A plunger is reciprocable in a barrel to force fuel to the injection nozzles. The portion of the barrel defining with the plunger the compression chamber is movable and is formed with a port leading to the unpressurized fuel reservoir in such a manner that the plunger uncovers the port at the end of its fuel injection stroke to overflow fuel from the compression chamber to the reservoir and terminate fuel injection. The position of the movable portion of the barrel and thereby the port relative to the plunger determines the fuel injection volume. A closed loop electronic servo system is provided to position the movable part of the barrel in accordance with the required fuel injection volume.