Abstract: A constant-stroke, variable by-pass, plunger-type, jerk pump for injecting fuel-oil into a working cylinder of an internal combustion engine, wherein the plunger comprises a passage-way opening at one end thereof into the working chamber of said plunger and adapted to communicate at its other end with at least one duct connected to the fuel supply and opening into the pump cylinder housing said plunger to temporarily cut off the fuel discharged from said chamber to the fuel injector during the upward stroke of the plunger towards its top dead center, the first fuel discharge phase enabling the fuel to be pre-injected into the engine cylinder whereas the second fuel discharge phase provides for the main injection of fuel thereinto.