A fuel injection arrangement for a piston engine, the arrangement comprising an injector nozzle (1) for injecting fuel into the combustion chamber (6) of the cylinder. A Theological actuator (15) is arranged in connection with the injector nozzle (1), the actuator comprising a space (18) containing Theological fluid and an apparatus (19, 20) by means of which the viscosity of the Theological fluid can be changed for controlling the fuel injection.