Abstract:
A charge forming device includes a body with a fuel and air mixing passage, a bypass passage that communicates with the fuel and air mixing passage, a throttle valve and a choke valve. The throttle valve is movable between idle and wide open positions and operable to control at least in part the fluid flow through the fuel and air mixing passage. The choke valve is movable between an open position permitting a substantially free flow of air into the fuel and air mixing passage and a closed position at least substantially restricting air flow into the fuel and air mixing passage. A bypass valve is movable between an open position and a closed position to selectively permit fluid flow through the bypass passage. The bypass valve is movable toward its open position when the throttle valve is displaced at least a threshold amount away from its idle position.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to internal combustion engines and more particularly to a charge forming device for such engines. 
     BACKGROUND OF THE INVENTION 
     Small internal combustion engines may be used in various devices including recreational vehicles and garden implements such as chain saws, lawn mowers and string trimmers. Some of these devices have pull cord type starting systems that require a retractable cord to be pulled by a user of the device to start it. In a recoil starter mechanism, pulling the cord rotates a recoil pulley which, through a one way clutch, rotates a crank shaft of the engine to start the engine. 
     SUMMARY OF THE INVENTION 
     A charge forming device includes a body defining at least part of a fuel and air mixing passage and a bypass passage that communicates with the fuel and air mixing passage, and a throttle valve and a choke valve. The throttle valve is carried by the body for movement between idle and wide open positions and operable to control at least in part the fluid flow through the fuel and air mixing passage. The choke valve is operably associated with the fuel and air mixing passage, and movable between an open position permitting a substantially free flow of air into the fuel and air mixing passage and a closed position at least substantially restricting air flow into the fuel and air mixing passage. A bypass valve associated with the bypass passage is movable between an open position and a closed position to selectively permit fluid flow through the bypass passage. The bypass valve is movable toward its open position when the throttle valve is displaced at least a threshold amount away from its idle position. 
     In one implementation, the charge forming device includes a carburetor and at least one bypass valve is carried by the throttle valve and at least one bypass valve is carried by the choke valve. The bypass valves permit air flow therethrough when the choke valve is moved toward its closed position and the throttle valve is moved toward its wide open position, such as during a choke assisted start of an engine at wide open throttle. Of course, other arrangements of the bypass, throttle and choke valves may be utilized, as desired for different applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, features and advantages of the present invention will be apparent from the following detailed description of preferred embodiments and best mode, appended claims and accompanying drawings in which: 
         FIG. 1  is a side view of one embodiment of a charge forming device with a throttle valve in its idle position; 
         FIG. 2  is a side view of the charge forming device of  FIG. 1  illustrating a choke valve in its open position; 
         FIG. 3  is a plan view of the charge forming device; 
         FIG. 4  is a side view of the charge forming device illustrating the throttle valve in its wide open position; 
         FIG. 5  is a fragmentary side view illustrating the choke valve in its closed position; 
         FIG. 6  is a fragmentary sectional view illustrating an air bypass passage in the charge forming device with the choke valve in its closed position and the throttle valve in its idle position; 
         FIG. 7 . is a fragmentary sectional view like  FIG. 6  illustrating the choke valve in its closed position and the throttle valve in its wide open position; 
         FIG. 8 . is a fragmentary sectional view like  FIG. 6  illustrating the choke valve in its open position and the throttle valve in its idle position; 
         FIG. 9  is a fragmentary sectional view like  FIG. 6  illustrating the choke valve in its open position and the throttle valve in its wide open position; and 
         FIG. 10  is a sectional view of a main body of the charge forming device illustrating a fuel and air mixing passage and the air bypass passage formed in the body. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring in more detail to the drawings,  FIGS. 1-4  illustrate a charge forming device  10 , such as a carburetor, that has a main body  12  including a central block  14  and one or more plates  16  attached to the block  14 . The carburetor  10  may be a diaphragm-type carburetor that includes a flexible fuel pumping diagram and a flexible fuel metering diaphragm (not shown). In that regard, the carburetor may be constructed generally as disclosed in U.S. Pat. No. 4,752,420, the disclosure of which is incorporated herein by reference in its entirety. 
     The main block  14  includes a fuel and air mixing passage  18  formed therethrough and communicating with a fuel metering chamber (not shown). Fuel from the metering chamber is provided into an air flow through the fuel and air mixing passage  18  to provide a fuel and air mixture to an operating engine. At least one air bypass passage  20  is also provided, preferably in the main block  14 . In the implementation shown, two bypass passages  20  are formed in the carburetor  10 . The bypass passages  20  may be disposed on generally diametrically opposed sides of the fuel and air mixing passage  18  and are adapted to permit air to flow therethrough to, at least under certain engine conditions, permit additional air flow to the engine, as will be discussed in more detail herein. 
     The carburetor body  12  preferably also includes an outwardly extending stop  22  which as shown, is carried by a plate  16  attached to the block  14  of the carburetor body. A protrusion  24  may include a threaded bore in which a threaded idle adjustment screw  26  is received. A spring  28  may be disposed about the shank of the idle adjustment screw  26  between its head and the protrusion  24 . An end of the idle adjustment screw  26 , opposite its head, extends through the protrusion  24  and is adapted to engage a position limiting lever  30  of a throttle valve  32  to define the idle position of the throttle valve  32 . Accordingly, advancing or retracting the idle adjustment screw  26  relative to the protrusion  24  permits adjustment of the idle position of the throttle valve  32 . The protrusion  24  itself may provide a stop  34  ( FIG. 3 ) engageable by the throttle valve lever  30  to define the wide open position of the throttle valve  32  by limiting rotation of the throttle valve  32  away from its idle position. 
     In the implementation shown, the carburetor throttle valve  32  is a butterfly type valve that includes a valve shaft  36  rotatably carried in a bore  38  extending in the block  14 , through the fuel and air mixing passage  18  and through each bypass passage  20 . The throttle valve  32  also includes a valve head  40  which may be a flat disc fixed to the shaft  36 , such as by a fastener  41 , for corotation with the shaft. The throttle valve  32  may further include the position limiting lever  30  as previously recited, and at its opposite end, a start assist lever  42  extending generally radially outwardly from the shaft  36  and fixed to the shaft for rotation therewith. A spring  44  may be disposed about the throttle valve shaft  36  with one end of the spring engaged with the carburetor body  12  and its other end engaged with the position limiting lever  30  to yieldably bias the throttle valve  32  to its idle position wherein the valve head  40  substantially prevents fluid flow out of the fuel and air mixing passage  18 . 
     The carburetor  10  may also include a choke valve  46 , as is known in the art. The choke valve  46  is disposed upstream of the throttle valve  32  and preferably includes a shaft  48  rotatably carried by the carburetor such as in a bore  50  formed in the block  14  and extending through the fuel and air mixing passage  18  and the bypass passages  20 . The choke valve shaft  48  may extend parallel to the throttle valve shaft  36 . The choke valve  46  preferably also includes a start assist lever  52  disposed on the same side of the carburetor as the start assist lever  42  of the throttle valve  32 . A return spring  54  can be disposed about the choke valve shaft  48  and engaged at one end with the carburetor body  12  and at its other end with the start assist lever  52  to yieldably bias the choke valve  46  to its open position wherein a substantially unrestricted flow of air is permitted into the fuel and air mixing passage  18 . Opposite the start assist lever  52 , the choke valve shaft may extend out of the carburetor body  12  and be retained thereto by a suitable retainer, such as a clip or other fastener  58 . The choke valve  46  may also be of a butterfly type having a flat disc valve head  56  fixed to the choke valve shaft  48  and being complementary shaped to the adjacent portion of the fuel and air mixing passage  18  to substantially close the fuel and air mixing passage  18  when the choke valve  46  is rotated to its closed position, shown in  FIG. 5 . The choke valve may not fully close the fuel and air mixing passage  18  to permit a limited, calibrated air flow therethrough when closed or in a “start” position. This may be done by providing a hole in the valve head  56 , or by providing a peripheral gap between the valve head  56  and carburetor body through which air may flow into the fuel and air mixing passage  18 . 
     As best shown in  FIG. 10 , both the throttle valve  32  and the choke valve  46  preferably include at least one bypass valve  60 , 62 , respectively, that selectively permit or control the rate of air flow through the bypass passages  20 . In the implementation shown, two bypass valves  60 , 62  are carried by each of the throttle valve  32  and choke valve  46 . The bypass valves  60 , 62  may be defined or include one or more recesses  64 , 66  formed in the respective valve shafts  36 , 48 . The recesses  64  extend radially inwardly about one half of the thickness of the shafts  36 , 48  and axially a distance generally equal to the width of the bypass passages  20 , but of course other dimensions could be used. In one implementation, the recesses  66  are formed on either side of a land  67  of the choke valve shaft  48 . As also shown in  FIG. 10 , the bypass passages  20  may extend completely through the block  14  from the choke valve side  68  through the throttle valve side  70 . However, the end of the bypass passages  20  at the throttle valve side  70  may be closed off by a gasket when the carburetor  10  is mounted to the engine. Accordingly, the air flow through the bypass passages  20  may follow the arrows in  FIG. 10  which show the air flow entering the fuel and air mixing passage  18  through the bore  38  in which the throttle valve shaft  36  is carried. Of course, the bypass valves  60 , 62  may take other forms, such as relatively flat discs acting as valve heads fastened to the shafts  36 , 48  for rotation with the shafts or otherwise actuated by rotation of the shafts  36 , 48 , for example. 
     Accordingly, when the bypass valves  60 , 62  of both the throttle valve  32  and choke valve  46  are opened, or permit air flow therethrough, air flows through the bypass passages  20  from the choke valve side  68  of the carburetor  10  toward the throttle valve side  70 , with that air flow being provided into the fuel and air mixing passage  18  for delivery to the engine. In the implementation shown, the bypass valves  62  associated with the choke valve  46  are open only when the choke valve  46  is closed, or at least substantially closed. The bypass valves  60  of the throttle valve  32  are open when the throttle valve  32  is in its wide open position, or relatively near its wide open position. In one implementation, the air flow through the bypass passages  20  is permitted when the choke valve  46  is rotated at least ⅔ of the way from its open position toward its closed position, and the throttle valve  32  is rotated at least ⅔ of the way from its idle position toward its wide open position. The bypass passage valves  60 , 62  are preferably opened and closed, and moved between their opened and closed positions, as a function of the position of the throttle valve  32  and choke valve  46 , although they may be otherwise moved such as by an operator of the device with which the carburetor is used. In one form, the bypass valves  60 , 62  are coupled to the throttle valve  32  and choke valve  46  and are driven between their open and closed positions by movement of the throttle and choke valves. In the implementation shown, the bypass valves  60 , 62  are carried by the throttle valve  32  and choke valve  46  for corotation with these valves. 
     As best shown in  FIG. 6 , when the choke valve  46  is closed or in a start position wherein the choke valve is substantially closed, the bypass valves  62  associated therewith are open, permitting air flow therethrough and toward the throttle valve side  70  of the carburetor  10 . However, when the throttle valve  32  is in its idle position, or a fast idle position such as may be employed during starting of an engine with which the carburetor is used, the bypass valves  60  associated with the throttle valve  32  are in their closed position, preventing air flow therethrough. This prevents or substantially restricts the flow of additional air to the fuel and air mixing passage  18  which would otherwise make the fuel and air mixture undesirably lean for starting and initial warming up of the engine. As shown in  FIG. 8 , when the choke valve  46  is in its open position, the bypass valves  62  associated with the choke valve  46  are closed, or prevent the flow of air through the bypass passages  20 . In  FIG. 8 , the throttle valve  32  is shown in its idle position and the bypass valves  60  associated therewith also prevent air flow through the bypass passages  20 . In  FIG. 9 , the choke valve  46  is open so that its bypass valves  62  prevent air flow therethrough The throttle valve  32  is in its wide open position, but even though the bypass valves  60  associated therewith are open, no or substantially no air flows through the bypass passages  20  because the bypass valves  62  of the choke valve  46  are closed. 
     Accordingly, in this implementation, the bypass passages  20  provide air flow into the fuel and air mixing passage  18  only when, as shown in  FIG. 7 , the choke valve  46  is in or near its closed position such that the bypass valves  62  associated therewith are open, and the throttle valve  32  is in or near its wide open position, such that the bypass valves  60  associated therewith are also open. The choke and throttle valves may be in this position, for example, when the engine becomes flooded during attempted starts with the throttle valve  32  in its idle position. After this occurs, the throttle valve  32  may be moved to its wide open position permitting increased air flow therethrough, in an attempt to start the engine with a leaner fuel and air mixture. However, with the choke valve  46  closed, there is not much air flow through the fuel and air mixing passage  18 . Accordingly, in this situation, the bypass air passages  20  are open and air flows therethrough and into the fuel and air mixing passage  18  to provide a relatively lean mixture and facilitate starting a flooded engine. In this manner, air flow is provided even though the choke valve  46  is closed or substantially closed. 
     This carburetor  10  may be used, for example, with an engine having an easy start system wherein the choke valve  46  is automatically applied upon pulling a pull cord to start the engine. Such systems couple the choke valve  46  to mechanisms that are moved upon pulling the pull cord. A representative example of such an easy start pull cord system is disclosed in U.S. patent application Ser. No. 11/285,554, now U.S. Pat. No. 7,275,508 which was filed on Nov. 21, 2005, the disclosure of which is incorporated herein by reference in its entirety. 
     In such systems, the choke valve  46  may be pulled closed as a function of the force resisting pulling of the pull cord such that the choke valve  46  becomes fully closed up to a top dead center position of a piston in the engine and after top dead center, the choke valve  46  may be moved towards its idle position by its return spring  54 . When the throttle valve  32  is in its idle position, the choke valve  46  will slip or move back to its start position which may be rotatably spaced or inclined from its fully closed position. The start assist levers  52 , 42  on both the choke valve  46  and throttle valve  32 , respectively, may become engaged with each other to define a start position of both the choke valve  46  and the throttle valve  32 . When the throttle valve  32  is in its wide open position upon attempted start of the engine, the start assist levers  42 , 52  of the throttle and choke valves  32 , 46  may or may not engage after the force closing the choke valve is reduced. If the levers,  42 , 52  do not engage in this situation, the choke valve  46  may return to its open position after the piston moves past top dead center if the force of the return spring  54  is greater than the force tending to close the choke valve  46 . 
     In any event, as the choke valve  46  is moved sufficiently towards its closed position when the engine pull cord is pulled to start the engine, the bypass valves  62  associated with the choke valve  46  are open. If the throttle valve  32  is in its wide open position, or sufficiently close thereto, its bypass valves  60  are also opened so that an air flow may occur through the bypass passages  20  and into the fuel and air mixing passage  18 . Of course, the flow rate of air through the bypass passages  20  during a wide open throttle engine start, combined with the flow of air through the choke valve  46 , can be calibrated for a particular engine or application to provide a desired fuel and air mixture to support starting and continued operation of the engine.