Variable venturi carburetor
BACKGROUND OF THE INVENTION 
     This invention relates to a variable venturi carburetor for an internal combustion engine which controls the air-fuel ratio by air bleed control operation. 
     In a conventional carburetor, the air-fuel ratio is controlled by varying the amount of bleed air admitted into an air chamber in the fuel jet of the carburetor. Such an air chamber is the annular space defined between the first orifice formed by the fuel metering portion of the fuel jet and by the fuel metering needle and the second orifice formed by the fuel-air mixture discharge portion of the fuel jet and by the fuel metering needle. The amount of bleed air is controlled by the actuator which is operated in response to the output signal transmitted from a computer into which the signals transmitted from sensors for sensing the engine running conditions are inputted. In such a control system, the fuel injected from the fuel metering portion and the bleed air delivered through an air bleed passage are mixed in the said annular space and the fuel-air mixture is discharged into the venturi portion. The flow rate of the fuel or the bleed air and the relative flow rate thereof are varied with the engine running conditions. In general, there arises a case where the flow rate is unstable with respect to a certain value of gas or liquid flow rate and relative flow rate thereof. Under these conditions, when the fuel-air mixture is metered by the said second orifice, the mixture is disadvantageously variably discharged. To avoid this drawback, the inside diameter of the discharge portion may be enlarged so as for the fuel-air mixture not to variably discharged. However, in this case, there arises a problem that &#34;bleed sensitivity&#34;, that is, the variation in the air-fuel ratio relative to the variation in the restriction created by the actuator, is reduced. This is caused by the fact that the pressure variation within the annular space relative to the constant variation in the restriction created by the actuator is decreased with an increase in the annular opening area of the second orifice formed at the discharge portion. 
     In another conventional variable venturi carburetor including a fuel jet provided with a jet portion, the jet portion is provided with an air bleed aperture through which bleed air is delivered into the annular restriction opening defined between the fuel metering needle and the jet portion. In this case, the amount of bleed air may be controlled by the sensors, computer and actuator as mentioned in connection with the preceding variable venturi carburetor, thereby permitting the air-fuel ratio to be controlled. As the amount of induced air flowing through the venturi portion of the air intake passageway is decreased, the said annular restriction opening area at the jet portion is decreased, and therewith the &#34;bleed sensitivity&#34; is increased. When the amount of induced air is further decreased until the engine reaches the idle condition, the air bleed aperture into the jet portion is nearly closed by the fuel metering needle (see FIG. 6). Accordingly, it should be apparent that the air-fuel ratio cannot be controlled by the air bleeding operation as depicted by the solid line in FIG. 7. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a primary object of the present invention to provide a variable venturi carburetor which obviates the above mentioned drawbacks and is designed to control the air-fuel ratio by air bleeding operation. 
     It is another object of the present invention to provide a variable venturi carburetor which assures a constant &#34;bleed sensitivity&#34; sufficient to control the air-fuel ratio at a constant level irrespective of the amount of induced air flowing through the air intake passageway. 
     It is a further object of the present invention to provide a variable venturi carburetor which improves a control efficiency of the air-fuel ratio and stabilizes a drivability of engine, thereby increasing fuel economy. 
     According to this invention, the variable venturi carburetor is adapted to control the air-fuel ratio by air bleeding operation which includes an actuator for controlling the amount of bleed air in response to the engine running conditions and a fuel jet having a jet portion and an air bleed portion provided downstream of and adjacent to the jet portion. The air bleed portion has an outlet opening adapted for delivering bleed air into the fuel passage and the inside diameter of the air bleed portion is designed to be slightly larger than that of the jet portion. Especially, at engine idle operation, the restriction area of the air bleed portion is designed to become larger than that adjusted by the actuator and the inside diameter of the fuel passage downstream of the air bleed portion is designed to become large in such a manner that air and fuel flows may not be influenced by the restricting operation of the air bleed portion. 
     Various general and specific objects, advantages and aspects of the invention will become apparent when reference is made to the following detailed description of the invention considered in conjunction with the related accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic vertical sectional view of a variable venturi carburetor, employing teachings of the present invention, along with an associated engine; 
     FIG. 2 is an enlarged vertical sectional view of the variable venturi carburetor shown in FIG. 1; 
     FIG. 3A is a vertical sectional view of the essential part of the variable venturi carburetor according to the first preferred embodiment; 
     FIG. 3B is a cross-sectional view taken along the line III--III of FIG. 3A; 
     FIG. 4A is a vertical sectional view similar to FIG. 3A according to the second preferred embodiment; 
     FIG. 4B is a cross-sectional view taken along the line IV--IV of FIG. 4A; 
     FIG. 5 is a graph illustrating change in the restriction area of the jet portion and the air bleed portion in relation to change in the amount of induced air according to the present invention; 
     FIG. 6 is a graph illustrating change in the amount of bleed air in a venturi carburetor of a conventional type wherein an air bleed opening is provided at a jet portion, in relation to change in the restriction area adjusted by an actuator; and 
     FIG. 7 is a graph illustrating the characteristics of the air-fuel ratio in relation to change in the amount of induced air according to the prior art and the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1 and 2, reference numeral 1 designates a carburetor body 1 of a variable venturi type having a float chamber 2, an air intake passage 3, a throttle valve 4 and a venturi portion 5. Reference numeral 6 designates a fuel passage communicating with the float chamber 2 and the venturi portion 5. The fuel passage 6 is provided with a fuel jet 7. The venturi portion 5 is defined upstream of the throttle valve 4 by the inside wall 3a of the air intake passage 3 and the bottom end 8a of a suction piston 8. A suction chamber 9 is defined by a cylindrical portion 1a of the carburetor body 1 and the suction piston 8 slidably inserted into the cylindrical portion 1a. A compression spring 8b is inserted in the suction chamber 9 and serves to normally urge the suction piston 8 toward the inside wall 3a of the air intake passage 3. A vacuum communication port 9a is provided at the bottom end 8a of the suction piston 8 and adapted to communicate with the suction chamber 9 and the venturi portion 5. An atmospheric pressure chamber 10 is defined by the sliding flange portion 8c of the suction piston 8 and the carburetor body 1 and provided with an atmospheric pressure communication port 10a in the vicinity of the inlet of the air intake passage 3, whereby ambient air is induced through the port 10a. A fuel metering needle 11 is fixed to the bottom end 8a of the suction piston 8 at its center portion. The free end of the metering needle 11 is laterally reciprocatingly inserted in the interior of the fuel jet 7. 
     Reference numeral 12 designates an air bleed passage having an inlet 12a provided in the vicinity of the inlet of the air intake passage 3 and an outlet 12b provided downstream of the jet portion or fuel metering portion 7a of the fuel jet 7. The fuel jet 7 is provided with an air bleed portion 7b downstream of the jet portion 7a. The air bleed portion 7b including the outlet 12b of the air bleed passage 12 serves as an air flow restricting means. The inside diameter of the air bleed portion 7b is designed to be slightly larger than that of the jet portion 7a as seen in FIGS. 3A and 3B. 
     As shown in FIGS. 1 and 2, on the way of the air bleed passage 12 to the fuel passage 6, there are mounted a needle 13 for varying the annular area of the air bleed passage 12, its associated seat 14 and an actuator 15 therefor. The actuator 15 is controlled by output signals from a computer 16 to which are transmitted input signals from an oxygen sensor 17 and an engine coolant temperature sensor 18 adapted for sensing the conditions of engine 19, so as to drive the needle 13. 
     The annular area defined by the bleed air restricting portion 7b and the metering needle 11 is preset to minimum at idle engine operation and also preset to be larger than the annular area defined by the needle 13 and the seat 14 which area is controlled by the actuator 15 to the value required for the air-fuel ratio control. 
     FIGS. 4A and 4B show another preferred embodiment of the present invention in which reference numerals 107, 107a, 112 and 112b designate a fuel jet, jet portion, air bleed passage and outlet of the air bleed passage, respectively. The inside diameter of the jet portion 107a is equal to that of the upstream end of the air bleed portion 107b, which is substantially conical in such a manner that the inside diameter of the air bleed portion 107b progressively increases from its upstream end to its downstream end opening into the venturi portion 5. The tapering angle of the conical air bleed portion 107b is designed to be larger than that of the metering needle 11. 
     As shown in FIG. 3B, four outlets 12b of the air bleed passage 12 are arranged on the circumference of the air bleed portion 7b in equally spaced relation with each other. Similarly, in FIG. 4B, eight outlets 112b of the air bleed passage 112 are arranged on the circumference of the air bleed portion 107b in equally spaced relation with each other. In both FIGS. 3B and 4B, total of the opening areas of the outlets 12b and 112b which open into the fuel passage 6 is preset to be larger than the annular area defined by the needle 13 and the seat 14. However, it will be noted that the arrangement and the number of the outlets 12b and 112b may be determined as desired. 
     In operation of the first preferred embodiment illustrated in FIGS. 1 through 3, the liquid fuel stored in the float chamber 2 is sucked up through the fuel passage 6 into the fuel jet 7 by a constant vacuum created in the venturi portion 5. The sucked fuel is metered by the fuel metering jet 7a and then injected through the downstream end or the left end of the fuel jet 7 to the venturi portion 5. At this time, ambient air induced through the air bleed passage 12 is delivered through the outlet 12b of the air bleed portion 7b. As the result, the negative pressure at the fuel metering jet 7a varies from the constant vacuum prior to the air bleeding operation due to the positive pressure of the bleed air. Accordingly, the metered fuel at the fuel metering jet 7a is controlled by varying the amount of bleed air delivered through the outlet 12b of the air bleed portion 7b, and thereby the air-fuel ratio can be controlled. Such control operation of bleed air is carried out in response to the engine conditions by the oxygen sensor 17, engine coolant temperature sensor 18, computer 16 and actuator 15, thereby permitting the air-fuel ratio to be maintained at an optimum value responsive to the engine conditions. 
     To accomplish the air-fuel ratio control by such air bleeding operation, the fuel jet 7 is not provided with a restriction means downstream of the air bleed portion 7b. As the result, unstable flow of air-fuel mixture is not created as seen in the prior art, but air and fuel are separately delivered to the venturi portion 5 under the state of laminar flow. In the case that the annular space S defined between the air bleed outlet 12b and the fuel metering needle 11 is large, variation of the air-fuel ratio relative to variation of the annular opening area defined between the bleed air metering needle 13 and the seat 14, that is, &#34;bleed sensitivity&#34; is decreased. However, in this embodiment, the annular space S is preferably restricted by the bleed air restricting portion 7b, thereby permitting the &#34;bleed sensitivity&#34; to be remained at the optimum level. Since the inside diameter of the bleed air restricting portion 7b is designed to be larger than that of the fuel metering portion 7a, even when the amount of induced air flowing through the intake passageway 3 is small, necessary annular opening area defined between the bleed air restricting portion 7b and the fuel metering needle 11 may be remained, thereby preventing the &#34;bleed sensitivity&#34; from becoming excessive, and also preventing the air bleed outlet 12b from shutting off at idle operation as seen in FIGS. 5 and 7. 
     The operation of the second preferred embodiment is substantially identical with that of the first preferred embodiment except that the inside diameter of the bleed air restricting portion 107b is designed to gradually increase from the left end of the fuel metering portion 107a to the discharge end opening into the venturi portion 5, thereby resulting in superior atomization of the fuel-air mixture to be discharged into the venturi portion 5 in comparison to the first preferred embodiment. 
     Having thus described the preferred embodiments of the invention it should be understood that numerous structural modifications and adaptations may be restored to without departing from the spirit of the invention.