Abstract:
A fuel shut off solenoid device of a carburetor has a solenoid chamber which typically fills with fuel. When the solenoid device is energized, fuel flows from a fuel chamber into a mixing passage of the carburetor to mix with air. During the energized state, heat from the solenoid tends to vaporize the fuel within the solenoid chamber. Also when energized, the solenoid device is held in a retracted position whereby a head at a distal end of the shaft mates with or seals to a washer which in turn seals to an upward face of the encasement of the fuel shut off solenoid device. Thus the potential migration of large vapor bubbles from the solenoid chamber to the mixing passage of the carburetor is eliminated, providing a smoother idling or running engine at light loads.

Description:
FIELD OF THE INVENTION 
     This invention relates to a carburetor for small engines, and more particularly to a carburetor having a fuel shut off solenoid device. 
     BACKGROUND OF THE INVENTION 
     The use of solenoid devices to control a variety of fuel flow transients within a carburetor of a small engine is known. One particular application consists of a fuel shut off solenoid device of a carburetor capable of blocking fuel flow from entering a mixing passage of the carburetor when an ignition switch is turned off, thereby preventing engine dieseling and after boom. When the ignition switch is on, the solenoid device is energized and thereby held in a retracted position. If retracted, fuel flows from a fuel bowl, through a main tube or nozzle where the fuel premixes with air, and into the carburetor mixing passage to mix with more air. When the ignition switch is off, the solenoid device is de-energized and a head at a distal end of a shaft of the solenoid device is extended upward thereby isolating the fuel bowl from the main tube and effectively cutting off fuel flow. 
     The solenoid device is typically mounted in an upright position below the carburetor body. A solenoid chamber defined by an encasement of the device is usually disposed below the fuel bowl. When the head extends, the shaft of the solenoid device moves upward out of the solenoid chamber and the head mates with the bottom side of the main tube to cut off fuel flow. Because the solenoid chamber is located beneath the fuel bowl of the carburetor and a clearance exists between the shaft and the encasement of the solenoid device, fuel migrates via gravity into the solenoid chamber. 
     SUMMARY OF THE INVENTION 
     Although the migrating fuel was thought to be useful in cooling the energized solenoid device, it has been found that heat emitted by the coil of the energized solenoid device vaporizes the fuel contained within the solenoid chamber. The heat generated by the solenoid valve heats the fuel thereby creating vapor bubbles which migrate up through the main nozzle, interfering with steady or smooth operation of the engine. The bubbles interfere with the mixing of fuel and air causing a noticeably rough engine idling or light load condition. Accordingly, the present invention is a carburetor having a fuel shut off solenoid device which does not inject fuel vapor into the liquid fuel. 
     A carburetor body of the carburetor has an inner sidewall defining a mixing or lower chamber disposed above the fuel shut off solenoid device. A fuel chamber containing a constant level of fuel is defined by a fuel bowl engaged to an outward or underside of the carburetor body. Fuel flows through an orifice communicating between the fuel chamber and the lower chamber. The lower chamber communicates with an elongated main tube or nozzle defining an enriched fuel bore and extending longitudinally upward from the lower chamber and tranversely into a mixing passage. The main tube has a mating surface on a lower end facing downward and extending radially outward thereby engaging the sidewalls of the lower chamber. 
     The fuel shut off solenoid device has an encasement which threadably engages a bottom portion of the carburetor body. The encasement defines a solenoid chamber which houses an extendable shaft having a head at a distal end disposed above the encasement. The solenoid chamber contains migrating fuel thought to cool the solenoid coil. The shaft is disposed vertically within the solenoid chamber and extends upward through an outward face extending radially outward from an inner brim which circles the shaft. Mounted on top of the outward face and also circling the shaft, is a washer. When the solenoid is energized and in a retracted position, a head of the shaft engages to an outward face of the washer. The outward face of the encasement engages to the inward or opposite face of the washer. 
     The head of the shaft has an annular trailing surface expanding radially outward from an inner perimeter edge congruent to the surface of the shaft, to a peripheral edge. The trailing surface of the head confronts the outward face of the encasement. The peripheral edge of the trailing surface has a diameter larger than the diameter of a hole of the washer. 
     A clearance is defined radially between the shaft and the inner brim of the encasement. Fuel flows or migrates through the clearance between the lower chamber of the carburetor body and the solenoid chamber. While energized, the fuel shut off solenoid device has a tendency to heat the fuel in the solenoid chamber thereby creating vapor bubbles which can interfere with the idle or light load fuel mixture of the carburetor. The engagement of the washer between the trailing surface of the head and the outward face of the energized solenoid device partially blocks or stops the migration of fuel vapor bubbles from the solenoid chamber into the lower chamber. 
     Objects, features and advantages of this invention include the elimination of fuel vapors migrating from the solenoid chamber into the lower chamber of the carburetor body, a smoother operating engine, particularly noticeable during engine idling or light load conditions, and which is rugged, durable, economical to manufacture and assemble, and has a long useful service life. 
    
    
     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 broken cross sectional side view of a carburetor according to the present invention; 
     FIG. 2 is a partial cross sectional view of the carburetor taken along line  2 — 2  of FIG. 1; 
     FIG. 3 is a longitudinal cross sectional view of the fuel shut off solenoid of the carburetor; 
     FIG. 4 is a partial cross sectional view of the carburetor taken along line  4 — 4  of FIG. 2; 
     FIG. 5 is a plain top view of a washer of the fuel shut off solenoid device; and 
     FIG. 6 is a side view of the washer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring in more detail to the drawings, FIG. 1 illustrates a carburetor  10  embodying the present invention with a carburetor body  12  having a mixing passage  14  through which air flows in the direction of the arrows. An air inlet portion  16  of the mixing passage  14  is positioned downstream of an air filter unit (not shown). The air inlet portion  16  houses a pivoting choke plate  18  having a pivotal axis  19  perpendicular to the longitude of the mixing passage  14 . The choke plate  18  is substantially closed during cold engine start conditions thereby controlling or limiting the air intake. Downstream of the air inlet portion  16  is a fuel and air mixture outlet portion  18  of the mixing passage  14 . The outlet portion  18  houses a pivoting throttle plate  20 , similar to the choke plate  18 , but which controls the amount of fuel and air mixture entering a running engine. With the engine running, the air pressure at the air inlet portion  16  is near atmospheric minus the pressure drop across the air filter unit (not shown). 
     Referring to FIGS. 1 and 2, the longitude or axis of the mixing passage  14  is preferably horizontal. A fuel bowl  22  engages the carburetor body  12  from beneath thereby defining a fuel chamber  24  between them. The fuel chamber  24  maintains a consistent level of fuel via a float mechanism. In operation, fuel flows from the fuel chamber  24  through an orifice  30  and into a lower or mixing chamber  26  of the carburetor body  12 . A preferably cylindrical side wall  28  of the carburetor body  12  defines in part the lower chamber  26 . The orifice  30  penetrates a dividing portion of the carburetor body  12  through the side wall  28  thereby communicating between the fuel chamber  24  and the lower chamber  26 , as shown in FIGS. 2 and 4. 
     During engine operation under non-idle conditions, fuel and air flows upward via negative pressure from the lower chamber  26 , through a bore  32  defined by an elongated main or nozzle tube  34 , and into the mixing passage  14  between the choke plate  18  and the throttle plate  20 . An upper end portion  38  of the main tube  34  extends substantially perpendicular into the mixing passage  14 . The main tube  34  has an outer surface  36  which engages the carburetor body  12  at the upper end portion  38  of the main tube  34 . The carburetor body  12  and the tube  34  define an upper annular chamber  40  disposed above the lower chamber  26  and beneath the upper end portion  38  of the main tube  34 . The main tube  34  has a lower end  42  which flares radially outward to sealably engage the carburetor body side wall  28  beneath the upper chamber  40 , thereby isolating the lower chamber  26  from the upper chamber  40 . The lower chamber  26  is generally filled with fuel and the upper chamber  40  is approximately half filled with fuel during steady state engine operating conditions. 
     Air enters into the upper chamber  40 , through a choke bore  44  which communicates with the air inlet portion  16  of the mixing passage  14  at the downstream side of the choke plate  18  and upstream from the protruding upper end portion  38  of the main tube  34 , shown in FIG.  1 . In operation, the upper chamber  40  is slightly below atmospheric pressure and fuel and air flows from the upper chamber  40  into the bore  32  through a plurality of transverse holes  46  which penetrate the wall of the main tube  34  near the lower end  42 . An overly rich fuel-to-air mixture flows through the bore  32  and into the mixing passage  14  to mix with additional air. In operation, because the fuel bore  32  is below atmospheric pressure, the combination of choke bore  44 , upper chamber  40  and plurality of holes  46  function together (as a fuel pump) to cause fuel to flow from the fuel chamber  24  into the mixing passage  14  for mixing with flowing air between the choke plate  18  and the throttle plate  20 . 
     During engine idle running conditions, fuel flows not via the “fuel pump” but from the lower portion of the bore  32  into an idle fuel feed tube  48  by a vacuum drawn from the intake manifold, not shown. Feed tube  48  extends transversely across the mixing passage  14  between the choke and throttle plates  18 ,  20  and generally longitudinally into the main tube  34  through the upper end  38 . A distal or intake nozzle end  50  of feed tube  48  terminates slightly above the flared lower end  42  of the main tube  34 . 
     Referring to FIGS. 2-4, turning off the ignition of the running engine causes a fuel shut-off solenoid device  52  to isolate fuel flow from the lower chamber  30  into the enriched-fuel bore  32 , preventing engine dieseling and after boom. The solenoid device  52  mounts to carburetor body  12  from beneath and has a shaft  54  which moves vertically from an energized or retracted position  56  (shown in FIG. 2) to a de-energized or extended position  58  (shown in FIG. 1) into the lower chamber  26 . A mid portion of the shaft  54  moves transversely through an outward face  60  of an encasement  62  of the solenoid device  52 . In assembly, the outward face  60  defines the bottom of the lower chamber  26 , and the encasement  62  defines a solenoid chamber  64  which houses a substantial portion of the shaft  54 . An electrical coil  66  is encased within the encasement  62  and winds about the solenoid chamber  64 . 
     When the electrical coil  66  is energized, the shaft  54  is moved to and retained in the retracted position  56  and fuel is free to flow from the lower chamber  26  to the enriched-fuel bore  32 . However, when the electrical coil  66  is de-energized the shaft  54  is moved to and retained in the extended position  58 . When extended, a head  68  of at a distal end of the shaft  54  engages a downward facing mating surface  70  formed by the radial flaring of the lower end  42  of the main tube or nozzle  34 . The nozzle end  50  of the idle fuel feed tube  48  is suspended slightly above the head  68 . Therefore, fuel flow is not completely isolated from the idle fuel feed tube  48  when the head  68  engages the mating surface  70 . Of course, if tolerances can be achieved within a reasonable manufacturing cost, it is preferable to seal off the nozzle end  50  in addition to the main tube  34  utilizing the head  68 . 
     Fuel migrates from the lower chamber  26  into the solenoid chamber  64  through a clearance  72  defined radially between an inner brim  74  of the outward face  60  of the encasement  62  and a cylindrical surface  75  of the shaft  54 . The fuel within the solenoid chamber  64  cools the constantly energized solenoid device  52  of a running engine. 
     The head  68  of the shaft  54  flares laterally outward thereby forming a trailing face  76 . The trailing face  76  is preferably annular and is defined radially between an inner perimeter edge  78  which is congruent to the cylindrical surface  75  of the shaft  54  and a peripheral edge  80  of the radially enlarged head  68 . Preferably, the trailing face  76  is substantially parallel to the outward face  60  of the encasement  62 . When shaft  54  is in retracted position  56 , the trailing face  76  is interconnected sealably to the outward face  60  to prevent the release of vaporized fuel or bubbles from the solenoid chamber  64  into the lower chamber  26 . 
     Referring to FIGS. 3,  5  and  6 , when in use heat generated by the electrical coil  66  within the solenoid  52  creates vapor bubbles within the solenoid chamber  64 . Without a sealing engagement between the head  68  and the encasement  62  of the solenoid  52 , large bubbles would be emitted through the clearance  72  and into the lower chamber  26  causing rough idle or light load conditions of the running engine. To complete the sealing engagement, preferably a washer  82  is utilized about the shaft  54  between the head  68  and the outward face  60  of the encasement  62 . The washer  82  has an inner perimeter edge  84  which is slightly larger than the inner perimeter edge  78  of the shaft  54 . This permits the washer  82  to move freely up and down the shaft  54  without interfering with the extending and retracting movement of the shaft  54 . The inner perimeter edge  84  however is smaller than the peripheral edge  80  of the head  68 . Therefore, when the shaft  54  is in the retracted position  56  the trailing face  76  mates with the upward surface of the washer  82 , and the lower surface of the washer  82  mates with the outward face  60  of the encasement  62 . In short, preferably the diameter of the hole  86  of the washer  82  is larger than the diameter of the shaft  54  and smaller than the outside diameter of the face  76  of the head  68 . 
     Preferably, the head  68  is an elastomer grommet, and the washer  82  is of a non-corrosive material having a low heat capacity such as plastic and provides a seal with the face  60  of the encasement  62 . 
     In one embodiment of the invention, utilizing a fuel cut-off solenoid valve manufactured by Bicron, Inc. (Walbro Engine Corporation part number 76-521) and utilizing a Walbro Engine Corporation Carburetor Assembly part number LMK-106, a central hole  86  defined by the inner perimeter edge  84  of the washer  82  has a diameter  88  equal to 0.136 plus or minus 0.005 inches. An outer diameter  90  of the washer  82  is equal to 0.300 plus or minus 0.005 inches, and the thickness length  92  of the washer  82  is 0.031 plus or minus 0.003 inches. The washer is made of plastic. 
     While the forms of the invention herein disclosed constitute a presently preferred embodiment many others are possible. For instance, the trailing face  76  of the head  68  or elastomer grommet can seal directly to the outward face  60  of the solenoid  52  thereby eliminating the need for the washer  82 . Regardless, it is not intended herein to mention all the possible equivalent forms or ramifications of the invention. It is further understood that the terms used herein are merely descriptive rather than limiting, in that various changes may be made without departing from the spirit or scope of this invention.