Aircraft fuel system for gas turbine engine
A propulsion system for an aircraft includes a gas turbine engine and a fuel tank, wherein the fuel includes at least a proportion of a sustainable aviation fuel—SAF—having a density between 90% and 98% of the density, ρ K , of kerosene and a calorific value between 101% and 105% the calorific value CV K , of kerosene. The engine includes a combustor; and a fuel pump arranged to supply a fuel thereto at an energy flow rate, C, the pump being arranged to output fuel at a volumetric flow rate, Q, the percentage of fuel passing through the pump not provided to the combustor being referred to as a spill percentage. The fuel include X % SAF, where X % is in the range from 5% to 100%, and has a density, ρ F , and a calorific value CV F . The propulsion system is arranged so: the fuel-change spill ratio, R s , of: R s = spill percentage at cruise using kerosene spill percentage at cruise using the fuel with X % S A F is equal to: Q - ( C / ( CV K × ρ K ) ) Q - ( C / ( CV F × ρ F ) ) .
1. A propulsion system for an aircraft comprising a gas turbine engine and at least one fuel tank arranged to store fuel to power the gas turbine engine, wherein the stored fuel comprises at least a proportion of a sustainable aviation fuel—SAF—having a density of between 90% and 98% of the density, ρ K , of kerosene and a calorific value of between 101% and 105% the calorific value CV K , of kerosene,
the gas turbine engine comprising:
a combustor; and
a fuel pump arranged to supply a fuel to the combustor at an energy flow rate, C, the fuel pump having an inlet arranged to receive fuel from the at least one fuel tank and an outlet arranged to output fuel at a pump output volumetric flow rate, Q, a proportion of the output fuel being provided to the combustor and a remaining portion of the fuel being recirculated, a percentage of the fuel passing through the pump which is not provided to the combustor being referred to as a spill percentage, and wherein the fuel supplied to the pump comprises X % SAF, where X % is in the range from 5% to 100%, with any remainder of the fuel being kerosene, and has a density, ρ F , and a calorific value CV F
and wherein the propulsion system is arranged such that:
the fuel-change spill ratio, R s , of:
R
s
=
spill
percentage
at
cruise
using
kerosene
spill
percentage
at
cruise
using
the
fuel
with
X
%
S
A
F
is equal to:
Q
-
C
/
(
C
V
K
×
ρ
K
)
Q
-
C
/
(
C
V
F
×
ρ
F
)
;
and
R s is less than or equal to 1.04.
2. The propulsion system of claim 1 , wherein the gas turbine engine has a maximum take-off thrust in the range from 400 kN to 500 kN, and is arranged such that R s is in the following range at cruise:
Q
-
(
Q
-
5
.
8
0
)
/
(
0.014
×
C
V
K
×
ρ
K
)
Q
-
(
Q
-
6
.
3
1
)
/
(
0.014
×
C
V
F
×
ρ
F
)
≤
R
s
≤
(
Q
-
(
Q
-
6
.
3
1
)
/
(
0.014
×
CV
K
×
ρ
K
)
Q
-
(
Q
-
5
.
8
0
)
/
(
0.014
×
C
V
F
×
ρ
F
)
)
where Q is measured in litres per second, CV in MJ/kg, and ρ in kg per litre.
3. The propulsion system of claim 1 , wherein the gas turbine engine has a maximum take-off thrust in the range from 300 kN to 350 kN, and is arranged such that R s is in the following range at cruise:
Q
-
(
Q
-
4.37
)
/
(
0.027
×
CV
K
×
ρ
K
)
Q
-
(
Q
-
4.87
)
/
(
0.027
×
CV
F
×
ρ
F
)
≤
R
s
≤
(
Q
-
(
Q
-
4.87
)
/
(
0.027
×
CV
K
×
ρ
K
)
Q
-
(
Q
-
4.37
)
/
(
0.027
×
CV
F
×
ρ
F
)
)
where Q is measured in litres per second, CV in MJ/kg, and ρ in kg per litre.
4. The propulsion system of claim 1 , wherein the gas turbine engine is arranged such that R s ≥1.003.
5. The propulsion system of claim 1 , wherein the gas turbine engine is arranged such that R s ≥1.014.
6. The propulsion system of claim 1 , wherein R s is varied based on at least one of ambient temperature, altitude, and stage of cruise.
7. The propulsion system of claim 6 , wherein R s decreases by less than 0.15% between the beginning and end of cruise, for a constant temperature and altitude.
8. The propulsion system of claim 6 , wherein R s decreases by at least 0.11% when altitude increases by at least 600 m.
9. A propulsion system for an aircraft comprising a gas turbine engine and at least one fuel tank arranged to store fuel to power the gas turbine engine, wherein the stored fuel comprises at least a proportion of a sustainable aviation fuel—SAF—having a density of between 90% and 98% of the density, ρ K , of kerosene and a calorific value of between 101% and 105% the calorific value CV K , of kerosene,
the gas turbine engine comprising:
a combustor; and
a fuel pump arranged to supply a fuel to the combustor, the fuel pump having an inlet arranged to receive fuel from the at least one fuel tank, the fuel supplied to the pump comprising X % SAF, with any remainder of the fuel being kerosene, and an outlet arranged to output the fuel, a proportion of the output fuel being provided to the combustor and a remaining portion of the fuel being recirculated, a percentage of the fuel passing through the pump which is not provided to the combustor being referred to as a spill percentage,
and wherein the propulsion system is arranged such that:
the fuel-change spill ratio, R s , of:
R
s
=
spill
percentage
at
cruise
using
kerosene
spill
percentage
at
cruise
using
a
fuel
with
X
%
S
A
F
,
where X % is at least 30%, is greater than or equal to 1.003.
10. The propulsion system of claim 9 , wherein
R
s
≥
1
+
X
1
0
0
0
0
.
11. The propulsion system of claim 9 , wherein X % is 50%, and wherein the fuel-change spill ratio is at least 1.0066.
12. The propulsion system of claim 9 , wherein X % is 100%, such that the fuel is pure SAF, and wherein the fuel-change spill ratio is at least 1.0138.
13. The propulsion system of claim 9 , wherein R s varies based on at least one of ambient temperature, altitude, and stage of cruise.
14. The propulsion system of claim 13 , wherein R s decreases by less than 0.15% between the beginning and end of cruise, for a constant temperature and altitude.
15. The propulsion system of claim 13 , wherein R s decreases by at least 0.11% when altitude increases by at least 600 m.
16. The propulsion system of claim 9 , wherein the gas turbine engine is arranged such that R s ≤1.04.
17. The propulsion system of claim 9 , wherein the gas turbine engine is arranged such that R s ≥1.003.
18. The propulsion system of claim 9 , wherein the gas turbine engine is arranged such that R s ≥1.014.
19. A method of operating an aircraft comprising a propulsion system, the propulsion system comprising a gas turbine engine, and at least one fuel tank arranged to store fuel to power the gas turbine engine, wherein the stored fuel comprises at least a proportion of a sustainable aviation fuel—SAF—having a density of between 90% and 98% of the density, ρ K , of kerosene and a calorific value of between 101% and 105% the calorific value CV K , of kerosene, the gas turbine engine comprising a fuel pump arranged to supply a fuel from at least one fuel tank to a combustor of the gas turbine engine at an energy flow rate, C, the fuel pump having an inlet arranged to receive fuel from the at least one fuel tank and an outlet arranged to output fuel at a pump output volumetric flow rate, Q, a proportion of the output fuel being provided to the combustor and a remaining portion of the fuel being recirculated, a percentage of the fuel passing through the pump which is not provided to the combustor being referred to as a spill percentage, the method comprising:
supplying a fuel from at least one fuel tank to the gas turbine engine, the fuel supplied to the gas turbine engine comprising X % SAF, where X % is in the range from 5% to 100%, with any remainder of the fuel being kerosene, and wherein the fuel has a density, ρ F , and a calorific value CV F ; and
controlling the propulsion system such that:
the fuel-change spill ratio, R s , of:
R
s
=
spill
percentage
at
cruise
using
kerosene
spill
percentage
at
cruise
using
the
fuel
with
X
%
S
A
F
is equal to:
Q
-
(
C
/
(
C
V
K
×
ρ
K
)
)
Q
-
(
C
/
(
C
V
F
×
ρ
F
)
)
;
and
R s is less than or equal to 1.04.