Abstract:
An automotive air conditioning compressor with a capacity control valve improves oil retention in the crankcase with a crankcase to suction chamber passage that is formed through the central shaft. The passage inlet opening in the shaft is located within a central chamber inset into the cylinder block, and is thereby sheltered from the main chamber of the crankcase, although still open to the crankcase. So sheltering the inlet of the shaft passage between crankcase and suction chamber isolates the inlet from the greater turbulence and higher velocity gradients within main chamber of the crankcase. Less oil is thus forced through the passage and back out of the crankcase.

Description:
PRIOR APPLICATION  
       [0001]    This application claims the benefit of prior Provisional Patent Application Serial No. 60/335,344 filed Nov. 2, 2001. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to variable capacity air conditioning compressors in general, and specifically to such a compressor with improved crankcase oil retention.  
         BACKGROUND OF THE INVENTION  
         [0003]    Piston driven automotive air conditioning compressors with variable capacity generally vary the piston stroke by allowing the angle of a nutating piston driving plate to change relative to the centerline of a drive shaft. Smaller angles yield a shorter nutation and shorter piston stroke, and larger angles create a loner piston stoke. The tilting plate may be of the unitary type that directly drives the pistons (swashplate), or a compound type that indirectly drives the pistons (wobble plate). In either case, a plate tilt mechanism consists of several sliding and pivoting members located behind the pistons and within the main hollow body of the compressor housing, the so called “crankcase” volume. All rubbing interfaces within the compressor and the crankcase, including the tilt mechanism, require sufficient lubrication for proper operation, and this depends on lubricant being carried to different parts of the compressor by the refrigerant in which it is entrained. To the extent that lubricant is well retained within the crankcase, these sliding interfaces are well lubricated.  
           [0004]    The compressor pumping capacity can be controlled by allowing the plate to shift to a different angle, rather than externally physically moving it along the shaft. This is done by controlling the net pressure differential between the front or head of the pistons and the rear of the pistons. The back of the pistons face the inner volume or crankcase, while the heads of the pistons face the pressure in a suction chamber, and the two pressures between which the differential exists can be referred to as crankcase and suction pressure. When there is substantially a zero crankcase-suction pressure differential, there is no net resistance preventing the piston from moving back as far as it can, so that the plate is allowed to shift to its largest angle relative to the shaft centerline, creating the longest piston stroke. At the highest pressure differential, there is the highest net resistance to the piston backstroke, so the plate shifts to the smallest angle relative to the shaft centerline, creating the shortest stroke of the piston.  
           [0005]    A capacity control valve in the compressor body controls the net pressure balance on the piston by controlling refrigerant gas flow into or out of the crankcase. The valve can be responsive to both suction pressure and discharge pressure to control selective communication of compressor discharge and suction chambers with the crankcase, thereby controlling the net pressure balance on the pistons (and thereby controlling the effective piston stroke and capacity). The controlled refrigerant flow requires the provision of a flow passage for gas flow from the crankcase to the control valve and ultimately to the suction chamber, and, in swashplate compressors, such crankcase to suction passages typically been bored through the back of the cylinder block, the structural member in which the piston cylinders are formed. This is further described below in the description of FIG. 1. As such, the inlet opening of the crankcase to suction cavity passage flow passage has been directly and clearly exposed to the crankcase, and thereby directly exposed to the greatest swirl and velocity of refrigerant gas. Oil in the compressor which would otherwise be retained can be easily blown out. In a wobble plate compressor, the equivalent crankcase to valve to suction chamber flow passage is bored through the central shaft and part of the plate tilt mechanism, with the inlet opening to the passage located even more deeply into the crankcase volume and even more exposed.  
         SUMMARY OF THE INVENTION  
         [0006]    The subject invention provides a variable capacity compressor in which the initial portion of the crankcase-to control valve-to suction chamber passage is bored through the central drive shaft, rather than through the cylinder block, but in which the inlet opening is not exposed directly to the main portion of the crankcase. Instead, the inlet opening is sheltered within a central cylinder block bore, inset from the plane of the back of the cylinder block, and therefore isolated from the more turbulent main portion of the crankcase. Within the sheltered and isolated volume surrounding the inlet passage, the refrigerant is less turbulent, carries less entrained lubricant, and, therefore, less lubricant is forced out of the crankcase with the flow of refrigerant through the passage. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a cross section of a prior art compressor;  
         [0008]    [0008]FIG. 2 is a cross section of a compressor incorporating the differently configure crankcase to suction flow passage of the invention;  
         [0009]    [0009]FIG. 3 is a graph showing a comparison of the performance, in terms of crankcase lubricant retention, of the FIG. 1 and FIG. 2 type of compressor. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]    Referring first to FIG. 1, a variable capacity compressor of the swashplate type has a generally cylindrical housing  10  and a central drive shaft  12  and a rear head, indicated generally at  14 , within which various chambers and bores are cast and machined. Contained within housing  10 , near the rear end cap  14  is a cylinder block, indicated generally at  16 , the front face of which abuts substantially flat against a valve plate  29 , but for a notch  17  that serves a purpose described below. Block  16  is bored to accommodate several pistons, one of which is indicated at  18 . The pistons  18  are arrayed about drive shaft  12 , which drives a tiltable swash plate mechanism  22  to, in turn, reciprocate the pistons  18  back and forth, over a stroke length determined by the angle of the mechanism  22  relative to the shaft  12 . The mechanism  22  is designed to assure that the forwardmost point of the piston stroke is approximately always the same, but the rearmost point will vary, as described further below. At the center of cylinder e end of shaft  12  concentrically within bore  24 , and for one end of a return spring  28  for the tilt mechanism  22 . Bore  24  is inset from a plane P generally defined by the back of cylinder block and, conventionally, has no purpose other than that just described. The end of shaft  12  is spaced away from valve plate  29 , which is sandwiched between rear head  14  and cylinder block  16 .  
         [0011]    Still referring to FIG. 1, rear head  14  contains a peripheral intake or suction chamber  30 , out of which each piston  18  draws refrigerant from a non illustrated evaporator. Rear head  14  also contains a central discharge chamber  32 , into which each piston  18  pushes compressed refrigerant vapor, which then flows to a non illustrated condenser. Outboard of the plane P and the is an internal volume that is referred to as the crankcase  34 , within which the mechanism  22  is enclosed. All rubbing interfaces located within the crankcase  34  require adequate lubrication, lubricant which is carried by the inflow of refrigerant vapor, but which can be carried out by the outflow of vapor, as well. The inflow of vapor from discharge chamber  32  to crankcase  34 , and the outflow of refrigerant vapor (and lubricant) from crankcase  34  to suction chamber  30 , is controlled by a capacity control valve, indicated generally at  36 , located in rear head  14 . This controlled gas inflow and outflow balance thereby controls the pressure within crankcase  34 , relative to the pressure in suction chamber  30 , so as to control the net force balance on the reciprocating pistons  18 , and ultimately to control their stroke length. Specifically, when increased capacity and stroke length is required, more vapor is routed out of crankcase  34  into the suction chamber  30 , and less or no vapor routed in from discharge chamber  32 , so that a reduced net piston force balance is established. Conversely, when reduced capacity and stroke length are needed, less or no vapor is routed out of crankcase  34  into suction chamber  30 , and more vapor is pumped in from discharge chamber  32 , so that a higher net piston force balance is created. Further description of just how valve  36  works may be found in co assigned U.S. Pat. No. 4,428,718, hereby incorporated by reference. This control scheme obviously requires a physical flow passage between the crankcase  34  and the two chambers  30  and  32 , described in more detail next.  
         [0012]    Still referring to FIG. 1, in the embodiment disclosed, the flow path out of crankcase  34  consists of a initial passage, indicated at  38 , bored through cylinder block  16 , opening across notch  17  to a hole  39  through valve plate  29  and then into a passage  40  formed in rear head  14 . Notch  17  is not a necessary part of the vapor flow path per se, and is, in fact, intended to make the flow path more tortuous, to try to reduce lubricant loss from crankcase  34 . Rear head passage  40  then opens below a suction control valve portion  42  of valve  36 , and ultimately into suction chamber  30 . The flow path between discharge chamber  32  and crankcase  34  likewise consists of passage  44  bored through cylinder block  16 , opening into a shorter passage  46  in rear head  14  that opens below a discharge control valve portion  48  of control valve  36 , and ultimately into discharge chamber  32 . As shown by the arrows, gas flow is always out of crankcase  34  and into suction chamber  30 , when there is flow, and that flow rate is regulated by the portion  42  of control valve  36 . Likewise, gas flow is always out of discharge chamber  32  and into crankcase  34 , when there is flow, and that flow rate is regulated by the portion  48  of control valve  36 . Since they are bored through the cylinder block  16 , the inlet opening from the two flow paths to the crankcase  34  is directly exposed to the crankcase  34 . In the case of the “from discharge” flow path, this is not a problem, since gas and lubricant flow would be always into crankcase  34 , when there was flow. In the case of the “to suction” flow, however, the direct presentation of the inlet of the passage  38  to the crankcase  34 , and to the most turbulent flow within crankcase  34 , does allow a direct and efficient flow path of lubricant out of crankcase  34 . As noted above, an alternate crankcase to suction flow path found in the prior art is one formed through the drive shaft  12  and through the central part of the tilt mechanism of a wobble plate, which thus has an inlet that is located even deeper within the crankcase  34 , as may be seen in U.S. Pat. No. 4,428,718 noted above. Consequently, a high charge of lubricant in the system is necessary to assure that enough lubricant will be retained within crankcase  34  at all times to assure adequate lubrication of the various rubbing interfaces located within it.  
         [0013]    Referring next to FIG. 2, a preferred embodiment of the invention includes the same basic components and parts, which are labeled with the same number primed. The discharge to crankcase flow path is the same, with the same passage  44 ′ opening through cylinder block  16 , and the capacity control valve  36 ′ works the same way. Now, however, the initial part of the flow path out of crankcase  34 ′ is formed in a new manner. An initial flow passage  50  is bored through the end of drive shaft  12 ′, with an outlet through the end face of the end of shaft  12 ′ and into the central bore  24 ′, and with an inlet opening  52  that is bored at a right angle thereto, axially spaced from the end face of shaft  12 ′. The inlet opening  52  is sheltered within the central bore  24 ′, inset from the plane P and isolated from the turbulence within the main volume of the crankcase  34 ′. Gas flow from the main volume of crankcase  24 ′ can flow into one end of the central bore  24 ′, into the inlet  52 , out the passage  50 , through the other end of the bore  24 ′ and, conveniently, through the pre existing notch  17 ′ and ultimately through valve plate hole  39 ′ and into the same passage  40 ′ in rear head  14 ′. So, notch  17 ′ now acts to assist, rather than retard, vapor flow. Rear head passage  40 ′, as before, opens into suction chamber  30 ′ in a controlled fashion across the control valve portion  42 ′. Gas flow out of crankcase  34 ′ to the suction chamber  30 ′ is just as efficient as in the prior design, if not more so, but lubricant is not blown out as readily. This is due in part to a centrifugal slinging action out of the inlet opening  52  in the spinning shaft  12 ′, but mostly to the sheltered, isolated location of the inlet opening  52 , protected from the turbulence and high velocity gradients within the crankcase  34 ′. An additional advantage is thereby garnered from the central bore  24 ′ at essentially no extra cost.  
         [0014]    Referring next to FIG. 3, comparative test results are shown for a six cylinder variable capacity compressor of the type shown in FIGS. 1 and 2 above. Each was used in a system with a fixed orifice refrigerant expansion valve (“O/T”), and the total system charge was 6 ounces of a lubricant called RL-897, well know to those skilled in the art. Several different tests were run, as shown by the legend, including high speed tests with high and low loading, a test of bearing durability, and a long term durability test of a type required by German testing standards (“VDA”), also known to those skilled in the art. Lubricant retention in the crankcase was measured and was significantly higher for the compressor shown in FIG. 2. Clearly, this was due to the new location of the crankcase to suction flow passage shown, given the fact that that was the only structural change.  
         [0015]    Variations in the particular form of the passage  50  shown. For example, the shaft passage could be drilled as a single passage at an angle, so that the inlet opening was not part of a separate leg of an L shaped passage as shown at  52 . Or, the second leg  52  could itself be at a slight angle, or consist of two or more separate bores, or both. In any event, the inlet opening or openings to the through-shaft flow passage would be sheltered within the central bore  24 ′ in the cylinder block  16 ′, giving the same improved crankcase oil retention. The flow path out of the end of the central bore  24 ′ could be otherwise provided, as by a larger valve plate hole  39 ′, or a notch formed into valve plate  29 , instead of the pre existing notch  17 ′.