Abstract:
The quantitative determination of two contaminants of a fluid is carried out in the following steps: detection of permittivity ε and viscosity v of the fluid; determination of the quantity of the first contaminant  11   e  on the basis of a difference between detected permittivity Δε and a first preset value  10   e ; estimation of viscosity  12   n  on the basis of the quantity of the first contaminant  11   e ; and determination of the quantity of the second contaminant on the basis of a difference between detected viscosity v and estimated viscosity  12   n.

Description:
BACKGROUND INFORMATION  
       [0001]     Although the present invention is described below with reference to the determination of the soot and fuel concentrations in an aged motor oil, the present invention is not limited thereto, but rather generally relates to methods which use two physical variables to determine the quantity of two contaminants in a fluid. Motor oils are used in internal combustion engines to lubricate moving parts to reduce the friction and abrasion of metal surfaces moving relative to one another. Motor oil is subject to multiple aging processes which make it necessary to change the oil after a certain period of time. In this regard, it is extremely useful to provide a sensor device and a method which can detect the condition of the oil in an internal combustion engine during operation.  
         [0002]     The condition of the motor oil is influenced in different ways by a wide range of aging processes. The effect of some aging processes on the properties and composition of the motor oil is known. Moreover, these aging processes may be identified by certain processes in an engine or equipment connected to the engine. Conversely, possible adjustment errors or defects in the motor or connected equipment may therefore be determined from the condition of the oil. This information is used in oil analyses which determine the composition of a used oil through complex physical-chemical laboratory process steps and, on the basis of this composition, enable conclusions to be drawn as to the condition of an engine. However, these oil analyses have the disadvantage that they cannot be carried out locally in a vehicle without a great deal of effort.  
       SUMMARY OF THE INVENTION  
       [0003]     The method according to the present invention for the quantitative detection of two contaminants in a fluid involves the following steps: detection of two physical variables of the fluid, one physical variable being the viscosity and the second an electrical variable; determination of the quantity of the first contaminant on the basis of a difference between the detected physical variable and a first preset value; estimation of the second physical variable on the basis of the quantity of the first contaminant; and determination of the quantity of the second contaminant on the basis of a difference between the detected second variable and the estimated second physical variable.  
         [0004]     An advantage of the present method is that two physical variables of the fluid are detectable in a vehicle during operation without requiring a great deal of measurement work. This also enables the condition of the motor oil to be monitored during vehicle operation.  
         [0005]     According to a preferred embodiment, the electrical variable is the permittivity or specific resistance of the fluid.  
         [0006]     According to an embodiment, fuel is determined as one contaminant and soot and/or oxidized components of the fluid as the other contaminant.  
         [0007]     According to a further embodiment, the fluid includes oil.  
         [0008]     According to a further embodiment, the quantity of the first and the second contaminants is determined by a first and a second linear dependency on the difference between the first and the second physical variable.  
         [0009]     According to a further embodiment, the second physical variable is estimated via a third linear dependency on the quantity of the first contaminant.  
         [0010]     According to a further embodiment, the first physical variable is determined for a fluid without using the first and the second contaminants as the preset value.  
         [0011]     According to a further embodiment, the preset value and linear coefficients for the first, second and third linear dependencies are stored for different oil temperatures, and the temperature of the contaminated oil is determined. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  shows a schematic representation of the influence of contaminants on the physical variables of a motor oil.  
         [0013]      FIGS. 2 through 4  show graphical representations of three steps of an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]     Motor oils as well as other oils are used in a variety of ways to lubricate components moving relative to one other. These oils have a wide range of applications in internal combustion engines and transmissions. The physical-chemical properties of the oils change due to thermal and chemical influences. This “aging” of the oil usually increases the viscosity. If the viscosity exceeds a critical value, this may result in higher abrasion or, in the worst case, cause the piston to seize. If the viscosity drops below a critical value, this may result in the removal of the lubricating film between the moving parts, e.g., between the piston and the cylinder. It is therefore necessary to change the oil before this occurs. According to conventional methods, the oil is changed after a predetermined period of engine operation, e.g., vehicle mileage, or after a predetermined period of time. A sensor which detects the quality or condition of the motor oil may display a required early oil change or, if necessary, indicate that the oil is still in proper condition.  
         [0015]     A wide range of processes influence the condition of the oil. When thermally activated, the motor oil oxidizes, particularly in the case of spark ignition engines. Oxidation produces aldehydes, ketones and carboxylic acids in the oil. Due to polymerization and polycondensation reactions, paint-, resin- and sludge-like deposits of a higher molecular weight which are largely oil-insoluble form from these starting materials. The oil-soluble aging products cause the oil viscosity to increase. In addition to oxidation, soot is the main cause of increased viscosity in diesel engines. This soot is produced in the form of an unwanted waste product during diesel combustion and enters into motor oil circulation in low concentrations. Exhaust treatment equipment causes a greater increase of soot in the oil, particularly if the soot-particulate filter is clogged and needs to be cleaned.  
         [0016]     In internal combustion engines and, in particular, diesel engines, low concentrations of uncombusted fuel enter the motor oil. Because the oil and fuel mix well and the fuel also has a much lower viscosity than the oil, even low concentrations of fuel in the oil reduce the viscosity thereof.  
         [0017]     Because the soot or oxidation and the fuel in the oil influence viscosity in such a way that they may at least partially compensate for each other, only a qualitative and uncertain statement on the condition of the oil is obtained by determining only the viscosity as a measure.  
         [0018]     In  FIG. 1 , the influences of oxidation  1 , soot concentration  2 , water  3  and fuel  4  in an oil are plotted schematically against viscosity v. In addition to the previously described dependency of the viscosity on the contaminants, permittivity ε is likewise dependent on these contaminants, albeit in a different manner. Thus, the addition of water to the motor oil causes an emulsion to form which substantially increases permittivity ε, while leaving viscosity v unchanged. In contrast, fuel  4  largely results in a change in viscosity v and leaves permittivity ε unchanged. Soot  2  in the oil and oxidation  1  of the oil both increase viscosity v and permittivity ε, oxidation  1  having a less pronounced influence on permittivity ε than soot concentration  2 .  
         [0019]     According to one embodiment of the present invention, permittivity ε and viscosity v are used to quantitatively determine the concentration of soot  2  and the quantity of fuel  4  in the used oil. The method largely takes place in three steps and is described with reference to  FIGS. 2, 3  and  4 . In  FIG. 2 , the permittivity for used oil  11   e  and clean oil  10   e  is plotted over temperature T. Temperature T 0  and permittivity ε of contaminated oil  11   e  are determined in a first method step. Permittivity ε of clean oil at determined temperature T 0  is read from precompiled value tables. Resulting difference Δε between permittivity ε of clean oil  10   e  and contaminated oil  11   e  is attributable mainly to the contribution of soot  2  and oxidation  1  of the oil. Since the contaminants in the total oil are present only in minute concentrations, it may be assumed that the change in permittivity Δε is proportional to the mass percentage of soot  2  or of oxidized components  1  of the old oil in the total oil. According to the diagram in  FIG. 1 , an expected change in viscosity r, as shown in  FIG. 3 , is derived from the determined concentration of soot  2  or oxidized oil  1  from the diagram in  FIG. 1 . The estimated change in viscosity r is assumed to be proportional to the concentration of soot  2  or oxidized oil  1 .  FIG. 3  shows a schematic representation of the viscosity for a clean oil  10   n  in comparison with the viscosity of an oil  12   n  contaminated with soot  2  for a soot concentration. Viscosity v of contaminated oil  11   n  is determined in a further step. This generally yields a difference Δv between estimated viscosity  12   n  and detected viscosity  11   n  of the oil. This difference Δv is attributable to concentrations of fuel  4  in the contaminated oil. Assuming that difference Δv is proportional to the concentration of fuel  4  in the contaminated oil, this concentration, in turn, may be determined via a predetermined proportionality constant.  
         [0020]     The proportionality constants needed to determine the contaminants and estimate the viscosity may be determined ahead of time in laboratory trials and stored in a memory device for the calculations.  
         [0021]     In a diesel engine having a soot-particulate filter, a greater increase in the soot concentration of the oil may be a sign that the soot-particulate filter is clogged. The condition of the oil may therefore be used as an indication that the soot-particulate filter needs to be cleaned, e.g., by burn-off or post-injection. Furthermore, a greater increase in fuel concentration in the oil also indicates a malfunction of the soot-particulate filter.  
         [0022]     Although the present invention was described with reference to a particularly preferred embodiment, it is not limited thereto. In particular, the sequence of detected physical variables is not rigidly specified. It is therefore also possible to begin by detecting the viscosity and then estimate a permittivity on the basis of the determined viscosity, and to determine the concentration of the fuel and soot on the basis of the difference between the estimated and detected permittivity.  
       LIST OF REFERENCE NUMERALS  
       [0023]    
       
         
               
               
             
           
               
                   
               
               
                   
               
             
             
               
                 1 
                 oxidation 
               
               
                 2 
                 soot 
               
               
                 3 
                 water 
               
               
                 4 
                 fuel 
               
               
                 ε 
                 permittivity 
               
               
                 10e 
                 permittivity of clean oil 
               
               
                 11e 
                 permittivity of contaminated oil 
               
               
                 v 
                 viscosity 
               
               
                 10n 
                 viscosity of clean oil 
               
               
                 11n 
                 viscosity of contaminated oil 
               
               
                 12n 
                 estimated viscosity 
               
               
                 t 
                 time axis