The picture above shows the measurement cell of the sensor. It is basically a cylindrical capacitor consisting of two tubes, the inner and outer electrode. The fuel flows through the gaps between. An AC voltage is applied to the electrodes. The relative permittivity of the fuel changes with the ethanol content, which leads to a change in the capacity accordingly (picture on the left). As the capacitance increases with increasing ethanol content, the oscillation frequency in the system will decrease and vice versa.
The analog frequency output is tied to a microprocessor which determines the capacitance by working backwards from the measured frequency. Due to the correlation between ethanol’s permittivity and the fluid’s temperature, the microprocessor corrects the capacitance for temperature variations in the fuel according to the output from the temperature measurement (NTC thermistor). In addition, errors caused by the different conductivities of gasoline and ethanol are corrected.
The frequency of the single-wire output of the microprocessor to the ECM indicates the percentage of ethanol in the fuel. If required by the OEM the pulse width of the signal will indicate the fuel temperature. The ECM adjusts the injection according to the blend of the fuel, which leads to a stoichiometric combustion.
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