Patent Publication Number: US-8973623-B2

Title: Gas filling system, and vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase application of International Application No. PCT/IB2011/000069, filed Jan. 18, 2011, and claims the priority of Japanese Application No. 2010-018248, filed Jan. 29, 2010, the content of both of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a gas filling system that fills gas from a gas station into, for example, a vehicle-mounted fuel tank, and also relates to a vehicle equipped with such a fuel tank. 
     2. Description of the Related Art 
     Known gas stations as mentioned above include a hydrogen station that fills hydrogen gas into a hydrogen tank of a fuel cell vehicle (see, for example, Japanese Patent Application Publication No. 2005-069327 (JP-A-2005-069327)) and Japanese Patent Application Publication No. 2007-321572 (JP-A-2007-321572)). In Japanese Patent Application Publication No. 2005-069327 (JP-A-2005-069327), the vehicle and the hydrogen station are electrically connected via an adaptor, and information about the maximum service pressure of the hydrogen tank of the vehicle is grasped at a hydrogen station side in order to control hydrogen filling so that the maximum service pressure will not be exceeded. 
     While hydrogen is being filled, it is desirable to monitor the temperature and the pressure in the hydrogen tank which rise in association with the hydrogen filling. In conjunction with this respect, consideration has been given to a construction in which, through the use of a communication equipment, signals regarding the temperature and the like in the fuel tank are sent from the vehicle side and the hydrogen station as need arises, and the hydrogen station automatically stops the hydrogen filling before the allowable temperature (reference value) of the hydrogen tank is exceeded (see, Japanese Patent Application Publication No. 2007-321572 (JP-A-2007-321572)). 
     By the way, the hydrogen filling sometimes requires a long time, depending on conditions. For example, during the daytime under a blazing sun, the hydrogen tank is exposed to an environment whose temperature is higher than normal temperature. If, in view of this situation, the hydrogen filling is performed at slower filling speed than at normal temperature so that the temperature of the hydrogen tank will not exceed its allowable temperature, the filling time becomes long. If during such a long-time filling, data about, for example, temperature in the hydrogen tank, and the like, is transmitted from the vehicle side to the hydrogen station side as need arises, there is a possibility that the electric power source of the vehicle, which is at rest, may lack power. 
     SUMMARY OF THE INVENTION 
     The invention provides a gas filling system and a vehicle that reduce the electric power consumed during the gas filling. 
     A gas filling system in accordance with a first aspect of the invention includes: a vehicle having a fuel tank, a transmitter, and a control device that is connected to the transmitter; and a gas station having a receiver that receives a signal from the transmitter, wherein the control device changes a transmission cycle of the signal that is transmitted from the transmitter to the receiver during gas filling from the gas station to the fuel tank, according to a state quantity regarding an interior of the fuel tank. 
     A vehicle in accordance with a second aspect of the invention includes: a fuel tank into which gas is filled from a gas station that has a receiver; a transmitter; and a control device that is connected to the transmitter, and that changes a transmission cycle of a signal transmitted from the transmitter to the receiver during the gas filling from the gas station to the fuel tank, according to a state quantity regarding an interior of the fuel tank. 
     According to the first and second aspects of the invention, the signal transmission cycle used during the gas filling is not fixed, but is changed according to a state quantity regarding the interior of the fuel tank. Due to this construction, for example, in the case where the interior of the fuel tank has such a state quantity that the filling time will be long, selective use of a long transmission cycle will reduce the electric power needed for the transmission from the transmitter in a single operation of gas filling as a whole. Hence, the electric power consumption during gas filling can be reduced. 
     In the first and second aspect of the invention, the state quantity regarding the interior of the fuel tank may include temperature and pressure in the fuel tank, and if the temperature in the fuel tank is greater than or equal to a first threshold value and the pressure in the fuel tank is less than or equal to a second threshold value, the control device may make the transmission cycle longer than if the temperature in the fuel tank is less than the first threshold value or if the pressure in the fuel tank is greater than the second threshold value. 
     According to this construction, in the former case, in which the filling time will be longer than in the latter case, a longer transmission cycle can be used to reduce the electric power consumption during the gas filling. In particular, since the temperature and the pressure in the fuel tank are both compared with their respective threshold values, the state in the fuel tank can be precisely grasped, so that the length of the transmission cycle can be appropriately determined. 
     It is to be noted herein that the first threshold value and the second threshold value each vary depending on the kinds of gas. In an example case where hydrogen gas is filled, the first threshold value is 60° C., and the second threshold value is 2 MPa. 
     The vehicle may further include: a temperature sensor that detects the temperature in the fuel tank; a pressure sensor that detects the pressure in the fuel tank, and the temperature in the fuel tank that is compared with the first threshold value may be the temperature that the temperature sensor detects before the gas filling starts, and the pressure in the fuel tank that is compared with the second threshold value may be the pressure that the pressure sensor detects before the gas filling starts. 
     According to this construction, the length of the signal transmission cycle during gas filling is determined on the basis of the temperature and the pressure in the fuel tank which are grasped prior to the start of gas filling. This makes it possible to use a long signal transmission cycle from the start of gas filling, so that it becomes possible to further reduce the electric power consumption. 
     The signal transmitted from the transmitter to the receiver during the gas filling may include detection information about the temperature and the pressure in the fuel tank that the temperature sensor and the pressure sensor detect during the gas filling. 
     According to this construction, it is possible to detect the temperature and the pressure in the fuel tank during the gas filling at the side of the gas station while restraining the electric power consumption needed for the transmission performed by the transmitter. 
     The gas station may have a station-side control device that is connected to the receiver, and the station-side control device may control the gas filling into the fuel tank based on the detection signal that the receiver receives. 
     According to this construction, at least one condition in the gas filling suitable to the pressure and the temperature in the fuel tank during the gas filling, for example, at least one of the filling speed and the amount of filling, can be changed. 
     In the foregoing construction, during the gas filling, if the receiver does not receive the signal from the transmitter during a predetermined cycle, the station-side control device may determine that a transmission error has occurred, and the control device may set the transmission cycle at less than half of the predetermined cycle. 
     According to this construction, it becomes possible to perform a process in terms of control in which it is determined that there is no abnormality occurring in the transmitter despite actual occurrence of a transmission error if the occurrence of the transmission error is the first occurrence, even in the case where a long transmission cycle is used. 
     The vehicle may have an electricity storage device that supplies electric power to the temperature sensor, the pressure sensor, the transmitter and the control device. 
     Since measures are provided to reduce the electric power consumption during the gas filling as mentioned above, the power shortage of the electricity storage device during gas filling can be restrained. This restrains, for example, occurrence of an event that the temperature sensor or the like cannot input information. 
     The control device may use a first transmission cycle if an elapsed time following start of the gas filling is less than or equal to a third threshold value, and the control device may use a second transmission cycle that is longer than the first transmission cycle if the elapsed time exceeds the third threshold value. 
     According to this construction, in the case of a long-time gas filling that lasts longer than the third threshold value, the electric power consumption during gas filling can be reduced. 
     The transmitter and the receiver may be infrared communication instruments. 
     This construction reduces the electric power consumption during the gas filling for which infrared communication is used. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, advantages, and technical and industrial significance of this invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a schematic diagram of a gas filling system in accordance with an embodiment of the invention; 
         FIG. 2  is a construction diagram of a gas filling system in accordance with the embodiment of the invention; 
         FIG. 3  is a flowchart showing an operation procedure that a gas filling system in accordance with the embodiment of the invention performs before starting filling gas; 
         FIG. 4  is a flowchart showing an operation procedure of determining a transmission cycle in accordance with a first modification of the embodiment of the invention; and 
         FIG. 5  is a flowchart showing an operation procedure of determining a transmission cycle in accordance with a second modification of the embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiments of the invention will be described hereinafter with reference to the drawing. The following description will be made in conjunction with an example of a gas filling system in which hydrogen gas is filled from a hydrogen station into a fuel tank of a fuel cell vehicle equipped with a fuel cell system. Incidentally, the fuel cell system includes a fuel cell that generates electricity by electrochemical reaction between a fuel gas (e.g., hydrogen gas) and an oxidizing gas (e.g., air). Besides, the filling of hydrogen gas is one manner of supplying hydrogen gas from the hydrogen station into the fuel tank. 
     As shown in  FIG. 1 , a gas filling system  1  includes, for example, a hydrogen station  2  as a gas station, and a vehicle  3  that is supplied with hydrogen gas from the hydrogen station  2 . 
     As shown in  FIG. 2 , the vehicle  3  includes a fuel tank  30 , a receptacle  32 , a pressure sensor  36 , a temperature sensor  38 , a display device  42 , a communication instrument  44 , and a control device  46 . The fuel tank  30  is a fuel gas supply source for the fuel cell, and, for example, is a high-pressure tank that is capable of storing hydrogen gas at 35 MPa or 70 MPa. In the case where a plurality of fuel tanks  30  are mounted in the vehicle  3 , the fuel tanks  30  are connected in parallel to the fuel cell. The hydrogen gas in the fuel tank  30  is supplied to the fuel cell  50  through a supply channel  52 . On the other hand, the supply of hydrogen gas to the fuel tank  30  is carried out by the hydrogen station  2  releasing hydrogen gas into a filling channel  34  through a receptacle  32 . The filling channel  34  is made up of a gas piping provided outside the fuel tank  30 , and a channel portion that is provided in a valve assembly (not shown) that is attached to an opening portion of the fuel tank  30 . Besides, the filling channel  34  is provided with a check valve  35  for restraining reverse flow of the hydrogen gas. 
     The pressure sensor  36  detects the pressure of the hydrogen gas released from the hydrogen station  2 , and is provided in the filling channel  34 . For example, the pressure sensor  36  is provided in the foregoing gas piping that is provided at a downstream side of the check valve  35  and that is immediately upstream of the fuel tank  30 , and detects a pressure that substantially reflects the pressure of the hydrogen gas within the fuel tank  30  (hereinafter, referred to as “tank pressure”). The temperature sensor  38  is provided in a channel portion in the valve assembly, and is disposed within the fuel tank  30 . The temperature sensor  38  detects a temperature that reflects the temperature of hydrogen gas within the fuel tank  30  (hereinafter, referred to as “tank temperature”). Incidentally, in other embodiments, the pressure sensor  36  may also be disposed within the fuel tank  30 . Besides, the position in the fuel tank  30  at which the temperature sensor  38  is disposed is not particularly limited as long as the position allows the tank temperature to be substantially detected. It is preferable that the temperature sensor  38  be provided near a blowout opening that leads hydrogen gas into the fuel tank  30 . 
     The display device  42  is able to be used, for example, as a portion of a car navigation system, and displays various kinds of information in its display screen. The communication instrument  44  is provided for communicating with the hydrogen station  2 , and, for example, as a communication interface for wireless communication such as an infrared communication and the like. The communication instrument  44  is built in the receptacle  32  or is fixed within a lidded box of the vehicle  3  so as to be capable of communication when a filling nozzle  12  of the hydrogen station  2  is connected to the receptacle  32 . 
     The control device  46  is constructed as a microcomputer that has therein a CPU, a ROM and a RAM. The control device  46  controls the vehicle  3 . The CPU executes desired computations according to control programs so as to perform various processes and controls, such as changing the transmission cycle of the communication instrument  44 . The ROM stores control programs and control data that are processed by the CPU. The RAM is used mainly as various workspaces for control processes. The control device  46  is connected to the pressure sensor  36 , the temperature sensor  38 , a display device  42 , a communication instrument  44 , etc., and sends, for example, information detected by the pressure sensor  36  and the temperature sensor  38 , to the hydrogen station  2  by using the communication instrument  44 . 
     A high-voltage DC/DC converter  54  that controls the output voltage of the fuel cell  50  is connected to the fuel cell  50 . Various high-voltage accessories  56  that are used for the operation of the fuel cell  50  are electrically connected between the fuel cell  50  and the high-voltage DC/DC converter  54 . The high-voltage accessories  56  are accessories that need high voltage (e.g., a voltage above 12 V), for example, an air compressor that pressure-feeds an oxidizing gas to the fuel cell  50 , a hydrogen pump that supplies an hydrogen off-gas to the fuel cell  50  in a circulating manner. The electricity storage device  58  is a chargeable-dischargeable secondary cell that functions as a high-voltage electricity storage device. The electricity storage device  58  is connected to, for example, a traction motor of the vehicle  3  via the high-voltage DC/DC converter  54 , in parallel with the fuel cell  50 . 
     A low-voltage DC/DC converter  60  is electrically connected between the high-voltage DC/DC converter  54  and the electricity storage device  58 . The low-voltage DC/DC converter  60  lowers the voltage of a portion of the direct-current power of a high-voltage circuit side, and supplies the lowered-voltage power to a low-voltage circuit side. The lowered-voltage direct-current power is partially used to charge the low-voltage battery  62 , and is partially used as power to drive low-voltage accessories  64 . The low-voltage battery  62  functions as an electricity storage device of low voltage (e.g., 12 V), and is a secondary cell that is represented by, for example, a nickel metal hydride cell, a lithium ion cell. The low-voltage accessories  64  are various appliances that are driven at low voltage, and are different from the high-voltage accessories  56 . What are regarded as the low-voltage accessories  64  are, for example, the pressure sensor  36 , the temperature sensor  38 , the display device  42 , the communication instrument  44 , etc. The low-voltage accessories  64  and the control device  46  are also driven when supplied with electric power from the low-voltage battery  62  as a power supply source. For example, while the vehicle  3  is in an ignition-off state (hereinafter, referred to as “IGOFF”), the fuel cell  50  does not generate electricity, and therefore the power for the low-voltage accessories  64  and the control device  46  is supplied from the low-voltage battery  62 . 
     The hydrogen station  2  includes a control device  5  that controls various appliances and the like that are provided at the hydrogen station  2 , a communication instrument  6  for communicating with the vehicle  3 , a display device  7  that displays various kinds of information in its display screen, and an outside air temperature sensor  8  that detects the outside air temperature at the location where the hydrogen station  2  is installed. The communication instrument  6  is of a type that is pursuant with the communication instrument  44  of the vehicle  3 , and sends and receives various kinds of information to and from the communication instrument  44 . The display device  7  displays information such as the filling speed (filling flow rate) and the filling amount during the filling operation, and the like. Besides, the display screen of the display device  7  is equipped with an operation panel on which an operating person or a user (hereinafter, collectively referred to as “user”) is allowed to input a desired filling condition. That is, the user is allowed to designate a complete filling of the tank (complete filling), a desired filling amount, a desired amount of money to be paid for a single operation of filling, etc., as desired filling conditions, via the operation panel of the display device  7 . Incidentally, in other embodiments, it is also permissible to adopt a construction that is able to receive filling conditions input from a user by using a device different from the display device  7 . 
     Besides, the hydrogen station  2  has a cluster of tanks (a gas supply source)  11  that stores hydrogen gas, a filling nozzle  12  that releases hydrogen gas toward the fuel tank  30  mounted in the vehicle  3 , and a gas channel  13  that connects the cluster of tanks  11  and the filling nozzle  12 . The filling nozzle  12  is also called filling coupling, and is connected to the receptacle  32  of the vehicle  3  when hydrogen gas is filled into the vehicle  3 . The filling nozzle  12  and the receptacle  32  form a connecting unit that connects the hydrogen station  2  and the fuel tank  30 . Besides, the filling nozzle  12  is provided with a pressure sensor  9  and a temperature sensor  10  that detect the pressure and the temperature of hydrogen gas that the hydrogen station  2  supplies to the fuel tank  30 . Due to the provision of these sensors  9  and  10  on the filling nozzle  12 , the actual pressure and the actual temperature of the hydrogen that is actually supplied from the hydrogen station  2  to the fuel tank  30  can be acquired by a simple construction. Besides, it is preferable that the temperature sensor  10  be provided at a distal end of the filling nozzle  12  (a fuel tank  30 -side portion of the filling nozzle  12 ). 
     The gas channel  13  is provided with a compressor  14 , a pressure accumulator  15 , a pre-cooler  16 , a flow control valve  17 , a flow meter  18  and a dispenser  19 . The compressor  14  compresses hydrogen gas from the cluster of tanks  11 , and discharges it. The pressure accumulator  15  stores hydrogen gas whose pressure has been raised to a predetermined pressure by the compressor  14 . The pre-cooler  16  cools the hydrogen gas from the pressure accumulator  15  whose temperature is approximately equal to room temperature to a predetermined low temperature (e.g., −20° C. or −40° C.). The flow control valve  17  is an electrically driven valve, and adjusts the amount of flow of hydrogen gas from the pressure accumulator  15  in accordance with a command from the control device  5 . Due to this, the filling flow rate (filling speed) of hydrogen gas to the fuel tank  30  is controlled. This controlled filling flow rate is measured by the flow meter  18 . In response to a result of the measurement, the control device  5  performs a feedback control of the flow control valve  17  so as to achieve a desired filling flow rate. Incidentally, it is also possible to use a flow rate control device other than the flow control valve  17 . The dispenser  19  is sends out hydrogen gas to the filling nozzle  12 . For example, if a trigger lever of the filling nozzle  12  is pulled, the dispenser  19  is actuated to become able to discharge hydrogen gas from the filling nozzle  12  toward the fuel tank  30 . Incidentally, although not shown in the drawings, a shutoff valve that opens the gas channel  13  during the filling operation is provided in the pressure accumulator  15  or at a downstream side of the pressure accumulator  15 . 
     The control device  5  is constructed as a microcomputer that has therein a CPU, a ROM and a RAM. The CPU executes desired computations according to control programs so as to perform various processes and controls. The ROM stores control programs and control data that are processed by the CPU. The RAM is used mainly as various workspaces for control processes. The control device  5  is electrically connected to the communication instrument  6 , the display device  7 , the outside air temperature sensor  8 , the pressure sensor  9 , the temperature sensor  10 , the flow control valve  17 , the flow meter  18 , the pressure accumulator  15 , etc., which are connected by a control line shown by a dashed one-dotted line in  FIG. 2 . For example, the control device  5  recognizes the pressure and the temperature that the pressure sensor  36  and the temperature sensor  38  detect, as the pressure and the temperature in the fuel tank  30  (i.e., the tank pressure and the tank temperature) of the vehicle  3 , and accordingly controls the filling of hydrogen gas. Specifically, the control device  5  controls the degree of opening of the flow control valve  17  on the basis of the information regarding the tank pressure and the tank temperature of the vehicle  3  side which the control device  5  receives from the communication instrument  6 . Besides, the control device  5  sends information that is obtainable at the hydrogen station  2  to the communication instrument  44  of the vehicle  3 , by using the communication instrument  6 . 
     Thus, the communication instruments  6  and  44  function as receivers and transmitters that receive and transmit signals that include various kinds of information from and to each other. As the communication instruments  6  and  44 , it is possible to use infrared communication instruments that each include a transmission element and a reception element. The transmission element and the reception element of each communication instrument may be constructed of, for example, an infrared light-emitting diode and a photodiode, respectively. 
     In the gas filling system  1  described above, when hydrogen gas is to be filled into the vehicle  3 , the filling nozzle  12  is firstly connected to the receptacle  32 , and then the dispenser  19  is actuated while the connection therebetween is maintained. Then, hydrogen gas is released from the filling nozzle  12  toward the fuel tank  30 , and is therefore filled into the fuel tank  30 . In the gas filling system  1  of this embodiment, before the filling is started, the state quantity regarding an interior of the fuel tank  30  is grasped. If the state quantity satisfies a predetermined condition, the cycle of the transmission from the vehicle  3  to the communication instrument  44  of the hydrogen station  2  is elongated (i.e., the next transmission timing is delayed). In this manner, the electric power consumption during the gas filling is minimized. 
     Next, an operation procedure of determining the transmission cycle of the communication instrument  44  in the gas filling system  1  will be described with reference to  FIG. 3 . The procedure shown in  FIG. 3  is executed before the filling of hydrogen gas from the hydrogen station  2  into the fuel tank  30  is started. 
     Firstly, the electric power source of the control device  46  of the vehicle  3  is turned on (step S 1 ). This is carried out by supplying electric power from the low-voltage battery  62  to the control device  46 . Examples of the trigger of turning on the electric power source of the control device  46  includes detecting that the operation of connecting the filling nozzle  12  and the receptacle  32  has been performed, by means of a sensor (not shown). It is also permissible to detect that a fuel cover of a lidded box of the vehicle  3  has been opened, by means of a sensor, through the use of a method different from the foregoing method. When the electric power source of the control device  46  is turned on, the pressure sensor  36 , the temperature sensor  38  and the communication instrument  44  are activated. The activation of these devices and the like is carried out by the supply of electric power from the low-voltage battery  62 . 
     Then, after the activation of the pressure sensor  36  and the temperature sensor  38  is completed, a tank pressure P 1  and a tank temperature T 1  (hereinafter, sometimes referred to as “initial tank pressure” and “initial tank temperature”, respectively) are input as state quantities of the fuel tank  30  occurring before the filling starts. Information about the tank pressure P 1  and the tank temperature T 1  that are input is temporarily stored into, for example, the RAM of the control device  46 . On the other hand, simultaneously with or prior or subsequently to the completion of the activation of the pressure sensor  36  and the temperature sensor  38 , the activation of the communication instrument  44  is completed. At this time, if the operation of connecting the filling nozzle  12  and the receptacle  32  has been completed, a state in which wireless communication is possible between the hydrogen station  2  and the vehicle  3  is established. 
     Next, in step S 2 , it is determined whether or not the tank temperature T 1  is greater than or equal to a threshold value T th . This determination is carried out by the control device  46 . The threshold value T th  is determined beforehand, by factoring in the filling time, and is stored beforehand in the ROM of the control device  46 . 
     Concretely, through an evaluation test or a simulation performed beforehand, it is possible to grasp the approximate lengths of the filling time corresponding to various tank temperatures and various tank pressures. For example, through a prior evaluation, it has been ascertained that when the tank temperature of a fuel tank is greater than or equal to 60° C. and the tank pressure thereof is less than or equal to 2 MPa, the filling time required is greater than or equal to 40 minutes. The filling time tends to be longer the higher the tank temperature. Therefore, when the tank temperature is about normal temperature, a long filling time that is greater than or equal to 40 minutes is not required. In view of what has been described above, the threshold value T th  in this embodiment is set at a tank temperature that has been ascertained to require a long filling time (e.g., 40 minutes or more) through a prior evaluation or the like. An example of the threshold value T th  is 60° C. 
     However, the threshold value T th  varies depending on the characteristics (heat dissipation characteristic) of the fuel tank  30  and also on the reference temperature (allowable temperature) of the fuel tank  30 , the cooling capability of the pre-cooler  16 , etc. That is, differences in these factors vary the filling time. For example, the heat dissipation characteristic of the fuel tank  30  is better in the case where aluminum is used as a liner of the fuel tank  30  than in the case where a resin (polyethylene or the like) is used as the liner. Therefore, in the former case, it is possible to increase the threshold value T th . Besides, the rise of the tank temperature caused by the filling can be made less the lower the temperature of hydrogen gas that can be achieved by the pre-cooler  16 . Therefore, it is also possible to make the threshold value T th  larger the higher the cooling capability of the pre-cooler  16 . Therefore, while the threshold value T th  in step S 2  can be a fixed value (60° C. as in the foregoing example), the threshold value Tth can also be varied on the basis of a characteristic of the fuel tank  30  or the like. Incidentally, it should be apparent that in the case where the gas to be filled in is a gas that is different in kind from hydrogen gas, the threshold value T th  used is also different from that used in the case where the gas to be filled in is hydrogen gas. 
     If it is determined in step S 2  that the tank temperature T 1  is less than the threshold value T th  (NO in step S 2 ), it is considered that the filling time required is not very long. Therefore, the control device  46  determines that a transmission cycle T A  is to be used, without taking the tank pressure P 1  into account (step S 3 ). On the other hand, if the tank temperature T 1  is higher than or equal to the threshold value T th  (YES in step S 2 ), it is considered that a long filling time will be required. Therefore, the control device  46  compares the tank pressure P 1  with the threshold value P th  (step S 4 ). 
     The threshold value P th , similar to the threshold value T th , is determined beforehand by taking the filling time into account, and is stored beforehand in the ROM of the control device  46 . In this case, the filling time tends to be long the lower the tank pressure. The threshold value P th  in this embodiment is set at a tank pressure that has been ascertained to require a long filling time (e.g., 40 minutes or more) through a prior evaluation or the like. An example of the threshold value P th  is 2 MPa. It is to be noted herein that the threshold value P th , similar to the threshold value T th , may be a fixed value, or may also be varied on the basis of characteristics (heat dissipation characteristic) of the fuel tank  30 , the reference temperature (allowable temperature) of the fuel tank  30 , or the cooling capability of the pre-cooler  16 , etc. 
     If in step S 4  it is determined that the tank pressure P 1  is greater than the threshold value P th  (NO in step S 4 ), it is considered that the filling time will not be long, and therefore it is determined to use a signal transmission cycle T A  (step S 3 ). On the other hand, if the tank pressure P 1  is less than or equal to the threshold value P th  (YES in step S 4 ), it is considered that the filling time will be long. Therefore, the control device  46  determines to use a signal transmission cycle T B  that is longer than the transmission cycle T A  (step S 5 ). 
     The transmission cycles T A  and T B  will be described. The term “transmission cycles T A  and T B ” refers to a cycle period of a signal that the communication instrument  44  transmits to the communication instrument  6 . That is, the time duration from when the communication instrument  44  transmits a signal to the communication instrument  6  to when the communication instrument  44  transmits the next signal to the communication instrument  6  is the transmission cycle. The transmission cycles T A  and T B  are used for the communication between the communication instrument  44  and the communication instrument  6  during the filling, and are stored beforehand in a storage portion such as the ROM of the control device  46  or the like. In an example, the transmission cycle T A  is 100 ms, and the transmission cycle T B , which is longer than the transmission cycle T A , is 200 ms, 300 ms or 400 ms. However, if there is a standard of the maximum length of the transmission cycle during the filling, the transmission cycles T A  and T B  are set within a range of the cycle that satisfies the standard maximum length and that does not affect the filling. 
     It is preferable to determine whether or not a transmission error of the communication instrument  44  has occurred. As an index for the determination, it is advisable to use the presence/absence of reception of a signal from the communication instrument  6  during a fixed period. For example, if during a fixed period (predetermined cycle) of 500 ms, the communication instrument  6  does not receive a signal from the communication instrument  44 , it can be determined that a transmission error of the communication instrument  44  has occurred. This determination can be performed by the control device  5  during the filling. 
     Therefore, in the case where the control device  5  has a reference for determining whether a transmission error of the communication instrument  44  has occurred, it is preferable to set the transmission cycles T A  and T B  at less than half of the predetermined cycle period that is used according to the foregoing reference. Concretely, in the foregoing example, in the case where the predetermined cycle period is 500 ms, it is preferable that the transmission cycle T B  be set at 200 ms, which is less than 250 ms. This setting makes it possible for the control device  5  to perform a process in terms of control in which a transmission error, if occurring for the first time, is assumed to be no occurrence of abnormality in the communication instrument  44 . This setting is particularly useful in the case where the reliability of communication is low. Incidentally, in the case where the reliability of communication is high, the transmission cycle T B  may be set at a half or more of the predetermined cycle period. 
     After the transmission cycle T A  or T B  is determined (step S 3  or S 5 ), the communication during the filling is performed at the determined transmission cycle. More specifically, after the determination of the transmission cycle, information about the initial tank pressure and the initial tank temperature is transferred to the control device  5  of the hydrogen station  2 . Next, the control device  5  looks up the initial tank pressure and the initial tank temperature that are received in a filling speed map, and starts the filling so as to achieve a filling speed that corresponds to the initial tank pressure and temperature. 
     During the filling, the tank pressure and the tank temperature are input as needed, and a signal that contains information about the tank pressure and the tank temperature that are input is transmitted from the communication instrument  44  to the communication instrument  6  at the determined transmission cycle T A  or T B . Owing to this, the control device  5  monitors the tank pressure and the tank temperature during the filling. Then, the control device  5  looks up the tank pressure and the tank temperature that the control device  5  monitors in the filling speed map, and controls the degree of opening of the flow control valve  17  so as to change the filling speed according to need. Incidentally, during the filling, the vehicle  3  is in an IGOFF state, so that the electric power for the communication instrument  44  is supplied by the low-voltage battery  62 . 
     As the filling operation completes a filling that satisfies a desired filling condition that the user has designated, the supply of hydrogen gas to the fuel tank  30  is stopped, and the filling ends. After that, the filling nozzle  12  is detached from the receptacle  32 , and the power source of the pressure sensor  36 , the temperature sensor  38  and the communication instrument  44  is turned off, that is, the supply of electric power thereto from the low-voltage battery  62  ends. Then, the electric power source of the control device  46  is also turned off. 
     Operation and effects of the foregoing embodiment will be described. During the communication-based filling of the fuel tank  30 , the electric power source used for the communication instrument  44  and the like in the vehicle  3  is the low-voltage battery  62 . Therefore, if the transmission cycle T A , which is relatively short, is used during the filling whose filling time is long, there arises possibility of power shortage of the low-voltage battery  62 . 
     In this embodiment, in the case where it is determined from the initial tank temperature and the initial tank pressure that the filling time will be long, the transmission cycle T B , which is relatively long, is used during the filling. Therefore, during the entire single operation of filling, the electric power needed for the communication via the communication instrument  44  decreases, so that the electric power consumption during the filling can be reduced. Therefore, it is possible to restrain the low-voltage battery  62  from entering a so-called dead state. 
     Incidentally, although in the foregoing embodiment, a transmission cycle is determined before the filling is started, it is also possible to perform steps S 2  to S 4  in  FIG. 3  after the filling is started. In this case, the length of the transmission cycle is determined from the state quantity (tank temperature, tank pressure) regarding the interior of the fuel tank  30  after the filling is started, and is then used. 
     MODIFICATIONS 
     Modifications of the foregoing embodiment will be described. The following modifications are applicable to each other. 
     First Modification 
     A first modification shown in  FIG. 4  is different from the foregoing embodiment mainly in that the transmission cycles T A  and T B  are determined on the basis of the elapsed time in the filling. 
     Firstly, after the same process as in step S 1  in  FIG. 3  is performed (step S 11 ), a filling speed corresponding to the initial tank pressure and the initial tank temperature is selected from a filling speed map, and the filling of the fuel tank  30  is accordingly started (step S 12 ). Next, the elapsed time following the start of the filling (hereinafter, referred to as “elapsed filling time”) is measured by a timer (not shown), and it is checked whether or not the elapsed filling time is greater than a predetermined threshold value (third threshold value) (step S 13 ). This checking can be performed by the control device  46  of the vehicle  3 . The predetermined threshold value can be arbitrarily set; for example, it can be set at 20 minutes or 40 minutes. 
     If in step S 13  it is checked that the elapsed filling time is less than or equal to the predetermined threshold value (NO in step S 13 ), it is determined to use the transmission cycle T A , and the filling is accordingly continued (step S 14 ). On the other hand, if the elapsed filling time is longer than the predetermined threshold value (YES in step S 13 ), it is determined to use the transmission cycle T B , which is longer than the transmission cycle T A , and the filling is accordingly continued (step S 15 ). Therefore, according to the first modification, in the case of a long-time filling whose filling time exceeds the predetermined threshold value, the electric power consumption during the filling can be reduced. 
     Incidentally, in step S 13 , it is also permissible to compare an activation time of the communication instrument  44  with a threshold value (e.g., 20 minutes or 40 minutes). In this case, when the filling is started in step S 12 , the hydrogen station  2  has not grasped the initial tank pressure nor the initial tank temperature yet, and therefore the filling is started at a filling speed that is set by taking safety into account. For example, the filling is started by using as an initial tank pressure the pressure hydrogen gas supplied from the hydrogen station  2  (e.g., detected at the nozzle  12 ) or a higher pressure, and using as an initial tank temperature the outside air temperature (a value detected by the outside air temperature sensor  8 ) or a temperature below the detected value. 
     The first modification can be performed independently of the operation procedure shown in  FIG. 3 , or can also be incorporated into the procedure shown in  FIG. 3 . In the latter case, the process of steps S 12  to S 15  shown in  FIG. 15  is incorporated after the process of steps S 3  and S 5  shown in  FIG. 3 . 
     Second Modification 
     A second modification shown in  FIG. 5  is different from the foregoing embodiment mainly in that the transmission cycles T A  and T B  are determined on the basis of the voltage of the low-voltage battery  62 , which is a battery for accessories. 
     Firstly, in step S 21 , the same process as in step S 1  in  FIG. 3  is performed. Next, in step S 22 , the voltage of the low-voltage battery  62  (the state of charge thereof) is detected by an SOC sensor (not shown), and it is checked whether or not the detected voltage of the low-voltage battery  62  is less than or equal to a predetermined threshold value (step S 22 ). This checking can be performed by the vehicle 3-side control device  46 . The predetermined threshold value for use for comparison in step S 22  can be arbitrarily set; for example, it can be set at a voltage that corresponds to 20% or 30% of the full voltage (full state or charge). 
     If in step S 22  it is checked that the voltage of the low-voltage battery  62  is greater than the predetermined threshold value (NO in step S 22 ), it is determined that the dead battery will not occur during the filling, and it is determined to use the transmission cycle T A  (step S 23 ). On the other hand, if the voltage of the low-voltage battery  62  is less than or equal to the predetermined threshold value (YES in step S 22 ), it is determined that the dead battery will occur during the filling, and it is determined to use the transmission cycle T B , which is longer than the transmission cycle T A . Therefore, according to the second modification, the electric power consumption during the filling can be reduced. 
     The second modification can be performed independently of the procedure shown in  FIG. 3 , and can also be incorporated into the procedure shown in  FIG. 3 . In the latter case, the procedure shown in  FIG. 3  is performed before the filling is started, and during the filling, the process of steps S 22  to S 24  shown in  FIG. 5  is performed by monitoring the voltage of the low-voltage battery  62 . 
     Third Modification 
     It is also possible to adopt a construction in which the functions of the vehicle 3-side control device  46  is to two control devices, and in which both control devices receive electric power from the low-voltage battery  62 . For example, in a construction in which a fuel-cell control device for use for the electricity generation control of the fuel cell  50  and a filling control device for use for the control of communication-based filling are used, it suffices that the filling control device is equipped with functions needed for the foregoing communication-based filling which are performed by the control device  46  in the foregoing embodiment (functions that are needed in order to execute the procedures shown in  FIGS. 3 to 5 ). In this construction, the process of “turning on the power source of a vehicle-side control device” which is shown in steps S 1 , S 11  and S 21  in  FIGS. 3 to 5  is performed only for the filling control device, and does not need to be performed for the fuel-cell control device. 
     The gas filling system and the vehicle of the invention can be applied not only to hydrogen gas, but also to other fuel gases such as natural gas and the like. Besides, the invention is applicable not only to wheeled vehicles but also to other mobile units equipped with fuel tanks as an object of the filling of a fuel gas from outside, such as aircrafts, ships and boats, robots, etc.