A fuel system comprises a spill valve 38 operable to relieve the fuel pressure differential across at least one positive displacement pump 16, 18 , a shut-off valve 32 operable to control the supply of fuel to an engine 84 , and a two stage servo-valve 62 operable to control the operation of the shut-off valve 32 , the spill valve 38 having a control chamber 40 the fluid pressure within which is controlled so as to be held at an intermediate level during engine shut-down.
1. A fuel system comprising a spill valve operable to relieve the fuel pressure differential across at least one positive displacement pump, a shut-off valve operable to control the supply of fuel to an engine, and a two stage servo-valve operable to control the operation of the shut-off valve, the spill valve having a control chamber, and spill control means operable to control the fluid pressure within the control chamber so as to hold the fluid pressure at an intermediate level during engine shut-down.
2. A fuel system according to claim 1 , wherein the spill control means comprises a port provided on the shut-off valve, the control chamber of the spill valve communicating with said port.
3. A fuel system according to claim 2 , wherein the port is arranged to open upon movement of the shut-off valve to terminate the supply of fuel to the engine.
4. A fuel system according to claim 2 , wherein the port is arranged to close, or partially close, upon movement of the shut-off valve to terminate the supply of fuel to the engine.
5. A fuel system according to claim 2 wherein the control chamber of the spill valve communicates with a fluid line connected through a restriction with a source of fuel at high pressure, the port being connected to the fluid line intermediate the control chamber and the restriction.
6. A fuel system according to claim 5 , wherein the port controls communication between the fluid line and a source of fuel at low pressure.
7. A fuel system according to claim 5 , wherein the port controls communication between the fluid line and a source of fuel at high pressure.
8. A fuel system according to claim 7 , wherein the fluid line is further connected to a port of the two-stage servo-valve and the two stage servo-valve is operable to connect the fluid line to a source of fuel at low pressure, the port of the two stage servo-valve and the port of the shut-off valve serving as a fluid pressure potentiometer network.