WO1997008443A1 - Fuel evaporators - Google Patents

Abstract

An evaporator (50) for the supply of an air/fuel mixture to a small engine consists of a flow duct (51) accommodating a stack of flat plates (52) having non-porous, non-reticulated surfaces and spaced apart by spacers (53) so as to define narrow passages for flow of air therebetween. Each of the spacers (53) is provided with through holes (54) extending through parts of the spacer (53) which have recesses (55) in their front and rear faces, the through holes (54) being aligned with corresponding holes (not shown) in the plates (52) to form fuel passages such that fuel supplied along the passages is emitted from the recesses (55) onto the evaporation surfaces transversely of the air flow supplied to the inlet of the flow duct (51) in the direction of the arrow (57), so that a fuel film (56) is produced on each evaporation surface having a thickness which reduces downstream of the supply point. Furthermore the air flow in the direction of the arrow (57) results in variation of the local air flow velocity (V) so as to produce a shear stress in the fuel film (56) which assists downstream spreading of the fuel, and evaporation of fuel from the fuel film into the air flow to produce a substantially uniform air/fuel mixture.

Description

"Fuel Evaporators"

This invention relates to evaporators for evaporation of fuel into an air flow,

and is concerned more particularly, but not exclusively, with carburettor metering

systems for small gasoline engines.

Small gasoline engines are typically single cylinder four-stroke engines of low

cost, and are used in applications such as lawnmowers and outboard motors. Such

single cylinder engines draw in air/fuel mixture intermittently, and this causes a

problem in relation to fuel metering which is not encountered to the same extent in

multi-cylinder engines as are typically used in the automotive field. Furthermore

such single cylinder engines, unlike automotive engines, operate either at a governor

controlled speed or with a fixed relationship between speed and load, and in addition

usually have fixed ignition timing. Existing carburettor metering systems for such

small engines have a tendency to produce inhomogeneous air/ fuel mixtures containing

unevaporated fuel droplets which tend to increase the quantity of hydrocarbons in the

exhaust emissions. It is desirable for carburettor metering systems to produce highly

homogenous air/fuel mixtures as this not only tends to decrease the hydrocarbons in

the exhaust emissions, but also allows engine operation with a leaner air/fuel mixture

at part load, thus reducing emissions of oxides of nitrogen and of carbon monoxide. Generally carburettor metering systems produce a pressure difference related

to air flow, and use this pressure difference to propel fuel from a constant pressure

fuel reservoir into the air flow, usually as a more or less atomised spray. In order to

produce an air/fuel mixture of highly uniform consistency, with the above mentioned

benefits in terms of exhaust emissions, it is known to supply the fuel to a fabric wick

through which the warmed air is passed to produce a substantially dry fuel vapour.

International Published Application No. WO 94/16211 discloses an evaporator

consisting of a stack of plates which are spaced apart so as to define narrow air

passages therebetween and so as to provide porous evaporation surfaces for fuel along

the sides of the passages, the evaporation surfaces being formed from sintered metal

or from fabric. The bottoms of the plates are immersed in liquid fuel so that the fuel

is caused to diffuse over the evaporation surfaces by capillary action, and the air flow

along the passages causes fuel to be evaporated from the evaporation surfaces into the

air flow to produce a homogeneous air/fuel mixture. However such an evaporator

suffers from a number of disadvantages in practice when produced at a sufficiently

low cost to render it feasible in a small engine application. One of the main

disadvantages is that, under high transients, it is possible for droplets of fuel to break

away from the evaporation surfaces into the air flow, thus causing inhomogeneity of

the resulting air/fuel mixture and a consequent increase in hydrocarbons in the

exhaust emissions. It is an object of the invention to provide a novel fuel evaporator which can

be produced at low cost and which provides uniform consistency of the air/fuel

mixture for different engine loads.

The invention is defined in the accompanying claims.

In order that the invention may be more fully understood, several

embodiments of the invention will now be described, by way of example, with

reference to the accompanying drawings, in which:

Figure 1 shows a fuel evaporator in accordance with the invention;

Figures 2, 3 and 4 are diagrams illustrating the supply of fuel to the

evaporation surfaces of the evaporator;

Figures 5, 6, and 7 show different spacer arrangements for the evaporator;

Figures 8, 9, 10 and 11 show a carburettor metering system in accordance

with the invention incoφorating a fuel valve and an air control valve;

Figure 12 is a graph illustrating the operation of the fuel valve;

Figure 13 is a diagram of an alternative carburettor metering system in

accordance with the invention, in vertical section;

Figure 14 is a section taken along the line A-A in Figure 13;

Figures 15 and 16 are diagrammatic sections through details of an alternative

evaporator which may be used in the system of Figure 13, Figure 16 shows a section taken along the line C-C in Figure 15;

Figure 17 is a diagrammatic section through a further altemative evaporator

which may be used in the system of Figure 13;

Figure 18 is a graph of air flow and fuel flow against pressure difference in

use of an evaporator according to the invention; and

Figures 19 and 20 show a modification of the carburettor metering system of

Figures 8 to 11.

The evaporators to be described below are intended to produce a highly

homogenous mixture of gasoline vapour in air for supplying to a small gasoline

engine, typically a single cylinder 4-stroke engine of low cost, and will be described

when used in a carburettor system for supplying such an engine However similar

evaporators may also be used to supply larger engines, such as multi-cylinder

automotive engines, and it is also possible to utilise such evaporators for supplying

an air/fuel mixture to other types of combustion apparatus, such as furnaces.

The illustrated carburettor metering system is provided for supplying an

air/fuel mixture to a single cylinder lawnmower engine and is dimensioned to fit

within the space available in such an application. In such governor controlled engines

the mixture quantity varies in a known way with load and can thus be used to control

the mixture strength so as to minimise exhaust emissions. Current legislation in California specifies emissions of carbon monoxide, and of combined hydrocarbons

and oxides of nitrogen. For lean engine operation with air/fuel ratios greater than

about 17: 1 the nitrogen oxide emissions fall as the mixture is made leaner, while, at

very lean mixtures, hydrocarbon emissions start to rise. Carbon monoxide emissions

are low and practically constant for ratios greater than about 16: 1. Thus the total

emissions are low over a reasonable range of mixture strengths, and this allows some

tolerance in optimising the mixture strength for low emissions. The legislation

specifies limits based on tests at idle, quarter load, half load, three quarter load and

full load, and a typical engine might require air/fuel ratios of 17: 1, 18: 1, 19: 1 and

12: 1 respectively at these load conditions.

Figure 1 shows an evaporator 50 consisting of a flow duct 51 of generally

rectangular cross-section within which a stack of flat brass plates 52 having non¬

porous, non-reticulated surfaces is accommodated. The plates 52 are spaced apart by

brass spacers 53 disposed between each pair of adjacent plates 52 so as to define

narrow passages for flow of air therebetween, the figure being broken away to show

four such spacers 53 which would normally be disposed between two of the plates 52.

Each of the spacers 53 is provided with two through holes 54 extending through

recessed parts of the spacer 52 which have recesses 55 in their front and rear faces,

the through holes 54 being aligned with corresponding holes (not shown) in the plates

52 so that four pairs of passages extend through the complete stack of plates 52. If required wires may be threaded through the passages in order to accurately align the

plates 52 with one another, although the primary function of the passages is to permit

supply of fuel along the passages so that fuel is emitted from the recesses 55 onto the

evaporation surfaces transversely of each of the spacers 53 in both directions and thus

transversely of the air flow supplied to the inlet of the flow duct 51 in the direction

of the arrow 57.

Such fuel supply to the evaporation surface of a plate 52 from a supply recess

55, as shown diagrammatically Figure 2, results in the fuel spreading laterally by a

combination of the momentum of the fuel passing through the recess 55 and surface

wetting, so that a fuel film 56 is produced having a thickness which reduces

downstream of the supply point. Furthermore the air flow in the direction of the

arrow 57 results in variation of the local air flow velocity V in the manner shown

graphically in broken lines by the velocity distribution curve 58, resulting in a

velocity gradient in the air flow downstream of the fuel supply point producing a

shear stress in the fuel film 56 which assists downstream spreading of the fuel. In

practice the effect of gravity on fuel spreading is small compared with the air flow

shear stress. The object of the fuel supply system is to supply fuel to the evaporation

surfaces close to the point of air entry and at a substantially uniform rate. To this end

each supply point should offer a small consistent restriction to flow of fuel

therethrough. Figure 3 shows the supply of fuel to the evaporation surface at one side of a

spacer 53 by way of the recess 55, it being understood that a corresponding supply

of fuel to the evaporation surface will also be provided from the recess 55 at the other

side of the spacer 53 (and similar fuel supply will be provided at the opposite face of

the spacer 53 to the evaporation surface on the opposite side of the passage).

Figure 4 shows corresponding supply of fuel to an evaporation surface in a

modified arrangement in which spacers 59 are provided which are similar to the

spacers 53 but which do not include recesses 55. Instead each of the plates 52 is

provided with slots 60 extending outwardly from the holes through the plates 52 to

positions beyond the sides of the spacers 59 so that fuel supply to the evaporation

surfaces takes place from the ends of the slots 60 extending through the plates 52

themselves (rather than through recesses in the spacers).

Figure 5 is a section in the vicinity of one wall 62 of the flow duct 51 showing

a milled fuel manifold 63 having an inlet 64 for the supply of fuel thereto from a fuel

reservoir (not shown). From the fuel manifold 63 fuel is supplied along the passage

65 extending through the stack of plates 52 to permit fuel supply to the evaporation

surfaces of the individual plates 52 in the manner already described. As shown in

Figure 5 the spacers 53' are in the form of washers provided with grooves (not

shown) in both faces for fuel supply to the evaporation surfaces, this representing a modification of the spacer arrangement shown in Figure 1. The plates 52 are held

in register by a guide wire 66 extending through the passage 65 with clearance.

A further possible spacer arrangement is shown in Figures 6 and 7 in which

each pair of adjacent plates 52 is separated by a single unitary spacer member 70.

Each spacer member 70 has parts 71 provided with through holes 72 and front and

rear recesses 73 in a generally similar manner to the spacers 53 of Figure 1 , the parts

71 being interconnected by integral web parts 74 of reduced thickness so as to provide

the required throughflow cross-section for the input of air as shown by the arrows 75

and for the output of the air/fuel mixture as shown by the arrows 76. Locating nibs

77 may be provided on the spacer members 70, as shown in Figure 7, in order to

correctly locate the plates 52 relative to the spacer members 70 so that the holes 72

in the spacer parts 71 are aligned with the holes 78 in the plates 52. Fuel supply to

the evaporation surfaces through the recesses 73 occurs in the manner previously

described. Numerous other spacer arrangements are possible within the scope of the

invention, and in particular spacers may be constituted by integral portions of the

plates.

Figures 8 and 9 show a carburettor metering system 1 comprising a housing

2 having two parts 3 and 4. Figure 8 shows a vertical section through the housing

part 3 which incorporates an air control valve 5, a fuel valve 6, an air cleaner 7 and a float bowl 8. The air control valve 5 comprises an air valve gate 9 in the form of

an arm 10 pivotally connected at one end to the housing part 3 by a pivot hinge or

flexure 11, and an air valve seat 12 in the form of a sill located at some distance from

the pivot hinge or flexure 11. The arm 10 extends substantially across the whole of

the top of the housing part 3, but is provided with sufficient clearance at its edges to

permit it to pivot within the housing part 3 and to permit passage of air into or out

of the space of the above arm 10 during movement of the gate 9. The restricted

passage for air flow between the space above the arm 10 and the space downstream

of the air control valve 5 restricts the rate of movement of the gate 9 and thus

provides damping so as to prevent oscillation and permit extra fuel supply for

acceleration. Furthermore the gate 9 includes a gate member 14 having the shape of

an arc centered about the pivot point and depending downwardly from the arm 10.

The gate member 14 has a cut-out 15, as may be seen in the detail of the gate

member 14 shown in Figure 10, having a profiled edge 13 which defines with the

straight edge of the sill 12 an air flow orifice 16. It will be appreciated that, as the

gate 9 is pivoted by air pressure acting against gravity and/or an optional spring 17,

as shown in broken lines in Figure 8, the gate member 14 is moved upwardly or

downwardly in the direction of the arrow 18 in Figure 10 relative to the seat 12 in

order to vary the throughflow cross-section of the orifice 16.

A small governor-controlled engine typically has an air flow range of 3 or 4 to 1, and such an air valve gate may be satisfactorily utilised to provide the required

metering to produce a substantially constant air/fuel mixture or more likely an air/fuel

mixture having a ratio which varies with load in order to minimise emissions. In a

governor-controlled engine the air flow rate varies with load, and in such a case the

shape of the profiled edge 13 defining the air flow orifice can be modified to provide

any required variation of air/fuel mixture strength against air flow. The gate profile

can also be modified to accommodate the use of the optional spring 17 so that the

pressure difference across the gate varies with flow.

The fuel valve 6 comprises a fuel valve body 20 incoφorating an axial bore

21 having a constant circular cross-section along its length and a fuel outlet 22 at its

lower end, as well as a transverse passage 23 intersecting the bore 21 and providing

a fuel inlet. The fuel valve 6 also incoφorates a fuel valve member 25 having a

constant circular cross-section along its length substantially matching the cross-section

of the bore 21 into which the valve member 25 extends, but having a slightly smaller

diameter to enable the valve member 25 to move freely up and down within the bore

21. The valve member 25 has an integral extension member 26 having a head 27

which engages the lower surface of the pivotal arm 10 of the air valve gate 9 such

that pivotal movement of the gate 9 causes corresponding movement of the valve

member 25 within the bore 21 so as to vary the length of an annular passage 28

provided between the outside wall of the valve member 25 and the inside wall of the bore 21 for fuel flowing from the transverse passage 23 along the bore 21 to the fuel

outlet 22. The head 26 of the extension member 27 is maintained in contact with the

air valve gate 9 by a compression spring 26A.

Fuel is supplied to the transverse passage 23 from the float bowl 8 by way of

a fuel supply passage 29, and the passage of fuel along the bore 21 to the fuel outlet

22 is restricted by the presence of the cylindrical valve member 25 by an amount

which depends on the length by which the lower end of the valve member 25 extends

below the transverse bore 23, which may be referred to as the length of overlap.

Since the air pressure at the free surface of the fuel within the float bowl 8 is equal

to the air pressure upstream of the air control valve 5 and since the air pressure at the

location to which the fuel is supplied downstream of the fuel valve 6 is equal to the

air pressure downstream of the air control valve 5, the pressure difference driving the

fuel through the fuel valve 6 corresponds to the pressure difference across the air

control valve 5 required to lift the air valve gate 9 against gravity and/or the spring

17.

The pressure difference across the air control valve 5 will be approximately

proportional to the square of the ratio of the air mass flow to the area of the orifice

16 for air flow through the valve, whereas the fuel flow through the fuel valve 6 will

be proportional to the pressure difference across the valve divided by the length of overlap (since other geometric factors will remain constant as the length of overlap

varies). If the pressure difference across the fuel valve 6 is constant (that is affected

only by gravity and not by spring pressure), the length of overlap multiplied by the

area of the throughflow orifice through the air control valve 5 will need to be constant

if the air/fuel ratio is to remain constant. Such constancy requires that the width of

the cut-out 15 of the air control valve 5 at the level of the sill formed by the air valve

seat 12 should be proportional to the square of the reciprocal of the length of overlap.

Such a relationship is shown graphically in Figure 12 which plots the width of the

cut-out 15 at the level of the sill against the length of overlap.

The air which passes through the air control valve 5 then passes into the

housing part 4 by way of an air outlet 30 as shown in Figure 9, and from there passes

through an evaporator 31 consisting of a series of parallel plates 32 spaced apart so

as to define narrow passages 33 therebetween and providing evaporation surfaces 34

along the sides of the passages 33. Figure 11 is an exploded view of one of the plates

32 showing that each plate 32 is in fact made up of three parts bonded together (or

possibly formed integrally), namely a manifold portion 33 sandwiched between two

perforated outer portions 34 and 35. The manifold portion 33 has a T-shaped

aperture extending therethrough constituting a manifold chamber 36, and each of the

perforated outer portions 34 and 35 has a number of fine pores 37 extending

therethrough and communicating with the manifold chamber 36. Each of the pores 37 is formed by a laser in the portions 34 and 35, which are preferably metal sheets

or foils, and has a diameter of 20-40 microns, and the pores 37 are spaced about 2

mm apart, about 10 pores being spaced along a line in each portion. Furthermore

each perforated portion 34 or 35 is provided with an aperture 38 extending

therethrough and communicating with the upright portion of the T-shaped manifold

chamber 36. A lip 39 surrounds the aperture 38 in at least one of the portions 34 and

35 so that, when the plates 32 are bonded together, the lips 39 bridge the gaps

between the plates and have the effect of forming a continuous fuel inlet duct

supplying fuel to each of the manifold chambers 36. In addition nipples 40 on at least

one of the portions 34 and 35 of each plate 32 serve to space apart the plates 32 with

the required narrow passages 33 therebetween.

In operation of the evaporator 31 fuel from the outlet 22 of the fuel valve 6

is supplied to the manifold chambers 36 by way of the apertures 38, and at the same

time air passing through the air outlet 30 is caused to enter the narrow passages 33

between the plates 32 by way of the evaporator inlet 42 (see Figure 9). As a result

of the pressure difference across the fuel valve 6, fuel from the manifold chambers

36 is caused to seep through the pores 37 so as to form a thin fuel film on the

evaporation surfaces 34 on the outsides of the plates 32. The air flow along the

narrow passages 33 between the plates 32, which occurs generally in the direction of

the arrow 43 in Figure 11 , causes evaporation of fuel from the evaporation surfaces 34 so that a highly homogenous air/fuel mixture is outputted through the evaporator

oudet 44 towards an exit tube 45 of the system as shown in Figure 9. The small size

of the pores 37 substantially eliminates the production of droplets in the air flow

which would otherwise cause inhomogeneities in the air/fuel mixture. Any fuel from

the fuel films on the evaporation surfaces 34 which passes towards the bottom of the

evaporation surfaces without being evaporated into the air flow may be collected and

conducted to the float bowl 8 for recirculation within the system. The evaporator 31

may be replaced by an evaporator having non-porous, non-reticulated evaporation

surfaces as described above with reference to Figures 1 to 7 if required.

As described in detail in International Published Application No. WO

94/16211, the exit tube 45 incorporates a valve member (not shown) which serves to

change the system between two modes of carburettor operation, namely lean

operation, which is provided up to about three quarters load, and rich operation in

which additional fuel may be supplied by way of a fixed fuel restrictor provided for

starting and full load operation in parallel with the fuel valve 6. In lean operation the

throttle is progressively opened by the governor up to full throttle as the load is

increased. Thereafter, as the load is further increased, change over to rich operation

takes place, and the throttle is closed as additional fuel is introduced in order to

prevent a stepwise increase in torque. Further increase in load leads to progressive

opening of the throttle again, up to full throttle. Figure 19 shows a variant 1 ' of the carburettor metering system previously

described with reference to Figures 8 to 12, the variant incoφorating an air control

valve 5' and a fuel valve 6', and the parts of this variant being indicated in Figure 19

by the same reference numerals as the corresponding parts in Figure 8 with primes

added thereto. In this case the complete housing part 3' is sealed and contains an

evaporator 31 ' supported by shoulders 150 above an exit tube 45'. A main air inlet

150 is partially covered by the gate member 14' of the air control valve 5' , and the

upper face of the gate 9" is subjected to the housing pressure while the lower face of

the gate 9' is subjected to the outside pressure applied by way of an air restrictor 152

whose function is to damp out oscillations of the gate 9' and to delay movement of

the gate 9' in response to air flow changes, thus providing temporary mixture

enrichment during acceleration and improving the response to transient demands. If

required, the pivoted gate 9' can be replaced by a bellows or piston arrangement.

Figure 20 shows three possible fuel valve arrangements, each of which will require

a corresponding profile of the cut-out in the gate member 14' of the air control valve

5 ' so that the relationship between air flow and air/fuel ratio is appropriate for the

application. In A the fuel valve member 25' has a constant circular cross-section

along its length whereas, in C, part of the fuel valve member 25' is tapered with a

conical angle matching the conical angle of a section of the bore within which the

tapered part is displaceable so as to define therebetween an annular passage having

a length which varies in proportion to the width of the passage (and such that, when the valve member is in the closed position, the width and length of the passage is

substantially zero. In C a tapered part of the fuel valve member 25' is displaceable

to vary the throughflow cross-section through a restriction 154.

Figure 13 shows an alternative carburettor metering system 101 for supplying

an air/fuel mixture to a governor-controlled single cylinder lawnmower engine, the

system comprising an evaporator 102 for receiving fuel from a fuel reservoir 103 and

air from an air inlet 104 and for supplying air/ fuel mixture to an engine by way of

an outlet 106 in dependence on the position of a throttle 105. The fuel reservoir 103

incoφorates a float 107 for controlling the level 108 of fuel in the reservoir 103 by

means of a valve 109.

The evaporator 102 consists of a flow duct 110 of generally rectangular cross-

section within which a stack of generally parallel metal plates 111 is accommodated.

The plates 111 are spaced apart by spacers (not shown) which define narrow passages

113 for flow of air between the plates 111. Each of the plates 111 has a cross-

sectional shape as shown in Figure 1 having a thickness which is at a maximum close

to the air inlet 104 to the evaporator 102 and which decreases progressively towards

the outlet 106 of the evaporator 102. Thus the evaporation surfaces 114 provided on

both sides of the plates 111 (apart from the outermost plates which have such an

evaporation surface on only one side) are shaped so as to ensure that the air passages 113 have a cross-section which is at a minimum close to the air inlet 104 and which

increases progressively towards the outlet. Each air passage 113 is in the form of a

venturi having a throat 115 in the vicinity of a fuel supply pipe 112 by means of

which fuel is supplied to the plates 111.

In operation of the carburettor metering system, air drawn in through the inlet

104 passes through a gate valve 116 which is in the form of a hinged flap to be lifted

by the air flow against its own weight and/or a light spring and which provides a

substantially constant pressure difference in the air flow. The air then flows along

the narrow passages 113 between the plates 111 of the evaporator 102, and, at the

throat 115 of each venturi, provides a pressure depression which causes fuel to flow

from the fuel supply pipe 112 onto the evaporation surfaces 114, the fuel being

supplied to the fuel supply from the fuel reservoir 103 by way of a restrictor 117.

A fuel film of large surface area is thus provided over the evaporation surfaces 113

of the plates 111, and the passage of the air, which has been warmed prior to entry

into the inlet 104, over the evaporation surfaces 114 results in evaporation of fuel into

the air flow.

The pressure applied to the fuel surface within the float-controlled fuel

reservoir 103 is related to the air pressure at the inlet 104 with a result that the sum

of the pressure difference across the fuel restrictor 117 and the hydrostatic pressure difference (due to the difference between the level of fuel in the fuel supply pipe 112

and the fuel level 108 in the fuel reservoir 103) is the same as the sum of the air

pressure difference across the gate valve 116 and the pressure drop at the venturi

throats 115. Thus the evaporation of fuel into the air flow is proportional to the air

flow, and a substantially constant air/fuel mixture of highly uniform consistency is

obtained.

Such a carburettor metering system may be used to supply a substantially

constant air/fuel mixture or more likely an air/fuel mixture having a ratio which

varies with load in order to minimise emissions. The provision of the gate valve 116

enables the fuel supply pipe 102 to be arranged at a level higher than the fuel level

108 in the fuel reservoir 103 so as to prevent leakage of fuel in the evaporator 102

in the absence of the required air pressure.

Figures 15 and 16 show sections through a modified carburettor 120 which

may be used in such a system. As will be apparent from Figure 15, showing a

section taken along the line D-D in Figure 16, the plates 121 are of constant thickness

along their lengths. However the narrow passages 122 between the plates 121

nevertheless constitute Venturis by virtue of the fact that, as will be appreciated from

Figure 16 showing a section taken along the line C-C in Figure 15, adjacent plates

121 are spaced apart by spacers 123 providing shaped surfaces 124 on opposite sides of the intervening passage 122 which define a throat 125 close to the air inlet at which

the passage cross-section is at a minimum. In this case fuel is supplied to the plates

123 in the vicinity of the throats 125 of the Venturis by means of fuel supply pipes

126.

Figure 17 is a sectional view, similar to that of Figure 16, of a further

modified evaporator 130 which may be used in such a system. In this case the plates

of the evaporator are again of substantially constant thickness as shown in Figure 15.

However each plate is separated by a shaped central spacer 132 such that air flows

occur in parallel along passage portions 133 and 134 on the two sides of the spacer

132. It will be appreciated that each passage portion 132 and 133 constitutes a

venturi, and that accordingly the required control of fuel supply may be obtained by

supplying fuel to the evaporator plates in the vicinity of the throat of each venturi.

Figure 18 is a graph showing both the air flow (solid line) and the fuel flow

(broken line) within the evaporator against the applied pressure difference ΔP across

the evaporator. Air flow is initiated at a particular pressure difference sufficient to

overcome the resistance of the gate valve 116, and fuel flow is initiated at a pressure

difference sufficient to overcome the hydrostatic pressure due to the difference in

level between the fuel supply pipe 112 of the evaporator and the fuel level in the fuel

reservoir. It will be appreciated that the fuel flow will be substantially proportional to the air flow over a substantial pressure range R with the result that the required

substantially constant mixture strength will be obtained over a wide range of load in

a small governor-controlled engine.

Claims

1. An evaporator for evaporation of fuel into an air flow, the evaporator

comprising a series of plates which are spaced apart so as to define narrow passages

therebetween and which provide non-porous, non-reticulated evaporation surfaces

along opposite sides of the passages, fuel inlet means for supplying fuel to the plates

in such a manner as to form a fuel film on the evaporation surfaces, air inlet means

for providing an air flow along the passages in such a manner as to spread the fuel

film along the evaporation surfaces and so as to evaporate fuel from the fuel film into

the air flow to produce an air/fuel mixture, and outiet means for output of the air/fuel

mixture from the passages remotely of the fuel inlet means and the air inlet means.

2. An evaporator according to claim 1, wherein the fuel inlet means is adapted

to supply the fuel to each plate through a plurality of apertures opening on each

evaporation surface and spaced apart over the evaporation surface in a direction

transverse to the air flow along the passages.

3. An evaporator for evaporation of fuel into an air flow, the evaporator

comprising a series of plates which are spaced apart so as to define narrow passages

therebetween and which provide evaporation surfaces along opposite sides of the

passages, fuel inlet means incorporating a series of small apertures through which fuel is introduced into the passages in such a manner as to form a fuel film on the

evaporation surfaces, air inlet means for providing an air flow along the passages in

order to produce an air/fuel mixture by evaporation of fuel from the fuel film into the

air flow along the passages, and outlet means for output of the air/fuel mixture from

the passages remotely of the fuel inlet means and the air inlet means.

4. An evaporator according to claim 3, wherein at least one of the plates

comprises a manifold portion defining a fuel chamber to which fuel is supplied, and

at least one perforated outer portion through which the apertures extend so that fuel

from the fuel chamber is introduced through the apertures into the passage at the or

each side of the plate.

5. An evaporator according to any preceding claim, wherein the passages have

a cross-section which is at a minimum close to the air inlet means and which increases

progressively towards the outlet means.

6. An evaporator according to claim 5, wherein each of the passages is in the

form of a venturi extending between the air inlet means and the outlet means.

7. An evaporator according to claim 5 or 6, wherein the plates are shaped so as

to have a thickness which is at a maximum close to the air inlet means and which decreases progressively towards the outlet means so as to ensure that the cross-section

of the passages increases towards the outlet means.

8. An evaporator according to claim 5, 6 or 7, wherein each passage is delimited

laterally by shaped spacers on opposite sides of the passage which define a throat

close to the air inlet means at which the passage cross-section is at a minimum and

a diverging passage portion in which the passage cross-section increases progressively

towards the outlet means.

9. An evaporator according to claim 5, 6, 7 or 8, wherein each passage is

divided into two passage portions by a shaped central spacer spacing apart the

adjacent plates such that air flows occur in parallel along the passage portions

between the air inlet means and the outlet means and the cross-section of each passage

portion is at a minimum close to the air inlet means and increases progressively

towards the outlet means.

10. A carburettor metering system comprising a fuel valve for varying the supply

of fuel into an air flow, the fuel valve comprising a fuel valve body incoφorating an

axial bore having a fuel outlet or inlet at one end, and a transverse passage

intersecting the bore and providing a fuel inlet or outlet, and an elongate fuel valve

member axially movable within the bore, and an air control valve for varying the air flow, the air control valve incorporating an air valve seat, and an air valve gate which

is movable relative to the air valve seat to vary the throughflow cross-section through

the air control valve and which is coupled to the fuel valve member so as to vary the

fuel flow from the fuel outlet into the air flow downstream of the air control valve.

11. A system according to claim 10, wherein the axial bore in the fuel valve body

has a constant cross-section along at least part of its length and the fuel valve member

has a constant cross-section along at least part of its length matching said cross-

section of the axial bore so as to provide an annular passage of variable length

between the outside wall of the fuel valve member and the inside wall of the bore for

fuel flowing from the fuel inlet towards the fuel outlet.

12. A system according to claim 11, wherein the air valve gate and/or the air

valve seat has a profiled edge such that the total throughflow cross-section of at least

one air flow orifice defined between the air valve gate and the air valve seat varies

substantially in proportion to the reciprocal of the length of the annular passage within

the fuel valve for a required substantially constant air/ fuel mixture strength.

13. A system according to claim 10, 11 or 12, wherein the air valve gate is

pivotally connected at one end to a fixed housing and is provided at its opposite end

with a depending lip which overlaps a sill on the housing constituting the air valve seat so that the air valve gate is pivotable under air pressure acting on the air valve

gate so as to vary the throughflow cross-section for passage of air between the air

valve gate and the air valve seat.

14. A system according to claim 10, 11, 12 or 13, wherein the fuel valve member

has an integral extension member having a head which is engaged by the air valve

gate such that movement of the air valve gate causes corresponding movement of the

fuel valve member.