Lo relacionado al examen de admisión

magnetic susceptibility)

Lubricant consists of long-chain hydrocarbons. As lubricant ages, it oxidizes and undergoes a slow increase in polarity. Lubricant oxidation is a chain reaction initiates by appearance of colloidal carbon, giving rise to black coloration and solid deposits and by breakage of hydrocarbon compound, into a free radical due to high shear force between engine components. The two degradation mechanisms are expected to affect electro- magnetic properties of lubricant such as permittivity, electrical conductivity, and magnetic susceptibility.

1.3.2.1

Permittivity

Permittivity or dielectric constant is a measure of the extent to which it concentrates electrostatic lines of flux. It is the ratio of the amount of stored electrical energy when a potential is applied, relative to the permittivity of a vacuum. The relative static permittivity is the same as the relative permittivity evaluated for a frequency of zero. The

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exact effect of lubricant oxidation on dielectric constant is unclear. It has been suggested that the main effect may be to raise the dielectric constant, since the oxidized lubricant will acquire a dipole character. This process implies that a correlation between lubricant aging process and dielectric constant should exist.

1.3.2.1.1 Sensing permittivity using shear vibration

Jakoby et al. [55] described a multi-sensors device to measure the lubricant's viscosity, permittivity, temperature, and level. The viscosity sensor was a microacoustic device utilizing shear-polarized oscillations. In this study, the viscosity measurements of engine lubricant (fresh, used and contaminated) were compared to permittivity sensor reading for better understanding of correlation between physical properties of engine lubricant. Also, the lubricant viscosity was studied as a function of accumulated mileage and temperature signal. This study concluded that it is imperative to study more than one physical- mechanical property of lubricant to help evaluate the monitoring of lubricant condition.

1.3.2.1.2 Sensing permittivity using mass-sensitive quartz crystal

microbalance

Duchowski and Ringholm [35] introduced a multi-sensor array, which consisted of mass- sensitive quartz crystal microbalance (QCM), resistance temperature detection (RTD), capacitor with polyimide dielectric and an inner digital capacitor sensor to measure relative viscosity, temperature, moisture and dielectric, respectively. The preliminary results indicated that the multi-sensor array was able to measure relative viscosity, temperature, dielectric constant and contaminants in the engine lubricant. However, the lack of response to characterizing aging processes in different fluid classes was considered to be the next step into the applicability of this sensor.

1.3.2.1.3 Sensing of permittivity using tuning fork quartz resonator

Furthermore, Dobrinski et al. [32] developed a combo-sensor capable of measuring

several independent lubricant parameters continuously. It comprised an ultrasonic transducer for the lubricant level detection as well as a tuning fork mechanical resonator for the lubricant condition measurement. Both elements were realized as small multi-

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chip modules, which are combined within a mechanical flange. The raw data was transmitted to an external electronic control unit (ECU) and a complementary software module provided the lubricant condition for user. This study concluded the proposed sensor was successful to fulfill its purpose to monitor the lubricant level and condition, and also to provide a lubricant management system for vehicle to provide lubricant change based on the operating condition and driving habit and for different types of vehicles.

1.3.2.1.4 Sensing permittivity using capacitive sensor

Na et al. [82] designed a capacitive sensor with interdigit structure to monitor changes in

the dielectric constant of the engine lubricant and fabricated the sensor by using semiconductor fabrication technology. This study concluded that the dielectric constant change of engine lubricant resulted in the capacitance change of the sensor. It also stated that the dielectric constant of engine lubricant changed as it deteriorated and the fabricated sensor could measure the degree of deterioration.

Turner and Austin [109] reported a study in which changes to the dielectric and magnetic properties of the lubricant were assessed as methods of measuring the degradation of engine lubricant. The relationship between lubricant use, measured by accumulated mileage, and lubricant viscosity was also measured. A parallel plate capacitor was constructed using brass discs with 1 mm separation on a nylon core. The air capacitance of this device was 170 pF. The magnetic susceptibility balance reading was used to measure the changes in magnetic property of the lubricant. The conclusions from this work were that simple distance travelled (miles/kilometers) was not a good indicator of the state of a lubricant, as estimated by measuring its viscosity. The magnetic characteristics of lubricating oil (i.e. its magnetic permeability) do change as the lubricant degrades, but the measurements were poorly correlated with viscosity and did not seem to offer much promise as the basis of a lubricant monitoring system. The dielectric properties of lubricating lubricant were reasonably well correlated with viscosity, and it was proposed that this could form the basis of a useful sensing technique.

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In a study by Zhang et al. [122] a chemical sensor which was consisted of bulk

acoustic wave quartz resonator and a capacitor formed between a grounded quartz resonator electrode and an additional electrode was developed to monitor the soot content in the heavy-duty diesel engine. Two series of diesel lubricant samples, low and high soot concentration, were tested. The results showed that mass-sensitive quartz crystal microbalance (QCM) sensor was capable of detecting soot concentration in the samples regardless of its concentration or agglomeration, while the permittivity sensor exhibited different sensing pattern in detecting soot at higher concentration. It was also observed soot agglomeration had effect on the permittivity sensor reading.

1.3.2.2

Sensing electrical conductivity

Electrical conductivity or specific conductivity is a measure of a material's ability to conduct electric current. When an electrical potential difference is applied across a conductor, its movable charges flow, giving rise to an electric current. The conductivity is defined as the ratio of the current density to the electric field strength.

1.3.2.2.1 Sensing electrical conductivity using tuning fork quartz

resonator

Bennett et al. [13] designed and fabricated a miniature, solid state lubricant condition

sensor based on a crystal tuning fork. The sensor provided direct measurement of the critical physical properties of viscosity, density, dielectric permittivity, and AC conductance of lubricants. This study demonstrated the capabilities of a tuning fork flexural resonator based sensor to provide in-situ characterization of engine lubricant oil. The results compare favorably with those obtained through laboratory analysis.

1.3.2.2.2 Sensing electrical conductivity using capacitive sensor

In a study by Basu et al. [11] progress of development work to design a lubricant

monitoring sensor was described. This study, capacitive sensor and impedance measurement methods were utilized to measure the TAN and TBN contents and also the lubricant level inside the lubricant pan on-line and correlate the sensing parameters to lubricant viscosity. Several laboratory and road tests were conducted and the preliminary

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results indicated of capability of the proposed sensor to monitor engine lubricant condition using capacitive and impedance methodologies.

1.3.2.2.3 Sensing electrical conductivity using corrosion sensor

Agoston et al. [2] proposed a methodology to monitor the corrosion effect of engine lubricant as a measure to monitor the condition of lubricant in biogas-fuelled engines. The sensor was consisted of four copper resistive films with thickness of 100 nm, 300 nm, 600 nm, and 1000 nm to monitor the rate of corrosion. The rate of the corrosion depends on the current corrosiveness of the lubricant. The corrosive material loss was monitored electrically by the resistance of the copper films. This study identified two major corrosion mechanisms which takes place in biogas-fuelled engines lubricant: a quasi-uniform corrosion and a blotched type of corrosion. The first kind represents etch- type corrosion. The second type is often combined with surface depositions and chemical conversion of the copper layer to mechanically weak compounds, like copper sulfide. In case of the non-uniform, blotched corrosion, the applied resistive read-out method suffered from lacking reproducibility because corrosion blotches can lead to an unpredictable and early loss of electric contact due to their random distribution over the surface.

1.3.2.2.4 Sensing electrical conductivity using CNT

Moon et al. [81] have developed a new approach that examined the use of multiwall

carbon nanotube (CNT) to monitor lubricant degradation. Oxidation of the engine lubricant in the internal combustion engine led to an increase in the electrical conductivity of CNTs used as a sensing parameter in this study. In this work, the correlation between TAN formed from the lubricant degradation and CNT sensor was used to monitor the life of the engine lubricant. This study concluded that the engine lubricant sensor output corresponded to TAN could withstand the maximum temperature of 160˚C of engine lubricant. This study also showed the CNT sensor can be applied to other type lubricants.

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1.3.2.3

Impedance

Electrical impedance, or simply impedance, describes a measure of opposition to alternating current (AC). Electrical impedance extends the concept of resistance to AC circuits, describing not only the relative amplitudes of the voltage and current, but also the relative phases. When the circuit is driven with direct current (DC) there is no distinction between impedance and resistance; the latter can be thought of as impedance with zero phase angle. Impedance is defined as the frequency domain ratio of the voltage to the current. In other words, it is the voltage–current ratio for a single complex exponential at a particular frequency ω. In general, impedance will be a complex number, with the same units as resistance, for which the SI unit is the ohm. For a sinusoidal current or voltage input, the polar form of the complex impedance relates the amplitude and phase of the voltage and current.

1.3.2.4

Sensing Impedance using electrochemical impedance

spectroscopy

Lvovich and Smiechowski [73,74] examined the application of electrochemical impedance spectroscopy (EIS) for analysis of the electrochemical properties of industrial lubricants and focuses on establishing a relationship between lubricant chemical composition and EIS data. Theory on the planar interdigit electrode sensors was combined with experimental results to come to a better description of a sensor for highly resistive industrial fluid monitoring. The experiments were conducted on an impedance/dielectric analyzer. Electrochemical data were generated on several series of lubricant drains from engine field tests. Impedance spectra for lubricants were investigated over a range of temperatures, electrode geometries, potentials, and degradation states. Analysis of the spectra was divided into high, medium and low frequency regimes. The modeling and analysis of the EIS data as a combination of changes in bulk, adsorption, diffusion and charge transfer resistances, capacitances and constant phase element (CPE) parameters allowed studying the conditions of a lubricant. Experimental results demonstrated that the diffusive process could often be modeled as Warburg impedance with a finite double layer. The electrochemical structure of a typical industrial lubricant through the use of an equivalent circuit (EC) model was presented.

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The study proposed the use of this model for analysis of the impedance results as a reflection of chemical composition and changes in the system.