EP1989407B1 - Method of improving an engine using a lubricating composition comprising a colloidal dispersion of a rare earth to catalyse the combustion of soot - Google Patents

Abstract

Engines equipped with an exhaust line fitted with a particulate filter, in which the particulates contained in the exhaust gas are trapped on such filter, and wherein the trapped particulates are periodically burned off, are operated by improvedly catalyzing the combustion of said particles utilizing a composition containing mixture of a lubricating oil and a colloidal dispersion as the engine-lubricating composition, which dispersion includes particles of at least one compound of at least one rare earth and an amphiphilic agent.

Description

The present invention relates to a method of operating a motor using a lubricant composition comprising a colloidal dispersion of a rare earth to catalyze the combustion of soot.

It is known that during the combustion of diesel fuel in a diesel engine, carbonaceous products tend to form soot, which is considered harmful for both the environment and health. We have been looking for a long time at techniques that make it possible to reduce the emission of these soot or carbonaceous particles. The same problem arises for gasoline engines operating in lean-burn engines (lean-bum engines) which also emit such particles.

A satisfactory solution now used in mass production is to collect the particles on a filter which is regenerated regularly to prevent clogging. Regeneration of the filter is all the more facilitated that the auto-ignition temperature of the soot is low which can be obtained by introducing a catalyst in the very heart of the soot during combustion. This technology, known as "Fuel Borne Catalysis" or FBC is also widely used. The sooted soot has a sufficiently low autoignition temperature to be frequently reached during normal engine running or during specific regeneration cycles.

So FR 2 833 862 discloses a colloidal iron dispersion useful as an adjunct of soot combustion for a diesel engine. This dispersion is implemented in the fuel.

DE 41 33 137 A1 discloses a lubricating oil that contains an additive for the combustion of soot for a diesel engine. The nature of this additive is not specified.

Although the FBC technology is satisfactory, there is a need for other alternative technologies so as to have the widest possible range of solutions and thus be able to respond to the problem of reducing the emission of harmful particles whatever the conditions in which this problem arises.

The object of the invention is therefore to propose such a new technology.

• d'une huile lubrifiante;
• d'une dispersion colloïdale qui comprend des particules d'un composé d'au moins une terre rare et un agent amphiphile.

For this purpose, the invention relates to a method of operating an engine capable of producing exhaust gases containing particles and equipped with an exhaust pipe provided with a particulate filter, in which the particles are trapped. on said filter and the trapped particles are periodically burned and characterized in that catalyzing the combustion of said particles, a lubricant composition of the engine is used which results from mixing:

• a lubricating oil;
• a colloidal dispersion which comprises particles of a compound of at least one rare earth and an amphiphilic agent.

The method of the invention has the advantage of eliminating the presence of a specific reservoir for the soot combustion catalyst and a device for metering it in the fuel unlike the method using the FBC technology.

Other features, details and advantages of the invention will appear even more fully on reading the description which follows, as well as various concrete but non-limiting examples intended to illustrate it.

In rare earth is meant in this same description the elements of the group consisting of yttrium, scandium and elements of the periodic classification of atomic number included between 57 and 71 inclusively.

The composition of the invention comes from a mixture of two essential elements: the lubricating oil and the colloidal dispersion.

Lubricating oils are well known to those skilled in the art. It can be recalled that these products contain a base oil with lubricating properties. This base oil may be a mineral oil derived from oils, based in particular on paraffins, aromatics or isoparaffins and mixtures of these compounds. The mineral oil can be obtained by vacuum distillation of a crude oil, the distillate obtained is then hydrocracked, hydrotreated and in a second time dewaxed and / or hydroisomerized so as to improve the properties such as viscosity and those of flow. the base oil thus obtained.

These base oils may also be synthetic oils based on polyalphaolefins or organic esters.

The viscosity index of the mineral oils can be, for example, between 90 and 100 (index measured according to the ASTM D2270 standard), that of the hydrotreated products between 120 and 130, and this index can be greater than 140 for the synthetic oils based on of polyalphaolefins and can even reach 200 for those based on organic esters.

In a manner also known, the lubricating oils further contain various additives which can be classified into three groups: those intended to improve the chemical stability of the oil or to inhibit the effects of degradation products, those which improve the rheological properties and those that protect metal surfaces and have an anti-wear effect.

In the first group are antioxidant additives based for example on phenols, substituted arylamines or sulfur compounds or also dialkyl-dithiophosphates zinc. There are also detergent additives such as salts of organic acids or of phenols and divalent metals and dispersing additives of the organic surfactant type.

The additives of the second group are those which act on the pour point of the oils and are of the oligomeric type possessing alkyl chains, or the so-called anticongelating products of the alkylnaphthalene type, the polyacrylates of long-chain alcohols or else of the alkylated polystyrene type. . This second group also contains additives improving the viscosity index. These additives are based on hydrocarbon polymers (ethylene-propylene copolymers for example) or ester-functional polymers (polymethacrylate type). Finally, in this second group of additives are also anti-foaming products for example based on silicones.

The third group of additives includes products with anti-wear effect. They are generally organic products containing sulfur, chlorine or phosphorus, such as dithiophosphoric derivatives or phosphomolybdate derivatives.

The second essential element of the composition of the invention is the colloidal dispersion.

The term "colloidal dispersion" designates in the present description any system consisting of fine solid particles of colloidal dimensions based on a compound of a rare earth, in suspension in a liquid phase, said particles being able, in addition, to optionally contain residual amounts of bound or adsorbed ions such as, for example, nitrates, acetates, citrates or ammoniums. By colloidal dimensions is meant dimensions of between about 1 nm and about 500 nm. The particles may more particularly have an average size of at most about 250 nm, in particular at most 100 nm, preferably at most 20 nm and even more preferably at most 15 nm. It will be appreciated that in such dispersions the rare earth compound may be either, preferably, completely in the form of colloids, or in the form of colloids and partly in the form of ions.

The particle size of which is mentioned above and for the remainder of the description, unless otherwise indicated, is determined by transmission electron microscopy (TEM), in a conventional manner, on a previously dried sample deposited on a supported carbon membrane. on a copper grid.

The rare earth may be chosen more particularly from cerium, lanthanum, yttrium, neodymium, gadolinium and praseodymium. Cerium can be chosen especially.

According to a first variant of the invention, the colloidal dispersion is characterized in that it comprises particles of a compound of cerium and another rare earth.

According to another variant, the colloidal dispersion of the invention is characterized in that it comprises particles based on a compound of cerium, possibly another rare earth, and iron.

More particularly, when the particles of the dispersion of the invention are based on a compound of several elements, that is to say cerium, other rare earth and / or iron, these elements are mixed within each particle, these elements are generally in the form of mixed oxides and / or hydrated mixed oxides (oxyhydroxides).

In the case of a colloidal dispersion of cerium, this element is preferably mainly in the form of cerium IV. For example, the content of cerium III with respect to cerium IV (content expressed by the atomic ratio Ce III / Ce total) is generally 'not more than 40%. It can vary according to the embodiments of the dispersions used and thus be at most 20%, more particularly at most 10%, and even more particularly at most 1%.

In the case of the first variant mentioned above, the rare earth other than cerium may be more particularly lanthanum or praseodymium. Of course, the present variant covers the case where the particle is a compound of cerium and several other rare earths in combination.

The proportion of rare earth other than cerium is preferably at least 10%, more particularly at least 20%, and even more particularly at most 50%, in moles relative to the total number of moles of cerium. and rare earth oxide.

In the case of the second variant, the proportion of cerium is preferably at most 50%, more particularly at most 20% and even more particularly at most 10%, this proportion being expressed in moles of oxide. of cerium CeO ₂ relative to the total number of moles of cerium oxide and Fe ₂ O ₃ iron oxide.

Both variants can be combined, ie the particles can be cerium compounds, at least one other rare earth and iron.

As indicated above, the particles of the colloidal dispersion are suspended in a liquid phase which is here an organic phase.

This organic phase may consist of the lubricating base oil described above or it may also be a mixture of this base oil with another organic phase, miscible with this oil. Indeed and as will be seen later, the lubricant composition of the invention can be obtained by mixing the lubricating oil with a previously prepared colloidal dispersion. In this case, this dispersion comprises an organic phase which may be a hydrocarbon, more particularly apolar.

Examples of organic phase include aliphatic hydrocarbons such as hexane, heptane, octane, nonane, inert cycloaliphatic hydrocarbons such as cyclohexane, cyclopentane, cycloheptane, aromatic hydrocarbons such as as benzene, toluene, ethylbenzene, xylenes, liquid naphthenes. Also suitable are the Isopar or Solvesso type petroleum fractions (trademarks registered by the company EXXON), in particular Solvesso 100 which essentially contains a mixture of methylethyl and trimethylbenzene, Solvesso 150 which contains a mixture of alkylbenzenes, in particular dimethylbenzene and of tetramethylbenzene and Isopar which contains mainly iso- and cyclo-paraffinic hydrocarbons at C-11 and C-12. It can also be made, as other oil cuts, those type of Petrolink ^® Petrolink company or type Isane ^® company Total.

Chlorinated hydrocarbons such as chloro- or dichlorobenzene, chlorotoluene can also be used for the organic phase. The aliphatic and cycloaliphatic ethers as well as ketones, for example diisopropyl ether, dibutyl ether, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone and mesityl oxide, may be envisaged.

The esters can be envisaged, but they have the disadvantage of risking being hydrolysed. Mention may be made, as esters that may be used, of those resulting from the reaction of acids with C1 to C8 alcohols and in particular the palmitates of secondary alcohols, such as isopropanol. There may be mentioned butyl acetate as an example.

Of course, the organic phase may be based on a mixture of two or more hydrocarbons or compounds of the type described above.

Finally, as indicated above, the choice of the organic phase among the examples that have just been given will be based on its compatibility or miscibility with the lubricating oil.

In addition, the colloidal dispersion comprises an amphiphilic agent.

This amphiphilic agent is at least partly in interaction, either by grafting or by electrostatic binding, with the particles of the rare earth compound and, optionally, iron.

This agent may be more particularly an acid.

The acid is more particularly chosen from organic acids which comprise at least 6 carbon atoms, more particularly from 10 to 60 carbon atoms, preferably from 10 to 50 carbon atoms and even more preferably from 15 to 25 carbon atoms. carbon.

These acids can be linear or branched. They may be aryl, aliphatic or arylaliphatic acids, optionally carrying other functions provided that these functions are stable in the environments where it is desired to use the dispersions according to the present invention. For example, it is possible to use, for example, acids aliphatic carboxylic acids, aliphatic sulfonic acids, aliphatic phosphonic acids, alkylarylsulfonic acids and alkylarylphosphonic acids having from about 10 to about 40 carbon atoms, whether natural or synthetic. It is of course possible to use the acids in mixture.

It is also possible to use carboxylic acids whose carbon chain carries ketonic functions, such as the alpha-substituted pyruvic acids of the ketone function. It can also be alpha-halo carboxylic acids or alpha hydroxycarboxylic acids. The chain attached to the carboxylic group may carry unsaturations. The chain can be interrupted by ether or ester functions provided that the lipophilicity of the carrier chain of the carboxylic group is not greatly impaired.

By way of example, mention may be made of tall oil fatty acids, soya oil, tallow, linseed oil, oleic acid, linoleic acid, stearic acid and its isomers, pelargonic acid, capric acid, lauric acid, myristic acid, dodecylbenzenesulfonic acid, 2-ethylhexanoic acid, naphthenic acid, hexoic acid, toluenesulphonic acid, acid toluene phosphonic acid, lauryl sulfonic acid, lauryl phosphonic acid, palmityl sulfonic acid, and palmityl phosphonic acid.

ou encore les phosphates de dialcoyle polyoxyéthylénés de formule :

dans lesquelles :

• ■ R¹, R², R³, identiques ou différents représentent un radical alkyl linéaire ou ramifié, notamment de 2 à 20 atomes de carbone; un radical phényle; un radical alkylaryl, plus particulièrement un radical alkylphényl, avec notamment une chaîne alkyle de 8 à 12 atomes de carbone; un radical arylalkyle, plus particulièrement un radical phénylaryl;
• ■ n le nombre d'oxyde d'éthylène pouvant aller de 0 à 12 par exemple;
• ■ M représente un atome d'hydrogène, de sodium ou de potassium.

As amphiphilic agent, mention may also be made of polyoxyethylenated alkyl ether phosphates. Here we mean the organophosphates of formula:

or else the polyoxyethylenated dialkoyl phosphates of formula:

in which :

• R ¹ , R ² and R ³ , which may be identical or different, represent a linear or branched alkyl radical, in particular from 2 to 20 carbon atoms; a phenyl radical; an alkylaryl radical, more particularly an alkylphenyl radical, with in particular an alkyl chain of 8 to 12 carbon atoms; an arylalkyl radical, more particularly a phenylaryl radical;
• The number of ethylene oxide, for example, ranging from 0 to 12;
• M represents a hydrogen, sodium or potassium atom.

The radical R ¹ may especially be a hexyl, octyl, decyl, dodecyl, oleyl or nonylphenyl radical.

• les poly-oxy-éthylène alkyl (C8-C10) éthers phosphates Rhodafac® RA 600
• le poly-oxyéthylène tridécyl éther phosphate Rhodafac® RS 710 ou RS 410
• le poly-oxy-éthylène oléocétyl éther phosphate Rhodafac® PA 35
• le poly-oxy-éthylène nonylphenyl éther phosphate Rhodafac® PA 17
• le poly-oxy-éthylène nonyl(ramifié) éther phosphate Rhodafac® RE 610.

Examples of this type of amphiphilic compound include those marketed under the Lubrophos® and Rhodafac® brands sold by Rhodia, and in particular the products below:

• poly (oxy-ethylene alkyl) (C8-C10) phosphate ethers Rhodafac® RA 600
• poly-oxyethylene tridecyl ether phosphate Rhodafac® RS 710 or RS 410
• Rhodafac® PA poly-oxy-ethylene oleoketyl ether phosphate 35
• poly-oxy-ethylene nonylphenyl ether phosphate Rhodafac® PA 17
• Rhodafac® RE 610 polyoxyethylene ethylene nonyl (branched) ether phosphate.

• r = nombre de mole d'agent amphiphile/nombre de mole de composé E E désignant ici la ou les terres rares ou l'ensemble terre(s) rare(s) et fer.

The amount of the amphiphilic agent present in the dispersion can be defined by the molar ratio r:

• r = number of moles of amphiphilic agent / number of moles of EE compound here denoting the rare earth or rare earths and iron.

This molar ratio may be between 0.2 and 1, preferably between 0.4 and 0.8.

The dispersion that can be used in the process of the invention may be in a specific embodiment.

According to this embodiment, the dispersion is such that at least 90% of the particles are monocrystalline. By "monocrystalline" particles are meant particles which, when one examines the dispersion by TEM (High Resolution Transmission Electron Microscopy), appear individualized and consist of a single crystallite.

The cryo-MET technique can also be used to determine the aggregation state of the elementary particles. It makes it possible to observe by transmission electron microscopy (TEM) samples kept frozen in their natural environment which is either water or organic diluents such as aromatic or aliphatic solvents such as for example Solvesso and Isopar or else certain alcohols such as ethanol.

Freezing is carried out on thin films of about 50 to 100 nm thick either in liquid ethane for aqueous samples or in liquid nitrogen for the others.

By cryo-MET the state of dispersion of the particles is well preserved and representative of that present in the real medium.

According to this embodiment, the particles have a fine particle size and tightened. Indeed, they have a d ₅₀ of between 1 and 5 nm, preferably between 2 and 3 nm.

Generally and by way of example only, the concentration of the rare earth and possibly iron dispersion is between 1 and 40% by weight of rare earth oxide (s) or of rare earth oxide (s) and Fe ₂ O ₃ iron relative to the total weight of the dispersion.

Finally, in general and by way of example for the colloidal dispersions of a compound of a rare earth, possibly a rare earth and iron and their preparation, it will be possible to refer to the whole of the description of each patent application EP 0671205 , WO 01/10545 and WO 97/19022 .

The lubricating composition for use in the process of the invention may be prepared by mixing a lubricating oil with a colloidal dispersion of a rare earth compound, or a rare earth compound and iron. This mixing can be done in proportions that are not critical and that can vary in a wide range. By way of example, these proportions may be such that the content of rare earth, possibly rare earth and iron, in the lubricating composition, expressed as metal element, and coming from the colloidal dispersion is at most 15% by weight. mass of the entire composition, more particularly at most 10%, proportions of only a few percent being possible.

It is observed that the composition thus obtained is stable, that is to say that there is no decantation of the dispersion and therefore no deposition of the cerium or iron particles at the bottom of the tank containing the lubricant composition. Moreover, this stability is maintained even when the lubricant composition is exposed to a high temperature which is the case during operation of the engine for which the composition is used as a lubricant. In addition, it is unexpectedly found that the use of this composition in the operation of the engine does indeed catalyze the combustion of soot.

The method of the invention applies to an engine which, during its operation, is capable of producing harmful particles, such as soot, and which are found in the exhaust gas. It may be more particularly a diesel engine or a gasoline engine operating in lean mixture.

This method applies to an engine which, in a known manner, is equipped with a line or muffler in which is integrated a particulate filter. Conventionally, this filter comprises a filter filter type filter ceramic or silicon carbide through which circulate the exhaust gas. However, it may also be one or more sieves in wire cloth or a filter type foam ceramic or fibrous material.

The method of the invention aims to catalyze the combustion of particles or soot trapped on the particulate filter. In the case of the invention, the compound of the rare earth or iron and rare earth is used as a catalyst for the combustion of these soot and it is provided by the lubricating composition and not by the fuel as in the processes of the prior art. The lubricant composition, prepared prior to its use in the engine and thus comprising the dispersion of rare earth or iron and rare earth, is introduced into the engine oil tank for example during a drain. The lubricant composition thus passes into the engine lubrication circuit. It is found that the rare earth or rare earth compound and the iron itself introduced by the lubricating composition is found in the soot and can thus contribute to catalyzing their combustion.

Of course, it is possible, without departing from the scope of the present invention, to implement the above method while using for the operation of the engine a fuel which also contains a catalyst of soot combustion. This catalyst may also be a colloidal dispersion as described above. It is also possible to implement the method of the invention in a system in which the exhaust pipe is equipped with a catalyzed particulate filter. This type of filter is well known, it is a filter in which is incorporated, during its manufacture, a catalyst for oxidation of particles or soot.

Examples will now be given.

EXAMPLE 1

This example relates to the preparation of a lubricant composition that can be used in a process according to the invention.

For this preparation, a colloidal dispersion based on cerium prepared according to a process of the type of that of example 4 of the patent application is used. EP 671205 but with a temperature of 160 ° C for the autoclave treatment. The organic phase of this dispersion is Isopar and the amphiphilic agent is isostearic. The content of ceric nitrate involved is adjusted so as to obtain a colloidal dispersion containing 25% by weight of cerium metal. The colloids have an average size of 5 nm.

To this dispersion is added a commercial oil (Total Activa Diesel 10W40) so as to obtain a lubricating composition containing 43% by weight of this commercial oil and 57% by weight of the colloidal dispersion.

EXAMPLE 2

This example relates to the preparation of a second lubricating composition that can be used in a process according to the invention.

For this preparation, a colloidal dispersion based on cerium and iron in a metal molar ratio of 50/50, prepared according to a process of the type of Example 3 of the patent application, is used. WO 01/10545 . The organic phase of this dispersion is Isopar and the amphiphilic agent is isostearic acid. The dispersion contains 10% by weight of metal (cerium and iron). The colloids have a size of between 3 and 4 nm and are perfectly individualized.

To this dispersion is added a commercial oil (Total Activa Diesel 10W40) so as to obtain a lubricating composition containing 28% by weight of this commercial oil and 72% by weight of the colloidal dispersion.

EXAMPLE 3

This example relates to a catalytic oxidation of soot test carried out in the presence of a lubricant composition that can be used in a process according to the invention. The catalytic properties of soot oxidation are measured by thermogravimetric analysis. A Setaram thermobalance equipped with a quartz boat is used in which a sample containing approximately 20 mg of sample is placed.

The sample consists of a mixture of 20% by weight of the lubricating composition of Example 1 and 80% by weight of carbon black. The carbon black used to simulate the soot emitted by a diesel engine is carbon black marketed by Cabot under the reference Elftex 125. The lubricating composition and carbon black mixture is homogenized through a mixture with a spatula. The dough thus obtained is dried beforehand in a ventilated oven at 60 ° C. and then up to 120 ° C.

20 mg of the sample thus prepared and treated are introduced into the nacelle of the thermobalance then a gas stream is circulated consisting of an air / water mixture in respective volume proportions of 87 and 13%. After 30 minutes at 150 ° C., the rise in temperature is started up to 900 ° C. with a ramp of 10 ° C./min, and the mass loss of the sample is recorded as a function of temperature.

EXAMPLE 4

This example is carried out according to the same protocol as Example 3 but with the lubricant composition described in Example 2.

COMPARATIVE EXAMPLE 5

This example relates to a soot oxidation test carried out in the presence of a lubricant composition of the prior art. The test is carried out according to the same protocol but using Total Activa Diesel 10W40 pure commercial oil. The sample thus evaluated is therefore composed of a mixture of 20% by weight of the pure commercial oil and 80% by weight of carbon black.

The results are given in Table 1: they are expressed in soot half-oxidation temperature (T50% (soot)) corresponding to the temperature required to obtain half of the mass loss measured between 200 and 900 ° C. Table 1 Example T50% (soot) in ° C 3 510 ° C 4 515 ° C. 5 comparative 610 ° C Sows alone 620 ° C

It is found that the addition of pure commercial oil has little or no effect on the soot half conversion temperature while the use of a lubricating composition containing a commercial oil and a colloidal dispersion containing cerium or cerium and iron can significantly lower the soot combustion temperature.

EXAMPLE 6

In this example, the colloidal dispersion of cerium and iron of Example 2 is used again and 10 grams of this dispersion at 10% by weight of cerium and iron metal are added to 160 g of a motor oil (Elf Prestigrade 15W40) so that to obtain a lubricating composition containing 94% mass of this commercial oil and 6% mass of the commercial colloidal dispersion. The iron metal content of this lubricating composition is thus 0.20% by weight while that in cerium metal is 0.45% by weight.

This lubricating composition is then introduced into a partially clogged container, itself placed in a ventilated chamber maintained at 110 ° C. The iron and cerium content of the composition in the upper part of the container is then regularly measured by a chemical assay technique (ICP).

Table 2 below gives the iron and cerium contents thus measured after different residence times in the chamber at 110 ° C. Table 2 Time to
heating at 110 ° C
in days % cerium % wt iron 0 0.45 0.20 1 0.45 0.20 3 0.45 0.20 6 0.45 0.20 10 0.45 0.20 13 0.45 0.20 17 0.45 0.20 28 0.45 0.20 38 0.45 0.20 55 0.45 0.20 79 0.45 0.20

It is therefore found that the thermal stability of this lubricant composition is very important given that the iron and cerium contents do not change during 79 days of continuous heating at 110 ° C. This stability period determined under these conditions can be considered sufficient to ensure the stability of the lubricant composition between two oil changes of the engine oil circuit.

Claims (10)

1. Method of operating an engine capable of producing exhaust gases that contain particulates and equipped with a muffler provided with a particulate filter, in which the particulates are trapped on said filter and the combustion of the trapped particulates is carried out periodically, and in which, with a view to catalyzing the combustion of said particulates, use is made, as a lubricating composition for the engine, of a composition which results from the mixture of:

   - a lubricating oil; and

   - a colloidal dispersion;

   characterized in that the colloidal dispersion comprises particles of a compound of cerium and iron and an amphiphilic agent, at least 90% of these particles being single-crystal particles and having a D₅₀ value between 1 and 5 nm.
2. Method according to Claim 1, characterized in that the colloidal dispersion further comprises particles of a compound of a rare earth chosen from lanthanum, yttrium, neodymium, gadolinium or praseodymium.
3. Method according to Claim 1 or 2, characterized in that the amphiphilic agent is an acid.
4. Method according to Claim 3, characterized in that the aforementioned acid is a carboxylic acid comprising from 10 to 60 carbon atoms, more particularly from 15 to 25 carbon atoms.
5. Method according to either of Claims 3 and 4, characterized in that the acid is chosen from the fatty acids of tall oil, of soybean oil, of tallow or of linseed oil, oleic acid, linoleic acid, stearic acid and isomers thereof, pelargonic acid, capric acid, lauric acid, myristic acid, dodecylbenzenesulfonic acid, 2-ethylhexanoic acid, naphthenic acid, hexoic acid, toluenesulfonic acid, toluenephosphonic acid, laurylsulfonic acid, laurylphosphonic acid, palmitylsulfonic acid and palmitylphosphonic acid.
6. Method according to one of the preceding claims, characterized in that the particles of the colloidal dispersion have a D₅₀ value between 2 and 3 nm.
7. Method according to one of the preceding claims, characterized in that the content of cerium and of iron, optionally of the aforementioned rare earth, expressed as metal element, originating from the colloidal dispersion in the lubricating composition is at most 15% by weight of the total composition.
8. Method according to one of the preceding claims, characterized in that a lubricating composition is used that comprises the lubricating oil and the colloidal dispersion and which has been prepared prior to its use in the engine.
9. Method according to one of the preceding claims, characterized in that the engine is a diesel engine or a petrol engine operating in lean burn mode.
10. Method according to one of the preceding claims, characterized in that it is carried out with an engine operating with a fuel which contains a catalyst for combustion of the particulates or with an engine equipped with a muffler which comprises a catalyzed particulate filter.