Abstract:
It is intended to effectively prevent blow-by with no need for large changes in typical structures of two-cycle internal combustion engines. A main scavenging passage ( 24 ) for supplying air-fuel mixture from a crankcase to a combustion chamber for scavenging purposes has a branch scavenging passage ( 26 ) that extends upward aslant toward an intake port ( 14 ). The main scavenging passage ( 24 ) communicates with a first scavenging port ( 20 ) located nearer to an exhaust port ( 16 ). The branch scavenging passage ( 26 ) communicates with a second scavenging port ( 22 ). A mean cross-sectional area of the branch scavenging passage ( 26 ) is smaller than that of the main scavenging passage ( 24 ). Cross-sectional area of a portion ( 24   b ) next to an inlet port ( 24   a ) of the main scavenging passage ( 24 ) opening to the crankcase is smaller than the sum of cross-sectional areas of the first and second scavenging ports ( 20, 22 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese Patent Application No. 2011-174936, filed Aug. 10, 2011, which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to a two-stroke internal combustion engine, and more specifically relates to a two-stroke internal combustion engine that is capable of reducing the blow-by of air-fuel mixture. 
     2. Description of Related Art 
     Two-stroke internal combustion engines, composed of only a small number of parts, are lightweight and compact. Therefore, they are conveniently used as power sources of chain saws and brush cutters. Two-stroke internal combustion engines, in general, have a structure in which a piston opens and closes exhaust ports of a cylinder in its up-and-down movements in the cylinder. Since such engines are configured to discharge exhaust gas from the combustion chamber while supplying air-fuel mixture into the combustion chamber, they involve the problem that the mixture charged into the combustion chamber and yet unburned is discharged outside. This is the problem so-called “blow-by”. The blow-by of air-fuel mixture not only deteriorates the fuel consumption but also invites an increase of unburned component (HC=hydrocarbon) in the exhaust gas. 
     Japanese Laid-open Patent Publication No. S59-170423 A (No. 170423 of the year 1984) has an object to diminish the “blow-by” of air-fuel mixture, and proposes to provide a plurality of scavenging ports opening into the combustion chamber, thereby introducing air-fuel mixture from some of the scavenging ports remoter from an exhaust port into the combustion chamber and introducing fresh air from the others of the scavenging ports nearer to the exhaust port. According to this proposal, since the fresh air is introduced into the combustion chamber in addition to air-fuel mixture, and works to scavenge the combustion chamber, the blow-by amount of air fuel mixture is reduced. This method of scavenging is called “stratified scavenging”. 
     Japanese Laid-open Patent Publication No. S59-170423 A (No. 170423 of the year 1984) proposes another method of stratified scavenging. The proposal of this publication is explained below in greater detail. An invention disclosed in this publication relies on the theory that, to reduce the blow-by phenomenon in a two-stroke internal combustion engine, new air (air-fuel mixture) introduced into the combustion chamber and the burnt gas remaining in the combustion chamber should preferably be prevented from merging. From this standpoint, this publication proposes the invention related to an engine in which scavenging ports are provided at positions symmetrical of an imaginary line connecting the center of the cylinder bore and the center of the exhaust port. At each side of the imaginary line, the scavenging port is composed of a pair of divisional scavenging ports separated by a partition wall, which regulates the directions of air-fuel mixture flowing out of the individual scavenging ports. The engine further has a cavity acting as a scavenging-airflow attenuator at a location opposite from the exhaust port about the center of the cylinder bore. A first one of each pair of divisional scavenging ports, located closer to the exhaust port is oriented away from the exhaust port, that it, upward. In contrast, a second one of each pair of divisional scavenging ports, located closer to the scavenging-airflow attenuating cavity, is oriented toward that cavity. 
     According to the invention of the publication No. S59-170423 A, scavenging airflows exit from the right and left second divisional scavenging ports in which the partition walls regulate the flow directions toward the scavenging-airflow attenuating cavity. These scavenging airflows hit each other in the scavenging-airflow attenuating cavity and hit the inner wall of the scavenging-airflow attenuating cavity. Thus, the scavenging airflows are attenuated in flow rate and hence prevented from diffusion toward the exhaust port by the scavenging-airflow attenuating cavity. On the other hand, the scavenging airflows exiting from the first divisional scavenging ports flow toward the top of the cylinder while striking one another and expelling the burnt gas into the exhaust port. In this manner, it is possible to make a layered distribution of gases for stratified scavenging, in which the scavenging gas, which is an air-fuel mixture introduced into the combustion chamber through the first and second divisional scavenging ports, distributes in a space in the cylinder apart from the exhaust port, which is a region in the combustion chamber apart from the exhaust port. On the other hand, the burnt gas distributes in a region next to the exhaust port. 
     Japanese Laid-open Patent Publication No. S60-156933 (No. 156933 of the year 1985) focuses attention to the role of the scavenging passage, which makes communication between the scavenging port opening to the combustion chamber and the crankcase, in a two-stroke internal combustion engine, and proposes an improvement to solve the blow-by problem mentioned above. More specifically, this publication proposes to provide main scavenging passages and sub scavenging passages separated from main scavenging passages by partition walls respectively. The main scavenging passages are continuous from first scavenging ports and the sub scavenging ports are continuous from second scavenging ports. Thus, this proposal uses second scavenging airflows of a higher velocity from the second scavenging ports to control first scavenging airflows from the first scavenging ports. In short, the publication No. S60-156933 proposes to control the flow directions of the first scavenging airflows by using the second scavenging airflows flowing from the second scavenging ports at a higher velocity. Thus, it discloses an embodiment, as a typical example, in which the second scavenging airflows prevent that the first scavenging airflows partly flow into the exhaust port by circulatory shunt. 
     U.S. Pat. No. 6,848,398 aims higher output power and lower emission, and proposes to regulate angles of sidewalls of a scavenging port approximately rectangular in cross section in a two-stroke internal combustion engine. 
     The Inventors made researches on the blow-by phenomenon relative to locations of scavenging ports opening to the combustion chamber.  FIG. 10  schematically illustrates a typical one of known two-stroke internal combustion engines. Reference numeral  1  in  FIG. 1  indicates an exhaust port. A pair of first and second scavenging ports  2  and  3  are provided, respectively, on the right and left symmetrical positions of an imaginary line CL that connects the center O of cylinder bore and the widthwise center of the exhaust port  1 . These first and second scavenging ports  2  and  3  are oriented away from the exhaust port. This engine with multiple scavenging ports is a so-called four-flow scavenging engine having four scavenging ports  2 ,  2 ,  3 ,  3  in total. 
       FIG. 11  illustrates a three-dimensional aspect of the first and second scavenging passages  4  and  5 , which extend longitudinally (in parallel to the axial line of the cylinder bore) from the crankcase to the combustion chamber, and the first and second scavenging ports  2 ,  3 , which are upper ends of the first and second scavenging passages  4 ,  5 . As understood from  FIG. 11 , in the conventional two-stroke internal combustion engine of the multi-scavenging-port type, each scavenging port ( 2 ,  3 ) is associated with a scavenging passage of its own, and each scavenging passage is substan3tially independent from each other.  FIG. 12  shows a layout of the exhaust port  1 , intake port  8 , first and second scavenging passages  4  and  5  relative to the cylindrically shaped cylinder bore  7 . 
       FIGS. 13 and 14  are diagrams similar to  FIG. 10 , in which  FIG. 13  shows a version with the first and second scavenging ports  2  and  3  being remoter from the exhaust port  1 , and  FIG. 14  shows a version with the first and second scavenging ports  2  and  3  being closer to the exhaust port  1 . In both figures, arrows indicate directions of scavenging airflows. 
     In the layout of  FIG. 13  with the scavenging ports  2  and  3  being remoter from the exhaust port  1 , gas exchange is difficult to take place in the in-cylinder region DS between the first scavenging port  2  and the exhaust port  1 . In the layout of  FIG. 14 , in contrast, with the scavenging ports  2  and  3  being nearer to the exhaust port  1 , the scavenging airflows supplied from the first scavenging ports  2  are partly liable to shunt and escape through the exhaust port  1  in the former half of each exhaust stroke of the engine. 
     It is therefore an object of the present invention to provide a two-stroke internal combustion engine capable of effectively preventing the blow-by phenomenon without the need for significant modifications in its typical structure. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a two-stroke internal combustion engine configured to expel burnt gas externally of a combustion chamber through an exhaust port while introducing air-fuel mixture into the combustion chamber from a crankcase through scavenging passages, comprising: 
     first scavenging ports opening to said combustion chamber and oriented away from said exhaust port; 
     main scavenging passages making communication between each said first scavenging port and said crankcase; 
     second scavenging ports opening to said combustion chamber at positions remoter from said exhaust port than said first scavenging ports respectively, and oriented away from said exhaust port; and 
     branch scavenging passages branched from said main scavenging passages and extending aslant away from said exhaust port up to each said second scavenging port, 
     wherein said branch scavenging passages have a mean cross-sectional area smaller than a mean cross-sectional area of said main scavenging passages, and
         wherein each said first scavenging port and each said second scavenging port have opening areas which are in total larger than a cross-sectional area of each said main scavenging passage at an inlet portion thereof next to said crankcase.       

     With this structure of the invention, since the scavenging passages open at the ports to the combustion chamber with a wider area than that at the port to the crankcase, scavenging airflows entering into the combustion chamber from the scavenging ports have a lower flow velocity than in conventional engines. Additionally, since the branch scavenging passage is thinner than the main scavenging passage, velocity of a second scavenging airflow from the second scavenging port in communication with the branch scavenging passage is higher than the velocity of a first scavenging air flow from the second scavenging port. Furthermore, the branch scavenging passage extending aslant contributes to improving directivity of the second scavenging airflow from the second scavenging port. 
     Because of the above-mentioned mechanism, the first scavenging airflow from the first scavenging port closer to the exhaust port is drawn toward the second scavenging airflow from the second scavenging port remoter from the exhaust port. This contributes to reducing the short-cut phenomenon that part of the first scavenging airflow escapes from the exhaust port to the exterior in an initial stage of each exhaust stroke. Further, because the flow velocity of the first and second scavenging airflows ejected from the first and second scavenging ports is relatively slow, and because the first scavenging airflow ejected from the first scavenging port is drawn away from the exhaust port by the second scavenging airflow flowing relatively faster from the second scavenging port, the first scavenging airflow from the first scavenging port moves away from the exhaust port, next hits the inner wall of the cylinder bore, and thereby changes its flow direction toward the exhaust port. Therefore, the traveling distance of the first scavenging airflow from the first scavenging port up to the exhaust port is elongated. This contributes to preventing the blow-by, which will otherwise occur in a later half of each exhaust stroke (see  FIG. 2  referred to in later explanation). 
     Intended effect of preventing the blow-by by the present invention in first and second halves of each exhaust stroke can be attained by simply modifying a conventional engine typically of a four-flow scavenging type. Of course, the second scavenging airflow to be supplied from the second scavenging port may be either air-fuel mixture from the crankcase or fresh air, which may be supplied through the branch scavenging passage. 
     Other objects and features of this invention will become apparent from detailed explanation of preferred embodiments, which follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a cylinder bore of an engine taken as an embodiment, in which an exhaust port, intake port and multi-scavenging ports opening to the cylinder bore are shown as well to explain scavenging passages continuous to the multi-scavenging ports. 
         FIG. 2  is a diagram for explaining functions of a scavenging system provided in the embodiment of  FIG. 1 . 
         FIG. 3  is a diagram three-dimensionally showing scavenging passages and scavenging ports of the scavenging system provided in the embodiment of  FIG. 1   
         FIG. 4  is a diagram for explaining a cross-sectional area of a main scavenging passage at a port opening to a crankcase. 
         FIG. 5  is a diagram for explaining scavenging passages in a first modification of the embodiment of  FIG. 1 . 
         FIG. 6  is a diagram for explaining scavenging passages in a second modification of the embodiment of  FIG. 1 . 
         FIG. 7  is a diagram for explaining scavenging passages in a third modification of the embodiment of  FIG. 1 . 
         FIG. 8  is a diagram for explaining scavenging passages in a fourth modification of the embodiment of  FIG. 1 . 
         FIG. 9  is a diagram for explaining scavenging passages in a fifth modification of the embodiment of  FIG. 1 . 
         FIG. 10  is diagram for explaining a scavenging system used in a conventional two-stroke engine. 
         FIG. 11  is a diagram three-dimensionally showing scavenging passages and scavenging ports continuous from the scavenging passages and opening to the combustion chamber in a conventional two-stroke engine. 
         FIG. 12  is a diagram showing a cylinder bore of a conventional two-stroke engine, in which an exhaust port, intake port and multi-scavenging ports opening to the cylinder bore are shown as well to explain scavenging passages continuous to the multi-scavenging ports. 
         FIG. 13  is a diagram for explaining problems with positioning scavenging ports in a conventional two-stroke engine close to the intake port. 
         FIG. 14  is a diagram for explaining problems with positioning multi-scavenging ports in a conventional two-stroke engine close to the exhaust port. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Some embodiments of the invention is explained below with reference to the drawings. 
       FIGS. 1 through 3  show an embodiment of the invention. As shown in  FIG. 1 , an air-cooled single-cylinder two-stroke internal combustion engine  10  has a cylinder bore  12  that may be, for example, an aluminum die cast product. An intake port  14  and an exhaust port  16  are formed at diametrically opposite positions of the cylinder bore  12 . Air-fuel mixture introduced from the intake port  14  is charged in the crankcase (not shown). 
     With reference to  FIG. 2 , a pair of the first and second scavenging ports  20  and  22  is provided at each of axisymmetric positions about an imaginary line CL that connects the center of the cylinder bore O and the center of the exhaust port  16 . The first and second scavenging ports  20  and  22  are opened and closed by strokes of piston (not shown). These features of the two-stroke internal combustion engine  10  according to the embodiment of the invention so far depicted are identical to those of conventional engines of the four-flow scavenging type. 
     With reference to  FIG. 3  that illustrates a scavenging system provided in the embodiment, the first scavenging ports  20  closer to the exhaust port  16  are communicated with the crankcase (not shown) by main scavenging passages  24  formed in the cylinder block (not shown) to extend longitudinally. The first scavenging ports  20  are oriented away from the exhaust port  20  as in the conventional engines. 
     Still referring to  FIG. 3 , the two-stroke internal combustion engine  10  has branch scavenging passages  26  branched from the main scavenging passages  24  and extending aslant toward the intake port  14 . Each branch scavenging passage  26  has an upper wall  26   a  and a lower wall  26   b  both extending aslant from the main scavenging passage  24  toward the intake port  14  approximately in parallel to each other. The branch scavenging passages  26  may have any cross-sectional geometry like the main scavenging passage  24  above. Each branch scavenging passage  26  is smoothly continuous at its top end to the second scavenging port  22 , which opens to the combustion chamber just like the first scavenging port  20 . Like in conventional engines, the second scavenging port  22  is oriented away from the exhaust port  16 . 
     Let each main scavenging passage  24  have a mean cross-sectional area S 1  ( FIG. 3 ) in its entire length from its inlet port  24   a  opening to the crankcase up to the first scavenging port  20  opening to the combustion chamber, and let the branch scavenging passage  26  have a mean cross-sectional area S 2  ( FIG. 3 ) in its entire length from the branched point up to the second scavenging port  22  opening to the combustion chamber. When these mean cross-sectional areas S 1  an S 2  are compared, the mean cross-sectional area S 2  of the branch scavenging passage  26  is smaller than the mean cross-sectional area S 1  of the main scavenging passage  24 . More specifically, the mean cross-sectional area S 2  of the branch scavenging passage  26  is approximately 0.56 to 0.75 times the mean cross-sectional area S 1  of the main scavenging passage  24 . More preferably, minimum cross-sectional area of the branch scavenging passage  26  is about 0.29 to 0.38 times a minimum cross-sectional area of the main scavenging passage  24 . That is, the branch scavenging passage  24  defines a thinner passage than the main scavenging passage  26 . 
     In the engine  10  having the above-explained structural features of the scavenging passages, like in conventional engines, the piston (not shown), in its strokes, opens and closes the exhaust port  16 , first and second scavenging ports  20 ,  22 , thereby charging air-fuel mixture into the combustion chamber from the crankcase and scavenging the combustion chamber with the air-fuel mixture introduced therein. In the engine  10  according to the embodiment, however, the inlet  24   a  ( FIG. 3 ) of the main scavenging passage  24  opening to the crankcase not only acts as the port for introducing air-fuel mixture from the crankcase to generate the first scavenging airflow to be supplied to the combustion chamber from the first scavenging port  20 , but also acts as a port for introducing air-fuel mixture from the crankcase to generate the second scavenging airflow to be supplied to the combustion chamber from the second scavenging port  22 . That is, in the scavenging system of the engine  10  according to the embodiment, as shown in  FIG. 3 , air-fuel mixture in the crankcase enters into the main scavenging passage  24  from its inlet port  24   a  at the crankcase, and the air-fuel mixture is distributed to the branch scavenging passage  26  on the way to the first scavenging port  20  via the main scavenging passage  24 . 
     Therefore, in the scavenging system according to the embodiment, the total opening area (S 4 +S 5  in  FIG. 3 ) of the multiple scavenging ports opening to the combustion chamber, i.e., the first scavenging port  20  and the second scavenging port  22 , is larger than the cross-sectional area S 3  of the passage at the cross section ( FIG. 4 ) of the common inlet passage portion  24   b  opening to the crankcase. More specifically, in the embodiment, the total opening area (S 4 +S 5 ) of the first scavenging port  20  and the second scavenging port  22  is approximately 1.2 to 1.4 times the cross-sectional area S 3  of the passage at the inlet portion  24   b . As a result, velocities of the first and second airflows  28  and  30  ( FIG. 2 ) from the first and second scavenging ports  20  and  22  are lower than those in conventional engines. 
     In addition, the branch scavenging passage  26  continuous to the second scavenging port  22  closer to the intake port  14  extends upward aslant from the main scavenging passage  24  closer to the exhaust port  16  toward the intake port  14  as already explained. Since this extending direction of the branch scavenging passage  26  is common to the orientation of the second scavenging port  22 , the branch scavenging passage  26  enhances directional control of the second scavenging airflow  30  from each second scavenging port  22 . 
     As a result of the enhanced directional control of the second scavenging airflows  30  from the second scavenging ports  22 , the first scavenging airflows  28  from the first scavenging ports  20  nearer to the exhaust port  16  are drawn toward or into the second scavenging airflows  30 . Because of these motions, it is possible to diminish the short-cut phenomenon that the first scavenging airflows  28  partly flow out to the exterior from the exhaust port in an initial stage of each exhaust stroke. 
     Further, the flow speed of the first and second scavenging airflows  28  and  30  supplied from the first and second scavenging ports  20  and  22  into the combustion chamber is relatively slow because their total opening area of the first and second scavenging ports  20  and  22  is larger than the cross-sectional area of the common passage portions  24   a . Moreover, as the first scavenging airflows  28  from the first scavenging ports  20  nearer to the exhaust port  16  continuous to the main scavenging passage  24  are drawn toward the intake port  14  by the second scavenging airflows  30  from the second scavenging ports  22 , the first scavenging airflows  28  flowing from the first scavenging ports  22  at a relatively slow speed shift toward the intake port  14  and next change their flow directions by bouncing at the inner wall of the cylinder bore  12 . This results in substantially elongating the travel distances of the first scavenging airflows  28  up the exhaust port  16 . The relatively low flow speed of the first and second scavenging airflows  28 ,  30  and the elongation of the traveling distances of the first scavenging airflows  28  contribute to prevention of the blow-by in the latter half of each exhaust stroke. 
     To evaluate the effect of the invention, the Inventors prepared a prototype engine and compared it with existing engines. The Inventors could confirm approximately 1.3% to 3.3% increase of the and approximately 30% reduction of HC by the present invention. 
     An engine taken as the embodiment has been explained heretofore. The engine, however, can be modified in various respects. For example, regarding the first and second scavenging ports  20 ,  22 , So far explained has been the embodied example of the subject invention. As for the first and second scavenging ports  20  and  22 , angles of the sidewalls  20   a ,  20   b  ( FIG. 2 ) of the first and second scavenging ports  20 ,  22 , which are crossing angles of the sidewalls  20   a ,  20   b  of each first scavenging port  20  and/or crossing angles of the sidewalls  22   a ,  22   b  of each second scavenging port  22  relative to the imaginary line CL ( FIG. 2 ) connecting the center O of the cylinder bore  12  and the center of the exhaust port  16 , may be regulated as taught by U.S. Pat. No. 6,848,398. Regarding the crossing angles, detailed teaching of U.S. Pat. No. 6,848,398 is incorporated herein, and the present specification omits its explanation. 
     The explanation of the embodiment was made above by taking the example in which the main scavenging passages  24  and the branch scavenging passages  26  are integrally formed in the cylinder block. However, these passages  24  and/or  26  may be defined by using elements separate prepared from the cylinder block. For example, removable passage-forming member may be fixed to the cylinder block to define the main scavenging passages  24  and the branch scavenging passages  26 . It is also possible to pipe members removably connected to the cylinder block to define the main scavenging passages  24  and the branch scavenging passages  26 . 
       FIGS. 5 to 9  are diagrams for explaining some modified structures.  FIG. 5  illustrates a version in which the lower end of each main scavenging passage  24  is offset toward the exhaust port  16  to incline the main scavenging passage  24 .  FIG. 6  illustrates a version in which the main scavenging passage inclines more largely to a degree where the lower end thereof vertically aligns with the exhaust port  16 . 
       FIG. 7  depicts a structure in which multiple branch scavenging passages ( 26 A,  26 B) are provided to branch from each main scavenging passage  24  such that they communicate with the main scavenging passage  24  at its vertically distant positions. Although the main scavenging passage  24  is shown to extend upright in  FIG. 7 , it may be inclined as shown in  FIGS. 5 and 6 . 
       FIG. 8  shows a version in which multiple branch scavenging passages  26  ( 26 A,  26 B) branch from each main scavenging passage  24  like in the version of  FIG. 7 .  FIG. 8 , however, depicts that a single branch scavenging passage  26 A may branch directly from, and communicate with, the main scavenging passage  24 , and the single branch scavenging passage  26 A may divide into multiple branches to form the other branch scavenging passage(s)  26 B. 
       FIG. 9  schematically shows an example of scavenging by fresh air. This is illustrates as scavenging the combustion chamber by supplying fresh air to the branch scavenging passage  26  and introducing it into the combustion chamber through the branch scavenging passage  26 . Regarding the supply of fresh air, this example may be modified to supply fresh air to the main scavenging passage  24  and introducing it into the combustion chamber from the first scavenging port  20 , or from both the first and second scavenging ports  20 ,  22 . 
     In the embodiment and modified examples explained above, it is effective to determine slanting angles (elevation angles) of the first scavenging ports  20  nearer to the exhaust port  16  and the second scavenging ports  22  nearer to intake port with respect to a horizontal plane of the cylinder bore such that the elevation angle of the second scavenging ports  22  are larger than the elevation angle of the first scavenging ports  20 . This structure of the second scavenging ports  22  having a relatively large elevation angle contributes to three-dimensionally scavenging of burnt gas from the combustion chamber. 
     INDUSTRIAL APPLICABILITY 
     The present invention is suitable for use as a power source of portable work machine or compact work machine such as a chain saw, brush cutter, hedge trimmer or blower.