Abstract: An injector for injecting the working fluid of a piston vapour-engine. It incorporates an injector nozzle with injector nozzle-needle, needle actuating plunger and a solenoid controlled valve. It is characterised by the working fluid being supplied continuously to a working-fluid reservoir (4), while fluid for servocontrol purposes is supplied continuously to a servo-fluid reservoir (10). The injector nozzle-needle (1) is connected to the plunger (2) which passes through the working-fluid reservoir (4) and also through the servo-fluid reservoir (10). The top of the plunger (2) is capped by a plunger pressure-face (6), which is located in the servo-control chamber (11). The servo-control chamber (11) is connected by means of a feed-orifice (12) to the servo-fluid reservoir (10). The plunger (2) has a pressure-shoulder (7) located in the servo-fluid reservoir (10).