Fuel system and ecology valve for use therein

1. An aerospace fuel system comprising a pressure raising valve operable to control the delivery of fuel from a fuel delivery line to an engine manifold line, and an ecology valve communicating via an ecology port with the engine manifold line but not restricting fuel flow along the engine manifold line, the ecology valve comprising a piston movable within a bore, the piston and bore together defining a control chamber and an ecology chamber, the volume of the ecology chamber being dependent upon the position of the piston, the ecology chamber communicating with the engine manifold line via the ecology port, the pressure within the control chamber being controllable to control the position occupied by the piston, the ecology valve further comprising seal means whereby, when the piston occupies a first position in which it engages the seal means, only a first, relatively small, effective area of the piston is exposed to a fuel pressure within the engine manifold line and the volume of the ecology chamber is relatively small, a second, larger, effective area of the piston being exposed to the fuel pressure within the engine manifold line, and the ecology chamber being of increased volume, when the piston occupies a second position in which it does not engage the seal, the movement of the piston from its first position to its second position drawing fuel from the engine manifold line into the ecology chamber through the ecology port.

2. A system according to claim 1 , wherein the pressure in the control chamber is controlled by valve means.

3. A system according to claim 2 , wherein the valve means is defined by a porting arrangement provided in the pressure raising valve.

4. A system according to claim 1 , wherein the ecology valve includes a resilient biasing means biasing the piston towards a second position in which the ecology chamber is of relatively large volume, and the control chamber is of relatively small volume.

5. A system according to claim 4 , wherein the resilient biasing means is in the form of a spring.

6. A system according to claim 1 , wherein a restricted flow path is provided between the ecology chamber and a low pressure line.

7. A system according to claim 6 , wherein a valve arrangement seals the restricted flow path when the piston occupies its second position.

8. A system according to claim 7 , wherein the valve arrangement comprises a seal member carried by the piston.

9. An ecology valve communicating via an ecology port with the engine manifold line but not restricting fuel flow along the engine manifold line, the ecology valve comprising a piston movable within a bore, the piston and bore together defining a control chamber and an ecology chamber, the volume of the ecology chamber being dependent upon the position of the piston, the ecology chamber communicating with the engine manifold line via the ecology port, the pressure within the control chamber being controllable to control the position occupied by the piston, the ecology valve further comprising seal means whereby, when the piston occupies a first position in which it engages the seal means, only a first, relatively small, effective area of the piston is exposed to a fuel pressure within the engine manifold line and the volume of the ecology chamber is relatively small, a second, larger, effective area of the piston being exposed to the fuel pressure within the engine manifold line, and the ecology chamber being of increased volume, when the piston occupies a second position in which it does not engage the seal, the movement of the piston from its first position to its second position drawing fuel from the engine manifold line into the ecology chamber through the ecology port.