GB2167135A - Improvements in mixing fluids - Google Patents

Abstract

In a fluid mixing device, for example a down-draught carburetor for an internal combustion engine, a primary throttle valve (9) formed as an aerofoil (12) is provided to increase the mixing of the fluid stream. <IMAGE>

Description

SPECIFICATION Improvements in mixing fluids The present invention relates to improvements in the mixing of fluids particularly, but not exclusively, for mixing fluids in different phases.

The mixing of fluids in different phases is a continuing problem, and particularly so when a really homogenous mixture is required. This is particularly so in the field of internal combustion engines where reliably obtaining an intimate admixture of vapourized fuel and air is a key to improved engine performance. This has recently become particularly desirable because on the one hand it has been found necessary to increase fuel eceonomy, and on the other hand to decrease the proportion of partial combustion products in exhaust gases particularly NOx. Various methods have been attempted to achieve either or both of these ends.

One favoured approach is to increase combustion efficiency to a degree such that the combustion products contain virtually no partial combustion products at all. This is difficult to achieve at low engine speeds, but has the advantage of both providing a cleaner exhaust and increasing fuel efficiency when attained.

To this end various attempts have been made to increase the degree of mixing between the atomised fuel and combustion air stream in a carburetor. As a result, carburetors have become progressively more complex both to maintain and to manufacture.

Another problem which cannot be overcome by substantive redesign is that there are in existence today a large number of carburetors which by design fail to adequately mix atomised fuel and combustion air.

The use of aerofoil sections in the intake passages of carburetors is known. These have, however, previously been unsuccessful for a number of reasons.

One such is to be found in GB-A-280293 wherein a perforate wing is spring biased to a desired acute angle relative to the axis of the air intake passage. This wing is acted upon by air pressure induced by opening a throttle valve downstream thereof. The fuel jet nozzle is contained within the perforation in the wing and hence the proximity of the edges of the perforation tend to impede the acceleration of the fuel droplets and hence an uneven mixing results.

Further, in US-A-3671208 there is revealed a fluid mixing apparatus, particularly a carburetor, which includes a fixed aerofoil in the intake tube. Undoubtedly this arrangement assist in fluid mixing but because it is fixed it does not materially assist fuel atomisation at low speeds i.e. does not materially assist in reduction of MO,.

US-A-3176704 provides a shaped throttle valve in a carburetor having a substantially upturned V-shaped addition to one edge of the throttle butterfly valve assembly. This tends to act somewhat as an aerofoil at high engine speeds, but at low speeds it has a detrimental effect because the sharp edge of the V-shape tends to smoothly divide the flow at low opening angles and hence negate the aerofoil effect at low speeds.

Other shapes of butterfly valve are also known, for example from US-A-4420438 which reveals butterfly valves which are essentially tear-drop in cross-section. This arrangement tends merely to divide the flow, and any extra mixing which takes place is on the basis of an acceleration of the fluid streams rather than on the action of an aerofoil as in the present invention.

Thus in the present invention higher mixing turbulence can be caused at lower speeds than in any other known arrangement.

The present invention seeks to alleviate this and other fluid mixing problems by providing an aerofoil within a fluid mixing channel, said aerofoil preferably being rotatable to a position substantially coincident with the direction of fluid flow at high fluid speeds.

In one particular embodiment the invention provides a novel design of butterfly throttle valve which assists in mixing the atomised fuel and combustion air to improve combustion efficiency in an internal combustion engine.

According, therefore, to the present invention there is provided a carburetor comprising an air intake passage, a jet for fuel supply, and a throttle valve downstream of said jet, characterised in that the valve is a butterfly valve in the form of a complete aerofoil.

The aerofoil of the present invention is defined as a true aerofoil section which causes positive pressure over the upper curved surface thereof.

The trailing edge of the aerofoil when used in a carburetor is preferably directed towards the downstream side of fluid flow.

The invention also extends to a method of intimately admixing a fluid stream which comprises subjecting said stream to the influence of an aerofoil as defined herein within a mixing channel, characterised in that the aerofoil is rotatable about a medium axis from a position perpendicular to the stream to a position co-incident with it thereby to achieve maximum mixing turbulence for a given gluid stream velosity.

One embodiment of the present invention will now be described, by way of illustration only, with reference to the accompanying drawing wherein the Figure shows th primary section of a down-draught carburetor assembly in accordance with the present invention.

With reference to the Figure a down-draught carburetor is provided of the serial venturi type. The precise operation of carburetors of this type is well known and will not be specifically described in detail. However in general such a carburetor comprises a primary main nozzle (1), a primary main air bleed (2), a primary slow jet (3), an anti-dieselling solenoid (4), a primary slow air bleed (5), a fuel float (6), primary emulsion tube (7), primary main jet (8), primary throttle valve (9), serial venturi (11) and a choke valve (10).

This primary system is adapted for use with a secondary system which cuts in at higher speeds and is similarly provided with somewhat modified elements. In general fuel is delivered via the primary main nozzle into the smaller upper venturi (11) while air is drawn in past the choke valve (10) and flows over the serial venturi towards the primary throttle valve (9). The primary throttle valve (9) is provided with a butterfly which is in the form of a true aerofoil (12).

When a carburetor arranged as in the Figure is utilised in a motor vehicle increases in power and in miles per gallon are experienced and exhaust gases are less noxious.

The mechanism by which the improved carburetor operates is not wholly understood, however, it is believed that the mechanism for the complete aerofoil section operates as follows: With the primary throttle valve substantially closed the aerofoil has little effect over that which would be experienced with a standard butterfly valve. As the throttle opens to an intermediate position, however, turbulence is created as a result of an increase in negative pressure to one side of the butterfly aerofoil.

In its more fully opened positions a negative pressure will be increasingly created on one side of the air induction pipe adjacent the butterfly while the mixed fluid flowing past the butterfly to the aerofoil side will have an extra speed imparted to it. Accordingly, turbulence is created downstream of the aerofoil which results in increased mixing of the atomised fuel and air at a position where the operating temperature is higher than at the venturi. This increases the admixture of the fuel and air.

Improvments in mixing also occur at inter mediate throttle positions by virtue of the tur bulence caused downstream of the aerofoil at lower fluid passage speeds.

It will be appreciated that at these lower speeds with the throttle in its partially opened condition mixing is conventionally somewhat problematic, and there is a tendency therefore for a mix containing more atomised fuel than is strictly necessary to enter the combustion chambers. By increasing the turbulence at the primary throttle this problem is alleviated.

In addition to internal combustion engines for vehicles the present invention may be be neficially applied to standing engines, or to engines of the type used in small horticultural devices such as chain saws etc.

Although the present invention has been described with reference to internal combustion engines for vehicles it is equally applicable to other mixing systems wherein mixtures of fluid streams are required to be intimately admixed.

Claims (6)

1. A carburetor comprising an intake passage, a jet for fuel supply, and a throttle valve downstream of said jet Characterised in that the valve is a butterfly valve in the form of a complete aerofoil as defined.

2. A carburetor according to Claim 1 wherein the trailing edge of the aerofoil is directed to the downstream side of fluid flow.

3. A method of intimately admixing a fluid stream which comprises subjecting said stream to the influence of an aerofoil as defined, within a mixing channel, characterised in that the aerofoil is rotatable about a medium axis from a position perpendicular to the fluid stream, to a position coincident therewith.

4. A method according to Claim 3 wherein the trailing edge of the aerofoil is directed to the downstream side of the fluid flow.

5. A carburetor substantially as hereinbefore set forth with reference to, and/or as illustrated in, the Figure of the accompanying drawing.

6. A method substantially as hereinbefore set forth with reference to, and as illustrated in, the Figure of the accompanying drawing.