GB1568057A

GB1568057A - Stirling cycle engines

Info

Publication number: GB1568057A
Application number: GB4679775A
Authority: GB
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1975-11-12
Filing date: 1975-11-12
Publication date: 1980-05-21

Description

(54) IMPROVEMENTS IN OR RELATING TO STIRLING CYCLE ENGINES (71) We, UNITED KINGDOM ATOMIC ENERGY AUTHORITY, of 11 Charles II Street, London SW1Y 4QP, a British Authority, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to stirling cycle heat engines in which a liquid system is displaceable for varying the volume of a gas in hot and/or cold chamber(s) of the engine, and more particularly to Fluidyne (Registered Trade Mark) Stirling cycle heat engines. Examples of such Fluidyne engines are shown in British Patent Specification Nos. 1,329,567 and 1,507,678 to which reference is made for further detailed information.

One of the problems associated with Stirling cycle heat engines is that of ensuring an efficient transfer of heat to and from the working gas in the hot and cold chambers respectively during its operational cycle.

One solution that has been adopted has been the provision of heat exchangers attached to the outside walls of the chambers to transfer heat by conduction through the walls and by convection in the chambers. This reliance on convection within the chambers has disadvantages, particularly where relatively large chambers are concerned, and the invention is therefore concerned with providing an improved means of extracting heat from and imparting heat to the working gas in such chambers.

According to one aspect of the present invention, in a Stirling cycle heat engine having hot and cold variable volume chambers adapted for containing a working gas and intercommunicating through a regenerator, and a liquid system in which liquid is displaceable and arranged for varying the gas volume in each of the chambers, a heat transfer cooling means is provided for cooling the gas flowing from the hot chamber, and the cooling means is arranged to be cooled by the liquid.

Preferably, the liquid is constrained to flow over the cooling means by a pump means at the cold chamber, the pump means being operated by gas pressure changes in the Fluidyne.

Preferably, the cooling means is disposed at or near the cold chamber.

Preferably, a float is provided adapted to float on and substantially cover that portion of the surface of the liquid arranged to vary the volume of the hot chamber so as to separate said liquid from the hot chamber.

According to another aspect of the present invention, a Stirling cycle heat engine comprises hot and cold variable volume chambers adapted for containing a working gas and intercommunicating through a regenerator, a liquid system in which liquid within at least one of the chambers is displaceable for varying the gas volume in the one chamber, means adapted for heating or cooling the liquid so that it is hotter or colder respectively than the gas in the one chamber, and a heat transfer means in the one chamber arranged so that as the liquid and gas oscillate in operation of the engine, they alternately flow over the heat transfer means and thereby transfer heat from one to the other.

According to a further aspect of the present invention there is provided a method of operating a Stirling cycle heat engine having hot and cold variable volume chambers adapted for ccntaining a working gas and intercommunicating through a regenerator and having liquid within at least one of the chambers for varying the gas volume in the one chamber by oscillation of the liquid, which method includes the steps of producing a temperature difference between the liquid and the gas at the one chamber by means acting on the liquid, and arranging that as the liquid and gas oscillate in the one chamber they alternately flow over a heat transfer means whereby heat is transferred from one to the other.

In one arrangement in accordance with the invention, the liquid is arranged to be heated, or cooled, by means immersed in the liquid.

In another arrangement in accordance with the invention, a portion of the liquid is arranged to be circulated through an external heating or cooling means and then fed back to the engine.

To enable the present invention to be more readily understood attention is directed by way of example only to the accompanying drawings, in which: Figure 1 shows in median diagrammatic section a Fluidyne Stirling cycle engine incorporating one form of the invention Figure la shows a diagrammatic view on the line I-I in Figure l; Figure 2 shows in median diagrammatic section a Fluidyne Stirling cycle engine but having an alternative form of the invention; Figure 3 shows in median diagrammatic section a multi-cylinder Fluidyne Stirling cycle engine incorporating one form of the invention; Figure 4 shows in median diagrammatic section a representation of another alternative Fluidyne engine;; Figure 4a shows a sectional representation to an enlarged scale on the line IV-IV of Figure 4; Figure 5 shows in median diagrammatic section the Fluidyne engine of Figure 4 having a modified heat transfer cooling means, and Figure 5a shows in median fragmentary section an alternative arrangement at the cold chamber of the engine of Figure 5.

Referring now to Figure 1, the Fluidyne Stirling cycle heat engine shown comprises two U-tubes 1 and 2, each having limbs of the same cross-section partially filled with liquid 3 (e.g. oil) and 4 (e.g. water) respectively. The space above the liquid 3 in the left-hand limb of the U-tube 1 provides a hot variable volume chamber 5, and the corresponding space in the right-hand limb of this U-tube provides a part 6a of a cold variable volume chamber. The space above the liquid 4 in the left-hand limb of the Utube 2 provides another part 6b of the cold variable volume chamber, and a pipe 11 which connects the part chambers 6a and 6b completes the volume of the cold variable volume chamber. The right-hand limb of Utube 2 is open to atmosphere.The hot chamber 5 and the cold chamber parts 6a and 6b contain a working gas (e.g. air, or helium) and are intercommunicating through a regenerator 12.

An extended surface heat transfer means in the form of a hexagonal cross-section, aluminium honeycomb section 15, is disposed in each of the chambers 5, 6a, and 6b, so as to divide them into a plurality of cavities 16 (not shown in Figure 1) aligned generally parallel with the sides of the Utubes 1 and 2 respectively, but so as to leave a small gap 17 at the top of each chamber 5, 6a or 6b. A sectional view of the honeycomb section 15 on the line I-I is shown to an enlarged scale in Figure la, to which reference is made, and illustrates the crosssection of the cavities 16 formed in each of the chambers 5, 6a and 6b. A honeycomb section 15 of about .003" thick aluminium sheet material, or slightly thinner, and arranged to provide hexagonal section cavities 16 of about 6 mm between parallel flat surfaces has been found satisfactory.

Such a honeycomb section 15 is a compromise between providing an extended surface heat transfer area and avoiding a drag on the liquid in the cavities 16 from capillary forces and viscosity.

Referring again to Figure 1, an electric resistance heating coil 18 is disposed in the liquid 3 near the hot chamber 5, and tubular cooling coils 19, through which cold water is constrained to flow, are similarly disposed in the liquids 3 and 4 near the cold chamber parts 6a and 6b respectively, the direction of flow being indicated by the arrows.

In operation of the engine, as explained more fully in British Patent Specification No. 1,329,567, as the liquid 3 in U-tube 1 oscillates, gas is displaced between the hot chamber 5 and cold chamber parts 6a and 6b flowing through the regenerator 12.

Oscillation of the liquid in U-tube 2 provides the additional volume change required for operation of the engine on a Stirling cycle, and is in the nature of a reservoir system which by its connection to the cold chamber part 6a is capable of changing the total volume of gas within the hot chamber 5 and cold chamber parts 6a and 6b. As the Utubes 1 and 2 are of the same cross-sectional area, the volume changes in the hot chamber 5 and - cold chamber parts 6a and 6b will have equal amplitude and 90" phase difference if the liquid 4 in U-tube 2 lags 450 behind the liquid 3 in U-tube 1 and moves through times the amplitude of the liquid 3 in U-tube 1 to provide optimum performance, although other operating conditions are possible and may be suitable for particular conditions. The pressure change in the gas then has a component in phase with the velocity of the liquid 4 in the U-tube 2 which can be used to overcome viscous fraction in U-tube 2 and to do external work. In order to maintain the amplitude and phase of the liquid 4 in Utube 1 against viscous losses, some of the output power should be returned to the engine so that the liquids 3 and 4 are correctly tuned and resonate in the required phase relationship. Some examples of such engines tuned as aforesaid are shown in British Patent Specification No. 1,329,567 to which reference should be made for further more detailed information.

The liquid' 3 is heated by the heating coil 18 to a temperature higher than that of the gas in the hot chamber 5. During a cycle of the engine, as the level of liquid 3 rises in the hot chamber 5, it flows in the cavities 16 and heats the honeycomb section 15, gas being displaced from the cavities 16 so that it flows through the gap 17 into the regenerator 12 and the cold chamber parts 6a and 6b. As the level of liquid 3 subsequently falls in the hot chamber 5, gas flowing into the chamber from the regenerator 12 flows into the cavities 16 and is heated by the honeycomb section 15 as it flows over its exposed extended surface.

In a similar manner but in the reverse direction in the cold chamber parts 6a and 6b, liquids 3 and 4 are cooled by the cooling coils 19 to a temperature below that of the gas in the cold chamber parts 6a and 6b. As the level of liquid rises in the cold chamber parts 6a and 6b, it flows in the cavities 16 and cools the honeycomb section 15, gas being displaced from the cavities 16 so that it flows through the gap 17 into the regenerator 12 and the hot chamber 5. As the level of liquid 3 and 4 subsequently falls in the cold chamber parts 6a and 6b, gas flowing into them from the regenerator 12 flows into the cavities 16 and is cooled by the honeycomb section 15 as it flows over its exposed extended surface.

The upper level 'u' and lower level '1' to which the liquids 3 and 4 rise and fall in the chambers 5, and part chambers 6a and 6b are shown as chain dotted lines in Figure 1.

The honeycomb section 15 in each chamber 5, 6a and 6b, is preferably arranged, as shown, so that it extends below the lower level '1' to maximise the surface area available for heat transfer between the liquids 3 and 4, and the gas, and also to avoid disturbance caused by liquid dripping from the edges of the cavities 16 in the event of the lower edge of the honeycomb section 15 being exposed. The upper edge of the honeycomb section 15 may be arranged as shown so that it coincides with the upper level 'u', but on the other hand may extend just past or be just below the upper level 'u' without substantially affecting the efficiency of operation of the engine.

An alternative means of heating the liquid 3 may be used, such as a tubular heater through which hot fluid is passed.

In another alternative arrangement liquids 3 and 4 from the U-tubes 1 and 2 may be withdrawn from the U-tubes I and 2 and passed through an external heating or cooling source and then fed back to the respective U-tube 1 or 2, and such an arrangement is shown in Figure 2 to which reference is now made. The engine shown in Figure 2 is similar in all other respects to the engine shown in Figure 1. At the left-hand limb of U-tube 1, liquid 3 is arranged to be circulated through a duct 21 by a pump 22, then through a heat exchanger 23 where it is heated, and then back to the U-tube 1 near the hot chamber 5. Similarly liquids 3 and 4 are each circulated through a duct 24, by a pump 25, then through a heat exchanger 26 where they are cooled, and fed back to the U-tubes 1 and 2 respectively near the cold chamber parts 6a and 6b.If a source of hot or cold liquid 3 or 4 is available, this may be fed through the U-tubes 1 and 2 through the ducts 21 or 24 respectively, instead of recirculating a portion of the liquid 3 or 4 through a heat exchanger 23 or 26. For example, when the engine is used as a water pump, cold water from the outlet of the pump may provide a required source of cold water to be fed through the duct 24 of the Utube 2 or may be passed through the heat exchangers 26.

The invention has been described for simplicity in relation to a "Fluidyne" Stirling cycle engine of basic construction, but it is naturally applicable to other forms of "Fluidyne" Stirling cycle heat engines, for example, those described in British Patent Specification No. 1,329,567.

The invention is also applicable for example to the multicylinder Fluidyne Stirling cycle heat engines described in British Specification No. 1,507,678 (Application No. 50113/74) and an example of such an application in a basic form of engine is shown in Figure 3.

Referring now to Figure 3, a multicylinder Fluidyne heat engine is shown in which there are three pairs of cylinders provided by three U-tubes 31, 32 and 33.

Each U-tube 31, 32 and 33, contains liquid (e.g. water) up to a level such as to define at each end of its limbs a chamber 5 or 6 containing gas and referred to respectively as hot or cold chambers. The hot chambers 5 and cold chambers 6 of the U-tubes 31, 32 and 33 are connected in series by regenerator ducts 12 as shown in the Figure.

As in the engine shown in Figure 1, a honeycomb section 15 is disposed in each hot and cold chamber 5 and 6 respectively, and a heating coil 18 and cooling coil 19 are disposed in the liquid in each U-tube 31, 32 and 33. For further detailed information on the multi-cylinder Fluidyne Stirling cycle heat engine, reference should be made to the aforedescribed British Patent Specification No. 1,507,678 (Application No. 50113/74).

In operation of the engine, the liquid in each U-tube 31, 32 and 33 is arranged to oscillate with a natural frequency of oscillation which is the same for each tube and can be caused to go into oscillation 1200 out of phase with each other. Heat is supplied to the gas in the hot chambers 5, and removed from the gas in the cold chambers 6, via the honeycomb section 15 as described in relation to the engine shown in Figure 1. Instead of the use of a heating coil 18 and cooling coil 19, an external heating and cooling source may be used as described in relation to the engine shown in Figure 2.

Naturally, in a Fluidyne Stirling cycle heat engine, the gas in the hot chamber may be heated via an internal heating means immersed in the liquid in the limb of the Utube, and the gas in the cold chamber may be cooled via an external cooling source through which liquid is circulated from the U-tube, and similar combinations of internal and external heating and cooling of the liquid are possible. If desired only one limb of a U-tube may be adapted to incorporate the invention, but this is most likely to be the limb containing the cold chamber.

Alternative liquids, for example, heat transfer liquids such as Dowtherm (Registered Trade Mark), may be used in the U-tubes 1 and 2 in Figure 1, and the Utubes 31, 32 and 33 in Figure 3, their selection depending on the power and efficiency required from the engine.

Generally if a relatively high vapour pressure liquid is used in a U-tube having a hot variable volume chamber (e.g. U-tube 1 in Figure 1) the engine thermodynamically operates less on a ideal Stirling cycle and more on a vapour cycle than if a relatively low vapour pressure liquid is used, and although more output power may be obtained with the higher vapour pressure liquid the engine would have a lower efficiency. The use of liquids having a relatively low viscosity is also desirable in order to reduce mechanical losses in the engine. Although aluminium is the preferred material for the honeycomb section 15 it may be made from alternative materials provided that they are compatible with the liquid in the U-tube and are relatively corrosion resistant.

As an alternative to the use of a pump for circulating liquid through an external heat exchanger, such circulation may be achieved, for example, by natural convection, or by using the pressure variations in the liquid inside the U-tube in conjunction with valve means.

The wall of the U-tube may be corrugated locally along its length to provide a surface extending into the hot or cold chambers, and thereby provide a heat transfer surface as an alternative to the honeycomb section. Other arrangements for providing a heat transfer means in the hot and cold chambers may be used, such as fins which may also be disposed on the outside wall of the cold chamber to assist in dissipating heat from the cold chamber and in the smaller Fluidyne engines may provide sufficient coqling to enable internal or external cooling means for the liquid to be dispensed with.

Honeycomb sections having cavities with cross-sections such as circular or rectangular may be used as an alternative to the use of hexagonal cross-section cavities, the criteria being to present an extended surface for efficient heat transfer.

In the Fluidyne engines of Figures 1, 2 and 3 evaporation of liquid into the hot chamber may seriously reduce the efficiency of the engine. In the engine of Figure 4, to which reference is now made, the liquid is arranged to be separated from the hot chamber by a float to reduce such evaporation, and an alternative arrangement for transforming heat from the gas to the liquid is shown.

Referring now to Figure 4, the Fluidyne engine shown comprises a U-tube 1 having limbs of unequal length, and an output tube 2 having a smaller bore than the U-tube 1 and connected to the underside of the Utube 1 substantially coaxially with the lefthand limb thereof. The U-tube 1 and output tube 2 are partially filled with a liquid 4 (e.g.

water). The space above the liquid 4 in the left-hand and longer limb of the U-tube 1 provides a hot variable volume chamber 5, and the corresponding space in the righthand and shorter limb provides a cold variable volume chamber 6. The right-hand limb of the output tube 2 is open to atmosphere. The hot chamber 5 and the cold chamber 6 contain a working gas (e.g.

helium or air) and are intercommunicating through a regenerator 12. A hollow float 50 of poor thermal conductivity and made from a material such as stainless steel is disposed on the surface of the liquid 4 below the hot chamber 5 to separate the liquid 4 from the hot chamber 5 and define the upper and lower limits of the stroke of the engine in the hot chamber 5. The float 50 substantially covers the surface of the liquid 4 to leave an annular clearance gap, typically of about 0.1" between the float 50 and the inside surface of the U-tube 1 to reduce the surface area of the liquid 4 from which evaporation into the hot chamber 5 can occur.

A stabilising means in the form of a weight 51 at the end of a rod 52 rigidly connected to the centre of the base of the float 50 is suspended below the float 50 to keep it substantially vertical throughout the stroke of the engine and inhibit any rotational oscillatory movement in the direction of the sides of the U-tube 1.

The effect of the float 50 is to separate the hot working gas from the liquid 4 in that limb of the U-tube 1 containing the hot chamber 5, so that the liquid 4 receives less heat from the hot chamber 5 and consequently reduces the rate of evaporation of the liquid 4. The float 50 desirably has a weight to length ratio with respect to the liquid 4 such that the portion of the float 50 projecting above the surface of the liquid 4 exceeds the length of the stroke of the engine in the hot chamber 5 so that the liquid 4 is therefore confined to portions of the U-tube 1 not warmed by the working gas in the hot chamber 5.

In order to heat the gas flowing from the regenerator 12 into the hot chamber 5, a heating means in the form of a heater assembly 55 is disposed in the gas flow path between the regenerator 12 and the hot chamber 5. The heater assembly 55 as shown in section in Figure 4a to which reference is now made, comprises a hollow rectangular structure 56 of copper having a plurality of inwardly extending heat exchange fins 57 projecting from its inside surface to increase the area of contact with the gas. An electric resistance heating coil 58 is disposed around the outside of the structure 56.

The right-hand limb of the U-tube 1 is similar to that of U-tube 1 of Figure 1. The tubular cooling coil 19 may be dispensed with in the smaller Fluidynes if sufficient heat dissipation from the vicinity of the cold chamber can be achieved by the aforedescribed alternative heat transfer arrangements. A gas output tube 59 may be connected to the cold chamber 59 to provide one way of deriving power from the gas pressure changes in the Fluidyne.

The engine of Figure 4 operates on a Stirling cycle in a similar manner to that aforedescribed in relation to the engine of Figure 1. As the liquid 4 in U-tube 1 oscillates, gas is displaced between the hot chamber 5 and cold chamber 6 and flows through the heater assembly 55 and the regenerator 12. Oscillation of the liquid in the output tube 2 provides the additional volume change required for operation of the engine on a Stirling cycle, and which by its connection to the liquid in the U-tube 1 is capable of changing the total volume of gas within the hot chamber 5 and cold chamber 6. The liquid 4 in the output tube 2 also feeds energy into the liquid 4 in the lefthand limb of the U-tube 1 to maintain the oscillatory movement of the liquid 4 in the U-tube 1.

As the liquid 4 in the U-tube 1 oscillates, the upper and lower limits of the stroke of the engine (shown as chain dotted lines in Figure 4) are defined in the hot chamber 5 by the float 50, whilst the liquid 4 in the Utube I below the hot chamber 5 remains relatively cool so that evaporation thereof is minimised. In the cold chamber 6 as described in relation to the engine of Figure 1, the gas flows through the honeycomb section 15 into the cold chamber 6 and is cooled thereby, and during the next halfcycle the liquid 4 rises in the cold chamber 6 and flows over and therefore cools the honeycomb section 15.

To reduce heat transfer through the float 50 between the hot chamber 5 and the liquid 4, the float may be filled as shown in Figure 1 with a thermal insulating material 53, such as carbon foam, or as an alternative, transverse baffles (not shown) may be incorporated in the float 50 to inhibit thermal convection and radiation. As an alternative to the use of a weight 51, downwardly extending baffles (not shown) may be provided below the base of the float 50, or combined with the use of the weight 51.

Alternative floats 50 may be used, for example, instead of using a metal casing, a solid float made from a closed cell rigid foam material may be used provided it is capable of withstanding the working temperature of the hot chamber 5. The weight 51 may be dispensed with when the float 50 is made from a material which is unlikely to create too much friction if it rubs against the U-tube 1 during the stroke of the engine. A float 50 having a lower portion of an impermeable material (e.g. a metal casing) and an upper portion of carbon foam may be used.

The float 50 may be arranged so that it just, or almost impinges on the underside of the end of the U-tube 1 at the upper limit of the stroke in the hot chamber 5 so as to minimise the unused volume of the hot chamber 5.

As an alternative to using an electric resistance heating coil 58 to heat the heater assembly 55, an alternative heat source may be used, for example an oil, gas, or a solid fuel burner, or solar energy.

Use of the float 50 permits a liquid having a relatively high vapour pressure such as water to be used in the U-tube containing the hot chamber, whereas hitherto a liquid such as oil has had to be used to avert difficulties caused by evaporation into the hot chamber.

Power may also be derived from the Fluidyne engine of Figure 4 by use of the oscillations of the liquid in the output tube 2. The space above the liquid in the output tube 2 may be closed to introduce a resilient force acting on the liquid 4.

It will be appreciated that the float 50 has the effect of raising the hot chamber 5 by the height of the float 50 above the liquid, and since the liquid will remain at the same level in each limb of the U-tube 1, the limb containing the cold chamber 6 should be made shorter than the hot chamber limb so as not to increase the dead volume in the cold chamber 6.

The use of a float may also be incorporated in an alternative form of Fluidyne engine, and a modification of the engine of Figure 4 is shown in Figure 5 to which reference is now made.

In Figure 5 the Fluidyne engine shown is similar to the engine shown in Figure 4 but has an extended surface heat transfer cooling means in the form of a hexagonal cross-section aluminium honeycomb section 60 similar to the heat transfer means 15 of Figure 4 disposed in the cold chamber 6 above the uppermost level of the liquid 4 and in the gas flow path between the regenerator 12 and the cold chamber 6.

A pump 61 is disposed axially in the cold chamber 6, and comprises a hollow cylindrical shell 62 having an inlet tube 63 extending downwardly into the liquid 4 from the base of the shell 62, and an outlet tube 64 extending upwardly from below the centre of the shell 62 through the top of the shell 62 and the honeycomb section 60 and terminating in a liquid distributor 65 extending across the upper surface of the honeycomb section 60. The distributor 65 is provided with a multiplicity of fine holes (not shown) at the underside thereof so as to direct a spray of liquid 4 over the honeycomb section 60. Non-return valves 68 and 69 are disposed in the inlet and outlet tubes 63 and 64 respectively and are arranged to permit only upward flow of liquid 4. A volume of the working gas is arranged to be trapped inside the shell 62 above the lower end of the outlet tube 64.

The engine of Figure 5 operates on a Stirling cycle in a similar manner to that of the engine of Figure 4. Initially, during the operating half-cycle in which the pressure of the gas in the cold chamber 6 rises above the average value, liquid flows through the inlet tube 63 past the non-return valve 68 into the shell 62 compressing the gas therein between the top of the shell 62. During the next half-cycle as the gas pressure falls in the cold chamber 6, the gas in the shell 62 can expand to equalise the gas pressures in the shell 62 and the cold chamber 6, liquid 4 being ejected from the outlet tube 64 and the distributor 65 to flow over and thereby cool the honeycomb section 60. This sequence is then repeated during succeeding cycles.

One of the advantages of the arrangement shown in Figure 5 is that the viscous drag on the liquid in the cold chamber 6 is considerably reduced compared with the arrangements in Figures 1 to 4 in which all the liquid in the cold chamber 6 has to flow through the honeycomb section 60 as it oscillates in the U-tube I to vary the volume of the cold chamber 6. At the same time the advantage is retained of having the honeycomb section 60 cooled by the liquid.

As the working parts of the arrangement shown in Figure 5 are within a sealed enclosure, there are no problems of providing external connections.

External cooling fins (not shown) and a few internal fins if desired may be provided around the U-tube 1 at the cold chamber 6 to enhance the loss of heat to the environment. If desired when relatively large amounts of heat have to be dissipated from the cold chamber 6, for example with the larger Fluidyne engines, means for cooling the liquid 4 may be provided as described in relation to the engines of Figures 1 to 4.

It will be appreciated that the longer the stroke and greater the pressure variations in the cold chamber 6, and hence the greater the heat flux, the greater the amount of liquid 4 flowing through the distributor 65 over the honeycomb section 60 for each cycle of the Fluidyne.

Gas flow tubes (not shown) may be provided through the distributor 64 to avoid introducing a serious resistance in the gas flow from the regenerator 12 to the honeycomb section 60.

A similar arrangement of honeycomb section 60 and pump 61 could be used in the hot chamber 5 of the engine of Figure 5, the float 50 being dispensed with, although such an arrangement might tend to increase the total heat loss and hence reduce the efficiency.

Although the honeycomb section 60 has been shown in the U-tube 1 in Figure 5, it may be installed above the cold chamber 6 of the U-tube 1 in a similar manner to the heater assembly 55 above the hot chamber 5, and may still be cooled by operation of the pump 61 as shown in Figure 5a to which reference is made.

It will be appreciated that the arrangement of honeycomb section 60 and pump 61 may be incorporated in an alternative form of Fluidyne engine.

WHAT WE CLAIM IS: 1. A Stirling cycle heat engine having hot and cold variable volume chambers adapted for containing a working gas and intercommunicating through a regenerator, and a liquid system in which liquid is displaceable and arranged for varying the gas volume in each of the chambers, wherein a heat transfer cooling means is provided for cooling the gas flowing from the hot chamber, and the cooling means is arranged to be cooled by the liquid.

2. A Stirling cycle heat engine comprising hot and cold variable volume chambers adapted for containing a working gas and

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. made shorter than the hot chamber limb so as not to increase the dead volume in the cold chamber 6. The use of a float may also be incorporated in an alternative form of Fluidyne engine, and a modification of the engine of Figure 4 is shown in Figure 5 to which reference is now made. In Figure 5 the Fluidyne engine shown is similar to the engine shown in Figure 4 but has an extended surface heat transfer cooling means in the form of a hexagonal cross-section aluminium honeycomb section 60 similar to the heat transfer means 15 of Figure 4 disposed in the cold chamber 6 above the uppermost level of the liquid 4 and in the gas flow path between the regenerator 12 and the cold chamber 6. A pump 61 is disposed axially in the cold chamber 6, and comprises a hollow cylindrical shell 62 having an inlet tube 63 extending downwardly into the liquid 4 from the base of the shell 62, and an outlet tube 64 extending upwardly from below the centre of the shell 62 through the top of the shell 62 and the honeycomb section 60 and terminating in a liquid distributor 65 extending across the upper surface of the honeycomb section 60. The distributor 65 is provided with a multiplicity of fine holes (not shown) at the underside thereof so as to direct a spray of liquid 4 over the honeycomb section 60. Non-return valves 68 and 69 are disposed in the inlet and outlet tubes 63 and 64 respectively and are arranged to permit only upward flow of liquid 4. A volume of the working gas is arranged to be trapped inside the shell 62 above the lower end of the outlet tube 64. The engine of Figure 5 operates on a Stirling cycle in a similar manner to that of the engine of Figure 4. Initially, during the operating half-cycle in which the pressure of the gas in the cold chamber 6 rises above the average value, liquid flows through the inlet tube 63 past the non-return valve 68 into the shell 62 compressing the gas therein between the top of the shell 62. During the next half-cycle as the gas pressure falls in the cold chamber 6, the gas in the shell 62 can expand to equalise the gas pressures in the shell 62 and the cold chamber 6, liquid 4 being ejected from the outlet tube 64 and the distributor 65 to flow over and thereby cool the honeycomb section 60. This sequence is then repeated during succeeding cycles. One of the advantages of the arrangement shown in Figure 5 is that the viscous drag on the liquid in the cold chamber 6 is considerably reduced compared with the arrangements in Figures 1 to 4 in which all the liquid in the cold chamber 6 has to flow through the honeycomb section 60 as it oscillates in the U-tube I to vary the volume of the cold chamber 6. At the same time the advantage is retained of having the honeycomb section 60 cooled by the liquid. As the working parts of the arrangement shown in Figure 5 are within a sealed enclosure, there are no problems of providing external connections. External cooling fins (not shown) and a few internal fins if desired may be provided around the U-tube 1 at the cold chamber 6 to enhance the loss of heat to the environment. If desired when relatively large amounts of heat have to be dissipated from the cold chamber 6, for example with the larger Fluidyne engines, means for cooling the liquid 4 may be provided as described in relation to the engines of Figures 1 to 4. It will be appreciated that the longer the stroke and greater the pressure variations in the cold chamber 6, and hence the greater the heat flux, the greater the amount of liquid 4 flowing through the distributor 65 over the honeycomb section 60 for each cycle of the Fluidyne. Gas flow tubes (not shown) may be provided through the distributor 64 to avoid introducing a serious resistance in the gas flow from the regenerator 12 to the honeycomb section 60. A similar arrangement of honeycomb section 60 and pump 61 could be used in the hot chamber 5 of the engine of Figure 5, the float 50 being dispensed with, although such an arrangement might tend to increase the total heat loss and hence reduce the efficiency. Although the honeycomb section 60 has been shown in the U-tube 1 in Figure 5, it may be installed above the cold chamber 6 of the U-tube 1 in a similar manner to the heater assembly 55 above the hot chamber 5, and may still be cooled by operation of the pump 61 as shown in Figure 5a to which reference is made. It will be appreciated that the arrangement of honeycomb section 60 and pump 61 may be incorporated in an alternative form of Fluidyne engine. WHAT WE CLAIM IS:

1. A Stirling cycle heat engine having hot and cold variable volume chambers adapted for containing a working gas and intercommunicating through a regenerator, and a liquid system in which liquid is displaceable and arranged for varying the gas volume in each of the chambers, wherein a heat transfer cooling means is provided for cooling the gas flowing from the hot chamber, and the cooling means is arranged to be cooled by the liquid.

intercommunicating through a regenerator, a liquid system in which liquid within at least one of the chambers is displaceable for varying the gas volume in the one chamber, means adapted for heating or cooling the liquid so that it is hotter or colder respectively than the gas in the one chamber, and a heat transfer means in the one chamber arranged so that as the liquid and gas oscillate in operation of the engine they alternately flow over the heat transfer means and thereby transfer heat from one to the other.

3. A Stirling cycle heat engine comprising hot and cold variable volume chambers adapted for containing a working gas and intercommunicating through a regenerator, a liquid system in which liquid is displaceable and arranged for varying the gas volume in each of the chambers, a heat transfer cooling means at the cold chamber arranged so that as the liquid and gas in the cold chamber oscillate in operation of the engine said gas and liquid alternately flow over the heat transfer means and transfer heat from one to the other, and a float adapted to float on and substantially cover that portion of the surface of the liquid arranged to vary the volume of the hot chamber so as to separate said liquid from the hot chamber.

4. An engine as claimed in any one of the preceding claims, wherein the liquid is arranged to be cooled by cooling means immersed in the liquid.

5. An engine as claimed in Claim 1 or Claim 2, wherein the liquid is arranged to be heated by means immersed in the liquid.

6. An engine as claimed in any one of the preceding Claims, wherein a portion of the liquid is arranged to be circulated through an external cooling or heating means and then fed back to the engine.

7. An engine as claimed in any one of the preceding Claims, wherein the engine is adapted to drive a water pump, and a portion of the liquid in the engine is arranged to be cooled by water discharged from the pump.

8. An engine as claimed in claim 1 or Claim 2, wherein a float is provided and adapted to float on and substantially cover that portion of the surface of the liquid arranged to vary the volume of the hot chamber so as to separate said liquid from the hot chamber.

9. An engine as claimed in Claim 3 or Claim 8, wherein that portion of the float projecting above the surface of the liquid exceeds the length of the engine stroke in the hot chamber.

10. An engine as claimed in Claim 3 or Claims 8 or 9, wherein the float is adapted to inihibit the transfer of heat therethrough between the hot chamber and the liquid.

11. An engine as claimed in any one of the preceding Claims, wherein at least a portion of the heat transfer means is disposed above the uppermost level of the stroke of the liquid in the cold chamber and the liquid is constrained to flow over said portion by pump means disposed in the engine.

12. An engine as claimed in Claim 11, wherein the pump means is operated by gas pressure changes in the engine.

13. An engine as claimed in any one of the preceding Claims, wherein a gas outlet is provided at the cold chamber for deriving power from the engine.

14. An engine as claimed in Claim 3 or any one of Claims 8 to 10, wherein a heating means to heat the gas is provided in the gas flow path between the hot chamber and the regenerator.

15. An engine as claimed in any one of the preceding Claims, wherein the heat transfer means comprises an extended surface metal honeycomb section.

16. A method of operating a Stirling cycle heat engine having hot and cold variable volume chambers adapted for containing a working gas and intercommunicating through a regenerator and having liquid within at least one of the chambers for varying the gas volume in the one chamber by oscillation of the liquid, which method includes the steps of producing a temperature difference between the liquid and the gas at the one chamber by means acting on the liquid, and arranging that as the liquid and gas oscillate in the one chamber they alternately flow over a heat transfer means whereby heat is transferred from one to the other.

17. A Stirling cycle heat engine substantially as hereinbefore described with reference to Figures 1 and la, or Figure 2, or Figure 3, or Figures 4 and 4a, or Figure 5, or Figures 5 and 5a of the accompanying drawing.