AIR CONDITIONING SYSTEMS FOR VEHICLES

[0001] This application claims priority to and the benefit of Japanese Patent Application No. 2005-377877 filed on Dec. 28, 2005, the disclosure of which is incorporated herein by reference.

[0002] 1. Field of the Invention

[0003] The present invention relates to air conditioning systems for vehicles, and specifically, to air conditioning systems for vehicles using a natural refrigerant, such as carbon dioxide.

[0004] 2. Description of Related Art

[0005] Use of freon-based refrigerants in air conditioning systems for vehicles has been restricted due to environmental concerns. As a result, natural refrigerants, such as carbon dioxide, are being used as a replacement for freon-based refrigerants. Carbon dioxide refrigerant is non-toxic and non-combustible, but the temperature of refrigerant discharged from a compressor is relatively high, and the theoretical energy efficiency of carbon dioxide as a refrigerant is relatively low. To improve the energy efficiency of refigerant, a heat exchanger is provided for performing heat exchange between refrigerant at an exit side of a radiator and refrigerant drawn into a compressor, as shown in Japanese Patent Publication No. H11-193967 A. The resulting heat exchange between higher pressure refrigerant and lower pressure refrigerant reduces both the specific enthalpy of the lower pressure refrigerant at the exit side of the radiator and the pressure elevation of the higher pressure refrigerant at the compressor, thereby improving the efficiency of the refrigeration cycle.

[0006] Nevertheless, when the outside air temperature exceeds a critical temperature of carbon dioxide (about 32° C.), which may occur frequently when air conditioning is desired, during an idling operation, a thermal load on a refrigeration cycle, in particular, a radiator (e.g, a gas cooler) thereof, increases, and a temperature of refrigerant at a suction side of a compressor rises, which may result in superheating of the refrigerant. In such a condition, the heat radiation ability of an internal heat exchanger is reduced, and it may become difficult to lower the temperature of refrigerant at the exit side of the radiator sufficient for adequate air conditioning or cooling effect. Even if the refrigerant is passed through a pressure reducer and then evaporated by an evaporator in such a superheated state, the refrigerating ability of the air conditioning system may be significantly decreased.

[0007] Therefore, a need has arisen for air conditioning systems for vehicles that overcomes these and other shortcomings of the related art. A technical advantage of the present invention is that a second refrigeration cycle may be provided for performing heat exchange with refrigerant at an exit side of a radiator in the first refrigeration cycle, the refrigerant at the exit side of the radiator is not cooled by the refrigerant in the same cycle, but the temperature of the refrigerant at the exit side of the radiator is lowered using a new heat medium disposed in a second refrigeration cycle, which may be a cascade cycle relative to the first cycle, thereby increasing the refrigeration ability during an idling operation at elevated outside air temperatures. Another technical advantage of the present invention is that energy of the compressed refrigerant may be harnessed to power a compressor in the second refrigeration cycle, thereby obviating the need for an additional power source for the second refrigeration cycle. This may further improve an energy efficiency of the air conditioning system. Yet another technical advantage of the present invention is that the heat exchanger may use an opposing flow of refrigerant from the second refigerant cycle, instead of air cooling-type arrangement, to reduce the refrigerant temperature, thereby reducing tube length and providing a heat exchanger of reduced size that is suitable for mounting within the restricted space of a vehicle.

[0008] According to an embodiment of the invention, air conditioning systems for vehicles may comprise a first refrigeration cycle, an expander, and a second refrigeration cycle. The first refrigeration cycle may comprise a first compressor for compressing refrigerant, a first radiator fluidly connected to the compressor and configured to radiate heat from the refrigerant, a pressure reduction mechanism configured to reduce a pressure of refrigerant from the first radiator; an evaporator configured to evaporate an amount of refrigerant, and a gas/liquid separator configured to separate evaporated refrigerant from refrigerant in a liquid state. The expander may be disposed on a first fluid communication path between an outlet of the first radiator and an inlet of the first pressure reduction mechanism. The second refrigeration cycle, which may be a cascade cycle relative to the first refrigeration cycle, may comprise a second compressor powered by a pressure reduction and expansion energy of refrigerant resulting from an adiabatic expansion of the refrigerant carried out by the expander.

[0009] According to another embodiment of the invention, a method is provided for improving the energy efficiency of a natural refrigerant utilized in a vehicle air conditioner. The method may comprise the steps of performing a first refrigeration cycle and of performing a second refrigeration cycle. The first refrigeration cycle may comprise the substeps of compressing a first, natural refrigerant, radiating heat from the compressed natural refrigerant, expanding the compressed natural refrigerant, harnessing energy resulting from the expansion of the natural refrigerant, reducing a pressure of the radiated, compressed natural refrigerant; evaporating an amount of natural refrigerant, and separating evaporated refrigerant from refrigerant in a liquid state. The second refrigeration cycle, which may be a cascade cycle relative to the first refrigeration cycle, may comprise the substep of compressing a second refrigerant using energy harnessed from the expansion of the natural refrigerant.

[0010] Other objects, features, and advantages of the present invention will be apparent to persons of ordinary skill in the art in view of the foregoing detailed description of the invention and the accompanying drawings.

[0011] For a more complete understanding of the present invention, needs satisfied thereby, and the objects, features, and advantages thereof, reference now is made to the following description taken in connection with the accompanying drawings.

[0012] FIG. 1 is a circuit diagram of a refrigeration cycle in an air conditioning system for a vehicle, according to an embodiment of the present invention.

[0013] FIG. 2 is a cross-sectional view of the expander and the second compressor of FIG. 1, according to an embodiment of the present invention.

[0014] FIG. 3 is a perspective view showing an example of a double tube heat exchanger of FIG. 1, according to an embodiment of the present invention.

[0015] FIGS. 4A and 4B are a plan view and an elevational view, respectively, of the integrated radiator, according to an embodiment of the present invention.

[0016] Preferred embodiments of the present invention, and their features and advantages, may be understood by referring to FIGS. 1-4B, like numerals being used for corresponding parts in the various drawings.

[0017] Referring to FIG. 1, a refrigeration cycle 1 may be a refrigeration cycle for cooling an interior of a vehicle. In this first refrigeration cycle 1, a first compressor 11 for compressing refrigerant and a first radiator 12, which may be a gas cooler, for radiating heat from refrigerant may be provided. An expander 13 for reducing a pressure of and expanding refrigerant volume may be provided at a downstream position of first radiator 12 in the refrigerant flow direction. Expander 13 may be configured to harness energy resulting from the pressure reduction and expansion part of a pressure reducing process for refrigerant, which may be due to adiabatic expansion at expander 13 that is isoentropic, thereby permitting harnessing of the resultant energy. Generally, using a known expansion valve for expansion of refrigerant may result in an isoenthalpic change that is not as suitable for harnessing to perform work.

[0018] A heat exchanger 14 may be provided at a position downstream of expander 13, heat exchanger 14 may serve as an evaporator in a second refrigeration cycle 2 and may provide a heat exchange between refrigerant in first refrigeration cycle 1 and refrigerant in second refrigeration cycle 2. In FIG. 1, heat exchanger 14 is depicted as a double tube heat exchanger, which is described later. Second refrigeration cycle 2 may be a cascade cycle for performing heat exchange with first refrigeration cycle 1. A first pressure reducer 15, which may be an expansion valve or a throttle valve, may be provided in first refrigeration cycle 1 for reducing the pressure of refrigerant after passing through heat exchanger 14. An evaporator 16 then may evaporate an amount of refrigerant reduced in pressure by first pressure reducer 15. At a position downstream of evaporator 16, a gas/liquid separator 17 may be provided for separating the evaporated refrigerant from the refrigerant remaining in a liquid state, so that evaporated refrigerant may be sent to first compressor 11. Liquid refrigerant may be stored in the gas/liquid separator 17. A natural refrigerant, such as carbon dioxide, is preferred for the refrigerant for first refrigeration cycle 1. A hydrofluorocarbon (“HFC”) refrigerant, hydrocarbon (“HC”) refrigerant, or a natural refrigerant may be used for second refrigeration cycle 2, but HFC134a is preferred when a natural refrigerant is used as the refrigerant for first refrigeration cycle 1.

[0019] The pressure reduction and expansion energy recovered by expander 13 may be harnessed to power a second compressor 21 of second refrigeration cycle 2. In second refrigeration cycle 2, refrigerant may be compressed by second compressor 21, and the heat may be radiated from the compressed refrigerant by second radiator 22. Second radiator 22 may be formed as a structure integrated with first radiator 12. Refrigerant radiated by second radiator 22 may be passed through second pressure reducer 23, which also may be an expansion valve or a throttle valve, and thereafter, refrigerant may be evaporated by double tube heat exchanger 14 due to a heat exchange with refrigerant in first refrigerant cycle 1. As a result of heat exchange between the refrigerant in first refrigeration cycle 1 and the refrigerant in second refrigeration cycle 2 at double tube heat exchanger 14, the temperature of refrigerant may be reduced from the potentially superheated state at the outlet of first radiator 12. Alternatively, an orifice tube may be used in lieu of second pressure reducer 23 to reduce costs. Nevertheless, in consideration of load variation in second refrigeration cycle 2, it is preferred to provide a buffer function to double tube heat exchanger 14.

[0020] In view of the first law of thermodynamics, i.e., the law of energy conservation, the exchanged quantities of heat in heat exchanger 14 between first refrigeration cycle 1 and second refrigeration cycle 2 may be substantially or about the same. Further, the energy harnessed from the adiabatic expansion, i.e., isoentropic expansion, at expander 13 may be substantially or about equal to the power inputted to second compressor 21.

[0021] Referring to FIG. 2, second compressor 21 may be connected directly to expander 13. Impellers 41, which may be turbine impellers similarly in an exhaust gas turbine supercharger, may be provided in each of expander 13 and second compressor 21, and further may be coupled by a shaft 43. Each of respective impellers 41 may be disposed within casings 42. A pressure reducing process 44 of first refrigeration cycle 1 may occur in a route from first radiator 12 to heat exchanger 14, and an associated compression process 45 of second refrigeration cycle 2 may occur in a route from heat exchanger 14 to second radiator 22. As described above, because the pressure reduction and expansion energy recovered at pressure reducing process 44 may be used for compression process 45, an additional power source is not necessary for driving and circulating refrigerant of second refrigeration cycle 2, and the recovered energy may be effectively harnessed.

[0022] Referring to FIG. 3, heat exchanger 14 may be a double tube heat exchanger 51. The heat exchange between the refrigerant passed through expander 13 of first refrigeration cycle 1 and the refrigerant passed through second pressure reducer 23 of second refrigeration cycle 2 may be performed at a counter flow condition. By using a double tube, heat exchanger 51, as compared with another known air cooling-type heat exchanger, a necessary tube length may be shortened, which is advantageous for mounting heat exachanger 51 within a space of a vehicle that is restricted in shape and dimension. Further, increase of the efficiency of heat exchange between refrigerant of first and second refrigerant cycles 1, 2 may be expected due to the counter flow structure. Nevertheless, the structure of heat exchanger 14 is not limited to the structure of double tube, heat exchanger 51, which may be any suitable structure for permitting heat exchange between refrigerant of first and second refrigerant cycles 1, 2.

[0023] Referring to FIGS. 4A and 4B, first radiator 12 and second radiator 22 may be integrated with each other to form an integrated radiator 61. Because the temperature of refrigerant discharged from a compressor is relatively high, i.e. potentially superheated above a critical point, in a case of carbon dioxide refrigerant, integrated radiator 61 may comprise a structure for increasing an area for heat exchange, such as by increasing the number of paths or turns. An integrated structure for the respective radiators may be desirable for mounting an air conditioning system including such an integrated radiator arrangement within the restricted space of a vehicle interior.

[0024] In air conditioning systems for vehicles thus constructed, even during an idling operation at an elevated outside air temperature, the refrigeration ability of evaporator 16 may be increased because the temperature of refrigerant at the exit side of first radiator 12 of first refrigeration cycle 1 is reduced by the heat exchange with refrigerant of second refrigeration cycle 2 at heat exchanger 14. Further, because the refrigerant drawn into first compressor 11 is not heated, the specific volume of refrigerant vapor drawn into the compressor may be reduced and the volume efficiency of the compressor is increased, thereby reducing the compressor's power requirements. Moreover, the second refrigeration cycle 2 may be driven without a new power source because the compressor power of first refrigeration cycle 1 may be recovered by expander 13 and may be utilized as the power source for second refrigeration cycle 2. Therefore, the coefficient of performance as a whole of the refrigeration cycle may be increased, and a refrigeration cycle with a significantly increased energy efficiency may be achieved.

[0025] Air conditioning systems for vehicles according to the present invention may be suitable, in particular, for air conditioning systems comprising a refrigeration cycle using carbon dioxide refrigerant, which has a relatively low critical point and may be prone to superheating.

[0026] While the invention has been described in connection with preferred embodiments, it will be understood by those of ordinary skill in the art that other variations and modifications of the preferred embodiments described above may be made without departing from the scope of the invention. Other embodiments will be apparent to those of ordinary skill in the art from a consideration of the specification or practice of the invention disclosed herein. The specification and the described examples are considered as exemplary only, with the true scope and spirit of the invention indicated by the following claims.