Abstract:
An optical fiber sensor includes: an optical fiber; a light source portion; and a light receiving portion. The optical fiber includes: a core that includes a grating that generates a clad mode upon receipt of light; a clad that covers the core; and a fiber jacket that covers the clad, wherein a part of the fiber jacket corresponding to an area where the grating is formed is removed so that the clad is contactable with the fuel. The light source portion includes a light cutting element that emits light, whose wavelength is within a wavelength band of the cladding mode toward the optical fiber. The light receiving portion that detects intensity of the light transmitted through the grating. The optical fiber, the light receiving portion and the light source portion are arranged linearly.

Description:
This application claims priority from Japanese Patent Application No. 2009-193058 filed on Aug. 24, 2009, the entire subject matter of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an on-vehicle fuel property detection device, among members including a fuel supply apparatus, which is mounted in a fuel tank of a vehicle or the like, and which pressurizes and supplies fuel to an injector that injects fuel into an engine. Specifically, this invention relates to the detection device using an optical fiber. 
     2. Description of the Related Art 
     Recently, alcohol-blended fuel mixed with bioethanol has become widely used at an accelerated rate as an alternative to gasoline. However, in such alcohol-blended fuel, an optimum ignition timing, an optimum air-fuel ratio and the like vary with a concentration of ethanol contained in the fuel. Thus, in order to achieve appropriate engine control by a computer, i.e., an electronic control unit (ECU), it is necessary to accurately detect the concentration of alcohol contained in the fuel. 
     It is known, as one of means therefor, to detect the property of a liquid by an optical fiber sensor having an optical fiber, a light source and a light receiving portion (see, e.g., WO2006/126468 (Embodiment 20)). The optical fiber includes of a core having an area in which a grating is formed and a clad. The optical fiber is disposed at a position at which at least a part of the area in which the grating is formed is immersed in the liquid. The light source outputs light, whose wavelength is within a band of wavelengths of light corresponding to a cladding mode to be caused due to the grating of the area in which the grating is formed, to be incident on the optical fiber. The light receiving portion detects the intensity of light which is incident on the optical fiber from the light source and transmitted by the grating of the area. 
     WO2006/126468 discloses that the property of fuel stored in a fuel tank is accurately detected by providing an optical fiber probe in the fuel tank (in the case of Embodiment 20) or outside the fuel tank (in the case of Embodiment 19). Between these cases, the case of providing the optical sensor probe in the fuel tank as in Embodiment 20 has an advantage in that the detected property is insusceptible to temperature and heat, as compared with the case of providing the optical sensor probe outside the fuel tank, more particularly, in the vicinity of a vehicular engine room. However, on the other hand, the optical fiber used in the optical sensor probe described in WO2006/126468 is bent like a letter “U” regardless of which of the transmission type and the reflection type the optical fiber sensor is. The influence of the bending of the optical fiber superimposes on the properties of the fuel. Consequently, the optical fiber sensor described in WO2006/126468 has a problem of increasing an error of the detected properties. 
     SUMMARY OF THE INVENTION 
     The invention is accomplished to solve the above problem. An object of the invention is to obtain an on-vehicle fuel property detection device using an optical fiber sensor, which reduces a detection error of detected properties while a structure, in which an optical fiber sensor is provided in a fuel tank, in order to make the properties insusceptible to temperature and heat. 
     According to one aspect of the invention, there is provided an optical fiber sensor, which is provided in a fuel tank of a vehicle, and which detects a property of fuel in the fuel tank, the optical fiber sensor comprising: an optical fiber comprising: a core that comprises a grating that generates a clad mode upon receipt of light; a clad that covers the core; and a fiber jacket that covers the clad, wherein a part of the fiber jacket corresponding to an area where the grating is formed is removed so that the clad is contactable with the fuel; a light source portion comprising a light cutting element that emits light, whose wavelength is within a wavelength band of the cladding mode toward the optical fiber; and a light receiving portion that detects intensity of the light transmitted through the grating, wherein the optical fiber, the light receiving portion and the light source portion are arranged linearly. 
     As described above, according to the invention, the on-vehicle fuel property detection device can be obtained, which uses the optical fiber sensor that is compact and has a simple structure, and that accurately measures the concentration of alcohol contained in fuel without being affected by temperature and heat. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating the configuration of an on-vehicle fuel control system according to Embodiment 1 of the invention; 
         FIG. 2  is a diagram detailedly illustrating an A-portion shown in  FIG. 1 ; 
         FIG. 3  is an enlarged diagram of a B-portion shown in  FIG. 2 , which illustrates an optical fiber sensor; 
         FIGS. 4A to 4C  are perspective diagrams illustrating the appearance of the B-portion shown in  FIG. 3 ; 
         FIG. 5  is an enlarged diagram of a C-portion shown in  FIG. 3 ; 
         FIG. 6  is a graph illustrating an output characteristic of the optical fiber sensor according to Embodiment 1 of the invention; 
         FIG. 7  is a table illustrating the refractive index of fuel; and 
         FIG. 8  is cross-sectional diagram taken along line D-D shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     An object of the invention is to accurately detect a concentration of alcohol contained in fuel in order to achieve appropriate ECU control. First, upon describing a mechanism for measuring the concentration of alcohol contained in fuel, a configuration and an operation of an entire fuel control system are first described with reference to  FIG. 1 .  FIG. 1  is a diagram illustrating the configuration of an on-vehicle fuel control system. 
     In  FIG. 1 , reference numeral  100  designates a fuel property detection device (hereinafter referred to as an optical fiber sensor) implemented by an optical fiber sensor. Reference numeral  101  designates an engine of an automobile or the like. Reference numeral  102  designates a fuel injection valve. Reference numeral  103  designates a fuel tank. Reference numeral  104  designates a fuel pump. Reference numeral  106  designates a high-pressure filter for filtering fuel  117  sucked up from the fuel pump  104  via a fuel supply pipe  105 . Reference numeral  107  designates a fuel distribution pipe. Reference numeral  108  designates a fuel pressure regulator. Reference numeral  109  designates a fuel return pipe. Reference numeral  110  designates an air-fuel-ratio sensor. Reference numeral  111  designates an ignition plug. Reference numeral  112  designates an engine speed sensor. Reference numeral  113  designates an intake pressure sensor. Reference numeral  114  designates a throttle valve. Reference numeral  115  designates an air cleaner. Reference numeral  116  designates a control unit including an ECU, to which signals output from the optical fiber sensor  100 , the air-fuel-ratio sensor  110 , the engine speed sensor  112 , the intake pressure sensor  113 , and the like are input. The control unit  116  drives the fuel injection valve  102 , the ignition plug  111 , and the like, based on controlled variables corresponding to the input signals. 
     Next, a series of operations of the fuel control system are described hereinafter. When the fuel  117  is supplied to the fuel tank  103 , the engine  101  is started. Simultaneously, the fuel  117  is pressurized by the fuel pump  104 . Thus, the fuel  117  flows into the fuel distribution pipe  107  through the fuel supply pipe  105  and the high-pressure filter  106 . Apart of the fuel  117  is supplied to the engine  101  from the fuel injection valve  102 . The rest of the fuel  117  is returned to the fuel tank  103  through the fuel pressure regulator  108  and the fuel return pipe  109 . Incidentally, the fuel pressure regulator  108  always maintains the pressure of the fuel  117  in the pipes up to the fuel distribution pipe  107  at a constant value, regardless of an amount of fuel consumption of the fuel injection valve  102 . The presence/absence of alcohol mixed in the fuel  117  is detected by the optical fiber sensor  100  attached to the fuel pump  104 . When alcohol mixed in the fuel is present, the rate of content of alcohol is measured by the optical fiber sensor  100 , as will be described below. When the measured rate of content of alcohol is input to the control unit  116 , the control unit  116  grasps the state of the engine according to signals output from the engine speed sensor  112  and the intake pressure sensor  113 , and the control unit  116  changes an amount of fuel supplied to the engine by controlling the valve opening time of the fuel injection valve  102 . On the other hand, an air-fuel ratio is detected by the air-fuel-ratio sensor  110 . The control unit  116  performs the feedback control of the air-fuel-ratio so that the air-fuel-ratio reaches a target value corresponding to the state of the engine at that time. In addition, the control unit  116  controls the ignition timing of the ignition plug  111  according to the state of the engine. Accordingly, optimum engine control according to the type of fuel supplied to a vehicle becomes possible. 
     Next, the attachment of the optical fiber sensor  100  to the fuel pump  104  is described below with reference to  FIG. 2 . Incidentally,  FIG. 2  is a diagram detailedly illustrating an A-portion shown in  FIG. 1 , what is called a fuel supply apparatus. As illustrated in  FIG. 2 , the fuel pump  104  sucks and pressurizes the fuel  117  through a filter  50  and feeds the pressurized fuel  117  into the fuel injection valve  102  (see  FIG. 1 , that is a part of injector) through the fuel supply pipe  105 . Generally, when the fuel pump  104  is mounted in the fuel tank  103 , the fuel pump  104  is removably supported therein by a stay  52  fixed to a plate  51  that blocks a hole  103   a  provided in the fuel tank  103 . The stay  52  is provided with an arm  53  on which the optical fiber sensor  100  is provided so that the direction of the optical fiber sensor  100  is perpendicular to a liquid surface of the fuel  117 . 
     A pair of sensing lines  54  respectively extending from a light source portion  3  and a light receiving portion  5 (to be described below) provided in the optical fiber sensor  100  are connected to a control portion  55  in which signals from the sensing lines  54  are converted into optimum signals representing the property of the fuel  117 . Then, the optimum signals are connected via a signal line  56  to a connector  57  which is connected to the control unit  116 . The pair of sensing lines  54  differs in length between the optical fiber sensor  100  and the control portion  55  from each other due to the structure of the optical fiber sensor  100 . In  FIG. 2 , the control portion  55  is added to the fuel pump  104  such that the control portion  55  is one member including the fuel supply apparatus. Alternatively, the control portion  55  may be installed in or outside the fuel tank  103 . For example, when the fuel tank  103  is installed in the fuel tank  103 , the number of members can be reduced by incorporating the function of the connector  57  to the control portion  55 . The fuel supply apparatus maybe configured by incorporating the high-pressure filter  106  and the fuel pressure regulator  108  illustrated in  FIG. 1  thereinto, i.e., in the form of what is called a fuel pump module. Specifically, when the fuel pressure regulator  108  is incorporated into the fuel supply apparatus, the fuel is not returned to the fuel tank  103  through the fuel return pipe  109 , and thus this case has a merit that the temperature resistance of the optical fiber sensor  100  can be improved. 
     Next, the internal structure of the optical fiber sensor  100  is described hereinafter with reference to  FIGS. 3 to 4C .  FIG. 3  is an enlarged diagram of a B-portion shown in  FIG. 2 .  FIGS. 4A to 4C  are perspective diagrams illustrating the appearance of the B-portion shown in  FIG. 3 . In  FIG. 3 , reference numeral  1  designates an optical fiber. Reference numeral  3  designates a light source portion including a light emitting element  2  disposed at a first end portion of the optical fiber  1 . Reference numeral  5  designates a light receiving portion including a light receiving element  4  disposed at a second end portion of the optical fiber  1 . A light emitting diode, a laser diode or the like can be used as the light emitting element  2 . A spectral analyzer, a photodiode or the like can be used as the light receiving element  4 . The light source portion  3  and the light receiving portion  5  are airtightly connected to the optical fiber  1  penetrating through opening portions  6   a  of a pipe  6 . The light source portion  3  and the light receiving portion  5  are immersed in the fuel  117 , as shown in  FIG. 2 . Thus, each of the light source portion  3  and the light receiving portion  5  has an airtight structure. 
     The airtight structure is obtained by performing a welding connection of each of the opening portions  6   a  or by applying a fusion structure using glass thereon. Preferably, each of the opening portions  6   a  functioning as a part of a connection portion is sealed with low-melting-point glass by way of example. Preferably, the pipe  6  is formed of metal when the opening portions  6   a  are sealed. In addition, in consideration of the fact that the pipe  6  is immersed in the fuel  117 , similarly to the light source portion  3  and the light receiving portion  5 , preferably, the pipe  6  is formed of a stainless steel. Obviously, it is necessary that the optical fiber  1  is contacted with the fuel  117 . Thus, as shown in  FIG. 3  or  FIG. 4A , the pipe  6  can maintain the continuity thereof (in plain words, the pipe  6  can hold the optical fiber  1 ) by providing a spiral fuel introduction hole  6   b  therein. The fuel introduction hole  6   b  may be formed into a shape illustrated in  FIG. 4B  or  4 C. Specifically, the shape illustrated in  FIG. 4C  facilitates forming the pipe  6  of resin and reduction in weight of the optical fiber sensor  1 . When the pipe  6  is formed of resin, it is useful that the sealing of the opening portions  6   a  with low-melting-point glass is performed at the light source portion  3  and the light receiving portion  5 , and that the light sealing portion  3  and the light receiving portion  5  are connected to the pipe  6  by, e.g., screwing. Even when the pipe  6  is formed of resin, similarly in consideration of the fact that the pipe  6  is immersed in the fuel  117 , preferably, the material of the pipe  6  is a polyacetal resin. 
     Hereinafter, a principle of detecting the property of fuel is described with reference to  FIGS. 5 to 7 . Incidentally,  FIG. 5  is an enlarged diagram of a C-portion shown in  FIG. 3 . 
       FIG. 6  is a graph illustrating an output characteristic of the optical fiber sensor.  FIG. 7  is a table illustrating the refractive index of fuel. The optical fiber  1  includes a core  10  in which light emitted from the light source portion  3  (see  FIG. 3 ) propagates, a clad  11  which covers the core  10  to confine light in the core  10 , and a fiber jacket  12  covering the core  10  and the clad  11  for protection. In order to detect the property of the fuel  117  around the optical fiber sensor  100 , a part of the fiber jacket  12  is removed so that the clad  11  is contacted directly with the fuel  117 . Inorganic glass such as quartz glass, or plastic materials such as polymethylmethacrylate, can be used as the materials of the core  10  and the clad  11 . A highly gasoline-resistant resin, such as a fluororesin, can be used as the material of the fiber jacket  12 . 
     The principle of detecting the property of fuel utilizes a phenomenon that the intensity of a light beam in “a cladding mode” caused when a grating  13  reflects or transmits the light beam propagating in the core  10 , which varies depending upon the refractive index of fuel contacted with an outer peripheral part of the clad  11 . That is, in a part of the core  10  not formed the grating  13  the light beam propagating therein repeats reflection on the boundary surface between the core  10  and the clad  11 , so that the light beams propagate only in the core  10 . However, when the light beam reaches the grating  13 , the light beam is split into a first light beam  14  which is transmitted by the grating  13  and propagates in the core  10 , a second light beam  15  which undergoes a Bragg reflection at the grating  13  and propagates in the core  10  in a opposite direction, and a third light beam  16  which leaks out of the core  10  and propagates in the clad  11 . The intensity of the first light beam  14  which is transmitted through the grating  13  and propagates in the core  10 , and the third light beam  16  which leaks out of the core  10  and propagates in the clad  11 , can be detected by the light receiving portion  5  (see  FIG. 3 ) located at the second end of the optical fiber  1  in the direction of propagation of the light beams. 
     Here, the wavelength characteristic curve of the intensity of the transmitted light in the cladding mode has periodic loss peaks. Because the optical fiber  1  is immersed in the fuel  117 , the height of each periodic loss peak varies depending upon the refractive index of the fuel  117 . In the alcohol-blended fuel, it has already been known that the refractive index of the fuel varies depending upon the concentration of ethanol contained in the fuel, shown in  FIG. 7 . Thus, it is detected that the loss peaks of the transmission spectra of light in the cladding mode vary depending on the refractive index of the fuel, the concentration of alcohol contained in the fuel can be estimated by detecting the refractive index of the liquid. 
     That is, a total amount of the intensity of light transmitted through the grating  13  changes depending upon the property (refractive index) of the fuel contacted with the outer peripheral part of the clad  11 . Accordingly, the property (refractive index) of the fuel can be detected by an amount of light received by the light receiving element  4 . The control portion  55  (see  FIG. 3 ) converts the amount of light detected by the light receiving element  4  into a voltage signal and outputs the voltage signal. As shown in  FIG. 6 , an output voltage (V) of the control portion  55  has a substantially inverse proportion relationship with the refractive index of the fuel. That is, as shown in  FIGS. 6 and 7 , when the concentration of alcohol contained in the fuel increases, the refractive index of the fuel decreases, while the output voltage increases. An estimated value of the refractive index is calculated from the value of the output voltage (V) of the control portion  55 . Specifically, the refractive index is estimated from the output voltage. Then, the properties of the fuel, e.g., the presence/absence of alcohol mixed in the fuel and the rate of content of the alcohol when the alcohol mixed in the fuel is present, can be grasped by the estimated refractive index. In other word, because such an output voltage corresponding to such a refractive index of the fuel is obtained, the valve opening time of the fuel injection valve  102  and the ignition timing of the ignition plug  111  are controlled according to such an output voltage. Consequently, optimal engine control can be implemented. 
     The shape of the optical fiber sensor  100 , which results in the optimal engine control, is described hereinafter in more detail. As is apparent from  FIGS. 2 and 3  that have already been described, the optical fiber sensor  100  is configured so that the light source portion  3  and the light receiving portion  5  in addition to the optical fiber  1  are immersed in the fuel  117 . Because it is unnecessary to bend the optical fiber  1 , an error of the output voltage obtained corresponding to the property of the fuel is extremely small. The light source portion  3  and the light receiving portion  5  need to maintain airtightness. However, because the light source portion  3  and the light receiving portion  5  are immersed in the fuel, change in the temperature thereof is relatively small. Accordingly, the invention can have a considerable ripple effect that a sensor stable in temperature is obtained. 
     Because the optical fiber sensor  100  is attached to the arm  53  installed on the stay  52 , when the optical fiber sensor  100  is immersed in the fuel, an immersing operation is very easily achieved. When the optical fiber sensor  100  is attached to the fuel pump  104 , the fuel pump  104  and the optical fiber sensor  100  according to the invention are configured such that the area of a projection of the fuel pump  104  and the optical fiber sensor  100  on a plane including the hole  103   a  is less than the area of the hole  103   a  as shown in  FIG. 8.That  is, the area (S 3 ) of a cross-section of the optical fiber sensor  100  is less than a value obtained by subtracting the area (S 2 ) of a maximum cross-section part of the fuel pump  104  from the area (S 1 ) of the hole  103   a . Thus, even in the case of a fuel supply apparatus having a fuel property detection device, the fuel supply apparatus can smoothly be mounted in the fuel tank  103  without being damaged. 
     Hereinafter, another embodiment will be described. The relationship between the output voltage of the optical fiber sensor  100  and the refractive index of the fuel  117  has shown in  FIG. 6 . For example, when the fuel is not contacted with the grating  13 , an output voltage having an output characteristic curve indicated with a dashed line added to  FIG. 6  is obtained. When the output voltage which is about 5 V is input to the control portion  55 , the output voltage can be used to turn on an alarm lamp indicating occurrence of a “small-remaining-fuel-amount” state in which the liquid level of the fuel contained in the fuel tank  103  is lower than the position of the optical fiber sensor  100 . 
     The optical fiber sensor  100  according to the invention has been described as a member of the fuel supply apparatus attached to the fuel pump  104 . However, the mode for carrying out the invention is not limited thereto. For example, even when the optical fiber sensor  100  is used in a stand-alone mode, similarly to shown in FIG. 41 of WO2006/126468, it is apparent that the advantages can similarly be obtained. In addition, although the optical fiber sensor  100  has been described as a fuel property detection device, similarly, it is apparent that the optical fiber sensor  100  can be applied to a liquid level detection device by causing the optical fiber sensor  100  itself extending in a direction perpendicular to the liquid surface of the fuel. 
     While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.