A fuel system comprises a supply unit operable to deliver a metered supply of fuel to a supply manifold, a staging valve operable to control the supply of fuel from the supply manifold to a pilot outlet and a main outlet, and control means operable to control the operation of the staging valve to control whether fuel is delivered through the pilot and/or main outlet thereof.
1. A fuel system comprising:
a supply unit configured to deliver a metered supply of fuel to a supply manifold;
a plurality of staging valves, each configured to control the supply of fuel from the supply manifold to a respective pilot injector or pilot injector part of a combined injector via a pilot outlet and a respective main injector or main injector part of a combined injector via a main outlet; and
a control means configured to control the operation of the plurality of staging valves to control whether fuel is delivered through the pilot and/or main outlet thereof and wherein the control means comprises a control unit configured to control a pressure difference between the fuel pressure within a control manifold and that within the supply manifold, the plurality staging valves being configured to respond to the said pressure difference.
2. The fuel system according to claim 1 , wherein the plurality of staging valves are fuel-draulically controlled.
3. The fuel system according to claim 1 , wherein the control unit comprises a pump configured to raise the fuel pressure within the control manifold above that within the supply manifold, and to maintain the aforementioned pressure difference therebetween.
4. The fuel system according to claim 3 , wherein a restricted flow path is provided between the control manifold and the supply manifold.
5. The fuel system according to claim 4 , wherein a reduction in the operating speed of the pump, in combination with the presence of the restricted flow path, allows the said pressure difference to be reduced, whilst increasing the pump operating speed raises the pressure difference.
6. The fuel system according to claim 1 , wherein the plurality of staging valves are electrically controlled.
7. The fuel system according to claim 6 , wherein the plurality of staging valves include an electric motor configured to drive the plurality of staging valves to positions in which fuel is delivered from the supply manifold to the pilot and/or main outlet thereof.
8. The fuel system according to claim 1 , wherein at least one of the staging valves is designed in such a manner that there is constant communication between the pilot outlet and the supply manifold and another of the staging valves is designed in such a manner that communication between the supply manifold and the pilot outlet can be significantly restricted.
9. The fuel system according to claim 1 , further comprising pilot and main outlet lines connecting the pilot and main outlets to the pilot and main injectors or the pilot and main injector parts of a combined injector, and low-pressure check valves provided in the pilot and main outlet lines.
10. A fuel system comprising:
a supply unit configured to deliver a metered supply of fuel to a supply manifold;
a plurality of staging valves, each configured to control the supply of fuel from the supply manifold to a respective pilot outlet and a respective main outlet, at least one of the staging valves is designed in such a manner that there is constant communication between the respective pilot outlet and the supply manifold and another of the staging valves is designed in such a manner that communication between the supply manifold and the respective pilot outlet can be significantly restricted, and
a control means configured to control the operation of the plurality of staging valves to control whether fuel is delivered through the pilot outlet and/or main outlet thereof.