Fuel compositions

1. A gas oil fuel composition, the composition comprising:

A petroleum derived gas oil,

a Fischer-Tropsch derived fuel consisting of a Fischer-Tropsch derived gas oil, and

at least 2% by weight of a compound according to formula (I):

wherein:

R 1 to R 4 are each independently hydrogen or a C 1-10 alkyl group, where such alkyl groups may be the same as or different from one another; and

X is a nitrogen- or oxygen-containing group.

2. A method of reducing the cetane number of a gas oil fuel composition, said method comprising adding at least 2% by weight of a compound according to formula (I):

wherein:

R 1 to R 4 are each independently hydrogen or a C 1-10 alkyl group, where such alkyl groups may be the same as or different from one another; and

X is a nitrogen- or oxygen-containing group,

to a fuel composition comprising a petroleum derived gas oil and a Fischer-Tropsch derived fuel consisting of a Fischer-Tropsch derived gas oil.

3. A process for the preparation of a gas oil fuel composition, which process comprises blending at least 2% by weight of a compound according to formula (I):

wherein:

R 1 to R 4 are each independently hydrogen or a C 1-10 alkyl group, where such alkyl groups may be the same as or different from one another; and

X is a nitrogen- or oxygen-containing group,

at least one Fischer-Tropsch derived fuel component consisting of a Fischer-Tropsch derived gas oil,

and at least one petroleum derived gas oil fuel component.

4. A method of operating a fuel consuming system, the method comprising the step of reducing the cetane number of a fuel composition comprising a petroleum derived gas oil and a Fischer-Tropsch derived fuel component consisting of a Fischer-Tropsch derived gas oil before operating the fuel consuming system by adding at least 2% by weight of a compound according to formula (I):

wherein:

R 1 to R 4 are each independently hydrogen or a C 1-10 alkyl group, where such alkyl groups may be the same as or different from one another; and

X is a nitrogen- or oxygen-containing group;

to said fuel composition, and then introducing into the system said fuel composition.

5. The composition of claim 1 wherein said petroleum derived gas oil fuel composition comprises at least one base fuel.

6. The composition of claim 5 wherein said at least one base fuel comprises a diesel base fuel.

7. The composition of claim 1 wherein said compound according to formula (I) is 1,2,3,4-tetrahydroquinoline.

8. The composition of claim 1 , wherein the Fischer-Tropsch derived fuel has a sulfur content of less than 5 mg/kg.

9. The composition of claim 1 wherein the gas oil fuel composition has a sulfur content of less than 15 ppmw.

10. The method of claim 2 , wherein the Fischer-Tropsch derived fuel has a sulfur content of less than 5 mg/kg.

11. The method of claim 2 , wherein the gas oil fuel composition has a sulfur content of less than 15 ppmw.

12. The process of claim 3 , wherein the Fischer-Tropsch derived fuel has a sulfur content of less than 5 mg/kg.

13. The process of claim 3 , wherein the gas oil fuel composition has a sulfur content of less than 15 ppmw.

14. The method of claim 4 , wherein the Fischer-Tropsch derived fuel has a sulfur content of less than 5 mg/kg.

15. The method of claim 4 , wherein the gas oil fuel composition has a sulfur content of less than 15 ppmw.

16. The composition of claim 1 , wherein the compound according to formula (I) is present in an amount between 2% and 5% by weight.

17. The method of claim 2 , wherein the method further comprises the step of adding between 2% and 5% by weight of the compound according to formula (I).

18. The process of claim 3 , wherein the process further comprises the step of blending between 2% and 5% of a compound according to formula (I).

19. The method of operating a fuel consuming system according to claim 4 , the method adding between 2% and 5% of the compound according to formula (I).