GB2033483A - Piston air valve constant suction carburettor - Google Patents

Abstract

The piston 13 engages an opposing induction passage bridge 32 prior to engine starting to seal the passage 3, 6 upstream of the fuel nozzle 27 and a reduced or notched portion 35 of the needle 33 cooperates with the jet 28, thereby to provide a rich starting mixture. The effective force acting to lift the piston 13 may be reduced when the induction passage is closed by connecting the chamber 20 to the passage downstream of the piston by a valve (42), Figs. 8 to 10 (not shown), operated by the actuator piston 14 or a solenoid. <IMAGE>

Description

SPECIFICATION Variable venturi carburettor The present invention relates to a variable venturi carburettor.

It is well known that carburettors, for inter nal combustion engines can be selected from various kinds of fixed- and variable-venturi carburettors. Due to their various advantages, such as good response characteristics to change in the air-fuel mixture, and absence of a branched arrangement of a slow speed system and a main system and so forth, variable venturi carburettors have gradually been installed in an increasingly wide range of cars.

However, there are various problems still to be solved with variable venturi carburettors.

Among them is the presently troublesome procedure of the supply and viscosity adjustment of the oil for the oil damper which is usually provided in order to restrict self-excitation and over-shoot of the suction piston, which is the venturi varying member of the carburettor. One proposal aimed at lessening this drawback involves a gas-responsive mechanical design such as disclosed, for example, in the present Applicant's British Patent Application No. 7913568 (Serial No.

2027812) which teaches a variable venturi type carburettor having a venturi between an air inlet and a mixing chamber, a piston movable, to adjust the venturi, within a suction chamber and having a gas tight and sliding contact with the suction chamber, a fixed flange engaging the inner peripheral surface of the piston to define a gas damper chamber within the piston, the piston and suction chamber also defining a vacuum chamber communicating with the mixing chamber, there being a pressure transmission delaying passage between the vacuum chamber and damper chamber, and a spring urging the piston in a direction to reduce the venturi.

In this construction without an oil damper, a kind of limited fixed venturi construction is employed wherein a stopper is generally provided in the venturi section to avoid complete closing thereof by the suction piston. This is in order to stabilize the air-fuel ratio in a rich condition during the period of low negative pressure in the carburettor which occurs at the time of low temperature starting until the speed is reached. It follows that during low temperature starting or idling, the negative pressure may not rise sufficiently for the suction of fuel from the main nozzle from the float chamber to be sufficient to provide the rich air-fuel ratio.To cope with this problem, a choke valve can be disposed on the upstream side of the venturi section to elevate the negative pressure, while a starter nozzle is interposed between the choke valve and the suction piston, so as to increase the amount of fuel when the choke valve is operated.

However, these counter-measures inevitably increase the height of the carburettor by the height of the choke valve and thus lead to a different problem in that the height of the engine compartment may have to be correspondingly increased. In addition to the complicated construction due to the provision of the starter nozzle, there occurs another problem of insufficient power or unstable driving performance because low temperature acceleration and warming-up driving must be controlled by the single choke valve. If a controlor adjustment-mechanism is added so as to compensate for these drawbacks, there occur again the problems of a still further complicated construction and constly maintenance.

According to the present invention there is provided a variable venturi carburettor including a spring urged suction piston movable within a suction chamber to vary the venturi gap, the suction chamber communicating with the mixing chamber and the suction piston carrying a metering needle inserted in a metering jet wherein the suction piston is so shaped as to close the venturi section upstream of the metering needle at the time of engine stop.

With the present invention starting of the engine is carried out while the venturi section of the suction piston is fully closed and the low negative pressure occurring in the mixing chamber due to turning the engine over is reliably applied to the gap between the metering needle and the metering jet which is preferarbly wider than the gap between them when the engine is idling. Hence, the engine starts operating with a rich fuel ratio. A flange is preferably defined between the suction piston and the barrel so that, during the period prior to idling, the lift of the suction piston is made smaller with the passage of time thereby to obtain an enriched air-fuel ratio.

Alternatively, atmospheric pressure is communicated with the negative pressure to the suction chamber in order to enable driving at a set air-fuel ratio with a suitable lift quantity.

In order that the invention may be more clearly understood, the following description is given, merely by way of example, with reference to the accompanying drawings in which: Figure 1 is a longitudinal sectional view useful for explaining a first embodiment of the invention; Figure 2 is a partial enlarged schematic view of the first embodiment; Figure 3 is a schematic view of the upper face of the venturi section; Figure 4 is a schematic view for explaining the first embodiment of Fig. 2 at the time of idling; Figures 5a and 5b are schematic views respectively for explaining the upper face in part and the side face of the metering needle, respectively; Figure 6 is schematic view of another embodiment corresponding to the showing of Fig. 2; Figures 7a and 7b are schematic views for explaining the suction piston of another embodiment and its action;; Figure 7c is a schematic view similar to Fig.

7a and showing still another embodiment of suction pistion; Figure 8 is a longitudinal sectional view similar to Fig. 1 of a still further embodiment; and Figures 9 and 10 are a partial enlarged view and a schematic view for explaining the embodiment of Fig. 8 and its idle state.

In the embodiment shown in Figs. 1 to 4, reference numeral 1 generally indicates an oildamperless variable venturi carburettor. This has a truncated conical suction bore 3 and a cylindrical, hollow barrel 4 which continues from the bore 3 bored into the upper portion of a casing 2 of the carburettor. At the downstream end of the barrel 4, there is formed a mixing chamber 6 which is below a central venturi section 5, and a throttle valve 7 is dispoed below the mixing chamber 6.

To one side of the casing 2 is secured a housing 11 in which a suction chamber 10 is defined. A bush 9 having a guide hole 8 bored at its center extends inside the housing 11. The suction chamber 10 communicates with the mixing chamber 6 via a negative pressure passage 1 2. Alternatively communication may be through a choke orifice. Slidably fitted in the chamber 10 via a pressure transmission delaying labyrinth 1 5 on a flange section 14 is a suction piston 13, which is movable to control the size of the venturi. A return spring 1 7 is interposed between the rear face of the suction piston 1 3 and the inner face of the housing 11, and a rod 18 which is rightly pushed into the head 1 6 of the suction piston is slidably inserted into the guide hole 8.

Reference numeral 1 9 designates an air communication hole which is open to the suction bore 3 of the casing 2 and communicates with an air chamber 20 between the front face of the flange section 14 of the suction piston 1 3 and the casing 2.

A housing 22 having a nozzle hole 21 is disposed on the other side of the casing from the suction chamber 10 and piston 1 3. On a downwards extension of this housing 22 is disposed a float chamber 25 which houses a float 26 and includes a suction pipe 23 opening towards and near the bottom of the chamber, immersed in fuel shown at 24.

The nozzle hole 21 is positioned centrally with respect to the suction piston 1 3 and has, at its inner end, a main nozzle 27. A metering jet 28 inwardly of the main nozzle 27 relative to the housing 22 communicates with an air bleed 29 which is open at the suction bore 3.

Reference numeral 30 indicates an idles port which opens at the region of the throttle valve 7 and communicates with a rear portion of the main nozzle 27 via an idle path 31.

Reference numeral 32 indicates a bridge which is secured in the barrel 4 on the upstream side of the main nozzle 27, so that the front face of the head 1 6 of the suction piston 1 3 comes into intimate contact with this bridge 32 when the engine is stopped to thereby close the venturi section 5.

A metering needle 33 is fitted to the center of the head 1 6 of the suction piston 13, having its base pressed into the tip of the rod 18, and is located so that it is centered with respect to the main nozzle 27 and the metering jet 28.

The metering needle 33 is generally tapered from its base towards its tip with a predetermined degree of taper, but it has a narrow portion 35 which is in register with the metering jet 28 when the engine is stopped, or when the head 1 6 of the suction piston 1 3 is otherwise in contact with the bridge 32, as shown in Figs. 2 and 4, so that the gap between it and the metering jet 28 is, in such a situation, greater than usual. The portion 35 has a cross sectional area smaller than that of an adjacent idle portion 36 of the needle which leads to the tip.

The narrow sectional area portion 35 is shown exaggerated in the drawings for illustration purposes. It can be of cylindrical form as shown, and thus extend to the idle portion 36, or alternatively, a notch 37 may be provided as shown in Figs. 5a and 5b so that the overall outer envelope of the portion 35 corresponds to the taper of the needle.

As to the axial length of the narrow sectional area portion 35, the design is preferably such that when a lift quantity of the head 1 6 of the suction piston 1 3 relative to the bridge 32 becomes idle lift, i.e. when the gap between them is that obtained on engine idling, the portion 35 comes out of the metering jet 28 and the idle portion 36 of the metering needle 33 is then in register with the metering jet 28.

In the above-described construction, Figs.

1, 2 and 3 illustrate the state where the engine is stopped. When the engine is started, and begins to turn over at low tem perature, the head 1 6 of the suction piston 1 3 contacts the bridge 32 and closes the venturi section 5 as described already. Accordingly, upon low speed starting, a negative pressure is reliably generated in the mixing chamber 6.

The negative pressure occurring in the mix ing chamber 6 at the time of starting gener ates a negative suction pressure in the relatively large gap between the metering jet 28 and the narrow sectional area portion 35 of the metering needle 35. Therefore, a relatively rich air-fuel mixture is sucked from the main nozzle 27 into the mixing chamber 6, the richness depending on the sizes of the narrow sectional area portion 35, the metering jet 28, the air bleed 29 and the main nozzle 27.

Thereafter, idling condition can be attained in a reliable manner at the time of starting, even if the engine turns over only a few times, from the perfectly closed state of the venturi section 5 of the suction piston 1 3.

Also, of course, the negative pressure oc curring in the mixing chamber 6 at the com mencement of starting is effective to produce a negative pressure in the suction chamber 10 via the negative pressure path 1 2 whereby the balance of the suction force and the spring 1 7 on the suction piston 1 3 is altered thereby producing a lift of the suction piston.

After the shift to the idle state, the variable venturi section 5 is defined between the head 1 6 of the suction piston 1 3 and the bridge 32 and depends in width on the balance of the forces referred to, and to the leak gas pressure of the orifice, the choke and so forth, thus producing the venturi negative pressure.

During idling, the idle portion 36 of the metering needle 33, that is the larger sectional area portion continuing from the narrow sectional area portion 35, comes in register with the metering jet 28 as a result of the withdrawl of the metering needle 33 with movement of the suction piston 1 3. Thus the gap between the metering needle 33 and the metering jet 28 becomes smaller. Accordingly, the air-fuel mixture is prevented from becoming over-rich and a mixture having a predetermined air-fuel ratio is sucked into the mixing chamber 6, thereby enabling the maintenance of suitable idling.

Once the idle state is attained, the negative pressure inside the mixing chamber 6 increases. Hence, the negative pressure inside the suction chamber 10 also increases via the negative pressure path and the lift increases.

Prevention of self-exitation and over-shoot during this period are effected by the spring 1 7 in mutual cooperation with the air chamber 20, the suction negative pressure and the labyrinth seal 1 5.

Although it is desired to obtain an air-fuel mixture having a rich air-fuel ratio at the time of initial starting, by letting the narrow sectional area portion 35 of the metering needle 33 reliably come to the position corresponding to the metering jet 28, when the venturi is closed, this can lead to the disadvantage that the lift of the suction piston is very small during idling with the narrow sectional area portion 35 remaining at the position in register with the metering jet 28 on account, for instance of error during assembly. A way of avoiding this problem is as follows. As shown in Fig. 6, groove 38 is formed at the upper tip portion of the head 1 6 of the suction piston 1 3 while the bridge here shown at 32' is arranged to have an overlap portion 39 in the slit 38.This arrangement increases the difference between the lift quantity at the time of starting and that at the time of idling. Even if there is any deviation during assembly, this arrangement enables the small sectional area portion of the metering needle 33 reliably to come out of the metering jet 28 and allows the idle portion 36 to keep its place, thereby preventing difficulty due to an over-rich airfuel mixture at the time of idling.

In the embodiment shown in Figs. 7a and 7b, the design of the head 1 6 of the suction piston 1 3 is similar to that of the embodiment shown in Figs. 1 to 4, insofar as the head 1 6 is applied to the bridge 32 to seal it at starting. However, a pair of flanges 40, are formed to extend from the head 1 6 so that the lift at the time of idling is quite large. This arrangement helps to prevent the over-rich airfuel mixture occurring at the time of idling and thus provides the same additional effect as the embodiment shown in Fig. 6.

In the abovementioned embodiment, the inner edges of the flanges 40, 40 are linear with a predetermined angle between them in accordance with the interrelationship between the narrow sectional area portion 35 of the metering needle 33, the size of the metering jet 28 and the venturi section 5. It is possible to have the edges shaped to extend along the curve of a quadratic function, the curve of a trigometrical function or the curve of an exponential function. Furthermore, it is possible to employ such a design in a situation wherein the angle between the inner edges of the pair of flanges 40', 40' is made large, with parallel slits 41 being interposed between the base portions of these flanges 40', 40', as shown in Fig. 7c, so as to reduce the lift at the time of starting and to enlarge the lift at the time of idling.These flanges may be positioned alternatively on the barrel on the side of the bridge 32 so as to extend towards the bridge.

In the embodiment shown in Figs. 8, 9 and 10, an operation valve 42 is in a portion of the side wall casing 2 of the mixture chamber 6. This is designed to open at a low negative pressure, to communicate with the air chamber 20 and with the negative pressure path 1 2. The valve 42 has a casing having a communication hole 43 to open to the air chamber 20 and also having a communication hole 44 to communicate with the negative pressure path 1 2 and to face an orifice 45.

An operation piston 47 has a push rod 46 which extends from the front portion of the piston and is able to come into, and out of, contact with the flange 14 of the suction piston 1 3. This piston 47 is incorporated in the casing in such a manner that it is centered with respect to the communication hole 43 and is allowed to slide via a return spring 48.

At engine start, therefore, the air from the air communication hole 1 9 enters the air chamber 20 as shown in Fig. 9 and the flange 1 4 of the suction piston 1 3 which is at or near its fully closed position and causes the push rod 46 to retreat against the return spring 48 so that the operation piston 47 opens the communication holes 43 and 44.

Consequently, the air chamber 20 is communicated with the negative pressure path 1 2 and the incoming atmospheric pressure from the air chamber 20 weakens the negative pressure of the mixing chamber 6 flowing through the orifice 49 and the negative pressure path and acting on the suction chamber 10. Hence, the narrow sectional area portion 35 of the metering needle 33 is reliably caused to be in register with the metering jet 28 so that the rise of the negative pressure inside the suction chamber 10 is restricted, the slide-back of the suction piston 1 3 is restrained and the lift is relatively reduced thereby to provide a rich air-fuel mixture.

After the shift to idling, the communication negative pressure in the negative pressure path 1 2 exhibits a rapid increase along with the rise of the negative pressure of the mixing chamber 6. Hence, supply of air from the air chamber 20 is relatively reduced and the negative pressure in the suction chamber 10 rises, whereby the suction piston 1 3 moves so as to give an idling lift in accordance with the set balance.

In consequence, the operation piston 47 is caused to advance by means of the force of the return spring 48 together with the push rod 46. During idling it finally closes the communication hole 43. As the hole 43 is closed, the negative pressure inside the suction chamber 10 increases rapidly and the lift quantity changes rapidly to the idle state or to another suitable lift. Consequently, the idle portion 36 of the metering needle 33 is caused to immediately come in register with the metering jet 28 so as to avoid an over-rich mixture.

Accordingly, in this embodiment, too, the full closing of the venturi section 5, the small lift and the medium lift can be suitably established at the time of starting and idling.

Incidentally, in the embodiment shown in Figs. 8, 9 and 10, a suitable electromagnetic valve may be interposed between the communication holes 43 and 44 in order to control the negative pressure by communicating the air chamber 20 with the suction chamber 10 during starting. Alternatively, an air suction electromagnetic valve may be added to the suction chamber 10 itself.

It goes without saying that the invention is not specifically limited to the above-described embodiments but can be adapted to conventional oil damper system SU carburettors.

As described above, the present invention resides essentially in that the suction piston of the variable verturi fully closes the venturi section at the time of engine stop. Negative pressure is reliably produced inside the mixing chamber at the time of starting the rich airfuel mixture, so that without using a choke valve or a starter nozzle, rich starting mixtures are available with a simple, small and cheap unit. As there is no choke valve instabilities are reduced, while a leaner mixture is automatically provided at idling speeds, and fast idling can be achieved.

The flange extending across the venturi is disposed on at least one of the suction piston head and the venturi barrel and this arrangement makes it possible to make the idling lift relatively large, thereby avoiding over-rich mixtures when idling. Hence, it is also possible to employ effective measures for coping with the exhaust gas using a suitable air-fuel mixture ratio.

The operation valve which when provided operates at a low negative pressure and communicates with the air chamber is interposed in the negative pressure path communicating with the suction chamber, and makes it possible to minimize the lift of the suction piston at the time of starting. This also means that it is possible to make the most of the negative pressure with respect to the main jet at the time of starting so as to enable suction of the rich air-fuel mixture. The air-fuel ratio is adjusted upon idling, however, by the rapid increase in lift due to the rapid retreat of the suction piston, this in turn being caused by the rapid increase in the negative pressure at the time of idling.

Claims (5)

1. A variable venturi carburettor including a spring urged suction piston movable within a suction chamber to vary the venturi gap, the suction chamber communicating with the mixing chamber and the suction piston carrying a metering needle inserted in a metering jet wherein the suction piston is so shaped as to close the venturi section upstream of the metering needle at the time of engine stop.

2. A variable venturi carburettor according to claim 1, wherein said metering needle is generally tapered from its base portion, at the suction piston, to its tip and has a section of reduced cross-sectional area which is in register with a fuel metering jet at the time of engine stop, such portion being removed from the jet upon engine idling.

3. A variable venturi carburettor according to claim 1 or 2, wherein there is a flange extending across the venturi on one at least of said suction piston head and a barrel defining said venturi section.

4. A variable venturi carburettor according to claim 1, 2 or 3 including an air chamber equipped with an operation valve so as to communicate with a negative pressure path from the mixing chamber to the suction cham ber, said operation valve opening when the negative pressure is low.

5. A variable venturi carburettor constructed and arranged substantially as herein before described with reference to and as illustrated in Figs. 1 to 4 of the accompanying drawings, or modified substantially as hereinbefore described with reference to and as illustrated in any others of the accompanying drawings.