EP0699739A2

EP0699739A2 - Engine oil composition

Info

Publication number: EP0699739A2
Application number: EP95113870A
Authority: EP
Inventors: Yasushi c/o Japan Energy Corp. Naitoh; Kenyu Akiyama
Original assignee: Japan Energy Corp; Toyota Motor Corp
Current assignee: Toyota Motor Corp; Eneos Corp
Priority date: 1994-09-05
Filing date: 1995-09-04
Publication date: 1996-03-06
Anticipated expiration: 2015-09-04
Also published as: DE69515093D1; DE69515093T2; JPH0873878A; EP0699739A3; EP0699739B1; JP2971748B2

Abstract

An engine oil composition is composed of: (1) at least one oil selected from the group consisting of a mineral oil and a synthetic lubricant as a base oil; (2) a molybdenum dithicarbamate in an amount of 50 to 2000 ppm by weight when calculated as molybdenum (Mo), relative to the total weight of the engine oil composition; (3) zinc dithiophosphate in an amount of 0.01 to 0.2 wt% when calculated as phosphorus (P), relative to the total amount of the engine oil composition; and (4) an ashless organic polysulfide compound in an amount of 0.01 to 0.4 wt% when calculated as sulfur (S), relative to the total amount of the engine oil composition.

Description

Background of the Invention

(1) Field of the Invention

The present invention relates to an engine oil composition for automobiles. More particularly, the invention relates to the long life and fuel-saving engine oil composition which can suppress the friction loss in the engine to a low level for a long time.

(2) Related Art Statement

With the progress of the engines, the automobile engine oil compositions (hereinafter referred to briefly as "engine oil compositions") have been required to possess various performances such as wear resistance, oxidation stability, and detergent dispersibility. Recently, in order to suppress the earth from getting warmer due to increase in the content of CO₂ in the atmosphere, how to improve the mileage of the automobiles is an important problem. Accordingly, the fuel saving has been also strongly required with respect to the engine oils.
Ordinarily, the engine oil composition is composed of a mixture of a base oil purified from petroleum, added with additives ouch as a detergent, an antioxidant, an anti-wear agent, and a viscosity index improver. In order to increase the fuel efficiency (mileage) of the engine oil, for example, the viscosity of the engine oil is lowered by decreasing the viscosity of the base oil or changing the viscosity index improver. However, friction cannot be reduced in the case of the above ordinary engine oil composition in such an area as a boundary lubricating condition where the viscosity does not contributes to mitigation of the friction. Consequently, a friction modifier (FM) has recently come to be added so as to reduce the wearing in the boundary lubricating area. With respect to the friction modifiers, it is known that organic molybdenum compound such as molybdenum dithiocarbamate (MoDTC) and oxymolybdenum organo phosphodithioate sulfide (MoDTP) are highly effective as described in JP-B 3-23595.
However, as the time passes, each of the above organic molybdenum compounds used in the engine oil composition is consumed. Therefore, though the fresh engine oil composition gives a low fuel consumption rate, such a low fuel consumption rate of the engine oil composition is deteriorated with the lapse of time. In order to lessen the above drawback, it may be considered that the addition amount of the organic molybdenum compound in a fresh oil is increased. However, if the addition amount of the organic molybdenum compound is merely increased, the cost of the product becomes higher, which is economically unfavorable. Further, among the organic molybdenum compounds, MoDTP contains phosphorus, so that a phosphorus compound may deposit on the surface of an exhaust gas catalyst to deteriorate the catalytic activity. Therefore, the addition amount of the MoDTP cannot be increased beyond a given level.
On the other hand, since MoDTC contains no phosphorus, increase in its addition amount does not cause decrease in the catalytic activity. However, since MoDTC has a small friction-mitigating effect, it may be considered that MoDTC is used in combination with zinc dithiophosphate (ZnDTP) as an anti-wear agent so as to supplement the wear-mitigating effect of the former. ZnDTP has been frequently used, as antioxidant and anti-wear agent, in the engine oil compositions. However, since ZnDTP contains phosphorus and gives adverse influence upon the exhaust gas catalyst as mentioned above, its addition amount is limited so that good friction-mitigating effect cannot unfavorably be maintained for a long time. Further, it is proposed that MoDTC be used in combination with a sulfur-based extreme pressure additive (See JP-B 5-83599). This combination does not afford adverse effect upon the exhaust gas catalyst, but it encounters a practically great problem upon the engine oil composition in that wear largely occurs in the valve train system.

Summary of the Invention

Under the circumstances, it is an object of the present invention to enable the engine oil composition to maintain the friction loss at a low level even when the engine oil composition is used for a long time.
Furthermore, it is another object of the present invention to enable the engine oil composition to maintain the friction loss at a low level for a long time, while the addition amount of the friction modifier is kept at the same level as formerly employed.
It is still another object of the present invention to enable the engine oil composition to maintain the friction loss at a low level for a long time without affording adverse influence upon the catalytic activity for exhaust gases.
Having made strenuous investigation to accomplish the above-mentioned objects, the present inventors discovered that the combination of MoDTC and ZnDTP with a polysulfide compound can remarkably prolong the performance of the low fuel consumption rate, that is, can maintain the friction-mitigating effect of the engine oil for a long time without affording adverse influence upon the exhaust gas catalyst. Based on this discovery, the inventors have accomplished the present invention.
That is, the present invention relates to the engine oil composition comprising (1) at least one oil selected from the group consisting of a mineral oil and a synthetic lubricant as a base oil; (2) a molybdenum dithicarbamate in an amount of 50 to 2000 ppm by weight when calculated as molybdenum (Mo), relative to the total weight of the engine oil composition; (3) zinc dithiophosphate in an amount of 0.01 to 0.2 wt% when calculated as phosphorus (P), relative to the total amount of the engine oil composition; and (4) an ashless organic polysulfide compound in an amount of 0.01 to 0.4 wt% when calculated as sulfur (S), relative to the total amount of the engine oil composition. This engine oil composition is a long life and low fuel consumption engine oil composition which can maintain the friction loss at a low level for a long time.
These and other objects, features and advantages of the invention will be apparent from the following description of the invention with the understanding that some modifications, variations and changes of the same could easily be made by the skilled person in the art.

Detailed description of the invention

The base oil to be used in the engine oil composition according to the present invention is a mineral oil and/or a synthetic oil. As the base oil, which is used, in the engine oil composition, as a base component occupying a great part of the engine oil composition, any base oil may be used. Specifically, as the mineral oil, use may be made of a lubricant base oil which is producing by obtaining a cut through distilling an ordinary pressure distillation residue of such as a paraffinic crude oil under reduced pressure, treating the resulting cut through extraction with a solvent such as furfural, purification by hydrogenation and dewaxing with a solvent such as MEK/toluene, a lubricant base oil produced by obtaining a deasphalted oil by deasphalting the above pressure-reduced distillation residue and treating it by any of the above appropriate processes, a highly purified base oil obtained through isomerization of slack wax and dewaxing an appropriate cut of the isomerized oil with a solvent of MEK/toluene, or an appropriate mixture thereof.
As the synthetic oil, use may be made of an α-olefin oligomer, a diester synthesized from a dibasic acid such as an adipic acid and a primary alcohol, a polyol ester synthesized from a higher alcohol such as neopentyl glycol, trimethylol propane or pentaerithritol and a monobasic acid, an alkyl benzene or a polyoxyalkylene glycol or an appropriate mixture thereof. Further, needless to say, a mixed oil obtained by appropriately combining the mineral oil with the synthetic oil may be used as a base oil for the engine oil composition according to the present invention.
The molybdenum dithiocarbamate (MoDTC) to be used as an additive in the present invention is a compound expressed by the following formula (1)

In the formula (1), R₁ through R₄ independently denote a straight-chain or branched-chain alkyl group or a straight-chain or branched-chain alkenyl group having four to eighteen carbons; and X₁ through X₄ independently denote an oxygen atom or a sulfur atom, the ratio between the number of the oxygen atom or atoms and that of the sulfur atom or atoms with respect to X₁ through X₄ being 1/3 to 3/1. As R₁ through R₄, the alkyl group is preferred. More specifically, butyl group, 2-ethylhexyl group, isotridecyl group or stearyl group may be recited. These four R₁ through R₄ existing in one molecule may be identical with or different from each other. Further, two or more MoDTCs differing in terms of R₁ through R₄ may be used in a mixed state.
MoDTC is used in the addition amount of 50 to 2000 ppm by weight, preferably 300 to 1000 ppm by weight, when calculated as molybdenum (Mo), relative to the total weight of the engine oil composition. If the addition amount is less than 50 ppm by weight, the friction-reducing effect is small, whereas if it is more than 2000 ppm by weight, the friction-reducing effect is saturated and the cost increases.
The zinc dithiophosphate (ZnDTP) to be used as an additive in the present invention is a compound expressed by the formula (2):

In the formula (2), R₅ and R₆ independently denote a straight-chain or branched chain alkyl group or a straight-chain or branched chain aryl group having three to eighteen carbon atoms. As R₅ and R₆, an alkyl group, particularly, a primary alkyl group is preferred from the standpoint that the friction-mitigating performance must be maintained for a long time. More specifically, for example, propyl group, butyl group, pentyl group, hexyl group, octyl group and lauryl group may be recited. These two R₅ and R₆ existing in one molecule may be identical with or different from each other. Further, two or more kinds of ZnDTPs differing in terms of R₅ and R₆ may be used in a mixed state.
ZnDTP is added in an amount of 0.01 to 0.2 wt%, preferably 0.04 to 0.2 wt%, more preferably 0.04 to 0.1 wt% when calculated as phosphorus (P), relative to the total amount of the engine oil composition. If the addition amount is less than 0.01 wt%, the wear-preventing performance of the engine oil composition for the valve train system is deteriorated. On the other hand, if it is more than 0.2 wt%, influence of the phosphorus component upon the catalytic activity for the exhaust gas becomes greater.
The ashless organic polysulfide compound to be used in the present invention includes organic compounds expressed by the following formulae, such as sulfides of oils or fats or polyolefins, in which a sulfur atom group having two or more sulfur atoms adjoining and bonded together is present in a molecular structure.
R₇-Sx-R₈

OHC-R₉-S-S-R₁₀-CHO
R₇-C(O)O-R₉-S-S-R₁₀-OC(O)-R₈
R₇-OC(S)-S-S-C(S)O-R₈
In the above formulae, R₇ and R₈ independently denote a straight-chain, branched-chain, alicyclic or aromatic hydrocarbon group in which a straight chain, a branched chain, an alicyclic unit and an aromatic unit may be selectively contained in any combined manner. An unsaturated bond may be contained, but a saturated hydrocarbon group is preferred. Among them, alkyl group, aryl group, alkylaryl group, benzyl group, and alkylbenzyl group are preferred. R₉ and R₁₀ independently denote a straight-chain, branched-chain alicyclic or aromatic hydrocarbon group which has two bonding sites and in which a straight chain, a branched chain, an alicyclic unit and an aromatic unit may be selectively contained in any combined manner. An unsaturated bond may be contained, but a saturated hydrocarbon group is preferred. Among them, alkylene group is preferred. R₁₁ and R₁₂ independently denote a straight-chain or branched-chain hydrocarbon group. "x" and "y" denote independently an integer of two or more.
Specifically, for example, mention may be made of sulfurized sperm oil, sulfurized pinene oil, sulfurized soybean oil, sulfurized polyolefin, dialkyl disulfide, dialkyl polysulfide, dibenzyl disulfide, di-tertiary butyl disulfide, polyolefin polysulfide, thiadiazol type compound such as bis-alkyl polysulfanyl thiadiazole, and sulfurized phenol. Among these compounds, dialkyl polysulfide, dibenzyl disulfide, and thiadiazol type compound are preferred. Particularly, bis-alkyl polysulfanyl thiadiazole is preferred.
As the lubricant additive, a metal-containing compound such as Ca phenate having a polysulfide bond is used. However, since this compound has a large coefficient of friction, it is not suitable. To the contrary, the above organic polysulfide compound is an ashless compound containing no metal, and exhibits excellent performance in maintaining a low coefficient of friction for a long time when used in combination with MoDTC and ZnDTP.
The above ashless organic polysulfide compound (hereinafter referred to briefly as "polysulfide compound") is added in an amount of 0.01 to 0.4 wt%, preferably 0.1-0.3 wt%, more preferably 0.2-0.3 wt%, when calculated as sulfur (S), relative to the total amount of the engine oil composition. If the addition amount is less than 0.01 wt%, it is difficult to attain the intended effect, whereas if it is more than 0.4 wt%, there is a danger that corrosive wear increase. Needless to say, only one kind of the above polysulfide compound may be used, and two kinds of such polysulfide compounds may also be used in combination.
In order to ensure the performance suitable for the intended use, engine oil additives other than the above may be appropriately added to the engine oil composition according to the present invention so as to improve the total performance. As such engine oil additives, mention may be made of so-called metallic detergents such as sulfonate, phenate and salicylate of alkaline earth metals such as Ca, Mg and Ba and alkali metals such as Na, ashless dispersants such as alkenyl succinic acid imide, succinic acid esters and benzylamine, phenolic anti-oxidant such as bisphenol, amine-based anti-oxidant such as diphenylamine, and viscosity index improvers such as olefin copolymer or polymetacrylate. Further, other engine oil additives such as a pour point depressant, anti-corrosion agent and antifoaming agent may be appropriately added.

(Experiments)

The present invention will be explained in more detail with reference to Examples and Comparative Examples.

A lubricant in each of Examples and Comparative Examples was prepared by using Mineral Oils 1 or 2 having the following properties as a base oil.

Table 1

	Mineral oil 1	Mineral oil 2
Density (15°C)g/cm³	0.862	0.821
Dynamic viscosity (40°C)mm²/s	17.7	19.7
Dynamic viscosity (100°C)mm²/s	3.78	4.51
Viscosity index	99	147
Flow point (°C)	-15.0	-15.0
Content of saturated component (%)	76.5	98.8

As additives, the following were used.

(1) MoDTC:

Compound having the above-mentioned formula (1) in which R₁ through R₄ are all 2-ethylhexyl groups.

(2-1) Sulfur-based additive 1

Sulfur-based additive 1 means an additive containing the polysulfide compound used in the present invention, and includes a thiadiazole type polysulfide compound having the following formula. The content of sulfur in the sulfur-based additive is 36 wt%.

In the formula R₁₃ and R₁₄ independently denote the same meanings as R₇ and R₈ do, respectively.

(2-2) Sulfur-based additive 2

Sulfur-based additive 2 means an additives containing a sulfurized oil and fat type polysulfide compounds, and the content of sulfur in the sulfur-based additive 2 is 10.5 wt%.

(2-3) Sulfur-based additive 3:

Sulfur-based additive 3 means an additives containing a dibenzyl disulfide, and the content of sulfur in the sulfur-based additive 3 is 25.5 wt%.

(3-1) ZnDTP1

ZnDTP1 is a primary alkyl compound of the above formula (2) in which R₅ and R₆ are 2-ethylhexyl groups.

(3-2) ZnDTP2

ZnDTP2 means secondary alkyl compounds of the above formula (2) in which R₅ and R₆ are isopropyl groups or isohexyl groups or a mixture of these compounds each having the respective two above alkyl groups.

(4) Additive package

Additive package includes metallic detergent, ashless dispersant, phenolic anti-oxidant, amine-based anti-oxidant, viscosity index improver, anti-corrosion agent and antifoaming agent.
The above mentioned base oils and additives were selectively mixed at recipes shown in Table 3, thereby preparing long life and low fuel consumption engine oil compositions according to the present invention as Examples 1 through 5. In the same manner, base oils and additives were selectively mixed at recipes shown in Table 5, thereby preparing engine oil composition as Comparative Examples 1 through 8. In Tables 3 and 5, figures for the ingredients are compounding rates based on the unit "wt %" except that the foaming agent is based on the unit "wt ppm".
The Engine oil compositions thus prepared as Examples and Comparative Examples were evaluated with respect to the friction performance and wear characteristic in the valve train system according to the following methods.

(1) Friction performance

With respect to fresh lubricants and used ones, the coefficient of friction was measured under the following conditions by using an SRV tester. As test pieces, a ball made of SUJ-2 (bearing steel material, Japanese Industrial Standards), and having 10 mm in diameter and a disc made of SUJ-2 were used. Table 2

Test conditions

Break in conditions Actual test conditions

Load (N) 10 200

Amplitude (mm) 1.5 1.5

Frequency (Hz) 50 50

Temperature (°C) 40 80

Time (min) 10 30
The coefficient of friction is the average coefficient of friction determined in the friction test during the final 20 minutes.
The used oil compositions are oil compositions obtained when the oil was subjected to running in simulation with an actual car driving. The engine was operated under an AMA running mode at an oil temperature of 100°C and a water temperature of 100°C, and the engine oil composition was sampled after the lapse of 160 hours (corresponding to 4000 km) and 400 hours (corresponding to 10000 km). The thus obtained used oil compositions were subjected to the above friction test.

(2) Valve train system wearing test

Each engine oil composition was subjected to the valve train system wear test according to JASO (Japanese Automobile Standards organization) M328-91. Then, scuffing of a rocker arm was evaluated, and a worn amount of a cam nose was measured.

Evaluation results in Examples 1 through 5 are shown in Table 4, and those in Comparative Examples 1 through 8 are shown in Table 6. In Tables 4 and 6, scuffing of the rocker arm was evaluated by using a figure between 1 to 10.0, "1" and "10.0" being the lowest and the highest, respectively.

Table 3

	Example 1	Example 2	Example 3	Example 4	Example 5
Mineral oil 1	84.5	83.1	84.3	-	85.0
Mineral oil 2	-	-	-	84.5	-
MoDTC additive	2.0	2.0	2.0	2.0	2.0
Content of Mo in oil composition	0.08	0.08	0.08	0.08	0.08
Sulfur-based additive 1	0.6	-	-	0.6	0.6
Sulfur-based additive 2	-	2.0	-	-	-
Sulfur-based additive 3	-	-	0.8	-	-
Content of Sulfur in oil composition	0.22	0.21	0.20	0.22	0.22
ZnDTP 1	1.5	1.5	1.5	1.5	-
ZnDTP 2	-	-	-	-	1.0
Content of P in oil composition	0.095	0.095	0.095	0.095	0.090
Metallic detergent	2.0	2.0	2.0	2.0	2.0
Ash-based dispersant	4.0	4.0	4.0	4.0	4.0
Phenolic anti-oxidant	0.8	0.8	0.8	0.8	0.8
Amine-based anti-oxidant	0.4	0.4	0.4	0.4	0.4
Viscosity index improver	4.0	4.0	4.0	4.0	4.0
Corrosion inhibitor	0.2	0.2	0.2	0.2	0.2
Antifoaming agent (ppm)	5	5	5	5	5

Table 4

		Example 1	Example 2	Example 3	Example 4	Example 5
Dynamic viscosity (40°C)mm²/sec		53.5	54.5	52.5	51.4	54.3
Dynamic viscosity (100°C)mm²/sec		9.4	9.5	9.3	9.8	9.5
Viscosity index		160	159	161	180	160
Coefficient of friction	fresh oil composition	0.045	0.043	0.044	0.042	0.040
	used oil composition (160 hrs)	0.044	0.047	0.046	0.041	0.050
	used oil composition (400 hrs)	0.066	0.063	0.067	0.059	0.072
Wear of valve-moving system (rocker arm scuffing): Merit rating		9.0	8.6	8.7	8.6	9.2
Wear of cam nose µm		3	4	5	4	3

Examples 1 through 3 in Table 3 are engine oil compositions which all used Mineral Oil 1 and also employed a thiadiazole compound, a sulfurized oil and fat type compound and dibenzyl disulfide as the polysulfide compound, respectively. Example 4 is the same engine oil composition as in Example 1 except that Mineral Oil 1 was replaced by more highly purified Mineral Oil 2. In Example 5, a secondary alkyl type was used as ZnDTP.
In Table 5, Comparative Example 1 is an engine oil composition containing no polysulfide compound, and Comparative Example 2 is an engine oil composition containing much ZnDTP. Comparative Example 3 is an engine oil composition containing no ZnDTP, and Comparative Example 4 is the same engine oil composition as Comparative Example 1 except that the base oil was replaced by highly purified Mineral Oil 2. Comparative Example 5 is an engine oil composition containing no MoDTC, and Comparative Example 6 is an engine oil composition containing neither MoDTC nor polysulfide compound, and Comparative Example 7 is an engine oil composition containing neither ZnDTP nor polysulfide compound. Comparative Example 8 is an engine oil composition containing neither MoDTC nor ZnDTP.
Comparison between Examples and Comparative Examples in Table 4 and Table 6 reveals that particularly the coefficients of friction of the engine oil compositions in Examples are clearly smaller as compared with those in Comparative Examples after deterioration for 400 hours, though the former do not almost differ from the latter with respect to the fresh engine oil compositions, i.e., changes in the coefficient of friction of the engine oil compositions in Examples are smaller than those in Comparative Examples even after long-term use.
For example, Comparison between Example 1 and Comparative Example 1, between Example 2 and Comparative Example 2 and between Example 4 and Comparative Example 4 reveals that when the polysulfide compound was used in combination, the coefficient of friction particularly after the passage of 400 hours remarkably decreased. Comparison between Example 3 and Comparative Example 3 reveals that in Comparative Example 3, since no ZnDTP was used in combination, the coefficient of friction after the passage of 400 hours was not only high, but also the worn amount of the cam nose consipicuously increased. Comparison between Example 5 and Comparative Examples 5 and 6 reveals that in Comparative Examples 5 and 6, since no MoDTC was used in combination, the coefficient of friction was high from the beginning. In Comparative Example 7, since neither ZnDTP nor polysulfide compound were used in combination, the coefficient of friction with the passage of 400 hours was not only high, but also the worn amount of the cam nose conspicuously increased. In Comparative Example 8, since neither MoDTC nor ZnDTP were used in combination, the coefficient of friction was not only high from the beginning, but also the worn amount of the cam nose was extremely high.
The engine oil composition of the present invention is characterized in that MoDTC and ZnDTP are combined with the ashless organic polysulfide compound in the respectively specified addition amounts, and that a low coefficient of friction can be maintained in a long-term use even without addition of a large amount of particularly MoDTP or ZnDTP. Therefore, when the engine oil composition according to the present invention is charged into and used in the automobile, splendid effects can be exhibited with respect to fuel consumption saving and environmental maintenance.

Claims

An engine oil composition comprising:
(1) at least one oil selected from the group consisting of a mineral oil and a synthetic lubricant as a base oil;

(2) a molybdenum dithicarbamate (MoDTC) in an amount of 50 to 2000 ppm by weight when calculated as molybdenum (Mo), relative to the total weight of the engine oil composition;

(3) zinc dithiophosphate (ZnDTC) in an amount of 0.01 to 0.2 wt% when calculated as phosphorus (P), relative to the total amount of the engine oil composition; and

(4) an ashless organic polysulfide compound in an amount of 0.01 to 0.4 wt% when calculated as sulfur (S), relative to the total amount of the engine oil composition.
The engine oil composition claimed in claim 1, wherein said molybdenum dithiocarbamate (MoDTC) is a compound expressed by the following formula (1)
in which R₁ through R₄ independently denote a straight-chain or branched-chain alkyl group or a straight-chain or branched-chain alkenyl group having four to eighteen carbons; and X₁ through X₄ independently denote an oxygen atom or a sulfur atom, the ratio between the number of the oxygen atom or atoms and that of the sulfur atom or atoms with respect to X₁ through X₄ being 1/3 to 3/1.
The engine oil composition claimed in claim 2, wherein said R₁ through R₄ independently denote the alkyl group.
The engine oil composition claimed in claim 2, wherein said R₁ through R₄ independently denote butyl group, 2-ethylhexyl group, isotridecyl group or stearyl group.
The engine oil composition claimed in any one of claims 1 to 4, wherein said MoDTC is used in the addition amount of 300 to 1000 ppm by weight, when calculated as molybdenum (Mo), relative to the total weight of the engine oil composition.
The engine oil composition claimed in any one of claims 1 to 5, wherein said zinc dithiophosphate (ZnDTP) is a compound expressed by the formula (2):
in which R₅ and R₆ independently denote a straight-chain or branched chain alkyl group or a straight-chain or branched chain aryl group having three to eighteen carbon atoms.
The engine oil composition claimed in claim 6, wherein said R₅ and R₆ independently denote an alkyl group.
The engine oil composition claimed in claim 6, wherein said R₅ and R₆ independently denote a primary alkyl group.
The engine oil composition claimed in claim 6, wherein said R₅ and R₆ independently denote propyl group, butyl group, pentyl group, hexyl group, octyl group or lauryl group.
The engine oil composition claimed in claim 6, wherein said ZnDTP is added in an amount of 0.04 to 0.2 wt%.
The engine oil composition claimed in any one of claims 1 to 10, wherein said ashless organic polysulfide compound is an organic compound selected from the group consisting of organic compounds expressed by the following formulae.
R₇-Sx-R₈
OHC-R₉-S-S-R₁₀-CHO
R₇-C(O)O-R₉-S-S-R₁₀-OC(O)-R₈
R₇-OC(S)-S-S-C(S)O-R₈
in which R₇ and R₈ independently denote a straight-chain, branched-chain, alicyclic or aromatic hydrocarbon group in which a straight chain, a branched chain, an alicyclic unit and an aromatic unit may be selectively contained in any combined manner; R₉ and R₁₀ independently denote a straight-chain, branched-chain alicyclic or aromatic hydrocarbon group which has two bonding sites and in which a straight chain, a branched chain, an alicyclic unit and an aromatic unit may be selectively contained in any combined manner; R₁₁ and R₁₂ independently denote a straight-chain or branched-chain hydrocarbon group; and "x" and "y" denote independently an integer of two or more.
The engine oil composition claimed in claim 11, wherein said ashless organic polysulfide compound is selected from the group consisting of sulfurized sperm oil, sulfurized pinene oil, sulfurized soybean oil, sulfurized polyolefin, dialkyl disulfide, dialkyl polysulfide, dibenzyl disulfide, di-tertiary butyl disulfide, polyolefin polysulfide, thiadiazol type compound, and sulfurized phenol.
The engine oil composition claimed in claim 12, wherein said ashless organic polysulfide compound is the thiadiazol type compound.
The engine oil composition claimed in claim 12, wherein said ashless organic polysulfide compound is dialkyl polysulfide and bidenzyl disulfide are preferred.
The engine oil composition claimed in any one of claims 11 to 14, wherein said ashless organic polysulfide compound is added in an amount of 0.1-0.3 wt% when calculated as sulfur (S), relative to the total amount of the engine oil composition.
The engine oil composition claimed in any one of claims 1 to 15, wherein said molybdenum dithicarbamate (MoDTC) in an amount of 300 to 1000 ppm by weight when calculated as molybdenum (Mo), relative to the total weight of the engine oil composition; said zinc dithiophosphate in an amount of 0.04 to 0.2 wt% when calculated as phosphorus (P), relative to the total amount of the engine oil composition; and said ashless organic polysulfide compound in an amount of 0.1 to 0.3 wt% when calculated as sulfur (S), relative to the total amount of the engine oil composition.