Stirling engines are well known in the prior art and normally comprise two reciprocating pistons, one called a displacer piston and one called a power or working piston which are coupled together by means of a crankshaft from which the power output is drawn.

The significant feature of a Stirling engine is that it receives heat through the wall of the cylinder (rather than internally as in an internal combustion engine) and heat is removed from the cooler of the engine. As a consequence of the resulting temperature difference between the heater and cooler, some of the thermal energy is converted into mechanical energy by the engine, appearing as output shaft rotational energy, and can be applied to a mechanical load.

The majority of Stirling engines comprise a two piston arrangement of a displacer piston and a working piston and Stirling engines are commonly arranged such that the displacer piston has a relatively long stroke, the stroke being larger than the diameter of the piston. This is the accepted practice for Stirling engines.

The Harwell Laboratories in the United Kingdom produced a Stirling engine which replaced the working piston of a normal Stirling engine with a steel diaphragm member which flexes instead of reciprocating. However, a significant improvement over this arrangement is described in GB-A-2298903, in which the diaphragm is made from a non-metallic flexible material. This type of Stirling engine uses a free displacer piston which can oscillate freely in

a cylinder and allows the possibility of selection of a resilient diaphragm with a resonant frequency which enhances the performance of the engine by being complementary to the natural frequency of the spring displacer piston, such that a synergistic effect is achieved. Moreover, the displacer piston engine is reliably self-starting, has only two moving parts which are not connected, is simple in design and quiet in operation. The engine is inexpensive to produce and requires little maintenance. Thus, such an engine can advantageously be used in a combined heat and power system.

With a conventional power piston, leakage of the working fluid between the piston and cylinder must be minimised by the use of piston rings or other seals.

These introduce sliding friction losses which reduce the output power of the engine and also cause seal wear and ultimately failure of the seals. When a flexible diaphragm which is preferably made of rubber is used as the piston, sliding seals are unnecessary, and so the life of the engine can be increased and additionally the power output increased. In particular the rubber diaphragm can be designed so that it will never fail due to internal stresses caused by flexing.

Secondly, free piston Stirling engines are potentially self-starting engines if sliding friction losses are made very small. With a rubber diaphragm piston, sliding friction losses do not exist and also, losses within the rubber material due to its flexing are small. If displacer piston sliding friction losses are also absent, which is the case with the engine under consideration, a self-starting engine results and this in turn obviates the need for a starting system and reduces costs.

Thirdly, a rubber diaphragm piston is self centering which is advantageous. Some engines require

piston centering systems for correct operation, which adds to the manufacturing cost. The disadvantages of using a rubber diaphragm power piston are that engine dead spaces are increased.

In general the advantages of using a rubber diaphragm as a power piston greatly outweigh its disadvantage.

In a pressurised free piston Stirling engine it is necessary to equalise the pressure on both sides of the diaphragm piston, since otherwise the diaphragm centre will be displaced from its mean position and the engine will not operate effectively. The space between the rubber diaphragm and the displacer piston is commonly termed the compression space, whereas the space on the other side of the rubber diaphragm, which is nearly always enclosed, is termed the bounce space.

With a rubber diaphragm engine the enclosed bounce space must be connected by a small bore capillary tube to the compression space so that the mean pressure across the diaphragm is made zero. In this manner the central undeflected position of the diaphragm is defined, no matter what the mean working fluid pressure. Cyclic perturbations of pressure across the diaphragm due to the engine's operation, are unaffected by the presence of the capillary tube since the gas flow rate along it is small. Thus in operation the diaphragm deflects in both directions but its central position is always fixed. The high frequency oscillation of the diaphragm makes it important to ensure that it is clamped securely and evenly in position.

In a known arrangement, the clamping of the diaphragm can be simply carried out by passing bolts through the diaphragm and the adjacent flanges of the engine housing, around the perimeter of the diaphragm.

It is important, however, when clamping the rubber diaphragm with this system that the bolts are

tightened equally and to a specific and known degree.

If the bolts are insufficiently tightened, then the diaphragm is incorrectly clamped around its edge and its natural resonant frequency will be incorrect and gas leakage will occur. On the other hand, over- tightening of the bolts will produce excessive stresses upon the diaphragm and will result in a shorter diaphragm life. Unequal tightening of the bolts will produce a variable diaphragm thickness around its perimeter and result in some or all of these deleterious effects.

A simple method of achieving the correct and equal degree of tightening in current use is to use a tubular metal spacer around each bolt. The height of each tubular spacer is predetermined and is equal to the desired working thickness of the diaphragm. When clamping the diaphragm the procedure is to tighten the bolts until the applied torque increases rapidly, signifying shortening of the bolts to the tubular spacer length and clamping of the diaphragm to the correct thickness.

However, with this system a sealing problem can arise since the diaphragm is made from rubber, which is a flexible material. Under pressurisation from both sides, the diaphragm will bulge at its perimeter and leakage of working fluid will occur to the atmosphere along either or both of the diaphragm's pressurised surfaces.

It is an object of the present invention therefore to provide a clamping arrangement for the diaphragm in which the aforementioned problems are prevented.

The invention therefore provides a Stirling engine having a displacer piston reciprocal in a cylinder, said cylinder being defined by a cylinder block, a first chamber on a first side of the displacer piston and a second chamber on a second side

of the displacer piston, wherein the second chamber is closed by a flexible diaphragm made substantially of an elastomeric material, and a housing attached to the cylinder block with the periphery of the diaphragm being located between two parallel radially extending annular flanges provided on the cylinder block and housing respectively, fastening means being provided at a plurality of spaced locations around the flanges to clamp the diaphragm therebetween, wherein spacer means are provided to maintain a predetermined distance between the flanges when the fastening means are fully tightened and a peripheral transverse edge of the diaphragm is covered by a constraining band, which is provided in continuous contact with each flange so as to wholly enclose and seal the peripheral edge of the diaphragm therebetween.

Preferably the spacer means are individual tubular elements located round each fastening means.

The constraining band is preferably a transverse lip extending from the radially extending flange of the housing to contact the cylinder block flange.

The spacer means are preferably provided by the constraining band.

The constraining band may be of metal.

The spacer means may be of metal.

The fastening means are preferably bolts.

In a preferred embodiment of the invention the majority of the diaphragm is composed of a natural or synthetic elastomeric material.

The majority of the diaphragm is preferably composed of natural or silicone rubber.

The invention will now be described, by way of example only, with reference to the accompanying drawings in which:- Fig. 1 is a schematic cross-sectional side elevation'of a known Stirling engine having a flexible

diaphragm ; Fig. 2 is an enlarged view of the diaphragm clamping area of Fig. 1 showing a clamping arrangement according to the present invention; and Figs. 3 to 5 are alternative embodiments of the clamping arrangement of Fig. 2.

Referring first to Fig. 1 the Stirling engine comprises a cylindrical cylinder block 10 defining a cylinder 11.

A free displacer piston 12 is provided to move reciprocally in the cylinder 11.

A cylinder head closes off the top of the cylinder 11 and defines, with the cylinder block 10 and an upper plate of the displacer piston 12, a variable valve chamber A. The cylinder head abuts a heat exchanger block 13, which is heated by means of heater tubes 14.

Attached to the cylinder block 10 is a flexible elastomeric diaphragm 16, preferably made of natural rubber or another flexible material having similar characteristics. The diaphragm 16 defines, with the cylinder block 10 and a bottom plate of the displacer piston 12, a variable volume chamber B.

By virtue of the oscillating motions of the displacer piston 12 and elastomeric diaphragm 16, the working fluid (a gas such as air, nitrogen, hydrogen, helium, CO2 or other well-known in the art of Stirling Engine fabrication) is forced to oscillate back and forth between chambers A and B.

Interconnecting the two chambers A and B is a system of heat exchanges through which the working fluid flows. For the direction A to B, the working fluid passes firstly through the heater tubes 14, next through a regenerator element (not shown but familiar to those skilled in the technology of Stirling Engines) and then through the cooler tubes (again not

shown but familiar to those versed in the art) to end up in chamber B. The working fluid follows the reverse path on its return to A in the next cycle of its oscillation motion. The regenerator is located at the top of the cylinder 11 whereas the cooler tubes are located lower down within the cylinder block 10 (as described in GB-A-2298903). Both cooler tubes and cylinder block 10 are cooled by water which enters and leaves the system via ports 15.

A plunger 17 is connected to the diaphragm 16 to move with the diaphragm 16. The plunger 17 is connected to a linear alternator 18 which is located in a housing 26 defining a bounce chamber 22.

The method of operation of such a Stirling engine is described in particular in GB-A-2298903, on which the embodiment of Fig. 1 is based.

Referring now to Fig. 2, this shows an enlargement of the diaphragm clamping area 19. The diaphragm 16 is clamped between parallel radially extending annular flanges 20,21 provided at a lower end of the cylinder block 10 and an upper end of the alternator housing 22 respectively. Bolts 23 pass through the flanges 20,21 on the diaphragm 16.

Tubular metal spacers 24 are positioned around each bolt 23 to ensure that the correct and equal tightening of the bolt 23 is achieved. The height of each tubular spacer 24 is, as mentioned above, predetermined and is equal to the desired working thickness of the diaphragm 16.

To avoid the problems outlined above, however, and to prevent the occurrence of bulging of the diaphragm 16 and thus permit sealing to high working pressures, the peripheral transverse edge of the diaphragm 16 is covered with a band 25 of a constraining material, such as steel. The band 25 contacts at least a portion of the ends of both flanges 20,21, so as to enclose the periphery of the

diaphragm 16 therebetween. As rubber has a Poisson's constant of nearly 0.5 it is thus relatively incompressible, and so sealing is effective as the band 25 fully encloses the diaphragm 16 both circumferentially and longitudinally.

In a further embodiment, as shown in Fig. 3, it is possible to combine the constraining band 25 with the flange 21 of the alternator housing 26 or (not shown) with the flange 20 of the cylinder housing.

This embodiment may be advantageous in terms of production costs and ease of assembly.

As a further alternative approach, as shown in Fig. 4, the band 25 may also assume the functional role of the tubular metal spacers 24 of Figs. 2 and 3.

Thus the bolts 23 are tightened just sufficiently to compress the rubber diaphragm 16 to the same width as the band 25.

In the same way, tubular spacers 24 may be omitted from the arrangement of Fig. 3 and the band 25, which extends from the alternator housing flange 21, is used to determine the correct diaphragm compression, as shown in Fig. 5. The arrangement of Fig. 5 can also be modified by inverting the arrangement so that the flange 21 is located above the cylinder block 10.

Use of the above-mentioned arrangements allow the inside of the engine to be pressurised to at least 10 bar gauge pressure and higher, whilst the outside of the engine is at atmospheric pressure, with little need for sealing further than that shown above.