GB2086988A

GB2086988A - Variable venturi carburetor

Info

Publication number: GB2086988A
Application number: GB8133748A
Authority: GB
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 1980-11-10
Filing date: 1981-11-09
Publication date: 1982-05-19
Also published as: DE3143395A1; JPS5781148A; GB2086988B; DE3143395C2

Abstract

In a variable choke carburetor a metering needle 2 slides against the inside of a metering jet 3, in which a plurality of bleed holes 5 or a bleed slit (5') (Figs. 13, 14, not shown) is defined at least on the portion of the metering jet 3 against which the metering needle 2 slides. An air bleed path communicates with the holes 5 or slit (5'). <IMAGE>

Description

SPECIFICATION Variable venturi carburetor The invention relates to an air bleed control technique for a metering jet of a variable venturi carburetor.

As is well known in the art, in comparison with a fixed venturi carburetor, the variable venturi carburetor has various excellent characteristics such as good transient characteristics, a large air intake quantity, excellent inter-cylinder distribution performance, low engine height, and so forth. Because of these characteristic features, the variable venturi carburetor has gained a wide application from sports cars to ordinary passenger cars. However, the variable venturi carburetor has the problem of unsteady bleed sensitivity.

Namely, in order to reduce changes in the fuel flow quantity with respect to changes in the fuel temperature, the prior art device has a mechanism in which a metering needle 2 disposed at the head of a suction piston 1 is equipped with air bleed holes 5 is communicated with an air bleed path 4 at a portion where it measures the fuel in cooperation with a metering jet 3 in order to bleed air, as illustrated in Figures 1 a and 1 b. To obtain a stable fuel supply, the metering needle 2 is held against the inner surface of the metering jet 3 as shown in the drawings.

Furthermore, the bleed holes 5 are generally open on the side opposite to the pushing side of the metering needle 2 as shown in the drawings so as to constantly maintain the bleed function.

In a low intake air range, however, since the closed cross-sectional area occupied by the metering needle 2 inside the metering jet 3 is great, the bleed sensitivity characteristic curve C, in the low intake air range is by far sharper than those C2, C3 of the medium and high intake air ranges as shown in Figure 2 in which the abscissa represents the bleed hole area S and the ordinate represents the air-fuel ratio A/F. It can be understood that the latter curves have a dull sensitivity because the cross-sectional area of the metering needle is relatively small.

This can also be appreciated from experimental data of an actual car shown in Figure 3. Namely, the bleed sensitivity curve C,' of the low intake air range of the intake air quantity 2g/s is sharper than the curves C2, and C3' of the medium and high intake air ranges of the intake air quantities of 20g/s and 40g/s, respectively.

Accordingly, in such an embodiment, the airfuel ratio is not very controllable and an extremely complicated system would be required in order to make the bleed sensitivity as constant as possible.

Eventually, change in the air-fuel ratio is unavoidable and poor reproducibility as well as low accuracy must be tolerated.

For the abovementioned reasons, a comparison is made between a conventional embodiment in which the bleed holes 5 are disposed on the metering needle 2 on the opposite side to the support side of the metering jet 3 as shown in Figure 4 and an embodiment in which the bleed holes 5 are also disposed on the support sliding contact side as shown in Figure 5 in order to examine the relationship between the angle of rotation O' of the metering needle with respect to its revolving sliding contact with the metering jet 3 and the area change ratio S' of the bleed hole 5.

Whereas the relationship in the embodiment shown in Figure 4 exhibits a sharp change as represented by solid line a in Figure 6, that in the embodiment shown in Figure 5 hardly exhibits any changes as represented by dot-and-chain line b in the same drawing. Similar results are also obtained for the change in air-fuel ration (A/F)' as shown in Figure 7. This means that the smoothing effect of the bleed holes 5 disposed at least on the support sliding contact portion is extremely effective.

Accordingly, when the metering needle slides and moves back and forth with respect to the metering jet in accordance with the intake air quantity thereby to change its cross-sectional area, the embodiment equipped with the bleed holes on the support sliding contact side is more effective than the embodiment not equipped with the same, or the conventional embodiment such as shown in Figures 1 a and 1 b. The former also makes it possible to absorb production tolerances and assembly error.

In view of the problem of the air bleed of the metering jet in the conventional variable venturi carburetor, the present invention is directed to provide a variable venturi carburetor in which at least the bleed holes or slits are disposed on the metering needle on its support sliding contact side with respect to the metering jet on the basis of the effects of the abovementioned sampling contrast and in which the bleed sensitivity is kept constant as the air-fuel ratio does not rapidly change even when the bleed hole area changes.

Figure 1 a is a schematic view useful for explaining the operation of the metering needle and the metering jet on the basis of the conventional technique; Figure 1 b is a schematic view useful for explaining the bleed hole of the metering jet; Figure 2 is a diagram showing the characteristic curve of the bleed sensitivity based on the conventional technique; Figure 3 is a diagram showing experimental data on the bleed sensitivity; Figure 4 is a sectional view showing a model of the bleed hole in accordance with the conventional technique; Figure 5 is a sectional view showing a model of the bleed hole in accordance with the novel technique; Figure 6 is a diagram showing the relationship between the angle of rotation of the metering needle of the models shown in Figures 4 and 5 and the bleed hole area change ratio;; Figure 7 is a diagram showing the relationship between the angle of rotation of the metering needle of the models of Figures 4 and 5 and the air-fuel ratio change; Figures 8 to 14 illustrate embodiments of the invention of the present application, wherein: Figure 8 is a sectional view showing the overall construction of an embodiment of the present invention; Figure 9 is a sectional view taken along line A-A of Figure 8; Figure 10 is a diagram showing the bleed sensitivity data of the embodiment of Figure 8; Figure 11 is a diagram showing the relationship between the intake air quantity and the air-fuel ratio in accordance with the present invention in comparison with that of the conventional apparatus; Figure 12 is a sectional view of another embodiment corresponding to the embodiment shown in Figure 9;; Figure 13 is a sectional view of still another embodiment; Figure 14 is a sectional view taken along line B-B of Figure 13.

Next, embodiments of the present invention will be described by referring to Figures 8 through 11. Incidentally, the same reference numerals are used as in Figures 1 a through 7 to identify the same constituents.

Reference numeral 6 represents an air damper type variable venturi carburetor in accordance with the present invention. A venturi portion 10 is formed between the throttle valve 8 of its barrel 7 and the air horn 9 upstream of the valve 8. A suction chamber 10 is formed on one side of the venturi portion 10. Inside the suction chamber 10 are disposed an atmospheric chamber 12 communicating with the air horn 9 and a suction piston 14 formed in a negative pressure chamber 13. They can retract against a suction spring 1 5. A mixing chamber 1 6 and the negative pressure chamber 13 are communicated with each other through a suction hole 1 7.

The metering needle 2 is disposed at the head of the suction piston 1 so as to extend forwardly and to loosely penetrate through a main nozzle 1 9 formed on a bridge 18 that is disposed on the opposite side of the venturi portion 10. The metering needle 2 is brought into sliding contact with the lower side, as viewed in the drawing, of the inner surface of the metering jet 3' that constitutes the gist of the invention of this application. The tip of the needle faces a well 23 that is connected, via a fuel pipe 22, to a float chamber 21 having a float 20.

The metering jet 3' has an annular path 26 that is communicated, via the air bleed path 4, with a known low temperature compensation device connected to the air horn 9 via a path 24. The annular path 26 has six bleed holes 5 in all. Three of these holes are equidistantly disposed on the side of the metering jet 3' that the metering needle 2 makes sliding contact with, and the rest are on the non-sliding contact side.

In this embodiment, the bleed holes 5 on the sliding contact side are essential. According to the design theory and experiments, it is preferred that the inner circumferential length L of the metering jet 3' and the sum Fd of the diameters d of the bleed holes 5 satisfy the relation 0.5 < Fd/L < 0.8 and that the number of the bleed holes be at least six.

If the abovementioned value is below 0.5, the distance between the bleed holes 5 becomes greater and the area of the bleed hole changes sd that the change is the air-fuel ratio eventually becomes too great and the bleed sensitivity is not constant. If the value is above 0.8, on the other hand, the bleed hole area becomes so great that the measuring operation by two metering jets and metering needles is virtually effected, thereby losing the function of the single metering jet.

Hence, the fuel flow adjustment becomes more complicated and fuel flow can no longer be obtained with a high level of accuracy.

In the abovementioned construction, when the throttle valve 8 is at a small open angle or in a low suction air range, the mixing chamber 1 6 is at a low negative pressure and the negative pressure in the negative pressure chamber 13 of the suction chamber 11 via the suction hole 1 7 is also small. Accordingly, due to the balance between the negative pressure, the atmospheric pressure of the atmospheric chamber 1 2 and the suction spring 15, the suction piston 1 is opened with small lift.Hence, the change in the open area in each bleed hole 5 on the sliding contact side of the metering needle 2 with respect to the metering jet 3' is small, thereby restricting the air bleed quantity as well as the fuel increase due to a large cross-sectional area and ensuring a small bleed quantity, that is, a proper air-fuel ratio. The fuel is sucked by the negative pressure of the venturi portion 10 and is fed to the mixing chamber 16.

On the other hand, when the throttle valve 8 is opened at a large open angle, the negative pressure becomes greater and the suction piston 1 moves backward with great lift so that the metering needle 2 also moves backward, thereby a decreasing the cross-sectional area. At the same time, the area of each bleed hole 5 on the sliding contact side is increased so that the fuel quantity increases and the bleed quantity becomes greater.

In this case, the air-fuel ratio, that is, the bleed sensitivity, is kept substantially constant.

The relationship between the area S of each bleed hole 5 and the air-fuel ratio A/F in this process is illustrated in Figure 1 0. Namely, the bleed sensitivity curves C1", C2" and C3" of low, medium and high intake air quantities of 2g/s, 20g/s and 40g/s, respectively, substantially coincide with one another as shown in the drawing.

Accordingly, as shown in Figure 11 , the air-fuel ratio A/F can be kept constant with respect to the intake air quantity V. In other words, the bleed sensitivity can be kept constant as represented by the curves D of the Invention of this application.

Thus, the present invention eliminates the necessity of the conventional complicated construction, represented by the curves D', that varies the bleed hole area in accordance with the bleed quantity (M; large) or (N; small).

In comparison with the abovementioned embodiment, in the embodiment shown in Figure 12, the bleed holes 5 are disposed on the lower half of the metering jet 3", that is, only on the sliding contact side of the metering needle 2.

When the lift quantity is small or when the degree of taper of the metering needle 2 is great, the area change of the bleed hole functions more effectively in accordance with this embodiment.

In embodiments shown in Figures 13 and 14, a bleed slit 5' is formed in the annular path 26 of the metering jet 3"' on the sliding contact side of the metering needle 2. This arrangement substantially satisfies the aforementioned formula and is capable of keeping the bleed sensitivity constant extremely accurately and extremely sensitively because the bleed changes smoothly in response to smooth changes in the lift. Moreover, it is more machinable and provides a high level of accuracy.

Needless to say, the present invention is not limited to the foregoing embodiments, and various other embodiments are also possible, for example, by combining the bleed holes with the slit.

As described in the foregoing, in accordance with the invention of this application, in the variable venturi carburetor, the metering needle extending forward from the suction piston is brought into sliding contact with the inside of the metering jet to supply the fuel stably, and the bleed holes connected to the air bleed path are disposed on at least the sliding contact side of the metering needle in such an arrangement that the bleed hole area changes linearly in accordance with the displacement of the metering needle in response to the lift of the suction piston corresponding to the intake air quantity, and this keeps the bleed sensitivity constant in cooperation with the increase or decrease in the fuel feed quantity due to the change in the cross-sectional area of the metering needle inside the metering jet.

Above all, in the prior art apparatuses, the bleed sensitivity changes extremely sensitively in the low air intake range, but in accordance with the present invention, it becomes dull in the same way as in the medium and high intake air ranges and the air-fuel ratio setting can be automatically controlled as it is set.

Since the bleed slit is disposed on the sliding contact side of the metering jet with respect to the metering needle, the bleed area changes smoothly in response to a smooth retraction of the metering needle so that the bleed sensitivity characteristics are further improved. Moreover, the bleed slit can be made without the necessity of taking into account the gaps of the bleed holes or the error of the hole diameter and consequently, massproduction is feasible with a high level of accuracy.

Further, in accordance with the invention of this application, since the air-fuel ratio control under the operating condition in response to the intake air quantity can automatically be controlled by the air bleed quantity, response is quick and accurate control is possible. Hence, the air-fuel ratio control can be made with a high level of accuracy comparable to that of EFi, and further improved control can be made in combination with an 2 sensor.

Claims

1. A variable venturi carburetor of the type in which a suction piston moving back and forth with respect to a suction chamber formed in a venturi portion moving in accordance with an intake air quantity and equipped with a metering needle loosely penetrates through a main nozzle in sliding contact with a metering jet, wherein plural bleed holes are provided on said metering jet at least on the side of its sliding contact with respect to said metering needle and are communicated with an air bleed path.

2. The variable venturi carburetor as defined in claim 1 wherein plural bleed holes are uniformly disposed around the entire circumference of said metering jet.

3. The variable venturi carburetor as defined in claim 1 wherein said bleed holes are disposed in plural numbers on the hemi-circumferential portion on the side of said metering jet with which said metering needle makes sliding contact.

4. The variable venturi carburetor as defined in any of claims 1 through 3 wherein the diameter d of said bleed hole and the inner circumferential length L of said metering jet satisfy the relation 0.5 < d/L < 0.8.

5. The variable venturi carburetor as defined in any of claims 1 through 4 wherein the number of said bleed holes is up to six.

6. The variable venturi carburetor as defined in any of claims 1 through 5 wherein slits are communicated between said bleed holes.

7. In a variable venturi carburetor of the type in which a suction piston moving back and forth with respect to a suction chamber formed in a venturi portion, moving in accordance with an intake air quantity and equipped with a metering needle loosely penetrates through a main nozzle in sliding contact with a metering jet, the improvement wherein bleed slits are formed on said metering jet at least on the slide of its sliding contact with respect to said metering needle and are communicated with an air bleed path.

8. The variable venturi carburetor as defined in claim 7 wherein the length I of said slit and the inner circumferential length L of said metering jet satisfy the relation 0.5 ~ I/L S 0.8.

9. A carburetor constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in Figures 5 to 14 of the accompanying drawings,

10. An internal combustion engine incorporating at least one carburetor according to any one of the preceding claims.