The invention is particularly, although not exclusively, applicable to processing of exhaust gases from internal combustion engines. In many countries regulations now govern the emission of environmentally undesirable gases including CO, NOX and VOC, as well as particulate combustion products, by motor vehicle exhausts. At present new motor vehicles are typically fitted with three-way catalytic converters utilizing the precious metals platinum, rhodium and palladium to catalyse reactions in which undesirable exhaust gases are converted to less damaging compounds. Problems for the industry include (1) the expense and uncertain supply of the required precious metals; (2) the serious reduction in fuel economy (as much as 10%) caused by use of the catalytic converter; (3) its limited lifetime and susceptibility to poisoning and other deleterious effects in operation; and (4) the need to attain a working temperature in the region of 300°C before the catalyst becomes effective, as a result of which the catalyst typically does not function well on short journeys with a cold engine.

It has previously been proposed to utilise microwave energy in exhaust gas processing.

US patent 4934141 (Ollivon et al) discloses a device for"microwave elimination of carbon particles"in exhaust gas wherein the gas is passed through a filter which is arranged in a microwave cavity, the stated purpose of the microwave power being only to clean the filter. There is no suggestion that microwaves should be used to create a plasma in the exhaust gas.

US patent 5782085 (Steinwandel et al) proposes removal of nitrous oxides from oxygen rich exhaust gas by feeding into the gas a"reactive nitrogen-containing plasma jet". A nitrogen gas source separate from the exhaust itself is seemingly required, the plasma jet being formed by microwave action on this gas which is subsequently ejected through a nozzle into the exhaust gas stream. It is believed that installation of such a system in a motor vehicle would be problematic. A somewhat similar proposal, utilising a microwave plasma jet for exhaust gas processing, has been made by Al-Shamma'a, Wylie, Lucas and Pau in a paper in the Journal of Physics (J. Phys. D: Appl. Phys. 34 (2001) 2734-2741).

US patent 6422002 (Whealton et al) discloses application of pulsed microwave fields directly to a catalyst material for exhaust gas treatment. The proposed apparatus has a microwave waveguide intersecting a quartz tube carrying the gases under treatment (quartz seemingly being chosen for its properties with respect to the microwave fields). Frequencies of 100 MH : z and higher are used.

Adapting the device to mass production and installation in a motor vehicle appears problematic. Whealton is considered by the inventors to exemplify the approach which researchers have typically taken in this field, which is to provide a passage or chamber for the exhaust gas, this passage being formed of dielectric material. So far as the inventors are aware this approach has not led to production of a practical device.

P atent Office searches have also made the applicants aware of published international patent application PCT/AU/00325 (publication number WO 00/62904) which t eaches passing effluent gases through a"treatment zone"and directing microwave energy to the zone to establish a plasma in it, in which effluent gases are ionised, permitting their recombination.

The provision of a practical exhaust gas reactor using microwave energy is an object of the present invention.

In accordance with a first aspect of the present invention there is an exhaust gas reactor comprising a microwave source, a microwave-resonant reactor chamber having an electrically conductive boundary, and means for coupling microwaves from the source into the reactor chamber, wherein the reactor chamber is formed as or communicates with a combustion chamber, the reactor chamber receiving in either case exhaust gases released by combustion in the combustion chamber and communicating with an outlet through which the exhaust gases pass to reach the atmosphere, the exhaust gases being consequently subject to microwave energy and being thereby converted, in an interaction zone of the processor, into a plasma whereby chemical reactions in the exhaust gas are promoted.

In accordance with a second aspect of the present invention there is a method of processing exhaust gases comprising receiving the exhaust gases in a microwave- resonant reactor cavity, out of which the exhaust gases subsequently pass to reach the atmosphere, and coupling microwaves into the cavity thereby to convert the exhaust gases, in an interaction zone, into a plasma and so to promote chemical reactions in the exhaust gas.

The present invention is found to be remarkably effective in generating plasma and so promoting the necessary reactions in the exhaust gases, yet provides a potentially very simple, robust and highly economical construction.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a perspective illustration of an exhaust gas reactor embodying the present invention; Figure 2 is a perspective illustration of a reactor chamber of the same processor, a cavity wall being partly cut away to reveal the cavity interior and electric field lines being schematically indicated; Figure 3 is a perspective illustration of an alternative reactor chamber, a cavity wall again being cut away to reveal the cavity interior; Figure 4 is a perspective illustration of a further reactor chamber, a cavity wall again being shown cut away to reveal the cavity interior; Figure 5 is a diagram of a switching circuit for controlling electrical input to the reactor; Figures 6 and 7 are simplified diagrams of doubler circuits for providing a high voltage supply to drive the reactor; Figure 8 is a highly schematic diagram of a complete circuit for driving the reactor; Figure 9 is a graph showing variation of reactor input power and concentration of certain gaseous emissions (vertical axis) with time (horizontal axis); Figure 10 is a perspective illustration of a further exhaust gas reactor embodying the present invention; Figure 11 is a perspective illustration of still a further exhaust gas reactor embodying the present invention; and Figure 12 is a perpsective illustration of a filter construction used in the reactor shown in Figure 10.

The reactor illustrated in the drawings is a working test apparatus and has been used in experimental trials for processing of exhaust gases from the internal combustion engine of a motor car. However it is anticipated that in the process of developing a production version of the processor various aspects of its construction will be modified.

The reactor uses a microwave plasma, created by use of microwave energy within a chamber which acts as a microwave resonator, to reduce the levels of polluting gases emitted in the exhaust from a combustion chamber (or typically the several combustion chambers formed by the cylinders of an internal combustion engine). It is found that, in an"interaction zone"within the reactor, the effect is to generate plasma and to promote reactions by which environmentally undesirable combustion products are converted into less damaging compounds. Examples of such reactions are: 2 NO-+N2+02 2CO+O2e2CO2<BR> C7Hl6+1102o7C02+8H20 Looking at Figure 1, the illustrated reactor uses a microwave source in the form of a magnetron 2, a wholly familiar device to those skilled in microwave engineering whose construction and function will therefore not be considered here.

The magnetron used in the illustrated experimental processor was of a type conventionally used in a domestic microwave oven, a highly economical unit which nonetheless proved suitable for the purpose. The reactor further comprises a wall defining a reactor chamber 6. The wall is conductive, being formed in the illustrated embodiment of metal. Figure 1 shows the reactor chamber to be formed by a length of rectangular section metal tube 4. The dimensions of the reactor chamber are selected w ith r eference t o t he m agnetron o perating frequency a nd b ased o n t he physical requirements for resonance within the cavity. In the illustrated version the rectangular cavity 6 has, in cross section, a shorter dimension of 43 millimetres (half of a wavelength, at the magnetron frequency of 2. 45GHz) and a longer dimension of 86 millimetres (one wavelength) which, the walls of the cavity being conductive, thereby forming nodes in the microwave pattern, meets the requirements for resonance.

Microwaves generated by the magnetron 2 must be coupled into the reactor chamber 6. While the magnetron could be connected via electrical cabling to an antenna in the reactor chamber 6, the approach adopted in the illustrated embodiment is to couple the magnetron's output into the reactor chamber 6 through a waveguide 7. The waveguide in this embodiment takes the form of a hollow, conductive enclosure having the same depth and width as the reactor chamber 6 itself. For ease of installation in a motor vehicle, the illustrated waveguide is flexible by virtue of a concertina-type formation of its walls. That is, the walls of the waveguide 7 are folded back and forth, changes in the angles of the folds allowing the waveguide as a whole to flex.

Provision of the waveguide, between the magnetron 2 and the reactor chamber 6, allows these two parts to be remote from one another, which is considered potentially advantageous in automotive applications. The magnetron could for example be mounted in a car boot, the waveguide leading downward to the reactor chamber 6, which could be mounted on the car underside to form part of the exhaust manifold.

Earlier prototypes of the reactor have used a"matched load"coupled to the reactor chamber or waveguide and performing an impedance matching function, maximising microwave power to the reactor chamber. Optimised design of the waveguide and chamber are considered to make this unnecessary, however, and the illustrated embodiment does not incorporate this feature.

Similarly, earlier embodiments have used movable metal stubs (not illustrated) which project an adjustable distance into the chamber 6. Adjustment of these stubs allowed fine tuning of the performance of the processor, their effect being to alter the wave pattern within the cavity 6. A production system may use such features to adjust performance, but it is preferred to rely instead on optimised design of the cavity to provide a suitable wave pattern. Modes of microwave propagation are schematically indicated by arrows 8,10 in Figure 2 Throughput of exhaust gases is provided for by means of an input port 18 and an outlet tube 20 both communicating with the chamber 6. In Fig. 1 a box 14 schematically represents a combustion chamber. This may be a cylinder (or more typically s everal cylinders) o f an internal c ombustion engine. T he invention i s however applicable to other combustion processes-the exhaust gases could for example be output from a furnace or from another type of engine. The combustion chamber's exhaust output is led into the reactor chamber 6 through the input port 18 and reactions take place therein, as explained above, reducing the concentration of certain unwanted constituents. The outlet tube 20 leads eventually to the atmosphere.

The illustrated example, being intended for motor vehicle use, has a muffler box 21 which receives the gas from the outlet tube 20 and releases it to the atmosphere. The illustrated construction, in which the reactor chamber 6 is elongate and intersects the exhaust manifold, is well suited to mounting on the underside of a motor vehicle.

Figures 2,3 and 4 show openings 23,25 in the upper and lower walls of the reactor chamber through which the exhaust gases pass. The illustrated embodiments all incorporate, in the vicinity of these openings and hence in the flow path of the gas, arrangements for locally intensifying electrical field. It can be seen in Figure 2 that a conductive (metal) spike 22 is arranged concentrically with, and in proximity to, the outlet opening 25. As is well known, electric field strength is increased in the vicinity of a conductive body with a small radius of curvature, such as the point of the spike 22. The effect of the spike is therefore to locally intensify electrical field, which is beneficial since it promotes striking of the plasma discharge.

The principles of plasma generation will be familiar to the skilled person.

Microwaves cause oscillation of electrons in the exhaust gas and given sufficient electron energy (of the order of 20eV) electrons are capable upon collision of ionising gas modules. The free electrons thereby created increase the likelihood of ion- producing collisions and so promote plasma generation. The process is self reinforcing and consequently easier to maintain than it is to initiate or"strike". In the illustrated reactor the high electrical field strength in the vicinity of the spike 22 creates the conditions required for striking the plasma discharge in that region. The plasma i s t hen f ound t o s pread more widely within the chamber 6. A field o f approximately 30 kilovolts per centimetre is considered suitable to achieve striking, although this is dependent on various operating parameters.

In Figure 3 an alternative formation for promoting striking of plasma discharge is illustrated, comprising support structures 27,29 lying across the respective openings 23,25 to support respective spikes 31,33 between which a high electric field is created. The support structures, which are cruciform in the illustrated embodiment, do not inhibit passage of gas but allow the structures used to intensify the field-the spikes 31,33-to be positioned centrally in the gas flow, providing maximum field strength in the region of the reactor most important to its function.

In Figure 4 a further alternative formation for locally intensifying the field is seen to comprise an upstanding support structure 35 mounting a conductive spike 37.

The support structure is in this embodiment tripedal although it will be clear that its function could be carried out by a different type of structure.

It should be understood that different means could be used to promote striking, e. g. appropriate formation of the chamber wall, to locally intensify the electrical field, or design of the reactor chamber to provide a sufficiently high Q factor to achieve striking. It may be that, given suitable chamber design and input power, it is not necessary to locally intensify field strength in order to achieve striking of the plasma.

The microwave power input to the chamber 6 is modulated (or more specifically, in the illustrated reactor, pulsed). By modulating input power, an increased instantaneous power level can be provided for a given average power level. It is found that, for a given average input power, the size of the plasma discharge and of the interaction zone, in which the desirable chemical reactions take place, is increased by the power modulation.

It is believed that power modulation is also beneficial in avoiding problems due to"quenching"of the plasma discharge. The continuous throughput of exhaust material, and particularly the presence of water molecules in the exhaust gases, has been observed in previous experimental studies to tend to quench-ie halt-the plasma discharge. The illustrated reactor has been found to be less susceptible to this problem and to produce reliable plasma discharge in trials on motor vehicle exhaust gases. The inventors have conjectured that the high instantaneous microwave power provided by the reactors embodying their invention may break down water molecules into hydrogen and oxygen, whose presence assists in sustaining the plasma. Hence the presence of water, problematic in other systems, may actually be beneficial in the illustrated reactor. Striking of plasma discharge is also promoted by the high instantaneous power provided by virtue of the power modulation.

Excessively high average input powers and correspondingly high effective temperatures in the exhaust gases would potentially favour undesirable reactions in the exhaust gases. By modulating input power the high instantaneous power required to achieve and sustain plasma discharge is provided without excessive average power. The frequency of power modulation is sufficiently high to ensure plasma is continuously present throughout the modulation cycle. Frequencies in the region of 10-100 kHz are considered suitable.

Modulation of microwave power also provides a convenient means of controlling power input. The illustrated processor provides for control of power input to the reactor in dependence upon demand. The demand may be determined in a variety of ways. The illustrated reactor senses the pollutant concentration in its output through NO and/or CO sensors housed in a branch 26 of the outlet tube 20.

In fact two further sensors are incorporated in the illustrated arrangement at 39 and 41, making it possible to monitor exhaust gas properties upstream, downstream and within the reactor. This is useful for research purposes in the prototype system although a more simple arrangement may well be used in a production version. The sensors'outputs are led to control electronics which set the input microwave power, which is thus adjusted as necessary e. g. to achieve a target pollutant level in the output gases. An alternative, or complementary, control mode would be to control microwave power in dependence on engine operating parameters such as throttle position, temperature etc. which influence the nature and composition of the exhausted material. This control could be implemented by the engine controller.

A temperature sensor may also be provided to protect the device from overheating and to influence the power control.

In the illustrated reactor, power control is achieved by adjustment of the power modulation and more specifically by variation of the mark-space ratio of the (pulsed) power input to the magnetron.

A circuit 500 for switching the power input to the magnetron, to provide pulsing of the microwave signal, is illustrated in Fig. 5 and is a high voltage switch capable of handling 1900 volts. The illustrated circuit uses a DC to DC converter 501 receiving a 5 volt DC input to provide a 12 volt DC supply for the switching circuit itself. In a production version installed in a motor car another type of power supply may be selected. The high voltage switch is controlled by a signal generator 502 which provides a logic signal, controlling the HV switch, to the input of an opto- isolator. The opto-isolated output is fed to an input of a high speed, integrated circuit MOSFET/IGBT driver 506, an output of which is connected through a 10 ohm resistor 508 to the gate of a high voltage IGBT (integrated gate bipolar transistor) 510. The IGBT 510 serves to switch the voltage applied to the magnetron and the component used in trials is capable of handling 2500 volts although a series combination of zener diodes 512 clamps voltage across the IGBT at no more than 1900 volts. Two or more such circuits can when necessary be cascaded to handle larger voltages, the opto-isolator inputs being connected-typically in series-to switch concurrently. Two stage circuits have been used to control a magnetron anode voltage of 3 kV. A refinement would be to add a resistor/capacitor number network across each IGBT to aid voltage sharing.

Circuitry used to provide the high voltage required to drive the magnetron is illustrated in Figs 6 and 7. The illustrated circuits have been designed to run on a mains supply and used in trials with static internal combustion engines. Again, motor vehicle applications are expected to require somewhat different circuitry. Fig. 6 shows a voltage doubler circuit 600 for use with a single phase AC supply, applied to primary windings 602 of a step-up transformer whose secondary windings 604 are connected on one side to earth and on the other side via a doubler capacitor 606 and diode 608 to earth. During the capacitor charging time there is no voltage applied to the magnetron 609. Rather than take a path through ground and up to the plate of the magnetron, the current swings up through the diode. The voltage across the capacitor rises with the transformer secondary voltage to the peak supply voltage (2800 volts).

As the transformer secondary voltage begins to decrease from its maximum positive value the diode prevents capacitor discharge and the doubler capacitor 606 remains at the peak supply voltage.

Subsequently the transformer secondary (output) voltage swings into the negative half-cycle and increases in a negative direction to the negative peak of the supply voltage (2800 volts). The transformer secondary and the charged capacitor are now essentially two EMFs in series. The 2800 volts across the transformer winding adds to the 2800 volts stored in the capacitor and the sum voltage of 5600 volts is applied to the magnetron cathode 610 to drive the magnetron 609.

There are two fundamental characteristics of this high voltage output that should be noted. First, because a voltage doubler is also a rectifier, the output is a DC voltage. Second, the resulting output voltage that is applied to the magnetron tube is actually a pulsed DC voltage. This is because the doubler generates an output only during the negative half-cycle of the transformer's output (secondary) voltage.

Hence, the magnetron tube is, in fact, pulsed on and off at the supply frequency (e. g.

50 Hz, in the case of a domestic UK mains supply.

To remove the supply frequency pulsing of the high voltage, a three phase supply can be used with the circuit 700 illustrated in Fig. 7. Similarly to the previous voltage double circuit 600, this uses a step-up transformer 702 one side of whose secondary 705 is led to ground via a doubler capacitor 704 and diode 706. A bleeder resistor 708 is incorporated in the circuit 700, in parallel with the double capacitor 704. The inclusion of a second diode 708, connected in series between the capacitor 704 and the output 712 through which high voltage is supplied to the magnetron, prevents the common voltage point from affecting the charge/discharge cycle to the voltage doubler and so enables use with a three phase supply.

A substantially constant DC high voltage output is provided by the doubler circuit 700.

The entire magnetron drive circuit 800 is schematically indicated in Fig. 8, incorporating the high voltage switching circuit 500 and the three phase doubler circuit 700 to drive the magnetron, labelled 802 in this drawing. The three phase supply is indicated at 804.

The illustrated circuits have been used for testing purposes with mains electrical supplies. It will be apparent that some adaptation will be required for motor vehicle applications.

In tests the above described reactor has proven highly effective. The reactor has been used with a 1.8 litre petrol engine of a Mitsubishi Gallant (Registered trade mark) motor car and also with diesel and LPG engines, none of them using a conventional catalytic converter. It is found that the resultant emissions readily satisfy not only current European Emissions standards but also the stricter standards expected to be introduced in the European Community in 2006, the engines being operated on the appropriate test cycle for engine speed and load. Performance was particularly good with the diesel engine. As well as gaseous emission, regulations govern emission of particulate material. The forthcoming European standard for particulate e mission i s 2 gm/mile for a d iesel e ngine ; an o utput o f o nly 1 gm o f particulates per mile was found in tests on the present reactor. The dramatic effect of the reactor on gaseous emissions can be recognised in Fig. 9 in which power (900) to the reactor is switched on and off and the corresponding variation in SO2 (902j and CO (904) concentrations are shown.

Compared with a conventional catalytic converter, the illustrated reactor has several important benefits.

Gas flow from the exhaust is not obstructed. Such obstructions directly relate to the efficiency losses associated with the catalytic converter. In this respect the illustrated reactor offers a fuel saving of the order of 10%. The microwave power required by the illustrated reactor amounts to roughly 2% of the engine power, in the test arrangements, so that overall an 8% fuel saving is achieved.

The reactor is expected to be cheaper to manufacture than the conventional catalytic converter and does not require expensive rare earth materials-platinum, rhodium, palladium.

Tests show that the reactor works almost immediately following engine start- up, without need of a warm up period (an important consideration for numerous short journeys in which the catalytic converter may not reach its operating temperature).

The reactor is able to efficiently oxidise HC and carbon particles including the micro-particles which are a major health concern.

The inventors consider that combustion of unburnt fuel in the exhaust manifold, promoted by the reactor, contributes significantly to its efficiency. It is conjectured that perhaps two thirds of the required energy is provided in this way, reducing the energy input required of the microwave source.

The arrangement illustrated in Figure 10 differs from that of Figure 1 in that the source of microwaves is a unit 1000 formed separately from the rest of the reactor system 1002 and coupled to it through cables 1004,1006. A launcher section 1007, formed as a metal-walled box section, receives microwave energy from the cable 1004 whence it propagates through a tapered metal-walled waveguide 1008 leading to a metal-walled resonant cavity which is defined by a plate 1010 with a through- going circular opening (not seen in the drawing). The cavity receives exhaust gas from an input conduit 1012 whose terminal flange 1013 is bolted to the underside of the plate 1010 and outputs it through an exhaust passage 1014, which again terminates in a flange 1015 bolted to the upper face of the plate 1010. These parts are shown exploded in Figure 10 so that they can be separately discerned. The arrangement is intended for installation in a motor vehicle and the output passage 1014 leads to an exhaust muffler 1016, which is itself coupled to the microwave source through cable 1006. The microwave source once more uses a magnetron and is in this embodiment air-cooled.

Arranged across the mouth of the exhaust passage can be seen a filter structure 1017 which is illustrated in more detail in Figure 12. It comprises a filter element 1018 which is arranged across the passage for exhaust gases and serves to retain particulate material. The filter element in this particular embodiment is a woven metal mesh for filtering out particles with dimensions down to approximately 10 micrometres. In the absence of a filter, the reactor would have limited capacity to dispose of carbon particulate material because residence time in the plasma zone would be too brief for some particles to be broken down. In the present embodiment particles are captured by the filter and then burnt.

The filter element 1018 is mounted in a circular opening of a filter plate 1020 which is bolted to the upper face of the cavity plate 1010, being sandwiched between plate 1010 and the flange 1015 of the exhaust passage.

The present embodiment comprises a second filter 1022 within the exhaust muffler 1016 and formed as a porous ceramic filter for capturing particles including those of less than 10 micrometres size. The microwave output 1006 to the muffler 1016 serves to cause dielectric heating in the ceramic, thereby destroying the smaller particulate material. In this embodiment provision is made to switch microwave power between the two cables 1004,1006 as required to perform the dual cleaning action. Alternatively provision may be made to vary the relative power levels through the two cables.

If it becomes necessary in future to remove even ultra fine carbon particles, small enough to pass through the two filters of the illustrated embodiment, then the exhaust gases could be passed through a microwave plasma which has a sufficient transit time for removal of these ultra fine particles.

Figure 11 illustrates a further embodiment of the present invention in which the function of the microwave launcher is performed by a tapered waveguide 1100 which receives microwave energy from microwave source 1102 through cable 1104.

The exhaust is omitted from this drawing but the inlet conduit is seen at 1106 and opening 1108 in filter plate 1110 can be seen.

In current prototypes a proportional integral controller (PIC) has been used to control microwave power, based upon feedback relating to composition of the exhausted material and/or upon temperature. Hence power can be optimised for reduction of both gases and particulates. Networked microcontroller software, written in Delphi (registered trade mark), has been designed and implemented with the capability of monitoring and logging the exhaust gases, particulates and emissions from the reactor system. This package has the ability to be incorporated into a vehicle's networked management system. It provides the facility to display data concerning exhaust emissions. Such information can be provided to the driver, e. g. through a dashboard display. For example the driver may be provided with information relating to emission of an y of CO, NO, 02, HC and particulate material.

This information could be in the form of a digital readout, perhaps with averaging, or of a graph.

It will be appreciated that the above described embodiments serve merely as examples of ways the present invention can be implemented. Production systems are expected to differ in various respects due for example to packaging and mounting requirements in motor vehicles. The magnetron used as a microwave source in the embodiments is considered highly suitable for the purpose but progress in this field- for example, the anticipated development of solid state microwave generators-may mean that other types of source will be used for the reactor. The actual form of the microwave chamber may be varied without departing from the scope of the invention.

While applications of the invention to internal combustion engines are considered highly important, the inventors consider the invention to be potentially applicable to jet engines and other devices in which combustion produces exhaust gases. The reactor chamber may, with suitable selection of microwave frequency to achieve resonance, be formed not in the exhaust gas manifold but in the combustion chambers (cylinders) themselves.