WO1999034098A1

WO1999034098A1 - Method and apparatus for removing particulates from the exhaust stream of an internal combustion engine

Info

Publication number: WO1999034098A1
Application number: PCT/GB1998/003914
Authority: WO
Inventors: Allyn Boyes; Andrew Ian Briggs
Original assignee: Notetry Limited
Priority date: 1997-12-24
Filing date: 1998-12-24
Publication date: 1999-07-08
Also published as: TW413711B; GB2332631B; GB2332631A; GB2332631A9; AU1775999A; GB9809101D0

Abstract

A method of removing particulates from the exhaust gas stream of an internal combustion engine (20) comprising the steps of combining the exhaust gas stream with a vaporised liquid; cooling the gas stream and vaporised liquid so that the vaporised liquid condenses onto the particulates; passing the gas stream and condensed liquid through a separating device (50) so that the condensed liquid and particulates are removed from the gas stream; removing the particulates from the condensed liquid; and recirculating the condensed liquid for reintroduction to the exhaust gas stream in order to effect removal of further particulates therefrom; characterised in that the liquid is an oil and all of the condensed oil is collected and recirculated for reintroduction into the gas stream so as to provide a transportable system suitable for use in motor vehicles. The invention also provides apparatus for removing particulates from the exhaust gas stream of an internal combustion engine (20) comprising; a flow path (21, 22, 23) for carrying the gas stream between an inlet and an outlet; introduction means (30) for introducing to the gas stream either a vaporised liquid or an atomized liquid which is subsequently vaporised; cooling means (40) for cooling the gas stream and vaporised liquid so as to condense the vaporised liquid onto the particulates entrained within the gas stream; a separating device (50) located downstream of the cooling means (40) for separating the condensed liquid and particulates from the gas stream; particulate removing means (80) for removing the particulates from the condensed liquid; and recirculation means (25, 26, 27, 70) for recirculating the condensed liquid back to the introduction means; characterised in that the liquid is an oil and the recirculation means (25, 26, 27, 70) are adapted to recirculate all of the condensed oil back to the introduction means (30) so that the apparatus (10) is transportable.

Description

Method and Apparatus for Removing Particulates from the Exhaust Stream of an Internal Combustion Engine

The invention relates to a method and apparatus for removing particulates from the exhaust stream of an internal combustion engine. Particularly, but not exclusively, the invention relates to a method and apparatus for removing particulates from the exhaust of a diesel engine.

Many gas streams have particulates entrained within them and it is often desirable to remove those particulates, particularly when the gas stream is likely to be vented to the atmosphere. The most common methods of separation of the particulates from the gas stream use either mesh filters or pore filters, through which the gas stream passes so that the particulates are entrained in the mesh or on the surface of the ceramic, or inertial separators such as a centrifugal separator in which the heavy particulates are flung against the walls of a chamber in order to separate them from the gas stream. Both methods have disadvantages. Mesh or pore filters become choked or clogged and require periodic cleaning or replacement. This is particularly problematic for fine particles as the filters are often more prone to blockage. This reduces the useful life of such filters. Another problem, when such a filter is being used to remove soot particles from the exhaust stream of a vehicle, is that soot particles cake the face of the filter. Although the soot may be removed from the surface of clogged ceramic filters by burning the soot off, this, however, requires the vehicle to be out of service or new filters to be fitted. Centrifugal separators can be more bulky and more expensive to manufacture and install than a comparable mesh or ceramic filter. Further, such a separator is less efficient when dealing with very fine particulates and has great difficulty separating particulates having a diameter below a specific size (around 1 μm) without significantly increasing the cost of the separator to an unacceptable degree.

An example of a gas stream having very fine particulates entrained within it and from which it would be desirable to remove at least some of those particulates is the exhaust of an internal combustion engine, particularly a diesel engine. Very fine carbon particulates are entrained within a diesel exhaust and the volume of particulates increases with the load conditions of the engine. It is well recognized that these particulates can be harmful to the population and to the environment if released into the atmosphere. Some proposals have been made to remove such particulates from combustion engine exhausts, but the small size of the particulates, 0.1 μm and below, makes centrifugal separators unsuitable. Mesh-type or particularly pore filters able to cope with the separation of particulates of the relevant size would, if manufactured to be sufficiently small to be installed in a vehicle, prevent the necessary free flow of the exhaust and would also become blocked very quickly. Centrifugal separators do not operate efficiently to remove particulates of such a small size because the particulates have low mass/inertia. The forces required to effect separation are outside the current range of efficient cyclone design.

It is known to increase the separation efficiency of a separation system designed to remove particulates from a gas stream by enlarging the particulates so that they can be more easily separated using conventional equipment. The particulates are enlarged or grown by injecting steam into the gas stream and cooling the stream so that water vapour condenses on and around the particulates. The grown particulates are then removed from the gas stream in a conventional way, such as by using a wet scrubber. Full details of this type of arrangement are given in GB 1 435 192 Teller and US 5 176 723 Liu.

The main problem associated with separation systems of this type is the fact that they make use of water to grow the particulates and the water is then released or expelled from the system. This means that a sufficient supply of water must be made available to operate the system. When the system is static this presents no problem. However, the requirement for connection to a water supply or reservoir makes this type of system completely unsuitable for use in a vehicle. An object of the present invention is to provide a method and apparatus for removing particulates from the exhaust stream of an internal combustion engine which is more efficient at removing very fine particulates than known methods and apparatus. Another object of the invention is to provide a method and apparatus which is suitable for removing carbon particulates from the exhaust of a diesel engine. A further object of the invention is to provide a method and apparatus for reducing the paniculate content of a gas stream having an enhanced efficiency without affecting the general combustion apparatus.

The invention provides a method as set out in Claim 1 and apparatus as set out in Claim 18. Advantageous features are set out in the subsidiary claims.

Using an oil to condense onto the particulates effectively increases the diameter and the mass of each paniculate. The increase is sufficient to ensure that the particles, now suspended within a droplet of condensed oil behave like the equivalent relatively large particles. They can therefore be efficiently separated, using suitable separation means, from the gas stream. The separation means can be centrifugal or another form of separator or a filter. The removal of the particulates from the oil and the recirculation of the oil for reintroduction means that the system can be made self-contained and therefore transportable. This allows the system to be applied to the exhaust of an internal combustion engine forming part of a vehicle. The vaporised oil preferably possesses the necessary qualities of a low vapour pressure liquid at relatively elevated temperatures. Preferably, the oil has a boiling point within the range of 250 - 350°C at normal atmospheric pressure and is inert at these temperatures. Such an oil may be a mineral oil or vegetable oil. A system according to the invention is capable of significantly reducing the content of carbon particulates in a diesel exhaust prior to emission of the gas stream into the atmosphere without introducing unacceptable increases in the volume or cost of the exhaust system. This results in fewer pollutants entering the atmosphere with obvious beneficial results. An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:-

Figure 1 is a schematic representation of apparatus according to the invention and shown connected to the exhaust of an internal combustion engine;

Figure 2a shows, in cross section and on an enlarged scale, injection means forming part of the apparatus shown in Figure 1 ;

Figure 2b and 2c show in cross section alternative injection means;

Figure 3a shows, in cross section and on an enlarged scale, cooling and condensing apparatus forming part of the apparatus shown in Figure 1 ;

Figure 3b shows an alternative cooling and condensing apparatus;

Figure 4a shows in horizontal cross section a cyclonic separator forming part of the apparatus shown in Figure 1 ;

Figure 4b shows in cross section a collecting chamber associated with the cyclonic separator shown in Figure 4;

Figure 5 illustrates a filtration separator capable of replacing the cyclonic separator illustrated in Figure 4;

Figure 6 illustrates in cross section filtration means forming part of the apparatus shown in Figure 1 ;

Figure 7 is a schematic representation of test apparatus; Figures 8a and 8b show the particle distribution resulting from a first series of tests; and

Figure 9 shows the particle distribution resulting from a second series of tests.

The apparatus shown in Figure 1 illustrates how the invention can be utilized in after- treatment of exhaust products containing unacceptable levels of particulates, for example in conjunction with an internal combustion engine in order to reduce the paniculate content of the exhaust therefrom. The apparatus 10 is shown connected to the exhaust of an engine 20 via a conduit 21. The engine 20 can be any form of internal combustion engine, although it is envisaged that the invention will be particularly applicable to diesel engines or to other compression-ignition engines.

The apparatus 10 essentially consists of a series of conduits connecting a plurality of elements together. The inlet conduit 21 carries the hot exhaust, in which carbon particulates are entrained, from the engine 20 to injection means 30 in which an atomized oil is injected into the exhaust stream. The atomized oil is then vaporised by the hot exhaust gas. Alternatively, the oil can be vaporised prior to injection into the exhaust stream. The exhaust stream is then carried via conduit 22 through cooling and condensing apparatus 40. The effect of the cooling and condensing apparatus 40 is to allow the oil vapour in the exhaust stream to condense on and around the carbon particulates entrained within the exhaust stream. The result is that a significant proportion of the carbon particulates leave the cooling and condensing apparatus 40 suspended within a droplet of oil. This effectively increases the mass and diameter of each respective carbon paniculate so that, when the exhaust stream enters a separator 50, for example a cyclonic separator, the relatively large droplets can be separated easily from the exhaust gas. The exhaust gas exits the cyclonic separator 50 via an outlet conduit 23 whilst the separated droplets, with the carbon particulates suspended therein, drop down into a collecting chamber 60 via a conduit 24. An outlet conduit 25 carries the collected oil, with the carbon particulates suspended therein, to a pump 70 which feeds the oil to filtration means 80 via another conduit 26. In the filtration means 80, the carbon particulates can be separated effectively from the oil which is then recirculated back to the injection means 30 via a conduit 27 in order to be reintroduced into the exhaust stream and revaporised in order to effect further separation of carbon particulates.

Having described the apparatus and its manner of operation in broad terms, the individual components will now be described in more detail.

Figure 2a illustrates in cross section the injection means 30 of the apparatus 10. The injection means 30 include a chamber 32 having a cross sectional area which is somewhat larger than that of the inlet conduit 21. The enlarged cross section of the chamber 32 allows the fluids introduced thereinto to mix well by providing a sufficient residence time within the chamber 32. The inlet conduit 21 communicates with the chamber 32 on a first side thereof and the conduit 22 communicates with the chamber 32 on the opposite side thereof. Located on the lower side of the chamber 32, and projecting thereinto, is an injector nozzle 34. The injector nozzle 34 is illustrated simply in the drawings and can take the form of an aerosol injector, a spray atomizer or an engine-type diesel injector. All three forms of injector are well known and require no further description here. All that is required is that the injector nozzle 34 is capable of introducing to the chamber 32 a fine mist or spray of the oil to be vaporised and ensuring good turbulent mixing to help reduce the length of mixing time required. The oil mixes with the exhaust gas within the chamber 32 and quickly vaporises before exiting with the exhaust gas via conduit 22. In Figure 2a, the carbon particulates entrained within the exhaust gas are illustrated using reference numeral 12. For illustrative purposes, the vaporised oil is illustrated using reference numeral 14. It will be understood that the oil may be introduced in the form of a very fine mist or a true vapour. The illustration is not intended to imply that droplets of oil must be introduced via the injector nozzle 34.

The oil which is introduced into the chamber 32 via the injector nozzle 34 is a mineral oil having a boiling point of around 300°C at atmospheric pressure. Other oils can be used. (It will be understood that the term "oil" is here used to mean any liquid having a high boiling point and suitable for use in an application of this sort). The oil is introduced to the chamber 32 via the nozzle 34 and is heated by the hot exhaust gas to effect vapourisation. In the event that the exhaust gas temperature is too low to evaporate the injected mist, the mist may be heated by, for example, an electric heater located downstream of the injector 34 to cause the mist to evaporate. Conversely, if the exhaust gas temperature is too high, this would cause the injected oil mist to burn rather than to just vaporise. In these circumstances, it will be necessary to cool the exhaust gas prior to injecting the mist.

An alternative form of injection means 30 is illustrated in Figure 2b. In this embodiment, eight mjector nozzles 34a are arranged around an annular support 36 and these nozzles are in direct communication with the conduit 27 by means of which the oil to be vaporised is carried to the injection means 30. The injector nozzles 34a surround a central chamber 32a which need not be any larger in cross section than the inlet conduit 21 or the conduit 22. The fact that a plurality of mjector nozzles 34a are provided, and their spacing around the chamber 32a, means that mixing of the vaporised liquid with the exhaust gas is swift and effective.

A further alternative form of injection means is illustrated in Figure 2c. In this embodiment, and injector nozzle 34 is arranged in the path of the conduit 12. The outlet of the injector nozzle 34 is directed down stream to improve the mixing of the mist and exhaust gas.

Figures 3a and 3b illustrate alternative cooling and condensing means 40. In Figure 3a, the conduit 22 is surrounded by a chamber 42 having an inlet 44 and outlet 46. The chamber 42 is sealed around the conduit 22 so that a coolant 48 can be made to flow through the chamber 42 thus having a cooling effect on the contents of the conduit 22. The coolant 48 will pass through some form of second heat exchange device after leaving the exit 46 and before returning to the inlet 44. Any appropriate form of heat exchange device can be used, including a simple array of air cooled fins. When the apparatus 10 used in conjunction with the internal combustion engine of a vehicle, the vehicle's onboard cooling system can also be used.

The effect of cooling the exhaust stream flowing along conduit 22 is to encourage the vaporised oil to condense within the conduit 22. At least some of the vaporised oil will condense onto the carbon particulates also entrained within the exhaust stream. As the condensation proceeds, a significant proportion of the carbon particulates become surrounded by and suspended within droplets of the condensed oil. Each carbon paniculate therefore increases its diameter and mass, effectively making it many times larger than its true size.

The same effect can be achieved by the provision of a cooling coil 42a wound around the conduit 22 as illustrated in Figure 3b. The coil 42a has an inlet 44 and an outlet 46 and carries the coolant 48. Again, some sort of heat exchange device is required to be positioned between the outlet 46 and the inlet 44 so that the recirculating coolant 48 enters the coil 42a at a lower temperature than that at which it leaves the coil.

A suitable coolant would be any water/glycol mix.

The conduit 22 carries the exhaust stream, with the carbon particulates suspended within droplets entrained therein, to the tangential inlet of a cyclonic separator 50. The cyclonic separator 50 is shown in horizontal cross section in Figure 4a. The tangential arrangement of the inlet 52 causes the exhaust stream to swirl around the conically tapering wall 54 and the consequent acceleration of the exhaust stream causes the droplets 16 to be separated from the exhaust stream. The exhaust stream passes towards the central axis of the cyclonic separator 50 and exits the cyclonic separator 50 via the outlet pipe 23. The separated droplets of oil with the carbon particulates suspended therein drop down under the force of gravity into the conduit 24 and fall into the collected chamber 60. The collecting chamber 60 is illustrated in Figure 4b. It is essentially a simple container in which the oil 62 is collected. The oil 62 has carbon particulates suspended therein. The container 60 has an outlet 64 to which the conduit 25 is connected.

Use of a cyclonic separator will cause a certain loss of heat from the exhaust stream as well as losses along the length of the conduit. Therefore, a separate upstream heat exchanger as described above may not necessarily need to be provided.

It will be appreciated that the cyclonic separator 50 can easily be replaced by a simple filtration separator 50a such as that illustrated in Figure 5. The filtration separator 50a consists of a simple mesh or filter 52a through which the exhaust stream is directed. The droplets 16 collect on the mesh and subsequently fall into the collecting chamber 60 positioned beneath the filtration separator 50a. Periodic replacement of the mesh 52a may be required.

The conduit 25 carries the oil 62 to a pump 70 which is a simple pump and requires no further description. The pump 70 can be supplied with its own motor or, alternatively, it can be connected to a vehicle's onboard systems. The purpose of the pump 70 is to return the oil 62 to the injection means 30 via the filtration means 80. The conduit 26 carries the oil from the pump 70 to the filtration means 80.

The filtration means 80 are illustrated in Figure 6. The filtration means 80 consist of a chamber 82 having a filtration cartridge 84 suspended therein. The conduit 26 carries the oil 62 directly to the interior of the filtration cartridge. The carbon particulates 12 are then retained in the filtration cartridge 84 whilst the oil droplets 18 are returned via conduit 27 to the injection means 30. The filtration cartridge 84 is advantageously removable and replaceable in order that deposits of carbon particulates can be safely disposed of and so that the filtration efficiency of the filter cartridge 84 can be maintained. To remove the cartridge 84, the upper surface 82a of the chamber 82 can be made removable and the cartridge itself can be made releasable from the conduit 26. It will be appreciated that the condensed oil is to be revaporised. The injection means 30 may therefore include means for heating and compressing the oil before it is injected into the exhaust stream or alternatively heating means can also be located in the conduit 27 separately from the injection means 30 to heat the atomized oil. Standard equipment can be used to achieve this.

It is widely recognized that the concentration of carbon particulates appearing in the exhaust of a diesel engine is dependent upon the loading of the diesel engine. Engines operated at high load produce more carbon particulates. It is therefore advantageous if the apparatus described above also incorporates means for adjusting the amount of oil injected into the exhaust stream in accordance with the expected concentration of carbon particulates. In a preferred embodiment of the invention, therefore, either a manual adjusting device is included in the injection means, perhaps in the form of a calibrated valve for adjusting the rate of flow of the oil to the injection means 30, or else means are provided for detecting a parameter capable of indicating of the level of carbon particulates in the exhaust stream and for adjusting the amount of oil injected into the stream in response to the parameter. For example, a control system can be incorporated into a vehicle which monitors the loading of the engine and a signal can be passed directly to the injection means 30 in order to control the amount of oil injected into the exhaust stream. Other ways of monitoring the loading of the engine will be apparent to a reader skilled in the art and these too can be used to control the amount of injected oil.

It is envisaged that the oil which is vaporised and then condensed on and around the carbon particulates can be mineral oil or even a simple vegetable oil. Many such oils boil and recondense at a temperature of around 300°C. Internal combustion engines generally have exhaust temperatures of somewhere around 350°C, although temperatures of around 500°C can be reached with high loads. Assuming that the exhaust stream enters the conduit 21 at a temperature of approximately 350°C, it is envisaged that the injected oil would be a suitable mineral or vegetable oil which is atomized then heated by the hot exhaust gas to vapourise it. In the event of high loads in which the exhaust gas is too hot and causes the atomized oil to burn, the exhaust gas is cooled prior to its introduction into the injection means 30. As the mixture of exhaust gases and vaporised oil passes through the cooling and condensing apparatus, the mixture is cooled to approximately 280°C, or approximately 20-40°C below the boiling point of the oil at atmospheric pressure. This is sufficient to ensure that substantially all of the vaporised oil is condensed thus allowing the oil to be separated from the gas stream and recirculated for further use. Vegetable oils which are considered to be suitable for use are refined and deodorised soyabean or rapeseed oil. Sesame oil (refined and deodorised) is also believed to be suitable because of its non- viscous properties. Standard mineral oils, such as the lubricating oils already used in many motor vehicles, are equally suitable, as are more inert oils such as synthetic carbon-hydrogen oils, silicon-based oils and perfluorinated carbon oils. The more inert the oil, the less likely it is to degrade during operation of the process described above.

Alternatively, an oil can be used which has a boiling point at atmospheric pressure which is substantially higher than the exhaust temperature of the engine. If the oil is heated under pressure to a temperature above its boiling point and then injected into the exhaust stream at the temperature, thus causing vapourisation, the mixing of the vapour with the cooler exhaust stream will cause the oil to condense. The presence of separate cooling and condensing means is then unnecessary.

Tests have been carried out to confirm that particulates can be "grown" in a manner as described above, using two different engines; a Perkins 4 litre, 4 cylinder turbocharged diesel engine and a Cummins 5.9 litre, 6 cylinder turbocharged direct injected diesel engine. The test apparatus is illustrated in Figure 7. The results of the tests are shown graphically in Figures 8a and 8B and Figure 9, respectively.

The first series of tests were carried out on a Perkins 4 litre, 4 cylinder turbocharged diesel engine type 1004-4.

The test apparatus as shown in Figure 7 comprises the Perkins engine 101 which is loaded by an eddy current dynamometer 102 with water cooling. The exhaust pipe 108 of the engine 101 comprises a stainless steel pipe having a diameter of 75mm. Injection means 104 is provided downstream of the exhaust pipe 108 which atomizes oil fed from a reservoir 103 and introduces the atomized oil into the pipe 108. The injection means 104 comprises a Taylor scientific series smoke-aerosol generator for atomizing the oil. The oil droplets are then introduced into the exhaust gas stream by a Bird and Tolle type BTS176 air ejector.

A sample 106 is provided downstream of the injection means 104 approximately 2.2m from the point of injection. The sampler 106 comprises an isokinetic probe which is located centrally in the exhaust pipe and a PALAS dilution system which is set to dilute the sampled stream at 100 to 1. The sampler 106 is connected to a particle size measuring device 107 which is an electrical low pressure impactor manufactured by DEKATI. The impactor measures the particle diameter and numbers of particles in twelve bands from 30nm to lOμm. The data is stored on a computer for later analysis. The exhaust gas is then finally passed out into the atmosphere.

During the tests, the engine was operated at 750RPM full torque with a load of 288nM. This gave an exhaust gas flow of approximately 35 htre/s. The temperature at the point of injection was measured and maintained at 382°C. The temperature at the sample point was 270°C. The drop in temperature from the point of injection to the sample point was a result of losses to atmosphere over the length of the pipe 108 from the injection point to the sample point. The drop in temperature was sufficient to condense the oil droplets and therefore a cooler 105, shown in Figure 7, was not required. Shell Ondina EL oil was used and injected into the exhaust gas stream at a rate of approximately lOOmg/hour.

The resulting particle distribution before and after oil injection is shown in the graphs of Figures 8a and 8b. As can be clearly seen, the distribution of the particle shifts so that there is an increase in the number of larger particles so that inertial separation will be more effective. A second series of tests were carried out on a Cummins 5.9 litre, 6 cylinder turbocharged direct injected diesel engine.

The test apparatus comprises a Cummins engine 101 connected to a load 102 which is a water brake. The exhaust gas from the engine 101 is carried through an exhaust pipe 108. The pipe 108 comprises a tee section 109 in which a portion of the exhaust gas is vented to the atmosphere under the control of valves so that a controlled flow rate of exhaust gas is passed through injection means 104 and a cooler 105. The flow rate is set to approximately 18 litre/s. The exhaust pipe 108 is a stainless steel pipe having a diameter of 50mm.

The test apparatus further comprises injection means 104 immediately downstream of the tee section 109 and further downstream a cooler 105. The length of the pipe 108 between the injection point and the exit of the cooler 105 is approximately 2.5m. The injection means 104 corresponds to that utilized in the first tests described above. The cooler 105 is of the type shown in Figures 3a and 3b and the coolant was water with a flow rate of approximately 10 litres per minute. Immediately downstream of the cooler 105, the exhaust gas passes into a Richard Ohver dilution tunnel from which a sample is extracted by an isokinetic probe. The sample is then passed to a particle size measuring device 107 as utilized in the first tests described above.

The oil droplets are generated by a plurality of proprietary nebulisers, for example 3 or 4 nebulisers, to provide a flow of 42 mg/hour. The engine was operated at 1200RPM with a load of 540nM. The exhaust gas produced had a temperature at the injection point of 430°C and a temperature at the sample point of 200-250°C. Shell Ondina EL oil was used and injected into the exhaust gas stream at a rate of 42mg/hour.

The resulting particle distribution before and after oil injection is shown in the graph of Figure 9. As can be clearly seen, the distribution of the particle shifts so that there is an increase in the number of larger particles so that inertial separation will be more effective.

Claims

1. A method of removing particulates from the exhaust gas stream of an internal combustion engine comprising the steps of:

combining the exhaust gas stream with a vaporised liquid;

cooling the gas stream and vaporised liquid so that the vaporised liquid condenses onto the particulates;

passing the gas stream and condensed liquid through a separating device so that the condensed liquid and particulates are removed from the gas stream;

removing the particulates from the condensed liquid; and

recirculating the condensed liquid for reintroduction to uic ..╬¢w╬▒ust gas stream in order to effect removal of further particulates therefrom;

characterised in that the liquid is an oil and all of the condensed oil is collected and recirculated for reintroduction into the gas stream so as to provide a transportable system suitable for use in motor vehicles.

2. A method as claimed in Claim 1 , wherein the oil is introduced as a liquid to the gas stream at a point at which the temperature of the gas stream is above the boiling point of the oil so that the oil is vaporised on introduction of the oil to the gas stream.

3. A method as claimed in Claim 2, wherein the oil is introduced to the gas stream as an atomized spray.

4. A method as claimed in Claim 2 or 3, wherein the temperature of the gas stream is at least 20┬░C above the boiling point of the oil when the oil is introduced thereto.

5. A method as claimed in any one of the preceding claims, wherein the amount of vaporised oil introduced to the gas stream is controlled in dependence upon the expected concentration of particulates in the gas stream.

6. A method as claimed in any one of the preceding claims, wherein the gas stream and vaporised oil are cooled to a temperature at least 10┬░C below the boiling point of the oil.

7. A method as claimed in Claim 6, wherein the gas stream and vaporised oil are cooled to a temperature at least 20┬░C below the boiling point of the oil.

8. A method as claimed in any one of the preceding claims, wherein the gas stream and vaporised oil are passed through a heat exchanger in order to effect cooling.

9. A method as claimed in Claim 8, wherein the coolant used to cool the heat exchanger is the same as the coolant used to cool the internal combustion engine.

10. A method as claimed in any one of the preceding claims, wherein the condensed oil and particulates are separated from the gas stream by means of a centrifugal separator.

11. A method as claimed in Claim 10, wherein the condensed oil and particulates are separated from the gas stream by means of a cyclonic separator.

12. A method as claimed in Claim 10 or 1 1, wherein the centrifugal separator acts as a silencer to reduce the acoustic energy of the gas stream.

13. A method as claimed in any one of the preceding claims, wherein the particulates are removed from the condensed oil by means of filtration.

14. A method as claimed in any one of Claims 1 to 12, wherein the particulates are removed from the condensed oil by means of a centrifugal separator.

15. A method as claimed in Claim 14, wherein the particulates are removed from the condensed oil by means of a liquid cyclone.

16. A method as claimed in any one of the preceding claims, wherein the oil has a boiling point of between 200┬░C and 400┬░C.

17. A method of removing particulates from the exhaust gas stream of an internal combustion engine substantially as hereinbefore described with reference to any one of the embodiments shown in the accompanying drawings.

18. Apparatus for removing particulates from the exhaust gas stream of an internal combustion engine comprising:

a flow path for carrying the gas stream between an inlet and an outlet;

introduction means for introducing to the gas stream either a vaporised liquid or an atomized liquid which is subsequently vaporised;

cooling means for cooling the gas stream and vaporised liquid so as to condense the vaporised liquid onto the particulates entrained within the gas stream;

a separating device located downstream of the cooling means for separating the condensed liquid and particulates from the gas stream; paniculate removing means for removing the particulates from the condensed liquid; and recirculation means for recirculating the condensed liquid back to the introduction means; characterised in that the liquid is an oil and the recirculation means are adapted to recirculate all of the condensed oil back to the introduction means so that the apparatus is transportable.

19. Apparatus as claimed in Claim 18, wherein the introduction means comprise a spray atomizer.

20. Apparatus as claimed in Claim 18, wherein the introduction means comprise an aerosol injector.

21. Apparatus as claimed in Claim 18, wherein the introduction means comprise a diesel-type injector.

22. Apparatus as claimed in anyone of Claims 18 to 21 , wherein the introduction means comprise a valve for adjusting the amount of oil introduced to the gas stream.

23. Apparatus as claimed in any one of Claims 18 to 22, wherein the cooling means comprise a heat exchanger surrounding the flow path.

24. Apparatus as claimed in Claim 23, wherein the heat exchanger contains a coolant comprising water, air or a water-ethylene-glycol mixture.

25. Apparatus as claimed in Claim 23 or 24, wherein the heat exchanger contains the same coolant as that which is used to cool the internal combustion engine.

26. Apparatus as claimed in any one of Claims 18 to 25, wherein the separating device is a centrifugal separator.

27. Apparatus as claimed in Claim 26, wherein the centrifugal separator is a cyclonic separator.

28. Apparatus as claimed in any one of Claims 18 to 27, wherein the separating device includes a collector for collecting the condensed oil and particulates separated from the gas stream.

29. Apparatus as claimed in Claim 28, wherein the recirculation means comprise a conduit for returning the condensed oil from the collector to the introduction means.

30. Apparatus as claimed in Claim 29, wherein the particulate removing means are located in the conduit.

31. Apparatus as claimed in any one of Claims 18 to 30, wherein the particulate removing means comprise a filter having at least one removable filter cartridge.

32. Apparatus as claimed in any one of Claims 18 to 30, wherein the particulate removing means comprise a cyclonic separator.

33. Apparatus for removing particulates from the exhaust gas stream of an internal combustion engine substantially as hereinbefore described with reference to any one of the embodiments shown in the accompanying drawings.