GB2188681A - Regenerative heat engines and heat pumps - Google Patents

Abstract

A heat engine comprises a regenerator 3 on one side of which is a hot chamber 5 and on the other side a cold chamber 8, each equipped with a piston to enable the temperature, pressure and volume of a quantity of gas contained therein to be altered as required. The movement of the pistons is so arranged that the work done on the gas when it passes through the regenerator from the hot to the cold chamber is substantially equal to the work done by the gas when it passes through in the opposite direction. Means of stabilising the temperatures of the chambers are provided. Heat is extracted and net work obtained from expansion of the gas in one or both chambers. The hot chamber may be provided with fuel injection and ignition means for internal combustion (Fig. 8, not shown). The cycle may be reversed and the engine mechanically driven so that it acts as a heat pump. <IMAGE>

Description

SPECIFICATION Regenerative heat engines and heat pumps This invention relates to regenerative heat engines and heat pumps using a gaseous working substance and performing a complete cycle of operations.

Heat engines and heat pumps which use a regenerator to take in heat from a gaseous working substance at one stage of the cycle and give it back to the gas at another stage are familiar to thermodynamicists. The Stirling engine is an example. Here the gas takes in heat from an isothermal source whilst expanding and doing work on a piston. The gas is then passed at constant volume through a regenerator wherein it deposits heat and thus falls in temperature. Next the gas is compressed isothermally, the heat of compression being rejected to a sink. Finally, it is passed through the regenerator in the reverse direction, again at constant volume, to recover from the regenerator the heat that was previously deposited therein, so as to restore the initial state of the gas and complete the cycle of operations.

The Stirling engine is reversible and if mechanically driven may be made to act as a pump, taking in heat from a source at one temperature and delivering heat to a sink at a higher temperature.

The Ericsson engine is also regenerative, but here heat exchange between the regenerator and the gas takes place at constant pressure.

Like the Stirling engine, it is reversible and can then act as a heat pump.

Stirling and Ericsson engines have the disadvantage that the thermodynamic efficiency depends on the temperature range within which they work and is always less than unity. The co-efficient of performance of the pumps is likewise temperature-dependent.

A basic heat engine according to the present invention comprises two chambers separated by a regenerator. Pistons slide within the chambers to permit their volume to be altered as required. One chamber absorbs heat from a source and forms the hot chamber.

The other is at a lower temperature and forms the cold chamber. Three operations take place. First a quantity of gas contained within the hot chamber is allowed to expand isothermally, taking in heat from the source and doing work against the resistance of the hot piston, whilst the cold piston is at rest.

Second, by appropriate movement of both pistons the gas is passed through the regenerator into the cold chamber, depositing heat in the regenerator in such a way that the pressure and volume of the gas change in a predetermined manner and the gas attains the temperature of the cold chamber. Third, the gas is returned through the regenerator by the action of the pistons so that the gas follows a different pressure and volume path, to reach the temperature, pressure and volume of the gas before isothermal expansion and so complete the cycle.The movement of the pistons is arranged to make the work done on the gas when it passes through the regenerator from the hot to the cold chamber equal to the work done by the gas when it passes through in the opposite direction, so that the change in the internal energy plus the work done is the same for each alternate passage through the regenerator.

In the following description of the invention reference is made to cycles and machines illustrated by drawings, as follows: Figure la is the P-V diagram of an ideal cycle in which the working substance passes through the regenerator first at constant volume and then at constant pressure.

Figures ib and ic are the indicator diagrams of the cycle shown in Fig. 1a.

Figure 2a is the P-V diagram of an ideal cycle in which the gas passes through the regenerator at a continuously varied pressure and volume.

Figures 2b and 2c are the indicator diagrams for the cycle depicted in Fig. 2a.

Figure 3a is the P-V diagram for a cycle using low-temperature stabilisation by heat sink.

Figures 3b and 3c are the indicator diagrams for the cycle in Fig. 3a.

Figure 4a is the P-V diagram of a cycle using two heat sources.

Figures 4b and 4c are the indicator diagrams for the cycle depicted in Fig. 4a.

Figure 5 illustrates an engine arrangement suitable for small powers and slow-running machines.

Figure 6 shows an alternative engine design, suitable for large and fast-running machines.

Figure 7a is the P-V diagram of a cycle which coinbines internal combustion and regenerative action.

Figures 7b and 7c are the indicator diagrams for the cycle shown in Fig. 7a.

Figure 8 is a schematic diagram of an engine arrangement using internal combustion plus regenerative action.

Figure 9a is the P-V diagram for the cycle of a heat pump.

Figures 9b and 9c are the indicator diagrams for the cycle shown in Fig. 9a.

Figure 10 is a schematic diagram of a heat pump or heat engine suitable for producing liquid air.

The P-V diagram of Fig. la shows the changes inpressure and volume during the cycle when the working substance, typically a gas, passes through the regenerator, first at constant volume and then at constant pressure. First, the gas in the hot chamber absorbs heat from a source and expands isothermally from a to b. The gas then passes from b to c at constant volume and from c to d at constant pressure whilst depositing heat in the regenerator. Third, the gas passes from d to e at constant volume and from e to a at constant pressure, picking up heat from the regenerator and completing the cycle. The work done on the gas during its passage through the regenerator is represented by the area subtended by c-d.The gas paths through the regenerator are designed so that this area is made the same as that subtended by e-a, and is therefore equal to the work done by the gas on returning through the re generator. It follows that the area enclosed by abcde is equal to the area subtended by a-b, so that the net work done during the cycle is equal to the heat abosrbed isothermally along the path a-b. Fig. 1b is the indicator diagram for the cold chamber of an engine using the cycle in Fig. 1a. The area bounded by a'b'c' d'e' represents the work done on the gas. Fig.

1c is the indicator diagram for the hot cham ber of the same engine. The area bounded by a"b"c"d"e" represents the work done by the gas. The difference between these two areas represents the net work done by the engine.

The P-V diagram of Fig. 2a shows the changes in pressure and volume during the cycle when the working substance passes through the regenerator at varying pressure and volume. First, the gas in the hot chamber absorbs heat from a source and expands isothermally from a to b. Then the gas passes from b to c at varying pressure and volume whilst depositing heat in the regenerator.

Third, the gas passes through the regenerator by a different pressure and volume path, c-a, absorbing heat from the regenerator and completing the cycle. The work done on the gas during its passage through the regenerator is represented by the area subtended by b-c.

The pressure and volume changes of the gas passing through the regenerator are designed so that this area is equal to that subtended by c-a and is therefore equal to the work done by the gas during its return through the regenerator. The area enclosed by abc is equal to the area subtended by a-b, so that the net work obtained is equal to the heat absorbed isothermally along the path a-b Fig. 2b is the indicator diagram for the cold chamber of an engine using the cycle in Fig. 2a. The area bounded by a'b'c' represents the work done on the gas. Fig. 2c is the indicator diagram for the hot chamber of the same engine. The area bounded by a"b"c" represents the work done by the gas. The difference between the two areas represents the work done by the machine.

In the ideal cycle depicted in Figs. 1a and 2a the machines are assumed to work between the temperature limits of the isothermal along which heat is taken in and the lowest point of the cycle: d in Fig. 1a and c in Fig.

2a. In a practical device, particularly a fastrunning machine, in order to absorb enough energy from the source it may be necessary to use a separate heater and take in heat at constant pressure or constant volume and then expand adiabatically down to the isother mal temperature. Heat absorption at constant pressure is generally most convenient, and this is illustrated by the dotted lines aaO in Fig.

2a and a"aO" in Fig. 2c, subsequent adiabatic expansion being along aob and aO"b".

To allow for in-leak to or out-leak from the regenerator and conduction along it, it is necessary to stabilise the lowest temperature of operation of an engine at the intended level. Temperature stabilisation may be achieved in several ways. If the temperature of the source is above ambient, the working substance may be compressed isothermally at the lowest temperature of operation, the heat thus generated then being absorbed by a heat sink such as a supply of air or cold water, as in Stirling or Ericsson engines. Adiabatic compression followed by heat absorption at constant pressure may be used instead. The P-V diagram is shown in Fig. 3a and the indicator diagrams in Figs. 3b and 3c. The full lines cd and c'd' are isothermals.The dotted lines cod and c0,d' are constant pressure paths; ccO and c'c0, are adiabats.

A feature of the present invention is that if the above means of temperature stabilisation is adopted the path along the lower isotherm (or the equivalent adiabatic plus isochoric or isobaric path) may be such that the loss of entropy of the working substance during heat rejection to the sink is less than the gain in entropy from the heat source.

Another method of stabilisation is to raise the temperature of the heat rejected at the lower isotherm to that of the surroundings by means of a heat pump, and then dissipate it.

A better way is to reverse the path along the lower isotherm and allow the working substance to expand and absorb heat from a secondary source. The P-V diagram for such a cycle is depicted in Fig. 4a, and the indicator diagrams in Figs. 4b and 4c. By way of example, the primary source of heat may be at 1200 K and the secondary source at the ambient temperature of 300 K, or the primary source at 300 K and the secondary source at the temperature of liquid air, circa 75 K. Instead of isothermal expansion, absorption of heat at constant pressure, followed by adiabatic expansion, may be used to stabilise the lower temperature of operation, in a similar manner to that described above for heat absorption by a heat sink. If the working substance is air and the lowest temeprature of operation that of the surroundings, the air may be drawn into the cold chamber at the beginning of the cycle, subjected to the required sequence of operations and discharged from the cold chamber at the end. Fig. 7a is the P-V diagram for such a cycle and Figs. 7b and 7c are the indicator diagrams.

An engine arrangement according to my invention, suitable for small powers and slowrunning machines, is depicted in Fig. 5. It comprises a main cylinder 1 communicating with an auxiliary cylinder 2 via a regenerator 3. The cylinder head and upper part of the main cylinder 1 are provided with external fins 4 parallel to the axis of the machine. These accept heat from an external supply of hot gas entering at A and transfer it to the working substance in the expansion space 5 via the internal fins 6 in the cylinder head, parallel to the machine axis. The hot gas leaves at B.

The head of the auxiliary cylinder 2 is similarly provided with internal fins 7 parallel to the machine axis; these transfer heat from the working substance in the compression space 8 to the external fins 9. The latter are maintained at a stable temperature by a stream of air or water which enters at C and leaves at D. A main piston 10 slides within the cylinder 1 and turns the crank shaft 11 by means of the connecting rod 12 and the crank 13. The crown of the piston 10 is provided with fins 14 parallel to the axis of the machine. These are arranged to interpenetrate the fins 6 in the cylinder head as the piston 10 moves into the top dead centre position, so as to displace the maximum amount of working substance from the expansion space 5 through the regenerator 3 to the compression space 8.Thin discs of reflective metal 15 are fitted to the underside of piston 10 to reduce loss of heat from expansion space 5.

Sliding within the auxiliary cylinder 2 is a piston 16, the crown of which is provided with fins 17, parallel to the machine axis, which interpenetrate the fins 7 as the piston 16 moves into its topmost position. The piston 16 is attached to a rod 18 which slides through a crankcase opening 19 and bears on a cam 20 via a cam follower 21, shrouded to prevent rotation of the piston 16. Cam 20 is attached to crankshaft 11. A spring 22 fitted over the piston rod 18 holds the cam follower 21 against the cam 20 so that vertical movement of piston 16 follows the contour of cam 20. The contour of the latter and its angular relation to the crank 13 are designed so as to produce as far as practicable the piston movements required by the indicator diagrams.

By way of example the operation of the machine depicted in Fig. 5 will be described, using the cycle shown in Fig. 3a, the cold space indicator diagram of Fig. 3b and the hot space diagram of Fig. 3c. The working substance is preferably helium.

Hot gas at a temperature in excess of 1200 K enters at A and leaves at B, heating the fins 4 and 6 and the helium in hot space 5 to 1200 K. Air or water enters at C and leaves D, cooling the fins 7 and 9 and the helium in cold space 8 to 300 K. Points a and b in Fig.

3a lie on the isotherm at 1200 K; points c and d lie on the 300 K isotherm.

Referring to Figs. 3b and 3c, at point a' the auxiliary piston 16 is at the top of its stroke, with the fins 7 and 17 interpenetrating each other so as to enclose the minimum volume of helium in the cold space 8. The main piston 10 encloses a volume of 4 units of helium in the hot space 5, at a pressure of 4 units, as indicated by point a".

Operation a-b: The helium in the hot space 5 expands isothermally absorbing heat and doing work on the piston 10, which moves to bottom dead centre and rotates the crankshaft 11 via connecting rod 12. The auxiliary piston 16 remains stationary and the pressure in both the hot and the cold spaces falls to P=3.2 units, as indicated by points b' and b".

Operation b-c: As the crankshaft continues to rotate piston 10 moves to top dead centre, driving the helium from the hot space 5 into the cold space 8 and depositing heat in the regenerator 3. The fins 14 interpenetrate the fins 6 to enclose near-zero volume of gas in the hot space 5 (point c" in Fig. 3c). Cam 20 turns and allows piston rod 18 to descend so that piston 16 reaches the bottom of its stroke (c' in Fig. 3b). The volume of the cold space is now a maximum at V=2.2 units. The pressure in the hot and cold spaces drops to 1.8 units.

Operation c-d: Continued rotation of the crankshaft 11 causes the cam 20 to turn and push the piston rod 18 upwards so that the piston 16 compresses the helium in the cold space 8 isothermally to P=2, V=2 (point d'), the heat of compression being absorbed by the fins 7 and 9 and carried away by the fluid flowing through from C to D. Piston 10 remains substantially at top dead centre (point dt Operation d-a: Further rotation of the crankshaft 11 turns the cam 20 so that the piston 16 advances to the top of its stroke. The fins 17 interpenetrate the fins 7 and all the helium in the cold space 8 is driven into the hot space 5, absorbing heat from the regenerator 3 as it does so. The piston 10 descends, the pressure rises to P=4 and the volume of helium in the hot space reaches V=4, so completing the cycle.

The theoretical net work obtained from the engine is represented by the area abcd of Fig.

3a. This is equal to the net work done by the gas in the hot space, represented by the area a"b"c"d" of Fig. 3c, less the work done on the gas in the cold space, represented by the area a'b'c'd' of Fig. 3b.

An alternative engine design, suitable for large and fast running machines, is illustrated in Fig. 6. It comprises a main cylinder 1 communicating with an auxiliary cylinder 2 via a finned tubular heater 3 and regenerator 4. The heater 3 takes in heat from a supply of hot gas, A, which after passing through the heater 3 flows over external fins 5 on the head and upper part of the cylinder 1, parallel to the machine axis, and leaves at B. The fins and heater tubes transfer heat to the working substance in expansion space 6. The head of cylinder 1 is provided with internal fins 7, parallel to the machine axis, to enhance heat transfer to the working substance. The head of auxiliary cylinder 2 is similarly provided with internal fins 18, parallel to the machine axis. The fins 18 transfer heat from the working substance in compression space 9 to external fins 10.The fins 10 are maintained at a stable temperature by a stream of air or water which enters at C and leaves at D. A main piston 11 slides within cylinder 1 and turns the crankshaft 12 by means of connecting rod 13 and crank 14. The crown of piston 11 is provided with fins 15 parallel to the axis of the machine. The fins 15 are disposed so as to interpenetrate the fins 7 in the head of cylinder 1 as piston 11 moves into the top dead centre position, so as to displace the maximum amount of working substance from expansion space 6 through the regenerator 4 and into compression space 9. Thin discs of reflective metal 16 are fitted to the under side of the crown of piston 11 to reduce loss of heat from expansion space 6.

Sliding within auxiliary cylinder 2 is piston 17, the fins 8 of which interpenetrate the cylinder head fins 18 as piston 17 moves into the top position. Piston 17 is attached to a rod 19 which slides through a crankcase opening 20 and carries at its lower end a clevis 21 which straddles the mainshaft 12.

The clevis 21 is fitted with pins or rollers 22 which engage mirror-image grooves in the faces of twin cams 23 and 24. The shape of the cam grooves and the angular relation of the cams 23 and 24 are designed so as to produce as far as practicable the piston movements required by the indicator diagrams.

The operation of the machine illustrated in Fig. 6 will now be described. The working substance is helium, though air or hydrogen may be used as alternatives. The volume occupied by the heater is assumed to be negligible.

Hot gas at a temperature in excess of 1200 K enters at A, passes over heater tubes 3 and leaves at B, heating fins 5 and 7 and the helium in hot space 6 to 1280 K. Air or water enters at C and leaves at D, cooling the fins 10 and 18 and the helium in cold space 9 to 300 K. In Fig. 3a, ab and cd are isothermals at temperatures of 1200 K and 300 K respectively, and aob and cOc are adiabats, whilst aaO and cod are constant pressure lines; bc and da are the regenerator paths. The engine is designed so that the areas subtended by bc and da are equal.

At the point a the auxiliary piston 17 is at the top of its stroke with the fins 8 and 18 interpenetrating each other so as to enclose near-zero volume of helium in the cold space 9. The main piston 11 encloses a volume of 4 units of helium in the hot space 6 at a pressure of 4 units.

Operation a-a0: As piston 11 descends helium from the regenerator 4 at 1200 K picks up heat from the heater 3; the temperature of the gas increases to 1280 K and the volume to 4.27 units.

Operation aO-b: The helium in hot space 6 expands adiabatically doing work on piston 11, which moves to bottom dead centre and rotates crankshaft 12 via connecting rod 13.

Auxiliary piston 17 remains stationary and the pressure in the hot and cold spaces falls to P=3.2 units.

Operation b-c: As the crankshaft 12 continues to rotate piston 11 moves to top dead centre, driving helium from the hot space 6 into the cold space 9 and depositing heat in the regenerator 4. Fins 15 interpenetrate fins 7 to enclose substantially zero helium in the hot space 6. Rotation of the crankshaft 12 turns cams 23 and 24 and causes piston rod 19 to descend so that piston 17 reaches the bottom of its stroke at c' (Fig. 3b). The volume of the cold space is now a maximum at V=2.2 units and the pressure in the hot and cold spaces drops to 1.8 units.

Operation c-cO: Continued rotation of crankshaft 12 causes cams 23 and 24 to turn and push piston rod 19 upwards so that piston 17 compresses the helium in cold space 9 adiabatically to V=2, P=2.04 (cO in Fig. 3a and c0, in Fig. 3b), the heat of compression being absorbed by the fins 8 and 18 and carried away by the fluid flowing through from C to D. Piston 11 remains substantially at top dead centre (c" and d" in Fig. 3c).

Operation co-d: The fluid flowing through from C to D removes heat from the gas in the cold space at constant pressure and the volume of the gas in the cold space 9 falls to 2 units.

Operation d-a: Further rotation of crankshaft 12 causes piston 1 7 to advance to the top of its stroke (a' in Fig. 3b). The fins 8 interpenetrate fins 18 and all the helium in cold space 9 is driven into hot space 6, absorbing heat from regenerator 4 and heater 3 as it does so. Meanwhile piston 11 descends, and the pressure in the hot space 6 rises to P=4 and the volume to V=4 (a" in Fig. 3c), so completing the cycle.

The theoretical net work obtained from the engine is represented by the area aaobccOd of Fig. 3a. This is equal to the net work done by the gas in the hot space, represented by the area a"a"Ob"c"d" in Fig. 3c, less the work done on the gas in the cold space, represented by the area a'b'c'c',d' in Fig. 3b.

In a heat engine according to the present invention the type of regenerative cycle described above and depicted in Fig. 3a can be combined with a Carnot cycle or with internal combustion engine cycles like the Otto and Diesel, by matching the heat rejected from the one to the heat accepted by the other along a common isotherm. A regenerative Otto cycle is shown in Fig. 7a, with cf as the isotherm.

Air is drawn into the cold chamber, pushed through the regenerator, heated to isothermal temperature, then further compressed, burnt with fuel and expanded down to the isothermal again, the combustion products being finally passed back through the regenerator and discharged to the atmosphere.

A schematic arrangement for a machine using internal combustion plus regeneration is shown in Fig. 8. It comprises a main cylinder 1 containing a hot space D communicating with an auxiliary cylinder 2 containing a cold space C, via a regenerator 3. The cylinder head of auxiliary cylinder 2 is fitted with a spring-controlled valve 4 for admitting air and a spring-controlled valve 5 to allow the release of combustion products to the atmosphere. Cams 7 and 8 operate valves 4 and 5 respectively. The cams 7 and 8 are attached to camshaft 6. Sliding within cylinder 2 and enclosing the cold space C is a piston 9, to which is attached a piston rod 10 passing through a crankcase opening 11. Piston rod 10 bears on a cam 12 via a cam follower 13.

Cam 12 is fixed to a crankshaft 14. Camshaft 6 is driven from crankshaft 14 by gears, chains or other means, at half the speed of the crankshaft. Cylinder 1 is fitted with a fuel injector 15, which injects into hot space D of cylinder 1 a measured quantity of fuel at a pre-determined point in the cycle by means of a pump and metering device, as in established internal combustion engine practice. An ignition device 16, such as a spark plug, is also fitted to cylinder 1, timed and activated as in present internal combustion engine practice. If compression ignition is used the plug may be dispensed with. A primary piston 1 7 slides within cylinder 1 and turns the crankshaft 14 by means of the connecting rod 18 and the crank 19.

A secondary piston 20 also slides within cylinder 1. Piston 20 is attached to a piston rod 21 sliding through a crankcase opening 22. Piston rod 21 carries at its upper end a clevis 23 which straddles the camshaft 6. The clevis 23 is fitted with pins or rollers 24 which engage mirror-image grooves in the faces of twin cams 25 and 26. The shape of the cam grooves and the angular relation of the cams 25 and 26 to the camshaft 6 are such as to produce, as far as practicable, the required movements of the secondary piston 20. Similarly, the shape of cam 12 and its angular relation to crankshaft 14 are designed to give the required movements of piston 9.

The angular relation between camshaft 6 and crankshaft 14 is so arranged that the movements of pistons 9, 17 and 20 and of the valves 4 and 5 are substantially those required at the various stages in the cycle.

The operation of the engine described above and illustrated in Fig. 8 will now be dealt with. The appropriate cycle is shown in Fig. 7a, the cold space indicator diagram in Fig. 7b and the hot space diagram in Fig. 7c.

The working substance is air, which enters at A, the products of combustion leaving at B, as shown in Fig. 8. The volume excursions of the pistons 9, 17 and 20 (Fig. 8) are referred to the datum E, which is level with the top of the regenerator. Movements above E are regarded as positive and those below this datum as negative. Thus for piston 17 T.D.C. is zero and B.D.C. at -2.5 units.

In Fig. 7a, a is at 300 K, the ambient temperature, a -aO represents the induction and exhaust paths, abc and fga are the regenerator paths and cf is the isothermal, whilst cd and ef are adiabats and de is the constant volume combustion path. The isothermal is at 1200 K.

At point a in the cycle piston 9 is at the bottom of its travel and the cold space C encloses a volume of 1 unit of air at atmospheric pressure and a temperature of 300 K.

Piston 17 is at the top of its stroke, level with the datum E. Piston 20 is also level with datum E. Together, pistons 17 and 20 enclose zero volume in hot space D.

Operation a-b: As crankshaft 14 and camshaft 6 turn, piston 9 rises from V - 1 to V= -0.2 and pushes air through regenerator 3 into hot space D, picking up heat as it does so. Piston 17 descends and piston 20 remains stationary, so that the volume of air in the hot space D becomes 0.8 units, the pressure 2.5 units and the temperature 1200 K (b' and b" in Figs. 7b, 7c).

Operation b-c: Further rotation of crankshaft 14 and camshaft 6 causes piston 9 to rise to the level of the datum E, so as to enclose zero volume in cold space C and push the remaining air through regenerator 3 into hot space D. Meanwhile, piston 20 rises to V=+0.7 and piston 17 descends to V=-0.9, giving a hot space volume of 1.6 units (c" in Fig. 7c).

Operation c-d: As the crankshaft 14 and the camshaft 6 rotate, piston 9 remains stationary and piston 17 descends to a volume of V - 1.0 units. Meanwhile piston 20 moves down to datum level E, compressing the gas in hot space D adiabatically to a volume of 1 unit, a pressure of 5.45 and temperature of 1635 K.

Operation d-e: At point d fuel is introduced into hot space D via the injector 15 and ignited by the spark plug 16. The temperature of the gas in the hot space D rises to 1890 K and the pressure to 6.3 units. Pistons 9, 17 and 20 are at rest.

Operation e-f: The burnt gas in space D expands adiabatically and piston 1 7 descends to a volume of V---2.5. The pressure falls to P-- 1.6 and the temperature in hot space D to 1200 K. Pistons 9 and 20 remain stationary.

Operation f-g: Piston 17 pushes part of the combustion products through the regenerator 3 from hot space D into cold space C, depositing heat in regenerator 3. Meanwhile piston 9 retracts to give a volume of -0.5 in the cold space C, at atmospheric temperature and pressure. Piston 20 remains at rest.

Operation g-a: Piston 17 ascends and drives the remaining combustion products in hot space D through the regenerator 3 into cold space C at constant pressure. Piston 20 remains stationary and the volume of gas in the hot space D is zero. Meanwhile piston 9 descends to receive the combustion products at a volume of V= -1.0.

Operation a-a0: Rotation of camshaft 6 actuates exhaust valve 5, which opens. Piston 9 rises and pushes the exhaust products in cold space C through port B. Piston 17 descends to B.D.C. Piston 20, actuated by cams 25 and 26, descends to follow piston 17, so that together they enclose zero volume of hot space D. At a0 exhaust valve 5 closes.

Operation a0-a: Inlet valve 4 opens. Piston 9 descends to a a volume of V= -1.0, drawing in air at atmospheric pressure. Pistons 17 and 20 move to the datum level E, thus completing the cycle.

The theoretical net work obtained from the engine is represented by the area abcdefg of Fig. 7a. This is equal to the net work done by the gas in the hot space D, represented by the area a"b"c"d"e"f"g" in Fig. 7c, less the work done on the gas in the cold space C, represented by the area a'b'c'(d'e')f'g' in Fig.

7b.

The cycles depicted in Figs. 3a, 4a and 9a all meet the requirement that the work done on the gas (the working substance) during its passage through the regenerator in one direction is equal to that done by the gas during its passage in the reverse direction. Hence they are reversible, and a machine based on these cycles may act as an engine and produce mechanical work when supplied with heat, or as a heat pump if driven mechanically, as in the Stirling type of machine. For optimum performance some details of construction and operation may require altering if the role is changed, but the essential principle remains.

A schematic arrangement of a machine suitable for use as a heat pump producing liquid air or as an engine deriving work from two heat sources (ambient air and liquefying air) is shown in Fig. 10. It comprises a main cylinder 1 communicating with an auxiliary cylinder 2 via a regenerator 3. The cylinder head and upper part of the cylinder 1 are provided with external fins 4 parallel to the axis of the machine. A stream of air or water enters at A and leaves at B, and when acting as a pump the fins 4 absorb heat from the working substance in space 5 via the internal fins 6 parallel to the machine axis. When acting as a heat engine the fins 4 and 6 transfer heat to the working substance in space 5. The cylinder head of auxiliary cylinder 2 is similarly provided with internal fins 7 parallel to the machine axis.The fins 7 transfer heat to the working substance in space 8 from external fins 9 on the head of auxiliary cylinder 2. The external fins 9 extract heat from from the column of incoming air which enters at C.

As the air liquefies it is drawn off at D. A main piston 10 reciprocates within cylinder 1 via connecting rod 11, crank 12 and crankshaft 13. The crown of piston 10 is provided with fins 14 parallel to the axis of the machine. The fins 14 are designed to interpenetrate internal fins 6 when the piston 10 is in the top dead centre position, so as to displace the maximum amount of working substance from the space 5 through the regenerator 3 and into space 8. Sliding within auxiliary cylinder 2 is piston 15, the crown of which is provided with fins 16 parallel to the machine axis. Fins 16 are designed to interpenetrate fins 7 as the piston 15 moves into its topmost position and so displace the maximum amount of working substance.The piston 15 is attached to a rod 17 which slides through a crankcase opening 18 and bears on a cam 19 via a cam follower 20, shrouded to prevent rotation of the piston 15. A spring 21 fitted over the piston rod 17 holds the cam follower 20 against the cam 19 so that vertical movement of piston 15 follows the contour of the cam 19. Cam 19 is attached to crankshaft 13, and the shape of cam 19 and its angular relation to the crank 12 are designed so as to produce, as far as practicable, the movements of pistons 10 and 15 required by the indicator diagrams. When operating as an engine with two heat sources the appropriate cycle is that depicted in Fig. 4a, and the indicator diagrams are those in Figs. 4b and 4c. When designed as a heat pump for producing liquid air, the cycle shown in Fig. 9a is relevant, and the indicator diagrams are those in Figs. 9b and 9c. The column of incoming air is freed from water vapour by passage through a labyrinth of perforated metal plates 22 before reaching the external fins 9.

The water vapour condenses on the plates 22, which are removed and defrosted as required. An insulating jacket 23 is fitted around auxiliary cylinder 2 and its extension 24 so as to reduce in-leak of heat from the surroundings.

The operation of an engine deriving work from two heat sources will now be described, using the cycle shown in Fig. 4a, the indicator diagrams of Figs. 4b and 4c, and the engine depicted in Fig. 10. In Fig. 4a, ab and cd are isothermals at temperatures of 300 K and 75 K respectively, and ad and bc are the regenerator paths. The engine is designed so that the areas subtended by ad and bc are equal. The net work done by the working substance is represented by the area subtended by the isothermal ab plus the area subtended by the isothermal cd. The working substance is helium, though hydrogen may be used as an alternative. The pressure in the gas spaces 5 and 8 of Fig. 10 always exceeds that in the crankcase. At point a' of Fig. 4b and a" of Fig. 4c the piston 15 is in its topmost position and expansion space 8 occupies zero volume.Piston 10 is approaching the lower end of its stroke to enclose a volume of 4 units of gas at a temperature of 300 K and a pressure of P=4.

Operation a-b: Piston 15 remains at rest.

The helium in gas space 5 expands isothermally to B.D.C. at a volume of 5 units, taking in heat from the stream of air or water entering at A and leaving at B, via the fins 4 and 6.

Operation b-c: As crankshaft 13 revolves the piston 10 rises to T.D.C., pushing all the gas in expansion space 5 through regenerator 3, where it deposits heat, into gas space 8.

Meanwhile, piston 15 descends to a volume of V=2.0 at a pressure of 2.0 units (c' in Fig.

4b) and a temperature of 75 K.

Operation c-d: Piston 10 remains substantially at T.D.C. Cam 19 turns so that piston rod 17 and piston 15 descend and the gas in space 8 expands isothermally to a volume of 2.2 units at a pressure of P=1.8. As the helium expands it absorbs heat from the gas in expansion space 8, which in turn takes in heat from the column of air in C, via fins 7 and 9. As liquid air forms it is removed at D.

Operation d-a: As the crankshaft continues to rotate the piston 15 rises to its topmost position and drives all the helium in space 8 through the regenerator 3 into space 5, picking up heat as it does so. Piston 10 retracts to a volume of 4 units at a pressure of 4 units and a temperature of 300 K, thus completing the cycle.

The theoretical net work produced by the engine is represented in Fig. 4a by the area abe less the area cde. This is equal to the net work done by the helium in space 5, represented in Fig. 4c by the area a"b"e" minus area c"d"e", less the work done on the helium in space 8, represented in Fig. 4b by the area a'b'e' minus the area c'd'e'. The total net work is equal to the sum of the areas subtended by the isothermals ab and cd.

In the foregoing, although ambient air and liquefying air have been chosen as the primary and secondary heat sources, with appropriate modification to the engine other sources may be used: for example, if hot gases of combustion and the surrounding air are used as heat sources the engine illustrated in Fig. 5 and a cycle similar to that shown in Fig. 4a would be suitable.

The operation of a heat pump suitable for producing liquid air, as depicted in Fig. 10, will now be described, using the cycle shown in Fig. 9a and the indicator diagrams of Figs.

9b and 9c. In Fig. 9a, ab and cd are isothermals at temperatures of 75 K and 300 K respectively, and ad and bc are the regenerator paths. The pump is designed so that the areas subtended by ad and bc are made equal, and the net work down by the pump on the working substance is represented by the area subtended by the isothermal dc less the area subtended by the isothermal ab. The crankshaft of the pump is driven by external power and the preferred working substance is helium.

At point a of Fig. 9a the helium in space 8 is at 75 K and occupies a volume of 1 unit at 1 unit of pressure (a" in Fig. 9c). The piston 10 is at top dead centre to give zero volume in the space 5 (a' in Fig. 9b).

Operation a-b: Piston 10 remains at top dead centre. The helium in space 8 expands isothermally, falling in pressure to P=0.7 at a volume of 1.5 units, and extracts heat from the column of air at C, via the fins 7 and 8.

As it forms around the fins 9, liquid air is withdrawn at D.

Operation b-c: The piston 15, actuated by the cam 19, rises to its topmost position and pushes all the helium in space 8 through the regenerator into the space 5, picking up heat as it does so. Piston 10 descends to bottom dead centre at a volume of 2.5 units and the pressure in space 5 rises to 1.6 units.

Operation c-d: Piston 15 remains at rest, enclosing zero volume in space 8. Piston 10 ascends to reach a volume of V=2.0 units and compresses the helium isothermally to a pressure of 2.0 units, the heat of compression being absorbed by the fins 4 and 6 and carried away by the stream of air or water entering at A and leaving at B.

Operation d-a: Piston 10 rises to top dead centre and pushes the helium in space 5 through the regenerator 3, wherein it deposits heat, into space 8. Piston 15 descends to a volume of V=1.0 and a pressure of 1.0 (a" in Fig. 9c), thus completing the cycle.

The theoretical work required to operate the pump and produce liquid air is represented by the area abcd in Fig. 9a. This is equal to the net work down on the helium in the space 5, represented by the area a'b'c'd' of Fig. 9b, less the work done by the helium in the space 8, represented by the area a"b"c"d" in Fig. 9c.

When used for refrigeration, in a regenerative pump according to the present invention the upper temperature may be fixed by the room temperature or that of a supply of cooling water, and the lower temperature by the value reached when the heat pumped out is equal to the heat leaking in. If the pump is used for heating purposes the lower operating temperature may be that of the atmosphere or a supply of cooling water and the upper temperature that reached when the heat delivered by the pump is balanced by dissipation to the immediate surroundings.

In an engine or heat pump according to the present invention it is provided that the methods of control may be one or more of those already known to the art. For example, in engines similar to those illustrated in Figs. 5 and 6 the pressure of the gas in the working spaces may be altered by means of a feed pump and release valve; or the phase angle between the cam and crank may be changed.

In a regenerative internal combustion engine (Fig. 8) throttling of the air or fuel may be adopted. Intermittent operation or speed variation are suitable for heat pumps based on the machines shown in Figs. 5 and 10.

In any practical embodiment of the present invention minor departures from the theoretical conditions may exist. For example, within the regenerator the work done on the gas may differ slightly from that done by it; heat absorption may be polytropic rather than isothermal; pistons may not follow cam profiles exactly. The basic principles are unaffected.

Claims (14)

1. A basic heat engine comprising two chambers separated by a regenerator, and pistons sliding within the chambers to permit their volume to be altered as required. One chamber absorbs heat and forms the hot chamber and the other is at a lower temperature and forms the cold chamber. A quantity of gas contained within the hot chamber is allowed to expand isothermally, taking in heat from the hot chamber and doing work against the piston, whilst the cold piston is at rest.

By appropriate movement of both pistons the gas is passed through the regenerator in such a way that the pressure and volume of the gas change in a pre-determined manner and the gas attains the temperature of the cold chamber. The gas is then returned through the regenerator by the action of the pistons so that the gas follows a different pressure and volume path, to reach the temperature, pressure and volume of the gas before isothermal expansion and so complete the cycle. The movement of the pistons is arranged to make the work done on the gas when it passes through the regenerator from the hot to the cold chamber equal to the work done by the gas when it passes through in the opposite direction, so that the change in internal energy plus the work done is the same for each alternate passage through the regenerator.

A heat engine as claimed in claim 1, wherein isothermal expansion of the gas in the hot chamber is replaced by constant pressure or constant-volume absorption of heat from the chamber, followed by adiabatic expansion down to the equivalent isotherm, as described herein with reference to Figs.

2 and 3 of the accompanying drawings.

3. A heat engine as claimed in Claims 1 and 2, wherein the lower temperature of the regenerator is stabilised by compressing the gas isothermally and absorbing the heat so produced in a heat sink such as a supply of cooling water or the surrounding air.

4. A heat engine as claimed in Claims 1 and 2, wherein the lower temperature of the regenerator is stabilised by adiabatic compression followed by heat absorption at constant pressure or volume by means of a heat sink such as a supply of water or air.

5. A heat engine as claimed in Claims 1, 2, 3 and 4, wherein the path along the lower isotherm (or the equivalent adiabatic plus isochoric or isobaric path) as described herein with reference to Fig. 3, is such that the loss of entropy of the working substance during heat rejection to the sink is less than the gain in entropy from the heat source.

6. A heat engine as claimed in Claims 1, 2 and 3, wherein the heat rejected at the lower isotherm or equivalent path is raised in temperature to that of the surroundings by means of a heat pump and then dissipated.

7. A heat engine as claimed in Claim 1, wherein the temperature of the heat source is above that of the surroundings and the working substance is air, the latter being discharged and replaced at the end of each cycle, so fixing the lower temperature of operation.

8. A heat engine as claimed in Claims 1 and 2, wherein the regenerative cycle is combined with another cycle, by arranging that the heat rejected from the other cycle is accepted by the regenerative cycle by matching along a common isothermal.

9. A heat engine as claimed in Claim 1, wherein an internal combustion engine cycle is combined with a regenerative cycle, as described herein and illustrated in the accompanying drawings in Figs. 7 and 8.

10. A heat engine as claimed in Claim 1, wherein heat is extracted from two sources at different temperatures, as described herein and illustrated in Figs. 4 and 10.

11. A heat engine as claimed in Claim 1, wherein heat is taken in from two sources, one at the upper temperature of the regenerator and the other at the lower temperature, during separate isothermal expansions of the working substance, as described herein and illustrated in Figs. 4, 5 and 10.

12. A heat engine as claimed in Claims 1 and 11, wherein heat is extracted from the surrounding air and also from liquefying air, to produce liquid air and net work, as described herein and illustrated in Figs. 4 and 10.

13. A heat pump wherein the cycle claimed in Claim 1 is reversed and heat is taken in from a source at one temperature and delivered to a sink at a higher temperature by the expenditure of net work, as described herein and illustrated in Figs. 5 9 and 10.

14. A heat pump or heat engine as claimed in Claims 1 to 13 wherein the work done on the gas when it passes through the regenerator in one direction may differ slightly from the work done by the gas when it passes through in the reverse direction.