Staged combustion

1. A method of operating a staged combustion system in an aircraft gas turbine engine, comprising:

providing pilot injectors for rich combustion and main injectors for lean combustion in said staged combustion system;

identifying an atmospheric condition;

determining a ratio of pilot fuel flow rate for the pilot injectors to main fuel flow rate for the main injectors in response to the atmospheric condition;

wherein the determining step comprises:

identifying the atmospheric condition to the effect that a model indicates that increasing an optical depth of a condensation trail produced by the engine would reduce a net warming impact of the condensation trail;

evaluating a ratio of pilot fuel flow rate to main fuel flow rate that varies soot production to increase ice particle formation;

wherein the evaluating step comprises:

obtaining a measurement of an ambient temperature from a temperature sensor;

in response to the ambient temperature being greater than a transition temperature, evaluating the ratio of pilot fuel flow rate to main fuel flow rate that increases an index of soot emissions;

in response to the ambient temperature being less than or equal to the transition temperature and a current index of soot emissions being greater than a transition value, evaluating the ratio of pilot fuel flow rate to main fuel flow rate that increases the index of soot emissions;

in response to the ambient temperature being less than or equal to the transition temperature and the current index of soot emissions being less than or equal to the transition value, evaluating the ratio of pilot fuel flow rate to main fuel flow rate that decreases the index of soot emissions; and

injecting fuel by the pilot injectors and the main injectors in accordance with said ratio to control the index of soot emissions caused by combustion of fuel therein.

2. The method of claim 1 , in which the index of soot emissions is the quantity of soot particles discharged from the combustor per unit mass of fuel injected by both the pilot injectors and the main injectors.

3. The method of claim 1 , further comprising detecting the atmospheric condition using a measurement device, which comprises one or more of:

a humidity sensor configured to sense an atmospheric humidity;

an optical sensor configured to sense the formation, or not, of the condensation trail.

4. The method of claim 1 , further comprising inferring the atmospheric condition using location data in combination with one or more of:

satellite imagery;

weather forecasts.

5. The method of claim 1 , in which the determining step comprises:

identifying the atmospheric condition to the effect that the model indicates that reducing or increasing the optical depth of the condensation trail produced by the engine on the basis of a time-integrated effect of a persistent condensation trail over the lifespan of the persistent condensation trail given a current atmospheric condition and a predicted future atmospheric condition would reduce the net warming impact of the persistent condensation trail.

6. A method of operating a staged combustion system in an aircraft gas turbine engine, comprising:

providing pilot injectors for rich combustion and main injectors for lean combustion in said staged combustion system;

identifying an atmospheric condition;

determining a ratio of pilot fuel flow rate for the pilot injectors to main fuel flow rate for the main injectors in response to the atmospheric condition;

wherein the determining step comprises:

identifying the atmospheric condition to the effect that a model indicates that reducing or increasing an optical depth of a condensation trail produced by the engine on the basis of a time-integrated effect of a persistent condensation trail over the lifespan of the persistent condensation trail given a current atmospheric condition and a predicted future atmospheric condition would reduce the net warming impact of the persistent condensation trail; and

injecting fuel by the pilot injectors and the main injectors in accordance with said ratio to control the index of soot emissions caused by combustion of fuel therein.

7. The method of claim 6 , in which the index of soot emissions is the quantity of soot particles discharged from the combustor per unit mass of fuel injected by both the pilot injectors and the main injectors.

8. The method of claim 6 , further comprising detecting the atmospheric condition using a measurement device, which comprises one or more of:

a humidity sensor configured to sense an atmospheric humidity;

an optical sensor configured to sense the formation, or not, of the condensation trail.

9. The method of claim 6 , further comprising inferring the atmospheric condition using location data in combination with one or more of:

satellite imagery;

weather forecasts.

10. A gas turbine engine for an aircraft, comprising:

a staged combustion system having pilot injectors and main injectors;

a fuel metering system configured to control fuel flow to the pilot injectors and the main injectors;

a fuel system controller configured to: identify an atmospheric condition; determine a ratio of pilot fuel flow rate for the pilot injectors to main fuel flow rate for the main injectors in response to the atmospheric condition;

wherein the fuel system controller is configured to determine the ratio of pilot fuel flow rate to main fuel flow rate by:

identifying the atmospheric condition to the effect that reducing or increasing an optical depth of a condensation trail produced by the engine on the basis of a time-integrated effect of a persistent condensation trail over the lifespan of the persistent condensation trail given a current atmospheric condition and a predicted future atmospheric condition would reduce the net warming impact of the persistent condensation trail; and

inject fuel by the pilot injectors and the main injectors in accordance with said ratio to control an index of soot emissions caused by combustion of the fuel therein.

11. The gas turbine engine of claim 10 , in which the index of soot emissions is the quantity of soot particles discharged from the combustor per unit mass of fuel injected by both the pilot injectors and the main injectors.

12. The gas turbine engine of claim 10 , further comprising a measurement device configured to detect the atmospheric condition, the measurement device comprising one or more of:

a hygrometer configured to sense an atmospheric humidity;

an optical sensor configured to sense the formation, or not, of the condensation trail.

13. The gas turbine engine of claim 10 , in which the fuel system controller is configured to infer the atmospheric condition from one or more of:

satellite imagery;

weather forecasts.