AU2007338023A1

AU2007338023A1 - Heat engine

Info

Publication number: AU2007338023A1
Application number: AU2007338023A
Authority: AU
Inventors: Robert Lloyd
Original assignee: CBD LABS Pty Ltd
Current assignee: CBD LABS Pty Ltd
Priority date: 2006-12-21
Filing date: 2007-12-20
Publication date: 2008-07-03
Also published as: DE102006061911A1; WO2008077899A2; WO2008077899A3

Description

Thermal Engine Technical.Field The -present 'invention relates to an-engine driven using thermal energy. Background of the Invention Internal combustion engines are widely-used for transportation,-generation of electricity and other purposes. These engines use chemical energy, which 'is released:by combustion of a fuel in the. engine. Combustion generates expanding gases, which provide mechanical power todrive the engine. Such -engines however produce by-products, many of which are noxious and/or are pollutants. In addition,.the hydrocarbon fuels for these engines are a non-renewable resource. Many attempts have been made to.overcome the disadvantages of internal combustion engines, but none to date. hasbeen totally satisfactory..Amongst the proposed alternatives are electric engines, hydrogen fuel cell engines and hydrogen combustion- engines. A disadvantage with fuel-cell powered vehicles is that they are expected to cost substantially.more-than conventional cars when they become.available. Alternatives to petroleum fuels, such as hydrogen, are alsolikely to be'relatively expensive, and will require major infrastructure to provide convenient supply:for vehicles. A further consideration is vehicle range. A typical new car-with a conventional internal combustion engine-has a range between refuellings of approximately 550km. *If modified t reduce weight anid roliing resistance, the rangemay be extended to about 850km. Electric cars are unable approach either range, and ;barring an unexpected breakthrough in.battery.technology, ,it is unlikely.they ever will. Light-weighted fuel-cell cars with hydrogen stored on - board have ranges of around 650km, comparable to current.cars but about one third less than a similarly light-weighted conventional car. Fuel-cell cars that ireform"hydrogen-from-gasoline-do-befter-(1350km) Another disadvantage with technologies seeking to displace the internal combustion engine is that drivers consistently demand higher performance from new vehicles. Electric cars can only match internal combustion engine powered cars in this regards for-about one hour. Quick starts and high speeds quickly drain the powerfrom the batteries of-electric cars, greatly reducing their range. A final consideration is safety. In the event of an accident, acid-filled batteries may leak and pressurised canisters of hydrogen may explode. Recharging batteries and refilling pressurised tanks present significant safety risks that few people are currently accustomed to managing. The extensive light 1 weighting proposed to.offset the .added weight of fuel cells:and:batteries makes those vehicles -less safe in a.collision with-other cars oristationary objects. An alternative form of energy that.could be used to drive an engine is thermal energy. There is therefore need for an engine that:can be driven using thermal energy, and does not require-combustion -of fuels. There is a further need for such. an-engineithat scan te made'by conversion of existing engines that currently are driven using fuel combustion. Object of the invention It-is the object of the present -invention to overcome or substantially ameliorate at least one -of the above disadvantages. It is a further object to satisfy at least.one:of the abovementioned.needs. Summary ofthe tInvention In a broadest aspect of the invention there-is provided an engine comprising: - means for forming a mixture of a heat transfer material at a first temperature with a fluid at a second temperature, the first temperature being -higher than the second temperature, and - means for converting an energy of expansion of the mixture to usable mechanical energy. The means for'forming the mixture may comprise a mixing chamber. In an embodiment there is provided an engine comprising: a)- a mixing chamber for iorrming a mixture of a heat transfer material at a first.temperature with a fluid at a second temperature, the first temperature being;higher than the second temperature, -and b) an energy converter for converting an energy of -expansion of the mixture in the mixing chamber to usable mechanical energy. The fluid and-the heat transfer material should be unreactive ;towards each other at-the first temperature-and-are-stable-at-thefirsttemperature . The energy converter may be coupled to the mixing chamber. In this context, the term "un reactive" refers to a situation in which the two substances do not react with -each other chemically to generate a new chemical entity. The term "stable" refers to .a situation in which the substance does not react chemically to generate.a new chemical entity, e.g. it does not decompose. The engine may additionally comprise a heater for heating the heat transfer material to at least the first temperature. The energy converter may comprise a motor connected to the mixing chamber, said motor being capable of being powered by the mixture as the mixture expands. The fluid may be a non-oxidising gas. The second temperature may be between about 0*C and about 100 0 C. The 2 first temperature may be greater than about:200 0 C. The difference:between the second temperature and the first temperature may- begreater than about 25 Celsius degrees. The engine may additionally comprise the heat transfer-material ,and/or the fluid. Each may, independently, be located inithe mixing chamber o'rin some other-part of the engine or in both the mixing chamber or in some other part of the engine.'The heat transfer-material may comprise-apluralityof solidparticles. The particles may-have:a mean size of less than-about 10-microns. They maybe substantially spherical. They may'-for example be carbon particles having a percentage of -carbon of greater than 95%.by weight or by volume.'The .heat transfer-material -and thefluid may be such that, when the heat transfer material at the first temperature is mixed with the fluid at the second temperature to form a mixture, the mixture:is capable of expanding to provide mechanical energy. The engine may further comprise a storage vessel for the-heat transfer -material. The-storage vessel may comprise a heater for-heating the heat transfer material. The engine may further comprise-a fluid storage vessel for the fluid. The mixing chamber may comprise an accumulator for maintaining .an approximately constant pressure on mixing the fluid and the- heat transfer material. The engine may further comprise a delivery controller for-controlling the. rate of delivery of the fluid and/or of the heat transfer materialto the mixing chamber. The engine may further comprise a cooler for cooling the mixing -chamber. The energy converter may comprise -either a piston-driven motor or a turbine or a movable yart of either a piston-driven motor or a turbine. The engine may additionally.comprise a recycling system for recycling at least one of the heattransfer material andthefluid -for reuse in-the engine. The recycling system may comprise one or more of: f c) a separator for at least partially separating-the heat transfer material and the fluid; -- d)----a-cooler-for-cooling-the-fluid;e) a conveyor for conveying the heat transfer material to a storage vessel; -and f) tubing and/or pipework connecting the above components, and connecting -the recycling system to other components of the engine. The fluid may be a gas at the second temperature and the recycling system may further comprise a compressor for compressing the gas after it has exited the separator. In another embodiment there is provided an engine comprising: 3 a mixing chamber for forming a mixture of a heat transfer material at a first temperature with a fluid at a -second temperature, the-first temperature.being higher than-the second temperature, - an energy converter for converting an energy-of expansion of the mixture-to usable mechanical energy, and a recycling system for recycling at least one of the heat transfer material and the fluid. The invention -also provides a method- for operating-an engine, said.engine comprising (i) a mixing chamber for forming -a mixture of a heat transfer material ata.first temperature with:a fluid at a-second temperature, the first temperature being higher than the second temperature, 'and (ii) an -energy converter for converting an energy of expansion of the mixture in -the mixing chamber to usable mechanical energy, said method comprising: a) heating the heat transfer material-to at least'the first temperature; b) preparing.a mixture of the-heat transfer material at the -firsttemperature with-a fluid, said fluid being at a second temperature which is less than the-first temperature;. and c) converting an energy.of expansion of the mixture in the mixing chamber to usable-mechanical energy; wherein-the fluid and the heat transfer material are unreactive towards each other at the first temperature and are stable at the first temperature. Step c) may comprise using expansion of the mixture -to power a motor, said expansion being a consequence of step:b). Expansion of the mixture may be due to-heating of.a gas, volatilisation of a liquid orto both heating of a gas.and volatilisation of a liquid. Step-c) may comprise using the energy. of expansion to power one or-more pistons:which are:part of a piston-driven motor, or to -power a turbine. The method may-additionally comprise -controlling the rate of entry of the ----- fluid-and/or-of-the-heat-transfer-material-into-the-mixing-chamber.~The-methodmay additionally comprise the step of recycling at least one of the heat transfer material-and the fluid for reuse in the engine. The step of recycling may comprise one or more of: g) at least partially separating -the heat transfer material and -the fluid, h) cooling the fluid, and i) conveying the heat transfer material to a storage vessel.. Step a) may comprise heating the heat transfer material to a temperature of between about 2000C and about-4000C. The second temperature may be . between about 0"C and about 100 0 C. 4 The invention also,provides a system comprising an engine and a controller for controlling at least one operating.parameter. of the -engine, -said engine comprising.(i) a mixing chamber for forming admixture of a heat-transfer material at a firstitemperature with a.fluid at a second temperature, the first temperature being higher than the second temperature, and (ii) an energy converter for converting an energy of-expansion of the mixture in -the mixing chamber to usable mechanical -energy. The at -least one operating parameter maybe selected from the:group consistingof: - the first temperature, e the second temperature, e the temperature.of a heater for heating the heat transfer material to the first temperature, - the rate of entry of the fluid into the mixing chamber, * the rate of entry of the heat transfer.material into the mixing -chamber, the pressure in'the mixing chamber, and * a combination of any one or more of these. There is also provided a -vehicle or an electrical generator comprising an engine, said engine comprising (i) -amixing chamber for forming a mixture of a heat transfer material at a first temperature with a fluid at a second temperature, the first temperature being higher than the-second temperature, and-(ii) an energy converter for converting an -energy of expansion of the mixture in the mixing chaiiber to usable lrnechanical energy. There is also:provided a.mixture for use in an engine, said mixture comprising: e a plurality of carbon particles, said particles having a mean particle size of less than 10 microns, and ea fluid, e-said-carbon-particles-being-at-a temperatureoff-tsst~2O0 0 *C-a dthT nature of the fluid and of the carbon particles being such that they are stable, and do not react with-each other, at the temperature of the mixture. The specific gravity of the mixture at the temperature of the carbon particles and at a pressure of one atmosphere may be greater than about 0.5. The proportion of carbon particles in the mixture may be greater than about 30% by volume. In a first aspect-of the invention there is provided an engine comprising: - a heater for heating the heat transfer material to at least a first temperature; 5 - a mixing chamber for preparing a mixture of the.heat:transfer material at the first temperature and alfluid at a-second. temperature, the first temperature being higher than the second -temperature; and - a motor:connected to the mixing chamber, said motor being capable of being powered-by the mixture as :the.mixture expands. The fluid and -the heat transfer material are unreactive-towards each other at the-first -temperature and are stable at-the first temperature. The fluid may be a non-:oxidising fluid, or.it may be a mixture:of non-oxidising fluids. At the second temperature, the -fluid may be a gas, a liquid or a vapour, *or.itmay.be a mixture of these..At the first temperature the fluid may be a gas ora vapour .or.a mixture thereof. The fluid may comprise for example-nitrogen,.water -or a water/nitrogen mixture. The second temperature -may be between-about 0"C and about 100"C, and may be for example about 50 0 C, and the-first temperature may be greater than about 200 0 C, and may be for example about 300"C. The difference between the second temperature and the first temperature may-be greater than:about 25 Celsius degrees and maybe for-example about 250-Celsius degrees. The heat transfer material maybe a solid and may comprise a plurality -of particles. The-particles may have a mean size of less than about 10 microns, and commonly have a-mean-size of less than about 1 micron. The. particles may-have a specific gravity of greater than about 1, and commonly-have a -specific gravity greater than about.2. The particles may be spherical or they may have -some other shape (e.g. ovoid, oblate spherical, torroidal etc.), or-they may have a rniixtureof shiapes. At least someof the particles may be*carbon particles and the carbon maybe in the form of graphite and may be in the form of:pyrolytic graphite. The carbon particles may be purified.carbon particles and they may have apercentage of carbon of greater than about 95% by-weight or by volume. * Alternatively the-heat transfer material may be a liquid which-is -not substantially volatile at the first temperature. ----The-engine-may-also-comprise-a storage-vessel-fdrth hettransfer material, Which:may be a hopper, a bin, a container, a funnel, a silo; a storage chamber -or some other type of storage vessel. The heater may comprise for example an electrical heater, a heat exchanger, a boiler, a fumace, a gas fired heater or a burner, a heating block, or may. be for example an apparatus according to WO .95125416 which is hereby incorporated by reference. The heater may be located between the storage vessel and the mixing chamber, or the storage vessel may be located between the heater and the mixing chamber. Alternatively, the storage vessel may be heated and the heater and the storage vessel may thus be coincident. The heater may be located in the mixing chamber or adjacent to the mixing chamber or non-adjacent to the mixing chamber, and it 6 may be integralwith or separate from the mixing- chamber. The engine may comprise one-or-more tubes or-pipes.connecting'the heater to the mixing chamber, the storage vessel:to.the:mixing chamber, and/or the'storage vessel to the:heater. The :mixing chamber may comprise an accumulator formaintaining an approximately constant pressure on mixing:the fluid and-the heat transfer material. Alternatively there may be an:accumulator separate from.the mixing chamber. The engine:may additionally have a delivery:controller for controlling the rate of.delivery of the fluid and/or of the .heat transfer material. The -delivery controllermay comprise one or more valves, stopcocks, spigots, taps, variable tube constrictions or other suitable means. There may be cooler for cooling the mixing chamber. Suitable coolers include for example. aheatsexchanger, a chiller, afrefrigeration urit, a .cooler or vanes for air. cooling, or more than one of these, .however other types of cooler may be used. The motor may be any motor that is capable -of being driven by expansion of a fluid, and:may.be for example a piston-driven motor or a turbine. The motor maybe a movable part of a turbine, for example the rotor of a turbine, whereby the mixing chamber is also part of the turbine. The turbine may be located in a chamber. The engine may additionally comprise a recycling system to recycle at least one of the heat transfer material-and the fluid, for reuse in the engine. The recycling system may comprise one or more of: - a separator for ai least partially separating the heat transfer material and the fluid, - a cooler for cooling the fluid, - -a conveyor for-conveying the heat transfer material to a storage vessel, and tubing and/or pipework connecting the above components, and conhectinfg'the recycling system to other components of the engine. The recycling system may also comprise one or more additional components, for example those described in the embodiments of this aspect of the invention. The tubing and/or pipework may comprise a tube or a pipe connecting the motor to the separator, for example connecting- the mixing chamber to the separator, or connecting an accumulator to the separator, or connecting a cylinder of a piston motor to the separator, or connecting a chamber having the rotor of a turbine to the separator. It may additionally comprise one or more of a tube or a pipe connecting the separator to a cooler or to a compressor, a tube or a pipe connecting a compressor to a cooler, a tube or a pipe connecting a compressor to a storage vessel, a tube or a pipe connecting a cooler to a storage vessel, a tube or a pipe connecting a compressor toga condenser, a tube or a pipe connecting a condenser to a storage-vessel, a tube or a pipe connecting the separator to a conveyor for conveying the heat transfer material -to a storage vessel, and a tube or a pipe- connecting -the conveyor to a storage vessel. The engine may additionally comprise tubes or pipes connecting a storage vessel -for the fluid to the:mixing chamber,:and connecting a storage -vessel for the heat transfer material 'to the-mixing chamber and/or to the -heater. The tubes or pipes may have valves for controlling the passage of material.through the pipes. A suitable separator may comprise for example a filter, a mesh, a screen, a flotation device, asprecipitator, a settling:device or a centrifugal separator. The conveyor for conveying the heat transfer material to storage vessel may comprise aiscrew-conveyor, a conveyor belt, a. pump -or a. gravity feed 'system and..may comprise more thari one of-these. The cooler may comprise for example a heat-exchanger, a chiller,a condenser, a refrigeration unit or vanes -for air cooling. There maybe more than one cooler. In -a firstembodiment -of the first aspect, the fluid is a gas at the second temperature. The gas may be a non-oxidising gas and it-may be an inert gas. The gas may be, for example, nitrogen, carbon dioxide, .helium, neon or argon, or it may be a mixture of non-oxidising gases. In-this embodiment-the fluid -may-be stored in a gas container before entering the -mixing .chamber, for example in a compressed gas cylinder, a bladder, a-balloon, a.gas pressure vessel-or some other suitabIe gas container. The -enginemay comprise a recycling system as described earlier in this specification. The recycling system may also comprise a compressor for compressing the gas afterlit has exited the separator. There may be a cooler.betweenthe separator and.the compressor, and there may be a cooler betweenthe compressor :and the gas container. In a second embodiment of the first aspect, the fluid is a liquid at the -second -temperature-said-liquid-being-capable-of forming-a-vapour-on -contactwith 'the heat transfer material-at the'first temperature. The liquid may be a non oxidising liquid and 'may be for example water or a volatile organic liquid. In this embodiment the fluid is stored in 'a liquid container before entering the mixing chamber, for example in a reservoir, a tank, a chamber, a bottle, a cask, a vat, a vessel or some other liquid container. In-this embodiment, the engine may comprise a recycling system as described earlier in this specification. The recycling system may also comprise a condenser for condensing the vapour that exits the separator, and there may be means to return the liquid to the liquid container. 8 In a third embodiment of the first aspect, thefluid is a.rnixture.of a gas. as described in the first-embodiment and a liquid as described -in the-second embodiment. The gas and the liquid may be unreactive towards-eadh other at-the first temperature and are stable at the first temperature. The fluid may be for example a spray, an aerosol, a-mist,-a fog-or some other..gas/liquid~mixture. The liquid is stored in a:liquid container, for example a reservoir, -a tank,:a chamber, a bottle, a.cask, a-vat, a vessel or some other liquid container. The gas-mayibe stored in a -gas. container,for. example a-compressed gas cylinder, a bladder, a balloon, a-gas pressure vessel or-some-other suitable gas -container. In-this embodiment there may be means to generate.a gas/liquid mixture, and said means may be-a spray nozzle, -a spray head, a-nebuliser, an atomiser or some other means for generating a gas/liquid mixture. The -engine-may comprise a recycling system. The-recycling system may comprise a condenser for at'least partially condensing the vapour that exits the-separator, and for..at least partially separating the liquid from the.gas. Alternatively the separator may-comprise a condenser. Theremay also be a compressor for compressing the gas -after -it has exited-the condenser. In a. particular embodiment of the first aspect, -the engine comprises: - a heaterforheating-a heat transfer material to at least-a -first temperature, - - a mixing chamber for preparing a mixture of the heat transfer material at the first temperature and a gas at a second temperature, wherein the fifst temperature is higheithan the second temperature, the heat transfer material.comprising a plurality of spherical carbon particles with mean diameter less than 1 micron, and the gas being unreactive towards the heat transfermaterial at least up to 200*C, -a motor connected to the mixing chamber, said motor being capable of being.:powered -by .the mixture as the mixture expands, and - optionally,-a gas-container, for-example a gas pressure vesselfor storing the gas before entering the mixing chamber, In use, the gas may be at a-temperature of between about 0*C and about 1 00*C, commonly about 50*C, on entering the mixing chamber and the heat transfer material may be at a temperature of between about 150*C and about - 400*C, preferably about 300 0 C, on entering the mixing chamber. The motor may be a piston-driven motor or a turbine or a turbine rotor. The engine of this embodiment additionally comprises a recycling system to recycle the heat transfer material and the gas, for reuse in the engine. In this embodiment the recycling system comprises: 9 - a separator, for-example a centrifugal separator, for at-least -partially separating the heat transfer material and -the gas, - - a compressor for compressing the-gas after it has exited the separator, - a cooler between the-separator and the:compressor, and optionally a second coolerbetween the compressor and Ahegas container, - a.conveyor,-for example.a screw conveyor, for conveying the heat transfer material to a-storage vessel, and - tubing and/or. pipework connecting the components of-the engine. In a further embodiment there is provided an -engine. comprising: a heater for heating a heat -transfer-material to at least a first temperature, a mixing chamber for preparing a mixture of the heattransfer material at the first temperature:and a gas at a second;temperature, wherein the first temperature -is higher-than the second temperature,.the heat transfer. material comprising a:plurality -of spherical carbon particless with mean -diameter-less than'-1 micron, and:the gas:being unreactive -towards the heat transfermaterial at: least. up to 200 0 C, a motor connected to -the mixing chamber, -said motor being capable of being -powered -by thermixture as-the mixture -expands, optionally, a gas container for storing the gas 'before-entering the mixing chamber, a separator-for at least partially separating the, heat transfer material aid the gas, - a compressor for compressing the gas after it has exited the-separator, - a cooler between the separator and the compressor, and optionally a second cooler between the compressor and-the-gas container, f - a- conveyor for conveying the heat transfer material.toa-storage vessel, and - --- tubingandorpipeworkconnectingthe-compofetits-of the engine. - In a second aspect of the invention there is provided system comprising: - a heater for heating the heat transfer material to at least a first temperature, - an engine comprising a mixing chamber for forming -a mixture of the heat transfer material at the first temperature and a fluid at a second temperature, the first temperature being higher than the second temperature, and - optionally, a recycling system to recycle at least one of the heat transfer material and the fluid, 10 wherein the motor is capable of being powered by themixture as the mixture expands. In a third aspect of the invention there is provided a method for operating an engine, comprising the steps of: - heating a heat.transfer material to at:least a first temperature, - preparing mixture of the heat transfer material at the first temperature.with a fluid, said:fluid being:at a second -temperature which is less than the first temperature, and - using expansion of the mixture to power a'motor, said expansion being a consequence of the mixing. The step of heating .may be.by means -of a-heater, which may comprise for example an electrical heater, a heat exchanger, a boiler, a furnace, a gas fired heater or a burner, a heating block, or may be for example an apparatus according to WO 95/25416. The expansion of the fluid may be:due to heating of a gas, or it may be due to -volatilisation of a liquid or it may be. due to both heating of a gas and volatilisation of a liquid. The expansion.may be used for.example to power one or more pistons which are part of a piston-driven motor, or it may be used to power a turbine or some other motor capable of being powered :by expansion of a fluid. The method may additionally comprise controlling the rate of entry of the fluid and/or of the heat transfer material into the mixing -chamber. The controlling may be by means of one or more.valves, stopcocks, spigots, taps, variable tube constrictions or other suitable means. The method may additionally-comprise recycling at least one of the heat transfer material and the fluid for reuse in the engine. The recycling may comprise one or more of: - at'least-partially separating the heat transfer material and the fluid, - cooling the fluid, and -- conveying-the-heat-transfer materiarto-a-storage vessel. The recycling may also comprise one-or more additional steps which are described subsequently in this specification. In a first embodiment of the third aspect, the fluid is a gas at the second temperature. The gas may be a non-oxidising gas and it may be an inert gas. The gas may be, for example, nitrogen, carbon dioxide, helium, neon or argon, or it may be a mixture of non-oxidising gases. In this embodiment, the process may comprise recycling at least one of the heat transfer material and the fluid for reuse in the engine, as described earlier in this specification. The recycling may also comprise compressing the gas after it has been at least partially separated from the heat transfer material. The step of cooling the fluid may be conducted at 11 one or more times selected from before the compressing, during the compressing and after the compressing. In a-second embodiment of the -third aspect, the-fluid is a liquid at the second.temperature, said liquid being capable of-forming a vapour-on contact with the heat transfer material at the first temperature. The liquid maybe a non oxidising liquid and -may -be for example water or:a volatile organic liquid. knthis embodiment, the processimay compriserecycling at least. one ofthe heat transfer material and'the liquid for reuse in the-engine, as described earlier in this specification. The cooling-may comprise condensing-the vapour after it has been at least partially separated from the heat-transfer material, and the recycling.may also comprise -returning.the liquid to liquid container. In a third embodiment of the third aspect, the fluid- is a mixture of a gas as described -in the first embodimrent and a liquid as described in 'the second embodiment. The gas.and the liquid are unreactive towards each other at the first temperature and are stable at the first temperature. The-fluid may be-for example a-spray or an aerosol, a mist, a fog, a foam or some -other gas/liquid mixture. In this.embodiment the process comprises generating a gas/liquid mixture,.-and said forming may be by means of a -spray nozzle or an atomiser or some other means for generating a gas/liquid:mixture. In this embodiment, the.process may comprise recycling at.least one of the heat:transfer material and the fluid for reuse in the engine, as described -earlier in this specification. The recycling may comprise at least -partially condensing the vapour after the fluid has been at least partially separated fromthe heat transfer material, and rnay comprise returning the liquid to a liquid container. The recycling may also comprise compressing the gas -after it:has been at least partially separated from the heat transfer material. The step of-cooling the fluid may be conducted atone or more times selected * from before the condensing, during the condensing, afterthe condensing, before the compressing, -during the compressing and after the compressing. In-a-particular-embodiment- therreth-bd-o-nipisas-th-e-steps -6f: - heating a heat transfer material to a temperature of between about 200 0 C and about 400*C, commonly about 300*C, said heat transfer material comprising a plurality.of spherical graphite particles with mean diameter less than about 1 micron; - preparing a mixture of the heat transfer material and a gas which is unreactive towards the heat transfer material up to about 400 0 C, said gas being at a temperature of between about 0*C and about 1 00*C, commonly about 50*C; and - using expansion of the mixture to power a motor, said expansion being a consequence of heating of the gas. 12 The method additionally comprises 'recycling the heat transfer material and the gas for reuse-in the engine. The recycling comprises: - at -least- partially separating the heat transfer material and the fluid, for example using a centifugal separator, - compressing the gas.after-the separating, - cooling the gas after the separating. and before the compressing, and optionally also cooling the gas:after the compressing, and - conveying the-heat transfer material to a storage vessel. In a further embodiment the method comprises: - 'heating a -heat transfer material to a temperature of between-about 200 0 C and about 400*C, commonly about 300*C, said heat-transfer material comprising a plurality of spherical graphite particles with mean diameter less than 1 micron; - preparing a mixture of the heat transfer material and a gas which is unreactive towards the heat transfer-material up-to about 400*C, said gas being:at a temperature of between about 0*C and about 100*C, commonly about 50*C; .and using expansion of the mixture to power a motor, said expansion being a consequence of heating of the gas,. - at-least partially separating the heat transfer material and the fluid, - compressing the gas after the separating, - cooling the gas after the separating and before the.compressing, and option lly also cooling'the * as after the compressing, -and - conveying the heat transfer material 'to a storage vessel, for 'example using a screw conveyor. In a fourth aspect of the invention'there is provided a system comprising an engine according to the invention and -a controller for controlling .at least one operating parameter of the engine. The at least one operating parameter may-be - selected'from-the-group-consisting-ofthefirst-temperaturethe second--~- ---- temperature, the temperature of the heater, the rate of entry of the fluid into the mixing chamber, the rate of entry of the heat transfer material into the mixing chamber, the.pressure in the mixing chamber, the-timing of transfer of the mixture to the motor, and a combination of any one or more of these. Other operating parameters may be.controlled by the controller. The controller may be a computer, a programmable logic controller (PLC) or some other type of controller. This invention also encompasses an engine or a system when operated using the method of the third aspect, the method of the third aspect when used to operate an engine or a 'system, and also a vehicle or an electrical generator comprising an engine or a system according to the invention. 13 In a.fifth aspect of-the invention there is. provided a mixture for use in an engine, said mixture comprising: - .a plurality of carbon particles, said.particles having a mean particle size of less .than about 10 microns, and - a fkiid, said mixture being at a temperature of It least about.200 0 C, -and the nature of the fluid.and of the carbon.particles being such that they. are stable, and do not react with each other,.at the temperature of the mixture. The mixture may be homogeneous or it may be heterogeneous, The specific gravity of the mixture at the first temperature and 1 atmosphere pressure maybe greater than about 0.5: The proportion of carbon particles in -the mixture may be greaterthan about 30% by volume. The fluid may b6e a non-oxidising fluid. The fluid may-be -a gas or a vapour, or may:bera.mixture of-fluids each of which may be a gas or a vapour. The carbon particles may be purified carbon particles.and they may have a percentage of carbon of greater-than aboutV95%. They may comprise primarily graphite and the graphite may be pyrolytic graphite. They may-have a mean particle size of.less than about 10 microns and commonly have amean size less than about 1 micron. They may be spherical, or they may be some other shape, e.g. ovoid, oblate spherical or toroidal. In a -sixth aspect of the invention there:is a process for making a-mixture for use in an engine, said.process comprising mixing a fluid having a second temperature before -said mixing and a plurality of carbon particles, said particles hving a mean particle sizeofless than about 10 microns and having a first temperature before said mixing, wherein: - the nature of the fluid is such that, at the first temperature, the fluid -is stable, and the fluid and the carbon particles do not react with each 5 other, - the fluid.is a gas or a vapour at. the first temperature, and - -the-first-temperature-is-greater-than-th-e-d-tnperture. Detailed Description of the Invention For the.purpose of this specification, a fluid is taken to refer to either a gas or -a liquid, and a-gas is taken to refer to a gas, a vapour or a mixture of a gas and a vapour. In this specification, the term "stable" refers to a situation in which the substance does not react chemically to generate a new chemical entity, e.g. it does not decompose. It will be understood that minor reaction (e.g. decomposition) may be tolerated, and thus stable" should be taken to refer to at least about 95% stability, or at least about 96, 97, 98, 99, 99.5, 99.6, 99.7, 99.8 or 99.9% stability, i.e. the substance reacts or decomposes to a degree less than about 5, 4, 3, 2, 1, 0.5, 0.4, 0.3, 0.2 or 0.1%. 14 The-engine -of the present-invention is operated by forming a mixture of a heat transfer material and a fluid -in a mixing:chamber, wherein the heat transfer material is at a -first temperature on entering the mixing chamber and -the fluid is at a second temperature on entering the mixing chamber, the. second temperature being -lower:than -the first -temperature. The fluid-is heated by the heat-transfer material, said heating resulting in-expansion of the mixture. Said expansion may be used to provide power to a:motor-which is part of the engine. The-expansion may be a consequence:of thermal expansion-of a-gas or of vaporisation-of.a liquid or of both. The expansion may be not due to.a chemical interaction between the heat transfer material and the fluid. The heat transfer material and the fluid may betincapable of reacting with each other under the conditions pertaining in-the mixing chamber. They may be inert towards each other. Expansion of a-gas may -approximate 'the ideal gas law wherein P.V/T is a constant. Thus an increase in temperature results in an increase in volume at constant pressure. When a'liquid vaporises, the resulting vapour occupies a far greater volume than the original'liquid-at the-same pressure. In both cases the resulting expansion can be used t6 power a motor, In the engine of the present invention, expansion of the fluid is caused by transfer of heat energy from the heat transfermaterial. Since the heat transfer material remains in contact with-the fluid throughout the time-they are present in the mixing chamber, expansion energy is continually generated and consequently power is generated throughout the period in which the-fluid and heat transfer material are resent -in the mixing chamber. This contrasts, for example; with an internal combustion engine, in which -an initial ignition provides energy,. but power decreases.as the cylinder (mixing-chamber) expands due -to-movement of the piston therein. Commonly, power generationin the-engine of the -present invention remains at least about 70%.of maximum power throughout-the cycle, or at -least about 75, 80, 85 or 90% of maximum. -- - - --- - The-engine-of the-present-irvention-mayoperateat-aii-riiately -oristit pressure. The engine may have an accumulator to accommodate the initial expansion of the mixture, -which can then be transferred to the engine as required. For example, in an engine having a piston motor, the expanding mixture can be supplied to each cylinder -when the piston is at or -about top dead centre. Each cylinder may have a separate accumulator, however a single accumulator may be configured to serve more than -one cylinder of a piston motor. If the motor is a turbine motor, there may also be an accumulator as described above, although it is not necessary to have an accumulator in that case. In an engine having a turbine motor with no accumulator, the expanding mixture may pass - 15 directly from the mixing chamber to vanes of the turbine, causing continuous rotation of the turbine. The mixing chamber may comprise one or-more pistons such that the mixture powers the mixture as the mixture expands. The mixing -chamber may be coupled to a chamber comprising one.or more pistons such that the rMixture powers the mixture as the-mixture expands. The mixing chamber may be coupled to the chamber comprising one:ormore pistons by at least-one tube; pipe, hole or other coupling means. The-tube, pipe, hole or other coupling means may comprise a one-way valve to allow the mixture to pass from the mixing chamber to-the-chamber comprising one or more pistons. The mixing -chamber may be coupled to a chamber having at least one turbine blade. The mixing-chamber may be coupled to the chamber having at least one turbine blade -by at least one tube, pipe, hole-or other coupling means,. and-the tube, pipe, hole or other coupling means may be oriented so as to direct the mixture towards the at least one turbine blade. The specific gravity of -the mixture or -fluid and heat transfer material at-the first temperature and Iatmosphere'pressure may be greater than about 0.5, or greater than about 0.6,.0.7, 08, 0.9, 1, 1.1, 1.2, 1.3, 1.4, or 1.5. The specific gravity of the mixture may be between about 0.5 and about 1.5 or between about 0.6 and about 1.3 or between about 0.7 and about 1.1 or between about 0'8 and about 1 or between about 0:5 and about 1 or between about 0.5 and about 0:8 or between about 0.8 and about 1 or between about I and about 1.5-or between aboui 1.2 -and about 1.5 and it may be about 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4 or 1.5. The proportion of'heat transfer material in the mixture may be greater than about 30% by volume, or greater than about 35, 40, 45, 50, 55; 60, 65, 70 or 75% at thefirst temperature and 1 atmosphere pressure, and may be between about 30 and about 80% or between about 40:and -about 75% or between about 50 and about 70%.or between.about 30% and about 50% or - betweenabout 30%-and-sb~t 40%-o-b~6tween ab5of50% d~bo~UE66% or between about 60% and about 80% , and may be about 30, 35,-40, 45, 50, 55, 60, 65, 70, 75 or 80% by volume at the first temperature and 1 atmosphere pressure. The fluid may be a non-oxidising fluid. At the second temperature the fluid may be a gas, and may be an inert gas, and may be, for example,.nitrogen, carbon dioxide, helium, neon or argon, or it may be a mixture of non-oxidising gases. Alternatively at the second temperature the fluid may be a liquid which is capable of forming a vapour on contact with the heat transfer material at the first temperature. The liquid may be a non-oxidising liquid and may be for example 16 water or a volatile organic liquid. If the liquid or the vapour is flammable, it is preferable to at least partially exclude oxygen from the engine. The ratio of heat transfer material entering the mixing chamber to-liquid entering the mixing chamber-is commonly between about 10 4 :1 and about 104:5 by volume,-or between 10 4 :2.and about 10:4 and maybe about 10 4 :1, 104:2, 104:3, 1O4:4 or 104:5 by volume. As a further alternative, at the second temperature the'fluid-may be a mixture-of a gas :and aliquid. The gas and the liquid areunreactive towards each other at the first temperature andiare stable at the first temperature.The fluid-may be for example a spray or an aerosol or a mist or a-fog or-a foam or:some other gas/liquid Mixture. The proportion of liquid in the gas/liquid mixture may be between about 0.1% and about"10% by volume, or betweenlabout 0.2% and about 5% or betveen:about-0.3% and about 4% or between-about:0:4:and about 2% or between about 0.5% and about 1% by volume; and maybe about 0.1, 0.2, 0.3,-0.4, 0.5,0.6:0.7, 0.8, 0.9, 1, 2, 3, 4, 5,-6, 7, -8, 9 or 10% by volume,-or it may be some..other proportion. The fluid is commonly stable in the presence of the heat transfer. material at the-first temperature, and does not react chemiically with the heat transfer material at-the first temperature. The heat transfer material may be a solid and may comprise a plurality of particles. At least some of the particles may be carbon particles. The carbon particles may be purified carbon particles and they may have a percentage of carbon of greater than about 95% or greater than about 96, 97, 98, 99, 99:5, .996, 99.,.99.8, 99.9, 99.9599.99, 99.95 or99.9999% on a weight, volume or mole basis, and they may have percentage of carbon of about 95% or about96, 97, 98,-99, 99.5, 99.6; 99.7, 99.8, 99.9, 99.95 99.99, 99.995, 99.999:or 100%.on a weight, volume or mole basis. The carbon particles may comprise primarily graphite, and may be greater than about 70% graphite by mass or by volume, or greater than about 75,80, 85, .90,'95 or 99% graphite -by mass or-by volume, and - maytbeabout 75780 85 90 95 96 97,98 9999 99-9-r 100% graphite by mass or by volume. The graphite maybe pyrolytic graphite. The carbon.particles may have a mean particle size of less than about 10 microns or less than about 5,.2, 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2 or 0.1 microns. Theymay have a mean particle size of between about 0.1 and about 10 microns, or between about 0.2 and about 5 microns or between about 0.3 and about 3 microns or between about 0.4 and about 2 microns or between about 0.5 and about 1 micron or between about 0.1 and about 5 microns or between about 0.1 and about 1 micron or between about-0.5 and about 10 microns or between about 1 and about 10 microns or between about 1 and about 5 microns, and they may have a mean particle size of about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.5, 2, 3, 4, 5, 6, 17 7, 8, 9 or 10 microns.,For the purposes -ofithis specification the.particle size of an individual particle is-taken to.be-the maximum dimension of'hat,-particle. The particles:may.be spherical, or they may be some other -shape, for example ellipsoid, toroid, oblate spherical,.ovoid, conical, truncated cone.:shaped, dome shaped, cylindrical-or polyhedral, for example cubic, rectangular prismoidal, triangular prismoidal or.polyhedral -with between.4 and 60-or more-faces, or they maybe.some other shape, or they-maybe an irregular shape, or they -may be-a mixture of different shapes. The particles may be porous or non-porous:and may be hollow-or non-hollow. The method of the invenion comprises the.step of-mixing-the heat transfer material at-the first temperature with a fluid, said fluid being at a second temperature which is less.than-the -first temperature. The difference between the firsttemperature and -the -second temperature maybe greater than about 25 Celsius degrees, or greater than about 50, 75, 100, 125, 150, 175,200, 250, 300, 400, 500, 750, 1000, 1500, 2000, 2500 or 3000 Celsius degrees. Said difference may bebetween about 25-and about 3000 Celsius-degrees or between about 30 and about 2000 Celsius. degrees or between:about 35 and-about 1000 Celsius degrees or between about 40 and about 500 Celsius degrees or between about 50 and about 200 Celsius:degrees or between about 25:and;about 200 Celsius degrees. or-between about 25 and:about 100 Celsius degrees or between about 100 and about 500.Celsius-degrees or between about 200 and about 400 .Celsius degrees or between about 1000 and -about 3000 Celsius degrees or between about.2000 and about 300O Celsius diiiegreesand maybe about 50, 75, 100, 125, 150, 175, 200, 250, 300, 400,'500, 750,1000,;1250, 1500, 1750,.2000, 2250 2500, 2750 or 3000 Celsius degrees. The second temperature maybe between about 0*C and about 100*C, or between,'about 1-0*C-and about 90 0 C or-between aboUt'200C and.about 800C or between about 30*C and-about 70*C or between about 50 0 C and about 60*C or between.abut'OOC -and about 50*Cror between - about 0"C-a nd-about 25*C-o-tween abbiUt-25 0 C ard-ab-iit 0 "C or between about 50 0 C and about 1000C or between about 75*C and about 100*C, and may be about 0, 5, 10, 15, 20, 25, 30,35, 4.0,45, 50, 60, 70, .80, 90 or 100 0 C, although it-may- in some cases be belowabout 0*C or above about 100*C.The first temperature may be greater than about 200 0 C, or greater than about 220, 240, 260, 280, 300, 320, 340, 360, 380, 400, 450, 500, 600, 700, 800, 900, 1000, 1250, 1500, 1750, 2000, 2500 or 3000 0 C, or may be.between about 2000C and about 30000C, or between about .2000C and about 2500 0 C, or between about - 200"C and about 2000*C or between about 200"C and about 15000C or between 2000C and about 1000 0 C or between about 200*C and about 500 0 C or between about 200 0 C and about 350 0 C or between about 250*C and about 3500C or 18 between about 250 0 C and about 500*C or between about 3000C and about 5000C or between about 3000C and about 400*C or may be. about.200 0 C, or about 220, 240, 260,280, 300, 320, 340, '360, 380, 400, 450, 500, 700, 700, 800, 900, 1000, 1250, 1500, 1750, 2000, 2500.or 3000 0 C. The heat transfer material maybe heated to-the first temperature by means of a heater which may be for example an electrical heater, a heat exchanger, a boiler, a furnace, a gas fired heater:or a burner, a heating block, or maybe for example an apparatus according to WO 95/25416. The heater may comprise a high temperature heat reservoir. ard a low temperature heat reservoir, whereby the liowtemperature heat -reservoir is used to heat the heat transfer material and the high-temperature heat reservoir is -used to-heat thelow temperature heat -reservoir. The high temperature heat reservoir may be at a temperature of between about 1500*C and about 30000C, orbetween about . 17500C and about .2750*C or between about 20000C and about 25000C or between about 20000C and about 30000C or between about 15000C and about 2500*C, and may be at about 1500, 1.600, 1700, 1800, 1900, 2000, 2100, 2200, 2300, 2400, 2500, 2600, 2700, 2800, 2900 or 30000C. The -low temperature heat reservoir may be at a temperature oftbetween about 2000C and about 15000C, and or between about 2000C and about 1000*C, or between about 200*C and about 500*C or between about 200 0 C and about 4000C or between about 5000C and about 15000C or between about 1000C and about 1500"C or between about 3000C and about 1000*C or between about 3000C and about 500 0 C, and may be at about 200 210, 220, 230, 240,250,*260, 270, 280, 290, 300, 350, 400, 450, 500, 600, 700, 800, .900, 1000, 1100, 1200,1300,1400 or 1500"C. The high temperature heat reservoir.may be connected to the low temperature heat reservoir by a thermal-conductor, and the-thermal conductor may comprise a switch to enable the thermal -connection between the high and low temperature heat reservoirs to be interrupted. The switch may be activatableby a thermostat --- or-by-a-temperature-sensor,-for-example:a-thermometer-or-atkermocouple- ~ The method may additionally comprise recycling at least one of the heat transfer material and the fluid for reuse in the engine. The recycling may comprise one or more of: - at least partially separating the heat transfer material and the fluid, - cooling the fluid, and - conveying the heat transfer material to a storage vessel. At least partially separating the heat transfer material and the fluid may be performed in a separator, for example a filter, a mesh, a screen, a flotation device, a precipitator, a settling device or a centrifugal separator, or any other separator suitable for separating a solid particulate material from a fluid. 19 If the fluid is a gas at the second temperature, it may then be cooled using a cooler which maybe for example a heat exchanger, a chiller, a refrigeration unit, vanes for air cooling or any other suitable type of cooler or by a combination of such coolers. It may then be compressed using a compressor. Compressing a gas commonly increases its temperature, and so a second.cooling step may also be performed-before retuming the compressed gas to a storage:vessel. The storage vessel may be-for.example a compressed:gas cylinder, a bladder, a balloon, a gas pressure vessel or-some other suitable gas container. If the fluid is a-liquid at the second temperature, then it may be cooled following the -step of at least. partially separating,-to a temperature at or.below the temperature at .which it condenses. Said temperature-may be the second temperature, or it may be some temperature between the second and the first temperature. Following at least partial condensation of the fluid to a liquid,-the liquid may be returned to the liquid container, which may be for example a reservoir, a tank, a chamber, a bottle,.a cask, a vat, a vessel or some other liquid container. There maybe a-second cooling step to cool the liquid to-about the second temperature. The second cooling step should be performed: before the. liquid.enters the mixing chamber, and may be.performed before or after the liquid .is retumed to the liquid container. If the fluid comprises a:mixture of a first fluid and a second fluid, the first -fluid being a gas at the:second temperature and the second fluid.being a.liquid at the second temperature, then the mixture may be cooled following the.at least partiallyseparating, to a temperature at or below.the temperature at which the second fluid condenses. Said temperature may be the second -temperature, or it may be some temperature between the second and -the first temperature. Following atleast partial condensation of the second fluid to a liquid, the liquid may be returned to the liquid .container, which may be forexample a reservoir, a tank, a chamber, a bottle, a cask or some other suitable liquid container. The first -fluid-maybe-compressed-usirg-a-coinpressor, and6otiially a second coodlfiig step may also be performed, 'before returning the compressed gas to a storage vessel. If the second fluid -is a blend, then the blend is commonly cooled to a temperature at or below which the lowest condensing component of the blend condenses. For example if the second temperature is 50 0 C and the fluid comprises a gas at the second temperature as well as a first.liquid which condenses at 70 0 C and a second liquid which condenses at 80 0 C, then the step of cooling should comprise cooling to a temperature at or below 70 0 C. There may be a second cooling step to cool the liquid to about the second temperature. The second cooling step should be performed before the liquid enters the mixing 20 chamber, and may be performed before or after the liquid is returned to the liquid container. As the fluid is cooled in the recycling process, its volume is reduced. This may give rise to a pressure drop, thereby assisting. passage.of hot fluid and.heat transfer material from the motor to the recycling -system. In the mixing chamber of the present invention, the heat transfer material at the first temperature mixes with.the fluid at the second:temperature, where the first-temperature is higher than the second temperature. Thus heat transfer results in loss of heat energy (commonly cooling) from the heat transfer. material and gain in heat energy (heating and/or phase change) of the fluid. When the heat transfer material is-separated from the fluid, it is reheated to at. least the first temperature for reuse. Thus heat energy retained in the. heat transfer material is retained in the engine. The fluid on the other hand is returned lo the second temperature, which may result in a loss of heat energy. This loss is lost:from. the engine, hoviever it is relatively minor, since the thermal mass of the fluid is commonly far smaller than the thermal mass of the heat transfer material. Thus the present invention provides an efficient energy conversion, since very'little energy is lost in its-operation. The motor of the invention may be any suitable motor that is-capable of being powered by a mixture .of the fluid and the heat transfer material as the mixture expands, and may be for example a piston motor-or a turbine motor. Particularly .in the case of a. piston motor, but also optionally in.the case:of a turbine motor, an accumulator may be used in order to take up changes in volume of the mixture without substantially changing the-pressure. This may-be particularly:useful when the motor requires,an intermittent supply of.the mixture, as is the case for a piston motor. The design of the accumulator may be such -that it inhibits separation of the mixture. The engine of the present invention has several advantages, which include: - the components of the mixture used in the engine do not react together chemically, and can thus be reused; - the mixture used in the engine has a high specific gravity relative to the fluid alone, and consequently in motion the mixture has a relatively high momentum, providing greater power; - - separation of the mixture-used in the engine is relatively easy using, for example a centrifugal separator, due to the difference in specific gravity between the fluid and the heat transfer material at the first temperature; and - graphite particles that enter the motor can act as a lubricant. 21 Brief Description of -the Drawings* A preferred form of -the present-invention will now be described by way of example with reference to -the accompanying drawings wherein: Figure 1 is a diagrammatic representation -of an engine according to the present invention; Figure 2 is a diagrammatic representation of a recycling system for an engine according -to the present -invention, wherein-the fluid-is a gas at-the second temperature; Figure 3 is a diagrammatic representation of a recycling system for-an engine according to the present invention, Wherein the fluid is a liquid at the second temperature; Figure 4 is a diagrammatic representation of an engine:according to the present invention,-wherein the fluid is a mixture of a gas and a liquid.at the second temperature; Figures 5a to g show a dismantled turbine motor for use in the :present invention; Figure 5h shows an expanded viewof the vanes of the turbine motor of Figs. 5a to.5g; -Figure 6a is a diagrammatic representation of a portion of an engine according to the present invention, showing an accumulator having weights to control pressure and a delivery controller for controlling the rate of delivery-of th -fluid and -of the heat transfer material; Figure 6b is a diagrammatic representation of a portion of an engine according to the present invention, showing a gas driven accumulator and:a delivery-controller for controlling-the rate of delivery of the-fluid and of the heattransfer material; and Figure 7 is a diagrammatic representation of-a turbine similar to a steam -turbine, suitable for use in the present:invention. Detailed Description of the 'Preferred:Embodiments With reference to Fig. 1, engine 100comprises.heater 105 for heating a heat transfer materialiiifg ha-fiibef10 for i-iig-heat transfer material and a fluid, and motor 115 connected to mixing chamber 110, motor 115 being capable of being powered by a mixture.of the fluid and the heat transfer material as the mixture expands. Engine 1.00 also comprises hopper 120 for storing the heat transfer material, and vessel 125 foir storing the fluid. If the fluid is a gas, vessel 125 is a gas container, for example a gas pressure vessel, and if the fluid is liquid, vessel 125 is a liquid container, for example a reservoir. Heater 105 may comprise for example an electrical heater or a burner, a heating block, or may conveniently be for example an apparatus according to WO 95/25416. In an apparatus according to WO 95/25416, a mass of graphite is held at between 2000*C and 3000*C, commonly at about 2500 0 C, and may be used as a heat 22 reservoir to heat the heat transfer material, or may be used to heat a secondary heat reservoir, held at-between about250 0 C and 3500C, commonly at about 300*C, which may then be used-to heat the heat transfer material. Heater 105. is located between hopper 120.and mixing chamber 110. Mixing chamber 11 Ois located close to motor 11:5 to.minimise power losses between them. Motor 115 may be any motor that is capable-of being driven by.expansion of a fluid, and is conveniently a piston-driven motor or a turbine. Engine 100 additionally.has valve 130 and mechanism 135 for controlling the rate of entry of.the gas and:of the -heat transfer material respectively into niixing chamber 110. Pipe 112 connects hopper 120 to mechanism 135, and heater 105, for:heating the heat transfer material, surrounds a portion of pipe 11.2... Pipe 11-4 connects vessel 125 to 110, and valve 130 is located in pipe 114. Pipe 116 connects mixing chamber 110 to motor 115. Engine 100 additionally comprises recycling system 150 to recycle the'heat transfer material and the fluid for reuse. Pipes 118 and 122 connect.motor 115 and hopper 120 respectively to recycling system 150, and pipe 124 connects recycling system 150 to vessel 125. In a preferred embodimenf the heat transfer material comprises a plurality of particles. The particles preferably have a mean size of less than about 1 micron, and preferably between about 0.5 and 1 micron. The.particles are preferably spherical and are composed of pyrolytic graphite with a purity of greater than about 95% and preferably greater than about .99% by weight. The fluid may be an inert gas, and:it may be, for example, nitrogen, carbon dioxide, helium, neon or argon. A commonly:used gas is nitrogen. Alternatively'the fluid may be a vaporisable liquid which .is unreactive towards the heat transfer material at thetemperature of the:heated transfer mateiial, and may be for example water. A method for-operating engine 100 comprises heating the heat transfer material, which has been stored -in hopper 120, to between about 250*C and about 350 0 C, and. preferably to about 3000C, -by.passing the heat transfer material through heater 105bby means of pipe 112 Valve 130 and mechanism 135 are used-to control the rate-of entry of thefluid and the heat transfer material respectively into mixing chamber 110. If the fluid is a gas, the proportion of heat transfer material to the total amount of heat transfer material plus fluid entering mixing chamber 110 is controlled to between about 50% and about 80% by volume, commonly between about 60% and 70% by volume. If the fluid is a liquid, the ratio of heat transfer material entering mixing chamber 110 to liquid entering mixing chamber 110 is commonly between about 104:1 and about 104:5 by volume and may be about 104:2 by volume. The heated heat transfer material mixes with the fluid in mixing chamber 110, said fluid being at between about 00C and about 50*C, and preferably at about 250C. The expansion of the fluid/heat 23 transfermaterial-mixture, which is caused either-by heating of a gas or by vaporisation of a liquid by the heat transfer material, causes the mixture to pass rapidly through pipe 11.6 to motor 115, and provides power-which is used to drive motor 115. The method additionally comprises recycling the heat transfer material and the gas for reuse in engine 100. Recycling system 150 separates the-fluid from the heat transfer material, and returns heat transfer material to hopper 120 through:pipe 122 and fluid to vessel 125'through:pipe 124. With -reference to Fig. 2, recycling system 200 comprises separator 210 for at least partially separating-the heat transfer-material and the.gas, compressor 220 for compressing-the gas after it has exited separator 210, and heat exchangers 230 and .240 for cooling the gas. Separator 210 -is connected by pipe 118 to motor 115 of Fig. 1. Separator:210. may..be 'for example a centrifugal separator. Heat exchanger 230 is located between separator 210 and compressor 220, and is connected to them by pipes 252 and 254 respectively, and heat exchanger 240 is located between compressor 220 and gas'pressure vessel 125 of Fig. 1, and is connected to then by pipes 256 and 124-respectively. Recycling system 200 additionally comprises conveyor 250 -for conveying the -heat transfer material-to hopper 120 of -Fig. 1, and.pipe 2 58 joiing-separator.210 and conveyor 250. Conveyor 250 may comprise for example a screw conveyor or *a conveyor belt, and is connected to hopper 120 of Fig. 1 by pipe* 122. In a preferred embodiment-the heat transfer material comprises a plurality of particles. The:particles preferably have a mean-size of less than about I micron, and -preferably'between about 0:5-and 1 micron. The-particles'are preferably spherical and are composed of graphite with a purity-of -greater than about 95% and preferably greater thanabout 99% by weight. The gasIsis preferably an inert-gas, and it may be,. for example, nitrogen, carbon dioxide, helium, neon or argon. A commonly used gas is nitrogen. --- Irf-operation, a mixture of heafttransfer material:and gas enters recycling system 200 through pipe 118 and passes to separator 210, which at least partially separates the heat transfer material and the gas. The gas then 'passes through pipe 252 to heat exchanger 230 which cools the gas. It then passes through pipe 254 to compressor 220 where it is compressed to a pressure between 1 and 2 atmospheres, commonly about 1.5 atmospheres. It then passes through pipe 256 to heat exchanger 240, where the heat generated by the compressing is removed. Finally the cooled, compressed gas passes through pipe 124 to storage vessel 125 of Fig. 1. The heat transfer material that is. separated by separator 210 passes through pipe 258 to screw conveyor 250, which conveys it via pipe 122 to hopper 120 of Fig. 1. 24 With reference to Fig. 3, recycling system 300 comprises separator 210 for at least.partially separating the heat:transfer material and the vaporised liquid, and condenser 330 is provided to at least partially condense the vaporised -liquid to a liquid state. Separator 210 is connected by pipe 118 to motor 115 of Fig. 1. Separator 210 may be for exarnple a centrifugal separator, and may optionally be heated to prevent condensation. Pump 340 is provided to pump the liquid through pipe 124 to storage vessel 125 of Fig: 1. Pipe 352 connects.separator 210.to condenser 330 and pipe 354 connects condenser'330 to pump .340. Recycling system 300 -additionally comprises conveyor 250.for conveying the heat transfer material to hopper 120 of Fig. 1, pipe 258joining separator 210 and conveyor 250. Conveyor.250 may comprise for example a screw conveyor or a conveyor belt, and is connected to hopper 120 of Fig. 1.by.pipe 122. In a preferred embodiment the heat transfer material.comprises.a plurality of particles. The particles -preferably have a mean size of less than about 1 micron, and preferably between about 0.5 and 1 micron. The particles are preferably spherical and are composed of graphite with a purity of greater than about 95% and preferably greater than about.99% by weight. The liquid may be any liquid that is stable under the conditions -pertaining in the engine, and which is capable of vaporizing in contact with the heat transfer material, and may for example. be water. In operation, a mixture of heat transfer material and vaporised liquid enters recycling system 300 throughpipe 118 and passes to separator 210, which at least partially separates the heat transfer material and'the vaporised liquid. The vaporised liquid then passes through pipe 352 to condenser 330 which at least partially condenses the vaporised liquid to-a liquid-state. The resulting liquid then passes through.pipe 354 to-pump 340 which pumps it-through*pipe 124 to storage vessel 125 of Fig. 1. The heat transfer material that is separated by separator 210 passes through pipe 258 to screw conveyor 250, which conveys it ---via-pipe-1-22-to-hopper-1 20 of-Fig.-l. With reference to Fig. 4, engine 400 comprises heater 105 for heating a heat transfer material, atomiser 440 for generating a spray of the liquid and the gas, mixing chamber 110 for mixing the heat transfer material and the spray, and motor 115 connected to mixing chamber 110, motor 115 being capable of being powered by the mixture of the gas and the liquid and the heat transfer material as the gas expands and the liquid vaporises. Engine 400 also comprises hopper 120 for storing the heat transfer material, gas pressure vessel 425 for storing the gas and reservoir 430 for storing the liquid. Pump .435 is provided to pump liquid from reservoir 430 to atomiser 440. Heater 105 may comprise for example an electrical heater or a burner, or may conveniently be for example an apparatus 25 according -to WO .95/25416. In an apparatus according to WO 95/25416, -a mass of graphite is held at between 2000 0 C and 3000*C, commonly at about 25000C, and may be used as a heat reservoir to -heat the heat transfer material, or maybe used to heat-a secondary heat:reservoir,. held at between about 250"C and 350 0 C, commonly at about 300"C, which may then -be used to heat the heat transfer material. Heater 105 is located between hopper 120 and mixing chamber 110. Mixing chamber 110 is located close to:motor 115 to minimise power losses between them. 1Votor 115. may be.any motor that- is capable of being driven by expansion of a fluid, and is conveniently a piston-driven -motor -or a turbine. Engine 400 additionally has mechanism 135 for controlling the-rate of entry of the heat transfer-material into mixing -chamber 110. The rate of entry of the spray into mixing chamber 110 is controlled by atomiser 440. Pipe 112 connects hopper 120 to mechanism 135, and heater 105,:for heating the heat transfer material, surrounds a portion of pipe 112. Pipes 412.and-414 connect gas pressure vessel 425 and reservoir 430 respectively to atomiser 440..Pipe 116 connects mixing chamber 110 to motor 115. Engine 400 additionally comprises recycling system 450 to recycle the heat transfermaterial, the gas and the liquid.for reuse. Recycling system .450 comprises separator 455 for at least partially separating the heat transfer material from the vapour and the gas, condenser 460 for condensing the vapour that exits separator 455, thereby at-least.partally separating the liquid from the gas, and pump 465 for returning the liquid'to reservoir 430. Separator 455 may be .for example a centrifugal separator. Separator 455 is connected by pipe 118 to-motor 115 and by pipe 452 to condenser 460. Condenser460 is connected to compressor 470 .by pipe 454 and to pump 465 by pipe -456. Pipe .458 connects compressor.470 to heat exchanger 480. Pipe 462 .connects heat exchanger 480:to gas pressure vessel 425 and pipe 464 c6nnects pump 465 to reservoir 430. Compressor-470 is also provided, for compressing the gas after it- has been at least partially separated from'the liquid --- in-condenser-460-and :heat-exchanger-480~s -praide-d for cooligthe gas. Recycling system 450 additionally comprises conveyor 250 for conveying the heat transfer material to hopper 120, pipe 258 joining :separator 455 and conveyor 250. Conveyor 250 may comprise for- example a screw conveyor or a conveyor belt, and is connected to hopper 120 by pipe 122. In a preferred embodiment the heat transfer material comprises a plurality of particles. The particles preferably have a mean size of less than about 1 micron, and preferably between about 0.5 and 1 micron. The particles are preferably spherical and are composed of graphite with a purity of greater than about 95% and preferably greater than about 99% by weight. The gas is preferably an inert gas, and it may be, for example, nitrogen, carbon dioxide, 26.

helium, neon or argon. A commonly used gas is nitrogen. The liquid may be any liquid that is-stable under the conditions pertaining.in the engine, and which is capable of vaporising in contact-with the'heat transfer material, and may for example be water. A method for operating engine 400 comprises heating the heat transfer material, which hasbeen stored.in hopper 120, to between about 250 0 C and about 350 0 C, and-preferably to about 300 0 C, by passing the heat-transfer .material through heater 105'by means of pipe 112. Mechanism 135 is -used to control the rate of entry of the heat transfer material into- mixing chamber'110. Gas passes from pressure vessel 425 through pipe 412 to atomiser 440, and pump 435.pumps liquid from reservoir 430 through pipe 414 to atomiser 440. Atomiser 440 is used for generating a -spray of the liquid and the gas. The ratio of liquid in the spray is commonly'between about -0.1% and about 10% -by volume, and may be about 1% by volume. The rato of heat transfer material to spray entering.mixing chamber 310 is commonly'between about 4:1 and about 20:1 by volume and-may be between about 5:1 and about 10:1 by volume. The heated heat transfer material then mixes with the spray in mixing chamber 110, .the spray being at-between;about 0*C and about 50 0 C, and preferably at about 25 0 C. The expansion -of the spray/heat transfer material mixture, which is caused'by.heating of the gas -portion -of the spray by the heat transfer material and by vaporisation of the liquid portion of the spray, causes the mixture to pass rapidly to motor 115, and provides power to motor 115. The method additionally:comprises recycling the heat transfer material, the gas and the liquid using recycling system 450. A mixture of heat transfer material, gas and vaporised liquid enters -recycling system 450 -from motor 115 through pipe 118 and passes to separator 455, which at'least partially separates the heat transfer material from the gas and the vaporised liquid. The gas and vaporised liquid then pass through pipe 452 to condenser 460 which at least partially c-dens-es -thevpodiie-d-liCijid-tifa liqjilid ste_ The resulting liquid then passes through pipe 456 to pump 465 which pumps it through pipe 464 to reservoir 430. The gas that exits condenser 460 then passes through pipe 454 to compressor 470 where it is compressed to a pressure between 1 and 2 atmospheres, commonly about 1.5 atmospheres. It then passes through pipe 458 to heat exchanger 480, where the heat generated by the compressing is removed. Finally the cooled, compressed gas passes through pipe 462 to gas pressure vessel 425. The heat transfer material that is separated by separator 455 passes through pipe 258 to screw conveyor 250, which conveys it via pipe 122 to hopper 120. 27 Figures 5a to g .show a dismantled turbine motor 500 for use in an engine according to the -present invention..In Figures 5a to g, the motor*500 comprises mixing. chamber 510 for mixing the heat transfer material and fluid, :and rotor 520, which, when motor 500'is assembled, fits into space .549. Space.549..is connected tomixing chamber 510 by holes 530.. Mixing chamber 510 -is annular in shape.and has entry port 525for.admitting the heattransfer material and the fluid. Rotor 520 has radial vanes 540 which enable rotor 520 toturn about shaft 545 powered bythe mixture expanding in mixing chamber 51 0..Holes 530 are angled in -order to direct the expanding mixture to faces 54.7 ofivanes 540. Engine 500 also has :exit port 550 to allow mixture to pass from space 549 to a -separator (not shown). In operation, fluid at:a temperature of between about and 50 0 C and heat transfer material at a temperature of about 350 0 C enter mixing chamber 5.10 through entry port.525.- The resultant expansion of the mixture propels it through holes 530.so that it impacts on faces 547.of vanes 540, which are part of rotor 520. The momentum of the mixture is transferred to rotor 520, causing it to continue -to rotate. The mixture then exits through port 550, from which it passes to a separator (not-shown) for recycling. Figure 5h shows an expanded view of the vanes of the turbine motor.500 of Figs. 5a to 5g. In Figure 5h, rotor 520 is designed to-rotate in the:directiori of arrow 552. Arrows-555:show the direction of flow of the. mixture from mixing chamber 510 through holes 530,to space 549. Arrows 560 show.the direction of flow of the mixture after it has impacted on faces 547. Thus in operation,. fluid at a temperature of betweenabout 0 and 50 0 C and heat transfer material at a temperature of about 350*C enter mixing chamber 510 and pass through holes :530. Holes 530 direct the mixture onto faces 547, and the momeritum of the mixture causes rotor 520 to rotate in the direction of arrow 552. The rotation .of rotor 520,propels the mixture in the direction-of arrows 560, which lead.it:towards the exit port(not:shown in Fig. 5h). Cooling of the mixture in-the-recycling system vlt(not-hhown)-causes-eamsubsequentirefdition-li-6diffe iiture, Which enhances the flow of mixture from space 549 to the exit port. Figure 6a is a diagrammatic representation of a portion of an engine according to the present invention, showing an accumulator 600 having weights 605 attached to lever 610 to control pressure in chamber 615, and feed.system 630 for controlling the rate of delivery of a fluid and a heat transfer material to chamber .615. In Figure 6a, accumulator 600 has lever 610 is pivotably attached to shaft 620, and is.connected via cable 612 to housing 614. Lever 610 is connected by shaft 620 to insulated piston 625 in order to control the movement thereof. Chamber 615 is located inside the lower portion of housing 614. Chamber 615 has a-curved region 612 to encourage circulation of a mixture of 28 fluid and heat transfer material and to. inhibit.separation of the mixture. Housing 614 has mixture-entry port 616 to admit fluid and heat transfer material from feed system 630 to chamber 615,.and has mixture exit port:617-leading from chamber 615 to a motor (not shown). Feed system 630 comprises graphite entty pipe 635 connected to mixture pipe 640 by graphite. rotary valve 645, and gas entry pipe 650 connected to mixture pipe 640 by gas rotary valve 655. Both graphite valve 645 and gas valve 655 are located on valve:shaft 660 which is driven by valve motor 665, so that rotation.of shaft 660 causes coordinated openings:and closures of valves 645 and 655. Mixture pipe 640:is connected-to mixture.entry port 616, and is provided with volume controller 670 which is capable-of moving along shaft 660 in order to accommodate changes in pressure in.rrixture pipe 640. In operation, valve motor 665 rotates valve shaft 660 and thereby causes graphite valve 645 to transfer a fixed quantity of hot graphite from graphite entry pipe 635.irito mixture pipe 640. Further rotation of shaft 660:causes graphite valve 645 to close, sealing -graphite entry-pipe 635 against back pressure, and also causes gas. valve 655 to transfer a fixed volume of -gas into mixture pipe 640. The volume of gas sweeps the graphite in pipe 640:into chamber 615 through entry port 616, causing piston 625 to move upwards in order to maintain approximately constant pressure in chamber 615. When required by the motor to which exit port 617 is connected, the mixture of gas and graphite is propelled out of chamber 615 to the motor, and piston 625 moves downwards due to the pressure provided by-weights 605 and lever"610, thereby reducing the volume of chamber 615 and maintaining approximately constant.pressure therein. Gas volume- controller 670 is- capable of moving along shaft 660 in order-to maintain approximately.constant .pressure in :mixture .pipe 640. Thus-expansion of the gas in.pipe 640 can be accommodated byrnovement of controller,670 towards motor 665. The-speed of:valve motor 665 controls -the -rate of provision of gas and of

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graphite-to-the-motor-to-which-exit-port617-is connected. Figure 6b is a diagrammatic representation of a portion of an-engine according to-the present invention, showing gas driven accumulator'700 and feed system 630 for controlling the rate of delivery of a fluid and of a heat-transfer material to chamber 615. In Figure 6b, accumulator 700 comprises pressure adjusting chamber 705, mixing chamber 615 and insulated piston 625 within housing*714. Chamber 615 has a curved region 612 to encourage circulation of a mixture of fluid- and heat transfer material and to inhibit separation of the mixture. Housing 714 has gas inlet 710 and gas outlet 715 for permitting ingress and egress of gas to and from chamber 705. The gas in chamber 705 allows movement of piston 625 in order to maintain the pressure in chamber 615 at an 29 approximately constant value. Gas outlet 715 is connected to gas entry pipe 650 of feed system 630. Housing 714 also has mixture entry port 616 to admit fluid and heat transfer material from feed-system 630 to chamber 615, and mixture exit port 617 leading from chamber 615 to a motor(not shower). Feed system 630 comprises graphite entry pipe 635 connected to mixture pipe 640 by graphite rotary valve 645, and gas entry pipe 650 connected to mixture pipe 640 by gas rotary valve 655. Both graphite valve 645 and gas valve 655 are located on valve shaft 660 which- is driven by valve- motor 665, so that rotation of shaft 660 causes coordinated openings and closures of valves 645 and 655. High -pressure gas pipe 750 is provided in order to -provide make up gas to provide the required pressure in chamber 615 during operation. Mixturepipe 640 is also provided with volume controller 670 which is capable of moVing along shaft 660 in order to accommodate changes in pressure-inmixture pipe 640. In operation, valve motor 665 rotates valve shaft 660 and thereby causes graphite valve 645 to transfer a fixed quantity.of -hot graphite from graphite entry pipe 635 into mixture pipe 640. Further rotation of shaft 660 causes graphite valve 645 to close, sealing graphite entry pipe-35-against back pressure, and also causes gas valve 655 to transfer a fixed volume of gas into mixture pipe 640. If necessary, extra gas is provided through high pressure gas -pipe 750. The gas sweeps the graphite in pipe 640Dinto-chamber 615 of accumulator 700 through entry port 616, causing piston 625 to move upwards in. order to maintain approximately constant pressure in chamber 615. When required by the motor to which exit port 617 is connected, the mixture of gas :and graphite is propelled out of chamber 615 to the motor, and piston :625 moves downwards under the pressure provided by the gas in chamber 705, thereby:reducing the volume of chamber 615 and maintaining .approximately:constant pressure therein. Gas volume controller 670 is capable of moving along shaft 660 in order to maintain approximately constant pressure-in mixture pipe 640. Thus expansion of the gas - -in-pipe640-can-be-accommodatedby-movemerit-of controlldr-670-towards motor- 665. The speed- of -valve motor 665 controls the rate of provision of gas and of graphite to the motor to which exit port 617 is connected. Figure 7 shows a turbine similar to a steam turbine, suitable for use in the present invention. In Figure 7, turbine 800 comprises housing 810 having inlet 815 for admitting a heated mixture of heat transfer material and gas. The heated mixture moves through turbine 800 in the direction of arrows 820. Turbine 800 has axial shaft 825 with movable blades 830 to 834 attached thereto. Blades 830 to 834 are shaped so that when the mixture impacts thereon from the direction of inlet 815,-a force is generated which tends to cause the blade to turn in the direction of arrow 835. The shape of blades 830 to 834 is also such as to allow 30 the mixture to move to the downstream side of the blade after it has impacted thereon. Blades 830.to 834 may be for example-propeller-shaped blades. Turbine 800 additionally has fixed blades 840 to 844, which have holes (not shown) for directing the mixture onto movable blades 830 to 834 respectively at right angles in order to transfer the maximum amount of force to movable blades 830 to 834. There is a fixed. blade 840 to 844 upstream (i:e. on the inlet side) of each movable blade 830 to 834 respectively' In operation, heated heat transfer material and cool gas.are mixed in a mixing chamber (not shown) and the mixture of gas and heat transfer-material expands into turbine 800 through inlet 815. The mixture passes through holes in fixed blade 840 so as to impact on movable blade 830 at right angles. After passing movable blade.830, the mixture passes through holes in fixed -blade 841 so as to impact on movable blade 831 at right angles. In similarfashion, the mixture passes successively through blades 842, 832,;843, 833,,844 and 843 in the direction of arrows 820, and finally-exits turbine 800 for separation in a separator (not shown). As the mixture impacts on movable blades 830 to 834, it imparts momentum to those blades, causing shaft -835 to turn in the direction of arrow 835. 31

Claims

1. An engine comprising: * a mixing chamber for forming a mixture of a heat transferimaterial at a - first temperature with a fluid at a second temperature, the first temperature being.higher than the second -temperature, and * an energy converter for converting an energy of expansion of the mixture in the mixing chamber to usable mechanical energy.

2. The engine of claim 1 additionally comprising a heater for heating the heat transfer: material to at least the first temperature.

3. The -engine of claim 1 or claim 2 wherein the energy converter comprises a motor connected to themixing chamber, said motor being capable of 'being powered.by the mixture as the Mixture-expands.

4. The-engine of any one of claim '1 to 3 wherein, the -fluid 'is a non oxidising gas.

5. The.engine of any-ond of claim 1 to 4 wherein the second temperature-is between about 0*C and about 100 0 C.

6. The engine of any one of claims 1 to 5-wherein'the first'tdmperature is greater than about 200 0 C.

7. The -engine -of any one of claims.1 to'6 wherein the.difference between the second temperature and the'first temperature is greater than about 25 Celsius degrees.

8. The engine of any one of claims 1 to 7 wherein the heat transfer material comprises a plurality of solid particles.

9. The engine of claim 8 wherein the particles have a mean size of less than about'10 microns.

10. The engine of claim 8 or claim 9 wherein 'the particles are substantially -spherical.

11. The engine of-any 'one of claims 8 to 10 Wherein the particles -are carbon-particleshaving-a percentageof--rbohf g§reatehan g5*7oby weight or by volume.

12. The engine of any one of claims 1 to 11 further comprising a storage vessel for the heat transfer material.

13. The engine of claim 12 wherein the storage vessel comprises a heater for heating the heat transfer material.

14. . The engine of any one of claims 1 to 13 further comprising a fluid storage vessel for the fluid.

15. The engine of any one of claims 1 to 14 wherein the mixing chamber comprises an accumulator for maintaining an approximately constant pressure on mixing the fluid and the heat transfer material. 32

16. The. engine of any one of claims .1 to 15 further comprising a delivery controller for controlling the rate of delivery of the fluid and/or of the heat transfer; material to the mixing chamber.

17. The engine of any one of claims 1 to 16 further comprising a cooler for.cooling the mixing chamber.

'18. The engine of any one of claims to 17 wherein the energy converter comprises either a piston-driven motor or a:turbine.or a-mcvable part of -either a piston-driven motoror a-'turbine.

19. The engine of any one of claims 1 to 18 additionally comprising a recycling system for recycling-at least one of the heat transfer material and the fluid for reuse in the-engine.

20. 'The engine of claim 19 wherein the recycling system comprises one or more of: " a separator for at least partially separating the heat.transfer material and the fluid; * a cooler for cooling the fluid; * a conveyor for conveying the heat transfer material to a storage vessel; and tubing and/or pipework -connecting the above components, -and connecting the recycling system to other components of the engine.

21. The engine of claim 19 or blaim.20-wherein.the. fluid is a-gas at.the second temperature and the recycling system further comprises a compressor for dompressing the gas after it. has exited the separator.

22. A method-for operating an engine, said engine comprising (i) a mixing chamber for forming -a mixture of a heat transfer material.at a first temperature with a fluid at a second temperature, the first temperature being higher than the-second temperature, and (ii) an energy converter.for converting an energy of expansion of the mixture in the mixing.chamber to -- usablemechanical-energysaid-method-comprising: a) 'heating the heat transfer material to at least the first temperature; . b) preparing a mixture of the heat transfer material at the first temperature with a fluid, said fluid being at a second temperature which is less than the first temperature; and c) converting an energy of expansion of the mixture in the mixing chamber to usable mechanical energy; wherein the fluid and the heat transfermaterial are unreactive towards each other at the first temperature and are stable at the first temperature. 33

23. The method of claim 22 wherein step c) comprises using expansion of the mixture to power a motor, said expansion being a consequence of step b).

24. The method of claim 22 or claim 23 wherein expansion of the. mixture is due to heating of a gas, volatilisation of a liquid or to both heating of. a gas and volatilisation of a liquid.

25. The method of any one of claims 22 .to 24 wherein step c) comprises using the energy of expansion to power one or more pistons which are part of a piston-driven motor, or to power a turbine.

26. The method of any one of claims 22 to 25 additionally comprising controlling the rate of entry of the fluid and/or of the heat transfer material into' the mixing chamber.

27. The method of any one of claims 22 to 26 additionally comprising the step of recycling at least one of the heat transfer material and the fluid for reuse in the engine.

28. The method of claim 27 wherein the step of recycling comprising one or more of: d) at least partially separating the heat transfer material and the fluid, e) cooling the fluid, and f) conveying the heat transfer material to a storage vessel.

29. The method of any one of claims 22 to 28 wherein step a) comprises heating the heat transfer material to a temperature of between about 2000C and about 400*C.

30. The method of any one of claims 22 to 29 wherein the second temperature is between about 0*C and about 100*C.

31. A system comprising an engine and a controller for controlling at least one operating parameter of the engine, said engine comprising (i) a mixing chamber for forming a mixture of a heat transfer material at a first terrifp-ratur-eWith a-flUidat a second-terperat~ufethefist temperature being higher than the second temperature, and (ii) an energy converter for converting an energy of expansion of the mixture in the mixing chamber to usable mechanical energy.

32. The system of claim 31 wherein the at least one operating parameter is selected from the group consisting of: * the first temperature, * the second temperature, e the temperature of a heater for heating the heat transfer material to the first temperature, * the rate of entry of the fluid into the mixing chamber, 34 e the rate of entry of the heat transfer material into the mixing chamber, a the pressure. in the mixing chamber, and * a combination of any -one or more of these.

33. A vehicle or an electrical generator comprising an engine, said engine comprising (i) a mixing chamber for forming a mixture of a heat transfer material at a first temperature with a fluid at a second temperature, the first temperature:being higher than the-second temperature,.and (ii) an energy converter for converting:an energy of expansion of the mixture in the mixing chamber to usable mechanical energy.

34. A mixture-for use in an engine, said mixture comprising: 0 a plurality of carbon particles, said particles having a mean particle size of less than about 10 microns, and 0 a fluid, a said. carbon particles being at a temperature of at least 200 0 C, and the nature of the:fluid and of the carbon -particles being such that they are stable, and do not react with each other, at the temperature of the - mixture. 34. The mixture of claim 34 wherein the specific gravity of the mixture at the temperature of the carbon particles and at a pressure of one atmosphere is greater than about 0.5.

35. The mixture of claim 34 or claim 35 wherein the proportion of carbon particles in the mixture is greater than about 30%.by volume. 35