Method of determining closing time of needle valve of a fuel injector

A method of controlling a solenoid controlled fuel injector including a solenoid controlled actuator adapted to control a needle valve by the movement of a needle to and from a valve seat of the needle valve, said method comprising the steps of:

• a) providing a reference signal, wherein the reference signal is indicative of the voltage across the solenoid against time of the injector during an operational cycle where injection does not occur;
• b) activating the injector by sending an activation pulse to the solenoid, and from a timepoint after the end of the activation pulse, measuring the voltage signal across the terminals of the solenoid to provide a solenoid voltage signal;
• c) determining a difference signal based on the difference between the reference signal of step a) and the measured voltage signal of step b);
• d) differentiating the difference signal of step c) to provide a first differential signal;
• e) from step d) providing a normalized signal in which the first differential signal is translated into absolute value and normalized around zero;
• f) differentiating the first differential signal of step d) to provide a second differential signal;
• g) multiplying the values of the signals from step f), step e) and step c) to provide a plot;
• h) determining the maximum peak value of the plot from step g) and determining the valve closing time from the time point of the determined maximum peak;
• and
• i) subsequently controlling the fuel injector based on the result of step h).