Abstract:
A sensing apparatus for determining a property of a fuel such as a gasoline and ethanol blend known as flex fuel includes an acetal plastic tube with an inlet, an outlet and a fuel passage in between. One property is a dielectric constant. A pair of semi-circular shaped sensing plates are placed around the tube in concentric relation therewith, leaving the fuel passage unobstructed. A processing circuit on a printed circuit board (PCB) is located near and connected with the sensing plates. The circuit applies an excitation signal, senses a capacitance, and generates an output signal indicative of a property of the fuel. A shield for reducing EMI surrounds and encloses the sensing plates and the PCB. The sensed capacitance will increase with increasing concentration of ethanol in the fuel flowing through the passage. An interface connector allows the sensing apparatus to output the indicative signal to an engine controller.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to sensors and more particularly to a fuel sensor having sensing plates that do not obstruct a fuel passage. 
       BACKGROUND OF THE INVENTION 
       [0002]    Due to the fact that ethanol is a renewable fuel, and for other reasons as well, the use of ethanol and ethanol blends (i.e., ethanol and gasoline) continues to grow. For example, flexible fuel vehicles are known that are designed to run on gasoline as a fuel or a blend of up to 85% ethanol (E85). Properties of such fuels, such as its conductivity or dielectric constant, can be used to determine the concentration of ethanol in the gasoline/ethanol blend and can also be used to determine the amount of water mixed in with the fuel. Experimental data shows that the fuel dielectric constant is directly proportional to the ethanol concentration but relatively insensitive to water contamination, provided that the water concentration is below about 1% since the dielectric constant of water is around 80 at 25° C. (i.e., surveys show that the water concentration on most U.S. Flex fuel stations is below 1%). On the other hand, fuel conductivity is very sensitive to water concentration. For example, ethanol has a dielectric constant of around 24 at 25 degrees Celsius while gasoline has a dielectric constant of around 2 at the same temperature. Determining the properties of such fuels is important for operation of a motor vehicle since an engine controller or the like can use the information regarding the composition, quality, temperature and other properties of the fuel to adjust air/fuel ratio, ignition timing and injection timing, among other things. Additionally, increasingly strict emissions-compliance requirements have only further strengthened the need for an accurate flexible fuel sensor. 
         [0003]    As added background, most sensor technologies for fuel property sensing require in-situ signal processing electronics to convert the relatively small sensing signals to a suitably strong electrical signal that can be used by an external circuit, such as an engine controller, to define the measured fuel property of interest. For example only, a capacitive sensor, which is configured to apply an excitation signal to spaced apart sensing plates, induces a relatively small response signal, thus requiring local electronics to preserve the signal-to-noise ratio. 
         [0004]    It is also known that most in-situ sensors (e.g., capacitive, inductive or magnetic technologies) do not require direct contact or exposure to the fuel in order to assess the relevant fuel properties. Nonetheless, these sensors generally benefit from the physical isolation from the fuel, since contact with the fuel can often degrade the performance of the sensor. While it is known to use coatings to isolate various sensor components from contact with the fuel, such coatings may induce stress and/or degrade the signal-to-noise ratio of the sensing approach. 
         [0005]    Fuel passage obstruction is another shortcoming of conventional fuel sensors, particularly capacitance-based approaches. More specifically, to measure the capacitance of the fuel, conventional sensors are known to use plates with different shapes, but in all such applications these plates are inside the fuel line (i.e., the fuel passage). This makes the construction of such sensors more complex and poses a potential for obstructing the fuel flow. Additionally, this approach imposes stricter requirements to protect the plates from corrosion by the ethanol, as described above. 
         [0006]    There is therefore a need for a fuel sensor that minimizes or eliminates one or more of the problems set forth above. 
       SUMMARY OF THE INVENTION 
       [0007]    The invention is directed to a fuel sensing apparatus where the sensing plates are placed outside the fuel passage so that no obstruction to fuel flow is produced. Additionally, the sensing plates and signal processing electronics are located away from any contact with the fuel, reducing the risk of degradation due to corrosion, without the use of any coatings or the like, which simplifies the design. 
         [0008]    An apparatus is provided for use in sensing one or more properties of a fuel. The apparatus includes a tube and first and second sensing plates. The tube extends along a longitudinal axis and has a hollow interior defining a fuel passage between a fuel inlet and a fuel outlet of the tube. The sensing plates are disposed radially outwardly of the tube on its outer surface tube, leaving the fuel passage unobstructed between inlet and outlet, and also isolating the plates from contact with the fuel. The tube and the sensing plates are preferably in a concentric relationship, with the tube preferably comprising acetal thermoplastic material. 
         [0009]    In a preferred embodiment, the sensing plates include a plurality of apertures configured to cooperate with a corresponding plurality of protuberances projecting from the tube to align and retain the sensing plates to the tube. A pair of spacer wheels, enlarged in diameter relative to the tube, extend radially outwardly from the tube at axially opposing ends. A generally cylindrical, hollow shield is located radially outwardly of the tube and is sized to engage and fit on the spacer wheels, where the shield and the spacer wheels cooperate to form a cavity. The cavity encloses the sensing plates and is configured in size and shape so as to be able to house a processing circuit on a printed circuit board (PCB). The processing circuit is therefore located near to and is electrically coupled with the sensing plates and is arranged to determine a characteristic (e.g., a capacitance) of the structure between the plates, which is mainly, in a preferred embodiment, determined by the concentration of ethanol in the fuel flowing through the passage. The processing circuit is configured to generate an output signal indicative of one or properties of the fuel (e.g., dielectric constant). 
         [0010]    Other features, aspects and advantages are presented. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will now be described by way of example, with reference to the accompanying drawings: 
           [0012]      FIG. 1  is a top, perspective view of an embodiment of an obstructionless inline flexible fuel sensing apparatus according to the invention. 
           [0013]      FIG. 2  is an exploded view of the fuel sensing apparatus of  FIG. 1 . 
           [0014]      FIG. 3  is a perspective of a tube portion of the fuel sensing apparatus of  FIG. 1  as viewed in the direction of line  3 - 3  in  FIG. 2 . 
           [0015]      FIG. 4  is a perspective view of a connector portion of the fuel sensing apparatus of  FIG. 2 . 
           [0016]      FIG. 5  is a cross-sectional view of a concentric tube and sensing plate assembly taken substantially along line  5 - 5  in  FIG. 2 . 
           [0017]      FIG. 6  is a simplified schematic diagram showing the fixed and variable capacitive contributions provided by the tube, and variable ethanol concentration fuel, respectively. 
           [0018]      FIG. 7  is a diagram showing how the capacitance of a fuel flowing through the fuel sensing apparatus of  FIG. 1  varies with ethanol concentration. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]    Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views,  FIG. 1  is a perspective view of an apparatus  10  for sensing one or more properties of a fuel, such as a dielectric constant of a gasoline/ethanol blend. The sensing apparatus  10 , as shown, is an in-line type fuel sensing apparatus that is coupled between a source of fuel, such as a fuel tank  12 , and a destination, such as various fuel delivery apparatus  14  associated with an automotive vehicle internal combustion engine (not shown). The sensing apparatus  10 , generally, includes a pair of sensing plates surrounding an inner tube, in a concentric manner, which are connected to a closely-located electrical circuit with signal processing capability so as to generate an output signal  16 . The sensing plates around the inner tube will form a capacitor. The material between the plates includes a fixed portion, namely the tube walls, which have a fixed dielectric constant. However, the dielectric constant of the fuel flowing through the fuel line will vary, depending on the composition of the fuel itself. The total effective capacitance will be mainly driven by the variable portion. The circuit will measure the capacitance for purposes of generating the signal  16 . The output signal  16  is indicative of one or more sensed physical properties of the fuel, such as dielectric constant or conductivity. The output signal  16  may then be provided to, for example only, an electronic engine controller  18  or the like for use in, as known in the art, and as described in the Background, fuel delivery control. 
         [0020]      FIG. 2  is an exploded view showing in greater detail the sensing apparatus  10  and its constituent parts described generally above. The sensing apparatus  10  includes a tube  20 , a first sensing plate  22 , a second sensing plate  24 , a shield  26 , an electrical processing circuit  28  on a printed circuit board (PCB)  30  and an electrical connector  32 . The stack-up assembly, as will be described, is generally concentric, starting with the tube  20  as the innermost component, then the plates  22 ,  24 , and then the shield  26 . 
         [0021]      FIG. 3  is an enlarged perspective view showing the tube  20  in greater detail. The tube  20  extends along a main, longitudinal axis labeled “A”. The tube  20  is preferably unitary (i.e., one piece) in construction, solid and continuous, and comprises plastic or other material that is resistant to degradation in the presence of various fuels including gasoline/ethanol blends. In one embodiment, the tube  20  is formed using an engineering plastic, such as a thermoplastic material known as acetal (or sometimes polyacetal). Acetal material exhibits desired chemical resistance properties with respect to the fuel that is contemplated to flow through the sensing apparatus  10 . 
         [0022]    As shown, the tube  20  includes an inlet  34 , an outlet  36  and a fuel passage  38  (also shown in  FIG. 5 ) formed in between. It should be appreciated that the inlet and outlet designations here are arbitrary, the principal of operation being applicable to fuel flows in either direction through the fuel passage  38 . The inlet  34  and the outlet  36  each include a respective interface that is suitable for connection to a fuel hose or tube or other mechanism, as per the requirements of any particular application. For example only, as illustrated, the inlet  34  and the outlet  36  each include respective O-ring seals  40 ,  42 . Of course, other variations are possible. Significantly, the fuel passage  38  is unobstructed between the inlet  34  and the outlet  36 . The sensing plates  22  and  24  are located outside of the tube  20  and hence out of the fuel passage  38 , which is unlike the construction of conventional fuel sensors. 
         [0023]    The tube  20  further includes an outer surface  44  spaced from the fuel passage  38  (i.e., by the wall thickness of the tube). The tube  20  is substantially circular in radial cross-section (best shown in  FIG. 5 ). The tube  20  also includes a plurality of protuberances  46  configured to cooperate with a corresponding plurality of apertures  48  ( FIG. 2 ) in the sensing plates  22  and  24  configured to align and retain the sensing plates  22 ,  24  with respect to the tube  20 . The protuberances  46  may be snaps or heat stakes, or other conventional approaches for forming projections. 
         [0024]    The tube  20  also includes a pair of spacer wheels  50  disposed on axially opposing ends  52  and  54  of the tube  20 . Each spacer wheel  50  has a first outside diameter  56  that is larger than an outside diameter  58  of the tube  20 . The spacer wheels  50  generally are configured to accommodate the shield  26  and form a fully enclosed sensing apparatus  10 . It is preferred that the tube  20  as inclusive of the spacer wheels  50  be unitary (one-piece molded). The spacer wheels  50  may be formed with a radially-outermost sleeve, which if an outer edge is crimped, may be useful to hold the shield  26  in place. 
         [0025]    Referring again to  FIG. 2 , the sensing plates  22  and  24  are generally semi-circular in shape and sized so as to snugly fit radially outwardly directly on the tube  20 . The sensing plates  22  and  24  are preferably formed of an electrically-conductive material to which a copper wire or other conductor can be electrically-connected to (e.g., soldered), such as various thin plated metals and alloys known in the art for constructing sensing plates. For example, typical embodiments of the present invention may use a copper-based alloy (e.g., brass) for the sensing plates. The apertures  48  in the plates  22 ,  24  sized and located in correspondence with protuberances  46  so as to facilitate assembly of the plates to the tube  20 . Upon assembly, the sensing plates  22  and  24  engage the outer surface  44  of the tube  20  wherein the sensing plates  22  and  24  and the tube  20  are in a concentric relationship with each other. This is best shown in  FIG. 5 . 
         [0026]    The shield  26  is configured to reduce electromagnetic interference (EMI). More specifically, one function performed by the shield  26  is to minimize or eliminate the effect that stray or external electromagnetic interference may otherwise have on the sensing plates  22  and  24 . A second function performed by the shield  26  is to minimize or eliminate any electromagnetic emissions produced by the excitation of the sensing plates  22  and  24  from propagating outwards from the sensing apparatus  10 . As to construction, the shield  26  may comprise electrically-conductive material such as various metals and be coupled to a ground terminal of the interface connector  32 , either directly via internal conductors or indirectly via a connection on the PCB  30 . In the illustrated embodiment, the shield  26  is generally disposed radially outwardly of the tube  20 , circumferentially continuous, and has an axial length sufficient to span the spacer wheels  50 . The shield  26  is hollow and has an interior surface configured to engage and fit on the outside diameter of the spacer wheels  50 . The shield  26  and the spacer wheels  50  cooperate to enclose the sensing plates  22  and  24 . In addition, the shield  26  and the spacer wheels  50  cooperate to form a closed cavity  60  (i.e., the radially-outwardly extending space between the sensing plates/tube, on the one hand, and the interior surface of the shield  26 , on the other hand. 
         [0027]      FIG. 5  is a cross-sectional view of the sensing apparatus  10  taken substantially along line  5 - 5  in  FIG. 2 . As shown, the circuit  28  on the PCB  30  is electrically coupled to the sensing plates  22  and  24 . Such a connection may be made using, conventionally, either separate wires or through suitably configured extensions of the sensing plates themselves that would terminate directly on the PCB. The PCB  30  is preferably located close to the sensing plates  22  and  24 , and in the preferred embodiment, the PCB  30  is disposed within the cavity  60  of the sensing apparatus  10 . The cavity  60  is thus configured in size and shape to at least house the printed circuit board (PCB)  30 . While this will be described in greater detail below, generally, to perform its function, the signal processing circuit  28  is configured to apply suitable excitation signals to the sensing plates  22  and  24  and to detect and process the resulting induced signals to develop the output signal  16  indicative of a physical property of the fuel. The close proximity of the circuit  28  to the sensing plates improves the signal-to-noise ratio of the detected induced signal. 
         [0028]    Referring to  FIGS. 2 and 4 , the interface connector  32  may comprise conventional construction approaches and materials, and may include a plurality of electrical terminals. In one embodiment, the connector  32  may include power, ground and output signal electrical terminals designated by reference numerals  62 ,  64  and  66 , respectively ( FIG. 4 ). Leads from these terminals  62 ,  64  and  66  are electrically connected to the circuit  28  on the PCB  30 . In the embodiment where the PCB  30  is situated in the cavity  60 , the leads  62 ,  64  and  66  from the connector  32  may pass through a series of axially-extending apertures  68  located in a main wall of one of the spacer wheels  50 , as shown in  FIG. 3  enclosed in a dashed-line box. The leads may then be connected to the PCB  30  using conventional means (e.g., soldering). 
         [0029]      FIG. 6  is a simplified schematic diagram showing a simplified equivalent circuit  70  representing the sensing apparatus  10 . It should be understood that in the present disclosure, a pair of sensing plates  22  and  24 , with fuel flowing in the fuel passage  38 , will appear to the electronics on PCB  30  as a complex load (e.g., a parallel combination of a resistor and a capacitor). More specifically, the tube  20  and the two sensing plates form a relatively small value capacitor, which is designated C 1  in  FIG. 5 . Generally speaking, the value of C 1  is fixed. When fuel flows through the fuel passage  38 , an additional capacitance is added to the complex load, which is variable and depends on the particular properties of the fuel. This variable capacitance is designated C 2  in  FIG. 5 . As described, the greater the ethanol concentration, the greater is the composite dielectric constant of the fuel blend. Since capacitance is determined based generally on plate geometry, spacing (which are fixed), and the dielectric constant of the material between the plates (which may vary here), it can be seen that the sensed capacitance C 2  increases with higher concentrations of ethanol in a gasoline/ethanol blend. There is an additional resistive component, which is also variable, and is designated R in  FIG. 5 . This complex impedance comprises a real component part (resistive) and an imaginary component part (capacitive), which can be deconstructed and correlated to a conductivity and a dielectric constant, useful physical properties of the fuel. In particular, a dielectric constant can be derived from sensed capacitance using known relationships. The art is replete with approaches for measuring the complex impedance, or components thereof, for purposes of ascertaining one or more physical properties of the fuel, for example, as seen by reference to U.S. application Ser. No. 10/199,651 filed Jul. 19, 2002, now U.S. Pat. No. 6,693,444 B2 entitled “CIRCUIT DESIGN FOR LIQUID PROPERTY SENSOR” issued Feb. 17, 2004 to Lin et al., owned by the common assignee of the present invention, and hereby incorporated by reference in its entirety herein. 
         [0030]      FIG. 7  is a chart showing the increase in sensed capacitance with increasing concentrations of ethanol in a gasoline/ethanol blend (e.g., a Flex Fuel). As shown, trace  72  represents a curve-fit relationship between particular measured plotted points. It should be understood that suitable a configuration of the signal processing circuit  28  may be employed to obtain a desired relationship of the output signal  16  and the variable concentration fuel. Alternatively, the controller  18  may be suitably configured to process a raw signal  16  to obtain or extract the desired information of the fuel properties. 
         [0031]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.