a fuel injection device of for internal combustion engines , particularly a unit fuel injector , having a pump piston preferably driven via a drive cam and having a free piston hydraulically driven by the pump piston , the free piston divides a pump chamber from a pressure chamber , from which a pressure line leads to an injection nozzle of the engine . via a magnetic valve , the fuel to be injected is first metered into the pressure chamber during the intake stroke of the pump piston and subsequently , in the control stroke , an injection quantity determining the supply onset is metered into the pump chamber . the pump piston with spaced annular grooves serves as a reversal device . during the metering stroke , a maintenance pressure that is lower than the metering pressure of the fuel and is generated via a pressure control valve is maintained in the pump chamber .