A computer-implemented method for controlling direct injection of hydrogen fuel into an engine cylinder of a vehicle from a hydrogen tank via an injector system. The method includes determining a present hydrogen pressure in the hydrogen tank; determining a present requested engine torque and speed; determining a target injection pressure for injecting hydrogen into the engine combustion chamber using a model the target injection pressure is the injection pressure that, according to the model, minimizes the mechanical work of the compressor, that provides an in-cylinder pressure sufficient for the engine to provide the requested engine torque and speed, that provides NOx emission below a predetermined maximum NOx emission level, and that reduces engine fuel consumption, and adjusting the injector pressure according to the determined target injection pressure.