GB1593199A

GB1593199A - Carburettor for an internal combustion engine

Info

Publication number: GB1593199A
Application number: GB4731677A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1977-11-14
Filing date: 1977-11-14
Publication date: 1981-07-15

Description

(54) CARBURETTOR FOR AN INTERNAL COMBUSTION ENGINE (71) I, JACK KENNETH IBBOTT, a citizen of Australia, of 4-17-7 Nishi Azabu, Minato-ku, Tokyo, Japan, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a carburettor in which the vaporization rate of volatile fuels to be fed into combustion chambers in an internal combustion engine is increased.

The vaporization rate of volatile fuels, such as petrol, which are to be fed to combustion chambers in the engine is closely related to the amount of carbon monoxide(CO) and hydrocarbon(HC) discharged through the exhaust pipe from a motor vehicle, and also to the fuel economy. The discharged carbon monoxide and hydrocarbon cause pollution of air, which, as is well known, is very harmful to human health.

Many proposals have been made for increasing the vaporization rate of the volatile fuels in a carburettor, such as to provide a mesh, foraminous material, or a porous material of coarse open porosity within the carburettor for breaking the fuel into a fine spray as it is drawn through the said material, as shown in U.S. Patents 1,327,233, 1,857,565 and 2,149,460 and British Patent 245,254.

However, these proposals do not provide the desired effect and in many cases have proved to cause a reduction in engine power and thus unsatisfactory in use.

According to one aspect of the invention there is provided a carburettor for an internal combustion engine, the carburettor comprising: venturi means into which passes air, the venturi including a throat and a portion diverging in the downstream direction from the throat, the dynamic pressure in the venturi means decreasing in the downstream direction from the throat when the carburettor is in use; main fuel delivery orifice means for spraying liquid fuel into the interior of the venturi means adjacent the throat so that in use a portion of the sprayed fuel is converted into vapour which joins with the said air to form an air and vaporized fuel mixture and a further portion of the sprayed fuel is thrown in liquid form against the interior walls of the diverging portion of the venturi; and a butterfly throttle valve positioned down stream of the venturi means and comprising a disc having first and second surfaces arranged so that except when the valve is wide open they face respectively towards and away from the venturi means, the static pressure within the carburettor adjacent tbe first surface being, in use, greater than the static pressure adjacent the second surface;; means being provided at and only at the venturi means and/or the said disc for vaporizing liquid fuel impinging thereon, the said means comprising porous material having a mean pore size of less than 12 , the porous material being capable of being wetted by and absorbing the fuel in liquid form so that the wetted porous material prevents the passage therethrough of the air and vaporized fuel mixture, the liquid fuel absorbed by the porous material being released therefrom in vapour form in the downstream direction.

According to a second aspect of the invention there is provided a carburettor for an internal combustion engine, the carburettor comprising: a throttle butterfly valve disc made of porous material having a mean pore size of less than 12 1; and a carburettor venturi coated over a portion thereof with a porous material having a mean pore size of less than 12 L, the coated portion extending longitudinally from a first part of the venturi to a second part of the throat whose diameter is larger than that of the said first part.

According to a third aspect of the invention there IS provided a carburettor for an internal combustion engine, the carburettor compris mg: a throttle butterfly valve disc made of porous material having a mean pore size of less than 12 ; and a venturi tube made over a portion thereof of porous material having a mean pore size of less than 12 1, the said portion extending longitudinally from a first part of said ven turi tube to a second part of the said venturi tube whose diameter is larger than that of the said first part.

The liquid fuel sprayed into the carburettor from a fuel delivery orifice is partly vapourized by the venturi action, while a part of the fuel fed therein remains still in the liquid state.

The vapourized portion of the fuel is intimately mixed with the air and therefore it is not desirable or necessary to pass this air/fuel mixture through any vapourization device in view of the restricted flow caused therethrough. The fuel in the liquid state causes an increase of the amount of carbon monoxide and hydrocarbon in the exhaust gas and poor fuel economy. Accordingly, in the present invention only that fuel which is still in the liquid state passes through a vapourization device and the mixture of air and vapourized fuel does not pass therethrough.

The porous material used in the invention readily absorbs liquid fuel by capillary action and will then not allow the mixture of air and vapourized fuel to pass therethrough.

Such porous material does not release the absorbed liquid so readily but readily releases the absorbed liquid in the form of a gas or vapour. In tests using porous materials having different pore sizes, it was noted that the finer the pore size of the porous material the stronger the capillary action it exerted to absorb volatile liquid fuel rapidly, to retain the liquid fuel strongly and to release the fuel in the vapour state rapidly. The invention uses porous material having a mean pore size of less than 12 ,a. However, a mean pore size of less than 2 LLt iS preferable.

As used therein, the term "porous material" means a material having minute openings having a mean diameter which is less than 12 , and being in the nature of minute cells or spaces formed between particles of solid substance, the said minute cells being interconnected by minute spaces.

In the present invention the porous material is present at a part or parts where a difference in pressure is developed within the carburettor from a relatively high dynamic or static pressure to a relatively low dynamic or static pressure in the direction of flow of the airfuel stream by a displacement of linear flow of the air-fuel stream.

The porous material may be any substance so long as it is stable, retains its form at the temperature prevailing and is unaffected by the fuel. Substances which may be used include Plaster of Paris, gypsum, ceramic, sintered metal, any substance which can be formed into a fine powder and then by means, for example, of heat or pressure be formed into a compacted mass retaining the desired pore size, or a substance like Plaster of Paris which will form itself into the required porous mass when treated with some setting agent which in the case of Plaster of Paris is the simple procedure of mixing the dry Plaster of Paris powder with water, and allowing it to set.

When the porous material is wetted or loaded with a liquid (fuel) it acquires the characteristics of a non-porous material, that is, the liquid filling the pores of the porous material forms a "blockage" and will prevent the passage of any gas or air therethrough unless considerable pressure is applied sufficient to dislodge the liquid contained in the material pores. Such pressure is in excess of any pressure which might exist under normal operating conditions within a carburettor.

It is not desired that the liquid fuel be forced from out of the porous material in the form of a liquid spray. Rather, the porous material must strongly retain the liquid and allow it to be released in the form of a gas or vapour.

The fine porosity of the material promotes rapid conversion of the liquid fuel into the gas or vapour state.

The smaller the pore size the better the conversion from liquid state to vapour state.

It is believed that when the liquid fuel is absorbed into the porous material the liquid is dispersed throughout the pores and broken into individual parts of pore size. With such finely dispersed fuel the conversion from liquid state to gas or vapour state more readily takes place. The porous material is of such fine pore size that it can be considered almost as a catalyst in the physical sense or promoting the physical change from liquid state to gas or vapour state.

It is important that the porous material is used only in those areas or parts of the carburettor where a difference in pressure is developed from a relatively high static or dynamic pressure to a relatively low static or dynamic pressure in the direction of travel of the air-fuel stream by a displacement of linear flow of the air-fuel stream. The porous material may be used as a longitudinal or transverse communication link between the areas of high and low pressure in the direction of the air stream through the carburettor.

The use of porous material in the primary and secondary venturis may be cited as examples of longitudinal communication between a high pressure area of smaller diameter and a low pressure area of larger diameter. The use of porous material for the butterfly disc valve is an example of transverse communication between the high pressure area of the upstream (carburettor side) and the low pressure area of the down-stream (intake passage) side. The use of such porous material in any area where there is no communication between a relatively high pressure area and a relatively low pressure area will have no effect or even a loss of performance below that of a standard carburettor.

As can be noted from the above disclosure, preferable parts of the carburettor where the porous materials are provided are as follows: (1) Primary venturi tube, (2) Carburettor throat, or secondary ven turi, (3) Butterfly throttle valve.

For this purpose the primary venturi tube and the throttle valve butterfly are formed or made of the porous material, while the secondary venturi is coated with the porous material.

When the all of the above parts are formed of or coated with the porous material, the vapounzation rate of the fuel is most greatly improved since the vaporization rate thereof is cumulatively enhanced thereby. However, a considerable improvement in the vaporization rate of the fuel is also obtained when either only the venturi tube or only the throttle valve butterfly are formed of the porous material.

Reference is now made to the requirements when using the porous material in the various parts of the carburettor mentioned above.

a) Primary venturi.

It is important that the pore size of the material be very fine to prevent the passage of air through the porous part. In this primary venturi there is a region of relatively high dynamic pressure at the small diameter part thereof and a region of relatively low dynamic pressure at the larger diameter part downstream thereof. However, there is also a region of relatively high pressure on the outside surface of the primary venturi and if the material of the primary venturi were to allow the passage of air therethrough transversely this would lower the efficiency of the venturi action because the difference in pressure developed between the small diameter area and the larger diameter area would be reduced.

Furthermore, if the pore size is large enough to permit the passage of air therethrough this will defeat the object of the invention, since air passing through the wet porous material would carry with it wet or liquid fuel in the form of bubbles or spray and to some extent condensation of the liquid fuel particles would occur and combination into larger drops of liquid. To achieve peak performance the porous material of the primary venturi should have such fine pore size that when wet with the liquid fuel absorbed into the pores it will resist the passage of air therethrough and hold the liquid fuel strongly. The effect of the fine pore size combined with a lowering of pressure provides efficient gasification of the absorbed liquid fuel.The primary venturi should only be porous in that part which expands in diameter in the direction of the air stream, that is from the smaller diameter to the larger diameter, and should not be porous at the mouth of the primary venturi where the diameter is changing from a larger diameter to a small diameter in the direction of the air stream. The use of the porous material at the mouth of the primary venturi where the air is entering has a negative effect in that it partly cancels out the effect of the porous material used at the expanding part where the air is being expanded and the dynamic pressure being lowered.

b) Secondary venturi.

In a relatively lower air speed area as compared with the primary venturi there is a smaller difference in pressure between the relatively high pressure and relatively low pressure areas. The porous material must therefore be in the form of a very thin layer or film when the communication between the relatively high pressure area and the relatively low pressure area is in the longitudinal sense.

Also, as in the case of the primary venturi the porous layer should only communicate between the smaller diameter area and the larger diameter area in the direction of the air stream and not be applied at the mouth of the throat where the air is entering.

It has been found that any use of the porous material, in a thin or thick layer, in the linear parts of the carburettor where there was no communication between a relatively high pressure and a relatively low pressure, that is, where there is no displacement of the linear flow of the air fuel stream, such as the straight sides barrel following the large venturi or throat, causes a drastic drop in performance and the engine becomes very sluggish.

c) Butterfly throttle valve.

In the - case of the butterfly valve the porous material is used in the transverse sense with the high pressure area on one side of the disc of porous material and the low pressure area on the other side of the disc of porous material. The pore size must be homogeneous for optimum conditions and must, according to the invention, be less than 22 p. This is especially important for the butterfly valve at which the maximum pressure difference within the carburettor occurs, the pressure being as much as 14.7 psi at the high pressure side and as little as 5 psi at the low pressure side when the engine is idling or running down hill with the throttle closed.The porous material should have a high resistance to "blow through", that is, the pressure difference across the material should not cause the absorbed liquid petrol in the pores of the porous material to be blown through and out the low pressure sides in the form of a liquid spray.

The accompanying drawing is a partially sectioned perspective view showing a fuel feeding device for an internal combustion engine, the device incorporating the present invention.

The fuel feeding device shown in the drawing comprises a carburettor 1 which is connected to a float chamber 2 from which fuel 3 in the float chamber 2 is supplied into the carburettor 1 through a supply passage 4 opening into the carburettor 1 via a main fuel delivery orifice 5 or another supply passage 6 opening into the carburettor idle and intermediate fuel delivery orifices 7 and 8.

A primary venturi tube 9 is concentrically supported within the carburettor 1 by radial arms 10, through one of which the tip portion of the supply passage extends, and the main fuel delivery orifice 5 opens in a throat in the axial centre of the primary venturi tube 9.

The primary venturi tube 9 has an intemal shape which rapidly converges to a relatively small diameter then slowly expands. The outside form is linear while the internal shape defines a throat. The internal shape of the carburettor 1 below the primary venturi tube 9 is similar to the internal shape of the primary venturi tube 9, thereby forming a secondary venturi 11. Provided below the carburettor throat 11 is a disc-shaped butterfly throttle valve 12 which is rotatably supported within the carburettor 1 by a shaft 13. The idle and intermediate fuel delivery orifices 7 and 8 open into the carburettor 1 at places adjacent to the valve 12.

The present invention is applied to such a known fuel feeding device as described above without alterating or modifying the shape and configuration of the device in substance.

Porous material 1 la having a mean pore size of less than 12 z1 and preferably less than 2 y is provided or coated on the internal surface of the secondary venturi 11, while the primary venturi tube 9 and the throttle valve butterfly 12 are formed of porous material having a mean pore size of less than 12 ,u.

In the primary venturi tube 9 a reduction in the dynamic pressure develops in the expanding part thereof when air is drawn through the primary venturi tube 9 from the smaller diameter portion in the direction of the relatively slowly expanding diameter. This reduction in dynamic pressure is caused by the displacement of the linear air flow, and the pressure reduction draws liquid fuel through the main fuel delivery orifice 5 causing the liquid fuel to be sprayed into the air stream passing through the primary venturi tube 9.In view of the increasing diameter of that part of the primary venturi tube 9, the stream of air entering at the small diameter end is caused to expand (displacement of the linear flow) as it passes through the part of the primary venturi tube 9 of increasing diameter, this expansion causes the liquid fuel in the spray emitted from the main fuel delivery orifice 5 to be thrown on to the inside walls of the expanding part of the primary venturi tube 9 as shown by arrows A, and also because of the reduction in dynamic pressure as mentioned above some part of the fuel emitted from the main fuel delivery orifice 5 is converted into a vapour.

In a known venturi tube made of solid metal, that liquid part of the fuel spray which is thrown on to the inside wall of the primary venturi tube drains down the walls and drips off the lower edge of the primary venturi tube.

In this manner part of the effect of the primary venturi tube and fuel delivery orifice 5 in breaking the liquid fuel into a spray is neutralized because of the spray being condensed back into large drops of liquid fuel.

When that part 9a of the primary venturi tube 9 which is of increasing diameter in the direction of the air stream therethrough, is made of a very finely porous material of less than 12 P mean pore size in place of the stan dard solid metal, some of the liquid fuel which is thrown on to the inside wall 9a is absorbed into the porous material. Because there is a drop in dynamic pressure downstream from a relatively high dynamic pressure at the throat the liquid fuel so absorbed will be released from the porous wall in the form of a gas or vapour as it progresses towards the relatively lower dynamic pressure.

The absorbed liquid fuel will not condense and drip off the lower edge of the primary venturi tube 9 as in the case of the standard solid metal.

Thus, when that part 9a of the primary venturi tube 9 extending from the smaller diameter to the larger diameter in the direction of the air stream therethrough is formed or made of very finely porous material of less than 12 mean pore size the vaporization rate of the fuel passing therethrough is greatly improved as compared with that attainded by a standard primary venturi tube made of solid metal. Furthermore condensation of the fuel into large drops is also prevented.

The secondary venturi 11 has a similar shape to the internal shape of the primary venturi tube 9 and the action is the same as that of the primary venturi tube 9 in that the dynamic pressure decreases from the throat thereof as one moves in a downstream direction therefrom. The normal action of the secondary venturi 11 is to further vaporize the fuel coming from the primary venturi tube by the dynamic pressure reduction therein.

As the air-fuel stream leaves the primary venturi tube 9 the liquid fuel is thrown against the walls of the secondary venturi 11 and drains down the metal walls of the secondary venturi.

When the inner wall of the secondary venturi 11 is coated or provided with a porous material 1 la having a mean pore size of less than 12 ,,a, only in a part extending downstream from the throat thereof, the fuel spray thrown against the material 1 la of the throat as shown by arrows B will be absorbed into the porous material and be released, as shown by arrows C, in the form of a gas or vapour from the porous material 1 la of the secondary venturi 11 as it progresses downstream. This action is the same as previously described for the primary venturi tube 9.

Thus, when the secondary venturi 11 is coated or provided with the finely porous material ila of less than 12 s mean pore size, the vaporization rate of the fuel is improved as compared with that attained by a standard secondary venturi 11 having solid inner walls, and furthermore, the fuel is restrained from dripping or draining down the secondary venturi walls into the intake manifold of the engine.

In actual experiments the porous material used was Plaster of Paris mixed to a very thin consistency with water and lighly painted over the required area with a brush. Attempts to apply a thicker layer of the porous material over the area resulted in an downgrading of the performance. The indication used for the preferable thickness of the porous layer was that the layer would become transparent when wet with fuel. Any thickness greater than this which would not become transparent when wet with fuel resulted in a loss of efficiency. Preferably the thickness of the porous layer is less than 0.3 mm.

The butterfly throttle valve is used to control the amount of fuel and air delivered into the internal combustion engine, and accord ingly controls the speed of the engine. A known butterfly valve is normally a solid metal disc attached to a shaft 13, whereby the rotation of the shaft 13 causes the 'butterfly valve to move in such a manner as to control the amount of air-fuel mixture being drawn into the internal combustion engine.

According to the present invention, the butterfly valve 12 may be composed entirely of very finely porous material of less than 12 Sr mean pore size. In this case the butter fly valve still controls the speed of the internal combustion engine as with the normal metal disc butterfly valve.However, as the butter fly valve is made of very finely porous material having a mean pore size of less than 12 y liquid fuel carried in the air-fuel stream coming from the upstream parts of the carburettor will impinge on and be absorbed into the upper surface as shown by arrows D, the surface of the porous butterfly valve 12 facing towards the oncoming air-fuel stream, and as the pore size of the material of the butterfly valve is very small the absorbed liquid fuel will substantially restrict the flow of air and fuel vapour therethrough the air and fuel vapour will pass around the edge of the butterfly valve 12 as shown by arrow E.

Because there is a difference in pressure between the upper surface of the porous butterfly valve and the lower surface thereof, the lower surface being at a lower pressure, the liquid fuel impinging on and being absorbed into the upper surface will be released from the lower surface in the form of a gas or vapour.

Thus, the use of a butterfly valve of such porous material greatly improves the vaporization rate of the fuel compared with that achieved by a buterfly valve made of solid metal.

The device as shown in the drawing was subjected to tests to check the amount of CO and HC emissions in the exhaust gas and driving distance per unit of fuel. The tests were carried out using a 1600 cc Isuzu (Registered Trade Mark) Florian with and without the porous material in the carburettor.

The porous material used in the present device was Plaster of Paris having a mean pore size of less than 12 p. The porous material was coated over the required area of the carburettor venturi to a thickness of about 0.2 mm. The butterfly valve was made of Plaster of Paris formed over a metal web frame for structural strength, the thickness of the valve being approximately 2.5 mm.

In the tests, gas sensors for CO and HC were installed in the car and meters reading the relative amounts of each gas were installed in front of the driver to show instant readings of gas emission under the particular driving conditions. These meter readings were only relative and did not show actual percentages, parts per million, or grmtkilometre. Another meter showing an instant reading kilometres/ litre was also installed, and in this case the meter was accurately calibrated and showed actual readings of kilometres/litre.

Test Results

Standard carburettor without porous carburettor with material porous material (metre readings) (metre readings) CO emission engine idling 3 1.5 to 2 driving (40 km/hr.) 10+ 3.5 to 4 HC emission engine idling 3 2 accelerating 10+ 4 driving (40 kn/hr.) 9 3 Kilometres/litre 7 to 8 10 to 11 (city driving) In the above table, the meter reading shown as 10+ means that the meter indicator had passed the maximum reading and was hard against the stop.These readings could have been of the order of 15 or more had the meter scale been extended.

It was found in the tests that the use of porous material in the carburettor enabled the fuel delivery orifices to be reduced in size by about 30% for the same engine power and performance as obtained with the standard carburettor. This was because of the increase in the amount of fuel being vaporized.

After using a carburettor according to the invention for approximately 500 kilometres it was found that the spark plugs had become clean and had no deposit of black carbon.

These spark plugs had been previously used with a standard carburettor and had developed a heavy deposit of carbon on the metal base part.

It was also noted in the tests that the black carbon deposit on the inside of the exhaust pipe had changed and the colour had become a medium brown. This indicated the absence of free carbon in the engine exhaust and the brown colour was from the additives in the fuel.

As will be apparent from the above, by using the invention, the liquid fuel coming into and out of the carburettor is effectively gasified or vaporized by the very finely porous material, and the gasified or vaporized fuel is intimately mixed with the air-fuel stream, so that the airfuel mixture fed into the combustion chambers in the engine will be almost completely burnt with the result that the emissions of CO and HC in the exhaust gas are considerably reduced and the economy of the vehicle using the present device is considerably increased.

WHAT WE CLAIM IS: 1. A carburettor for an internal combustion engine, the carburettor comprising: venturi means into which passes air, the venturi including a throat and a portion diverging in the downstream direction from the throat, the dynamic pressure in the venturi means decreasing in the downstream direction from the throat when the carburettor is in use; main fuel delivery orifice means for spraying liquid fuel into the interior of the venturi means adjacent the throat so that in use a portion of the sprayed fuel is converted into vapour which joins with the said air to form an air and vaporized fuel mixture and a further portion of the sprayed fuel is thrown in liquid form against the interior walls of the diverging portion of the venturi; and a butterfly throttle valve positioned downstream of the venturi means and comprising a disc having first and second surfaces arranged so that except when the valve is wide open they face respectively towards and away from the venturi means, the static pressure within the carburettor adjacent the first surface being,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Test Results Standard carburettor without porous carburettor with material porous material (metre readings) (metre readings) CO emission engine idling 3 1.5 to 2 driving (40 km/hr.) 10+ 3.5 to 4 HC emission engine idling 3 2 accelerating 10+ 4 driving (40 kn/hr.) 9 3 Kilometres/litre 7 to 8 10 to 11 (city driving) In the above table, the meter reading shown as 10+ means that the meter indicator had passed the maximum reading and was hard against the stop.These readings could have been of the order of 15 or more had the meter scale been extended. It was found in the tests that the use of porous material in the carburettor enabled the fuel delivery orifices to be reduced in size by about 30% for the same engine power and performance as obtained with the standard carburettor. This was because of the increase in the amount of fuel being vaporized. After using a carburettor according to the invention for approximately 500 kilometres it was found that the spark plugs had become clean and had no deposit of black carbon. These spark plugs had been previously used with a standard carburettor and had developed a heavy deposit of carbon on the metal base part. It was also noted in the tests that the black carbon deposit on the inside of the exhaust pipe had changed and the colour had become a medium brown. This indicated the absence of free carbon in the engine exhaust and the brown colour was from the additives in the fuel. As will be apparent from the above, by using the invention, the liquid fuel coming into and out of the carburettor is effectively gasified or vaporized by the very finely porous material, and the gasified or vaporized fuel is intimately mixed with the air-fuel stream, so that the airfuel mixture fed into the combustion chambers in the engine will be almost completely burnt with the result that the emissions of CO and HC in the exhaust gas are considerably reduced and the economy of the vehicle using the present device is considerably increased. WHAT WE CLAIM IS:

1. A carburettor for an internal combustion engine, the carburettor comprising: venturi means into which passes air, the venturi including a throat and a portion diverging in the downstream direction from the throat, the dynamic pressure in the venturi means decreasing in the downstream direction from the throat when the carburettor is in use; main fuel delivery orifice means for spraying liquid fuel into the interior of the venturi means adjacent the throat so that in use a portion of the sprayed fuel is converted into vapour which joins with the said air to form an air and vaporized fuel mixture and a further portion of the sprayed fuel is thrown in liquid form against the interior walls of the diverging portion of the venturi; and a butterfly throttle valve positioned downstream of the venturi means and comprising a disc having first and second surfaces arranged so that except when the valve is wide open they face respectively towards and away from the venturi means, the static pressure within the carburettor adjacent the first surface being,

in use, greater than the static pressure adjacent the second surface; means being provided at and only at the venturi means and/or the said disc for vaporizing liquid fuel impinging thereon, the said means comprising porous material having a mean pore size of less than 12 , , the porous material being capable of being wetted by and absorbing the fuel in liquid form so that the wetter porous material prevents the passage therethrough of the air and vaporized fuel mixture, the liquid fuel absorbed by the porous material being released therefrom in vapour form in the downstream direction.

2. A carburettor as claimed in claim 1, wherein the porous material is provided at the venturi means, from the throat longitudinally througthout the downstream diverging portion only.

3. A carburettor as claimed in claim 2, wherein the venturi means comprises a primary venturi tube and a secondary venturi, only the primary venturi tube being provided with the porous material, at a part thereof which extends from the throat throughout the downstream diverging portion.

4. A carburettor as claimed in claim 3, wherein the said part of the venturi tube is made of the porous material.

5. A carburetor as claimed in claim 2, wherein the venturi means comprises a primary venturi tube and a secondary venturi, only the secondary venturi being provided with porous material, at a part thereof which extends from the throat throughout the downstream diverging portion.

6. A carburettor as claimed in claim 5, wherein the porous material is coated over the said part of the secondary venturi to a thickness of less than 0.3 mm.

7. A carburettor as claimed in any preceding claim, wherein the butterfly valve disc is made of the porous material.

8. A carburettor for an internal combustion engine, the carburettor comprising: a throttle butterfly valve disc made of porous material having a mean pore size of less than 12 ,U; and a carburettor venturi coated over a portion thereof with a porous material having a mean pore size of less than 12 Y, the coated portion extending longitudinally from a first part of the venturi to a second part of the throat, whose diameter is larger than that of the said first part.

9. A carburettor as claimed in claim 8, further comprising a venturi tube made over a portion thereof of porous material having a mean pore size of less than 12 s, the said portion of the venturi tube extending longitudinally from a first part thereof to a second part thereof whose diameter is larger than that of the said first part.

10. A carburettor for an internal combustion engine, the carburettor comprising: a throttle butterfly valve disc made of porous material having a mean pore size of less than 12us; and a venturi tube made of a portion thereof of porous material having a mean pore size of less than 12 ,u, the said portion extending longitudinally from a first part of said venturi tube to a second part of said venturi tube whose diameter is larger than that of the said first part.

11. A carburettor for an internal combustion engine, substantially as herein described with reference to the accompanying drawing.