SE541778C2

SE541778C2 - Stirling engine comprising an exhaust gas catalyst

Info

Publication number: SE541778C2
Application number: SE1850008A
Authority: SE
Inventors: Stefan Larsson-Mastonstråle
Original assignee: Maston AB
Priority date: 2018-01-02
Filing date: 2018-01-02
Publication date: 2019-12-17
Also published as: SE1850008A1; WO2019135698A1

Abstract

A Stirling engine comprising a crank case (1) with a crank shaft (2) arranged therein, a displacer cylinder (3), a working cylinder (8), a heater device comprising a combustion chamber (15) arranged at an end of said displacer cylinder (3), wherein the combustion chamber (15) is defined by a tubular wall element, and has a first end which is connected to the displacer cylinder (3) and a second end which is remote from the displacer cylinder (3), and a burner arrangement for generating a flame inside the combustion chamber (15). The Stirling engine further comprises an exhaust gas catalyst element which is arranged inside the combustion chamber (15).

Description

Stirling engine comprising an exhaust gas catalyst TECHNICAL FIELD The present invention relates to a Stirling engine comprising: - a crank case with a crank shaft arranged therein, - a displacer cylinder with a reciprocatingly arranged displacer piston therein, said displacer piston being connected to said crank shaft via a connecting rod extending through a first end of said displacer cylinder, and wherein the displacer cylinder defines a hot chamber and a cool chamber separated by the displacer piston, - a working cylinder with a reciprocatingly arranged working piston therein, said working piston being connected to said crank shaft via a connecting rod extending through a first end of the working cylinder, - a heater device comprising a combustion chamber arranged at a second end of said displacer cylinder opposite to said first end, - a first heat exchanger and a second heat exchanger, said first heat exchanger comprising at least one tube that extends from a displacer cylinder head provided at said second end of the displacer cylinder into the combustion chamber and out of the combustion chamber to the second heat exchanger, which is comprised by a regenerator provided outside the combustion chamber and outside the displacer cylinder, and wherein the hot chamber defined by the displacer cylinder is in fluid communication with a second end of the working cylinder through the first and second heat exchangers, - wherein the combustion chamber is defined by a tubular wall element, and has a first end which is connected to the displacer cylinder and a second end which is remote from the displacer cylinder, and - wherein there is provided a burner arrangement for generating a flame inside the combustion chamber.

A regenerator is referred to as an internal heat exchanger and temporary heat store placed between the hot chamber of the displacer cylinder and the working cylinder such that the working fluid passes through it first in one direction then the other, taking heat from the fluid in one direction, and returning it in the other. It can be as simple as metal mesh or foam, and benefits from high surface area, high heat capacity, low conductivity and low flow friction. Its function is to retain within the system the heat that would otherwise be exchanged with the environment at temperatures intermediate to the maximum and minimum cycle temperatures.

BACKGROUND ART External combustion engines of Stirling type are well known. They may be of three different types, which are named alpha, beta and gamma and differ from each other with regard to how the displacer cylinder, working cylinder and the displacer piston and the working piston are arranged in relation to each other and to the crank shaft that is driven by the working piston.

Essential to the function of a Stirling engine is that a working medium is heated, preferably by a burner flame in a combustion chamber. The fuel used in the combustion chamber may be in a gaseous state or a solid state. The working gas is conducted through a heat exchanger that may comprise one or more tubes that extend from the hot chamber of the displacer cylinder into the combustion chamber, out of the combustion chamber and to a regenerator. The regenerator is located outside the combustion chamber and is the individual component that distinguishes Stirling engines from other types of external combustion engines.

The burner in the combustion chamber may be arranged such that a flame is directed from an inner periphery of the combustion chamber wall towards a centre part of the combustion chamber. Preferably, the tube or tubes of the heat exchanger are not directly touched by the flame, and should thus be arranged such that they extend around the assumed position of the flame.

As an alternative, the burner flame may be directed tangentially along the inner periphery of the combustion chamber wall, wherein the heat exchanger tube or tubes may be arranged in the centre of the combustion chamber.

Combustion of fuel through the flame generated by the burner arrangement is generally not total. Unburned material is therefore present in the exhaust gases. For environmental and for combustion efficiency reasons, the amount of unburned fuel and material having negative impact on the environment leaving the Stirling engine should be minimized.

THE OBJECT OF THE INVENTION It is an object of the present invention to present a Stirling engine having a combustion chamber design that promotes an improved energy yield and that promotes low emission of unburned fuel and material having negative impact on the environment.

SUMMARY OF THE INVENTION The object of the present invention is achieved by means of the initially defined Stirling engine, which is characterised in that it comprises an exhaust gas catalyst element which is arranged inside the combustion chamber. By arranging the catalyst element inside the combustion chamber, and not as expected in an exhaust gas conduit downstream the combustion chamber, the catalyst element can be used as a heat absorbing and heat emitting element that contributes to an improved energy yield inside the combustion chamber. The at least one tube of the first heat exchanger is located between the assumed path of the flame and the catalyst element, such that heat from the flame that is transferred to the catalyst element will be reflected by the catalyst element back towards said at least one tube.

Thereby, the efficiency of the heating of the at least one tube will be improved, the energy yield will be improved, and, as a result thereof, less unburned fuel and environmentally negative material will leave the combustion chamber with exhaust gases.

According to one embodiment, the catalyst element extends from said second end of the combustion chamber. Preferably, the catalyst element extends over at least 50%, preferably, over at least 75% of the total length of the combustion chamber, as seen from said second end thereof to a head of the displacer cylinder.

According to one embodiment, the exhaust gas catalyst element comprises a cylindrical body having a gas-permeable cylinder wall having a longitudinal centre axis which is parallel with a longitudinal centre axis of the combustion chamber.

According to one embodiment, the exhaust gas catalyst element comprises a cylindrical body having gas-permeable cylinder wall that has a longitudinal centre axis which is coaxial with a longitudinal centre axis of the combustion chamber.

According to one embodiment, the Stirling engine comprises means configured to affect said flame to follow an inner periphery of the tubular wall element that defines the combustion chamber in a generally tangential direction.

According to one embodiment, the Stirling engine comprises an exhaust gas outlet in said second end of the combustion chamber, wherein said means comprises a fan provided in said second end of the combustion chamber, which fan is configured to suck exhaust gas from the centre region of the combustion chamber out of the combustion chamber through said outlet in a longitudinal direction of the combustion chamber thereby generating a vortex flow of gas in a centre region of the combustion chamber towards said second end of the combustion chamber.

According to one embodiment, the exhaust gas catalyst element comprises a cylindrical body that extends from said second end of the combustion chamber and that has a gas-permeable cylinder wall, wherein the end of the cylindrical body closest to said second end of the combustion chamber encircles the exhaust gas outlet. At said second end of the combustion chamber, the end of the catalyst element is preferably attached to an end wall of the tubular wall element that defines the combustion chamber. The catalyst element preferably extends from said second end beyond the location of the burner arrangement and ends close to a head of the displacer cylinder. Thereby, exhaust gases from the flame will be forced to pass through the cylinder wall of the catalyst element in order to reach the exhaust gas outlet.

According to one embodiment, said fan has a rotational axis which is parallel with a longitudinal centre axis of the combustion chamber.

According to one embodiment, said fan has a rotational axis which is coaxial with a longitudinal centre axis of the combustion chamber.

According to one embodiment, at least in a region where the first heat exchanger is provided in the combustion chamber, the combustion chamber has a generally circular cross section as seen in the direction of a longitudinal axis thereof.

According to one embodiment, said burner arrangement comprises an opening in the tubular wall element configured for introduction of solid fuel into the combustion chamber, and there is provided an air inlet configured for introduction of air into the combustion chamber for combustion together with said solid fuel.

According to one embodiment, the opening for introduction of solid fuel is arranged in the tubular wall element in said region and at a position approximately 45°-100° from a bottom line of the combustion chamber, and there is provided a receiver element at the bottom of tubular wall element, said receiver element being configured to receive solid fuel falling down from the opening for introduction of solid fuel, and wherein said air inlet for the introduction of air is provided in the region of said receiver element.

According to one embodiment, said means configured to affect the flame comprises a baffle plate that is connected to and extends from the inner periphery of the tubular wall element from a level above the opening for introduction of solid fuel, and which follows the inner periphery of the tubular element with a predetermined distance to said inner periphery from said point to a point beyond said receiver element, such that a flame generated by combustion of solid fuel in said receiver element will be given a tangential direction along the inner periphery of the tubular wall element from said receiver element towards a gap defined between a free end of the baffle plate and the inner periphery of the tubular wall element opposite the end of the baffle positioned at said level above the opening for introduction of solid fuel.

According to one embodiment, the Stirling engine comprises a tubular exhaust gas flow controller element with openings in the mantle surface thereof, said tubular exhaust gas flow controller element extending from the second end of the combustion chamber and having a longitudinal centre axis which is coaxial with a rotational axis of said fan. In absence of the exhaust gas flow controller element, the exhaust gas will have a tendency to take the shortest path from the flame initiation region to the outlet. The openings of the exhaust gas flow controller element contribute to balance the mass flow of exhaust gases in the longitudinal direction of the combustion chamber. Thereby, a flame that extends tangentially along the inner periphery of the tubular wall element will be prevented from being directed radially inwards and confronting components such as the first heat exchanger. Preferably, the exhaust gas flow controller element extends over at least 50%, preferably, over at least 75% of the total length of the combustion chamber.

According to one embodiment, the exhaust gas catalyst element is arranged radially outside the tubular exhaust gas flow controller element. Preferably, the exhaust gas catalyst element extends from the second end of the combustion chamber and has a length corresponding to the length of the tubular exhaust gas flow controller element.

According to one embodiment, the first heat exchanger comprises a plurality of tubes that together define a gas-permeable tubular body which has a longitudinal centre axis that is parallel with a longitudinal centre axis of the combustion chamber. Preferably, the first heat exchanger extends over at least 50%, preferably, over at least 75% of the total length of the combustion chamber. Preferably, the tubes of the first heat exchanger are provided with flanges for improved heat exchange. Preferably, such flanges are annular flanges attached to the outer periphery of the tubes and extending in a plane which is perpendicular to the centre axis of the tube to which they are attached.

According to one embodiment, the longitudinal centre axis of said tubular body is coaxial with the longitudinal centre axis of the combustion chamber.

According to one embodiment, the first heat exchanger comprises a plurality of tubes, wherein each tube is a U-shaped tube that extends from the displacer cylinder head into the combustion chamber and back towards a first end wall of the combustion chamber.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention are shown in the annexed drawing, on which: Fig. 1 is a view from above of a Stirling engine according to the invention provided with schematically shown heater device, Fig. 2 is a cross-section according to I-I in fig. 1, still with the heater device shown schematically, Fig. 3 is a view corresponding to fig. 1, but with the heater device removed from the rest of the engine, Fig. 4 is an end view as seen from the left to the right in the following fig. 5, and Fig. 5 is a cross-section according to IV-IV in fig. 4, Fig. 6 is a side view of a heater device according to the invention with a fuel container arranged on top thereof, Fig. 7 is a cross-section according to VI-VI in fig. 6, Fig. 8 is an end view of a fan arranged on the heater device, Fig. 9 is a cross section according to VIII-VIII in fig. 8, Fig. 10 is a detailed side view of an integrated exhaust gas catalyst element and exhaust gas flow control element positioned inside the heater device, Fig. 11 is a perspective view of the integrate catalyst and flow control element shown in fig.

, Fig. 12 is an end view of a an alternative embodiment of an exhaust gas catalyst element and exhaust gas flow control element as separate parts, Fig. 13 is a cross-section according to XII-XII in fig. 12, Fig. 14 is a perspective view of the exhaust gas catalyst element and exhaust gas flow control element as separate parts shown in figs. 12 and 13, Fig. 15 is a view from above of a part of the Stirling engine, comprising a first heat exchanger, Fig. 16 is a cross section according to XV-XV in fig. 15, and Fig. 17 is a perspective view of the part shown in fig. 15.

DETAILED DESCRIPTION Figs. 1-3 show an embodiment of a Stirling engine according to the present invention. The Stirling engine shown is of gamma type and comprises a crank case 1 with a crank shaft 2 arranged therein, and a displacer cylinder 3 with a reciprocatingly arranged displacer piston 4 therein. The displacer piston 4 is connected to the crank shaft 2 via a connecting rod 5 extending through a first end of said displacer cylinder 3. During operation of the Stirling engine, the displacer cylinder 3 defines a hot chamber 6 and a cool chamber 7 separated by the displacer piston 4.

The Stirling engine further comprises a working cylinder 8 with a reciprocatingly arranged working piston 9 therein, said working piston 9 being connected to the crank shaft 2 via a connecting rod 10 extending through a first end of the working cylinder 8. A working cylinder chamber 11 defined by the working cylinder 8 is divided by the working piston 9 into a first part 12, through which said connecting rod 10 extends, and a second part 13 configured to house a working gas during operation of the Stirling engine. The second part 13 of the working cylinder chamber 11 is in fluid communication with the hot chamber 6 of the displacer cylinder 3 for the transportation of the working gas between said second part 13 of the working chamber 11 and the hot chamber 6 of the displacer cylinder 3 during operation of the engine.

To the crank shaft 2 there is connected an electric generator 48, via which electric power can be transferred from the Stirling engine.

A heater device 14, shown more in detail in fig. 5, is arranged at a second end of the displacer cylinder 3 opposite to said first end and configured to heat a working gas which is present in the hot chamber 6 of the displacer cylinder 3 and which is in fluid communication with the second part 13 of the working cylinder chamber 11. In the embodiment shown the heater device 14 comprises a combustion chamber 15 which is arranged at the second end of said displacer cylinder 3 opposite to said first end.

Furthermore, the Stirling engine comprises a first heat exchanger 16 and a second heat exchanger 17. The first heat exchanger 16 comprises plurality of tubes 18 that extend from a displacer cylinder head 19 provided at said second end of the displacer cylinder 3 into the combustion chamber 15 and out of the combustion chamber 15 to the second heat exchanger 17. The second heat exchanger 17 is comprised by a regenerator provided outside the combustion chamber 15 and outside the displacer cylinder 3. In the embodiment shown the engine also comprises a third heat exchanger 20 formed by a cooler arranged between the regenerator 17 and the working cylinder chamber 11, a first transition flow element 21 provided between said first and second heat exchangers 16, 17, and a second transition flow element 22 provided between the third heat exchanger 20 and the working cylinder 8. The cooler 20 comprises a body with channels 46 for the conduction of the working gas through said body and with further channels 47 which form part of a cooling medium circuit for active cooling of the cooler 20.

The hot chamber 6 defined by the displacer cylinder 3 is in fluid communication with a second end, i.e. the above-defined second part 13, of the working cylinder chamber 11 through a channel comprising the first heat exchanger 16, the second heat exchanger 17, the third heat exchanger 20, the first transition flow element 21 and the second transition flow element 22.

The combustion chamber 15 is defined by a tubular wall element 23, and has a first end which is connected to the displacer cylinder 3 and a second end which is remote from the displacer cylinder 3. There is provided a burner arrangement 21 for generating a flame inside the combustion chamber 15. The heater device 14 comprises means configured to affect said flame to follow an inner periphery of the tubular wall element 23 in a generally tangential direction. A tangential direction is preferably referred to as a direction perpendicular to a longitudinal direction of the tubular wall element 23.

The heater device 14 comprises an exhaust gas outlet 24 in said second end of the combustion chamber 15 and said means configured to affect the flame comprises a fan 25 provided in said second end of the combustion chamber 15. The fan 25 is configured to suck exhaust gas from the centre region of the combustion chamber 15 out of the combustion chamber 15 through said outlet 24 in a longitudinal direction of the combustion chamber thereby generating a vortex flow of gas in a centre region of the combustion chamber 15 towards the second end of the combustion chamber 15.

The fan 25, shown in fig. 5 and more in detail in fig. 9, has a rotational axis x which is coaxial with a longitudinal centre axis of the combustion chamber 15, and the combustion chamber 15 has a generally circular cross section as seen in the direction of a longitudinal axis X thereof. In a preferred operational position of the Stirling engine, the longitudinal axis X of the combustion chamber is generally horizontal.

The burner arrangement 21 comprises an opening 26 in the tubular wall element configured for introduction of solid fuel into the combustion chamber 15, and there is provided an air inlet 27 configured for introduction of air into the combustion chamber for combustion together with said solid fuel. The opening 26 for introduction of solid fuel is arranged in the tubular wall element 23 at a position approximately 90° from a bottom line of the combustion chamber 15 under the provision that the heater device is in an operational position in which the longitudinal axis X thereof extends generally horizontally. There is provided a receiver element 28 at the bottom of the tubular wall element 23, and the receiver element 28 is configured to receive solid fuel falling down from the opening 26 for introduction of solid fuel. The air inlet 27 for the introduction of air is provided in the region of said receiver element 28. The receiver element 28 comprises means for removal of ash therefrom during operation of the engine.

Preferably, there is provided a solid fuel container 29 on top of the heater device 14, from which solid fuel, preferably pellets of a bio material, can be introduced into the combustion chamber through said opening 26 for introduction of solid fuel. A tube 30 extending from the bottom of the solid fuel container 29 to the opening 26 provides for the transportation of fuel from the container 29 to the combustion chamber 15. A feeding screw mechanism may be arranged in the tube 30 or in the container 29 for the purpose of feeding the solid fuel towards or through the tube 30 towards said opening 26.

Said means configured to affect the flame also comprises a baffle plate 31 that is connected to and extends from the inner periphery of the tubular wall element 23 from a level above the opening 26 for introduction of solid fuel. The baffle plate 31 follows the inner periphery of the tubular element 23 with a predetermined distance to said inner periphery from said point to a point beyond said receiver element 28, such that a flame generated by combustion of solid fuel in said receiver element 28 will be given a tangential direction along the inner periphery of the tubular wall element 23 from said receiver element 28 towards a gap 32 defined between a free end 33 of the baffle plate 31 and the inner periphery of the tubular wall element 23 opposite the end of the baffle plate 31 positioned at said level above the opening 26 for introduction of solid fuel. Accordingly, the design and position of the baffle plate 31, in combination with the position of the opening 26 for introduction of solid fuel and the receiver element 28 promotes the generation of a flame directed away from the opening 26. The free end 33 of the baffle plate 31 faces the inner periphery of the tubular wall element 23 at position were a line drawn from the bottom line of the tubular wall element has an angel in the range of 30°-45° relative to a horizontal plane, under provision that the engine is in an operational position in which the longitudinal axis X of the combustion chamber is generally horizontal.

The heater device further comprises a tubular exhaust gas flow controller element 34 which is provided with openings 35 in the mantle surface thereof and which, preferably, is made of metal. The tubular exhaust gas flow controller element 34 extends from the second end of the combustion chamber 15 and has a longitudinal centre axis which is coaxial with the rotational axis x of the fan 25. The tubular exhaust gas flow controller element 34 will prevent the flame from deflecting away from its extension along the inner periphery of the tubular wall element 23 by affecting the flow of the exhaust gases, and promoting the vortex flow generated by the fan 25. It will also radiate heat from the flame back towards the tubes 18 of the first heat exchanger 16, which, as will be explained later, is arranged circumferentially around the exhaust gas flow controller element 34.

The heater device 14 also comprises an exhaust gas catalyst element 36 which is arranged inside the combustion chamber 15, and is attached to and extends from the second end of the tubular wall element 23 and the combustion chamber 15. The exhaust gas catalyst element 36 comprises a cylindrical body having a gas-permeable cylinder wall and having a longitudinal centre axis which is coaxial with the longitudinal centre axis X of the combustion chamber 15. The exhaust gas catalyst element 36 is arranged radially outside the tubular exhaust gas flow controller element 34. The end of the exhaust gas catalyst element 36 closest to said second end of the combustion chamber 15 encircles the exhaust gas outlet 15, such that exhaust gases from the flame will be forced to pass through the cylinder wall of the catalyst element 36 to reach the exhaust gas outlet 24. The catalyst element 36 extends from the second end of the combustion chamber 15 beyond the location of the solid fuel receiver element 28 towards the displacer cylinder head 19, such that it extends over at least 75 % of the length of the combustion chamber 15 as seen from said second end to the displacer cylinder head 19.

The plurality of tubes 18 of the first heat exchanger 16 define a gas-permeable tubular body which has a longitudinal centre axis that is parallel with the longitudinal centre axis X of the combustion chamber 15. The term "gas permeable" is here referred to as meaning that gas can flow through gaps provided between the tubes 18. Each tube 18 is a U-shaped tube that extends from the displacer cylinder head 19 into the combustion chamber 15 and back towards a first end wall 37 of the combustion chamber 15. The end wall is 37 is an annular wall arranged on outer periphery of the displacer cylinder 3. On each of the tubes 18 there are provided flanges 38 configured to improve the heat exchange between the medium in the combustion chamber 15 and the working gas provided inside the tubes 18. The flanges 38 extend generally circumferentially around the respective tube 18 in a plane generally perpendicular to a centre axis of the respective tube 18. The exhaust gas catalyst element 36 is arranged radially inside the tubular body defined by the tubes 18 of the first heat exchanger 16. There is a gap between the outer periphery of the tubular body defined by the tubes 18 and the inner periphery of the tubular wall element 23, such that the flame can, and will, extend in said gap with being in direct contact with the tubes 18.

The heater device further comprises a recuperator 39 comprising an air conduit 40 in fluid communication with ambient air and with the combustion chamber 15 via said air inlet 27. The recuperator 39 further comprises an exhaust gas channel 41 in fluid communication with the combustion chamber 15 via the fan 25, wherein the recuperator 39 is configured to operate as a heat exchanger in which air to be introduced into the combustion chamber 15 via said air inlet 27 is preheated through heat exchange with said exhaust gas in said recuperator 39. The air conduit 40 and the exhaust gas channel 41 are separated by a wall 42, which is preferably made of metal and through which heat is exchanged between the air and the hot exhaust gas. The recuperator 39 has the shape of a tubular body attached to and enclosing the outer periphery of the tubular wall element 23 along a major part of the length of the latter.

The fan 25 has a first set of blades 43 which force exhaust gas from the combustion chamber 15 into the exhaust gas channel 41 of the recuperator 39, and a second set of blades 44 that force ambient air into the air conduit 40 in the recuperator 39, wherein the first and second set of blades are separated by a heat conducting plate 45. In this embodiment the blades 43 of the first set of blades and the blades 44 of the second set of blades are formed by the same blades, wherein the separating heat conducting plate 45 is actually subdivided into a plurality of segments that together form a wall that separates the air conduit 40 from the exhaust gas channel 41.

The tubular wall element 23 that defines the combustion chamber 15 comprises an inner wall element 23a and a thermally insulating coating 23b provided on the outer periphery of the inner wall element 23a. The coating 23b differs from the inner wall element 23a in that it has a substantially lower thermal emissivity than the inner wall element 23a. In the specific embodiment presented here, the inner wall element 23a comprises steel and the coating 23b comprises a ceramic compound, preferably HOSPed zirconium dioxide.

It should be noted that the heater device and combustion chamber that has been disclosed hereinabove has been disclosed as applied to a though preferred but still only exemplifying embodiment of the rest of the Stirling engine. Accordingly, it should be emphasized that the scope of protection claimed is not primarily limited to this specific embodiment, but as defined in the annexed patent claims. For example, the Stirling engine of which the heater device as defined hereinabove is part may be of alfa, beta or gamma type as long as person skilled in the art will not suffer from undue burden when applying the principles of the disclosed heater device to any type of Stirling engine. However, the gamma type of Stirling engine is, for the moment being, preferred.

Claims

1. A Stirling engine comprising: - a crank case (1) with a crank shaft (2) arranged therein, - a displacer cylinder (3) with a reciprocatingly arranged displacer piston (4) therein, said displacer piston (4) being connected to said crank shaft (2) via a connecting rod (5) extending through a first end of said displacer cylinder (3), and wherein the displacer cylinder (3) defines a hot chamber (6) and a cool chamber (7) separated by the displacer piston (4), - a working cylinder (8) with a reciprocatingly arranged working piston (9) therein, said working piston (9) being connected to said crank shaft (2) via a connecting rod (10) extending through a first end of the working cylinder (8), - a heater device comprising a combustion chamber (15) arranged at a second end of said displacer cylinder (3) opposite to said first end, - a first heat exchanger (16) and a second heat exchanger (17), said first heat exchanger (16) comprising at least one tube (18) that extends from a head (19) of the displacer cylinder (3) provided at said second end of the displacer cylinder (3) into the combustion chamber (15) and out of the combustion chamber (15) to the second heat exchanger (17), which is comprised by a regenerator provided outside the combustion chamber (15) and outside the displacer cylinder (3), and wherein the hot chamber defined by the displacer cylinder (3) is in fluid communication with a second end of the working cylinder through the first and second heat exchangers, - wherein the combustion chamber (15) is defined by a tubular wall element (23), and has a first end which is connected to the displacer cylinder (3) and a second end which is remote from the displacer cylinder (3), and -wherein there is provided a burner arrangement (26, 27, 28) for generating a flame inside the combustion chamber (15), said Stirling engine being characterised in that it comprises an exhaust gas catalyst element (36) which is arranged inside the combustion chamber (15) and that the at least one tube (18) of the first heat exchanger (16) is located between an assumed path of the flame and the exhaust gas catalyst element (36), such that heat from the flame that is transferred to the exhaust gas catalyst element (36) will be reflected by the exhaust gas catalyst element (36) back towards said at least one tube (18).

2. A Stirling engine according to claim 1, characterised in that the exhaust gas catalyst element (36) comprises a cylindrical body having gas-permeable cylinder wall having a longitudinal centre axis which is parallel with a longitudinal centre axis (X) of the combustion chamber (15).

3. A Stirling engine according to claim 1 or 2, characterised in that the exhaust gas catalyst element (36) comprises a cylindrical body having gas-permeable cylinder wall that has a longitudinal centre axis which is coaxial with a longitudinal centre axis (X) of the combustion chamber (15).

4. A Stirling engine according to any one of claims 1-3, characterised in that it comprises means (25, 31, 34) configured to affect said flame to follow an inner periphery of the tubular wall element (23) that defines the combustion chamber (15) in a generally tangential direction.

5. A Stirling engine according to claim 4, characterised in that it comprises an exhaust gas outlet (24) in said second end of the combustion chamber (15) and that said means comprises a fan (25) provided in said second end of the combustion chamber (15), which fan (25) is configured to suck exhaust gas from the centre region of the combustion chamber (15) out of the combustion chamber (15) through said exhaust gas outlet (24) in a longitudinal direction of the combustion chamber (15) thereby generating a vortex flow of gas in a centre region of the combustion chamber (15) towards said second end of the combustion chamber (15).

6. A Stirling engine according to claim 5, characterised in that said fan (25) has a rotational axis which is parallel with a longitudinal centre axis (X) of the combustion chamber (15).

7. A Stirling engine according to claim 5 or 6 characterised in that said fan (25) has a rotational axis which is coaxial with a longitudinal centre axis (X) of the combustion chamber (15).

8. A Stirling engine according to any one of claims 5-7, characterised in that the exhaust gas catalyst element (36) comprises a cylindrical body that extends from said second end of the combustion chamber (15) and that has a gas-permeable cylinder wall, wherein the end of the cylindrical body closest to said second end of the combustion chamber (15) encircles the exhaust gas outlet (24).

9. A Stirling engine according to any one of claims 1-8, characterised in that, at least in a region where first heat exchanger (16) is provided in the combustion chamber (15), the combustion chamber (15) has a generally circular cross section as seen in the direction of a longitudinal axis (X) thereof.

10. A Stirling engine according to any one of claims 1-9, characterised in that said burner arrangement comprises an opening (26) in the tubular wall element (23) configured for introduction of solid fuel into the combustion chamber (15), and wherein there is provided an air inlet (27) configured for introduction of air into the combustion chamber (15) for combustion together with said solid fuel.

11. A Stirling engine according to claim 10, characterised in that the opening (26) for introduction of solid fuel is arranged in the tubular wall element (23) in said region and at a position approximately 45°-100° from a bottom line of the combustion chamber (15), and that there is provided a receiver element (28) at the bottom of tubular wall element (23), said receiver element (28) being configured to receive solid fuel falling down from the opening (26) for introduction of solid fuel, and wherein said air inlet (27) for the introduction of air is provided in the region of said receiver element (28).

12. A Stirling engine according to claim 10 or 11, characterised in that said means comprises a baffle plate (31) that is connected to and extends from the inner periphery of the tubular wall element (23) from a level above the opening (26) for introduction of solid fuel, and which follows the inner periphery of the tubular wall element (23) with a predetermined distance to said inner periphery from said point to a point beyond said receiver element (28), such that a flame generated by combustion of solid fuel in said receiver element (28) will be given a tangential direction along the inner periphery of the tubular wall element (23) from said receiver element (28) towards a gap (32) defined between a free end (33) of the baffle plate and the inner periphery of the tubular wall element (23) opposite the end of the baffle plate (31) positioned at said level above the opening (26) for introduction of solid fuel.

13. A Stirling engine according to any one of claims 1-12, characterised in that it comprises a tubular exhaust gas flow controller element (34) with openings (35) in the mantle surface thereof, said tubular exhaust gas flow controller element (34) extending from the second end of the combustion chamber (15) and having a longitudinal centre axis which is coaxial with a rotational axis of said fan (25).

14. A Stirling engine according to claim 13 characterised in that the exhaust gas catalyst element (36) is arranged radially outside the tubular exhaust gas flow controller element (34).

15. A Stirling engine according to any one of claims 1-14, characterised in that the first heat exchanger (16) comprises a plurality of tubes (18) that together define a gas-permeable tubular body which has a longitudinal centre axis that is parallel with a longitudinal centre axis (X) of the combustion chamber (15).

16. A Stirling engine according to claim 15, characterised in that the longitudinal centre axis of said tubular body is coaxial with the longitudinal centre axis (X) of the combustion chamber (15).

17. A Stirling engine according to claim 15 or 16, characterised in that the first heat exchanger (16) comprises a plurality of tubes (18), wherein each tube (18) is a U-shaped tube that extends from a head (19) of the displacer cylinder (3) into the combustion chamber (15) and back towards a first end wall (37) of the combustion chamber (15).