Abstract:
A separator unit of an oil mist separator comprises a front unit including a perforated plate and a plurality of holding studs extending rearward from the perforated plate, a rear unit placed behind the perforated plate and including a base plate that is to be intimately put on a rear surface of the perforated plate and has an opening sized to permit insertion of the holding studs of the front unit, a plurality of connecting studs extending rearward from the base plate and an impinge plate held by leading ends of the connecting studs, and a fiber material piece put on the impinge plate, wherein when the front unit is coupled with the rear unit having the holding studs inserted through the opening of the base plate, the fiber material piece is sandwiched between the leading ends of the holding studs and the impinge plate.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to oil mist separators for an internal combustion engine, and more particularly to the oil mist separators of a type that separates oil mist from blow-by gas produced in the internal combustion engine. 
         [0003]    2. Description of the Related Art 
         [0004]    In current days, for protecting the environment from pollution, internal combustion engines for motor vehicles are usually provided with a blow-by gas processing system through which the blow-by gas leaking from the combustion chambers to the crankcase is led into an intake system of the engine for burning unburnt components of the blow-by gas. Since the blow-by gas passing through the crankcase contains oil mist that would lower the performance of the intake system, an oil mist separator is usually provided in for example a cylinder head cover for separating and removing the oil mist from the blow-by gas. 
         [0005]    Some of such oil mist separators are disclosed in Japanese Laid-open Patent Application (tokkai) 2009-121281 and Published International Patent Application 2013/179829. The oil mist separator disclosed in the former generally comprises a separator chamber, a partition wall installed in the separator chamber, a plurality of openings formed in the partition wall and an impinge wall installed in the separator chamber at a position downstream of the partition wall. In operation of the engine, the blow-by gas is forced to pass through the openings. With this passing, the blow-by gas is highly accelerated before impinging against the impinge wall, and thus the oil mist in the blow-by gas is suitably separated and removed. The other oil mist separator disclosed in the latter is substantially the same as that of the former except that in the latter, the impinge wall is lined or covered with a fiber laminate for improving the oil mist separation and removing. 
       SUMMARY OF THE INVENTION 
       [0006]    It has been revealed that the oil mist separator disclosed in the latter (viz., Published International Patent Application 2013/179829) exhibits a satisfied oil mist separation performance due to provision of the fiber laminate on the impinge wall. 
         [0007]    However, in the oil mist separator of this publication, fixing the fiber laminate to the impinge wall is entirely made by only bonding or welding, and thus, it sometimes occurs that the fiber laminate comes off from the impinge wall particularly after long use. Furthermore, the work for bonding or welding the fiber laminate to a desired position of the impinge wall is not easy. 
         [0008]    Accordingly, an object of the present invention is to eliminate the above-mentioned drawbacks of the known oil mist separator. 
         [0009]    In accordance with a first aspect of the present invention, there is provided an oil mist separator which comprises a housing body through which an oil mist containing gas flows, the housing body including upper and lower members that are coupled to constitute therebetween a gas flow passage for the oil mist containing gas; and a separator unit installed in the gas flow passage, the separator unit comprising a front unit including a perforated plate and a plurality of holding studs extending rearward from the perforated plate; a rear unit placed behind the perforated plate, the rear unit including a base plate that is to be intimately put on a rear surface of the perforated plate and has an opening sized to permit insertion of the holding studs of the front unit, a plurality of connecting studs extending rearward from the base plate and an impinge plate held by leading ends of the connecting studs; and a fiber material piece put on a front surface of the impinge plate, wherein when the front unit is coupled with the rear unit having the holding studs passed through the opening of the base plate, the fiber material piece is sandwiched between the leading ends of the holding studs and the impinge plate. 
         [0010]    In accordance with a second aspect of the present invention, there is provided an oil mist separator which comprises a housing body through which an oil mist containing gas flows, the housing body including upper and lower members that are coupled to constitute therebetween a gas flow passage for the oil mist containing gas and have at respective inner surfaces mounting grooves that constitute an endless mounting groove when the upper and lower members are coupled; and a separator unit installed in the gas flow passage, the separator unit comprising a front unit including a perforated plate and a plurality of holding studs extending rearward from the perforated plate; a rear unit placed behind the perforated plate, the rear unit including a base plate that is to be intimately put on a rear surface of the perforated plate and has an opening sized to permit insertion of the holding studs of the front unit, a plurality of connecting studs extending rearward from the base plate and an impinge plate held by leading ends of the connecting studs; and a fiber material piece put on a front surface of the impinge plate, wherein when the front unit is coupled with the rear unit having the holding studs passed through the opening of the base plate, the fiber material piece is sandwiched between the leading ends of the holding studs and the impinge plate, and wherein outer peripheral portions of the perforated plate and the base plate are put in the endless mounting groove of the housing body. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Other objects and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings, in which: 
           [0012]      FIG. 1  is a longitudinally sectioned view of an oil mist separator of the present invention; 
           [0013]      FIG. 2  is an enlarged sectioned and perspective view of an essential part of the oil mist separator of the present invention, which is taken from a front-side position of the oil mist separator; 
           [0014]      FIG. 3  is a view similar to  FIG. 2 , but which is taken from a rear-side position of the oil mist separator; 
           [0015]      FIG. 4  is a perspective view of a separator unit employed in a first embodiment of the present invention; 
           [0016]      FIG. 5  is a sectioned view of the oil mist separator showing a perforated plate installed therein; 
           [0017]      FIG. 6  is a sectioned view of the oil mist separator showing an impinge plate installed therein; 
           [0018]      FIG. 7  is an enlarged sectioned view of the separator unit taken along the line A-A of  FIG. 1 ; 
           [0019]      FIG. 8  is an enlarged sectioned view of the separator unit taken along the line B-B of  FIG. 1 ; 
           [0020]      FIG. 9  is a longitudinally sectioned enlarged view of the separator unit; 
           [0021]      FIG. 10  is a characteristic graph showing both a pressure loss of the oil mist separator of the present invention with respect to a gas flow rate and for its comparison, that of a comparative example; 
           [0022]      FIG. 11  is a characteristic graph showing an oil mist trapping performance of the oil mist separator of the present invention with respect to the gas flow rate and for its comparison, that of the comparative example; 
           [0023]      FIG. 12  is a characteristic graph showing an oil mist trapping performance of the oil mist separator of the present invention with respect to a pressure flow and for its comparison, that of the comparative example; 
           [0024]      FIG. 13  is a side view of a separator unit employed in a second embodiment of the present invention; 
           [0025]      FIG. 14  is a front view of the separator unit employed in the second embodiment of the present invention; 
           [0026]      FIG. 15  is a sectioned view of the separator unit taken along the line C-C of  FIG. 13 ; and 
           [0027]      FIG. 16  is a sectioned view of the separator unit taken along the line D-D of  FIG. 13 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    In the following, a first embodiment of the present invention will be described in detail with reference to  FIGS. 1 to 9 . 
         [0029]    As is seen from  FIG. 1 , an of mist separator  1  of the present invention comprises an elongate housing part (or upper cover)  2  that has an opened bottom portion, and a separator cover (or lower cover)  3  that is connected to the opened bottom part of the elongate housing part  2 . 
         [0030]    As seen in  FIG. 1 , the elongate housing part  2  is arranged and constructed to form a part of an upper wall of a cylinder head cover  4  of synthetic resin. 
         [0031]    The separator cover  3  is made of synthetic resin and separately molded from the elongate housing part  2 . Bonding and/or welding may be used for connecting the separator cover  3  to the elongate housing part  2 . Although, in the illustrated embodiment, the elongate housing part  2  is constructed to integrally form a part of the cylinder head cover  4 , the housing part  2  may be provided separately from the cylinder head cover  4 . 
         [0032]    In the illustrated embodiment, the oil mist separator  1  is arranged to extend in a direction perpendicular to a cylinder row of an associated internal combustion engine (not shown). In other words, the oil mist separator  1  extends in the width direction of the engine. 
         [0033]    As shown in  FIG. 1 , between the housing part  2  and the separator cover  3 , there is defined an elongate separator chamber  5  that has a rectangular cross section (see  FIG. 5 ). 
         [0034]    At one longitudinal end part of the elongate separator chamber  5 , there is provided a blow-by gas inlet port  6  and at the other longitudinal end part of the separator chamber  5 , there is provided a blow-by gas outlet port  7 . Thus, in operation, the blow-by gas led into the elongate separator chamber  5  from the inlet port  6  is forced to flow longitudinally in the separator chamber  5  toward the outlet port  7 . 
         [0035]    In the illustrated embodiment, the elongate separator chamber  5  is arranged and shaped to extend generally perpendicular to an imaginary plane in and along which respective center axes of the cylinders of the engine extend. However, considering a case in which the engine is mounted in an engine room of a motor vehicle in an inclined attitude, the elongate separator chamber  5  may be arranged and shaped to extend in a direction inclined to the imaginary plane. Preferably, under a rest condition of the motor vehicle, the elongate separator chamber  5  extends in parallel with a horizontal plane. 
         [0036]    In the illustrated embodiment, the blow-by gas inlet port  6  is provided in the separator cover  3 . Although not shown in the drawings, the blow-by gas inlet port  6  is connected to or communicated with an interior of the engine. While, the blow-by gas outlet port  7  is provided in a downstream end wall  8  of the housing part  2 . It is to be noted that the directional terms “upstream” and “downstream” mentioned in the following are to be understood with respect to a normal flow of the blow-by gas in the elongate separator chamber  5 . 
         [0037]    As is seen from  FIG. 1 , at a longitudinally middle portion of the separator chamber  5 , there is arranged a separator unit  10  that is tightly supported between the housing part  2  and the separator cover  3 . As is seen from this drawing, due to provision of this separator unit  10 , the elongate separator chamber  5  is divided into two chambers which are an upstream chamber  11  having the blow-by gas inlet port  6  connected thereto and a downstream chamber  12  having the blow-by gas outlet port  7  connected thereto. At the bottom wall of the downstream chamber  12 , there is integrally provided a drain pipe  13 . More specifically, the drain pipe  13  extends downward from the separator cover  3 . 
         [0038]    Thus, in operation of the engine, entire amount of the blow-by gas is forced to pass through the separator unit  10  during flowing in the elongate separator chamber  5  from the blow-by gas inlet port  6  to the blow-by gas outlet port  7 . As will be described in detail hereinafter, when the blow-by gas passes through the separator unit  10 , oil mist is effectively separated from the blow-by gas. The oil mist or oil thus separated forms oil drops. The oil drops drop onto the bottom wall of the downstream chamber  12  and flow to the drain pipe  13 . The oil thus led to the drain pipe  13  is discharged to the outside of the oil mist separator  1 . The blow-by gas thus free of the oil mist is led to the blow-by gas outlet port  7  and then to an intake system of the associated internal combustion engine for burning unburnt components of the blow-by gas. 
         [0039]    In the following, the detail of the separator unit  10  will be described with reference to the drawings, particularly  FIGS. 2 to 9 . 
         [0040]    As is seen from  FIGS. 2, 3 and 5 , the separator unit  10  generally comprises a rectangular perforated plate  20  of synthetic resin that is formed with six small circular openings (orifices)  21  for increasing the flow speed of the blow-by gas, a rear frame  25  of synthetic resin that is equipped with an impinge plate  26  against which the speed increased blow-by gas from the rectangular perforated plate  20  violently impinge, and a fiber material piece  35  that is seated on the impinge plate  26  for improving the oil mist separation. 
         [0041]    As is seen from  FIGS. 3 and 4 , the rear frame  25  is formed with a base plate  27  that is integrally formed with the impinge plate  26 . As will be described in detail hereinafter, the base plate  27  and the impinge plate  26  are integrally connected to each other through four connecting studs  31 . Due to provision of the four connecting studs  31 , the base plate  27  and the impinge plate  26  are axially spaced from each other while showing a parallel arrangement therebetween. 
         [0042]    As is seen from the drawings, the base plate  27  is rectangular in shape and sized to be neatly and tightly put in the separator chamber  5 . 
         [0043]    As is best seen from  FIG. 4 , the rectangular base plate  27  is formed at its central portion with a circular opening  28  and at lower ends of its side edges with projected portions  29 . 
         [0044]    As is seen from  FIGS. 4 and 6 , the impinge plate  26  is circular and flat in shape and has an outer diameter substantially the same as the diameter of the circular opening  28  of the base plate  27 . The circular impinge plate  26  is formed at its center portion with a small circular opening  30 . 
         [0045]    As is seen from  FIG. 4 , each of the connecting studs  31  has a rectangular cross section and extends in a direction perpendicular to both the rectangular base plate  27  and the circular impinge plate  26 . As shown, a front (or upstream) end of each connecting stud  31  is integrally connected to a peripheral edge of the circular opening  28  and a rear (or downstream) end of the stud  31  is integrally connected to an outer periphery of the circular impinge plate  26 . Due to provision of the four connecting studs  31 , four rectangular connecting openings  32  are provided between the impinge plate  26  and the base plate  27 , as shown. 
         [0046]    As is seen from  FIGS. 4 and 8 , the fiber material piece  35  is shaped circular and has an outer diameter substantially the same as the diameter of the circular opening  28  of the base plate  27 . In other words, the diameter of the circular fiber material piece  35  is substantially the same as that of the circular impinge plate  26 . 
         [0047]    The circular fiber material piece  35  is formed at a central potion thereof with a small circular opening  36  whose diameter is substantially the same as that of the small circular opening  30  of the impinge plate  26 . 
         [0048]    As a material of the circular fiber material piece  35 , polyester fiber, acrylic fiber, aramid fiber, PPS (polyphenylenesulfide) fiber and/or the like is used. Non-woven fabrics, fabrics such as fleece or the like are also usable as the material of the piece  35 . 
         [0049]    As is well seen from  FIGS. 3 and 4 , the rectangular perforated plate  20  has substantially the same shape as the above-mentioned rectangular base plate  27 . Thus, the perforated plate  20  has projected portions  22  at lower ends of its side edges. 
         [0050]    As is seen from  FIG. 5 , the six circular openings (orifices)  21  of the perforated plate  20  are circularly arranged. 
         [0051]    As is best seen from  FIGS. 7 and 9 , the six circular openings  21  are cylindrical holes defined in a cylindrically swelled portion  23  integrally formed on a back surface of the perforated plate  20 . With such cylindrical holes  21 , a straight running stability of the blow-by gas running in the holes  21  is increased. 
         [0052]    As is seen from  FIGS. 4 and 7 , the perforated plate  20  is formed with six holding studs  24  that extend rearward in such a manner as to surround the cylindrically swelled portion  23  (see  FIG. 7 ). Each stud  24  has a circular cross section. 
         [0053]    As will be understood from  FIGS. 4 and 8 , these six holding studs  24  are circularly arranged. For the reasons that will become apparent hereinafter, an imaginary circle defined by most-outer cylindrical parts of the six holding studs has a diameter that is slightly smaller than the diameter of the circular opening  28  of the base plate  27 . With this arrangement, upon assembly, each top of the six holding studs  24  presses an outer peripheral portion of the circular fiber material piece  35  against the impinge plate  26  neatly set in the rear frame  25 . As is seen from  FIG. 4 , each holding stud  24  extends perpendicular from the perforated plate  20  beyond the cylindrically swelled portion  23 . 
         [0054]    The projected part of each holding stud  24  beyond the cylindrically swelled portion  23  is somewhat smaller in length than a distance between the base plate  27  and the impinge plate  26 . With this arrangement, upon assembly, the outer peripheral portion of the circular fiber material piece  35  is suitably pressed by the tops of the six holding studs  24 , and thus, the circular fiber material piece  35  is stably held on the impinge plate  26 . That is, as will be understood from  FIGS. 2 and 3 , when the perforated plate  20  and the base plate  7  are intimately joined for assembling the separator unit  10 , the tops of the six holding studs  24  take given positions to suitably press or compress the outer peripheral portion of the circular fiber material piece  35 . Thus, by varying the length of the projected part of each holding stud  24 , compression ratio of the outer peripheral portion of the circular fiber material piece  35  is varied. The compression ratio is determined considering both a satisfied oil mist separation performance expected from the oil mist separator  1  and an assured holding of the circular fiber material piece  35  on the impinge plate  26 . The six holding studs  24  are equally spaced from one another so as to evenly compress or press the entire of the outer peripheral portion of the circular fiber material piece  35 . 
         [0055]    When the separator unit  10  is properly assembled, the cylindrically swelled portion  23  and the six holding studs  24 , which are integral with and extend from the perforated plate  20 , are neatly received in the circular opening  28  of the base plate  27  and the perforated plate  20  and the base plate  27  are intimately joined as is seen from  FIGS. 1 to 3 . Under this condition, as is seen from  FIG. 3 , the cylindrically swelled portion  23  is positioned away from the circular fiber material piece  35  and the six holding studs  24  press the outer peripheral portion of the circular fiber material piece  35  against the impinge plate  26 . Due to compression of the outer peripheral portion by the studs  24 , the circular fiber material piece  35  is assuredly positioned in a radial direction on the impinge plate  26 . Furthermore, due to the pressing work of the studs  24 , the outer peripheral portion of the circular fiber material piece  35  is compressed at a suitable compression ratio. 
         [0056]    The separator unit  10  thus assembled in the above-mentioned manner is then set in the elongate separator chamber  5  in such a manner as is shown in  FIG. 1 . 
         [0057]    For setting the separator unit  10  in the separator chamber  5 , the housing part  2  and the separator cover  3  are prepared in a separated manner. 
         [0058]    As is seen from  FIGS. 1, 2 and 3 , for setting the separator unit  10  in the separator chamber  5 , the housing part  2  and the separator cover  3  have at their inner walls mounting grooves  2   b  and  3   b  that constitute an endless groove extending in and around a cylindrical inner surface of the separator chamber  5 , as will be understood from  FIGS. 2 and 3 . That is, for forming the groove  2   b , two elongate and spaced banks  2   a  are provided on the inner surface of the housing part  2  and for forming the other groove  3   b , two elongate and spaced banks  3   a  are provided on the inner surface of the separator cover  3 . 
         [0059]    As is seen from  FIG. 2 , for setting the separator unit  10  in the separator chamber  5 , a peripheral portion of the intimately joined flat unit consisting of the perforated plate  20  and base plate  27  is snugly put in the mounting groove  2   b , and then the separator cover  3  is temporally connected to the lower open part of the housing part  2  having a lower part of the peripheral portion of the intimately joined flat unit snugly put in the other mounting groove  3   b . Then, as is mentioned hereinabove, a suitable bonding technique is used for tightly connecting the housing part  2  and the separator cover  3 . 
         [0060]    Once the separator unit  10  is properly set in the separator chamber  5  in the above-mentioned manner, the separator unit  10  is suppressed from making a backlash due to a biasing force produced by the circular fiber material piece  35  that is kept compressed by the holding studs  24 . Of course, the width of the mounting grooves  2   b  and  3   b  is determined so as to suppress or at least minimize the backlash of the separator unit  10 . 
         [0061]    In the following, operation of the oil mist separator  1  will be described with the aid of  FIGS. 1 and 9 . 
         [0062]    As is seen in  FIG. 1  and mentioned hereinabove, under operation of an associated internal combustion engine, the blow-by gas is forced to flow in the separator chamber  5  of the oil mist separator  1  as is indicated by arrows. During this flow, the blow-by gas is treated by the separator unit  10  in the following manner. 
         [0063]    As is seen from  FIG. 9 , the blow-by gas flowing from the blow-by gas inlet port  6  (see  FIG. 1 ) is forced to flow in the six small circular openings (orifices)  21  of the perforated plate  20 . During this, due to reduction in passage area, the flowing speed of the blow-by gas is increased. The blow-by gas thus increased in flowing speed is forced to impinge against the impinge plate  26  after passing through the fiber material piece  35 . During this flow, oil mist in the blow-by gas is mainly separated by the fiber material piece  35  and some of the separated oil mist get stuck onto the impinge plate  26 , and on the impinge plate  26 , the separated oil mist or oil forms larger oil drops. The larger oil drops fall down onto the inner wall of the separator cover  3  by their gravity from lower ends of the fiber material piece  35  and impinge plate  26 , and the oil thus accumulated is forced to flow down toward the drain pipe  13  (see  FIG. 1 ) and drained into the outside of the oil mist separator  1 . The blow-by gas thus free of the oil mist is forced to run in radially outward direction from the four rectangular connecting openings  32  (see  FIG. 3 ) and then toward the blow-by gas outlet port  7  after changing its flowing direction. Part of the oil mist free blow-by gas is forced to flow toward the blow-by gas outlet port  7  through the smaller center opening  36  of the fiber material piece  35  and the smaller center opening  30  of the impinge plate  26 . 
         [0064]    In the following, advantages of the oil mist separator  1  of the present invention will be described. 
         [0065]    As will be understood from  FIGS. 2 and 4 , once the perforated plate  20  and the base plate  27  are intimately and correctly put together, the holding studs  24  press the outer peripheral portion of the circular fiber material piece  35  against the impinge plate  26 . Thus, the circular fiber material piece  35  can be tightly attached to the impinge plate  26  without usage of any bonding material. Due to provision of the four connecting studs  31 , undesired lateral slippage or displacement of the circular fiber material piece  25  on the impinge plate  26  is suppressed or at least minimized. 
         [0066]    As will be understood from  FIGS. 1, 2 and 3 , the separator unit  10  can be easily and tightly set and mounted in the separator chamber  5  by only putting the outer peripheral portions of the intimately jointed two plates (viz., perforated plate  20  and base plate  27 ) into the mounting grooves  2   b  and  3   b . This setting and mounting are made without usage of any bonding material. More specifically, there is no need of bonding the outer peripheral portions of the two plates  20  and  27  to the housing part  2  and separator cover  3 . 
         [0067]    If the separator cover  3  is detachably connected to the housing part  2  by using mechanical binders or the like, the separator unit  10  can be dismounted from the separator chamber  5  with ease. In this case, the circular fiber material piece  35  can be easily replaced with a new one. 
         [0068]    In the above-mentioned first embodiment, the perforated plate  20  has the same size as the base plate  27 . However, if desired, the size of the perforated plate  20  may be reduced to that of the circular fiber material piece  35 . In this case, the base plate  27  is provided with a holder for holding the smaller perforated plate  20 . 
         [0069]    For examining the performance of the oil mist separator of the above-mentioned first embodiment of the present invention, various tests were carried out together with comparative examples. Each comparative example had the same construction as the corresponding oil mist separator of the first embodiment except that in the comparative example, the circular fiber material piece  35  was not employed. 
         [0070]      FIG. 10  shows the results of the test for examining a relationship between a gas flow rate and a pressure loss. As is seen from this graph, the pressure loss of the first embodiment relative to the gas flow rate is clearly low as compared with that of the comparative example. 
         [0071]      FIG. 11  shows the results of the test for examining a relationship between the gas flow rate and an oil mist trapping performance. As is seen from this graph, the oil mist trapping performance of the first embodiment relative to the gas flow rate is clearly high as compared with that of the comparative example. 
         [0072]      FIG. 12  shows the results of the test for examining a relationship between a pressure flow and the oil mist trapping performance. As is seen from this graph, the oil mist trapping performance of the first embodiment relative to the pressure flow is clearly high as compared with that of the comparative example. That is, in the invention, the oil mist trapping performance and the pressure loss performance, which have a so-called trade-off relation to each other, are obtained in a high level of establishment. 
         [0073]    In the following, a separator unit  100  employed in an oil mist separator of a second embodiment of the present invention will be described with reference to  FIGS. 13 to 16 . 
         [0074]    As is seen from these drawings, especially  FIG. 13 , like in the above-mentioned first embodiment 1, the separator unit  100  employed in the second embodiment comprises a perforated plate  120  that has three small circular openings  121  which are a center one and two side ones that are somewhat higher in position than the center one. 
         [0075]    The perforated plate  120  is rectangular in shape and integrally formed with both a rectangularly swelled portion  123  projected rightward in  FIG. 13  from a central portion of the perforated plate  120  and four holding studs  124  projected rightward in  FIG. 13  from four corner portions of the perforated plate  120 . 
         [0076]    As is seen from the drawings, especially  FIG. 13 , the separator unit  100  further comprises a rectangular base plate  127  that is formed at its central portion with a rectangular opening  128 . 
         [0077]    As is seen from  FIGS. 13, 15 and 16 , the rectangular base plate  127  is formed with four connecting studs  131  that extend rightward in  FIG. 13  from four corner portions of the rectangular base plate  127 . As is seen from  FIG. 13 , a rectangular impinge plate  126  is integrally connected to right (or leading) ends of the four connecting studs  131 . Thus, the rectangular base plate  127 , the four connecting studs  131  and the rectangular impinge plate  126  constitute a generally a rectangular parallelepiped body. 
         [0078]    As is seen from  FIG. 15 , the rectangular opening  128  of the rectangular base plate  127  is so sized as to receive the rectangularly swelled portion  123  of the rectangular perforated plate  120  as well as the four holding studs  124 . 
         [0079]    As is seen from  FIG. 13 , upon assembly of the separator unit  100 , a rectangular fiber material piece  135  is put on the rectangular impinge plate  126  and pressed against the impinge plate  126  by the leading ends of the four holding studs  124 . That is, as will be understood from  FIG. 13 , when the perforated plate  120  is intimately mated with the base plate  127  having the four holding studs  124  passed through the rectangular opening  128 , the leading ends of the four holding studs  124  automatically press four peripheral portions of the rectangular fiber material piece  135  against the impinge plate  126 . 
         [0080]    Although not shown in the drawings, the rectangular base plate  127  is provided with a holder for holding the rectangular perforated plate  120 . When the separator unit  100  is set in the separator chamber  5  (see  FIG. 1 ), the outer peripheral portion of the base plate  127  is intimately and snugly put in the mounting grooves  2   b  and  3   b  formed in the housing part  2  and the separator cover  3 . 
         [0081]    The entire contents of Japanese Patent Application 2014-255668 filed Dec. 18, 2014 are incorporated herein by reference. 
         [0082]    Although the invention has been described above with reference to embodiments of the invention, the invention is not limited to such embodiments as described above. Various modifications and variations of such embodiments may be carried out by those skilled in the art, in light of the above description.