Integral finned heater and cooler for stirling engines
The Government of the United States of America has rights in this invention pursuant to Contract No. DEN 3-32 awarded by the U.S. Department of Energy. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed towards an integral heat exchanger and improved piston and cylinder design for use in an engine, particularly a Stirling engine. 
     BACKGROUND OF THE INVENTION 
     With the renewed interest in Stirling engines there has been ever increasing attempts to improve engine efficiency and reduce cost so as to produce a cost effective product. One of the means of improvements in this regard includes variations in the design of the piston and cylinder head as for example can be seen in U.S. Pat. Nos. 4,271,669 issued June 9, 1981; 3,899,888 issued Aug. 19, 1975; and 3,636,719 issued Jan. 25 1972. These representative engines are an attempt to maximize the isothermal change in condition and thereby improve the degree of thermal efficiency of the engine. However, while such arrangements may be more efficient than prior devices, the designs are relatively complicated, difficult to manufacture, expensive or subject to other disadvantages such as excessive dead volume. This inhibits the engine cycle performance, limiting their application. 
     Accordingly, current Stirling production engines usually utilize a separate heater or cooler volume in addition to the main expansion volume. This practice has the disadvantage of increasing the overall volume resulting in relatively unproductive (thermodynamically) working gas volume and requiring alternative multiple parts which increase the failure probabilities and raise costs. 
     In my copending application entitled &#34;Integral Finned Heater/Cylinder Head for Stirling Engines&#34; filed concurrently herewith, under Ser. No. 423,526, and incorporated by reference, there is disclosed a cylinder head and piston which are formed into an interlocking relationship via a conical finned piston and correspondingly finned internal surface of the cylinder head at the hot end of the cylinder. The piston is hollow having a regenerator therein through which a working gas can pass eliminating a portion of the excess working volume attributable to a separate regenerator. 
     The present invention is directed towards further improving on this concept. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide for an engine cylinder with a mating piston which is also a thermally efficient heater and cooler, yet relatively simple in construction and with relatively fewer, more durable parts that do not require additional volume of working gas. 
     It is a further object of the invention to provide an engine cylinder and a mating piston which may include a regenerator and a finned cooler which minimizes dead volume and manufacturing expense. 
     The present invention provides a cylinder head and piston for an engine, particularly a Stirling engine, which are formed in an interlocking, meshing or nesting relationship via a conical finned piston and a corresponding conical finned internal surface of the cylinder head. The interlocking fins provide large surface areas having close proximity to the working gas to allow for both superior heating of the working gas in the hot end of the working chamber and cooling of the working gas in the cold end of the chamber. The fins also serve to provide flow channels or guides for the combustion gas and cooling fluid about the cylinder and working gas therein. 
     Further, the oblique motion of the piston fins with the interlocking head fins produce an effectively narrow, deep passage for efficient heat transfer yet allow for a shallower, relatively easy construction of the fins. Also, this motion allows for an effective longer piston stroke for a given fin height without the disengagement of mating fins so as to maximize the thermal transfer. 
     Further, the large surface to volume ratio maximizes heat exchange and the cone-like configuration improves structural integrity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Thus by the aforenoted invention its objects and advantages and others will be realized, the description of which should be taken in conjunction with the following drawings wherein: 
     FIG. 1 is a partially cross sectional view of a cylinder head with mating piston having an integral heat exchanger, regenerator and finned cooler, incorporating the teachings of the present invention; 
     FIG. 2 is a sectional view of the piston incorporating the teachings of the present invention; 
     FIG. 3 is a top sectional view of the piston taken along lines 3--3 of FIG. 2; 
     FIG. 4 is a bottom sectional view of the piston taken along lines 4--4 of FIG. 2; 
     FIG. 5 is an enlarged partial view of the mating relationship between the cylinder head and the piston in the hot end of the working chamber; and 
     FIG. 6 is an enlarged partial view of the mating relationship between the cylinder head and the piston at the cold end of the working chamber. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference now to the drawings, there is shown in FIG. 1 a cylinder head 10, particularly one for a Stirling engine, which comprises cylinder housings 12 and 14. Disposed in the housings 12 and 14 are pistons 16 and 18 respectively, operationally coupled in a typical Stirling engine manner. Note that while only two cylinders are shown, they are used merely as an example since the present invention may be incorporatedd into a single or multi-cylinder engine (preferably 3-6 cylinders, for proper phase angles) with slight modifications, as will be apparent to those skilled in the art. Pistons 16 and 18 are identically constructed as illustrated in FIG. 2 and comprise an upper conical member 20 coupled to a cylindrical portion 22 which may be integral therewith, which in turn is coupled to a lower conical member 24. Upper and lower conical members 20 and 24 are to be positioned in the hot and cold end respectively of the cylinder as will be apparent. 
     Located on the entire exterior surface 26 of the conical member 20 are a plurality of vertical or axially-orientated piston fins 28, which are seen more clearly with reference to FIGS. 3 and 5. The piston fins 28 extend the length of the conical member 20 forming a tip or apex 30 and terminating at portion 22. The individual piston fins 28 themselves may be solid and integral with the conical member 20 if so desired. The fins are generally formed in a wedge shape starting from a radially outward edge 32 and flaring radially inwardly to a base 34 on the exterior surface 26. Between the adjacent bases 34 of the fins 28 there is provided a series of perforations 36 (see particularly FIG. 5) along the length of the conical member 20 to allow access between the piston exterior surface 26 and the conical hollow interior surface 38, which is defined by a conical hollow therein. Advantageously, located within the hollow is a regenerator 40 which may be constructed of standard regenerator material. 
     The ingress and egress of working fluid to the regenerator 40 is assisted (aside from the perforations 36) by way of the piston coupling to the interior of cylindrical portion 22. Access from the interior to the exterior surface 42 of the cylindrical portion 22 is via channel 44 with opening(s) 46 for the channel 44 disposed between adjacent sealing rings 48 and 49, positioned on exterior 42 which provide a sliding seal between the cylindrical portion 22 and the adjacent cylindrical interior surface 50 of the head. 
     The lower conical member 24 is shown solid and integral with cylindrical portion 22. This member 24 also includes a plurality of fins 52 on its outer exterior surface 54 as shown by FIGS. 4 and 6. Similarly, these fins 52 extend the length of the conical member 24 from cylindrical portion 22 to shaft 56, which is coupled to a crank shaft, for example, (not shown). A sealing ring 58 is provided and is slidably disposed between shaft 56 and channel surface 59 to effect a sliding seal therebetween. 
     In the cylinder head 10, the respective cylinder housings 12 and 14 may be enclosed by a pressure vessel with respective combustion gas inlet and outlet ports 60 and 62 provided through which passes hot combustion gas or liquid from the combustor or heater (not shown). 
     The internal cavity surface 64 of the upper or hot end of the cylinder housing, is formed of a conical hollow larger than the adjacent upper portion of the piston to allow the movement therein of conical member 20. On surface 64 is positioned a plurality of hollowed vertical fins 66 which are coupled to the inlet port 60 and outlet port 62. These fins 66 correspond in a mating or nesting-like fashion to piston fins 32 in an alternating fashion as shown most clearly in FIG. 5. Note that the fins 66 are supported by the pressure vessel (not shown) of cylinder head 10 which serves to pick up the hoop stress thereon. Due to radial loading of the pressure on the head&#39;s interior, the fin walls 68 can be fabricated out of relatively thin material. This allows for effective and more nearly isothermal transfer from the combustion gas passing through the interior of fins 66 to the working gas which is positioned in the space between fins 28 and 66, generally indicated at 70. Note that the interlocking fins 28 and 66 provide large working surfaces within close proximity to the working gas so as to provide for efficient heat exchange from the combustion gas. Fins 66 also provides a flow channel for the combustion gas. 
     In this regard, as can be seen in FIG. 1, combustion gas enters the head 10 via port 60 and passes through fins 66 and out port 62. The heat exchanged through fins 66 causes a heating or expansion of the working gas in the adjacent cylinder head 12 in space 70, causing the working gas to expand against the piston forcing it downward. Due to the perforations 36 in the piston a short path to the next element, that being the regenerator 40 is established advantageously avoiding excessive shear pressure which would result if the heated working gas was required to travel from the piston nose tip 30 along the entire outside of the piston 12. 
     The piston fin 28 and head fin 66 arrangement (i.e., conically and obliquely engaged) allows the piston to have a relatively short stroke for longer engagement therebetween providing obviously more effective thermal coupling. 
     With reference now to the lower or cold end of the cylinder housing there is provided an interior cavity or chamber surface 72 which is similarly formed out of a conical hollow larger than the adjacent portion of the piston to allow movement therein of conical member 24. On surface 72 there are disposed hollow cooling fins 74 in which passes a cooling fluid via inlet and outlet ports 76 and 78 (see particularly FIGS. 1 and 6). Note, the advantages of this arrangement are the same as that set forth in the discussion regarding the hot end and the piston may be integrally constructed as a monolithic structure to reduce its cost. 
     The cold end of the cylinder receives cyclic working gas from the hot end of the adjacent cylinder via channel(s) 80. During each piston cycle, one end of channel 80 is disposed adjacent opening 46 of channel 44, between seals 48 and 49, and an opposite end of 80 is disposed adjacent chamber 72, below seal 48. The working gas now passes through a cooling space 82 which is defined by fins 52 and 74 which results in cooling the working gas. In operation, the working gas is shuttled back forth from the heated end of one cylinder to the cooler end of another by the pistons which reciprocate axially within the housing generating the periodic pressure wave in the working gas. The working gas movement results in useful work within typical Stirling engine principles. 
     Thus by the aforenoted invention its objects and advantages are readily realized and although a preferred embodiment has been disclosed and described in detail herein, its scope should not be limited thereby, rather its scope should be determined by that of the appended claims.