Abstract:
A piston-cylinder unit including a piston ( 103 ) that is fluid pressure supported and movable in a linear manner in a cylinder ( 2 ), wherein the cylinder ( 2 ), a face wall ( 116 ) of the piston ( 103 ) and a face wall ( 12 ) of the cylinder define a compression cavity ( 18 ) which is at a minimum size in a portion of a top dead center (TDC) of the piston ( 103 ), wherein the compression cavity ( 18 ) is connected in a fluid transferring mariner with a bearing gap ( 19 ) which is formed between a cylinder inner circumferential wall ( 14 ) and a piston outer circumferential wall ( 136 ), wherein a plurality of fluid outlet nozzles ( 32′, 34 ′) are arranged in at least one cross-sectional plane (Q 2 , Q 3 ) of the cylinder ( 2 ) in the cylinder inner circumferential wall ( 14 ) along a circumference, which fluid outlet nozzles ( 32′, 34 ′) open into the bearing gap ( 19 ) and are connected with a supply conduit for a pressurized fluid, and wherein a plurality of fluid outlet nozzles ( 130 ′) which open into the bearing gap ( 19 ) are arranged in at least one cross-sectional plane (Q 1 ) of the piston ( 103 ) adjacent to the piston face wall ( 116 ) in the piston outer circumferential wall ( 136 ) along the circumference, characterized in that the fluid outlet nozzles ( 130 ′) in the piston outer circumferential wall ( 136 ) are also connected with the supply conduit for the pressurized fluid.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of International patent application PCT/DE2013/100171 claiming priority from German patent applications DE 10 2012 104 163.6, filed on May 11, 2012, DE 10 2012 104 164.4, filed on May 11, 2012, and DE 10 2102 104 165.2, filed on May 11, 2102, all of which are incorporated in their entirety by this reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The instant invention relates to a piston cylinder unit with a piston that is fluid pressure supported and moveable in a linear manner in a cylinder according to the preamble of patent claim  1 . 
       BACKGROUND OF THE INVENTION 
       [0003]    In piston cylinder units of this type there is a risk when a pressure in a compression cavity is greater than a pressure in a bearing gap that the piston is laterally tilted from a position that is coaxial with the cylinder wherein the tilting is caused by fluid from the compression cavity that enters the bearing gap asymmetrically. This changes a thickness of the bearing gap at least partially and rapidly reduces a load bearing capability of the fluid pressure bearing between the cylinder and the piston. In particular when the piston-cylinder unit is configured as a compressor tilt stability for the piston has to be provided. 
         [0004]    DE 10 2004 061 904 A1, whose disclosure is incorporated in its entirety by this reference discloses a piston-cylinder unit which provides increased tilting stability for the piston. This known piston cylinder unit is illustrated in  FIG. 1  as prior art. This figure illustrates a longitudinal sectional view through a piston cylinder unit  1  with a cylinder  2  and a piston  3 . The cylinder is provided with a cylinder bore hole  10  in which the piston  3  is moveable back and forth in a direction of a longitudinal axis X of the cylinder bore hole  10  and received freely supported. The piston  3  is connected through a piston rod  4  with an input or an output which are not illustrated. The cylinder face wall  12  that is configured at a cylinder head  23  and which forms the face side termination of the cylinder bore hole  10 , the inner circumferential wall of the cylinder bore hole  10  and the piston face wall  16  define the cylinder volume and define a pressure cavity  18 . 
         [0005]    An inlet channel  22  provided with a schematically illustrated valve  20  leads into the cylinder face wall  12 . An outlet channel  24  is also provided in the cylinder face wall  12 , wherein the outlet channel  24  also includes a respective outlet valve  26 . Also this outlet channel leads into the cylinder bore hole  10 . 
         [0006]    When the piston is moved in  FIG. 1  to the right up to the dashed position of the piston  3  where it reaches its top dead center TDC where it reverses its movement direction, the fluid that is included in the cylinder volume  18  and which is for example gaseous is compressed when the piston-cylinder unit is a compressor. The cylinder volume  18  then forms a compression cavity. When the outlet valve  26  opens the compressed fluid flows out of the compression cavity  18  through the outlet channel  24 , for example to downstream consumers. 
         [0007]    A portion of the expelled fluid is conducted out of the outlet channel  24  through a connection channel  28  that is provided in the cylinder head  23  and in the housing  21  of the cylinder  2  into ring channels  30 ,  32 ,  34  which are also provided in the housing  21  of the cylinder  2  and which envelop the cylinder bore hole  10  in an annular manner. The ring channels  30 ,  32 ,  34  are offset from one another in a direction of the longitudinal axis X of the cylinder bore hole. Each of the ring channels  32 ,  33 ,  34  is provided with a plurality of micro holes  30 ′,  32 ′,  34 ′ which are evenly distributed over a circumference of the cylinder bore hole  10  and respectively connect the ring channel  32 ,  33 ,  34  with an interior of the cylinder bore hole  10  and thus penetrate the inner circumferential wall  14  of the cylinder. The micro holes  30 ′  32 ′,  34 ′ of each ring channel  30 ,  32 ,  34  thus respectively form an annular nozzle arrangement  30 ″,  32 ″  34 ″. Pressurized fluid, advantageously pressurized gas which is conducted through the connection channel  28  into the ring channels  30 ,  32 ,  34  can thus exit through the micro holes  30 ′,  32 ′  34 ′ and can form a fluid cushion for example a gas cushion laterally supporting the piston in a bearing gap  19  between a cylinder side bearing surface  15  on an inner circumferential wall  14  of the cylinder  2  and a piston side support surface  38  on an outer circumferential wall of the piston. 
         [0008]    The first ring channel  30  that is most proximal to the cylinder face wall  12  and includes associated micro holes  30 ′ is arranged in a portion in which the piston only covers the micro holes  30 ′ only when the piston is arranged proximal to the compression position, thus the top dead center, thus when the cylinder volume  18  is minimized. In this case the piston  3  covers the forward first micro holes  30 ′ with the bearing surface  38  in a front portion  3 ″. This way it is assured that the piston section which is adjacent to the piston face wall  16  is laterally stabilized in its position proximal to the top dead center TDC, so that a risk that the piston is laterally displaced by fluid entering the bearing gap from the compression cavity is essentially excluded. 
         [0009]    The second ring channel  22  is arranged so that micro holes  32 ′ associated therewith are always covered by the moving piston  3  so that the micro holes  32 ′ help to form the supporting gas cushion between the inner circumferential wall  14  of the cylinder  2  and the outer circumferential wall  36  of the piston  3  over an entire axial movement path of the piston  3 . 
         [0010]    The third ring channel  34  is the furthest away from the cylinder face wall  12 . The micro holes  34 ′ associated with the third ring channel  34  are thus covered by the piston  3 , thus by the support surface  38  in the rear portion  3 ′ of the piston only when the piston  3  is in its retracted position in which the cylinder volume is at a maximum. 
         [0011]    This known piston-cylinder unit supports the piston in its forward circumferential portion also in its top dead center position but it cannot be excluded that pressurized fluid that enters the bearing gap from the compression cavity  18  imparts a lateral force upon the piston because a distance between the piston face wall and the impact location of the pressurized bearing fluid exiting from the micro holes  30 ′ varies at the piston circumference due to the piston movement. 
         [0012]    DE 10 2008 007 661 A1 shows a linear compressor with a piston-cylinder unit whose piston is driven by a linear motor to perform a reciprocating movement. The piston is gas pressure supported in the cylinder and the cylinder wall is provided with a plurality of nozzle openings for this purpose. The piston is provided with a plurality of slanted bore holes or radial slots at its face side, wherein the slanted bore holes or radial slots extend from a base of the piston to a circumference of the piston. A pressure balancing between the spaces on both sides of the piston shall be provided through the bore holes or slots. 
         [0013]    From DE 81 32 123 U1 a gas pressure support of a piston-cylinder unit is known, wherein a fluid connection is provided between the compression cavity and a pressure cavity of the gas bearing. 
         [0014]    U.S. Pat. No. 5,140,915 A illustrates and describes a gas supported piston in a piston-cylinder unit in which circumferential grooves are provided in the forward end section wherein the circumferential grooves are introduced into the circumferential wall in an insulated manner. These circumferential grooves are configured to insulate the gas bearing from an oscillating pressure in the compression cavity. 
         [0015]    JP 2002 349 435 A discloses a linear compressor with an air supported piston which is provided with a circumferential groove in its center section in axial direction. This circumferential groove provides pressure compensation along the circumference of the piston and thus pressure compensation acting in circumferential direction in the bearing gap. When compressed air moves in this known linear compressor from the compression cavity into the bearing gap at a location of the bearing gap, forces which might cause a tilting of the piston are quickly compensated by the pressure compensation caused by the circumferential groove, so that the piston quickly moves back into its position that is coaxial with the cylinder axis or in an ideal case it does not even leave this position. This circumferential groove does not only weaken the undesirable transversal force, but also the air bearing which reduces load bearing capability of the air bearing. 
         [0016]    A piston cylinder unit is known from U.S. Pat. No. 2,907,304 which forms a linear drive device that is actuatable by a fluid. The cylinder wall is provided with a plurality of openings through which a pressurized fluid is introduced into the cylinder cavity. Furthermore switchable fluid outlets are provided at both face sides of the cylinder housing so that alternating opening of respective outlet valves generates a linear movement of the piston. 
       BRIEF SUMMARY OF THE INVENTION 
       [0017]    Thus, it is the object of the present invention to provide a piston cylinder unit which assures a lateral support of the piston in a portion of the piston face wall even in highly dynamic applications, thus when the piston-cylinder unit runs with a high operating frequency even when the piston is in the portion of its top dead center and thus a pressure in the compression cavity is maximized. 
         [0018]    The object is thus achieved through a piston-cylinder unit including a piston that is fluid pressure supported and movable in a linear manner in a cylinder, wherein the cylinder, a face wall of the piston and a face wall of the cylinder define a compression cavity which is at a minimum size in a portion of a top dead center of the piston, wherein the compression cavity is connected in a fluid transferring manner with a bearing gap which is formed between a cylinder inner circumferential wall and a piston outer circumferential wall, wherein a plurality of fluid outlet nozzles are arranged in at least one cross-sectional plane of the cylinder in the cylinder inner circumferential wall along a circumference, which fluid outlet nozzles open into the bearing gap and are connected with a supply conduit for a pressurized fluid, wherein a plurality of fluid outlet nozzles which open into the bearing gap are arranged in at least one cross-sectional plane of the piston adjacent to the piston face wall in the piston outer circumferential wall along the circumference, and wherein the fluid outlet nozzles in the piston outer circumferential wall are also connected with the supply conduit for the pressurized fluid. 
         [0019]    In this piston cylinder unit a plurality of fluid outlet nozzles is arranged in at least one cross sectional plane of the cylinder in the inner circumferential wall of the cylinder along the circumference, wherein the fluid outlet nozzles lead into the bearing gap and in at least one cross sectional plane of the piston adjacent to the piston face wall a plurality of fluid outlet nozzles is arranged in the piston outer circumferential wall along the circumference wherein the fluid outlet nozzles lead into the bearing gap. 
         [0020]    According to the invention also the fluid outlet nozzles in the piston outer circumferential wall are connected with the supply conduit for the pressurized fluid. 
         [0021]    This way the piston independently of its position in the cylinder is always supported against the cylinder inner circumferential wall at a front side of the piston that is adjacent to its piston face wall through pressurized fluid exiting from the piston side fluid outlet nozzles. The fluid outlet nozzles are thus covered in any piston position by the opposite surface at the cylinder inner circumferential wall and a distance between the fluid outlet nozzles and the edge of the bearing surface, thus the piston face wall is constant in any piston position. Thus, the piston is much more tilt stable proximal to its top dead center, thus at maximum compression in the compression cavity, than in solutions that are known in the art. 
         [0022]    An advantageous embodiment of this piston cylinder unit according to the invention is characterized in that the at least one cross sectional plane of the piston with the fluid outlet nozzles is arranged in any position of the piston that moves back and forth during operation between the at least one cross sectional plane of the cylinder with the fluid outlet nozzles and the cylinder face wall. 
         [0023]    This advantageous embodiment provides that the front section that is adjacent to the piston face wall is always supported by the pressurized fluid flowing out of the piston side fluid outlet nozzles, whereas the rear piston section is supported by the pressurized fluid which exits from the cylinder side fluid outlet nozzles. 
         [0024]    This object is also achieved with a piston cylinder unit including a piston that is fluid pressure supported and movable in a linear manner in a cylinder, wherein the cylinder, a face wall of the piston and a face wall of the cylinder define a compression cavity which is at a minimum size in a portion of a top dead center of the piston, wherein the compression cavity is connected in a fluid transferring manner with a bearing gap which is formed between a cylinder inner circumferential wall and a piston outer circumferential wall, wherein a plurality of fluid outlet nozzles are arranged in at least one cross-sectional plane of the cylinder in the cylinder inner circumferential wall along the circumference, which fluid outlet nozzles open into the bearing gap, wherein the piston is provided with a ventilation groove configured as a circumferential groove into which a ventilation conduit opens, wherein the ventilation groove is configured in a circumferential section of the piston that is adjacent to the piston face wall, and wherein the ventilation conduit reduces pressurized fluid entering the ventilation groove to a pressure level which is lower than a pressure in the compression cavity when the piston is in its top dead center or when it moves towards the top dead center in proximity to the top dead center. 
         [0025]    This piston cylinder unit according to the invention includes a piston that is moveable in a linear manner and which is fluid pressure supported in the cylinder, wherein the cylinder, a face wall of the piston and a cylinder face wall define a compression cavity which is at a minimum in a portion of the top dead center of the piston. This compression cavity is in fluid connection with a bearing gap formed between a cylinder interior circumferential wall and a piston exterior circumferential wall. In at least one transversal plane of the cylinder a plurality of fluid outlet nozzles leads into a bearing gap, wherein the fluid outlet nozzles are arranged in the cylinder circumferential wall along the circumference. 
         [0026]    It is provided in order to achieve the object according to the invention that the piston is provided with a circumferential groove that leads into a venting conduit wherein the venting groove is configured in a circumferential section of the piston adjacent to the piston face wall and wherein the venting conduit vents pressurized fluid entering the venting groove to a pressure level that is lower than the pressure in the compression cavity when the piston is in its top dead center or proximal to the top dead center moving towards the top dead center. 
         [0027]    By providing a venting conduit of this type in the circumferential groove not only a pressure balancing along the circumference of the piston is provided beyond the teachings of the prior art, thus along the circumference of the bearing gap but beyond that pressurized fluid entering the circumferential groove is vented to a lower pressure level. Therefore pressurized fluid entering the bearing gap from the compression cavity does not pose any barrier for the pressure fluid entering through the micro holes into the bearing gap. This prevents that the load bearing capability of the bearing is degraded when the piston is proximal to the top dead center or in top dead center and the pressure in the compression cavity is much higher than the bearing fluid pressure. 
         [0028]    Through arranging the air ventilation groove in a circumferential section of the piston that is adjacent to the piston face wall it is facilitated that pressurized fluid entering into the bearing gap from the compression cavity is already vented immediately after entering the bearing gap so that transversal forces impacting the piston are minimized. 
         [0029]    Thus the bearing-pressure fluid can also flow in a direction of the circumferential groove which significantly improves the load bearing capability of the fluid pressure bearing between the piston and the cylinder. 
         [0030]    Advantageously the vented air groove is in fluid connection with a space where the lower pressure lever prevails. 
         [0031]    Advantageously a pressure compensation circumferential groove is provided between the piston face wall and the ventilation groove, wherein the ventilation groove has the effect that the pressure in the bearing gap along the piston circumference is always compensated and that there is no asymmetric pressure distribution. Thus, the piston always maintains its centered position. 
         [0032]    In another advantageous variation of this second embodiment of the invention the piston has a piston section with reduced diameter in the portion of the piston face wall. The ventilation groove is thus provided in the remaining piston portion with a diameter that is not reduced. 
         [0033]    Providing the piston section with reduced diameter in the portion of the piston face wall has the effect that the compressed fluid enters from the compression cavity into the ring cavity enveloping the piston section with reduced diameter when the pressure of the compressed fluid in the compression cavity is higher than the pressure in the bearing gap so that the piston is stabilized in its centered position. 
         [0034]    It is particularly advantageous when the diameter of the piston section with reduced diameter increases in axial direction of the piston starting from the piston face wall. The compressed fluid entering from the compression cavity into the ring cavity enveloping the piston section with reduced diameter develops radial forces like in an outlet throttled fluid bearing. Thus, it is advantageous when the tightest location of the ring cavity, thus the transition from the piston section with reduced diameter to the piston section with non-reduced diameter is arranged in front of the first fluid outlet nozzles of the pressure fluid bearing for the piston. 
         [0035]    The increase of the diameter in the piston section with reduced diameter can be advantageously linear or also nonlinear. 
         [0036]    It is also advantageous when a plurality of fluid outlet nozzles is arranged in the piston exterior circumferential wall along a circumference at least in a cross sectional plane of the piston on a side of the ventilation groove that is oriented away from the piston face wall, wherein the fluid outlet nozzles lead into the bearing gap. This way the piston is always supported at its front side adjacent to the piston face wall relative the cylinder circumferential wall irrespective of its position in the cylinder through pressurized fluid exiting from the piston side fluid outlet nozzles. The fluid outlet nozzles are thus covered in each piston position by the opposite surface at the cylinder inner circumferential wall and a distance between the fluid outlet nozzles and an edge of the bearing surface, thus the piston face wall is constant in any position of the piston. Thus, the piston even close to top dead center, thus under maximum compression in the compression cavity, is much more tilt stable than in any prior art solution. 
         [0037]    Thus it is advantageous when the at least one cross sectional plane of the piston with the fluid outlet nozzles is arranged in any position of the piston moving back and forth during operations between the at least one cross sectional plane of the cylinder with the fluid outlet nozzles and the cylinder face wall. This advantageous embodiment provides that the forward section that is adjacent to the piston face wall is always supported by the pressurized fluid flowing out of the piston side fluid outlet nozzles, whereas the rear piston section is supported by the pressurized fluid which exits from the cylinder side fluid outlet nozzles. 
         [0038]    The object is furthermore achieved in a piston cylinder unit including a piston that is fluid pressure supported and movable in a linear manner in a cylinder, wherein the cylinder, a face wall of the piston and a face wall of the cylinder define a compression cavity which is at a minimum size in a portion of a top dead center of the piston, wherein the compression cavity is connected in a fluid transferring manner with a bearing gap which is formed between a cylinder inner circumferential wall and a piston outer circumferential wall, wherein a plurality of fluid outlet nozzles are arranged in the cylinder inner circumferential wall along a circumference at least in a cross sectional plane of the cylinder where the fluid outlet nozzles open into the bearing gap, and wherein a section of the bearing gap that is adjacent to the compression cavity has a greater radial extension than a section of the bearing gap that is oriented away from the compression cavity, at least when the piston approaches top dead center. 
         [0039]    In order 
         [0000]    to achieve the object is provided in this piston cylinder unit that a plurality of fluid outlet nozzles is arranged in the cylinder inner circumferential wall along the circumference in at least one cross sectional plane of the cylinder, wherein the fluid outlet nozzles lead into the bearing gap, and wherein a section of the bearing gap that is adjacent to the compression cavity has a greater redial extension than the section of the bearing gap that is oriented away from the compression cavity at least when the piston approaches drop dead center. 
         [0040]    The configuration of the piston cylinder unit wherein a section of the bearing gap that is adjacent to the compression cavity has a greater radial extension than the section of the bearing gap oriented away from the compression cavity has the effect that the compressed fluid from the compression cavity enters the section of the bearing gap with greater radial extension along the entire piston circumference when the pressure of the compressed fluid in the compression cavity is greater than the pressure in the bearing gap which stabilizes the piston in its centered position. The compressed fluid entering in this section of the bearing gap with greater radial extension from the compression cavity develops radial forces in this portion like an outlet throttled fluid bearing, as soon as the pressure of the compressed fluid is greater than the pressure in the bearing gap. 
         [0041]    Advantageously the section of the bearing gap with greater radial extension is formed by a piston section with reduced diameter. Tests have shown that it is already sufficient when the diameter differential between the piston section with reduced diameter and the remaining piston section is less than 5%, advantageously less than 1% of the non-reduced piston diameter. 
         [0042]    It is particularly advantageous when the diameter of the piston section with reduced diameter increases from the piston face wall in axial direction of the piston. The compressed fluid entering from the compression cavity into the ring cavity enveloping the piston section with reduced diameter develops radial forces like in an outlet throttled fluid bearing. Thus, it is helpful when the tightest spot of the ring cavity, thus the transition from the piston section with reduced diameter to the piston section with non-reduced diameter is arranged in front of the first fluid outlet nozzles of the pressure fluid bearing for the piston. 
         [0043]    The increase of the diameter in the piston section with reduced diameter can be advantageously linear or nonlinear. 
         [0044]    Alternatively the section of the bearing gap with greater radial extension can also be formed by a cylinder section with expanded diameter. 
         [0045]    Thus it is advantageous when the diameter of the cylinder section with increased diameter decreases starting from the cylinder face wall in axial direction of the cylinder. 
         [0046]    This decrease of the diameter in the cylinder section with expanded diameter is advantageously linear, however it can also be nonlinear. 
         [0047]    An advantageous embodiment of this piston cylinder unit according to the invention is characterized in that in at least one cross sectional plane of the piston, adjacent to the piston face wall or to the face side piston section with reduced diameter, a plurality of fluid outlet nozzles is arranged in the piston outer circumferential wall along the circumference, wherein the fluid outlet nozzles lead into the bearing gap. This way the piston, irrespective of its position in the cylinder is always supported relative to the cylinder inner circumferential wall through pressurized fluid exiting from the piston side fluid outlet nozzles in a forward portion of the piston that is adjacent to the section of the bearing gap with greater radial extension. The fluid outlet nozzles are thus covered in each piston position by the opposite surface at the cylinder inner circumferential wall and the distance between the fluid outlet nozzles and the edge of the bearing surface of the piston, thus the piston face wall or the transition between the piston outer circumference into the section with reduced diameter is constant in any piston position. Thus, the piston is much more tilt stable than in the solutions known in the art also proximal to top dead center, thus under maximum compression in the compression cavity. 
         [0048]    Thus, it is advantageous when at least a cross sectional plane of the piston with the fluid outlet nozzles in any position of the piston moving back and forth during operations is arranged between the at least one cross sectional plane of the cylinder in the fluid outlet nozzles and the cylinder face wall. This advantageous embodiment provides that the forward piston section is always supported by pressurized fluid flowing out of the piston side outlet nozzles, whereas the rear piston section is supported by the pressurized fluid which exits from the cylinder side outlet nozzles. 
         [0049]    Another advantageous variation of the third embodiment of the piston cylinder unit according to the invention is characterized in that the piston in a circumferential section that is adjacent to the piston face wall or the piston section with reduced diameter is provided with at least one circumferential groove. This circumferential groove forms a circumferential pressure compensation groove which provides that pressure differences along the circumference of the bearing gap which can be generated for example through asymmetrically entering pressurized fluid from the compression cavity are directly balanced so that the piston remains in its position that is centered about the cylinder longitudinal axis X and is not laterally displaced. 
         [0050]    It is furthermore advantageous when at least one circumferential groove of the piston is configured as a ventilation groove into which a ventilation conduit leads. Thus, pressurized fluid entering into the bearing gap from the compression cavity can be ventilated through the ventilation groove and the ventilation conduit. 
         [0051]    Thus, the ventilation conduit is advantageously connected in a fluid conducting manner with a space in which a fluid pressure prevails which is lower than the pressure in the compression cavity when the piston is in its top dead center or moves towards top dead center. This prevents that the load bearing capability of the bearing degrades when the piston is proximal to top dead center or in top dead center and the pressure in the compression cavity is much higher than the bearing fluid pressure. 
         [0052]    Advantageously the ventilation groove is configured in a circumferential section of the piston that is adjacent to the piston face wall. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0053]    The invention is subsequently described in more detail based on embodiments with reference to the drawing figure, wherein: 
           [0054]      FIG. 1  illustrates a prior art piston cylinder unit; 
           [0055]      FIG. 2  illustrates a piston cylinder unit according to a first embodiment of the invention; 
           [0056]      FIG. 3  illustrates a first variant of a second embodiment of a piston cylinder unit according to the invention; 
           [0057]      FIG. 4  illustrates a second variant of the second embodiment of the piston cylinder unit according to the invention; 
           [0058]      FIG. 5  illustrates a third variant of the second embodiment of the piston cylinder unit according to the invention; 
           [0059]      FIG. 6  illustrates a piston of the third variant of the second embodiment with a conical front end section; 
           [0060]      FIG. 7  illustrates a piston of the third variant of the second embodiment with concave front end section; 
           [0061]      FIG. 8  illustrates a piston of the third variant of the second embodiment with convex forward end section; 
           [0062]      FIG. 9  illustrates a fourth variant of the second embodiment of the piston cylinder unit according to the invention; 
           [0063]      FIG. 10  illustrates a first variant of a third embodiment of the piston cylinder unit according to the invention; 
           [0064]      FIG. 11  illustrates a piston of the first variant of the third embodiment with conical forward end section; 
           [0065]      FIG. 12  illustrates a piston of the first variant of the third embodiment with concave forward end section; 
           [0066]      FIG. 13  illustrates a piston of the first variant of the third embodiment with convex forward end section; 
           [0067]      FIG. 14  illustrates a piston cylinder unit according to the invention with a piston of the first variant of the third embodiment which includes a ventilation groove; 
           [0068]      FIG. 15  illustrates the variant of  FIG. 14  wherein the piston includes an additional pressure compensation circumferential groove; 
           [0069]      FIG. 16  illustrates a piston cylinder unit of the first variant of the third embodiment with a piston which is provided with a piston side fluid pressure bearing in the forward piston section; 
           [0070]      FIG. 17  illustrates the piston cylinder unit of  FIG. 16 , wherein the piston is provided with an additional ventilation groove; and 
           [0071]      FIG. 18  illustrates a second variant of the third embodiment of the piston cylinder according to the invention with a cylinder section with expanded diameter. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0072]      FIG. 1  illustrates the prior art piston cylinder unit according to DE 10 2004 061 904 A1 that is already described in the introduction of the description. 
         [0073]      FIG. 2  illustrates a first embodiment of the piston cylinder unit according to the invention, wherein the same reference numerals are used for the elements in  FIG. 2  that are identical to the elements in  FIG. 1 . 
         [0074]    The piston  103  is arranged in a center position between its bottom dead center UT and its top dead center OT. The second ring channel  32  and the third ring channel  34  are arranged in the cylinder similar to the piston cylinder unit illustrated in  FIG. 1 . The position of the micro holes  32 ′ associated with the second ring channel  32  and forming fluid outlet nozzles in the cross sectional plane Q 2  and the position of the micro holes  34 ′ associated with the third ring channel  34  and forming fluid outlet nozzles in the cross sectional plane Q 3  and the distance between the second annular nozzle arrangement  32 ″ and the third annular nozzle arrangement  34 ″ in axial direction are selected so that the micro holes  32 ′ and  34 ′ are covered by the circumferential wall  136  of the piston  103  during the entire axial movement of the piston  103 . The two cylinder side air bearings, namely the second air bearing formed by the second annular nozzle arrangement  32 ″ and the third air bearing formed by the third annular nozzle arrangement  34 ″ are thus active during the entire piston movement and support the piston  103  in a rear piston section  103 ′ and in a forward piston section  103 ″ in radial direction. 
         [0075]    The first air bearing contrary to the embodiment of  FIG. 1  is not configured in the cylinder but in the piston  103 . Thus, the piston  103  is provided with micro holes  130 ′ distributed over the circumference, evenly offset and forming fluid outlet nozzles in the piston outer circumferential wall  136  in a cross sectional plane Q 1  directly adjacent to the piston face wall  116 , wherein the micro holes lead into a ring channel  130  configured in an interior of the piston  103  and form a first forward annular nozzle arrangement  130 ″. The ring channel  130  in the interior of the piston  103  is connected through a channel  131  extending in an interior of the piston rod  104  and through a non-illustrated supply conduit with the connection channel  28 . The pressurized fluid flowing into the connection channel  28  is thus also conducted into the ring channel  130  in the interior of the piston and flows from the first micro holes  130 ′ into the bearing gap  19 . 
         [0076]    This way a fluid bearing, for example an air bearing is formed in the most forward portion of the forward piston section  103 ″ of the piston  103  by the annular nozzle arrangement  130 ″ provided at this location, wherein the air bearing supports the piston  103  radially directly adjacent to the piston face wall  16  relative to the cylinder inner circumferential wall  14  forming the bearing surface  15 . Since this most forward fluid bearing moves with the piston the forces applied in this area for radially supporting the piston  103  are almost constant over the entire piston movement. Laterally deflecting the piston transversal to the longitudinal axis X is therefore almost impossible even when fluid compressed in the compression cavity  18  penetrates under pressure into the bearing gap  19 . 
         [0077]      FIG. 3  illustrates a second embodiment of the piston cylinder unit according to the invention, wherein identical reference numerals are used for the elements in  FIG. 3  that are identical with  FIG. 1 . 
         [0078]    The piston  203  is illustrated in a center position between its bottom dead center UT and its top dead center TDC. The second ring channel  32  and the third ring channel  34  are arranged in the cylinder similar to the piston-cylinder unit illustrated in  FIG. 1 . A position of the micro holes  32 ′ associated with the second ring channel  32  forming fluid outlet nozzles in the cross sectional plane Q 2  and a position of the micro holes  34 ′ forming fluid outlet nozzles in the cross sectional plane Q 3  and associated with the third ring channel and a distance between the second annular nozzle arrangement  32 ″ and the third annular nozzle arrangement  34 ″ in axial direction are selected so that the micro holes  32 ′ and  34 ′ are covered during the entire axial movement of the piston  203  by the outer circumferential wall  236  of the piston  230 . The two cylinder side air bearings, namely the second air bearing formed by the second annular nozzle arrangement  32 ′ and the third air bearing formed by the third annular nozzle arrangement  34 ″ are thus active during the entire piston movement and support the piston  203  in a rear piston section  203 ′ and in a forward piston section  203 ″ in radial direction. 
         [0079]    The piston  203  is provided with a circumferentially extending ventilation groove  233  in a forward piston section  203 ″ in the piston outer circumferential wall  236  directly adjacent to the piston face wall  216 , wherein a ventilation opening  233 ′ leads into the ventilation groove  233  wherein the ventilation opening is provided with a fluid connection through a channel  233 ″which extends in an interior of the piston rod  204  with a space in which a fluid pressure prevails which is lower than the pressure in the compression cavity  18  when the piston  203  is in its top dead center TDC or moves towards its top dead center TDC; at least the pressure provided in the ventilation air groove  233  must be lower than the pressure in the bearing gap  19  in front and behind the ventilation groove  233 . 
         [0080]      FIG. 4  illustrates a variation of the embodiment according to  FIG. 3  in which another circumferential groove  235  is configured in the piston outer circumferential wall  236  between the piston face wall  216  and the ventilation groove  233  directly adjacent to the piston face wall  216 . This additional circumferential groove  235  forms a pressure balancing circumferential groove which provides that a pressure compensation along the circumference of the piston  203  is provided in the pressurized fluid entering the bearing gap  19  on one side from the compression cavity  18  so that the piston remains in its centered position with reference to the cylinder axis X and is not displaced laterally. 
         [0081]      FIG. 5  illustrates another variant of the piston  203  provided with the ventilation groove  233  in which the piston  203  in its forward piston section  203 ″ in the portion of the piston face wall  216  includes a piston section  237  with reduced diameter. This piston section  237  with reduced diameter is offset from the ventilation groove  233  in axial direction so that the ventilation groove  233  is configured in the remaining portion of the forward piston portion  203 ″ with a non-reduced diameter. 
         [0082]    By providing the piston section  237  with reduced diameter an annular gap  19 ′ is provided between the cylinder inner circumferential wall  14  and the outer circumferential wall  237 ′ of the piston section  237  with reduced diameter, wherein a radial extension of the annular gap, thus its radial thickness is greater than a thickness of the bearing gap  19 . When compressed fluid exits during the compression movement of the piston  203  from the compression cavity  18  into the forward ring cavity  19 ′ the pressurized fluid entering the annular gap  19  centers the piston  203 . 
         [0083]    In the variant according to  FIG. 5  the piston section  237  with the reduced diameter is configured cylindrical. The piston section  237  however can also be configured with increasing diameter starting from the piston face wall  216  in axial direction of the piston. This can be for example implemented as piston section with a conical circumferential contour  239  as illustrated in  FIG. 1 , wherein the increase of the diameter in the piston section  237  with reduced diameter is linear. The increase of the diameter in the piston section  237  with reduced diameter however can also be nonlinear as illustrated in  FIGS. 7 and 8 . Thus, the piston section can also have a concave circumferential contour  239 ′ ( FIG. 7 ) or a convex circumferential contour  239 ″ ( FIG. 8 ). 
         [0084]    The configuration of the piston  203  with the forward piston section  237  with reduced diameter can also be provided in the variant illustrated in  FIG. 4  of the piston with an additional pressure compensation circumferential groove  235 . 
         [0085]    By the same token as illustrated in  FIG. 9  the piston  203  that is provided according to the invention with the ventilation groove  233  can be additionally provided with a forward piston side fluid bearing in its embodiments according to  FIGS. 3-8  described herein. 
         [0086]    Thus, the piston  203  is provided with micro holes  230 ′ distributed over the circumference and forming fluid outlet nozzles evenly offset from one another in a cross sectional plane Q 1 ′ in the piston exterior circumferential wall  236  directly adjacent to the ventilation groove  233  but axially offset therefrom on a side of the ventilation groove  233  that is oriented away from the piston face wall  216 . These micro holes  230 ′ lead into a ring channel  230  configured in an interior of the piston  203  and form a first forward annular nozzle arrangement  203 ″. The ring channel  240  in the interior of the piston  203  is connected with the connection channel  28  through a channel  231  that also extends in an interior of the piston rod  204  and through a non-illustrated supply conduit. The pressurized fluid flowing into the connection channel  28  is thus also run into the ring channel  230  in the interior of the piston  203  and flows from the first micro holes  230 ′ into the bearing gap  19 . 
         [0087]    This way a fluid bearing, for example an air bearing is also formed in the forward piston section  203  by the annular nozzle arrangement  230 ″ provided at this location wherein the fluid bearing supports the piston  203  in the forward piston section  203 ″ in radial direction against the cylinder inner circumferential wall  14  forming the bearing surface  15 . Since the forward fluid bearing moves with the piston the forces applied in this portion for the radial support of the piston  203  are almost constant over the entire piston movement. A lateral displacement of the piston transversal to the longitudinal axis X is therefore almost impossible even when an asymmetrical entry of compressed fluid from the compression cavity  18  into the bearing gap should occur in spite of the additional measures described supra (pressure compensation circumferential groove  235 , piston section  237  with reduced diameter). 
         [0088]      FIG. 10  illustrates a third embodiment of the piston cylinder unit according to the invention, wherein identical reference numerals are used for elements of  FIG. 10  that are identical with  FIG. 1 . 
         [0089]    The piston  303  is illustrated in a center position between its bottom dead center UT and its top dead center TDC. The second ring channel  32  and the third ring channel  34  are arranged in the cylinder similar to the piston-cylinder unit illustrated in  FIG. 1 . The position of the micro holes  32 ′ associated with the second ring channel  32  and forming fluid outlet nozzles in the cross sectional plane Q 2  and the position of the micro holes  34 ′ associated with the third ring channel  34  and forming fluid outlet nozzles in the cross sectional plane Q 3  and the distance between the second annular nozzle arrangement  32 ″ and the third annular nozzle arrangement  34 ″ in axial direction are selected so that the micro holes  32 ′ and  34 ′ during the entire axial movement of the piston  303  are covered by the exterior circumferential wall  336  of the piston  303 . The two cylinder side air bearings, namely the second air bearing formed by the second annular nozzle arrangement  32 ″ and the third air bearing formed by the third annular nozzle arrangement  34 ″ are thus active during the entire piston movement and support the piston  303  in a rear piston section  303 ′ and in a forward piston section  303 ″ in radial direction. 
         [0090]    The piston  303  is provided with a piston section  337  with reduced diameter in its forward piston section  303 ″ in the portion of the piston face wall  316 , wherein the bearing gap  19  in this section forms an annular gap  19 ′ with a greater radial extension than the section of the bearing gap  19  oriented away from the compression cavity  18 . 
         [0091]    Providing the piston section  337  with reduced diameter provides an annular gap  19 ′ between the cylinder inner circumferential wall  14  and the outer circumferential wall  337 ′ of the piston section  337  with reduced diameter, wherein the radial extension of the annular gap, thus its radial thickness is greater than the radial thickness of the bearing gap  19 . When pressurized fluid enters into the forward annular gap  19 ′ from the compression cavity  18  during the compression movement of the piston  303  the pressurized fluid entering the annular gap  19  centers the piston  303 . 
         [0092]    In the embodiment according to  FIG. 10  the piston section  337  is configured cylindrical with a reduced diameter. However, the piston section  337  can also be provided with an increased diameter in axial direction of the piston starting from the piston face wall  316 . This can be implemented for example as a piston section with a conical circumferential contour  339  as illustrated in  FIG. 11  wherein the increase of the diameter in the piston section  337  with reduced diameter is linear. This increase of the diameter in the piston section  337  with reduced diameter, however, can also be nonlinear as illustrated in  FIGS. 12 and 13 . The piston section  337  can also include a concave circumferential contour  339 ′ ( FIG. 12 ) or a convex circumferential contour  339 ″ ( FIG. 13 ). 
         [0093]      FIG. 14  illustrates another variant of the piston  303  provided with the piston section  337  with reduced diameter. The piston  303  in its forward piston section  303 ″ adjacent to the piston section  337  with reduced diameter is provided with a ventilation groove  333  extending along the circumference wherein the ventilation groove leads into a ventilation opening  333 ′ which is in fluid connection through a channel  333 ″ in the interior of the piston rod  304  with a cavity in which a fluid pressure is provided which is lower than the pressure in the compression cavity  18  when the piston  303  is in its top dead center TDC or when it moves towards the top dead center TDC; at least the pressure prevailing in the ventilation groove  333  has to be lower than the pressure in the bearing gap  19  in front and behind the ventilation groove  333 . The ventilation groove  333  is offset in axial direction from the piston section  337  with reduced diameter so that the ventilation groove  333  is not configured with the reduced diameter in the remaining portion of the forward piston portion  303 ″. 
         [0094]      FIG. 15  illustrates a variation of the embodiment according to  FIG. 14  in which an additional circumferential groove  335  is configured in the piston outer circumferential wall  336  adjacent to the piston section  337  with reduced diameter between the piston section  337  with reduced diameter and the ventilation groove  333 . This additional circumferential groove  335  forms a pressure balancing circumferential groove which provides a pressure balancing in the pressurized fluid entering from the compression cavity  18  on one side into the bearing gap  19 , wherein the pressure balancing is provided along the circumference of the piston  303  so that the piston remains in its centered position with reference to the cylinder axis X and is not laterally displaced. 
         [0095]      FIG. 16  illustrates another alternative embodiment of the piston cylinder unit according to the invention in which the piston  303  includes a piston side fluid bearing in its forward piston section  303 ″ adjacent to the piston section  337  with reduced diameter. 
         [0096]    For this purpose the piston  303  is provided with micro holes  330 ′ that are distributed over the circumference and evenly offset from one another and which form fluid outlet nozzles in a transversal plane Q 1 ″ in the piston outer wall  336  directly adjacent to the piston section  337  with reduced diameter but offset there from. These micro holes  330 ′ lead into a ring channel  330  configured in an interior of the piston  303  and form a first forward annular nozzle arrangement  330 ″. The ring channel  330  in the interior of the piston  303  is connected with the connection channel  28  through a channel  331  extending in an interior of the piston rod  304  and through a non illustrated supply conduit. The pressurized fluid flowing into the connection channel  28  is also conducted into the ring channel  330  in an interior of the piston  303  and flows from the first micro holes  330 ′ into the bearing gap  19 . 
         [0097]    As illustrated in  FIG. 17  the piston  303  that is illustrated in  FIG. 16  and which is provided with the piston side air bearing can be additionally provided with a ventilation groove  333  as described in the context of  FIGS. 14 and 15 . In addition to or as an alternative to the ventilation groove  333  also the pressure compensation circumferential groove  335  can be provided which is described in the context with  FIG. 15 . The ventilation groove  333  and also the pressure compensation circumferential groove  335  are configured between the piston section  337  with reduced diameter and the forward annular nozzle arrangement  330 ″ in the portion of the piston  303  which does not have a reduced diameter. 
         [0098]    This way a fluid bearing, for example a gas or air bearing is also formed in the forward piston section  303 ″ by the annular nozzle arrangement  330 ″ provided at this location, wherein the gas or air bearing supports the piston  303  in the forward piston section  303 ″ in radial direction relative to the cylinder inner circumferential wall  14 . Since this forward fluid bearing moves with the piston, forces applied for a radial support of the piston  303  in this area are almost constant over the entire piston movement. A lateral displacement of the piston transversal to the longitudinal axis X is therefore almost impossible even in case compressed fluid enters in an asymmetric manner from the compression cavity  18  into the bearing gap in spite of the additional measures recited supra, thus the pressure compensation circumferential groove  335  and piston section  337  with reduced diameter. 
         [0099]    Eventually  FIG. 18  illustrates a second variation of the third embodiment of the piston cylinder unit according to the invention in which the section  19 ″ of the bearing gap  19  with greater radial extension is formed by a forward portion  10 ′ of the cylinder bore hole  10  that is arranged proximal to the cylinder face wall  12  in which portion the cylinder bore hole  10 ′ is increased in diameter towards the cylinder face wall  12  (cylinder section  2 ′). This portion  10 ′ of the cylinder bore hole with increasing or increased diameter envelops at least a portion of the forward piston section  303 ′ of the piston  303  when the piston as represented in  FIG. 18  in dashed lines is in its top dead center TDC. 
         [0100]    In the variant of the third embodiment of the piston cylinder unit illustrated in  FIG. 18  it is not required that the piston is provided with the piston section  337  with reduced diameter in the portion of its piston face wall, though this is not impossible either. 
         [0101]    Also in the variant according to  FIG. 18 , the piston  303  can be provided with a ventilation groove  333 , a pressure compensation circumferential groove  335 , a piston side fluid bearing (forward annular nozzle arrangement  330 ″) or with combinations thereof as has already been described in conjunction with the first variant of the third embodiment. 
         [0102]    The piston cylinder unit according to the invention, and this also applies for all embodiments, forms an element of a linear compressor in an advantageous embodiment, wherein the compressed fluid is a gas, for example air. The fluid bearings are thus configured as gas pressure bearings, for example air bearings. An advantageous embodiment is a refrigeration system linear compressor wherein the fluid is a gaseous refrigerant. 
         [0103]    The invention is not limited to the embodiments recited supra which only provide a general description of the core idea of the invention. Within the scope of the invention the device according to the invention can also be provided in embodiments that differ from the embodiments recited supra. The device can thus in particular include features which represent a combination from the respective individual features of the patent claims. 
         [0104]    Reference numerals in the patent claims, the description and the drawings are intended for better comprehension of the invention and do not limit the scope thereof.