Stratified scavenging two-stroke internal combustion engine and carburetor thereof

Abstract

A carburetor achieves better emission characteristics while improving a combustion state by increasing a delivery ratio of a stratified scavenging engine and reducing intake resistance. No dividing wall is provided between a throttle valve 204 and a choke valve 242, and a gap 244 between the valves 204 and 242 is opened. Intake air in both of an upper region and a lower region of the choke valve 242 flows into an air-fuel mixture passage 246 in a lower region of the fully-open throttle valve 204. A main nozzle 202 is arranged so as to be inclined toward the throttle valve 204.

Claims

1. A stratified scavenging two-stroke internal combustion engine comprising: a piston valve-type engine body; a carburetor being interposed between the engine body and an air cleaner; an intake member connecting the carburetor and the engine body, the intake member including a fresh air passage and an air-fuel mixture passage, and the air-fuel mixture passage is capable of communicating with a crankcase of the engine body; wherein the carburetor includes an intake air passage receiving air filtered by the air cleaner, a throttle valve that is arranged in the carburetor and is composed of a plate butterfly valve, and a main nozzle or a main port discharging fuel toward a plate surface of the throttle valve in a fully-open state, wherein the engine body includes a scavenging passage that feeds an air-fuel mixture in the crankcase to a combustion chamber to perform scavenging, an air port that receives fresh air from the fresh air passage, a piston groove that is provided in a peripheral surface of a piston so as to fill an upper portion of the scavenging passage with the fresh air from the air port, and an air-mixture port that communicates the air-fuel mixture passage with the crankcase characterized in that an upstream side of the throttle valve is composed of an open space with no dividing wall, in the carburetor, an opening timing of the air-fuel mixture port is set to be earlier than a timing at which the air port and the scavenging passage communicate with each other via the piston groove, wherein the crankcase starts to be charged with the air-fuel mixture from the air-fuel mixture passage before the timing at which the upper portion of the scavenging passage starts to be charged with the fresh air through the piston groove and the scavenging passage communicating with each other during an upstroke of the piston, whereby a whole amount of the fuel discharged from the main nozzle or the main port flows into the air-fuel mixture passage in the fully-open state of the throttle valve and that, once released into the air-fuel mixture passage, the whole amount of the fuel discharged from the main nozzle or the main port stays in the air-fuel mixture passage until introduced into the crankcase during the upstroke of the piston, wherein the carburetor includes no venturi portion on an upstream side of the throttle valve, and no air valve on an upstream side of the throttle valve for adjusting a quantity of the fuel to the intake air passage through the main nozzle or the main port, wherein the fuel is discharged obliquely and toward a downstream side of the carburetor from the main nozzle or the main port.

2. The stratified scavenging two-stroke internal combustion engine of claim 1, wherein the main nozzle is arranged so as to be inclined toward a downstream side of the carburetor.

3. The stratified scavenging two-stroke internal combustion engine of claim 1, wherein a discharge port of the main nozzle is located at a position not interfering with a rotation locus of an outer end edge of the throttle valve, and adjacent to the rotation locus.

4. The stratified scavenging two-stroke internal combustion engine of claim 1, a distance between a shaft of the throttle valve and a downstream-side end surface of the carburetor is or less of a radius of the throttle valve in plan view.

5. The stratified scavenging two-stroke internal combustion engine of claim 1, wherein the main port opens in a top portion of a local bulge that projects from a wall surface defining the intake air passage.

6. The stratified scavenging two-stroke internal combustion engine of claim 1, further comprising a flow rectifying element arranged adjacent to and immediately upstream of the main nozzle or the main port for rectifying a flow of intake air passing through the main nozzle or the main port.

7. The stratified scavenging two-stroke internal combustion engine of claim 1, further comprising a choke valve that is arranged upstream of the throttle valve and is composed of a butterfly valve, wherein an interval between the choke valve and the throttle valve is an open space with no dividing wall.

8. The stratified scavenging two-stroke internal combustion engine of claim 7, wherein intake air flowing through an upper region of the choke valve flows into a lower region of the throttle valve through the open space between the choke valve and the throttle valve, and the fuel discharged from the main nozzle or the main port is directed toward a position P where first intake air flowing into the lower region of the throttle valve through the open space joins second intake air flowing through a lower region of the choke valve, or a downstream side of the position.

9. The stratified scavenging two-stroke internal combustion engine of claim 7, wherein the choke valve and the throttle valve are arranged close to each other up to a position where the choke valve and the throttle valve do not interfere with each other, and a distance between a shaft of the choke valve and a shaft of the throttle valve is equal to a value obtained by adding a radius of the choke valve and a radius of the throttle valve.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a diagram for explaining a principle of a carburetor of the present invention.

(2) FIG. 2 shows a diagram for explaining a modification included in the carburetor of the present invention.

(3) FIG. 3 shows a diagram for explaining another modification included in the carburetor of the present invention.

(4) FIG. 4 shows a diagram for explaining the principle of the present invention by employing a carburetor including a main port as an example, in which a throttle valve is in a fully-open state.

(5) FIG. 5 shows a diagram for explaining the principle of the present invention by employing the carburetor including the main port as an example, in which the throttle valve is in a partially-open state.

(6) FIG. 6 shows a diagram for explaining a modification of a dividing wall of an intake member connected to the carburetor including the main port, in which the throttle valve is in a fully-open state.

(7) FIG. 7 shows a diagram for explaining the modification of the dividing wall of the intake member connected to the carburetor including the main port, in which the throttle valve is in a partially-open state.

(8) FIG. 8 shows a diagram for explaining the principle of the present invention by employing a carburetor including a main nozzle as an example, in which a throttle valve is in a fully-open state.

(9) FIG. 9 shows a diagram for explaining the principle of the present invention by employing the carburetor including the main nozzle as an example, in which the throttle valve is in a partially-open state.

(10) FIG. 10 shows a diagram for explaining inclination of the main nozzle in the carburetor including the main nozzle that is arranged so as to be inclined.

(11) FIG. 11 shows a perspective diagram of a carburetor with no venturi portion of an embodiment.

(12) FIG. 12 shows a sectional diagram for explaining an internal structure of the embodiment.

(13) FIG. 13(I) shows a diagram for explaining that a throttle valve and a choke valve can be arranged close to each other up to a position where the throttle valve and the choke valve do not interfere with each other. FIG. 13(II) shows an example in which the throttle valve and the choke valve are arranged slightly apart from each other.

(14) FIG. 14 shows a diagram for explaining an advantage obtained when a throttle valve shaft and a downstream-side end surface of the carburetor are located adjacent to each other when the carburetor of the embodiment is viewed in plan view.

(15) FIG. 15 shows a diagram for explaining that a throttle valve shaft and a downstream-side end surface of a conventional carburetor are apart from each other when the conventional carburetor is viewed in plan view, and a gap is thereby generated downstream of a throttle valve in a fully-open state.

(16) FIG. 16 shows a diagram for explaining an example in which a flow rectifying element is arranged immediately upstream of a main nozzle included in the carburetor of the embodiment.

(17) FIG. 17 shows a diagram for explaining a preferred position toward which fuel discharged from the main nozzle or the main port is directed in the carburetor including the throttle valve and the choke valve.

(18) FIG. 18 shows a sectional diagram of a conventional and typical carburetor, and shows the carburetor in an idle operation state.

(19) FIG. 19 shows a sectional diagram of the conventional and typical carburetor, and shows the carburetor in a partial operation state.

(20) FIG. 20 shows a sectional diagram of the conventional and typical carburetor, and shows the carburetor in a high-speed operation state.

(21) FIG. 21 shows a schematic diagram of a carburetor disclosed in U.S. Pat. No. 7,201,120 B2.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

(22) In the following, a carburetor according to the present invention will be described based on the accompanying drawings.

(23) FIGS. 4 and 5 show diagrams for explaining one specific example of the carburetor according to the present invention. Reference numeral 2 shown in FIGS. 4 and 5 denotes a throttle valve-type carburetor. The carburetor 2 is incorporated in a conventionally-known stratified scavenging two-stroke internal combustion engine. The stratified scavenging two-stroke engine has various configurations. Since a mechanism and an action of the stratified scavenging engine are described in detail in Japanese Patent Laid-Open No. 2002-227653 and International Publication No. WO 98/57053, Japanese Patent Laid-Open No. 2002-227653 and International Publication No. WO 98/57053 are incorporated herein.

(24) An outline of the stratified scavenging two-stroke engine is as follows. The stratified scavenging two-stroke engine includes a scavenging passage that communicates with a crankcase and a combustion chamber similarly to a general two-stroke engine. The crankcase is filled with an air-fuel mixture. The air-fuel mixture in the crankcase is introduced into the combustion chamber through the scavenging passage. The stratified scavenging two-stroke engine is characterized in that leading air containing no fuel component is introduced into the combustion chamber immediately before the air-fuel mixture in the crankcase is introduced into the combustion chamber in a scavenging stroke, that is, at an initial stage of the scavenging stroke.

(25) Referring to FIGS. 4 and 5, the carburetor 2 includes an intake air passage 4. A throttle valve 6 is arranged in the intake air passage 4. The throttle valve 6 can swing about a shaft 6a. An amount of intake air changes depending on an opening degree of the throttle valve 6, so that engine power is controlled. In the drawings, an arrow indicates a flow direction of the intake air. Air filtered by an air cleaner is fed to the carburetor 2. A downstream end of the carburetor 2 is connected to an engine body via an intake member 12.

(26) The intake member 12 is a member that connects the carburetor 2 and the engine body, and that constitutes an intake passage. The intake member 12 may be composed of a single member that is continuous in a longitudinal direction, or may be composed of a plurality of members.

(27) The intake air passage 4 may include a venturi portion similarly to the conventional carburetor, or may not include the venturi portion as in an embodiment described later. In a case in which the intake air passage 4 includes a venturi portion having a conventional height, it is preferable to suppress an entire projecting height by inclining a main nozzle toward a downstream side of the flow direction.

(28) A main port 14 is arranged immediately upstream of the throttle valve 6 so as to face the intake air passage 4. The main port 14 preferably opens obliquely toward the downward side of the flow direction of the intake air. It is also preferable to form a small bulge 15 on a wall surface of the intake air passage 4, and locate the opening of the main port 14 in the vicinity of a top portion of the local small bulge 15 as shown in the drawings.

(29) FIG. 4 shows a state of the carburetor 2 in a high-speed operation. In the high-speed operation, that is, when the throttle valve 6 is in a fully-open state, an air-fuel mixture passage 16 is formed by the throttle vale 6 and a dividing wall 12a of the intake member 12 located downstream of the throttle vale 6 similarly to the conventional carburetor. Fuel discharged from the main port 14 is guided by the throttle valve 6 to pass through the air-fuel mixture passage 16 to fill the crankcase of the engine body.

(30) FIG. 5 shows a state of the carburetor 2 in a partial operation. In the partial operation, the throttle valve 6 is in a partially-open state. The fuel discharged from the main port 14 partially flows into a fresh air passage 20 similarly to the conventional carburetor through a gap 18 between a downstream end of the partially-open throttle valve 6 and the dividing wall 12a of the intake member 12. A large portion of the fuel discharged from the main port 14 is fed to the engine body through the air-fuel mixture passage 16.

(31) Reference character D2 in FIGS. 6 and 7 denotes a distance between the shaft 6a of the throttle valve 6 and a downstream end of the intake air passage 4. It should be understood that the distance D2 is smaller than that in the conventional carburetor. That is, the throttle valve shaft 6a is positioned adjacent to the downstream end of the intake air passage. The distance D2 is about 2.5 to 6.0 mm, preferably about 2.6 to 5.0 mm, and most preferably about 2.8 to 4.0 mm when the throttle valve shaft 6a has a diameter of 5.0 mm.

(32) The gap 18 between the partially-open throttle valve 6 and the dividing wall 12a of the intake member 12 can be decreased by the arrangement of the throttle valve shaft 6a. In other words, a degree of separation between the air-fuel mixture and fresh air in the partial operation can be controlled by a size of the relatively small gap 18. The distance D2 between the throttle valve shaft 6a and the downstream end of the intake air passage 4 may be determined so as to set the degree of separation in the partial operation to a desired value.

(33) It goes without saying that the technical concept of setting the distance D2 to a small value is not limited to the embodiment of the present invention, and can be also widely and generally applied to the stratified scavenging two-stroke internal combustion engine.

(34) Of course, an arrangement position of the main port 14 may be set to a position at which the main port 14 can work similarly to the conventional carburetor in an idle range, a partial range, and a high-speed range. To be more specific, the arrangement position of the main port 14 is set to a position satisfying the following conditions. (1) Fuel is discharged from the main port 14 in the partial operation and the high-speed operation. (2) No fuel is discharged from the main port 14 in an idle operation. The conditions (1) and (2) are the same as those of the conventional carburetor.

(35) As for the dividing wall 12a of the intake member 12, a step portion 12b is formed at an upstream end portion of the dividing wall 12a. When the throttle valve 6 is seated in the step portion 12b, the throttle valve 6 comes into a fully-open state.

(36) As a modification of the dividing wall 12a, the dividing wall 12a without the step portion 12b may extend to an upstream end of the intake member 12 as shown in FIGS. 6 and 7.

(37) FIGS. 8 and 9 show diagrams for explaining one specific example of the carburetor according to the present invention. A carburetor 22 shown in FIGS. 8 and 9 basically corresponds to the above carburetor 2 shown in FIGS. 4 and 5. In a description of the carburetor 22 shown in FIGS. 8 and 9, the same elements as those described with reference to FIGS. 4 and 5 are assigned the same reference numerals, and a description thereof is omitted.

(38) While the carburetor 2 shown in FIGS. 4 and 5 includes the main port 14 that opens in the wall surface defining the intake air passage 4, the carburetor 22 shown in FIGS. 8 and 9 includes a main nozzle 24 that is located projecting from the wall surface of the intake air passage 4 instead of the main port 14. The main nozzle 24 is preferably positioned in a state in which the main nozzle 24 is inclined toward the downstream side of the flow direction of the intake air. An angle of the inclination is indicated by in FIG. 10.

(39) A length and the inclination angle of the main nozzle 24 are set based on a condition that the main nozzle 24 does not interfere with the swinging throttle valve 6. In consideration of an angle at which a work of press-fitting the main nozzle 24 into a hole in the wall surface defining the intake air passage 4 can be performed with no difficulty when the carburetor 22 is produced, the inclination angle of the main nozzle 24 is set to an angle smaller than the angle at which the main nozzle 24 can be press-fitted, and an angle at which the fuel is stably discharged from the main nozzle 24. The inclination angle is specifically 0 to 50, preferably 10 to 40, and most preferably 10 to 35.

(40) In the above carburetor disclosed in U.S. Pat. No. 7,201,120 B2, the inclination angle of the main nozzle is larger than 30.

(41) FIG. 8 shows a state of the carburetor 22 in the high-speed operation. FIG. 9 shows a state of the carburetor 22 in the partial operation. Since an action of the carburetor 22 shown in FIGS. 8 and 9 is substantially the same as that of the carburetor 2 described with reference to FIGS. 4 and 5, a description thereof is omitted.

(42) FIG. 11 and the drawings after of show a throttle valve-type carburetor 30 of the embodiment. The carburetor 30 of the embodiment is applied to the stratified scavenging two-stroke internal combustion engine. The two-stroke engine has a single cylinder, and its displacement is 20 cc to 120 cc. In a description of the carburetor 30 of the embodiment, the same elements as those described with reference to FIG. 4 or the like are assigned the same reference numerals, and a description thereof is omitted. Specific examples of a portable working machine equipped with the engine of the embodiment include a chain saw, a trimmer, a power blower, an engine-type pump, a small generator, and an agricultural chemical sprayer.

(43) From FIG. 11 (a perspective diagram of the carburetor 30), a person skilled in the art would immediately understand that the carburetor 30 is compact. FIG. 12 shows a sectional diagram of the carburetor 30. Referring to FIG. 12, the carburetor 30 includes a choke valve 32 in addition to the throttle valve 6. As is immediately understood in comparison with the conventional example in FIG. 18 or the like that has been previously referred to, the carburetor 30 of the embodiment does not include a venturi portion (reference numeral 104 in FIG. 18) in the intake air passage 4.

(44) An arrow in FIG. 12 indicates the flow direction of the intake air. The throttle valve 6 and the choke valve 32 are arranged adjacent to each other. A center distance D1 (FIG. 12) between the throttle valve shaft 6a and a choke valve shaft 32a is substantially equal to a value obtained by adding a radius of the throttle valve 6 and a radius of the choke valve 32. When both the throttle valve 6 and the choke valve 32 are in a fully-open state, the throttle valve 6 and the choke valve 32 form substantially continuous surfaces flush with each other.

(45) The main nozzle 24 is positioned immediately upstream of the throttle valve 6. The main nozzle 24 is arranged between the throttle valve 6 and the choke valve 32 arranged adjacent to each other.

(46) The main nozzle 24 located immediately upstream of the throttle valve 6 is positioned so as to be inclined. In the embodiment, the inclination angle of the main nozzle 24 is 25. The main nozzle 24 is inclined in a direction in which a distal end thereof separates from a plate surface of the choke valve 32 in a fully-open state, and is directed toward a plate surface of the throttle valve 6 in a fully-open state. Because of the configuration, a dividing wall between the throttle valve 6 and the choke valve 32 becomes unnecessary. That is, it is possible to prevent the fuel discharged from the main nozzle 24 from entering the fresh air passage from the upstream side of the throttle valve 6 even when the dividing wall is not provided upstream of the throttle valve 6.

(47) A downstream-side end surface 30a of the carburetor 30 is located adjacent to the shaft 6a of the throttle valve 6. Similarly, an upstream-side end surface 30b of the carburetor 30 is located adjacent to the shaft 32a of the choke valve 32.

(48) The carburetor 30 of the embodiment of the above configuration has a smaller length dimension L1 in the flow direction of the intake air than that in the conventional carburetor 100 (FIG. 18).

(49) Reasons why the dimension L1 can be made smaller are as follows. (1) The venturi portion 104 (FIG. 18) described in the conventional carburetor does not exist. (2) The distance D1 between the throttle valve shaft 6a and the choke valve shaft 32a is small. (3) The throttle valve shaft 6a is located adjacent to the downstream-side end surface 30a of the carburetor 30. That is, the distance D2 described in FIG. 6 and so on is small. To be more specific, the distance D2 is 3.2 mm. A bore size of the carburetor 30 is 17.5 mm. (4) The choke valve shaft 32a is located adjacent to the upstream-side end surface 30b of the carburetor 30.

(50) In the carburetor 30 of the embodiment, the venturi portion 104 (FIG. 18) does not exist as described above. Accordingly, the throttle valve 6 and the choke valve 32 can be arranged close to each other up to a position where the throttle valve 6 and the choke valve 32 do not interfere with each other.

(51) FIG. 13 show diagrams for explaining the arrangement of the throttle valve 6 and the choke valve 32. FIG. 13(I) shows an example in which the throttle valve 6 and the choke valve 32 are arranged slightly apart from each other.

(52) FIG. 13(II) shows a diagram for explaining that the throttle valve 6 and the choke valve 32 can be arranged close to each other up to a position where the throttle valve 6 and the choke valve 32 do not interfere with each other. It goes without saying that the main nozzle 24 arranged between the throttle valve 6 and the choke valve 32 is set to a position and a projecting amount where the main nozzle 24 does not interfere with the throttle valve 6 and the choke valve 32.

(53) As described above, the venturi portion 104 is an essential element in the conventional carburetor 100 (FIG. 18). In the conventional carburetor 100, a configuration in which the main port 110 or the main nozzle is arranged at the top portion of the venturi portion 104 is employed, and the configuration has been considered as essential.

(54) The inventors of the this application have verified that the venturi portion that has been conventionally considered as essential is not essential in the carburetor applied to the stratified scavenging engine. Based on the verification result, the carburetor 30 of the embodiment does not include the venturi portion. Accordingly, a degree of freedom regarding the arrangement position of the main port 14 or the main nozzle 24 is higher than that of the prior art. In other words, a distance between the throttle valve 6 and the main port 14 or the main nozzle 24 can be set to be small.

(55) Also, since the main nozzle 24 can be positioned immediately upstream of the throttle valve 6, it is not necessary to set the inclination angle of the main nozzle 24 to be large in order to direct the fuel discharged from the main nozzle 24 toward the plate surface of the fully-open throttle valve 6. The inclination angle of the main nozzle 24 can be set to an angle that has already been actually produced. In the embodiment, the inclination angle is 25.

(56) Referring to FIG. 14, the distance D2 between the downstream-side end surface 30a of the carburetor 30 and the throttle valve shaft 6a is smaller than that of the conventional carburetor. Accordingly, the length dimension L1 of the carburetor 30 can be made smaller. That is, the carburetor 30 can be made more compact than that of the conventional carburetor. FIG. 15 shows the conventional example.

(57) As is well understood from FIG. 15 (the conventional example), since a downstream-side end surface 100a of the carburetor 100 is spaced from a throttle valve shaft 106a, a gap G is formed downstream of the throttle valve 106 in a fully-open state. Reference numeral 12 in FIG. 15 denotes the intake member described above. The intake member 12 is arranged between the carburetor 100 and the engine body. Reference numeral 12a denotes the dividing wall. The air-fuel mixture passage 16 and the fresh air passage 20 are divided by the dividing wall 12a (FIG. 4).

(58) Referring to FIG. 15 (the prior art), when the fully-open throttle valve 106 is viewed in plan view, the gaps G on both sides of the throttle valve 106 are one of causes for decreasing the degree of separation between the air-fuel mixture and the fresh air in the high-speed operation.

(59) Referring to FIG. 14, the distance D2 between the downstream-side end surface 30a of the carburetor 30 of the embodiment and the throttle valve shaft 6a is small. Accordingly, the gaps G on both sides of the throttle valve 6 can be made as small as possible. It goes without saying that the gap G becomes smaller as the distance D2 is decreased. An area of the gap G is also reduced in a quadratic curve as the distance D2 is reduced.

(60) When the distance D2 between the throttle valve shaft 6a and the downstream-side end surface 30a adjacent thereto is or less of the radius of the throttle valve 6, a gas flow passing through the gap G can be considered as almost zero. In the embodiment, the throttle valve shaft 6a is arranged at a position of 3.2 mm from the downstream-side end surface 30a. The numeric value of 3.2 mm is smaller than of the radius of the throttle valve 6. Accordingly, the gas flow through the gap G can be ignored in the high-speed operation, that is, when the throttle valve 6 is in a fully-open state. The degree of separation between the air-fuel mixture and the fresh air can be thereby increased.

(61) That is, in accordance with the carburetor 30 of the embodiment, by decreasing the distance D2 between the downstream-side end surface 30a and the throttle valve shaft 6a, it is possible to eliminate the necessity for extending the dividing wall 12a of the intake member 12 in order to fill the gaps G, and it is thereby possible to provide the simple and compact carburetor for the stratified scavenging engine.

(62) The present invention has been specifically described above. The present invention includes various specific aspects and modifications included in the invention defined by the claims. FIG. 16 shows a diagram for explaining an example in which an element 36 that rectifies the air flow is arranged immediately upstream of the main nozzle 24. The air flowing through the intake air passage 4 passes through the main nozzle 24 in a state in which the flow is partially rectified by the flow rectifying element 36. Accordingly, a discharge amount of the fuel discharged from the main nozzle 24 can be made stable.

(63) The flow rectifying element 36 may be formed by raising a portion of the wall surface defining the intake air passage 4. The flow rectifying element 36 may be formed by building up a portion of the wall surface defining the intake air passage 4. The flow rectifying element 36 may be formed by a member integrated with the main nozzle 24.

(64) Of course, the flow rectifying element 36 may be also provided immediately upstream of the main port 14 described with reference to FIGS. 4 and 5. To be more specific with respect to the example in FIGS. 4 and 5, a portion of the local small bulge 15 where the main port 14 opens may be formed in a shape exerting a flow rectifying effect.

(65) Preferred arrangement of the main nozzle 202 is described by using the carburetor 240 including the throttle valve 204 and the choke valve 242 as an example with reference to FIGS. 1 to 3. FIG. 17 shows the same carburetor 240 as that shown in FIG. 1.

(66) Referring to FIG. 17, fresh air Ar(1) entering the air-fuel mixture passage 246 through the gap 244 between the throttle valve 204 and the choke valve 242, and drawn air Ar(2) entering the air-fuel mixture passage 246 join each other at a point P of the air-fuel mixture passage 246. The fuel discharged from the main nozzle 202 is preferably directed toward the joining point P or the downstream side thereof. To be more specific, the joining point P is located ahead (the downstream side) of a longitudinal intermediate position of the gap 244 formed between the throttle valve 204 and the choke valve 242. That is, the discharge port of the main nozzle 202 is directed toward the downstream side of the longitudinal intermediate position of the gap 244. Of course, the arrangement position and the inclination angle of the main nozzle 202 are preferably set such that the air-fuel mixture passage 246 defined by the lower surface of the throttle valve 204 can receive the whole fuel discharged from the main nozzle 202.

(67) The same applies to the carburetor that employs the main port instead of the main nozzle 202.

REFERENCE SIGNS LIST

(68) 2 One specific example of a carburetor according to the present invention 4 Intake air passage 6 Throttle valve 6a Throttle valve shaft 12 Intake member 12a Dividing wall of the intake member 14 Main port 16 Air-fuel mixture passage 18 Gap between the throttle valve in a partially-open state and the dividing wall 20 Fresh air passage 22 Another specific example of the carburetor according to the present invention 24 Main nozzle Inclination angle of the main nozzle 30 Carburetor of Embodiment 32 Choke valve 32a Choke valve shaft L1 Length dimension from an upstream end to a downstream end of the carburetor D1 Center distance between the throttle valve shaft and the choke valve shaft 36 Flow rectifying element