Abstract:
A method for sealing a working zone in a cylinder of a thermal cycle engine. The method includes providing a seal ring within a ring groove of the piston, and, additionally, applying a backing force on the seal ring by means of a backing ring having a width that varies with circumferential displacement from a fiducial circumferential position on the backing ring. The backing force is provided by means of a backing ring that has a non-circular shape in an uncompressed condition.

Description:
[0001]    This application claims priority from U.S. provisional patent application serial No. 60/299,696, filed Jun. 20, 2001, entitled “Piston Ring Improvements for a Stirling Engine” and bearing attorney docket number 2229/108, the disclosure of which is incorporated herein, in its entirety, by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention pertains to methods for sealing a working zone within a cylinder of a thermal cycle engine, and to piston rings for a thermal cycle engine and more particularly to rings that provide sealing for non-lubricated pistons.  
         BACKGROUND OF THE INVENTION  
         [0003]    Stirling cycle machines, including engines and refrigerators, have a long technological heritage, described in detail in Walker,  Stirling Engines,  Oxford University Press (1980), incorporated herein by reference. The principle underlying the Stirling cycle engine is the mechanical realization of the Stirling thermodynamic cycle: isovolumetric heating of a gas within a cylinder, isothermal expansion of the gas (during which work is performed by driving a piston), isovolumetric cooling, and isothermal compression.  
           [0004]    Stirling cycle engines have not generally been used in practical applications, due to several daunting engineering challenges to their development. These involve such practical considerations as efficiency, vibration, lifetime, and cost.  
           [0005]    The advantages of a welded structure for containment of the high pressure working gas employed in a Stirling engine emphasize the need for long-lived components. The piston seals, which are intended to prevent the flow of working gas past the piston, must seal against the wall of the cylinder while in motion parallel to the wall. The seal must be provided in the face of wear of the rings due to friction, compounded in the case of Stirling engines which, due to the extreme operating temperatures, are typically not lubricated. Thus a unique requirement exists for a mechanism to seal a moving interface under extreme thermal conditions in the absence of lubrication.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with preferred embodiments of the present invention, a method is provided for sealing a working zone in a cylinder of a thermal cycle engine. The method includes providing a seal ring within a ring groove of the piston, and, additionally, applying a backing force on the seal ring by means of a backing ring having a width that varies with circumferential displacement from a fiducial circumferential position on the backing ring. In accordance with an alternate embodiment of the invention, the backing force is provided by means of a backing ring that has a non-circular shape in an uncompressed condition.  
           [0007]    In accordance with further embodiments of the invention, the seal ring, in either of the above cases, may be closed at a single lap-joint for providing a pressure seal. Additionally, the motion of the piston within the cylinder of a thermal cycle engine may be guided, in accordance with other embodiments of the invention, by providing a guide ring surrounding the piston at a position displaced axially from a plane in which sealing against the wall of the cylinder is achieved. A further embodiment of the invention combines the seal ring as described with a separate guide ring.  
           [0008]    In accordance with another embodiment of the invention, a piston seal for sealing a working zone in a cylinder of the thermal cycle engine includes a seal ring disposed within a ring groove of a piston capable of driven motion within the cylinder and a backing ring for applying a backing force on the seal ring. The backing ring is in contact with an inner surface of the seal ring and has a width that varies with circumferential displacement from a fiducial circumferential position on the backing ring. In accordance with another embodiment of the invention the backing ring has a non-circular shape in an uncompressed condition. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which:  
         [0010]    [0010]FIG. 1 is a side view in cross section of a Stirling cycle engine in which the embodiments of the present invention may advantageously be applied;  
         [0011]    [0011]FIG. 2 is a partial cross sectional view of a piston with a seal ring, backup ring, and guide ring, in accordance with preferred embodiments of the present invention;  
         [0012]    [0012]FIG. 3 a  shows a top view of a circular backup ring, shown as uncompressed and under compression;  
         [0013]    [0013]FIG. 3 b  is a diagram indicating the radial thrust forces exerted by the seal ring on the cylinder wall by virtue of the ring embodiment of FIG. 3 a;    
         [0014]    [0014]FIG. 4 is a perspective view of a seal ring and a backup ring of tapered width in accordance with an embodiment of the present invention;  
         [0015]    [0015]FIG. 5 a  shows a top view of an oval backup ring, in accordance with an embodiment of the present invention, showing a circular shape upon compression;  
         [0016]    [0016]FIG. 5 b  is a diagram indicating the radial thrust forces exerted by the seal ring on the cylinder wall by virtue of the ring embodiment of FIG. 5 a;  and  
         [0017]    [0017]FIG. 6 shows a perspective view of the guide ring, in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0018]    Many mechanical layouts of Stirling cycle engines are known in the art, and the particular Stirling engine designated generally by numeral  10  is shown merely for illustrative purposes. A piston  121  (otherwise referred to herein as a “expansion piston”) and a second piston (also known as an “compression piston,” and not shown in FIG. 1) move in phased reciprocating motion within separate, interconnected, cylinders. Piston seals  14  prevents the flow of a working fluid contained within expansion volume  98  from escaping around piston  121 . Other key components of the Stirling cycle engine are labeled and are described in detail in related application, U.S. Ser. No. 09/517,245, filed Mar. 2, 2000, which is incorporated herein by reference.  
         [0019]    Expansion volume  98  is surrounded on its sides by expansion cylinder liner  115 , disposed, in turn, inside heater head  100  and typically supported by the heater head. The expansion piston  121  travels along the interior of expansion cylinder liner  115 . As the expansion piston travels toward closed end  120  of heater head  100 , the working fluid within the heater head is displaced and caused to flow through flow channels defined by the outer surface of the expansion cylinder liner  115  and the inner surface of heater head  100 .  
         [0020]    [0020]FIG. 2 shows a partial cross section of piston  121 , driven along central axis  20  of cylinder, or cylinder liner  22 . Seal ring  24  provides a seal against wall  26  of the cylinder. Wall  26  is typically a hardened metal (preferably 60-62 RC) such as steel finished to a smooth surface finish. A backing ring  28  (also referred to herein as a ‘back-up ring’) is sprung to provide a thrust force against the seal ring thereby providing sufficient contact pressure to ensure sealing around the entire outward surface of the ring. Seal ring  24  and backing ring  28  may together be referred to as a composite ring.  
         [0021]    The material of seal ring  24  is chosen by considering a balance between the coefficient of friction of the seal ring against the cylinder wall and the wear on the ring it engenders. In applications in which piston lubrication is not possible, such as at the high operating temperatures of a Stirling cycle engine, the use of engineering plastic rings is preferred. The preferred composition is a nylon matrix loaded with a lubricating and wear-resistant material. Examples of such lubricating materials include PTFE/silicone, PTFE, graphite, etc. Examples of wear-resistant materials include glass fibers and carbon fibers. Examples of such engineering plastics are manufactured by LNP Engineering Plastics, Inc. of Exton, Pa. Backing ring  28  is preferably a metal.  
         [0022]    The fit between seal ring  24  and seal ring groove  30  is preferably a clearance fit (˜0.002″), while the fit of backing ring  28  is preferably a looser fit, of the order of ˜0.005″. Seal ring  24  provides a pressure seal against cylinder wall  26  and also one of the surfaces  32  of the seal ring groove  30 , depending on the direction of the pressure difference across the ring and the direction of piston travel.  
         [0023]    [0023]FIGS. 3 a  and  3   b  show that if the backing ring is essentially circularly symmetrical, but for gap  30 , it will assume, upon compression, an oval shape, as shown by dashed backing ring  36 . The result may be an uneven thrust force exerted on the seal ring, and thus an uneven pressure of the seal ring against the cylinder wall, causing uneven wear of the seal ring and premature failure of the seal.  
         [0024]    A solution to the problem of uneven thrust force, in accordance with an embodiment of the invention, is a backing ring  28  having a cross-section varying with circumferential displacement from the gap  30 , as shown in FIG. 4. A tapering of the width is shown from the position denoted by numeral  40  to the position denoted by numeral  42 . Also shown in FIG. 4 is a lap joint  44  providing for circumferential closure of seal ring  24 . As some seals will wear significantly over their lifetime, backing ring  28  should supply an even pressure of a range of movement. The tapered backing ring  28  shown in FIG. 4 may provide this advantage.  
         [0025]    [0025]FIG. 5 illustrates another solution to the problem of uneven thrust force of the seal ring against the cylinder wall, in accordance with a further embodiment of the present invention. As shown in FIG. 5 a,  backing ring  50  is fashioned in an oval shape, so that upon compression within the cylinder, the ring assumes the circular shape shown by dashed backing ring  52 . A constant contact pressure between the seal ring and the cylinder wall may thus be provided by an even radial thrust force of the backing ring, as depicted in FIG. 6 a.    
         [0026]    Referring again to FIG. 2, a guide ring  50  may also be provided, in accordance with embodiments of the present invention, for bearing any side load on piston  121  as it moves up and down the cylinder. Guide ring  50  is also preferably fabricated from an engineering plastic material loaded with a lubricating material. A perspective view of guide ring  50  is shown in FIG. 6. An overlapping joint  52  is shown and may be diagonal to the axis  54  of the ring, for example.  
         [0027]    The devices and methods described herein may be applied in other applications besides the Stirling engine in terms of which the invention has been described. The described embodiments of the invention are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.