Initially, fuel will be injected in relatively large droplets since the pressure in the engine cylinder is still relatively low. However, in the case of a compression ignition engine, immediately ignition of the fuel in the cylinder occurs, there is a rapid increase in cylinder pressure which acts on face 31 of the piston 30. This

The maximum fuel charge is determined by the spacing of the bleed bores 50, 51 which effectively also provides a maximum engine speed limiter. In particular, when the bleed bores 50 and 51 align, the fuel pressure in the high pressure chamber 45 is immediately relieved through the bleed bores 50, 51 and this pressure drop is immediately conveyed to the delivery chamber 65 so that the needle valve 70 immediately closes.

The external control of the pressure relief through the outlet 20 of the injector, e.g. by means of the governor means shown in FIG. 2, not only controls the point of opening movement of the piston assembly 30, 35 but also controls the low pressure side in chamber 37 during an injection operation. If the pressure relief through outlet 20 is retarded, the movement of piston assembly 30, 35 is limited by the relief of pressure in the low pressure chamber 37 and also the opening movement of the needle valve 70 is resisted by the retarded relief of pressure in control chamber 78 acting against face 77 of the shank 75 of the needle valve 70. Thus low pressure side hydraulic lock up controls termination of the fuel injection operation. Alternatively, the termination of the injection operation occurs when the maximum fuel charge is injected and the bleed bores 50, 51 align and cause an immediate high pressure side pressure drop. In either case, the delivery needle valve 70 closes the orifice 68. The delivery valve 56 also will immediately move towards its closed position under the action of spring 58 so that the shoulder 59 reaches the end of passage 60 thus closing off communication between the high pressure chamber 45 and the delivery chamber 65. Because the delivery valve 56 continues to move beyond the point at which shoulder 59 reaches the end of passage 60, the fluid pressure in delivery chamber 65 can continue to be relieved preventing opening of needle valve 70 until high pressure is again built up in delivery chamber 65. These combined actions of hydraulic lock up of the low pressure side or high pressure side pressure relief, together with the two stage movement of the delivery valve 56 ensure immediate and positive termination of the fuel injection.