Lubricating Oil Composition And Method For Producing Same

Abstract

The lubricating oil composition of the present invention is a lubricating oil composition containing a lubricating base oil, a metal-based detergent, a molybdenum dialkyldithiocarbamate compound, and a boron-containing compound, wherein the metal-based detergent contains a calcium salicylate and at least one selected from the group consisting of a calcium sulfonate and a magnesium-based detergent; the lubricating oil composition arbitrarily contains an ashless friction modifier; the content of a calcium atom is 0.12 to 0.16 mass % on the basis of the whole amount of the lubricating oil composition; the content of a molybdenum atom is 0.05 to 0.10 mass % on the basis of the whole amount of the lubricating oil composition; and a predetermined coefficient X is less than 0.050.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a lubricating oil composition, for example, a lubricating oil composition for internal combustion engine to be used in a gasoline engine.

BACKGROUND OF THE INVENTION

[0002] In recent years, the environmental regulation is strengthened, and fuel consumption reduction of automobile engines is required. Thus, a supercharged direct injection engine mounted with a direct injection mechanism and a supercharging mechanism is being introduced. In the supercharged direct injection engine, by increasing a torque at a low rotational speed, the displacement can be decreased while keeping a power, and therefore, the fuel consumption can be improved.

[0003] Meanwhile, in engine oils that are used for automobile engines, various additives are generally blended. For example, for the purpose of decreasing an intermetal friction coefficient to improve the fuel consumption, it is known to blend a molybdenum dialkyldithiocarbamate. In addition, for the purposes of insurance of detergency, wear prevention, prevention of sludge generation, and so on, it is also known that a metal-based detergent containing calcium or magnesium, various phosphorus-containing compounds, and an ashless detergent dispersant such as a borated dispersant, etc., are blended.

[0004] Among gasoline engines, in supercharged direct injection engines, abnormal combustion called low speed pre-ignition is liable to occur. Thus, for the purpose of suppressing the frequency of occurrence of low speed pre-ignition, various improvements are made in engine oils. For example, in PTL 1, it is known to regulate blending amounts of various additives such that the quantity of calcium, the quantity of magnesium, the quantity of molybdenum, the quantity of phosphorus, and the quantity of nitrogen contained in an engine oil are satisfied with a specified relational expression. In specific examples of the engine oil disclosed in PTL 1, the blending amount of molybdenum is restricted into 0.01 to 0.04 mass % while setting the quantity of calcium or the quantity of magnesium to be fixed amounts.

CITATION LIST

Patent Literature

[0005] PTL 1: WO 2015/114920

SUMMARY OF THE INVENTION

[0006] However, when the blending amount of molybdenum is restricted as in the examples disclosed in PTL 1, there is a case where it becomes difficult to decrease the intermetal friction coefficient.

[0007] In addition, it is known that in order to suppress the low speed pre-ignition, it is effective to reduce the amount of a calcium component in an engine oil. However, in an engine oil containing a molybdenum dialkyldithiocarbamate, when the amount of a calcium component is reduced, the friction coefficient may increase. Furthermore, in an engine oil containing a molybdenum dialkyldithiocarbamate, in which the amount of a molybdenum component is a predetermine amount or more, the high-temperature detergency is frequently worsened. For that reason, a large amount of a detergent dispersant may be blended, but detergent dispersants often increases the friction coefficient.

[0008] That is, in an engine oil in which the amount of a molybdenum component is a predetermined amount or more, and the amount of a calcium component is reduced, it is difficult to achieve both an improvement of high-temperature detergency and a decrease of friction coefficient that are compatible with each other.

[0009] Under the foregoing circumstances, the present invention has been made. A problem of the present invention is to provide a lubricating oil composition, in which though it contains a molybdenum dialkyldithiocarbamate, with the amount of a molybdenum component being a predetermined amount or more and has a low amount of a calcium component, a low friction coefficient can be realized while ensuring high-temperature detergency.

[0010] The present inventors made extensive and intensive investigations. As a result, it has been found that in a lubricating oil composition containing a molybdenum dialkyldithiocarbamate compound, with the amount of a molybdenum component being a predetermined amount or more, and having a low amount of a calcium component, by regulating a soap component derived from a calcium-based detergent, the quantity of a magnesium component derived from a magnesium-based detergent, the quantity of a boron component derived from a boron-containing compound, and the content of an ashless friction modifier so as to satisfy a predetermined relational expression, the aforementioned problem can be solved, leading to accomplishment of the present invention. The present invention provides the following lubricating oil composition.

[0011] A lubricating oil composition containing a lubricating base oil, a metal-based detergent, a molybdenum dialkyldithiocarbamate compound, and a boron-containing compound, wherein

[0012] the metal-based detergent contains

[0013] a calcium salicylate and

[0014] at least one selected from the group consisting of a calcium sulfonate and a magnesium-based detergent,

[0015] the lubricating base oil arbitrarily contains an ashless friction modifier,

[0016] in the lubricating oil composition, the content of a calcium atom is 0.12 to 0.16 mass % on the basis of the whole amount of the lubricating oil composition; and the content of a molybdenum atom is 0.05 to 0.10 mass % on the basis of the whole amount of the lubricating oil composition, and

[0017] X calculated according to the following expression (1) is less than 0.050:

X=3.8.times.10.sup.-2-2.7.times.10.sup.-4.times.[A]-4.2.times.10.sup.-1x- [B]+5.4.times.10.sup.-1x[C]-5.2.times.10.sup.-3.times.[D]+7.3.times.10.sup- .-3.times.[E] (1)

wherein

[0018] [A] denotes a proportion (mass %) occupied by a soap component derived from a calcium sulfonate in the total amount of soap components derived from a calcium salicylate, a calcium sulfonate, and a calcium phenate;

[0019] [B] denotes the content (mass %) of the magnesium-based detergent as converted into a magnesium atom;

[0020] [C] denotes the content (mass %) of a boron atom contained in the lubricating oil composition;

[0021] [D] denotes the content (mass %) of the ashless friction modifier; and [E] denotes E calculated according to the following expression:

E=([A]+1).times.([B]+0.001) (2)

the values of [B] to [D] being a value on the basis of the whole amount of the lubricating oil composition.

[0022] In addition, the present invention further provides the following method for producing a lubricating oil composition.

[0023] A method for producing a lubricating oil composition including: blending a lubricating base oil with at least a metal-based detergent, a molybdenum dialkyldithiocarbamate compound, and a boron-containing compound, to obtain a lubricating oil composition, wherein

[0024] the metal-based detergent contains

[0025] a calcium salicylate and

[0026] at least one selected from the group consisting of a calcium sulfonate and a magnesium-based detergent,

[0027] the lubricating base oil is further arbitrarily blended with an ashless friction modifier,

[0028] in the lubricating oil composition, the content of a calcium atom is 0.12 to 0.16 mass % on the basis of the whole amount of the lubricating oil composition, and the content of a molybdenum atom is 0.05 to 0.10 mass % on the basis of the whole amount of the lubricating oil composition, and

[0029] X calculated according to the following expression (1) is less than 0.050:

X=3.8.times.10.sup.-2-2.7.times.10.sup.-4.times.[A]-4.2.times.10.sup.-1.- times.[B]+5.4.times.10.sup.-1.times.[C]-5.2.times.10.sup.-3.times.[D]+7.3.- times.10.sup.-3.times.[E] (1)

wherein

[0030] [A] denotes a proportion (mass %) occupied by a soap component derived from a calcium sulfonate in the total amount of soap components derived from a calcium salicylate, a calcium sulfonate, and a calcium phenate;

[0031] [B] denotes the content (mass %) of the magnesium-based detergent as converted into a magnesium atom;

[0032] [C] denotes the content (mass %) of a boron atom contained in the lubricating oil composition;

[0033] [D] denotes the content (mass %) of the ashless friction modifier; and

[0034] [E] denotes E calculated according to the following expression:

E=([A]+1).times.([B]+0.001) (2)

the values of [B] to [D] being a value on the basis of the whole amount of the lubricating oil composition.

[0035] The present invention provides a lubricating oil composition, in which though it contains a molybdenum dialkyldithiocarbamate compound, with the amount of a molybdenum component being a predetermined amount or more, and has a low amount of a calcium component, a low friction coefficient can be realized while ensuring high-temperature detergency.

DETAILED DESCRIPTION OF THE INVENTION

[0036] The present invention is hereunder described with reference to embodiments.

[0037] The lubricating oil composition of the present embodiment contains a lubricating base oil, a metal-based detergent, a molybdenum dialkyldithiocarbamate compound, and a boron-containing compound and further arbitrarily contains an ashless friction modifier. The metal-based detergent contains a calcium salicylate and at least one selected from the group consisting of a calcium sulfonate and a magnesium-based detergent.

[0038] In the lubricating oil composition, the contents of various additives are regulated such that the content of a calcium atom in the composition is 0.12 to 0.16 mass % on the basis of the whole amount of the lubricating oil composition, the content of a molybdenum atom in the composition is 0.05 to 0.10 mass % on the basis of the whole amount of the lubricating oil composition, and X calculated according to the following expression (1) is less than 0.050:

X=3.8.times.10.sup.-2-2.7.times.10.sup.-4.times.[A]-4.2.times.10.sup.-1.- times.[B]+5.4.times.10.sup.-1.times.[C]-5.2.times.10.sup.-3.times.[D]+7.3.- times.10.sup.-3.times.[E] (1)

wherein

[0039] [A] denotes a proportion (mass %) occupied by a soap component derived from a calcium sulfonate in the total amount of soap components derived from a calcium salicylate, a calcium sulfonate, and a calcium phenate;

[0040] [B] denotes the content (mass %) of the magnesium-based detergent as converted into a magnesium atom;

[0041] [C] denotes the content (mass %) of a boron atom contained in the lubricating oil composition;

[0042] [D] denotes the content (mass %) of the ashless friction modifier; and [E] denotes E calculated according to the following expression:

E=([A]+1).times.([B]+0.001) (2)

the values of [B] to [D] being a value on the basis of the whole amount of the lubricating oil composition.

[0043] In the present embodiment, in the case where the calcium amount and the molybdenum amount in the composition fall within the aforementioned fixed ranges, the value of "X" calculated in the aforementioned expression (1) becomes a value approximate to an intermetal friction coefficient as measured in a block-on-ring test as mentioned later. That is, as the value of "X" is smaller, it becomes possible to decrease the intermetal friction coefficient, and good fuel consumption properties are readily obtained. Meanwhile, when the value of "X" is 0.050 or more, the friction coefficient also becomes high, so that it becomes difficult to improve the fuel consumption.

[0044] Here, the smaller the value of X, the larger the effect for decreasing the friction coefficient is. For that reason, X is preferably 0.045 or less, more preferably 0.042 or less, and still more preferably 0.040 or less.

[0045] Although a lower limit value of X is not particularly limited, from the viewpoints of a balance with other performances required for the lubricating oil composition, and so on, it is preferably 0.015 or more, and more preferably 0.030 or more.

[0046] In the present embodiment, when the content of a calcium atom or the content of a molybdenum atom falls outside the aforementioned range, the value of X might not become a value approximate to the friction coefficient, and even when the value of X is less than 0.050, the value of the friction coefficient might not become thoroughly low.

[0047] Furthermore, when the calcium amount is more than the aforementioned upper limit value, for example, in the case of being used for an engine oil of a supercharged direct injection engine, it becomes difficult to suppress the frequency of occurrence of low speed pre-ignition. When the molybdenum amount is more than the aforementioned upper limit value, faults, such as worsening of high-temperature detergency, etc., are liable to be generated.

[0048] From those viewpoints, the content of a calcium atom in the lubricating oil composition is preferably 0.12 to 0.16 mass %, more preferably 0.12 to 0.15 mass %, and still more preferably 0.12 to 0.14 mass % on the basis of the whole amount of the lubricating oil composition. The content of a molybdenum atom in the lubricating oil composition is preferably 0.05 to 0.09 mass %, and more preferably 0.05 to 0.08 mass %.

[0049] Furthermore, from the viewpoint of ensuring the high-temperature detergency good while keeping the friction coefficient thoroughly low, the content of a boron atom that is contained in the lubricating oil composition is preferably 0.01 to 0.15 mass %, more preferably 0.01 to 0.10 mass %, and still more preferably 0.01 to 0.05 mass % on the basis of the whole amount of the lubricating oil composition. It is preferred that all of the boron atom in the lubricating oil composition is derived from a boron-containing compound (boron-based detergent dispersant) as mentioned later.

[0050] A total content of the calcium atom and the magnesium atom in the lubricating oil composition is preferably 0.3 mass % or less on the basis of the whole amount of the lubricating oil composition. When such a total content is 0.3 mass % or less, the amount of an ash component is low, so that there is less concern of clogging a post-processing device of exhaust gas, such as a gasoline particulate filter. The total content is more preferably 0.25 mass % or less, and still more preferably 0.20 mass % or less.

[0051] The total content of the calcium atom and the magnesium atom in the composition is 0.12 mass % or more, preferably 0.13 mass % or more, and more preferably 0.14 mass % or more. By making this total content high, it becomes easy to regulate a base number of the lubricating oil composition as mentioned later to 4 mgKOH/g or more.

[0052] The content of the magnesium atom in the lubricating oil composition is preferably 0.14 mass % or less, more preferably 0.10 mass % or less, and still more preferably 0.08 mass % or less on the basis of the whole amount of the lubricating oil composition.

[0053] It is preferred that the whole amount of the calcium atom and the magnesium atom in the lubricating oil composition is derived from a metal-based detergent as mentioned later.

[0054] Each of the components that are contained in the lubricating oil composition is hereunder described in more detail.

[Lubricating Base Oil]

[0055] The lubricating base oil may be either a mineral oil or a synthetic oil, and a mixed oil of a mineral oil and a synthetic oil may also be used.

[0056] Examples of the mineral oil include a mineral oil refined by subjecting a lubricating oil distillate that is obtained by distilling under reduced pressure an atmospheric residue given by atmospheric distillation of crude oil, to one or more treatments of solvent deasphalting, solvent extraction, hydro-cracking, solvent dewaxing, catalytic dewaxing, hydrorefining, and the like; a mineral oil produced by isomerizing a mineral oil-based wax or a wax (GTL WAX) produced by the Fischer-Tropsch process, etc.; and the like.

[0057] Examples of the synthetic oil include polyolefins, such as polybutene, an .alpha.-olefin homopolymer or copolymer (e.g., an ethylene-.alpha.-olefin copolymer), etc.; various esters, such as a polyol ester, a dibasic acid ester, a phosphate ester, etc.; various ethers, such as a polyphenyl ether, etc.; polyglycols; alkylbenzenes; alkylnaphthalenes; and the like.

[0058] The lubricating base oils may be used singly or may be used in combination of two or more thereof.

[0059] A kinematic viscosity at 100.degree. C. of the lubricating base oil is preferably 2 to 15 mm.sup.2/s, more preferably 2 to 10 mm.sup.2/s, and still more preferably 3 to 5 mm.sup.2/s.

[0060] When the kinematic viscosity at 100.degree. C. of the lubricating base oil is the aforementioned lower limit value or more, an evaporation loss is small, and hence, it is preferred. On the other hand, when the kinematic viscosity is the aforementioned upper limit value or less, a power loss to be caused due to viscous resistance is not excessively large, so that a fuel consumption improving effect is readily obtained.

[0061] The lubricating base oil has an amount of a paraffin component (also referred to as "% Cp") by n-d-M ring analysis of preferably 70% or more, more preferably 75% or more, and still more preferably 80% or more. When the amount of a paraffin component of the lubricating base oil is the aforementioned lower limit value or more, it is easy to render the oxidation stability and so on good. Furthermore, when the lubricating base oil has a kinematic viscosity at 100.degree. C. of 3 to 5 mm.sup.2/s and a % Cp of 80% or more, it is easy to render the oxidation stability better.

[0062] Furthermore, in order to improve the fuel consumption properties while suppressing a change in viscosity due to a change in temperature, a viscosity index of the lubricating base oil is preferably 80 or more, more preferably 90 or more, and still more preferably 100 or more.

[0063] In the lubricating oil composition, in the case of using a mixed oil made of a combination of two or more base oils, it is also preferred that the kinematic viscosity, viscosity index, and % Cp of the mixed oil fall within the aforementioned ranges, respectively.

[0064] The content of the lubricating base oil in the lubricating oil composition is preferably 65 to 95 mass %, more preferably 70 to 95 mass %, and still more preferably 70 to 90 mass % on the basis of the whole amount of the lubricating oil composition.

[Metal-Based Detergent]

[0065] The metal-based detergent that is used in the present embodiment contains a calcium salicylate. In the present embodiment, by using a calcium salicylate, the high-temperature detergency is readily improved.

[0066] The content of the calcium salicylate as converted into a calcium atom is preferably 0.08 to 0.16 mass %, more preferably 0.10 to 0.16 mass %, and still more preferably 0.10 to 0.14 mass % on the basis of the whole amount of the lubricating oil composition.

[0067] The metal-based detergent further contains, in addition to the calcium salicylate, at least one of a calcium sulfonate and a magnesium-based detergent. In the present embodiment, by using a calcium sulfonate and/or a magnesium-based detergent as the metal-based detergent, it becomes easy to decrease the friction coefficient while rendering the high-temperature detergency good.

[0068] Although two of these calcium sulfonate and magnesium-based detergent may be used in combination, only one of them may also be used.

[0069] Examples of the magnesium-based detergent include a magnesium salicylate, a magnesium sulfonate, and a magnesium phenate. It is preferred to use a magnesium sulfonate. These compounds may be used singly or may be used in admixture of two or more thereof. Although a base number of the magnesium-based detergent is not particularly limited, it is preferably 10 to 500 mgKOH/g, more preferably 200 to 450 mgKOH/g, and still more preferably 300 to 450 mgKOH/g.

[0070] Here, in the case where the calcium sulfonate is contained in the lubricating oil composition, the content of the calcium sulfonate as converted into a calcium atom is preferably 0.01 to 0.05 mass %, more preferably 0.01 to 0.04 mass %, and still more preferably 0.01 to 0.03 mass % on the basis of the whole amount of the lubricating oil composition.

[0071] In the case where the magnesium-based detergent is contained in the lubricating oil composition, the content of the magnesium-based detergent as converted into a magnesium atom is preferably 0.01 to 0.14 mass %, and more preferably 0.01 to 0.10 mass % on the basis of the whole amount of the lubricating oil composition. Then, in the case where the magnesium-based detergent is used in combination with the calcium sulfonate, its content as converted into a magnesium atom is still more preferably 0.02 to 0.06 mass %, whereas in the case where the magnesium-based detergent is used singly without being used in combination of the calcium sulfonate, its content is still more preferably 0.04 to 0.08 mass %.

[0072] The metal-based detergent may further contain a metal-based detergent other than the calcium salicylate, the calcium sulfonate, and the magnesium-based detergent. Examples of such a metal-based detergent include a calcium phenate, and a metal-based detergent other than the calcium-based detergent and the magnesium-based detergent. Examples of the metal detergent other than the calcium-based detergent and the magnesium-based detergent include a barium-based detergent, a sodium-based detergent, and a potassium-based detergent.

[0073] However, it is preferred that the metal-based detergent that is contained in the lubricating oil composition consists of a metal-based detergent selected from the group consisting of a calcium-based detergent and a magnesium-based detergent.

[0074] In the present embodiment, by regulating the amount of a soap component contained in the aforementioned calcium-based detergent, it is possible to adjust the value of the friction coefficient. Specifically, by controlling a proportion occupied by a soap component derived from the calcium sulfonate (hereinafter also referred to as "sulfonate soap component") in the total amount of soap components derived from the calcium salicylate, the calcium sulfonate, and the calcium phenate to be high, it is possible to decrease the friction coefficient.

[0075] For that reason, in the aforementioned expression (1), [A] denoting the proportion of the sulfonate soap component is given a minus coefficient and is subtracted from the value of X. The coefficient "-2.7.times.10.sup.4" to be multiplied on the coefficient [A] is produced taking into consideration any influence which the sulfonate soap component gives to the friction coefficient in the lubricating oil composition of the present embodiment through experiments and analyses thereof.

[0076] It is preferred that the sulfonate soap component (namely, the calcium sulfonate) is contained in the lubricating oil composition in order to easily decrease the friction coefficient, but the sulfonate soap component may not be contained in the lubricating oil composition. Even when the lubricating oil composition does not contain the sulfonate soap component, by regulating [B] to [E], it is possible to allow X to fall within the aforementioned range.

[0077] In the case where the sulfonate soap component is contained, the proportion occupied by the sulfonate soap component in the total amount of soap components derived from the calcium salicylate, the calcium sulfonate, and the calcium phenate is preferably 10 to 75 mass %, more preferably 20 to 70 mass %, and still more preferably 25 to 65 mass %.

[0078] In the lubricating oil composition, the magnesium-based detergent tends to decrease the friction coefficient. For that reason, in the aforementioned expression (1), [B] denoting the amount of magnesium derived from the magnesium-based detergent is given a minus coefficient and is subtracted from the value of X. The coefficient "-4.2.times.10.sup.-1" to be multiplied on the coefficient [B] is produced taking into consideration any influence which magnesium gives to the friction coefficient in the lubricating oil composition of the present embodiment through experiments and analyses thereof.

[0079] Furthermore, in the case where the lubricating oil composition contains both the sulfonate soap component (namely, the calcium sulfonate) and the magnesium-based detergent, the friction coefficient tends to become high due to an interaction between these components. The factor [E] in the expression (1) expresses this interaction. In the present lubricating oil composition, it has been found through experiments and analyses thereof that the friction coefficient becomes high approximately in proportion to a value obtained by multiplying the factor [E] by "7.3.times.10.sup.-3", and therefore, "7.3.times.10.sup.3.times.[E]" is added to the value "X" in the expression (1).

[0080] Examples of the aforementioned calcium sulfonate or magnesium sulfonate include calcium salts or magnesium salts of an alkyl aromatic sulfonic acid, the alkyl aromatic sulfonic acid being obtained through sulfonation of an alkyl aromatic compound having a weight average molecular weight of preferably 300 to 1,500, and more preferably 400 to 700.

[0081] Examples of the calcium phenate or magnesium phenate include calcium salts or magnesium salts of an alkylphenol, an alkylphenol sulfide, or a Mannich reaction product of alkylphenol.

[0082] Furthermore, examples of the calcium salicylate or magnesium salicylate include calcium salts or magnesium salts of an alkylsalicylic acid.

[0083] The alkyl group that is contained in each of these calcium-based detergents or magnesium-based detergents is an alkyl group having preferably 4 to 30 carbon atoms, and more preferably 10 to 26 carbon atoms, and such an alkyl group may be either linear or branched. Such an alkyl group may be a primary alkyl group, a secondary alkyl group, or a tertiary alkyl group.

[0084] As each of the aforementioned calcium-based detergents or magnesium-based detergents, substantially neutral materials (neutral salts) that are obtained by allowing the aforementioned alkyl aromatic sulfonic acid, alkylphenol, alkylphenol sulfide, Mannich reaction product of alkylphenol, or alkylsalicylic acid, or the like to react directly with an oxide or hydroxide of an alkaline earth metal (namely, calcium or magnesium) or the like; by once converting it into an alkali metal salt, such as a sodium salt, a potassium salt, etc., and then substituting the alkali metal salt with an alkaline earth metal salt; or other means, can be used.

[0085] Alternatively, materials obtained by heating the above-obtained neutral salt and an excessive alkaline earth metal salt or alkaline earth metal base in the presence of water may also be used, or materials obtained by allowing the above-obtained neutral salt to react with a carbonic acid salt or boric acid salt of an alkaline earth metal in the presence of a carbon dioxide gas, or other means may also be used.

[0086] As for the calcium-based detergent, when the neutral salt is used among those described above, it is possible to increase the proportion of the soap component in the calcium-based detergent. By regulating the amount of the calcium salt, calcium base, boric acid salt, carbon dioxide gas, or the like to be allowed to act on the calcium-based sulfonate, calcium-based phenate, or calcium-based salicylate, the amount of the soap component that is contained in the calcium-based detergent can also be regulated.

[Molybdenum Dialkyldithiocarbamate Compound]

[0087] As the molybdenum dialkyldithiocarbamate compound (hereinafter also referred to simply as "molybdenum compound") that is used in the present embodiment, specifically, a compound represented by following general formula (I) is exemplified.

##STR00001##

[0088] In the general formula (I), each of R.sup.1 to R.sup.4 represents an alkyl group having 4 to 22 carbon atoms, and R.sup.1 to R.sup.4 may be the same as or different from each other; and each of X.sup.1 to X.sup.4 represents a sulfur atom or an oxygen atom.

[0089] In the general formula (I), when the number of carbon atoms of the alkyl group is 4 or more, the oil solubility of the molybdenum compound is good. When the number of carbon atoms of the alkyl group is 22 or less, the melting point is relatively low, the handling properties are good, and the friction-reducing ability is high. From these viewpoints, the number of carbon atoms of the alkyl group represented by each of R.sup.1 to R.sup.4 is preferably 4 to 18, and more preferably 8 to 13.

[0090] The alkyl group represented by each of R.sup.1 to R.sup.4 may be any of a branched alkyl group and a linear alkyl group. Preferred examples of the alkyl group include an n-octyl group, a 2-ethylhexyl group, an isononyl group, an n-decyl group, an isodecyl group, a dodecyl group, a tridecyl group, an isotridecyl group, and the like.

[0091] From the viewpoints of solubility in the base oil, storage stability, and friction-reducing ability, as for the molybdenum compound represented by general formula (I), it is preferred that R.sup.1 and R.sup.2 are the same alkyl group, R.sup.3 and R.sup.4 are the same alkyl group, and the alkyl groups of R.sup.1 and R.sup.2 and the alkyl groups of R.sup.3 and R.sup.4 are different from each other.

[0092] In the general formula (I), each of X.sup.1 to X.sup.4 represents a sulfur atom or an oxygen atom, and X.sup.1 to X.sup.4 may be the same as or different from each other. A ratio between the sulfur atom and the oxygen atom is preferably 1/3 to 3/1, and more preferably 1.5/2.5 to 3/1 in terms of (sulfur atom)/(oxygen atom). When the ratio falls within the aforementioned range, good performances are obtained from the standpoints of corrosion resistance and solubility in the lubricating base oil. All of X.sup.1 to X.sup.4 may be a sulfur atom or an oxygen atom, alternatively.

[0093] In the present embodiment, it is preferred that all of the molybdenum atoms in the lubricating oil composition are those derived from the aforementioned molybdenum compound. For that reason, in the lubricating oil composition, the content of the aforementioned molybdenum compound as converted into a molybdenum atom is preferably 0.05 to 0.10 mass %, more preferably 0.05 to 0.09 mass %, and still more preferably 0.05 to 0.08 mass % on the basis of the whole amount of the lubricating oil composition.

[Boron-Containing Compound]

[0094] The boron-containing compound that is used in the present embodiment is a detergent dispersant (boron-based detergent dispersant). Specifically, examples thereof include a borated dispersant, an alkali metal boric acid salt, a borated epoxide, a boric acid ester, a borated aliphatic amine, and a borated amide. The boron-containing compounds may be used singly or may be used in admixture of two or more thereof.

[0095] By using such a boron-containing compound, the high-temperature detergency of the lubricating oil composition can be made good.

[0096] The borated dispersant is an ashless dispersant. More specifically, examples of the borated dispersant may include borated polyalkenyl succinic anhydrides; borated non-nitrogen-containing derivatives of a polyalkylene succinic anhydride; borates of basic nitrogen compounds (borated basic nitrogen compounds) selected from the group consisting of succinimides, carboxylic acid amides, hydrocarbyl monoamines, hydrocarbyl polyamines, Mannich bases, phosphonoamides, thiophosphonoamides, and phosphoramides, thiazoles, mercaptobenzothiazoles, (for example, 2,5-dimercapto-1,3,4-thiadiazoles) and derivatives of these, triazoles (for example, alkyltriazoles and benzotriazoles), copolymers which contain a carboxylate ester with one or more additional polar functional groups, such as amines, amides, imines, imides, etc. (for example, products produced through copolymerization of a long chain alkyl acrylate or methacrylate with a monomer having the aforementioned functional group); and mixtures thereof. The borated dispersant is preferably a borated succinimide.

[0097] Examples of the borated succinimide include ones obtained through boration of an alkenyl or alkyl succinic monoimide or an alkenyl or alkyl succinic bisimide.

[0098] As the alkenyl or alkyl succinic monoimide, a compound represented by the following general formula (II) is exemplified. As the alkenyl or alkyl succinic bisimide, a compound represented by the following general formula (III) is exemplified.

##STR00002##

[0099] In the general formulae (II) and (III), each of R.sup.21, R.sup.23, and R.sup.24 is an alkenyl group or an alkyl group, and a weight average molecular weight of each of them is preferably 500 to 3,000, and more preferably 900 to 3,000.

[0100] When the weight average molecular weight of each of R.sup.21, R.sup.23, and R.sup.24 is 500 or more, the solubility in the lubricating base oil can be made good. When it is 3,000 or less, it is expected that the effect obtained by the present compound is appropriately exhibited. R.sup.23 and R.sup.24 may be the same as or different from each other.

[0101] Each of R.sup.22, R.sup.25, and R.sup.26 is an alkylene group having 2 to 5 carbon atoms, and R.sup.25 and R.sup.26 may be the same as or different from each other. a denotes an integer of 1 to 10, and b denotes 0 or an integer of 1 to 10.

[0102] Here, a is preferably 2 to 5, and more preferably 2 to 4. When a is 2 or more, it is expected that the effect obtained by the borated succinimide is readily obtained. When a is 5 or less, the solubility in the lubricating base oil becomes much more good.

[0103] b is preferably 1 to 6, and more preferably 2 to 6. When b is 1 or more, it is expected that the effect obtained by the present compound is appropriately exhibited. When b is 6 or less, the solubility in the lubricating base oil becomes much more good.

[0104] Examples of the alkenyl group may include a polybutenyl group, a polyisobutenyl group, and an ethylene-propylene copolymer. Examples of the alkyl group include ones resulting from hydrogenation of the foregoing alkenyl groups. Suitable examples of the alkenyl group include a polybutenyl group and a polyisobutenyl group. As the polybutenyl group, ones resulting from polymerization of a mixture of 1-butene and isobutene or high-purity isobutene are suitably used. Representative examples of the alkyl group that is suitable include ones resulting from hydrogenation of a polybutenyl group or a polyisobutenyl group.

[0105] The borated succinimide can be obtained by, for example, allowing a polyolefin to react with maleic anhydride to obtain an alkenyl succinic anhydride, further allowing a polyamine to react with a boron compound to obtain an intermediate, and allowing this intermediate to react with the alkenyl succinic anhydride to achieve imidation. It is possible to produce the monoimide or bisimide by altering a ratio between the alkenyl succinic anhydride or alkyl succinic anhydride and the polyamine.

[0106] The borated succinimide can also be produced by treating a non-boron-containing alkenyl or alkyl succinic monoimide or alkenyl or alkyl succinic bisimide with a boron compound.

[0107] Examples of the boron compound that is used herein include a boric acid, a boric acid salt, a boric acid ester, and the like. Examples of the boric acid include orthoboric acid, metaboric acid, paraboric acid, and the like. Examples of the boric acid salt used herein include ammonium borates, such as ammonium metaborate, ammonium tetraborate, ammonium pentaborate, ammonium octaborate, etc., and the like. Examples of the boric acid ester used herein include monomethyl borate, dimethyl borate, trimethyl borate, monoethyl borate, diethyl borate, triethyl borate, monopropyl borate, dipropyl borate, tripropyl borate, monobutyl borate, dibutyl borate, tributyl borate, and the like.

[0108] Although the alkali metal atom that is contained in the alkali metal boric acid salt is not particularly limited so long as it is an alkali metal atom, it is preferably a potassium atom or a sodium atom, and more preferably a potassium atom. The boric acid salt used in the alkali metal boric acid salt is an electrically positive compound (salt) that contains boron and oxygen and is arbitrarily hydrated. Examples of the boric acid salt include a salt of a boric acid ion (BO.sub.3.sup.3-), a salt of a metaboric acid ion (BO.sub.2.sup.-), and the like. The boric acid ion (BO.sub.3.sup.3-) may form various polymer ions, such as a triboric acid ion (B.sub.3O.sub.5.sup.-), a tetraboric acid ion (B.sub.4O.sub.7.sup.2-), a pentaboric acid ion (B.sub.5O.sub.8.sup.-), etc.

[0109] Specific examples of the alkali metal boric acid salt include sodium tetraborate, sodium pentaborate, sodium hexaborate, sodium octaborate, sodium cliborate, potassium metaborate, potassium triborate, potassium tetraborate, potassium pentaborate, potassium hexaborate, potassium octaborate, and the like. The alkali metal boric acid salt may also be a hydrate. Among the alkali metal boric acid salts, from the viewpoint of an improvement of detergency at a high temperature and the viewpoint of solubility in the base oil, potassium triborate (KB.sub.3O.sub.5) and a hydrate thereof (KB.sub.3O.sub.5.nH.sub.2O (n is a number of 0.5 to 2.4) are preferred.

[0110] Examples of the borated epoxide include ones obtained from a reaction product between one or more boron compounds and at least one epoxide. The epoxide is an aliphatic epoxide having generally 8 to 30 carbon atoms, preferably 10 to 24 carbon atoms, and more preferably about 12 to 20 carbon atoms. Suitable examples of the aliphatic epoxide include dodecene oxide, hexadecene oxide, and the like, and mixtures thereof.

[0111] Examples of the boric acid ester used as the boron-containing compound include ones obtained by allowing one or more boron compounds and one or more suitable oil-soluble alcohols to react with each other. As the alcohol, ones having generally 6 to 30 carbon atoms, and preferably 8 to 24 carbon atoms can be used. The boric acid ester may also be a borated phospholipid.

[0112] Examples of the borated aliphatic amine include ones obtained by allowing one or more boron compounds and one or more aliphatic amines, such as an amine having 14 to 18 carbon atoms, etc., to react with each other. The borated aliphatic amine can be produced by allowing the amine and the boron compound to react with each other at a temperature ranging from 50 to 300.degree. C., and preferably from 100 to 250.degree. C. in an equivalent ratio of the amine to the boron compound of 3/1 to 1/3.

[0113] Examples of the borated amide include a borated amide obtained from a reaction product of a linear or branched, saturated or unsaturated monovalent fatty acid having 8 to 22 carbon atoms, urea, and a polyalkylene polyamine; and a boron compound.

[0114] Specific examples of the boron compound that is used in producing the borated epoxide, the boric acid ester, the borated aliphatic amine, or the borated amide may include boron oxide, boron oxide hydrate, boron trioxide, boron trifluoride, boron tribromide, boron trichloride, boron acids such as boronic acid, boric acid, tetraboric acid, and metaboric acid, a boron amide, and various esters of boronic acid.

[0115] Among the foregoing boron-containing compounds, borated dispersants are preferred, and above all, it is preferred to use a borated succinimide. The borated succinimide may be used singly, but may also be used in combination with at least one selected from the aforementioned boron-containing compounds other than the borated succinimide.

[0116] In the lubricating oil composition of the present embodiment, the boron-containing compound improves the high-temperature detergency, whereas the boron component becomes a cause to increase the friction coefficient. However, so long as the relational expression of the expression (1) is satisfied, it is possible to keep the friction coefficient sufficiently low. It has been found through experiments and analyses thereof that the boron component increases the friction coefficient approximately in proportion to a value obtained by multiplying the content of a boron atom [C] by "5.4.times.10.sup.-1", and therefore, the value of "5.4.times.10.sup.-1.times.[C]" is added to the value of "X" as in the expression (1).

[0117] From the viewpoint of ensuring the high-temperature detergency good while keeping the friction coefficient sufficiently low, the content of the boron-containing compound as converted into a boron atom in the lubricating oil composition is preferably 0.01 to 0.15 mass %, more preferably 0.01 to 0.10 mass %, and still more preferably 0.01 to 0.05 mass % on the basis of the whole amount of the lubricating oil composition.

[0118] Furthermore, from the viewpoint of ensuring the high-temperature detergency while making the friction coefficient low, the content of the boron atom that is contained in the lubricating oil composition is preferably 0.1 to 0.8, more preferably 0.2 to 0.7, and still more preferably 0.3 to 0.6 relative to the content of the molybdenum atom in the lubricating oil composition.

[Ashless Friction Modifier]

[0119] The lubricating oil composition in the present embodiment may contain an ashless friction modifier, but it may not contain an ashless friction modifier. When the lubricating oil composition contains an ashless friction modifier, it becomes possible to make the friction coefficient lower. Meanwhile, when no ashless friction modifier is contained, it becomes easy to make the high-temperature detergency good.

[0120] Examples of the ashless friction modifier include an ester-based ashless friction modifier and an amine-type ashless friction modifier. The ashless friction modifiers may be used singly or may be used in combination of two or more thereof.

[0121] Examples of the ester-based ashless friction modifier include esters of a fatty acid and an aliphatic alcohol. Examples of the fatty acid include aliphatic monocarboxylic acids having a linear or branched hydrocarbon group having 6 to 30 carbon atoms. The number of carbon atoms of this hydrocarbon group is preferably 8 to 24, and more preferably 10 to 20. The hydrocarbon group as referred to herein refers to a hydrocarbon moiety from which a carboxyl group of the fatty acid is eliminated.

[0122] As the aliphatic alcohol, an aliphatic polyhydric alcohol is used. The ester of a fatty acid and an aliphatic alcohol may be a partial ester in which only a part of the alcohol is esterified, or may be a complete ester in which the alcohol is entirely esterified. In general, a partial ester is used.

[0123] Examples of the aforementioned linear or branched hydrocarbon group having 6 to 30 carbon atoms include alkyl groups, such as a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, an octadecyl group, a nonadecyl group, an eicosyl group, a pentaeicosyl group, a docosyl group, a tricosyl group, a tetracosyl group, a pentacosyl group, a hexacosyl group, a heptacosyl group, an octacosyl group, a nonacosyl group, a triacontyl group, etc.; alkenyl groups, such as a hexenyl group, a heptenyl group, an octenyl group, a nonenyl group, a decenyl group, an undecenyl group, a dodecenyl group, a tridecenyl group, a tetradecenyl group, a pentadecenyl group, a hexadecenyl group, a heptadecenyl group, an octadecenyl group, a nonadecenyl group, an eicosenyl group, a heneicosenyl group, a docosenyl group, a tricosenyl group, a tetracosenyl group, a pentacosenyl group, a hexacosenyl group, a heptacosenyl group, an octacosenyl group, a nonacosenyl group, a triacontenyl group, etc.; hydrocarbon groups having two or more double bonds; and the like. The aforementioned alkyl group, alkenyl group, and hydrocarbon group having two or more double bonds include all of linear structures and branched structures which are possible, and the position or positions of the double bond or bonds in the alkenyl group and the hydrocarbon group having two or more double bonds are arbitrary.

[0124] Specifically, examples of the fatty acid include saturated fatty acids, such as caproic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, lignoceric acid, etc.; and unsaturated fatty acids, such as myristoleic acid, palmitoleic acid, oleic acid, linoleic acid, etc. Of those, unsaturated fatty acids are preferred, an oleic acid is more preferred.

[0125] The aforementioned aliphatic polyhydric alcohol is a dihydric to hexahydric alcohol, and examples thereof include ethylene glycol, glycerin, trimethylolpropane, pentaerythritol, sorbitol, and the like, with glycerin being preferred.

[0126] That is, the ester-based ashless friction modifier is preferably an ester of glycerin and the aforementioned aliphatic monocarboxylic acid. The foregoing ester may be a complete ester or may be a partial ester. Above all, a partial ester obtained through reaction of glycerin and the aforementioned unsaturated fatty acid is more preferred. Specifically, examples thereof include monoesters, such as glycerin monomyristate, glycerin monopalmitate, glycerin monooleate, etc.; and diesters, such as glycerin dimyristate, glycerin dipalmitate, glycerin dioleate, etc.

[0127] Examples of the amine-type ashless friction modifier include an aliphatic amine compound. The foregoing aliphatic amine compound is an amine compound having a linear or branched hydrocarbon group having 6 to 30 carbon atoms. The number of carbon atoms of the hydrocarbon group of this amine compound is preferably 8 to 24, and more preferably 10 to 20. Examples of the hydrocarbon group having 6 to 30 carbon atoms are same as that enumerated as the hydrocarbon group for the fatty acid as mentioned above.

[0128] Examples of the aliphatic amine compound may include aliphatic monoamines or alkylene oxide adducts thereof, alkanolamines, aliphatic polyamines, imidazoline compounds, and the like. Specifically, examples thereof include aliphatic amine compounds, such as laurylamine, lauryldiethylamine, lauryldiethanolamine, dodecylodipropanolamine, palmitylamine, stearylamine, stearyltetraethylenepentamine, oleylamine, oleylpropylenediamine, oleyldiethanolamine, N-hydroxyethyl oleyl imidazoline, etc.; and adducts of an amine alkylene oxide such as N,N-dipolyoxyalkylene-N-alkyl (or alkenyl) (number of carbon atoms: 6 to 28) to the aliphatic amine compound or the like.

[0129] The ashless friction modifier is preferably an ester-based ashless friction modifier. Above all, as mentioned above, an ester of glycerin and a fatty acid is more preferred, and in particular, glycerin monooleate and glycerin dioleate, which are a partial ester of glycerin and oleic acid, are still more preferred.

[0130] In the lubricating oil composition in the present embodiment, it has been found through experiments and analyses thereof that the ashless friction modifier decreases the friction coefficient approximately in proportion to a value obtained by multiplying its content [D] by "5.2.times.10.sup.3", and therefore, in the expression (1), the value of "5.2.times.10.sup.-3.times.[D]" is subtracted from "X".

[0131] In the case where the ashless friction modifier is contained in the lubricating oil composition, the ashless friction modifier is contained in an amount of preferably 0.2 to 1.8 mass %, more preferably 0.2 to 1.7 mass %, and still more preferably 0.2 to 1.5 mass % on the basis of the whole amount of the lubricating oil composition.

[Non-Boron-Containing Ashless Detergent Dispersant]

[0132] The lubricating oil composition in the present embodiment may contain a non-boron-containing succinimide as a non-boron-containing ashless detergent dispersant. When the lubricating oil composition contains the non-boron-containing succinimide, it becomes easy to more improve the high-temperature detergency.

[0133] Examples of the succinimide include an alkenyl or alkyl succinic monoimide and an alkenyl or alkyl succinic bisimide Examples of the alkenyl or alkyl succinic monoimide include the compounds represented by the aforementioned general formula (II). Examples of the alkenyl or alkyl succinic bisimide include the compounds represented by the aforementioned general formula (III).

[0134] The non-boron-containing succinimide is contained in an amount of preferably 0.1 to 10 mass %, more preferably 0.5 to 5 mass %, and still more preferably 1 to 4 mass % on the basis of the whole amount of the lubricating oil composition.

[Other Additives]

[0135] The lubricating oil composition may further contain additives for lubricating oil, such as a viscosity index improver, a pour-point depressant, an anti-wear agent, an antioxidant, an antifoaming agent, etc. These additives may be used singly or may be used in combination of two or more thereof.

(Viscosity Index Improver)

[0136] Examples of the viscosity index improver include a polymethacrylate-based viscosity index improver, an olefin-based copolymer, such as an ethylene-propylene copolymer, etc., a styrene-based copolymer, such as a styrene-diene hydrogenated copolymer, etc., and the like. Of those, a polymethacrylate-based viscosity index improver is preferred. The polymethacrylate-based viscosity index improver may be either a dispersion type or a non-dispersion type.

[0137] A weight average molecular weight of the polymethacrylate-based viscosity index improver is preferably 10,000 to 1,000,000, more preferably 100,000 to 800,000, and still more preferably 300,000 to 600,000. The weight average molecular weight is a value as measured by GPC and obtained using polystyrene as a calibration curve.

[0138] The viscosity index improver is contained in an amount of preferably 0.1 to 15 mass %, more preferably 0.5 to 12 mass %, and still more preferably 1 to 12 mass % on the basis of the whole amount of the lubricating oil composition.

(Pour-Point Depressant)

[0139] Examples of the pour-point depressant include an ethylene-vinyl acetate copolymer, a condensate of a chlorinated paraffin and naphthalene, a condensate of a chlorinated paraffin and phenol, a polymethacrylate, a polyalkylstyrene, and the like.

[0140] The pour-point depressant is contained in an amount of preferably 0.01 to 2 mass %, more preferably 0.05 to 1 mass %, and still more preferably 0.1 to 0.5 mass % on the basis of the whole amount of the lubricating oil composition.

(Anti-Wear Agent)

[0141] Examples of the anti-wear agent include a zinc dithiophosphate, a zinc phosphate, a zinc dithiocarbamate; sulfur-containing compounds, such as a disulfide, a thiocarbonate, a thiocarbamate, etc.; phosphorus-containing compounds, such as a diphosphite ester, a phosphate ester, a phosphonate ester, and amine salts thereof, etc.; sulfur and phosphorus-containing compounds, such as a thiophosphite ester, a thiophosphate ester, a thiophosphonate ester, and amine salts thereof, etc.; and the like. Of those, a zinc dithiophosphate is preferred, and as a more preferred specific example thereof, a zinc dialkyldithiophosphate is exemplified.

[0142] The anti-wear agent is contained in an amount of preferably 0.01 to 4 mass %, more preferably 0.05 to 3 mass %, and still more preferably 0.1 to 2 mass % on the basis of the whole amount of the lubricating oil composition.

(Antioxidant)

[0143] The lubricating oil composition may further contain an antioxidant. Examples of the antioxidant include an amine-type antioxidant, a phenol-based antioxidant, a sulfur-based antioxidant, a phosphorus-based antioxidant, and the like. Of those, an amine-type antioxidant and a phenol-based antioxidant are preferred. As such an antioxidant, an arbitrary compound can be properly selected and used among known antioxidants that are used as an antioxidant of conventional lubricating oils.

[0144] Examples of the amine-type antioxidant include diphenylamine-type compounds, such as diphenylamine, a dialkyl diphenylamine having an alkyl group having 3 to 20 carbon atoms, etc.; and naphthylamine-type compounds, such as .alpha.-naphthylamine, a phenyl-.alpha.-naphthylamine substituted with an alkyl group having 3 to 20 carbon atoms, etc.

[0145] Examples of the phenol-based antioxidant include monophenol-based compounds, such as 2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-ethylphenol, octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, etc.; diphenol-based compounds, such as 4,4'-methylenebis(2,6-di-tert-butylphenol), 2,2'-methylenebis(4-ethyl-6-tert-butylphenol), etc.; and the like.

[0146] Examples of the sulfur-based antioxidant include dilauryl-3,3'-thiodipropionate and the like; and examples of the phosphorus-based antioxidant include a phosphite and the like.

[0147] These antioxidants can be contained singly, or plural kinds thereof can be arbitrarily combined and contained. In general, a combination of two or more kinds is preferably used, and a combination of an amine-type antioxidant and a phenol-based antioxidant is more preferably used.

[0148] The content of the antioxidant is preferably 0.01 to 10 mass %, more preferably 0.1 to 5 mass %, and still more preferably 0.5 to 3 mass % on the basis of the whole amount of the composition.

(Antifoaming Agent)

[0149] Examples of the antifoaming agent include a silicone oil, a fluorosilicone oil, and the like. The content of the antifoaming agent is preferably 0.1 to 30 ppm by mass, more preferably 0.5 to 15 ppm by mass, and still more preferably 1 to 10 ppm by mass in terms of a value as converted into silicon.

[0150] In the present embodiment, the lubricating oil composition contains a lubricating base oil, a metal-based detergent, a molybdenum dialkyldithiocarbamate compound, and a boron-containing compound at least as mentioned above, but it is preferred that a lubricating oil composition consists of a lubricating base oil, a metal-based detergent, a molybdenum dialkyldithiocarbamate compound, a boron-containing compound, a non-boron-containing succinimide, and at least one selected from other additives as enumerated above, or consists of a lubricating base oil, a metal-based detergent, a molybdenum dialkyldithiocarbamate compound, a boron-containing compound, a non-boron-containing succinimide, an ashless friction modifier, and at least one selected from other additives as enumerated above, from the standpoint of practical use.

<Physical Properties of Lubricating Oil Composition>

[0151] It is preferred that the lubricating oil composition of the present embodiment has a base number of 4 to 10 mgKOH/g. When the base number of the lubricating oil composition is 4 mgKOH/g or more, it is possible to suppress the formation of a deposit in the inside of an engine at the time of high-temperature operation and to prevent accumulation of a sludge, thereby keeping the inside of the engine clean. In addition, it is possible to prevent corrosive wear from occurring by neutralizing an acidic material formed due to a degradation of the engine oil, or the like. When the base number is 10 mgKOH/g or less, it becomes easy to decrease the contents of calcium atom and magnesium atom in the lubricating oil composition.

[0152] From the foregoing viewpoints, the base number of the lubricating oil composition is more preferably 5 to 10 mgKOH/g, and still more preferably 6 to 10 mgKOH/g.

[0153] It is preferred that the lubricating oil composition of the present embodiment has a kinematic viscosity at 100.degree. C. of less than 12.5 mm.sup.2/s and a high-temperature high-shear viscosity (HTHS viscosity) at 150.degree. C. of less than 3.5 mPas. By allowing the lubricating oil composition to fall within the foregoing viscosity range, it becomes possible to ensure the lubricating properties in a supercharged direct injection engine and so on.

[0154] The kinematic viscosity at 100.degree. C. of the lubricating oil composition is more preferably 6.1 to 12.5 mm.sup.2/s, and still more preferably 6.1 to 9.3 mm.sup.2/s. Furthermore, the HTHS viscosity at 150.degree. C. is more preferably 1.7 to 3.2 mPas, and still more preferably 2.0 to 2.9 mPas.

[Production Method of Lubricating Oil Composition]

[0155] The production method of a lubricating oil composition according to an embodiment of the present invention is a method of blending a lubricating base oil with at least a metal-based detergent, a molybdenum dialkyldithiocarbamate compound, and a boron-containing compound, in which the content of calcium atom, the content of molybdenum atom, and X calculated according to the expression (1) fall within the aforementioned predetermined ranges. In the present production method, the lubricating base oil may or may not be blended with an ashless friction modifier. Furthermore, a non-boron-containing succinimide and/or other additive(s) may be blended. The details of these respective components and blending amounts thereof and the like are the same as the details of the respective components and the contents of the respective components as mentioned above, and therefore, descriptions thereof are omitted.

[0156] In addition, as mentioned above, the value of X is approximate to the value of the intermetal friction coefficient as measured in a block-on-ring test. In consequence, when the expression (1) is adopted, even if the lubricating oil composition is not actually produced, it is possible to expect the friction coefficient of the obtained lubricating oil composition from a formulation of the lubricating oil composition. Accordingly, it is possible that while the value of X according to the expression (1) is previously calculated, a formulation of the lubricating oil composition is determined such that the value of X falls within the aforementioned predetermined range, and then the lubricating oil composition is produced.

[Application of Lubricating Oil Composition]

[0157] Although the aforementioned lubricating oil composition can be used for lubricating oils of various applications, it is preferred to use it for lubrication in an internal combustion engine. Above all, the lubricating oil composition is more preferably used for a gasoline engine, and especially, it is suitably used for a supercharged direct injection engine mounted with a direct injection mechanism and a supercharging mechanism. When the aforementioned lubricating oil composition is used for such an application, it is able to exhibit excellent detergency and fuel consumption performance while preventing low speed pre-ignition from occurring.

[Friction-Reducing Method of Internal Combustion Engine]

[0158] The friction-reducing method of an internal combustion engine according to an embodiment of the present invention is a method of adding the aforementioned lubricating oil composition to an internal combustion engine to reduce the friction generated on a sliding surface of the internal combustion engine. The sliding surface is preferably between a metal surface and a metal surface.

[0159] According to the friction-reducing method of an internal combustion engine of the present embodiment, by regulating X calculated according to the expression (1) to a predetermined range while controlling the calcium atom and the molybdenum atom in the lubricating oil composition to fixed amounts, it is possible to make the friction-reducing effect good while ensuring the excellent detergency, thereby realizing excellent fuel consumption properties. In addition, the friction-reducing method of an internal combustion engine of the present embodiment is suitable in the case where the internal combustion engine is a gasoline engine, and especially, in the case where the internal combustion engine is a supercharged direct injection engine, it is also possible to prevent low speed pre-ignition from occurring.

Examples

[0160] Next, the present invention is described by reference to Examples, but it should be construed that the present invention is by no means limited by these Examples.

[0161] Measurement methods and evaluation method of various properties in the present specification are as follows.

(1) Kinematic Viscosity:

[0162] The kinematic viscosity is a value as measured using a glass-made capillary viscometer in conformity with JIS K2283:2000.

(2) Viscosity Index:

[0163] The viscosity index is a value as measured in conformity with JIS K2283:2000.

(3) HTHS Viscosity at 150.degree. C.:

[0164] The HTHS viscosity at 150.degree. C. is a viscosity as measured using a TBS viscometer (tapered bearing simulator viscometer) by the method of ASTM D4683 under conditions of shear rate: 10.sup.6 sec.sup.-1, rotational rate (motor): 3,000 rpm, rotor/stator clearance: 3 .mu.m, and oil temperature: 150.degree. C.

(4) Paraffin Component (% Cp) by Ring Analysis:

[0165] The % Cp expresses a proportion (percentage) of paraffin components as calculated by the ring analysis n-d-M method and measured in conformity with ASTM D-3238.

(5) Soap Component:

[0166] The calcium-based detergent was subjected to rubber membrane dialysis, a rubber membrane residue after the dialysis was treated with hydrochloric acid, and thereafter, a component extracted with diethyl ether was quantitatively determined as the soap component.

(6) Measurement Method of Each of Contents of Calcium Atom, Magnesium Atom, Phosphorus Atom, Molybdenum Atom, and Boron Atom:

[0167] The measurement was performed in conformity with JPI-5S-38-92.

(7) Base Number:

[0168] The base number of the lubricating oil composition was measured by a perchloric acid method of JIS K2501:2003. The base number of the metal-based detergent is one measured according to a perchloric acid method of JIS K2501:2003.

(8) Friction Coefficient (LFW-1 Test):

[0169] The intermetal friction coefficient was measured using a block-on ring tester (LFW-1). Specifically, the test conditions are as follows.

[0170] Test instrument:

[0171] Ring: S-10 standard material

[0172] Block: SUJ-2

[0173] Test conditions:

[0174] Oil temperature: 80.degree. C.

[0175] Load: 30 kgf

[0176] Rotation rate: 160 rpm

(9) Hot Tube Test:

[0177] The hot tube test was performed on the basis of JPI-5S-55-99. Specifically, 0.3 mL/hr of the lubricating oil composition and 10 mL/min of air were continuously flown for 16 hours into a glass tube having an inside diameter of 2 mm and kept at a temperature of 280.degree. C. A lacquer attached in the interior of the glass tube was compared with a color sample, and the merit rating was given such that the case where the lacquer was transparent was denoted as Score 10, whereas the case where the lacquer was colored black was denoted as Score 0. It is meant that the higher the merit rating, the higher the detergency is.

Examples 1 to 9 and Comparative Examples 1 to 3

[0178] A lubricating oil composition of each of the Examples and Comparative Examples was prepared in a formulation shown in Table 1. With respect to the resulting lubricating oil compositions, the respective properties and the contents of the respective atoms in the lubricating oil compositions were measured. In addition, the friction coefficient was measured by the LFW-1 test, and the hot tube test was also performed. These results are shown in Table 1.

[0179] The blending amount of the viscosity index improver was regulated such that the HTHS viscosity at 150.degree. C. of each of the lubricating oil compositions of the Examples and Comparative Examples was 2.3 mPas.

TABLE-US-00001 TABLE 1 Example 1 2 3 4 5 6 Formulation Lubricating base Balance Balance Balance Balance Balance Balance oil wt % Calcium-based 0.95 1.10 0.95 1.10 1.10 0.95 detergent 1 wt % Calcium-based 0.75 0.75 0.75 detergent 2 wt % Magnesium-based 0.32 0.32 0.32 0.54 0.32 detergent wt % Friction modifier 0.80 0.80 0.80 0.80 0.80 0.80 wt % Ashless friction modifier wt % Viscosity index Regulated Regulated Regulated Regulated Regulated Regulated improver wt % Pour-point 0.20 0.20 0.20 0.20 0.20 0.20 depressant wt % Additive 7.90 7.90 7.90 package A wt % Additive package 8.90 8.90 8.90 B wt % Additive package C wt % Properties of lubricating oil composition HTHS viscosity 2.3 2.3 2.3 2.3 2.3 2.3 (at 150.degree. C.) mPa s Base number 6.1 7.8 7.4 8.1 9.0 7.7 mgKOH/g Calcium amount 0.14 0.14 0.14 0.14 0.14 0.14 wt % Magnesium 0.00 0.03 0.03 0.03 0.05 0.03 amount wt % Total amount 0.14 0.17 0.17 0.17 0.19 0.17 of calcium and magnesium wt % Phosphorus 0.08 0.08 0.08 0.08 0.08 0.08 amount wt % Molybdenum 0.08 0.08 0.08 0.08 0.08 0.08 amount wt % Boron amount 0.03 0.03 0.03 0.04 0.04 0.04 wt % Boron amount/ 0.38 0.38 0.38 0.50 0.50 0.50 molybdenum amount Calcium amount 0.12 0.14 0.12 0.14 0.14 0.12 derived from detergent 1 wt % Calcium amount 0.02 0.00 0.02 0.00 0.00 0.02 derived from detergent 2 wt % Numerical expression A 60 0 60 0 0 60 B 0 0.03 0.03 0.03 0.05 0.03 C 0.03 0.03 0.03 0.04 0.04 0.04 D 0 0 0 0 0 0 E 0.061 0.031 1.891 0.031 0.051 1.891 X 0.038 0.042 0.039 0.047 0.039 0.045 Evaluation results of lubricating oil composition Friction coefficient 0.036 0.042 0.041 0.049 0.041 0.045 by LFW-1 test Merit rating 7.0 7.0 7.0 8.0 8.0 8.0 of hot tube test (at 280.degree. C.) Comparative Example Example 7 8 9 1 2 3 Formulation Lubricating base Balance Balance Balance Balance Balance Balance oil wt % Calcium-based 0.95 0.95 1.10 1.10 1.10 0.95 detergent 1 wt % Calcium-based 0.75 0.75 0.75 detergent 2 wt % Magnesium-based 0.54 0.32 0.54 detergent wt % Friction modifier 0.80 0.80 0.80 0.80 0.80 0.80 wt % Ashless friction 1.00 1.00 modifier wt % Viscosity index Regulated Regulated Regulated Regulated Regulated Regulated improver wt % Pour-point 0.20 0.20 0.20 0.20 0.20 0.20 depressant wt % Additive package 7.90 7.90 A wt % Additive package 8.90 8.90 8.90 B wt % Additive package 5.90 C wt % Properties of lubricating oil composition HTHS viscosity 2.3 2.3 2.3 2.3 2.3 2.3 (at 150.degree. C.) mPa s Base number 8.6 7.4 9.0 6.5 6.8 6.1 mgKOH/g Calcium amount 0.14 0.14 0.14 0.14 0.14 0.14 wt % Magnesium 0.05 0.03 0.05 0.00 0.00 0.00 amount wt % Total amount of 0.19 0.17 0.19 0.14 0.14 0.14 calcium and magnesium wt% Phosphorus 0.08 0.08 0.08 0.08 0.08 0.08 amount wt % Molybdenum 0.08 0.08 0.08 0.08 0.08 0.08 amount wt % Boron amount 0.04 0.03 0.04 0.03 0.04 0.00 Boron 0.50 0.38 0.50 0.38 0.50 0.00 amount/molybdenum amount Calcium amount 0.12 0.12 0.14 0.14 0.14 0.12 derived from detergent 1 wt % Calcium amount 0.02 0.02 0.00 0.00 0.00 0.02 derived from detergent 2 wt % Numerical expression A 60 60 0 0 0 60 B 0.05 0.03 0.05 0 0 0 C 0.04 0.03 0.04 0.03 0.04 0 D 0 1 1 0 0 0 E 3.111 1.891 0.051 0.001 0.001 0.061 X 0.045 0.034 0.034 0.054 0.060 0.022 Evaluation results of lubricating oil composition Friction coefficient 0.042 0.037 0.031 0.052 0.062 -- by LFW-1 test Merit rating of hot 8.0 6.5 7.0 7.0 8.0 2.0 tube test (at 280.degree. C.)

[0180] The details of the respective components in Table 1 are as follows.

[0181] Lubricating base oil: Mineral oil having a kinematic viscosity (at 100.degree. C.) of 4.0 mm.sup.2/s, a viscosity index of 130, and a % Cp of 87 mass %

[0182] Calcium-based detergent 1: Calcium salicylate

[0183] Calcium-based detergent 2: Calcium sulfonate

[0184] Magnesium-based detergent: Magnesium sulfonate having a base number of 410 mgKOH/g

[0185] Friction modifier: Molybdenum dialkyldithiocarbamate compound

[0186] Ashless friction modifier: Glycerin monooleate

[0187] Viscosity index improver: Polymethacrylate having a weight average molecular weight of 300,000

[0188] Pour-point depressant: Polymethacrylate having a weight average molecular of 50,000

[0189] Additive package A: Mixture of boron-based detergent dispersant, non-boron-containing alkenyl succinimide, zinc dialkyldithiophosphate, phenol-based antioxidant, and amine-type antioxidant

[0190] Additive package B: Same as Additive package A except for increasing an amount of the boron-based detergent dispersant

[0191] Additive package C: Mixture of non-boron-containing alkenyl succinimide, zinc dialkyldithiophosphate, phenol-based antioxidant, and amine-type antioxidant

[0192] As is clear from the results of Table 1, in Examples 1 to 9, by containing the metal-based detergent, the molybdenum dialkyldithiocarbamate compound, and the boron-containing compound in the lubricating oil composition and regulating the value of X to less than 0.050 while allowing the calcium atom amount and the molybdenum atom amount in the composition to fall within the predetermined ranges, the value of the friction coefficient could be made low while keeping the detergency.

[0193] On the other hand, in Comparative Examples 1 and 2, the metal-based detergent, the molybdenum dialkyldithiocarbamate compound, and the boron-containing compound were contained in the lubricating oil composition, and the calcium atom amount and the molybdenum atom amount in the composition were allowed to fall within the predetermined ranges; however, since the value of X was 0.050 or more, the friction coefficient became large, so that the lubricating oil composition having good fuel consumption properties could not be obtained.

[0194] In addition, as shown in Comparative Example 3, even if the value of X was regulated to less than 0.050, when the boron-containing compound was not contained, the merit rating of the hot tube test became low, and sufficient detergency could not be obtained.

Claims

1. A lubricating oil composition comprising: a lubricating base oil, a metal-based detergent, a molybdenum dialkyldithiocarbamate compound, and a boron-containing compound, wherein the metal-based detergent contains a calcium salicylate and at least one selected from the group consisting of a calcium sulfonate and a magnesium-based detergent, the lubricating base oil arbitrarily contains an ashless friction modifier, in the lubricating oil composition, the content of a calcium atom is 0.12 to 0.16 mass % on the basis of the whole amount of the lubricating oil composition; and the content of a molybdenum atom is 0.05 to 0.10 mass % on the basis of the whole amount of the lubricating oil composition, and X calculated according to the following expression (1) is less than 0.050: X=3.8.times.10.sup.-2-2.7.times.10.sup.-4.times.[A]-4.2.times.10.sup.-1.t- imes.[B]+5.4.times.10.sup.-1.times.[C]-5.2.times.10.sup.-3.times.[D]+7.3.t- imes.10.sup.-3.times.[E] (1) wherein [A] denotes a proportion (mass %) occupied by a soap component derived from a calcium sulfonate in the total amount of soap components derived from a calcium salicylate, a calcium sulfonate, and a calcium phenate; [B] denotes the content (mass %) of the magnesium-based detergent as converted into a magnesium atom; [C] denotes the content (mass %) of a boron atom contained in the lubricating oil composition; [D] denotes the content (mass %) of the ashless friction modifier; and [E] denotes E calculated according to the following expression: E=([A]+1).times.([B]+0.001) (2) the values of [B] to [D] being a value on the basis of the whole amount of the lubricating oil composition.

2. The lubricating oil composition according to claim 1, wherein a total content of the calcium atom and the magnesium atom in the lubricating oil composition is 0.3 mass % or less on the basis of the whole amount of the lubricating oil composition.

3. The lubricating oil composition according to claim 1, wherein the lubricating base oil has a kinematic viscosity at 100.degree. C. of 3 to 5 mm.sup.2/s and a % Cp by n-d-M ring analysis of 80% or more.

4. The lubricating oil composition according to claim 1, wherein the boron-containing compound is at least one selected from the group consisting of a borated dispersant, an alkali metal boric acid salt, a borated epoxide, a boric acid ester, a borated aliphatic amine, and a borated amide.

5. The lubricating oil composition according to claim 1, further comprising a non-boron-containing succinimide.

6. The lubricating oil composition according to claim 1, wherein the ashless friction modifier is at least one selected from the group consisting of an ester-based ashless friction modifier and an amine-type ashless friction modifier.

7. The lubricating oil composition according to claim 1, further comprising one or more of an additive for lubricating oil selected from the group consisting of a viscosity index improver, a pour-point depressant, an anti-wear agent, an antioxidant, and an antifoaming agent.

8. The lubricating oil composition according to claim 1, which has a kinematic viscosity at 100.degree. C. of less than 12.5 mm.sup.2/s and a high-temperature high-shear viscosity (HTHS viscosity) at 150.degree. C. of less than 3.5 mPas.

9. The lubricating oil composition according to claim 1, which has a base number of 4 to 10 mgKOH/g.

10. The lubricating oil composition according to claim 1, which is used for an internal combustion engine.

11. A method for producing a lubricating oil composition comprising: blending a lubricating base oil with at least a metal-based detergent, a molybdenum dialkyldithiocarbamate compound, and a boron-containing compound, to obtain a lubricating oil composition, wherein the metal-based detergent contains a calcium salicylate and at least one selected from the group consisting of a calcium sulfonate and a magnesium-based detergent, the lubricating base oil is further arbitrarily blended with an ashless friction modifier, in the lubricating oil composition, the content of a calcium atom is 0.12 to 0.16 mass % on the basis of the whole amount of the lubricating oil composition; and the content of a molybdenum atom is 0.05 to 0.10 mass % on the basis of the whole amount of the lubricating oil composition, and X calculated according to the following expression (1) is less than 0.050: X=3.8.times.10.sup.-2-2.7.times.10.sup.-4.times.[A]-4.2.times.10.sup.-1.t- imes.[B]+5.4.times.10.sup.-1.times.[C]-5.2.times.10.sup.-3.times.[D]+7.3.t- imes.10.sup.-3.times.[E] (1) wherein [A] denotes a proportion (mass %) occupied by a soap component derived from a calcium sulfonate in the total amount of soap components derived from a calcium salicylate, a calcium sulfonate, and a calcium phenate; [B] denotes the content (mass %) of the magnesium-based detergent as converted into a magnesium atom; [C] denotes the content (mass %) of a boron atom contained in the lubricating oil composition; [D] denotes the content (mass %) of the ashless friction modifier; and [E] denotes E calculated according to the following expression: E=([A]+1).times.([B]+0.001) (2) the values of [B] to [D] being a value on the basis of the whole amount of the lubricating oil composition.