Internal combustion engine system and a method for an internal combustion engine system

Abstract

The invention provides an internal combustion engine system (1) comprising—at least one combustor (3), and—a first expander (4) arranged to receive exhaust gases from at least one of the at least one combustor (3), and to expand and extract energy from the exhaust gases —characterized in that the system comprises a second expander (5) arranged to receive exhaust gases from the first expander (4), and to expand and extract energy from the exhaust gases.

Claims

1. A method for an internal combustion engine system comprising at least one combustor, a first expander arranged to receive exhaust gases from at least one of the at least one combustor, and to expand and extract energy from the exhaust gases, wherein the system comprises a crankshaft, and the combustor comprises a piston arranged to reciprocate in a cylinder, and to drive the crankshaft, wherein the first expander is a piston expander arranged to drive the crankshaft with the extracted energy, comprising providing an exhaust treatment device arranged to receive exhaust gases from the first expander, to process the received exhaust gases, the method comprising determining the temperature in the exhaust treatment device, and adjusting the swallowing capacity of the first expander to control the temperature of the process in the exhaust treatment device.

2. A method according to claim 1, wherein the internal combustion engine comprises a second expander arranged to receive exhaust gases from the first expander, and to expand and extract energy from the exhaust gases, wherein the processed exhaust gases are delivered to the second expander.

3. A method according to claim 1, comprising increasing the temperature of the exhaust treatment device by increasing the swallowing capacity of the first expander, and decreasing the temperature of the exhaust treatment device by decreasing the swallowing capacity of the first expander.

4. A method according to claim 1, wherein the first expander is arranged to receive the exhaust gases by means of an expander inlet valve, an expander variable valve actuation mechanism being arranged to actuate the expander inlet valve, wherein adjusting the swallowing capacity of the first expander comprises adjusting the expander variable valve actuation mechanism.

5. A method according to claim 3, comprising increasing the swallowing capacity of the first expander by delaying a closing event of the expander inlet valve, and decreasing the swallowing capacity of the first expander by advancing the closing event of the expander inlet valve.

6. A method according to claim 3, wherein the first expander is arranged to expel the exhaust gases by means of an expander exhaust valve, the expander variable valve actuation mechanism being arranged to actuate the expander exhaust valve, the method comprising controlling the expander variable valve actuation mechanism so as to adjust the expander exhaust valve to minimize the pressure difference across the expander exhaust valve at an opening event of the expander exhaust valve, and/or to minimize the pressure difference across the expander inlet valve at an opening event of the expander inlet valve.

7. A method according to claim 2, comprising adjusting the swallowing capacity of the second expander to control the temperature of the process in the exhaust treatment device.

8. A method according to claim 7, comprising increasing the temperature of the exhaust treatment device by decreasing the swallowing capacity of the second expander, and decreasing the temperature of the exhaust treatment device by increasing the swallowing capacity of the second expander.

9. A method according to claim 7, wherein adjusting the swallowing capacity of the first expander and/or the second expander is done in dependence on a rotational speed of the engine system and/or a requested torque for the engine system.

10. A method according to claim 7, comprising increasing the temperature of the exhaust treatment device by increasing the swallowing capacity of the first expander and decreasing the swallowing capacity of the second expander, and decreasing the temperature of the exhaust treatment device by decreasing the swallowing capacity of the first expander and increasing the swallowing capacity of the second expander.

11. A method according to claim 10, comprising determining a temperature of the exhaust gases and, with an increasing exhaust gas temperature, decreasing the swallowing capacity of the first expander and increasing the swallowing capacity of the second expander, and, with a decreasing exhaust gas temperature, increasing the swallowing capacity of the first expander and decreasing the swallowing capacity of the second expander.

12. A computer program comprising program code means for performing the steps of claim 1 when the program is run on a computer.

13. A computer readable medium carrying a computer program comprising program code means for performing the steps of claim 1 when the program is run on a computer.

14. A control unit arranged to perform the steps of claim 1.

15. A method for an internal combustion engine system comprising a combustor arranged to combust air and fuel, an expander arranged to receive, expand and extract energy from exhaust gases from the combustor, and to expel the exhaust gases from the expander by means of an expander exhaust valve, and wherein the system comprises a crankshaft, and the combustor comprises a piston arranged to reciprocate in a cylinder, and to drive the crankshaft, wherein the expander is a piston expander arranged to drive the crankshaft with the extracted energy, and providing an exhaust treatment device arranged to receive the expelled exhaust gases from the expander and to process the exhaust gases, the method comprising determining the temperature in the exhaust treatment device, and controlling the expander exhaust valve to control the temperature of the process in the exhaust treatment device.

16. A method according to claim 15, wherein the control of the expander exhaust valve comprises controlling the temperature of the process in the exhaust treatment device to be within the range of 300-450° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples. In the drawings:

(2) FIG. 1 is a partially sectioned side view of a vehicle in the form of a truck.

(3) FIG. 2 is a schematic perspective view of an engine system in the vehicle in FIG. 1.

(4) FIG. 3 is a schematic cross-sectional view of the engine system in FIG. 2.

(5) FIG. 4 is a flow diagram depicting steps in a method carried out in the system in FIG. 3.

(6) FIG. 5 is a flow diagram depicting steps in an alternative method carried out in the system in FIG. 3.

(7) FIG. 6 is a schematic cross-sectional view of an engine system according to an alternative embodiment of the invention.

(8) FIG. 7 is a schematic cross-sectional view of an engine system according to another alternative embodiment of the invention.

(9) FIG. 8 is a schematic cross-sectional view of an engine system according to yet a further embodiment of the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

(10) FIG. 1 shows a vehicle in the form of a truck, or a tractor for a semitrailer. It should be noted however that the invention is applicable to a variety of alternative types of vehicles, such as a car, a bus, or a working machine such as a wheel loader. The vehicle comprises an internal combustion engine system 1.

(11) As can be seen in the schematic FIG. 2 the engine system 1 comprises a multi-stage compression and expansion internal combustion engine. The engine comprises three combustors 3, in the form of cylinders with pistons, and three piston compressors 9. The system further comprises an air guide 901 arranged to guide compressed air from the compressors 9 to the combustors 3. The air guide is provided with an air buffer container 902, arranged to receive compressed air from the compressors 9, to provide an air buffer volume for the compressed air, and to deliver the compressed air to the combustors 3.

(12) The system further comprises three first piston expanders 4 arranged to expand exhaust gases from the combustors 3 and to extract energy from the expanded exhaust gases. An exhaust guide 701 is arranged to guide exhaust gases from the combustors 3 to the first expanders 4. The exhaust guide 701 comprises a pre-expander exhaust treatment device 7 described closer below.

(13) The system further comprises a post-expander exhaust treatment device 6 in the form of a selective catalytic reduction (SCR) catalyst. The SCR catalyst 6 is arranged to receive exhaust gases from the first expander 4 and to provide an exhaust treatment process to the received exhaust gases, which process reduces nitrogen oxides (NOx) as is known per se. Three second piston expanders 5 are arranged to receive and expand exhaust gases from the SCR catalyst 6 and to extract energy from the expanded exhaust gases.

(14) It is understood that the engine system may comprise any suitable number of combustors 3, compressors 9, first expanders 4, and second expanders 5. Herein, the engine is understood as comprising the combustors 3, the compressors 9, the first expanders 4 and the second expenders 5, and the engine system is understood as comprising the engine, the pre-expander exhaust treatment device 7, and the post-expander exhaust treatment device 6.

(15) Reference is made to FIG. 3 in which only one of the combustors 3, only one of compressors 9, and only one of the first expanders 4 are shown. The piston 301 of each combustor 3 is arranged to reciprocate in the respective cylinder 302, whereby the pistons are all arranged to drive a crankshaft 2 of the engine. For simplicity, the combustor 3, the compressor 9, and the first expander 4 are shown as all being located in the same cross-sectional plane; in a real implementation of the embodiment, the combustor 3, the compressor 9, and the first expander 4 are preferably offset in relation to each other along the crankshaft 2.

(16) The pistons 401 of the first expanders 4 are arranged to drive the crankshaft 2 with the energy extracted from the exhaust gases from the combustors 3. The pistons 501 of the second expanders 5 are arranged to drive the crankshaft 2 with the energy extracted from the exhaust gases from the first expanders 4 and the SCR catalyst 6. Further, the pistons 903 of the compressors 9 are all arranged to be driven by the crankshaft 2.

(17) The pre-expander exhaust treatment device 7 is arranged to provide an exhaust treatment process to the exhaust gases from the combustor 3. The pre-expander exhaust treatment device 7 comprises an oxidation catalyst 7, and a particulate filter 8 located downstream of the oxidation catalyst 7.

(18) The engine system comprises a control unit 15 arranged to control various function of the system as described below.

(19) The combustors 3 are provided with respective sets of combustor inlet and outlet valves 303, 304, arranged to be actuated by a combustor valve actuator assembly 306. The outlet valve 304 is herein also referred to as an exhaust valve 304. The combustor valve actuator assembly 306 may be arranged to actuate the combustor inlet and outlet valves 303, 304 in any manner known per se, e.g. with cams mounted on camshafts. The combustor valve actuator assembly 306 is controllable by the control unit 15, to adjust the timing and the maximum movements of the combustor inlet and outlet valves 303, 304, in any manner known per se. The combustor valve actuator assembly 306 is herein also referred to as a variable valve actuation mechanism or a variable valve timing mechanism 306.

(20) In addition, the combustors 3 are provided with respective fuel injectors 305 for injecting a fuel into the cylinders 302. The fuel may be of any suitable type, e.g. diesel, methane e.g. in liquid natural gas (LNG), gasoline, etc. The fuel injectors 305 are controllable by the control unit 15. In this example, the combustors 3 are arranged to provide a Diesel cycle to extract work from the air and fuel provided. However, the invention is equally applicable to engines in which the combustors are arranged to provide an Otto cycle, wherein the engine system may be provided with means for air mass flow control, such as variable inlet and outlet valves 913, 914 of the compressors 9, described further below, for controlling the air supply to the combustors 3. Alternatively, or in addition, the means for air mass flow control may comprise one or more throttles for controlling the air supply to the combustors 3. The engine system may be provided with spark plugs in the combustors.

(21) The first expanders 4 are provided with respective sets of first expander inlet and outlet valves 403, 404, arranged to be actuated by a first expander valve actuator assembly 406, including e.g. cams mounted on camshafts. The first expander valve actuator assembly 406 is controllable by the control unit 15, to adjust the timing and the maximum movements of the first expander inlet and outlet valves 403, 404. The first expander valve actuator assembly 406 is herein also referred to as a variable valve actuation mechanism or an expander variable valve timing mechanism 406.

(22) Similarly, the second expanders 5 are provided with respective sets of second expander inlet and outlet valves 503, 504, arranged to be actuated by a second expander valve actuator assembly 506, including e.g. cams mounted on camshafts. The second expander valve actuator assembly 506 is controllable by the control unit 15, to adjust the timing and the maximum movements of the second expander inlet and outlet valves 503, 504. The second expander valve actuator assembly 506 is herein also referred to as a variable valve actuation mechanism or an expander variable valve timing mechanism.

(23) In addition, the compressors 9 are provided with respective sets of said compressor inlet and outlet valves 913, 914, arranged to be actuated by a compressor valve actuator assembly 916, including e.g. cams mounted on camshafts. The compressor valve actuator assembly 916 is controllable by the control unit 15, to adjust the timing and the maximum movements of the compressor inlet and outlet valves 913, 914.

(24) The system also comprises three injectors 8 arranged to inject reductant for the SCR catalyst 6. Each injector 10 is arranged to inject the reductant downstream of the pre-expander exhaust treatment device 7, into a respective portion of the exhaust guide 701 leading to the respective first expander 4. The injectors 8 are controllable by the control unit 15, to control the timing, the flow and the duration of the redundant injections. Specifically, the control unit 15 is arranged to control the timing and duration of the reductant injections so that they are coordinated with the actuations of the first expander inlet valves 403, in order to enable good mixing of the reductant with the exhaust gases in the respective first expander 4. In some embodiments, each injector 8 is arranged to inject the reductant directly into the respective exhaust first expander 4.

(25) In some embodiments, a single reductant injector may be provided, e.g. where the engine system is provided with a single first expander 4 arranged to receive exhaust gases from a plurality of combustors 3. The single reductant injector may be thereby be arranged to inject the reductant upstream of, or into the single first expander.

(26) The control unit 15 is also arranged to receive signals from a mass air flow (MAF) sensor 141, located in an air intake system 921 located upstream of the compressors 9 and arranged to guide air to the compressors 9. The control unit 15 is arranged to determine the load of the engine during its operation in the vehicle, as is known per se, e.g. based on signals from the MAF sensor 702, an accelerator pedal position (APP) sensor, (not shown), indicating a requested torque for the engine, and/or a manifold absolute pressure (MAP) sensor (not shown) in the air intake system 921. The load of the engine may also be based on the rotational speed of the engine.

(27) The control unit 15 is further arranged to receive signals from a first temperature sensor 142, arranged at the pre-expander exhaust treatment device 7. The control unit 15 is arranged to determine the temperature in the pre-expander exhaust treatment device 7, based on signals from the first temperature sensor 142.

(28) The control unit 15 is also arranged to receive signals from a second temperature sensor 143, arranged at the post-expander exhaust treatment device 6. The control unit 15 is arranged to determine the temperature in the post-expander exhaust treatment device 6, based on signals from the second temperature sensor 143.

(29) Reference is made to FIG. 4 depicting steps in a method carried out in the system described above. During operation of the engine system in the vehicle, the control unit 15 determines S1 the load of the engine system, as described above.

(30) The method involves controlling the injector 8, to inject S4 redundant upstream of the first expander for the post-expander exhaust treatment device. The timing and duration of the reductant injections are coordinated with the actuations of the first expander inlet valve 403, in order to enable good mixing of the reductant with the exhaust gases in the expander.

(31) The method also involves controlling the temperature of the process in the post-expander exhaust treatment device 6. During operation of the engine system in the vehicle, the control unit 15 determines S5 the temperature of the post-expander exhaust treatment device 6, as described above.

(32) The method comprises comparing S501 the temperature of the post-expander exhaust treatment device 6 with a target temperature of the post-expander exhaust treatment device 6. The target temperature may be predetermined; it may be a fixed value or a value determined in dependence on one or more operational parameters of the engine system.

(33) If the determined temperature is below the target temperature, the first expander variable valve actuation mechanism 406 is adjusted S502 so as to delay, in the cycles of the first expander, a closing event of the first expander inlet valve 403, and the second expander variable valve actuation mechanism 506 is adjusted so as to advance, in the cycles of the second expander, a closing event of the second expander inlet valve 503. Thereby the swallowing capacity of the first expander is increased and the swallowing capacity of the second expander is decreased. Thereby the pressure and the temperature between the first and second expanders will be increased, whereby the temperature of the post-expander exhaust treatment device 6 will be increased.

(34) If the determined temperature is above the target temperature, the first expander variable valve actuation mechanism 406 is adjusted S503 so as to advance, in the cycles of the first expander, the closing event of the first expander inlet valve 403, and the second expander variable valve actuation mechanism 506 is adjusted so as to delay, in the cycles of the second expander, the closing event of the second expander inlet valve 503. Thereby the swallowing capacity of the first expander is decreased and the swallowing capacity of the second expander is increased. Thereby the pressure and the temperature between the first and second expanders will be decreased, whereby the temperature of the post-expander exhaust treatment device 6 will be decreased.

(35) Upon adjusting the first and second expander inlet valve closing timings, the first and second expander variable valve actuation mechanisms 406, 506 are controlled so as to adjust S504 the first and second expander exhaust valves 404, 504 to minimize the pressure difference across the expander exhaust valves 404, 504 at the opening events of the expander exhaust valves.

(36) Further the first and second expander exhaust valves 404, 504 are adjusted to minimize the pressure difference across the expander inlet valves 403, 503 at opening events of the expander inlet valves; this is done by adjusting the timing of the expander exhaust valve closing events, thereby controlling the degree of recompression in the first and second expanders.

(37) In alternative embodiments, instead of determining the post-expander exhaust treatment device temperature, the temperature of the exhaust gases may be determined, and the swallowing capacities of the first and second expanders may be adjusted based on the determined exhaust gas temperature.

(38) Reference is made to FIG. 5 depicting steps in a further method carried out in the system described above. During an operation of the engine system 1, the air guide 901 may present a pressure within the range of 8-12 bar and a temperature within the range of 250-350° C. The method involves controlling the temperature of the process in the pre-expander exhaust treatment device 7. During operation of the engine system in the vehicle, the control unit 15 determines S1 the load of the engine system, as described above.

(39) The method involves, as in the example described with reference to FIG. 4, controlling the injector 8, to inject S4 redundant upstream of the first expander for the post-expander exhaust treatment device. The timing and duration of the reductant injections are coordinated with the actuations of the first expander inlet valve 403, in order to enable good mixing of the reductant with the exhaust gases in the expander.

(40) The method also involves controlling the temperature of the process in the post-expander exhaust treatment device 6. During operation of the engine system in the vehicle, e.g. during a cold start event of the engine system, the control unit 15 determines S5 the temperature of the post-expander exhaust treatment device 6, as described above.

(41) Based on the determined post-expander exhaust treatment device temperature, and optionally the engine load, the first expander outlet valves 404 are controlled S6 in the first expander cycles so as to control the post-expander exhaust treatment device temperature. More specifically, by opening the first expander outlet valves 404 relatively early in the cycles, the expansion ratio of the first expanders 4 will be reduced, and some heat is distributed from providing power for the first expander crankshaft drive to providing heat to the post-expander exhaust treatment device 6. Thereby, the temperature of the post-expander exhaust treatment device 6 may be increased. Also, energy not absorbed by the first expanders 4 due to the reduced expansion ratio, may be absorbed by the second expanders 5. By opening the first expander outlet valves 404 relatively late in the cycles, the heat escape from the first expanders 4 may be reduced, whereby the temperature of the post-expander exhaust treatment device 6 may be decreased.

(42) During the engine system operation, the controller 14 periodically repeats said steps of engine load determination S1, reductant injection S4, post-expander exhaust treatment device temperature determination S5, and first expander outlet valve control S6.

(43) In the configuration described above with an oxidation catalyst 10 and a particulate filter 11, the pre-expander exhaust treatment device 7 suitably presents a temperature within the range of 550-800° C. and a pressure within the range of 10-25 bar. The methods described with reference to FIG. 4 and FIG. 5 provide effective manners of controlling the temperature of the post-expander exhaust treatment device 6, for keeping it within a range that is beneficial to the processes therein. In the configuration described above with an SCR catalyst, the post-expander exhaust treatment device 6 suitably presents a temperature within the range of 300-450° C. More generally, the methods provide an advantageous distribution and control of the temperatures along the path of the exhaust gases, so that exhaust treatment devices distributed along said path are provided with different temperatures which are each optimized for the respective exhaust treatment device.

(44) In this example, the compressor 9 may be arranged to compress the air to a volumetric ratio of 1:1-1:8, preferably 1:4-1:6, in dependence on the actuation of the variable compressor inlet and outlet valves 913, 914. The combustors 3 may be arranged to compress the air from the compressors to a ratio of 1:8-1:14, preferably 1:10-1:12, and to expand the gases in the combustors 3 by a ratio of 1:4-1:14, preferably 1:10-1:12, in dependence on the actuation of the variable combustor inlet and outlet valves 303, 304. The first expander 4 may be arranged to expand the exhaust gases with a ratio of 1:2-1:6, preferably 1:3-1:4, and the second expander 5 may be arranged to expand the exhaust gases with a ratio of 1:2-1:6, preferably 1:2-1:3, in dependence on the actuation of the variable first and second expander inlet and outlet valves 403, 404, 503, 504. Specifically, as suggested, the first expander expansion ratio may be adjusted to control the temperature of the post-expander exhaust treatment device 6. It should be noted that in embodiments of the invention, any of the compression and expansion ratios may be outside the intervals mentioned above.

(45) Reference is made to FIG. 6, showing an engine system according to an alternative embodiment of the invention. This embodiment shares essential features with the embodiment described with reference to FIG. 2-FIG. 5. However, some further advantageous features are also provided.

(46) In the embodiment in FIG. 6, each pair of a second expander 5 and a compressor 9 are integrated so as to share a piston 591 which is connected to the crankshaft 2. Thus, the compressors 9 and the second expanders 5 form three compressor and expander combinations 5, 9, in which the pistons 591 is arranged to reciprocate in shared cylinders 592.

(47) In FIG. 6 one of the compressor and expander combinations 5, 9 is shown. The compressor and expander combination 5, 9, herein also referred to as a combined compressor and expander 5, 9, presents on a first side of the piston 591 a first head 595, and compressor inlet and outlet valves 913, 914, arranged to be actuated by a compressor valve actuator assembly, (not shown), similarly to the compressor inlet and outlet valves 913, 914 in the embodiment in FIG. 3. The compressor and expander combination 5, 9 also presents on a second side of the piston 591 a second head 596, and second expander inlet and outlet valves 503, 504, arranged to be actuated by a second expander valve actuator assembly, (not shown), similarly to the second expander inlet and outlet valves 913, 914 in the embodiment in FIG. 3.

(48) The piston is connected to the crankshaft 2 via a connecting rod 593 and an extension rod 594. The extension rod connects the piston 591 with the connecting rod 593, and extends with a tight fit through an opening in the second head 596. Said tight fit may be accomplished, e.g. with a suitable arrangement of a sliding bush or piston rings.

(49) The compressor and expander combination 5, 9 is arranged to admit, by actuation of the compressor inlet valve 913, air to the shared cylinder 592, on the first side of the piston 591. The compressor and expander combination 5, 9 is further arranged to compress the air by the piston movement caused by the crankshaft rotation. The compressor and expander combination 5, 9 is further adapted to admit, by actuation of the second expander inlet valve 503, exhaust gases, received from the first expander 4 and the post-expander exhaust treatment device 6, on the second side of the piston 591. The compressor and expander combination 5, 9 is arranged to expand the admitted exhaust gases, thereby urging the piston 591 towards the first heat 595.

(50) By the urge of the piston 591 towards the first heat 595, in addition to driving the crankshaft 2, the compressor and expander combination 5, 9 is arranged to deliver energy for the air compression in a direct manner. More specifically, the exhaust gases provides a direct pressure on the shared piston 591 in turn serving to directly exert a direct pressure on the air. Thereby, mechanical losses for the transfer of this energy is substantially eliminated. Also, compared to providing the compressor and expander separately, the compressor and expander combination 5, 9 results in a reduced number of parts, since the piston 591 and the cylinder 592 are shared, thereby reducing the complexity and cost of the engine system 1.

(51) It should be noted that in alternative embodiments, the compressor 9 and the first expander 4 may be integrated so as to share a piston which is connected to the crankshaft 2, similarly to what is described with reference to FIG. 6.

(52) It should be noted that while the inlet and outlet valves 303, 304, 403, 404, 503, 504, 913, 914 in the drawings are depicted as poppet valves, any one of them may be provided in any suitable alternative form, such as a sleeve valve.

(53) Reference is made to FIG. 7, showing an engine system according to a further alternative embodiment of the invention. This embodiment shares essential features with the embodiment described with reference to FIG. 2-FIG. 5. However, some further advantageous features are also provided.

(54) In FIG. 3, the pistons of the compressors 9, the first expanders 4, and the second expanders 5 are indirectly connected to the crankshaft 2. However, the invention is applicable also to engine systems, where pistons of the compressors 9 and the first expanders 4 are indirectly connected to the crankshaft 2, via an additional crankshaft 2b and a chain or belt connection 201, as shown in FIG. 7. In FIG. 7, the pistons of the compressors 9 and the first expanders 4 are directly connected to the additional crankshaft 2b, and the chain or belt connection 201 is provided between the crankshafts 2, 2b. Any suitable type of connection between the crankshafts 2, 2b may be provided. For example, as an alternative to the chain or belt connection 201, a toothed gear wheel connection between the crankshafts 2, 2b may be provided.

(55) In should be noted that in the embodiment in FIG. 7, the second expander 5 comprises a piston arranged to drive a further crankshaft 2c connected to the additional crankshaft 2b, via a further chain or belt connection 202. The system also comprises an additional compressor 19 with a piston connected to the further crankshaft 2c. The additional compressor 19 is located upstream of the piston compressor 9, and is arranged to provide an additional compression step.

(56) Reference is made to FIG. 8, showing an engine system according to yet another embodiment of the invention. This embodiment shares essential features with the embodiment described with reference to FIG. 7. However, in the embodiment in FIG. 8, the second expander 5 comprises a turbine arranged to drive an additional compressor 19.

(57) The additional compressor 19 is provided in the form of a centrifugal compressor 19, which is directly connected by means of a shaft to the turbine of the second expander. The additional compressor is located upstream of the piston compressor 9, and is arranged to provide an additional compression step.

(58) It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.