Abstract:
A two-stroke engine includes a scavenging passage opening in a combustion chamber in a scavenging stroke to fill the combustion chamber with a working gas containing a fuel; and a scavenge filling chamber communicating through a communicating port with the scavenging passage and arranged to be filled with a non-working gas in a fuel weight concentration smaller than that of the working gas prior to the scavenging stroke. In the scavenging stroke, the scavenging passage and the scavenging filling chamber are made open in the combustion chamber and the non-working gas in the scavenge filling chamber is forced into the combustion chamber by the working gas inside the scavenging passage through the communicating port to scavenge the combustion chamber.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a two-stroke engine to be mounted, for example, on a lawn mower, a backpacked power sprayer, or the like and, more particularly, to a two-stroke engine achieving reduction of hydrocarbons (THC: Total Hydro Carbon: total amount of hydrocarbons) in exhaust gas. 
   2. Related Background Art 
   In a two-stroke engine mounted on a lawn mower or a backpacked power sprayer, an air-fuel mixture in a crank chamber is introduced through a scavenging port into a combustion chamber in a scavenging stroke to fill the combustion chamber while scavenging the combustion chamber. For this reason, the conventional two-stroke engines experienced so-called “blow-by”: the fresh charge gas (air-fuel mixture) introduced through the scavenging port into the combustion chamber directly blew through the exhaust port without staying in the combustion chamber. This blow-by mixture was sometimes released as unburned gas into the atmosphere without being cleaned up. In recent years, in order to reduce the blow-by of the air-fuel mixture, two-stroke engines performing so-called stratified scavenging were put into practical use (Japanese Patent Application Laid-Open No. 2001-140651 and Japanese Patent Application Laid-Open No. 2000-320338). 
   SUMMARY OF THE INVENTION 
   In the stratified scavenging, the exhaust gas refluxed from the exhaust system or the like, or a gas without fuel such as air introduced from the intake system (these refluxed exhaust gas and intake air will be collectively called non-working gas), and the fresh charge gas (hereinafter referred to as working gas) are introduced into the combustion chamber in the scavenging stroke. At this time, the non-working gas and the working gas are not homogeneously fully mixed, but fill the combustion chamber so as to form a laminar boundary, and only the non-working gas layer undergoes blow-by after scavenging, thereby preventing the blow-by of THC. 
   The ideal stratified scavenging is such that the entire blow-by gas in the scavenging stroke is the non-working gas without the blow-by component of the working gas consisting of the fresh charge air and the non-working gas completely blows across the combustion chamber without staying inside the combustion chamber. In the conventional stratified scavenging, however, a mixed layer in which the non-working gas and the working gas coexist (or are mixed) was present in the laminar boundary part between the non-working gas layer and the working gas layer. For this reason, where the mixed layer was included in the blow-by gas, it led to an increase in fuel consumption due to a decrease of trapping efficiency. It can also cause an increase in the atmospheric discharge amount of THC components to be cleaned up. Where the mixed layer stays in the combustion chamber, the non-working gas in the mixed layer goes together with the working gas through the combustion stroke, so as to cause a power drop due to a decrease of delivery ratio. 
   There were thus desires for such an improvement that this mixed layer was eliminated (or reduced) to make the laminar boundary clearer between the non-working gas layer and the working gas layer. This improvement can solve the aforementioned problem. Therefore, an object of the present invention is to provide a two-stroke engine capable of more efficiently reducing the blow-by of the air-fuel mixture. 
   A two-stroke engine ( 10 ) of the present invention comprises a scavenging passage ( 19 ) opening in a combustion chamber ( 12 ) in a scavenging stroke to fill the combustion chamber with a working gas containing a combustion fuel; and a scavenge filling chamber ( 18 ) communicating through a communicating port ( 21 ) with the scavenging passage ( 19 ) and arranged to be filled with a non-working gas in a fuel weight concentration smaller than that of the working gas prior to the scavenging stroke, wherein in the scavenging stroke the scavenging passage ( 19 ) and the scavenge filling chamber ( 18 ) are made open in the combustion chamber ( 12 ) and the non-working gas in the scavenge filling chamber ( 18 ) is forced into the combustion chamber ( 12 ) by the working gas inside the scavenging passage ( 19 ) through the communicating port ( 21 ) to scavenge the combustion chamber ( 12 ). 
   The two-stroke engine ( 10 ) of the present invention embraces, particularly, a Schnürle-method two-stroke engine. The Schnürle method is also called a collision reverse type, in which gas flows are introduced into the combustion chamber through a pair of scavenging ports disposed in symmetry on a transverse plane of the combustion chamber (a projection plane normal to the center axis of the combustion chamber) to collide with each other to form reverse vortices. The Schnürle-method two-stroke engine makes use of the reverse vortices to implement effective scavenging. 
   In the present invention, the non-working gas in the fuel weight concentration smaller than that of the working gas embraces a gas in the fuel weight concentration of 0, of course. The working gas is, for example, a gas introduced from a carburetor through an intake port ( 15 ) into a crank chamber ( 28 ) in an intake stroke and then introduced through the scavenging passage ( 19 ) into the combustion chamber ( 12 ), and in this process a desired amount of exhaust gas may be mixed therein (provided that the fuel weight concentration of the working gas should be larger than that of the aforementioned non-working gas). 
   The scavenge filling chamber ( 18 ) is filled with the non-working gas prior to the scavenging stroke, and in the scavenging stroke this non-working gas thus filling the chamber is introduced into the combustion chamber ( 12 ) to effect scavenging. However, this does not mean that the non-working gas should be continuously supplied from the scavenge filling chamber ( 18 ) throughout the entire period of the scavenging stroke. Namely, in a late stage of the scavenging stroke in which the blow-by rate of gas through an exhaust port ( 16 ) is lower, the working gas moving from the scavenging passage ( 19 ) through the communicating port ( 21 ) into the scavenge filling chamber ( 18 ) may be introduced from the scavenge filling chamber ( 18 ) into the combustion chamber ( 12 ) as the working gas is charged from the scavenging passage ( 19 ). 
   The scavenge filling chamber ( 18 ) is filled with the non-working gas prior to the scavenging stroke, and the scavenge filling chamber ( 18 ) is in communication through the communicating port ( 21 ) with the scavenging passage ( 19 ). As far as the working gas does not enter through the communicating port ( 21 ), the non-working gas in the scavenge filling chamber ( 18 ) is not mixed with the working gas. During or after the filling of the scavenge filling chamber ( 18 ) with the non-working gas, the working gas is unlikely to enter the interior of the scavenge filling chamber ( 18 ) formed as a filling room, through the communicating port ( 21 ), whereby the working gas is unlikely to be mixed with the non-working gas. Namely, during the filling of the scavenge filling chamber ( 18 ) with the non-working gas, the non-working gas in the scavenge filling chamber ( 18 ) is discharged through the communicating port ( 21 ) toward the scavenging passage ( 19 ) side and thus the working gas is prevented from moving from the scavenging passage ( 19 ) side to the scavenge filling chamber ( 18 ) side. After the filling with the non-working gas, the interior of the scavenge filling chamber ( 18 ) as a filling room is filled with the non-working gas, so as to prevent the inflow of the working gas through the communicating port ( 21 ). 
   For this reason, the working gas is not mixed with the non-working gas in the scavenge filling chamber ( 18 ) before the scavenging stroke, and in the scavenging stroke the non-working gas is introduced from the scavenge filling chamber ( 18 ) into the combustion chamber ( 12 ) and the working gas is introduced from the scavenging passage ( 19 ) into the combustion chamber ( 12 ), whereupon a laminar boundary becomes clear between the non-working gas layer and the working gas layer. The mixed layer is not formed (or is barely formed if any) because of creation of the clear laminar boundary, which makes it easier to implement the blow-by of the non-working gas only and to keep the working gas only staying in the combustion chamber ( 12 ). This prevents the working gas from being contained as a mixed layer during the blow-by, and prevents an increase of fuel consumption. It is also feasible to prevent the non-working gas from staying as a mixed layer in the combustion chamber ( 12 ) and causing a power drop due to a decrease of delivery ratio. 
   The two-stroke engine is preferably constructed as described below. The two-stroke engine ( 10 ) further comprises a cylinder block ( 11 ) forming the combustion chamber ( 12 ); a piston ( 33 ) to reciprocate in the combustion chamber; and a crank chamber ( 28 ) into which the working gas is to be introduced through an intake port ( 15 ). The scavenging passage ( 19 ) and the scavenge filling chamber ( 18 ) extend in the cylinder block so as to be adjacent to each other along an axial direction of the combustion chamber ( 12 ), and the scavenge filling chamber ( 18 ) has an aperture ( 18   a ) to open in the combustion chamber ( 12 ) when the piston ( 33 ) is located at a position near a bottom dead center. The scavenging passage ( 19 ) has an aperture ( 19   a ) to open in the combustion chamber ( 12 ) when the piston ( 33 ) is located at a position near the bottom dead center, and one end of the scavenging passage is in communication with the crank chamber ( 28 ). The communicating port ( 21 ) opens in a bulkhead ( 20 ) interposed between the scavenging passage ( 19 ) and the scavenge filling chamber ( 18 ) and its opening direction is perpendicular to a gas flow direction in the scavenging passage ( 19 ). 
   Since the opening direction of the communicating port ( 21 ) for communication between the scavenging passage ( 19 ) and the scavenge filling chamber ( 18 ) is perpendicular to the gas flow direction in the scavenging passage ( 19 ), the working gas on the scavenging passage ( 19 ) side is hardly mixed with the non-working gas on the scavenge filling chamber ( 18 ) side, so as to form no mixed layer in the aforementioned laminar boundary part (or barely form the mixed layer, if any), whereby the laminar boundary becomes clear. This makes it easier to implement the blow-by of the non-working gas only and to keep the working gas only staying in the combustion chamber ( 12 ). 
   The two-stroke engine is further preferably constructed as described below. A wall part of the scavenge filling chamber on the bottom dead center side of the piston ( 33 ) is formed by a gasket ( 24 ) sandwiched between the cylinder block ( 11 ) forming the combustion chamber ( 12 ) and a crank case ( 27 ) forming the crank chamber ( 28 ). This permits easy formation of the scavenge filling chamber ( 18 ) in such a manner that a hollow for the scavenge filling chamber ( 18 ) is formed in the cylinder block ( 11 ) and the crank case ( 27 ) just like the scavenging passage ( 19 ) and an opening portion of this hollow is closed by the gasket ( 24 ). There is neither need for increase in the number of parts nor for addition of an extra processing step, because the gasket ( 24 ) between the cylinder block ( 11 ) and the crank case ( 27 ) is utilized. 
   The two-stroke engine is further preferably constructed as described below. The intake port ( 15 ) and the exhaust port ( 16 ) are located at positions substantially opposite to each other with respect to a center of the combustion chamber ( 12 ) on a projection plane normal to the center axis of the combustion chamber ( 12 ) Each of the scavenge filling chamber ( 18 ) and the scavenging passage ( 19 ) comprises a pair arranged one on each side in symmetry with respect to a line connecting the intake port ( 15 ) to the exhaust port ( 16 ), and the pair of scavenge filling chambers ( 18 ) are placed on the exhaust port ( 16 ) side with respect to the pair of scavenging passages ( 19 ). 
   The non-working gas introduced from the scavenge filling chambers ( 18 ) into the combustion chamber ( 12 ) and the working gas introduced from the scavenging passages ( 19 ) into the combustion chamber ( 12 ) are discharged through the exhaust port ( 16 ) while scavenging the interior of the combustion chamber ( 12 ). Since the scavenge filling chambers ( 18 ) are located on the exhaust port ( 16 ) side with respect to the scavenging passages ( 19 ), the non-working gas introduced from the scavenge filling chambers ( 18 ) is introduced to the exhaust port ( 16 ) side, and the non-working gas becomes likely to be discharged as a blow-by, whereby it is feasible to effectively prevent the blow-by of the working gas. 
   The two-stroke engine is further preferably constructed as described below. The pair of scavenging passages ( 19 ) and the pair of scavenge filling chambers ( 18 ) each are oriented so that introduced gases therefrom into the combustion chamber ( 12 ) in the scavenging stroke collide with each other on the opposite side to the exhaust port ( 16 ). 
   The working gases introduced from the pair of scavenging passages ( 19 ) into the combustion chamber ( 12 ) collide with each other to form reverse vortices. The non-working gases introduced from the pair of scavenge filling chambers ( 18 ) into the combustion chamber ( 12 ) also collide with each other to form reverse vortices. The reverse vortices of the working gases are inhibited from blowing through the exhaust port ( 16 ) by the flows of the non-working gases introduced on the exhaust port ( 16 ) side and the existence of the reverse vortices thereof. The reverse vortices realize appropriate scavenging inside the combustion chamber ( 12 ). 
   The two-stroke engine is further preferably constructed as described below. The non-working gas is comprised essentially of an exhaust gas refluxed from an exhaust system. The non-working gas comprised essentially of the exhaust gas may be the exhaust gas itself, or may be a gas resulting from mixing of the exhaust gas with another gas containing no fuel (e.g., part of intake air, or air newly introduced from the outside, or the like). 
   Supply of the exhaust gas from the exhaust system into the scavenge filling chambers ( 18 ) in the case where the non-working gas is comprised essentially of the exhaust gas is implemented through a communication path ( 40 ) formed in the piston ( 33 ) and/or in a wall part of the combustion chamber ( 12 ) so as to bring the scavenge filling chambers ( 18 ) into communication with the exhaust port ( 16 ), when the piston ( 33 ) to reciprocate in the combustion chamber ( 12 ) is located near a top dead center. When the communication path ( 40 ) is formed in the surface of the piston ( 33 ) and/or in the wall part of the combustion chamber ( 12 ), it is formed as a groove; when it is formed inside the piston ( 33 ) and/or inside the wall part of the combustion chamber ( 12 ), it is formed as a hole. The communication path ( 40 ) may be formed as a groove in part and a hole in the other part. 
   Since the piston ( 33 ) is located near the top dead center immediately after the movement of the piston ( 33 ) from the bottom dead center toward the top dead center, a positive pressure from the exhaust downstream side acts at the exhaust port ( 16 ). When the exhaust port ( 16 ) is brought into communication with the scavenge filling chambers ( 18 ) through the aforementioned communication path ( 40 ) at this time, the positive pressure forces the exhaust gas into the scavenge filling chambers ( 18 ) to fill the scavenge filling chambers. Where the scavenging passages ( 19 ) are in communication with the interior of the crank chamber ( 28 ), a negative pressure acts inside the crank chamber ( 28 ) when the piston ( 33 ) is located near the top dead center. For this reason, this negative pressure acts on the scavenge filling chambers ( 18 ) through the scavenging passages ( 19 ) and the communicating ports ( 21 ) (establishing communication between the scavenging passages ( 19 ) and the scavenge filling chambers ( 18 )). This action of the negative pressure promotes the supply of the exhaust gas from the exhaust port ( 16 ) through the aforementioned communication path ( 40 ). In this structure, the inflow/outflow control of the non-working gas is performed by the pressure difference in conjunction with the movement of the piston ( 33 ) and there is no need for addition of an extra switching valve in the communication path ( 40 ), so as to simplify the structure. 
   The two-stroke engine is further preferably constructed as described below. The non-working gas is comprised essentially of intake air without fuel introduced from an exterior atmospheric space. The non-working gas comprised essentially of the intake air may be the atmospheric air itself, or a gas resulting from mixing thereof with another gas containing no fuel (e.g., newly introduced air different from air taken in for the purpose of combustion, an inert gas in storage, or the like). 
   Supply of the intake air into the scavenge filling chambers ( 18 ) in the case where the non-working gas is comprised essentially of the intake air is implemented through a communication path ( 41 ) formed in the piston ( 33 ) and/or in the wall part of the combustion chamber ( 12 ) so as to bring the scavenge filling chambers ( 18 ) into communication with an air passage ( 17 ) for supplying the intake air without fuel to the scavenge filling chambers ( 18 ), when the piston ( 33 ) to reciprocate inside the combustion chamber ( 12 ) is located near the top dead center. The communication path ( 41 ) is formed as a groove when it is formed in the surface of the piston ( 33 ) and/or the wall part of the combustion chamber ( 12 ); it is formed as a hole when it is formed inside the piston ( 33 ) and/or the wall part of the combustion chamber ( 12 ). The communication path ( 41 ) may also be formed as a groove in part and a hole in the other part. The air passage ( 17 ) to be brought in communication with the communication path ( 41 ) is a passage for supplying the intake air containing no fuel (e.g., the intake air before mixed with the fuel component). In this structure, there is no need for addition of an extra switching valve in the communication path ( 41 ), so as to simplify the structure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view (with the piston at the top dead center) showing the first embodiment of the two-stroke engine of the present invention. 
       FIG. 2  is a sectional view (with the piston at the bottom dead center) showing the first embodiment of the two-stroke engine of the present invention. 
       FIG. 3  is a sectional view along III—III line in  FIG. 1 . 
       FIG. 4  is a sectional view along IV—IV line in  FIG. 1 . 
       FIG. 5  is a sectional view along V—V line in  FIG. 1 . 
       FIG. 6  is a sectional view along VI—VI line in  FIG. 2 . 
       FIG. 7  is a sectional view (with the piston at the top dead center) showing the second embodiment of the two-stroke engine of the present invention. 
       FIG. 8  is a sectional view along VIII—VIII line in  FIG. 7 . 
       FIG. 9  is a sectional view along IX—IX line in  FIG. 7 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The first embodiment of the present invention will be described below with reference to the drawings.  FIG. 1  and  FIG. 2  are vertical sectional views of Schnürle-method two-stroke engine  10  of the present embodiment. In  FIG. 1  the piston  33  is at the top dead center and in  FIG. 2  the piston dJ 3  at the bottom dead center.  FIG. 3  is a sectional view along III—III line in  FIG. 1 ,  FIG. 4  a sectional view along IV—IV line in  FIG. 1 , and  FIG. 5  a sectional view along V—V line in  FIG. 1 .  FIG. 6  is a sectional view along VI—VI line in  FIG. 2  (of cylinder block  11  only). 
   The Schnürle-method two-stroke engine  10  is mounted, for example, on a lawn mower or on a backpacked power sprayer. A cylinder (combustion chamber)  12  is formed in a cylinder block  11 . The cylinder  12  extends along the center line of the cylinder block  11  and inside the cylinder block  11  and opens in the lower end face of the cylinder block  11 . A recess  13  is formed in the top part of the cylinder  12  and a discharge electrode of a spark plug not shown is placed inside the recess  13 . A spark plug mounting hole  14  is shown in  FIG. 6 . 
   The upper part of the cylinder  12  and the part of recess  13  function as a combustion chamber. An intake port  15  and an exhaust port  16  establish communication between the outside of the cylinder block  11  and the interior of the cylinder  12 . The intake port  15  and exhaust port  16  are formed at positions 180° apart in the circumferential direction of the cylinder  12  and in the peripheral wall of the cylinder block  11  so that the exhaust port  16  is located nearer the top dead center than the intake port  15  in the height direction of the cylinder  12 . Namely, where a projection plane is defined as a plane perpendicular to the center axis of the cylinder  12 , when the positions of the intake port  15  and exhaust port  16  are projected onto this projection plane, the intake port  15  and exhaust port  16  are located at the positions substantially opposite to each other with respect to the center of the cylinder  12 . 
   A plurality of cooling fins for heat radiation formed on the outer surface of the cylinder block  11  project outwards in parallel with each other in the radial direction-of the cylinder block  11  and near the top dead center of the cylinder block  11 . Scavenge filling chambers  18  and scavenging passages  19  have their respective apertures  18   a ,  19   a  opening inside the cylinder  12  near the top dead center. These apertures are located at much the same height as the exhaust port  16  is in the axial direction of the cylinder  12 , and are positioned to open in the combustion chamber as the piston  33  approaches the bottom dead center. 
   The scavenge filling chambers  18  and scavenging passages  19  all are formed in the cylinder block  11  outside the cylinder  12  and extend in the axial direction of the cylinder  12 . The scavenge filling chambers  18  and scavenging passages  19  are provided two each as paired. The pair of scavenge filling chambers  18  are placed one on each side in symmetry with respect to a line connecting the intake port  15  to the exhaust port  16  when their positions are projected onto the aforementioned projection plane. Likewise, the pair of scavenging passages  19  are also placed one on each side in symmetry with respect to the line connecting the intake port  15  to the exhaust port  16 . On the projection plane, the pair of scavenge filling chambers  18  are located nearer the exhaust port  16  than the pair of scavenging passages  19 . The pair of scavenge filling chambers  18  and the pair of scavenging passages  19 , as shown in  FIGS. 3 and 4  (which are not projected figures but sectional views), each are oriented so that on the aforementioned projection plane, introduced gases therefrom into the cylinder  12  collide with each other on the opposite side to the exhaust port  16 . 
   As seen from  FIG. 6 , the ends of the scavenge filling chambers  18  and scavenging passages  19  on the top dead center side are inclined toward the recess  13 . The gases to be supplied from the scavenge filling chambers  18  and from the scavenging passages  19  into the cylinder  12  are introduced in the directions toward the recess  13  in the top part of the cylinder  12 , so as to effect better scavenging inside the combustion chamber. With regard to the cylinder block  11 , the scavenge filling chambers  18  and the scavenging passages  19  both have their ends open on the bottom dead center side. However, concerning the scavenge filling chambers  18 , their open ends are closed by a gasket  24  sandwiched between the cylinder block  11  and crank case  27 . The scavenge filling chamber  18  and the scavenging passage  19  adjacent to each other are isolated from each other by bulkhead  20 , and this bulkhead  20  is provided with a communicating port  21  for communication between the scavenge filling chamber  18  and the scavenging passage  19 . The bulkhead  20  is located approximately in parallel with the flow of gas stream in the scavenging passage  19 . For this reason, the communicating port  21  formed in the bulkhead  20  opens approximately perpendicularly to this gas stream. 
   The crank case  27  has its upper surface joined to the lower surface of cylinder block  11 . A crank chamber  28  is formed inside the crank case  27 . The crank chamber  28  is always in communication with the scavenging passages  19  but is not in communication with the scavenge filling chambers  18  because of the aforementioned gasket  24 . The crank chamber  28  is brought into communication with the intake port  15  when the piston  33  is located near the top dead center (cf.  FIG. 1 ). A crank shaft  29  is rotatably journaled on the wall at both ends of the crank case  27 . The piston  33  is slidably fitted in the cylinder  12  and reciprocates inside the cylinder  12 . As the piston reciprocates inside the cylinder  12 , the volume of the combustion chamber increases and decreases. A connecting rod  35  is rotatably coupled at one end thereof to the piston  33  and rotatably coupled at the other end to the crank shaft  29 . 
   A pair of communication paths  40  are formed in groove shape in the lower end of the peripheral surface of the piston  33 . On the aforementioned projection plane, the communication paths  40  extend in the range from the exhaust port  16  to the scavenge filling chambers  18  on the circumference of the piston  33 . When the piston  33  is located near the top dead center, each communication path  40  gets its both ends communicating with the exhaust port  16  and with the aperture  18   a  of the scavenge filling chamber  18 , so as to establish mutual communication between the exhaust port  16  and the scavenge filling chamber  18  (cf.  FIG. 3 ). 
   The operation of the Schnürle-method two-stroke engine  10  of the present embodiment will be described. First, in a stroke in which the piston  33  moves from the bottom dead center to the top dead center, the volume of the combustion chamber decreases, while the volume on the crank chamber  28  side increases. The exhaust port  16  is closed by the piston  33  with the movement of the piston  33  from the bottom dead center to the top dead center, so as to compress the air-fuel mixture (a mixture of fuel and air) in the combustion chamber. With further movement of the piston  33  toward the top dead center, the intake port  15  comes into communication with the crank chamber  28 , and then the air-fuel mixture from the carburetor is introduced through the intake port  15  into the crank chamber  28  in tandem with the compression of the air-fuel mixture in the combustion chamber. 
   When further movement of the piston  33  brings the piston  33  to near the top dead center, discharge of the spark plug occurs to cause ignition and explosion of the fuel in the air-fuel mixture inside the combustion chamber and its explosive power moves the piston  33  toward the bottom dead center. When the piston  33  is located in the vicinity of the top dead center, the lower end of the piston  33  reaches the height of the scavenge filling chambers  18 , whereupon the scavenge filling chambers  18  come into communication through the communication paths  40  with the exhaust port  16  (the state of  FIG. 1 ). At this time, the scavenge filling chambers  18  are under action of the positive pressure from the exhaust system through the exhaust port  16  on the communication path  40  side and under action of the negative pressure in the crank chamber  28  in the intake stroke on the communicating port  21  side. Accordingly, the gas existing in each scavenge filling chamber  18  is discharged through the communicating port  21  to the scavenging passage  19  side and the exhaust gas from the exhaust port  16  is supplied into the scavenge filling chambers  18  to fill them. 
   After the ignition of the air-fuel mixture, the piston  33  starts to move toward the bottom dead center and this movement terminates the communication between the exhaust port  16  and the scavenge filling chambers  18  through the communication paths  40 . At this time, the scavenge filling chambers  18  are in communication with the crank chamber only through the communicating ports  21  and scavenging passages  19  having functioned to suck the gas out, and thus the exhaust gas (non-working gas) in the scavenge filling chambers  18  is prevented from being mixed with the air-fuel mixture (working gas) in the crank chamber  28 . Although the communicating ports  21  still remain opening definitely, the interior of the scavenge filling chambers  18  is already filled with the non-working gas and therefore the working gas does not flow thereinto through the communicating ports  21  (or the inflow thereof can be negligibly small). 
   In the two-stroke engine described in aforementioned [Patent Document 1], the portions corresponding to the scavenge filling chambers  18  in the present embodiment were also formed in the same structure as the scavenging passages  19  in the present embodiment, and were open at their one end in the crank chamber. For this reason, when the non-working gas (exhaust gas) was introduced from the exhaust system with the piston being located near the top dead center, the working gas (air-fuel mixture) was likely to flow into those portions from the end side of open end. Particularly, since the flow of gas at this time was directed along the scavenging passages, the working gas was likely to flow into those portions. 
   In contrast to it, the scavenge filling chambers  18  of the present embodiment are closed by the gasket as described above, at their end on the crank chamber  28  side. For this reason, the working gas is prevented from being mixed with the non-working gas filled in the scavenge filling chambers  18 . Since the communicating ports  21  open in the direction perpendicular to the direction along the scavenging passages  19  (the flow direction in the scavenging passages  19 ), there occurs little inflow/outflow of gas through the communicating ports  21 , and thus the working gas is prevented from being mixed with the non-working gas in the scavenge filling chambers  18 . 
   With further movement of the piston  33  toward the bottom dead center, the exhaust port  16  and each of the apertures  18   a ,  19   a  of the scavenge filling chambers  18  and the scavenging passages  19  on the top dead center side come to open in the combustion chamber. At this time, by virtue of the positive pressure in the crank chamber  28  arising with movement of the piston  33 , the working gas (air-fuel mixture) is introduced through the scavenging passages  19  into the combustion chamber. At the same time as it, the apertures  18   a  of the scavenge filling chambers  18  at the end near the top dead center become open, and then the working gas flows from the scavenging passage  19  side through the communicating port  21  into each scavenge filling chamber  18 . In conjunction therewith, the non-working gases (exhaust gases) filled in the scavenge filling chambers  18  are forced through the apertures  18   a  near the top dead center into the combustion chamber. 
   The working gas is not introduced through the scavenge filling chambers  18  into the combustion chamber until the non-working gases filled inside the scavenge filling chambers  18  are completely forced from the scavenge filling chambers  18  into the combustion chamber. For this reason, the working gas layer introduced through the scavenging passages  19  into the combustion chamber and the non-working gas layer introduced through the scavenge filling chambers  18  into the combustion chamber scavenge the interior of the combustion chamber while maintaining a clear boundary between them. Namely, no mixed layer of the two gases is formed between the non-working gas layer and the working gas layer (or the mixture is negligibly small if formed). The working gas streams introduced from the pair of scavenging passages  19  into the combustion chamber collide with each other on the opposite side to the exhaust port  16  because of the shape of the scavenging passages  19 , to form reverse vortices, and then move toward the exhaust port  16  while scavenging the interior. 
   At this time, on the exhaust port  16  side with respect to this working gas, the non-working gas streams introduced from the pair of scavenge filling chambers  18  into the combustion chamber also form reverse vortices in similar fashion to inhibit movement of the working gas toward the exhaust port  16 . This prevents the blow-by of the working gas and the non-working gas is first discharged through the exhaust port  16 . When the aforementioned clear laminar boundary part between the non-working gas layer and the working gas layer reaches the exhaust port  16 , the piston  33  starts rising to close the exhaust port  16 . This allows only the non-working gas to undergo blow-by, but does not allow the working gas to undergo blow-by. Since the laminar boundary is clear, no excessive non-working gas remains in the combustion chamber, and the non-working gas undergoes secure blow-by. 
   As described above, the communicating ports  21  open approximately perpendicularly to the flow of the gas streams introduced from the scavenge filling chambers  18  and from the scavenging passages  19  into the combustion chamber. For this reason, just as in the process of filling the scavenge filling chambers  18  with the non-working gas, there occurs no mixture of the non-working gas and the working gas during this scavenging stroke between the scavenge filling chambers  18  and the scavenging passages  19  through the communicating ports  21 . This also more effectively prevents disturbance of the laminar flow with the clear laminar boundary. 
   This configuration prevents the working gas from being mixed with the non-working gas filled in the scavenge filling chambers  18 , and thus the aforementioned mixed layer is not formed. For this reason, it becomes easy to make only the non-working gas undergo blow-by and to prevent the working gas from undergoing blow-by, and it thus becomes feasible to effectively reduce the amount of exhaust THC. An increase of trapping efficiency also permits a decrease of fuel consumption. Furthermore, the non-working gas remaining in the combustion chamber is also reduced, and the delivery ratio increases to raise an expectation of power increase as well. 
   In the present embodiment, even if an overflow occurs during the filling process of the non-working gas into the scavenge filling chambers  18 , the non-working gas will be trapped inside the scavenging passages  19 . Namely, the scavenging passages  19  serve like a buffer, also to prevent the non-working gas from being mixed with the working gas in the crank chamber  28 . Since the non-working gas overflowing into the scavenging passages  19  is first introduced into the combustion chamber prior to the inflow of the working gas in the scavenging stroke, the stratified scavenging flow is not disturbed and there arises no problem in terms of reduction of THC and securing of power. 
     FIGS. 7 to 9  show the second embodiment of the present invention. Many components in the present embodiment are identical or equivalent to those in the aforementioned first embodiment. For this reason, the identical or equivalent components to those in the aforementioned first embodiment will be denoted by the same reference symbols, without detailed description thereof.  FIG. 7  is a view corresponding to  FIG. 1  of the first embodiment.  FIG. 8  is a sectional view along VIII—VIII line in  FIG. 7  (a view corresponding to  FIG. 5 ), and  FIG. 9  a sectional view along IX—IX line in  FIG. 7  (a view corresponding to  FIG. 4 ). 
   In the first embodiment described above, the non-working gas filled in the scavenge filling chambers  18  was the exhaust gas refluxed from the exhaust system (or gas consisting primarily of the exhaust gas). In contrast to it, the present embodiment uses the intake air without fuel introduced from the exterior atmospheric space (or gas consisting primarily of the intake air). In the present embodiment, as shown in  FIG. 7 , an air passage  17  is provided on the top dead center side with respect to the intake port (intake passage)  15 . The air passage  17  has an end thereof opening in the inner surface of the cylinder  12 . The height of this opening part is approximately equal to those of the exhaust port  16  and the end apertures  18   a ,  19   a  of the scavenge filling chambers  18  and the scavenging passages  19 . 
   In the present embodiment, a pair of scavenge filling chambers  18  are placed on the side where the intake port  15  and air passage  17  are located. A pair of scavenging passages  19  are placed on the exhaust port  16  side with respect to the pair of scavenge filling chambers  18 . In fact, this configuration is realized by replacement of the closed portions of the open ends on the crank chamber  28  side by the gasket  24 . A pair of communication paths  41  corresponding to the communication paths  40  in the first embodiment are not formed on the exhaust port  16  side but formed on the intake port  15  and air passage  17  side. The pair of communication paths  41  are also formed in groove shape in the lower end part of the peripheral surface of the piston  33 . However, on the aforementioned projection plane (the same as in the first embodiment), the communication paths  41  in the present embodiment extend in the range from the aperture of the air passage  17  to the apertures  18   a  of the scavenge filling chambers  18 . When the piston  33  is located in the vicinity of the top dead center, each communication path  41  gets its both ends communicating with the aperture of the air passage  17  and with the aperture  18   a  of the scavenge filling chamber  18 , so as to establish mutual communication between the air passage  17  and the scavenge filling chamber  18 . 
   The air passage  17  is used to fill the scavenge filling chambers  18  with the non-working gas (intake air) through the communication paths  41  when the piston  33  is located near the top dead center. At this time, the scavenge filling chambers  18  are under action of the negative pressure in the crank chamber  28  in the intake stroke on the communicating port  21  side. This results in filling the scavenge filling chambers  18  with the non-working gas. At this time, it is preferable to make a positive pressure from the air passage  17  side act on the scavenge filling chambers  18 , in order to effect smoother filling with the non-working gas. A conceivable method of making the positive pressure act is a forced feed by means of a pump or the like. The pump may be an electrically driven one, or one using the power of the engine  10 . 
   Air may be introduced into the air passage  17  by branching it from the intake passage on the downstream side of an air filter, or by securing a new intake passage. The air passage  17  opens at its end in the combustion chamber when the piston  33  reaches the bottom dead center. At this time, the intake air is introduced from the air passage  17  into the combustion chamber. This intake air flow, together with the non-working gas filled inside the scavenge filling chambers  18 , forms the non-working gas layer. This configuration is also able to make a clear boundary part between the non-working gas layer and the working gas layer introduced from the scavenging passages  19  into the combustion chamber, and to implement reduction of discharge amount of THC, reduction of fuel consumption, and increase of power. 
   The present invention is by no means limited to the embodiments described above. For example, the above-described embodiments used the gasket  24  to close the scavenge filling chambers  18  on the crank chamber  28  side. This use of the gasket  24  enables easy formation of the scavenge filling chambers  18 , but the scavenge filling chambers may also be formed by any other technique than the use of the gasket. For example, the scavenge filling chambers may be closed on the crank case side by other components such as stop members. In that case, the positions of the stop members do not always have to be the crank-case-side ends of the scavenge filling chambers. Furthermore, the position of the aforementioned communicating port  21  can also be optionally set in consideration of the motion of the gas flow. 
   Since the present invention makes the laminar boundary clear between the non-working gas layer and the working gas layer in the scavenging stroke as described above, the mixed layer is not formed (or is barely formed, if any), and it thus becomes easy to make the non-working gas only undergo blow-by and to keep the working gas only staying in the combustion chamber. This prevents the working gas from being contained as a mixed layer in the blow-by and causing an increase of fuel consumption. The non-working gas is also prevented from remaining as a mixed layer in the combustion chamber and causing a power drop due to a decrease of delivery ratio.