Patent Publication Number: US-11384715-B2

Title: Mixture formation unit and two stroke engine having a mixture formation unit

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority of German patent application no. 10 2019 004 063.5, filed Jun. 8, 2019, the entire content of which is incorporated herein by reference. 
     FIELD OF THE DISCLOSURE 
     The invention relates to a mixture formation unit and to a two stroke engine having a mixture formation unit. 
     BACKGROUND OF THE DISCLOSURE 
     US 2014/0261329 A1 discloses a mixture formation unit, namely a carburetor in which the main fuel nozzle is arranged in a rectilinear channel. As a result, the main fuel nozzle can be pushed or pressed from outside into the base body of the carburetor in a simple manner. On the base body there are arranged a cover, which holds the control membrane, and also a cover of the fuel pump. If the cover, which holds the control membrane is mounted on the base body, the channel in which the main fuel nozzle is arranged is only accessible from the intake channel. 
     The channels of a carburetor are customarily at least partially produced using machining production methods. However, after mounting the cover of the regulating chamber, the channel in which the main fuel nozzle is arranged is accessible only from the intake channel. Therefore, cleaning of the carburetor after the assembly of all components is possible only to a limited degree. Particles which are situated in the fuel-conducting channels can become detached during operation and accumulate at undesired positions, for example at sensitive components such as valves or the like and thus interfere with the operation of the carburetor. 
     SUMMARY OF THE DISCLOSURE 
     It is an object of the invention to provide a mixture formation unit that has a high degree of robustness in operation and can be readily cleaned. 
     It is a further object of the invention to provide a two stroke engine having a mixture formation unit. 
     With regard to the mixture formation unit, this object can, for example, be achieved by a mixture formation unit having: a base body in which an intake channel section is formed; the base body having a first end side and a second end side; the intake channel section extending from the first end side of the base body to the second end side of the base body; at least one rectilinearly extending channel which opens into the intake channel section; and, wherein the at least one rectilinearly extending channel opens at the first end side of the base body. 
     With regard to the two stroke engine, the object can, for example, be achieved by a two stroke engine having: an intake channel having an intake channel section; a mixture formation unit having a base body in which the intake channel section is formed; the base body having a first end side and a second end side; the intake channel section extending from the first end side of the base body to the second end side of the base body; the mixture formation unit having at least one rectilinearly extending channel which opens into the intake channel section; the at least one rectilinearly extending channel opening at the first end side of the base body; a cylinder having a combustion chamber formed therein; a crankcase defining a crankcase interior; a crankshaft mounted in the crankcase; a piston configured to drive the crankshaft; the combustion chamber being delimited by the piston; the crankcase interior being connected in at least one position of the piston to the combustion chamber via at least one transfer channel; the intake channel being divided by a partition wall downstream of the intake channel section; and, the partition wall dividing the intake channel into a mixture channel for supplying a fuel/air mixture into the combustion chamber and into an air channel for supplying scavenging advance air to the at least one transfer channel. 
     There is provision that the channel which opens into the intake channel is configured as a rectilinearly extending channel and opens at an end side of the base body of the mixture formation unit. As a result, both ends of the channel are also still readily accessible after mounting add-on parts such as covers, a fuel pump or the control device of the mixture formation unit. Consequently, the channel can be completely cleaned and flushed through. A cleaning line can be connected in particular to the end side of the base body, resulting in good accessibility of the connection. 
     A component of the mixture formation unit can preferably be arranged in the channel. By virtue of the arrangement of the mouth opening of the channel at the first end side of the base body, the channel can be cleaned in a simple manner before mounting the component. In the event of functional disturbances, for example, the component arranged in the channel can subsequently be changed in a simple manner. Here, the first end side on which the channel opens can be both the upstream end side of the mixture formation unit and the downstream end side of the mixture formation unit. Advantageously, the mixture formation unit has at least one fuel opening which opens into the intake channel section and which is formed on a fuel nozzle. Here, fuel nozzle designates the component on which there is configured the constriction which forms the nozzle cross section. It is also possible for further functions to be realized in the fuel nozzle. The fuel nozzle is a component which can include a plurality of individual parts. The fuel opening can preferably be a main fuel opening and the fuel nozzle is a main fuel nozzle. Preferably, the component forms with the channel, in particular with the channel wall of the channel, an annular gap which is connected to the fuel opening. By virtue of the fact that the channel is configured as a rectilinear channel, it can be manufactured with a high degree of accuracy, for example by drilling or milling, thus resulting in defined dimensions for the annular gap. 
     The component arranged in the channel preferably has a check valve. Particles such as chips or the like which arise during the production and cannot be removed from the base body of the mixture formation unit can impair the sealing function of a valve plate of the check valve and thus considerably compromise the functioning. In particular for a check valve, it is therefore desirable to clean residues resulting from preceding machining methods, such as chips or the like. 
     In an alternative configuration, there is provision that the component is a fuel valve. The fuel valve preferably can have a valve plate which is movable between a stop and a valve seat. Here, too, chips or the like can adversely affect the sealing function of the valve plate. The fuel valve is in particular an electrically operated fuel valve, preferably an electromagnetic valve. 
     In an embodiment, the channel extends comparatively flat in the base body of the mixture formation unit. This results in an advantageous arrangement and good utilization of the customary available installation space in the mixture formation unit. The center axis of the channel advantageously encloses an angle of 0° to 30°, in particular from 0° to 25°, with the intake channel longitudinal axis in a section plane which contains the intake channel longitudinal axis and extends parallel to the center axis of the channel. The center axis of the channel can accordingly lie in one plane with the intake channel longitudinal axis or extend obliquely to the intake channel longitudinal axis. If the center axis of the channel extends obliquely to the intake channel longitudinal axis, the angle is measured in the section plane between a projection of the center axis of the channel perpendicular to the section plane and the intake channel longitudinal axis. 
     The intake channel section preferably has a venturi section. In particular a throttle element is mounted in the base body downstream of the venturi section. The throttle element is preferably arranged so as to be adjustable and serves for setting the free flow cross section of the intake channel section. The throttle element can advantageously be pivotable about a rotational axis. 
     The mixture formation unit is in particular a carburetor in which the fuel preparation occurs at least partially in the venturi section or downstream thereof. The first end side at which the channel opens is preferably the upstream end side of the base body. However, there can also be provision that the first end side at which the channel opens is the downstream end side of the base body. The throttle element is preferably a throttle flap. A choke element can advantageously be held in the base body upstream of the throttle element. The choke element is preferably a choke flap. With the choke element configured as a choke flap, there is sufficient installation space present in the installation channel section, with the result that the channel and the choke element can be arranged at least partially in the same cross section of the mixture formation unit. There can be provision that no partition wall section is arranged in the intake channel section upstream of the throttle element. In a preferred configuration, a partition wall section is arranged in the intake channel section upstream of the throttle element. A simple construction results if the component is pressed into the channel. Here, the component can be pressed directly into the channel. The outside circumference of the component and the channel may advantageously form an interference fit assembly and bear against one another. In an alternative configuration, there can be provision that the component is pressed into the channel with interposition of at least one seal. A plurality of seals can be advantageous, in particular in order to seal different regions on the outer circumference of the component with respect to one another. If the component has a valve, it can be particularly advantageous for the regions downstream and upstream of the valve to be separated from one another via at least one seal on the outer circumference of the component. The seal can be an O-ring, for example. However, another configuration of the seal can also be advantageous. 
     For a two stroke engine having a mixture formation unit, there is provision that the two stroke engine has a cylinder in which there is formed a combustion chamber which is delimited by a piston. The piston drives a crankshaft which is mounted so as to be rotatable in a crankcase. A crankcase interior is connected in at least one position of the piston to the combustion chamber via at least one transfer channel. The two stroke engine has an intake channel which, downstream of the intake channel section formed in the mixture formation unit, is divided by a partition wall into a mixture channel for the supply of fuel/air mixture into the combustion chamber and into an air channel for the supply of scavenging advance air to the at least one transfer channel. It has been shown that, in particular in a mixture formation unit for a stratified scavenging engine in which the intake channel section is divided into a mixture channel and an air channel, sufficient installation space is available for the rectilinear channel opening at an end side of the base body. 
     Upstream of the throttle element, there can be provided a partition wall section for subdividing the intake channel section into the mixture channel and the air channel. However, there can also be provision that no partition wall section for subdividing the intake channel section into the mixture channel and air channel is provided upstream of the throttle element. 
     The mixture formation device according to the disclosure can also be provided for a two stroke engine which does not have an air channel or for a two stroke engine which has an air channel routed separately from the mixture channel. The mixture formation device is also advantageous for a four stroke engine, in particular for a mixture-lubricated four stroke engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described with reference to the drawings wherein: 
         FIG. 1  shows a schematic illustration of a two stroke engine; 
         FIG. 2  shows a sectional illustration of one exemplary embodiment of a carburetor; 
         FIG. 3  shows a detail side view of the carburetor from  FIG. 2  in the direction of the arrow III in  FIG. 2 ; 
         FIG. 4  shows a sectional illustration of a further exemplary embodiment of a carburetor; and, 
         FIG. 5  shows a detail sectional illustration of a further exemplary embodiment of a carburetor. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The two stroke engine  1  schematically illustrated in  FIG. 1  has a cylinder  2  and a crankcase  4 . A combustion chamber  3  is formed in the cylinder  2 , and a crankcase interior  6  is formed in the crankcase  4 . The crankcase interior  6  and the combustion chamber  3  are separated by a piston  5  which is movable back and forth in the cylinder  2 . The crankcase interior  6  and the combustion chamber  3  are connected to one another via transfer channels  8  in predetermined piston positions, for example in the position of the piston  5  in the region of bottom dead center illustrated in  FIG. 1 . The transfer channels  8  open with transfer windows  9  into the combustion chamber  3 . The transfer windows  9  are opened or closed in dependence on the position of the piston  5  with respect to the combustion chamber  3 . The piston  5  drives in rotation a crankshaft  7  mounted rotatably in the crankcase  4 . The two stroke engine  1  can be for example the drive engine in a handheld work apparatus such as a power saw, a cut-off grinder, a blowing apparatus, a hedge trimmer, a spraying apparatus or the like, and the crankshaft  7  can serve to drive a tool of the work apparatus. In the case of a blowing apparatus or spraying apparatus, the tool is typically a fan which delivers an operating air flow. Instead of a two stroke engine  1 , the drive engine can also be a four stroke engine, in particular a mixture-lubricated four stroke engine. 
     The two stroke engine  1  has an intake tract with an air filter  49 , a mixture formation unit  13  and a connecting piece  41  for connecting the mixture formation unit  13  to the cylinder  2 . In the exemplary embodiment, the mixture formation unit  13  is a carburetor. Instead of the connecting piece  41 , it is possible to provide one or more arbitrary other parts for fluidic connection of mixture formation unit  13  with the cylinder  2  or the crankcase  4 . The air filter  49  has a filter element  39 . Downstream of the filter element  39  there is formed a clean space  50  from which an intake channel  10  leads. An intake channel section  11  is formed in the mixture formation unit  13 . A throttle element  17 , in the exemplary embodiment a throttle flap, is mounted in an adjustable manner in the intake channel section  11 . In the exemplary embodiment, the throttle element  17  is mounted with a throttle shaft  18 . Downstream of the throttle element  17 , the intake channel  10  is divided into a mixture channel  12  and an air channel  14 . The intake channel  10  has an intake channel longitudinal axis  32  which forms the longitudinal center axis of the intake channel  10 . The mixture channel  12  opens with a mixture channel opening  15  at the cylinder bore  55 . The mixture channel opening  15  is controlled by the piston  5 . The mixture channel opening  15  is opened toward the crankcase interior  6  in the region of top dead center of the piston  15 . The air channel  14  opens with at least one air channel opening  16  at the cylinder bore  55 . The air channel opening  16  is also controlled by the piston  5 . The piston  5  has at least one piston pocket  37  which connects the air channel opening  16  to the transfer windows  9  in the region of top dead center of the piston  5 . Via the air channel  14 , the air channel opening  16  and the transfer windows  9 , scavenging advance air is provided upstream in the transfer channels  8  in the region of top dead center of the piston  5 . The cylinder  2  has an outlet  40  from the combustion chamber  3 . 
     As  FIG. 1  also shows, a main fuel opening  27  and a plurality of secondary fuel openings  28  open into the intake channel section  11  in the mixture formation unit  13 . The main fuel opening  27  is formed on a main fuel nozzle  29 . The main fuel opening  27  opens into the intake channel section  11  in the region of a venturi section  34 . The mixture formation unit  13  has a base body  23  which has a first, upstream end side  24  and a second, downstream end side  25 . The main fuel nozzle  29  is arranged in a rectilinear channel  26  which extends from the first end side  24  into the intake channel section  11 . It is thus possible, during the production of the mixture formation unit  13  or after an exchange of the main fuel nozzle  29 , for a hose with cleaning fluid, such as for example air, to be connected to the first end side  24 , and the channel  26  and parts of the fuel system can be cleaned. An opening on the first end side  24  can also be advantageous for other channels of the mixture formation unit  13 . In the exemplary embodiment, the first end side  24  on which the channel  26  opens is the upstream end side. However, the first end side  24  on which the channel  26  opens can also be the downstream end side of the base body  23 . 
     The main fuel nozzle  29  may advantageously be pressed into the channel  26 . Here, the main fuel nozzle  29  can be pressed directly into the channel  26 , with the result that the outside circumference of the main fuel nozzle  29  is in contact with the wall of the channel  26 . Alternatively, the main fuel nozzle  29  can be pressed into the channel  26  with the interposition of at least one seal. For this purpose,  FIG. 2  depicts a seal  80  schematically with a dashed line. The seal  80  can be an O-ring, for example. A plurality of seals  80  can also be advantageous. 
     As  FIG. 1  shows, no further elements for subdividing the intake channel section  11  into a mixture channel  12  and air channel  14  are provided upstream of the throttle element  17 , nor is a choke element provided. 
     In the exemplary embodiment according to  FIG. 1 , the intake channel  10  is divided by a partition wall  35  into the mixture channel  12  and the air channel  14  downstream of the throttle element  17 . On the side facing the throttle element  17 , the partition wall  35  has a bearing surface  38  against which the throttle element  17  bears in the completely opened position. In a partially closed position of the throttle element  17 , an opening via which fuel can pass into the region situated upstream of the air channel  14  is formed between the throttle shaft  18  and the bearing surface  38 . 
     During operation of the two stroke engine  1 , fuel/air mixture is sucked from the mixture channel  12  into the crankcase interior  6  during the upward stroke of the piston  5  as soon as the mixture channel opening  15  opens. As long as the air channel opening  16  is connected to the transfer windows  9  via the piston pocket  37 , scavenging advance air is provided upstream in the transfer channels  8 . During the downward stroke of the piston  5 , the air/fuel mixture in the crankcase interior  6  is compressed and, as soon as the transfer windows  9  open, scavenging advance air first of all flows out of the transfer channels  8  and then fuel/air mixture flows out of the crankcase interior  6  into the combustion chamber  3 . The fuel/air mixture is compressed in the combustion chamber  3  during the upward stroke of the piston  5  and ignited by a spark plug  72  in the region of top dead center of the piston  5 . Preferably, the spark plug  72  is activated by a control unit  61  which also activates a fuel valve  60  ( FIG. 4 ). During the downward stroke of the piston  5 , the piston  5  first of all opens the outlet  40 , with the result that exhaust gases can flow out of the combustion chamber  3 . The transfer windows  9  are then opened and scavenging advance air flows into the combustion chamber  3  and scavenges the remaining exhaust gases out of the combustion chamber  3  through the outlet  40 . Fresh fuel/air mixture then flows into the combustion chamber  3  for the next combustion. 
       FIG. 2  shows a further exemplary embodiment of a mixture formation unit  13 . In all exemplary embodiments, the same reference signs denote elements which correspond to one another. The mixture formation unit  13  from  FIG. 2  likewise has a base body  23  with a first end side  24  and a second end side  25 . During operation, air flows from the first end side  24  to the second end side  25 , as is schematically illustrated by the arrow  51  in  FIG. 2 . The throttle element  17  is fixed to the throttle shaft  18  via a fastening screw  19 . With respect to the flow direction, a choke element  20  is arranged in the intake channel section  11  upstream of the throttle element  17 . The choke element  20  is configured as a choke flap and is fixed to a choke shaft  21  via a fastening screw  22 . The throttle element  17  is mounted so as to be pivotable about a rotational axis  76 , and the choke element  18  is mounted so as to be pivotable about a rotational axis  77 . A partition wall section  36  is arranged in the intake channel section  11  in the flow direction between the choke shaft  21  and the throttle shaft  18 . The partition wall section  36  separates the air channel  14  and mixture channel  12  from one another. 
     In the exemplary embodiment according to  FIG. 2 , a bearing surface  56  for the throttle element  17  is formed on the partition wall section  36 . The bearing surface  56  is arranged on that side of the partition wall section  36  facing the mixture channel  12 . A bearing surface  57  for the choke element  20  is formed on the side facing the air channel  14 . 
     In the exemplary embodiment according to  FIG. 2 , too, a channel  26  is provided in the base body  23 . The channel  26  can advantageously be configured as a rectilinear, continuous bore of constant diameter. The channel  26  may advantageously from the end side  24  up into the intake channel section  11 . In the exemplary embodiment, the channel  26  is not closed over its entire length over its entire circumference, but is open in the region adjoining the end side  24  toward the intake channel section  11 . There can also be provision that the channel  26  is configured to be circumferentially open over a subportion of its length in another direction. There can also be provision that the channel  26  is configured to be open over its entire length in one direction over a subportion of its circumference. Here, the wall delimiting the channel  26  can be configured to be for example approximately U-shaped in cross section. The channel  26  can be produced via drilling or milling or can be produced as a cast structure during the casting of the base body  23 . 
     The channel  26  has a center axis  33 . In the exemplary embodiment, the center axis  33  encloses an angle α, which is less than 90°, with the intake channel longitudinal axis  32 . In the exemplary embodiment, the angle α is greater than 0°. However, an angle of 0° can also be advantageous. The angle α is preferably from 0° to 30°, in particular from 0° to 25°. Here, the angle α is measured in a section plane which contains the intake channel longitudinal axis  32  and which extends parallel to the center axis  33  of the channel  26 . In the exemplary embodiment, the section plane contains both the intake channel longitudinal axis  32  and the center axis  33  and corresponds to the section plane illustrated in  FIG. 2 . Should the intake channel longitudinal axis  32  and the center axis  33  extend obliquely to one another, the angle α is measured between the intake channel longitudinal axis  32  and a projection of the center axis  33  into the section plane in a projection direction perpendicular to the section plane. 
     The base body  23  of the mixture formation unit  13  has a first longitudinal side  58  and a second longitudinal side  59 . The longitudinal sides  58  and  59  extend approximately parallel to the center axis  33  of the intake channel section  11 . A fuel pump  46  can advantageously be formed on the first longitudinal side  58 . The fuel pump  46  is delimited by the base body  23 , by a pump cover  47  fixed to the base body  23  and also by a pump membrane (not shown). The pump cover  47  is preferably screwed to the base body  23  via a fastening screw  48 . On the opposite longitudinal side  59 , a regulating chamber  42  and a compensation chamber  43  which are separated by a control membrane  44  may advantageously be formed. The control membrane  44  is held on the base body  23  by a regulating chamber cover  62  schematically illustrated in  FIG. 2 . The regulating chamber  42  can advantageously be coupled in a customary manner by a spring-loaded lever to an inlet valve which controls the fuel flow from the fuel pump  46  into the regulating chamber  42 . The regulating chamber  42  is connected to secondary fuel openings  28  via a check valve  45 . Additionally leading out of the regulating chamber  42  is a fuel channel  64  in which a fixed throttle  63  is arranged in the exemplary embodiment. Instead of the fixed throttle  63 , an adjustable throttle, for example, can be provided. 
     The main fuel nozzle  29  is arranged in the channel  26 . On the outside circumference of the main fuel nozzle  29  there is formed an annular gap  30  into which the fuel channel  64  opens. The annular gap  30  is delimited by a peripheral groove on the outside circumference of the main fuel nozzle  29  and by the wall of the channel  26 . In the main fuel nozzle  29  there is formed a transverse channel  65 , which, in the exemplary embodiment, extends perpendicular to the center axis  33 , and a longitudinal channel  66  which extends in the direction of the center axis  33  centrally through the main fuel nozzle  29 . The annular gap  30  is connected to the longitudinal channel  66  via the transverse channel  65 . The longitudinal channel  66  opens at a valve plate  52 . The valve plate  52  forms with a valve seat  54  a check valve  31 . In the closed state of the check valve  31 , the valve plate  52  bears against the valve seat  54 . In the case of excess pressure in the intake channel section  11  with respect to the regulating chamber  42 , the check valve  31  is closed. In the case of negative pressure in the intake channel section  11 , the valve plate  52  is lifted off the valve seat  54 . The check valve  31  has a stop  53  which delimits the maximum stroke of the valve plate  52 . The stroke of the valve plate  52  is preferably as small as possible. 
       FIG. 3  shows the mouth of the channel  26  on the first end side  24 . In the exemplary embodiment, the channel  26  is formed completely in the base body  23  and configured to be at least partially closed over its circumference. For this purpose, the base body  23  has a section  67  which projects into the intake channel section  11 . The section  67  reduces the free flow cross section in the mixture channel  12 . In the exemplary embodiment, the section  67  has a bevel  68 , with the result that the flow cross section increases at the section  67  in the direction of the arrow  51  ( FIG. 2 ). The bevel  68  is also illustrated in  FIG. 2 . In an alternative embodiment, the bevel  68  can also be dispensed with. P  FIG. 4  shows an exemplary embodiment of a mixture formation unit  13  whose construction substantially corresponds to the mixture formation unit  13  shown and described in  FIGS. 2 and 3 . The same reference signs denote elements corresponding to one another in all the figures. In the mixture formation unit  13  in  FIG. 4 , the channel  26  has been rotated with respect to the embodiment shown in  FIGS. 1 and 2 . The channel  26  extends parallel to the intake channel longitudinal axis  32 . The center axis  33  of the channel  26  and the intake channel longitudinal axis  32  enclose an angle of 0°, and thus extend in parallel. 
       FIG. 5  shows an exemplary embodiment of a mixture formation unit  13  in which a fuel valve  60  is arranged in the channel  26 . In terms of the further construction, the mixture formation unit  13  corresponds to the mixture formation unit  13  shown and described in  FIGS. 2 and 3 . The fuel valve  60  is an electromagnetic valve, preferably a valve which is open in the de-energized state. It can also be advantageous for the fuel valve  60  to be closed in the de-energized state. The fuel valve  60  likewise has a valve plate  52  which, however, is not acted upon by the prevailing pressure conditions, but rather by a spring  69  and an electromagnet  70 . If current flows through the electromagnet  70 , the valve plate  52  is drawn against an inlet opening  71  against the force of the spring  69  and closes the opening. In the de-energized state, the valve plate  52  is drawn toward a stop  53  and frees the inlet opening  71  in this position. To activate the electromagnet  70 , the fuel valve  60  is connected to a control unit  61 . The control unit  61  is in particular a control unit which also controls the ignition time of the two stroke engine  1  or of a four stroke engine. 
     In the exemplary embodiment, a check valve  81  is arranged in the fuel channel  64  which connects the regulating chamber  42  to the channel  26 . The check valve  81  closes in the flow direction from the channel  26  to the regulating chamber  42 . In the exemplary embodiment, the check valve  81  is arranged downstream of the fixed throttle  63 . Another arrangement of the check valve  81  can also be advantageous. 
     Another configuration of the fuel valve  60  can also be advantageous. Instead of the main fuel nozzle  29  or the fuel valve  60 , it is also possible for other components to be arranged in the channel  26 . In the channel  26  there can be arranged in particular a needle valve or a spring-loaded valve, as is used for example in a purger. 
     In the exemplary embodiments, the mixture formation unit  13  is configured as a carburetor. A carburetor delivers the fuel into the intake channel as a result of the negative pressure existing in the intake channel. In an alternative configuration, another mixture formation unit can also be provided. The mixture formation unit can in particular have a fuel valve which supplies fuel into the intake channel as a result of excess pressure of the fuel, in particular injects the fuel into the intake channel. 
     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.