The present invention relates to the method and use of engine lubricants in four-stroke engines, and more especially to the use of engine lubricants having an SAE class rating of less than SAE 5W in four-stroke engines.

Owing to the ever increasing environmental, legislative and economic pressures, engine lubricants are required that contribute to increased engine efficiency, ie greater mpg or kpl and lowered engine emissions, and decreased frequency between lubricant changes, ie less oil usage.

However, these requirements are difficult to meet, especially when using petroleum-based oils, eg mineral oils, as lubricants, as they impose conflicting requirements on the viscosity and volatility properties exhibited by such oils. For example, engine oils are required to allow easy cold engine starting at low ambient temperatures whilst ensuring good lubrication at high operating temperatures. This may be achieved by blending lubricant stocks of different viscosities. However, such formulations may not be sufficient to meet the operating temperature range requirements owing to disparate viscosity indices of the component lubricant stocks. This has led to the use of viscosity index improvers, often in relatively high amounts. Such viscosity index improvers are frequently polymeric in nature and may be broken down by the operating temperatures and shearing of the fluid in the engines, especially in high performance vehicles, leading to potential loss in viscosity and engine failure.

Other approaches use synthetic lubricant stocks such as specially processed mineral oils, α-olefin oligomers and polymers (hereinafter poly-α-olefins) and esters including monoesters, diesters, polyol esters and complex esters, with or without appropriate additives such as viscosity index improvers.

Such approaches are exemplified in, for example, DE Offenlegungsschrift 2133042, EP-A-0089709, EP-B-0792334, JP 1993331483A, US-A-4155861 and US-B2-6303548.

DE OL 2133042 discloses an engine lubricant of viscosity class 10W-20 to 5W-20 which consists of a mineral oil raffinate having a viscosity index of between 80 and 105 and a kinematic viscosity at 100°C of between 7.5cSt and 12cSt, an oil-soluble synthetic lubricating oil, such as a diester, having a kinematic viscosity at 100°C of between 3cSt and 5cSt and a Noack evaporation loss of between 3 and 10% and additives. A specific example of a 10W engine oil having a kinematic viscosity at 100 °C of 7sSt and a viscosity index of 116 and has a 5% additive packages is derived from 75% mineral oil having a kinematic viscosity at 100°C of 9cST, a viscosity index of 102 and a Noack evaporation loss of 6% and 25% di-n-decanol trimethyl adipate.

EP-A-0089709 discloses organic carbonic acid diesters derived from alcohols as components in engine lubricants.

EP-B-0792334 discloses an engine lubricant having at least one ester derived from a saturated branched chain aliphatic monohydric alcohol having at least 8 carbon atoms and a saturated branched chain aliphatic monocarboxylic acid having at least 10 carbon atoms.

JP 1993331483A discloses an engine oil in which reduced amounts of viscosity index improvers are required. The engine oil has 10% to 30% of a diester or polyol ester, 60% to 89% of an α-olefin oligomer, 1% to 20% of an ethylene α-olefin oligomer and 0.5% to 3% zinc dialkyl dithiophosphate as an anti-wear agent. The oil has a kinematic viscosity at 100°C of 4cSt or greater. A specific example incorporates diisodecyl adipate which has a kinematic viscosity at 100°C of 3.62cSt.

US-A-4155861 discloses lubricating oils based on mixed esters consisting of a monomeric diester of a dicarboxylic acid and a complex ester derived from a dicarboxylic acid (preferably branched) and hexanediol or trimethyl hexanediol. In the specific examples, the monomeric diester is n-octyl, n-decyl trimethyl adipate. The addition of the complex ester at levels of 1% to 10% to the n-octyl, n-decyl trimethyl adipate is said to result in engine oils in the SAE classes 5W/20, 5W/30 or 10W/40.

US-B2-6303548 discloses an SAE 0W-40 lubricant composition consisting of 5% to 80% of a mineral oil base stock, 5% to 90% of a poly-α-olefin which has a kinematic viscosity at 100°C in the range 3.5cSt to 4.5cSt and 1% to 30% of an ester derived from monocarboxylic acids and polycarboxylic acids with monohydroxyl alcohols and polyols together with a viscosity improver comprising a mixture of 3% to 7% of a polymethacrylate and 4% to 9% olefin copolymer or hydrogenated diene copolymer. A specific example uses di-isooctyl adipate. DE 10 2004 034202 discloses ester oils with a low volatility and a low viscosity.

US-A-2004198614 discloses a method of reducing intake valve deposits in a direct injection engine, GB716086 discloses synthetic lubricants having utility at both high and low temperatures, GB1293225 discloses a lubricant composition comprising at least one diester of tripropylene glycol as a base fluid and an antioxidant, GB828956 discloses a lubricant composition which is particularly designed for lubrication of turbine engines, US3194764 discloses a process for the preparation of complex esters having improved viscosity characteristics and low acid number and US3546116 discloses functional fluid compositions which exhibit improved oxidative stability together with improved resistance to the corrosion of metal surfaces

Some disadvantages of such lubricants include the inherent limitation imposed by the viscosity indices of the base oils (which impacts film thickness); and the inability to reduce viscosity without increasing volatility (ie increasing the Noack evaporation loss of the lubricant). Additionally, very low viscosity esters can also have high polarity which can lead to seal compatibility issues and potential wear issues due to competition with antiwear agents such as ZDDP when the esters are used at high dose rates, eg >15 wt%. For example, di-isooctyl adipate has an NPI of 41. In addition, low viscosity lubricants, which have been optimised to give low volatilities, can also suffer from either low viscosity indices (<125), poor low temperature flow properties or shorter drain intervals resulting from poor oxidative stability (from the use of components in which gem dimethyl branching is present).

It is an object of the present invention to provide method and use of an engine lubricant in four-stroke engines. It is a further object of the present invention to provide use of an engine lubricant which has an SAE class rating of less than 5W, more especially an SAE class rating of 0W or lower.

According to the present invention, there is provided a use and a method as defined in the claims.

As used in this specification in relation to the invention described and claimed, as required by the context, the term "wt%" indicates the percentage by weight of the component referred to as a percentage of the total weight of the engine lubricant. Where the context refers to a specific component, for example a Noack evaporation loss, the term "wt%" indicates the percentage by weight of the total weight of the component.

Preferably, said engine lubricant comprises at least 20 wt%, more preferably at least 25 wt% of said at least one diester. Said engine lubricant may comprise up to 75 wt%, more preferably up to 50 wt% and, up to 40 wt% of said at least one diester. In one embodiment, said engine lubricant comprises about 30 wt% of said at least one diester.

When said engine lubricant does not consist essentially of said at least one diester and said additives, the balance of said engine lubricant comprises lubricant components selected from API Groups III, IV and V lubricants and gas-to-liquid lubricants (GTL) and mixtures thereof. Examples of suitable Group III lubricants include mineral oils. Examples of suitable Group IV lubricants included poly-α-olefins derived from C₈ to C₁₂ α-olefins and having kinematic viscosities in the range 3.6 cSt to 8 cSt at 100°C. Examples of Group V lubricants include alkyl naphthalenes, alkyl benzenes and esters, for example esters derived from monohydric alcohols and/or polyols and monocarboxylic acids or polycarboxylic acids. Examples of alkyl naphthalenes include Synesstic™ 5 and Synesstic™ 12 alkyl naphthalenes available from Mobil. Examples of esters are Priolube 1976™ a monoester and Priolube 3970™ a TMP nC₈InC₁₀ polyol ester. GTL base stocks are made by conversion of natural gas (ie, methane and higher alkanes) to synthesis gas (carbon monoxide and hydrogen) and then via oligomerisation (eg the Fischer-Tropsch process) to higher molecular weight molecules that are hydrocracked to produce iso-paraffins in the required lubricant boiling/viscosity range. GTL base stocks are only just being commercialised and consequently there is little or no data relating to them that is freely available. As far as it is known, such GTL base stocks will have viscosity grades similar to poly-α-olefins.

In one embodiment, said engine lubricant consists essentially of said at least one diester, at least one Group V lubricant, especially an alkyl naphthalene or an ester other than said at least one diester such as a polyol ester or complex ester, and said additives.

Preferably, said at least one diester has a kinematic viscosity at 100°C of not more than 3.0cSt. Preferably, said at least one diester has a viscosity index of at least 140. Said at least one diester has a pour point of not more than about - 30°C, more particularly of not more than -35°C and especially not more than -40°C. Preferably, said at least one diester has a Noack evaporation loss of not more than 14.5 wt%, more preferably of not more than 14.0 wt%.

Preferably, said at least one diester has a flash point of at least 200°C, more preferably at least 210°C, more particularly at least 220°C and especially of about 230°C.

Preferably, said at least one diester has a non-polarity index (NPI), as described in EP-B-0792334, of more than 30 but less than 100, more preferably less than 80.

Preferably, said at least one diester is stable when held at -20 °C for one week. This low temperature stability may be tested by storing approximately 30ml of diester in a glass vial and placing the vial in a freezer unit at -20°C for one week, checking the sample at regular intervals and noting any signs of crystal formation or gelling.

Preferably, said at least one diester has a cold crank simulation (CCS) dynamic viscosity at -35°C of not more than 6200 cPs.

Preferably, said engine lubricant comprises only one of said at least one diester.

Where said engine lubricant comprises at least two of said at least one diester, each diester may be selected with different properties. Preferably, the properties of each diester are within the values of such properties as described above; alternatively, one or more of the properties of at least one diester may be outside the values of such properties as described above provided that the properties of the mixture of diesters are within the values of such properties as described above.

Preferably, said at least one diester is selected from the group consisting of:

1. a) reaction products of at least one C₆ to C₁₀, aliphatic dicarboxylic acid or an anhydride thereof with at least one primary C₈ to C₁₀, aliphatic monohydric alcohol, wherein, if said at least one acid is branched, then at least one of said at least one alcohol is linear and, if said at least one acid is linear, then at least one of said at least one alcohol is branched; and
2. b)  

Preferably, the diester, when derived from diacids or anhydrides thereof and monohydric alcohols, contains 17 to 36, more particularly 20 to 30 and especially 23 to 26 carbon atoms.

The reaction products of dicarboxylic acids and alcohols are reaction products of either branched acids with linear alcohols or linear acids with branched alcohols.

The branched chains of the branched acids and/or branched alcohols may be C₁ to C₄ alkyl, more preferably C₁ or C₂ alkyl and especially methyl. The branched acids are preferably not branched in the α-position but are preferably branched in the β-position. Preferably, the acids do not contain any gem branched groups, eg gem dimethyl or gem diethyl, and preferably contain only one or two branches, especially a single branch in the β-position.

The dicarboxylic acids include adipic acid, 3-methyl adipic acid and sebacic acid. The primary alcohols include 1-octanol, 1-decanol and mixtures thereof, 2-ethylhexanol and isononyl alcohol.

Preferred diesters are selected from the group consisting of di-isononyl adipate, di-n-octyl 3-methyl-adipate, di-2-ethylhexyl sebacate and mixtures thereof. More particularly, the diesters are selected from the group consisting of di-isononyl adipate, di-n-octyl 3-methyl-adipate and di-2-ethylhexyl sebacate and mixtures thereof.

As will be appreciated, the acids and alcohols used to make said diesters will be from commercial sources and may not necessarily comprise 100 wt% of the acid or alcohol. Such commercial products usually comprise a major proportion of the primary product together with other isomers and/or additional products of shorter or longer chain length. This may lead to variations in properties of the diesters which are reaction products. Such variations in properties are exemplified below in the Examples.

In one embodiment, said engine lubricant optionally may comprise esters selected from simple esters, diesters, not being diesters as hereinbefore described, and complex esters or mixtures thereof. Preferably, the weight ratio of said diesters to said optional esters will be between 100:0 to 60:40, more preferably between 100:0 to 75:25, more particularly between 99:1 and 80:20 and, especially between 95:5 to 85:15.

As previously described, said engine lubricant comprises not more than 15 wt% of additives, more especially not more than 10 wt% of additives. Typically, said additives are:

1. a) viscosity index improvers, for example alkyl methacrylate copolymers, olefin copolymers (OCP) and mixtures thereof, which are added in effective amounts, typically in the range 0.1 wt% to 6 wt%;
2. b) antioxidants, for example phenolic antioxidants, such as hindered phenols, and alkylated diphenyl amines and mixtures thereof, which are added in effective amounts, typically in the range 0.5 wt% to 1 wt%;
3. c) metal deactivators, for example metal dialkyldithiophosphates, thiadiazoles and triazoles (which may also function as corrosion inhibitors and extreme pressure additives), which are added in effective amounts, typically in the range 0.01 wt% to 0.5 wt%;
4. d) pour point depressants which are added in effective amounts, typically in the range 0.1 wt% to 1.0 wt%;
5. e) extreme pressure additives, for example zinc diaryl dithiophosphates (ZDDP), which are added in effective amounts, typically in the range 0.5 wt% to 3.0 wt%;
6. f) As friction modifiers, glycerol mono-oleate, which is added in the range 0.3 wt% to 1.3 wt%;
7. g) anti-foam agents, for example dimethyl polysiloxane, polyacrylate, which are added in effective amounts, typically in the range 1ppm to 100ppm;
8. h) multifunctional additives such as DDI (detergent-dispersion-inhibitor) packages;
9. i) and mixtures of such additives.

In engine lubricants according to the claims in which said diesters and, optionally, other esters are present in significant quantities, preferably as a major component of the engine lubricants, such engine lubricants may be free of some additives such as viscosity index improvers.

The combinations of additives used in engine lubricants and the amounts thereof may vary significantly; however, the total amount of all additives included in said engine lubricant according to the invention is subject to the upper limits of 15 wt% and more especially 10 wt%, as previously described.

The present invention includes the use of said engine lubricant as herein described in lubricating four-stroke engines and a method of lubricating a four-stroke engine comprising lubricating said engine with said engine lubricant as herein described.

The present invention further includes use of an SAE 0W engine lubricant, said engine lubricant comprising at least one diester as herein described. The features and embodiments herein described apply also mutatis mutandis to said SAE 0W engine lubricant.

The invention will now be further illustrated with reference to the following Example.

Samples 1 to 4 as identified in Table 1 below are diesters suitable for use in said engine lubricants according to the invention. Comparative samples 5 to 11 are also identified in Table 1. The properties of the samples are given in Table 2. Sample Linear Acid Branched Acid Linear Alcohol Branched Alcohol Poly(alkyl glycol) 1 Adipic - - Isononyl* - 2 Adipic - - Isononyl** - 3 - 3-methyl adipic 1-octanol - - 4 Sebacic - - 2-ethylhexanol - 5 Caprylic/capric (approximately 50:50 mixture) - - - PPG 225 6 Caprylic/capric (approximately 50:50 mixture) - - Trimethylol propane 7 Adipic - 1-octanol/1-decanol (approximately 50:50 mixture) - - 8 Heptanoic - Glycerol - - 9 Caprylic - - Guerbert iso-C₂₀ - 10 Adipic - - 2-propylheptanol - 11 - 2,2,4-methyl adipic - 2-ethylhexanol - *Commercially-sourced isononyl alcohol comprising <85 wt% isononyl alcohol.
**Commercially-sourced isononyl alcohol comprising at least 85 wt% isononyl alcohol. Sample Viscosity @ 40°C (cSt) Viscosity @ 100°C (cSt) Viscosity Index Flash Point (°C) Pour Point (°C) Noack Evaporation Loss (wt%) Stability at -20°C for 1 week NPI CCS -35°C (cPs) 1 12.0 3.3 156 210 -60 12 Pass 48 <500 2 10.5 3.0 159 230 -50 13 Pass 48 <500 3 8.9 2.8 169 227* -36** 14 Pass 47 <500 4 11.5 3.3 157 230 <-60 12 Pass 55 <500 5 10.6 3.0 149 228 -62 14? - <61 - 6 19.5 4.4 140 245 -51 3 Pass 60 3390 7 12.2 3.4 157 230 -26 11 Fail 48 <500 8 9.6 2.7 128 240 -56 14 Pass 34 <500 9 10.5 2.9 138 230 -28 12? - 118 >6200 10 11.4 3.0 122 210 <-38 15 Pass 48 <500 11 9.9 2.5 116 - <-60 22 Pass 48 - *Pre-1950's literature data.
**Precipitate started to form at about -36°C and continued to build up. At -48°C, the sample still had a liquid layer that was fluid.
? = modelled data based on flashpoint.