Abstract:
A two-stroke engine has a cylinder with a combustion chamber that is delimited by a reciprocating piston that drives with a connecting rod a crankshaft rotatably supported in a crankcase. In predetermined positions of the piston, the crankcase is connected by at least one transfer channel to the combustion chamber. The two-stroke engine has a mixture channel for supplying a fuel/air mixture and an air channel that supplies substantially fuel-free air to the transfer channel. In order to provide a simple adjustment of the air channel to different two-stroke engines of a model range, a component in which the air channel is formed has a throttle member that is arranged at an end face of the component. The throttle member throttles the air flow through the air channel in at least one operating state of the two-stroke engine.

Description:
BACKGROUND OF THE INVENTION 
   The invention concerns a two-stroke engine, in particular, for a hand-guided working tool such as a motor chainsaw, a cut-off machine or the like. The two-stroke engine comprises a cylinder in which a combustion chamber is disposed that is delimited by a reciprocating piston. The piston drives by means of a connecting rod a crankshaft rotatably supported in a crankcase. The crankcase, in predetermined positions of the piston, is connected by at least one transfer channel to the combustion chamber. The engine further comprises a mixture channel for supplying a fuel/air mixture and an air channel that supplies substantially fuel-free air to the transfer channel. 
   U.S. Pat. No. 6,450,135 B1 discloses a two-stroke engine that supplies substantially fuel-free air to the transfer channels arranged near the exhaust port. The substantially fuel-free air serves for scavenging the exhaust gas from the combustion chamber. The air that is contained in the transfer channels must be matched to the supplied quantity of fuel/air mixture. The supplied fuel quantity can be adjusted conventionally by means of an adjusting screw of a carburetor. In order to match the supplied air quantity to the operational state of the internal combustion engine, a throttle valve can be provided in the air channel. 
   The flow cross-section of the air channel is very small in two-stroke engines of small piston displacement. Mounting of the throttle valve is difficult in such a small channel. Since for different two-stroke engines different flow cross-sections of the air channel are required, it is necessary to provide air channels with different flow cross-sections for a cylinder model range with different piston displacements. This requires a significant expenditure in regard to tools for manufacturing the air channels as well as for stockholding the different channels. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to to provide a two-stroke engine of the aforementioned kind that enables a simple adjustment of the flow cross-section of the air channel. 
   In accordance with the present invention, this is achieved in that on one end face of a component in which the air channel is formed, a throttle member is arranged that throttles the air flow through the air channel in at least one operating state of the two-stroke engine. 
   In accordance with the present invention, this is achieved also in that a throttle member embodied as a fixed aperture is arranged in the air channel, wherein the flow cross-section of the aperture is matched to the displacement of the two-stroke engine. 
   The throttle member enables an adjustment of the air flow through the air channel without having to change the air channel itself. In this way, for all cylinders of a model range the same air channel can be used. Since the throttle member is provided on an end face of a component, it can be mounted on the air channel, or exchanged, in a simple way. 
   Preferably, the throttle member is arranged at the intake of the air channel. However, it can also be expedient to arrange the throttle member at the outlet of the air channel into the cylinder. The throttle member can be arranged, without having to change the air channel itself, at the intake into the air channel or the outlet from the air channel. However, it can also be provided that the throttle member is arranged between two components that delimit the air channel. In this case, the throttle member can be arranged in a simple way between the two present components that are present without having to change anything on the components that delimit the air channel. 
   Preferably, the flow cross-section in the throttle member can be variable. It was found that in two-stroke engines that require an adjustment of the flow cross-section the reduction of the flow-cross section is not needed in all operating states. For example, under full load the supply of a large quantity of substantially fuel-free air can be expedient in order to achieve a sufficient scavenging of the combustion chamber and to thus achieve minimal exhaust gas values. When employing a carburetor for supplying fuel, an enrichment of the mixture will result at high engine speed because of the flow conditions. This enrichment can be compensated by supplying a larger amount of air. At low engine speed or when accelerating, the supply of a reduced amount of substantially fuel-free air is required in order to be able to generate a combustible mixture in the combustion chamber. The adjustment of the flow cross-section can be realized in a simple way by adjustment of the flow cross-section of the throttle member. 
   Preferably, the flow cross-section of the throttle member is mechanically adjustable. However, it can also be expedient for the flow cross-section of the throttle member to be pneumatically adjustable. It is provided that the flow-cross-section of the throttle member is pressure-dependent. The flow cross-section of the throttle member changes accordingly in particular as a function of the pressure in the air channel. The pressure in the air channel is different for different operating states of the two-stroke engine. With increasing engine speed, the vacuum increases, i.e., the pressure is reduced. Accordingly, the vacuum can be used for the adjustment of the flow cross-section of the throttle member. However, the flow cross-section in the throttle member can also be dependent on the engine speed of the two-stroke engine. 
   It is provided that a throttle element is arranged in the mixture channel. The throttle element is in particular the throttle valve of a carburetor arranged in the mixture channel. The throttle element however can also be configured as a roll-type or barrel-type throttle (throttle barrel). Also, throttle elements of other configurations can be advantageous. Advantageously, the flow cross-section in the throttle member depends on the position of the throttle element in the mixture channel. In particular, the change of the flow cross-section of the throttle member takes place with delay, i.e., is dampened. 
   It is provided that in the air channel a throttle element is arranged in a component that delimits the air channel. The throttle element in the air channel can be, for example, a throttle valve whose position is coupled to the position of the throttle element in the mixture channel. In the case of a direct coupling of the throttle element in the air channel to the throttle element in the mixture channel, an optimal opening characteristics of the throttle valve in the air channel does not result. At low engine speed the two-stroke engine receives too much substantially fuel-free air while at high engine speed the supplied air is insufficient for proper combustion chamber scavenging. This additional adjustment can be achieved by a throttle member that is arranged upstream or downstream. 
   Advantageously, the throttle member throttles the air flow through the air channel in idle condition and at low engine speed of the two-stroke engine. Expediently, the throttle member throttles the air flow through the air channel upon accelerating the two-stroke engine. In these operating states the reduction of the flow cross-section by means of a throttle valve arranged in the air channel is not sufficient. The additional throttle member enables in a simple way a further reduction of the supplied air quantity. However, it can also be expedient to arrange the throttle member at the end face of a component delimiting the air channel in the case of an air channel in which no additional throttle element is arranged. 
   It is provided that the flow cross-section of the air channel is matched to the two-stroke engine by selecting a suitable throttle member. The two-stroke engine of a model range can be configured in accordance with a modular principle wherein the two-stroke engine has air channels that differ only in the selected throttle element. In this way, a model range can be built in a simple way. 
   A two-stroke engine that enables a simple adaptation of the flow cross-section of the air channel is also achieved by a two-stroke engine comprising a cylinder, in which a combustion chamber is formed that is delimited by a reciprocating piston wherein the piston drives by means of a connecting rod a crank shaft supported rotatably in a crankcase, wherein the crankcase in predetermined positions of the piston is connected by at least one transfer channel to the combustion chamber; comprising a mixture channel for supplying a fuel/air mixture; and comprising an air channel that supplies to the transfer channel substantially fuel-free air, wherein in the air channel a fixed aperture is arranged, wherein the flow cross-section of the aperture is matched to the displacement of the two-stroke engine. 
   The fixed aperture in the air channel enables an adjustment of the air flow passing through the air channel to the displacement of the two-stroke engine. Accordingly, the air channel itself must not be changed so that for cylinders of a model range with different displacement the same air channel with a different fixed aperture can be used. The aperture can be arranged at any location within the air channel. 
   Advantageously, the ratio of the flow cross-section of the aperture in square millimeters relative to the displacement of the two-stroke engine in cubic centimeters is smaller than 3.5. It was found that for such a configuration of the flow cross-section of the aperture relative to the displacement of the two-stroke engine an excellent adjustment in regard to the throughput of the two-stroke engine can be achieved. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a longitudinal section view of a two-stroke engine. 
       FIG. 2  is a schematic illustration of a section of the two-stroke engine of  FIG. 1  along the line II-II. 
       FIG. 3  is a schematic illustration of a first embodiment of a throttle member arranged in an air channel. 
       FIG. 4  is a schematic illustration of a second embodiment of a throttle member arranged in an air channel. 
       FIG. 5  is a schematic illustration of a third embodiment of a throttle member arranged in an air channel. 
       FIG. 6  is a schematic illustration of a fourth embodiment of a throttle member arranged in an air channel. 
       FIG. 7  is a schematic section illustration of a two-stroke engine at the level of the line II-II in  FIG. 1  showing a first arrangement of a throttle member. 
       FIG. 8  is a schematic section illustration of a two-stroke engine at the level of the line II-II in  FIG. 1  showing a second arrangement of a throttle member. 
       FIG. 9  shows a diagram that indicates the total throughput and fuel/air mixture throughput through the two-stroke engine as a function of the cross-sectional surface area of the throttle member. 
       FIG. 10  shows a diagram that indicates the air throughput and the fuel throughput through the two-stroke engine as a function of the cross-sectional surface area of the throttle member. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The two-stroke engine  1  illustrated in  FIG. 1  has a cylinder  2  in which a combustion chamber  3  is formed. The combustion chamber  3  is delimited by a piston  5  that drives by means of a connecting rod  6  a crankshaft  7  rotatably supported in a crankcase  4 . As also shown in  FIG. 2 , the two-stroke engine  1  has two opposed transfer channels  8  near the exhaust port that open with transfer ports  22  into the combustion chamber  3 . Remote from the exhaust port, two opposed transfer channels  9  are provided that open with transfer ports  23  into the combustion chamber. In the area of the bottom dead center of the piston  5  illustrated in  FIG. 1 , the transfer channels  8 ,  9  connect the crankcase  4  to the combustion chamber  3 . Exhaust port  25  for exhaust gas leads away from the combustion chamber  3 . 
   The two-stroke engine  1  has a mixture channel  10  that connects an air filter  15  to an intake  24  into the crank case  4 . The intake  24  is open in the area of the top dead center of the piston  5 . The mixture channel  10  extends within the carburetor  12  and an elastic intake pipe  20 . A choke valve  13  and a throttle valve  14  are arranged in the carburetor  12 . In the area of the throttle valve  14  fuel ports open into the mixture channel  10  and supply fuel to the air that has been taken in into the mixture channel  10 . 
   The two-stroke engine  1  has an air channel  11  that supplies the transfer channels  8  and  9  with substantially fuel-free air. A section of the air channel  11  is formed within a pipe section  26  in which a throttle valve  19  is pivotably supported. The position of the throttle valve  19  is coupled in particular to the position of the throttle valve  14  in the mixture channel  10 . The pipe section  26  extends parallel to the section of the mixture channel  10  that is disposed within the carburetor  12 . The pipe section  26  is secured on the carburetor  12  and can be formed as a monolithic part thereof. The mixture channel  10  and the air channel  11  are connected to the clean chamber  18  of the air filter  15 . The clean chamber  18  is separated by filter material  16  from the dirt chamber  17  of the air filter  15 . On the end face  46  of the pipe section  26  that faces the air filter  15  the throttle member  27  is secured. The throttle member  27  can be secured also in the air filter bottom or between the pipe section  26  and the air filter  15 . 
     FIG. 2  shows that the air channel  11  downstream of the pipe section  26  divides into two branches  32  and  33 . Each branch  32 ,  33  opens via an air channel port  34  at the cylinder bore  48 . The air channel ports  34  are advantageously arranged on the side of the transfer port  23  of the transfer channel  9  that is facing the crankcase  4 . The piston  5  has two piston recesses  21  that connect the air channel  11  in the area of the top dead center of the piston  5  to the transfer channels  8 ,  9 . The connection is realized via the air channel ports  34 , the piston recesses  21 , and the transfer ports  22  and  23 . As shown in  FIG. 2 , the sections  49  and  50  of the air channel  11  opening at the air channel ports  34  are formed within the cylinder  2 . 
   In operation of the two-stroke engine  1 , fuel/air mixture is sucked in through the intake  24  into the crankcase  4  in the area of the top dead center of the piston  5 . Through the air channel  11  and the piston recess  21  the transfer channels  8 ,  9  are flushed from the side facing the combustion chamber  3  with substantially fuel-free air. Upon downward stroke of the piston  5  the fuel/air mixture is compressed in the crankcase  4 . As soon as the transfer channels  8 ,  9  open toward the combustion chamber  3 , the air that is located upstream of the transfer channels  8 ,  9  flows into the combustion chamber  3  and flushes the exhaust gases within the combustion chamber  3  through the exhaust port  25  out of the combustion chamber  3 . The fuel/air mixture that flows into the combustion chamber  3  from the crankcase  4  is compressed in the subsequent upward stroke of the piston  5  within the combustion chamber  3  and ignited in the area of the top dead center by means of the spark plug  56  projecting into the combustion chamber  3 . As soon as the exhaust port  25  opens upon subsequent downward movement of the piston  5 , the exhaust gases flow out of the combustion chamber  3  and are scavenged out by means of the substantially fuel-free air flowing from the transfer channels  8 ,  9  into the combustion chamber  3 . 
   The quantity of substantially fuel-free air that is supplied to the transfer channels  8  and  9  depends on the flow cross-section of the air channel  11 . By means of the throttle valve  19  the flow cross-section is adjusted to the operating state of the two-stroke engine  1 . At low engine speed, the throttle valve  19  is substantially closed so that only a minimal amount of substantially fuel-free air is located upstream within the transfer channels  8  and  9 . At full load, the throttle valve  19  is completely open and impairs only minimally the flow cross-section in the air channel  11 . In this way, a large quantity of substantially fuel-free air is located upstream of the transfer channels  8  and  9 . The throttle member  27  is configured as a fixed aperture. Accordingly, the throttle member  27  reduces the air flow through the air channel  11  in any operating state of the two-stroke engine  1 . In this way, the effective flow cross-section of the air channel  11  can be reduced without the air channel  11  itself having to be changed in regard to its configuration. 
   Embodiments of throttle members are illustrated in  FIGS. 3 to 5 . The throttle member  28  illustrated in  FIG. 3  has a fixed aperture  29 . Relative to the flow direction  31  in the air channel  11 , a movable diaphragm  30  is arranged downstream of the fixed aperture  29 . The diaphragm  30  has a fixed end  90  with which it is secured to the aperture  29 . An opposed free end  91  is movable relative to the aperture  29 . The diaphragm  30  is arranged downstream of an opening  92  in the aperture  29 . As a function of the air mass flow through the opening  92 , the free end  91  is pushed away more or less from the aperture  29 . In this way, the diaphragm  30  throttles the air flow in the air channel  11  as a function of the air mass flow through the throttle member  28 . 
   In the throttle member  35  illustrated in  FIG. 4 , the throttle action is realized as a function of the pressure in the air channel  11 . The throttle member  35  has a throttle body  36  that projects into an opening  37  in the throttle member  35 . The opening  37  delimits the air channel  11 ; the flow cross-section of the opening  37  corresponds advantageously to the flow cross-section of the air channel  11 . The throttle body  36  is slidably supported in a housing  93  and is seal-tightly guided in a bore  94 . By means of a spring  38  the throttle body  36  is spring-loaded into the opening  37 . Between the throttle body  36  and the housing  93  an annular chamber  40  is formed in which a predetermined pressure, in particular, ambient pressure, is present. In the housing  93 , a chamber  95  is formed in which the spring  39  is arranged. The chamber  95  is separated from the annular chamber  40  by a diaphragm  39 . The throttle body  36  is secured on the diaphragm  39 . The air channel  11  communicates by means of a compensating bore  45  with the chamber  95 . The underpressure that is present in the air channel  11  is transmitted through the compensation bore  45  into the chamber  95 . The compensating bore  45  opens into the air channel  11  at the upstream side of the throttle body  36 . When the pressure drops in the air channel  11  and thus also within the chamber  95 , the force that is exerted by the annular chamber  40  onto the diaphragm  39  increases as a result of the constant pressure in the annular chamber  40 . In this way, the throttle body  36  is pulled in the direction toward the chamber  95  away from the opening  37 . The throttle body  36  has a cavity  42  in its interior; the cavity is filled with a damping medium  41 . A piston  43  that is fixedly secured to the housing  93  projects into the cavity  42 ; the cavity  42  is movable relative to the piston in the movement direction of the throttle body  36 . The piston  43  has a compensation opening  44  between the two ends of the piston  43  and the damping medium  41  flows through the opening upon movement of the piston  43 . In this way, the movement of the throttle body  36  is dampened. 
   With increasing engine speed of the two-stroke engine  1 , the under pressure in the air channel  11  increases and the absolute pressure therefore drops. This leads to the throttle body  36  of the throttle member  35  being pulled out of the opening  37  so that the flow cross-section in the air channel  11  increases and the sucked-in air quantity increases. At low engine speed the under pressure in the air channel  11  is minimal so that the throttle body  36  projects far into the opening  37  and greatly reduces the flow cross-section. In this way, it can be ensured that at low engine speed only a minimal quantity of substantially fuel-free air is supplied and that the fuel/air mixture that is introduced into the combustion chamber is sufficiently enriched in order to ensure combustion. 
   In the case of the throttle member  75  illustrated in  FIG. 5 , the change of the flow cross-section in the throttle member  75  is realized mechanically. In this case, the change of the flow cross-section is coupled to the position of the throttle valve  14  in the mixture channel  10 . For this purpose, a lever  78  is fixedly attached to the throttle shaft  74  of the throttle valve  14 . The lever  78  is preferably arranged outside of the mixture channel  10  on the throttle shaft  74 . The throttle member  75  has a fixed aperture  76  with an opening  82  that delimits the air channel  11 . A slide  77  is movably supported transversely to the flow direction  31  in the air channel  11  in the aperture  76 . The slide  77  is preferably arranged perpendicularly to the flow direction  31  in the air channel  11  but it can also be arranged angularly to the flow direction  31  in order to achieve beneficial geometric conditions for its actuation. The slide  77  has a bore  79  that, in the partially open position of the throttle valve  14  illustrated in  FIG. 5 , is arranged in a staggered position in the mixture channel  10  relative to the opening  82  of the apertures  76  so that the slide  77  reduces the flow cross-section of the opening  82 . The lever  78  has a pin  80  that projects into a slotted hole  81  in the slide  77 . Upon rotation of the throttle shaft  74 , the lever  78  moves the slide  77  by means of the pin  80 . Upon further opening of the throttle valve  14 , i.e., upon a rotation of the throttle shaft  74  in  FIG. 5  in the clockwise direction, the slide  77  is pulled downwardly, the bore  79  is pulled into the opening  82 , and the flow cross-section in the throttle member  75  is enlarged. Upon closing of the throttle valve  14 , i.e., upon rotation of the throttle shaft  74  in counterclockwise direction in  FIG. 5 , the opening  79  in the slide  77  is pushed out of the opening  82  so that the flow cross-section in the throttle member  75  is reduced more. In this way, the flow cross-section of the throttle member  75  is coupled to the position of the throttle valve  14  in the mixture channel  10 . 
   In the throttle member  85  illustrated in  FIG. 6 , a slide  87  with an opening  79  projects into the opening  82  of the aperture  76 . The slide  87  is secured by a sleeve  88  which is coupled in the longitudinal direction of the slide  87  to the webs  89 . Two of the webs  89  secure a body of inertia  86 , respectively, that is embodied as a centrifugal member and connected to the crankshaft  7  of the two-stroke engine. As a function of the speed of the crankshaft  7 , the body of inertia  86  is deflected more or less outwardly as a result of centrifugal force. By means of the webs  89  the movement of the body of inertia  86  is transmitted onto the sleeve  88 . With increasing engine speed, the inertia bodies  86  are accelerated radially outwardly. As a result of this movement, the sleeve  88  is moved in the longitudinal direction of the slide  87  such that the slide  87  is pulled out of the aperture  76 . In this way, the flow cross-section of the air channel  11  is reduced to a lesser degree by the slide  87 . With dropping engine speed, the bodies of inertia  86  are pulled radially inwardly by means of the springs  84  by which the bodies of inertia  86  are secured on the crankshaft  7 . In this way, the sleeve  88  is displaced in the longitudinal direction of the slide  87 . The slide  87  is pushed into the opening  82  of the aperture  76  so that the flow cross-section in the air channel  11  is throttled more. 
   The two-stroke engine  1  illustrated in  FIG. 7  has an air channel  51 . The air channel  51  is formed downstream of the air filter within the pipe section  54  in which a throttle valve  19  is pivotably supported. Downstream of the pipe section  54  the air channel  51  divides into two branches  52  and  53  that open via an air channel port  34  at the cylinder bore  48 , respectively. The two branches  52  and  53  are formed within a channel section  58 . A throttle member  55  is arranged in the air channel  51  for reducing the flow cross-section. The throttle member  55  is arranged at the downstream end face  47  of the pipe section  54  between the pipe section  54  and the channel section  58 . The throttle member  55  can be configured as a fixed aperture. However, the flow cross-section of the throttle member  55  can also be variable. For example, throttle members can be used that are embodied as disclosed in  FIGS. 3 to 6 . 
   The two-stroke engine  1  illustrated in  FIG. 8  has an air channel  61  that divides into two branches  62  and  63 . The two branches  62  and  63  are formed in a channel section  68 . The branch  62  is secured with its end face  66  on the cylinder  2  and the branch  63  with the end face  67 . A throttle member  64 ,  65  is secured to the end faces  66  and  67 , respectively, that reduces the flow cross-section of the air channel  61 . The throttle member  64  is arranged between the branch  62  and the section  49  of the air channel  61  formed within the cylinder  2 . The throttle member  65  is arranged between the branch  63  and the section  50  of the air channel  61  formed in the cylinder  2 . The throttle member  64  and  65  are embodied as fixed apertures. However, throttle members with variable flow cross-section, for example, those of  FIGS. 3 to 6 , can be used also. 
     FIGS. 9 and 10  show diagrams that illustrate the throughput M through the internal combustion engine as a function of the flow cross-section A of a throttle member in the air channel  11 ,  51 ,  61 . Both diagrams show in this connection the throughput M at a fixed engine speed of the two-stroke engine  1 . The throughput M is illustrated in both diagrams as a function of the flow cross-section A of an aperture. 
   The curve  70  in  FIG. 9  shows the total throughput of air and fuel/air mixture through the two-stroke engine. With increasing flow cross-section A, the total throughput increases. The curve  71  illustrates the mixture throughput through the two-stroke engine  1 . The latter drops with increasing flow cross-section A of the throttle member. In order to be able to achieve a certain output of the two-stroke engine  1 , the total throughput illustrated in curve  70  through the engine cannot decrease arbitrarily. A minimum throughput must be ensured. For this reason, the flow cross-section A through the throttle member cannot be selected to be arbitrarily small. At the same time, a sufficient supply of fuel to the two-stroke engine must be ensured. A high throughput of fuel/air mixture, illustrated by curve  71 , is however achieved by reduced flow cross-sections A. When arranging a throttle member  27  that is configured as a fixed aperture in the air channel  11 , the flow cross-section A represents an optimal value for these two requirements. In order to ensure a predetermined output of the two-stroke engine  1  and at the same time a satisfactory fuel supply, the ratio of flow cross-section A of the aperture in square millimeters relative to the displacement of the two-stroke engine  1  in cubic centimeters is less than 3.5. Advantageously, the ratio is 0.9 to 3.5, expediently 0.9 to 2.5, and, in particular, 2.1 to 3.2. Preferably, the ratio of the flow cross-section A in square millimeters to the displacement of the two-stroke engine in cubic centimeters is within a range of 2.1 to 3.2. The flow cross-section A is advantageously between a minimal flow cross-section  96  and a maximum flow cross-section  97  shown in the diagram in  FIG. 9 . 
   As shown in  FIG. 10 , the pure air throughput that is illustrated by curve  73  increases with increasing flow cross-section A of the aperture. When arranging a throttle member  27  that is configured as a fixed aperture in the pure air channel, it must be taken into account that the flow cross-section A of the aperture is sufficiently large so that the air located upstream in the transfer channels is sufficient for separating exhaust gases and the mixture that flows in from the crankcase. As illustrated in curve  72 , the fuel throughput through the aperture initially drops greatly and subsequently only weakly with increasing flow cross-section A. In order to achieve also at low engine speed a still sufficient enrichment of the fuel/air mixture, a certain fuel quantity must be supplied to the two-stroke engine. In order to achieve a sufficient enrichment as well as adequate scavenging action, in the arrangement of a fixed aperture in the air channel it is necessary to adjust the flow cross-section A of the aperture relative to the displacement of the two-stroke engine. It was found that the ratio of the flow cross-section A of the aperture in square millimeters to the displacement of the two-stroke engine  1  in cubic centimeters should be smaller than 3.5. In particular, the ratio should be 0.9 to 3.5, advantageously 0.9 to 3.2. The ratio is preferably 2.1 to 3.2. 
   In  FIGS. 9 and 10 , an advantageous minimal flow cross-section  96  and an advantageous maximum flow cross-section  97  for the flow cross-section A of the aperture is indicated, respectively. These flow cross-sections are dependent on the displacement of the two-stroke engine, respectively. The displacement of the two-stroke engine  1  is the volume that is displaced by the piston  5  upon movement between the bottom dead center and the top dead center. 
   This application incorporates by reference the entire disclosure of German priority application 10 2004 060 046.5 filed Dec. 14, 2004. 
   While specific embodiments of the invention have been shown and described in detail to illustrate the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.