Lubricating composition comprising a diester

1. A lubricating composition of grade according to the SAE J300 classification defined by formula (X)W(Y), wherein X represents 0 or 5; and Y represents an integer ranging from 4 to 20; said composition comprising at least one diester of formula (I):

R a —C(O)—O—([C(R) 2 ] n —O) s —C(O)—R b    (I)

wherein:

R independently represent a hydrogen atom or a linear or branched (C 1 -C 5 )alkyl;

s is 1, 2, 3, 4, 5 or 6;

n is 1, 2 or 3; it being understood that, when s is different from 1, n may be the same or different; and

R a and R b , which may be identical or different, independently represent saturated or unsaturated, linear or branched hydrocarbon groups having a linear chain of 6 to 18 carbon atoms;

provided that, when s is 2 and n, which are identical, are 2, at least one of the groups R represents a linear or branched (C 1 -C 5 )alkyl group; and

provided that, when s is 1 and n is 3, at least one of the groups R bonded to the carbon in the beta position of the oxygen atoms of the ester functions represents a hydrogen atom.

2. The composition as claimed in claim 1 , wherein R a and R b , which may be identical or different, have a linear sequence of 7 to 14 carbon atoms.

3. The composition as claimed in claim 1 , wherein R a and R b , which may be identical or different, represent C 6 to C 18 linear alkyl groups.

4. The composition as claimed in claim 1 , wherein R a and R b both represent n-octyl or n-undecyl groups.

5. The composition as claimed in claim 1 , wherein the diester is of the following formula (I′)

R a —C(O)—O—[C(R) 2 ] n —O)—([C(R′) 2 ] m —O) s-1 —C(O)—R b    (I′)

wherein:

R and R′ independently represent a hydrogen atom or a linear or branched (C 1 -C 5 )alkyl group;

s is 1, 2 or 3;

n is 2;

m is 2;

R a and R b , which may be identical or different, independently represent saturated or unsaturated, linear or branched hydrocarbon groups having a linear chain of 6 to 18 carbon atoms;

provided that, when s is 2, at least one of the groups R or R′ represents a linear or branched (C 1 -C 5 )alkyl group.

6. The composition as claimed in claim 5 , wherein the diester is of formula (I′) wherein:

s is 2,

one of the groups R represents a linear or branched (C 1 -C 5 )alkyl group; and

one of the groups R′ represents a linear or branched (C 1 -C 5 )alkyl group; the other groups R and R′ representing hydrogen atoms.

7. The composition as claimed in claim 5 , wherein:

s is equal to 1;

one of the groups R represents a linear or branched (C 1 -C 5 )alkyl group, the others representing hydrogen atoms.

8. The composition as claimed in claim 1 , wherein the diester is obtained by esterification reaction between a monopropylene or polypropylene glycol; and one or more carboxylic acids R a —COOH and R b —COOH.

9. The composition as claimed in claim 1 , said composition comprising from 1 to 30 wt % diester(s) of formula (I) based on the total weight of the composition.

10. The composition as claimed in claim 1 , said composition comprising one or more base oils selected from the oils of Groups II, III and IV of the API classification.

11. The composition as claimed in claim 1 , comprising one or more additives selected from friction modifying additives, anti-wear additives, extreme pressure additives, detergent additives, antioxidant additives, viscosity index improvers, pour point depressants, dispersing agents, antifoam agents, thickeners, and mixtures thereof.

12. The composition as claimed in claim 1 , comprising at least one friction-modifying additive.

13. The composition as claimed in claim 1 , of grade according to the SAE J300 classification selected from 0W4, 0W8, 0W12, 0W16, 0W20, 5W4, 5W8, 5W12, 5W16 and 5W20.

14. A method for lubricating an engine using a lubricating composition of grade according to the SAE J300 classification defined by formula (X)W(Y), wherein X represents 0 or 5; and Y represents an integer ranging from 4 to 20; said composition comprising at least one diester of formula (I):

R a —C(O)—O—([C(R) 2 ] n —O) s —C(O)—R b    (I)

wherein:

R independently represent a hydrogen atom or a linear or branched (C 1 -C 5 )alkyl group;

s is 1, 2, 3, 4, 5 or 6;

n is 1, 2 or 3; it being understood that, when s is different from 1, n may be the same or different; and

R a and R b , which may be identical or different, independently represent saturated or unsaturated, linear or branched hydrocarbon groups having a linear chain of 6 to 18 carbon atoms;

provided that, when s is 2 and n, which are identical, are 2, at least one of the groups R represents a linear or branched (C 1 -C 5 )alkyl group; and

provided that, when s is 1 and n is 3, at least one of the groups R bonded to the carbon in the beta position of the oxygen atoms of the ester functions represents a hydrogen atom.

15. A method for reducing the fuel consumption of the engine, using a lubricating composition of grade according to the SAE J300 classification defined by the formula (X)W(Y), wherein X represents 0 or 5, and Y represents an integer ranging from 4 to 20 and dedicated to an engine, comprising as additive a diester of formula (I)

R a —C(O)—O—([C(R) 2 ] n —O) s —C(O)—R b    (I)

wherein:

R independently represent a hydrogen atom or a linear or branched (C 1 -C 5 )alkyl group;

s is 1, 2, 3, 4, 5 or 6;

n is 1, 2 or 3; it being understood that, when s is different from 1, n may be the same or different; and

R a and R b , which may be identical or different, independently represent saturated or unsaturated, linear or branched hydrocarbon groups having a linear chain of 6 to 18 carbon atoms;

provided that, when s is 2 and n, which are identical, are 2, at least one of the groups R represents a linear or branched (C 1 -C 5 )alkyl group; and

provided that, when s is 1 and n is 3, at least one of the groups R bonded to the carbon in the beta position of the oxygen atoms of the ester functions represents a hydrogen atom.

16. A method for improving engine cleanliness, using a lubricating composition of grade according to the SAE J300 classification defined by the formula (X)W(Y), wherein X represents 0 or 5, and Y represents an integer ranging from 4 to 20, and dedicated to an engine, comprising as additive a diester of formula (I)

R a —C(O)—O—([C(R) 2 ] n —O) s —C(O)—R b    (I)

wherein:

R independently represent a hydrogen atom or a linear or branched (C 1 -C 5 )alkyl group;

s is 1, 2, 3, 4, 5 or 6;

n is 1, 2 or 3; it being understood that, when s is different from 1, n may be the same or different; and

R a and R b , which may be identical or different, independently represent saturated or unsaturated, linear or branched hydrocarbon groups having a linear chain of 6 to 18 carbon atoms;

provided that, when s is 2 and n, which are identical, are 2, at least one of the groups R represents a linear or branched (C 1 -C 5 )alkyl group; and

provided that, when s is 1 and n is 3, at least one of the groups R bonded to the carbon in the beta position of the oxygen atoms of the ester functions represents a hydrogen atom.