Patent Publication Number: US-8978631-B2

Title: Engine equipped with breather mechanism

Description:
FIELD OF THE INVENTION 
     The present invention relates to an engine in which lubricating oil is stored in a crankcase and which includes a breather mechanism for causing blow-by gas of the crankcase to flow out of the crankcase. Such an engine will hereinafter sometimes be referred to as “breather-mechanism-equipped engine”. 
     BACKGROUND OF THE INVENTION 
     In engines, gas of a combustion chamber flows through a gap between an engine cylinder and a piston into a crankcase as blow-by gas. Many of the engines include a breather device for causing the blow-by gas to flow out of the crankcase and then return to the combustion chamber, as disclosed for example in Japanese Patent No. 4089334. According to the breather device disclosed in the 4089334 patent, once the inner pressure of the crankcase exceeds a predetermined pressure due to the blow-by gas, a breather valve (lead valve) of the breather device opens, so that the blow-by gas is sucked into an air intake system via a breather passage. The blow-by gas thus sucked into the air intake system is returned back to the combustion chamber of the engine. 
     For stability during transportation or storage of the engine, it is sometimes desired that the engine be retained in a laterally laid-down position, i.e. in a position where the engine cylinder lies sideways. However, the engine disclosed in the 4089334 patent is constructed for use in a vertically upright position (i.e., position where the engine cylinder is located in an upper region of the engine). Thus, when the engine is placed in the laterally laid-down position, lubricating oil stored in the crankcase may enter the breather passage. Consequently, if the engine is driven after (particularly, immediately after) the engine is returned from the laterally laid-down position to the vertically upright position, the lubricating oil staying in the breather passage may be undesirably sucked into an air intake system of the engine. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing prior art problems, it is an object of the present invention to provide an improved breather-mechanism-equipped engine which can reliably prevent lubricating oil from being sucked into the air intake system as the engine is driven after (particularly, immediately after) the engine is returned to the vertically upright position from the laterally laid-down position intended for transportation or storage of the engine. 
     In order to accomplish the above-mentioned object, the present invention provides an improved engine in which lubricating oil is stored in a crankcase and which is equipped with a breather mechanism for causing blow-by gas of the crankcase to flow out of the crankcase, the breather mechanism comprising: a breather passage opening into the crankcase; a breather chamber communicating with the crankcase via the breather passage; and a breather valve provided at an inlet of the breather chamber, the breather valve being openable to allow the blow-by gas to flow out of the crankcase into the breather chamber via the breather passage once inner pressure of the crankcase exceeds a predetermined value. When the engine is placed in a laterally laid-down position, entry, into the breather chamber, of the lubricating oil is preventable by the breather passage. When the engine is placed in a vertically upright position, an outlet end portion of the breather passage, opening into the breather chamber, is located above an inlet end portion of the breather passage opening into the crankcase. 
     According to the present invention, when the engine is placed in the laterally laid-down position that is intended for transportation, storage, etc. of the engine, the lubricating oil can be reliably prevented by the breather passage from entering the breather chamber. When the engine is in the laterally laid-down position, the inlet end portion of the breather passage, opening into the crankcase, may be immersed in the lubricating oil stored in the crankcase. If the engine is returned from such a laterally laid-down position back to the vertically upright position, the lubricating oil would stay in the inlet end portion, so that the lubricating oil may be undesirably directed by the blow-by gas to the breather chamber. To avoid such an inconvenience, the outlet end portion of the breather passage in the present invention is disposed above (higher than) the inlet end portion when the engine is in the vertically upright position. 
     As known, the lubricating oil has a greater specific gravity than the blow-by gas, and thus, the residual lubricating oil staying in the inlet end portion cannot be lifted up to the outlet end portion by the blow-by gas. In this way, it is possible to reliably prevent a portion of the residual lubricating oil, staying in the inlet end portion, from entering the breather chamber and thereby prevent the lubricating oil from being sucked into the air intake system. 
     Meanwhile, the remaining portion of the residual lubricating oil is directed, together with the blow-by gas, to the outlet end portion via the breather passage, from which it further flows into the breather chamber via the opened breather valve. 
     Because the breather chamber has a greater space than the breather passage, the flow speed of the blow-by gas, having flown into the breather chamber, would decrease, so that the residual lubricating gas, having flown into the breather chamber together with the blow-by gas, is separated from the blow-by gas and drops down to a lower portion of the breather chamber. Thus, the residual lubricating oil, having flown into the breather chamber, can be prevented from being sucked into the combustion chamber of the engine, so that only the blow-by gas can be sucked into the combustion chamber. 
     Preferably, when the engine is placed in the laterally laid-down position lying on its one side (e.g., front side), the breather chamber is located above an engine block (and hence the crankcase), and the inlet end portion of the breather passage is immersed in the lubricating oil. Thus, if the engine is returned from the laterally laid-down position back to the vertically upright position, the lubricating oil would stay in the inlet end portion. According to the present invention, as noted above, the outlet end portion of the breather passage is disposed above (higher than) the inlet end portion when the engine is in the vertically upright position. Further, because the lubricating oil has a greater specific gravity than the blow-by gas, the residual lubricating oil staying in the inlet end portion cannot be lifted up to the outlet end portion by the blow-by gas. In this way, it is possible to prevent the residual lubricating oil, staying in the inlet end portion, from entering the breather chamber and thereby prevent the lubricating oil from being sucked into the air intake system. 
     Preferably, when the engine is placed in the laterally laid-down position lying on its another side (e.g., back side) opposite from the one side, the breather chamber is located below the engine block (and hence the crankcase), and the inlet end portion of the breather passage is located above the lubricating oil stored in the crankcase. This arrangement can prevent the lubricating oil from entering the breather passage via the inlet end portion. In this way, it is possible to prevent the residual lubricating oil from entering the breather chamber and thereby prevent the lubricating oil from being sucked into the air intake system. 
     Preferably, the breather valve includes a valve seat, and a valve body formed of an elastic material into a dome shape and mounted on the valve seat, the valve seat having a valve passage normally closed with the valve body. Thus, once the inner pressure of the crankcase (i.e., pressure of the blow-by gas) exceeds the predetermined value, the breather valve can be efficiently elastically deformed to allow the blow-by gas to efficiently flow out of the crankcase to the breather chamber. 
     The following will describe embodiments of the present invention, but it should be appreciated that the present invention is not limited to the described embodiments and various modifications of the invention are possible without departing from the basic principles. The scope of the present invention is therefore to be determined solely by the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain preferred embodiments of the present invention will be described in detail below, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is an overall perspective view of a power generator including a breather-mechanism-equipped engine according to a first embodiment of the present invention; 
         FIG. 2  is a side view of the power generator of  FIG. 1  placed in a laterally laid-down position; 
         FIG. 3  is a sectional view showing the breather-mechanism-equipped engine; 
         FIG. 4  is a sectional view taken along line  4 - 4  of  FIG. 3 ; 
         FIG. 5  is a sectional view taken along line  5 - 5  of  FIG. 3 ; 
         FIG. 6  is an enlarged sectional view of a section depicted at  6  in  FIG. 3 ; 
         FIGS. 7A and 7B  are views explanatory of an example manner in which the breather mechanism according to the first embodiment directs blow-by gas out of a crankcase; 
         FIG. 8  is a view explanatory of an example manner in which the breather mechanism according to the first embodiment directs the blow-by gas to a breather chamber; 
         FIG. 9  is a view explanatory of an example manner in which the breather mechanism according to the first embodiment behaves when the power generator and hence the engine is retained in a forward laterally-laid-down position; 
         FIG. 10  is a view explanatory of an example manner in which the breather mechanism employed according to the first embodiment behaves when the engine is placed in a vertically upright position back from the forward laterally-laid-down position; 
         FIG. 11  is a view explanatory of an example manner in which the breather mechanism according to the first embodiment behaves when the power generator and hence the engine is retained in a backward laterally-laid-down position; 
         FIG. 12  is a sectional view showing a breather mechanism according to a second embodiment of the present invention; and 
         FIG. 13  is a view explanatory of an example manner in which the breather mechanism according to the second embodiment directs the blow-by gas of the crankcase to the breather chamber. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is an overall perspective view of a power generator  10  including an engine  21  equipped with a first embodiment of a breather mechanism, and  FIG. 2  is a side view of the power generator  10  shown in  FIG. 1 . As shown in  FIGS. 1 and 2 , the power generator  10  includes: a housing case  11  having a substantially rectangular parallelepiped contour or shape; an engine/generator unit  12  accommodated in the case  11 ; left and right wheels  13  rotatably mounted on the case  11 ; and a towing handle  14  pivotably mounted on the case  11 . 
     The power generator  10  is towable via the left and right wheels  13  by a user or human operator pivoting the handle  14  upward and pulling the pivoted handle  14 . Further, the power generator  10  can be retained in a vertically upright position P 1  by means of the left and right wheels  13  and left and right lower foot portions  16 . The vertically upright position P 1  is where a cylinder  42  ( FIG. 3 ) of an engine  21  is located in an upper region of the power generator  10  and hence engine  21 . With the power generator  10  placed in such a vertically upright position P 1 , the engine/generator unit  12  can be driven. 
     Furthermore, the power generator  10  can also be retained in a laterally laid-down position P 2  lying on its front side (i.e., front side as viewed in the towing direction of the power generator  10 ) by means of the left and right wheels  13  and left and right upper foot portions  17 , as shown in  FIG. 2 . Such a position P 2  is where the engine cylinder  42  lies sideways and will hereinafter be referred to as “forward laterally-laid-down position P 2 ”. With the power generator  10  (and hence the engine  21 ) placed in such a laterally laid-down position P 2 , the power generator  10  (and hence the engine  21 ) can be retained stably during transportation, storage, etc. thereof. 
     The engine/generator unit  12  comprises an integral combination of the engine  21  mounted on a bottom portion  11   a  of the case  11  and a power generation section  22  drivable by the engine  21 . In the engine/generator unit  12 , the engine  21  rotates the rotor of the power generation section  22  around the outer periphery of the stator so that electric power can be generated. 
     As shown in  FIGS. 3 and 4 , the engine  12  is, for example, a four-cycle, single-cylinder engine which includes an engine body  24 , a breather mechanism  25  provided on the engine body  24 , and an air intake system  26  provided in fluid communication with the breather mechanism  25 . 
     The engine body  24  includes an engine block  31  integrally having a cylinder section  32  and a case section  33 , and a crank case section  34  ( FIG. 4 ) fixedly attached to the case section  33 . A crankcase  35  is constructed, in a hermetically sealed state, of the case section  33  and crank case section  34 . 
     Lubricating oil  37  is stored in the crankcase  35 , and the lubricating oil  37  is supplied, for example, to sliding components in the engine  21  to secure fluid lubrication of sliding surfaces of the components to thereby minimize friction between the sliding surfaces. 
     Further, the engine body  24  includes a crankshaft  41  rotatably mounted in the crankcase  35 , a piston  43  slidably mounted in a cylinder  42  of the cylinder section  32 , and a connecting rod  44  connecting between the piston  43  and the crankshaft  41 . 
     A combustion chamber is provided in an upper region of the cylinder  42 . During operation of the engine  21 , gas of the combustion chamber flows through a gap between the cylinder  42  and the piston  43  into the crankcase  35  as blow-by gas. 
       FIG. 3  shows the breather-mechanism-equipped engine  21  with the crank case section  34  detached therefrom to ease understanding of the construction of the engine  21 . The breather mechanism  25  is provided in the engine block  31 . 
     The breather mechanism  25  includes: a breather passage  51  communicating with the crankcase  35 ; a breather chamber  52  communicating with an outlet end portion  58   b  of the breather passage  51  opening into the breather chamber  52 ; a breather valve  53  disposed on an inlet end portion  59  of the breather chamber  52 ; and a main return passage  54  for returning the lubricating oil  37 , having flown into the breather chamber  52 , back to the crankcase  35 . 
     The breather passage  51  includes first to third breather passage sections  56 ,  57  and  58 . The first breather passage section  56  is formed in the case section  33  for communicating between the crankcase  35  and the second breather passage section  57 . The second breather passage section  57  is provided in the case section  33  for communicating between the first breather passage section  56  and the third breather passage section  58 . The third breather passage section  58  extends backward from the case section  33  for communicating between the second breather passage section  57  and the breather chamber  52 . 
     As shown in  FIG. 5 , the first breather passage section  56  extends substantially vertically when the engine  21  is in the vertically upright position P 1  (see  FIG. 3 ), and it has an inlet end portion  56   a  opening into the crankcase  35  and an outlet end portion  56   b  opening into the second breather passage section  57 . When the engine  21  is in the laterally laid-down position P 2  (see  FIG. 9 ), the lubricating oil  37  can enter the first breather passage section  56 . 
     Referring back to  FIG. 3 , the second breather passage section  57  extends substantially horizontally when the engine  21  is in the vertically upright position P 1 . The second breather passage section  57  has an entry portion  62  through which the lubricating oil  37  can enter the passage section  57  when the engine  21  is in the laterally laid-down position P 2  (see  FIG. 9 ). 
     The entry portion  62  has a sub return passage  63 , which communicates between the entry portion  62  and the crankcase  35  to return the lubricating oil  37 , having entered the second breather passage section  57 , back to the crankcase  35 . 
     The first breather passage section  56  and the entry portion  62  of the second breather passage section  57  together constitute an oil entry portion  61  (i.e., inlet end portion of the breather passage  51  opening into the crankcase  35 ) which the lubricating oil  37  can enter when the engine  21  is placed in the laterally laid-down position P 2 . When the power generator  10  is in the vertically upright position P 1 , the oil entry portion  61  is located over the lubricating oil  37 , i.e. above the fluid level  37   a  of the lubricating oil  37 . 
     As shown in  FIGS. 3 and 6 , the third breather passage  58  has an inlet end portion  58   a  communicating with an outlet end portion  57   a  of the second breather passage section  57 , and an outlet end portion  58   b  communicating with the inlet end portion  59  of the breather chamber  52 . 
     The third breather passage  58  is of a generally crank shape such that, when the engine  21  is in the vertically upright position P 1 , the third breather passage  58  extends substantially horizontally with the outlet end portion  58   b  located above the inlet end portion  58   a.    
     Namely, the third breather passage section  58  has a stepped portion  58   c  such that the outlet end portion  58   b  of the third breather passage section  58  is located higher than the oil entry portion  61  by a height H 1  (see  FIG. 7A ). 
     The lubricating oil  37  has a greater specific gravity than the blow-by gas. Thus, the lubricating oil  37  cannot be lifted up to the outlet end portion  58   b  by the blow-by gas directing the lubricating oil  37 . In this way, it is possible to prevent the lubricating oil  37  from entering the breather chamber  52 . 
     The breather chamber  52  is defined in a breather case  65  of a substantially rectangular parallelepiped shape. The breather chamber  52  is in communication with the crankcase  35  via the breather passage  51  and has an upper end portion  52   a  communicating with a lower end portion  67   a  of an air inlet chamber  67  via a communication passage  68 . When the power generator  10  is retained in the forward laterally laid-down position P 2 , as shown in  FIG. 9 , the breather chamber  52  is located above the engine block  31  and hence the crankcase  35 . 
     The inlet end portion  59  projects from the front surface of an inner wall portion  65   a  of the breather case  65  toward the engine block  31 , and it opens into a substantially middle interior portion of the breather chamber  52 . The outlet end portion  58   b  of the third breather passage section  58  is in communication with the inlet end portion  59 . 
     Further, a valve mounting portion  71  is formed on the reverse or back surface of the inner wall portion  65   a  of the breather case  65 , and the breather valve  53  disposed on the inlet end portion  59  of the breather chamber  52  is mounted to the valve mounting portion  71 . 
     As shown in  FIGS. 4 and 6 , the breather valve  53  is a lead valve that includes a valve seat  73  attached to the valve mounting portion  71  and a valve body  74   73 . 
     The valve body  74  of the breather valve  53  has an upper end portion attached to the valve seat  73 , and a lower end portion closing a valve passage  73   a  of the valve seat  73 . Namely, with the lower end portion of the valve body  74  kept in contact with a surface of the valve seat  73 , the breather valve  53  is normally kept in a closed state. 
     Once the inner pressure of the crankcase  35  exceeds a predetermined value so that a pressure difference ΔP between a primary-side pressure (i.e., front-side pressure) PR 1  and a secondary-side pressure (i.e., back-side pressure) PR 2  exceeds a predetermined setting value, the valve body  74  moves, as indicated by a black arrow in  FIG. 6 , to thereby open the valve passage  73   a  of the valve seat  73 . By the valve passage  73   a  being opened like this, the blow-by gas of the crankcase  35  flows into the breather chamber  52  through the breather passage  51 . 
     As shown in  FIGS. 3 and 6 , a return outlet end portion  77  is formed on a bottom portion  65   b  of the breather case  65  to project downward from the bottom portion  65   b  of the breather case  65 , and this return outlet end portion  77  opens into a lower end portion  52   b  of the breather chamber  52 . 
     The main return passage  54  is in communication with the return outlet end portion  77 . The main return passage  54  includes a first return passage section  81  provided substantially horizontally in the case section  33  of the engine block  31 , and a second return passage section  82  communicating at one end with the first return passage section  81  and at the other end with the return outlet end portion  77 . Thus, the lower end portion  52   b  of breather chamber  52  is in communication with the crankcase  35  via the main return passage  54 . 
     The first return passage section  81  is attached substantially horizontally to a rear wall portion  33   a  of the case section  33  and has a distal end portion  81   a  projecting forwardly beyond the rear wall portion  33   a  by a length L 1 . 
     Thus, when the power generator  10  is brought down to a backward laterally-laid-down position P 3  as shown in  FIG. 11 , the distal end portion  81   a  of the first return passage section  81  projects upwardly beyond the fluid level  37   a  of the lubricating oil  37  by a length L 2 . When the power generator  10  is in the vertically upright position P 1 , the first return passage section  81  extends substantially horizontally and is located above the fluid level  37   a  of the lubricating oil  37 . 
     In the breather mechanism  25 , the breather valve  53  opens once the inner pressure of the crankcase  35 , i.e. the pressure of the blow-by gas of the crankcase  35 , exceeds the predetermined value. Such opening of the breather valve  53  allows the blow-by gas of the crankcase  35  and the lubricating oil  37 , having entered the oil entry portion  61 , to be directed into (i.e. to flow into) the breather chamber  52  through the breather passage  51 . 
     Because the breather chamber  52  has a greater space than the breather passage  51 , the flow speed of the blow-by gas having flown into the breather chamber  52  would decrease, so that the residual lubricating gas  37  having flown into the breather chamber  52  together with the blow-by gas can be separated from the blow-by gas. The blow-by gas from which the lubricating gas  37  has been separated is sucked from the breather chamber  52  into the air inlet chamber  67  through the communication passage  68 . Meanwhile, the lubricating gas  37 , having been separated from the blow-by gas, drops down to the lower portion  52   b  of the breather chamber  52  and then is returned to the crankcase  35  through the main return passage  54 . 
     As shown in  FIGS. 3 and 4 , the air intake system  26  includes: the air inlet chamber  67  communicating with the upper end portion  52   a  of the breather chamber  52  via the communication passage  68 ; an air-fuel mixer  85  communicating with the air inlet chamber  67  via a communication passage  84 ; and an air cleaner  87  communicating with the air inlet chamber  67  via a communication passage  86 . External air is sucked from the air cleaner  87  into the air inlet chamber  67 , and the blow-by gas is sucked from the breather chamber  52  into the air inlet chamber  67 . 
     The mixer  85  has an inlet portion communicating with the air inlet chamber  67  and a fuel supply path (not shown), and an outlet portion communicating with the combustion chamber of the cylinder section  32 . The air-fuel mixer  85  mixes together the air and blow-by gas, sucked from the air inlet chamber  67 , and fuel sucked from the fuel supply path (not shown), and then it can direct a resultant mixture of the air and blow-by gas and fuel to the combustion chamber of the cylinder section  32 . 
     The following describe an example manner in which the breather mechanism  25  directs the blow-by gas of the crankcase  35  to the breather chamber  52 , i.e. allows the blow-by gas to escape to the breather chamber  52 , with reference to  FIGS. 7 and 8 . As shown in  FIG. 7A , gas of the combustion chamber flows through the gap between the cylinder  42  and the piston  43  into the crankcase  35  as blow-by gas as the engine  21  is driven. Once the inner pressure of the crankcase  35 , i.e. the pressure of the blow-by gas of the crankcase  35 , exceeds the predetermined value, the valve body  74  of the breather valve  53  pivotally moves as indicated by arrow A, so that the valve passage  73   a  is opened. 
     In response to such opening of the valve passage  73   a  of the breather valve  53 , the blow-by gas of the crankcase  35  flows through the first breather passage section  56  as indicated by arrow B and then flows into the second breather passage section  57  as indicated by arrow C, as clearly seen in  FIG. 7B . 
     Referring back to  FIG. 7A , the blow-by gas having flown into the second breather passage section  57  further flows into the third breather passage section  58  via the second breather passage section  57 , as indicated by arrow D. 
     As shown in  FIG. 8 , the blow-by gas, having flown through the third breather passage section  58 , flows into the breather chamber  52  via the opened breather valve  53  (valve passage  73   a ), as indicated by arrow E. Then, the blow-by gas, having flown into the breather chamber  52 , is sucked into the air inlet chamber  67  via the communication passage  68 , as indicated by arrow F. 
     As set forth above in relation to  FIGS. 7 and 8 , the breather mechanism  25  can direct the blow-by gas of the crankcase  35  into the air inlet chamber  67  by way of the breather chamber  52  and further direct efficiently the blow-by gas into the combustion chamber of the cylinder section  32  by way of the mixer  85 . 
     The following describe an example manner in which the breather mechanism  25  behaves when the power generator  10  (and hence the engine  21 ) is retained in the forward laterally-laid-down position P 2 , with reference to  FIGS. 9 and 10 . The power generator  10  is retained in the forward laterally-laid-down position P 2  with the engine  21  in the deactivated state, as shown in  FIG. 9  and  FIG. 2  as well. When the power generator  10  is in the forward laterally-laid-down position P 2  like this, the oil entry portion  61  (i.e., first breather passage section  56  and the entry portion  62  of the second breather passage section  57 ) is located below the fluid level  37   a  of the lubricating oil  37 . Namely, the oil entry portion  61  is immersed in the lubricating oil  37 , so that the lubricating oil  37  enters the oil entry portion  61 . 
     Further, when the power generator  10  is in the forward laterally-laid-down position P 2 , the breather chamber  52  is located above the engine block  31 . Thus, the lubricating oil  37 , having entered the oil entry portion  61 , can be prevented from entering the breather chamber  52 . 
     Furthermore, the first return passage section  81  of the main return passage  54  is located above the fluid level  37   a  of the lubricating oil  37 . Thus, the lubricating oil  37  can be prevented from entering the first return passage section  81 . When the power generator  10  is to be used, the power generator  10  is brought from the forward laterally-laid-down position P 2  back to the vertically upright position P 1  as indicated by arrow G. 
     When the power generator  10  is back in the vertically upright position P 1 , the oil entry portion  61  is located above the fluid level  37   a  of the lubricating oil  37 , as shown in  FIG. 10 . Thus, the lubricating oil  37 , having entered the oil entry portion  61 , is returned to the crankcase  35  by way of the first breather passage  51  and sub return passage  63  as indicated by arrow H. 
     The engine  21  may sometimes be driven after (particularly, immediately after) the power generator  10  is returned from the laterally laid-down position to the vertically upright position. In such a case, the engine  21  may sometimes be driven before all of the lubricating oil  37 , having entered in the oil entry portion  61 , is returned to the crankcase  35 . In response to the engine  21  being driven like this, the valve body  74  of the breather valve  53  operates to open the valve passage  73   a , and thus, the blow-by gas flows from the crankcase  35  into the oil entry portion  61 . Thus, the lubricating oil  37 , staying in the oil entry portion  61 , flows into the third breather passage section  58  by way of the second breather passage section  57 , as indicated by arrow I, together with the blow-by gas having flown into the oil entry portion  61 . 
     As noted above, the third breather passage section  58  has the stepped portion  58   c  such that the outlet end portion  58   b  of the third breather passage section  58  is located higher than the oil entry portion  61  by the height H 1  (see  FIG. 7A ). Furthermore, the lubricating oil  37  has a greater specific gravity than the blow-by gas. Thus, the lubricating oil  37  cannot be lifted up to the outlet end portion  58   b  by the blow-by gas directing the lubricating oil  37 . As a consequence, a portion of the residual lubricating oil  37 , having flown into the third breather passage section  58 , is prevented by the stepped portion  58   c  from flowing into the breather chamber  52 . 
     That portion of the residual lubricating oil  37 , having been prevented from flowing into the breather chamber  52 , is returned to the crankcase  35  by way of the second breather passage section  57  and sub return passage  63  as indicated by arrow H. 
     Meanwhile, the remaining portion of the residual lubricating oil  37  is directed, together with the blow-by gas, to the outlet end portion  58   b  by way of the stepped portion  58   c , from which it flows into the breather chamber  52  via the opened breather valve  53  as indicated by arrow J. 
     Because the breather chamber  52  has a greater space than the breather passage  51 , the flow speed of the blow-by gas having flown into the breather chamber  52  would decrease, so that the residual lubricating gas  37  having flown into the breather chamber  52  together with the blow-by gas is separated from the blow-by gas and drops down to the lower portion  52   b  of the breather chamber  52 . The residual lubricating gas  37 , having dropped down to the lower portion  52   b , is returned to the crankcase  35  through the main return passage  54  as indicated by arrow K in  FIG. 10 . 
     The blow-by gas, having flown through the third breather passage section  58 , flows, together with the residual lubricating gas  37 , into the breather chamber  52  via the opened breather valve  53  as indicated by arrow J, where the blow-by gas is separated from the residual lubricating gas  37 . The blow-by gas, having been separated from the residual lubricating gas  37 , is sucked from the breather chamber  52  into the air inlet chamber  67  via the communication passage  68  as indicated by arrow L. 
     As set forth above in relation to  FIGS. 9 and 10 , when the power generator  10  is retained in the forward laterally-laid-down position P 2 , the breather chamber  52  of the breather mechanism  25  is located above the engine block  31 , so that the lubricating oil  37  can be prevented from entering the breather chamber  52  via the breather passage  51 . Stated differently, when the power generator  10  is retained in the forward laterally-laid-down position P 2 , the breather mechanism  25  can prevent, by means of the breather passage  51 , the lubricating oil  37  from entering the breather chamber  52 . 
     Further, when the power generator  10  is retained in the forward laterally-laid-down position P 2 , the first return passage section  81  of the main return passage  54  is located above the fluid level  37   a  of the lubricating oil  37 . Thus, the lubricating oil  37  can be prevented from entering the breather chamber  52  by way of the main return passage  54 . 
     In addition, by the provision of the stepped portion  58  of the third breather passage section  58  and the breather chamber  52 , the breather mechanism  25  can prevent the lubricating oil  37 , staying in the oil entry portion  61 , from being sucked into the air intake system  26 . Thus, the breather mechanism  25  can prevent the lubricating oil  37 , staying in the oil entry portion  61 , from being sucked into the combustion chamber of the cylinder section  32  as the engine  21  is driven after having been returned to the vertical upright position P 1 . 
     The following describe an example manner in which the breather mechanism  25  behaves when the power generator  10  is retained in the backward laterally-laid-down position P 3  where it lies on its back side, with reference to  FIG. 11 . The power generator  10  is retained in the backward laterally-laid-down position P 3  with the engine  21  in the deactivated state, as shown in  FIG. 11 . When the power generator  10  is in the backward laterally-laid-down position P 3  like this, the breather chamber  52  is located below the engine block  31  and hence the crankcase  35 . 
     Thus, the oil entry portion  61  (i.e., first breather passage section  56  and the entry portion  62  of the second breather passage section  57 ) is located above the fluid level  37   a  of the lubricating oil  37 . The oil entry portion  61  constitutes an inlet end portion of the breather passage  51  opening into the crankcase  35 . With the oil entry portion  61  located above the fluid level  37   a  of the lubricating oil  37 , the lubricating oil  37  can be prevented from entering the oil entry portion  61 . 
     Further, the first return passage section  81  of the main return passage  54  has its distal end portion  81   a  projecting forwardly beyond the rear wall portion  33   a  by the length L 1 . Thus, when the power generator  10  is brought down to the backward laterally-laid-down position P 3  as shown in  FIG. 11 , the distal end portion  81   a  of the first return passage section  81  can project upwardly beyond the fluid level  37   a  of the lubricating oil  37  by the length L 2 . Therefore, the lubricating oil  37  can be prevented from entering the first return passage section  81 . 
     In the aforementioned manner, the breather mechanism  25  can prevent the lubricating oil  37  from entering the oil entry portion  61  and entering the first return passage section  81 , to thereby reliably prevent the lubricating oil  37  from entering the breather chamber  52 . 
     When the power generator  10  is to be used, the power generator  10  is brought from the backward laterally-laid-down position P 3  back to the vertically upright position P 1  as indicated by arrow M. After the power generator  10  is brought back to the vertically upright position P 1 , the engine  21  is driven, so that the blow-by gas of the crankcase  35  can be directed to the breather chamber  52  by way of the breather passage  51  and breather valve  53 . Then, the blow-by gas, having flown into the breather chamber  52 , can be directed to the combustion chamber of the cylinder section  32  by way of the mixer  85 . 
     When the power generator  10  is in the backward laterally-laid-down position P 3 , the oil entry portion  61  is located above the fluid level  37   a  of the lubricating oil  37  as set forth above in relation to  FIG. 11 , and thus, the lubricating oil  37  can be prevented from entering the oil entry portion  61  (breather passage  51 ) and hence from the breather chamber  52 . 
     Stated differently, when the power generator  10  is retained in the backward laterally-laid-down position P 3 , the breather mechanism  25  can prevent, by means of the breather passage  51 , the lubricating oil  37  from entering the breather chamber  52 . 
     Further, when the power generator  10  is in the backward laterally-laid-down position P 3 , the distal end portion  81   a  of the first return passage section  81  projects upwardly beyond the fluid level  37   a  of the lubricating oil  37 , and thus, the lubricating oil  37  can be prevented from entering the breather chamber  52  via the main return passage  54 . In addition, the lubricating oil  37  can be prevented from being sucked into the combustion chamber of the cylinder section  32  as the engine  21  is driven in the vertical upright position P 1 . 
     The following describe another or second embodiment of the breather mechanism  90  with reference to  FIGS. 12 and 13 . Similar elements to those in the first embodiment of the breather mechanism  25  are indicated by the same reference numerals and characters as used for the first embodiment and will not be described here to avoid unnecessary duplication. 
     As shown in  FIG. 12 , the second embodiment of the breather mechanism  90  includes a breather valve  92  constructed differently from the breather valve  53  employed in the first embodiment. The breather valve  92  includes a valve seat  93  mounted on the valve mounting portion  71  and a valve body  94   73 . 
     The valve seat  93  has a mounting hole  93   a  formed substantially centrally therein, and valve passages  95  formed therein radially outwardly of the mounting hole  93   a . A support shaft  94   a  of the valve body  94  is inserted through the mounting hole  93   a  of the valve seat  93 . 
     The valve body  94  is formed of an elastic material and has a dome-shaped portion  96 . The valve body  94  has a radially outer peripheral surface  96   a  held in contact with a surface of the valve seat  93 . Namely, the breather valve  92  is a so-called umbrella valve which closes the valve passages  95  by the radially outer peripheral surface  96   a  of the dome-shaped portion  96  of the valve body  94  being held in contact with the valve seat  93  and which opens the valve passages  95  by the valve body  94  being elastically deformed so that the radially outer peripheral surface  96   a  is brought out of contact with the valve seat  93 . 
     The breather valve  92  is normally kept closed, and it is opened once the inner pressure of the crankcase  35 , i.e. the pressure of the blow-by gas of the crankcase  35 , exceeds a predetermined value. Such opening of the breather valve  92  allows the blow-by gas of the crankcase  35  to be directed into (i.e. to flow into) the breather chamber  52  through the breather passage  51 . 
     Because the valve body  94  is formed of the elastic material into a dome shape as noted above, it can be elastically deformed with a minute pressure. Thus, the breather valve  92  can be reliably deformed to open the valve passages  95  once the inner pressure of the crankcase  35  exceeds the predetermined value. 
     The following describe an example manner in which the second embodiment of the breather mechanism  90  directs the blow-by gas of the crankcase  35  to the breather chamber  52  through operation of the breather valve  92 , with reference to  FIG. 13 . As shown in  FIG. 13 , once the inner pressure of the crankcase  35  (see  FIG. 3 ) exceeds the predetermined value as the engine  21  is driven, the valve body  94  of the breather valve  92  is elastically deformed to open the valve passages  95 . By the valve passages  95  being opened like this, the blow-by gas of the crankcase  35  and the lubricating oil  37  having entered the oil entry portion  61  flows into the breather chamber  52  by way of the breather passage  51  and breather valve  92  as indicated by arrow N. 
     Because the breather chamber  52  has a greater space than the breather passage  51 , the flow speed of the blow-by gas having flown into the breather chamber  52  would decrease, so that the lubricating gas  37 , having flown into the breather chamber  52  together with the blow-by gas, can be separated from the blow-by gas. The blow-by gas from which the lubricating gas  37  has been separated is sucked from the breather chamber  52  into the air inlet chamber  67  (see  FIG. 3 ) through the communication passage  68  as indicated by arrow O. Meanwhile, the lubricating gas  37 , having been separated from the blow-by gas, drops down to the lower portion  52   b  of the breather chamber  52  and is returned to the crankcase  35  (see  FIG. 3 ) through the main return passage  54 . 
     As set forth above in relation to  FIGS. 12 and 13 , the valve body  94  of the breather valve  92  is formed of the elastic material into the dome shape. Thus, the breather valve  92  can be reliably deformed so that the blow-by gas can be directed into the breather chamber  52  in an appropriate manner. 
     Further, the breather valve  92  employed in the second embodiment can achieve generally the same advantageous benefits as the breather valve  53  employed in the first embodiment. 
     It should be appreciated that the breather-mechanism-equipped engine  21  of the present invention may be modified variously as necessary without being limited to the above-described embodiments. 
     For example, whereas the first and second embodiments of the breather-mechanism-equipped engine  21  have been described above as applied to the power generator  10 , the present invention is not so limited, and the breather-mechanism-equipped engine  21  of the present invention may be applied to other working machines than power generators. 
     Furthermore, the shapes and constructions of the power generator  10 , engine  21 , breather mechanism  25 , crankcase  35 , breather passage  51 , breather chamber  52 , breather valves  53  and  92 , outlet end portion  58   b  of the breather passage  51 , inlet end portion  59  of the breather chamber  52 , oil entry portion  61 , valve seats  73  and  93 , valve bodies  74  and  94 , etc. are not limited to the aforementioned and may be modified as necessary. 
     The basic principles of the present invention are particularly suited for application to engines in which lubricating oil is stored in a crankcase and which is equipped with a breather mechanism that causes blow-by gas of the crankcase to flow out of the crankcase.