US20170292411A1

US20170292411A1 - Method of and Apparatus For Improved Utilization of the Thermal Energy Contained in a Gaseous Medium

Info

Publication number: US20170292411A1
Application number: US15/480,825
Authority: US
Inventors: Gerd Bauer
Original assignee: Individual
Current assignee: Individual
Priority date: 2016-04-07
Filing date: 2017-04-06
Publication date: 2017-10-12
Also published as: EP3228842A1; DE102016106389A1

Abstract

The present invention concerns a method of utilising the waste heat contained in the exhaust gas of an internal combustion engine, comprising a turbine (20). To provide an apparatus and a method of operating same which directly supplies additional drive energy which otherwise would be lost as waste heat, it is proposed according to the invention that the turbine is an inverse turbine connected downstream of the exhaust gas outlet of the internal combustion engine and comprising at the inlet side an expansion stage (23) and at the outlet side a subsequent compressor (21), wherein the expansion stage and the compressor of the inverse turbine are so designed that the downstream-disposed compressor of the inverse turbine generates at the outlet of the expansion stage (23) a reduced pressure (p1) below the ambient pressure (p0), wherein the outlet (2 b) of the compressor (21) is at the level of the ambient pressure and the compressor of the inverse turbine is driven by the turbine.

Description

The present invention concerns a method of and an apparatus for utilising the thermal energy contained in a gaseous medium, comprising a turbine driven by the exhaust gases.
In practically all technical processes, waste heat is generated, in particular in internal combustion engines, the exhaust gas of which is generally still at high temperatures and also has kinetic energy in the form of a flow speed or pressure drop in an exhaust pipe.
Attempts at at least partially utilising that energy contained in the waste heat as drive energy have in the past frequently failed because of practical considerations. For example in the case of a reciprocating piston engine the pistons and cylinder length would have to be drastically increased in order to more greatly cool down the temperature of the combustion gases before the outlet and reducing their flow speed. That in turn has consequences in regard to subsequent compression in the cylinder and in regard to the size of moving parts, which as a result makes the engine more expensive and at least partially nullifies again an increase in efficiency and better utilisation of the fuel.
In comparison the present invention adopts the approach of better utilising the energy contained in the exhaust gas from an internal combustion engine by means of an additional unit.
In principle a downstream-connected unit in the case of engines for example in the form of an exhaust gas turbocharger is known, in which hot exhaust gases drive a turbine which in turn drives a compressor which forces additional combustion air into the combustion chamber under increased pressure.
That causes more efficient combustion and increases the efficiency of an engine, but of course also more heavily stresses the engine components. In that respect the compressor is functionally disposed upstream of the intake of the internal combustion engine in order to increase the pressure of the gas, in particular the combustion air, as it passes into the intake of the engine. That admittedly increases the engine power but also the fuel consumption, although to a lesser degree than the power, so that an engine with a turbocharger can be of smaller size and requires less fuel with the same power output.
The turbine is disposed in the flow of working gas downstream of the actual internal combustion engine and the compressor, in which case the energy obtained therefrom serves to drive the compressor. Apart from partial exhaust gas recycling which is possibly provided however the exhaust gases flowing through the turbine as such are not again compressed by the compressor.
In comparison the object of the present invention is to provide an apparatus for an internal combustion engine and a method of operating the apparatus, which directly supplies additional drive energy which would otherwise be lost as waste heat, more specifically without increasing the fuel consumption.
In regard to the method that object is attained in that the exhaust gas acts on an inverse turbine (20) which at the inlet side comprises an expansion stage (23) and at the outlet side a subsequent compressor (21), wherein the expansion stage and the compressor of the inverse turbine are operated in such a way that the downstream-connected compressor of the inverse turbine generates at the outlet of the expansion stage (23) a reduced pressure (p1) below the ambient pressure (p0), wherein the outlet (2 b) of the compressor (21) is at the level of the ambient pressure and the compressor is driven by the turbine.
Such a mode of operation is achieved for example by suitable dimensioning of the expansion stage and compressor or also by suitable adaptation of the speeds of rotation of those assemblies.
In regard to the apparatus that object is attained in that connected downstream of the exhaust gas outlet (1 b) of the internal combustion engine is an inverse turbine which at the inlet side comprises an expansion stage (23) and at the outlet side a subsequent compressor (21), wherein the expansion stage and the compressor of the inverse turbine are so designed that the downstream-connected compressor of the inverse turbine generates at the outlet of the expansion stage (23) a reduced pressure (p1) below the ambient pressure (p0), wherein the outlet (2 b) of the compressor (2) is at the level of the ambient pressure and the compressor of the inverse turbine is driven by the expansion stage.
For use in conjunction with a gas turbine this means that the compressor of the inverse turbine, in relation to the compressor of the gas turbine, must be so dimensioned and arranged that—in contrast to an exhaust gas turbocharger—it does not generate an increased pressure, but only at its inlet side a reduced pressure in relation to its outlet side which in turn is at the level of the ambient pressure.
Similarly the inverse turbine in the case of an internal combustion engine is also to be adapted to the exhaust gas flow of the engine and is to be so operated that the reduced pressure is generated at the outlet of the expansion stage of the inverse turbine.
The inlet of the expansion stage in that case is connected to the exhaust gas passage of an upstream-connected internal combustion engine in order to utilise the energy contained in the exhaust gas for driving the turbine. The energy which is now converted by virtue of a higher pressure and temperature drop in the expansion stage, that is generated by the compressor, is only partly used by the compressor of the inverse turbine, which operates at a lower temperature level and which has to operate against a lower pressure drop or a slight pressure rise, more specifically only to ambient pressure. The remaining energy of the expansion stage can be utilised for driving further assemblies. That applies at least for exhaust gases which prior to passing into the expansion stage are at a pressure above the ambient pressure.
Particular advantages of the apparatus according to the invention will be further afforded from intercooling of the working gas between the expansion stage and the compressor because in that way the pressure drop across the expansion stage is increased without having to increase the power of the compressor for that purpose.
Here the term “turbine stage” or “expansion stage” is used to denote any turbine or any part of a turbine which converts energy contained in its working gas into kinetic energy by pressure relief and cooling. This can involve one or also a plurality of blade rings driven by the working gas and also a plurality of turbine stages or expansion stages can in turn again be viewed as a “turbine stage” in accordance with the invention.
To provide a clearer distinction here, in regard to the inverse turbine, the inlet stage thereof is referred exclusively as the “expansion stage”.
The invention is therefore particularly suitable for improving the efficiency of conventional turbines, in particular gas turbines, downstream of which the apparatus according to the invention is simply connected, wherein the expansion stage and the compressor of the inverse turbine can also be mounted on a shaft which is common with the upstream-connected machine, in particular a gas turbine.
By virtue of the arrangement of the turbine stage and the compressor being reversed in comparison with a conventional gas turbine the apparatus according to the invention is referred to herein as an “inverse turbine”. In particular the inverse turbine can be connected directly to the outlet of a conventional gas turbine and possibly also mounted on the same shaft. In that respect it is in particular also possible that the inlet part of the inverse turbine, that is referred to as the “expansion stage”, more specifically the rotor driven by the pressure relief of warm or hot gas, is in turn the final stage of a conventional gas turbine which is accordingly connected directly to a further compressor which is generally not present in gas turbines and which generates an additional reduced pressure at the outlet of the last turbine stage and thus increases the energy yield.
It will be noted that the additional energy yield generally becomes higher if a complete inverse turbine is connected downstream of the conventional gas turbine, that is to say an apparatus having its own turbine stage and a compressor which are matched to each other and which are generally of larger dimensions for optimum energy utilisation or—if possible—are to be operated at a higher speed of rotation than a typical last stage of a conventional gas turbine. In order for example to reduce the flow speed which in many gas turbines could otherwise be in the region of the speed of sound of the gas however operation of the inverse turbine is also possible with a speed of rotation which is the same as the upstream-connected turbine stage or lower than same. The inverse turbine however then has to be designed with a correspondingly larger volume.
As already mentioned the term “expansion stage” relates hereinafter exclusively to the first stage of the inverse turbine, in particular a rotor with turbine blades, between which a warm gas is relieved of pressure and cooled and in so doing drives the rotor. Conventional turbines, a term which is generally used to denote the combination of compressor, combustion chamber and rotor with a plurality of turbine stages, are referred to hereinafter as a “gas turbine” and the individual stages of the rotor are referred to as the “turbine stages”, “blade rings” or groups of blade rings.
As already mentioned generation of the reduced pressure or vacuum at the outlet of the expansion stage can also be markedly improved by the provision of an intercooler between the outlet of the expansion stage and the inlet of the compressor, for the gas passing therethrough.
In an embodiment the expansion stage and the compressor are so mounted on a common shaft that they rotate jointly at the same speed. In that case the amounts of gas which are passed through with a different pressure drop and at different temperatures can be adapted by the geometrical configuration of the expansion stage and the compressor, specifically by virtue of the diameter of the rotor and the number, size, arrangement and shape of the turbine and compressor blades.
Otherwise however it is also possible to produce the uniform and optimised gas through-flow while maintaining the desired pressure drop by the compressor being connected to the turbine shaft by way of an interposed transmission. In this case also the expansion stage and the compressor, possibly also the outlet stage of an upstream-connected gas turbine or an engine shaft can be mounted rotatably about the same axis and the relative rotatability of the individual assemblies is achieved in that they in part have hollow shafts which portion-wise surround the shaft of another assembly and are mounted rotatably thereon.
As already mentioned in the case of the inverse turbine the part played by the expansion stage can also be played by the last stage of a conventional gas turbine, which simplifies and shortens the structure somewhat, even if in that case the waste heat is still not put to optimum use.
Moreover the expansion stage of the inverse turbine can have devices for the combustion of additional fuel and, if present, possibly also incompletely burnt exhaust gases. In that case the fuel feed is preferably set for isothermal combustion. The downstream-connected expansion stage can accordingly achieve additional energy by further combustion of fuel and the constituents of the exhaust gas, that have not yet been converted. As the exhaust gas is still very hot when passing into the expansion stage of the inverse turbine the additional fuel feed can be adjusted and throttled virtually as desired without adversely affecting the combustion process.
In particular the expansion stage or the rotor of the inverse turbine can have their own combustion chamber.
For cooling the working gas, in particular between the expansion stage and the compressor, an embodiment provides that the expansion stage and/or the compressor have at least one diffuser, the interior of which has a coolant at least partially flowing therethrough. Corresponding diffusers could be provided for example at the outlet of the expansion stage or at the inlet of the compressor.
The invention also concerns a method of utilising waste heat in a gaseous medium with an apparatus according to one of the preceding claims, wherein the expansion stage is driven by the exhaust gas of an internal combustion engine and possibly the combustion of additional fuel, which serve as working gas for the expansion stage.
Without the combustion of additional fuel such a mode of operation would in principle be known from the operation of an exhaust gas turbocharger.
For direct utilisation of the drive energy from the turbine stage however it is provided according to the present invention that at least a part of the exhaust gas flow of an internal combustion engine is passed as a working gas through the expansion stage of the apparatus and the outlet of the compressor is held at the pressure level of the environment.
In a further configuration of the method according to the invention additional cooling of the working gas takes place between the expansion stage and the compressor.

Further advantages, features and possible uses of the present invention will be clearly apparent from the description hereinafter of a preferred embodiment and the related Figures.

FIG. 1 diagrammatically shows the structure of a conventional gas turbine with compressor, combustion chamber and turbine rotor,

FIG. 2 shows a diagrammatic view of the conventional gas turbine of FIG. 1 together with a downstream-connected inverse turbine,

FIG. 3 shows a thermodynamic TS diagram relating to the combination of FIG. 2,

FIG. 4 shows a diagrammatic view of a combination similar to the combination of FIG. 2 but with a further combustion chamber disposed upstream of the inverse turbine for complete combustion of exhaust gases, and

FIG. 5 shows the structure of an aircraft engine with downstream-connected inverse turbine.

FIG. 1 firstly shows the diagrammatic layout of a conventional gas turbine 1 which substantially comprises a compressor 11, a combustion chamber (not shown) and the turbine stage 13 and which are mounted in a suitable turbine housing for example on a common shaft.
A gaseous fuel or spray mist of fuel is injected into and fired in the combustion chamber so that the combustion gases drive the turbine stage 13. That in turn is carried on a common shaft 4 with the upstream-connected compressor 1 and drives same which thereby compresses additional combustion air and forces it into the combustion chamber.
The corresponding inverse turbine 20 which according to the invention could be connected downstream of the turbine 10 in FIG. 1 is shown in FIG. 2 and at the inlet side comprises an expansion stage 23, followed by an intercooler 22, and then a compressor 21.
The only diagrammatically illustrated intercooler can be implemented for example in the form of a heat exchanger, for example by cooling conduits which extend in the interior of one or more diffusers and an expansion stage or compressor housing.
The hot gas issuing from the last stage 13 of the turbine 10 is fed to the expansion stage 23 of the inverse turbine 20 which is driven thereby and which in turn by way of a common shaft 24 drives the compressor 21, the inlet side of which is connected to the outlet side of the expansion stage 23. As a result a reduced pressure is generated at the outlet of the expansion stage 23 so that the increased pressure drop causes an increase in power of the rotor of the expansion stage 23. A part of that additional power is used by the downstream-connected compressor 21 which however at a lower temperature level and with a lower pressure drop or a lesser pressure rise compresses the exhaust gas again to the ambient pressure and accordingly has a compressor outlet 21 b open to the environment. The lower temperature level achieved by the intercooler 22 allows an increase in the pressure drop.
FIG. 3 shows the additional energy gain idealised in a schematic temperature-entropy diagram (TS diagram).
Adiabatic compression by a compressor between points 1 and 2 in the TS diagram is followed by the feed of heat and the increase in pressure and temperature from the point 2 to the point 3. The following adiabatic relief would end at the point 4 without the downstream-connected inverse turbine, whereupon once again cooling would be effected with a reduction in pressure and temperature towards point 1.
The inverse turbine or the downstream-connected compressor of the inverse turbine allows greater adiabatic expansion at the point 5, from where in turn gas is cooled along the path from point 5 to point 6 and then is raised by the compressor from the point 6 to a somewhat higher temperature at point 7, from where the cycle can begin again at 1.
In that way an additional amount of energy which in the TS diagram corresponds to the area of the rectangle defined by the points 4, 5, 6 and 7 is obtained.
FIG. 2 shows the diagrammatic layout of an inverse turbine combined with a conventional turbine, wherein the inverse turbine, in the case of the typically lesser pressure and temperature drop in the exhaust gas, is of greater dimensions. That effect however is in part also compensated by the lower temperature of the exhaust gases so that in practice the inverse turbine often does not have to be of larger dimensions, or of only slightly larger dimensions, than the upstream-connected conventional gas turbine. Alternatively the speed of rotation of the inverse turbine 11 can be increased in relation to the speed of rotation of the upstream-connected gas turbine 10, for example by an interposed transmission, and, with a suitable design configuration, the speed of rotation of the compressor 21 can differ from that of the expansion stage 23.
A further example is a combination with a combustion chamber additionally provided between the conventional gas turbine and the inverse turbine, as shown in FIG. 4, wherein that variant is referred to herein as a “crossover turbine”, and wherein the additional combustion chamber is intended in particular to serve for more complete combustion of the exhaust gases from the combustion chamber of the first turbine, but in addition can also be supplied with further fuels. Because of the higher temperatures that permits a marked increase in power even if the dimensions of the inverse turbine are possibly to be correspondingly increased. Desirably the inverse turbine is mounted on a shaft 24 which is separate from the shaft 14 of the gas turbine in order generally to differently set the speeds of rotation of the conventional turbine and the inverse turbine, in consideration of the prevailing pressure, temperature and flow conditions.
FIG. 5 is a view which is admittedly still diagrammatic but rather closer to reality than the other Figures of essential components of an aircraft engine 100 with a downstream-connected inverse turbine.
The aircraft engine 100 shown in FIG. 5 has an outer housing 1 and an inner housing 2 which are connected together by way of a diffuser 21 which comprises guide blades 26 a which are distributed over the periphery and which join the inner housing 2 and the outer housing 1 together in the manner of spokes. A first shaft 14 carrying a plurality of different blade rings 111, 123 and 122 is rotatably mounted in the inner housing 2. In addition, mounted on the first shaft 14 in a recessed portion is a second shaft 24 which also carries a part of the rings 111 and blade rings 113. Two diagrammatically illustrated bearings 19 illustrate the rotatability of the shaft 24 with respect to the shaft 14. The rotatable mounting of the shaft 14 to the housing 2 and/or 1 is not shown here. One or more blade rings which are arranged in part on the shaft 14 and in part on the shaft 24 respectively belong to different compressor and turbine stages.
Arranged at the front end of the shaft 14 is a so-called fan 25, that is to say a blade ring of large diameter, which substantially fills up the inside diameter of the outer housing 1. The fan is driven by the first shaft 14. The diffuser comprises a ring of guide blades which however are arranged rigidly between the housing 1 and the inner housing 2 and which serve to orient the air flow generated by the fan optimally along the axis to produce a maximum amount of thrust.
The mounting of the shaft 14 in the inner housing is not shown here but can also be implemented for example in the region of a cooling chamber 22 and at the front end of the shaft 14 behind the fan and at the inner housing 2 respectively.
At least a part of the blades 26 a have inner cooling passages 27 which serve to cool a coolant in the interior of the diffuser blades which is passed by way of the inner housing 2 into the region of the cooling chamber 22 and in addition can possibly also circulate through rigid guide blades or spokes in the region of the cooling chamber 22. The flow of coolant is otherwise only diagrammatically shown in FIG. 5 outside the housing 2.
The individual components which occur in succession along the axis can respectively each have more or fewer blade rings than shown here and the axial length of the engine in relation to the diameter is here not necessarily reproduced in the correct relationship.
Substantially the engine comprises a low pressure compressor 11 a formed by blade rings 111 on the first shaft 14, a subsequent high pressure compressor 11 b which is formed by similar blade rings 111 on the second shaft 24, a combustion chamber 12 a which follows the high pressure compressor 11 b, a high pressure turbine portion 13 a equipped with blade rings 113 and finally followed by the low pressure turbine 23 a. The low pressure turbine 23 a would form the axially rearward end of a conventional aircraft engine, wherein, as stated, the number of blade rings 123 could also be greater and the low pressure turbine portion could be longer, so that for example it would also embrace the region 23 b.
The regions 23 a and 23 b however are here deliberately shown separately, wherein the portion 23 a is attributed to the low pressure turbine of the conventional engine while the region 23 b is to be viewed as the turbine portion of an inverse turbine which is finally also followed by a compressor 20 with blade rings 121. Disposed between the outlet of the low pressure turbine 23 a and the inlet of the inverse turbine 23 b is a further combustion chamber 15 which serves for complete combustion of constituents which hitherto have not been burnt. Optionally additional fuel could also be injected here. Disposed in the turbine portion 23 b of the inverse turbine and the compressor 21 of the inverse turbine there is also a cooling chamber serving to further cool the exhaust gases from the inverse turbine 23 b, which have already been considerably relieved of pressure but which are still hot, which in addition to the action of the compressor 21 generates a reduced pressure after the turbine portion 23 b and makes it possible to maintain the gas flow with a lower level of compression power.
The blade rings 111, 123 and 121 on the first shaft 14 are designed for low peripheral speeds while the blade rings 111 and 113 on the second shaft 24 which can be mounted on the inner shaft 14 or however in the region of the combustion chamber 12 a involve a higher peripheral speed.
The flow generated by the high pressure turbine however by way of the blade rings 123 drives the first shaft 14 which in turn drives the fan which generates the main part of the overall thrust.
With the additional relief of pressure of the working gas in the expansion stage 23 b of the inverse turbine and subsequent compression additional thrust is generated, which increases the efficiency of the engine by some percent and correspondingly reduces the fuel consumption.

LIST OF REFERENCES

1 outer housing
2 combustion chamber
3 turbine rotor
4 shaft
6 gas turbine
10 gas turbine
11 turbine rotor
11 a low pressure compressor
11 b high pressure compressor
12 a combustion chamber
13 compressor
13 a high pressure turbine portion
14 shaft
15 combustion chamber
16 first shaft
19 gas turbine
20 inverse turbine
21 compressor
21 b compressor outlet
22 intercooler
23 expansion stage
23 a low pressure turbine
23 b turbine
24 second shaft
25 fan
26 diffuser
26 a guide blades
27 passages
100 aircraft engine
111 running ring; blade ring
113 running ring; blade ring
121 running ring; blade ring
122 blade ring
123 running ring; blade ring

Claims

1. A method of utilising the waste heat contained in the exhaust gas of an internal combustion engine, comprising a turbine (10), characterised in that at least a part of the exhaust gas acts on an inverse turbine (20) which at the inlet side comprises an expansion stage (23) and at the outlet side a subsequent compressor (21), wherein the expansion stage and the compressor of the inverse turbine are operated in such a way that the downstream-connected compressor of the inverse turbine generates at the outlet of the expansion stage (23) a reduced pressure (p1) below the ambient pressure (p0), wherein the outlet (2 b) of the compressor (21) is at the level of the ambient pressure and the compressor is driven by the turbine.

2. A method according to claim 1 characterised in that it is applied to a gas turbine, wherein the pressure ratio of the compressor of the gas turbine is set to at least 10, and the pressure ratio between the outlet and the inlet of the expansion stage is set to at least 10.

3. A method according to claim 1 characterised in that the reduced pressure at the outlet of the expansion stage of the inverse turbine is promoted by intercooling, i. e. by means of an intercooler, in particular a heat exchanger.

4. A method according to claim 1 characterised in that the inverse turbine is operated at a speed of rotation which is different from the speed of rotation of the upstream-disposed turbine.

5. Apparatus for utilising the waste heat contained in the exhaust gas of an internal combustion engine, comprising a turbine (11, 13), characterised in that the turbine is an inverse turbine connected downstream of the exhaust gas outlet of the internal combustion engine and comprising at the inlet side an expansion stage (23) and at the outlet side a subsequent compressor (21), wherein the expansion stage and the compressor of the inverse turbine are so designed that the downstream-disposed compressor of the inverse turbine generates at the outlet of the expansion stage (23) a reduced pressure (p1) below the ambient pressure (p0), wherein the outlet (2 b) of the compressor (21) is at the level of the ambient pressure and the compressor of the inverse turbine is driven by the turbine.

6. Apparatus according to claim 5 characterised in that the expansion stage (1) and the compressor (2) rotate about the same axis.

7. Apparatus according to claim 5 characterised in that the compressor (2) is connected to the turbine shaft by way of an interposed transmission (5).

8. Apparatus according to claim 5 characterised in that the volume of the compressor (11) and the expansion chamber (13) of the turbine is respectively smaller by a factor of 1.2 to 4, than the volume of the compressor (21) and the expansion stage (23) of the inverse turbine.

9. Apparatus according to claim 5 characterised in that the expansion stage and/or the compressor have at least one diffuser, the interior of which has a coolant at least partially flowing therethrough.

10. Apparatus according to 9 claim 5 characterised in that the expansion stage (1) is connected downstream of the last stage of a gas turbine (6) and is driven by the exhaust gas from the gas turbine (6) and the pressure drop generated by the compressor (2).

11. A combination of a gas turbine (19) with an inverse turbine (20) according to claim 5 characterised in that devices for the feed of additional fuel are provided after the outlet of the gas turbine and preferably before or in the inverse turbine.

12. Apparatus according to claim 5 characterised in that the expansion stage (1) is connected to an exhaust gas outlet of an internal combustion engine and is driven by the exhaust gas of the internal combustion engine and the pressure drop generated by the compressor (2).

13. An aircraft engine characterised by an apparatus according to claim 5.

14. An aircraft engine according to claim 13 comprising an outer housing 1 and an inner engine housing 2, between which there are provided support elements and a stationary diffuser (26), wherein a fan (25) driven by a first turbine shaft (14) generates an axial air flow between the outer (1) and the inner housings (2), characterised in that provided at the end of the turbine shaft (14), that is remote from the fan, is an inverse turbine (23 b) with a concluding compressor (21) and a cooling device (22) arranged therebetween.

15. An aircraft engine according to claim 13 comprising a heat exchanger between the expansion stage and the compressor of an inverse turbine, wherein cooling conduits extend through the housing and possibly through diffusers provided in the cooling device and in diffuser blades and contain a coolant to cool exhaust gases in the cooling chamber (22).

16. A method according to claim 1 characterised in that it is applied to a gas turbine, wherein the pressure ratio of the compressor of the gas turbine is set to at least 15, and the pressure ratio between the outlet and the inlet of the expansion stage is set to at least 15.

17. Apparatus according to claim 6 characterised in that the expansion stage (1) and the compressor (2) are mounted on a common shaft.

18. Apparatus according to claim 8 characterised in that the volume of the compressor (11) and the expansion chamber (13) of the turbine is respectively smaller by a factor of 1.2 to 2.5 than the volume of the compressor (21) and the expansion stage (23) of the inverse turbine.