Abstract:
A vehicle fuel gas supply system includes a fuel line supplying a fuel gas to a gas engine, a shut-off valve and a pressure sensor disposed on said fuel line, and a control section. The pressure sensor is disposed closer to said gas engine in comparison with said shut-off valve. The control section closes the shut-off valve being opened, starts to count the time of a shut-off valve closing timer. Then, the control section opens the shut-off vale when the shut-off valve closing timer counts up, simultaneously therewith measures a pressure by the pressure sensor to store the pressure as an initial pressure, and starting to count the time of a diagnostic timer. Further, the control section measures a second pressure by the pressure sensor when the diagnostic timer counts up to store the second pressure as a final pressure value, and judges that the shut-off value is in a failure state when a pressure increase rate obtained from the initial pressure value and the final pressure value is smaller than a predetermined pressure increase rate threshold value.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a technology for judging that a shut-off valve used in a vehicle fuel gas supply system is in a failure state. 
     2. Description of the Related Art 
     In recent years, natural gas (NG) has been used as one of alternative fuels to gasoline and gas oil, and in a case where natural gas is used as fuel for vehicles, in particular, passenger vehicles, a compressed-gas cylinder (bomb) is mounted on a passenger vehicle. A natural gas compressed to in the order of 200 kg/cm 2  is then filled in the compressed-gas cylinder, and the natural gas so filled is then reduced in pressure by means of a pressure reducing valve to become a low pressure gas for supply to a combustion chamber. A natural gas as compressed to a high pressure like this is called a compressed natural gas (CNG). 
     Research on vehicles using compressed natural gas as a fuel has been progressing, and Japanese Patent Unexamined Publication No. Hei.7-189731 entitled “Residual Fuel Volume Display Device for Gas-Fueled Vehicles” proposes a technology for displaying the volume of residual fuel more accurately by correcting pressure and temperature fluctuations attributed to a compressible fluid. This publication discloses a CNG tank, a high pressure piping for supplying a compressed natural gas taken from the CNG tank, a first solenoid shut-off valve provided upstream of the high pressure piping, and a second solenoid shut-off valve provided downstream of the high pressure piping. The first and second solenoid shut-off valves are able to be closed by means of an ECU. 
     In general, used as the solenoid shut-off valves is a shut-off valve in which the valve is opened when a plunger (rod) is axially moved by virtue of the electromagnetic force of the solenoid, while the valve is closed when the plunger is returned by virtue of the compression force of a spring used. In order to allow the plunger to reciprocate in the axial directions, there must be provided a gap. However, if this gap is too large, the plunger may be moved in radial directions, causing “chattering.” To cope with this, the gap between the plunger and a component for supporting the plunger thereon needs to be as small as possible. 
     Natural gas consists mainly of methane gas, and since it is collected from the underground, the natural gas tends to contain water. Even if much labor is spent in producing a dried compressed natural gas, there exists a risk of water contained in air entering into a fuel tank mounted on a vehicle when the dried compressed natural gas is transferred into the fuel tank. In addition, it is well considered that water cannot be removed in a complete fashion even in producing a fuel line system. 
     There is a risk of the water so present generating rust at metallic portions of the solenoid valves. To cope with this, plungers are made of stainless steel or plated, but those countermeasures are not perfect. 
     There are considered the following two cases of failure in operation of the first solenoid shut-off valve. 
     The first case is where rust disturbs to open the shut-off valve cannot be opened because of rust. With the shut-off valve being left closed, a sufficient amount of fuel cannot be supplied to the engine, and therefore, although the engine can be started with fuel remaining within the fuel line, the engine stops soon thereafter. 
     The second case is where the shut-off valve from cannot be closed because of rust. With the shut-off valve being left opened, even if the engine is stopped, the compressed natural gas is kept supplied from the compressed gas cylinder to the high pressure piping, and therefore, there is a high risk of gas leakage while the engine is being stopped. In addition, in view of safety servicing of a vehicle, such a gas leakage is undesirable. 
     SUMMARY OF THE INVENTION 
     Then, an object of the present invention is to provide a technology for efficiently judging of a failure state of a solenoid shut-off valve used in a vehicle fuel gas supply system having a high probability of a risk of water entering into the fuel supply system. 
     With a view to attaining the aforesaid object, according to the present invention, there is provided a vehicle fuel gas supply system in which a shut-off valve and a pressure sensor are disposed in that order along a fuel line for supplying a fuel gas to a gas engine, comprising a control section for; 
     closing the shat-off valve which is being opened and starting to count the time of a shut-off valve closing timer, 
     opening the shut-off valve when the shut-off valve closing timer counts up, simultaneously therewith measuring a pressure with the pressure sensor and storing the measured pressure as an initial pressure value, and starting to count a diagnostic timer, 
     measuring a pressure with the pressure sensor when the diagnostic timer counts up and storing the measured pressure as a final pressure value, and 
     judging that the shut-off value is in a failure state when a pressure increase rate obtained from the initial pressure value and the final pressure value is smaller than a predetermined pressure increase rate threshold value. 
     With this fuel gas supply system constructed as described above, it is possible to judge on a failure state of the shut-off valve while the vehicle is being stopped or running. 
     The diagnosis of failure consists of the following two types; the shut-off valve is kept closed and cannot be opened, and the shut-off valve is locked to the closed side and a sufficient degree of opening cannot be attained. 
     According to the present invention, it is possible not only to diagnose that the shut-off valve properly operates at suitable timings but also to prevent a trouble that would be entailed by the failure of the shut-off valve. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing a vehicle to which the present invention is applied; 
     FIG. 2 is a drawing showing the principle of a fuel gas supply system according to the present invention; 
     FIG. 3 is an enlarged view of a part indicated by a reference numeral III in FIG. 2; 
     FIGS. 4A and 4B are drawings showing operations of a check valve and a first shut-off valve according to the present invention; 
     FIG. 5 is an enlarged view of a part indicated by a reference numeral V in FIG. 2; 
     FIG. 6 is a bottom view of a pressure control unit shown in FIG. 5; 
     FIG. 7 is a drawing showing the operational principle of the pressure control unit adopted in the present invention. 
     FIG. 8 is a fuel gas supply system drawing for use in explaining a failure judgement technology according to the present invention; 
     FIGS.  9 ( a )- 9 ( f ) are a timing chart showing a failure diagnosis of a shut-off valve according to the present invention; 
     FIG. 10 is a flowchart for detecting a failure of the first shut-off valve according to the present invention (part 1); 
     FIG. 11 is a flowchart for detecting a failure of the first shut-off valve according to the present invention (part 2); and 
     FIG. 12 is a flowchart for detecting a failure of the first shut-off valve according to the present invention (part 3). 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the accompanying drawings, a mode of carrying out the present invention will be described below. In the following description, a first, a second, . . . or a primary, a secondary, . . . are a nominal identification prefix that is given in the order fuel gas flows in a fuel gas supply system. 
     FIG. 1 is a diagram showing a vehicle to which the present invention is applied. The vehicle  10  is shown as being provided with a fuel gas supply system in which a CNG tank  12  filled with a compressed natural gas as a fuel gas is mounted at a rear part of a vehicle body  11  of this vehicle  10 , the compressed natural gas within the CNG tank is supplied to a combustion chamber of a gas engine  16  mounted at a front portion of the vehicle body  11  via high pressure piping  13 , a pressure control unit  60 , low pressure piping  14 , and an injector  15 . The other reference numerals will be described later on. 
     FIG. 2 is a diagram explaining the principle of the fuel gas supply system according to the present invention. In this fuel gas supply system  20 , a compressed natural gas can be filled in the CNG tank  12  from the outside via a filler port  21 , a filler pipe  22  and a check valve  40 , while the compressed natural gas so filled and stored in the CNG tank  12  can be sent to an intake port  26  via a first shut-off valve  50 , the high pressure piping  13 , a joint box  23 , a manual On/Off valve  24 , a filter  25 , the pressure control unit  60 , the low pressure piping  14  and the injector  15 , which are disposed along the high pressure piping  13 . 
     In addition, this system comprises a first pressure sensor  31  and a first temperature sensor  23  which are provided in the joint box  23 , a second pressure sensor  33  provided at an outlet of the pressure control unit  60 , a second temperature sensor  34  provided at the injector  15 , an injector driver  35  for controlling the injector  15  and a control section  36  for controlling the first shut-off valve  50  and a second shut-off valve (which will be described in detail later on) installed in the pressure control unit  60  so as to be opened or closed. 
     Namely, the pressure P 0  and temperature T 0  of the compressed natural gas in the high pressure piping  13  are measured by means of the first pressure sensor  31  and the first temperature sensor  32 , and information resulting from such measurements is inputted into the control section  36 . The pressure P 2  and temperature T 2  of the compressed natural gas in the low pressure piping  14  are measured by means of the second pressure sensor  33  and the second temperature sensor  34 , and information resulting from such measurements is inputted into the control section  36 . Then, the control section  36  controls the first shut-off valve  50  and the second shut-off valve installed in the pressure control unit  60  based on the inputted information to thereby judge whether or not those shut-off valves are in a failure state. 
     The compressed natural gas is a gas and a compressible fluid, and therefore, it follows the Boyle-Charles Law (PV/T=constant, where P is an absolute pressure, V is a volume or a capacity, and T is an absolute temperature). Here, the capacity of the flow path is constant, and therefore, V remains the same. With fluctuations in temperature, the pressure fluctuates in proportion thereto, and thus, there is a possibility that the pressure changes at all times, this causing a problem in control. To cope with this, the pressure P 0  is temperature corrected at the temperature T 0 . Similarly, the pressure P 2  is temperature corrected at the temperature T 2 . With such temperature corrections, any pressures can be regarded as a pressure referencing zero (0) degree, thereby making it possible for them to relatively be compared with each other. 
     FIG. 3 is an enlarged sectional view of a portion indicated by a circle denoted by reference numeral III in FIG.  2 . The check valve  40  and the first shut-off valve  50  are incorporated in a common valve cage  41 , and this valve cage  41  is screwed into an opening in the CNG tank  12 . In particular, the first shut-off valve  50  is called an in-tank shut-off valve because a main part thereof resides inside the CNG tank  12 . The construction of the check valve  40  and the first shut-off valve  50  will be described sequentially. 
     The check valve  40  is constructed such that a sleeve  42  is screwed into the valve cage  41 , that a valve element  43  is brought into abutment with a lower opening of the sleeve  42 , that the valve element  43  is pushed to a valve closing side by means of a spring  44 , that this spring  44  is supported by another sleeve  45  and a rod  46 , and that a flow path  47  and a throttle portion  48  are formed in the rod  46 . An operation of the check valve  40  will be described referring to FIG.  4 A. Reference numeral  49  denotes a stop plug, which is shown as being put in an open state. The plug  49  can be turned with a hexagon wrench so as to close an upper opening  41   a . This is used in periodically inspecting the filler pipe  22  for maintenance. 
     The first shut-off valve  50  is a solenoid controlled shut-off valve which is constructed such that a cylindrical solenoid holder  52  for supporting a solenoid  51  is screwed into the valve cage  41 , that a rod  53  is put through the solenoid holder  52 , that a valve element  55  is fixed to a tip of the rod  53  via a pin  54 , and that the valve element  55  is caused to confront a valve seat of the valve cage  41 . When the solenoid  51  is excited, the rod  53  is withdrawn so as to open the shut-off valve by virtue of the attracting action of the solenoid  51  resulting, while, when the excitation of the solenoid  51  is stopped, the shut-off valve is closed by virtue of the pushing action of a spring  56  activated when the attracting action of the solenoid  51  disappears. Reference numeral  57  denotes a port opened in the solenoid holder  52 . 
     An operation of the first shut-off valve  50  will be described referring to FIG.  4 B. Reference numeral  58  also denotes a stop plug, which is shown as being in an open state. This plug can also be turned with the hexagon wrench to close an upper opening  41   b.    
     FIGS. 4A and 4B show operations of the check valve and the first shut-off valve according to the present invention. 
     In FIG. 4A, when compressed natural gas under high pressure is supplied as shown by a while arrow, the valve element  43  is moved to a valve opening side by virtue of the pressure of the gas. As a result of this, the compressed natural gas reaches the CNG tank  12  as indicated by an arrow {circle around ( 1 )} via the throttle portion  48 . When the supply of the compressed natural gas is stopped, the valve element  43  is returned by virtue of the action of the spring  45 , whereby a reverse flow is prevented. 
     In FIG. 4B, when the solenoid  51  is excited, the valve element  55  is withdrawn to thereby produce a valve opened state, whereby the compressed natural gas stored in the CNG tank  12  flows as indicated by an arrow {circle around ( 2 )} through the port  57 . When the excitation of the solenoid  51  is stopped, the shut-off valve is closed by virtue of the action of the spring  56 . 
     Returning to FIG. 3, a gap  59  is provided between the solenoid holder  52  and the rod  53  so that the rod  53  can move relative to the solenoid holder  52 . If this gap  59  is too large, the rod  53  is allowed to swing in radial directions, and therefore, the gap  59  must be kept as small as possible. If natural gas is dried imperfectly, it eventually contains water, or, as shown in FIG. 4A, when compressed natural gas is filled in the CNG tank  12 , there is a risk of water contained in nearby air entering into the CNG tank  12 . 
     With the water present, the water may penetrate into the gap  59  and produce foreign matters thereat, slowing the movement of the rod  53 . 
     To cope with this, the present invention provides a technology that can cope with the slowed movement of the shut-off valve  50  resulting from a reason as described above. The technology will be described later. 
     FIG. 5 is an enlarged sectional view of a portion indicated by a circle denoted by reference numeral V in FIG.  2 . An operation thereof will be described later referring to FIG. 7, and therefore, the construction thereof will briefly be described here. 
     The pressure control unit  60  is an integrated body comprising the second shut-off valve  65 , a primary pressure reducing valve  70 , a safety valve  77  and a secondary pressure reducing valve  80 . Although a detailed description of the construction thereof will be omitted, the second shut-off valve  65  is a solenoid controlled shut-off valve using a solenoid  66  as a driving source, and the primary pressure reducing valve  70  is a pressure regulator comprising a diaphragm  71 , pressure regulating spring  72 , a back pressure chamber  73 , a back pressure inlet port  74  and a pressure regulating screw  75 . The safety valve  77  is a valve comprising a valve element  78  and a spring  79 , and the secondary pressure reducing valve  80  is a pressure regulator comprising a diaphragm  81 , a pressure regulating spring  82 , a back pressure chamber  83 , a back pressure inlet port  84  and a pressure regulating screw  85 . 
     FIG. 6 is a bottom view of the pressure control unit shown in FIG.  5 . The compressed natural gas entering into the pressure control unit  60  as indicated by an arrow {circle around ( 3 )} flows as indicated by an arrow {circle around ( 4 )} via an inner filter  86  and the second shut-off valve  65 . Returning to FIG. 5, the flow indicated by the arrow {circle around ( 4 )} passes through the primary pressure reducing valve  70 , flows upwardly as indicated by an arrow {circle around ( 5 )} and reaches the secondary pressure reducing valve  80 . 
     FIG. 7 is a diagram showing the operational principle of the pressure control unit adopted in the present invention. The primary pressure reducing valve  70  is intended to reduce the pressure P 0  to the pressure P 1 . To be specific, the pressure P 0  acts on an upper surface of the diaphragm  71  as viewed in the drawing, and the pressure P 1  and a pushing force of the spring  72  act on a lower surface of the diaphragm  71  as viewed in the drawing. When the opening degree of the valve is determined by a balance of three forces acting on the diaphragm, the natural gas is allowed to flow. If the pressure P 1  is increased higher than a set pressure therefor, the pressure inside the back pressure chamber  73  is increased to thereby push up the diaphragm  71 , whereby the valve is throttled. As a result of this, the pressure P 1  is reduced. In the event that the pressure P 1  is lower than the set pressure, on the contrary, the opening degree of the valve is increased, and the pressure P 1  is increased. Thus, the primary pressure reducing valve  70  can maintain the pressure P 1  at the predetermined set pressure. 
     In this embodiment, the pressure P 0  ranges 10 to 260 kg/cm 2 , and the pressure P 1  is 6 kg/cm 2 . Even if the pressure P 0  is remarkably changed, the pressure P 1  can be maintained so as to provide a constant pressure. 
     The secondary pressure reducing valve  80  is intended to reduce the pressure P 1  to the pressure P 2 , and a basic operation thereof is the same as that of the primary pressure reducing valve  70 , and therefore, a description thereof will be omitted. The pressure P 1  is 6 kg/cm 2  and the pressure P 2  is 2.6 kg/cm 2 . It is needless to say that the pressure P 2  can be maintained at the predetermined set pressure even if the pressure P 1  fluctuates. The pressure P 2  is detected by the second pressure sensor  33 . 
     The pressure values described above are illustrated as examples only, and the present invention is not limited to those numerical values. 
     When the solenoid  66  is excited, the second shut-off valve  65  is opened so as to allow the natural gas to flow as indicated by an arrow {circle around ( 6 )}, and when the excitation of the solenoid  66  is stopped, the shut-off valve is closed by virtue of the action of the spring  67 . 
     The safety valve  77  is provided so as to cope with a remarkable increase in the pressure P 1  that would be caused by a trouble taking place in the primary pressure reducing valve  70  and is constructed so as to be opened when such a remarkable increase actually occurs to thereby protect the low pressure piping  14  including the secondary pressure reducing valve  80 . 
     Next, described below will be a technology for judging of a failure of the shut-off valves used in the aforesaid fuel gas supply system according to the present invention. 
     FIG. 8 is a fuel gas supply system drawing for use in explaining a failure judgement technology according to the present invention and is a combination of the main part extracted from FIG. 2 and a starter switch  90  additionally shown therein in order to suffice a control flow that will be described below. The starter switch  90  comprises ACC-OFF contact  91 , ACC-ON contact  92 , IG-ON contact  93  and ST-ON contact  94 . With ACC-OFF contact  91  being selected, an accessory OFF state is produced, with ACC-ON contact  92  selected, an accessory ON state is produced, with IG-ON contact  93  selected, an ignition ON state is produced, and with ST-ON contact  94  selected, the rotation of a starter is initiated. 
     The other reference numerals have been described and therefore they will not be described here in order to avoid a repeated description thereof. However, as described referring to FIG. 3, there is a possibility that there is caused a trouble with the first shut-off valve  50  in which the shut-off valve is left “closed” and cannot be opened due to rust generated by water present in methane gas or entering thereinto during the production process. A means for judging on the trouble will be described below. 
     FIG. 9 is a timing chart showing a failure diagnosis of the shut-off valve according to the present invention. 
     Assume that a failure diagnostic signal is inputted at (a) of FIG.  9 . 
     A “first shut-off valve closing timer” is started at (b) of FIG.  9 . The set time for this timer is tc. 
     At (c) of FIG. 9, since the timer ends after the elapse of the set time tc, a diagnostic timer is started based on a signal indicating that the first shut-off valve closing timer has ended up. The set time for this diagnostic timer is t 1 . 
     As shown at (d) of FIG. 9, the second shut-off valve is kept opened. 
     As shown at (e) of FIG. 9, the first shut-off valve is closed when the diagnostic signal is inputted at (a) of FIG.  9  and is opened when the set time tc has elapsed. 
     Returning to FIG. 8, when the first shut-off valve is closed with the second shut-off valve being kept opened while the engine is running, since there is no supply of the compressed natural gas from the CNG tank  12 , the pressures in the high pressure piping  13  and the low pressure piping  14  become lower with passage of time. The change of the pressure P 0  of the high pressure piping  13  is shown in (f) of FIG. 9 (between 0 and tc along the axis of abscissas). 
     Then, when the first shut-off valve  50 , which is closed only for a time tc in FIG. 8, is re-opened, the supply of the compressed natural gas from the CNG tank  12  is resumed, and the pressures in the high pressure piping  13  and the low pressure piping  14  increase with passage of time. 
     What is important here is an aspect of the pressure recovery in a “pressure increase” region between time t:(0) to t 1  of the diagnostic timer illustrated by the axis of abscissas in (f) of FIG.  9 . 
     The pressure is expected to recover as soon as the first shut-off valve is fully opened, and the pressure recovers as illustrated by a thick solid line. 
     On the contrary, if the first shut-off valve is not opened sufficiently due to failure, the supply of compressed natural gas becomes insufficient, and the recovery of the pressure is delayed or impossible. Thus, the pressure recovers as illustrated by a broken line. 
     Namely, assuming that a pressure P 0  at a time t=0 of the diagnostic timer is an initial pressure value P 0 ( 1 ) and that a pressure P 0  at a time t=t 1  of the diagnostic timer is a final pressure value P 0 ( 2 ), the pressure increase rate at that time becomes (P 0 ( 2 )−P 0 ( 1 ))/t 1 . This value becomes large if the pressure recovers as illustrated by the thick solid line, while it becomes small if the pressure recovers as illustrated by the broken line. 
     A pressure increase rate threshold value is required in order to judge the pressure increase rate. To this end, a gas engine is actually run, and experiments are repeatedly carried out on the relationship between the pressure increase amount and the time required for recovery of the pressure so as to investigate the pressure increase rate, from which a “pressure increase rate threshold value ΔP(t)” is determined, the pressure increase rate threshold value ΔP(t) so determined being stored in advance in the control section. 
     Next, an example of a control flow following the above time chart will be described. 
     FIG. 10 is a flowchart (part 1) for detecting a failure of the first shut-off valve according to the present invention, and STXX indicates a step number. 
     ST 01 : first, it is judged whether or not the engine is in a cranking state, that is, whether or not the crank shaft of the engine is in a rotating state occurring before a complete explosion is caused to take place in the cylinders by an engine starter. To be specific, the number of rotations of the crank shaft is measured by a crank shaft rotation sensor (not shown), and in the event that the number of rotations so measured is greater than zero (0) and is smaller than 500 rpm (rotations /minute), then the answer is YES, and the flowchart jumps to ST 09 . 
     ST 02 : If NO in ST 01 , a complete explosion signal, that is, a signal indicating whether or not a complete cylinder combustion is occurring is investigated. Specifically, the complete explosion signal is judged as YES if the number of rotations of the crank shaft exceeds a predetermined number of rotations (for instance, 500 rpm). 
     ST 03 : If the complete explosion signal is judged as NO in ST 02 , this means that the engine has not yet been started, and therefore, whether or not the engine starter switch is positioned in the accessory ON state is checked. If No, in other words, if the starter switch is not in the accessory ON state, this means that the engine is not in a started state, and therefore the flowchart ends, that is, no further diagnosis is carried out. 
     ST 04 : In the event that the starter switch is confirmed to be in the accessory ON state in ST 03 , the control system is put in a stand-by state. To be specific, in FIG. 8, power is supplied to the control section  36  and the injector driver  35  from the power supply so as to activate the first and second pressure sensors  31 ,  33  and the first and second temperature sensors  32 ,  34 , and a diagnostic timer (not shown) is started. 
     ST 05 : Confirm that the pressure P 0  is normal which is detected by the first pressure sensor. To be specific, the pressure inside the high pressure piping when the engine is stopped is investigated in advance through an experiment and a normal range for the pressure P 0  is determined in advance. If the pressure measured in ST 05  is greatly lower than the normal range so determined, this means a possibility that there is occurring something abnormal in the sensor or that there is occurring a leakage in the piping or the second shut-off valve. In this case, the flowchart ends or depart from the flow. On the contrary, if the pressure P 0  is greatly higher than the normal range thereof, this means a possibility that there is occurring something abnormal in the sensor or that there is occurring a leakage in the first shut-off valve. At this time, too, the flowchart ends or depart therefrom. 
     ST 06 : Confirm that the temperature T 0  is normal which is detected by the first temperature sensor. Specifically, the temperature inside the high pressure piping when the engine is stopped is investigated in advance through an experiment and a normal range for the temperature T 0  is determined in advance. If the temperature T 0  measured in ST 06  is greatly lower than from the normal range so determined, this means a possibility that there is occurring something abnormal in the sensor, or in the piping or the second shut-off valve. In this case, the flowchart ends or depart from the flow. On the contrary, if the temperature T 0  is greatly higher than the normal range thereof, this means a possibility that there is occurring something abnormal in the sensor, or in the first shut-off valve. At this time, too, the flowchart ends or depart therefrom. 
     The present invention is intended for diagnosing the shut-off valves. Then, since the pressure sensor is diagnosed in ST 05  and the temperature sensor is so done in ST 06 , this makes the contents of the diagnosis substantial, thereby making it possible to improve the reliability of the results of the diagnosis. 
     ST 07 : A fuel meter is put in a display state. 
     ST 08 : The solenoids of the first shut-off valve  50  and the second shut-off valve  65  shown in FIG. 8 are excited so as to open the valves. This produces a state in which a fuel gas can be supplied from the compressed gas cylinder to the engine. 
     ST 09 : Check whether or not a complete explosion signal has been detected. If NO, wait until the signal can be detected. If YES, advance to (A). 
     ST 10 : If the complete explosion signal is detected in ST 02  and the vehicle is cruising, the answer is YES, and then jump directly to (A) bypassing ST 03  to ST 09 . 
     FIG. 11 is a flowchart (part 2) for detecting a failure of the first shut-off valve according to the present invention, which follows (A) in the previous flowchart. 
     ST 11 : Here, the fuel meter is updated before a failure diagnosis is carried out. This is because the display of the meter is not updated when the valve is judged as being in a failure state while the meter display is updated when the valve is judged as not being in a failure state. 
     ST 12 : The excitation of the solenoid of the first shut-off valve is stopped and the shut-off valve is “closed.” The second shut-off valve is left opened (refer to ST 08 ). 
     Simultaneously with this, the first shut-off valve closing timer is reset, and the integration of time of the timer is started. 
     ST 13 : Wait until the accumulated time becomes equal to or exceeds a set time tc. 
     ST 14 : If the time requirements are met (if the timer counts up), the first shut-off valve is “opened,” and simultaneously the diagnostic timer, which is another timer, is reset, and the integration of time tf of the diagnostic timer is started. 
     ST 15 : Pressure information is taken in from the first pressure sensor for storage. This pressure information is an initial pressure value of the pressure P 0  and is regarded as a pressure initial value P 0 ( 1 ). 
     ST 16 : The sampling of the pressure P 0  when the pressure information is taken in from the first pressure sensor is carried out by taking in the pressure information from the first pressure sensor. 
     ST 17 : The sampling of the pressure P 0  is continued until the integrated time tf of the diagnostic timer becomes equal to or exceeds a predetermined judging time (time required for judgement) t 1 . Every time the sampling is repeated, the pressure P 0  becomes a new value. If tf≧ti is satisfied (if the diagnostic timer counts up), move to the next step. The deciding method of the judging time t 1  is described later. 
     ST 18 : The pressure P 0  is taken in, and the pressure so taken in is regarded as a final value, this being called a final pressure value P 0  ( 2 ). 
     ST 19 : The pressure increase rate at this time is obtained by (P 0 ( 2 )−P 0 ( 1 ))/t 1 . In the event that this pressure increase rate is equal to or larger than the pressure increase rate threshold value ΔP(t) stored in advance in the control section, then advance to (B), while in the event it is smaller than ΔP(t), move to ST 20 . 
     ST 20 : Impart 1 to flag FL as a failure detection identification. 
     ST 11  to ST 20  described above is a diagnosis mode. 
     FIG. 12 is a flowchart (part 3) for detecting a failure of the first shut-off valve according to the present invention. The flowchart shows a flow that should follow (B) in the previous flowchart. 
     ST 21 : Check whether or not the flag indicates “1” which designates a failure, and if it does not so indicate, then advance directly to END, and on the contrary, if it does, then move to the following step. 
     ST 22 : Since the valve is judged as in a failure state, the fuel meter is not updated. This is because the meter display is updated when the valve is judged as not being in a failure state, while the display of the meter is not updated when the valve is judged as being in a failure state. 
     ST 23 : A failure of the first shut-off valve is displayed in a warning display section (announcement panel or instrument panel) by means of a lamp or the like. 
     When the warning is displayed, the driver drives the vehicle to a garage for repair to eliminate the failure so warned of without any delay. 
     The first shut-off valve  50  shown in FIG. 8 is opened and/or closed based on a command from the control section  36 . Therefore, whether or not the shut-off valve is in a failure state can be judged while the vehicle is being stopped or running when the first shut-off valve  50  is closed by a command from the control section  36  by adopting the present invention. 
     In addition, in this embodiment, since no diagnosis is designed to be carried out when the vehicle is not cruising, a failure diagnosis in an unstable condition can be avoided, thereby making it possible to prevent the deterioration in reliability of judgement. 
     Furthermore, the shut-off valve and the pressure sensor may be the second shut-off valve and the second temperature sensor, respectively. Namely, since the present invention functions through a combination of the shut-off valve and the pressure sensor on the downstream side (secondary side) thereof, it is possible to diagnose the first and second shut-off valves in an alternate fashion. However, the pressure increase rate threshold values have to be set, respectively. 
     In addition, as described in this embodiment, the application of the present invention to the first shut-off valve which is disposed close to the CNG tank is more advantageous in application thereof to the second shut-off valve, since the first shut-off valve is closer to the CNG tank than the second shut-off valve and hence is affected by water penetrating into the CNG tank than the second shut-off valve. 
     Moreover, the fuel gas may be of any kind of fuel gas including compressed natural gas, hydrogen gas and coal gas, and therefore there is no limitation to the kind thereof. 
     Constructed as described above, the present invention exhibits the following advantages. 
     According to the invention, the shut-off valve used in the fuel gas supply system can be diagnosed with respect to a failure in which the shut-off valve is left closed and cannot be opened while the vehicle is being stopped or driven. Thus, it is possible to diagnose that the shut-off valve operates properly at suitable timings, whereby a trouble can be prevented that is entailed by a failure of the shut-off valve. 
     While only certain embodiments of the invention have been specifically described herein, it will apparent that numerous modifications may be made thereto without departing from the spirit and scope of the invention. 
     The present disclosure relates to the subject matter contained in Japanese patent application No. Hei. 11-87266 filed on Mar. 29, 1999 which is expressly incorporated herein by reference in its entirety.