BACKGROUND ART Combustion in almost all internal combustion engines is an explosively rapid function, which, once initiated, cannot be controlled. In a gas turbine, the fuel is injected into a hot, turbulent air flow, not too dissimilar to the oil fired pressure-jet boiler. In a diesel engine, the aerosol droplets of fuel are caused, by the force of injection and induced turbulence, to move about in a combustion chamber which is heated up, during the compression stroke, due to adiabatic compression until such time as one droplet initiates a'slow'explosion. In a petrol engine, the droplets and vapour are drawn into the combustion chamber and compressed when the petrol vapour present is ignited by a spark. With both petrol and diesel engines, some of the fuel droplets impinge on the confining walls of the combustion chamber causing irregularities in combustion,

and some fuel droplets burn, from the surface of the droplet, within the combustion chamber leading to the formation of particulates. Piston engines suffer from crevice-entrapment of fuel in areas such as valve recesses and piston ring grooves. This can lead to unburned hydrocarbon being released during the exhaust stroke, which can lead to carbon deposits in piston ring clearance zones.

Further problems have occurred as designers have attempted to obtain optimum conditions for all aspects of engine operation-low RPM, mid range RPM and peak RPM-all with varying conditions of load. Many of the desired aspects for successful operation under any one condition require design features to be engineered into the hardware of the engine which cannot be modified as conditions vary.

In a diesel engine, the aim is to produce a homogeneous mixture throughout the combustion chamber, thereby to produce, when ignited, a smooth rise in the pressure, curve.

In practice, neither homogeneity nor smooth pressure rise is achieved, nor can it be achieved. As injected, the droplets must not be too small otherwise they will not have the inertia to be projected to the extremes of the combustion chamber. If the droplets are too large, they impinge on the combustion chamber and cylinder walls. Fuel concentration varies widely and the pressure curve cannot be timed correctly. Engine RPM will vary with operating conditions, but the rate of combustion is more or less

fixed. The uncontrolled pressure rise is anything but smooth, causing extreme shock loading in the engine structure.

In petrol engines, some of these problems are not so severe, as the more volatile fuel, being introduced during the intake stroke, has a longer residence time in the hot environment of the cylinder, and vaporisation is more complete. However, in most engines, droplet combustion still occurs, and there are wide variations in mixture strength within the combustion chamber. When stratification of combustion is intended, such variations are deliberate, but this is not without its problems.

Gas turbines and oil-fired boilers, being steady-state devices, suffer different problems, but many of these are the result of non-homogeneity of the fuel-air mixture during the combustion process. This results in unwanted emissions-hydrocarbon, soot/particulates, carbon monoxide (CO) and nitrogen oxides (NOx)-and a consequent loss of efficiency.

In all cases, the droplets of injected fuel have to evaporate from their surfaces to form a vapour so that combustion can occur. A droplet can only gain heat by radiation, the boundary layer of the droplet acting as an insulator. Heat is radiated into the droplet in the combustion chamber as the temperature rises rapidly,

causing some evaporation. The evaporation raises the partial pressure of fuel fractions in the gaseous boundary layer surrounding the droplet via a fractional evaporation process. When that boundary layer reaches a combustible concentration, it will auto-ignite or be ignited by combustion around it. After combustion occurs, the boundary layer surrounding that droplet is starved of oxygen and further combustion is incomplete or non-existent.

Turbulence can reduce the thickness of, or scavenge, the boundary layer, and, while the droplet is larger, this will occur. However, as the droplet is reduced in size and mass, the effect of turbulence is reduced.

This incomplete, anaerobic evaporation causes the droplet to be reduced to either unburned fuel liquid or a skeleton of. carbon with fuel'heavy ends'adsorbed onto it-ie. a particulate. Any combustion which does occur on the surface of the boundary layer surrounding the droplet is depleted in oxygen. Combustion will occur at the surface of the boundary layer, producing a very sooty flame. Since the mixture is not homogeneous, in the hotter, weaker zones, where nitrogen and oxygen coexist, nitrogen oxides will be formed. The foregoing presumes that the droplet does not impinge on any surface, which would alter the conditions of combustion dramatically depending on whether the surface was hot or cold.

SUMMARY OF THE INVENTION It would be desirable to be able to provide a fuel injection nozzle assembly which substantially overcame the aforementioned problems and in particular which inhibited or minimised the formation of particulates, unburned fuel, NOX and CO2.

According to the present invention there is provided a fuel injection nozzle assembly for a main combustion chamber characterised by a primary combustion chamber incorporating a primary fuel injection inlet, boundary means within the primary combustion chamber defining a resonant cavity region, excitation means for providing microwave radiation within the primary combustion chamber so as to be able to induce within the resonant cavity region, substantially adjacent the primary fuel injection inlet, an energisation region conducive to restricted oxidation, fuel evaporation and decomposition, primary air inlet means into the primary combustion chamber, and outlet means for the fuel products of the restricted oxidation, evaporation and decomposition region.

In this context the phrase'microwave radiation'is to be construed as radiation within the spectrum between normal radio waves and infrared, typically, but not limited to, radiation in the frequency range 1 gigahertz to 300 gigahertz (wavelength about lmm to 30 cms)

Preferably the resonant cavity region for the microwave radiation lies between the primary air inlet means and the outlet means.

Conveniently the excitation means for providing the microwave radiation is a microwave wave guide outlet, and the primary combustion chamber provides the boundary means for the microwave energy.

The outlet means may comprise a mesh or sieve plate.

The boundary means may comprise the outlet means and the excitation means which, between them, define a primary axis for the resonant cavity region, while the resonant cavity may be constructed such that at least one region of maximum microwave excitation on or substantially adjacent to the primary axis lies on the flow path of fuel products between the primary air inlet means and the outlet means.

In an alternative embodiment, the excitation means may be. an electrode or antenna provided at the end of the primary fuel injection inlet.

In such an assembly for an internal combustion engine main combustion chamber, the electrode or antenna may be connected to a metal wire guide connectable to a source of microwave radiation.

The primary combustion chamber may define a venturi enabling expansion of the fuel products from the primary fuel inlet to the outlet means, while the walls of the venturi may provide a conductive heat source to preheat the fuel mixture before exit through the outlet means.

In an assembly for an internal combustion engine main combustion chamber, the air inlet means may comprise passages in the primary combustion chamber for communication with the main combustion chamber and being of such a size and arranged in the nozzle assembly such that primary fuel emerging from the primary fuel injection inlet draws a proportionate amount of air through the passages into the primary combustion chamber to provide a sub- stoichiometric air-fuel ratio sufficient to incompletely burn the fuel to carbon monoxide while suppressing the further burning of the fuel to carbon dioxide and suppressing the oxidation of nitrogen.

In an assembly for an external combustion engine main combustion chamber, the primary combustion chamber may include fuel pre-heater pipework. in the walls thereof to recover heat from the primary combustion chamber.

BRIEF DESCRIPTION OF THE DRAWINGS Fig 1 shows a nozzle assembly according to the invention;

Fig 2 is a flow chart diagram of the phase controlled combustion process system in a nozzle assembly according to the invention; Fig 3 shows the nozzle assembly of Fig. 1 in a piston engine cylinder head; Fig 4 is a flow chart which shows the situation during combustion in the piston engine cylinder head shown in Fig. 3; Fig 5 shows typical pressure curves and illustrates the pressure curve achieved according to the invention, and Fig 6 shows an application of the invention to a gas turbine engine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Abbreviations used- NO. Any oxides of nitrogen CO-Carbon monoxide CO2-Carbon dioxide H20-Water or water vapour CH-Any hydrocarbon free radical formed by and present during combustion

The principle of the invention is illustrated with reference to Figs 1,2,3,4 and 5 for piston engines and Figs 6 for gas turbines and air-fired boilers. Actual dimensional parameters or the frequency of the electrical discharge are not defined, because different sizes of combustor for varying fuel flow rates, using different frequencies, will be required to accommodate various size systems. The basic concept involved is that the process of combustion of hydrocarbon is separated into the two major phases of oxidation in two separated zones. The primary combustion zone is where all hydrocarbon is converted to carbon monoxide, using a high frequency electrical discharge from a magnetron to force the evaporation of any fuel droplets remaining after injection. The secondary combustion zone is where secondary air is used to quench- cool and burn the carbon monoxide produced by the primary zone, oxidising it to carbon dioxide Piston Engine Fig 1 illustrates an injector nozzle 1, suitable for a piston engine, in the entrance of a venturi 2, which can have a mesh or sieve plate end piece 3. The injector nozzle 1 has a serrated outer edge to act as an electrode. The venturi 2, manufactured from a suitable metal, combination of metals, ceramic or ceramic/metal composite material, is designed with air vents 4 so that air can be metered from

the main combustion chamber 9 (as shown in Fig 3), in relation to the amount of fuel injected into the pre- combustion chamber 5 enclosed by the venturi 2. The aim is to produce a sub-stoichiometric mixture within the pre- combustion chamber. The venturi 2 also serves as a heat source to preheat fuel, by conduction, in the serrated injector nozzle 1. The venturi 2 is situated within the main combustion chamber 9 (as shown in Fig 3).

Referring to Figs 1 and 2, when preheated fuel is injected into the venturi 2, a proportionate quantity of air from the main combustion chamber 9 (as shown in Fig 3), is drawn into the venturi 2 along with the fuel vapour and droplets.

The air metering will be pre-set to provide a sub- stoichiometric air-fuel ratio of approximately 6: 1. The injector nozzle 1 is designed to produce droplets, as fine as possible to assist vaporisation, in a small cone of spread. The fuel being preheated to a temperature lower than its decomposition temperature and higher than its boiling point at the pressure in the combustion chamber, will flash evaporate as it leaves the injector nozzle 1, but some droplets may remain in the gas stream. The vapour, droplets and entrained air will pass into the centre zone of the venturi 2. The space within the venturi 2 will be heated due to adiabatic compression of the gas in the engine cylinder, and the venturi 2 will retain heat from the previous ignition. Auto-ignition will commence instantly. A sub-stoichiometric ratio of fuel/air is

required i. e. there is sufficient air to convert all of the fuel to CO but insufficient to oxidise the CO to C020r nitrogen to NO2.

A high frequency electrical discharge is directed into the venturi 2 from an externally fitted magnetron (not shown).

The electrical discharge potential is directed to the serrated end of the fuel injector nozzle 1 by means of a conducting metal wire guide (not shown). The internal shape of the venturi 2 with the mesh or sieve plate end piece 3 fitted to it is such that the high frequency electrical energy field created will resonate within the tuned confines of the venturi 2, between the serrated fuel injector nozzle 1 and the mesh or sieve plate end piece 3.

This discharge may manifest itself as a corona, a plasma or neither, depending on conditions inside the venturi 2. This is not important to the process. All fuel is injected into the electrical discharge field from the serrated fuel injector nozzle 1 and any droplets present will absorb energy from the discharge, raising their internal energy level, forcing them to evaporate (see Note A comprising the next paragraph). Auto-ignition will occur instantly. The rate and degree of molecular dissociation will be increased, thus speeding up the rate of combustion. The type of hydrocarbon (note :-'environmentally unfriendly' elements such as halogens, lead, zinc etc. should be avoided in order to maintain clean emissions) is not important provided it is a fluid capable of being sprayed

from the serrated injector nozzle 1 as a fine mist.

Note A-Liquid hydrocarbons are generally not polar and microwave energy will operate on a material only when there is present within the body of the liquid a polar or ionic molecular structure. In this process the liquid fuel has been preheated to a temperature slightly below its decomposition temperature so that some molecular cracking will be present. At the instant of cracking the molecular parts so formed exhibit polar properties due to the fractured bonds. The intense microwave field will cause violent spin or vibration to occur in the molecular parts thus causing rapid localised heating to the molecular parts and the molecules surrounding it. This is a chain reaction within the droplet, which will vaporise very rapidly from within its boundary.

Since the mixture within the pre-combustion chamber 5 is sub-stoichiometric, the heat generated by combustion, and the resultant temperature of combustion will be lower than that for a stoichiometric mixture. Due to the equilibrium dynamics of the reaction between carbon and oxygen and those between oxygen and nitrogen, in a zone restricted in oxygen, the carbon/oxygen equilibrium will favour a high conversion to CO, taking significant preference over the nitrogen/oxygen equilibrium which will favour a low conversion to NO,.

Therefore, when fuel is injected into the venturi 2, hot CO will pass from the pre-combustion chamber 5 via the mesh or sieve plate end piece 3 into the cooler air within the main combustion chamber 9 (Fig 3), being quench-cooled in the process. The CO will oxidise further to C02in a standing flame situated at the mesh or sieve plate end piece 3 (See Flow Diagram Fig 2). Within the boundary of that secondary flame there is no free oxygen, thus there is a strongly reducing gas mixture, inhibiting the nitrogen to NOx reaction.

Fig 3 shows the combustor in an engine cylinder head with the piston 8 at top dead centre. A pressure transducer (not shown) will be connected into the main combustion chamber 9. It will register the pressure profile, within the main combustion chamber 9 during injection, and communicate it to the engine management electronics. The engine management electronics can then modify the fuel injection profile during the combustion process to produce variations in the pressure profile within the cylinder in order to achieve maximum efficiency for the conditions prevailing at that instant in time.

In a piston engine the CO-CO2flame reaction will stay at the mesh or sieve plate end piece 3 (Fig 1) of the venturi 2. As the piston 8 descends, under the pressure created by initial compression and the additive pressure of combustion, the flame will remain at the venturi 2 outlet,

(See Fig 4) filling the cylinder only with gases of complete combustion and its original charge of clean air.

In Fig 5, the pressure curve 10 created by combustion in a conventional diesel engine, is superimposed upon the pressure curve 11 from this type of combustion. This combustion process oxidises the fuel as it is injected and therefore the rate of injection and consequent combustion can be controlled so that a smooth pressure curve 11 can be produced. This is very different to that produced by conventional injection pressure curve 10. Since the area under the smooth pressure curve 11 is much greater than the area under the conventional injection pressure curve 10, for the same peak cylinder pressure, there will be more torque produced by the new system. The lack of abrupt pressure changes with the new injection system would indicate that shock loads are unlikely to be imposed upon the engine structure, unlike conventional injection systems. The adiabatic pressure curve 12 is given for comparison.

Although energy has to be supplied to generate the electrical discharge, no energy is lost to the system because it is manifested within the combustion chamber 9 (Fig 3) as heat. In this way, initial ignition or the rate of combustion is not left to chance. By separating the two main combustion reactions, FUEL to CO and CO to CO2, better control can be maintained and, under such control,

the formation of unwanted products can be restricted or inhibited, ensuring enhanced fuel combustion.

Gas Turbine Fig 6 illustrates a serrated injector nozzle 13, suitable for a gas turbine or oil fired boiler, in the entrance of a primary combustion chamber 14 which can have a mesh or sieve plate end piece 15. The injector nozzle 13 has a serrated outer edge to act as an electrode. Fuel is supplied to the serrated injector nozzle 13 via a pre- heater pipe, arranged so that it recovers heat from the primary combustion chamber 14, or, alternatively, the fuel could be preheated electrically, before directing superheated fuel into the primary combustion chamber 14 via the injector nozzle 13. Air, from the compressor turbine (not shown) is metered into the primary combustion chamber 14 through the primary air control valve 16. A secondary control valve 17 directs air, from the compressor, into the CO gas coming from the mesh or sieve plate end piece 15 of the primary combustion chamber 14. When the high frequency electrical discharge from the magnetron 18 is switched on, it will be directed into the primary combustion chamber 14 which is designed to dimensions such that the electrical discharge resonates between the serrated edge of the injector nozzle 13 and the outlet mesh or sieve plate end piece 15. As stated, these dimensions will vary according to the fuel flow rates required.

In the embodiment of Fig. 6 fuel is discharged into the centre zone of a high-density discharge field. The discharge may form a corona or plasma, depending on the state of combustion. A corona could preheat the chamber to assist a cold start. Once combustion is established, the preheated fuel will be injected as a fine mist, which will flash evaporate and auto-ignition will occur instantly.

Some droplets could remain. The electrical discharge field will have little effect on the vapour and gases. However, the electrical discharge field will vibrate very vigorously, the polar or electrically charged molecules or radicals within any remaining superheated droplets in the fuel exiting the injector nozzle 13 (Fig 6). (see Note A above). The vibration of the droplets will elevate the energy level of the droplets causing evaporation. The hydrocarbon fuel vapour will partially oxidise rapidly at a temperature lower than that of complete combustion because the mixture in the primary combustion chamber 14 (Fig 6) is sub-stoichiometric.

Referring again to Fig 6, the compressed air supplied to the primary combustion chamber 14 is regulated by the primary air control valve 16, which is, in turn, controlled by the sensor 19 thus forming a closed loop control system.

The sensor 19 senses the flame colour temperature within the primary combustion chamber 14 which it compares with a pre-set optimum value. An electronic control system (not

shown) will vary the primary air control valve 16 so as to bring the flame colour to the optimum setting, thus maintaining a predetermined air-fuel ratio. This will enable the system to vary its settings automatically to compensate for variations in fuel rate and all external influences such as temperature, humidity and atmospheric pressure, so that the correct level of partial oxidation will occur within the primary combustion chamber 14. The setting required will achieve only partial oxidation of the hydrocarbon fuel such that all fuel is converted to CO.

This requires a sub-stoichiometric air-fuel ratio e. g. approximately 6-1. At this approximate level there will be just sufficient oxygen present to convert the hydrocarbons to CO, thus restricting any oxidation of the nitrogen present and preventing the further oxidation of CO to C02.

The gas mixture formed in the partial oxidation process will pass through the mesh or sieve plate end piece 15 of the primary combustion chamber 14 where it mixes, within the outer housing 20, with the air which has been metered in through the secondary air control valve 17. This secondary air supply is much cooler than the gas mixture passing from the primary combustion chamber 14, and rapid quench cooling will occur, as well as the further oxidation of CO to CO2 The reducing temperature of the gas emitted from the primary combustion chamber will move the chemical equilibrium towards a lower partial pressure of NOX and a

higher conversion of CO to CO2.

The resultant flame exiting the outer housing 20 will pass into the larger volume of the main combustion chamber where all oxidation will be completed prior to the gases passing into the exhaust turbine. As with the primary combustion chamber 14, the flame colour temperature within the main combustion chamber can be sensed by sensor 21 to create a closed loop control of the secondary air control valve 17.

Since it will be most unlikely that any unburned fuel can pass through the primary combustion chamber 14, and that the level of NO2 has been restricted to very low levels, the final emission from the turbine should be free from unwanted pollutant products of hydrocarbon combustion.

Additionally, all of the fuel will be fully combusted, yielding greater overall efficiency.

Oil-Fired Boilers Oil-fired boilers and similar types of burner will use a system similar to that outlined for the gas turbine. Design variations will be required to allow for the lower air inlet temperatures and pressures, but the principles remain the same. The accurate combustion control of this process will permit constant high temperatures required for the disposal of toxic waste materials.