MEMS for biofuel composition measurement based on thermal impedance spectroscopy

Abstract

Continuous monitoring of the composition of E85 biofuel is essential for a quick start and clean and efficient operation of Flex-Fuel Vehicles. The actual ethanol concentration in E85 fuel is in the range 50%–85% and fuel-line sensors are used for ethanol-gasoline composition measurement. However, also a small amount of water is typically present, which cannot be reliably detected using state-of-the-art capacitive fuel-line sensors. Thermal impedance spectroscopy has been investigated as a non-destructive technique to determine the composition of ternary mixtures of biofuels. The principle of the thermal conductivity detector has been extended for measuring both the thermal conductivity and the thermal capacity of biofuel in the range up to 10 kHz using an AC-operated polysilicon heater for injecting a sinusoidal heat flux, and another polysilicon strip at a well-defined spacing or thermopile sensors for measuring the in-phase and quadrature components of the resulting AC temperature difference. Measurements on the components are in reasonable agreement with simulations, with a −3 dB cut-off frequency at 422.5 Hz and 340.8 Hz for ethanol and gasoline, respectively. However, the cut-off frequency of water was found to be significantly lower than simulations due to its high surface tension, thus limiting access to the detector.