EP2288676A2 - Fuel compositions - Google Patents

Abstract

Use of a Fischer-Tropsch derived fuel component in a fuel composition, to reduce the rate of accumulation of acidic components in a lubricant fluid present in an internal combustion engine which is running, or is intended to be run, on the fuel composition. The Fischer- Tropsch derived fuel component may also be used to reduce the oxidation and/or nitration rate in the lubricant fluid, to reduce the rate of acid-induced engine wear in the engine and/or to reduce the frequency of lubricant fluid changes in the engine. The fuel composition is preferably a diesel fuel composition.

Description

FUEL COMPOSITIONS

The present invention relates to the use of certain types of fuel component in fuel compositions, for the purpose of reducing acid accumulation rates in engine lubricants . It is known to use lubricant fluids in engines, to reduce friction between, and hence wear on, moving parts.

During use of an engine, however, the properties of a lubricant can gradually deteriorate over time, to a point where its performance is impaired and it has to be replaced. Much of this lubricant degradation is due to contaminants that pass from the combustion chamber into the sump. For example, soot can accumulate in the lubricant, and as it does so it increases the viscosity of the fluid until it can no longer function effectively. Soot particles can also agglomerate and become abrasive, again contributing to engine wear.

A lubricant can also deteriorate due to oxidation. This results in an increase in its carboxylic acid content. In addition, nitration of the lubricant can occur as nitrogen oxides from the engine become dissolved in it, typically as nitrates and nitric and nitrous acids - this too results in an overall increase in acidity. The acidic components tend to be corrosive, and can therefore cause further engine wear. Although lubricants are typically formulated with a base "reservoir'^'' so as to neutralise acidic components which accumulate in them during use, too rapid an increase in acid content, or equally too rapid a reduction in base content, is clearly undesirable due to its potential impact on engine corrosion. Moreover, both oxidation and nitration can result in the formation of sludge precursor products. Oil oxidation gives rise to highly viscous agglomerated hydrocarbons which can adhere as a "sludge" to the internal engine surfaces, and can settle in areas of low oil flow rate. This reduces the effective oil volume and as a result increases wear. Thus, the accumulation of sludge in a lubricant can also impair its performance and shorten the interval between lubricant changes. The draining and replacement of an engine lubricant can be costly and time consuming. It would, therefore, be desirable to be able to reduce the rate of lubricant deterioration and hence to increase the interval between lubricant changes. It would also be desirable to be able to achieve these improvements without the need to alter the engine itself, and irrespective of the nature of the lubricant fluid.

According to a first aspect of the present invention there is provided the use of a Fischer-Tropsch derived fuel component in an internal combustion engine, for the purpose of reducing the rate of accumulation of acidic components in a lubricant fluid which is present in the engine whilst it is running either on the fuel component or on a fuel composition containing the component. According to a second aspect, the present invention provides the use of a Fischer-Tropsch derived fuel component in a fuel composition, for the purpose of reducing the rate of accumulation of acidic components in a lubricant fluid present in an internal combustion engine which is running, or is intended to be run, on the fuel composition.

The acidic components may in particular be derived from oxidation and/or nitration of the lubricant fluid. They may, for example, be selected from carboxylic acids, nitric and nitrous acids, and mixtures thereof. The "rate of accumulation" of such components is the rate of change (increase) in their concentration in the lubricant fluid. In cases the present invention is used to reduce the rate of accumulation of acidic components other than those present in soot, for example soot formed due to incomplete combustion.

Thus, according to a third aspect of the present invention there is provided the use of a Fischer-Tropsch derived fuel component in an internal combustion engine, for the purpose of reducing the rate of oxidation and/or nitration of a lubricant fluid which is present in the engine whilst it is running on the fuel composition. According to a fourth aspect, the present invention provides the use of a Fischer-Tropsch derived fuel component in a fuel composition, for the purpose of reducing the rate of oxidation and/or nitration of a lubricant fluid present in an internal combustion engine which is running, or is intended to be run, on the fuel composition.

In the present context, an internal combustion engine may be, for example, a spark ignition ("petrol") engine or a compression ignition ("diesel") engine. In an embodiment of the present invention, the engine is a diesel engine.

The fuel composition may be any fuel composition which is suitable for use in an internal combustion engine. It may, for example, be a gasoline (petrol) fuel composition or a diesel fuel composition, In addition to the Fischer-Tropsch derived fuel component, the fuel composition may contain one or more (typically diesel) base fuels, and/or one or more non-Fischer-Tropsch derived fuel components.

The Fischer-Tropsch derived fuel component may be used at a concentration, based on the overall fuel composition, of 1 %v/v or greater, or 2 or 5 or 10 %v/v or greater, for example 50 %v/v or greater or 80 or 90 %v/v or greater. Its concentration in the overall composition may be up to 100 %v/v, for example up to 99.8 %v/v or up to 99.5 %v/v or 99 %v/v. In an embodiment of the present invention, the fuel composition consists primarily of the Fischer-Tropsch derived fuel component; it may, for example, contain 98 %v/v or greater of the Fischer-Tropsch derived fuel component, or 99 %v/v or greater, or 99.5 or 99.8 %v/v or greater, optionally together with one or more fuel additives.

Thus, the present invention also embraces the use of a Fischer-Tropsch derived fuel component as a fuel composition in an internal combustion engine, for the purpose of reducing the rate of accumulation of acidic components in, and/or the rate of oxidation or nitration of, a lubricant fluid which is present in the engine whilst it is running on the fuel composition (i.e. on the Fischer-Tropsch derived fuel component} . In an embodiment of the present invention, the fuel composition in which the Fischer-Tropsch derived component is used is a diesel fuel composition which is suitable and/or adapted and/or intended for use in a diesel engine. It may, therefore, comprise one or more diesel base fuels, typically gas oils, which may be non- Fischer Tropsch derived (for example petroleum derived "distillate") fuels. It may, for example, be an automotive diesel fuel composition, for use in powering an automotive vehicle.

A diesel fuel composition prepared according to the present invention may contain, in addition to the Fischer-Tropsch derived fuel component, one or more diesel fuel components of conventional type. Such components will typically comprise liquid hydrocarbon middle distillate fuel oil(s), for instance petroleum derived gas oils. In general, such fuel components may be organically or synthetically derived, and are suitably obtained by distillation of a desired range of fractions from a crude oil. They will typically have boiling points within the usual diesel range of 150 to 410⁰C or 170 to 370⁰C, depending on grade and use. Typically, the fuel composition will include one or more cracked products, obtained by splitting heavy hydrocarbons.

A petroleum derived gas oil may, for instance, be obtained by refining and optionally (hydro) processing a crude petroleum source. It may be a single gas oil stream obtained from such a refinery process or a blend of several gas oil fractions obtained in the refinery process via different processing routes. Examples of such gas oil fractions are straight run gas oil, vacuum gas oil, gas oil as obtained in a thermal cracking process, light and heavy cycle oils as obtained in a fluid catalytic cracking unit and gas oil as obtained from a hydrocracker unit. Optionally, a petroleum derived gas oil may comprise some petroleum derived kerosene fraction. Such gas oils may be processed in a hydrodesulphurisation (HDS) unit so as to reduce their sulphur content to a level suitable for inclusion in a diesel fuel composition. Diesel fuel components contained in a composition prepared according to the present invention will typically have a density of from 750 to 900 kg/rn³, preferably from 800 to 860 kg/m³, at 15°C (ASTM D-4052 or EN ISO 3675) and/or a kinematic viscosity at 40⁰C (VK 40) of from 1.5 to 6.0 centistokes (mm^/s) (ASTM D-445 or EN ISO 3104) .

References in this specification to viscosity are, unless otherwise specified, intended to mean kinematic viscosity.

In a diesel fuel composition prepared according to the present invention, a base fuel may itself comprise a mixture of two or more diesel fuel components of the types described above. It may be or contain a so-called "biodiesel" fuel component such as a vegetable oil, hydrogenated vegetable oil or vegetable oil derivative (e.g. a fatty acid ester, in particular a fatty acid methyl ester) or another oxygenate such as an acid, ketone or ester. Such components need not necessarily be bio-derived.

A fuel composition prepared according to the present invention, in particular a diesel fuel and more particularly an automotive diesel fuel, suitably contains no more than 5000 ppmw (parts per million by weight) of sulphur, typically from 2000 to 5000 ppmw, or from 1000 to 2000 ppmw, or alternatively up to 1000 ppmw. The composition may, for example, be a low or ultra low sulphur fuel, or a sulphur free fuel, for instance containing at most 500 ppmw, preferably no more than 350 ppmw, most preferably no more than 100 or 50 or even 10 ppmw, of sulphur.

In the context of the present invention, the term "Fischer-Tropsch derived" means that a material is, or derives from, a synthesis product of a Fischer-ϊropsch condensation process. The term "non-Fischer-Tropsch derived" may be interpreted accordingly. A Fischer™Tropsch derived fuel or fuel component will therefore be a hydrocarbon stream in which a substantial portion, except for added hydrogen, is derived directly or indirectly from a Fischer-Tropsch condensation process .

A Fischer-Tropsch derived product may also be referred to as a GTL product.

The Fischer-Tropsch reaction converts carbon monoxide and hydrogen into longer chain, usually paraffiniC_f hydrocarbons: n{CO + 2H₂ ) = (-CH2 -)_n + nH₂O + heat, in the presence of an appropriate catalyst and typically at elevated temperatures (e.g. 125 to 300⁰C, preferably 175 to 250⁰C) and/or pressures (e.g. 0.5 to 10 MPa, preferably 1.2 to 5 MPa) . Hydrogen: carbon monoxide ratios other than 2:1 may be employed if desired. The carbon monoxide and hydrogen may themselves be derived from organic, inorganic, natural or synthetic sources, typically either from natural gas or from organically derived methane.

A Fischer-Tropsch derived fuel component of use in the present invention may be obtained directly from the refining or the Fischer-Tropsch reaction, or indirectly, for instance by fractionation or hydrotreating of the refining or synthesis product to give a fractionated or hydrotreated product. Hydrotreatment can involve hydrocracking to adjust the boiling range (see eg,

GB-B-2077289 and EP-A-0147873) and/or hydroisomerisation which can improve cold flow properties by increasing the proportion of branched paraffins. EP-A-0583836 describes a two-step hydrotreatment process in which a Fischer-Tropsch synthesis product is firstly subjected to hydroconversion under conditions such that it undergoes substantially no isomerisation or hydrocracking (this hydrogenates the olefinic and oxygen-containing components) , and then at least part of the resultant product is hydroconverted under conditions such that hydrocracking and isomerisation occur to yield a substantially paraffinic hydrocarbon fuel. The desired fraction(s), typically gas oil fraction (s), may subsequently be isolated for instance by distillation.

Other post-synthesis treatments, such as polymerisation, alkylation, distillation, cracking- decarboxylation, isomerisation and hydroreforming, may be employed to modify the properties of Fischer-Tropsch condensation products, as described for instance in US-A-4125566 and US-A-4478955.

Typical catalysts for the Fischer-Tropsch synthesis of paraffinic hydrocarbons comprise, as the catalytically active component, a metal from Group VIII of the periodic table of the elements, in particular ruthenium, iron, cobalt or nickel. Suitable such catalysts are described for instance in EP-A-0583836.

An example of a Fischer-Tropsch based process is the Shell™ "Gas-to-liquids" or "GTL" technology (formerly known as the SMDS (Shell Middle Distillate Synthesis) and described in "The Shell Middle Distillate Synthesis Process", van der Burgt et al, paper delivered at the 5th Synfuels Worldwide Symposium, Washington DC, November 1985, and in the November 1989 publication of the same title from Shell International Petroleum Company Ltd, London, UK) . In the latter case, preferred features of the hydroconversion process may be as disclosed therein. This process produces middle distillate range products by conversion of a natural gas into a heavy long chain hydrocarbon (paraffin) wax which can then be hydroconverted and fractionated. By virtue of the Fischer-Tropsch process, a

Fischer-Tropsch derived fuel or fuel component has essentially no, or undetectable levels of, sulphur and nitrogen. Compounds containing these heteroatoms tend to act as poisons for Fischer-Tropsch catalysts and are therefore removed from the synthesis gas feed. This can bring additional benefits to fuel compositions prepared in accordance with the present invention.

Generally speaking, Fischer-Tropsch derived hydrocarbon products have relatively low levels of polar components, in particular polar surfactants, for instance compared to petroleum derived fuels. This may contribute to improved antifoaming and dehazing performance. Such polar components may include, for example, oxygenates, and sulphur and nitrogen containing compounds. A low level of sulphur in a Fischer-Tropsch derived product is generally indicative of low levels of both oxygenates and nitrogen containing compounds, since all are removed by the same treatment processes.

Further, the Fischer-Tropsch process as usually operated produces no or virtually no aromatic components.

For use in the present invention, a Fischer-Tropsch derived fuel component may be any suitable component derived from a gas to liquid synthesis (hereinafter a GTL component) , or a component derived from an analogous Fischer-Tropsch synthesis, for instance converting gas, biomass or coal to liquid (hereinafter an XTL component} . A Fischer-Tropsch derived fuel component is preferably a GTL fuel component. It may be a BTL (biomass to liquid) component. In general, a suitable XTL component may be a middle distillate fuel component, for instance selected from naphtha, kerosene, diesel and gas oil fractions as known in the art; such components may be generically classed as synthetic process fuels or synthetic process oils. Preferably an XTL component for use in the present invention is a gas oil.

A Fischer-Tropsch derived gas oil will usually contain a majority (for instance 95 %v/v or greater) of components having boiling points within the typical diesel fuel ("gas oil") range, i.e. from about 150 to 400 "C or from 170 to 370 "C. It will suitably have a 90 %v/v distillation temperature of from 300 to 370 ^*C. A Fischer-Tropsch derived gas oil will typically have a density (ASTM D-4052 or EN ISO 3675) from 0.76 to 0.79 g/cm³ at 15° C; a cetane number (ASTM D-613) greater than 70, suitably from 74 to 85; a VK 40 (ASTM D-445 or EN ISO 3104) from 2 to 4.5, preferably from 2.5 to 4.0, more preferably from 2.9 to 3.7, centistokes (rnm²/s) ; and a sulphur content (ASTM D-2622 or EN ISO 20846) of 5 mg/kg or less, preferably of 2 rαg/kg or less.

In an embodiment, a Fischer-Tropsch derived gas oil used in the present invention is a product prepared by a Fischer-Tropsch methane condensation reaction using a hydrogen/carbon monoxide ratio of less than 2.5, preferably less than 1.75, more preferably from 0.4 to 1.5, and ideally using a cobalt containing catalyst. It may have been obtained from a hydrocracked Fischer-Tropsch synthesis product (for instance as described in GB-B-2077289 and/or EP-A-0147873) , or more preferably a product from a two stage hydroconversion process such as that described in EP-A-0583836 (see above) . In the latter case, preferred features of the hydroconversion process may be as disclosed at pages 4 to 6, and in the examples, of EP~A-058383β.

Suitably, in accordance with the present invention, a Fischer-Tropsch derived gas oil will consist of at least 70 %w/w, preferably at least 80 %w/w, more preferably at least 90 or 95 or 98 %w/w, most preferably at least 99 or 99.5 or even 99.8 %w/w, of paraffinic components. The weight ratio of iso-paraffins to normal paraffins will suitably be greater than 1:1 or 2:1, for example from 1:1 to 10:1 or from 1:1 to 5:1. The actual value for this ratio will be determined, in part, by the hydroconversion process used to prepare the gas oil from the Fischer-Tropsch synthesis product.

The olefin content of the Fischer-Tropsch derived gas oil is suitably 0.5 %w/w or lower. Its aromatics content is suitably 0.5 %w/w or lower.

According to the present invention, a mixture of two or more Fischer-Tropsch derived fuel components may be used in a fuel composition and/or in an internal combustion engine.

In the context of the present invention, "use" of a Fischer-Tropsch derived fuel component in an internal combustion engine means introducing the Fischer-Tropsch derived fuel component into the engine, typically into a combustion chamber. The Fischer-Tropsch derived fuel may be thus introduced as part of a fuel composition containing one or more other fuel components and/or additives. The use may involve running the engine on such a fuel composition, or on the Fischer-Tropsch derived fuel component alone. "Use" of a Fischer-Tropsch derived fuel component in a fuel composition means incorporating the Fischer-Tropsch derived fuel component into the composition, typically as a blend (i.e. a physical mixture} with one or more other fuel components (typically diesel base fuels) and optionally with one or more fuel additives. The Fischer-Iropsch derived fuel component is conveniently incorporated before the composition is introduced into an internal combustion engine which is to be run on the composition. Instead or in addition the use may involve running an internal combustion engine on the fuel composition containing the Fischer-Tropsch derived fuel component, typically by introducing the composition into a combustion chamber of the engine .

"Use" of a Fischer-Tropsch derived fuel component, in accordance with the present invention, may also embrace supplying such a component together with instructions for its use in a fuel composition and/or in an internal combustion engine in order to achieve one or more of the purpose (s) described herein, in particular to reduce the rate of accumulation of acidic components in, and/or the rate of oxidation or nitration of, a lubricant fluid in an internal combustion engine.

The Fischer-Tropsch derived fuel component may itself be supplied as a component of a formulation which is suitable for and/or intended for use as a fuel additive, in particular a diesel fuel additive, in which case the Fischer-Tropsch derived component may be included in such a formulation for the purpose of influencing the effect, of a fuel composition into which it is dosed, on the rate of accumulation of acidic components in a lubricant fluid in an internal combustion engine into which the fuel composition is, or is intended to be, introduced, and/or on the rate of oxidation and/or nitration of such a lubricant fluid. Thus, the Fischer-Tropsch derived fuel component may be incorporated into an additive formulation or package along with one or more fuel additives. It may, for instance, be combined, in an additive formulation, with one or more fuel additives selected from detergents, anti-corrosion additives, esters, poly alpha olefins, long chain organic acids, components containing amine or amide active centres, and mixtures thereof. In particular, it may be combined with one or more so-called performance additives, which will typically include at least a detergent.

The Fischer-Tropsch derived fuel component may be blended directly with one or more other components of the fuel composition, for example at the refinery. A fuel composition prepared according to the present invention, or a base fuel used in such a composition, may contain other components, for example one or more fuel additives.

If additivated (additive-containing), e.g. at the refinery, a fuel composition or component may contain minor amounts of one or more additives selected, for example, from anti-static agents, pipeline drag reducers, flow improvers (e.g. ethylene/vinyl acetate copolymers or acrylate/malelc anhydride copolymers) , lubricity additives, antioxidants and wax anti-settling agents.

Thus, a fuel composition may contain a minor proportion (preferably 1 %w/w or less, more preferably 0.5 %w/w (5000 ppmw) or less and most preferably 0.2 %w/w (2000 ppmw) or less), of one or more fuel additives. A fuel composition prepared according to the present invention may, for example, contain a detergent. Detergent-containing fuel additives are known and commercially available. Such additives may be added to fuels at levels intended to reduce, remove or slow the build up of engine deposits.

Examples of detergents suitable for use in fuel additives for the present purpose include polyolefin substituted succinimides or succinamides of polyamines, for instance polyisobutylene succinimides or polyisobutylene amine succinamides, aliphatic amines, Mannich bases or amines and polyolefin (e.g. polyisobutylene} maleic anhydrides. Succinimide dispersant additives are described for example in

GB-A-960493, EP-A-0147240, EP-A-0482253, EP-A-0613938 , EP-A-0557516 and WO-A-98/42808. Particularly preferred are polyolefin substituted succinimides such as polyisobutylene succinimides. A fuel additive mixture useable in a fuel composition prepared according to the present invention may contain other components in addition to a detergent. Examples - in particular for use in a diesel fuel composition - are lubricity enhancers; dehazers, e.g. alkoxylated phenol formaldehyde polymers; anti-foaming agents (e.g. polyether-modified polysiloxanes} ; ignition improvers (cetane improvers) (e.g. 2-ethylhexyl nitrate (EHN) , cyclohexyl nitrate, di-tert-butyl peroxide and those disclosed in US-A-4208190 at column 2, line 27 to column 3, line 21); anti-rust agents (e.g. a propane-1,2- diol semi-ester of tetrapropenyl succinic acid, or polyhydric alcohol esters of a succinic acid derivative, the succinic acid derivative having on at least one of its alpha-carbon atoms an unsubstituted or substituted aliphatic hydrocarbon group containing from 20 to 500 carbon atoms, e.g. the pentaerythritol diester of polyisobutylene-substituted succinic acid) ; corrosion inhibitors; reodorants; anti-wear additives; anti-oxidants (e.g. phenolics such as 2, 6-di-tert-butylphenol, or phenylenediamines such as N,N'-di-sec-butyl-p-phenylenediamine) ; metal deactivators; combustion improvers; static dissipator additives; cold flow improvers; viscosity improvers; and wax anti-settling agents.

Such a fuel additive mixture may contain a lubricity enhancer, especially when the fuel composition has a low (e.g. 500 ppmw or less) sulphur content. In the additivated fuel composition, the lubricity enhancer is conveniently present at a concentration of less than 1000 ppmw, preferably between 50 and 1000 ppmw, more preferably between 70 and 1000 ppmw. Suitable commercially available lubricity enhancers include ester- and acid-based additives. Other lubricity enhancers are described in the patent literature, in particular in connection with their use in low sulphur content diesel fuels, for example in: the paper by Danping Wei and H. A. Spikes, "The Lubricity of Diesel Fuels", Wear, III (1986) 217-235;

WO-A-95/33805 - cold flow improvers to enhance lubricity of low sulphur fuels;

WO-A-94 /17160 - certain esters of a carboxylic acid and an alcohol wherein the acid has from 2 to 50 carbon atoms and the alcohol has 1 or more carbon atoms, particularly glycerol monooleate and di-isodecyl adipate, as fuel additives for wear reduction in a diesel engine injection system;

US-A-5490864 - certain dithiophosphoric diester™ dialcohols as anti-wear lubricity additives for low sulphur diesel fuels; and

WO-A-98/01516 - certain alkyl aromatic compounds having at least one carboxyl group attached to their aromatic nuclei, to confer anti-wear lubricity effects particularly in low sulphur diesel fuels.

It may also be preferred for the fuel composition to contain an anti-foaming agent, more preferably in combination with an anti-rust agent and/or a corrosion inhibitor and/or a lubricity enhancing additive. Unless otherwise stated, the (active matter) concentration of each such additive component in the additivated fuel composition is preferably up to 10000 ppmw, more preferably in the range of 0.1 to 1000 ppmw, advantageously from 0.1 to 300 ppmw, such as from 0.1 to 150 ppmw .

The (active matter) concentration of any dehazer in the fuel composition will preferably be in the range from 0.1 to 20 ppmw, more preferably from 1 to 15 ppmw, still more preferably from 1 to 10 ppmw, advantageously from 1 to 5 ppmw. The (active matter) concentration of any ignition improver present will preferably be 2600 ppmw or less, more preferably 2000 ppmw or less, conveniently from 300 to 1500 ppmw. The (active matter) concentration of any detergent in the fuel composition will preferably be in the range from 5 to 1500 ppmw, more preferably from 10 to 750 ppmw, most preferably from 20 to 500 ppmw.

If desired one or more additive components, such as those listed above, may be co-mixed - preferably together with suitable diluent (s) - in an additive concentrate, and the additive concentrate may then be dispersed into a base fuel or fuel composition.

In the case of a diesel fuel composition, for example, the fuel additive mixture will typically contain a detergent, optionally together with other components as described above, and a diesel fuel-compatible diluent, which may be a mineral oil, a solvent such as those sold by Shell companies under the trade mark "SHELLSOL", a polar solvent such as an ester and, in particular, an alcohol, e.g. hexanol, 2-ethylhexanol, decanol, isotridecanol and alcohol mixtures such as those sold by Shell companies under the trade mark "LINEVOL", especially LINEVOL 79 alcohol which is a mixture of C7-9 primary alcohols, or a C12-14 alcohol mixture which is commercially available.

The total content of additives in the fuel composition may suitably be between 0 and 10000 ppmw, preferably below 5000 ppmw.

In this specification, amounts (concentrations, %v/v, ppmw, %w/w} of components are of active matter, i.e. exclusive of volatile solvents/diluent materials. In the context of the present invention, the lubricant fluid may be any lubricant fluid, typically an oil, which is suitable and/or adapted and/or intended for use in an internal combustion engine, in particular a diesel engine. Typical lubricant fluids are composed primarily of one or more base oils, which may be selected from any of the synthetic (lubricating) oils, mineral oils, natural oils or mixtures thereof. Mineral oils include liquid petroleum oils and solvent-treated or acid-treated mineral lubricating oils of the paraffinic, naphthenic or mixed paraffinic/naphthenic type, which may be further refined by hydrofinishing processes and/or dewaxing. Synthetic base oils include Fischer-Tropsch derived base oils, as well as olefin oligomers (PAOs) , dibasic acid esters, polyol esters and dewaxed waxy raffinates.

For use in an Internal combustion engine, a base oil will suitably contain less than 1 %wt, preferably less than 0.1 %wt, of sulphur, as determined, for instance, by ASTM D-2622, D-4294, D-4927 or D-3120. It will suitably have a viscosity index of more than 80, preferably of more than 120, as measured according to ASTM D-2270. It may conveniently have a VK 100 of from 3.8 to 26 centistokes (mm²/s) (ASTM D-445) .

A lubricant fluid for use in an internal combustion engine might suitably have a VK 100 of from 2 to 80 centistokes (mm²/s) , preferably from 3 to 70 centistokes (mm²/s) or from 4 to 50 centistokes (mm²/s) . Natural oils suitable for use as base oils include both animal and vegetable oils (e.g. castor or lard oil); liquid petroleum oils; and hydrorefined, solvent-treated or acid-treated mineral lubricating oils of the paraffinic, naphthenic and mixed paraffinic-naphthenic types. Oils of lubricating viscosity derived from coal or shale are also useful base oils.

Alkylene oxide polymers and interpolymers and derivatives thereof, in which the terminal hydroxyl groups have been modified by esterification, etherification, etc, constitute another class of known synthetic lubricating oils. These are exemplified by polyoxyalkylene polymers prepared by polymerisation of ethylene oxide or propylene oxide; the alkyl and aryl ethers of these polyoxyalkylene polymers (e.g. methyl- polyisopropylene glycol ether having an average molecular weight of 1000, diphenyl ether of polyethylene glycol having a molecular weight of 500-1000, diethyl ether of polypropylene glycol having a molecular weight of 1000- 1500} ; and mono-and polycarboxylic esters thereof, for example, the acetic acid esters, mixed C3-C8 fatty acid esters and the C13 oxo acid diester of tetraethylene glycol . Another suitable class of synthetic lubricating oils comprises the esters formed by reacting dicarboxylic acids (e.g. phthalic acid, succinic acid, alkyl succinic acids and alkenyl succinic acids, maleic acid, azelaic acid, suberic acid, sebasic acid, fumaric acid, adip±c acid, linoleic acid dimer, malonic acid, alkylmalonic acids, alkenyl malonic acids) with a variety of alcohols (e.g. butyl alcohol, hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol, ethylene glycol, diethylene glycol monoether, propylene glycol) .

Specific examples of these esters include dibutyl adipate, di (2-ethylhexyl) sebacate, di-n-hexyl fumarate, dioctyl sebacate, diisooctyl azelate, diisodecyl azelate, dioctyl phthalate, didecyl phthalate, dieicosyl sebacate, the 2-ethylhexyl diester of linoleic acid dimer, and the complex ester formed by reacting one mole of sebacic acid with two moles of tetraethylene glycol and two moles of 2- ethylhexanoic acid.

Esters useful as synthetic oils also include those made from C5 to C12 monocarboxylic acids and polyols and polyol ethers such as neopentyl glycol, trimethylolpropane, pentaerythritol, dipentaerythritol and tripentaerythritol.

Silicon-based oils, such as the polyalkyl-, polyaryl-, polyalkoxy, or polyaryloxysiloxane oils and silicate oils, comprise another useful class of synthetic lubricating oils; they include tetraethyl silicate, tetraisopropyl silicate, tetra- (2-ethylhexyl) silicate, tetra- (4-methyl~2-ethyl-hexyl) silicate, tetra- (p- tertbutylphenyl) silicate, hexa- (4-methyl~ 2-pentoxy) disiloxane, poly (methyl) siloxanes and poly (methylphenyl) siloxanes. Other synthetic lubricating oils include liquid esters of phosphorous-containing acids (e.g. tricresyl phosphate, trioctyl phosphate, diethyl ester of decylphosphonic acid) and polymeric tetrahydrofurans .

Lubricant fluids may typically contain additives as known in the art, for example oxidation inhibitors (antioxidants) , dispersants, seal fix or seal compatibility agents, and/or detergents. They may also include other lubricant additives that perform specific functions not provided by the main components. These additional additives include, but are not limited to, corrosion inhibitors, viscosity index improvers (or modifiers) , pour point depressants, zinc dialkyldithiophosphates, anti-wear agents, anti-foam agents, and/or friction modifiers. Suitable additives are described in US-A-5320765 and US-B-6528461. Suitable oxidation inhibitors include, for example, copper antioxidants, phenolic compounds and/or aminic compounds. Suitable dispersants include, for example, succinimides . Suitable detergents include, for example, salicylate, phenate and sulphonate detergents. Suitable anti-wear additives include zinc dithiophosphates.

Examples of lubricating base oils, and of additives for use in lubricant fluids, are described at pages 15 to 23 of WO-A-2007/128740, and also in WO-A-2004/0031X6. In the context of the present invention, "reducing" the rate of accumulation of acidic components in a lubricant fluid embraces any degree of reduction, including to zero (i.e. prevention of increase in concentration of the components) . The reduction may be as compared to the acidic component accumulation rate when using the fuel composition prior to incorporation of the Fischer-Tropsch derived fuel component. It may be as compared to the relevant rate measured when the same engine is run on an otherwise analogous fuel composition which is intended (e.g. marketed) for use in a (typically diesel) internal combustion engine, prior to adding a Fischer-Tropsch derived fuel component to it. The present invention may, for example, involve adjusting the effects of the fuel composition on acidic component accumulation rate, by means of the Fischer- Tropsch derived fuel component, in order to meet a desired target. Similarly, "reducing" the rate of oxidation or nitration in a lubricant fluid embraces any degree of reduction, including to zero (i.e. prevention of the relevant process) . The reduction may be as compared to the oxidation or nitration rate when using the fuel composition prior to incorporation of the Fischer-Tropsch derived fuel component. It may be as compared to the relevant rate measured when the same engine is run on an otherwise analogous fuel composition which is intended {e.g. marketed) for use in a (typically diesel) internal combustion engine, prior to adding a Fischer-Tropsch derived fuel component to it.

The present invention may, for example, involve adjusting the effects of the fuel composition on lubricant fluid oxidation and/or nitration rate, by means of the Fischer-Tropsch derived fuel component, in order to meet a desired target.

A reduction in the acidic component accumulation rate, or on the oxidation or nitration rate, of a lubricant fluid may also embrace mitigation, to at least a degree, of an increase in the relevant rate due to another cause (i.e. a cause other than the nature of the engine fuel) . The acidic component accumulation rate, or the oxidation or nitration rate, may be measured over a predetermined time period (i.e. engine running time), in particular a period which begins at the time of introduction of the (previously unused) lubricant fluid into the engine. The relevant rate may, for example, be measured over a period of 100 hours or more of engine running time, or 200 hours or more, or 250 hours or more, for example 300 or 400 or 500 hours or more, following the introduction of the lubricant fluid into the engine.

Alternatively the relevant rate may be measured over a predetermined engine running distance, in particular beginning at the time of introduction of the (previously unused) lubricant fluid into the engine. The rate may for example be measured over 5000 engine miles or more, or

8000 engine miles or more, or 10000 engine miles or more, or 13000 or 15000 engine miles or more, following the introduction of the lubricant fluid into the engine.

The relevant rate may accordingly be expressed as a change per unit engine running time or as a change per unit engine running distance.

The concentration of acidic components in a lubricant fluid may be assessed by measuring its acid number, for instance using the standard test method ASTM D-664 ("Standard test method for acid number of petroleum products by potentiometric titration") . From two or more such measurements, the rate of accumulation of acidic components over a particular period of time can be calculated. The base number of a lubricant fluid, and in turn the rate of decrease in its base number, can also provide an indication of the accumulation of acidic components in the fluid. Base number may be measured using the standard test method ASTM D-4739 ("Standard test method for base number determination by potentiometrie titration") or ASTM D-2896 ("Standard test method for base number of petroleum products by potentiometric perchloric acid titration") .

The present invention may, therefore, be used to reduce the rate of increase in the acid number of the lubricant fluid, and/or the rate of decrease in its base number . The degree of oxidation in a lubricant fluid may be measured by Fourier Transform Infra Red (FTIR) spectrometry, for example using the standard test method ASTM E-2412 ("Standard practice for condition monitoring of used lubricants by trend analysis using FTIR spectrometry") . Again, oxidation rate can be calculated from two or more such measurements over a particular time period.

The degree and/or rate of nitration in a lubricant fluid may also be measured by FTIR spectrometry, for example using ASTiVi E-2412.

Where the present invention is used to reduce the rate of accumulation of acidic components in the lubricant fluid, the reduction may be of at least 5 %, preferably of at least 10 %, for example of at least 15 or 20 or 25 or 30 or in cases even 35 or 40 %, compared to the rate observed when running the engine on the fuel composition prior to incorporation of the Fischer-Tropsch derived fuel component. The reduction may be as compared to the rate of accumulation of acidic components in the same lubricant fluid when the same engine is run on an otherwise analogous fuel composition which is intended (e.g. marketed) for use in a (typically diesel) internal combustion engine, prior to adding a Fischer-Tropsch derived fuel component to it.

Where the present invention is used to reduce the rate of decrease in base number of the lubricant fluid, the reduction may be of at least 0.5 or 1 %, preferably of at least 2 %, for example of at least 3 or 4 or 5 %, compared to that observed when running the engine on the fuel composition prior to incorporation of the Fischer- Tropsch derived fuel component. The reduction may be as compared to the rate of decrease in base number for the same lubricant fluid when the same engine is run on an otherwise analogous fuel composition which is intended (e.g. marketed) for use in a (typically diesel} internal combustion engine, prior to adding a Fischer-Tropsch derived fuel component to it.

Where the present invention is used to reduce the rate of oxidation in the lubricant fluid, the reduction may be of at least 5 %, preferably at least 10 or 15 or 20 % or in cases at least 25 or even 30 %, compared to the rate observed when running the engine on the fuel composition prior to incorporation of the Fischer-Tropsch derived fuel component. The reduction may be as compared to the rate of oxidation in the same lubricant fluid when the same engine is run on an otherwise analogous fuel composition which is intended (e.g. marketed) for use in a {typically diesel) internal combustion engine, prior to adding a Fischer-Tropsch derived fuel component to it. Where the present invention is used to reduce the rate of nitration in the lubricant fluid, the reduction may be of at least 5 %, preferably of at least 10 or 12 or 15 % or in cases at least 17 or 18 %, compared to the rate observed when running the engine on the fuel composition prior to incorporation of the Fischer-Tropsch derived fuel component. The reduction may be as compared to the rate of nitration in the same lubricant fluid when the same engine is run on an otherwise analogous fuel composition which is intended (e.g. marketed) for use in a (typically diesel) internal combustion engine, prior to adding a Fischer-Tropsch derived fuel component to it.

The present invention may further be used to reduce the rate of sulphation in the lubricant fluid, in which case the reduction may be of at least 5 or 10 %, preferably of at least 20 or 30 % or in cases at least 35 or even 40 %, compared to the rate observed when running the engine on the fuel composition prior to incorporation of the Fischer-Tropsch derived fuel component.

A Fischer-Tropsch derived fuel component may be used, in accordance with the present invention, for the purpose of reducing the frequency of lubricant fluid changes, and/or of increasing an interval between lubricant fluid changes. As described above, lubricant fluid changes are necessary whenever the properties and/or performance of the fluid deteriorate to such an extent as to impair its performance, and/or to impede satisfactory functioning of the engine which the fluid is used to lubricate, ϊn particular, the Fischer-Tropsch derived fuel component may be used to reduce the frequency of lubricant fluid changes which are necessary due to changes in the concentration of acidic components in the fluid, and/or to changes in its degree of oxidation or nitration.

Where the present invention is used to increase an interval between lubricant fluid changes needed, the increase may be of at least 10 or 20 %, preferably of at least 50 or 60 or 70 or 80 %, in cases of at least 90 or even 100 %, compared to the intervals required when running the engine on the fuel composition prior to incorporation of the Fischer-Tropsch derived fuel component. The increase may be as compared to the lubricant change interval for the same lubricant fluid when the same engine is run on an otherwise analogous fuel composition which is intended (e.g. marketed) for use in a (typically diesel) internal combustion engine, prior to adding a Fischer-Tropsch derived fuel component to it. The point at which a lubricant change is deemed necessary should be evaluated in each case using the same criteria, which will typically include the kinematic viscosity of the fluid (e.g. at 100⁰C) . Thus, if it is determined (for example by a user, or by a regulatory body, or by a lubricant fluid or engine manufacturer) that the fluid needs to be changed when a certain parameter reaches a predetermined value, the present invention may be used to increase the interval before that happens. The parameter in question may be, for example, the kinematic viscosity of the fluid, the concentration of acidic components which it contains, its degree of oxidation or nitration, and/or its acid or base number. The parameter may be the concentration of a contaminant - such as a metal - in the fluid. An interval between lubricant fluid changes may, for example, be measured in terms of engine running time and/or the distance covered by a vehicle powered by the engine.

The use of a Fischer-Tropsch derived fuel component in or as a fuel composition can bring other advantages in addition to those provided by the present invention. Fischer-Tropsch derived fuels are generally regarded as "cleaner" fuels: they tend to cause lower engine emissions. Fischer-Tropsch derived fuels also tend to have relatively high cetane numbers, in the context of their use in diesel engines, and often need lower levels of performance enhancing additives. They are, moreover, biodegradable and non-toxic, and hence relatively benign actors in environmentally sensitive areas such as nature reserves and parks. The present invention may be used to achieve one or more of these advantages in addition to those discussed above in connection with lubricant deterioration.

The accumulation of corrosive acidic components in a lubricant fluid can increase engine wear. Oxidation and nitration of the fluid, because they lead to increased acid levels, can therefore also impact on engine wear. Engine wear can also be increased due to sludge formation and the consequent impairment of lubricant performance, sludge formation again being linked with oxidation of the lubricant. A result of increased wear is that particles from components of the engine - in particular metals such as iron and copper - can accumulate as additional contaminants in the lubricant fluid.

Accordingly, a fifth aspect of the present invention provides the use of a Fischer-Tropsch derived fuel component in an internal combustion engine, for the purpose of reducing the rate of engine wear in the engine whilst it is running on the fuel composition.

A sixth aspect provides the use of a Fischer-Tropsch derived fuel component in a fuel composition, for the purpose of reducing the rate of engine wear in an internal combustion engine which is running, or is intended to be run, on the fuel composition.

In the context of the fifth and sixth aspects of the present invention, engine wear may in particular be acid™ induced engine wear, which is induced or exacerbated by the presence of acidic components in a lubricant fluid which is present in the engine. Instead or in addition, it may be wear which is induced or exacerbated by oxidation or nitration of such a lubricant fluid, and/or by sludge formation in the fluid. Engine wear may include the removal, for instance due to friction between moving parts or between a moving part and a lubricant fluid, of particles of, for example, metals such as iron and copper, which can then enter the lubricant fluid as contaminants (typically from the engine bearings} . It may include the corrosion of engine components due to the presence of corrosive materials, in particular acids, in the lubricant fluid. The degree, and thus also the rate, of engine wear may be assessed either by visually inspecting components of the engine, and/or by measuring the concentration of wear-derived contaminants in the lubricant fluid. Such contaminants may include metals such as copper, aluminium, chromium, tin, phosphorus and in particular iron, derived from engine components.

The present invention may therefore be used to reduce the rate of accumulation of a wear-induced contaminant, in particular iron, in the lubricant fluid. Again, the reduction in engine wear, in particular acid-induced engine wear, which is achievable using the present invention may be as compared to the rate of engine wear when using the fuel composition prior to incorporation of the Fischer-Tropsch derived fuel component. It may be as compared to the relevant rate measured when the same engine is run on an otherwise analogous fuel composition which is intended (e.g. marketed) for use in a (typically diesel) internal combustion engine, prior to adding a Fischer-Tropsch derived fuel component to it.

It has been found that the kinematic viscosity of a lubricant fluid can remain relatively stable when an engine is run on a fuel composition containing a Fischer-Tropsch derived fuel component. Thus, in accordance with the present invention, the Fischer-Tropsch derived fuel component may be used for any one of the purposes discussed above in connection with the first to the sixth aspects of the present invention, and at the same time for reducing variations (in particular increases) in the kinematic viscosity of the lubricant fluid, for example during a predetermined time period. According to a seventh aspect, the present invention provides the use of a fuel composition containing a Fischer-Tropsch derived fuel component, in an internal combustion engine, for one or more of the purposes described above in connection with the first to the sixth aspects of the present invention, in particular to reduce the rate of accumulation of acidic components in a lubricant fluid which is present in the engine, and/or to reduce the rate of oxidation or nitration in such a lubricant fluid. Again, the engine may be a diesel engine.

An eighth aspect provides a method of operating an internal combustion engine, and/or a system (for example an automotive vehicle} which is powered by such an engine, which method involves introducing into a combustion chamber of the engine a fuel composition containing a Fischer-Tropsch derived fuel component for one or more of the purposes defined in any one of the first to the sixth aspects of the present invention. The engine is preferably operated with the fuel composition for the purpose of reducing the rate of accumulation of acidic components in a lubricant fluid which is present in the engine, and/or its oxidation or nitration rate. Again the engine may in particular be a diesel engine. It may be of the direct injection type, for example of the rotary pump, in-line pump, unit pump, electronic unit injector or common rail type, or of the indirect injection type. Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and do not exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Preferred features of each aspect of the present invention may be as described in connection with any of the other aspects. Other features of the present invention will become apparent from the following examples. Generally speaking the invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings} . Thus, features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the present invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Moreover, unless stated otherwise, any feature disclosed herein may be replaced by an alternative feature serving the same or a similar purpose.

The following examples illustrate the use of Fischer-Tropsch derived fuel components in fuel compositions, in accordance with the present invention, and assess their effects on the properties of lubricant oils in engines running on the fuel compositions. Example 1

A diesel power unit was operated for a period of time on a conventional petroleum derived diesel fuel Fl, and subsequently on a Fischer-Tropsch derived (GTL) diesel fuel F2. Samples were taken from the lubricant oil used in the unit at weekly intervals following the change from conventional to GTL fuel, and their properties analysed in order to assess the effect of the fuel change on lubricant deterioration. Prior to the fuel change, oil samples were analysed only every 250 hours, to coincide with oil changes.

The power unit was a Caterpillar™ 3408C Marine Generator Set (DITA; direct injection; turbocharged and aftercooled} . Its engine capacity was 18 litres and its power rating 320 kW.

The two test fuels had the properties shown in Table 1 below. The conventional (petroleum derived) fuel Fl was a commercially available fuel obtained from Gulf Oil Nederland BV. The GTL fuel F2 was obtained from the Shell Bintulu plant. Table 1

The lubricant oil was a commercially available Caterpillar® product, Cat® DEO™ 15W-40. According to the manufacturer's specifications, this mineral oil has a VK 100 (ASTM D-445) of 14.2 centistokes, a pour point (ASTM D-97) of -30 ⁰C, a viscosity index (ASTM D-2270) of 141 and a total base number (ASTM D-2896) of 11.3 mgKOH/g.

During the initial stages of the experiment, the oil was changed every 250 hours. After a period of time running on the GTL fuel, however, it was determined that oil changes could instead be carried out at half that frequency, i.e. every 500 hours. This in itself illustrates the potential of the present invention to reduce lubricant fluid deterioration rates and hence to increase lubricant change intervals. The following properties were measured for each oil sample: a) kinematic viscosity at 100⁰C (VK 100, using the standard test method ASTM D-445) . b) degree of oxidation (by FTIR spectrometry) . c) degree of nitration (by FTIR spectrometry) . d} iron content, using ASTM D-5185 ("Standard test method for determination of additive elements, wear metals and contaminants in used lubricating oils and determination of selected elements in base oils by inductively coupled plasma atomic emission spectrometry (ICP-AES)") . e) copper content (ASTM D-5185) .

Copper and iron are generally believed to be present as a result of engine wear. Thus, lower contents will indicate reduced engine wear and hence increased lubricant oil performance. Other such "wear elements", including aluminium, chromium, lead, silicon, sodium and tin, were also analysed in the oil samples, again using ICP-AES (ASTM D-5185) . The results of all of these analyses are shown in

Tables 2a and 2b. The figures across the top of the table are engine running hours. Oil changes are highlighted in bold. VK 100 values are quoted in mm^/s. Oxidation and nitration values are in units of absorbance/cm. Table 2a

ch. = change; St. = start

Table 2b

The data show significantly lower oxidation and nitration rates in the lubricant oil when the power unit is fuelled by the GTL fuel than when it is running on the petroleum derived fuel Fl. Even 500 hours after the oil change at 7030 engine hours, the level of oxidation in the oil is lower than that observed at 250-hour oil changes using Fl. After 250 engine hours, the lubricant oxidation level was -29 units when using the petroleum derived fuel but only -21 units when using the GTL fuel; the GTL fuel thus causes a reduction of -27 % in the oxidation level. Similarly after 250 engine hours, the lubricant nitration level was -38 units when using the petroleum derived fuel but only -31 units when using the GTL fuel, a reduction of -18 %. Lubricant sulphation levels were also found to be significantly lower when using the GTL fuel: after 250 engine hours, the sulphation level was -37 units when using the petroleum derived fuel but only -22 units when using the GTL fuel, representing a reduction of -40 %. The oil iron content is also significantly lower when the motor is running on the GTL fuel. Even 500 hours after the oil change at 7030 engine hours, iron contents had not reached the levels found during use of the petroleum derived fuel. Since the iron content in the oil tends to reflect levels of engine wear, at least some of which is likely to be caused by increased oxidation levels in the oil (and hence acid-induced corrosion) , this shows that the oil continues to perform well, and to cause less wear, when used in conjunction with the GTL (Fischer-Tropsch derived) fuel, even after long periods of use. Similarly, copper levels in the oil appear to increase less rapidly when the motor is running on the GTL fuel than when it is running on the more conventional fuel. Low copper levels indicate that no problems are occurring in the cooling system and bearings of the power unit. Aluminium levels are also low, indicating low levels of piston wear.

The sulphur content of the oil is also significantly lower during use of the GTL fuel. Meanwhile the kinematic viscosity of the oil remains relatively stable during use of the GTL fuel, which is consistent with the observed lower oxidation and nitration levels and the lower contaminant concentrations. Since too great an increase in viscosity is the primary motivation for an oil change, the relatively low and consistent oil viscosities observed during use of the GTL fuel indicate the potential for longer intervals between oil changes. Lower viscosities may also be associated with improved fuel economy. Based on these measured properties, therefore, the lubricant oil could apparently be changed even less frequently than every 500 hours, when the engine is running on a Fischer-Tropsch derived fuel, without undue detriment to its performance or to likely levels of engine wear. Example 2

Two test cars equipped with diesel engines were fuelled with a conventional petroleum derived low sulphur diesel fuel F3 and a Fischer-Tropsch derived gas oil F4 respectively. At regular intervals, samples were removed from the lubricant oils used in the two engines, and their acid and base numbers analysed. The viscosities of the samples were also measured, as were their concentrations of certain wear elements.

The test cars were Peugeot™ 206 1.9 litre IDI passenger cars, registered in 2001. Their engines were light duty diesel engines using indirect injection (IDI) technology, both being low mileage at the start of the test (14535 miles for the car run on the petroleum derived fuel F3 and 18645 miles for the car run on the Fischer-Tropsch derived fuel F4) .

The two test fuels had the properties shown in Table 3 below. The petroleum derived fuel was a commercially available Hungarian diesel fuel (ex. Shell} . The F±scher™ Tropsch derived fuel was obtained from Shell (ex. Bintulu) and was dosed prior to the test with two additives: Stadis™ 450 (static dissipator additive, ex. Innospec, treat rate 2 ppmw) and Paradyne™ 655 (lubricity improver, ex. Infineum, treat rate 200 ppmw) .

Table 3

(Aromatics were measured using IP 156 ("Petroleum products and related materials - Determination of hydrocarbon types - Fluorescent indicator adsorption method" . ) The lubricant fluid used in both test engines was

Shell Helix™ Diesel SuperD 15W40 (ex. Shell) . The same batch of lubricant was used throughout the trial, to avoid inter-batch compositional variations.

Immediately before the start of the trial, both vehicles were run on a commercially available UK distillate diesel fuel for one week. At the end of this pre-conditioning period, all fuel was drained from the tanks, which were then refilled with the respective test fuels. The existing distillate diesel fuel was not flushed from the systems.

Each of the test cars was then driven for 18000 miles (somewhat higher than the typical oil drain interval) over a period of 11 months. Similar roads and conditions were used for both. Because the Fischer- Tropsch derived fuel F4 was a "summer grade" diesel, with cold flow properties which were incompatible with vehicle operation at sub-zero ambient temperatures, the trial design allowed for the cars to be garaged during periods of cold weather to reduce cold operability problems. No driveability issues were reported for either car.

At the start of the trial, each of the car engines was run briefly for 1 mile so as to circulate the lubricant oil before taking the first sample. Subsequently, lubricant samples were taken at 2000 mile intervals. As far as possible, the samples were taken at similar mileages for the two test cars.

Among others, the following properties were measured for each sample: a) kinematic viscosity at 100⁰C (VK 100, using the Standard test method ASTM D-445) . b) total acid number {TAN, ASTM D-664) , which is an indication of the amount of acids present in the lubricant and is expressed in milligrams of potassium hydroxide per gram of lubricant (mgKOH/g) . c) total base number (TBN, ASTM D-4739) . d) iron content (using ICP-AES (ASTM D-5185) ) .

The results are shown in Table 4 for the car running on the distillate diesel fuel (car A) and in Table 5 for the one running on the Fischer-Tropsch derived fuel {car B) . Note that no adjustment was made to the measured properties to account for the removal of oil samples and the occasional topping up of the oils so as to maintain sump levels. The first column of each table shows the as- new properties of the lubricant oil.

These data demonstrate superior retention of base number {i.e. a slower decrease in base number) for the oil in car B (the car running on the Fischer-Tropsch derived fuel) . This trend was apparent at the 10000-mile standard oil drain interval and continued throughout the trial. Similarly, the acid number remained generally lower for the oil in car B. Over the duration of the trial, TAN values increased at the rate of 0.052 mgKOH/g per 1000 miles for car A and at the rate of 0.033 mgKOH/g per 1000 miles for car B, a reduction of -38 % for the GTL-fuelled car. TBN values decreased, over the same period, at the rate of 0.094 mgKOH/g per 1000 miles for car A and 0.089 mgKOH/g per 1000 miles for car B, a reduction of ~5 %.

Thus, during the standard oil drain interval the Fischer-Tropsch derived fuel can lead to reduced oil degradation, and indeed this benefit can be seen to continue far beyond the standard oil drain interval. This in turn offers the potential for extending the oil drain interval in an engine fuelled using a Fischer-Tropsch derived fuel. A lower acid content in the oil in car B is believed to be at least partly due to lower oxidation and nitration rates in the oil, as demonstrated in Example 1.

A slower reduction in base number, and a correspondingly slower increase in acid number, is also likely to reduce the rate of engine wear due to acid corrosion (for instance by nitric and carboxylic acids) . This is consistent with the lower iron contents - and slower iron accumulation rates - observed for the lubricant oil used in car B, which indicate reduced wear in that engine.

Claims

C L A I M S

1. Use of a Fischer-Tropsch derived fuel component in a fuel composition, for the purpose of reducing the rate of accumulation of acidic components in a lubricant fluid present in an internal combustion engine which is running, or is intended to be run, on the fuel composition.

2. Use of a Fischer-Tropsch derived fuel component in a fuel composition, for the purpose of reducing the rate of oxidation and/or nitration of a lubricant fluid present in an internal combustion engine which is running, or is intended to be run, on the fuel composition.

3. Use of a Fischer-Tropsch derived fuel component in a fuel composition, for the purpose of reducing the rate of acid-induced engine wear in an internal combustion engine which is running, or is intended to be run, on the fuel composition.

4. Use of a Fischer-Tropsch derived fuel component in a fuel composition, for the purpose of reducing the frequency of lubricant fluid changes in an internal combustion engine which is running, or is intended to be run, on the fuel composition.

5. Use of a fuel composition containing a Fischer- Tropsch derived fuel component, in an internal combustion engine, for one or more of the purposes defined in claims 1 to 4.

6. Use according to any one of the preceding claims, wherein the fuel composition is a diesel fuel composition.

7. Use according to any one of the preceding claims, wherein the concentration of the Fischer-Tropsch derived fuel component in the fuel composition is 10 %v/v or greater.

8. Use according to any one of the preceding claims, wherein the Fischer-Tropsch derived fuel component is a gas oil.

9. A method of operating an internal combustion engine, and/or a system which is powered by such an engine, which method involves introducing into a combustion chamber of the engine a fuel composition containing a Fischer-Tropsch derived fuel component for one or more of the purposes defined in any one of the preceding claims.