[0001 ] The present invention relates to an engine and to a positive displacement machine having a rotary piston. More particularly, but not exclusively, the invention relates to a thermal engine, such as a Stirling engine, and to a positive displacement machine for use in such an engine. Aspects of the invention relate to an engine, to a positive displacement machine and to a method.

[0002] Co-pending patent application WO2005/124106, the contents of which are expressly incorporated herein by reference, discloses a heat engine or Stirling cycle engine including two positive displacement machines (PDMs), each comprising a casing having a circular cylindrical internal surface defining an operating chamber and an orbiting piston.

[0003] One of the PDMs is arranged to draw in a working fluid, such as ambient air, through an inlet and deliver it to the other of the PDMs, by means of the orbiting piston, via an intermediate duct. A heater is positioned in the intermediate duct and is arranged to heat the working fluid therein, causing it to increase in temperature and pressure.

[0004] The pressurised working fluid is applied to the operating chamber in the second PDM where it expands, driving the piston in its orbit around the cylinder so as to drive an output device such as a motor or generator. A kinematic connection is provided between the first and second PDMs such that the piston in the first PDM is driven by the piston in the second PDM.

[0005] It is an aim of the present invention to improve upon this type of heat engine. Embodiments of the invention may provide a heat engine having improved performance and efficiency. Further embodiments of the invention may provide an improved form of positive displacement machine for use with such a heat engine. Other aims and advantages of the invention will become apparent from the following description, claims and drawings.

[0006] Aspects of the present invention therefore provide an engine, an apparatus, a positive displacement machine and a method as claimed in the appended claims.

[0007] According to another aspect of the invention for which protection is sought, there is provided an engine comprising a first positive displacement machine, a second positive displacement machine, duct means connected between an outlet of the first positive displacement machine and an inlet of the second positive displacement machine, heater means for raising the temperature and pressure of a working fluid in the duct means and a kinematic connection between the first and second positive displacement machines, wherein the first and second positive displacement machines each comprise at least one orbiting or rotating piston and wherein the at least one piston in the first positive displacement machine is arranged to orbit or rotate out of phase with the at least one piston in the second positive displacement machine.

[0008] In an embodiment, the arrangement is such that, in use, the first positive displacement machine causes the working fluid to flow through the duct means to the second positive displacement machine, the heated working fluid drives the second positive displacement machine, and the second positive displacement machine drives the first positive displacement machine via the kinematic connection.

[0009] In an embodiment, the relative angular difference between the at least one piston in the first positive displacement machine and the at least one piston in the second positive displacement machine is between approximately 20° and approximately 120°. Preferably, the relative angular difference between the at least one piston in the first positive displacement machine and the at least one piston in the second positive displacement machine is between approximately 40° and approximately 90°. More preferably, the relative angular difference between the at least one piston in the first positive displacement machine and the at least one piston in the second positive displacement machine is approximately 50°.

[0010] In an embodiment, the at least one piston in the first positive displacement machine leads the at least one piston in the second positive displacement machine.

[001 1 ] In an embodiment, the first and second positive displacement machines each comprise a single rotary piston. Alternatively, the first and/or second positive displacement machines may comprises two or more rotary pistons in parallel.

[0012] The engine may comprise power generation means and a kinematic connection between the second positive displacement machine and the power generation means.

[0013] In an embodiment, an exhaust duct connects the outlet of the second positive displacement machine to a fluid inlet of the heater means. Advantageously, in this embodiment waste heat from the second positive displacement machine can be supplied to the heater means for heating the working fluid in the duct means.

[0014] The heater means may comprise at least one of a heat exchanger, a condenser, a heat pump, an electric heater, solar heating and a combustion heater. [0015] In an embodiment, the working fluid is air.

[0016] According to another aspect of the invention for which protection is sought, there is provided a rotary positive displacement machine comprising a casing having a circular cylindrical internal surface delimiting an operating chamber, a piston disposed in the operating chamber for rotation about a chamber axis being the axis of the said internal surface, the piston having a generally cylindrical external surface and being arranged such that a generatrix of the external surface is adjacent to the said internal surface and a diametrically opposite generatrix is spaced from the said internal surface, a vane member mounted on the casing, the vane member having a tip face which faces the external surface of the piston and means for maintaining the tip face of the vane member adjacent the external surface of the piston, wherein the external surface of the piston comprises a first region having a first radius of curvature and a second region having a second radius of curvature, the second radius of curvature being greater than the first radius of curvature.

[0017] In an embodiment, the second radius of curvature substantially corresponds to the radius of curvature of the internal surface. For example, the first region may have a greater curvature than the internal surface, whereas the second region may have a curvature that substantially conforms to the internal surface.

[0018] In an embodiment, the first region has a larger circumferential length than the second region. For example, the second region may subtend an arc of between approximately 10° to approximately 30°. Preferably, the second region subtends an arc of between 10° and 20°, more preferably 16°

[0019] In an embodiment, a shaft extends axially through the operating chamber. In one embodiment, the shaft extends coaxially through the operating chamber. The piston may be mounted eccentrically to the shaft.

[0020] In an embodiment, the vane member is mounted on the casing and is pivotable about a pivot axis parallel to the chamber axis. The vane member may be accommodated in a fluid inlet/outlet aperture in the casing and comprise a passageway communicating between the exterior of the casing and the operating chamber.

[0021 ] In an embodiment, the vane member comprises an arcuate face which is coaxial with the pivot axis and which has a length substantially equal to that of the piston. The vane member may also comprise end faces extending from the respective lateral ends of the arcuate face towards the pivot axis.

[0022] In an embodiment, one or all of the arcuate face, the end faces and the tip face comprise sealing faces with respect to corresponding surfaces of the casing aperture and the piston.

[0023] In an embodiment, the means for maintaining the tip face of the vane member adjacent the external surface of the piston comprises a linkage. All or part of the linkage may be disposed externally of the casing.

[0024] The linkage may comprise a first connecting member connected to the shaft and a second member connected to the vane member.

[0025] In an embodiment, a first end of the first connecting member is pivotally and eccentrically connected to the shaft, a second end of the first connecting member is pivotally connected to a first end of the second connecting member and the second connecting member is connected, for example rigidly connected, to the vane member.

[0026] A second end of the second connecting member may be pivotally connected to the casing. In an embodiment, the second connecting member is connected to the vane member at a point part way along the length thereof.

[0027] The arrangement may be such that rotation of the shaft causes an oscillatory or reciprocating movement of the vane member relative to the casing so as to maintain the tip face of the vane member adjacent the external surface of the piston.

[0028] In an embodiment, at least one of the first connecting member and the second connecting member is disposed externally of the casing.

[0029] In an embodiment, the linkage is connected to the vane member by an articulation having an articulation axis such that a plane containing the articulation axis and the axis of the said external surface passes through the region of sealing contact.According to a further aspect of the invention for which protection is sought, there is provided an engine as set out in any of the preceding paragraphs, comprising at least one positive displacement machine as set out in any of the preceding paragraphs.

[0030] Within the scope of this application it is envisaged that the various aspects, embodiments, examples, features and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings may be taken independently or in any combination thereof.

[0031 ] The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 illustrates schematically a known form of heat engine, such as that described in W02005/124106;

Figure 2 and 3 show, in perspective view, a known form of rotary positive displacement machine (PDM), such as that described in WO2005/124106;

Figure 4 is a side section through of a form of PDM embodying one aspect of the invention;

Figure 5 is a perspective view of the PDM of Figure 4;

Figure 6 illustrates the shape of a piston used in the PDM of Figure 4;

Figure 7 illustrates a vane assembly being part of the PDM of Figure 4;

Figure 8 illustrates the vane assembly of Figure 7 in situ;

Figure 9 illustrates a pair of the PDMs shown in Figure 4 operating together; and

Figure 10 illustrates an alternative arrangement of PDMs for use in an engine according to one aspect of the present invention.

[0032] Referring firstly to Figure 1 , a known form of heat engine is shown generally at 10. This form of heat engine is disclosed in detail in WO2005/124106, the contents of which are expressly incorporated herein by reference.

[0033] The heat engine 10, which operates as an open-cycle Stirling engine, comprises first and second positive displacement machines (PDMs) 12, 22, each of which includes a casing having a circular cylindrical internal surface defining an operating chamber, an inlet, an outlet and a generally circular piston which orbits within the operating chamber. A more detailed description of each PDM is provided below.

[0034] The inlet 13 of the first PDM 12 is connected to a supply of a working fluid which may, for example, comprise air so that the inlet is open to atmosphere. The outlet 14 of the first PDM 12 is connected to the inlet 23 of the second PDM 22 via an intermediate duct 16 in which a heater, in the form of a heat exchanger 18, is disposed. An outlet 24 of the second PDM 22 is connected to an exhaust 26. A kinematic connection 28 is provided between the first and second PDMs 12, 22 such that the piston 15 in the first PDM 12 is driven in an orbiting motion around the operating chamber A by orbital movement of the piston 25 in the operating chamber B of the second PDM 22. [0035] The orbital motion of the piston 15 in the first PDM 12 (hereafter referred to as the "cold piston") causes working fluid, such as ambient air, to be drawn into the operating chamber A through the inlet 13. The working fluid is then expelled from the operating chamber A through the outlet 14 and delivered to the second PDM 22 via the intermediate duct 16.

[0036] The heater 18, positioned in the intermediate duct 16, is arranged to heat the working fluid therein, causing it to increase in temperature and pressure. The pressurised working fluid is applied to the operating chamber B in the second PDM 22 through the inlet 23 thereof wherein it is allowed to expand, applying a force to the piston 25 (hereafter referred to as the "hot piston") which is caused to orbit within the cylinder.

[0037] The hot piston 25 is kinematically connected to an output device such as an electrical motor or generator (not shown, but connected to kinematic drive 28) for generating power. In addition, the hot piston 25 is linked to the cold piston 15 by a suitable kinematic connection 28 which may, for example, comprise a shaft, belt, chain or gears. By means of this kinematic connection 28, the cold piston 15 is driven in its orbital motion within the operating chamber A of the first PDM 12 by the orbital motion of the hot piston 25 within the operating chamber B of the second PDM 22 so that the supply of working fluid thereto is maintained.

[0038] After expansion in the second PDM 14, the still hot working fluid is exhausted from the outlet 24 through the exhaust 26 and may be used to provide heating.

[0039] In the arrangement of Figure 1 , the first and second PDMs 12, 22 are generally identical in size, such that their swept volumes are substantially equal. The cold and hot pistons 15, 25 orbit in-phase with each other and at a similar rate.

[0040] In order to maintain useful efficiency of operation, the first PDM 12 is arranged such that there is substantially no mechanical compression of the working fluid during the transfer to the second PDM 14. That is to say, the first PDM 12 does not act as a compressor but simply feeds the working fluid at substantially atmospheric pressure into the intermediate duct 16. Thus, any increase in pressure in the working fluid is caused solely by the heating of the fluid by the heat exchanger 18.

[0041 ] This is achieved, at least in part, by having both cold and hot pistons 15, 25 rotating in phase with each other, so that the changes in volume of the operating chamber A in the first PDM 12 during the transfer cycle are balanced by corresponding changes in the volume of the operating chamber B in the second PDM 22.

[0042] Since the pressure increase in the working fluid in the intermediate duct 16 is felt equally by the piston 15, 25 in each PDM 12, 22, it is necessary to shut off the supply of working fluid from the first PDM 12 in order to prevent the increasing pressure in the intermediate duct 16 from applying a reversing force on the cold piston 15 which could cause the mechanism to stall. A shut-off or non-return valve (not shown) is therefore provided in the intermediate duct 16 which closes at a predetermined point in the orbit of the cold piston 15 in the first PDM 12, for example at approximately 240° degrees rotation of the cold piston 15.

[0043] In order to enable work to be done by the engine 10, the pressurised working fluid must be able to expand in the operating chamber B of the second PDM 22 so as to drive the hot piston 25 in its orbit. The expansion phase of the hot piston 25 corresponds approximately to the final 120° of its orbital rotation, i.e. the portion of the orbit following closure of the non-return valve.

[0044] Since the hot piston 25 is kinematically connected to the cold piston 15, the latter is also driven in its orbit within the operating chamber A during the expansion phase of the hot piston 25. However, when the non-return valve is closed, continued orbiting of the cold piston 15 within the first PDM 12 caused by the driving force of the hot piston 25 orbiting in the second PDM 22 via the kinematic connection 28 causes the remaining volume of working fluid in the operating chamber A of the first PDM 12 to become compressed between the cold piston 15 and the non-return valve. If left unchecked, the compression of the working fluid in the operating chamber A of the first PDM 12 reaches a point where further rotation of the cold piston 15 is no longer possible and the mechanism stalls.

[0045] To prevent this occurrence, a venting valve (not shown) is provided in the intermediate duct 16 which vents the remaining fluid in the operating chamber A of the first PDM 12 to atmosphere when the shut-off valve closes, enabling the cold piston 15 to continue its orbiting motion within the cylinder, thereby permitting continued orbiting of the hot piston 25 within the second PDM 22.

[0046] Figures 2 and 3 together illustrate parts of the first PDM 12 (or the second PDM 22, which is substantially identical) in perspective view. This form of PDM 12 is disclosed in greater detail in WO2005/124106 and WO03/062604, the contents of both of which are expressly incorporated herein by reference. For convenience, like reference numerals indicate like parts between Figures 1 -3. [0047] The PDM 12 comprises a casing 31 having a peripheral wall 32 with a circular cylindrical internal surface 33. The internal surface 33 defines, at least in part, a cylinder within which an orbiting piston 15 is disposed. The orbiting piston 15 comprises a rotating inner part 15a, eccentrically mounted on an input/output drive shaft 39 and carrying at each end a shutter in the form of a flange or disc 36, and a non-rotating outer part 15b which orbits about the axis of the internal surface 33. The outer part 15b of the orbiting piston 15 has a circular cylindrical external surface 41 , and one generatrix is spaced from the internal surface 33.

[0048] A vane member 37 is accommodated in an aperture 38 in the casing which functions as a fluid inlet/outlet. The vane member 37 has passageways 37a communicating between the exterior of the casing 31 and the operating chamber A, an arcuate end wall 37b, transverse walls 37c extending from the respective ends of the end wall 37b, a forked arm 37d which is pivotally mounted on the casing 31 about a pivot axis 35, and a tip face (not visible) which is a sealing surface with respect to a recess 42 in the external surface 41 of the orbiting piston 15. A fixed appendage 43 to the outer part 15b is connected to the arm 37d by a bearing (not visible) at a position between the pivot axis 35 of the vane member 37 and its arcuate end wall 37b.

[0049] Each end disc 36 has a circular cylindrical periphery 47 with only a small clearance between itself and the internal surface 33 of the casing 31 . Each disc 36 has fluid inlet/outlet passages 53 for communicating between the operating chamber A and openings (not shown) in the casing 31 .

[0050] The outer part 15b of the orbiting piston 15 is provided with a plurality of compliant strips (not shown) extending in the axial direction and being equally spaced apart. Each strip is made of an elastomer, e.g. Viton or butyl rubber, and is mounted in a respective groove 56.

[0051 ] Rotation of the drive shaft 39 causes the inner part 15a of the piston 15 to rotate eccentrically within the cylinder which, in turn, causes the non-rotating outer part 15b to orbit (i.e. translate non-rotationally) therearound. As the outer part 15b orbits, the piston 15 performs a rolling motion relative to the cylinder such that the point of contact X (see Figure 1 ) between the piston 15 and the internal surface 33 of the cylinder sweeps around the circumference of the operating chamber A.

[0052] The point of contact X between the piston 15 and the internal surface 33 of the cylinder 31 and the point of contact Y (not shown) between the piston 15 and the vane member 37, together define the terminators of the operating chamber A, comprising the volume defined between the piston 15 and the internal surface 33. As the piston 15 orbits around the cylinder, the volume of the operating chamber A varies. In particular, the operating chamber A is at its maximum volume when the contact point between the piston 15 and the internal surface 33 of the cylinder is coincident with the trailing edge of the aperture 38 and reduces during further orbital motion of the piston 15 to a minimum when the contact points X and Y are coincident.

[0053] In operation, as the piston 15 orbits around the cylinder, working fluid contained in the operating chamber A defined between the two contact points X, Y is expelled through the passages 53 and openings in the casing 31 , into the intermediate duct 16. At the same time, working fluid is drawn into the volume behind the contact point X. When of the working fluid in the operating chamber A has been expelled (i.e. when the contact points X, Y are coincident), further orbital movement of the piston 15 moves the contact point X past the trailing edge of the aperture 18, sealing the newly drawn in working fluid into the operating chamber A, and the cycle begins again with the new working fluid being expelled from the cylinder via the passages 53.

[0054] This form of PDM can be used for either the first or second PDM 12, 22 in the engine of Figure 1 . As described above, in this form of engine, two in-phase PDMs 12, 22 are employed, the first PDM 12 operating to draw in the working fluid and to expel it (as described above) via the intermediate duct 16, in which a heat exchanger system 18 is located, to the second PDM 22. In the second PDM 22, the working fluid heated by the heat exchanger 18 expands within the operating chamber B to rotatingly drive the piston 25 within the cylinder. The piston 25 in the second PDM 22 rotates in-phase with the piston 15 in the first PDM 12 and is kinematically connected thereto so as to maintain the delivery of working fluid to the second PDM 22 through the intermediate duct 16.

[0055] As mentioned above, in the engine of Figure 1 , a venting valve must be provided to vent undelivered working fluid from the operating chamber A when the non-return valve is closed in order to permit expansion of the working fluid in the second PDM 22 and continued orbital movement of the piston 25.

[0056] The applicant has recognised that this venting valve adds cost and complexity to the design of the PDM and that, in addition, venting of the working fluid from the first PDM 12 in order to maintain rotation of the pistons 15, 25 results in a loss of energy and therefore a reduction in system efficiency, since the mass of fluid drawn into the first PDM 12 is greater than that delivered to, and received by, the second PDM 22.

[0057] The applicant has identified an elegantly simple, but non-obvious solution to this problem. In particular, the first and second PDM's 12, 22 are arranged such that the orbit of the cold piston 15 in the first PDM 12 leads that of the hot piston 25 in the second PDM 22 by a predetermined amount. This is in contrast to the prior art engine shown in Figure 1 in which both pistons 15, 25 orbit in-phase with each other.

[0058] In the illustrated embodiment, the cold piston 15 leads the hot piston 25 by approximately 120°, although this amount is not essential. It has been found that beneficial effects may be achieved by arranging the cold piston 15 to lead the hot piston 25 by any angle from approximately 20° to approximately 120°, with improved results being achieved by lead angles of approximately 40° to approximately 90°, and optimal results being obtained at approximately 50°.

[0059] The expansion phase of the hot piston 25, constituting approximately the final third of the piston's orbit within the second PDM 22, thus corresponds to the first third of orbit of the cold piston 15 within the first PDM 12. By having the cold piston 15 lead the hot piston 25 in their respective orbits, delivery of the working fluid from the operating chamber A of the first PDM 12 into the intermediate duct 16 can be completed by, or shortly after, the beginning of the expansion phase in the second PDM 22.

[0060] Unlike in the known engine of figure 1 , therefore, expansion of the working fluid in the operating chamber B of the second PDM 22 is able to continue since there is no fluid compression in the operating chamber A of the first PDM 12 which may cause a stall, this fluid having been fully delivered into the intermediate duct 16 by, or shortly after, the commencement of the expansion phase.

[0061 ] More specifically, in use, the non-return valve is closed at or close to the start of the expansion phase in the second PDM 22 in order to prevent a reversing force on the cold piston 15 caused by the increasing pressure of the heated fluid in the intermediate duct 16. By this point, however, the cold piston 15 has completed, or is nearing completion of, its orbit such that all of the working fluid contained in the delivery chamber A within the first PDM 12 has been delivered into the intermediate duct 16. Since there is little or no fluid remaining in the operating chamber A in the first PDM 12 when the non-return valve is closed, the expansion phase of the second PDM 22 can be started and completed without any venting of fluid being necessary.

[0062] Thus, unlike in the system of Figure 1 , the mass of working fluid drawn into the first PDM 12 is equal to that delivered to, and received by, the second PDM 22 over a complete cycle. The leading of the cold piston 15 relative to the hot piston 25 therefore improves the efficiency of the engine by eliminating the wasteful venting of working fluid during operation. [0063] It will be appreciated that by the apparently simple step of making the cold and hot pistons 15, 25 orbit or rotate out of phase, in particular with the cold piston 15 leading the hot piston 25 by a predetermined amount, venting of working fluid from the operating chamber A in the first PDM 12 is not required, thereby eliminating the complexity and expense of the venting valve and also the efficiency loss associated with the venting of fluid.

[0064] A further improvement to the system of Figure 1 comprises supplying the still-hot air exhausted from the second PDM 22 to the heat exchanger 18. This may be achieved by connecting the exhaust duct 26 to a fluid inlet (not shown) of the heat exchanger 18 and enables otherwise wasted heat exhausted from the second PDM 22 to supplement the heating effect of the heat exchanger 18 on the working fluid in the intermediate duct 16. In this case, the only substantial heat loss from the system is that lost from the heat exchanger 18 itself. Recycling the waste heat from the second PDM 22 further improves the efficiency of the engine.

[0065] While the form of PDM shown in Figures 2 and 3, is entirely suitable for use in both the engine of Figure 1 and the improved engine of the present invention described above, the applicant has recognised two potential disadvantages of using this form of PDM:

Firstly, since the PDM 22 may be required to operate at temperatures of between 150-200°C in order to achieve acceptable efficiency of the engine, the thermal loading on the bearing connecting the piston and vane in the second PDM 22 is very high. This can significantly reduce the operating life of the bearing, potentially requiring regular maintenance, repair or even replacement.

Secondly, the change from an in-phase orbital motion of the hot and cold pistons 15, 25 in the engine of Figure 1 to an operation whereby the cold piston 15 leads the hot piston 25 by a predetermined amount in the improved engine of the present invention causes variations in the instantaneous overall volume of the system to occur during the transfer cycle because the two identical piston geometries are out of phase with each other. These changes in volume may cause significant pressure fluctuations or variations within the system, potentially leading to thermal losses and reduced efficiency.

[0066] The applicant has resolved the above-mentioned problems by the development of a novel form of PDM, representing another aspect of the invention, an embodiment of which is shown in Figures 4 to 8 generally at 300. While the PDM 300 is similar in many respects to the PDM shown in Figures 2 and 3, for completeness it is described in detail hereinbelow.

[0067] The PDM 300 comprises a casing 310 defining a substantially enclosed cylinder, having a circular cylindrical wall 312 closed by two generally planar end walls 314, 316. A generally rectangular aperture 318 is formed in the cylindrical wall 312 and functions as a fluid inlet and/or outlet, as described below.

[0068] Disposed within the cylinder 310 is a drum-shaped piston 320 (equivalent to the piston 15 of the PDM shown in Figures 2 and 3) which is mounted on a shaft 321 that extends coaxially through the cylinder 310 for rotation therewith. The piston 320 has a generally cylindrical outer surface 322 and is substantially smaller in overall diameter than the cylinder 310. However, as best shown in Figure 6, the piston 320 features a "flat" portion 323 on its external surface, which portion has an increased radius of curvature that is comparable to that of the cylinder 310. The purpose of this flat portion 323 is described below.

[0069] Although not clearly shown in the drawings, the piston 320 is eccentrically mounted on the shaft 321 such that the flat portion 323 is in substantially sliding contact with the internal surface 312 of the cylinder 310 at all times during its rotation (shown by arrow R). The circumferential length of the flat portion 323 can be selected as desired but, in the illustrated embodiment, is approximately equal to the circumferential length of the aperture 318 which is thus substantially fully closed off during that part of the rotation at which the flat portion 323 is adjacent to the aperture 318.

[0070] The flat portion 323 is bounded by a leading edge 323a and a trailing edge 323b, either side of which the radius of curvature of the piston 320 decreases and the external surface 322 of the piston falls away from the internal surface 312 of the cylinder. It will be appreciated from the foregoing that only the flat portion 323 of the piston external surface 322 is in sliding contact with the internal surface 312 of the cylinder at any point during the rotation of the piston 320.

[0071 ] Rotation of the shaft 321 thus causes the piston 320 to rotate or sweep around the circumference of the cylinder 310 with the flat portion 323 of the outer piston surface 322 remaining in sliding contact with the internal surface 312 of the cylindrical wall 310 throughout its rotation.

[0072] The applicant has further recognised that the surface speed of the rotating piston 320 is higher relative to the casing 310 than in the case of the orbiting piston of Figure 1 , making sealing more difficult. As best shown in Figure 6, therefore, the piston 320 includes axial strips seated in grooves 328 provided on the flat portion 323 of the outer piston surface 322. The strips may be formed from a low friction material such as PTFE and are provided to accommodate manufacturing tolerances in the piston 320 and cylinder 310. Rotation of the piston 320 during first use of the PDM causes the strips to be worn down through frictional contact with the internal surface 312 of the cylindrical wall until only a light, sliding contact remains between internal surface 312 and the strips. The strips thus act as a sealing means between the piston 320 and the internal surface 312, providing a more effective sealing surface than that of Figure 1 .

[0073] Figure 6 also clearly shows that the cold piston 15 (320) leads the hot piston 25 (320) by approximately 120° as described above, and that each piston 320 has a flat portion which is in sliding contact with the internal surface 312 of the respective cylinder 310.

[0074] A vane member 330 is rockably disposed in the aperture 318 and comprises an arcuate vane 332, extending transversely to the cylinder 310, disposed at one end of a cranked arm 334. A cylindrical sleeve 335 is fixed to the other end of the cranked arm 334 and extends substantially orthogonally thereto. A vane axle 336 extends through the sleeve 335 and is pivotably mounted at either end, by means of bearings 338, to a pair of supports 340 that are fixed to, and project from, the casing 310. The vane member 330 is therefore rotatable relative to the casing 310.

[0075] As best shown in Figure 4, an operating chamber A is defined within the cylinder between the piston outer surface 322 and the internal wall 312 of the cylinder 310. More specifically, the operating chamber A, termed hereafter the delivery chamber, is defined, at least in part, by the portion of the outer piston surface 322 between the leading edge 323a of the flat portion 323 and the vane member 330.

[0076] Oscillatory motion of the vane member 330 is effected by means of a vane actuating mechanism comprising a connecting rod 342 which is eccentrically mounted at one end to the shaft 321 and extends externally of the casing 310 towards the vane member 330. The other end of the connecting rod 342 is pivotally connected, by means of a bearing 343, to one of a pair of plane parallel spaced coupling plates 344 which are fixedly mounted to the vane axle 336, externally of the supports 340 and the casing 310, for rotation therewith. The coupling plates 344 are connected to the cranked arm 334 at a point approximately midway between the vane 332 and the sleeve 335 via respective coupling rods 345.

[0077] It will be understood from the foregoing that rotation of the shaft 321 causes the vane member 330, connected to the shaft 321 via the eccentrically-mounted connecting rod 342 and the coupling plates 344, to pivot or oscillate relative to the cylinder 310 such that the lower portion of the vane 332 moves into and out of the aperture 318 in the manner of a so-called "nodding donkey" or reciprocating piston. Furthermore, the length of the connecting rod 342, and its eccentrically-mounted position relative to the piston 320, are such that the lower portion of the vane member 332 remains in sliding contact with the outer surface 322 of the piston 320 throughout its rotation within the cylinder 310. A sealing member (not shown) may be provided in the lower portion of the vane 332 to provide sealing contact between it and the piston 320.

[0078] Operation of the PDM 300 will now be described. As the piston 320 rotates within the cylinder, working fluid in the delivery chamber ahead of the leading edge 323a is expelled from the cylinder through orifices (not shown) disposed in the wall of the casing 310, while fresh working fluid is drawn into the operating chamber A behind the trailing edge 323b of the flat portion 323 through the aperture 318.

[0079] During rotation of the piston 320, the delivery chamber continuously changes in volume. For example, the delivery chamber may be at its greatest volume when the leading 323a of the flat portion 323 is coincident (circumferentially aligned) with the trailing edge 318b of the aperture 318. Once the leading edge 323a moves past the trailing edge 318b of the aperture 318, the volume of the delivery chamber between the leading edge 323a and the vane 332, starts to decrease. Thus working fluid contained in that volume is forced out of the delivery chamber A via the orifices in the casing 310 until the point of rotation is reached wherein the leading edge 323a is coincident with the vane 332.

[0080] On the other hand, when the trailing edge 323b of the flat portion 323 moves past the trailing edge 318b of the aperture 318, the volume of the operating chamber behind the trailing edge 323b, i.e. between the trailing edge 323b and the vane member 330, starts to increase, causing working fluid from atmosphere to be drawn into the cylinder through the aperture 318.

[0081 ] When the piston 320 has rotated to the point where leading edge 323a is substantially coincident with the vane member 330, all of the working fluid in the delivery chamber A ahead of the flat portion 323 has been exhausted through the orifices. Further rotation of the piston 320 moves the flat portion 323 past the vane member 330 and starts to close the aperture 318. During this time, the volume of the operating chamber behind the trailing edge 323b of the flat portion 323 continues to increase, drawing working fluid into the cylinder through the aperture until it is completely closed off by the flat portion 323.

[0082] At this point, the leading edge 323a is again coincident with the trailing edge 318b of the aperture 318 and the delivery cycle begins again with the newly ingested working fluid being expelled through the orifices in the casing 310. [0083] As described above, the out-of-phase orbit or rotation of the hot and cold pistons 15, 25 in the engine of the present invention gives rise to pressure fluctuations within the heat engine. The applicant has solved the above problem by adopting the form of PDM described above with reference to Figures 4-8. In this aspect of the present invention, therefore, the orbiting piston of the known PDM is replaced by a rotating piston having a flat portion 323 around a part of the circumference thereof at which the radius of curvature is similar to that of the cylinder. The presence of the flat portion on the piston surface reduces both the system volume changes and fluid leakage during the transfer process, thereby also reducing the pressure variations in the system due to the relative angular displacement of the two rotating pistons. In particular, the applicant has recognised that the volume change in the system during fluid transfer increases as the amount of angular lead between the pistons increases and that this internal rise and fall in volume is greater when the pistons are circular than it is when a portion of each piston's periphery has a flat portion 323. By adjusting the circumferential length of the flat portion in dependence on the relative angular displacement of the pistons, fluctuations in internal pressure caused thereby can be reduced and/or substantially eliminated.

[0084] In addition, the components of the vane actuating mechanism, that is the connecting rod 342, the coupling plates 344 and the bearing 343, are disposed outside of the casing 310 and are thus in a position of relatively low temperature, thereby prolonging the operating life of the bearing.

[0085] Figure 9 illustrates, in perspective view, an example of a practical arrangement for the first and second PDMs 12, 22. In this embodiment, the pistons 15, 25 of the first and second PDMs 12, 22 are mounted to a common shaft which is also connected to the generator. In this figure, the intermediate duct and the heat exchanger are not shown for clarity.

[0086] Figure 10 illustrates, in perspective view, an alternative example of a practical arrangement for the first and second PDMs 12, 22. In this embodiment, the second PDM 22 is formed from two half-sized PDMs 22a, 22b which are disposed on either side of a single first PDM 12. The combined swept volume of the pair of second PDMs 22a, 22b is substantially equal to the swept volume of the first PDM 12.

[0087] The pistons 15, 25 of the PDMs 12, 22a, 22b are mounted on a common shaft 321 which is also directly connected to the generator 400. The intermediate duct 16 is divided into two branches 16a, 16b to supply working fluid exiting the first PDM 12 to each of the second PDMs 22a, 22b. Each branch 16a, 16b of the intermediate duct 16 is provided with a respective heat exchanger 18a, 18b. [0088] It will be understood from the foregoing that the present invention addresses a number of problems or disadvantages associated with the prior art heat engine and positive displacement machine disclosed in, for example, WO2005/124106. These are summarised below:

[0089] Firstly, the change from an in-phase orbital motion of the hot and cold pistons 15, 25 to an arrangement in which the cold piston 15 angularly leads the hot piston 25 addresses the undesirable necessity of fluid venting from the operating chamber A in order to maintain operation of the engine. The novel and inventive arrangement described above enables continuous operation of the engine without any venting of working fluid, thereby eliminating the need for a separate venting valve and improving engine efficiency by eliminating wasted fluid.

[0090] Secondly, the adoption of rotating, rather than orbiting, pistons in the first and second PDMs 12, 22, wherein a portion of each piston's periphery or external surface 322 is provided with a flat region 323 having a radius of curvature similar or equal to that of the cylinder 310, addresses the problem of undesirable volume and pressure variations that occur within the system as a result of the use of out-of-phase pistons in the first and second PDMs 12, 22.

[0091 ] The flat portion 323 permits the cold piston to complete delivery of fluid, i.e. to shut off from the intermediate duct, and the hot piston to complete the expansion phase, earlier and later, respectively, than a fully circular piston. For example, where the flat portion 323 subtends an angle of 60°, the cold piston completes delivery and shuts off 30° earlier, and the hot piston finishes expansion 30° later, than the relative angular displacement would allow using fully circular pistons. The presence of the flat portion thus reduces the "effective" angular displacement of the pistons, whilst allowing the beneficial physical angular displacement to be maintained.

[0092] The flat portion also advantageously provides more time for the non-return valve to close without leak-back of fluid.

[0093] Thirdly, the provision of a linkage 342-344 external of the casing 310 to actuate the vane member 330 addresses the problem of thermal degradation of the bearing 343 between the piston 320 and the vane 332 by positioning the bearing in a region of relatively low temperature, thereby extending its operational life.

[0094] In addition, however, the applicant has discovered a further, surprising technical benefit provided by the present invention. [0095] In particular, during rotation of the cold piston 15 within the first PDM 12, the area of the working surface of the piston 15, that is to say the portion of the external surface 322 of the piston that acts to push the fluid into the intermediate duct 16, varies in size throughout its rotation. For example, at the beginning of the rotation, the surface area of the piston between the leading edge 323a and the vane member 330 is at its greatest, and decreases to zero as the piston rotates towards the end of its rotation.

[0096] Similarly, the working surface of the hot piston 25 within the second PDM 22, that is to say the area of the portion of the external surface of the piston against which the expanding fluid applies a rotational force, also varies in size throughout the operational cycle.

[0097] Although these changes are identical for both cold and hot pistons 15, 25 due to their identical geometry, because the pistons 15, 25 rotate out of phase in the present invention, the changes in size of the working surface area of each piston also occur out of phase. This contrasts with the arrangement of WO2005/124106 in which the variations in size of the working surface are of each piston occur synchronously due to their in-phase orbit, such that the working surface are of both pistons are always substantially equal in size.

[0098] In the present invention, therefore, at the beginning of the transfer cycle the working surface area of the cold piston 15 is greater in size than the working surface area of the hot piston 25, the latter lagging approximately 90° behind the former and approaching the end of its rotational cycle.

[0099] Because of these asynchronous changes in working surface area of the hot and cold pistons 15, 25, at only one point in the transfer cycle are the working surface areas of both pistons the same. Thus, at any given moment during the transfer cycle, the pressure of the fluid within the intermediate duct 16 is felt differently by the hot and cold pistons 15, 25, i.e. the magnitude of the force applied to each piston 15, 25 by the pressure is different.

[00100] For example, at the beginning of the transfer cycle, and for approximately the first 180° of rotation of the cold piston 15, the working surface area of the cold piston exceeds that of the hot piston 25. Thus any pressure increases in the working fluid in the intermediate duct 16 caused by heating result in a greater force being applied to the cold piston 15 than to the hot piston 25. It will be appreciated that the force applied to the cold piston 15 by the fluid pressure is in the reverse direction, i.e. generating negative work. [00101 ] If the pressure increase were high, this reversing force on the cold piston 15 could cause the mechanism to stall. At this point in the cycle, however, the temperature, and thus the pressure of the fluid is relatively low and so the reversing force on the cold piston 15 is negligible.

[00102] Moreover, as the pressure increases in the intermediate duct 16, the relative difference in the working surface area of the cold piston 15 and the hot piston 25 reduces until a point is reached (at approximately 180° of rotation of the cold piston 15) when the working surface area of the hot piston 25 becomes the greater. At this point, the positive force on the hot piston caused by fluid pressure in the intermediate duct is greater than the negative force applied to the cold piston and so positive work is done to rotate the engine.

[00103] Furthermore, when the leading edge 323a of the flat portion 323 of the cold piston

15 reaches the vane member 330, the working surface area of cold piston is substantially zero such that the cold piston 15 is effectively shut off from the fluid pressure in the intermediate duct 16. From this point in the cycle, therefore, all of the pressure in the intermediate duct is applied to the hot piston 25 which is still only approximately two thirds of the way around its rotation. During the final part of the rotation of the hot piston, therefore, positive work is being done.

[00104] In other words, during the period in the cycle at which the cold piston 15 has the largest working surface area, i.e. when the pressure in the intermediate duct 16 can have the greatest negative effect on the engine, the pressure in the intermediate duct 16 is at its minimum. On the other hand, during the period in the cycle at which the hot piston 25 has the largest working surface area, i.e. any pressure in the intermediate duct 16 has its greatest positive effect on the engine, the pressure in the intermediate duct is at its maximum.

[00105] The net amount of work done during a single cycle is therefore significantly greater than would be achieved through the use of in-phase rotating pistons and this unexpected beneficial effect further enhances the efficiency of the system.

[00106] It will be appreciated that the present invention provides an engine and positive displacement machine that are significantly improve in comparison with prior art systems such as those disclosed in WO2005/124106. Nevertheless, various modifications may be made to the engine and positive displacement machine disclosed herein without departing from the scope of the present invention, which scope is defined in the appended claims and statements of invention hereinabove.

[00107] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[00108] Features, integers and characteristics described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[00109] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.