Abstract:
A method for determining a level of water contamination in a fuel containing ethanol, including determining the ethanol concentration of the fuel; sensing the resistance of the fuel; determining a resistance limit of the fuel; and comparing the resistance to the resistance limit to provide the level of water contamination. Ethanol concentration is preferably obtained by comparing a measured capacitance to known values in a look-up table. The resistance limit can be determined by multiplying a resistance corresponding to the ethanol concentration by an alarm fraction. The resistance is obtained by a look-up table of resistance values at known water contamination levels. Reporting occurs when the measured resistance is at or below the resistance limit. Alternatively, the measured resistance is normalized with respect to the resistance with no water contamination and reporting occurs when −1.6667*normalized resistance+1.6667 approaches 1.0.

Description:
TECHNICAL FIELD  
         [0001]    The invention relates in general to sensing algorithms in automotive fuel blends.  
         BACKGROUND OF THE INVENTION  
         [0002]    Ethanol is a common replacement for gasoline in automotive fuel blends. However, fuel containing ethanol tends to absorb moisture from the atmosphere. Too much water condensed in the fuel causes phase separation of the gasoline and ethanol with the ethanol and water phase settled at the bottom of the fuel tank of an engine while the gasoline is at the top. If some of the ethanol and water phase is pumped through the fuel system to the engine, the engine may not ignite. Further, water corrodes engine components in time. Thus, water is a contaminant in fuel.  
           [0003]    Current fuel sensors are important in the adjustment of the air-to-fuel (A/F) ratio. Adjustment of the A/F ratio is necessary in a vehicle whenever its fuel composition changes, such as a change in the ethanol content of the fuel. This makes the presence of a real-time on-board fuel sensor to measure the ethanol content in the fuel desirable. While current fuel sensors can be used to estimate the ethanol content, none have the capability of monitoring the water contamination in fuel that can result in phase separation.  
         SUMMARY OF THE INVENTION  
         [0004]    Accordingly, a sensing algorithm, which can be added to current sensors, can monitor the water content in fuel. In doing so, it can signal the driver to prevent phase separation, i.e., the separation of gasoline from ethanol mixed with the water contaminant. Specifically, the present invention is a method for determining a level of water contamination in a fuel containing ethanol, comprising the steps of determining an ethanol concentration of the fuel; sensing a resistance of the fuel; determining a resistance limit of the fuel using the ethanol concentration; and comparing the resistance to the resistance limit to provide the level of water contamination.  
           [0005]    In a preferred aspect, the method also comprises the step of measuring a capacitance of the fuel, and the step of determining the ethanol concentration includes the step of comparing the capacitance to values on a look-up table, the look-up table including a plurality of capacitance values and ethanol concentrations corresponding to the plurality of capacitance values.  
           [0006]    In one aspect of the invention, the step of determining a resistance limit of the fuel using the ethanol concentration comprises the steps of comparing the ethanol concentration to values on a look-up table, the look-up table including a plurality of ethanol concentrations and resistance values corresponding to the plurality of ethanol concentrations, wherein each resistance value represents a known resistance of fuel at a known level of water contamination; and multiplying a known resistance corresponding to the ethanol concentration by an alarm fraction to obtain the resistance limit. In one variation of this aspect, the known level of water contamination is a level of water contamination just prior to a phase separation. In another variation of this aspect, the known level of water contamination is 0%.  
           [0007]    Any of the foregoing aspects can include the step of reporting when the resistance is at or below the resistance limit. Preferably, this step comprises the step of producing an alarm.  
           [0008]    In the aspect of the invention wherein the known resistance level corresponding to the ethanol concentration is at a level of water contamination of 0%, a further aspect includes that the alarm fraction is equal to 1.0 and the step of comparing the resistance to the resistance limit to provide the level of water contamination comprises the steps of calculating a normalized resistance by dividing the resistance by the resistance limit; and calculating a water contamination parameter, wherein the water contamination parameter=−1.6667*(normalized resistance)+1.6667. The inventive method according to this aspect can also include the step of reporting when the water contamination parameter is greater than an alarm value, wherein 0&lt;alarm value&lt;1.0.  
           [0009]    In yet another aspect of the invention, the step of calculating a resistance limit of the fuel using the ethanol concentration comprises the step of comparing the ethanol concentration to values on a look-up table, the look-up table including a plurality of ethanol concentrations and resistance values corresponding to the plurality of ethanol concentrations, wherein each resistance value represents a known resistance of fuel with no water contamination; and wherein a known resistance corresponding to the ethanol concentration is the resistance limit. In this aspect, the step of comparing the resistance to the resistance limit to provide the level of water contamination can include the steps of calculating a normalized resistance by dividing the resistance by the resistance limit; and calculating a water contamination parameter, wherein the water contamination parameter=−1.6667*(normalized resistance)+1.6667. Preferably, this aspect also includes the step of reporting when the water contamination parameter is greater than an alarm value, wherein 0&lt;alarm value≦1.0.  
           [0010]    The step of reporting when the water contamination parameter is greater than an alarm value can comprises the step of producing an alarm. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein:  
         [0012]    [0012]FIG. 1 is a block diagram of one aspect of a sensing algorithm according to the present invention;  
         [0013]    [0013]FIG. 2 is one sensor that can be used to measure resistance and capacitance in the present invention;  
         [0014]    [0014]FIG. 3 is a graph showing the capacitance of gasoline related to the ethanol content in the gasoline;  
         [0015]    [0015]FIG. 4 is a graph showing the resistance of gasoline related to the percentage of ethanol content in the gasoline; and  
         [0016]    [0016]FIG. 5 is a block diagram of a second aspect of the sensing algorithm according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]    The drawing, particularly FIGS. 1 and 5, show the method of the present invention. Specifically, FIGS. 1 and 5 each show a different aspect of the present inventive method of determining water contamination in fuel. The method incorporates an algorithm stored in preferably a conventional microcontroller of a vehicle, which includes such elements as a central processing unit (CPU), read only memory, random access memory, input/output control circuitry, and analog to digital conversion circuitry. The controller is activated upon application of ignition power to an engine and carries out a series of operations stored in an instruction-by-instruction format in memory for providing engine control, diagnostic and maintenance operations.  
         [0018]    A first aspect of the invention shown in FIG. 1 starts at step  10  and proceeds to step  12  where capacitance of the fuel is measured. Any type of sensor can be used to determine capacitance. Preferably, the sensor has two interdigitated sensing electrodes coupled to a coaxial cable, the sensor measuring both the capacitance and the resistance of the fuel.  
         [0019]    A block diagram of such a sensor  50  is shown in FIG. 2. The sensing element  58  of a sensor  50  is submerged in the fuel of an engine and excited, then the resistance and capacitance of the fuel are calculated from the induced current measured at the excitation frequency. Specifically, a sinusoidal wave generator  52  supplies a current from 10 kHz to 100 kHz to excite one electrode, or plate, of a sensing element  58 . The sinusoidal wave generator  52  generates a wave centered at the voltage Vdd/2, with a peak-to-peak amplitude of around 4 volts. The sinusoidal wave generator  52  is connected to the sensing element  58  at node  56  through a DC block capacitor  54 .  
         [0020]    Node  56  brings the DC voltage of the excitation plate of the sensing element  58  down to ground through a grounding resistor. At node  56 , the circuit of the sensor  50  bifurcates. One path supplies the excitation signal to a DC shift buffer  60 . The output from the shift buffer  60  is provided to the inverting input of a comparator functioning as a reference cross detector  62 . The non-inverting input is tied to Vdd/2. The output of the reference cross detector  62  is a reference input excitation signal for a pulse width modulated (PWM) generator  72 , to be hereinafter discussed.  
         [0021]    The other path from node  56  supplies the input stage of the sensor  50  through the sensing element  58 . As mentioned, one electrode of the sensing element  58  is connected to the sinusoidal wave generator  52 . The other electrode of the sensing element  58  is grounded through a resistor to bring the DC components of the signal to ground. Together with the ground provided for the excitation plate, this ground assures that the signal has no DC components. Also at this electrode, the shield of the shielded cable is grounded. The electrode is then connected through a series capacitor to the inverting input of an amplifier configured as a current-to-voltage converter  64 . Feedback is supplied through a feedback impedance, and the inverting input is raised to Vdd/2 through a resistor. The non-inverting input of the comparator is coupled to Vdd/2. The output of the current-to-voltage converter  64  is fed through a conventional amplifier  66 .  
         [0022]    The output of the amplifier  66 , which is the output signal of the input stage, is supplied to two components. First, the output signal is supplied to a peak detector  68  or any kind of an AC amplitude to DC converter that detects the magnitude of the peak of the signal, i.e., a magnitude output. Preferably, the magnitude output is filtered through an active low pass filter (not shown) before being combined with the phase output, to be hereinafter discussed. Second, the output signal of the input stage is supplied to the inverting input of a comparator functioning as a reference cross detector  70 . The non-inverting input is tied to Vdd/2. The output of the reference cross detector  70  is the input stage output signal, which is used as an input to the PWM generator  72 . As mentioned, the other input to the PWM generator  72  is a reference input excitation signal from the sinusoidal wave generator  52 . The output of the PWM generator  72  indicates the phase of the output signal from the input stage, i.e., a phase output. Preferably, the phase output is filtered through an active low pass filter (not shown) before being combined with the magnitude output. Given the magnitude output and the phase output, a controller can determine the resistance and capacitance of the fuel.  
         [0023]    Returning now to FIG. 1, once the capacitance is measured in step  12 , the ethanol content of the fuel is calculated from this measured capacitance in step  14 . FIG. 3 graphically shows the relationship between ethanol content (in percent) and the measured capacitance of gasoline (in volts) based upon experimental data. In step  14 , the measured capacitance can be used in a formula developed from such data, or used with a look up table developed using the data, to obtain ethanol content. Although the invention is described as determining ethanol content based on measuring capacitance, any other means for obtaining ethanol content is also contemplated within the scope of the invention, including the measurement of other parameters indicative of the ethanol content, direct measurement of ethanol content or user input.  
         [0024]    Returning now to FIG. 1, the resistance limit of the fuel containing the percentage ethanol content calculated in step  14  is determined in step  16 . The resistance limit represents the highest level of water contamination allowed in the fuel. Preferably, the resistance limit is determined from a look up table developed from data such as that graphically shown in FIG. 4, which was measured experimentally at 20 degrees Celcius. The resistance limit can be calculated from either the high resistance value, i.e., the resistance of the calculated ethanol content with no water, or the low resistance value, i.e., the resistance of the calculated ethanol content when separation occurs due to excessive water contamination. For example, in FIG. 4 the dotted line designated as “X” indicates the ethanol content determined in step  14 . The dotted line “Y 1 ” represents the logarithm of the high resistance value, and the dotted line “Y 2 ” represents the logarithm of the low resistance value. The resistance limit, as mentioned, is calculated from either of these values for resistance. Specifically, the resistance limit is calculated by determining either the logarithm of the high or the low resistance value from the look up table, then the logarithmic value is multiplied by an alarm fraction.  
         [0025]    The alarm fraction is a fraction representing either how far below the high resistance value the logarithm of the measured resistance can get, or how close to the logarithm of the low resistance value the measured resistance can get, before some corrective action should be taken due to the potential phase separation. For example, if the ethanol content X is 12.3%, and logarithm of the high resistance Y 1  is 0.5, the resistance limit can be calculated by multiplying an alarm fraction of −0.9, by example, times 0.5, providing a resistance limit of −0.45, where −0.9 represents how far below the high resistance value the measured resistance can get. Similarly, if the low resistance Y 2  of a fuel containing the ethanol content X of 12.3% is −0.5, the resistance limit can be calculated from the logarithm of the low resistance by multiplying an alarm fraction 0.9, for example, times −0.5, providing a resistance limit of −0.45, where 0.9 represents how close the logarithm of the low resistance value the measured resistance can get. Thus, the resistance limit represents a maximum allowed level of water contamination.  
         [0026]    Returning now to FIG. 1, the actual resistance of the fuel is measured in step  18 , preferably using the same sensor and circuit used in step  12  to measure capacitance. However, any circuit able to measure resistance of the fuel can be used. After the resistance is measured in step  18 , the measured resistance is compared to the resistance limit in step  20 . Specifically, the logarithm of the measured resistance is compared to see if it is greater than the resistance limit. For example, if the logarithm of the measured resistance is 0.3, this measured value is compared to, using the examples above, −0.45. When the measured value is not above the resistance limit, whichever way the resistance limit is calculated, some type of corrective action can be taken in step  22 . For example, an alarm can be produced. The algorithm then ends at step  24 . If in step  20 , however, the measured value is above the resistance limit, then the resistance is measured again in step  18  and the remainder of the steps are repeated until the engine is off. When the engine starts again, the algorithm starts again at step  10 .  
         [0027]    [0027]FIG. 5 shows another aspect of the present inventive method. Specifically, this aspect starts at step  30  and proceeds to step  32 , where the capacitance is measured as discussed in step  12 . Then, the ethanol content is calculated in step  34 , as discussed with regards to step  14 . In step  36 , using the ethanol content and the measured capacitance, a resistance is determined based upon the look up table as graphically represented in FIG. 4. In contrast to the aspect of FIG. 1, however, the resistance limit here is the unadjusted logarithm of the high resistance value Y 1 , that is, where the water content is 0%. In step  38 , the resistance of the fuel is measured as described previously with respect to step  18 .  
         [0028]    In step  40 , the logarithm of the measured resistance is normalized with respect to the logarithm of the high resistance value. For example, if the logarithm of the measured resistance is 0.3, and the logarithm of the high resistance value is 0.5, then the normalized resistance is 0.3/0.5=0.6. In step  42 , the water contamination parameter is calculated using the normalized resistance determined in step  40  according to the following formula:  
           w.c.p.=− 1.6667*(normalized resistance)+1.6667;  
         [0029]    wherein  
         [0030]    w.c.p. is the water contamination parameter. Ideally, the normalized resistance is equal to 1.0, and the water contamination parameter is equal to zero. The smaller the normalized resistance, the larger the water contamination parameter. When the water contamination parameter reaches 1.0, which is when the normalized resistance is at about 0.4, separation is likely.  
         [0031]    In step  44 , the water contamination parameter is compared to an alarm value according to the following formula:  
         [0032]    alarm value=1.0 x contamination percentage; wherein the contamination percentage ranges from 0 to 100 percent of the maximum allowed water contamination. Thus, the alarm value represents the closest the water contamination parameter can get to 1.0, representing likely separation, before corrective action is taken. For example, if the contamination percentage is 0.95, then the water contamination parameter is compared to 0.95 in step  44 . If the water contamination parameter is greater than 0.95, then an alarm is produced in step  46 , and the algorithm ends at step  48 . If, however, the water contamination parameter is less than or equal to 0.95, then the algorithm returns to measure the resistance of the fuel at periodic intervals in step  38  and continues to do so as long as the engine runs, or until the alarm is produced in step  46 . After the engine is turned off, the algorithm starts again at step  30  when the engine turns on.  
         [0033]    Thus are presented algorithms for sensing the water contamination for ethanol containing fuel. They can be used to monitor the water content in fuel and to prevent phase separation.  
         [0034]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, 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.