Engine with boundary layer turbine

Abstract

The engine (1) comprises a compressed gas source (3) and a fluid circuit connected to an output of the compressed gas source such that the compressed gas source can drive compressed gas through the circuit. The output of the compressed gas source is connected via the fluid circuit to a turbine component (5) connected to a rotating shaft (9) which in use serves as the output of the engine. The heat exchanger (13) is configured to receive the fluid in the circuit that has passed through the turbine component (5) and reduce the temperature and pressure of the fluid. The turbine component (5) comprises a boundary layer turbine comprising a plurality of axially aligned disks (51), each disk having an exhaust (53) located towards the centre of the disk.

Description

[Technical field]

[0001] The present invention relates to an engine which uses circulating compressed gas to drive a turbine to provide an energy output. [Background technology]

[0002] A wide variety of engines are known, ranging from traditional internal combustion engines to engines powered by compressed air, wind, water and other sources.

[0003] All such engines are driven by the need to provide energy output while minimizing energy consumption and minimizing potentially harmful emissions from the environment. In addition, they are often driven by the need to easily control the output and a desire to extend useful life through the use of minimal moving components and the provision of components that are less susceptible to wear and tear in order to reduce maintenance needs.

[0004] The present invention seeks to provide an engine which is highly efficient, has a high degree of controllability over its output, is simple and easy to maintain, and has a long useful life with minimal maintenance. Summary of the Invention

[0005] According to the present invention there is provided an engine comprising a compressed gas source and a fluid circuit connected to an output of the compressed gas source such that compressed gas can be driven through the circuit by the compressed gas source, the output of the compressed gas source being connected through the fluid circuit to a turbine component which is connected to a rotating shaft which in use functions as an output of the engine, and a heat exchanger arranged to receive fluid in the circuit which has passed through the turbine component and reduce the temperature and pressure of the fluid, the turbine component comprising a boundary layer turbine comprising a plurality of axially aligned disks, each disk having an exhaust port arranged towards the centre of the disk.

[0006] Traditionally, engines that use turbines to provide energy from circulating gases have used bladed turbine components. Boundary layer turbines (also known as Tesla turbines) have been around for many years but are not commonly used. Boundary layer turbines are difficult to start and do not operate efficiently until they reach very high rotational speeds (e.g., well over 20,000 rpm). At such high speeds, there is a risk that the disks of the boundary layer turbine will deflect, so building a boundary layer turbine that can withstand efficient operating speeds is not considered commercially viable for many applications compared to bladed turbines.

[0007] However, applicants have realized that, contrary to what might be expected, the inclusion of a boundary layer turbine (as described herein) in an engine having a compressed gas source and a heat exchanger can provide a more viable and efficient energy source than existing engines that use bladed turbines. In use, gas is driven by the compressed gas source into the turbine components and across the surface of the turbine disk. Boundary layer effects cause nearby fluids to drag the surface of the disk, transferring energy to the disk and causing it to rotate. As the fluid loses energy, it spirals toward the center of the disk, where the exhaust is located. The amount of energy available from a boundary layer turbine is significantly greater than a standard bladed turbine. This is because energy is transferred over the entire length of the disk spiral, which (for a given size turbine) is significantly longer than the distance the fluid travels as it passes over the blades of a bladed turbine. This also , which means that a boundary layer turbine of significantly smaller size can be used instead of a standard bladed turbine to output the same amount of energy.

[0008] A further advantage of boundary layer turbines is that they can tolerate gases, steam and liquids, but for convenience and clarity, the description will refer to gases (or more generally, fluids) moving through the fluid circuit. The terms "turbine disk" and "disk" are used interchangeably, and references to a "disk" component should be taken to refer to one disk of the plurality of axially aligned disks described above (unless otherwise stated, such as a solid disk described below). Axially aligned disks means that the major planes formed by each of the disks (i.e., by the diameter and circumference of the disk) are parallel. A rotating shaft that functions as the output of the engine in use means that the rotating shaft outputs its rotation to another component connected to the shaft (e.g., a generator, described below), and that the rotating shaft outputs mechanical rotational power.

[0009] The compressed gas source may include a compressor, and the output of the heat exchanger is connected to an input of the compressor such that the fluid circuit is a closed fluid circuit. Additionally or alternatively, the compressed gas source may comprise a gas storage vessel.

[0010] The fact that the compressed gas source comprises a compressor and that the fluid circuit is a closed fluid circuit ensures that there is an absolute minimum of fluid loss in the circuit, i.e. the fluid can be circulated throughout the engine for long periods with a minimum of moving parts, there is no consumption of the fluid used in the circuit and there is very little environmental damage.

[0011] A gas storage vessel is a vessel for storing gas at a pressure high enough to drive turbine components when the gas is released from the vessel. When the compressed gas source comprises a gas storage vessel, the vessel may be included in the engine in addition to or as an alternative to the compressor. The gas storage vessel serves as a store of potential energy that is used to drive the turbine components and generate engine power on demand. Preferably, the gas storage vessel is removably connected to the engine, and such a vessel can be removed from the engine (e.g., when the compressed gas is depleted) and replaced with another (or the same refilled) gas storage vessel.

[0012] If the engine includes a gas storage vessel but no compressor, the gas is output from the vessel to drive turbine components and passes through a heat exchanger before exiting the open fluid circuit (to another vessel for storage or to the ambient environment).

[0013] When an engine includes a gas storage vessel and a compressor, the engine's utility is increased. For example, when the compressed gas source is a compressor with a closed fluid circuit, the engine can operate as described above, efficiently producing power with minimal fluid losses. In situations where additional engine power is desired, the gas storage vessel can be used to temporarily increase the flow rate of fluid through the turbine components, thereby further increasing engine power. The engine can also include mechanisms such as relief valves to regulate the fluid circuit by venting fluid (introduced from the gas storage vessel) from the fluid circuit.

[0014] At least one of the disks may include a plurality of grooves arranged radially around an edge of the disk to induce fluid to move in a spiral manner toward the center of the disk.

[0015] In this manner, the grooves are configured to further direct the fluid from the turbine inlet to achieve a longer spiral path (i.e., the path of travel of the fluid from the turbine inlet to the center of the disk). As the gases move towards the exhaust, the contact area between the gases and the turbine disks increases, which improves the efficiency of the turbine components, especially when the engine is first started (i.e. at low speeds).

[0016] At least one disc of the plurality of discs may include a plurality of airfoils radially disposed about the disc to induce fluid to spiral toward a center of the disc.

[0017] In this way, the airfoils act similarly to grooves to guide the fluid across the surface of the disk once it enters the turbine component. Preferably, the airfoils are located near the edge of the disk. The cumulative weight of multiple airfoils around the disk helps the disk rotate and prevent deflection. Preferably, the airfoils are constructed of a high density material, such as titanium.

[0018] The airfoil may be disposed adjacent to the groove such that the airfoil and groove collectively function to direct fluid flow across the turbine disk.

[0019] The airfoils may extend between and connect adjacent disks, thus attaching adjacent disks together to improve the strength of the boundary layer turbine component and further reduce disk deflection.

[0020] The airfoil may be an early arcuate airfoil or a double wedge airfoil, with the early arcuate airfoil having been found to be most efficient at subsonic speeds and the double wedge airfoil most efficient at supersonic speeds.

[0021] The boundary layer turbine may further comprise a solid disk disposed between two adjacent disks of the plurality of disks, the solid disk being in axial alignment with the plurality of disks.

[0022] Preferably, the solid disc is of similar construction to a plurality of discs without any outlets.

[0023] The solid disk being similar in structure to the disks without exhaust ports means that the solid disk is structurally the same as the turbine disks included in the boundary layer turbine, except that the solid disk does not include an exhaust port. For example, if the boundary layer turbine includes turbine disks having airfoils connecting adjacent disks, the solid disk will also be similarly configured and include airfoils connecting the solid disk to the adjacent disks.

[0024] In this manner, the solid disk further contributes to the rotation of the boundary layer turbine while preventing the merging of the fluid streams at the disk exhaust, which would create turbulence and waste energy, and the inclusion of a solid disk further improves the efficiency of the turbine component.

[0025] Preferably, the solid disc is centrally located along the plurality of discs, which has been found to maximize reduction in turbulence.

[0026] Optionally, the solid disk may comprise sector elements disposed towards a center of the solid disk, the sector elements configured to accelerate the flow of fluid away from the solid disk and through the turbine component.

[0027] In these examples of the invention, the solid disk includes sector elements and does not include exhaust ports. Except for the fact that the sector elements are structurally similar to turbine disks, in certain instances, sector elements may be raised features on the surface of the solid disk that encourage fluid to flow away from the solid disk as it rotates.

[0028] The output of the compressed gas source may be connected to the turbine component using multiple turbine inlets.

[0029] Connecting the output of the compressed gas source to the turbine component using multiple turbine inlets means that fluid from the compressed gas source can enter (e.g., drive) the turbine component at multiple discrete points. Multiple turbine inlets can be provided between a pair of disks. Increasing the number of turbine inlets increases the number of fluid flow paths that spiral across the surface of the disks, thereby increasing the energy transferred to a single disk.

[0030] The multiple turbine inlets may be equally spaced around the circumference of the turbine component, in this manner the boundary layer effect forces are evenly distributed across the surface of the disk.

[0031] The outlet of each of the disks may include a curved or angled edge toward the direction of flow of the fluid circuit. Configuring the outlet in this manner reduces turbulence of the fluid through the outlet, thereby increasing turbine components. For example, the edge of the outlet may be tapered or arcuate toward the opening of the outlet. A check valve may be disposed between the outlet of the heat exchanger and the input of the compressed gas source and configured to allow fluid to flow only in a direction from the heat exchanger to the compressed gas source.

[0032] In this manner, controlling the flow of the expanded circulating fluid optimizes performance and improves the efficiency of the engine for a given load.

[0033] The compressed gas may be any suitable gas, such as ammonia, or more preferably, for reasons of safety and operational efficiency, carbon dioxide.

[0034] The engine may further include an electric generator connected to the output shaft.

[0035] At least a portion of the electrical power generated by the generator may be used to power a compressed gas source, which may refer to starting and / or driving the compressed gas source.

[0036] The engine may further comprise a control component for feedback controlling the electrical power generated from the generator to the compressed gas source, the control component being connected to the generator, meaning that a portion of the electrical power output from the generator is fed back to the compressed gas source, and the control component providing the ability to control the feedback of power to the engine system in order to optimize the operation of the engine.

[0037] The engine may further comprise energy recovery means associated with the heat exchanger for recovering thermal energy from the heat exchanger and providing additional energy output from the engine, thereby enabling the thermal energy to be converted into electrical energy for starting and / or driving a compressed gas source, thereby improving the efficiency of the engine.

[0038] Aspects of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0039] [Figure 1] 1 is a schematic diagram of an engine according to the present invention; [Diagram 2] FIG. 2 is a schematic diagram showing the flow of circulating gas within the engine of FIG. 1. [Diagram 3] 1 illustrates a turbine disk for a boundary layer turbine according to an example of the present invention. [Figure 4] 4 shows a turbine disk of a boundary layer turbine according to a further example of the invention. [Diagram 5] 4 illustrates a turbine disk for a boundary layer turbine according to another example of the present invention. [Figure 6] 4 illustrates a turbine disk for a boundary layer turbine according to yet another example of the present invention. [Figure 7A] 1 illustrates a boundary layer turbine according to an example of the present invention. [Figure 7B] 1 illustrates a boundary layer turbine according to an example of the present invention. [Figure 8] 4 shows a portion of a boundary layer turbine according to a further example of the invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] FIG. 1 shows a schematic diagram of an engine 1. The compressor provides a compressed gas source 3 which, in use, compresses the gas in the circuit and drives it from the compressed gas source outlet 4 towards the turbine component 5. The compressor in this example is electrically powered, but may have alternative power sources. The diameter of the compressed gas source outlet 4 controls the pressure and flow rate of the compressed gas when it reaches the turbine 5. High efficiency is achieved in this part of the fluid circuit when the pressure and temperature of the gas are at a critical point. To achieve this, a heater (not shown) for heating the compressed gas can be incorporated adjacent to the compressed gas source 3. As the gas enters the turbine component 5 from the compressed gas source outlet 4, it passes through a disk 51, in which it expands and drives a series of disks 51 and a central shaft 9 on which the disks 51 are mounted. The shaft 9 serves as the output of the engine 1 and may be connected to a mechanical component to be driven, for example via a gearbox. The turbine component 5 and the compressed gas source 3 are separate components (i.e. separate component bodies) and are fluidly connected.

[0041] In this example, it is shown connected to a generator 10 which is linked to control components 11 and 12. These control components receive power from the generator 10 and can control the output load of the shaft 9, the operating power of the generator 10, and potentially also the feedback of the generated power from the generator 10 to the compressor, again to control the overall operating power of the engine and optimize its energy efficiency. The compressor and the generator 10 are separate components and are electrically connected. Additionally or alternatively, the generator 10 may be mechanically connected to the compressor such that it can act as a motor to power (i.e. start and / or drive) the compressor. It is also possible to integrate components from the generator 10 and the turbine components into a single unit to reduce the size and mechanical losses associated with connecting the two.

[0042] In one example, the generator produces 400 Hz AC for high efficiency, but also has stepping components to step down to 50 Hz for use with standard electrical components.

[0043] After passing through the turbine disks 51, the reduced pressure fluid leaves the turbine component 5 through a heat exchanger 13. In this example, the heat exchanger 13 is a series of tubes, which function to dissipate the residual heat in the fluid. The heat exchanger 13 recovers the thermal energy and potentially converts it into other forms of energy, potentially electrical energy, to improve the efficiency of the engine's electrical output, or indeed to allow for supply to a control component or compressed gas source 3. The output of the heat exchanger 13 is then fed thereafter, optionally through a further pressure reducing component 14, and optionally through a check valve 15, to be compressed again by the compressor. The check valve 15 ensures a unidirectional fluid flow, which helps to ensure the correct flow of fluid through the system, especially at start-up, and can be used to regulate the power output of the engine via the fluid flow, thus optimizing the efficiency of the engine.

[0044] The check valve 15 can also be used to regulate the pressure differential of the compressed gas source 3 to optimize its operating efficiency.

[0045] Relief valves (not shown) may be located at appropriate points in the fluid circuit to relieve unwanted pressure build-up if necessary for safety reasons.

[0046] In one operating example, the pressure difference of the compressed gas source 3 is controlled at about 2 bar (60 bar at the input and 58 bar at the output of the compressed gas source) using CO2 as the compressed gas.

[0047] Figure 2 shows in schematic form the flow of fluids through the engine 1, with corresponding numbering of the fluid positions in the closed circuit in the associated components as outlined in Figure 1. As will be appreciated from the above, by simple control of the compressed gas source 3 and its output through the compressed gas source outlet 4, the generator 10 can be powered to optimize the output of the engine 1 with a high level of efficiency and minimal moving parts, reducing maintenance and improving operation. In this configuration, by proper operation of the compressed gas source 3, feedback of generated power to the compressed gas source 3, proper energy recovery from the heat exchanger 13, and proper control of the check valve 15, the engine operates with minimal noise output, yet can be controlled to have a variety of power outputs with minimal adaptation.

[0048] In the above examples, a compressor is used as the compressed gas source 3. Alternatively, or in addition to a compressor or another compressed gas source 3, a gas storage vessel may be used as the compressed gas source 3.

[0049] 3-8 provide further details of the turbine component 5. Surprisingly, it has been found that a boundary layer turbine can be configured to provide a higher efficiency output to the engine than a conventional bladed turbine. The turbine component 5 is a boundary layer turbine (also known as a Tesla turbine) comprising a plurality of disks 51 connected to a rotating shaft 9. The disks 51 are axially aligned with one another along the rotating shaft 9 such that the planes formed by each disk 51 are parallel to one another, and the disks 51 are rotatable about the axis of the rotating shaft 9. Each disk 51 includes at least one exhaust port 53 located near the center of the disk 51. The exhaust port 53 is an opening in the disk 51 that allows fluid to pass from one side of the disk 51 to the other. Preferably, the edges of the exhaust port 53 are curved or angled (e.g., tapered or arcuate) to improve the flow of fluid through the exhaust port 53 by reducing turbulence. That is, the edges of the exhaust port 53 are curved or angled toward the direction of the flow of fluid through the disk 51.

[0050] FIG. 3 shows a simplified example of a disk 51 of a turbine component 5. Fluid from the compressed gas source outlet 4 passes through a turbine inlet 52 (such as a nozzle) and crosses the surface of the disk 51 to enter the turbine component 5. Due to boundary layer effects, as the fluid moves across the surface of the disk 51, it drags the disk 51 and transfers energy to the disk 51. This causes the disk 51 to rotate and the fluid path 100 to spiral toward the center of the disk 51 and the exhaust port 53. Although only one turbine inlet 52 is shown in the example of FIG. 3, the turbine component 5 can include multiple turbine inlets 52 arranged around the circumference of the disk 51. Preferably, the turbine inlets 52 are evenly spaced around the circumference of the disk 51 to distribute boundary layer effect forces evenly across the disk 51. Because the turbine component 5 includes multiple disks 51, the turbine inlets 52 may be arranged such that a single inlet 52 spans some or all of the disks 51 (i.e., one to many), such that a single inlet 52 corresponds to a single gap between two disks 51 (i.e., one to one), or such that multiple inlets 52 correspond to a single gap between two disks 51 (i.e., many to one).

[0051] The disk 51 of FIG. 4 includes grooves 54 arranged radially around the outer edge of the face of the disk 51. The grooves 54 are curved in shape to direct the fluid flow path further towards the center of the disk 51, and are evenly distributed around the circumference of the disk 51 so that the fluid flow path remains constant (as long as the velocity of the fluid as it leaves the turbine inlet 52 remains constant). Preferably, the grooves 54 and inlet 52 are configured to direct the fluid flow path to complete more loops around the center of the disk 51 before the fluid reaches the exhaust outlet 53, thereby increasing the length of the fluid flow path and therefore the amount of energy transferred from the fluid to the disk 51.

[0052] 5 and 6 both show the disk 51 further comprising airfoils 55 arranged radially around the face of the disk 51. In these examples, the disk 51 also includes grooves 54 at the edge of the disk 51, with the airfoils 55 arranged on the inboard side of the grooves 54 (i.e., the airfoils 55 are arranged closer to the center of the disk 51 than the grooves 54), although in other examples the disk 51 may include the airfoils 55 but not the grooves 54. The airfoils 55 protrude from the surface of the disk 51 and further direct the fluid flow to move in a spiral manner around the disk 51. As discussed above with respect to the grooves 54, the airfoils 55 are preferably configured to maximize the number of times the fluid flow path orbits the exhaust port 53.

[0053] The cumulative weight of the airfoils 55 near the edge of the disk 51 aids in rotation, so to ensure balance of the disk 51, it is important that the airfoils 55 are evenly distributed around the disk 51. The configuration of the airfoils 55 is selected to increase maximum rotational speed and increase torque on the central shaft 9. While many different configurations are suitable for use with the disk 51, an earlier bowed airfoil design and a double wedge airfoil design have been found to be most effective for subsonic and supersonic speeds, respectively. For example, at an operating frequency of a given turbine component 5 where the turbine disk 51 has a larger diameter and the airfoil 55 is located near the edge of the disk 51 (e.g., 500 Hz or 30,000 rpm), the airfoil 55 moves faster than the speed of sound and therefore a double wedge airfoil design must be used (e.g., an airfoil at the edge of a 250 mm diameter disk 51 at 500 Hz rotates at approximately 390 m / s). In another example, with a smaller diameter turbine disk 5 at the same operating frequency, the airfoil 55 does not move faster than the speed of sound and therefore an earlier bowed airfoil design is used (e.g., an airfoil at the edge of a 100 mm diameter disk 51 at 500 Hz rotates at approximately 160 m / s).

[0054] 7A and 7B, the disks 51 are closely spaced with airfoils 55 extending between and connecting adjacent disks 51. Connecting the disks 51 through the airfoils 55 in this manner strengthens the turbine component 5 and prevents the disks 51 from flexing at high rotational speeds. Locating adjacent disks 51 closely together increases the surface effect between the fluid and the disks 51, thereby increasing drag in a given volume and improving the efficiency of the turbine component 5.

[0055] It has been found that if the turbine component 5 does not include a solid disk 56, the fluid flows will merge at the outlet 53 of one of the turbine disks 51, creating turbulence and reducing efficiency. If the turbine inlets 52 and disks 51 are evenly distributed, this is usually the most central disk 51. Therefore, by introducing a solid disk 56 (i.e. a disk that does not include an outlet), the flow paths are directed only away from the solid disk 56, preventing the fluid flows from merging and reducing turbulence. An example of such a turbine component 5 is shown in FIG. 8, where the distance between the turbine disk 51 and the solid disk 56 is exaggerated to more clearly show the difference between the turbine disk 51 and the solid disk 56.

[0056] 7A and 7B, the turbine disk 51 and the solid disk 56 are preferably in intimate contact with each other and equally spaced along the turbine shaft 9. The solid disk 56 is the same size as the turbine disk 51 and is axially aligned with said turbine disk 51 such that the solid disk 56 still contributes to the rotation of the central shaft 9. Preferably, the solid disk 56 also includes the same surface modifications as the turbine disk 51 (e.g., grooves 54 and airfoils 55), such that the configuration of the solid disk 56 is similar to that of the turbine disk 51, except for the absence of the exhaust port 53.

[0057] 8, there is a relatively smooth region in the area corresponding to the location of the exhaust of the turbine disk 51. In another example, the solid disk 56 is provided with a fan-like element (not shown) configured to accelerate the flow of fluid (in particular the turbulent flow) through the turbine component 5 in a direction away from the solid disk 56.

Claims

1. An engine, a compressed gas source and a fluid circuit connected to an output of the compressed gas source such that compressed gas can be driven through the circuit by the compressed gas source, the output of the compressed gas source being connected through the fluid circuit to a turbine component connected to a rotating shaft which, in use, functions as the output of the engine; a heat exchanger positioned to receive the fluid in the circuit that has passed through the turbine component and to reduce the temperature and pressure of the fluid; Equipped with the turbine component comprises a boundary layer turbine comprising a plurality of axially aligned disks; an engine wherein each disc includes an outlet vent located toward the center of said disc.

2. 2. The engine of claim 1, wherein the source of compressed gas comprises a compressor, the output of the heat exchanger being connected to the input of the compressor such that the fluid circuit is a closed fluid circuit.

3. The engine of claim 1 , wherein the source of compressed gas comprises a gas storage vessel.

4. 2. The engine of claim 1, wherein at least one disk of the plurality of disks includes a plurality of grooves arranged radially around an edge of the disk to induce fluid to move spirally toward a center of the disk.

5. 2. The engine of claim 1, wherein at least one disk of the plurality of disks comprises a plurality of airfoils radially arranged around the disk for directing fluid to spiral toward a center of the disk.

6. At least one of the plurality of disks has a plurality of grooves radially arranged around the edge of the disk to induce fluid to move in a spiral pattern toward the center of the disk; The engine of claim 5 , wherein the airfoil is positioned adjacent to the groove.

7. The engine of claim 5 , wherein the airfoils extend between and connect adjacent disks.

8. 6. An engine according to claim 5, wherein the airfoil is an early arcuate airfoil or a double wedge airfoil.

9. 2. The engine of claim 1, wherein the boundary layer turbine further comprises a solid disk disposed between two adjacent disks of the plurality of disks, the solid disk being axially aligned with the plurality of disks and structurally mimicking the plurality of disks without the outlet vent.

10. The engine of claim 9 , wherein the solid disc is centrally located along the plurality of discs.

11. 10. The engine of claim 9, wherein the solid disk further comprises a sector element disposed toward a center of the solid disk, the sector element configured to accelerate a flow of fluid leaving the solid disk.

12. The engine of claim 1 , wherein the output of the compressed gas source connects to the turbine component using multiple turbine inlets.

13. The engine of claim 12 , wherein the plurality of turbine inlets are equally spaced around the circumference of the turbine component.

14. The engine of claim 1 , wherein the outlet vent of each disk of the plurality of disks includes a curved or angled edge toward the direction of flow of the fluid circuit.

15. 3. The engine of claim 2, further comprising a check valve disposed between the output of the heat exchanger and the input of the compressed gas source, the check valve configured to allow fluid to flow only in a direction from the heat exchanger to the compressed gas source.

16. The engine of claim 1 , wherein the compressed gas is one of carbon dioxide or ammonia.

17. The engine of claim 1 further comprising a generator connected to the output shaft.

18. 18. The engine of claim 17, wherein at least a portion of the electrical power generated by the generator is used to power the source of compressed gas.

19. 20. The engine of claim 18, further comprising a control component for controlling feedback of electrical power generated from the generator to the source of compressed gas.

20. An engine according to any preceding claim, further comprising energy recovery means associated with the heat exchanger for recovering thermal energy from the heat exchanger and providing additional energy output from the engine.

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