CA1226443A - Stirling-cycle, reciprocating, thermal machines - Google Patents

Abstract

ABSTRACT
Mechanical arrangements with primary application to single-acting, multiple-piston, Stirling-cycle machines pro-viding a dramatic reduction in mechanical complexity and production cost. Two specific new machines are disclosed in detail, a single-acting, two-piston "ducted axle" machine and a quasi double-acting, four-piston, "drum cam" machine. A
power level control subsystem inherent in the ducted axle machine permits engine output to vary from maximum positive through zero to maximum negative under full load conditions by means of the simple rotation of a single moving part. New thermodynamic working fluids are disclosed which have a higher dynamic heat transfer coefficient than either hydrogen or helium and are both chemically inert and easily liquefied.
A different power level control subsystem for Stirling-cycle prime movers based on the thermophysical properties of these fluids greatly simplifies changing the average system working pressure. A new construction of the Stirling regenerator employs the anisotropic properties of materials such as pyro-lytic graphite to achieve superior performance. Advantageous specific applications of copper matrix composites, manganese-copper alloys, and structural ceramics are disclosed.

Description

BACKGROUND OF Toll No or ~226443 1. Focal or the lnvcnti-~n:
This invention relates to Stirling-cycle engines, also known as regenerative thermal machines and more particularly to a set of new mechanical arrangements for the construction of a family of multiple-piston, Stirling-cycle machines.
These machines embody a practical approximation to the well known Stirling thermodynamic cycle; employ unique design and arrangement of components and materials to achieve an ultimate mechanical simplicity; and perform with high efficiency in the production of both mecltanical power (i.e., heat engines or prime movers, compressors, fluid pumps) and refrigeration it refrigerators, air conditioners, heat pumps, gas liquefiers).
A Stirling-cycle engine is a machine which operates on a closed regenerative thermodynamic cycle, with periodic comprcs-soon and expansion of a gaseous working fluid at different temperature levels, and where the flow is controlled by volume changes in such a way as to produce a net conversion of heat to work or vice versa. The regenerator is a device which in prior art takes the Norm of a porous mass of metal in an insulated duct. This mass takes up heat from the working fluid during one part of the cycle, temporarily stores it within the machine until a later part of the cycle, and subsequently returns it to the working fluid prior to the start of the next cycle. Thus the regenerator may be thought of as an oscilla-tory thermodynamic sponge, alternately absorbing and releasing heat with-complete reversibility and no loss.
A reversible process for a thermodynamic system is an ideal process, which once having taken place, can be reversed without causing a change in either tile system or its surround-ins. Regenerative processes are reversible in that they involve reversible heat transfer and storage; their importance derives from the fact that idealized reversible heat transfer is closely approximated by the regenerators of actual machines.

AYE i Thus the Stirling engine is the only practical example of a reversible heat engine which can lye operated either as a prime mover or as a heat pump.

2. Description of the Prior art:
The Stirling-cycle engine was first conceived and reduced to practice in Scotland 164 years ago. A hot-air, closed-cycle prime mover based on the principle was patented by the Reverent Raft Stirling in 1~17 as an alternative to the explosively dangerous steam engine. Incredibly, this event occurred early in the Age of Steam, lung before the invention of the internal combustion engine and several years before the first formal exposition ox the Issue of Thermodynamics.
Air was the first and only working fluid in early lath century machines, iris hydrogen an helium have been the preferred working fluids for modern machines. In Britain, Europe, and the united States thousands of regenerative hot air prime movers in a variety of shapes and sizes were widely used throughout the lath century. The smaller engines were reliable, reasonably efficient for their time and, most important, safe compared with contemporary-reciprocating steam engines. The larger engines were less reliable, however, because they tended to overheat and often succumbed unexpectedly to prom-lure material failure.
By 1920, therefore, the electric motor and the steam turbine had almost universally and completely replaced the hot air prime mover in the marketplace. Until recently there was little incentive or opportunity to reconsider the commercial exploitation ox the Stirling engine's numerous potential advantages. This was partly because of the primacy of diesel and gasoline internal combustion engines in an era of abundant and cheap fuel supplies, and partly because the state of the art in many areas of related technology was inadequate. Since World War II, however, there have been unprecedented advances in the general technologies of machine design, heat transfer, materials science, system analysis and simulation, manufac-luring methods, and Stirling engine development.
Today, in comparison to their conventional internal combustion counterparts, all modern Stirling prime movers are external combustion engines which consistently demonstrate, in the laboratory, higher efficiency, multifoil capability, lower exhaust emissions, quieter operation, equivalent power density, and superior torque characteristics. Nevertheless, none of these engines is in mass production anywhere in the world today. The reason for this is that contemporary Stirling engines have been created ~argely-by adapting traditional methods and designs from the more familiar internal combustion engine technology hose, and are therefore too complex.
Patchwork adaptation of the old as a shortcut path to the new is a process which inexorably produces a hodgepodge arrangement of excessive mechanical complexity and which Inevitably results in high production costs. In the design of innovative high-technology devices, the easy or obvious soul-lion to a design problem at the component level often leads to an unacceptably complex ramification at the system level.
Despite clearly superior technical performance characteristics, therefore, contemporary Stirling engines are invariably not cost competitive from the standpoint of economical mass production.
SUGARY OF THE INVENTION
The invention comprises fundamental concepts and mechanic eel components which are combined to form a new family of Stirling-cycle machines, specifically including the following:
(1) a single-acting, two-piston engine having stationary, coaxial, in-line cylinders and employing a pair of cylindrical' face cams affixed to the pistons to drive a centrally disposed flywheel element about a hollow shaft, herein termed a "dueled axle which also serves as the regenerator housing; (2) an engine power level control subsystem associated with that I

~226443 dueled axle machine by which the instantaneous phase angle between the periodic reciprocating motions of the pistons is readily adjusted as a function of power demand; 13) a quasi double-acting, multiple-piston engine having an annular and parallel array of cylinder and regenerator volumes and employing a single cylindrical drum cam to control the alone-said instantaneous phase angle and to transfer mechanical work into or out of the Messianic; (4) an engine power level control subsystem associated with that drum cam machine (or any other Stirling engine) by which the mean system working pressure is conveniently varied by the hydraulic injection or ejection of condensed working fluid through a reservoir cooler element;
(5) working fluids other than hydrogen, helium, or air, namely certain fluorine compounds exemplified by sulfur hexafluoride, perfluorobutane, perfluoropropane, and octafluorocyclobutane, which provide an increased dynamic heat transfer coefficient yet are nonflammable, nontoxic, and easily liquefied; (6) a regenerator which employs materials having-anisotropic symmetry, such as pyrolytic graphite, to achieve an isotropic thermal conductivity and large spcci~ic heat capacity by a closely packed assemblage of perforated disks disposed within an insulated tubular duct; and (7) the application of dispel-; soon strengthened copper composites in conjunction with a high-expansion, low-conductivity, manganese copper alloy in one class of machines, and the application of advanced silicon carbide structural ceramic in conjunction with boron carbide structural ceramic in another, for the design of heat transfer elements having improved performance and increased cost effectiveness.
The primary significance of the present invention is that it represents a radical departure from traditional mechanic eel arrangements and methods. It thereby achieves a striking reduction in overall complexity and cost, both at the system and at the component level. Indeed, in its simplest and perhaps SUE
most useful form, the dueled axle machine, the invention can be functionally accomplished with as few as five moving parts.
Yet the same device can be scaled to virtually any size for application to products of enormous diversity, and it can be adapted to run on any fuel, whether gas, liquid, solid, or hybrid, or on any other heat source, including solar energy.
The invention constitutes a rare and special come bination of superior technical performance, broad market potential, and economic mass producibility, and therefore port tends a new era of more thermally efficient and cost effective power products. These include noiseless propane powered lawn mowers, thermal battery powered automobiles, Bahamas powered fishing vessels, solar powered irrigation pumps, and nuclear powered navy warships. And since the Stirling-cycle engine is thermodynamically reversible, the invention will find countless other applications in the realm of refrigerators, heat pumps, air conditioners, and the like.
STATEMENT OF THE OBJECTS OF THE INVENTION
It is a primary object of the invention to provide a new and improved family of Stirling-cycle machines which are mechanically uncomplicated and economical to produce on a large scale, and wherein the hot and cold regions of each machine are inherently located at extreme diametrically oppo-5 tie ends.
It is another object ox the invention to provide a single-acting, two-piston engine which possesses stationary, coaxial, in-line cylinders, and which employs a pair of cylindrical face cams affixed to the pistons to drive or be driven by a centrally disposed flywheel element about a hollow shaft or axle, said hollow shaft also being the regenerator duct of the machine, hence the name "dueled axle" machine.
It is a further object of the invention to provide an engine power level control subsystem in conjunction with the aforesaid dueled axle machine by means of which the instant ~22~i4~3 Tunis phase angle between the periodic motions of the expand soon piston and the compression piston is readily adjusted as a direct function of power demand.
It is a still further object of the invention to pro-vise a power and speed controlled dueled axle engine analogous to a synchronous electric motor-generator or converter to be used in a thermally conservative power plant involving regenera-live braking and waste heat reclamation in conjunction with a thermal energy storage device (thermal accumulator) in order to accomplish certain specialized applications, chief among which is that of an automotive prime mover.
It is yet another object of the invention to provide a single-acting, quasi double-acting, multiple-piston engine series, each of which is comprised by an annular and parallel array of cylinders and regenerator volumes, and which employs a single cylindrical drum cam to control the phase relationships between each of the interconnected and periodically varying working volumes of a multiplicity of stages and to transfer mechanical work into or out of such machines, hence the name "drum cam" machines.
It is a still further object of the invention to pro-vise an engine power level control subsystem in conjunction with those drum cam machines (or in conjunction with any other regenerative thermal machines) by means of which the mean operating pressure of the cycle is conveniently and automatic gaily varied as a direct function of power demand by the hydraulic injection or ejection of condensed working fluid through a special reservoir cooler element.
It is a further object of the invention to provide a thermodynamic working fluid other than the usual hydrogen, helium, or air which possesses a critical temperature which is both somewhat above the minimum ambient temperature of the available heat sink yet somewhat below the designated heat rejection temperature of the cycle as maintained within the ~ZZ6443 engine cooler an which possesses in audition a critical pressure substantially byway the mean operating pressure of the cycle, and which is also nonflammable, nontoxic, inexpen-size, inert, a oily dynamic heat transfer agent, and of low viscosity.
It is a further object of the invention to provide a novel form of regenerator, designed to incorporate certain materials such as pyrolytic graphite, which possess anisotro-pie symmetry in addition two the desirable physical properties of low density and high heat capacity, thereby inherently exhibiting a high thermal conductivity in directions normal to the flow of working fluid and a low thermal conductivity in the direction of the flow within the same contiguous mass.
It is yet another object of the-invention to provide a : substantial increase in performance and efficiency through the deliberate and judicious utilization of advanced composite materials such as dispersion strengthened copper, graphite reinforced polymers, and the new generation of structural ceramics exemplified by silicon carbide, silicon nitride, boron carbide, and boron nitride for the construction of high performance Stirling engine heat transfer components.
BRIEF DESCRIPTION OF 'rye DRAWINGS
: Other objects, advantages, and novel features of the invention will become readily apparent upon consideration of the following detailed description when read in conjunction with the acco1npanying drawings wherein .
j FIG. 1 is an illustration of the operational sequence of events during one complete cycle of an idealized single-acting two-piston Stirling engine used in the prime mover mode;
FIG. I and FIG. I are schematics which illustrate the idealized pressure-volume and temperature-entropy diagrams of the thermodynamic cycle of the working fluid in the same machine depicted by FIG. l; FIG. I is a pressure-volume diagram which depicts the working of an actual machine;

AYE
FIG. 3 shows a comparison of the basic features of a piston of the dueled axle machine according to my invention (FIG. I) Whitehall a piston of a representative prior art Stirling-cycle machine (FIG I);
FIG. 4 is a perspective view which illustrates the overall functional configuration of the dueled axle machine;
FIG. 5 is a sectional view of the dueled axle machine;
FIG. 6 is an exploded perspective view of the dueled axle machine;
FIG. 7 is a partial exploded perspective view of the compression piston and cylinder assembly which illustrates one arrangement for achieving rotation of that assembly about the dueled axle axis relative to the expansion piston and cylinder assembly;
FIG. -8 illustrates how said rotation alters the rota-live orientation of the cylindrical face cams on the piston to achieve various output torque levels by adjusting the instant Tunis piston phase angle;
FIG. 9 is a schematic representation of the prior art Ryan double-acting multi~le-piston mechanical arrangement;
FIG. 10 is a schematic illustration of how one compression/expansion/regeneration stage of a single-acting quasi double-acting arrangement according to my invention may be derived from a simple conceptual transformation of such a stage of the inn double-acting arrangement;
FIG. 11 is a schematic representation of a multi-stage, single-acting, quasi double-acting mechanical arrangement according to my invention;
FIG. 12 is a perspective view of the drive assembly and interconnected working volumes of a drum cam machine;
FIG. 13 is an offset sectional view of the drum cam machine of FIG. 12;

FIG. 14 is a partially exploded perspective view which illustrates the overall functional configuration of the drum cam machine of FIGS. 12 anal 13;
FIG. 15 is a schematic representation of means for controlling the power level of a Stirling-cycle machine, for example, the drum cam machine, by adjusting the mean operating pressure;
FIG. 16 is a bar graph comparison of the dynamic heat transfer coefficient calculated for various gaseous working fluids relative to air.;
FIG. 17 is an illustration of the construction of a lo regenerator using an isotropic perforated disks; and FIG. 18 depicts some of the unique elevated temperature mechanical properties of GLIDCOP dispersion strengthened copper composite.
DESCRIPTION OF THE PREFERRED EMBODIMENTS

_ Attention is directed to FIG. l wherein numeral 1 designates an idealized version of a two-piston Stirling-cycle prime mover. A conceptually constant mass of pressurized gaseous working fluid occupies the working volume between the compression piston 2 and the expansion piston 3. The total working volume is comprised by compression space 4, regenera-ion 5, and expansion space 6. A portion of compression space is continually cooled by cooler 7, while a portion of expand soon space 6 is continually heated by heater 8. Arrows 9 are intended to represent the input of heat by conduction, convect lion, or radiation. Escape of fluid from the working volume is prevented my tile piston seals 10.
During the compression stroke (between positions I and II) the working fluid is compressed isothermally by piston 2 at the minimum temperature level of the cycle. Heat is con-tunnel rejected at this temperature through cooler 7; the pressure rises slightly and the total working volume decreases to a minimum. During the forward displacement (cold-side to hot-side transfer) stroke (between positions II and III) regenerator 5 yields stored heat to the working fluid as it is ~Z26443 transferred to expansion space 6 with the volume remaining constant. The temperature and pressure rise to their maximum levels.
During the expansion stroke (between positions III and IV) the working fluid expands isothermally at the maximum temperature level of the cycle, doing work on piston 3. The temperature level is maintained by the input of heater 8; the pressure drops and the total working volume increases to a maximum. During the reverse displacement (hot-side to cold-side transfer) stroke (between positions IV and I) regenerator 5 recovers heat from the working fluid as it is transferred to compression space 4 with the volume remaining constant. The temperature and pressure return to the starting levels of the cycle.
A clearer understanding of the foregoing may be obtained by referring to the diagrams of FIG. aye) and FIG. I wherein the same complete cycle is presented in terms of the pressure-volume diagram and the temperature-entropy diagram for the working fluid. For each process as depicted by the curves between the indicated position numtter, I-II, II-III, III-IV, and IV-I, the area under a curve on the P-v diagram is a representative measure of the mechanical work added to or removed from the system during the process. Similarly, the area under a curve on a T-s diagram is a measure of the heat transferred to or rejected from the working fluid during the process.
Actual machines differ fundamentally from the idealized versions in that the motion of each piston is continuous and smooth, rather than discontinuous and abrupt. This causes the indicated processes of FIG. I and FIG. I to overlap one another, and results in P-v diagrams which are smooth con-tenuous curves devoid of sharp corners as shown by FIG. I.

Thus the piston motion of actual machines is smoothly periodic to the point of being sinusoidal, and the working fluid is likewise distributed in a periodically time-variant manner throughout the total working volume. Thy instantaneous phase angle between the relative motions of the two pistons is a critical parameter in the operation ox real machines and usually has a value on the order of 90 degrees. It it well-known, therefore, that various arrangements to accomplish phase angle adjustment may be used to effect continuous power level control. See e.g., US. Patent 2,465,139 and 3,482,457.
The processes of compression and expansion in real machines are not strictly isothermal, which constitutes another major departure from the ideal. The provision of heater 8 adjacent to expansion space 6 and of cooler 7 adja-cent to compression space 4 effects only a crude approximation to-the isothermal condition. additionally, the presence of these elements tends to increase the unswept or dead volume ratio, which has a critically adverse effect on performance.
Moreover, the working fluid is heated or cooled, not only when flowing in the correct direction between regenerator 5 and either expansion space 6 or compression space 4, hut also when flowing in the respective opposite directions. Nevertheless, extant Stirling engines exhibit comparable power densities and significantly greater efficiencies than conventional internal combustion engines of all types.
One favorable embodiment of the present invention may be characterized as a single-acting, two-piston engine with stationary, coaxial, in-line cylinders and a dueled axle.
Although at first glance it would seem to have a similar arrangement to various classical opposed piston designs, close examination by those familiar with the art will reveal this new dueled axle design to be unique. The essential novelties and fundamental differences of the dueled axle machine, come pared to those of all prior art multiple-piston engine designs, derive from the form, function, and mode of operation of the pistons themselves.

Sue Attention is now directed to the drawings of FIG. 3 wherein FIG. I shows the form of a piston 20 of a dueled axle machine as compared to that of a piston 11 from an oared-nary multiple piston machine. It may be seen that piston 20 has no connecting rod 14 as does piston if; that piston 20 incorporates a special cam surface 26 normal to the axis of reciprocation 13; and that there is an axial bore 22 through the top of piston 20. This last condition permits the pistons of a dueled axle machine to be operated in a coaxial arrange-mint surrounding, and at the opposite ends of, a tubularregenerator/shaft combination.
These and other details of the construction and opera-lion of the dueled axle machine may be discerned by referring to the illustrations contained in FIG. 4, FIG. 5, FIG. 6, and FIG. 7. The regenerator/shaft or dueled axle 40 serves as the structural backbone of the machine and also provides an inter-natty disposed conduit for the regenerator element 42. Dueled axle 40 is coaxial with the machine axis of symmetry I
extends from one end of the machine to the other between cooler 25 and heater 35, and provides the axle about which the centrally disposed flywheel drive element 45 revolves. The rotational motion of the flywheel drive element is guided by a pair of radial bearings 46 and a pair of thrust bearings 48.
- As shown in the sectional view, FIG. 5, a pair of stay shunner coaxial in-line right-circular cylinders comprises the housing of the dueled axle machine. Compression cylinder 16 encloses compression space Andy all other compression elements, is closed and sealed at one end by cooler 25, and is threaded to receive cooler head 29. Expansion cylinder 18 en-closes expansion space 37 and all other expansion elements, is closed and sealed at one end by heater 35, and is 'threaded to receive heater head 39. As exemplified in FIG. 4, heater head 39' may take on a variety of forms to accommodate various come busters, collectors, thermal accumulators, or other sources ISSUE
of heat. Roth cylinders (shown in FOE. 5) are c-quipped with a plurality of radial and annular slots 53 disposed within their interior wall which are designed to mate (as shown in FIG. 7 for the compression piston and cylinder assembly) will- cores-pounding tabs 54 on both the compression disk element 28 and the expansion disk 38.
The disk elements are designed to be affixed to the extreme opposite ends of the dueled axle I and together with it comprise the interior frame or structural support of the machine. Various mechanical fastener means may be used, depending upon whether it is desired to prohibit or permit relative rotation of either disk about dueled axle 40. In the first case a threaded retainer 41 and a splint 43 are provided;
in the second case the retainer 44 is a heavy duty snap ring mated to groove 55. In either case the disk elements serve to constrain the longitudinal placement of both cylinders in relation to dueled axle 40 and to direct the flow of working fluid to and prom the periphery of cooler 25 and heater 35.
It may be seen that compression piston 20 incorporates internally a coccal disposed cylindrical face cam 24 having a cam surface 26 which is oriented in such a way as to face, and to maintain a particular angular position with respect to, the corresponding cam surface 36 of cylindrical face cam 34 similarly fixed within expansion piston 30. The pressure forces of the working fluid hold each cam against low-friction cam follower assemblies 56 mounted upon flywheel drive element 45.
I The reciprocating motion of the pistons within the cylinders I and along the dueled axle is coupled to and converted into or from rotational motion of flywheel drive element 45, and vice versa, by opposed cam surfaces 26 and 36. Angular rotation of the pistons within the bore is prevented without restricting their axial reciprocation by means of longitudinal slots sun which reengaged by piston guide assemblies 52.
Reciprocating seals 21 and 23 prevent the escape of 1 ~226443 working fluid from compulsion space 27, while reciprocating seals 31 and 33 similarly contain the working fluid within expansion space 37. Rotating seals 49 contain a separate quantity of gaseous buffer fluid within buffer space 47, which is partially pressurized to reduce the magnitude of the static loading on the cams. Holes 51 within flywheel drive element 45 conjoin the compression and expansion sections of buffer space 47. During the aforementioned forward and reverse displacement strokes, therefore, working fluid is alternately shifted from compression space 27, over the rounded periphery of compression disk element 28, through the radial flow passe-goes of cooler 25, through regenerator element 42 within dueled axle 40, through the radial flow passages of heater 35, over the rounded periphery of expansion disk element 38, to expand soon space 37, and back again. Working fluid is introduced into the working volume by means of tank valve 17; buffer fluid is introduced into the buffer space by means of tank valve 19~
One revolution of flywheel drive element 45 corresponds to one complete thermodynamic cycle. When the machine is employed as a prime mover, work is done on flywheel drive eye-mint 45 to increase its kinetic energy during the foremen-toned expansion stroke of each cycle; likewise, work is done on the working fluid by flywheel drive element 45 during the compression and displacement phases of each cycle. Net power output may be transferred from flywheel drive element 45 to the indicated output shaft 57 by means of any common mechanic eel transmission such as a V-belt (designated by numeral 58 in FIG. 4), chain, or gear drive assembly. Since the Stirling prime mover is not self-starting, an external starter device (not shown) would normally be an adjunct to the power trays-mission subsystem.
When the machine is to be employed as a refrigerator or a heat pump, power from an external source such as an electric ~226443 motor would be input through shaft 57 and transmitted to flywheel drive element 45 by similar means. It would of course be necessary in this case to alter the configuration of cooler head 29 and heater head 39 to conform to the specific heat transfer requirements of any given practical application. But when driven in this manner, the complete thermodynamic fever-sublet of the dueled axle machine would produce the desired effects within the heat exchange elements at each end of the machine. That is, given the same mechanical configuration and orientation, heat would be extracted from the surrounds of heater 35 within expansion cylinder 18 and would be ejected to the surrounds of cooler 25 within compression cylinder 16, since the flow of heat generally progresses in one direction in a regenerative thermal machine.
A significant consequence of the dueled axle machine arrangement is to permit, in one embodiment of the invention, a uniquely uncomplicated power level control method. It should be recalled at this point that the instantaneous phase angle between the sinusoidal time-variant reciprocations of the pistons in a Stirling engine is a critical performance parameter. It will be readily appreciated by those skilled in the art that the instantaneous phase angle of the dueled axle machine depends only upon the relative angular displacement of compression cam 24 with respect to that of expansion cam 34 for a given rotational direction of flywheel Arrive element 45.
As shown in FIG. 7, the angular alignment of compression cylinder 16 with respect to that of expansion cylinder 18, and therefore the angular position of compression cam 24 with respect to that of expansion cam 34 (since the pistons are slaved in rotation to the cylinders by guide assemblies 52), can be facilitated by design.

hut it is an important teaching of this invention that it is possible to alter the operation of the dueled axle -machine from positive torque, through zero torque, to negative lZ26443 torque by mechanical leans, even white the machine is running under a maximum load condition, by simply rotating one cylinder with respect to the other. This is illustrated schematically in FIG. 8; the desired angular adjustment may be accomplished virtually independent of the load by electric, pneumatic, hydraulic, or any other suitable mechanical means. yin this manner the instantaneous phase angle, and consequently the power level of the machine, may be either manually or automatic gaily controlled with precision by an enormous variety of mechanical, thermal, electronic, or other conventional and well-known feedback methods. With this type of power level control one can thereby optimize engine performance and efficiency for any given speed and torque requirements inherent in the nature of the system application. This is a natural and synergistic consequence of the deceptively simple employment of cylinder-eel face cams in the manner indicated, an essential and distinctive aspect of the seemingly artless character of the dueled axle machine.
' Moreover, yet another important specific teaching of this invention is that speed controlled engines analogous to a synchronous electric motor-generator or converter may be developed on this basis for specialized applications. That is, the engine would act either as a prime mover or as a heat pump depending on whether the engine is driving the load or the load is driving the engine at a selected excitation frequency.
This type of device could have a striking impact on the tech-neology of transportation from the standpoint of total system energy efficiency and conservation. The effective utilization of the aforesaid negative torque mode for regenerative braking and reversible heat reclamation in the largely stop-and-go environment of the automotive prime mover may constitute a technological breakthrough of astounding economic significance.
Another favorable embodiment of the invention may be characterized as a single-acting, quasi double-acting, four-~226443 piston engine having an annular and parallel array of sullenness and regenerators interconnected in series and incorporating a cylindrical drum cam drive element. That is, the machine has four in-line cylinder pairs arranged within and symmetrically about a cylindrical annuls which also contains four regenera-ion ducts exterior from, parallel to, and alternately inter-aspersed among the cylinders. These are all interconnected in series so as to form a folded serpentine arrangement in con-junction with four double-ended pistons within the aforesaid cylindrical annuls. The drive shaft is coaxial with and integral to a right-circular cylindrical drum cam mechanism which is interior to and symmetrical with the said annular array of components.
Although the present machine is strictly a single-acting multiple-piston engine, it retains a superficial resemblance to certain well-known contemporary double-acting Stirling engines, hence the term quasi double-acting. Alien-lion is now directed to the drawings of FIG. 4 wherein numeral 61 designates a schematize version of the modern double-acting Stirling engine of the type due to Ryan and descried by prior art US.

Patent No. 2,579,702 of December 1951. As it appreciated by those familiar with the art this machine is double-acting because each piston 62 simultaneously works directly against the low temperature working fluid in compression space 64 below, and against the high temperature working fluid in expansion space 66 above seal 90. It may be seen that in the Ryan arrange-mint multiple cylinders are interconnected, so that the first swept volume of lower compression space 64 of one cylinder is connected to the last swept volume of the upper expansion space 66 of an adjacent cylinder by means of a flow path comprising the last swept volume of space 64 jacketed by cooler 6?, a no-generator 65~ and the first swept volume of space 66 jacketed by heater 68 in series. The jacketed portion of compression space 64 is continually cooled by cooler 67 and the jacketed ~22~443 ( portion of expansion space ho is continually heater by heater 68; arrows 69 are intended to represent the input ox heat by conduction, convection and radiation.
Each separate working volume so interconnected keenest-lutes a stage wherein both the constant volume displacement functions and the compression and expansion functions of the Stirling cycle are independently accomplished as long as pistons 62 are constrained by the design of the drive mocha-noisome (not shown) to move with a suitable phase shift in their displacement. In the case of four-cylinder Ryan type engines as illustrated the proper phase shift is 90 degrees which is normally accomplished by means of well-known swish plate or crankshaft type drive mechanisms. The Ryan arrangement has the advantage that the number of moving parts associated with a given cylinder is only one per cycle, compared with two per cycle in other prior art designs. It also affords a reason-nobly compact mechanical arrangement when the cylinders are arranged parallel to one another in a cylindrical annuls and the pistons are coordinated by means of a swish plate mocha-noisome internal to the annular volume.
Inherent in the design of the Ryan engine, however, is the requirement for each piston 62 to operate in the aforesaid double-acting mode. This is disadvantageous in that both the piston and the cylinder wall constitute an undesirably short thermal conduction path from the normally hot to the normally cold regions of the machine. Since any heat flow between these regions represents a total thermal loss to the system and lowers the overall thermal efficiency, this circumstance necessitates in prior art machines the utilization of a long insulating piston cap 63 or other device as more fully explained by prior art US. Patent No. 3,496,270 of February 1970 to Nolan and exacerbates an already difficult high pressure reciprocating seal design problem. In addition, the mechanical necessity of extending the piston rod through the wall of compression space 64 along axis ~20-~226443 ( of reciprocatiol7 13 requires the laminate of the drive mechanism, whether of the swish plate or crankshaft type, at the opposite end ox the machine from floaters 68. This in turn relegates the locus of coolers 67 to a position which is undesirably proximate to heaters 68, and which precludes their natural collocation in an uncomplicated manner.
Attention is now directed to the drawings of FIG. 10, FIG. 11, FIG. 12, JIG. 13, and FIG. 14 wherein a preferred embodiment of the drum cam machine of the present invention is illustrated. It may be understood that the drum cam machine represents a fundamental departure from the Renewal arrangement in that the number of cylinders is doubled, and the pistons are incorporated in rigid-body pairs at the opposite ends of double-ended connecting rods in the manner of a dumbbell. As depicted in FIG. 10 the desired configuration can be imagined to derive from a simple conceptual transformation of the Ryan arrangement which preserves the manner in which the cylinders, now become disjoint cylinder pairs, are interconnected, but which replaces the double-acting character of the Ryan approach with a single-acting, quasi double-acting mode. It may be seen that the new arrangement disposes of the undo-sizable aspects of the Ryan arrangement without disturbing the cyclic phase relationship inherent in equivalent working volumes.
Referring now to FIG. 11, it may be recognized that numeral 71 designates a schematized version of the quasi double-acting drum cam machine of the present invention.
Those skilled in the art will appreciate that this machine is single-acting because only one surface of each piston 72 works directly against working fluid pressure, whether at low them-portray in compression space 74 below or at high temperature in expansion space 76 above the seals 90, the other surface being substantially at atmospheric. Yet, as in the Ryan arrangement, multiple cylinders are fluid flow interconnected, '~226443 so that the lower compression space 74 of one cylinder is con-netted to the upper expansion space 76 of an adjacent cylinder by means of a flow path past cooler 77 through regenerator 75, and past heater 78 in series. And, as before, a portion of compression space 74 is continually cooled by cooler 77, while a portion of expansion space 76 is continually heated by heater 78; arrows 79 are intended to represent the input of heat by conduction, convection, ox radiation.
In the arrangement of my invention, however, the then-met conduction path through pistons 72 from each hot expansion - space 76 to each cold compression space 74 is now seen to be considerably lengthened, notwithstanding the complete elimina-lion of the use of an insulative piston cap 63. Neither is there any thermal loss occasioned by conduction through the Snow disjoint cylinder walls. In audition, it becomes possible by this arrangement to connect the drive mechanism in the ; vicinity of the midpoint 80 of each connecting rod 81, the locus of which is now centrally disposed within the machine.
This permits the natural separation and convenient collocation of coolers 77 and heaters 78 in positions which are at extreme diametrically opposite ends of the machine. This serves to minimize thermal losses and thereby improve the overall then-met efficiency while at the same time it permits a sub Stan-trial simplification in the design and construction of the heat exchange elements as compared to the prior art.
Any number of piston-cylinder pairs can be intercom-netted in the manner illustrated for four cylinders in FIG.
11, as long as the proper phase shift is maintained between each set. This can be accomplished, as shown in FIG. 12, by.
taking the drive from sets 82 of paired rotary gudgeon pins located at the mid-point 80 of each double-ended piston con-netting rod 81. Each gudgeon pin assembly 82 issue low Eric-lion cam follower mechanism which is mated in reloaded into-mate contact with the protruding surfaces of cylindrical drum ~ZZ6443 cam 83 and the linear surfaces of longitudinal cams 91.
Cylindrical drum cam 83 mechanically converts uniform angular rotation of drive shaft 85, guided by low friction bearings 92 and 92', into simple harmonic reciprocation of each double-ended piston 72 and vice versa. The proper phase relationship is cinematically determined by the relative angular position of each piston-cylinder pair about the machine axis of sum-metro 15.
As shown in FIG. 13, each gudgeon pin assembly 82 con-sits of pyres drum cam followers 84 and matching opposite-lye directed pairs ox longitudinal cam followers 86 supported by means of precision low friction. needle or roller bearings 87. Spring 88 serves to maintain the requisite reloading force, while balls 89 decouple the rotation of drum cam followers 84 from the rotation of longitudinal cam followers 86. All longitudinal cam followers 86 are constrained to move back and forth within longitudinal cams 91, which are parallel to both machine axis of symmetry 15 and reciprocation axes 13, and which serve to prevent the relative rotation of pistons 72 within the bore. Follower axes of rotation 15' are oriented radially with respect to the machine axis of symmetry 15.
Attention is now directed to FIG. 14 wherein the overall functional configuration of the drum cam machine is illustrated. It should be apparent that all compression spaces 74 are collected within a single stationary right-circular cylindrical "compression block" 94, made of material having ! comparatively to-: thermal conductivity. Likewise all expand t soon spaces 76 are collected within a single stationary right-circular cylindrical "expansion block" 96, also made of material having comparatively low thermal conductivity.
Compression block 94 and expansion block 96 are conjoined by the four regenerator housings US and also by the four longitu-dial cams 91. it the extreme opposite ends of both compression block 94 and expansion block 96, Azores of shallow segmented lZ26443 annular repressions 93 keynote each piston-cylinder working volume with an adjacent regenerator duct it and serve as a housing for the internal heat transfer surfaces of either cooler 77 or heater 78. Working fluid is conveyed into each piston-cylinder working volume by means of tank valves 99 located on the periphery of compression block 94.
Thus it may be seen that the individual heat exchange elements of each ox the aforedescribed separate but intercom-netted working volumes are naturally and conveniently cello-acted within a single component, cooler 77 or heater 78. These now consist of a flanged plate made of material possessing comparatively high thermal conductivity, each having a plural lily of radial flow passages on the exterior face and a plural lily of segmented annular flow passages on the interior face.
Cooler 77 serves upon assembly and in conjunction with cooler head 97 to close and connect compression volumes 74 with adja-cent regenerators 75 and to transfer heat from the internal work-in fluid to an exterior sink. Heater 78 serves upon assembly and in conjunction with heater head 98 to close and connect ox-pension volumes 76 with adjacent regenerators 75 and to transfer heat from an exterior source to the internal working It is therefore an important teaching of this invention that the drum cam machine so constructed thereby-effects the aforesaid inter-connection of stages in a uniquely compact annularly folded son-pontoon head-to-tail arrangement, clearly illustrated by Fig. 12.
The drum cam machine design is an arrangement which involves a minimum number of separate components, and wherein the hot and cold regions of the machine are inherently located at extreme diametrically opposite ends. It should be readily apparent to those skilled in the art that the collocation of cooler elements within a compact cooler head at one end of the drum cam machine, and of heater elements within a similarly compact heater head at the other end of the mushiness the highly desirable effect of reducing heat losses from conduction and radiation to improve toe overrule thermal efficiency of thy machine. But it assay leads to-a substantial simplification in the design and mantlfacture of not only the heat transfer eye-mints but also of other mecl-allica1 components of the machine as well.¦ For example, both compression block 94 and expansion block 96 may now be very conveniently constructed from identi-eel mass-produced precision investment castings. This could be of crucial importance with respect to economical production in a high volume application, by producing an important savings in the cost of materials andJlabor. similar economy of pro-diction might also be realized for some applications through the design and fabrication of identical cooler head assemblies 97 and heater head assemblies 98.
Yet another favorable embodiment of the invention is an engine power level control subsystem to be used in conjunction with the operation of either the drum cam machine or any other regenerative thermal machine, by means of which the mean system operating pressure can be rapidly and automatically varied as a function of power demand. In the case off modern but conventional high performance Stirling-cy~le engine, the operating power level is normally controlled by simultaneously adjusting both the quantity of heat input to the heater head and the mean working pressure of the cycle, since it is these variables which most directly influence the power level. The first of these is accomplished by means of a combustion control subsystem quite similar in function to the familiar accelerator I throttle linkage of the automotive internal combustion engine, I except that the time response is much slower due to the large thermal mass of heat transfer components. -The second, however, currently requires a complex pressure control subsystem consisting of pressurized heavy wall stainless steel hydrogen gas bottles; sophisticated son-voactuated high pressure wow control valves; an array of essential and specially designed check valves, stop valves, ~22~443 bypass valves, relief valve;, assay, anal to like; and a high capacity hydrogen-compatihle compressor as described in prior are patents filed under Subclass 521 of Class 60. (See Pug. Us Puppets TV ~,699,700 of October 1972 to Bennethum, 3,827,241 of August 1974 to Almstrom, or 4,030,297 of June 1977 to Katz). In short, the control sub-systems currently required Jo vary the power level of a Stilling prime mover are both complicated and Cole and they represent a critical stumbling block to the economical use and the widespread acceptance of these engines. This is particularly true today with respect to the highly competitive worldwide automotive market.
One of the primary technical ramifications of the usual working pressure control subsystem in such applications may be described as an inherent accelerator time lag or "hesitation.
The time response of existing compressor-dependent systems is poor, because the compressor must handle a comparatively large volume of gas in order to effect a small change in the mean system pressure. This is fundamentally unavoidable in such a system, as the bulk modulus of a gas is by nature extremely low. It is therefore an accepted practice to accomplish a rapid transition from low power demand to high power demand by maintaining the gas reservoir at a pressure substantially greater than the maximum working pressure of the cycle. The force of this greater pressure is then relied upon to rapidly inject additional gas into the working volume whenever an increase in power output is indicated.
As will be appreciated by those familiar with the art, the transition in the reverse direction from a high power demand condition to a low power demand condition is less easily achieved. Since the time required is typically on the order of 5 to 20 seconds, depending on the engine's speed and the degree of change required, additional control valves are sometimes employed to temporarily short circuit the normal flow path between the cylinders to achieve a more rapid decrease in power output. The basic feasibility of this type -foe-of power level control subsystem has teen demonstrated in the laboratory, but it is an undesirable solution to the general problem because it forces the system design in the wrong direction toward aft even greater complexity and cost.
Attention is now directed to the schematic illustration of FIG, 15 wherein a novel power level control subsystem is depicted which is deliberately intended to operate in a single-component two-phase mode. This system is similar to the prior art in that it operates on the well-known principle that a change in the steady state power level of a Stirling engine is virtually a direct linear function of a change in the mean operating pressure of the gaseous working fluid con-twined therein. But it is radically different from previous systems in that the working fluid is intended to undergo a change in phase whenever it is added to or withdrawn from the working volume. Thus it is yet another important specific teaching of this invention that a rapid transition from a con-diction of low power demand to some other condition of high power demand may best be accomplished by the rapid injection of working fluid into the working volume in the form of a Yin-tally incompressible liquid.
It may be seen that one preferred embodiment of such a power level control subsystem is comprised by a servo actuated variable displacement hydraulic pump 100, a power demand control mechanist or accelerator 102, free piston 104, fluid reservoir 105, reservoir cooler 106, and a plurality of ! Stirling cycle engine coolers 77. When an increased demand ! for power output is indicated by a change in the setting of accelerator 102, pump 100 forces hydraulic fluid 101 into reservoir 105 which in turn forces piston 104, sealed by O-ring 103, to move to the right. This action causes the rapid injection of condensed liquid working fluid 107 Pinto the working volume at points of entry contiguous with coolers 77.

The change in mean operating pressure is immediate, because .226~43 the introduction of nearly incompressible liquid medium in this manner instantly lowers the total working volume available to the gaseous medium. As the injected liquid is subsequently evaporated by heat transferred within engine coolers 77, the available volume gradually returns to its former value, but only as the pressure level is increased by the generation and presence of additional vapor. Thus both the increased mean operating pressure and therefore a higher output power level are sustained.
lo The reverse transition from a condition of high power demand to a condition of low power demand is also rapidly accomplished by a reverse change in the setting of accelerator 102. This occurs by means of the simple venting of a portion of the gaseous working fluid back to reservoir 105 when pump l00 reverses its direction of flow, since reservoir l05 is maintained at a pressure below the minimum working pressure of the system, and at a temperature less than that of-engine coolers 77. Thus it should be apparent to those skilled in the art that two essential prerequisites for the successful implementation of the proposed power control system are the presence of an additional heat exchanger shown in FIX. 15 as reservoir cooler 106 and the utilization of a working fluid which possesses a critical temperature, To, between the normal operating temperature of coolers 77 and that of reservoir cooler 106.
Each engine cooler 77 is maintained at a temperature sufficiently low for good Stirling-cycle thermodynamic efficiency, but at a temperature somewhat above To so that its function from the standpoint of power control is always that of an evaporator. On the other hand, reservoir cooler 106 and therefore reservoir 105 must be maintained at a temperature somewhat below To so that its function is always that of a condenser. Thus the pressure in reservoir l05 is the saturated vapor pressure of the condensed working fluid at that temperature.

122~443 It is perhaps appropriate at this point to emphasize that the liquid phase is thcrc~ore present within the working volume of the machine only in the region of each engine cooler dead volume (excluding most of compression space 74) and only for short periods of time during increasing power output transients. A reduction in the overall system complexity and cost, coupled with a dramatic increase in the system time response, can be expected to result from this approach to the design of a Stirling engine power level control device. From a practical engineering standpoint, the business of effecting and controlling the flow of a high bulk modulus liquid by means of a low capacity positive displacement pump is prefer-able to the business of effecting and controlling the equivalent mass flow in the form of a low bulk modulus gas by means of a high capacity compressor.
Another favorable embodiment of the invention is the utilization of alternative working fluids which provide increased performance, greater safety, and improved reliability.
From an historical standpoint there appear to be only three working fluids of significant interest for Duplication in regenerative thermal machines: air, helium, and hydrogen.
Air was and still is of interest primarily because of its unit vernal availability. But helium and hydrogen are the normal working fluids of choice because their thermophysical proper-ties are such as to permit high rates of heat transfer and flow to occur, with relatively low viscous flow losses, come pared to air.
Under standard conditions of temperature and pressure, for example, relative values for the thermal conductivity, heat capacity, and gaseous viscosity are 6.45/5.47/1.00,14.31/5.16/
1.00, and 0.47/1.06/1.00 for hydrogen, helium, and air respect lively. In terms of engine performance, therefore, hydrogen is better than helium, and is also very much less expensive.
jut hydrogen is dangerously combustible in the presence of air AYE
or oxygen, tends to destructively em~rittle common engine construction materials, and it the most difficult of all eye-mints to confine under pressure.
The preeminence of hydrogen and helium as working media for modern Stirling-cycle machines derives from the need to maximize the rates of heat transfer and minimize flow losses, or drag, in high performance applications. The complexities of fluid flow phenomena which include unsteady heat, mass, and momentum transport are analytically intrac-table for even a simple boundary geometry. Real world enjoy-needing design and development must therefore rely heavily on empirical methods and equations, which in some cases are well developed and reasonably accurate.
It is known, for example, that the rate at which heat is exchanged between a solid wall and a fluid can be described by the equation: Oh A do. Here Q is the heat flow rate, A
is the heat transfer surface area, do is some characteristic temperature difference, and h is a heat transfer coefficient defined empirically for various types of flow. Thus, for a fixed boundary condition and given surface temperatures, maxim mum rates of heat transfer result from high heat transfer coefficients. It is also well known that toe heat transfer coefficient for heat exchange between a fluid and a long tube (length/diameter greater than 10) may be accurately described by the following empirically derived equation:
h = 0.026 Ed V d) ( ) Here k is the fluid thermal conductivity, D is the tube diameter, V is the velocity of flow, d is the fluid density is the absolute viscosity, and Cup is the specific heat capacity.
Therefore, it is yet another important specific teaching of this invention that by means of this relationship, and by choosing a representative flow velocity and tube ~226443 diameter or purr s ox coln~rir,on, it can ye demonstrate that certain gases other Loan hydrogen or helium are superior heat transfer media. Results for some selected fluorine compounds, namely sulfur hexafluoride, perfluoropropane, and octafluorocyclobutane are illustrated by the graph of FIG. 16.
Although the indicated values were derived from data for these gases at room temperature and atmospheric pressure, a similar calculation made for the nominal operating conditions of a Stirling cycle prime mover, i.e., a temperature of 750~C and a pressure of 20 Ma (2900.74 Asia), indicates that the heat transfer coefficient for sulfur hexafluoride is nearly twice (1.8) that for hydrogen under the same conditions.
The three alternative working fluids suggested by FIG.
16 are nontoxic, nonflammable, and easily liquefied under pressure at room temperature, which leads to improved safety and ease of handling. They are also chemically and thermally stable, and generally possess a much higher molecular weight compared to hydrogen or helium. According to Graham's Law the rate at which gases tend to' diffuse through very small open-ins is inversely proportional to the square root of the density. Thus these high molecular weight gases present a far less difficult reciprocating seal design problem compared to either hydrogen or helium, and a far greater quantity of makeup fluid can be stored in a given volume as a liquefied gas than as a pressurized gas.
Another reason often given for choosing hydrogen or helium over other gases as a working fluid is that their low viscosity automatically results in the lowest possible viscous flow losses. But the empirically derived dimensionless pane-meter known as the Reynolds Number, which is well known to those skilled in the science of fluid mechanics to equal Density x Speed x Size / Viscosity, is a measure of the ratio of inertial forces in a flow to viscous forces in the flow.
This means that at tow Reynolds Number flows, viscous effects ~22644~ !

in the slow dominate the inertial effects in the flow and vice versa. But it is Allah well known that most common examples of high performance machines, including Stirling-cycle engines, operate generally in a high Reynolds Number regime where inert trial effects are known to dominate viscous effects.
And although the fluid viscosity surely is not irrele-vent in high Reynolds Number flows, an approximate experimental law for drag in such flows is known to be: Drag = (Spud x Density x (Suzuki. This means that the choice of a working medium which possesses a higher heat transfer coefficient will, for a given heat flow fate or power level, require a lower fluid velocity (Speed) anywhere a smaller wetted heat transfer surface area (Size), and will therefore result in a lower fluid drag notwithstanding the value of the fluid viscosity, and to a lesser extent the density of the medium.
It is for these reasons that the invention proposes the utilization of the indicated compounds, and others, as alter-native working fluid media in the Stirling-cycle engine. The essence of this concept is the novel recognition that such working fluids should be selected primarily with regard to whether or not they exhibit a high dynamic heat transfer coefficient, outstanding chemical and physical inertness, and the requisite critical properties to facilitate liquefaction under normal operating conditions. In addition are the tenor-mows advantages of simplicity and safety inherent in the storage and handling of a nontoxic, nonflammable, inert liquid at low pressures, when compared with the storage and handling of dangerously flammable and chemically active hydrogen at high pressures. Thus, it is apparent that this teaching of my invention is away from the teaching of the prior art and was not at all obvious at that time.

Yet another novel concept of the-invention is a con-struction of the all-important regenerator, the most critical component of all regenerative thermal machines. As explained ~2Z6443 in prior art US. Patent No. 3,960,204 of June 1976 Jo Horn, it is important to minimize longitudinal thermal conductivity of all regenerators. My concept proposes the utilization of the unique physical prop-arty known as bulk an isotropy, which is displayed by certain well-known materials such as pyrolytic graphite, for the construction of an advanced reyel-erator in the manner illustrated by FIG. 17. It may be seen that regenerator 115 is nothing more than an ordered or stacked assemblage of per-forayed disk elements 120 contained within a tubular duct 125 which possesses a comparatively low thermal conductivity The perforations 130, which may take many different forms, are designed so as to maximize the ratio of the perimeter of the perforation to toe cross sectional area of toe perforation.
The basic purpose of this approach is to maximize both the capacity and the rate of heat transfer with respect to the material of the regenerator, while at the same time to mini-mite working fluid flow losses and longitudinal thermal con-ductility losses within the regenerator.
Pyrolytic graphite is a polycrystalline form of carbon having a high degree of molecular orientation. It possesses no binder, has a very high purity, and may exceed 98.5~. of the theoretical density for carbon. Tile material is usually pro-duped by chemical vapor deposition onto a substrate which is maintained at an elevated temperature. Such deposits possess great high temperature strength, exceptional thermophysical properties and phenomenal an isotropic symmetry. That is, they naturally and consistently exhibit one value for physical constants as measured in the plane of the deposit and compared to the value for the same constant as measured across the plane of the deposit.
It is a most remarkable, but nevertheless well-known fact that the thermal conductivity of pyrolytic graphite in the plane of the deposit is about equal to that of copper at room temperature (4.2 watts/cm2/C/cm); but the conductivity ~ZZ6443 I-across the plane of the deposit is reduced by almost 200 to 1 ~0.025 watts/cm2/(/cm). The corresponding values at 1000C
are known to be similarly anomalous (1.25 watts/cm2/C/cm and 0.012 watts/cm2/C/cm) and the value of the specific heat at 750C (1382F) is known to be approximately 0.42 cal/g/C, which is among the highest values for all structural enjoy-needing materials.
It is therefore another important specific teaching- of this invention that a number of perforated disks 120 may be made of this or a similar material to have a comparatively large biaxial thermal conductivity (i.e., in the plane of the disk), yet to have a comparatively small axial thermal conduct tivity (i.e., across the thickness of the disk). The India acted assemblage of said perforated disks 120 would therefore comprise, when placed within the insulative cylindrical con-trainer l25, a remarkably efficient regenerator. It should be apparent that such a device would quickly and effectively transfer and store large amounts of heat to and from a fluid flowing within the internal duct formed by the superimposed perforations 130 due to the favorable thermal properties in the biaxial (or radial) direction, but would maintain a high temperature gradient in the direction of flow because of the low value of thermal conductivity in that direction.
Pyrolytic graphite also has a great difference in linear thermal expansion coefficients between the directions within the plane of the deposit and the direction per pen-declare to the plane of the deposit. The average coefficient of linear thermal expansion from room temperature to 1000C is known to be 1.3 x 10-6 cm/cm/C in the plane of deposit and 22.0 x 10-6 cm/cm/C across the plane of deposit. The latter value should be matched by the wall of the containing vessel, in order to preclude or minimize thermal stresses; fortunately, it is reasonably close to that of many structural alloys of interest, including certain alloys of aluminum, manganese, and copper.

3lZz6443 This leads to other advantageous concepts for use in the various embodiments of my invention, namely, specific applications of certain recent advances in the field of materials technoloc1y to improved Stirling-cycle machine disagree.
A classic problem associated with the design of any thermal machine or heat engine, which must by definition accommodate interior regions at different temperature levels, arises from the characteristic behavior of materials at elevated temperatures. 'rho ramifications of this problem are perhaps most often encountered in the form of those two serious and inevitable physical effects: heat rupture and differential expansion.
It is well known that the strength, hardness, creep resistance, and other mechanical properties of all engineering materials, with the exception of graphite and other forms of carbon, diminish with increasing temperature. Thus ail material structures possess a maximum use temperature. It is also well known that a fixed joint between components which have different linear thermal expansion coefficients generally produces undesirable and potentially destructive thermal stresses in a structure undergoing a large temperature change.
Therefore the intelligent design of thermal machines demands the selection of those materials which possess the requisite use temperature, but which also exhibit closely matched then-met expansion properties with respect to other materials placed in adjoining proximity.
In this regard, the materials chosen for the design of the heat transfer components and of the heater head components in a Stirling prime mover present the greatest challenge.
These should ideally possess either high or low thermal con-ductility and high strength at a nominal use temperature of at least 750C (1382F) as well as a closely matched thermal expansion coefficient compared to that of any adjacent come potent or components. Pure copper has the most desirable thermal conductivity ox any ox the common engineering materials, but its notorious loss of strength and creep resistance at high temperatures precludes its use in such applications. Certain copper alloys have improved high temperature mechanical prop-reties, beryllium copper for example, but their corresponding thermal properties are typically no better than those of high temperature steels, which are stronger and often less expensive.
It is an important specific teaching of this invention, therefore, to use a new materials technology develop-mint of the type exemplified by a product of the Gladden Metals Division of SCM Corporation known as GLIDCOP. GLIDCOP is a dispersion strengthened copper composite material offering both high temperature strength and high thermal conductivity.
It consists of a high purity copper with submicroscopic par-tides of insoluble aluminum oxide finely distributed throughout the copper matrix. Dispersion strengthening offers one of the most promising methods of improving the elevated ; temperature properties of copper without seriously degrading its thermal conductivity.
The strengthening mechanism in GLIDCOP is a finely disk pursed phase that acts as a beefier to dislocation movement in the composite material. In GLIDCOP and other materials of similar nature, but different origin, the dispersed phase remains insoluble in the copper matrix, and hence no overawing in the usual sense can occur at elevated temperatures as it does in heat treatable alloys. The dispersed phase particles interfere with dislocation movement, raise the recrystallize-lion temperature, and exert a powerful effect on elevated them-portray strength and hardness. The graphs of FIG. 18 illustrate some of the unique elevated temperature mechanical properties of GLIDCOP. The terms ~L-20 and AL-35 refer to materials having .20 and .35 weight percent aluminum present as oxide, while the term CA-182 refers to a standard and well-known high temperature copper alloy.

~2~6~43 f It is appropriate at this print to reemphasize that the material for the in~ulative components of the heater head and the expansion block of a Stirling engine should have, in con-junction with the adjacent heater, a closely matched thermal expansion coefficient and the lowest possible thermal conduct tivity. It is, therefore, yet another important specific teaching of this invention that the use of eutectic or near-eutectic manganese-copper alloys can satisfy both of these requirements and provide a high degree of vibration damping capacity as well. That is, referring back to FIG. 14 for example, it is proposed that heater 78 should be made of GLIDCOP, whereas both expansion block 96 and heater head 98 should be made of manganese-copper eutectic alloy to achieve maximum utility with minimum thermal stress or strain.
Since the Stirling-cycle engine, according to the Cannot principle and the well-known Laws of Thermodynamics, achieves maximum efficiency by virtue of a large difference in temperature between the expansion volume and the compression volume, there is a strong incentive to raise the normal operating temperatures of the heater head and expansion block components in prime movers beyond the normal limits of oared-nary materials. Recent advances in the research and develop-mint of high temperature structural ceramics promise to greatly extend the performance limitations of current-Stirling-cycle rime movers. It is well known, for example, that hot-pressed and reaction-bonded silicon carbide, silicon nitride, and the oxygen-substituted silicon nitride compounds called SALONS
retain high-strength at temperatures as high as 1400C (2552DF).
Advanced structural ceramics are also attractive choices because of their low density, high strength-to-weight ratio, low cost compared to the superalloy, and excellent hot gas corrosion resistance. But the promise of these materials will be ultimately realized only for conceptual designs which retain sufficient component level simplicity to allow economical mass ~ZZ6443 production - an nbsolutcly essential prerequisite or success in the market. the simplicity inherent in the various embody-mints of this invention from the standpoint of the quantity, shape, and arrangement of components may permit, for the first time in history, the mass production and competitive introduce lion of a ceramic-enhanced Stirling-cycle engine in the varied and numerous world markets for high performance prime movers presently dominated by the less efficient and highly pollutions internal combustion engine.
In this regard, it is yet another important specific teaching of this invention that an ideal combination of both mechanical and thermal properties is to be found in the use of silicon carbide, Sick for the heat conducting components in conjunction will- boron carbide, B4C, for the heat insulating components of an advanced ceramic-enhanced Stirling-cycle prime mover. The coefficient of linear thermal expansion (from 0 - 1000C) for these materials is very closely matched (4.5 x 10-6 cm/cm/C), while the ratio of their thermal conductivities is nearly 80 to 1. moron carbide would also be an excellent choice for piston and cylinder construction because of its low density and extreme hardness; it it well known to resist Abram size wear better than any other readily available engineering material.
OPERATION AND SCOPE OF THE INVENTION
Since the closed cycle Stirling prime mover operates solely on the basis of the difference in temperature in the working fluid between the hot expansion space and the cold compression space, the development of useful power output is not specific to the source of heat available for use. There-fore the design of the heat source can be any one of a large variety of possible types. A rather simple combustion system can be produced, for example, which will cleanly and efficiently burn various kinds of both liquid fuels and gaseous fuels with-out any modification whatsoever r Thus it will be appreciated AYE
by those familiar with the rut that a single prime mover may be made to operate on regular or premium gasoline, diesel oil, alcohol, crude oil, lubricating oil, olive oil, salad oil, propane, butane, and natural gas.
It should also be appreciated that through the inter-muddier of a suitable heat transport system, a heat pipe exchange unit for example, virtually any heat source at a surf-ficiently high temperature can be adapted, including Rhodes-lopes, nuclear reactors, solar collectors, thermal storage devices, and the burning of coal, wood, or even municipal solid waste. The heat pipe is a well-known device for passive heat transfer in which a fluid within a sealed envelope vaporizes when heated and condenses when cooled, transferring heat by vapor transport before being returned to the heat source as liquid again, generally by capillary action. The historical development, theory of operation, and details of GQnst~ucti~n of the heat pipe are amply set forth in prior art US. Patents Nos.: 235034~ of June 1964 to Gauger and No. 3,~29,759 of January 1966 Jo Grover.

The heat pipe is an amazingly simple device with no moving parts and it can transfer large quantities of heat between small temperature differences. Its effective thermal conductivity is hundreds of times better than that of any solid conductor, including copper, for toe same volume. It is yet another important specific teaching of this invention, therefore, that the use of heat pipes in the design of both the heater exchange elements and the cooler exchange elements, which is now greatly facilitated by the aforementioned swooper-lion and collocation of these elements unique to the present approach, is indicated for very high performance Stirling-cycle machines. Referring again to FIG. 6 and FIG. 14, for example, heaters 35 and 78 and coolers 25 and 77 could be substantially hollow instead of solid structures containing both working fluid and wick common to the heat pipe for improved heat transfer.

It is important at this point to reemphasize the fact that each small segment of a well-designed regenerator trays-lens heat to an from the working fluid with minimal tempera-lure differences. Thus all stages in the regenerator are reversible in an actual thermodynamic sense. Therefore, the entire machine cycle is reversible in function; that is, the direction of flow of heat and work can be reversed. The Stirling engine is truly unique in that it is the only pray-tidal example of a thermodynamically reversible machine.
It should be thoroughly understood, therefore, that many of the design concepts disclosed herein or Stirling prime movers are also applicable to the design and development of Stirling refrigerators, heat pumps, air conditioners, and the like. It is another important specific teaching of this invention that machines of this kind would be appreciably more efficient than conventional vapor cycle reciprocating refrig-orators or thermally-activated absorption refrigerators, with a substantial savings in size and weight. In addition, a hybrid device obtained from the combination of a Stirling prime mover mechanically coupled to a Stirling heat pump will permit both multifoil and non fuel powered refrigeration units to be developed and applied to specialized applications.
In view of the foregoing it should be readily apparent to those skilled in the art that the operation of the present invention may be accomplished by means of and in the context of an enormous variety of diverse applications. In fact, Yin-tally every market in the world which is currently occupied by the application of a reciprocating internal combustion prime mover, or by the application of a conventional vapor cycle, absorption, or other type of refrigeration device, is subject to improvement by virtue of the diligent application of the teachings of this invention.

These include but are by no means limited to the following: automotive prime movers, marine prime movers,

3.,~Z~443 aeronautical praline movers, astronautical prime movers, industrial prime movers, military prime movers, agricultural prime movers, multifoil prime movers, non fuel prime movers, portable prime movers, biomedical prime movers, refrigerators, air conditioners, cryogenic cooling engines, residential heat pumps, industrial heat pumps, military heat pumps, water coolers, air compressors, other gas compressors, remote electric generators, portable electric generators, stationary electric generators, hydroelectric power converters, nuclear power converters, radioisotope power converters, solar power converters, geothermal power converters, ocean thermal power converters, Bahamas power converters, solid waste power con-venters, small cogeneration power plants, large cogeneration power plants, remote fluid pumps, portable fluid pups, stay shunner fluid pumps, remote power tools, portable power tools, outdoor power tools, underwater power tools, toys and novelties.
Obviously many modifications and variations of the present invention may occur to those skilled in the art in the light of the above teachings. Indeed, every possible applique-lion of the reciprocating single-acting multiple-piston Stirling-cycle machines set forth herein is, in and of itself, a unique and special variation of this invention. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A reciprocating, single-acting, multiple-piston, Stirling-cycle thermal machine which includes a frame; at least one pair of stationary, coaxial, in-line, right circular cylinders mounted in said frame, said cylinders being thermally isolated from each other, open at their adjacent ends and closed at their remote ends, one of said cylinders enclosing an expansion space of the machine and the other enclosing a compression space of the machine;
an external heat source; an external heat sink; regenerators for each cylinder pair; a heater for each expansion space comprising a heat exchanger element closing the remote end of the expansion space and serving to thermally conductively connect said source to the machine working fluid; a cooler for each compression space comprising a heat exchanger element closing the remote end of the compression space and serving to thermally conductively connect said sink to the machine working fluid; a piston arranged to reciprocate within each of said cylinders, locked against rotation relative thereto and sealing the open end thereof, the pistons of each coaxial pair of cylinders being mechanically linked; cam and cam follower means operatively connecting said pistons to a rotary drive mechanism turning about an axis coaxial with the principal machine axis of symmetry, the symmetry plane of said drive mechanism normal to its axis of rotation being coincident with the plane of symmetry of the reciprocating parts of the machine; the machine working fluid being contained in one or more isolated stages comprising an expansion space, a heater, a regenerator, a cooler, and a compression space connected in series by passages permitting oscillating flow between said spaces.

2. A machine according to claim 1 having only two cylinders and pistons in which the frame includes a ducted axle coaxial with the axis of reciprocation carrying the wall structure of one of said cylinders attached to each of its outer ends, held against longitudinal displacement along said axle by a retainer member permitting working fluid to flow around it, the piston in each cylinder having a central bore in its outwardly-facing top through which said axle extends in sliding, gas-tight relation, the adjacent interiors of each piston carrying a harmonic displacement cylindrical face cam, said cam surfaces being oriented normal to the axis of reciprocation and occupying a specified relative angular position about the machine axis of symmetry within plus or minus 90° one with respect to the other, the hollow center of said ducted axle forming the passage connecting the heater and cooler and containing the regenerator.

3. A machine according to claim 2 in which one of the cylinder wall structures is fixed against rotation relative to the frame and the other is held by retainer means which permit such rotation.

4. A machine according to claim 3 including means for effecting rotation of one cylinder structure relative to the other whereby the instantaneous phase angle between the periodic reciprocating motions of the pistons can be adjusted.

5. A machine according to claim 4 connected to a vehicular load wherein the external heat source includes thermal energy storage means and the range of phase angle adjustment includes a negative torque mode.

6. A machine according to claim 1 having at least four pairs of cylinders in which the frame includes an expansion block holding all expansion spaces in an annular array, and a compression block holding all compression spaces in a like annular array, with the axes of reciprocation of each pair of cylinders parallel to and symmetrically disposed about the principal machine axis of symmetry, the pistons in each pair of cylinders being joined by a rigid connecting rod carrying at its midpoint the cam follower means, said cam follower engaging a right circular cylindrical drum cam holding the input-output shaft, said cam and shaft being guided by journal bearing means in the frame for rotation about an axis coaxial with the machine axis of symmetry, the machine working fluid being contained in a number of stages corresponding to the number of cylinder pairs, with the expansion space of each stage being in one cylinder pair and the compression space of that stage being in another cylinder pair.

7. A machine according to claim 6 having four pairs of cylinders in which the regenerators are contained in tubular passages parallel to, annularly arrayed among, and coterminous with the cylinders, each extending into segmented depressions within the expansion and compression blocks, each segmented depression serving to connect one cylinder and one regenerator tube end of a stage and to collocate and accommodate heaters within the expansion block and coolers within the compression block, respectively.