Patent Publication Number: US-2015069641-A1

Title: Carburetor for air scavenged engine

Description:
REFERENCE TO CO-PENDING APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/876,504 filed Sep. 11, 2013, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to a carburetor for an air scavenged engine. 
     BACKGROUND 
     A carburetor is used to provide a combustible charge or mixture of fuel and air to an internal combustion engine. The carburetor meters liquid fuel for mixing with air to adjust a fuel-to-air ratio, according to varying engine requirements during engine startup, idle, steady-state operation, and changes in load and altitude. 
     A diaphragm-type carburetor is typically used with small two-stroke internal combustion engines commonly used in hand-held power tools such as chain saws, weed trimmers, leaf blowers, and the like. In the diaphragm carburetor, a body defines a mixing passage with an air inlet and a downstream fuel-and-air mixture outlet. A throttle valve is disposed in the fuel-and-air mixing passage downstream of the air inlet for controlling delivery of a primary fuel-and-air mixture to the engine. 
     A scavenging-type of diaphragm carburetor may be used with some engines to reduce scavenging losses or blow-through of some of the fuel-and-air mixture out of engine exhaust ports. A scavenging carburetor is known to have a fuel-and-air mixture passage and a separate scavenging air passage that both communicate at one end of the carburetor with a clean air source at atmospheric pressure, such as an air filter. 
     SUMMARY 
     A fuel and air supply device for an engine includes at least one carburetor body having a main bore and an air passage and an air valve operably associated with the air passage to control air flow through the air passage. In at least some implementations, the air passage has a portion with a reduced dimension compared to a different portion of the passage. This may facilitate, among other things, a varying external size of the device and facilitate its use within constrained spaces. 
     A fuel and air supply device for an engine may include at least one carburetor body having a main bore and an air passage and an air valve. The air passage may have an inlet end through which air enters the air passage and an outlet end from which air exits the air passage. And in at least some implementations, the air valve is carried at least partially within the air passage to control air flow through the air passage. Also in at least some implementations, the air passage has a portion with a reduced dimension compared to a different portion of the passage. The air passage may also have a first air flow area that is defined in the area of the air valve and is determined by deducting the size of the obstruction caused by the air valve from the flow area of the air passage, and the minimum air flow area in the air passage downstream of the air valve is at least equal to the first air flow area. The air passage may further be noncircular downstream of the air valve to reduce the height of a portion of the air passage without reducing the air flow area of the air passage downstream of the air valve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description of preferred embodiments and best mode will be set forth with reference to the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view of a portion of a carburetor showing a main bore and an air passage; 
         FIG. 2  is a fragmentary end view of the carburetor showing an inlet side of the air passage; and 
         FIG. 3  is a fragmentary end view of the carburetor showing an outlet side of the air passage. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring in more detail to the drawings,  FIGS. 1-3  show a carburetor  10  that may be used to supply a fuel and air mixture to an engine to support combustion in the engine. The carburetor  10  as shown is a diaphragm type carburetor having a main body  11  with a main bore  12  through which a fuel and air mixture is delivered to the engine and an air passage  14  through which air is supplied to the engine. In at least one implementation, the air supplied to the engine from the air passage  14  may be used to scavenge or help remove exhaust gasses from the engine. The flow of the fuel and air mixture through the main bore  12  may be controlled, at least in part, by a throttle valve  16  and optionally a choke valve  17 . And the flow of air through the air passage  14  may be controlled, at least in part, by an air valve  18  operably associated with the air passage. This type of carburetor  10  may be referred to as a stratified scavenging carburetor, or an air scavenging carburetor. The carburetor  10  may be constructed and may operate as disclosed in U.S. Pat. No. 6,896,245 issued on May 24, 2005, the disclosure of which is incorporated herein by reference in its entirety. 
     In at least certain implementations, the main bore  12  may be formed in the main carburetor body  11 . The air passage  14  may be formed in a separate scavenging air body  19  which may be formed separately from the main body  11  and attached thereto, such as by one or screws or in some other way. The air body  19  may also be formed from the same piece of material as the main body  11 , if desired. When formed separately, a carburetor component, such as a fuel pump assembly  21 , may be located between the main bore  12  and the air passage. In the embodiment shown in  FIG. 1 , the fuel pump assembly  21  includes a diaphragm and a sealing gasket (both denoted by reference number  23  in the figures) located between the main body  11  and the air body  19 . The air valve  18  may be carried by the air body  19  and the throttle valve  16  may be carried by the main body  11 . 
     As shown in  FIG. 3 , the throttle valve  16  may include an actuating lever  20  coupled to a throttle valve shaft  22  to rotate the throttle valve shaft  22 , and a throttle valve head  26  may be carried on the valve shaft  22  and within the main bore  12 . The throttle valve shaft  22  may be rotatably received in a bore that intersects the main bore  12 . The throttle valve head  26 , in the implementation shown, is a thin disc or butterfly-type valve head. The valve head  26  may be rotated from a first position closing or substantially closing the main bore  12  to a second position wherein the valve head  26  may be fully opened as shown in  FIG. 1 . In at least some implementations, the valve head  26  when in its second position may be parallel, or nearly parallel, to fluid flow through the main bore  12 . The throttle valve  16  could be of a different type and construction, as desired. By way of one non-limiting example, the throttle valve  16  could be a rotary throttle valve such as is shown in U.S. Pat. No. 6,394,425 or a different type of valve. 
     In the implementation shown, the air valve  18  is constructed similarly to the throttle valve  16 . In this way, the air valve  18  may have an actuating lever  30  coupled to an air valve shaft  32  to rotate the air valve shaft  32 , and an air valve head  34  may be carried on the air valve shaft  32  and at least partially within the air passage  14 . The air valve shaft  32  may be rotatably received in a bore  36  that intersects the air passage  14 . The air valve shaft  32  may be oriented at the same angle as the throttle valve shaft  22 , if desired, to facilitate coupling of the valves  16 ,  18  and actuation of the air valve  18  by the throttle valve  16 . Due to the angle needed for the air valve shaft  32  in at least certain implementations, it may be desirable to offset the shaft  32  relative to the air passage  14  to provide clearance for one or more other carburetor components. In the example shown, the fuel pump is disposed between the main bore  12  and air passage  14  and the axis of the air valve shaft  32  is offset from a central axis  35  (shown in  FIG. 1 ) of the air passage (here it is shown as being raised relative to a central axis) to provide clearance for the fuel pump. 
     The air valve head  34 , in the implementation shown, is a thin disc or butterfly-type valve head. The air valve head  34  is connected to the shaft  32  in a position so that it may be rotated from a first position closing or substantially closing the air passage to a second position wherein the air valve head may be fully opened (in at least some implementations, the air valve head when in its second position may be parallel, or nearly parallel, to fluid flow through the air passage as generally shown in  FIG. 1 ). 
     In more detail, and as shown in  FIG. 1 , the air valve shaft  32  may be generally cylindrical and include a flat section  38  extending in the area of the air passage  14 . The air valve head  34  may be connected to the flat section  38  of the air valve shaft  32 , such as by a screw  40 , adhesive, weld or other connector or connection feature. The air valve head  34 , in at least some implementations, may be sized to fully or at least substantially close the air passage  14 . Accordingly, when the air valve  18  is in its first position, usually associated with idle engine operation, the air valve fully or substantially prevents air flow through the air passage  14 . This substantially limits or prevents the flow of air through the air passage  14  and to the engine to avoid leaning out the fuel and air mixture delivered to the engine during certain engine operating conditions, such as engine starting or idle engine operation. 
     The air passage  14  has a portion with a reduced dimension compared to a different portion of the air passage  14 . In at least some implementations, a portion of the air passage  14  has a first dimension in a first plane  42  perpendicular to the air flow direction in the air passage  14  and a different portion of the air passage has a smaller dimension taken in a second plane  43  spaced from and parallel to the first plane  42 . The air passage  14  may have at least a portion with a noncircular shape in cross-section taken in a plane (e.g. second plane  43 ) perpendicular to the direction of air flow through the air passage. 
     In the nonlimiting implementation shown, the air passage  14  has an inlet end  44  through which air from a filter enters the air passage and an outlet end  46  from which air exits the air passage and is delivered to the engine. In this implementation, the air passage  14  is noncircular downstream of the air valve  18  to reduce the height of a portion of the air body  19  without reducing the air flow area of the air passage downstream of the air valve. In the example shown, the air passage  14  is oblong or generally oval in shape downstream of the air valve  18 , and circular in the area of and upstream of the air valve. The circular inlet section  48  works well with a circular air valve head  34  which may be easier to manufacture at least when no or only very limited air flow is desired when the air valve is in its first, closed position. Downstream of the inlet section  48 , the air passage  14  tapers down in height (e.g. dimension perpendicular to air flow in the air passage) and could optionally become wider in the dimension parallel to air flow although it need not become wider. Where the air passage  14  does not become wider along its length it may be possible to form the passage with a single core that is pulled from the carburetor from the inlet side during manufacturing of the air body  19 . 
     In at least some implementations, the flow area in the air passage  14  downstream of the air valve  18  is the same as or increased relative to the flow area of the inlet section  48  so that the flow area of the air passage downstream of the air valve does not create any significant restriction or resistance to air flow compared to the inlet section  48 . That is, the area of the air passage  14  including the air valve  18  provides the same or a greater restriction than the portion of the air passage downstream of the air valve. The portion of the air passage  14  downstream of the air valve  18  may have a lesser cross-sectional area (perpendicular to air flow) than the inlet section  48 , in at least certain implementations. Of course, the air passage downstream of the air valve  18  may have a lesser flow area than the maximum flow area of the inlet section so that the downstream portion can cause a restriction to air flow therethrough, which may be desirable in certain applications. 
     In an engine that operates at or near WOT, the obstruction provided by the air valve  18 , including the valve shaft  32 , valve head  34 , and any fastener  40  connecting the two, can be calculated as the surface area of those components perpendicular to the air flow through the air passage  14 . The available flow area for air to pass through the air passage  14  and around the air valve  18  may be defined as a first flow area and may be determined as the difference between the total surface area of the air passage and the area of the obstructions in the air passage (e.g. the area of the air valve). The maximum flow area in the inlet section  48  occurs when there is a minimum restriction (e.g. minimum surface area obstruction) provided by the air valve  18 . This may occur when the air valve  18  is fully opened and the valve head  34  is parallel (or nearly so) to the direction of air flow. Then, in designing the air passage  14 , the portion downstream of the air valve  18  can be made to have a minimum flow area that is at least the same as the maximum flow area in the inlet section. 
     In at least some implementations, the height of the air passage  14  at its outlet end  46  may be up to about 85% less than the height at the inlet end  44 . In one of many presently preferred embodiments, the height of the inlet end  44  of the air passage  14  is 16 mm and the height at the outlet end  46  of the air passage is 13.5 mm, with a generally smooth transition between the heights provided at least in part by a sloped or tapered upper wall  50  of the air passage, as shown in  FIG. 1 . The smooth transition may prevent undue disruption of the air flow. The lower wall  52  of the air passage  14 , which is positioned adjacent to the main carburetor body  11 , may be generally uniform in thickness. Although, in the implementation shown in  FIGS. 1-3 , there is an increasing thickness of the lower wall  52  providing a more gradual slope that also reduces the height of the air passage  14 . 
     Hence, the reduction in height of the air passage  14  need not be uniform along the length of the air passage, need not be consistent along the length, and need not be the same between the upper and lower walls  50 ,  52  of the air passage. Likewise, the side walls  54  ( FIG. 3 ), between and joining the upper and lower walls  50 , 52 , need not flare outwardly at all, but if they do, they need not do so to the same extent or at the same rate so that opposite sides may take a different shape along at least a portion of the length of the air passage. With a round or generally round air passage  14 , the side walls  54 , and upper and lower walls  50 ,  52  will blend into one another and might not appear as discrete portions of the air passage. Regardless, in at least certain implementations, the air passage shape changes downstream of the inlet section to alter a dimension of the air passage as desired. The above descriptions have focused on reducing the height of the air passage  14 , but the width could also be reduced, if desired. In at least some implementations, a dimension of the air passage  14  at the outlet  46  is less than the corresponding dimension of the air passage at the inlet  44 . For example, in some carburetors a portion of the air passage  14  is circular in cross-section taken perpendicular to the direction of air flow through the air passage and a different portion of the air passage has a dimension that is less than the diameter of the circular portion. Further, where the air passage  14  is circular at the inlet  44 , a portion of the outlet  46  of the air passage may have a dimension that is less than the diameter of the inlet  44 . 
     Given that the walls defining the air passage  14  will have certain minimum thickness requirements, reducing the dimension of a part of the air passage will permit the air body  19  to have a reduced size at least in the area corresponding the reduced dimension portion of the air passage. Thus, changing the shape of the air passage  14  may permit the carburetor to fit into confined spaces on and relative to an engine. This provides greater design freedom and also permits different carburetors to be used on existing engines within existing space constraints. Where the air body  19  is separately formed from and later attached to the main carburetor body  11 , as shown in the drawings, different air valve bodies may be mounted to the same main body  11  to permit that main body and its components (e.g. fuel metering assembly, fuel pump assembly, throttle valve, etc) to be used on different engines. When the minimum air flow area of the air passage  14  downstream of the air valve is at least as large as the maximum air flow area of the inlet section  48 , then no significant reduction in air flow should occur due to the changed air passage shape downstream of the inlet section and the carburetor may readily be used on an engine without having to reconfigure or recalibrate other components or the engine. 
     While the change in shape of the air passage is shown and described as being gradual via tapered walls, the change in the air passage may be accomplished in other ways, including one or more discrete steps or by any other way (including going from a passage bounded by round or generally round walls to a polygon shaped passage, or the like). Also, the air valve shaft may be offset relative to the air passage and need not extend through the center of the passage or its widest portion. For example, the air valve shaft may be closer to one of the air passage upper and lower walls  50 , 52  than the other, as desired. 
     While the forms of the invention herein disclosed constitute presently preferred embodiments, many others are possible. It is not intended herein to mention all the possible equivalent forms or ramifications of the invention. It is understood that the terms used herein are merely descriptive, rather than limiting, and that various changes may be made without departing from the spirit or scope of the invention.