The movement of the governor 90 in FIG. 2 can be achieved by any suitable means such as a mechanical adjustment of the position of governor 90. Alternatively the governor 90 could be moved by a DC electric motor or linear motor enabling electronic control of the fuel injection. In this way, it is possible to infinitely vary the fuel injection by controlling governor 90 in a continuous manner, thereby controlling continuously the low pressure side of the injector which in turn enables control of the point in an operating cycle at which movement of the piston assembly 30, 35 is allowed to commence. In general terms, the hydraulic control of the low pressure side of the piston assembly 30, 35 of the injector enables precise control of the point of commencement of the stroke of the piston assembly 30, 35 which controls the amount of fuel injected, up to a maximum charge determined by the spacing of the bleed bore 50 and 51.

In operation of the injector in an internal combustion engine, the increasing pressure on the front face 31 of the low pressure piston 30 during the compression state of the engine will tend to move the piston assembly 30, 35 against the action of both the main spring 36 and the fluid pressure in chamber 37. If the pressure relief from the low pressure chamber 37 through outlet 20 is permitted, the piston assembly 30, 35 retracts to compress fuel in high pressure chamber 45. The fuel flows through fuel passage 55, past delivery valve 56 and into delivery chamber 65. The pressure in chamber 65 causes the needle valve 70 to open against the action of both delivery spring 71 and the pressure in low pressure chamber 37 which, in turn, is in communication with control chamber 78 so that fuel injection through orifice 68 commences.