US20230383210A1

US20230383210A1 - Use of dialkylene glycol ester to reduce friction in vehicles with hybrid engines

Info

Publication number: US20230383210A1
Application number: US18/032,599
Authority: US
Inventors: Gael Robineau; Nicolas BORD; Paula USSA ALDANA
Original assignee: TotalEnergies Onetech SAS
Current assignee: TotalEnergies Onetech SAS
Priority date: 2020-10-20
Filing date: 2021-10-19
Publication date: 2023-11-30
Also published as: FR3115291B1; CN116472330A; FR3115291A1; WO2022084320A1; EP4232534A1

Abstract

The present invention relates to the use of a diester of formula (I) in a lubricant composition for a vehicle equipped with a hybrid engine, to reduce friction when the engine is used at low speed and/or when cold.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. National Phase Application under 35 U.S.C. § 371 of International Patent Application No. PCT/EP2021/078956 filed Oct. 19, 2021, which claims priority of French Patent Application No. 20 10736 filed Oct. 20, 2020. The entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to the use of dialkylene glycol ester in a lubricant composition for vehicles equipped with a hybrid engine, to reduce the coefficient of friction when using the engine at low speed and/or when cold.

BACKGROUND

Improving the performance of lubricants is a constant area of interest. In particular, to meet increasing environmental requirements, it is increasingly sought to improve the performance of lubricants and in particular those for new generation vehicles, which include hybrid vehicles.
However, few compositions have a particular focus on use thereof in a hybrid vehicle. Hybrid vehicles have particular constraints and in particular when used at low speed or when cold i.e. over specific periods which could be termed «start of cycle» periods, after vehicle start-up. Over these cycles, the temperatures of the oils used in a hybrid vehicle are low, lower than the temperatures in a vehicle with an internal combustion engine, and it would be an essential advantage to propose a formulation exhibiting better performance during these particular periods of use.
Against all expectations, the inventors have found that it is possible to gain access to lubricant compositions having increased efficiency in Fuel-Eco terms for cycles of use called low speed or cold cycles.

SUMMARY

It is therefore one object to the present invention to propose an additive for a lubricant composition for a vehicle equipped with a hybrid engine, to reduce the coefficient of friction when the engine is used at low speed.
A further object of the invention is also to propose an additive for a lubricant composition for a vehicle equipped with a hybrid engine, to reduce the coefficient of friction during cold cycle use of the engine.
Other objects will become apparent on reading the following description of the invention.

DETAILED DESCRIPTION

These objects are achieved with the present invention which relates to the use of a diester of formula (I) in a lubricant composition for vehicles equipped with a hybrid engine, to reduce the coefficient of friction when the engine is used at low speed and/or when cold.
R^a—C(O)—O—([C(R)₂]_n—O)_s—C(O)—R ^b (1)

- where:
- R, each independently, are a hydrogen atom or a linear or branched alkyl group having 1 to 5 carbon atoms, in particular a methyl, ethyl or propyl group, preferably methyl;
- s is 1 or 2;
- n is 1, 2 or 3, on the understanding that when s differs from 1, the n values can be the same or different;
- R^aand R^b, the same or different, are each independently linear or branched, saturated or unsaturated hydrocarbon groups, having a linear sequence of 6 to 18 carbon atoms;
- provided that, when s is 2, and n all the same are 2, at least one of the R groups is a linear or branched alkyl group having 1 to 5 carbon atoms; and provided that when s is 1 and n is 3, at least one of the R groups linked to the carbon at beta position of the oxygen atoms of the ester functions is a hydrogen atom.

The present invention also relates to a method for reducing the coefficient of friction when a hybrid engine is used at low speed and/or when cold , comprising the lubricating of said engine with a lubricant composition comprising at least one base oil and a diester of formula (I)
R^a—C(O)—O—([C(R)₂]_n—O)_s—C(O)—R^b (I)

- where:
- R, each independently, are a hydrogen atom or a linear or branched alkyl group having 1 to 5 carbon atoms, in particular a methyl, ethyl or propyl group, preferably methyl;
- s is 1 or 2;
- n is 1, 2 or 3, on the understanding that when s differs from 1, the n values can be the same or different;
- R^aand R^b, the same or different, are each independently linear or branched, saturated or unsaturated hydrocarbon groups having a linear sequence of 6 to 18 carbon atoms;
- provided that when s is 2 and n all the same are 2, at least one of the R groups is a linear or branched alkyl group having 1 to 5 carbon atoms; and
- provided that when s is 1 and n is 3, at least one of the R groups linked to the carbon at beta position of the oxygen atoms of the ester functions is a hydrogen atom.

In the present invention, by «low speed» it is meant a specific engine speed. Engine speed designates the speed of rotation of the engine and is expressed in revolutions per minute. Low speed according to the invention is characterized by a number of revolutions per minute (rpm) of between 600 and 2000 rpm, preferably between 900 and 1500 rpm.
In the present invention, by «when cold» it is meant use of the engines at start-up and during phase 1 of the WLTC cycle, at which time the engine is at a temperature lower than 50° C., preferably between 20 and 50° C., preferably between 25 and 40° C.
The reduction in the coefficient of friction is to be construed as a reduction in comparison with the observation which would be made with a lubricant composition not comprising the diester of the invention.
Preferably, in the diester of formula (I) of the invention, when s differs from 1, all the n values are the same.
Preferably, in the diester of formula (I) of the invention, n is 2 or 3, preferably 2.
Preferably, in the diester of formula (I) of the invention, at least one of the R groups is a linear or branched alkyl group having 1 to 5 carbon atoms, preferably 1 to 4 carbon atoms, and more preferably is methyl, ethyl or propyl, advantageously methyl.
In one particular embodiment, the diester of formula (I) of the invention is a diester of 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′)

- where
- R and R′, each independently, are a hydrogen atom or a linear or branched alkyl group having 1 to 5 carbon atoms, preferably a methyl, ethyl or propyl group, preferably a methyl group;
- s is 1 or 2;
- n is 2;
- m is 2;
- Ra and Rb, the same or different, are each independently linear or branched, saturated or unsaturated hydrocarbon groups having a linear sequence of 6 to 18 carbon atoms;
- provided that when s is 2, at least one of the groups R or R′ is a linear or branched alkyl group having 1 to 5 carbon atoms.

Advantageously, at least one of the groups R or R′ in the diester of formula (I′) is a linear or branched alkyl group having 1 to 5 carbon atoms, preferably 1 to 4 carbon atoms, preferably methyl, ethyl or propyl, advantageously methyl.
Preferably, in the compounds of formula (I) and (I′), Ra and Rb have a linear sequence of 7 to 14 carbon atoms, preferably 8 to 12 carbon atoms, more preferably 8 to 11 carbon atoms, in particular 9 or 11 carbon atoms.
In one particular embodiment, in formulas (I) or (I′), s is 2.
The diester of the invention is preferably a compound of formula (I′a):
R^a—C(O)—O—([C(R)₂]_n—O)—([C(R′)₂]_m—O)—C(O)—R^b (I′a)

- where:
  - R and R′ are each independently a hydrogen atom or a linear or branched alkyl group having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, preferably methyl;
  - n is 2;
  - m is 2;
  - Ra and Rb, the same or different, are each independently linear or branched, saturated or unsaturated hydrocarbon groups having a linear sequence of 6 to 18 carbon atoms;
- provided that at least one of the groups R or R′ is a linear or branched alkyl group having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, preferably methyl.

Preferably, at least one of the R groups is a linear or branched alkyl group having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, preferably methyl; and at least one of the R′ groups is a linear or branched alkyl group having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, preferably methyl.
Further preferably, in the diester of formula (I′a) one of the R groups is a linear or branched alkyl group having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, preferably methyl; and one of the R′ groups is a linear or branched alkyl group having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, preferably methyl; the other R and R′ groups representing hydrogen atoms.
In one particular embodiment, the diester of the invention is a compound of formula (I″a)
R^a—C(O)—O—CHR¹—CHR²—O—CHR³—CHR⁴—O—C(O)—R^b (I″a)

- where:
  - one of the groups R¹and R²is a linear or branched alkyl group having 1 to 5 carbon atoms, the other being a hydrogen atom;
  - one of the groups R³and R⁴is a linear or branched alkyl group having 1 to 5 carbon atoms, the other being a hydrogen atom;
  - Ra and Rb, the same or different, are such as previously defined.

Preferably, in the compounds of formula (I″a):

- one of the groups R¹and R²is a methyl, ethyl or propyl group, preferably methyl, the other being a hydrogen atom;
- one of the groups R³and R⁴is a methyl, ethyl or propyl group, preferably methyl, the other being a hydrogen atom.

In another alternative embodiment, in the compounds of formula (I) or (I′), s is 1 and the diesters of the invention have the formula (I′b):
R^a—C(O)—O—([C(R)₂]_n—O)—C(O)—^b (I′b)

- where:
  - R, each independently, are a hydrogen atom or a linear or branched alkyl group having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, preferably methyl;
- n is 2;
- R^aand R^b, the same or different, are each independently linear or branched, saturated or unsaturated hydrocarbon groups having a linear sequence of 6 to 18 carbon atoms.

Preferably, in formula (I′b), at least one of the R groups is a linear or branched alkyl group having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, preferably methyl.
Preferably, in formula (I′b), one of the R groups is a linear or branched alkyl group having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, preferably methyl, the others being hydrogen atoms.
In the present invention, it is to be understood that the expression «having x to y carbon atoms» also includes the limits x and y.
In the present invention, it is to be understood that by «linear sequence of x to y carbon atoms» it is meant a saturated or unsaturated carbon chain, preferably saturated, having x to y carbon atoms following after each other, any carbon atoms which may be present at the branches of the carbon chain not being taken into account in the number of carbon atoms (x-y) forming the linear sequence.
In one particular embodiment, in formulas (I), (I′), (I′a), (I″a) or (I′b), Ra and Rb the same or different are of plant, animal or petroleum origin.
In one particular embodiment, in formulas (I), (I′), (I′a), (I″a) or (I′b), Ra and Rb the same or different are saturated groups.
In one particular embodiment, in formulas (I), (I′), (I′a), (I″a) or (I′b), Ra and Rb the same or different are linear groups. In particular, Ra and Rb the same or different are saturated linear hydrocarbon groups having 6 to 18 carbon atoms, preferably 7 to 17 carbon atoms, more preferably 7 to 14 carbon atoms, further preferably 8 to 12 carbon atoms and in particular 9 or 12 carbon atoms.
In another preferred embodiment, in formulas (I), (I′), (I′a), (I″a) or (I′b), Ra and Rb the same or different are saturated linear alkyl groups having 6 to 18 carbon atoms, preferably 7 to 17 carbon atoms, more preferably 7 to 14 carbon atoms, further preferably 8 to 12 carbon atoms and in particular 9 or 12 carbon atoms.
Preferably, Ra and Rb are the same.
The diesters of formula (I) can be commercially available diesters or they can be prepared following the synthesis methods described in the literature and known to persons skilled in the art, in particular according to the methods described in WO201925446.
Preferably, the diester of formula (I) is added in a proportion of 1 to 40% by weight relative to the total weight of the lubricant composition, preferably 10 to 30 %.
The base oils used in the lubricant compositions of the invention can be oils of mineral or synthetic origin belonging to Groups I to V in the classes defined by the API classification (or the equivalents thereof in the ATIEL classification (Table 1), or mixtures thereof.

TABLE 1

Content of		Viscosity
saturated		Index
substances	Sulfur content	(VI)

Group I	<90%	>0.03%	80 ≤ VI < 120
Mineral oils
Group II	≥90%	≤0.03%	80 ≤ VI < 120
Hydrocracked oils
Group III	≥90%	≤0.03%	≥120
Hydro-isomerized
oils

Group IV	Polyalphaolefins (PAO)
Group V	Esters and other bases not included in Groups I to IV

The mineral base oils of the invention include any type of base oil obtained by atmospheric and vacuum distillation of crude petroleum, followed by refining operations such as solvent extraction, deasphalting, solvent dewaxing, hydrotreatment, hydrocracking, hydroisomerization and hydrofinishing.
Mixtures of synthetic and mineral oils can also be used.
The base oils of the lubricant composition of the invention can also be selected from among synthetic oils such as some esters of carboxylic acids and alcohols, and polyalphaolefins. The polyalphaolefins used as base oil are obtained for example from monomers having 4 to 32 carbon atoms, for example from octene or decene, and in which viscosity at 100° C. is between 1.5 and 15 mm2.s-1 according to standard ASTM D445. The molecular weight average thereof is generally between 250 et 3000 according to standard ASTM D5296.
The lubricant composition of the invention may comprise at least 50% by weight of base oil relative to the total weight of the composition. More advantageously, the lubricant composition of the invention comprises at least 60% by weight, or even at least 70% by weight of base oils relative to the total weight of the lubricant composition. More preferably, the lubricant composition of the invention comprises from 75 to 97% by weight of base oils relative to the total weight of the composition.
The composition of the invention may also comprise at least one additive.
Numerous additives can be used in the lubricant compositions of the invention.
The preferred additives for the lubricant composition of the invention are selected from among detergent additives, friction modifying additives differing from the molybdenum compounds defined above, extreme pressure additives, dispersants, pour point activators, defoaming agents, thickeners, and mixtures thereof.
Preferably, the lubricant compositions of the invention comprise at least one extreme pressure additive, or a mixture.
Antiwear additives and extreme pressure additives protect against surface frictions by forming a protective film adsorbed on surfaces.
There exists a wide variety of antiwear additives. Preferably, for the lubricant compositions of the invention, the antiwear additives are selected from among additives comprising phosphorus and sulfur such as alkylthiophosphate metals, in particular zinc alkylthiophosphate, and more specifically zinc dialkyldithiophosphate or ZnDTP. The preferred compounds have the formula Zn((SP(S)(OR)(OR′))2, where R and R′, the same or different, are each independently an alkyl group, preferably an alkyl group having 1 to 18 carbon atoms.
Amine phosphates are also antiwear additives that can be used in the lubricant compositions of the invention. However, the phosphorus atoms contributed by these additives may act as poison for the catalytic systems of motor vehicles since they generate ash. It is possible to minimise these effects by substituting part of the amine phosphates by additives not contributing phosphorus such as polysulfides for example, in particular sulfur- containing olefins.
Advantageously, the lubricant compositions of the invention may comprise from 0.01 to 6 % by weight, preferably from 0.05 to 4% by weight, more preferably from 0.1 to 2% by weight of antiwear and extreme pressure additives, relative to the total weight of the lubricant composition.
Advantageously, the lubricant compositions of the invention comprise from 0.01 to 6 by weight, preferably from 0.05 to 4% by weight, more preferably from 0.1 to 2% by weight of antiwear additives (or anti-wear compound) relative to the total weight of the lubricant composition.
Advantageously, the compositions of the invention may comprise at least friction modifying additive differing from the molybdenum compounds of the invention. The friction modifying additives can be selected in particular from among compounds contributing metal elements and ash-free compounds. Among those compounds contributing metal elements mention can be made of complexes of transition metals such as Mo, Sb, Sn, Fe, Cu, Zn in which the ligands can be hydrocarbon compounds comprising atoms of oxygen, nitrogen, sulfur or phosphorus. The ash-free friction modifying additives are generally of organic origin and can be selected from among the monoesters of fatty acids and polyols, alkoxylated amines, alkoxylated fatty amines, fatty epoxides, fatty epoxide borates, fatty amines or the esters of glycerol acid. In the invention, the fatty compounds comprising at least one hydrocarbon group have 10 to 24 carbon atoms.
Advantageously, the lubricant composition of the invention may comprise from 0.01 to 2% by weight, or from 0.01 to 5% by weight, preferably from 0.1 to 1.5% by weight or from 0.1 to 2% by weight of friction modifying additives differing from the molybdenum compounds of the invention, relative to the total weight of the lubricant composition.
Advantageously, the lubricant composition of the invention may comprise at least one antioxidant additive.
Antioxidant additives generally delay degradation of the lubricant composition. This degradation most often translates as the formation of a deposit, the presence of sludge or by an increase in the viscosity of the lubricant composition.
Antioxidant additives generally act as radical scavengers or hydroperoxide decomposers. Among the antioxidants routinely used, mention can be made of antioxidants of phenolic type, antioxidants of amine type, antioxidants containing sulfur and phosphorus. Some of these antioxidants, for example those comprising sulfur and phosphorus, may generate ash. Phenolic antioxidant additives can be ash-free or they can be in the form of neutral or basic salts. The antioxidant additives can particularly be selected from among sterically hindered phenols, sterically hindered phenol esters, sterically hindered phenols comprising a thioether bridge, diphenylamines, diphenylamines substituted by at least one C₁to C₁₂alkyl group, N,N′-dialkyl-aryl-diamines, and mixtures thereof.
Preferably, in the invention, the sterically hindered phenols are selected from among compounds comprising a phenol group in which at least one of the carbon atoms in the vicinity of the carbon atom carrying the alcohol function is substituted by at least one C₁to C₁₀alkyl group, preferably a C₁to C₆alkyl group, preferably a C₄alkyl group, preferably a tert-butyl group.
Amine compounds are another class of antioxidant additives which can be used, optionally in combination with phenolic antioxidant additives. Examples of amine compounds are aromatic amines, for example aromatic amines having the formula NRaRbRc where Ra is an optionally substituted aliphatic group or aromatic group, Rb is an optionally substituted aromatic group, Rc is a hydrogen atom, an alkyl group, an aryl group or a group of formula RdS(O)zRe where Rd is an alkylene or alkenylene group, Re is an alkyl group, an alkenyl group or aryl group and z is 0, 1 or 2.
Alkyl-phenols containing sulfur or the alkali or alkaline-earth metal salts thereof can also be used as antioxidant additives.
Other classes of antioxidant additives are copper-containing compounds, for example copper thio- ou dithio-phosphate, salts of copper and carboxylic acids, dithiocarbamates, sulfonates, phenates, copper acetylacetonates. The salts of copper I and II, the salts of succinic acid or anhydride can also be used.
The lubricant compositions of the invention may also comprise any type of antioxidant known to persons skilled in the art.
Advantageously, the lubricant composition comprises at least one ash-free antioxidant additive.
Also advantageously, the lubricant composition of the invention comprises from 0.1 to 2% by weight of at least one antioxidant additive, relative to the total weight of the composition.
The lubricant composition of the invention may also comprise at least one detergent additive.
Detergent additives generally allow a reduction in the formation of deposits on the surface of metal parts, by dissolving secondary products of oxidation and combustion.
The detergent additives able to be used in the lubricant compositions of the invention are generally known to persons skilled in the art. The detergent additives can be anionic compounds comprising a long lipophilic hydrocarbon chain and hydrophobic head. The associated cation can be a metal cation of an alkali or alkaline-earth metal.
The detergent additives are preferably selected from among the alkali or alkaline-earth metal salts of carboxylic acids, sulfonates, salicylates, naphthenates, and phenate salts. The alkali and alkaline-earth metals are preferably calcium, magnesium, sodium or barium.
These metal salts generally comprise the metal in stoichiometric amount or in excess, i.e. in an amount greater than the stoichiometric amount. They are then overbased detergents; the excess metal imparting the overbased nature to the detergent additive is generally in the form of an oil-insoluble metal, for example carbonate, hydroxide, oxalate, acetate, glutamate, preferably carbonate.
Advantageously, the lubricant composition of the invention may comprise from 0.5 to 8%, or from 2 to 4% by weight of overbased detergent additive relative to the total weight of the lubricant composition.
Also advantageously, the lubricant composition of the invention may also comprise a pour point depressant additive.
By slowing the formation of wax crystals, the pour point depressant additive generally improves the cold-start properties of the lubricant composition of the invention.
As examples of pour point depressant additives, mention can be made of alkyl polymethacrylates, polyacrylates, polyarylamides, polyalkylphenols, polyalkylnaphthalene, alkyl polystyrenes.
Advantageously, the lubricant composition of the invention may also comprise a dispersant.
Dispersants can be selected from among Mannich bases, succinimides, and derivatives thereof.
Also advantageously, the lubricant composition of the invention may comprise from to 10% by weight of dispersant relative to the total weight of the lubricant composition.
Advantageously, the lubricant composition of the invention may further comprise at least one other additional polymer improving the viscosity index. As examples of additional polymer improving the viscosity index, mention can be made of polymeric esters, the homopolymers or copolymers, whether or not hydrogenated, of styrene, butadiene, and isoprene, polymethacrylates (PMA). Also advantageously, the lubricant composition of the invention may comprise from 1 to 15% by weight of additive improving the viscosity index, relative to the total weight of the lubricant composition.
The lubricant composition of the invention may also comprise at least one thickening agent.
The lubricant composition of the invention may also comprise a defoaming agent or demulsifying agent.
Preferably, the lubricant composition of the invention further comprises at least one antiwear agent, in particular zinc-based, and particularly ZnDTP.
In the present invention, by hybrid motor vehicle or vehicles equipped with a hybrid engine, it is meant vehicles having recourse to two separate sources of energy for powering.
In particular, hybrid vehicles associate an internal combustion engine with an electric motor, said electric motor taking part in vehicle traction.
The invention also relates to a method for reducing friction when a hybrid engine is used at low speed and/or when cold, which comprises the addition of a diester of the invention to the lubricant composition of said engine.
The invention also relates to a method for reducing friction when a hybrid engine is used at low speed and/or when cold, which comprises the lubrication of said engine with a lubricant composition comprising a diester of the invention.
The present application is described below with the aid of nonlimiting examples.

Example 1: Preparation of Lubricant Compositions

The following lubricant compositions (CL: compositions of the invention, and CC: comparative compositions) were prepared following methods known to persons skilled in the art.

TABLE 2

CC1	CC2	CL1	CC3	CL2
(wt. %)	(wt. %)	(wt. %)	(wt. %)	(wt. %)

Additive package	12.7	12.7	12.7	12.7	12.7
MoDTC	0.5	0.5	0.5	0.5	0.5
PPD	0.2	0.2	0.2	0.2	0.2
Antioxidant	0.5	0.5	0.5	0.5	0.5
Viscosity	0.4	0.8	0.8	1.3	1.5
modifier
PAO	0	30	0	0	0
Ester of	0	0	30	0	20
formula (I)
Base oil	85.7	55.3	55.3	84.8	64.6
Grade	0W12	0W12	0W12	0W12	0W12

TABLE 3

CC4	CL3	CC5	CL4	CC6
(wt. %)	(wt. %)	(wt. %)	(wt. %)	(wt. %)

Additive package	13	13	12.6	12.6	12.6
MoDTC	0.5	0.5	0.5	0.5	0.5
PPD	0.2	0.2	0.2	0.2	0.2
Viscosity	1.3	4	4	4	4
modifier
PAO	0	0	0	0	20
Ester of	0	10	0	20	0
formula (I)
Base oil	85	72.3	82.7	62.7	62.7
Grade	0W12	0W12	0W12	0W12	0W12

TABLE 4

CC7	CL5	CC8
(wt. %)	(wt. %)	(wt. %)

Additive package	10.1	10.1	10.1
MoDTC	0.4	0.4	0.4
Viscosity modifier	6.7	6.7	5.8
Ester of formula (I)	0	20	0
Base oil	82.8	62.8	83.7
Grade	0W20	0W20	0W20

Example 2: Results of FMEP Tests

A FMEP test bed (Friction Mean Effective Pressure) was used to measure engine friction. The engine was driven by a generator and the torque to be supplied by this generator was measured (this torque is therefore the image of engine friction).
In this type of test, the engine does not therefore burn any fuel, it is just driven in exactly the same manner as in a vehicle engine braking situation.
The properties of the test bed in terms of engine speed and temperature were the following:

- Speed: 900*-6000 rpm
- Oil temperature: 35-110° C.**
- Water temperature: 37-90° C.**

4 thermal conditions were tested:

- Engine oil temperature=37° C./Temperature of coolant=37° C.
- Engine oil temperature=50° C./Temperature of coolant=50° C.
- Engine oil temperature=80° C./Temperature of coolant=80° C.
- Engine oil temperature=110° C./Temperature of coolant=90° C.

The sequences of tested engine speeds were the following:
900, 1450, 2000, 2500, 3000, 3500, 4000, 4500
For the test, complete mapping of engine friction was determined according to the different rotation speeds and different oil and water temperatures.
The friction mapping obtained at the end of the test was composed of 8 engine speeds and 4 temperatures.
The time required for this mapping was about 11 h.
The results are expressed as a gain in mean coefficient of friction.
This mapping was then compared with that of a so-called reference oil which was measured before and after the testing of the candidate oil to determine any drift from the test mean and integrate the same in the interpretation of results.
The composition of the so-called reference oil was as follows:

	TABLE 5

	PAO	10.00
	PAO	10.00
	MoDTC	0.50
	Pour point depressant	0.20
	Base oil	61.70
	Additive package	12.70
	Amine antioxidant	0.50
	Polymer improving the viscosity index	4.40

It is with reference to this oil that the percentages for gain in friction were calculated.
The results in terms of gain in friction compared with the reference oil are given in Tables 6 to 8 below.

TABLE 6

CC1 (%)	CC2 (%)	CL1 (%)	CC3 (%)	CL2 (%)

30° C.	1.42	1.06	3.38	1.20	3.35
50° C.	1.7	1.71	2.48	1.85	2.72

TABLE 7

CC4 (%)	CL3 (%)	CC5 (%)	CL4 (%)	CC6 (%)

30° C.	1.44	3.55	3.01	4.19	2.66
50° C.	1.27	2.93	2.67	2.88	2.25

TABLE 8

CC7 (%)	CL5 (%)	CC8 (%)

30° C.	3.89	6.25	0.2
50° C.	2.78	3.98	0.54

The results show the capability of the ester of the invention to improve gains in friction over temperatures of between 30° C. and 50° C.

Claims

1-7. (canceled)

8. A method for reducing friction when using a hybrid engine at low speed and/or when cold comprising the addition, to the lubricant composition of said engine, of a diester of formula (I)

R^a—C(O)—O—([C(R)₂]_a—O)_b—C(O)—R^b (I)

where:

R, each independently, are a hydrogen atom or a linear or branched alkyl group having 1 to 5 carbon atoms;

s is 1 or 2;

n is 1, 2 or 3, on the understanding that when s differs from 1, the n values can be the same or different;

R^aand R^b, the same or different, are each independently linear or branched, saturated or unsaturated hydrocarbon groups, having a linear sequence of 6 to 18 carbon atoms;

provided that when s is 2 and n all the same are 2, at least one of the R groups is a linear or branched alkyl group having 1 to 5 carbon atoms; and

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

9. The method according to claim 8, wherein the diester is a diester of formula (I), where when s differs from 1, all the n values are the same.

10. The method according to claim 8, wherein the diester is a diester of formula (I) where n is 2 or 3.

11. The method according to claim 8, wherein the diester is a diester of formula (I) where at least one of the R groups is a linear or branched alkyl group having 1 to 5 carbon atoms.

12. The method according to claim 8, wherein the diester of formula (I) is a diester of following formula (I′):

R^a—C(O)—O—([C(R)₂]_n—O)—([C(R′)₂]_m—O)_b-1—C(O)—R^b (I′)

where:

R and R′, each independently, are a hydrogen atom or a linear or branched alkyl group having 1 to 5 carbon atoms;

s is 1 or 2;

n is 2;

m is 2;

R^aand R^b, the same or different, are each independently linear or branched, saturated or unsaturated hydrocarbon groups having a linear sequence of 6 to 18 carbon atoms;

provided that when s is 2, at least one of the groups R or R′ is a linear or branched alkyl group having 1 to 5 carbon atoms.

13. The method to claim 8, wherein the diester is a compound of formula (I″a)

R^a—C(O)—O—CHR¹—CHR²—O—CHR³—CHR⁴—O—C(O)—R^b (I″a)

where:

one of the groups R¹and R²is a linear or branched alkyl group having 1 to 5 carbon atoms, the other being a hydrogen atom;

one of the groups R³and R⁴is a linear or branched alkyl group having 1 to 5 carbon atoms, the other being a hydrogen atom,

R^aand R^b, the same or different, are such as defined in claim 8.

14. The method according to claim 8, wherein the diester of formula (I) is added in a proportion of 1 to 40% by weight relative to the total weight of the lubricant composition.