Use of an emissions sensor to control fuel flow staging

1. A fuel system comprising:

a fuel actuation arrangement that is configured to split a metered flow of fuel into at least a first stream and a second stream; and

a control unit that controls the operation of the fuel actuation arrangement, wherein

the control unit derives a calculated combustion flame temperature value using a combination of an out gut of a temperature sensor sensitive to a gas temperature downstream of a high pressure compressor of an associated engine and an output of a gas sensor sensitive to at least one parameter of a composition of a gas downstream of a combustor of the engine; and

the control unit is configured to control the operation of the fuel actuation arrangement using the calculated combustion flame temperature value.

2. A system according to claim 1 , wherein the temperature sensor is sensitive to a temperature at an exit of the high pressure compressor.

3. A system according to claim 1 , wherein the parameter to which the gas sensor is sensitive is a content of the gas downstream of the combustor of at least one of the following: oxygen, carbon dioxide, or carbon monoxide.

4. A system according to claim 1 , wherein the gas sensor is located adjacent an exit of a low pressure turbine stage.

5. A system according to claim 4 , wherein the gas sensor is located at a gas path of a low pressure turbine outlet guide vane or its exit.

6. A system according to claim 1 , wherein the gas sensor is of the laser-detector type.

7. A system according to claim 6 , wherein the gas sensor comprises a probe located in the engine, and an analysis device located remotely from the probe.

8. A system according to claim 7 , wherein the gas sensor includes one or more optical fibres connecting the probe to the analysis device.

9. A system according to claim 8 , wherein a first optical fibre is used to carry a laser output to the probe, and a second optical fibre carries a measurement back to the analysis device.

10. A system according to claim 1 , wherein the gas sensor output and the temperature sensor output are used as inputs to a combustor temperature rise calculation software routine to derive a difference value representative of a difference between a sensed temperature and the calculated combustion flame temperature.

11. A system according to claim 10 , wherein the calculated combustion flame temperature is calculated by adding the temperature sensor output and the difference value together.

12. A method of controlling an operation of a fuel system, the method comprising the steps of:

sensing a parameter of a composition of an engine gas downstream of a combustor of an engine;

sensing a temperature of the engine gas downstream of a high pressure compressor of the engine;

deriving, on a control unit, a calculated combustion flame temperature value based on a combination of the sensed gas composition parameter and the sensed temperature; and

controlling, with the control unit, the operation of a fuel actuation arrangement to split a fuel supply into a first stream and a second stream, based on the calculated combustion flame temperature value.

13. A method according to claim 12 , wherein a combustor temperature rise calculation software routine is used to derive a difference value from the sensed gas composition parameter and the sensed temperature.

14. A method according to claim 13 , wherein the difference value is used in combination with the sensed temperature to derive the calculated combustion flame temperature value.