Abstract:
A heater head assembly for a multi-cylinder heat engine the stirling engine, such as a multi-cylinder Stirling engine, having a plurality of regenerators and cylinders. Each regenerator has a regenerator manifold and each cylinder has a cylinder manifold. First identical cast heater tubes connect the regenerator manifold to first heater tube openings in a heater head manifold. Second identical cast heater tubes connect second heater tube openings in the heater head manifold to the cylinder manifold. The first and second heater tubes are parallel with respect to each other and form a pair of partial concentric staggered arrays. The heater tubes are rotationally asymmetric, having fin sections with less surface area upstream than downstream and thicknesses which decrease radially away from the central passageways of said heater tubes.

Description:
BACKGROUND AND SUMMARY 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. 
     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 prime mover or engine power units for hybrid electric applications. Such applications place extreme demands on Stirling engine design. Due to the wide acceptance of spark ignition and Diesel engines, to gain acceptance, a Stirling engine must show significant advantages over those types, such as a dramatic enhancement in fuel efficiency or other advantages. In addition, reliability and the ability to manufacture such an engine at a low cost are of paramount importance in automotive applications. Similar demands are present in other fields of potential use of a Stirling engine such as stationary auxiliary power units, marine applications, solar energy conversion, etc. 
     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 concentrated solar energy, mechanical energy can be delivered by the engine. This energy can be used to generate electricity or be directly mechanically coupled to a load. In the case of a motor vehicle application, a Stirling engine could be used to directly drive traction wheels of the vehicle through a mechanical transmission. Another application in the automotive environmental is for use with a so-called “hybrid” vehicle in which the engine drives an alternator for generating electricity which charges storage batteries. The batteries drive the vehicle through electric motors coupled to the traction wheels. Perhaps other technologies for energy storage could be coupled to a Stirling engine in a hybrid vehicle such as flywheel or thermal storage systems, etc. 
     The Assignee of the present application, Stirling Thermal Motors, Inc. has made significant advances in the technology of Stirling machines for a number of years. Examples of such innovations include the development of a compact and efficient basic Stirling machine configuration employing a parallel cluster of double acting cylinders which are coupled mechanically through a rotating swashplate. In many applications, a swashplate actuator is implemented to enable the swashplate angle and therefore the piston stroke to be changed in accordance with operating requirements. 
     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. For such applications, for example in motor vehicles, great demands are placed on reliability and cost. It is well known that motor vehicle manufacturers around the world have made great strides in improving the reliability of their products. The importance of a vehicle engine continuing to operate reliably cannot be overstated. If a Stirling engine is to be seriously considered for motor vehicle applications, it must be cost competitive with other power plant technologies. This is a significant consideration given the mature technology of the spark ignition and Diesel internal combustion engines now predominately found in motor vehicles today. 
     During the past several decades, significant improvements in exhaust pollution and fuel economy have been made for spark ignition and Diesel engines. However, there are fundamental limits to the improvements achievable for these types of internal combustion engines. Due to the high temperature intermittent combustion process which takes place in internal combustion engines, pollutants are a significant problem. Particularly significant are NO x  and CO emissions. Although catalytic converters, engine control, and exhaust treatment technologies significantly improve the quality of emissions, there remains room for improvement. Fuel efficiency is another area of concern for the future of motor vehicles which will require that alternative technologies be studied seriously. It is expected that the ultimate thermal efficiency achievable with the spark ignition internal combustion engines is on the order of 20%, with Diesel engines marginally exceeding this value. However, in the case of Stirling engines, particularly if advanced ceramic or other high temperature materials are implemented, thermal efficiencies in the neighborhood of 40% to 50% appear achievable. The external combustion process which could be implemented in an automotive Stirling engine would provide a steady state combustion process which allows precise control and clean combustion. Such a combustion system allows undesirable pollutants to be reduced. 
     In view of the foregoing, there is a need to provide a Stirling cycle engine having design features enabling it to be a viable candidate for incorporation into large scale mass production such as for automobiles and for other applications. The present invention relates to features for a Stirling engine which achieves these objects and goals. 
     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 planar 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 rod to atmosphere. Sliding contact rod seals provide adequate sealing for many applications. For example, in an automotive engine such an approach might be used. The sliding contact seal would, however, inevitably allow some leakage of working fluid, if only on a molecular level. In solar energy conversion or other applications where the engine must operate over an extremely long life, other types of sealing technology may be necessary to provide a hermetic, i.e., non-leaking, seal. In the engine of this invention, if other rod sealing approaches are required, it would be a simple matter to insert a plate between the drive case and cylinder block which supports a bellows or other type of hermetic sealing element. Thus the same basic engine componentry could be implemented for various applications. 
     The Stirling engine of the present invention further includes a number of features which enable it to be manufactured efficiently in terms of component costs, processing, and parts assembly. The drive case and cylinder block feature a number of bores and passageways which can be machined at 90° from their major mounting face surfaces, thus simplifying machining processes. Designs which require castings to be machined at multiple compound angles and with intersecting passageways place more demands on production machinery, tools, and operators, and therefore negatively impact cost. 
     The Stirling engine according to this invention provides a number of features intended to enhance its ease of assembly. An example of such a feature is the use of a flat top retaining plate which mounts the cylinder extensions and regenerator housings of the engine in place on the cylinder block. The use of such flat surfaces and a single piece retaining plate simplifies machining and assembly. The retaining plate design further lowers cost by allowing a reduction in the high temperature alloy content of the engine. Furthermore, the one-piece retaining plate provides superior component retention as compared with separate retainers for each cylinder extension and regenerator housing. 
     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. For example, in many of the Assignee&#39;s prior designs, the entire drive case was subject to such pressures. For such designs, the entire housing might be considered a “pressure vessel” by certifying organizations and others critically evaluating the engine from the perspective of safety concerns. Thus, the burst strength of the housing may need to be dramatically increased. This consideration would greatly increase the cost, weight, and size of the machine. 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. 
     As a means of enhancing the degree of control of operation of the Stirling engine of this invention, a variable piston stroke feature is provided. In order to achieve this, some means of adjusting the swashplate angle is required. In many past designs, hydraulic actuators were used. These devices, however, consume significant amounts of energy since they are always activated and tend to be costly to build and operate. This invention encompasses two versions of electric swashplate actuators. A first version features a rotating motor which couples to the swashplate drive through a planetary gear set. A second embodiment incorporates a stationary mounted motor which drives the actuator through a worm gear coupled to a pair of planetary gear sets. In both cases, a high gear reduction is achieved, which through friction in the mechanically coupled element, prevents the actuator from being back-driven and thus a swashplate angle can be maintained at a set position without continuously energizing the drive motor. Power is applied to the drive motor only when there is a need to change the swashplate angle and hence piston stroke. 
     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 cross heads include sliders which engage both sides of the swashplate. The clearance between the sliders and the swashplate surfaces is very critical in order to develop the appropriate hydro-dynamic lubricant film at their interfaces. An innovative approach to providing a means of adjusting the cross head bearing clearances is provided in accordance with the present invention. 
     This invention further encompasses features of the piston assemblies which include a sealing approach which implements easily machined elements which provide piston sealing. A pair of sealing rings are used and they are subjected to fluid forces such that only one of the sealing rings is effective in a particular direction of reciprocation of the piston. This approach reduces friction, provides long ring life and enhances sealing performance. 
     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 excellent thermal efficiency. The surfaces of the preheater exposed to combustion gases can be coated with a catalyst material such as platinum, palladium or other elements or compounds which enable the combustion process to be further completed, thus generating additional thermal energy. The catalyst further reduces exhaust emissions as they do for today&#39;s internal combustion engines. 
     The Stirling engine of this invention incorporates a heater head assembly with a number of tubes which are exposed to combustion gases enabling the heat of combustion to be transferred to the working gas within the engine. The typical approach toward constructing such a heater head assembly is to painstakingly bend tubing to the proper configuration with each tube having a unique shape. Such an approach is ill-suited for volume production. The requirement of using bent tubing also places significant limitations on heater head performance. Material selections are limited since it must have adequate ductility to enable tube stock formed in straight runs or coils to be bent to the proper shape. Such tubing also has a uniform wall thickness and cannot readily be incorporated with external fins to enhance heat transfer area without welding or brazing additional parts to the outside of the tube. These steps add to cost and complexity. Moreover, when brazing materials are used, temperature limits are placed on the heater tubes to avoid failure of these joints. This temperature limitation also reduces thermal efficiency which tends to increase with combustion temperature. In accordance with this invention, cast heater tubes are provided which can be made in multiples of the same configuration connected together through a heater head. The cast material allows the heater tubes to be subjected to much higher temperatures. In addition, special configurations can be provided to enhance performance. For example, fins of various cross-sectional shape can be provided. Also, the fins need not have a rotationally symmetric configuration, but instead can be designed to consider the fluid mechanics of the fluids moving across them. Through appropriate fin design, it is believed possible to cause the entire perimeter of the heater tubes to be a near uniform temperature despite the fact that fluids are flowing transversely across them. Temperature gradients associated with prior heater tube designs place significant thermal stresses on the tubes, which over time, lead to mechanical fatigue failure. 
     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 by connecting together the four volumes through capillary tubes. 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. 
     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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal cross-sectional view through a Stirling engine in accordance with this invention; 
     FIG. 1A is a longitudinal cross-sectional view of the heater assembly of the engine according to this invention; 
     FIG. 1B is a partial cross-sectional view of a bellows rod seal incorporated into a modified form of this invention showing the bellows in an extended condition; 
     FIG. 1C is a view similar to FIG. 1B but showing the bellows compressed; 
     FIG. 2 is an end view of the drive case assembly taken from the output shaft end of the drive case, particularly showing the cross head components; 
     FIG. 3 is an enlarged cross-sectional view taken from FIG. 1 showing in greater detail the cross head assembly of the engine of this invention; 
     FIG. 4 is a partial cross-sectional view showing an electric swashplate actuator in accordance with a first embodiment of this invention; 
     FIG. 5 is a longitudinal cross-sectional view through a Stirling engine according to this invention showing an alternate embodiment of a electric swashplate actuator in accordance with this invention; 
     FIG. 6 is a top view of the cross head body showing the guide rods in section; 
     FIG. 7 is a view partially in elevation and partially in section of the cross head body shown in FIG. 6; 
     FIG. 8 is a top view of the cross head adjuster sleeve; 
     FIG. 9 is a cross-sectional view taken along line  9 — 9  of FIG. 8; 
     FIG. 10 is an end view of the cylinder block component taken from the end of the drive case assembly; 
     FIG. 11 is a longitudinal cross-sectional view through the piston assembly; 
     FIG. 12 is an enlarged partial cross-sectional view particularly showing the piston ring assembly of this invention; 
     FIG. 13 is a top view of the cooler assembly; 
     FIG. 14 is a side view partially in section of the cooler assembly; 
     FIG. 15 is a plan view of retainer plate of this invention; 
     FIG. 16 is a plan view of a cylinder extension locking C-ring; 
     FIG. 17 is a cross sectional view taken along line  17 — 17  from FIG. 16; 
     FIG. 18 is a plan view of a manifold component of the heater head assembly of this invention; 
     FIG. 19 is a cross-sectional view taken along line  19 — 19  of FIG. 18; 
     FIG. 20 is a longitudinal cross-sectional view of a heater tube from the heater head assembly; 
     FIG. 21 is an enlarged partial cross-sectional view showing particularly the fin configuration of the heater tube; 
     FIG. 22 is a plan view of one of the fins of the heater tube shown in FIG. 20; 
     FIG. 23 is a plan view of an alternate configuration of a fin shape for a heater tube according to this invention; 
     FIG. 24 is a cross-sectional view through the unloader valve; 
     FIG. 25 is a top view of the air preheater; 
     FIG. 26 shows a sheet of metal material from which the air preheater is formed; 
     FIG. 27 is a side view of the air preheater shown in FIG. 25; 
     FIG. 28 is an enlarged side view particularly showing the alternately welded configuration of the adjacent leaves of the preheater. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A Stirling engine in accordance with this invention is shown in a completely assembled condition in FIG.  1  and is generally designated by reference number  10 . Stirling engine  10  includes a number of primary components and assemblies including drive case assembly  12 , cylinder block assembly  14 , and heater head assembly  16 . 
     Overall Construction 
     Drive case assembly  12  includes a housing  18  having a pair of flat opposed mating surfaces  20  and  22  at opposite ends. Mating surface  20  is adapted to receive drive shaft housing  28  which is bolted to the drive case housing  18  using threaded fasteners  29 . Mating surface  22  is adapted to be mounted to cylinder block assembly  14 . Drive case housing  18  has a hollow interior and includes a journal  24  for mounting a drive shaft bearing. Arranged around the interior perimeter of drive case housing  18  is a series of cross head guide rods  26 . A pair of adjacent guide rods  26  is provided for each of the four cross heads of the engine (which are described below). As will be evident from a further description of Stirling engine  10 , it is essential that adjacent guide rods  26  be parallel within extremely close tolerances. 
     One end of each guide rod  26  is mounted within bores  30  of drive case housing  18 . The opposite ends of guide rods  26  are received in bores  32  of drive shaft housing  28 . The mounting arrangement for guide rods  26  is shown in FIGS. 1 and 3. One end of each guide rod  26  has a conical configuration bore  36  which terminates at a blind threaded bore. In addition, a series of slits are placed diametrically through the end of guide rods  26  at bore  36  so that guide rod end has limited hoop strength. Cone  34  is inserted within conical bore  36 . A threaded fastener such as cap screw  38  is threaded into the threaded bore at the end of guide rod  26 . By torquing threaded fastener  38 , cone  34  is driven into bore  36  causing the end of guide rod  26  to expand into mechanical engagement with bore  32 . This is achieved without altering the concentricity between the longitudinal axis of guide rod  26  and guide rod bores  30  and  32 . Cap  40  seals and protects bore  32  and retains lubricating oil within the drive case. 
     Centrally located within drive shaft housing  28  is journal  44  which provides an area for receiving spherical rolling bearing assembly  46  which is used for mounting drive shaft  50 . At the opposite end of drive shaft  50  there is provided spherical roller bearing assembly  52  mounted in journal  24 . Spherical bearing configurations are provided for bearing assemblies  46  and  52  to accommodate a limited degree of bending deflection which drive shaft  50  experiences during operation. Drive case housing  18  also provides a central cavity within which oil pump  56  is located. Oil pump  56  could be of various types but a rotor type would be preferred. Through drilled passageways, high pressure lubricating oil is forced into spray nozzle  58  which sprays a film of lubricant onto the piston rods  260  (described below). In addition, lubricant is forced through internal passages within drive shaft  50 , as will be explained in greater detail later. 
     Drive case  18  further defines a series of four counter-bored rod seal bores  60 . At a position which would correspond with the lower portion of drive case  18 , a sump port  62  is provided. The lubrication system of engine  10  can be characterized as a dry sump type with oil collecting in the interior cavity of drive case  18  being directed to oil pump and returned via suction of oil pump  56 , where it is then pumped to various locations and sprayed as mentioned previously. 
     Drive shaft  50  is best described with reference to FIG.  1 . Drive shaft  50  incorporates a variable angle swashplate mechanism. Drive shaft  50  includes an annular swashplate carrier  66  which is oriented along a plane tipped with respect to the longitudinal axis of drive shaft  50 . Swashplate  68  in turn includes an annular interior cavity  70  enabling it to be mounted onto swashplate carrier  66 . Bearings enable swashplate  68  to be rotated with respect to drive shaft swashplate carrier  66 . Swashplate disc  72  is generally circular and planer but is oriented at an angle inclined with respect to that of swashplate cavity  70 . By rotating swashplate  68  with respect to drive shaft  50 , the angle defined by the plane of disc  72  and the longitudinal axis of drive shaft  50  can be changed from a position where they are perpendicular, to other angular orientations. Thus, rotation of drive shaft  50  causes disc  72  to rotate about an inclined axis. This basic swashplate configuration is a well known design implemented by the Assignee in prior Stirling engine configurations. Drive shaft  50  includes splined end  74  enabling it to be coupled to a load, which as previously stated, may be of various types. Two embodiments of actuators for changing the swashplate angle in a desired manner will be described later. 
     Swashplate Actuator 
     A first embodiment of an electric swashplate actuator in accordance with this invention is best shown with reference to FIGS. 1 and 4, and is generally designated by reference number  110 . Actuator  110  uses a DC torque motor, a planetary gear system, and bevelled gears to accomplish control over swashplate angle. With this embodiment of electric swashplate actuator  110 , it is necessary to communicate electrical signals to rotating components. To achieve this, two pairs of slip ring assemblies  112  are provided. Two pairs are provided for redundancy since it is only necessary for one pair to apply electrical power. Each slip ring assembly  112  includes a pair of spring biased brushes  114  mounted to a carrier  116  attached to drive shaft housing  28 . Electrical signals are transmitted into slip rings  118  directly attached to drive shaft  50 . Electrical conductors are connected to slip rings  118  and run through bearing mount  120  which is keyed to drive shaft  50  such that relative rotation is not possible between these two parts. Bearing mount  120  is connected with motor stator  122  having a number of permanent magnets (not shown) mounted thereto. The motor rotor  124  is journalled onto drive shaft  50  using needle bearing elements  126  such that they can rotate relative to one another. Electrical signals are transmitted to rotor  124  and its windings  128  via a second set of brushes  130 . Accordingly, through the application of DC electrical signals through slip ring assemblies  112 , electrical signals are transmitted to rotor windings  128  and thus the rotor can rotate relative to drive shaft  50 . By applying voltage in the proper polarity, rotor  124  can be rotated in either direction as desired. 
     Actuator rotor  124  includes an extension defining sun gear  132 . Three planet gears  134  mesh with sun gear  132  and also with teeth formed by stator extension  122  defining a ring gear which is fixed such that it does not rotate relative to shaft  50 . Thus, as rotor  124  rotates relative to shaft  50 , planet gears  134  orbit. Planet gears  134  feature two sections, the first section  138  meshing with sun gear  132 , and a second section  139  having a fewer number of teeth meshing with ring gear  140 . Revolution of the planet gear  134  causes rotation of ring gear  140  relative to drive shaft  50 . Ring gear  140  is directly coupled to a bevel gear  142  which engages a bevel gear surface  144  of swashplate  68 . As explained previously, relative rotation of swashplate  68  relative to drive shaft  50  causes an effective change in swashplate angle. 
     In normal operation, electric actuator  110  is not energized, therefore, sun gear  132  is stationary relative to drive shaft  50 . Ring gear  140  is driven by swashplate  68  and both rotate at the same speed. Planet gears  134  carry the torque from ring gear  140  to sun gear  132  and stator ring gear  136 . These then carry the torque to bearing mount  120  which in turn carries the torque to shaft  50 . Therefore, except when actuated, there is no movement of the gears of electric actuator  110  relative to one another. 
     Now with reference to FIG. 5, a second embodiment of an electric swashplate actuator according to this invention is shown and is generally designated by reference number  160 . The primary distinction of electric actuator  160  as compared with electric actuator  110  is the use of a stationary motor which avoids the requirement of slip rings for communicating power to motor windings. Electric actuator  160  includes a stationary mounted driving electric motor (not shown) which drives worm gear  164  meshing with worm wheel  166 . Worm wheel  166  can rotate freely relative to drive shaft  50  through a pair of anti-friction bearings  168 . Worm wheel  166  is coupled to carrier arm  170 . Shaft  172  is mounted to carrier arm  170  and drives planet gear  174  having a larger diameter toothed segment  176  and a smaller diameter toothed segment  178  which can rotate relative to shaft  172 . Larger diameter planet gear segment  176  meshes with fixed gear  182  which is keyed or otherwise fixed to drive shaft  50  for rotation therewith. The smaller diameter planet gear segment  178  meshes with idler gear  184  which rotates relative to the shaft on bearings  186 . Idler gear  184  engages with another planet gear set having planetary gears  188  having a smaller diameter segment  192  and a larger diameter segment  193 . Planet gear  188  rotates about shaft  194 . Shaft  194  is grounded to drive case housing  18 . Larger diameter planet gear segment  193  meshes with sun gear  198  which is fixed to collar  200  which rotates relative to shaft  50  on bearings  202 . Collar  200  is connected to bevel gear  204  which meshes with swashplate bevel gear  144 . 
     In normal operation, the actuator driving motor is not turning. Accordingly, worm  164  and worm wheel  166  are both stationary relative to drive case  18 . Sun gear  198  is driven by the swashplate and both rotate at the same speed. Sun gear  198  causes the driven planet gear  188  to rotate about its axis which is held stationery to the drive case  18 . This in turn causes idler gear  184  to rotate relative to shaft  50 . The speed of idler gear  184  relative to the shaft is dependant on the sizes of the gears used. Fixed gear  182  meshes with the planetary gear  174 . Because fixed gear  182  and sun gear  198  are the same size, planet gear  174  does not revolve around the drive shaft axis. However, when worm  164  is rotated, a gear reduction acting through the two planetary gear sets causes bevel gear  204  to rotate relative to drive shaft  50 , thus changing the swashplate angle. 
     Cross Head Assembly 
     Details of cross head assembly  220  are best shown with references to FIGS. 2,  3  and  6  through  9 . Cross head body  222  forms a caliper with a pair of legs  224  and  226  connected by center bridge  228 . Each of legs  224  and  226  define a pair of guide bores  230 . Preferably, journal bearings are installed within guide bores  230  such as porous bronze graphite coated bushings  232 . Bushings  232  enable cross head body  222  to move smoothly along guide rods  26 . Cross head leg  224  also forms stepped cross head slider cup bore  234  a portion of which is threaded. Leg  226  forms slider cup bore  236  which also has a conical section  238 . Within bores  234  and  236  are positioned slider cups  240  and  242 , respectively. Slider cups  240  and  242  form semi-spherical surfaces  244  and  246 . Slider elements  248  and  250  also define spherical outside surfaces  252  and  254 , respectively, which are nested into slider cup surfaces  244  and  246 , respectively. Opposing flat surfaces  256  and  258  are formed by the slider elements and engage swashplate disc  72 . As will be explained in more detail below, a hydro-dynamic oil film is developed between spherical flat surfaces  256  and  258  as they bear against disc  72  to reduce friction at that interface. In a similar manner, a hydro-dynamic oil film is developed between slider cup spherical surfaces  244  and  246 , and slider spherical outside surfaces  252  and  254 . 
     Piston rods  260  extend between associated pistons and slider cup  242 . Piston rod  260  has a threaded end  262  which meshes with slider cup threaded bore  264 . The end of piston rod  260  adjacent threaded end  262  forms a conical outside surface  266  which is tightly received by cross head bore conical section  238 . Thus, the relative position between slider cup  242  and cross head leg  224  is fixed. However, slider cup  240  is provided with means for adjusting its axial position within cross head body bore  234  such that precise adjustment of the clearances of the hydro-dynamic films is achievable. Slider cup  240  includes an extended threaded stud  270 . In the annular space surrounded threaded stud  270  are adjuster sleeve  272  and cone  274 . As best shown in FIGS. 8 and 9, sleeves  272  define an inside conical surface  276  and an outside threaded surface  273 . Two perpendicular slits are formed diametrically across sleeve  272 , one from the upper surface and one from the bottom surface and render the sleeve compliant in response to hoop stresses. Adjustment of the clearances for the hydro-dynamic films is provided by changing the axial position of slider cup  240  in bore  234  which is done by rotating sleeve  272 , causing it to advance into slider cup bore  234 , due to the threaded engagement of the sleeve in the bore. Once the gaps are adjusted properly, nut  278  is threaded onto stud  270  which forces cone  274  into engagement with sleeve conical surface  276 , causing the sleeve to radially expand. This action forces the sleeve into tight engagement with cross head bore  234 , keeping it from rotating, thus fixing the position of cup  240 . 
     Rod Seals 
     As shown in FIG. 1, piston rod seal assembly  290  includes housing  292  mounted within rod seal bore  60 . Rod seal assembly  290  further includes spring seal actuator  294  which urges an actuating collar  296  against sealing bushing  298 . Seal actuator spring  294  is maintained within housing  292  through installation of an internal C-clip  300 . Due to the conical surfaces formed on collar  296  and bushing  298 , seal actuator spring  294  is able to cause the bushing to exert a radially inward squeezing force against piston rod  260 , thus providing a fluid seal. Preferably, collar  296  is made of an elastomeric material such as a graphite filled Teflon™ material. 
     An alternate embodiment of a rod seal assembly is shown in FIGS. 1B and 1C. Bellows seal assembly  570  provides a hermetic rod seal. Bellows element  572  has its stationary end mounted to base  574 , whereas the opposite end is mounted to ring  576 . Bellows seal assembly  570  is carried by block  578  clamped between cylinder block assembly  14  and drive case assembly  12 . FIG. 1B shows the bellows seal element in an extended position whereas FIG. 1C shows the element compressed. The design of engine  10  readily allows the sliding contact rod seal  290  to be replaced by bellows seal assembly  570  without substantial reworking of the engine design. 
     Lubrication System 
     Oil lubrication of machine  10  takes place exclusively within drive case assembly  12 . As mentioned previously, sump port  62  provides a collection point for lubrication oil within drive case housing  18 . Through a sump pick-up (not shown), oil from sump port  62  enters oil pump  56  where it is forced at an outlet port through a number of lubrication pathways. Some of this oil sprays from nozzle  58  onto piston rods  260  and cross head guide rods  26 . Another path for oil is through a center passage  310  within drive shaft  50 . Through a series of radial passageways  312  in drive shaft  50 , oil,is distributed to the various bearings which support the drive shaft. Oil is also ported to swashplate  68  surfaces. The oil then splashed onto the sliding elements of the cross head assembly including slider cups  240  and  242 , and slider elements  248  and  250 . The exposed surfaces of these parts during their operation are coated with oil and thus generate a hydrodynamic oil film. 
     Cylinder Block 
     Cylinder block assembly  14 , best shown in FIGS. 1 and 10, includes a cylinder block casting  320  having a pair of opposed parallel flat mating surfaces  322  and  324 . Mating surface  322  enables cylinder block casting  320  to be mounted to drive case housing mating surface  22 . Bolts  326  hold these two parts together. Stirling engine  10  according to the present invention is a four cylinder engine. Accordingly, cylinder block casting  320  defines four cylinder bores  328  which are mutually parallel. As shown in FIG. 1, cylinder bores  328  define a larger diameter segment through which piston assembly  330  reciprocates, as well as a reduced diameter clearance bore section for rod seal assembly  290 . Four cooler bores  332  are also formed in cylinder block casting  320  and are mutually parallel as well as parallel to cylinder bores  328 . Cylinder bores  328  are arranged in a square cluster near the longitudinal center of cylinder block casting  320 . Cooler bores  332  are also arranged in a square cluster but lie on a circle outside that of cylinder bores  328 , and are aligned with the cylinder bores such that radials through the center of cooler bores  332  pass between adjacent cylinder bores. In that Stirling engine  10  is a double acting type, cylinder block casting  320  including working gas passageways  334  which connect the bottom end of cooler bore  332  to the bottom end of an adjacent cylinder bore  328  as shown in FIG.  10 . Cylinder block casting  320  further forms coolant passageways  336  which provide for a flow of liquid coolant through coolant bores  332  in a diametric transverse direction. 
     Piston Assembly 
     Piston assembly  330  is best shown with reference to FIGS. 11 and 12. Piston base  350  forms a conical bore  352  which receives a conical end  354  of piston rod  260 . Nut  356  combined with friction at the conical surfaces maintains the piston rod fixed to piston base  350 . An outer perimeter groove  358  of the piston base receives bearing ring  360  which serves to provide a low friction surface engagement with the inside of cylinder bore  328 . Bearing ring  360  is preferably made of an low friction elastomeric material such as “Rulon™” material. Dome base  362  is fastened onto piston base  350  through threaded engagement. Dome  364  is welded or otherwise attached to dome base  362 . Dome  364  and dome base  362  define a hollow interior cavity  366  which is provided to thermally isolate opposing ends of piston assembly  330 . 
     Located between piston base  350  and dome base  362  are a number of elements which comprise piston ring assembly  368  which provides a gas seal around the perimeter of piston assembly  330  as it reciprocates in its bore. Sealing washer  370  is clamped between piston base  350  and dome base  362  and is a flat with opposing parallel lapped surfaces. A number of radial passageways  378  are drilled through washer  370 . On opposing sides of sealing washer  370  are provided sealing rings  380  and  382  preferably made of “Rulon™” type elastomeric low friction material. Sealing rings  380  and  382  contact cylinder bore  328  to provide gas sealing. Acting at the inside diameter of sealing rings  380  and  382  are spring rings  384  and  386  which are split to provide radial compliance. Spring rings  384  and  386  are provided to outwardly bias sealing rings  380  and  382 , urging them into engagement with the cylinder bore. 
     At a number of circumferential locations, passageways  388  are drilled radially into dome base  362 . In a similar manner, passageways  390  are formed within piston base  350 . A pair of O-rings  392  and  394  are clamped against opposing face surfaces of sealing washer  370 . At axial location aligned with sealing washer  370 , piston base  350  defines one or more radial passageways  396  communicating with piston dome interior cavity  366  which functions as a gas accumulator. 
     As piston assembly  330  reciprocates within its bore the two sealing rings  380  and  382  provide a gas seal preventing cycle fluid from leaking across the piston assembly. Sealing rings  380  and  382  are pressure actuated such that only one of the two rings is providing a primary seal at any time. Specifically, sealing ring  380  provides a gas seal when the piston is moving downwardly (i.e. toward swash plate  68 ) whereas sealing ring  382  is pressure actuated when the piston is moved in an upward direction. Since Stirling engine  10  is of the double acting variety, piston assembly  330  is urged to move in both its reciprocating directions under the influence of a positive fluid pressure differential across the piston assembly. Thus, just after piston assembly  330  reaches its top dead center position, a positive pressure is urging the piston downwardly. This positive pressure acts on sealing ring  380  urging it into sealing contact with the upper surface of sealing washer  370 . The lower sealing ring  382  however, is not fluid pressure actuated since it is urged away from sealing contact with sealing washer  370  since passageway  390  provides for equal pressure acting on the upper and lower sides of the ring. In the upward stroke of piston assembly  330  a positive pressure is urging the piston to move upwardly and thus sealing ring  382  seals and sealing ring  380  is not fluid pressure actuated as described previously. As this reciprocation occurs, piston cavity  366  is maintained at the minimum cycle pressure. This assures that the radial clearance space between sealing rings  380  and  382  is at a low pressure, thus providing a pressure differential for pressure actuating the seal rings into engagement with the inside diameter of the piston bores, thus providing a fluid seal. 
     Cooler Assembly 
     Cooler assembly  400  is best shown with reference to FIGS. 13 and 14 and is disposed within cylinder block cooler bores  332 . Cooler assembly  400  comprises a “shell and tube” type heat exchanger. As shown, housing  402  includes pairs of perimeter grooves at its opposite ends which receive sealing rings  405  for sealing the assembly within cooler bore  332 . Housing  402  also forms pairs of coolant apertures  408  within housing  402 . A number of tubes  410  are arranged to extend between housing ends  412  and  414 . Tubes  410  can be made of various materials and could be welded or brazed in place within bores in housing ends  410  and  414 . As a means of reducing flow loses of the Stirling cycle working gas, the ends of the inside diameters of tubes  410  are counter bored or flared to form enlarged openings. The Stirling cycle working gas is shuttled back and forth between the ends  412  and  414  of the cooler housing and passes through the inside of tubes  410 . A coolant, preferably a liquid is pumped in a cross flow manner through block coolant passages  336  and housing apertures  408  to remove heat from the working gas. 
     Cylinder Extensions 
     Cylinder block assembly  14  further includes tubular cylinder tops or extensions  420  which form a continuation of the cylinder block bores  328 . At their open ends, tubular cylinder extensions  420  form a skirt which allows them to be accurately aligned with cylinder bores  328  by piloting. O-ring seal  422  provides a fluid seal between cylinder block bores  328  and tubular cylinder extensions  420 . Cylinder extensions  420  at their opposing ends form cylinder extension manifolds  424  which will be described in more detail below. Cylinder extension manifolds  424  are often simply referred to as cylinder manifolds. 
     Regenerator Housings 
     Cup shaped regenerator housings  430  are provided which are aligned co-axially with cooler bores  332 . Regenerator housings  430  define an open end  432  and a closed top  434  having regenerator housing manifold  436  for communication with the heater assembly. Regenerator housing manifolds  436  are often simply referred to as regenerator manifolds. Within regenerator housing  430  is disposed regenerator  444 , which in accordance with known regenerator technology for Stirling engines, is comprised of a material having high gas flow permeably as well as high thermal conductivity and heat absorption characteristics. One type of regenerator uses wire gauze sheets which are stacked in a dense matrix. 
     Retainer Plate 
     Retainer plate  448  is best shown in FIG.  15  and provides a one-piece mounting structure for retaining tubular cylinder extensions  420  and regenerator housings  430  in position. Retainer plate  448  forms cylinder extension bores  450  and regenerator housing bores  452 . Cylinder extension bores  450  have a diameter slightly larger than the largest diameter at the open end of tubular cylinder extension  420  and the bore is stepped as shown in FIG.  1 . In a similar fashion, regenerator housing bores  452  are also enlarged with respect to the open end of regenerator housing  430  and are also stepped. Retainer plate  448  is designed so that the open ends of tubular cylinder extensions  420  and regenerator housings  430  can be inserted as an assembly through their associated plate bores. This is advantageous since the configuration of cylinder extension  420  and the heater head assembly  16  attached to the cylinder extension and regenerator housing  430  would not permit top mounting. For assembly, retainer plate  448  is first positioned over cylinder extensions  420  and regenerator housings  430 . Thereafter, semi-circular cylinder extension locking C-rings  454  shown in FIGS. 1,  16  and  17 , and regenerator housings locking C-rings  456  are placed around the associated structure and allow retaining plate  448  to clamp these components against cylinder block mounting face  324 , in a manner similar to that of an internal combustion engine valve stem retainer. Mounting bolts  458  fasten retainer plate  448  to cylinder block body  320 . The use of a one-piece retaining plate provides rapid assembly and securely mounts the various components in an accurately aligned condition. 
     Cylinder extension  420  interact with cylinder block mating surface  324  to accurately pilot the center of the cylinder extensions with respect to cylinder block cylinder bores  328 . However, the need for such accurate alignment does not exist for regenerator housings  430 , and therefore, a face seal is provided allowing some degree of tolerance for misalignment between the regenerator housings and cooler bores  332 . In this way, assembly is simplified by reducing the number of ports which must be simultaneously aligned. 
     Heater Head Assembly 
     Heater head assembly  16  provides a means of inputting thermal energy into the Stirling engine working gas and is shown in FIG. 1A. A combustor (not shown) is used to bum a fossil fuel or other combustible material. Alternatively, heat can be input from another source such as concentrated solar energy, etc. In Stirling engine  10  according to this invention, combustion gases flow axially toward central heat dome  470  where it is deflected to flow in a radial direction. An array of heater tubes  478  is arranged to conduct heat from the hot gas as it flows radially out of the engine. Heater tubes  478  are arranged to form an inner band  480  and an outer band  482 . The tubes of inner band  480  have one end which fits within cylinder extension manifold  424  and the opposite end fitting into heater tube manifold segment  484 , which is also referred to as the heater head manifold. Although heater tube manifold segment  484  is referred to as the heater head manifold, it should be noted that in the disclosed embodiment, the heater tube manifold segment has an equivalent number of inlets and outlets. In this embodiment heater tube manifold segment  484  could also be referred to simply as a head. As best shown in FIGS. 1A,  18  and  19 , the tubes of inner bands  480  are arranged in a staggered relationship as are the tubes of outer band  482 , thus enhancing heat transfer to the heater tubes. Heater tube manifold segment  484  has internally formed passageways such that the inner-most tubes of inner band  480  are connected with the inner-most band of outer tubes  482  through passageways  486 . In a similar manner, the outer groups of inner and outer bands are connected via internal passageways  488 . The tubes of the outer band  482  are connected with heater tube manifold segment  484  and the regenerator housing manifold  436 . 
     Each of tubes  478  defining heater tube inner band  480  and outer band  482  are identical except the outer band tubes are longer. Tubes  478  are preferably made from a metal casting process which provides a number of benefits. The material which can be used for cast heater tubes can be selected to have higher temperature tolerance characteristics as compared with the deformable thin-walled tubes typically used. As shown in FIGS. 20 and 21, heater tubes  478  have projecting circular fins  492 . The cross-section of the fins shown in FIG. 21 reveals that they can have a thickness which decreases along their length with rounded ends. Various other cross-sectional configurations for fins  492  can be provided to optimize heat transfer characteristics. In addition to optimizing the longitudinal cross-sectional shape of the fins, modifications of their perimeter shape can be provided. FIG. 22 shows a circular outside perimeter shape for fins  492 . Using a casting process for forming heater tubes  478 , other shapes to be provided. For example, FIG. 23 shows a generally dart shaped platform configuration. The configuration can be tailored to the gas flow dynamics which occur around the tubes. For example, it is known that for tubes arranged perpendicular to the gas flow direction, the upstream side surface  496  of the tubes tends to absorb more heat than the downstream or back side  498  of the tubes. For conventional tubes, this leads to significant thermal gradients which produce mechanical stresses on the heater tubes which can in turn lead to their failure over time. The platform provided in FIG. 23 may be advantageous to increase heat absorption on the backside  498  to maintain more constant tube temperature for gas flowing in the direction of arrow  492  since more fin area is exposed on the downstream side where heat transfer is less efficient. 
     Tubes  478 , heater head  484 , cylinder extension manifold  424  and regenerator housing manifold  436  are preferably cast from superalloy metallic materials. Superalloys have been developed for very high temperature applications where relatively high stresses are encountered (such as tensile, thermal, vibratory and shock stresses) and oxidation resistance is often required. Superalloys are routinely used in jet-engine applications, such as for casting turbine blades. By casting all of the components of heater head assembly  16  from the same superalloy material, problems which could be caused by differences in material properties, such as differences in thermal expansion, can be avoided. Applicants believe that nickel-based, cobalt-based, and iron-based superalloys offer the best performance characteristics for the inventive heater head assembly. The preferred superalloy for the components of the heater head assembly is Inconel 713C. This alloy is nickel-based and includes significant proportions of chromium, aluminum and molybdenum. The operating temperature of heater head components cast from Inconel 713C is approximately 1000° C., approximately 200° C. higher than the operating temperatures of heater head assemblies manufactured utilizing conventional bent tube techniques. 
     Pressure Balancing 
     As in conventional Stirling cycle engines employing multiple double acting cylinders, in the case of the four cylinder engine shown in connection with this invention, four distinct isolated volumes of working gas such as hydrogen or helium are present in the engine. One of the volumes is defined by the expansion space above piston dome  364  which in turn flows through heater tubes  478 , regenerator  444 , cooler assembly  400 , and cylinder block passageway  334  to the lower end of an adjacent cylinder bore  328 . In a similar manner, three additional discrete volumes of gas are defined. Each of the gas volumes undergo shuttling between a compression space defined at the lower end of piston cylinder bore  28  in cylinder block casting  320 , and an expansion space defined within tubular cylinder extension  420 . Thus, the gases are shuttled between these spaces as occurs in all Stirling engines. Gases passing through heater head assembly  16  absorb heat and expand in the expansion space and are cooled by cooler assembly  400  before passing into the compression space. 
     In order to minimize imbalances in the operation of engine  10 , the mean pressure of the four distinct gas volumes needs to be equalized. This is achieved through the use of working fluid ports  500  positioned at the lower-most end of cylinder block cooler bore  332 , best shown in FIG. 10, each of which are exposed to the separate gas volumes. Fitting  502  is installed in a port and from it are three separate tube elements. A first small capillary tube  504  communicates with pressure transducer block  506  having individual pressure transducers for each of the gas volumes, enabling those pressures to be measured. Capillary tube  508  communicates with manifold block  510  having an internal cavity which connects each of the individual capillary tubes  508  together. The function of manifold block  510  is to “leak” together the volumes for equalization of any mean pressure imbalances which may occur between them. A low restriction passageway connecting these cycle volumes together would unload the engine and would constitute an efficiency loss. Therefore, tubes  508  have a restricted inside diameter and thus the flow rate through these tubes is restricted. However, over time, pressure imbalances are permitted to equalize through fluid communication between the volumes. 
     Unloader Valve 
     In the event of a mechanical failure or other condition which leads to a leakage of working gas from the engine, a severe imbalance condition can result. For example, if only one or more of the enclosed gas volumes leaks to atmosphere, potentially destructive loads would be placed on the mechanical components of engine  10 . In order to preclude this from occurring, conduits  518  communicate with unloader valve  520  as shown with reference to FIG.  24 . As shown, unloader valve includes housing  522  within internal stepped bore  524 . A series of pipe fittings  526  are provided which communicate with individual diameter sections of stepped bore  524  via passageways  528 . Each of fittings  526  communicates with the separate gas volumes via conduits  518 . Spool  530  is positioned within stepped bore  524  and is maintained in the housing by cap  532 . A series of grooves  534  provided on the various diameter sections of spool  530  and retain O-rings  536 . Spool  530  is urged in the right-hand direction as viewed in FIG. 24 by coil spring  538 . An additional port is provided at fitting  540  which communicates with manifold block  510  via conduit  541  and is exposed to the engine mean pressure. This pressure signal passes through passageway  542  and acts on the full end area of spool  530 . During normal engine operation, individual diameter sections of stepped bore  524  are exposed to the mean pressure of the four enclosed gas volumes. Each of these pressure signals produces a resultant net force on spool  530  urging it toward the right-hand direction which is assisted by the compliance of spring  538 . In a normal operating condition, these pressures produce forces added to the spring compliance pushing shuttle spool  530  to the right-hand position as shown. However, in the event of the mechanical failure of engine  10  causing a leakage of working fluid, one (or more) of the passageways  528  experiences a loss in pressure. In this event, the net force acting to retains spool  530  in position is reduced and the equilibrium condition is unbalanced to move the shuttle in the left-hand direction under the influence of the engine mean cycle pressure through passageway  542 . When this occurs, the various O-rings  536  unseat from their associated sealing surfaces and thus all of the gas volumes are vented together inside housing  522 , rendering the engine incapable of producing mechanical output power and thus protecting the engine from destructive imbalance forces. 
     Air Preheater 
     Combustion gases which pass through heater tube inner and outer banks  480  and  482  still are at an elevated temperature and have useful heat energy which can be recovered to enhance the thermal efficiency of engine  10 . This is achieved through the use of air preheater  550  which has an annular ring configuration and surrounds heater tube outer bank  482 . Air preheater  550  is formed from sheet metal stock having a high temperature capability. The stock first begins with a flat sheet  552  which may have local deformations as shown in FIG. 26 such as dimples  554 , and is bent in an accordion-like fashion about fold lines  556 . After sheet  552  is corrugated, its ends are welded to define the annular preheater configuration shown in FIGS. 25,  27 , and  28 . FIG. 28 shows that these corrugations are pinched together and welded at the axial ends of the preheater. Upper end  558  is formed with adjacent layers pinched together and welded as shown. Bottom end  560  has layers which are pinched together but alternate with those pinched together at top end  558 . This arrangement provides the gas flow direction shown in FIG. 1A in which combustion gas flow is shown by cross-hatched arrows and fresh combustion air by clear arrows. Combustion gases passing through heater head assembly  16  are deflected by baffle  562 . The hot gases then enter the inside diameter of air preheater  550 . Since the upper end  558  of these wraps are sealed, the gas is forced to flow downwardly as shown by the arrows. After passing through air preheater  550  these gases are vented or are further treated downstream. Fresh combustion air enters at the radially outer side of air preheater  550  and is constrained to flow in an axial direction through baffle  564 . Combustion inlet air travels upwardly in an axial direction as shown by the upward directed arrows and is thereafter conveyed to a fuel combustor (not shown). Heat is transferred through the thin sheet metal forming air heater  550 . 
     As a means of further enhancing thermal efficiency of engine  10 , the inside surface of air preheater  550  exposed to combustion gases can be coated with a catalyst material such as platinum or palladium, or other catalyst materials. This thin layer  566  encourages further combustion of hydro-carbons within the combustion gases which has the two-fold benefits of reducing emissions as well as increasing the combustion gas temperature thereby increasing combustor inlet air temperature and efficiency. 
     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 in the following claims.