Patent Publication Number: US-9410513-B2

Title: Engine configured to drive a diaphragm fuel pump using pressure fluctuation in a crank chamber of the engine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2011-028702, filed Feb. 14, 2011, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to an engine configured to drive a diaphragm fuel pump using the pressure fluctuation in a crank chamber of the engine. 
     2. Description of the Related Art 
     Recently, due to increasing public awareness regarding environmental issues, enhancement of emission control and so forth, a two-stroke engine has been taken over by a four-stroke engine, as a drive engine for a working machine such as a brush cutter, a chain saw and a backpack blower being carried by the user&#39;s hand or carried on the user&#39;s shoulder. 
     Some two-stroke engines use the pressure fluctuation in an intake port as a power source to drive a fuel pump (diaphragm fuel pump) as disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 2005-140027 (Patent literature 1) and Japanese Patent Application Laid-Open Publication No. HEI9-158806 (Patent literature 2). However, most two-stroke engines use the pressure fluctuation in a crank chamber. In this case, a positive pressure and a negative pressure generated in the crank chamber are often used as a power source to drive a diaphragm chamber in a diaphragm fuel pump, as disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 11E13-189363 (Patent literature 3), Japanese Patent Application Laid-Open Publication No. 2003-172221 (Patent literature 4) and Japanese Patent Application Laid-Open Publication No. 2001-207914 (Patent literature 5). 
     In the cases of Patent literature 1 and patent literature 2, that is, if a diaphragm fuel pump in a four-stroke engine is driven by using the pressure fluctuation in an intake port as a power source, there is a problem that the diaphragm fuel pump cannot acquire sufficient power because the pressure in the intake port changes only once while a crankshaft rotates twice. In addition, in the cases of Patent literature 3, Patent literature 4 and Patent literature 5, that is, if a diaphragm fuel pump is driven by using the pressure fluctuation in a crank chamber, it is possible to acquire power by which the pressure changes once while a crankshaft rotates once, and consequently solve the above-described problem. However, a positive pressure in the crank chamber affects the inside of a diaphragm chamber, and therefore the oil from the crank chamber enters the diaphragm chamber and a path in communication with the diaphragm chamber. As a result, the pressure fluctuation cannot be transferred to the diaphragm chamber, and this may cause eventually the diaphragm fuel pump failure. 
     SUMMARY 
     The present invention was achieved in view of the above-described background. It is therefore an object of the present invention to provide an engine configured to be able to acquire sufficient pressure fluctuation to drive a diaphragm fuel pump and prevent oil from entering a diaphragm chamber. 
     To solve the above-described problem, an engine includes: a crank chamber in which pressure fluctuation occurs; and a carburetor including a diaphragm fuel pump. The diaphragm fuel pump includes a pump chamber configured to suck in and eject fuel; and a diaphragm chamber to which a pressure that drives the pump chamber is supplied. The diaphragm chamber and the crank chamber communicate with one another in a state in which a negative pressure is created in the crank chamber. 
     It is preferred that the engine further includes a communicating path configured to allow communication between the diaphragm chamber and the crank chamber. An atmospheric pressure opening path configured to communicate with a space under atmospheric pressure is connected to the communicating path. 
     It is preferred that the engine further includes a communicating path configured to allow communication between the diaphragm chamber and the crank chamber. An atmospheric pressure opening path configured to communicate with a space under atmospheric pressure is connected to the diaphragm chamber. 
     It is preferred that the engine further includes a communicating path configured to allow communication between the diaphragm chamber and the crank chamber. An opening of the communicating path in the crank chamber side is formed near a position in which a termination portion of a skirt part in a piston is located when the piston is located at a top dead center. 
     It is preferred that the engine further includes a communicating path configured to allow communication between the diaphragm chamber and the crank chamber. An opening of the communicating path in the crank chamber side is formed in a position closer to a crankshaft than a position in which a piston ring is located when the piston is located at a bottom dead center. 
     It is preferred that the opening of the communicating path in the crank chamber side is formed in a position near the position in which the piston ring of the piston is located when the piston is located at the bottom dead center. 
     It is preferred that the engine further includes a communicating path configured to allow communication between the diaphragm chamber and the crank chamber. An orifice is formed in an opening of the communicating path in the crank chamber side. 
     It is preferred that the engine further includes a communicating path configured to allow communication between the diaphragm chamber and the crank chamber. An orifice is formed in an atmospheric pressure opening path, the atmospheric pressure opening path being connected to one of the communicating path and the diaphragm chamber to allow communication with a space under atmospheric pressure. 
     It is preferred that the engine further includes a communicating path configured to allow communication between the diaphragm chamber and the crank chamber. An orifice is formed in an atmospheric pressure opening path, the atmospheric pressure opening path being connected to one of the communicating path and the diaphragm chamber to allow communication with a space under atmospheric pressure. 
     It is preferred that the engine is a four-stroke engine. 
     According to the present invention, it is possible to provide an engine configured to be able to acquire sufficient pressure fluctuation to drive a diaphragm fuel pump and prevent oil from entering a diaphragm chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration schematically showing Embodiment 1 of the present invention; 
         FIG. 2  is an illustration showing the position of a crank chamber side opening; 
         FIG. 3  is an illustration showing the structure of a carburetor using a diaphragm fuel pump; 
         FIG. 4  is an illustration showing a nozzle; 
         FIG. 5  is a cross sectional view taken along line A-A′ of  FIG. 4 ; 
         FIG. 6  is an illustration showing an effect of Embodiment 1; 
         FIG. 7  is an illustration showing Embodiment 2; 
         FIG. 8  is an illustration showing Embodiment 3; and 
         FIG. 9  is an illustration showing Embodiment 4. 
     
    
    
     DETAILED DESCRIPTION 
     &lt;Embodiment 1&gt; 
     Now, preferred Embodiment 1 of an engine according to the present invention will be explained with reference to  FIG. 1 .  FIG. 1  is an illustration schematically showing Embodiment 1 of the present invention. Here, a four-stroke engine  1  is shown in  FIG. 1  where a piston is located near the top dead center (TDC). 
     As shown in  FIG. 1 , the four-stroke engine  1  includes a cylinder part  3 , a crank case  5  mounted under the cylinder part  3  and an oil tank  15  provided below the crank case  5 . The cylinder part  3  has a cylindrical space to slidably move a piston  9  upward and downward in  FIG. 1 . Then, the piston  9  is fitted into the space with a gap to slidably move upward and downward in  FIG. 1 . A crank chamber  7  is defined by the cylinder part  3 , the crank case  5  and the piston  9 . That is, the crank chamber  7  is an approximately cylindrical space defined by the side surface of the cylinder part  3 , the piston  9  and the crank case  5 . The volume of the inner space of this crank chamber  7  varies as the piston  9  slidably moves. A combustion chamber  8  is defined by the cylinder head  26 , the cylinder part  3  and the piston  9 . The oil tank  15  to store oil is provided separately from the crank case  5 . 
     A check valve  17  is provided between the oil tank  15  and the crank case  5  to allow oil to flow only in the direction from the crank case  5  (crank chamber  7 ) to the oil tank  15 . Here, a negative pressure is created in the crank chamber  7  as the piston  9  moves from the bottom dead center (BDC) to TDC. By contrast with this, a positive pressure is created in the crank chamber  7  as the piston  9  moves from TDC to BDC. Although a negative pressure is easily created in the crank chamber  7  because the check valve  17  is provided, the pressure in the crank chamber  7  can rise only up to a positive pressure that overcomes the elasticity of a spring and so forth used in the check valve  17 . Then, the elasticity of a spring and so forth used in the check valve  17  is relatively poor, so that the pressure in the crank chamber can only increase to a positive pressure a little. Here, the pressure in the crank chamber  7  changes once while a crankshaft  13   a  rotates once. This is different from the pressure in an intake port or an exhaust port, which changes only once while the crankshaft  13   a  rotates twice. 
     A crank  13  is rotatably supported in the crank case  5 . This crank  13  is formed by the crankshaft  13   a  which is the center of rotation, counterweight and so forth. The piston  9  and the crank  13  are connected one another via a connecting rod  11 . The connecting rod  11  is rotatably connected to both the piston  9  and the crank  13 . This configuration allows the piston  9  to reciprocally and slidably move in the cylinder part  3 . 
     A cylinder head  26  is provided on the upper wall of the cylinder part  3 . The cylinder head  26  is provided with an intake port  27  that allows communication with the carburetor  25  and an exhaust port  33  that allows communication with an exhaust muffler (not shown). The cylinder head  26  is also provided with an intake valve  29  to open and close the intake port  27 . In addition, the cylinder head  26  is provided with an exhaust valve  31  to open and close the exhaust port  33 . 
     An air cleaner  21  is provided outside the carburetor  25 . A filter  23  is disposed in the air cleaner  21 . The filter  23  allows air to pass through to remove dust and so forth in the air. 
     The carburetor  25  is an apparatus to mix fuel into the air having passed through the air cleaner  21 . To be more specific, the carburetor  25  can control mixing of the air and fuel and also control the total amount of the air-fuel mixture. The carburetor  25  has a diaphragm fuel pump  109  to mix fuel into the air. This diaphragm fuel pump  109  is driven using pressure fluctuation as power. 
     With the present embodiment, a diaphragm chamber  110  in the diaphragm fuel pump  109  is connected to the crank chamber  7  via a communicating path  104  to supply power. Here, the diaphragm fuel pump  109  is provided with a diaphragm  108  whose position changes in response to pressure fluctuation. 
     A crank chamber side opening  103  is provided in the communicating path  104  in the crank chamber  7  side. Then, an atmospheric pressure opening path  107  is connected to the communicating path  104 . One end of the atmospheric pressure opening path  107  has an air cleaner side opening  117  which opens in the air cleaner  21  (the space after the air has passed through the filter  23 ). The other end of the atmospheric pressure opening path  107  opens on the way of the route of the communicating path  104 . Here, with respect to the connecting point between the communicating path  104  and the atmospheric pressure opening path  107 , the communicating path  104  in the diaphragm chamber  110  side is referred to as a diaphragm chamber side communicating path  113 , and the communicating path  104  in the crank chamber  7  side is referred to as a crank chamber side communicating path  105 . 
     By providing the atmospheric pressure opening path  107 , even if oil and so forth enters the communication path  104 , it is possible to eject the oil and so forth to the crank chamber  7  when a negative pressure is created in the crank chamber  7 . It is because the air cleaner side opening  117  in the atmospheric pressure opening path  107  opens in a space under atmospheric pressure. Therefore, when a negative pressure is created in the crank chamber  7 , the air enters the crank chamber side opening  103  from the air cleaner side opening  117  to eject the oil having flown into the communicating path  104 . Here, note that the pipeline resistance of the atmospheric pressure opening path  107  should not be set too low in order to prevent the performance of the diaphragm fuel pump  109  from degrading. It is because too low pipeline resistance of the atmospheric pressure opening path  107  causes a situation in which the air not in the diaphragm chamber  110  side but in the atmospheric pressure opening path  107  side is sucked too much when a negative pressure is created in the crank chamber  7 . 
     An air cleaner side orifice  111  is provided to set the pipeline resistance of the atmospheric pressure opening path  107 . This air cleaner side orifice  111  increases pipeline resistance. In order to increase pipeline resistance, there are several methods, for example, a method of setting the length of a pipeline long, a method of setting the entire pipeline thin, a method of folding a pipeline more than once and so forth. Here, combinations of the above-described methods are possible to provide a synergistic effect. In addition, the air cleaner side orifice  111  does not need to be always provided near the air cleaner side opening  117  because it is used to set pipeline resistance. For example, the air cleaner side orifice  111  may be provided in the center of the atmospheric pressure opening path  107 , the communicating path  104  side and so forth. 
     A crank chamber side orifice  115  is provided in the crank chamber side opening  103 . This crank chamber side orifice  115  serves to control pressure fluctuation to drive the diaphragm fuel pump  109 . In addition, the crank chamber side orifice  115  is provided to reduce the amount of oil and so forth flowing from the crank chamber  7  into the communicating path  104 . 
     The atmospheric pressure opening path  107  opens in the space (the cleaned side) after the air has passed through the filter  23  in the air cleaner  21 . Therefore, it is possible to flow the cleaned air not containing dust and so forth into the atmospheric pressure opening path  107 . 
       FIG. 2  is an illustration showing the position of the crank chamber side opening  103 . Here, in  FIG. 2 , the piston  9  located at TDC is indicated by the solid line, and the piston  9  located at BDC is indicated by the broken line. 
     Here, piston  9  includes a piston head  9   a  and a skirt part  9   b  following the piston head  9   a . A termination portion  9   c  is formed at the end of the skirt part  9   b  in the crank chamber  7  side. 
     With the present embodiment, as shown in  FIG. 2 , the crank chamber side opening  103  of the communicating path  104  in the crank chamber  7  side is formed to open in the position near the position where the termination portion  9   c  of the skirt part  9   b  in the piston  9  is located when the piston  9  is located at TDC. This prevents oil and so forth from entering the communicating path  104  and the diaphragm chamber  110  due to a positive pressure created in the crank chamber  7  (crank case  5 ). Moreover, the crank chamber side opening  103  of the communicating path  104  in the crank chamber  7  side is formed to open in the position closer to the crankshaft  13   a  than the position in which the termination portion  9   c  is located when the piston  9  is located at TDC. By forming the crank chamber side opening  103  in this position, it is possible to close the communicating path  104  when a positive pressure is created in the crank chamber  7 , and consequently supply substantially only a negative pressure to the communicating path  104 . 
     An annular piston ring  52  is fitted into a portion of the side surface of the piston  9  in the combustion chamber  8  side. This piston ring  52  is formed by a compression ring  53  and an oil ring  51 . The compression ring  53  needs to always be tightly attached to the cylinder part  3  because it is provided to separate the combustion chamber  8  from the crank chamber  7 . In addition, the compression ring  53  needs to lubricate to prevent abrasion because it slidably moves. Therefore, there is much more oil in the gap portion between the cylinder part  3  and the piston  9  in the combustion chamber  8  side than in the region between the compression ring  53  and the oil ring  51 . There is blowby gas and so forth in the gap portion. Therefore, when the piston  9  moves to place the crank chamber side opening  103  between the compression ring  53  and the oil ring  51 , oil, blowby gas and so forth may enter the communicating path  104  from the crank chamber side opening  103 . As the present embodiment, the crank chamber side opening  103  of the communicating path  104  in the crank chamber  7  side is formed in the position closer to the crankshaft  13   a  than the position in which the oil ring  51  is located when the piston  9  is located at BDC. This prevents oil and so forth from entering the communicating path  104  from the crank chamber side opening  103 . 
     If the crank chamber side opening  103  is formed in the position apart from the position in which the oil ring  51  in the piston  9  is located when the piston  9  is located at BDC, it is required to increase the length of the skirt part  9   b  accordingly, and consequently increase the size of the piston  9 . Therefore, with the present embodiment, the crank chamber side opening  103  is formed near the position in which the oil ring  51  in the piston  9  is located when the piston  9  is located at BDC to reduce the size of the piston  9  and prevent oil and so forth from entering the communicating path  104 . 
     Here, with the present embodiment, the crank chamber side opening  103  is formed in the position near the position in which the termination portion  9   c  of the skirt part  9   b  in the piston  9  is located when the piston  9  is located at TDC as shown in  FIG. 2 . In this case, even if a negative pressure is applied to the communicating path  104 , the diaphragm fuel pump  109  cannot exhibit sufficient performance unless there is the atmospheric pressure opening path  107 . It is because the crank chamber side opening  103  is closed by the skirt part  9   b  before the pressure returns to a positive pressure after the piston  9  has arrived at TDC and the pressure in the communicating path  104  has been minimized. This causes a situation in which the pressure in the communicating path  104  keeps a certain negative pressure, and therefore it is not possible to generate sufficient pressure fluctuation. Then, when the piston  9  arrives at TDC by the next stroke, the pressure can only change from the certain negative pressure to the minimum pressure. The diaphragm fuel pump  109  is driven according to the magnitude of pressure fluctuation, and therefore cannot work if the magnitude of pressure fluctuation is small. Therefore, with the present embodiment, a configuration is adopted where the atmospheric pressure opening path  107  is provided and the air is supplied to the communicating path  104  while the crank chamber side opening  103  is closed by the skirt part  9   b  in the piston  9  to make the pressure fluctuation in the diaphragm chamber  110  greater. Here, with the configuration according to the present embodiment, the period of time over which the crank chamber side opening  103  is closed is substantially longer than the period of time over which the crank chamber side opening  103  is open. Therefore, even if the pipeline resistance of the atmospheric pressure opening path  107  increases to some extent, it is possible to supply a sufficient amount of the air to the communicating path  104 . By this means, it is possible to generate a sufficient magnitude of pressure fluctuation in the communicating path  104 . 
       FIG. 3  is an illustration showing the structure of the carburetor  25  using the diaphragm fuel pump  109 . 
     As shown in  FIG. 3 , the carburetor  25  includes a carburetor body  1102 . The communicating path  104  which allows communication with the crank chamber  7 , is formed in the carburetor body  1102 . This communicating path  104  faces the diaphragm chamber  110 , which is one side (the upper part in the figure) of the diaphragm fuel pump  109 . A pump chamber  1108  is formed in the other side (the lower part in the figure) of the diaphragm fuel pump  109 . A fuel inlet  1112  communicates with the pump chamber  1108  via an inlet valve  1110 , and a metering chamber  118  in a metering diaphragm  1120  communicates with the pump chamber  1108  via an outlet valve  1114  and a needle valve  1116 . Here, the fuel inlet  1112  is connected to a fuel tank (not shown). The crank chamber side opening  103  of the communicating path  104  in the crank chamber  7  side is formed in the cylinder part  3  which defines the crank chamber  7 . 
     The pressure in the crank chamber  7  varies according to a change in its volume. As described above, only a negative pressure of the varying pressure affects the diaphragm chamber  110  via the communicating path  104 . Then, the diaphragm fuel pump  109  is driven by the negative pressure affecting the diaphragm chamber  110 . To be more specific, a negative pressure affects the diaphragm chamber  110  in the diaphragm fuel pump  109 , and therefore the negative pressure affects the pump chamber  1108  side when the diaphragm  108  bends to the diaphragm chamber  110  side. The negative pressure in the pump chamber  1108  allows the inlet valve  1110  to open while the outlet valve  1114  is closed, and therefore fuel is sucked from the fuel inlet  1112  into the pump chamber  1108 . Next, in this state, when the negative pressure affecting the diaphragm chamber  110  in the diaphragm fuel pump  109  changes to a positive pressure, the elastic force of the diaphragm  108  forces the diaphragm  108  to return to the original state. Therefore, a positive pressure affects the pump chamber  1108  side. Then, when the motion of the diaphragm  108  causes the positive pressure to affect the pump chamber  1108  side, the outlet valve  1114  opens while the inlet valve  1110  remains closed to eject the fuel from the pump chamber  1108 . This ejected fuel is supplied to the metering chamber  1118  in the metering diaphragm  1120  via the needle valve  1116 . 
     The metering chamber  1118  is separated from a back pressure chamber  1122  by the metering diaphragm  1120 . The pressure of the four-stroke engine  1  affects the back pressure chamber  1122 . The metering diaphragm  1120  is driven by the difference in pressure between the four-stroke engine  1  and the metering chamber  1118 . Here, a path is not shown in the figure, which allows communication between the back pressure chamber  1122  and the space under a negative pressure in the engine. The metering diaphragm  1120  is connected to the above-described needle valve  1116  via a control lever  1124 , and operates to open and close the needle valve  1116 . To be more specific, when the metering chamber  1118  is filled with fuel, the pressure in the metering chamber  1118  rises and the metering diaphragm  1120  bends to the back pressure chamber  1122  side. At this time, the elastic force of a control lever spring  1126  causes the control lever  1124  to rotate such that one end (the left side in the figure) of the control lever  1124  is pushed down and the other end (the right side in the figure) is pushed up. This rotation of the control lever  1124  causes the needle valve  1116  to push up and breaks the communication between the pump chamber  1108  and the metering chamber  1118 . 
     A path  1128  is formed in the carburetor body  1102  to connect between the intake port  27  formed in the cylinder part  3  and the air cleaner  21 . This path  1128  has a large diameter part  1128   a  in the upper stream side (the air cleaner  21  side) and a smaller venturi part  1128   b  in the downstream side (the intake port  27  side) than the large diameter part  1128   a . The venturi part  1128   b  includes a throttle valve  1130  to change its opening. The axis of rotation of the throttle valve  1130  is orthogonal to the path  1128 . By operating a rotating lever  1130   a , the throttle valve  1130  rotates, sliding upward and downward in the figure to change the opening of the venturi part  1128   b  according to the degree of rotation. 
     In addition, this throttle valve  1130  is provided with a first adjuster screw  1131  which is coaxial with the axis of rotation of the throttle valve  1130  to fine-tune the amount of fuel mixed into the air flowing through the path  1128 . This first adjuster screw  1131  is provided with a second adjuster screw  1132  which is coaxial with the axis of rotation of the first adjuster screw  1131 . The second adjuster screw  1132  is provided to extend upward and downward in the figure. The outer diameter of the second adjuster screw  1132 , which is approximately the same as the inner diameter of the nozzle  1134  described later, reduces from the top to the bottom in two steps. A switching part  1132   a  to switch a main jet  1136  described later is provided on the tip of the second adjuster screw  1132 . In the figure, the first adjuster screw  1131  moves downward, rotating in one direction (to tighten the screw) with respect to the throttle valve  1130 , and, on the other hand, moves upward, rotating in the other direction (to loosen the screw) with respect to the throttle valve  1130 . Likewise, in the figure, the second adjuster screw  1132  moves downward, rotating in one direction (to tighten the screw) with respect to the first adjuster screw  1131 , and, on the other hand, moves upward, rotating in the other direction (to loosen the screw) with respect to the first adjuster screw  1131 . 
     The nozzle  1134  is provided in the carburetor body  1102  to face the second adjuster screw  1132 . The tip of the second adjuster screw  1132  is inserted into a nozzle tip  1134   a  of the nozzle  1134 . In addition, the nozzle  1134  includes a hole  1134   b  which opens in the path  1128 . A bottom  1134   c  in communication with the hole  1134   b  faces the metering chamber  1118 . Here, the main jet  1136  and a main check valve  1138 , which serve as a mixture ratio adjusting means and a fuel adjusting mechanism, are provided between the hole  1134   b  and the metering chamber  1118 . 
       FIG. 4  is an illustration showing the nozzle  1134 . Here,  FIG. 5  is a cross sectional view taken along line A-A′ of  FIG. 4 . 
     As shown in  FIG. 4  and  FIG. 5 , the main jet  1136  includes a first main jet part  1136   a  and a second main jet part  1136   b.  The first main jet part  1136   a  has a predetermined opening area to allow communication between the hole  1134   b  of the nozzle  1134  and the metering chamber  1118 . The second main jet part  1136   b  has a larger opening area than of the first main jet part  1136   a  to allow communication between the hole  1134   b  of the nozzle  1134  and the metering chamber  1118 . One of the first main jet part  1136   a  and the second main jet part  1136   b  of the main jet  1136  is closed by the switching part  1132   a  in the second adjuster screw  1132 , and the other allows communication between the hole  1134   b  of the nozzle  1134  and the metering chamber  1118 . By rotating the second adjuster screw  1132  with respect to the first adjuster screw  1131 , it is possible to switch between open and close of the first main jet part  1136   a  and the second main jet part  1136   b  of the main jet  1136 . That is, by rotating the second adjuster screw  1132  with respect to the first adjuster screw  1131  according to fuel to be used, it is possible to deliver fuel to one of the first main jet part  1136   a  and the second main jet part  1136   b  of the main jet  1136 . 
       FIG. 6  is an illustration showing an effect of the present embodiment. 
     As the piston  9  reciprocates between TDC and BDC, the pressure in the crank chamber  7  fluctuates as shown in the solid line and the broken line in  FIG. 6A . On the other hand, the pressure in the intake port  27  changes only once while the crankshaft  13   a  rotates twice as shown in  FIG. 6B . Therefore, it is not appropriate to use the pressure in the intake port  27  as the power source for the diaphragm fuel pump  109 . As the configuration with the present embodiment, the crank chamber side opening  103  of the communicating path  104  in the crank chamber  7  side is formed to open in the position near the position in which the termination portion  9   c  of the skirt part  9   b  in the piston  9  is located when the piston  9  is located at TDC. By this means, the pressure in the crank chamber  7  acts near the crank chamber side opening  103  as shown in the solid line in  FIG. 6A . However, in this configuration, if there is no atmospheric pressure opening path  107 , the pressure in the communicating path  104  can only fluctuate as shown in  FIG. 6C . Under such a circumstance, the diaphragm fuel pump  109  cannot work satisfactorily because it is driven according to the magnitude of pressure fluctuation. Therefore, the atmospheric pressure opening path  107  is connected to the communicating path  104  to allow the air in the space under atmospheric pressure to be supplied to the communicating path  104 . By this means, the pressure in the communicating path  104  is returned to nearly atmospheric pressure, so that it is possible to make pressure fluctuation greater as shown in  FIG. 6D . Here, broken line a shown in  FIG. 6D  shows the pressure fluctuation in a case in which the air cleaner side orifice  111  is not provided in the air cleaner side opening  117  of the atmospheric pressure opening path  107 . Meanwhile, solid line b shown in  FIG. 6D  shows the pressure fluctuation in a case in which the air cleaner side orifice  111  is provided in the air cleaner side opening  117  of the atmospheric pressure opening path  107 . As described above, by providing the air cleaner side orifice  111 , it is possible to adequately increase the pipeline resistance of the atmospheric opening path  107  to prevent the air from being sucked more than necessary from the atmospheric pressure opening path  107  when the crank chamber  7  and the communicating path  104  communicate with one another. Here, the air cleaner side orifice  111  is not always required, but a case is possible where the pipeline is thinned, lengthened, bent and the like to control pipeline resistance. However, with the above-described methods, it is not easy to control pipeline resistance. Therefore, it is preferable to provide the air cleaner side orifice  111 . 
     Moreover, by providing the atmospheric pressure opening path  107 , it is possible to eject oil and so forth having entered the communicating path  104  by an ejector effect. Here, for this, it is preferable to increase a speed at which the airflows from the atmospheric pressure opening path  107  to the communicating path  104 . 
     &lt;Embodiment 2&gt; 
       FIG. 7  is an illustration showing Embodiment 2. 
     The atmospheric pressure opening path  107  does not communicate with the communicating path  104  but communicates with the diaphragm chamber  110  in the diaphragm fuel pump  109 . Here, in this case, it is preferable to provide the air cleaner side orifice  111  in the air cleaner side opening  117  of the atmospheric pressure opening path  107 . 
     &lt;Embodiment 3&gt; 
       FIG. 8  is an illustration showing Embodiment 3. 
     As shown in  FIG. 8 , a configuration is possible where the communicating path  104  is provided to directly communicate with the crank case  5 . Moreover, in this case, a configuration is possible where the communicating path  104  branches into a second communicating path  119  to let out the positive pressure created in the communicating path  104 . By this configuration, it is possible to provide a mechanism that drives the diaphragm fuel pump  109  with a simpler structure. 
     Moreover, it is more preferable to allow communication between the second communicating path  119  and the oil tank  15  and provide a second check valve  121  in the oil tank  15  side. Here, in this case, the elastic force of a spring and so forth used in the second check valve  121  to let out the positive pressure created in the communicating path  104  is smaller than in the check valve  17 . By this configuration, it is possible to substantially provide only a negative pressure to the diaphragm fuel pump  109  with a simpler structure. 
     &lt;Embodiment 4&gt; 
       FIG. 9  is an illustration showing Embodiment 4. 
     As shown in  FIG. 9 , the crank chamber side orifice  115  is not provided in the crank chamber side opening  103 , but a one-way valve  123  (check valve, or lead valve) that prevents the flow from the crank chamber  7  side and permits the flow in the backward direction may be provided in the crank chamber side communicating path  105 . By this configuration, it is possible to prevent oil from entering the route of the communicating path  104 . 
     &lt;The Configurations and Effects of the Embodiments&gt; 
     The four-stroke engine  1  according to the present invention includes the crank chamber  7  in which pressure fluctuation occurs, and the carburetor  25 . The carburetor  25  includes the diaphragm fuel pump  109 . The diaphragm fuel pump  109  includes the pump chamber  1108  that sucks in and ejects fuel, and the diaphragm chamber  110  to which the pressure that drives the pump chamber  1108  is supplied. The diaphragm chamber  110  and the crank chamber  7  communicate with one another in a state in which a negative pressure is created in the crank chamber  7 . By this configuration, it is possible to prevent oil from entering the communicating path  104  from the crank chamber  7 . 
     The communicating path  104  is provided to allow communication between the diaphragm chamber  110  and the crank chamber  7 . The atmospheric pressure opening path  107  communicating with a space under atmospheric pressure is connected to the communicating path  104 . By this configuration, it is possible to prevent oil from entering the communicating path  104  with a simple mechanism. In addition, it is possible to make the pressure fluctuation in the diaphragm  110  greater. 
     The communicating path  104  is provided to allow communication between the diaphragm chamber  110  and the crank chamber  7 . The atmospheric pressure opening path  107  that allows communication with a space under atmospheric pressure, is connected to the diaphragm  110 . By this configuration, even if oil and so forth enter the diaphragm chamber  110 , it is possible to eject the oil and so forth from diaphragm  110  and the communicating path  104  . In addition, it is possible to make the pressure fluctuation occurs in the diaphragm chamber  110  greater. 
     The communicating path  104  is provided to allow communication between the diaphragm chamber  110  and the crank chamber  7 . The crank chamber side opening  103  of the communicating path  104  in the crank chamber  7  side is formed near the position in which the termination portion  9   c  of the skirt part  9   b  in the piston  9  is located when the piston  9  is located at TDC. By forming the crank chamber side opening  103  in this position, a positive pressure is not applied to the communicating path  104 , and therefore it is possible to prevent oil from entering the communication path  104  from the crank chamber  7 . 
     The communicating path  104  is provided to allow communication between the diaphragm chamber  110  and the crank chamber  7 . The crank chamber side opening  103  of the communicating path  104  in the crank chamber  7  side is formed in the position closer to the crankshaft  13   a  than the position in which the piston ring  52  is located when the piston  9  is located at BDC. By forming the crank chamber side opening  103  in this position, the movement trajectory of the piston ring  52  does not overlap the crank chamber side opening  103 , and therefore, it is possible to prevent the oil wiped with the piston  9  from entering the communicating path  104 . 
     The crank chamber side opening  103  of the communicating path  104  in the crank chamber  7  side is formed in the position near the position in which the piston ring  52  of the piston  9  is located when the piston  9  is located at BDC. By this configuration, it is possible to reduce the size of the piston  9  and prevent oil and so forth from entering the communicating path  104 . 
     The communicating path  104  is provided to allow communication between the diaphragm chamber  110  and the crank chamber  7 . The crank chamber side orifice  115  is formed in the crank chamber side opening  103  of the communicating path  104  in the crank chamber  7  side. By this configuration, it is possible to prevent oil and so forth from entering the communicating path  104  from the crank chamber  7 . 
     The communicating path  104  is provided to allow communication between the diaphragm chamber  110  and the crank chamber  7 . The air cleaner side orifice  111  is formed in the atmospheric pressure opening path  107  that is connected to one of the communicating path  104  and the diaphragm chamber  110  to allow communication with a space under atmospheric pressure. By this configuration, it is possible to adequately control the pressure fluctuation in the diaphragm chamber  110 . That is, with this air cleaner side orifice  111 , it is possible to adequately control the timing the pressure in the diaphragm chamber  110 , which is a negative pressure, returns to atmospheric pressure. 
     The communicating path  104  is provided to allow communication between the diaphragm chamber  110  and the crank chamber  7 . The atmospheric pressure opening path  107  is connected to one of the communicating path  104  and the diaphragm chamber  110  to allow communication with a space under atmospheric pressure. The atmospheric pressure chamber opening path  107  opens in the cleaned side of the air cleaner  21 . By this configuration, it is possible to prevent dust from entering the pipeline of the atmospheric pressure opening path  107 . The engine according to the embodiments is applicable to a working machine such as a chain saw and a concrete cutter which generate a dust storm. 
     Although the four-stroke engine has been described as an example, it is possible to provide the same effect with a two-stroke engine. 
     In addition, the present invention is not limited to the above-described embodiments, but may have various modified structures and configurations.