Patent Publication Number: US-2021171854-A1

Title: Lubricant composition for industrial engines with increased fe potential

Description:
The present invention relates to the field of multipurpose lubricants which may be used in the various components of automotive vehicles, notably in the engine of a vehicle, the transmission or the hydraulic circuit. More precisely, the invention relates to the field of lubricants for industrial machines, such as civil engineering machines, typically equipped with industrial diesel engines. The present invention is directed in particular toward proposing the use of specific polymers which improve the viscosity index for the purpose of developing lubricant compositions which show enhanced “FE (fuel economy) potential” over time or CIFE (continuously increasing fuel economy), as explained hereinbelow. This terminology covers lubricants whose FE potential increases in the course of use, which is reflected not only by the fact that the viscosity of the lubricant does not increase significantly in the course of its use in the industrial diesel engine, but also that it is less than the viscosity of the same lubricants before their use. 
     Lubricant compositions, also referred to as “lubricants”, are commonly used in engines for the main purposes of reducing the friction forces between the various metal parts in motion in the engines, the transmission and the hydraulic circuit. They are also efficient for preventing premature wear or even damage of these parts, and in particular of their surface. 
     To do this, a lubricant composition is conventionally composed of a base oil which is generally combined with several additives intended for stimulating the lubricant performance of the base oil, such as polymers which improve the viscosity index and friction-modifying additives. 
     In the field of industrial engines, a single lubricant composition is used directly in several types of application, in particular in the various components of automotive vehicles such as the engines, the transmission devices (gearboxes and transfer boxes), the hydraulic circuits and other secondary components without necessitating modification; in other words, the composition of this fluid is directly suitable for the various types of use in question. 
     Thus, a multipurpose lubricant composition must from the outset meet particular viscosity constraints associated with the fact that the functioning of the various components gives rise to particular viscosities of said lubricant composition in the course of its use. In other words, these constraints make it necessary to target compromises in terms of viscosity and, as a corollary, in the choice of the polymers, which has an impact on the viscosity index. 
     Furthermore, industrial diesel engines are often subjected to harsh or even drastic use. 
     Having available a single lubricant composition or multipurpose composition for lubricating different components of a vehicle, relative to the use of several multipurpose oils, offers advantages notably in terms of ease of maintenance and storage, of servicing of the vehicle or of a fleet of vehicles, of conditioning and of logistics. This is particularly true for large fleets of civil engineering vehicles, which are often used on isolated work sites and subjected to inclement climatic conditions and which do not have suitable storage devices. 
     Finally, added to the need to meet these intrinsic constraints due to the architecture of industrial engines and to the unique use for the various components which constitute them, and also to a potentially prolonged use of these engines, is the need to find lubricant compositions whose viscosity decreases in the course of its use. 
     Lubricant compositions known as “fuel-eco” (FE) (meaning fuel economy) lubricants are known, using polymers with a high viscosity index (VI) and low shear, which have notably been developed for the lubrication of industrial equipment used, for example, in civil engineering or in mines and quarries. These compositions afford a saving in fuel consumption. 
     Thus, when used, the lubricants of the prior art conventionally undergo an increase in viscosity, which has a negative impact on the FE nature of the lubricants. 
     In the case of these lubricants of FE nature, the viscosity of the lubricant composition is reduced, thus enabling FE to be achieved. However, this FE property is not enhanced over time. Effectively, the viscosity of the fluid decreases due to the shear of the polymer, but this is compensated for in service by the appearance of soot and oxidation products, which increase the overall viscosity of the lubricant. 
     GB1575449 discloses a copolymer of conjugated diene and of aromatic vinyl which can be used as a viscosity index enhancer, notably since it improves the oxidation stability of lubricant compositions. 
     WO 2013/066915 discloses a lubricant oil composition comprising a base oil of lubricant viscosity, a viscosity modifier with a low shear stability index, and a viscosity modifier with a high shear stability index. 
     These prior art documents are not directed toward improving the FE potential over time, during the use of the lubricant composition, notably under stress, such as the shear stresses that are conventionally encountered during the use of a lubricant composition in an industrial vehicle, notably a diesel engine industrial vehicle. 
     In other words, there is a need for polymers which improve the viscosity index, for the preparation of multipurpose lubricant compositions whose viscosity decreases in the course of the use of an industrial vehicle, notably of a diesel engine industrial vehicle, and the viscosity of which is lower after use than that of these same lubricant compositions before use, and in particular for all three of the applications, namely the engine, the transmission and the hydraulic circuit. 
     It follows therefrom that the decrease in viscosity that may be observed in the course of the use of the lubricant compositions which correspond to these properties increases over time. 
     Such lubricant compositions, the preparation of which is targeted in the context of the present invention, may thus be termed lubricant compositions with continuously increasing FE (CIFE) properties. 
     In the context of the present invention, said FE properties are also referred to as the FE potential or fuel economy potential. 
     Thus, the invention is directed toward the use of at least one polymer which improves the viscosity index, chosen from hydrogenated copolymers of diene and of aromatic vinyl, in a lubricant composition for improving the fuel economy potential of the lubricant composition in the course of its use during the lubrication of the various components of an industrial vehicle, notably of a diesel engine industrial vehicle, such as the engine, the gearbox and the hydraulic circuit. 
     The invention is directed, precisely, toward proposing the use of at least one polymer which improves the viscosity index, chosen from hydrogenated copolymers of diene and of aromatic vinyl, for the purpose of preparing a lubricant composition intended for lubricating the various components of an industrial vehicle, notably of a diesel engine industrial vehicle, such as the engine, the gearbox and the hydraulic circuit, characterized in that the measured viscosity of said lubricant composition decreases in the course of its use for lubricating said vehicle. 
     The invention is also directed toward proposing the use of at least one polymer which improves the viscosity index, chosen from hydrogenated copolymers of diene and of aromatic vinyl, in a lubricant composition for decreasing the viscosity of said lubricant composition in the course of the use of said lubricant composition during the lubrication of the various components of an industrial vehicle, notably of a diesel engine industrial vehicle, such as the engine, the gearbox and the hydraulic circuit, said lubricant composition undergoing at least one thermal shear during its use. 
     The lubricant composition thus obtained may be used for lubricating the various components of an industrial vehicle and in particular the engine of an industrial vehicle, notably of a diesel engine industrial vehicle, such as the machines used in civil engineering or in mines and quarries. Said lubricant composition thus has a viscosity profile suited to the conditions of use required in each target component, namely the engine, the gearbox and the hydraulic circuit. For the purposes of the present invention, an industrial vehicle is to be distinguished from a motor vehicle. Typically, the conditions of use impose long-term mechanical stresses, such as mechanical shear and thermal shear. In the context of the present invention, the term “thermal shear” means thermal stresses or thermal shear stresses. 
     This thermal shear typically arises during exposure to at least 70° C., in particular at least 90° C., more particularly at least 100° C., even more particularly from 170 to 300° C., for example from 90 to 250° C. or, for example, from 100 to 200° C. 
     The inventors have discovered that the polymer defined in the present invention in a lubricant composition can reduce the viscosity of said lubricant composition during its use, and can do so even when the lubricant composition undergoes at least thermal shear during its use, and more particularly thermal shear and mechanical shear. 
     According to a particular embodiment of the invention, the lubrication under the conditions of use comprising at least thermal shear lasts at least 24 hours, for example at least 30 hours, or even at least 40 hours, 80 hours or 120 hours. 
     According to another embodiment of the invention, the polymer is used in order to reduce the viscosity of the lubricant composition on conclusion of the dynamic road cycle, notably over a period of at least 80 hours, in particular of at least 180 hours and even more particularly of at least 250 hours, for instance that described for step 2 of the engine test of example 3 of the experimental section. 
     Contrary to all expectation, the inventors have discovered that the lubricant composition obtained in accordance with the invention has, on conclusion of prolonged use in an industrial vehicle, a viscosity lower than that of a fresh lubricant composition, this being the case under normal conditions of use. Such normal conditions of use are, for example, understood as being favorable to shear stresses, and more particularly without supplying any external oxygen, i.e. other than the oxygen of the ambient air. Typically, the use targeted in the present invention is to be distinguished from a use for improving the oxidation stability. 
     In other words, the present invention is directed toward proposing the use of at least one polymer which improves the viscosity index, chosen from hydrogenated copolymers of diene and of aromatic vinyl, in a lubricant composition for decreasing the viscosity of said lubricant composition in the course of the use of said lubricant composition during the lubrication of the various components of an industrial vehicle, notably of a diesel engine industrial vehicle, such as the engine, the gearbox and the hydraulic circuit, said lubricant composition undergoing at least one thermal shear during its use, without supplying external oxygen. 
     The examples hereinbelow thus demonstrate that the composition in accordance with the invention, as obtained on conclusion of the use, which is the subject of the present invention, makes it possible to conserve the grade according to the classification SAEJ300 after prolonged use in a diesel engine industrial vehicle. 
     To model and prove this property, the inventors thus in particular demonstrated that the compositions obtained with the use of the copolymers for improving the viscosity in accordance with the present invention
         (i) have a kinematic viscosity after firing at 150° C. for 504 hours lower than that of the composition before firing, and   (ii) have a kinematic viscosity after the Bosch-90 cycles cycle lower than that of the composition before this test,   (iii) enable CIFE to be achieved after an endurance test performed on an industrial engine, notably a diesel industrial engine.       

     The inventors also demonstrated the decrease in the viscosity of said lubricant composition in the course of these two tests (i) and (ii) notably as illustrated in example 3. 
     The inventors also demonstrated that the decrease in the viscosity of said lubricant composition in the course of test (iii), notably as illustrated in example 4, enables CIFE to be achieved. 
     Thus, as also emerges from the examples below and notably from example 2, the hydrogenated copolymers of diene and of aromatic vinyl are the only polymers improving the viscosity index which have this property of gradually decreasing the viscosity of said lubricant composition in the course of the use in a diesel engine industrial vehicle and thus of producing lubricant compositions that enable CIFE to be achieved. 
     The present invention also relates to the use of a composition comprising at least one base oil and at least one polymer which improves the viscosity index, chosen from hydrogenated copolymers of diene and of aromatic vinyl, for lubricating the various components of an industrial vehicle, and notably of a diesel engine industrial vehicle, such as the engine, the gearbox and the hydraulic circuit, in particular the engine of an industrial vehicle, notably of a diesel engine industrial vehicle, characterized in that the measured viscosity of said lubricant composition decreases in the course of its use for lubricating said vehicle. 
     According to one embodiment of the invention, the polymer is used in order to reduce the viscosity of the lubricant composition by at least 4%, preferably by at least 8%, more preferably by at least 10%, preferentially by at least 12% after conditioning the lubricant composition at 150° C. for 504 hours. 
     According to one embodiment of the invention, the polymer is used in order to reduce the viscosity of the lubricant composition by at least 5%, preferably by at least 10%, more preferably by at least 12%, preferentially by at least 15% on conclusion of the dynamic road cycle, for instance that described for step 2 of the engine test of example 3 of the experimental section. 
     The invention also relates to a process for lubricating the various components of an industrial vehicle, and notably of a diesel engine industrial vehicle, such as the engine, the gearbox and the hydraulic circuit, in particular the engine of an industrial vehicle, notably of a diesel engine industrial vehicle, comprising the placing of said components in contact with a lubricant composition comprising at least one base oil and at least one polymer which improves the viscosity index, chosen from hydrogenated copolymers of diene and of aromatic vinyl, characterized in that the measured viscosity of said lubricant composition decreases in the course of the lubrication of said components, said lubricant composition undergoing at least one thermal shear in the course of the lubrication, more particularly undergoing at least one thermal shear and at least one mechanical shear, in particular without supplying external oxygen. 
     As indicated above, according to another embodiment of the invention, the lubrication in the course of the process comprising at least the thermal shear lasts at least 24 hours, for example at least 30 hours, or even at least 40 hours, 80 hours or 120 hours. 
     As indicated above, according to another embodiment of the invention, the polymer makes it possible to reduce the viscosity of the lubricant composition on conclusion of the dynamic road cycle, notably over a period of at least 80 hours, in particular of at least 180 hours and even more particularly of at least 250 hours, for instance that described for step 2 of the engine test of example 3 of the experimental section. 
    
    
     
         FIG. 1  illustrates the behavior of the viscosity of compositions in accordance and not in accordance with the invention at 100° C. after Bosch 90 cycles tests (example 2). 
         FIGS. 2 and 3  illustrate the CIFE behavior of the compositions in accordance with the invention during the endurance test performed on an industrial diesel engine and relate to example 3 (viscosity measurement curves). 
     
    
    
     In the context of the invention, the lubricant compositions under consideration are graded according to the SAEJ300 classification, defined by the formula (X)W(Y), in which X represents 5, 10 or 15 and Y represents 30 or 40. 
     This SAEJ300 classification defines the viscosity grades of new engine oils notably by measuring their kinematic viscosities at 100° C. 
     The grade qualifies a selection of lubricant compositions specifically intended for industrial vehicle use and which notably meet quantified specificities with respect to various parameters such as the multipurpose nature with respect to the various components, the cold start viscosity, the cold pumpability, the low-shear kinematic viscosity and the high-shear dynamic viscosity at high temperature. 
     An engine oil is of grade 30 according to SAEJ300 if its kinematic viscosity at 100° C. is from 9.3 to 12.5 cSt. 
     An engine oil is of grade 40 according to SAEJ300 if its kinematic viscosity at 100° C. is from 12.5 to 16.3 cSt. 
     The ACEA standards define in detailed manner a certain number of additional specifications for engine oils, and notably impose the maintenance of a certain viscosity level for the oils in service subjected to shear in the engine. 
     Thus, according to the sequence ACEA E7 or E9, the kinematic viscosity of grade 30 and 40 engine oils, measured at 100° C., after the Bosch-90 cycles test, must be, respectively, greater than 9.3 and 12.5 cSt. 
     These lubricant compositions in accordance with the present invention have a kinematic viscosity at 100° C. of greater than 9.3 cSt, preferably in the range from 9.3 to 12.5 cSt after the Bosch-90 cycles test according to the standard CEC-L-14-A-93 for a starting oil of grade 30. 
     These lubricant compositions in accordance with the present invention have a kinematic viscosity at 100° C. of greater than 13.0 cSt, preferably in the range from 13.0 to 15.0 cSt after the Bosch-90 cycles test according to the standard CEC-L-14-A-93 for a starting oil of grade 40. 
     Other characteristics, variants and advantages of the lubricant compositions in accordance with the invention will emerge more clearly on reading the description and the examples that follow, which are given as nonlimiting illustrations of the invention. 
     In the continuation of the text, the expressions “between . . . and . . . ”, “ranging from . . . to . . . ” and “varying from . . . to . . . ” are equivalent and are intended to mean that the limits are included, unless otherwise mentioned. 
     In the context of the present invention, the standard CEC-L-14-A-93 (or ASTM D6278) defines the tests representative of the shear conditions in the engine, known as the Bosch-90 cycles test. 
     Without further mention in the continuation of the text, the term “Bosch-90 cycles” refers to said standard. 
     To characterize the lubricant composition in accordance with the present invention, the Applicant defined the representative shear conditions of the engine. 
     Lubricant Composition in Accordance with the Invention 
     Polymer for Improving the Viscosity Index Chosen from Hydrogenated Copolymers of Diene and of Aromatic Vinyl 
     In the context of the present invention, the diene may be a conjugated diene comprising from 4 to 20 carbon atoms, preferably from 2 to 12 carbon atoms. 
     In particular, the diene may be a conjugated diene comprising from 2 to 20 carbon atoms, preferably from 4 to 12 carbon atoms. 
     Preferably, the diene may be chosen from butadiene, isoprene, piperylene, 4-methylpenta-1,3-diene, 2-phenyl-1,3-butadiene, 3,4-dimethyl-1,3-hexadiene and 4,5-diethyl-1,3-octadiene. 
     Advantageously, the diene may be an isoprene or a butadiene. 
     In the context of the present invention, the aromatic vinyl may comprise from 8 to 16 carbon atoms. 
     Preferably, the aromatic vinyl may be chosen from styrene, alkoxystyrene, vinylnaphthalene and alkylvinylnaphthalene. Typically, the alkoxy and alkyl groups comprise from 1 to 6 carbon atoms. 
     Advantageously, the aromatic vinyl is styrene. 
     Advantageously, the polymer in accordance with the invention may be chosen from a hydrogenated copolymer of isoprene and styrene (HCIS), a hydrogenated copolymer of isoprene, butadiene and styrene, a hydrogenated copolymer of butadiene and styrene (HCBS), and a mixture thereof. 
     According to a preferred embodiment, the polymer in accordance with the invention may be chosen from a hydrogenated copolymer of isoprene and styrene (HCIS), a hydrogenated copolymer of butadiene and styrene (HCBS), and a mixture thereof. 
     According to this preferred embodiment, the copolymer used in the present invention is not a copolymer of isoprene, butadiene and styrene. Still according to this preferred embodiment, the copolymer used in the present invention is not a terpolymer. 
     For example, the hydrogenated copolymers of isoprene and styrene and the hydrogenated copolymers of isoprene, butadiene and styrene for the purposes of the invention are described in patent application EP 2 363 454 and the structures and definitions of these polymers as described in EP 2 363 454 are incorporated into the description of the present patent application. 
     In the context of the present invention, the hydrogenated copolymer of diene and styrene may be a block copolymer or a star copolymer. 
     In the context of the present invention, the polymers according to the present invention may have a number-average molecular mass from about 10 000 to 700 000, preferably from about 30 000 to 500 000. The term “number-average molecular mass” as used herein denotes the number-average weight measured by gel permeation chromatography (GPC) with a polymer standard, after hydrogenation. 
     According to a preferred embodiment, the HCIS and HCBS copolymers do not comprise any monomer additional to the monomers, respectively, of hydrogenated isoprene and styrene and of hydrogenated butadiene and styrene. 
     According to a particular embodiment, the polymer is a hydrogenated copolymer of isoprene and styrene (HCIS). 
     For example, among the HCIS copolymers that are suitable for use in the present invention, mention may be made of the copolymers having the formula (I) or (II) below: 
     
       
         
         
             
             
         
       
     
     with R1, R2, R3 and R4: (hydrogenated) isoprene/styrene/isoprene copolymers, l, m, n and o are, independently of each other, integers greater than or equal to 0 such that the number-average molar mass of the copolymer ranges from 10 000 to 700 000. 
     These copolymers of formula (II) are star copolymers, obtained by reaction of isoprene/styrene/isoprene block copolymers with divinylbenzene followed by hydrogenation, according to techniques known to those skilled in the art. 
     For the purposes of the invention, hydrogenated copolymers of isoprene and styrene (HCIS) or hydrogenated copolymers of isoprene, butadiene and styrene that may notably be mentioned include those sold under the names linear SV154, star SV300 (pure or diluted in the form SV301), star SV260 (pure or diluted in the form SV 261) by the company Infineum and Lz 7306 by the company Lubrizol. 
     According to a particular embodiment, the polymer is a hydrogenated copolymer of butadiene and styrene (HCBS). 
     For example, among the HCBS copolymers that are suitable for use in the present invention, mention may be made of the copolymers having the formula (I′) or (II′) below: 
     
       
         
         
             
             
         
       
     
     with R1′, R2′, R3′ and R4′: (hydrogenated) butadiene/styrene/butadiene copolymers, l, m, n and o are, independently of each other, integers greater than or equal to 0 such that the number-average molar mass of the copolymer ranges from 10 000 to 700 000. 
     These copolymers of formula (II′) are star copolymers, obtained by reaction of butadiene/styrene/butadiene block copolymers with divinylbenzene followed by hydrogenation. 
     As HCBS copolymers, mention may notably be made of those sold under the name Lz 7408 (pure or diluted in the form Lz 7418A) by the company Lubrizol or Hitec 6005 by the company Afton Chemicals. 
     Thus, according to a particular embodiment of the invention, the hydrogenated copolymer of isoprene and styrene (HCIS) and the hydrogenated copolymer of butadiene and styrene (HCBS) are of star type. 
     In particular, the content of polymer(s) for improving the viscosity index in the lubricant composition according to the invention is from 0.1% to 10% by weight, relative to the total weight of the lubricant composition, preferably from 0.1% to 8%, more preferentially from 0.1% to 5%, even more preferentially from 0.1% to 2%. This amount is understood as an amount of polymer active material. Specifically, the polymer used in the context of the present invention may be in the form of a dispersion in a mineral or synthetic or pure oil. 
     In particular also, a composition used according to the invention may comprise from 1% to 25% by weight, preferably from 2% to 20% by weight, more preferentially from 4% to 20% by weight of polymer(s) for improving the viscosity index diluted in a base oil, relative to the total weight of the composition. 
     It falls to a person skilled in the art to adapt the content of copolymer as defined above to be used in a lubricant composition. 
     Thus, according to a particular embodiment, the present invention also relates to the use of a composition comprising at least one base oil and a polymer which improves the viscosity index, chosen from a hydrogenated copolymer of isoprene and styrene (HCIS) and a hydrogenated copolymer of butadiene and styrene (HCBS), for lubricating the various components of an industrial vehicle, and notably of a diesel engine industrial vehicle, such as the engine, the gearbox and the hydraulic circuit, in particular the engine of an industrial vehicle, notably of a diesel engine industrial vehicle, characterized in that the measured viscosity of said lubricant composition decreases in the course of its use for lubricating said vehicle, said lubricant composition undergoing at least one thermal shear during its use, more particularly undergoing at least one thermal shear and at least one mechanical shear, in particular without supplying any external oxygen. 
     The copolymers defined above may be mixed with one or base oils, in particular as defined below, to form a ready-to-use lubricant composition. Alternatively, they may be added alone or as a mixture with one or more other additives, as defined below, as additives intended to be added to a mixture of base oils for improving the properties of the lubricant composition. 
     According to one embodiment of the invention, the use in accordance with the present invention is characterized in that the lubricant composition comprises a base oil from groups I to V, more particularly II or III, and optionally an additive pack and optionally a pour-point enhancer. 
     Base Oil 
     The base oils used in the lubricant formulation according to the present invention are oils, of mineral, synthetic or natural origin, used alone or as a mixture, belonging to groups I to V according to the API classification (table A), or the equivalents thereof according to the ATIEL classification, or mixtures thereof, one of the characteristics of which is that they are insensitive to shear, i.e. their viscosity is not modified under shear. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE A 
               
               
                   
                   
               
               
                   
                 Content of 
                 Sulfur 
                 Viscosity 
               
               
                   
                 saturates 
                 content 
                 index (VI) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Group I 
                  &lt;90% 
                 &gt;0.03% 
                 80 ≤ VI &lt; 120 
               
               
                 Mineral oils 
               
               
                 Group II 
                 ≥90% 
                 ≤0.03% 
                 80 ≤ VI &lt; 120 
               
               
                 Hydrocracked oils 
               
               
                 Group III 
                 ≥90% 
                 ≤0.03% 
                 ≥120 
               
               
                 Hydrocracked or 
               
               
                 hydroisomerized 
               
               
                 oils 
               
            
           
           
               
               
            
               
                 Group IV 
                 Poly-α-olefins (PAO) 
               
               
                 Group V 
                 Esters and other bases not included in groups I to IV 
               
               
                   
               
            
           
         
       
     
     The mineral base oils include all types of bases obtained by atmospheric and vacuum distillation of crude oil, followed by refining operations such as solvent extraction, deasphalting, solvent deparaffinning, hydrotreating, hydrocracking, hydroisomerization and hydrofinishing. 
     The synthetic base oils may be esters of carboxylic acids and of alcohols or poly-α-olefins or polyalkylene glycols. The poly-α-olefins used as base oils are obtained, for example, from monomers comprising 4 to 32 carbon atoms, for example from decene, octene or dodecene, and with a viscosity at 100° C. of between 1.5 and 15 mm 2 ·s −1  according to the standard ASTM D445. Their average molecular mass is generally between 250 and 3000 according to the standard ASTM D5296. 
     The polyalkylene glycols are obtained by polymerization or copolymerization of alkylene oxides comprising from 2 to 8 carbon atoms, in particular from 2 to 4 carbon atoms. 
     Mixtures of synthetic and mineral oils may also be used. 
     There is generally no limit as regards the use of different lubricant bases to produce the lubricant compositions according to the invention, other than the fact that they must have properties, notably in terms of viscosity, viscosity index, sulfur content and oxidation resistance, which are suitable for use for the various components of an industrial vehicle, such as the engine, the gearbox and the hydraulic circuit, in particular for industrial vehicle engines. Needless to say, they must also not affect the properties afforded by the oil(s) with which they are combined. 
     According to a particular embodiment, the lubricant composition in accordance with the present invention, the use of which is the subject of the present invention, uses a base oil from group II. 
     They represent in the lubricant composition in accordance with the invention at least 50% by weight, relative to the total weight of the composition, in particular at least 60% by weight and more particularly between 60% and 90% by weight. 
     Additives 
     The composition in accordance with the present invention may also comprise additives or “an additive pack” according to the terminology conventionally used in the field of multipurpose lubricant compositions. 
     The additive packs used in the lubricant formulations in accordance with the invention are conventional and also known to a person skilled in the art and meet performance levels defined, inter alia, by the ACEA (Association des Constructeurs Européens d&#39;Automobiles) and/or the API (American Petroleum Institute). 
     A lubricant composition according to the invention may thus comprise one or more additives chosen from friction-modifying additives, antiwear additives, extreme-pressure additives, detergent additives, antioxidant additives, viscosity index (VI) enhancers other than the hydrogenated copolymers of diene and of aromatic vinyl, pour-point depressant (PPD) additives, dispersants, antifoams, thickeners, and mixtures thereof. 
     As regards the friction-modifying additives, they may be chosen from compounds providing metal elements and ash-free compounds. 
     Among the compounds providing metal elements, mention may be made of complexes of transition metals such as Mo, Sb, Sn, Fe, Cu or Zn, the ligands of which may be hydrocarbon-based compounds comprising oxygen, nitrogen, sulfur or phosphorus atoms. 
     The ash-free friction-modifying additives are generally of organic origin and may be chosen from fatty acid monoesters of polyols, alkoxylated amines, alkoxylated fatty amines, fatty epoxides, borate fatty epoxides, fatty amines or fatty acid esters of glycerol. According to the invention, the fatty compounds comprise at least one hydrocarbon-based group comprising 10 to 24 carbon atoms. 
     According to an advantageous variant, a lubricant composition according to the invention comprises at least one friction-modifying additive, in particular based on molybdenum. 
     In particular, the molybdenum-based compounds may be chosen from molybdenum dithiocarbamates (Mo-DTC), molybdenum dithiophosphates (Mo-DTP), and mixtures thereof. 
     According to a particular embodiment, a lubricant composition according to the invention comprises at least one Mo-DTC compound and at least one Mo-DTP compound. A lubricant composition may notably comprise a molybdenum content of between 1000 and 2500 ppm. 
     Advantageously, such a composition makes it possible to make additional fuel savings. 
     Advantageously, a lubricant composition according to the invention may comprise from 0.01% to 5% by weight, preferably from 0.01% to 5% by weight, more particularly from 0.1% to 2% by weight or even more particularly from 0.1% to 1.5% by weight, relative to the total weight of the lubricant composition, of friction-modifying additives, advantageously including at least one molybdenum-based friction-modifying additive. 
     As regards the antiwear additives and the extreme-pressure additives, they are more particularly directed toward protecting the friction surfaces by forming a protective film adsorbed onto these surfaces. A wide variety of antiwear additives exists. 
     Antiwear additives chosen from polysulfide additives, sulfur-based olefin additives or phospho-sulfur-based additives, such as metal alkylthiophosphates, in particular zinc alkylthiophosphates and more specifically zinc dialkyldithiophosphates or ZnDTP, are most particularly suitable for use as lubricant compositions according to the invention. The preferred compounds are of formula Zn((SP(S)(OR)(OR′)) 2 , in which R and R′, which may be identical or different, independently represent an alkyl group preferentially including from 1 to 18 carbon atoms. 
     Advantageously, a lubricant composition according to the invention may comprise from 0.01% to 6% by weight, preferentially from 0.05% to 4% by weight and more preferentially from 0.1% to 2% by weight, relative to the total weight of the composition, of antiwear additives and of extreme-pressure additives. 
     As regards the antioxidant additives, they are essentially dedicated toward retarding the degradation of the lubricant composition in service. This degradation may notably be reflected by the formation of deposits, the presence of sludges, or an increase in the viscosity of the lubricant composition. They act notably as free-radical inhibitors or hydroperoxide destroyers. Among the commonly used antioxidant additives, mention may be made of antioxidants of phenolic type, antioxidant additives of amine type and phospho-sulfur-based antioxidant additives. Some of these antioxidant additives, for example the phospho-sulfur-based antioxidant additives, may be ash generators. The phenolic antioxidants additives may be ash-free or may be in the form of neutral or basic metal salts. The antioxidants additives may notably be chosen from sterically hindered phenols, sterically hindered phenol esters and sterically hindered phenols comprising a thioether bridge, diphenylamines, diphenylamines substituted with at least one C 1 -C 12  alkyl group, N,N′-dialkyl-aryl-diamines, and mixtures thereof. 
     Preferably, the sterically hindered phenols are chosen from compounds comprising a phenol group, in which at least one carbon vicinal to the carbon bearing the alcohol function is substituted with at least one C 1 -C 10  alkyl group, preferably a C 1 -C 6  alkyl group, preferably a C 4  alkyl group, preferably with a tert-butyl group. 
     Amine compounds are another class of antioxidant additives that may be used, optionally in combination with the phenolic antioxidants additives. Examples of amine compounds are aromatic amines, for example the aromatic amines of formula NR 5 R 6 R 7  in which R 5  represents an optionally substituted aliphatic or aromatic group, R 6  represents an optionally substituted aromatic group, R 7  represents a hydrogen atom, an alkyl group, an aryl group or a group of formula R 8 S(O) z R 9  in which R 8  represents an alkylene group or an alkenylene group, R 9  represents an alkyl group, an alkenyl group or an aryl group and z represents 0, 1 or 2. 
     Sulfurized alkylphenols or the alkali metal or alkaline-earth metal salts thereof may also be used as antioxidant additives. 
     The lubricant composition according to the invention may contain any type of antioxidant additive known to those skilled in the art. Advantageously, the lubricant composition comprises at least one ash-free antioxidant additive. 
     Advantageously also, a lubricant composition according to the invention may comprise from 0.1% to 2% by weight, relative to the total weight of the composition, of at least one antioxidant additive. 
     As regards the detergent additives, they generally make it possible to reduce the formation of deposits on the surface of metal parts by dissolving the oxidation and combustion by-products. 
     The detergent additives that may be used in a 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-based 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 preferentially chosen from alkali metal or alkaline-earth metal salts of carboxylic acids, sulfonates, salicylates and naphthenates, and also phenate salts. The alkali metals and alkaline-earth metals are preferentially calcium, magnesium, sodium or barium. These metal salts generally comprise the metal in a stoichiometric amount or in excess, thus in an amount greater than the stoichiometric amount. They are then overbased detergent additives; the excess metal giving the overbased nature to the detergent additive is then generally in the form of a metal salt that is insoluble in the base oil, for example a carbonate, a hydroxide, an oxalate, an acetate or a glutamate, preferentially a carbonate. 
     A lubricant composition according to the invention may comprise from 0.5% to 8% by weight and preferably from 0.5% to 4% by weight of detergent additive relative to the total weight of the lubricant composition. 
     Advantageously, a lubricant composition according to the invention may comprise less than 4% by weight of detergent additive(s), in particular less than 2% by weight, notably less than 1% by weight, or may even be free of detergent additive. 
     As regards the pour-point depressant (PPD) additives, they make it possible, by slowing down the formation of paraffin crystals, to improve the cold-weather behavior of the lubricant composition according to the invention. 
     Examples of pour-point depressants that may be mentioned include polyalkyl methacrylates, polyacrylates, polyarylamides, polyalkylphenols, polyalkylnaphthalenes and polyalkylstyrenes. 
     As regards the dispersants, they ensure the holding in suspension and the removal of insoluble solid contaminants constituted by the oxidation by-products that are formed when the lubricant composition is in service. They may be chosen from Mannich bases, succinimides and derivatives thereof. 
     In particular, a lubricant composition according to the invention may comprise from 0.2% to 10% by weight of dispersant(s) relative to the total weight of the composition. 
     Additional viscosity index (VI) enhancers, other than the hydrogenated copolymers of diene and of aromatic vinyl, may also be present in a lubricant composition in accordance with the present invention. These viscosity index (VI) enhancers may be present in a composition in accordance with the present invention in contents which do not disrupt the effect desired in the context of the present invention, namely the CIFE effect. These additional viscosity index (VI) enhancers, in particular the additional viscosity index-enhancing polymers, make it possible to ensure good cold-weather behavior and a minimal viscosity at high temperature. Examples of viscosity index-enhancing polymers that may be mentioned include polymeric esters, homopolymers or copolymers of olefins, such as ethylene or propylene, polyacrylates and polymethacrylates (PMA). 
     In particular, a lubricant composition according to the invention may comprise from 1% to 15% by weight of additional viscosity index-enhancing additive(s) relative to the total weight of the lubricant composition. 
     The antifoam additives may be chosen from polar polymers such as polymethylsiloxanes or polyacrylates. 
     In particular, a lubricant composition according to the invention may comprise from 0.01% to 3% by weight of antifoam additive(s) relative to the total weight of the lubricant composition. 
     The additive packs ready to be incorporated into a lubricant composition comprise between 20% and 30% by weight of a diluent consisting of base oil. The weight percentage of additive pack relative to the weight of the lubricant composition in accordance with the invention is at least 5%, the diluent being included in this percentage. 
     According to one embodiment, the lubricant composition in accordance with the invention comprises from 10% to 25% by weight, notably from 10% to 20% by weight and more particularly from 13% to 18% by weight of an additive pack, relative to the weight of the composition. 
     Characterization of the Lubricant Composition in Accordance with the Invention 
     Preferably, a composition in accordance with the present invention has a kinematic viscosity at 100° C. of between 9.3 and 16.3 cSt measured by the standard ASTM D445 (grade SAE 30 and 40). 
     According to a particular embodiment, the grade according to the classification SAEJ300 of a lubricant composition according to the invention is chosen from 5W30, 10W30, 10W40 and 15W40. 
     According to a particular embodiment, a composition in accordance with the present invention has a viscosity index VI of between 140 and 165. 
     In the context of the present invention, the viscosity index is measured according to the standard ASTM D2270-93, as is the case in example 1 below. According to a particular embodiment of the invention, the use, which is the subject of the invention, is also characterized in that the measured kinematic viscosity of said lubricant composition decreases by at least 0.5 mm 2 /s, preferably by at least 0.6 mm 2 /s, even more preferably by at least 0.8 mm 2 /s, for example by at least 1 mm 2 /s, when said lubricant composition is used in the test described below, relative to the initial kinematic viscosity before using said lubricant composition in said test: 
     150 g of lubricant composition are placed in a ventilated oven heated at 150° C. for 504 hours. On conclusion of this test, a sample of the lubricant composition is taken and the kinematic viscosity of this composition at 100° C. according to the standard ASTM D445-97 (mm 2 /s) is measured. 
     Examples of this decrease in kinematic viscosity observed for the compositions in accordance with the present invention after the thermal stability test are given in example 2. 
     Use of the Copolymers for Preparing a Lubricant Composition 
     The lubricant compositions in accordance with the invention find a particularly advantageous application as lubricants for the various components of an industrial vehicle, such as the engines, the transmission systems (gearbox and transfer box), the hydraulic circuits and other secondary components, and notably for an industrial vehicle engine, in particular a diesel engine. 
     They make it possible, by virtue of their viscosity properties, not only to lubricate these various components but also to extend the intervals between oil changes and to achieve fuel savings. 
     Process for Preparing a Lubricant Composition 
     A lubricant composition in accordance with the invention may be prepared according to the conventional methods known to those skilled in the art. 
     The invention will now be described by means of the examples that follow, which are, needless to say, given as nonlimiting illustrations of the invention. 
     EXAMPLES 
     Example 1: Preparation of the Lubricant Compositions 
     Table 1 below shows the detail of the lubricant compositions according to the invention (LC) and of the comparative compositions (CC), for which the contents are expressed as mass percentages, and also the physicochemical properties thereof. 
     The lubricant compositions are obtained by simple mixing at room temperature of the following components: 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 CC1  
                 CC2  
                 LC1  
                 LC2  
                 LC3  
                 LC4  
                 LC5  
                 LC6  
                 CC3  
                 CC4 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Base oil 1  (1)    
                 5.1  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
               
               
                 Base oil 2  (2)    
                 34.5  
                 33.3  
                 34.5  
                 34.8  
                 25.1  
                 26.7  
                 38.9  
                 39.4  
                 35.3  
                 36.2  
               
               
                 Base oil 3  (2)    
                 44  
                 39.6  
                 44  
                 44  
                 44  
                 44  
                 44  
                 44  
                 44  
                 44  
               
               
                 Additive pack  (3)    
                 16.2  
                 16.2  
                 16.2  
                 16.2  
                 16.2  
                 16.2  
                 16.2  
                 16.2  
                 16.2  
                 16.2  
               
               
                 Pour point  
                 0.2  
                 0.2  
                 0.2  
                 0.2  
                 0.2  
                 0.2  
                 0.2  
                 0.2  
                 0.2  
                 0.2  
               
               
                 depressant  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 additive  (4)    
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Polymer 1  (5)    
                 0  
                 10.7  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
               
               
                 Polymer 2  (6)    
                 0  
                 0  
                 0  
                 0  
                 0  
                 12.8  
                 0  
                 0  
                 0  
                 0  
               
               
                 Polymer 3  (7)    
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0.7  
                 0  
                 0  
                 0  
               
               
                 Polymer 4  (8)    
                 0  
                 0  
                 0  
                 4.8  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
               
               
                 Polymer 5  (9)    
                 0  
                 0  
                 5.1  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
               
               
                 Polymer 6  (10)    
                 0  
                 0  
                 0  
                 0  
                 14.5  
                 0  
                 0  
                 0  
                 0  
                 0  
               
               
                 Polymer 7  (11)    
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0.2  
                 0  
                 0  
               
               
                 Polymer 8  (12)    
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
                 4.3  
                 0  
               
               
                 Polymer 9  (13)    
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
                 0  
                 3.4  
               
               
                 KV 40° C.  
                 54.3  
                 92.5  
                 84.2  
                 84.9  
                 77.1  
                 74.9  
                 86.0  
                 79.3  
                 70.3  
                 75.6  
               
               
                 ASTM D445-97  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 (mm 2 /s)  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 KV 100° C.  
                 8.4  
                 12.1  
                 12.3  
                 12.4  
                 12.2  
                 12.4  
                 12.4  
                 12.4  
                 12.3  
                 12.4  
               
               
                 ASTM D445-97  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 (mm 2 /s)  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Viscosity index  
                 128  
                 123  
                 142  
                 142  
                 155  
                 164  
                 140  
                 153  
                 175  
                 163  
               
               
                 (VI) ASTM  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 D2270-93 
               
               
                   
               
               
                   (1)  Base oil 1 is a base oil from group I (kinematic viscosity at 100° C. measured according to the standard ASTM D445 = 5.30 mm 2 /s) commercially available, for example, from the company TOTAL under the trade name 150 NS 
               
               
                   (2)  Base oil 2 is a base oil from group II (kinematic viscosity at 100° C. measured according to the standard ASTM D445 = 4.10 mm 2 /s) commercially available, for example, from the company Chevron under the trade name 100R 
               
               
                 Base oil 3 is a base oil from group II (kinematic viscosity at 100° C. measured according to the standard ASTM D445 = 6.4 mm 2 /s) commercially available, for example, from the company Chevron under the trade name 220R 
               
               
                   (3)  A conventional additive pack comprising, at least, a dispersant, detergents, an antiwear agent, antioxidants and friction modifiers 
               
               
                   (4)  A pour point depressant additive which is a conventional polymethacrylate polymer commercially available from the company Evonik under the trade name Viscoplex ®  
               
               
                   (5)  Polymer 1 (outside the invention) is a polyisobutylene polymer commercially available from the company Ineos under the trade name Indopole ® H300 
               
               
                   (6)  Polymer 2 is a hydrogenated styrene-butadiene polymer commercially available from the company Lubrizol under the trade name Lz ® 7418 
               
               
                   (7)  Polymer 3 is a hydrogenated styrene-butadiene polymer commercially available from the company Afton under the trade name Hitec ® 6005 
               
               
                   (8)  Polymer 4 is a star hydrogenated isoprene-styrene polymer commercially available from the company Infineum under the trade name SV ® 301 
               
               
                   (9)  Polymer 5 is a star hydrogenated isoprene-styrene polymer commercially available from the company Infineum under the trade name SV ® 261 
               
               
                   (10)  Polymer 6 is a linear hydrogenated isoprene-styrene polymer commercially available (from the company Infineum under the trade name SVC) 154 
               
               
                   (11)  Polymer 7 is a hydrogenated isoprene-styrene polymer commercially available from the company Lubrizol under the trade name Lz ® 7306 
               
               
                   (12)  Polymer 8 is a polymethacrylate polymer commercially available from the company Evonik under the trade name Viscoplex ® 6-950 
               
               
                   (13)  Polymer 9 is a polymethacrylate polymer commercially available from the company Evonik under the trade name Viscoplex ® 6-850 
               
               
                   (14)  Polymer 10 is a polymethacrylate polymer commercially available from the company Sanyo Chemical under the trade name AClub ® V10-70 
               
            
           
         
       
     
     Example 2: Compared Viscosity Behavior for Illustrating the Decrease in Viscosity in the Course of its Use 
     The present examples were performed for the purpose of demonstrating the selection made from among the viscosity index-enhancing polymers, for preparing lubricant compositions which have CIFE properties as targeted in the context of the present invention. 
     The tests performed are the following:
         Thermal stability at 150° C.       

     150 g of lubricant composition are placed in a ventilated oven heated at 150° C. for 504 hours. On conclusion of this test, a sample of the lubricant composition is taken and the kinematic viscosity of this composition at 100° C. is measured according to the standard ASTM D445-97 (mm 2 /s). 
     The kinematic viscosities of the comparative compositions and of the compositions according to the invention as described in table 1, which were first subjected to the thermal stability test as described above, were measured and collated in table 2 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 KV 100° C. 
                 KV 100° C. 
               
               
                   
                 ASTM D445-97 
                 ASTM D445-97 (mm 2 /s) 
               
               
                   
                 (mm 2 /s) before 
                 after thermal stability 
               
               
                   
                 thermal stability test 
                 test at 150° C. 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 CC1 
                 8.4 
                 8.6 
               
               
                   
                 CC2 
                 12.1 
                 13.1 
               
               
                   
                 LC1 
                 12.3 
                 10.6 
               
               
                   
                 LC2 
                 12.4 
                 10.3 
               
               
                   
                 LC3 
                 12.2 
                 10.9 
               
               
                   
                 LC4 
                 12.4 
                 10.8 
               
               
                   
                 LC5 
                 12.4 
                 11.8 
               
               
                   
                 LC6 
                 12.4 
                 10.8 
               
               
                   
                 CC3 
                 12.3 
                 12.7 
               
               
                   
                 CC4 
                 12.4 
                 13.1 
               
               
                   
                   
               
            
           
         
       
     
     It emerges from these results that the compositions according to the invention have a kinematic viscosity at 100° C., measured according to the standard ASTM D445-97 after the thermal stability test, which decreases over time relative to their kinematic viscosities measured before the stability test. 
     It also emerges from these results that the comparative compositions have a kinematic viscosity at 100° C., measured according to the standard ASTM D445-97 after the thermal stability tests, which increases over time relative to their kinematic viscosities measured before the stability tests. 
     These results illustrate the change in the decrease in viscosity of the compositions according to the invention as a function of time and, consequently, the behavior as required according to the present invention, namely the decrease in viscosity as a function of time of the compositions in accordance with the invention, during a thermal shear, in contrast with the comparative compositions for which an increase in viscosity is observed during a thermal shear. 
     These results also demonstrate the impact of the chemistry of the polymers on the viscosity profile of the lubricant compositions during a thermal shear. 
     Specifically, the polymers according to the invention make it possible to obtain compositions whose viscosity decreases during a thermal shear, in contrast with the polymers outside the invention, which, when they are in a lubricant composition, do not make it possible to reduce the viscosity of said composition during a thermal shear; quite to the contrary, the viscosity of said composition increases. 
     Mechanical Stability 
     Viscosity Measurement at 100° C. of the Oil Sheared after Bosch 90 Cycles Tests (KV100° C. Bosch 90 Cycles) 
     The compositions described in example 1 were subjected to a mechanical shear (Bosch 90 cycles injector test). 
       FIG. 1  illustrates the phenomenon of the decrease in viscosity of the compositions as a function of the Bosch cycle number. 
     This figure also illustrates the behavior as required according to the present invention, including after a mechanical shear. 
     As a general conclusion to example 2, following the thermal and mechanical shears, it is observed that only the lubricant compositions in accordance with the invention have viscosity values lower than that of the same composition before shear. 
     Consequently, the kinematic viscosities of the compositions according to the invention after thermal and mechanical stability tests do not increase over time, but quite to the contrary decrease. This demonstrates that the compositions according to the invention correspond to the CIFE properties. Specifically, the more the viscosity of a composition increases, the more the various lubricated components of the engine consume energy and, consequently, fuel. 
     Example 3: Engine Tests 
     Engine tests were performed on the lubricant compositions as described in example 1. 
     Test Principle 
     The engine tests are performed on a Volvo D11 €5 engine (440 HP), for which the thermal management of the oil is deliberately set at 118° C. of oil sump temperature, in order to be representative of hot running conditions and thus to promote the shear of the lubricant compositions via the thermal effect. 
     Each lubricant composition test is characterized as follows:
         Step 1: fresh oil, measurement of fuel consumptions on a WHSC normalized cycle (World Harmonized Stationary Cycle, 13 stabilized mode points, full regime).   Step 2: aging of the lubricant composition over an endurance cycle, which consists in reproducing on an engine test bed a dynamic road cycle representative of road use, which was recorded under real conditions by a heavy-duty vehicle OEM. The test lasted 300 hours. The dynamism of the test road cycle is favorable to shear of the tested lubricant compositions by a mechanical effect. The fuel consumption is monitored in dynamic mode throughout the endurance test as a guide. Intermediate oil samples are taken during the test to perform various measurements (kinematic viscosity at 100° C. in particular, shown in  FIGS. 2 and 3 ).   Step 3: after the endurance test, the lubricant composition present in the test engine is once again measured according to the WHSC normalized cycle in order to characterize the fuel consumptions after the endurance test. The fuel consumption results for each of the 13 measurement mode points (regime/load) are compared with the results obtained from step 1, in order to evaluate the CIFE performance of the tested lubricant composition.       

     It is thus the result obtained from step 3 which will characterize the CIFE potential of the tested lubricant composition relative to a reference lubricant tested under the same conditions (steps 1, 2, 3). The savings in fuel consumption are established on the entire engine field. 
     Results Composition LC2 was evaluated before and after the endurance test on the WHSC cycle.  FIG. 2  shows the measurement curve for the viscosity at 100° C. of this composition LC2 during the engine test. A 0.87% saving in fuel consumption was measured on the sheared oil which underwent the endurance test relative to the oil before the endurance test. This saving is significant relative to the threshold of the method for discrimination between two products (0.34%). 
     A comparative lubricant composition CC5 was then evaluated according to the same criteria. 
     Said comparative lubricant composition is detailed in table 3 below: 
     The contents are expressed as mass percentages. 
     
       
         
           
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 CC5 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Base oil 2  (2)   
                 30 
               
               
                   
                 Base oil 3  (2)   
                 38.6 
               
               
                   
                 Additive pack  (3)   
                 16.2 
               
               
                   
                 Pour point depressant additive  (4)   
                 0.2 
               
               
                   
                 Polymer 10  (14)   
                 15 
               
               
                   
                 KV 40° C. 
                 82.25 
               
               
                   
                 ASTM D445-97 (mm 2 /s) 
               
               
                   
                 KV 100° C. 
                 12.39 
               
               
                   
                 ASTM D445-97 (mm 2 /s) 
               
               
                   
                 Viscosity index (VI) 
                 147 
               
               
                   
                 ASTM D2270-93 
               
               
                   
                   
               
            
           
         
       
     
     The meanings of the components defined by indices are those given in example 1. 
     A 0.25% excess fuel consumption was measured on the sheared oil which underwent the endurance test relative to the oil before the endurance test.  FIG. 3  shows the measurement curve for the viscosity at 100° C. of this comparative composition during the engine test. 
     This example very clearly shows the difference in behavior in terms of viscosity and more particularly the satisfaction of the CIFE characteristic required in the context of the invention, for the lubricant compositions in accordance with the invention, namely those comprising at least one polymer for improving the viscosity index chosen from hydrogenated copolymers of diene and of aromatic vinyl, compared with lubricant compositions comprising polymers of another nature which improve the viscosity index. 
     Example 4: Compared Viscosity Behavior for Illustrating the Decrease in Viscosity in the Course of its Use in the Gearbox and in the Hydraulic Circuit 
     The present examples were performed for the purpose of demonstrating the selection made from among the viscosity index-enhancing polymers, for preparing lubricant compositions which have CIFE properties when they are used in the gearbox and in the hydraulic circuit. 
     It is known that the temperatures prevailing in the gearbox and in the hydraulic circuit are lower than those in the engine and generally do not exceed 110° C. It is also known that the interval between oil changes is longer for the gearbox and the hydraulic circuits when compared with that for the engine. Consequently, this thermal stability test performed at a lower temperature and over a period corresponding to the interval between oil changes makes it possible to demonstrate the viscosity behavior of the compositions according to the invention in gearboxes and hydraulic circuits. 
     The tests performed are the following: 
     Thermal Stability at 80° C. 
     150 g of lubricant composition are placed in a ventilated oven heated at 80° C. for 1008 hours. On conclusion of this test, a sample of the lubricant composition is taken and the kinematic viscosity of this composition at 100° C. is measured according to the standard ASTM D445-97 (mm 2 /s). 
     Thermal Stability at 100° C. 
     150 g of lubricant composition are placed in a ventilated oven heated at 100° C. for 1008 hours. On conclusion of this test, a sample of the lubricant composition is taken and the kinematic viscosity of this composition at 100° C. is measured according to the standard ASTM D445-97 (mm 2 /s). 
     The kinematic viscosities of the comparative compositions and of the compositions according to the invention as described in table 1, which were first subjected to the thermal stability test as described above, were measured and collated in table 4 below. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                 KV 100° C. 
                 KV 100° C. 
               
               
                   
                 KV 100° C. 
                 ASTM D445-97 
                 ASTM D445-97 
               
               
                   
                 ASTM D445-97 
                 (mm 2 /s) after 
                 (mm 2 /s) after 
               
               
                   
                 (mm 2 /s) before 
                 thermal stability 
                 thermal stability 
               
               
                   
                 thermal stability test 
                 test at 80° C. 
                 test at 100° C. 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 CC2 
                 12.5 
                 12.5 
                 12.5 
               
               
                 LC1 
                 12.3 
                 12.1 
                 11.8 
               
               
                 LC2 
                 12.3 
                 11.8 
                 11.0 
               
               
                   
               
            
           
         
       
     
     It emerges from these results that the compositions according to the invention have a kinematic viscosity at 100° C., measured according to the standard ASTM D445-97 after the thermal stability test, which decreases over time relative to their kinematic viscosities measured before the stability test. 
     It also emerges from these results that the comparative composition has a kinematic viscosity at 100° C., measured according to the standard ASTM D445-97 after the thermal stability tests, which remains constant over time relative to its kinematic viscosity measured before the stability tests. 
     These results illustrate the change in the decrease in viscosity of the compositions according to the invention as a function of time and, consequently, the behavior as required according to the present invention, namely the decrease in viscosity as a function of time of the compositions in accordance with the invention, during a thermal shear, in contrast with the comparative composition for which maintenance of the viscosity during a thermal shear is observed. 
     These results also demonstrate the impact of the chemistry of the polymers on the viscosity profile of the lubricant compositions during a thermal shear. 
     Specifically, the polymers according to the invention make it possible to obtain compositions whose viscosity decreases during a thermal shear, in contrast with the polymers outside the invention, which, when they are in a lubricant composition, do not make it possible to reduce the viscosity of said composition during a thermal shear. 
     In conclusion, following the thermal shears, it is observed that only the lubricant compositions in accordance with the invention have viscosity values lower than that of the same composition before shear. 
     Consequently, this demonstrates that the compositions according to the invention correspond to the CIFE properties when the composition is used in the gearbox and in the hydraulic circuit. Specifically, the more the viscosity of a composition increases, the more the various lubricated components of the gearbox and of the hydraulic circuit consume energy and, consequently, fuel. 
     Example 5 
     Compositions LC1 and LC2 according to the invention underwent a KRL shear test for 3 hours and 20 hours according to the standard CEC-L-45-A-99. This test is representative of the shear conditions of gearboxes when it is performed over a period of 20 hours and of the conditions of the hydraulic circuit when it is performed over 3 hours. The viscosities before the test and after the test were measured at 100° C. and at 40° C. (standard ASTM D445-97), and are collated in table 5 below, in which the viscosities are indicated in mm 2 /s. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 LC1 
                 LC2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 KV 100° C. before the KRL test 
                 12.3 
                 12.4 
               
               
                   
                 KV 100° C. after the KRL 3 h test 
                 9.1 
                 9.0 
               
               
                   
                 KV 100° C. after the KRL 20 h test 
                 8.3 
                 8.2 
               
               
                   
                 KV 40° C. before the KRL test 
                 84.2 
                 84.9 
               
               
                   
                 KV 40° C. after the KRL 3 h test 
                 59.8 
                 59.4 
               
               
                   
                 KV 40° C. after the KRL 20 h test 
                 54.0 
                 53.2 
               
               
                   
                   
               
            
           
         
       
     
     It emerges from these results that the compositions according to the invention have a kinematic viscosity at 100° C., measured according to the standard ASTM D445-97 after the KRL shear test, which decreases over time relative to their kinematic viscosities measured before the shear test. 
     These results illustrate the change in the decrease in viscosity of the compositions according to the invention as a function of time and, consequently, the behavior as required according to the present invention, namely the decrease in viscosity as a function of time of the compositions in accordance with the invention, during a shear such as that which a lubricant composition undergoes in a gearbox and a hydraulic circuit, notably of an industrial vehicle, in particular of a diesel engine industrial vehicle.