Capacitive fuel composition sensor with padding capacitor

Claim 1

In a fuel composition sensor for a fuel mixture in a fuel supply line for a combustion engine wherein the fuel mixture comprises two fuels of different dielectric constants in unknown relative concentrations, the sensor comprising a sensor capacitor in the fuel line adapted for fuel flow therethrough so that the fuel mixture provided to the engine comprises a dielectric which determines the sensor capacitance thereof and the sensor capacitance varies in a linear manner with fuel composition, a reference capacitor with a constant reference capacitance in series with the sensor capacitor to form therewith a voltage divider having an output junction and circuit means for producing a predetermined fast voltage change across the series combination of the reference and sensor capacitors to produce a corresponding voltage change at the output junction determined by the voltage divider ratio of the reference capacitance to the sum of the sensor and reference capacitances and thus varying in a non-linear manner with fuel composition, the improvement comprising: a padding capacitor having a constant padding capacitance in parallel with the sensor capacitor so that the voltage change at the output junction is determined by the voltage divider ratio of the reference capacitance to the sum of the sensor, reference and padding capacitances, the padding capacitance having a value such that the maximum value of the voltage divider ratio is substantially less than one, whereby the voltage change at the output junction varies essentially linearly with fuel composition.