Abstract:
A diaphragm-type carburetor includes a fuel well communicating with a lower end of a fuel nozzle, a constant-pressure fuel chamber communicating with the fuel well through an outlet bore, and a fuel pump incorporated in fuel passages defined between an inlet bore of the constant-pressure fuel chamber and a fuel tank. A fuel introduction control valve controls the introduction of fuel into the constant-pressure fuel chamber. The fuel introduction control valve includes a valve seat member which communicates at its lower end with the fuel well through a bypass passage extending above the constant-pressure fuel chamber. Thus, when fuel vapor is introduced into the constant-pressure fuel chamber, it immediately passes towards the fuel nozzle, whereby the extreme reduction in air-fuel ratio of a fuel-air mixture can be avoided.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a diaphragm-type carburetor, and in particular, to an improvement in a diaphragm-type carburetor including a constant-pressure fuel chamber having an outlet bore communicating with a lower end of a fuel nozzle through a fuel jet and a check valve, a fuel pump incorporated in a fuel passage which communicates between an inlet bore in the constant-pressure fuel chamber and a fuel tank for providing fuel for the constant-pressure fuel chamber in response to a pulsation pressure in a pulsation pressure generating source, and a fuel introduction control valve for controlling the introduction of the fuel into the constant-pressure fuel chamber by opening or closing the inlet bore in the constant-pressure fuel chamber. The fuel introduction control valve is provided with a cylindrical valve seat member mounted on an upper wall of the constant-pressure fuel chamber and having the inlet bore in its upper end, and a valve member lifted and lowered within the valve seat member to open and close the inlet bore. 
     2. Description of the Related Art 
     A diaphragm-type carburetor is already known, as disclosed, for example, in Japanese Patent Application Laid-Open No. 1-151758. 
     In such a carburetor, fuel delivered to a constant-pressure fuel chamber by operation of a fuel pump is often subjected to pressure pulsation received from the fuel pump, heat or vibration received from an engine or the like to produce fuel vapor. The fuel vapor introduced into the constant-pressure fuel chamber is ejected from a fuel nozzle along with the fuel, because the constant-pressure fuel chamber has no air vent (to enable the operating attitude of the engine in all directions). In the prior art, however, the fuel vapor may stagnate in the constant-pressure chamber depending on the operational attitude of the engine. When the operational attitude of the engine is changed, a large amount of stagnating fuel vapor may be ejected at one time from the fuel nozzle to extremely reduce the fuel-air ratio of the fuel-air mixture, thereby causing misoperation of the engine. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a diaphragm-type carburetor of the above-described type, wherein when the fuel vapor is introduced into the constant-pressure fuel chamber, the fuel vapor immediately moves towards the fuel nozzle without stagnating in the constant-pressure fuel chamber, whereby the extreme reduction in fuel-air ratio of the fuel-air mixture caused by the fuel vapor can be avoided. 
     To achieve the above object, according to the present invention, there is provided a diaphragm-type carburetor comprising a constant-pressure fuel chamber having an outlet bore communicating with a lower end of a fuel nozzle through a fuel jet and a check valve. A fuel pump is incorporated in a fuel passage for permitting communication between an inlet in the constant-pressure chamber and a fuel tank for providing fuel to the constant-pressure fuel chamber in response to a pulsation pressure in a pulsation pressure generating source. A fuel introduction control valve controls the introduction of the fuel into the constant-pressure fuel chamber by opening and closing an inlet bore in the constant-pressure fuel chamber. The fuel introduction control valve is provided with a cylindrical valve seat member mounted on an upper wall of the constant-pressure fuel chamber and has the inlet bore at an upper end thereof. A valve member is raised and lowered within the valve seat member for opening and closing the inlet bore, wherein a fuel well is defined between the outlet bore in the constant-pressure fuel chamber and the check valve, and the lower end of the valve seat member is in communication with the fuel well through a bypass passage extending above the constant-pressure fuel chamber. 
     With the above feature, when the valve member of the fuel introduction control valve opens the inlet bore of the valve seat member, the fuel delivered from the fuel pump is introduced into the constant-pressure fuel chamber through the valve seat member. In this case, if fuel vapor is contained in the fuel, the fuel vapor rises up in the bypass passage connected to the lower end of the valve seat member to enter the fuel well, as soon as it passes through the valve seat member. Thus, the fuel vapor can be ejected promptly from the fuel nozzle along with the other fuel. Therefore, the amount of the fuel vapor ejected from the fuel nozzle is very small and hardly varies the fuel-air ratio of the fuel-air mixture, and hence, the normal operation of the engine can be ensured. 
     The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical sectional front view of a diaphragm-type carburetor of the present invention. 
     FIG. 2 is a sectional view taken along a line  2 — 2  in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIG. 1, a diaphragm-type carburetor C is mounted in a hand-held type engine carried on a portable working machine adapted to be used in all-direction attitudes, such as a mowing-off machine. A carburetor body  1  of the carburetor C includes a horizontal intake passage  2  connected to an intake port (not shown) of the engine, and a bottomed cylindrical valve guide bore  3  extending in a vertical direction perpendicular to the intake passage  2 . A rotary-type throttle valve  4  is rotatably and slidably received in the valve guide bore  3 , and a cap  5  for closing the valve guide bore  3 , is secured to the carburetor body  1 . A spring  6  is mounted under compression between the throttle valve  4  and the cap  5  for biasing the throttle valve  4  toward a bottom of the valve guide bore  3 . The throttle valve  4  has a throttle bore  9  provided so that the area of communication with the intake passage  2  is increased in response to the rotation of the throttle valve  4  in an opening-degree increasing direction. 
     The throttle valve  4  has a valve stem  4   a  extending through the cap  5 , and an operating arm  7  is secured to the valve stem  4   a  by a sleeve  8  fitted in a hollow in the valve stem  4   a.    
     A boss  10  is provided in the bottom of the valve guide bore  3  to protrude into the throttle bore  9 , and a fuel nozzle  11  is mounted to the boss  10  and rises in the throttle bore  9 . A needle valve  12 , threadedly mounted in the sleeve  8 , is inserted into the fuel nozzle  11 . 
     An annular slant  7   a  is formed on a lower surface of the operating arm  7  and the operating arm  7  is supported by a ball  13  mounted on an upper surface of the cap  5 . When the operating arm  7  is rotated in a direction to open the throttle valve  4 , it is pushed up by the ball  13 , and with this pushing, the throttle valve  4  is displaced upwards along with the needle valve  12  against the biasing force of the spring  6 , thereby increasing the opening degree of the fuel nozzle  11 . 
     A stopper bolt  14  is threadedly mounted in the cap  5  for regulation of advancing and retracting movement, and is adapted to abut against the operating arm  7  to define an idle opening degree of the throttle valve  4 . 
     A pressure plate  15 , a resilient packing  16  and a bottom plate  17  are coupled to a lower surface of the carburetor body  1  in a sequentially superposed manner. A fuel pipe  21  connected to a fuel tank T is connected to a joint  22  which projects from a lower surface of one side of the bottom plate  17 . An upstream fuel passage  23   a  in carburetor body  1  is connected to the joint  22 , and a pump chamber  29  in diaphragm-type fuel pump  24  in bottom plate  17 . A downstream fuel passage  23   b  is provided in the carburetor body  1  and is connected to the pump chamber  29 , and a constant-pressure fuel chamber  26  is provided in the bottom plate  17  and is connected to the downstream fuel passage  23   b.    
     The diaphragm-type fuel pump  24  has a diaphragm  27  formed by a portion of packing  16 . An operating chamber  28  and the pump chamber  29  faced by upper and lower surfaces of the diaphragm  27 , are formed on the carburetor body  1  and the bottom plate  17 , respectively. An intake valve  30  utilizing a portion of the packing  16 , and a fuel filter  31  located upstream of the intake valve  30 , are mounted in the upstream fuel passage  23   a , and a discharge valve  32  likewise utilizing a portion of the packing  16 , is mounted in the downstream fuel passage  23   b . The operating chamber  28  communicates with a pulsation pressure generating source P, e.g., the inside of a crank chamber or an intake pipe through a conduit  34 . 
     As shown in FIGS. 1 and 2, a fuel introduction control valve  35  is mounted in the constant-pressure fuel chamber  26  for controlling the introduction of fuel from the downstream fuel passage  23   b  into the constant-pressure fuel chamber  26 . The fuel introduction control valve  35  is comprised of a cylindrical valve seat member  37  mounted on the bottom plate  17  on one side of the constant-pressure fuel chamber  26 , so that an inlet bore  36  in an upper end wall faces the downstream fuel passage  23   b . A valve member  38  is vertically movably received in the valve seat member  37  to open and close the inlet bore  36 , and an operating lever  40  which is swingably carried on a support shaft  39 , is supported on the bottom plate  17  with one end engaged with a lower end of the valve member  38 . A valve spring  41  biases the operating lever  40  in a direction to close the valve member  38 , and a diaphragm  42  is mounted on a lower surface of the bottom plate  17  so as to form a bottom surface of the constant-pressure fuel chamber  26 . An urging element  42   a  is mounted at a central portion of the diaphragm  42  to abut against the other end of the operating lever  40  for movement away from such other end. The diaphragm  42  has a peripheral edge fastened to the bottom plate  17  along with a cover  43  which covers the diaphragm  42 . The cover  43  is provided with an air vent  44  for applying atmospheric pressure to a lower surface of the diaphragm  42 . 
     A fuel well  45  is defined in the bottom plate  17  and is located above the other end of the constant-pressure fuel chamber  26 . The fuel well  45  communicates at its lower portion with the constant-pressure fuel chamber  26  through an outlet bore  47  and at its upper portion with a lower end of the fuel nozzle  11  through a check valve  48  and a fuel jet  49 . 
     Further, a bypass passage  50  is provided in the bottom plate  17  and passes above the constant-pressure fuel chamber  26  to permit the lower end of the valve seat member  37  to communicate with the fuel well  45 . 
     The operation of the embodiment will be described below. 
     When the engine is operated, a pulsation pressure in the pulsation pressure generating source P is applied to the operating chamber  28  in the fuel pump  24  to vibrate the diaphragm  27 . When the diaphragm  27  is flexed toward the operating chamber  28 , the pump chamber  29  is increased in volume, thereby pumping fuel in the fuel tank T through the intake valve  30  and the upstream fuel passage  23   a . When the diaphragm  27  is flexed toward the pump chamber  29 , the pump chamber  29  is reduced in volume, thereby delivering the fuel therein toward the downstream fuel passage  23   b  through the discharge valve  32 . 
     In this case, if the fuel in the constant-pressure fuel chamber  26  does not reach a defined amount, the diaphragm  42  is displaced upwards under the action of the atmospheric pressure to swing the operating lever  40  in a clockwise direction as viewed in FIG. 1 against the biasing force of the valve spring  41 , thereby pulling down the valve member  38  to open the inlet bore  36 . Therefore, the fuel in the downstream fuel passage  23   b  is introduced into the constant-pressure fuel chamber  26 . When the fuel introduced into the constant-pressure fuel chamber  26  reaches the defined amount, the diaphragm  42  is lowered to pull the urging element  42   a  away from the operating lever  40 . Then, the operating lever  40  pushes up the valve member  38  by the action of the biasing force of the valve spring  41 , thereby closing the inlet bore  36 . Thus, the introduction of the fuel into the constant-pressure fuel chamber  26  is stopped. In this manner, the defined amount of fuel is constantly stored in the constant-pressure fuel chamber  26  during operation of the engine and passes through the outlet bore  47  to fill the fuel well  45 . 
     On the other hand, in the intake passage  2  and the throttle bore  9 , a negative pressure is produced around the fuel nozzle  11 . The fuel in the fuel well  45  rises sequentially in the check valve  48 , the fuel jet  49  and the fuel nozzle  11  and is ejected into the throttle bore  9  by the action of the negative pressure. The ejected fuel is drawn into the engine, while being mixed with air passed through the intake passage  2  and the throttle bore  9  to produce a fuel-air mixture. The amount of fuel-air mixture into the engine is regulated by increasing or decreasing the opening degree of the throttle valve  4 . 
     When fuel delivered from the fuel pump  24  to the downstream fuel passage  23   b  is subjected to a pressure pulsation provided by vibration of the diaphragm  42 , heat or vibration from the engine, or the like to produce a fuel vapor, the fuel vapor is delivered along with the fuel from the inlet bore  36  through the valve seat member  37  to the constant-pressure fuel chamber  26 , when the valve member  38  of the fuel introducing valve  35  is opened. However, because the lower end of the valve seat member  37  is in communication with the fuel well  45  through the bypass passage  50  extending above the constant-pressure fuel chamber  26 , the fuel vapor which is lighter than the fuel, rises up in the bypass passage  50  to the fuel well  45  as soon as it passes through the valve seat member  37 , and is then ejected promptly from the fuel nozzle  11  along with the fuel in the fuel well  45 . Therefore, the amount of fuel vapor ejected from the fuel nozzle  11  is very small and varies only slightly the fuel-air ratio of the fuel-air mixture and hence, normal operation of the engine can be ensured. 
     Although the embodiment of the present invention has been described in detail, it will be understood that the present invention is not limited to the above-described embodiment, and various modifications in design may be made without departing from the spirit and scope of the invention defined in claims. For example, the throttle valve  4  may be constructed into a butterfly type.