Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/26—Oils; Viscous liquids; Paints; Inks
  • G01N33/28—Oils, i.e. hydrocarbon liquids
  • G01N33/2888—Lubricating oil characteristics, e.g. deterioration
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
  • G01N27/026—Dielectric impedance spectroscopy

Definitions

• the present invention relates to a method for measuring the efficiency of a lubricating oil, using a device comprising a measuring cell capable of receiving such an oil, said cell comprising two metal electrodes isolated from each other, and connected to a measurement circuit.
• the invention also relates to the applications of this method and of this device to monitoring the reserve of effectiveness of a lubricating oil, in particular with regard to the kinematic viscosity KV40, the TAN (Total Acid Number) and the content of wear metals, and the production of a drain indicator for a vehicle.
• EP 60 588 which relates to a device for measuring the variation of an impedance, the dielectric component of which essentially consists of a lubricant.
• patent EP 80 632 which relates to an apparatus for detecting the degree of deterioration of a lubricating oil for an engine, comprising an electrical circuit provided with a detector comprising a pair of electrodes immersed in the lubricant , so as to form a detector capacitor whose capacity varies as a function of the dielectric constant of the lubricant.
• US Pat. No. 5,523,692 describes an oil deterioration detector, based on potential difference measurements between two electrodes.
• US Patent 5,274,335 describes a vehicle probe which allows, by electrochemical type measurements, on the one hand to distinguish a lubricant for two-stroke engine from a lubricant for four-stroke engines, and on the other hand to detect the contamination of this oil with ethylene glycol antifreeze.
• This probe includes two electrodes positioned in the lubricant to be tested, and to which a triangular alternating current of constant frequency is applied. The resulting current (in particular the value of its voltage) is compared to predetermined threshold values.
• This probe uses the electrochemical technique of voltammetry, and works at constant frequency.
• Electrochemical impedance spectroscopy is a non-destructive technique allowing the evaluation of the dielectric properties of an electrochemical system, in this case a metal / electralyte system, from measurements carried out over a whole range of frequencies.
• This technique can be applied to poorly conductive media, such as lubricating oils.
• This technique has already been implemented with a capacitive detection device, for measuring variations in the characteristics of a lubricant, in particular as a function of its wear, as described in the article by W.
• patent application EP 1 014 082 describes a device for monitoring the quality of various lubricants (in particular engine oils, transmission fluids, hydraulic fluids), which operates as follows: an alternating current is applied to two electrodes immersed in the lubricant, and the intensity of the resulting current is measured. Two intensity measurements are made, the first at a given high frequency, and the second at a given low frequency. The first measurement is compared as it is with reference values (values that have been determined beforehand, at the same frequency, on “acceptable” fluids), and the difference between the first and second measurements is compared with a value threshold. If the two criteria (first measurement, and difference between the two measurements) are within predetermined limits, the lubricant is considered valid.
• impedance measurements of the lubricant in contact with two electrodes are carried out successively in two different frequency ranges (at high and low frequency).
• Software simulates an electrical circuit providing an equivalent impedance profile, and two impedance values are calculated, one at high and the other at low frequency.
• Ro value determined at high frequency, is used: this value is directly compared with those obtained on new oil and on contaminated oil, which makes it possible to determine a rate of contamination of the oil. in glycol.
• the Applicant had the idea of representing the results obtained by electrochemical impedance spectroscopy of the measurement of the impedance of a lubricant in a measurement cell (metal / electrolyte system) in the Nyquist plane, it that is to say, to plot the variation curve of the imaginary part of the complex impedance, Im (Z) (on the ordinate) as a function of the real part of the complex impedance, Re (Z) (on the abscissa), for a wide range of frequencies, from high to low frequencies.
• the curve can be broken down into at least two separate parts:
• the Applicant has also established that, according to the typical current response of the lubricant, it is possible to select and simulate an equivalent electrical circuit providing an impedance profile coinciding with the response of the metal / lubricant system, and to calculate, in each of the two frequency ranges mentioned above, at least two parameters representative of two types of properties of the lubricant.
• the present invention therefore relates to a method and a device allowing, by simple and rapid measurements, the evaluation of the properties of a lubricant, accounting on the one hand for the degradation of the volume properties of the lubricating oil during its aging during its service, for example due to the degradation of the various chemical additives contained in the lubricant, and on the other hand accounting for the surface properties of the lubricant (metal / lubricant interface), for example the anti-wear properties of this lubricant.
• the simultaneous comparison of these two properties with pre-established values makes it possible to assess the state of remaining effectiveness of this lubricant.
• the method and the device according to the invention thus make it possible to predict, by simple and rapid measurements, the behavior of a lubricant during its period of use and to discriminate the changes occurring in its volume properties and in its properties with respect to surfaces.
• the subject of the present invention is a method for measuring the efficiency of a lubricant by electrochemical impedance spectroscopy, comprising: - the introduction of the lubricant into a measurement cell comprising two metal electrodes, the power supply of said electrodes by at least one voltage of variable frequency, measuring the impedance of the lubricant-electrode system in the high frequency range and in the low frequency range, the simulation, by means of software, of a circuit electrical equivalent providing an impedance profile coinciding with that measured using the cell, the calculation, from said simulated equivalent electrical circuit, of at least one first parameter determined in the high frequency range and representative of the changes in volume properties of the lubricant, and of at least one second parameter determined in the low frequency range and representative of the properties lubricant surface sides.
• the invention allows in particular the determination of the following properties of the lubricant: - volume properties: viscosity, pollution, oxidation, resistance to aging;
• the method according to the invention advantageously comprises the drawing of the lubricant impedance diagrams in the Nyquist plane.
• the present invention also relates to a device specially designed for the implementation of the above method.
• the invention also relates to a device for measuring the efficiency of a lubricant by electrochemical impedance spectroscopy, using a measuring cell capable of receiving a sample of said lubricant, and comprising two metal electrodes which are connected to a voltage supply circuit of variable frequency and for measuring the impedance of the lubricant-electrode system, characterized in that said device also comprises software capable of simulating an equivalent electrical circuit providing an impedance profile coinciding with that measured using the cell, means for measuring impedance in the high frequency range and in the low frequency range, and means making it possible to calculate, from said simulated equivalent electrical circuit, at least a first parameter determined in the high frequency range and representative of the changes in volume properties of the lubricant, and a u at least one second parameter determined in the low frequency range and representative of the surface properties of the lubricant.
• the voltage supply and measurement circuit advantageously comprises a potentiostat capable of delivering a sinusoidal voltage of variable frequency, and a frequency response analyzer making it possible to measure the impedance of the lubricant-electrode system, the potentiostat and the analyzer being connected to a computer which makes it possible to plot the lubricant impedance diagrams in the Nyquist plane.
• the equivalent electrical circuit is mainly of the resistive and / or capacitive type, which makes it possible to calculate parameters representative of the lubricant, as regards its volume properties of on the one hand, and surface on the other.
• the implementation of the invention is described in more detail with regard to the determination of the aging resistance in service of a lubricant and of its anti-wear properties, it being understood that the scope of the he invention cannot be limited to these two particular applications.
• those skilled in the art will be able to adapt the teaching described below in order to determine many other volume and surface properties of a lubricant.
• the software calculates the values of a parameter or resistance Ri of the corresponding part of the equivalent electrical circuit, representing the characteristics of the resistance to aging of the lubricant during its service.
• the variation of the resistance Ri as a function of the thermal aging time of the lubricant passes through a maximum value Rimax, then decreases to reach a level R ⁇ pa iier, which extends more or less depending on the performance of the oil, then Ri increases relatively strongly, the ratio R ⁇ ma ⁇ / Ri a ⁇ er having a good correlation with the value of a composite index of high temperature resistance of the lubricant, calculated from tests on engine test bench.
• the software can then calculate the values of a resistance R and a capacity C2 (or Y2 representing an element of constant phase, in the absence of pure capacity) of the corresponding part of the equivalent electrical circuit, representing the anti-wear properties of the lubricant.
• the parameter R2C2 or R2Y2 appears to have a good correlation with the value of a composite index of protection against wear of the lubricant, calculated on the basis of tests on an engine test bench.
• the calculation of the parameters Ri and / or R as a function of the duration of use of the lubricant, and the comparison of the variation of these values with data acquired on the value of these parameters for durations of aging in predetermined service of said lubricant make it possible to determine its critical duration of use.
• the electrodes of the measuring cell are preferably made of stainless steel, carbon steel, alloy steel, cast iron or any other conductive material.
• the electrodes of the measuring cell can be new or thermochemically oxidized in the presence of the lubricant, or tribochemically worn in the presence of the lubricant.
• the spacing e between the electrodes is manually adjustable, if necessary by a micrometer, and is advantageously between 50 and 400 ⁇ m, for example between 100 and 400 ⁇ m.
• At least one of the electrodes is mobile, so that the distance e between the electrodes can be fixed, by means of an automatic control system, to a first value el at the time of the admission of the lubricant into the measuring cell, then to a second value e2 at the time of the impedance measurements, e2 being between 50 and 400 ⁇ m, and el being greater than e2.
• This second embodiment has substantial advantages. Indeed, it allows dynamic measurements to be made, and regular automatic monitoring of a lubricant during its service. Furthermore, the cell is less subject to fouling phenomena by the particles carried by the lubricant, due to its very simple geometry and the flushing effect obtained during variations in the inter-electrode distance.
• the impedance measurements are carried out at a controlled temperature of the cell, advantageously between -20 ° C and 150 ° C, preferably between 0 ° C and 100 ° C.
• Another object of the invention relates to the application of the method or the device according to the invention to the production of a drain indicator for lubricating oils for engine, with long drain intervals.
• the indicator may be of the type independent of the vehicle, and it may be of the type on board the vehicle, or mounted on a stationary engine.
• Another subject of the invention relates to the application of the method or the device according to the invention to the production of an indicator of the reserve of transmission oil, manual or automatic, or hydraulic oil, of the type on board or not on the vehicle.
• Another application of the method or the device according to the invention relates to the production of an indicator of pollution of a lubricant by a more conductive compound, such as an aqueous solution or a solution of coolant.
• FIG. 2 schematically shows a measurement circuit connected to the electrodes of the measurement cell of Figure 1;
• FIG. 4 illustrates the correlation between the high temperature resistance index of lubricants and the Rimax / Ripaiier criterion. for different lubricants
• FIG. 8 shows a sectional view of the measuring cell of which at least one of the electrodes is movable and controlled by an automatic system.
• a measuring cell according to the invention used in the device according to the invention is shown in Figure 1.
• it consists of two cylindrical electrodes
• the electrodes are arranged at inside a main insulating body 3 in Teflon (registered trademark).
• a micrometer 4 here makes it possible to adjust the spacing e between the electrodes.
• the spacing can vary from 100 to 400 ⁇ m, and is chosen to 200 ⁇ m to measure the properties of the metal / lubricant interface.
• An opening 5 on the top of the cell allows the introduction of the lubricant to be tested.
• An electrical measurement circuit 10 shown in FIG. 2, is connected to this cell. It consists of a potentiostat 6 which is a variable frequency voltage signal generator which is applied to the electrodes 1, and a frequency response analyzer 7 of the Solartron type.
• the potentiostat and the analyzer are connected to a computer 8 via a communication and data acquisition card.
• the computer 8 draws the impedance diagram in the Nyquist plane, that is to say the variation curve of the imaginary part Im (Z) (on the ordinate) of the complex impedance Z, as a function of the real part Re (Z) (on the abscissa) of said impedance Z, for a wide range of frequencies which can go from 20 kHz to lmHz.
• This curve can be broken down into at least two distinct parts:
• a first semicircle in the high frequency range (between approximately 20 kHz and 1 Hz), which characterizes the intrinsic volume properties of the lubricant and in particular its conductivity, and makes it possible for example to account for its degradation by oxidation,
• a second semicircle in the low frequency range (between approximately 1 Hz and 10 mHz), which characterizes the surface properties of the lubricant (state of the metal / lubricant interface, evolution of the formation of a protective film on the electrode), for example the anti-wear properties of the lubricant.
• the first semicircle can be modeled by an equivalent electrical circuit composed of a resistor Ri and a capacitor Ci in parallel, the simulation of which with the specific software "Boukamp" provides impedance values coinciding with those obtained with the measuring cell, and allows the Ri values to be calculated for a given lubricating oil as a function of its aging.
• the second semicircle can also be modeled by an equivalent electrical circuit composed of a resistance R2 and a capacity C2 or, in the absence of a pure capacity, of an element of constant phase.
• the test results which follow illustrate a first mode of exploitation of the invention.
• the measuring cell as described above, is equipped with stainless steel electrodes.
• the measurements are carried out on samples of engine oils which have undergone artificial aging at 175 ° C extended up to 400 hours or strong thickening of the oil, which are taken at regular intervals and introduced into the maintained measuring cell. at 30 ° C in an oven for 30 minutes.
• the parameter Ri (which is inversely proportional to its conductivity) measured with the device according to the invention, in the high frequency range, passes through a maximum Rimax, which corresponds to a neutralization of the basic compounds initially present in the oil, then decreases to reach a Ripa ⁇ er plateau, which extends more or less depending on the performance of the oil, then Ri increases relatively strongly, in relation to l increase in viscosity of the degraded oil.
• the Rimax / Ripaiier criterion is physically dependent on the variation in volume conductivity of the oil, which is inter alia linked to its oxidation state.
• the Applicant has highlighted a correlation between this criterion and the resistance to aging of the oil.
• the high temperature withstand performance of a lubricating oil for engines is judged through a test on a 96 hour TU3M engine on a bench at high temperature and full load (150 ° C, 5500 rpm - 50 kW). This test is described in the CEC L-55-T-95 method and is included in the international specifications for engine oils.
• the engine is dismantled and the pistons are subject to a merit rating (out of 100 points) according to a conventional grid relating to the freedom of the segments and the cleanliness of the segment grooves and particular of the first segment throat which is the most exposed to the highest temperatures.
• a merit rating (out of 100 points) according to a conventional grid relating to the freedom of the segments and the cleanliness of the segment grooves and particular of the first segment throat which is the most exposed to the highest temperatures.
• the measurement of the variation of the kinematic viscosity at 40 ° C. of the oil at the end of the test also makes it possible to quantify the resistance to oxidation of the oil.
• An oil is judged to be good according to the level claimed when the total piston merit is not less than 5 or 10 points than that of the reference oil RL 194 which generally gives a merit of between 60 and 80.
• FIGS. 5 and 6 A second mode of operation of the device according to the invention is illustrated by FIGS. 5 and 6 and by the following test results.
• the measuring cell is equipped with FT 25 cast iron electrodes, a material similar to that of a conventional motor, which have been immersed beforehand for 24 hours at 30 ° C in the oil to be tested, in order to form an interfacial film. on their surface.
• This film can also be formed using a tribometer.
• the parameter R 2 as a function of the aging time of the oil goes through a maximum, then decreases (see Figure 6).
• the parameter R2Y2 calculated from the results of the measurements carried out on the different oils, and which is proportional to the resistivity and inversely proportional to the thickness of the interfacial reactive film formed by the oil additives on the metal surfaces, has a good correlation with the anti-wear properties of this oil.
• the adhesive wear protection performance of the distribution members of an engine with an overhead camshaft of an oil are judged by means of a bench test on a 100 hour TU3M petrol engine cut in half. phases: one of 40 hours at low temperature and low speed (40 ° C and 1500 rpm) and the other of 60 hours at high temperature and high speed (100 ° C and 3000 rpm).
• the cylinder head is dismantled and the cams are subject to a quantification of their wear by a metrological measurement of their loss of profile (in micrometers) while the rocker shoes make up for their part subject to a visual merit rating (from 0 to 10) according to a conventional reference grid.
• FIG. 5 obtained from the results obtained for oils of technologies and different grades in this TU3 wear test, expresses the correlation between the value of a composite engine wear index, calculated as follows: Merit shoe - wear (in ⁇ m) / 10, and that of the parameter R2Y2 described above. It is noted that all the oils demonstrating performances superior to those of the reference oil lead to a parameter lower than 6.10 " 6 , which gives a predictive character of the antiwear performances on engine of the electrochemical method.
• Another object of the invention relates to the determination of the critical duration of use of an oil.
• the parameter Ri passes through a maximum Rimax then decreases to reach a level R ⁇ pa iier, which extends more or less in depending on the performance of the oil, then Ri increases relatively strongly.
• the R2 parameter also goes through a maximum, for another duration independent of that of Rimax, then decreases (see Figure 6).
• the oil can be considered to have exceeded its effective time either when Ri begins to increase significantly after its plateau value, or when the criterion R2 decreases below a predetermined threshold, fixed for example by the residual mean value of this oil change criteria during previous vehicle tests.
• FIG. 6 therefore illustrates the case of two oils, namely an oil 1 for which the failure or longevity is determined by R2, that is to say a criterion which rather accounts for the affinity with respect to surfaces and anti-wear protection conferred by the oil additives, which appears during the observation time (80 h), and an oil 2 for which this is not the case.
• R2 the failure or longevity is determined by R2
• a limit or threshold value the value of which has was arbitrarily set at 10% of the value of R2ma ⁇ , or 2.48 .10 8 . ⁇ .
• FIG. 7 shows the case of an oil for which the failure is determined by the sudden increase in Ri.
• the parameter R2 cannot be measured before an already significant degree of aging of the oil: the longevity of such a lubricant is only governed by the variations in Ri.
• the deferral of the usual criteria for monitoring an engine oil in service such as the kinematic viscosity KV at 40 ° C, (measured in mm 2 / s), the TAN (Total Acid Number) (expressed in mg of KOH / g) and the content of wear metals (expressed in ppm), shows that they are well correlated with the evolution of Ri.
• the cell shown in FIG. 8 is particularly suitable for producing a drain indicator on board a vehicle. It comprises two electrodes: a fixed electrode lb and a mobile electrode la, both provided with seals 12. They are connected by means of respective electrical connections l ia and 1 lb to a voltage supply circuit of variable frequency and measuring the impedance of the lubricant-electrode system.
• the mobile electrode 1a is actuated by means of a rod 4, the displacements of which are guided by a sliding guide 14.
• the rod 4 is connected to an automatic control system not shown.
• Two openings 5 provided on the fixed electrode 1b allow the entry and exit of the lubricant to be tested.
• the lubricant to be tested penetrates into the measuring cell. Then the movable electrode la approaches the fixed electrode lb up to a minimum value of spacing e2 (for example of 70 ⁇ m), adjusted by means of the stops 13, causing the exit of only part of the lubricant.
• the impedance measurement is carried out at the distance value e2
• Another application of the device according to the invention relates to the production of an indicator of pollution of a lubricating oil by a more conductive compound, such as water or coolant.

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• 2001
  • 2001-02-12 FR FR0101862A patent/FR2820824B1/fr not_active Expired - Fee Related
• 2002
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