Abstract:
A method and system is provided for reducing chatter in multi-capacity compressors having disengageable eccentric structures. The multi-capacity fluid compressor includes a compression chamber having a discharge end and an inner surface. The compressor also includes a compression member having a disengageable eccentric structure allowing the compressor to provide discrete compression capacities. A valve portion is disposed adjacent to the discharge end of the compression chamber and is arranged and disposed to discharge a compressed fluid when the compression member has completed. A discharge arrangement is arranged and disposed to discharge at least a portion of fluid remaining in the compression chamber at the completion of the compression cycle. The discharge of at least a portion of the fluid remaining in the compression chamber reduces or eliminates forces on the disengageable eccentric structure to limit rotational acceleration of the disengageable eccentric structure.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to multi-capacity compressors having disengageable eccentric structures. More specifically, the present invention relates to a system and method for reducing noise in a multi-capacity compressor caused by a disengageable eccentric structure. 
   BACKGROUND OF THE INVENTION 
   Compressor capacity in refrigerant compressors may be varied, especially in multi-cylinder refrigerant compressors, by providing a two position eccentric cam rotatably mounted on the crankpin. The cam is angularly adjustable in response to reversing the direction of rotation of the crankpin by the crankshaft drive motor. One direction of rotation results in the positioning of the eccentric cam having a more eccentric rotation path to provide compression in a corresponding cylinder, while the opposite direction of rotation results in the position of the eccentric cam having a circular rotational path to provide a different amount of compression or no compression in the cylinder. The use of the two position eccentric cam (i.e., the disengageable eccentric cam) allows the compressor to have variable capacity by effectively removing compression in one of the cylinders for one direction of rotation and allows the compressor to maintain efficiency, while under varying load requirements. 
   One type of eccentric cam is described in U.S. Pat. No. 4,479,419, hereinafter the &#39;419 Patent. The angular positioning of the cam (i.e., the eccentric cam) on the crankpin is accomplished by providing a pair of drive stops which are angularly spaced on a portion of the crankpin, and a dog provided on the cam. These stops and the dog are angularly positioned with respect to each other such that upon rotation of the crankshaft in one direction a first stop will engage one side of the dog and rotate the cam to a first prescribed angular position on the crankpin to produce one piston stroke length. Conversely, reversing the rotation of the crankshaft disengages the dog from the first stop and causes the cam to rotate and engage the opposite side of the dog to a second stop, which also rotates the cam to a second prescribed angular position on the crankpin to produce another piston stroke length. 
   A compressor operates by drawing gas into a chamber and compressing the gas during a compression cycle. The end of the compression cycle is when the discharge of gas from the compression chamber ends and drawing of the gas into the chamber begins. Reciprocating compressors having disengageable eccentric structures typically include a piston that compresses gas inside a compression cylinder or chamber. A protrusion on the eccentric cam, called a dog, engages a stop on the crankshaft to facilitate rotation of eccentric cam structure. At the completion of the compression cycle, the compressed gas is discharged from the compression cylinder through a discharge valve in a valve plate at one end of the cylinder. The end of the compression cycle in a reciprocating compressor corresponds approximately to the top dead center position of the piston (i.e., the maximum length the piston extends into the compression cylinder). A volume of gas, commonly referred to as reexpansion gas, is not discharged from the compression cylinder and remains in the clearance space of the cylinder (i.e., the space between the valve plate and piston) at the completion of the compression cycle. The reexpansion gas remaining in the cylinder exerts force on the piston. In reciprocating compressors using a disengageable eccentric cam, a force on the piston from the reexpansion gas transfers through the piston assembly to the disengageable eccentric cam. The eccentric cam is accelerated to a rotational velocity greater than the velocity of the crankpin, which results in a slight disengagement of the disengageable eccentric cam&#39;s dog from the stop on the crankpin. The crankpin continues to rotate and the eccentric portion returns to the same velocity as the crankpin. The eventual reengagement of the stop on the crankpin with the dog on the disengageable eccentric cam occurs with substantial momentum and impact, thus producing noise, commonly referred to as chatter. Chatter is a metallic clacking or clicking noise generated by the rapid and forceful reengagement of the stop and dog. 
   Rotary compressors having disengageable eccentric structures are also susceptible to noise in the form of chatter. Rotary compressors include a roller having an eccentric crank mounted on a crankshaft. A protrusion on the eccentric crank, called a dog, engages a stop on the crankpin to facilitate rotation of the roller structure. The roller compresses gas inside a compression cylinder. At the completion of the compression cycle, the compressed gas is discharged from the compression cylinder through a discharge valve positioned along the inner surface of the cylinder. Like in the reciprocating compressor, a volume of reexpansion gas is not discharged from the compression cylinder and remains in the cylinder at the completion of the compression cycle. The reexpansion gas remaining in the cylinder exerts force on the roller, causing the roller and eccentric crank to accelerate to a rotational velocity greater than the crankpin. The crankpin continues to rotate and the roller and eccentric crank return to the same velocity as the crankshaft. The eventual reengagement of the stop on the crankpin with the dog on the disengageable eccentric crank occurs with substantial momentum and impact, thus producing the chatter. 
   The problem of chatter is not limited to reciprocating and rotary compressors. Any type of compressor having a disengageable eccentric structure may be susceptible to the problem of chatter. 
   One attempt to address the problem of disengagement and reengagement of the stop and dog includes placing locking mechanisms for the disengageable eccentric structure on the disengageable eccentric cam. For example, U.S. Pat. No. 6,092,993, herein incorporated by reference, utilizes various latching mechanisms that mechanically hold the disengageable eccentric cam and the crankpin stop together while the crankpin is rotating. However, the latching means requires additional components and/or machining on the rotating crankpin and disengageable eccentric cam to maintain engagement. Also shown in U.S. Pat. No. 6,092,993, is the attempt to address the problem of disengagement and reengagement of the stop and dog using inertial mass to hold disengageable eccentric structure against the crankpin stops. The addition of mass to the eccentric cam shifts the center of gravity of the eccentric cam and acts to provide additional force to maintain engagement while the crankpin is rotating. However, cam inertia is generally ineffective to prevent disengagement, particularly from the force against the disengageable cam caused by reexpansion gas. 
   What is needed is a method and/or system for reducing noise and chatter in variable capacity compressors with disengageable eccentric structures resulting from reexpansion gas remaining in the cylinder at the completion of the compression cycle. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a method and system for reducing noise in multi-capacity compressors having disengageable eccentric structures. The noise created by rapid engagement and disengagement of the disengageable eccentric structure with the crankpin is reduced or eliminated by decreasing the amount of reexpansion gas present in the compression chamber of the compressor at or near the completion of the compression cycle. 
   The present invention includes a multi-capacity fluid compressor including a compression chamber having a discharge end and an inner surface. The compressor also includes a compression member having a disengageable eccentric structure allowing the compressor to provide a plurality of discrete compression capacities. A valve portion is disposed adjacent to the discharge end of the compression chamber and is arranged and disposed to discharge a compressed fluid when the compression member has completed a compression cycle. A discharge arrangement is arranged and disposed to discharge at least a portion of fluid remaining in the compression chamber at the completion of the compression cycle by the compression member. The discharge of at least a portion of the fluid remaining in the compression chamber reduces or eliminates forces on the disengageable eccentric structure to limit rotational acceleration of the disengageable eccentric structure. 
   Another embodiment of the present invention includes a multi-capacity fluid compressor including a compression chamber having a discharge end and an inner surface. The compressor further includes a compression member having a disengageable eccentric structure allowing the compressor to provide a plurality of discrete compression capacities. The compression member is arranged and disposed to travel along a portion of the inner surface to vary the volume of the compression chamber. A valve portion is disposed adjacent to the discharge end of the compression chamber and is arranged and disposed to discharge a compressed fluid when the compression member has completed a compression cycle. An opening is disposed in one of the components selected from the group consisting of the valve portion, the compression member, the inner surface and combinations thereof. The opening is configured to discharge at least a portion of fluid remaining in the compression chamber at the completion of the compression cycle by the compression member. The discharge of at least a portion of fluid remaining in the compression chamber reduces or eliminates forces on the disengageable eccentric structure to limit rotational acceleration of the disengageable eccentric structure. 
   A method for reducing chatter in multi-capacity compressors comprising the steps of providing a multi-capacity compressor having a compression chamber having a discharge end and an inner surface. The compressor further includes a compression member having a disengageable eccentric structure that allows the compressor to provide a plurality of discrete compression capacities. The compression member is arranged and disposed to travel along a portion of the inner surface to vary the volume of the compression chamber. A valve portion is disposed adjacent to the discharge end of the cylinder and is arranged and disposed to discharge compressed fluid. An opening is disposed in one of the components selected from the group consisting of the valve portion, the compression member, the inner surface and combinations thereof. The method further includes compressing a fluid by decreasing the volume of the compression chamber with the compression member. A volume of compressed fluid is discharged from the valve portion when the compression member has completed compressing the fluid. Thereafter at least a portion of fluid remaining in the compression chamber is removed through the opening to reduce or eliminate forces on the disengageable eccentric structure to prevent rotational acceleration of the disengageable eccentric structure. 
   The method and/or system according to the present invention may be utilized with any type of compressor having a portion of the compression mechanism disengageable from the driving member during operation susceptible to chatter. In particular, the present invention is suitable for use with a multi-capacity reciprocating compressor or a multi-capacity rotary compressor. 
   The method and/or system according to the present invention reduces noise in a compressor having a disengageable eccentric structure without additional noise reducing components and/or machining of the rotating crankpin and disengageable eccentric structure. Further, the system according to the present invention is capable of reducing noise in a compressor having a disengageable eccentric structure with little or no loss in efficiency. 
   The method and/or system according to the present invention also reduces the number of disengagement and reengagements of the dog on the disengageable eccentric structure and the stop on the crankpin, decreasing the wear on the components and increasing the operational life of the system. 
   Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates schematically a refrigeration system used with the present invention. 
       FIGS. 2A and 2B  illustrate disengageable eccentric cams on rotating crankshaft assemblies. 
       FIG. 3  illustrates a reexpansion discharge assembly according to one embodiment of the invention. 
       FIG. 4  illustrates a reexpansion discharge assembly according to another embodiment of the invention. 
       FIG. 5  illustrates a reexpansion discharge assembly according to still another embodiment of the invention. 
       FIG. 6  illustrates a reexpansion discharge assembly according to still another embodiment of the invention. 
       FIG. 7  illustrates a reexpansion discharge assembly according to still another embodiment of the invention. 
     Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A system to which the invention may be applied is illustrated, by means of example, in  FIG. 1 . As shown, the HVAC, refrigeration, or chiller system  100  includes a compressor  101 , a condenser  103  and an evaporator  107 . The conventional HVAC or refrigeration system includes many other features that are not shown in  FIG. 1 . The features not shown have been purposely omitted to simplify the drawing for ease of illustration. 
   The compressor  101  compresses a refrigerant vapor and delivers it to the condenser  103 . The compressor  101  is preferably a reciprocating compressor, however the compressor according to the present invention is not limited to a reciprocating compressor. Any type of compressor that uses a portion of the compression mechanism disengageable from the driving member during operation (i.e., a disengageable eccentric cam structure) may utilize the present invention. Other suitable compressor types include, but are not limited to, rotary compressors, scotch yoke compressors, and scroll compressors. The refrigerant vapor delivered by the compressor  101  to the condenser  103  enters into a heat exchange relationship with a fluid, e.g., air or water, and undergoes a phase change to a refrigerant liquid as a result of the heat exchange relationship with the fluid. The condensed liquid refrigerant from condenser  103  flows though an expansion device  105  to an evaporator  107 . The liquid refrigerant in the evaporator  107  enters into a heat exchange relationship with another fluid, e.g. air or water, to remove heat from the air or water. The refrigerant liquid in the evaporator  107  undergoes a phase change to a refrigerant vapor as a result of the heat exchange relationship with the air or water. The vapor refrigerant in the evaporator  107  exits the evaporator  107  and returns to the compressor  101  by a suction line to complete the cycle. It is to be understood that any suitable configuration of condenser  103  and evaporator  107  can be used in the system  100 , provided that the appropriate phase change of the refrigerant in the condenser  103  and evaporator  107  is obtained. 
   The capacity of compressor  101  directly affects the amount of cooling provided by the refrigerant in the evaporator  107 . For example, when a two-stage reciprocating compressor is operated in a maximum capacity mode, compressor  101  operates at full capacity and provides maximum cooling in the evaporator  107 . When the two-stage reciprocating compressor is operated in a reduced capacity mode, the amount of cooling provided in the evaporator  107  is similarly reduced. 
   The multi-capacity gas compressor according to the invention includes a plurality of compression chambers, where each of the compression chambers has a discharge end, an inner surface, and a compressing component (e.g. a piston or compression roller). The compressing component is positioned in the compression chamber adjacent to the inner surface and is mounted to allow travel within the compression chamber, either axially or circumferentially. The position of the compressing component determines the volume of the compression chamber. Accordingly, the travel of the piston or roller increases or decreases the volume of the compression chamber. A reexpansion gas discharge system is positioned adjacent to the discharge end of the compression chamber, e.g., a cylinder. The reexpansion gas discharge can include an opening in the discharge end, the compressing component, the inner surface, or any combination thereof. The opening allows discharge of at least a portion of gas remaining in the compression chamber (i.e., reexpansion gas) after the compression cycle is complete. 
     FIGS. 2A and 2B  illustrate a camshaft assembly  200  for a reciprocating compressor according to the present invention. The camshaft assembly  200  includes a crankpin  201 , a disengageable eccentric cam  203 , a dog  205  extending from the disengageable eccentric cam  203  and a first stop surface  207 .  FIG. 2A  illustrates the camshaft assembly  200  rotating in a rotational direction  211  that causes the disengageable eccentric cam  203  to rotate on the crankpin  201  and engage a first side of the dog  205  to the first stop surface  207 . The rotational direction shown in  FIG. 2A  represents the direction of full compressor capacity. The disengageable cam  203  has an eccentric shape when rotated in this direction, which permits an attached piston to compress gas in the compression chamber thereby providing additional compression capacity. During operation, the engagement of the first side of the dog  205  and the first stop surface  207  is a result of the rotation of the crankpin  201 . However, during operation, a force resulting from reexpansion gas in the compression cylinder causes the disengageable cam  203  to accelerate to a rotational velocity sufficient to cause disengagement at the point of contact with the first side of the dog  205  and the first stop surface  207 . Noise, in the form of chatter, results from the disengageable eccentric cam  203  returning to the velocity of the crankpin  201  and reengaging the first stop surface  207 . 
     FIG. 2B  illustrates the camshaft assembly  200  rotating in a rotational direction  213  that causes the disengageable eccentric cam  203  to rotate on the crankpin  201  and engage a second side of the dog  205  to a second stop surface  209 . The disengageable cam  203  has a substantially circular shape when rotated in this direction, which reduces or eliminates the motion of the piston, thereby reducing or eliminating the ability of the piston to compress gas in the compression chamber thereby providing an different compression capacity than shown in  FIG. 2A . Although  FIG. 2B  shows a substantially circular disengageable eccentric cam  203  shape when rotating in one direction, the geometry of the disenageable eccentric cam  203  is not limited to a circular shape and may be an eccentric or other geometry that permits the piston to compress gas in the compression chamber at a different capacity than the capacity shown in  FIG. 2A . During operation, the engagement of the second side of the dog  205  and the second stop surface  209  is a result of the rotation of the crankpin  201 . The rotational direction illustrated in  FIG. 2B  results in the compressor operating at a reduced capacity. In addition, little or no noise is produced by the disenageable eccentric cam  203  from disengagement of the disengageable eccentric cam  203  from the dog  205  having the reduced or eliminated compression capacity because the presence of reexpansion gas is reduced or eliminated due to the reduced motion of the piston. 
     FIGS. 3 through 6  illustrates various embodiments of a piston assembly  300  incorporating the reexpansion gas discharge assembly of the invention. Piston assembly  300  illustrates a compressor piston at or near the completion of the compression stroke. The piston assembly  300  includes a piston head  301  inside of a cylinder wall  303 . The piston head  301  includes a piston ring groove  305 , which is capable of receiving a piston ring. A valve plate  307  is positioned adjacent to the cylinder wall  303  to form a cylindrical space. The piston head  301  is allowed to travel within the cylindrical space formed by the valve plate  307  and the cylinder wall  303 . The space between the piston head  301  and the valve plate  307  is the compression chamber. As the compressor completes the compression stroke, the piston head  301  forms a clearance space between the piston head  301  and the valve plate  307 . At or near the completion of the compression stroke reexpansion gas  309  remains in the space between the end of the piston head  301  and the valve plate  307 . 
     FIG. 3  illustrates an embodiment of a reexpansion gas discharge assembly according to the present invention, where the piston head  301  includes a piston exhaust passage  311  that allows reexpansion gas  309  to pass through the piston head  301  to a cylinder wall passage  313 . The reexpansion gas  309  flows from the compression chamber in the cylinder, as discharged reexpansion gas  315 , to the suction side of the compressor  101  through cylinder wall passage  313 . 
     FIG. 4  illustrates an alternate embodiment of the reexpansion gas discharge assembly. The piston assembly  300  includes an exhaust indentation  401  in the cylinder wall  303  of the piston assembly  300 . The exhaust indentation  401  is positioned along the length of the piston head  301  so that at or near the completion of the compression stroke, a piston ring positioned in the piston ring groove  305  releases reexpansion gas  309  through the exhaust indentation  401 , as discharged reexpansion gas  403 . In other words, the reexpansion gas can bypass the piston ring through exhaust indentation  401  and does not remain in the compression chamber, but is vented to the suction side of the compressor. 
     FIG. 5  illustrates a further alternate embodiment of the reexpansion gas discharge assembly. The piston assembly  300  includes an exhaust passage  501  in the valve plate  307  that allows reexpansion gas  309  to discharge through the exhaust passage  501 , as discharged reexpansion gas  503 . The exhaust passage  501  is positioned to discharge the discharged reexpansion gas  503  to the suction side of the compressor. The discharged reexpansion gas  503  is permitted to discharge from the reexpansion gas discharge assembly at a constant rate. The size of the exhaust passage  501  is such that reexpansion gas is removed from the compression chamber with little or no loss in compressor efficiency. The exhaust passage  501  has little or no loss in compressor efficiency because the exhaust passage  501  is configured and arranged to exhaust reexpansion gas  309 , which is not part of discharge gas exhausted from the compression chamber as part of the compression cycle. Since the discharged reexpansion gas  503  is not part of the discharge gas, there is little or no efficiency loss due to the exhaust passage  501 . 
     FIG. 6  illustrates an alternate embodiment of the reexpansion gas discharge assembly. The piston assembly  300  includes a valve  601  in the valve plate  307  that is activated by the piston head  301  that allows reexpansion gas  309  to discharge through the valve  601 , as discharged reexpansion gas  603 . The valve  601  is positioned to discharge the discharged reexpansion gas  603  to the suction side of the compressor. 
     FIG. 7  illustrates an embodiment of a rotary compression assembly  700 , for a rotary compressor, having a reexpansion gas discharge assembly according to the present invention. The rotary compression assembly  700  includes an inlet channel  701  that carries inlet gas  702  into a compression chamber  705  via inlet port  703 . The compression assembly  700  also includes a roller  707  and a disengageable eccentric crank  709  mounted on a crankpin  711 . During a compression cycle, the crankpin  711  rotates the roller  707  and the disengageable eccentric crank  709  inside the compression chamber  705 . As the roller  707  rotates inside the compression chamber  705 , a blade  719  separates the inlet port  703  from the discharge port  713 . The blade  719  is in sliding contact with the roller  707  and substantially prevents the passage of gas during compressor operation. The disengageable eccentric crank  709  includes roller  707 , which provides a small clearance when the crankpin  711  is rotated in one direction and a larger clearance when rotated in the opposite direction. The rotary compressor operates at a larger capacity (i.e., compresses a greater quantity of gas) when the roller or rollers provide a small clearance and a smaller capacity (i.e., compresses a smaller quantity of gas) when the roller or rollers provide a larger clearance. The clearance is measured as the smallest distance between the roller  707  and the surface of the compression chamber  705 . When the clearance is small the roller  707  may contact a surface of the compression chamber  705  or may be sufficiently close to the surface of the compression chamber  705  to substantially prevent leakage of fluid through the clearance space between the roller  707  and the compression chamber  705 . However, when the clearance is large, at least some leakage between the roller  707  and the surface of the compression chamber  705  is permitted. When the crankpin  711  rotates in the direction of the smaller clearance (i.e., clockwise, as shown in  FIG. 7 ), the gas inside the compression chamber is compressed by the roller  707  as the crankpin  711 , the disengageable eccentric crank  709  and the roller  707  rotate. At the end of the compression cycle, the compressed gas exits the compression chamber  705  through the discharge port  713 , as discharge gas  717  and is carried through a discharge channel  715  to the discharge of the compressor. A reexpansion gas discharge opening  723  is positioned to discharge reexpansion gas  721  present in the compression chamber after the completion of the compression cycle. The reexpansion gas discharge opening  723  may be open to the compression chamber during the entire cycle to constantly bleed reexpansion gas from the compression chamber, or the reexpansion gas discharge opening  723  may be positioned and/or configured to open at or near the completion of the compression cycle. In either embodiment, the reexpansion gas is removed from the compression chamber and is discharged to the suction side of the compressor. 
     FIGS. 3-7  show and describe that reexpansion gas  309  is discharged to the suction of the compressor. Although the reexpansion gas  309  may be discharged to the suction of the compressor, the present invention is not limited to discharging to the suction of the compressor. The reexpansion gas  309  may be discharged to any location that has a lower pressure than the reexpansion gas  309  in the compression chamber at or near the end of the compression stroke. For example, the reexpansion gas  309  may be discharged to any area, such as chambers, ports or cavities having a fluid pressure lower than the fluid pressure of the reexpansion gas  309  in the compressor system. 
   The discharge of reexpansion gas  309  from the compression chamber either takes place through an opening in the compression chamber or through a valve activated at or near the completion of the compression cycle. The opening in the chamber according the present invention includes openings that allow constant passage of at least some gas, or openings that allow passage of gas only at predetermined positions of the compressing component in the compression cycle. 
   In one embodiment having an opening allowing the constant passage of at least some gas, the gas is permitted to discharge from the compression chamber to either an opening in the valve plate  307 , as illustrated in  FIG. 5 , an opening in the cylinder wall  303 , as illustrated in  FIG. 7 , or an opening in the piston head  301 . The opening allows the discharge of reexpansion gas from the clearance space in the compression chamber in order to reduce or eliminate the force on the disengageable eccentric structure. 
   In one embodiment having an opening that allows passage of gas only at predetermined positions of the compression member, the piston assembly  300  may include a passage in the piston head  301  that aligns with a passage in the cylinder wall, as illustrated in  FIG. 3 , at or near the completion of the compression cycle to allow discharge of reexpansion gas  309  from the compression chamber. Alternatively, the cylinder may include an exhaust indentation  401  (i.e., cavity), as illustrated in  FIG. 4 , that permits discharge of reexpansion gas  309  from the compression chamber at or near the end of the compression cycle when the piston ring is positioned at or near the exhaust indentation  401 . The discharge of reexpansion gas  309  takes place when the piston ring travels past the exhaust indentation  401  in the compression cycle. Alternatively, the cylinder may also include a valve  601 , as illustrated in  FIG. 6 , that opens at a predetermined position of the compressing component in the compression cycle to allow discharge of reexpansion gas from the cylinder. Valve  601  may be operated in any manner that opens the valve  601  at or near the completion of the compression cycle. For example, the valve  601  may be positioned in a location in which the valve  601  is opened by contact with the piston head  301  when the piston head  301  substantially reaches the completion of the compression cycle. Although  FIG. 6  depicts a mechanically operated valve, the invention is not limited to a valve that is mechanically operated. Any valve that is capable of opening at a predetermined point in the compression cycle is suitable for use in the present invention. Alternative valves include, but are not limited to, pneumatic valves or solenoid valves. 
   In accordance with one embodiment, the present invention is directed to a reciprocating compressor. The compressor includes a reversible motor for rotating in a forward and a reverse direction and a block with a plurality of cylinders and the associated compression chambers each having a single piston. One or more of the pistons include a disengageable eccentric cam system between the motor and the piston or pistons for driving the piston or pistons at a full stroke between a bottom position and a top dead center position when the motor is operated in the forward direction. The piston with the disengageable eccentric cam is driven at a reduced stroke between an intermediate position and the bottom position when the motor is operated in the reverse direction. The structure supporting the cylinders includes an opening for the appropriate cylinders that allows the discharge of reexpansion gas at or near the completion of the compression cycle (i.e., at or near the top dead center position of the stroke). Alternatively, the structure supporting the cylinders may include a valve  601  that is opened for the appropriate cylinders at or near the end of the compression cycle to discharge at least a portion of the reexpansion gas. 
   In accordance with another embodiment of the present invention, the invention is directed to a two-stage reciprocating compressor. In this embodiment, the compressor includes a reversible motor for rotating in either a forward or a reverse direction and a structure for supporting one or more cylinders having a single cylinder, an associated single compression chamber, and a single piston. A mechanical system is provided between a motor and the single piston for driving the piston within the cylinder between a bottom position and a top dead center position when the motor is operated in the forward direction. The space formed within the cylinder by the piston is the compression chamber. When a reduced capacity is desired, the piston is driven at a reduced stroke between an intermediate position and the top dead center position by operating the motor in a reverse direction. In order to discharge reexpansion gas, an opening is provided in the structure supporting the cylinders permitting discharge of reexpansion gas at or near the completion of the compression cycle (i.e., at or near the top dead center position of the stroke). Alternatively, the structure having the cylinder may include a valve  601  that is opened at or near the end of the compression cycle to discharge at least a portion of the reexpansion gas  309 . 
   In accordance with still another embodiment of the present invention, the invention is directed to a rotary compressor. The compressor includes a reversible motor for rotating in a forward and a reverse direction and a plurality of compression chambers and associated compression rollers. One or more of the compression rollers are mechanically connected to a disengageable eccentric structure driven by the crankpin and the motor. The disengageable eccentric system includes a compression roller or rollers that provide a small clearance when the motor is operated in one direction and a larger clearance when operated in the opposite direction. The rotary compressor operates at a larger capacity (i.e., compresses a greater quantity of gas) when the roller or rollers provide a small clearance and a smaller capacity (i.e., compresses a smaller quantity of gas) when the roller or rollers provide a larger clearance. The cylinders containing the compression rollers include an opening that allows the discharge of reexpansion gas remaining in the cylinder at or near the completion of the compression cycle (i.e., at or near the point where the discharge of gas is complete and the drawing in of gas begins). Alternatively, the structure that includes the cylinder may include a valve  601  that is opened at or near the end of the compression cycle to discharge at least a portion of the reexpansion gas  309 . 
   The compressor according to the present invention is not limited to reciprocating compressors or rotary compressors. Any type of compressor that uses a portion of compression mechanism disengageable from the driving member during operation (i.e., a disengageable eccentric structure) may utilize the present invention. Other suitable compressor types include, but are not limited to, scotch yoke compressors, and scroll compressors. 
   In accordance with a further embodiment of the present invention, the compressor having the system for reducing the amount of reexpansion gas in the compression chamber at or near the end of the compression cycle may be used in a variety of commercial or residential applications utilizing a refrigeration cycle. For example, the present invention may be utilized in a heating, ventilating, and air conditioning (“HVAC”) system to condition air within an enclosure. The HVAC system includes a two-stage compressor having an opening in a compression cylinder and/or compression component to discharge reexpansion gas. The compressor is operable at either a first stage with a first capacity or at a second stage with a second, reduced capacity. 
   According to another embodiment, the invention is directed to a refrigerator appliance that includes a two-stage compressor having an opening in the compression cylinder and/or compression component to discharge reexpansion gas. The compressor is operable at either a first stage with a first capacity or at a second stage with a second, reduced capacity. Preferably, the compressor is continuously operated in the reduced capacity mode until a high cooling demand, such as opening the door or introducing a load of relatively warm perishables, is placed on the refrigerator. When high demand is required, the compressor may be switched to the first, increased, capacity to compensate for the increased demand. 
   While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.