Abstract:
A two-stroke engine has a cylinder and a piston reciprocating in the cylinder and delimiting a combustion chamber. The piston has at least one piston recess. A crankshaft is rotatably arranged in a crankcase connected to the cylinder. A connecting rod connects piston and crankshaft. Transfer channels connect the crankcase to the combustion chamber in pre-defined piston positions. The transfer channels each have a transfer port opening into the combustion chamber. A device for supplying fuel and an air channel supplying combustion air are provided. In predetermined piston positions, the air channel is connected by the piston recess to the transfer ports. The air channel opens into the cylinder bore opposite the exhaust for the exhaust gases. The air channel, the piston recess, and the transfer channels define an air passage that provides approximately uniform distribution of air from the air channel into the transfer channels.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to a two-stroke engine comprising a cylinder in which a combustion chamber is provided that is delimited by a reciprocating piston. The piston drives by means of a connecting rod the crankshaft rotatably supported in a crankcase. In pre-defined piston positions, the crankcase is connected to the combustion chamber by means of at least two transfer channels wherein each transfer channel opens with a transfer port into the combustion chamber. The engine comprises a device for supplying fuel and an air channel for supplying combustion air. In predetermined piston positions, the air channel is connected by a piston recess provided at the piston to the transfer ports of the two transfer channels. An exhaust for the exhaust gases generated in the combustion chamber is provided wherein the air channel opens into the cylinder bore on a side opposite the exhaust. The air channel, the piston recess, and the transfer channels define an air passage.  
         [0002]     US 2003/0217712 A1 discloses a two-stroke engine in which combustion air is stored within the transfer channels; the transfer channels separate the exhaust gases within the combustion chamber from the fresh mixture that flows in from the crankcase. The air is supplied through an air channel that opens into the cylinder bore. In the area of the top dead center of the piston, the air channel is connected by means of a piston recess provided within the piston to two neighboring transfer channels. The air can flow through the piston recess into the transfer channels. The air channel and the mixture channel supplying the mixture to the two-stroke engine are arranged on a side of the cylinder opposite the exhaust of the combustion chamber. The air channel opens approximately horizontally into the cylinder bore. When air is flowing into the transfer channel proximal to the exhaust, the flow direction of the incoming air is deflected less than when flowing into the transfer channel that is remote from the exhaust and adjacent to the air channel. This causes a predominant portion of the scavenging air to be supplied to the exhaust-near transfer channels (transfer channels proximal to the exhaust) so that the scavenging air can pass through the exhaust-near transfer channel into the crankcase. At the same time, a complete filling with air of the transfer channels remote from (distal to) the exhaust is not achieved. This can lead to a deterioration of the exhaust gas values.  
       SUMMARY OF THE INVENTION  
       [0003]     It is an object of the present invention to provide a two-stroke engine of the aforementioned kind that is of a simple configuration and has minimal exhaust gas values.  
         [0004]     In accordance with the present invention, this is achieved in that the air passage is designed such that an approximately uniform distribution of the air into the transfer channels results.  
         [0005]     Because of the uniform distribution of the air into the transfer channels, excellent scavenging of the combustion chamber and excellent separation of the exhaust gases from the fresh mixture being supplied from the crankcase can be achieved. By means of the constructive configuration of the air passage it can be ensured that in any operating state an excellent uniform filling of the transfer channels can be achieved.  
         [0006]     Advantageously, the air flows into the piston recess in such a way that a distribution of the incoming air into the transfer channels in a ratio between 60% to 40% and 40% to 60% results. For such a distribution, excellent scavenging of the transfer channels and thus excellent separation of the exhaust gases exiting the combustion chamber from the incoming mixture can be achieved so that minimal exhaust gas values result. It is provided that the air channel opens into the cylinder bore such that the air flowing out of the air channel into the piston recess flows essentially against the ceiling of the piston recess.  
         [0007]     By flowing against the ceiling of the piston recess, it can be achieved that the component of the flow direction that is directed toward the transfer channel near the exhaust is reduced. In this way, the exhaust-remote transfer channel is supplied with more air and the exhaust-near transfer channel is supplied with less air in comparison to a horizontal flow. This leads to a uniform distribution of the air into the exhaust-near and the exhaust-remote transfer channels.  
         [0008]     It is provided that the air that leaves the air channel flows into the piston recess at an angle of less than 90 degrees, in particular, at an angle between 30 degrees and 60 degrees, relative to the upright or longitudinal direction of the cylinder. It was found that a uniform distribution of the air into the transfer channels can be achieved for such a flow angle.  
         [0009]     Advantageously, means for distributing the air are provided in the cylinder. As a result of the relative movement of cylinder and piston, means for distributing the air provided at the piston recess are effective only in predetermined piston positions or they must extend across the entire height of the piston recess. In contrast to this, means for distributing the air provided in the cylinder, i.e., in the air channel and the transfer channels, are effective in any piston position. It is provided that means for distributing the air into the transfer channels are arranged in the air channel upstream of the air channel port. By arranging means for distributing the air in the air channel, the flow direction of the air flowing into the piston recess can be affected. In this way, the flow conditions in the piston recess and thus the distribution of the air into the transfer channels are affected.  
         [0010]     However, it can also be advantageous that at least one transfer channel has means for affecting the distribution of air into the transfer channels. The means for distributing the air are particularly arranged in the exhaust-near transfer channel and configured as a throttle. The throttle in the exhaust-near transfer channel, in particular in the area of the exhaust-near transfer port, causes an increase of the flow resistance in the exhaust-near transfer channel and thus a reduction of the amount of air supplied to the exhaust-near transfer channel. Advantageously, the piston recess has means for distributing the air into the transfer channels. In particular, the air channel opens into the cylinder bore on a side of the cylinder opposite the exhaust. In order to reduce the amount of air that is supplied to the exhaust-near transfer channel, it is provided that in at least one piston position the exhaust-near transfer port is at least partially closed while the exhaust-remote transfer port is completely open relative to the piston recess. By reducing the free flow cross-section in the exhaust-remote transfer channels, the resulting distribution of air into the transfer channels can be affected additionally in a simple way. Expediently, one piston position in which the exhaust-near transfer port is at least partially closed while the exhaust-remote transfer port is open completely toward the piston recess is the top dead center of the piston.  
         [0011]     A partial closure of the exhaust-near transfer channels can be achieved in a simple way in that the piston skirt closes off a section of the exhaust-near transfer port that extends in the circumferential direction of the piston. This can be achieved by an appropriate adaptation of the shape of the piston recess. For this purpose, it is provided that at least one section of the exhaust-near vertical or upright edge of the piston recess is displaced relative to the transfer port of the exhaust-near transfer channel into the interior of the piston recess. By means of this displacement of the edge of the piston recess, the transfer port is partially or completely closed so that a reduction of the supplied amount of air results and the air that continues to flow in from the air channel is completely directed into the exhaust-remote transfer channel. However, it can also be expedient that the piston skirt closes off a section of the exhaust-near transfer port that is positioned in the upright or longitudinal direction of the cylinder. When closing a section of the transfer port positioned in the upright direction, an approximately plane air front can be achieved in the transfer channel despite the partially closed transfer port. This ensures that the transfer channel can be completely scavenged so that the mixture coming from the crankcase is separated completely by combustion air from the exhaust gases in the combustion chamber.  
         [0012]     Closure of the transfer port can be achieved in a simple way also in that the bottom edge of the piston recess in the area of the exhaust-near transfer port is displaced relative to the bottom edge in the area of the exhaust-remote transfer port in the direction toward the topside of the piston. Expediently, the stay or web that is formed between the piston recess and the bottom side of the piston is wider in the area of the exhaust-near transfer port than in the area of the exhaust-remote transfer port. In this way, it can be ensured that the exhaust-near transfer port is closed partially or even entirely by the piston skirt. Opening of the transfer port toward the crankcase is prevented by the widened stay.  
         [0013]     For shortening the duration in which the exhaust-remote transfer port is connected by means of the piston recess to the air channel, it can be provided that the ceiling of the piston recess in the area of the exhaust-near transfer port is displaced relative to the ceiling in the area of the exhaust-remote transfer port in the direction toward the crankcase. This results in a delayed opening and earlier closing of the exhaust-near transfer port.  
         [0014]     In the case of an internal combustion engine in which the air channel opens into the cylinder bore at a side of the cylinder opposite the exhaust, more air is supplied to the exhaust-remote transfer channel than to the exhaust-near transfer channel because of the flow direction. In this arrangement, the amount of air supplied to the exhaust-remote transfer channel must be reduced by means of the aforementioned measures, wherein the aforementioned measures for the exhaust-near transfer channel must be employed for the exhaust-remote transfer channel and vice versa.  
         [0015]     Expediently, the connecting rod is secured by a piston pin on the piston; the piston pin is arranged approximately at the level of the ceiling of the piston port and projects at a piston pin boss to the exterior of the piston. Because the piston recess extends into the area of the piston pin, a comparatively long connection between air channel and the transfer channels can be ensured so that a sufficient amount of air can flow into the transfer channels. Advantageously, the piston pin boss is arranged in the piston recess. In this way, the piston recess can be designed to be large so that a large quantity of air can be supplied to the transfer channels. However, it can also be expedient to separate the piston pin boss by a stay or web from the piston recess. In this case, the surface area of the piston recess is reduced by the piston pin boss. Advantageously, the stay closes at least partially the exhaust-near transfer port in at least one piston position. In this way, the area of the piston pin boss can be used also to reduce the amount of air that is supplied to the exhaust-near transfer channel.  
         [0016]     Expediently, the piston recess has a ramp for deflecting the flow in the piston recess into the exhaust-remote transfer port. A ramp can be arranged simply in the piston recess. By means of the design of the ramp, the distribution of air into the transfer channels can be affected in a simple way. The ramp can be arranged in the rear wall of the piston recess or can extend from the ceiling or from the bottom edge of the piston recess into the piston recess.  
         [0017]     A mixture channel for supply of a fuel/air mixture to the cylinder communicates with the crankcase. Advantageously, at least a section of the length of the air channel extends on the side of the mixture channel that is facing the crankcase. In this way, a flow direction against the ceiling of the piston recess can be achieved in a simple way. It is provided that the air channel opens with an air channel port into the cylinder bore and that the air channel port is arranged on the side of the exhaust-remote transfer port that is facing the crankcase. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0018]      FIG. 1  is a schematic illustration of a longitudinal section of a two-stroke engine.  
         [0019]      FIG. 2  is a schematic section view of a two-stroke engine at the level of the transfer ports showing the piston in the area of the top dead center.  
         [0020]      FIG. 3  is a schematic illustration of a piston with channels opening into the piston recesses.  
         [0021]      FIG. 4  is a schematic perspective illustration of an air channel, piston recess, and transfer channels.  
         [0022]      FIG. 5  is a developed view of one half of the cylinder bore and the piston arranged therein in a first piston position.  
         [0023]      FIG. 6  is a developed view of one half of the cylinder bore and the piston arranged therein in a second piston position.  
         [0024]      FIG. 7  is a developed view of one half of the cylinder bore and the piston arranged therein in a third piston position.  
         [0025]      FIG. 8  is a developed view of one half of the cylinder bore and the piston arranged therein in a fourth piston position.  
         [0026]      FIG. 9  shows a diagram illustrating the surface area of the transfer ports as a function of the crank angle.  
         [0027]      FIG. 10  is a developed view of a cylinder bore and piston of another embodiment.  
         [0028]      FIG. 11  is a developed view of a cylinder bore and piston of yet another embodiment.  
         [0029]      FIG. 12  is a schematic perspective illustration of air channel, piston recess, and transfer channels of yet another embodiment.  
         [0030]      FIG. 13  is a schematic perspective illustration of air channel, piston recess, and transfer channels of yet another embodiment.  
         [0031]      FIG. 14  is a schematic perspective illustration of air channel, piston recess, and transfer channels of yet another embodiment.  
         [0032]      FIG. 15  is a schematic perspective illustration of air channel, piston recess, and transfer channels of yet another embodiment.  
         [0033]      FIG. 16  is a schematic perspective illustration of air channel, piston recess, and transfer channels of yet another embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]     The two-stroke engine  1  illustrated in  FIG. 1  comprises a cylinder  2  in which a combustion chamber  3  is formed. A spark plug  8  projects into the combustion chamber  3 . The combustion chamber  3  is delimited by a piston  5  that reciprocates within the cylinder  2 . The piston  5  drives by means of a connecting rod  6  the crankshaft  7  that is rotatably supported in a crankcase  4 . The piston  5  is connected to the connecting rod  6  by means of a piston pin  25 . The piston  5  moves within the cylinder  2  in the direction of the longitudinal axis  17  of the cylinder. The two-stroke engine  1  has an intake  9  through which a fuel/air mixture is supplied to the crankcase  4 . The intake  9  is connected by a mixture channel  41  to an air filter  42  through which ambient air is sucked in. A section of the mixture channel  41  is formed within carburetor  43  in which fuel is supplied to the combustion air. An exhaust  10  extends away from the combustion chamber  3 ; exhaust gases can escape from the combustion chamber  3  through the exhaust  10 .  
         [0035]     The two-stroke engine  1  has two exhaust-near transfer channels  11  and two exhaust-remote transfer channels  13  (see also  FIG. 2 ) that open by means of transfer ports  12  and  14  into the combustion chamber  3  and connect the combustion chamber  3  to the crankcase  4  when the piston  5  is in the area of the bottom dead center (illustrated in  FIG. 1 ). One of the exhaust-near transfer channels  11  and one of the exhaust-remote transfer channels  13  are positioned adjacent to one another on either side of the center plane  46  dividing the exhaust  10  and the intake  9  (see  FIG. 2 ). Below the exhaust-remote transfer port  14 , the air channel port  16  of the air channel  15  opens into the cylinder bore  39 . The air channel  15  is connected to the air filter  42 . External to the cylinder  2 , the air channel  15  extends on the side of the mixture channel  41  that is facing the crankcase  4 . In the area of the cylinder  2 , the air channel  15  has two branches. Advantageously, the air channel  15  divides between the air filter  42  and the cylinder  2  into the two branches or divides within the cylinder wall of the cylinder  2  into the two branches.  
         [0036]     As illustrated in  FIG. 2 , the piston  5  has two symmetrically arranged piston recesses  18  that are formed as depressions in the piston skirt  33 . By means of the two piston recesses  18  the transfer ports  12  and  14  are connected to the air channel port  16 . In the area of the bottom dead center (illustrated in  FIG. 2 ) of the piston  5 , the air flows in the air channel  15  in the flow direction  20  into the piston recess  18  and from there into the transfer channels  11  and  13 . Each piston recess  18  connects one exhaust-near transfer channel  11  and one exhaust-remote transfer channel  13  to one branch of the air channel  15 .  
         [0037]     In operation of the two-stroke engine  1 , a fuel/air mixture is sucked into the crankcase  4  through the intake  9  when the piston  5  is in the area of the top dead center. In this position of the piston  5 , the transfer channels  11  and  13  are connected by the piston recesses  18  to the air channel  15  so that essentially fuel-free air can flow from the air channel  15  into the transfer channels  11 ,  13 . In this way, the fuel/air mixture located in the transfer channels  11  and  13  is flushed into the crankcase  4 . Upon downward stroke of the piston  5 , the mixture is compressed within the crankcase  4 . As soon as the transfer ports  12  and  14  open toward the combustion chamber  3 , initially the air that is stored within the transfer channels  11  and  13  flows into the combustion chamber  3 . The stored air scavenges the exhaust gases from the previous combustion cycle out of the combustion chamber  3  through the exhaust  10 . Fuel/air mixture from the crankcase  4  flows in subsequently. Upon upward stroke of the piston  5  the mixture is compressed in the combustion chamber  3  and is ignited in the area of the top dead center by the spark plug  8 . Because of the combustion, the piston  5  is accelerated in the direction toward the crankcase  4 . As soon as the exhaust  10  opens, the exhaust gases flow out of the combustion chamber  3  and are scavenged by the air flowing in from the transfer channels.  
         [0038]     In order to achieve an excellent scavenging result, the transfer channels  11  and  13  should be largely or completely filled with substantially fuel-free air from the air channel  15 . As shown in  FIG. 2 , the air flows out of the air channel  15 , viewed in section of the cylinder  2 , in the direction toward the transfer ports  12  of the exhaust-near transfer channels  11 . In the case of an approximately horizontal flow direction of the air into the piston recess  18 , this leads to a large portion of the scavenging air flowing into the exhaust-near transfer channels  11 . In this way, a complete flushing of the exhaust-remote transfer channels  13  cannot be ensured. In order to achieve that the exhaust-remote transfer channels  13  are also supplied satisfactorily with substantially fuel-free air, it is provided that the air from the air channel flows against the ceiling  19  of the piston recess  18 .  
         [0039]     In  FIG. 3 , the flow directions are schematically illustrated. In this connection, the two branches of the air channel  15  are rotated into the plane of the exhaust-remote transfer channels  13  for improved illustration. As shown in  FIG. 3 , the air flows out of the air channel  15  in the flow direction  20  into the piston recess  18  that is positioned relative to the cylinder bore  39  at an angle α that is smaller than 90 degrees. The angle α is advantageously within a range of 30 degrees to 60 degrees. Accordingly, the air that flows into the piston recess  18  is directed essentially against the ceiling  19  of the piston recess  18  that is facing the topside  34  of the piston  5 . The air flow rebounds at the ceiling  19  and is deflected into the transfer ports  14  of the exhaust-remote transfer channels  13  and into the transfer ports  12  of the exhaust-near transfer channels  11  (not illustrated in  FIG. 3 ). Because the air channel  15  opens into the cylinder bore  39  at an angle α of less than 90 degrees, it is achieved that the portion of the flow direction  20  oriented in the direction toward the exhaust-near transfer channel  11  is reduced. In this way, a uniform distribution of the scavenging air into both transfer channels  11  and  13  can be achieved. The angle α at which the air channel  15  opens into the cylinder bore  39  is advantageously selected such that the air is distributed in a ratio of 40% to 60% and 60% to 40% to the exhaust-remote transfer channel  13  and the exhaust-near transfer channel  11 , respectively. This uniform distribution ensures excellent scavenging of the combustion chamber  3 .  
         [0040]      FIG. 4  shows the transfer channels  11  and  13 , the piston recess  18 , and the air channel  15  in a perspective view. In this illustration, only the walls that delimit the channels are illustrated. The piston recess  18  is shown in a position of the piston in which the transfer channels are still closed by the piston skirt and no connection to the air channel  15  is present. As illustrated in  FIG. 4 , the piston pin boss  26  where the piston pin  25  projects to the exterior of the piston  5 , is arranged at the level of the ceiling  19  of the piston recess  18 . Between the piston pin boss  26  and the piston recess  18  a stay  40  is arranged that separates the piston pin boss  26  from the piston recess  18 . As illustrated in  FIG. 4 , the piston pin boss  26  is arranged in the area of the transfer port  12  of the exhaust-near transfer channel  11 . In the area of the piston pin boss  26  the piston recess  18  has a recessed portion  27 . Upon further upward movement of the piston  5 , first a small section, i.e., that half of the transfer port  12  facing away from the exhaust-remote transfer channel  13 , is released by the piston recess  18 . The other half of the transfer port  12  is covered by the piston pin boss  26  and the stay  40 . The transfer port  14  of the exhaust-remote transfer channel  13  is only minimally covered by the piston pin boss  26  and the stay  40 . In this way, the amount of air that is supplied to the exhaust-remote transfer channel  13  is increased and the amount of air supplied to the exhaust-near transfer channel  11  is reduced. As illustrated in  FIG. 4 , the air channel  15  opens into the piston recess  18  at an angle of less than 90 degrees so that the air entering the piston recess  18  flows substantially against the ceiling  19  of the piston recess  18 . In this way, an increase of the quantity of air supplied to the exhaust-remote transfer channel  13  can be achieved also.  
         [0041]     In  FIGS. 5 through 8  a developed view of the piston skirt  33  in the cylinder bore  39  is illustrated. The piston  5  illustrated in FIGS.  5  to  8  has a piston recess  28  whose ceiling  19  has a bulge  30  for the piston pin boss  29  illustrated in  FIG. 5  in dashed lines. The piston pin boss  29  is arranged accordingly in the piston recess  28  so that the piston pin boss  29  does not cover the transfer ports  12  and  14 . In the area of the exhaust-remote transfer port  14  a bottom edge  22  of the piston recess  28  is separated by the stay  24  from the bottom side  35  of the piston. The stay  24  has a width b. In the area of the exhaust-near transfer port  12  on the bottom edge  21  of the piston recess  28  a stay  23  having width a is provided. The width a is significantly greater than the width b. The bottom edge  21  is displaced relative to the bottom edge  22  by a displacement f in the direction toward the topside  34  of the piston  5 . This causes the exhaust-near transfer port  12  to be closed by the bottom edge  21  while the exhaust-remote transfer port  14  is still completely open toward the piston recess  28 . The width a ensures that the transfer port  12  is closed by the piston skirt  33  and does not open into the crankcase  4 .  
         [0042]     In  FIG. 5 , the piston  5  is illustrated in the area of the bottom dead center. The piston recess  28  is arranged below the air channel port  16  and below the transfer ports  12  and  14 . In the position illustrated in  FIG. 6 , the piston  5  is shown before reaching its top dead center. The transfer ports  12  and  14  and the air channel port  16  are located in the area of the piston recess  28  so that air from the air channel port  16  can flow in through the piston recess  28  into the transfer ports  12  and  14 . As shown in  FIG. 6 , in the area of the exhaust-near transfer port  12  the ceiling  19  is displaced in the direction toward the bottom side of the piston by a displacement e. This causes the exhaust-near transfer port  12  to be open completely later than the exhaust-remote transfer port  14 . Upon downward stroke of the piston  5 , the exhaust-near transfer port  12  is closed before the exhaust-remote transfer port  14  is closed. The height c of the piston recess  28  in the area of the exhaust-near transfer port  12  is only minimally greater than the height of the transfer port  12  measured in the direction of the longitudinal axis  17  of the cylinder. In this way, the exhaust-near transfer port  12  is open completely only for a short period of time. The height d of the piston recess  28  in the area of the exhaust-remote transfer port  14  is significantly greater than the height c.  
         [0043]     As illustrated in  FIG. 7 , upon further upward movement of the piston  5  the exhaust-near transfer port  12  is covered by the bottom edge  21  so that the transfer port  12  is partially closed by the piston skirt  33 .  
         [0044]     As shown in  FIG. 8 , in the area of the top dead center of the piston  5  the exhaust-near transfer port  12  is almost completely closed by the piston skirt  33 . The exhaust-near transfer port  12  is arranged in the area of the stay  23  and is covered by it.  
         [0045]     In  FIG. 9  the surface area A of the transfer port  12  of the exhaust-near transfer channel  11  that is open toward the piston recess  28  is illustrated as a function of the crank angle. Upon upward movement of the piston  5  the transfer port  12  is opened increasingly until it is completely open at a crank angle of approximately 315 degrees. This situation is illustrated in  FIG. 6 . Upon further upward movement, the transfer port  12  begins to close again starting at a crank angle of approximately 330 degrees as illustrated by the line  45  in  FIG. 9 . The closing of the transfer port  12  corresponds to the illustrations of  FIGS. 7 and 8 . The reduction of the flow cross-section of the exhaust-near transfer channel  11  into the piston recess  28  is affected by the bottom edge  21  of the piston recess  28  that is displaced in the direction toward the piston topside  34 . In  FIG. 9 , the line  44  indicates the course of the flow cross-section without the displaced bottom edge  21 , i.e., for a transfer port that is completely open at the bottom dead center of the piston.  
         [0046]     One embodiment is illustrated in  FIG. 10  as a developed view of a piston skirt  33 . A piston recess  30  is arranged In the piston skirt  33 . The exhaust-near upright edge  36  of the piston recess  38  has at the side facing the bottom side  35  of the piston a section  37  that is displaced relative to the upright edge  36  in the direction toward the interior of the piston recess  38 . The exhaust-near transfer port  12  is closed in the area of the section  37  by the piston skirt  33  when the piston  5  is in the area of the top dead center. In this way, the flow cross-section into the exhaust-near transfer channel  11  is reduced.  
         [0047]     In the embodiment illustrated in  FIG. 11 , the piston recess  48  has a ramp  49  that extends from the ceiling  19  of the piston recess  48  into the interior of the piston recess  48 . The ramp  49  is positioned adjacent to the bulge  30  in the area of the piston pin boss  29 . Accordingly, the ramp  49  extends within an area of the circumference of the piston  5  that, when the piston  5  is in the area of the top dead center, is arranged on a side of the transfer port  14  facing the transfer port  12 . The ramp  49  effects a deflection of the flow in the piston recess  48  into the transfer port  12  and thus causes a reduction of the amount of air supplied to the transfer port  12  and to the exhaust-near transfer channel  11 . It can be advantageous to arrange a ramp, in addition or instead of the ramp  49 , at the rear wall of the piston recess  48  or in the area of the bottom edge  22  of the piston recess  48 .  
         [0048]     The schematic illustration in  FIG. 12  shows the arrangement of an exhaust-near transfer channel  31  at the piston recess  18 . The exhaust-near transfer channel  31  has a step  32  upstream of its transfer port  12 . The step  32  is arranged at the wall of the transfer channel  31  facing the crankcase  4  and causes a reduction of the flow cross-section of the transfer port  12 . In this way, the step  32  provides a throttle of the transfer channel  31 . By throttling the transfer channel  31  the amount of air supplied to the transfer channel  31  is reduced and the amount of air that is supplied to the exhaust-remote transfer channel  31  is increased. In addition, the air channel  15  is designed such that the air flowing into the piston recess  18  flows substantially against the ceiling  19  of the piston recess  18 . However, when throttling the exhaust-near transfer channel  31 , the air channel  15  can also open approximately horizontally into the piston recess  18 .  
         [0049]     The embodiment illustrated in  FIG. 13  shows an air channel  55  that is provided upstream of its air channel port  56  with a ramp  57 . The ramp  57  is arranged at the side of the air channel  55  facing the crankcase  4  and causes a change of the flow direction of the air flowing into the piston recess  18  in the direction toward the ceiling  19  of the piston recess  18 . In this way, the air channel  55  can open into the cylinder bore  39  at a large angle relative to the longitudinal axis  17  of the cylinder. The air channel  55  can open at an angle of approximately 90 degrees into the cylinder bore  39 . By means of the ramp  57  the flow into the piston recess  18  can be deflected such that a uniform distribution of the air into the transfer channels  11  and  13  results. The ramp  57  can also be arranged at the walls of the air channel  55  extending in the circumferential direction of the cylinder bore  39  or at the wall of the air channel  55  that is facing the combustion chamber  3 . The distribution of the air into the transfer channels  11  and  13  can be affected by the flow direction in the air channel  55  in the area of the air channel port  56  in such a way that a uniform distribution of the air into the transfer channels results.  
         [0050]     In FIGS.  14  to  16  additional embodiments for the configuration of the piston recesses of a piston are illustrated. The pistons  5  illustrated in  FIGS. 14 through 16  each have piston recesses that are arranged and configured symmetrically relative to the center plane  46  illustrated in  FIG. 2 . For improved illustration, the radii of the piston recesses are shown in only one of the piston recesses, respectively.  
         [0051]     The piston  5  illustrated in  FIG. 14  has two piston recesses  58 . The piston recesses  58  have a depth t that is measured in radial direction relative to the longitudinal axis  17  of the cylinder. The depth t indicates the radial spacing of the rear wall  61  of the piston recesses from the piston skirt  33 . The depth t indicates in this connection the maximum depth of the piston recesses  58 . The ceiling  59  of the piston recesses  58  passes at a radius r into the rear wall  61 . The radius r corresponds approximately to the depth t of the piston recess  58 . The bottom  60  of the piston recess  58 , starting at the piston skirt, also extends at a radius q. The radius q is greater than the depth t of the piston recess  58 .  
         [0052]     In the embodiment of  FIG. 15 a  piston  5  with piston recesses  68  is illustrated. The ceiling  69  of the piston recesses  68  passes at a radius o into the rear wall  71  of the piston recesses  68 . The radius o is smaller than the depth t of the piston recess  68 . The ceiling  69  of the piston recesses  68 , starting at the piston skirt  33 , initially extends approximately perpendicularly to the longitudinal axis  17  of the cylinder and then passes at a radius o into the rear wall  71 . The bottom  70  extends at a radius s that corresponds approximately to the depth t of the piston recess  68 . The radius s extends from the piston skirt  33  to the rear wall  71 .  
         [0053]     In  FIG. 16 , a piston  5  with piston recesses  78  is illustrated. The ceiling  79 , the rear wall  81 , and the bottom  80  extend at a continuous radius p. The radius p corresponds to the depth u of the piston recess  78 . The piston recess  78  is accordingly curved at a continuous radius.  
         [0054]     It can be expedient that the bottom of the piston recess passes at a smaller radius into the rear wall than the ceiling of the piston recess. Advantageously, the bottom and/or the ceiling of the piston recess pass at a radius o, p, q, r, s into the rear wall  61 ,  71 ,  81  that is between 50% and 150% of the depth t, u of the piston recess  58 ,  68 ,  78 . By a suitable selection of the radii, the distribution of air into the transfer channels can be affected such that an approximately identical distribution results. The pistons  5  illustrated in FIGS.  14  to  16  corresponds essentially in other respects to the piston  5  illustrated in  FIG. 3 .  
         [0055]     It is possible to employ other means for affecting the distribution of air into the transfer channels. The means can be provided in the air channel, in the piston recess, and in the transfer channels. It can also be expedient that only individual means for distributing the air are provided. Instead of the ramp in the air channel or a step in the transfer channel, other means for distributing the air can be advantageous. Also, by means of the flow direction into the transfer channels, the distribution of the air into the transfer channels can be affected. The distribution of air into the transfer channels can also be achieved in that the flow resistance in one of the transfer channels, in particular, in the exhaust-remote transfer channel, is reduced. In particular, the means for distributing the air are provided in the cylinder.  
         [0056]     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.