GB2194596A

GB2194596A - Reversible Stirling engine

Info

Publication number: GB2194596A
Application number: GB08719852A
Authority: GB
Inventors: Graham Walker
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1986-09-03
Filing date: 1987-08-21
Publication date: 1988-03-09
Also published as: GB8719852D0

Abstract

The engine has at least three power cylinders 1 and, associated with each, a cooler 13, regenerator 14 and heater 15. The engine has a reversing means which comprises a rotatable spool valve 21. The spool valve comprises an outer shell 2 and an inner spool 23. Ports in the outer shell 22 communicate with the compression spaces of the power cylinders and with the coolers. The inner spool 23 has two diametrally opposite sets of passages which coact in register with the ports in the shell 22 such that in one position of the spool 23 the engine rotates in a forward sense and when the spool 23 is rotated by 180 DEG to a second position the engine rotates in the reverse sense. <IMAGE>

Description

SPECIFICATION Stirling engine with reversing means This invention relates to Stirling engines, and especially to means for reversing such engines having a plurality of power cylinders.

A Stirling engine is a reciprocating prime mover of the external combustion kind, and has popularly been known as a hot air engine.

A Stirling engine operates on closed cycle in which the same working fluid eg air, is compressed and expanded at different temperature levels and at different locations, ie the cold compression space and hot expansion space.

General information concerning Stirling engines may be found, by way of example, in the book "Stirling Engines" by G Walker, published by Oxford University Press 1980.

The present invention is concerned especially with a sub-class of Stirling engines, ie the Siemens-Stirling engine. This is an engine having three or more power cylinders; ie it is provided with three or more units, each a basic Stirling engine system, the units being coupled mechanically. The hot expansion space of one cylinder is connected, through heat exchangers-the heater, the regenerator and the cooler-to the cold compression space of another cylinder.

Many different means have been devised for reversing Stirling engines. The means for reversing, according to the present invention, have the advantage of being simple, compact, potentially highly reliable and easy to maintain.

According to the invention, there is provided a Stirling engine having: a) three or more power cylinders; b) a cooler, a regenerator and a heater associated with each power cylinder; and c) reversing means; wherein the reversing means comprises a rotatable spool valve the said spool valve having a cylindrical outer shell co-acting sealably with an inner spool; the said outer shell having ports therein and the said ports communicating on the one hand each with the compression space of a power cylinder and on the other with a cooler; and the said inner spool having two diametrally opposite sets of passages formed therein to co-act in register with the ports in the outer shell; whereby when the said sets of passages are in register with the said ports in one position of the spool the engine tends, in use, to rotate in a forward sense, and when the spool is rotated from said one position through substantially 1800 to a second position the passages and ports are in register in such a different manner that the engine tends to rotate in the reverse sense.

In one particular embodiment of the invention, applied to a Siemens-Stirling engine having three power cylinders, one set of passages in the spool is arranged, in a first position thereof, to connect the compression space of cylinder 1 to the cooler associated with cylinder 2, and the compression space of cylinder 2 to the cooler associated with cylinder 3, while a passage diametrally opposite to said one set of passages connects the compression space of cylinder 3 to the cooler associated with cylinder 1; and when the spool is rotated through substantially 1800 to a second position the passages in the spool connect the compression space of cylinder 1 to the cooler associated with cylinder 3, the compression space of cylinder 2 to the cooler associated with cylinder 1 and the compression space of cylinder 3 to the cooler associated with cylinder 2; whereby the sense of rotation of the engine tends to be reversed as between the first and second positions of the spool.

The invention further provides a rotatable spool valve wherein the spool valve has a cylindrical outer shell co-acting sealably with an inner spool; the said outer shell having ports therein and the said ports being suitable for communication on the one hand each with the compression space of a power cylinder and on the other with a cooler; and the said inner spool having two diametrally opposite sets of passages formed therein to co-act in register with the ports in the outer shell.

The invention will be further described, by way of example, by reference to the accompanying drawings, in which: Figure 1 is a schematic general arrangement of a three cylinder Siemens-Stirling engine; Figure 2 is a cross-section through a power cylinder and associated components of the engine shown in Fig. 1; Figures 3 and 4 are schematic diagrams of passages through a spool valve according to the invention; and Figure 5 is an isometric illustration of passages through a spool for a spool valve as in Figs. 3 and 4.

Referring to Fig. 1 of the drawings, a Siemens-Stirling engine has three power cylinders 1, 2 and 3. Each cylinder is provided with a domed, hollow piston 4. Each piston is connected by a pin 5, and connecting rod 6, to a crank 7, 8, 9, on a crank-shaft 10. The cranks 7, 8, 9 are set at an angle, one to the others, of 1200. Each piston and crank is so proportioned that the crank can rotate inside the piston; each piston being provided with slots, 11, in the walls so that it may reciprocate in the cylinder. The engine is provided with a flywheel 12.

Associated with power cylinder 1 is a complex of cooler 13, regenerator 14 and heater 15 (Fig. 2). The cooler is of the Giaque-Hampson kind, being provided with finned, smallbore tubes 16, (here shown endwise), arranged at close pitch. The tubes carry cold liquid which extracts heat from gas passing over the tubes. The regenerator comprises a porous metal matrix, in the form of wire or strips, indicated by 17, which acts as a heat reservoir. The heater has an outer containment vessel 18. Inside this vessel is a nest of tubes 19 which conduct gas between the regenerator 17 and the manifold 20, communicating with the expansion space of the cylinder. Heat is supplied to the outer surfaces of the tubes by condensing sodium, contained by the vessel 18, and generated by boiling sodium in a separate conventional boiler (not illustrated).To reduce heat loss to surroundings, the heaters may be covered with lagging material (not shown).

The compression space under the piston 4 of the cylinder 1 is connected, for the transfer of gas, to the cooler associated with cylinder 2. The compression space under the piston of cylinder 2 is connected to the cooler associated with cylinder 3; and the compression space under the piston of cylinder 3 is connected back to the cooler associated with cylinder 1. Thus the piston in each cylinder acts at the same time as displacer for transferring gas to another cylinder through its associated regenerator.

In an engine required to rotate in one sense only, and to function consistently as a prime mover, the above mentioned gas connections, between compression spaces and coolers, may be made permanently. However, if the engine is required to reverse its sense of rotation, eg in use for vehicle propulsion; or to be used intermittently as a heat pump, then provision must be made to carry the gas connections between compression spaces and coolers through some reversing means.

In the present invention the reversing means is a rotatable spool valve 21, having a cylindrical shell 22, and an inner spool 23, the spool having passages formed therein, and the outer shell having ports formed therein, for making the necessary gas connections. The inner spool 23 is rotatable within the outer shell 22 about an axis of rotation 24 along the centre of the inner spool 23.

Fig. 3 illustrates, in diagrammatic manner, the gas connections, through the spool valve, between compression spaces and coolers, for an arbitrary forward sense of rotation of the engine. For this setting of the spool valve, Fig. 3a represents a view of the spool 23 from above, on the assumption that the cylinder manifolds 20 (Fig. 2) are above the crankshaft 10; while Fig. 3b represents the bottom of the spool (in that setting thereof) but still as viewed from above-that is as though through the substance of the spool. Fig. 4, in similar manner, illustrates the gas connections for rotation of the engine in a reverse sense; the spool 23 having been rotated about its longitudinal axis 24 through 1800 in relation to the setting shown in Fig. 3.

Referring to Fig. 3a, a duct 25 connects the compression space of cylinder 1 to the cooler associated with cylinder 2, through the passage 26 in the spool, suitable porting being provided in the outer shell 22 of the spool valve communicating with the appropriate ducts. Similarly a duct 27 connects the compression space of cylinder 2 to the cooler associated with cylinder 3, through the passage 28 in the spool. On the diametrally opposite (lower) side of the spool (Fig. 3b) the ducts 29, 30 connect the compression space of cylinder 3 to the cooler associated with cylinder 1 through the passage 31 in the spool.

Referring to Fig. 4a, with the spool 23 shown in the position for reverse sense of rotation of the engine, the ducts 25 and 27 connect the compression space of cylinder 1 to the cooler associated with cylinder 3, through the passage 31 in the spool. On the diametrally opposite (lower) side of the spool (Fig. 4b) the duct 29 connects the compression space of cylinder 3 to the cooler associated with cylinder 2 through the passage 28; similarly the duct 30 connects the compression space of cylinder 2 to the cooler associated with cylinder 1 through the passage 26 in the spool.

The construction of the spool 23, and the passages formed therein is clarified further by Fig. 5 showing the spool in two positions, differing one from the other by rotation through an angle of 1800.

Reversing means of the kind described has the advantage of operating at low temperatures, the gas passing through the spool valve 21 always having been cooled by passing through the coolers 13. Conduction of heat to the compression space through the piston 4 is reduced by constructing the crown of the piston with an internal thermal barrier, eg of ceramic material.

Since the only motion of the spool is rotation through 180 , reversing may be effected simply, according to the physical size of the engine, eg by a lever, an arrangement of worm and wheel, or a pnuematic or electric actuator.

The invention is applicable to engines having three, four or five power cylinders, or in multiples of those numbers.

Claims

1. A Stirling engine having: a) three or more power cylinders; b) a cooler, a regenerator and a heater associated with each power cylinder; and c) reversing means; wherein the reversing means comprises a rotatable spool valve.

2. A Stirling engine according to claim 1 wherein the spool valve has a cylindrical outer shell co-acting sealably with an inner spool;.

3. A Stirling engine according to claim 2 wherein the outer shell has ports therein, the said ports communicating on the one hand each with the compression space of a power cylinder and on the other with a cooler.

4. A Stirling engine according to claim 3 wherein the inner spool has two diametrally opposite sets of passages formed therein to co-act in register with the ports in the outer shell; whereby when the said sets of passages are in register with the said ports in one position of the spool the engine tends, in use, to rotate in a forward sense, and when the spool is rotated from said one position through substantially 180 to a second position the passages and ports are in register in such a different manner that the engine tends to rotate in the reverse sense.

5. A Stirling engine according to claim 4 wherein the engine is a Siemens-Stirling engine having three power cylinders and wherein one set of passages in the spool is arranged, in a first position thereof, to connect the compression space of cylinder 1 to the cooler associated with cylinder 2, and the compression space of cylinder 2 to the cooler associated with cylinder 3, while a passage diametrally opposite to said one set of passages connects the compression space of cylinder 3 to the cooler associated with cylinder 1; and when the spool is rotated through substantially 1800 to a second position the passages in the spool connect the compression space of cylinder 1 to the cooler associated with cylinder 3, the compression space of cylinder 2 to the cooler associated with cylinder 1 and the compression space of cylinder 3 to the cooler associated with cylinder 2; whereby the sense of rotation of the engine tends to be reversed as between the first and second po scions of the spool.

6. A rotatable spool valve wherein the spool valve has a cylindrical outer shell coacting sealably with an inner spool.

7. A rotatable spool valve according to claim 6 wherein the outer shell has ports therein, the said ports being suitable for communication on the one hand each with the compression space of a power cylinder and on the other with a cooler.

8. A rotatable spool valve according to claim 7 wherein the inner spool has two diametrally opposite sets of passages formed therein to co-act in register with the ports in the outer shell.

9. A Stirling engine as hereinbefore described with reference to Figs. 1 and 2 of the attached drawings.

10. A rotatable spool valve as hereinbefore described with reference to Figs. 3 to 5 of the attached drawings.