Patent Publication Number: US-10767649-B2

Title: Hermetic compressor with cylinder having elliptical inner circumferential surface, roller, and at least one vane

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     Pursuant to 35 U.S.C. § 119(a), this application claims the benefit of an earlier filing date of and the right of priority to Korean Application No. 10-2016-0182836, filed in Korea on Dec. 29, 2016, the contents of which are incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Field 
     A hermetic compressor, and more particularly, to a vane rotary compressor is disclosed herein. 
     2. Background 
     Generally, a rotary compressor is a compressor having a structure in which a roller and a vane contact each other, and a compression space of a cylinder is divided into a suction chamber and a discharge chamber on the basis of the vane. In such a general rotary compressor (hereinafter referred to as a “rotary compressor”), a vane performs a linear motion while a roller performs an orbiting motion, and a refrigerant is suctioned, compressed, and discharged as a suction chamber and a discharge chamber form a compression chamber having its volume changed. 
     Contrary to such a rotary compressor, there is a vane rotary compressor having a structure in which a vane inserted into a roller performs a rotary motion together with the roller, and a structure in which a compression chamber is formed as the vane is withdrawn by a centrifugal force and a back pressure. In such a vane rotary compressor, a plurality of vanes is rotated together with a roller, and the vanes slide as front end surfaces thereof contact an inner circumferential surface of a cylinder. This may cause a frictional loss to be increased in comparison to a general rotary compressor. 
     Such a vane rotary compressor may be formed such that an inner circumferential surface of a cylinder may have a circular shape. However, recently, a vane rotary compressor having a hybrid cylinder (hereinafter, referred to as a “hybrid rotary compressor”) has been introduced, capable of reducing a frictional loss and enhancing a compression efficiency as an inner circumferential surface of a cylinder has an elliptical shape or a combination shape of an ellipse and a circle. 
       FIG. 1  is a cross-sectional view of a compression part of a vane rotary compressor in accordance with the conventional art.  FIG. 2  is a schematic view for explaining a shape of an inner circumferential surface of a hybrid cylinder in the compression part of  FIG. 1 . 
     As shown, the conventional hybrid cylinder is formed as a symmetrical elliptical cylinder an inner circumferential surface of which is symmetrical on the basis of a first center line (L 1 ) passing through a neighboring position between an inner circumferential surface of the cylinder  1  and an outer circumferential surface of a roller  2  (hereinafter, referred to as a “first contact point” (P 1 )) and passing through a center (Oc) of the cylinder  1 , and on the basis of a second center line (L 2 ) perpendicular to the first center line (L 1 ) and passing through the center (Oc) of the cylinder  1 . That is, as shown in  FIG. 2 , the inner circumferential surface of the cylinder  1  includes a first ellipse  1   a  which is at an upper side on the basis of the first center line (L 1 ), and a second ellipse  1   b  which is at a lower side on the basis of the first center line (L 1 ). The first ellipse  1   a  has a symmetrical shape on the basis of the second center line (L 2 ), and the second ellipse  1   b  has a symmetrical shape on the basis of the second center line (L 2 ). 
     The roller  2  is eccentric from the center (Oc) of the cylinder  1 , and a center (Or) of the roller  2  is concentric with a center (Os) of a rotary shaft  3 . Accordingly, even while the roller  2  is being rotated, the contact point (P 1 ) between the cylinder  1  and the roller  2  is maintained at a same position. 
     An outer circumferential surface of the roller  2  has a circular shape, and a plurality of vane slots  21  is formed on the outer circumferential surface of the roller  2  in a circumferential direction. As vanes  4  are slidably inserted into the vane slots  21 , a compression space  11  of the cylinder  1  is divided into a plurality of compression chambers  11   a , 11   b , 11   c.    
     Back pressure chambers  22  that pressurize the vanes  4  towards the inner circumferential surface of the cylinder  1  by introducing oil (or a refrigerant) towards rear surfaces of the vanes  4  are formed at inner ends of the vane slots  21  corresponding to the rear surfaces of the vanes  4 . Accordingly, if the roller  2  is rotated, the vanes  4  are withdrawn from the roller  2  by a centrifugal force and a back pressure to contact the inner circumferential surface of the cylinder  1  at a contact point (P 2 ). The contact point (P 2 ) between the vanes  4  and the cylinder  1  moves along the inner circumferential surface of the cylinder  1 . 
     On the basis of the first contact point (P 1 ) between the cylinder  1  and the roller  2 , a suction opening  12  is formed at one side of the inner circumferential surface of the cylinder  1 , and discharge openings  13   a , 13   b  are formed at another side thereof. 
     The vane rotary compressor has an over-compression because its compression period is shorter than that of a general rotary compressor. Due to the over-compression, a compression loss occurs. Accordingly, in the conventional cylinder  1 , in order to solve such over-compression, a compressed refrigerant is partially and sequentially discharged through a plurality of discharge openings  13   a , 13   b  formed along a compression path (a compression direction). 
     The discharge openings  13   a , 13   b  may include a sub discharge opening  13   a  (or a first discharge opening) positioned at an upstream side on the basis of the compression path, and a main discharge opening  13   b  (or a second discharge opening) positioned at a downstream side. Discharge valves  51 ,  52  are installed outside the discharge openings  13   a , 13   b.    
     In the conventional vane rotary compressor, as aforementioned, in order to solve over-compression, the plurality of discharge openings  13   a , 13   b  is formed on the inner circumferential surface of the cylinder  1 , along the compression path. However, if the discharge opening  13   a  (especially, the sub discharge opening) has a very large inner diameter, leakage may increase among the compression chambers  11   a , 11   b , 11   c . Accordingly, the inner diameter of the discharge opening  13   a  cannot be sufficiently obtained, and the over-compression cannot be solved. This may lower a compression efficiency. 
     Further, in the conventional vane rotary compressor, as the inner circumferential surface of the cylinder  1  is formed in a symmetrical shape, a volume diagram of the compression chambers cannot be variously controlled. As a result, there is a limitation in moving a suction completion time or a compression starting time towards the first contact point. 
     Furthermore, in the conventional vane rotary compressor, a compression starting time at the compression space of the cylinder  1  is delayed, and thus, a compression period becomes short. This may increase a pressure difference between the compression chambers. As a result, refrigerant leakage between the compression chambers may be increased, and a frictional loss may be increased between the cylinder and the vanes. 
     Also, in the conventional vane rotary compressor, as the compression starting time at the compression chambers of the cylinder  1  is delayed, a gradient of the compression period is sharply increased. This may lower a compression efficiency due to over-compression. 
     Additionally, in the conventional vane rotary compressor, as the cylinder  1  and the roller  2  linearly-contact each other at the first contact point (P 1 ), a sealing area is reduced. This may cause refrigerant leakage between the compression chamber which forms the suction chamber, and the compression chamber which forms the discharge chamber. This may cause a suction loss or a compression loss. 
    
    
     
       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 compression part of a vane rotary compressor in accordance with the conventional art; 
         FIG. 2  is a schematic view for explaining a shape of an inner circumferential surface of a hybrid cylinder in the compression part of  FIG. 1 ; 
         FIG. 3  is a longitudinal sectional view of a vane rotary compressor having a hybrid cylinder according an embodiment; 
         FIG. 4  is a cross-sectional view of a compression part applied to  FIG. 3 ; 
         FIGS. 5A to 5D  are sectional view showing processes to suction, compress and discharge a refrigerant in a cylinder according to an embodiment; 
         FIG. 6  is a schematic view for explaining a shape of an inner circumferential surface of a cylinder according to an embodiment; 
         FIGS. 7A-7C  shows graphs comparing suction completion times with each other according to a shape of an ellipse which forms an inner circumferential surface of a cylinder; and 
         FIG. 8  is an enlarged sectional view of the cylinder shown in  FIG. 4  according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a vane rotary compressor according to an embodiment will be explained with reference to the attached drawings. Where possible, like reference numerals have been used to indicate like elements, and repetitive disclosure has been omitted. 
       FIG. 3  is a longitudinal sectional view of a vane rotary compressor having a hybrid cylinder according to an embodiment.  FIG. 4  is a cross-sectional view of a compression part applied to  FIG. 3 . 
     As shown in  FIG. 3 , in the vane rotary compressor according to an embodiment, a motor part or motor  200  is installed in a casing  100 , and a compression part or device  300  connected to the motor part  200  by a rotary shaft  230  is installed at one side of the motor part  200 . The motor part  200  may include a stator  210  and a rotor  220 . The casing  100  may be categorized into a horizontal type or a vertical type according to an installation aspect of the compressor. The vertical type has a structure that the motor part and the compression part are disposed at upper and lower sides in an axial direction, whereas the horizontal type has a structure that the motor part and the compression part are disposed at right and left or lateral sides. 
     The compression part  300  may include a cylinder  330  having a compression space  410  by a main bearing  310  and a sub bearing  320  installed at both sides in an axial direction. The cylinder  330  according to this embodiment may be formed such that an inner circumferential surface thereof has an elliptical shape rather than a circular shape. The cylinder  330  may be formed as a symmetrical ellipse having a pair of long and short axes, or may be formed as an asymmetrical ellipse having a plurality of pairs of long and short axes. Such a cylinder having an asymmetrical elliptical shape is called a hybrid cylinder, and this embodiment is related to a vane rotary compressor to which a hybrid cylinder is applied. 
     As shown in  FIG. 4 , the hybrid cylinder  330  according to this embodiment (hereinafter, referred to as a “cylinder”) may have an outer circumferential surface  331  formed in a circular shape. However, the outer circumferential surface  331  of the cylinder  330  may be formed in a non-circular shape, if it can be fixed to an inner circumferential surface of the casing  100 . The main bearing  310  or the sub bearing  320  may be fixed to the inner circumferential surface of the casing  100 , and the cylinder  330  may be coupled to the bearing fixed to the casing  100  by, for example, a bolt. 
     An empty space portion or space which forms a compression space  333  and includes an inner circumferential surface  332  is formed at a middle part or portion of the cylinder  330 . The empty space portion is sealed by the main bearing  310  and the sub bearing  320 , thereby forming the compression space  333 . A roller  340 , which is discussed hereinafter, may be rotatably coupled to the compression space  333 . 
     A suction opening  334  and discharge openings  335   a , 335   b  may be formed at both sides of the inner circumferential surface  332  of the cylinder  330  in a circumferential direction, on the basis of a point where the inner circumferential surface  332  of the cylinder  330  and an outer circumferential surface  341  of the roller  340  almost contact each other. The suction opening  334  may be directly connected to a suction pipe  120  which penetrates the casing  100 , and the discharge openings  335   a , 335   b  may be indirectly connected to a discharge pipe  130  which may be penetratingly-coupled to the casing  100  to communicate with an inner space  110  of the casing  100 . Thus, a refrigerant may be directly suctioned into the compression space  333  through the suction opening  334 . On the other hand, the compressed refrigerant may be discharged to the inner space  110  of the casing  100  through the discharge openings  335   a , 335   b , and then discharged to the discharge pipe  130 . Accordingly, the inner space  110  of the casing  100  maintains a high pressure state which forms a discharge pressure. 
     An additional suction valve is not installed at the suction opening  334 , whereas discharge valves  336   a , 336   b  that open and close the discharge openings  335   a , 335   b  may be installed at the discharge openings  335   a , 335   b . The discharge valves  336   a , 336   b  may be implemented as reed valves, one ends of which may be fixed and another ends of which may be formed as free ends. However, the discharge valves  336   a , 336   b  may be variously implemented as piston valves, for example, rather than reed valves. 
     If the discharge valves  336   a , 336   b  are implemented as reed valves, valve grooves  337   a , 337   b  to mount the discharge valves  336   a , 336   b  may be formed on an outer circumferential surface of the cylinder  330 . Accordingly, as a length of the discharge openings  335   a , 335   b  is minimized, a dead volume may be reduced. As shown in  FIG. 4 , the valve grooves  337   a , 337   b  may be formed in a triangular shape so as to obtain a flat valve seat surface. 
     A plurality of the discharge openings  335   a , 335   b  may be formed along a compression path (a compression direction). For convenience, the discharge openings  335   a , 335   b  may be sorted as a sub discharge opening (or a first discharge opening)  335   a  positioned at an upstream side on the basis of the compression path, and a main discharge opening (or a second discharge opening)  335   b  positioned at a downstream side. 
     However, the sub discharge opening is not necessarily required, but may be selectively provided. For example, in this embodiment, if the inner circumferential surface  332  of the cylinder  330  reduces an over-compression of a refrigerant as a compression period is formed to be long, which is discussed hereinafter, the sub discharge opening may not be formed. However, in order to minimize an over-compression amount of a refrigerant to be compressed, the sub discharge opening  335   a  may be formed at a front side of the main discharge opening  335   b , for example, at an upstream side of the main discharge opening  335   b  on the basis of the compression direction. 
     The roller  340  may be rotatably provided at the compression space  333  of the cylinder  330 . The roller  340  may have a circular outer circumferential surface, and the rotary shaft  230  may be integrally coupled to a center of the roller  340 . As a result, the roller  340  has a center (Or) consistent with a center of the rotary shaft  230 , and the roller  340  is rotated around the center (Or) together with the rotary shaft  230 . 
     The center (Or) of the roller  340  is eccentric from a center (Oc) of the cylinder  330 , that is, a center of an inner space of the cylinder  330 , so that one side of the outer circumferential surface  341  of the roller  340  almost contacts the inner circumferential surface  332  of the cylinder  330 . When it is assumed that a point of the cylinder  330  to which one side of the roller  340  almost contacts is a first contact point (P 1 ), the first contact point (P 1 ) may be at a position corresponding to a short-axis of an ellipse formed as a first center line (L 1 ) passing the center (Oc) of the cylinder  330  contacts the inner circumferential surface  332  of the cylinder  330 . 
     Vane slots  342  may be formed on the outer circumferential surface  341  of the roller  340  in a circumferential direction, and vanes  351 , 352 , 353  may be slidably coupled to the vane slots  342 . The vane slots  342  may be formed in a radial direction on the basis of the center (Or) of the roller  340 . However, in this case, it is difficult to sufficiently obtain a length of the vanes. Thus, the vane slots  342  may be formed with a predetermined inclination angle in the radial direction, for obtainment of the vane length. 
     The vanes  351 , 352 , 353  may be inclined in a reverse direction to a rotational direction of the roller  340 . That is, front end surfaces of the vanes  351 , 352 , 353 , which contact the inner circumferential surface  332  of the cylinder  330 , may be inclined towards the rotational direction of the roller  340  such that a compression starting angle may be towards the rotational direction of the roller  340  for early-start of compression. 
     Back pressure chambers  343  for pressurizing the vanes  351 , 352 , 353  towards the inner circumferential surface  332  of the cylinder  330  by introducing oil (or a refrigerant) towards a rear side of the vanes  351 , 352 , 353  may be formed at inner ends of the vane slots  342 . The back pressure chambers  343  may be sealed by the main bearing  310  and the sub bearing  320 . The back pressure chambers  343  may independently communicate with a back pressure passage (not shown). However, the back pressure chambers  343  may communicate together with the back pressure passage. 
     The vanes  351 , 352 , 353  may include a first vane  351  closest to the first contact point (P 1 ) on the basis of the compression direction, a second vane  352  secondly-closest to the first contact point (P 1 ), and the third vane  353  farthest from the first contact point (P 1 ). In this case, the first and second vanes  351 ,  352  may be spaced from each other, the second and third vanes  352 ,  353  may be spaced from each other, and the third and first vanes  353 ,  351  may be spaced from each other, by a same circumferential angle. 
     Thus, when a compression chamber formed by the first and second vanes  351 ,  352  is a first compression chamber  333   a , a compression chamber formed by the second and third vanes  352 ,  353  is a second compression chamber  333   b , and a compression chamber formed by the third and first vanes  353 ,  351  is a third compression chamber  333   c , all the compression chambers  333   a , 333   b , 333   c  have a same volume at a same crank angle. 
     The vanes  351 , 352 , 353  may be formed to have an approximate rectangular parallelepiped shape. Both ends of the vane in a lengthwise direction may include a front end surface contacting the inner circumferential surface  332  of the cylinder  330 , and a rear end surface facing the back pressure chamber. 
     The front end surfaces of the vanes  351 , 352 , 353  may be curved so as to linearly-contact the inner circumferential surface  332  of the cylinder  330 . The rear end surfaces of the vanes  351 , 352 , 353  may be flat so as to evenly receive a back pressure by being inserted into the back pressure chambers  343 . 
     In the vane rotary compressor having a hybrid cylinder, if power is supplied to the motor part  200  to rotate the rotor  220  of the motor part  200  and the rotary shaft  230  coupled to the rotor  220 , the roller  340  is rotated together with the rotary shaft  230 . Then, the vanes  351 , 352 , 353  are withdrawn from or inserted into the vane slots  343  by a centrifugal force generated when the roller  340  is rotated, and by a back pressure formed at a rear side of the vanes  351 , 352 , 353 . As a result, the front end surfaces of the vanes  351 , 352 , 353  contact the inner circumferential surface  332  of the cylinder  330 . 
     Then, the compression space  333  of the cylinder  330  forms compression chambers having a same number as the vanes  351 , 352 , 353 , by the plurality of vanes  351 , 352 , 353 . Each of the compression chambers  333   a , 333   b , 333   c  has its volume changed by a shape of the inner circumferential surface  332  of the cylinder  330  and an eccentric state of the roller  340  while moving along a rotation of the roller  340 . A refrigerant filled in each of the compression chambers  333   a , 333   b , 333   c  is suctioned, compressed, and discharged while moving along the roller  340  and the vanes  351 , 352 , 353 . 
     This will be explained hereinafter.  FIGS. 5A to 5D  are sectional view showing processes to suction, compress, and discharge a refrigerant in the cylinder according to an embodiment. 
     As shown in  FIG. 5A , until before the first vane  351  passes through the suction opening  334  and the second vane  352  reaches a suction completion time, a volume of the first compression chamber  333   a  is continuously increased. As a result, a refrigerant is continuously introduced into the first compression chamber  333   a  from the suction opening  334 . 
     As shown in  FIG. 5B , if the second vane  352  reaches the suction completion time (or a compression starting angle), the first compression chamber  333   a  is in a sealed state to move towards the discharge openings together with the roller  340 . In this process, the volume of the first compression chamber  333   a  is continuously decreased. As a result, the refrigerant in the first compression chamber  333   a  is gradually compressed. 
     As shown in  FIG. 5C , if the first vane  351  passes through the first discharge opening  335   a  and the second vane  352  does not reach the first discharge opening  335   a , the first compression chamber  333   a  communicates with the first discharge opening  335   a , and the first discharge valve  336   a  is opened by a pressure of the first compression chamber  333   a . Then, the refrigerant in the first compression chamber  333   a  is partially discharged to the inner space  110  of the casing  100  through the first discharge opening  335   a . As a result, the pressure of the first compression chamber  333   a  is lowered to a predetermined value. If the first discharge opening  335   a  is not provided, the refrigerant of the first compression chamber  333   a  further moves towards the second discharge opening  335   b , the main discharge opening without being discharged out. 
     As shown in  FIG. 5D , if the first vane  351  passes through the second discharge opening  335   b  and the second vane  352  reaches a discharge starting angle, the second discharge valve  336   b  is opened by the pressure of the first compression chamber  333   a . As a result, the refrigerant of the first compression chamber  333   a  is discharged to the inner space  110  of the casing  100  through the second discharge opening  336   b.    
     The above processes are equally repeated at the second compression chamber  333   b  between the second and third vanes  352 ,  353 , and at the third compression chamber  333   c  between the third and first vanes  353 ,  351 . Accordingly, in the vane rotary compressor according to this embodiment, a discharge operation is performed three times per single rotation of the roller  340 , that is, six times if a discharge operation from the first discharge opening is included. 
     However, if the hybrid cylinder has an inner circumferential surface formed in a symmetrical shape, a suction period is relatively long, and a compression period becomes short. This may cause a pressure difference at each compression chamber to be increased, resulting in leakage of a refrigerant to a space between the cylinder  330  and the vanes. Also, if a back pressure with respect to the vanes is increased, a frictional loss may be increased between the cylinder  330  and the vanes. Further, as the compression period becomes short, a gradient of the compression period also becomes steep. This may increase an over-compression amount, resulting in lowering a compression efficiency. 
     The hybrid cylinder according to this embodiment may prevent or solve over-compression by lowering a pressure difference between the compression chambers and by making the compression period have a gradual gradient, by decreasing the suction period of the compression chambers and by increasing the compression period. 
       FIG. 6  is a schematic view for explaining a shape of the inner circumferential surface of the cylinder according to an embodiment. As shown, the hybrid cylinder according to this embodiment may be formed such that an inner circumferential surface thereof may have an elliptical shape. In this case, center points (O′,O″) of the ellipse may be spaced apart from a center (Oc) of the cylinder  330  by a predetermined gap, in an eccentric manner. For a reduced suction period and an increased compression period, the center points (O′,O″) of the ellipse may be positioned at the suction opening  334  and the discharge openings  335   a , 335   b  on the basis of a second center line (L 2 ) perpendicular to a first center line (L 1 ) passing through the first contact point (P 1 ) and the center (Oc) of the cylinder  330 . 
     Further, if the center point (O′) of the ellipse where the suction opening is formed is farther from the center (Oc) of the cylinder  330  than the center (O″) of the ellipse where the discharge openings are formed, between the center points (O′,O″) which constitute the inner circumferential surface  332  of the cylinder  330 , the compression starting angle may be towards the suction opening as the suction period becomes short or decrease. This may be more effective to restrict an over-compression. Further, the center point (O″) of the ellipse where the discharge openings are formed may be closer to the center point (O′) of the ellipse where the suction opening is formed, or may be farther from the center (Oc) of the cylinder  330  than the center point (O′) of the ellipse. In this case, the compression period may become long or increase and the compression gradient may become gradual. This may be effective to reduce a compression loss. 
     For example, in the cylinder according to this embodiment, it is assumed that a line passing through the first contact point (P 1 ) where the inner circumferential surface  332  of the cylinder  330  and the outer circumferential surface  341  of the roller  340  are closest to each other and passing through the center of the cylinder  330 , is defined as a first center line (L 1 ). And it is assumed that a line perpendicular to the first center line (L 1 ) and passing through the center of the cylinder  330  is defined as a second center line (L 2 ). In this case, the inner circumferential surface  332  of the cylinder  330  may have an asymmetrical shape on the basis of the first and second center lines (L 1 , L 2 ). 
     That is, it is assumed that the inner circumferential surface  332  of the cylinder  330  includes a first ellipse (or partial ellipse)  332   a  positioned at one side of the first center line (L 1 ), and a second ellipse (or partial ellipse) 332   b  positioned at another side of the first center line (L 1 ). And it is assumed that the first ellipse has a first center point (O′), and the second ellipse has a second center point (O″). In this case, the first center point (O′) and the second center point (O″) are spaced apart from the center (Oc) of the cylinder  330  in the same direction, on the first center line (L 1 ). 
     The first ellipse  332   a  may be formed as two ellipses (or partial ellipses)  332   a ′,  332   a ″ having a same sum of distances to two focal points for every point thereon. Center points of the two ellipses  332   a ′,  332   a ″ are overlapped with each other to form the same center point (O′). The ellipse  332   a ′ positioned at a relatively short distance from the first contact point (P 1 ) than the ellipse  332   a ″ positioned at a relatively long distance from the first contact point (P 1 ) may be formed such that a distance between two focal points thereon is relatively large. That is, the first ellipse  332   a  may be formed in an asymmetrical shape, on the basis of a center line passing through the first center point (O′) and perpendicular to the first center line (L 1 ) (hereinafter, referred to as a “third center line L 3 ”), even if the sum of distances to two focal points for every point thereon is the same. 
     In this case, it is assumed that a section from the first contact point (P 1 ) to an intersection point between the two ellipses  332   a ′, 332   a ″, that is, the third center line (L 3 ) is defined as a first quadrant (Q 1 ), and a section from the third center line (L 3 ) to the first center line (L 1 ) in the rotational direction of the roller  340  is defined as a second quadrant (Q 2 ). The ellipse  332   a ′ in the first quadrant (Q 1 ) and the ellipse  332   a ″ in the second quadrant (Q 2 ) have a same length of a long axis, but have different lengths of short axes. That is, as the ellipse  332   a ″ in the second quadrant (Q 2 ) has the longer short axis than the ellipse  332   a ′ in the first quadrant (Q 1 ), an eccentricity of the ellipse  332   a ′ in the first quadrant (Q 1 ) is larger than that of the ellipse  332   a ″ in the second quadrant (Q 2 ). 
     With such a configuration, a suction volume in the first quadrant (Q 1 ) is increased, and a suction completion time becomes short or decreases. This may allow the suction opening  334  to move towards the contact point. 
     If the suction completion time becomes short or decreases, a compression starting time becomes early, resulting in increasing a compression period. If the compression period is increased, a motor efficiency may be enhanced, and thus, a compression efficiency of the compressor may be enhanced. Further, as a linear velocity in the first quadrant (Q 1 ) and the second quadrant (Q 2 ) is reduced, a frictional loss on the inner circumferential surface  332  of the cylinder  330  corresponding to the first ellipse  332   a  may be reduced. 
     Like the first ellipse  332   a , the second ellipse  332   b  may be formed as two ellipses (or partial ellipses)  332   b ′,  332   b ″ having a same sum of distances to two focal points for every point thereon. Center points of the two ellipses  332   b ′,  332   b ″ are overlapped with each other to form the same center point (O′). The ellipse positioned at a relatively short distance from the first contact point (P 1 ) than the ellipse positioned at a relatively long distance from the first contact point (P 1 ) may be formed such that a distance between two focal points thereon is relatively large. That is, the second ellipse  332   b  may be formed in an asymmetrical shape, on the basis of a center line passing through the second center point (O″) and perpendicular to the first center line (L 1 ) (hereinafter, referred to as a “fourth center line L 4 ”), even if the sum of distances to two focal points for every point thereon is the same. 
     In this case, it is assumed that a section from the first center line (L 1 ) to an intersection point between the two ellipses  332   b ′, 332   b ″, that is, the fourth center line (L 4 ) in the rotational direction of the roller  340  is defined as a third quadrant (Q 3 ), and a section from the fourth center line (L 4 ) to the first contact point (P 1 ) in the rotational direction of the roller  340  is defined as a fourth quadrant (Q 4 ). The ellipse  332   b ′ in the third quadrant (Q 3 ) and the ellipse  332   b ″ in the fourth quadrant (Q 4 ) have a same length of a long axis, but have different lengths of short axes. That is, as the ellipse  332   b ″ in the fourth quadrant (Q 4 ) has a smaller short axis than the ellipse  332   b ′ in the third second quadrant (Q 3 ), an eccentricity of the ellipse  332   b ′ in the third quadrant (Q 3 ) is smaller than that of the ellipse  332   b ″ in the fourth quadrant (Q 4 ). 
     Accordingly, as a compression gradient in the third quadrant (Q 3 ) and the fourth quadrant (Q 4 ) becomes gradual, an over-compression amount may be reduced. This may enhance a compression efficiency. Further, as a linear velocity in the third quadrant (Q 3 ) and the fourth quadrant (Q 4 ) is reduced, a frictional loss on the inner circumferential surface  332  of the cylinder  330  corresponding to the second ellipse  332   b  may be reduced. 
       FIG. 7  shows graphs comparing suction completion times with each other according to a shape of an ellipse which forms the inner circumferential surface of the cylinder. An ellipse with respect to each quadrant may be defined as follows. Referring to  FIG. 6 , on the first ellipse  332   a ′ in the first quadrant (Q 1 ) and the first ellipse  332   a ″ in the second quadrant (Q 2 ), it may be assumed that a long-axis radius of the first ellipse  332   a  corresponding to the third center line (L 3 ) is A, a short-axis radius of the first ellipse  332   a ′ in the first quadrant (Q 1 ) is B1, and a short-axis radius of the first ellipse  332   a ″ in the second quadrant (Q 2 ) is B2. In this case, the short-axis radius of the first ellipse  332   a  in the first quadrant (Q 1 ) with respect to the long-axis radius of the first ellipse  332   a  may satisfy a formula, 0.5≤B1/A≤0.7. And the short-axis radius of the first ellipse  332   a ″ in the second quadrant (Q 2 ) with respect to the long-axis radius of the first ellipse  332   a  may satisfy a formula, 0.7≤B2/A≤0.9. 
     On the second ellipse  332   b ′ in the third quadrant (Q 3 ) and the second ellipse  332   b ″ in the fourth quadrant (Q 4 ), it may be assumed that a short-axis radius of the second ellipse  332   b ′ in the third quadrant (Q 3 ) is B3, a short-axis radius of the second ellipse  332   b ″ in the fourth quadrant (Q 4 ) is B4, and a radius of the cylinder  330  at the fourth center line (L 4 ) is C. In this case, the short-axis radius (B3) of the second ellipse  332   b ′ in the third quadrant (Q 3 ), with respect to the radius of the cylinder  330  may satisfy a formula, 1.0≤B3/C≤1.2. Further, the short-axis radius (B4) of the second ellipse  332   b ″ in the fourth quadrant (Q 4 ), with respect to the radius of the cylinder  330  may satisfy a formula, 0.8≤B4/C≤1.0. 
     Referring to  FIG. 7  under such conditions, when the radiuses in the respective quadrants are defined as the following examples, results are obtained as shown in the following table. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                   
                 Reference 
                 Example 
                 Example 
                 Example 
                 Example 
                 Example 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Items 
                 Example 
                 {circle around (1)} 
                 {circle around (2)} 
                 {circle around (3)} 
                 {circle around (4)} 
                 {circle around (5)} 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Quadrant  
                 B1/ 
                 0.7 
                 0.6 
                 0.5 
                 0.7 
                 0.7 
                 0.5 
               
               
                 1 
                 A 
                   
                   
                   
                   
                   
                   
               
               
                 Quadrant  
                 B2/ 
                 0.7 
                 0.8 
                 0.9 
                 0.7 
                 0.7 
                 0.9 
               
               
                 2 
                 A 
                   
                   
                   
                   
                   
                   
               
               
                 Quadrant  
                 B3/ 
                 1 
                 1 
                 1 
                 1.1 
                 1.2 
                 1.2 
               
               
                 3 
                 C 
                   
                   
                   
                   
                   
                   
               
               
                 Quadrant  
                 B4/ 
                 1 
                 1 
                 1 
                 0.9 
                 0.9 
                 0.8 
               
               
                 4 
                 C 
               
               
                   
               
            
           
         
       
     
     That is, it was shown that suction completion times of ellipses corresponding to examples {circumflex over (1)}, {circumflex over (2)} and {circumflex over (5)} are earlier than that of an ellipse corresponding to the reference example. Suction completion times of ellipses corresponding to examples {circumflex over (3)} and {circumflex over (4)} are equal to that of the ellipse corresponding to the reference example. 
     Further, it was shown that compression starting times of the ellipses corresponding to the examples {circumflex over (1)}, {circumflex over (2)} and {circumflex over (5)} are earlier than that of an ellipse corresponding to the reference example, as the suction completion times thereof become early. Compression starting times of the ellipses corresponding to the examples {circumflex over (3)} and {circumflex over (4)} are slightly earlier than that of the ellipse corresponding to the reference example. 
     When the ellipses in the respective quadrants (Q 1 ,Q 2 ,Q 3 ,Q 4 ) which form the inner circumferential surface  332  of the cylinder  330  are defined, it may be seen that the suction starting time becomes early and thus a compression is performed early. 
     In the aforementioned embodiments, the first center point (O′) is formed to be farther from the center (Oc) of the cylinder  330 , than the second center point (O″). However, the second center point (O″) may be formed to be farther from the center (Oc) of the cylinder  330 , than the first center point (O′). This may be selectively applied according to whether the compressor is in a cooling mode or a heating mode. 
     As the inner circumferential surface of the cylinder is formed in an asymmetrical shape having four ellipses, a volume diagram may be variously controlled. As a result, a suction period and a compression period may be properly controlled to enhance a compression efficiency. Accordingly, the inner circumferential surface of the cylinder may be formed to have a larger number of ellipses than the aforementioned ellipses. 
     A sealing section  338  having a same curvature radius (Rc) as a curvature radius (Rr) of the roller  340  may be further formed on the inner circumferential surface  332  of the cylinder  330  including the first contact point (P 1 ).  FIG. 8  is an enlarged sectional view of the cylinder shown in  FIG. 4  according to another embodiment. 
     That is, the ellipse  332   a ′ corresponding to the first quadrant (Q 1 ) and the ellipse  332   b ″ corresponding to the fourth quadrant (Q 4 ) are connected to each other at a region corresponding to the first contact point (P 1 ) among the inner circumferential surface  332  of the cylinder  330 . However, a curvature radius (Rc) at this region is larger than a curvature radius (Rr) of the roller  340  formed by the outer circumferential surface  341  of the roller  340 , similar to the curvature radius (Rc) at other regions. Accordingly, the outer circumferential surface  341  of the roller  340 , and the inner circumferential surface  332  of the cylinder  330  linearly contact each other even at the first contact point (P 1 ). 
     The first compression chamber  333   a  which forms a suction pressure and the third compression chamber  333   c  which forms a discharge pressure are formed at both sides of the first contact point (P 1 ). Accordingly, a sealing force of the first contact point (P 1 ) should be higher than that of another region. For this, an oil film on the first contact point (P 1 ) should be widely formed and should be maintained stably. However, if the outer circumferential surface  341  of the roller  340  and the inner circumferential surface  332  of the cylinder  330  linearly contact each other even at the first contact point (P 1 ), oil is not kept at the first contact point (P 1 ). This may cause an oil film not to be formed, thereby not sealing a space between the two compression chambers. 
     However, as shown in  FIG. 8 , if a curvature radius (Rc 1 ) of the inner circumferential surface  332  of the cylinder  330  at a sealing section  338  of the first contact point (P 1 ) is smaller than a curvature radius (Rc 2 ) of the inner circumferential surface  332  of the cylinder  330  at another region, and is the same as the curvature radius (Rr) of the roller  340  formed by the outer circumferential surface  341  of the roller  340 , the roller  340  and the cylinder  330  come into planar-contact with each other at the sealing section  338 . Then, the sealing section  338  of the first contact point (P) serves as a sealing section to keep a predetermined amount of oil therein. As a result, an oil film  338   a  may be formed at the sealing section  338 , thereby enhancing a sealing effect between the compression chambers  333   a ,  333   c.    
     Embodiments disclosed herein provide a vane rotary compressor capable of excluding a sub discharge opening except for a main discharge opening, or capable of minimizing the number of sub discharge openings or an inner diameter of the sub discharge opening and effectively reducing over-compression. Further, embodiments disclosed herein provide a vane rotary compressor capable of variously controlling a volume diagram by forming an inner circumferential surface of a cylinder in an asymmetric shape, and capable of enhancing a compression efficiency by properly changing a suction period and a compression period. Furthermore, embodiments disclosed herein provide a vane rotary compressor capable of preventing refrigerant leakage between compression chambers and capable of reducing a frictional loss between a cylinder and a vane, by reducing a pressure difference between the compression chambers by reducing a suction period and increasing a compression period by changing a shape of an inner circumferential surface of the cylinder. 
     Embodiments disclosed herein also provide a vane rotary compressor capable of enhancing a compression efficiency by reducing an over-compression amount by making a gradient of a compression period gradual. Embodiments disclosed herein provide a vane rotary compressor capable of preventing refrigerant leakage between a suction chamber and a discharge chamber by obtaining a wide sealing area at a region near a cylinder and a roller. Embodiments disclosed herein further provide a vane rotary compressor having a structure that an inner circumferential surface of a cylinder is formed in an elliptical shape, and a long axis of the cylinder is eccentrically spaced apart from a center of the cylinder by a predetermined distance. A center line in a direction of the long axis of the cylinder, perpendicular to a center line in a direction of a short axis of the cylinder may be formed in plurality in number. 
     Embodiments disclosed herein provide a hermetic compressor that may include a cylinder having an inner circumferential surface which forms a compression chamber formed in an elliptical shape; a roller provided to be eccentric from the inner circumferential surface of the cylinder, and configured to change a volume of the compression chamber by being rotated; and a vane formed to be withdrawn towards the inner circumferential surface of the cylinder when the roller is rotated, and configured to divide the compression chamber into a plurality of spaces. On the basis of a contact point where the inner circumferential surface of the cylinder and an outer circumferential surface of the roller are closest to each other, and a first center line passing through a center of the cylinder, an ellipse positioned at one side of the first center line and forming the inner circumferential surface of the cylinder may be defined as a first ellipse, a center point of the first ellipse may be defined as a first center point, an ellipse positioned at another side of the first center line and forming the inner circumferential surface of the cylinder may be defined as a second ellipse, a center point of the second ellipse may be defined as a second center point. Under these assumptions, the first center point and the second center point may be spaced apart from the center of the cylinder. 
     The first center point and the second center point may be positioned on the first center line. The first center point and the second center point may be positioned on the first center line, at different separation distances from the center of the cylinder. 
     The first center point and the second center point may be positioned on a same side, on the basis of a second center line passing through the center of the cylinder and perpendicular to the first center line. The first center point may be farther from the center of the cylinder than the second center point. 
     The first ellipse may be formed as two ellipses having a same sum of distances to two focal points for every point thereon. The ellipse positioned at a relatively short distance from the contact point than the ellipse positioned at a relatively long distance from the contact point may be formed such that a distance between two focal points thereon is relatively large. 
     The second ellipse may be formed as two ellipses having a same sum of distances to two focal points for every point thereon. The ellipse positioned at a relatively short distance from the contact point than the ellipse positioned at a relatively long distance from the contact point is formed such that a distance between two focal points thereon is relatively large. 
     Each of the first ellipse and the second ellipse may be formed as two ellipses having a same sum of distances to two focal points for every point thereon. The first ellipse positioned at a relatively short distance from the contact point than the first ellipse positioned at a relatively long distance from the contact point may be formed such that a distance between two focal points thereon is relatively large. The second ellipse positioned at a relatively short distance from the contact point than the second ellipse positioned at a relatively long distance from the contact point may be formed such that a distance between two focal points thereon is relatively large. 
     A line passing through the first center point in a direction perpendicular to the first center line may be defined as a third center line, and a line passing through the second center point in a direction perpendicular to the first center line may be defined as a fourth center line. A region of the first ellipse may be divided into first and second quadrants in a rotational direction of the roller by the first and third center lines, and a region of the second ellipse may be divided into third and fourth quadrants by the first and fourth center lines. The ellipses in the four quadrants may be formed to have different distances between two focal points thereof. 
     The contact point may be included in the first quadrant on the basis of the third center line, and a distance between two focal points of an ellipse in the first quadrant including the contact point may be formed to be larger than that of an ellipse in the second quadrant. The contact point may be included in the fourth quadrant on the basis of the fourth center line, and a distance between two focal points of an ellipse in the fourth quadrant including the contact point may be formed to be larger than that of an ellipse in the third quadrant. A section having a same curvature radius as the roller may be further formed at a peripheral section including the contact point among the inner circumferential surface of the cylinder. 
     Embodiments disclosed herein provided a hermetic compressor that may include a cylinder having an inner circumferential surface which forms a compression chamber formed in an elliptical shape; a roller provided to be eccentric from the inner circumferential surface of the cylinder, and configured to change a volume of the compression chamber by being rotated; and a vane formed to be withdrawn towards the inner circumferential surface of the cylinder when the roller is rotated, and configured to divide the compression chamber into a plurality of spaces. A position where the inner circumferential surface of the cylinder and an outer circumferential surface of the roller are closest to each other may be a contact point, a line passing through the contact point and a center of the cylinder may be a first center line, and a line perpendicular to the first center line and passing through the center of the cylinder may be a second center line. Under the assumptions, the inner circumferential surface of the cylinder may be formed to have an asymmetrical shape on the basis of the first and second center lines. 
     The inner circumferential surface of the cylinder may be divided into four quadrants by the first and second center lines, and the quadrants corresponding to each other among the four quadrants may be formed to be asymmetrical with each other on the basis of the second center line. 
     The vane rotary compressor according to embodiments may have at least the following advantages. 
     Firstly, as a compression period is formed to be long, a pressure difference between the compression chambers may be lowered, in a state that a sub discharge opening is excluded or the number of sub discharge openings or an inner diameter of the sub discharge opening is minimized. With such a configuration, the number of processes with respect to the sub discharge opening, and the number of valves for opening and closing the sub discharge openings may be reduced, resulting in lowering the fabrication costs. 
     Further, as the inner circumferential surface of the cylinder is formed in an asymmetrical shape having three or more ellipses, a volume diagram may be variously controlled. As a result, a suction period and a compression period may be properly controlled to enhance a compression efficiency. 
     Furthermore, as the inner circumferential surface of the cylinder is formed such that the suction period is short and the compression period is long, over-compression in the compression chambers may be prevented or reduced. This may enhance a compression efficiency. 
     Also, as the inner circumferential surface of the cylinder is formed such that a suction completion time and a compression starting time are towards a suction opening, the compression period becomes long, and thus, a compression difference between the compression chambers is reduced. Besides, as the compression period has a gradual gradient, an over-compression amount may be reduced. 
     Further, as a wide sealing area is obtained at a region near the cylinder and the roller, refrigerant leakage between a suction chamber and a discharge chamber may be prevented. This may reduce a suction loss or a compression loss. 
     Further scope of applicability of embodiments will become more apparent from the detailed description given. However, it should be understood that the detailed description and specific examples, while indicating embodiments, are given by way of illustration only, since various changes and modifications within the spirit and scope will become apparent to those skilled in the art from the detailed description. 
     As the present features may be embodied in several forms without departing from the characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalents of such metes and bounds are therefore intended to be embraced by the appended claims. 
     It will be understood that when an element or layer is referred to as being “on” another element or layer, the element or layer can be directly on another element or layer or intervening elements or layers. In contrast, when an element is referred to as being “directly on” another element or layer, there are no intervening elements or layers present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that, although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. 
     Spatially relative terms, such as “lower”, “upper” and the like, may be used herein for ease of description to describe the relationship of one element or feature to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation, in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “lower” relative to other elements or features would then be oriented “upper” relative the other elements or features. Thus, the exemplary term “lower” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Embodiments of the disclosure are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of the disclosure. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the disclosure should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     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.