Abstract:
A variable displacement type compressor circulates a fluid in an air conditioning circuit. The compressor has a compression mechanism and a displacement control valve. The compression mechanism compresses the fluid. The displacement control valve controls discharge amount of the fluid of the compressor. In a first predetermined range of discharge pressure, suction pressure decreases at a first variation as the discharge pressure increases. In a second predetermined range of the discharge pressure that is higher than the first predetermined range, the suction pressure varies at a second variation as the discharge pressure increases. The second variation is constituted of at least one of a third variation that is smaller than the first variation and at which the suction pressure decreases as the discharge pressure increases, a fourth variation at which the suction pressure increases as the discharge pressure increases, and substantially zero.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to a variable displacement type compressor adapted for use in an air conditioner, and more specifically to a displacement control system for the variable displacement type compressor which makes it possible to achieve the desired air conditioning performance by appropriately controlling the displacement of the compressor.  
           [0002]    A variable displacement type compressor, for example, for use in an automotive air conditioner has incorporated therein a control valve for controlling discharge amount of a refrigerant. In the compressor equipped with such a control valve, operating performance of the air conditioner varies depending on the relationship between the discharge pressure and suction pressure of the refrigerant. FIG. 12 shows three different operating regions A, B and C of the compressor, indicated by shaded areas, in connection with the relationship between the discharge pressure Pd and suction pressure Ps of the refrigerant. The region A represents a region where the compressor is operating under a low cooling load and with low discharge pressure Pd. In such an operating state, a mist tends to be formed on the interior surface of vehicle windshield with an increase in the suction pressure Ps and hence the refrigerant pressure at the outlet of an evaporator connected in the air conditioning system, thus offering a problem of insufficient de-misting performance of the air conditioner. In the region B where the compressor is operating similarly under a low cooling load and with a low discharge pressure Pd, the evaporator tends to be frosted with a decrease in the suction pressure Ps. In the region C where the compressor is in operation under a high cooling load and with high discharge pressure, cooling performance of the air conditioner tends to be reduced with an increase in the suction pressure Ps. Thus, there has been a demand for a control system of an air conditioner which is designed in view of the above problems.  
           [0003]    Various displacement control valves for an air conditioning system are disclosed in Japanese Unexamined Patent Publications No. 4-321779. No. 6-123279 and No. 7-119642, which are designed to achieve the desired air conditioning performance by appropriately controlling the displacement of a variable displacement type compressor. A compressor using such displacement control valve can prevent the aforementioned problems by achieving Pd-Ps characteristic as represented by a curve in FIG. 12.  
           [0004]    According to the Pd-Ps characteristic curve of FIG. 12, however, the suction pressure is decreased excessively in the control region of high cooling loads ant the compressor is operated continuously, with the result that the engine for driving the compressor is applied with an excessive load and the coolant in a radiator of the engine is heated accordingly.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention relates to a variable displacement type compressor, an air conditioning system equipped with such a compressor and a method for controlling the displacement of such a compressor which will not impose an excessive load on an engine while the compressor is running under a high load.  
           [0006]    According to the present invention, a variable displacement type compressor circulates a fluid in an air conditioning circuit. The fluid is drawn into a suction region before compression. The pressure in the suction region is defined as suction pressure. The fluid is discharged to the discharge region after compression. The pressure in the discharge region is defined as discharge pressure. The suction region is connected to the discharge region. The compressor has a compression mechanism and a displacement control valve. The compression mechanism compresses the fluid. The displacement control valve controls discharge amount of the fluid of the compressor. In a first predetermined range of the discharge pressure the suction pressure decreases at a first variation as the discharge pressure increases. In a second predetermined range of the discharge pressure that is higher than the first predetermined range the suction pressure varies at a second variation as the discharge pressure increases. The second variation is constituted of at least one of a third variation that is smaller than the first variation and at which the suction pressure decreases as the discharge pressure increases, a fourth variation at which the suction pressure increases as the discharge pressure increases, and substantially zero.  
           [0007]    The present invention also provides a method for controlling displacement in a variable displacement type compressor that circulates a fluid in an air conditioning circuit. The fluid is drawn into a suction region before compression. The pressure in the suction region is defined as suction pressure. The fluid is discharged to the discharge region after compression. The pressure in the discharge region is defined as discharge pressure. The suction region is connected to the discharge region. The method comprises the steps of decreasing the suction pressure at a first variation as the discharge pressure increases in a first predetermined range of the discharge pressure, setting a third variation that is smaller than the first variation and at which the suction pressure decrease as the discharge pressure increases in a second predetermined range of the discharge pressure that is higher than the first predetermined range, setting a fourth variation at which the suction pressure increases as the discharge pressure increases in the second predetermined range, setting a second variation by using at least one of the third variation, the fourth variation, and substantially zero in the second predetermined range, and varying the suction pressure at the second variation as the discharge pressure increases in the second predetermined range. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:  
         [0009]    [0009]FIG. 1 is a longitudinal sectional view illustrating a variable displacement swash plate type compressor  100  according to a preferred embodiment of the present invention;  
         [0010]    [0010]FIG. 2 is an enlarged longitudinal sectional view illustrating a displacement control valve  30 , which is shown in FIG. 1, under a low-load control range R 3  according to the preferred embodiment of the present invention;  
         [0011]    [0011]FIG. 3 is an enlarged perspective view illustrating a valve box  60 , a valve body  40  and a partial discharge pressure correction rod  41  in FIG. 2 according to the preferred embodiment of the present invention;  
         [0012]    [0012]FIG. 4 is an enlarged longitudinal sectional view illustrating a displacement control valve  30 , which is shown in FIG. 1, under an intermediate-load control range R 1  according to the preferred embodiment of the present invention;  
         [0013]    [0013]FIG. 5 is an enlarged longitudinal sectional view illustrating a displacement control valve  30 , which is shown in FIG. 1, under a high-load control range R 2  according to the preferred embodiment of the present invention;  
         [0014]    [0014]FIG. 6 is a graph illustrating a Pd-Ps characteristic curve when the displacement control valve  30  according to the preferred embodiment of the present invention is used;  
         [0015]    [0015]FIG. 7 is a schematic view illustrating various forces acting in the displacement control valve  30  during compressor operation in the low-load control range R 3  according to the preferred embodiment of the present invention;  
         [0016]    [0016]FIG. 8 is a schematic view illustrating various forces acting in the displacement control valve  30  during compressor operation in the intermediate-load control range R 1  according to the preferred embodiment of the present invention;  
         [0017]    [0017]FIG. 9 is a schematic view illustrating various forces acting in the displacement control valve  30  during compressor operation in the high-load control range R 2  according to the preferred embodiment of the present invention;  
         [0018]    [0018]FIG. 10 is a longitudinal sectional view illustrating a displacement control valve  130  according to another preferred embodiment of the present invention;  
         [0019]    [0019]FIG. 11 is a cross sectional view illustrating a spring  80 , spring washers  81  and  82 , a through hole  82   a,  a valve body  40  and a partial discharge pressure correction rod according to yet another preferred embodiment of the present invention; and  
         [0020]    [0020]FIG. 12 is a graph illustrating a Pd-Ps characteristic curve when a displacement control valve according to a prior art is used. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]    The following will describe a preferred embodiment of a variable displacement type compressor according to the present invention while having reference to the accompanying drawings. It is noted the following description will deal with a variable displacement swash plate type compressor adapted for use in an automotive air conditioning system.  
         [0022]    Referring firstly to FIG. 1, the variable displacement swash plate type compressor  100  (referred to merely as “compressor” hereinafter) includes a cylinder block  1  having formed therein a plurality of cylinder bores  1   a  arranged around the central axis of the cylinder block  1  and each receiving therein a reciprocally movable piston  18 . A front housing  2  is sealingly fastened to the front end of the cylinder block  1 , and a rear housing  5  is similarly fastened to the rear end of the cylinder block  1  with a valve plate assembly  6  interposed therebetween. The cylinder block  1  and the front housing  2  cooperate to define a crank chamber  9  as a crank chamber pressure region in which a wobble plate  15  and its associated parts are disposed as will be described in detail in later part hereof.  
         [0023]    The rear housing  5  has formed therein a suction chamber  3  as a suction region or a suction pressure region into which refrigerant before compression is drawn and a discharge chamber  4  as a discharge region or a discharge pressure region into which refrigerant compressed in tho respective cylinder bores  1   a  is discharged. The valve plate assembly  6  is formed therethrough with a suction port for providing communication between the suction chamber  3  and each cylinder bore  1   a  through a suction valve  3   a  and also with a discharge port for communication between the discharge chamber  4  and each cylinder bore  1   a  through a discharge valve  4   a.  A retainer  4   b  is fixed in the discharge chamber  4  so as to limit the maximum opening of the discharge valve  4   a.  On the rear side of the rear housing  5  is provided a displacement control valve  30  serving as the displacement control means of the present invention which will be described in detail in later part hereof.  
         [0024]    A first supply passage  20  extends through the cylinder block  1  and the rear housing  5  for communication between the crank chamber  9  and the displacement control valve  30 . A bleed passage  21  having therein an orifice  21   a  is formed in the cylinder block  1  for communication between the crank chamber  9  and the suction chamber  3 . Furthermore, the rear housing  5  has formed therein a pressure sensing passage  22  and a second supply passage  23  for communication of the suction chamber  3  and the discharge chamber  4  with the displacement control valve  30 , respectively, as will be described more in detail with reference to FIG. 2.  
         [0025]    As shown in FIG. 1, a drive shaft  8  is disposed in the crank chamber  9  and rotatably supported in the cylinder block  1  and the front housing  2  by bearings  1   b  and  2   b  arranged in the cylinder block  1  and the front housing  2 , respectively. The drive shaft  8  is connected at the front end thereof to a vehicle engine by way of a suitable clutching means such as electromagnetic clutch (not shown). A shaft seal  2   a  is provided between the drive shaft  8  and the front housing  2 . It is noted that the compressor  100  may dispense with the clutch so that the drive shaft  8  is driven constantly by means of a belt and pulley arrangement.  
         [0026]    A rotor  7  is fixedly mounted on the drive shaft  8  for rotation therewith in the crank chamber  9  with a thrust bearing  2   c  disposed between the rotor  7  and the inner wall of the front housing  2 , and a sleeve  19  is axially slidably mounted on the drive shaft  8  adjacent to the rotor  7 . The rotor  7  is formed with an elongated through-hole  7   b  through which a pin  11   a  of a swash plate  11  is inserted slidably in the elongated through-hole  7   b.  The swash plate  11  is rotatable with the drive shaft  8  and pivotally supported by a pair of trunnion pins  19   a  projecting from opposite sides of the sleeve  19  so that, as the drive shaft  8  is rotated, the swash plate  11  makes a nutational motion about the drive shaft  8  at an inclination angle. A wobble plate  15  is fitted to the swash plate  11  by way of a thrust bearing  12 , a plane bearing  10 , a race  13  and a thrust washer  14 , and a guide rod  16  extends in the crank chamber  9  to prohibit rotation of the wobble plate  15 . Each of the pistons  18  received in the cylinder bores  1   a  is connected to the wobble plate  15  by a rod  17 . In operation, the wobble plate  15  makes a wobbling movement in response to the nutational motion of the swash plate  11  and the pistons  18  connected to the wobble plate  15  are caused to move reciprocally in their associated cylinder bores  1   a.  Refrigerant is drawn from the suction chamber  3  into the cylinder bore  1   a  during the suction stroke of the piston  18  and then compressed in and then discharged out of the cylinder bore  1   a  during the discharge stroke of the piston  18 , thus compressed refrigerant being discharged into the discharge chamber  4 .  
         [0027]    Displacement of the compressor  100  depends on the length of stroke of the piston  18  and such stroke length varies with the inclination angle of the swash plate  11 . To be more specific, the stroke length of the piston  18  and hence the displacement is increased with an increase of the angle at which the swash plate  11  is inclined with respect to a plane perpendicular to the axis of the drive shaft  8 , and vice versa. This inclination angle of the swash plate  11  during compressor operation is determined by the pressure differential between the pressure in the cylinder bores  1   a  and in the crank chamber  9 , and this pressure differential is adjusted by the displacement control valve  30 .  
         [0028]    The following will describe the structure of the displacement control valve  30  while having reference to FIGS. 2 and 3.  
         [0029]    Referring to firstly FIG. 2, the displacement control valve  30  includes a main valve portion  33 , a cylindrical housing  31  fixed at one end thereof to one end of the main valve portion  33 , and a cap  38  fixed to the other end of the main valve portion  33 . An adjusting portion  32  is screwed into the other end of the cylindrical housing  31  by way of an O-ring, and an insert  37  is disposed in the cap  38 .  
         [0030]    The main valve portion  33 , the cylindrical housing  31  and the adjusting portion  32  cooperate to define a suction pressure chamber  51  as a suction region which is in communication with the suction chamber  5  via the aforementioned pressure sensing passage  22  in the rear housing  5  and radial passages  51  a formed in the cylindrical housing  31 . Thus, suction pressure Ps prevails in the suction pressure chamber  51  of the displacement control valve  30 . Within the suction pressure chamber  51  is disposed a bellows  36  having one end thereof fixed to the adjusting portion  32  and the other end thereof engaged with a rod  35  which is slidably disposed in an axial bore formed in the main valve portion  33 . The bellows  36  has therein a spring  30   a  urging the bellows  36  in the direction indicated by arrow F 1  and the bellows interior is maintained under vacuum. F 1  represents the sum of the elastic force of the bellows  36  and the urging force of the spring  36   a  both acting in the same arrow direction. The bellows  36 , which serves as the suction pressure Ps sensitive means of the invention, has an effective pressure sensing area S 1  to which suction pressure Ps acts in the direction opposite to the arrow direction F 1 . It is noted that any suitable means such as diaphragm may be used in place of the bellows  36  an the suction pressure Ps sensitive means of the invention.  
         [0031]    The rod  35  is slidable in the axial bore in the main valve portion  33  by contraction or expansion of the bellows  36 . The main valve portion  33  has formed therein at an intermediate position thereof an axial bore  20   b  into which the distal end of the rod  35  extends and first radial supply ports  20   a  extending radially from the axial bore  20   b.  The first supply ports  20   a  are in communication with the aforementioned first supply passage  20  formed through the cylinder block  1  and the rear housing  5  for communication with the crank chamber  9 . The axial bore  20   b  is formed with a cross sectional area S 2 .  
         [0032]    The main valve portion  33  and the insert  37  define therebetween a discharge pressure chamber  52  as a discharge region which is in communication with the discharge chamber  4  through the second supply passage  23  formed in the rear housing  5  and second radial supply ports  23   a  which are formed in the main valve portion  33 . The first supply passage  20 , the first supply port  20   a,  the second supply passage  23  and the second supply port  23   a  constitute communication routes of the variable displacement type compressor according to the present invention.  
         [0033]    The insert  37  and the cap  38  have defined therebetween a crank pressure chamber  53  as a crank chamber pressure region which is in communication with the crank chamber  9  of the compressor  100  by way of a communication passage  33   a  formed in the main valve portion  33 .  
         [0034]    The insert  37  is formed at the axial center thereof with an axial bore through which a discharge pressure correction rod  41 , which serves as the discharge pressure sensitive means of the invention, is slidably inserted. This rod  41  has a flange portion  41   a  disposed in the discharge pressure chamber  52  and a stem portion  41   b  passing through the insert  37  and extending into the crank pressure chamber  53 . A spring  12  having a spring constant k 2  is provided in the crank pressure chamber  53  for urging the correction rod  11  toward the discharge pressure chamber  52  as indicated by arrow  70  with force F 2 . The stem portion  41   b  of the correction rod  41  has a cross sectional area S 3 .  
         [0035]    As shown in FIGS. 2 and 3, a valve box  60  serving as the rod supplementing member of the invention is disposed within the discharge pressure chamber  52 , and a valve body  40  in the form of a spherical ball serving as the valve means of the invention and part of the correction rod  41  including its flange portion  41   a  and part of the stem portion  41   b  adjacent to the flange portion  41   a  are incorporated within the valve box  60 . A spring  63  with a spring constant k 3  is disposed between the flange portion  41   a  and the inner end of the valve box  60 . Part of the valve body  40  which protrudes out of the valve box  60  through its first opening  61  is contactable with the rod  35 . The discharge pressure correction rod  41  is movable axially through the opposite second opening  62  of the valve box  60 . The discharge pressure correction rod  41 , the spring  42  and the valve box  60  constitute the urging means of the present invention. Reference numeral  39  designates a valve seat for the valve body  40 .  
         [0036]    It is noted that the cross sectional areas of S 1 , S 2  and S 3  of the bellows  36 , the axial bore  20   b  and the stem portion  41   b  of the correction rod  41 , respectively, are provided such that S 1 &gt;S 3 &gt;S 2 .  
         [0037]    The compressor  100  having incorporated therein such displacement control valve  30  is disposed in a refrigeration circuit together with a condenser, expansion valve, evaporator, etc. (not shown). When the drive shaft  8  is driven to rotate by vehicle engine, the swash plate  11  is rotated at an inclined angle by the rotor  7  that is fixed on and hence rotatable with the drive shaft  8 . The wobble plate  15  fitted to the swash plate  11  makes a wobbling movement at the inclined angle of the swash plate  11 , which causes the pistons  18  to move reciprocally in their associated cylinder bores  1   a  for a stroke length corresponding to the inclined angle of the wobble plate  15 . In so doing, refrigerant flowing from the evaporator to the suction chamber  3  is drawn into the cylinder bore  1   a  then in suction stroke. Refrigerant introduced in the cylinder bore  1   a  is compressed by the piston  18  and then discharged into the discharge chamber  4 .  
         [0038]    As is apparent from the foregoing description, the displacement control valve  30  is provided as an internal control mechanism of the compressor  100  wherein the valve body  40  of the displacement control valve  30  is operable by way of the bellows  36  as the suction pressure sensitive means and the discharge pressure correction rod  41  as the discharge pressure sensitive means, respectively.  
         [0039]    The displacement control valve  30  thus constructed is configured such that a Pd-Ps characteristic curve as indicated by a solid line in FIG. 6 is achieved, as compared with a curve of a dotted line achievable by prior art.  
         [0040]    Referring now specifically to FIG. 6 showing two Pd-Ps characteristic curves, wherein the solid line curve shows Pd-Ps characteristic achievable by use of the displacement control valve  30  of the illustrated embodiment, while the dotted line curve represents similar characteristic of the prior art control valves. In the diagram of FIG. 6, symbols T 1  and T 2  depict infection points of Pd-Ps characteristic curve of the displacement control valve  30 , so that the curve may be divided into three line sections L 1 , L 2  and L 3  by such inflection points T 1  and T 2 .  
         [0041]    The displacement control valve  30  is configured to operate as follows. In the low-load control region R 3  (or the third mode in the invention) corresponding to the line section L 3  where the compressor  100  is operating under a low discharge pressure Pd and hence with a low displacement, suction pressure Ps increases with an increase in discharge pressure Pd. In the intermediate-load control range R 1  (or the first mode in the invention) corresponding to the line section L 1  between the inflection points T 1  and T 2  where discharge pressure Pd is in a middle range, the suction pressure Ps decreases with an increase of discharge pressure Pd. In the high-load control range R 2  corresponding to the line section L 2  (or the second mode in the invention) where the compressor  100  is operating under a high discharge pressure Pd and hence with a high displacement, suction pressure Ps is maintained substantially at a constant level irrespective of a change of discharge pressure Pd. In the control range R 2 , suction pressure Ps is prevented from being dropped.  
         [0042]    As shown in FIG. 6, the Pd-Ps characteristic describes a curve so that it avoids interference with any of the shaded region A where a mist tends to be produced, the region B where evaporator frosting tends to occur and the region C where cooling performance tends to be decreased. In other words, the compressor  100  operating according the Pd-Ps characteristic curve can forestall these three problems.  
         [0043]    As appreciated from FIG. 6, suction pressure Ps in the intermediate-load control range R 1  between the inflection points T 1  and T 2  is generally raised or set higher than that of the characteristic curve attainable by the prior art control valves as indicated by a dotted line, without interfering with the operating region C. Furthermore, suction pressure Ps in the high-load control range R 2  of the Pd-Ps characteristic curve is maintained substantially while avoiding interference with the operating region C. As is apparent from comparison with the dotted line, maintenance of a substantially constant suction pressure Ps in the high-load control range R 2  is accomplished by providing the inflection point T 2  between the line sections L 1  and L 2  so as to differentiate the inclinations of the line sections L 1  and L 2 .  
         [0044]    According to the Pd-Ps characteristic curve of the displacement control valve  30  wherein suction pressure Ps is set higher than heretofore in the intermediate-load control range R 1 , fuel consumption can be improved. Additionally, suppressing a decrease of suction pressure Ps in the high-load control range R 2  helps not only to improve the fuel consumption but also to prevent temperature rise of coolant in a vehicle radiator.  
         [0045]    The following will describe the operation of the displacement control valve  30  in the control ranges R 3 , R 1  and R 2  with reference to FIGS. 2, 4,  5  and  6 .  
         [0046]    [0046]FIG. 2 shows a state of the displacement control valve  30  when the compressor  100  is operating under a low load in the control range R 3 . The discharge pressure correction rod  41  is urged in the direction of the arrow  70  by the spring  42 , and the valve body  40  is pushed by the correction rod  41  accordingly to be seated on the valve seat  39 , so that the axial bore  20   b  which is in communication with the crank chamber  9  through the first supply passage  20  in the cylinder block  1  is shut off from the discharge pressure chamber  52 . That is, the discharge chamber  4  and the crank chamber  9  are shut off from each other. In this state of FIG. 2, the spring  63  has one end thereof free from contact with its adjacent inner surface of the valve box  60  and hence provides no urging action. Because the crank chamber  9  and the suction chamber  3  are in communication with each other by way of the bleed passage  21  having therein the orifice  21   a,  part of the refrigerant in the crank chamber  9  flows into the suction chamber  3 . Since flowing of refrigerant under high pressure from the discharge chamber  4  into the crank chamber  9  is shut off, crank chamber pressure Pc is reduced and the back pressure acting on the pistons  18  is reduced, accordingly. Therefore, the inclination angle of the wobble plate  15  is increased thereby to increase the stroke length of the pistons  18 , with the result that the displacement is increased. In this state of the displacement control valve  30 , the valve body  40  is not lifted off from the valve seat  39  unless suction pressure Ps in the suction pressure chamber  51  is substantially reduced relatively to the force F 1 . Therefore, suction pressure Ps is increased with a build-up of discharge pressure Pd. At this state, the crank chamber pressure Pc and the suction pressure Ps are maintained substantially to be equal to each other.  
         [0047]    From the schematic diagram of FIG. 7 showing various forces acting in the displacement control valve  30  during compressor operation in the low-load control range R 3 , the equilibrium state of such forces can be expressed by equation (1), and transforming this equation (1), the relationship between suction pressure Ps and discharge pressure Pd can be expressed by equation (2), as follows.  
         F 1 −S 1 ·Ps+S 2 ·Ps−S 2 ·Pd+S 3 ·Pd−S 3 ·Ps−F 2 =0  (1)  
         Ps=−[(S 2 −S 3 )/(S 1 −S 2 +S 3 )]·Pd+(F 1 −F 2 )/(S 1 −S 2 +S 3 )  (2)  
         [0048]    Expressing the equation (2) in a coordinate system with discharge pressure Pd and suction pressure Ps represented by abscissa and ordinate, respectively, as shown in FIG. 6, the inclination of the line is determined by −(S 2 −S 3 )/(S 1 −S 2 +S 3 ). Since the cross-sectional areas of S 1 , S 2  and S 3  are such that S 1 &gt;S 3 &gt;S 2 , the inclination of the line, or the manner in which suction pressure Ps varies with discharge pressure in the Pd-Ps characteristic line, is positive. That is, the displacement control valve  30  provides Pd-Ps characteristic as shown by the line section L 3  of FIG. 6 in the low-load control range R 3 .  
         [0049]    Referring to FIG. 4 showing a state of the displacement control valve  30  when the compressor  100  is operating under an intermediate load in the control range R 1 , the discharge pressure correction rod  41  is moved in the direction of an arrow  72  with an increase of the discharge pressure Pd while overcoming the urging force of the spring  42 . Thus, the pressing force to keep the valve body  40  in closed position by the correction rod  41  is cancelled. In this state of the discharge pressure correction rod  41 , the spring  63  is merely moved in the direction of the arrow  72  with the correction rod  41 , exerting no urging force. The valve body  40  is moved off the valve seat  39  and, therefore, the first supply port  20   a  and the second supply port  23   a  become in communication with each other, thereby allowing refrigerant under a high pressure in the discharge chamber  4  to flow into the crank chamber  9  through the second supply passage  23 , the second supply port  23   a,  the first supply port  20   a  and the first supply passage  20 . As a result, crank chamber pressure Pc is increased and the back pressure acting on the pistons  18  is increased accordingly, so that the inclination angle of the wobble plate  15  is decreased. Thus, the stroke length of the pistons  18  is shortened, causing the displacement to be reduced. Since the suction pressure Ps in the suction pressure chamber  51  acts against F 1 , the force to open the valve body  40  is decreased with an increase of the suction pressure Ps.  
         [0050]    From the schematic diagram of FIG. 8 showing various forces acting in the displacement control valve  30  while the compressor is operating in the intermediate-load control range R 1 , the equilibrium state of such forces can be expressed by equation (3), and transforming this equation (3), the relationship between suction pressure Ps and discharge pressure Pd can be expressed by equation (4), as follows.  
         F 1 −S 1 ·Ps+S 2 ·Ps−S 2 ·Pd=0  (3)  
         Ps=−{S 2 /(S 1 −S 2 )}·Pd+F 1 /(S 1 −S 2 )  (4)  
         [0051]    Expressing the equation (4) in a coordinate system with the discharge pressure Pd and the suction pressure Ps represented by abscissa and ordinate, respectively, the inclination of the line is determined by −S 2 /(S 1 −S 2 ). Since the cross-sectional area S 1  is greater than S 2 , or S 1 &gt;S 2 , the inclination of the Pd Ps characteristic line in the control range R 1  is negative. That is, the displacement control valve  30  provides Pd-Ps control characteristic as shown by the line section L 1  of FIG. 6 in the intermediate-load control range R 1 . The variation of suction pressure Ps with respect to discharge pressure Pd in the control range R 1  is referred to as the first variation in the invention.  
         [0052]    Now referring to FIG. 5 showing a state of the displacement control valve  30  when the compressor  100  is operating under a high load in the control range R 2 , the discharge pressure correction rod  41  is moved further in the direction of the arrow  72  with a buildup of the discharge pressure Pd while overcoming the urging force of the spring  42 . With the correction rod  41  thus moved, the spring  63  begins to be compressed and to act against the force F 2 . After the spring  63  has been fully compressed, the valve box  60  and the valve body  40  are moved together with the discharge pressure correction rod  41  in the direction of the arrow  72  which causes the valve body  40  to open. That is, in the high-load control range R 2 , the valve body  40  is moved in its opening direction by cooperative action of the correction rod  41 , the valve box  60  and the spring  63 .  
         [0053]    From the schematic diagram of FIG. 9 showing various forces acting in the displacement control valve  30  during compressor operation in the high-load control range R 2 , the equilibrium state of such forces is expressed by equation (5), and transforming this equation (5), the relationship between suction pressure Ps and discharge pressure Pd is expressed by equation (6), as follows.  
         F 1 −S 1 ·Ps+S 2 ·Ps−S 2 ·Pd−S 3 ·Ps+k 3 ·x 3 −k 2 ·x 2 +S 3 ·Pd−F 2 =0  (5)  
         [0054]    [0054]               F1   -     S1   ·   Ps     +     S2   ·   Ps     -     S2   ·   Pd     -     S3   ·   Ps     +       k   3     ·     x   3       -       k   2     ·     x   2       +     S3   ·   Pd     -   F2     =   0           (   5   )               Ps   =         -     {       (     S2   -   S3     )     /     (     S1   -   S2   +   S3     )       }       ·   Pd     -       (         k   2     ·     x   2       -       k   3     ·     x   3         )     /     (     S1   -   S2   +   S3     )       +       (     F1   -   F2     )     /     (     S1   -   S2   +   S3     )                 (   6   )                                 
         [0055]    In the equation (6), the urging forces of the springs  42  and  63  are expressed by k 2 ·x 2  and k 3 ·x 3 , respectively, wherein x 2  and x 3  represent the distances by which the respective springs  42  and  63  are compressed.  
         [0056]    In the illustrated embodiment, the displacement control valve  30  operates in the high-load control range R 2  such that Pd-Ps characteristic is represented by the line section L 2  which is substantially flat as shown in FIG. 6. The variation of suction pressure Ps with respect to discharge pressure Pd in the control range R 2  is referred to as the second variation in the invention. It is noted that, since the values x 2  and x 3  are dependent on discharge pressure Pd, the urging forces k 2 x 2  and k 0 x 0  of the springs  42  and  63 , respectively, vary with discharge pressure Pd. Therefore, the inclination of the line section L 2  depends on the values of the first and second terms of the right side of the equation (6).  
         [0057]    As is now apparent from the foregoing, fuel consumption can be improved by elevating the suction pressure Ps in the intermediate-load control range R 1  and also engine is not overloaded by suppressing a decrease of the suction pressure Ps in the high-load control range R 2 . In this control range R 2 , an excessive temperature rise of coolant of a vehicle engine can be prevented successfully. Thus, the proof stress of the compressor  100  can be ensured. As is apparent from FIG. 6, problems such as mist formation on the windshield surface and evaporator frosting which tends to occur in the low-load control range H 3  can be forestalled.  
         [0058]    Additionally, the displacement control valve  30  makes it possible to rationally control displacement of the compressor  100  in each of the control ranges R 1 , R 2  and R 3 .  
         [0059]    As is understood by those skilled in the art the present invention is not limited to the above illustrated specific embodiment, but it can be practiced in various forms and changes, as exemplified below.  
         [0060]    While in the above embodiment controlling is performed such that suction pressure Ps is maintained substantially constant irrespective of an increase in discharge pressure Pd in the high-load control range R 2 , controlling of suction pressure Ps in this range may be changed as required. For example, setting may be made such that change of suction pressure Ps with respect to a rise of discharge pressure Pd occurs in any combination of manners of changes which include decreasing of suction pressure Ps at a third variation which is smaller than the first variation in the intermediate-load control range R 1 , increasing of suction pressure Ps at a fourth variation and maintaining suction pressure Ps substantially constant.  
         [0061]    Referring to FIG. 10 showing a modification of the present invention, a modified displacement control valve  130  is provided, wherein like reference numerals or symbols designate like elements or parts of the displacement control valve  30  of the preferred embodiment.  
         [0062]    The cylindrical housing  31  is formed with third supply ports  23   b  communicating with the second supply passages  23  in the main valve body  33  and a discharge pressure chamber  152  as a discharge region where discharge pressure Pd prevails due to the communication of the above third supply port  23   b  and the second supply passage  23 . In this embodiment, the first supply passage  20 , the first supply port  20   a,  the second supply passage  23  and the second supply port  23   a  and the third supply port  23   b  constitute the communication routes of the variable displacement type compressor according to the present invention. A discharge pressure correction rod  141  is disposed within the discharge pressure chamber  152 . The stem portion  141   b  of this correction rod  141  has a cross-sectional area S 4 . In the high-load control region R 1 , the discharge pressure correction rod  141  acts on the bellows  36  in the direction of an arrow  170 , or rightward as seen in FIG. 10, against the urging force of a spring  142  disposed in the discharge pressure chamber  152  and having a spring constant k 4 . These discharge pressure correction rod  141  and the spring  142  are referred to as the urging means of the invention. In operation in the high-load control range R 2 , the discharge pressure correction rod  141  and the spring  142  cooperate to urge the valve body  40  in the direction that cause the valve to open. In the control ranges R 1  and R 3 , the displacement control valve  130  operates in the same manner as the counterpart  30  of the preferred embodiment.  
         [0063]    Equilibrium state of forces acting in the displacement control valve  130  in the control range R 2  is expressed by equations (7) and (8) below, wherein k 1  represents the spring constant of the spring  36   a  and x 1  represents the distances by which the bellows  36  is contacted.  
         Ps=−{S 1 /(S 1 +S 2 )}Pd+(k 1 ·x 1 )/(S 1 +S 2 )+F 1 /(S 1 +S 2 )  (7)  
         F 1 +k 1 ·x 1 =(Pd−Ps)·S 4 −k 4 ·x 4   (8)  
         [0064]    In the equation (8), the contraction distance x 4  and hence the urging force k 4 ·x 4  of the spring  142  is dependent on discharge pressure Pd, the inclination of the line section L 2  for the high-load control range R 2  depends on the values of the first and second terms of the right side of the equation (7).  
         [0065]    Referring to FIG. 11 showing a further modification of the present invention, this modification differs from the preferred embodiment in the structure of valve box as the rod supplementing member of the invention. The rod supplementing member of FIG. 11 includes a spring  80  and spring washers  81  and  82 . The discharge pressure correction rod  41  is disposed passing through a is through-hole  82   a  formed in the spring washer  82 . This rod supplementing member can perform the same function as the counterpart comprising the valve box  60  and the spring  63  of the preferred embodiment. Additionally, the spring  80  and the spring washer  82  may be substituted by a single member including a modified spring.  
         [0066]    The present examples and preferred embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein but may be modified within the scope of the appended claims.