Fuel injector

A fuel injector (100; 200; 300) for an internal combustion engine is disclosed. The injector includes a slave piston (130; 230; 330) provided with a control bore (144; 244; 344) and associated with a valve needle (110; 210; 310), such that movement of the slave piston causes movement of the valve needle. An end of the slave piston remote from the valve needle is exposed to fuel pressure in a control chamber (152; 252; 352) of the injector, in use. A control piston (148; 348) is operable to move relative to the slave piston so as to control fuel flow through first and second control ports that connect the control bore to a high-pressure fuel supply and a low-pressure drain respectively. By suitable positioning of the control piston, the valve needle can be biased in an opening direction and in a closing direction and, optionally, the valve needle can be held in one or more intermediate positions between its fully-lifted and fully-seated positions. Proportional control of the position and velocity of the valve needle is possible, and a low-force actuator can be used to control the valve needle.