A fuel
system comprises a pressure raising valve operable to control communication between a
fuel delivery line and an engine manifold line, and an
ecology valve comprising a
piston movable within a bore, the
piston and bore together defining a control chamber and an
ecology chamber, the
ecology chamber communicating with the engine manifold line, 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, a second, larger, effective area being exposed to the fuel pressure within the engine manifold line when the piston occupies a position in which it does not engage the seal. An ecology valve for use in the fuel
system is also described.