US 20050056083 A1
Activation energy, W, is determined from oil conductivity measurements to thereby provide engine oil condition from a known relationship between viscosity and W. Changes of W at a given temperature as the oil ages are reflective of changes in viscosity of the oil at the same given temperature, wherein changes in W at different temperatures are reflective of changes of viscosity at those respective temperatures as the oil ages. To determine viscosity, the temperature dependence of the oil's conductivity is measured to deduce the value of W at a given temperature. W is monitored as the oil ages. W may also be determined through the ratio of the oil conductivities at two different temperatures by techniques well known in the art by which the viscosity may be determined as the oil ages.
1. A method for determining engine lubricating oil condition comprising the steps of:
determining a relationship between condition of the oil and viscosity of the oil;
determining a relationship between activation energy of the oil and viscosity of the oil;
measuring temperature dependent conductivity, S(T) of a selected oil over a predetermined range of temperatures;
calculating an activation energy, W, of the oil at a selected temperature responsive to said step of measuring;
determining the condition of the oil from the relationships between activation energy, viscosity, and condition; and
periodically repeating said steps to thereby determine the condition of the oil as the oil ages.
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The present invention relates to a method for determining engine oil condition from knowledge of its activation energy derived from measurements of its conductivity.
Three of the important properties of an internal combustion engine lubricating oil, herein simply referred to as “oil”, that are worth sensing are the viscosity, the condition of the additive package, and the total acidity of the oil. Lubricating oil used in internal combustion engines for lubrication of moving components deteriorates by the depletion of the additives and the increase in the acidity of the oil, as measured by a quantity called the total acid number (TAN). The depletion of the additives and the increase in the acidity of the oil, in some combination, are sensed in gasoline engines by measuring the electrical conductivity of the oil. As the additive package varies from oil to oil, it has proven necessary to monitor the actual variation of the electrical conductivity of each particular oil filling as it ages in the engine. Oil in diesel engines is degraded by the same mechanisms as in gasoline engines, but with the additional presence of soot particles, which increases as the oil ages. During usage of a diesel engine, the crankcase oil gradually builds up soot which is a combustion product in the combustion chamber of the engine and which is transferred in small amounts to the crankcase oil. When the soot builds up to an unacceptable amount, say about four percent by mass or weight of the oil, the lubricating quality of the oil is inhibited. Thus, it is necessary to change the crankcase oil whenever the soot content reaches an unacceptable value.
The prior art also describes a number of techniques that measure the dielectric constant with a sensor built like a capacitor. The capacitor like sensor includes two metal electrodes with the lubricating oil acting as the dielectric between the electrodes. The two metal electrodes take the form of two parallel plates or two concentric cylinders. Most of these sensors determine the permittivity of the oil through a measurement of the capacitance between the metal electrodes. Sensors that measure the loss tangent, essentially the ratio of the electrical conductivity of the oil to the dielectric constant, have also been proposed
Delphi Corporation possesses a design for a gasoline engine oil contaminant sensor that measures the electrical conductivity of the oil using D.C. or a low frequency (below one kHz). The sensor consists of two metal electrodes, which can be parallel plates or concentric cylinders or rings. The conductivity is determined through a measurement of the electrical resistance between the electrodes. This sensor mainly detects the changes in the concentration of ions in the oil. In this regard, fresh oil is slightly basic. As the oil ages, the combustion products create acidic ions in the oil. At first, the acids neutralize the bases and the conductivity decreases. As the oil ages further, the increase in acidic ions makes the conductivity rise again. This makes for a very good oil quality sensor in gasoline engines.
Delphi Corporation also possesses a method that measures the electrical conductivity of diesel engine oil at high frequencies (one MHz to ten MHz) to determine soot concentration utilizing a sensor having the same geometry as the D.C. sensor for gasoline engines as described above and can be used to measure the electrical conductivity of diesel engine oil using D.C. or low frequencies (below one kHz).
It is also known in the art that the viscosity of internal combustion engine oils increases as the oil ages. Internal combustion engine oil condition can, therefore, be determined by monitoring the viscosity of the oil. The prior art describes a number of techniques for the measurement of viscosity in engine oil utilizing viscosimeters. Most viscosimeters are based on a measurement of the shear force associated with the displacement of the oil. In order to make viscosity measurements of oil on operating vehicles, it is necessary to provide a measuring system which is sufficiently inexpensive to incorporate on automotive vehicles made in large numbers and sufficiently rugged to withstand the engine operating environment. Moreover, a method of measuring viscosity must be valid for many types of oil, both natural and synthetic, and containing many different types of additives.
It would be more economical to an engine/automotive manufacturer to use existing oil quality sensors based on the electrical conductivity of the oil to somehow use this quantity as an indication of the oil viscosity than to measure the viscosity using existing viscosimeters based upon a shear force measurement of the oil.
Accordingly, what is needed in the art is a more robust method to determine oil condition utilizing an indication of oil viscosity which is independent of the brand of oil.
The present invention is a method by which the condition of internal combustion engine oil is determined using electrical conductivity measurements of the oil at, preferably, D.C. or low frequencies (that is, frequencies less than two kHz).
According to the method of the present invention, an activation energy can be determined from oil conductivity measurements which is related to the oil viscosity. Changes of the activation energy at a given temperature as the oil ages are reflective of changes in viscosity of the oil at the same given temperature, wherein changes in the activation energy at different temperatures are reflective of changes of viscosity at those respective temperatures as the oil ages.
As a result, to provide an indication of viscosity or changes in viscosity of oil as it ages, it is possible to simply measure the temperature dependence of the oil's conductivity, deduce the value of the activation energy at a given temperature and monitor the activation energy as the oil changes, wherein the activation energy is related to the viscosity at a given temperature. The activation energy may also be determined through the ratio of the conductivities at two different temperatures by techniques well known in the art by which the viscosity may be determined as the oil ages.
It is, therefore, possible to determine the condition of internal combustion engine oil by monitoring the value or change in value of the activation energy thereby determining when the oil should be replaced with fresh oil.
Accordingly, it is one object of the present invention to measure the electrical conductivity of engine oil at DC or low frequencies to determine the activation energy thereof.
This and additional objects, features and advantages of the present invention will become clearer from the following specification of a preferred embodiment.
It is well known in the art that the electrical conductivity S of a fluid is given by:
In electrolytes the mobility u is directly related to the viscosity η. This is understood by considering the motion of an ion of charge q under an electric field E. This ion, of radius r is accelerating under a force F:
Unfortunately, as the oil ages, the number of ions N and their charge q, which depends on their ionization state, change along with changes in viscosity. Therefore, the electrical conductivity S is not a straightforward measure of viscosity.
As is also well known in the art, the viscosity of most fluids varies with temperature and the temperature dependent viscosity η(T) can be expressed as:
As a result, to determine an indication of the viscosity or changes in the viscosity of oil as it ages, it is possible to simply measure the temperature dependence of the oil's conductivity, deduce the value of W at a given temperature through equation 6 and monitor W as the oil changes wherein W is related to the viscosity as previously described. The activation energy W may also be determined through the ratio of the conductivities at two different temperatures by equation 6 by techniques well known in the art, yielding:
Most oils have viscosity index improvers in their additive package that are activated at higher temperatures to increase the high temperature viscosity above that determined by equation 5. Therefore, the temperatures selected for the present invention must be such as to avoid the activation of the viscosity index improvers within the oil. Suggested temperatures for the present invention are T1=40 degrees Celsius and T2=60 degrees Celsius.
Referring now to the drawing,
It is, therefore, possible to determine the condition of internal combustion engine oil by monitoring the value or change in value of the activation energy W, thereby determining when the oil should be replaced with fresh oil. For example, if W reaches or exceeds a value (threshold), for instance, of 40,000 Joule/mole then the oil should be replaced with fresh oil or, if a change in W of, for example, a sixty per cent increase from the value of W when the oil was fresh (i.e. from 28,000 to about 44,000 Joule/mole) occurs then the oil should be replaced with fresh oil.
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.