Engine fuel control system

An engine fuel control system including:

• a fuel metering valve operable to control a flow of fuel between a supply line and a delivery line which delivers fuel to burners of an engine;
• a fixed displacement main pump configured to receive fuel from a low pressure source and deliver the fuel at a first high pressure to the supply line;
• a variable displacement augmenter pump which is configured to receive fuel from the low pressure source and deliver the fuel at a second high pressure to one or more fuel-pressure operated auxiliary engine devices, the variable displacement augmenter pump being configured to provide a fuel delivery flow rate which is variable to compensate for fuel flow demands imposed by the one or more fuel-pressure operated auxiliary engine devices;
• and
• a start valve which is configured to be actuated at low engine speeds to open a flow path which diverts fuel delivered by the variable displacement augmenter pump away from the one or more fuel-pressure operated auxiliary engine devices to the supply line to augment the fuel delivered thereto by the fixed displacement main pump, the start valve being configured to be actuated at higher engine speeds to shut the flow path;
• wherein the variable displacement augmenter pump has a servo-controller configured to vary its pump flow rate;
• and
• wherein the engine fuel control system further includes a pressure drop control valve which is configured to sense a pressure differential across the variable displacement augmenter pump, the pressure drop control valve being arranged to control the servo-controller such that the pump flow rate of the variable displacement augmenter pump is varied to maintain a pressure differential across the variable displacement augmenter pump at a predetermined level, and being configured to spill, at the higher engine speeds, a portion of the fuel delivered by the variable displacement augmenter pump away from the one or more fuel-pressure operated auxiliary engine devices to the supply line to augment the fuel delivered thereto by the fixed displacement main pump.