Carburetor for stratified scavenging two-stroke engine

Abstract

The present invention is to prevent mixing of fresh air and an air-fuel mixture when a throttle valve is fully opened and increase a delivery ratio. A main nozzle (30) is surrounded by a tunnel-like air flow guiding member (52). The air flow guiding member (52) is opened at its front and back sides. A whole amount of fuel discharged via a main nozzle (30) is sent to a downstream side by an air flow created by the air flow guiding member (52). When a throttle valve (22) and a choke valve (24) are both in a fully-opened state, fresh air flows into an air-fuel mixture passage (12) through a gap between these valves (22, 24).

Claims

1. A carburetor for a stratified scavenging engine, the carburetor comprising: an intake air passage configured to receive air filtered by an air cleaner and create an air-fuel mixture; a throttle valve arranged in the intake air passage, the throttle valve being a butterfly valve, wherein, in response to the throttle valve being placed in a fully-opened state, a fresh air passage and an air-fuel mixture passage are formed by the throttle valve, air coming out of the fresh air passage being supplied to an upper portion of a scavenging passage of the stratified scavenging engine, and an air-fuel mixture coming out of the air-fuel mixture passage being supplied to a crankcase of the stratified scavenging engine; and a first air flow guiding member configured to adjust a flow of gas passing through the fresh air passage and/or the air-fuel mixture passage and direct the flow of gas toward a downstream side, wherein the carburetor further comprises a main nozzle or a main port for supplying fuel to the intake air passage and at least one idle port for supplying fuel to the intake air passage with the at least one idle port located on a side downstream of the main nozzle or main port, wherein the first air flow guiding member is arranged in the fresh air passage and configured to adjust the flow of the gas passing through the fresh air passage, an upper surface of the first air flow guiding member is positioned above a shaft of the throttle valve, the main nozzle or main port is arranged in an intermediate position in a longitudinal direction of the first air flow guiding member.

2. The carburetor for a stratified scavenging engine according to claim 1, wherein the air coming out of the fresh-air passage is supplied via a piston groove to the upper portion of the scavenging passage.

3. The carburetor for a stratified scavenging engine according to claim 1, further comprising a second air flow guiding member being arranged to face a main nozzle or a main port for supplying fuel to the intake air passage.

4. The carburetor for a stratified scavenging engine according to claim 3, further comprising a third air flow guiding member being arranged in the air-fuel mixture passage, wherein the third air flow guiding member is formed in a plate-like shape; and a surface of the third air flow guiding member in contact with the air-fuel mixture passage is positioned to be lower than a shaft of the throttle valve.

5. The carburetor for a stratified scavenging engine according to claim 4, wherein the third air flow guiding member is inclined downward from an upstream side toward the downstream side.

6. The carburetor for a stratified scavenging engine according to claim 3, wherein the second air flow guiding member is formed in a shape of a tunnel extending in a flow direction of intake air.

7. The carburetor for a stratified scavenging engine according to claim 6, wherein an inlet of the second air flow guiding member is larger than an outlet of the second air flow guiding member.

8. The carburetor for a stratified scavenging engine according to claim 1, further comprising a choke valve which is a butterfly valve.

9. The carburetor for a stratified scavenging engine according to claim 8, wherein the first air flow guide member is arranged in the fresh air passage; and a surface of the first air flow guiding member in contact with the fresh air passage is positioned above a shaft of the choke valve and a shaft of the throttle valve.

10. The carburetor for a stratified scavenging engine according to claim 8, further comprising an additional air flow guiding member that is arranged in the air-fuel mixture passage, wherein the throttle valve includes a shaft; the additional air flow guiding member is formed in a plate-like shape; the additional air flow guiding member is positioned such that the additional air flow guiding member resides below and near the throttle valve; and a surface of the additional air flow guiding member in contact with the air-fuel mixture passage is positioned lower than the shaft of the throttle valve.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic illustration of a stratified scavenging two-stroke engine that incorporates a carburetor according to the present invention;

(2) FIG. 2 is perspective view of an appearance of the carburetor according to the embodiments;

(3) FIG. 3 illustrates a carburetor according to a first embodiment, in which FIG. 3(A) is a longitudinal sectional view of the carburetor and FIG. 3(B) is an end view of the carburetor viewed from an upstream side;

(4) FIG. 4 illustrates a carburetor according to a second embodiment, in which FIG. 4(A) is a longitudinal sectional view of the carburetor and FIG. 4(B) is an end view of the carburetor viewed from the upstream side;

(5) FIG. 5 illustrates a carburetor according to a third embodiment, in which FIG. 5(A) is a longitudinal sectional view of the carburetor and FIG. 5(B) is an end view of the carburetor viewed from the upstream side;

(6) FIG. 6 illustrates a carburetor according to a fourth embodiment, in which FIG. 6(A) is a longitudinal sectional view of the carburetor and FIG. 6(B) is an end view of the carburetor viewed from the upstream side;

(7) FIG. 7 illustrates a carburetor according to a fifth embodiment, in which FIG. 7(A) is a longitudinal sectional view of the carburetor and FIG. 7(B) is an end view of the carburetor viewed from the upstream side;

(8) FIG. 8 illustrates a carburetor according to a sixth embodiment, in which FIG. 8(A) is a longitudinal sectional view of the carburetor and FIG. 8(B) is an end view of the carburetor viewed from the upstream side;

(9) FIG. 9 illustrates a carburetor according to a seventh embodiment, in which FIG. 9(A) is a longitudinal sectional view of the carburetor and FIG. 9(B) is an end view of the carburetor viewed from the upstream side;

(10) FIG. 10 illustrates a carburetor according to an eighth embodiment, in which FIG. 10(A) is a longitudinal sectional view of the carburetor and FIG. 10(B) is an end view of the carburetor viewed from the upstream side;

(11) FIG. 11 illustrates a carburetor according to a ninth embodiment, in which FIG. 11(A) is a longitudinal sectional view of the carburetor and FIG. 11(B) is an end view of the carburetor viewed from the upstream side;

(12) FIG. 12 illustrates a carburetor according to a tenth embodiment, in which FIG. 12(A) is a longitudinal sectional view of the carburetor and FIG. 12(B) is an end view of the carburetor viewed from the upstream side;

(13) FIG. 13 illustrates a carburetor according to an eleventh embodiment, in which FIG. 13(A) is a longitudinal sectional view of the carburetor and FIG. 13(B) is an end view of the carburetor viewed from the upstream side;

(14) FIG. 14 illustrates a carburetor according to a twelfth embodiment, in which FIG. 14(A) is a longitudinal sectional view of the carburetor and FIG. 14(B) is an end view of the carburetor viewed from the upstream side;

(15) FIG. 15 illustrates a carburetor according to a thirteenth embodiment, in which FIG. 15(A) is a longitudinal sectional view of the carburetor and FIG. 15(B) is an end view of the carburetor viewed from the upstream side;

(16) FIG. 16 illustrates a carburetor according to a fourteenth embodiment, in which FIG. 16(A) is a longitudinal sectional view of the carburetor and FIG. 16(B) is an end view of the carburetor viewed from the upstream side;

(17) FIG. 17 illustrates a carburetor according to a fifteenth embodiment, in which FIG. 17(A) is a longitudinal sectional view and FIG. 17(B) is an end view of the carburetor viewed from the upstream side;

(18) FIG. 18 illustrates a carburetor according to a sixteenth embodiment, in which FIG. 18(A) is a longitudinal sectional view of the carburetor and FIG. 18(B) is an end view of the carburetor viewed from the upstream side;

(19) FIG. 19 illustrates a carburetor according to a seventeenth embodiment, in which FIG. 19(A) is a longitudinal sectional view of the carburetor and FIG. 19(B) is an end view of the carburetor viewed from the upstream side; and

(20) FIG. 20 illustrates a conventional and typical carburetor, in which FIG. 20(A) is a plan view of a throttle valve in a fully-opened state and FIG. 20(B) is a longitudinal sectional view of the carburetor.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

(21) Preferred embodiments of the present invention are described below with reference to the accompanying drawings.

(22) FIG. 1 schematically illustrates a stratified scavenging two-stroke engine that incorporates a carburetor according to the present invention. An engine 1 includes a piston-valve-type engine body 2, a carburetor 4, and an air cleaner 6, and the engine body 2 and the carburetor 4 are coupled to each other via an intake member 8. The intake member 8 may be configured in its longitudinal direction as one single component, or may be configured by more than one component.

(23) The engine body 2 is a stratified scavenging type engine. There are various specific configurations of stratified scavenging two-stroke engines. The mechanism and functionality of stratified scavenging engines are described in detail in Japanese Patent Laid-Open No. JP2002-227653 (U.S. Pat. Appln. No. 2002/139326 A1)and International Publication No. WO 98/57053, (U.S. Pat. No. 6,289,856)the contents of which are incorporated herein by reference.

(24) The features of a stratified scavenging two-stroke engine may be summarized as follows: The stratified scavenging two-stroke engine includes, as in the case of typical two-stroke engines, a scavenging passage whose lower end communicates with a crankcase and whose upper end communicates with a combustion chamber. Further, the crankcase is filled with an air-fuel mixture. The air-fuel mixture in the crankcase is introduced via the scavenging passage into the combustion chamber.

(25) Referring to FIG. 1 again, the intake member 8 includes a partition wall 8a extending and continuing in its longitudinal direction. The intake member 8 includes a fresh air passage 10 and an air-fuel mixture passage 12 which are partitioned by the partition wall 8a. The fresh air passage 10 can communicate with an upper portion of the above-described scavenging passage via a piston groove 19 or a reed valve. The air-fuel mixture passage 12 communicates with the crankcase 14, and the communication between the air-fuel mixture passage 12 and the crankcase 14 is controlled by a piston 16.

(26) The engine body 2 is configured to introduce, in a scavenging stroke, leading air (fresh air) that does not contain any fuel component into the combustion chamber 18 immediately before introducing the air-fuel mixture of the crankcase 14 into a combustion chamber 18, in other words, at an early stage of the scavenging stroke.

(27) FIG. 2 is a perspective view of an appearance of the carburetor 4 according to the embodiments. Examples of portable working machines that incorporate the engine of the embodiments may include a chain saw, a trimmer, a power blower, an engine type pump, a small generator, and agrochemical sprayer, etc. Referring to FIG. 2, those skilled in the art would readily understand that the carburetor 4 is compact.

(28) Various exemplary carburetors 4 are illustrated in FIGS. 3 to 6 in which reference numerals 401 to 404 are assigned to the carburetors 4 according to various exemplary embodiments, respectively. The carburetors 401 to 404 according to the first to fourth embodiments illustrated in FIGS. 3 to 6 include a choke valve 24 and a throttle valve 22 that are arranged in an intake air passage 20, and a venturi section 26 provided between the choke valve 24 and the throttle valve 22.

(29) Ports that discharge fuel into the intake air passage 20 may include as in the case of a conventional carburetor, a main nozzle 30 pertaining to a main system and slow ports 32 pertaining to a slow system. The main nozzle 30 is positioned in position near the top of the venturi section 26. The main nozzle 30 may be substituted by a main port. The slow ports 32 are positioned in position near the circumference of the throttle valve 22 in its fully-closed state. The slow ports 32 comprise first to third idle ports 34-1 to 34-3.

(30) The first idle port 34-1 is called a primary idle port. The first idle port 34-1 is positioned on the downstream side in the direction of air flow. The third idle port 34-3 is positioned on the upstream side in the direction of air flow. The second idle port 34-2 is positioned between the first idle port 34-1 and the third idle port 34-3.

(31) In an idle operation, the throttle valve 22 is placed in a fully-closed state. In this state, fuel is supplied via the first idle port 34-1. In a partial operation, the throttle valve 22 is placed in a half-opened state. In this state, the fuel is supplied not only via the first to third idle ports 34-1 to 34-3 but also via the main nozzle 30.

(32) In a high-speed operation, the throttle valve 22 is placed in a fully-opened state. The high-speed operation is called full throttle (full-open) operation. In the high-speed operation, as in the case of the above-described partial operation, the fuel is supplied via the first to third idle ports 34-1 to 34-3 and the main nozzle 30. In the high-speed operation, a large amount of fuel is supplied to the intake air passage 20. Accordingly, in the high-speed operation, the fuel that is supplied via the main nozzle 30 positioned at the top of the venturi section 26 accounts for the primary part of the whole amount of fuel supplied to the intake air passage 20.

(33) In the carburetors 401 to 404 according to the first to fourth embodiments illustrated in FIGS. 3 to 6, a fresh air passage 10 is formed on upper side of the intake passage 20 and an air-fuel mixture passage 12 is formed on down side of the intake passage 20 by the throttle valve 22 and the choke valve 24 that are both in the fully-opened state. The carburetors 401 to 404 according to the first to fourth embodiments include a first air flow guiding member 50 between the throttle valve 22 and the choke valve 24. The air flow guiding member 50 is arranged in the fresh air passage 10.

(34) The first air flow guiding member 50 illustrated in FIGS. 3 to 6 includes a horizontal plate part 50a positioned above and near the throttle valve 22 in the fully-opened state. The horizontal plate part 50a is in parallel with the plate surface of the throttle valve 22 in the fully-opened state. In addition, the horizontal plate part 50a extends in a transverse direction with respect to the intake air passage 20.

(35) In the illustrated carburetors 401 to 404, both ends of the horizontal plate part 50a in its width direction are in abutment with the wall surface defining the intake air passage 20. As a variation of the first air flow guiding member 50, the first air flow guiding member 50 may include a horizontal plate part 50a and a suspended support part 50b (indicated by virtual lines) extending upward from the both ends in the width direction of the horizontal plate part 50a.

(36) FIGS. 3 to 6 indicate that the horizontal plate part 50a may take various length dimensions (the length defined with respect to the air flow direction A). As can be appreciated from FIGS. 3(A), 4(A), 5(A) and 6(A) illustrating the cross sections of the throttle valve 22 and the choke valve 24 that are both in the fully-opened state, the upstream end of the horizontal plate part 50a is positioned such that the upstream end does not interfere with the downstream-side edge of the choke valve 24. The horizontal plate part 50a extends to the upstream-side end of the throttle valve 22, and, in the plan view, the downstream-side end of the horizontal plate part 50a overlaps with at least the upstream-side end of the throttle valve 22.

(37) The carburetor 401 according to the first embodiment (FIG. 3) has a length dimension such that the downstream-side end of the horizontal plate part 50a extends to the upstream-side end of the throttle valve 22. The carburetors 402 and 403 according to the second and third embodiments (FIGS. 4 and 5) have a length dimension such that the downstream-side end of the horizontal plate part 50a extends to the downstream side to a larger extent than that of the carburetor 401 according to the first embodiment. The carburetor 404 according to the fourth embodiment (FIG. 6) has a length dimension such that the downstream-side end of the horizontal plate part 50a extends to an extent that it reaches the vicinity of the throttle valve shaft 22a.

(38) As can be appreciated from FIGS. 3(A) to 6(A), the upper surface of the plate-like horizontal plate part 50a is positioned above the throttle valve shaft 22a.

(39) When the first air flow guiding member 50 includes the horizontal plate part 50a having its length extending in the gas flow direction A and the plate-like suspended support part 50b (indicated by virtual lines) extending upward from the both ends in the width direction of the horizontal plate part 50a, then, as can be best appreciated from FIGS. 3(A) to 6(B), the upstream-side edge of the plate-like suspended support part 50b may take an arc shape that is concave with respect to the downstream side (FIGS. 4 and 6) or may take a straight shape (FIGS. 3 and 5) such that the plate-like suspended support part 50b does not interfere with opening/closing movement of the choke valve 24.

(40) In addition, the upstream-side edge of the plate-like suspended support part 50b may have a contour shape extending from the horizontal plate part 50a and being inclined toward the upstream side (FIGS. 4 to 6), or may have a contour shape that extends upward from the horizontal plate part 50a (FIG. 3).

(41) The downstream-side edge of the plate-like suspended support part 50b may have a contour shape extending from the horizontal plate part 50a and being inclined toward the downstream side (FIGS. 4 to 6), or may have a contour shape that extends upward from the horizontal plate part 50a (FIG. 3).

(42) The right and left suspended support parts 50b illustrated by virtual lines in FIGS. 3(B) to 6(B) may be parallel to each other as illustrated, or may have dimensions widening upward.

(43) According to the carburetors 401 to 404 of the first to fourth embodiments (FIGS. 3 to 6), the flow direction of the air passing through the fresh air passage 10 is adjusted to correspond to the direction along the axis 20a of the intake air passage 20 of the carburetors 401 to 404 by virtue of the horizontal plate part 50a of the first air flow guiding member 50. Since the upper surface of the horizontal plate part 50a is positioned above the throttle valve shaft 22a, the fresh air guided by the horizontal plate part 50a is allowed to be directed to the engine without being disturbed by the throttle valve shaft 22a.

(44) In the piston-valve-type two-stroke engine body 2, the crankcase 14 is first filled with the air-fuel mixture in the process of the piston 16 leaving the bottom dead center and moving up, and then fresh air is supplied to the scavenging passage. Accordingly, in the fresh air passage 10 and the air-fuel mixture passage 12 in the carburetors 401 to 404 according to the first to fourth embodiments, the flow of air-fuel mixture is first created in the air-fuel mixture passage 12, and then the flow of air is created in the fresh air passage 10.

(45) Referring to FIGS. 3(A) to 6(A), when the flow of the air-fuel mixture in the air-fuel mixture passage 12 is generated, air enters the air-fuel mixture passage 12 from the fresh air passage 10 via a gap between the downstream-side end of the choke valve 24 and the horizontal plate part 50a. This implies that the at least delivery ratio of the engine body 2 is increased.

(46) FIGS. 7 and 8 illustrate carburetors 405 and 406 according to fifth and sixth embodiments. The carburetors 405 and 406 according to the fifth and sixth embodiments include a second air flow guiding member 52, and the second air flow guiding member 52 is arranged in the air-fuel mixture passage 12. The second air flow guiding member 52 has a tunnel shape that extends straight in the flow direction A of the intake air. The cross-sectional shape of the second air flow guiding member 52 may be defined as appropriate.

(47) The illustrated carburetors 405 and 406 according to the fifth and sixth embodiments include the above-described first air flow guiding member 50 whose functions and effects have been described above.

(48) The second air flow guiding member 52 is arranged in a position in association with the main nozzle 30. The main nozzle 30 may be a main port. When described with reference to the carburetors illustrated in FIGS. 7 to 9, the main nozzle 30 is arranged in an intermediate portion in the longitudinal direction of the second air flow guiding member 52.

(49) The second air flow guiding member 52 sends downstream the whole amount of the fuel discharged from the main nozzle 30 by virtue of the air flow created by the second air flow guiding member 52. As long as this function is effective, any relative positions of the main nozzle 30 and the second air flow guiding member 52 with respect to each other may be employed. For example, the second air flow guiding member 52 may be arranged such that the main nozzle 30 is positioned near the upstream end of the second air flow guiding member 52.

(50) The downstream end of the second air flow guiding member 52 may be positioned between the choke valve 24 and the throttle valve 22 (FIG. 7), or may be positioned at a position at which it overlaps with the upstream-side end of the throttle valve 22 (FIG. 8). It should be noted that it is necessary to design the length of the second air flow guiding member 52 such that the downstream end of the second air flow guiding member 52 does not interfere with opening/closing of the throttle valve 22.

(51) A third air flow guiding member 54, which is illustrated in FIG. 7 as one example, may be provided in the carburetors 405 and 406 according to the fifth and sixth embodiments. The third air flow guiding member 54 has a plate-like shape arranged in the air-fuel mixture passage 12. Specifically, the third air flow guiding member 54 is arranged below and near the choke valve 24, and arranged in parallel with the choke valve 24 in its fully-opened state.

(52) The lower surface of the plate-like third air flow guiding member 54 is positioned below the choke valve shaft 24a. The upstream-side end of the third air flow guiding member 54 may optionally be positioned near the choke valve shaft 24a. The third air flow guiding member 54 may optionally extend to the downstream side to a larger extent than the downstream-side edge of the choke valve 24. It should be noted that it is necessary to design the length of the third air flow guiding member 54 such that the downstream end of the third air flow guiding member 54 does not interfere with opening/closing of the throttle valve 22.

(53) The above-described third air flow guiding member 54 may be provided in the carburetors 401 to 404 of the above-described first to fourth embodiments.

(54) The carburetors 405 and 406 according to the fifth and sixth embodiments (FIGS. 7 and 8) makes it possible to prevent the whole amount of the fuel discharged via the main nozzle 30 from being diffused from the air-fuel mixture passage 12 to the fresh air passage 10 in the carburetor by virtue of the tunnel-shaped second air flow guiding member 52. In other words, the whole amount of the fuel discharged from the main nozzle 30 is allowed to be directed from the air-fuel mixture passage 12 in the carburetor to the engine side.

(55) In addition, by virtue of the plate-like third air flow guiding member 54 arranged below and near the choke valve 24, it is made possible to adjust the flow of gas in the air-fuel mixture passage 12 in the carburetor such that the gas flows along the axis 20a of the intake air passage 20, and thereby prevent entry of the air-fuel mixture flowing in the air-fuel mixture passage 12 into the fresh air passage 10 in the carburetor.

(56) Although the carburetors 405 and 406 of the above-described fifth and sixth embodiments (FIGS. 7 and 8) include the first air flow guiding member 50 in the fresh air passage 10, the first air flow guiding member 50 may be omitted. Examples in which the first air flow guiding member 50 is omitted are illustrated in FIGS. 9 to 12.

(57) A carburetor 407 according to a seventh embodiment (FIG. 9) includes the above-described second air flow guiding member 52. With regard to the second air flow guiding member 52, the top plate part 52a is provided in parallel with the choke valve 24, but the top plate member 52a may be obliquely provided with respect to the axis 20a of the intake air passage 20 (FIG. 10). Specifically, as in the case of a carburetor 408 according to an eighth embodiment illustrated in FIG. 10, the top plate part 52a may be obliquely provided such that the top plate part 52a becomes more spaced away from the axis 20a as it extends toward the downstream side.

(58) FIG. 11 illustrates a carburetor 409 according to a ninth embodiment. The carburetor 409 according to the ninth embodiment includes the above-described second and third air flow guiding members 52 and 54. The top plate part 52a of the second air flow guiding member 52 illustrated in FIG. 11 is in parallel with the choke valve 24, but there may be provided the inclined top plate part 52a described with reference to FIG. 10.

(59) The carburetor 409 according to the ninth embodiment further includes a fourth air flow guiding member 56. The fourth air flow guiding member 56 has a plate-like shape, and extends in parallel with the throttle valve 22 such that it resides below and near the throttle valve 22. The lower surface of the fourth air flow guiding member 56 is positioned below the throttle valve shaft 22a.

(60) By virtue of the plate-like fourth air flow guiding member 56, the flow of the air-fuel mixture in the air-fuel mixture passage 12 is adjusted. It will be appreciated that the fourth air flow guiding member 56 may be provided in other carburetors such as the carburetors 401 to 408 according to the first to eighth embodiments.

(61) FIG. 12 illustrates a carburetor 410 according to a tenth embodiment. The carburetor 410 according to the tenth embodiment includes a fifth air flow guiding member 58 arranged in the air-fuel mixture passage 12. The fifth air flow guiding member 58 has a configuration integrating the above-described second air flow guiding member 52 and the third air flow guiding member 54. Specifically, the fifth air flow guiding member 58 includes a horizontal plate part 58a extending below and near the choke valve 24, and the horizontal plate part 58a extends from the vicinity of the choke valve shaft 24a to the vicinity of the upstream edge of the throttle valve 22.

(62) The fifth air flow guiding member 58 includes a relatively short tunnel section 58b fixed to the horizontal plate part 58a. The tunnel section 58b is positioned to face the main nozzle 30. The lower surface of the tunnel section 58b is configured by a curved surface that is downward convex when the carburetor 410 is viewed in its longitudinal section.

(63) In a modified example, the fifth air flow guiding member 58 may include right and left upright walls 58c, so that the above-described horizontal plate part 58a is coupled to the upper ends of the upright walls 58c.

(64) The main nozzle 30 of the carburetors 407 to 410 according to the above-described seventh to tenth embodiment may be substituted by a main port.

(65) FIGS. 13 to 16 illustrate carburetors 411 to 414 according to eleventh to fourteenth embodiments that do not include a choke valve. Referring to FIG. 13, the carburetor 411 according to the eleventh embodiment includes the above-described second air flow guiding member 52 having the tunnel shape in the air-fuel mixture passage 12. The second air flow guiding member 52 may take any cross-sectional shape and length as appropriate, as discussed in the foregoing. The main nozzle 30 is positioned facing the second air flow guiding member 52. The main nozzle 30 may be substituted by a main port. The reference numeral 26 denotes the above-described venturi section, and the main nozzle 30 is arranged in the venturi section 26.

(66) FIG. 14 illustrates the carburetor 412 according to the twelfth embodiment. The carburetor 412 according to the twelfth embodiment includes, in addition to the second air flow guiding member 52, the plate-like third and fourth air flow guiding members 54 and 56 as described above. It will be appreciated that either of the third and fourth air flow guiding members 54 and 56 may be omitted.

(67) The tunnel-like second air flow guiding member 52 may take, as described above, any cross section and length as appropriate. An example thereof is illustrated in FIG. 15. The second air flow guiding member 52 provided in the carburetor 413 of the thirteenth embodiment illustrated in FIG. 15 has a length that extends from the upstream end of the carburetor 413 to a position where the second air flow guiding member 52 does not interfere with the throttle valve 22. In addition, the upstream end, which is an inlet, of the second air flow guiding member 52 has a shape widening from the upper end of the opening of the intake air passage 20 toward the lower end thereof. In the carburetor 413 according to the thirteenth embodiment, as can be appreciated from FIG. 15(B), the inlet and the outlet of the second air flow guiding member 52 have an inverted V shape, but their shapes are not limited to this exemplary shape. The second air flow guiding member 52 has a shape like a slope in which the ridge is inclined frontward and downward when viewed in a lateral direction.

(68) According to the carburetor 413 of the thirteenth embodiment, the inlet of the second air flow guiding member 52 is larger than the outlet (downstream end) thereof, so that it is made possible to take in larger amount of air and thereby create the air-fuel mixture using the larger amount of air. In addition, the uniformity of the flow direction of the air-fuel mixture coming out of the second air flow guiding member 52 is effectively maintained.

(69) FIG. 16 illustrates the carburetor 414 according to the fourteenth embodiment. The carburetor 414 according to the fourteenth embodiment includes the above-described fifth air flow guiding member 58 (FIG. 12). Any position of the upstream end of the horizontal plate part 58a of the fifth air flow guiding member 58 may be employed as appropriate; for example, the upstream end of the horizontal plate part 58a may protrude outward from the upstream end of the carburetor 414 as illustrated in this figure. It will also be appreciated that the upstream end of the horizontal plate part 58a may coincide with the upstream end of the carburetor 414.

(70) The main nozzle 30 of the carburetors 411 to 414 (FIGS. 13 to 16) without the choke valve according to the above-described eleventh to fourteenth embodiments may be substituted by a main port. In addition, although the carburetors 411 to 414 according to the eleventh to fourteenth embodiments include the venturi section 26, the venturi section 26 may be omitted. Experiments demonstrate that the carburetors 411 to 414 sufficiently exert their effective functions in the absence of the venturi section.

(71) FIGS. 17 to 19 illustrate carburetors 415 to 417 that do not include a venturi section. FIG. 17 illustrates the carburetor 415 according to the fifteenth embodiment. The carburetor 415 according to the fifteenth embodiment adopts the above-mentioned main port. The reference numeral 36 in FIG. 17 denotes the main port. In the carburetor 415 according to the fifteenth embodiment, the second air flow guiding member 52 is arranged to face the main port 36.

(72) FIG. 18 illustrates a carburetor 416 according to a sixteenth embodiment. The second air flow guiding member 52 included in the carburetor 416 according to the sixteenth embodiment has a tapered shape, so that the outlet on the downstream side is smaller than the inlet on the upstream side. By virtue of this configuration, it is made possible to effectively maintain the uniformity of the direction in which the air-fuel mixture coming out of the second air flow guiding member 52 flows, which is the flow direction of the air-fuel mixture. In addition, the carburetor 416 according to the sixteenth embodiment includes the above-described plate-like third air flow guiding member 54 positioned on the upstream side of the throttle valve 22. The downstream end of the third air flow guiding member 54 is positioned at a position that is near the throttle valve 22 but does not interfere with the throttle valve 22. Any position of the upstream end of the third air flow guiding member 54 may be employed as appropriate; for example, the upstream end of the third air flow guiding member 54 may be positioned at a position in alignment with the upstream end of the carburetor 416. Meanwhile, in this embodiment, the upstream end of the third air flow guiding member 54 is positioned at a position such that the upstream end protrudes more upstream than the upstream end of the carburetor 416.

(73) FIG. 19 illustrates a carburetor 417 according to a seventeenth embodiment. The carburetor 417 according to the seventeenth embodiment includes, in addition to the second air flow guiding member 52, the above-described plate-like third air flow guiding member 54. In addition, the second air flow guiding member 52 provided in the carburetor 417 according to the seventeenth embodiment is relatively elongated, and the downstream-side outlet of the second air flow guiding member 52 is smaller than the upstream-side inlet thereof.

(74) FIGS. 17 to 19 mentioned above disclose the carburetors 415 to 417 according to the fifteenth to seventeenth embodiments that do not include the venturi section. Nevertheless, these carburetors 415 to 417 according to the fifteenth to seventeenth embodiments may include the venturi section. In addition, although the carburetors 415 to 417 according to the fifteenth to seventeenth embodiments include the main port 36, the carburetors 415 to 417 may include the above-described main nozzle 30 instead of the main port 36.

(75) In any one of the above-described carburetors 401 to 417, when the throttle valve 22 is fully opened, a portion of the air of the intake air that entered the intake air passage 20, that is, a portion of air flowing through the fresh air passage 10 is allowed to enter the air-fuel mixture passage 12 from the upstream side of the throttle valve 22. Thus, it is made possible to increase the amount of air-fuel mixture supplied to the engine.

(76) While the invention has been described with reference to the specific exemplary embodiment, it will be apparent to those skilled in the art that various changes and modifications can be made to the specific embodiment without departing from the spirit and scope of the invention as defined in the claims.

REFERENCE SIGNS LIST

(77) A Flow of gas in an intake air passage 1 Stratified scavenging engine 4 Carburetor 10 Fresh air passage 12 Air-fuel mixture passage 14 Crankcase 16 Piston 18 Combustion chamber 20 Intake air passage 20a Axis of intake air passage 22 Throttle valve 22a Throttle valve shaft 24 Choke valve 24a Choke valve shaft 26 Venturi section 30 Main nozzle 36 Main port 50 First air flow guiding member 52 Second air flow guiding member (tunnel shape) 54 Plate-like third air flow guiding member 56 Plate-like fourth air flow guiding member 58 Fifth air flow guiding member