WO2006005711A1

WO2006005711A1 - Lubricating oil composition

Info

Publication number: WO2006005711A1
Application number: PCT/EP2005/053238
Authority: WO
Inventors: Peter Sant; Mark Philip Wakem
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2004-07-08
Filing date: 2005-07-07
Publication date: 2006-01-19
Also published as: US20060009366A1; EP1765963A1; JP5767182B2; JP2012246501A; MY145889A; JP2008505994A; ZA200700089B; BRPI0513049A; CN1981020A

Abstract

A lubricating oil composition for internal combustion engines which composition is phosphorus-free and which composition comprises at least 60 % wt. of base oil, wherein said base oil is selected from Group I, Group II, Group III and Group IV base oils or mixtures thereof, and at least 1.4 % wt. of one or more antioxidants selected from the group of aminic antioxidants and/or phenolic antioxidants, based on the total weight of the lubricating oil composition.

Description

LUBRICATING OIL COMPOSITION

The present invention relates to a lubricating oil composition, in particular to a lubricating oil composition which is suitable for lubricating camless internal combustion engines. One of the most critical areas for a lubricating oil composition in an internal combustion engine is the sliding cam/follower contact in the valve train. A variety of methods for eliminating the cam mechanism have been proposed, with electrical and hydraulic actuation being two of the most popular.

The elimination of the cam actuation for the valves is not for the benefit of the lubricating oil composition, but in order to have much better control over the valve operation. These systems will allow independent control of opening, closing and maximum lift of the inlet and exhaust valves, in each individual cylinder of a multi-cylinder engine.

This opens the possibility for a wide rang-e of engine improvements; improved exhaust emissions, improved torque curve and lower fuel consumption, for example. Unusual operating modes are possible, such as cylinder cut-out and eight- or twelve-stroke cycle, which can lead to improved performance as well.

Examples of "camless" internal combustion engines are disclosed in U.S. Patents 5,255,641; 5,311,711;

5,367,990; 5,373,817; 5,377,631; 5,404,844; 5,419,301; 5,456,221; 5,456,222; 5,562,070; 5,572,961; 5,615,646; 5,619,965; 5,694,893; 5,709,178; 5,758,625; 5,970,956; and 6,024,060. The benefit of these systems for the lubricating oil composition is that no special anti-wear additive is required for valve train protection. Hence, organometallic phosphorus antiwear additives such as zinc dithiophosphate (ZnDTP) are not required. Simply making this change in isolation not only leads to a reduction in treat cost but also gives benefits in other areas.

For example, ZnDTP contributes to the sulphated ash content, sulphur content and phosphorus content in a lubricating oil composition. In view of the adverse affects that the sulphated ash, sulphur and phosphorus concentrations of lubricating oil compositions may have on vehicle exhaust after-treatment device-s, it is therefore prudent to develop lubricating oil compositions with reduced sulphated ash, sulphur and/or phosphorus concentrations therein. Thus, reducing or avoiding the use of ZnDTP in lubricating oil compositions is a method to reduce the sulphur and phosphorus content therein.

In addition, the anti-wear films developed by ZnDTPs may cause increased friction, leading to extra power loss.

Accordingly, WO-A-02/24843 describes a method of operating a camless internal combustion engine which comprises: (A) operating said engine using a normally liquid or gaseous fuel composition; and

(B) lubricating said engine using a low-phosphorus or phosphorus-free lubricating oil composition, said low- phosphorus or phosphorus-free lubricating oil composition optionally containing an extreme-pressure additive comprised of metal and phosphorus, provided the amount of phosphorus contributed to said low-phosphorus or phosphorus-free lubricating oil composition by said extreme-pressure additive does not exceed about 0.08% by weight based on the weight of said low-phosphorus or phosphorus-free lubricating oil composition.

However, the removal of ZnDTP from lubricating oil compositions can lead to increased cylinder bore wear. EP-A-1338643 discloses reduced phosphorus lubricating oil compositions for use as passenger car engine lubricants.

Said compositions comprise a major amount of Group II-IV and ester base oils; an overbased calcium or magnesium salicylate detergent; an oil-soluble organo- molybdenum compound; an ashless dispersant; and a supplemental antioxidant.

Said supplemental antioxidant is said to reduce the tendency of base oils to deteriorate in service which deterioration can be evidenced by the products of oxidation such as sludge and varnish-like deposits on metal surfaces and by viscosity growth.

Whilst said supplemental antioxidant is said to be present in an amount of from 0.1 to 5.0 wt%, said supplemental antioxidant is preferably present in an amount of 0.25 to 1.0 wt%. In this regard, the Example in EP-A-1338643 employs supplemental antioxidant in an amount of 0.50 %wt.

US-A-5439605 describes various low ash and light ash formulations for use as motor oils.

The base oils present in said formulations may optionally contain antioxidant in an amount from about 0.5 % to about 1.0 %.

In some embodiments, said formulations are indicated to not contain phosphorus.

However, neither EP-A-1338643 nor US-A-5439605 are concerned with the development of phosphorus-free lubricating oils compositions for camless engines which -A-

have advantageous anti-wear properties, in particular giving reduced cylinder bore wear.

It is therefore desirable to develop phosphorus-free lubricating oil compositions for camless engines which have beneficial anti-wear properties.

There have now been surprisingly found in the present invention, lubricating oil compositions suitable for camless engines which exhibit advantageous anti-wear properties, in particular giving reduced cylinder bore wear.

Accordingly, the present invention provides a lubricating oil composition for internal combustion engines which composition is phosphorus-free and which composition comprises at least 60% wt. of base oil, wherein said base oil is selected from Group I, Group II, Group III and Group IV base oils or mixtures thereof, and at least 1.4 % wt. of one or more antioxidants selected from the group of aminic antioxidants and/or phenolic antioxidants, based on the total weight of the lubricating oil composition.

By "phosphorus-free" in the present invention, is meant that the lubricating oil composition does not comprise any phosphorus-containing compounds therein.

In a preferred embodiment of the present invention, said one or more antioxidants are present in an amount of at least 1.6 % wt., more preferably in an amount of at least 1.7 % wt., based on the total weight of the lubricating oil composition.

The lubricating oil composition of the present invention may comprise one or more aminic antioxidants.

Examples of aminic antioxidants which may be conveniently used include alkylated diphenylamines, phenyl-α-naphthylamines, phenyl-β-naphthylamines and alkylated α-naphthylamines. ~" O —

Preferred aminic antioxidants include dialkyldiphenylamines such as p,p'-dioctyl-diphenylamine, p,p'-di-α-methylbenzyl-diphenylamine and N-p-butylphenyl- N-p'-octylphenylamine, monoalkyldiphenylamines such as monq-t-butyldiphenylamine and mono-octyldiphenylamine, bis (dialkylphenyl) amines such as di-(2,4- diethylphenyl) amine and di (2-ethyl-4-nonylphenyl) amine, alkylphenyl-1-naphthylamines such as octylphenyl-1- naphthylamine and n-t-dodecylphenyl-1-naphthylamine, 1- naphthylamine, arylnaphthylamines such as phenyl-1- naphthylamine, phenyl-2-naphthylamine, N-hexylphenyl-2- naphthylamine and N-octylphenyl-2-naphthylamine, phenylenediamines such as N,N'-diisopropyl-p- phenylenediamine and N,N'-diphenyl-p-phenylenediamine, and phenothiazines such as phenσthiazine and 3,7- dioctylphenothiazine.

Preferred aminic antioxidants include those available under the following trade designations: "Sonoflex OD-3" (ex. Seiko Kagaku Co.), "Irganox L-57" (ex. Ciba Specialty Chemicals Co.) and phenothiazine (ex. Hodogaya Kagaku Co.) .

The lubricating oil composition of the present invention may comprise one or more phenolic antioxidants. Examples of phenolic antioxidants which may be conveniently used include C7-C9 branched alkyl esters of 3,5-bis (1, 1-dimethyl-ethyl) -4-hydroxy-benzenepropanoic acid, 2-t-butylphenol, 2-t-butyl-4-methylphenol, 2-t- butyl-5-methylphenol, 2,4-di-t-butylphenol, 2, 4-dimethyl- β-t-butylphenol, 2-t-butyl-4-methoxyphenol, 3-t-butyl-4- methoxyphenol, 2, 5-di-t-butylhydroquinone, 2, 6-di-t-butyl- 4-alkylphenols such as 2, 6-di-t-butylphenol, 2,6-di-t- butyl-4-methylphenol and 2, 6-di-t-butyl-4-ethylphenol, 2, 6-di-t-butyl-4-alkoxyphenols such as 2, 6-di-t-butyl-4- methoxyphenol and 2, 6-di-t-butyl-4-ethoxyphenol, 3_f5-di-t- butyl-4-hydroxybenzylmercaptooctylacetate, alkyl-3- (3,5- di-t-butyl-4-hydroxyphenyl)propionates such as n- octadecyl-3- (3, 5-di-t-butyl-4-hydroxyphenyl)propionate, n- butyl-3- (3, 5-di-t-butyl-4-hydroxyphenyl)propionate and 2'- ethylhexyl-3- (3,5-di-t-butyl-4-hydroxyphenyl)propionate, 2, 6-d-t-butyl-α-dimethylamino-p-cresol, 2,2'-methylene- bis (4-alkyl-6-t-butylphenol) such as 2,2 '-methylenebis (4- methyl-6-t-butylphenol, and 2,2-methylenebis (4-ethyl-6-t- butylphenol) , bisphenols such as 4, 4 '-butylidenebis (3- methyl-β-t-butylphenol, 4, 4 '-methylenebis (2, 6-di-t- butylphenol) , 4,4 '-bis (2, β-di-t-butylphenol) , 2,2-(di-p- hydroxyphenyl)propane, 2,2-bis (3, 5-di-t-butyl-4- hydroxyphenyl)propane, 4, 4 '-cyclohexylidenebis (2, 6-t- butylphenol) , hexamethyleneglycol-bis[3- (3,5-di-t-butyl-4- hydroxyphenyl) propionate] , triethyleneglycolbis [3- (3-t- butyl-4-hydroxy-5-methylphenyl)propionate] , 2,2'-thio- [diethyl-3- (3, 5-di-t-butyl-4-hydroxyphenyl)propionate] , 3, 9-bis{1, l-dimethyl-2- [3- (3-t-butyl-4-hydroxy-5-methyl- phenyl)propionyloxy] ethyl}2, 4,8, 10- tetraoxaspiro[5,5]undecane, 4,4 '-thiobis (3-methyl-6-t- butylphenol) and 2,2 '-thiobis (4, 6-di-t-butylresorcinol) , polyphenols such as tetrakis [methylene-3- (3,5-di-t-butyl- 4-hydroxyphenyl)propionate]methane, 1, 1, 3-tris (2-methyl-4- hydroxy-5-t-butylphenyl)butane, 1,3, 5-trimethyl-2, 4, 6- tris (3,5-di-t-butyl-4-hydroxybenzyl)benzene, bis- [3,3'- bis (4 '-hydroxy-3'-t-butylphenyl)butyric acid] glycol ester, 2- (3' ,5'-di-t-butyl-4-hydroxyphenyl)methyl-4- (2",4"-di-t- butyl-3"-hydroxyphenyl)methyl-6-t-butylphenol and 2,6- bis (2 '-hydroxy-3 '-t-butyl-5 '-methylbenzyl) -4-methylphenol, and p-t-butylphenol - formaldehyde condensates and p-t- butylphenol - acetaldehyde condensates.

Preferred phenolic antioxidants include those available under the following trade designations: "Irganox L-135" (ex. Ciba Specialty Chemicals Co.), "Anteeji DBH" (ex. Kawaguchi Kagaku Co.,), "Yoshinox SS" (ex. Yoshitomi Seiyaku Co.), "Antage W-400" (ex. Kawaguchi Kagaku Co.), "Antage W-500" (ex. Kawaguchi Kagaku Co.), "Antage W-300" (ex. Kawaguchi Kagaku Co.), "Ionox 220AH" (ex. Shell Japan Co.), bisphenol A, produced by the Shell Japan Co., "Irganox L109" (ex. Ciba Speciality Chemicals Co.), "Tominox 917" (ex. Yoshitomi Seiyaku Co.), "Irganox L115" (ex. Ciba Speciality Chemicals Co.), "Sumilizer GA80" (ex. Sumitomo Kagaku), "Antage RC" (ex. Kawaguchi Kagaku Co.), "Irganox LlOl" (ex. Ciba Speciality Chemicals Co.), "Yoshinox 930" (ex. Yoshitomi Seiyaku Co.), "Ionox 330" (ex. Shell Japan Co.) .

The amount of base oil incorporated in the lubricating oil composition of the present invention is preferably present in an amount of at least 60 % wt., more preferably in an amount in the range of from 60 to 98 % wt., most preferably in an amount in the range of from 75 to 90 % wt., with respect to the total weight of the lubricating oil composition. By "Group I" base oil, "Group II" base oil, "Group III" base oil and "Group IV" base oil in the present invention are meant base oils according to the definitions of American Petroleum Institute (API) categories I, II, III and IV. Such API categories are defined in API Publication 1509, 15^th Edition, Appendix E, April 2002.

Group I base oils contain less than 90 % saturates (according to ASTM D2007) and/or greater than 0.03 % sulphur (according to ASTM D2622, D4294, D4927 or D3120) and have a viscosity index of greater than or equal to 80 and less than 120 (according to ASTM D2270) .

Group II base oils contain greater than or equal to 90 % saturates and less than or equal to 0.03 % sulphur and have a viscosity index of greater than or equal to 80 and less than 120, according to the aforementioned ASTM methods.

Group III base oils contain greater than or equal to 90 % saturates and less than or equal to 0.03 % sulphur and have a viscosity index of greater than 120, according to the afore-mentioned ASTM methods.

Group IV base oils are polyalphaolefins (PAO) . There are no particular limitations regarding the Group I to IV base oils used in the present invention, and various conventional known Group I to IV base oils selected from mineral oils and synthetic lubricating oils may be conveniently used.

Mineral oils include liquid petroleum oils and solvent-treated or acid-treated mineral lubricating oil of the paraffinic, naphthenic, or mixed paraffinic/naphthenic type which may be further refined by hydrofinishing processes and/or dewaxing.

Naphthenic base oils have low viscosity index (VI) (generally 40-80) and a low pour point. Such base oils are produced from feedstocks rich in naphthenes and low in wax content and are used mainly for lubricants in which colour and colour stability are important, and VI and oxidation stability are of secondary importance.

Paraffinic base oils have higher VI (generally >95) and a high pour point. Said base oils are produced from feedstocks rich in paraffins, and are used for lubricants in which VI and oxidation stability are important.

Fischer-Tropsch derived base oils may be conveniently used as the base oil in the lubricating oil composition of the present invention, for example, the Fischer-Tropsch derived base oils disclosed in EP-A- 776959, EP-A-668342, WO-A-97/21788, WO-00/15736, WO- 00/14188, WO-00/14187, WO-00/14183, WO-00/14179, WO- 00/08115, WO-99/41332, EP-1029029, WO-01/18156 and WO- 01/57166.

Synthetic processes enable molecules to be built from simpler substances or to have their structures modified to give the precise properties required.

Synthetic lubricating oils include hydrocarbon oils such as olefin oligomers (PAOs) (Group IV base oils) and dewaxed waxy raffinate.

Synthetic Group III hydrocarbon base oils sold by the Royal Dutch/Shell Group of Companies under the designation "XHVI" (trade mark) may be conveniently used.

Preferably, the base oil used in the present invention is constituted from mineral oils and/or synthetic base oils which contain more than 80% wt of saturates, preferably more than 90 % wt., as measured according to ASTM D2007.

It is further preferred that the base oil used in the present invention contains less than 1.0 % wt., preferably less than 0.1 % wt. of sulphur, calculated as elemental sulphur and measured according to ASTM D2622, ASTM D4294, ASTM D4927 or ASTM D3120.

Preferably, the viscosity index of base oil used in the present invention is more than 80, more preferably more than 120, as measured according to ASTM D2270. Preferably, the base oil used in the present invention has a kinematic viscosity in the range of from

2 to 80 mm²/s at 100 ⁰C, more preferably of from 3 to 70 mm²/s, most preferably of from 4 to 50 mm²/s.

The lubricating oil composition of the present invention preferably has a sulphated ash content of not greater than 1.3 wt. %, more preferably not greater than 1.1 wt. % and most preferably not greater than 1.0 wt. %, based on the total weight of the lubricating oil composition. The lubricating oil composition of the present invention preferably has a sulphur content of not greater than 1.2 wt. %, more preferably not greater than 0.8 wt. % and most preferably not greater than 0.05 wt. %, based on the total weight of the lubricating oil composition. The lubricating oil composition of the present invention may further comprise additional additives such as anti-wear additives, detergents, dispersants, friction modifiers, viscosity index improvers, pour point depressants, corrosion inhibitors, defoaming agents and seal fix or seal compatibility agents, provided that said additive components do not contain any phosphorus therein.

Suitable phosphorus-free anti-wear additives include boron-containing compounds such as borate esters, borated fatty amines, borated epoxides, alkali metal (or mixed alkali or alkaline earth metal) borates and borated overbased metal salts.

Said boron-containing anti-wear additives may be conveniently added to the lubricating oil composition of the present invention in an amount in the range of from 0.1 to 3.0 wt. %, based on the total weight of lubricating oil composition.

Typical detergents that may be used in the lubricating oil of the present invention include one or more salicylate and/or phenate and/or sulphonate detergents .

However, as metal organic and inorganic base salts which are used as detergents can contribute to the sulphated ash content of a lubricating composition, in a preferred embodiment of the present invention, the amounts of such additives are minimised.

The one or more salicylate and/or phenate and/or sulphonate detergents may be conveniently added in an amount in the range of from 0.01 to 20.0 wt. %, more preferably from 0.10 to 10.0 wt. %, based on the total weight of lubricating oil composition.

In order to maintain a low sulphur level, salicylate detergents are preferred. Thus, in a preferred embodiment, the lubricating oil composition of the present invention may comprise one or more salicylate detergents.

It is preferred that the one or more salicylate and/or phenate and/or sulphonate detergents, independently, have a TBN (total base number) in the range of from 10 to 500 mg.KOH/g, more preferably in the range of from 30 to 350 mg.KOH/g and most preferably in the range of from 50 to 300 mg.KOH/g, as measured by ASTM D2894. The lubricating oil compositions of the present invention may additionally contain an ash-free dispersant which is preferably admixed in an amount in the range of from 5 to 15 % wt., based on the total weight of the lubricating oil composition. Typical dispersants that may be conveniently employed in the lubricating oil composition of the present invention, include ash-free alkenyl- or alkyl- succinimides and polyalkenyl succininic acid esters or derivatives thereof. Said ash-free dispersants may be borated. The dispersants may have a high molecular weight (for example, of greater than 2000) or a low molecular weight (for example, of less than 2000, preferably less than 1200) .

Dispersants that may be conveniently employed in the lubricating oil composition of the present invention include those described in EP-A-1167497 and Japanese Patent Nos. 1367796, 1667140, 1302811 and 1743435. Preferred friction modifiers that may be conveniently used include fatty acid amides, more preferably unsaturated fatty acid amides.

The total amount of unsaturated fatty acid amide compound added is preferably from 0.05 to 0.35 % wt., based on the total weight of the lubricating oil composition.

Examples of viscosity index improvers which may conveniently used in the lubricating oil composition of the present invention include the styrene-butadiene copolymers, styrene-isoprene stellate copolymers and the polymethacrylate-based and ethylene-propylene copolymers and the like disclosed in Japanese Patent Nos . 954077, 1031507, 1468752, 1764494 and 1751082. Such viscosity index improvers may be conveniently employed in an amount in the range of from 1 to 20 % wt., based on the total weight of the lubricating oil composition. Similarly, dispersing-type viscosity index improvers comprising copolymerized polar monomer containing nitrogen atoms and oxygen atoms in the molecule may also be used therein. Polymethacrylates such as those as disclosed in Japanese Patent Nos. 1195542 and 1264056 may be conveniently employed in the lubricating oil compositions of the present invention as effective pour point depressants. Furthermore, compounds such as alkenyl succinic acid or ester moieties thereof, benzotriazole-based compounds and thiodiazole-based compounds may be conveniently used in the lubricating oil composition of the present invention as corrosion inhibitors. Compounds such as dimethyl polycyclohexane, polyacrylate may be conveniently used in the lubricating oil composition of the present invention as defoaming agents. Compounds which may be conveniently used in the lubricating oil composition of the present invention as seal fix or seal compatibility agents include, for example, commercially available aromatic esters. The lubricating oil compositions of the present invention may be conveniently prepared by admixing the one or more antioxidants selected from the group of aminic antioxidants and/or phenolic antioxidants, and, optionally, one or more further additives that are usually present in lubricating oils, for example as herein before described, with a mineral and/or synthetic base oil.

Lubricating oil compositions of the present invention display reduced wear, in particular reduced cylinder bore wear. Accordingly, in a further embodiment of the present invention, there is provided the use of a lubricating oil composition as hereinbefore described to reduce wear, preferably cylinder bore wear, in an internal combustion engine, in particular in a camless internal combustion engine.

In another embodiment of the present invention, there is provided a method ^'of lubricating an internal combustion engine, in particular a camless internal combustion engine comprising applying a lubricating oil composition as hereinbefore described thereto.

The present invention is described below with reference to the following Examples, which are not intended to limit the scope of the invention in any way.

EXAMPLES Formulations

Tables 1 and 2 indicate the formulations that were tested. Standard conventional detergents, dispersants, pour point depressants and viscosity modifiers were used therein.

The arainic antioxidant used was that available under the trade designation "Irganox L-57" ex. Ciba Specialty Chemicals Co. (p,p'-dioctyl-diphenylamine) .

The phenolic antioxidant used was that available under the trade designation "Irganox L-135" ex. Ciba Speciality Chemicals Co. (octyl 3- (3,5-di-t-butyl-4- hydroxylphenyl)propionate) .

Apart from the formulations of Examples 2 and 3, all of the formulations described in Tables 1 and 2 were nominally SAE 10W40 viscosity grade oils, as they were minor formulations variants of the baseline oil. The formulations tested in Examples 2 and 3 were SAE 5W30 viscosity grade oils.

Table 3 indicates the physical characteristics of the formulations that were tested.

TABLE 1

Hydrogenated styrene-isoprene viscosity index modifier (6%) and pour point depressant (1.9%) in diluent oil. Group I base oil. Group II base oil. Group III base oil

TABLE 2

Additive Example 3 Example 4

Antifoam 793 ppm 793 ppm

Magnesium/Calcium Detergents (% Wt.) 2.20 2.20

Ashless Dispersant⁽¹⁾ (% Wt.) 8.00 8.00

Phenolic Antioxidant (% Wt.) 4.00 4.0

Viscosity Index Modifier 2.90 - concentrate (B) ^<2> ⁽% Wt.)

Viscosity Index Modifier 2.90 — concentrate (C) ^<3) (% Wt.)

Dispersant-Viscosity Index Modifier 2.00 2.00

(% Wt.)

Supplementary Additive Package⁽⁴⁾ 1.00 1.00

(% Wt.)

Base Oil

PAO ⁽⁵⁾ 5.00 5.00

XHVI - 8.0 ⁽⁶⁾ {% Wt.) - 50.00

XHVI - 5.2 ⁽⁷⁾ (% Wt.) 67.00 27.80

XHVI - 4.0 ⁽⁸⁾ (% Wt.) 5.00 -

TOTAL 100

PIB-MALA polyamine dispersant.

Hydrogenated styrene-isoprene viscosity index modifier (6%) in diluent oil.

Hydrogenated polyisoprene viscosity index modifier (15%) in diluent oil.

Supplementary additive package containing a mixture of conventional additives:- friction modifier, corrosion inhibitor and seal fix agent.

5 Group IV base oil. 6, 7 , 8 Group III base oils. TABLE 3

* Kinematic Viscosity

** Low temperature Cranking Viscosity

Screening rig test

In order to test the lubricating oil compositions described in Tables 1 to 3, screening rig tests were used to simulate a camless engine environment. In said screening rigs, the oil system for the valve train was separated form the rest of the internal combustion engine, thereby allowing the lower part of the internal combustion engine to be lubricated with a "camless" type lubricant as it did not come into contact with the cam-follower area.

A screening rig test was developed by Shell Global Solutions (UK) using a Renault Megane 1.6 litre gasoline engine. The test cycle mimicked short distance stop-start driving, in a similar manner to the ASTM Sequence V test. The Renault Megane 1.6 litre gasoline engine was modified to separate the oil supply and return to the overhead camshaft valve train in the cylinder head from their normal routes in and out of the cylinder block. The cylinder head was lubricated by a separate external electric pump in a circuit with reservoir and temperature control. A conventional fully-formulated lubricant was used in this circuit.

The test oil was used in the remainder of the ^■ original engine circuit below the head. The modifications were confined to the cylinder head, so that it could be re-used. It was possible to use the head for several test runs before it required overhauling. A new short motor was used for each test, which was stripped and measured beforehand, and the piston ring gaps were increased to give higher blow-by flow rate for increased severity.

The standard test duration was 288 hours (12 days), running on a four hour cycle. The engine was stopped once a day for oil sampling and levelling/top-up. At the end of test, the engine was dismantled and components measured for wear calculations.

The engine used for the second series of tests was a Ford Zetec SE 1.7 litre DOHC, as used in the Puma Coupe. It was modified in a similar manner to the Megane engine in order to simulate camless operation. The rest of the test installation and method was the same as for the Megane test, except that all the tests were run to a duration of 576 hours (24 days) .

The screening rig testing is a publicly available commercial testing service that is available ex. Shell Global Solutions (UK), Cheshire Innovation Park, P.O. Box 1, Chester CHl 3SH, UK. (Email: [email protected]) .

Results and Discussion

Some of the formulations described in Tables 1 and 2 were tested using the afore-mentioned screening rig tests and the results obtained thereon are included in the following tables. (i) Screening rig using Renault Megane 1.6 litre gasoline engine

A standard test duration of 288 hours was used (i.e. the same duration as the ASTM Sequence VE test upon which the screening rig test was based) for the results in Table 4.

TABLE 4

Average of two test runs It is evident from the above results for Comparative Example 2 that when ZnDTP is removed from the baseline lubricating oil composition of Comparative Example 1, there is an increase in the depth of the wear step in the cylinder bore at top ring reversal (TRR) .

However, it is apparent from Example 1 that the addition of a greatly increased amount of antioxidant not only reduces the bore wear step depth to levels below those observed in Comparative Example 2, but said increased amount of antioxidant also surprisingly reduces the thrust side bore wear step depth to levels below that observed in the ZnDTP-containing formulation of Comparative Example 1.

In addition, the formulations of Examples 1 and 2 were also tested under an extended test duration of 576 hours. The results for the extended testing are given in Table 5 below.

TABLE 5

* 576 hours test duration ** 288 hours test duration

It is further evident from the above results that even when the formulation of Example 1 was tested for an extended period, the bore wear depth was still much lower than for the formulation of Comparative Example 2 which has much lower amounts of antioxidant therein. (ϋ) Screening rig using Ford Zetec SE 1.7 litre DOHC engine

As stated above, the test operational details for this rig were the same as those used with the Megane engine, except that all tests were run to the double duration (576 hours) .

The wear step measurement technique was slightly different, in that the total cross-sectional area of the material removed in the wear step was measured, rather than the maximum depth of the step.

Also the information is for the anti-thrust side, rather than the thrust. .

TABLE 6

As demonstrated above in (i) , the formulation of Example 2 surprisingly exhibits advantageous anti-wear properties. Table 6 indicates the total cross-sectional area of material removed in the wear step of a screening rig using Ford Zetec SE 1.7 litre DOHC engine using the formulation of Example 2.

It is apparent from Table 6 that the formulations of Example 3 and 4 (which comprised phenolic antioxidant rather than aminic antioxidant) had even lower bore wear than the formulation of Example 2. In addition, by comparing the results in Table 6 for Examples 3 and 4, it is apparent that the higher viscosity grade formulation (i.e. the formulation of Example 4) gives lower wear.

Claims

C L A I M S

1. A lubricating oil composition for internal combustion engines which composition is phosphorus-free and which composition comprises at least 60 % wt. of base oil, wherein said base oil is selected from Group I, Group II, Group III and Group IV base oils or mixtures thereof, and at least 1.4 % wt. of one or more antioxidants selected from the group of aminic antioxidants and/or phenolic antioxidants, based on the total weight of the lubricating oil composition.

2. Lubricating oil composition according to Claim 1, wherein said one or more antioxidants are present in an amount of at least 1.6 % wt., based on the total weight of the lubricating oil composition.

3. Lubricating oil composition according to Claim 1 or 2, wherein said one or more antioxidants are aminic antioxidants.

4. Lubricating oil composition according to Claim 3, wherein said aminic antioxidants are selected from alkylated diphenylamines, phenyl-α-naphthylamines, phenyl- β-naphthylamines and alkylated α-naphthylamines.

5. Lubricating oil composition according to Claim 1 or

2, wherein said one or more antioxidants are phenolic antioxidants.

6. Lubricating oil composition according to Claim 5, wherein said phenolic antioxidants are selected from 2, 6- di-t-butylphenol, C7-C9 branched alkyl esters of 3,5- bis (1, 1-dimethyl-ethyl) -4-hydroxy-benzenepropanoic acid, and 4, 4 ^r-methylenebis (2, 6-di-t-butylphenol) .

7. Method of lubricating an internal combustion engine comprising applying a lubricating oil composition according to any one of Claims 1 to 6 thereto.

8. Method according to Claim 1, wherein the internal combustion engine is a camless internal combustion engine.

9. Use of a lubricating oil composition according to any one of Claims 1 to 6 to reduce wear in an internal combustion engine.

10. Use according to Claim 9, wherein the internal combustion engine is a camless internal combustion engine,