Abstract: A hydraulically operated fuel injector for compression-ignition engines which includes an electronically or pneumatically operated hydraulic pilot-control valve which is supplied with fuel under substantially constant pressure from a pump driven directly or indirectly by the engine. The pilot-control valve is connected by supply and exhaust passages to a slave valve which in turn controls the admission of fuel to a charging cavity located adjacent to an outlet valve to the combustion chamber. The slave valve is a stepped diameter piston which functions as a pressure amplifier for the fuel supply to the combustion chamber. Fuel from the pump which is received at relatively low pressure through the slave valve is amplified to high pressures sufficient to overcome compression and combustion pressures of the engine and the resistance of spring seated injector outlets and/or the fuel supply orifice(s).