A fuel system comprises a first, variable displacement fuel pump operable to supply fuel through a metering valve to an engine, a control servo operable to control the operation of the first pump, a pressure drop control valve (PDCV) responsive to a pressure drop across the metering valve, a second fuel pump, and a spill valve movable between an open position in which substantially all of the fuel supplied by the second fuel pump is returned to an inlet side thereof, and a closed position, or partially closed position, wherein a proportion of the fuel supplied by the second pump is delivered to the engine.
1. A fuel system comprising a first variable displacement fuel pump operable to supply fuel through a metering valve to an engine, a control servo operable to control the operation of the first pump, a pressure drop control valve (PDCV) responsive to a pressure drop across the metering valve, a second fuel pump arranged in parallel with said first fuel pump, and a spill valve movable between an open position in which substantially all of the fuel supplied by the second fuel pump is returned to an inlet side of the second fuel pump, and a closed position, or partially closed position, wherein a proportion of the fuel supplied by the second fuel pump is delivered to the engine, via the metering valve, on normal use, to supplement that delivered by the first pump, and wherein the PDCV is operable to control the operation of both the control servo and the spill valve.
2. A system according to claim 1 , wherein the second pump comprises a fixed displacement pump.
3. A system according to claim 1 , wherein the spill valve comprises a spool slidable within a bore and having a first end surface exposed to metering valve inlet pressure, and a second end surface exposed to the fuel pressure within a first control chamber, the PDCV being operable to control the first control chamber pressure.
4. A system according to claim 1 , wherein the control servo comprises a piston slidable within a bore and having a first surface exposed to a reference pressure, and a second surface exposed to the fuel pressure within a second control chamber, the PDCV being operable to control the second control chamber pressure.
5. A system according to claim 4 , wherein the reference pressure is governed by a pressure potentiometer network connected between metering valve inlet pressure and low pressure.
6. A system according to claim 1 , wherein the PDCV comprises a spool slidable within a bore between a first position in which a first control port thereof is open and a second control port thereof is closed, and a second position in which the second control port is open and the first control port is closed.
7. A system according to claim 6 , wherein the first control port communicates with the first control chamber and the second control port communicates with the second control chamber.
8. A system according to claim 6 , wherein the spool of the PDCV defines a gallery which communicates through an internal passage with a chamber defined by one end of the spool and the associated bore.