fig1 is a diagrammatic view of a vapour compression system 1 according to an embodiment of the invention . the vapour compression system 1 comprises a compressor 2 , a condenser 3 , an expansion valve 4 and an evaporator 5 arranged along a refrigerant path . during operation , refrigerant flowing in the refrigerant path is compressed in the compressor 2 . the compressed refrigerant is supplied to the condenser 3 , where it condenses , the refrigerant leaving the condenser 3 thereby being in a substantially liquid state . the refrigerant is then supplied to the expansion valve 4 where it is expanded , thereby forming a mixed state refrigerant , i . e . a mixture of gaseous and liquid refrigerant is supplied from the expansion valve 4 to the evaporator 5 . in the evaporator 5 , the liquid part of the refrigerant is evaporated while exchanging heat with a secondary fluid flow , such as an air flow , across the evaporator 5 , illustrated by arrows 6 . finally , the refrigerant is once again supplied to the compressor 2 , thereby completing the cycle . fig2 a is a side view of an expansion valve 4 according to a first embodiment of the invention . the expansion valve 4 comprises a first valve member 7 and a second valve member 8 . the first valve member 7 and the second valve member 8 are arranged in such a manner that they may perform relative movements . this will be described further below with reference to fig4 and 5 . a compressible spring 9 is arranged between the first valve member 7 and the second valve member 8 , thereby biasing the first valve member 7 and the second valve member 8 in a direction away from each other . in fig2 a the first valve member 7 and the second valve member 8 are arranged in a first relative position , where a distance is defined between the first valve member 7 and the second valve member 8 . it is clear from fig2 a that it is possible to compress the compressible spring 9 further , thereby moving the first valve member 7 and the second valve member 8 towards each other , against the spring force of the compressible spring 9 . fig2 b is a cross sectional view of the expansion valve of fig2 a . in fig2 b it can be seen that the first valve member 7 is provided with an opening 10 defining a fluid passage through the first valve member 7 , the opening 10 having a first diameter . when the first valve member 7 and the second valve member 8 are in the first relative position shown in fig2 a and 2 b , the fluid flow through the expansion valve 4 is determined by the diameter of the opening 10 . the first valve member 7 is further provided with a conical portion 11 extending towards the second valve member 8 . the conical portion 11 of the first valve member 7 is capable of guiding a mating conical portion 12 formed on the second valve member 8 . thereby it is ensured that the conical portions 11 , 12 are arranged in abutment when the first valve member 7 and the second valve member 8 are moved towards each other . this will be described in further detail below with reference to fig3 a and 3 b . the conical portion 11 of the second valve member 8 is also provided with an opening 13 . the opening 13 of the second valve member 8 has a smaller diameter than the opening 10 of the first valve member 7 . however , when the first valve member 7 and the second valve member 8 are in the first relative position shown in fig2 a and 2 b , the diameter of the opening 13 of the second valve member 8 is not limiting for the fluid flow through the expansion valve 4 , because fluid is allowed to flow past the conical portion 11 of the second valve member 8 , since the conical portion 11 is attached to the remaining parts of the second valve member 8 by means of a number of ribs 14 . fig3 a is a side view of the expansion valve 4 according to the first embodiment of the invention , shown in fig2 a and 2 b . in fig3 a the first valve member 7 and the second valve member 8 are arranged in a second relative position , where the valve members 7 , 8 are as close to each other as possible , actually in abutment with each other . it is clear from fig3 a that the compressible spring 9 is completely compressed in this position . fig3 b is a cross sectional view of the expansion valve 4 according to the first embodiment of the invention , the expansion valve 4 being in the second position shown in fig3 a . from fig3 b it is clear that the first valve member 7 and the second valve member 8 are arranged completely in abutment in this relative position . in particular , it is clear that the conical portion 12 of the second valve member 8 is arranged inside the conical portion 11 of the first valve member 7 in such a manner that the opening 10 defined by the first valve member 7 and the opening 13 defined by the second valve member 8 are arranged adjacent to each other . thereby a fluid flow passing through the expansion valve 4 when the valve members 7 , 8 are in this relative position , must pass through the opening 13 defined by the second valve member 8 . since the diameter of the opening 13 defined by the second valve member 8 is smaller than the diameter of the opening 10 defined by the first valve member 7 , the opening degree of the expansion valve 4 is reduced as compared to the situation illustrated in fig2 a and 2 b . fig4 is a cross sectional view of an expansion valve 4 according to the first embodiment of the invention , the expansion valve 4 being arranged in a refrigerant path 15 of a vapour compression system . the expansion valve 4 is arranged immediately upstream relative to an evaporator ( not shown ), i . e . it is arranged in the liquid line of the evaporator . in fig4 the first valve member 7 and the second valve member 8 of the expansion valve 4 are in the first relative position which is also illustrated in fig2 a and 2 b . the second valve member 8 is arranged substantially immovably relatively to the refrigerant path 15 , and the first valve member 7 is arranged movably relatively to the second valve member 8 . the position of the first valve member 7 is determined by the differential pressure across the expansion valve 4 of the refrigerant flowing in the refrigerant path 15 . refrigerant flowing in the refrigerant path 15 and through the expansion valve 4 flows in a direction from the first valve member 7 towards the second valve member 8 , i . e . along the direction indicated by arrow 16 . thus , when the differential pressure across the expansion valve 4 is increased , the first valve member 7 will be forced towards the second valve member 8 , against the spring force of the compressible spring 9 . as long as the valve members 7 , 8 are arranged with a mutual distance , the opening degree of the expansion valve 4 will be determined by the diameter of the opening 10 defined by the first valve member 7 , as described above with reference to fig2 b . thus , the opening degree of the expansion valve 4 is , in this case , as large as possible . in the situation illustrated in fig4 the first valve member 7 is arranged far from the second valve member 8 , i . e . the differential pressure across the expansion valve 4 is , in this case , relatively low . thus , fig4 illustrates that a low differential pressure across the expansion valve 4 results in a large opening degree of the expansion valve 4 . fig5 is a cross sectional view of the expansion valve 4 according to the first embodiment of the invention , arranged in a refrigerant path 15 , similarly to the situation shown in fig4 . the expansion valve 4 is arranged immediately upstream relative to an evaporator ( not shown ), i . e . it is arranged in the liquid line of the evaporator . however , in fig5 the first valve member 7 and the second valve member 8 are arranged in the second relative position illustrated in fig3 a and 3 b , i . e . the first valve member 7 and the second valve member 8 are arranged substantially in abutment with each other . as described below with reference to fig3 b , the opening degree of the expansion valve is , in this situation , determined by the diameter of the opening 13 of the second valve member 8 . since the diameter of the opening 13 of the second valve member 8 is smaller than the diameter of the opening 10 of the first valve member 7 , the opening degree of the expansion valve 4 in the situation illustrated in fig5 is smaller than the opening degree of the expansion valve 4 in the situation illustrated in fig4 . the position of the first valve member 7 as close as possible to the second valve member 8 indicates that the differential pressure across the expansion valve 4 is relatively high . thus , fig5 illustrates that a high differential pressure across the expansion valve 4 automatically results in a small opening degree of the expansion valve 4 . fig6 and 7 are exploded views of the expansion valve 4 according to the first embodiment of the invention , seen from two different angles . the first valve member 7 , the second valve member 8 and the compressible spring 9 are clearly seen . furthermore , details of the first valve member 7 and the second valve member 8 can be seen , such as the conical sections 11 , 12 and the openings 10 , 13 . fig8 is a cross sectional view of an expansion valve 4 according to a second embodiment of the invention . the expansion valve 4 is arranged in a refrigerant path 15 of a vapour compression system . the expansion valve 4 comprises a first valve member 7 and a second valve member 8 . the second valve member 8 is arranged substantially immovable relatively to the refrigerant path 15 , and the first valve member 7 is arranged movably relative to the second valve member 8 . a compressible spring 9 is arranged between the first valve member 7 and the second valve member 8 , biasing the valve members 7 , 8 in a direction away from each other . the second valve member 8 is provided with an opening 17 defining a fluid passage through the expansion valve 4 . the first valve member 7 comprises a protruding element 18 extending in a direction towards the second valve member 8 . the protruding element 18 has a conical shape , i . e . the diameter of the protruding element 18 varies along a longitudinal direction of the protruding element 18 . during normal operation of the vapour compression system , refrigerant flows in the refrigerant path 15 and through the expansion valve 4 along a direction from the first valve member 7 towards the second valve member 8 , i . e . along the direction indicated by arrow 16 . thus , if the differential pressure across the expansion valve 4 increases , the first valve member 7 will be forced in a direction towards the second valve member 8 , against the spring force of the compressible spring 9 . similarly , if the differential pressure across the expansion valve 4 decreases , the first valve member 7 will move in the opposite direction , away from the second valve member 8 . in fig8 the protruding element 18 of the first valve member 7 is arranged in the opening 17 of the second valve member 8 , thereby blocking a part of the fluid passage defined by the opening 17 . due to the conical shape of the protruding element 18 , the relative position between the first valve member 7 and the second valve member 8 determines how large a part of the fluid passage is blocked by the protruding element 18 . thereby the relative position also determines the size of the remaining passage . accordingly , the relative position determines the fluid flow through the expansion valve 4 , and thereby the opening degree of the expansion valve 4 . in fig8 the first valve member 7 is arranged in a position which defines a relatively large opening degree of the expansion valve 4 , indicating that the differential pressure across the expansion valve 4 is relatively low . a stopping member 19 is arranged in the refrigerant path 15 upstream relatively to the expansion valve 4 . the function of the stopping member 19 will be described in further detail below with reference to fig1 . fig9 is a cross sectional view of the expansion valve 4 according to the second embodiment of the invention . in fig9 the differential pressure across the expansion valve 4 is higher than in the situation illustrated in fig8 . accordingly , the first valve member 7 has been moved towards the second valve member 8 , against the spring force of the compressible spring 9 . thereby the protruding element 18 has been moved further into the opening 17 , and a larger part of the fluid passage defined by the opening 17 is blocked by the protruding element 18 . accordingly , the opening degree of the expansion valve 4 is smaller in the situation illustrated in fig9 than in the situation illustrated in fig8 . thus , fig8 and 9 illustrate that the opening degree of the expansion valve 4 automatically decreases when the differential pressure across the expansion valve 4 increases . similarly , the opening degree of the expansion valve 4 automatically increases when the differential pressure across the expansion valve 4 is decreased . fig1 is a cross sectional view of the expansion valve 4 according to the second embodiment of the invention . in fig1 the fluid flow through the expansion valve 4 has been reversed . thus , refrigerant flowing through the expansion valve 4 flows along a direction from the second valve member 8 towards the first valve member 7 , i . e . along the direction illustrated by arrow 20 . this has caused the first valve member 7 to be pushed away from the second valve member 8 and into abutment with the stopping member 19 . the stopping member 19 prevents the first valve member 7 from being moved further along this direction . when the first valve member 7 is in this position , the protruding element 18 is no longer arranged inside the opening 17 . thereby the opening 17 is not blocked by the protruding element 18 , and the opening degree of the expansion valve 4 is defined by the diameter of the opening 17 . this is significantly larger than the opening degrees illustrated in fig8 and 9 , and the fluid flow through the expansion valve 4 is in essence unrestricted . the reverse flow situation illustrated in fig1 can occur in vapour compression systems which can be selectively operated in an air condition mode or a heat pump mode . in this case the vapour compression system comprises a compressor , two expansion valves and two heat exchangers . the heat exchangers are both capable of operating as an evaporator or as a condenser , depending on the flow direction in the system . each of the expansion valves is capable of controlling the flow of refrigerant to one of the heat exchangers , when the respective heat exchanger operates as an evaporator . however , when a given heat exchanger operates as a condenser , fluid flow through the corresponding expansion valve should not be restricted . this is obtained by the expansion valve 4 according to the second embodiment of the invention , and the expansion valve 4 illustrated in fig8 - 10 is therefore suitable for use in a vapour compression system which is capable of selectively operating in an air condition mode or a heat pump mode . fig1 is an end view of the expansion valve 4 according to the second embodiment of the invention . the shape of the first valve member 7 can be seen , and it is clear that refrigerant is allowed to flow past the first valve member 7 . fig1 is a graph illustrating opening degree , od , of an expansion valve according to the first embodiment of the invention as a function of differential pressure , δp , across the expansion valve . in the situation illustrated in fig1 , two distinct levels of the opening degree are defined , a high level at low differential pressures , and a low level at high differential pressures . at a threshold value 21 of the differential pressure , the opening degree changes abruptly between the two distinct levels . this corresponds to the behaviour of the expansion valve according to the first embodiment of the invention , and described above with reference to fig2 a - 7 . fig1 is a graph illustrating opening degree , od , of an expansion valve according to the second embodiment of the invention as a function of differential pressure , δp , across the expansion valve . in the situation illustrated in fig1 , the opening degree decreases substantially linearly as the differential pressure increases . two illustrative points 22 , 23 on the curve are marked . these could , e . g ., illustrate the measurement points used for evaluating the seer value of the vapour compression system . the graph of fig1 could , e . g ., originate from the expansion valve 4 according to the second embodiment of the invention , described above with reference to fig8 - 11 . although various embodiments of the present invention have been described and shown , the invention is not restricted thereto , but may also be embodied in other ways within the scope of the subject - matter defined in the following claims .