A controller for determining an injection strategy for a fuel injector within a hydrogen fuel injection system within an engine, the controller comprising: an input arranged to receive an engine operating parameter and a pressure signal associated with the pressure of hydrogen fuel available for injection from the injector in the fuel injection system; a processor arranged to determine an injection strategy for the injector in dependence on the received pressure signal and engine control parameter, the determined injection strategy comprising one or more injector control signals to control operation of the fuel injector; an output arranged to output the one or more control signals to the fuel injector.

Operating a fuel injector in a fuel system for an engine includes energizing a first solenoid actuator to move a spill valve from a first position to a second, closed position, and energizing a second solenoid actuator to move a fuel injection valve in the fuel injector from a closed position to an open position. A pull-in tier of a waveform energizing the second solenoid actuator is generated via a first current produced by a boosted voltage power supply and a second current produced by a lower voltage power supply. Arrival timing of the valve at the open position is detected based on a property of the second current, such that a valve arrival timing error may be used to trim the fuel injector. Related methodology and control logic is also disclosed.