Abstract:
A two-stroke engine, comprising: a crankcase; a cylinder connected to said crankcase; a combustion chamber defined by a reciprocally mounted piston within said cylinder; a crankshaft mounted rotatably in said crankcase and drivingly connected with said piston; at least one overflow passage connecting the crankcase and the combustion chamber when the piston is positioned in the lower dead center region; a connecting stub attached to said cylinder which contains an intake passage that opens into the crankcase and supplies fuel and combustion air, said intake passage including therein an air-fuel mixture passage, wherein said connecting stub contains at least one section of the mixture passage; at least one fuel opening for supplying fuel into said mixture passage; and a guiding rib positioned within said connecting stub and oriented longitudinally in the mixture passage and protruding into the mixture passage.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from prior German Patent Application No. 10 2010 054 838.3, filed Dec. 16, 2010 the entire contents of which are incorporated herein by reference in their entirety. 
       BACKGROUND 
       [0002]    The invention relates to a two-stroke engine of the type having a cylinder, in which a combustion chamber is formed, wherein the combustion chamber is bounded by a piston which drives a crankshaft mounted rotatably in a crankcase, wherein the crankcase is connected, via at least one overflow passage, to the combustion chamber when the piston is in the region of its lower dead center, with an air-fuel mixture passage which opens into the crankcase and which is guided in an intake passage, for supplying fuel and combustion air and into which fuel is supplied via at least one fuel opening, and wherein at least one section of the mixture passage is guided in a connecting stub. 
         [0003]    DE 10 2007 037 009 A1 discloses a connecting stub for a two-stroke engine, the intake passage of which is divided into an air passage and a mixture passage. 
         [0004]    It has been shown that, in particular during idling, the running behavior in a two-stroke engine of this type may be inadequate because fuel can accumulate in the connecting stub and is then supplied to the crankcase in an undefined manner. Dead regions in the connecting stub, in which there is insufficient flow of air, may also cause fuel to accumulate, the fuel then not being available to the internal combustion engine. 
         [0005]    It is known to provide the connecting stub with grooves which conduct the fuel accumulated in the connecting stub away to the crankcase. Structures, in which fuel can accumulate, such as pyramid-shaped elevations or ribs running in the circumferential direction, are also known. 
       SUMMARY OF THE INVENTION 
       [0006]    It is one object of the invention to provide a two-stroke engine of the type discussed above, which has improved running behavior, even during idling. 
         [0007]    This and other objects are achieved by a two-stroke engine, comprising: a crankcase; a cylinder connected to the crankcase; a piston reciprocally mounted within the cylinder to define a combustion chamber in the cylinder; a crankshaft mounted rotatably in the crankcase and drivingly connected with the piston; at least one overflow passage connecting the crankcase and the combustion chamber when the piston is located in the region of its lower dead center position; a connecting stub attached to the cylinder and having therein an intake passage that opens into the crankcase and supplies fuel and combustion air, this intake passage including therein an air-fuel mixture passage, wherein at least one section of the mixture passage extends in the connecting stub; at least one fuel opening for supplying fuel into the mixture passage; and a guiding rib positioned within the connecting stub and running in the longitudinal direction of the mixture passage and protruding into the mixture passage. Further objects, features and advantages of the present invention will become apparent from the detailed description of preferred embodiments of the invention which is set forth below, when considered together with the figures of drawing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    An exemplary embodiment of the invention is explained below with reference to the drawings, in which: 
           [0009]      FIG. 1  shows a schematic sectional illustration of a two-stroke engine, 
           [0010]      FIG. 2  shows a perspective sectional illustration through a carburetor and connecting stub of the two-stroke engine from  FIG. 1 , 
           [0011]      FIG. 3  shows a partially sectioned, perspective illustration of the connecting stub from  FIG. 2 , 
           [0012]      FIG. 4  and  FIG. 5  show side views of the connecting stub, 
           [0013]      FIG. 6  shows a section through the connecting stub along the line VI-VI in  FIG. 5 , 
           [0014]      FIG. 7  shows a side view of the connecting stub, 
           [0015]      FIG. 8  shows a section along the line VIII-VIII in  FIG. 7 , 
           [0016]      FIG. 9  shows the detail IX from  FIG. 8  in an enlarged illustration, 
           [0017]      FIG. 10  shows a partially sectioned view along the line X-X in  FIG. 8 , 
           [0018]      FIG. 11  shows the detail XI from  FIG. 10 , 
           [0019]      FIG. 12  shows the detail XII from  FIG. 10 , 
           [0020]      FIG. 13  shows a side view in the direction of the arrow XIII in  FIG. 10 , 
           [0021]      FIG. 14  shows a perspective illustration of an intermediate ring, 
           [0022]      FIG. 15  shows a side view of the intermediate ring from the side facing the connecting stub, 
           [0023]      FIG. 16  shows a section along the line XVI-XVI in  FIG. 15 , 
           [0024]      FIG. 17  shows a side view in the direction of the arrow XVII in  FIG. 15 , 
           [0025]      FIG. 18  shows a schematic side view of the intermediate ring from the side facing the carburetor. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0026]    It has surprisingly been shown that the formation of regions of turbulence and dead regions in the connecting stub can be substantially prevented by a guiding rib which runs in the longitudinal direction of the mixture passage and protrudes into the mixture passage. A uniform flow in the mixture passage is thereby achieved in a simple manner. This is advantageous, in particular, in two-stroke engines in which, during idling, there is low flow through the mixture passage, for example, in the case of two-stroke engines in which the intake passage is separated into an air passage and a mixture passage which are controlled by a common throttle element. By means of the guiding rib, a local accumulation of the wall film forming in the connecting stub can be largely avoided, even when there is low air flow. The guiding rib is advantageously arranged in such a manner that fuel is deposited in the form of a wall film essentially only on one longitudinal side of the guiding rib. The guiding rib prevents the wall film from spreading out over the entire circumference of the mixture passage. 
         [0027]    The height of the guiding rib is advantageously small with reference to the diameter of the intake passage. A height of the guiding rib of approximately 5% to approximately 25% of the diameter of the intake passage at the upstream end of the connecting stub has proven advantageous. In particular, the height of the guiding rib is preferably approximately 10% to approximately 20% of the diameter of the intake passage at the upstream end of the connecting stub. 
         [0028]    For thorough mixture preparation, it is provided, according to another aspect of the invention, that a secondary passage into which at least one idling fuel opening opens is formed in the mixture passage. In this case, the secondary passage advantageously opens laterally into the mixture passage in a manner offset in the circumferential direction with respect to the guiding rib by an angle about the longitudinal axis of the intake passage with reference to the direction of flow in the connecting stub. This ensures that fuel flows essentially along only one side of the guiding rib. Despite the comparatively low height of the guiding rib, it is possible largely to prevent fuel from being deposited in the form of a wall film on the connecting stub on that side of the guiding rib which faces away from the secondary passage. The angle here is advantageously selected in such a manner that there is a distance between the secondary passage and guiding rib, as seen in the direction of flow. The distance here can be small and can be considerably less than the width of the secondary passage. In this case, the secondary passage is advantageously partially bounded by the outer wall of the connecting stub, and therefore the fuel/air mixture leads from the secondary passage into the connecting stub close to the wall. The guiding rib is advantageously at a distance from the upstream end of the connecting stub, which distance approximately corresponds to the length of that section of the secondary passage which is guided in the connecting stub. The guiding rib is not required in those regions of the mixture passage in which fuel does not flow during idling, and therefore the guiding rib begins only at the exit from the secondary passage. 
         [0029]    A section of the intake passage is advantageously formed in a carburetor. Downstream of the carburetor, the intake passage is divided in particular into an air passage and the mixture passage. The air passage here is advantageously connected to at least one overflow passage and serves to feed advance scavenging air into the overflow passages. At least one fuel opening in the carburetor opens into the intake passage. A throttle element, in particular a throttle valve, is mounted pivotably in the carburetor, said throttle valve controlling the combustion air quantity supplied to the air passage and the mixture passage. In particular in the case of two-stroke engines, in which the intake passage is divided into an air passage and a mixture passage, the flow through the mixture passage is very small during idling, since the air passage is also partially opened by the throttle element on account of the joint control of both passages. For two-stroke engines of this type, the arrangement of a guiding rib has proven particularly advantageous. 
         [0030]    The guiding rib on the outer wall of the mixture passage is advantageously arranged lying opposite a separating wall separating an air passage and mixture passage and divides the outer wall into a first circumferential section and a second circumferential section. In this case, the secondary passage advantageously opens into one of the two circumferential sections. In particular, the secondary passage opens into the larger of the two circumferential sections. 
         [0031]    In order to improve the temporary storage of fuel in the connecting stub, which is deposited in the form of a wall film, the connecting stub is provided with pyramid-shaped elevations in the mixture passage. Pyramid-shaped elevations are preferably also provided in the air passage, since, in particular during idling, fuel may also overflow into the air passage. The elevations are advantageously oriented and offset with respect to one another in such a manner that intersecting passages which are inclined with respect to the longitudinal axis of the intake passage are formed between the elevations. Elevations in the connecting stub are advantageously arranged on the entire radially outer, inner wall surface of the intake passage, i.e., inside surfaces located on the outer wall of the mixture passage and on the outer wall of the air passage, and also on both sides of the separating wall. The elevations therefore advantageously extend substantially over the entire inner circumference of the mixture passage and air passage. The elevations in the air passage advantageously extend over less than half of the length of the air passage guided in the connecting stub, in particular over less than one third of the length of the air passage in the connecting stub. The elevations in the mixture passage advantageously extend over less than half, in particular over less than one third of the length of the mixture passage guided in the connecting stub. Owing to the fact that the elevations extend only over a subsection of the length of the connecting stub, the durability of the elastic connecting stub during operation is increased. In this case, the elevations are arranged in particular in a central region of the connecting stub. The elevations in the air passage and the elevations in the mixture passage are advantageously arranged axially offset with respect to one another in the direction of flow. This also increases the durability of the elastic connecting stub during operation. 
         [0032]    The mixture passage expediently has different cross-sectional shapes at the upstream end and at the downstream end of the connecting stub. 
         [0033]    Turning now to the drawings,  FIG. 1  schematically shows a two-stroke engine  1 , as can be used, for example, for driving a tool in hand-guided working implements, such as motor-driven saws, abrasive cutting-off machines, brush cutters or the like. The two-stroke engine  1  has a cylinder  2 , in which a combustion chamber  3  is formed. The combustion chamber  3  is bounded on one side by a piston  5  which is mounted such that it moves to and fro in the cylinder  2  and, via a connecting rod  6 , drives a crankshaft  7 , which is mounted rotatably in a crankcase  4 . In the lower dead center of the piston  5 , the interior of the crankcase  4  is connected to the combustion chamber  3  via overflow passages  14  which open by means of overflow apertures  15  into the combustion chamber  3 . An outlet  16  for exhaust leads out of the combustion chamber  3 . The two-stroke engine  1  has an intake passage  61  which is connected to an air filter  18  and via which combustion air is sucked in. A section of the intake passage  61  is formed in carburetor  17 . A choke valve  24  with a choke shaft  25 , and also, downstream of the choke valve  24 , a throttle valve  22  with a throttle shaft  23  is mounted pivotably in the carburetor  17 , which is designed in the exemplary embodiment as a diaphragm-type carburetor. Instead of the throttle valve  22 , a different throttle element may alternatively be provided, and instead of the choke valve  24 , a different choke element may alternatively be provided. Downstream of the throttle valve  22 , the intake passage  61  is separated by a separating wall  10  into an air-fuel mixture passage  8  and an air passage  9 . A separating wall section  26  is arranged between the throttle valve  22  and choke valve  24 . A main fuel opening  20  and a plurality of idling fuel openings  21  in the carburetor  17  open into the mixture passage  8 . The idling fuel openings  21  open into the mixture passage  8  downstream of the main fuel opening  20 . In the region of the main fuel opening  20 , a Venturi  19  is formed in the intake passage  61 . 
         [0034]    The mixture passage  8  opens with a mixture inlet  11  on the cylinder  2  and the port is controlled by the piston  5 . The air passage  9  opens with an air inlet  12  on the cylinder  2 . The piston  5  has one or more piston recesses  13  which connect the air inlet  12  in the region of the upper dead center of the piston  5  to the overflow apertures  15 . The air passage  9  may alternatively be divided into two branches which each open with a separate air inlet  12  on the cylinder  2 . 
         [0035]    During operation, a fuel/air mixture is sucked up into the crankcase  4  via the mixture inlet  11  during the upward stroke of the piston  5 . In the region of the upper dead center, largely fuel-free combustion air from the air passage  9  is temporarily stored in the overflow passages  14 . During the downward stroke of the piston  5 , the fuel/air mixture in the crankcase  4  is compressed and enters the combustion chamber  3  in the region of the lower dead center of the piston  5 . In the process, the air temporarily stored in the overflow passages  14  first of all flows into the combustion chamber  3 . During the subsequent upward stroke of the piston  5 , the fuel/air mixture is once again compressed in the combustion chamber  3  and ignited in the region of the upper dead center of the piston  5 . During the subsequent downward stroke of the piston  5 , the outlet  16  is opened, and the exhaust gases flow out of the combustion chamber  3  and are expelled by the combustion air flowing in subsequently via the overflow passages  14 . 
         [0036]    The combustion air flows in the intake passage  61  in a direction of flow  58  from the air filter  18  to the cylinder  2 . A connecting stub  28  is arranged between the carburetor  17  and cylinder  2 , the connecting stub being composed of an elastic material, for example, rubber or an elastomeric plastic, and in which both the mixture passage  8  and the air passage  9  are guided. A shielding element  27  which bounds a secondary passage  37  is arranged in the mixture passage  8 , adjacent to the throttle valve  22 . At least one idling fuel opening  21  opens into the secondary passage  37 . The secondary passage  37  is arranged in the mixture passage  8  and is separated from the latter by the shielding element  27 . 
         [0037]      FIG. 2  shows the configuration in detail. An intermediate ring  36  is arranged between the carburetor  17  and connecting stub  28 , said intermediate ring being held in a sealing manner in both the carburetor  17  and in the connecting stub  28 . The shielding element  27  is preferably integrally formed on the intermediate ring  36 . The intermediate ring  36  is advantageously composed of a dimensionally stable plastic. 
         [0038]    As  FIG. 2  shows, the carburetor  17  preferably has a control chamber  29  which is separated from a compensation chamber  31  via a diaphragm  30 . The fuel is metered to the intake passage  61  via the control chamber  29 . As  FIG. 2  also shows, the separating wall section  26  has a cutout or recess  41  on the side facing the air passage  9 , against which the choke valve  24  bears. In the completely open position, the choke valve  24  preferably adjoins the separating wall section  26  in an approximately flush manner. The separating wall section  26  extends upstream, preferably nearly as far as the choke shaft  25 . The separating wall section  26  is at a distance from the throttle shaft  23 . On the side facing the mixture passage  8 , the separating wall section  26  has a cutout or recess  40  which is formed on a narrow border of the separating wall section  26  and against which the throttle valve  22  bears in the fully open position. An opening  53  is formed between the separating wall section  26  and the throttle shaft  23 , via which opening the air passage  9  and the mixture passage  8  are connected to each other in the closed and partially open position of the throttle valve  22 . 
         [0039]    The throttle valve  22  has an opening  59 , the border of which, in the closed position of the throttle valve  22 , is arranged on the shielding element  27  in an approximately flush manner such that combustion air from the region upstream of the throttle valve  22  can enter the secondary passage  37  through the opening  59 . As  FIG. 2  also shows, the shielding element protrudes both into the carburetor  17  and into the connecting stub  28 . 
         [0040]    As  FIG. 2  shows, the intake passage  61  is separated in the connecting stub  28  by the separating wall  10  into an air passage  9  and mixture passage  8 . Both in the air passage  9  and in the mixture passage  8 , elevations  38  which are of approximately pyramid-shaped design and at which precipitated fuel can accumulate are arranged in a central region of the connecting stub  8 . The fuel is gradually output again by the elevations  38  to the combustion air flowing past, thus avoiding a surge-like overflowing of fuel, for example if the two-stroke engine  1  is pivoted. The elevations in the air passage  9  are arranged upstream of the elevations in the mixture passage  8 . The elevations  38  in the air passage  9  and in the mixture passage  8  do not overlap in the direction of flow  58 , and therefore in each cross section located perpendicularly to the longitudinal center axis  65 , elevations  38  are provided in the mixture passage  8  or in the air passage  9 , or no elevations  38  are provided, i.e., elevations  38  are not provided both in the air passage  9  and in the mixture passage  8  in any cross section. 
         [0041]    As  FIG. 2  also shows, a guiding rib  39  which runs approximately in the direction of the longitudinal axis  65  of the intake passage is arranged in the mixture passage  8  on the passage side opposite the separating wall  10 . In  FIGS. 1 and 2 , the mixture passage  8  is arranged below the air passage  9 . However, in the actual installed position, the mixture passage  8  is advantageously arranged above the air passage  9  with reference to the direction of gravitational force. 
         [0042]    The connecting stub  28  has a carburetor connection flange  32 , by which said connecting stub is held on the carburetor  17 . The carburetor connection flange  32  is held on the end side of the carburetor  17  via clamping elements (not shown). For connection to the cylinder  2 , the connecting stub  28  has an engine connection flange  33 . The engine connection flange  33  has fastening openings  43  for fastening means, for example, bolts, with which the engine connection flange  33  can be screwed to the cylinder flange. In order to increase the strength, the engine connection flange  33  has a reinforcing element  35  which is injected into the material of the connecting stub  28 . An encircling seal  34 , which completely surrounds the mouth openings of the air passage  9  and mixture passage  8  and thus results in good sealing, is injected onto the end side. Two stubs or short feed pipes  44 , of which one is shown in the sectional illustration in  FIG. 2 , are preferably integrally formed on the connecting stub  28 . The stubs  44  protrude beyond the engine connection flange  33  into the cylinder flange and bound the air passage  9 . This results in a favorable shaping, and the cylinder flange can easily be removed from the mold during the production of the cylinder  2  by die-casting. 
         [0043]    As  FIG. 3  shows, the guiding rib  39  has an upstream, carburetor-side end  46  which is offset from the carburetor connection flange  32  into the interior of the connecting stub  28 . In addition, the guiding rib  39  has a downstream, engine-side end  55  which lies in the plane of the engine connection flange  33 . The carburetor-side end  46  lies approximately at the height of the end of the shielding element  27 . 
         [0044]    The guiding rib  39  divides the outer wall  73  of the mixture passage  8  ( FIG. 7 ) into a first circumferential section  62  and a second circumferential section  63 . The shielding element  27  is offset in the circumferential direction in relation to the guiding rib  39 , and therefore the secondary passage  37  opens at the first circumferential section  62 . The guiding rib  39  serves to guide the flow toward the cylinder  2  in the direction of the longitudinal axis  65  of the intake passage. At the same time, the wall film of fuel precipitated in the first circumferential section  62  is prevented from migrating into the second circumferential section  63 . This is advantageous, in particular during idling. The fuel and the combustion air are guided directly to the cylinder  2  by the guiding rib  39 . This prevents the fuel from being distributed throughout the entire connecting stub  28 . As a result, fuel cannot pass into dead regions not having an air flow, and therefore an accumulation of fuel and an undefined, surge-like introduction of fuel into the crankcase  4  are avoided. In addition, the guiding rib  39  projecting into the mixture passage  8  evens out the flow in the mixture passage  8  and prevents turbulence in the flow. 
         [0045]    As  FIG. 4  shows, a seal  45  which is integrally formed on the connecting stub  28  is provided on the carburetor connection flange  32 . 
         [0046]      FIG. 5  shows the arrangement of the elevations  38 . As  FIG. 5  shows, elevations  38  are provided both in the air passage  9  and in the mixture passage  8 . The separating wall is formed in the connecting stub  28  by a separating wall section  74 , which is formed integrally on the connecting stub  28 . Elevations  38  are also arranged on both sides of the separating wall section  74 . 
         [0047]    As  FIG. 5  shows, the intake passage  61  on the carburetor connection flange  32  has a diameter d. The diameter d is therefore measured at the carburetor-side end  76  of the connecting stub  28 . The height h of the guiding rib  39  is significantly smaller than the diameter d of the intake passage  61 . The height h is advantageously approximately 5% to approximately 25%, in particular approximately 15% to approximately 20% of the diameter d of the intake passage  61 . On the carburetor connection flange  32 , the connecting stub  28  has a receptacle or notch  56  into which an element for positionally securing the intermediate ring  36  protrudes. 
         [0048]      FIG. 6  shows the configuration of the guiding rib  39  and the arrangement of the elevations  38 . The carburetor-side end of the guiding rib  39  is at a distance a from the connection surface  47  of the carburetor connection flange  32 . The elevations  38  are arranged in such a manner that passages which each run at an inclination with respect to the longitudinal axis  65  of the intake passage ( FIG. 2 ) and which intersect are formed between the pyramid-shaped elevations  38 . As a result, the accumulated fuel can be readily and uniformly conducted away to the combustion air flowing past. At the same time, a relatively large quantity of fuel can be picked up and temporarily stored. The passages  64  are also shown in  FIG. 8 . 
         [0049]    As  FIG. 7  shows, the connecting stub  28  on the carburetor connection flange  32  has an entry opening  70  into the mixture passage  8  and an entry opening  71  into the air passage  9 . Both entry openings  70 ,  71  have an approximately semicircular cross section. The entire intake passage  61  has a round cross section which is divided by a separating wall section  74  into the two semicircular entry openings  70  and  71 . The mixture passage  8  has an outer wall  73  which is bounded by a curved wall section of the intake passage  61  and on which elevations are arranged. Elevations  38  are also provided on the curved outer wall  72  of the air passage  9 . (See also  FIG. 5 ) As  FIG. 8  shows, elevations  38  are also provided on the separating wall section  74 , on the side facing the mixture passage  8 . Elevations  38  are also arranged on the separating wall section  74 , on the opposite side facing the air passage  9 . As  FIG. 8  shows, the elevations  38  extend over a length e of the mixture passage  8 , which length is significantly smaller than that length g of the mixture passage  8  which is shown in  FIG. 10 . The length e is advantageously less than a half, in particular less than a third, of the length g. 
         [0050]    As  FIG. 10  also shows, elevations  38  which extend over a length e′, which is measured in the direction of the longitudinal axis  65  of the intake passage, are arranged in the air passage  9 . The length e′ of the elevations  38  in the air passage  9  is advantageously less than half of the length f of the air passage  9 , in particular less than one third of the length f. In this case, the lengths measured in the air passage  9  and mixture passage  8  are average lengths which are advantageously measured at the connecting line of the centers of the cross-sectional areas of the particular passage. 
         [0051]    As  FIG. 9  shows, the elevations  38  are of pyramid-shaped design and have a triangular cross section. In this case, the side edges of the pyramid-shaped elevations  38  are arranged in a diamond-shaped manner on the passage wall. The intersecting passages  64  are formed by the side walls which are inclined with respect to the longitudinal axis  65  of the intake passage ( FIG. 8 ). 
         [0052]    Respective seals  45  and  34 , which surround the mouth openings at the respective connection surface, are arranged on the connection flanges  32  and  33 . As  FIGS. 10 and 11  show, the seal  45  which consists of the material of the connecting stub  28  and is integrally injection-molded on the connecting stub is arranged on the carburetor connection flange  32 . Grooves  67  which surround the seal  45  and permit good contact pressure and lateral yielding of the seal  45  are provided on both sides of the seal  45 . The seal  45  jointly surrounds both entry openings  70  and  71 . The separating wall  74  does not begin until shortly after the connection surface  75  of the carburetor connection flange  32 . 
         [0053]    As  FIG. 12  shows, the seal  34  on the engine connection flange  33  is surrounded by grooves  66 . The seal  34  surrounds the mouth openings  68  and  69 , which are shown in  FIG. 10 , of the mixture passage  8  and air passage  9 , individually, such that the mouth openings  68  and  69  are separated from each other by the seal  34 . 
         [0054]    This is also shown in  FIG. 13 . As  FIG. 13  also shows, the mouth opening  69  of the air passage  9  has an approximately semicircular flow cross section which corresponds approximately to the cross section at the entry opening  71 . The shape of the flow cross section of the mouth opening  68  of the mixture passage  8  is approximately rectangular, wherein the guiding rib  39  protrudes into the rectangular cross section. The mixture passage  8  accordingly has different cross-sectional shapes on the carburetor connection flange  32  and on the engine connection flange  33 . This enables a good connection geometry to be achieved. Owing to the different cross-sectional shapes, vortices which arise can be largely prevented by the guiding rib  39 . 
         [0055]      FIGS. 14 to 17  show in detail the configuration of a preferred embodiment of the intermediate ring  36 . The intermediate ring  36  has an outwardly protruding positioning lug  54  which is arranged in the receptacle  56  of the connecting stub  28  ( FIG. 5 ). As the figures show, the shielding element  27  is of curved design, wherein the concave side bounds the secondary passage  37 . On the side opposite the shielding element  27 , the secondary passage  37  is bounded by the outer wall  73  ( FIG. 7 ) of the mixture passage  8 . This results in a very small flow cross section of the secondary passage  37 . The secondary passage  37  is separated from the mixture passage  8  only by the shielding element  27  integrally formed on the intermediate ring  36 . The shielding element  27  projects on both sides beyond the annular section of the intermediate ring  36  and projects into the carburetor  17  and the connecting stub  28 . As the figures show, a separating wall section  50  is integrally formed on the intermediate ring  36 . As  FIG. 16  shows, a bearing surface  57  for the throttle valve  22  is formed on the separating wall section  50 . On the section protruding into the connecting stub  28 , the separating wall section  50  is of flattened design, and therefore the flattened portion of the separating wall section  50  bears against the separating wall section  74  ( FIG. 7 ) in the connecting stub  28  and thus leads to an increase in stability. 
         [0056]    On the side bordering the air passage  9 , the intermediate ring  36  has a thickened portion  51 . As shown schematically in  FIG. 16 , when the throttle valve  22  is slightly open, for example, during idling, a gap is formed between the edge of the throttle valve  22  and the intermediate ring  36 , through which gap the combustion air flows. That side of the thickened portion  51  which faces the throttle valve  22  is formed in a radius  52 , and therefore the air flowing past between the throttle valve  22  and the intermediate ring  26  is directed toward the mixture passage  8 . In the process, the combustion air flows through the opening  53  formed between the throttle valve  22  and the separating wall section  50 . 
         [0057]    As  FIG. 16  shows, the intermediate ring  36  has a first fastening section  48  which protrudes into the carburetor  17  and bears an outwardly protruding web or protrusion lip  42 , with which the fastening section  48  is held in a sealing manner in the carburetor  17 . The web  42  is provided to compensate for tolerances and is deformed or sheared off during fitting such that the fastening section  48  always sits in a sealing manner in the carburetor  17  even in the event of unfavorable tolerance pairings. The second fastening section  49 , which lies downstream and protrudes into the connecting stub  28 , is of partially conically tapering design, and therefore the connecting stub  28  can be pushed in a readily sealing manner onto the intermediate ring  36 . 
         [0058]    As  FIG. 16  shows, the intermediate ring  36  has an insertion length b into the connecting stub  28 , which length approximately corresponds to the distance a ( FIG. 6 ) of the carburetor-side end  46  of the guiding rib  39 . The shielding element  27 , and therefore the secondary passage  37 , has a length l which is approximately 25% to approximately 150% of the diameter c of the throttle valve  22 . A length l of the secondary passage  37  of approximately 40% to approximately 100% of the diameter c of the throttle valve  22  is considered to be particularly advantageous.  FIG. 16  also shows the entry opening  60  into the secondary passage  37  at the upstream end of the secondary passage  37 . In the idling position of the throttle valve  22  that is shown in  FIG. 16 , the throttle valve  22  is adjacent to the entry opening  60 . In this case, the opening  59  in throttle valve  22  is arranged on the entry opening  60 , and therefore combustion air can flow into the secondary passage  37  through the opening  59 . 
         [0059]      FIG. 18  shows the intermediate ring  36  and the guiding rib  39  in an installed position looking from the carburetor  17 . 
         [0060]    The mixture passage  8  is arranged above the air passage  9  in the direction of gravitational force. In the connecting stub  28 , the guiding rib  39 , which is shown schematically in  FIG. 18  in order to clarify the arrangement, is offset laterally in the circumferential direction with respect to the secondary passage  37 . The guiding rib  39  encloses an angle α with the secondary passage  37 , which is sufficient to ensure that fuel flows along only one side of the guiding rib. This angle is advantageously from approximately 10° to approximately 45°. In this case, the angle α is measured between the connecting line extending from the center of the secondary passage  37  and the connecting line at the center of the guiding rib  39 , in each case the lines connecting to the longitudinal axis  65  of the intake passage. As  FIG. 18  also schematically shows, the circumferential wall section  63 , into which the secondary passage  37  opens, is significantly smaller than the circumferential wall section  62 . The flow guiding rib  39  does not directly adjoin the shielding element  27 , as seen in the direction of flow  58 . A distance m is preferably formed between the shielding element  27  and the flow guiding rib  39 . The distance here can be small and can be considerably less than the width of the secondary passage.