The present invention relates to a lubricating composition, in particular for use as a gas engine oil.

In power generation, gas engines operate continuously near full load conditions, shutting down only for maintenance and/or change of lubricant. As a result, the lubricant in use is exposed to a sustained high temperature environment. This high temperature environment may cause relatively severe lubricant oxidation and nitration processes, which lead to base number depletion, increased viscosity and reduced engine cleanliness with a consequential increase in fuel consumption.

Moreover, gas engines fuelled with non-natural gas (such as sour gas) present additional challenges for the lubricant to neutralise acidic combustion products in the lubricant or other acidic compounds already present in the fuel.

In commercial available low ash gas engine oil products typically sulphated ash values of about 0.5 wt.% and TBN (Total Base Number) values of at maximum about 7 mg KOH/g are used. Examples of such commercial available products are Mobil Pegasus 605 and Mobil Pegasus 1005, which are available from Exxon Mobil Corporation.

According to the Technical Data Sheets thereof, Mobil Pegasus 605 (version 01-2006) contains a sulphated ash content (according to ASTM D 874) of 0.52 and a TBN value (according to ASTM D 2896) of 4.9, and Mobil Pegasus 1005 contains a sulphated ash content of 0.5 and a TBN value of 5. It is an object of the present invention to improve the oxidation and nitration stability and base number retention of lubricating compositions, especially for use in gas engine oils.

It is another object of the present invention to provide alternative lubricating compositions for use in a gas engine.

One or more of the above or other objects can be obtained by the present invention by providing a lubricating composition comprising a base oil and one or more additives, wherein the composition has: - a sulphated ash content (according to ASTM D 874) of at least 0.1 wt.% and at most 0.60 wt.%, preferably at most 0.55 wt.%, more preferably at most 0.50 wt.%; and a total base number (TBN) value (according to ASTM D 2896) of at least 8.0 mg KOH/g and at most 15.0 mg KOH/g. In this respect it is noted that although

WO 2007/117776 discloses a lubricating composition containing less than 1.0 wt.% sulphated ash and a total base number of at least about 8.5 mg KOH/g, (and although paragraph [0032] of WO 2007/117776 indicates that the total ash content in certain embodiments may be 0.1-0.7 wt.%) in the Examples of WO 2007/117776 no actual suggestion or teaching has been made of formulations containing at most 0.60 wt.% sulphated ash and at the same time a TBN value of at least 8.0 mg KOH/g. Also, as can be learned from paragraph [0003] thereof, WO 2007/117776 suggests to add certain selected nitrogen-containing dispersants to boost the TBN value. However, it has been found that this results in a weak TBN retention when compared to the use of detergents. On the website of http: /atlantis- usa. coπi/indexl/index. htm a disclosure was made of an ashless gas engine oil having a sulphated ash content of 0.02 wt.% and a Total Base Number value of 13. However, there is no proof that this information was available before 12.08.2009, i.e. after the priority date of this application.

It has now surprisingly been found according to the present invention that the lubricating compositions according to the present invention may exhibit improved oxidation stability, base number retention and oxidation nitration stability. This results in longer ODIs (oil drain intervals), which is highly desirable in view of less downtime and lower maintenance costs of the gas engine. Also, the lubricating compositions according to the present invention may exhibit desirable seal compatibility properties.

The sulphated ash content of the lubricating composition according to the present invention is above 0.1 wt.%. However, preferably the sulphated ash content is above 0.3 wt.%, more preferably above 0.4 wt.%, even more preferably above 0.45 wt.%.

According to a preferred embodiment of the invention, the base number value is at least 8.5 mg KOH/g, preferably at least 9.0 mg KOH/g. Typically, the base number is below 15.0 mg KOH/g, preferably below

12.0 mg KOH/g, preferably below 10.0 mg KOH/g.

Further it is preferred that the composition has a calcium content (according to ASTM D 4951) of at most 0.15 wt.%. Typically the calcium content is above

0.05 wt.%, more preferably above 0.08 wt.%, even more preferably above 0.10 wt.%.

According to an especially preferred embodiment according to the present invention, the lubricating composition comprises a detergent being the reaction product of an acidic organic compound, a boron compound, and a basic organic compound. In this respect specific reference is made to US 2005/0172543 and in particular paragraphs [0025] - [0077] thereof, the teaching of which is hereby incorporated by reference. Preferably, the above detergent has a molecular weight of below 650. Preferably, the above detergent is present in an amount of from 0.5 to 4.0 wt.%, preferably from 1.0 to 3.0 wt.% based on the weight of the total lubricating composition. It is even more preferred that the detergent has the general formula (I) below

wherein a is l or Z; and wherein K" and K" are independently selected hydrocarbyl groups, provided that, where a is 2, the groups R ¹ and R ² are independently selected.

The above hydrocarbyl group, which is preferably an alkyl group, may contain from 1 to 50 carbon atoms, preferably from. 12 to 30 carbon atoms, most preferably 14 to 18 carbon atoms. Further it is preferred according to the present invention that the lubricating composition has a P- content (according to DIN 51363 T2) of at most 0.04 wt.%. Typically the P~content is above 0.01 wt.%.

Suitably, the lubricating composition according to the present invention has a boron content of above 100 ppm (according to ASTM D 4951) , preferably above 200 ppm; furthermore, the boron content is typically below 600 ppm, more preferably below 500 ppm.

Also it is preferred that at least 50% of the TBN of the composition is provided by non-metal containing additives (preferably detergents) , preferably at least 55%, more preferably at least 60%, even more preferably at least 65%, most preferably at least 70%. According to the present invention, the contribution of TBN by a non- metal containing additive is calculated from the TBN measured for the single additive concentrate, multiplied by the percentage added to the lubricant composition and divided by the the total TBN of the finished lubricant composition. The at least one non-metal containing additive may also be referred to as an ashless additive, since it will typically not produce any sulfated ash when subjected to the conditions of ASTM D 874. An additive is referred to as "non-metal containing" if it does not contribute metal content to the lubricant composition. It is recognized, of course, that a non-metal containing additive will normally eventually be mixed with other material in a lubricating composition and certain of the other materials may be metal containing. If this happens, some of the metal ions from the other material may become associated with the non-metal containing material. However, such in-situ association is not intended to negate the identity of the additive in question as a non- metal containing additive. Thus, the additive is, more strictly speaking, non-metal containing prior to mixing with other components. As indicated above, in a preferred embodiment, at least 50% of the TBN of the composition is provided by non-metal containing detergents.

There are no particular limitations regarding the base oil used in lubricating composition according to the present invention, and various conventional mineral oils, synthetic oils as well as naturally derived esters such as vegetable oils may be conveniently used.

The base oil used in the present invention may conveniently comprise mixtures of one or more mineral oils and/or one or more synthetic oils; thus, according to the present invention, the term "base oil" may refer to a mixture containing more than one base oil. 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.

Suitable base oils for use in the lubricating oil composition of the present invention are Group I-III mineral base oils, Group IV poly-alpha olefins (PAOs) , Group I-III Fischer-Tropsch derived base oils and mixtures thereof.

By "Group I", "Group II", "Group III" and "Group IV" base oils in the present invention are meant lubricating oil base oils according to the definitions of American Petroleum Institute (API) for category III and IV. These API categories are defined in API Publication 1509, 15th Edition, Appendix E, April 2002. Fischer-Tropsch derived base oils are known in the art. By the term "Fischer-Tropsch derived" is meant that a base oil is, or is derived from, a synthesis product of a Fischer-Tropsch process. A Fischer-Tropsch derived base oil may also be referred to as a GTL (Gas-To-Liquids) base oil. Suitable Fischer-Tropsch derived base oils that may be conveniently used as the base oil in the lubricating composition of the present invention are those as for example disclosed in EP 0 776 959, EP 0 G68 342, WO 97/21788, WO 00/15736, WO 00/14188, WO 00/14187, WO 00/14183, WO 00/14179, WO 00/08115,

WO 99/41332, EP 1 029 029, WO 01/18156 and WO 01/57166.

Synthetic oils include hydrocarbon oils such as olefin oligomers (including polyalphaolefin base oils; PAOs) , dibasic acid esters, polyol esters, polyalkylene glycols (PAGs) , alkyl naphthalenes and dewaxed waxy isomerates. Synthetic hydrocarbon base oils sold by the Shell Group under the designation "Shell XHVI" (trade mark) may be conveniently used.

Poly-alpha olefin base oils (PAOs) and their manufacture are well known in the art. Preferred poly- alpha olefin base oils that may be used in the lubricating compositions of the present invention may be derived from linear C ₂ to 0 ₃₂ , preferably C$ to Ci«, alpha olefins. Particularly preferred feedstocks for said poly™ alpha olefins are 1-octene, 1-decene, 1-dodecene and 1- tetradecene .

Preferably, the base oil as used in the lubricating composition according to the present invention comprises a base oil selected from the group consisting of a poly- alpha olefin base oil and a Fischer-Tropsch derived base oil or a combination thereof.

The total amount of base oil incorporated in the lubricating composition of the present invention is preferably present in an amount in the range of from 60 to 99 wt. %, more preferably in an amount in the range of from 65 to 98 wt.% and most preferably in an amount in the range of from 70 to 95 wt.%, with respect to the total weight of the lubricating composition.

Typically, the kinematic viscosity at 100 ⁰ C (according to ASTM D 445) of the composition is between 9.3 and 26.1 cSt, preferably above 9.3 and below 16.3.

The lubricating composition according to the present invention may further comprise one or more additives such as anti-oxidants, anti-wear additives, dispersants, detergents, overbased detergents, extreme pressure additives, friction modifiers, viscosity modifiers, pour point depressants, metal passivators, corrosion inhibitors, demulsifiers, anti-foam agents, seal compatibility agents and additive diluent base oils, etc. As the person skilled in the art is familiar with the above and other additives, these are not further discussed here in detail. Specific examples of such additives are described in for example Kirk-Othmer Encyclopedia of Chemical Technology, third edition, volume 14, pages 477-526. Anti-oxidants that may be conveniently used include aminic and phenolic antioxidants. Examples of suitable antioxidants are phenyl-naphthylamines and diphenylamines . Anti-wear additives that may be conveniently used include zinc-containing compounds such as zinc dithiophosphate compounds selected from zinc dialkyl-, diaryl- and/or alkylaryl™ dithiophosphates, molybdenum- containing compounds, boron-containing compounds and ashless anti-wear additives such as substituted or unsubstituted thiophosphoric acids, and salts thereof.

Examples of such molybdenum-containing compounds may conveniently include molybdenum dithiocarbamates, trinuclear molybdenum compounds, for example as described in WO 98/26030, sulphides of molybdenum and molybdenum dithiophosphate .

Boron-containing compounds that may be conveniently used include borate esters, borated fatty amines, borated epoxides, alkali metal (or mixed alkali metal or alkaline earth metal) borates and borated overbased metal salts.

The dispersant used is preferably an ashless dispersaπt. Suitable examples of ashless dispersants are polybutylene succinimide polyamines and Mannic base type dispersants . The detergent used is preferably an overbased detergent or detergent mixture containing e.g. salicylate, sulphonate and/or phenate-type detergents.

Examples of viscosity modifiers which may conveniently be used in the lubricating composition of the present invention include the styrene-butadiene stellate copolymers, styrene-isoprene stellate copolymers and the polymethacrylate copolymer and ethylene-propylene copolymers. Dispersant-viscosity modifiers may be used in the lubricating composition of the present invention. _ Q —

Preferably, the composition contains at least 0.1 wt. % of a pour point depressant. As an example, alkylated naphthalene and phenolic polymers, polymethacrylates, maleate/furnarate copolymer esters may be conveniently used as effective pour point depressants. Preferably not more than 0.3 wt . % of the pour point depressant is used. 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 composition of the present invention as corrosion inhibitors.

Compounds such as polysiloxanes, dimethyl polycyclohexane and polyacrylates may be conveniently used in the lubricating composition of the present invention as defoaming agents.

Compounds which may be conveniently used in the lubricating composition of the present invention as seal fix or seal compatibility agents include, for example, commercially available aromatic esters. The lubricating compositions of the present invention may be conveniently prepared by admixing the one or more additives with the base oil(s) .

The above-mentioned additives are typically present in an amount in the range of from 0.01 to 35.0 wt.%, based on the total weight of the lubricating composition, preferably in an amount in the range of from 0.05 to 25.0 wt.%, more preferably from 1.0 to 20.0 wt.%, based on the total weight of the lubricating composition.

In another aspect, the present invention provides the use of a lubricating composition according to the present invention, in particular in a gas engine, in order to improve one or more of: oxidation stability (in particular according to the IP-48/97 (2004) test); and - base number retention (in particular according to the IP-48/97 (2004) test).

Also, the present invention may result in an improvement of engine cleanliness. The lubricating compositions according to the present invention are useful for lubricating apparatus generally, but in particular for use as engine oils for internal combustion engines. These engine oils include passenger car engines, diesel engines, marine diesel engines, gas engines, two- and four-cycle engines, etc., and in particular gas engines.

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

Various lubricating compositions for use in a gas engine were formulated. Table 1 indicates the composition and properties of the fully formulated gas engine oil formulations that were tested; the amounts of the components are given in wt.%, based on the total weight of the fully formulated formulations . All tested gas engine oil formulations were formulated as SAE 40 formulations meeting the so-called SAE J300 Specifications (as revised in May 2004; SAE stands for Society of Automotive Engineers) .

All the tested gas engine oil formulations contained a combination of a base oil, an additive package and a detergent, which additive package was the same in all tested compositions. Examples 2 and 3 additionally contained a conventional PIB-based thickener in order to meet the viscosity requirements of SAE 40. The additive package contained a combination of additives including anti-oxidants, zinc-based anti-wear additives, an ashless dispersant, a pour point depressant, a corrosion inhibitor and a metal passivator. "Base oil 1" was a commercially available Group II base oil having a kinematic viscosity at 100 ⁰ C (ASTM D445} of approx. 12.4 cSt (IrUn ² S ^"1 ). Base oil 1 is commercially available from e.g. Chevron Products Company (San Ramon, CA, United States) (under the trade designation "Chevron 600 R").

"Base oil 2" was a Fischer-Tropsch derived base oil ("GTL 8") having a kinematic viscosity at 100 ⁰ C (ASTM D445) of approx. 8 cSt (ItIm ² S ^"1 ) . This GTL base oil may be conveniently manufactured by the process described in e.g. WO 02/070631, the teaching of which is hereby incorporated by reference.

"Base oil 3" was a commercially available Group III base oil having a kinematic viscosity at 100 ⁰ C (ASTM D445) of approx. 8 cSt (HIm ² S ^"1 ) . Base oil 3 is commercially available from e.g. SK Energy (Ulsan, South Korea) (under the trade designation "Yubase 8") .

"Detergent 1" was an ashless detergent having general formula (I) as described above.

"Detergent 2" was a conventional overbased salicylate detergent having a TBN-value of about 230.

"Detergent 3" was a conventional overbased detergent having a TBN-value of about 150.

The compositions of Examples 1-3 and Comparative Examples 1-3 were obtained by mixing the base oils with the additive package and detergent (s) using conventional lubricant blending procedures . Table 1

ccording to ASTM D 874

According to ASTM D 2896. The contribution of TBN by a non-metal containing detergent is calculated from the

TBN measured for the single additive concentrate, multiplied by the percentage added to the lubricant composition and divided by the the total TBN of the finished lubricant composition.

According to ASTM D 445

^According to ASTM D 4951 n.d. = not determined

_ i 3 __

Oxidation Stability

In order to demonstrate the oxidation stability properties of the present invention, measurements were performed according to the industry standard test of IP- 48/97 (2004), apart from that a test time of 48 hours

(instead of 12 hours) was used. The measured values for viscosity, base number retention, TAN (Total Acid Number} and pH retention at the end of the test according to IP- 48/97 (2004) are indicated in Table 2 below. Seal Compatibility

In order to demonstrate the seal compatibility properties of the present invention, measurements were performed according to the VDA 673 301 / M 3273 (MAN) test. It was found that the values of tensile strength, elongation rupture, hardness {Shore A) and volume variation for Examples 1-3 remained well within the limits according to M 3277 and M 3477 (as established in November 2005) .

Table 2

_ o —

Discussion

As can be learned from Table 1, the present invention surprisingly allows formulating suitable gas engine oil compositions having a low sulphated ash content and a high TBN-value (which TBN-value is much higher than that of commercially available gas engine oils; as reflected in Comparative Examples 1 and 2) .

Table 2 shows that the values for viscosity (both at 40°C and 100 ⁰ C), TAN and base number retention for the compositions according to the present invention were significantly improved when compared with Comparative Example 1 {containing the same amount of detergent - see Table 1) and Comparative Example 2. This is a clear indication of desirable oxidation stability and base number retention properties for the compositions according to the present invention.

Further, it was found (see Table 2} that the compositions of Examples 1-3 outperformed Comparative Example 1 in terms of pH level; typically values below 4 indicate oxidation .

When compared with Comparative Example 3 (containing a sulphated ash content of below 0.1 wt.%, viz. 0.02 wt. %) the compositions of Examples 1-3 showed significantly improved TAN control and base number retention.

Further it can be learned from Table 2 that the composition of Examples 1-3 outperformed Comparative Examples 1 and 3 in terms of final pH.

Furthermore, desirable oxidation nitration stability properties were found for Examples 1-3.

Also, desirable seal compatibility values were obtained using Example 1 according to the present invention. - -

Furthermore, it was found that the lubricating compositions according to the present invention showed desirable engine cleanliness properties.