The invention relates to an injector including a needle mounted in a nozzle and having an end defining a valve, the needle being connected at the other end to an actuator including first, second and third portions, the first and third portions being provided on either side of the second portion, the three portions being tightened together in order to form a block having two axially opposite limits, the first portion being connected with the needle at one of said limits, and an excitation means for vibrating the second portion according to setpoint period τ. According to the invention, the length between the two limits of the block is such that the propagation time T of the acoustic waves generated by the vibrations of the second portion of the actuator and running along that length meets the equation: T=n*[τ/2], where n is an integer positive multiplication coefficient different from zero.