Patent Publication Number: US-10329975-B2

Title: Oil separation device for internal combustion engine

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 15/033,943 entitled OIL SEPARATION DEVICE FOR INTERNAL COMBUSTION ENGINE, filed May 3, 2016, the disclosure of which is fully incorporated herein by reference. U.S. patent application Ser. No. 15/033,943 is a National Stage Entry of PCT Application Serial No. PCT/JP2014/005604 filed Nov. 7, 2014. PCT Application Serial No. PCT/JP/2014/005604 claims benefit to Japanese Patent Application No. 2014-150489 filed Jul. 24, 2014 and Japanese Patent Application No. 2013-232596 filed Nov. 8, 2013. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an oil separation device (breather system) for an internal combustion engine, and in particular to an oil separation device for separating oil mist from blow-by gas. 
     BACKGROUND ART 
     In the field of internal combustion engines, it is known to provide an oil separation device in a blow-by passage for returning the blow-by gas in a crankcase chamber back to an intake system. For instance, in the arrangements disclosed in Patent Documents 1 and 2, a head cover formed by combining a plurality of members is internally provided with a gas liquid separation chamber in which a plurality of baffle plates are positioned. The blow-by gas containing oil mist changes the flow direction thereof by colliding with the baffle plates as the blow-by gas travels from the inlet to the outlet of the gas liquid separation chamber. When the blow-by gas changes the flow direction thereof, the oil contained in the blow-by gas is caused to adhere to the baffle plates owing to the inertia thereof, and is thereby removed from the blow-by gas. 
     PRIOR ART DOCUMENT(S) 
     Patent Document(s) 
     Patent Document 1 JP4043825B 
     Patent Document 2 JP4353473B 
     SUMMARY OF THE INVENTION 
     Task to be Accomplished by the Invention 
     In an oil separation device configured to remove oil by means of a plurality of baffle plates, oil separation performance can be improved by increasing the number of baffle plates, increasing the degree of tortuousness of the passage for the blow-by gas and/or increasing the length of the passage for the blow-by gas. However, these approaches increase the flow resistance at the same time, and the resulting reduction in the velocity of the blow-by gas flow imposes a limit on the improvement in oil separation performance. 
     The present invention was made in view of such a problem of the prior art, and has a primary object to provide an oil separation device for an internal combustion engine with an improved oil separation performance. 
     Means for Accomplish the Task 
     To achieve such an object, the present invention provides an oil separation device ( 10 ) for an internal combustion engine ( 1 ), comprising: a gas liquid separation passage ( 56 ) internally defined by a passage forming member ( 41 ,  42 ) including a lower wall ( 56 A), an upper wall ( 56 B) and a pair of side walls ( 56 C,  56 D), and extending in a horizontal direction; a gas inlet ( 54 ) provided on one end of the gas liquid separation passage and a gas outlet ( 63 ) provided on another end of the gas liquid separation passage; a plurality of lower partition walls ( 56 H) projecting upward from the lower wall and disposed in parallel to one another; and a plurality of upper partition walls ( 56 J) projecting downward from the upper wall and disposed in parallel to one another; wherein each lower partition wall extends in a first direction which is tilted with respect to a lengthwise direction of the gas liquid separation passage in plan view, and each upper partition wall extends in a second direction crossing the first direction so that a spiral passage extending in the lengthwise direction of the gas liquid separation passage is defined by the lower partition walls and the upper partition walls; and wherein the spiral passage with a certain turn is defined by the lower partition walls and the upper partition walls for causing a swirl flow as gas flows from the gas inlet to the gas outlet, and the lower wall is inclined with respect to a horizontal plane such that an upstream part of the lower wall is lower than a downstream part of the lower wall when viewed in a direction of the swirl flow. 
     According to this arrangement, a spiral passage is defined in the gas liquid separation chamber, and the blow-by gas flowing through the spiral passage is converted into a vortex flow. Thereby, the oil contained in the gas is adhered to the lower wall, the upper wall and the side walls by the centrifugal force, and is separated from the gas. Because the upper partition walls and the lower partition walls form a spiral passage for conducting the gas along a spiral path so that the flow resistance can be minimized as opposed to the labyrinth passage, and reduction in the flow velocity of the gas can be minimized. Because the lower wall is inclined so that the bottom surfaces opposes the swirl flow created in the gas flowing from the gas inlet to the gas outlet, the adherence of the oil in the gas is promoted, and the oil separation performance can be improved. 
     In this invention, one of the side walls ( 56 C) located on the upstream part may define an acute angle with respect to the lower wall. 
     Thereby, the gas flowing along the spiral passage is bent at an acute angle at the boundary between the one side wall and the bottom wall as the gas flows along the one side wall and the bottom wall. 
     In this invention, it may be arranged such that the other side wall ( 56 D) located on a downstream side extends in parallel with the one side wall, and the upper wall extends in parallel with the lower wall so that the air gas separation passage is provided with a parallelepiped cross section extending perpendicular to the lengthwise direction. 
     Thereby, the gas flowing along the spiral passage is bent at an acute angle at the boundary between the other side wall and the upper wall as the gas flows along the other side wall and the upper wall. Thereby, the adherence of the oil to the upper wall is promoted. The oil that has deposited on the upper wall flows down to the upstream side of the lower wall along the upper wall and the side wall, or directly dropping onto the lower wall. 
     In this invention, each lower partition wall may define a gap with respect to the one side wall ( 56 C) on an upstream side. 
     According to this arrangement, the oil that has been collected on the upstream side of the lower wall can move in the lengthwise direction of the gas liquid separation passage by passing through the gaps defined between the one side wall and the lower partition walls. 
     In this invention, preferably, a lower end surface of each upper partition wall is located higher than an upper end surface of each lower partition wall, and each upper partition wall crosses at least one of the lower partition walls in plan view. 
     According to this arrangement, because the upper partition walls and the lower partition walls do not interfere with one another so that relatively large numbers of upper partition walls and lower partition walls can be arranged in the gas liquid separation chamber. By increasing the numbers of the upper partition walls and lower partition walls, the number of turns of the swirl flow for a given length of the spiral passage can be increased. By thus increasing the number of turns of the swirl flow traveling from the gas inlet to the gas outlet, the oil separation performance can be improved. 
     In this invention, preferably, a lower end surface of each upper partition wall includes a part that contacts the upper end surface of the corresponding lower partition wall. 
     According to this arrangement, because the lower end surfaces of the upper partition walls and the upper end surfaces of the lower partition walls abut one another, the spiral passage is better defined so that the blow-by gas flowing through the gas liquid separation passage can be converted into a vortex flow in an even more reliable manner. 
     In this invention, the gas liquid separation passage may include a narrowed section at least in a part thereof, the narrowed section ( 56 G) having a reduced cross section in comparison to adjoining part. 
     Thereby, the flow velocity of the blow-by gas is increased at the narrowed section so that the oil can be separated even more favorably by the centrifugal force. 
     In this invention, preferably, the passage forming member includes a first cover member ( 41 ) and a second cover member ( 42 ) jointly forming a part of a head cover of the internal combustion engine, and the first cover member includes at least the lower wall and the lower partition walls while the second cover member includes at least the upper wall and the upper partition walls. 
     According to this arrangement, the gas liquid separation passage having the upper partition walls and the lower partition walls can be formed with a highly simple structure. 
     Another aspect of the present invention provides a breather system (oil separation device:  201 ) of an internal combustion engine (E) provided with a PCV circuit for returning blow-by gas produced in an internal combustion engine fitted with a supercharger to an intake passage ( 202   x ) by using an intake negative pressure produced in the intake passage to be used for combustion once again, the breather system comprising, a breather chamber (R 3 ) provided on top of a head cover ( 214 ) of the internal combustion engine and communicating with a breather passage (R 0   a , R 0   b ) for conducting blow-by gas produced in the internal combustion chamber, the breather chamber including a first gas liquid separation chamber (R 2   a ) extending in a cylinder row direction and communicating with the breather passage, a second gas liquid separation chamber (R 2   b ) extending in the cylinder row direction along the first gas liquid separation chamber and communicating with an interior of the internal combustion engine via the intake passage, and a communication passage (R 2   c ) communicating the first gas liquid separation chamber with the second gas liquid separation chamber at one end of the internal combustion engine with respect to the cylinder row direction; a PCV valve ( 241 ) provided in the communication passage for adjusting a flow rate of the blow-by gas; and a one-way valve ( 242 ) provided at another end of the internal combustion engine with respect to the cylinder row direction for expelling the blow-by gas from the second gas liquid separation chamber; wherein the second gas liquid separation chamber includes a spiral passage ( 261 ) for causing a swirling flow of the blow-by gas in the second gas liquid separation chamber. 
     According to this arrangement, because the PCV valve separates the breather chamber into two gas liquid separation chambers, the part of the oil mist in the form of relatively large particles is captured by the PCV valve, and removed therefrom as the blow-by gas flows through the PCV valve. The part of the oil mist in the form of relatively small particles is converted into oil mist in the form of relatively large particles as the blow-by gas flows through narrow passage of the PCV valve by adhering to one another before being introduced into the second gas liquid separation chamber. The oil mist in the form of relatively large particles can be readily separated from the blow-by gas in the second gas liquid separation chamber. Because the blow-by gas flows through the second gas liquid separation chamber as a swirl flow including a vertical motion owing to the spiral passage defined in the second gas liquid separation chamber, the small particles of oil contained in the blow-by gas are caused to adhere to one another and converted into large particles of oil under the centrifugal force created by the swirl flow so that the gas liquid separation performance can be improved. Because the blow-by gas outlet of the second gas liquid separation chamber is provided with the one way valve, when the intake passage is placed under a positive pressure condition owing to the operation of the supercharger, the reverse flow of fresh air from the intake passage to the breather chamber can be avoided so that the adjustment of the pressure in the interior of the internal combustion engine can be performed favorably, and the scavenging performance of the internal combustion engine can be improved. 
     According to a preferred embodiment of the present invention, in the breather system of an internal combustion engine, a fresh air chamber (R 3 ) extends in the cylinder row direction along the breather chamber in an upper part of the head cover of the internal combustion engine for introducing fresh air into an interior of the internal combustion engine and scavenging an interior of the internal combustion engine by using an intake negative pressure in the internal combustion engine, and the fresh air chamber communicates with an interior of the internal combustion engine at one end thereof with respect to the cylinder row direction and with a part of the intake passage on an upstream side of the supercharger at another end thereof with respect to the cylinder row direction, and is provided with a spiral passage ( 262 ) for causing a swirl flow of the fresh air and the blow-by gas conducted in the fresh air chamber. 
     According to this arrangement, when the blow-by gas in the internal combustion engine is supplied to the intake passage by the intake negative pressure, the negative pressure in the interior of the internal combustion engine causes fresh air to be introduced from the fresh air chamber to the interior of the internal combustion engine so that the interior of the internal combustion engine can be favorably scavenged. When the supercharger is in operation or when the engine is operated under a high load and a high rpm condition, the intake passage is placed under positive pressure condition by the compressor of the supercharger. The one way valve in the breather chamber closes at such a time so that the blow-by gas ceases to flow in the breather chamber, and there is a fear that the blow-by gas might flow into the fresh air chamber. However, the fresh air chamber including the spiral passage functions as a gas liquid separation chamber, and a favorable gas liquid separation function is performed so that the blow-by gas free from oil is introduced into the intake passage. Therefore, deposition of oil in the intake passage can be prevented, and the pressure in the internal combustion engine can be favorably controlled so that the scavenging performance for the interior of the internal combustion engine can be favorably maintained. 
     Preferably, the spiral passage comprises a lower rib ( 261   b ,  262   b ) extending upward from a bottom surface of the corresponding chamber provided with the spiral passage and an upper rib ( 261   a ,  262   a ) extending downward from a ceiling surface of the corresponding chamber, the lower rib and the upper rib crossing each other so as to present a letter X shape in plan view. 
     According to this arrangement, because the ribs of the spiral passage cause the blow-by gas to form a swirl flow including the motion in the vertical direction, even when the length of the internal combustion engine in the cylinder row direction is short, and the cross sectional area of each chamber is small, an adequate gas liquid separation performance can be achieved. 
     According to another embodiment of the present invention, the breather system of the internal combustion engine further comprises a guide passage (R 1   a ) provided in the head cover of the internal combustion engine for communicating the breather chamber with a breather passage for conducting blow-by gas produced in the internal combustion engine, the guide passage communicating with the first gas liquid separation chamber at an end part of the first gas liquid separation chamber on the other end side of the cylinder row direction; a first oil return portion ( 214   e ) formed in a part of a bottom wall of the first gas liquid separation chamber on the other end side of the cylinder row direction; and a second oil return portion ( 2140  formed in a part of a bottom wall of the second gas liquid separation chamber on the one end side of the cylinder row direction or in a part of a bottom wall of the communication passage adjacent to the second gas liquid separation chamber. 
     According to this arrangement, when the supercharger is in operation and the intake passage is under a positive pressure condition, the one way valve provided in the breather chamber is closed so that the flow of the blow-by gas essentially ceases. Even at such a time, the oil separated from the blow-by gas in the guide passage and the first gas liquid separation passage can be quickly returned to the interior of the internal combustion engine via the first oil return hole. The oil collected in the second gas liquid separation passage can be quickly returned to the interior of the engine via the second oil return hole. When the engine is operated at a low or medium rotational speed, and the supercharger is not in operation, the intake passage is placed under negative pressure condition. At such a time, because the flow velocity of the blow-by gas flowing into the first gas liquid separation passage is relatively low, even though the first oil return hole is provided adjacent to the guide passage, the separated oil located adjacent to the first oil return hole is not blown up by the blow-gas so that very little of the separated oil is mixed into the blow-by gas. Because the first oil return hole is provided remote from the PCV valve, the oil in the first gas liquid separation chamber is prevented from stagnating in an area adjoining the PCV valve, and is prevented from obstructing the flow of the blow-by gas in the PCV valve. 
     According to another embodiment of the present invention, the first gas liquid separation chamber comprises a hole portion serving as an oil return hole and a blow-by gas inlet hole formed in a bottom wall of an end of the first gas liquid separation chamber on the other end side of the cylinder row direction, and an auxiliary opening ( 21   ag ) for introducing the blow-by gas into the first gas liquid separation chamber, the auxiliary opening being provided in a part of the first gas liquid separation chamber located higher than the hole portion serving as an oil return hole and a blow-by gas inlet hole in a state where the internal combustion engine is mounted on a vehicle and different from an upstream side of a downward flow of oil separated from the blow-by gas. 
     According to this arrangement, even when the oil return hole portion is blocked by the returned oil, the blow-by gas can be introduced into the breather chamber via the auxiliary opening located near the oil return hole portion in a favorable manner. 
     Preferably, a bottom surface of the first gas liquid separation chamber is inclined downward from one end side of the cylinder row direction to another end side of the cylinder row direction in a state where the internal combustion engine is mounted on a vehicle; and wherein a bottom surface of the second gas liquid separation chamber is inclined downward from the other end side of the cylinder row direction to the one end side of the cylinder row direction in a state where the internal combustion engine is mounted on a vehicle. 
     According to this arrangement, because the bottom surfaces of the first gas liquid separation chamber and the second gas liquid separation chamber are inclined downward toward the respective oil return hole portions, the oil separated from the blow-by gas in the first and second gas liquid separation chambers can be quickly returned to the interior of the internal combustion engine via the respective oil return hole portions. 
     Preferably, the PCV valve is provided along a length of the second gas liquid separation chamber in a part of the communication passage adjacent to a side wall of the second gas liquid separation chamber, and the second gas liquid separation chamber is provided with a rib ( 235 ) opposing a blow-by gas outlet of the PCV valve and spaced from the blow-by gas outlet by a prescribed gap. 
     According to this arrangement, because the PCV valve serving also as a communication passage between the first and second gas liquid separation passages extends along the length of the second gas liquid separation passage, the width of the head cover with respect to the direction perpendicular to the cylinder row direction can be minimized, and a compact design of the breather chamber and a high gas liquid separation performance can be achieved at the same time. The blow-by gas that has been expelled from the blow-by gas outlet of the PCV valve to the second gas liquid separation passage acquires an increased velocity by passing through a narrow passage of the PCV valve, and collides with the rib provided opposite to the blow-by gas outlet of the PCV valve with the result that the fine oil particles of the oil mist are deposited on the rib, and flows downward as droplets of larger sizes. 
     In the breather system for an internal combustion engine according to the present invention, preferably, the breather passage and the guide passage include a plurality of breather passages and a plurality of communication passages, and the breather passages are formed in parts of a bottom of the oil pan of the internal combustion engine having mutually different depths. 
     Because the breather passages are provided in the parts of the oil pan having different depths, even when the internal combustion engine E is inclined owing to the cornering of the vehicle or the inclination of the road surface, and part of the breather passages are submerged in the oil, the blow-by gas can still be introduced into the breather chamber via the remaining breather passages so that the pressure in the internal combustion engine E can be favorably adjusted, and an increase in friction owing to the increase in the blow-by gas in the crankcase chamber can be avoided. 
     According to a preferred embodiment of the present invention, the breather passage functions as an oil level gauge insertion hole, and the upper end of the breather passage communicates with the guide passage by branching out from the oil level gauge insertion hole, and the head cover includes an oil level gauge guide hole ( 214   h ) opposing a front end of the oil level gauge insertion hole and a guard member covering a part located between the oil level gauge guide hole and the oil level gauge insertion hole. 
     According to this arrangement, because a same, single hole serves as both the breather passage and the oil level gauge insertion hole, the manufacturing work is simplified, and the size of the internal combustion engine can be reduced. Also, owing to the presence of the guard members, the smearing of the upper end of the oil level gauge insertion hole with the oil mist in the blow-by gas flowing through the breather passage can be avoided, and the oil dropping from the oil return hole is prevented from splashing onto the upper end of the oil level gauge insertion hole. In other words, when the oil level gauge is inserted into the oil level gauge insertion hole or is removed from the oil level gauge insertion hole, oil is prevented from being deposited on the gauge part of the oil level gauge in the upper end of the oil level gauge insertion hole so that the oil level can be favorably measured without suffering from such interferences. 
     The oil level gauge may further comprise a bulging part ( 272 ) for closing a part of the oil level gauge insertion hole located above a part thereof where the breather passage branches out. 
     According to this arrangement, even when oil should be deposited on the upper end of the oil level gauge insertion hole, the oil is deposited on the bulging part, and is thereby prevented from being deposited on the gauge part of the oil level gauge so that the oil level can be favorably measured at all times. 
     Effect of the Invention 
     The oil separation device for an internal combustion engine arranged as discussed above can improve oil separation performance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
         FIG. 1  is a diagram of an internal combustion engine fitted with an oil separation device given as a first embodiment of the present invention; 
         FIG. 2  is a plan view of a head cover incorporated with the oil separation device of the first embodiment; 
         FIG. 3  is a sectional view taken along line III-III of  FIG. 2 ; 
         FIG. 4  is a sectional view taken along line IV-IV of  FIG. 3 ; 
         FIG. 5  is a sectional view taken along line V-V of  FIG. 4 ; 
         FIG. 6  is a sectional view taken along line VI-VI of  FIG. 4 ; 
         FIG. 7  is a sectional view taken along line VII-VII of  FIG. 4 ; 
         FIG. 8  is a sectional view taken along line VIII-VIII of  FIG. 4 ; 
         FIG. 9  is a diagram of an internal combustion engine given as a second embodiment of the present invention; 
         FIG. 10  is a perspective view of a head cover incorporated with a breather chamber and a fresh air chamber; 
         FIG. 11  is a plan view of the head cover incorporated with the breather chamber and the fresh air chamber; 
         FIG. 12  is a sectional view of the head cover incorporated with the breather chamber and the fresh air chamber, showing the flow of blow-by gas under a negative pressure condition; 
         FIG. 13 a    is a view as seen in the direction of arrow X 1  in  FIG. 11 ; 
         FIG. 13 b    is a view as seen in the direction of arrow X 2  in  FIG. 11 ; 
         FIG. 14 a    is a view as seen in the direction of arrow X 3  in  FIG. 11 ; 
         FIG. 14 b    is a view as seen in the direction of arrow X 4  in  FIG. 11 ; 
         FIG. 15  is a sectional view of the head cover incorporated with the breather chamber and the fresh air chamber, showing the flow of blow-by gas under a positive pressure condition; 
         FIG. 16  is a view as seen in the direction of arrow X 5  in  FIG. 11 , showing an oil level gauge insertion hole and an oil level gauge; 
         FIG. 17  is a perspective view of the head cover as seen from below; 
         FIG. 18 a    is a view as seen in the direction of arrow X 6  in  FIG. 17 ; 
         FIG. 18 b    is a view as seen in the direction of arrow X 7  in  FIG. 17 ; 
         FIG. 18 c    is a view as seen in the direction of arrow X 8  in  FIG. 17 ; 
         FIG. 19  is a plan view of a breather system given as another embodiment of the present invention; and 
         FIG. 20  is a view as seen in the direction of arrow X 9  in  FIG. 19 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     Preferred embodiments of the present invention as applied to automotive internal combustion engines are described in the following with reference to the appended drawings. 
     First Embodiment 
     The internal combustion engine of the first embodiment consists of an in line, four-cylinder reciprocating engine. As shown in  FIG. 1 , the engine  1  includes a cylinder block  2 , a cylinder head  3  attached to the upper part of the cylinder block  2 , a head cover  4  attached to an upper part of the cylinder head  3  and an oil pan  5  attached to a lower part of the cylinder block  2 . The head cover  4  is provided with a pair of oil separation devices  10  for removing oil from gas circulating therein. 
     The cylinder block  2  defines four cylinders  8  provided with axial center lines which are mutually parallel to one another and disposed in series on a common hypothetical plane. The direction along which the cylinders are disposed are called as a cylinder row direction, and the direction perpendicular to both the cylinder row direction and the lateral direction is called as a fore and aft direction. The cylinders  8  are referred to as the first, second, third and fourth cylinders from the left most one to the right most one in  FIG. 2 . 
     Each cylinder  8  opens out at the upper surface of the cylinder block  2  at the upper end thereof, and communicates with a crankcase chamber  11  defined in a lower part of the cylinder block  2  at the lower end thereof. Each cylinder  8  receives a piston  14  in a slidable manner, and the piston  14  is connected to a crankshaft  13  via a connecting rod  12 . The axial line of the crankshaft  13  extends in the lateral direction in  FIG. 2 . 
     The cylinder head  3  is elongated in the cylinder row direction (in the lateral direction in  FIG. 2 ), and is provided on the lower surface thereof with four combustion chamber recesses  16  corresponding to the respective cylinders  8 . The cylinder head  3  is also provided with intake ports  18  extending from the respective combustion chamber recesses  16  to the rear side of the cylinder head  3 , and exhaust ports  19  extending from the respective combustion chamber recesses  16  to the front side of the cylinder head  3 . 
     The intake system  21  of the internal combustion engine  1  includes an air inlet  22 , an air cleaner  23 , a compressor  24 A of a turbocharger, a throttle valve  25  and an intake manifold  26  in that order from the upstream end. The exhaust system  31  of the internal combustion engine  1  includes an exhaust manifold  32 , a turbine  24 B of the turbocharger, a catalytic converter (not shown in the drawings), a muffler (not shown in the drawings) and an exhaust outlet (not shown in the drawings) in that order from the upstream end. The exhaust manifold  32  is connected to the cylinder head  3 , and communicate with the exhaust ports  19 . 
     The oil pan  5  consists of a box having an open upper end, and is connected to the lower part of the cylinder block  2  to define an oil chamber  33  for storing engine oil. 
     The cylinder block  2  and the cylinder head  3  extend vertically, and define an oil return passage  35 , a first blow-by gas passage  36  and a gauge passage  37 , each having a lower end opening out to the crankcase chamber  11  and an upper end opening out at the upper surface of the cylinder head  3 . The oil return passage  35  is configured to return the oil that is collected on the upper surface of the cylinder head  3  to the crankcase chamber  11  and the oil chamber  33 . The gauge passage  37  is a passage for receiving an oil level gauge  51 . The first blow-by gas passage  36  extends in a front left part of the first cylinder  8 . The gauge passage  37  is located laterally between the second cylinder  8  and the third cylinder  8  in a forwardly offset manner. 
     As shown in  FIGS. 2 to 4 , the head cover  4  includes a first cover member  41  and a second cover member  42  that are connected to each other. The first cover member  41  is provided with an upper wall  41 A and a side wall  41 B extending downward from the side edge of the upper wall  41 A along the entire circumference so as to define a box opening out in the lower end thereof. The first cover member  41  is connected to the cylinder head  3  so as to abut the upper peripheral part of the cylinder head  3  at the lower end of the side wall  41 B, and cover the entire upper part of the cylinder head  3 . A valve actuation chamber  44  is defined between the first cover member  41  and the cylinder head  3  to accommodate a per se known valve actuation mechanism including a camshaft and rocker arms. 
     As shown in  FIGS. 2 and 4 , the upper wall  41 A of the first cover member  41  is formed with plug holes  45 A to  45 D for receiving spark plugs, respectively, at positions corresponding to the upper ends of the cylinders  8 . Each plug hole  45 A to  45 D is passed through the upper wall  41 A. The plug holes  45 A to  45 D are called as the first to the fourth plugs in that order from the left. Each plug hole  45 A to  45 D is closed by inserting the corresponding spark plug therein. 
     The upper wall  41 A of the first cover member  41  is formed with a recess  47  which is recessed downward and located in front of the fourth plug hole  45 D. A plurality of through holes  47 A are passed through the bottom wall of the recess  47 . 
     As shown in  FIG. 4 , a gauge hole  48  consisting of a through hole is formed in a part of the upper wall  41 A of the first cover member  41  which is located between the second plug hole  45 B and the third plug hole  45 C in a forwardly offset manner. The lower end of the gauge hole  48  is connected to the upper end of the gauge passage  37 . The oil level gauge  51  is provided with a plug (not shown in the drawings) adjacent to the base end thereof to close the gauge hole  48  by coming into contact with the wall of the gauge hole  48  when the oil level gauge  51  is fully inserted in the gauge hole  48 . 
     A first gas inlet hole  53  consisting of a through hole is formed in a part of the upper wall  41 A of the first cover member  41  which is located to the front left of the first plug hole  45 . The lower end of the first gas inlet hole  53  is connected to the upper end of the first blow-by gas passage  36 . 
     A vent hole  54  is formed in a part of the upper wall  41 A of the first cover member  41  located between the third plug hole  45 C and the fourth plug hole  45 D in a forwardly offset manner. The vent hole  54  is provided to the left of the recess  47  for communicating the upper face side of the upper wall  41 A and the valve actuation chamber  44 . 
     The second cover member  42  is connected to the upper surface of the upper wall  41 A of the first cover member  41 , and between the upper wall  41 A of the first cover member  41  and the second cover member  42  are defined a first gas liquid separation passage  56 , a second gas liquid separation passage  57 , a second blow-by gas passage  58 , a third blow-by gas passage  59  and an oil feed passage  60 . In other words, the first cover member  41  and the second cover member  42  serve as passage forming members for forming these passages  56  to  60 . The first gas liquid separation passage  56  and the second gas liquid separation passage  57  jointly form the oil separation device  10 . 
     As shown in  FIGS. 2 to 4 , the first gas liquid separation passage  56  extends laterally in front of the gauge hole  48 . The left end of the first gas liquid separation passage  56  is located between the first and second plug holes  45 A and  45 B, and the right end of the first gas liquid separation passage  56  is located between the third and fourth plug holes  45 C and  45 D, with respect to the lateral direction. The upper end of the vent hole  54  communicates with the lower part of the right end of the first gas liquid separation passage  56 . 
     The first gas liquid separation passage  56  is defined by various walls, including a lower wall  56 A, an upper wall  56 B, a front side wall  56 C, a rear side wall  56 D, a left side wall  56 E and a right side wall  56 F. The lower wall  56 A is formed by the upper wall  41 A of the first cover member  41 , and the upper wall  56 B is formed by the second cover member  42 . The front side wall  56 C, rear side wall  56 D, left side wall  56 E and right side wall  56 F are defined by at least one of the first cover member  41  and the second cover member  42 . 
     A middle part of the rear side wall  56 D of the first gas liquid separation passage  56  is curved forward to avoid interference with the gauge hole  48 . Therefore, the first gas liquid separation passage  56  includes a narrow section  56 G having a narrower width (with respect to the fore and aft direction) than the adjoining parts, and has a smaller cross sectional area in the narrow section  56 G than the adjoining part of the first gas liquid separation passage  56 . 
     The left side wall  56 E of the first gas liquid separation passage  56  is formed with a gas communication port  63 . As shown in  FIG. 1 , the gas communication port  63  is connected to a node between the air cleaner  23  and the compressor  24 A of the intake system  21  via a gas passage  64  formed by a hose or a tube. As will be discussed hereinafter, the gas communication port  63  functions as a fresh air inlet for introducing fresh air from the intake system  21  to the first gas liquid separation passage  56  as indicated by a white arrow in  FIG. 2 , and a blow-by gas outlet for expelling blow-by gas from the first gas liquid separation passage  56  to the exhaust system  31  as indicated by a solid arrow in  FIG. 2 . 
     As shown in  FIGS. 2 to 4 , the second gas liquid separation passage  57  extends laterally behind the row of the first to the fourth plug holes  45 A to  45 D. The left end of the second gas liquid separation passage  57  is located in a position corresponding to the first plug hole  45 A, and the right end of the second gas liquid separation passage  57  is located in a position corresponding to the fourth plug hole  45 D. 
     The second gas liquid separation passage  57  is defined by various walls, including a lower wall  57 A, an upper wall  57 B, a front side wall  57 C, a rear side wall  57 D, a left side wall  57 E and a right side wall  57 F. The lower wall  57 A is formed by the upper wall  41 A of the first cover member  41 , and the upper wall  57 B is formed by the second cover member  42 . The front side wall  57 C, rear side wall  57 D, left side wall  57 E and right side wall  57 F are defined by at least one of the first cover member  41  and the second cover member  42 . 
     As shown in  FIG. 4 , the left half of the front side wall  57 C of the second gas liquid separation passage  57  is rearwardly offset relative to the right half thereof. Thereby, the right half of the second gas liquid separation passage  57  is wider with respect to the fore and aft direction than the left half thereof, and is hence provided with a larger cross sectional area than the left half thereof. The rear side wall  57 D of the second gas liquid separation passage  57  consists of a serpentine wall extending in the lateral direction, presenting a wavy shape in plan view. Thereby, the second gas liquid separation passage  57  is provided with a plurality of narrowed sections  57 G having a narrower width with respect to the fore and aft direction. The second gas liquid separation passage  57  has a smaller cross sectional area in the narrowed sections  57 G than in the remaining part thereof. 
     A gas outlet port  65  is formed in the right end of the rear side wall  57 D of the second gas liquid separation passage  57 . The gas outlet port  65  is connected to the downstream side of the intake system  21  or, more specifically, to the intake manifold  26  via a blow-by gas supply passage  66  formed by a hose or a tube. As indicated by the solid arrow in  FIG. 2 , the gas outlet port  65  functions as a blow-by gas outlet for expelling the blow-by gas from the second gas liquid separation passage  57  to the side of the intake system  21 . 
     As shown in  FIG. 4 , the blow-by gas passage  58  extends from a terminal end thereof located in a front left part of the first plug hole  45 A rightward along a front part of the first plug hole  45 A, and after bending rearward, extends rearward between the first plug hole  45 A and the second plug hole  45 B. Thereafter, the blow-by gas passage  58  extends leftward in front of the second gas liquid separation passage  57 , and reaches the left side of the left side wall  57 E of the second gas liquid separation passage  57 . This terminal end (the second terminal end) of the second gas liquid separation passage  57  adjoins the left end (the first terminal end) of the second gas liquid separation passage  57 . 
     The first gas inlet hole  53  communicates with a lower part of one of the terminal ends of the second blow-by gas passage  58 . The lower wall of the second blow-by gas passage  58  is inclined such that the first terminal end in the front is lower than the second terminal end in the rear. Thereby, the liquid that has collected on the lower wall at the second terminal end of the second blow-by gas passage  58  is caused to flow along the inclined lower wall to the first gas inlet hole  53  located in the first terminal end of the second blow-by gas passage  58  under the action of the gravity. 
     The third blow-by gas passage  59  extends from a first terminal end thereof located ahead of the gauge hole  48  and behind the first gas liquid separation passage  56 , rearward between the gauge hole  48  and the first gas liquid separation passage  56 , and further between the second plug hole  45 B and the third plug hole  45 C. Thereafter, the third blow-by gas passage  59  bends leftward, and after extending leftward between the second plug hole  45 B and the second gas liquid separation passage  57 , is connected to the second blow-by gas passage  58  at the second terminal end thereof. The first terminal end of the third blow-by gas passage  59  is connected to the gauge hole  48  via a second gas inlet port  67  consisting of a passage extending radially from the gauge hole  48 . One end of the second gas inlet port  67  is located below the plug portion of the fully inserted oil level gauge  51  in the gauge hole  48 . Therefore, when the oil level gauge  51  is fully inserted in the gauge hole  48 , communication between the gauge hole  48  and the second gas inlet port  67  can be maintained. 
     The lower wall of the third blow-by gas passage  59  is inclined such that the one end thereof located in the front is lower than the other end thereof located in the back. Thereby, the liquid that has collected on the lower wall at the other end of the third blow-by gas passage  59  flows along the inclined lower wall, and reaches the second gas inlet port  67  located in the one end of the third blow-by gas passage  59  under the action of the gravity. 
     As shown in  FIGS. 4 and 6 , a PCV valve  70  is fitted into a through hole formed in the left side wall  57 E of the second gas liquid separation passage  57  located between the second blow-by gas passage  58  and the second gas liquid separation passage  57 . The PCV valve  70  includes a housing defining an internal passage communicating the second blow-by gas passage  58  and the second gas liquid separation passage  57  with each other, a valve seat provided in the inner passage and facing the second gas liquid separation passage  57 , a valve member configured to be seated on the valve seat and a biasing member for urging the valve member onto the valve seat. The PCV valve  70  is initially closed by the valve member being seated on the valve seat under the biasing force of the biasing member. When the pressure on the side of the second gas liquid separation passage  57  is lower than the pressure on the side of the second blow-by gas passage  58 , the valve member is lifted from the valve seat to permit the flow of gas from the side of the second blow-by gas passage  58  to side of the second gas liquid separation passage  57 . 
     The PCV valve  70  is passed through the wall of the second cover member  42  defining the second blow-by gas passage  58 , and extends to the left side wall  57 E of the second gas liquid separation passage  57 . Therefore, the PCV valve  70  can be installed in the left side wall  57 E of the second gas liquid separation passage  57  from outside. 
     As shown in  FIGS. 2 and 4 , the oil feed passage  60  is defined by the recess  47  formed in the upper wall  41 A of the first cover member  41  and the second cover member  42  covering the recess  47 . The part of the second cover member  42  covering the recess  47  is provided with a tube portion  73  projecting upward. The tube portion  73  internally defines a passage which is open on both ends thereof. As shown in  FIG. 3 , the upper opening of the tube portion  73  is provided with a detachable cap  74  so that oil may be filled into the upper open end of the tube portion  73  by removing the cap  74  from the tube portion  73 . The oil that has been filled into the tube portion  73  flows onto the upper surface of the cylinder head  3  via the through holes  47 A formed in the bottom wall of the recess  47 , and then flows into the oil chamber  33  via the oil return passage  35 . 
     As shown in  FIG. 7 , the upper surface of the lower wall  56 A of the first gas liquid separation passage  56  is inclined with respect to the horizontal surface such that the front part (on the side of the front side wall  56 C) is lower than the rear part (on the side of the rear side wall  56 D). The lower surface of the upper wall  56 B of the first gas liquid separation passage  56  is in parallel with the upper surface of the lower wall  56 A. The front side wall  56 C and the rear side wall  56 D of the first gas liquid separation passage  56  extends vertically. Therefore, the cross section (extending perpendicularly to the lengthwise or lateral direction of the first gas liquid separation passage  56 ) is provided with a parallelepiped shape. The upper surface of the lower wall  56 A of the first gas liquid separation passage  56  is inclined with respect to the horizontal surface so that the right end is lower than the left end. 
     As shown in  FIGS. 3 to 5  and  FIG. 7 , a plurality of lower partition walls  56 H project upward from the upper surface of the lower wall  56 A of the first gas liquid separation passage  56 . Each lower partition wall  56 H consists of a plate member, and has a certain horizontal length. The lower partition walls  56 H extend in parallel with one another in plan view, and are spaced from one another by a regular spacing with respect to the lateral direction. Each lower partition wall  56 H extends in a first direction which is tilted with respect to the lateral direction in plan view. More specifically, each lower partition wall  56 H extends in the forwardly and rightward direction (in the first direction) so that the front end is positioned to the right of the rear end. 
     The rear end of each lower partition wall  56 H is connected to the rear side wall  56 D of the first gas liquid separation passage  56 . Meanwhile, the front end of each lower partition wall  56 H defines a free end spaced from the front side wall  56 C, defining a gap with respect to the front side wall  56 C. The front end of each lower partition wall  56 H is curved in the rightward direction with a progressively increasing curvature toward the front end thereof. The vertical dimension of each lower partition wall  56 H is about one half of the distance between the lower wall  56 A and the upper wall  56 B. 
     A plurality of upper partition walls  56 J project downward from the lower surface of the upper wall  56 B of the first gas liquid separation passage  56 . Each upper partition wall  56 J consists of a plate member, and has a certain horizontal length. The upper partition walls  56 J extend in parallel with one another in plan view, and are spaced from one another by a regular spacing with respect to the lateral direction. Each upper partition wall  56 J extends in a second direction which is tilted with respect to the lateral direction in plan view. More specifically, each upper partition wall  56 J extends in the forwardly and leftward direction (in the second direction) so that the front end is positioned to the left of the rear end. The first and second directions are symmetric with respect to a line of symmetry extending in the lateral direction. 
     The rear end of each upper partition wall  56 J is connected to the rear side wall  56 D of the first gas liquid separation passage  56 , and the front end of each upper partition wall  56 J is connected to the front side wall  56 C of the first gas liquid separation passage  56 . The vertical dimension of each upper partition wall  56 J is about one half of the distance between the lower wall  56 A and the upper wall  56 B. In plan view, each upper partition wall  56 J crosses at least one of the lower partition walls  56 H. As shown in  FIGS. 5 and 7 , at each of the intersections between the upper partition walls  56 J and the lower partition walls  56 H, the lower end surface of the corresponding upper partition wall  56 J and the upper end surface of the corresponding lower partition wall  56 H abut each other. In other words, the lower end surface of each upper partition wall  56 J includes a part that contacts the upper end surface of the corresponding lower partition wall  56 H. 
     As shown in  FIGS. 4 and 7 , the upper partition walls  56 J and the lower partition walls  56 H jointly define a spiral passage of a clockwise turn in the first gas liquid separation passage  56 . Thus, when gas flows from the vent hole  54  to the gas communication port  63 , as shown by arrows  100  in  FIGS. 4 and 7 , the gas flows leftward and forward along the first upper partition wall  56 J, downward along the front side wall  56 C, leftward and rearward along the subsequent lower partition wall  56 H, and upward along the rear side wall  56 D, or flows in a clockwise spiral pattern. Also, when gas flows from the gas communication port  63  to the vent hole  54 , as shown by arrows  101  in  FIG. 4 , the gas flows rightward and rearward along the upper partition wall  56 J, downward along the rear side wall  56 D, rightward and forward along the subsequent lower partition wall  56 H, and upward along the front side wall  56 C, or flows in a clockwise spiral pattern. 
     As shown in  FIGS. 5 and 7 , a plurality of ribs  56 K extending laterally project downward from the lower surface of the upper wall  56 B of the first gas liquid separation passage  56 , and are arranged in the fore and aft direction at a regular interval. The projecting length of the ribs  56 K is smaller than the downward projecting length of the upper partition walls  56 J. 
     As shown in  FIGS. 3, 4, 6 and 8 , the upper surface of the lower wall  57 A of the second gas liquid separation passage  57  is inclined with respect to the horizontal surface such that the front part (on the side of the front side wall  57 C) is lower than the rear part (on the side of the rear side wall  57 D). The lower surface of the upper wall  57 B of the second gas liquid separation passage  57  is in parallel with the lower surface of the lower wall  57 A. The front side wall  57 C and the rear side wall  57 D of the second gas liquid separation passage  57  extend vertically. Therefore, the cross section (extending perpendicularly to the lengthwise or lateral direction of the second gas liquid separation passage  57 ) is provided with a parallelepiped shape. The upper surface of the lower wall  57 A of the second gas liquid separation passage  57  is inclined with respect to the horizontal surface so that the left end is lower than the right end. 
     A plurality of lower partition walls  57 H project upward from the upper surface of the lower wall  57 A of the second gas liquid separation passage  57 . Each lower partition wall  57 H consists of a plate member, and has a certain horizontal length. The lower partition walls  57 H extend in parallel with one another in plan view, and are spaced from one another by a regular spacing with respect to the lateral direction. Each lower partition wall  57 H extends in a third direction which is tilted with respect to the lateral direction in plan view. More specifically, each lower partition wall  57 H extends in the forwardly and leftward direction (in the third direction) so that the front end is positioned to the left of the rear end. 
     The front end of each lower partition wall  57 H defines a free end spaced from the front side wall  57 C, defining a gap with respect to the front side wall  57 C. Meanwhile, the rear end of each lower partition wall  57 H defines a free end spaced from the rear side wall  57 D, defining a gap with respect to the rear side wall  57 D. The front end of each lower partition wall  57 H is curved in the leftward direction with a progressively increasing curvature toward the front end thereof, and the rear end of each lower partition wall  57 H is curved in the rightward direction with a progressively increasing curvature toward the rear end thereof. The vertical dimension each lower partition wall  57 H is about one half of the distance between the lower wall  57 A and the upper wall  57 B. 
     A plurality of upper partition walls  57 J project downward from the lower surface of the upper wall  57 B of the second gas liquid separation passage  57 . Each upper partition wall  57 J consists of a plate member, and has a certain horizontal length. The upper partition walls  57 J extend in parallel with one another in plan view, and are spaced from one another by a regular spacing with respect to the lateral direction. Each upper partition wall  57 J extends in a fourth direction which is tilted with respect to the lateral direction in plan view. More specifically, each upper partition wall  57 J extends in the forwardly and rightward direction (in the fourth direction) so that the front end is positioned to the right of the rear end. The third and fourth directions are symmetric with respect to a line of symmetry extending in the lateral direction. 
     The rear end of each upper partition wall  57 J is connected to the rear side wall  57 D of the second gas liquid separation passage  57 , and the front end of each upper partition wall  57 J is connected to the front side wall  57 C of the second gas liquid separation passage  57 . The vertical dimension of each upper partition wall  57 J is about one half of the distance between the lower wall  57 A and the upper wall  57 B. In plan view, each upper partition wall  57 J crosses at least one of the lower partition walls  57 H. As shown in  FIGS. 6 and 8 , at each of the intersections between the upper partition walls  57 J and the lower partition walls  57 H, the lower end surface of the corresponding upper partition wall  57 J and the upper end surface of the corresponding lower partition wall  57 H abut each other. In other words, the lower end surface of each upper partition wall  57 J includes a part that contacts the upper end surface of the corresponding lower partition wall  57 H. 
     As shown in  FIGS. 4 and 8 , the upper partition walls  57 J and the lower partition walls  57 H jointly define a spiral passage of a counter clockwise turn in the second gas liquid separation passage  57 . Thus, when gas flows from the PCV valve  70  to the gas outlet port  65 , as shown by arrows  102  in  FIGS. 4 and 8 , the gas flows rightward and forward along the second upper partition wall  57 J, downward along the front side wall  57 C, rightward and rearward along the subsequent lower partition wall  57 H, and upward along the rear side wall  57 D, or flows in a clockwise spiral pattern. 
     A baffle wall  75  projects rearward from the rear side of the left end part of the front side wall  57 C of the second gas liquid separation passage  57  so as to oppose the opening of the PCT valve  70  on the side of the second gas liquid separation passage  57 . An oil discharge hole  76  is passed downward through a part of the front left end part of the lower wall  57 A of the second gas liquid separation passage  57  located to the right of the baffle wall  75 . The oil discharge hole  76  communicates the second gas liquid separation passage  57  with the valve actuation chamber  44 . 
     The flow of the blow-by gas and the flow of the fresh air in the internal combustion engine  1  discussed above are described in the following. Under a low load condition of the internal combustion engine  1 , the turbocharger is not in operation. Under this condition, the part of the intake system  21  on the downstream side of the throttle valve  25  is placed in a negative pressure condition during the downward stroke of the pistons  14 , and is therefore lower in pressure than the upstream side of the throttle valve  25 . The negative pressure on the downstream side of the throttle valve  25  is supplied to the second gas liquid separation passage  57  via the blow-by gas supply passage  66 , and opens the PCV valve  70 . As a result, the blow-by gas in the crankcase chamber  11  flows into the second blow-by gas passage  58  via at least one of the paths or the path that passes through the first blow-by gas passage  36  and the first gas inlet hole  53  and the path that passes through the gauge passage  37 , the second gas inlet port  67  and the third blow-by gas passage  59 . Thereafter, the blow-by gas passes through the PCV valve  70 , the second gas liquid separation passage  57  and the blow-by gas supply passage  66 . See the solid arrows in  FIG. 1 . 
     The oil mist contained in the blow-by gas is removed from the blow-by gas by adhering to the wall of the various passages, in particular the second gas liquid separation passage  57 . As the blow-by gas flows in the second gas liquid separation passage  57  as a spiral flow centered around the central axial line extending in the lengthwise direction, the centrifugal force pushes the oil mist radially outward, and causes the oil mist to adhere to the various walls  57 A to  57 F, and the lower partition walls  57 H and the upper partition walls  57 J. 
     At the same time as the blow-by gas in the crankcase chamber  11  is expelled to the intake system  21 , the fresh air on the upstream side of the throttle valve  25  in the intake system  21  flows into the crankcase chamber  11  via the gas passage  64 , the gas communication port  63 , the first gas liquid separation passage  56 , the vent hole  54 , the valve actuation chamber  44  and the oil return passage  35  in that order. Thereby, the crankcase chamber  11  is ventilated. See the white arrows in  FIG. 1 . 
     Under a high load condition of the internal combustion engine  1 , the turbocharger is in operation so that the part of the intake system  21  downstream of the compressor  24 A is under a positive pressure which is higher than the pressure on the upstream side of the compressor  24 A. The positive pressure on the downstream side of the compressor  24 A is supplied to the second gas liquid separation passage  57  via the blow-by gas supply passage  66 , and closes the PCV valve  70 . As a result, the blow-by gas in the crankcase chamber  11  flows into the upstream side of the compressor  24 A of the intake system  21  via the oil return passage  35 , the valve actuation chamber  44 , the vent hole  54 , the first gas liquid separation passage  56 , the gas communication port  63  and the gas passage  64  in that order, instead of flowing through the first blow-by gas passage  36  and the gauge passage  37 . See the solid arrows in  FIG. 1 . In other words, under a high load condition, the blow-by gas flows in the opposite direction to the fresh air, or reverses the flow of the fresh air through the gas passage  64 , the communication port  63 , the first gas liquid separation passage  56 , the vent hole  54 , the valve actuation chamber  44  and the oil return passage  35 , in that order. 
     The oil mist contained in the blow-by gas is removed from the blow-by gas by adhering to the wall of the various passages, in particular the first gas liquid separation passage  56 . As the blow-by gas flows in the first gas liquid separation passage  56  as a spiral flow centered around the central axial line extending in the lengthwise direction, the centrifugal force pushes the oil mist radially outward, and causes the oil mist to adhere to the various walls  56 A to  56 F, and the lower partition walls  56 H and the upper partition walls  56 J. 
     The action and the advantages of the oil separation device  10  of the internal combustion engine  1  of the first embodiment are described in the following. Because the first gas liquid separation passage  56  and the second gas liquid separation passage  57  are formed as spiral passages, the blow-by gas that flows these passages is converted into a spiral flow. As a result, the oil contained in the gas is caused to adhere to the various walls  56 A to  56 F and  57 A to  57 F, the lower partition walls  56 H and  57 H and the upper partition walls  56 J and  57 J, and is thereby separated from the gas. The lower partition walls  56 H and  57 H and the upper partition walls  56 J and  57 J define spiral passages so that the flow resistance is reduced, and the decrease in the flow velocity of the gas can be minimized as compared to the case where labyrinth passages are used. 
     The lower wall  56 A of the first gas liquid separation passage  56  is inclined in a downward direction as the gas flows from the vent hole  54  (functioning as a gas inlet) to the gas communication port  63  (functioning as a gas outlet), and therefore opposes the spiral flow of the gas with a clockwise turn. This promotes the adherence of the oil in the blow-by gas on the lower wall  56 A so that the oil separation performance can be improved. The oil that has adhered to the lower wall  56 A is conducted toward the upstream part (front part) on the inclined lower wall  56 A under the gravitational force, and is collected therein. Likewise, in the second gas liquid separation passage  57 , the lower wall  57 A is inclined in a downward direction as the gas flows from the PCV valve  70  (functioning as a gas inlet) to the gas outlet port  65 , and therefore opposes the spiral flow of the gas with a clockwise turn. This promotes the adherence of the oil in the blow-by gas on the lower wall  57 A so that the oil separation performance can be improved. The oil that has adhered to the lower wall  57 A is caused to flow toward the upstream part (rear part) on the lower wall  57 A under the gravitational force. 
     In the first gas liquid separation passage  56 , because the lower wall  56 A and the front side wall  56 C define an acute angle therebetween, the gas flowing through the spiral passage is sharply bent along the front side wall  56 C and the lower wall  56 A at the boundary between the front side wall  56 C and the lower wall  56 A so that the adherence of the oil in the blow-by gas to the lower wall  56 A is promoted even further. Similarly, in the second gas liquid separation passage  57 , because the lower wall  57 A and the front side wall  57 C define an acute angle therebetween, the gas flowing through the spiral passage is sharply bent along the front side wall  57 C and the lower wall  57 A at the boundary between the front side wall  57 C and the lower wall  57 A so that the adherence of the oil in the gas to the lower wall  57 A is promoted even further. 
     Because the cross section of the first gas liquid separation passage  56  is in the shape of a parallelepiped, and the rear side wall  56 D and the upper wall  56 B define a sharp angle, the adherence of oil to the upper wall  56 B is promoted. The gas that flows along the upper wall  56 B collides with the ribs  56 K, and thereby effectively sheds the oil contained therein. The oil that has adhered to the upper wall  56 B and the ribs  56 K drops onto the lower wall  56 A, and is thence collected in the front part of the lower wall  56 A. 
     Because the cross section of the second gas liquid separation passage  57  is in the shape of a parallelepiped, adherence of oil onto the upper wall  57 B is promoted owing to the fact that an acute angle is defined between the rear side wall  57 D and the upper wall  57 B. The oil that has adhered to the upper wall  57 B drips along the upper wall  57 B and the front side wall  57 C to the lower wall  57 A, or drops onto the lower wall  57 A, and is collected in the front part of the lower wall  57 A. 
     In the first gas liquid separation passage  56 , because a gap is defined between the front end of each lower partition wall  56 H and the front side wall  56 C, the oil that has been collected in the front part of the lower wall  56 A can move in the lateral direction. In the first embodiment, because the upper surface of the lower wall  56 A of the first gas liquid separation passage  56  is inclined such that the right end of the lower wall  56 A is lower than the left end thereof, the oil that has been collected in the front part of the lower wall  56 A can flow rightward, and be expelled to the valve actuation chamber  44  via the vent hole  54 . 
     In the second gas liquid separation passage  57 , because a gap is defined between the front end of each lower partition wall  57 H and the front side wall  57 C, the oil that has been collected in the front part of the lower wall  57 A can move in the lateral direction. In the first embodiment, because the upper surface of the lower wall  57 A of the second gas liquid separation passage  57  is inclined such that the left end of the lower wall  57 A is lower than the right end thereof, the oil that has been collected in the front part of the lower wall  57 A can flow leftward, and be expelled to the valve actuation chamber  44  via the oil discharge hole  76 . 
     The lower partition walls  56 H and  57 H, and the upper partition walls  56 J and  57 J of the first and second gas liquid separation passages  56  and  57  are arranged such that the lower end surface of each upper partition wall  56 J,  57 J is located higher than the upper end surface of the corresponding lower partition wall  56 H,  57 H, and in plan view, each upper partition wall  56 J,  57 J crosses at least one of the lower partition walls  56 H,  57 H. Because the lower partition walls  56 H and  57 H, and the upper partition walls  56 J and  57 J do not interfere with one another, the number of the lower partition walls  56 H and  57 H, and the upper partition walls  56 J and  57 J in the first and second gas liquid separation passages  56  and  57  can be increased. By increasing the number of the lower partition walls  56 H and  57 H, and the upper partition walls  56 J and  57 J, the number of turns of the spiral passage for a given length of the passage can be increased. Therefore, the number of turns of the gas that passes through the first and second gas liquid separation passages  56  and  57  can be increased, and the oil separation performance can be improved. 
     Because the lower surfaces of the upper partition walls  56 J and  57 J have parts that contact the upper surfaces of the lower partition walls  56 H and  57 H, the outer profiles of the spiral passages can be even better defined by the lower partition walls  56 H and  57 H and the upper partition walls  56 J and  57 J. Thereby, the blow-by gas conducted through these spiral passages can be converted into a spiral flow in a highly reliable manner. 
     The flow velocity of the blow-by gas in the first and second gas liquid separation passages  56  and  57  is increased at each of the narrowed sections  56 G and  57 G, and the centrifugal force acting on the oil in the blow-by gas increases in such regions. Thereby, the oil separation performance can be improved. 
     The first and second gas liquid separation passages  56  and  57  can be defined in a highly simple manner by defining them between the first and second cover members  42  which are joined to each other. In particular, because the lower walls  56 A and  57 A, and the lower partition walls  56 H and  57 H are formed by the first cover member  41 , and the upper walls  56 B and  57 B, and the upper partition walls  56 J and  57 J are formed by the second cover member  42 , the spiral passages having relatively complex configurations can be formed in a highly simple manner. 
     Second Embodiment 
     In the second embodiment, the internal combustion engine of the present invention consists of an automotive, in-line, four-cylinder engine (L4 engine). The directions mentioned in the following description are based on the directions of the vehicle shown in  FIG. 10 . 
     In the second embodiment, the vertical direction corresponds to the plumb vertical direction of the engine when installed on a vehicle body. The first end of the cylinder row corresponds to the left side of the vehicle on which the engine is mounted, and the second end of the engine corresponds to the right side of the vehicle on which the engine is mounted or the side on which the chain case of the engine is provided. The upper side corresponds to the side where the head cover is provided, and the lower side corresponds to the side where the oil pan of the engine is provided. The front side corresponds to the side where the exhaust camshaft is provided, and the rear side corresponds to the side where the intake camshaft is provided. The cylinder row direction corresponds to the lengthwise direction of the cylinder head  213 . 
     As shown in  FIG. 9 , the engine E of the illustrated embodiment includes an engine main body essentially consisting of a cylinder block  211 , a lower block  212 , a cylinder head  213 , a head cover  214  and an oil pan  215 . 
     &lt;Cylinder Block&gt; 
     The cylinder block  211  mainly defines cylinder bores and a crankcase chamber although not shown in the drawings. The cylinder block  211  is internally provided with pistons, connecting rods and a crankshaft. 
     &lt;Lower Block&gt; 
     The lower block  212  internally defines a crankcase chamber in cooperation with the cylinder block  211 , and is positioned under the cylinder block  211 . In the illustrated embodiment, the crankshaft is rotatably supported by a plurality of upper crank journals formed on the lower surface of the cylinder block  211  and a plurality of lower crank journals formed on the upper surface of the lower block  212 . 
     &lt;Cylinder Head&gt; 
     The cylinder head  213  defines combustion chambers by recesses formed on the bottom surface thereof in parts corresponding to the cylinder bores formed in the cylinder block  211 , in cooperation with the top surfaces of the piston slidably received in the cylinder bores. The cylinder head  213  is internally formed with intake ports and exhaust ports communicating with the combustion chambers, and is provided with intake valves and exhaust valves for selectively closing the intake ports and the exhaust ports, respectively. A valve actuation chamber is defined on the top side of the cylinder head for receiving rocker arms and other components of a valve actuation mechanism. In particular, the valve actuation chamber accommodates an intake camshaft and an exhaust cam shaft which are actuated by the crankshaft. 
     &lt;Head Cover&gt; 
     The head cover  214  is a cover member covering the upper side of the cylinder head  213  to define the valve actuation chamber. A gas liquid separation chamber is defined inside the head cover  214  or on the top side of the head cover  214  by the head cover  214  alone or in combination with other members. 
     &lt;Oil Pan&gt; 
     The oil pan  215  receives the oil dripping thereinto after lubricating various parts of the internal combustion engine E, and stores the received oil. The oil pan  215  is connected to the lower end of the lower block  212 , and is internally provided with an oil strainer via which oil is supplied to an oil pump for supplying oil under pressure to various parts of the internal combustion engine E. 
     The internal combustion engine E is provided with a supercharger. An intake passage  202   x  of the engine E is provided with an air cleaner  202   a , a compressor  202   b  forming a part of the turbocharger, a throttle valve  202   c  and an intake manifold  202   d.    
     &lt;Blow-by Gas&gt; 
     Blow-by gas is a mixture of un-combusted gas, which is produced in the combustion chambers and has leaked into the crankcase chamber, and oil mist. The blow-by gas is conducted to a breather system  201  (gas liquid separation passage, oil separation device) of the internal combustion engine E via a breather passage. 
     In  FIG. 9 , the solid arrows indicate the flow of blow-by gas and fresh air when the intake passage  202   x  is in a negative pressure condition or when the supercharger is not in operation. The white arrows indicate the flow of blow-by gas when the intake passage  202   x  is in a positive pressure condition or when the supercharger is in operation. The dotted arrows indicate the flow of oil that has been separated from the blow-by gas. 
     In this engine E, the fresh air that has been admitted via the air cleaner  202   a  is compressed by the compressor  202   b , and is supplied to the main body of the internal combustion engine E via the throttle valve  202   c  and the intake manifold  202   d.    
     Owing to the negative pressure produced in the intake passage  202   x  during the intake stroke, the blow-by gas that has leaked from the combustion chambers to the crankcase chamber in the internal combustion engine E is introduced into the intake passage  202   x  via the breather passages R 0   a  and R 0   b . When the blow-by gas flows down the breather chamber R 2 , oil is separated from the blow-by gas, and the blow-by gas freed from the oil is returned to the combustion chambers to be combusted once again via the intake manifold  202   d . Because the pressure in the internal combustion engine drops when the blow-by gas is returned to the intake system, part of the fresh air introduced into the intake passage is supplied to the interior of the internal combustion engine E via a fresh air chamber R 3  to scavenge the interior of the internal combustion engine E. 
     Under a high load condition of the internal combustion engine E or under a medium load and high rpm condition of the internal combustion engine E, the part of the intake passage  202   x  downstream of the compressor  202   b  is placed under a positive pressure condition. Therefore, a one way valve  242  of the breather chamber R 2  is closed so that the returning flow of the blow-by gas from the breather chamber R 2  ceases. Under this condition, owing to the positive pressure condition of the intake passage  202   x , essentially no blow-by gas is returned to the intake passage  202   x . However, when the internal combustion engine E is operated in a high rpm condition for a prolonged period of time, the internal pressure of the internal combustion engine may rise to such an extent that a small amount of blow-by gas may flow through the fresh air chamber R 3  to be introduced into a part of the intake passage  202   x  upstream of the compressor  202   b . However, by this time, the blow-by gas is thoroughly freed from the oil in the spiral passage  262 , and is introduced into the intake passage  202   x  via a fresh air passage  202   y.    
     &lt;Gas Liquid Separation Passage&gt; 
     In the second embodiment, as shown in  FIGS. 10 and 11 , the breather system  201  is integrally formed in an upper part of the head cover  214  which is provided with a breather chamber upper  230 , a PCV valve  241 , a one way valve  242  and a fresh air chamber upper  250 . The head cover  214 , the breather chamber upper  230  and the fresh air chamber upper  250  form the guide passages R 1   a  and R 1   b , the breather chamber R 2  and the fresh air chamber R 3  as shown in  FIG. 12 . In the second embodiment, the breather chamber R 2  extends laterally in a rear end part of the head cover  214  of the internal combustion engine E, and the fresh air chamber R 3  extends laterally in a front end part of the head cover  214  of the internal combustion engine E. 
     &lt;Guide Passage&gt; 
     As shown in  FIG. 12 , the guide passage R 1   a  is a blow-by gas passage communicating the breather passage R 0   a  and the first gas liquid separation chamber R 2   a  with each other. When the internal combustion engine E is mounted on a vehicle, the upstream side of the guide passage R 1   a  extends in the fore and aft direction. The guide passage R 1   a  is formed by welding a guide passage upper  221   a  integrally formed with the fresh air chamber R 3  to a guide passage lower  221   b  integrally formed with the breather chamber upper  230 . 
     The guide passage R 1   b  is a blow-by gas passage communicating the breather passage R 0   b  and the first gas liquid separation chamber R 2   a  with each other. When the internal combustion engine E is mounted on a vehicle, the upstream side of the guide passage R 1   b  extends laterally and the downstream part of the guide passage R 1   b  extends in the fore and aft direction. The guide passage R 1   b  is formed by welding a guide passage lower  214   a   2  to a guide passage upper  222  integrally formed with the breather chamber upper  230 . 
     As shown in  FIG. 9 , the breather passages R 0   a  and R 0   b  are provided in parts of the oil pan  215  having mutually different depths. In the second embodiment, the breather passage R 0   a  is provided in a relatively deep part of the oil pan  215 , and the breather passage R 0   b  is provided in a relatively shallow part of the oil pan  215 . 
     &lt;Breather Chamber&gt; 
     As shown in  FIG. 12 , the breather chamber R 2  has the function to separate oil from the blow-by gas introduced from the guide passages R 1   a  and R 1   b  when the intake passage  202   x  is under a negative pressure condition, and return the blow-by gas freed from the oil to the combustion chambers via the intake manifold  202   d  while the separated oil is retuned to the interior of the internal combustion engine E. The breather chamber R 2  is formed by welding the breather chamber upper  230  to a breather chamber lower  214   b.    
     The breather chamber lower  214   b  forms the lower part of the breather chamber R 2  or the bottom wall, the front and rear side walls, and the left and right side walls of the breather chamber R 2 , and is integrally formed with the upper wall of the head cover  214 . 
     The breather chamber upper  230  forms an upper part of the breather chamber R 2 , and includes an upper main body  231 , a partition wall portion  232 , valve mounting portions  233  and  234  and a rib  235 . 
     The upper main body  231  forms the upper wall and an upper part of the front and rear side walls and the left and right side walls. The partition wall portion  232  separates the interior of the breather chamber R 2  into the first gas liquid separation chamber R 2   a , the second gas liquid separation chamber R 2   b  and the communication passage R 2   c.    
     The valve mounting portion  233  is formed in a part of the upper main body  231  on the one end side with respect to the cylinder row direction, and consists of a hole for receiving a PCV valve  241 . The valve mounting portion  234  is formed in a part of the upper main body  231  on the other side with respect to the cylinder row direction, and consists of a hole for receiving a one way valve  242 . 
     The rib  235  extends upright from the partition wall portion  232 , and opposes a blow-by gas outlet  241   a  of the PCV valve  241  mounted on the valve mounting portion  233 . 
     The breather chamber R 2  includes the first gas liquid separation chamber R 2   a , the second gas liquid separation chamber R 2   b  and the communication passage R 2   c.    
     &lt;First Gas Liquid Separation Chamber&gt; 
     The first gas liquid separation chamber R 2   a  is a blow-by gas passage extending linearly along the cylinder row direction in plan view. The bottom surface of the first gas liquid separation chamber R 2   a  inclines downward from the one end side of the cylinder row direction to the other end side of the cylinder row direction. A first oil return hole  214   e  is formed in a part of the bottom wall of the first gas liquid separation chamber R 2   a  located on the other end side of the cylinder row direction. The first oil return hole  214   e  communicates with the valve actuation chamber in the cylinder head  213 . 
     &lt;Second Gas Liquid Separation Chamber&gt; 
     The second gas liquid separation chamber R 2   b  is a blow-by gas passage extending linearly along the cylinder row direction in plan view. The second gas liquid separation chamber R 2   b  extends along and behind the first gas liquid separation chamber R 2   a  in a state where the internal combustion engine is mounted on a vehicle. The bottom surface of the second gas liquid separation chamber R 2   b  inclines downward from the other end side of the cylinder row direction to the one end side of the cylinder row direction. See  FIG. 13   a.    
     As shown in  FIG. 13 b   , the second gas liquid separation chamber R 2   b  is provided with a spiral passage  261  which is defined by a plurality of upper ribs  261   a  and a plurality of lower ribs  261   b  to cause a swirl motion to the blow-by gas flowing through the second gas liquid separation chamber R 2   b . As shown by broken lines in  FIG. 11 , these ribs  261   a  and  261   b  oppose one another so that each upper rib  261   a  oppose the corresponding lower rib  261   b  at the respective middle points so as to present the shape of letter X in plan view. 
     The upper ribs  261   a  extend downward from the inner surface of the breather chamber upper  230  of the breather chamber R 2 , and the lower ribs  261   b  extend upward from the inner surface of the breather chamber lower  214   b  of the breather chamber R 2 . The upper and lower ribs  261   a  and  261   b  are tilted so as to cross one another at the respective middle points, thereby presenting the shape of letter X in plan view. Owing to these ribs  261   a  and  261   b , the blow-by gas flowing through the breather chamber R 2  is converted into a swirling vortex flow. 
     &lt;Communication Passage&gt; 
     The communication passage R 2   c  is a blow-by gas passage extending in the shape of letter U between a part of the first gas liquid separation chamber R 2   a  on the one end side of the cylinder row direction and a part of the second gas liquid separation chamber R 2   b  on the one end side of the cylinder row direction. The bottom surface of the part of the communication passage R 2   c  adjoining the second gas liquid separation chamber R 2   b  is more downwardly recessed than the bottom surface of the other end part of the communication passage R 2   c  and the bottom surface of the second gas liquid separation chamber R 2   b , and a second oil return hole  214   f  (See  FIG. 13 b   ) is formed in this recess. 
     &lt;PCV Valve&gt; 
     As shown in  FIG. 12 , the PCV (positive crankcase ventilation) valve  241  is provided in the part of the communication passage R 2   c  adjoining the second gas liquid separation chamber R 2   b , and extends in the lengthwise direction of the second gas liquid separation chamber R 2   b . The PCV valve  241  regulates the flow rate of the blow-by gas that flows through the communication passage R 2   c , and is configured to open when the negative pressure in the intake passage  202   x  drops below a prescribed level, and open by a degree corresponding to the level of the negative pressure. When the negative pressure is lost, the PCV valve closes. The PCV valve  241  has the function to improve the oil separation performance by the fact that the elastic member urging the valve member as well as the valve member vibrates owing to the collision with the blow-by gas, and the resulting increase in the turbulence of the blow-by gas causes the blow-by gas to more actively collide with the valve member, the elastic member and the communication passage wall in the PCV valve  241 . The blow-by gas that has been introduced into the PCV valve  241  from the communication passage R 2   c  is expelled from a blow-by gas outlet  241   a  of the PCV valve  241  to the second gas liquid separation chamber R 2   b.    
     &lt;One Way Valve&gt; 
     The one way valve  242  is provided in a part of the second gas liquid separation chamber R 2   b  on the other end side of the cylinder row direction, and the downstream side of the one way valve  242  is communicated with the intake manifold  202   d . The one way valve  242  opens when the intake passage  202   x  is under a negative pressure condition to permit the ejection of blow-by gas from the second gas liquid separation chamber R 2   b  to the intake manifold  202   d , and closes when the intake passage  202   x  is under a positive pressure condition to prevent the reverse flow of fresh air from the intake passage  202   x.    
     &lt;Fresh Air Chamber&gt; 
     The fresh air chamber R 3  has the function to introduce a part of the fresh air introduced into the intake passage  202   x  into the interior of the internal combustion engine E for a scavenging action by making use of the fact that the pressure of the interior of the internal combustion engine E drops when the intake passage  202   x  is under a negative pressure condition, and the blow-by gas is drawn into the interior of the internal combustion engine E. When the intake passage  202   x  is under a positive pressure condition, the blow-by gas in the interior of the internal combustion engine E is returned to the intake manifold  202   d  while the oil contained in the blow-by gas is removed. The end of the fresh air chamber R 3  on the one end side of the cylinder row direction is in communication with the part of the intake passage  202   x  upstream of the compressor  202   b  via the fresh air passage  202   y  for conducting the fresh air and the blow-by gas. The end of the fresh air chamber R 3  on the other end side of the cylinder row direction is in communication with the interior of the internal combustion engine E via the intake manifold  202   d  for conducting the fresh air and the blow-by gas. 
     The fresh air chamber R 3  is formed by welding or otherwise bonding the fresh air chamber upper  250  to the fresh air chamber lower  214   c.    
     The fresh air chamber lower  214   c  is integrally formed on the upper end of the head cover  214 , and forms the lower part of the fresh air chamber R 3  or the bottom wall, and the lower part of the front and rear side walls, and the right and left side walls of the fresh air chamber R 3 . 
     The fresh air chamber upper  250  forms the upper part of the fresh air chamber R 3 , and includes an upper main body  251 , a pipe mounting portion  252  and a partition wall  253 . 
     The upper main body  251  forms the upper wall, and the upper part of the front and rear side walls, and the right and left side walls of the fresh air chamber R 3 . The pipe mounting portion  252  consists of a hole formed in the rear wall of the upper main body  251 , and is connected to a pipe defining the fresh air passage  202   y  communicating the fresh air chamber R 3  with the upstream end of the compressor  202   b  for conducting blow-by gas. The partition wall  253  separates the fresh air chamber upper  250  from the guide passage upper  221   a  integrally formed with the fresh air chamber upper  250 . 
     &lt;Spiral Passage&gt; 
     As shown in  FIG. 14 a   , the fresh air chamber R 3  is provided with the spiral passage  262  which includes a plurality of upper ribs  262   a  and a plurality of lower ribs  262   b  for causing a swirl or vortex flow to the blow-gas flowing through the fresh air chamber R 3 . 
     The upper ribs  262   a  extend downward from the inner surface of the breather chamber upper  230  of the fresh air chamber R 3 , and the lower ribs  262   b  extend upward from the inner surface of the fresh air chamber lower  214   c  of the fresh air chamber R 3 . The upper and lower ribs  262   a  and  262   b  are tilted so as to cross one another at the respective middle points, thereby presenting the shape of letter X in plan view. Owing to these ribs  262   a  and  262   b , the blow-by gas flowing through the fresh air chamber R 3  is converted into a swirling vortex flow. 
     &lt;Mode of Operation&gt; 
     The mode of operation of the breather system  201  of the internal combustion engine E is described in the following regarding the case of a negative pressure condition in the intake passage and a positive pressure condition in the intake passage, in that order. 
     &lt;Gas Liquid Separation in the Intake Passage Under Negative Pressure Condition&gt; 
     In  FIG. 12 , the solid arrows indicate the flows of the fresh air and the blow-by gas. In  FIGS. 13 a , 13 b , 14 b    and  16 , and the solid arrows indicate the flow of the blow-by gas, and the broken line arrows indicate the flow of the oil separated from the blow-by gas. 
     As shown in  FIG. 12 , the blow-by gas that has leaked from the combustion chambers to the crankcase chamber is introduced into the first gas liquid separation passage R 2   a  of the breather chamber R 2  via the breather passage R 0   a  and the guide passage R 1   a  (See  FIG. 14 b   ), and is also introduced into the first gas liquid separation chamber R 2   a  of the breather chamber R 2  via the breather passage R 0   b  and the guide passage R 1   b  (See  FIG. 16 ). The oil separated from the blow-by gas flowing through the guide passage R 1   a  is returned to the interior of the internal combustion engine E via the first oil return hole  214   e.    
     The blow-by gas that has been introduced into the first gas liquid separation chamber R 2   a  passes through the first gas liquid separation chamber R 2   a  from the end thereof on the one end side of the cylinder row direction to the end thereof on the other end side of the cylinder row direction before being introduced into the communication passage R 2   c . The oil that has been separated from the blow-by gas that flows through the first gas liquid separation chamber R 2   a  flows along the bottom surface of the first gas liquid separation chamber R 2   a  from the one end side of the cylinder row direction to the other end side of the cylinder row direction, and is then returned to the interior of the internal combustion engine E via the second oil return hole  214   f  (See  FIG. 13 a   ). 
     The blow-by gas that has been introduced into the communication passage R 2   c  flows therein toward the PCV valve  241  provided in the communication passage R 2   c.    
     Of the oil mist contained in the blow-by gas introduced into the PCV valve  241 , the part in the form of relatively large particles is removed from the blow-by gas as the blow-by gas flows through the passage in the PCV valve  241 . The oil separated from the blow-by gas as the blow-by gas flows through the PCV valve  241  is expelled downward from the blow-by gas outlet  241   a  of the PCV valve  241 , and is then returned to the interior of the internal combustion engine E via the second oil return hole  214   f  (See  FIG. 13 b   ). 
     Of the oil mist contained in the blow-by gas introduced into the PCV valve  241 , the part in the form of relatively small particles is converted into oil mist in the form of relatively large particles as the blow-by gas flows through the PCV valve  241  because the small particles adhere to one another as they pass through the narrow passage of the PCV valve  241 , and the oil mist in the form of relatively large particles is introduced into the second gas liquid separation chamber R 2   b . The oil mist in the form of relatively large particles can be relatively easily separated into the gas and the oil. The blow-by gas introduced into the second gas liquid separation chamber R 2   b  flows from the one end side of the cylinder row direction to the other end side of the cylinder row direction. 
     The blow-by gas that flows through the second gas liquid separation chamber R 2   b  flows as a swirling vortex flow owing to the spiral passage  261  defined in the second gas liquid separation chamber R 2   b . The oil mist in the blow-by gas is converted into oil mist in the form of relatively large particles owing to the mutual adherence of the oil particles under the action of the centrifugal force, and is separated from the blow-by gas. The oil that has been separated from the blow-by gas flowing through the second gas liquid separation chamber R 2   b  flows along the bottom surface of the second gas liquid separation chamber R 2   b  from the other end side of the cylinder row direction to the one end side of the cylinder row direction, and is then returned to the interior of the internal combustion engine E via the second oil return hole  214   f  (See  FIG. 13 b   ). 
     &lt;Positive Pressure Condition in the Intake Passage&gt; 
     In  FIG. 15 , the solid arrows indicate the flows of the blow-by gas. In  FIG. 14 a   , the solid arrows indicate the flow of the blow-by gas, and the broken line arrows indicate the flow of the oil separated from the blow-by gas. 
     When the intake passage  202   x  is under a positive pressure condition, the one way valve  242  remains open. In this case, gas liquid separation by the breather chamber R 2  does not take place. When the rpm of the internal combustion engine is high, and the supercharger is in operation, the intake passage  202   x  is under a positive pressure condition. However, when the pressure in the internal combustion engine is raised by the supercharger, blow-by gas is introduced into the fresh air chamber R 3 , and after the oil is separated from the blow-by gas, the blow-by gas is returned to a part of the intake passage  202   x  upstream of the compressor  202   b  of the supercharger. In other words, as shown in  FIG. 15 , the blow-by gas in the internal combustion engine E is introduced into the fresh air chamber R 3  via the oil return hole  214   d . The blow-by gas introduced into the fresh air chamber R 3  flows therein from the other end side of the cylinder row direction to the one end side of the cylinder row direction, and is returned to a part of the intake passage  202   x  upstream of the compressor  202   b  of the supercharger via the pipe mounting portion  252 . 
     The blow-by gas that flows through the fresh air chamber R 3  flows as a swirling vortex flow owing to the spiral passage  262  defined in the fresh air chamber R 3 . The oil mist in the blow-by gas is converted into oil mist in the form of relatively large particles owing to the mutual adherence of the oil particles under the action of the centrifugal force, and is separated from the blow-by gas. The oil that has been separated from the blow-by gas flowing through the fresh air chamber R 3  flows along the bottom surface of the fresh air chamber R 3  from the one end side of the cylinder row direction to the other end side of the cylinder row direction, and is then returned to the interior of the internal combustion engine E via the oil return hole  214   d  (See  FIG. 14 a   ). 
     &lt;Oil Level Gauge Insertion Hole and Oil Level Gauge&gt; 
     The oil level gauge insertion hole and oil level gauge in the breather system  201  of the internal combustion engine E are described in the following. As shown in  FIG. 16 , the breather passage R 0   a  serves also as an oil level gauge insertion hole R 4  for receiving the oil level gauge  270 . The breather passage R 0   a  branches out from the oil level gauge insertion hole R 4  in a part thereof adjacent to the upper end of the oil level gauge insertion hole R 4 , and is connected to the guide passage R 1   a  via a branch passage R 0   a   1  for conducting the blow-by gas. 
     The head cover  214  is provided with an oil level gauge guide hole  214   h  formed in a part thereof opposing the upper end of the oil level gauge insertion hole R 4 . 
     The oil level gauge  270  includes a gauge part  271 , a bulging part  272 , a large diameter part  273 , a flange part  274  and a handle part  275 . 
     The gauge part  271  is inserted into the oil level gauge insertion hole R 4 , and indicates the level of oil in the oil pan  215  according to the way the oil is deposited on the free end (lower end) of the gauge part  271 . 
     The bulging part  272  closes the oil level gauge insertion hole R 4  at a part thereof higher than the branching point of the branch passage R 0   a   1  when the oil level gauge  270  is fully inserted in the oil level gauge insertion hole R 4 . The flange part  274  is an annular flange for minimizing the gap between the oil level gauge insertion hole R 4  and the bulging part  272 . 
     The large diameter part  273  is provided on the base end side of the gauge part  271 , and is configured to be received in the oil level gauge guide hole  214   h  when the oil level gauge  270  is fully inserted in the oil level gauge insertion hole R 4 , and is provided with a plurality of seal members on the outer periphery thereof for sealing the gap between the large diameter part  273  and the inner circumferential surface of the oil level gauge guide hole  214   h.    
     The flange part  274  is provided in the end of the large diameter part  273  remote from the gauge part  271 , and is configured to engage the peripheral part of the upper end of the oil level gauge guide hole  214   h  when the oil level gauge  270  is fully inserted in the oil level gauge insertion hole R 4 . 
     The handle part  275  is provided in the end of the flange part  274  remote from the large diameter part  273 , and consists of a ring by which the user holds the oil level gauge  270 . 
     As shown in  FIGS. 17 and 18 , the head cover  214  is provided with guard members  214   i  and  214   j . The guard member  214   i  consists of a tubular member projecting downward from the bottom surface of the head cover  214  to separate the oil level gauge guide hole  214   h  from the oil level gauge insertion hole R 4 . The guard member  214   j  consists of a tubular member projecting downward from the bottom surface of the head cover  214  to cover a lower end of the oil return hole  214   d.    
     The guide passage R 1   a  on the side of the head cover  214  opens out in a part adjoining a cam lobe which is integrally formed with the camshaft and rotates in the second embodiment for compact design of the internal combustion engine E. The guard members  214   i  and  214  prevent the oil that is splashed within the cam actuation chamber by the cam lobe from being drawn into the guide passage R 1   a  and the oil return hole  214   d  along with the blow-by gas. 
     In the second embodiment, the guide passage R 1   a  on the side of the head cover  214  is provided with an enlarged cross sectional area in comparison to the oil level gauge insertion hole R 4  in order to promote the flow of blow-by gas into the branch passage R 0   a   1  branching out from the oil level gauge insertion hole R 4 . Therefore, the guard member  214   i  prevents the splashed oil from being mixed into the blow-by gas, and the blow-by gas can be favorably introduced into the breather chamber R 2  so that the scavenging performance and the gas liquid separation performance can be further improved. 
     Because the bulging part  272  provided in the oil level gauge  270  is located immediately above the point of the oil level gauge insertion hole R 4  from which the branch passage R 0   a   1  branches out, the blow-by gas flowing down the oil level gauge insertion hole R 4  is prevented from flowing toward the oil level gauge guide hole  214   h , and is favorably diverted to the side of the branch passage R 0   a   1 . Even when the oil splashed in the valve actuation chamber reaches the area surrounding the oil level gauge guide hole  214   h , the splashed oil is prevented from smearing the gauge part  271  when inserting and/or removing the oil level gauge  270  by causing the oil mist to be deposited on the bulging part  272 . This is an additional function of the bulging part  272 . 
     In the breather system  201  of the internal combustion engine of the second embodiment, the breather chamber R 2  is essentially separated into two gas liquid separation chambers by the PCV valve  241  so that the part of the oil mist with relatively large particle sizes is removed when the blow-by gas flows through the PCV valve  241 , and the part of the oil mist with relative small particle sizes is changed into that with relative large particle sizes owing to the mutual adherence of the particles as the oil mist passes through the narrow passage in the PCV valve  241  before being introduced into the second gas liquid separation chamber R 2   b . The oil mist with increased particle sizes can be readily separated from the blow-by gas in the second gas liquid separation chamber R 2   b . Also owing to the spiral passage  261  in the second gas liquid separation chamber R 2   b , the blow-by gas passes through the second gas liquid separation chamber R 2   b  as a swirling vortex flow as the blow-by gas flows up and down in the second gas liquid separation chamber R 2   b  so that the oil mist in the blow-by gas is converted into that with large particle sizes under the action of the centrifugal force, and the gas liquid separation performance can be improved. 
     In the breather system  201  of the internal combustion engine E, when the blow-by gas in the internal combustion engine E is supplied to the intake passage  202   x  by the intake negative pressure, the negative pressure in the interior of the internal combustion engine E admits the fresh air from the fresh air chamber R 3  into the interior of the internal combustion engine E so that the interior of the internal combustion engine E can be favorably scavenged. Furthermore, when the intake passage  202   x  is placed under a positive pressure condition by the compressor  202   b  of the supercharger when the engine E is operating under a high load and a high rpm condition, and the supercharger is therefore in operation, the one way valve  242  of the breather chamber R 2  closes so that the blow-by gas does not flow into the breather chamber R 2 , thereby causing a fear that the blow-by gas might flow into the fresh air chamber R 3 . However, because the fresh air chamber R 3  is provided with a spiral passage, and is therefore capable of removing oil from the blow-by gas in a favorable manner, only the blow-by gas freed from oil is introduced into the intake passage  202   x . Therefore, pollution of the intake passage  202  with the oil can be avoided, and the pressure in the interior of the internal combustion engine is prevented from rising excessively so that the scavenging performance for the interior of the internal combustion engine can be maintained in a favorable manner. 
     In the breather system  201  of the internal combustion engine E, because the ribs  261   a ,  261   b ,  262   a  and  262   b  in the spiral passages  261  and  262  cause the swirling movement of the blow-by gas as the blow-by gas flows up and down in the spiral passages  261  and  262 , even when the length of the internal combustion engine along the cylinder row direction is small, and the passage diameter of the breather chamber R 2  and the fresh air chamber R 3  is small, an adequate gas liquid separation performance can be achieved owing to the use of the centrifugal force. 
     In the breather system  201  of the internal combustion engine E, when the intake passage  202   x  is placed under a positive pressure condition owing to the operation of the supercharger, and the one way valve  242  provided in the breather chamber R 2  closes so that the flow of the blow-by gas has substantially ceased, the oil separated from the blow-by gas in the guide passage R 1   a  and the first gas liquid separation chamber R 2   a  can be quickly returned to the interior of the internal combustion engine E via the first oil return hole  214   e , and the oil collected in the second gas liquid separation chamber R 2   b  can be quickly returned to the interior of the internal combustion engine E via the second oil return hole  214   f.    
     When the engine is operating at a low rpm or a medium rpm, and the supercharger is not in operation, the intake passage  202   x  is under a negative pressure condition. Because the velocity of the flow of the blow-by gas into the first gas liquid separation chamber R 2   a  is relative low, even though the first oil return hole  214   e  is provided in a part of the first gas liquid separation chamber R 2   a  adjoining the guide passage R 1   a , very little of the oil separated from the blow-by gas is stirred up by the blow-by gas in the part surrounding the first oil return hole  214   e , and is thereby mixed back into the blow-by gas. Also, because the first oil return hole  214   e  is provided remote from the PCV valve  241 , the oil in the PCV valve  241  is prevented from being deposited in a part adjoining the PCV valve  241  and obstructing the flow of the blow-by gas through the PCV valve  241 . 
     In the breather system  201  of the internal combustion engine E, because the bottom surfaces of the first gas liquid separation chamber R 2   a  and the second gas liquid separation chamber R 2   b  are inclined downward toward the respective oil return holes  214   e  and  214   f , the oil separated from the blow-by gas in the first gas liquid separation chamber R 2   a  and the second gas liquid separation chamber R 2   b  can be quickly returned to the interior of the internal combustion engine E via the oil return holes  214   e  and  214   f.    
     In the breather system  201  of the internal combustion engine E, the PCV valve  241  having the additional function to serve as a communication passage between the first gas liquid separation chamber R 2   a  and the second gas liquid separation chamber R 2   b  extends along the lengthwise direction of the second gas liquid separation chamber R 2   b  so that the width of the head cover  214  with respect to the direction perpendicular to the cylinder row direction can be minimized, and a compact design of the breather chamber R 2  and a high gas liquid separation performance can be achieved at the same time. In the breather system  201  of the internal combustion engine E, the blow-by gas that has been expelled from the PCV valve  241  to the second gas liquid separation chamber R 2   b  attains a high velocity as the blow-by gas passes through the narrow passage in the PCV valve  241  and flows out of the blow-by gas outlet  241   a . The blow-by gas that leaves the PCV valve  241  via the blow-by gas outlet  241   a  at high velocity collides with a rib  235  provided opposite to the blow-by gas outlet  241   a  so that the oil mist adheres to the rib  235 , and increases the particle sizes thereof, and the gas liquid separation performance can be improved even further. 
     In the breather system  201  of the internal combustion engine E, because the breather passages Roa and Rob are provided in the parts of the oil pan  215  having different depths, even when the internal combustion engine E is inclined owing to the cornering of the vehicle or the inclination of the road surface, and part of the breather passages are submerged in the oil, the blow-by gas can still be introduced into the breather chamber R 2  via the remaining breather passages so that the pressure in the internal combustion engine E can be favorably adjusted, and an increase in friction owing to the increase in the blow-by gas pressure in the crankcase chamber can be avoided. 
     In the breather system  201  of the internal combustion engine E, because a same, single hole serves as both the breather passage R 0   a  and the oil level gauge insertion hole R 4 , the manufacturing work is simplified, and the size of the internal combustion engine E can be reduced. In the breather system  201  of the internal combustion engine E, owing to the presence of the guard members  213   i  and  214   j , the smearing of the upper end of the oil level gauge insertion hole R 4  with the oil mist in the blow-by gas flowing through the breather passage R 0   a  can be avoided, and the oil dripping from the oil return hole  214   d  is prevented from splashing onto the upper end of the oil level gauge insertion hole R 4 . In other words, when the oil level gauge  270  is inserted into the oil level gauge insertion hole R 4  or is removed from the oil level gauge insertion hole R 4 , oil is prevented from being deposited on the gauge part  271  of the oil level gauge  270  in the upper end of the oil level gauge insertion hole R 4  so that the oil level can be favorably measured without suffering from such interferences. 
     In the breather system  201  of the internal combustion engine E, even when oil should be deposited on the upper end of the oil level gauge insertion hole R 4 , the oil that could otherwise interfere with the measurement of the oil level is deposited on or wiped out by the bulging part  272 , and is thereby prevented from being deposited on the gauge part  271  of the oil level gauge  270  so that the oil level can be favorably measured at all times. 
     OTHER EMBODIMENTS 
     The breather system of an internal combustion engine of another embodiment of the present invention is described in the following in regard to the breather system  201  discussed above.  FIG. 19  is a plan view of the breather system of another embodiment of the present invention.  FIG. 20  is a view from the direction indicated by arrow X 9  in  FIG. 19 . In the following description, the one end side and the other end of side of the cylinder row direction are reversed from those of the preceding embodiment. 
     In the breather system  201 B of the internal combustion engine E of the embodiment illustrated in  FIG. 19 , the breather chamber R 2  is provided with a relatively large volume, and the PCV valve  241  extends perpendicularly to the direction of the blow-by gas flow in the gas liquid separation chambers R 2   a  and R 2   b , instead of extending along the length of the gas liquid separation chambers R 2   a  and R 2   b . In the breather system  201 B, the first gas liquid separation chambers R 2   a  is also provided with a spiral passage  263  similar to the spiral passages  261  and  262 . In this embodiment, the first gas liquid separation chambers R 2   a  is located behind the second gas liquid separation chambers R 2   b.    
     As shown in  FIG. 20 , when the internal combustion engine E is laterally mounted on a vehicle, the bottom surfaces of the chambers R 2  and R 3  may be inclined with respect to the direction perpendicular to the cylinder row direction so that the oil separated from the blow-by gas in the first gas liquid separation chambers R 2   a  is caused to flow downward through the PCV valve  241  toward the side of the second gas liquid separation chamber R 2   b . Thereby, the oil separated in the first gas liquid separation chambers R 2   a  is prevented from being stored therein, and can be quickly returned to the side of the oil pan  215  (See  FIG. 1 ). 
     In the breather system  201 B illustrated in  FIG. 19 , no special guide passage for guiding the blow-by gas from the breather passage to the breather chamber R 2  is provided, and a same hole serves both as the oil return hole  214   e  for returning oil from the first gas liquid separation chambers R 2   a  and the blow-by gas introduction hole (breather passage) for introducing blow-by gas into the first gas liquid separation chambers R 2   a . In other words, the oil return hole  214   e  serves both as the oil return hole and the blow-by gas inlet hole. 
     The breather passages and the oil return hole  214   e  are typically formed in parts of the internal combustion engine which are not used for other purposes, and are often provided in a plurality of locations. Therefore, it has been a common practice to use a same hole or passage as an oil return passage and a breather passage. 
     In an internal combustion engine E equipped with a supercharger, as is the case with the present invention, the rotational speed is high, and the amount of blow-by gas is great. Therefore, when the oil return passage and the breather passage share a same passage, if the oil to be returned is large in quantity, the oil return hole  214   e  could be blocked, and the blow-by gas may not flow freely into the breather chamber R 2 . Therefore, in the breather system  201 B illustrated in  FIG. 19 , an auxiliary opening  214   g  is formed in a position higher than the oil return hole  214   e  along the inclined direction and different from the upstream side of the oil return hole  214   e  of the oil return hole  214   e  with respect to the flow of the oil separated from the blow-by gas along the bottom surface of the first gas liquid separation chambers R 2   a.    
     Thereby, the auxiliary opening  214   g  is located substantially above the upper end of the breather passage in the internal combustion engine E so that the blow-by gas can be favorably introduced into the breather chamber R 2  via the auxiliary opening  214   g  which is located adjacent to the oil return hole  214   e  which could be blocked by the returning oil. 
     The embodiments of the present invention have been described above, but the present invention is not limited by such embodiments, and can be modified and substituted without departing from the spirit of the present invention. In the first embodiment, the first and second gas liquid separation passages  56  and  57  were both arranged along the cylinder row direction (lateral direction), but may also extend in any other horizontal direction such as in the fore and aft direction. The position of the PCV valve  70  can be changed, and may be provided between the second gas liquid separation passage  57  and the intake system  21 . The internal combustion engine E may also be mounted on a vehicle in other orientations, beside from that shown in the drawings. The second oil return hole  214   f  may be provided in a part of the bottom wall of the second gas liquid separation chamber R 2   b  on the one end side of the cylinder row direction. 
     GLOSSARY OF TERMS 
     
         
           1  internal combustion engine 
           4  head cover 
           10  oil separation device 
           11  crankcase chamber 
           21  intake system 
           24 A compressor 
           25  throttle valve 
           26  intake manifold 
           33  oil chamber 
           35  oil return passage 
           36  first blow-by gas passage 
           37  gauge passage 
           41  first cover member 
           42  second cover member 
           44  valve actuation chamber 
           48  gauge hole 
           51  oil level gauge 
           53  first gas inlet hole 
           54  vent hole (gas inlet) 
           56  first gas liquid separation passage 
           56 A lower wall 
           56 B upper wall 
           56 C front side wall 
           56 D rear side wall 
           56 G narrowed section 
           56 H lower partition wall 
           56 J upper partition wall 
           57  second gas liquid separation passage 
           57 A lower wall 
           57 B upper wall 
           57 C front side wall 
           57 D rear side wall 
           57 G narrowed section 
           57 H lower partition wall 
           57 J upper partition wall 
           58  second blow-by gas passage 
           59  third blow-by gas passage 
           63  gas communication port (gas outlet) 
           64  gas passage 
           65  gas outlet port 
           66  blow-by gas supply passage 
           67  second gas inlet port 
           70  PCV valve (gas inlet) 
           76  oil discharge hole 
           201  breather system of internal combustion engine (oil separation device) 
           202   b  compressor 
           214  cylinder head 
           215  oil pan 
           235  rib 
           241  PCV valve 
           242  one way valve 
           261 ,  262 ,  263  spiral passage 
           261   a ,  262   a  upper rib (upper partition wall) 
           261   b ,  262   b  lower rib (lower partition wall) 
           270  oil level gauge 
           272  bulging portion 
         E internal combustion engine 
         R 0   a  breather passage 
         R 0   a   1  branch passage 
         R 0   b  breather passage 
         R 1   a  guide passage 
         R 1   b  guide passage 
         R 2  breather chamber 
         R 2   a  first gas liquid separation passage 
         R 2   b  second gas liquid separation passage 
         R 2   c  communication passage 
         R 3  fresh air chamber 
         R 4  oil level gauge insertion hole