WO2005066483A1

WO2005066483A1 - A displacer piston

Info

Publication number: WO2005066483A1
Application number: PCT/GB2005/000043
Authority: WO
Inventors: John Stephen Williams; William Henry Cross
Original assignee: Microgen Energy Limited
Priority date: 2004-01-08
Filing date: 2005-01-07
Publication date: 2005-07-21
Also published as: GB0400360D0; JP2007518018A; EP1706625A1

Abstract

A method of constructing a displacement piston for a Sterling engine. The piston comprises a hollow housing (1) and a stack of baffles (3). A first baffles (3A) is inserted into the hollow housing (1) and is permanently fixed in place. Further baffles have been inserted and each is located on the previously inserted baffles that is not permanently fixed to the housing. The final baffle is then inserted and is permanently fixed to the housing.

Description

A DISPLACER PISTON

The present invention relates to a displacer piston for a Stirling engine, and in particular for a linear free piston Stirling engine.

Such a piston is employed to reciprocate in a Stirling engine to transfer gas between the hot and cold spaces thereby causing a power piston to reciprocate inside an alternator coil to generate electricity.

It is essential to the efficient operation of the Stirling engine that the heat is transferred between hot and cold spaces via the gas flowing through a regenerator, and not directly through the displacer piston body. In order to achieve this, a displacer piston comprises a hollow casing which is fitted with a number of internal baffles which aid the heat management within the displacer. The baffles are spaced at regular intervals along the displacer piston to minimise internal stresses and, in order to minimise operational noise from within the engine, they are individually spot welded into position. This is a time consuming process requiring accurate quality control to ensure that all baffles are secure before the displacer piston is sealed within the engine.

The present invention aims at simplifying the construction of the displacer piston to provide a component which is suitable for mass production.

According to a first aspect of the present invention, there is provided a method of constructing a displacer piston for a Stirling engine, the displacer piston comprising a hollow housing and a stack of baffles, the method comprising the steps of inserting a first baffle into the hollow housing and permanently fixing it in place, inserting at least one further baffle and locating it on the previously inserted baffle, but not permanently fixing it to the housing, inserting a final baffle and permanently fixing it to the housing. Such an arrangement reduces the number of permanent fixings (e.g. spot welds) and provides baffles which are self-locating ensuring that the correct spacing and a secure fixing are achieved. Preferably, each baffle has a reduced diameter portion sized to fit within an adjacent baffle. This allow each baffle to align with respect to the previous baffle, as well as aligning with respect to the housing. Preferably, the housing and baffles are each provided with at least one respective through hole. This provides a gas path into the piston so that if the Stirling engine is filled with the working gas, this working gas can fill the displacer piston.

The space between the baffles may be filled with insulation, but in order to simplify the construction and keep the weight to a minimum, the space between adjacent baffles is preferably filled only with gas.

The first baffle may be inserted to a predetermined depth within the housing on a jig which is moved to a predetermined position. Alternatively, the first baffle may locate on a shoulder.

The first aspect of the present invention also extends to a displacer piston for a Stirling engine comprising a hollow housing and a stack of baffles, wherein the baffles at either end of the stack are permanently fixed to the housing and wherein the intermediate baffles are located on adjacent baffles and are not permanently fixed to the housing.

According to a second aspect of the invention there is provided a method of constructing a displacer piston for a Stirling engine comprising a hollow housing and a stack of baffles, the method comprising inserting each baffle into the housing and snap fitting each baffle into a respective groove within the housing.

With such an arrangement, no permanent fixing of the baffles to the housing is required.

Preferably, the radially outermost edge of each baffle has a tapered profile enabling each baffle to readily pass grooves which are provided for later fitted baffles .

The second aspect of the invention also extends to a displacer piston for a Stirling engine comprising a hollow housing with a plurality of spaced grooves in its internal wall, and a respective baffle which is a snap fit into each groove. Preferably, the outer peripheries of each baffle have a tapered profile. Examples of displacer pistons in accordance with the present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a cross-section through a baffle according to a first aspect of the present invention;

Fig. 2 is a cross-section through a displacer piston according to a first aspect of the present invention;

Fig. 3 is a view similar to Fig. 1 of a second aspect of the present invention; and

Fig. 4 is a view similar to Fig. 2 of the second aspect of the present invention.

The first aspect of the present invention will described with reference to Figs. 1 and 2. The displacer piston comprises a hollow housing known as a can 1 which is open at one end. The opposite end is provided with a number of holes 2 (typically 8-12 having a diameter of 0.2 mm) for reasons described below. A series of baffles 3 are arranged along approximately two thirds of the length of the can. Each baffle is also provided with a number of holes 4 (typically 6-8 having a diameter of 1 mm) for reasons described below. The housing and baffles may be made of stainless steel (e.g. type 304). Alternatively, they may be aluminium or high temperature plastic. Each baffle 3 has a circular open cup shape configuration as shown in Fig. 1. Each baffle is provided with an annular shoulder 5 on which an adjacent baffle can locate. In order to assemble the displacer piston of Fig. 2, a first baffle 3A is inserted into the can on a j ig which locates it at the correct position. In this position, the baffle 3A is spot welded in place as represented by arrows 'A' . Alternatively, a different fixing, such as a high temperature adhesive could be used. Each subsequent baffle is then inserted into the can and locates on shoulder 5, but is not welded into place. Finally, the last baffle 3B is placed into the can and locates on the shoulder 5 of the previous baffle. This final baffle 3B is then spot welded in place as represented by arrows ' B ' .

With the baffles in place, a displacer body 6 is inserted into the open end of the can 1 with location grooves 7 ensuring a firm, sealed connection. The displacer body 6 combines elongate hollow axial tubular member 8 which, in use, extends through the power piston (not shown) . The displacer body 6 also has a female screw thread for a flexible rod (not shown) which, in use, extends along the tubular element 8 and is supported via a resilient member on the engine casing to allow the displacer to reciprocate as is well known in the art. Once the engineer is assembled, it is filled with a working gas, such as helium. The working gas enters the can 1 via holes 2 and subsequently passes through the holes 4 into the spaces between the baffles 3. However, due to the small size of the holes and the fact that there is no gas path through the displacer body 6, the holes do not have a detrimental effect on the operation of the displacer in use. A second example of the invention will now be described with reference to Figs . 3 and 4.

The basic structure of the displacer piston is the same as shown in Figs. 1 and 2, with the exception that the can is provided with a series of internal annular grooves 10 axially spaced along the can and each baffle is provided with a tapered lip 11 at its radially outmost edge. In order to assemble this displacer piston, each baffle 2 is pushed into the can 1 from the open end. The tapered lip 11 allows the baffle to snap in and out of the grooves 10 as it travels to the correct location. This time, it will snap into the correct groove and, other than the first baffle, will also locate on the shoulder 5 of the previously inserted baffle.

Claims

1. A method of constructing a displacer piston for a Stirling engine, the displacer piston comprising a hollow housing and a stack of baffles, the method comprising the steps of inserting a first baffle into the hollow housing and permanently fixing it in place, inserting at least one further baffle and locating it on the previously inserted baffle, but not permanently fixing it to the housing, inserting a final baffle and permanently fixing it to the housing.

2. A method according to claim 1, wherein each baffle has a reduced diameter portion sized to fit within an adjacent baffle, to locate the baffle in position.

3. A method according to claim 1 or claim 2, wherein the housing and baffles are each provided with a respective through hole .

4. A method according to any preceding claim, wherein the space between adjacent baffles is filled only with gas.

5. A method according to claim 1, wherein the first baffle is inserted to a predetermined depth within the housing on a jig which is moved to a predetermined position.

6. A displacer piston for a Stirling engine comprising a hollow housing and a stack of baffles, wherein the baffles at either end of the stack are permanently fixed to the housing and wherein the intermediate baffles are located on adjacent baffles and are not permanently fixed to the housing.

7. A piston according to claim 6, wherein each baffle has a reduced diameter portion sized to fit within an adjacent baffle, to locate the baffle in position.

8. A piston according to claim 6 or claim 7, wherein the housing and baffles are each provided with a respective through hole.

9. A piston according to any of claims 6 to 8 , wherein the space between adjacent baffles is filled only with gas.

10. A method of constructing a displacer piston for a

Stirling engine comprising a hollow housing and a stack of baffles, the method comprising inserting each baffle into the housing and snap fitting each baffle into a respective groove within the housing.

11. A method according to claim 10, wherein the radially outermost edge of each baffle has a tapered profile enabling each baffle to readily pass grooves which are provided for later fitted baffles.

12. A method according to claim 10 or claim 11, wherein the housing and baffles are each provided with a respective through hole .

13. A method according to any of claims 10 to 12, wherein the space between adjacent baffles is filled only with gas.

14. A displacer piston for a Stirling engine comprising a hollow housing with a plurality of spaced grooves in its internal wall, and a respective baffle which is a snap fit into each groove .

15. A piston according to claim 14, wherein the housing and baffles are each provided with a respective through hole.

16. A piston according to claim 14 or 15, wherein the space between adjacent baffles is filled only with gas.