Lubricant Oil Composition For Internal Combustion Engine

Abstract

A lubricant oil composition for internal combustion engines with which further reduction of friction can be achieved to provide excellent fuel efficiency is provided. (A) A lubricant oil base oil having a kinematic viscosity at 100.degree. C. of 2.0 to 5.0 mm.sup.2/s; (B) a molybdenum-based friction modifier in an amount of 0.005 to 0.2 mass % in terms of the mass of the molybdenum relative to the total mass of the composition; (C) a metal-based detergent in an amount of 0.01 to 1 mass % in terms of the mass of the metal relative to the total mass of the composition; and (D) 0.01 to 10 mass % of at least one compound selected from amino acids having a C.sub.6-24 alkyl, alkenyl, or acyl group, and/or derivatives of the amino acids.

Description

TECHNICAL FIELD

[0001] The present invention relates to a fuel-efficient lubricant oil composition for internal combustion engines.

BACKGROUND ART

[0002] The trend for improving the fuel efficiency of automobiles since the wake of the oil crisis remains an important issue in light of resource protection and environmental protection, and the need for improved fuel efficiency has been higher. The conventional approach for improving the fuel efficiency of automobiles includes vehicle weight reduction, improvement of engine combustion, and reduction of friction in engines and the drive train. Low engine friction is achieved through improving the valve train mechanism, reducing the surface roughness of sliding members, and using a fuel-efficient lubricant oil composition for internal combustion engines (engine oil).

[0003] Among them, the use of a fuel-efficient engine oil is gaining the acceptance in the market because of its high cost effectiveness. As an effort to achieve high fuel efficiency through engine oil, there have been studies of low-viscosity oils directed to reducing a frictional loss under fluid lubricating conditions of components such as pistons and bearing portions. There is also proposed adding a friction modifier such as an organic molybdenum compound to reduce a frictional loss in the mixed or boundary lubrication of components such as the valve train.

[0004] Various such fuel-efficient engine oils have been proposed. For example, PTL 1 proposes an engine oil composition in which specific additives (e.g., an alkali earth metal salicylate-based detergent, and a molybdenum dithiocarbamate-based friction modifier) are added in specific amounts in a base oil having a kinematic viscosity at 100.degree. C. of 2 to 8 mm.sup.2/s and containing 15 mass % of aromatic. PTL 2 proposes a lubricant oil composition for internal combustion engines in which a molybdenum-based friction modifier or an ester- or amine-based ashless friction modifier, and overbased Ca salicylate are mixed in a lubricant oil base oil containing an ester-based lubricant oil base oil having a kinematic viscosity at 100.degree. C. of 3 to 8 mm.sup.2/s. PTL 3 proposes a lubricant oil composition for internal combustion engines in which oxymolybdenum dithiocarbamate sulfide is combined with ashless friction modifiers such as acid amide compounds, aliphatic partial ester compounds, and/or aliphatic amine compounds.

[0005] However, a problem of the molybdenum-based friction modifier is that increasing its content produces only a limited friction reducing effect. There is also a stability problem due to formation of precipitates. Another drawback is that use of the molybdenum-based friction modifier with ester- or amine-based ashless friction modifiers hardly improves the friction reducing effect. In today's environment where the demand for more fuel efficient lubricants continues to increase, the conventional engine oils are insufficient in terms of fuel efficiency.

[0006] On the other hand, sarcosine and aspartic acid derivatives are known examples of ashless friction modifiers (for example, PTL 4 to 6). However, adding of these modifiers in a lubricant oil composition for internal combustion engines, and a synergistic effect on reducing friction by using these modifiers with molybdenum-based friction modifiers have not been known. Such an effect also has not been anticipated by a skilled artisan.

CITATION LIST

Patent Literature

[0007] PTL 1: JP-A-8-302378

[0008] PTL 2: JP-A-2005-41998

[0009] PTL 3: JP-A-2008-106199

[0010] PTL 4: JP-A-9-316475

[0011] PTL 5: JP-A-2008-179669

[0012] PTL 6: JP-A-2005-290181

SUMMARY OF INVENTION

Technical Problem

[0013] The present invention is intended to provide a solution to the foregoing problems, and it is an object of the present invention to provide a lubricant oil composition for internal combustion engines with which further reduction of friction can be achieved to provide excellent fuel efficiency.

Solution to Problem

[0014] The present invention is a lubricant oil composition for internal combustion engines, the lubricant oil composition comprising:

[0015] (A) a lubricant oil base oil having a kinematic viscosity at 100.degree. C. of 2.0 to 5.0 mm.sup.2/s;

[0016] (B) a molybdenum-based friction modifier in an amount of 0.005 to 0.2 mass % in terms of the mass of the molybdenum relative to the total mass of the composition;

[0017] (C) a metal-based detergent in an amount of 0.01 to 1 mass % in terms of the mass of the metal relative to the total mass of the composition; and

[0018] (D) 0.01 to 10 mass % of at least one compound selected from amino acids having a C.sub.6-24 alkyl, alkenyl, or acyl group, and/or derivatives of the amino acids.

[0019] It is preferable that the (C) metal-based detergent contains at least a salicylate-based detergent. It is preferable that the lubricant oil composition for internal combustion engines has a kinematic viscosity at 100.degree. C. of 4.0 to 12.5 mm.sup.2/s. It is preferable that the lubricant oil composition for internal combustion engines further comprises zinc dialkyl dithiophosphate (ZnDTP) in an amount of 0.02 to 0.2 mass % in terms of the mass of the phosphorus relative to the total mass of the composition.

Advantageous Effects of Invention

[0020] The lubricant oil composition for internal combustion engines of the present invention has notable effects, including low frictional coefficient, and excellent fuel-efficient performance.

DESCRIPTION OF EMBODIMENTS

(A) Lubricant Oil Base Oil

[0021] The lubricant oil base oil of the present invention is not particularly limited, as long as it is a lubricant oil base oil having a kinematic viscosity at 100.degree. C. of 2.0 to 5.0 mm.sup.2/s. Any lubricant oil base oil, regardless of mineral or synthetic, which is used with a lubricant oil composition for common internal combustion engines may be used.

[0022] The kinematic viscosity at 100.degree. C. of the lubricant oil base oil is preferably 2.5 to 4.5 mm.sup.2/s, more preferably 3.0 mm.sup.2/s or more, further preferably 3.5 mm.sup.2/s or more.

[0023] When the kinematic viscosity at 100.degree. C. is less than 2.0 mm.sup.2/s, oil film formation at lubricating portions becomes insufficient, the lubricity suffers, and the evaporative loss of the lubricant oil base oil increases. On the other hand, a kinematic viscosity above 5. 0 mm.sup.2/s lowers the fuel-efficient effect, and the viscosity characteristics at low temperature deteriorate.

[0024] As used herein, "kinematic viscosity at 100.degree. C." means a kinematic viscosity at 100.degree. C. as defined by ASTM D-445 standards.

[0025] The lubricant oil base oil of the present invention has a viscosity index of preferably 90 or more, more preferably 100 or more. A viscosity index of abase oil below 90 increases the low-temperature viscosity, and may cause poor starting performance. As used herein, "viscosity index" means a viscosity index measured according to JIS K2283-1993.

[0026] The lubricant oil base oil of the present invention may be a mineral oil-based base oil or a synthetic base oil, or a mixture of two or more mineral oil-based base oils, or a mixture of two or more synthetic base oils, or may be a mixture of a mineral oil-based base oil and a synthetic base oil, as long as the foregoing physical properties for the lubricant oil base oil are satisfied. The two or more base oils in the mixture may have any desired mixture ratio.

[0027] The mineral oil-based base oil may be, for example, a paraffin-based lubricant oil base oil or a naphthene-based lubricant oil base oil obtained after a lubricant oil fraction from the atmospheric distillation and vacuum distillation of crude oil is purified by using an appropriate combination of purification processes such as solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, catalytic dewaxing, hydrorefining, sulfuric acid treatment, and clay treatment.

[0028] Example of the synthetic base oil include poly-.alpha.-olefins (for example, such as polybutene, 1-octene oligomer, 1-decene oligomer, and ethylene-propylene oligomer) or hydrides thereof, isobutene oligomers or hydrides thereof, isoparaffin, alkylbenzene, alkylnaphthalene, diesters (for example, such as dibutyl maleate, ditridecyl glutarate, di-2-ethylhexyladipate, diisodecyladipate, ditridecyladipate, and di-2-ethylhexylsebacate), copolymers of .alpha.-olefins and diesters, polyolesters (for example, such as trimethylolpropane caprylate, trimethylolpropane pelargonate, pentaerythritol-2-ethylhexanoate, and pentaerythritol pelargonate), dialkyl diphenyl ether, and polyphenyl ether.

[0029] When the lubricant oil base oil of the present invention is a mineral oil-based base oil, the lubricant oil base oil has a saturated hydrocarbon content of preferably 90% or more. As used herein, "saturated hydrocarbon content" means a measured value according to ASTM D-2007.

[0030] The base oil is preferably selected from those that fall under group III or higher categories in the Base Stock Categories of API (American Petroleum Institute), or from base oils obtained through isomerization of waxes.

[0031] The producing process of the base oil is not particularly limited. Preferably, the base oil is produced by desulfurization and hydrocracking of an atmospheric residual oil obtained through atmospheric distillation of crude oil, followed by fractionation of the resulting oil into a set viscosity grade, or by solvent dewaxing or catalytic dewaxing of the residual oil, and, as required, solvent extraction and hydrogenation. Among them, the base oil is preferably obtained through catalytic dewaxing.

[0032] In recent years, the lubricant oil base oil also includes petroleum-based wax isomerized lubricant oil base oils, which are produced by hydrogen isomerization of by-product petroleum-based wax from a dewaxing process, wherein the dewaxing process is one part of producing processes of a base oil including further vacuum distillation of an atmospheric distillation residual oil and fractionation into the desired viscosity grade, subsequent processes such as solvent purification and hydrorefining, and further subsequent solvent dewaxing; and also includes GTL-based wax isomerized lubricant oil base oils produced through isomerization of a GTL WAX (gas-to-liquid wax) produced by using techniques such as the Fischer-Tropsch process. The producing process of the wax isomerized lubricant oil base oil, in this case, is basically the same as the producing process of the hydrocracked base oil.

[0033] The % C.sub.A of the base oil is not particularly limited, but is preferably less than 3, more preferably 2 or less, further preferably 1 or less, most preferably substantially 0. With a % C.sub.A above 10, improvement of heat resistance, one of the objectives of the present invention, becomes insufficient.

[0034] Here, "% C.sub.A" is a value measured by using a method (n-d-M ring analysis) according to ASTM D3238-85.

[0035] The sulfur content in the base oil is not particularly limited, but is preferably 0.03 mass % or less, more preferably 0.01 mass % or less. Particularly preferably, the base oil is substantially sulfur free. Lower sulfur contents mean higher purity, and a less likelihood of causing the sludge solubility problem.

[0036] The method used to measure sulfur content is not particularly limited. Typically, for example, the JIS K2541-1996 method is used.

(B) Molybdenum-Based Friction Modifier

[0037] Examples of the molybdenum-based friction modifier of the present invention include molybdenum dithiocarbamate (MoDTC) , and molybdenum dithiophosphate (MoDTP) . Specific examples of the molybdenum dithiocarbamate include the compounds represented by the following general formula (1). Specific examples of the molybdenum dithiophosphate include the compounds represented by the following general formula (2).

##STR00001##

[0038] In general formulae (1) and (2), R.sup.1 to R.sup.8 each independently represent C.sub.1-24 hydrocarbon groups, a, b, c, and d each independently represent any one of integers of 0 to 4, which satisfies a+b=4, and c+d=4.

[0039] Preferred examples of the C.sub.1-24 hydrocarbon groups represented by R.sup.1 to R.sup.8 in general formulae (1) and (2) each independently include linear or branched C.sub.1-24 alkyl groups, C.sub.5-13 cycloalkyl groups or linear or branched C.sub.5-13 alkylcycloalkyl groups, linear or branched C.sub.3-24 alkenyl groups, C.sub.6-18 aryl groups or linear or branched C.sub.6-18 alkylaryl groups, and C.sub.7-19 arylalkyl groups. The alkyl groups and the alkenyl groups may be primary, secondary, or tertiary.

[0040] Other preferred examples of the molybdenum-based friction modifier in the lubricant oil composition of the present invention include organic molybdenum complexes as reaction products of basic nitrogen compounds such as succinimide, acidic molybdenum compounds such as molybdenum trioxide, and sulfur compounds such as hydrogen sulfide and phosphorus pentasulfide.

[0041] The content of the molybdenum-based friction modifier in the lubricant oil composition of the present invention is 0.005 mass % to 0.2 mass o, preferably 0.01 mass % or more in terms of the mass of the molybdenum element relative to the total mass of the composition. A notable fuel-efficient effect cannot be obtained when the content of the molybdenum-based friction modifier is less than 0.005 mass % in terms of the mass of the molybdenum element. On the other hand, with a molybdenum-based friction modifier content exceeding 0.2 mass % in terms of the mass of the molybdenum element, the extra content does not provide a proportional improvement in the fuel-efficient effect. These contents should thus be avoided.

[0042] The lubricant oil composition of the present invention can preferably use molybdenum dithiophosphate and molybdenum dithiocarbamate. However, it is particularly preferable to use molybdenum dithiocarbamate because it can greatly improve the fuel-efficient performance from low temperature to high temperature in synergy with other components.

(C) Metal-Based Detergent

[0043] The metal-based detergent of the present invention may be any of compounds commonly used for lubricants. For example, overbased compounds of oil-soluble metal salts having a linear or branched hydrocarbon group, and an OH group and/or a carbonyl group may be used. It is also possible to use overbased metal salts such as alkali earth metal sulfonate, alkali earth metal carboxylate, alkali earth metal salicylate, alkali earth metal phenate, and alkali earth metal phosphonate; and overbased metal salts obtained through reaction of alkali earth metal hydroxide or oxide, and, as required, boric acid or boric anhydride. Examples of the alkali earth metal include magnesium, calcium, and barium, of which calcium is preferred. More preferred for use as the overbased metal salts are oil-soluble metal salts of OH group- and/or carbonyl group-containing compounds overbased with alkali earth metal borate or alkali earth metal carbonate. For fuel efficiency, it is preferable to use alkali earth metal salicylate, more preferably alkali earth metal salicylate overbased with alkali earth metal borate.

[0044] The metal-based detergent of the present invention preferably has a base number of 50 mgKOH/g or more, more preferably 100 mgKOH/g or more, further preferably 120 mgKOH/g or more, particularly preferably 140 mgKOH/g or more. The base number is preferably 300 mgKOH/g or less, more preferably 200 mgKOH/g or less. With a base number of less than 50 mgKOH/g, the increased viscosity decreases the fuel efficiency, and the friction reducing effect from the addition of the metal-based detergent tends to become insufficient. With a base number above 300 mgKOH/g, it tends to inhibit the effects of other components such as an antiwear additive, and the friction reducing effect tends to become insufficient. As used herein, "base number" is a measured value according to JIS K 2501 5.2.3.

[0045] The metal-based detergent used in the present invention may be produced by using any method. For example, the metal-based detergent can be obtained through reaction of the oil-soluble metal salt, alkali earth metal hydroxide or oxide, and, as required, boric acid or boric anhydride at 20 to 200.degree. C. for 2 to 8 hours in the presence of water, an alcohol (such as methanol, ethanol, propanol, and butanol) , and a dilute solvent (such as benzene, toluene, and xylene) , followed by heating at 100 to 200.degree. C., and removal of water, and, as required, the alcohol and the dilute solvent. Specific reaction conditions are appropriately selected according to such factors as the type of the raw material, and the amount of the reactant. For details of the producing process, see, for example, JP-A-60-116688, and JP-A-61-204298. The oil-soluble metal salt overbased with alkali earth metal borate produced as above has a total base number of typically 100 mgKOH/g or more, and can preferably be used for the lubricant oil composition of the present invention.

[0046] The metal-based detergent of the present invention has a metal ratio of preferably 4.0 or less, more preferably 3.0 or less, further preferably 2.0 or less. A metal ratio above 4.0 has the possibility of reducing the friction torque, and specifically, the fuel efficiency may become insufficient. The metal ratio of the metal-based detergent is adjusted to preferably 1.0 or more, more preferably 1.1 or more, further preferably 1.5 or more. With a metal ratio of less than 1.0, the kinematic viscosity and the low-temperature viscosity of the lubricant oil composition for internal combustion engines increase, and may cause problems in fuel efficiency or starting performance.

[0047] As used herein, "metal ratio" is represented by metallic element valency.times.metallic element content (mol %)/soap group content (mol %) in the metal-based detergent. The metallic element includes calcium, and magnesium. The soap group includes a sulfonic acid group, a phenol group, and a salicylic acid group.

[0048] The linear or branched hydrocarbon group of the metal-based detergent of the present invention is preferably an alkyl group or an alkenyl group. The alkyl or alkenyl group has preferably 8 or more carbon atoms, more preferably 10 or more carbon atoms, further preferably 12 or more carbon atoms. The number of carbon atoms is preferably at most 19. Sufficient oil solubility cannot be obtained with less than 8 carbon atoms which is not preferable. The alkyl or alkenyl group may be linear or branched, and is preferably linear. The alkyl or alkenyl group may be primary alkyl or alkenyl group, secondary alkyl or alkenyl group, or tertiary alkyl or alkenyl group. When the alkyl or alkenyl group is secondary alkyl or alkenyl group, or tertiary alkyl or alkenyl group, branching occurs preferably only at carbon atoms attached to an aromatic group.

[0049] The content of the metal-based detergent is 0.01 mass % or more, preferably 0.03 mass % or more, more preferably 0.05 mass % or more, and is 1 mass % or less, preferably 0.5 mass % or less, more preferably 0.4 mass % or less, further preferably 0.3 mass % or less, particularly preferably 0.25 mass % or less, most preferably 0.22 mass % or less in terms of the mass of the metallic element relative to the total mass of the lubricant oil composition. When the content is less than 0.01 mass %, the friction reducing effect from the addition of the metal-based detergent tends to become insufficient, and the lubricant oil composition often fails to provide sufficient fuel efficiency, heat and oxidation stability, and cleaning performance. On the other hand, when the content exceeds 1 mass %, the friction reducing effect from the addition of the metal-based detergent tends to become insufficient, and the fuel efficiency of the lubricant oil composition tends to become insufficient.

[0050] The content of the boron-containing metal-based detergent is preferably 0.01 mass % or more, more preferably 0.03 mass % or more, further preferably 0.04 mass % or more, particularly preferably 0.05 mass % or more, and is preferably 0.2 mass % or less, more preferably 0.10 mass % or less, further preferably 0.08 mass % or less, particularly preferably 0.07 mass % or less in terms of the mass of the boron element relative to the total mass of the lubricant oil composition. When the content is less than 0.01 mass %, the friction reducing effect from the addition of the metal-based detergent tends to become insufficient, and the fuel efficiency, heat and oxidation stability, and cleaning performance of the lubricant oil composition tend to become insufficient. On the other hand, when the content exceeds 0.2 mass %, the friction reducing effect from the addition of the metal-based detergent tends to become insufficient, and the fuel efficiency of the lubricant oil composition tends to become insufficient.

[0051] The boron-containing metal-based detergent has an (MB1)/(MB2) ratio of preferably 1 or more, more preferably 2 or more, further preferably 2.5 or more, where (MB1) is the weight of the metallic element contained in the detergent, and (MB2) is the weight of the boron element contained in the detergent. An (MB1)/(MB2) ratio of less than 1 is not preferable because it may lead to poor fuel efficiency. The (MB1)/(MB2) ratio is preferably 20 or less, more preferably or less, further preferably 10 or less, particularly preferably 5 or less. An (MB1)/(MB2) ratio of above 20 is not preferable because it may lead to poor fuel efficiency.

(D) Ashless Friction Modifier

[0052] In the present invention, the ashless friction modifier is at least one compound selected from amino acids having a C.sub.6-24 alkyl, alkenyl, or acyl group, and/or derivatives of such amino acids. Examples of such compounds include the compounds represented by the following general formula (3).

##STR00002##

[0053] Herein, R.sup.9 is a C.sub.6-24 alkyl, alkenyl, or acyl group, R.sup.10 is a C.sub.1-4 alkyl group or hydrogen, and R.sup.11 is hydrogen or a C.sub.1-10 alkyl group. The alkyl group may be linear or branched, or may contain a cyclic structure. The carbon atoms may be substituted with heteroatoms, or may be modified with functional groups such as a hydroxyl group, a carboxyl group, and an amino group. R.sup.12 is a C.sub.1-4 alkyl group or hydrogen, n is 0 or 1, X is a functional group having active hydrogen, a hydrocarbon having such a functional group, a metal salt or an ethanolamine salt of such a functional group, or a methoxy group.

[0054] For considerations such as solubility in the base oil, R.sup.9 in general formula (3) is more preferably an alkyl, alkenyl, or acyl group of 11 or more carbon atoms. For considerations such as storage stability, the number of carbon atoms is more preferably 20 or less. From the standpoint of friction reducing effect, the alkyl, alkenyl, or acyl group is preferably linear. Specific examples of such alkyl, alkenyl, and acyl groups include alkyl group such as hexyl group, heptyl group, octyl group, nonyl group, decyl group, undecyl group, dodecyl group, tridecyl group, tetradecyl group, pentadecyl group, hexadecyl group, heptadecyl group, octadecyl group, nonadecyl group, icosyl group, heneicosyl group, docosyl group, tricosyl group, and tetracosyl group (these alkyl groups may be linear or branched), alkenyl group such as hexenyl group, heptenyl group, octenyl group, nonenyl group, decenyl group, undecenyl group, dodecenyl group, tridecenyl group, tetradecenyl group, pentadecenyl group, hexadecenyl group, heptadecenyl group, octadecenyl group, nonadecenyl group, icocenyl group, heneicosenyl group, docosenyl group, tricosenyl group, and tetracosenyl group (these alkenyl groups may be linear or branched, and the double bond may occur at any position), and acyl group having a ketone group at the terminal of these alkyl or alkenyl groups.

[0055] For considerations such as storage stability, R.sup.10 in general formula (3) is more preferably an alkyl group of 4 or less carbon atoms, further preferably 3 or less carbon atoms, particularly preferably 2 or less carbon atoms.

[0056] The alkyl group represented by R.sup.11 may be linear or branched, or may contain a cyclic structure. The carbon atoms may be substituted with heteroatoms, or may be modified with functional groups such as a hydroxyl group, a carboxyl group, and an amino group. From the standpoint of friction reducing effect and solubility in the base oil, the alkyl group is more preferably of 2 or less carbon atoms, further preferably of 1 or less carbon atom, particularly preferably hydrogen.

[0057] For considerations such as storage stability, R.sup.12 is more preferably alkyl group of 4 or less carbon atoms, further preferably 3 or less carbon atoms, particularly preferably 2 or less carbon atoms, most preferably hydrogen.

[0058] Preferred examples of the functional group with active hydrogen represented by X in general formula (3) include a hydroxyl group, and an amino group. The amino group is preferably a primary or secondary amine, particularly preferably a primary amine. Examples of the metal salts of the active hydrogen group include metal salts of a hydroxyl group. Preferably, --COX in general formula (3) is a carboxyl group.

[0059] Specific examples of the hydrocarbons having a hydroxyl group corresponding to the functional group having active hydrogen include: dihydric alcohols such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,2-butanediol, neopentyl glycol, 1,6-hexanediol, 1,2-octanediol, 1,8-octanediol, isopreneglycol, 3-methyl-1,5-pentanediol, sorbite, catechol, resorcin, hydroquinone, bisphenol A, bisphenol F, hydrogenated bisphenol A, hydrogenated bisphenol F, and dimerdiol; trihydric alcohols such as glycerine, 2-(hydroxymethyl)-1,3-propanediol, 1,2,3-butanetriol, 1,2,3-pentanetriol, 2-methyl-1,2,3-propanetriol, 2-methyl-2,3,4-butanetriol, 2-ethyl-1,2,3-butanetriol, 2,3,4-pentanetriol, 2,3,4-hexanetriol, 4-propyl-3,4,5-heptanetriol, 2,4-dimethyl-2,3,4-pentanetriol, 1,2,4-butanetriol, 1,2,4-pentanetriol, trimethylolethane, and trimethylolpropane; tetrahydric alcohols such as pentaerythritol, erythritol, 1,2,3,4-pentanetetrol, 2,3,4,5-hexanetetrol, 1,2,4,5-pentanetetrol, 1,3,4,5-hexanetetrol, diglycerin, and sorbitan; penthydric alcohols such as adonitol, arabitol, xylitol, and triglycerine; hexahydric alcohols such as dipentaerythritol, sorbitol, mannitol, iditol, inositol, dulcitol, talose, and allose; and polyglycerines or dehydrocondensation products thereof.

[0060] Examples of the metals of the hydroxyl metal salts include alkali metals, alkali earth metals, and zinc. Examples of the alkali metals and the alkali earth metals include sodium, potassium, magnesium, and calcium. Preferred for improving the persistence of the frictional effect are alkali earth metals, and zinc.

[0061] The metal salts are preferably carboxylates with a carboxyl structure representing --COX in general formula (3).

[0062] For considerations such as improvement of the persistence of the frictional effect, the ashless friction modifier of the present invention is preferably at least one kind of compounds selected from the compounds of general formula (3). The one kind of compounds selected from the compounds of general formula (3) may be used alone, or as a mixture of two or more kinds of compounds.

[0063] Preferred examples of the compounds represented by general formula (3) include N-acyl sarcosine, particularly N-oleoyl sarcosine in which R.sup.9 is a C.sub.18 acyl group, R.sup.10 is methyl group, R.sup.11 is hydrogen, X is hydroxyl group, and n is 0, and N-lauroyl-N-methyl-.beta.-alanine in which R.sup.9 is lauroyl group (C.sub.12 acyl group) , R.sup.10 is methyl group, R.sup.11 is hydrogen, R.sup.12 is hydrogen, X is hydroxyl group, and n is 1.

[0064] The content of the ashless friction modifier is 0.01 to 10 mass %, preferably 5 mass % or less, more preferably 2 mass % or less relative to the total mass of the composition. A content above 10 mass % is not preferable because the extra content does not provide a further improvement in frictional characteristics, but results in poor storage stability. The content is preferably 0.05 mass % or more, more preferably 0.1 mass % relative to the total mass of the composition. A content below 0.01 mass % is not preferable because it is ineffective at improving the frictional characteristics.

(E) Antiwear Agent

[0065] The lubricant oil composition for internal combustion engines of the present invention preferably contains zinc dialkyl dithiophosphate (ZnDTP) of the following general formula (4) as an antiwear agent, in addition to the foregoing additives.

##STR00003##

[0066] R.sup.13 to R.sup.16 in the general formula (4) each independently represent hydrogen, and at least one of R.sup.13 to R.sup.16 is a linear or branched C.sub.1-24 alkyl group. The alkyl group maybe primary, secondary, or tertiary.

[0067] In the present invention, the zinc dialkyl dithiophosphates may be used either alone or in a combination of two or more kinds thereof. For improved wear resistance, it is, however, preferable to use a zinc dithiophosphoate having a primary alkyl group (primary ZnDTP), or a zinc dithiophosphoate having a secondary alkyl group (secondary ZnDTP), particularly a zinc dialkyl dithiophosphate containing secondary ZnDTP as a main component.

[0068] The content of the zinc dialkyl dithiophosphate in the lubricant oil composition of the present invention is preferably 0.02 to 0.2 mass %, more preferably 0.03 to 0.1 mass % in terms of the mass of the phosphorus content relative to the total mass of the composition. A phosphorus content of less than 0.02 mass % is insufficient in terms of wear resistance and high-temperature cleaning performance. Above 0.2 mass %, the exhaust gas catalyst causes serious catalyst poisoning, which is not preferable.

[0069] The lubricant oil composition for internal combustion engines of the present invention may appropriately contain other additives, as required, provided that the addition of such additional components is not detrimental to the objects of the present invention. Examples of such additives include viscosity index improvers, pour point depressants, antioxidants, wear inhibitors or extreme pressure agents, friction modifiers, dispersants, anti-rusting agents, surfactants or demulsifiers, and defoaming agents.

[0070] The viscosity index improvers may be, for example, non-dispersive viscosity index improvers or dispersive viscosity index improvers. Specific examples include non-dispersive or dispersive polymethacrylate and olefin copolymers, polyisobutene, polystyrene, ethylene-propylene copolymer, styrene-diene copolymer, and hydrides thereof. The weight-average molecular weights of these agents are typically 5,000 to 1,000,000. For improved fuel-efficient performance, it is, however, preferable to use viscosity index improvers having a weight-average molecular weight of 100,000 to 1,000,000, preferably 200,000 to 900,000, particularly preferably 400,000 to 800,000. In the present invention, it is preferable for improved fuel efficiency that the viscosity index improver is a poly(meth)acrylate-based viscosity index improver containing 30 to 90 mol % of the structure unit represented by the following general formula (5) , 0.1 to 50 mol % of the structure unit represented by the following general formula (6) , and a hydrocarbon main chain in a proportion of 0.18 or less.

##STR00004##

[0071] In the general formula (5) , R.sup.17 represents hydrogen or a methyl group, R.sup.18 represents a linear or branched hydrocarbon group of 6 or less carbon atoms. In general formula (6) , R.sup.19 represents hydrogen or a methyl group, and R.sup.20 represents a linear or branched hydrocarbon group of 16 or more carbon atoms.

[0072] The viscosity index improver preferably has a diesel injector PSSI (permanent shear stability index) of 30 or less.

[0073] With a PSSI of above 30, the shear stability suffers, and the initial fuel efficiency may decrease to maintain certain levels of kinematic viscosity or HTHS viscosity after use.

[0074] As used herein, "diesel injector PSSI" means a permanent shear stability index of a polymer calculated with measured data from ASTM D6278-02 (Test Method for Shear Stability of Polymer Containing Fluids Using a European Diesel Injector Apparatus) according to ASTM D6022-01 (Standard Practice for Calculation of Permanent Shear Stability Index).

[0075] Examples of the pour point depressants include polymethacrylate-based polymers, alkylated aromatic compounds, fumarate-vinyl acetate copolymers, and ethylene-vinyl acetate copolymers that are compatible with the lubricant oil base oil used.

[0076] The detergent dispersant may be, for example, succinimide, benzylamine, alkylpolyamine, polybuteneamine, or modified products thereof with boron compounds or sulfur compounds, or an alkenyl succinic acid ester.

[0077] The detergent dispersant is preferably a mono or bis succinimide, more preferably a bis succinimide, particularly preferably a boron-free bis succinimide.

[0078] The detergent dispersant has a molecular weight of preferably 1000 or more, more preferably 5000 or more, further preferably 7000 or more, even more preferably 9000 or more. The molecular weight is preferably 30000 or less, more preferably 25000 or less, further preferably 20000 or less. Cleaning performance may become insufficient when the molecular weight is 1000 or less. On the other hand, the fuel efficiency of the engine oil composition may greatly decrease with a molecular weight exceeding 30000.

[0079] The content of the detergent dispersant is preferably 0.1 to 15 mass %, more preferably 0.5 to 10 mass %, further preferably 1.0 to 8 mass % relative to the total mass of the engine oil composition. Cleaning performance may become insufficient when the detergent dispersant content is less than 0.1 mass %. On the other hand, the fuel efficiency of the engine oil composition may greatly decrease with a content exceeding 15 mass %.

[0080] The N content in the detergent dispersant is preferably 0.1 or more, more preferably 0.3 or more, further preferably 0.4 or more, even more preferably 0.5 or more. The N content is preferably 2.0 or less, more preferably 1.0 or less, further preferably 0.8 or less. Cleaning performance may become insufficient when the N content is 0.1 or less. On the other hand, the fuel efficiency of the engine oil composition may greatly decrease with the N content exceeding 2.0.

[0081] The antioxidant may be a phenol- or amine-based compound or any other compound, provided that it is selected from those commonly used for lubricants. Examples thereof include alkylphenols such as 2, 6-di-tert-butyl-4-methylphenol;

[0082] bisphenols such as methylene-4,4-bis(2,6-di-tert-butyl-4-methylphenol); naphthylamines such as phenyl-.alpha.-naphthylamine; dialkyldiphenylamines; and phenothiazines.

[0083] Examples of the extreme pressure additives and the antiwear agents include phosphorus compounds such as phosphoric acid esters, phosphorous acid esters, and salts thereof; and sulfur compounds such as disulfides, sulfurized olefins, and sulfurized grease.

[0084] The anti-rusting agents may be, for example, alkenyl succinic acid, alkenyl succinic acid ester, polyalcohol ester, petroleum sulfonate, or dinonylnaphthalene sulfonate.

[0085] The corrosion inhibitors may be, for example, benzotriazole-, thiadiazole-, or imidazole-based compounds.

[0086] The defoaming agents may be, for example, silicone compounds such as dimethyl silicone, and fluorosilicone.

[0087] These additives may be added in any amounts. Typically, the content of the defoaming agent is 0.0005 to 0.01 mass %, the content of the viscosity index improver is 0.05 to 20 mass %, the content of the corrosion inhibitor is 0.005 to 0.2 mass %, and the content of other additive is 0.05 to 10 mass % relative to the total mass of the composition.

[0088] The lubricant oil composition for internal combustion engines of the present invention has a kinematic viscosity at 100.degree. C. of preferably 4.0 mm.sup.2/s or more, more preferably 6.0 mm.sup.2/s or more, further preferably 6.1 mm.sup.2/s or more, most preferably 6.2 mm.sup.2/s or more. The kinematic viscosity at 100.degree. C. is preferably 12. 5 mm.sup.2/s or less, more preferably 9.3 mm.sup.2/s or less, further preferably 8.5 mm.sup.2/s or less. As used herein, "kinematic viscosity at 100.degree. C." is a kinematic viscosity at 100.degree. C. as defined by ASTM D-445. Insufficient lubricity may result when the kinematic viscosity at 100.degree. C. is less than 4.0 mm.sup.2/s. With a kinematic viscosity at 100.degree. C. above 12.5 mm.sup.2/s, it may not be possible to obtain the necessary low-temperature viscosity, and sufficient fuel-efficient performance.

[0089] The lubricant oil composition has a kinematic viscosity at 40.degree. C. of preferably 4 to 50 mm.sup.2/s, preferably 40 mm.sup.2/s or less, more preferably 35 mm.sup.2/s or less. The kinematic viscosity at 40.degree. C. is preferably 15 mm.sup.2/s or more, more preferably 18 mm.sup.2/s or more, further preferably 20 mm.sup.2/s or more, particularly preferably 22 mm.sup.2/s or more, most preferably 25 mm.sup.2/s or more. As used herein, "kinematic viscosity at 40.degree. C." is a kinematic viscosity at 40.degree. C. as defined by ASTM D-445. Insufficient lubricity may result when the kinematic viscosity at 40.degree. C. is less than 4 mm.sup.2/s. With a kinematic viscosity at 40.degree. C. above 50 mm.sup.2/s, it may not be possible to obtain the necessary low-temperature viscosity, and sufficient fuel-efficient performance.

[0090] The lubricant oil composition has a viscosity index of preferably 120 to 400, more preferably 190 or more, further preferably 200 or more, particularly preferably 230 or more, most preferably 240 or more. With a viscosity index of less than 120, it may become difficult to improve fuel efficiency while maintaining the 150.degree. C. HTHS viscosity. A viscosity index above 400 may result in poor evaporativity, and may cause problems due to the insufficiency of the solubility of the additives, or of compatibility with the sealant.

[0091] In order to improve fuel efficiency while preventing the low-viscosity problem and maintaining durability, it is effective to increase the HTHS viscosity at 150.degree. C. (HTHS viscosity is also known as "high-temperature high-shear viscosity"), and to decrease the kinematic viscosity at 40.degree. C., kinematic viscosity at 100.degree. C., and HTHS viscosity at 100.degree. C. It is, however, very difficult to satisfy all these conditions with conventional lubricant oils.

[0092] The lubricant oil composition has a HTHS viscosity at 100.degree. C. of preferably 5.5 mPas or less, more preferably 5.0 mPas or less, further preferably 4.7 mPas or less, particularly preferably 4.5 mPas or less, most preferably 4.4 mPas or less. The HTHS viscosity at 100.degree. C. is preferably 3.0 mPas or more, further preferably 3.5 mPas or more, particularly preferably 4.0 mPas or more, most preferably 4.1 mPas or more. As used herein, "HTHS viscosity at 100.degree. C. " is a high-temperature high-shear viscosity at 100.degree. C. as defined by ASTM D4683. Insufficient lubricity may result when the HTHS viscosity at 100.degree. C. is less than 3.0 mPas. With a HTHS viscosity at 100.degree. C. above 5.5 mPas, it may not be possible to obtain the necessary low-temperature viscosity, and sufficient fuel-efficient performance.

[0093] The ratio of HTHS viscosity at 150.degree. C. to HTHS viscosity at 100.degree. C. (HTHS viscosity at 150.degree. C./HTHS viscosity at 100.degree. C.) in the lubricant oil composition of the present invention is preferably 0.45 or more, more preferably 0.475 or more, further preferably 0.50, even more preferably 0.515 or more, particularly preferably 0.53 or more. With a ratio of HTHS viscosity at 150.degree. C. to HTHS viscosity at 100.degree. C. of less than 0.45, it may not be possible to obtain the necessary low-temperature viscosity, and sufficient fuel-efficient performance.

EXAMPLES

[0094] The present invention is described below in greater detail using Examples and Comparative Examples. However, the present invention is not limited to the following examples. (Examples 1 to 3, and Comparative Examples 1 to 7)

(A) Lubricant Oil Base Oil

[0095] The hydrocracked lubricant oil base oils of the following properties were used by being mixed in the proportions shown in Table 1.

[0096] (A-1) Kinematic viscosity at 40.degree. C.: 19.6=.sup.2/s; kinematic viscosity at 100.degree. C.: 4.2 mm.sup.2/s; viscosity index: 122; sulfur content: less than 10 ppm; % C.sub.P: 80.7; % C.sub.N: 19.3; % C.sub.A: 0

[0097] (A-2) Kinematic viscosity at 40.degree. C.: 13.5 mm.sup.2/s; kinematic viscosity at 100.degree. C.: 3.2 mm.sup.2/s; viscosity index: 112; sulfur content: less than 10 ppm; % C.sub.P: 72.6; % C.sub.N: 27.4; % C.sub.A: 0

[0098] The following additives were added to the lubricant oil base oils in the proportions shown in Table 1 to prepare lubricant oil compositions.

(B) Molybdenum-Based Friction Modifier

[0099] Molybdenum dithiocarbamate of general formula (1) in which R.sup.1 to R.sup.4 are C.sub.8 or C.sub.13 alkyl group, and a and b are 2. The molybdenum element concentration: 10 mass %; sulfur content: 11 mass %

(C) Metal-Based Detergent

[0100] (C-1) Overbased Ca salicylate

[0101] Metal ratio: 2.3; C.sub.14-18 alkyl group; Ca content: 6.2 mass %; base number: 180 mgKOH/g

(C-2) Overbased boric acid Ca salicylate

[0102] Metal ratio: 2.5; C.sub.14-18 alkyl group; Ca content: 6.8 mass %; B content: 2.7 mass %; base number: 190 mgKOH/g

(C-3) Overbased boric acid Ca salicylate

[0103] Metal ratio: 1.5; C.sub.14-28 alkyl group; Ca content: 5.0 mass %; B content: 1.8 mass %; base number: 140 mgKOH/g

(D) Ashless Friction Modifier

[0104] (D-1) Oleoyl sarcosine [0105] (D-2) N-Lauroyl-N-methyl-P-alanine [0106] (D-3) N-Lauroyl sarcosine [0107] (D-4) Oleoyl-N-methyl-P-alanine [0108] (D-5) Alkylamine ethylene oxide adduct [0109] (D-6) Oleylamine [0110] (D-7) Glycerine monooleate [0111] (D-8) Oleylamide [0112] (D-9) Oleylurea

(E) Other Additives

(E-1) ZnDTP

[0113] Secondary alkyl group; 4 and 6 carbon atoms; Zn content: 7.8 mass %; P content: 7.2 mass %; S content: 15 mass %

(E-2) Non-dispersive PMA-based viscosity index improver (Mw =380,000; PSSI=25) (E-3) Polybutenyl succinimide

[0114] Molecular weight: 9000; N content: 0.6 mass %

(E-4) Antioxidant, defoaming agent (dimethylsilicone), and others

[0115] The lubricant oil compositions prepared as above were each measured for friction torque by a motoring friction test performed under the following conditions. The average friction torque of each lubricant oil composition was calculated, and percentage improvement relative to the average friction torque of Comparative Example 1 was determined (Percentage improvement=average friction torque of Examples 1 to 7 and Comparative Examples 2 to 7/average friction torque of Comparative Example 1). The results (%) are presented in Table 1, along with the physical properties of the lubricant oil compositions.

(Test Conditions)

[0116] Test Engine: Inline 4-cylinder 1800-cc engine with roller locker arms

[0117] Oil temperature: 100.degree. C.

[0118] Engine speed: 1000 rpm

TABLE-US-00001 TABLE 1 Example Comparative Example 1 2 3 4 5 6 7 1 2 3 4 5 6 7 Base oil (mass % (A-1) 70 70 70 70 70 70 70 70 70 70 70 70 70 70 relative to the total (A-2) 30 30 30 30 30 30 30 30 30 30 30 30 30 30 mass of base oil) Kinematic viscosity 40.degree. C. 17.0 17.0 17.0 17.0 17.0 17.0 17.0 17.0 17.0 17.0 17.0 17.0 17.0 17.0 of base oil (mm.sup.2/s) 100.degree. C. 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 Viscosity index of base oil 120 120 120 120 120 120 120 120 120 120 120 120 120 120 Additives (mass % (B) 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.3 0.3 0.8 relative to (C-1) 2.5 2.5 2.5 the total mass (C-2) 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 of composition) (C-3) 3.8 (D-1) 1.0 0.5 1.0 1.0 (D-2) 1.0 (D-3) 1.0 (D-4) 1.0 (D-5) 1.0 (D-6) 1.0 (D-7) 1.0 (D-8) 1.0 (D-9) 1.0 (E-1) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 (E-2) 15.5 15.5 15.5 15.5 15.5 15.5 15.5 15.5 15.5 15.5 15.5 15.5 15.5 15.5 (E-3) 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 (E-4) 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 Kinematic viscosity 40.degree. C. 31.5 31.5 31.5 31.5 31.5 31.5 31.5 31.5 31.5 31.5 31.5 31.5 31.5 31.5 of lubricant oil 100.degree. C. 8.1 8.1 8.1 8.1 8.1 8.1 8.1 8.1 8.1 8.1 8.1 8.1 8.1 8.1 composition (mm.sup.2/s) Viscosity index of 248 248 248 248 248 248 248 248 248 248 248 248 248 248 lubricant oil composition HTHS viscosity 100.degree. C. 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 of lubricant oil 150.degree. C. 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 composition (mPa s) Percentage vs. Comp. 1.11 1.10 0.93 1.81 0.2 0.6 1.88 0.00 -1.14 -3.95 -0.11 -1.60 -1.14 -0.9 improvement Example 1 of motoring friction torque (%)

[0119] As can been seen in these results, there was no friction reducing effect in Comparative Examples 2 to 6, in which the molybdenum-based friction modifier was used with the ashless friction modifier that did not contain a C.sub.6-24 hydrocarbon group, a nitrogen atom, and a carboxyl group within the molecule. In contrast, the lubricant oil compositions that used the ashless friction modifier containing a C.sub.6-24 hydrocarbon group, a nitrogen atom, and a carboxyl group within the molecule clearly showed a friction reducing effect in synergy with the modifier. As clearly demonstrated above, the lubricant oil composition for internal combustion engines of the present invention has notable effects, specifically low frictional coefficient, and excellent fuel-efficient performance.

INDUSTRIAL APPLICABILITY

[0120] The lubricant oil composition for internal combustion engines of the present invention can preferably be used as a fuel-efficient engine oil for, for example, gasoline engines, and diesel engines.

Claims

1. A lubricant oil composition for internal combustion engines, the lubricant oil composition comprising: (A) a lubricant oil base oil having a kinematic viscosity at 100.degree. C. of 2.0 to 5.0 mm.sup.2/s; (B) a molybdenum-based friction modifier in an amount of 0.005 to 0.2 mass % in terms of the mass of the molybdenum relative to the total mass of the composition; (C) a metal-based detergent in an amount of 0.01 to 1 mass % in terms of the mass of the metal relative to the total mass of the composition; and (D) 0.01 to 10 mass % of at least one compound selected from amino acids having a C.sub.6-24 alkyl, alkenyl, or acyl group, and/or derivatives of the amino acids.

2. The lubricant oil composition for internal combustion engines according to claim 1, wherein the (C) metal-based detergent contains at least a salicylate-based detergent.

3. The lubricant oil composition for internal combustion engines according to claim 1, wherein the lubricant oil composition has a kinematic viscosity at 100.degree. C. of 4.0 to 12.5 mm.sup.2/s.

4. The lubricant oil composition for internal combustion engines according to claim 1, further comprising zinc dialkyl dithiophosphate (ZnDTP) in an amount of 0.02 to 0.2 mass % in terms of the mass of the phosphorus relative to the total mass of the composition.

5. The lubricant oil composition for internal combustion engines according to claim 2, wherein the lubricant oil composition has a kinematic viscosity at 100.degree. C. of 4.0 to 12.5 mm.sup.2/s.

6. The lubricant oil composition for internal combustion engines according to claim 2, further comprising zinc dialkyl dithiophosphate (ZnDTP) in an amount of 0.02 to 0.2 mass % in terms of the mass of the phosphorus relative to the total mass of the composition.

7. The lubricant oil composition for internal combustion engines according to claim 3, further comprising zinc dialkyl dithiophosphate (ZnDTP) in an amount of 0.02 to 0.2 mass % in terms of the mass of the phosphorus relative to the total mass of the composition.

8. The lubricant oil composition for internal combustion engines according to claim 5, further comprising zinc dialkyl dithiophosphate (ZnDTP) in an amount of 0.02 to 0.2 mass % in terms of the mass of the phosphorus relative to the total mass of the composition.