Abstract:
To detect the volume of gas remaining in a gas bottle or a fixed tank of a system for supplying liquefied gas as fuel for a combustion engine, a positive temperature coefficient (or PTC) probe ( 16 ) is placed in the gas feed line ( 12 ) between the gas source and the combustion engine. The PTC probe ( 16 ) detects the change of phase, from the liquid phase of the liquefied gas to the gaseous phase, in the feed line ( 12 ) and delivers an electrical signal to an electronic circuit that triggers a visual or audible signal in response to the detection of the phase change.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to European Application No. 04100362.5 filed Feb. 2, 2004, and which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a system for supplying liquefied gas as fuel to a combustion engine, comprising a line for supplying gas to the engine, a liquefied gas source, a minimum liquefied-gas level detector and a means for indicating detection of the minimum level. 
     Many industrial vehicles use liquefied petroleum gas as fuel. The fuel is stored on the vehicle either in a changeable bottle or in a fixed tank. In the first case, the driver changes the bottle when the fuel is about to run out, while in the second case the driver must drive to a filling station in order to fill the tank. 
     These two storage modes each have a major drawback: 
     When the fuel is stored in a bottle, the driver has no information about the fuel level remaining in the bottle. To avoid running out of fuel, the driver has no other alternative but to regularly change the bottle, which generates substantial waste. 
     When the fuel is stored in a tank, the tank must be filled in a filling station. The filling is carried out on the basis of the differential pressure principle: the pressure of the gas contained in the station&#39;s tank “pushes” the liquid phase of the fuel into the vehicle&#39;s tank where the pressure is lower. If the pressure of the gas remaining in the tank is above the pressure of the gas in the filling station, filling is impossible. This may occur when the vehicle is running in a high temperature environment (for example a tank exposed to sunshine). 
     To remedy this problem there exists in the prior art an end-of-fuel-supply detection system that detects the drop in gas pressure. This system is placed in the fuel supply line downstream of the fuel source and upstream of the pressure regulator. When the fuel is about to run out, the pressure of the fuel source drops and a pressure-activated switch turns on a warning light on the dashboard. This system is not reliable as the pressure is directly proportional to temperature—therefore the time remaining before running out of fuel is random and may vary from a few minutes to a few seconds. In most cases, the warning light is turned on at the moment when the engine stalls for lack of sufficient fuel. Since the system is not reliable, the driver ignores the information displayed on the dashboard. 
     Other manufacturers place a second bottle on the vehicle. This method is not very practical since the vehicle&#39;s engine stops when the bottle is empty. If this occurs during a maneuver with a heavy load, the vehicle stops suddenly, and this may be hazardous. In addition, it turns out that some drivers do not automatically replace the empty bottle, the breakdown then occurring when the second bottle is empty. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the invention is to provide a system for supplying liquefied gas as fuel to a combustion engine with a minimum level detector that is reliable and does not have the abovementioned disadvantages of the prior solutions. 
     According to the invention, the liquefied gas supply system of the aforementioned type is characterized in that the detector is a means arranged in the supply line for detecting the occurrence of the change from the liquid phase of the liquefied gas to the gaseous phase in the supply line. 
     The detector is placed downstream of the fuel source and upstream of the vaporizer/pressure regulator in which the gas is vaporized by hot water and the pressure of the gas is reduced before being supplied to the motor. The detector that detects the liquid or gaseous phase of the fuel is immersed in the stream of fuel, preferably directly at the outlet of the tank (or of the bottle). The detector is preferably a component of the PTC (positive temperature coefficient) type, the electrical resistance of which element increases proportionally with the temperature. 
     According to a preferred embodiment, this component is connected to an electronic circuit driven by a microprocessor. The latter continuously controls the intensity of the current flowing through the electrical resistance element of the PTC component. 
     The current flowing through the PTC component causes the latter to heat up. As long as the element is swept by a flow of liquid fuel, the heat is dissipated immediately and the temperature remains relatively low. When the flow becomes a gas flow, the heat is no longer absorbed by the flow and the temperature of the component increases, which causes a variation in its electrical resistance. This variation is immediately detected by the microprocessor, which causes the warning light on the vehicle&#39;s dashboard to be turned on or an audible signal to be generated. As long as the tank (or bottle) contains a sufficient volume of fuel in the liquid phase, the warning light on the dashboard remains off or the audible signal is not triggered. 
     In the case of operation with fuel in a bottle, the driver will have to replace it within the travel time permitted by the amount of fuel remaining in the bottle. 
     In the case of storage in a fixed tank, the driver will also be warned when the minimum liquid fuel level is reached. In addition, the system may include in this case a means of detecting the pressure in the tank, and the electrical circuit may include a means of generating a visual or audible signal at the dashboard of the vehicle when the pressure in the tank has decreased down to a value allowing the tank to be filled. This avoids the driver having to drive to a filling station with a tank under high pressure, since filling is possible only if the pressure in the tank is below the filling pressure generated by the station&#39;s pump, as mentioned above. 
     The minimum fuel level detector according to the invention has the following advantages: 
     1. it is independent of the ambient temperature; 
     2. there is sufficient time to complete the current maneuver and reach a filling centre for filling the tank or for replacing the bottle; 
     3. it is possible to use the bottle right to the last drop of fuel (a major financial saving); 
     4. in the case of a fixed tank, the driver returns to the filling centre only when filling is possible (therefore saving time because unnecessary round trips are avoided). 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIGS. 1 and 2  show the operating principle of the system according to the present invention; 
         FIG. 3  shows the detection device with the PTC component in vertical section; 
         FIG. 4  is a diagram of the electronic circuit according to a first embodiment; and 
         FIG. 5  is a diagram of the electronic circuit according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The operating principle will now be explained in greater detail with reference to  FIGS. 1 and 2 . 
     The fuel supply system with a minimum fuel level detector includes a fuel source  10 , for example a liquefied gas tank or bottle. A supply line  12  extends from the fuel source  10  to the engine (not shown). A dip tube  14  forming part of the supply line  12  is fastened to the service tap (not shown) of the bottle or tank, and it allows the fuel to be led from the bottle or tank to a pressure regulator (not shown). A detection device  16  provided with a PTC component or probe is located in the supply line, preferably at the outlet of the bottle or fixed tank upstream of the pressure regulator (not shown). 
     In  FIG. 1 , the lower end of the dip tube  14  is in the liquefied gas. The gaseous phase is at the pressure p 1  and at the temperature t 1  dependent on the ambient temperature. The pressure pushes the liquid into the dip tube  14  and fills the pipe  12  as far as the pressure regulator (not shown). The PTC probe ( FIG. 3 ) of the detection device  16  for detecting the change of phase (liquid to gas) of the fuel inserted in the supply line  12  is swept by the fuel in liquid form. 
     When the liquid level has reached a level below the end of the dip tube  14 , as shown in  FIG. 2 , the supply line  12  becomes empty of the liquid fuel phase and the gaseous flow that bathes the PTC probe will no longer be sufficient to maintain the temperature of the probe below the detection threshold. The microprocessor of the electronic circuit ( FIG. 4 ) detects this rise in temperature and generates a light or audible signal in order to warn the driver that the vehicle is running on the gaseous phase of the fuel. 
     If the fuel is stored in a bottle, the driver will then drive to the place of storage in order to replace it. 
     If the fuel is stored in a fixed tank, the driver must continue to use the vehicle. The gas pressure in the tank will progressively drop and the driver will be warned by a second signal emitted by a pressure detection means ( FIG. 5 ) when the pressure in the tank has reached the threshold allowing it to be filled. 
     When the liquid level is below the end of the dip tube  14 , the little liquid that remains will evaporate. This evaporation will lower the temperature within the tank, which will speed up the drop in pressure. As mentioned above, the sensor or the pressure detection means will trigger the second signal, inviting the driver to drive to a filling station when the pressure is low enough to allow filling. 
     Consequently, whatever the source or the method of storing the fuel (bottle or fixed tank), the system according to the invention will warn the driver that he is close to running out of fuel by detecting the change of phase (liquid to gas) of the fuel in the supply line. The fuel has a low combustibility when it is in gaseous form, while the operating time left will be 5 to 10 minutes depending on the pressure within the bottle (or tank). 
       FIG. 3  shows an example of the detection device  16  inserted into the supply line  12 . This detection device  16  has a body  22  provided with a fuel passage  24 , which passes through it. The fuel passage  24  is connected on one side to that upstream part of the supply line  12  which is connected to the fuel source  10  and is connected on the other side to that downstream part of the fuel line  12  which goes towards the pressure regulator and the engine (these not being shown). The fuel passage  24  has a vertical part  24   a  and a horizontal part  24   b.    
     A pressure-reducing safety valve  28  having a body  30  provided with a central passage  32  communicates with the horizontal part  24   b  of the fuel passage  24 . The safety valve  28  is provided with a seat disc  34  carrying a sealing washer  36 . A helical spring  38  pushes the seat disc  34  downwardly in order to engage the sealing washer  36  with a fixed seat of the body of the valve  28 . The other end of the spring  38  presses on an adjustable disc  40  screwed into a threaded part of the body of the valve  28 . A cap  42  closes off the upper end of the safety valve  28 . When the pressure in the passage  24  passing through the device  22  exceeds a predetermined value, the sealing washer  36  is lifted off its seat under the effect of the pressure, with a view to reducing the overpressure. 
     The PTC probe or sensor  44  is located in a second vertical passage  28  that communicates with the horizontal passage  24   b.  This probe  44  is housed in a central bore of a support body  46  which is screwed into a tubular retaining screw  48  which is in turn screwed into a threaded part at the lower end of the second vertical passage  28  of the body  22  of the device  16 . Seals  45 ,  47  and  49  are provided between: (1) the probe  44  and its support  46 ; (2) the support  46  and the retaining screw  48 ; and (3) the screw  48  and the body  22 . The probe  44  has an enlarged top part that extends, at its upper end, upwards beyond the support  46  so as to be swept by the fuel flowing from the fuel source towards the engine. At its lower end, the PTC probe  44  is provided with a thinned rod connected by electrical conductors  50  to the electronic circuit  52 . The electronic circuit  52  controls the intensity of the current flowing through the PTC probe  44  and detects a variation in the electrical resistance of the PTC element caused by an increase in its temperature when it is no longer swept by a flow of liquid fuel. Detection of the change in electrical resistance of the PTC probe  44  immediately causes a visual signal  54  or an audible signal  54  to be issued at the vehicle&#39;s dashboard. 
       FIG. 4  shows an example of the electronic circuit  52  according to a first embodiment. The electronic circuit  52  is supplied via the power supply  56  for the modules of the circuit  52 , such as the current regulator  58 , the microprocessor  60  and the visual or audible signal means. The microprocessor  60  is operated at a frequency delivered by a clock circuit  64  in the usual manner known in the prior art. The power supply  56  communicates with the microprocessor  60  for the purpose of ensuring that a constant electrical current flows through the PTC element  44 , as described above. The electrical voltage caused by the flow of this current through the PTC element  44  is applied to a detector circuit  66  and picked up by the latter. The detector circuit  66  compares the voltage level at the PTC element  44  with a reference value, and transmits the result of this operation in digital form to the microprocessor  60 . Depending on the result, the microprocessor  60  causes an output element  62  to trigger a visual or audible signal by means of a signaling means  54 . 
     The operating mode of this electronic circuit  52  is as follows. The current regulator  58  delivers a constant current to the PTC element  44 , which generates an electrical voltage across the terminals of the latter that is proportional to the electronic resistance of the PTC element  44 . Assuming that the liquid gas level is high enough, that is to say the PTC element  44  is swept by the liquid, preventing the PTC element  44  from heating up, the resistance of this element is relatively low. When the gas level in the bottle or tank drops, the PTC element  44  is no longer cooled by the liquid gas. Thus, the constant current delivered by the current regulator  58  to the PTC element causes the latter to heat up, and this results in a change in its electrical resistance proportional to its temperature. This change in resistance can be measured by the electrical voltage detected by the detector circuit  66 . When the temperature reaches a predetermined level, the detector circuit  66  indicates this to the microprocessor  60 , which triggers the visual or audible signal. 
     In  FIG. 5 , a pressure sensor  68  has been added to the electronic circuit  52  according to a preferred embodiment. This sensor  68  is also supplied by the electrical current source  56 . The pressure detected, in the form of an electrical voltage, is transmitted to a second detector circuit  70 . The detector circuit  70  compares the pressure level at the sensor  68  with a reference value and sends the result of this operation in digital form to the microprocessor  60 . Depending on the result, the microprocessor  60  causes a second output element  72  to trigger a second visual or audible signal by means of a second signaling means  74 . 
     The operating mode of this modified electronic circuit  52  is as follows. The electrical current source  56  delivers a constant voltage to the pressure sensor  68 , which generates an electrical voltage across the terminals of the latter that is proportional to the pressure in the tank. Assuming that the pressure in the tank is relatively high, the voltage across the terminals of the sensor is also relatively high. When the pressure in the tank drops, the voltage across the terminals of the sensor  68  also drops. Thus, this change in pressure can be measured by the electrical voltage detected by the detector circuit  70 . When the pressure reaches a predetermined level, the detector circuit  70  indicates this to the microprocessor  60 , which triggers the second visual or audible signal. 
     It is clearly understood that although the examples of the embodiments described above refer to a circuit based on a microprocessor, equivalent functions could also be produced by analogue circuits. 
     In view of the above, it will be seen that the several objects and advantages of the present invention have been achieved and other advantageous results have been obtained.