WO2005124493A1

WO2005124493A1 - Pressure reducing device and pressure reducing system

Info

Publication number: WO2005124493A1
Application number: PCT/IB2005/001652
Authority: WO
Inventors: Masahiko Kimbara; Nobuo Kobayashi; Makoto Tsuzuki
Original assignee: Toyota Jidosha Kabushiki Kaisha; Kabushiki Kaisha Toyota Jidoshokki
Priority date: 2004-06-15
Filing date: 2005-06-14
Publication date: 2005-12-29
Also published as: JP2006003943A

Abstract

In a pressure reducing device (290, 291. 292, 293, 294) which can reduce a fluctuating pressure of a fluid in a passage, the effect of the passage resistance that occurs when the pressure is low is reduced, and the fluid is made to flow reliably. When the pressure of the fluid flowing in the pressure reducing device (290, 291, 292, 293, 294) is higher than a predetermined release pressure, a normal pressure reducing state where the pressure of the fluid is reduced and the fluid with the reduced pressure is made to flow out of the pressure reducing device (290, 291, 292, 293. 294) is achieved. On the other hand, when the pressure of the fluid flowing in the pressure reducing device (290, 291, 292, 293, 294) is equal to or lower than the predetermined release pressure, a pressure non-reducing state where the pressure of the fluid is not reduced and the fluid with the unchanged pressure is made to flow out of the pressure reducing device (290, 291, 292, 293, 294) is achieved.

Description

PRESSURE REDUCING DEVICE AND PRESSURE REDUCING SYSTEM

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The invention relates to a pressure reducing device and a pressure reducing system for reducing a fluctuating pressure of a fluid in a passage. 2. Description of the Related Art [0002] Various types of pressure reducing devices for reducing a fluctuating pressure of a fluid in a passage are known. For example, there is a known technology disclosed in Japanese Utility Model Application Publication No. 06-81005. In this technology, a pressure reducing device is used for reducing a pressure of hydrogen gas in a hydrogen tank and then supplying the hydrogen gas having a reduced pressure to a fuel cell stack, in a fuel cell system which generates electric power by a reaction of hydrogen gas and oxygen gas. [0003] Generally, hydrogen gas having a considerably high pressure is supplied into a hydrogen tank of a fuel cell system. As the hydrogen gas is consumed, the pressure in the tank is reduced. In the pressure reducing device disclosed in Japanese Utility Model Application Publication No. 06-81005, when the pressure in the hydrogen tank is reduced and a pressure of a fluid flowing in the pressure reducing device is reduced, a volume flow rate of the fluid per unit mass flow rate increases. Accordingly, an effect of a resistance of a passage to a flow of the fluid in the pressure reducing device increases, the resistance being generated when the fluid passes the passage (hereinafter, this resistance will be referred to as a "passage resistance"). As a result, when the pressure of the fluid is low, the fluid cannot pass through the pressure reducing device, and a considerable amount of hydrogen gas remains in the hydrogen tank. [0004] Such a problem occurs not only in a hydrogen gas pressure reducing device which is used in a fuel cell system, but also in a pressure reducing device which can reduce a fluctuating pressure of a fluid in a passage. SUMMARY OF THE INVENTION [0005] The invention is made in order to solve the above-mentioned problem. It is therefore an object of the invention to provide a technology for reducing an effect of passage resistance that occurs when a pressure of a fluid in a passage is low, thereby enabling the fluid to flow through the passage reliably, the technology being applicable to a pressure reducing device which can reduce the fluctuating pressure of the fluid in the passage. [0006] In order to achieve the above-mentioned object, there is provided a following pressure reducing device. The pressure reducing device which can reduce a fluctuating pressure of a fluid in a passage, is characterized in that, when the pressure of the fluid flowing in the pressure reducing device is higher than a predetermined release pressure, a normal pressure reducing state where the pressure of the fluid is reduced and the fluid with the reduced pressure is made to flow out of the pressure reducing device is achieved, and when the pressure of the fluid flowing in the pressure reducing device is equal to or lower than the predetermined release pressure, a pressure non-reducing state where the pressure of the fluid is not reduced and the fluid with the unchanged pressure is made to flow out of the pressure reducing device is achieved. [0007] With the pressure reducing device, when the pressure of the fluid flowing in the pressure reducing device is equal to or lower than the predetermined release pressure, the pressure non-reducing state where the pressure of the fluid is not reduced and the fluid with the unchanged pressure is made to flow out of the pressure reducing device is achieved. It is therefore possible to reduce the effect of the passage resistance that occurs when the pressure of the fluid is low, thereby enabling the fluid to flow through the passage reliably. [0008] In the pressure reducing device, in the normal pressure reducing state, a line indicating a relationship between the pressure of the fluid, which is obtained when the fluid is flowing in the pressure reducing device and the pressure of the fluid, which is obtained when the fluid is flowing out of the pressure reducing device may be linear, the pressure of the fluid, which is obtained when the fluid is flowing out of the pressure reducing device increasing with an increase in the pressure of the fluid, which is obtained when the fluid is flowing in the pressure reducing device. [0009] In the pressure reducing device, in the normal pressure reducing state, the pressure of the fluid, which is obtained when the fluid is flowing out of the pressure reducing device may be equal to or higher than one thirds of the pressure of the fluid, which is obtained when the fluid is flowing in the pressure reducing device, and lower than the pressure of the fluid, which is obtained when the fluid is flowing in the pressure reducing device. [0010] The pressure reducing device may include a housing; a high-pressure chamber formed in the housing, at a position on an upstream side in the passage formed in the pressure reducing device; a low-pressure chamber formed in the housing, at a position on a downstream side in the passage formed in the pressure reducing device; a piston which is housed in the hosing so as to be slidable between the high-pressure chamber and the low- pressure chamber, and which has a first pressure-receiving surface that receives the pressure of the fluid in the high-pressure chamber, a second pressure-receiving surface that receives the pressure of the fluid in the low-pressure chamber and whose area is larger than an area of the first pressure-receiving surface, and a through-passage that introduces the fluid in the high-pressure chamber into the low-pressure chamber; a valve portion which increases/decreases passage resistance in a region from the high pressure chamber to the low pressure chamber in accordance with sliding of the piston, thereby changing a state of the pressure reducing device between the normal pressure reducing state and the pressure non-reducing state; and an elastic body which urges the piston in a direction in which the passage resistance decreases. [0011] With this structure, the pressure reducing device can achieve the normal pressure reducing state and the pressure non-reducing state. [0012] In order to achieve the above-mentioned object, there is provided a following pressure reducing system. The pressure reducing system which is formed by connecting "n" ("n" is an integral number that is equal to or larger than 2) units of the pressure reducing devices to each other in series, is characterized in that a relationship between the pressure of the fluid, which is obtained when the fluid is flowing in the pressure reducing system, and the pressure of the fluid, which is obtained when the fluid is flowing out of the pressure reducing system, is set such that the predetermined release pressure varies with each pressure reducing device. [0013] With the pressure reducing system, the differential pressure applied to a seal portion of each of the pressure reducing devices can be reduced. As a result, leakage of the fluid can be suppressed. In addition, the pressure resistance performance of the seal portion can reduced, and therefore the response to the change in the pressure can be improved. Accordingly, the pressure reduction accuracy is improved. When the pressure of the fluid flowing in each pressure reducing device is lower than the corresponding release pressure, the pressure reducing device is brought into the pressure non-reducing state. Accordingly, a range of the inflow pressure, in which the pressure reducing device operates in the normal pressure reducing state is limited to a small range, and the effect of the passage resistance that occurs when the pressure is low in each pressure reducing device can be reduced. As a result, the pressure reduction accuracy of the pressure reducing system can be further improved. In addition, the fluid is allowed to flow reliably, even when the pressure is low. [0014] In the pressure reducing system, the relationship between the pressure of the fluid, which is obtained when the fluid is flowing in the pressure reducing system, and the pressure of the fluid, which is obtained when the fluid is flowing out of the pressure reducing system, may be set such that the predetermined release pressure of the pressure reducing device becomes higher toward an upstream side in the passage formed in the pressure reducing system. [0015] With this structure, the variable pressure reducing devices are sequentially brought into the pressure non-reducing state, starting from the pressure reducing device on the uppermost stream side, which requires the high pressure resistance performance and has relatively low pressure reduction accuracy. Therefore, the pressure reducing accuracy of the pressure reducing system can be improved. [0016] In the pressure reducing system, the "n" units of the pressure reducing devices may be configured such that, when the pressure of the fluid, which is obtained when the fluid is flowing in the pressure reducing system is the maximum value, the pressure of the fluid, which is obtained when the fluid is flowing out of the pressure reducing device located "m"th place ("m" is an integral number that is equal to or larger than 1 and that is equal to or lower than "n") from a downstream side in the passage formed in the pressure reducing system becomes substantially equal to m/(m+l) of the pressure of the fluid, which is obtained when the fluid is flowing in the pressure reducing device located "m"th place from the downstream side in the passage formed in the pressure reducing system. [0017] With this structure, the maximum value of the pressure reduction amount of each pressure reducing device can be reduced. Thus, the pressure reduction accuracy can be further improved, and the durability of each pressure reducing device can be increased. [0018] In the pressure reducing system, there may be further provided a second pressure reducing device which is provided in the passage formed in the pressure reducing system, at a position downstream of the "n" units of pressure reducing devices, and which reduces the pressure of the fluid flowing in the second pressure reducing device to a pressure substantially equal to a predetermined pressure and then makes the fluid with the reduced pressure flow out of the second pressure reducing device. [0019] With this structure, the pressure reducing system can supply the fluid having a predetermined pressure. Also, since the pressure reducing devices are provided upstream of the second pressure reducing device, the range of the pressure of the fluid flowing in the second pressure reducing device can be limited to a small range, and the pressure reduction accuracy of the second pressure reducing device can be improved. Also, the differential pressure applied to a seal portion of the second pressure reducing device can be reduced. As a result, leakage of fluid can be suppressed. In addition, the pressure resistance performance of the seal portion can reduced, and therefore the response to the change in the pressure can be improved. As a result, the pressure reduction accuracy can be further improved. [0020] In the pressure reducing system, a ratio of an area of the second pressure- reducing surface to an area of the first pressure-reducing surface in the pressure reducing device may become lower toward the upstream side in the passage formed in the pressure reducing system. [0021] With this structure, the difference in the pressure reduction amount among the variable pressure reducing devices can be reduced. In addition, the pressure reduction accuracy of the pressure reducing system can be improved. [0022] In the pressure reducing system, each of the pressure reducing devices may further have an intermediate chamber which is isolated from the high-pressure chamber and the low-pressure chamber, and which is formed between an inner wall of the housing and a side wall of the piston, and the intermediate chamber may be communicated with the low-pressure chamber of another pressure reducing device that is located downstream of the pressure reducing device. [0023] With this structure, leakage of the fluid to the outside of the pressure reducing system can be suppressed. Also, the differential pressure among the intermediate chamber, the high-pressure chamber, and the low-pressure chamber can be reduced, and the differential pressure applied to the seal portion can be reduced. [0024] In the pressure reducing system, the low-pressure chamber of each of the pressure reducing devices may be formed as a space that also serves as the high-pressure chamber of another pressure reducing device which is located immediately downstream of the pressure reducing device. [0025] With this structure, the number of the components can be reduced, the size of the device can be reduced, and the structure can be simplified. [0026] Note that the invention can be realized in various other embodiments. For example, the invention can be realized in, for example, a pressure reducing device, a pressure reducing system, a pressure reducing method, and a fuel cell system in which a pressure reducing device or a pressure reducing system is used. BRIEF DESCRIPTION OF THE DRAWINGS [0027] The above-mentioned embodiment and other embodiments, objects, features, advantages, technical and industrial significance of this invention will be better understood by reading the following detailed description of the exemplary embodiments of the invention, when considered in connection with the accompanying drawings, in which: FIG. 1 is a view schematically showing a structure of a fuel cell system for a vehicle, which includes a pressure reducing system according to a first embodiment of the invention; FIG. 2 is a view schematically showing a structure of the pressure reducing system according to the first embodiment; FIGS. 3 A and 3B are views for describing a pressure reducing operation of a variable pressure reducing valve; FIGS. 4 A and 4B are views for describing a pressure reducing operation of a constant pressure reducing portion; FIG. 5 is a graph showing pressure reducing characteristics of the pressure reducing system; FIG. 6 is a view schematically showing a structure of a pressure reducing system according to a second embodiment of the invention; and FIG. 7 is a view schematically showing a structure of a pressure reducing system according to a third embodiment of the invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS [0028] In the following description, the invention will be described in more detail in terms of exemplary embodiments. [0029] Hereafter, a first embodiment of the invention will be described in detail. FIG. 1 is a view schematically showing a structure of a fuel cell system 10 for a vehicle, which includes a pressure reducing system according to the first embodiment. The fuel cell system 10 generates electric power by an electrochemical reaction of hydrogen and oxygen. The generated electric power is used as power for a vehicle. The fuel cell system 10 includes a fuel cell stack 20, an airline 30, and a fuel line 40. [0030] The fuel cell stack 20 is a stacked body formed by stacking multiple cells (not shown). Each cell is formed by laminating a separator, a hydrogen electrode (hereinafter, referred to as an "anode"), an electrolyte membrane, an oxygen electrode (hereinafter, referred to as a "cathode"), and a separator in this order. The cell generates electric power by an electrochemical reaction of the hydrogen supplied through a groove formed in the separator and the oxygen contained in air. [0031] The airline 30 is a passage through which air is supplied to the fuel cell stack 20. The airline 30 includes a filter 32, a compressor 34, a humidifier 36, and a pipe 38 that connects the filter 32, the compressor 34, and the humidifier 36 to each other. The air taken in from the outside of the fuel cell system 10 through the filter 36 is compressed by the compressor 34, and humidified by the humidifier 36, and then supplied to the cathode of each of the multiple cells constituting the fuel cell stack 20. The exhaust gas that has been used for the reaction in the fuel cell stack 20 is discharged from an exhaust pipe located downstream of the fuel cell stack 20 to the outside of the fuel cell system 10. [0032] The fuel line 40 is a passage through which hydrogen gas serving as fuel is supplied to the fuel cell stack 20. The fuel line 40 includes a hydrogen tank 42, a pressure reducing system 44, a shut valve 46, and a pipe 48 that connects the hydrogen tank 42, the pressure reducing system 44, and the shut valve 46 to each other. The pressure of the hydrogen gas stored in the hydrogen tank 42 is reduced by the pressure reducing system 44, and then the hydrogen gas having the reduced pressure is supplied to the anode of each of the multiple cells constituting the fuel cell stack 20. The pressure in the hydrogen tank 42 is approximately 70 MPa, which is considerably high, when the hydrogen tank 42 is filled with the hydrogen gas. The pressure in the hydrogen tank 42 is reduced as the hydrogen gas flows out of the hydrogen tank 42. The pressure reducing system 44 reduces the pressure of the hydrogen gas, which flows in the pressure reducing system 44 from the hydrogen tank 42 through the pipe 48, to approximately 0.3 MPa, thereby preventing an excessively high pressure from being applied to the electrolyte membranes in the fuel cell stack 20. The shut valve 46 can permit/interrupt a flow in the fuel in the pipe 48. Also, the exhaust gas that has been used for the reaction in the fuel cell stack 20 is returned to the fuel line 40 by a hydrogen circulating pump 50. [0033] The electric power generated by the fuel cell stack 20 is output to an inverter 60, and used for driving a motor 70 for running of the vehicle. When the amount of generated electric power is larger than the amount of electric power required for running of the vehicle, and therefore there is excessive electric power, the excessive electric power is supplied to a storage battery 90 through a DC/DC converter 80 and then stored in the storage battery 90. The electric power stored in the storage battery 90 is used when electric power shortage occurs, for example, when the vehicle is accelerated suddenly. [0034] FIG. 2 is a view schematically showing a structure of the pressure reducing system 44 in the first embodiment. FIG. 2 is a vertical cross-sectional view of the pressure reducing system 44. The pressure reducing system 44 is formed by connecting two portions, that are, a variable pressure reducing portion 200 and a constant pressure reducing portion 300 to each other in series. The variable pressure reducing portion 200 is positioned upstream of the constant pressure reducing portion 300 (i.e., the variable pressure reducing portion 200 is closer to the hydrogen tank 42 than the constant pressure reducing portion 300 is). The hydrogen gas flowing from the hydrogen tank 42 into the pressure reducing system 44 initially passes through the variable pressure reducing portion 200, then passes through the constant pressure reducing portion 300, and flows into the fuel cell stack 20. [0035] The variable pressure reducing portion 200 in the first embodiment serves as four variable pressure reducing valves that are connected to each other in series. In this case, the pressure of the hydrogen gas flowing out of the variable pressure reducing valve (hereinafter, referred to as an "outflow pressure" where appropriate) (i.e, secondary pressure) changes in accordance with a change in the pressure of the hydrogen gas flowing in the variable pressure reducing valve (hereinafter, referred to as an "inflow pressure" where appropriate) (i.e, primary pressure). The variable pressure reducing portion 200 is not configured such that the variable pressure reducing portion 200 can be clearly divided into portions each of which serves as one variable pressure reducing valve. In the following description, however, the portion which serves as one variable pressure reducing valve is referred to as a variable pressure reducing valve 290, for convenience of description. The four variable pressure reducing valves 290 included in the variable pressure reducing portion 200 are referred to as a first variable pressure reducing valve 291, a second variable pressure reducing valve 292, a third variable pressure reducing valve 293, and a fourth variable pressure reducing valve 294, from the upstream side (from the side close to the hydrogen tank 42) toward the downstream side. [0036] The variable pressure reducing portion 200 includes a housing 210, a valve portion 220, a piston 230, and a lid portion 240. The housing 210 is formed as one portion shared by the four variable pressure reducing valves 290. Meanwhile, one valve portion 220, one piston 230, and one lid portion 240 are provided in each of the variable pressure reducing valves 290. Note that the valve portion 220 is not provided in the fourth variable pressure reducing valve 294, and a valve portion 320 of the constant pressure reducing portion 300, to be described later in detail, performs the function as the valve portion 220. In the following description, the term "valve portions 220" signifies not only the valve portions 220 of the first to third variable pressure reducing valves 291 to 293 but also the valve portion 320 in the fourth variable pressure reducing valve 294, unless otherwise stated. [0037] The housing 210. has four concave portions 211 in each of which the piston 230 is housed. Each concave portion 211 has a shape that is formed by stacking a small diameter concave portion 212 having a cylindrical shape with a small diameter and a large diameter concave portion 213 having a cylindrical shape with a diameter larger than the diameter of the small diameter concave portion 212 on each other on the same axis. The axes of the four concave portions 211 are parallel to each other. The positional relationship between the small diameter concave portion 212 and the large diameter concave portion 213 in each of the first concave portion 211 and the third concave portion 211 from the upstream side in the pressure reducing system 44 is opposite to the positional relationship between the small diameter concave portion 212 and the large diameter concave portion 213 in each of the second concave portion 211 and the fourth concave portion 211 from the upstream side in the pressure reducing system 44. Namely, in each of the first concave portion 211 and the third concave portion 211 from the upstream side in the pressure reducing system 44 (i.e, the right side in FIG. 2), the large diameter concave portion 213 is located on the small diameter concave portion 212. In contrast to this, in each of the second concave portion 211 and the fourth concave portion 211 from the upstream side in the pressure reducing system 44, the small diameter concave portion 212 is located on the large diameter concave portion 213. Note that the diameters of the small diameter concave portions 212 of the respective four concave portions 211 need not be equal to each other. Similarly, the diameters of the large diameter concave portions 213 of the respective four concave portions 211 need not be equal to each other. [0038] Hole portions 215 through which the valve portions 220 are fitted are formed in the housing 210. Each hole portion 215 is formed in the small diameter concave portion 212 at a position on the opposite side of the large diameter concave portion 213. The hole portion 215 is formed in a substantially cylindrical shape such that communication between the outside of the pressure reducing system 44 and the small diameter concave portion 212 is permitted. A female screw is formed in an inner surface of the housing 210, which defines the hole portion 215. Note that the hole portion 215 located on the lowermost stream side (i.e., on the far left side) is a hole portion through which the valve portion 320 of the constant pressure reducing portion 300 is inserted, instead of the valve portion 220. A female screw is not formed in an inner surface of the housing 210, which defines this hole portion 215. [0039] In addition, opening portions 216 through which the pistons 230 are inserted into the concave portions 211 are formed in the housing 210. Each opening portion 216 is formed in the large diameter concave portion 213 at a position on the opposite side of the small diameter concave portion 212. The opening portion 216 is formed in a substantially cylindrical shape such that communication between the outside of the pressure reducing system 44 and the large diameter concave portion 213 is permitted. [0040] The valve portion 220 has a substantially cylindrical body portion 221, and a substantially cone-shaped tip portion 222 which is formed at the tip of the body portion 221. A male screw is formed on the outer surface of the body portion 221. The male screw is fitted in the female screw formed in the inner surface of the housing 210, which defines the hole portion 215. [0041] The piston 230 is formed into a shape that is realized by stacking a small diameter piston portion 231 having a cylindrical shape with a small diameter and a large diameter piston portion 232 having a cylindrical shape with a diameter larger than the diameter of the small diameter piston portion 231 on each other on the same axis. The diameters of the small diameter piston portions 231 of the four pistons 230 are substantially equal to the diameters of the corresponding small diameter concave portions 212 of concave portions 211 in which the respective four pistons 230 are fitted. Similarly, the diameters of the large diameter piston portions 232 are substantially equal to the diameters of the corresponding large diameter concave portions 213 of the concave portions 211 in which the respective four pistons 230 are fitted. A through-passage 233 is formed through the piston 230 along the axes of the small diameter piston portion 231 and the large diameter piston portion 232. Grooves in which O-rings are fitted are formed in the outer surfaces of the small diameter piston portion 231 and the large diameter piston portion 232. [0042] The lid portion 240 has a cylindrical shape and has a diameter substantially equal to the diameter of the opening portion 216. A taper portion 241 is formed along the periphery of one of circular flat surfaces of the lid portion 240. Also, a groove in which an O-ring is fitted is formed in the outer surface of the lid portion 240. [0043] Assembly of the variable pressure reducing portion 200 is performed as follows. First, each valve portion 220 is inserted into the corresponding hole portion 215 from the outside of the housing 210 and fixed such that the tip portion 222 protrudes into the small diameter concave portion 212 of the housing 210 (this step is not performed for the concave portion 211 on the lowermost stream side). Next, each piston 230 with the O-rings fitted in the grooves formed in the small diameter piston portion 231 and the large diameter piston portion 232 is inserted from the corresponding opening portion 216, and then slidably housed in the corresponding concave portion 211 of the housing 210. At this time, a ring-shaped space (hereinafter, referred to as an "intermediate chamber 201") is formed between the outer surface of the small diameter piston portion 231 and the inner surface of the large diameter concave portion 213, in the large diameter concave portion 213. The intermediate chamber 201 is a space reserved to enable the piston 230 to slide. A spring 250 is provided in the intermediate chamber 201, and the spring 250 urges the piston 230 toward the opening portion 216. Finally, the lid portion 240 with the O-ring fitted in the groove is fitted in the opening portion 216 such that the surface in which the taper portion 241 is forms faces outward, and the lid portions 240 is fixed by crimping a part of a member of the housing 210, which is located around the lid portion 240. [0044] In the assembled variable pressure reducing portion 200, a high-pressure chamber 202 is formed between a bottom surface of each small piston portion 231 and the corresponding valve portion 220, and a low-pressure chamber 203 is formed between a bottom surface of each large diameter piston portion 232 and the corresponding lid portion 240. The through-passage 233 permits communication between the high-pressure chamber 202 and the low-pressure chamber 203. One unit of variable pressure reducing valve 290 is formed from the high-pressure chamber 202, the low-pressure chamber 203, the piston 230, the valve portion 220, the intermediate chamber 201, and the spring 250. [0045] A communication passage 208 permits communication between the low- pressure chamber 203 of each variable pressure reducing valve 290 and the high-pressure chamber 202 of the vaiiable pressure reducing valve 290 that is located immediately downstream of the variable pressure reducing valve 290. For example, the communication passage 208 permits communication between the low-pressure chamber 203 of the first variable pressure reducing valve 291 and the high-pressure chamber 202 of the second variable pressure reducing valve 292. Note that, in the fourth variable pressure reducing valve 294, the high-pressure chamber 202 is communicated with tire through-passage 233, and the through-passage 233 is communicated with the low-pressure chamber 203 and an inflow passage 323 of the constant pressure reducing portion 300. An inflow port 217 for connecting the pipe 48 of the fuel line 40 to the pressure reducing system 44 is formed in the housing 210 so as to be communicated with the high-pressure chamber 202 of the first variable pressure reducing valve 291. A passage is formed in the variable pressure reducing portion 200, the passage starting from the inflow port 217, passing through the high-pressure chambers 202 and the low-pressure chambers 203 formed in the respective variable pressure reducing valves 290 from the upstream side toward the downstream side in the variable pressure reducing portion 200, and being connected to the inflow passage 323 of the constant pressure reducing portion 300. A taper screw for connecting the pipe 48 to the pressure reducing system 44 is formed at the inflow port 217. [0046] A communication passage 209 permits communication between the intermediate chamber 201 of each variable pressure reducing valve 290 and the low- pressure chamber 203 of the variable pressure reducing valve 290 that is located immediately downstream of the variable pressure reducing valve 290. Therefore, the intermediate chamber 201 of each variable pressure reducing valve 290 is supplied with the pressure in the low-pressure chamber 203 of the variable pressure reducing valve 290 that is located immediately downstream of the variable pressure reducing valve 290. For example, the communication passage 209 permits communication between the intermediate chamber 201 of the first variable pressure reducing valve 291 and the low- pressure chamber 203 of the second variable pressure reducing valve 292. In this case, therefore, the intermediate chamber 201 of the first variable pressure reducing valve 291 is supplied with the pressure in the low-pressure chamber 203 of the second variable pressure reducing valve 292. Note that the intermediate chamber 201 of the fourth variable pressure reducing valve 294 is communicated with a first low-pressure chamber 303 of the constant pressure reducing portion 300 by an intermediate passage 430 and a relay member 420. [0047] The constant pressure reducing portion 300 in the first embodiment serves as a constant pressure reducing valve. In this case, the pressure of the hydrogen gas flowing out of the constant pressure reducing valve (hereinafter, referred to as the "outflow pressure" where appropriate) (i.e, secondary pressure) is substantially constant regardless of the degree of pressure of the hydrogen gas flowing in the constant pressure reducing valve (hereinafter, referred to as the "inflow pressure" where appropriate) (i.e, primary pressure). The constant pressure reducing portion 300 includes a housing 310, a piston 330, and a lid portion 340. [0048] The housing 310 has a concave portion 311 in which the piston 330 is housed. The concave portion 311 is formed in a shape that is obtained by stacking a small diameter concave portion 312 having a cylindrical shape with small diameter and a large diameter concave portion 313 having a cylindrical shape with a diameter that is larger than the diameter of the small diameter concave portion 312 on each other on the same axis. [0049] The housing 310 has a spring housing portion 314 that is a ring-shaped space in which a spring 350 is housed. The spring housing portion 314 is formed so as to surround the small diameter concave portion 312. The housing 310 has a substantially cylindrical opening portion 316, in which the piston 330 is inserted, in the concave portion 311. The opening portion 316 is formed in the large diameter concave portion 313 at a position on the opposite side of the small diameter concave portion 312. [0050] The housing 310 has the valve portion 320 in the small diameter concave portion 312. The valve portion 320 is located on the opposite side of the large diameter concave portion 313. As mentioned above, the valve portion 320 performs the function as the valve portion 220 for the fourth variable pressure reducing valve 294 of the variable pressure reducing portion 200. The valve portion 320 has a substantially cylindrical body portion 321, and a substantially cone-shaped tip portion 322 that is formed at the tip of the body portion 321. A groove in which an O-ring is fitted formed in an outer surface of the body portion 321. The inflow passage 323, which permits communication between an opening formed at the tip of a cone-shaped portion of the tip portion 322 and the small diameter concave portion 312, is formed along the axis of the valve portion 320. [0051] The piston 330 is formed in a shape that is obtained by stacking a small diameter piston portion 331 having a cylindrical shape with a small diameter and a large diameter piston portion 332 having a cylindrical shape with a diameter that is larger than the diameter of the small diameter piston portion 331 on each other on the same axis, and further providing a substantially cone-shaped tip portion 334 at the small diameter piston portion 331 at a position on the opposite side of the large diameter piston portion 332. The small diameter piston portion 331, the large diameter piston portion 332, and the tip portion 334 are stacked on each other on the same axis. The piston 330 has a communication passage 333 that permits communication between an opening formed in a side surface of the tip portion 334 and an opening formed in a bottom surface of the large diameter piston portion 332. A groove in which an O-ring is fitted is formed in an outer surface of each of the small diameter piston portion 331 and the large diameter piston portion 332. [0052] The lid portion 340 has a cylindrical shape, and has a diameter substantially equal to the diameter of the opening portion 316. A taper portion 341 is formed along the periphery of one of circular flat surfaces of the lid portion 340. Also, a groove in which an O-ring is fitted is formed in the outer surface of the lid portion 340. [0053] Assembly of the constant pressure reducing portion 300 is performed as follows. First, the piston 330 with the O-rings fitted in the grooves of the small diameter piston portion 331 and the large diameter piston portion 332 is inserted from the opening portion 316. Then, the piston 330 is slidably housed in the concave portion 311 of the housing 310. A portion of the large diameter concave portion 313, which is located between the bottom surface of the large diameter piston portion 332 and the upper end of the small diameter concave portion 312, and the spring housing portion 314 forms a space (hereinafter, referred to as an "intermediate chamber 301). The spring 350 is provided in the intermediate chamber 301. The spring 350 urges the piston 330 toward the opening portion 316. Finally, the lid portion 340 with the O-ring fitted in the groove is fitted in the opening portion 316 such that the surface in which the taper portion 341 is formed faces outward, and the lid portions 340 is fixed by crimping a part of a member of the housing 310, which is located around the lid portion 340. [0054] In the assembled constant pressure reducing portion 300, a first low-pressure chamber 303 is formed in the space in the small diameter concave portion 312. The first low-pressure chamber 303 is equivalent to a part of the space in the small diameter concave portion 312; the part of the space not being occupied by the small diameter piston portion 331 and the tip portion 334 of the piston 330. A second low-pressure chamber 304 is formed in the space of the large diameter concave portion 313. A second low- pressure chamber 304 is formed between the piston 330 and the lid portion 340. The communication passage 333 of the piston 330 permits communication between the first low-pressure chamber 303 and the second low-pressure chamber 304. [0055] An outflow port 317 for connecting the pipe 48 of the fuel line 40 to the pressure reducing system 44 is formed in the housing 310 so as to communicate with the second low-pressure chamber 304. A taper screw for connecting the pipe 48 to the pressure reducing system 44 is provided in the outflow port 317. The intermediate passage 309 permits communication between the intermediate chamber 301 and the outside of the pressure reducing system 44, and therefore the atmospheric pressure is introduced into the intermediate chamber 301. [0056] The variable pressure reducing portion 200 and the constant pressure reducing portion 300 are connected to each other by a fastening bolt 410. Thus, the variable pressure reducing portion 200 and the constant pressure reducing portion 300 are integrated with each other. At this time, the valve portion 320 of the constant pressure reducing portion 300 is inserted into the hole portion 215 of the variable pressure reducing portion 200 located on the lowermost stream side (far left side in FIG. 2). The opening of the inflow passage 323, which is formed in the tip portion 322 of the valve portion 320, faces the through-passage 233 formed in the piston 230 of the fourth variable pressure reducing valve 294. [0057] As mentioned above, communication between the intermediate chamber 201 of the fourth variable pressure reducing valve 294 and the first low-pressure chamber 303 is permitted by the intermediate passage 430. In order to secure gas-tightness of the intermediate passage 430, the hollow and substantially cylindrical relay member 420 with two O-rings fitted in grooves is provided in the intermediate passage 430 at a position near a joint surface at which the variable pressure portion 200 and the constant pressure reducing portion 300 are connected to each other. [0058] The hydrogen gas flowing from the hydrogen tank 42 into the pressure reducing system 44 flows into the first variable pressure reducing valve 291 of the variable pressure reducing portion 200 through the inflow port 217. The hydrogen gas then passes through the second variable pressure reducing valve 292, the third variable pressure reducing valve 293, and the fourth variable pressure reducing valve 294. The hydrogen gas then flows into the constant pressure reducing portion 300 through the inflow passage 323, and flows out of the pressure reducing system 44 through the outflow port 317. In the pressure reducing system 44, each of the pressure reducing valves 290 in the variable pressure reducing portion 200 and the constant pressure reducing portion 300 can perform a pressure reducing operation for reducing the pressure of the hydrogen gas. [0059] FIGS. 3A and 3B are views showing the pressure reducing operation of the variable pressure reducing valve 290. FIG. 3 A shows the workings of the pressure reducing operation of one pressure reducing valve 290. FIG. 3B shows the characteristics of the pressure reducing operation of one pressure reducing valve 290. [0060] As shown in FIG. 3A, in the variable pressure reducing valve 290, the tip portion 222 of the valve portion 220 and the through-passage 233 of the piston 230 are arranged on the same axis. The positional relationship between the valve portion 220 and the piston 230 is set such that the tip portion 222 of the valve portion 220 is inserted in the through-passage 233 when the piston 230 comes close to the valve portion 220. Therefore, a cross sectional area of a passage through which the hydrogen gas flows from the high-pressure chamber 202 into the through-passage 233 (hereinafter, referred to as a "valve portion passage area") increases/decreases according to the movement of the piston 230, and the passage resistance decreases/increases in accordance with an increase/decrease in the valve portion passage area. [0061] As the piston 230 comes close to the valve portion 220, the valve portion passage area decreases, and the passage resistance increases. In the variable pressure reducing valve 290, a direction in which the piston 230 comes close to the valve portion 220 (downward direction in FIG. 3A) is referred to as a "valve closing direction". When the piston 230 reaches a position at which the piston 230 contacts the valve portion 220, an opening of the through-passage 233, which faces the high-pressure chamber 202, (hereinafter, the opening will be referred to as a "passage port 234") is blocked by the tip portion 222. At this time, the valve portion passage area becomes zero, and the passage resistance becomes the maximum value. Also, movement of the fluid from the high- pressure chamber 202 into the through-passage 233 is interrupted. [0062] In contrast to this, when the piston 230 moves away from the valve portion 220, the valve portion passage area increases, and the passage resistance decreases. In the variable pressure reducing valve 290, the direction in which the piston 230 moves away from the valve portion 220 (upward direction in FIG. 3 A) is referred to as a "valve opening direction ". [0063] The pressure of the hydrogen gas in the high-pressure chamber 202 (hereinafter, referred to as the "primary pressure"), the pressure of the hydrogen gas in the intermediate chamber 201, and the force of the spring 250 are applied to the piston 230 of the variable pressure reducing valve 290, as the force applied in the valve opening direction. Meanwhile, as the force applied in the valve closing direction, the pressure of the hydrogen gas in the low-pressure chamber 203 (hereinafter, referred to as the "secondary pressure") is applied to the piston 230. In the variable pressure reducing valve 290, the piston 230 moves such that a balance between the total force applied in the valve opening direction and the total force applied in the valve closing direction is maintained, whereby the pressure reducing operation is performed. [0064] When the primary pressure is high, the force of the spring 250 is set so as to become sufficiently small as compared with the force applied in the valve opening direction due to the primary pressure and the force applied in the valve closing direction due to the secondary pressure. Also, the pressure in the intermediate chamber 201 of the variable pressure reducing valve 290 is equal to the pressure in the low-pressure chamber 203 of the variable pressure reducing valve 290 immediately downstream of the variable pressure reducing valve 290. For example, the pressure in the intermediate chamber 201 of the first variable pressure reducing valve 291 is equal to the pressure in the low-pressure chamber 203 of the second variable pressure reducing valve 292. In each variable pressure reducing valve 290, the pressure in the intermediate chamber 201 is lower than the pressure in the low-pressure chamber 203. In addition, an area of a pressure-receiving surface of the piston 230, which faces the high-pressure chamber 202 (hereinafter, referred to as a "primary side pressure-receiving surface") is smaller than an area of a pressure- receiving surface of the piston 230, which faces the low-pressure chamber 203 (hereinafter, referred to as a "secondary side pressure-receiving surface"). Accordingly, if the primary pressure is equal to the secondary pressure, the total force applied in the valve closing direction becomes larger than the total force applied in the valve opening direction due to the difference in the area between the pressure-receiving surfaces. As a result, the piston 230 moves in the valve closing direction. If the piston 230 moves in the valve closing direction, the valve portion passage area decreases and the passage resistance increases. Accordingly, the pressure of the hydrogen gas is reduced when the hydrogen gas flows from the high-pressure chamber 202 into the through-passage 233, and the secondary pressure is reduced. At this time, the secondary pressure is a value at which the balance between the total force applied in the valve opening direction and the total force applied in the valve closing direction is maintained. In the variable pressure reducing valve 290, even when the primary pressure changes, the piston 230 moves so as to increase/decrease the valve portion passage area (namely, decrease/increase the passage resistance) such that the balance between the total force applied in the valve opening direction and the total force applied in the valve closing direction is maintained, whereby the pressure reducing operation is performed. The state where the variable pressure reducing valve 290 performs the pressure reducing operation is referred to as a "normal pressure reducing state". [0065] In FIG. 3B, the horizontal axis indicates the primary pressure, and the vertical axis indicates the secondary pressure. FIG. 3B shows the change in the secondary pressure in accordance with the change in the primary pressure in the variable pressure reducing valve 290. A constant pressure line indicated by a dashed line shows the state where the primary pressure is equal to the secondary pressure. For example, when the primary pressure is Pis, the secondary pressure becomes POs due to the pressure reducing operation of the variable pressure reducing valve 290. When the primary pressure is reduced, the secondary pressure is also reduced while the balance between the total force applied in the valve opening direction and the total force applied in the valve closing direction is maintained. As shown in FIG. 3B, when the primary pressure is higher than a release primary pressure Plk, to be described later, the variable pressure reducing valve 290 is in the normal pressure reducing state. When the variable pressure reducing valve 290 is in the normal pressure reducing state, the line indicating the relationship between the primary pressure and the secondary pressure is linear, the secondary pressure increasing with an increase in the primary pressure. A ratio of the secondary pressure to the primary pressure (hereinafter, referred to as a "pressure reducing ratio") is set based mainly on a ratio of the area of the secondary side pressure-receiving surface to the area of the primary side pressure-receiving surface (hereinafter, referred to as a "pressure- receiving area ratio") in the piston 230. [0066] When the primary pressure is reduced, the primary pressure and the secondary pressure becomes equal to each other at a certain time point. In this state, the force of the spring 250 relatively increases due to the reduction of the primary pressure and the reduction of the secondary pressure that is caused by the reduction of the primary pressure, and balance between the total force applied in the valve opening direction and the total force applied in the valve closing direction is maintained even if the primary pressure and the secondary pressure are equal to each other. The primary pressure in this state is referred to as a "release primary pressure Plk". When the primary pressure is equal to or lower than the release primary pressure Plk, the valve portion passage area of the piston 230 of the variable pressure reducing valve 290 is sufficiently large. Accordingly, the piston 230 moves to a position at which there is almost no passage resistance when the hydrogen gas flows from the high-pressure chamber 202 into the through-passage 233. Therefore, the variable pressure reducing valve 290 does not perform the pressure reducing operation, and the primary pressure and the secondary pressure become substantially equal to each other. Such a state of the vaiiable pressure reducing valve 290 is referred to as a "pressure non-reducing state". The pressure non-reducing state includes the state where the minimum pressure loss occurs when the hydrogen gas flows from the high-pressure chamber 202 into the through-passage 233. [0067] The pressure reducing ratio and the release primary pressure Plk when the variable pressure reducing valve 290 is in the normal pressure reducing state can be arbitrarily set by adjusting the pressure-receiving area ratio and the force of the spring 250. For example, the pressure reducing ratio can be increased by increasing the pressure- receiving area ratio. Also, the release primary pressure Plk can be increased by increasing the force of the spring 250. [0068] FIGS. 4 A and 4B are views showing the pressure reducing operation of the constant pressure reducing portion 300. FIG. 4A shows the workings of the pressure reducing operation of the constant pressure reducing portion 300. FIG. 4B shows the characteristics of the pressure reducing operation of the constant pressure reducing portion

300. [0069] As shown in FIG. 4A, in the constant pressure reducing portion 300, the tip portion 334 of the piston 330 and the inflow passage 323 are arranged on the same axis. The positional relationship between the valve portion 320 and the piston 330 is set such that the tip portion 334 of the piston 330 is inserted into the inflow passage 323 when the piston 330 comes close to the inflow passage 323. Accordingly, as the piston 330 moves, an area of a passage through which the hydrogen gas flows from the inflow passage 323 into the first low-pressure chamber 303 (hereinafter, referred to as a "valve portion passage area") increases/decreases, and the passage resistance decreases/increases in accordance with the increase/decrease in the valve portion passage area. [0070] As the piston 330 comes closer to the inflow passage 323, the valve portion passage area decreases, and the passage resistance increases. In the constant pressure reducing portion 300, a direction in which the piston 330 comes close to the inflow passage 323 (downward direction in FIG. 4A) is referred to as a "valve closing direction". When the piston 330 reaches a position at which the piston 330 contacts the inflow passage 323, an opening of the inflow passage 323, which faces the first low-pressure chamber 303 (hereinafter, the opening will be referred to as a "passage port 324") is blocked by the tip portion 334. At this time, the valve portion passage area becomes zero, and the passage resistance becomes the maximum value. Also, movement of the fluid from the inflow passage 323 into the first low-pressure chamber 303 is interrupted. [0071] In contrast to this, as the piston 330 moves away from the inflow passage 323, the valve portion passage area increases and the passage resistance decreases. In the constant pressure reducing portion 300, a direction in which the piston 330 moves away from the inflow passage 323 (upward direction in FIG. 4A) is referred to as a "valve opening direction". [0072] As the force applied in the valve opening direction, the pressure of the hydrogen gas in the inflow passage 323 (hereinafter, referred to as the "primary pressure"), the pressure of the hydrogen gas in the first low-pressure chamber 303, the pressure of the air in the intermediate chamber 301, and the force of the spring 350 are applied to the piston 330 of the constant pressure reducing portion 300. Meanwhile, as the force applied in the valve closing direction, the pressure of the hydrogen gas in the second low-pressure chamber 304 (hereinafter, referred to as the "secondary pressure") is applied to the piston 330. In the constant pressure reducing portion 300, the piston 330 moves such that the balance between the total force applied in the valve opening direction and the total force applied in the valve closing direction is maintained, whereby the pressure reducing operation is performed. Since the passage resistance in the region from the first low- pressure chamber 303 to the second low-pressure chamber 304 through the communication passage 333 is considerably low, the pressure in the first low-pressure chamber 303 is equal to the pressure in the second low-pressure chamber 304 (namely, the secondary pressure). [0073] In the constant pressure reducing portion 300, the force of the spring 350 mainly constitute the force applied to the piston 330 in the valve opening direction. Since the pressure receiving surface of the piston 330, which faces the inflow passage 323 and the pressure-receiving surface of the piston 330, which faces the first low-pressure chamber 303 are considerably small, the force applied to the piston 330 due to the pressure in the inflow passage 323 and the pressure in the first low-pressure chamber 303 is considerably small. Also, since the pressure in the intermediate chamber 301 is the atmospheric pressure, the force applied to the piston 330 due to the pressure in the intermediate chamber 301 is also considerably small. [0074] Accordingly, in the constant pressure reducing portion 300, the valve portion passage area is increased/decreased (namely, the passage resistance is decreased/increased) due to the movement of the piston 330 such that the balance between the force of the spring 350 and the secondary pressure (the pressure in the second low-pressure chamber 304) is maintained, whereby the pressure reducing operation is performed. Since the force of the spring 350 does not change, the secondary pressure becomes a predetermined value which hardly changes. [0075] In FIG. 4B, the horizontal axis indicates the primary pressure, and the vertical axis indicates the secondary pressure. FIG. 4B shows the change in the secondary pressure due to the change in the primary pressure in the constant pressure reducing portion 300. Even when the primary pressure is reduced from the Pit, the secondary pressure is maintained at the predetermined pressure POt which hardly changes. The secondary pressure of the constant pressure reducing portion 300 can be arbitrarily set by adjusting the area of the pressure-receiving surface of the piston 330, which faces the second low-pressure chamber 304, and the force of the spring 350. [0076] FIG. 5 is a graph showing the pressure reducing characteristics of the pressure reducing system 44. In FIG. 5, the horizontal axis indicates the pressure in the hydrogen tank 42, and the vertical axis indicates the secondary pressure of each pressure reducing portion of the pressure reducing system 44. FIG. 5 shows the change in the secondary pressure of each pressure reducing portion in accordance with the change in the pressure in the hydrogen tank 42. The pressure reducing portions of the pressure reducing system 44 are pressure reducing valves 290 in the variable pressure reducing portion 200, and the constant pressure reducing portion 300. In FIG. 5, VI indicates the change in the secondaiy pressure of the first variable pressure reducing valve 291, V2 indicates the change in the secondary pressure of the second variable pressure reducing valve 292, V3 indicates the change in the secondary pressure of the third variable pressure reducing valve 293, V4 indicates the change in the secondary pressure of the fourth variable pressure reducing valve 294, and C indicates the change in the secondary pressure of the constant pressure reducing portion 300. Since the primary pressure of the first variable pressure reducing valve 291 is equal to the pressure in the hydrogen tank 42, the primary pressure of the first variable pressure reducing valve 291 is shown by the constant pressure line indicated by the dashed line in FIG. 5. The primary pressure of each of the other pressure reducing portions is equal to the secondary pressure of the pressure reducing portion located immediately upstream of the pressure reducing portion. For example, the primary pressure of the second pressure reducing valve 292 is equal to the secondary pressure of the first pressure reducing valve 291. Accordingly, the primary pressure of the second variable pressure reducing valve 292 is indicated by VI. [0077] As shown in FIG. 5, the pressure of the hydrogen gas flowing in the pressure reducing system 44 is progressively reduced by the pressure reducing portions. For example, when the pressure in the hydrogen tank 42 is the maximum value PTmax, the pressure of the hydrogen gas is reduced to PO1 by the first variable pressure reducing valve 291. Then, the pressure of the hydrogen gas is furtlier reduced to the PO2 by the second variable pressure reducing valve 292, then reduced to the PO3 by the third variable pressure reducing valve 293, then reduced to the PO4 by the fourth variable pressure reducing valve 294, and then reduced to the POc by the constant pressure reducing portion 300. [0078] The line indicating the relationship between the primary pressure and the secondary pressure of the variable pressure reducing valve 290 is linear, the secondary ΔΔ pressure increasing with an increase in the primary pressure. Therefore, the secondary pressure of each variable pressure reducing valve 290 is reduced with a reduction in the pressure in the hydrogen tank 42. Note that the secondary pressure of the constant pressure reducing portion 300 is substantially maintained at the predeterniined value POc even if the pressure in the hydrogen tank 42 is reduced. [0079] When the pressure in the hydrogen tank 42 is reduced to PTl, the primary pressure of the first variable pressure reducing valve 291 reaches the release primary pressure Plk, and the first variable pressure reducing valve 291 is brought into the pressure non-reducing state. Namely, the release primary pressure Plk of the first variable pressure reducing valve 291 is set to PTl . When the pressure in the hydrogen tank 42 is equal to or lower than PTl, the first variable pressure reducing valve 291 does not perform pressure reducing operation. Therefore, the hydrogen gas, which has flowed in the pressure reducing system 44, flows in the high-pressure chamber 202 of the second variable pressure reducing valve 292 while the pressure of the hydrogen gas is maintained. The pressure of the hydrogen gas is reduced by the second variable pressure reducing valve 292 and each of the pressure reducing portions located downstream of the second variable pressure reducing valve 292. [0080] When the pressure in the hydrogen tank 42 is reduced to PT2, the second variable pressure reducing valve 292 is brought into the pressure non-reducing state. When the pressure in the hydrogen tank 42 is reduced to PT3, the third pressure reducing valve 293 is brought into the pressure non-reducing state. When the pressure in the hydrogen tank 42 is reduced to PT4, the fourth variable pressure reducing valve 294 is brought into the pressure non-reducing state. Namely, the release primary pressure Plk of the second variable pressure reducing valve 292, the release primary pressure Plk of the third variable pressure reducing valve 293, and the release primary pressure Plk of the fourth variable pressure reducing valve 294 are set to PT2, PT3, and PT4, respectively. [0081] In the pressure reducing system 44 in the first embodiment, each pressure reducing valve 290 is brought into the pressure non-reducing state when the primary pressure is equal to or lower than the set release primary pressure Plk. Therefore, the range of the primary pressure in which each pressure reducing valve 290 operates in the normal pressure reducing state is limited to a predetermined range. For example, as shown in FIG. 5, a range ΔPI1 in which the first variable pressure reducing valve 291 operates in the normal pressure reducing state is a range in which the primary pressure is from PTmax to PTl. Similarly, each of the range ΔPI2 in which the second variable pressure reducing valve 292 operates in the normal pressure reducing state, the range ΔPI3 in which the third variable pressure reducing valve 293 operates in the normal pressure reducing state, and the range ΔPI4 in which the fourth variable pressure reducing valve 294 operates in the normal pressure reducing state is limited to a predetermined range. Generally, as the range of the primary pressure in which the pressure reducing operation is performed becomes smaller, the accuracy of pressure reduction (hereinafter, referred to as "pressure reduction accuracy") is improved. Therefore, in the pressure reducing system 44 in the first embodiment, the pressure reduction accuracy of each variable pressure reducing valve 290 can be improved. Therefore, the pressure reduction accuracy of the entire pressure reducing portion 200 can be also improved. Also, the range of the primary pressure in which each variable pressure reducing valve 290 operates in the normal pressure reducing state is the range excluding the region where the pressure is low. Accordingly, the effect of the passage resistance when the pressure is low can be reduced. As a result, the pressure reduction accuracy of each variable pressure reducing valve 290 can be further improved. In addition, the hydrogen gas is allowed to flow reliably, and therefore the amount of hydrogen gas remaining in the hydrogen tank 42 can be reduced. [0082] In the pressure reducing system 44 in the first embodiment, the relationship between the primary pressure and the secondary pressure is set such that the release primary pressure Plk of the variable pressure reducing valve 290 becomes higher toward the upstream side of the pressure reducing system 44. Generally, as the pressure resistance performance required of the pressure reducing device becomes higher, the response to the change in the primary pressure becomes lower, and therefore, the pressure reduction accuracy is reduced. In the pressure reducing system 44 in the first embodiment, the variable pressure reducing valves 290 are sequentially brought into the pressure non-reducing state in the order of the pressure resistance performance, starting from the first variable pressure reducing valve 291 having the highest pressure resistance performance. Namely, the variable pressure reducing valve 290 are sequentially made to stop the pressure reducing operation in the order of the pressure reduction accuracy, staring from the pressure reducing valve 290 having the lowest pressure reduction accuracy. Therefore, the pressure reducing accuracy can be further improved. [0083] In the pressure reducing system 44 in the first embodiment, as mentioned above, the line indicating the relationship between the primary pressure and the secondary pressure of each variable pressure reducing valve 290 is linear, the secondary pressure increasing with an increase in the primary pressure. Therefore, as shown in FIG. 5, as the primary pressure of each variable pressure reducing valve 290 is reduced, the secondary pressure is also reduced. Accordingly, the difference in the pressure reduction amount among the variable pressure reducing valves 290 and the constant pressure reducing portion 300 can be reduced. Thus, the differential pressure applied to the seal portion of each of the variable pressure reducing valves 290 and the constant pressure reducing portion 300 can be reduced. As a result, leakage of hydrogen gas can be suppressed. In addition, the pressure resistance performance of the seal portion can reduced, and therefore the response to the change in the pressure can be improved. Accordingly, the pressure reduction accuracy is improved. [0084] In the pressure reducing system 44 in the first embodiment, the secondary pressure of the constant pressure reducing portion 300 is substantially maintained at the predetermined value POc even when the pressure in the hydrogen tank 42 is reduced. Therefore, it is possible to supply the hydrogen gas having the predetermined pressure to the fuel cell stack 20. Also, since the variable pressure reducing valves 290 are provided upstream of the constant pressure reducing portion 300, the range of the pressure flowing in the constant pressure reducing portion 300 can be limited to a predetermined range. As a result, the pressure reduction accuracy of the constant pressure reducing portion 300 can be improved. Also, the differential pressure applied to the seal portion of the constant pressure reducing portion 300 can be reduced, and leakage of hydrogen gas can be suppressed. In addition, the pressure resistance performance of the seal portion can be reduced, and therefore the response to the change in the pressure can be improved. As a result, the pressure reduction accuracy can be further improved. [0085] In the pressure reducing system 44 in the first embodiment, preferably, a pressure reduction ratio (a ratio of the secondary pressure to the primary pressure in the normal pressure reducing state) of each variable pressure reducing valve 290 is equal to or higher than one-to-three and lower than one-to-one. More preferably, the pressure reduction ratio is equal to or higher than one-to-two and lower than one-to-one. As the pressure reducing ratio becomes higher, the secondary pressure tends to increase (namely, pressure reduction amount tends to become smaller). Also, preferably, the pressure reduction ratio of the variable pressure reducing valve 290 becomes higher toward the upstream side of the pressure reducing system 44. Therefore, preferably, the pressure- receiving surface area ratio (the ratio of the secondary side pressure-receiving area to the primary side pressure-receiving area in the piston 230) of the variable pressure reducing valve 290 becomes lower toward the upstream side of the pressure reducing system 44. Thus, the difference in the pressure reduction amount among the variable pressure reducing valves 290 can be reduced, and therefore the pressure reduction accuracy can be improved. [0086] In the pressure reducing system 44 in the first embodiment, when the pressure in the hydrogen tank 42 is the maximum value, preferably, the pressure reduction amounts of the pressure reducing portions in the pressure reducing system 44 (ΔPlmax, ΔP2max, ΔP3max, ΔP4max, and ΔPcmax in FIG. 5) become substantially equal to each other (for example, the difference is within ± 20%). When the pressure in the hydrogen tank 42 is the maximum value, the pressure reduction amount of the pressure reducing system 44 becomes the maximum value. Therefore, if the pressure reduction amounts of the pressure reducing portions are substantially equal to each other, the maximum value of the pressure reduction amount of each pressure reducing portion can be reduced. Thus, the pressure reduction accuracy can be further improved, and the durability of each pressure reducing portion can be increased. In order to make the pressure reduction amounts of the pressure reducing portions equal to each other, the secondary pressure of the variable pressure reducing valve 290 located "m"th place ("m" is an integral number that is equal to or larger than 1) from the downstream side is made to be m/(m+l) of the primary pressure. Accordingly, for example, preferably, the secondary pressure of the variable pressure reducing valve 290 located "m"th place ("m" is an integral number equal to or larger than 1) from the downstream side is in the range of +- 20% of m/(m+l) of the primary pressure. [0087] In the pressure 'reducing system 44 in the first embodiment, the intermediate chamber 201 of each of the variable pressure reducing valves 290 excluding the fourth variable pressure reducing valve 294 is communicated with the low-pressure chamber 203 of the variable pressure reducing valve 290 located immediately downstream of the variable pressure reducing valve 290. For example, the intermediate chamber 201 of the first variable pressure reducing valve 291 is communicated with the low-pressure chamber 203 of the second variable pressure reducing valve 292. The intermediate chamber 201 of the fourth variable pressure reducing valve 294 is communicated with the first low- pressure chamber 303 of the constant pressure reducing portion 300. The intermediate chamber 201 of the variable pressure reducing valve 290 is communicated with the chamber in the pressure reducing system 44, instead of being communicated with the outside of the pressure reducing system 44. Thus, leakage of the hydrogen gas in the variable pressure reducing valve 290 to the outside can be suppressed. As compared with the case where the intermediate chamber 201 is communicated with the outside of the pressure reducing system 44, the differential pressure among the intermediate chamber 201, the high-pressure chamber 202, and the low-pressure chamber 203 can be reduced, and the differential pressure applied to the seal members such as an O-ring can be reduced. In addition, air is not introduced into the intermediate chamber 201, and therefore hydrogen and oxygen do not contact each other. Accordingly, a possibility of occurrence of a reaction of hydrogen and oxygen in the variable pressure reducing valve 290 can be suppressed. [0088] Hereinafter, a second embodiment of the invention will be described. FIG. 6 is a view schematically showing a structure of a pressure reducing system 44a according to the second embodiment. FIG. 6 is a vertical cross sectional view of the pressure reducing system 44a. The structure of the pressure reducing system 44a according to the second embodiment shown in FIG. 6 is the same as the structure of the pressure reducing system 44 according to the first embodiment shown in FIG. 2, except for the structure of a variable pressure reducing portion 200a. [0089] The variable pressure reducing portion 200a in the pressure reducing system 44a in the second embodiment has a portion which serves as four variable pressure reducing valves 290a, as in the case of the first embodiment. For the sake of convenience, FIG. 6 shows only two variable pressure reducing valves 290a, that are, a third variable pressure reducing valve 293 a and a fourth variable pressure reducing valve 294a. In actuality, the variable pressure reducing portion 200a includes a second variable pressure reducing valve 292a located upstream of the third variable pressure reducing valve 293 a, and a first variable pressure reducing valve 291a located upstream of the second variable pressure reducing valve 292a. A high-pressure chamber 202a of the first variable pressure reducing valve 291a is communicated with the outside of the pressure reducing system 44 through an inflow port 217a (not shown). [0090] The variable pressure reducing valve 290a in the second embodiment includes the high-pressure chamber 202a, a first low-pressure chamber 204a, a second low-pressure chamber 205a, an intermediate chamber 201a, a piston 230a, and a spring 250a. The piston 230a has a substantially cone-shaped tip portion 234a which faces the high-pressure chamber 202a. When the piston 230a comes close to the high-pressure chamber 202a, the tip portion 234a is inserted into the substantially cylindrical high-pressure chamber 202a. Therefore, as the piston 230a slides, the cross sectional area of a passage through which the hydrogen has flows from the high-pressure chamber 202a into the first low-pressure chamber 204a increases/decreases, and therefore the passage pressure decreases/increases. Also, communication between the first low-pressure chamber 204a and the second low- pressure chamber 205a is permitted by a through-passage 233a formed in the piston 230a. Also, communication between the first low-pressure chamber 204a of the variable pressure reducing valve 290a and the high-pressure chamber 202a of the variable pressure reducing valve 290a located immediately downstream of the variable pressure reducing valve 290a is permitted by a communication passage 206a and a communication port 207a. For example, communication between the first low-pressure chamber 204a of the first variable pressure reducing valve 291a and the high-pressure chamber 202a of the second variable pressure reducing valve 292a is permitted by the communication passage 206a and the communication port 207a. [0091] Therefore, the variable pressure reducing valve 290a in the second embodiment is different from the variable pressure reducing valve 290 in the first embodiment in that the pressure of the hydrogen gas is reduced by using the passage pressure that occurs when the hydrogen gas flows from the high-pressure chamber 202a into the first low-pressure chamber 204a. Also, the second embodiment is different from the first embodiment in that, as the force applied to the piston 230a in the valve opening direction, the pressure in the first low-pressure chamber 204a is applied to the piston 230a in addition to the pressure in the high-pressure chamber 202a, the pressure in the intermediate chamber 201a, and the force of the spring 250. The second embodiment is the same as the first embodiment except for the above-mentioned aspects. In the pressure reducing valve 290a in the second embodiment as well, the piston 230a moves so as to increase/decrease the valve portion passage area (namely, decrease/increase the passage pressure) such that the balance between the total force applied in the valve opening direction and the total force applied in the valve closing direction is maintained, whereby the pressure reducing operation is performed. Also, the pressure reducing characteristics of each pressure reducing portion are the same as those in the first embodiment shown in FIG. 5. Accordingly, in the pressure reducing system 44a in the second embodiment, the pressure reduction accuracy can be improved, as in the case of the pressure reducing system 44 in the first embodiment. [0092] Hereinafter, a third embodiment of the invention will be described. FIG. 7 is a view schematically showing a structure of a pressure reducing system 44b according to the third embodiment. FIG. 7 is a vertical cross sectional view of the pressure reducing system 44b. The pressure reducing system 44b according to the third embodiment shown in FIG. 7 is the same as the pressure reducing system 44 according to the first embodiment shown in FIG. 2, except for the structure of a variable pressure reducing portion 200b. [0093] The pressure reducing portion 200b in the third embodiment is different from the pressure reducing portion 200 according to the first embodiment in that a chamber corresponding to the low-pressure chamber 203 of one variable pressure reducing valve 290 and a chamber corresponding to the high-pressure chamber 202 of the variable pressure reducing valve 290 immediately downstream of the one variable pressure reducing valve 290 are^' formed as one and the same space. Namely, for example, an intermediate chamber 280b formed between a first variable pressure reducing valve 291b and a second variable pressure reducing valve 292b serves as a low-pressure chamber for the first variable pressure reducing valve 291b and a high-pressure chamber for the second variable pressure reducing valve 292b. [0094] In the variable pressure reducing portion 200b in the third embodiment, in order to realize the above-mentioned structure, pistons 230b are provided on the same axis. Also, each piston 230b has a substantially cone-shaped tip portion 237b on the downstream side. When the adjacent pistons 230b come close to each other, the tip portion 237b is inserted into a through-passage 233b of the piston 230b. Therefore, as the pistons 230b relatively move, the cross sectional area of a passage through which the hydrogen gas flows from a high-pressure chamber 202b or the intermediate chamber 280b into the through-passage 233b increases/decreases, and therefore the passage resistance decreases/increases. A spring 250b is provided in the high-pressure chamber 202b or the intermediate chamber 280b. [0095] In the variable pressure reducing portion 200b in the third embodiment as well, the pistons 230b relatively move thereby increasing/decreasing the valve portion passage area (namely, decreasing/increasing the passage resistance) such that the balance between the total force applied in the valve opening direction and the total force applied in the valve closing direction is maintained in each variable pressure reducing valve 290, whereby the pressure reducing operation is performed. Also, the pressure reducing characteristics of each pressure reducing portion is the same as those in the first embodiment shown in FIG. 5. Accordingly, in the pressure reducing system 44b in the third embodiment as well, the pressure reduction accuracy can be improved, as in the case of the pressure reducing system 44 in the first embodiment. [0096] In addition, in the pressure reducing system 44b in the third embodiment, the number of the components can be reduced, the size of the device can be reduced, and the structure can be simplified. [0097] In the pressure reducing system 44b in the third embodiment, the diameter of a large diameter piston portion 232b of the piston 230b of each variable pressure reducing valve 290b is substantially equal to the diameter of a small piston portion 231b of the piston 230b of the variable pressure reducing valve 290b immediately downstream of the variable pressure reducing valve 290b. For example, the diameter of the large diameter piston portion 232b of the piston 230b of the first variable pressure reducing valve 291b is substantially equal to the diameter of the small piston portion 231b of the piston 230b of the second variable pressure reducing valve 292b. Therefore, the structure of a housing 210b can be simplified. [0098] In the pressure reducing system 44b in the third embodiment, communication between an intermediate chamber 201b of each pressure reducing valve 290b and a low- pressure chamber 203b (the intermediate chamber 280b) of the variable pressure reducing valve 290b located immediately downstream of the variable pressure reducing valve 290b is permitted by an intermediate passage 238b formed in the piston 230b. For example, communication between the intermediate chamber 201b of the first pressure reducing valve 291b and the low-pressure chamber 203b of the second variable pressure reducing valve 292b is permitted by the intermediate passage 238b formed in the piston 230b. Thus, the structure of the housing 210b can be further simplified, and the size of the device can be reduced. [0099] Note that the invention is not limited to the above-mentioned embodiments, and the invention may be realized in various other embodiments within the scope of the invention. For example, the following modification can be made. [0100] In the above-mentioned embodiments, the pressure reducing system 44 is formed by connecting the variable pressure reducing portion 200 and the constant pressure reducing portion 300 in series. However, the pressure reducing system 44 may have another structure. For example, the pressure reducing system 44 may be formed from only the variable pressure reducing portion 200. [0101] In the above-mentioned embodiments, the variable pressure reducing portion 200 of the pressure reducing system 44 has a portion which serves as the four variable pressure reducing valves 290. However, the number of the variable pressure reducing valves 290 can be arbitrarily set. Also, the pressure reducing system 44 may be formed from a single variable pressure reducing valve 290. [0102] The structure of the variable pressure reducing valve 290 and the structure of the constant pressure reducing portion 300 in each of the above-mentioned embodiments are merely one example. Each of the variable pressure reducing valve 290 and the constant pressure reducing portion 300 may have another structure. U [0103] In the above-mentioned embodiments, the case where the pressure reducing system is used in the fuel cell system for a vehicle is described. However, the invention may be applied to a pressure reducing system used in another type of fuel cell system, for example, a stationary fuel cell system, and a pressure reducing system used for elements other than a fuel cell system, as long as the system is a pressure reducing system in which the fluctuating pressure of the fluid in the passage is reduced. [0104] In the above-mentioned embodiments, the spring 250 is used to urge the piston

230 in the valve opening direction. However, another type of elastic body such as rubber may be used to urge the piston 230 in the valve opening direction. In each of the first embodiment and the second embodiment, the spring 250 is provided in the intermediate chamber 201. However, the spring 250 may be provided in another location. [0105] The view showing the characteristics of the pressure reducing system in the above-mentioned embodiments is merely one example. The pressure reducing system may have other characteristics.

Claims

CLAIMS:

1. A pressure reducing device which can reduce a fluctuating pressure of a fluid in a passage, characterized in that, when the pressure of the fluid flowing in the pressure reducing device (290, 291, 292, 293, 294) is higher than a predetermined release pressure, a normal pressure reducing state where the pressure of the fluid is reduced and the fluid with the reduced pressure is made to flow out of the pressure reducing device (290, 291, 292, 293, 294) is achieved, and when the pressure of the fluid flowing in the pressure reducing device (290, 291, 292, 293, 294) is equal to or lower than the predetermined release pressure, a pressure non- reducing state where the pressure of the fluid is not reduced and the fluid with the unchanged pressure is made to flow out of the pressure reducing device (290, 291, 292, 293, 294) is achieved.

2. The pressure reducing device according to claim 1, wherein, in the normal pressure reducing state, a line indicating a relationship between the pressure of the fluid, which is obtained when the fluid is flowing in the pressure reducing device (290, 291, 292, 293, 294) and the pressure of the fluid, which is obtained when the fluid is flowing out of the pressure reducing device (290, 291, 292, 293, 294) is linear, the pressure of the fluid, which is obtained when the fluid is flowing out of the pressure reducing device (290, 291, 292, 293, 294) increasing with an increase in the pressure of the fluid, which is obtained when the fluid is flowing in the pressure reducing device (290, 291, 292, 293, 294).

3. The pressure reducing device according to claim 2, wherein, in the normal pressure reducing state, the pressure of the fluid, which is obtained when the fluid is flowing out of the pressure reducing device (290, 291, 292, 293, 294) is equal to or higher than one thirds of the pressure of the fluid, which is obtained when the fluid is flowing in the pressure reducing device (290, 291, 292, 293, 294), and is lower than the pressure of the fluid, which is obtained when the fluid is flowing in the pressure reducing device (290, 291, 292, 293, 294).

4. The pressure reducing device according to any one of claims 1 to 3, characterized by comprising: a housing (210); a high-pressure chamber (202) formed in the housing (210), at a position on an upstream side in the passage formed in the pressure reducing device; a low-pressure chamber (203) formed in the housing (210), at a position on a downstream side in the passage formed in the pressure reducing device; a piston (230) which is housed in the hosing (210) so as to be slidable between the high- pressure chamber (202) and the low-pressure chamber (203), and which has a first pressure-receiving surface that receives the pressure of the fluid in the high-pressure chamber (202); a second pressure-receiving surface that receives the pressure of the fluid in the low-pressure chamber (203) and whose area is larger than an area of the first pressure-receiving surface; and a through-passage (233) that introduces the fluid in the high-pressure chamber (202) into the low-pressure chamber (203); a valve portion (220) which increases/decreases passage resistance in a region from the high pressure chamber (202) to the low pressure chamber (203) in accordance with sliding of the piston (230), thereby changing a state of the pressure reducing device (290, 291, 292, 293, 294) between the normal pressure reducing state and the pressure non-reducing state; and an elastic body (250) which urges the piston (230) in a direction in which the passage resistance decreases.

5. A pressure reducing system which is formed by connecting "n" ("n" is an integral number that is equal to or larger than 2) units of the pressure reducing devices (290, 291, 292, 293, 294) according to any one of claims 1 to 4, to each other in series, characterized in that a relationship between the pressure of the fluid, which is obtained when the fluid is flowing in the pressure reducing system (200), and the pressure of the fluid, which is obtained when the fluid is flowing out of the pressure reducing system (200), is set such that the predetermined release pressure varies with each pressure reducing device (290, 291, 292, 293, 294).

6. The pressure reducing system according to claim 5, wherein the relationship between the pressure of the fluid, which is obtained when the fluid is flowing in the pressure reducing system (200), and the pressure of the fluid, which is obtained when the fluid is flowing out of the pressure reducing system (200), is set such that the predetermined release pressure of the pressure reducing device (290, 291, 292, 293, 294) becomes higher toward an upstream side in the passage formed in the pressure reducing system.

7. The pressure reducing system according to claim 6, wherein, the "n" units of the pressure reducing devices (290, 291, 292, 293, 294) are configured such that, when the pressure of the fluid, which is obtained when the fluid is flowing in the pressure reducing system (200) is a maximum value, the pressure of the fluid, which is obtained when the fluid is flowing out of the pressure reducing device (290, 291, 292, 293, 294) located "m"th place ("m" is an integral number that is equal to or larger than 1 and that is equal to or lower than "n") from a downstream side in the passage formed in the pressure reducing system becomes substantially equal to m/(m+l) of the pressure of the fluid, which is obtained when the fluid is flowing in the pressure reducing device(290, 291, 292, 293, 294) located "m"th place from the downstream side in the passage formed in the pressure reducing system.

8. The pressure reducing system according to any one of claims 5 to 7, characterized by further comprising: a second pressure reducing device (300) which is provided in the passage formed in the pressure reducing system, at a position downstream of the "n" units of pressure reducing devices (290, 291, 292, 293, 294), and which reduces the pressure of the fluid flowing in the second pressure reducing device (300) to a pressure substantially equal to a predetermined pressure and then makes the fluid with the reduced pressure flow out of the second pressure reducing device (300).

9. The pressure reducing system which .is formed by connecting "n" ("n" is an integral number that is equal to or larger than 2) units of the pressure reducing devices (290, 291, 292, 293, 294) according to claim 4, to each other in series, characterized in that; a ratio of an area of the second pressure-reducing surface to an area of the first pressure- reducing surface in the pressure reducing device (290, 291, 292, 293, 294) becomes lower toward the upstream side in the passage formed in the pressure reducing system.

10. The pressure reducing system which is formed by connecting "n" ("n" is an integral number that is equal to or larger than 2) units of the pressure reducing devices (290, 291, 292, 293, 294) according to claim 4, to each other in series, characterized in that each of the pressure reducing devices (290, 291, 292, 293, 294) further has an intermediate chamber (201) which is isolated from the high-pressure chamber (202) and the low-pressure chamber (203), and which is formed between an inner wall of the housing

(210) and a side wall of the piston (230), and the intermediate chamber (201) is communicated with the low-pressure chamber (203) of another pressure reducing device (290, 292, 293, 294) that is located immediately downstream of the pressure reducing device (290, 291, 292, 293).

11. The pressure reducing system which is formed by connecting "n" ("n" is an integral number that is equal to or larger than 2) units of the pressure reducing devices (290, 291,

292, 293, 294) according to claim 4, to each other in series, characterized in that the low-pressure chamber (203) of each of the pressure reducing devices (290, 291, 292,

293, 294) is formed as a space that also serves as the high-pressure chamber (202) of another pressure reducing device (290, 292, 293, 294) which is located immediately downstream of the pressure reducing device (290, 291, 292, 293).