A method for determining the opening for piezo servo-driven injectors may include measuring a voltage and a charge at an end of a charging process of the piezo actuator; determining a provisional scaling factor by calculating Q/U; determining the maximum of CVORL and using the instant at which the maximum occurs as an approximation value of the valve opening instant; measuring the charge and voltage at the approximation of the valve opening instant; solving the equation: 1.25×F2/E×C=(U×C−Q)2 Where F=force acting on the piezo actuator, E=electrical parameter of the piezo actuator; determining a new scaling factor from the solution of the equation; calculating the difference (U×C−Q) and determining a maximum of said value; and using the instant at which said maximum occurs as valve opening instant.