In the extension 40 there is provided a bleed bore 50 and extending through the high pressure barrel 44 is a bleed bore 51 which opens into the low pressure chamber 37. If the stroke of the piston assembly 30, 35 is sufficient for the bleed bore 50 to align with the bleed bore 51, the fuel within the high pressure chamber 45 is immediately placed in communication with the low pressure chamber 37 and the fuel pressure in high pressure chamber 45 will immediately drop so that there will be insufficient pressure for fuel injection to continue as will be described later. Thus the longitudinal separation between the bleed bore 50 and the bleed bore 51 effectively defines the maximum fuel charge that can be injected during one stroke of the piston assembly 30, 35 and this, in turn, effectively limits the speed of running of the associated engine to a predetermined maximum determined by the maximum fuel charge.

Running longitudinally through the extension 40 of the high pressure piston 35 is a fuel passage 55 along which pressurised fuel from the high pressure chamber 45 travels as the piston assembly 30, 35 moves under the action of the cylinder pressure. The fuel passes a non-return delivery valve 56 which is shown resting against shoulder 57 under the action of spring 58. In operation, high pressure fuel moves valve 56 away from shoulder 57 against the action of spring 58. Fuel flows past the valve 56 only when it has moved sufficiently for the shoulder 59 of the valve to move past the end of passage 60 formed on the inside surface of the high pressure piston 35. With this arrangement, when delivery valve 56 is closing, fuel flow past the valve 56 is stopped when the shoulder 59 reaches the end of the passage 60, after which the valve 56 continues to move by a further limited extent until the valve 56 reaches shoulder 57. This continued movement of valve 56 after the valve has closed off fuel flow relieves pressure on the downstream side of the valve 56 for a purpose which will be described later.