Abstract:
A variable capacity automotive refrigerant compressor is provided with a pressure equalization passage between the crankcase volume and the suction passage in the manifold to prevent a pressure imbalance between the two that could otherwise cause a reduction in crankcase lubricant retention during extended periods of system inactivity.

Description:
TECHNICAL FIELD 
   This invention relates to a refrigerant compressor with improved oil retention. 
   BACKGROUND OF THE INVENTION 
   Automotive air conditioning systems typically include a front mounted refrigerant condenser, an underhood refrigerant compressor, and an evaporator contained in an HVAC housing that is essentially inside the passenger compartment. The main inner volume of the compressor, the so called crankcase, is substantially hollow, but numerous moving components are either contained in it, or exposed to it, such as the central drive shaft (and support bearings), swash plate, and reciprocating pistons. During operation, refrigerant vapor running through the system (and the compressor) carries entrained oil, which reaches and lubricates the various moving part interfaces. When the compressor sits for extended periods of non operation, it is desirable that a substantial pool of lubricant remain at the bottom of the crankcase, to be available to lubricate the interfaces during start up. 
   Observation made prior to the subject invention found that, surprisingly, lubricant appeared to be actively leaving the compressor crankcase during periods of vehicle and compressor inactivity, and moving to the condenser, where it would not be immediately available at compressor start up. This appeared to be an incremental, rather than a precipitous process, but was still a concern, and the cause was not readily apparent. Detailed analysis found an explanation for this phenomenon, which was a pressure imbalance between the main crankcase volume of the compressor, and the suction cavity in the adjacent manifold. This imbalance was creating a condition by which liquid refrigerant and lubricant, which is miscible in the refrigerant, was subject to a combination of internal siphoning and pushing forces that pushed and pulled the liquid out of the compressor. Although the problem was better understood, a solution was not immediately apparent. 
   SUMMARY OF THE INVENTION 
   The subject invention provides a solution to the problem analyzed above comprising a means to equalize and reduce the pressure differential that initiates the liquid refrigerant-lubricant migration. A small pressure equalization passage is provided at a high point within the compressor, between the crankcase and suction chamber in the manifold. This reduces the tendency of the liquid refrigerant-oil mixture to be pulled and or pushed out of the crankcase and into the manifold, which could otherwise ultimately result in the migration described above. The size of the equalization hole is small enough to not significantly affect ordinary compressor operation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross sectional view of a compressor of the type described above, showing the various volumes or spaces within, and showing the general location of an evaporator and a condenser schematically, 
       FIG. 2  shows an enlarged view of a circled section of  FIG. 1 ; 
       FIG. 3  shows a sectional view taken along the plane indicated by line  3 - 3  in  FIG. 2 ; 
       FIG. 4  shows the compressor during an extended period of system activity, at a point in the mid afternoon, 
       FIG. 5  shows the same compressor at a point late in the afternoon. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring first to  FIG. 1 , a standard variable capacity compressor has almost identical features to compressor  10 , which is modified, according to the invention, as indicated below. Typical features of compressor  10  include a relatively massive main compressor body, basically a horizontal cylinder, with an internal crankcase  12  adjacent to a refrigerant manifold  14 , the two separated by a valve plate  16 . Most of the moving parts of compressor  10  are contained within crankcase  12 , including reciprocating pistons  18  (moved by a non illustrated swash plate), drive shaft  20 , and drive shaft support bearings  22 . Valve plate  16  supports one way suction reed valves  24  that pass refrigerant vapor from a manifold suction chamber  26  to the pistons  18 , and opposed one way discharge reed valves  27  that pass compressed refrigerant vapor from pistons  18  into a manifold discharge chamber  28 . Both the suction chamber  26  and discharge chamber  28  are convoluted spaces, but the suction chamber is basically radially outboard relative to the manifold  14 , and the discharge chamber  28  central, with each kept sealed from the other by a sealing gasket  30 , clamped tightly between the outside of valve plate  16  and the inside of manifold  14 . Several conventional through bolts  31  that clamp head  14  in place necessarily pass through valve plate  16  and gasket  30 , a factor that is significant later in the description. Near the bottom or “6 o&#39;clock” position of manifold  14 , a control valve cavity  32  contains a non illustrated control valve which provides for selective communication of vapor pressure between suction chamber  26  and the crankcase  12 , so as to adjust a relative pressure balance between the vapor pressures acting on the front and rear of the pistons  18 , thereby controlling their stroke. What is significant here is not the operation of the control valve per se, but rather the fact that it is located at a low point relative to the crankcase  12 , and also that manifold  14  includes both a crankcase to valve cavity passage  34  and a valve suction chamber to valve cavity bleed orifice  36  that allow the refrigerant vapor flows necessary to the operation of the compressor  10 . The centrally located discharge chamber  28  has a discharge port  38 , which, significantly, is located above the valve cavity  32 . Discharge port  38  is connected by conventional refrigerant lines to a condenser, indicated schematically at  40 , which is mounted behind the vehicle grill, generally lower than the outlet port  38 . Condenser  40  is exposed to lower directed, morning sun rays, but more shielded later in the day, and is relatively light weight, so that it both cools and warms relatively rapidly. In addition, the suction chamber  26  has an inlet port  42 , located generally above discharge port  38 , and connected by refrigerant lines to an evaporator, indicated schematically at  44 . Evaporator  44  is located typically inside an HVAC housing that is at least partially inside the vehicle cabin, is exposed to the same greenhouse effect solar warming, and also capable or relatively rapid warming. The relative location and inherent characteristics of these three main components, compressor  10 , condenser  40 , and evaporator  44 , as well as the internal structures of compressor  10 , were found to contribute to the previously unappreciated lubricant migration phenomenon noted above. Lubricant that migrates to and is temporarily retained in the condenser  40  is not available at compressor start up, and will not re enter the system and the compressor fully until the system has been running for a time. As a consequence, a larger system charge of lubricant is required than would otherwise be necessary if the crankcase retention of lubricant during prolonged periods of system inactivity could be somehow improved. An obvious solution is the addition of a check valve in the refrigerant line between the discharge port  38  and the condenser  40 . This would add cost and pressure drop to the system, however. 
   Referring next to  FIGS. 2 and 3  the invention provides a more elegant and less costly solution to the problem, a solution that is, in fact, cost free as disclosed. The sealing gasket  30  referred to above has a simple notch  46  molded into it at the high point where the upper through bolt  31  passes through it. The operation of notch  46  is described next. 
   Referring next to  FIG. 4 , the compressor is shown at a point in the mid afternoon, during a period of prolonged system inactivity, such as might occur when the vehicle sits in a parking lot for several days consecutively. With gasket  30  having been provided with the notch  46  as described above, a small vapor flow passage is thereby created at a high point within crankcase  12  into suction chamber  26 . Being at a high point, vapor will reach it, but liquid will not, unlike the passages  34  and  36  at the lower location. Consequently, a pressure differential does not develop between crankcase  12  and suction chamber  14  to drive outflow of liquid from crankcase  12  into manifold  14 . Instead, vapor is able to equalize between the two, and the liquid level remains the same in crankcase  12  and manifold  14 . While the pressure equalization passage provided by gasket notch  46  is small, the process described is relatively slow, so the small passage is more than large enough to allow pressures to equalize, but still small enough not to effect compressor operation later. It provides a significant “slow leak” of pressure, but an insignificant “fast leak.” 
   Referring next to  FIG. 5 , the same compressor  10  is illustrated at a point later in the afternoon. Again, the level of liquid in crankcase  12  and manifold  14  remain the same, due to the equalization of pressure allowed between the two. Refrigerant vapor has flowed out of the compressor  10  to condense in the relatively cooler condenser  40 . In addition, some drainage of liquid out of discharge chamber  28 , through discharge port  38  into the system line to condenser  40  is lower and runs “downhill,” in effect. But what has not occurred is the more serious, pressure differential driven migration action described above, so the retention of oil in crankcase  12  is much improved. For example, one test showed the oil retained over a 15 day period of system activity to have been improved dramatically, from essentially none to approximately 28 milliliters. As noted, this is an improvement in retention that allows the initial lubricant charge to be minimized. 
   Once the analysis of the previously unappreciated, and fairly complex, oil migration process was complete, the improvement described above was devised, as an alternative to the obvious expedient of a check valve. Unlike the check valve, it deals more with the root cause than with the result of the problem, alleviating the pressure differential, rather than blocking its action, as a simple check valve would do. Other means of providing the pressure equalization passage could be provided, such as a dedicated, drilled hole, so long as it was at a similar location, high within the compressor  10 , at a point where vapor is seen, but liquid is not. Therefore, it will be understood that it is not intended to limit the invention to just the embodiment disclosed.