by a low pressure feed pump 2 which delivers fuel at a pressure pb the fuel leaving the pump 2 together with fuel from a return line which will be referred to hereinafter are delivered via a line 21 to a high pressure pump 3 whose output is adjustable according to the drive speed in accordance with the invention the controlled delivery from the pump 3 is equal to the sum of the delivery needed to supply the engine fuel injectors and the maximum possible delivery which may be consumed by the hydromechanical devices which use the pressurized fuel as the hydraulic driving fluid consequently the delivery variations to be accommodated by the pump 3 are merely the variations in the delivery to the engine fuel injectors these variations are slow enough to be controlled without difficulty more particularly they are compatible with the pump response time the fuel from the pump 3 passes through a line 22 to a self cleaning filter 4 which in known manner has an outlet 5 for unfiltered fuel and an outlet 6 for filtered fuel the unfiltered fuel is delivered to a unit 9 for metering fuel to the engine fuel injectors the filtered fuel from the outlet 6 is directed through a heater 7 before being supplied for use by servovalves operating mechanisms of the engine in accordance with the invention a device 10 is provided to maintain a constant servovalve feed pressure whatever delivery is taken by the servovalves which drive the engine mechanisms the device 10 therefore receives via line 8 a fuel delivery which is substantially equal to the maximum possible delivery required if.