IES84834Y1 - A carburetor - Google Patents

Abstract

ABSTRACT This invention relates to a carburetor. In two stroke engines, fuel flows from the engine cylinder out of the exhaust port at the time of scavenging. To reduce this effect, some two~stroke engines have an air passage separate from the air and fuel mixture. The air passage is connected to the scavenging passage that links the crankcase to the combustion chamber such that, under the control of an air supply valve, the combustion gas is exhausted by the pure air from the air passage into the combustion chamber at the beginning of the scavenging cycle when the scavenging port opens and then the air/fuel mixture is introduced.

Description

A Carburetor This invention relates to a carburetor.

In two stroke engines, fuel flows from the engine cylinder out of the exhaust port at the time of scavenging. To reduce this effect, some two~stroke engines have an air passage separate from the air and fuel mixture. The air passage is connected to the scavenging passage that links the crankcase to the combustion chamber such that, under the control of an air supply valve, the combustion gas is exhausted by the pure air from the air passage into the combustion chamber at the beginning of the scavenging cycle when the scavenging port opens and then the air/fuel mixture is introduced.

The above configuration requires a mechanism between the carburetor throttle valve and the air supply valve to allow coordinated opening of the two valves. An example is described in US Patent 6,928,996 which has a lost-motion connection linking the two valves. However, such a configuration places significant restrictions on the carburetor structure due to the use of large levers and links and the positioning of the components, hence reducing the degree of design freedom and making it difficult to incorporate in existing carburetors. Also, due to production tolerances in the linking mechanism and in the carburetor, it is difficult to establish the proper opening relationship between the air supply valve and the throttle valve. This may unbalance the air/fuel ratio in a particular load region, hence reducing the performance of the engine.

P785591]-'_-200/spec as filed (10.02.06) It is an object of the invention to provide a construction of carburetor in which these disadvantages may be avoided or mitigated.

Accordingly, the present invention provides a carburetor comprising an air/fuel mixing passage, a throttle valve rotatable about an axis transverse the mixing passage, an air passage separate from the air/fuel mixing passage, an air supply valve rotatable about an axis transverse the air passage, and a lost motion mechanism connecting the throttle valve and the air supply valve for progressively opening the air supply valve, after a predetermined angular delay, in response to a progressive opening of the throttle valve, the lost motion mechanism comprising a first lever coupled for substantially non-slip rotation with the throttle valve, a second lever rotatable relative to the throttle valve and entrained by the first lever after the latter has rotated through a predetermined dwell angle, and a connection having substantially no play between the air supply valve and the second lever, the first lever being angularly adjustable relative to the throttle valve so as to permit adjustment of the dwell angle to compensate for production tolerances in the carburetor.

Preferably the connection between the air supply valve and the second lever comprises a third lever coupled for substantially non-slip rotation with the air supply valve and a link coupling the second and third levers substantially without play.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: P78559IE00/spec as filed (10.02.06) FIGS 1(a) and 1(b) are cross—sectiona1 and side views respectively of an embodiment of carburetor according to the invention in idle operation.

FIGS 2(a) and 2(b) are cross—sectional and side views respectively of the carburetor of FIG 1 in intermediate load operation.

FIGS 3(a) and 3(b) are cross—sectional and side views respectively of the carburetor of FIG 1 in full load operation.

FIG 4 is a perspective exploded view of the lost motion mechanism in the carburetor of FIG 1.

FIGS 1 to 3 illustrate a diaphragm carburetor of the kind typically, but not necessarily, used with small two—stroke engines of the type used on hand—held products such as chain saws, trimmers, garden blowers and concrete saws.

The carburetor comprises a main carburetor body 1 defining an air/fuel mixing passage 5 having an air intake side 6, an engine outlet side 8 and a Venturi 19 (FIG 3). A butterfly—type throttle valve 2 is mounted within the mixing passage 5 between the air intake side 6 and the engine outlet side 8. The throttle valve 2 is fixed to a shaft 3 for rotation about an axis transverse the mixing passage 5. In use of the carburetor the throttle valve 2 is rotated, against the bias of a conventional throttle valve return spring, from the closed position of FIG 1 to the fully open position of FIG 3 by a throttle mechanism not visible in the drawings. The carburetor also includes P78559IE00/spec as filed (10.02.06) a butterfly—type choke valve 7 upstream of the throttle valve 2 which is rotatable about an axis parallel to the rotational axis of the shaft 3. A fuel metering chamber 17 supplies fuel from a fuel pump 24 into the carburetor mixing passage 5 via one or more fuel discharge ports 16, 18, 20 (see also FIGS 2 and 3) according to the rotational position of the throttle valve 2.

The construction and operation of such carburetors is very well known to those skilled in the art and, therefore, will not be further described except to the extent necessary for an understanding of the present embodiment.

The carburetor also includes a body 9 defining an air passage 10 for introducing air to an exhaust port of the engine prior to the supply of an air/fuel mixture to a scavenging port. The body 9 may be integral with the main carburetor body 1 or assembled to it. A butterfly—type air supply valve 11 is mounted within the air passage 10, the air supply valve 11 being fixed to a shaft 12 for rotation about an axis transverse the air passage 10 and substantially parallel to the axis of rotation of the throttle shaft 3.

A throttle lever 4 is mounted on an end of the throttle shaft 3 projecting through the wall of the body 1. As will be described with reference to FIG 4, splines on the end of the shaft 3 and press—fit lever 4 ensure that the lever 4 co—rotates with the shaft 3, and hence with the throttle valve 2, without slippage. A secondary lever 14 is also mounted on the projecting end of the throttle shaft 3, but in this case the lever 14 is rotatable on the shaft 3. The P78S59IEOO/spec as filed (10.02.06) throttle lever 4 has a projection 4A for engaging the secondary lever 14, as will be described.

An air supply lever 13 is fixedly mounted on an end of the shaft 12 projecting through the wall of the body 9 so that the lever 13 co—rotates with the shaft 12, and hence with the air supply valve 11, without slippage. A return spring 23 surrounds the shaft 12 with one end 23a of the spring 23 fastened to the air supply lever 13 and the other end 23b of the spring 23 fastened to the body 9 so that the air supply valve 11 is always biased to return to the closed position when opened. The end 23b can alternatively so be fastened to the body 1 or to a component on the body 1 or to a component on the body 9. The air supply lever 13 and the secondary lever 14 are connected by a link 15. The connections at both ends of the link 15 are free from play.

As will now be described, the levers 4, 13 and 14 together with the link 15 constitute a lost motion mechanism connecting the throttle valve 2 and the air supply valve 11 for progressively opening the air supply valve, after a predetermined angular delay, in response to a progressive opening of the throttle valve.

FIG 1 shows the carburetor 1 in idle operation. The throttle valve 2 is partially cracked and the air supply valve 11 is closed. Engine suction is transmitted through the primary idle fuel discharge port 16 to the fuel- metering chamber 17 to allow fuel to flow.

As the throttle valve 2 is progressively opened by clockwise rotation from the idle position the throttle lever 4 rotates with it (clockwise refers to the View seen P78559IEOO/spec as filed (10.02.06) in the drawings). Initially the air supply valve 11 remains closed. However, after a predetermined angular delay, corresponding to rotation of the throttle lever 4 through a predetermined dwell angle 9, the projection 4A of the throttle lever 4 comes to bear against the secondary as the throttle valve 2 continues to lever 14. Thereafter, open, the projection 4A entrains the secondary lever 14 so that the latter rotates in synchronism with the throttle valve 2. This pulls on the link 15 to rotate the air supply lever 13 counterclockwise which in turn This is progressively opens the air supply valve 11. shown in FIGS 2 and 3.

FIG 2 shows the carburetor in intermediate load operation.

The throttle valve 2 has opened to a part—throttle position. In this position the engine speed increases and more fuel is supplied to the engine by valving in the secondary idle discharge ports 18 located immediately behind the throttle valve 2. FIG 2 shows the state where the projection 4A of the throttle lever 4 has just come into contact with the secondary lever 14 but has not yet moved it; hence the air supply valve 11 remains closed.

FIG 3 shows the carburetor in full load operation. The throttle valve 2 has now reached its full open position.

In this position the air velocity through the venturi 19 increases and fuel is metered through the main fuel discharge port 20 in accordance with the power requirements of the engine. The throttle lever 4 has now entrained and rotated the secondary lever 14, the rotated secondary 14 has pulled the link 15, and the pulled link 15 has rotated the air supply lever 13 so that both the throttle valve 2 and the air supply valve II are in the fully open position.

P78559IEO0/spec as filed (10 02.06) FIG 4 shows the throttle shaft 3, the secondary lever 14, the throttle lever 4, and a retaining screw 21. The secondary lever 14 is assembled to the throttle shaft 3 for free rotation thereon and has a hole 22 for play—free connection to the upper end of the link 15. The throttle lever 4 has an internal cylindrical bore 4B which is press—fit onto a complementary splined portion 30 of the throttle shaft 3, and is held there by the retaining screw 21. As the angular delay between the opening of the throttle valve 2 and the opening of the air supply valve 11 is critical to the function of the engine performance, this arrangement allows the angular position of the throttle lever 4 to be adjusted relative to the shaft 3, and thus relative to the throttle valve 2, to overcome the stack-up of production tolerances due to the other components and machining, e.g. air supply body, gaskets, diaphragms, linkage, levers, carburetor body, etc. With this arrangement the throttle valve 2 and the air supply valve 11 can be accurately set to ensure that initially the throttle valve can open to a predetermined angle without opening the air supply valve. Further rotation of the throttle valve causes a resultant opening of the air supply valve regardless of the stack—up of production tolerance.

The advantages of the above embodiment are: - As both ends of the link 15 are free from play, secondary lever 14 and air supply lever 13 can be manufactured in small sizes hence reducing cost. - with the secondary lever 14 and the air supply lever 13 being small in size, the accuracy and tolerances of the 7 P78559IEOO/spec as filed (10.02.06) opening of the throttle valve 2 and the air supply valve 11 are greatly improved due to the link 15 being close to the centre of the throttle shaft 3 and air supply shaft 12. Large levers increase inaccuracy due to the greater distances from the shaft centres.

- With the throttle lever 4 being small in size Contact surface 25 (see FIG 4) is close to the centre of the throttle shaft 3 hence increasing the accuracy of Contact in lost motion between the throttle lever 4 and the secondary lever 14.

— The angle of lost motion can be easily set by adjusting the angular position of the throttle lever 4 without affecting the position of the secondary lever 14. This will ensure that stack-up of production tolerances will not effect the predetermined angles of the valves and the function of the engine.

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

P78559IEOO/spec as filed (10.02.06)

Claims (3)

Claims

1. A carburetor comprising an air/fuel mixing passage, a throttle valve rotatable about an axis transverse the mixing passage, an air passage separate from the air/fuel mixing passage, an air supply valve rotatable about an axis transverse the air passage, and a lost motion mechanism connecting the throttle valve and the air supply valve for progressively opening the air supply valve, after a predetermined angular delay, in response to a progressive opening of the throttle valve, the lost motion mechanism comprising a first lever coupled for substantially non-slip rotation with the throttle valve, a second lever rotatable relative to the throttle valve and entrained by the first lever after the latter has rotated through a predetermined dwell angle, and a connection having substantially no play between the air supply valve and the second lever, the first lever being angularly adjustable relative to the throttle valve so as to permit adjustment of the dwell angle to compensate for production tolerances in the carburetor.

2. A carburetor as claimed in claim 1, wherein the connection between the air supply valve and the second lever comprises a third lever coupled for substantially non—slip rotation with the air supply valve and a link coupling the second and third levers substantially without play.

3. A carburetor as claimed in claim 1 or 2, wherein the second lever has a common rotational axis with the first lever.