Patent Publication Number: US-2017356316-A1

Title: Gas-liquid separation device for blow-by gas in engine

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2016-115970 filed on Jun. 10, 2016. The entire contents of this application are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     As a gas-liquid separation device for blow-by gas in a vehicle engine, conventionally, there exists a device provided in a head cover as described in, for example, Japanese Patent No. 2647951. The engine disclosed in Japanese Patent No. 2647951 is mounted in the engine room of a vehicle. The engine is mounted on a vehicle body in a state in which a cylinder head is located above a cylinder block. 
     The cylinder head of the engine includes a valve gear including an intake valve, an exhaust valve, an intake camshaft, an exhaust camshaft, and the like. The valve gear is housed in a valve gear chamber between the cylinder head and the head cover. The intake valve and the exhaust valve extend through the bottom of the valve gear chamber. 
     A gas-liquid separation device for blow-by gas is provided on a portion of the head cover facing the bottom of the valve gear chamber and corresponding to an intake camshaft. The gas-liquid separation device for blow-by gas includes a gas-liquid separation chamber separated from the valve gear chamber and extending parallel to the camshaft, and a plurality of partition walls that define a blow-by gas passage having a serpentine shape in the gas-liquid separation chamber. The blow-by gas inlet of the gas-liquid separation chamber defines an opening in the upper portion of the valve gear chamber. On the other hand, the blow-by gas outlet of the gas-liquid separation chamber is connected to an intake air passage via a pipe. 
     The blow-by gas inlet of the gas-liquid separation chamber defines an opening in the upper portion of the valve gear chamber in a direction toward the cylinder head. For this reason, the negative pressure (to be referred to as an intake negative pressure hereinafter) in the intake air passage is propagated to the blow-by gas passage in the gas-liquid separation chamber via the pipe, and blow-by gas in the valve gear chamber is drawn from the blow-by gas inlet to the gas-liquid separation chamber. 
     The intake negative pressure is also propagated from the gas-liquid separation chamber to a crank chamber on the cylinder block side via the valve gear chamber. For this reason, the blow-by gas in the crank chamber and mist-like oil particles (to be referred to as an oil mist hereinafter) mixed into the blow-by gas are drawn from the cylinder block side to the valve gear chamber by the negative intake pressure. Due to the momentum of suction to the valve gear chamber, the blow-by gas and the oil mist tend to directly move upward in the valve gear chamber and stay near the blow-by gas inlet of the gas-liquid separation chamber. 
     On the other hand, in the valve gear chamber, a camshaft rotates at a high speed, and an oil mist is scattered from the valve gear chamber by centrifugal force. A portion of the scattered oil mist is scattered in the vicinity of the blow-by gas inlet in the upper portion of the valve gear chamber, and drawn into the gas-liquid separation chamber via the blow-by gas inlet together with the blow-by gas and oil mist from the cylinder block side. 
     The blow-by gas drawn into the gas-liquid separation chamber flows through the blow-by gas passage having a serpentine shape. At this time, oil contained in the blow-by gas adheres to the wall surfaces or partition walls of the blow-by gas passage, and the oil is captured. The oil is able to flow from an oil return hole in the bottom of the gas-liquid separation chamber to the valve gear chamber. 
     In the gas-liquid separation device for blow-by gas in the engine shown in Japanese Patent No. 2647951, the amount of oil mist contained in the blow-by gas drawn into the gas-liquid separation chamber is large. The amount of oil mist probably increases because of the following two phenomena. As the first phenomenon, the oil mist drawn from the cylinder head side to the valve gear chamber readily stays on the upper side of the valve gear chamber. As the second phenomenon, the oil mist that has scattered according to the rotation of the camshaft is also drawn into the blow-by gas inlet. 
     As a result, in the gas-liquid separation device for blow-by gas in the engine shown in Japanese Patent No. 2647951, the gas-liquid separation chamber needs to be bulky to remove the large amount of oil mist drawn into the gas-liquid separation chamber. There are two reasons. As the first reason, a gas-liquid separation chamber needs to have a large volume to provide a long blow-by gas passage. As the second reason, a complex gas-liquid separation structure needs to be used. 
     However, in the engine mounted in the engine room of a vehicle, since the height of the head cover is limited by the engine hood, there is a limitation in provided a large gas-liquid separation chamber. In addition, since the complex gas-liquid separation structure requires many parts and man-hours to assemble, the manufacturing cost is high. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide a gas-liquid separation device for blow-by gas in an engine that is able to remove an oil mist contained in the blow-by gas while using a compact and simple gas-liquid separation structure. 
     According to a preferred embodiment of the present invention, a gas-liquid separation device for blow-by gas in an engine includes a head cover that defines a valve gear chamber together with a cylinder head, a gas-liquid separation chamber in the head cover and including a blow-by gas inlet and a blow-by gas outlet, a partition wall in the gas-liquid separation chamber to change a direction of blow-by gas that flows between the blow-by gas inlet and the blow-by gas outlet, and a blow-by gas suction passage connected to the blow-by gas inlet and extending from the gas-liquid separation chamber to the valve gear chamber, wherein an upstream end of the blow-by gas suction passage defines an opening in a vicinity of or adjacent to a bottom wall of the valve gear chamber. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view showing an engine including a gas-liquid separation device for blow-by gas in an engine according to a preferred embodiment of the present invention in a state in which a cylinder head is cut away, and a crank case is not illustrated. 
         FIG. 2  is a side view of the engine. 
         FIG. 3  is a plan view of a cylinder head, which illustrates only the two ends of the cylinder head. 
         FIG. 4  is a sectional view of a gas-liquid separation chamber viewed from the axial direction of a crankshaft, a cutaway position thereof is a position indicated by a line IV-IV in  FIG. 3 . 
         FIG. 5  is a sectional view of the main portion in  FIG. 2  taken along a line V-V. 
         FIG. 6  is a sectional view of the gas-liquid separation device in  FIG. 2  taken along a line VI-VI. 
         FIG. 7  is a longitudinal sectional view of the gas-liquid separation device along line VII-VII in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A gas-liquid separation device for blow-by gas in an engine according to preferred embodiments of the present invention will now be described in detail with reference to  FIGS. 1 to 7 . 
     An engine  1  shown in  FIG. 1  is preferably a DOHC 4-cylinder engine, for example, mounted on a vehicle. As shown in  FIG. 2 , the engine  1  includes a cylinder block  3  with four cylinder holes  2 , a cylinder head  4  attached to the upper end of the cylinder block  3 , a transmission case  5  attached to front ends of the cylinder block  3  and the cylinder head  4 , an oil pan  7  attached to a crank case  6  at the lower end of the cylinder block  3 , a head cover  8  attached to the upper end of the cylinder head  4 , and a gas-liquid separation device  9  for blow-by gas in the head cover  8 . 
     A piston  10  is movably fitted in each of the four cylinder holes  2 . Each piston  10  is connected to a crankshaft  12  via a connecting rod  11 . As shown in  FIG. 1 , the crankshaft  12  is housed in a crank chamber  13  located between the crank case  6  and the oil pan  7  and rotatably supported by the crank case  6  (cylinder block  3 ). An explanation will be made while defining one end side in the axial direction of the crankshaft  12  where the transmission case  5  is located on the front end of the engine  1 . In addition, a direction perpendicular to the sheet surface of  FIG. 1 , that is, a direction perpendicular to an axis C 1  of the crankshaft  12  and a cylinder axis C 2  will be referred to as the left-and-right direction of the engine  1 . 
     As shown in  FIG. 1 , the cylinder head  4  includes a concave portion  14  that defines a combustion chamber, and is located at a position corresponding to the cylinder hole  2  of each cylinder. The cylinder head  4  also includes an intake port  15  and an exhaust port  16  that define openings in the concave portion  14 . In addition, the cylinder head  4  includes an intake valve  17  and an intake camshaft  18 , and an exhaust valve  19  and an exhaust camshaft  20 . The intake valve  17  is driven by the intake camshaft  18  to open/close the intake port  15 . The exhaust valve  19  is driven by the exhaust camshaft  20  to open/close the exhaust port  16 . 
     The engine  1  according to the present preferred embodiment is mounted in the engine room of a vehicle body (not shown) at an incline such that the intake camshaft  18  is located higher than the exhaust camshaft  20 . 
     The other end (upstream end) of the intake port  15  is connected to an intake device  21  on one side (the right side as viewed from the front of the engine) of the cylinder head  4  in the left-and-right direction. The intake device  21  includes a throttle valve  23  connected to the upstream end of the intake port  15  via a downstream intake air passage  22 , and an air cleaner  25  connected to the throttle valve  23  via an upstream intake air passage  24 . The throttle valve  23  is opened/closed in synchronism with an accelerator operation performed by the driver. 
     The other end (downstream end) of the exhaust port  16  is connected to an exhaust device (not shown) on the other side of the cylinder head  4  in the left-and-right direction. 
     The intake valve  17  and the exhaust valve  19  preferably each include two valves per cylinder, for example. The intake valves  17  and exhaust valves  19  are inserted into intake valve holes  26  and exhaust valve holes  27  in the cylinder head  4  as shown in  FIG. 3 , and are movably supported therein. The holes  26  and  27  are located in a bottom wall  28  (to be described below) of the cylinder head  4 . 
     As shown in  FIG. 2 , the intake camshaft  18  and the exhaust camshaft  20  extend parallel or substantially parallel to the crankshaft  12  and are rotatably supported by the cylinder head  4  with front ends (the ends on the front end of the engine) projecting from the cylinder head  4 . The intake camshaft  18  and the exhaust camshaft  20  are supported by a plurality of journals  29  (see  FIG. 3 ) provided in the cylinder head  4 . The journals  29  are located where the intake camshaft  18  and the exhaust camshaft  20  project from the cylinder head  4  to the front end of the engine, between each pair of intake valve holes  26  in each cylinder, and between each pair of exhaust valve holes  27  in each cylinder. 
     The front ends of the intake camshaft  18  and the exhaust camshaft  20  projecting from the cylinder head  4  are connected to the crankshaft  12  via a transmission gear  31 , as shown in  FIG. 2 . The transmission gear  31  transmits a rotation using a wrap transmission member  32  such as a timing chain or a timing belt. The transmission gear  31  is housed in the transmission case  5  attached to front ends of the cylinder block  3  and the cylinder head  4 . The transmission case  5  preferably is box shaped or substantially box shaped and defines openings in two directions, that is, toward the upper side and toward the rear end of the engine. The opening of the transmission case  5  in the direction toward the upper side is closed by the head cover  8  (to be described below). The opening of the transmission case  5  in the direction toward the rear end of the engine is closed by the cylinder block  3  and the cylinder head  4 . 
     As shown in  FIG. 4 , the cylinder head  4  is fixed to the cylinder block  3  by a plurality of head bolts  33 , for example, extending through the bottom wall  28 . The head bolts  33  are screwed into the cylinder block  3  and extend through the bottom wall  28 . The head bolts  33  are provided at a plurality of locations along the axial direction (the left-and-right direction in  FIG. 3 ) of the crankshaft  12 , as shown in  FIG. 3 , and on both sides in the left-and-right direction (the vertical direction in  FIG. 3 ) perpendicular or substantially perpendicular to the axial direction of the crankshaft  12  and the cylinder axis C (see  FIG. 1 ). The above-described plurality of locations in the axial direction of the crankshaft  12  include locations at the two ends of the cylinder head  4  and locations between the cylinders. 
     The cylinder head  4  according to the present preferred embodiment includes the bottom wall  28 , an intake-side vertical wall  34  extending upward from the bottom wall  28  and surrounding the intake camshaft  18 , an exhaust-side vertical wall  35  extending upward from the bottom wall  28  and surrounding the exhaust camshaft  20 , and a ceiling wall  36  that connects the vertical walls  34  and  35 , as shown in  FIG. 4 . The intake-side vertical wall  34  and the exhaust-side vertical wall  35  are preferably integral with the bottom wall  28 . The head cover  8  (to be described below) is attached to the upper ends of the intake-side vertical wall  34  and the exhaust-side vertical wall  35 . The bottom wall  28  and the intake-side vertical wall  34  define an intake-side valve gear chamber  37  together with the head cover  8 . The bottom wall  28  and the exhaust-side vertical wall  35  define an exhaust-side valve gear chamber  38  together with the head cover  8 . 
     As shown in  FIGS. 3 and 5 , openings  41   a  at the downstream end of a communication path  41  are located adjacent to the head bolts  33  in a portion of the bottom wall  28  of the cylinder head  4  defining the intake-side valve gear chamber  37 . The openings  41   a  at the downstream end of the communication path  41  are provided at a plurality of positions adjacent to the head bolts  33  between the cylinders. As shown in  FIG. 2 , the communication path  41  includes first to third branches  42  to  44  and an upstream portion  45 . The first to third branches  42  to  44  extend parallel or substantially parallel to the cylinder axis C 2  from the openings  41   a  at the downstream end to the side of the cylinder block  3 . The upstream portion  45  connects the first to third branches  42  to  44  and extends in the cylinder block  3  toward the crank chamber  13 . 
     As shown in  FIG. 5 , the upstream portion  45  is located along one side wall  46  of the cylinder block  3  in the left-and-right direction. The width of the upstream portion  45  in the axial direction of the crankshaft  12  is larger than the width of the engine  1  in the left-and-right direction (the thickness direction of the side wall  46 ). The upstream end (the lower end in  FIG. 5 ) of the upstream portion  45  defines an opening in the crank chamber  13 . For this reason, the intake-side valve gear chamber  37  communicates with the crank chamber  13  via the communication path  41 . 
     An oil return passage  48  is provided on the other side wall  47  of the cylinder block  3 . The lower end of the oil return passage  48  defines an opening in the crank chamber  13 . The upper portion of the oil return passage  48  extends from the cylinder block  3  into the cylinder head  4 . The upper end of the oil return passage  48  defines an opening in a portion of the bottom wall  28  of the cylinder head  4  defining the exhaust-side valve gear chamber  38 . Openings  48   a  at the upper end of the oil return passage  48  are provided in the bottom wall  28  between the cylinders and at the two ends of the crankshaft  12  in the axial direction, as shown in  FIG. 3 . The openings  48   a  are located at the lowermost positions in the valve gear chambers (the intake-side valve gear chamber  37  and the exhaust-side valve gear chamber  38 ) between the cylinder head  4  and the head cover  8 . 
     The head cover  8  is preferably molded into a predetermined shape using a plastic material, for example. In the head cover  8 , as shown in  FIG. 4 , an intake-side cover  51  connected to the intake-side vertical wall  34 , an exhaust-side cover  52  connected to the exhaust-side vertical wall  35 , and a case cover  53  (see  FIG. 2 ) connected to the upper end of the transmission case  5  are integral with each other. 
     The intake-side cover  51  defines the intake-side valve gear chamber  37  together with the intake-side vertical wall  34  and the bottom wall  28  of the cylinder head  4 . The exhaust-side cover  52  defines the exhaust-side valve gear chamber  38  together with the exhaust-side vertical wall  35  and the bottom wall  28  of the cylinder head  4 . As shown in  FIG. 4 , the intake-side valve gear chamber  37  and the exhaust-side valve gear chamber  38  communicate with each other via a communication chamber  54  located between the ceiling wall  36  and the bottom wall  28  of the cylinder head  4 . In the present preferred embodiment, a series of spaces defined by the intake-side valve gear chamber  37 , the exhaust-side valve gear chamber  38 , and the communication chamber  54  define the valve gear chamber. 
     As shown in  FIG. 2 , the transmission case cover  53  of the head cover  8  is connected to the upper end of the transmission case  5 . A transmission chamber  55  surrounded by the transmission case  5 , the cylinder head  4 , and the cylinder block  3  is located under the transmission case cover  53 . The transmission gear  31  that transmits the rotation of the crankshaft  12  to the intake camshaft  18  and the exhaust camshaft  20  is housed in the transmission chamber  55 . The front end of the cylinder head  4  on the front end of the engine is connected to the transmission chamber  55 . The transmission chamber  55  communicates with the intake-side valve gear chamber  37  via the space above the intake camshaft  18  and also communicates with the exhaust-side valve gear chamber  38  via the space above the exhaust camshaft  20 . Hence, the ends of the intake-side valve gear chamber  37  and the exhaust-side valve gear chamber  38  on the front end of the engine communicate with the crank chamber  13  via the transmission chamber  55 . 
     As shown in  FIG. 4 , the intake-side cover  51  of the head cover  8  includes the gas-liquid separation device  9  for blow-by gas. The gas-liquid separation device  9  guides blow-by gas in the engine  1  to the intake device  21 , and includes a gas-liquid separation chamber  61  that removes mist-like oil contained in the blow-by gas, and a fresh air chamber  62  that introduces fresh air into the crank chamber  13  to ventilate the engine  1 . 
     The gas-liquid separation chamber  61  and the fresh air chamber  62  are located between a box body  63  that defines an opening in the direction toward the lower side and a cover body  64  that closes the opening of the box body  63 . In the present preferred embodiment, the box body  63  corresponds to a main body that is integral with the head cover, and the cover body  64  corresponds to a cover body attached to the head cover. The box body  63  is integral with the rear end of the intake-side cover  51  on the rear end of the engine and projects toward the upper side of the front end (the end on the front end of the engine) of the intake-side cover  51 . 
     A partition  65  (see  FIGS. 4 and 6 ), a first partition wall  68  (see  FIGS. 4 and 7 ), a second partition wall  69  (see  FIGS. 6 and 7 ), and a third partition wall  70  are provided in the box body  63 . The partition  65  separates the gas-liquid separation chamber  61  and the fresh air chamber  62 . The first partition wall  68  divides the gas-liquid separation chamber  61  into a lower chamber  66  and an upper chamber  67 . The second partition wall  69  divides the lower chamber  66  into an upstream portion  66   a  and a downstream portion  66   b.  The third partition wall  70  divides the fresh air chamber  62  into an upstream portion  62   a  and a downstream portion  62   b.  The partition  65 , the first partition wall  68 , the second partition wall  69 , and the third partition wall  70  are integral with the box body  63  by, for example, integral molding. 
     As shown in  FIG. 6 , the partition  65  includes a horizontal wall  65   a  and a vertical wall  65   b.  The horizontal wall  65   a  extends from the upper side portion (the upper side portion in  FIG. 6 ) of the box body  63  close to the exhaust camshaft  20  into the box body  63  and divides the box body  63  in the axial direction of the crankshaft  12 . The vertical wall  65   b  extends in the box body  63  from the distal end of the horizontal wall  65   a  to the rear end of the engine. Hence, the gas-liquid separation chamber  61  is provided in the box body  63  on the rear end of the engine and on one side close to the exhaust camshaft  20 . 
     A blow-by gas outlet  71  is provided at the end of the upper chamber  67  of the gas-liquid separation chamber  61  on the rear end of the engine, as shown in  FIG. 7 . One end of a blow-by gas hose  73  is attached to the blow-by gas outlet  71  via a PCV (Positive Crankcase Ventilation) valve  72 . The other end of the blow-by gas hose  73  is connected to the downstream intake air passage  22 , as shown in  FIG. 1 . The PCV valve  72  opens when the magnitude of an intake negative pressure propagated from the downstream intake air passage  22  via the blow-by gas hose  73  exceeds a predetermined threshold. The upstream portion  62   a  (see  FIG. 6 ) of the fresh air chamber  62  is located at a position adjacent to the upper chamber  67  of the gas-liquid separation chamber  61  in the left-and-right direction of the engine  1 . A fresh air inlet  74  is provided at the end of the upstream portion  62   a  on the rear end of the engine, as shown in  FIG. 4 . The fresh air inlet  74  communicates with the upstream intake air passage  24  via a fresh air hose  74   a  (see  FIG. 1 ). 
     As shown in  FIG. 7 , a gap S 1  is provided between the first partition wall  68  and the horizontal wall  65   a  of the partition  65 . 
     A gap S 2  is provided between the distal end (see  FIG. 6 ) of the second partition wall  69  and a side wall  63   a  of the box body  63 . 
     A gap S 3  is provided between the distal end of the third partition wall  70  and the side wall  63   a  of the box body  63 . 
     As shown in  FIGS. 4 and 7 , the cover body  64  includes a main plate  75  that closes the opening of the box body  63 , a first partition plate  76  and a second partition plate  77  which project upward from the main plate  75 , and a duct  78  projecting downward from the end of the main plate  75  on the rear end of the engine. The main plate  75 , the first partition plate  76 , the second partition plate  77 , and the duct  78  are preferably integral and made of a plastic material by, for example, integral molding. The cover body  64  is attached to the box body  63  by, for example, welding the main plate  75  to the box body  63 . 
     As shown in  FIGS. 6 and 7 , a blow-by gas inlet  81  is provided at the end on the rear end of the engine in a portion of the main plate  75  defining the lower chamber  66  of the gas-liquid separation chamber  61 . In addition, a fresh air outlet  82  is provided at the front end of the engine in a portion of the main plate  75  defining the downstream portion  62   b  of the fresh air chamber  62 . The fresh air outlet  82  is provided in a passage  75   a  projecting downward from the main plate  75 , and defines an opening in the upper portion of the intake-side valve gear chamber  37  in the direction toward the front end of the engine. As shown in  FIG. 2 , the fresh air outlet  82  is provided at a portion in the intake-side valve gear chamber  37  close to the transmission chamber  55 . 
     The first partition plate  76  is located on the rear end of the engine (upstream side) with respect to the second partition wall  69  in the gas-liquid separation chamber  61 , and divides the upstream portion  66   a  of the lower chamber  66  to the side of the blow-by gas inlet  81  and the side of the second partition wall  69 . As shown in  FIG. 6 , an oil return hole  83  is provided in the main plate  75  between the first partition plate  76  and the second partition wall  69 . The oil return hole  83  is located at a portion of the main plate  75 , which is the lowest plate in a state in which the engine  1  is mounted in a vehicle body (not shown). As shown in  FIG. 1 , the engine  1  according to the present preferred embodiment is mounted in a vehicle body such that the exhaust-side valve gear chamber  38  is located lower than the intake-side valve gear chamber  37 . For this reason, the oil return hole  83  is provided at the end of the main plate  75  close to the exhaust-side valve gear chamber  38 . 
     A plurality of through holes  76   a  are provided in the first partition plate  76 . In the present preferred embodiment, the first partition plate  76  and the above-described second partition wall  69  correspond to a partition wall. 
     The second partition plate  77  divides the downstream portion  62   b  of the fresh air chamber  62  to the side of the third partition wall  70  and the side of the fresh air outlet  82 . A plurality of through holes  77   a  (see  FIG. 5 ) are provided in the second partition plate  77  as well. 
     As shown in  FIG. 4 , the duct  78  extends downward from the blow-by gas inlet  81  of the gas-liquid separation chamber  61 . The interior of the duct  78  defines a blow-by gas suction passage  84  connected to the blow-by gas inlet  81  and extends from the gas-liquid separation chamber  61  into the intake-side valve gear chamber  37 . The lower end (the upstream end of the blow-by gas suction passage  84 ) of the duct  78  defines an opening near or adjacent to the bottom wall  28  through which the head bolts  33  extend in the cylinder head  4 . The opening of the lower end of the duct  78  is preferably located between the intake camshaft  18  and the bottom wall  28  of the cylinder head  4  when viewed from the axial direction of the crankshaft  12 , and preferably, near or adjacent to the head bolts  33 . The opening of the lower end of the duct  78  opens toward the bottom wall  28 . 
     As shown in  FIG. 2 , the lower end of the duct  78  is provided at a position spaced apart from the openings  41   a  at the downstream ends of the first to third branches  42  to  44  in the axial direction of the crankshaft  12 . The lower end of the duct  78  according to the present preferred embodiment is located at the rear end (the end on the rear end of the engine) of the intake-side valve gear chamber  37  with one end connected to the transmission chamber  55  on the side opposite to the transmission chamber  55 . For this reason, the blow-by gas suction passage  84  according to the present preferred embodiment is located at the end of the intake-side valve gear chamber  37  in the axial direction of the intake camshaft  18  at a position overlapping the intake camshaft  18  when viewed from the axial direction of the intake camshaft  18 , as shown in  FIG. 4 . 
     In the gas-liquid separation device  9 , at the time of engine operation, an intake negative pressure is propagated from the downstream intake air passage  22  to the PCV valve  72  via the blow-by gas hose  73 . If the magnitude of the negative pressure exceeds a predetermined threshold, the PCV valve  72  opens, and the negative pressure is propagated into gas-liquid separation chamber  61 . The negative pressure is propagated from the gas-liquid separation chamber  61  to the intake-side valve gear chamber  37  via the duct  78  (blow-by gas suction passage  84 ) and further propagated to the crank chamber  13  via the communication path  41 . 
     On the other hand, at this time, since the negative pressure is also propagated from the fresh air outlet  82  in the intake-side valve gear chamber  37  to the upstream intake air passage  24  via the fresh air chamber  62  and the fresh air hose  74   a,  fresh air in the upstream intake air passage  24  is introduced from the fresh air outlet  82  into the intake-side valve gear chamber  37 . The fresh air flows in the intake-side valve gear chamber  37  toward the transmission chamber  55  in the direction in which the fresh air outlet  82  is directed. 
     When the intake negative pressure is propagated from the communication path  41  to the crank chamber  13 , the blow-by gas in the crank chamber  13  is drawn into the upstream portion  45  of the communication path  41 . The blow-by gas contains an oil mist. 
     When the blow-by gas is thus drawn into the communication path  41 , and the pressure in the crank chamber  13  is reduced, the gas containing fresh air in the intake-side valve gear chamber  37  flows into the crank chamber  13  via the transmission chamber  55 . As a result, the crank chamber  13  is ventilated. 
     The blow-by gas drawn into the upstream portion  45  of the communication path  41  is drawn out from the upstream portion to the intake-side valve gear chamber  37  via the first to third branches  42  to  44  by the intake negative pressure. The blow-by gas drawn into the intake-side valve gear chamber  37  and the oil mist mixed therein directly advance to the opposite side of the cylinder block  3  due to momentum. The blow-by gas and the oil mist collide against the walls of the upper portion of the valve gear chamber defined by the cover body  64 , the head cover  8 , and the like and then turn toward the bottom of the intake-side valve gear chamber  37 . At this time, the oil mist, which has a mass larger than that of the gas, is left behind and stays in the upper portion of the intake-side valve gear chamber  37 , or is captured by the walls that the mist collides against. For this reason, the amount of oil mist mixed with the gas decreases. The gas and the oil mist which is turned by the upper portion of the intake-side valve gear chamber  37  flows toward the bottom of the valve gear chamber  37  and collides against the bottom wall  28  of the intake-side valve gear chamber  37 . The oil mist is captured even when colliding against the bottom wall  28 . As a result, the scattered amount of oil mist decreases in the vicinity of the bottom wall  28 . 
     In the intake-side valve gear chamber  37 , the intake camshaft  18  rotates at a high speed, and the oil mist is scattered from there by centrifugal force. Even when scattered toward the bottom wall of the intake-side valve gear chamber  37 , the oil mist never directly enters the blow-by gas suction passage  84 . 
     According to the present preferred embodiment, since the blow-by gas suction passage  84  extends in the intake-side valve gear chamber  37 , and the upstream end of the blow-by gas suction passage  84  defines an opening near or adjacent to the bottom wall  28  of the valve gear chamber  37 , blow-by gas containing only a little amount of oil mist is drawn into the blow-by gas suction passage  84 . 
     The blow-by gas drawn into the blow-by gas suction passage  84  flows from the blow-by gas inlet  81  of the gas-liquid separation chamber  61  into the upstream portion  66   a  of the lower chamber  66  and passes through a number of through holes  76   a  in the first partition plate  76 . At this time, a portion of the oil mist contained in the blow-by gas adheres to the first partition plate  76 . The blow-by gas that has passed through the through holes  76   a  flows into the upper chamber  67  via the gap S 2  between the second partition wall  69  and the box body  63  and the gap S 1  between the first partition wall  68  and the partition  65 . When the blow-by gas passes through the gaps S 2  and S 1 , the direction of the flow of the blow-by gas is reversed. Hence, the particles of oil mist readily adhere to the side wall  63   a  of the box body  63  or the first partition wall  68  and the second partition wall  69 . 
     The oil separated from the blow-by gas in the gas-liquid separation chamber  61  flows down from the oil return hole  83  to the intake-side valve gear chamber  37 . In the intake-side valve gear chamber  37 , oil scatters from the lubricated portion of the intake camshaft  18 . The oil flowing down from the oil return hole  83  flows into the relatively low exhaust-side valve gear chamber  38  together with other oil in the intake-side valve gear chamber  37  and returns to the oil pan  7  via the oil return passage  48  in the exhaust-side valve gear chamber  38 . 
     The blow-by gas flowing into the upper chamber  67  of the gas-liquid separation chamber  61  is drawn from the blow-by gas outlet  71  into the downstream intake air passage  22  via the PCV valve  72  and the blow-by gas hose  73 . 
     If the rotational speed of the engine  1  reaches a high rotation/high load operation range, the throttle valve  23  fully opens. Hence, the pressure in the downstream intake air passage  22  increases, and the PCV valve  72  closes. In this operation state, the intake negative pressure may be propagated from the upstream intake air passage  24  to the intake-side valve gear chamber  37  via the fresh air chamber  62 , and the blow-by gas in the intake-side valve gear chamber  37  may be drawn into the upstream intake air passage  24  via the fresh air chamber  62 . In this case, the direction of the flow of the blow-by gas is changed by the second partition plate  77 , the third partition wall  70 , and the partition  65  provided in the fresh air chamber  62 , and the oil mist contained in the blow-by gas adheres to these members and the inner wall of the fresh air chamber  62  and is captured. 
     The gas-liquid separation device  9  according to the present preferred embodiment uses a simple gas-liquid separation structure including only a plurality of partitions in the compact gas-liquid separation chamber  61  in the box body  63  and the cover body  64 . Since the blow-by gas suction passage  84  connected to the blow-by gas inlet  81  of the gas-liquid separation chamber  61  defines an opening near or adjacent to the bottom wall  28  where the scattered amount of oil mist is small, the amount of oil mist drawn from the blow-by gas suction passage  84  into the gas-liquid separation chamber  61  decreases. 
     According to the present preferred embodiment, since the amount of oil mist drawn into the gas-liquid separation chamber  61  is small, the gas-liquid separation chamber  61  is made compact, and the structure is simplified. 
     Hence, according to the present preferred embodiment, it is possible to provide a gas-liquid separation device for blow-by gas in an engine that removes an oil mist contained in blow-by gas while using a compact and simple gas-liquid separation structure. 
     The upstream end of the blow-by gas suction passage  84  according to the present preferred embodiment opens near or adjacent to the bottom wall  28  and in a direction toward the bottom wall  28 . For this reason, it is more difficult to draw the oil mist into the blow-by gas suction passage  84 . It is therefore possible to provide a high performance gas-liquid separation device for blow-by gas. 
     The upstream end of the blow-by gas suction passage  84  is provided at a position spaced apart from the openings  41   a  of the communication path  41  on the downstream side in the axial direction of the crankshaft  12 . It is therefore possible to prevent the blow-by gas drawn into the intake-side valve gear chamber  37  from being directly drawn into the blow-by gas suction passage  84 . 
     The blow-by gas suction passage  84  according to the present preferred embodiment is provided at the end of the intake-side valve gear chamber  37  in the axial direction of the intake camshaft  18  at a position overlapping the intake camshaft  18  when viewed from the axial direction of the intake camshaft  18 . For this reason, the oil mist scattered from the intake camshaft  18  that rotates at a high speed hardly enters the blow-by gas suction passage  84 . It is therefore possible to further reduce the oil mist drawn into the gas-liquid separation chamber  61 . 
     Additionally, in the present preferred embodiment, when the blow-by gas in the crank chamber  13  is drawn into the gas-liquid separation chamber  61  via the communication path  41 , the intake-side valve gear chamber  37 , and the blow-by gas suction passage  84 , fresh air is introduced from the transmission chamber  55  to the crank chamber  13 . Hence, it is possible to provide a gas-liquid separation device for blow-by gas in an engine that ventilates the crank chamber  13 . 
     The gas-liquid separation chamber  61  according to the present preferred embodiment includes the box body  63  and the cover body  64  with the duct  78 . For this reason, since the gas-liquid separation chamber  61  and the blow-by gas suction passage  84  are defined by combining members having simple shapes, it is possible to inexpensively provide a gas-liquid separation device for blow-by gas in an engine. 
     In the present preferred embodiment, the upstream end of the blow-by gas suction passage  84  opens at a location between the intake camshaft  18  and the bottom wall  28  when viewed from the axial direction of the crankshaft  12 . 
     For this reason, the blow-by gas is drawn from an area where the scattered amount of oil mist is small into the blow-by gas suction passage  84 . Hence, the amount of oil mist drawn from the blow-by gas suction passage  84  into the gas-liquid separation chamber  61  decreases. 
     In the above preferred embodiments, an example in which the gas-liquid separation device  9  for blow-by gas preferably is provided in a 4-cylinder engine  1  has been described. However, the gas-liquid separation device  9  for blow-by gas is not restricted by the number of cylinders. For this reason, the number of cylinders of the engine  1  may be appropriately changed. 
     The gas-liquid separation device is preferably located at the highest position in a state in which the engine is mounted in a vehicle. In a preferred embodiment, the gas-liquid separation device  9  is provided on the intake side. If the exhaust side is higher, the gas-liquid separation device  9  is preferably provided on the exhaust side. 
     The inventor of the preferred embodiments of the present invention considered that the gas-liquid separation chamber can be made compact if the amount of oil mist in the blow-by gas drawn in by the gas-liquid separation device is small. It was also discovered as a result of experiments that the amount of oil mist scattered in the valve gear chamber increases or decreases depending on the location. As a result of repetitive experiments, the present inventor discovered that the scattered amount of oil mist is small in the vicinity of the bottom wall of the valve gear chamber and conceived the idea of the present invention. 
     According to various preferred embodiments of the present invention, since the blow-by gas suction passage extends into the valve gear chamber, and the upstream end of the blow-by gas suction passage defines an opening near or adjacent to the bottom wall of the valve gear chamber, blow-by gas containing only a little amount of oil mist is introduced into the gas-liquid separation chamber. If the amount of oil mist drawn into the gas-liquid separation chamber is small, the gas-liquid separation chamber is able to be made compact, and the structure simplified. 
     The reason why the scattered amount of oil mist decreases in the vicinity of the bottom wall of the valve gear chamber is not clear. However, the following assumption is possible. 
     The blow-by gas drawn from the cylinder block side into the valve gear chamber by the action of the intake negative pressure and the oil mist mixed therein directly advance to the opposite side of the cylinder block due to momentum. The blow-by gas and the oil mist collide against the walls of the upper portion of the valve gear chamber defined by the bottom of the gas-liquid separation chamber, the head cover, and the like and then turn toward the bottom of the valve gear chamber. At this time, the oil mist, which has a mass larger than that of the gas, is left behind and stays in the upper portion of the valve gear chamber, or is captured by the walls that the mist collides against. For this reason, the amount of oil mist mixed in the gas decreases. The gas and the oil mist turned by the upper portion of the valve gear chamber flows toward the bottom of the valve gear chamber and collides against the bottom wall of the valve gear chamber. The oil mist is captured even when colliding against the bottom wall. As a result, the scattered amount of oil mist decreases in the vicinity of the bottom wall. 
     Note that in the valve gear chamber, the camshaft rotates at a high speed, and the oil mist is scattered therefrom by centrifugal force. Even when the oil mist is scattered toward the bottom wall of the valve gear chamber, it never directly enters the oil return passage. 
     Hence, according to preferred embodiments of the present invention, it is possible to provide a gas-liquid separation device for blow-by gas in an engine that removes an oil mist contained in blow-by gas while using a compact and simple gas-liquid separation structure. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.