Patent Publication Number: US-10774703-B2

Title: Oil mist separator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. § 119 of Japanese Application No. 2016-212238 filed on Oct. 28, 2016, the disclosure of which is expressly incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to an oil mist separator which separates oil mist in blow-by gas generated in an engine. 
     2. Related Art 
     Conventionally, there has been used an oil mist separator which separates oil mist contained in blow-by gas generated in an engine and returns the oil to an oil pan of the engine. Concerning this oil mist separator, for example, the oil mist separator described in JP 2015-4330 A is known, and the oil mist separator described therein comprises a housing which is provided with an upstream-side half-split body in which an inflow port for blow-by gas is formed and a downstream-side half-split body in which an outflow port for blow-by gas is formed, and is configured in such a manner that an oil separating part is held between the upstream-side half-split body and the downstream-side half-split body. 
     However, the oil mist separator described in JP 2015-4330 A has a single structure in which the oil separating part is provided in only one position, and thus oil basically cannot be collected at a collection rate according to pressure loss. Contrary to this, in JP 2009-121281 A, a partition wall is provided in approximately the center in the axial direction of a housing, and an oil separating part is provided on the upstream side and downstream side thereof, respectively, i.e., two oil separating parts are provided in series in a flowing direction of blow-by gas. The oil mist separator provided with the two oil separating parts has a large oil collection rate as compared with that of the oil mist separator of JP 2015-4330 A which is provided with one oil separating part. 
     In the oil mist separator described in JP 2009-121281 A, however, the blow-by gas having flown into the housing is jetted out from an inflow port having a large flow channel area as compared with the nozzle which is provided on the partition wall directly toward an upstream-side primary collision plate, and thus collides with the primary collision plate at a low rate because of its low jetting rate, so that the oil collection efficiency by the oil separating parts is small. 
     Also, the oil mist separator described in JP 2009-121281 A is provided with two oil separating parts, and thus drain pipes for discharging oil are accordingly provided for the respective oil separating parts. Therefore, when the oil mist separator is installed, for example, in a cylinder head cover, the drain pipes sometimes interfere with peripheral members which are disposed below the oil mist separator, thereby causing failure in installation. Therefore, a broad space is sometimes required below the oil mist separator. 
     SUMMARY 
     An object of embodiment of the present invention is to provide an oil mist separator comprising a multiple-structure oil separating part in which the oil collection rate can be increased. In addition to the above-mentioned problem, another object of the embodiment of the present invention is to provide an oil mist separator which can prevent drain pipes from interfering with peripheral members so that the installation space can be reduced. 
     One aspect of the present embodiments provides 1 an oil mist separator comprising: a housing which is provided with an inflow port and an outflow port for blow-by gas and a drain pipe through which oil is discharged; and an oil separating part which is mounted in the housing to separate oil mist in the blow-by gas, wherein the oil separating part includes a plurality of oil separating parts which are disposed in series in a flowing direction of the blow-by gas, and wherein the oil separating parts each comprise: a partition wall which defines chambers formed for the respective oil separating parts in the housing and is provided with nozzles jetting out blow-by gas; and a collision part with which the blow-by gas jetted out from the nozzles collides. 
     In a further aspect, the nozzles may serve also as passages for discharging the oil separated by the oil separating part which is provided in a predetermined one of the chambers toward the drain pipe which is provided in another one of the chambers. 
     In a further aspect, the oil separating parts may include a first oil separating part and a second oil separating part which is arranged on the downstream side of the first oil separating part in a flowing direction of the blow-by gas, wherein the partition plate which constitutes the first oil separating part may be arranged in the vertical direction in the housing, wherein the partition plate which constitutes the second oil separating part may be arranged in the horizontal direction in the housing so that a flow channel is formed between the partition plate and the bottom part of the housing, wherein the drain pipe may be provided in the chamber in which the first oil separating part is provided, and wherein the nozzles of the partition plate which constitutes the second oil separating part may serve also as passages for discharging the oil separated by the second oil separating part toward the drain pipe. 
     In a further aspect, the oil separating parts may include a first oil separating part and a second oil separating part which is arranged on the downstream side of the first oil separating part in a flowing direction of the blow-by gas, wherein the partition plate which constitutes the first oil separating part may be arranged in the vertical direction in the housing, wherein the partition plate which constitutes the second oil separating part may be arranged in the vertical direction in the housing, wherein the drain pipe may be provided in the chamber in which the second oil separating part is provided, and wherein at least one of the nozzles of the partition plate which constitutes the second oil separating part may be provided in the lowermost part of the partition plate and serves also as a passage for discharging the oil separated by the first oil separating part toward the drain pipe. 
     In a further aspect, a flowing resistance part which increases the flowing resistance of the blow-by gas may be provided around the upper side of the drain pipe in order to prevent backflow of the oil discharged into the drain pipe. 
     In a further aspect, the drain pipe may be provided inside with a check valve which prevents backflow of the discharged oil. 
     According to the oil mist separator of this embodiment, the oil separating part includes a plurality of oil separating parts which are disposed in series in a flowing direction of the blow-by gas, and the oil separating parts each comprise: a partition wall which defines chambers formed for the respective oil separating parts in the housing and is provided with nozzles jetting out blow-by gas; and a collision part with which the blow-by gas jetted out from the nozzles collides. Thus, in the respective oil separating parts, the blow-by gas is jetted out from the nozzles formed as quite small openings in the partition wall toward the collision part at a high speed. Thus, the collection rate of the oil contained in the blow-by gas can be increased. 
     When the nozzles serve also as passages for discharging the oil separated by the oil separating part which is provided in a predetermined one of the chambers toward the drain pipe provided in another one of the chambers, the oil separated by the oil separating part and remaining on the bottom surface of the chamber flows through the passages (i.e., nozzles) into the drain pipe which is provided in the other chamber, and is discharged. Thus, no drain pipe has to be installed in the predetermined chamber which is provided with the oil separating part. As a result, it is possible to avoid the interference of the drain pipe with peripheral members located blow the oil mist separator and to reduce the installation space for the oil mist separator. 
     Further, when the oil separating parts include a first oil separating part and a second oil separating part; the partition plate which constitutes the first oil separating part is arranged in the vertical direction in the housing; the partition plate which constitutes the second oil separating part is arranged in the horizontal direction in the housing so that a flow channel is formed between the partition plate and the bottom part of the housing; the drain pipe is provided in the chamber in which the first oil separating part is provided; and the nozzles of the partition plate which constitutes the second oil separating part serve also as passages for discharging the oil separated by the second oil separating part toward the drain pipe, the oil separated by the second oil separating part and remaining on the partition plate flows through the passages (i.e., nozzles) to the flow channel and then into the drain pipe, and is discharged. Thus, no drain pipe has to be installed in the chamber which is provided with the second oil separating part. 
     When the oil separating parts include a first oil separating part and a second oil separating parts; the partition plate which constitutes the first oil separating part is arranged in the vertical direction in the housing; the partition plate which constitutes the second oil separating part is arranged in the vertical direction in the housing; the drain pipe is provided in the chamber in which the second oil separating part is provided; and at least one of the nozzles of the partition plate which constitutes the second oil separating part is provided in the lowermost part of the partition plate and serves also as a passage for discharging the oil separated by the first oil separating part toward the drain pipe, the oil separated by the first oil separating part and retaining on the lower surface of the chamber flows through the passage (i.e., nozzle) into the drain pipe, and is discharged. Thus, no drain pipe has to be installed in the chamber which is provided with the first oil separating part. 
     When a flowing resistance part which increases the flowing resistance of the blow-by gas is provided around the upper side of the drain pipe in order to prevent backflow of the oil discharged into the drain pipe, the oil can be prevented from flowing back and being mixed in the blow-by gas again and returned to the intake system. 
     Further, when the drain pipe is provided inside with a check valve which prevents backflow of the discharged oil, the oil can be prevented from flowing back and being mixed in the blow-by gas again and returned to the intake system, and at the same time, the drain pipe can be shortened, thereby making it possible to avoid the interference of the drain pipe with peripheral members located below the oil mist separator and to reduce the installation space for the oil mist separator. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The present invention is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein: 
         FIG. 1  is a vertical cross-sectional view showing an oil mist separator of an embodiment of the present invention; 
         FIG. 2  is a vertical cross-sectional view showing a body portion of an oil mist separator in a first embodiment; 
         FIG. 3  is a graph of comparison between the oil mist separator of the present invention and a conventional oil mist separator in terms of the collection rate; 
         FIGS. 4A and 4B  show a vertical cross-sectional view for comparison between the conventional oil mist separator and the oil mist separator of the first embodiment in terms of the length of a drain pipe; 
         FIG. 5  is a vertical cross-sectional view showing a body portion of an oil mist separator in a second embodiment; 
         FIG. 6  is a vertical cross-sectional view showing a body portion of an oil mist separator in a third embodiment; and 
         FIG. 7  is a vertical cross-sectional view showing a flowing resistance part provided around the upper side of the drain pipe. 
     
    
    
     DETAILED DESCRIPTION 
     The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description is taken with the drawings making apparent to those skilled in the art how the forms of the present invention may be embodied in practice. 
     First Embodiment 
     First, a detailed description will be given of an oil mist separator of the first embodiment according to the present invention based on drawings. 
     The blow-by gas generated in an engine is returned to an intake system and re-combusted, after separation of the oil contained in the blow-by gas during passage through the oil mist separator. On the other hand, the separated oil is discharged from a drain pipe, and recovered in the engine. The oil mist separator of the first embodiment is provided in the middle of a flow channel for blow-by gas, and installed in a cylinder head cover. The oil mist separator is integrally provided, on the upstream side, with a pre-separator  2  which traps large oil droplets in blow-by gas first by means of an oil separating part  2   a  having a labyrinth structure, as shown in  FIG. 1 . Incidentally, the configuration of a body portion of an oil mist separator, except the pre-separator  2 , will be described in each of the following embodiments. 
       FIG. 2  shows a body portion of an oil mist separator  1 A, and the body portion of the oil mist separator  1 A is integrally bonded to the pre-separator  2 , but for convenience of description, the left and right end parts of the oil mist separator  1 A are designated as an inflow port  12  and an outflow port  13 , respectively, on an axis line schematically shown in  FIG. 2 . The oil mist separator  1 A comprises a housing  11  which is provided with a drain pipe  27  through which oil is discharged and an oil separating part which is disposed in the housing  11  and separates oil mist in the blow-by gas generated in an engine. 
     The housing  11  is formed of a synthetic resin in a cylindrical shape, and comprises the inflow port  12  and the outflow port  13 . The housing  11  is provided inside with a first partition wall  21  in the vertical direction and a second partition wall  31  in the horizontal direction, and thus comparted into an upstream-side first chamber  22  and a downstream-side second chamber  32 . Two oil separating parts are provided; an upstream-side first oil separating part  23  is provided in the first chamber  22 , and a downstream-side second oil separating part  33  is provided in the second chamber  32 . Although the first oil separating part  23  and the second oil separating part  33  are different in terms of the installation direction and the like, they are disposed in series in a flowing direction of blow-by gas as a whole. Hereinafter, the respective members will be further described. 
     The first partition wall  21  is erected vertically so as to block a flow channel for blow-by gas in a position which is spaced apart from the inflow port  12  by a predetermined distance to the downstream side. Further, a first filter  24  which is made of, for example, a fibrous body such as a nonwoven fabric, paper, woven fabric or knitted fabric, a resin continuous foamed body, or a porous material is disposed in the vertical direction in a position which is spaced apart from the first partition wall  21  by a predetermined distance to the downstream side in the first chamber  22 , and a first collision plate  25  is bonded along the back surface, i.e., downstream-side surface of the first filter  24 . A plurality of first nozzles  26  which comprise pores jetting blow-by gas toward the first filter  24  and the first collision plate  25  are provided in a position opposite to the first filter  24  in the first partition wall  21 . The first partition wall  21  and the first filter  24  and first collision plate  25  (exemplified as “collision part” according to the present invention) constitute the impactor filter type first oil separating part  23 . In addition, a drain pipe  27 , which discharges the oil separated by the first oil separating part  23  and remaining in the bottom part of the first chamber  22 , is provided in the bottom part located below the first filter  24  in the first chamber  22 . The drain pipe  27  is provided, in its lower end opening, with a blocking plate which blocks this opening, and a through hole having a small diameter is formed in this blocking plate so that the oil flowing and pooled inside flows downward from this through hole, and is recovered in a drain pan within the engine. 
     A second chamber  32  is formed, via a second partition wall  31 , on the downstream side adjacent to the first chamber  22 . The second partition wall  31  is provided in the horizontal direction in the bottom part of the second chamber  32 , and a flow channel  38  which communicates with the first chamber  22  is formed below the second partition wall  31 . Further, a second filter  34  having the same specification as that of the first filter  24  is disposed in the horizontal direction in a position which is spaced apart upward from the second partition wall  31  by a predetermined distance in the second chamber  32 . Also, a second collision plate  35  is bonded along the back surface, i.e., upper surface of the second filter  34 . The second partition wall  31  blocks a flow channel  38  which is located below the second partition wall  31 , and is provided with a plurality of second nozzles  36  which comprise pores jetting blow-by gas toward the second filter  34  and the second collision plate  35  in a position opposite to the second filter  34 . These second nozzles  36  function also as passages  37  for discharging the oil separated by the second oil separating part  33  toward the drain pipe  27  provided in the first chamber  22 . The second partition wall  31  and the second filter  34  and second collision plate  35  (exemplified as “collision part” according to the present invention) constitute the impactor filter type second oil separating part  33 . A partition  39  is erected on the bottom surface of the second chamber  32  closer to the downstream, and a gap  40  through which blow-by gas passes is formed between the upper end part of the partition  39  and the ceiling surface of the second chamber  32 . 
     Next, separation of the oil in blow-by gas by the oil mist separator  1 A of the first embodiment configured in the above-mentioned manner will be described. 
     The blow-by gas generated in the engine and flowing through the flow channel flows from the inflow port  12  of the oil mist separator  1 A as the body portion into the housing  11 , after separation of large oil droplets in the oil separating part  2   a  of the pre-separator  2 . 
     Then, blow-by gas is jetted out from the first nozzles  26  of the first partition wall  21  toward the first filter  24  and first collision plate  25  in the first oil separating part  23  of the first chamber  22 . At this time, the first nozzles  26  are formed of quite small openings, and thus blow-by gas is accelerated when passing through this portion. Then, blow-by gas collides with the front surface of the first filter  24  which is spaced apart from the first partition wall  21  by a predetermined distance, and a part thereof passes through the inside of the first filter  24 , and collides with the first collision plate  25 . By this collision, the oil mist in blow-by gas is trapped by the first filter  24  and the first collision plate  25 , separated from blow-by gas, and dropped onto the bottom surface of the first chamber  22  along the first filter  24  and the first collision plate  25 . Also, a part of blow-by gas having passed through the inside of the first filter  24  is trapped by the filtration of the first filter  24  while passing therethrough, and similarly dropped onto the bottom surface of the first chamber  22  by its own weight. The oil dropped onto the bottom surface of the first chamber  22  flows into the drain pipe  27  of the first chamber  22 , returns to the oil pan in the engine, and is recovered. Thus, a considerable portion of the oil is separated from the blow-by gas flowing in the first chamber  22  by the first oil separating part  23 . On the other hand, the blow-by gas having collided with the first filter  24  and the first collision plate  25  and the blow-by gas flowing around to the lower side without colliding with these members flow from the bottom part of the first chamber  22  to the lower side of the second partition wall  31  of the second chamber  32  along the flow channel  38 . 
     Next, the blow-by gas flowing to the side of the second chamber  32  is jetted out from the second nozzles  36  in the second partition wall  31  arranged in the horizontal direction toward the second filter  34  and the second collision plate  35  located above. Then, the blow-by gas collides with the lower surface of the second filter  34  in a similar manner as the first oil separating part  23 , and at the same time, a part thereof passes through the inside of the second filter  34 , and collides with the second collision plate  35 . Thus, the oil mist remaining in the blow-by gas from which a considerable amount of the oil mist has been separated and removed by the first oil separating part  23  is trapped by the second oil separating part  33 , separated from the blow-by gas, dropped onto the upper surface of the second partition wall  31 , and pooled in this portion. Also, the blow-by gas having collided with the second filter  34  and the second collision plate  35  and the blow-by gas flowing around to the side without colliding with these members flow out of the outflow port  13  through the gap  40  formed between the ceiling surface of the second chamber  32  and the upper end part of the partition  39 , and are returned to the intake system of the engine. 
     On the other hand, the oil dropped onto the second partition wall  31  in the second chamber  32  and pooled during operation of the engine is not sucked into the second chamber  32  by negative pressure because the negative pressure in the second chamber  32  becomes equal to the atmospheric pressure when the engine stops, passes through the inside of the second nozzles  36  which function also as the passages  37  in the second partition wall  31  and falls into the lower flow channel  38  which communicates with the first chamber  22 , by its own weight, further flows through the flow channel  38  into the drain pipe  27  which is provided in the bottom part of the first chamber  22 , and is returned in the engine. In other words, the oil separated by the first oil separating part  23  and the oil separated by the second oil separating part  33  join together and are discharged to the drain pipe  27  which is provided in the first chamber  22 . In the drawing, dotted lines indicate oil flow. 
     In this way, the oil mist separator  1 A in which the oil in blow-by gas is separated by the twin oil separating part structure provides an equivalent or higher collection rate as compared with a conventional oil mist separator in which only one oil separating part is arranged in a housing, as shown in  FIG. 3 .  FIG. 3  shows results obtained by measuring the collection rates for the respective particle diameters of particles collected by the entire oil mist separator under the condition of the same pressure loss for an oil mist separator comprising only one oil separating part and an oil mist separator comprising two oil separating parts. As is evident from  FIG. 3 , both of the oil mist separators provide an approximately equivalent collection rate in a region with a large particle diameter, but the oil mist separator comprising two oil separating parts provides a larger collection rate in any other particle diameter regions. 
     Next, the drain pipe  27  provided in the first chamber  22  will be compared, in terms of length, with a conventional drain pipe including one oil separating part. 
     Since the pressure on the outflow side of the oil mist separator is negative pressure during operation of the engine, the oil discharged in the drain pipe may be sucked into the housing, mixed in blow-by gas again and brought out through the outflow port  13  to the intake system. Therefore, the drain pipe is set to have a length which allows the drain pipe to store therein a predetermined volume of oil and prevents oil from being mixed in blow-by gas again by suction by negative pressure, in consideration of the pressure difference between the pressure by the load of the oil and the negative pressure on the discharge side. Since the negative pressure on the outflow side becomes larger in proportion to the pressure loss level of the oil mist separator, it is necessary to elongate the drain pipe accordingly for increasing the volume of oil to be stored. 
     Now, the drain pipe provided in the conventional oil mist separator comprising only one oil separating part is assumed to be set so as to have a length corresponding to the pressure loss of the entire oil mist separator, i.e., a length L 1  as shown in  FIG. 4A . On the other hand, the drain pipe  27  provided only in the first chamber  22  of the oil mist separator  1 A of the first embodiment according to the present invention has a length corresponding to the pressure loss of the first oil separating part  23  of the first chamber  22 . Assuming that the first oil separating part  23  and the second oil separating part  33  are set to have the same specification, the pressure loss of the first oil separating part  23  is half the pressure loss of the entire oil mist separator  1 A. Thus, as shown in  FIG. 4B , when the length of the drain pipe  27  of the first embodiment according to the present invention is defined as L 2 , L 2 =L 1 ×(½) is satisfied. A half length of the drain pipe of the conventional oil mist separator is enough for the drain pipe  27 . 
     Next, the action of the above-mentioned oil mist separator  1 A of the first embodiment will be described. 
     The oil mist separator  1 A includes two oil separating parts which are disposed in series in a flowing direction of blow-by gas, and thus has an increased oil collection rate and an improved collection performance under the condition of the same pressure loss, as compared with the oil mist separator which is provided with one oil separating part, as mentioned above. 
     In addition, the second nozzles  36  which jet out blow-by gas are provided in the second partition wall  31  on the downstream side. The second nozzles  36  function also as the passages  37  for discharging the oil separated by the second oil separating part  33  toward the drain pipe  27  which is provided in the first chamber  22 , and thus the oil separated by the second oil separating part  33  and remaining on the upper surface of the second partition wall  31  which serves also as the bottom surface of the second chamber  32  passes through the second nozzles  36  which serve also as the passages  37 , and is guided into the drain pipe  27  which is provided in the first chamber  22 , and discharged, when the engine stops or the flow rate of blow-by gas is lowered. Thus, the drain pipe  27  does not have to be installed in the second chamber  32  in which the second oil separating part  33  is provided, and one drain pipe is enough in the entire oil mist separator  1 A whereas the conventional oil mist separator comprising two oil separating parts requires two drain pipes. As a result, if a drain pipe is also installed in the second chamber  32 , it is possible to avoid the interference of this drain pipe with peripheral members located below the oil mist separator  1 A, and to reduce the installation space for the oil mist separator  1 A. Further, the number of drain pipes can be decreased to one, with the result that reduction in cost and simplification of the configuration of the oil mist separator  1 A are achieved. 
     Also, since the oil mist separator  1 A comprises two oil separating parts, a half length of the drain pipe of the conventional oil mist separator comprising one oil separating part is enough for the drain pipe  27  in the first chamber  22 . Thus, it is possible to reduce and save the space occupied by the drain pipe  27 , and to reduce the interference between the drain pipe  27  and the peripheral members located below. 
     The oil mist separator  1 A of the above-mentioned embodiment includes impactor filter type oil separating parts which are built in the housing  11 , but the present invention can similarly employ impactor type oil separating parts which cause blow-by gas jetted out from the nozzles at a high speed to collide with a collision wall (exemplified as “collision part” according to the present invention) to trap and separate the oil contained therein. 
     Second Embodiment 
     Then, an oil mist separator of the second embodiment according to the present invention will be described. The oil mist separator of the second embodiment is identical with the oil mist separator  1 A of the first embodiment in terms of comprising two oil separating parts and one drain pipe, but is different therefrom, for example, in terms of the position where the drain pipe is installed. 
     In  FIG. 5 , a housing  11  of an oil mist separator  1 B is provided with a first partition wall  21  in the vertical direction on the upstream side and a second partition wall  31  similarly in the vertical direction on the downstream side, and thus comparted into an upstream-side first chamber  22  and a downstream-side second chamber  32 . An oil separating part is composed of an upstream-side first oil separating part  23  provided in a first chamber  22  and a downstream-side second oil separating part  33  provided in a second chamber  32 , which are disposed in series along a flowing direction of blow-by gas. 
     More specifically, the first partition wall  21  is erected vertically so as to block a flow channel for blow-by gas in a position which is spaced apart from an inflow port  12  by a predetermined distance to the downstream side. Also, a first filter  24 , which is similar to the filter of the first embodiment, is disposed in the vertical direction in a position which is spaced apart from the first partition wall  21  by a predetermined distance to the downstream side in the first chamber  22 , and further, a first collision plate  25  is bonded along the back surface, i.e., downstream-side surface of the first filter  24 . A plurality of first nozzles  26  which jet blow-by gas toward the first filter  24  and the first collision plate  25  are provided in a position opposite to the first filter  24  in the first partition wall  21 . The first partition wall  21  and the first filter  24  and first collision plate  25  (exemplified as “collision part” according to the present invention) constitute the impactor filter type first oil separating part  23 . A gap  28 , through which the blow-by gas jetted out from the first nozzles  26  in the first partition wall  21  and colliding with or passing through the first filter  24  and the blow-by gas flowing around to the lower side of the first filter  24  without colliding with or passing through the first filter  24  flow, is provided between the lower end part of the first filter  24  and the bottom surface of the first chamber  22 . 
     A second partition wall  31 , which is provided in the downstream-side second chamber  32  adjacent to the first chamber  22 , is erected vertically so as to block the flow channel in a position which is spaced apart from the first collision plate  25  by a predetermined distance. Also, a second filter  34 , which is made of a material similar to that for the first filter  24 , is disposed in the vertical direction in a position which is spaced apart from the second partition wall  31  by a predetermined distance to the downstream side in the second chamber  32 , and further, a second collision plate  35  is bonded along the back surface of the second filter  34 . A plurality of second nozzles  36  which jet out blow-by gas toward the second filter  34  and the second collision plate  35  are provided in a position opposite to the second filter  34  in the second partition wall  31 , and at least one of these second nozzles  36  is provided in the lowermost part of the second partition wall  31 . This second nozzle  36  functions also as a passage  37  for discharging the oil separated by the upstream-side first oil separating part  23  toward a drain pipe  41  which is provided in the downstream-side second chamber  32  (which will be described later), as with the second nozzle  36  which is provided in the second partition wall  31  of the first embodiment. The second partition wall  31  and the second filter  34  and second collision plate  35  (exemplified as “collision part” according to the present invention) constitute the impactor filter type second oil separating part  33 . 
     A gap  42 , through which the blow-by gas jetted out from the second nozzles  36  in the second partition wall  31  and colliding with or passing through the second filter  34  and the blow-by gas flowing around to the lower side of the second filter  34  without colliding with or passing through the second filter  34  flow, is provided between the lower end part of the second filter  34  and the bottom surface of the second chamber  32 . Further, a drain pipe  41  which discharges the oil separated by the second oil separating part  33  is provided vertically downward at the outflow-side end part in the bottom surface of the second chamber  32 . 
     The thus-configured oil mist separator  1 B of the second embodiment is not provided with any drain pipe in the first chamber  22 , but the second nozzle  36 , which is formed in the lowermost part of the second partition wall  31 , functions also as the passage  37  through which the separated oil flows. Hence, the oil dropped onto the bottom surface of the first chamber  22  and pooled during operation of the engine is not sucked by negative pressure in the first chamber  22 , flows along the bottom surface of the first chamber  22  to the downstream side by its own weight, passes through the inside of the second nozzle  36  which serves also as the passage  37  in the lowermost part of the second partition wall  31 , is guided into the second chamber  32 , further flows on the bottom surface of the second chamber  32  to the downstream side, and flows into the drain pipe  41 , when the engine stops. In other words, the oil separated by the first oil separating part  23  and the oil separated by the second oil separating part  33  join together and are discharged to the drain pipe  41  which is provided in the second chamber  32 . 
     By virtue of this, the drain pipe  41  does not have to be installed in the first chamber  22  which is provided with the first oil separating part  23 . As a result, as with the oil mist separator of the first embodiment, if a drain pipe is also installed in the first chamber  22 , it is possible to avoid the interference of this drain pipe with peripheral members located below the oil mist separator  1 B, and to reduce and save the installation space for the oil mist separator  1 B. Further, the number of drain pipes can be decreased to one, with the result that reduction in cost and simplification of the configuration of the oil mist separator  1 B are achieved. Further, in general, among peripheral members of the engine which are located below the oil mist separator, a large number thereof are located immediately below the center part of the oil mist separator, and a small number thereof are located on the end part side of the oil mist separator. However, the drain pipe  41  of the second embodiment is provided at the outflow-side end part of the second chamber  32 , and thus the interference with the peripheral members of the engine can be further reduced. 
     Third Embodiment 
     Next, an oil mist separator of the third embodiment according to the present invention will be described. The oil mist separator of the third embodiment comprises two oil separating parts as with the oil mist separators of the first and second embodiments, but is different therefrom, for example, in that two drain pipes are provided. 
     In  FIG. 6 , a housing  11  of an oil mist separator  1 C is provided inside with a first partition wall  21  in the vertical direction on the upstream side and a second partition wall  31  in the vertical direction on the downstream side, and thus comparted into an upstream-side first chamber  22  and a downstream-side second chamber  32 , as with the second embodiment. An oil separating part is composed of an upstream-side first oil separating part  23  provided in the first chamber  22  and a downstream-side second oil separating part  33  provided in the second chamber  32 , which are disposed in series along a flowing direction of blow-by gas. The respective oil separating parts have specification and configuration similar to those of the respective oil separating parts of the second embodiment. 
     On the other hand, the oil mist separator  1 C of the third embodiment is provided with a drain pipe in the bottom part of the first chamber  22  and the bottom part of the second chamber  32 , respectively. Further, a drain pipe  41  in the second chamber  32  is provided with a check valve  43  which prevents backflow of the oil once discharged in the drain pipe  41 . The drain pipe  41  is provided, in its lower end opening, with a blocking plate which blocks this opening, and a through hole having a small diameter is formed in this blocking plate. A check valve  43  (not shown) is mounted on the blocking plate so as to hold this blocking plate vertically. Further, in the check valve  43 , an opening/closing valve which freely blocks the through hole to be openable/closable is mounted in the lower portion of the blocking plate. When the pressure on the side of the oil mist separator  1 C is negative pressure, the valve body, which is made of a soft material, of the opening/closing valve is sucked to the blocking plate side and blocks the through hole. When the pressure within the oil mist separator  1 C is equal to the atmospheric pressure upon stop of the engine, the valve body of the opening/closing valve recovers in a direction spaced apart from the blocking plate and brings the through hole in an open state. Hence, the oil pooled in the drain pipe  41  flows down from the through hole by its own weight. By virtue of this, the oil once separated and recovered during operation of the engine is prevented from flowing back from the drain pipe  41  and being mixed in blow-by gas again and returned to the intake system. 
     Also, a plurality of second nozzles  36  are formed opposite to the second filter  34  in the second partition wall  31 , but no second nozzle  36  is provided in the lowermost part of the second partition wall  31 . The oil mist separator  1 C of the third embodiment is different from the oil mist separator  1 B of the second embodiment in these respects. 
     In the thus-configured oil mist separator  1 C of the third embodiment, blow-by gas is jetted at a high speed from the nozzles in the respective partition walls which constitute the oil separating part toward the respective filters and collision plates so that the oil in the blow-by gas is separated, as with the oil mist separators of the first and second embodiments. 
     Incidentally, the check valve  43  is provided on the side of the second chamber  32  in the drain pipe  41 , but does not have to be provided. In the case where no check valve  43  is provided, assuming that the first oil separating part  23  and the second oil separating part  33  have similar specification, the drain pipe  41  on the side of the second chamber  32  has a length corresponding to the pressure loss of the entire oil mist separator  1 C, and thus is twice as long as the drain pipe  27  on the side of the first chamber  22 . Here, a considerable amount of the oil contained in blow-by gas is separated in the first oil separating part  23 , and a small amount of oil mist remains in the blow-by gas flowing into the second chamber  32 . Thus, the drain pipe  41  on the side of the second chamber  32  would actually be short. The check valve  43  may be provided also in the drain pipes of the other embodiments. 
     In the above-mentioned respective embodiments, a flowing resistance part  51  which increases the pressure loss of blow-by gas and also increases the flowing resistance may be provided around the upper side of the discharge port of the drain pipe, in order to prevent backflow of the oil discharged into the drain pipe. Specifically, the flowing resistance part  51  can be formed, for example, by adding a horizontal plate  52  having a flowing hole  53  for blow-by gas below the second chamber  32  around the upper side of the discharge port of the drain pipe  41 , as shown in  FIG. 7 , or adding a labyrinth structure in which a plurality of partition plates are alternately disposed in the flow channel, though not shown. 
     Further, the oil mist separators of the above-mentioned respective embodiments have a twin structure in which two oil separating parts are disposed in series in a flowing direction of blow-by gas, but the present invention, when implemented, may employ a structure in which three or more oil separating parts are disposed. 
     Furthermore, the first oil separating part  23  and the second oil separating part  33  of the respective embodiments are formed in the same specification or of the same material, and employ the same oil separation system, but may be different from each other. 
     In addition, the respective partition walls of the respective embodiments are provided with a plurality of nozzles, but may be provided with only one nozzle. 
     A drain pipe for discharging the trapped oil into the engine may be provided also in the pre-separator  2 , as shown in  FIG. 1 , which is provided adjacent to the upstream side of each of the oil mist separators and traps large oil droplets in blow-by gas. 
     Also, the oil mist separators of the respective embodiments are installed in the cylinder head cover, but the oil mist separator of the present invention can similarly be applied to structures in which it is installed externally in a flow channel outside the engine. 
     It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to exemplary embodiments, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular structures, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims. 
     The present invention is not limited to the above-described embodiments, and various variations and modifications may be possible without departing from the scope of the present invention. 
     The present invention is widely utilized as a technique of separating oil mist in blow-by gas generated in an engine.