Fuel delivery

A method of operating a gas turbine engine for an aircraft, the gas turbine engine comprising a staged combustion system having pilot fuel injectors and main fuel injectors, the staged combustion system being operable in a pilot-only range of operation and a pilot-and-main range of operation, the method comprising:

• regulating fuel delivery to the pilot and main fuel injectors from a first fuel source containing a first fuel having a first fuel characteristic and a second fuel source containing a second fuel having a second fuel characteristic, the second fuel characteristic being different from the first fuel characteristic, wherein
• the regulating of the fuel delivery comprises delivering fuel to the pilot fuel injectors during at least part of the pilot-only range of operation having a different fuel characteristic from fuel delivered to one or both of the pilot and main fuel injectors during at least part of the pilot-and-main range of operation,
• the first fuel is a sustainable aviation fuel (SAF), the first fuel characteristic is associated with a level of nonvolatile particulate matter (nvPM) production which is less than that of the second fuel characteristic, and the fuel delivered to the pilot fuel injectors during at least part of the pilot-only range of operation is associated with a level of nvPM production which is less than that of the fuel delivered to one or both of the pilot and main fuel injectors during at least part of the pilot-and-main range of operation,
• the regulating of the fuel delivery comprises delivering fuel from the first fuel source to the pilot fuel injectors during operation in at least part of the pilot-only range of operation, and delivering fuel from the second fuel source to the pilot fuel injectors during the pilot-and-main range of operation
• the regulating of the fuel delivery comprises switching delivery of fuel to the pilot fuel injectors between fuel from the first fuel source and fuel from the second fuel source at one or more operating points within, or at a boundary of, the pilot-only range of operation, according to a mode signal from an electronic engine controller indicative of an operation range change of the staged combustion system,
• the regulating of the fuel delivery further comprises switching delivery of fuel to the pilot fuel injectors between fuel from the first fuel source and fuel from the second fuel source at a threshold point within the pilot-only range of operation, and
• the threshold point is:
• a) a threshold fuel flow rate at which the production of nvPM by the gas turbine engine passes a threshold amount of the nvPM produced by the gas turbine engine during operation in which the pilot fuel injectors are delivered fuel having the second fuel characteristic;
• and/or
• b) a predefined threshold fuel flow rate which is less than the fuel flow rate at the boundary between pilot-only operation and pilot-and-main operation by a predefined amount.

A gas turbine engine for an aircraft, comprising:

• a staged combustion system having pilot fuel injectors and main fuel injectors, the staged combustion system being operable in a pilot-only range of operation and a pilot-and-main range of operation;
• a fuel delivery regulator arranged to control delivery of fuel to the pilot and main fuel injectors, the fuel delivery regulator being arranged to receive fuel from a first fuel source containing a first fuel having a first fuel characteristic and a second fuel source containing a second fuel having a second fuel characteristic, the second fuel characteristic being different from the first fuel characteristic;
• and
• an electronic engine controller configured to control the fuel delivery regulator, wherein
• the fuel delivery regulator is arranged to deliver fuel to the pilot fuel injectors during at least part of the pilot-only range of operation having a different fuel characteristic from fuel delivered to one or both of the pilot and main fuel injectors during at least part of the pilot-and-main range of operation,
• the first fuel is a sustainable aviation fuel (SAF), the first fuel characteristic is associated with a level of nonvolatile particulate matter (nvPM) production which is less than that of the second fuel characteristic, and the fuel delivered to the pilot fuel injectors during at least part of the pilot-only range of operation is associated with a level of nvPM production which is less than that of the fuel delivered to one or both of the pilot and main fuel injectors during at least part of the pilot-and-main range of operation,
• the fuel delivery regulator is arranged to deliver fuel from the first fuel source to the pilot fuel injectors during operation in at least part of the pilot-only range of operation, and deliver fuel from the second fuel source to the pilot fuel injectors during the pilot-and-main range of operation,
• the fuel delivery regulator is arranged to switch delivery of fuel to the pilot fuel injectors between fuel from the first fuel source and fuel from the second fuel source at one or more operating points within, or at a boundary of, the pilot-only range of operation, according to a mode signal from the electronic engine controller indicative of an operation range change of the staged combustion system,
• the fuel delivery regulator comprises a fuel blender arranged to receive a supply of fuel from both the first and second fuel sources and output fuel from the first fuel source, fuel from the second fuel source, or a blend thereof,
• the fuel blender is arranged to deliver fuel to the pilot fuel injectors, and
• the fuel blender is arranged to deliver the blend of fuel from the first fuel source and fuel from the second fuel source to the pilot fuel injectors during at least part of the pilot-only range of operation.