Abstract:
An external combustion engine comprises a mass of compressible working fluid; a fluidic piston in fluid communication with said working fluid; and a second piston in hydraulic communication with said fluidic piston and in fluid communication with said working fluid.

Description:
BACKGROUND  
         [0001]    Stirling engines refer to a specific class of external combustion heat engines that convert heat differentials into mechanical energy with relatively high conversion efficiencies. Such efficiencies for a class of optimized Stirling engines can surpass most known air-breathing internal combustion engines, and utilize a regenerator to store or release fluid heat during the engine cycle. Such Stirling engines can approach the efficiency of an ideal Carnot cycle.  
           [0002]    External combustion engines can be used in a variety of applications. For example, as prime movers, cooling systems, cryogenic coolers, heat pumps or pressure generators in a variety of design forms and operating sizes. Current applications range in size from large power generators to miniature engines for artificial hearts. Stirling engines thus far developed vary in output power from as little as a few watts to as much as 1 MW (1 MW=1341 hp). High temperature Stirling engines may operate at temperatures exceeding 1000 K, with mean working fluid pressures in the range of one atmosphere to as high as 20 MPa (1 MPa≈10 atmospheres).  
           [0003]    Conventional Stirling engines utilize the roughly steady-state expansion of a highly compressed fixed number of light molecular mass working fluid, such as helium, hydrogen or air, in contact with a heat source at a substantially fixed temperature for their power stroke; followed by forced convection heat transfer or gas cooling by contact with a heat sink to generate engine speeds ranging from low to high frequency, typically measured in revolutions per minute.  
           [0004]    Since the essential ingredient needed to operate a Stirling engine is an appropriate external heat source such as, for example, solar, natural gas, fossil fuel, oil, coal, waste heat or geothermal energy; this makes the Stirling engine well suited for not only terrestrial applications but also for large scale space and underwater applications, including spacecraft and submarines.  
           [0005]    Any conventional (non-rotary) type Stirling engine requires simple components for its operation. It requires internal pistons as the means for displacing and compressing the working fluid therein and to generate output power. The pistons receive work during their up-stroke (compression), and generate greater work during their down-stroke (expansion), followed by a transfer of heat at some temperature by the working fluid to the surrounding heat sink. The power pistons are usually equipped with high performance fluid rings to assure and maintain a high pressure differential between their upper and lower faces.  
           [0006]    Regenerators, which are placed between the hot and cold heat exchangers, optimally recycle the heat supply and transfer process by acting as thermodynamic sponges. Their function is to receive heat from the working fluid during the fluid passage from the high to low temperature space, and to release heat to the working fluid during the fluid passage back from the low to high temperature space.  
           [0007]    Generally, the system efficiency and the cyclic work output are functions of both thermodynamic variables, such as pressure, and the internal volumetric compression ratio. From a thermodynamic standpoint, an ideal reversible four-path Stirling cycle when depicted in the pressure (P) versus volume (V) diagram consists of two isothermal (constant temperature) and two isochoric (constant volume) processes in sequence. When depicted in the temperature (T) versus entropy (S) diagram, the heat energy transfer in the process is proportional to the area enclosed in the T-S diagram (?         SdT). Likewise, the work done by the engine is proportional to the area enclosed on the P-V diagram (?         PdV).  
           [0008]    Disadvantages of conventional Stirling engines include the use of relatively expensive and heavy materials, such as Inconel® and other alloy steels, for the high temperature structural components (e.g., pistons, cylinders and regenerators). In addition, the use of seals at the piston connecting rod is a serious factor for limiting the useful life of the engine and is a well-known cause for downgrading the engine&#39;s overall efficiency. Further, these seals are specialized designs and are correspondingly expensive to produce, and typically do not perform an adequate function in preventing leakage of high-pressure working fluid.  
           [0009]    Conventional high temperature Stirling engines generally operate at high rotational velocities of typically about 3000 RPM. This is one prime reason for the reduction of the regenerator efficiency, and causes a marked increase in frictional losses from the high velocity motion of the working fluid. Other adverse effects due to high temperatures, typically about 720° C., include the high heat losses due to the blackbody radiation. Although the low to middle temperature types of Stirling engines (&lt;450° C.) can alleviate some of these losses, the price to pay is a lower Carnot efficiency. There also remain numerous other drawbacks, deficiencies, and disadvantages associated with conventional Stirling engines. One disadvantage is the premature failure of the seal between the connecting rod, which exhibits complex translational and rotational motions, and the mechanical drive linkage, despite expensive seal designs. A further disadvantage is the mechanical coupling of all adjacent pistons that results in a fixed phase angle relationship, which prevents optimization of the engine.  
           [0010]    Since a Stirling engine is a device based on an oscillatory and forced convection of the working fluid, parasitic losses of the engine are related to the frequency of the operation. The higher the frequency the worse are certain performance losses. For a fixed target power, the lower frequency engine may be preferable. Reduction of unnecessary dead space volumes that do not participate in power generation and overall operation is desirable in the design of an optimized Stirling engine.  
         SUMMARY  
         [0011]    The above-described and other features are exemplified by the following Figures and description in which an external combustion engine comprises a mass of compressible working fluid; a fluidic piston in fluid communication with the working fluid; and a second piston in hydraulic communication with the fluidic piston and in fluid communication with the working fluid.  
           [0012]    These and other features are further exemplified by an external combustion engine comprising at least four upright cavities disposed substantially equidistant from a central upright axis, a compressible working fluid in fluid communication between each pair of adjacent cavities; and at least one linkage in reciprocal phase communication between each pair of alternate cavities. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    Referring now to the drawings wherein like elements are numbered alike in the several Figures:  
         [0014]    [0014]FIG. 1 is a schematic representation of a prior art Stirling engine;  
         [0015]    [0015]FIG. 2 is a schematic representation of a first exemplary embodiment Fluidic-Piston engine;  
         [0016]    [0016]FIG. 3 is a schematic representation of a second exemplary embodiment Fluidic-Piston engine;  
         [0017]    [0017]FIG. 4 is a schematic representation of a third exemplary embodiment Fluidic-Piston engine; and  
         [0018]    [0018]FIG. 5 is a schematic representation of a fourth exemplary embodiment Fluidic-Piston engine. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0019]    As shown in FIG. 1, a prior art double-acting multi-cylinder Stirling engine is indicated generally by the reference numeral  10 . The engine  10  has one reciprocating element  12   a - 12   d  per cylinder  14   a - 14   d , respectively, and is generally designed such that the compression space  16   a - 16   d  of each cylinder  14   a - 14   d , respectively, is connected through an associated heat exchanger  18   a - 18   d , respectively, to the expansion space  20   a - 20   d  in an adjacent cylinder  14   a - 14   b , respectively, via associated passageways  22   a - 22   d ,  24   a - 24   d . As used herein, the term heat exchanger shall include a cooler, a regenerator or a heater, or any combination thereof.  
         [0020]    Compression space  16   a  of cylinder  14   a  is connected through regenerator  18   a  via passageways  22   a  and  24   a  to the expansion space  20   b  of the adjacent cylinder  14   b . The three adjacent cylinders  14   b - 14   d  are similarly connected. Here, each piston  12   a - 12   d  has two functions: (1) to transmit the power to the output shafts  26   a - 26   d , respectively; and (2) to cause fluid motion to and from the two sets of variable spaces  16   a - 16   d , and  20   a - 20   d . This configuration is known in the art as the Rider configuration. The connecting rods  28   a - 28   d , which are coupled to the output shafts  26   a - 26   d , respectively, are sealed from the environment, which tends to decrease the engine&#39;s efficiency as will be further described. Conventional Rider configuration Stirling engines depend on some mechanical linkage or crankshaft  30  to transmit power from the connecting rods  28   a - 28   d . Such linkages include crankshafts, rhombic drives, swash plates and slider-cranks. Because the pistons  12   a - 12   d  are mechanically linked via the crankshaft  30 , for example, the thermodynamic state of one cylinder may not be optimized relative to an adjacent cylinder.  
         [0021]    Turning now to FIG. 2, an exemplary embodiment Fluidic-Piston engine is indicated generally by the reference numeral  100 . Engine  100  is an external combustion engine that includes a plurality of rigid pistons  141   a - 141   d  disposed upright in a plurality of corresponding chambers, cavities or cylinders  140   a - 140   d , respectively. The cylinders  140   a - 140   d  are formed in a cylinder block (not shown), which may include cylinder heads  146   d  or  146   a - 146   c  that are also known as heater heads. As used herein, “cylinder block” shall include a single block or a multi-component structure, which, when coupled together, functions as the cylinder block of the engine  100 . Each rigid piston  141   a - 141   d  is in contact along its bottom surface with a corresponding substantially incompressible fluidic piston  142   a - 142   d , respectively. The cylinders  140   a - 140   d  are physically arranged in a two-by-two matrix with each other such that each cylinder is vertically oriented and equidistant from each of two adjacent cylinders.  
         [0022]    Although the fluidic pistons  142   a - 142   d  are preferably liquid, other relatively incompressible fluids, including gels, may be used. The material of the fluidic piston may be chosen from water, glycols (such as ethylene glycol, propylene glycol or other glycols), DOWTHERM®, aromatics, silicones, and similar substantially incompressible fluids for serving the function of the fluidic piston described herein. In general, the material of the fluidic piston will preferably exhibit substantial incompressibility relative to a working fluid, high boiling point, high breakdown temperature (including auto-ignition and/or dissociation temperatures), low density, low viscosity change under design conditions, low viscosity at lower temperatures, low vapor pressure and immiscibility with the chosen working fluid. Functions of the fluidic piston may include displacing a volume of working fluid, lubricating piston and engine seals, and preventing high-pressure gaseous working fluid from leaking between the internal passageways of the engine and an external environment by means of a gas to liquid barrier. Each cylinder  140   a - 140   d  defines a compression space  143   a - 143   d , respectively, defined by the volume above each fluidic piston  142   a - 142   d , respectively, and below each regenerator  144   a - 144   d , respectively; and an expansion space  145   d  or  145   a - 145   c  defined by the volume above each rigid piston  141   a - 141   d , respectively, and above each regenerator  144   d  or  144   a - 144   c , respectively. The compression space  143   a - 143   d  of each cylinder  140   a - 140   d , respectively, is thermodynamically coupled to a corresponding heat exchanger or regenerator  144   a - 144   d , respectively, and connected to the expansion space  145   a - 145   d , respectively, of an adjacent cylinder  140   b - 140   d  or  140   a , respectively, via associated passageways or tubes  147   a - 147   d , respectively. For example, the compression space  143   a  of cylinder  140   a  is connected through passageways or tubes  148   a  to the regenerator  144   a , and, in turn, through the tubes  147   a  to the expansion space  145   a  of the adjacent cylinder  140   b.    
         [0023]    Hence, the pistons  141   a - 141   d  that move away under force from the expanding working fluid are called power or expansion pistons, and the pistons  142   a - 142   d  that move to compress the working fluid are called compression pistons. In alternate embodiments, a single piston may be substituted to fulfill the functions of an expansion piston and a compression piston. The compression  143   a - 143   d  and expansion spaces  145   d  or  145   a - 145   c  are occupied during the engine cycle by a substantially fixed mass of working fluid, such as, for example, air, hydrogen, helium, or other working fluids having suitable heat-transfer and thermodynamic properties for serving the functions of the working fluid described herein.  
         [0024]    The exemplary engine  100  includes a second set of transfer pistons  149   a - 149   d , which are directly and rigidly connected to pistons  141   a - 141   d , respectively, in an axial relationship. Pistons  149   a - 149   d  are connected to pistons  141   a - 141   d , respectively, via shafts  150   a - 150   d , respectively, passing through shaft ports  151   a - 151   d , respectively, which may be formed entirely within the cylinder block to effectively seal the shaft. The second set of pistons  149   a - 149   d  are housed within chambers or cylinders  152   a - 152   d , respectively.  
         [0025]    Each cylinder  152   a - 152   d  includes a volumetric space  153   a - 153   d , for hydraulic fluid to ingress and egress during each stroke. For example, piston  141   a  in the expansion mode is rigidly connected to piston  149   a , which, in turn, transfers the force of expansion through an articulated mechanical linkage  155   a  to piston  149   c . Thus, the pistons  141   a  and  141   c  are maintained in an opposite phase relationship of 180 degrees. A second linkage (not shown) similarly transfers expansion forces between pistons  149   b  and  149   d  and thus maintains the pistons  141   b  and  141   d  in an opposite phase relationship.  
         [0026]    Although alternate pistons such as  141   a  and  141   c  are maintained in a phase relationship of 180 degrees, the phase relationship between adjacent pistons such as between pistons  141   a  and  141   b  is not fixed at 90 degrees, and is gas dynamically variable between about 0 to about 180 degrees during portions of an engine cycle. It shall be recognized that the instantaneous phase relationship between one pair of adjacent pistons is equal to the instantaneous phase relationship between the other pair of adjacent pistons. Thus, if piston  141   a  leads piston  141   b  by 135 degrees, for example, at a point in its expansion stroke; then piston  141   c  leads piston  141   d  by 135 degrees at an opposite point in its compression stroke. These phase differentials between adjacent pistons will tend towards 90 degrees or ¼ engine cycle at about a mid-point of the travel of each piston  141   a - 141   d  due to the balancing effect of the linkages  155   a  and  155   b.    
         [0027]    Unlike the crankshaft  30  of FIG. 1, the linkage  155   a  need not make a full rotation to guide the pistons  141   a ,  141   c  through a complete cycle. Thus, the stroke length and corresponding compression ratio for each piston  141   a - 141   d  is not fixed by the linkage  155   a , but may automatically vary from a relatively small stroke up to the limits of travel of the pistons  141   a - 141   d  according to the thermodynamic conditions in the working fluid.  
         [0028]    At the same time that piston  141   a  displaces the fluidic piston  142   a , which compresses and displaces the working fluid in the compression space  143   a , piston  149   c  displaces the hydraulic fluid from the volume space  153   c  through valve  154   c  that controls the flow of hydraulic fluid into and out of the cylinder  152   c . This hydraulic fluid can then be used to drive a conventional hydraulic motor (not shown) for production of rotational motion.  
         [0029]    Each fluidic piston  142   a - 142   d  is substantially incompressible and at least partially disposed in the corresponding cylinder  144   d  or  144   a - 144   c , respectively, which is formed in the cylinder block (not shown). Each fluidic piston  142   a - 142   d  also extends into the tube set  148   a - 148   d , respectively, during a portion of each thermodynamic cycle, thereby displacing substantially all of the working fluid from the tube set  148   a - 148   d  into the corresponding heat-exchanger  144   a - 144   d , respectively, when the corresponding rigid piston  141   a - 141   d , respectively, is at the bottom of its travel.  
         [0030]    Engine  100  may further include conventional features such as piston rings connected to the rigid pistons  141   a - 141   d  and/or  149   a - 149   d , a cooling system coupled to the cylinder block and/or cylinder heads  146   d  or  146   a - 146   c , and magnetic couplings coupled to the pistons  141   a - 141   d  for generating power. These items have not been shown in the drawings for the sake of clarity.  
         [0031]    According to this exemplary Fluidic-Piston engine embodiment, since the non-adjacent pistons of opposite phase are linked to each other via the linkages  155   a , the effect is that any piston in the power or expansion mode belonging to a given cylinder becomes, during a given half-cycle time, the complementing driving means for the compression or displacement mode of the oppositely disposed piston. Thus, as specifically shown in FIG. 2, pistons  142   a  and  142   b  are in the power or expansion mode, while pistons  142   c  and  142   d  are in the compression or displacement mode. The arrows on the pistons  149   a - 149   d  indicate the directional motion of each oppositely disposed piston at this particular phase in the engine cycle. Due to the inherent operational symmetry of the engine  100 , the reversing of this process occurs during the second half-cycle time for the engine. As such, any pair of linked pistons belonging to two opposite phase cylinders will oscillate substantially in reciprocal register with one another.  
         [0032]    Accordingly, the engine  100  provides a continuously gas dynamically variable inter-cylinder phase relationship as well as a dynamically variable stroke length and compression ratio, each of which are either actively controlled or inherently varied by the thermodynamic conditions in the working fluid to optimize the efficiency of the thermodynamic cycle.  
         [0033]    As may be recognized by those of ordinary skill in the pertinent art, the pistons  141   a - 141   d  and/or  149   a - 149   d  may be substituted with other displacement devices, such as, for example, rotors in a Wankel type rotary engine chamber, for fulfilling the functions of the pistons described herein without departing from the teachings of this disclosure.  
         [0034]    As shown in FIG. 3, an alternate exemplary embodiment Fluidic-Piston engine is indicated generally by the reference numeral  200 . Engine  200  is an external combustion engine similar to engine  100  of FIG. 2, and therefore like reference numerals preceded by the numeral “2” are used to indicate like elements having like functionality. Engine  200  includes a plurality of rigid pistons  241   a - 241   d  disposed in a plurality of corresponding cylinders  240   a - 240   d , respectively. The cylinders  240   a - 240   d  are formed in a cylinder block (not shown), which may include cylinder heads  246   d  or  246   a - 246   c . Each rigid piston  241   a - 241   d  is in contact along its bottom surface with a corresponding fluidic piston  242   a - 242   d , respectively.  
         [0035]    Each cylinder  240   a - 240   d  defines a compression space  243   a - 243   d , respectively, defined by the volume above each fluidic piston  242   a - 242   d , respectively, and below each regenerator  244   a - 244   d , respectively; and an expansion space  245   d  or  245   a - 245   c  defined by the volume above each rigid piston  241   a - 241   d , respectively, and above each regenerator  244   d  or  244   a - 244   c , respectively. The compression space  243   a - 243   d  of each cylinder  240   a - 240   d , respectively, is thermodynamically coupled to a corresponding heat exchanger or regenerator  244   a - 244   d , respectively, and connected to the expansion space  245   a - 245   d , respectively, of an adjacent cylinder  240   b - 240   d  or  240   a , respectively, via associated passageways  247   a - 247   d , respectively. For example, the compression space  243   a  of cylinder  240   a  is connected through passageways  248   a  to the regenerator  244   a , and, in turn, through the passageways  247   a  to the expansion space  245   a  of the adjacent cylinder  240   b.    
         [0036]    The exemplary engine  200  includes a second set of pistons  249   a - 249   d , which are directly connected to pistons  241   a - 241   d , respectively, in an axial relationship. Pistons  249   a - 249   d  are connected to pistons  241   a - 241   d , respectively, via shafts  250   a - 250   d , respectively, passing through shaft ports  251   a - 251   d , respectively, which may be formed entirely within the cylinder block to effectively seal the shaft. The second set of pistons  249   a - 249   d  are housed within cylinders  252   a - 252   d , respectively. Each cylinder  252   a - 252   d  includes a volumetric space  253   a - 253   d , for hydraulic fluid to ingress and egress during each stroke.  
         [0037]    The compressive force is transferred to each piston  249 a 249   d  by the expansive force applied to the oppositely phased piston  249   c - 249   d  and  249   a - 249   b , respectively, acting on a hydraulic fluid circuit  255   a - 255   d , respectively. Thus, the fixed phase relationship is maintained between oppositely phased cylinders  240   a  and  240   c , as well as between  240   b  and  240   d , while using a portion of the kinetic energy produced by the expansion cycle of one cylinder to produce the work required by the compression cycle of an oppositely phased cylinder  240   c - 240   d  or  240   a - 240   b , respectively. Due to the inherent operational symmetry of the engine  200 , the reversing of this process occurs during the second half-cycle time for the engine. Therefore, any pair of linked pistons belonging to two oppositely phased cylinders will oscillate substantially in reciprocal register with one another.  
         [0038]    Turning now to FIG. 4, an alternate exemplary embodiment Fluidic-Piston engine is indicated generally by the reference numeral  300 . The Fluidic-Piston engine  300  primarily differs from engine  200  in that the power valve  254  of FIG. 3 has been relocated from the expansion side of paired power piston  249  of FIG. 3 to the compression side of power piston  241  of FIG. 3. However, new reference numerals will be used for clarity.  
         [0039]    Engine  300  includes a plurality of rigid pistons  36 la- 361   d  disposed in a plurality of corresponding cavities or cylinders  360   a - 360   d , respectively. The cylinders  360   a - 360   d  are formed in a cylinder block (not shown), which may include cylinder heads  371   a - 371   d . Each rigid piston  361   a - 361   d  is in contact along its bottom surface with an hydraulic fluid in a volumetric space  362   a - 362   d . The volumetric spaces  362   a - 362   d  are open to hydraulic valves  363   a - 363   d , respectively, for hydraulic fluid to ingress and egress by pumping action during each stroke. This pumped hydraulic fluid is used by a hydraulic motor (not shown) to generate rotary or other designed output motion.  
         [0040]    Each cavity  360   a - 360   d  defines a compression space  366   a - 366   d , respectively, defined by the volume above a fluidic piston  365   a - 365   d , respectively, and below a regenerator  367   b - 367   d  or  367   a , respectively; and an expansion space  368   a - 368   d  defined by the volume above each rigid piston  361   a - 361   d , respectively, and above each regenerator  367   a - 367   d , respectively. The compression space  366   a - 366   d  of each cavity  360   a - 360   d , respectively, is thermodynamically coupled to the corresponding heat exchanger or regenerator  367   a - 367   d , respectively, and open to the expansion space  368   a - 368   d , respectively, of an adjacent cylinder  360   d  or  360   a - 360   c , respectively, via associated passageways  370   a - 370   d , respectively. For example, the compression space  366   a  of cavity  360   a  is open through passageways  370   a  to the regenerator  367   a , and, in turn, to the expansion space  368   a  of the adjacent cylinder  360   d.    
         [0041]    The exemplary engine  300  includes a second set of pistons  364   a - 364   d , which are directly rigidly connected to pistons  361   a - 361   d , respectively, in an axial relationship. Pistons  364   a - 364   d  are connected to pistons  361   a - 361   d , respectively, via shafts  373   a - 373   d , respectively, passing through shaft ports  372   a - 372   d , respectively, which may be formed entirely within the cylinder block to effectively seal the shaft. The second set of pistons  364   a - 364   d  is housed within cavities or cylinders  369   a - 369   d , respectively. Each cylinder  369   a - 369   d  includes a volumetric space spanned by the fluidic piston  365   a - 365   d , respectively, for the fluidic piston to ingress and egress therefrom during each stroke. The compressive force is transferred to each piston  365   a - 365   d  by the expansive force applied to the corresponding piston  364   a - 364   d , respectively.  
         [0042]    The compressive force is transferred to each piston  364   a - 364   d  by the expansive force applied by the corresponding piston  364   c - 364   d  or  364   a - 364   b , respectively, acting through a hydraulic fluid circuit  375   a - 375   b , respectively. Thus, the fixed phase relationship is maintained between oppositely phased pistons  361   a - 361   d  while using a portion of the kinetic energy produced by the expansion cycle of one cylinder to produce the work required by the compression cycle of an oppositely phased cylinder  360   c - 360   d  or  360   a - 360   b , respectively. Due to the inherent operational symmetry of the engine  300 , the reversing of this process occurs during the second half-cycle time for the engine. Therefore, any pair of linked pistons belonging to two oppositely phased cylinders will oscillate substantially in reciprocal register with one another.  
         [0043]    Thus, in the engine  300 , the piston  364   a - 364   d  is acted on by both the direct expansive force of  361   a - 361   d  in the expansion mode, and the hydraulic fluid from the hydraulic passageways  375   a - 375   b  resulting from the expansive force of pistons  361   c - 361   d  or  361   a - 361   b , respectively.  
         [0044]    This embodiment exhibits improved efficiency because the power output to the hydraulic motor is not first transferred to the coupled piston while incurring frictional losses as in engine  200 , but is received directly from the compression side of piston  364   a - 364   d  while the lesser power required to compress the oppositely phased piston is transferred indirectly via hydraulic passageways  375   a - 375   b , thereby incurring reduced frictional losses due to the reduced mass flow or power transfer. Alternate embodiments based on engine  300  may also utilize the articulated mechanical linkage  155   a  as described for engine  100 .  
         [0045]    As shown in FIG. 5, an alternate exemplary embodiment Fluidic-Piston engine is indicated generally by the reference numeral  400 . Engine  400  is an external combustion engine similar to engine  100  of FIG. 2, and therefore like reference numerals preceded by the numeral “4” are used to indicate like elements having similar functionality. Engine  400  includes a rigid power piston  441  disposed in a first cylinder  440 , and a rigid compression piston  464  disposed in a second cylinder  478 . The cylinders  440 ,  478  are formed in a cylinder block (not shown), which is connected at its upper surface to a cylinder head  446 . Alternatively, the cylinder head  446  may be formed as an integral part of the cylinder block (not shown). The rigid power piston  441  is connected at its lower end to the upper end of a first connecting rod  480 , which, in turn, is connected at its lower end to a crankshaft  482 . The crankshaft  482  is connected to the lower end of a second connecting rod  484  having a fixed rotational phase delay of about 90 degrees relative to the first connecting rod  480 . The second connecting rod  484  is connected at its upper end to the lower end of the rigid compression piston  464 . The rigid compression piston  464  is adjacent at its upper surface to a fluidic piston  442 .  
         [0046]    The cylinder  478  defines a compression space  443  in the volume above the fluidic piston  442  and below a regenerator  444 , which is located at the upper end of the cylinder  478  in this exemplary embodiment. The volume above the rigid piston  441  and above the regenerator  444  defines an expansion space  445 . The expansion space includes the volumes within the elongated hot heat exchanger  447 , which are open at their first ends to the portion of the expansion space immediately above the rigid power piston  441 , and at their second ends to the upper end of the regenerator  444 . The compression space  443  of the cylinder  478  is thermodynamically coupled to the regenerator  444  via a cold heat exchanger  448 . The hot heat exchanger  447  includes three elongated tubes in this embodiment, although any number of shaped passageways may be used in alternate embodiments for fulfilling the functions of the hot heat exchanger described herein. The cold heat exchanger  448  includes five elongated tubes in this embodiment, although any number of shaped passageways may be used in alternate embodiments for fulfilling the functions of the cold heat exchanger described herein.  
         [0047]    In operation, an external combustion heat source is applied to the hot heat exchanger  447  to cause heating of a substantially isochoric volume of working fluid as the power piston  441  travels from about the top of its travel down to about the mid-point of its travel while the fluidic piston travels from about the half-displacement point of its travel to the top of its travel. Next, substantially isothermal expansion of the working fluid takes place as the power piston  441  travels from about the mid-point of its travel to about the bottom of its travel while the fluidic piston  442  travels from about the top of its travel to about the half-displacement point of its travel. Following this expansion, substantially isochoric cooling of this volume of working fluid takes place as the power piston  441  travels from about the bottom of its travel to about the mid-point of its travel while the fluidic piston  442  travels from about the half-displacement point of its travel to about the bottom of its travel, thereby cooling a substantial portion of the working fluid by heat conduction and convection with the cold heat exchanger  448 . Finally, substantially isothermal compression takes place as the power piston  441  travels from about the mid-point of its travel to about the top of its travel while the fluidic piston  442  travels from about the bottom of its travel to about the half-displacement point of its travel, thereby completing the cycle.  
         [0048]    Thus, the fluidic piston  442  substantially increases the thermodynamic efficiency of practical embodiments of Fluidic-Piston Stirling engines by expelling the working fluid from the internal passages of the cold heat exchanger during the substantially isochoric heating phase of the cycle. In alternate embodiments, such as, for example, those utilizing fluidic pistons having very low thermal conductivity and density, the fluidic pistons may also displace working fluid from the regenerators. The fluidic piston  442  is maintained separate from the working fluid by means of gravity. Thus, the fluidic piston has a specific density greater than that of the working fluid, and the cylinder  478  is oriented vertically with the fluidic piston  442  disposed below the working fluid.  
         [0049]    When the expanding working fluid forces the power piston  441  downwards, it causes the connecting rod  480  to apply positive work to the crankshaft  482 . The crankshaft  482  has a finite rotational inertia resulting in storage of kinetic energy in the crankshaft  482 . Once the power piston  411  reaches the bottom of its travel, it begins moving upward by means of an upward force applied by the crankshaft  482  through the connecting rod  480 . At the same time, the fluidic piston begins moving downwards by means of a downward force applied by the high-pressure working fluid through the connecting rod  484  and the compression piston  464 . Once the compression piston  464  reaches the bottom of its travel, it begins to move upward by means of an upward force applied by the crankshaft  482  through the connecting rod  484 . This combination of the upward motion of the power piston  441  and the downward motion of the fluidic piston  442  results in a substantially isochoric phase wherein the volume of the working fluid remains substantially constant while its temperature is reduced via thermodynamic transfer of heat to the regenerator  444  and the cold heat exchanger  448 .  
         [0050]    Although the fluidic piston is a compression piston in the above-described exemplary embodiment, it is contemplated that the fluidic piston may serve as an expansion piston in alternate embodiments. As may be recognized by those of ordinary skill in the pertinent art, other possible configurations of the fluidic piston  442  are contemplated as new fluids are encountered for serving the purposes of the working fluid and/or the fluidic piston. For example, a magnetorheological fluid may be used as the fluidic piston  442  in combination with a magnetic compression piston  464  in order to substantially eliminate the reliance on gravity to keep the fluidic piston intact as shown in this exemplary embodiment. Other fluids may also be substituted for the fluidic piston  442  that would generally exhibit substantial incompressibility, low viscosity, and low thermal conductivity in order to fulfill the functions of the exemplary fluidic piston described herein.  
         [0051]    As may also be recognized by those of ordinary skill in the pertinent art, an alternate fluidic piston may be added directly above the power piston  441  along with geometrical or other minor modifications, if necessary, to prevent the alternate fluidic piston from substantially mixing with the working fluid and/or substantially entering the regenerator  444 . However, the type of fluid used for the alternate piston would have more stringent thermal breakdown requirements than that of the fluidic piston  442  due to its direct contact with the hot heat exchanger  447  and the heated working fluid therein.  
         [0052]    A balancing of the overall engine torque for a four-cycle engine constrains the “optimal” minimum number of like cylinders to four. However, a lesser total number of cylinders is also realizable in alternate embodiment engines if the dynamic balancing requirement is relaxed, as is permissible for lower speed engines. In addition, although the exemplary engine  100  shows piston sets that are substantially identical, the coupling of two un-identical pistons (such as, for example, having different diameters and/or temperature ranges) that share a common linkage is possible and is contemplated as part of this disclosure. In addition, those of ordinary skill in the pertinent art will recognize, based on the teachings herein, that an Ericsson type engine may be provided by incorporating associated pressure valves between the individual passageways  147   a - 147   d  and  148   a - 148   d.    
         [0053]    According to these exemplary embodiments of the FluidicPiston engine, the components of engines  100 ,  200 ,  300  and  400 , such as the heat exchangers and regenerators, may be formed of a high temperature conductive material to increase the overall efficiency by allowing faster and more complete heat transfer to occur between the various critical components. A current example of such a high temperature conductive material is Inconel® or nickel-based alloy steel. In addition, the Fluidic-Piston engine operates at a relatively low frequency in order to reduce viscous losses and enhance the heat exchanger efficiency; typically from an upper frequency of about 50 Hz, with an upper frequency of about 35 Hz desired, and an upper frequency of about 20 Hz more desired; to a lower frequency of about 0.5 Hz, with a lower frequency of about 2.5 Hz desired, and a lower frequency of about 5 Hz more desired.  
         [0054]    Thus, a Fluidic-Piston engine is configured for increased efficiency and useful life. The engine includes mechanically and/or hydraulically coupled piston pairs. A first set of pistons is disposed in a first set of cylinders and a second set of pistons is disposed in a second set of cylinders. A piston in the second set of pistons is mechanically and/or hydraulically connected to a piston in the first set of pistons in an opposing relation, thereby defining a pair of connected pistons. The pair of pistons is thus adapted to oscillate in reciprocal register with one another. Thus, a piston in an expansion mode drives an oppositely disposed piston in a displacement mode. Because adjacent pistons operate independently in that no direct mechanical or hydraulic coupling exists, adjacent pistons are driven based on substantially optimized operating parameters.  
         [0055]    Accordingly, an advantage of embodiments of the Fluidic-Piston engine is the reduced leakage of working fluid at the engine seals due to the additional sealing provided by the liquid to gas barrier of the fluidic piston to working fluid interface. Another advantage of embodiments of the Fluidic-Piston engine is reduced packaging requirements due to the similar vertical orientation of all power pistons made possible by the provision of the non-rigid interconnections between cylinders. Yet another advantage of the Fluidic-Piston engine is increased engine efficiency due to the optimization of the instantaneous phase angle difference permissible between a power piston and an adjacent compression piston acting on the same mass of working fluid. Another, more specific, advantage of the Fluidic-Piston engine is increased kinetic energy output of the engine due to substantially complete evacuation of the working fluid from the cold heat exchanger during the compression and heating phases of the engine cycle. Another more specific advantage of the Fluidic-Piston engine is reduced frictional losses and wear of the seals due to lubrication from the fluid.  
         [0056]    While the invention has been described with reference to exemplary embodiments, it will be understood by those of ordinary skill in the pertinent art that various changes may be made and equivalents may be substituted for the elements thereof without departing from the scope of the disclosure. In addition, numerous modifications may be made to adapt the teachings of the disclosure to a particular object, material or situation without departing from the essential scope thereof. Therefore, it is intended that the Claims not be limited to the particular embodiments disclosed as the currently preferred best modes contemplated for carrying out the teachings herein, but that the Claims shall cover all embodiments falling within the true scope and spirit of the disclosure.