CARBURETOR AND VALVE MECHANISM

Abstract: A valve mechanism (60) for opening and closing a fluid passage (90) in a carburetor comprises, a movable member (69) for selectively opening and closing the fluid passage in the carburetor, and means capable of biasing the member (69) into a position to close the fluid passage. A two-position cam means (76,77) is provided which acts between the carburetor and the movable member (69) so that a first position of the cam means defines a closed condition of the fluid passage and a second position of the cam means defines an open condition of the fluid passage.

Description: The present invention relates to a carburetor comprising a main body portion defining a throttle means comprising a venturi and a throttle bore and having an air intake side and an engine outlet side; a fuel pump; a fuel inlet supply channel for conveying fuel from the fuel pump to the throttle means; a throttle shutter mounted within the throttle means between the air intake side and the engine outlet side; a metering chamber for supplying fuel from the fuel pump into the throttle means via a main discharge port and at least one idle discharge port, the main discharge port opening into the throttle means on the air intake side of the throttle shutter; and a choke feed passage having an inlet port and an outlet port; said inlet port is in direct fluid communication with the fuel inlet supply channel; said outlet port is in direct fluid communication with the throttle bore; said inlet port is in direct fluid communication with said outlet port; means mounted directly in a housing is provided for selectively opening and closing the choke feed passage which means comprises a movable member; and means capable of biasing the member into a position to close the choke feed passage. Such a carburetor is disclosed in Patent Specification No. US-A-4,554,896 which forms the basis of the precharacterizing portion of appended claim 1 and which relates to a cold enrichment device for an internal combustion engine having a pair of parallel flow paths communicating with the engine for delivering cold enrichment fuel. The cold enrichment fuel is delivered downstream of the carburetor into the manifold induction passage, the crankcase chamber or the scavenge passages. The invention has particular application with carburetors of the HU or HS type used in chainsaws. It has been the practice on chainsaws to which such carburetors have been fitted, to fit a choke within the air filter housing or in the carburetor body which consists essentially of a shutter blade which is selectively positionable in front of the carburetor venturi. This, however, has its disadvantages. In particular, since the choke is operated from the main fuel discharge port (main jet), if the latter is incorrectly adjusted great difficulty may be encountered starting the engine due to the incorrect air/fuel ratio. In published European Patent Specification EP-A-0,253,469 the contents of which are incorporated herein by reference, there is described a carburetor having an improved choking mechanism. This choking mechanism comprises a choke feed passage extending from the metering chamber to the throttle bore, the choke feed passage opening into the throttle bore on the opposite side of the throttle shutter to the main discharge port, and means for selectively opening and closing the choke feed passage. When the throttle shutter is in a partially opened position and the choke feed passage is open, a lower pressure in the throttle bore on the engine outlet side of the throttle shutter than on the air intake side, causes fuel to be drawn from the metering chamber primarily through the choke feed passage into the throttle bore on the engine outlet side of the throttle shutter. Although this mechanism operates reasonably satisfactorily it has one disadvantage in that the throttle shutter angle is very critical and has to be set very accurately to achieve the required vacuum to draw sufficient fuel through the choke feed passage. Patent Specification No. US-A-4616805 discloses a valve for shutting off the fuel supply from a tank when an engine is not in use. The valve comprises a non-circular shaft which is sprung such that one end normally closes a fuel line aperture. The other end of the shaft has a protruding radial pin which rests against the helical cam surface of a rotatable collar. Thus, when rotating the collar, the pin rides up the cam surface and the shaft is moved so as to open the valve. Patent Specification No. US-A-2510393 discloses a fluid valve comprising two coaxial cams having helical surfaces. A valve member is attached to one of the cams and a spring biases that cam so as to maintain the valve member in a closed state. In operation, a handle is provided to rotate one cam against the other, thereby causing the cams to ride up against one another and move the valve member to an open state. According to the invention there is provided a carburetor characterised in that the outlet of the choke feed passage opens upstream of the throttle shutter and in that the means for selectively opening and closing the choke feed passage is provided in said carburetor main body portion; there is provided a two-position rotatable cam means acting between the carburetor body portion and the movable member, said cam means comprises a first cam surface securable to the carburetor body portion and a second cam surface engaged with the movable member, such that a first position of the cam means defines a closed condition of the choke feed passage and a second position of the cam means defines an open condition of the choke feed passage, such that when the throttle shutter is in a partially open position and the choke feed passage is open, the fuel pump causes fuel to be injected through the choke feed passage into the throttle means upstream of the throttle shutter. Preferably the choke feed passage comprises a bore formed in the main body portion of the carburetor, extending from the fuel inlet supply channel to the throttle bore. Preferably the opening is located in the region of the notional intersection of the venturi and throttle bore. Preferably the movable member is axially slidable during movement of the cam means between its two positions. Preferably one of the cam surfaces is rotatable relative to the other cam surface. Preferably the biasing means comprises a coil spring. Preferably said first cam surface is provided on a bushing securable in the carburetor body portion and said second cam surface is provided on a control level engaged with the movable member. Preferably the biasing means is located in a recess in the bushing and acts between a collar on the movable member and a sealing member also located in the recess in the bushing. One advantage of the invention as compared to the prior art is that a direct choke system is used, i.e. it is substantially independent of the setting of the main jet adjustment. Consequently, the engine is easier to prime and choking will always work irrespective of the main jet setting. A particular advantage of the invention is that the throttle shutter angle is not critical in choking operation because the fuel is injected under pressure from the pump, rather than solely under the influence of vacuum. Embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross-section of an embodiment of a carburetor according to the present invention in its choked condition; Figure 2 is a cross-sectional view of a valve mechanism in a closed condition according to the invention but not showing the control lever; Figure 3 is a diagrammatic view of the position of the control level and bushing in the closed condition of the valve mechanism; and Figure 4 is a diagrammatic view of the position of the control level and bushing in an open condition of the valve mechanism. Referring now to Figure 1, there is shown a cross-sectional diagram of a carburetor according to the present invention, and the parts indicated by the reference numerals in Figure 1 are identified in the following list: 1. Filtering screen. 2. Venturi. 3. Fuel chamber. 4. Pulse chamber. 5. Fuel pump diaphragm. 5a Pump diaphragm inlet valve. 5b Pump diaphragm outlet valve. 6. Fuel pump body. 7. Fuel pump gasket. 8. Fuel inlet. 9. Impulse channel. 10. Throttle shutter. 11a. Primary idle discharge port. 11b. Secondary idle discharge port. 12. Main carburetor body. 13. Idle fuel adjustment. 14. Metering chamber. 15. Idle fuel adjustment orifice. 16. Diaphragm cover. 17. Metering diaphragm. 18. Atmospheric chamber. 19. Atmospheric vent. 20. Main fuel adjustment orifice. 21. Inlet tension spring. 22. Fulcrum pin. 23. Diaphragm gasket. 24. Inlet control lever. 25. Main fuel adjustment. 26. Inlet needle. 27. Main nozzle discharge port. 28. Fuel inlet supply channel. Since such carburetors are well known in the art, a full description of the operation thereof is not considered necessary and reference should be made to European Patent Specification EP-A-0,253,469 for a full description. The following description will therefore describe the main difference between the mechanism described in European Patent Specification EP-A-0,253,469 and the present invention. Referring now to Figure 1 there is shown an embodiment of a Carburetor according to the present invention having a different choking mechanism. In Figure 1, parts which serve a like function to parts described in European Patent Specification EP-A-0,253,469 have been given the same reference numerals. It will be noted that the choke feed hole 31 described in European Patent Specification EP-A-0,253,469 has been omitted together with its associated parts, the bore 34, and plunger 32. In the present invention, a choke feed passage 40 extends from the fuel inlet supply channel 28 into the throttle bore 2 a . The choke feed passage 40 is formed in a solid part of the carburetor body. The choke feed passage 40 has an opening 41 which is located in the air intake side of the throttle bore 2 a and more particularly in the region of the notional intersection 42 of the venturi 2 and throttle bore 2 a . It has been found that the location of the opening 41 at the notional intersection of the venturi 2 and throttle bore 2 a provides a particularly effective operation of the carburetor and of an engine on which it is used. Suitable valve means are provided for selectively opening and closing the choke feed passage 40 that being an on/off valve mechanism as described below. To choke the engine, the throttle shutter 10 is partially but not fully cracked, and the choke feed passage 40 is opened. This is the condition of the carburetor in Figure 1. As the engine is cranked, engine vacuum is transmitted to the metering chamber 14 through the idle discharge ports 11 a and 11 b creating a low pressure on the fuel side 14 of the diaphragm 17. Atmospheric pressure in the chamber 18 forces the diaphragm 17 upwards to open the inlet needle 26. This permits fuel to enter the metering chamber 14 and from there to enter the venturi 2/throttle bore 2 a , through the orifice 15 and idle discharge ports 11 a and 11 b for supply to the engine. In addition fuel is injected through the choke feed passage 40 from the fuel inlet supply channel 28, under pressure from the fuel pump 5, into the throttle bore 2 a . When the engine has reached its operating temperature, the carburetor is operated in normal fashion by opening up the throttle shutter 10 and simultaneously closing the choke feed passage 40, whereby the main discharge port 27 comes into operation. The basic difference between the present choking mechanism and that described in European Patent Specification EP-A-0,253,469 is that in the present mechanism fuel is injected under pressure directly from the pump 5 rather than being solely induced from the metering chamber under vacuum. This positive injection of fuel under pressure has been found to provide an increased reliability of operation of the carburetor. Of course, the vacuum induced in the carburetor when the engine is cranked also assists in the drawing of fuel into the carburetor via the choke feed passage 40. In addition, it will be appreciated that the setting of the throttle shutter 10 is not critical as it is in the mechanism of the abovementioned European Patent Specification. It is to be understood that the diameter of the orifice of the choke feed passage 40 must be properly selected to ensure optimum engine starting and continued running with the choke on. Referring now to Figures 2 to 4 of the drawings, there is shown therein the valve mechanism generally indicated at 60 according to the invention. The valve mechanism 60 comprises a carburetor body portion 12 having an inlet port 28, a valve seat 63 and an outlet port 41. A plastic bushing 65 is a push fit into a recess 66 in the carburetor body portion 12. The bushing 65 has a recess 67 opening into a larger recess 68 and a needle or valve member 26 is axially slidable in the recesses 67, 68. The needle or valve member 26 has integrally formed thereon a collar 70 which is a sliding fit in recess 68. An 'O' ring seal 71 is located in the recess 68 at the opposite end of the recess 68 from the collar 70 of the needle 69. A compression spring 72 acts between the 'O' ring 71 and the collar 70 to resiliently bias the tip 73 of the needle 26 against the valve seat 63. The tip 73 of the needle 26 is preferably of a hardened rubber material. The 'O' ring 71 is thus compressed by the coil spring 72 to provide an excellent fluid seal between the needle 26 and the bushing 65 particularly when the needle is moved to the left in Figure 2. The free end 74 of the needle 26 is engaged with or fixed to control lever 75. The control lever 75 and the bushing 65 have cooperating slanted cam surfaces 76, 77 respectively. In an off or closed position of the valve mechanism 60, as shown in Figure 2, the control lever 75 is in the position of Figure 3 with the two cam surfaces 76, 77 just slightly spaced apart from each other. To open the valve mechanism the control lever 75 is rotated through 180° to the position shown in Figure 4 in which the needle 26 is moved axially away from the valve seat 63 to permit fluid flow from the inlet port 62 to the outlet port 41 via a channel 90. As the control lever 75 is rotated, the cam surfaces 76, 77 engage each other and the control lever 75 and needle 26 are moved to the left in Figure 2 so that the tip 73 of the needle 26 moves away from the valve seat 63. As the needle 26 moves away from the valve seat 63 the coil spring 72 is compressed by the collar 70. The control lever 75 may be rotated manually or automatically. To close the valve mechanism again, the control lever 75 is again rotated to the position of the Figure 3. The valve mechanism is thus used in a carburetor for selectively opening and closing a choke feed passage or hole during operation of an engine as is used for example on a chainsaw. Thus, in operation, the valve mechanism 60 is mounted on the main body portion 12 of the carburetor and during choking operation of an engine, with the valve mechanism 60 open, fuel from the fuel inlet supply channel 28 enters inlet port 28 and is injected into the carburetor via the choke feed passage 40 and the fuel outlet port 41. Thus, the valve mechanism 60 provides a simple mechanism which is readily operated, to control the flow of fuel through the choke feed passage 40.

Claims: A carburetor comprising a main body portion (12) defining a throttle means comprising a venturi (2) and a throttle bore (2 a ) and having an air intake side and an engine outlet side; a fuel pump (5); a fuel inlet supply channel (28) for conveying fuel from the fuel pump (5) to the throttle means (2,2 a ); a throttle shutter (10) mounted within the throttle means (2,2 a ) between the air intake side and the engine outlet side; a metering chamber (14) for supplying fuel from the fuel pump (5) into the throttle means (2,2 a ) via a main discharge port (27) and at least one idle discharge port, the main discharge port (27) opening into the throttle means (2,2 a ) on the air intake side of the throttle shutter (10); and a choke feed passage (40) having an inlet port (28) and an outlet port (41); said inlet port (28) is in direct fluid communication with the fuel inlet supply channel (28); said outlet port (41) is in direct fluid communication with the throttle bore (2 a ); said inlet port (28) is in direct fluid communication with said outlet port (41); means (60) mounted directly in a housing is provided for selectively opening and closing the choke feed passage (40) which means (60) comprises a movable member (26); means capable of biasing the member (26) into a position to close the choke feed passage (40); characterised in that the outlet (41) of the choke feed passage opens upstream of the throttle shutter (10) and in that the means (60) for selectively opening and closing the choke feed passage (40) is provided in said carburetor main body portion (12); there is provided a two-position rotatable cam means acting between the carburetor body portion (12) and the movable member (26), said cam means (75) comprises a first cam surface (76) securable to the carburetor body portion (12) and a second cam surface (77) engaged with the movable member (26), such that a first position of the cam means (75) defines a closed condition of the choke feed passage (40) and a second position of the cam means (75) defines an open condition of the choke feed passage (40), such that when the throttle shutter (10) is in a partially open position and the choke feed passage (40) is open, the fuel pump causes fuel to be injected through the choke feed passage (40) into the throttle means upstream of the throttle shutter (10). A carburetor as claimed in claim 1 wherein the choke feed passage (40) comprises a bore (2 a ) formed in the main body portion (12) of the carburetor, extending from the fuel inlet supply channel (28) to the throttle means. A carburetor as claimed in claims 1 or 2 wherein the choke feed passage (40) defines an opening which is located in the region of the notional intersection of the venturi (2) and the throttle bore (2 a ). A carburetor as claimed in any of claims 1-3 wherein the movable member (26) is axially slidable during movement of the cam means (75) between its two positions. A carburetor as claimed in any of claims 1-4 wherein one of the cam surfaces (76) is rotatable relative to the other cam surface (77). A carburetor as claimed in any of claims 1-5 wherein the biasing means comprises a coil spring (72). A carburetor as claimed in any of claims 1-6 wherein said first cam surface (77) is provided on a bushing (65) securable in the carburetor body portion (12) and said second cam surface (76) is provided on a control level engaged with the movable member (26). A carburetor as claimed in claim 6 wherein the biasing means is located in a recess in the bushing (65) and acts between a collar on the movable member (26) and a sealing member also located in the recess in the bushing.

Inventors: BOWLES, ROGER A., KENNY, BRIAN P.
Applicants: TILLOTSON LIMITED
Application year: 1988
Main classification: F02M1
IPC codes: F02M17, F02M19, F02M1