Patent Publication Number: US-6981851-B2

Title: Piston stroke limiting device for a reciprocating compressor

Description:
CROSS REFERENCE TO PRIOR APPLICATIONS 
   This is a U.S. national phase application under 35 U.S.C. §371 of International Patent Application No. PCT/BR01/00122 filed Oct. 4, 2001, and claims the benefit of Brazilian Application No. PI 0004859-3, filed Oct. 5, 2000. The International Application was published in English on Apr. 11, 2002 as International Publication No. WO 02/29251 A1 under PCT Article 21(2). 
   FIELD OF THE INVENTION 
   The present invention refers to a piston stroke limiting device for a reciprocating compressor, particularly of the type driven by a linear motor and used in small refrigerating appliances, such as refrigerators, freezers, water fountains, etc. 
   BACKGROUND OF THE INVENTION 
   In the reciprocating compressors with a linear motor, the gas compression mechanism occurs by the axial movement of approximation and spacing of a piston inside a cylinder, in relation to a cylinder head, which is mounted to an end of the cylinder and in which are positioned suction and discharge valves for controlling the gas inlet and gas outlet in the cylinder. 
   The piston is driven by an actuator, which carries a magnetic component driven by a linear motor. The piston is connected to a resonant spring and forms with the latter and with the magnetic component the resonant assembly of the compressor. This resonant assembly driven by the linear motor has the function of performing a linear reciprocating movement, making the movement of the piston inside the cylinder exert a compression action on the gas admitted by the suction valve, until said gas is discharged through the discharge valve to the high pressure side of the refrigeration system to which the compressor is mounted. 
   Variations in the operating conditions of the compressor, or variations in the voltage supply thereof may cause the resonant assembly to be displaced beyond a certain predetermined limit, from which the gas mass of the dead compression volume still existing in the compression chamber begins to actuate as an extra impeller to the piston, increasing the amplitude of its operating cycle, and making the piston top collide with the cylinder head, causing damages to the compressor. 
   Several forms of controlling the amplitude of the piston stroke have already been presented, either electronic (U.S. Pat. Nos. 5,156,005; 5,342,176; 5,496,153; 5,450,521; 5,592,073), which control the current supplied to the motor as a function of the piston position, or mechanical (usually employed in Stirling machines), such as the inclusion of relief gas channels, or mechanical or pneumatic springs. Electronic commands, which are able to precisely control the position of the piston, are extremely expensive, besides presenting an inertia that is inherent to the system, as it occurs with the mechanical solutions. Such inertia allows the occurrence of occasional impacts. 
   SUMMARY OF THE INVENTION 
   Thus, it is an object of the present invention to provide a piston stroke limiting device for a reciprocating compressor, particularly of the type driven by a linear motor, which device, at the end of the compression stroke, minimizes the impelling effects on the piston resulting from the gas mass in the compression chamber that does not reach the discharge chamber at the end of compression. 
   This and other objects are achieved by a piston stroke limiting device for a reciprocating compressor, said compressor including a piston reciprocating inside a cylinder, which is closed by a cylinder head and defines, between the latter and the top of the piston, a compression chamber, said device comprising: a driving element, which is provided in one of the parts defined by the piston and the cylinder head, and which projects to the inside of the compression chamber, in order to touch the other of said parts when the piston surpasses a predetermined nominal position at the end of the compression stroke; at least one relief passage, which is provided with a valve seat and disposed in one of the parts defined by the piston and the cylinder head, communicating the compression chamber with a region of the compressor that is subject to a substantially lower operational pressure; and a relief valve, which is constantly forced to a closed condition, in which it is seated on the respective valve seat, in order to be selectively displaced to an open condition by action of the driving element, when the piston surpasses said predetermined nominal position at the end of the compression stroke. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described below, with reference to the attached drawings, in which: 
       FIG. 1  is a schematic longitudinal diametrical sectional view of part of a reciprocating compressor with a linear motor, constructed according to the prior art; 
       FIG. 2  is a schematic longitudinal diametrical sectional view of part of a reciprocating compressor with a linear motor, constructed according to a first way of carrying out the present invention; 
       FIG. 3  is a schematic longitudinal diametrical sectional view of part of a reciprocating compressor with a linear motor, constructed according to a second way of carrying out the present invention; and 
       FIG. 4  is a schematic longitudinal diametrical sectional view of part of a reciprocating compressor with a linear motor, constructed according to a third way of carrying out the present invention. 
   

   DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
   The present invention will be described in relation to a reciprocating compressor used in refrigeration systems and driven by a linear motor, this motor-compressor assembly being mounted inside a hermetic shell  1  connecting the compressor to, for example, a refrigeration system. In the illustrated constructions, the compressor presents a piston  2  provided inside a cylinder  3 , which is closed by a cylinder head  4  and defines, between the latter and the piston top, a compression chamber  5 . 
   In the illustrated constructions in which the compressor is of the type driven by a linear motor, the piston  2  is coupled to an actuating means  6 , which is generally tubular, external to the cylinder  3 , and carries a magnetic component  7 , which is axially impelled by energization of the linear motor. The spacing and approximation movements of the piston  2  inside the cylinder  3  in relation to the cylinder head  4  determines, respectively, the suction operation and the discharge operation of the gas in the compression cycle of the compressor. The compression stroke end condition of the piston is defined by a predetermined nominal position of the piston inside the compressor chamber  5 , which position, indicated in  FIGS. 2–4  by a dashed line, if surpassed, causes the previously mentioned deficiencies. 
   According to the illustration in  FIG. 1 , in the cylinder head  4  is provided a suction orifice, in which is mounted a suction valve  8 , selectively communicating the inside of the compression chamber  5  with the inside of a suction chamber  10 , and a discharge orifice, in which is mounted a discharge valve  9 , selectively communicating the inside of the compression chamber  5  with the discharge chamber  20 , said suction and discharge valves  8 ,  9  controlling the gas inlet and gas outlet in the cylinder  3 . 
   The piston  2  is connected to a resonant spring  30 , forming therewith and with the magnetic component  7 , a resonant assembly. 
   According to the present invention, in order to minimize the effects resulting from the impelling action of the non-compressed gas to the discharge chamber  20  at the end of compression, the present invention has a piston stroke limiting device for a reciprocating compressor, particularly of the type driven by a linear motor, said device comprising a driving element  40 , provided in one of the parts defined by the piston  2  and the cylinder head  4 , and projecting to the inside of the compression chamber  5 , in order to touch the other of said parts when the piston  2  surpasses a predetermined nominal position at the end of the compression stroke; at least one relief passage  50 , which is provided with a valve seat  51 , and disposed in one of the parts defined by the piston  2  and the cylinder head  4 , communicating the compression chamber  5  with a region of the compressor subject to a lower operational pressure, such as for example, the suction chamber  10 , or the inside of the hermetic shell  1  of the compressor; and a relief valve  60 , which is constantly forced to a closed condition, for instance by a spring  70 , which is seated on the part of the piston  2  or the cylinder head  4  that defines the valve seat  51 , and which is seated, at said closed condition, on said valve seat  51 , in order to be selectively displaced to a open condition, by action of the driving element  40 , when the piston  2  surpasses said predetermined nominal position at the end of the compression stroke. Although a construction has been described and illustrated, in which the piston stroke limiting device has only one driving element  40  actuating on a respective relief valve  60 , the concept presented herein foresees the provision of a plurality of driving elements and the respective relief valves. 
   According to an illustrated constructive form of the present invention, the driving element  40  presents an extension corresponding to the distance between the stroke end predetermined nominal position of the piston  2  and the cylinder head  4 . 
   According to an illustrated constructive form of the present invention, the piston  2  carries on the top thereof the driving element  40 . 
   In a constructive option illustrated in  FIG. 2 , the piston  2  affixes, for example incorporating in a single piece, the driving element  40 . In this construction, the relief passage  50  is provided in the cylinder head  4 , the valve seat  51  being defined on a face of said cylinder head  4  turned to the inside of the suction chamber  20 . In this construction, the relief valve  60  is mounted inside said suction chamber  10 , from a seating surface  11  provided inside said suction chamber  10  and which affixes the spring  70  that is constantly forcing the relief valve  60  to a closed position of the relief passage  50 . In the constructive option illustrated in  FIG. 2 , the relief valve  60  is in the form of a flat disc. 
   According to the illustrations of  FIGS. 3 and 4 , the driving element  40  is affixed, for instance incorporated to the relief valve  60 , having its axis coinciding with the axis of said relief valve  60 , and coinciding with the axis of piston  2 . In these constructions, the relief valve  60  is in the form of a substantially tapered body. 
   In the construction illustrated in  FIG. 3 , the relief valve  60  is mounted inside the suction chamber  10 , on a seating support  11  thereof, and the relief passage  50  is defined in this construction in the cylinder head  4 . 
   In the illustrated construction of  FIG. 4 , the relief valve  60  carrying the driving element  40  is mounted, through the spring  70 , on a support  80 , which is affixed or incorporated inside the piston  2 , spaced from the top thereof, by a certain predetermined distance, in the construction therein, the relief passage  50  that defines the valve seat  51  for the relief valve  50  is provided on the top of the piston  2 . In this construction, the relief valve  60  seats, in its closed condition, on a valve seat  51 , defined on the top of the piston  2  top and opened to the inside of the hermetic shell  1 . 
   The support  80  further presents at least one another relief passage  50 , communicating the inside of the compression chamber  5  with the inside of the shell  1 , when the relief valve is opened against the action of the spring  70 , in the conditions where the piston  2  surpasses said predetermined nominal position at the compression stroke end.