2-stroke engine

Abstract

To improve scavenging efficiency in a cylinder at a two stroke engine.SOLUTION: In a two stroke engine, a scavenging passage connected to at least one scavenging port 16 forms a variable scavenging passage 14(ch). An upper end of the variable scavenging passage 14(ch) has a guide surface 50 which defines a discharge direction of scavenging gas discharged from the variable scavenging port 16(ch) connected to the variable scavenging passage 14(ch) on a horizontal surface. The guide surface 50 includes: an upper guide part 50(H) which defines a first discharge direction of the scavenging gas; and a lower guide part 50(L) which defines a second discharge direction of the scavenging gas. At the upper end of the variable scavenging passage 14(ch), an effective cross sectional area of the scavenging passage in a position of the upper guide part 50(H) is different from an effective cross sectional area of the scavenging passage in a position of the lower guide part 50(L), and the effective cross sectional area of the scavenging passage in the position of the lower guide part is relatively large.SELECTED DRAWING: Figure 5

Description

TECHNICAL FIELD

[0001] The present invention relates to a two-stroke engine. BACKGROUND ART

[0002] In a conventional general two-stroke engine, during the scavenging stroke, scavenging inside the cylinder is performed using an air-fuel mixture pre-compressed in a crankcase. A two-stroke engine includes a scavenging passage communicating with the crankcase and a combustion chamber. The upper end opening of the scavenging passage, that is, the scavenging port, is opened and closed by a piston that reciprocates between a top dead center and a bottom dead center. The exhaust port is also opened and closed by the movement of this piston.

[0003] When the piston descends in the combustion stroke, the exhaust port and the scavenging port are opened near the bottom dead center of the piston, and scavenging inside the cylinder is started at the same time as the scavenging port opens. The scavenging port communicates with the crankcase via the scavenging passage. As soon as the scavenging port opens, the pre-compressed air-fuel mixture is discharged as scavenging gas from the scavenging port into the cylinder.

[0004] Conventionally known two-stroke engines that pre-compress the air-fuel mixture in the crankcase have the problem of "air-fuel mixture short-circuiting" that occurs during the scavenging stroke. "Air-fuel mixture short-circuiting" is a phenomenon in which the air-fuel mixture discharged from the scavenging port, that is, the scavenging gas, is directly discharged from the exhaust port without contributing to scavenging. This short-circuiting phenomenon not only pollutes the environment because unburned air-fuel mixture is discharged, but also reduces the charging efficiency (η tr ) and worsens the fuel consumption rate.

[0005] To address the problem of air-fuel mixture blow-through, "reverse scavenging" was proposed as a scavenging method and has become the mainstream method for current two-stroke engines. Reverse scavenging is performed by directing the air-fuel mixture, i.e., the scavenging gas, discharged from the scavenging port towards the cylinder wall on the intake side, which is opposite the exhaust port. A reverse scavenging two-stroke engine is disclosed, for example, in Patent Document 1. As can be seen from Patent Document 1, the scavenging gas discharged from the scavenging port is directed towards the cylinder wall on the intake side. Then, the scavenging gas reverses direction within the cylinder and moves towards the exhaust port.

[0006] Here, Patent Document 1 proposes that the cross-sectional shape of the upper end of the scavenging passage be made into a roughly triangular shape with the scavenging port as one side, thereby directing the scavenging gas discharged from the scavenging port toward the cylinder wall on the intake side.

[0007] Amid growing concerns about environmental issues, exhaust gas regulations are becoming stricter. In response to this, various proposals have been made to prevent blow-through of the air-fuel mixture. The scavenging port opens as the piston descends, and the effective opening area increases. Patent documents 2 and 3 disclose an invention that changes the direction of the air-fuel mixture, or scavenging gas, discharged from the scavenging port during this process.

[0008] To make the explanation easier to understand, the surface that expands along the up-and-down motion of the piston is called the "vertical surface," and the surface that expands in a horizontal direction perpendicular to it is called the "horizontal surface."

[0009] Patent Document 2 discloses an invention in which the ceiling wall surface at the upper end of the scavenging passage, i.e., the portion near the scavenging port, has a three-dimensional shape, and the discharge direction of the scavenging gas is changed on a vertical plane by the three-dimensional shape of the ceiling wall surface. According to this invention, when the scavenging port begins to open, the scavenging gas is directed upward. Furthermore, as the piston descends and the effective opening area of ​​the scavenging port expands, the scavenging gas is gradually directed downward.

[0010] Patent Document 3 discloses an invention in which the ceiling wall surface of the upper end portion of the scavenging passage, that is, the portion near the scavenging port, is divided into a first surface on the intake side of the cylinder and a second surface on the exhaust side, and the first surface on the intake side has a relatively larger inclination angle that directs the scavenging gas upward compared to the second surface on the exhaust side. In relation to the shape of this scavenging passage, the upper edge of the scavenging port has a stepped shape, and at the upper edge of the scavenging port, the first half upper edge on the intake side is located higher than the second half upper edge on the exhaust side.

[0011] According to the invention of Patent Document 3, when the piston descends and reaches the upper edge of the first half on the intake side of the scavenging port, causing the scavenging port to begin opening, the air-fuel mixture, or scavenging gas, discharged from the scavenging port is directed relatively upward. Furthermore, when the piston descends and reaches the upper edge of the second half on the exhaust side of the scavenging port, thereafter, the scavenging gas discharged from the scavenging port is directed relatively downward by the second surface, which has a relatively smaller inclination angle. In other words, the invention disclosed in Patent Document 3, like that in Patent Document 2, discloses an invention that changes the discharge direction of the scavenging gas on a vertical plane. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] JP Patent No. 5553552 [Patent Document 2] JP JP 60-145417 Publication [Patent Document 3] Japanese Patent Publication No. 2001-182541 [Overview of the Initiative] [Problems that the invention aims to solve]

[0013] With exhaust gas regulations becoming increasingly stringent, in order to minimize the emission of unburned gases and improve engine output, intake efficiency (η) is crucial. trFurther improvements are needed. From this perspective, considering the proposals in Patent Documents 2 and 3, when viewing the combustion chamber from above, there is a possibility that dead areas may occur where the scavenging gas discharged from the scavenging port does not reach, and further improvements are expected.

[0014] The present invention aims to improve the scavenging efficiency within the cylinder of a two-stroke engine. The present invention can be suitably applied to a reverse-scavenging engine.

[0015] Furthermore, in some two-stroke engines, a fuel injection system is used instead of a carburetor for the purpose of more precise fuel control. Regarding the placement of the fuel injection system in these engines, in addition to (1) placing the fuel injection system in the engine's intake system, (2) placing the fuel injection system in the crankcase, (3) placing the fuel injection system in the cylinder is also possible. Engines in which the fuel injection system is placed in the cylinder are called "direct injection engines". In a direct injection two-stroke engine, air is supplied to the crankcase and this air is pre-compressed in the crankcase. The pre-compressed air is then introduced into the cylinder as scavenging gas through the scavenging passage and scavenging port and used for scavenging within the cylinder.

[0016] In describing the present invention, the term "fresh air" is used to refer collectively to the air-fuel mixture or air supplied into the cylinder as scavenging gas through the scavenging passage and scavenging port. In other words, the term "fresh air" includes either the air-fuel mixture or air, or a combination of both flowing into the cylinder in sequence. [Means for solving the problem]

[0017] The above technical problems are addressed in the present invention. A piston positioned inside the cylinder, which reciprocates between top dead center and bottom dead center and defines the combustion chamber, An exhaust port for discharging the burnt gas from the combustion chamber, which opens in the cylinder and is opened and closed by the piston, A crank chamber that receives fresh air and pre-compresses the fresh air by the downward movement of the piston, A plurality of scavenging passages communicating with the combustion chamber and the crankshaft chamber during a scavenging stroke and connected to a plurality of scavenge ports for discharging fresh air precompressed in the crankshaft chamber into the combustion chamber as scavenging gas, wherein the scavenge ports are opened and closed by the piston, a reverse scavenging two-stroke engine, wherein the scavenging gas discharged from the scavenge ports is directed toward a cylinder wall surface on an intake port side located on a radially opposite side of the cylinder from the exhaust port, and the scavenging gas discharged from the scavenge ports reverses on the cylinder wall on the intake port side and travels toward the exhaust port to scavenge the combustion chamber, among the plurality of scavenging passages, the scavenging passage communicating with at least one of the scavenge ports constitutes a variable scavenging passage, the scavenge port communicating with the variable scavenging passage constitutes a variable scavenge port, an upper end portion of the variable scavenging passage has, on a side wall surface on the intake port side located on a radially opposite side of the cylinder from the exhaust port, a guide surface that defines a discharge direction, on a horizontal plane extending in a lateral direction perpendicular to a vertical plane extending along the vertical movement of the piston, of the scavenging gas discharged from the variable scavenge port, the guide surface comprises at least an upper guide portion that defines a first discharge direction of the scavenging gas in the discharge direction on the horizontal plane at an initial stage of the scavenging stroke and is located on a top dead center side of the piston, and a lower guide portion that is located on a bottom dead center side of the piston below the upper guide portion and defines a second discharge direction of the scavenging gas, at the upper end portion of the variable scavenging passage, an effective cross-sectional area of the scavenging passage at a position of the upper guide portion is different from an effective cross-sectional area of the scavenging passage at a position of the lower guide portion, as the scavenging stroke progresses, on the horizontal plane the Inverted scavenging type directed towards the intake side flow direction of the scavenging gas Direction approaching the central axis of the cylinder changes and the flow velocity changes. This object is achieved by providing the two-stroke engine characterized by the above.

[0018] According to the present invention, during the progress of the scavenging stroke, the flow direction of the scavenging gas can be changed and the flow velocity can be controlled. Specifically, in the middle to late period of the scavenging stroke, for example, by setting the effective cross-sectional area of the lower guide portion to be larger than that of the upper guide portion, a relatively gentle flow can be formed. This not only prevents the short-circuiting of the scavenging gas discharged from the scavenging port to the exhaust port, but also enables effective gas exchange in the cylinder by the scavenging gas.

[0019] The present invention is typically applied to reverse scavenging engines. In a preferred embodiment of the present invention, the guide surface is configured as a vertically stepped surface. The stepped surface may have two vertical steps, three steps, or more than three steps. Further, the guide surface may be configured as a curved stepless surface.

[0020] In a preferred embodiment of the present invention, when the guide surface at the upper end of the scavenging passage connected to the scavenging port has, for example, two vertical steps, the first discharge direction of the scavenging gas defined by the upper guide portion formed at the upper part of the guide surface is directed toward the intake side more than the second discharge direction of the scavenging gas defined by the lower guide portion formed at the lower part of the guide surface.

[0021] The operational effects and other objects of the present invention will become apparent from the following detailed description of preferred embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] [Figure 1] It is a schematic diagram of the single-cylinder two-stroke engine of the embodiment. [Figure 2] It is a schematic diagram of the intake system of a two-stroke engine including the engine of the embodiment. [Figure 3] It is a cross-sectional view taken along line A-A in Fig. 1. [Figure 4] It is a cross-sectional view taken along line B-B in Fig. 1. [Figure 5] It is a schematic diagram for explaining the guide surface formed at the upper end of the variable scavenging passage. [Figure 6]This is a cross-sectional view along the line VI-VI in Figure 5. [Figure 7] This is a perspective view of the cylinder block of the engine in the embodiment. [Figure 8] This is a cross-sectional view along the line VIII-VIII in Figure 7. [Figure 9] This is a cross-sectional view along the line IX-IX in Figure 7. [Figure 10] Figure 5 is a schematic diagram illustrating a modified example of the guide surface shown, which consists of three stages. [Figure 11] This is a cross-sectional view along the line XI-XI in Figure 10. [Figure 12] Figure 5 is a schematic diagram illustrating a modified version of the guide surface shown, which is a stepless guide surface. [Figure 13] This is a cross-sectional view along the line XIII-XIII in Figure 12. [Figure 14] This diagram corresponds to Figure 3 and illustrates an example in which the present invention is applied to the upper end of a scavenging passage connected to the exhaust-side first scavenging port, which is opposite the exhaust-side second scavenging port of a four-flow scavenging cylinder. [Figure 15] This figure illustrates an example in which the present invention is applied to the upper end of the scavenging passage connected to the exhaust-side second scavenging port and the upper end of the scavenging passage connected to the intake-side first scavenging port. [Figure 16] This figure illustrates an example of applying the present invention to the upper end of a scavenging passage connected to an exhaust-side second scavenging port located on one side of a cylinder, and to the upper end of a scavenging passage connected to an intake-side second scavenging port. [Figure 17] This figure illustrates an example in which the present invention is applied to the upper end of the scavenging passage connected to the first and second scavenging ports, which are opposite each other, on the exhaust side of a cylinder. [Figure 18] This figure illustrates an example in which the present invention is applied to the upper end of the scavenging passages connected to the three scavenging ports, excluding the scavenging passage connected to the intake-side first scavenging port. [Figure 19] This figure illustrates an example of applying the present invention to the upper end of a scavenging passage connected to a scavenging port located on one side of a two-flow scavenging cylinder. [Figure 20]This figure illustrates an example in which the present invention is applied to the upper ends of the scavenging passages connected to the scavenging ports located on both sides of a two-flow scavenging cylinder. [Figure 21] This diagram illustrates an example in which the guide surface is composed of a first and second dividing surface, where the angle between the first and second dividing surfaces toward the cylinder is acute. [Figure 22] This diagram illustrates an example in which the guide surface is composed of a first and second dividing surface, where the angle between the first and second dividing surfaces toward the cylinder is obtuse. [Modes for carrying out the invention] [Examples]

[0023] Preferred embodiments of the present invention will be described below with reference to the attached drawings. Figure 1 is a schematic diagram of an engine 2 of an embodiment included in a single-cylinder two-stroke engine system 100. Engine 2 is suitably applied to portable work machines such as chainsaws, blowers, and brush cutters. Referring to Figure 1, the illustrated engine 2 has a piston 6 fitted into a cylinder 4, which reciprocates between top dead center and bottom dead center. A spark plug 10 is disposed in a combustion chamber 8 defined by the piston 6.

[0024] The combustion chamber 8 and the crankcase 12 are connected by a scavenging passage 14 during the scavenging stroke. The scavenging passage 14 has a rectangular scavenging port 16 at its upper end, which is opened and closed by the piston 6. The engine 2 has four scavenging ports 16, and the upper edges of the rectangular scavenging ports 16 are positioned at the same height level so that the opening timing of the four scavenging ports is synchronized. The lower end of each scavenging port 16 opens to the crankcase 12, and this open lower end constitutes a scavenging gas inlet 18. A two-stroke engine with four scavenging ports 16 is called a "four-stroke scavenging engine".

[0025] Reference numeral 20 indicates the intake port, through which the air-fuel mixture is supplied to the crankcase 12. Reference numeral 22 indicates the exhaust port, which is located on the opposite side from the intake port 20. Engine 2 is a piston valve engine. That is, the intake port 20 and exhaust port 22 are opened and closed by the piston 6. The air-fuel mixture is supplied to the crankcase 12 through the intake port 20, while the burnt gases from the combustion chamber 8 are discharged through the exhaust port 22.

[0026] Figure 2 is a schematic diagram of the intake system 24 included in the two-stroke engine system 100. The intake system 24 has an air cleaner 26 at its upstream end, and the air purified by the air cleaner 26 is supplied to the vaporizer 28. Fuel is supplied to the vaporizer 28 from the fuel tank 30, and a fuel-air mixture is produced by the vaporizer 28.

[0027] Engine 2 is a stratified scavenging engine. The carburetor 28 has a first passage 28a for generating the air-fuel mixture and a second passage 28b through which air received from the air cleaner 26 passes. The first passage 28a, which is the air-fuel mixture generation passage, forms part of the air-fuel mixture passage 32 leading to the crankcase 12. The air-fuel mixture generated in the carburetor 28 is supplied to the crankcase 12 through the air-fuel mixture passage 32 and pre-compressed in the crankcase 12.

[0028] The second passage 28b through which air passes constitutes part of the leading air passage 34 that supplies leading air to the scavenging passage 14. The piston 6 has a piston groove 6a on its circumferential surface. The leading air received from the air cleaner 26 is supplied to the upper end of the scavenging passage 14 via the piston groove 6a. The piston groove 6a is described in detail in US2016 / 0376979A1, so a description of the piston groove 6a is omitted here. A reed valve may be used instead of the piston groove 6a. The reed valve is described in detail in JP JP A 2000-337154.

[0029] In the stratified scavenging engine 2, lead air is supplied to the combustion chamber 8 at the beginning of the scavenging stroke, and then the air-fuel mixture from the crankcase 12 is supplied to the combustion chamber 8.

[0030] Figure 3 is a cross-sectional view along line A-A in Figure 1. Figure 4 is a cross-sectional view along line B-B in Figure 1. As can be seen from Figures 3 and 4, the engine 2 is a "reverse scavenging" type that directs the scavenging gas discharged from each of the four scavenging ports 16 towards the intake side of the cylinder.

[0031] Referring to Figures 3 and 4, the four rectangular scavenging ports 16 are arranged two on each side of the cylinder. To identify each scavenging port 16, of the two scavenging ports 16 located on the right side of the drawing, the one located on the exhaust port 22 side is called the "exhaust-side first scavenging port," and this exhaust-side first scavenging port is given the reference numeral "16(ex1)." Of the two scavenging ports 16 located on the right side of the drawing, the one located on the intake port 20 side is called the "intake-side first scavenging port," and this intake-side first scavenging port is given the reference numeral "16(in1)."

[0032] Of the two scavenging ports 16 located on the left side of the drawing, the scavenging port 16 located on the exhaust port 22 side is called the "exhaust-side second scavenging port," and this exhaust-side second scavenging port is designated with the reference numeral "16(ex2)." Of the two scavenging ports 16 located on the left side of the drawing, the scavenging port 16 located on the intake port 20 side is called the "intake-side second scavenging port," and this intake-side second scavenging port is designated with the reference numeral "16(in2)."

[0033] The arrows 40 in Figures 3 and 4 indicate the direction of the scavenging gas discharged from each scavenging port 16. From Figures 3 and 4, it can be seen that the scavenging gas is discharged toward the intake side, opposite to the exhaust port 22. In other words, the upper ends of each rectangular scavenging port 16 of the engine 2 and the scavenging passages 14 connected thereto are substantially the same in their basic configuration as the upper ends of each scavenging port and scavenging passage connected thereto described in, for example, Japanese Patent Publication No. 2000-34927, which is included in a known reversing scavenging engine.

[0034] Of the four scavenging ports 16 included in the engine 2 shown in Figure 1, the present invention is applied only to the upper end of the scavenging passage 14 associated with the exhaust-side second scavenging port 16 (ex2). Therefore, the upper ends of the scavenging passages 14 associated with the exhaust-side first scavenging port 16 (ex1), intake-side first scavenging port 16 (in1), and intake-side second scavenging port 16 (in2), other than the exhaust-side second scavenging port 16 (ex2), are the same as in the conventional design. The scavenging passages 14 and scavenging ports 16 to which the present invention is applied are referred to as "variable scavenging passages 14 (ch)" and "variable scavenging ports 16 (ch)."

[0035] Figure 5 is a schematic diagram of the upper end of the variable scavenging passage 14(ch), and Figure 6 is a cross-sectional view along the line VI-VI in Figure 5. At the upper end of the variable scavenging passage 14(ch), the side wall surface on the cylinder intake side constitutes the guide surface 50. The guide surface 50 has the function of directing the scavenging gas toward the intake side. The guide surface 50 is composed of an upper guide portion 50(H) and a lower guide portion 50(L). Reference numeral 52 in Figure 6 indicates a stepped portion. The stepped portion 52 is formed between the upper guide portion 50(H) and the lower guide portion 50(L), and the stepped portion 52 is provided in the middle of the guide surface 50 in the vertical direction.

[0036] Referring to Figures 5 and 6, it is preferable that the stepped portion 52 extends in a direction perpendicular to the axis of the cylinder 4, that is, the direction of movement of the piston 6. In other words, it is preferable that the stepped portion 52 be located on a horizontal plane. This allows the scavenging gas to be introduced into the cylinder without disturbing the flow of scavenging gas discharged from the variable scavenging port 16(ch) when switching from the upper guide portion 50(H) to the lower guide portion 50(L) as the piston 6 descends and the opening of the variable scavenging port 16(ch) increases. Although the stepped portion 52 shown is composed of a vertical wall, it may also be composed of an inclined wall or a curved wall.

[0037] As can be clearly seen by referring to Figure 3, in the cross-sectional shape of the upper end of the variable scavenging passage 14(ch), the upper guide portion 50(H) substantially forms the hypotenuse of a roughly triangular shape with the variable scavenging port 16(ch), that is, the exhaust-side second scavenging port 16(ex2), as one side. As mentioned above, Figure 3 is a cross-sectional view along the line A-A in Figure 1. As can be seen from Figure 1, the cutting line A-A is such that the descending piston 6 is located slightly below the upper edge of the rectangular scavenging port 16. In other words, Figure 3 shows the cross-sectional shape of the upper end of the scavenging passage 14(ch) immediately after the scavenging stroke has started in the variable scavenging passage 14(ch) leading to the variable scavenging port 16(ch).

[0038] As can be seen from Figure 3, the upper guide portion 50(H) is composed of a surface that extends tangentially to the inner wall of the cylinder 4 from the exhaust side to the intake side. That is, the upper guide portion 50(H) is composed of an inclined surface having an angle θ(ex-H). The upper guide portion 50(H) with an inclination angle θ(ex-H) directs the scavenging gas tangentially to the inner wall of the cylinder 4. Here, the inclination angle θ refers to the inclination angle of the guide surface 50 with respect to a straight line parallel to the straight line L connecting the exhaust port 22 and the intake port 20 when the combustion chamber 6 is viewed from above.

[0039] As a result, in the initial stages of the scavenging stroke, from immediately after the scavenging port 16 is opened by the descending piston 6 to the middle section of the scavenging port 16 in the vertical direction, the scavenging gas guided by the upper guide portion 50(H) is directed away from the exhaust side of the cylinder and tangentially to the inner wall of the cylinder 4. The scavenging gas whose discharge direction is defined by the upper guide portion 50(H) is indicated by the white arrow 42 (Figure 3). This configuration prevents unburned gas, which is likely to be generated in the initial stages of scavenging, from blowing into the exhaust port 22. Furthermore, due to the approximately triangular cross-sectional shape of the upper end of the scavenging passage formed by the directionality of the upper guide portion 50(H), the scavenging gas discharged from the variable scavenging port 16(ch) equipped with the guide surface 50 has a higher flow velocity compared to the scavenging gas discharged from other scavenging ports 16. Therefore, the scavenging gas flow can be controlled in a more diverse manner not only by directionality but also by differences in flow velocity. Furthermore, by applying the present invention to a portion of the multiple scavenging passages, the directional and velocity changes of these passages can prevent collisions between scavenging gases discharged from opposing scavenging ports 16, thereby preventing a portion of the scavenging gas from short-circuiting to the exhaust port 22.

[0040] On the other hand, the lower guide portion 50(L) shown in Figure 4, as can be clearly seen by referring to Figure 4, has substantially the same cross-sectional shape as, for example, the exhaust-side first scavenging port 16(ex1) in the upper end of the variable scavenging passage 14(ch) connected to the variable scavenging port 16(ch), i.e., the exhaust-side second scavenging port 16(ex2). The lower guide portion 50(L) is composed of an inclined surface having an angle θ(ex-L) similar to that of conventional designs, and has a roughly parallelogram shape in cross-section. As can be seen by comparing Figure 3 and Figure 4, the absolute value of the inclination angle θ(ex-L) of the lower guide portion 50(L) is larger than the inclination angle θ(ex-H) of the upper guide portion 50(H).

[0041] As mentioned above, Figure 4 is a cross-sectional view along the line B-B in Figure 1. As can be seen from Figure 1, the B-B cutting line indicates that the descending piston 6 is located slightly above the lower edge of the rectangular scavenging port 16. Figure 4 shows the shape of the upper end of the variable scavenging passage 14(ch) just before the piston 6 closes the variable scavenging port 16(ch), that is, the exhaust-side second scavenging port 16(ex2). In other words, Figure 4 shows the state in which the piston 6 is located below the vertical midpoint of the rectangular scavenging port 16, that is, the middle to latter half of the scavenging stroke. During the middle to latter half of the scavenging stroke, the exhaust port 22 is wide open and the combustion of the gas in the cylinder has spread sufficiently, so the combustion chamber is filled with already burned gas. In a cylinder in this state, as can be seen from Figure 4, the downward guide portion 50(L) guides the scavenging gas from the middle of the variable scavenging port 16(ch) in the vertical direction to the lower edge as the descending piston 6 moves. The scavenging gas is directed in the opposite direction to the exhaust port 22 and away from the tangential direction of the inner wall of the cylinder 4, towards the central part of the cylinder 4.

[0042] During the middle to later stages of the scavenging stroke, the scavenging gas replaces the already-burned gas that has spread throughout the entire combustion chamber, including the central part. Furthermore, because the cross-sectional area is larger than that of the upper guide section 50(H), a relatively gentle flow is formed. As a result, the scavenging gas discharged from each scavenging port 16 does not short-circuit to the exhaust port 22, and the scavenging gases discharged from opposing scavenging ports 16 gently mix, replacing the gas throughout the entire combustion chamber. This ensures that scavenging is performed evenly and without excess throughout the entire cylinder.

[0043] As can be seen from the above explanation, during the scavenging stroke, the direction of the scavenging gas discharged from the variable scavenging port 16(ch), that is, the exhaust-side second scavenging port 16(ex2), changes. In the first process, until the variable scavenging port 16(ch) is half-open, the scavenging gas is directed tangentially to the inner wall of the cylinder 4. Then, in the second process, from the half-open state until the exhaust-side second scavenging port 16(ex2) closes, the gas is directed away from the tangential direction to the inner wall of the cylinder 4, that is, towards the central axis of the cylinder 4.

[0044] Thus, in a horizontal plane viewed from above the combustion chamber 8, during the downward movement of the piston 6, the direction of the scavenging gas discharged from the variable scavenging port 16(ch), i.e., the exhaust-side second scavenging port 16(ex2), is changed from the initial to the later stages of scavenging. This prevents initial "blow-through" and allows the scavenging gas to reach the entire combustion chamber 8, thereby increasing scavenging efficiency. Furthermore, by making the flow direction of the scavenging gas different for only a portion of the multiple scavenging passages 14 (the variable scavenging passage 14(ch)), collisions between scavenging gases discharged from opposing scavenging ports 16 are avoided. This prevents the scavenging gases from losing direction after the collision and short-circuiting to the exhaust port 22, allowing each scavenging gas discharged from the multiple scavenging ports 16 to maintain its respective flow path, scavenge the combustion chamber 8, and then proceed to the exhaust port 22. In the two-stroke engine 2 of this embodiment, the air-fuel mixture is pre-compressed in the crankcase 12 and used as scavenging gas, thus preventing blow-through of the air-fuel mixture and improving the intake efficiency (η tr This can improve the air supply efficiency (η). tr By improving this aspect, combustion efficiency can be increased.

[0045] The engine system 100 relates to a stratified scavenging engine. In a stratified scavenging engine, the scavenging passage 14 discharges lead air into the combustion chamber 8 ahead of the air-fuel mixture at the beginning of the scavenging stroke. The four scavenging ports 16, including the exhaust-side second scavenging port 16 (ex2) to which the present invention is applied, have a rectangular shape common to conventional stratified scavenging engines, and the height level of the upper edge of the rectangular scavenging port 16, i.e., the timing of the start of scavenging, is substantially the same. In other words, when lead air is introduced from the lead air passage 34 through the piston groove to each scavenging passage 14 during the intake stroke, substantially the same amount of sufficient lead air can be filled into all scavenging passages 14, thereby increasing scavenging efficiency without impairing the conventionally known stratified scavenging effect.

[0046] Furthermore, as can be clearly seen by referring to Figures 3 and 4, when comparing the cross-sectional shape of the upper end of the variable scavenging passage 14(ch) formed by the upper guide section 50(H) with the cross-sectional shape formed by the lower guide section 50(L), the effective cross-sectional area of ​​the passage is different. This difference adds diversity to the flow velocity of the scavenging gas discharged from the variable scavenging port 16(ch), and in terms of directionality and time, the scavenging gases discharged facing each other can be distributed evenly within the combustion chamber 8 without colliding. In this embodiment, the cross-sectional area of ​​the scavenging passage formed by the upper guide section 50(H) is set to 37% of the cross-sectional area of ​​the scavenging passage formed by the lower guide section 50(L), but this "37%" value can be optimized by adjusting it within the range of 20-70% depending on the balance with the engine displacement and the required amount of leading air.

[0047] Figures 7 to 9 show specific examples of the engine 2 described with reference to Figures 1 and 3 to 5. Figure 7 shows the cylinder block 60 that constitutes the engine 2. In the cylinder block 60, the portion corresponding to the upper part of the four scavenging ports 16 and the scavenging passages 14 connected thereto is made up of a pair of scavenging passage forming caps 62 that are screwed to the cylinder block 60. Reference numeral 64 in Figure 7 indicates a screw hole.

[0048] Figure 8 is a cross-sectional view taken along the line VIII-VIII in Figure 7, and is the cross-sectional view corresponding to Figure 3 described above. Figure 9 is a cross-sectional view taken along the line IX-IX in Figure 7, and is the cross-sectional view corresponding to Figure 4 described above. As can be seen from these drawings, the guide surface 50 is formed by the scavenging passage forming cap 62. In this embodiment, the upper ends of all four scavenging passages are formed by the scavenging passage forming cap 62, but it is also possible to form only the variable scavenging passage 14(ch) with the scavenging passage forming cap 62.

[0049] As described above, the side wall surface on the cylinder intake side at the upper end of the variable scavenging passage 14(ch) connected to the rectangular variable scavenging port 16(ch) constitutes a guide surface 50, and this guide surface 50 directs the scavenging gas toward the intake side. During the scavenging stroke, the direction of the scavenging gas changes on the horizontal plane. Figures 10 to 13 are diagrams illustrating modified examples of the guide surface 50 shown in Figure 5. The guide surface 50 shown in Figure 5 is composed of two guide sections, an upper guide section 50(H) and a lower guide section 50(L), but it may also be composed of three guide sections, upper and lower, as shown in Figures 10 and 11.

[0050] Figure 11 is a cross-sectional view along the line XI-XI in Figure 10. Referring specifically to Figures 10 and 11, the guide surface 54 of the first modified example has an intermediate guide section 54(M) between the upper guide section 54(H) and the lower guide section 54(L). The upper guide section 54(H) and lower guide section 54(L) included in the first modified example are composed of surfaces having, for example, the same inclination angle θ as the upper guide section 50(H) and lower guide section 50(L) included in the guide surface 50 of Figure 5 described above. The intermediate guide section 54(M) preferably has an inclination angle θ intermediate between the upper guide section 54(H) and the lower guide section 54(L), but is not limited to this and may be different. This allows the direction of the scavenging gas to be changed in three stages on the horizontal plane during the scavenging stroke.

[0051] As described above, the intermediate guide section 54(M) preferably has an inclination angle θ intermediate between the upper guide section 54(H) and the lower guide section 54(L), but is not limited to this. For example, the inclination angle θ of the intermediate guide section 54(M) may be set to the same value as the inclination angle θ of the lower guide section 50(L) included in the guide surface 50 of Figure 5, while the inclination angle θ of the lower guide section 54(L) may be set to a different value from the inclination angle θ of the intermediate guide section 54(M). Alternatively, the inclination angles of the upper guide section 54(H), intermediate guide section 54(M), and lower guide section 54(L) may be set to sequentially and progressively different values.

[0052] As another example, the inclination angle θ of the intermediate guide section 54(M) may be set to the same value as the inclination angle θ of the upper guide section 50(H) included in the guide surface 50 of Figure 5, and the inclination angles θ of the upper guide section 54(H) and the lower guide section 54(L) may be set to the same value as, for example, the inclination angle θ of the lower guide section 50(L) included in the guide surface 50 of Figure 5. Of course, the inclination angles θ of the upper guide section 54(H) and the lower guide section 54(L) may be different.

[0053] As a further modification of the guide surface 50, it may be configured with a multi-stage guide section, such as four or five stages. This allows the direction of the scavenging gas to be changed in multiple stages on the horizontal plane during the scavenging stroke. Another modification 56 of the guide surface 50 is a curved, stepless guide surface, as shown in Figures 12 and 13. Figure 13 is a cross-sectional view along the line XIII-XIII in Figure 12. This allows the direction of the scavenging gas to be changed steplessly on the horizontal plane during the scavenging stroke.

[0054] Although embodiments of the present invention applied to a stratified scavenging engine have been described above, the present invention can also be applied to two-stroke engines that do not include leading air in the scavenging gas, that is, engines that use a pre-compressed fuel-air mixture in the crankcase 12 as the scavenging gas.

[0055] Furthermore, the present invention can be suitably applied to two-stroke engines that employ a fuel injection system instead of a carburetor 28. For example, when the present invention is applied to a direct-injection two-stroke engine, pre-compressed air in the crankcase is used as scavenging gas. By applying the present invention to this direct-injection two-stroke engine, the scavenging efficiency in the cylinder can be improved.

[0056] The following describes modified examples relating to the upper end of the variable scavenging port 16(ch) and the variable scavenging passage 14(ch) connected thereto to which the present invention is applied, with reference to Figures 14 to 18. These modified examples are, of course, applicable to stratified scavenging engines and engines that use a pre-compressed fuel-air mixture in the crankcase 12 as scavenging gas. Referring to Figure 3 above, in the above-described embodiment, the present invention was applied to the upper end of the scavenging passage 14 connected to the exhaust-side second scavenging port 16(ex2). Alternatively, as shown in Figure 14, the present invention may be applied to the upper end of the scavenging passage 14(ch) connected to the exhaust-side first scavenging port 16(ex1) which faces the exhaust-side second scavenging port 16(ex2). As a further modification, the present invention may be applied to the upper end of the scavenging passage 14 connected to the intake-side first or second scavenging port 16(in1 or in2) instead of the exhaust-side first or second scavenging port 16(ex1 or ex2).

[0057] Figure 15 shows an example in which the present invention is applied to the upper end of the scavenging passage 14(ch) connected to the exhaust-side second scavenging port 16(ex2) and the upper end of the scavenging passage 14(ch) connected to the intake-side first scavenging port 16(in1). As a modified example, the present invention may also be applied to the upper end of the scavenging passage 14 connected to the exhaust-side first scavenging port 16(ex1) and the upper end of the scavenging passage 14 connected to the intake-side second scavenging port 16(in2).

[0058] Figure 16 shows an example in which the present invention is applied to the upper end of the scavenging passage 14(ch) connected to the exhaust-side second scavenging port 16(ex2) located on one side of the cylinder, and to the upper end of the scavenging passage 14(ch) connected to the intake-side second scavenging port 16(in2). As a modified example, the present invention may also be applied to the upper end of the scavenging passage 14 connected to the exhaust-side first scavenging port 16(ex1) and to the upper end of the scavenging passage 14 connected to the intake-side first scavenging port 16(in1).

[0059] Figure 17 shows an example in which the present invention is applied to the upper end of the scavenging passage 14(ch) connected to the first and second scavenging ports 16(ex1) and 16(ex2), which are opposite to each other, on the exhaust side of the cylinder. As a modified example, the present invention may also be applied to the upper end of the scavenging passage 14 connected to the first and second scavenging ports 16(in1) and 16(in2), which are opposite to each other, on the intake side of the cylinder.

[0060] The present invention may also be applied to the upper ends of each scavenging passage 14 connected to three of the four scavenging ports 16 contained in a cylinder, excluding one. Figure 18 shows an example in which the present invention is applied to the upper ends of the scavenging passages 14(ch) connected to three scavenging ports 16(ch), excluding the scavenging passage 14 connected to the intake-side first scavenging port 16(in1). The one scavenging port 16 to which the present invention is not applied may be the intake-side second scavenging port 16(in2) or the exhaust-side first or second scavenging port 16(ex1 or ex2).

[0061] The present invention is suitably applicable to a two-flow scavenging engine having one scavenging port 16 on each side of the cylinder. Figure 19 shows an example in which the present invention is applied to the upper end of a scavenging passage 14 connected to a scavenging port 16(2) located on one side of the cylinder. Figure 20 shows an example in which the present invention is applied to the upper ends of scavenging passages 14 connected to scavenging ports 16(1) and 16(2), respectively, located on both sides of the cylinder.

[0062] In the examples shown in Figures 17, 18, and 20, the opposing exhaust-side scavenging ports 16(ex1), 16(ex2), 16(1), and 16(2) are variable scavenging ports 16(ch) associated with the variable scavenging passage 14(ch) to which the present invention is applied. To explain using the example in Figure 17 as a representative example, it is preferable to set the inclination angle θ(ex-H1) of the upper guide surface 50(H) associated with the exhaust-side first scavenging port 16(ex1) and the inclination angle θ(ex-H2) of the upper guide surface 50(H) associated with the exhaust-side second scavenging port 16(ex2) to different values ​​so that the scavenging flow 42(1) discharged from the exhaust-side first scavenging port 16(ex1) and the scavenging flow 42(2) discharged from the exhaust-side second scavenging port 16(ex2) do not collide. This configuration prevents collisions between scavenging gases discharged from opposing scavenging ports 16, which would cause a short circuit to the exhaust port 22. Instead, the scavenging gases discharged from each scavenging port 16 maintain their respective flow paths while scavenging the combustion chamber before proceeding to the exhaust port 22.

[0063] The explanation above, with reference to Figures 15 and 16, has been given regarding the upper guide section 50(H), and the same explanation applies to the lower guide section 50(L).

[0064] As can be seen from the drawings, each of the guide surfaces 50(H), 50(L), 54(H), 54(M), and 54(L) described above is composed of a straight-line surface, but it may also be composed of a curved surface when viewed from above. Furthermore, at least one of the guide surfaces 50(H), 50(L), 54(H), 54(M), and 54(L) may have a curved surface shape with multiple dividing surfaces. To illustrate the curved surface shape by denoting the dividing surfaces with reference numeral 58, Figures 21 and 22 show an example composed of two dividing surfaces 58(1) and 58(2), but this is merely an example, and it may be composed of three or more dividing surfaces.

[0065] Figure 21 shows an example where the angle α1 between the first and second dividing surfaces 58(1) and 58(2), that is, the angle formed by the first and second dividing surfaces 58(1) and 58(2) toward the inside of the cylinder 4, is acute, and Figure 22 shows an example where the angle α2 between the first and second dividing surfaces 58(1) and 58(2), that is, the angle formed by the first and second dividing surfaces 58(1) and 58(2) toward the inside of the cylinder 4, is obtuse. The guide surfaces 50(H), 50(L), 54(H), 54(M), and 54(L) formed by the first and second dividing surfaces 58(1) and 58(2) as exemplified in Figures 21 and 22 can discharge scavenging gas in two directions, as indicated by the white arrows 42(1) and 42(2) in Figures 21 and 22. [Explanation of Symbols]

[0066] Engine of 2 Embodiments 4 cylinders 6 pistons 8 Combustion chamber 12 Crank chamber 14. Scavenging passage 14(ch) Variable scavenging passage 16 scavenging ports 16(ch) Variable Scavenging Ports 22 exhaust ports 50 Guide surface 50(H) Upper guide section 50(L) Lower guide section 60 Cylinder Block 62. Scavenging passage forming cap 64 screw holes

Claims

1. A piston positioned inside the cylinder, which reciprocates between top dead center and bottom dead center and defines the combustion chamber, An exhaust port for discharging the burnt gas from the combustion chamber, which opens in the cylinder and is opened and closed by the piston, A crank chamber that receives fresh air and pre-compresses the fresh air by the downward movement of the piston, The apparatus is provided with a plurality of scavenging passages connected to a plurality of scavenging ports that communicate with the combustion chamber and the crank chamber and discharge fresh air pre-compressed in the crank chamber as scavenging gas into the combustion chamber during the scavenging stroke, and the scavenging ports are opened and closed by the piston, In a reversing scavenging type two-stroke engine, the scavenging gas discharged from the scavenging port is directed towards the cylinder wall on the intake port side, which is located radially opposite to the exhaust port of the cylinder, so that the scavenging gas discharged from the scavenging port reverses direction at the cylinder wall on the intake port side and proceeds toward the exhaust port to scavenge the combustion chamber. Of the plurality of scavenging passages, at least one scavenging passage connected to the scavenging port constitutes a variable scavenging passage. The scavenging port connected to the variable scavenging passage constitutes a variable scavenging port. At the upper end of the variable scavenging passage, on the side wall surface on the intake port side, which is located radially opposite to the exhaust port of the cylinder, there is a guide surface that defines the discharge direction of the scavenging gas discharged from the variable scavenging port, which extends horizontally in a direction perpendicular to the vertical plane that extends along the up-and-down movement of the piston. The guide surface comprises at least an upper guide portion that defines a first discharge direction of the scavenging gas in the horizontal discharge direction at the beginning of the scavenging stroke and is located on the top dead center side of the piston, and a lower guide portion located below the upper guide portion on the bottom dead center side of the piston and defines a second discharge direction of the scavenging gas. At the upper end of the variable scavenging passage, the effective cross-sectional area of ​​the scavenging passage at the position of the upper guide portion is different from the effective cross-sectional area of ​​the scavenging passage at the position of the lower guide portion. A two-stroke engine characterized in that, as the scavenging stroke progresses, the direction of the scavenging gas flow directed towards the intake side of the reverse scavenging system on the horizontal plane changes in a direction that approaches the central axis of the cylinder and the flow velocity changes.

2. The two-stroke engine according to claim 1, wherein the effective cross-sectional area of ​​the scavenging passage at the position of the upper guide portion is 20 to 70% of the effective cross-sectional area of ​​the scavenging passage at the position of the lower guide portion.

3. The two-stroke engine according to claim 1 or 2, wherein the guide surface is composed of multiple stages in the vertical direction, including the upper guide portion and the lower guide portion.

4. The two-stroke engine according to claim 1 or 2, wherein the guide surface is composed of a stepless surface that is curved in the vertical direction, including the upper guide portion and the lower guide portion.

5. The aforementioned two-stroke engine has four of the aforementioned scavenging ports, The two-stroke engine according to claim 1, wherein the scavenging passage connected to at least one of the four scavenging ports is the variable scavenging passage.

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