Patent Publication Number: US-10774705-B2

Title: Oil separator unit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2018-157527 filed on Aug. 24, 2018, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to an oil separator unit configured to separate oil mist contained in blowby gas. 
     DESCRIPTION OF THE RELATED ART 
     There have been known so-called impactor filter-type oil separator units, which include an upstream plate with multiple through holes through which blowby gas passes, a downstream plate that is hit by the blowby gas that has passed through the through holes, and a fiber material disposed between the pair of plates such that there is a gap between the fiber material and the surface of the upstream plate. Such an oil separator unit is described in, for example, Specification of United States Patent Application Publication No. 2011/0179755 (US2011/0179755A). In a unit described in US2011/0179755A, a pair of plates and a fiber material are formed in approximately rectangular shapes elongated in the up-down direction, the pair of plates are coupled through a pair of upper and lower connecting parts, and the position in the up-down direction of the fiber material is regulated by the pair of upper and lower connecting parts. 
     However, in the case of the unit that regulates the position in the up-down direction of the fiber material using the pair of upper and lower connecting parts, such as US2011/0179755A, it is difficult to dispose the pair of plates at high positional accuracy. Therefore, it is difficult to sufficiently separate oil mist. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is an oil separator unit configured to separate an oil mist contained in a blowby gas generated in an internal combustion engine. The oil separator unit includes: an upstream member disposed on an upstream side in a flow direction of the blowby gas and having an opening through which the blowby gas passes; a downstream member disposed on a downstream side in the flow direction of the blowby gas and having a hit portion hit by the blowby gas; and a porous member including a first surface facing the upstream member and a second surface facing the downstream member and formed in a substantially rectangular shape with a predetermined thickness so as to trap the oil mist contained in the blowby gas that has passed through the through hole. The upstream member includes an upstream surface facing the first surface, and a pair of recesses formed in substantially U-shapes along circumference portions in both end sides on the upstream surface. The downstream member includes a pair of flange parts formed in substantially U-shapes so as to fit in the pair of recesses. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, features, and advantages of the present invention will become clearer from the following description of embodiments in relation to the attached drawings, in which: 
         FIG. 1  is a diagram showing schematically a configuration of an oil separator system incorporating an oil mist separator according to an embodiment of the invention; 
         FIG. 2  is an exploded perspective view of the oil mist separator according to the embodiment of the present invention; 
         FIG. 3  is a simplified sectional view showing a major part of the assembled configuration of the oil mist separator according to the embodiment of the present invention; 
         FIG. 4A  is an exploded perspective view of the oil separator unit according to the embodiment of the present invention, seen from a rear-left side; 
         FIG. 4B  is an exploded perspective view of the oil separator unit according to the embodiment of the present invention, seen from a front-left side; 
         FIG. 5  is a perspective view showing the assembled configuration of the oil separator unit according to the embodiment of the present invention; 
         FIG. 6  is a front view of an upstream plate constituting the oil separator unit according to the embodiment of the present invention; 
         FIG. 7  is a front view of a downstream plate constituting the oil separator unit according to the embodiment of the present invention; 
         FIG. 8  is a side view showing the assembled configuration of the oil separator unit according to the embodiment of the present invention; 
         FIG. 9A  is a front view of an oil separator unit according to a modification of the embodiment; and 
         FIG. 9B  is a sectional view taken along line B-B of  FIG. 9A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, an embodiment of the present invention will be described with reference to  FIGS. 1 to 9B . An oil separator unit according to an embodiment of the present invention configures a part of an oil mist separator.  FIG. 1  is a drawing showing a schematic configuration of an oil separator system including the oil mist separator according to the present embodiment and schematically shows an upper portion of an engine  1  as an internal combustion engine. In  FIG. 1 , the front-rear direction and up-down direction of a vehicle having the engine  1  mounted thereon are shown by arrows. The front-rear direction corresponds to the vehicle length direction, and the up-down direction corresponds to the vehicle height direction (gravity direction). 
     As shown in  FIG. 1 , the engine  1  is a V-type engine (e.g., V-6 engine), in which multiple cylinders are disposed in a V-shape in a side view. Specifically, the engine  1  is formed as a V-type engine in which a pair of front and rear head portions  3 F and  3 R are disposed on a cylinder block  2  so as to be inclined forward and rearward, respectively, with respect to a vertical line and thus form a pair of front and rear banks  4 F and  4 R. 
     The head portions  3 F and  3 R each include a cylinder head mounted on the upper end of the cylinder block  2 , a cylinder head cover mounted on the upper end of the cylinder head, and the like. A crankcase  5  is disposed under the cylinder block  2 , and an oil pan is disposed under the crankcase  5 . Lubricating oil is stored into the oil pan through a crank chamber  5   a  in the crankcase  5 . 
       FIG. 1  shows the configuration of the intake system of the engine  1 . This intake system includes an air cleaner  6 , an intake passage  7 , and an intake manifold  8  and is configured to supply intake air to the combustion chambers of the cylinders of the engine  1 . A compressor  9   b  forming a turbocharger  9  with a turbine  9   a  is disposed on the intake passage  7 , and a throttle valve  10  that controls the amount of intake air supplied to the engine  1  is disposed downstream of the compressor  9   b . In a state in which the engine rotational speed is low (during non-turbocharging), the pressure in the intake manifold  8  becomes negative, and intake air is supplied to the engine  1 . On the other hand, in a state in which the engine rotational speed is high (during turbocharging), intake air is compressed by the turbocharger  9  and thus the pressure in the intake manifold  8  becomes positive. 
     The oil separator system  100  according to the present embodiment includes a pair of oil mist separators  101  ( 101 F,  101 R) mounted on the upper ends of the pair of front and rear head portions  3 F and  3 R, a passage  11  connecting the front oil mist separator  101 F and the intake manifold  8 , and a passage  12  branched from the intake passage  7  between the air cleaner  6  and compressor  9   b  and connected to the rear oil mist separator  101 R. An open/close valve  13  is disposed on one end of the passage  11 , that is, the oil mist separator  101 F-side end thereof. The open/close valve  13  is configured to be opened when the pressure in the intake manifold  8  is negative and to be closed when the pressure is positive. The open/close valve  13  is also configured to, when opened, permit the gas flow from the oil mist separator  101 F to the passage  11  and prohibit the flow in the opposite direction. 
     The crank chamber  5   a  communicates with the internal spaces SP ( FIG. 3 ) of the oil mist separators  101 F and  101 R through communication holes (not shown) provided in the cylinder block  2  and cylinder heads. Blowby gas containing unburned components is leaked from the combustion chambers of the cylinders of the engine  1  into the crank chamber  5   a  during operation of the engine. Oil mist floating in the crank chamber  5   a  is mixed into the blowby gas. In  FIG. 1 , for convenience, the blowby gas containing unburned components is represented by arrows A 1  and B 1 , and the blowby gas containing the oil mist is represented by arrows A 2  and B 2 . 
     When the pressure in the intake manifold  8  is negative, the open/close valve  13  is opened, and the blowby gas is sucked into the intake manifold  8  through the front oil mist separator  101 F, open/close valve  13 , and passage  11 , as shown by arrows A 1 , A 2 , and A 3  (solid lines) in  FIG. 1 . In this process, the oil mist contained in the blowby gas is separated and removed by the oil mist separator  101 F, and the blowby gas not containing the oil mist is supplied to the intake manifold  8 . At this time, as shown by an arrow A 4  (solid line), fresh air is supplied into the crank chamber  5   a  through the passage  12  and the internal space of the rear oil mist separators  101 R and thus the crank chamber  5   a  is ventilated. 
     On the other hand, when the pressure in the intake manifold  8  is positive, the open/close valve  13  is closed, and the blowby gas is sucked into the intake passage  7  from upstream side of the compressor  9   b  through the oil mist separator  101 R and passage  12 , as shown by arrows B 1 , B 2 , and B 3  (dotted lines) in  FIG. 1 . In this process, the oil mist contained in the blowby gas is separated and removed by the oil mist separators  101 R. Then, the blowby gas not containing the oil mist is supplied to the intake manifold  8  through the compressor  9   b  and throttle valve  10 . 
     The configuration of the oil mist separators  101  ( 101 F,  101 R) will be described. The front oil mist separator  101 F and rear oil mist separator  101 R have the same basic configuration.  FIG. 2  is an exploded perspective view of an oil mist separator  101  (a perspective view seen from a front-lower side).  FIG. 3  is a simplified sectional view showing a major part configuration of the assembled oil mist separator  101 . As in  FIG. 1 , in  FIGS. 2 and 3 , the front-rear direction and up-down direction are represented by arrows. In  FIG. 2 , the left-right direction is also represented by arrows. The left-right direction corresponds to the vehicle width direction. In  FIG. 3 , a part of a cylinder head  16  is shown. 
     Since the engine  1  is a V-type engine, the upper surface of the cylinder head having the oil mist separator  101  mounted thereon is inclined in the front-rear direction with respect to a horizontal plane. For this reason, a direction perpendicular to the upper surface (inclined surface) of the cylinder head is not the up-down direction in an exact sense, but an approximate up-down direction. However, although it is mounted on the inclined surface, the oil mist separator  101  sufficiently works and therefore the direction perpendicular to the upper surface of the cylinder head is handled as the up-down direction (gravity direction) below. That is, not only the direction in which the oil drops but also the direction in which the oil flows along the wall surface of a plate part  51  ( FIG. 4A ) (to be discussed later) due to the gravity is handled as the gravity direction. 
     As shown in  FIGS. 2 and 3 , the oil mist separator  101  includes a lower case  20  mounted on the upper surface of the cylinder head  16  with a gasket  15  therebetween, an upper case  30  fixed to the upper surface of the lower case  20 , and an oil separator unit (simply referred to as the “separator unit”)  102  contained in a space formed by the cases  20  and  30 . The lower case  20  and upper case  30  form a part of a cylinder head cover. These cases also form a part of the head portion  3 F or  3 R in  FIG. 1  with a cylinder head  16 . The lower case  20  is formed in an approximately tabular shape on the whole. The upper case  30  is formed in a housing shape having an open at its bottom on the whole, and the bottom is covered by the case  20 . 
     The lower case  20  and upper case  30  are formed by molding resin. A flange  21  is disposed over the entire periphery of the lower case  20 . A flange  31  is disposed over the entire periphery of the lower end of the upper case  30  so as to correspond to the flange  21 . The lower case  20  and upper case  30  are integrated with each other, for example, by vibration welding the upper surface of the flange  21  and the lower surface of the flange  31  and thus form a space SP therein. The lower case  20  and upper case  30  may be integrated with each other using a different technique. 
     A partition  32  extends in the left-right direction in the upper case  30 , and the space SP is partitioned into a rear first space SP 1  and a front second space SP 2  by the partition  32 . An approximately U-shaped notch  33  is formed in the partition  32  upward from the bottom surface thereof so as to correspond to the shape of the periphery of the separator unit  102 . A recessed slit  33   a  is disposed over the entire periphery of the notch  33 , and the periphery of the separator unit  102  is fitted to the slit  33   a  from below, as described later. In this state, the cases  20  and  30  are welded together. Thus, the separator unit  102  is sandwiched between the cases  20  and  30  with the position thereof regulated by the slit  33   a . In this way, the oil mist separator  101  is assembled. The assembled oil mist separator  101  is fixed to the upper surface of the cylinder head  16  using multiple bolts that penetrate the flanges  21  and  31 . 
     A through hole  22  formed in an approximately rectangular shape in a plan view is provided in the left end of the lower case  20  so as to penetrate the lower case  20  in the up-down direction. The first space SP 1  and the crank chamber  5   a  under the cylinder block  2  communicate with each other through the through hole  22 . Thus, the blowby gas can be caused to flow from the crank chamber  5   a  into the first space SP 1  through the through hole  22 . The blowby gas, which has flown into the first space SP 1 , flows into the second space SP 2  while the oil mist is separated therefrom by the separator unit  102 . An approximately cylindrical pipe  34  is protruded forward from the upper end of the upper case  30 . Through the pipe  34 , the second space SP 2  and passage  12  ( FIG. 1 ) or open/close valve  13  ( FIG. 1 ) communicate with each other. Thus, the blowby gas can be caused to flow out of the second space SP 2  through the pipe  34 . 
     The lower case  20  is provided with a swelling portion  23  that swells downward. As shown in  FIG. 3 , the swelling portion  23  is located directly in front of the notch  33  and forms a recessed space SP 3  under the second space SP 2 . A small-diameter through hole  24  is provided in the lowest portion of the swelling portion  23  so as to penetrate the lower case  20  in the up-down direction. The second space SP 2  and crank chamber  5   a  communicate with each other through the through hole  24 . Thus, the oil separated by the separator unit  102  is guided to the space SP 3  and then returned to the crank chamber  5   a  through the through hole  24 . 
     The configuration of the separator unit  102  according to the present embodiment will be described in detail below.  FIGS. 4A and 4B  are exploded perspective views of the separator unit  102  seen from a rear-left side and a front-left side).  FIG. 5  is a perspective view showing the assembled separator unit  102  (seen from a rear-left side). For convenience, in these drawings, the up-down direction, front-rear direction, and left-right direction are defined as in  FIGS. 1 to 3 , and the configurations of the separator unit  102  will be described in accordance with this definition. The separator unit  102  is a small assembly having an overall length of several centimeters order in the up-down direction and is assembled without using fastening members, such as a bolt. Hereafter, the up-down direction, front-rear direction, and left-right direction may be referred to as the height direction, thickness direction, and width direction, respectively. The separator unit  102  is formed in the height direction, thickness direction, and width direction. 
     The separator unit  102  is formed as an impactor filter type. As shown in  FIGS. 4A and 4B , the separator unit  102  includes an upstream plate  40  disposed on the upstream side, that is, on the rear side of the flow direction of the blowby gas, a downstream plate  50  disposed on the downstream side, that is, on the front side of the flow direction of the blowby gas, and a fiber member  60  disposed between the upstream plate  40  and downstream plate  50 . 
     The fiber member  60  has a front surface  61  and a rear surface  62 , an upper surface  63  and a lower surface  64 , and a left surface  65  and a right surface  66 . The fiber member  60  is formed in an approximately rectangular parallelepiped shape on the whole. The length in the up-down direction (longitudinal direction) of the fiber member  60  is longer than the length in the left-right direction (lateral direction) (width) thereof. That is, the fiber member  60  is formed so as to be elongated in the up-down direction. The rear surface  62  of the fiber member  60  is opposed to the upstream plate  40 , and the front surface  61  thereof is opposed to the downstream plate  50 . 
     The upstream plate  40  includes an approximately rectangular plate part  41  having a predetermined thickness in the front-rear direction and multiple ( 10  in the drawings) spacers  42  protruded forward from the front surface  41   a  of the plate part  41 . As shown in  FIG. 4A , the rear surface  41   b  of the plate part  41  is formed so as to be flat or approximately flat. The upstream plate  40  is integrally molded from resin. Also, the upstream plate  40  is formed so as to be bilaterally symmetrical on the whole.  FIG. 6  is a front view of the upstream plate  40 . In  FIG. 6 , the position of the fiber member  60  when the separator unit  102  is assembled is shown by a chain double-dashed line. 
     As shown in  FIGS. 4A and 6 , the length in the up-down direction (longitudinal direction) of the plate part  41  is longer than the length in the left-right direction (lateral direction) (width) thereof. That is, the plate part  41  has a constant or approximately constant width in the left-right direction and is formed so as to be elongated in the up-down direction. In the central portion in the left-right direction of the plate part  41 , a through hole  43  elongated in the up-down direction with a constant width is formed. The upper and lower ends of the through hole  43  are formed in arc shapes (semicircular shapes). 
     As shown in  FIGS. 4B and 6 , the spacers  42  are disposed so as to be equally spaced in the up-down direction from the upper end to the lower end of the through hole  43  and to be bilaterally symmetrical with respect to the through hole  43 . The spacers  42  are formed in tabular shapes and extend in parallel with each other in the left-right direction. For convenience, the spacers  42  ( 421  to  425 ) may be referred to as first spacers  421 , second spacers  422 , third spacers  423 , fourth spacers  424 , and fifth spacers  425  in the top-to-bottom order. 
     The front surfaces  42   a  of the left and right spacers  421  to  425  configure support surfaces that support the fiber member  60  by contacting the rear surface  62  of the fiber member  60 . The protruding lengths of the spacers  421  to  425 , that is, the lengths from the front surface  41   a  of the plate part  41  to the front surfaces  42   a  of the spacers  421  to  425  are equal to each other. The front surfaces  42   a  of the spacers  421  to  425  are located on the same virtual plane extending in the up-down and left-right directions. 
     Cylindrical protrusions  42   b  having the same height (length) are protruded from the central portions in the left-right direction of the front surfaces  42   a  of the left and right spacers  421  to  425 . The inner end surfaces in the left-right direction of the spacers  421  to  425  are located on planes extending from the opening end surfaces (left and right end surfaces) of the through hole  43 . Accordingly, the left end surfaces of the right spacers  421  to  425  are located on the same plane, and the right end surface of the left spacers  421  to  425  are located on the same plane. For this reason, the lengths in the left-right direction of the front surfaces  42   a  of the spacers  421  to  425  are equal to each other. 
     Guides  44  protruding forward than the front surfaces  42   a  of the spacers  42  are connected to the outside surfaces in the left-right direction of the left and right first, third, and fifth spacers  421 ,  423 , and  425  of the spacers  421  to  425 . The protruding lengths of the guides  44  are longer than the protruding lengths of the protrusions  42   b , as shown in  FIG. 4B , and shorter than the thickness (the length in the front-rear direction) of the fiber member  60 , as shown in  FIG. 5 . For example, the protruding lengths of the guides  44  are set within a range that is ½ to ⅔ of the thickness of the fiber member  60 . 
     As shown in  FIG. 6 , the right end surfaces  44   a  of the guides  44  of the left first, third, and fifth spacers  421 ,  423 , and  425  are located on the same plane, and the left end surfaces  44   b  of the guides  44  of the right first, third, and fifth spacers  421 ,  423 , and  425  are also located on the same plane. The lengths from the right end surfaces  44   a  to the left end surfaces  44   b  of the guides  44  are equal to the width (the length in the left-right direction) of the fiber member  60 . For this reason, the right end surfaces  44   a  configure support surfaces that support the left surface  65  of the fiber member  60  by contacting the left surface  65 , and the left end surfaces  44   b  configure support surfaces that support the right surface  66  of the fiber member  60  by contacting the right surface  66 . 
     A plate-shaped guide  45  is protruded forward from the front surface  41   a  of the plate part  41  below the through hole  43 . The protruding length (the length in the front-rear direction) of the guide  45  is equal to the lengths from the front surface  41   a  of the plate part  41  to the front end surfaces of the guides  44 . The upper surface  45   a  of the guide  45  configures a support surface that supports the lower surface  64  of the fiber member  60  by contacting the lower surface  64 . 
     A pair of upper and lower recesses (concave portions)  46  and  47  having a predetermined depth are provided in the front surface  41   a  of the plate part  41 . The upper recess  46  is formed in an approximate U-shape along the outside shape of the plate part  41  so as to extend from the upper end of the front surface  41   a  of the plate part  41  through the left and right outer sides of the left and right first spacers  421  to the left and right outer sides of the left and right second spacers  422 . The lower recess  47  is formed in an approximate U-shape along the outside shape of the plate part  41  so as to extend from the lower end of the front surface  41   a  of the plate part  41  through the left and right outer sides of the left and right fifth spacers  425  to the left and right outer sides of the left and right fourth spacers  424 . The bottom surfaces  46   a  and  47   a  of the recesses  46  and  47  are formed on the same virtual plane extending in the up-down and left-right directions. As shown in  FIG. 4B , the area between the left and right lower ends of the recess  46  and the left and right upper ends of the recess  47  is protruded forward than the recesses  46  and  47 . This area may be referred to as a protrusion (convex portion)  48 . 
     As shown in  FIGS. 4B and 6 , engagement portions are partially disposed on the approximately U-shaped peripheral surface (side surface) of the recess  46 , that is, on a boundary surface  46   b  between the bottom surface  46   a  of the recess  46  and the front surface  41   a  along the bottom surface  46   a . More specifically, an engagement recess (concave portion)  461  that is approximately rectangularly recessed from a left boundary surface  46   b  is disposed adjacent to the left end surface of the left second spacer  422 , an engagement recess (concave portion)  462  that is approximately rectangularly recessed from a right boundary surface  46   b  is disposed adjacent to the right end surface of the right second spacer  422 , and an engagement recess (concave portion)  463  that is approximately rectangularly recessed from an upper boundary surface  46   b  is disposed above the through hole  43 . The left end surface of the guide  44  of the left first spacer  421  and the right end surface of the guide  44  of the right first spacer  421  are located on the same or approximately the same planes as the boundary surfaces  46   b.    
     Engagement portions are partially disposed on the approximately U-shaped peripheral surface (side surface) of the recess  47 , that is, on a boundary surface  47   b  between the bottom surface  47   a  of the recess  47  and the front surface  41   a  along the bottom surface  47   a . More specifically, an engagement recess (concave portion)  471  that is approximately rectangularly recessed from a left boundary surface  47   b  is disposed adjacent to the left end surface of the left fourth spacer  424 , an engagement recess (concave portion)  472  that is approximately rectangularly recessed from a right boundary surface  47   b  is disposed adjacent to the right end surface of the right fourth spacer  424 , and an engagement protrusion (convex portion)  473  that is approximately rectangularly protruded from a lower boundary surface  47   b  is disposed adjacent to the lower surface of the guide  45  below the through hole  43 . The left end surface of the guide  44  of the left fifth spacer  425  and the right end surface of the guide  44  of the right fifth spacer  425  are located on the same or approximately the same planes as the boundary surfaces  47   b.    
     As shown in  FIGS. 4A and 4B , the downstream plate  50  includes the approximately rectangular plate part  51  having a predetermined thickness in the front-rear direction, a pair of upper and lower flanges  52  and  53  disposed behind the plate part  51 , and multiple connecting parts  54  that couple the plate part  51  and flanges  52  and  53 . As shown in  FIG. 4B , the front surface  51   a  of the plate part  51  is formed so as to be flat or approximately flat. The downstream plate  50  is integrally molded from resin. Also, the downstream plate  50  is formed so as to be bilaterally symmetrical on the whole.  FIG. 7  is a rear view of the downstream plate  50 . In  FIG. 7 , the position of the fiber member  60  when the separator unit  102  is assembled is shown by a chain double-dashed line. 
     As shown in  FIGS. 4A and 7 , the length in the up-down direction (longitudinal direction) of the plate part  51  is longer than the length in the left-right direction (lateral direction) (width) thereof. That is, the plate part  51  has a constant or approximately constant width in the left-right direction and is formed so as to be elongated in the up-down direction. More specifically, the shape of the plate part  51  approximately matches the shape of the front surface  41   a  of the plate part  41 ) of the upstream plate  40 , that is, the shape of the front surface  41   a  except for the recesses  46  and  47 . The plate part  51  configures a hit portion that is hit by the blowby gas that has passed through the through hole  43  of the upstream plate  40  and the fiber member  60 . 
     Multiple (3 in the drawings) columnar ribs  55  having approximately rectangular cross-sections protrude rearward from the rear surface  51   b  of the plate part  51  so as to be equally spaced in the left-right direction. For convenience, the ribs  55  ( 551  to  553 ) may be referred to as the first rib  551 , second rib  552 , and third rib  553  in the left-to-right order. The ribs  551  to  553  are extended in parallel with each other in the up-down direction, and the extending direction thereof is perpendicular to the extending direction (left-right direction) of the spacers  421  to  425  ( FIG. 6 ). The lengths (widths) in the left-right direction of the ribs  551  to  553  are equal to each other. 
     The second rib  552  is located in the front of the through hole  43  and on the central portion in the left-right direction of the plate part  51 . In the assembled separator unit  102 , the first rib  551  is located approximately in the same position in the left-right direction as the line of the protrusions  42   b  ( FIG. 6 ) provided at the left spacers  421  to  425  of the upstream plate  40 , and the third rib  553  is located approximately in the same position in the left-right direction as the line of the protrusions  42   b  provided at the right spacers  421  to  425  of the upstream plate  40 . In other words, the second rib  552  is disposed so as to correspond to the position of the through hole  43 , the first rib  551  is disposed so as to correspond to the positions of the left spacers  421  to  425 , and the third rib  553  is disposed so as to correspond to the positions of the right spacers  421  to  425 . 
     The rear surfaces  55   a  of the ribs  551  to  553  configure support surfaces that support the fiber member  60  by contacting the front surface  61  of the fiber member  60 . The protruding lengths of the ribs  551  to  553 , that is, the lengths from the rear surface  51   b  of the plate part  51  to the rear surfaces  55   a  of the ribs  551  to  553  are equal to each other. The rear surfaces  55   a  of the ribs  551  to  553  are located on the same virtual plane extending in the up-down and left-right directions. The protruding lengths of the ribs  551  to  553  are shorter than the protruding lengths of the spacers  421  to  425 . 
     Multiple (five) cylindrical protrusions  55   b  having the same heights are disposed in equally spaced positions in the up-down direction on each of the rear surfaces  55   a  of the first rib  551  and third rib  553 . The protrusions  55   b  are formed in the same shape as the protrusions  42   b  of the spacer  42  of the upstream plate  40 . Also, in the assembled separator unit  102 , the protrusions  55   b  are disposed in the same positions in the up-down and left-right directions as the protrusions  42   b.    
     The upper flange  52  is formed in an approximate U-shape, and the inner peripheral surface  52   a  thereof is approximately the same as the shape of the outer peripheral surface  51   c  (edge) of the plate part  51 . The outer peripheral surface  52   b  thereof is approximately the same as the shape of the outer peripheral surface  41   c  ( FIG. 4A ) of the plate part  41  of the upstream plate  40 . The lower flange  53  is formed in an approximate U-shape, and the inner peripheral surface  53   a  thereof is approximately the same as the shape of the outer peripheral surface  51   c  of the plate part  51 . The outer peripheral surface  53   b  thereof is approximately the same as the shape of the outer peripheral surface  41   c  of the plate part  41  of the upstream plate  40 . The rear surfaces  52   c  and  53   c  of the flanges  52  and  53  are formed so as to be flat and are located on the same virtual plane extending in the up-down and left-right directions. The lengths (thicknesses) in the front-rear direction of the flanges  52  and  53  are approximately equal to the depths of the recesses  46  and  47  ( FIG. 4B ) of the upstream plate  40 . 
     The connecting parts  54  are formed in tabular shapes having approximately rectangular cross-sections. As shown in  FIG. 7 , the connecting parts  54  include connecting parts  541  to  543  having one ends (rear ends) fixed to the left and right lower ends and upper end of the inner peripheral surface  52   a  of the flange  52 . The connecting parts  541  to  543  are extended in the front-rear direction, and the other ends (front ends) thereof are fixed to the left and right lower ends and upper end of the plate part  51 . Thus, both ends and central portion in the longitudinal direction of the flange  52  extending in an approximate U-shape are supported by the connecting parts  541  to  543 . The connecting parts  541  to  543  have convex cross-sectional shapes corresponding to the shapes of the engagement recesses  461  to  463  ( FIG. 6 ) of the plate part  41  of the upstream plate  40 . Thus, when the separator unit  102  is assembled as shown in  FIG. 5 , the flange  52  is fitted to the recess  46 , and the connecting parts  541  to  543  are fitted to the engagement recesses  461  to  463 . 
     In this fitted state, the right end surface  541   a  of the connecting part  541  and the left end surface  542   a  of the connecting part  542  shown in  FIG. 7  are located on the same or approximately the same planes as the right end surface  44   a  of the left guide  44  and the left end surface  44   b  of the right guide  44  of the upstream plate  40  shown in  FIG. 6 . Thus, as with the end surfaces  44   a  and  44   b  of the guides  44 , the end surfaces  541   a  and  542   a  configure support surfaces that support the left surface  65  and right surface  66  of the fiber member  60 . That is, the connecting parts  541  and  542  are located in the same position in the up-down direction as the second spacers  422  of the upstream plate  40  and outside the second spacers  422  in the left-right direction, as well as serve as support portions that support the fiber member  60 , as with the guides  44 . The lower surface  543   a  of the connecting part (central connecting part)  543  configures a support surface that supports the upper surface  63  of the fiber member  60 . 
     Also, as shown in  FIG. 7 , the connecting parts  54  include connecting parts  544  to  547  having one ends (rear ends) fixed to the left and right upper ends and left and right lower ends and of the inner peripheral surface  53   a  of the flange  53 . The connecting parts  544  to  547  extend in the front-rear direction. The other ends (front ends) of the connecting parts  544  and  545  are fixed to the left and right ends of the plate part  51 , and the other ends of the connecting parts  546  and  547  are fixed to the left and right lower ends of the plate part  51 . Thus, both ends and the central portion in the longitudinal direction of the flange  53  extending in an approximate U-shape are supported by the connecting parts  544  to  547 . 
     The connecting parts  544  and  545  have convex cross-sectional shapes corresponding to the shapes of the engagement recesses  471  and  472  ( FIG. 6 ) of the plate part  41  of the upstream plate  40 . The lengths in the up-down direction of the connecting parts  546  and  547  are equal to the length in the up-down direction of the engagement protrusion  473 , and the width in the left-right direction between the connecting parts  546  and  547  is equal to the width of the engagement protrusion  473 . Thus, when the separator unit  102  ( FIG. 5 ) is assembled, the flange  53  is fitted to the recess  47 , the connecting parts  544  and  545  are fitted to the engagement recesses  471  and  472 , and the connecting parts  546  and  547  are fitted to the recesses  47  on the left and right sides of the engagement protrusion  473 . 
     In this fitted state, the right end surface  544   a  of the connecting part  544  and the left end surface  545   a  of the connecting part  545  shown in  FIG. 7  are located on the same or approximately the same planes as the right end surface  44   a  of the left guide  44  and the left end surface  44   b  of the right guide  44  of the upstream plate  40  shown in  FIG. 6 . Thus, as with the end surfaces  44   a  and  44   b  of the guides  44 , the end surfaces  544   a  and  545   a  configure support surfaces that support the left surface  65  and right surface  66  of the fiber member  60 . That is, the connecting parts  544  and  545  are located in the same position in the up-down direction as the fourth spacers  424  of the upstream plate  40  and outside the fourth spacers  424  in the left-right direction, as well as serve as support portions that support the fiber member  60 , as with the guides  44 . 
     The upper surfaces  546   a  and  547   a  of the connecting parts (central connecting parts)  546  and  547  shown in  FIG. 7  are located on the same or approximately the same plane as the upper surface  45   a  of the guide  45  of the upstream plate  40  shown in  FIG. 6 . Thus, as with the upper surface  45   a  of the guide  45 , the upper surfaces  546   a  and  547   a  configure support surfaces that support the lower surface  64  of the fiber member  60 . 
       FIG. 8  is a side view (left side view) of the assembled separator unit  102 . As shown in  FIG. 8 , the length L 1  from the rear surfaces  55   a  of the ribs  55  of the downstream plate  50  to the rear surfaces  52   c  and  53   c  of the flanges  52  and  53  is longer than the length L 2  from the bottom surfaces  46   a  and  47   a  of the recesses  46  and  47  of the upstream plate  40  to the front end surfaces of the guides  44 . Thus, when the separator unit  102  is assembled, the rear surfaces  52   c  and  53   c  of the flanges  52  and  53  contact the bottom surfaces  46   a  and  47   a  of the recesses  46  and  47 . The length L 1  is determined by the length of the connecting parts  54 , or the like. Also, the length L 1  is set such that the length L 3  from the rear surfaces  55   a  of the ribs  55  to the front surfaces  42   a  of the spacers  42  is the same or approximately the same as the thickness (the length in the front-rear direction) of the fiber member  60  with the rear surfaces  52   c  and  53   c  in contact with the bottom surfaces  46   a  and  47   a.    
     In a side view of the separator unit  102  shown in  FIG. 8 , the connecting parts  541  and  542  (only  541  is shown) fixed to the left and right lower ends of the flange  52  are extended in the front-rear direction in the same position in the up-down direction as the second spacers  422 . The connecting parts  544  and  545  (only  544  is shown) fixed to the left and right upper ends of the flange  53  are extended in the front-rear direction in the same position in the up-down direction as the fourth spacers  424 . That is, in a side view of the separator unit  102 , the connecting parts  541  and  542 , and  544  and  545  are extended in the front-rear direction so as to be overlaid on the spacers  422  and  424  and cover the spacers  422  and  424  from outside. 
     The fiber member  60  is a type of air-permeable porous material and has elasticity (flexibility or contractibility). Specifically, the fiber member  60  is configured as a main component, fiber, including a fiber mass consisting of natural fiber, synthetic fiber, or the like, a fiber assembly, such as nonwoven fabric or composition, and the like. 
     A method for assembling the oil mist separator  101  according to the present embodiment will be described. First, the separator unit  102  is assembled. Specifically, the fiber member  60  is fitted to the inside of the guides  44  of the left and right spacers  42  protruding from the front surface  41   a  of the upstream plate  40  shown in  FIG. 4B . Thus, the left and right surfaces  65  and  66  of the upper end, the left and right surfaces  65  and  66  of the central portion, and the left and right surfaces  65  and  66  of the lower end of the fiber member  60  are supported by the guides  44  of the first, third, and fifth spacers  421 ,  423 , and  425 . That is, the left and right surfaces  65  and  66  over the whole in the length direction (up-down direction) of the fiber member  60  are supported by the guides  44 . Also, the lower surface  64  of the fiber member  60  is brought into contact with the upper surface  45   a  of the guide  45 , thereby determining the position in the up-down direction of the fiber member  60 . 
     Then, the upper and lower flanges  52  and  53  of the downstream plate  50  shown in  FIG. 4A  are fitted to the recesses  46  and  47  around the spacers  42  of the upstream plate  40 , thereby mounting the downstream plate  50  to the upstream plate  40 . Since the flanges  52  and  53  have approximate U-shapes, the position in the up-down and left-right directions of the downstream plate  50  with respect to the upstream plate  40  is regulated. Thus, the downstream plate  50  can be easily mounted to the upstream plate  40  without misalignment. 
     At this time, the pair of left and right connecting parts  541  and  542  supporting the flange  52  by extending the front-rear direction are fitted to the engagement recesses  461  and  462  on the left and right sides of the second spacers  422 . The pair of left and right connecting parts  544  and  545  supporting the flange  53  by extending the front-rear direction are fitted to the engagement recesses  471  and  472  on the left and right sides of the fourth spacers  424 . Thus, the left and right surfaces  65  and  66  of the fiber member  60  are supported by the guides  44 , as well as the end surfaces  541   a ,  542   a ,  544   a , and  545   a  ( FIG. 7 ) of the connecting parts  54 . That is, the connecting parts  541 ,  542 ,  544 , and  545  are members that support the flanges  52  and  53 , as well as are members that support the fiber member  60 , as with the guides  44 . 
     The connecting part  543  supporting the upper end of the flange  52  by extending the front-rear direction is fitted to the engagement recess  463  above the through hole  43  of the upstream plate  40 . The connecting parts  546  and  547  supporting the lower end of the flange  53  by extending the front-rear direction are fitted to the recesses  47  on the left and right sides of the engagement protrusion  473 . Thus, the upper and lower surfaces (upper surface  63 , lower surface  64 ) of the fiber member  60  are supported by the lower surface  543   a  of the connecting part  543  and the upper surfaces  546   a  and  547   a  of the connecting parts  546  and  547 . As a result, when the downstream plate  50  is mounted to the upstream plate  40 , the left and right surfaces  65  and  66  and upper and lower surfaces  63  and  64  of the fiber member  60  are supported by the guides  44  and  45  and the connecting parts  54 , and the position in the up-down and left-right directions of the fiber member  60  is regulated. 
     With the downstream plate  50  mounted to the upstream plate  40 , the fiber member  60  is supported between the front surfaces  42   a  of the spacers  42  of the upstream plate  40  and the rear surfaces  55   a  of the ribs  55  of the downstream plate  50 . In this case, the protrusions  42   b  and  55   b  disposed on the front surfaces  42   a  and rear surfaces  55   a  press the front and rear surfaces (front surface  61 , rear surface  62 ) of the fiber member  60 . Thus, the positions of the front and rear surfaces  61  and  62  of the fiber member  60  are regulated by the protrusions  42   b  and  55   b , allowing the fiber member  60  to be held stably. 
     As shown in  FIG. 8 , the plate part  41  of the upstream plate  40  and the flanges  52  and  53  of the downstream plate  50  configure a part of the periphery  102   a  of the separator unit  102 . The protrusions  48  ( FIG. 4B ) interposed between the left and right lower ends of the flange  52  and the left and right upper ends of the flange  53  also configure a part of the periphery  102   a  of the separator unit  102 . Thus, the entire periphery  102   a  of the separator unit  102  has a constant thickness (the length in the front-rear direction). As a result, the recessed and protruding shapes allow the flanges  52  and  53  to be easily positioned, as well as increases the rigidity of the periphery  102   a  of the assembled separator unit  102 . 
     As shown in  FIG. 2 , the periphery  102   a  of the separator unit  102  thus assembled (temporarily assembled) is fitted to the slit  33   a  of the notch  33  of the upper case  30  from below. Then, the upper surface of the flange  21  of the lower case  20  is fixed to the lower surface of the flange  31  of the upper case  30  by vibration welding. Thus, the assembly of the oil mist separator  101  is completed. 
     As shown in  FIG. 3 , a recessed slit  20   a  is provided in a position corresponding to the notch  33  of the upper surface of the lower case  20 . When the lower case  20  is mounted, the lower end of the separator unit  102  is fitted to the slit  20   a . More specifically, as shown in  FIGS. 4A and 4B , flange surfaces  41   d  and  53   d  are formed over the entire area in the left-right direction of the lower end of the rear surface  41   b  of the plate part  41  of the upstream plate  40  and over the entire area in the left-right direction of the lower end of the front surface of the flange  53  of the downstream plate  50 , respectively. The flange surface  53   d  is formed from the lower end of the flange  53  to the lower surfaces of the connecting parts  546  and  547 , and the flange surface  41   d  is formed in the same length in the up-down direction as the flange surface  53   d . The flange surfaces  41   d  and  53   d  are fitted to the slit  20   a.    
     Fitting the separator unit  102  to the slits  33   a  and  20   a  as described above allows the separator unit  102  to be accurately positioned with respect to the cases  20  and  30  and mounted thereon, as well as allows the separator unit  102  to be firmly fixed to the cases  20  and  30 . In the assembled oil mist separator  101 , the lower end of the downstream plate  50 , that is, the lower surface of the plate part  51  and the lower surfaces of the connecting parts  54  ( 546 ,  547 ) is in contact with the upper surface of the lower case  20 . Thus, the flow of the blowby gas below the downstream plate  50  is suppressed. 
     Next, there will be described an example of the operation of the oil mist separator  101  according to the present embodiment. As shown in  FIG. 3 , the oil mist-containing blowby gas that has flown into the first space SP 1  through the through hole  22  passes through the through hole  43  of the upstream plate  40  of the separator unit  102  and flows into the fiber member  60  through the rear surface  62  of the fiber member  60 . This blowby gas flows at increased speed due to narrowing of the flow path by the through hole  43 , passes through the fiber member  60  from the rear surface  62  to the front surface  61 , and flows out of the fiber member  60  through the front surface  61 , and then hits the plate part  51  of the downstream plate  50 . 
     At this time, the oil mist contained in the blowby gas is trapped by the fiber of the fiber member  60  by adhering thereto. The trapped oil mist is coarsened into oil masses while moving rearward along the flow of the gas, and the oil masses drop down along the gaps between the ribs  55  of the rear surface  51   b  of the plate part  51 . Or, before reaching the rear surface  51   b , the oil masses grow into large droplets and drop down. In this way, the oil components are separated from the blowby gas. The separated oil is guided into the space SP 3  of the swelling portion  23  of the lower case  20  and then returned to the crank chamber  5   a  through the through hole  24 . 
     On the other hand, the oil mist-removed blowby gas, which has passed through the fiber member  60 , mostly flows in the left-right direction through the gap between the plate part  51  and the front surface  61  of the fiber member  60  and flows into the second space SP 2 . A part of the oil mist-removed blowby gas flows into the second space SP 2  through the left and right surfaces  65  and  66  of the fiber member  60 . As shown in  FIG. 8 , the connecting parts  541  and  544  on the left and right sides of the fiber member  60  are disposed in positions that are overlaid on the spacers  422  and  424 . This allows for suppression of an reduction in the flow-path area of the blowby gas flowing out to the left and right sides of the fiber member  60 , allowing for efficient flow of the blowby gas. In this case, the oil separated from the blowby gas drops along the ribs  55 . Thus, the ribs  55  can suppress the outflow of the oil through the left and right sides of the rear side of the plate part  51  along the flow of the blowby gas, that is, the ribs  55  can suppress the involution of the oil. The blowby gas that has flown into the second space SP 2  flows out through the pipe  34  and is returned to the combustion chambers of the engine  1 . 
     As shown in  FIG. 1 , during non-turbocharging, fresh air is introduced into the internal space of the oil mist separator  101 R disposed on the head portion  3 R on the rear side of the engine  1  through the passage  12 . In this case, the flow in a direction opposite to that described above generates at the separator unit  102 . That is, the fresh air flows from the second space SP 2  to the first space SP 1  through the separator unit  102 . At this time, the connecting parts  54  on the left and right sides of the fiber member  60 , or the like can smoothly guide the fresh air to the first space SP 1  as straightening plates. 
     The present embodiment can produce the following advantageous effects: 
     (1) The separator unit  102  is configured to separate the oil mist contained in the blowby gas generated in the engine  1 . More specifically, the separator unit  102  includes the upstream plate  40  that is disposed on the upstream side in the flow direction of the blowby gas and has the through hole  43  through which the blowby gas passes, the downstream plate  50  that is disposed on the downstream side in the flow direction of the blowby gas and includes the plate part  51  hit by the blowby gas, and the fiber member  60  that has the rear surface  62  opposed to the upstream plate  40  and the front surface  61  opposed to the downstream plate  50 , is formed in an approximately rectangular shape having a predetermined thickness (substantially rectangular parallelepiped shape), and traps the oil mist contained in the blowby gas that has passed through the through hole  43  ( FIGS. 4A, 4B ). The upstream plate  40  includes the front surface  41   a  of the plate part  41  opposed to the rear surface  62  of the fiber member  60 , and the pair of upper and lower recesses  46  and  47  formed in substantially U-shapes and provided at circumference portions in the upper and lower end sides of the front surface  41   a  ( FIG. 4B ). The downstream plate  50  includes the pair of upper and lower flanges  52  and  53  fitted in the recesses  46  and  47  ( FIG. 4B ). 
     Due to this configuration, it is possible to enhance the rigidity of the separator unit  102 , in particular, the rigidity of the periphery  102   a  and to retain the downstream plate  50  in a state of being accurately positioned with respect to the upstream plate  40 . In other words, the upstream plate  40  and the downstream plate  50  can be easily held integral with each other by merely overlaying the flanges  52  and  53  on the plate part  41 . Accordingly, the upstream plate  40 , the downstream plate  50  and the fiber member  60  can be assembled at high positional accuracy, and therefore, it is possible to sufficiently separate the oil mist. 
     (2) The downstream plate  50  further includes the rear surface  51   b  of the plate part  51  facing the front surface  61  of the fiber member  60 , and the connecting parts  54  extending along the flow direction (front-rear direction) of the blowby gas that has passed through the through hole  43  and having one ends (rear ends) connected to the flanges  52  and  53  and the other ends (front ends) connected to the rear surface  51   b  of the plate part  51  ( FIG. 4A ). By extending the connecting parts  54  in the front-rear direction in this way, the flow direction of the blowby gas is restricted and the flow of the blowby gas in the front-rear direction after passing the through hole  43  is promoted. Further, positional shift of the fiber member  60  in the width direction (left-right direction) is prevented and separating effect of the oil mist enhances. Furthermore, when fresh air is introduced into the oil mist separator  101  during non-turbocharging, straightening effect is obtained by the connecting parts  54 . 
     (3) The connecting parts  54  include connecting parts  541 ,  542 ,  544  and  545  that have rear ends connected to both ends in the longitudinal direction of the flange  52  and  53  extending in the approximate U-shape, i.e., the left and right lower ends of the flange  52  and the left and right upper ends of the flange  53  and front ends connected to the rear surface  51   b  of the plate part  51  ( FIG. 7 ). Therefore, even if the flanges  52  and  53  are provided away from the plate part  51 , the downstream plate  50  can possess the high rigidity. 
     (4) The upstream plate  40  includes the spacers  42  projected from the front surface  41   a  of the plate part  41  to support the rear surface  62  of the fiber member  60  ( FIG. 4B ). The connecting parts  541 ,  542 ,  544  and  545  are extended in parallel with projecting direction of the spacer (front-rear direction) so as to overlap with the spacers  42  ( 422  and  424 ) in a side view of the separator unit  102  ( FIG. 8 ). Therefore, it is possible to suppress a reduction in the flow-path area of the blowby gas flowing out to the left and right sides of the fiber member  60  due to the connecting parts  54  and to achieve an efficient flow of the blowby gas. Further, it is possible to suppress a reduction in the flow-path are when fresh air is introduced into the fiber member  60  from the left and right sides and to achieve a smooth flow of the fresh air. 
     The above embodiment can be modified into various forms, and modifications will be described below.  FIG. 9A  is a front view of a separator unit  102 A according to a modification of the present embodiment, and  FIG. 9B  is a sectional view taken along an axis (line B-B) in the center in the left-right direction of  FIG. 9A . The modification differs from the above embodiment in the configuration of a plate part  41  of an upstream plate  40 . That is, as shown in  FIGS. 9A and 9B , a protrusion  411  that protrudes rearward from the entire periphery of a through hole  43  is disposed on the rear surface  41   b  of the plate part  41 , and the length (thickness) in the front-rear direction of the plate part  41  is increased due to the protrusion  411 . In other words, the protrusion  411  configures a thick portion that increases the thickness of the through hole  43  along the flow direction of the blowby gas. 
     Thus, the length of the through hole  43  in the front-rear direction is increased, and such a through hole  43  produces favorable blowby-gas straightening effects. That is, the blowby gas is straightened by the through hole  43  lengthened in the front-rear direction and thus flows forward. Thus, the entire blowby gas that has passed through the through hole  43  can be caused to flow into the fiber member  60 . As a result, the oil mist separation effects can be increased. A thick portion may be configured by disposing a protrusion that surrounds the through hole  43 , on the front surface  41   a  of the plate part  41  rather than on the rear surface  41   b.    
     Although, in the above embodiment, the fiber member  60  having the rear surface  62  (a first surface) facing the upstream plate  40  and the front surface  61  (a second surface) facing the downstream plate  50  is disposed between the upstream plate  40  and downstream plate  50 , a porous member other than the fiber member  60  may be used. For example, a network structure or the like formed from a metal, ceramic, or the like may be used as a porous member. 
     Although, in the above embodiment, the upstream plate  40  having the front surface  41   a  facing the rear surface  62  of the fiber member  60  (an upstream surface) is disposed on the downstream side of the flow direction of the blowby gas as an upstream member, the upstream member need not be configured as described above. For example, instead of single through hole  43 , multiple blowby-gas passage holes may be provided in the plate part  41  as an opening. Such opening needs not be the through hole and may be a notch extending from the periphery (the upper end or lower end) of the plate part  41 . Although, in the above embodiment, the downstream plate  50  having the rear surface  51   b  (a downstream surface) of the plate part  51  facing the front surface  61  of the fiber member  60  is provide at downstream side than upstream plate  40  in the flow direction of the blowby gas as a downstream member, the downstream member is not limited to the above configuration. 
     Although, in the above embodiment, the plate parts  41  and  51  and the fiber member  60  are configured to be elongated in the up-down direction, they may be configured to be elongated in the left-right direction or configured in a square shape. Therefore, a downstream member may be provided with a pair of left and right flange parts, instead of a pair of up and down flange parts, and corresponding to such a configuration, a upstream member may be provided with a pair of left and right recesses. In the above embodiment, the recesses  46  and  47  of the upstream plate  40  and the flanges  52  and  53  of the downstream plate  50  are formed in an approximate U-shape, respectively. However, as long as flange parts are fitted in recesses passing through outsides of a porous member when assembling a separator unit, the flange parts and recesses are not limited to the above configuration. Although, in the above embodiment, the connecting parts  54  are connected to both ends in the longitudinal direction of the flanges  52  and  53  extending in approximate U-shape, a connecting position of connecting parts are not limited to the above configuration. 
     In the above embodiment ( FIG. 8 ), the connecting parts  541 ,  542 ,  544  and  545  in the left and right outsides of the second spacers  422  and the fourth spacers  424  are configured to cover all of the spacers  422  and  424  in a side view. However, as long as the connecting parts are configured so as to overlap with the spacers  422  and  424 , the connecting parts are limited to the above configurations. The connecting parts  541 ,  542 ,  544  and  545  may be configured to cover a part of the spacers  422  and  424 . For example, the length (thickness) of the spacers  422  and  424  in the up-down direction may be longer than the length of the connecting parts  541 ,  542 ,  544  and  545  in the up-down direction. 
     Although, in the above embodiment, the separator unit  102  is fitted to the slits  33   a  and  20   a  of the upper case  30  and lower case  20 , the separator unit need not be mounted using this method. For example, the separator unit may be fixed to the cases by welding or the like. Although, in the above embodiment, the oil mist separators  101  including the separator units  102  are disposed above the engine  1 , they may be disposed in other positions. Although, in the above embodiment, the separator unit  102  is applied to the V-type engine of the vehicle having a turbocharger, it can also be applied to non-V-type-engines and engines having no turbocharger in a similar manner. 
     The above embodiment can be combined as desired with one or more of the above modifications. The modifications can also be combined with one another. 
     According to the present invention, a downstream member can be assembled with an upstream member at a high positional accuracy, and therefore, it is possible to sufficiently separate an oil mist contained in a blowby gas. 
     Above, while the present invention has been described with reference to the preferred embodiments thereof, it will be understood, by those skilled in the art, that various changes and modifications may be made thereto without departing from the scope of the appended claims.