Patent Publication Number: US-6659432-B2

Title: Carburetor arrangement

Description:
BACKGROUND OF THE INVENTION 
     Work apparatus having an internal combustion engine as a drive motor include a carburetor for generating an air/fuel mixture which is so adjusted that the air/fuel ratio is optimal for the running of the engine in the operationally warm condition. The work apparatus can include, for example, a motor-driven chain saw, a brushcutter, a cutoff machine or the like. For a cold start, however, a part of the fuel of the air/fuel mixture condenses on the cold walls of the engine and, as a consequence thereof, the air/fuel mixture is too lean. As a consequence, the engine does not start or starts only poorly. 
     A choke element is widely used for improving the cold start performance. With the choke element, the intake channel of the engine is partially closed in the region of the carburetor in such a manner that an enrichment of the air/fuel mixture occurs. 
     In a great many known carburetor configurations, a choke flap is integrated into the carburetor, which, however, requires a considerable constructive complexity. In other known configurations, a choke element is provided outside of the carburetor in the region of the intake opening in the air filter housing which lies upstream of the intake channel. This choke element, for example, can be pressed against the intake opening with a pivot arm. Here, a precise seating of the choke element on the intake opening is required for a precise reproducibility of the adjustment conditions for the cold start. The external arrangement of the choke element and especially its journaling, for example, on a wall of the forward-mounted air filter housing, can lead to tolerance problems. As a consequence of these tolerance problems, the choke element lies, in the closed state, with inadequate sealing in the region of the intake opening. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a carburetor arrangement with improved insensitivity to tolerances in the region of the choke element. 
     The carburetor arrangement of the invention is for an internal combustion engine including an engine of a work apparatus including a motor-driven chain saw, brushcutter, cutoff machine or the like. The carburetor arrangement includes: a carburetor defining an intake channel; an air filter housing including an air filter arranged upstream of the intake channel; the air filter housing having a base wall defining an intake opening communicating with the intake channel; and, a choke assembly including: a carrier; a choke element arranged on the carrier so as to permit the choke element to at least partially cover the intake opening; and, means for holding the choke element so as to permit limited relative movement thereof. 
     In the above, the choke element is held on a separate choke carrier. The mounting of the choke carrier is designed for limited movement. The limiting of the movability leads to a coarse alignment of the choke element with respect to the intake opening. The mobility itself makes possible an automatic adaptation of the position of the choke element relative to the intake opening. As a practical matter, the movable mounting is so configured that the choke element is laterally displaceable on the choke carrier in a direction, which lies in the plane of the intake opening, and is laterally displaceable preferably in desired directions lying in the above-mentioned plane. Position tolerances, which lie laterally and in elevation, can, in this way, be compensated in a simple manner without a costly highly precise manufacture being necessary. 
     In an advantageous further embodiment of the invention, the choke element is pivotally held about one and preferably about any number of axes lying in the plane of the intake opening. In this way, angle tolerances between the plane of the choke element and the plane of the intake opening are automatically compensated. Accordingly, the complexity of an excessively high-precision manufacture is unnecessary. 
     In addition to the compensation of manufacturing tolerances, the above mentioned embodiments also easily make possible a compensation of, for example, thermal expansions, even, under some circumstances considerable, thermal expansion of an air filter housing, which is made of plastic and on which the choke carrier is journalled with the choke element. 
     In a practical further embodiment of the invention, a guard to prevent rotation or twisting of the choke element with respect to the longitudinal axis of the intake channel is provided. Especially in connection with aerodynamic ancillary devices (such as form bodies or air bores on the choke element), the alignment of the ancillary devices with respect to the carburetor is ensured. 
     The above-described connection of the choke element to the choke carrier provides for limited movement and is easily realizable by means of a snap connection having play. The constructive and manufacturing complexity is low and a limited movement in the above-described degrees of freedom is made possible in a simple manner in a corresponding configuration of the play. The snap connection additionally permits a rapid and cost-effective assembly. 
     In a constructively simple and effective embodiment of the snap connection, the choke element has a latch lug which can latch into a corresponding latch clamp of the choke carrier. A holding hook is advantageously provided on the latch lug and this holding hook engages behind an edge of the choke carrier in the assembled condition and thereby reliably prevents an axial slippage of the latch lug out of the latch clamp without affecting the assembly operation. The latch hook is advantageously so configured that it permits a limited axial play. 
     To simplify the assembly operation, the latch clamp has at least one assembly bevel whereby the latch clamp is automatically widened when the latch lug is introduced so that the latch lug can snap into the latch clamp. The above-described protection against rotation or twisting is constructively simply configured in that a holding lug is provided on the choke element at a radial spacing to the snap connection. The holding lug engages in a corresponding receptacle opening on the choke carrier. 
     To achieve a reliable and precise alignment of the choke element with respect to the intake opening to be covered, it is practical to provide a centering device by means of which the choke element is automatically aligned with respect to the intake direction. In a simple and effective embodiment, this automatic alignment is achieved in that a peripherally extending centering bevel, which is adapted to the contour of the intake opening, is provided on the choke element. The centering bevel can, for example, be in the form of a truncated cone. The peripherally extending centering bevel has an automatic centering action when the throttle element closes and furthermore forms a reliable self-adapting sealing seat for the choke element. In an advantageous variation, the intake opening has a rounded edge and the above-described centering bevel is configured as a concave chamfer. The rounded edge of the intake opening leads, in the opened state of the choke element, to a low intake flow resistance which contributes to improving the engine power. In the closed state of the choke flap, an areal contact engagement, and therefore a reliable seal, results from the connection of the rounded edge with the contact-engaging concave chamfer. 
     The choke carrier is advantageously configured as a pivot arm. With a simple pivot movement, the choke element is brought to the intake opening and only low tolerances must be compensated. The choke element is moved approximately in the direction of the channel axis especially with a pivot axis of the pivot arm which lies transversely to the longitudinal axis of the intake channel. In this way, a precise preadjustment is already provided and the centering bevel, which is described above, is cleanly introduced into the intake opening. 
     In a practical embodiment, the pivot arm is journalled in a wall of the air filter housing. This arrangement does not require a separate device for journaling and furthermore makes possible a common removal of the choke element with the air filter housing whereby the carburetor is, for example, exposed for service work with few hand motions. 
     The choke element is advantageously configured as an injection molded part and especially of plastic. In this way, complex shapes can be produced in a cost-effective manner under large-series manufacturing conditions, which, in combination with the tolerance insensitivity of the choke arrangement, leads to an excellent choke action in the closed state of the choke element and to a low intake resistance in the opened state. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the drawings wherein: 
     FIG. 1 is a schematic overview of an internal combustion engine having a choke element mounted in the air filter housing; 
     FIG. 2 is a detail enlargement of the arrangement of FIG. 1 in the region of the intake opening and of the choke element; and, 
     FIG. 3 is a detail view of the pivot arm of FIG. 2 wherein the pivot arm carries the choke element. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     FIG. 1 shows an internal combustion engine  1  having a cylinder  26  and a motor housing  36 . A crankshaft (not shown) rotates a fan wheel  29  about a crankshaft axis  28 . On one side of the cylinder  26 , an exhaust-gas muffler  27  and, on the opposite-lying side, a carburetor  2 , are provided. The carburetor  2  supplies the engine  1  with an air/fuel mixture in the flow direction indicated by arrow  37 . The carburetor  2  is connected via an intake channel  3  to an air filter  4 . The intake channel  3  opens upstream (that is, against the direction identified by the arrow  37 ) with an intake opening  6  into a housing base  7  of the air filter housing  5 . The intake opening  6  is at least partially closable by means of a choke element  8 . For this purpose, the choke element  8  is pivotally guided on a choke carrier  9  in the form of a pivot arm  23 . The pivot arm  23  is pivotally journalled about a pivot axis  24  and this axis lies transversely to the channel axis  11  of the intake channel  3 . Outside of the air filter housing  5 , the pivot arm  23  has an actuating lever  32 . With the movement of the actuating lever  32  in the direction of the double arrow  30 , the choke element  8  can be moved in the direction of double arrow  31 , whereby the choke element  8  is pressed against the intake opening  6  or is lifted away from the intake opening  6 . The choke element  8  can also be configured as a laterally pivotable slider with the pivot axis  24  lying approximately parallel to the channel axis  11 . A linear or parallelogram-shaped displacement of the choke element  8  can also be provided. 
     FIG. 2 is an enlarged view and shows details of the arrangement of FIG. 1 in the region of the intake opening  6  and of the choke element  8 . In the embodiment shown, the intake opening  6  has a circularly-shaped cross section in a plane E; however, another desired cross section can also be practical. A peripherally extending edge  21  of the intake opening  6  is configured so as to be rounded. In the embodiment shown, the region of the intake opening  6 , the housing base  7  and the air filter housing  5  are all formed as one piece with a lower wall  25 . However, a multi-piece configuration can also be practical. The pivot arm  23  for the choke element  8  is journalled on the air filter housing  5  about the pivot axis  24  in the region of the wall  25  and the actuating lever  32  extends out from the air filter housing  5 . 
     On its side facing toward the intake opening  6 , the choke element  8  has an aerodynamically shaped body  35 . A centering bevel  20  is provided, which extends about the choke element  8 , and this centering bevel can have the form of a truncated cone and, in the embodiment shown, is a concave chamfer  22  adapted to the rounded edge  21 . The rounded edge  21  and the concave chamfer  22  conjointly define an automatic centering device for aligning the choke element  8  relative to the intake opening  6 . 
     The choke element  8  is fixed on the choke carrier  9  by means of a snap connection  12  having play associated therewith. With this configuration, a journaling of the choke element  8  providing limited movement results with respect to a linear displaceability in all directions lying in the plane E of the intake opening  6  as well as a limited pivotability about desired axes lying in the plane E. Depending upon the application, a limiting of the degree of freedom of movement can be practical, for example, by the arrangement of a pivot axis or cardan joint or a linear guide. 
     A guard  10  is provided for aligning the choke element  8  having an aerodynamically shaped body  35  with respect to the intake opening  6 . In the embodiment shown, this guard  10  comprises a holding lug  17  on the choke element  8  and a receiving opening  19  on the choke carrier  9  for the holding lug  17 . The holding lug  17  is radially spaced relative to the snap connection  12 . With this arrangement, a rotation or twisting of the choke element  8  relative to the channel axis  11  is prevented. 
     In the embodiment shown, the snap connection  12  comprises a latch lug  13  on the choke element  8  with the latch lug being held in a latch clamp  14  described in greater detail below. 
     The latch lug  13  includes a holding hook  15  at its end facing away from the choke element  8 . The holding hook  15  engages, with play, behind the pivot arm  23  in the region of the latch clamp  14  and, in this way, permits a limited axial movability while simultaneously preventing the latch lug from slipping out. The choke element  8  with its aerodynamically shaped body  35  is manufactured as one piece with the latch lug  13 , the holding hook  15  and the holding lug  17  utilizing injection molding. Depending upon the application, a configuration in cast metal or as a machined part can also be practical. 
     FIG. 3 shows details for the pivot arm  23  of FIG. 2 formed as a single piece. The pivot arm  23  includes a lever shaft  34  having an end on which the actuating lever  32  is formed for pivoting about the pivot axis  24 . The receiving opening  18  in the form of a slot is arranged in the pivot arm  23  for receiving the holding lug  17  (FIG.  2 ). In the region of the free end of the pivot arm  23 , an approximately cylindrical latch opening  33  is provided for receiving the latch lug  13  (FIG.  2 ). The diameter of the latch opening  33  is greater than the diameter of the latch lug  13  whereby a defined journaling with play results. The latch opening  33  is fork shaped in the direction of the free end of the pivot arm  23  and includes latch clamps  14  on respective sides and each latch clamp  14  has an inwardly directed assembly bevel  16 . 
     It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.