Background of the invention

[0001] The present invention relates to an arrangement in accordance with the preamble of claim 1 in a hot air engine.

[0002] The hot air engine is a motor type known per se, whose power originates from a temperature difference provided in a gas (conventionally air or helium) contained in a tightly sealed container. The engine comprises a so-called hot end and a so-called cold end, between which the gas inside the engine moves cooling and heating alternately. By the effect of thermal expansion of the gas, in the container there is produced variation in pressure, which, in turn, is transformed to mechanical work by means of a work piston and a displacer controlled by the engine. The operation of the work piston is controlled conventionally by the displacer motion, and their action generates a movement that advantageously actuates a separate mechanism or an electric generator, for instance.

[0003] The hot air engine involves a substantial problem, which is to be solved in this connection. The problem relates to the displacer, which has conventionally been manufactured of metal in order to ensure the resistance of the displacer when subjected to high temperatures and variations in temperature. This has resulted in the displacer being heavy, and consequently, the efficiency of the engine suffers. In addition, to machine a metal displacer of this kind such that it fits in its cylinder is a particularly demanding task.

[0004] Another problem associated with the conventional technology is, how to achieve sufficient tightness when manufacturing a sealed cylin- der/displacer combination in pursuit of improved efficiency of the hot air engine. Sealing of the structure, efficiency deteriorated by friction resulting therefrom, lubrication solutions necessary for preventing friction and wear of seals, they all cause problems that are difficult to solve and increase the engine's need for maintenance.

Brief description of the invention

[0005] The object of the present invention is to provide an arrangement that solves the above-described prior art problems in a completely novel manner. [0006] In particular, the present problems may be solved by combining the characteristics as disclosed in the characterizing part of claim 1. Preferred embodiments of the invention are disclosed in the dependent claims.

[0007] In this connection, the term "hot air engine" is not deemed to limit the invention just to the apparatuses having ordinary air as the working gas, but the invention may be utilized with a wide variety of gas mixtures.

[0008] The terms "upward", "downward", "in upper position", "in lower position" and so forth used in the specification illustrate the features of the invention in directions that are relative to the arrangement of the invention in the hot air engine as it is presented in the attached figures.

[0009] The invention provides considerable advantages. Thus, the structure of the present hot air engine enables a substantially frictionless interface between a displacer and a cylinder surrounding the displacer. In this manner the almost frictionless structure permits elimination of problems posed by conventional structures. Thanks to the implementation of the interface no separate sealing is needed, nor the lubrication associated therewith. As there are no seals, it is also possible to avoid wear in the structures of the cylinder and the displacer, and consequently, the need for maintenance required by conventional hot air engines is largely avoidable.

[0010] The hot air engine of the invention is designed such that the gap between he_dispjacejrjand_^ possible. Thus, the air or other gas in the cylinder may be conveyed from the hot end to the cold end, or vice versa, through a separate heat exchanger without any bypass flow that would be significant to the operation of the apparatus.

[0011] Other advantages of the invention are presented in the following in connection with a more detailed description of a particular embodiment of the invention.

Brief description of the figures

[0012] In the following, some preferred embodiments of the invention are explained in closer detail with reference to the accompanying drawing, in which

Figure 1 shows schematically the operating process of a hot air engine and

Figure 2 shows schematically a longitudinal cross section of the present arrangement. Detailed description of a preferred embodiment

[0013] The present figures do not show the arrangement in the hot air engine in scale but the figures are schematic, illustrating the structure and operation of the preferred embodiment in principle. The structural parts indicated by reference numerals in the attached figures correspond to the structural parts marked with reference numerals in this specification.

[0014] The operating process of the hot air engine may be implemented by various arrangements. The attached prior art example in accordance with Figure 1 is from an article titled "Design analysis method for Stirling engines" written by H. Snyman, T.M. Harms and J.M. Strauss and published in the periodical "Journal of Energy in Southern Africa", vol. 19, issue 3, August 2008.

[0015] A hot air engine of this kind comprises a cylinder 1 , a dis- placer 3 moving therein along the same axis 2 as a work piston 4 comprising a working rod 5. The displacer divides the cylinder into two parts separate from one another, a hot end 6 appearing higher up in the figures and a cold end 7 lower in the figures. The work piston is arranged to move in the so-called cold end that is connected to the hot end through a pipe line 8, which in turn is furnished with a regenerator 9 that recovers and releases heat. The apparatus works as follows:

1. The cold end 7 of the cylinder 1 is cooled at constant temperature from outside, whereby the gas accumulated in the cold end and the displacer 3 as well as in the constant volume defined by the extreme positions of the work piston 4 and contracting therein as a result of cooling is conveyed, along with the return and compression movement of the work piston towards the displacer, back towards the hot end 6 of the cylinder along the pipeline 8. The work piston returns to its upper position guided by the return movement of the working rod. Figures (a) and (b).

2. The displacer is controlled in the hot end 6 by the effect of the warming and expanding gas to move towards the work piston that is in its upper position, whereby the rest of the gas in the cold end is compressed towards the hot end 6 of the cylinder along the pipe line 8. Figure (c ).

3. In addition to the gas heated in the regenerator 9 locating in the pipe line 8, heat is conveyed from outside into the hot end 6 of the cylinder 1 , whereby the gas in the constant volume defined by the lower position of the displacer continues to heat up. The heated gas expands at constant temperature and is compressed back to the cold end 7 on the other side of the displacer and simultaneously transfers part of its thermal energy to the regenerator 9. The hot and expanded gas thus produces a force that is exerted on the work piston 4 and makes it move. The kinetic energy of the work piston movement retreating from the displacer and being directed downwardly in the figure is recovered in a manner known per se through the working rod 5. Figure (d).

4. At the end of the cycle, the displacer 3 is controlled to its upper position in the hot end 6 as shown in Figure (a), which forces the rest of the gas therein towards the cold end.

[0016] Figure 2, in turn, shows a hot air engine 10 in accordance with the present arrangement. In the figure, reference numeral 1 1 denotes an engine cylinder, in which there is a hot end, denoted by reference numeral 12, and opposite thereto a cold end, denoted by reference numeral 13. In the cylinder there is arranged a displacer 14 with a control rod 15 connected thereto. The control rod is arranged to control a power unit known per se that is not described separately in this connection. The displacer simultaneously controls a work piston 16, which may be, for instance, bellows advantageously made of rubber. The work piston, in turn, controls a working rod 17, which is also connected to the above-mentioned power unit, which may be a linear generator producing electric energy, for instance. In this connection, said solution is not described separately.

[0017] The hot air engine 10 further comprises a gas flow circuit 18, which includes a heater 19 and a cooler 20 for the gas circulating therein. Heat is also recoverable from the gas with a separate regenerator 21.

[0018] By affecting the volume and temperature of the gas in the hot end 12 and the cold end 13 of the hot air engine in the above-described manner it is possible to provide utilizable work movements of the control rod 15 and the working rod 17.

[0019] Due to the high temperatures used for controlling the hot air engine, manufacture of a displacer 14 has usually been difficult. So, selected manufacturing materials have been such that are highly heat resistant but difficult and expensive to machine. Because these materials, as a rule, are also heavy, the efficiency of the hot air engine declines, sometimes even signifi- cantly. Another problem associated with the previous solutions is a need for sealing a interface between the cylinder 11 and the displacer 14 by various sealing solutions causing more or less friction.

[0020] By providing the interface between the cylinder 11 and the displacer 14 with a clearance, it is possible to avoid use of seals. Because the gas in the cylinder is to be conveyed between the hot end 12 and the cold end

13 through a separate flow circuit, said clearance has to be as small as possible, and consequently its precise machining is a challenging task in manufacturing.

[0021] It was found, however, that by manufacturing the displacer

14 of cellular material resistant to high temperatures it is possible to improve the efficiency of the hot air engine and at the same time to simplify the manufacture of the displacer. The efficiency improves, because the displacer manufactured in a novel manner is considerably light, and consequently the energy required for its control is also considerably low. On the other hand, machining of these cellular materials is considerably simpler than machining of previous metal-based or ceramic-based materials, whereby it is possible to achieve significant economic savings.

[0022] The present solution is particularly well suited for devices operating at temperatures lower than 500 °C, whereby it is possible to use jtandard materials as the ^jLUj turaLroatexial&^fQr-th -device . . . With selected manufacturing materials the fitting between the displacer 14 and the cylinder 11 , which usually poses difficulties, is simpler and more economical than conventionally.

[0023] A suitable material for manufacturing the displacer 14 is a high temperature resistant polyurethane - so-called PIR insulation material (polyisocyanurate) - or a polyimide foam. The displacer may also be manufactured of foamed metals resistant to higher temperatures.

[0024] These cellular materials are suitable for being used as the displacer 14 either as such, after machining, or coated when they will have a layer that closes the outermost, machined cellular surface. When material to be molded, extruded or foamed is used, a solid outer surface is obtainable by manufacturing the displacer in a particular mold. When necessary, the surface of the displacer may also be coated with a ceramic coating or a nano-coating, which provide the outer surface of the displacer with a protective layer improving its heat resistance. A heat resistant paint (not solvent-based, however) also protects the cellular insulation material and both binds and seals the cellular surface.

[0025] The thermal load to which the displacer 14 is subjected being at its highest on the end surface 22 facing the its hot end, said end surface can be furnished with a separate heat shield 23. This heat shield can be manufactured of material suitable for the purpose, such as ceramic casting, steel plate or the like. The thermal load being high, or the cellular material requiring a particularly effective protection, an insulating material 24, which may be conventional, readily deformable material known per se, can be arranged between the heat shield and the surface of the displacer facing the hot end. This material may be rock wool or mineral wool, ceramic wool or felt, or an aerogel, for instance. The heat shield and the insulation, if any, are connected to the displacer by mechanical methods known per se, with fastenings or by gluing or casting.

[0026] On the surface of the displacer 14 there may also be arranged a layer consisting of a ceramic felt. The felt layer is thus arranged, in particular, on the end surface of the displacer and on the cylindrical outer surface 25 delimited between the hot end 12 and the cold end 13. Particularly well the felt layer will be attached to the cellular material, when the mould in which the displacer is manufactured is lined therewith.

[0027] The^eramic^eJt layej^rranged _in .the_„displacer forms the necessary heat shield on the end surface of the displacer. The felt layer on the outer surface enhances, for its part, the heat resistance of the displacer made of polyurethane and polyimide, while the mechanical strength of the displacer is increased. The ceramic felt used on the outer surface further allows minimization of bypass flow between the hot and cold ends 12 and 13.

[0028] It is to be understood that the above description and the related figures are only intended to illustrate the present solution. The solution is thus not restricted to the embodiment described above or defined in the claims, but it will be obvious to a person skilled in the art that a variety of variations and modifications are possible within the scope of the inventive idea defined in the accompanying claims.