Abstract:
A carburetor for an internal combustion engine with an accelerator fuel pump in the carburetor having a piston actuated by a cam on a throttle valve shaft and a ball between them. The axis of a pump chamber in which the piston is slidably received is offset from and eccentric to the axis of rotation of the throttle shaft so that little fuel is supplied to the operating engine by the accelerator pump as the shaft is rotated to move the throttle valve from its idle position to an intermediate position and most of the fuel supplied by the accelerator pump to the engine is delivered as the shaft is rotated to move the throttle valve from the intermediate position to its wide open throttle position. This provides a proper fuel mixture to the engine to accelerate it and avoids the problem of supplying an overly rich fuel mixture to the engine during acceleration and particularly if the operator moves the throttle valve several times back and forth between the idle and intermediate positions before moving the throttle valve to the wide open position to accelerate the engine.

Description:
REFERENCE TO RELATED APPLICATION 
     Applicants claim the priority of Japanese patent application, Ser. No.  11-023917 , filed Feb. 1, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to carburetors for internal combustion engines and more particularly to an accelerator pump of the carburetor. 
     BACKGROUND OF THE INVENTION 
     Some carburetors for gasoline fueled small engines such as two-stroke engines for handheld power tools such as chain saws, weed trimmers, leaf blowers and the like have carburetors with an internal accelerator pump which supplies additional fuel to the operating engine as the throttle valve of the carburetor is opened from its essentially closed or idle position toward its completely wide open throttle position. This additional fuel is needed to smoothly and rapidly accelerate the engine without stumbling particularly when it is under a load. Many prior accelerator devices have a positive displacement pump with a piston actuated by rotation of a shaft of a throttle valve through a wide variety of mechanical cam and linkage arrangements. One problem with these positive displacement accelerator pumps is they supply an excess quantity of fuel producing an overly rich fuel mixture for the engine upon initial opening of the throttle from its idle position and particularly if the throttle is opened or advanced to only an intermediate position which is less than the wide open throttle position. This is particularly a problem with a handheld power tool because many operators tend to rather rapidly partially open and close the throttle several times before fully opening and maintaining the throttle at its wide open position for a period of time during which a power tool is in actual use and its engine is under a substantial load. 
     SUMMARY OF THE INVENTION 
     In a carburetor with a shaft rotatable to move a throttle valve in a fuel and air mixing passage between an essentially closed or idle position and a fully open or wide open throttle position, an accelerator pump which delivers most of the accelerating fuel only after the throttle has been partially opened to an intermediate position and is then further advanced toward its wide open position so that an excessively rich fuel mixture is not provided for accelerating the engine. Preferably, the accelerator pump has a piston slidably received in a cylinder bore and movable to an advanced position to dispense a quantity of accelerating fuel and to a retracted position to refill the cylinder with fuel. In response to rotation of the throttle shaft to move the throttle valve from its idle to its wide open position, the piston is advanced by a cam which is preferably a face on the throttle shaft engaging a ball received in the bore between the piston and the shaft. So that the extent to which the piston is advanced is small as to throttle valve and shaft are moved from the idle position to an intermediate position and the extent of travel is significantly greater as the shaft and throttle are further advanced from the intermediate position to the wide open throttle position, the axis of the cylinder bore and hence the piston and the path of travel of the center of the ball are all eccentric to or offset and spaced from the axis of rotation of the throttle shaft so that these axes do not intersect. Preferably, these axes are at right angle to each other and the piston is yieldably biased toward its retracted position and into engagement with the ball and the ball into engagement with the cam by a spring or other biasing means. 
     OBJECTS, FEATURES AND ADVANTAGES OF THIS INVENTION 
     Objects, features and advantages of this invention include providing a carburetor with an accelerator pump in which most of the accelerating fuel is delivered only as the throttle valve and shaft is advanced from an intermediate position toward its wide open position, supplies only sufficient fuel for accelerating an operating engine without providing an overly rich fuel mixture for acceleration, does not provide an overrich fuel mixture even when an operator rapidly moves the throttle valve between its idle and intermediate positions several times, is rugged, durable, reliable, of relatively simple design and economical manufacture and assembly and in service has a long useful life without any maintenance or repair. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of this invention will be apparent from the following detailed description of the preferred embodiments and best mode, appended claims, and accompanying drawings in which: 
     FIG. 1 is a full sectional view of a diaphragm carburetor with a first embodiment of an accelerator pump of this invention; 
     FIG. 2 is a fragmentary sectional side view of the accelerator pump of FIG. 1 with the throttle valve and shaft shown in their idle position; 
     FIG. 3 is a fragmentary sectional plan view of the accelerator pump of FIG. 1 taken at a right angle to the sectional view of FIG. 2 with the throttle valve and shaft shown in their idle position; 
     FIG. 4 is a fragmentary sectional side view of the accelerator pump of FIG. 1 with the component parts shown in the position they assume when the throttle valve and shaft are in their wide open throttle position; 
     FIG. 5 is a fragmentary sectional side view of the accelerator pump of FIG. 1 with the throttle shaft and ball shown in solid line in the throttle valve idle position, in chain line in the throttle valve intermediate position, and in dashed line in the wide open throttle valve position; 
     FIG. 6 is a graph showing in solid line the extent of advancement of the piston of the accelerator pump of FIG. 1 as a function of the extent of rotation of the throttle shaft and valve from their idle position to their wide open throttle position and in dashed line the extent of advancement of a piston of a prior art accelerator pump as a function of the extent of rotation of its throttle shaft and valve from their idle position to their wide open throttle position; 
     FIG. 7 is a fragmentary sectional side view of a modified fuel pump of this invention in the carburetor of FIG. 1 with the throttle valve and shaft shown in their idle position; 
     FIG. 8 is a fragmentary sectional side view of the accelerator pump of FIG. 7 with the throttle shaft, ball and piston shown in their respective positions when the throttle shaft and valve are in their wide open throttle position; and 
     FIG. 9 is a fragmentary sectional side view of the accelerator pump of FIG. 7 with the throttle shaft and ball shown in solid line in the throttle valve idle position, in chain line in the throttle valve intermediate position, and in dashed line in the throttle valve wide open throttle position. 
    
    
     DETAILED DESCRIPTION 
     Referring in more detail to the drawings, FIG. 1 illustrates a diaphragm type carburetor  1  for an internal combustion gasoline fueled engine with an accelerator pump D embodying this invention. The diaphragm carburetor  1  also has a manual primer pump assembly A, a fuel supply pump assembly B, and a fuel metering system C each of which, if desired, may be of conventional construction. 
     When the engine is operating, the fuel pump assembly B supplies fuel to the metering system C of the carburetor. The fuel pump B has a flexible diaphragm or membrane  10  received and sealed between an upper face of the carburetor body  16  and a lower face of an upper cover  5  and defining in part a fuel pump chamber  13  and a pulse chamber  12  to which pressure and vacuum pulses in the crankcase of a two-cycle operating engine are introduced through a passage  18  to flex or actuate the diaphragm  10 . Flexing of the diaphragm  10  draws fuel from a fuel tank (not shown) through inlet passage  19  and a one-way check valve  15  into the pump chamber  13  and supplies the fuel under pressure through an outlet passage  60 , one-way check valve  14  and a screen  17  to the fuel metering system C through its flow control valve  20 . 
     The fuel metering system C has a flexible diaphragm or membrane  53  received and sealed between a lower face of the carburetor body and a lower cover  56  to define a fuel chamber  54  on one side of the diaphragm and an atmospheric air chamber  55  on the other side of the diaphragm which communicates with the atmosphere exteriorly of the carburetor through a port  57  in the lower cover. The flow valve  20  is opened and closed to control the admission of fuel to chamber  54  by movement of the diaphragm which is operably connected to the valve by a lever  50  connected adjacent one end to the valve  20  and adjacent the other end bears on a projection  62  attached to the center of the diaphragm and between its ends is pivotally mounted on a support shaft  49 . The valve  20  is yieldably biased to its closed position by a spring  52  bearing on the lever  50 . 
     The carburetor has an air and fuel mixing passage  24  with an air inlet  62 , a restricted venturi section  23  downstream of the inlet, usually a choke valve (not shown) between them, and downstream of the venturi an outlet  66  which communicates with an intake passage of the engine. A throttle valve  25  is received in the mixing passage downstream of the venturi and is mounted on a throttle shaft  26  extending transversely through the passage and journalled for rotation in the body  16 . 
     In operation of the carburetor, fuel is supplied from the metering chamber  54  to a high speed fuel nozzle  39  opening into the mixing passage  24  via a check valve  44 , passage  43 , adjustable fuel regulating needle valve  42 , passage  38 , and check valve  37 . Fuel is also supplied to a series of low speed fuel nozzles or ports  45  which open into the mixing passage  24  both upstream and downstream of the throttle valve in its idle or closed position, via a branch passage  48 , adjustable low speed fuel regulating needle valve  47  and passage  46 . In operation, air flowing through the mixing passage  24  creates a pressure differential causing fuel to flow through the low speed nozzle  45  downstream of the throttle valve  25  (in its idle position) into the mixing passage and in the engine under idle and near idle operating conditions and to flow through the high speed nozzle  39  into the mixing passage  24  and the engine when the engine is in the range from near idle to wide open throttle operating conditions. This pressure differential acts on the diaphragm  53  to open and close the valve  20  to maintain a predetermined quantity of fuel in the metering chamber  54  and at a substantially constant pressure when the engine is operating to supply fuel to the low and high speed nozzles. 
     When the engine is not operating and in preparation for starting it, a primer pump A may be manually actuated to expel any air and/or fuel vapor from the fuel chamber  54  before starting the engine. The primer pump has a flexible rubber dome or syringe  2  attached and sealed to the upper cover  5  by a retainer plate  4  and enclosing a complex mushroom shaped valve  3  with a suction or inlet valve  3   a  communicating through a passage  7  with an upper portion of the fuel metering chamber  54  and an outlet or discharge valve  3   b  communicating through a passage  8  with an upper portion of the fuel tank. The primer pump A is actuated by manually repeatedly alternately pressing down or collapsing and releasing the dome. When the collapsed dome is released, air and fuel vapor in the fuel chamber flow through the passage  7  and the valve  3   a  and enters the dome  2  and, as the dome is pressed or collapsed, the air and fuel vapor therein flows through the valve  3   b  and passage  8  and is discharged into the fuel tank. 
     As shown in FIGS. 1-3, in accordance with this invention, preferably the accelerator device D is provided inside the carburetor body adjacent the throttle shaft  26  in an area spaced or remote from or outside of the mixing passage  24 . The accelerator pump D has a piston  33  slidably received in a blind bore  68  forming a pump cylinder or chamber  30  which communicates through passages  36  and  40  with both the high speed fuel nozzle  39  and the metering chamber  54  through the needle valve  42 , passage  43  and check valve  44 . Preferably, the other end of the bore  68  is closed by a plug  27  press fit therein. In use, the piston is actuated by a cam  28  on the throttle shaft which engages a spherical ball  32  disposed between them and received in a recess  33   a  in an end of the piston. Preferably, the recess  33   a  has a conical or spherical shape to retain the center of the ball  32  coincident with the axis  70  of the piston  33  and the bore  68 . A seal is provided between the piston and the bore by an O-ring  35  and the piston  33  is yieldably biased towards its retracted position and into engagement with the ball  32  which in turn is urged into engagement with the cam  28  by a spring  34  received in the chamber  30  and bearing on the piston. Preferably, the cam  28  consists of a cut-away face  28   a  and a peripheral face  28   b  of the throttle shaft. As shown in FIG. 3, preferably the face  28   a  is a cylindrical surface with an axis which is perpendicular to the axis of the throttle shaft and a radius which is larger than the radius of the ball  32 . 
     In accordance with the invention, as shown in FIGS. 2 and 5, the axis  70  of the pump cylinder  30  is offset and spaced from or located eccentrically with respect to the axis of rotation of the throttle valve shaft  26  by a distance a so that upon initial rotation of the throttle valve from its idle position to an intermediate position the displacement of the ball  32  and piston  33  is very small and most of the displacement of the ball and piston occurs as the throttle valve is further rotated from its intermediate position toward and to its wide open throttle position. In FIG. 5, the intermediate position where the throttle valve  25  has been rotated from its idle position about 30° counterclockwise is shown by a chain line and the position where the throttle valve has been rotated from its idle position about 75° to its fully open or wide open throttle position is shown by a dashed line. It is apparent that the amount of travel or displacement S 3  of the piston  33  when the throttle valve  25  is rotated from its idle position to the intermediate position is very small compared to the amount of travel or displacement T 3  of the piston when the throttle valve is rotated from its idle position to its wide open throttle position. As indicated by the solid line  72  in the graph of FIG. 6, with this accelerator pump D, the amount of advancement or travel S 3  of the piston from the idle position to the intermediate position of 30° of rotation θ of the throttle valve  25  is very small compared to the amount of advancement or travel S 1  of the piston of a conventional prior art accelerator pump as indicated by the dashed line  71 . Accordingly, compared to prior art devices, the accelerator pump D will deliver little fuel when the throttle is advanced to the intermediate position and thus will not supply an overly rich fuel mixture to the engine. 
     A modification of the accelerator pump D is illustrated in FIGS. 7-9 in which the cut-away cam face  28   a  is a semi-spherical surface and the other components are the same as those of the first embodiment of FIGS. 2-5. As will be apparent to skilled persons, the cam face  28   a  may also have other configurations such as an arcuate surface with radii or a cylindrical surface with a radius larger than the radius of the ball. The axis of the cylindrical surface may be substantially parallel to the axis of the throttle shaft. As shown in FIG. 9, with these modifications, the amount of displacement or travel S 4  of the piston  33  when the throttle valve  25  is rotated 30° counterclockwise from its idle position to its intermediate position is also small compared with the amount of advancement or travel S 1  of the piston of a prior art accelerator pump and relative to the total displacement or travel T 4  of the piston with these modifications when the throttle valve is rotated 75° counterclockwise from its idle position to its wide open throttle position. In FIG. 9, the position of the throttle shaft  26  and ball  32  when the throttle valve  25  is in its idle position is shown in solid line, when the throttle valve is in its intermediate position is shown in chain line, and when the throttle valve is in its wide open throttle position is shown in dashed line. Thus, rotation of the throttle valve from its idle position to its intermediate position causes the accelerator pump to supply only a very small quantity of fuel so that an overly rich fuel mixture is not supplied to the engine. 
     In use of the carburetor on an operating engine, in all embodiments of the accelerator pump, as the operator initially advances the throttle valve from its idle position to its intermediate position, the piston  33  is advanced only a small amount and thus the pump delivers only a small quantity of additional fuel to the operating engine so that it does not receive an overly rich mixture and when the throttle valve is further opened from its intermediate position toward its wide open throttle position, the cam and ball arrangement advances the piston  33  a comparatively large amount to discharge a relatively larger quantity of fuel from the pump chamber and through the high speed fuel nozzle  39  into the mixing passage to provide an enriched fuel mixture to accelerate the operating engine. As the throttle shaft  26  is rotated to move the throttle valve from its idle position to its wide open throttle position, preferably the contact point of the ball  32  with the cam face  28   a  moves from one side of the face  28   a  through the center of the face and toward the edge of the face or toward the peripheral face  28   b  of the cam. 
     When the throttle is moved from its wide open position toward its idle position, the spring  34  moves the piston  33  toward its retracted position which draws fuel from the metering chamber  54  into the pump chamber  30  through the interconnecting passages to refill the accelerator pump chamber with fuel. Even if the operator repeatedly opens and closes the throttle valve between its idle and intermediate positions, the accelerator pump will supply relatively little fuel to the engine and thus the fuel mixture supplied to the engine will not be too rich. Thus, as frequently occurs, the operator rapidly and repeatedly “snapping” the throttle partially open and closed, as a prelude to moving the throttle to its wide open position to accelerate the engine will not adversely affect engine performance and acceleration.