Patent Publication Number: US-6662765-B2

Title: Two-stroke internal combustion engine

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates to a two-stroke internal combustion engine which is suited for use in a portable power working machine, and in particular to a two-stroke internal combustion engine which is capable of minimizing the quantity of so-called blow-by or the quantity of air-fuel mixture to be discharged without being utilized for the combustion. 
     2. Description of the Related Art 
     An ordinary small air-cooled two-stroke internal combustion engine which is conventionally used in a portable power working machine, such as a chain saw or brush cutter, is constructed such that an ignition plug is disposed at the head portion of the cylinder, and an intake port, a scavenging port and an exhaust port, which are opened and closed by a piston, are provided at the trunk portion of the cylinder. According to this two-stroke internal combustion engine, one cycle of the engine is accomplished by two strokes of the piston without undergoing a stroke exclusively assigned to the intake or exhaust. 
     More specifically, in the ascending stroke of the piston, air-fuel mixture is introduced from the intake port into a crank chamber disposed below the piston. During the subsequent descending stroke, the air-fuel mixture is pre-compressed, and compressed gas mixture is then blown into a combustion actuating chamber (though it may be also called a combustion chamber, actuating chamber, cylinder chamber, etc., these chambers are generically referred to as combustion actuating chamber in the present specification) which is disposed above the piston, thereby enabling waste combustion gas to be discharged from the exhaust port. In other words, since the scavenging of the waste combustion gas is effected by making use of the gas flow of the air-fuel mixture, the unburned air-fuel mixture is more likely to be mixed into the combustion gas (exhaust gas), thereby increasing the quantity of so-called blow-by or the quantity of air-fuel mixture to be discharged into the atmospheric air without being utilized for the combustion. Because of this, two-stroke internal combustion engines are not only inferior in fuel consumption but also disadvantageous in that a large amount of poisonous components such as HC (unburned components in fuel) and CO (incomplete combustion components in fuel) are included in the exhaust gas when compared to four-stroke engines. Therefore, even if the two-stroke engine is small in capacity, the influence of these poisonous components on the environmental contamination should not be disregarded. 
     With a view to addressing these problems, various proposals on the two-stroke internal combustion engine have been suggested. The proposals have included the introduction of air into the combustion actuating chamber prior to the introduction of air-fuel mixture so as to scavenge the combustion gas (for example, Japanese Patent Unexamined Publications H9-125966 and H5-33657). 
     In such conventional air-preintroduction type two-stroke internal combustion engines, however, air inlet passageways or check valves are required to be separately installed, so the resulting engine may be complicated in structure, and inevitably result in a sharp increase in manufacturing cost. 
     Additionally, since air is caused to be excessively introduced into the combustion actuating chamber, the combustion stability may easily deteriorate, thus creating problems, such as difficulties in the engine setting. Accordingly, there exists a need in the art for a two-stroke engine which can overcome the aforementioned disadvantages associated with the conventional two-stroke engines. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an air-preintroduction type two-stroke internal combustion engine which is capable of minimizing the quantity of so-called blow-by or the quantity of air-fuel mixture discharged without being utilized for combustion. 
     Another object of the present invention is to provide an air-preintroduction type two-stroke internal combustion engine with improved fuel consumption and engine power. 
     Yet another object of the present invention is to provide an air-preintroduction type two-stroke internal combustion engine capable of reducing the amount of poisonous components in the exhaust gas. 
     Still another object of the present invention is to reduce the manufacturing cost of an air-preintroduction type two-stroke internal combustion engine, without extensively altering its conventional structure. 
     These and other objects of the present invention, which will become apparent with reference to the disclosure herein, are accomplished by the two-stroke internal combustion engine according to the present invention, which includes one or more pairs of Schnürle-type scavenging passageways, each allowing the combustion actuating chamber disposed above a piston to communicate with the crank chamber. The scavenging passageways are symmetrically provided on both sides of the longitudinal section that figuratively divides an exhaust port into two equal parts. A through-hole or a communication groove is provided at a skirt portion of the piston to thereby enable a scavenging inlet port provided at an upper end of the scavenging passageway to communicate with the crank chamber. In the descending stroke of the piston, combustion exhaust gas originating from the combustion actuating chamber is enabled to be introduced, via a scavenging outlet port provided at the downstream end of the scavenging passageway, into the scavenging passageway while closing the scavenging inlet port at the skirt portion of the piston. In synchronization with the descending stroke of the piston, the air-fuel mixture may be introduced from the crank chamber, via the through-hole or the communication groove, into the scavenging passageway. The combustion exhaust gas existing inside the scavenging passageway is designed to be introduced into the combustion actuating chamber prior to the introduction thereinto of the air-fuel mixture. 
     In a preferred embodiment, in the descending stroke of the piston, the scavenging outlet port is opened a moment after the exhaust port has been opened, which is followed by the opening of the scavenging inlet port through the through-hole or the communication groove. 
     Alternatively, the two-stroke internal combustion engine may include a pair of first Schnürle-type scavenging passageways located close to the exhaust port and a pair of second Schnürle-type scavenging passageways located away from the exhaust port, each passageway allowing the combustion actuating chamber, disposed above the piston, to communicate with the crank chamber. The scavenging passageways are symmetrically provided on both sides of the longitudinal section that figuratively divides the exhaust port into two equal parts. A through-hole or a communication groove is provided at a skirt portion of the piston to thereby enable a second scavenging inlet port, provided at an upper end of the second Schnürle-type scavenging passageway, to communicate with the crank chamber. The upper end of the first scavenging passageway communicates via a throttle passageway with the second scavenging passageway. In the descending stroke of the piston, combustion exhaust gas originating from the combustion actuating chamber is introduced, via the first scavenging outlet port provided at the downstream end of the first scavenging passageway, into the first scavenging passageway, while the second scavenging inlet port is closed at the skirt portion of the piston. In synchronization with the descending stroke of the piston, the air-fuel mixture fed from the crank chamber is subsequently introduced into the second scavenging passageway via the through-hole or the communication groove, as well as through the second scavenging inlet port, and also into the first scavenging passageway through the throttle passageway. The combustion exhaust gas existing inside the first scavenging passageway is designed to be introduced into the combustion actuating chamber prior to the introduction thereinto of the air-fuel mixture. 
     In a preferred embodiment, in the descending stroke of the piston, the scavenging outlet port is opened a moment after the exhaust port has been opened, which is followed by the opening of the second scavenging inlet port via the through-hole or the communication groove and is followed by the opening of the second scavenging outlet port. 
     According to the preferred embodiments of two-stroke internal combustion engine of the present invention as described above, the air-fuel mixture supplied from the air-fuel mixture-generating means, e.g., a carburetor, is received and stored in the crank chamber in the ascending stroke of the piston. 
     When the air-fuel mixture inside the combustion actuating chamber, disposed above the piston, explodes and burns after being ignited, combustion gas is generated and the piston is pushed downward. In this descending stroke of the piston, the air-fuel mixture existing inside the crank chamber is compressed by the piston, and at the same time, the exhaust port is opened to permit the combustion exhaust gas to be discharged from the exhaust port. 
     As the piston further descends, the scavenging port, provided at the downstream end of each of the scavenging passageways, is opened. At this moment, the scavenging inlet port of each of the scavenging passageways is closed by the skirt portion of the piston. Since the combustion gas (combustion exhaust gas) pressure existing inside the combustion actuating chamber due to the aforementioned explosive burning is higher than the pressure of the air-fuel mixture inside the scavenging passageways, part of the combustion exhaust gas is permitted to blow down from the scavenging outlet port and hence is permitted to be introduced into and stored in the scavenging passageways. 
     As the piston still further descends, the scavenging inlet port is opened via the through-hole or the communication groove provided at the skirt portion of the piston, thereby allowing the air-fuel mixture that has been pre-compressed in the crank chamber to be introduced into the scavenging passageway. As a result, the combustion exhaust gas existing inside the scavenging passageway is pushed out by the air-fuel mixture introduced as mentioned above into the scavenging passageway, thereby enabling the combustion exhaust gas to be blown out from the scavenging outlet port toward the combustion actuating chamber. Due to the scavenging gas flow of the combustion exhaust gas thus blown out, the combustion exhaust gas existing inside the combustion actuating chamber is pushed out therefrom toward the exhaust port. Furthermore, following the combustion exhaust gas flow, the air-fuel mixture is also permitted to flow into the combustion actuating chamber, thereby completely forcing the combustion exhaust gas that has been introduced in advance into the combustion actuating chamber to flow toward the exhaust port. 
     In the preferred embodiment, the scavenging (by making use of the combustion exhaust gas) is performed by deliberately delaying the scavenging-initiating timing from the exhaust-initiating timing (the timing to open the exhaust port). In succession to this scavenging, a high-pressure fresh air (air-fuel mixture), which has been pre-compressed, is enabled to be introduced at a stretch into the combustion actuating chamber. 
     According to the conventional two-stroke internal combustion engine where the air-fuel mixture is employed as a scavenging flow, a fairly large quantity of air-fuel mixture is permitted to blow by, since the portion indicated by the hatched region (area) in FIG. 13 corresponds to the quantity of so-called blow-by of fresh air (air-fuel mixture). 
     According to the preferred embodiments of the two-stroke internal combustion engine of the present invention as described above, the quantity of blow-by of air-fuel mixture can be greatly reduced since the combustion exhaust gas is employed in place of the air-fuel mixture as a scavenging flow, which results in that the portion indicated by the hatched region (area) in FIG. 12 corresponds to the quantity of so-called blow-by of fresh air (air-fuel mixture). Therefore, according to the two-stroke internal combustion engine of the present invention, the quantity of so-called blow-by or the quantity of air-fuel mixture to be discharged without being utilized for the combustion can be reduced to a minimum, thus making it possible to improve the fuel consumption and engine power, and to minimize the amount of poisonous components in the exhaust gas. 
     Furthermore, the two-stroke internal combustion engine of the present invention can be manufactured by slightly modifying the piston (drilling a through-hole, etc. therein), so that the conventional engine structure would not be significantly altered. Therefore, it is possible to reduce the manufacturing cost of the engine to a minimum. 
     According to the prior art, it has been generally tried to narrow the timings of suction and exhaust (timing area) so as to reduce the amount of exhaust gas as a whole (the engine power would be inevitably lowered) in order to minimize the presence of poisonous substances in the exhaust gas (exhaust emission). According to the present invention, it is no longer absolutely required to reduce the amount of exhaust gas. Moreover, since air is no longer permitted to be introduced into the engine, it is now possible to prevent combustion stability from deteriorating and to facilitate the engine setting. 
     In accordance with the invention, the objects as described above have been met, and the need in the art for a two-stroke engine that has low amount of poisonous components in the exhaust gas, improved fuel consumption and combustion stability, has been satisfied. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a longitudinal sectional view schematically illustrating one embodiment of the two-stroke internal combustion engine according to the present invention, wherein the piston is positioned at the top dead center; 
     FIG. 2 is a longitudinal sectional view schematically illustrating a state wherein the piston of the two-stroke internal combustion engine of FIG. 1 is slightly descended from the top dead center; 
     FIG. 3 is a longitudinal sectional view schematically illustrating a state wherein the piston of the two-stroke internal combustion engine of FIG. 1 is descended to such an extent that the exhaust port thereof is begun to open; 
     FIG. 4 is a longitudinal sectional view schematically illustrating a state wherein the piston of the two-stroke internal combustion engine of FIG. 1 is descended to such an extent that the scavenging outlet port thereof begins to open; 
     FIG. 5 is a longitudinal sectional view schematically illustrating a state wherein the piston of the two-stroke internal combustion engine of FIG. 1 is descended to such an extent that the through-hole of the piston begins to open to the scavenging inlet port; 
     FIG. 6 is a longitudinal sectional view schematically illustrating a state wherein the piston of the two-stroke internal combustion engine of FIG. 1 is positioned at the bottom dead center; 
     FIG. 7 is a longitudinal sectional view schematically illustrating a state wherein the piston of the two-stroke internal combustion engine of FIG. 1 is slightly ascended from the bottom dead center; 
     FIG. 8 is a longitudinal sectional view schematically illustrating a state wherein the piston of the two-stroke internal combustion engine of FIG. 1 is ascended to such an extent that the scavenging outlet port as well as the scavenging inlet port thereof are closed; 
     FIG. 9 is a cross-sectional view illustrating the two-stroke internal combustion engine shown in FIG. 1; 
     FIG. 10 is a perspective view illustrating one example of the piston to be employed in the two-stroke internal combustion engine shown in FIG. 1; 
     FIG. 11 is a perspective view illustrating another example of the piston to be employed in the two-stroke internal combustion engine shown in FIG. 1; 
     FIG. 12 is a graph illustrating the characteristics of the two-stroke internal combustion engine shown in FIG. 1; 
     FIG. 13 is a graph illustrating the characteristics of the two-stroke internal combustion engine according to the prior art; 
     FIG. 14 is a schematic longitudinal sectional view, taken along a plane splitting the exhaust port of the two-stroke internal combustion engine according to a second embodiment of the present invention, wherein the piston is positioned at the top dead center; 
     FIG. 15 is a schematic longitudinal sectional view, taken along a plane orthogonally intersecting the exhaust port of the two-stroke internal combustion engine shown in FIG. 14, wherein the piston is positioned at the top dead center; 
     FIG. 16 is a longitudinal sectional view schematically illustrating a state wherein the piston of the two-stroke internal combustion engine of FIG. 15 is descended to such an extent that the exhaust port thereof begins to open; 
     FIG. 17 is a longitudinal sectional view schematically illustrating a state wherein the piston of the two-stroke internal combustion engine of FIG. 15 is descended to such an extent that the first scavenging outlet port thereof begins to open; 
     FIG. 18 is a longitudinal sectional view schematically illustrating a state wherein the piston of the two-stroke internal combustion engine of FIG. 15 is descended to such an extent that the second scavenging outlet port thereof begins to open; 
     FIG. 19 is a longitudinal sectional view schematically illustrating a state wherein the piston of the two-stroke internal combustion engine of FIG. 15 is ascended from the bottom dead center to such an extent that the first scavenging outlet port as well as the second scavenging outlet port thereof are closed; and 
     FIG. 20 is a cross-sectional view illustrating the two-stroke internal combustion engine shown in FIG.  14 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Specific embodiments of a two-stroke internal combustion engine according to the present invention will be further explained with reference to the drawings. 
     FIGS. 1 through 8 respectively illustrate a longitudinal sectional view schematically illustrating one embodiment of the two-stroke internal combustion engine according to the present invention. FIG. 9 is a schematic cross-sectional view taken along the exhaust port. 
     For easier explanation, in FIGS. 1 through 8, an exhaust port  34  and an intake port  33  are shown such that they are disposed on the same side. Also, in FIG. 9, the exhaust port  34  and the intake port  33  are shown such that they are positioned at the same level. However, as a matter of fact, the intake port  33  is preferably displaced, in the horizontal direction, from the exhaust port  34  at an angle of 180° and also placed, in the vertical direction, lower than the exhaust port  34 . Furthermore, scavenging passageways  30  are both preferably displaced from the intake port  33  at an angle of 90°. 
     Referring to FIGS. 1-8, the two-stroke internal combustion engine  1  is formed of a small air-cooled two-stroke gasoline engine of binary scavenging type, which is adapted to be employed in a portable working machine. The engine  1  includes a cylinder  12  in which a piston  20  is fittingly inserted, and a crankcase  14  axially supporting a crank shaft  22  for reciprocally moving the piston  20  up and down via a connecting rod  24 . The cylinder  12  is provided, on the outer circumferential wall thereof, with a large number of cooling fins (not illustrated ), and, at the head portion thereof, with a squish-dome shape (semi-spherical) combustion chamber  15   a  constituting the combustion actuating chamber  15 . An ignition plug  17  is protruded into the combustion chamber  15   a.    
     The exhaust port  34  is attached to one side (the left side in FIG. 9) of the cylinder&#39;s trunk portion and the intake port  33  is attached to the other side (the right side in FIG. 9) of the trunk portion. A pair of right and left scavenging passageways  30  are symmetrically provided on both sides of the longitudinal section F—F (FIG. 9) which figuratively divides the exhaust port  34  into two equal parts so as to permit the combustion actuating chamber  15 , disposed above the piston  20 , to communicate with the crank chamber  18  defined by the crankcase  14 . A pair of scavenging outlet ports  30   a  both opening to the combustion actuating chamber  15  are provided, respectively, at the upper end (a downstream end) of each of the scavenging passageways  30 , and a pair of scavenging inlet ports  30   b  both opening to the crank chamber  18  are provided, respectively, at the lower end (an upstream end) of each of the scavenging passageways  30 . 
     The scavenging outlet ports  30   a  are disposed at a level which is lower than the exhaust port  34  by a distance of “h”, so that the scavenging outlet ports  30   a  are enabled to open a moment later, i.e., a crank angle corresponding to the distance of “h” (for example, about 10 degrees) after the opening of the exhaust port  34  in the descending stroke of the piston  20 . 
     As shown in FIG. 10, the piston  20  is provided, at the skirt portion  20 A thereof, with a rectangular through-hole  25 , so as to enable the scavenging inlet ports  30   b , formed at the upstream ends of the scavenging passageways  30 , to communicate with the crank chamber  18 . 
     Alternatively, a horizontally inclined U-shaped or an L-shaped communication groove  26  as shown in FIG. 11 may be provided in place of the through-hole  25 , in the skirt portion  20 A of the piston  20 . 
     In the ascending stroke of the piston  20 , an air-fuel mixture M is introduced from an air-fuel mixture-generating means  40 , e.g. a carburetor, into the crank chamber  18  disposed below the piston  20  and reserved through the intake port  33  in the crank chamber  18 . 
     As shown in FIG. 1, when the air-fuel mixture M, which is compressed and existing inside the combustion actuating chamber  15  disposed above the piston  20 , is ignited and explodes, combustion exhaust gas E is generated and the piston  20  begins to descend (see FIG.  2 ). In the descending stroke of the piston  20 , the air-fuel mixture M existing inside the crank chamber  18  is compressed by the piston  20 , and at the same time, as shown in FIG. 3, the exhaust port  34  is opened at first to thereby permit the combustion exhaust gas E to be discharged into the atmosphere through the exhaust port  34 . 
     As the piston  20  descends, the scavenging outlet ports  30   a  provided at the downstream end of the scavenging passageways  30  open, as shown in FIG.  4 . At this moment, the scavenging inlet ports  30   b  of the scavenging passageways  30  are kept closed by the skirt portion  20 A of the piston  20 . Since the combustion gas (combustion exhaust gas) E, generated from the aforementioned explosive burning and existing inside the combustion actuating chamber  15 , has higher pressure than the pressure acting on the air-fuel mixture M inside the scavenging passageways  30 , part of the combustion exhaust gas E is permitted to be blown down from the scavenging outlet ports  30  and hence permitted to be introduced into and stored in the scavenging passageways  30 . 
     When the piston  20  further descends, the scavenging inlet ports  30   b  are opened through the through-hole  25  provided at the skirt portion  20 A of the piston  20  as shown in FIG. 5, thereby allowing an air-fuel mixture M that has been pre-compressed in the crank chamber  18 , to be introduced, via the through-hole  25  and the scavenging inlet ports  30   b , into the scavenging passageways  30 . As a result, the combustion exhaust gas E existing inside the scavenging passageways  30  and the combustion actuating chamber  15  is pushed out therefrom toward the exhaust port  34  by the scavenging flow of the air-fuel mixture M that has been blown out of the scavenging outlet ports  30   a  during a period starting from the moment when the piston  20  begins to ascend (FIG. 7) from the bottom dead center (FIG. 6) until the scavenging outlet ports  30   a  are closed, as shown in FIG.  8 . In other words, following the outflow of the combustion exhaust gas E, the air-fuel mixture M is also permitted to be introduced into the combustion actuating chamber  15 , from the scavenging passageways  30 , thereby enabling all of the combustion exhaust gas E, including the gas that has been introduced in advance into the combustion actuating chamber  15 , to be blown out toward the exhaust port  34  (see also FIG.  9 ). 
     According to the present invention, the scavenging (by making use of the combustion exhaust gas E) is performed by deliberately delaying the scavenging-initiating timing from the exhaust-initiating timing (the timing to open the exhaust port  34 ). Following the scavenging, a high-pressure fresh air (air-fuel mixture) M that has been pre-compressed is introduced at a stretch into the combustion actuating chamber  15 . 
     In this case, according to the conventional two-stroke internal combustion engine where the air-fuel mixture is employed as a scavenging flow, since the portion indicated by the hatched region (area) in FIG. 13 corresponds to the quantity of so-called blow-by of fresh air (air-fuel mixture), a fairly large quantity of air-fuel mixture is permitted to blow by. Conversely, according to the two-stroke internal combustion engine  1  of the present invention, when the combustion exhaust gas is employed in place of the air-fuel mixture as a scavenging flow, the quantity of blow-by of air-fuel mixture can be greatly reduced, as indicated by the hatched region (area) in FIG. 12, corresponding to the quantity of so-called blow-by of fresh air (air-fuel mixture). 
     Therefore, according to the two-stroke internal combustion engine  1  of the present invention, the quantity of so-called blow-by or the quantity of air-fuel mixture to be discharged without being utilized for the combustion can be reduced to a minimum, thus making it possible to improve fuel consumption and engine power, and minimize the amount of poisonous components in the exhaust gas. 
     Furthermore, the two-stroke internal combustion engine of the present invention can be manufactured by slightly modifying the piston (drilling a through-hole, etc. therein), so that the conventional structure of the engine would not be significantly altered. Therefore, it is possible to also reduce the manufacturing cost of the engine. 
     Additionally, according to the prior art, it has been generally tried to narrow the timings of suction and exhaust (timing area) so as to reduce the amount of exhaust gas as a whole (the engine power would be inevitably lowered) in order to minimize the existence of poisonous substances in the exhaust gas (exhaust emission). According to the present invention, it is no longer absolutely required to reduce the amount of exhaust gas. Furthermore, since air is no longer permitted to be introduced into the engine, it is now possible to prevent the combustion stability from deteriorating and to facilitate the engine setting. 
     FIG. 14 illustrates a second embodiment of the two-stroke internal combustion engine according to the present invention, wherein the exhaust port  34  and the intake port  33  are both split into two parts. FIGS. 15 through 19 respectively show a schematic longitudinal sectional view of the two-stroke internal combustion engine shown in FIG.  14 . FIG. 20 is a schematic cross-sectional view taken along a line passing through the exhaust port  34 . 
     For easier explanation, in FIGS. 15 through 19, a longitudinal sectional view passing through a second scavenging passageway  32  is shown on the left side, while a longitudinal sectional view passing through a first scavenging passageway  31  is shown on the right side, where these sectional views are combined with each other in each figure. Preferably, a pair of the first scavenging passageways  31 , as well as a pair of the second scavenging passageways  32  are disposed on the right and left sides, respectively. Further, in FIG. 20, the exhaust port  34  and the intake port  33  are shown such that they are positioned at the same level. Preferably, the intake port  33  is placed, in the vertical direction, lower than the exhaust port  34 . 
     In FIGS. 14 through 20, the portions or members which correspond in structure with the counterparts of the two-stroke internal combustion engine  1  according to the aforementioned first embodiment will be identified by the same reference numbers, thereby omitting a duplicated explanation thereof. Therefore, in the following explanation, only the features which differ from the aforementioned first embodiment will be explained. 
     In the two-stroke internal combustion engine  2  according to the second embodiment shown in these figures, a pair of first scavenging passageways  31  which are disposed close to the exhaust port  34 , and a pair of second scavenging passageways  32  which are disposed away from the exhaust port  34 , both pairs of scavenging passageways forming the Schnürle-type scavenging system, are symmetrically provided on both sides of the longitudinal section F which figuratively divides the exhaust port  34  into two equal parts so as to permit the combustion actuating chamber  15 , disposed above the piston  20 , to communicate with the crank chamber  18 . Additionally, the piston  20  is provided, at the skirt portion  20 A thereof, with a through-hole  25  in the same manner as in the aforementioned first embodiment so as to enable the second scavenging inlet ports  32   b  formed at the upstream end of each of the second scavenging passageways  32  to communicate with the crank chamber  18 . 
     Furthermore, the upstream end of the first scavenging passageway  31  communicates, via a throttle passageway  31   e , with the corresponding second scavenging passageway  32 . 
     In this case, the first scavenging outlet ports  31   a  are disposed at a level which is lower than the exhaust port  34  by a distance of “h 1 ”, while the second scavenging outlet ports  32   a  are disposed at a level which is lower than the exhaust port  34  by a distance of “h” and also lower than the first scavenging outlet ports  31   a  by a distance of “h 2 ”. 
     According to the two-stroke internal combustion engine  2  of this embodiment constructed as described above, in the ascending stroke of the piston  20 , an air-fuel mixture M is introduced from an air-fuel mixture-generating means  40 , e.g., a carburetor, into the crank chamber  18  disposed below the piston  20  and reserved in the crank chamber  18 . 
     As shown in FIGS. 14 and 15, when the air-fuel mixture M, existing inside the combustion actuating chamber  15  disposed above the piston  20 , is ignited and explodes, combustion exhaust gas E is generated and the piston  20  begins to descend. In the descending stroke of the piston  20 , the air-fuel mixture M existing inside the crank chamber  18  is compressed by the piston  20 , and at the same time, as shown in FIG. 16, the exhaust port  34  is opened first to permit the combustion exhaust gas E to be discharged into the atmosphere. 
     When the piston  20  further descends, the first scavenging outlet ports  31   a  provided at the downstream end of the scavenging passageways  31  are opened as shown in FIG.  17 . At this moment, the second scavenging inlet ports  32   b  of the second scavenging passageways  32  are kept closed by the skirt portion  20 A of the piston  20 . Therefore, since the combustion gas (combustion exhaust gas) E, generated from the aforementioned explosive burning and existing inside the combustion actuating chamber  15 , has much higher pressure than the air-fuel mixture M inside the first scavenging passageways  31 , part of the combustion exhaust gas E is permitted to be introduced into and stored in the scavenging passageways  30  from the first scavenging outlet ports  31   a.    
     Subsequently, when the piston  20  still further descends, the second scavenging inlet ports  32   b  are opened via the through-hole  25  as shown in FIG. 18, and at the same time, the second scavenging outlet ports  32   a  are also opened. 
     As a result, the air-fuel mixture M is permitted to flow from the crank chamber  18  so as to be introduced, via the through-hole  25  and the second scavenging inlet ports  32   b , into the second scavenging passageways  32 , and also into the first scavenging passageways  31  through the throttle passageways  31   e . In this case, the combustion exhaust gas E existing inside the first scavenging passageways  31  is permitted to be introduced as a scavenging flow into the combustion actuating chamber  15  prior to the introduction of the air-fuel mixture M into the combustion actuating chamber  15 . Concurrently, the air-fuel mixture M existing inside the second scavenging passageways  32  is also introduced into the combustion actuating chamber  15 . 
     In this case, as clearly shown in FIG. 20, since the scavenging flow of the combustion exhaust gas E being fed from the first scavenging passageways  31  is located closer to the exhaust port  34  than the scavenging flow of the air-fuel mixture M being fed from the second scavenging passageways  32 , the combustion exhaust gas E that has been introduced into the combustion actuating chamber  15  is pushed out toward the exhaust port  34  by the air-fuel mixture M. The period of this scavenging is continued starting from the moment when the piston  20  begins to ascend from the bottom dead center until all of the first and second scavenging outlet ports  31   a  and  32   a  are entirely closed as shown in FIG.  19 . 
     As a result of the aforementioned structure, in the two-stroke internal combustion engine  2  of this embodiment, the quantity of so-called blow-by, or the quantity of air-fuel mixture to be discharged without being utilized for combustion can be reduced to a minimum, thus making it possible to improve fuel consumption and the engine power, and to minimize the amount of poisonous components in exhaust gas. 
     Additionally, according to the prior art, it has been generally tried to narrow the timings of suction and exhaust (timing area) so as to reduce the amount of exhaust gas as a whole in order to minimize the amount of poisonous substances in the exhaust gas (exhaust emission). According to the present invention, however, it is no longer absolutely required to reduce the amount of exhaust gas. Furthermore, since air is no longer permitted to be introduced into the engine, it is now possible to prevent the combustion stability from deteriorating, and to facilitate the engine setting. 
     As seen from the above explanation, it is possible, according to the present invention, to minimize so-called blow-by, or the quantity of air-fuel mixture discharged without being utilized for combustion, without extensively altering the structure of the conventional engine. Additionally, it is possible to improve the fuel consumption and engine power, minimize the amount of poisonous components in the exhaust gas, and reduce the manufacturing cost of the engine. 
     Although the invention has been described herein by reference to specific embodiments thereof, it will be understood that such embodiments are susceptible of modification and variation without departing from the inventive concepts disclosed. All such modifications and variations, therefore, are intended to be included within the spirit and scope of the appended claims.