IES81113B2 - A carburetor - Google Patents

Abstract

A diaphragm carburetor has a metering chamber 16, a mixing passage (venturi)12 and at least one fuel path between the metering chamber and the mixing passage. The fuel path includes a fuel chamber 18, at least one hole 22 leading to the fuel chamber for supplying fuel from the metering chamber to the fuel chamber, and a fixed jet 24 for supplying fuel from the fuel chamber to the mixing passage. A rotatable adjustment member 30 with an inclined end face 44 is provided for blocking the hole 22 to a variable extent so as to permit adjustment of the rate of fuel flow into the mixing passage between a minimum and a maximum, the minimum being greater than zero.

Description

A CARBURETOR The present invention relates to a carburetor, for example a diaphragm type carburetor.

A typical conventional diaphragm type carburetor comprises a carburetor body defining a mixing passage having an air intake side and an engine outlet side, a fuel pump, a throttle shutter mounted within the carburetor mixing passage between the air intake side and the engine outlet side, and a metering chamber for supplying fuel from the fuel pump into the carburetor mixing passage via a high speed fuel circuit comprising an adjustable high speed needle valve and a low speed/idle fuel circuit comprising an adjustable low speed needle valve. The needle valves are adjusted by respective screw heads which are accessible on the outside of the carburetor.

In such a carburetor the volume per second of fuel delivered to the engine is adjustable, for low speed operation via the low speed needle valve and for high speed operation via the high speed needle valve. Adjustment is normally factory set by the engine manufacturer to give the desired engine performance/air fuel ratios.

With such a system adjustment can be made within a broad band from no fuel flow when an adjusting needle pi28848/epc is screwed fully in (the needle tip closes the associated orifice) to fully open when the needle tip is fully out of the orifice. In this case the orifice diameter controls the maximum volume per second of fuel flow.

With the advent of emission regulations applicable to IC engines, and in particular to two stroke engines which are regulated by the type of carburetor mentioned above, it is necessary to have a system which limits the amount of adjustment on the high speed and low speed adjusting needles. This is commonly achieved by fixing a cap to the screw head which has a suitably located integral projection which stops against an independent fixed member thereby limiting the rotation of the screw between corresponding maximum rich and maximum lean limits complying with the emissions regulations.

However, such a system is generally expensive to produce, difficult to pre-set or set on a fully assembled engine and, being generally made from plastic components, is easily tampered with or removed entirely allowing for non-limited adjustment of both the low and high speed needles. A carburetor having a fixed (i.e. non-adjustable) jet is an alternative option but, due to engine manufacturing variations, it is not possible to guarantee compliance with emission regulations. pi28848/epc Accordingly, the invention provides a carburetor having a metering chamber, a mixing passage, and at least one fuel path from the metering chamber to the mixing passage, the said one fuel path including a fuel chamber, at least one hole leading to the fuel chamber for supplying fuel from the metering chamber to the fuel chamber, and a fixed jet for supplying fuel from the fuel chamber to the mixing passage, the carburetor further including an adjustment member for blocking the said one hole to a variable extent so as to permit adjustment of the rate of fuel flow into the mixing passage.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: FIG. 1 is an end view of a carburetor according to a first embodiment of the invention; FIG. 2 is a side view of the carburetor of Fig. 1; FIG. 3 is a cross sectional view taken along the line A - A of Fig. 2 showing the high speed adjustment member in the fully closed position; FIG. 4 is a cross sectional view taken along the line A - A of Fig. 2 showing the high speed adjustment member in the fully open position,· pi28848/epc FIG. 5 is a cross sectional view taken along line A - A of Fig. 2 showing the high speed adjustment member in the half open position; FIG. 6 is a cross sectional view taken along line B - B of Fig. 2 showing the low speed adjustment member in the fully open position,· FIG. 7 is an elevation, plan view and cross sectional view of the adjustment member of Figs. 1 to 6; FIG. 8 is an isometric view of the adjustment member; FIG. 9 is a cross-sectional view, similar to Fig. 4, of a second embodiment of the invention showing the high speed adjustment needle in an open position; FIG. 10 is a cross-sectional view of the second embodiment showing the high speed adjustment needle in a closed position; FIG. 11 is a cross sectional view, similar to Fig. 4, of a third embodiment of the invention showing the high speed adjustment member in the position of maximum partial closure; pi28848/spc FIG. 12 is a cross sectional view of the third embodiment showing the high speed adjustment member in the fully open position; FIG. 13 is a cross sectional view, similar to Fig. 3, of a fourth embodiment of the invention showing the high speed adjustment member in the fully closed position; and FIG. 14 is a cross sectional view of the fourth embodiment showing the high speed adjustment member in the fully open position.

Referring now to Figs. 1 to 8 of the drawings, a diaphragm type carburetor comprises a carburetor body 10 defining a mixing passage (venturi) 12 having an air intake side and an engine outlet side, a fuel pump (not shown) within the body 10, a throttle shutter 14 mounted within the carburetor mixing passage 12 between the air intake side and the engine outlet side, and a diaphragm-operated metering chamber 16 for supplying fuel from the fuel pump into the carburetor mixing passage. The construction and operation of such carburetors is very well known to those skilled in the art and, therefore, the following description will concentrate on the differences from the conventional carburetor. ΡΪ28848/spc Fuel is delivered from the metering chamber 16 to the mixing passage 12 via a high speed fuel circuit (Figs. 3, 4 and 5) and a low speed/idle fuel circuit (Fig. 6) . The high speed circuit comprises a fuel chamber 18 formed in the carburetor body 10 intermediate the metering chamber 16 and the mixing passage 12, the fuel chamber 18 being of constant circular cross-section throughout its length and extending fully to the exterior surface of the body 10. Two fuel supply holes 20, 22 respectively lead from the metering chamber 16 to the intermediate fuel chamber 18, each terminating in the circular sidewall of the latter. -A fixed, i.e. non-adjustable, high speed (main) jet 24 is located at the internal end of and coaxial with the axis of the fuel chamber 18 for supplying fuel from the fuel chamber 18 to the mixing passage 12 via a channel 26 and a main nozzle 28. The jet 24 may be either permanently secured in position or removable by means of a thread.

A high speed adjustment member 30 is mounted in the fuel chamber 18. The member 30 comprises a cylindrical body 32 of circular cross-section which is a snug fit within the fuel chamber 18 so as to closely engage the circular sidewalls of the fuel chamber 18. The body 32 is rotatable in the fuel chamber 18 about its own axis which is coincident with the axis of fuel chamber. The body 32 has a screw head 34 externally of the carburetor body 10 to permit rotation of the body 32 by pi28848/epc a screwdriver. The adjusting member 30 is held against axial movement in the fuel chamber 18 by a pin 36 fixed in the carburetor body 10 and whose free end engages a circumferential groove 38 in the body 32. A further circumferential groove 40 accommodates a rubber O ring 42 to provide an air tight seal against the sidewall of the chamber 18 and also to provide a suitable amount of friction which will allow the adjustment member 30 to be rotated via the screw head 34 but will hold the adjustment member 30 at any desired angular setting.

The free end of the body 32 has a face 44 inclined at an angle to the axis of rotation of the body such that, in one angular position of the adjustment member 30, i.e. the fully closed position shown in Fig. 3, the angled face 44 is turned away from the hole 22 so that the hole 22 is completely covered by the body 32 and no fuel can flow through the hole 22 from the metering chamber 16 to the fuel chamber 18. However, by rotating the body 32 through 180 degrees to the fully open position shown in Fig. 4, the angled face 44 is turned towards the hole 22 thereby fully uncovering it and allowing fuel to flow through it to the fuel chamber 18. Fig. 5 shows the adjustment member 30 rotated to a partially open position between the fully open and fully closed positions, wherein the hole 22 is only partially covered. ->s a ' a / -,r· A stop web 46, Figs. 7 and 8, is provided in the groove 38 of the adjustment member 30 to halt the rotational movement of the body 32 substantially at the fully open or the fully closed position of the adjustment member 30. This is achieved by the pin 36 coming into contact with the web 46. In all angular positions of the adjustment member 30 between its fully open and fully closed positions the hole 20 is unaffected and remains fully uncovered. As shown by the dashed lines 48 in Fig. 7, the rate of change or sensitivity of the adjustment can be altered by changing the angle the face 44 of the adjustment member makes with the axis of the body 32, or by otherwise altering the profile of the face 44.

During carburetor operation negative pressure is transmitted from the mixing passage 12 to the metering chamber 16 via the nozzle 28, channel 26, main jet 24, fuel chamber 18, hole 20 and, when the adjustment member 30 is in a partially or fully open position, the hole 22. The jet 24 is calibrated (sized) to give a required fuel flow which corresponds to a point between the maximum rich and maximum lean limits (which may be the mean value) as determined by emissions requirements. Hole 20 is an enleanment orifice and is calibrated to alter and reduce the flow of fuel through the jet 24. Hole 12 is an enrichment orifice and is calibrated to alter and increase the flow of fuel through the jet 24. In the maximum rich condition both pi28848/spc holes 20 and 22 may contribute to the maximum rich limit. Adjustment between the maximum lean limit and maximum rich limit is achieved by opening and closing the hole 22.

The low speed/idle fuel circuit of the carburetor of Figs. 1 and 2 is constructed in substantially the same way as the high speed fuel circuit. This is seen in Fig. 6 where components equivalent to those of the high speed fuel circuit of Figs. 3 to 5 have been given the same reference numerals plus 100. The low speed/idle fuel circuit operates in the same manner: as the high speed circuit except that channel 126 feeds to a bypass chamber 150 which is sealed from the metering chamber 16. Bypass holes 152 and 154 feed directly to the mixing passage 12. It will be understood that the bypass holes 152, 154 of the low speed/idle fuel circuit feed into the mixing passage 12 at a location downstream of the throttle shutter when the latter is closed, whereas the nozzle 28 of the high speed fuel circuit feeds into the mixing passage at a location upstream of the throttle shutter.

Figs. 9 and 10 illustrate the high speed fuel circuit of a second embodiment of carburetor according to the invention, and in these figures components equivalent to those of Figs. 3 to 5 have been given the same reference numerals plus 200. It will be understood that except for the differences described below, the px28848/spc second embodiment may be the same as the first embodiment. In the second embodiment, the fuel chamber 218 again has two fuel supply holes, a first hole 220 which, like the hole 20 in Figs. 3 to 5, opens onto the sidewall of the chamber 218, and a second hole 222, equivalent to hole 22, which is formed as the seat of a needle valve coaxial with the main jet 224. Fuel is fed to the hole 222 from a further hole 262 which is large enough to have no restrictive influence on the hole 222.

A conventional valve needle 260 is threaded into the carburetor body 10 such that as the needle 260 is rotated its tip moves axially towards or away from the valve seat 222. The needle 260 is rotatable from a position where the hole 222 is fully closed, Fig. 10, to a position where the hole 222 is open to a variable extent, Fig. 9, depending on the distance of the tip of the needle 260 from the hole. For all positions of the needle 260 the hole 220 is unaffected, and remains fully open at all times.

A third embodiment of a carburetor according to the invention, Figs. 11 and 12, is a modification of the embodiment of Figs. 1 to 8 in which, in the high speed fuel circuit, the two holes 20 and 22 have been replaced by a single, larger hole 300. In this case rotation of the adjusting member 30 covers the hole 300 to a variable extent, but never completely blocks it. pi28848/spc In particular, the adjusting member 30 is rotatable through 180 degrees between a first position, Fig. 12, where the hole 300 is fully uncovered, and a second position, Fig. 11, where the hole 300 is partially covered to a maximum extent. Between these positions the hole 300 will be partially covered to a decreasing extent as the adjustment member 30 is rotated from the second to the first position.

It will be appreciated that when the adjusting member is in the Fig. 11 position, the fuel flow through the partially covered hole 300 will correspond to that through the hole 20 in the first embodiment, whereas when it is in the Fig. 12 position the fuel flow through the hole 300 corresponds to that through both holes 20 and 22 in the first embodiment.

A fourth embodiment of a carburetor according to the invention, Figs. 11 and 12, is also a modification of the embodiment of Figs. 1 to 8. In this case, in the high speed fuel circuit, the hole 20 into the fuel chamber 18 has been omitted so there is only the one hole 22 which can be fully covered or fully uncovered according to the angular position of the adjustment member 30. However, the fourth embodiment has a further fuel path 400 from the metering chamber 16 to the mixing passage 12, the path 400 containing a further fixed jet 402 at the end thereof which opens into the metering chamber. The fuel path 400 and the pi28848/spc fuel path comprising the hole 22, fuel chamber 18 and fixed jet 24 meet downstream of the jets 24, 402 at the nozzle 28.

In this fourth embodiment the lean limit is when fuel is provided to the nozzle 28 only by the fixed jet 400, the hole 22 being fully covered by the body 32 (Fig. 13) . The rich limit is when the hole 22 is fully uncovered (Fig. 14) so that fuel is supplied to the nozzle by both fixed jets 24 and 402. Adjustment between the rich and lean limits is achieved by rotating adjustment member 30 to an intermediate angular position so that the hole 22 is partially covered to an adjustable extent.

The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.

Claims (5)

1. A carburetor having a metering chamber, a mixing passage, and at least one fuel path from the metering chamber to the mixing passage, the said one fuel path including a fuel chamber, at least one hole leading to the fuel chamber for supplying fuel from the metering chamber to the fuel chamber, and a fixed jet for supplying fuel from the fuel chamber to the mixing passage, the carburetor further including an adjustment member for blocking the said one hole to a variable extent so as to permit adjustment of the rate of fuel flow into the mixing passage.

2. A carburetor as claimed in claim 1, wherein the adjustment member is rotatable, such rotation blocking the said hole to a variable extent according to the angular position of the adjustment member.

3. A carburetor as claimed in claim 2, wherein the said at least one hole is present in a sidewall of the fuel chamber, and wherein rotation of the adjustment member covers the said hole to a variable extent according to the angular position of the adjustment member.

4. A carburetor as claimed in claim 2, wherein the adjustment member is a needle mounted for axial movement as it is rotated, the needle co-operating with the at least one hole such that rotation of the needle blocks the hole to a variable extent.

5. Ά carburetor as claimed in any preceding claim, wherein the carburetor is a diaphragm type carburetor