BACKGROUND OF THE INVENTION 
   The present invention relates to a two-cycle engine, especially for a portable, manually-guided implement such as a power chain saw, a cut-off machine, or the like. 
   WO 99/18338 (EP 0 971 110 A1) discloses a two-cycle engine to which combustion air is supplied via an air channel. The air channel opens via a diaphragm valve into a transfer channel. To achieve low exhaust gas values, the ratio of the previously stored air to the fuel/air mixture supplied to the crankcase is 0.7 to 1.4. The diaphragm valve opens and closes due to different pressure levels in the air channel and crankcase. The air quantity supplied to the transfer channels is thus dependent upon the existing pressure conditions. Since these pressure conditions vary with respect to the speed, too much air is supplied via the transfer channels to such an internal combustion engine at low speeds. The fuel/air mixture supply to the combustion chamber therefore becomes lean, resulting in a poor operating characteristic in the low speed range. 
   It is therefore an object of the present invention to provide a two-cycle engine of the aforementioned general type according to which, at advantageous operating characteristics, good exhaust gas values are achieved in all speed ranges. 


   
     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 longitudinal cross-sectional view through a two-cycle engine; 
       FIG. 2  is a cross-sectional view taken along the line II—II in  FIG. 1 ; and 
       FIG. 3  is a graph that plots the fraction of the mass flow supplied to the transfer channel versus the speed. 
   


   SUMMARY OF THE INVENTION 
   The two-cycle engine of the present invention comprises a cylinder in which is formed a combustion chamber that is delimited by a reciprocating piston that, via a connecting rod, drives a crankshaft that is rotatably mounted in a crankcase, wherein an inlet is provided for supplying a fuel/air mixture to the crankcase, wherein an outlet is provided for discharging exhaust gas from the combustion chamber, wherein at least one transfer channel is provided that, in prescribed positions of the piston, fluidically connects the crankcase with the combustion chamber, wherein the transfer channel opens via an inlet window into the combustion chamber, and via an opening window into the crankcase, wherein the transfer channel, in a region along the length thereof, has a closed configuration relative to the cylinder, wherein an air channel is provided for conveying essentially fuel-free air, wherein the air channel is fluidically connected, in a port-controlled manner, with at least one transfer channel, and wherein at a rated speed a quantity of air flowing from the air channel into the transfer channel during a piston stroke has a volume that is at least 75% of the volume of the transfer channel between the inlet window and the opening window. 
   Due to the port control of the fluidic connection of air channel and transfer channel, the channels are interconnected independently of the pressure conditions and only as a function of the control time. For a favorable combustion, it is provided that the air quantity flowing into the transfer channel, at the rated speed, has a volume that corresponds to at least 75% of the volume of the transfer channel between the inlet window and opening window. This quantity of previously stored air leads to a good separation of exhaust gases and subsequently flowing-in fuel/air mixture. At the same time, too lean of a fuel/air mixture flowing into the combustion chamber is avoided. 
   The fraction of the mass flow supplied to the transfer channel via the air channel during a piston stroke is expediently 0 to 80% of the mass flow supplied to the two-cycle engine during the piston stroke. In particular, the fraction is less than 50%. The percentage of the mass flow supplied to the transfer channel via the air channel during a piston stroke, to the total mass flow supplied to the two-cycle engine during the piston stroke, is advantageously approximately constant over the entire operating range of the two-cycle engine. Since the angle cross-section during port control of the connection of transfer channel and air channel decreases only slightly relative to the speed and approximately constantly, it is possible to advantageously realize this by means of the port control. In this connection, the angle cross section designates the. integral of the progress of the surface area of the narrowest cross-section of the connection of air channel versus the crankshaft angle, whereby integration occurs over one rotation of the crankshaft. 
   The fluidic connection of air channel and transfer channel in particular permits in principle a flow from air channel into the transfer channel, and a flow in the opposite direction. Flow is permitted depending on the dynamic conditions of the traveling air mass flow according to the transient pressure situation. In prescribed positions of the piston, air channel and transfer channel are expediently fluidically connected via a piston window. The port control can be realized in a straightforward manner via a piston window. In prescribed positions of the piston, two transfer channels that are disposed symmetrically relative to the central plane are advantageously connected with an air channel. In particular, four transfer channels are disposed symmetrically relative to the central plane. In prescribed positions of the piston, four transfer channels are expediently connected with an air channel. As a result, it is possible to realize a good separation of exhaust gas and subsequently flowing-in fuel/air mixture. 
   Further specific features of the present invention will be described in detail subsequently. 
   DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring now to the drawings in detail, the two-cycle engine  1  schematically illustrated in  FIG. 1  has a cylinder  2  in which is formed a combustion chamber  3 . The combustion chamber  3  is delimited by a reciprocating piston  5  that moves between an upper dead center position and a lower dead center position. By means of a connecting rod  6 , the piston  5  drives a crankshaft  7  that is rotatably mounted in the crankcase  4 . By means of the intake channel  9 , fuel/air mixture that is prepared in the carburetor  8  is supplied to the crankcase  4  via the inlet  11 . In prescribed positions of the piston, such as in the piston position illustrated in  FIG. 1 , the crankcase  4  and the combustion chamber  3  are fluidically connected with one another via four transfer channels  12 ,  15 , two of which are illustrated in FIG.  1 . The transfer channel  15  that is disposed close to the outlet  10  opens via an inlet window  16  into the combustion chamber  3 , and via an opening window  23  into the crankcase  4 . The transfer channel  12  that is disposed remote from the outlet  10  opens via an inlet window  13  into the combustion chamber  3  and via an opening window  22  into the crankcase  4 . In the region of the inlet windows  13 ,  16 , an air channel  17  opens into the cylinder  2  via an air channel window  18 , which is illustrated in FIG.  2 . Formed in the piston skirt  30  of the piston  5  is a piston window  21  which, as indicated in the cross-sectional view of  FIG. 2 , fluidically connects the air channel  17  with the inlet windows  13 ,  16  of the transfer channels  12 ,  15  in prescribed positions of the piston. The air channel window  18  is expediently offset relative to the inlet windows  13 ,  16  in a direction toward the crankcase  4 . 
   During the upward stroke of the piston  5  in a direction toward the combustion chamber  3 , fuel/air mixture is drawn into the crankcase  4  via the inlet  11 . During the subsequent downward stroke, the fuel/air mixture is compressed in the crankcase  4 . While the air channel  17  is fluidically connected via the piston window  21  with the transfer channels  12  and  15 , largely fuel-free air flows into the transfer channels  12  and  15  via the air channel and the piston window  21 . The largely fuel-free air is stored ahead of the fuel/air mixture from the crankcase  4 . During the subsequent downward stroke of the piston  5 , the air previously stored in the transfer channels  12  and  15 , and subsequent thereto the fuel/air mixture, flows out of the crankcase  4  into the combustion chamber  3  and displaces the exhaust gases from the combustion chamber  3  through the outlet  10 , which in particular is disposed approximately across from the inlet  11 . During the subsequent upward stroke of the piston  5 , the fuel/air mixture in the combustion chamber  3  is compressed, and in the region of the upper dead center position of the piston  5  is ignited by the spark plug  14 . In the downward stroke, the exhaust gases are displaced toward the outlet  10  by the in-flowing air and the fuel/air mixture. 
   As illustrated in the cross-sectional view of  FIG. 2 , two transfer channels  12  that are remote from the outlet  10 , and two transfer channels  15  that are disposed close to the outlet, are disposed symmetrically relative to the central plane  20 , which approximately centrally divides the inlet  10  and outlet  11 , and includes the longitudinal central axis  19  of the cylinder  2 . In the region of their longitudinal extension parallel to the central axis  19  of the cylinder  2 , the transfer channels  12 ,  15  are separated by a wall  24  or  25  from the cylinder  2 . The walls  24 ,  25  extend between the inlet windows  13 ,  16  and the opening windows  22 ,  23 . In prescribed positions of the piston  5 , the inlet windows  13  and  16  of the transfer channels  12  and  15  are fluidically connected with the air channel  17  via the piston window  21 , which is indicated by dashed lines in  FIG. 2 ; the air channel  17  is divided into two branches that are disposed symmetrically relative to the central plane  20 . In this connection, the air channel  17  opens via a respective air channel window  18  into the cylinder  2 . 
   In  FIG. 3 , the fraction or percentage x of the mass flow supplied to the transfer channels  12 ,  15  during a piston stroke via the air channel  17  is plotted versus the engine speed n. The fraction x is indicated in percentage, and the speed n is indicated in revolutions per minute. The lines  27  and  28  designate limiting curves of the distribution of the fraction x during the connection of air channel and transfer channels via a diaphragm valve. When using a diaphragm valve, the fraction x plotted versus the speed n is generally disposed in the region  29  that is disposed between the limiting curves  27  and  28  and is illustrated in cross-hatching. As illustrated in  FIG. 3 , as the speed increases, the fraction x decreases. At the rated speed N of about 9000 rpm, the fraction x is approximately between 40 and 50%. With this, there is achieved a favorable ratio of the previously stored quantity of air to the fuel/air mixture that is supplied to the crankcase. At lower speeds, however, the fraction x increases. As a result, at lower speeds the mixture becomes much leaner. The line  26  represents the curve of the fraction x plotted versus the speed with port-control of the connection of air channel  17  and transfer channels  12 ,  15 . The fraction x is approximately constant over the entire operating range of the two-cycle engine  1 . In the illustrated embodiment, the fraction x is between 40 and 45%. This results in good exhaust gas values at high speeds. At low speeds, too lean of a mixture is avoided. 
   The quantity of air that during a piston stroke flows into the transfer channel  12 ,  15  at the rated speed N expediently has a volume that corresponds at least to 75% of the volume of the transfer channel  12 ,  15  between the inlet windows  13 ,  16  and the opening windows  22 ,  23 . At low previously stored volumes, an adequate separation of exhaust gases and subsequently flowing-in fuel/air mixture cannot be ensured. The fraction x of the mass flow that during a piston stroke is supplied to the transfer channel  12 ,  15  via the air channel  17  is expediently 0 to 80% of the mass flow that is supplied to the two-cycle engine  1  during the piston stroke. Favorable exhaust gas values and good true-running characteristics of the engine in the entire speed range result in particular at a fraction x of less than 50%. 
   Due to the port control of the connection of air channel and the transfer channels, the angle cross-section is independent of resonance influences in the intake channel. The angle cross-section thus decreases only slightly and in a constant manner versus the speed. In particular, the fraction x versus the speed does not decrease with port control, but rather is largely constant. In contrast to the diaphragm valve, the piston window can in principle have flow therethrough in both directions, so that pressure differences during the connection of air channel and transfer channel can be compensated for in both directions. 
   It can be expedient for the air channel to open out only into the transfer channels that are close to the outlet. It can also be expedient to have the connection for the air channel with the two transfer channels that are remote from the outlet. In particular, the air channel is connected via the piston windows with all four symmetrically disposed transfer channels. The connection of air channels and transfer channels need not be effected via a piston window, but rather can, for example, also be port controlled by the crank web. 
   The specification incorporates by reference the disclosure of German priority document DE 102 23 071.4 filed May 24, 2002. 
   The present invention is, of course, in no way restricted to the specific disclosure of the specification and drawings, but also encompasses any modifications within the scope of the appended claims.
A two-cycle engine includes a reciprocating piston reciprocable in a combustion chamber and interconnected with a crankshaft in a crankcase. A transfer channel selectively fluidly connects the crankcase with the combustion chamber so a fuel/air mixture in the crankcase enters the combustion chamber for discharging exhaust gas from the combustion chamber. The transfer channel has a constructive volume between an inlet window into the combustion chamber and an opening window into the crankcase such that the volume of essentially fuel-free air that is drawn into the transfer channel during an intake stroke is no more than 75% of the constructive volume of the transfer channel.