A system comprises a pressure raising valve having a valve member movable within a bore between a closed position and an open position in which fuel supplied to the pressure raising valve from a metering valve, in use, is delivered to an engine manifold, the valve member being urged towards its closed position by the fuel pressure within a control chamber, the pressure raising valve including a low pressure port which communicates with the control chamber when the valve member is in its closed position, the valve member closing the low pressure port upon movement thereof away from its closed position beyond a predetermined distance.
1. A fuel system comprising a pressure raising valve having a valve member movable within a bore between a closed position in which the valve member closes an outlet of the pressure raising valve and so no fuel is supplied from the pressure raising valve, in use, to an engine manifold, and fully open position in which the valve member does not obstruct the outlet and so fuel is delivered to the engine manifold, the valve member being urged towards its closed position by the fuel pressure within a control chamber connected, in use, to a high pressure fuel line, the pressure raising valve including a low pressure port connected to low pressure at start-up and which communicates with the control chamber when the valve member is in its closed position such that the pressure within the control chamber at start-up is at a first, low pressure and so allows opening of the pressure raising valve at a reduced system pressure, the valve member closing the low pressure port upon commencement of movement thereof away from its closed position towards the fully open position to open the outlet thereby breaking the connection of the control chamber to low pressure and so allowing, the pressure within the control chamber to rise, to a second, higher pressure the low pressure port being closed by the valve member when the valve member occupies an intermediate position between the closed and fully open positions.
2. A system according to claim 1 , wherein the pressure raising valve comprises a pressure raising and shut-off valve (PRSOV).
3. A system according to claim 1 , wherein the valve member is spring biased towards its closed position.
4. A system according to claim 1 , wherein the low pressure pot comprises a plurality of openings formed in a sleeve.
5. A system according to claim 4 , wherein the openings are of rectangular shape.
6. A system according to claim 1 , further comprising a flow resistor formed in a low pressure line communicating with the low pressure port.
7. A system according to claim 1 , further comprising a fluid pressure potentiometer network communicating with the control chamber.