Two Alternative, Dielectric-Effect, Flexible-Fuel Sensors

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. One advantage of the resonant-cavity design is that, since the inner and outer conductors are short-circuited at one end, both conductors can be at d.c. ground potential. Another advantage is that, when used with a circuit that senses the frequency location of the resonance peak corresponding to the cavity being half a wavelength long, the resonance frequency is relatively insensitive to the presence of impurities that increase the conductivity of fuel blends.