Patent Publication Number: US-11384725-B2

Title: Engine decompression device and engine

Description:
CROSS REFERENCE TO PRIOR APPLICATION 
     This application is a National Stage Patent Application of PCT International Patent Application No. PCT/JP2018/025600 (filed on Jul. 5, 2018) under 35 U.S.C. § 371, which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a decompression device configured to improve startability of an engine and the engine including the decompression device. 
     BACKGROUND ART 
     A decompression device is known in which a decompression lift is applied to an intake valve or an exhaust valve to temporarily open the intake valve or the exhaust valve to enable smooth rotation of a crankshaft and improve startability of an engine (for example, see Patent Literature 1). The intake valve or the exhaust valve is in a closed position when the engine is started. 
     For example, a decompression device  150  according to the related art shown in  FIG. 11  includes: a camshaft  125  including an intake valve cam  125   b  and an exhaust valve cam  125   c ; a decompression weight  151  that is rotatably provided via a pivot  125   e  provided on the camshaft  125 ; a decompression spring  152  configured to bias the decompression weight  151 ; and a decompression shaft  156  including an engagement pin  153  that is guided by a guide groove  151   a  provided in the decompression weight  151 , a decompression cam  154  that is provided on one cam surface of the intake valve cam  125   b  and the exhaust valve cam  125   c  so as to advance and retreat, and a connection portion  155  that connects the engagement pin  153  and the decompression cam  154 . 
     In the decompression device  150  configured in this way, when the engine is started, the decompression cam  154  is located in an advanced position where the decompression cam  154  protrudes from the cam surface, while providing a decompression lift to the intake valve or exhaust valve (hereinafter, appropriately referred to as decompression operation). On the other hand, after the engine has been started, as the decompression weight  151  rotates against a biasing force of the decompression spring  152  due to a centrifugal force, the decompression shaft  156  rotates such that the decompression cam  154  moves to a retracted position where the decompression cam  154  retracts from the cam surface, and the decompression lift for the intake valve or the exhaust valve is released (hereinafter, appropriately referred to as decompression release). 
     RELATED ART LITERATURE 
     Patent Literature 
     Patent Literature 1: JP-A-H08-177437 
     SUMMARY OF THE INVENTION 
     Problem that the Invention is to Solve 
     As shown on an upper side of  FIG. 12 , in an engine, when the engine is stopped, a piston may not overcome a compression top dead center (a compression TDC), and reverse rotation may occur. In particular, in a working machine (for example, a lawn mower) provided with a clutch having a small inertia, when the engine is stopped, reverse rotation is likely to occur when the engine is stopped. 
     As shown on a lower side of  FIG. 12 , if reverse rotation occurs when the engine is stopped, a force acts on the decompression cam  154  from a lifter  127  in a direction of moving the decompression cam  154  from the advanced position to the retracted position, so that the engine may be stopped in a state in which the decompression shaft and the decompression weight  151  are moved to a decompression release side due to this force. In this engine stopped state, the decompression device  150  does not function normally when the engine starts next time, and a starting load (for example, a recoil pulling load) becomes excessive, so there is room for improvement. 
     The present invention provides an engine decompression device and an engine that are capable of preventing decompression release due to reverse rotation when the engine is stopped. 
     Means for Solving the Problem 
     The present invention provides an engine decompression device including: 
     a camshaft including an intake valve cam and an exhaust valve cam; 
     a decompression weight that is rotatably provided via a pivot provided on the camshaft; 
     a decompression spring configured to bias the decompression weight; and 
     a decompression shaft including an engagement pin that is guided by a guide groove formed in the decompression weight, a decompression cam that is provided on one cam surface of the intake valve cam and the exhaust valve cam so as to advance and retreat, and a connection portion that connects the engagement pin and the decompression cam, 
     in which, when the decompression weight rotates against a biasing force of the decompression spring due to a centrifugal force, the decompression shaft rotates such that the decompression cam moves from an advanced position where the decompression cam protrudes from the cam surface to a retracted position where the decompression cam is retracted from the cam surface, and 
     in which the decompression weight is formed with a rotation restricting groove that restricts rotation of the decompression shaft when a force acts in a direction in which the decompression cam moves on the decompression shaft from the advanced position to the retracted position when the engine is stopped and that is continuous with the guide groove. 
     The present invention provides an engine including: 
     a valve mechanism configured to operate an intake valve and an exhaust valve in accordance with rotation of a crankshaft; and 
     the decompression device, 
     in which the valve mechanism includes:
         a timing gear to be fixed to the crankshaft;   the camshaft that rotates in conjunction with rotation of the timing gear;   a pair of lifters that abut against the intake valve cam and the exhaust valve cam;   a pair of rocker arms in which one end portions of the pair of rocker arms abut against the intake valve or the exhaust valve;   a pair of push rods configured to connect the pair of lifters to the other end portions of the pair of rocker arms; and   a pair of valve springs each configured to bias a respective one of the intake valve and the exhaust valve in a closing direction.       

     Advantages of the Invention 
     According to the present invention, when a force acts in a direction in which the decompression cam moves on the decompression shaft from the advanced position to the retracted position, the rotation of the decompression shaft is restricted by the rotation restricting groove formed in the decompression weight continuously with the guide groove, so that it is possible to prevent decompression release due to reverse rotation of the engine when the engine is stopped. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross sectional view of an engine according to an embodiment of the present invention. 
         FIG. 2  is a perspective view of the engine whose top cover is removed, as viewed from an obliquely front and upper side. 
         FIG. 3  is an exploded perspective view of the engine as viewed from the obliquely front and upper side. 
         FIG. 4  is an exploded perspective view of an engine body as viewed from the obliquely front and upper side. 
         FIG. 5  is a plan view of the engine whose crankcase cover is removed. 
         FIG. 6  is a cross sectional view taken along a line A-A in  FIG. 5 . 
         FIG. 7  is a perspective view of a valve mechanism of the engine as viewed from the obliquely front and upper side. 
         FIG. 8  is an exploded perspective view of the valve mechanism of the engine as viewed from the obliquely front and upper side. 
         FIG. 9  is an exploded perspective view of an engine decompression device. 
         FIG. 10A  is an explanatory view showing the decompression device (a decompression operation state) when the engine is started. 
         FIG. 10B  is an explanatory view showing the decompression device (the decompression operation state→a decompression release state) immediately after the engine has been started. 
         FIG. 10C  is an explanatory view showing the decompression device (the decompression release state) after the engine has been started. 
         FIG. 10D  is an explanatory view showing the decompression device (the decompression operation state) when the engine is reversely rotated. 
         FIG. 11  is an explanatory view showing a decompression device in the related art. 
         FIG. 12  is an explanatory view showing a movement of the decompression device in the related art when the engine is reversely rotated. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     An embodiment of the present invention will be described below with reference to  FIGS. 1 to 8 . An engine E according to the present embodiment is a small-sized general-purpose engine provided in a walk-behind lawn mower or the like, and is an OHV engine. For simplicity and clarity of description in the present specification, an axial direction of a crankshaft  2  is defined as an upper-lower direction, a direction which is orthogonal to the upper-lower direction and in which a cylinder portion  1   b  extends is defined as a front-rear direction, and a direction orthogonal to the upper-lower direction and the front-rear direction is defined as a left-right direction. In the drawings, a front side of the engine E is indicated as Fr, a rear side of the engine E is indicated as Rr, a left side of the engine E is indicated as L, a right side of the engine E is indicated as R, an upper side of the engine E is indicated as U, and a lower side of the engine E is indicated as D. 
     As shown in  FIGS. 1 to 3 , the engine E according to the present embodiment includes: an engine body  1  including a crankcase portion  1   a  and the cylinder portion  1   b ; the crankshaft  2  that is rotatably supported by the crankcase portion  1   a  in the upper-lower direction; a piston  4  that is slidably fitted to the cylinder portion  1   b  and is connected to the crankshaft  2  via a connecting rod  3 ; an intake valve  5 , an exhaust valve  6 , and a spark plug  7  that are provided on a head portion  1   c  of the cylinder portion  1   b ; a head cover  8  configured to cover the head portion  1   c  of the cylinder portion  1   b ; a valve mechanism  9  configured to operate the intake valve  5  and the exhaust valve  6  in accordance with rotation of the crankshaft  2 ; a flywheel  10  that is connected to an upper end portion of the crankshaft  2 ; a recoil starter  11  that is provided above the flywheel  10  and is configured to start the engine E; a top cover  12  configured to cover an upper part of the engine E; a fuel tank  13  configured to store fuel; an air cleaner  14  configured to purify air; a carburetor  15  configured to generate a mixed gas including fuel and air and to supply the mixed gas into the cylinder portion  1   b ; a muffler  16  configured to discharge an exhaust gas discharged from the cylinder portion  1   b  while silencing the discharging; a governor mechanism  17  (see  FIGS. 5 and 6 ) configured to automatically open and close a throttle valve (not shown) of the carburetor  15  in accordance with a rotation speed of the crankshaft  2 ; and an auto choke mechanism  18  configured to automatically open and close a choke valve (not shown) of the carburetor  15  in accordance with a temperature of the engine body  1 . 
     [Engine Body] 
     As shown in  FIG. 4 , the engine body  1  includes a crankcase body  19 , a crankcase cover  20 , and a cylinder unit  21 . 
     As shown in  FIGS. 4 to 6 , the crankcase body  19  includes a bottom part  19   a , and a cylindrical portion  19   c  that is formed integrally with the bottom part  19   a  at a lower end portion of the cylindrical portion  19   c  and includes a case opening portion  19   b  at an upper end portion of the cylindrical portion  19   c . On a center part of the bottom part  19   a , a first crankshaft through hole  19   d  is formed through which a lower end side of the crankshaft  2  is inserted. On a front surface portion of the cylindrical portion  19   c , a cylinder insertion hole  19   e  is formed through which a cylinder base portion  21   a  of the cylinder unit  21  is inserted. 
     As shown in  FIG. 4 , the crankcase cover  20  is configured to cover the case opening portion  19   b  of the crankcase body  19  and constitutes the crankcase portion  1   a  of the engine body  1  together with the crankcase body  19 . On a center part of the crankcase cover  20 , a second crankshaft through hole  20   a  is formed through which an upper end side of the crankshaft  2  is inserted. Returning to  FIG. 1 , the crankshaft  2  is rotatably supported between a second bearing  22  provided adjacent to the second crankshaft through hole  20   a  of the crankcase cover  20  and a first bearing  23  provided adjacent to the first crankshaft through hole  19   d  of the crankcase body  19 . 
     The crankcase cover  20  is detachably attached to an upper end portion of the crankcase body  19  via a plurality of bolts B 1 . Specifically, a plurality of bolt through holes  20   b  through which the bolts B 1  are inserted from above are formed at a peripheral portion of the crankcase cover  20 . On the other hand, a plurality of bolt fastening holes  19   f  to which the bolts B 1  are fastened from above are formed at the upper end portion of the crankcase body  19 . By fastening the bolts B 1  to the bolt fastening holes  19   f  via the bolt through holes  20   b , the crankcase cover  20  can be attached to the crankcase body  19 . Conversely, by releasing the fastening of the bolts B 1  to the bolt fastening holes  19   f , the crankcase cover  20  can be removed from the crankcase body  19 . 
     According to the crankcase body  19  and the crankcase cover  20 , during maintenance of the engine E, an inside of the crankcase body  19  can be accessed from above by removing the crankcase cover  20 . In particular, when the crankshaft  2  is replaced, the crankshaft  2  can be easily replaced by removing the crankcase cover  20  and extracting the crankshaft  2 . 
     As shown in  FIGS. 4 to 6 , the cylinder unit  21  includes the cylinder base portion  21   a  that is inserted to the cylinder through hole  19   e  of the crankcase body  19  from the front side and is to be positioned inside the crankcase body  19 , and a cylinder block  21   b  that extends forward from the cylinder base portion  21   a  and is to be positioned outside the crankcase body  19 . The cylinder unit  21  alone constitutes the cylinder portion  1   b  of the engine body  1 , and a front end portion of the cylinder block  21   b  constitutes the head portion  1   c . Inner circumferential surfaces of cylindrical portions of the cylinder base portion  21   a  and the cylinder block  21   b  constitute a cylinder bore  21   c  that is a sliding surface with the piston  4 , and a large number of cooling fins  21   d  protrude from an outer peripheral portion of the cylinder block  21   b.    
     According to the cylinder unit  21 , a plurality of types of cylinder units  21  having different bore diameters are provided, so that it is possible to provide the engine body  1  having different exhaust amounts simply by replacing the cylinder unit  21  while sharing the crankcase body  19  and the crankcase cover  20 . 
     The cylinder unit  21  is detachably attached to the crankcase body  19  via a plurality of bolts B 2 , B 3 . For example, a plurality of bolt through holes (not shown) through which the bolts B 2  are inserted from the front side are formed at a rear end portion of the cylinder block  21   b . On the other hand, a plurality of bolt fastening holes  19   g  to which the bolts B 2  are fastened from the front side are formed at a front end portion of the crankcase body  19 . By fastening bolts B 2  to the bolt fastening holes  19   g  via the bolt through holes of the cylinder block  21   b , the cylinder unit  21  can be attached to the crankcase body  19 . Conversely, by releasing the fastening of the bolts B 2  to the bolt fastening holes  19   g , the cylinder unit  21  can be removed from the crankcase body  19 . 
     However, in the engine body  1  according to the present embodiment, when the cylinder unit  21  is detachably attached to the crankcase body  19  via the plurality of bolts B 2 , B 3 , the bolts B 3  on an upper end portion side are fastened to the cylinder unit  21  from the inside of the crankcase body  19 . Specifically, a plurality of bolt through holes  19   h  through which the bolts B 3  are inserted frontward from the inside of the crankcase body  19  are formed at the front end portion of the crankcase body  19 . On the other hand, a plurality of bolt fastening holes (not shown) to which the bolts B 3  are fastened from the rear side are formed at the rear end portion of the cylinder block  21   b . The bolts B 3  are fastened to the bolt fastening holes of the cylinder block  21   b  via, the bolt through holes  19   h  of the crankcase body  19 . 
     According to this attachment structure of the cylinder unit  21 , it is not required to form a space for fastening the bolts B 3  from the front side on at least the upper end portion side of the cylinder block  21   b . Therefore, the cylinder unit  21  can be attached to the crankcase body  19  without interfering with an external structure (for example, the cooling fins  21   d ) of the cylinder block  21   b , and a cooling performance and the like of the engine E can be improved. 
     [Valve Mechanism] 
     As shown in  FIGS. 6 to 8 , the valve mechanism  9  includes: a timing gear  24  to be assembled to the crankshaft  2  in an integrally rotatable manner; a camshaft  25  rotatably supported on the bottom portion  19   a  of crankcase body  19 ; a pair of lifters  27  that are swingably supported on the bottom part  19   a  of the crankcase body  19  via stepped bolts  26 ; a pair of rocker arms  29  which are swingably supported on the front end portion of the cylinder block  21   b  via rocker arm shafts  28 , and one end portions of which abut against a front end portion of the intake valve  5  or the exhaust valve  6 ; a pair of push rods  30  that are accommodated in a push rod accommodation portion  21   e  formed on a lower part of the cylinder unit  21 , and connect each of the lifters  27  to a respective one of the other end portions of the pair of rocker arms  29 ; and a pair of valve springs  31  each configured to bias a respective one of the intake valve  5  and the exhaust valve  6  in a closing direction. 
     The camshaft  25  includes a gear portion  25   a  that meshes with the timing gear  24  and are driven to rotate at a speed reduction ratio of ½ by the timing gear  24 , and a pair of cam portions  25   b ,  25   c  that press the pair of lifters  27  alternately in accordance with the rotation drive of the gear portion  25   a . When the cam portions  25   b ,  25   c  press the lifter  27 , the other end portion of the corresponding rocker arm  29  is pressed via the push rod  30 , and the intake valve  5  or the exhaust valve  6  connected to the one end portion of the rocker arm  29  is opened. On the other hand, when the pressing of the lifter  27  by the cam portion  25   b  is released, the intake valve  5  or the exhaust valve  6  is closed due to the biasing force of the valve spring  31 . In the present embodiment, the cam portion  25   b  functions as an intake valve cam configured to open and close the intake valve  5 , and the cam portion  25   c  functions as an exhaust valve cam configured to open and close the exhaust valve  6 . 
     The camshaft  25  according to the present embodiment is provided below the cylinder base portion  21   a  of the cylinder unit  21 . When the camshaft  25  is provided in this way, the inside of the crankcase body  19  can be accessed from above only by removing the crankcase cover  20  even without removing the camshaft  25  during the maintenance of the engine E. 
     [Configuration of Decompression Device] 
     Next, a decompression device  50  provided in the camshaft  25  will be described with reference to  FIGS. 9 and 10A to 10D . 
     The camshaft  25  is formed with a circular recess  25   d  on an upper surface of the camshaft  25 , and the decompression device  50  is provided in the recess  25   d . The decompression device  50  according to the present embodiment includes: a decompression weight  51  that is rotatably provided via a pivot  25   e  provided on the camshaft  25 ; a decompression spring  52  configured to bias the decompression weight  51 ; a decompression shaft  56  including an engagement pin  53  that is guided by a guide groove  51   a  provided in the decompression weight  51 , a decompression cam  54  that is provided on a cam surface of the cam portion  25   b  and the cam portion  25   c  so as to advance and retreat, and a connection portion  55  that connects the engagement pin  53  and the decompression cam  54 ; and a hold plate  57  configured to cover the recess  25   d  while holding the decompression weight  51 , the decompression spring  52 , and the decompression shaft  56 . 
     In the decompression device  50  configured in this way, when the engine E is started, the decompression cam  54  is located in an advanced position where the decompression cam  54  protrudes from the cam surface of the cam portion  25   b  or the cam portion  25   c , while providing a decompression lift to the intake valve  5  or the exhaust valve  6 . On the other hand, after the engine E has been started, as the decompression weight  51  rotates against a biasing force of the decompression spring  52  due to a centrifugal force, the decompression shaft  56  rotates such that the decompression cam  54  moves to a retracted position where the decompression cam  54  retracts from the cam surface of the cam portion  25   b  or the cam portion  25   c , and the decompression lift for the intake valve or the exhaust valve is released. Hereinafter, the recess  25   d  of the camshaft  25 , the decompression weight  51 , the decompression spring  52 , and the decompression shaft  56  will be described in detail. 
     The recess  25   d  of the camshaft  25  includes, in addition to the above-described pivot  25   e , a decompression shaft support hole  25   f  that rotatably supports the decompression shaft  56  and exposes the decompression cam  54  to the cam surface of the cam portion  25   b  or the cam portion  25   c  so that the decompression cam  54  can advance and retreat, a convex portion  25   g  that defines a rotation range of the decompression shaft  56  (the connection portion  55 ), and an inner peripheral wall portion  25   h  that defines a rotation limit position of the decompression weight  51  in a decompression release direction. 
     The decompression weight  51  is a metal plate member having an arcuate shape along the inner peripheral wall portion  25   h  of the camshaft  25 , and includes a fitting hole  51   b  that rotatably fits to the pivot  25   e  of the camshaft  25 , an outer peripheral portion  51   c  that abuts against the inner peripheral wall portion  25   h  of the camshaft  25  when the decompression is released, an inner peripheral portion  51   d  opposite the outer peripheral portion  51   c , a guide groove  51   a  that engages with the engagement pin  53  of the decompression shaft  56 , and a rotation restricting groove  51   e  that is continuous with the guide groove  51   a  and is provided at a connection portion between the guide groove  51   a  and the inner peripheral portion  51   d.    
     The decompression spring  52  is a torsion coil spring and is provided on the pivot  25   e  of the camshaft  25 . The decompression spring  52  biases the decompression weight  51  toward the inner peripheral side by engaging the camshaft  25  on one end side of the decompression spring  52  and engaging the decompression weight  51  on the other end side of the decompression spring  52 . 
     The decompression weight  51  configured in this way is rotatable between a rotation position (hereinafter, appropriately referred to as a decompression operation position) where the rotation restricting groove  51   e  abuts against the engagement pin  53  and a rotation position (hereinafter, appropriately referred to as a decompression release position) where the outer peripheral portion  51   c  abuts against the inner peripheral wall portion  25   h  of the camshaft  25 . When the engine E is started, the decompression weight  51  is maintained at the decompression operation position due to a biasing force of the decompression spring  52 . On the other hand, after the engine E has been started, the decompression weight  51  is rotated to the decompression release position against the biasing force of the decompression spring  52  due to a centrifugal force. 
     The guide groove  51   a  is provided on a distal end side away from a rotation fulcrum point (the pivot  25   e ) of the decompression weight  51 , and engages with the engagement pin  53  of the decompression shaft  56  to interlock the decompression shaft  56  with the rotation of the decompression weight  51 . More specifically, when the decompression weight  51  is located at the decompression operation position, the guide groove  51   a  rotates the decompression shaft  56  to a rotation position where the decompression cam  54  protrudes from the cam surface of the cam portion  25   b  or the cam portion  25   c . On the other hand, when the decompression weight  51  is located at the decompression release position, the guide groove  51   a  rotates the decompression shaft  56  to a rotation position where the decompression cam  54  is retracted from the cam surface of the cam portion  25   b  or the cam portion  25   c.    
     The rotation restricting groove  51   e  restricts rotation of the decompression shaft  56  when a force in a direction in which the decompression cam  54  moves from the advanced position to the retracted position acts on the decompression cam  54  from the lifter  27  (when the engine E is reversely rotated as described later). Specifically, the rotation restricting groove  51   e  includes a restricting surface  51   f  orthogonal to a virtual line L (see  FIG. 10D ) connecting the pivot  25   e  and the engagement pin  53  when the decompression cam  54  is in the advanced position. When a force in a direction in which the decompression cam  54  moves from the advanced position to the retracted position acts on the decompression cam  54  from the lifter  27 , the engagement pin  53  abuts against the restricting surface  51   f . At this time, since a vector for rotating the decompression weight  51  does not act on the decompression weight  51 , the rotation of the decompression shaft  56  is restricted. 
     As described above, the decompression shaft  56  rotates between the decompression operation position and the decompression release position in conjunction with the rotation of the decompression weight  51 . The decompression cam  54  provided on the decompression shaft  56  includes a circumferential surface  54   a  and a flat surface  54   b  obtained by cutting out a part of the circumferential surface  54   a . When the decompression shaft  56  is located at the decompression operation position, the circumferential surface  54   a  of the decompression cam  54  is protruded from the cam surface of the cam portion  25   b  or the cam portion  25   c . On the other hand, when the decompression shaft  56  is located at the decompression release position, the decompression cam  54  is retracted on the cam surface of the cam portion  25   b  or the cam portion  25   c  by aligning the flat surface  54   b  of the decompression cam  54  with the cam surface of the cam portion  25   b  or the cam portion  25   c.    
     [Operation of Decompression Device] 
     Next, operation of the decompression device  50  accompanying the start and stop of the engine E will be described with reference to  FIGS. 10A to 10D . In  FIGS. 10A to 10D , the cam portions  25   b ,  25   c  are indicated by solid lines. However, the cam portions  25   b ,  25   c  are located on an opposite side of the recess  25   d.    
     As shown in  FIG. 10A , when the engine E is started (before starting), the decompression weight  51  is located at the decompression operation position due to a biasing force of the decompression spring  52 . At this time, the engagement pin  53  of the decompression shall  56  is located in the rotation restricting groove  51   e  of the decompression weight  51  and is pushed by the decompression weight  51  in a direction of an arrow in  FIG. 10A , thereby holding the decompression shaft  56  in the decompression operation position. Therefore, when the engine E is started, the decompression cam  54  provided on the decompression shaft  56  is located in the advanced position where the decompression cam  54  protrudes from the cam surface of the cam portion  25   b  or the cam portion  25   c , and provides a decompression lift to the intake valve  5  or the exhaust valve  6 , thereby improving startability of the engine E. 
     As shown in  FIG. 10B , immediately after the engine E has been started, the decompression weight  51  rotates toward the decompression release position against a biasing force of the decompression spring  52  due to a centrifugal force. At this time, the engagement pin  53  of the decompression shaft  56  is located in the guide groove  51   a  of the decompression weight  51  and is pushed by the decompression weight  51  in a direction of an arrow in  FIG. 10B , thereby rotating the decompression shaft  56  toward the decompression release position. As shown in  FIG. 10C , after the engine E has been started, the decompression cam  54  provided on the decompression shall  56  is moved to the retracted position where the decompression cam  54  is retracted from the cam surface of the cam portion  25   b  or the cam portion  25   c , thereby releasing the decompression lift of the intake valve  5  or the exhaust valve  6 . 
     When the engine E is stopped, the decompression weight  51  rotates toward the decompression operation position due to a biasing force of the decompression spring  52 . At this time, the engagement pin  53  of the decompression shaft  56  is located in the guide groove  51   a  of the decompression weight  51  and is pushed by the decompression weight  51  in a direction of an arrow in  FIG. 10C , thereby rotating the decompression shaft  56  toward the decompression operation position. Therefore, after the engine E has been stopped, a state returns to the decompression operation state shown in  FIG. 10A , and next startability of the engine E is improved. 
     When the engine E is stopped, the piston  4  may not overcome a compression top dead center, and reverse rotation may occur. When the engine E is reversely rotated at a time of stopping, a force in the direction in which the decompression cam  54  is moved from the advanced position to the retracted position acts on the decompression cam  54  from the lifter  27 . This force attempts to rotate the decompression shaft  56  in the decompression release direction. However, when the engagement pin  53  of the decompression shaft  56  moves from a position in  FIG. 10C  to a position in  FIG. 10D , the engagement pin  53  of the decompression shaft  56  abuts against only the restricting surface  51   f  of the rotation restricting groove  51   e  of the decompression weight  51 . 
     That is, as described above, the rotation restricting groove  51   e  includes the restricting surface  51   f  orthogonal to the virtual line L connecting the pivot  25   e  and the engagement pin  53  when the decompression cam  54  is in the advanced position in the decompression operating state. When a force in the direction in which the decompression cam  54  moves from the advanced position to the retracted position acts on the decompression cam  54  from the lifter  27 , the engagement pin  53  abuts against the restricting surface  51   f . Therefore, a force from the engagement pin  53  to the decompression weight  51  acts only in a direction of an arrow in  FIG. 10D . Therefore, a vector for rotating the decompression weight  51  does not act on the decompression weight  51 , and the rotation of the decompression shaft  36  is restricted. Therefore, decompression release due to reverse rotation of the engine E when the engine E is stopped is prevented. 
     The above-described embodiment can be appropriately modified, improved, or the like. For example, in the above-described embodiment, a decompression device of a small-sized general-purpose engine provided in a walk-behind lawn mower or the like is shown. However, the decompression device according to the present invention is not limited to being applied to the small-sized general-purpose engine, and can be applied to various engines. 
     The present specification describes at least the following matters. Corresponding components in the above-described embodiment are shown in parentheses. However, the present invention is not limited thereto. 
     (1) An engine decompression device (the decompression device  50  of the engine E) including: 
     a camshaft (the camshaft  25 ) including an intake valve cam (the cam portion  25   c ) and an exhaust valve cam (the cam portion  25   b ); 
     a decompression weight (the decompression weight  51 ) that is rotatably provided via a pivot (the pivot  25   e ) provided on the camshaft; 
     a decompression spring (the decompression spring  52 ) configured to bias the decompression weight; and 
     a decompression shaft (the decompression shaft  56 ) including an engagement pin (the engagement pin  53 ) that is guided by a guide groove (the guide groove  51   a ) formed in the decompression weight, a decompression cam (the decompression cam  54 ) that is provided on one cam surface of the intake valve cam and the exhaust valve cam so as to advance and retreat, and a connection portion (the connection portion  55 ) that connects the engagement pin and the decompression cam, 
     in which, when the decompression weight rotates against a biasing force of the decompression spring due to a centrifugal force, the decompression shaft rotates such that the decompression cam moves from an advanced position where the decompression cam protrudes from the cam surface to a retracted position where the decompression cam is retracted from the cam surface, and 
     in which the decompression weight is formed with a rotation restricting groove (the rotation restricting groove  51   e ) that restricts rotation of the decompression shaft when a force acts in a direction in which the decompression cam moves on the decompression shaft from the advanced position to the retracted position when the engine is stopped and that is continuous with the guide groove. 
     According to (1), when a force acts in a direction in which the decompression cam moves on the decompression shaft from the advanced position to the retracted position, the rotation of the decompression shaft is restricted by the rotation restricting groove formed in the decompression weight continuously with the guide groove, so that it is possible to prevent decompression release due to reverse rotation of the engine when the engine is stopped. 
     (2) The engine decompression device according to (1), 
     in which the rotation restricting groove includes a restricting surface (the restricting surface  51   f ) orthogonal to a virtual line (the virtual line L) connecting the pivot and the engagement pin when the decompression cam is in the advanced position, and 
     in which, when a force acts in a direction in which the decompression cam moves on the decompression shaft from the advanced position to the retracted position, the engagement pin abuts against the restricting surface. 
     According to (2), when a force acts in a direction in which the decompression cam moves on the decompression shaft from the advanced position to the retracted position, the engagement pin abuts against the restricting surface, so that a vector for rotating the decompression weight does not act on the decompression weight, and rotation of the decompression shaft is restricted. 
     (3) An engine (the engine E) including: 
     a valve mechanism (the valve mechanism  9 ) configured to operate an intake valve (the intake valve  5 ) and an exhaust valve (the exhaust valve  6 ) in accordance with rotation of a crankshaft (the crankshaft  2 ); and 
     the decompression device according to (1) or (2), 
     in which the valve mechanism includes:
         a timing gear (the timing gear  24 ) to be fixed to the crankshaft;   the camshaft that rotates in conjunction with rotation of the timing gear;   a pair of lifters (the lifters  27 ) that abut against the intake valve cam and the exhaust valve cam;   a pair of rocker arms (the rocker arms  29 ) in which one end portions of the pair of rocker arms abut against the intake valve or the exhaust valve;   a pair of push rods (the push rods  30 ) configured to connect the pair of lifters to the other end portions of the pair of rocker arms; and   a pair of valve springs (the valve springs  31 ) each configured to bias a respective one of the intake valve and the exhaust valve in a closing direction.       

     According to (3), when a force acts in a direction in which the decompression cam moves on the decompression shaft from the advanced position to the retracted position by the lifters when the engine is stopped, rotation of the decompression shaft is restricted by the rotation restricting groove. Accordingly, it is possible to prevent decompression release due to reverse rotation of the engine when the engine is stopped. 
     DESCRIPTION OF REFERENCE NUMERALS AND CHARACTERS 
     
         
         
           
             E engine 
               2  crankshaft 
               5  intake valve 
               6  exhaust valve 
               9  valve mechanism 
               24  timing gear 
               25  camshaft 
               25   b  cam portion (intake valve cam) 
               25   c  cam portion (exhaust valve cam) 
               25   e  pivot 
               27  lifter 
               29  rocker arm 
               30  push rod 
               31  valve spring 
               50  decompression device 
               51  decompression weight 
               51   a  guide groove 
               51   e  rotation restricting groove 
               51   f  restricting surface 
               52  decompression spring 
               53  engagement pin 
               54  decompression cam 
               55  connection portion 
               56  decompression shaft 
             L virtual line