A method for controlling the displacement of a stack of a piezoelectric actuator adapted for use in a fuel injector in order to control the fuel injection rate and volume. The method comprises driving a varying current through the stack during an opening phase including a first opening stage and a second opening stage; driving the stack during the first opening stage with a first opening current level for a first opening time period and driving the stack during the second opening stage with a second opening current level for a second opening time period. The second opening current level is less than the first opening current level and the current levels are selected so that the fuel injection rate in the opening phase includes a first portion corresponding to the first opening stage having a gradient that is steeper than a second portion corresponding to the second opening stage.