REDUCTION OF DIESEL EMISSIONS

Abstract: Diesel engine particulate emissions can be reduced by the use of a crankcase lubricating composition having one or more of characteristics or components (a) to (e): (a) a relatively low high shear viscosity and a relatively low volatility; (b) a base oil giving a reduced ignition delay compared with that given by 150 solvent neutral; (c) a zinc content of zero or of not more than 0.06 mass % on an active ingredient basis; (d) a total sulphated ash (SASH) level of zero, or if one or more ash-producing additives are present, SASH level of not more than 0.8 mass % and a TBN to SASH ratio of less than 8.5; (e) at least one metal-containing detergent in which the metal has a first ionization potential lower than that of magnesium.

Description: Reduction of Diesel Emissions The invention relates to the reduction of the amount of particulate matter emitted by diesel. engines. In the operation of a typical diesel engine, fuel and a small proportion of lubricant (typically 0.0025 parts by mass of lubricant for 1 part by mass of fuel) are consumed, and "breathable" gases (nitrogen, water, carbon dioxide, and oxygen), gaseous pollutants (primarily carbon monoxide, unburnt hydrocarbons, nitrogen oxides and sulphur oxides) and particulate pollutants (primarily unburnt oil, carbon, unburnt fuel and inorganic materials) are produced, the proportions of the breathable gases, gaseous pollutants and particulate pollutants typically being approximately 99.9 mass %, 0.1 mass % and 0.0006 mass % respectively. Concern about the environment has resulted in a great deal of work being carried out in an attempt to reduce the emission of gaseous and particulate pollutants by diesel engines. In particular, engine manufacturers have devoted, and are devoting, very considerable time and effort to the production of engines which emit lower amounts of pollutants, such engines sometimes being referred to as "low emission engines". The applicants have surprisingly found that a significant reduction in the amount of particulate material emitted by a diesel engine can be obtained by the use in the engine of appropriate lubricating oils. In. particular, the applicants have found that crankcase lubricating oils can be formulated to give reduced emission of particulate materials by diesel engines. The term "particulate material" is used herein in the sense commonly used in this art, that is, to refer to materials emitted from an exhaust as finely divided solids or liquids (that is, as materials which can be removed by filtration) rather than as gas s. Other terms sometimes used in connection with such materials are "particulate emissions", "exhaust particulate" and "diesel particulate" . The lubricating compositions according to the invention may be referred to as giving rise to "low particulate emissions" or as "low emission oils". The present invention provides the use of a lubricating composition, having at least one of characteristics or components (a) to (e) below, for reducing the amount of particulate material emitted by a diesel engine: (a) a relatively low high shear viscosity and a relatively low volatility; (b) a base oil giving a reduced ignition delay compared with that given by 150 solvent neutral; (c) a zinc content of zero or of not more than 0.06 mass % on an active ingredient basis; (d) a total sulphated ash (SASH) level, as measured by ASTM D874, of zero or, if one or more ash-producing additives are present, a SASH level of not more than 0.8 mass % and a ratio of Total Base Number (TBN) , measured by ASTM D2896, to SASH level, measured by ASTM D874, of less than 8.5; (e) at least one metal-containing detergent, suitable for use in lubricating oils, in which the metal has a first ionize- -On potential lower than that of magnesium. The invention also provides the use of a lubricating composition having at least two, advantageously more than two, and preferably all, of characteristics or components (a) to (e) above, for reducing the amount of particulate material emitted by a diesel engine. The invention also provides a crankcase lubricating composition suitable for use in diesel engines which has at least two, advantageously more than two, and preferably all, of characteristics or components (a)' to (e) above. The invention also provides a process for the reduction of the amount of particulate material emitted by a diesel engine in which the diesel engine is lubricated with a lubricating composition having at least one, desirably at least two, and preferably all of the characteristics (a) to (e) above. The reduction in diesel particulate obtainable in accordance with the invention is normally obtained with only a relatively small, or no, increase in the emission of nitrogen oxides; indeed, in some cases nitrogen oxide emissions may be reduced. This is in contrast to the applicants' earlier observations that a decrease in particulate emissions could only be obtained at the cost of a significant increase in the emission of nitrogen oxides. In a first aspect of the invention, referred to above as (a) , a lubricating composition having a relatively low high shear viscosity and a relatively low volatility is used for reducing diesel particulate emissions. Accordingly, the present invention provides the use, for reducing the amount of particulate material emitted by a diesel engine, of a lubricating composition comprising a base oil and a viscosity modifier, the viscosity modifier being such that a mixture of the base oil and the viscosity modifier has a high shear viscosity (measured by Test Method CEC-L-36- T-84) and a volatility (measured by Test Method CEC-L-40-T-87) lower than that of a mixture, in the same proportions by mass, of the same base oil and a standard viscosity modifier (as hereinafter defined) . The invention also provides a process for lubricating a diesel engine with such a lubricating composition to obtain reduced diesel particulate emissions. To obtain the standard viscosity modifier used as a comparison in accordance with the first aspect of the invention, a polymer solution, containing 14 mass % polymer, obtained as described in Example 1 of U.S. Specification No. 4 137 185, was further diluted with S130N to 12.6 mass % polymer. The entire disclosure of U.S. Specification No. 4 137 185 is incorporated herein by reference. Solvent 130 Neutral Mineral Oil (S130N) is an oil which is derived by the vacuum distillation of crude oil followed by solvent extraction, dewaxing, and finishing, and has a viscosity index, measured by ASTM D2270, of 100 or more, a pour point, measured by ASTM D97, of -15°C or .lower, and a kinematic viscosity at 40°C of 4.4 to 4.8 cSt (4.4 to 4.8 x 10~ 6 m 2 /s) . A lubricating composition for use in accordance with the first aspect of the invention or used in a process of the first aspect of the invention preferably has a high shear viscosity of less than 4.5 cP (4.5 mPas) . In some cases a high shear viscosity of less than 4 is desirable. The high shear viscosity is measured by Test Method CEC-L-36-T-8 . This method uses a Ravenfield viscometer in which the lubricating composition is fed to the gap between a small angle conical rotor and a correspondingly-shaped outer stator. The viscosity is measured at a shear rate of 10 ~6 seconds and a temperature of 150°C. v The employment in accordance with the first aspect of the invention of a lubricating composition which preferably has as low a high shear viscosity as can be attained without affecting to an unacceptable extent other properties of the oil, is in contrast to the generally accepted view in the industry that the high shear viscosity should be relatively high. The lubricating composition of the first aspect of the invention advantageously also has a low shear viscosity which is relatively high. The low shear viscosity (measured at 100°C by ASTM D445) is advantageously at least 10 cSt (10 ~5 m 2 /s) . The volatility of the lubricating composition of the first aspect of the invention (the mass percent of the composition lost by evaporation as measured by Test Method CEC-L-40-T-87, in which a sample is maintained at 250°C for 1 hour using Noack apparatus) is advantageously less than 15%, (or, in some cases, less than 12.5), preferably less than 11%, and especially less than 8%. The volatility measured as indicated above is referred to herein as the Noack volatility. The invention also provides the use, for reducing the amount of particulate material emitted by a diesel engine, of an XW40 viscosity grade lubricating composition having a high shear viscosity (measured by Test Method CEC-L-36-T-84) of less than 4.5 cP (4.5 mPas) and a volatility (measured by Test Method CEC-L-40-T-87) of less than 15%, and further provides a process for lubricating a diesel engine with such a lubricating composition. The invention also provides the use, for reducing the amount of particulate material emitted by a diesel engine, of an X 30 viscosity grade lubricating composition having a high shear viscosity (measured by Test Method CEC-L-36-T-84) of less than 4 cP (4 mPas) and a volatility (measured by Test Method CEC-L- 40-T-87) of less than 12.5%, and further provides a process for lubricating a diesel engine with such a lubricating composition. A lubricating composition suitable for the first aspect of the invention is advantageously prepared by incorporating in a base oil a viscosity modifier additive comprising a solution of a polymer in the same or a different base oil. The polymer used in preparing an additive of the above- mentioned type advantageously has a shear stability index (percentage polymer breakdown as defined in IP procedure 294) of at least 25, preferably at least 30. The shear stability index of the polymer is advantageously in the range of from 30 to 50, and is preferably in the range of from 30 to 40. The use in the invention of a polymer having a specified minimum shear stability index is in contrast to the generally accepted view in the industry that the shear stability index of polymeric additives for lubricating oils should be as low as possible. Examples of polymers which may be prepared in a form in which they have a shear.stability index such that they are suitable for use in accordance with the invention are olefin copolymers, for example, ethylene/propylene copolymers, polystyrene, styrene copolymers, for example, styrene/isoprene and styrene/butadiene copolymers, and polymethacrylates. Suitable polymers typically have number average molecular weights of from 30,000 to 80,000, preferably 40,000 to 60,000, as determined by gel permeation chromatograph . The polymers may, if desired, contain groups to impart characteristics to them in addition to their viscosity modifying properties. Thus, for example, the polymers may have dispersant as well as viscosity modifying properties. The base oil in which the polymer is dissolved to give a viscosity modifier suitable for the first aspect of the invention is advantageously one having a relatively low volatility, pour point and low temperature viscosity, and is preferably a synthetic oil. Synthetic base oils include alkyl esters of dicarboxylic acids, polyglycols and alcohols; poly- α-olefins, including polybutenes; alkyl benzenes; organic esters of phosphoric acids; and polysilicone oils. Particularly preferred synthetic oils for use in preparing the viscosity modifier are poly-α-olefins and other base oils giving a reduced ignition delay as discussed later in this specificatio . A particularly suitable viscosity modifier for the first aspect of the invention may be made by the direct dissolution of an olefin copolymer rubber in an ester or, preferably, a poly-α-olefin base oil. Preferred pόly-α-olefin base oils are those having a viscosity of 4 to 8 cSt (4 x 10" 6 to 8 x 10" 6 m 2 /s) . The proportion of the polymer to the base oil in a viscosity modifier suitable for the first aspect of the invention is in general dictated largely by handling considerations, and can be determined by routine experiment. For guidance, a viscosity modifier comprising an olefin copolymer dissolved in a poly-α-olefin may comprise 5 to 15 mass % polymer and 95 to 85 mass % poly-α-olefin. In one particular case, a additive comprising about 7.5 mass % polymer and about 92.5 mass % poly-α-olefin was found to be advantageous. The base oil in which the viscosity modifier additive (polymer plus a base oil) is dissolved may be any oil of lubricating viscosity. The oil will conveniently have a viscosity of about 2.5 to about 12 cSt (about 2.5 x 10 -6 to about 12 x 10 -6 m 2 /s) and advantageously about 2.5 to about 9 cSt (about 2.5 x 10" 6 to 9 x 10" 6 m 2 /s) at 100°C. Preferred oils are those such that the final lubricating composition has a viscosity in the range of from 3 to 7 cSt (3 x 10~ 6 to 7 x 10 -6 m 2 /s) , at 100°C and a Noack volatility in the range of from 2 to 15%, preferably 4 to 8%. The viscosity modifier additive is preferably used in such a proportion as to impart to the lubricating composition the high shear viscosity, the volatility and/or the low shear viscosity indicated above. The lubricating composition may, and normally will, contain one or more further additives, for example, those mentioned below as being suitable for use in lubricants. The invention also provides a viscosity modifier additive suitable for use in a lubricating composition for diesel engines which comprises a solution of a polymer, preferably an olefin copolymer, having a shear stability index of at least 30, in a synthetic base oil, preferably a poly-α-olefin. The invention further provides the use of such an additive, in admixture with a base oil, for reducing the amount of particulate material emitted by a diesel engine. The invention also provides a lubricating composition comprising such an additive and a base oil. This invention provides a process for the reduction of the amount of particulate material emitted by a diesel engine, in which the diesel engine is lubricated with such a lubricating composition.In the case of an XW40 grade lubricating composition, the high shear viscosity is preferably less than 4.5 cp (4.5 mPas) and the volatility is preferably less than 15%. For a XW30 lubricating composition, the high shear viscosity is preferably less than 4 and the volatility is preferably less than 12.5%. In a second aspect of the invention, referred to above as (b) , a lubricating composition having a base oil giving a reduced ignition delay compared with that given by 150 solvent neutral is used for reducing diesel particulate emissions. Accordingly, the invention also provides the use, for reducing the amount of particulate material emitted by a diesel engine, of a lubricating composition which comprises a base oil and one or more additives, the base oil giving a reduced ignition delay (measured as hereinafter defined) compared with that given by 150 solvent neutral. The invention also provides a process for lubricating a diesel engine with such a lubricating composition to obtain reduced diesel particulate emissions. 150 solvent neutral, which is derived by the vacuum distillation of crude oil followed by solvent extraction, dewaxing, and finishing, has a viscosity index, measured by ASTM D2270, of 100 or more, a pour point, measured by ASTM D97, of -8°C or lower, and a kinematic viscosity at 40°C of 5.1 to 5.5 cSt (5.1 to 5.5 x 10" 6 m 2 /s) . The ignition delay of a diesel engine is a measure of the time which elapses between the introduction of a quantity of fuel into the engine and the ignition of that fuel. The applicants have surprisingly found that the use of a lubricating composition having a base oil giving a reduced ignition delay compared with 150 solvent neutral makes it possible to obtain a reduction in diesel particulate emissions. For the purposes of the second aspect of the invention, the ignition delay is measured using a Mercedes Benz OM 366 (1988 model year) diesel engine operating at 1,680 rpm without a load. The engine is provided with means for measuring the gas pressure within each cylinder and means for measuring the amount of particulate matter emitted from the engine. From the cylinder gas pressure it is possible to determine when combustion of the fuel charge commences, and thus to arrive at the ignition delay. The normal ignition delay for the engine when it is operating under the above conditions on fuel alone is 31.2 degrees of crankshaft rotation. Substances to be tested, in this case base oils for lubricating compositions, are introduced into the engine, while it is running under the above conditions, via the air inlet system, the amount of the substance introduced being controlled using a valve. When comparing the ignition delays given by two different substances, substantially equal masses of the substances should be introduced into the engine. Particularly preferred base oils for the second aspect of the invention are base oils comprising synthetic oils of lubricating viscosity, for example, poly-α-olefins and poly-internal-ole ins. Examples of suitable poly-α-olefins are polyisobutene and poly-n-butene (poly-1-butene) . Poly-α-olefins having viscosities of about 4 to 8 cSt (4 x 10 -6 to 8 x 10 -6 m 2 /s) at 100°C are particularly suitable. The base oil used in the second aspect of the invention need not consist solely of a preferred base oil as indicated above. Thus, for example, the base oil may also comprise a conventionally used base oil, for example 150 solvent neutral, and/or a further substance, for example, brightstock. Brightstock typically has a viscosity index, measured by ASTM D2270, of from 92 to 96, a pour point, measured by ASTM D97, of -5°C or lower, and a kinematic viscosity of 450 to 500 cSt (4.50 x 10" 4 to 5.00 x 10 ~4 m 2 /s) at 40°C. Brightstock may be obtained from the residue remaining after crude oil has been subjected to vacuum distillation to remove lubricant basestock fractions. The so-called vacuum residue is treated with a solvent to remove asphalt, and the resulting oil from which the asphalt has been removed is treated, for example, by solvent extraction, dewaxing, and finishing, to improve the viscosity index, the pour point, and the colour. In a particularly preferred base oil for use in the second aspect of the invention, part of a conventional natural or synthetic lubricating oil base oil is replaced by a mixture of poly-n-butene (poly-1-butene) and brightstock, the poly-n-butene advantageously being present in a range of from 30:70 to 70:30 parts by mass, advantageously 45:55 to 55:45 parts by mass. The poly-n-butene/brightstock mixture advantageously forms 15 to 30 mass % of the fully formulated lubricating composition. The invention also provides a base oil suitable for use in lubricating compositions for diesel engines which comprises a mixture of poly-n-butene (poly-1-butene) and brightstock, preferably in the proportions indicated above, especially in - li ¬ the advantageous proportions indicated above. The invention further provides a lubricating oil in which the base oil consists at least partly of such a mixture, the use of such a mixture for reducing the amount of particulate material emitted by a diesel engine, and a process for the reduction of the amount of particulate material emitted by a diesel engine in which the engine is lubricated with such a lubricating oil. In a further aspect of the invention, referred to above as (c) , a lubricating composition which is free from zinc compounds or contains not more than 0.06 mass % of zinc on an active ingredient basis, based on the total mass of the composition, is used for reducing diesel particulate emissions. This third aspect of the invention provides such a use and a process for lubricating a diesel engine with such a composition to obtain reduced diesel particulate emissions. Zinc compounds used in lubricating compositions, for example as antiwear and/or antioxidant additives, include zinc dithiocarbamates and zinc dihydrocarbyl dithiophosphates (ZDDPs) . Especially preferred ZDDPs for use in oil-based compositions are those of the formula Zn[SP(S) (OR) (OR 1 )] 2 wherein R and R 1 may be the same or different hydrocarbyl radicals containing from 1 to 18, and preferably 2 to 12, carbon atoms, for example, alkyl, alkenyl, aryl, aralkyl, alkaryl and cycloaliphatic radicals. Particularly preferred as R and R 1 radicals are alkyl radicals having 2 to 8 carbon atoms. Examples of radicals which R and R 1 may represent are ethyl, n-propyl, i-propyl, n-butyl, i-butyl, sec-butyl, amyl, n-hexyl, i-hexyl, n-heptyl, n-octyl, 2-ethylhexyl, decyl, dodecyl, octadecyl, phenyl, butylphenyl, cyclohexyl, methylcyclopentyl, propenyl and butenyl radicals. In order to obtain oil solubility, the total number of carbon atoms in R and R 1 will generally be about 5 or greater. ZDDPs are typically used in a proportion of 0.8 to 1.6 mass % of the fully formulated oil (on an active ingredient basis), corresponding to about 0.06 to 0.14 mass% of zinc (depending on the nature of R and R 1 ) . The applicants have surprisingly found that, by reducing the level of zinc to zero or less than 0.06 mass %, diesel particulate emissions can be reduced. Advantageously, the lubricating composition contains at most 0.035 mass % zinc (on an active ingredient basis) . In an especially preferred embodiment of this aspect of the invention, the lubricating compositions are substantially free of zinc, or contain no zinc. Where a lubricating composition contains low or zero amounts of zinc compounds, for example, ZDDPs, it will normally be necessary to include other additives to impart adequate antiwear and/or antioxidant properties to the composition. Examples of additives which may be used for this purpose in lubricating compositions which contain low or zero levels of zinc and/or phosphorus may be found in European Patent Specifications No. 280 579 A and 280 580 A, the disclosures of which are incorporated herein by reference. Examples of additives which may be incorporated in lubricating compositions to impart adequate antioxidant performance when ZDDPs are absent or present at only low levels are oil-soluble copper compounds, the compositions preferably containing 5 to 500 parts per million by mass (ppm) of added copper in oil- soluble form. The proportion of added copper in the compositions will generally be within the range of 10 to 400 ppm, typically 10 to 300 ppm, preferably 10 to 200 ppm, for example, 60 to 200 ppm. Copper compounds may if desired be used as supplementary antioxidants even when ZDDPs are present. The copper may be blended into the oil as any suitable oil- soluble copper compound. By oil-soluble it is meant that the compound is oil-soluble under normal blending conditions in the oil or additive package. The copper may be in the cuprous or cupric form. Suitable copper salts include oil-soluble copper salts of synthetic or natural carboxylic acids. Examples of suitable carboxylic acids include CQ to Ciβ fatty acids, for example, stearic or palmitic acid; unsaturated acids, for example, oleic acid; branched carboxylic acids, for example, naphthenic acids of molecular weight of from 200 to 500, neodecanoic acid or 2-ethylhexanoic acid; and alkyl or alkenyl-substituted dicarboxylic acids, for example, polyalkene-substituted succinic acids, for example, octadecenyl succinic acids, dodecenyl succinic acids and polyisobutenyl succinic acids. Other suitable copper salts include oil-soluble copper dithiocarbamates of the general formula (RR 1 NCSS) n Cu, where n is 1 or 2 and R and R 1 are the same or different hydrocarbyl radicals containing 1 to 18, preferably 2 to 12 carbon atoms, for example, alkyl, alkenyl, aryl, aralkyl, alkaryl and cycloalkyl radicals. Other copper-and sulphur-containing compounds, for example, copper mercaptides, disulphides and thioxanthates are also suitable for use in accordance with the invention. Copper sulphonates, phenates and acetylacetonates may also be used, as may any oil-soluble copper compound mentioned elsewhere in this specification. Alternatively, the copper may be introduced into the oil in an oil-insoluble form provided that in the finished lubricant composition the copper is in the form of an oil-soluble compound. The term "added copper" is intended to exclude copper present in the oil as a result of accumulation of copper in the oil during use, for example, by wear or corrosion of copper-containing components. Examples of other antioxidants which may be used in accordance with the invention are phenolic antioxidants and, especially, amine-containing antioxidants, for example, aromatic amines, especially aromatic amines which contain at least one aryl or arylene group directly attached to at least one nitrogen atom. Advantageous aromatic amines are N-aryl amines and N,N'— rylene diamines, and diphenylamines and phenylene diamines are especially preferred. Such antioxidants may be used in place of the copper-containing antioxidants or in conjunction with copper-containing antioxidants. Examples of additives which may be incorporated in lubricating compositions to enhance their antiwear and/or antioxidant properties are oil-soluble sulphur compounds. Sulphur- containing compounds which may be used in accordance with the present invention are dithiocarbamates , mercaptides, sulphurized unsaturated organic compounds, including sulphurized ole ins; sulphurized Diels-Alder products; and, particularly, sulphurized unsaturated alcohols and esters, for example sperm oil substitutes; sulphides, including di- and poly-sulphides; thioethers; thiophenols; thioxanthates (including copper thioxanthates as indicated above); sulphurized esters; thioesters; thioamides; thiazoles, for example benzothiazoles and, particularly, mercaptobenzothiazoles, and thiadiazoles. The proportion of a sulphur-containing compound used in accordance with the present invention may be, for example, 0.5 to 1 mass % of active ingredient based on the fully formulated oil. A composition which is free from ZDDPs, or which contains only very low proportions of ZDDPs, preferably contains a bearing corrosion inhibitor to inhibit corrosion effects on bearings such as Cu/Pb bearings, where, for example, copper staining and/or high weight loss can be a problem. Such additives have been found to enhance the antiwear performance of the oil. Preferred corrosion inhibitors for use in accordance with the invention are borate esters and thiadiazole mercaptans, for example, the borate esters and thiadiazole mercaptans (including derivatives thereof) described in European Specifications Nos. 280 579 A and 280 580 A. In a further aspect of the invention, referred to above as (d) , a lubricating composition having a total sulphated ash (SASH) level, as measured by ASTM D874, of zero or, if one or more ash-producing additives are present, having a SASH level of not more than 0.8 mass % and a ratio of Total Base Number (TBN) , measured by ASTM D 2896, to SASH level, as measured by ASTM D874, of less than 8.5, is used for reducing diesel particulate emissions. Accordingly, the invention also provides the use, for reducing the amount of particulate material emitted by a diesel engine, of a lubricating, composition which comprises a base oil and one or more lubricating oil additives, the composition having a total sulphated ash (SASH) level, as measured by ASTM D874, of zero or, if one or more ash-producing additives are present, having a SASH level of not more than 0.8 mass % and a ratio of Total Base Number (TBN) , measured by ASTM D2896, to SASH level, as measured by ASTM D874, of less than 8.5. The invention also provides a process for lubricating a diesel engine with such a lubricating composition to obtain reduced diesel particulate emissions. Lubricating compositions used in diesel engines typically have a TBN of the order of 10, whereas the TBN to SASH ratio indicated above will normally require the use of compositions of lower TBN. To obtain satisfactory results using a lubricating composition of low TBN it may be desirable in some cases to use a fuel with a relatively low sulphur content, for example, a sulphur content of not more than 0.5 mass %, based on the fuel. Advantageously, the TBN of a lubricating composition for this aspect of the invention is in the range of from 1 to 8, preferably in the range of 3 to 6. A TBN of this order of magnitude may, for example, be obtained by the incorporation in the composition of approximately 0.3 to 0.8 mass %, preferably 0.45 to 0.65 mass %, of a metal-containing detergent having a TBN in the range of from 250 to 500, the percentages being given on an active ingredient basis. Examples of metal-containing detergents which may be used in this aspect of the invention include the metal-containing detergents (including magnesium-containing detergents) mentioned in accordance with aspect (e) . Where a lubricating composition is essentially ashless, or contains only low levels of ash-producing additives, the composition must normally contain ashless additives which provide the composition with adequate dispersant, antioxidant, and corrosion inhibiting properties. Examples of ashless diesel lubricating oil compositions are given, for example, in European Specification No. 311 318 A, the disclosure of which is incorporated herein by reference, but other suitable compositions will be apparent to those skilled in the art. The compositions in the European specification comprise at least 3 wt. % of at least one ashless dispersant, at least 2 wt. % of at least one sulphurized alkyl phenol oxidation inhibitor, and at least 0.1 wt. % of at least one organo- sulphur copper corrosion inhibitor of the formula: wherein R 4 and R 5 are straight or branched chain alkyl, cyclic, alicylic, aryl, alkylaryl or arylalkyl radicals having from 2 to 30 carbon atoms, and w and z are numbers from 1 to 8. The European specification indicates that the lubricating oils disclosed therein are particularly useful in the crankcase of diesel engines having cylinders wherein the distance between the piston's top land and the cylinder wall liner is made small to minimize the amount of particulates generated in the cylinder's firing chamber. The lubricating compositions according to this aspect of the present invention are useful both in such engines and in conventional engines. In a further aspect of the invention, referred to above as (e) , a lubricating composition having at least one metal- containing detergent in which the metal has a first ionization potential lower than that of magnesium, that is lower than 7.646 ev, is used for reducing diesel particulate emissions. Accordingly, the invention also provides the use, for reducing the amount of particulate material emitted by a diesel engine, of a lubricating composition which comprises a base oil and at least one metal-containing detergent, suitable for use in a lubricating composition, in which the metal has a first ionization potential lower than that of magnesium. The invention also provides a process for lubricating a diesel engine with such a lubricating composition to obtain reduced diesel particulate emissions. The metal-containing detergents with which the invention is concerned are salts of an acidic organic compound, the salts • having a hydrophilic portion and a hydrophobic portion such that they are capable of acting as a surfactant. Thus, the salts contain a polar head (the salt-forming group) and one or more organophilic groups which together have sufficiently high molecular weight to form a hydrophobic tail to give the salts detergent (surfactant) properties. Such detergents may be neutral or basic (overbased) . Examples of metal-containing detergents suitable for use as additives in an oil-based composition are the neutral or overbased oil-soluble sulphonates, phenates, sulphurized phenates, thiophosphonates, salicylates, naphthenates and other carboxylates of the alkali or alkaline earth metals or magnesium, for example, sodium, lithium, calcium, barium and magnesium. The most commonly used metals are calcium and magnesium, mixtures of calcium and magnesium, and mixtures of calcium and/or magnesium with sodium. Overbased calcium and magnesium alkyl sulphonates and alkyl phenates are particularly widely used as detergents in oil-based compositions. Sulphonic acids for use in the preparation of oil soluble sulphonate detergents are typically obtained by sulphonation of alkyl-substituted aromatic hydrocarbons, for example, those obtained from the fractionation of petroleum by distillation and/or extraction, or by the alkylation of aromatic hydrocarbons, for example, benzene, toluene, xylene, naphthalene, or biphenyl. Alkylation of aromatic hydrocarbons may be carried out, in the presence of a catalyst, with alkylating agents having from 3 to more than 50 carbon atoms, such as, for example, haloparaffins, olefins that may be obtained by dehydrogenation of paraffins, and polyolefins, for example, polymers of ethylene, propylene, and/or butene. The alkaryl sulphonates usually contain from 9 to 70 or more carbon atoms, preferably from 16 to 50 carbon atoms, per alkyl-substituted aromatic moiety. The metal compounds which may be used in neutralizing these alkaryl sulphonic acids to provide the sulphonates include the oxides, hydroxides and alkoxides, for example, calcium hydroxide or magnesium oxide. Hydrocarbon solvents and/or diluent oils may also be included, as well as promoters and viscosity control agents, for example, formates and halides. The highly basic metal sulphonates are usually produced by neutralizing an alkaryl sulphonic acid with a large excess of metal base over that required for complete neutralization and ■ thereafter forming a dispersed carbonate complex by reacting the excess metal base with carbon dioxide to provide the desired overbasing. Neutral or slightly basic metal sulphonates may be used in place of the alkaryl sulphonic acid. The reaction mixture may include organic solvents, for example, toluene, xylene, hexane, chlorobenzene; other materials, for example, alcohols, water, amines, and/or salts of organic or inorganic acids, which serve to promote the overbasing process; and diluent oil. Volatile materials and undispersed solids are removed in the final stages of the process. Processes which use a metal alkoxide as the starting metal compound can proceed by a somewhat different route in which carbonation-of the alkoxide to give an alkoxide- carbonate complex is followed by hydrolysis of the complex to give the metal carbonate. These reactions may be carried out in the presence of alkaryl sulphonate, solvents and diluents. Highly basic metal sulphonates may have a TBN from 50 to 500, preferably 250 to 450, and contain about 10 to about 35 wt. % alkaryl sulphonate. The following patents provide illustrative examples of sulphonates and/or of processes for making them: EP 0 000 264, EP 0 000 318, EP 0 025 328, EP 0 047 126, EP 0 121 024, EP 0 015 341, EP 0 013 807, EP 0 013 808, EP 0 212 922 Examples of sulphurized alkyl phenols which may be used for preparing sulphurized phenate detergents are phenols of the general structure: where R is an alkyl radical, n is an integer from 0 to 4 and x is an integer rom 1 to 4. All the R groups will normally be the same, but this is not essential. The average number of carbon atoms in all of the R groups is preferably at least about 9 in order to ensure adequate solubility in oil. The individual R groups may contain from 5 to 40, preferably 8 to 20 carbon atoms. Alkylation of phenol may be carried out with, for example, alkylating agents of the types used to alkylate aromatic hydrocarbons in the manufacture of alkaryl sulphonates. Dihydroxybenzenes may be used in place of phenol. Sulphurization may be effected, for example, by reaction of the alkyl phenol with sulphur chloride or by reaction with sulphur. In the latter case, the alkyl phenol is usually present as the metal salt, although other sulphurization promoters may be used, for example, amines. Highly basic metal phenates may be made by methods similar to those used to prepare highly basic metal sulphonates. Highly basic metal phenates may have a TBN from, for example, 100 to 400, preferably about 200 to 350. The following patents provide illustrative examples of phenates and/or of processes for making them: EP 0094 814, GB 1 470 338, GB 1 551 819, GB 2 055 855, GB 2 055 886, GB 1597 482. Highly basic metal salicylates, naphthenates and thio- phosphonates may also be used in oil-based compositions and may be prepared by methods similar to those used to prepare highly basic sulphonates and phenates. Examples of metals with an ionization potential lower than that of magnesium are calcium, sodium, lithium and potassium. Detergents comprising any of these metals may be used in accordance with the invention. Because of their ready accessibility and other desirable properties, calcium- containing detergents are preferred. In accordance with this aspect of the invention, it is not essential that the lubricating compositions be completely free of magnesium-containing detergents. Thus one or more detergents in which the metal has a lower ionization potential than magnesium may be used in admixture with a magnesium- containing detergent. Preferably, however, magnesium- containing detergents are absent. The TBN of a detergent to be used in accordance with this aspect of the invention is advantageously in the range of from 0 to 500, preferably 0 to 300. Calcium-containing detergents with TBNs of 0 to- 50 are particularly preferred for use in accordance with this aspect of the invention. Metal-containing detergents are typically included in crankcase lubricating compositions in proportions in the range of from 0.01 to 6, preferably 0.01 to 4 mass % active ingredient based on the fully formulated composition, to impart a TBN of the order of 10 to the composition. If desired, a lower proportion of detergent, to give a TBN to SASH ratio in accordance the fourth aspect of the invention, may be used. As indicated earlier, a lubricating composition may if desired have two or more of characteristics or components (a) to (e) above. The characteristics or components, or combinations thereof, to be used in any particular case will depend on the performance required for the lubricating composition in question and, in particular, the level of the reduction in particulate emissions required. Additional additives may be incorporated in the compositions to enable them to meet particular requirements. Examples of additives which may be included in lubricating oil compositions are detergents and metal rust inhibitors, viscosity index improvers, corrosion inhibitors, oxidation inhibitors, friction modifiers, dispersants, anti-foaming agents, antiwear agents, and pour point depressants. Specific examples of these additives are given later in this specification. A lubricating composition typically comprises a lubricating oil (base oil), normally in a major proportion, and one or more additives, normally in a minor proportion. In the lubricating compositions with which the invention is concerned, the additive(s) is or are typically dissolved or dispersed in the base oil. The base oil is typically one suitable for use as a crankcase lubricating oil. The compositions of the invention are suitable for use in any type of compression-ignited internal combustion engine, for example, automobile and truck diesel engines, and marine and railroad diesel engines. Synthetic base oils include alkyl esters of dicarboxylic acids, polyglycols and alcohols; poly-α-olefins, including polybutenes; alkyl benzenes; organic esters of phosphoric acids; and polysilicone oils. Natural base oils include mineral lubricating oils which may vary widely as to their crude source, for example, as to whether they are paraffinic, naphthenic, mixed, or paraffinic- naphthenic, as well as to features used in their production, for example, as to the distillation range chosen, and as to whether they are, for example, straight run or cracked, hydrofined or solvent extracted. More specifically, natural lubricating oil base stocks which can be used may be straight mineral lubricating oil or distillates derived from paraffinic, naphthenic, asphaltic, or mixed base crude oils. Alternatively, if desired, various blended oils may be employed as well as residual oils, particularly those from which asphaltic constituents have been removed. The oils may be refined by any suitable method, for example, using acid, alkali, and/or clay or other agents such, for example, as aluminium chloride, or they may be extracted oils produced, for example, by solvent extraction with solvents, for example, phenol, sulphur dioxide, furfural, dichlorodiethyl ether, nitrobenzene, ' or crotonaldehyde. The lubricating oil basestock conveniently has a viscosity of about 2.5 to about 12 cSt (about 2.5 x 10 -6 to about 12 x 10 -6 m 2 /s) and preferably about 2.5 to about 9 cSt. (about 2.5 x 10 -6 to about 9 x 10" 6 m 2 /s) at 100°C. Mixtures of synthetic and natural base oils may be used if desired. Additives used in accordance with the invention are oil- soluble or (in common with certain of the other additives referred to below) are dissolvable in oil with the aid of a suitable solvent, or are stably dispersible materials. Oil- soluble, dissolvable, or stably dispersible as that terminology is used herein does not necessarily indicate that the materials are soluble, dissolvable, miscible, or capable of being suspended in oil in all proportions. It does mean, however, that the materials are, for instance, soluble or stably dispersible in oil to an extent sufficient to exert their intended effect in the environment in which the oil is employed. Moreover, the additional incorporation of other additives may also permit incorporation of higher levels of a particular additive, if desired. Additives can be incorporated into a base oil in any convenient way. Thus, they can be added directly to the oil by dispersing or by dissolving them in the oil at the desired level of concentration. Such blending can occur at room temperature or at an elevated temperature. Detergents and metal rust inhibitors include those mentioned above in connection with the fifth aspect of the invention, and also magnesium-containing detergents. Viscosity index improvers (or viscosity modifiers) impart high and low temperature operability to a lubricating oil and permit it to remain shear stable at elevated temperatures and also exhibit acceptable viscosity or " fluidity at low temperatures. Suitable compounds for use as viscosity modifiers are generally high molecular weight hydrocarbon polymers, including polyesters, and viscosity index improver dispersants, which function as dispersants as well as viscosity index improvers. Oil soluble viscosity modifying polymers generally have weight average molecular weights of from about 10,000 to 1,000,000, preferably 20,000 to 500,000, as determined by gel permeation chromatography or light scattering methods. Suitable viscosity modifiers include, among others, those described above in connection with the first aspect of the invention. Representative examples of suitable viscosity modifiers are polyisobutylene, copolymers of ethylene and propylene, polymethacrylates, ethacrylate copolymers, copolymers of an unsaturated dicarboxylic acid and a vinyl compound, interpolymers of styrene and acrylic esters, and partially hydrogenated copolymers of styrene/ isoprene, styrene/butadiene, and isoprene/butadiene, as well as the partially hydrogenated homopolymers of butadiene and isoprene. Corrosion inhibitors, also known as anti-corrosive agents, v reduce the degradation of metallic parts contacted by the lubricating oil composition. Illustrative of corrosion inhibitors are phospho-sulphurized hydrocarbons and the products obtained by reaction of a phosphosulphurized hydrocarbon with an alkaline earth metal oxide or hydroxide, preferably in the presence of an alkylated phenol, and also preferably in the presence of carbon dioxide. Phosphosulphurized hydrocarbons may be prepared by reacting a suitable hydrocarbon, for example, a terpene or a heavy petroleum fraction or a C 2 to Cς olefin polymer such, for example, as polyisobutylene, with from 5 to 30 mass % of a sulphide of phosphorus for 1/2 to 15 hours, at a temperature in the range of about 65 to about 315°C. Neutralization of the phosphosulphurized hydrocarbon may be effected in any suitable manner, for example, in the manner taught in U.S. Patent No. 1,969,324. Oxidation inhibitors, or antioxidants, reduce the tendency of mineral oils to deteriorate in service, evidence of such deterioration being, for example, the production of varnish¬ like deposits on metal surfaces and of sludge, and viscosity growth. Suitable oxidation inhibitors include ZDDPs, aromatic amines, for example alkylated phenylamines and phenyl α- napthylamine, hindered phenols, alkaline earth metal salts of sulphurized alkyl-phenols having preferably C5 to C3.2 alkyl side chains, e.g., calcium nonylphenyl sulphide; barium octylphenyl sulphide; and phosphosulphurized or sulphurized hydrocarbons . Other oxidation inhibitors or antioxidants which may be used in lubricating oil compositions comprise oil-soluble copper compounds. The copper may be blended into the oil as any suitable oil-soluble copper compound. By oil-soluble it is meant that the compound is oil-soluble under normal blending conditions in the oil or additive package. The copper may be in the cuprous or cupric form. The copper may, for example, be in the form of a copper dihydrocarbyl thio- or dithio- phosphate. Alternatively, the copper may be added as the copper salt of a synthetic or natural carboxylic acid. Examples of suitable acids include CQ to Ciβ fatty acids, such, for example, as stearic or palmitic acid, but unsaturated acids such, for example, as oleic acid or branched carboxylic acids such, for example, as naphthenic acids of molecular weights of from about 200 to 500, or synthetic carboxylic acids, are preferred, because of the improved handling and solubility properties of the resulting copper carboxylates . Also useful are oil-soluble copper dithiocarbamates of the general formula R c R (NCSS) 2 Cu, where z is 1 or 2, and R c and R d are the same or different hydrocarbyl radicals containing from 1 to 18, and preferably 2 to 12, carbon atoms, and including radicals such, for example, as alkyl, alkenyl, aryl, aralkyl, alkaryl and cycloaliphatic radicals. Particularly preferred as R c and R d groups are alkyl groups of from 2 to 8 carbon atoms. Thus, the radicals may, for example, be ethyl, n-propyl, i-propyl, n-butyl, i-butyl, . S ________butyl, amyl, n-hexyl, i-hexyl, n-heptyl, n-octyl, decyl, dodecyl, octadecyl, 2-ethylhexyl, phenyl, butylphenyl, cyclohexyl, methylcyclopentyl, propenyl, or butenyl radicals. In order to obtain oil solubility, the total number of carbon atoms (i.e. the carbon atoms in R c and R ) will generally be about five or greater. Copper sulphonates, phenates, and acetylacetonates may also be used. Examples of useful copper compounds are copper Cu 1 and/or Cu 11 salts derived from an alkenyl succinic acids or anhydride. The salts themselves may be basic, neutral or acidic. They may be formed by reacting (a) polyalkylene succinimides (having polymer groups of M n of 700 to 5,000) derived from polyalkylene-polyamines, which have at least one free carboxylic acid group, with (b) a reactive metal compound. Suitable reactive metal compounds include those such, for example, as cupric or cuprous hydroxides, oxides, acetates, borates, and carbonates or basic copper carbonate. Examples of these metal salts are Cu salts derived from polyisobutenyl succinic anhydride, and Cu salts of polyisobutenyl succinic acid. Preferably, the copper is in its divalent form, Cu 11 . The preferred substrates are polyalkenyl succinic acids in which the alkenyl group has a molecular weight greater than about 700. The alkenyl group desirably has a M n from about 900 to 1,400, and up to 2,500, with a M n of about 950 being most preferred. Especially preferred is polyisobutylene succinic anhydride or acid. These materials may desirably be dissolved in a solvent, for example, a mineral oil, and heated in the presence of a water solution (or slurry) of the metal-bearing material to a temperature of about 70°C to about 200°C. Temperatures of 100°C to 140°C are normally adequate. It may be necessary, depending upon the salt produced, not to allow the reaction mixture to remain at a temperature above about 140°C for an extended period of time, e.g., longer than 5 hours, or decomposition of the salt may occur. The copper antioxidants (e.g., Cu-polyisobutenyl succinate, Cu-oleate, or mixtures thereof) will generally be employed in an amount of from about 5 to 500 ppm by weight of the copper, in the final lubricating composition. Friction modifiers and fuel economy agents which are compatible with the other ingredients of the final oil may also be included. Examples of such materials are glyceryl monoesters of higher fatty acids, for example, glyceryl mono- oleate, esters of long chain polycarboxylic acids with diols, for example, the butane diol ester of a dimerized unsaturated fatty acid, and oxazoline compounds. Dispersants maintain oil-insoluble substances, resulting from oxidation during use, in suspension in the fluid, thus preventing sludge flocculation and precipitation or deposition on metal parts. So-called ashless dispersants are organic materials which form substantially no ash on combustion, in contrast to the metal-containing (and thus ash-forming) detergents described above. Suitable dispersants include, for example, derivatives of long chain hydrocarbon-substituted carboxylic acids in which the hydrocarbon groups contain 50 to 400 carbon atoms, examples of such derivatives being derivatives of high molecular weight hydrocarbyl-substituted succinic acid. Such hydrocarbon-substituted carboxylic acids may be reacted with, for example, a nitrogen-containing compound, advantageously a polyalkylene polyamine, or with an ester. Such nitrogen-containing and ester dispersants are well known in the art. Particularly preferred dispersants are the reaction products of polyalkylene amines with alkenyl succinic anhydrides. In general, suitable dispersants include oil soluble salts, amides, imides, oxazolines and esters, or mixtures thereof, of long chain hydrocarbon-substituted mono and dicarboxylic acids or their anhydrides; long chain aliphatic hydrocarbons having a polyamine attached directly thereto; and Mannich condensation products formed by condensing about 1 molar proportion of a long chain substituted phenol with about 1 to 2.5 moles of formaldehyde and about 0.5 to 2 moles of a polyalkylene polyamine. In these dispersants long chain hydrocarbon groups are suitably derived from polymers of a C 2 to C 5 monoolefin, the polymers having a molecular weight of about 700 to about 5000. As indicated above, a viscosity index improver dispersant functions both as a viscosity index improver and as a dispersant. Examples of viscosity index improver dispersants suitable for use in accordance with the invention include reaction products of amines, for example polyamines, with a hydrosarbyl-substituted mono- or dicarboxylic acid in which the hydrocarbyl substituent comprises a chain of sufficient length to impart viscosity index improving properties to the compounds_ In general, the viscosity index improver dispersant may be, for example, a polymer of a C 4 to C 24 unsaturated ester of vinyl alcohol or a C 3 to C 10 unsaturated mono-carboxylic acid or a C 4 to C 10 di-carboxylic acid with an unsaturated nitrogen-containing monomer having 4 to 20 carbon atoms; a polymer of a C 2 to C 2 0 olefin with an unsaturated C 3 to C 10 mono- or di-carboxylic acid neutralised with an amine, hydroxyamine or an alcohol; or a polymer of ethylene with a C 3 to C 20 olefin further reacted either by grafting a C 4 to C 20 unsaturated nitrogen - containing monomer thereon or by grafting an unsaturated acid onto the polymer backbone and then reacting carboxylic acid groups of the grafted acid with an amine, hydro y amine or alcohol. Examples of dispersants and viscosity index improver dispersants which may be used in accordance with the invention may be found in European Patent Specification No. 24146 B, the disclosure of which is incorporated herein by reference. Antiwear agents include zinc dithiocarbamates and zinc dihydrocarbyl dithiophosphates (ZDDPs), for example, those ZDDPs mentioned earlier in this specification. Pour point depressants, otherwise known as lube oil flow improvers, lower the minimum temperature at which the fluid will flow or can be poured. Such additives are well known. Typical of those additives which improve the low temperature fluidity of the fluid are C to Ciβ dialkyl fumarate/vinyl acetate copolymers, polymethacrylates, and wax naphthalene. Foam control can be provided by an antifoamant of the polysiloxane type, for example, silicone oil or polydimethyl siloxane. Some of the above-mentioned additives can provide a multiplicity of effects; thus for example, a single additive may act as a dispersant-oxidation inhibitor. This approach is well known and need not be further elaborated herein. Compositions when containing the above-mentioned additives are typically blended into the base oil in amounts which are effective to provide their normal function. Representative effective amounts of such additives, if present, are illustrated as follows: Additive Mass a.i.* Mass % a.i. (Broad) (Preferred) Detergents/Rust inhibitors 0.01-6 0.01-4 Viscosity Modifier 0.01- : 6 0.01-4 Corrosion Inhibitor 0.01-5 0.01-1.5 Oxidation Inhibitor 0.01-5 0.01-1.5 Dispersant 0.1-20 0.1-8 Pour Point Depressant 0.01-5 0.01-1.5 Anti-Foaming Agent 0.001-3 0.001-0.15 Anti-wear Agents 0.01-6 0.01-4 Friction Modifier 0.01-5 0.01-1.5 Mineral or Synthetic Balance Balance Base Oil * Mass % active ingredient based on the final oil. When a plurality of additives are employed it may be desirable, although not essential, to prepare additive concentrates comprising the additives (the concentrate being referred to herein as an additive package) whereby several additives can be added simultaneously to the base oil to form the lubricating oil composition. Dissolution of the additive concentrate into the lubricating oil may be facilitated, for example, by mixing accompanied with heating, but this is not essential. The concentrate or additive package will typically be formulated to contain the additive(s) in proper amounts to provide the desired concentration in the final formulation when the additive package is combined with a predetermined amount of base lubricant. Thus, one or more additives can be added to small amounts of base oil or other compatible solvents along with other desirable additives to form additive packages containing active ingredients in an amount, based on the additive package, of, for example, from about 2.5 to about 90 mass %, and preferably from about 5 to about 75 mass %, and most preferably from about 8 to about 50 mass % by weight, additives in the appropriate proportions with the remainder being base oil. The final formulations may employ typically about 10 to 15 mass % of the additive-package with the remainder being base oil. The following Examples illustrate the invention. Example 1 Two XW40 lubricating compositions, A and B, comprising a base oil, a detergent inhibitor formulation, and a viscosity modifier were prepared. Each composition comprised a total of 81 mass % base oil (61% poly-α-olefin and 20% synthetic diester) , 9.7 mass % detergent inhibitor, and 9.3 mass % viscosity modifier. The base oil and detergent inhibitor were the same in each case. In composition A the viscosity modifier was a standard viscosity modifier as defined earlier in this specification, while in composition B a viscosity modifier (an olefin copolymer dissolved in a synthetic basestock) in accordance with the first aspect of the invention was used. As the same base oil and detergent inhibitor, in the same proportions, were used in compositions A and B, any effect that these may have had on the viscosity and volatility of the compositions was the same in each case. The high shear viscosity (by CEC-L-36-T-84) , low shear viscosity (by ASTM D445) and volatility (by CEC-L-40-T-87) of compositions A and B were as follows: 1 cP = 1 mPas 1 cSt = 10 ~6 m 2 /s Each of compositions A and B was used in the crankcase of a 1988 Opel Kadett vehicle fitted with a 1.7 litre diesel engine running over the European ECE 15 emissions test cycle. The particulate and nitrogen oxide emissions are indicated below. Each of the figures given below is the average of the results obtained in four tests. Composition A B Particulate 0.1500 0.1324 Matter (g/km) Nitrogen oxides 0.801 0.749 (g/km) The results show that reducing the high shear viscosity and volatility in accordance with the present invention gave a significant reduction in both the particulate emissions and the nitrogen oxide emissions. Example 2 Two lubricating compositions were prepared using the same viscosity modifier and detergent inhibitor but different base oils. The proportions by mass of the three components in each case were: viscosity modifier -16.0%, detergent inhibitor -11.5%, base oil -72.5%. In one composition (C) , the base oil was a 80:20 mix (by mass) of a poly-α-olefin and a diester, while in the other composition (D) , the diester was replaced by a base oil which gave a reduced ignition delay compared with that given by 150 solvent neutral: when the ignition delay given by 150 solvent neutral was 32 degrees of crankshaft rotation, that given by the diester was 32.2 degrees, while that given by the base oil used for composition D was 27.6 degrees. The high shear viscosity (by CEC-L-36T-84) , low shear viscosity (by ASTM D445) and volatility (by CEC-L-40-T-87) of the supplemental basestocks used for compositions C and D were as follows: Composition High shear 4.31 4.40 viscosity (cp) Low shear 16.5 17.0 viscosity (cSt) Noack volatility 5.5 7.4 (mass %) 1 cP = 1 mPas 1 cSt = 10 -6 m 2 /s Each of compositions C and D was tested over the European Regulation 49 emissions cycle used to assess the emissions from Heavy Duty Truck engines. The engine used was a Mercedes Benz OM 366 LA engine (1990 model year) . The results obtained are given below. In each case the average of two results is given. Composition Particulate 0.4560 0.4225 matter (g/kWh) Nitrogen 9.76 9.92 oxides (g/kWh) Example 3 Two lubricating compositions, each comprising a base oil of lubricating viscosity, a viscosity modifier, and a detergent inhibitor system, were blended. The proportions of the three above-mentioned components were the same in each case. One of the lubricating compositions (E) also contained ZDDP in an amount to give the proportion of zinc indicated below. The other composition (F) contained no ZDDP. The two lubricating compositions were tested over the ECE 15 European passenger car emissions test cycle (with a hot start) using a VW Passat Turbo engine (1990 model year) . The emissions of particulate matter and nitrogen oxides are given below. In each case the average of four results is given. The slight rise in nitrogen oxide emissions found when using composition F was more than offset by the reduction in particulate emissions. Example 4 Two lubricating compositions, each comprising a mineral base oil of lubricating viscosity, a viscosity modifier, a pour point depressant, and a detergent inhibitor, were prepared. The proportions by mass of the four components in each case were: base oil -77.5%, viscosity modifier -10.5%, pour point depressant -0.2%, and detergent inhibitor -11.8%. The compositions differed primarily in the SASH level and the ratio of TBN to SASH level imparted by the detergent inhibitor. One oil (G) had a TBN to ash ratio of 8.95:1, while the other (H) had a much lower ratio of 7.67:1. The two lubricating compositions had the following characteristics: 1 cP — 1 mPas 1 cSt = 10 -6 m 2 /s The two lubricating compositions were tested using the U.S. Heavy Duty transient emissions cycle. The engine used was a Volvo TD 121/2 (1990 model year) modified to incorporate high pressure fuel injection. The emissions of particulate matter and nitrogen oxides are given below. In each case the average of six results is given. Composition G H Particulate 0.155 0.143 matter (g/bhp.h) Nitrogen oxides 4.67 4.56 (g/bhp.h) It will be seen that there was a reduction in both the particulate emissions and in the nitrogen oxide emissions. Example 5 Two lubricating compositions, each comprising a base oil, a viscosity modifier, a pour point depressant, and a detergent system, were prepared. The proportions by mass of the four components were: base oil -79.3%, viscosity modifier -9.0%, pour point depressant -0.2%, detergent system -11.5%. The base oil, viscosity modifier and ZDDP were the same for each of the compositions, as were the proportions of the base oil, the viscosity modifier, ZDDP, and the detergent system. In one composition (J) , the metal present in the greatest proportion (by mass) in the detergent system was magnesium, while in the other composition (K) the predominant metal was calcium. The two compositions were tested over the European Regulation 49 emissions cycle used to assess the emissions from Heavy Duty Truck engines. The engine used was a Mercedes Benz OM 366 LA engine (1990 model year) . Each of the figures given below is the average of the results given in three tests. Composition J K Particulate 0.508 0.455 matter (kWh) Nitrogen 9.35 9.87 oxides (kWh)

Claims: £I__ M£: 1. The use, for reducing the amount of particulate material emitted by a diesel engine, of a lubricating composition comprising a base oil and a viscosity modifier, the viscosity modifier being such that a mixture of the base oil and the viscosity modifier has a high shear viscosity (measured by Test Method CEC-L-36-T-84), and a volatility (measured by Test Method CEC-L-40-T-87) , lower than that of a mixture, in the same proportions by mass, of the same base oil and a standard viscosity modifier (as hereinbefore defined) . 2. An invention as claimed in claim 1, wherein the high shear viscosity is less than 4 cP (4 mPas) . 3. An invention as claimed in claim 1 or claim 2, wherein the lubricating composition has a volatility of less than 12.5 %. 4. An invention as claimed in any one of claims 1 to 3, wherein the lubricating composition has a low shear viscosity, measured by ASTM D445, of at least 10 cSt (10 -5 m 2 /s) . 5. An invention as claimed in any one of claims 1 to 4, wherein the viscosity modifier comprises a solution in a base oil of a polymer having a shear stability index of at least 25. 6. An invention as claimed in 5, wherein the polymer has a shear stability index in the range of from 30 to 40. 7. An invention as claimed in any one of claims 1 to 4, wherein the viscosity modifier comprises a solution in a synthetic base oil of a polymer as specified in any one of claims 5 or 6, and the proportion by mass of the polymer to the synthetic base oil is in the range of from 5:95 to 15:85. 8. The use, for reducing the amount of particulate material emitted by a diesel engine, of an XW40 viscosity grade lubricating composition having a high shear viscosity (measured by Test Method CEC-L-36-T-84) of less than 4.5 cP (4.5 mPas) and a volatility (measured by Test Method CEC-L-40-T-87) of less than 15%. 9. The use, for reducing, the amount of particulate material emitted by a diesel engine, of an XW30 viscosity grade lubricating composition having a high shear viscosity (measured by Test Method CEC-L-36-T-84) of less than 4 cP (4 mPas) and a volatility (measured by Test Method CEC-L- 40-T-87) of less than 12.5%. 10. A viscosity modifier additive suitable for use in a lubricating composition for diesel engines which comprises a solution, in a synthetic base oil, of a polymer having a shear stability index of at least 30. 11. The use, for reducing the amount of particulate material emitted by a diesel engine, of a lubricating composition which comprises a base oil and one or more additives, the base oil giving a reduced ignition delay (measured as hereinbefore defined) compared with that given by 150 solvent neutral. 12. An invention as claimed in claim 11, wherein the base oil comprises a mixture of poly-1-butene and brightstock. 13. An invention as claimed in claim 12, wherein the ratio of poly-n-butene to brightstock is in the range of from 30:70 to 70:30 parts by mass. 14. A base oil suitable for use in a lubricating composition which comprises a mixture of poly-1-butene and brightstock. 15. The use, for reducing the amount of particulate material emitted by a diesel engine, of a lubricating composition which comprises a base oil and one or more lubricating oil additives, the composition being free from zinc compounds or containing not more than 0.06 mass % of Zn on an active ingredient basis, based on the total mass of the composition. 16. An invention as claimed in claim 15, wherein the composition is free or substantially free from zinc compounds and comprises 5 to 500 parts per million by mass (ppm) of added copper present in oil-soluble form, one or more added oil-soluble sulphur-containing compounds, and a bearing corrosion inhibitor. 17. The use, for reducing the amount of particulate material emitted by a diesel engine, of a lubricating composition which comprises a base oil and one or more lubricating oil additives, the composition having a total sulphated ash (SASH) level, as measured by ASTM D874, of zero or, if one or more ash-producing additives are present, having a SASH level of not more than 0.8 mass % and a ratio of Total Base Number (TBN) , measured by AS * TM D2896, to SASH level, as measured by ASTM D874, of less than 8.5. 18. An invention as claimed in claim 17, wherein the ratio of TBN to the SASH level is less than 5. 19. The use, for reducing the amount of particulate material emitted by a diesel engine, of a lubricating composition which comprises a base oil and at least one metal- containing detergent, suitable for use in a lubricating composition, in which the metal has a first ionization potential lower than that of magnesium. 20. An invention as claimed in claim 19, wherein the metal- containing detergent comprises calcium phenate and/or calcium sulphonate. 21. The use, for reducing the amount of particulate material emitted by a diesel engine, of a lubricating composition having two or more of the characteristics or components specified in claims 1, 8, 9, 11, 15, 17 and 19. 22. The use, for reducing the amount of particulate material emitted by a diesel engine, of a lubricating composition having two or more of the characteristics or components specified in claims 1 to 20. 23. A crankcase lubricating composition suitable for use in diesel engines which comprises at least two of the characteristics or components specified in claims 1, 8, 9, 11, 15, 17 and 19. 2 . A process for reducing the amount of particulate material emitted by a diesel engine, in which the engine is lubricated by a lubricating composition comprising a base oil and a viscosity modifier, the viscosity modifier being such that a mixture of the base oil and the viscosity modifier has a high shear viscosity (measured by Test Method CEC-L-36-T-84) , and a volatility (measured by Test Method CEC-L-40-T-87) , lower than that of a mixture, in the same proportions by mass, of the same base oil and a standard viscosity modifier (as hereinbefore defined) . 25. An invention as claimed in claim 24, wherein the high shear viscosity is less than 4 cP (4 mPas) . 26. An invention as claimed in claim 24 or claim 25, wherein the lubricating composition has a volatility of less than 12.5 %. 27. An invention as claimed in any one of claims 24 to 26, wherein the lubricating composition has a low shear viscosity, measured by ASTM D445, of at least 10 cSt (10 -5 m 2 /s) . 28. An invention as claimed in any one of claims 24 to 27, wherein the viscosity modifier comprises a solution in a base oil of a polymer having a shear stability index of at least 25. 29. An invention as claimed in 28, wherein the polymer has a shear stability index in the range of from 30 to 40. 30. An invention as claimed in any one of claims 24 to 27, wherein the viscosity modifier comprises a solution in a synthetic base oil of a polymer as specified in claim 28 or 29, and the proportion by mass of the polymer to the synthetic base oil is in the range of from 5:95 to 15:85. 31. A process for reducing the amount of particulate material emitted by a diesel engine, in which the engine is lubricated by an XW40 viscosity grade lubricating composition having a high shear viscosity (measured by Test Method CEC-L-36-T-84) of less than 4.5 cP (4.5 mPas) and a volatility (measured by Test Method CEC-L-40-T-87) of less than 15%. 32. A process for reducing the amount of particulate material emitted by a diesel engine, in which the engine is lubricated by an XW30 viscosity grade lubricating composition having a high shear viscosity (measured by Test Method CEC-L-36-T-84) of less than 4 cP (4 mPas) and a volatility (measured by Test Method CEC-L-40-T-87) of less than 12.5%. 33. A process for reducing the amount of particulate material emitted by a diesel engine, in which the engine is lubricated by a lubricating composition which comprises a base oil and one or more additives, the base oil giving a reduced ignition delay (measured as hereinbefore defined) compared with that given by 150 solvent neutral. 34. An invention as claimed in claim 33, wherein the base oil comprises a mixture of poly-1-butene and brightstock. 35. An invention as claimed in claim 34, wherein the ratio of poly-n-butene to brightstock is in the range of from 30:70 to 70:30 parts by mass. 36. A process for reducing the amount of particulate material emitted by a diesel engine, in which the engine is lubricated by a lubricating composition which comprises a base oil and one or more lubricating oil additives, the composition being free from zinc compounds or containing not more than 0.06 mass % of Zn on an active ingredient basis, based on the total mass of the composition. 37. An invention as claimed in claim 36, wherein the composition is free or substantially free from zinc compounds and comprises 5 to 500 parts per million by mass (ppm) of added copper present in oil-soluble form, one or more added oil-soluble sulphur-containing compounds, and a bearing corrosion inhibitor. 38. A process for reducing the amount of particulate material emitted by a diesel engine, in which the engine is lubricated by a lubricating composition which comprises a base oil and one or more lubricating oil additives, the composition having a total sulphated ash (SASH) level, as measured by ASTM D874, of zero or, if one or more ash- producing additives are present, having a SASH level of not more than 0.8 mass % and a ratio of Total Base Number (TBN) , measured by ASTM D2896, to SASH level, as measured by ASTM D874, of less than 8.5. 39. An invention as claimed in claim 38, wherein the ratio of TBN to the SASH level is less than 5. 40. A process for reducing the amount of particulate material emitted by a diesel engine, in which the engine is lubricated by a lubricating composition which comprises a base oil and at least one metal-containing detergent, suitable for use in a lubricating composition, in which the metal has a first ionization potential lower than that of magnesium. 41. An invention as claimed in claim 40, wherein the metal- containing detergent comprises calcium phenate and/or calcium sulphonate. 42. A process for reducing the amount of particulate material emitted by a diesel engine, in which the engine is lubricated by a lubricating composition having two or more of the characteristics or components specified in claims 24, 31, 32, 33, 36, 38 and 40. 43. A process for reducing the amount of particulate material emitted by a diesel engine, in which the engine is lubricated by a lubricating composition having two or more of the characteristics or components specified in claims 24 to 40.

Inventors: DOWLING MICHAEL, DOWLING, MICHAEL
Applicants: EXXON CHEMICAL PATENTS INC, EXXON CHEMICAL PATENTS INC.
Application year: 1992
Main classification: C10M177
IPC codes: C10M167, C10M169, C10M177, F02B3