US 20030010200 A1
A piston engine has at least one piston which is mounted to be movable in a cylinder in a linear manner towards a cylinder head and a deformable element extends around the piston between the piston and the cylinder.
1. A piston engine comprising:
a cylinder having a cylinder head;
drive means for moving the piston in the cylinder in a linear manner toward and away from the cylinder head; and
a deformable element extending around the piston between the piston and the cylinder wherein the deformable element forms (1) a sealed working space for the piston and (2) a linear guide for the piston.
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 The present invention relates to a piston engine, especially a compressor or Stirling engine, having at least one piston which is mounted to be movable in a cylinder in a linear manner toward a cylinder head, the piston being connected to at least one drive element, especially a crankshaft.
 Such piston engines are known and obtainable in the market in very different shapes and designs. They are known as conventional Stirling engines, compressors that are driven by means of a medium or that compress any desired medium.
 A disadvantage of such piston engines that they usually do not operate without oil and are subject to severe wear. In addition, sealing problems frequently arise with excessive energy consumption.
 Frequently, such piston engines exhibit large dead volumes, which is likewise undesirable, as these impair the efficiency. In addition, these are subject to a high degree of wear, caused by friction, which is likewise undesirable.
 A further disadvantage is that these piston engines exhibit low efficiencies as a result of sealing problems and have only a limited pressure resistance.
 Furthermore, combustion engines and compressors are known in which pistons are reciprocally movable in cylinders in a conventional manner with oil lubrication. In the food industry, however, oil lubrication of the piston in piston engines is undesirable, as it also is in equipment used in medical technology. In Stirling engines, also, oil-lubricated pistons are highly disadvantageous, as their regenerators, like many small heat exchangers, would be rendered useless by coking or oil mist after a short service life.
 In the oil-free, conventional piston engines, especially coated cylinder and piston running surfaces and piston rings made from Teflon alloys are generally used, but these are subject to severe wear and then have only a short service life. In addition, they are also suitable and usable only within limited temperature ranges.
 A further problem with conventional piston engines is that the sealing of the cylinder relative to the crankcase, especially for the compression of noble gases such as, for example, helium and other gases requires an enormous expenditure of energy, particularly in the case of Stirling engines and in the case of compressors. It is therefore frequently necessary either for gas losses arising to be continuously replaced or for the crankcase additionally to be hermetically sealed and of pressure-resistant design, which is again undesirable.
 In order to prevent gas loss in coolant compressors, it is conventionally customary to install the motor and compressor units in a sealed housing, which requires elaborate and costly apparatus.
 It is an object of the present invention to provide a piston engine of the type referred to initially which eliminates the above-mentioned disadvantages and by means of which efficiencies can be improved, the dead volumes reduced, friction eliminated and the use of lubricant avoided.
 The foregoing object is achieved wherein an elastic, deformable element extending around the piston is inserted between piston and cylinder or cylinder head.
 In the present invention, the elastically deformable elements which guarantee a contact-free movement of a piston in a cylinder are inserted between piston and cylinder or cylinder wall and form a hermetically sealed pressure-resistant and leaktight working space. At the same time, the elastically deformable element forms a linear guide for the piston, which is reciprocally movable without contact in the cylinder or along the cylinder wall. The element preferably consists of spring elements, which are formed from a stack of rings in the form of spring washers, or the like. These spring elements are reciprocally connected, preferably welded or connected in a pressure-resistant manner by means of hard solder or even adhesively bonded, to one another, alternately at the internal and external radius. The piston is preferably coaxially fixed on the cylinder via the spring elements in such a way that the gas volume between the spring elements is likewise expelled during the compression movement of the piston. As a result, there now remains a very small dead volume in the cylinder which approximately corresponds to the minimum play between piston and cylinder wall, as a result of which a high sealing ratio can be achieved. Since, with this arrangement of the spring elements, no dead volume or unused stroke space arises, the number of stacked spring elements or spring washers can be adapted to the desired piston stroke as required.
 In the shaping of the spring washers, it is likewise possible for these to achieve a large spring travel for each element, to guarantee high pressure resistance and also to withstand long-term loading with high stroke frequencies.
 When the individual spring elements, especially the spring washers, are compressed, the internal radius becomes smaller and the external radius larger, as a result of which the material is primarily compressively stressed in the radial direction, which results in very high rigidity. The effect of this is to ensure that the piston can be reciprocally moved in the cylinder or within the cylinder wall in a linearly centered manner and without contact.
 In order to connect spring washers fixedly to one another, they have to be so shaped that they possess a flat connecting surface at the internal and at the external radius, as a result of which the rigidity can be further increased and/or a spring travel can then be restricted. This is also intended to come within the scope of the present invention.
 However, in order to obtain a large spring travel, spring elements of cross-sectionally sinusoidal form may be used, so that a resilience in the radial direction is obtained which withstands enormous tensile and/or compressive stresses and permits a stroke movement by means of low bending forces.
 In the case of the present invention, a piston engine is provided which permits a large number of embodiments and possible applications. As an amount of play is provided between piston and cylinder wall, it is unnecessary for any costly close tolerances to be observed during manufacture, which significantly minimizes the production costs. Cylinder and piston can therefore be produced particularly cost-effectively from, for example, deep-drawn steel sheets. In addition to cylindrical shapes, other shapes, such as for example conical shapes, are also possible. The corresponding piston bases and cylinder heads can then be adapted in accordance with that shape and, to minimize the moved masses, especially of stable shapes, be formed as cones or shapes of hemispherical form. This is also intended to be included within the concept of the present invention.
 Further advantages, features and details of the invention will be apparent from the description of preferred examples of embodiment that follows and by reference to the drawings, in which:
FIG. 1 shows a diagrammatically illustrated cross section through a component of the piston engine;
FIGS. 2a to 2 d show diagrammatically illustrated examples of embodiment of components of the piston engine in accordance with FIG. 1;
FIGS. 3a to 3 d show diagrammatically illustrated cross sections through possible examples of embodiment of elastic elements, especially spring elements;
FIG. 4 shows a diagrammatically illustrated partial cross section through a further example of embodiment of a piston engine, especially a Stirling engine.
 According to FIG. 1, a piston engine R1 according to the invention comprises a crankcase 1 in which two pistons 2, arranged opposite one another, are reciprocally movable along a cylinder 3, especially along a cylinder wall without contact and kept apart from one another by a spacer 4. At least one piston 2 is connected via a connecting rod 5 to a working element, preferably to a crankshaft 6.
 At its end faces, the cylinder wall 3 is adjoined at each end by a cylinder head 7. In a preferred example of embodiment, inlet lines 8 and outlet lines 9 open into the cylinder head 7, but these may also be designed as inlet valves or outlet valves.
 It is important in the present invention that the at least one piston 2 is reciprocally movable in a linear manner in the cylinder wall 3, as represented by the double-arrow direction X. For this purpose, according to the invention, an elastically deformable element 10 is inserted between the cylinder head 7 and the piston 2, this being fixedly connected, preferably welded, at one end to the cylinder head 7 and fixedly connected, especially welded, at the other end to a flange 11 or shoulder 12.
 It is important in the present invention that the elastic element 10 coaxially encompasses a piston wall 13 and completely and hermetically seals off a working space 14. The working space 14 is defined by the cylinder head 7, the piston 2 and the elastic element 10. The crankcase 1, by contrast, is free of pressure and may or may not be open to the environment.
 The elastic element 10 additionally has the function of providing a bearing for, or centering, the piston 2, without contact to be linearly movable in the X direction shown so that the piston 2 is mounted to be movable in the double-arrow direction X shown in a manner completely sealed off from the cylinder head 7. Preferably, the elastic element 10 is welded fixedly to the cylinder head 7 or the cylinder wall 3 at one end and to the flange 11 or shoulder 12 of the piston 2 at the other end.
 If, as shown in FIG. 1, the upper piston 2 moves toward the upper cylinder head 7, a dead volume of the working space 14 at top dead center 15 can be reduced, or is virtually zero.
 At the same time, the elastic element 10 is compressed by the movement of the piston 10 toward the cylinder head 7, so that a dead volume of the element 10 is likewise reduced, or is virtually zero.
 Preferably, the element 10 is formed from individual spring elements 16, which can be compressed.
 When the individual spring elements 16 lie one upon the other in the compressed state, they can simultaneously form a stop, especially a damper of the top dead center 15, for the piston 2.
 It is important, however, that the individual spring elements 16 are welded or soldered fixedly to one another and circumferentially, or adhesively bonded to one another. These are connected to one another in a manner resistant to high temperatures and pressure.
 As a result, the piston 2 can be reciprocally moved relative to the cylinder head 7 in the cylinder 3 with complete freedom from friction, absence of contact and freedom from play.
 As a result, a high pressure can be built up in the working space 14, the piston 2 being capable of creating an oil-free compression in the working space 14.
 A further advantage of the present invention is that an absolute seal is formed between the piston 2 and the elastic element 10 and the cylinder head 7, the piston 2 being simultaneously linearly movable, radially centered and mounted within the elastic element 10.
 In a further example of embodiment of the present invention, as shown in FIGS. 2a to 2 d, individual pistons 2 are guided within cylinder walls 3 with adjoining cylinder head 7, the shape of the piston 2 approximately corresponding to the shape of the cylinder head 7.
 In this case, as is shown in particular in FIG. 2a, the piston 2 may be flat and may be moved onto a flat cylinder head 7 in the double-arrow direction X shown, in order, for example, to compress the volume in the working space 14 or, in the event of superatmospheric pressure in the working space 14, to drive the piston 2. The present invention is not confined hereto.
FIGS. 3a to 3 d each show examples of embodiment of the elastically deformable element 10, the elastic element 10 being composed in each case of individual spring elements 16. Preferably, spring elements 16 in the form of spring washers are laid alternately one upon the other and circumferentially and fixedly welded to one another alternately at their external radius 17 and internal radius 18 so that they are connected to one another in a pressure-resistant and hermetically sealed manner.
 In this case, the individual spring elements 16 can also be adhesively bonded or hard-soldered to one another. The invention is not confined hereto.
 The spring elements 16 may also be provided in the form of flat spring washers and spring elements 16 of curved form. The invention is not confined hereto. Preferably, however, spring elements 16 are used which are of the same form and consistency, so that when subjected to pressure they rest one upon the other and no volume is formed between them. As a result, the dead volume in the compressed state or at top dead center 15 of the piston 2 close to the cylinder head 7 can likewise be substantially reduced.
 In the last example of embodiment of the present invention, as shown in FIG. 4, the piston engine R2 is illustrated as a Stirling engine having two pistons, the crankcase 1 being formed without pressure and the pistons 2 being reciprocally movable toward the latter in the cylinder walls 3 in an oil-free manner.
 If the elastically deformable element 10 is compressed, no dead volume is formed, which substantially increases the efficiency of the piston engine R1, R2 and, in addition, the piston 2 is guided to be reciprocally movable with complete freedom from friction in the cylinder 3 toward the cylinder head 7 and is radially mounted by the element 10.
 The upper cylinder head 7 is adjoined by a heater 19, which is adjoined by a regenerator 20 and then by a cooler 21, which is in connection with the second cylinder head 7.
 The more detailed manner of operation of the Stirling engine will not be dealt with here; this is described and known in the prior art in many different forms and embodiments.
 It is important, however, in the present invention that, as a result of the mode of operation of the piston 2, especially as a result of its mounting by means of the element 10 and its guidance, a dead volume is minimized. The piston 2 can be moved toward the cylinder head 7 without play or contact. The piston engine of this type can be used, for example, as a compressor, as a Stirling engine or as a heat pump, and especially as an oil-free compressor. It also serves to compress gases in, for example, the chemical industry, where no oil-containing components need be present in the engine.
 It may also serve as a cooling unit, in which case any coolant can be processed or compressed here. This is likewise intended to be included in the scope of the present invention.