Variable minimum pressure system

1. A fuel system comprising:

a fuel pump configured to deliver fuel through a supply line to a pressure raising valve, the pressure raising valve including a control chamber, fuel pressure within the control chamber controlling a minimum pump pressure differential, and

an actuator control servo-valve configured to control a supply of fuel from the supply line to an actuator control line, wherein the actuator control servo-valve further controls the pressure of fuel applied to the control chamber of the pressure raising valve;

wherein the control chamber communicates with a control line, and a pressure potentiometer network which includes low and high pressure restrictors configured to hold the pressure in the control line at an intermediate level between a low fuel pressure at the inlet of the fuel pump and a high fuel pressure at the outlet of the fuel pump;

wherein the actuator control servo-valve includes a port arrangement whereby the control line pressure can be raised above said intermediate level; and

wherein the actuator control servo-valve is configured to interchangeably apply said high pressure and said low pressure to the actuator control line, and further is configured to raise the pressure in the control line above said intermediate level when said high pressure is applied to the actuator control line, and also to raise the pressure in the control line above said intermediate level when said low pressure is applied to the actuator control line.

2. A system according to claim 1 , further comprising a shut-off servo-valve arrangement operable, when engine shut-down is required, to raise the control chamber pressure so as to close the pressure raising valve.

3. A system according to claim 1 , wherein a single actuator control line is provided, the actuator control servo-valve controlling the fuel pressure therein by controlling the connection thereof to high and low pressure sources.

4. A system according to claim 1 , wherein a pair of actuator control lines provided, the actuator control servo-valve controlling which of these control lines is connected to high pressure and which is connected to low pressure at any given time.

5. A system according to claim 1 , further comprising an actuator including a chamber in communication with the actuator control line.

6. A system according to claim 1 , wherein the actuator control servo-valve comprises a two stage servo-valve.

7. A system according to claim 6 , wherein the two-stage servo-valve comprises an electromagnetically controlled first stage operable to control the position of a spool of a second stage thereof.