Patent Publication Number: US-2019178122-A1

Title: Oil Separators

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese patent application serial number 2017-236918 filed Dec. 11, 2017, which is hereby incorporated herein by reference in its entirety for all purposes. 
     BACKGROUND 
     The present disclosure relates to an oil separator, and more specifically, it relates to an oil separator to trap oil mist contained in blow-by gas. 
     A conventional positive crankcase ventilation (PCV) system, which is employed in an internal combustion engine like an automobile engine, is known in the art. Air pollution may result when blow-by gas (un-combusted gas) leaks and/or is discharged from a gap between a piston ring and a cylinder wall of the engine to the exterior atmosphere during operation of the engine. To prevent such leakage of the blow-by gas, the PCV system collects the blow-by gas and then returns the collected blow-by gas to an air intake system. The returned blow-by gas then undergoes re-combustion within the engine. The blow-by gas contains oil mist, which is lubricant oil such as engine oil dispersed as micro particles. The PCV system includes an oil separator designed to trap the oil mist contained in the blow-by gas and prevent the oil mist from flowing into the air intake system. The oil separator is provided in the middle of a flow passage that connects a crankcase and an air-intake duct. 
     A conventional labyrinth-type oil separator  101 , as illustrated in  FIG. 6 , is disclosed, for example, in Japanese Laid-Open Patent Publication No. 2009-68471. As illustrated in  FIG. 7 , the oil separator  101  includes a lower base  110 , a middle base  130 , and an upper base  140 , where these bases  110 ,  130 ,  140  are attached to each other by vibration welding. The oil separator  101  includes a liquid oil trapping chamber  150 , a primary oil-mist trapping chamber  151 , and a secondary oil-mist trapping chamber  152 , where each of the chambers  150 ,  151 ,  152  is defined between two of the bases  110 ,  130 ,  140 . The liquid oil trapping chamber  150  contains a compartmented interior space therein, which is capable of trapping liquid oil with a relatively large particle size (not shown). The primary oil-mist trapping chamber  151  is located downstream of the liquid oil trapping chamber  150  and contains a compartmented interior space therein, which is capable of trapping oil mist with a relatively small particle size (not shown). 
     As illustrated in  FIG. 7 , the secondary oil-mist trapping chamber  152  is arranged adjacent to the primary oil-mist trapping chamber  151  via a separation plate  131  of the middle base  130 . The secondary oil-mist trapping chamber  152  is located downstream of the primary oil-mist trapping chamber  151  and contains compartmentalized interior space, which is capable of trapping oil-mist with a relatively small particle size. The separation plate  131  has a first communication port  132 , a second communication port  133 , and a return port  134 . The first communication port  132  allows the liquid oil trapping chamber  150  and the primary oil-mist trapping chamber  151  to communicate with each other. The second communication port  133  allows the primary oil-mist trapping chamber  151  and the secondary oil-mist trapping chamber  152  to communicate with each other. The return port  134  allows communication between the primary oil-mist trapping chamber  151  and the secondary oil-mist trapping chamber  152 , so as to allow the oil mist trapped by the secondary oil-mist trapping chamber  152  to return to the primary oil-mist trapping chamber  151 . 
     Containing more than one oil-mist trapping chambers (the primary oil-mist trapping chamber  151  and the secondary oil-mist trapping chamber  152  in this example), the oil separator provides a passage having a long length for the blow-by gas, which has flown into the oil separator  101 . Such a configuration of the oil separator  101 , with a long length of passage, can enhance trapping efficiency of the oil mist with a small diameter. The return port  134  allows the oil-mist trapped in the secondary oil-mist trapping chamber  152  to merge with the oil mist trapped in the primary oil-mist trapping chamber  151 . The merged, trapped oil may be collected together with the liquid oil trapped in the liquid oil trapping chamber  150 . This configuration allows the oil-mist trapped in the secondary oil-mist trapping chamber  152  to be collected without a dedicated collecting passage. As a result, the size of the oil separator  101  may be reduced while the oil separator  101  may efficiently trap and collect the oil mist with a small particle size. 
     As illustrated in  FIG. 8 , the return port  134  is located along the bottom surface  151   a  of the primary oil-mist trapping chamber  151 . Additionally, the return port  134  is located along the lowermost portion  152   b  of the bottom surface  152   a  of the secondary oil-mist trapping chamber  152 . As a result, the return port  134  may facilitate return of the oil-mist trapped in the secondary oil-mist trapping chamber  152  to the primary oil-mist trapping chamber  151 . 
     The blow-by gas, however, may contain a massive amount of the liquid oil with a large particle size. In this case, the liquid oil with a large particle size may flow from the liquid oil trapping passage  150  into the primary oil-mist trapping chamber  151  through the first communication port  132 . The liquid oil with a large particle size may subsequently flow through the return port  134  (i.e., shortcut) from the primary oil-mist trapping chamber  151  into the secondary oil-mist trapping chamber  152  without passing through the second communication port  133 , and may finally flow out of the oil separator  101  through an outlet port  113  at the lower base  110 . As a result, the liquid oil with a large particle size may undesirably flow into an internal combustion engine located downstream of the oil separator  101  and may be combusted, which may cause a failure of the internal combustion engine. 
     SUMMARY 
     According to one aspect of the present disclosure, an oil separator for trapping oil mist contained in blow-by gas includes a separation plate, an upstream oil-mist trapping chamber, and a downstream oil-mist trapping chamber. The upstream and downstream oil-mist trapping chambers are divided by the separation plate and are positioned adjacent to each other. The upstream oil-mist trapping chamber communicates with a flow inlet for blow-by gas. The downstream oil-mist trapping chamber communicates with the upstream oil-mist trapping chamber and a flow outlet for blow-by gas. A return port is configured to return oil mist trapped in the downstream oil-mist trapping chamber back into the upstream oil-mist trapping chamber. The return port is formed through the separation plate. The return port is positioned above the bottom surface of the upstream oil-mist trapping chamber and is positioned at substantially the same vertical level as the lowermost portion of the bottom surface of the downstream oil-mist trapping chamber. 
     The oil contained in blow-by gas is separated from the blow-by gas and drops in both of the upstream and downstream oil-mist trapping chambers. The oil dropped in the downstream oil-mist trapping chamber is allowed to pass through the return port to the upstream oil-mist trapping chamber. On the other hand, the oil dropped in the upstream oil-mist trapping chamber is prevented from passing through the return port and flowing into the downstream oil-mist trapping chamber. Consequently, the liquid oil gathers in the upstream oil-mist trapping chamber and does not accumulate in the downstream oil-mist trapping chamber. Consequently, the liquid oil is prevented from flowing out of the downstream oil-mist trapping chamber through the flow outlet and subsequently being discharged to the outside. As a result, the liquid oil is prevented from flowing to a device(s) such as an internal combustion engine positioned downstream of the oil separator and from being combusted in the internal combustion engine. In this way, this configuration may prevent a device(s) located downstream of the oil separator from being broken down. 
     According to another aspect of the present disclosure, the separation plate includes a wall surface facing opposite to the upstream oil-mist trapping chamber. The wall surface may be formed with a rib in the vicinity of the return port. The rib is configured to prevent blow-by gas from flowing into the return port through the return port. Consequently, blow-by gas is prevented from flowing from the upstream oil-mist trapping chamber through the return port into the downstream oil-mist trapping chamber. Thus, the blow-by gas tends not to flow through the return port but through a communication port that communicates the upstream oil-mist trapping chamber with the downstream oil-mist trapping chamber, from the upstream oil-mist trapping chamber to the downstream oil-mist trapping chamber. 
     According to another aspect of the present disclosure, the rib may have a substantially U-shaped form. For example, the rib may extend along the return port. Therefore, the blow-by gas may be prevented from flowing from the upstream oil-mist trapping chamber through the return port into the downstream oil-mist trapping chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an oil separator according to one exemplary embodiment. 
         FIG. 2  is an exploded view of the oil separator of  FIG. 1 . 
         FIG. 3  is a front view of a middle base of the oil separator of  FIG. 2 . 
         FIG. 4  is a cross-sectional view of the oil separator of  FIG. 1  taken along a line IV-IV of  FIG. 3 . 
         FIG. 5  is a schematic view of an interior of the oil separator of  FIG. 1 . 
         FIG. 6  is a perspective view of a conventional oil separator. 
         FIG. 7  is an exploded view of the conventional oil separator of  FIG. 6 . 
         FIG. 8  is a schematic view of an interior of the conventional oil separator of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     As previously described, in some conventional oil separators, the liquid oil with a large particle size may undesirably flow into an internal combustion engine located downstream of the oil separator and may be combusted. Thus, there has been a need of an oil separator capable of: preventing liquid oil contained in blow-by gas from flowing into a device located downstream of the oil separator, and thereby suppressing a failure of the device even when the blow-by gas contains a massive amount of the liquid oil with a large particle size. 
     Hereinafter, an exemplary embodiment will be described with reference to  FIGS. 1 to 5 . As illustrated in  FIGS. 1 and 2 , an oil separator  1  is a labyrinth-type separator, and comprises a lower base  10 , a middle base  30 , and an upper base  40 . 
     As illustrated in  FIG. 2 , the lower base  10  is a casing member having a recess  10   a  at a bottom and an opening  10   b  facing the middle base  30 . A first flow inlet  11  and a second flow inlet  12  are formed on a lower portion of the recess  10   a  to allow blow-by gas (not illustrated) to flow into the oil separator  1 . A flow outlet  13  is formed on an upper portion of the lower base  10  to allow the blow-by gas to flow out of the oil separator  1 . A first collision wall  14 , a second collision wall  15 , a third collision wall  16 , a fourth collision wall  17 , a fifth collision wall  18 , and a sixth collision wall  19  are formed within the recess  10   a.    
     As illustrated in  FIGS. 3 and 4 , the middle base  30  is a panel member sized and shaped to cover the opening  10   b  of the lower base  10 . A lower region of the middle base  30  defines a liquid oil trapping chamber  50  in cooperation with the lower base  10 . The upper region of the middle base  30  defines a secondary oil-mist trapping chamber  52  in cooperation with the lower base  10 . The middle base  30  defines the secondary oil-mist trapping chamber  52  and has a separation plate  31 , which separates the secondary oil-mist trapping chamber  52  from a primary oil-mist trapping chamber  51 . The primary oil-mist trapping chamber  51  and the secondary oil-mist trapping chamber  52  are arranged adjacent to each other, to the left and right of the separation plate  31 , respectively. A first communication port  32 , which allows the liquid oil trapping chamber  50  to communicate with the primary oil-mist trapping chamber  51 , is formed on the right-most region of the separation plate  31 . A second communication port  33 , which allows the primary oil-mist trapping chamber  51  to communicate with the secondary oil-mist trapping chamber  52 , is formed on the uppermost region of the separation plate  31 . A return port  34  is formed at the separation plate  31 . The return port  34  allows small particle size oil mist (not illustrated) trapped in the secondary oil-mist trapping chamber  52  to return to the primary oil-mist trapping chamber  51 . 
     As illustrated in  FIGS. 2 and 5 , the primary oil-mist trapping chamber  51  has a bottom surface  51   a . The location of the return port  34  within the chamber  51  is located at a higher level than (e.g., vertically above) the bottom surface  51   a . Thus, the return port  34  is spaced vertically above and apart from the bottom surface  51   a  of the primary oil-mist trapping chamber  51 . The secondary oil-mist trapping chamber  52  has a bottom surface  52   a , and the return port  34  is positioned at substantially the same level (e.g., same vertical height) as the lowermost portion  52   b  of the bottom surface  52   a . Unlike the conventional oil separator, the return port  34  is not located along the bottom surface  51   a  of the primary oil-mist trapping chamber  51 . That is, the return port  34  is located only along the bottom surface  52   a  of the secondary oil-mist trapping chamber  52 . The separation plate  31  has a wall surface  31   a  facing the primary oil-mist trapping chamber  51 . A rib  35  is formed near the return port  34  of the wall surface  31   a . In this embodiment, the rib  35  has a substantially U-shape such that it prevents the blow-by gas from flowing into the return port  34 . 
     As illustrated in  FIG. 2 , the upper base  40  is a cover member sized and shaped to cover the separation plate  31  of the middle base  30 . The primary oil-mist trapping chamber  51  is disposed between the upper base  40  and the separation plate  31 . 
     As illustrated in  FIG. 2 , the lower base  10 , the middle base  30 , and the upper base  40  are assembled to each other by vibration welding, etc. The liquid oil trapping chamber  50  and the secondary oil-mist trapping chamber  52  above the liquid oil trapping chamber  50  are positioned between the lower base  10  and the middle base  30 . The liquid oil trapping chamber  50  and the secondary oil-mist trapping chamber  52  are defined by partitions of the lower base  10  and the middle base  30 . The primary oil-mist trapping chamber  51  is positioned between the middle base  30  and the upper base  40 . The liquid oil trapping chamber  50  is a space that traps large particle size liquid oil (not illustrated) contained in the blow-by gas. 
     As illustrated in  FIG. 2 , the primary oil-mist trapping chamber  51  is located above the liquid oil trapping chamber  50 . The primary oil-mist trapping chamber  51  is hence located downstream of the liquid oil trapping chamber  50  in a flow passage for the blow-by gas. The primary oil-mist trapping chamber  51  is a space that traps small particle size oil mist contained in the blow-by gas. The secondary oil-mist trapping chamber  52  is located at substantially the same vertical level (e.g., same vertical height) as the primary oil-mist trapping chamber  51 , and it is adjacent to the primary oil-mist trapping chamber  51  in the horizontal left-to-right direction. The primary oil-mist trapping chamber  51  is separated from the secondary oil-mist trapping chamber  52  by the separation plate  31  of the middle base  30 . The secondary oil-mist trapping chamber  52  is located downstream of the primary oil-mist trapping chamber  51  in the flow passage of the blow-by gas. The secondary oil-mist trapping chamber  52  is a space that traps small particle size oil mist contained in the blow-by gas. 
     The lower base  10 , the middle base  30 , and the upper base  40  illustrated in  FIGS. 1 and 2  are different members made of rigid synthetic resin such as polypropylene. The oil separator  1  may be attached in the middle of a flow passage (not illustrated) that connects a crankcase (not illustrated) to an air-intake duct (not illustrated). For example, attaching portions  20  of the lower base  10  and attaching portions  36  of the middle base  30  are each attached to a corresponding component forming the flow passage via a metal collar (not illustrated). 
     As illustrated in  FIGS. 2 and 5 , the first flow inlet  11  and the second flow inlet  12 , which are formed in the lower region of the lower base  10 , each communicate with the flow passage in the crankcase. As a result, when the blow-by gas flows from the crankcase, it enters through the first flow inlet  11  and/or the second flow inlet  12  into the liquid oil trapping chamber  50 . The blow-by gas, which thereby flows into the liquid oil trapping chamber  50 , then collides with not only interior wall  10   c  of the recess  10   a  of the lower base  10 , but also collision walls  14 ,  15 ,  16 ,  17  and the interior surface of the middle base  30 . 
     As illustrated in  FIG. 2 , the first collision wall  14  extends upward from an intermediate region of the recess  10   a  between the flow inlets  11 ,  12  in the left-to-right direction. The collision wall  14  extends upward and obliquely above the first flow inlet  11 . The second collision wall  15  extends obliquely below and inward in the horizontal direction from a right side of the interior wall  10   c . The third collision wall  16  extends inward in the horizontal direction from a left side of the interior wall  10   c . The third collision wall  16  extends substantially parallel to the first collision wall  14 , and is positioned above the first collision wall  14 . The fourth collision wall  17  is located above the third collision wall  16 , and extends obliquely upward and rightward from an intermediate portion of the collision wall  16 . In this manner, the collision wall  17  forms the right portion of a bottom surface of the secondary oil-mist trapping chamber  52 . The collision wall  18  extends continuously leftward and upward from the collision wall  17  to form a left portion of the bottom surface of the secondary oil-mist trapping chamber  52 . The collision wall  18  extends obliquely upward and leftward from the collision wall  17  to connect with a left side of the interior wall  10   a . The sixth collision wall  19  extends downward from an upper wall surface of the interior wall  10   a.    
     As illustrated in  FIG. 2 , the large particle size liquid oil contained in the blow-by gas collides with the interior wall  10   c , the collision walls  14 ,  15 ,  16 ,  17 , and the interior surface of the middle base  30 , and thereby adheres to the corresponding surfaces. The large particle size liquid oil adhered to the interior wall  10   c , collision walls  14 ,  15 ,  16 ,  17 , and the interior surface of the middle base  30  drops under its own weight (via gravity), and accumulates on the bottom surface  50   a  of the liquid oil trapping chamber  50 . The accumulated liquid oil then flows through the flow inlets  11 ,  12  and is collected. As a result, the liquid oil trapping chamber  50  serves to trap the large particle size liquid oil contained in the blow-by gas. 
     Referring to  FIG. 2 , the blow-by gas flows from the liquid oil trapping chamber  50 , through the first communication port  32 , and into the primary oil-mist trapping chamber  51 . The blow-by gas collides with the wall surface  31   a  of the separation plate  31  and an interior surface of the upper base  40 , etc. The small particle size oil mist contained in the blow-by gas collides with the surfaces and adheres thereto. The small particle size oil mist adhered to the wall surface  31   a , etc. drops due under its own weight (via gravity), and then accumulates on the bottom surface  51   a  of the primary oil-mist trapping chamber  51 . The small particle size oil mist subsequently returns to the liquid oil trapping chamber  50  through the first communication port  32 , and then is collected together with the large particle size liquid oil. Thus, the primary oil-mist trapping chamber  51  serves to trap the small particle size oil mist contained in the blow-by gas. 
     Referring to  FIG. 2 , the blow-by gas flows from the primary oil-mist trapping chamber  51  through the second communication port  33  into the secondary oil-mist trapping chamber  52 . Here, the blow-by gas collides with each of the wall surfaces in the secondary oil-mist trapping chamber  52 . For example, the blow-by gas collides with the interior wall  10   c  of the recess  10   a  of the lower base  10  within the chamber  52 , as well as each of the collision walls  17 ,  18 ,  19 , and a back surface  31   b  of the separation plate  31 , etc. The small particle size oil mist contained in the blow-by gas collides with the surfaces and adheres thereto. The small particle size oil mist adhered to the surfaces drops under its own weight (via gravity), and then accumulates on the bottom surface  52   a  of the secondary oil-mist trapping chamber  52 . The small particle size oil mist returns to the primary oil-mist trapping chamber  51  through the return port  34 , and is collected together with the small particle size oil mist trapped in the primary oil-mist trapping chamber  51 . Thus, the secondary oil-mist trapping chamber  52  serves to trap the small particle size oil mist contained in the blow-by gas. 
     As illustrated in  FIG. 2 , a flow outlet  13  is formed on the upper right region of the lower base  10 . The flow outlet  13  communicates with the communication flow passage of the air-intake duct. Therefore, the blow-by gas flown into the oil separator  1  flows from the secondary oil-mist trapping chamber  52  through the flow outlet  13  in the air-intake duct. As described above, as blow-by gas flows into the oil separator  1 , large particle size liquid oil and small particle size oil mist contained in the blow-by gas are separated from the blow-by gas and discharged out of the flow inlets  11 ,  12 . In this way, small particle size liquid oil and oil mist are prevented from flowing from the flow outlet  13  of the oil separator  1  into the air-intake duct. As a result, the blow-by gas without the liquid oil and oil mist that were initially input into the separator  1 , can be returned as an output from the separator  1  to the internal combustion engine located downstream of the oil separator  1 , to be combusted again in the internal combustion engine. 
     As described above, the oil separator  1  includes a plurality of oil-mist trapping chambers, for example, a primary oil-mist trapping chamber  51  and a secondary oil-mist trapping chamber  52 . Therefore, the total length traversed by the blow-by gas passage within the oil separator  1  is relatively long. Such a long length flow passage through the oil separator offers the potential to enhance the efficiency at which the small particle size oil mist contained in the blow-by gas is trapped. 
     As illustrated in  FIG. 2 , the return port  34  allows the oil mist trapped in the secondary oil-mist trapping chamber  52  to be returned into the primary oil-mist trapping chamber  51 . The first communication port  32  allows the oil mist trapped in the secondary oil-mist trapping chamber  52  and the oil mist trapped in the primary oil-mist trapping chamber  51  to be returned into the liquid oil trapping chamber  50 . The flow inlets  11 ,  12  allow the above-described oil mist and the liquid oil trapped in the liquid oil trapping chamber  50  to be discharged. In this way, both oil mist and liquid oil can be collected. Therefore, it is possible to collect trapped oil mist with the secondary oil-mist trapping chamber  52  without providing a dedicated collecting passage, thereby reducing the amount of structural components needed. In addition, the small particle size oil mist can be efficiently trapped and the size of the oil separator  1  can be reduced. 
     As described above, the oil separator  1  includes a separation plate  31 , an upstream oil-mist trapping chamber (primary oil-mist trapping chamber  51 ), and a downstream oil-mist trapping chamber (secondary oil-mist trapping chamber  52 ) as illustrated in  FIG. 2 . The trapping chambers are divided by the separation plate  31  and arranged adjacent to each other in the left-to-right direction. The upstream oil-mist trapping chamber  51  fluidly communicates with the flow inlets  11 ,  12  for blow-by gas via the liquid oil trapping chamber  50 . The downstream oil-mist trapping chamber  52  fluidly communicates with the upstream oil-mist trapping chamber  51 , as well as with the flow outlet  13  for blow-by gas. A return port  34  is formed on the separation plate  31 , serving to return the oil mist trapped in the downstream oil-mist trapping chamber  52  to the upstream oil-mist trapping chamber  51 . The return port  54  is located at a position vertically above the bottom surface  51   a  of the upstream oil-mist trapping chamber  51  and is positioned at substantially the same vertical height as the lowermost portion of the bottom surface  52   a  of the downstream oil-mist trapping chamber  52 . 
     The oil contained in blow-by gas is separated from the blow-by gas by the separator  1 , and gathers as droplets in both the upstream oil-mist trapping chamber  51  and the downstream oil-mist trapping chamber  52 . The oil that drops and accumulates in the downstream oil-mist trapping chamber  52  passes through the return port  34  and is allowed to return to the upstream oil-mist trapping chamber  51 . On the other hand, the oil that drops and accumulates in the upstream oil-mist trapping chamber  51  is not allowed to flow through the return port  34  into the downstream oil-mist trapping chamber  52 . Consequently, liquid oil is not accumulated in the downstream oil-mist trapping chamber  52 , and as a result, the separated liquid oil is prevented from flowing out of the downstream oil-mist trapping chamber  52  through the flow outlet  13  so as to be discharged outside of the oil separator  1 . As a result, the liquid oil can be prevented from flowing to a device(s) such as an internal combustion engine positioned downstream of the oil separator  1  and from being combusted again in the internal combustion engine. In this way, the failure of a device(s) located downstream of the oil separator  1  can be prevented. 
     As illustrated in  FIG. 2 , the separation plate  31  includes the wall surface  31   a  facing opposite to the upstream oil-mist trapping chamber  51 . The rib  35  is formed in the vicinity of the return port  34  at the wall surface  31   a  so that the rib  35  prevents blow-by gas from flowing into the return port  34 . Therefore, blow-by gas is prevented from flowing from the upstream oil-mist trapping chamber  51  into the downstream oil-mist trapping chamber  52  through the return port  34 . Thus, the blow-by gas tends not to flow through the return port  34  but through the communication port  33 , which helps to flow the gas from the upstream oil-mist trapping chamber  51  into the downstream oil-mist trapping chamber  52 . 
     As illustrated in  FIGS. 2 and 3 , the rib  35  has a substantial U-shaped configuration. For example, the rib  35  extends circumferentially as an arc around the return port  4 . Therefore, the blow-by gas is prevented from flowing from the upstream oil-mist trapping chamber  51  through the return port  34  into the downstream oil-mist trapping chamber  52 . 
     As illustrated in  FIGS. 2 and 3 , the rib  35  has a right portion standing upright between the return port  34  and the first communication port  32 . Therefore, the right portion of the rib  35  prevents the blow-by gas from directly flowing from the first communication port  32  into the return port  34 . The rib  35  has an upper portion extending from the right portion along the upper edge of the return port  34  and a lower portion extending from the right portion along the lower edge of the return port  34 . As a result, the blow-by gas is more reliably prevented from flowing directly into the return port  34  from the first communication port  32 , and rather flows around the return port  34 . 
     The rib  35  has a substantially U-shape as illustrated in  FIG. 2 . Alternatively, the rib  35  may have various other shapes such as a substantially V-shape or a horseshoe shape, to deflect the gas from flowing into the return port  34 . The rib has preferably a shape that obtains the same effect as that of the substantially U-shaped rib  35 . 
     As described-above, the lower base  10 , the middle base  30 , and the upper base  40  are made of resin. Alternatively, the lower base  10 , the middle base  30 , and the upper base  40  may be made of separate metal members that are integrally connected. Such a metal oil separator generally has a higher strength and heat resistance as compared to the resin oil separator  1 . As described-above, the lower base  10 , the middle base  30 , and the upper base  40  are made as separate members. Alternatively, two or all of these bases may be formed in one member. 
     As described-above, the oil separator  1  has two oil-mist trapping chambers  51 ,  52 . Alternatively, the oil separator  1  may have more than two oil-mist trapping chambers. 
     The various examples described above in detail with reference to the attached drawings are intended to be representative of the present disclosure and are thus non limiting embodiments. The detailed description is intended to teach a person of skill in the art to make, use and/or practice various aspects of the present teachings and thus does not limit the scope of the disclosure in any manner. Furthermore, each of the additional features and teachings disclosed above may be applied and/or used separately or with other features and teachings in any combination thereof, to provide improved oil separators, and/or methods of making and using the same.