Abstract: A fuel injection system comprising a fuel injection nozzle having a fuel injection orifice, and a vibrator to produce atomization of the fuel injected by the nozzle, the nozzle being equipped at the inlet side of its nozzle orifice with a liquid retaining ball valve which is arranged to normally close the nozzle orifice and thus prevent the injection of fuel by the nozzle and which is adapted to move away from the nozzle orifice when the vibrator is activated and thus allow the injection of fuel by the nozzle, the valve being situated in a housing in the nozzle and the housing having an aperture at a position where the valve tends to move when the vibrator is activated, thereby to allow the fuel to enter the housing through this aperture and force the valve towards the nozzle orifice when the nozzle is not being vibrated. Usually, the fuel will be introduced into the housing by passing along the passage and through the small passages in the housing.