WO2010105288A1

WO2010105288A1 - Thermal engine using an external heat source

Info

Publication number: WO2010105288A1
Application number: PCT/AU2010/000295
Authority: WO
Inventors: Ivan Voropaev
Original assignee: Ivec Pty Ltd
Priority date: 2009-03-15
Filing date: 2010-03-15
Publication date: 2010-09-23

Abstract

An engine that utilises an external heat source, such as a solar energy, geothermal energy, waste heat, and/or burning of a fuel is provided. The engine has a piston located in a cylinder heated by the external heat source. Upon injection of a liquid into the heated cylinder, the liquid is vaporised, increasing pressure and providing power to the piston. Once the piston stroke is complete, the vapour is released and the piston returns to a position ready for injection again. In an embodiment, a cooling cylinder is provided which condenses the vapour into a liquid for injection into the heated cylinder. The engine may be utilised in conjunction with a steam turbine to convert water into steam, replacing a boiler while also providing additional work.

Description

THERMAL ENGINE USING AN EXTERNAL HEAT SOURCE

FIELD OF THE INVENTION

The invention relates to a thermal engine. More particularly, the invention relates to an engine and method of generating mechanical energy that utilise an external heat source to move a piston by expanding a fluid within a chamber.

BACKGROUND ART

[Mere reference to background art herein should not be construed as an admission that such art constitutes common general knowledge in relation to the invention.]

Various types of engines which convert potential energy into kinetic energy are known. For example, steam engines and internal combustion engines. The internal combustion engine burns stored chemical energy in a fuel, such as gasoline or diesel, and is particularly well known due to its extensive use in automobiles. In essence, a fuel air mixture is injected into a cylinder with a movable piston, compressed by moving the piston into the chamber, ignited (e.g. by a spark), and the resulting expansion of the ignited fuel pushes the piston. However, the internal combustion engine has undesirable side effects, primarily relating to the use of non-renewable fossil fuels and the exhaust emissions.

Steam engines on the other hand utilise water vapour in the form of high pressure steam to drive a piston. The steam is generated in a boiler, which is typically heated by burning some form of fuel (e.g. wood, coal, gas, or the like). Smaller heat sources are generally not suitable for powering a steam engine as a high quantity of energy is required to convert water in a boiler to high pressure steam. Furthermore, the boilers, and other high pressure components, can fail, causing an explosion. For example, the boiler may be over pressurised, insufficient water in the boiler may cause it to overheat and fail, the boiler itself may fail due to poor constructions and/or maintenance, and/or steam may escape at points of weakness (e.g. the pipework) reducing pressure and efficiency, and also possibly cause harm such as, for example, scalding.

It is an aim of this invention to provide a thermal engine which overcomes or ameliorates one or more of the disadvantages or problems described above, or which at least provides a useful alternative. SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided an engine utilising an external heat source, the engine comprising: at least one piston movably located within a cylinder together defining a chamber of variable internal volume, wherein at least a portion of the cylinder is heated by the external heat source; a fluid inlet to the chamber adapted to inject a fluid into the chamber when the internal volume of the chamber is low; and a fluid outlet from the chamber adapted to release at least a portion of the contents of the chamber when the internal volume of the chamber is high.

Preferably the piston is movable linearly within the cylinder. However, other piston/cylinder arrangements could be utilised such as, for example, a rotary piston (e.g. a rotor in a rotary engine arrangement). The engine may comprise a plurality of pistons within respective cylinders. Preferably the pistons are mechanically connected. Even more preferably, the pistons are connected to a crankshaft. Similarly to a crankshaft in an internal combustion engine, the pistons may have their expansion stroke offset to allow a the expansion stroke of a piston to provide power to another piston in a different stroke (e.g. in a compression stroke). The engine may comprise a flywheel to maintain a constant angular velocity of the crank shaft and/or to provide inertia, when the engine is running.

The external heat source may be any suitable source (or plurality of sources) such as fuel sources (e.g. burning wood, coal, gas, diesel, electrical, or the like), renewable sources (e.g. solar, geothermal, or the like), and/or heat from other heat sources (e.g. exhaust and/or heat losses from an internal combustion engine, power generators, heat losses from equipment, brine from processing, or the like). Preferably the external heat source provides constant heat.

The heat source preferably provides sufficient thermal energy to spontaneously convert the fluid from a liquid state to a gas state. The fluid may be any suitable working fluid that absorbs heat from the cylinder. In an embodiment the fluid is water, injected in its liquid state and converted to its gaseous state, steam, upon injection into the heated chamber. Other fluids could also be utilised (either individually, or in combination) such as refrigerant. Preferably the quantity of fluid injected into the chamber is selected to provide a suitable expansion when subjected to the heat. The quantity of fluid may depend on the fluid used, the temperature of the cylinder and/or chamber, and the internal volume of the chamber (particularly when the volume is low). Preferably, the quantity of fluid injected into the chamber when the internal volume of the chamber is low is between 10% and 90% of the lowest volume of the chamber. Even more preferably, the quantity of fluid injected is between 40 and 60% of the lowest volume of the chamber and, in an embodiment, the quantity of fluid injected is approximately 50% of the lowest volume of the chamber. Preferably the fluid being injected is a liquid preheated to at or near its vaporisation temperature (e.g. water preheated to approximately 100°C). The fluid may be preheated utilising energy/heat from the released contents.

In an embodiment, the fluid is water preheated to 100°C, the cylinder is heated by a natural gas external heat source to approximately 1000°C, the internal volume of the chamber is approximately 100ml at its lowest, and a 50ml quantity of the water is injected into the chamber. When subjected to the high temperature of the cylinder, the injected water instantly generates high pressure steam and moves the piston away, increasing the internal volume of the chamber. As the piston moves, and the internal volume of the chamber increases, the pressure and temperature inside the cylinder decrease. However, in comparison to a standard internal combustion engine the temperature drop is slower because the steam is continuously being heated by the external heat source. Once the piston is at maximum displacement (i.e. the chamber is at its maximum internal volume) the steam is released from the cylinder.

The engine may further comprise a cooling cylinder. The cooling cylinder may be fluidly connected with the fluid inlet and/or fluid outlet. Fluid outlet from the cylinder may be expelled into the cooling cylinder where it is cooled and injected in the fluid inlet. Preferably the outlet fluid is gaseous (e.g. steam), and may be condensed back to a liquid (e.g. water) in the cooling cylinder. The condensed liquid may then be injected via the liquid inlet to the heated cylinder where it is vaporised into a gas. The cooling cylinder may comprise a fluid inlet which allows a cooling fluid (preferably a liquid) to be injected to assist in the cooling process. The cooling fluid may be water, or other suitable fluid such as, for example, refrigerant. The cooling cylinder may have a control valve to control the pressure in the cooling cylinder. For example, the control valve may vent high pressures into the atmosphere to reduce the pressure in the cooling cylinder. More than one fluid outlet and/or fluid inlet may be utilised. The fluid outlet(s) may have a valve to ensure fluid can only flow from the piston cylinder to the cooling cylinder, and not vice versa. The fluid outlet(s) (or a portion thereof) may be located at a position that can only outlet fluid when the volume of the chamber is high and/or at a position that allows fluid to be outlet even when the volume of the chamber is low. A plurality of fluid inlets may be utilised to inject the fluid into the chamber from various positions, preferably improving the vaporisation process. The inlet(s) and outlet(s) are preferably located in a wall portion of the cylinder, but it is appreciated that either or both could be located elsewhere such as, for example, in the piston. (

A secondary chamber defined by the cylinder and the other side of the piston to the first chamber may be utilised. This secondary chamber may be utilised in a substantially similar way to the first ('primary') chamber, but operating in opposing phases. Alternatively, the secondary chamber may be utilised as a pump (e.g. a vacuum pump) to ventilate the cooling cylinder. The piston cylinder and/or cooling cylinder may be thermally insulated to reduce energy losses. They may be insulated using any suitable means such as, for example, using a thermally insulated material and/or layers (e.g. gas or vacuum layers).

The cooling cylinder may have a piston. Preferably the piston in the cooling chamber reciprocates with the piston in the heated cylinder. In an embodiment, the outlet of the heated cylinder is an inlet to the cooling cylinder. The inlet to the heated cylinder may be an outlet from the cooling cylinder. In this arrangement, the fluid is heated in the heated cylinder, expanding as it vaporises, and is then transferred to the cooling cylinder, contracting as it condenses. The cooling cylinder may also have an inlet for cooling fluid, such as cold water, to be injected to assist in cooling.

In an embodiment, the cylinder(s) are located in a liquid tank, preferably a water tank. The liquid tank is utilised as a thermal insulator and regulator, with the liquid absorbing heat loses from the heated piston cylinder. The fluid injected into the chamber of the piston cylinder may be drawn directly from the tank. The size of the tank is preferably determined to have a suitable amount of heat exchange between the heated piston cylinder(s) and exhaust and/or cooling cylinder(s). An exhaust pipe from the outlet of the piston cylinder(s) and/or from the cooling cylinder(s) may be utilised, and may be located inside the tank to reduce the temperature of exhaust gases. In steam turbine applications, such as for power generation, water is heated to steam which then drives a steam turbine. The conversion of water to steam typically takes place in a boiler. Instead of, or at least in addition to, the boiler, the engine may be utilised to heat water to steam for the steam turbine. Advantageously, this conversion of water to steam (which subsequently drives a steam turbine) produces additional work which can be utilised (e.g. for additional power generation). Furthermore, the engine may be used as a heat exchanger, drawing workable mechanical energy from otherwise wasted heat energy (e.g. where heat is otherwise expelled or cooled to atmosphere, as in many industrial and power generation applications).

According to a second aspect of the invention, there is provided a method of converting thermal energy into mechanical energy using an engine, the method comprising the steps of:

(i) heating a portion of a cylinder from an external heat source, wherein the cylinder contains a piston that together define a chamber of variable volume; (ii) injecting a fluid into the chamber when the internal volume of the chamber is low;

(iii) allowing the fluid within the chamber to expand due to an increase in temperature and change of state from a liquid to a gas, the fluid expansion causing the piston to move and the internal volume of the chamber to increase; (iv) releasing at least a portion of the contents of the chamber; and

(v) moving the piston to a position where the internal volume of the chamber is low.

Preferably the step of heating a portion of a cylinder from an external heat source is continuous, being carried out simultaneously with steps (ii) to (v). The heat source may be a periodic or fluctuating heat source, but preferably provides a substantially constant transfer of heat in use.

The timing of the injection of fluid into the chamber is preferably at or near a point when the piston is positioned such that internal volume of the chamber is at its least. The quantity, and/or type, of fluid being injected is may be selected to maximise its expansion.

The method may further comprise the step of preheating the fluid prior to injecting the fluid into the chamber. Preferably the fluid is preheated to a temperature that is at or near the temperature of vaporisation. The method may further comprise the step of cooling the released contents of the chamber. The released contents of the chamber may be condensed, and used as at least a portion of the fluid being injected. In such cases, the temperature of the fluid may instead be cooled prior to injecting, preferably to a temperature that is at or near the temperature of vaporisation.

The release of at least a portion of the contents of the chamber may be when the internal volume of the chamber is high. Alternatively, the release may be carried out as the piston returns to a position where the internal volume of the chamber is low and, before the step of injecting the fluid, the release is stopped.

The step of moving the piston to a position where the volume of the chamber is low may be actuated by one or more other pistons (e.g. in their expansion phase, or 'power stroke'), by momentum/inertia, and/or by any other suitable means. The step of releasing at least a portion of the contents of the chamber may comprise injecting a cooling fluid to assist in condensing the contents.

In order that the invention may be more readily understood and put into practice, one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a diagrammatic view of an embodiment of the invention.

Figure 2 illustrates a diagrammatic view of the embodiment of the invention illustrated in figure 1 in a different position.

Figure 3 illustrates a diagrammatic view of the embodiment of the invention illustrated in figure 1 connected to a load.

Figure 4 illustrates a pressure volume (PV) diagram illustrating a thermodynamic cycle according to an embodiment of the invention.

Figure 5 illustrates a diagrammatic view of an embodiment of the invention.

Figure 6 illustrates a diagrammatic view of the embodiment of the invention illustrated in figure 5 in a different position. Figure 7 illustrates a temperature entropy (T-s) diagram illustrating a thermodynamic cycles according to an embodiment of the invention.

Figure 8 illustrates a pressure volume (PV) diagram illustrating a thermodynamic cycles according to an embodiment of the invention

Figure 9 illustrates a diagrammatic view of an embodiment of the invention.

Figure 10 illustrates a diagrammatic view of an embodiment of the invention with multiple cylinders.

Figure 11 illustrates a diagrammatic view of an embodiment of the invention used in conjunction with an internal combustion engine.

Figure 12 illustrates a diagrammatic view of an embodiment of the invention utilising both sides of a piston within a cylinder.

Figure 13 illustrates a diagrammatic view of an embodiment of the invention having the engine drive a steam turbine.

DESCRIPTION OF PREFERRED EMBODIMENT(S) Figures 1 and 2 illustrates an embodiment of the invention having a cylinder 10 being heated by a constant heat source 11 (although the constant heat source is illustrated as a sun, and therefore may be indicative of the heat source being solar, any suitable heat source(s) could be utilised). The cylinder contains a piston 12 that is linearly movable within the cylinder 10 to define a chamber 13 of variable volume (depending on the location of the piston 12 within the cylinder 10).

The heat source 11 is external to the cylinder 10, and typically heats at least one side of the cylinder, in turn conducting heat to the internal walls of the cylinder, and therefore to the chamber 13. The piston 12 has a connecting rod 14 which may be engaged with a movable element to transfer movement of the piston to another mechanism.

The cylinder 10 has a fluid inlet 15 and a fluid outlet 16. The fluid inlet 15 injects fluid 17 into the chamber 13 when the internal volume of the chamber is low, as illustrated in figure 1. The fluid outlet 16 releases at least a portion of the contents of the chamber 13 after the fluid injected into the chamber 13 has expanded, as illustrated in figure 2. In the illustrated embodiment, the fluid 17 is water, but no limitation is meant thereby.

The water 17 is injected as a liquid into the cylinder 10 which is heated by the heat source 11 (as in figure 1). The sudden change in temperature causes the water to vaporise, changing state to steam 17'. The change in state causes the fluid to expand, increasing pressure and pushing the piston 12 out of the cylinder 10 (as in figure 2). As the heat source 11 is constant, the pressure of the steam 17' generally increases continually over the expansion phase (as opposed to an internal combustion engine where the pressure increase drops after ignition). The illustrated cylinder has thermal insulation 18 which reduces heat losses from the cylinder 10.

Once the piston reaches the top (i.e. when the volume of the chamber is high) the steam 17' is released out of the fluid outlet 16 (the fluid outlet has a valve to prevent steam released into the cooling cylinder 19 from getting back into the cylinder 10). The piston 12 can then return to a position where the chamber 10 has a low internal volume, as illustrated in figure 1, and the cycle can repeat. The steam 17' that is released is transferred to a cooling cylinder 19 (or condenser) that cools the steam 17', condensing it back to water 17. The water 17 can then be utilised for injection through the inlet 15 to the cylinder 10.

Although the outlet 16 is illustrated as being relatively high in relation to the inlet 15, the location of the outlet 16 could also be at a different location (e.g. on a similar level to the inlet 15). The inlet and outlet preferably have valves to determine when fluid can be transferred and/or the direction of fluid transferral.

Figure 3 illustrates the piston 12 being connected to a crankshaft 20 by the connecting rod 14. The crankshaft 20 is connected to a load 21, such as a generator. More than one cylinder/piston arrangement 100 may be utilised to provide power to the crankshaft 20, and (if run out of phase) to provide power during the return stroke of the piston 12.

Figure 4 shows a pressure volume (PV) diagram illustrating pressure and volume of the chamber 13 in the cylinder 10 over a cycle. At point 'A' the piston has just been injected with water 17, and therefore from A-B the piston 12 moves, increasing the volume of the chamber 13 while decreasing the pressure. At 'B', the outlet 16 is opened, allowing at least a portion of the steam 17' to escape, causing a rapid drip in pressure to 'C. The piston 12 then moves back into the cylinder 10, decreasing the volume of the chamber 13 and providing a slight increase in pressure until it reaches its lowest point at 'D'. Water 17 is injected into the chamber 13, with the sudden vaporisation causing a sudden increase in pressure, and the cycle repeats. The water 17 may be injected briefly or, more preferably, during the expansion phase of the chamber, to provide continual expansion of the contents in the chamber (e.g. the injection occurs all the way from the minimum volume of the chamber to the maximum volume of the chamber).

To minimise losses, the fluid 17 is preheated (or only cooled down to) a temperature at or near its vaporisation temperature. Assuming the fluid 17 is water, the temperature before injection is approximately 100°C. The external heat source 11 may be a natural gas burner. The combustion temperature of natural gas is approximately 2000⁰C, but some of this heat will be lost in heating the cylinder, and conducting the heat to the chamber. In an embodiment, the chamber has a 10OmL volume when the piston is at its lowest (i.e. when the chamber is at minimum volume). The injection of 5OmL of water (preheated to 100°C) into the heated chamber 13 instantly generates high pressure steam, moving the piston 12 up. Once the piston is at its highest (i.e. when the chamber is at maximum volume) the steam is outlet to the cooling cylinder 19.

The cooling cylinder 19 may have a control valve to allow it to maintain a certain pressure (or pressure range) by, for example, venting excess pressure into the atmosphere. The opposite side of the piston 12 may also be utilised as a vacuum pump to ventilate the cooling cylinder 19. The efficiency of the engine primarily depends on the external heat source 11, the heat exchange process (e.g. the efficiency in transferring the heat to the chamber 13), and the quality of thermal insulation 18.

Figures 5 and 6 illustrate diagrammatic views of an embodiment of the invention wherein the cooling cylinder 19 has a reciprocating piston 22. The steam 17' is transferred from the heated cylinder 10 to the cooling cylinder 19, during the downward/contraction phase of the piston 12. As the steam 17' cools and condenses, it creates a vacuum and draws the piston 22 back into the cooling cylinder 19. To assist in cooling/condensing the steam 17', a cooling fluid 17" may be injected into the cooling cylinder which, in the illustrated embodiment, is the same fluid used for expansion in the heated cylinder 10, water 17. Water exhausted form the cooling cylinder 19 may be split into two parts (of not necessarily equal volume). One part can be utilised back into the heated cylinder 10 as the fluid to be injected, and the other part to a radiator to cool itself as much as possible (for injection into the cooling cylinder 19 during the cooling phase). The radiator size is preferably defined by the temperature of the water from the radiator to be injected into the cooling cylinder 19. Exhausted gas from the heated cylinder 10 may also be utilised to preheat the water 17 before injection into the heated cylinder 10. Advantageously, this arrangement results in energy conversion during both the vaporisation and condensation phases. It is envisaged that such an arrangement could be used as a replacement for certain internal combustion engines, such as diesel generators/engines on ships and trains, and smaller engines could be utilised in hybrid cars.

As the respective pistons within the heated cylinder 10 and cooling cylinder 19 generate power at the same time (e.g. during expansion in the heated cylinder, and condensation in the cooling cylinder), more than one pair of cylinder/piston arrangements may be used to maintain a substantially constant torque (and to provide power during the non-power phases of the pistons). To negate, or at least reduce, frictional forces between the pistons and the cylinders, a small quantity of lubricant could be added to the water prior to, or simultaneously with, injecting it into the cylinder(s).

Figure 7 shows temperature entropy (T-s) diagram and figure 8 shows a pressure volume (PV) diagram, illustrating a thermodynamic cycle. The grey arrows illustrate changes in the heated cylinder/chamber, and the black arrows illustrate changes in the cooling cylinder/chamber. Following the grey arrows, water 17 injected into the heated cylinder 10 creates an increase in temperature and pressure (l->2), as the chamber volume increases (due to the piston moving up) the entropy and volume increases (2->3), as the chamber volume decreases (due to the piston moving down) the entropy and volume decreases (3->4).

The total thermal efficiency can be approximated by considering the heat source 11 to have an efficiency to approximately 80%, and the heated and cooling cylinders 10 and 19 to have 90% efficiency, resulting in a total efficiency of -64.8% (0.9*0.9*0.8). A 50% from heat to wheel/wire is considered readily achievable.

Illustrated in figure 9 is an embodiment where the cylinder 10 is located within a fluid, namelv a tank of water 30. The tank, of water 30 acts as a thermal insulator and regulator. Both heated cylinders 10 and cooling cylinders 19 can be positioned maintained within the tank 30, with the engine drawing fluid/water 17 directly from the tank 30. Figure 10 illustrates an embodiment having two heated cylinders 10 and two cooling cylinders 19, all contained within a single tank 30. The water in the tank 30 absorbs heat losses from the cylinders. The size of the water tank 30 is preferably determined by the heat exchange between the heated cylinder(s) 10 and cooling cylinder(s) 19. Exhaust gases (e.g. steam/vapour) can be vented inside the tank to reduce their temperature.

The embodiment illustrated in figure 11 is coupled to an internal combustion engine 40, having four standard fuel driven cylinders 41. The hot exhaust from the internal combustion engine 40 (typically in the range of 500°C to 700°C) is used to provide the external heat source to the cylinders 10, which are arranged in a reciprocating arrangement with cooling cylinders 19. This allows some of the thermal losses from the internal combustion engine (which are typically 50% or more) to provide additional work. A 300°C to 400°C drop in temperature of the exhaust gases utilising the embodiment of the invention is expected to provide approximately 30% more power. Advantageously, the cooling of the exhaust gases reduces the generation of greenhouse gases and pollution.

Figure 12 illustrates an embodiment with the heating and cooling sharing the same cylinder. The piston 50 has a one way valve that transfers the steam from the heated chamber 13 to the cooling side of the cylinder. Water is injected via inlets 51 to both sides of the piston simultaneously to initiate vaporisation and condensation in the heated and cooled sides of the cylinder, respectively. Condensed water in the cooling side of the cylinder may then be transferred back to the heated side. To optimise efficiency, the maximum possible ratio of are to hight is maintained (i.e. larger area, smaller height). Additionally, achieving high movement rates (e.g. revolutions per minute) allows for faster vaporisation with less time for heat losses.

The embodiment in figure 13 illustrates and engine 60 having cylinders (not illustrated in figure 13) heated by an external heat source 11. In the illustrated embodiment, the external heat source is transferred from a fluid inlet 61 (e.g. 250° brine heated by, for example, geothermal means). After the heat is transferred/exchanged from the fluid 61, it is outlet 63 at a reduced temperature. A working fluid (e.g. water) is heated and vaporised in the engine 60, which is then fed to a steam turbine 62 (generally instead of directly to a cooling cylinder, as in some previous embodiments). The steam turbine 62 generates power, as usual (and is known in the art), and the exhaust from the steam turbine 62 is fed to a condenser 64, where is it cooled and condensed. The condensed fluid is then used as a feed 65 into the engine 60 as a working fluid ready for vaporisation.

In this configuration, the engine 60 works like a boiler, but unlike a boiler the engine produces additional useful work that can be utilised for power generation.

Laboratory tests prove that the engine 60 can achieve close to regular internal combustion engine efficiencies, without thermal insulation and only using water as the working fluid (it is envisaged that other working fluids may improve the efficiency).

For example, using the configuration illustrated in figure 13 and assuming: the engine 60 has good thermal insulation, the external heat source 11 is content at 250°C, with a working fluid of HFC-365mfc (having a critical temperature of 187.7°C and a critical pressure of 2754kPA), and a steam turbine having:

Rated Power 5000 kW HP Steam Pressure 20 bar abs

HP Steam Temperature 200 °C Exhaust Steam Pressure 1 bar abs

Full Load Isentropic Efficiency 15.0 %

Full Load Specific Steam Consumption 50.0 kg/kWh

then rough steam consumption is estimated to be 70 kg/s. To create a 70 kg/s steam supply for turbine, it is anticipated the engine 60 will generate a 3MW power output, that also can be used as additional source for electricity generation.

It is to be understood that the terminology employed above is for the purpose of description and should not be regarded as limiting.

The foregoing embodiments are intended to be illustrative of the invention, without limiting the scope thereof. The invention is capable of being practised with various modifications and additions as will readily occur to those skilled in the art.

Accordingly, it is to be understood that the scope of the invention is not to be limited to the exact construction and operation described and illustrated, but only by the following claims which are intended, where the applicable law permits, to include all suitable modifications and equivalents within the spirit and concept of the invention. Throughout this specification, including the claims, where the context permits, the term "comprise" and variants thereof such as "comprises" or "comprising" are to be interpreted as including the stated integer or integers without necessarily excluding any other integers.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. An engine utilising an external heat source, the engine comprising: at least one piston movably located within a cylinder together defining a chamber of variable internal volume, wherein at least a portion of the cylinder is heated by the external heat source; a fluid inlet to the chamber adapted to inject a fluid into the chamber when the internal volume of the chamber is low; and a fluid outlet from the chamber adapted to release at least a portion of the contents of the chamber when the internal volume of the chamber is high.

2. An engine as claimed in claim I, wherein the fluid inlet injects water.

3. An engine as claimed in any preceding claim, wherein the fluid outlet releases water vapour or steam.

4. An engine as claimed in any preceding claim, further comprising a cooling cylinder in fluid communication with the fluid outlet, wherein the cooling cylinder receives and cools the released portion of the contents.

5. An engine as claimed in claim 4, wherein the cooling cylinder receives a gas, and condenses the gas to a liquid.

6. An engine as claimed in claim 4 or 5, wherein the cooling cylinder comprises a piston movably located within the cooling cylinder.

7. An engine as claimed in any one of claims 4 to 6, wherein the cooling cylinder has a fluid inlet adapted to inject a fluid to assist in cooling the received contents.

8. An engine as claimed in any preceding claim, further comprising a plurality of pistons within respective cylinders.

9. An engine as claimed in claim 8, wherein the plurality of pistons are mechanically connected to a crankshaft.

10. An engine as claimed in any preceding claim, wherein the external heat source provides a substantially constant source of heat.

11. An engine as claimed in any preceding claim, wherein the external heat source comprises at least one of: solar energy, geothermal energy, waste heat, and burning of a fuel.

12. An engine as claimed in any preceding claim, wherein the fluid outlet is in fluid communication with a steam turbine.

13. An engine as claimed in claim 12, wherein the released contents are fed directly to the steam turbine.

14. An engine as claimed in claim 12 or 13, wherein exhaust from the steam turbine is fed into a condenser, which subsequently provides the fluid for injection into the chamber.

15. An engine as claimed in any preceding claim, wherein at least the cylinder is thermally insulated to reduce heat losses.

16. An engine as claimed in any preceding claim, wherein the cylinder(s) are located in a tank of liquid.

17. An engine as claimed in any preceding claim, wherein the heat of the released contents is utilised to preheat the fluid being inlet.

18. A method of converting thermal energy into mechanical energy using an engine, the method comprising the steps of:

(i) heating a portion of a cylinder from an external heat source, wherein the cylinder contains a piston that together define a chamber of variable volume;

(ii) injecting a fluid into the chamber when the internal volume of the chamber is low;

(iii) allowing the fluid within the chamber to expand due to an increase in temperature and change of state from a liquid to a gas, the fluid expansion causing the piston to move and the internal volume of the chamber to increase; (iv) releasing at least a portion of the contents of the chamber; and (v) moving the piston to a position where the internal volume of the chamber is low.

19. A method as claimed in claim 18, wherein the step of heating a portion of a cylinder from an external heat source is continuous, being carried out simultaneously with steps (ii) to (V).

20. A method as claimed in claim 18 or 19, further comprising the step of preheating the fluid prior to injecting the fluid into the chamber.

21. A method as claimed in claim 20, wherein the fluid is at least partially preheated by the heat of the released contents.

22. A method as claimed in claim 20 or 21, wherein the fluid is preheated to a temperature that is at or near the temperature of vaporisation of the fluid.

23. A method as claimed in any one of claims 18 to 22, wherein the step of releasing at least a portion of the contents of the chamber comprises releasing the portion of the contents to a cooling cylinder.

24. A method as claimed in claim 23, wherein the method further comprises the step of condensing at least a portion of the contents released to the cooling cylinder.

25. A method as claimed in claim 22 or 24, wherein the method further comprises the step of injecting a cooling fluid into the cooling cylinder.

26. A method as claimed in any one of claims 22 to 25, wherein the step of injecting a fluid into the chamber comprises injecting fluid from the cooling cylinder.