Abstract:
In a vent control valve for a fuel tank, a housing receiving a float assembly therein is provided with a communication hole passed across a wall of the housing, and an opening of the communication hole facing an interior of the housing is directly exposed to an interior of the housing when the float assembly is in a low position and overlaps with a peripheral wall of the float assembly when the float assembly is in a high position. By suitably selecting the size of the communication hole and/or the number thereof, a desired delay in the dropping of the liquid fuel level in the housing following the filling up of the fuel tank can be achieved. Thereby, overfilling of the fuel tank can be avoided.

Description:
TECHNICAL FIELD 
     The present invention relates to a vent control valve for a fuel tank, in particular to a vent control valve that allows fuel vapor in a fuel tank to be properly vented out of the fuel tank by an appropriate flow rate and still avoids the fuel tank to be overfilled even when the vertical dimension of the vent control valve is relatively small. 
     BACKGROUND OF THE INVENTION 
     Gasoline is one of most commonly used fuels for motor vehicles. Gasoline has a relative low vapor pressure under normal condition, and vaporizes to a significant extent in the fuel tank depending on the temperature and pressure of the gasoline within the fuel tank. The fuel vapor is required to be vented in order to control the internal pressure of the fuel tank within a prescribed level, and the vented gasoline vapor is recovered by a canister filled with charcoal or other absorbent substance in order to avoid polluting the atmosphere. The fuel vapor captured in the canister is conducted to the intake system of the engine to be burned in the engine. 
     The canister is connected to the fuel tank via a pipe, and the fuel tank end of the pipe is connected to an upper part of the fuel tank to avoid liquid fuel from being forwarded to the canister. However, the vehicle is often subjected to violent motions due to irregular road surfaces, and the accelerations/decelerations and cornering movements of the vehicle, and may be parked on a slanted road surface. Therefore, the fuel tank end of the pipe leading to the canister is provided with a cutoff valve which is provided with a float valve configured to close the communication to the canister when the liquid fuel level rises beyond a certain level. 
     The fuel tank is additionally provided with an arrangement for preventing the overfilling the fuel tank that includes a float valve. This float valve closes when the liquid fuel level reaches a certain level, and causes the internal pressure of the fuel tank to rise. This causes the liquid fuel to rise in the filler pipe, and this in turn causes the fill limit sensor of the fuel pump nozzle to be activated. This arrangement controls excessive filling of fuel in the fuel tank, and ensures an adequate gas phase space within the gas tank. It has been proposed to combine the float valve for controlling the communication with the canister and the float valve for preventing the overfilling of the fuel tank into a single integrated valve. See Japanese patent No 3931291 (patent document 1) and Japanese patent 3948194 (patent document 2), for instance. 
     In the float valve disclosed in patent document 1, a small opening is formed in a case (housing) receiving a float therein to communicate the interior of the fuel tank with a pipe leading to a canister. This small opening is provided at a height higher than the liquid fuel level when the fuel tank is filled up so that even when the lower opening of the housing is closed by the liquid fuel, the fuel vapor is allowed to be vented to the canister. 
     In addition to the small opening, the housing of the float valve is provided with a skirt extending vertically in a lower part thereof. Thereby, when the liquid fuel level exceeds the level of the lower opening of the skirt, an increase in the internal pressure of the fuel tank causes the fuel liquid level within the housing (or skirt) to rise higher than the liquid fuel level of the remaining part of the fuel tank, and this in turn causes the float to rise. Once the float has risen high enough to close the port communicating with the canister, the internal pressure of the fuel tank rises sharply, and this activates the fill limit switch of the fuel pump nozzle. Thereafter, the fuel vapor is gradually introduced into the housing via the small opening, and this causes the pressure in the housing to drop. This in turn causes the float to drop and the communication with the canister to be established again. If this occurs too soon, the liquid fuel level in the filler pipe may drop too quickly and the fill limit sensor of the fuel nozzle to be released depending on the way the fuel is filled so that overfilling of the fuel tank could occur. 
     The problem of overfilling can be avoided by reducing the opening area of the small opening in the housing as this would delay the dropping of the liquid fuel level in the housing (hence the opening of the port leading to the canister). However, to ensure fuel vapor to be forwarded to the canister at an adequate flow rate, the opening area of the small opening is required to be reasonably large. If the opening area of the small opening is too small, the pressure inside the housing can be readily reduced, and the resulting rise in the liquid fuel level in the housing causes the float valve to close with the result that the fuel vapor is prevented from being vented to the canister at an adequate flow rate. 
     Also, there is an increasing demand to reduce the height of the fuel tank for the purpose of efficiently utilizing the space available in a vehicle, and this in turn requires the height of the float valve to be minimized. This means a reduced height for the skirt, and this increases the sensitivity of the float valve to the change in the liquid fuel level in the housing. In other words, it becomes more difficult to ensure proper venting of fuel vapor and to prevent overfilling of the fuel tank at the same time. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of such problems of the prior art, a primary object of the present invention is to provide a vent control valve for a fuel tank that has a relatively short skirt, but can adequately delay the fall of the liquid fuel level in the skirt after a filled up state of the fuel tank is detected. 
     A second object of the present invention is to provide a vent control valve for a fuel tank that can ensure fuel vapor to be properly vented out of the fuel tank but can prevent overfilling of the fuel tank in an effective manner. 
     According to the present invention, such an object can be at least partly accomplished by providing a vent control valve for a fuel tank, comprising: a cylindrical housing extending vertically, and configured to be attached to an upper wall of a fuel tank, the housing having an open lower end and a closed upper end defining a port communicating an interior of the housing with an external part of the fuel tank; a float assembly received in the housing and guided for a vertical movement therein, defining a prescribed gap between an outer profile of the float assembly and an inner circumferential wall of the housing; and a valve member provided in an upper part of the float assembly to close the port of the cylindrical housing when the float assembly has floated to a prescribed height; wherein the housing is provided with a communication hole passed across a wall of the housing, an opening of the communication hole facing an interior of the housing being directly exposed to an interior of the housing when the float assembly is in a low position and overlapping with a peripheral wall of the float assembly when the float assembly is in a high position. 
     By suitably selecting the size of the communication hole and/or the number thereof, a desired delay in the dropping of the liquid fuel level in the housing following the filling up of the fuel tank can be achieved. Thereby, overfilling of the fuel tank can be avoided. As the float assembly drops to such an extent as to clear the communication hole, the fuel vapor is allowed to flow more freely into the interior of the housing via the communication hole, and this causes an increase in the speed of the float to drop. Thereby, the venting of the fuel tank can be resumed at an appropriately earlier stage following the filled up state of the fuel tank. Thus, the closing and opening of the communication with the exterior of the fuel tank can be accomplished even when the vertical dimension of the housing is relatively small. A proper selection of a gap between an outer surface of the float and a wall surface at which the communication hole opens out contributes to the proper timing of the closing and opening the communication with the exterior of the fuel tank. 
     According to a preferred embodiment of the present invention, a plurality of vertically extending ribs are provided on an inner circumferential surface of the housing to guide the float assembly for a vertical movement. These ribs allow the vertical movement of the float assembly to be effected in a smooth manner. In particular, if at least one of the ribs is provided with a laterally enlarged portion, and the communication hole is passed across a thickness of the lateral enlarged portion, the inner surface of the housing at which the communication hole opens out can be brought close to the opposing surface of the float assembly so that a favorable control of the communication by the communication hole can be achieved. 
     The float assembly may comprise a lower float and an upper float connected to each other so as to permit a small displacement between them, and the valve member is provided on the upper end of the upper float. The float assembly may further comprise a middle float which is connected to the lower float so as to permit a small displacement between them, and the valve member comprise a rubber seal having a central hole passed vertically across the rubber seal, the rubber seal including an annular upper part configured to cooperate with a peripheral part of the port and a lower lip seal configured to close the central hole in cooperation with an upper surface of the middle float. 
     According to a particularly preferred embodiment of the present invention, an upper part of the lower float is formed with a reduced diameter portion and a radial flange formed at an upper end of the reduced diameter portion, and the upper float is provided with an upper horizontal wall and a peripheral wall depending from a peripheral part of the upper horizontal wall, a lower edge of the peripheral wall being provided with an engagement portion engaging the radial flange of the lower float. 
     The upper horizontal wall of the upper float may be provided with a central opening having the rubber seal fitted therein. The middle float may comprise an upper horizontal wall and a plurality of legs depending from the upper horizontal wall and received in openings formed in the radial flange of the lower float so as to guide a vertical movement of the middle float relative to the lower float. 
     For an optimum control of the buoyancy of the flow float, a compression spring may be interposed between the lower float and a shoulder piece extending radially inwardly from an inner circumferential surface of the housing. 
     If the cylindrical housing is provided with two or more communication holes at a regular angular interval, the effective cross sectional area of the communication holes can be kept substantially constant even when there is a gap between the inner circumference of the cylindrical housing and the outer circumference of the float assembly and the float assembly shifts laterally. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Now the present invention is described in the following with reference to the appended drawings, in which: 
         FIG. 1  is a perspective view of a vent control valve for a fuel tank embodying the present invention; 
         FIG. 2  is a vertical sectional view taken along line A-A of  FIG. 1 ; 
         FIG. 3  is an exploded vertical sectional view of the vent control valve; 
         FIG. 4  is an enlarged fragmentary perspective view of a part of  FIG. 3  indicated by arrow X in  FIG. 3 ; 
         FIG. 5  is a view similar to  FIG. 2  when the liquid fuel level is lower than the lower edge of a skirt of the vent control valve; 
         FIG. 6  is a view similar to  FIG. 5  when the liquid fuel level in the skirt is high enough for a float valve to close a port communicating the interior of the fuel tank with a canister; 
         FIG. 7  is an enlarged fragmentary view of a part of  FIG. 6  indicated by circle B; 
         FIG. 8  is a view similar to  FIG. 5  when the liquid fuel level in the skirt is dropping but the port communicating the interior of the fuel tank with a canister is still kept closed; and 
         FIG. 9  is a view similar to  FIG. 2  showing a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1 to 3 , the vent control valve  10  is attached to an upper wall of a fuel tank  60 , and comprises an upper body  20  and a lower body. The upper body  20  is provided with a vent passage  21  internally defined therein and communicating with a charcoal canister external to the fuel tank  60  via a pipe not shown in the drawings, and a flange  22  for connecting the upper body  20  to the a fuel tank  60 . The lower body is generally cylindrical in shape, and comprises a valve case  23  fitted into a lower cavity of the upper body  20  and a skirt  50  fitted onto the lower end of the valve case  23  and forming a lower extension of the valve case  23 . The skirt  50  has a lower edge  503  defining a horizontal plane. The lower body is referred to as a housing in the following description. 
     The vent passage  21  of the upper body  20  includes a horizontal section having an open outer end and an inner end communicating with the lower cavity of the upper body  20 . The upper body  20  is integrally molded by plastic material. The flange  22  may be attached to the fuel tank  60  by welding, bonding or other means that ensures a gas tight and mechanically secure connection. 
     The valve case  23  is cylindrical in shape and has a horizontal upper wall  232  or has the shape of an inverted cup. The upper wall  232  of the valve case  23  is formed with a central opening  231  communicating the interior of the valve case  23  with the cavity of the upper body  20 . The upper end of the valve case  23  is fitted into a lower opening of the upper body  20 , and is welded, bonded or mechanically attached thereto, and the lower end of the valve case  23  is connected to the skirt  50  which opens out toward the interior of the fuel tank  60 . 
     As best shown in  FIG. 4 , the inner circumferential wall of the valve case  23  is provided with a plurality of vertical ribs  233  extending substantially over the entire height of the valve case  23  and arranged at a regular interval along the circumference of the valve case  23 . These ribs  233  provide a guide for a float assembly for a smooth vertical movement thereof as will be described hereinafter. The upper end of one of the ribs  233  is laterally enlarged, and this enlarged portion  234  is formed with a communication hole  24  communicating the interior of the valve case  23  with the exterior thereof or the interior of the fuel tank  60 . The inner surface  235  of the enlarged portion  234  may be a part of a cylindrical surface concentric to the valve case  23 , or may be planar. Only one communication hole  24  is provided in the illustrated embodiment, but two or more communication holes  24  may also be formed (along with so many enlarged portions  234 ). 
     The valve case  23  receives a float assembly therein, and the float assembly comprises, from top to bottom, an upper float  26  centrally fitted with a seal rubber  25  at an upper end thereof, a middle float  27  and a lower float  28  in that order. 
     As shown in  FIGS. 2 and 3 , the upper float  26  has the shape of an inverted cup including a substantially horizontal upper wall  263  and a peripheral wall  262  depending from the peripheral edge of the upper wall  263 , and the upper wall  263  is formed with a central primary opening  261  and a plurality of secondary openings  264  arranged around the central primary opening  261 . The lower end of the peripheral wall  262  of the upper float  26  is provided with a radially inwardly directed annular bead  265 . The upper end of the lower float  28  is formed as a reduced diameter portion  281 , and an upper end of the reduced diameter portion  281  is provided with a radial flange  285  which is received within the upper float  28 , and is retained thereby the annular bead  265 . Thus, the radial flange  285  may be forced into the upper float  26  but prevented from being pulled out of the upper float  26  during normal use. The peripheral wall  262  of the upper float  26  is given with an outer diameter so as to define a prescribed gap t to the opposing wall of the enlarged portion  234 . 
     The seal rubber  25  has the shape of an inverted hat, and includes a central thick portion  252  formed with a vertically extending through hole  251  and a peripheral annular groove  253  on the peripheral side wall thereof, and an annular upper part in the form of an annular disk  255  extending horizontally from an upper part of the central thick portion  252 . The central thick portion  252  is fitted into the central opening  261  of the upper float  26 , and the surrounding edge of the upper wall of the upper float  26  is resiliently fitted into the peripheral annular groove  253  of the seal rubber  25 . The upper surface of the peripheral annular disk  255  of the seal rubber  25  defines an annular seal surface  254  that cooperates with an annular bead formed around the communication hole  231  in the upper wall of the valve case  23 . 
     The middle float  27  essentially consists of a disk member having a plurality of legs depending from the outer periphery thereof at a regular interval along the circumference thereof. Each of the legs fits into a corresponding opening formed in the upper flange of the lower float  28  for a vertical guided movement, and is provided with a claw that allows the leg to be forced into the opening but prevents the leg to be pulled out of the opening during normal use. The middle float  27  is provided with a plurality of openings extending across the thickness thereof. The upper surface of the middle float  27  is planar, and is configure to cooperate with ,a lip seal  252   a  formed in the lower end of the central thick portion  252  of the rubber seal  25 . When the middle float  27  is pushed against the lip seal  252   a , the through hole  251  is closed. 
     The lower float  28  comprises a cylindrical main body  284  having a central bore  282  extending substantially over the entire height thereof. The reduced diameter portion  281  provided with the flange  285  extends from the upper end of the lower float  28 . An annular slot  286  is formed concentrically in the wall of the lower float  28 , and extends from the lower end of the cylindrical main body  284  into a certain depth which is short of the upper end of the cylindrical main body  284 . A compression coil spring  29  is received in the annular slot  286 . A plurality of shoulder pieces  501  extend radially inward from the inner circumferential surface  502  of the skirt  50 . The compression coil spring  29  is interposed between these shoulder pieces  501  and the upper end of the annular slot  286 , and resiliently urges the lower float  28  upward. The spring force of this compression spring  29  assists the buoyant force of the lower float  28  such that the float valve can be closed only when the liquid fuel level has exceeded a prescribed level, and the spring force alone is not adequate to close the float valve. 
     The upper float  26  is connected to the lower float  28  so as to be moveable axially relative each other or toward and away from each other by a small stroke. The bead  265  formed in the lower end of the peripheral wall  262  of the upper float  26  allows the upper flange  285  of the lower float  28  to be forced into the interior of the upper float  26 , but keeps the upper flange  285  of the lower float  28  to be retained within the upper float  26 . The middle float  27  is interposed between the upper float  26  and lower float  28 , and the legs thereof are loosely fitted into the openings of the upper flange  285  of the lower float  28  so that the middle float  27  is axially moveable by a certain stroke within the space defined between the upper float  26  and lower float  28 . 
     The skirt  50  is cylindrical in shape, and has a lower edge on a common horizontal plane. The upper end of the skirt  50  is connected to the valve case  23  via a mechanical engagement in the illustrated embodiment, but welding or bonding may also be used for the same purpose. 
     The mode of operation of this vent control valve is described in the following with reference to  FIGS. 5 to 8 . 
       FIG. 5  shows the situation where the liquid fuel level  70  in the fuel tank  60  is lower than the lower edge  503  of the skirt  50 . The fuel vapor that may be present in the fuel tank  60  is allowed to enter the interior of the housing (valve casing  23  and skirt  50 ) from the lower end of the skirt  50 , and to pass into the vent passage  21  via the annular space defined between the lower float and opposing inner circumferential surface of the housing and the central opening  231  of the valve case  23 . The ribs  233  center the lower float  25  in the valve case  23 , and ensure the annular space to be defined between them. The communication hole  24  is also available for this flow of fuel vapor, but owing to a limited cross sectional area it provides, contributes very little in venting the fuel vapor in this situation. At this time, the pressure P 1  in the fuel tank, pressure P 2  in the valve case  23  and pressure P 3  in the cavity of the upper body  20  are related such that P 1 &gt;P 2 &gt;P 3 . 
       FIG. 6  shows the situation where the liquid fuel level  70  in the fuel tank  60  has reached the lower edge  503  of the skirt  50 , and the internal pressure of the fuel tank  60  has pushed the liquid fuel level in the housing to such a high level that the upper float  26  and middle float  27  are pushed upward by the lower float  28  and the rubber seal  25  is pushed against the valve seat of the central opening  231  of the valve case  23 . At the same time, the middle float  27  is pushed against the lower lip of the rubber seal  25 . As best shown in  FIG. 7  showing the details of the area surrounding the communication hole  24 , the outer circumferential surface of the peripheral wall  262  of the upper float  26  overlaps with the communication hole  24  formed in the valve case  23  at this time. As mentioned earlier, the prescribed gap t is formed between the enlarged portion  234  of the valve case  23  and opposing surface of the peripheral wall  262  of the upper float  26 . 
     As shown in  FIG. 8 , the fuel vapor within the fuel tank  60  is allowed to enter the interior of the valve case  23  via the communication hole  24  and the gap t, but this path is significantly constricted as opposed to the case where the liquid fuel level is lower as illustrated in  FIG. 5 . However, the pressure P 2  in the valve case  23  rises, and eventually becomes substantially equal to the pressure P 1  in the fuel tank  60 . This in turn causes the liquid fuel level  71  in the valve case  23  to drop toward the liquid fuel level of the fuel tank  60 . Even after the lower float  28  has dropped by a certain distance, the upper float  26  remains in the same position, and the rubber seal  25  continues to close the central opening  231 . The middle float  27  also remains in the same position, and continues to close the lower lip of the rubber seal  25 . 
     However, as the liquid fuel level in the housing drops further, the middle float  27  drops and opens the lower lip of the rubber seal  25 . This allows the interior of the housing to communicate with the canister (which is substantially at the atmospheric pressure), and the fuel vapor in the fuel tank  60  to be vented to the canister, however, at a limited flow rate, as illustrated in  FIG. 8 . In time, the lower float  28  drops even further, and ultimately pulls the rubber seal  25  away from the central opening  231  along with the upper float  26 , this fully equalizes the internal pressure of the housing with the remaining part of the fuel tank  60 . In short, the float assembly assumes the state illustrated in  FIG. 5 . 
     Thus, when the liquid fuel level in the fuel tank  60  has reached a tank full level, the communication with the canister is closed, and is reestablished with some time delay so that the overfilling of the fuel tank  60  can be avoided. A further drop of the lower float  28  causes the upper float  26  to drop clear of the communication hole  24 . This causes a sudden increase in the opening area for the communication between the interior of the housing and interior of the fuel tank  60 , and this regains the full capacity of the canister to absorb the fuel vapor. Therefore, the excessive rise in the internal pressure of the fuel tank can be avoided. 
     According to the illustrated embodiment, because the opening of the float valve following the filling of the fuel tank to a substantially full capacity takes place gradually and with a suitable time delay owing to the partial blockage of the communication hole, overfilling of the fuel tank can be avoided. And, owing to the subsequent prompt establishment of full communication between the fuel tank and canister achieved by the full exposure of the communication hole, proper venting of the fuel tank can be accomplished at the same time even when the vertical dimension of the housing or skirt is relatively small. 
     The communication hole  24  may also be formed in the housing at such a position that the lower float overlaps with the communication hole when the liquid fuel level in the housing has reached the high position. 
       FIG. 9  illustrates a second embodiment of the present invention. This embodiment differs from the previous embodiment in that the valve case  23  is provided with a pair of communication holes  24 , instead of one, at diametrically opposite positions or at a regular angular interval of 180 degrees. According to this embodiment, the effective cross sectional area of the communication holes  24  can be kept substantially constant even when there is a gap between the inner circumference of the valve case  23  (in particular the ribs  233  thereof) and the outer circumference of the upper float  26  and the upper float  26  shifts laterally as seen in  FIG. 9 . If the upper float  26  moves or shifts toward one of the communication holes  24 , the effective cross sectional area of the one communication hole  24  diminishes but that of the other communication hole  24  increases. It is also within the purview of the present invention that three or more communication holes  24  are provided in the valve case  23 , preferably at a regular angular interval. For instance, when three communication holes  24  are formed in the valve case  23 , they may be advantageously formed at an angular interval of 120 degrees, or, in other words, at diametrically opposing positions. 
     Although the present invention has been described in terms of a preferred embodiment thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims. 
     The contents of the original Japanese patent application on which the Paris Convention priority claim is made for the present application are incorporated in this application by reference.