WO2017021521A1 - Lubricating compositions for preventing or reducing pre-ignition in an engine - Google Patents

Abstract

The invention relates to the use of a lubricating composition comprising at least one base oil and at least one polyalkylene glycol (PAG) produced by polymerisation or copolymerisation of alkylene oxides comprising between 3 and 8 carbon atoms, at least one of which being a butylene oxide, for preventing or reducing pre-ignition in an engine, preferably in a vehicle engine.

Description

 LUBRICATING COMPOSITIONS FOR PREVENTING OR REDUCING PRE-IGNITION

 IN A MOTOR

The invention relates to the use of a lubricating composition to prevent or reduce pre-ignition in an engine. More particularly, the invention relates to the use of a lubricating composition comprising at least one base oil and at least one polyalkylene glycol (PAG) for preventing or reducing pre-ignition in an engine, preferably in a vehicle engine. , in particular of a motor vehicle.

 The present invention also relates to a method for preventing or reducing the pre-ignition in an engine, preferably in a vehicle engine, especially a motor vehicle, implementing this lubricant composition.

 The present invention also relates to the use of a polyalkylene glycol (PAG) in a lubricant composition to prevent or reduce pre-ignition in an engine, preferably in a motor vehicle, especially motor vehicles.

Technological background

Under ideal conditions, normal combustion in a spark-ignition engine occurs when a mixture of fuel, and in particular fuel and air, is ignited in the combustion chamber inside the cylinder by the production of a fuel. spark from a spark plug. Such normal combustion is generally characterized by expanding the flame front through the combustion chamber in an orderly and controlled manner.

 However, in some cases, the air / fuel mixture may be prematurely ignited by a source of ignition prior to ignition by the spark of the spark plug, leading to a phenomenon known as pre-ignition.

 However, it is preferable to reduce or even eliminate the pre-ignition, as this generally results in the presence of a sharp increase in temperatures and pressures in the combustion chamber, and thus have a significant negative impact on the efficiency. and the overall performance of an engine. In addition, pre-ignition can cause significant damage to the cylinders, pistons, spark plugs and valves in the engine and in some cases can even lead to engine failure or even engine failure.

Recently, low-speed pre-ignition (LSPI) has been identified, particularly by car manufacturers, as a potential problem for smaller engines (or downsized engines). LSPI typically occurs at low speeds and high loads, and can cause severe piston damage and / or the cylinders. This phenomenon is described in particular by Dahnz et al (SAE Int.J.Engines, volume 3, issue 1, 2010-01 -0355).

 Thus it would be desirable to have a lubricant that can prevent or reduce the risk of pre-ignition, including the LSPI.

Solutions for decreasing the calcium content or increasing the zinc dithiophosphate or molybdenum dithiocarbamate content in a lubricant have been described (Takeuchi et al, "Investigation of Engine Oil Effect on Abnormal Combustion in Turbocharged Direct Injection - Spark Ignition Engines, SAE Int J. Fuels Lubr 5 (3): 1017-1024, 2012, Hirano et al, "Investigation of Engine Oil Effect on Abnormal Combustion in Turbocharged Direct Injection - Spark Ignition Engines (Part 2)," SAE Technical Paper 2013-01 -2569, 2013). However, these solutions are still insufficient to significantly reduce pre-ignition and difficult to implement, especially in countries where a high level of basicity is required on fuels. In addition, problems of lubricant stability or compatibility with post-treatment systems are associated with these solutions.

 The application WO 2015023559 describes a method for reducing the pre-ignition by adding, in a lubricant, an additive for retarding ignition, said additive being chosen from organic compounds comprising at least one aromatic ring. However, these light organic compounds could cause an excessive increase in lubricant volatility.

An object of the present invention is therefore to provide compounds as well as a lubricating composition comprising these compounds which overcomes all or part of the aforementioned drawbacks.

Another object of the present invention is to provide a lubricant composition for preventing or reducing pre-ignition in an engine and whose formulation is easy to implement. Another object of the present invention is to provide a lubricant composition for preventing or reducing pre-ignition in an engine while maintaining satisfactory or improved lubrication properties.

Another object of the present invention is to provide a lubrication method for preventing or reducing pre-ignition in an engine. Summary of the invention

The invention thus relates to the use of a lubricating composition comprising at least one base oil and at least one polyalkylene glycol (PAG) obtained by polymerization or copolymerization of alkylene oxides comprising from 3 to 8 carbon atoms , including at least one butylene oxide to prevent or reduce pre-ignition in an engine.

Surprisingly, the Applicant has found that the presence of at least one PAG obtained by polymerization or copolymerization of alkylene oxides comprising from 3 to 8 carbon atoms, of which at least one butylene oxide in a lubricating composition makes it possible to the lubricating composition, when implemented in an engine, to prevent or reduce the pre-ignition in the engine.

 Thus, the present invention makes it possible to formulate lubricant compositions having both good stability and good prevention or pre-ignition reduction properties when used in an engine.

Advantageously, the lubricant compositions according to the invention have good pre-ignition prevention or reduction properties once implemented in an engine without the need to associate them with other technical solutions to prevent or reduce the meadow. ignition, and in particular technical solutions requiring to reduce the calcium content or causing an excessive increase in the volatility of the lubricant composition.

Advantageously, the lubricant compositions according to the invention have good prevention or pre-ignition reduction properties once implemented in an engine and whose formulation is easy to implement.

In one embodiment of the invention, the PAG is a PAG of formula (A)

R '2. x-1 R ^L 2. x

 (AT)

in which • Yi and Y ₂ represent, independently of one another, a hydrogen atom or a hydrocarbon group, preferably a CC pack ₃₀ alkyl or alkylphenyl;

 N represents an integer greater than or equal to 2, preferably less than 500, more preferably from 2 to 200, still more preferably from 2 to 60 or from 5 to 30 or from 7 to 20, advantageously from 7 to 15;

 X represents one or more integers ranging from 1 to n;

The groups R _2x -i and R _2x represent, independently of each other, a hydrogen atom or a hydrocarbon group comprising between 1 and 6 carbon atoms, preferably a linear or branched CC ₆ alkyl group, the sum of the numbers of carbon atoms of R _2x -i and R _2x is 2 for at least one value of x.

In another embodiment of the invention, the PAG is a PAG of formula (A) in which:

Y ₂ represents a hydrogen atom;

• R _2x -i and R _2x , identical or different, represent

independently a hydrogen atom or a d-C ₂ -alkyl group.

 •

 In another embodiment of the invention, the PAG is a PAG of formula (B)

 in which

R1 represents a linear or branched C ₈ -C ₁₂ -alkyl group;

 P represents a number ranging from 2 to 10, preferably from 2 to 6;

 Q represents a number ranging from 2 to 10, preferably from 2 to 6.

In another embodiment of the invention, the weight content of PAG, relative to the total weight of the lubricating composition, ranges from 5 to 50%, preferably from 10 to 40%, advantageously from 15 to 30%. In another embodiment of the invention, the lubricating composition further comprises an additive selected from friction modifiers, detergents, antiwear additives, extreme pressure additives, viscosity index improvers, dispersants and the like. , antioxidants, pour point improvers, defoamers, thickeners and mixtures thereof.

 In another embodiment of the invention, the lubricant composition is used to prevent or reduce pre-ignition in a vehicle engine, preferably a motor vehicle.

 In another embodiment of the invention, the lubricant composition is used to prevent or reduce low speed pre-ignition (LSPI) in an engine, preferably a vehicle, preferably a motor vehicle.

 The invention also relates to a method for preventing or reducing pre-ignition in an engine, said method comprising at least one step of contacting a mechanical part of the engine with a lubricant composition as defined above.

The invention also relates to the use of a polyalkylene glycol (PAG) according to the invention in a lubricant composition comprising at least one base oil to prevent or reduce the pre-ignition in an engine.

Detailed description of the invention

The percentages given below correspond to percentages by mass of active ingredient.

The lubricant composition used according to the invention comprises at least one polyalkylene glycol (PAG) obtained by polymerization or copolymerization of alkylene oxides comprising from 3 to 8 carbon atoms, of which at least one butylene oxide.

The PAG may be a polymer or a copolymer (random or block) of alkylene oxides, which may in particular be prepared according to the methods described in application WO 2009/134716, page 2 line 26 to page 4 line 12, for example by etching an alcohol initiator on the epoxy bond of an alkylene oxide and propagating the reaction.

In one embodiment of the invention, the PAG is obtained by polymerization or copolymerization of alkylene oxides comprising from 3 to 8 carbon atoms, of which at least one butylene oxide, in particular chosen from 1,2-oxide. butylene or 2,3-butylene oxide, preferably 1,2-butylene oxide. Among these PAGs, copolymers of butylene oxide and propylene oxide are particularly preferred.

 The application WO 201 1/01 1656, paragraphs [01 1] to [014] describes a mode of preparation, the characteristics, and the properties (in particular of solubility and miscibility in the base oils) of such PAG oxide copolymers. butylene and propylene oxide.

 These PAGs can be prepared by reacting one or more alcohols with a mixture of butylene oxide and propylene oxide.

 Advantageously, the PAGs are prepared with a mixture of butylene oxide and propylene oxide in which the weight ratio between butylene oxide and propylene oxide is from 3: 1 to 1: 3, preferably from 3: 1 to 1: 1.

 In a preferred embodiment of the invention, PAGs are prepared from alcohol having 8 to 12 carbon atoms. 2-Ethylhexanol and dodecanol, alone or in a mixture, and in particular dodecanol, are particularly preferred.

According to a preferred embodiment of the invention, the PAGs according to the invention are such that their molar carbon to oxygen ratio is greater than 3: 1, preferably ranging from 3: 1 to 6: 1. This confers in particular on said PAG polarity and viscosity index properties particularly suitable for use in motor oil.

In another embodiment of the invention, the PAGs are prepared with a mixture of butylene oxide and propylene oxide in which the mass ratio between butylene oxide and propylene oxide is greater than 3: 1, preferably greater than 5: 1, more preferably greater than 8: 1, advantageously from 8: 1 to 10: 1.

In another embodiment of the invention, the PAG is obtained by polymerization of butylene oxides only, in particular chosen from 1,2-butylene oxide or 2,3-butylene oxide, preferably 1, 2-butylene oxide.

In another embodiment of the invention, the PAG is a PAG of formula (A)

 ¾-1

 (AT)

in which

Y ₁ and Y ₂ represent, independently of one another, a hydrogen atom, or a hydrocarbon group, preferably a C 1 -C ₃₀ alkyl or alkylphenyl group; n represents an integer greater than or equal to 2, preferably less than 500, more preferably from 2 to 200, still more preferably from 2 to 60 or from 5 to 30 or from 7 to 20, advantageously from 7 to 15;

 x represents one or more integers ranging from 1 to n;

the groups R _2x -i and R _2x represent, independently of one another, a hydrogen atom or a hydrocarbon group comprising between 1 and 6 carbon atoms, preferably a linear or branched CrC ₆ alkyl group,

the sum of the carbon number of R _2x- i and R _2x is equal to 2 for at least one value of x.

In a preferred embodiment, the groups R _2x- and R _2x represent, independently of each other, a linear C ₁ -C ₆ alkyl group.

In another preferred embodiment of the invention, at least one of R _2x -i and R _2x represents a hydrogen atom. _2x preferably R represents a hydrogen atom.

In another preferred embodiment of the invention, the sum of the carbon number of R _2x-1 and R _2x is from 1 to 6 and the sum of the numbers of carbon atoms from R _2x-1 and R _2x equals 2 for at least one value of x.

In another preferred embodiment of the invention, the PAG is a PAG of formula (A) in which:

Y ₂ represents a hydrogen atom;

- R _2x- and R _2x , identical or different, independently represent a hydrogen atom or a group CrC ₂ -alkyl.

Advantageously, the PAG is a PAG of formula (B)

(B)

in which

- R1 represents a linear or branched C ₈ -C ₁₂ -alkyl group;

 p represents a number ranging from 2 to 10, preferably from 2 to 6;

q represents a number ranging from 2 to 10, preferably from 2 to 6. The PAGs are preferably light PAGs characterized by a kinematic viscosity measured at 100 ° C. according to ASTM D445 ranging from 1 to 12 mm ² / s, preferably from 2 to 7 mm ² / s.

Preferably, the viscosity index (Viscosity Index in English or VI measured according to standard NFT 60136) of the PAGs used in the invention is greater than or equal to 100, preferably greater than or equal to 120. Preferably, the lubricating composition used according to the invention comprises a content by weight of PAG, relative to the total weight of the lubricating composition, ranging from 5 to 50%, preferably from 10 to 40%, advantageously from 15 to 30%.

As examples of PAG that may be used in the lubricant composition used according to the invention, mention may be made of the PAGs described in the documents WO 2012/070007 or WO 2013/164457.

The lubricating composition used according to the invention also comprises at least one base oil.

In general, the lubricant composition used according to the invention may comprise any type of mineral, synthetic or natural lubricating base oil, animal or vegetable, known to those skilled in the art.

 The base oils used in the lubricant compositions according to the invention may be oils of mineral or synthetic origins belonging to groups I to V according to the classes defined in the API classification (or their equivalents according to the ATIEL classification) (Table A) or their mixtures.

 Content Content in Saturated Sulfur Viscosity Index (VI)

Group I

 <90%> 0.03% 80 <VI <120 Mineral oils

 Group II

 ≥90% <0.03% 80 <VI <120 Hydrocracked oils

 Group III

 Hydrocracked oils ≥90% <0.03% ≥120

or hydro-isomerized Group IV Polyalphaolefines (PAO)

Esters and other bases not included in groups 1

Group V

 at IV

 Table A

The mineral base oils according to the invention include all types of base oils obtained by atmospheric and vacuum distillation of crude oil, followed by refining operations such as solvent extraction, desalphating, solvent dewaxing, hydrotreatment, hydrocracking , hydroisomerization and hydrofinition.

 Mixtures of synthetic and mineral oils can also be used.

There is generally no limitation on the use of different lubricating bases for producing the lubricating compositions used according to the invention, except that they must have properties, in particular viscosity, viscosity index, sulfur, oxidation resistance, adapted for use for engines or for vehicle transmissions.

The base oils of the lubricant compositions used according to the invention may also be chosen from synthetic oils, such as certain carboxylic acid esters and alcohols, and from polyalphaolefins. The polyalphaolefins used as base oils are, for example, obtained from monomers comprising from 4 to 32 carbon atoms, for example from octene or decene, and whose viscosity at 100 ° C. is between 1, 5 and 15 mm ² . s ^"1 according to the ASTM D445 standard, their average molecular weight is generally between 250 and 3000 according to the ASTM D5296 standard.

 Preferably, the base oils of the present invention are chosen from the above base oils whose aromatic content is between 0 and 45%, preferably between 0 and 30%. The aromatic content of the oils is measured according to UV Burdett method.

Advantageously, the lubricant composition used according to the invention comprises at least 50% by weight of base oils relative to the total mass of the composition. More advantageously, the lubricant composition used according to the invention comprises at least 60% by weight, or even at least 70% by weight, of base oils relative to the total mass of the composition.

More particularly advantageously, the lubricant composition used according to the invention comprises from 60 to 99.5% by weight of base oils, preferably from 70 to 99.5% by weight of base oils, relative to the total mass of the composition. Many additives can be used for this lubricant composition used according to the invention.

 The preferred additives for the lubricant composition used according to the invention are chosen from friction modifiers, detergents, anti-wear additives, extreme pressure additives, viscosity index improvers, dispersants, antioxidants, pour point improvers, defoamers, thickeners and mixtures thereof. Preferably, the lubricant composition used according to the invention comprises at least one antiwear additive, at least one extreme pressure additive or their mixtures.

Anti-wear additives and extreme pressure additives protect friction surfaces by forming a protective film adsorbed on these surfaces.

There is a wide variety of anti-wear additives. In a preferred manner for the lubricating composition according to the invention, the anti-wear additives are chosen from phosphosulfur additives such as metal alkylthiophosphates, in particular zinc alkylthiophosphates, and more specifically zinc dialkyldithiophosphates or ZnDTPs. The preferred compounds have the formula Zn ((SP (S) (OR ² ) (OR ³ )) ₂ , in which R ² and R ³ , which may be identical or different, independently represent an alkyl group, preferably an alkyl group containing from 1 to 18 carbon atoms.

 Amine phosphates are also anti-wear additives which can be used in the lubricating composition according to the invention. However, the phosphorus provided by these additives can act as a poison of the catalytic systems of automobiles because these additives are ash generators. These effects can be minimized by partially substituting amine phosphates with non-phosphorus additives, such as, for example, polysulfides, especially sulfur-containing olefins.

 Advantageously, the lubricant composition according to 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 relative to the mass. total lubricating composition, antiwear additives and extreme pressure additives.

Advantageously, the lubricant composition according to the invention may comprise at least one friction-modifying additive. The friction modifying additive may be chosen from a compound providing metal elements and an ash-free compound. Among the compounds providing metal elements, mention may be made of transition metal complexes such as Mo, Sb, Sn, Fe, Cu and Zn, the ligands of which may be hydrocarbon compounds comprising oxygen, nitrogen, sulfur or phosphorus. The ashless friction modifier additives are generally of organic origin and may be selected from monoesters of fatty acids and polyols, alkoxylated amines, alkoxylated fatty amines, fatty epoxides, borate fatty epoxides; fatty amines or fatty acid glycerol esters. according to the invention, the fatty compounds comprise at least one hydrocarbon group comprising from 10 to 24 carbon atoms.

 Advantageously, the lubricant composition according to 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 0.1 at 2% by weight relative to the total mass of the lubricant composition, friction modifier additive.

 Advantageously, the lubricant composition according to the invention may comprise at least one antioxidant additive.

 The antioxidant additive generally serves to retard the degradation of the lubricating composition in service. This degradation can notably result in the formation of deposits, the presence of sludge or an increase in the viscosity of the lubricant composition.

 Antioxidant additives act in particular as radical inhibitors or destroyers of hydroperoxides. Among the antioxidant additives commonly used, mention may be made of antioxidant additives of phenolic type, antioxidant additives of amine type, antioxidant phosphosulfur additives. Some of these antioxidant additives, for example phosphosulfur antioxidant additives, can be ash generators. Phenolic antioxidant additives may be ash-free or may be in the form of neutral or basic metal salts. The antioxidant additives may especially be chosen from sterically hindered phenols, sterically hindered phenol esters and sterically hindered phenols comprising a thioether bridge, diphenylamines, diphenylamines substituted by at least one C 1 -C 12 alkyl group, and N, N '-dialkyl-aryl diamines and mixtures thereof.

Preferably, according to the invention, the sterically hindered phenols are chosen from compounds comprising a phenol group in which at least one vicinal carbon of the carbon bearing the alcohol function is substituted with at least one alkyl group containing ₁₀ carbon atoms, preferably an alkyl group containing CrC ₆ , preferably a C ₄ alkyl group, preferably by the ter-butyl group.

Amino compounds are another class of antioxidant additives that can be used, optionally in combination with phenolic antioxidant additives. Examples of amine compounds are the aromatic amines, for example the aromatic amines of formula NR ⁴ R ⁵ R ⁶ in which R ⁴ represents an optionally substituted aliphatic or aromatic group, R ⁵ represents an optionally substituted aromatic group R ⁶ represents a hydrogen atom, an alkyl group, an aryl group or a group of formula R ⁷ S (O) _z R ⁸ in which R ⁷ represents an alkylene group or an alkenylene group, R ⁸ represents an alkyl group, a alkenyl group or an aryl group and z represents 0, 1 or 2. Sulfurized alkyl phenols or their alkali and alkaline earth metal salts can also be used as antioxidant additives.

 Another class of antioxidant additives is copper compounds, for example copper thio- or dithio-phosphates, copper and carboxylic acid salts, dithiocarbamates, sulphonates, phenates, copper acetylacetonates. Copper salts I and II, succinic acid or anhydride salts can also be used. The lubricant composition according to the invention may contain all types of antioxidant additives known to those skilled in the art.

 Advantageously, the lubricating composition comprises at least one ash-free antioxidant additive.

 Also advantageously, the lubricating composition according to the invention comprises from 0.5 to 2% by weight relative to the total weight of the composition, of at least one antioxidant additive.

 The lubricant composition according to the invention may also comprise at least one detergent additive.

 The detergent additives generally make it possible to reduce the formation of deposits on the surface of the metal parts by dissolving the secondary oxidation and combustion products.

 The detergent additives that can be used in the lubricant composition according to the invention are generally known to those skilled in the art. The detergent additives may be anionic compounds comprising a long lipophilic hydrocarbon chain and a hydrophilic head. The associated cation may be a metal cation of an alkali metal or alkaline earth metal.

 The detergent additives are preferably chosen from the alkali metal or alkaline earth metal salts of carboxylic acids, the sulphonates, the salicylates, the naphthenates and the 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, therefore in an amount greater than the stoichiometric amount. It is then overbased detergent additives; the excess metal bringing the overbased character to the detergent additive is then generally in the form of an oil insoluble metal salt, for example a carbonate, a hydroxide, an oxalate, an acetate, a glutamate, preferably a carbonate .

Advantageously, the lubricant composition according to the invention may comprise from 2 to 4% by weight of detergent additive relative to the total mass of the lubricating composition. Also advantageously, the lubricant composition according to the invention may also comprise at least one pour point depressant additive. By slowing the formation of paraffin crystals, pour point depressant additives generally improve the cold behavior of the lubricant composition according to the invention.

 As examples of pour point depressant additives, mention may be made of alkyl polymethacrylates, polyacrylates, polyarylamides, polyalkylphenols, polyalkylnaphthalenes and alkylated polystyrenes.

 Advantageously, the lubricant composition according to the invention may also comprise at least one dispersing agent.

 The dispersing agent may be chosen from Mannich bases, succinimides and their derivatives.

 Also advantageously, the lubricating composition according to the invention may comprise from 0.2 to 10% by weight of dispersing agent relative to the total mass of the lubricating composition.

 The lubricating composition of the present invention may also comprise at least one viscosity index improving additive. Examples of additives improving the viscosity number include polymeric esters, homopolymers or copolymers, hydrogenated or non-hydrogenated, styrene, butadiene and isoprene, polyacrylates, polymethacrylates (PMA) or alternatively olefin copolymers, in particular ethylene / propylene copolymers.

The lubricating composition according to the invention can be in various forms. The lubricant composition according to the invention may in particular be an anhydrous composition.

Preferably, this lubricating composition is not an emulsion. The lubricant composition defined above is used to prevent or reduce preignition in an engine.

Motor according to the invention more particularly means the vehicle engines, such as:

- motor vehicle engines, including petrol and diesel engines, but also gas and petrol engines (dual fuel gas / gasoline engines) and gas and diesel engines (dual fuel gas engines) / diesel);

- the engines of heavy goods vehicles, and more specifically the engines of heavy goods vehicles running on gas. By engine according to the invention is also meant 4-stroke engines, and more specifically 4-stroke marine engines, preferably 4-stroke marine engines operating on gas. In a preferred embodiment of the invention, the lubricant composition is used to prevent or reduce pre-ignition in a vehicle engine, preferably a motor vehicle.

Pre-ignition according to the invention includes the phenomenon of low frequency vibration producing a sound effect of humming (or "Rumble" in English).

 By pre-ignition according to the invention is more particularly meant the low speed pre-ignition (LSPI).

 In a preferred embodiment of the invention, the lubricant composition is used to prevent or reduce low speed pre-ignition (LSPI) in an engine, preferably in a vehicle engine, preferably a motor vehicle.

The invention also relates to a method for preventing or reducing pre-ignition in an engine, said method comprising at least one step of contacting a mechanical part of the engine with a lubricant composition as defined above.

All of the features and preferences described for the above use also apply to this method.

The subject of the invention is also the use of a polyalkylene glycol (PAG) obtained by polymerization or copolymerization of alkylene oxides comprising from 3 to 8 carbon atoms, of which at least one butylene oxide in a lubricating composition comprising at least one base oil to prevent or reduce pre-ignition in an engine.

The set of characteristics and preferences relating to the PAG and the lubricating composition described above also applies to this use.

Claims

1. Use of a lubricating composition comprising at least one base oil and at least one polyalkylene glycol (PAG) obtained by polymerization or copolymerization of alkylene oxides comprising from 3 to 8 carbon atoms, of which at least one butylene oxide for prevent or reduce pre-ignition in an engine. 2. Use according to claim 1 wherein the PAG is a PAG of formula (A)

 R 2x-1  (AT)  in which Y ₁ and Y ₂ represent, independently of one another, a hydrogen atom, or a hydrocarbon group, preferably a C 1 -C ₃₀ alkyl or alkylphenyl group;  n represents an integer greater than or equal to 2, preferably less than 500, more preferably from 2 to 200, still more preferably from 2 to 60 or from 5 to 30 or from 7 to 20, advantageously from 7 to 15;  x represents one or more integers ranging from 1 to n; the groups R _2x -i and R _2x represent, independently of one another, a hydrogen atom or a hydrocarbon group comprising between 1 and 6 carbon atoms, preferably a linear or branched CrC ₆ alkyl group, the sum of the numbers of carbon atoms of R _2x -i and R _2x is 2 for at least one value of x. 3. Use according to claim 1 or 2 wherein the PAG is a PAG of formula (A) in which: Y ₂ represents a hydrogen atom; - R _2x -i and R _2x , identical or different, independently represent a hydrogen atom or a group dC ₂ -alkyl. 4. Use according to any preceding claim wherein the PAG is a PAG of formula (B)

(B)  which one • R 1 represents a C ₂ -C ₈ group linear or branched alkyl;  P represents a number ranging from 2 to 10, preferably from 2 to 6;  Q represents a number ranging from 2 to 10, preferably from 2 to 6. 5. Use according to any one of the preceding claims wherein the weight content of PAG, based on the total weight of the lubricant composition, is from 5 to 50%, preferably from 10 to 40%, advantageously from 15 to 30% by weight. %. Use according to any of the preceding claims wherein the lubricating composition further comprises an additive selected from the group consisting of friction modifiers, detergents, antiwear additives, extreme pressure additives, index improvers and the like. viscosity, dispersants, antioxidants, pour point improvers, defoamers, thickeners and mixtures thereof. 7. Use according to any one of the preceding claims for preventing or reducing pre-ignition in a vehicle engine, preferably a motor vehicle. 8. Use according to any one of the preceding claims for preventing or reducing the low speed pre-ignition (LSPI) in an engine, preferably a vehicle, preferably a motor vehicle. 9. Use of a polyalkylene glycol (PAG) obtained by polymerization or copolymerization of alkylene oxides comprising from 3 to 8 carbon atoms, of which at least one butylene oxide in a lubricating composition comprising at least one base oil for prevent or reduce pre-ignition in an engine.