Control valve for a stirling engine

A control valve for a double acting type Stirling engine of the type having two or more piston assemblies reciprocating in bores with each of the piston assemblies separating isolated volumes of a working gas. The control valve provides the functions of a check valve which allows an internally defined volume within the engine to be held at a low pressure near the minimum pressure of the cycle volumes. The valve may further be actuated to unload the engine to provide low starting torque and to unload the engine in case of an engine or load malfunction. The control valve further provides a controlled leakage path for working fluid flowing between the cycle volumes and the minimum pressure volume which acts as part of a pressure balancing system for the Stirling engine which maintains the volume or mass of a working fluid in a balanced condition between the cycle volumes.

FIELD OF THE INVENTION

This invention is related to a heat engine and particularly to an improved Stirling cycle engine incorporating numerous refinements and design features intended to enhance engine performance, manufacturability, and reliability.

BACKGROUND AND SUMMARY OF THE INVENTIONS

The basic concept of a Stirling engine dates back to a patent registered by Robert Stirling in 1817. Since that time, this engine has been the subject of intense scrutiny and evaluation. Various Stirling engine systems have been prototyped and put into limited operation throughout the world. One potential application area for Stirling engines is for automobiles as a prime mover or engine power unit for hybrid electric applications. Other fields of potential use of a Stirling engine such as stationary auxiliary power units, marine applications and solar energy conversion.

Stirling engines have a reversible thermodynamic cycle and therefore can be used as a means of delivering mechanical output energy from a source of heat, or acting as a heat pump through the application of mechanical input energy. Using various heat sources such as combusted fossil fuels or biogases, or concentrated solar energy, mechanical energy can be delivered by the engine. This energy can be used to generate electricity or can be directly mechanically coupled to a load.

The Assignee of the present application, Stirling Biopower, Inc. and its predecessor company have made significant advances in the technology of Stirling machines through a number of years. Although the Assignee has achieved significant advances in Stirling machine design, there is a constant need to further refine the machine, particularly if the intended application is in large volume production.

The Stirling engine of the present invention bears many similarities to those previously developed by Assignee and its predecessor company, including those described in U.S. Pat. Nos. 4,439,169; 4,481,771; 4,532,855; 4,579,046; 4,615,261; 4,669,736; 4,836,094; 4,885,980; 4,707,990; 4,996,841 4,977,742; 4,994,004; and 5,074,114, which are hereby incorporated by reference. Basic features of many of the Stirling machines described in the above referenced patents are also implemented in connection with the present invention.

The Stirling engine in accordance with the present invention has a so-called “modular” construction. The major components of the engine, comprising the drive case and cylinder block, are bolted together along mating surfaces. Piston rod seals for the pistons traverse this mating plane. A sliding rod seal can be used which is mounted either to the drive case or cylinder block. The rod seal controls leakage of the high pressure engine working gas at one end of the piston connecting rod to atmosphere.

In many past designs of Stirling engines, a large volume of the engine housing is exposed to the high working pressures of the working gas. In accordance with the engine of the present invention, the high pressure working fluid is confined to the extent possible to the opposing ends of the cylinder bores and the associated heat transfer devices and passageways. Thus the high pressure gas areas of the Stirling engine of this invention are analogous to that which is encountered in internal combustion engines, and therefore this Stirling engine can be thought of in a similar manner in terms of consideration for high pressure component failure. This benefit is achieved in the present invention by maintaining the drive case at a relatively low pressure which may be close to ambient pressure, while confining the high pressure working fluid within the cylinder block and the connected components including the cylinder extension, regenerator housing, and heater head.

The pistons of the engine are connected to cross heads by piston rods. The cross heads of the engine embrace the swashplate and convert the reciprocating movement of the piston connecting rods and pistons to rotation of the swashplate. The Stirling engine of this invention implements a pair of parallel guide rods mounted within the drive case for each cross head. The cross heads feature a pair of journals which receive the guide rods.

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

In the Stirling engine of the type according to the present invention employing four double acting cylinders, there are four discrete volumes of working gas which are isolated from one another (except by leakage across the pistons). In order to enable the engine to operate smoothly and with minimal force imbalances, the mean pressure of each of these four volumes need to be equalized. In accordance with this invention, this is achieved in part by connecting together the four volumes through small orifices. In addition, a system is provided for determining that the mean pressure in each cycle is within a predetermined range. Upon the occurrence of a component failure causing leakage, a significant imbalance could result which could have a destructive effect on the engine. The Stirling engine according to this invention features a pressure control system which unloads the engine upon the occurrence of such failure.

The Stirling engine in accordance with the present invention features a control valve component which, in part, provides the unloading feature mentioned previously. The control valve also provides one of the intended working gas leakage paths which forms part of the pressure balancing system in accordance with the present invention.

A critical component in the Stirling engine of the type described previously involves providing highly reliable seals between the high pressure displacer pistons and the low pressure drive case of the machine. Separating these two volumes is a piston rod seal assembly. Each piston connecting rod reciprocates through a piston rod seal which needs to reliably seal against the piston rod to maintain a low loss rate of working gas to the atmosphere. Absolute sealing of gas leakage through this area is likely not achievable. However, the piston rod seal assembly in accordance with the present invention provides low levels of leakage and reduces contamination of the working gas through “pumping” of lubricating oil in the drive case region.

Another critical design feature for enhancing efficiency of the Stirling engine comes from the design of the piston assembly. The displacer piston separates the hot and cold fluid spaces of the engine and reacts against gas pressures in these areas to deliver mechanical power. Thermal conduction losses across the piston between the hot and cold spaces need to be minimized to enhance efficiency. Moreover, a highly reliable sliding gas seal is required between the piston rings and the cylinder bore. In addition to constituting a thermal loss, such leakage across the piston seals further results in a net mass exchange of working gases between the individual cycle volumes of the Stirling engine. Significant differences in leakage across the piston seals can result in rapidly changing gas volumes in the cycle volumes. Although means are provided in accordance with this invention for reducing such imbalances, it is desirable to reduce the rate at which these imbalances occur.

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

DETAILED DESCRIPTION OF THE INVENTION

Stirling engine in accordance with this invention is shown in an assembled condition inFIG. 1and is generally designated by reference number10. Stirling engine10includes a number or primary components and assemblies including drive case assembly12, cylinder block assembly14, and heater assembly16(best shown inFIG. 4).

Overall Construction

Drive case assembly12includes a housing18having a pair of generally flat opposed mating surfaces20and22at opposite ends. Mating surface22is adapted to be mounted to cylinder block assembly14. Drive case housing18has a hollow interior and includes a journal24for mounting a drive shaft bearing. Arranged around the interior perimeter of drive case housing18is a series of cross head guides26. A pair of adjacent guides26is provided for each of the four cross head assemblies56of the engine (which are described below). As will be evident from a further description of Stirling engine10, it is essential that adjacent guides26have running surfaces which are parallel within extremely close tolerances.

At one end of drive shaft40there is provided journal bearing24. Drive case housing18also provides a central cavity within which oil pump44is located. Oil pump44could be of various types such as a gerotor type. Through drilled passageways45, high pressure lubricating oil is forced into spray nozzles which spray a film of lubricant onto the piston rods (described below). In addition, lubricant is forced through internal passages41within drive shaft40to provide lubrication for swashplate52.

At a lower portion of drive case18, a sump port50is provided. The lubrication system of engine10can be characterized as a sump type with oil collecting in the interior cavity of drive case12being directed to oil pump44by suction, where it is then pumped to various locations and sprayed as mentioned previously.

Driveshaft40supports swashplate52which is generally circular and planar but is oriented at an angle inclined with respect to the axis of rotation of the driveshaft. Rotation of driveshaft40causes swashplate52to rotate about the axis of rotation in an inclined plane. This basic swashplate configuration is a well known design implemented by the Assignee and its predecessor company in prior Stirling engine configurations. Attached to an end of drive shaft40is an output coupler54enabling connection to a mechanical load, which as previously stated, may be of various types. Flywheel53rotates with drive shaft40. Inductive pickup55is positioned near the outer diameter of flywheel53and responds to teeth or gaps in the outer diameter to provide electrical signals related to flywheel rotation.

Cylinder block assembly14defines a series of four counter bored rod seal bores48with passageways37and35connecting between them (shown inFIG. 5). A number of components attached to mounting surface49including regenerator housings122and cylinder extensions116which are described in more detail in the following sections. Cylinder block assembly14further defines four cylinder bores94aligned with the rod seal bores48. Additional components are attached to cylinder block surface90. Cylinder extension116and regenerator housings122are connected with cylinder block surface90which are bolted to the cylinder block.

Cross Head Assembly

With continued reference toFIGS. 1 and 2, cross head body58forms a caliper with a pair of legs60and62connected by center bridge64. Each of legs60and62define surfaces for running along guides26. Cross head leg62also forms slider cup bore72. Within bore72is positioned slider cup74which forms a semispherical surface75. Crosshead leg60has a surface machined with a semispherical surface76. Slider elements78and80also define spherical outside surfaces which are nested into the mating slider cup surfaces75and76. Opposing flat surfaces82and84are formed by the slider elements and engaged swashplate52. The swashplate surfaces may be made conical or crowned to generate line contact with the slider elements.

Cylinder Block

Cylinder block assembly14, best shown inFIGS. 1 and 3, includes a cylinder block casting86having a pair of opposed parallel generally flat mating surfaces88and90. Mating surface88enables cylinder block casting86to be mounted to drive case housing mating surface22. Bolts92hold these two parts together. Stirling engine10according to the present invention is a four cylinder engine. Accordingly, cylinder block casting86defines four cylinder bores94which are mutually parallel. As shown inFIG. 1, cylinder bores94define a larger diameter segment through which piston assembly96reciprocates, as well as a reduced diameter clearance bore section for rod seal assembly98. Four cooler bores102are also formed in cylinder block casting86and are mutually parallel as well as parallel to cylinder bores94. Cylinder bores94are arranged in a square cluster but lie on a circle outside that of cooler bores102as best shown inFIG. 5. In that Stirling engine10is a double acting type, cylinder block casting86including working gas passageways (not shown) which connect the bottom end of regenerator bore102to the bottom end of an adjacent cylinder bore94. Cylinder block casting86further forms coolant passageways which provide for a flow of liquid coolant through cylinder block14. Pressure transducer ports34enable mounting of pressure transducers (not shown) for measuring cycle pressure at the bottom of each piston assembly96.

Cooler Assembly

Cooler assemblies (not shown) are disposed within cylinder block cooler bores. Cooler assemblies may comprise a shell and tube type heat exchanger with a number of tubes arranged to extend between ends of the housing. The Stirling cycle working gas is shuttled back and forth between the ends of the cooler housing and passed to the inside of the tubes. A coolant, preferably a liquid, is pumped in a cross flow manner through block coolant passages107and through the cooler assemblies to remove heat from the working gas.

Cylinder Extensions

Cylinder block assembly14further mounts tubular tops or extensions116which form a continuation of cylinder block bores94. At their open ends, tubular cylinder extensions116form a skirt117which allows them to accurately align with cylinder bores94by piloting. Seals118provide a fluid seal between cylinder block bores94and tubular cylinder extensions116. Cylinder extensions116at their opposite ends form a heater tube manifold120.

Regenerator Housing

Cup shaped regenerator housings122are provided which are aligned coaxially with cooler bores102. Regenerator housing122define an open end and a closed top126having manifold128for communication with the heater assembly16. Within regenerator housing122is disposed a regenerator (not shown) which, in accordance with known regenerator technology for Stirling engines, is comprised of a material having high gas flow as well as high thermal conductivity and heat absorption characteristics.

Heater Assembly

Heater assembly16provides a means for inputting thermal energy into the Stirling cycle working gas and is shown inFIG. 4. A combustor (not shown) is used to burn a fossil fuel and other combustible material. Alternatively, heat can be inputted from another source such as concentrated solar energy, or other sources. In Stirling engine10, combustion gases flow axially toward the center of the engine where it is deflected to outwardly flow in a radial direction. An array of heater tubes136is arranged to conduct heat from the high gases that flow radially out of the engine. Heater tubes136are arranged to form an inner band and outer band with heat absorbing fins140therebetween. Heater tube manifolds120and128have internal passageways which connect the inner and outer bands of heater tubes136.

Air Preheater

Combustion gases which pass through heater tubes136are still at an elevated temperature and have useful heat energy which can be recovered to enhance the thermal efficiency of engine10. This is achieved through the use of an air preheater (not shown) which has an annular ring configuration and surrounds the outer bank of heater tubes136. The air preheater transfers waste heat from the exhaust gases.

Piston Assembly

Piston assembly96is shown in assembled condition inFIG. 6. As illustrated, piston assembly96principally comprises piston dome146, piston base148, and ring assembly150. Piston assembly96slides within cylinder liner144which is mounted within cylinder block bores94.

Piston dome146forms a hollow dome top152and a machined base section154. Dome top152has a hollow interior156. Since piston assembly96separates hot and cool spaces formed by the working gas, it is desirable to minimize heat exchange between the top and bottom ends of piston assembly96(with the top end at the right hand side ofFIG. 6and the bottom end at the left hand side). Heat transfer through piston assembly96between its ends represents a thermal efficiency loss for Stirling engine10. Accordingly, dome top152has a thin wall construction to minimize conductive heat transfer and interior156is hollow to minimize conductive heat paths. Piston base section154has a machined outer stepped diameter. The end of base section154forms threaded section158for assembly with piston base148.

Piston base148has a machined bore section forming internal threaded section162which meshes with external threaded section158to enable piston dome146and base section154to be threaded together and assembled as shown inFIG. 6. Seal164, located in seal groove160, provides sealing connection between these components.

Referring also toFIG. 7, the outer surface of piston base148features bearing groove166which receives elastomeric piston bearing168. Bearing168has a thickness sufficient to bear directly against the inside surface (or bore)145of cylinder liner, keeping the metal components of piston assembly96from directly contacting the inside bore surface of cylinder liner144. Bearing168does not provide gas sealing with bore145(i.e., it acts as a bearing and not a seal). The lower end of piston base section148provides a region for installation of piston ring assembly150which is sandwiched in place by fastening piston base plate170using cap screws172. Ring assembly150is shown in greater detail inFIG. 7.

Ring assembly150is fit within annular groove174. Ring assembly150forms a pair of ring sets including upper ring set176and lower ring set178. Ring sets176and178are positioned symmetrically on opposite radial surfaces of piston base land184which is held into position through clamping engagement between piston base plate170and piston base148. Piston base land184is preferably formed of steel material and features an annular outer groove186. Upper ring set176includes rings177and179. Lower ring set178includes rings181and183. The rings of ring sets176and178are preferably formed of an elastomeric material. Expander rings180, also formed of an elastomeric material, are placed radially inboard of the ring sets176and178, and have a sealing lip182which bears against ring177of the upper set and181of the lower set. Rings sets176and178, expander rings180, and bearing168may all be formed from a PEEK (polyetheretherketone) material.

Ring177of upper ring set176and ring181of lower ring set178, both illustrated inFIG. 7afeature a radial split185which provides a small leakage path for working gas to pass across the rings in a controlled manner. Rings179and183however are solid and do not feature a radial split. The radial splits185of rings177and181prevent pressure imbalances from occurring which otherwise can lead to pumping conditions.

Piston base section148forms a central tapered bore188. Piston rod192has a tapered upper end187which fits within tapered bore188. Piston rod192may have a threaded end190which can be engaged by an assembly tool to press-fit the piston rod tapered end into piston base taper bore188. Piston rod192can be press-fit into tapered bore188and, after that operation is completed, these parts are connected. Piston dome146may be threaded in an assembled condition with base154.

Rod Seal Assembly

Rod seal assembly98is best shown with reference toFIGS. 8,9, and10. As illustrated, piston rod192is shown passing through rod seal assembly98. As illustrated, piston rod192includes a central drilled bore196extending from its threaded end190to a point within rod seal assembly98intersecting with radial passageway194. Passageways196and194communicate with piston dome interior156and to a hollow interior cavity210of the rod seal assembly98with the cavities maintained at a cycle minimum pressure, as described previously. Rod seal assembly98includes additional principal components including housing202, cap seal assembly204, and base seal assembly206.

Housing202is formed of a rigid material such as steel and forms a recessed stepped bore208at its end facing piston assembly96. Interior bore210forms a hollow interior volume and a stepped bore for accommodating base seal assembly206. Radial passageways212are provided to communicate the interior cavities210of each of four rod seal assemblies98through passageways37drilled into the block. Interior cavities210are connected together with piston cavities156and define a volume at near minimum cycle pressure, designated later in this description as volume282inFIG. 12. Radial passageway214provides a flow passageway for lubricating oil to be sprayed against the outer surface of piston rod192for providing lubrication and cooling. The outer diameter of housing202features a number of grooves and locating features to enable it to be sealed and mounted in position within rod seal bores48of block14.

Cap seal assembly204is shown in detail inFIG. 9and is maintained in position by fastening cap seal plate216using screws. Cap seal assembly204is trapped within the hollow interior cavity provided when cap seal plate216is fastened and positioned. As shown, at its end closest to piston assembly96, cap seal assembly includes cap seal spacer220having an L-shaped cross-sectional configuration. Wave springs222are loaded into the radial cavity formed by cap seal spacer220and are used to place an axial load against other components of cap seal assembly204. Cap seal spacer220bears against cap seal224which has an inner diameter226which bears against the outer surface of piston rod192. Cap seal224has a semi-circular outer groove228and coil spring230is positioned around cap seal224and places a radially inward compressive force on the cap seal for enhanced sealing against the piston rod. Cap seal224features a radial split232which provides an intended gas leakage path. Cap seal224bears against cap seal seat washer234. Cap seal assembly204is used to provide a wiping function along the outer diameter of piston rod192. This provides a reduction in the pressure variation across cap seal assembly204to provide more effective gas sealing for the remaining components of the seal assembly98. Cap seal assembly204further provides an oil wiping function to remove lubricating oil from the outer surface of piston rod192to prevent contamination of the cycle gases.

Base seal assembly206, best shown inFIG. 10, is positioned within housing interior bore210. A pair of annular spring seats236and238is provided which produce an axial loading force for base seal assembly206. Spring seats236and238feature posts240and241which locate coil springs242. As illustrated, spring seat236bears against a shoulder within interior bore210to provide the axial loading force. Seal plug244is located in position within the housing bore and is maintained there through a snap ring246installed within groove248. Seal plug244further features passageway214for the transfer of lubricating oil to spray against piston rod192. The radial end face surface254of plug244is polished. Bearing against it is washer256made from a polyimide material and inner O-ring258. Washer256and O-ring258permit seat260to shift relative to plug244with low friction. This allows seal seat260to stay centered up with rod192during operation of the engine10.

Seal seat260features a concave semi-spherical surface262and a projecting inner post264which traps O-ring258in this trapped position. Rod seal266forms a projecting tubular section268and head270forming a convex semi-spherical surface272which further forms seal groove274which retains seal276. Ideally, seal groove274is formed such that its side surfaces are tangent to the semi-spherical surface272of seal266. The semi-spherical surface262of the seal seat260and its mating surface272of seal266enable the seal to respond to bending of piston rod192during operation of engine10, as well as adjust to any misalignments of the piston rods with respect to rod seal assembly98. The elastomeric components of cap seal assembly204, including cap seal224and rod seal266may be formed from a PTFE (polytetrafluoroethylene) material. Although surfaces262and272are described as “semi-spherical”, other matching concave and convex shapes deviating from purely spherical may also be used.

Pressure Equalization System and Control Valve Assembly

In operation of Stirling engine10, it is important that the total mass of working fluid contained in the four enclosed volumes of the engine are maintained to have closely equal masses of working fluid. This is necessary to prevent average pressure differences between the enclosed volumes and therefore force imbalances from occurring in the engine. There are unavoidable losses of working gas through heater head assembly16and other leakage paths, as well as minute leakage across piston assembly96and across rod seal98. Consequently, the Stirling engine in accordance with this invention provides a mechanism for allowing equalization in the mass of working gas existing in the four separate cycle volumes (each bounded at the top of one piston assembly96and the bottom of an adjacent piston assembly). In addition, it is desirable to reduce the starting torque required acting on driveshaft40. This enables smaller capacity starting motors having lower torque outputs to be used for starting the engine. These systems are best described with reference toFIGS. 11 and 12. Cycle volumes of working gas278(typically hydrogen and helium) are designated in the figures as cycle volumes #0, #1, #2, and #3.

FIG. 11shows diagrammatically a system providing pressure balance. As the diagram illustrates, the gas of each of the cycles are represented in the figure as cycle volumes278designated individually as “cycle #0”, etc. These cycle volumes are interconnected through a number of pathways. Two pressure volumes are formed within engine10including passageways37drilled into cylinder block14in a square arrangement when viewing the engine as shown inFIG. 5which are connected with rod seal internal cavities210and piston interiors156to collectively form a minimum pressure volume282. Passageways35having restrictors286, which may be in the form of capillary tubes having a diameter for example of 0.4 mm, communicate with the working fluid space formed at the bottom of piston assemblies96and are therefore exposed to cyclically varying cycle pressure. The internal volume of passageways35at their point of intersection past restrictors286in the center space39of the engine form a mean pressure volume280.

As stated previously, a minute leakage occurs between cycle volumes #0, #1, #2, and #3across the rings of piston assemblies96. This leakage pathway is designated diagrammatically inFIG. 11as restrictor284. Passageways35drilled within cylinder block14provide a common volume through which cycle leakage occurs through restrictors286. This allows a small net flow of working gas to be maintained within the mean pressure volume280. Since the pressure applied to restrictors286cycles between the cycle volumes278maximums and minimums, a small net flow periodically occurs in both directions through the restrictors and thus maintaining volume280at near mean cycle pressure (here “mean” refers not only to a pressure which is the average of the minimum and maximum pressures, but to any intermediate pressure between the cycle minimums and maximums). Restrictors286are represented diagrammatically inFIG. 11. As stated previously, rod seal volumes210are held a minimum pressure volume282. Valve port294is exposed to minimum cycle pressure volume282through rod seal passageway215, and to cycle pressure278in a radial space235surrounding rod seal assembly98(connected together through passageways35and restrictors286). Housing ports294are aligned with cap seal passageway215in cap seal housing202. A radial space235between cap seal housing202and the cylinder block86communicates with the bottom of the pistons96and therefore undergoes the cyclically varying gas pressure of the working gas volumes.

Valve assemblies288are provided for each of the cycle volumes and are described in more detail as follows. Diagrammatically, valve assemblies288act as a solenoid actuated check valve290. Valve assembly288also produces a leakage path through valve orifice292acting as a restrictor. When valve288is electrically actuated, a free flow between the cycle volumes278into minimum pressure volume282occurs. This minimizes engine starting torque and allows piston assemblies96to be reciprocated with low starting torque.

Valve288is illustrated in detail inFIG. 12. Valve assemblies288are each installed within ports294in cylinder block86with plug293extending into cap seal passageway215. As illustrated inFIG. 11, valve assemblies288control fluid movement between the minimum pressure volume282and the cycle volumes278. These two pressure volumes are separated through sealing provided by O-ring300around the head end295of plug293. Valve assembly288includes valve body302having a threaded end304, allowing it to be fixed into position within the associated ports294. Valve body302forms an internally stepped bore306. Sleeve assembly308is fastened in position relative to valve body302by installing threading cap310. Within sleeve assembly308is moveable plunger312which is held in a normal position against seat314by coil spring316. Coil winding320surrounds sleeve assembly308. When electric current is passed through winding320, plunger312is caused to move away from seat314, allowing free passage of fluid between the volumes282and278, thus effectively connecting together the cycle volumes. This free passage of gas between cycles reduces starting torque and can rapidly reduce power output in a condition of a failure of an engine component or other need to quickly unload the engine. Since plunger312is spring loaded into engagement with seat314, higher pressure in passageway296urges plunger312to move away from sealing engagement with seat314and thus the valve assembly288acts as a check valve290in conditions where current is not flowing into coil windings320. In one embodiment, valve288has a plunger lift-off (or cracking) pressure of less than about 1.0 Mpa (i.e., plunger312unseats at that pressure difference). Unless plunger312is actuated, gas flow in a reverse direction (from cycle volumes278to minimum pressure valve282) is inhibited (although a controlled “leak” occurs across restrictor292).

Referring back toFIG. 11, whenever the minimum pressure volume282is not greater than the lowest pressure occurring in the cyclical pressure variations of any of the cycle volumes278by more than the lift-off pressure of plunger312, no fluid is transferred through valve assembly288. If however, the minimum pressure experienced in any one of the cycle volumes278is less than the pressure of minimum pressure volume282by more than the lift-off pressure, a net force acts on valve assembly plunger312to urge it to open. A spring force applied to plunger by coil spring316is adjusted such that if this difference in pressure exceeds a predetermined amount (the lift-off pressure), plunger312unseats allowing fluid to be moved out of minimum pressure volume282, thus maintaining at its desired low pressure value in that volume. Orifice292, shown inFIG. 12and diagrammatically inFIG. 11provides a controlled leak between the cycle volumes278and minimum pressure volume282. This mechanism is another way in which gas is exchanges between the cycle volumes278which maintains the mass of gas in each cycle #0, #1, #2, and #3to be balanced as engine10is operating.

As mentioned previously, if valve assembly288is actuated, the “short circuit” or free flow of gas of cycle volumes278to one another interrupts the thermodynamic cycle of operation of Stirling engine10but permits low starting torque to put the mechanical components of the engine in motion, and also provides the unloading feature mentioned previously. During a prolonged period after stopping operation of engine10, the various pressure volumes in the engine with tend to equalize in pressure. Once engine10is operated and valve assembly288is de-energized (allowing plunger312to seat) the cycle volumes278undergo their pressure variations from a minimum to a maximum level in a cyclical manner. As mentioned previously, whenever any one of cycle volumes278goes to a pressure level less than the existing pressure in minimum pressure volume to282by an amount exceeding the check valve lift-off pressure, minimum pressure volume282is “pumped down” to a steady state pressure which is slightly greater than the minimum pressure experienced in the cycle volumes278. Thus during operation, if any one of the cycles #0, #1, #2, or #3exhibits a pressure imbalance with the other cycles in which its minimum pressure during cyclical variation is below that of the other cycles, a net flow of working gas into that cycle will occur as its respective check valve290operates. The continuous leakage paths provided by each of restrictors292causes a net periodic flow through each of the restrictors which is another means by which the volume or mass of working gas in each of the cycle volumes278are equalized during operation of the engine. Another mechanism for the exchange of gas between the cycle volumes278occurs by the leakage path to the center space39of the engine which is held at the mean pressure as mentioned previously. A constant shuttling of minute quantities of gas occurs through each of the restrictors286during operation of the engine. Since the mean pressure volume280communicates with each of the cycle volumes278, this mechanism provides a means of exchanging gas between the cycle volumes. It is acknowledged that any leakage of working gas in cycles278has the effect of reducing the magnitude of maximum and minimum pressures which leads to an efficiency penalty for the engine. However, by maintaining the leakage gas through restrictors288and286to minute levels, any degradation in performance becomes negligible.

It is to be understood that the invention is not limited to the exact construction illustrated and described above, but that various changes and modifications may be made without departing from the spirit and scope of the invention as defined by the following claims.