting the inner bore surface of the cylinder liner 144. The support 168 does not provide a gas seal with the aperture 145 (i.e., it acts as a support rather than a seal). The lower end of the piston base portion 148 provides an area for mounting a piston ring assembly 150, which piston ring assembly 150 is clamped in place by tightening the piston base plate 170 with cap screws 172. The ring assembly 150 is shown in more detail in figure 7.

The ring assembly 150 fits into the annular recess 174. The ring assembly 150 is formed with a pair of ring sets including an upper ring set 176 and a lower ring set 178. The ring sets 176 and 178 are symmetrically disposed on opposite radial surfaces of a piston base land 184, the piston base land 184 being held in place by a clamping engagement between the piston base plate 170 and the piston base 148. Piston base land 184 is preferably formed of a steel material and has an annular outer groove 186. Upper ring set 176 includes rings 177 and 179. Lower ring set 178 includes rings 181 and 183. The rings in ring sets 176 and 178 are preferably formed of an elastomeric material. An expansion ring 180, also formed of an elastomeric material, is disposed radially inward of the ring sets 176 and 178 and has a sealing lip 182 bearing against the upper and lower sets of rings 177 and 181. The ring sets 176 and 178, the expansion ring 180, and the support 168 may all be formed of a PEEK (polyetheretherketone) material.

The rings 177 and 181 of the upper and lower ring sets 176 and 178 shown in fig. 7a have radial splits 185, the radial splits 185 providing a small leak path for the working gas to pass through the rings in a controlled manner. However, rings 178 and 183 are solid and free of radial splits. The radial split 185 of the rings 177 and 181 prevents a pressure imbalance from occurring which could otherwise cause a suction situation.

The piston base portion 148 is formed with a central tapered bore 188. The piston rod 192 has a tapered upper end 187 that fits into the tapered bore 188. The piston rod 192 may have a threaded end 190, the threaded end 190 being engageable by an assembly tool to press fit the piston rod tapered end into the piston base tapered bore 188. The piston rod 192 can be press fit into the tapered cavity 188 and these components are connected after the operation is completed. Piston dome 146 may be threadably engaged with base 154 in an assembled condition.

Rod seal assembly

The rod seal assembly 98 is best shown with reference to fig. 8, 9 and 10. As shown, a piston rod 192 is shown passing through the rod seal assembly 98. As shown, the piston rod 192 includes a central bore 196 that extends from the threaded end 190 of the piston rod 192 to a location within the rod seal assembly 98 that intersects the radial passage 194. Passages 196 and 194 communicate with piston dome interior 156 and to hollow interior chamber 210 of rod seal assembly 98 to maintain the chamber at a cyclic minimum pressure, as previously described. The stem seal assembly 98 includes additional major components, including a housing 202, a cap seal assembly 204, and a base seal assembly 206.

Housing 202 is formed of a rigid material, such as steel, and has a recessed stepped bore 208 formed at its end facing piston assembly 96. The internal bore 210 forms a hollow internal volume and a stepped bore for receiving the base seal assembly 206. Radial passages 212 are provided to communicate with the internal cavity 210 of each of the four rod seal assemblies 98 through the passages 37 drilled into the cylinder block. The inner chamber 210 is connected to the piston chamber 156 and defines a volume near the minimum cycle pressure, which is later referred to in this description as volume 282 in fig. 12. The radial passages 214 provide flow passages for spraying lubricating oil onto the outer surface of the piston rod 192 to provide lubrication and cooling. The outer diameter of the housing 202 features a plurality of grooves and locating features to enable it to be sealed and mounted in place within the rod seal bore 48 of the cylinder block 14.

The lid seal assembly 204 is shown in detail in fig. 9 and is maintained in place by tightening the lid seal plate 216 with screws. The cap seal assembly 204 tapers into a hollow interior cavity provided when the cap seal plate 216 is secured and positioned. As shown, the lid seal assembly includes a lid sealing gasket 220 having an L-shaped cross-sectional configuration at its end closest to the piston assembly 96. The wave spring 222 is loaded into the radial cavity formed by the lid sealing gasket 220 and serves to place an axial load on the other components of the lid seal assembly 204. The lid sealing gasket 220 bears against a lid seal 224, the lid seal 224 having an inner diameter 226 bearing against the outer surface of the piston rod 192. The cap seal 224 has a semi-circular outer groove 228, and a coil spring 230 is disposed around the cap seal 224 and places a radially inward compressive force on the cap seal for enhancing the seal against the piston rod. The cap seal 224 has a radial split 232 that provides the desired gas leakage path. The lid seal 224 bears against a lid seal seat 234. The cap seal assembly 204 is used to provide a wiping function along the outer diameter of the piston rod 192. This reduces the pressure variation across the lid seal assembly 204, thereby providing a more effective gas seal for the remaining components of the seal assembly 98. The cap seal assembly 204 also provides an oil wiping function to remove lubricating oil from the outer surface of the piston rod 192, thereby preventing contamination of the circulating gas.

The base seal assembly 206, best shown in fig. 10, is disposed within the housing internal cavity 210. A pair of annular spring seats 236 and 238 are provided that generate an axial loading force for the base seal assembly 206. The spring seats 236 and 238 have posts 240 and 241 that locate coil springs 242. As shown, the spring seat 236 bears against a shoulder located within the internal cavity 210 to provide an axial load force. A sealing plug 244 is disposed in place within the housing bore and is maintained in this position by a retaining ring 246 that fits within a groove 248. The sealing plug 244 also has a passage 214 for delivering lubricant to spray the piston rod 192. The radial end surface 254 of the plug 244 is polished. An O-ring 258 and a gasket 256 made of a polyimide material bear against the radial end surface 254. The washer 256 and O-ring 258 allow the seat 260 to move with low friction relative to the plug 244. This allows seal carrier 260 to remain centered on stem 192 during operation of engine 10.

The seal holder 260 has a concave hemispherical surface 262 and a protruding inner post 264, the protruding inner post 264 capturing the O-ring 258 in the capture position. The stem seal 266 is formed with a projecting tubular portion 268 and a head 270, the head 270 being formed with a convex hemispherical surface 272, the convex hemispherical surface 272 further being formed with a seal groove 274 that retains a seal 276. Ideally, the sealing groove 274 is formed such that its side surface is tangent to the hemispherical surface 272 of the seal 266. Hemispherical surface 262 of seal holder 260 and surface 272 of seal 266 that mates therewith enable the seal to respond to bending of piston rod 192 during operation of engine 10, as well as accommodate any misalignment of the piston rod relative to rod seal assembly 98. The elastomeric components of the cap seal assembly 204, including the cap seal 224 and the stem seal 266, may be formed of a PTFE (polytetrafluoroethylene) material. Although surfaces 262 and 272 are described as "hemispherical," other mating concave and convex shapes that deviate from a pure spherical shape may be used.

Pressure balancing system and control valve assembly

In operation of the stirling engine 10, it is important to maintain the total mass of working fluid contained in the four enclosed volumes of the engine to have a near equal mass of working fluid. This is necessary to prevent an average pressure difference between the enclosed volumes and thus to prevent a force imbalance in the engine. There is an inevitable loss of working gas through the heater head assembly 16 and other leakage paths, and a small loss through the piston assembly 96 and through the rod seal 98. The stirling engine according to the present invention therefore provides a mechanism for enabling the mass of working gas present in four separate circulation volumes, each bounded at the top of one piston assembly 96 and the bottom of the adjacent piston assembly, to be equalised. In addition, it is desirable to reduce the starting torque required to act on drive shaft 40. This enables a smaller capacity starter motor with a lower torque output to be used to start the engine. These systems are best described with reference to fig. 11 and 12. The circulation volumes of the working gas 278 (typically hydrogen and helium) are indicated in the figures as circulation volumes #0, #1, #2 and # 3.

Figure 11 diagrammatically shows a system for providing pressure equalization. As shown, the gas in each cycle is represented in the figure by a cycle volume 278 individually designated as "cycle # 0" and so forth. The circulation volumes are interconnected by a plurality of paths. Two pressure volumes are formed in the engine 10, the engine 10 including passages 37 drilled into the cylinder block 14 in a square arrangement when viewing the engine as shown in fig. 5, the passages 37 connecting with the rod seal cavity 210 and the piston interior 156 to collectively form a minimum pressure volume 282. The channel 35 with flow restrictor 286 (which may be in the form of a capillary tube having a diameter of, for example, 0.4 mm) communicates with the working fluid space formed at the bottom of the piston assembly 96, thereby being exposed to cyclically varying cyclic pressures. The internal volume of the passages 35 in the central space 39 of the engine at the location where they intersect the flow restrictor 286 forms a mean pressure volume 280.

As previously described, a slight leakage occurs between the circulation volumes #0, #1, #2, and #3 through the rings of the piston assembly 96. This leakage path is schematically indicated in fig. 11 by a flow restrictor 284. The passages 35 drilled into the cylinder block 14 provide a common volume through which cyclic leakage occurs through the flow restrictor 286. This allows a small net flow of working gas to be maintained within the mean pressure volume 280. Because the pressure applied to the flow restrictor 286 cycles between the maximum and minimum values of the circulation volume 278, a small net flow occurs periodically in both directions through the flow restrictor and thereby maintains the volume 280 close to the average circulation pressure (here, "average" refers to not only the pressure of the average of the minimum and maximum pressures, but also any intermediate pressure between the minimum and maximum values of the circulation). The flow restrictors 286 are schematically illustrated in fig. 11. As previously described, the rod seal volume 210 is maintained at the minimum pressure volume 282. The valve port 294 is exposed to the minimum circulating pressure volume 282 through the rod seal channel 215 and to the circulating pressure 278 in the radial space 235 (connected together through the channel 35 and the flow restrictor 286) surrounding the rod seal assembly 98. The housing port 294 is aligned with the cap seal channel 215 in the cap seal housing 202. The radial space 235 between the head seal housing 202 and the cylinder block 86 communicates with the bottom of the piston 96 and is thereby subjected to cyclically varying gas pressures of working gas volume.

Valve assembly 288 is provided for each cycle volume and is described in detail below. Briefly, the valve assembly 288 functions as a solenoid actuated check valve 290. Valve assembly 288 also creates a leak path through valve port 292, which acts as a flow restrictor. When the valve 288 is electrically actuated, free flow into the minimum pressure volume 282 between the circulation volumes 278 occurs. This minimizes the starting torque of the engine and allows the piston assembly 96 to reciprocate with a low starting torque.

The valve 288 is shown in detail in fig. 12. Valve assemblies 288 are each mounted in a port 294 in cylinder block 86, and plug 293 extends into head seal passage 251. As shown in fig. 11, the valve assembly 288 controls the movement of fluid between the minimum pressure volume 282 and the circulation volume 278. The two pressure volumes are separated by the seal provided by the O-ring 300 around the head end 304 of the plug 293. Valve assembly 288 includes a valve body 302 having a threaded end 304, allowing it to be secured in position within an associated port 294. The valve body 302 is formed with an internal stepped bore 306. The sleeve assembly 308 is secured in place relative to the valve body 302 by mounting a threaded cap 310. A movable plunger (plunger)312 is located within the sleeve assembly 308, the movable plunger 312 being held in a normal position against a seat 314 by a coil spring 316. Coil windings 320 surround sleeve assembly 308. When current is passed through the windings 320, the plunger 312 is caused to move away from the seat 314, allowing free passage of fluid between the volumes 282 and 278, thereby effectively connecting the circulating volumes together. This free passage of gas between cycles reduces starting torque and enables rapid reduction of power output in the event of engine component failure or other conditions requiring rapid engine unloading. Because plunger 312 is spring loaded into engagement with seat 314, the higher pressure in passage 296 urges plunger 312 to move out of sealing engagement with seat 314 and thus valve assembly 288 acts as check valve 290 in the event that current does not flow into coil winding 320. In one embodiment, valve 288 has a plunger lift (or separation) pressure of less than about 1.0MPa (i.e., plunger 312 is unseated at this pressure differential). Unless the plunger 312 is actuated, gas flow in the opposite direction (from the circulation volume 278 to the minimum pressure valve 282) is inhibited (although controlled "leakage" occurs through the flow restrictor 292).

Referring back to fig. 11, whenever the minimum pressure volume 282 is at a pressure that is less than the minimum pressure present in any of the cyclical pressure variations of the circulation volume 278 by more than the lift pressure of the plunger 312, no fluid is communicated through the valve assembly 288. However, if the minimum pressure experienced in any of the circulation volumes 278 is less than the pressure of the minimum pressure volume 282 by more than the lift pressure, a net force acts on the valve assembly plunger 312 to force the plunger 312 open. The spring force applied to the plunger by the coil spring 316 is adjusted so that in the event that the pressure differential exceeds a predetermined amount (lift pressure), the plunger 312 unseats, allowing fluid to flow out of the minimum pressure volume 282, thereby maintaining its desired low pressure value in that volume. The port 292, shown in fig. 12 and schematically in fig. 11, provides a controlled leak between the circulation volume 278 and the minimum pressure volume 282. This mechanism is another way of exchanging gases between the cycle volumes 278, which maintains the mass of gases in each cycle #0, #1, #2, and #3 in balance while the engine 10 is operating.

As previously described, if the valve assembly 288 is actuated, the "short circuiting" of the circulation volumes 278 to each other or the free flow of gas interrupts the thermodynamic cycle of operation of the stirling engine 10 but allows for a low starting torque to place the mechanical components of the engine in motion, and also provides the aforementioned unloading feature. During a long period of time after engine 10 stops operating, the different pressure volumes in the engine tend to equalize in pressure. Once engine 10 is operated and valve assembly 288 is de-energized (allowing plunger 312 to seat), circulation volume 278 undergoes its pressure change from a minimum level to a maximum level in a cyclical manner. As previously described, whenever any one of the circulation volumes 278 reaches a pressure level that is less than the pressure present in the minimum pressure volume 282 by a value that exceeds the check valve lift pressure, the minimum pressure volume 282 is "depressurized" to a steady state pressure that is slightly greater than the minimum pressure experienced in the circulation volume 278. Thus during operation, if any of cycles #0, #1, #2, or #3 exhibits a pressure imbalance with the other cycles, where its minimum pressure is lower than the minimum pressure of the other cycles during the change of cycle, then when its corresponding check valve 290 operates, a net flow of working gas into that cycle is generated. The continuous leak path provided by each flow restrictor 292 results in a periodic net flow through each flow restrictor, which is another way of equalizing the volume or mass of working gas in each cycle volume 278 during operation of the engine. Another mechanism for gas exchange between the circulation volumes 278 results from the leakage path of the central space 39 of the engine, which central space 39 is maintained at an average pressure as previously described. A minute amount of constant back and forth movement of gas occurs through each flow restrictor 286 during operation of the engine. Because the mean pressure volume 280 is in communication with each cycle volume 278, the mechanism provides a means of gas exchange between the cycle volumes. It will be appreciated that any leakage of working gas in the cycle 278 has the effect of reducing the magnitude of the maximum and minimum pressures, which results in a loss of efficiency of the engine. However, by maintaining the gas leakage through the restrictors 288 and 286 at a slight level, any degradation in performance becomes negligible.

It is to be understood that the invention is not limited to the particular constructions shown and described above, but that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.

Claims (102)

1. A piston assembly for a stirling cycle engine of the type having two or more of said piston assemblies reciprocating within a cylinder bore, each of said piston assemblies separating isolated cycle volumes of a working gas contained within said engine, said piston assembly comprising:

a piston crown formed with a top end and a bottom end, the crown having a hollow interior;

a base attached to the piston dome base end;

a link attached to the base;

an upper ring set and a lower ring set disposed within the annular groove for sealing the cylinder bore; and

a land disposed within the annular groove between the upper ring set and the lower ring set, a first ring of the upper ring set disposed on a side of the land facing the dome top end portion and a second ring of the upper ring set disposed below the first ring and adjacent the land, a first ring of the lower ring set disposed on a side of the land facing the base portion and a second ring of the lower ring set disposed between the first ring of the lower ring set and the land.

2. A piston assembly for a stirling cycle engine in accordance with claim 1 wherein the first ring of the first or second ring sets is formed with a radial split.

3. A piston assembly for a stirling cycle engine in accordance with claim 1 wherein the first rings of the first and second ring sets are formed with radial splits.

4. A piston assembly for a stirling cycle engine in accordance with claim 1 wherein the second ring of the upper or lower ring sets is a solid circular ring.

5. A piston assembly for a stirling cycle engine in accordance with claim 1 wherein the second rings of the upper and lower ring sets are solid rings.

6. A piston assembly for a stirling cycle engine in accordance with claim 1 wherein the first or second ring of the upper or lower ring sets is formed of an elastomeric material.

7. A piston assembly for a Stirling cycle engine according to claim 6, wherein the elastomeric material comprises PEEK.

8. A piston assembly for a stirling cycle engine in accordance with claim 1 further comprising an expansion ring disposed radially inward of the upper or lower ring sets, formed of an elastomeric material, and having a lip engaging one of the first rings.

9. A piston assembly for a stirling cycle engine in accordance with claim 1 further comprising upper and lower expansion rings disposed radially inward of the upper and lower ring sets, respectively, each formed of an elastomeric material, and each having a lip engaging the first rings of the upper and lower ring sets, respectively.

10. A piston assembly for a stirling cycle engine in accordance with claim 1 wherein the land ring is formed with an outer diameter surface having a radial clearance with the surface of the cylinder bore.

11. A piston assembly for a stirling cycle engine in accordance with claim 1 further comprising: a piston base formed with at least a portion of the first annular groove portion of the annular groove for the upper ring set; and a piston base plate formed with at least a part of the second annular groove portion for the lower ring group of the annular groove, the land being sandwiched between the piston base and the piston base plate.

12. A piston assembly for a stirling cycle engine in accordance with claim 10 further comprising threaded fasteners clamping the piston base plate to the land ring and the piston base.

13. A piston assembly for a stirling cycle engine in accordance with claim 1 further comprising: the piston base is formed with a support groove, and a piston support is disposed in the support groove and engaged with a surface of the cylinder bore.

14. A piston assembly for a stirling cycle engine in accordance with claim 1 further comprising: the piston base is formed with a tapered bore for receiving the connecting rod.

15. A piston assembly for a stirling cycle engine in accordance with claim 1 further comprising the piston dome and the piston base attached together by meshing threads.

16. The piston assembly of claim 1, wherein the working gas is hydrogen or helium.

17. A piston assembly for a stirling cycle engine of the type having two or more of said piston assemblies reciprocating within a cylinder bore, each of said piston assemblies separating isolated cycle volumes of a working gas contained within said engine, said piston assembly comprising:

a piston dome formed with a top end and an open bottom end, the dome having a hollow interior;

a base attached to the piston dome at the bottom end;

a link attached to the base;

the upper ring group and the lower ring group are arranged in the annular groove; and

a land disposed within the annular groove between the upper ring set and the lower ring set, the land having an outer diameter surface with a radial clearance from a surface of the cylinder bore, a first ring of the upper ring set disposed on a side of the land facing the dome top end portion and the second ring of the upper ring set disposed below the first ring and adjacent the land, a first ring of the lower ring set disposed on a side of the land facing the base and a second ring of the lower ring set disposed between the first ring of the lower ring set and the land; and

an upper expansion ring and a lower expansion ring, each disposed radially inward of the upper ring set and the lower ring set, respectively, each formed of an elastomeric material, and each having a lip that engages the first ring of each of the upper ring set and the lower ring set, respectively.

18. A piston assembly for a stirling cycle engine in accordance with claim 17 wherein the first ring of the first or second ring sets is formed with a radial split.

19. A piston assembly for a stirling cycle engine in accordance with claim 17 wherein the first rings of the first and second ring sets are formed with radial splits.

20. A piston assembly for a stirling cycle engine in accordance with claim 17 wherein the second ring of the upper or lower ring sets is a solid ring.

21. A piston assembly for a stirling cycle engine in accordance with claim 17 wherein the second rings of the upper and lower ring sets are solid rings.

22. A piston assembly for a stirling cycle engine in accordance with claim 17 wherein the first or second ring of the upper or lower ring sets is formed of an elastomeric material.

23. A piston assembly for a stirling cycle engine in accordance with claim 22 wherein the elastomeric material comprises PEEK.

24. A piston assembly for a stirling cycle engine in accordance with claim 17 further comprising: the piston base formed with at least a portion of the first annular groove portion of the annular groove for the upper ring set; and a piston base plate formed with at least a part of the second annular groove portion for the lower ring group of the annular groove, the land being sandwiched between the piston base and the piston base plate.

25. A piston assembly for a stirling cycle engine in accordance with claim 24 further comprising threaded fasteners clamping the piston base plate to the land ring and the piston base.

26. A piston assembly for a stirling cycle engine in accordance with claim 17 further comprising: the piston base is formed with a support groove, and a piston support is disposed in the support groove and engaged with a surface of the cylinder bore.

27. A piston assembly for a stirling cycle engine in accordance with claim 17 further comprising: the piston base is formed with a tapered bore for receiving the connecting rod.

28. A piston assembly for a stirling cycle engine in accordance with claim 17 further comprising attaching the piston dome and the piston base together at mating threads.

29. The piston assembly of claim 17, wherein the working gas is hydrogen or helium.

30. A rod seal assembly for a stirling cycle engine, the stirling cycle engine being of the type having two or more piston assemblies reciprocating within a cylinder bore, each of the piston assemblies separating an isolated circulation volume of a working gas contained within the engine, each of the piston assemblies being attached to a connecting rod which oscillates in a linear reciprocating motion during operation of the engine, the rod seal assembly sealing the connecting rod to control leakage of the working gas from the circulation volume, the rod assembly comprising:

a rigid housing;

a cap seal assembly adjacent the piston assembly, the cap seal assembly being retained by the housing and having a cap seal engaged with the connecting rod;

a base seal assembly spaced from the piston assembly, the base seal assembly being retained by the housing, having a rod seal engaged with the connecting rod; and

the housing is formed with an interior cavity between the lid seal assembly and the base seal assembly.

31. A rod seal assembly for a stirling cycle engine in accordance with claim 30 further comprising: the base seal assembly has means for allowing the stem seal to be displaced relative to the housing during operation of the engine.

32. A rod seal assembly for a stirling cycle engine in accordance with claim 30 further comprising: the cap seal has a radial split.

33. A rod seal assembly for a stirling cycle engine in accordance with claim 30 further comprising: the lid seal assembly has a lid sealing gasket and a first spring for placing an axial load on the lid sealing gasket and the lid seal.

34. A rod seal assembly for a stirling cycle engine in accordance with claim 30 further comprising: the lid seal assembly has a second spring for placing a radial load on the lid seal.

35. A rod seal assembly for a stirling cycle engine in accordance with claim 30 further comprising the cap seal formed of PTFE material.

36. A rod seal assembly for a stirling cycle engine in accordance with claim 30 further comprising the rod seal formed of PTFE material.

37. A rod seal assembly for a stirling cycle engine in accordance with claim 30 further comprising: the base seal assembly having a sealing plug disposed at an end of the housing opposite the cap seal assembly, and a sealing seat formed with a concave hemispherical surface; the rod seal has an internal bore for the connecting rod and forms a convex hemispherical surface bearing against a concave hemispherical surface of the seal seat, and first and second spring seats have a spring located therebetween and disposed in the housing to place a compressive force on the rod seal.

38. A rod seal assembly for a stirling cycle engine in accordance with claim 37 further comprising: the sealing plug is formed with a planar end surface, a gasket disposed between the sealing seat and the sealing plug and in contact with the planar end surface, the gasket enabling relative radial movement between the sealing plug and the sealing seat.

39. A rod seal assembly for a stirling cycle engine in accordance with claim 38 further comprising: the stem seal has an O-ring groove formed in the convex hemispherical surface and an O-ring disposed in the O-ring groove and in contact with the convex hemispherical surface.

40. A rod seal assembly for a stirling cycle engine in accordance with claim 30 wherein the rod has a central passage communicating the interior of the piston assembly with the internal cavity.

41. A rod seal assembly for a Stirling cycle engine according to claim 40, wherein the interior of the piston assembly and the housing internal cavity maintain a minimum pressure near the cycle volume during operation of the engine.

42. A rod seal assembly for a Stirling cycle engine according to claim 41, wherein the engine has a plurality of the rod seal assemblies and each of the housing internal chambers are connected together to have equal working gas pressure.

43. The rod seal assembly of claim 30 wherein the working gas is hydrogen or helium.

44. A rod seal assembly for a stirling cycle engine, the stirling cycle engine being of the type having two or more piston assemblies reciprocating within a cylinder bore, each of the piston assemblies separating an isolated circulation volume of a working gas contained within the engine, each of the piston assemblies being attached to a connecting rod which oscillates in a linear reciprocating motion during operation of the engine, the rod seal assembly sealing the connecting rod to control leakage of the working gas from the circulation volume, the rod assembly comprising:

a rigid housing;

a cap seal assembly adjacent the piston assembly, the cap seal assembly being retained by the housing and having a cap seal that seals the connecting rod;

a base seal assembly spaced from the piston assembly, the base seal assembly being retained by the housing, the base seal assembly having a sealing plug disposed at an end of the housing opposite the cap seal assembly, a seal seat forming a convex hemispherical surface, a stem seal having an inner bore for the connecting rod and a concave hemispherical surface bearing against the convex hemispherical surface of the seal seat, a spring disposed in the housing to place a compressive force on the stem seal; and

the housing is formed with an interior cavity between the lid seal assembly and the base seal assembly.

45. A rod seal assembly for a Stirling cycle engine according to claim 44, further comprising: the base seal assembly has means for allowing the stem seal to be displaced relative to the housing during operation of the engine.

46. A rod seal assembly for a Stirling cycle engine according to claim 44, further comprising: the lid seal has a radial split and a lid sealing gasket, and a first spring for placing an axial load on the lid sealing gasket and the lid seal.

47. A rod seal assembly for a Stirling cycle engine according to claim 46, further comprising: the lid seal assembly has a second spring for placing a radial load on the lid seal.

48. A rod seal assembly for a Stirling cycle engine according to claim 44, further comprising the cover seal formed of PTFE material.

49. A rod seal assembly for a Stirling cycle engine according to claim 44, further comprising the rod seal formed of PTFE material.

50. A rod seal assembly for a Stirling cycle engine according to claim 44, further comprising: the base seal assembly has a first spring seat and a second spring seat with a spring therebetween disposed in the housing to place a compressive force on the stem seal to force the convex hemispherical surface against the concave hemispherical surface.

51. A rod seal assembly for a Stirling cycle engine according to claim 44, further comprising: a sealing plug formed with a planar end surface, a gasket disposed between and in contact with the sealing seat and the sealing plug, the gasket enabling relative radial movement between the sealing plug and the sealing seat.

52. A rod seal assembly for a Stirling cycle engine according to claim 44, further comprising: the stem seal has an O-ring groove formed in the convex hemispherical surface and an O-ring disposed in the O-ring groove and in contact with the convex hemispherical surface.

53. A rod seal assembly for a Stirling cycle engine according to claim 44, wherein the rod has a central passage communicating the interior of the piston assembly with the internal cavity.

54. A rod seal assembly for a Stirling cycle engine according to claim 53, wherein the interior of the piston assembly and the housing internal cavity maintain a minimum pressure near the cycle volume during operation of the engine.

55. A rod seal assembly for a Stirling cycle engine according to claim 53, wherein the engine has a plurality of the rod seal assemblies and each of the housing internal chambers are connected together to have equal working gas pressure.

56. The rod seal assembly of claim 44 wherein the working gas is hydrogen or helium.

57. A working gas control system for a double-acting stirling cycle engine having two or more piston assemblies reciprocating within a cylinder bore, each of the piston assemblies separating an isolated circulation volume of working gas contained within the engine, the circulation volume undergoing a cyclic pressure change from a minimum circulation pressure to a maximum circulation pressure during operation of the engine, each of the piston assemblies being connected to a connecting rod and having a rod seal assembly for sealing the connecting rod, the control system comprising:

a first volume defined by the hollow interior of the piston and communicating with a second passage through a first passage in the connecting rod, the first volume adapted to approach the minimum cycle pressure during operation of the engine;

a check valve connecting the circulation volume with the first volume such that during operation of the engine, the working gas flows from the first volume to the circulation volume to maintain the pressure in the first volume near the minimum circulation pressure;

a second volume adapted to approach an average cycle pressure between the minimum and maximum cycle pressures during operation of the engine and communicating with the cycle volume through a third passage having a first flow restrictor.

58. The working gas control system of claim 57 further comprising: the engine has a cylinder block assembly and the second and third passages formed by the cylinder block assembly.

59. The working gas control system of claim 57 further comprising: the check valve inhibits the working gas from flowing from the circulation volume to the first volume.

60. The working gas control system of claim 59, further comprising: the check valve is a controllable check valve that is actuatable to allow the working gas to flow in both directions between the circulation volume and the first volume.

61. A working gas control system according to claim 59, wherein the check valve is a valve assembly having an electrically actuated solenoid for actuating the check valve.

62. The working gas control system of claim 57 further comprising a second flow restrictor connecting the first volume to the circulation volume.

63. A working gas control system according to claim 62, wherein the check valve is in the form of a valve assembly having a valve port including the second flow restrictor.

64. A working gas control system according to claim 59, wherein the check valve permits flow from the first volume to the circulation volume when the gas pressure in the circulation volume is less than the gas pressure of the first volume minus the lift pressure of the check valve, the check valve prohibiting gas flow from the first volume to the circulation volume.

65. A working gas control system according to claim 59, wherein the check valve may be inoperative to permit the working gas to flow in both directions between the first volume and the circulation volume.

66. A working gas control system according to claim 57, wherein the rod seal assembly first passage communicates with an interior cavity of the rod seal assembly, the rod seal assembly interior cavity communicating with the second passage, wherein the interior cavity forms part of the first volume.

67. A working gas control system according to claim 57, wherein the first flow restrictor is provided in the third channel.

68. The working gas control system of claim 57 further comprising a third flow restrictor representing leakage of working gas past piston rings of the piston assembly.

69. A working gas control system according to claim 57, wherein the working gas is hydrogen or helium.

70. A working gas control system according to claim 57, wherein a check valve and a first flow restrictor are provided in association with each of the piston assemblies of the engine.

71. The working gas control system of claim 57 further comprising: a plurality of ports are formed in a cylinder block assembly of the engine, each of the plurality of ports communicating with a circulation volume and the first volume, and a valve mounted in the port provides the second flow restrictor and the check valve.

72. A working gas control system for a double-acting stirling cycle engine having two or more piston assemblies reciprocating within a cylinder bore, each of the piston assemblies separating an isolated circulating volume of working gas contained within the engine, the circulating volume undergoing cyclic pressure changes from a minimum circulating pressure to a maximum circulating pressure during operation of the engine, each of the piston assemblies being connected to a connecting rod and each having a rod seal assembly for sealing the connecting rod, the control system comprising:

the rod seal assembly having an internal cavity;

a first volume defined by the hollow interior of the piston and communicating with a rod seal assembly interior cavity and a second channel through a first channel in the connecting rod;

a check valve associated with each of the circulation volumes and the first volume, the check valve permitting flow from the first volume to the circulation volume when the gas pressure in the circulation volume is less than the gas pressure in the first volume minus a lift pressure of the check valve, the check valve inhibiting flow of working gas from the first volume to the circulation volume, the check valve causing the gas pressure in the first volume to be maintained near the minimum circulation pressure during operation of the engine;

a second volume adapted to approach an average cycle pressure between the minimum and maximum cycle pressures during operation of the engine, a first flow restrictor connecting the cycle volume with the second volume; and

a second flow restrictor connecting the first volume with the circulation volumes such that during operation of the engine working gas flows between the circulation volumes by flow through the first and second flow restrictors.

73. The working gas control system of claim 72 further comprising: the engine has a cylinder block assembly and the second and third passages formed by the cylinder block assembly.

74. The working gas control system of claim 72 further comprising: the check valves are each controllable check valves that can be actuated to allow the working gas to flow in both directions between the associated circulating volume and the first volume.

75. A working gas control system according to claim 72, wherein the check valve is a valve assembly having an electrically actuated solenoid for actuating the check valve.

76. A working gas control system according to claim 72, the check valves being in the form of valve assemblies each having a valve port including the second flow restrictor.

77. A working gas control system according to claim 72, wherein the check valve may be inoperative to permit gas flow in both directions between the first volume and the circulation volume.

78. The working gas control system of claim 72 wherein the first flow restrictor is disposed in the third channel.

79. The working gas control system of claim 72 further comprising a third flow restrictor representing leakage of working gas past piston rings of the piston assembly.

80. A working gas control system according to claim 72, wherein the working gas is hydrogen or helium.

81. The working gas control system of claim 72 further comprising: a plurality of ports are formed in a cylinder block assembly of the engine, the ports each communicating with a circulation volume and the first volume, and a valve mounted in the port provides the second flow restrictor and the check valve.

82. A method of controlling a working gas for a double-acting stirling cycle engine having two or more piston assemblies reciprocating within a cylinder bore, each of the piston assemblies separating an isolated circulation volume of working gas contained within the engine, the circulation volume undergoing a cyclic pressure change from a minimum circulation pressure to a maximum circulation pressure during operation of the engine, each of the piston assemblies being connected to a connecting rod and having a rod seal assembly for sealing the connecting rod, the method comprising:

forming a first volume defined by the hollow interior of the piston and communicating with a second passage through a first passage in the connecting rod, the first volume adapted to approach the minimum cycle pressure during operation of the engine;

providing a check valve connecting the circulation volume with the first volume such that during operation of the engine, the working gas flows from the first volume to the circulation volume to maintain the pressure in the first volume at a near minimum level;

forming a second volume adapted to approach an average cycle pressure between the minimum and maximum cycle pressures during operation of the engine and communicating with the cycle volume through a third passage having a first flow restrictor, wherein the working gas of the cycle volume is exchanged between the cycle volumes during operation of the engine to provide control of the volume of the working gas in the cycle volume.

83. The method of claim 82, further comprising a second flow restrictor disposed between the first volume and the circulation volume for allowing the working gas to flow between the circulation volume and the first volume to further enable control of the volume of the working gas in each circulation volume.

84. The method of claim 82, further comprising a controllable valve for opening gas flow between the first volume and the circulation volume to unload the engine by reducing pressure changes in the circulation volume.

85. A control valve for a double-acting stirling cycle engine of the type having two or more piston assemblies reciprocating within a cylinder bore, each of the piston assemblies separating an isolated cycle volume of working gas contained within the engine, the cycle volume undergoing a cycle pressure change from a minimum cycle pressure to a maximum cycle pressure, the engine having a valve port for connection to the control valve, the control valve being in communication with one of the cycle volumes and with a pressure volume, the control valve comprising:

a valve body formed with a first passage for communication with the circulation volume and a second passage for communication with the pressure volume;

a plunger movable between a closed position sealing the seat against the seat, wherein flow between the first and second passages is restricted, and an open position displaced from the seat, wherein flow between the passages is permitted;

a spring urging the plunger to the closed position such that the plunger is moved to the open position when the pressure in the first passage is less than the pressure in the second passage by more than a predetermined pressure differential; and

an actuator acting on the plunger to move the plunger to the open position against the urging force of the spring to connect the passages.

86. The control valve of claim 85, wherein the valve body includes a plug having a bore forming the second passage and an end of the plug forms the seat.

87. The control valve of claim 86, wherein the first passage of the valve body surrounds the plug.

88. The control valve of claim 86, wherein the plug is formed with a third passage forming a restricted flow connection between the first and second passages.

89. The control valve of claim 86, wherein the actuator is an electric solenoid.

90. A control valve for a double-acting stirling cycle engine of the type having two or more piston assemblies reciprocating within a cylinder bore, each of the piston assemblies separating an isolated cycle volume of working gas contained within the engine, the cycle volume undergoing cyclic pressure changes from a minimum cycle pressure to a maximum cycle pressure, each of the piston assemblies being connected to a connecting rod and having a rod seal assembly for sealing the connecting rod, the engine having a valve port for receiving the control valve, the control valve being in communication with the cycle volume and with a pressure volume, the control valve comprising:

a valve body formed with a first passage for communication with the circulation volume;

a valve plug having a hollow internal passage forming a second passage for communicating with the pressure volume;

a plunger movable between a closed position sealing the seat against the seat, wherein flow between the circulation volume and the pressure volume is restricted, and an open position displaced from the seat, wherein flow between the circulation volume and the first volume is permitted;

a spring urging the plunger to the closed position such that the plunger is moved to the open position when the pressure in the first passage is less than the pressure in the second passage by more than a predetermined pressure differential; and

an actuator acting on the plunger to move the plunger to the open position against the urging force of the spring to connect the passages.

91. The control valve of claim 90, wherein one end of the plug forms the seat.

92. The control valve of claim 90, wherein the first passageway of the valve body surrounds the plug.

93. The control valve of claim 90, wherein the plug forms a third passage forming a restricted flow connection between the first and second passages.

94. The control valve of claim 90, wherein the actuator is an electric solenoid.

95. A double-acting stirling cycle engine comprising:

two or more piston assemblies reciprocating in cylinder bores, each of the piston assemblies separating an isolated cycle volume of working gas contained within the engine, the cycle volume undergoing a cyclic pressure change from a minimum cycle pressure to a maximum cycle pressure;

the engine having a cylinder block formed with a valve port for each of the circulation volumes, the valve ports each communicating with one of the circulation volumes and with a pressure volume;

a control valve disposed within each of the valve ports and formed with a first passage for communication with the circulation volume and a second passage for communication with the pressure volume, the control valve further having: a plunger movable between a closed position sealing the seat against the seat, in which flow between the first and second passages is restricted, and an open position displaced from the seat, in which flow between the passages is permitted; a spring urging the plunger to the closed position such that the plunger moves to the open position when the pressure in the first passage is less than the pressure in the second passage by more than a predetermined pressure differential; an actuator acting on the plunger to move the plunger to the open position against the urging force of the spring to connect the passages.

96. A stirling engine in accordance with claim 95, wherein in operation of the engine the control valve acts to cause the pressure in the engine pressure volume to have a pressure close to the minimum cycle pressure.

97. A stirling engine in accordance with claim 95, wherein each of the control valve pressure volumes is connected to the pressure volume.

98. A stirling engine in accordance with claim 95, wherein the control valve has a second passage providing a restricted flow path between the first and second passages allowing the working gas to flow between the circulation volume and the pressure volume.

99. A stirling engine in accordance with claim 95, wherein the circulation volume is connected to the pressure volume when the control valve is open, which reduces the magnitude of the difference between the minimum and maximum circulation pressures.

100. A stirling engine in accordance with claim 95, wherein the engine includes a rod seal assembly sealing a connecting rod coupled to the piston, the rod seal assembly having a housing with a hollow interior cavity communicating with the pressure volume.

101. A stirling engine in accordance with claim 95, wherein the engine port communicates with the rod seal internal cavity.

102. A stirling engine in accordance with claim 101 wherein the rod seal internal cavity communicates with a seal port aligned with the valve port and the control valve has a plug with a head disposed in the seal port.