Abstract:
A two-cycle engine having forward scavenging is provided. Mixture drawn into the crankcase via a butterfly valve carburetor is conveyed into a combustion chamber via transfer channels in the cylinder. An air duct is connected via a controllable connection with a transfer channel to supply essentially fuel-free air thereto during a load state of the engine. To convey a fuel quantity adapted to drawn-in air during idling and partial load, yet during full throttle to achieve separated supply of air and mixture, a dividing wall extends in the direction of flow of air in the carburetor intake duct. In the pivot region of the butterfly valve, a connecting aperture in the dividing wall is closed in full throttle by a completely open butterfly valve. During idling and partial load the connecting aperture is open so that a uniform pressure can form in the intake duct in conformity with drawn-in air.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a two-cycle engine, especially as a drive engine in a portable, manually-guided tool or implement such as a power chain saw, a brush cutter, a trimmer, a cut-off machine, etc. 
     A two-cycle engine of this type is known from DE 199 00 445 A1. A combustion chamber formed in the cylinder is connected to the crankcase via transfer passages, the mixture required for combustion being conveyed to the crankcase. In order to ensure that as-little uncombusted fuel as possible is lost through the exhaust or outlet during the scavenging of the combustion chamber, the transfer passages close to the exhaust are connected to an air duct and fuel-free air is drawn in through the transfer passages during the intake stroke. The air is then held at the front of the transfer passages and enters first the next time the mixture transfers into the combustion chamber. The mixture flowing out of the crankcase follows some time later and the scavenging losses flowing out of the exhaust during the scavenging of the combustion chamber come largely from the forward positioned scavenging air. 
     In practice, a number of problems occur during the metering of the fuel required to operate the internal combustion engine by a carburetor. For example, at idle it is necessary to guarantee that the air duct is fully closed in order to prevent the idle mixture becoming too lean in an uncontrolled manner in the combustion chamber as a result of the air flowing into it. During acceleration, too, the opening of the air duct renders the mixture too lean as a result of which the speed of the internal combustion engine increases only reluctantly to the desired level. 
     On the other hand, it is important to guarantee that the air duct remains as free as possible from fuel at full throttle in order that the significant reduction in exhaust gas emissions which the forward positioned scavenging air is designed to achieve can be obtained. 
     The invention is based on the object of designing a two-cycle engine of the aforementioned type in such a manner that it is possible to reliably prevent the mixture in the combustion chamber from becoming too lean at idle and part throttle while retaining the advantageous effects of the supply of fuel-free air with which to scavenge the combustion chamber at full throttle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This object, and other objects and advantages of the present invention, will appear more clearly from the following specification in conjunction with the accompanying schematic drawings, in which: 
         FIG. 1  is a schematic view of a two-cycle engine with port-controlled forward scavenging air positioning and a single-flow carburetor. 
         FIG. 2  a schematic section along the line marked II—II in FIG.  1 . 
         FIG. 3  a schematic view of a section of a membrane-controlled system with forward scavenging air positioning as illustrated in FIG.  2 . 
         FIG. 4  a schematic sectional view through a carburetor with a throttle valve and a choke valve. 
         FIG. 5  a schematic view of the front face of a carburetor with an eccentrically positioned butterfly valve shaft. 
     
    
    
     SUMMARY OF THE INVENTION 
     A dividing wall in the intake duct of the carburetor divides the venturi along its longitudinal center line into an intake duct section and an air duct. Here the dividing wall is essentially provided along the entire length of the intake duct from one front face of the carburetor body to its other front face in such a manner that even fuel precipitating due to return pulsation upstream of the butterfly or throttle valve is unable to simply pass into the air duct. A connecting aperture is formed in the dividing wall in the pivot region of the throttle valve. At full throttle the throttle valve closes the connecting aperture in the dividing wall in such a manner that the dividing wall, which extends as far as the upstream front face, opposes any transfer of fuel upstream of the throttle valve. The dividing wall preferably extends as far as the base of an air filter fitted upstream of the carburetor, expediently into the air filter housing and in particular as far as the filter element itself. The extension of the dividing wall upstream of the throttle valve into the filter housing achieves a functional division of air duct and mixture duct on the intake side. 
     The design disclosed in the invention ensures that the pressure prevailing in the venturi at idle and part throttle corresponds to the joint pressure in the air duct and the mixture duct. The volume of fuel conveyed into the venturi in accordance with this joint underpressure is thus proportional to the volume of air conveyed, irrespective of whether it is conveyed to the combustion chamber via the mixture duct or the air duct. This prevents the mixture from becoming too lean at both idle and part throttle. 
     Similarly, if a choke valve is provided this arrangement guarantees that the underpressure prevailing due to the adjustment of the choke is the same throughout the entire system in such a manner that under choke conditions, too, a volume of fuel adapted to the volume of air drawn in is conveyed and mixed with the air. 
     In order to achieve a dry, i.e. largely fuel-free, air duct at full throttle, the aperture edge of the connecting aperture and the edge of the valve overlap. Here the overlapping aperture edge can be designed as a seat for the edge of the valve and the aperture edge can also have a seal. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The two-cycle engine  1  illustrated schematically in  FIG. 1  is used as a small-volume drive engine preferably in manually operated, portable tools such as, for example, chain saws, brush cutters, parting-off grinders, etc. The displacement of an internal combustion engine of this type lies within a range of 18 cm 3  and 500 cm 3 . 
     The two-cycle engine  1  has a cylinder  2  in which is provided a combustion chamber  3  which is delimited by a reciprocating piston  5 . Via a connecting rod  6 , the piston  5  drives a crankshaft  4  which is mounted in a crankcase  4  in such a manner that it can rotate. 
     An inlet  20 , which in the illustrated embodiment is controlled by the piston skirt  30  opens into the crankcase  4 . In the embodiment shown, the inlet  20  is therefore opened and closed dependent upon the stroke position of the piston  5 . It can be useful to provide a membrane or diaphragm control system instead of the piston port control system illustrated. The inlet  20  then opens into the crankcase  4  outside the piston stroke area, it being necessary to position a membrane valve which opens in the direction of the crankcase  4  in the inlet  20 . The opening of the inlet  20  is then controlled by underpressure. 
     The crankcase  4  is connected to the combustion chamber  3  via transfer passages  12 ,  15 , these transfer passages—see FIG.  2 —being designed as straight or handle-shaped passages in the side wall of the cylinder. In the version illustrated, two transfer passages  12  and two transfer passages  15  are provided, one of each on either side of a plane of symmetry  19 . The transfer passages  15  are located close to an outlet or exhaust  10  which conveys exhaust gases out of the combustion chamber  3  and are also referred to as exhaust transfer passages  15 . The transfer passages  12  are positioned some distance from the exhaust  10  and are referred to as exhaust-distant transfer passages  12 . As illustrated in the section shown in  FIG. 2 , the plane of symmetry  19  divides the cylinder  2  into symmetrical halves and runs roughly centrally through the exhaust  10  and the inlet  20 . 
     The end of each transfer passage  12 ,  15  facing the cylinder head  11  opens into the combustion chamber  3  via a transfer window or port  13 ,  16 . The transfer ports  13 ,  16  are controlled by the piston  5  as it reciprocates, the transfer ports  13 ,  16  being open in a lower piston position close to bottom dead center (BDC) illustrated in FIG.  1  and being closed in an upper piston position between BDC and top dead center (TDC). The ends of the transfer passages  12 ,  15  facing the crankcase  4  are open in both the lower and the upper piston positions. 
     Furthermore, the transfer passages  12 ,  15  can also be connected to an air duct  8  which opens into an air port  9  in the wall of the cylinder  2 . A connecting port  14  is formed in the piston skirt  30  at the level of the air port  9  and, as illustrated in  FIG. 2 , extends from the air port  9  opposite the exhaust  10  in both directions around the circumference of the piston covering a circumferential angle of some 120° such that in the corresponding piston stroke position the transfer ports  13 ,  15  communicate with the connecting port  14 , the connecting port  14  being designed such that it also connects with the air port  9  of the air duct  8  in this piston stroke position. Thus, when the piston  5  rises towards TDC, a connection is made between the air duct  8  and the transfer ports  13 ,  15  and due to the underpressure prevailing in the crankcase  4  at the time, medium is drawn in from the air duct  8  through the transfer passages  12 ,  15 . 
     The air duct  8  and an inlet duct  21  leading to the inlet  20  are connected separately to a mixture formation device which is a carburetor  17  in the embodiment shown. The carburetor  17  is expediently a diaphragm carburetor of the type predominantly used in drive engines in portable, manually operated tools. In the carburetor body  18  is a common intake duct  22  with a venturi  23 . Also positioned in the intake duct  22  is a throttle or butterfly valve  24  which is mounted on a throttle shaft  25  in the carburetor body  18  in such a manner that it is able to rotate. The common intake duct  22  is divided by means of a partition or dividing wall  31  which extends along the longitudinal center line  43  in the direction of the air flow  26 . The fuel feeders, in the embodiment illustrated idle jets  27  and a main fuel jet  28 , are located on one side of the dividing wall  31  which extends essentially from one front face  29   a  to the other front face  29   b  of the carburetor body  18  along the entire length l of the intake duct  22 . Here the part of the duct which contains the fuel feeders  27 ,  28  forms an intake duct section  32  which is connected to the inlet duct  21 . The other part of the duct forms an air duct  33  which is connected to the air duct  8  of the air port  9 . In the area of rotation of the throttle valve  24  is a connecting aperture  34  in the dividing wall  31  which forms a connection between the intake duct section  32  and the air duct  33 . This connection creates identical pressure conditions on both sides of the dividing wall  31  when the connecting aperture  34  is open. When the connecting aperture  34  is open, the diaphragm carburetor  17  therefore conveys a volume of fuel which is always proportional to the volume of air drawn in via the jets  27 ,  28 . 
     In the part throttle position illustrated in  FIG. 1 , the throttle valve is located half open transverse to the longitudinal center line  43  in the intake duct, the axis of rotation of the throttle valve being located exactly in the plane of the dividing wall  31 . In this throttle valve position, the connecting aperture  34  is partially open and the fuel drawn in through the fuel jets  27  therefore enters both the intake duct section  32  and the air duct  33  via the open connecting aperture  34 . At idle and/or part throttle, both the air duct  8  and the inlet duct  21  therefore convey a fuel/air mixture, it being possible, due to the arrangement of the jets in the intake duct section  32 , for the fuel/air mixture conveyed in the inlet duct  21  to be richer than that conveyed in the air duct  8  into which fuel is only allowed to enter via the partially opened connecting aperture  34 . 
     Downstream of the carburetor  17  the intake duct section  32  is connected to the inlet  20  via the inlet duct  21 , and the air duct  33  is connected to the air port  9  via the connecting or air duct  8 . Downstream of the carburetor  17  the air ducts  8 ,  33  therefore run separately from the mixture ducts  21 ,  32 . 
     When the internal combustion engine is in operation, as the piston  5  rises towards TDC the transfer ports  13 ,  16  and the exhaust  10  are closed. The rising piston  5  opens the inlet  20  and at the same time or a few crank angle degrees later connects the air port  9  to the transfer ports  13 ,  16  via the connecting port  14 . Thus at the same time as the air duct  8  is connected to the transfer passages  12 ,  15  or slightly earlier, the inlet  20  to the crankcase  4  is opened, allowing the mixture to flow into the crankcase  4 . When the air port  9  of the connecting port is connected to the transfer windows  13 ,  16 , a fuel-lean mixture or largely fuel-free air is drawn in and flows down through the transfer ports  13 ,  16  to the crankcase  4 . The transfer passages  12 ,  15  thus fill with lean mixture or with largely fuel-free air, the transfer passages  15  close to the exhaust preferably being filled with air. 
     Following ignition, the piston  5  descends to BDC again, the flow connection between the transfer passages  12 ,  15  and the air duct  8  being interrupted and the inlet  20  being closed. Since the piston  5  is descending, the mixture drawn into the crankcase  4  is compressed and, as the piston-controlled transfer ports  13 ,  16  are opened, flows into the combustion chamber  3 , filling it with fresh mixture for the next compression stroke. Here the fuel-lean or fuel-free air is positioned forward of the rich mixture in the crankcase  4  and scavenging losses flowing out through the open exhaust  10  are therefore largely formed by the fuel-lean mixture and the fuel-free air. 
     At full throttle, the throttle valve  24  is fully open as illustrated in the example of a diaphragm or membrane-controlled forward scavenging air positioning system shown in FIG.  3 . When the throttle valve  24  is fully open it lies roughly-parallel to the longitudinal center line  43  such that the air duct  33  and the intake duct section  32  are completely separate from each other since the throttle valve  24  preferably seals the connecting aperture  34 . In order to achieve this, the connecting aperture  34  is designed with a slightly smaller throughput section than that of the valve  24  itself. The aperture edge  35  of the connecting aperture  34  and the edge  36  of the throttle valve  24  overlap one another, thereby achieving a sealed fit. Here the aperture edge  35  is expediently designed as a seat for the edge  36  of the valve, the aperture edge  35  expediently bearing a seal  37 . The seal is preferably a rubber seal which may be provided in the form of a gasket or a tied-in seal. This guarantees that the air duct  8  is dry, i.e. free of fuel, at full throttle and thus that scavenging losses which occur during the scavenging of the combustion chamber  3  comprise exclusively fuel-free air. 
     In order to guarantee that the air duct  8 ,  33  remains free of fuel at full throttle, the dividing wall  31  is designed to extend upstream of the carburetor  17  as far as the base  40  of an air filter  41 . If the dividing wall  31 ′ ( FIG. 3 ) is taken into the air filter housing, preferably extended into the area of the filter element  42 , it is possible to prevent fuel precipitating in the air filter  41  as a result of air pulsation in the intake train from transferring to the air duct  33 . 
     While in the embodiment illustrated in  FIGS. 1 and 2  the connection between the air ducts  8 ,  33  and the transfer passages is controlled by piston ports,  FIG. 3  shows a connection between the air duct  8  and at least the transfer passages  15  close to the exhaust port via a distributor duct  38  and a non-return valve which is designed as a membrane valve  39  in the embodiment. The distributor duct  38  can be designed as an external duct, a hose connection or a duct integrated into the cylinder  2 . As the piston  5  rises, underpressure is created in the crankcase  4  and also in the transfer passages  12 ,  15  due to the fact that these transfer passages  12 ,  15  are open to the crankcase  4 . Due to the pressure difference thus created at the membrane valve  39 , the membrane valve  39  opens and fuel-lean mixture/fuel-free air is drawn into the transfer passage  15  close to the exhaust via the membrane valve  39 . As the piston  5  descends, the overpressure which builds up in the crankcase  4  closes the membrane valve  39 . It can also be useful to connect the transfer passages  12  to the air duct via a non-return valve such as a membrane valve, e.g. via a controlled connection to the distributor duct  38 . 
     In the embodiment illustrated in  FIG. 4 , a choke valve  44  is provided upstream of the throttle valve  24  and is mounted on a choke shaft  45  in the carburetor  17  or the carburetor body  18  in such a manner that it can rotate. The choke shaft  44  is located in the plane of the dividing wall  31 ,  31 ′. The choke valve  44  is associated with a further connecting aperture  46  in the dividing wall  31 , whereby when the choke valve  44  is in the open position illustrated in  FIG. 4  the further connecting aperture  46  is largely closed by the choke valve  44 . Here it is possible to provide sealing measures such as those which have already been described in relation to the throttle valve  24 . This design guarantees that when the choke and the partially opened throttle valve  24  are actuated, the higher intake underpressure produced takes effect in both the air duct and the mixture duct, the pressure conditions in the venturi are therefore identical and a volume of fuel proportional to the volume of air drawn in is metered. 
     It can be expedient to position the dividing wall  31 ,  31 ′ in the carburetor body  18  eccentrically in relation to the intake duct  22  thereby giving the air duct  33  and the mixture duct  32  different cross sectional areas. In this case, the throttle shaft  25  and a choke shaft  45  continue to be located approximately in the plane of the dividing wall  31 , but slightly offset relative to the center of the intake duct  22  as shown in FIG.  5 . The ratio A/L between the cross sectional area of the intake duct section  32  and the cross sectional area of the air duct  33  lies roughly within a range of 0.5 to 1.9 and preferably within a range of 0.54 to 1.86. This means that the cross sectional area of the air duct can be between 65% and 35% of the total cross sectional area of the intake duct  22 .