Patent Publication Number: US-2016237876-A1

Title: Air-cooled engine and engine working machine

Description:
TECHNICAL FIELD 
     The present invention mainly relates to an air-cooled engine used as a power source for a portable working machine such as a brush cutter and an air blower and for a working machine such as a generator, and in particular to a cooling structure for the air-cooled engine. 
     BACKGROUND ART 
     A small-sized air-cooled engine is widely used as a power source for a small-sized working machine such as a brush cutter and a chain saw.  FIG. 11  is a perspective view illustrating an outline of a brush cutter  101  as one example of an engine working machine. As illustrated in  FIG. 11 , a small-sized air-cooled engine  110  is mounted in the engine working machine. The engine working machine includes a hollow main pipe  105 , and an engine  110  is provided on one end of the main pipe  105 , and a rotary blade  106  is provided on the other end of the main pipe  105 . A driving shaft (not illustrated) is built inside the main pipe  105 . By rotating the driving shaft by the engine  110 , the rotary blade  106  is rotated. In the vicinity of the rotary blade  106 , a scatter protection cover  106   a  for preventing scattering of cut glass is provided on the other end of the main pipe  105 . The engine working machine  101  is carried by an operator through a shoulder suspending belt (not illustrated) etc. A handle  104  that is operated by an operator is attached in the vicinity of a longitudinal center portion of the main pipe  105 . The handle  104  is formed in substantially a U shape in a front view. A rotation speed of the engine is controlled by the operator through a throttle lever  107  attached to the vicinity of a grip portion  103 . Operation of the throttle lever  107  is transmitted to a throttle valve in a carburetor of the engine through a throttle wire  118 . In the vicinity of a distal end  129  of the grip portion  103 , a switch (not illustrated) for stopping the engine  110  is provided. 
     In the small-sized air-cooled engine used as a power source for a portable working machine such as a brush cutter and a chain saw and for a working machine such as a generator, a cooling fan is provided on one end of the driving shaft, and a cooling air path is formed by a cover that covers the cooling fan and the engine body. In the cooling air path, cooling air flows from the cooling fan side to the opposite side of the cooling fan. A muffler is attached to a cylinder and the cylinder is covered with a muffler cover  136  that forms a muffler cooling room of the muffler. For example, in the engine working machine described above, gas exhausted from a muffler and air after cooling of a cylinder are exhausted to a rear side that is an opposite side of a position of an operator with respect to the engine  110 . However, since a high-temperature exhaust gas flows inside the muffler, a surface of the muffler becomes a high temperature (for example, 300 to 500 degrees C.). Therefore, the muffler cover  136  is likely to receive heat damages such as erosion due to receiving radiation heat from the muffler and heat by natural convection. In particular, in a visible region from the outside, the heat damages (such as burnt, discoloration and transformation) result in an appearance defect, therefore, there has been a problem that the product value is remarkably deteriorated. 
     To deal with such a problem, as disclosed in Patent Literature 1, a muffler and a muffler cover are cooled by supplying cooling air to a space between the muffler and the muffler cover, thereby reducing the heat damages such as erosion. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Application Laid-Open Publication No. 2013-068140 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, although the cooling air is supplied to the space between the muffler and the muffler cover, a temperature cannot be sufficiently reduced, in particular, on an outer surface of the muffler cover because the cooling air also becomes a high temperature due to absorption of heat in the muffler by the time that the muffler is cooled. As a result, there has been a problem that the effect for reducing the heat damages is not sufficient. 
     It is an object of the present invention to provide a cooling structure capable of reducing the heat damages and preventing deterioration of the product value by sufficiently cooling a muffler cover, in particular, the temperature on an outer surface of the muffler cover along with a muffler, in the small-sized air-cooled engine that supplies the cooling air to the space between the muffler and the muffler cover. 
     Solution to Problem 
     An air-cooled engine of the present invention includes: a cylinder; a crank case that is attached to the cylinder and rotatably supports a driving shaft; a cooling fan that is attached to the driving shaft; a muffler that is attached to an exhaust port of the cylinder and exhausts exhaust gas passing inside from an exhaust exit; a fan case that covers the cooling fan; a cylinder cover that covers the cylinder to form a cylinder room; and a muffler cover that covers the muffler to form a muffler room, and the air-cooled engine is characterized in that the muffler cover includes a first muffler cover that covers at least some of the muffler and a second muffler cover that covers at least some of the first muffler cover, and some of cooling air generated by the cooling fan is supplied to at least one of a first space between the muffler and the first muffler cover and a second space between the first muffler and the second muffler cover, and an air guiding portion is provided inside the muffler cover, the air guiding portion guiding cooling air that is supplied to the first space or the second space to an exhaust exit side of the muffler. 
     Advantageous Effect of Invention 
     A small-sized air-cooled engine includes a first muffler cover that covers at least some of a muffler through a clearance and a second muffler cover that covers at least some of the first muffler cover through a clearance. A muffler cooling room is formed by the first muffler cover and the second muffler cover, and some of cooling air generated by a cooling fan is supplied to the muffler cooling room. Since the cooling air is supplied to at least one of a first space between the muffler and the first muffler cover and a second space between the first muffler cover and the second muffler cover, the muffler, the first muffler cover and the second muffler cover can be cooled by the cooling air, while the second muffler cover is protected from radiation heat by the muffler. Since the cooling air is guided in an exhaust direction of exhaust gas from the muffler by an air guiding portion, it is possible to effectively reduce heat damages of the second muffler cover. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an exploded perspective view of a small-sized air-cooled engine according to an embodiment of the present invention, 
         FIG. 2  is a front view of the small-sized air-cooled engine according to an example of the present invention, 
         FIG. 3  is a right side view of  FIG. 2 , 
         FIG. 4  is a cross-sectional view taken along the line C-C of  FIG. 3 , 
         FIG. 5  is a cross-sectional view taken along the line B-B of  FIG. 2 , 
         FIG. 6  is a cross-sectional view taken along the line A-A of  FIG. 2 , 
         FIG. 7  is a perspective view illustrating an inner surface of a second muffler cover illustrated in  FIG. 1 , 
         FIG. 8  is a perspective view illustrating an inner surface of a third muffler cover illustrated in  FIG. 1 , 
         FIG. 9  is a perspective view illustrating an inner surface of a first muffler cover illustrated in  FIG. 1 , 
         FIG. 10  is a perspective view of a divider plate illustrated in  FIG. 1 , and 
         FIG. 11  is a perspective view illustrating an appearance of a brush cutter as an engine working machine. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A structure of an air-cooled engine (engine) that is an embodiment of the present invention will be described. Herein, the air-cooled engine includes a two-stroke engine body having a cylinder, a crankcase, etc. and a cylinder cover that covers the cylinder, etc. In the engine body, a cooling fan is fixed to a driving shaft, and the cylinder is cooled in the cylinder cover by cooling air generated by rotation of the cooling fan. Further, in the cylinder, a plurality of radiation fins are formed on an outer surface of a cylinder portion formed in substantially a cylinder shape, and a combustion chamber is formed inside the cylinder portion. 
     This engine is applied to a portable working machine, such as a brush cutter and an air blower, carried by an operator, and the engine having the above structure is mounted in the engine working machine. Therefore, practically, a reducer, etc. for driving the engine working machine are connected to a driving shaft, and the structures for fixing the engine to the engine working machines (for example, operation tube, handle and cutting blade of brush cutter) are provided in the engine. 
     However, the connection structures between the engine and the engine working machines are not directly related to the present invention, and such structures are the same as those conventionally known. Therefore, the explanation of such structures will be omitted, but the structures and functions relating to cooling of the engine will be mainly described. 
       FIG. 1  is an exploded perspective view of a small-sized air-cooled engine (engine)  100  according to an embodiment of the present invention. The engine  100  includes a crankcase  4  that rotatably supports a driving shaft  21 , and a cylinder  2  is provided at an upper part of the crankcase  4 . An upper part of the cylinder  2  is covered with a cylinder cover  1 . A piston  17 , etc. (not illustrated in  FIG. 1 ) are provided inside the cylinder  2 , and in a surface of the cylinder  2 , a plurality of fins are arranged in substantially parallel in a vertical direction, so as to enhance cooling efficiency. An ignition plug (not illustrated) for igniting air-fuel mixture in the cylinder  2  is provided at an upper part of the cylinder  2 , and a plug cover  20  is attached to the ignition plug. The ignition plug is electrically connected to an ignition device  19  ( FIG. 4 ) attached to the cylinder  2  by a high-tension code (not illustrated), and the ignition plug emits sparks inside the cylinder  2  by operation of the ignition device  19 . A cooling fan  18  is fixed to the driving shaft  21  that is driven in the engine  100 , thereby generating cooling air. 
       FIG. 2  is a front view of the engine  100 ,  FIG. 3  is a right side view of  FIG. 2 ,  FIG. 4  is a cross-sectional view taken along the line C-C of  FIG. 3 , and  FIG. 5  is a cross-sectional view taken along the line B-B of  FIG. 2 . 
     At a lower part of the crankcase  4 , a fuel tank  5  in which fuel is accumulated is attached. An intake port  23  for guiding the air-fuel mixture in which the fuel and air are mixed into the cylinder  2  is provided on the right side of the cylinder  2  when seen from the front. An intake tube  22  is attached to the intake port  23 , and a carburetor  10  that generates the air-fuel mixture is also attached to the intake port  23 . Further, an air cleaner  6  is fixed to the right side of the carburetor  10 , so as to remove dust, etc. in air that is guided into the carburetor  10 . The air cleaner  6  is fixed in a state that a filter element (not illustrated) for removing dust, etc. is housed. The carburetor  10  and the fuel tank  5  are connected to each other by a fuel passage (not illustrated) formed of rubber tube, etc., and the fuel is supplied to the carburetor. The fuel is supplied to the fuel tank  5  by removing a tank cap  30  attached to a fill opening. 
     For purification and sound deadening of exhaust gas, a muffler  8  is connected to an exhaust port  3  of the cylinder  2  by screw members  52  through a divider plate  7 , and the exhaust gas is discharged outside through the muffler  8 . The divider plate  7  is formed of, for example, sheet-shaped material that covers steel sheet with black lead, and functions as a gasket between the muffler  8  and the exhaust port  3 . The divider plate  7  is extended in a radially outward direction of the exhaust port  3  more widely than the exhaust port  3 , and a cylinder cooling room  34  and a muffler cooling room  35  which are formed inside the cylinder cover  1  by the cylinder cover  1  are divided by the divider plate  7 . As illustrated in  FIG. 10 , in the divider plate  7 , a first communication port  24  is defined by a notch portion  24   a  and a second communication port  25  is defined by a notch portion  25   a . The cylinder cooling room  34  and the muffler cooling room  35  are communicated with each other by both of the first communication port  24  and the second communication port  25 . 
     The muffler  8  is formed in a substantially box shape, and includes an exhaust entrance and an exhaust exit  14  which are attached to the exhaust port  3 . As illustrated in  FIG. 5 , a divider plate  15  that divides inner spaces is provided inside the muffler  8 , and a catalyst  16  is attached to the divider plate  15  by weld. The catalyst  16  purifies exhaust gas by combusting harmful components in the exhaust gas. 
     As illustrated in  FIG. 2 , when a projecting side of the driving shaft  21  is a front of the engine  100 , the air cleaner  6  is disposed on the right side of the engine  100  and the muffler  8  is disposed on the left side of the engine  100 . The fuel tank  5  is disposed below the engine  100 . 
     As illustrated in  FIG. 2 , a muffler cover  36  that covers at least some of the muffler  8  is provided around the muffler  8 , and the muffler  8  is housed inside the muffler cover  36 . The muffler cover  36  includes a first muffler cover  11  that is disposed opposed to the muffler  8 , a second muffler cover  12  that is disposed opposed to the first muffler cover  11  and outside the first muffler cover  11 , and a third muffler cover  13  that is disposed at the exhaust exit  14  side of the second muffler cover  12 . 
     The first muffler cover  11  is formed of metal such as aluminum and steel. As illustrated in  FIGS. 5 and 9 , the first muffler cover  11  is formed in substantially a box shape such that the right side surface and the rear side surface are open, and in particular, includes a left wall portion opposed to the left side surface of the muffler  8  and a front wall portion opposed to the front side surface of the muffler  8  that is a cooling fan side. As described later, the front wall portion includes an air guiding portion  28 A that guides cooling air to the exhaust exit side. 
     The second muffler cover  12  is formed of resin having heat resistance. As illustrated in  FIG. 7 , the second muffler cover  12  is formed in substantially a box shape such that one face is open, and the second muffler cover  12  is attached to the cylinder cover  1  in a state that the open face is directed to the cylinder  2 . A surface of the second muffler cover  12  is formed in a netlike shape by a plurality of ribs. A wall surface is provided between adjacent ribs so as to reduce leakage of cooling air to the outside and effectively guide the cooling air, and through-holes are provided between some of the adjacent ribs as air windows  43 . That is, the second muffler cover  12  includes air guiding portions  28 B for cooling air at the left side surface and the front side surface. Further, at the rear side surface of the second muffler cover  12 , an air exhaust port  50  is formed at a position corresponding to the exhaust exit  14  of the muffler  8  as an opening. 
     The third muffler cover  13  is formed of resin having heat resistance. As illustrated in  FIG. 8 , the third muffler cover  13  is formed in a box shape such that one face is open as in the second muffler cover  12 , and the third muffler cover  13  is attached to the rear side surface of the second muffler cover  12  in a state that the open face is directed to the rear side surface of the muffler  8 . As illustrated in  FIG. 7 , screw boss portions  29  having female screws are provided at rear side wall portions of the second muffler cover  12 . The third muffler cover  13  is attached to the rear side surface of the second muffler cover  12  by screwing screw members  44  illustrated in  FIG. 1  and coupling the screw members to the screw boss portions  29 . Further, at the rear side surface of the third muffler cover  13 , an opening  49  is formed at a position corresponding to the exhaust exit  14  of the muffler  8 , as in the second muffler cover  12 . 
     The first muffler cover  11  is fastened to the second muffler cover  12  by the screw members  37 , and the third muffler cover  13  is fastened to the second muffler cover  12  by the screw members  44 . As illustrated in  FIG. 5 , an outside wall of the third muffler cover  13 , that is, a left-side wall portion  47  is formed so as to cover some of the left wall portion on the exhaust exist  14  side of the second muffler cover  12 . The first muffler cover  11  has a side wall opposed to the muffler  8 , and a first space  32 A is formed between the first muffler cover  11  and the muffler  8 . The first muffler cover  11  is not in contact with the muffler  8 . Aside from the second muffler cover  12 , the first muffler cover  11  may commonly be attached by screw members that attach the muffler  8  and the cylinder  2 . In this case, it is preferable to form the first space  32 A between the first muffler cover  11  and the muffler  8  by using a spacer, etc., thereby attaching the first muffler cover  11  and the muffler  8  to the cylinder  2 . Further, a second space  32 B is formed between the first muffler cover  11  and the second muffler cover  12 . The first muffler cover  11  and the second muffler cover  12  are in contact with each other only at fasting portions by the screw members  37 . 
     As described above, the muffler cooling room  35  is divided into the first space  32 A and the second space  32 B by the first muffler cover  11 . A small clearance is formed between the third muffler cover  13  and the second muffler cover  12 , and the third muffler cover  13  and the second muffler cover  12  are in contact with each other only at fasting portions by the screw members  44 . In a state that the divider plate  7  and the first muffler cover  11  are assembled, the notch portion  24   a  formed in the divider plate  7  and an end portion  41  of the first muffler cover  11  are intersected with each other in a directional view vertical to a divider plate illustrated in  FIG. 6 . Accordingly, the notch portion  24   a  is disposed ranging over both of the first space  32 A and the second space  32 B. Further, the notch portion  25   a  is located within a range of the first muffler cover  11 . Therefore, the first communication port  24  communicates with an inside and an outside of the first muffler cover  11 . In other words, the first communication port  24  is connected ranging over each of a first cooling air entrance  31 A and a second cooling air entrance  31 B, and the cylinder cooling room  34 , each of the first space  32 A and the second space  32 B are communicated by the divider plate  7 . Internal spaces of the second muffler cover  12  are divided into the first space  32 A and the second space  32 B, the first space  32 A formed between the first muffler cover  11  and the muffler  8 , the second space  32 B formed between the first muffler cover  11  and the second muffler cover  12 . Further, in a first mixture space  26 , the first space  32 A and the second space  32 B are communicated with each other through at least the front side surface and the left side surface of the muffler  8 . Note that the cooling air passing through the second communication port  25  more flows into the first space  32 A at an upper dead point of the cylinder than the first communication port  24 . 
     The cooling fan  18  is disposed at one end portion of the driving shaft  21 , that is, at a tip portion of the driving shaft  21 , and the cooling fan  18  is housed in a fan case  9 . As illustrated in  FIG. 4 , the fan case  9  is formed in a volute shape such that the distance from a center of the cooling fan  18  is gradually larger toward the cylinder  2  side, and when the driving shaft  21  (cooling fan  18 ) rotates in an anticlockwise direction, cooling air CA is generated toward the cylinder  2 . 
     As illustrated in  FIGS. 4 and 5 , the ignition device  19  is attached to the cylinder  2  so as to be opposed to the cooling fan  18 . A magnetic body (not illustrated) is housed in the cooling fan  18 . The ignition device  19  generates electric power by rotation of the magnetic body (not illustrated) along with the cooling fan  18 . As described above, the ignition plug (not illustrated) emits sparks inside the cylinder  2  by electric power generated by the ignition device  19 . Further, as illustrated in  FIGS. 3 and 5 , a start-up device  38  is provided on the other end side of the driving shaft  21 , that is, at a rear end portion side in a driving shaft direction. A holding portion  39  is provided in the start-up device  38 . A clutch mechanism (not illustrated) is housed in the start-up device  38 ; therefore, an operator can manually start the driving shaft  21  by pulling the holding portion  39 . 
     As illustrated in  FIG. 5 , a piston  17  is housed inside the cylinder  2  and the crankcase  4 . The piston  17  and the driving shaft  21  are connected with each other through the clutch mechanism (not illustrated), and the piston  17  is reciprocated inside the cylinder  2  by rotation of the driving shaft  21 . Therefore, an operator operates the holding portion  39  to rotate the driving shaft  21 , thereby reciprocating the piston  17 , and the air-fuel mixture is combusted by compressing the air-fuel mixture and emitting sparks inside the cylinder  2 . As a result, it is possible to start the engine  100 . When the engine  100  is started, the cylinder  2  and the muffler  8  increase in temperature due to the combustion and the heat of exhaust gas, and the increase in temperature is reduced by cooling air that is generated by rotation of the cooling fan  18 . 
     Hereinafter, cooling of the cylinder  2  will be described. In  FIG. 4 , cooling air CA 1  is generated when the driving shaft  21  (cooling fan  18 ) rotates anticlockwise. The cooling air CA 1  flows from the bottom to the top toward the cylinder  2 . The cylinder  2  is housed in the cylinder cooling room  34  formed by the cylinder cover  1  and the divider plate  7 . As illustrated in  FIG. 5 , the cooling air CA 1  flows in a horizontal direction while cooling the cylinder  2 , and the cooling air CA 1  is exhausted from an air window  40  provided in the cylinder cover  1 . 
     Hereafter, cooling of the muffler  8  will be described. In  FIG. 4 , the cooling air CA 1  is generated when the driving shaft  21  (cooling fan  18 ) rotates anticlockwise, and some of the cooling air CA 1  is supplied to the muffler cooling room  35  from the first communication port  24  and the second communication port  25  which are provided in the divider plate  7 . At this time, in the muffler cooling room  35 , the first communication port  24  and the second communication port  25  are provided on the side closest to the cooling fan  18 , and on the cooling fan  18  side, the cooling air is divided by the cylinder  2 , thereby supplying fresh cooling air before cooling of the cylinder  2 , that is, the cooling air of which the temperature is not increased to the muffler cooling room  35 . 
     As illustrated in  FIGS. 5 and 6 , in the muffler cooling room  35 , the end portion  41  of the first muffler cover  11  is disposed opposed to the first communication port  24 . By the end portion  41 , the cooling air is divided into first cooling air CA 2 A that flows in the first space  32 A and second cooling air CA 2 B that flows in the second space  32 B. In other words, some of the cooling air flowing into the muffler cooling room  35  from the cylinder cooling room  34  through the first communication port  24  is guided into the first cooling air entrance  31 A from the first communication port  24 , and other of the cooling air is guided into the first cooling air entrance  31 B from the first communication port  24  in a similar manner. Further, the second communication port  25  is covered with the first muffler cover  11 , and all of the third cooling air CA 3  supplied from the second communication port  25  to the muffler cooling room  35  flow into the first space  32 A by a guide portion  51  formed as a portion of the first muffler cover  11 . 
     In the first space  32 A, the first cooling air CA 2 A flows toward the exhaust exit  14 , while cooling a surface of the muffler  8  by the air guiding portion  28 A provided in the first muffler cover  11 . 
     In the second space  32 B, the second cooling air CA 2 B flows toward the exhaust exit  14 , while cooling an outer surface of the first muffler cover  11  and an inner surface of the second muffler cover  12  by the air guiding portion  28 B provided in the second muffler cover  12 . In this way, the respective air guiding portions  28 A and  28 B guide the cooling air to the exhaust exit side of the muffler  8 . In the first space  32 A and the second space  32 B, the cooling air flows in substantially the same direction. 
     Note that, as illustrated in  FIG. 7 , a plurality of air windows  43  are provided in the second muffler cover  12  besides the air guiding portions  28 B, and it is therefore possible to prevent heat accumulation in the second space  32 B during the operation of the engine  100  and after stopping the engine  100 . 
     In the first muffler cover  11 , an end portion  42  is extended in a direction substantially identical with an opening direction of the exhaust exit  14 , while the exhaust exit  14  side is open. A clearance is formed between the exhaust exit  14  side of the end portion  42  and the second muffler cover  12 , and the first mixture space  26  is formed through the clearance. Therefore, as illustrated in  FIG. 5 , as to the first cooling air CA 2 A that flows out from a first cooling-air exit  33 A formed between the end portion  42  and the first muffler cover  11 , the second cooling air CA 2 B that flows out from a second cooling-air exit  33 B formed between the end portion  42  and the second muffler cover  12  and exhaust gas EX that flows out from the exhaust exit  14 , the first cooling air CA 2 A, the second cooling air CA 2 B and the exhaust gas EX are mixed with each other when flowing into the first mixture space  26 , and the air temperature is equalized. 
     Therefore, as to the first cooling air CA 2 A and the exhaust gas EX of which the air temperatures are remarkably increased after cooling of a surface of the muffler  8 , the first cooling air CA 2 A and the exhaust gas EX do not run into a wall surface of the second muffler cover  12  while keeping the increased temperatures, and it is possible to effectively reduce the heat damages such as erosion, of the second muffler cover  12 . 
     The third muffler cover  13  is further provided on the exhaust exit  14  side of the second muffler cover  12 . The third muffler cover  13  covers at least some of the mixture space side of the second muffler cover  12  through a clearance, thereby forming a second mixture space  27  between the second muffler cover  12  and the third muffler cover  13 . In this way, since the second mixture space  27  is formed by the third muffler cover  13 , the air temperature is further equalized in the mixture gas G including the first cooling air CA 2 A, the second cooling air CA 2 B, the third cooling air CA 3  and the exhaust gas EX. Therefore, the local high-temperature portions of air temperature are eliminated. This enables the heat damages in the second muffler cover  12  and the third muffler cover  13  to be effectively reduced. Further, in the third muffler cover  13 , since a divider wall  48  that forms the second mixture space  27  is provided inside the sidewall portion  47  and a divider wall  46  that is communicated into the divider wall  48  is provided inside an upper wall portion  45 , the mixture gas G is not in contact with the sidewall portion  47  and the upper wall portion  45 , and it is possible to effectively reduce the surface temperatures in the sidewall portion  47  and the upper wall portion  45 . 
     Since the divider plate  7  includes the second communication port  25  that allows the cylinder cooling room  34  and the second space  32 B to communicate with each other and since the cooling air is supplied to at least the second space  32 B, the air not directly in contact with the muffler  8  can be directed in an exhaust direction of exhaust gas in the muffler  8 . It is therefore possible to reduce the temperature around the muffler  8  more effectively. 
     When the cooling air is supplied to both of the first space  32 A and the second space  32 B, even though the first muffler cover  11  increases in temperature by the first cooling air of which the temperature is increased after cooling of the muffler  8  in the first space  32 A, the first muffler cover  11  can be cooled by the second cooling air flowing in the second space  32 B. This enables interruption of cooling of the muffler, which is caused by heat accumulation in the first muffler cover  11 , to be reduced. This also enables an increase in temperature in the second muffler cover  12  to be reduced more effectively. For example, when the first cooling air CA 2 A flows toward an upstream side of the second cooling air CA 2 B, the first cooling air CA 2 A of which the temperature is increased after cooling of the muffler flows to an upstream side of the second cooling air CA 2 B in the first muffler cover  11 , so that the temperature is increased in the upstream side of the second cooling air CA 2 B. Therefore, at the entrance, the second cooling air CA 2 B increases in temperature by the first muffler cover  11 , resulting in deterioration of the cooling effects. 
     In the first space  32 A and the second space  32 B, the cooling air flows in substantially the same direction, therefore, the first cooling air CA 2 A and the second cooling air CA 2 B of which the temperatures are increased after cooling of the muffler  8  and the first muffler cover  11  do not flow toward the other upstream side, and it is possible to obtain high cooling effects. For example, when the first cooling air CA 2 A flows toward an upstream side of the second cooling air CA 2 B, the first cooling air CA 2 A of which the temperature is increased after cooling of the muffler flows to an upstream side of the second cooling air CA 2 B in the first muffler cover  11 , so that the temperature is increased in the upstream side of the second cooling air CA 2 B. Therefore, at the entrance, the second cooling air CA 2 B increases in temperature by the first muffler cover  11 , resulting in deterioration of the cooling effects. 
     Since there is provided the mixture space  26  that allows the first cooling-air exit  33 A and the second cooling-air exit  33 B to communicate with each other, the air temperature can be equalized by mixing the first cooling air CA 2 A of which the temperature is further increased in the mixture space  26  and the second cooling air CA 2 B having a lower temperature than that of the first cooling air CA 2 A. This enables a local increase in temperature by the first cooling air CA 2 A to be reduced. It is therefore possible to effectively reduce the heat damages such as erosion, of the second muffler cover  12 . Further, the temperature in the muffler cooling air exhausted from the second muffler cover  12  can be reduced. 
     Since the exhaust exit  14  of the muffler  8  is communicated with the mixture space  26 , the exhaust gas can be mixed with the first cooling air CA 2 A and the second cooling air CA 2 B. This enables a temperature of the exhaust gas to be reduced. Since the first communication ports  24  and  25  are disposed on the cooling fan side in a driving shaft direction and since the first cooling air entrance  31 A and the second cooling air entrance  31 B are also disposed on the cooling fan side, the cooling air can be readily removed from the cooling fan  18 , and it is possible to readily increase air volume of the first cooling air and the second cooling air. 
     Since the cylinder cooling room  34  is communicated with the first space  32 A and the second space  32 B through the common notch portion  24   a  provided in the divider plate  7 , it is possible to improve cooling efficiency with a simple structure. Further, since cooling air before cooling of the cylinder  2  is supplied to the muffler cooling room  35 , the colder cooling air can be supplied to the muffler cooling room  35 , and it is possible to obtain high cooling effects. 
     Since the first muffler cover  11  is attached to the second muffler cover  12 , the first muffler cover  11  is not directly in contact with the muffler  8 , and it is possible to effectively reduce an increase in temperature in the first muffler cover  11 . Since the third muffler cover  13  is provided in the second muffler cover  12 , the third muffler cover  13  covering some of the mixture space side of the second muffler cover  12  through a clearance, volume of the mixture space  27  can be readily increased, and it is possible to further promote a mixture of the first cooling air CA 2 A, the second cooling air CA 2 B and the exhaust gas EX. 
     Since a local increase in temperature of the driving source is reduced, the engine working machine having the air-cooled engine mounted therein can be improved in terms of workability. 
     The present invention is not limited to the foregoing embodiment and various modifications and alterations can be made within the scope of the present invention. For example, in the present embodiment, the first muffler cover  11  is formed of metal, but may be formed of resin in view of an increase in temperature. Further, as to the first muffler cover  11  and the second muffler cover  12 , a rib etc. extending from the second muffler cover  12  are used as an alternative to the first muffler cover  11 , or it is also possible to integrally form the first muffler cover  11  and the second muffler cover  12  as a single member by adopting the first muffler cover  11  formed of resin and attaching the first muffler cover  11  to the second muffler cover  12  by welding or the like. 
     Further, in the present embodiment, as means for dividing the cooling air, which is provided from the first communication port  24  to the muffler cooling room  35 , into the first space  32 A and the second space  32 B, the end portion  41  of the first muffler cover  11  is disposed opposed to the first communication port  24 . However, the first communication port  24  may be divided into a communication port that is open toward the first space  32 A and a communication port that is open toward the second space  32 B.