Method and apparatus for determining a frequency compensated capacitance of piezoelectric elements

The invention describes a method and apparatus for a fuel injection system with a piezoelectric element (10, 20, 30, 40, 50 or 60) for controlling the amount of injected fuel by applying a voltage to the piezoelectric element (10, 20, 30, 40, 50 or 60), characterized in that the fuel injection system comprises a control unit (D) for adjusting the applied voltage based upon a nonlinear relationship between the applied voltage and the charging time.