Patent Publication Number: US-9890772-B2

Title: Linear compressor

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application claims priority under 35 U.S.C. 119 and 35 U.S.C. 365 to Korean Patent Application No. 10-2014-0091878, filed in Korea on Jul. 21, 2014, which is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     1. Field 
     A linear compressor is disclosed herein. 
     2. Background 
     Cooling systems are systems in which a refrigerant is circulated to generate cool air. In such a cooling system, processes of compressing, condensing, expanding, and evaporating the refrigerant may be repeatedly performed. For this, the cooling system may include a compressor, a condenser, an expansion device, and an evaporator. Also, the cooling system may be installed in a refrigerator or air conditioner, which is a home appliance. 
     In general, compressors are machines that receive power from a power generation device, such as an electric motor or turbine, to compress air, a refrigerant, or various working gases, thereby increasing in pressure. Compressors are being widely used in home appliances or industrial fields. 
     Compressors may be largely classified into reciprocating compressors, in which a compression space, into and from which a working gas, such as a refrigerant, may be suctioned and discharged, is defined between a piston and a cylinder to allow the piston to be linearly reciprocated into the cylinder, thereby compressing the working gas; rotary compressors, in which a compression space into and from which a working gas, such as a refrigerant, may be suctioned and discharged, is defined between a roller that eccentrically rotates and a cylinder to allow the roller to eccentrically rotate along an inner wall of the cylinder, thereby compressing the working gas; and scroll compressors, in which a compression space into and from which a working gas, such as a refrigerant, may be suctioned and discharged, is defined between an orbiting scroll and a fixed scroll to compress the working gas while the orbiting scroll rotates along the fixed scroll. In recent years, a linear compressor, which is directly connected to a drive motor, and in which a piston is linearly reciprocated, to improve compression efficiency without mechanical loss due to movement conversion and having a simple structure, is being widely developed. 
     The linear compressor may suction and compress a refrigerant while a piston is linearly reciprocated in a sealed shell by a linear motor, and then, discharge the refrigerant. The linear motor may be configured to allow a permanent magnet to be disposed between an inner stator and an outer stator. The permanent magnet may be linearly reciprocated by an electromagnetic force between the permanent magnet and the inner (or outer) stator. Also, as the permanent magnet operates in a state in which the permanent magnet is connected to the piston, the permanent magnet may suction and compress the refrigerant while being linearly reciprocated within the cylinder and then discharge the refrigerant. 
     The present Applicant filed for a patent (hereinafter, referred to as a “prior document”) and registered the patent with respect to the linear compressor, as Korean Patent No. 10-1307688, filed in Korea on Sep. 5, 2013, and entitled “linear compressor”, which is hereby incorporated by reference. The linear compressor according to the prior document includes a shell that accommodates a plurality of components. A vertical height of the shell may be somewhat high, as illustrated in  FIG. 2  of the prior art document. Also, an oil supply assembly to supply oil between a cylinder and a piston may be disposed within the shell. 
     When the linear compressor is provided in a refrigerator, the linear compressor may be disposed in a machine chamber, which may be provided at a rear side of the refrigerator. In recent years, a major concern of customers is increasing an inner storage space of the refrigerator. To increase the inner storage space of the refrigerator, it may be necessary to reduce a volume of the machine room. Also, to reduce the volume of the machine room, it may be important to reduce a size of the linear compressor. However, as the linear compressor disclosed in the prior document has a relatively large volume, the linear compressor is not suitable for a refrigerator for which an increase in the inner storage space is desired or sought. 
     Further, to reduce the size of the linear compressor, it may be necessary to reduce a size of a main component of the linear compressor. In this case, a surface of the linear compressor may deteriorate. To compensate for the deteriorated performance of the linear compressor, it may be necessary to increase a drive frequency of the compressor. However, the more the drive frequency of the linear compressor is increased, the more a friction force due to oil circulating in the linear compressor increases, deteriorating performance of the linear compressor. 
     The prior document discloses a feature in which a discharge valve spring that supports a discharge valve is provided as a coil spring. When the coil spring is applied to the discharge valve spring, the discharge valve may rotate with respect to the coil spring, causing abrasion of the discharge valve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will be described in detail with reference to the following drawings in which like reference numerals refer to like elements, and wherein: 
         FIG. 1  is a cross-sectional view of a linear compressor according to an embodiment; 
         FIG. 2  is a cross-sectional view of a suction muffler of the linear compressor of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of a discharge cover and a discharge valve of the linear compressor of  FIG. 1 ; 
         FIG. 4  is an exploded perspective view of a cylinder and a frame of the linear compressor of  FIG. 1 ; 
         FIG. 5  is a cross-sectional view illustrating a state in which the cylinder and a piston are coupled to each other according to an embodiment; 
         FIG. 6  is a perspective view of the cylinder of the linear compressor of  FIG. 1 ; 
         FIG. 7  is an enlarged cross-sectional view illustrating a portion A of  FIG. 5 ; 
         FIG. 8  is a perspective view of a discharge valve coupled to the discharge cover according to an embodiment; 
         FIG. 9  is an exploded perspective view of the discharge cover and the discharge valve of  FIG. 8 ; 
         FIG. 10  is a view of a valve spring according to an embodiment; 
         FIG. 11  is a cross-sectional view of a discharge valve assembly according to an embodiment; 
         FIG. 12  is a cross-sectional view illustrating an effect of the discharge valve assembly according to an embodiment; 
         FIG. 13  is a cross-sectional view illustrating a flow of a refrigerant in the linear compressor of  FIG. 1 ; and 
         FIG. 14  is a view illustrating a state in which the discharge valve is opened when the linear compressor of  FIG. 1  operates. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, exemplary embodiments will be described with reference to the accompanying drawings. The embodiments may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, alternate embodiments included in other retrogressive inventions or falling within the spirit and scope will fully convey the concept to those skilled in the art. 
       FIG. 1  is a cross-sectional view of a linear compressor according to an embodiment. Referring to  FIG. 1 , the linear compressor  100  according to an embodiment may include a shell  101  having an approximately cylindrical shape, a first cover  102  coupled to one or a first side of the shell  101 , and a second cover  103  coupled to the other or a second side of the shell  101 . For example, the linear compressor  100  may be laid out in a horizontal direction. Also, in the linear compressor  100 , the first cover  102  may be coupled to a right side of the shell  101 , and the second cover  103  may be coupled to a left side of the shell  101 . Each of the first and second covers  102  and  103  may be understood as one component of the shell  101 . 
     The linear compressor  100  may include a cylinder  120  provided in the shell  101 , a piston  130  linearly reciprocated within the cylinder  120 , and a motor  140  that serves as a linear motor that applies a drive force to the piston  130 . When the motor  140  operates, the piston  130  may be linearly reciprocated at a high rate. The linear compressor  100  according to this embodiment may have a drive frequency of about 100 Hz, for example. 
     The linear compressor  100  may include a suction port  104 , through which a refrigerant may be introduced, and a discharge port  105 , through which the refrigerant compressed in the cylinder  120  may be discharged. The suction port  104  may be coupled to the first cover  102 , and the discharge port  105  may be coupled to the second cover  103 . 
     The refrigerant suctioned in through the suction port  104  may flow into the piston  130  via the suction muffler  150 . Thus, while the refrigerant passes through the suction muffler  150 , noise may be reduced. The suction muffler  150  may include a first muffler  151  coupled to a second muffler  153 . At least a portion of the suction muffler  150  may be disposed within the piston  130 . 
     The piston  130  may include a piston body  131  having an approximately cylindrical shape, and a piston flange  132  that extends from the piston body  131  in a radial direction. The piston body  131  may be reciprocated within the cylinder  120 , and the piston flange  132  may be reciprocated outside of the cylinder  120 . 
     The piston  130  may be formed of an aluminum material, such as aluminum or an aluminum alloy, which is a nonmagnetic material. As the piston  130  may be formed of the aluminum material, a magnetic flux generated in the motor  140  may be transmitted into the piston  130 , preventing the magnetic flux from leaking outside of the piston  130 . Also, the piston  130  may be manufactured by a forging process, for example. 
     The cylinder  120  may be formed of an aluminum material, such as aluminum or an aluminum alloy, which is a nonmagnetic material. Also, the cylinder  120  and the piston  130  may have a same material composition, that is, a same kind and composition. 
     As the cylinder  120  may be formed of the aluminum material, a magnetic flux generated in the motor  140  may be transmitted into the cylinder  120  to prevent the magnetic flux from leaking outside of the cylinder  120 . Also, the cylinder  120  may be manufactured by an extruding rod processing process, for example. 
     Also, as the piston  130  may be formed of a same material (aluminum) as the cylinder  120 , the piston  130  may have a same thermal expansion coefficient as the cylinder  120 . When the linear compressor  100  operates, a high-temperature (a temperature of about 100° C.) environment may be created within the shell  100 . Thus, as the piston  130  and the cylinder  120  have the same thermal expansion coefficient, the piston  130  and the cylinder  120  may be thermally deformed by a same degree. As a result, the piston  130  and the cylinder  120  may be thermally deformed with sizes and in directions different from each other to prevent the piston  130  from interfering with the cylinder  120  while the piston  130  moves. 
     The cylinder  120  may be configured to accommodate at least a portion of the suction muffler  150  and at least a portion of the piston  130 . The cylinder  120  may have a compression space P, in which the refrigerant may be compressed by the piston  130 . A suction hole  133 , through which the refrigerant may be introduced into the compression space P, may be defined in or at a front portion of the piston  130 , and a suction valve  135  to selectively open the suction hole  133  may be disposed on a front side of the suction hole  133 . A coupling hole, to which a predetermined coupling member may be coupled, may be defined in or at an approximately central portion of the suction valve  135 . 
     A discharge cover  200  that defines a discharge space or discharge passage for the refrigerant discharged from the compression space P, and a discharge valve assembly  220 ,  230 ,  240  coupled to the discharge cover  200  to selectively discharge the refrigerant compressed in the compression space P may be provided at a front side of the compression space P. The discharge valve assembly  220 ,  230 ,  240  may include a discharge valve  220  to introduce the refrigerant into the discharge space of the discharge cover  200  when a pressure within the compression space P is above a discharge pressure, a valve spring  230  disposed between the discharge valve  220  and the discharge cover  200  to apply an elastic force in an axial direction, and a stopper  240  that restricts deformation of the valve spring  230 . 
     The term “compression space P” may refer to a space defined between the suction valve  135  and the discharge valve  220 . Also, the suction valve  135  may be disposed on or at one or a first side of the compression space P, and the discharge valve  220  may be disposed on the other or a second side of the compression space P, that is, a side opposite of the suction valve  135 . 
     The term “axial direction” may refer to a direction in which the piston  130  is reciprocated, that is, a transverse direction in  FIG. 1 . Also, in the axial direction, a direction from the suction port  104  toward the discharge port  105 , that is, a direction in which the refrigerant flows, may be defined as a “frontward direction”, and a direction opposite to the frontward direction may be defined as a “rearward direction”. On the other hand, the term “radial direction” may be refer to a direction that is substantially perpendicular to the direction in which the piston  130  is reciprocated, that is, a horizontal direction in  FIG. 1 . 
     The stopper  240  may be seated on the discharge cover  200 , and the valve spring  230  may be seated at a rear side of the stopper  240 . The discharge valve  220  may be coupled to the valve spring  230 , and a rear portion or rear surface of the discharge valve  220  may be supported by a front surface of the cylinder  120 . 
     For example, the valve spring  230  may include a plate spring. As the valve spring  230  may be provided as a plate spring, rotation of the discharge valve  220  in a state in which the discharge valve  220  is coupled to the valve spring  230  may be prevented when compared to a structure in which the coil spring is provided according to the related art. 
     An insertion protrusion  222  of the discharge valve  220  and an insertion hole  232  of the valve spring  230  may be eccentrically disposed with respect to each other. A central portion of the discharge valve  220  and a central portion of the insertion protrusion  222  of the discharge valve  220  coupled to the valve spring  230  may be eccentrically disposed, that is, spaced apart from each other. On the other hand, a central portion of the valve spring  230  and a central portion of the insertion hole  232  of the valve spring  230  to which the insertion protrusion  222  is coupled may be eccentrically disposed, that is, spaced apart from each other, which will be described hereafter. 
     While the piston  130  is linearly reciprocated within the cylinder  120 , when the pressure of the compression space P is below the discharge pressure and a suction pressure, the suction valve  135  may be opened to suction the refrigerant into the compression space P. On the other hand, when the pressure of the compression space P is above the suction pressure, the refrigerant may be compressed in the compression space P in a state in which the suction valve  135  is closed. 
     When the pressure of the compression space P is above the discharge pressure, the valve spring  230  may be deformed to open the discharge valve  220 . The refrigerant may be discharged from the compression space P into the discharge space of the discharge cover  200 . When the discharge of the refrigerant is completed, the valve spring  230  may provide a restoring force to the discharge valve  220  to close the discharge valve  220 . 
     Also, the refrigerant flowing into the discharge space of the discharge cover  200  may be introduced into a loop pipe  165 . The loop pipe  165  may be coupled to the discharge cover  200  to extend to the discharge port  105 , thereby guiding the compressed refrigerant of the discharge space into the discharge port  105 . For example, the loop pipe  165  may have a shape that is wound in a predetermined direction and extends in a rounded shape. Also, the loop pipe  165  may be coupled to the discharge port  105 . 
     The linear compressor  100  may further include a frame  110 . The frame  110  may fix the cylinder  120  and be coupled to the cylinder  120  by a separate coupling member, for example. The frame  110  may be disposed to surround the cylinder  120 . That is, the cylinder  120  may be accommodated within the frame  110 . Also, the discharge cover  200  may be coupled to a front surface of the frame  110 . 
     At least a portion of the high-pressure gas refrigerant discharged through the opened discharge valve  220  may flow toward an outer circumferential surface of the cylinder  120  through a space at a portion at which the cylinder  120  and the frame  110  are coupled to each other. The refrigerant may be introduced into the cylinder  120  through a gas inflow (see reference numeral  122  of  FIG. 7 ) and a nozzle (see reference numeral  123  of  FIG. 7 ), which may be defined in the cylinder  120 . The introduced refrigerant may flow into a space defined between the piston  130  and the cylinder  120  to allow an outer circumferential surface of the piston  130  to be spaced apart from an inner circumferential surface of the cylinder  120 . Thus, the introduced refrigerant may serve as a “gas bearing” that reduces friction between the piston  130  and the cylinder  120  while the piston  130  is reciprocated. That is, in this embodiment, a bearing using oil is not applied. 
     The motor  140  may include outer stators  141 ,  143 , and  145  fixed to the frame  110  and disposed to surround the cylinder  120 , an inner stator  148  disposed to be spaced inward from the outer stators  141 ,  143 , and  145 , and a permanent magnet  146  disposed in a space between the outer stators  141 ,  143 , and  145  and the inner stator  148 . The permanent magnet  146  may be linearly reciprocated by a mutual electromagnetic force between the outer stators  141 ,  143 , and  145  and the inner stator  148 . Also, the permanent magnet  146  may be provided as a single magnet having one polarity, or a plurality of magnets having three polarities. 
     The permanent magnet  146  may be coupled to the piston  130  by a connection member  138 , for example. In detail, the connection member  138  may be coupled to the piston flange  132  and be bent to extend toward the permanent magnet  146 . As the permanent magnet  146  is reciprocated, the piston  130  may be reciprocated together with the permanent magnet  146  in the axial direction. 
     The motor  140  may further include a fixing member  147  to fixing the permanent magnet  146  to the connection member  138 . The fixing member  147  may be formed of a composition, in which a glass fiber or carbon fiber may be mixed with a resin. The fixing member  147  may be provided to surround an outside of the permanent magnet  146  to firmly maintain a coupled state between the permanent magnet  146  and the connection member  138 . 
     The outer stators  141 ,  143 , and  145  may include coil winding bodies  143  and  145 , and a stator core  141 . The coil winding bodies  143  and  145  may include a bobbin  143 , and a coil  145  wound in a circumferential direction of the bobbin  145 . The coil  145  may have a polygonal cross-section, for example, a hexagonal cross-section. The stator core  141  may be manufactured by stacking a plurality of laminations in the circumferential direction and be disposed to surround the coil winding bodies  143  and  145 , for example. 
     A stator cover  149  may be disposed on or at one side of the outer stators  141 ,  143 , and  145 . One or a first side of the outer stators  141 ,  143 , and  145  may be supported by the frame  110 , and the other or second side of the outer stators  141 ,  143 , and  145  may be supported by the stator cover  149 . The inner stator  148  may be fixed to a circumference of the frame  110 . Also, in the inner stator  148 , the plurality of laminations may be stacked in the circumferential direction outside of the frame  110 . 
     The linear compressor  100  may further include a support  137  that supports the piston  130 , and a back cover  170  spring-coupled to the support  137 . The support  137  may be coupled to the piston flange  132  and the connection member  138  by a predetermined coupling member, for example. 
     A suction guide  155  may be coupled to a front portion of the back cover  170 . The suction guide  155  may guide the refrigerant suctioned through the suction port  104  to introduce the refrigerant into the suction muffler  150 . 
     The linear compressor  100  may include a plurality of springs  176  which may be adjustable in natural frequency to allow the piston  130  to perform a resonant motion. The plurality of springs  176  may include a first spring supported between the support  137  and the stator cover  149 , and a second spring supported between the support  137  and the back cover  170 . 
     The linear compressor  100  may additionally include plate springs  172  and  174  disposed, respectively, on or at both sides of the shell  101  to allow inner components of the compressor  100  to be supported by the shell  101 . The plate springs  172  and  174  may include a first plate spring  172  coupled to the first cover  102 , and a second plate spring  174  coupled to the second cover  103 . For example, the first plate spring  172  may be fitted into a portion at which the shell  101  and the first cover  102  are coupled to each other, and the second plate spring  174  may be fitted into a portion at which the shell  101  and the second cover  103  are coupled to each other. 
       FIG. 2  is a cross-sectional view of a suction muffler of the linear compressor of  FIG. 1 . Referring to  FIG. 2 , the suction muffler  150  according to this embodiment may include the first muffler  151 , the second muffler  153  coupled to the first muffler  151 , and a first filter  310  supported by the first and second mufflers  151  and  153 . 
     A flow space, in which the refrigerant may flow may be defined in each of the first and second mufflers  151  and  153 . The first muffler  151  may extend from an inside of the suction port  104  in a direction of the discharge port  105 , and at least a portion of the first muffler  151  may extend to an inside of the suction guide  155 . The second muffler  153  may extend from the first muffler  151  to an inside of the piston body  131 . 
     The first filter  310  may refer to a component disposed in the flow space to filter foreign substances. The first filter  310  may be formed of a material having a magnetic property. Thus, foreign substances contained in the refrigerant, in particular, metallic substances, may be easily filtered. For example, the first filter  310  may be formed of stainless steel, and thus, may have a magnetic property to prevent the first filter  310  from rusting. As another example, the first filter  310  may be coated with a magnetic material, or a magnet may be attached to a surface of the first filter  310 . 
     The first filter  310  may be provided in a mesh-type structure and have an approximately circular plate shape. Each of the filter holes may have a diameter or width less than a predetermined diameter or width. For example, the predetermined size may be about 25 μm. 
     The first muffler  151  and the second muffler  153  may be assembled with each other using a press-fit manner, for example. Also, the first filter  310  may be fitted into a portion into which the first and second mufflers  151  and  153  are press-fitted and then may be assembled. 
     For example, a groove may be defined in one of the first muffler  151  or the second muffler  153 , and a protrusion inserted into the groove may be disposed on the other one of the first muffler  151  or the second muffler  153 . The first filter  310  may be supported by the first and second mufflers  151  and  153  in a state in which both sides of the first filter  310  are disposed between the groove and the protrusion. In a state in which the first filter  310  is disposed between the first and second mufflers  151  and  153 , when the first and second mufflers  151  and  153  move in a direction that approach each other and then are press-fitted, both sides of the first filter  310  may be inserted and fixed between the groove and the protrusion. 
     As described above, as the first filter  310  is provided on the suction muffler  150 , foreign substances having a size greater than a predetermined size in the refrigerant suctioned in through the suction port  104  may be filtered by the first filter  310 . Thus, the first filter  310  may filter the foreign substances from the refrigerant acting as the gas bearing between the piston  130  and the cylinder  120  to prevent the foreign substances from being introduced into the cylinder  120 . As the first filter  310  is firmly fixed to the portion at which the first and second mufflers  151  and  153  are press-fitted, separation of the first filter  310  from the suction muffler  150  may be prevented. 
       FIG. 3  is a cross-sectional view of a discharge cover and a discharge valve of the linear compressor of  FIG. 1 .  FIG. 4  is an exploded perspective view of a cylinder and a frame of the linear compressor of  FIG. 1 . 
     Referring to  FIGS. 3 and 4 , the linear compressor  100  according to this embodiment may further include the discharge valve  220 , which may be selectively opened to discharge the refrigerant compressed in the compression space P. A rear surface of the discharge valve  220  may be disposed to contact a front portion of the cylinder  120 . In a state in which the rear surface of the discharge valve  220  contacts the front portion of the cylinder  120 , the refrigerant within the compression space P may be compressed. When the pressure of the compression space P is above the discharge pressure, the rear surface of the discharge valve  220  may be spaced apart from the front portion of the cylinder  120  to open the discharge valve  220 . Thus, the compressed refrigerant may be discharged through the space. 
     The linear compressor  100  may further include the valve spring  230  coupled to a front portion of the discharge valve  220  to elastically support the discharge valve  220 , and the stopper  240  that restricts deformation of the valve spring  230  to a preset or predetermined degree or less. 
     When the discharge valve  220  is opened, the valve spring  230  may be deformed in a forward direction. In this process, the stopper  240  may interfere with the valve spring  230  at a front side of the valve spring  230  to prevent the valve spring  230  from being excessively deformed. 
     The linear compressor  100  may include a plurality of spacers  250  and  260  disposed, respectively, on one or a first side and the other or a second side of the stopper  240 . The plurality of spacers  250  and  260  may include a first spacer  250  disposed between the valve spring  230  and the stopper  240 , and a second spacer  260  disposed at a front side of the valve spring  230 . 
     The first spacer  250  may space the valve spring  230  from the stopper  240  by a preset or predetermined distance to secure a space in which the valve spring  230  may be deformed. The preset or predetermined distance may be determined by an adjustable thickness of the first spacer  250 . 
     The second spacer  260  may be disposed between the stopper  240  and the discharge cover  200  to stably support the stopper  240  on the discharge cover  200 . Thus, when a repetitive impact occurs between the valve spring  230  and the stopper  240 , damage to the stopper  240  by the discharge cover  200 , in particular, a phenomenon that occurs when the discharge cover  200  has a hardness greater than a hardness of the stopper  240  may be prevented. 
     The linear compressor  100  may include a second filter  320  disposed between the frame  110  and the cylinder  120  to filter a high-pressure gas refrigerant discharged through the discharge valve  220 . The second filter  320  may be disposed on a portion of a coupled surface at which the frame  110  and the cylinder  120  are coupled to each other. 
     The cylinder  120  may include a cylinder body  121  having an approximately cylindrical shape, and a cylinder flange  125  that extends from the cylinder body  121  in a radial direction. The cylinder body  121  may include the gas inflow  122 , through which the discharged gas refrigerant may be introduced. The gas inflow  122  may be recessed in an approximately circular shape along a circumferential surface of the cylinder body  121 . 
     A plurality of the gas inflow  122  may be provided. The plurality of gas inflows  122  may include gas inflows (see reference numerals  122   a  and  122   b  of  FIG. 6 ) disposed on or at one or a first side with respect to a center of the cylinder body  121  in an axial direction, and a gas inflow (see reference numeral  122   c  of  FIG. 6 ) disposed on or at the other or a second side with respect to the center of the cylinder body  121  in the axial direction. 
     A coupling part or portion  126  coupled to the frame  110  may be disposed on the cylinder flange  125 . The coupling portion  126  may protrude outward from an outer circumferential surface of the cylinder flange  125 . The coupling portion  126  may be coupled to a cylinder coupling hole  118  of the frame  110  by a predetermined coupling member, for example. 
     The cylinder flange  125  may have a seat surface  127  seated on the frame  110 . The seat surface  127  may be a rear surface of the cylinder flange  125  that extends from the cylinder body  121  in a radial direction. 
     The frame  110  may include a frame body  111  that surrounds the cylinder body  121 , and a cover coupling part or portion  115  that extends in a radial direction of the frame body  111  and coupled to the discharge cover  200 . 
     The cover coupling portion  115  may have a plurality of cover coupling holes  116  in which the coupling member coupled to the discharge cover  200  may be inserted and a plurality of the cylinder coupling holes  118  in which the coupling member coupled to the cylinder flange  125  may be inserted. The cylinder coupling holes  118  may be defined in positions that are recessed somewhat from the cover coupling portion  115 . 
     The frame  110  may include a recess  117  recessed in a backward direction from the cover coupling portion  115  to allow the cylinder flange  125  to be inserted therein. That is, the recess  117  may be disposed to surround an outer circumferential surface of the cylinder flange  125 . The recess  117  may have a recessed depth corresponding to a front/rear width of the cylinder flange  125 . 
     A predetermined refrigerant flow space may be defined between an inner circumferential surface of the recess  117  and the outer circumferential surface of the cylinder flange  125 . The high-pressure gas refrigerant discharged from the discharge valve  220  may flow toward an outer circumferential surface of the cylinder body  121  via the refrigerant flow space. The second filter  320  may be disposed in the refrigerant flow space to filter the refrigerant. 
     A seat having a stepped portion may be disposed on or at a rear end of the recess  117 . Also, the second filter  320 , which may have a ring shape, may be seated on the seat. 
     In a state in which the second filter  320  is seated on the seat, when the cylinder  120  is coupled to the frame  110 , the cylinder flange  125  may push the second filter  320  from a front side of the second filter  320 . That is, the second filter  320  may be disposed and fixed between the seat of the frame  110  and the seat surface  127  of the cylinder flange  125 . 
     The second filter  320  may prevent foreign substances in the high-pressure gas refrigerant discharged through the opened discharge valve  220  from being introduced into the gas inflow  122  of the cylinder  120  and be configured to absorb oil contained in the refrigerant. For example, the second filter  320  may include a felt formed of polyethylene terephthalate (PET) fiber, or an adsorbent paper. The PET fiber may have a superior heat-resistance and mechanical strength. A foreign substance having a size of about 2 μm or more, which may be contained in the refrigerant, may be blocked. 
     The high-pressure gas refrigerant passing through the flow space defined between the inner circumferential surface of the recess  117  and the outer circumferential surface of the cylinder flange  125  may pass through the second filter  320 . In this process, the refrigerant may be filtered by the second filter  320 . 
       FIG. 5  is a cross-sectional view illustrating a state in which the cylinder and the piston are coupled to each other according to an embodiment.  FIG. 6  is a perspective view of the cylinder of the linear compressor of  FIG. 1 .  FIG. 7  is an enlarged cross-sectional view illustrating a portion A of  FIG. 5 . 
     Referring to  FIGS. 5 to 7 , the cylinder  120  according to this embodiment may include the cylinder body  121  having an approximately cylindrical shape to form a first body end  121   a  and a second body end  121   b , and the cylinder flange  125  that extends from the second body end  121   b  of the cylinder body  121  in a radial direction. The first body end  121   a  and the second body end  121   b  may form both ends of the cylinder body  121  with respect to a central portion  121   c  of the cylinder body  121  in an axial direction. 
     The cylinder body  121  may include a plurality of the gas inflows  122  through which at least a portion of the high-pressure gas refrigerant discharged through the discharge valve  220  may flow. A third filter  330  as a “filter member” may be disposed on or in the plurality of gas inflows. 
     Each of the plurality of gas inflows  122  may be recessed from the outer circumferential surface of the cylinder body  121  by a predetermined depth and width. The refrigerant may be introduced into the cylinder body  121  through the plurality of gas inflows  122  and the nozzle  123 . 
     The introduced refrigerant may be disposed between an outer circumferential surface of the piston  130  and an inner circumferential surface of the cylinder  120  to serve as the gas bearing with respect to movement of the piston  130 . That is, an outer circumferential surface of the piston  130  may be maintained in a state in which the outer circumferential surface of the piston  130  is spaced apart from an inner circumferential surface of the cylinder  120  by the pressure of the introduced refrigerant. 
     The plurality of gas inflows  122  may include first and second gas inflows  122   a  disposed on or at one or a first side with respect to the central portion  121   c  in the axial direction of the cylinder body  121 , and a third gas inflows  122   c  disposed on the other or a second side with respect to the central portion  121   c  in the axial direction. The first and second gas inflows  122   a  and  122   b  may be disposed at positions closer to the second body end  121   b  with respect to the central portion in the axial direction of the cylinder body  121 , and the third gas inflow  122   c  may be disposed at a position closer to the first body end  121   a  with respect to the central portion  121   c  in the axial direction of the cylinder body  121 . That is, the plurality of gas inflows  122  may be provided in numbers that are not symmetrical to each other with respect to the central portion  121   c  in the axial direction of the cylinder body  121 . 
     Referring to  FIGS. 1 to 6 , the cylinder  120  may have a relatively high inner pressure at a side of the second body end  121   b  which is closer to a discharge-side of the compressed refrigerant when compared to the first body end  121   a  which is closer to a suction-side of the refrigerant. Thus, more gas inflows  122  may be provided to or at the side of the second body end  121   b  to enhance a function of the gas bearing, and relatively less gas inflows  122  may be provided to or at the side of the first body end  121   a.    
     The cylinder body  121  may further includes the nozzle  123  that extends from the plurality of gas inflows  122  toward the inner circumferential surface of the cylinder body  121 . The nozzle  123  may have a width or size less than a width or size of the gas inflow  122 . 
     A plurality of the nozzle  123  may be provided along the gas inflow  122  which may extend in a circular shape. Also, the plurality of nozzles  123  may be disposed to be spaced apart from each other. 
     The plurality of nozzles  123  may each include an inlet  123   a  connected to the gas inflow  122  and an outlet  123   b  connected to the inner circumferential surface of the cylinder body  121 . The nozzle  123  may have a predetermined length from the inlet  123   a  toward the outlet  123   b.    
     The refrigerant introduced into the gas inflow  122  may be filtered by the third filter  330  to flow into the inlet  123   a  of the nozzle  123  and then flow toward the inner circumferential surface of the cylinder  120  along the nozzle  123 . The refrigerant may be introduced into an inner space of the cylinder  120  through the outlet  123   b.    
     The piston  130  may operate to be spaced apart from the inner circumferential surface of the cylinder  120 , that is, may be lifted from the inner circumferential surface of the cylinder  120  by the pressure of the refrigerant discharged from the outlet  123   b . That is, the pressure of the refrigerant supplied into the cylinder  120  may provide a lifting force or pressure to the piston  130 . 
     A recessed depth and width of each of the plurality of gas inflows  122  and a length of the nozzle  123  may be determined to have adequate dimensions in consideration of a rigidity of the cylinder  120 , an amount of the third filter  330 , or an intensity in pressure drop of the refrigerant passing through the nozzle  123 . 
     For example, if the recessed depth and width of each of the plurality of gas inflows  122  are very large, or a length of the nozzle  123  is very short, the rigidity of the cylinder  120  may be weak. On the other hand, if the recessed depth and width of each of the plurality of gas inflows  122  are very small, an amount of the third filter  330  provided in the gas inflow  122  may be very small. Also, if the length of the nozzle  123  is too long, the pressure drop of the refrigerant passing through the nozzle  123  may be too large, and it may be difficult to perform the function as the gas bearing. 
     The inlet  123   a  of the nozzle  123  may have a diameter greater than a diameter of the outlet  123   b . In a flow direction of the refrigerant, a flow section area of the nozzle  123  may gradually decrease from the inlet  123   a  to the outlet  123   b.    
     In detail, if the diameter of the nozzle  123  is too small, an amount of refrigerant, which is introduced from the nozzle  123 , of the high-pressure gas refrigerant discharged through the discharge valve  220  may be too large, increasing flow loss in the linear compressor  100 . On the other hand, if the diameter of the nozzle  123  is too small, a pressure drop in the nozzle  123  may increase, reducing performance of the gas bearing. 
     Thus, in this embodiment, the inlet  123   a  of the nozzle  123  may have a relatively large diameter to reduce the pressure drop of the refrigerant introduced into the nozzle  123 . In addition, the outlet  123   b  may have a relatively small diameter to control an inflow amount of gas bearing through the nozzle  123  to a predetermined value or less. 
     The third filter  330  may prevent a foreign substance having a predetermined size or more from being introduced into the cylinder  120  and perform a function of absorbing oil contained in the refrigerant. The predetermined size may be about 1 μm, for example. 
     The third filter  330  may include a thread wound around the gas inflow  122 . The thread may be formed of a polyethylene terephthalate (PET) material and have a predetermined thickness or diameter, for example. 
     The thickness or diameter of the thread may be determined to have adequate dimensions in consideration of a rigidity of the thread. If the thickness or diameter of the thread is too small, the thread may be easily broken due to a very weak strength thereof. On the other hand, if the thickness or diameter of the thread is too large, a filtering effect with respect to the foreign substances may be deteriorated due to a very large pore in the gas inflow  122  when the thread is wound. 
     For example, the thickness or diameter of the thread may be several hundreds μm. The thread may be manufactured by coupling a plurality of strands of a spun thread having several tens μm to each other, for example. 
     The thread may be wound several times, and an end of the thread may be fixed through or by a knot. A wound number of the thread may be adequately selected in consideration of the pressure drop of the gas refrigerant and the filtering effect with respect to foreign substances. If the wound number of thread is too large, the pressure drop of the gas refrigerant may increase. On the other hand, if the wound number of thread is too small, the filtering effect with respect to foreign substances may be reduced. 
     Also, a tension force of the wound thread may be adequately controlled in consideration of a strain of the cylinder  120  and fixation of the thread. If the tension force is too large, deformation of the cylinder  120  may occur. On the other hand, if the tension force is too small, the thread may not be well fixed to the gas inflow  122 . 
       FIG. 8  is a perspective view of a discharge valve assembly coupled to the discharge cover according to an embodiment.  FIG. 9  is an exploded perspective view of the discharge cover and the discharge valve assembly of  FIG. 8 . 
     Referring to  FIGS. 8 and 9 , the linear compressor  100  according to this embodiment may include the discharge cover  200  coupled to a front portion of the frame  110  to define a discharge passage of the refrigerant discharged from the compression space P. The discharge cover  200  may include a cover body  200   a  that defines a discharge passage of the refrigerant discharged through the discharge valve  220 , a frame coupling part or portion  201  that extends from the cover body  200   a  in a radial direction and coupled to the frame  110 , and a pipe connection part or portion  202  that protrudes from the cover body  200   a  and discharges the refrigerant passing through the discharge passage of the discharge body  200   a  outside of the discharge cover  200 . The frame coupling portion  201  may be disposed on or at a rear surface of the discharge cover  200 , and the pipe connection portion  202  may be connected to the loop pipe  165 . 
     The discharge valve assembly  220 ,  230 ,  240  may be disposed on the discharge cover  200 . The discharge valve assembly may include the discharge valve  220 , the valve spring  230 , the stopper  240 , and the spacer  260 . The cover body  200   a  of the discharge cover  200  may include a plurality of steps  203  and  205  stepped in a forward direction from the frame coupling portion  201 . The plurality of steps  203  and  205  may include a first step  203  recessed in a backward direction from the frame coupling portion  201 , and a second step  205  further recessed from the first step  203  toward a resonance chamber  212 . 
     The cover body  200   a  may further include a step connection part or portion  203   a  that extends inward from the first step  203  in the radial direction and connected to the second step  205 . That is, in the cover body  200   a , the first step  203  may extend inward in the radial direction, and then, may be further recessed backward to form the second step  205 . 
     The first step  203  may include a discharge hole  204  to guide the refrigerant passing through the discharge passage of the cover body  200   a  into the pipe connection portion  202  to discharge the refrigerant from the discharge cover  200 . The discharge hole  204  may pass through at least a portion of the first step  203 . The refrigerant discharged through the discharge valve  220  may flow into the pipe connection portion  202  via the discharge hole  204 . 
     The cover body  200   a  may further include the resonance chamber  212 , which may be further recessed from the second step  205 , to define a space to reduce pulsation of the refrigerant. A plurality of the resonance chamber  212  may be provided. At least a portion of the refrigerant discharged through the discharge valve  220  may flow into the space of the resonance chamber  212 . 
     The cover body  200   a  may further include a seat  210  to partition the plurality of resonance chambers  212  and support the spacer  260 . The plurality of resonance chambers  212  may be further recessed forward from the seat  210  and disposed to be spaced apart from each other by the seat  210 . 
     A first guide groove  206  that guides at least a portion of the refrigerant discharged through the discharge valve  220  into the plurality of resonance chambers  212  may be defined in the cover body  200   a  as a “gas passage”. The first guide groove  206  may extend forward from the step connection portion  203   a  toward the second step  205 . At least portions of the step connection portion  203   a  and the second step  205  may be cut to define the first guide groove  206 . 
     A plurality of the first guide groove  206  may be provided to correspond to a number of the resonance chambers  212 . The plurality of first guide grooves  206  may be defined to be spaced apart from each other. As at least a portion of the refrigerant discharged through the opened discharge valve  220  may be introduced into the plurality of resonance chambers  212  along the first guide groove  206 , pulsation generated when the refrigerant flows while the linear compressor  100  operates may be reduced. 
     A second guide groove  207  that guides coupling of the stopper  240  may be defined in the cover body  200   a . The second guide groove  207  may guide coupling between the stopper  240  and a guide protrusion  243 . At least portions of the step connection portion  203   a  and the second step  205  may be cut define the second guide groove  207 . 
     A plurality of the first guide groove  207  may be provided to correspond to a number of the guide protrusions  243  of the stopper  240 . The plurality of second guide grooves  207  may be defined to be spaced apart from each other. 
     The discharge valve  220  may include a valve body  221  selectively attached to a front surface of the cylinder flange  125  of the cylinder  120 , and a valve recess  223  recessed in a forward direction from the valve body  221 . The valve recess  223  may be referred to as an “interference prevention groove” that prevents at least a portion of the piston  130  from interfering with the discharge valve  220  while the piston  130  move forward to compress the refrigerant. At least a portion of the piston  130  may include a coupling member that couples the suction valve  135  to the piston  130 . 
     The discharge valve  220  may further include an insertion protrusion  222  that protrudes in a forward direction from the valve body  221  and coupled to the valve spring  230 . The insertion protrusion  222  may be coupled to an insertion hole  232  defined in the valve spring  230 . 
     Each of the insertion protrusion  222  and the insertion hole  232  may have a noncircular cross-sectional shape. For example, the cross-sectional shape may be a polygonal shape. Thus, when the discharge valve  220  is opened or closed in a state in which the insertion protrusion  222  is inserted into the insertion hole  232 , it may prevent the discharge valve  220  from rotating. As a result, it may prevent the discharge valve  220  from being behaving unstably. In particular, if the gas bearing instead of the oil bearing is used in the linear compressor as described above, as there is no lubrication action for the discharge valve by the oil, abrasion of the discharge valve due to unstable behavior may be reduced. 
     The valve spring  230  may include a plate spring and have an approximately circular plate shape. The valve spring  230  may be coupled to a front portion of the discharge valve  220  to allow the discharge valve  220  to elastically move. The valve spring  230  may include a spring body  231  having a plurality of cutouts, and the insertion hole  232  defined in an approximately central portion of the spring body  231  and in which the insertion protrusion  222  of the discharge valve  220  may be inserted. 
     The spring body  231  may have a circular plate shape. The plurality of cutouts may have a spiral shape. Also, the valve spring  230  may be elastically deformed by the plurality of cutouts. 
     The valve spring  230  may include a spring recess  233  recessed from an outer circumferential surface of the spring body  231 . The spring recess  233  may guide a position of the guide protrusion  243  of the stopper  240 . 
     The stopper  240  may be disposed on or at a front side of the valve spring  230 . In detail, the stopper  240  may include a stopper body  241  that restricts deformation of the valve spring  230  while the valve spring  230  is deformed. The stopper body  241  may have an approximately circular plate shape. Also, when the valve spring  230  is deformed by a preset or predetermined degree or more, the stopper body  241  may be disposed at a position at which the stopper body  241  interferes with the valve spring  230 . 
     The stopper  240  may further include a valve avoidance groove  242  recessed forward from the stopper body  241 . The valve avoidance groove  242  may be recessed from an approximately central portion of the stopper body  241  to prevent the stopper body  241  from interfering with the insertion protrusion  222  of the discharge valve  220 . That is, when the insertion protrusion  222  moves forward while the discharge valve  220  is opened, the valve avoidance groove  242  may provide an interference avoidance space so that the stopper body  241  does not interfere with the insertion protrusion  222 . 
     The stopper  240  may further include a guide protrusion  243  that protrudes backward from a rear surface of the stopper body  241  to guide coupling of the discharge cover  200 . When the stopper  240  is coupled to the discharge cover  200 , the guide protrusion  243  may move into the cover body  200   a  along the second guide groove  207 . 
     The guide protrusion  243  may be coupled to the spring recess  233  of the valve spring  230  and a spacer groove  252  of the first spacer  250 . Thus, the valve spring  230  may be stably coupled to the stopper  240  and the first spacer  250 . 
     For example, the stopper  240  may be press-fitted into and fixed to the second guide groove  207  in a state in which the guide protrusion  243  is coupled to the spring recess  233  and the spacer groove  252 . Thus, the stopper  240  may be stably coupled to the discharge cover  200  without using a separate coupling member. 
     The first spacer  250  may be disposed between the valve spring  230  and the stopper  240  to space the valve  230  from the stopper  240 . The first spacer  250  may include a spacer body  251  having an approximately ring shape and a spacer groove  252  recessed from an outer circumferential surface of the spacer body  251  to guide a position of the guide protrusion  243  of the stopper  240 . 
     The second spacer  260  may be seated on the seat  210  of the cover body  200   a  to support the stopper  240 . That is, the second spacer  260  may be disposed between the seat  210  and the stopper  240  to prevent the stopper  240  from directly colliding with the discharge cover  200 . 
       FIG. 10  is a view of a valve spring according to an embodiment.  FIG. 11  is a cross-sectional view of a discharge valve assembly according to an embodiment.  FIG. 12  is a cross-sectional view illustrating an effect of the discharge valve assembly of  FIG. 11 . 
     Referring to  FIG. 10 , the valve spring  230  according to an embodiment may include the spring body  231  having a plurality of cutouts ( 230   a ) and the insertion hole  232  defined in the spring body  231  and in which the insertion protrusion  222  of the discharge valve  220  may be inserted. 
     The spring body  231  may have a circular plate shape. The plurality of cutouts  230   a  may have a spiral shape and be disposed to be spaced apart from each other. 
     A central portion C 2  of the insertion hole  232  may be spaced apart from a central portion C 1  of the spring body  231 . The central portion C 1  of the spring body  231  may refer to a geometric center and a center of weight thereof. Thus, when the spring body  231  has a circular plate shape, a distance from the central portion C 1  to an outer circumferential surface of the spring body  231  may be constant. 
     The central portion C 1  of the spring body  231  may be disposed at a position corresponding to a center of the cylinder body  121 . That is, as the cylinder body  121  has a cylindrical shape, when a central line that passes through the center of the cylinder body  121  extends forward, the central portion C 1  of the spring body  231  may be formed at a position at which the spring body  231  meets the spring body  231 . 
     The plurality of cutouts  230   a  may extend in an outer radial direction to form a spiral shape with respect to the central portion C 2  of the insertion hole  232 . The cutouts  230   a  may be spaced a same distance S from the central portion C 2  to extend in a spiral shape. 
     When two points at which a virtual extension line l 1  that passes through the central portion C 1  of the spring body  231  and the central portion C 2  of the insertion hole  232  meets an outer circumferential surface of the spring body  231  are defined as points C 3  and C 4 , a distance between the points C 1  and C 3  may be the same as a distance between the points C 1  and C 4 . On the other hand, a distance between the points C 2  and C 3  may be less than a distance between the points C 2  and C 4 . 
     As described above, as the central portion C 1  of the spring body  231  and the central portion C 2  of the insertion hole  232  may be spaced apart from each other, the valve spring  230  may have an asymmetrical shape. For example, the valve spring  230  may have an asymmetrical shape with respect to the virtual extension line l 1  or a virtual extension line l 2  that passes through the central portion C 1 , but does not pass through the central portion C 2 . 
     That is, as the plurality of cutouts  230   a  extend to have a predetermined pattern with respect to the central portion C 2  of the insertion hole  232 , the valve spring  230  may have an asymmetrical shape with respect to the central portion C 1  of the spring body  231 . In other words, the plurality of cutouts  230   a  may be disposed to have an asymmetrical shape with respect to the central portion C 1  of the spring body  231 . 
     Referring to  FIG. 11 , the discharge valve  220  may include the valve body  221  selectively closely attached to a front surface of the cylinder flange  125  of the cylinder  120  and the insertion protrusion  222  that protrudes forward from the valve body  221  and coupled to the valve spring  230 . 
     A virtual extension line  8  that passes through a center of the valve body  221  and a virtual extension line l 4  that passes through a center of the insertion protrusion  222  inserted into the insertion hole  232  of the valve spring  230  may be spaced apart from each other. The virtual extension lines l 3  and l 4  may refer to a virtual line that extends in an axial direction. 
     A length of a rear surface of the valve body  221  in a radial direction may have a value of a 1 +a 2 . The rear surface of the valve body  221  may refer to a surface that is closely attached to the cylinder  120 . 
     A distance from a point P 1  at which the virtual extension line  8  and the rear surface of the valve body  221  meet each other to one outer circumferential surface of the rear surface of the valve body  221  may have a value a 1 , and a distance from the point P 1  to the other outer circumferential surface of the rear surface of the valve body  221  may have a value a 2 . The value a 1  may be the same as the valve a 2 . 
     A distance from a point P 2  at which the virtual extension line l 4  and the rear surface of the valve body  221  meet each other to one point of the rear surface of the valve body  221  may have a value b 1 , and a distance from the point P 2  to the other outer point of the rear surface of the valve body  221  may have a value b 2 . The value a 2  may be greater than the valve b 1 . For example, the one point may be a lower end, and the other point may be an upper end in  FIG. 11 . 
     In summary, the central portion of the valve body  221  of the discharge valve  220  and the central portion of the insertion protrusion  222  may be disposed to be spaced apart from each other, that is, eccentrically disposed with respect to each other. This may corresponds to an idea in which the central portion C 1  of the spring body  231  and the central portion C 2  of the insertion hole  232  are disposed to be spaced apart from each other. 
     As described above, as the coupling portion at which the discharge valve  220  and the valve spring  230  are coupled to each other, that is, the centers of the insertion protrusion  222  and the insertion hole  232  are eccentrically disposed with respect to the centers of the valve body  221  and the spring body  231 , respectively, the discharge valve  220  may be inclinedly opened in one direction. 
     Referring to  FIG. 12 , when a pressure of the compression space P is above the discharge pressure, a predetermined force F due to a pressure of the refrigerant may act on the rear surface of the valve body  221 . As the distance b 2  from the point P 2  to the other point of the rear surface of the valve body  221  is greater than b 2 , a moment M in one direction, for example, a moment in a clockwise direction in FIG.  12  may be generated. Thus, the discharge valve  220  may be opened while a lower portion of the discharge valve  220  rotates. 
     As described above, as the discharge valve  220  is opened while a portion of the discharge valve  220  rotates, but while the whole of the discharge valve  220  rotates, when the discharge of the refrigerant is completed, and then the discharge valve  220  is closed, an impact applied to the cylinder  120  may be reduced. That is, when the discharge valve  220  is opened, the discharge valve  220  may be inclinedly disposed with respect to the radial direction of the linear compressor  100 . 
       FIG. 13  is a cross-sectional view illustrating a flow of a refrigerant in the linear compressor of  FIG. 1 .  FIG. 14  is a view illustrating a state in which the discharge valve is opened when the linear compressor operates according to an embodiment. 
     Referring to  FIGS. 13 and 14 , the refrigerant may be introduced into the shell  101  through the suction port  104  and flow into the suction muffler  150  through the suction guide  155 . The refrigerant may be introduced into the second muffler  153  via the first muffler  151  of the suction muffler  150  to flow into the piston  130 . With this process, suction noise of the refrigerant may be reduced. 
     A foreign substance having a predetermined size (about 25 μm) or more, which is contained in the refrigerant, may be filtered while passing through the first filter  310  provided on the suction muffler  150 . The refrigerant within the piston  130  after passing though the suction muffler  150  may be suctioned into the compression space P through the suction hole  133  when the suction valve  135  is opened. 
     When the refrigerant pressure in the compression space P is above the discharge pressure, the discharge valve  220  may be opened. Thus, the refrigerant may be discharged into the discharge space of the discharge cover  200  through the opened discharge valve  220 , flow into the discharge port  105  through the loop pipe  165  coupled to the discharge cover  200 , and be discharged outside of the linear compressor  100 . 
     When the discharge valve  220  is opened, the valve spring  230  may be elastically deformed in the forward direction. With this process, the stopper  240  may prevent the valve spring  230  from being deformed by a preset or predetermined degree or more. 
     In particular, with this embodiment, when the linear compressor  100  operates at a high frequency, an opening degree of the discharge valve  220 , that is, movement of the discharge valve  220  may increase. Thus, when the discharge valve  220  is closed, an impulse applied to the discharge valve  220  may increase to increase abrasion of the discharge valve  220 . In particular, when the gas bearing is applied without using oil, abrasion may increase. 
     Thus, in this embodiment, the discharge valve  220  may be elastically supported by the valve spring  230 , and the stopper  240  may be disposed on or at one side of the valve spring  230  to restrict the opening degree of the discharge valve  220 . Also, as the valve spring  230  has the asymmetrical shape, and the central portions of the discharge valve  220  and the insertion protrusion  222  are eccentrically disposed, when the discharge valve  220  is opened, the discharge valve  220  may be inclinedly disposed in one direction. As a result, when the discharge of the refrigerant is completed, and then the discharge valve  220  is closed, the impulse may be reduced, and thus, abrasion of the discharge valve  220  may be reduced. 
     At least a portion of the refrigerant within the discharge space of the discharge cover  200  may flow toward the outer circumferential surface of the cylinder body  121  via the space defined between the cylinder  120  and the frame  110 , that is, the inner circumferential surface of the recess  117  of the frame  110  and the outer circumferential surface of the cylinder flange  121  of the cylinder  120 . The refrigerant may pass through the second filter  320  disposed between the seat surface  127  of the cylinder flange  125  and the seat  113  of the frame  110 . With this process, a foreign substance having a predetermined size (about 2 μm) or more may be filtered. Also, oil of the refrigerant may be adsorbed onto or into the second filter  320 . 
     The refrigerant passing through the second filter  320  may be introduced into the plurality of gas inflows  122  defined in the outer circumferential surface of the cylinder body  121 . Also, while the refrigerant passes through the third filter  330  provided on the plurality of gas inflows  122 , a foreign substances having a predetermined size (about 1 μm) or more, which is contained in the refrigerant, may be filtered, and the oil contained in the refrigerant may be adsorbed. 
     The refrigerant passing through the third filter  330  may be introduced into the cylinder  120  through the nozzle  123  and be disposed between the inner circumferential surface of the cylinder  120  and the outer circumferential surface of the piston  130  to space the piston  130  from the inner circumferential surface of the cylinder  120  (gas bearing). The inlet  123   a  of the nozzle  123  may have a diameter greater than a diameter of the outlet  123   b . Thus, a refrigerant flow section area on the nozzle  123  may gradually decrease with respect to a flow direction of the refrigerant. For example, the inlet  123   a  may have a diameter greater two times than the diameter of the outlet  123   b.    
     As described above, the high-pressure gas refrigerant may be bypassed within the cylinder  120  to serve as the gas bearing with respect to the piston  130  which is reciprocated, thereby reducing abrasion between the piston  130  and the cylinder  120 . Also, as oil for the bearing is not used, friction loss due to oil may not occur even though the linear compressor  100  operates at a high rate. 
     Further, as the plurality of filters are provided in the passage of the refrigerant flowing in the linear compressor  100 , foreign substances contained in the refrigerant may be removed. Thus, the refrigerant acting as the gas bearing may be improved in reliability. Thus, it may prevent the piston  130  or the cylinder  120  from being worn by the foreign substances contained in the refrigerant. 
     Furthermore, as the oil contained in the refrigerant is removed by the plurality of filters, it may prevent friction loss due to the oil from occurring. The first, second, and third filters  310 ,  320 , and  330  may be referred to as a “refrigerant filter” in that the filters  310 ,  320 , and  330  filter the refrigerant that serves as the gas bearing. 
     According to embodiments disclosed herein, the linear compressor including the inner components may be decreased in size to reduce a volume of a machine room of a refrigerator and increase an inner storage space of the refrigerant. Also, a drive frequency of the linear compressor may be increased to prevent performance of the inner components from being deteriorated due to a decreasing size thereof. In addition, as the gas bearing is applied between the cylinder and the piston, a friction force occurring due to oil may be reduced. 
     Further, the discharge valve that selectively discharges the high-pressure gas compressed in the compression chamber may stably operate. In addition, an impulse occurring while the discharge valve operates may be reduced, reducing abrasion of the discharge valve. As a result, it may prevent foreign substances generated due to abrasion of the discharge valve from having an influence on the gas bearing. In particular, as the discharge valve is inclinedly opened in one direction, the impulse due to the impact with the cylinder may be reduced while the discharge valve is closed. 
     Furthermore, an opening degree of the discharge valve may be restricted by the stopper to reduce a time taken to close the discharge valve, thereby improving a response for operating the discharge valve. 
     Additionally, a resonance chamber may be provided in the discharge cover to reduce pulsation of the discharge gas, thereby reducing noise. 
     Also, as the plurality of filtering device is provided in the linear compressor, it may prevent foreign substances or oil contained in the compression gas (or discharge gas) introduced outside of the piston from the nozzle part of the cylinder from being introduced. Therefore, as blocking of the nozzle part of the cylinder may be prevented, as gas bearing effect may be effectively performed between the cylinder and the piston, and thus, abrasion of the cylinder and the piston may be prevented. 
     Embodiments disclosed herein provide a linear compressor in which abrasion of a discharge valve may be reduced. 
     Embodiments disclosed herein provide a linear compressor that may include a shell in which a discharge port is provided; a cylinder disposed in the shell to define a compression space for a refrigerant; a piston disposed to be reciprocated in an axial direction within the cylinder; a discharge valve disposed on or at one side of the cylinder to selectively discharge the refrigerant compressed in the compression space, the discharge valve including an insertion protrusion; and a valve spring coupled to the discharge valve to provide a restoring force to the discharge valve. The valve spring may include a spring body having a central portion (C 1 ) defined at a portion corresponding to a center of the cylinder; and an insertion hole defined in the spring body. The insertion hole may be coupled to the insertion protrusion of the discharge valve. The central portion (C 1 ) of the spring body may be spaced apart from a central portion (C 2 ) of the insertion hole. 
     The valve spring may have an asymmetrical shape with respect to a virtual extension line that passes through the central portion (C 1 ) of the spring body. A distance between one point (C 3 ) and the central portion (C 2 ) may be less than a distance between the other point (C 4 ) and the central portion (C 2 ) with respect to the two points (C 3 , C 4 ) at which a virtual extension line (l 1 ) that passes through the central portion (C 1 ) of the spring body and the central portion (C 2 ) of the insertion hole meets an outer circumferential surface of the spring body. The valve spring may have a spiral shape and may include at least one cutoff part or cutout that extends in an outer radial direction with respect to the central portion (C 2 ) of the insertion hole. 
     The discharge valve may further include a valve body that is closely attached to the cylinder, and the insertion protrusion may protrude from the valve body. A center of the valve body and a center of the insertion protrusion may be spaced apart from each other. A first virtual extension line (l 3 ) that passes through the center of the valve body, and a second virtual extension line (l 4 ) that passes through the center of the insertion protrusion may be spaced apart from each other. 
     The insertion protrusion may be eccentrically coupled to the valve body so that the discharge valve rotates in one direction when a pressure of the refrigerant is applied to the valve body. When the valve body is opened by the pressure of the refrigerant, the valve body may be disposed inclinedly with respect to a radius direction. 
     Each of the insertion protrusion of the discharge valve and the insertion hole of the valve spring may have a non-circular shape in section. The valve spring may include a plate spring. 
     The linear compressor may further include a frame that fixes the cylinder to the shell, and a discharge cover coupled to the frame. The discharge cover may have a resonance chamber to reduce pulsation of the refrigerant discharged through the discharge valve. 
     The linear compressor may further include a stopper coupled to the valve spring to restrict deformation of the valve spring. The linear compressor may further include a first spacer disposed between the valve spring and the stopper to space the valve spring from the stopper, and a second spacer disposed on the cover body to support the support. 
     The cylinder may include a nozzle part or nozzle disposed on an outer circumferential surface thereof to introduce at least a portion of the refrigerant discharged through the discharge valve. 
     Embodiments disclosed herein further provide a linear compressor that may include a shell in which a discharge port is provided; a cylinder disposed in the shell to define a compression space for a refrigerant; a piston disposed to be reciprocated in an axial direction within the cylinder; a discharge valve disposed on or at one side of the cylinder to selectively discharge the refrigerant compressed in the compression space, the discharge valve including a valve body and an insertion protrusion that is eccentrically coupled to the valve body; and a valve spring coupled to the discharge valve to provide a restoring force to the discharge valve. The valve spring may have an insertion hole coupled to the insertion protrusion of the discharge valve. When the refrigerant is discharged from the compression space, the discharge valve may be inclinedly opened with respect to a radial direction. 
     The valve spring may include a spring body having the insertion hole, and the insertion hole may be defined in or at a position which is eccentric from a central portion of the spring body. The valve spring may include a plurality of cutoff parts or cutouts having an asymmetrical shape with respect to the central portion of the spring body. The plurality of cutoff parts may have a spiral shape. 
     When the valve spring is deformed to a set or predetermined level or more, the linear compressor may further include a stopper that restricts the valve spring; a first spacer disposed on or at one or a first side of the stopper; and a second spacer disposed on or at the other or a second side of the stopper. 
     Any reference in this specification to “one embodiment,” “an embodiment,” “example embodiment,” etc., means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to effect such feature, structure, or characteristic in connection with other ones of the embodiments. 
     Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.