Abstract:
A variable venturi carburetor in which a biasing spring urges a metering needle to contact the metering jet during low air-intake operating conditions and including an expanded portion formed at the front end of the metering needle which is loosely contained in a large bore portion of a well during low air-intake operating conditions and which engages a small bore portion of the well during high air intake operating conditions to center the metering needle in the metering jet for maintaining bleed sensitivity constant while the amount of bleed air varies.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to controlling the air bleed in the metering jet by means of the metering needle in a variable venturi carburetor. 
     As is well known, carburetors of automotive engines can be classified into two categories: a fixed venturi type and a variable venturi type. The latter type of carburetor has come to be used partly on the sports type automobiles and also on regular passenger cars because of its advantages such as good transient response, large amount of intake air and the relatively small height of the unit. 
     The variable venturi carburetors, however, have various points that have yet to be improved, one of which is the variation of bleed sensitivity of the metering needle and the metering jet. 
     In the variable venturi carburetors, the amount of the fuel and bleed air to be fed into the venturi is adjusted by the cooperative action of the metering needle and the metering jet with the metering needle moving back and forth in the metering jet. This cooperative action is also utilized to increase fuel delivery during operation in cold condition. It is desirable in the variable venturi carburetor that the range of air-fuel ratio (bleed sensitivity) be constant through the entire operating condition from low air-intake operation to high air-intake operation. 
     With conventional variable carburetors, however, when the engine is running with a small amount of air being drawn in, the cross-sectional area of the metering needle in the metering jet increases, i.e., the annular effective opening of the metering jet becomes small, so that the shape of the annular effective opening becomes unstable due to vibration. This renders the fuel supply from the jet unstable, causing a large amount of bleed air to enter to reduce the negative pressure at the jet. This greatly affects the fuel flow from the float bowl. 
     A conventional practice to cope with this situation in FIG. 1 is to provide a spring 4 on one side of the needle 5 between the needle holder 3 disposed in the head 2 of the suction piston 1 and the base portion of the metering needle 5 loosely inserted into the main nozzle formed in the venturi 6 opposite to the suction piston head 2. This biasing spring 4 urges the metering needle 5 toward the direction indicated by the arrow to cause it to contact one side of the metering jet 8 to partly close the opening of the air bleed 9 so that the annular effective opening area defined by the metering needle 5 and the metering jet 8 can be maintained constant to stabilize the fuel supply from the main nozzle. 
     Although the biased metering needle 5 has the advantage of stabilizing the bleed sensitivity in the low air-intake range by throttling the air bleed, it does not work in the high air-intake range. That is, as the engine operating condition shifts into the high air-intake region and the metering needle 5 retracts, causing the engine to run at high speed, the bleed sensitivity becomes dull. Therefore, if the air bleed is throttled in the cold operating condition, the air-fuel mixture will not become sufficiently rich. This method has also another disadvantage that the metering needle wears out because of the vibration resulting from the high speed engine revolution. 
     To eliminate the above drawbacks, it may be conceived to equip the carburetor with a special bleed air control device. This, however, makes the structure of the carburetor complex, rendering the maintenance difficult, increasing the production cost. 
     SUMMARY OF THE INVENTION 
     In view of the problem with the bleed sensitivity of the metering needle in the variable venturi carburetor of the conventional technologies, it is the object of this invention to provide a variable venturi carburetor in which the low air-intake region the metering needle is biased to contact the metering jet and in the high air-intake region it is desengaged from and centered in the metering jet by the cooperative action between the expanded portion formed at the end of the metering needle and the large and small bores of the well, thus changing the air bleed opening area so as to maintain the bleed sensitivity constant through the entire range of intake air flow and to prevent the metering needle from wearing. 
     To achieve the above objective, the variable venturi carburetor of this invention is constructed to perform the following actions: in the low air-intake region, as the suction piston advances, the metering needle is biased by the biasing spring to press against one side of the metering jet with the expanded portion at the end of the metering needle floating in the large-bore portion of the well so as to reduce the air bleed area and maintain the shape of the effective jet opening area to ensure stable fuel delivery; and in the high air-intake region, as the suction piston is lifted up, the metering needle retracts and the expanded portion at its end moves into the small-bore portion of the well to cause the metering needle to float to the center of the metering jet increasing the air bleed area so as to maintain the bleed sensitivity constant and to prevent the metering needle from becoming worn out by vibration during high-speed revolution of the engine. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an explanatory drawing showing the air bleed control according to conventional technologies in the low air-intake operating condition; 
     FIG. 2 is a cross-sectional view of the variable carburetor of this invention showing the action of each component part during the low air-intake operating condition; 
     FIG. 3 is a partial enlarged view of the variable carburetor of this invention as illustrated in FIG. 2; 
     FIG. 4 is an explanatory drawing showing the action of each component part during the high air-intake operating condition; 
     FIG. 5 is an explanatory drawing of another embodiment of this invention; and 
     FIG. 6 is an explanatory drawing of still another embodiment of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the embodiment shown in FIGS. 2, 3 and 4, reference numeral 10 denotes a variable venturi carburetor of this invention which is of air damper type. A throttle valve 12 is installed at the downstream of the barrel 11 and a venturi 6 is formed between the throttle valve and the upstream air horn 13. Fixed to one side of the venturi 6 is a suction chamber 14 which has a suction piston 1 slidably installed therein. The suction piston 1 divides the suction chamber 14 into the atmospheric chamber 16 communicating with the air horn 13 through a passage 15 and the vacuum chamber 17. The suction piston has a rod 18 fixed thereto which is slidably supported in the rod guide 19 of the suction chamber 14. The suction piston 1 defines the size of venturi 6 as it is moved back and forth by the balancing action between forces of the suction spring 21, the atmospheric pressure in the atmospheric chamber 16 and the negative pressure in the vacuum chamber 17. 
     Rigidly installed in the head 2 of the suction piston 1 is a needle holder 3&#39; which contains a control plate 23 between the rear portion thereof and the fixed flange 24 of the metering needle 5. The control plate 23 has a projection 22 on the front side to urge the flange 24 and therefore the metering needle 5 by the biasing coil spring 4&#39; in the direction indicated by the arrow of FIG. 3. 
     The metering needle 5 is loosely inserted through the main nozzle formed in the bridge 25 on the other side of venturi 6 opposite to the suction piston 1 and is passed through the metering jet 8 into the well 26 in which the metering needle 5 has a disk fixed to its end by a stopper 28 to form the expanded portion 27. The metering needle 5 is biased by the spring 4&#39; to press against a part of the inner surface of the metering jet 8 which communicates with the air bleed 9. 
     As shown, the well 26 has a small-bore portion 30 on the metering jet side 8 and a large-bore portion 31 on the front side, the small and large-bore portions being connected by the tapered guide 29. The diameter of the expanded portion 27 is slightly smaller than that of the small-bore portion 30 so that it is slidable in the small-bore portion 30 and is loose in the large-bore portion. 
     The well 26 communicates with the float bowl 33 through the fuel pipe 32. 
     The metering jet 8 has a bleed hole 34 connected to the air bleed 9 which in turn is connected through the air bleed controller 35 of known construction to the passage 36 leading to the air horn 13. 
     With the above construction, in the low air-intake operating condition where the throttle valve 12 is opened at small degree, as shown in FIG. 2, the negative pressure in the vacuum chamber 17 coming from the intake manifold through the suction hole 20 is low, causing the suction piston 1 to reduce the effective area of the venturi 6 by the balancing action between the negative pressure, the atmospheric pressure in the atmospheric chamber 16 and the suction spring 21. As a result, the expanded portion 27 at the end of the metering needle 5 comes into the large-bore portion 31 of the well 26 in which it is floated. With the spring 4&#39; pressing the flange 24 against the projection 22, the metering needle 5 is biased to press against the inner side of the metering jet 8 to partly close the bleed hole 34. 
     This stabilizes the shape of the effective annular cross-sectional opening area as defined by the metering needle 5 and the metering jet 8, throttles the air bleed, brings the bleed sensitivity to a specified air-fuel ratio, and ensures stabilized delivery of fuel. 
     When, as shown in FIG. 4, the throttle valve 12 opens to a large degree and the negative pressure in the vacuum chamber 17 coming from the intake manifold through the suction hole 20 increases, the lift of the suction piston 1 increases retracting the metering needle 5. This causes the expanded portion 27 to move from the large-bore portion 31 of the well 26 into the small-bore portion 30 in which the expanded portion 27 is held centered. As a result, the metering needle 5 parts from the inner side of the metering jet 8 against which it was pressing until it is centered in the metering jet 8, with the result that the entire bleed hole 34 around the needle is open increasing the amount of bleed air. An increase in the amount of bleed air is accompanied by an increase in the amount of fuel which results from the increase in the effective annular opening area of the jet 8, thus maintaining the bleed sensitivity almost constant. 
     In the high air-intake region the engine is running at high speed, so that the metering needle 5 tends to be vibrated. However, the expanded portion 27 is held centered in the small-bore portion 30 of the well 26 with its circumference contacting the inner surface of the small-bore portion. This prevents the needle 5 from vibrating. Furthermore, the metering needle 5 is loosely centered in the metering jet 8 with a gap around the needle, so that it is prevented from hitting against the metering jet 8. This prevents wearing and deformation of the metering jet and keeps constant the shape of the effective opening area, thereby assuring the constant fuel flow and bleed air flow as well as the stable bleed sensitivity. 
     Although in this embodiment the expanded portion 27 is formed independently of the end of the metering needle 5 and secured to it by means of the stopper 28, the expanded portion 27&#39; may be formed integral with the needle 5 by cutting, as in another embodiment shown in FIG. 5, to obtain virtually the same effect. 
     The embodiment shown in FIG. 6 has a smooth tapered portion 29&#39; between the large-bore portion 31 and the small-bore portion 30 of the well 26 to improve the transient characteristic. It has another feature that the diameter d of the expanded portion 27&#34; is made very slightly smaller than the inner diameter D of the metering jet 8 to facilitate the assembling of the needle. 
     It should be noted that this invention is not limited to the embodiments described above and that various modifications may be made. For example, it is possible to form the expanded portion into a plate and taper it along the axis so that it can engage with the well having two different sizes of bore. 
     The structural features of this invention are: the biasing spring provided in the needle holder in the suction piston head of the variable venturi carburetor to bias the metering needle to press against a part of the inner side of the metering jet; the expanded portion formed at the end of the metering needle; and the well consisting of the large-bore portion and the small-bore portion with the large-bore portion corresponding to the low air-intake operation range and the small-bore portion to the high air-intake operation range. 
     These structural features produce the following effects or advantages. 
     In the low air-intake operating region, the expanded portion is inserted into the large-bore portion of the well and thus the metering needle is pressed against the metering jet by the biasing spring to partly close the bleed hole opening of the air bleed. This reduces the amount of bleed air, prevents the reduction of negative pressure at the jet, increase the amount of fuel and thereby maintains the bleed sensitivity constant. Since the shape of the effective opening area in the jet stays constant, the amount of fuel delivered will not change producing no variation in torque. 
     As the operating condition shifts into the high air-intake region and the suction piston has higher lift, the expanded portion moves into the small-bore portion of the well. This causes the metering needle to part from the metering jet until it is floated centered in the jet, leaving the entire bleed hole opened. As a result the amount of both fuel and bleed air increases to maintain the bleed sensitivity constant. 
     During the high air-intake operation the metering needle tends to vibrate due to the high revolution of the engine. However, since it is separated by the gap from the metering jet, they do not strike against each other, thus preventing wear of these components. This helps prevent the variation in the proportion between bleed air and fuel flow that would otherwise result from the change in the effective jet opening and therefore the bleed sensitivity can be maintained constant. 
     In summary this invention has the advantages of preventing the wear of the metering needle, maintaining the bleed sensitivity throughout the entire operating condition from the low air-intake region to the high air-intake region, and increasing the amount of fuel delivery during the cold operation by controlling the amount of the air bleed.