Patent Publication Number: US-8123829-B2

Title: Gas-liquid separation device of engine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a National Stage entry of International Application No. PCT/JP2006/312609, having an international filing date of Jun. 23, 2006; which claims priority to Japanese Application Nos.: JP 2005-183596 and JP 2005-183605, both filed Jun. 23, 2005, the disclosures of each of which is hereby incorporated in its entirety by reference. 
     TECHNICAL FIELD 
     The present invention relates to a gas-liquid separation device of an engine for separating oil mist from air in an engine case. 
     BACKGROUND ART 
     A conventional gas-liquid separation device is publicly known from the following Patent Publication 1 in which two mounting seats for mounting a breather case of a breather device having a gas-liquid separation function are provided on a ceiling wall and a peripheral wall of a crankcase of an engine, respectively, and the breather case is mounted on one of the two mounting seats which receives less oil droplets depending on the usage of the engine. 
     Patent Publication 1: Japanese Utility Model Publication No. 62-12820 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     The above-described conventional device has a disadvantage that the breather case projects from the surface of the crankcase to upsize the engine because a breather chamber is defined by a concave wall surface formed on the crankcase and the breather case mounted on the mounting seat, and also has a disadvantage that the shape of the crankcase is complicated because a concave wall surface is formed in the crankcase to partition a part of the breather chamber. 
     The present invention has been achieved in view of the above-mentioned circumstances, and has an object to provide a small light gas-liquid separation device of an engine which has a small number of components. 
     Means for Solving the Problems 
     In order to achieve the above object, according to a first feature of the present invention, there is provided a gas-liquid separation device of an engine for separating oil mist from air in an engine case, characterized in that a bearing holder comprising a bearing rotatably supporting a crankshaft is fixed so as to face an opening of the engine case, and a gas-liquid separation chamber is formed between a cover member covering the opening and the bearing holder. 
     The bearing corresponds to a ball bearing  67  in an embodiment of the present invention described later. 
     According to a second feature of the present invention, in addition to the first feature, a labyrinth is formed in the gas-liquid separation chamber by ribs projecting from at least one of the bearing holder and the cover member. 
     The ribs correspond to a fourth rib  66   d , a fifth rib  66   e , a first rib  68   a  and a second rib  68   b  in the embodiment of the present invention described later. 
     According to a third feature of the present invention, in addition to the second feature, the ribs projecting from the bearing holder and the ribs projecting from the cover member mutually overlap to form the labyrinth. 
     According to a fourth feature of the present invention, in addition to any of the first to third features, the air from which the oil mist is separated in the gas-liquid separation chamber is guided through a breather channel to a breather device to further perform gas-liquid separation. 
     According to a fifth feature of the present invention, in addition to the fourth feature, the breather channel is arranged on an upper part of the engine case. 
     According to a sixth feature of the present invention, in addition to the first feature, a part of the engine case is formed by a crank case having the opening on one side; a plurality of step portions facing the opening and aligned along a circumferential direction are formed on the inner peripheral wall of the crankcase; the opposite ends of the crankshaft are supported via bearings by the bearing holder which is fastened to the step portions and the other side wall of the crank case; and a reinforcement rib surrounding the plurality of step portions is formed integrally on an outer peripheral surface of the crankcase. 
     According to a seventh feature of the present invention, in addition to the sixth feature, a cylinder block is formed integrally on the crankcase to form the engine case, and an end of the reinforcement rib is connected integrally to the outer side wall of the cylinder block. 
     According to an eighth feature of the present invention, in addition to the sixth or seventh feature, an oil stirring chamber communicating with a crank chamber in the crankcase is defined between the bearing holder and the cover member, and a drive rotation member fixed on the crankshaft of a timing transmission system for valve operation is arranged in the oil stirring chamber. 
     According to a ninth feature of the present invention, in addition to the eighth feature, an oil slinger driven by the crankshaft to splash a lubricant oil stored in the oil stirring chamber is arranged in the oil stirring chamber, and a rib for guiding the lubricant oil splashed by the oil slinger to the timing transmission system side is formed in the bearing holder. 
     Effect of the Invention 
     With the arrangement of the first feature, the baring holder comprising the bearing rotatably supporting the crankshaft is fixed so as to face the opening of the engine case, and the gas-liquid separation chamber is formed between the cover member covering the opening and the bearing holder. Therefore, the bearing holder can be used as a part of a wall surface of the gas-liquid separation chamber to partition the gas-liquid separation chamber without increasing the number of components and without forming a special wall surface in the engine case. Consequently, the size and weight of the engine case can be reduced, the shape of the engine case can be simplified, and the cost can be reduced due to reduction of the number of components. 
     With the arrangement of the second feature, a labyrinth is formed by the rib projecting from at least one of the bearing holder and the cover member, so that gas-liquid separation can be effectively performed by the labyrinth. 
     With the arrangement of the third feature, the rib projecting from the bearing holder and the rib projecting from the cover member are made to mutually overlap to form the labyrinth, so that a complicated labyrinth can be formed with a simple arrangement to further increase the gas-liquid separation effect. 
     With the arrangement of the fourth feature, the air from which oil mist is separated in the gas-liquid separation chamber is introduced into the breather device through the breather channel to further perform gas-liquid separation, so that the consumption of oil can further be reduced. 
     With the arrangement of the fifth feature, the breather channel is arranged in the upper part of the engine case, thereby minimizing the amount of the remaining oil mist which is not removed in the gas-liquid separation chamber and enters the breather channel. 
     With the arrangement of the sixth feature, the reinforcement rib couple the plurality of step portions inside the crankcase to one another on the outer peripheral surface of the crankcase, so that the support rigidity of the bearing holder supported by the step portions, and hence the support rigidity of the crankshaft supported by the bearing holder can be effectively enhanced, resulting in reduced thickness and weight of the crankcase. 
     With the arrangement of the seventh feature, the end of the reinforcement rib is coupled integrally to the side wall of a cylinder block, so that the reinforcement function of the reinforcement rib is further improved, and the support rigidity of the bearing holder can be further enhanced. 
     With the arrangement of the eighth feature, a space between the bearing holder and the cover member can be effectively used for installation of the timing transmission system for valve operation, thereby contributing to decrease in the size of the engine. 
     With the arrangement of the ninth feature, the rib is formed in the bearing holder, so that the oil splashed by the oil slinger can be guided to the timing transmission system side, and the bearing holder can be easily molded together with the rib because the bearing holder is a relatively small component. 
     The above-mentioned object, other objects, characteristics, and advantages of the present invention will become apparent from a preferred embodiment, which will be described in detail below by reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of a general-purpose four-cycle engine. (first embodiment) 
         FIG. 2  is a view of  FIG. 1  viewed in the direction of arrow  2 . (first embodiment) 
         FIG. 3  is an enlarged sectional view taken along the  3 - 3  line in  FIG. 1 . (first embodiment) 
         FIG. 4  is a view of  FIG. 3  viewed in the direction of arrow  4 . (first embodiment) 
         FIG. 5  is an enlarged sectional view taken along the  5 - 5  line in  FIG. 4 . (first embodiment) 
         FIG. 6  is an enlarged sectional view taken along the  6 - 6  line in  FIG. 2 . (first embodiment) 
         FIG. 7  is an enlarged sectional view taken along the  7 - 7  line in  FIG. 6 . (first embodiment) 
         FIG. 8  is an enlarged sectional view taken along the  8 - 8  line in  FIG. 7 . (first embodiment) 
         FIG. 9  is an enlarged sectional view taken along the  9 - 9  line in  FIG. 6  and  FIG. 10 . (first embodiment) 
         FIG. 10  is an enlarged view taken along the  10 - 10  line and viewed in the direction of the arrow in  FIG. 2 . (first embodiment) 
         FIG. 11  is a view of a part of  FIG. 10 . (first embodiment) 
         FIG. 12  is a sectional view taken along the  12 - 12  line in  FIG. 10 . (first embodiment) 
         FIG. 13  is a longitudinal sectional plan view of the engine. (first embodiment) 
         FIG. 14  is a sectional view taken along the  14 - 14  line in  FIG. 13 . (first embodiment) 
         FIG. 15  is a sectional view taken along the  15 - 15  line in  FIG. 13 . (first embodiment) 
         FIG. 16  is an enlarged view of the periphery of a crankshaft of  FIG. 13 . (first embodiment) 
         FIG. 17  is a view of  FIG. 16  viewed in the direction of arrow  17 . (first embodiment) 
         FIG. 18  is a sectional view taken along the  18 - 18  line in  FIG. 14 . (first embodiment) 
         FIG. 19  is a sectional view taken along the  19 - 19  line in  FIG. 14 . (first embodiment) 
         FIG. 20  is a sectional view taken along the  20 - 20  line in  FIG. 18 . (first embodiment) 
         FIG. 21  is a sectional view taken along the  21 - 21  line in  FIG. 19 . (first embodiment) 
         FIG. 22  is a view taken along the  22 - 22  line and viewed in the direction of the arrow in  FIG. 20 . (first embodiment) 
         FIG. 23  is a view corresponding to  FIG. 22  with a driven pulley removed. (first embodiment) 
         FIG. 24  are views explaining how to attach the driven pulley to a camshaft. (first embodiment) 
     
    
    
     EXPLANATION OF REFERENCE NUMERALS AND SYMBOLS 
     
         
           11  engine case 
           11   e  breather chamber 
           11   k  opening 
           14  crankshaft 
           52  breather device 
           64  bearing 
           66  bearing holder 
           66   b ,  66   d ,  66   e ,  68   a ,  68   b  rib 
           67  bearing 
           68  cover member 
           70  oil stirring chamber 
           77  oil slinger 
           80  drive rotation member 
           82  labyrinth 
           83  gas-liquid separation chamber 
           102  crankcase 
           103  cylinder block 
           108 ,  108  step portion 
           109  crank chamber 
           116  reinforcement rib 
           137  timing transmission system 
           171  storing lubricant oil 
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     A preferred embodiment of the present invention is explained below with reference to the accompanying drawings. 
     Embodiment 1 
     As shown in  FIGS. 1 and 2 , a single-cylinder four-cycle engine E is arranged with a cylinder axis line L 1  slightly inclined so that a cylinder head  12  and a head cover  13  are high with respect to an engine case  11  integrally having a crankcase and a cylinder block. A crankshaft  14  projects from one of end surfaces of the engine case  11 . A recoil starter  16  for cranking a crankshaft  14  to start the engine is provided on the outer surface of a cover  15  covering the other end surface of the engine case  11 . A carburetor  17  is provided on the side part of the cylinder head  12 . An air intake channel  18  extending upward from the carburetor  17  is connected to an air cleaner  19 . A muffler  20  is mounted on the upper parts of the cylinder head  12  and the head cover  13  so as to align with the air cleaner  19 . A fuel tank  21  is mounted at a position closer to the crankcase than to the air cleaner  19  and the muffler  20 . 
     The fuel tank  21  is formed by integrally coupling the lower edge of a tank upper part  21   a , the upper edge of a tank lower part  21   b  and the upper edge of a tank holder  22  by a crimping portion  23 . A tank stay  24  is fixed by bolts  25  on four mounting bolts  11   a  projectingly provided on the engine case  11 . The outer peripheries of four rubber bushes  26  are supported on the upper surface of the tank stay  24 . A bolt  27  passing upward through the center of each rubber bush  26  passes through the tank holder  22  and a reinforcement plate  28 , and is fastened to a nut  29 , whereby the fuel tank  21  is supported above the engine case  11  in a vibration-isolating manner. 
     As shown in  FIG. 3  and  FIGS. 6 to 8 , an automatic fuel cock  30  automatically feeding fuel in the fuel tank  21  to carburetor  17  during operation of the engine E is mounted on the lower surface of the fuel tank  21 . The automatic fuel cock  30  comprises a first housing  31  and a second housing  32  which are integrally coupled to each other. A stay  31   a  (see  FIG. 6 ) protruding from the first housing  31  is fixed on the lower surface of the tank holder  22  by a bolt  33  and a nut  34 . At this time, the upper part of the automatic fuel cock  30  protrudes upward through an opening  22   a  (see  FIG. 7 ) of the tank holder  22 , and the lower part of the automatic fuel cock  30  protrudes downward through an opening  24   a  (see  FIGS. 3 and 6 ) of the tank stay  24 . 
     As best shown in  FIG. 8 , the first housing  31  of the automatic fuel cock  30  comprises: a fuel inlet joint  31   b ; a fuel outlet joint  31   c ; a valve seat  31   d  formed between the fuel inlet joint  31   b  and the fuel outlet joint  31   c ; and a disc-shaped diaphragm support portion  31   e . The second housing  32  comprises: a first negative pressure introduction joint  32   a ; a negative pressure chamber  32   b  communicating with the first negative pressure introduction joint  32   a ; and a disc-shaped diaphragm support portion  32   c . The fuel inlet joint  31   b  is connected to a joint  36  provided on the lower surface of the fuel tank  21  via a first fuel hose  35 . The fuel outlet joint  31   c  is connected to the carburetor  17  via a second fuel hose  37 . The first negative pressure introduction joint  32   a  is connected to a second negative pressure introduction joint  11   b  of the engine case  11  via a negative pressure tube  38  made of rubber. By using the negative pressure  38  made of rubber, the degree of freedom in the layout of the fuel tank  21  can be improved with respect to the engine case  11 . 
     An annular diaphragm support member  39  is sandwiched between the diaphragm support portion  31   e  of the first housing  31  and the diaphragm support portion  32   c  of the second housing  32 . The outer periphery of a first diaphragm  40  is fixed between the diaphragm support portion  31   e  of the first housing  31  and the diaphragm support member  39  via a seal member  41 . The outer periphery of a second diaphragm  42  is fixed between the diaphragm support portion  32   c  of the second housing  32  and the diaphragm support member  39  via a seal member  43 . The first and second diaphragms  40  and  42 , a spacer block  44  sandwiched between the central portions of the first and second diaphragms  40  and  42 , and a disk-shaped spring sheet  45  in contact with the rear surface of the second diaphragm  42  are fixed integrally by a rivet  46  passing through them. 
     A valve seat forming member  48  is fitted between the first negative pressure introduction joint  32   a  and the negative pressure chamber  32   b  of the second housing  32  via a spacer plate  47 . A valve spring  49  arranged between the valve seat forming member  48  and the spring sheet  45  urges a valve body  40   a  formed at the central part of the first diaphragm  40  in the direction to be seated on the valve seat  31   d  of the first housing  31 . Fixed to the valve seat forming member  48  by a bolt (not shown) are one end of a lead valve  50  capable of being seated on a valve seat  48   b  facing a through hole  48   a  passing through the center of the valve seat forming member  48 , and one end of a stopper  51  covering the outside thereof and regulating a range of motion of the lead valve  50 . A very small through hole  50   a  is formed in the lead valve  50  to provide communication between the first negative pressure introduction joint  32   a  and the negative pressure  32   b.    
     As apparent from  FIGS. 7 and 8 , a taper portion  32   d  for facilitating insertion of the negative pressure tube  38  is formed at the lower end of the first negative pressure introduction joint  32   a , and a reverse U-shaped notch  32   e  is formed on the taper portion  32   d . The negative pressure tube  38  comprises: a first coupling portion  38   a  extending in a vertical direction and inserted into the first negative pressure introduction joint  32   a ; a second coupling portion  38   b  extending in a vertical direction and inserted into the second negative pressure introduction joint  11   b ; and an intermediate portion  38   c  extending obliquely downward from the lower end of the first coupling portion  38   a  to the upper end of the second coupling portion  38   b . The negative pressure tube  38  is generally formed into the shape of a crank. A linear recessed portion  38   d  is formed on the bottom surface of the first coupling portion  38   a . A linear protrusion  11   c  is formed on the upper surface of the engine case  11  facing the bottom surface of the first coupling portion  38   a  of the negative pressure tube  38  so as to be engaged in the linear recessed portion  38   d , and this engagement between the protrusion  11   c  and the recessed portion  38   d  positions the negative pressure tube  38  in a direction of rotation about an vertical axis. 
     As apparent from  FIGS. 6 and 9 , a breather device  52  provided on the side surface of the engine case  11  comprises a breather chamber  54  surrounded by an annular peripheral wall  11   d  and a cover  53 . A breather chamber  11   e  is opened at one end of the breather chamber  54 . Fixed to the inner wall of the breather chamber  54  by a bolt  57  are one end of a lead valve  55  capable of being seated on a valve seat  11   f  formed in the opening of the breather channel  11   e , and one end of a stopper  56  regulating a range of motion of the lead valve  55 . A joint  53   a  is formed on the cover  53  such that the joint  53   a  faces the other end of the breather chamber  54  distant from the breather channel  11   e . The joint  53   a  is connected to an air intake system of the engine E via a breather pipe  58 . Two ribs  11   g  and  11   h  are projectingly provided in the breather chamber  54  to form a labyrinth  59  between the breather channel  11   e  and the joint  53   a . The bottom of the breather chamber  54  communicates with the inner space of the engine case  11  via an oil return hole  11   i . A communicating hole  11   j  passes through the interior of the second negative pressure introduction joint  11   b  to which the second coupling portion  38   b  of the negative pressure tube  38  is fitted, and communicates with the breather channel  11   e.    
     The structure of a gas-liquid separation device  61  of the engine E will now be described based on  FIGS. 9 to 12 . 
     The crankshaft  14  of the engine E has a pin portion  14   a  connected to a piston  63  via a connecting rod  62 . One journal portion  14   b  of the crankshaft  14  is supported on the engine case  11  via a ball bearing  64 , and the other journal portion  14   c  is supported on a bearing holder  66  fixed by six bolts  65  in the engine case  11  via a ball bearing  67 . A cover member  68  is fixed by nine bolts  69  in an opening  11   k  of the engine case  11  so as to cover the front surface of the bearing holder  66 . An oil stirring chamber  70  storing lubricant oil  171  on the bottom is defined between the cover member  68  and the bearing holder  66 . 
     Opposite ends of a primary balancer shaft  73  (see  FIG. 12 ) are supported between the engine case  11  and the bearing holder  66  via a pair of ball bearings  71  and  72 . A drive gear  74  provided on the crankshaft  14  engages with a driven gear  75  provided on the primary balancer shaft  73 , whereby the primary balancer shaft  73  rotates at a speed equal to the speed of rotation of the crankshaft  14 . 
     An oil slinger  77  is rotatably supported on the bottom of the oil stirring chamber  70  via a rotor shaft  76 . A driven gear  78  provided on the rotor shaft  76  is engaged with a drive gear  79  provided on the crankshaft  14 , whereby the oil slinger  77  is rotated by the crankshaft  14 . A timing belt  81  wound around a drive pulley  80  provided on the crankshaft  14  is connected to a driven pulley (not shown) provided on the cylinder head  12 . 
     As apparent from  FIGS. 10 and 11 , projectingly provided on the side surface of the bearing holder  66  are a first rib  66   a  surrounding a part of the outer periphery of the oil slinger  77 , a second rib  66   b  surrounding a part of the outer peripheries of the drive gear  79  and the drive pulley  80 , a third rib  66   c  leading to the end of the first rib  66   a  and extending along the lower surface of a chord on the lower side of the timing belt  81 , a fourth rib  66   d  communicating with the end of the second rib  66   b  and extending along the upper surface of a chord on the upper side of the timing belt  81 , and an independent fifth rib  66   e  extending obliquely in a direction opposite to a direction along which the fourth rib  66   d  extends obliquely from the vicinity of the connection between the second rib  66   b  and the fourth rib  66   d . A first rib  68   a  and a second rib  68   b  substantially parallel to the fourth rib  66   d  and the fifth rib  66   e  of the bearing holder  66  are projectingly provided on the side surface of the cover member  68   a.    
     A region surrounded by the first to fourth ribs  66   a  to  66   d  of the bearing holder  66  constitutes the oil stirring chamber  70 . A gas-liquid separation chamber  83  having a labyrinth  82  constituted by the fourth and fifth ribs  66   d  and  66   e  of the bearing holder  66  and the first and second ribs  68   a  and  68   b  of the cover member  68  is defined outside the first to fourth ribs  66   a  to  66   d . The upper part of the gas-liquid separation chamber  83  communicates with the breather device  52  via the breather channel  11   e  (see  FIG. 9 ). 
     The operation of the above-described arrangement will be described. 
     In  FIG. 10 , when the engine E is operated, the oil slinger  77  connected to the crankshaft  14  via the drive gear  79  and the driven gear  78  rotates in the oil stirring chamber  70 , and scoops up and splashes the oil accumulated on the bottom of the oil stirring chamber  70 . The splashed oil is guided by the first and second ribs  66   a  and  66   b  of the bearing holder  66  to an area between the third and fourth ribs  66   c  and  66   d  extending along the timing belt  81 , then deposited on the timing belt  81 , and fed to a valve-operating chamber of the cylinder head  12  to lubricate a valve-operating mechanism. The valve-operating mechanism and the lubrication thereof will be described in detail later. 
     Air-containing oil mist generated in the oil stirring chamber  70  passes through the labyrinth  82  constituted by the fourth and fifth ribs  66   d  and  66   e  of the bearing holder  66  and the first and second ribs  68   a  and  68   b  of the cover member  68  in the gas-liquid separation chamber  83 , and oil separated in this process falls along the first and second ribs  66   a  and  66   b  to be returned to the bottom of the oil stirring chamber  70 . 
     The bearing holder  66  comprising the ball bearing  67  supporting the crankshaft  14  is fixed so as to face the opening  11   k  of the engine case  11 . The gas-liquid separation chamber  83  is formed between the cover member  68  coupled to the opening  11   k  and the bearing holder  66 , thus using the bearing holder  68  as a part of the wall surface of the gas-liquid separation chamber  83 . Therefore, the number of components can be decreased as compared to a case where a part of the wall surface of the gas-liquid separation chamber  83  is constituted by a special member. Further, the size and weight of the engine case  11  can be reduced and the shape can be simplified as compared to a case where a part of the wall surface of the gas-liquid separation chamber  83  is constituted by a partition wall formed integrally with the engine case  11 . 
     Moreover, the labyrinth  82  is provided in the gas-liquid separation chamber  83 , thereby effectively separating the oil mist contained in the air in the engine case  11 . Particularly, the fourth and fifth ribs  66   d  and  66   e  projecting from the bearing holder  66  side, and the first and second ribs  68   a  and  68   b  projecting from the cover member  68  side are made to mutually overlap by a distance α (see  FIG. 9 ), thereby forming the complicated labyrinth  82  with a simple arrangement to further improve the gas-liquid separation effect. 
     In  FIG. 9 , the air from which the oil mist has been separated by the labyrinth  82  of the gas-liquid separation chamber  83  passes through the breather channel  11   e  and the lead valve  55  of the breather device  52 , and is fed to the breather chamber  54 . That is, a pressure pulsation generated with a reciprocation of the piston  63  is transmitted to the breather channel  11   e , the lead valve  55  is opened when the breather channel  11   e  has a positive pressure, and the lead valve  55  is closed when the breather channel  11   e  has a negative pressure, whereby the air in the breather channel  11   e  is fed to the breather chamber  54 . 
     In  FIG. 6 , the remaining oil which has not separated by the gas-liquid separation device  61  is also separated in the process that the air fed to the breather chamber  54  passes through the labyrinth  59  constituted by the ribs  11   g  and  11   h . Lastly, the air is fed back to the bottom of the engine case  11  through an oil return hole  11   i  provided on the bottom of the breather chamber  54 . Since gas-liquid separation is further performed for the air by the process that the air from which the oil mist has been separated by the gas-liquid separation device  61  is guided to the breather device  52  through the breather channel  11   e , the consumption of oil can be further reduced. The air from which the oil mist has been removed as described above still contains fuel vapor blowing from a combustion chamber into the engine case  11 , but the air containing the fuel vapor is fed back through the joint  53   a  of the cover  53  and the breather pipe  58  to the air intake system of the engine E where the fuel vapor is combusted together with a fuel-gas mixture, thereby preventing the fuel vapor from being emitted to the atmosphere. 
     In  FIG. 9 , the pressure pulsation in the engine case  11  is transmitted through the breather channel  11   e , the through hole  11   j  and the negative pressure tube  38  to the first negative pressure introduction joint  32   a  of the automatic fuel cock  30 . In  FIG. 8 , when the pressure transmitted to the first negative pressure introduction joint  32   a  of the automatic fuel cock  30  becomes a negative pressure, the lead valve  50  is separated from the valve seat  48   b  so that the negative pressure chamber  32   b  has the negative pressure; and conversely, when the pressure transmitted to the first negative pressure introduction joint  32   a  becomes a positive pressure, the lead valve  50  is seated on the valve seat  48   b  to keep the negative pressure in the negative pressure chamber  32   b . Since the negative pressure chamber  32   b  always has a negative pressure during operation of the engine E, the first and second diaphragms  40  and  42  are moved to the left against the resilient force of the valve spring  49 , and the valve body  40   a  formed in the first diaphragm  40  is separated from the valve seat  31   d . As a result, fuel in the fuel tank  21  is fed to the carburetor  17  through the fuel inlet joint  31   b , a gap between the valve seat  31   d  and the valve body  40   a , the fuel outlet joint  31   c  and the second fuel hose  37 . 
     When the engine E is stopped and the pressure pulsation in the breather channel  11   e  is eliminated, the lead valve  50  attracted in the right direction is seated on the valve seat  48   b  to seal the negative pressure chamber  32   b , because the first and second diaphragms  40  and  42  are urged in the right direction in  FIG. 8  by the resilient force of the valve spring  49 . However, air flows from the first negative pressure introduction joint  32   a  into the negative pressure chamber  32   b  through the very small through hole  50   a  provided on the valve seat  50 , and therefore the valve body  40   a  is seated on the valve seat  31   d  by the resilient force of the valve spring  49  to close the automatic fuel cock  30 . Thus, the fuel supply from the fuel tank  21  to the carburetor  17  can be automatically stopped when the engine E is stopped. 
     The negative pressure tube  38  is coupled to the first and second negative pressure introduction joints  32   a  and  11   b  according to the following procedure. The tank stay  24  is assembled beforehand to the tank holder  22  of the fuel tank  21  via the rubber bushes  26 , and further the automatic fuel cock  30  and the first fuel hose  35  are assembled beforehand to the tank holder  22 . The second coupling portion  38   b  of the negative pressure tube  38  is fitted beforehand to the second negative pressure introduction tube  11   b  of the engine case  11 . At this time, the recessed portion  38   d  on the bottom of the first coupling portion  38   a  of the negative tube  38  is engaged with the protrusion  11   c  of the engine case  11  (see  FIG. 7 ), whereby the negative pressure tube  38  can be positioned in the rotational direction. In this state, the fuel tank  21  is made to approach the engine case  11  of the fuel tank  21  from above; the first negative pressure introduction joint  32   a  of the automatic fuel cock  30  is fitted to the first coupling portion  38   a  of the negative tube  38 ; and the tank stay  24  is then fixed to the engine case  11  by the bolts  25 . The second fuel hose  37  communicating with the carburetor  17  is fitted to the fuel outlet joint  31   c  to complete the assembling. 
     As described above, since the negative pressure tube  38  can be connected to the first and second negative pressure introduction joints  32   a  and  11   b  by merely making the fuel tank  21  approach the engine case  11  from above, the mounting of the negative tube  38  is simplified. Further, the recessed portion  38   d  of the negative pressure tube  38  is engaged with the protrusion  11   c  of the engine case  11  to perform positioning, thereby facilitating the operation of fitting the first negative pressure introduction joint  32   a  of the automatic fuel cock  30  to the first coupling portion  38   a  of the negative pressure tube  38 . The negative pressure tube  38  once attached has a limited vertical movement and is never detached unless the fuel tank  21  is removed, thereby eliminating the need of fastening the end of the negative pressure tube  38  with a clip or the like to prevent detachment. 
     If the operation of attachment of the negative pressure tube  38  were carried out after fixing the fuel tank  21  to the engine case  11 , not only a workspace would be required for bending the negative pressure tube  38  to be fitted to the first and second negative pressure introduction joints  32   a  and  11   b , but also the negative tube  38  itself would be upsized, and therefore it would become impossible to place the fuel tank  21  close to the engine case  11  to upsize the entire engine E. 
     If oil mist in the engine case  11  were accumulated in the negative pressure tube  38  or in the first negative pressure introduction joint  32   a , the pressure pulsation of the breather channel  11   e  could not be transmitted to the negative pressure chamber  32   b  of the automatic fuel cock  30 , and thus the automatic fuel cock  30  could fall into defective operation. However, according to this embodiment, air from which a most part of the oil mist has been removed by the gas-liquid separation device  61  is fed to the breather channel  11   e , and the pressure pulsation of the breather channel  11   e  is guided to the automatic fuel cock  30 , thus preventing the defective operation of the automatic fuel cock  30 . 
     Particularly, the breather channel  11   e  for feeding air which has passed through the gas-liquid separation device  61  to the breather device  52  is provided on the upper part of the engine case  11 , thereby further effectively preventing the oil mist from entering the breather channel  11   e . Further, the pressure pulsation of the breather channel  11   e  is utilized to operate the automatic fuel cock  30 , thereby eliminating the need of forming a special channel for transmitting the pressure pulsation to the automatic fuel cock  30 . 
     Furthermore, the negative pressure tube  38  comprises: the first coupling portion  38   a  extending in a vertical direction and inserted into the first negative pressure introduction joint  32   a ; the second coupling portion  38   b  extending in a vertical direction and inserted into the second negative pressure introduction joint  11   b ; and the intermediate portion  38   c  extending obliquely downward from the lower end of the first coupling portion  38   a  to the upper end of the second coupling portion  38   b . Therefore, even if oil mist enters the inside of the negative pressure tube  38 , the oil mist is discharged to the breather channel  11   e  by gravitation without staying in the negative pressure tube  38 , thereby avoiding a situation where the pressure pulsation is not transmitted to the automatic fuel cock  30 . 
     Moreover, since the taper portion  32   d  is formed at the lower end of the first negative pressure introduction joint  32   a  of the automatic fuel cock  30 , the insertion of the negative pressure tube  38  into the first coupling portion  38   a  is facilitated. Also, the notch  32   e  is formed on the taper portion  32   d , and thus even if oil resides at the lower end of the first coupling portion  38   a  as shown by the chain line O in  FIG. 7  when the engine E is tilted, the first negative pressure introduction joint  32   a  is prevented from being clogged by the effect of the notch  32   e . Particularly, the notch  32   e  is opened toward the intermediate portion  38   c  side of the negative pressure tube  38 , and therefore the notch  32   e  is further reliably prevented from being immersed under the oil level. 
     If the first negative pressure introduction joint  32   a  is cut at a position of the upper end of the taper portion  32   d  (i.e. a position of the upper end of the notch  32   e ), also the effect same as that by provision of the notch  32   e  can be obtained, but in this case it becomes difficult to insert the negative pressure tube  38  due to the absence of the taper portion  32   d.    
     The automatic fuel cock  30  is operated not by an intake negative pressure of the engine E but by a larger negative pressure in the engine case  11 , and therefore only cranking by the recoil starter  16  can generate a sufficient negative pressure to feed fuel to the carburetor  17 . Particularly, by virtue of employment of two diaphragms, i.e. the first and second diaphragms  40  and  42 , the automatic fuel cock  30  can be reliably operated even with a small negative pressure. 
     Surroundings of the engine case  11  and the bearing holder  66  will now be described a little more in detail with reference to  FIGS. 13 to 16 . 
     The engine case  11  comprises: a crankcase  102  having a mounting seat  2   a  in its lower part; a cylinder block  103  integrally connected to the crankcase  102  and having an upwardly slanted cylinder bore  3   a ; and a cylinder head  12  jointed to the upper end surface of the cylinder block  103  via a gasket  104 . Four main coupling bolts  106 ,  106  arranged at four locations around the cylinder bore  3   a  and two auxiliary coupling bolts  107 ,  107  described later are used for joining, i.e. fastening the cylinder block  103  to the cylinder head  12 . 
     The crankcase  102  has its one side surface opened. A plurality of step portions  108  facing the open surface side and aligned along a circumferential direction are formed integrally on the inner peripheral wall slightly inward from the open surface. The bearing holder  66  is fixed to the step portions  108  by a plurality of bolts  65 . The opposite ends of the crankshaft  14  in a horizontal position are supported via the bearings  67  and  64  by the bearing holder  66  and the other sidewall of the crankcase  102 . The opposite ends of the primary balancer shaft  73  arranged adjacently in parallel to the crankshaft  14  are supported via the bearings  71  and  72  by the bearing holder  66  and the other side wall of the crankcase  102 . 
     As shown in  FIGS. 16 and 17 , on the outer peripheral surface of the crankcase  102 , a continuous reinforcement rib  116  is integrally formed so as to surround the plurality of step portions  108 , and the end of the reinforcement rib  116  is integrally connected to the outer wall of the cylinder block  103  integral with the crankcase  102 . 
     Thus, since the reinforcement rib  116  couples the plurality of step portions  108  inside the rib to one another on the outer peripheral surface of the crankcase  102 , the support rigidity of the bearing holder  66  supported by the step portions  108 , and hence the support rigidity of the crankshaft  14  supported by the bearing holder  66  can be effectively enhanced, resulting in reduced thickness and weight of the crankcase  102 . Particularly, as a result of integrally connecting the end of the reinforcement rib  116  to the outer wall of the cylinder block  103 , the reinforcement function of the reinforcement rib  116  is improved, and the support rigidity of the bearing holder  66  is enhanced. 
     The cover member  68  closing the open surface on one side of the crankcase  102  is jointed to the crankcase  102  by a plurality of bolts  69 . One end of the crankshaft  14  passes through the cover member  68  and projects outward as an output shaft portion. An oil seal  118  in close contact with the outer peripheral surface of the output shaft portion is attached to the cover member  68 . 
     Referring again in  FIG. 13 , the other end of the crankshaft  14  passes through the other side wall of the crankcase  102 , and an oil seal  119  in close contact with the other end of the crankshaft  14  is attached to the other side wall of the crankcase  102  so as to be adjacent to the outside of the bearing  64 . A fly wheel  121  serving also as a rotor of a generator  120  is fixed to the other end of the crankshaft  14 . A cooling fan  122  is provided on the outer surface of the fly wheel  121 . Further, at the other end of the crankshaft  14 , the recoil stator  16  supported by the crankcase  102  is arranged in a face-to-face manner. 
     In  FIGS. 13 and 15 , the piston  63  fitted to the cylinder bore  103   a  is connected to the crankshaft  14  via the connecting rod  62 . Formed on the cylinder head  12  are a combustion chamber  127  communicating with the cylinder bore  103   a , and an intake port  128   i  and an exhaust port  128   e  each opened in the combustion chamber  127 . An intake valve  129   i  and an exhaust valve  129   e  are mounted to the cylinder head  12  so as to open and close the opening ends of the intake and exhaust ports  128   i  and  128   e , respectively, opening to the combustion chamber  127 . Valve springs  130   i  and  130   e  are attached to the intake and exhaust valves  129   i  and  129   e , respectively, to urged them in a closing direction. The intake and exhaust valves  129   i  and  129   e  are opened and closed by a valve-operating system  135  operatable in association with the valve springs  130   i  and  130   e.    
     The valve-operating system  135  will be described with reference to  FIGS. 15 ,  16  and  18  to  24 C. 
     First, in  FIGS. 15 ,  16  and  18 , the valve-operating system  135  comprises: a cam shaft  136  supported in parallel to the crankshaft  14  by the cylinder head  12  and having an intake cam  136   i  and an exhaust cam  136   e ; a timing transmission system  137  coupling the crankshaft  14  and the cam shaft  136  to each other; an intake locker arm  138   i  interlocking the intake cam  136   i  and the exhaust valve  129   i  with each other; and an exhaust rocker arm  138   e  interlocking the exhaust cam  136   e  and the exhaust valve  129   e  with each other. 
     The cam shaft  136  has its opposite ends supported by a bag-shaped shaft bearing hole  139  formed on one side wall  12   a  of the cylinder head  12 , and a ball bearing  141  fitted to a bearing attachment hole  140  of the partition wall  12   b  of the intermediate portion of the cylinder head  12 . A single common rocker shaft  142  rockably supporting the intake and exhaust rocker arms  138   i  and  138   e  has its opposite ends supported by first and second support holes  143 ′ and  143  formed on the one side wall  12   a  and the partition wall  12   b , respectively. The first support hole  143 ′ of one side wall  12   a  is bag-shaped. The second support hole  143  of the partition wall  12   b  is through-hole-shaped. At the outer end of the second support hole  143 , a fixation bolt  144  having its front end contacting the outer end of the rocker shaft  142  is threadedly attached to the partition wall  12   b . Thus, the rocker shaft  142  is prohibited from moving in a thrust direction by the bag-shaped first support hole  143 ′ and the fixation bolt  144 . 
     The fixation bolt  144  integrally has, on its head, a flange seat  144   a  having a relatively large diameter. The fixation bolt  144  contacts the outer end surface of an outer lace  141   a  of the ball bearing  141  supporting the cam shaft  136 . 
     An inner lace  141   b  of the ball bearing  141  is press-fitted into the cam shaft  136 . Therefore, when the flange seat  144   a  of the fixation bolt  144  contacts the outer end of the outer lace  141   a  as described above, the cam shaft  136  is prohibited from moving in a thrust direction by the bag-shaped shaft bearing hole  139  and the flange seat  144   a.    
     Therefore, both the rocker shaft  142  and the cam shaft  136  can be prohibited from moving in a thrust direction by the single fixation bolt  144 , thus reducing the number of components, simplifying and downsizing the structure of the valve-operating system  135 , and contributing to an improvement in assemblability of the device  135 . 
     The timing transmission system  137  comprises: a toothed drive pulley  80  fixed on the crankshaft  14 ; a driven pulley  146  fixed on the cam shaft  136  and having teeth in the number twice as large as the number of teeth of the drive pulley  80 ; and an endless timing belt  81  wound around the drive and driven pulleys  80  and  146 . Thus, the rotation of the crank shaft  14  is transmitted to the cam shaft  136  with its rotational speed reduced by ½ by the timing transmission system  137 . With rotation of the cam shaft  136 , the intake and exhaust cams  136   i  and  136   e  rock the intake and exhaust rocker arms  138   i  and  138   e  against urging forces of the valve springs  130   i  and  130   e , thus opening and closing the intake and exhaust valves  129   i  and  129   e.    
     The timing transmission system  137  is housed in a timing transmission chamber  148  formed by sequentially connecting the oil stirring chamber  70  defined between the bearing holder  66  and the cover member  68 , an intermediate chamber  148   b  formed on the cylinder block  103  on one side of the cylinder bore  103   a , and an upper chamber  148   c  formed on one side of the cylinder head  12 . That is, the drive pulley  80  is arranged in the oil stirring chamber  70 , the driven pulley  146  is arranged in the upper chamber  148   c , and the timing belt  81  is arranged so as to pass through the intermediate chamber  148   b . As described above, the space between the bearing holder  66  and the cover member  68  is effectively used for installation of the timing transmission system  137 , thereby downsizing the engine E. 
     A valve-operating chamber  149  having its upper surface opened is formed between one side wall  12   a  and the partition wall  12   b  in the cylinder head  12 . The intake and exhaust cams  136   i  and  136   e  of the cam shaft  136 , the intake and exhaust rocker arms  138   i  and  138   e  and the other components are housed in the valve-operating chamber  149 . The upper open surface of the valve-operating chamber  149  is closed by the head cover  13  jointed to the cylinder head  12  by the bolt  153 . 
     The upper chamber  148   c  of the timing transmission chamber  148  and the valve-operating chamber  149  mutually communicate via an oil communication hole  175  (see  FIGS. 20 and 23 ) provided on the partition wall  12   b  and a plurality of oil communication grooves  176  (see  FIGS. 18 and 23 ) provided on the inner peripheral surface of the bearing attachment hole  140 . 
     In  FIGS. 18 to 21 , the outer end surface  12   c  of the cylinder head  12  is provided with an access window  155  opening the upper chamber  148   c  so as to be faced by the outer side face of the driven pulley  146 . Insertion of the driven pulley  146  into the timing belt  81  and mounting of the driven pulley  146  to the cam shaft  136  are carried out through the access window  155 . A lid body  157  closing the access window  155  is jointed by a plurality of bolts  158  to the outer end surface  12   c  via a seal member  156 . 
     As shown in  FIG. 18 , the outer end surface  12   c  of the cylinder head  12  to which the lid body  157  is jointed is formed to be a slanted surface  12   c  slanted so that at least a part of the outer periphery of the driven pulley  146  on the side opposite from the drive pulley  80  is exposed from the access window  155 , desirably exposed from the access window  155  over the half round of the driven pulley  146  on the side opposite from the drive pulley  80 . 
     A structure for attachment of the driven pulley  146  to the cam shaft  136  will now be described. 
     As shown in  FIG. 18 , the drive pulley  146  comprises: a bottomed cylindrical hub  146   a ; a web  146   b  radially extending from the hub  146   a ; and a toothed rim  146   c  formed on the outer periphery of the web  146   b . The hub  146   a  is fitted to the outer periphery of the outer end of the cam shaft  136  projecting to the upper chamber  148   c  side. The end wall of the hub  146   a  is provided with a bolt hole  160  occupying a position eccentric from the center thereof and a positioning groove  161  extending from one side of the bolt hole  160  to a side just opposite to the eccentricity direction. A first match mark  162   a  is engraved on the outer side surface of the rim  146   c . A second match mark  162   b  corresponding to the first match mark  162   a  is engraved on the outer end surface  12   c  of the cylinder head  12 . The web  146   b  is provided with a plurality of open holes  164  passing therethrough. 
     As shown in  FIGS. 18 and 23 , the outer end of the cam shaft  136  is provided with a screw hole  166  corresponding to the bolt hole  160 , and a positioning pin  167  corresponding to the positioning groove  161 . 
     Thus, when the crankshaft  14  is situated at a predetermined rotational position corresponding to a specified position (e.g. upper dead center) of the piston  63 , and the cam shaft  136  is situated at a position of a predetermined phase relationship with the crankshaft  14 , the first match mark  162   a  and the second match mark  162   b , the bolt hole  160  and the screw hole  166 , and the positioning groove  161  and the positioning pin  167  coincide, respectively, on a line L 2  passing through the centers of both the shafts  14  and  136 . 
     For attaching the driven pulley  146  to the cam shaft  136 , the crankshaft  14  is first fixed at a rotational position corresponding to the specified position of the piston  63 . Next, as shown in  FIG. 24(A) , the driven pulley  146  is inserted into the timing belt  81  already wound around the drive pulley  80  while aligning the first match mark  162   a  of the rim  146   c  with the second match mark  162   b  of the cylinder head  12 . Then, as shown in  FIG. 24(B) , the positioning pin  167  of the cam shaft  136  is fitted into the bolt hole  160  of the driven pulley  146 ; the driven pulley  146  is moved along with the timing belt  81  so as to guide the positioning pin  167  to the positioning groove  161 ; the cam shaft  136  rotates accordingly; the positioning pin  167  reaches the front end of the positioning groove  161 ; and then as shown in  FIG. 24(C) , the cam shaft  136  and the hub  146   a  are coaxially aligned, and at the same time the bolt hole  160  and the screw hole  166  mutually coincide. 
     As described above, the first and second match marks  162   a  and  162   b , the bolt hole  160  and the screw hole  166 , and the positioning groove  161  and positioning pin  167  are arranged all together on a line L 2  passing through the centers of the crankshaft  14  and the cam shaft  136 , by a remarkably simple operation of guiding the positioning pin  167  fitted into the bolt hole  160  to the positioning groove  161 . By visually observing this state, it can easily be confirmed that the crankshaft  14  and the cam shaft  136  have established a predetermined phase relationship. 
     As shown in  FIG. 18 , a mounting bolt  168  is passed through the bolt hole  160  and threadedly fitted and tightly fastened into the screw hole  166 , whereby the hub  146   a  is fixed to the cam shaft  136 . In this way, the timing transmission system  137  is attached to the crankshaft  14  and the cam shaft  136  which have been attached beforehand to the crankcase  102  and the cylinder head  12  in their predetermined phase relationship. 
     In this case, the bolt hole  160  and the screw hole  166  are arranged at positions eccentric from the centers of the hub  146   a  and the cam shaft  136 , and therefore the rotation of the driven pulley  146  can be reliably transmitted to the cam shaft  136  via the single eccentric mounting bolt  168 , and the mounting bolt  168  can be prevented from being loosened. 
     The screw hole  166  and the positioning pin  167  are arranged at positions eccentric in mutually opposite directions from the center of the cam shaft  136 , and therefore a sufficient amount of eccentricity can be given to each of the bolt hole  160  and the positioning groove  161  which are formed on the narrow end wall of the hub  146   a  of the driven pulley  146 , thereby improving the positioning effect of the positioning groove  161  on the positioning pin  167  and increasing the torque capacity of the mounting bolt  168 . 
     As described above, since the outer end surface of the cylinder head  12  in which the access window  155  is opened comprises the slanted surface  12   c , and a part of the outer periphery of the driven pulley  146  is exposed from the access window  155 , the part of the driven pulley  146  exposed to the outside of the access window  155  can be easily held by a tool or the like without being hindered by the cylinder head  12 , thereby easily carrying out the operation of attaching the driven pulley  146  to the cam shaft  136 , and also facilitating the detachment thereof. Thus, this can contribute to an improvement in assemblability and maintainability. 
     The side wall of the lid body  157  connected to the outer end surface  12   c , that is, the slanted surface  12   c  of the cylinder head  12  is formed so as to be slanted along the slanted surface  12 . With this arrangement, the engine case  11  obtains a head portion whose width is narrowing toward its tip end, thereby downsizing the engine E. 
     As shown in  FIGS. 19 to 21 , a pair of overhang portions  170 ,  170  overhanging to the outside of the access window  155  below the access window  155  is formed on the cylinder head  12 . The overhang portions  170 ,  170  are superimposed via the gasket  104  on the upper end surface of the cylinder block  103  outside the intermediate chamber  148   b , and fastened to the cylinder block  103  by the auxiliary coupling bolts  107 ,  107 . 
     By fastening with the auxiliary coupling bolts  107 ,  107 , contact pressures of the cylinder block  103  and the cylinder head  12  on the gasket  104  can be sufficiently increased also outside the intermediate chamber  148   b  housing the timing belt  81 . Moreover, a space accepting tools for manipulating the auxiliary coupling bolts  107 ,  107  can be sufficiently secured above the auxiliary coupling bolts  107 ,  107  by virtue of the presence of the slanted surface  12   c , thereby easily carrying out the operation of the auxiliary coupling bolts  107 ,  107 . This means that the amount of overhang of the overhang portions  170 ,  170  to the outside of the access window  155  can be reduced, and this also contributes to downsizing of the engine E. 
     The manipulation of the auxiliary coupling bolts  107 ,  107  is carried out before attaching the lid body  157 . 
     Lubrication of the valve-operating system  135  will now be described. 
     In  FIGS. 13 to 15  and  FIGS. 18 and 20 , the oil stirring chamber  70  of the timing transmission chamber  148  communicates with the inside of the crankcase  102 , i.e., the crank chamber  109 , through a plurality of step portions  108  on the inner wall of the crankcase  102  supporting the bearing holder  66 . A common lubricant oil  171  is stored in a certain amount in the crank chamber  109  and the oil stirring chamber  70 . 
     As shown in  FIG. 15 , the impeller-type oil slinger  77  driven via the gears  79  and  78  by the crankshaft  14  is arranged in the oil stirring chamber  70  such that a part of the oil slinger  77  is immersed in the oil  171  stored in the oil stirring chamber  70 . The oil slinger  77  rotates to splash the oil  171  to its surroundings. The rib  66   b  for guiding the splashed oil to the timing belt  81  side is formed integrally on the outer side surface of the bearing holder  66  so as to surround the oil slinger  77  and the periphery of the timing belt  81  on the drive pulley  80  side. The bearing holder  66  can be easily molded together with the rib  66   b  because the bearing holder  66  is a relatively small component. Further, the bearing holder  66  integrally has the rib  66   b  to enhance its rigidity, thereby effectively improving the support rigidity of the crankshaft  14 . 
     Thus, in the oil stirring chamber  70 , the oil splashed by the oil slinger  77  is guided to the timing belt  81  by the rib  66   b , and the oil deposited on the timing belt  81  is transferred to the upper chamber  148   c  by the belt  81 . When the timing belt  81  is wound around the drive pulley  146 , the oil is shaken off by a centrifugal force and splashed to the surroundings, collides against the surrounding walls to generate oil mist, and the upper chamber  148   c  is filled with the oil mist. Therefore, not only the entire timing transmission system  137  but also the ball bearing  141  of the cam shaft  136  can be lubricated. 
     Particularly, in the upper chamber  148   c , a part of the oil shaken off from the timing belt  81  collides against the slanted inner surface of the lid body  157 , and then bounces back to the web  146   b  of the driven pulley  146 . The oil passes through the open holes  164  of the driven pulley  146 , and splashes over the ball bearing  141 , thereby lubricating the ball bearing  141 . A part of the oil splashed over the ball bearing  141  is transferred to the valve-operating chamber  149  through the oil communication groove  176  on the outer periphery of the bearing  141 , and lubricates also from the ball bearing the valve-operating chamber  149  side. Thus, the ball bearing  141  is excellently lubricated. 
     As shown in  FIG. 14 , the bottom of the valve-operating chamber  149  communicates with the crank chamber  109  via a train of oil return channel  177  formed in the cylinder head  12  and the cylinder block  103  so as to extend along one side of the cylinder bore  103   a . The oil return channel  177  is inclined toward the crank chamber  109  so that the oil flows down from the valve-operating chamber  149  to the crank chamber  109 . 
     During operation of the engine E, a pulsation of pressure is generated in the crank chamber in association with the up-and-down movement of the piston  63 . When the pulsation pressure is transmitted to the valve-operating chamber  149  and the timing transmission chamber  148  through the oil return channel  177 , the oil communication hole  175  and the oil communication groove  176 , the oil mist travels between the valve-operating chamber  149  and the timing transmission chamber  148 . Therefore, the entire valve-operating system  135  can be effectively lubricated. 
     After the lubrication, the oil stored in the valve-operating chamber  149  flows down through the oil return channel  177  back into the crank chamber  109 . The bottom of the timing transmission chamber  148  is also inclined toward the oil stirring chamber  70 , and thus the oil stored in the upper chamber  148   c  flows down through the intermediate chamber  148   b  back into the oil stirring chamber  70 . 
     As described above, the operation of the oil slinger  77  and the timing transmission system  137 , and the pulsation pressure of the crank chamber  109  can be utilized to lubricate, by the oil mist, the insides of the mutually defined timing transmission chamber  148  and the valve-operating chamber  149  which are partitioned from each other. Therefore, an oil pump is unnecessary, thus simplifying and downsizing the structure of the engine E and reducing the cost. Moreover, the cam shaft  136  can maintain the overhead arrangement of the intake and exhaust valves  129   i  and  129   e , thereby ensuring a desired output performance of the engine. 
     The embodiment of the present invention has been described above, but various modifications in design can be made to the present invention within the scope of the invention. 
     For example, the general-purpose engine E has been described in the embodiment, but the present invention may be applied to an engine for any purpose. 
     In the embodiment, the ribs  66   d ,  66   e ,  68   a  and  68   b  forming the labyrinth  82  of the gas-liquid separation device  61  project from both the bearing holder  66  and the cover member  68 , but may project from only one of them. 
     The belt-type timing transmission system  137  may be replaced by a chain-type timing transmission system.