This invention relates to a machine which operates in the principle of a Stirling Engine and will be referred to herein as a Stirling Machine.

The basic principle of the Stirling machine is simple and easily explained in that it operates on the principle of the tendency of a gas to expand or rise in pressure when heated, but although the principle is simple, putting the theory into practise to produce a practical machine presents the engineer with a difficult and complex problem.

The Stirling machine can be explained in that a fixed amount of gas is contained in a working volume consisting of at least one space that is maintained at a high temperature and another at a lower temperature. By means of piston movements, some of the gas is transferred back and forth between hot and cold spaces; when more gas is in the hot space, the pressure rises and when the gas is transferred back to the cold space, the pressure falls again. The rising and falling of the pressure effects movement of at least one piston from which useful work can be extracted. In a linear Stirling machine having a linearly displaceable power piston, the machine also has a displacer piston which is responsible for transferring the gas. The principle of the Stirling machine can also be embodied in a rotary machine wherein the pistons are rotary members sweeping through lobed cavities into and out of which the air is charged and discharged.

Rotary pistons offer a number of advantages over and above their 15 near counterparts insofar as they provide for compact design, reduced sealing problems which are typically limited to the outgoing rotating shaft of the machine, and

moreover rotary pistons provide for a fully balanced machine at all operating conditions resulting in a silent and practially vibrationless operation. In addition to the above advantages associated with a rotary piston design there are also the advantages normally associated with Stirling machines such as: multifuel capability with external continuous conmbustion giving better control of emission; high thermal efficiency; low noise due to continuous combustion; a "flat" torque curve.

The present invention has particular application to rotary machines working on the Stirling principle. A particularly suitable example of a rotary piston Stirling machine is described in U.S. Patent 4,179,890 wherein epitrochoidal rotory pistons are contained in lobed chambers at respective ends of the machine, one of the ends being a hot end to which heat is applied, and the other being the cold end and housing the rotory piston from which the power of the machine is extracted. The present invention has particular application to a rotory piston Stirling machine having a hot end and a cold end each comprising a rotory piston in a lobed cavity or chamber and passage means connecting the chambers of the respective ends whereby gas contained in the chambers and in said passage means may be displaced back and forth between the chambers as the machine operates, and wherein the rotory pistons are interconnected by a means enabling the adjustment of the phase angle between the pistons.

The means permitting adjustment of the phase angle preferably is such as to permit reduction of the phase angle between the pistons from a maximum of 90° to a value in the order of 20% of the maximum phase angle.

The advantage of providing a means for adjustment of the phase angle in accordance with the invention it has been discovered is that with adjustment of the phase angle although the power input from the machine is varied, and from the 90° position (which is the recommended phase angle position for a two-piston rotory Stirling machine) the power reduces, and reduces substantially, there is not in fact the same extent of reduction in the thermal efficiency. It is therefore desireable that there should be a means of adjustment of the relative phase angle and such means is provided by the present invention.

By way of example, in a two-piston rotory Stirling machine running at a fixed speed of 3,250 rpm reduces by the order of 60-70% as between the position between which the rotors are at a 90° phase angle compared to the rotors being at a 35° phase angle, in fact the reduction in thermal efficiency is from 45% to 28%. At lower speeds, the reduction in thermal efficiency is even smaller. Thus, at a rotational speed of 1450 rpm the reduction in thermal efficiency is from the order of 65% to 58% although in fact the power output is halved as between the 90° phase angle position and the 35° phase angle position.

Moreover, it will also be understood that there are advantages to be gained from reversing the above principles of operation of the Stirling machine whereby power is supplied to the machine and heat extracted from the environment outside the machine since when the rotors are 90° out of phase, by driving the engine using an external workforce, it is possible to provide a refrigeration machine. This is because gas expanding at constant temperature in the cold end of the engine must be provided with an external heat supply. Thus during operation of the machine heat is absorbed from the environment at the cold end of the machine.

As a heat transfer machine the Stirling machine is well suited to operate at any temperature used in domestic/industrial refrigeration and heat pump applications. Also, the heat transfer control incorporated in this invention makes it possible to adjust the heat transfer and power input without any serious penalty in the performance as illustrated in Fig. 4 when the COP varies from 2.5 - 3.7 over a heat transfer range of 17 to 45 kw. In a heat pump/air conditioning system this would be very useful as the machine would be capable of operating at varying ambient temperatures and heat transfer demands.

By way of example, in a two-piston rotary Stirling machine running at a fixed speed of 750 rpm, wherein the co-efficient of performance (COP) is higher than at 1500 rpm due to reduced friction losses inside the machine, the heat pump performance is increased from 34% to 90% when the relative angular position between the rotors are changed from a 20° phase angle to a 90° phase angle. Preferably the working medium is helium, but the machine can be equally well operated on air although the COP may be somewhat reduced due to increased flow friction losses.

The application of a Stirling machine as a means of refrigerating an environment is currently particularly attractive as the gas used in a Stirling machine may be helium or air thus this appliction provides for a refrigeration system which obviates the need to use environmentally dangerous substances such as CFC's.

The means for permitting relative phase adjustment preferably comprises a gear train which may in one example comprise a gear cluster including a sun wheel, planet wheels in engagement therewith, and an annulus engaging the planet

wheels, the planet wheels of the respective clusters being mounted on common shafts. The annulus of one of the gear clusters is adjustable by suitable means such as a pneumatic ram, lever or the like which can be turned relative to the annulus of the other cluster thereby effecting an operation in the phase angle as between the respective rotory pistons.

Preferably the pistons are of epitrochoidal configuration, although this is not considered to be necessary to the principle of the invention.

Indeed, although specific reference has been made hereinbefore to rotory piston Stirling machines, it is believed that with suitable adaptation and modification, the invention may be applied to linear Stirling machines.

The passage means connecting the cavities of chambers may, as is conventional in many Stirling machines, be provided with a heater, a regenerator and a cooler. The purpose of- the regenerator is to provide a temperature gradient so that as the gas passes from the hot end towards the cold end, the gas cools gradually by giving up heat to the metal of the regenerator. Therefore, the gas leaves the regenerator already cooled, minimising the amount of heat to be rejected. On the return journey, the gas is gradually heated as it moves up the temperature gradient of the regenerator by picking up the heat that was deposited during the pass in the opposite direction. The gas emerges into the hot end already hot, minimising the heat to be supplied by the external source.

The additional heater and cooler, which are designed with high surface areas and narrow passages that minimise the heat conduction path through the gas, serve to add the heat absorbed during the expansion phase and to reject the heat

generated during the compression stage.

The rotory pistons co-operate with the cavities to define two chambers at each end of the machine and the respective hot chambers are connected with the respective cold chambers by means of said passage means.

The advantages of the Stirling machine are well known, and in a first conventional use, include principally the advantage that the machine is operated by the supply of external heat and there is therefore for example no internal combustion or the use of steam which has to expand from water to vapour to create the power source. The provision of the invention in providing for the adjustability of the phase between the pistons increases the machine's utility.

The advantages of the Stirling machine in a second less conventional use include principally the advantage that the machine is operated by an external power supply resulting in the machine acting as a heat pump, thus a means of refrigerating an environment is created without the need to resort to the use of CFC's.

For more details concerning the Stirling machine, its principle and its application, reference is made to the article published in

An embodiment of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, wherein:-

Fig. 1 is a graph of power output characteristics and thermal efficiency of a rotory Stirling machine according to the invention at a range of rotary speeds and relative phase angle displacement;

Fig. 2 is a sectional elevation of the Stirling machine according to the embodiment of the invention;

Fig. 3 is a sectional elevation through the respective rotors as indicated by the section on lines AA-BB shown in Fig. 2;

Fig. 4 shows a graph illustrating the coefficience of performance and heat pump efficiency of a Stirling machine co-operates as a heat extraction;

Fig. 5 is a diagram showing a first stage in a Stirling cycle wherein a Stirling machine is described as operating as a heat pump;

Fig. 6 is a diagram showing a second stage in a Stirling cycle wherein a Stirling machine is described as operating as a heat pump;

Fig. 7 is a diagram showing a third stage in a Stirling cycle wherein a Stirling machine is described as operating as a heat pump;

Fig. 8 is a diagram showing a fourth stage in a Stirling cycle wherein a Stirling machine is described as operating as a heat pump; and

Fig. 9 is a diagram showing a fifth stage in a Stirling cycle wherein a Stirling machine is described as operating as a heat pump.

Referring firstly to Fig. 2, the machine which is shown in diagrammatic form only, comprises a casing 10 defining a first end cavity 12 and a second end cavity 14. The cavities 12 and 14 are of the configuration shown in Fig. 3

so as to retain for efficient functioning epitrochoidal rotors 16 and 18.

The cavities 12 and 14 are such that when either rotor 16 or 18 which can rotate in the cavity is in a mid position, two chambers 16A, 16B and 18A and 18B are defined. These chambers respectively are connected by passage means in the form of ducts 20 and 22, duct 20, connecting chambers 16A and 18A, and duct 22 connecting chambers 16B and 18B. The machine thus far described is essentially conventional, and it will be noted that each of the ducts 20 and 22 is provided with the conventional heater 24, regenerator 26 and cooler 28.

In accordance with a first conventional use of the machine, one end of the machine is heated and is the hot end, and this typically will be the end having cavity 12 and rotor 16 whilst the other end is the cool end. When heat is applied to the hot end of the machine, and the rotor 16 is rotated so as to sweep cavity 12, the enigne will fire and run. The running of the machine is achieved in that the gas contained in the ducts 20 and 22 and in the chambers 16A, 16B, 18A and 18B will be subjected to expansion due to heating, and that expansion is used for displacing the rotor 18 to cause rotation of same in the cavity 14, and resulting in rotation of an output shaft 30 to provide power output from the machine.

Typically the working gas will be Helium and the internal nominal pressure is 30 bar.

It should be noted that each of the rotors 16, 18 is carried by the stub shaft 32, 34 for rotation thereon. The shaft 32 is shown as being coaxial with rotor 16, but in fact rotor 16 will be epitrochoidal in shape. The shaft 34 is clearly

offset relative to the rotor 18 as shown in Fig. 3. The stub shafts 32 and 34 are in turn connected to sun wheels 40 and 42 of gear wheel clusters 44 and 48 respectively connected with the rotors 16 and 18.

Each gear wheel cluster comprises a sun wheel 40, 42, planet wheels 50, 52, 54 and 56 and an annulus 60 and 62. Annulus 60 is formed in a casing 10 whilst annulus 62 is in fact a rotateable component which can be moved for adjusting the phase angle between the rotors 16 and 18 either whilst the machine is running, or when it is at rest.

In a two-rotor Stirling machine as shown in Figs. 2 and 3, it is usual for one rotor to be out of phase relative to the rotor by 90°, as this has been shown to be the optimum position for maximum power output, but it has been found that if the phase angle between the rotors is reduced then contrary to what might be expected, there is not the same reduction in thermal efficiency of the machine as there is indeed in the output power.

In accordance with a second application of the invention the use of a Stirling machine as a heat extraction means, or a refrigeration means is best explained with reference to the accompanying diagrammatic representations shown in Figs. 5 to 9. Figs. 5 to 9 show four stages in the cycle of a linear Stirling machine. However the following description though applies equally to a rotary Stirling machine whose rotors are adjustable with respect to their phase angle.

Fig. 5 illustrates the beginning of a compression stroke where most of the working gas is at the hot end of the machine and the average temperature of the gas is above ambient. Moving from Fig. 5 to Fig. 6 the hot gas is compressed at a constant temperature. In order to maintain

constant temperature during the compression stroke, heat has to be rejected in the hot end heat exchanger. At stage 2, illustrated in Fig. 6 the gas is displaced from the hot end of the machine to the cold end of the machine. During this displacement most of the heat in the gas is stored in the regenerator. After the gas is passed over the regenerator its average temperature will be less than ambient temperature. In state 3 represented in Fig. 7, the cool gas in the cold end of the engine is expanded, at constant temperature. In the expansion stroke, heat has to be transferred to the gas from the surrounding environment in order to maintain constant temperature. At state 4, illustrated in Fig. 8, the gas is at the cold end of the machine in an expanded state before it is displaced back to the first stage at constant volume, illustrated in Fig. 5. As the gas moves from the cold end to the hot end of the machine the heat stored in the regenerator is recovered and the average temperature of the gas is raised above ambient and the cycle is repeated.

In accordance with a second less conventional use of the machine, one end of the machine is heated and it is the hot end, and this typically will be the end having cavity 12 and rotor 16 whilst the other end is the cool end. It will be understood from the foregoing description that when work is applied to the machine via rotation of shaft 30 the rotor 18 is rotated so as to sweep cavity 14 whereby the gas containing volume of cavity 14 is increased, since this occurs at constant temperature heat must be absorbed from the surrounding environment. Thus the second use of the machine provides for a refrigeration means whereby heat can be extracted from the environment surrounding rotor 18.

The graphs taken from a power producing machine constructed as described in relation to Figs. 2 and 3 as portrayed in

Fig. 1, illustrate the machine performance of a power producing machine. Graphs 70 represent the power outputs for a series of machine speeds operating with a variable phase angle between the rotors. It will be seen that the maximum power is achieved at a phase angle between the rotors of 90° and a speed in the order of 3,250 rpm. When the phase angle between the rotors is set at 35°, the power output is considerably reduced, and it will be seen that maximum power at this phase angle occurs at a speed in the order of 2,250 rpm. Comparing the two characteristics of the machine operating at a phase angle of 90° with a machine operating at a phase angle of 35° does show however, that a reduction in the phase angle between the rotors results in a considerable reduction in power.

On the other hand, the upwardly extending thermal efficiency curves 80 shown in Fig. 1 illustrate that for an operating speed of 3,250 rpm, when the phase angle between the rotors is 90°, the thermal efficiency is just over 40%. At an operating speed of 2,250 rpm when the angle between the rotors is set at 35° the thermal efficiency is similarly just over 40%. Thus the striking feature to notice is that the thermal efficiency estimations over the speed range indicated are very close together, which means that for a reduction in phase angle there is little or no reduction in thermal efficiency.

This means that the machine can indeed operate satisfactorily over a wide range of phase angle settings as between rotors, and therefore over a wide range of power outputs without sacrificing too much thermal efficiency and hence the provision of the relative adjustmentability between the rotors to vary the phase angle represents an important step forward in the field of Stirling machines.

The graph shown in Fig. 4 is taken from a power operated machine constructed as described in relation to Figs. 2 and 3 and shows the heat performance of a power operated machine. Graph 90 illustrates heat consumption over a range of rotor phase angles and graph 92 shows the co-efficiency of power over a range of rotor phase angles. At a phase angle difference between the two rotors of 20° the heat delivered to the machine is approximately 17kw. In contrast that a phase angle difference between the two rotors of 90° the heat delivered is approximately 45kw. Thus by increasing the rotor phase angle difference it is possible to increase the heat delivered to the machine and the result of this is to increase the heat pump performance of same. The best coefficient of performance and highest heat transfer is developed when the phase angle between the compression space and the expansion phase is 90° the COP and heat transfer falling to zero when the pistons are in phase i.e. they just shift the gas from one side to the other without doing any compression or expansion of work. This property of the Stirling is employed in this invention to control the power input and the heat transfer capacity of the machine where the two rotors forming the cold space and the hot space can be phased at any angle between 10° and 110° to control the heat transfer capacity. Thus it can be seen that by driving the machine at variable phase angles one can increase the refrigeration capacity of the machine in order to accommodate the demands of a user.

The invention also has novelty in that epitrochoidal rotors are connected through a gear transmission. It is preferred that there are two epitrochoidal rotors connected by this transmission and that the transmission should be a sun wheel and planet arrangement, one rotor representing the hot end of the engine or refrigeration unit and the other end the cold. Whilst this preferred construction enables the angular phase

between the rotors to be adjusted as described herein, these features are also novel in themselves.

By way of additional disclosure extra drawings. Figs. 10 and 11, are included to show the layout of the engine with the epitrochoid rotors and shaft phasing epicyclic gear train used to control the power output, and Figs. 12 and 13 show the layout of the machine used to control the heat transfer capacity.

Essentially these figures illustrate the arrangement already described hereinbefore in relation to Figs. 2 and 4.