Abstract:
Soot content in Diesel engine lubrication oil is determined using electrical resistance measurements of the oil at high frequency. A sensor in the form of a capacitor is immersed in the oil, wherein the oil serves as a dielectric between the plates. The capacitance and resistance between the plates change as a function of engine oil condition. An inductor is placed in series with the sensor, and high frequencies are sweeped over a range to find resonance where the capacitive and inductive reactances cancel. At this frequency, the resistance of the oil is measured and the condition of the oil thereby determined.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates generally to Diesel engine oil contaminant sensors, and more particularly to a method for determining contamination of Diesel engine lubrication oil by measuring the resistance thereof at high frequency.  
         BACKGROUND OF THE INVENTION  
         [0002]    When engine oils become contaminated with the by-products of combustion, their value as a lubricant is greatly diminished. The main contaminate in engine oils during normal combustion is carbon. Diesel engines produce large amounts of carbon, referred to as soot, during combustion and the measurement of the percentage of soot in the diesel oil gives an indication of when the oil should be changed.  
           [0003]    Measurement attempts at DC and low frequency AC (ie., below about 1 kilo Hz) fail because the change in conductivity is very small for large changes in the percentages of soot. At high frequencies (ie., in the mega Hz range) the resistive losses due to the soot are measurable even at levels of less than one percent soot. The problem with the high frequency loss measurement is that the sensor, which defines a measurement volume, is a capacitor. Since the sensor has a capacitance associated with its physical shape, there is also a capacitive reactance associated with the sensor. The problem is the capacitive reactance is very low at these high frequencies (ie., on the order of between 1 to 6 hundred Ohms), and the resistance of the soot in oil which is in parallel with the capacitive reactance, is very high (ie., on the order of mega Ohms). There are methods that are used in a laboratory that can make the measurement, but the equipment is expensive and the setup must be nearly ideal (very short leads). The use of a network analyzer or vector voltmeter would be cost prohibitive, and an RF bridge measurement could be used if the resistance and capacitive reactance were near the same values. Thus, such a measurement is very difficult in the real world environment of an operating engine.  
           [0004]    Accordingly, what remains needed in the art is a method, applicable to real world engine operation environments, for determining contamination of Diesel engine lubrication oil by measuring the resistance thereof at high frequency.  
         SUMMARY OF THE INVENTION  
         [0005]    The soot content in Diesel engine lubrication oil is determined using electrical resistance measurements of the Diesel oil at high frequency which is independent of the brand of oil and immune to the effects of adding fresh oil with different dielectric or electrical properties than the original oil. “High frequency” is defined to be above about 2 mega Hz (for example, up to about 10 GHz).  
           [0006]    A sensor is provided which is immersed in the lubricating oil of a Diesel engine, such as at the bottom of the crankcase. The sensor is in the form of a capacitor with either parallel plates or concentric tubes defining a measurement volume (tubes and plates being referred to hereinafter simply as “plates”). The lubricating oil provides a high dielectric constant between the plates which form the capacitor. As carbon contaminates build up in the oil, the capacitor behaves like a capacitor with a resistor in parallel. The oil resistance across the plates due to the carbon contaminates is very high (ie., in the mega Ohm range). But, the capacitive reactance in the mega Hz frequency range is low (ie., on the order of hundreds of Ohms). Thus, in order to ascertain the oil resistance, the capacitive reactance must be cancelled.  
           [0007]    The present invention utilizes an inductor in series with the capacitor to thereby provide a resonant circuit. At resonance, the capacitive and inductive reactance vectors mutually cancel, leaving only the resistive component. In this regard, it is well known that at resonance the phase vectors of the capacitive reactance and inductive reactance mutually cancel, leaving just the resistive value. It is also known that the dielectric constant of different oils varies, and also the dielectric constant of any one oil changes as the additives are used up or breakdown. This means that while the inductance value remains constant, the capacitance value of the sensing element will be different with different oils and will change with time, and therefore, the resonate frequency will also change. Since the range of dielectric constants is known, the range of capacitance is also known, as is the frequency range, wherein a voltage controlled oscillator (VCO) is provided that will cover the range of frequencies due to dielectric change.  
           [0008]    In a first preferred form of the invention, a microcontroller outputs a stream of bit patterns to a digital to analog converter (D/A converter) which outputs a changing, ramp like, analog voltage. The analog voltage ramp is connected to the VCO control input. The output of the VCO is a sweep of frequencies over the range of interest. The output of the VCO is connected to the resonant circuit. As the microcontroller sweeps the frequency into the resonate circuit, the voltage signal proportional to the resonate circuit current is checked for a maximum value, because at resonance the current is at a maximum. The microcontroller stores the peak current and the voltage amplitude of the excitation and then calculates the relative resistance of the oil. The microcontroller then outputs a signal in a format that is required by external electronics.  
           [0009]    In a second, most preferred form of the invention, a phase locked loop (PLL) integrated circuit is utilized. The voltage controlled oscillator (VCO) within the PLL is set to free-run at a frequency that is in the range of frequencies expected due to the change of dielectric constant of the oil within the sensor. When phase information is presented to the PLL inputs, an internal error signal is generated if an out of phase condition exists. This error signal is filtered and connected to the VCO control pin, which changes the VCO frequency until the signals are in phase, at which time the PLL is locked and resoance is achieved. A current to voltage converter provides an output whereby the current at resonance may be determined. The VCO output is a constant amplitude square wave, thereby enabling the selection of a convenient drive voltage level for the resonant circuit by which the relative resistive loss introduced by the soot in the oil may be calculated, using the current at resonance, by the microcontroller and output in a form that is required by external electronics.  
           [0010]    Accordingly, it is an object of the present invention to measure the electrical resistance of Diesel engine lubrication oil at high frequency to thereby determine the amount of soot therein, wherein the measurement is independent of the brand of oil and immune to the effects of adding fresh oil with different dielectric or electrical properties than the original oil.  
           [0011]    This and additional objects, features and advantages of the present invention will become clearer from the following specification of a preferred embodiment. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1A depicts the engine placement of an oil sensor for resistance measurements according to the present invention.  
         [0013]    [0013]FIG. 1B depicts a detail view of the oil sensor of FIG. 1.  
         [0014]    [0014]FIG. 2 is a first example of an electrical circuit to measure resistance of Diesel engine lubrication oil at high frequencies.  
         [0015]    [0015]FIG. 3 is a second example of an electrical circuit to measure resistance of Diesel engine lubrication oil at high frequencies.  
         [0016]    [0016]FIG. 4 shows a computer simulation that graphs the current increase at resonance of an electrical circuit of FIG. 2 or  3 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    Referring now to the Drawing, FIG. 1A depicts an environment of placement and operation of a Diesel engine lubrication oil condition sensor  10 . The sensor  10  is located at the bottom of an oil pan  12  of a Diesel engine  14 . As shown at FIG. 1B, the oil sensor  10  has a cylindrical shell  16  having apertures  18  and an open top end  20 . Inside the shell  16  is a pair of concentrically arranged and mutually separated cylindrical capacitor plates  22 ,  24  which collectively form a capacitor  26 , each of which being connected to a respective portion of wiring, W.  
         [0018]    In operation of the sensor  10 , which sensor construction is known in the prior art, oil  28  in the oil pan  12  is sloshed, causing the oil to flowably fill the space separating the plates  22 ,  24 . As a result, the capacitance C and the resistance R (see FIG. 2) of the space between the plates  22 , 24  changes over time as the condition of the oil changes with hours of operation of the Diesel engine.  
         [0019]    [0019]FIG. 2 is a first example of an electrical circuit  50  to measure resistance of Diesel engine lubrication oil at high frequencies utilizing a series resonant LC circuit. The sensor  10  is modeled as the aforesaid capacitor  26  having a capacitance C with a resistor  52  have a resistance R in parallel therewith, wherein  1 /R represents the conductance of the physical configuration of the metal plates of the sensor  10  filled with Diesel engine lubrication oil to be measured, and wherein C represents the capacitance of the physical configuration of the metal plates ( 22 ,  24  of FIG. 1B) of the sensor  10  filled with the oil. An inductor  54  having an inductance L is chosen in accordance with the dimensions of the sensor  10  to provide an LC series resonant circuit  56  having resonance over a predetermined frequency range, for example between 2 MHz and 3 MHz.  
         [0020]    The Diesel engine lubrication oil provides a high dielectric constant for the capacitor  26 . As carbon contaminates (soot) build up in the oil, the capacitor  26  behaves like a capacitance C with a resistance R in parallel therewith. The resistance R due to the carbon contaminates is very high (ie., in the mega Ohm range), and the capacitive reactance of the capacitor  26  in the mega Hz frequency range is low (ie., on the order of hundreds of Ohms).  
         [0021]    It is well known in the art that at resonance the phase vectors of the capacitive reactance and inductive reactance cancel, leaving only a resistive value. It is also known that the dielectric constant of different Diesel engine oils varies, and also the dielectric constant of any one oil changes as the additives are used up or breakdown. This means the capacitance C of the sensor  10  will be different with different oils and will change with time and, therefore, the resonant frequency will also change. Since the range of dielectric constants is known, the range of capacitance C is also known, as is the frequency range over which resonance will occur. Thus, a voltage controlled oscillator (VCO)  58  of the electrical circuit  50  is preselected to cover the predetermined range of frequencies over which resonance of the resonant circuit  56  will occur due to changes of the dielectric constant of the capacitor  26 .  
         [0022]    In FIG. 2, a microcontroller  60  outputs a stream of bit patterns to a digital to analog converter (D/A converter)  62  which outputs a changing, ramp like, analog voltage. The analog voltage ramp is connected to a control input  58   a  of the VCO  58 . The output of the VCO  58  is a sweep of frequencies over the range of interest for providing resonance. The output of the VCO  58  is connected to the resonant circuit  56  and to an AC amplitude to DC converter  64 . The DC output of the AC amplitude to DC converter  64  is amplified by a DC Amplifier  66  and connected to an A/D input port  60   a  of the microcontroller  60  which monitors the AC voltage level VAC being fed into the resonant circuit  56 . The resonant circuit  56  consists of the aforementioned LC series resonant circuit  56 , comprising the inductor  54 , having a fixed inductance L, and the sensor  10 , wherein the sensor includes the capacitor  26  having a changing capacitance C and the resistor  52  having a changing resistance R in parallel therewith, the changing values of capacitance and resistance being related to the condition of the oil.  
         [0023]    The output of the resonant circuit  56  is connected to a current to voltage converter  68  which converts the currents flowing in the resonant circuit to a proportional AC voltage output V 0 . The output of the current to voltage converter  68  is connected to an AC amplitude to DC converter  70  whose output is amplified by DC Amplifier  72  and connected to an A/D input port  60   b  of the microcontroller  60  which monitors the AC voltage level V 0 . As the microcontroller  60  varies the input control voltage to the VCO  58 , the VCO output frequency is swept into the resonant circuit  56 . V 0  is monitored by the microprocessor  60  at the A/D port  60   b  until a maximum voltage is detected. At this maximum voltage, the VCO output is at the resonant frequency of resonant circuit  56 , and the current is at a maximum. In this regard, FIG. 4 shows a computer simulation plot  72  of how the current increases at resonance.  
         [0024]    The microcontroller  60  stores the maximum (peak) current and the voltage amplitude and then calculates the relative resistance of the oil. The microcontroller  60  then outputs a signal  74  in a format that is required by external electronics. In this regard, the microcontroller  60  has incorporated within it all parameters, constants, algorithms, and programs to effect the operation of the circuit  50  and the present invention utilizing the conductivity or conductance by techniques well known in the art.  
         [0025]    [0025]FIG. 3 is a second example of an electrical circuit  80 , which is the most preferred method of the present invention to measure resistance of Diesel engine lubrication oil at high frequencies, wherein a phase locked loop (PLL)  82  integrated circuit is used in conjunction with the aforedescribed resonant circuit  56 .  
         [0026]    A voltage controlled oscillator (VCO) incorporated within the PLL  82  is set to free-run at a frequency that is in the range of frequencies expected due to the change of dielectric constant of the oil within the sensor  10 , as previously described in FIG. 2. The VCO output of PLL  82  is buffered by a Buffer  84  to provide the required drive current to the resonant circuit  56 . The current flowing in the resonant circuit  56  is connected to the virtual ground input  86  of a current to voltage converter  88  through a DC blocking capacitor  90 . The voltage output V′ 0  of the current to voltage converter  88  is connected to the Phase Input  2  of the PLL  82  and to an AC amplitude to DC converter  92 . The output of the Buffer  84  is inverted by a Phase Inverter  94  to account for the phase inversion in output V′ 0  by the current to voltage converter  88 , and is connected to Phase Input  1  of the PLL  82 .  
         [0027]    The phase of the voltage output V′ 0  of the current to voltage converter  88  at Phase Input  2  of PLL  82  will lead or lag the VCO output of the PLL above or below resonance of the resonant circuit  56 , and will only be in phase with the voltage at Phase Input  1  at resonance, due to the fact that at resonance, the resonant circuit  56  is purely resistive. In this regard, at resonance, the voltage across the resonant circuit  56 , represented by the voltage at Phase Input  1  of the PLL  82 , is in phase with the current through the resonant circuit represented by the voltage V′ 0  at Phase Input  2  taking into account the phase shift produced by the current to voltage converter  88  and compensated for by the Phase Inverter  94 .  
         [0028]    When phase information is presented to the PLL  82  through input signals at Phase Input  1  and at Phase Input  2 , an internal error signal is generated within the PLL if an out of phase condition exists. This error signal is filtered and connected to an internal VCO control pin, which changes the VCO frequency until the input signals at Phase Input  1  and at Phase Input  2  are in phase, at which time the PLL  82  locks, the VCO frequency does not change and resonance is present.  
         [0029]    The output of the AC amplitude to DC converter  92  is fed to a DC Amplifier  96 , the output of which is connected to an A/D input  98   a  of a microcontroller  98 . A capacitor  100  serves as a DC blocking capacitor and passes the high frequencies from the VCO output of the PLL  82  and the Buffer  84  to the resonant circuit  56 . A resistance  102  establishes a ground reference for the high frequency AC voltage passed by the capacitor  100 . The VCO output of the PLL  82  is a constant amplitude square wave, thereby providing a constant voltage to the resonant circuit  56 .  
         [0030]    A convenient voltage amplitude can be selected for calculation by the microcontroller  98  along with the measured current of the resonant circuit  58  at resonance, represented by the voltage at the A/D input  98  of the microcontroller. The microcontroller  98  then calculates the relative resistive loss introduced by the soot in the oil, and thereupon outputs a signal  104  in a format required, by for example, an “Engine Management System,” that is related to the percentage of soot in the Diesel engine oil.  
         [0031]    The time required for the PLL  82  to achieve lock (resonance) is relatively short, for example, one millisecond. Hence, with a proper delay incorporated into the microcontroller  98  after being powered on, the voltage at the A/D input  98   a  of the microcontroller represents the current of the resonant circuit  56  at resonance. Having selected a convenient voltage amplitude for calculation by the microcontroller  98 , as described above, the microcontroller can calculate the resistance R of the sensor  10  due to the condition of the oil.  
         [0032]    As is known in the art, as the Diesel engine lubrication oil becomes contaminated with soot, the resistance decreases as a linear function of soot concentration, whereby the resistance R is greatest with fresh, clean (ie., soot free) Diesel engine lubrication oil. With fresh, clean Diesel engine lubrication oil in a vehicle, and with a proper delay incorporated into microcontroller  98  after being powered on, the resistance R is calculated and used as a reference resistance. Later calculated resistances are compared to this reference resistance by which the soot concentration may be ascertained.  
         [0033]    The microcontroller  98  has incorporated within it all parameters, constants, algorithms, and programs to effect the operation of the circuit  80  and the present invention utilizing the conductivity ratio and conductivity or conductance by techniques well known in the art.  
         [0034]    To those skilled in the art to which this invention appertains, the above described preferred embodiment may be subject to change or modification. Such change or modification can be carried out without departing from the scope of the invention, which is intended to be limited only by the scope of the appended claims.