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Experimental forms of two different types of engine oil viscosity sensors have been tested that use uniformly poled disks of piezoelectric PZT ceramic. In both cases, the disks were used to form electromechanical resonators functioning as the frequency-controlling element in a transistor oscillator circuit. The simpler type of sensor used only one disk, vibrating in a radial-longitudinal mode of vibration. In this mode, a disk 2.54 cm in diameter and 0.127 cm thick had a resonant frequency of approximately 90 kHz. The second type of sensor used two such disks bonded together by a conducting epoxy, with poling directions oriented in opposite directions. This composite resonator vibrated in a radially-symmetrical, flexural mode of vibration, with the lowest resonant frequency at approximately 20 kHz. The presence of tangential components of motion on the major faces of both resonators made them sensitive to the viscosity of fluids in which they were immersed.

This paper describes two types of dielectric-effect sensors that may be used as alternatives to a dielectric-effect sensor using a single capacitor. In the first type, three capacitors are mounted in a compact module inserted into a vehicle fuel line. The three capacitors are connected together to form an electrical pi-filter network. This approach provides a large variation of output signal as the fuel changes from gasoline to methanol. The sensor can be designed to operate in the 1 to 20 MHz frequency range. The second type of sensor investigated uses a resonant-cavity structure. Ordinarily, sensors based on resonant cavities are useful only if the operating frequency is several hundred MHz or higher. The high relative dielectric constant of methanol allows useful sensors to be built using relatively short lengths of metal tubing for the cavities. For example, a sensor built using a fuel rail only 38.7 cm long operated in a frequency range from 31 to 52 MHz.

There is a need for sensors that respond to chemical and physical properties of engine oil. In response to this need, an experimental design of an engine-mounted, capacitive sensor has been developed to monitor changes in the dielectric constant of the engine oil. The sensing element is a small, air-gap capacitor that is mounted in a spacer ring that fits between the oil filter and the engine block. Embedded in the same spacer ring is the associated circuitry. Experiments have been carried out with experimental capacitive oil sensors mounted on engines using a typical fully-formulated, factory-fill oil. The oil dielectric constant initially decreases and is less than the starting value while the anti-oxidant additives are active. After about 1600 km, the antioxidant additives become sufficiently depleted to allow oxidation products to accumulate and for the oil dielectric constant to increase at a steady rate of about 1% per 1000 km.