Reduced-weight fuel system for gas turbine engine, gas turbine engine having a reduced-weight fuel system, and method of providing fuel to a gas turbine engine using a reduced-weight fuel system
A fuel system ( 10 ) for a gas turbine engine ( 18 ) having a shaft ( 30 ) and requiring fuel to be supplied at a first rate for engine start up, the fuel system ( 10 ) including: a main fuel pump ( 14 ) for providing fuel to the gas turbine engine ( 18 ), the main fuel pump ( 14 ) having an output varying with shaft speed, the output being at a windmilling rate when the engine ( 18 ) is windmilling, the windmilling rate being less than the first rate; and an auxiliary fuel pump ( 40 ) for providing fuel to the gas turbine engine ( 18 ) and having an auxiliary output rate greater than or equal to the difference between the first rate and the windmilling rate. A method of operating such a fuel system ( 10 ) is also disclosed.
1. A fuel system for a gas turbine engine having a shaft and requiring fuel to be supplied at a first rate for engine start up, said fuel system comprising:
a main fuel pump for providing fuel to the gas turbine engine, said main fuel pump having an output varying with shaft speed, said output comprising a windmilling rate when the engine is windmilling, the windmilling rate being less than said first rate; and
an electric auxiliary fuel pump for providing fuel to the gas turbine engine having an auxiliary output rate greater than or equal to a difference between said first rate and said windmilling rate, wherein, the main and auxiliary fuel pumps are arranged to cooperate to supply sufficient fuel for starting during windmill operation.
2. The fuel system of claim 1 wherein said auxiliary output rate is greater than or equal to said first rate.
3. The fuel system of claim 1 wherein said electric auxiliary fuel pump comprises a fixed displacement pump.
4. The fuel system of claim 1 wherein said electric auxiliary fuel pump has an output of less than about 200 lb/hour.
5. The fuel system of claim 1 wherein said main fuel pump has a maximum output rate and wherein said auxiliary rate is less than about 2.5 percent of said maximum output rate.
6. The fuel system of claim 5 wherein said main fuel pump has a maximum output pressure and said electric auxiliary fuel pump has a maximum output pressure less than about one fifth of said main fuel pump maximum output pressure.
7. The fuel system of claim 1 wherein said windmilling rate is less than about half of said first rate.
8. A gas turbine engine comprising:
a rotatable shaft;
a plurality of vanes connected to the rotatable shaft;
a combustion chamber;
a main fuel pump providing fuel to the combustion chamber at a rate related to a rotation speed of the rotatable shaft;
said gas turbine engine requiring fuel to be supplied to the combustion chamber at a first rate for combustion to staff;
said main fuel pump supplying fuel to said combustion chamber at a second rate less than said first rate when said gas turbine engine is windmilling; and
an electric auxiliary fuel pump for providing fuel to the gas turbine engine at a third rate, said third rate being greater than or equal to the difference between said first rate and said second rate.
9. The gas turbine engine of claim 8 wherein said third rate is greater than or equal to said first rate, wherein, the main and auxiliary fuel pumps are arranged to cooperate to supply sufficient fuel for starting during windmill operation.
10. The fuel system of claim 8 wherein said electric auxiliary fuel pump comprises a fixed displacement pump.
11. The fuel system of claim 8 wherein said electric auxiliary fuel pump has an output of less than about 200 lb/hour.
12. The fuel system of claim 8 wherein said main fuel pump has a maximum output rate and wherein said auxiliary rate is less than about 2.5 percent of said maximum output rate.