via the branch line 26 the pressure on the plunger during this movement is influenced among other things by the design shape of the cam lobe and by inherent characteristics of the injector when a sufficient quantity of fuel has been injected the spill valve 20 is re opened and the needle control valve 25 is shifted back to the first position shown in the drawings this point marks a return to the original position in which the spill valve is open and the injector nozzle is closed according to the invention the spill valve 20 and the needle control valve 25 can be regulated by the engine control unit 27 in such a way that the fuel in the injector 10 causes the opposing force during the downward movement of the plunger to become greater than normal over a part of the working stroke of the plunger 13 this is done by regulating the needle control valve so that the injector nozzle is kept closed and regulating the hydraulic pressure between the plunger and the injector nozzle through opening and closing of the spill valve the hydraulic pressure which acts against the end of the plunger can thus be utilized in order to retard the downward movement of the plunger over a part of the working stroke which means that the force of the compression spring 16 does not need to be so great and that the plunger cam follower can still be made to follow the cam lobe without the risk of the contact between the cam lobe and the cam follower being interrupted if the engine is being used in engine braking mode that is to say without injection occurring it is possible to use a larger part of the cam lobe.