EP2649165A1 - Improvements relating to fuel economy - Google Patents

Abstract

The use of a viscosity increasing component in an diesel fuel composition is described for the purpose of improving the fuel economy of an engine or of a vehicle powered by such an engine. The viscosity increasing component is a viscosity index (VI) improving additive, such as a polystyrene-polyisoprene stellate copolymer. The diesel fuel may comprise a biofuel. Methods of using a viscosity increasing component for purposes of improving fuel economy, and methods of operating a compression ignition engine are also described,

Description

IMPROVEMENTS RELATING TO FUEL ECONOMY

Field of the Invention

The present invention relates to a method of

improving diesel fuel economy in a compression-ignition (diesel) engine; and in particular to the use of

viscosity increasing components in a diesel fuel

composition to give improvements in fuel economy.

Background of the Invention

Current emissions legislation in, for example, the US and Europe, set strict limits on the acceptable levels of polluting gases that are tolerated in the exhaust gas emissions of compression ignition engines.

The typical approach taken by engine and fuel manufacturers to improving the combustion process and reducing the production of undesirable exhaust gas emissions is to utilise some form of "advanced

combustion". Advanced combustion is an umbrella term that encompasses a number of different combustion modes, which typically involve one or more of the following features: fuel injection much advanced of Top Dead Centre (TDC) ; multiple fuel injections; high amounts of Exhaust Gas Recirculation (EGR) ; and high injection pressures. All of these modes generally attempt to achieve very low levels of NOx and soot (particulate matter) emissions through improved fuel-air mixing and reduced combustion

temperatures .

Many engines also include some form of after- treatment to reduce exhaust gas emissions to the level required by emissions regulations. Typical after- treatments include devices such as catalytic converters (e.g. for removing NOx emissions) and/or particulate filters (e.g. to remove soot from the exhaust gas

stream) .

Present means of controlling advanced combustion processes in an engine are based on monitoring various engine / combustion parameters, such as NOx production, and using an engine control unit to make adjustments to engine parameters to push the engine towards a set of conditions under which it is perceived that NOx

production during the combustion process will be

minimised. However, a problem, with adjusting such a sensitive process as advanced combustion, especially when it is pushed in the direction of minimal NOx production levels - under which conditions combustion can become unstable - is that incomplete combustion can occur, resulting in increased production of soot / particulate matter (PM) . Exhaust gas emissions from diesel engines can, therefore, be seen as a trade-off between NOx and PM emissions .

Thus, in modern diesel vehicles the engine is generally set-up to produce low NOx emissions and

consequentially high PM emissions; and a PM trap is employed to subsequently remove PM from the emissions in order to meet the low overall emissions criteria. It is known in the literature that this set-up results in lower engine efficiency and significantly increased fuel consumption. However, alternative emission reduction strategies are much more complicated and costly and have only been implemented on a large scale in heavy-duty vehicles. In fact, even were NOx after-treatments widely available for e.g. passenger cars, the reliability of PM filters would likely ensure that the engine set-up remains much the same for the foreseeable future. However, it should be appreciated that the control of advanced combustion and exhaust gas emissions is not merely a matter of air charge and engine controls, but also of fuel properties, such as cetane number, density, and presence of particular additives etc. Accordingly, during development, an engine's emissions are measured by reference to a tightly specified reference fuel, and engine set-up is, therefore, optimised to the properties of that reference fuel. A downside of this system is that any changes in the type of fuel that is used in an engine can have a significant impact on both engine performance (e.g. reduced efficiency) and emissions.

For example, to facilitate modern vehicle after- treatment technologies and to reduce vehicle emissions still further, fuel refineries have invested in

supplementary systems such as sulphur reducing

technologies. This has generally resulted in the

availability of lower density diesel fuels. The increased flexibility offered by these technologies has also enabled refineries to optimise "energy give-away" by further reducing fuel density nearer to the lower limit of the relevant full specification. The density of fuels in the market place has, therefore, gradually moved away from the reference fuels used for engine calibration. This shift in the properties of some fuels means that the type of fuel that is actually used in a particular engine can have a significant impact.

In addition, since the amount of fuel injected into the engine of a vehicle is largely controlled by volume, the reduction in density of fuels has lead to a reduced amount of combustible fuel in the engine cylinder and a consequential reduction in the effective amount of energy that can be converted. This drives the emissions of the engine further in the direction of low NOx and high PM, but to the detriment of engine efficiency and increased fuel consumption. Even though there may be a small benefit in NOx emission, it has now been appreciated that this set-up does not provide the optimum balance between fuel economy and exhaust emissions that would be intended by a vehicle manufacturer.

In view of the above operating procedures and problems, it is difficult to approach the optimum balance between fuel economy and exhaust gas emissions simply using standard refinery fuels. Accordingly, there is a need for improved fuels and methods for improving the fuel economy of engines.

Thus, this invention aims to overcome or alleviate at least one of the problems associated with the prior art .

Summary of the Invention

It has now surprisingly been found that increasing the viscosity of a fuel composition, and particularly a diesel fuel, can improve the fuel economy of an engine.

Without being bound by theory, it may be that the

increase in viscosity can compensate for the reduced density of typical refinery diesels in comparison to reference fuels; and the increase in viscosity may therefore enable both a reduction in fuel consumption and in CO₂ emissions. Even though the overall properties of a modified fuel according to the invention may not

necessarily be closer to those of the reference fuel, the combination of lower density and higher viscosity can improve fuel economy, and in some cases may even result in the same optimum balance between fuel consumption and exhaust emissions as achieved with the original reference fuel. In accordance with the invention, the viscosity of the fuel can be increased either directly at the

refinery, by adding high viscosity components or by adding viscosity increasing additives.

Accordingly, in a first aspect of the invention, there is provided the use of a viscosity increasing component in an diesel fuel composition, for the purpose of improving the fuel economy of an engine into which the fuel composition is or is intended to be introduced. The invention further relates to such a use for improving the fuel economy of a vehicle powered by such an engine.

Thus, the engine is preferably a diesel or

compression ignition engine. However, it is also

envisaged that the invention may be applicable to

gasoline fuel compositions and corresponding internal combustion engines of a non-compression ignition type.

The diesel engine may also be a turbo-charged diesel engine. The engine may be under the control of an engine management system (EMS} .

Any fuel viscosity increasing component (or agent} may be used in accordance with the invention. Exemplary such components include refinery components, such as high-viscosity fuel and/or oil fractions, or additives such as viscosity index (VI) improving additives. The viscosity increasing component may, therefore, be added to the fuel composition at the refinery or outside the refinery, such as prior to delivery to the point of sale or at the point of sale.

VI improving additives are well-known to the skilled person, and include compositions comprising block

copolymer, e.g. which contain one or more monomer blocks selected from ethylene, propylene, butylene, butadiene, isoprene and styrene monomers. A particularly suitable VI improving additive comprises a polystyrene-polyisoprene stellate copolymer, such as SV™ 200 (ex. Infineum,

Multisol and others) .

Suitably, the VI improving additive is used at a concentration in tire range of from 0.01 to 0.5% w/w, based on the total weight of the fuel composition. The VI improving additive may be used at a concentration of between: (i) 0.01% w/w and 2.0% w/w; {ii) 0.05% w/w and 1.0% w/w; or (iii) 0.1% w/w and 0.5 w/w; based on the total weight of the fuel composition.

Another suitable viscosity increasing component is a high viscosity diesel fuel or oil component, e.g. a refinery product, which has a higher kinematic viscosity than the base fuel to which it is (intended to be) added. According to one embodiment, the high viscosity component is used as a minor amount (e.g. less than 50% w/w of the total fuel composition) , with the base fuel being the major component of the fuel composition. Suitably, the high viscosity diesel fuel / oil component is used at a ^■ concentration of up to 20% of the total fuel composition; such as from approximately 1% to 10%. In a particularly suitable embodiment the amount of high viscosity

component is used at a concentration between

approximately 3% and 10%, or 5% and 8%.

In some embodiments the viscosity increasing

component is used in an amount sufficient to increase the kinematic viscosity of the fuel composition by (i) at least 0.2 mm²/s; (ii) 0.25 mm²/s to 1.0 mm²/s; or (iii) 0.32 mm²/s to 0.67 mm²/s; compared to the viscosity of the fuel composition prior to the addition of the

viscosity increasing component. The kinematic viscosity is measured under standard conditions, such as at 40°C.

As will be appreciated, the resultant or desired final kinematic viscosity of the fuel composition may be determined according to the desired properties of the fuel and/or by national or International regulations and standards. By way of example, in one embodiment, the kinematic viscosity at 40°C o1r the diesel fuel

composition comprising the viscosity increasing component may be up to 4.5 mm²/s such as between 2.0 mm²/s and 4.0 mm²/s; or between 3.0 ram²/s and 3.8 mm²/s.

For some applications it may be desirable for the diesel fuel composition to contain a biofuel. In such embodiments, the biofuel component may comprise fatty acid methyl esters (FAME) . In one embodiment there may be at least 2% w/w FAME in the fuel composition, based on the total weight of the fuel. In other embodiments there may be between 5% w/w and 50% w/w FAME; and in one suitable embodiment there is approximately 5% w/w FAME.

The fuel compositions may contain any number of additional useful additives known to the person of skill in the art. In some embodiments, two or more viscosity increasing components may be used, such as a VI improver and a high viscosity fuel or oil component. In another embodiment there may be two or more VI improvers of the same of different structural class.

The invention also relates to methods for improving / increasing the fuel economy of an engine or of a vehicle powered by such an engine. The method comprises introducing into a combustion chamber of the engine a fuel composition comprising a viscosity increasing component. A preferred fuel composition is a diesel fuel and a preferred engine is a compression ignition engine. It will be appreciated that all features and embodiments described in relation to the uses of the invention are applicable to the methods of the invention, unless otherwise stated. In yet another aspect, the invention relates to a method of operating a compression, ignition engine and/or a vehicle which is powered by such an engine. In this aspect, the method involves introducing rnto a combustion chamber of the engine a fuel composition obtained by the methods of the invention.

According to a specific application, the uses and/or methods of the invention may be for the purpose of reducing or mitigating a reduction in fuel economy that may, for example, be caused by the addition of a fuel component or additive that has been or is intended to be introduced into the fuel composition for any other purpose, e.g. for improving the emissions performance of the fuel concerned. Advantageously, the use of the invention causes a minimal deterioration, neutral or better emissions performance compared to that of the diesel fuel comprised in the fuel composition prior to addition of the viscosity increasing component. Likewise, the uses and/or methods of the invention suitably have minimal, or no detrimental impact on the performance of an engine powered by the fuel composition, compared to its performance prior to addition of the viscosity improving component.

In specific embodiments, the uses and methods of the invention may be for formulating fuels that give

demonstrably improved fuel economy in a particular engine, whilst falling within a desirable or

predetermined fuel standard, for example, the fuel composition may be a diesel fuel corresponding to the European Standard EN 590 (2000), for example an "ultra low sulphur diesel". Alternatively the uses and methods may be for ameliorating fuel economy losses that are associated with fuels or fuel blends which have a low volumetric energy, for example to give lower vehicle emissions, such as in a fuel or fuel blend containing a diesel fuel corresponding to the Swedish Class 1

standard.

In yet another aspect of the invention there is provided a method for the preparation of a fuel

composition conferring greater fuel economy on an engine, and especially a diesel fuel composition for use in a compression ignition engine. The method comprising adding a viscosity increasing component, such as defined herein, to the fuel composition; and blending the viscosity increasing component with the fuel composition to provide a fuel composition suitable for providing better fuel economy in a selected engine.

Brief Description of the Drawings

The invention is further illustrated by the

accompanying drawings in which:

Figure 1 illustrates the "Ne European Driving cycle" (NEDC) , which includes four consecutive "city cycles" (ECE) and one extra-urban "overland cycle"

(EUDC) ;

Figure 2 illustrates the NEDC measurements for fuel compositions of the invention against a control fuel composition;

Figure 3 illustrates the fuel economy benefits achieved in accordance with the invention, as a function of the concentration of viscosity increasing component in the fuel. Error bars represent 2-sided 95% confidence intervals;

Figure 4 illustrates the fuel economy improvements achieved using fuel compositions of the invention during each phase of the NEDC test driving protocol, with weight of fuel consumed (y-axis) against test run (x-axis) : (A) fuel usage over the entire NEDC test protocol; (B) fuel usage over Phase 1 of the test protocol; (C) fuel usage over Phase 2 of the test protocol; (D) fuel usage over Phase 3 of the test protocol; (E) fuel usage over Phase 4 of the test protocol; and (F) fuel usage over the EUDC phase of the test protocol.

Detailed Description of the Invention

In order to assist with the understanding of the invention several terms are defined herein.

"Viscosity Index" {or VI) is an arbitrary unit used to measure the change of kinematic viscosity with

temperature. It is generally used to characterise

lubricating oils in the automotive industry. Thus, the Viscosity Index highlights how a liquid's (or

lubricant's) viscosity changes with variations in

temperature. In general, the viscosity of a liquid decreases as its temperature increases. Many lubricant or fuel applications require the liquid to perform across a wide range of engine conditions: for example, at start-up when the liquid is at prevailing temperature of the environment, as well as when it is running (up to 200 °C /392 °F) . The higher the VI, the smaller the relative change in viscosity with temperature. Desirably, a fuel composition will not vary much in viscosity over its typical operating temperature range (i.e. it will have a relatively high VI) .

The reference temperatures at which viscosity is measured in accordance with the VI scale were chosen arbitrarily to be 37.8°C and 98.9°C (i.e. 100°F and 210°F) . Typically, however, kinematic viscosity

measurements are taken at approximately 40°C and/or approximately 100°C, unless otherwise indicated.

Conveniently, kinematic viscosity is measured using standardised testing procedures known to the person of skill in the art, such as ASTM D-445 or EN ISO 3104.

The term "viscosity increasing component" as used herein, encompasses any component that, when added to a fuel composition at a suitable concentration, has the effect of increasing the viscosity of the fuel

composition relative to its previous viscosity at one or more temperatures within the operating temperature range of the fuel. The term encompasses high viscosity fuel or oil components as well as natural or synthetic fuel or oil additives.

VI improvers (also known as viscosity modifiers) are additives that increase the viscosity of the fluid throughout the useful temperature range of the VI

improver. The useful operating temperature preferably overlaps at least a portion of the operating temperature range of a fuel composition in an engine.

VI improvers are polymeric molecules that are sensitive to temperature. At low temperatures, the molecule chains contract and so do not significantly impact on the fluid viscosity. However, at high

temperatures, the chains relax and a relative increase in viscosity occurs; although the actual viscosity will still decrease as temperature increases. Hence, the addition of VI improvers serves to slow down rather than halt the rate at which the viscosity decreases.

There are many types and structures of VI improvers. Higher molecular weight polymers make better thickeners but tend to have less resistance to mechanical shear. On the other hand, lower molecular weight polymers are more shear-resistant, but do not improve viscosity as

effectively at higher temperatures and, therefore, may be used in larger quantities to achieve the same effect at a desired temperature.

As used herein, an "increase" in the context of fuel viscosity embraces any degree of increase compared to a previously measured viscosity under the same or

equivalent conditions. Thus, the increase is suitably compared to the viscosity of the fuel composition prior to incorporation of the viscosity increasing (or

improving) component or additive. Alternatively, the viscosity increase may be measured in comparison to an otherwise analogous fuel composition (or batch or the same fuel composition) ; for example, which is intended (e.g. marketed) for use in an internal combustion engine, in particular a diesel engine, prior to adding a

viscosity increasing component to it.

The present invention may, for example, involve adjusting (i.e. increasing) the viscosity of the fuel composition, using the viscosity increasing component in order to achieve a desired target viscosity.

As noted, the viscosity increasing component is used in a sufficient quantity to increase the viscosity of ^'the fuel composition to which it is added as measured under the same conditions. The increase in kinematic viscosity may be measured at any suitable temperature, such as at 40°C or at 100°C. Conveniently, viscosity is measured at 40°C. Suitably, the viscosity increasing component is used in an amount to increase the viscosity by at least 0.05 mm²/s, at least 0.1 mm²/s, or at least 0.2 mm²/s. More suitably, the viscosity increase may be between 0.25 ram²/s and 2.0 mm²/s; or between 0.25 mm²/s and 1.0 mm²/s. In a preferred embodiment the viscosity increase is between 0.3 mm²/s and 0.8 mm/s, such as between 0.32 mm²/s and 0.67 mm²/s. In some cases it may be desirable to increase the viscosity by approximately 0.4 mm²/s, approximately 0.5 mm²/s, approximately 0.6 mm²/s or approximately 0.7 mrnVs.

Likewise, an "increase" in the context of fuel economy encompasses any amount of increase compared to the fuel economy of the same fuel composition prior to addition of the viscosity increasing component, as measured in the same or equivalent engine. Alternatively, the increase in fuel economy may be measured relative to an analogous fuel composition under the same or

equivalent conditions in the same or equivalent engine. Thus, the increase is suitably compared to the fuel economy of an engine or vehicle prior to incorporation of the viscosity increasing (or improving) component or additive .

The increase in fuel economy may be measured and/or reported in any suitable manner, such as a percentage increase, as an increase in distance travelled (e.g. km) for a set volume of fuel (e.g. L) , or as a reduction in fuel volume or mass to travel a particular distance under the same conditions (e.g. speed, workload). By way of example, the percentage increase may be at least 0.1%, such as at least 0.2%. Suitably, the percentage increase in fuel economy is at least 0.25%, or at least 0.5%. More suitably, the increase in fuel economy is at least 1.0%, at least 2.0% or at least 3.0%. In some particularly preferred embodiments, the increase in fuel economy is at least 5.0% or even at least 10%. However, it should be appreciated that any measurable improvement in fuel economy may provide a worthwhile advantage, particularly when it is considered how much fuel is used by vehicles throughout the world on a daily basis. The engine in which the fuel composition of the invention is used may be any appropriate engine. Thus, where the fuel is a diesel or biodiesel fuel composition, the engine is a diesel or compression ignition engine. Likewise, any type of diesel engine may be used, such as a turbo charged diesel engine, provided the same or equivalent engine is used to measure fuel economy with and without the viscosity increasing component.

Similarly, the invention is applicable to an engine in any vehicle. Generally, the invention is also applicable to any driving conditions, such as urban, extra urban and/or motorway / freeway / test track driving

conditions; although the invention may be particularly beneficial in certain engine type and/or under specific driving conditions.

In the context of the present invention, "use" of a viscosity increasing component in a fuel composition means incorporating the component into the composition, typically as a blend (i.e. a physical mixture) with one or more fuel components (typically diesel base fuels} and optionally with one or more fuel additives.

The viscosity increasing component is preferably incorporated into the fuel composition before the

composition is introduced into an engine which is to be run on the composition.

Accordingly, the viscosity increasing component may be dosed directly into (e.g. blended with) one or more components of the fuel composition or the base fuel at the refinery. For instance, it may be pre-diluted in a suitable fuel component, which subsequently forms part of the overall automotive fuel composition.

Alternatively, it may be added to an automotive fuel composition downstream of the refinery. For example, it may be added as part of an additive package containing one or more other fuel additives. This can be

particularly advantageous because in some circumstances it can be inconvenient or undesirable to modify the fuel composition at the refinery. For example, the blending of base fuel components may not be feasible at all

locations, whereas the introduction of fuel additives, at relatively low concentrations, can more readily be achieved at fuel depots or at other filling points such as road tanker, barge or train filling points,

dispensers, customer tanks and vehicles.

Accordingly, the "use" of the invention may also encompass the supply of a viscosity increasing component together with instructions for its use in an automotive fuel composition to achieve one of the benefits of the present invention (e.g. an increase in fuel economy in a particular internal combustion engine or in a particular vehicle) . The viscosity increasing component may

therefore 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. By way of example, the viscosity increasing component or additive may be incorporated into an additive formulation or package along with one or more other fuel additives. The one or more fuel additives may be selected from any useful additive, such as detergents, anti-corrosion additives, esters, poly-alpha olefins, long chain organic acids, components containing amine or amide active centres, and mixtures thereof, as is known to the person of skill in the art.

Instead, or in addition, the "use" of the invention may involve running an engine on the fuel composition containing the viscosity increasing component, typically by introducing the fuel composition into a combustion chamber of the engine.

In accordance with one embodiment of the present invention, two or more viscosity increasing components may be used in an automotive fuel composition to provide one or more of the effects of the invention described herei .

Viscosity Increasing Components

Viscosity increasing components may be high

viscosity fuel or oil derivatives or VI improving

additives .

When used, high viscosity fuel or oil derivatives may include group 3 lubricant base oils of higher

viscosity than the fuel into which they are to be

introduced. These compositions may be used at any

suitable concentration, depending on the desired effect.

VI improving additives tend to be synthetically prepared, and are therefore typically available with a well defined constitution and quality, in contrast to, for example, mineral derived viscosity increasing fuel components (refinery streams) , the constitution of which can vary from batch to batch. VI improving additives are also widely available, for use in lubricants, which can again make them an attractive additive for the new use proposed by the present invention. They are also often less expensive, in particular in view of the lower concentrations needed, than other viscosity increasing components such as mineral base oils.

The VI improving additive used in a fuel composition in accordance with the present invention may be polymeric in nature. It may, for example, be selected from: a) styrene-based copolymers, in particular block copolymers, for example those available as Kraton™ D or Kraton™ G additives (ex. Kraton) or as SV™ additives (ex.

Infrneum, Multisol or others) . Particular examples include copolymers of styrenic and ethylene/butylene monomers, for instance polystyrene-polyisoprene

copolymers and polystyrene-polybutadiene copolymers. Such copolymers may be block copolymers, such as SV™ 150 (a polystyrene-polyisoprene di-block copolymer) , or the Kraton™ additives (styrene-butadiene-styrene tri-block copolymers or styrene-ethylene-butylene block

copolymers) . They may be tapered copolymers, for instance styrene-butadiene copolymers. They may be stellate

{"star") copolymers, as for instance in SV™ 200 and SV™ 260 (styrene-polyisoprene star copolymers) ; b) other block copolymers based on ethylene, butylene, butadiene, isoprene or other olefin monomers, such as ethylene- propylene copolymers; c) polyisobutylenes (PIBs) ; d) polymethacrylates (PMAs) ; e) poly alpha olefins (PAOs); and f) mixtures thereof.

A VI improving additive may include one or more compounds of inorganic origin, for example zeolites.

Other examples of suitable VI improvers are

disclosed in Japanese Patents Nos . 954077, 1031507, 1468752, 1764494 and 1751082. Yet further examples include the dispersing-type VI improvers, which comprise copolymerised polar monomers containing nitrogen and oxygen atoms; alkyl aromatic-type VI improvers; and certain pour point depressants known for use as VI improvers .

Of the above, additives of type (a) and (b) , or mixtures thereof, may be preferred; and in particular additives of type (a) .

VI improving additives which contain, or ideally consist essentially of, block copolymers, may be preferred, as in general these can lead to fewer side effects such as increases in deposit and/or foam

formation. The VI improving additive may, for example, comprise a block copolymer which contains one or more olefin, monomer blocks, typically selected from ethylene, propylene, butylene, butadiene, isoprene and styrene monomers. A particularly preferred type of VI improver is a star copolymer based on styrene and isoprene; and a specifically preferred VI improver is SV™ 200, a

polystyrene-polyisoprene star copolymers.

The kinematic viscosity at 40°C (VK 40, as measured by ASTM D-445 or EN ISO 3104) of the VI improving

additive is suitably 40 mm²/s or greater, preferably 100 mm²/s or greater, more preferably 1000 mm/s or greater. Its density at 15°C (ASTM D-4052 or EN ISO 3675) is suitably 600 kg/m³ or greater, preferably 800 kg/m³ or greater. Its sulphur content (ASTM D-2622 or

EN ISO 20846) is suitably 1000 mg/kg or lower, preferably 350 mg/kg or lower, more preferably 10 mg/kg or lower.

The VI improving additive may be pre-dissolved in a suitable solvent, for example an oil such as a mineral oil or Fischer-Tropsch derived hydrocarbon mixture; a fuel component (which again may be either mineral or Fischer-Tropsch derived) compatible with the fuel composition in which the additive is to be used (for example a middle distillate fuel component such as a gas oil or kerosene, when intended for use in a diesel fuel composition) ; a poly alpha olefin; a so-called biofuel such as a fatty acid alkyl ester (FAAB) , a Fischer- Tropsch derived biomass-to-liquid synthesis product, a hydrogenated vegetable oil, a waste or algae oil or an alcohol such as ethanol; an aromatic solvent; any other hydrocarbon or organic solvent; or a mixture thereof. Preferred solvents for use in this context are mineral oil based diesel fuel components and solvents, and

Fischer-Tropsch derived components such as the ^vxXtL" components referred to below. Biofuel solvents may also be preferred in certain cases.

The concentration of the VI improving additive in the fuel composition may be up to 2% w/w, suitably up to 1.0% w/w and more suitably 0.5 %w/w. It may be 0.001% w/w or greater, suitably 0.01% w/w or greater, more suitably 0.05% w/w or greater, and still more suitably 0.1% w/w or greater. Suitable concentrations ranges may for instance be from approximately 0.001 to 2.0% w/w, 0.01 to 2.0% w/w, 0.01 to 1.0% w/w, 0.01 to 0.5% w/w, 0.05 to 1.0% w/w, 0.05 to 0.5% w/w, 0.1 to 0.5% w/w, or from 0.1 to 0.3% w/w. In some cases the amount of VI improving additive may be approximately from 0.15 to 0.25% w/w. Particularly useful concentrations of VI improver are approximately 0.1% w/w and 0.2% w/w based on the total weight of the fuel composition. In some advantageous embodiments, the VI improver is selected from the SV™ series of VI improvers (as described above) , and these are beneficially used in the range of 0.05 to 0.5% w/w. For example, it may be advantageous to use the SV™ 200 or SV™ 260 VI improvers in an amount of approximately 0.1%, 0.2% or 0.3% w/w. These concentrations are for the VI improving additive itself, and do not take account of any solvent (s) with which its active ingredient may be pre-diluted; and are based on the mass of the overall fuel composition. Where a combination of two or more VI improving additives is used in the composition, the same concentration ranges may apply to the overall combination of VI improving additives. It will be appreciated that amounts / concentrations may also be expressed as ppm, in which case 1% w/w corresponds to 10,000 ppm w/w.

The remainder of the composition will typically consist of one or more automotive base fuels, for

instance as described in more detail below, optionally together with one or more fuel additives. The

concentration of the VI improving additive used may depend on desirable fuel characteristics / properties, such as: the desired viscosity of the overall fuel composition; the viscosity of the composition prior to incorporation of the additive; the viscosity of the additive itself; and/or the viscosity of any solvent in which the additive is used. The relative proportions of the VI improving additive, fuel component (s) and any other components or additives present in a diesel fuel composition prepared according to the invention may also depend on other desired properties such as density, emissions performance and cetane number. Density of the overall fuel composition may in some cases be a

particularly relevant parameter.

Emission levels may be measured using standard testing procedures such as the European R49, ESC, OICA or ETC (for heavy-duty engines) or ECE+EUDC or MVEG (for light-duty engines) test cycles. Ideally emissions performance is measured on a diesel engine built to comply with the Euro II standard emissions limits (1996) or with the Euro III (2000), IV (2005) or even V (2008) standard limits.

Uses and Methods

Viscosity index improving additives (also referred to as VI improvers) are well known for use in lubricant formulations, where they are used to maintain viscosity as constant as possible over a desired temperature range by relatively increasing viscosity (i.e. slowing the decrease in viscosity) at higher temperatures. They are typically based on relatively high molecular weight, long chain polymeric molecules that can form conglomerates and/or micelles. These molecular systems expand at higher temperatures, thus further restricting their movement relative to one another and in turn increasing the viscosity of the system. Known VI improvers are typically included in lubricating oil formulations at

concentrations between 1 and 20% w/w. In WO 01/48120, however, certain of these types of additive are proposed for use in fuel compositions, in particular diesel fuel compositions, for the purpose of improving the ability of an engine to start at elevated temperatures. In US

2009/0241882, certain VI improving additives are

described for use in fuel compositions for the purpose of improving acceleration performance, which can be

manifested by an increase in engine power, and/or torque, and/or vehicle tractive effort at any given speed. They have not, however, to our knowledge, been proposed for use in improving the fuel economy of an engine and/or vehicle in which the engine is installed.

It has now been found that VI improving additives can significantly increase the viscosity of an automotive fuel composition, in particular a diesel fuel

composition, even when used at relatively low

concentrations; and that this can improve the fuel economy of an engine into which the composition is introduced. These fuel economy benefits may be observed under any type of driving condition, such as urban, extra urban, and highway, at low speed and/or at high speed. The invention is not, therefore, limited to specific driving conditions, although the fuel economy benefits may be more apparent under some particular conditions than others.

Likewise, the fuel economy benefits are not limited to particular types of engine, although diesel

compression ignition engines are preferred. Furthermore, the advantages of the invention may apply in turbo charged engines as well as in non-turbo engines.

Thus, the present invention can provide an effective way of improving the fuel economy of an internal

combustion engine by means of the fuel introduced into it .

While the amount of the viscosity increasing

component for use in accordance with the invention may vary depending of fuel type and/or engine type; a further benefit of the invention is that under some conditions the amount of VI improver needed to observe the benefit of the invention may be surprisingly low, such as at the level of typical fuel additives. This in turn can reduce the cost and complexity of the fuel preparation process. For example, it can allow a fuel composition to be altered, in order to improve fuel economy, by the

incorporation of additives downstream of the refinery, rather than by altering the content of the base fuel at its point of initial preparation. The blending of base fuel components may not be feasible at all locations, whereas the introduction of fuel additives, at relatively low concentrations, can more readily be achieved at fuel depots or at other filling points such as road tanker, barge or train filling points, dispensers, customer tanks and vehicles.

Moreover, an additive which is to be used at a relatively low concentration can naturally be

transported, stored and introduced into a fuel composition more cost effectively than can a fuel

component which needs to be used at concentrations of the order of tens of percent by weight.

The use of relatively low concentrations of VI improving additives can also help to reduce any

undesirable side effects: for example , impacting on distillation or cold flow properties, caused by their incorporation into a fuel composition.

Another aspect of the invention provides a method of operating an internal combustion engine and/or a vehicle powered by such an engine, which comprises introducing into a combustion chamber of the engine a fuel

composition prepared in accordance with the invention. The fuel composition is preferably introduced for one or more of the purposes described in connection with this invention. Thus, the engine is preferably operated with the fuel composition for the purpose of improving its fuel economy. The engine is in particular a diesel engine, and may be a turbo charged diesel engine. The diesel engine 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. It may be a heavy or a light duty diesel engine. For example, it may be an electronic unit direct injection (EUDI) engine.

Diesel Fuel Compositions

A further advantage of the present invention is that VI improving additives are designed specifically to increase viscosity at higher temperatures. Since

increases in engine power are generally linked to the conditions in the fuel injection system (which generally operates at high temperatures), and to fuel consumption, VI improving additives are believed capable of providing greater benefits than other more conventional viscosity increasing components in improving fuel economy,

particularly at higher speeds and powers.

Thus, it has surprisingly been found that, at least at the relatively low concentrations proposed for use in the present invention, a VI improving additive can increase the viscosity of a fuel composition, in

particular a diesel fuel composition, by an amount greater than that which theory would predict based on the viscosities of the individual components. The expected viscosity of a blend of two or more liquids having different viscosities can be calculated using a three- step procedure (see Hirshfelder et al., "Molecular Theory of Gases and Liquids", First Edition, Wiley (ISBN 0-471- 40065-3) ; and Maples (2000) , "Petroleum Refinery Process Economics", Second Edition, Pennwell Books (ISBN 0-87814- 779-9)). By way of example, it has been found (see e.g. WO 2009/118302) that a blend of 99% w/w of a sulphur-free diesel fuel having a VK 40 (kinematic viscosity at 40°C) of 2.15 mm²/s with 1% w/w of the VI improving additive SV™ 261 (which has a VK 40 of 16,300 mmVs) has an overall measured VK 40 of 3.19 mm²/s. In other words, incorporation of this VI improver increased the VK 40 of the diesel fuel by 0.44 mm²/s; whereas the formulae described in Hirshfelder et al., for example, would predict a theoretical VK 40 of 2.84 mm²/s, (i.e. an increase of only 0.09 mm²/s over the VK 40 of the diesel fuel alone) . Thus, based purely on theory, VI improving additives would not be expected significantly to increase the viscosity of a fuel composition at additive-level concentrations. SV™ 261 is a mixture of 15% w/w block copolymers (e.g. SV™ 260, also ex. Infineum) with 85% w/w mineral oil) . Due to the inclusion of the VI improving additive, a fuel composition prepared according to the present invention (in particular a diesel fuel composition) will suitably have a VK 40 of 2.0 mm²/s or greater, 2.5 mm²/s or greater, 2.7 mm²/s or greater, 2.8 mm²/s or greater, or preferably 2.9 mm²/s or greater. In some cases the VK 40 may be up to 4.5 mm²/s, up to 4.2 mm²/s, or up to 4.0 mm/s. Advantageously, the VK 40 of the fuel composition including the viscosity increasing component (VI improver or otherwise) is in the range of 3.0 mm²/s to 4.0 mm/s, such as 3.0 mm²/s to 3.8 mm²/s, 3.1 mm²/s to 3.7 mm²/s, or 3.2 mm/s to 3.6 mm²/s. In exceptional cases, however, for example in arctic diesel fuels, the VK 40 of the composition may be as low as 1.5 mm²/s, although it is preferably approximately 1.7 or 2.0 mm²/s or greater. It should be appreciated that references to viscosity herein are, unless otherwise specified, intended to mean

kinematic viscosity.

The composition preferably has a relatively high density for a diesel fuel composition, such as 830 kg/m³ or greater at 15°C (AST D-4052 or EN ISO 3675),

preferably 832 kg/m³ or greater, such as from 832 to 845 kg/m³ at 15 °C, which is the upper limit of the current EN 590 diesel fuel specification.

A diesel fuel composition prepared according to the present invention may in general be any type of diesel fuel composition suitable for use in a compression ignition (diesel) engine. It may contain, in addition to the VI improving additive, other standard diesel fuel components. It may, for example, include a major

proportion of a diesel base fuel, for instance of the type described below. In this context, a "major

proportion" means at least 50% w/w, and typically at least 85% w/w based on the overall composition. More suitably, at least 90% w/w or at least 95% w/w; and in some cases at least 98% w/w or at least 99% w/w of the fuel composition consists of the diesel base fuel.

Thus, in addition to the VI improving additive, a diesel fuel composition prepared according to the present invention may comprise 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. Such gas oils may be processed in a hydride- sulphurisation (HDS) unit so as to reduce their sulphur content to a level suitable for inclusion in a diesel fuel composition. They will typically have boiling points within the usual diesel range of 150 to 410°C or 170 to 370°C, depending on grade and use. In some cases, the fuel composition will include one or more cracked

products obtained by splitting heavy hydrocarbons.

A diesel base fuel may consist of or comprise a Fischer-Tropsch derived diesel fuel component, typically a Fischer-Tropsch derived gas oil. As used herein, the term "Fischer-Tropsch derived" means that a material is, or is obtained from, a synthesis product of a Fischer- Tropsch condensation process. 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. The Fischer-Tropsch process converts carbon monoxide and hydrogen into longer chains, which are usually paraffinic hydrocarbons. The carbon monoxide and hydrogen may themselves be derived from organic, inorganic, natural or synthetic sources, such as from natural gas or from organically derived methane .

A Fischer-Tropsch derived diesel 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. The desired

fraction (s), typically gas oil fraction (s) , may

subsequently be isolated e.g. by distillation. Other post-synthesis treatments, such as polymerisation, alkylation, distillation, cracking-decarboxylation, isomerisation and hydroreforming, may also be employed to modify the properties of Fischer-Tropsch condensation products, as is known in the art.

Fischer-Tropsch fuels may be derived by converting gas, biomass or coal to liquid (XtL) , specifically by gas to liquid conversion (GtL) , or from biomass to liquid conversion (BtL) . Any form of Fischer-Tropsch derived fuel component may be used as a base fuel in accordance with the invention.

Diesel fuel components contained in a composition prepared according to the present invention will

typically have a density of from 750 to 900 kg/m³, from 800 to 860 kg/m³, at 15°C (ASTM D-4052 or EN ISO 3675) and/or a VK 40 of from 1.5 to 6.0 mm²/s (ASTM D-445 or BN ISO 3104) .

In a diesel fuel composition prepared according to the present invention, the base fuel may itself comprise a mixture of two or more diesel fuel components of the types described above. In beneficial embodiments of the invention, the diesel fuel may consist of or comprise a so-called

"biodiesel" fuel component such as a vegetable oil, hydrogenaired vegetable oil or vegetable oil derivative (e.g. a fatty acid ester, in particular a fatty acid methyl ester, FAME) , or another oxygenate such as an acid, ketone or ester. Such components need not

necessarily be bio-derived.

Where the fuel composition contains a biodiesel component, the biodiesel component may be present in quantities of between 1% and 99% w/w, for example. In one embodiment the fuel comprises at least 2% w/w biodiesel, such as between 2% and 80% w/w. In some cases the

biodiesel is present at between 2% and 50% w/w, such as between 3% and 40% w/w, between 4% and 30% w/w, or between 5% and 20% w/w. In one beneficial embodiment the biodiesel component is FAME. In a preferred application FAME is present at approximately 5% w/w based on the total weight of the fuel composition.

In accordance with the present invention, a

viscosity increasing component, such as a VI improver may be used to increase the viscosity of a fuel composition. Thus, the base fuel(s) may have a relatively low

viscosity (e.g. less than 3.0 mm²/s) , and may then be "improved" by incorporation of the viscosity increasing component. A base fuel component which is perhaps not intrinsically beneficial for good engine fuel economy, e.g. because refining processes or additives have been used to optimise another important property of the fuel (such as exhaust gas emissions) , may thus be modified so as to improve fuel economy. Any detrimental effect that the additive or refining process might have been expected to have on fuel economy may be at least partially counteracted by increasing the viscosity of the fuel.

Likewise, the relatively lower expected fuel economy level may be a result of the operating conditions of the engine or vehicle concerned, for example, as may be controlled by an engine management system. Accordingly, the uses and methods of the invention may also go some way towards counteracting lower engine fuel economy resulting, at least in part, from engine operating conditions / parameters.

In the case of a diesel fuel composition, for example, the base fuel(s) consist of or comprise

relatively low viscosity components such as Fischer- Tropsch or mineral derived kerosene components, Fischer- Tropsch or mineral derived naphtha components, so-called "winter GtL" Fischer-Tropsch derived gas oils, low viscosity mineral oil diesel components or biodiesel components. Such base fuels may in some cases have a V 40 (ASTM D-445 or EN ISO 3104) that is below the maximum permitted by the European diesel fuel specification EN 590, for instance below 4.5 mm²/s, or below 3.5, 3.2 or 3.0 mm²/s. In cases they may have a VK 40 below the minimum permitted by EN 590, for example below 2.0 mmVs or even below 1.5 mm²/s. The VI improving additive may be pre-diluted in one or more such fuel components, prior to its incorporation into the final automotive fuel

compositio .

An automotive diesel fuel composition prepared according to the present invention will suitably comply with applicable current standard specificatio (s) such as, for example, EN 590 (for Europe) or ASTM D-975 (for the USA) . By way of example, the overall fuel composition may have a density from 820 to 845 kg/m³ at 15°C (ASTM D- 4052 or EN ISO 3675); a T95 boiling point (ASTM D-86 or EN ISO 3405) of 360°C or less; a measured cetane number (ASTM D-613) of 51 or greater; a VK 40 {ASTM D-445 or EN ISO 3104} from 2 to 4.5 mm²/s; a sulphur content (ASTM D- 2622 or EN ISO 20846) of 50 mg/kg or less; and/or a polycyclic aromatic hydrocarbons {PAH) content {IP 391 (mod) ) of less than 11% w/w . Relevant specifications may, however, differ from country to country and from year to year, and may depend on the intended use of the fuel compositio .

It will be appreciated, however, that diesel fuel composition prepared according to the present invention may contain fuel components with properties outside of these ranges, since the properties of an overall blend may differ, often significantly, from those of its individual constituents,

A diesel fuel composition prepared according to the present invention 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, beneficially no more than 350 ppmw, suitably no more than 100 or 50, or even 10 ppmw of sulphur.

An automotive fuel composition prepared according to the present invention, or a base fuel used in such a composition may contain one or more fuel additives, or may be additive-f ee. If additives are included (e.g. added to the fuel at the refinery) , it may contain minor amounts of one or more additives. Selected examples or suitable additives include (but are not limited to) :

anti-static agents; pipeline drag reducers; flow

improvers (e.g. ethylene/vinyl acetate copolymers or acrylate/maleic anhydride copolymers) ; lubricity

enhancing additives (e.g. ester- and acid-based

additives); 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) ; 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); corrosion inhibitors;

reodorants; anti-wear additives; antioxidants (e.g.

phenolics such as 2 , 6-di-tert-butylphenol ) ; metal

deactivators; combustion improvers; static dissipator additives; cold flow improvers (e.g. glycerol monooleate, di-isodecyl adipate) ; antioxidants; and wax anti-settling agents. The composition may for example contain a

detergent. Detergent-containing diesel fuel additives are known and commercially available. Such additives may be added to diesel fuels at levels intended to reduce, remove or slow the build up of engine deposits. In some embodiments, it may be advantageous 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 .

Where the composition contains such additives (other than the viscosity increasing components of the

invention) , it suitably contains a minor proportion (such as 1% w/w or less, 0.5% w/w or less, 0.2% w/w or less), of the one or more fuel additives, in addition to the viscosity increasing component { s ) . Unless otherwise stated, the (active matter) concentration of each such additive component in the fuel composition may be up to 10000 ppmw, such as in the range of 0.1 to 1000 ppmw; and advantageously from 0.1 to 300 ppmw, such as from 0.1 to 150 ppmw .

If desired, one or more additive components, such^* as those listed above, may be co-mixed (e.g. together with suitable diluent) in an additive concentrate, and the additive concentrate may then be dispersed into a base fuel or fuel composition. The viscosity increasing component, particularly the VI improver may, in

accordance with the present invention, be incorporated into such an additive formulation. Such a fuel additive mixture typically contains 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-i4 alcohol mixture which is commercially available) .

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

As used herein, amounts (e.g. concentrations, ppmw and %w/w) of components are of active matter, i.e.

exclusive of volatile solvents/diluent materials.

In one embodiment, the present invention involves adjusting the viscosity of the fuel composition, using the viscosity increasing component (e.g. a VI improving additive) , in order to achieve a desired target

viscosity. Suitably, the viscosity increasing component or VI improver increases the viscosity of the fuel composition by at least 0.05 mm²/s and less than 2.0 mm²/s, as previously noted. More suitably, the viscosity increase is between 0.25 mm²/s and 1.0 mm²/s, such as between 0.3 rom/s and 0.8 mm²/s. In some particular embodiments, the viscosity increase is approximately 0.32 mm²/s,

approximately 0.67 imVs, or any value in between these ranges .

The maximum viscosity of an automotive fuel

composition may often be limited by relevant legal and/or commercial specifications, such as the European diesel fuel specification EN 590 that stipulates a maximum VK 40 of 4.5 mm²/s, whilst a Swedish Class 1 diesel fuel must have a VK 40 of no greater than 4.0 mm²/s. Typical commercial automotive diesel fuels are currently

manufactured to far lower viscosities than these, however, such as around 2 to 3 mm²/s. Thus, the present invention may involve manipulation of an otherwise standard specification automotive fuel composition, using a VI improving additive, to increase its viscosity so as to improve the fuel economy of an engine into which it is, or is intended to be, introduced, while remaining within desired or legal viscosity ranges.

In some preferred embodiments, the density of the fuel composition is affected by less than 1%, such as less than 0.1% by addition of the viscosity increasing component, for example, as measured using the standard test method ASTM D-4052 or EN ISO 3675.

According to another aspect of the invention, there is provided a process for the preparation of an

automotive fuel composition, which process involves blending an automotive base fuel with a viscosity increasing component. The blending may be carried out for one or more of the purposes described herein.

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.

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. Thus, features of the "uses" of the invention are directly applicable to the "methods" of the invention. Moreover, unless stated otherwise, any feature disclosed herein may be replaced by an alternative feature serving the same or a similar purpose.

The invention will now be further illustrated by way of the following non-limiting examples.

Examples

Introduction

In these examples the results of a bench engine test program to evaluate the influence of fuel viscosity on diesel fuel economy is reported. Ά standard diesel was compared against the same diesel fuel containing

different concentrations of a viscosity increasing component. As the viscosity increasing component, a VI improver, in particular, ShellVis 200 ("SV200") was used.

1. Test Platform and Test Cycle

To assess the potential influence of fuel viscosity on diesel fuel economy a study was carried out using a Mercedes Benz 2.21 diesel bench engine (OM646.963L - the "OM646 engine"} . The OM646 engine is a common rail diesel engine installed on PAE test stand 007. Relevant

technical information / date for the OM646 test engine used is shown in Table 1 below.

Table 1

The OM646 engine has an "open" EMS and therefore the ability to have all relevant EMS data recorded. Prior to the start of tests the engine test bed was prepared and preconditioned with the aim of achieving good

repeatability of the individual tests. For this the engine is not stopped between repeat cycles but force- cooled to 28°C. To assist in identifying the fuel

consumption differences and the engine parameters that cause the effects, the following parameters were logged during the test programme: (i) oil temperature; (ii) coolant temperature; (iii) air temperature; (iv) fuel temperature; and (v) cell temperature.

1.1 Driving Cycle

As test cycle the non-transient New European Driving Cycle (NEDC) was selected (Figure 1}. The NEDC driving cycle consist of four repeated urban driving cycles {ECE} and an extra-urban driving cycle {EUDC) _r which accounts for higher speed driving modes. The NEDC is a widely recognised industry standard test cycle.

A detailed separate fuel consumption analysis of the initial ^cold start" and the following city cycles and the overland cycle was conducted.

For this program a single fuel economy test run consisted of 20 consecutive NEDCs with forced cooling down to 28 °C between each cycle. Therefore the NEDC cycles are not exact replications of the standard cold- start NEDC test, which has a minimum 6-hour soak period between tests and is cooled to approximately 23°C.

2. Test Fuels and Test Design

An overview of all test fuels used in the study is given in Table 2. As illustrated, fuel composition Al was obtained from base fuel composition AO by adding FAME to 5% w/w. Test fuels Bl and B2 were then obtained from fuel Al by adding VI improver SV200 at concentrations of 1000 mg/kg or 2000 mg/kg respectively. The resultant test fuels had absolute fuel viscosity increases at 40°C of 0.32 and 0.67 mm²/s, for Bl and B2, respectively, as shown in Table 4. These higher viscosity fuels (Bl and B2} were referenced against a zero-sulphur diesel(ZSD) base fuel Al.

Table 2

As described in section 1.1, the test results are the average fuel economy results over 20 cycles. In addition to the combined NEDC cycle results, separate fuel consumption data for the four 1 km ECE cycles

(Phases 1 to 4) and the EUDC cycle (Phase 5) were

determined. Table 3 shows the test sequence that was used for the assessment.

Table 3

Further analytical details of selected fuels are provided

in Table 4.

Table 4

Table 4 {continued)

2.1 Test Results

The measurements for the combined NEDC test are plotted in Figure 2 . There were no issues with data quality and all test results were used in the statistical analysis. These data illustrate that the engine using both test fuels of the inventio , containing viscosity increasing components, exhibited improved fuel economy in comparison to its performance when run on an otherwise identical control fuel lacking a viscosity increasing component.

The fuel economy benefits from each of the

individual phases of the test cycles were also analysed and reported in Table 5; and the same results are illustrated graphically in Figure 3. As illustrated,- at a concentration of 1000 mg/kg of SV200 additive, a fuel economy benefit of 0.15% was observed; while at a concentration of 2000 mg/kg of SV200 additrve, a fuel economy benefit of 0.59% could be identified (with a statistical significance of 99%) .

Table 5

More detailed test data of the program for the combined test cycle as well as for the individual sub- cycles are provided in Table 6. For each phase of the test protocol and for each fuel the weight of fuel used (in grams) is shown. A detailed illustration of these results is represented graphically in Figure 4, in which: graph A represents the fuel usage over the entire NEDC test protocol; graph B represents the fuel usage over Phase 1 of the test protocol; graph C represents the fuel usage over Phase 2 of the test protocol; graph D

represents the fuel usage over Phase 3 of the test protocol; graph E represents the fuel usage over Phase 4 of the test protocol; and graph F represents the fuel usage over the EUDC phase of the test protocol.

3. Conclusions

The objective of this study was to evaluate the influence of fuel viscosity on diesel fuel economy in the OM646 bench engine. As shown^" in Table 5, use of the commercially available viscosity improver, ShellVis 200 at 2000 mg/kg resulted in a fuel economy benefit compared to the control fuel of approximately 0.6% over the NEDC cycle at a significance level of 99%. Also as indicated, in some cycles the fuel economy benefit in the test was over 1% compared with a control fuel lacking the

viscosity increasing component. In these tests the fuel economy benefits observed during the urban driving cycles were on average higher with the higher concentration of VI improver (i.e. approximately 0.75%}, than during the extra-urban driving cycles. This effect was reversed, however, for the fuel with a lower concentration of VI improver. Notably, the fuel economy benefit was

consistently positive in all phases and with all tested concentrations of viscosity increasing component. In general, the benefits in fuel economy were greater using the higher concentration of VI improver than with the lower concentration (i.e. 1000 mg/kg) of VI improver.

This work represents the first correlation between viscosity of diesel fuel compositions and fuel economy. This relationship may be used in blending desirable fuel compositions, and for selecting fuels for blending based not only on desirable properties such as emissions performance, engine cleaning effect, power and/or acceleration; but also for fuel economy under all or particular driving conditions.

Claims

C L A I M S

1. Use of a viscosity increasing component in an diesel fuel composition, for the purpose of improving the fuel economy of an engine into which the fuel composition is or is intended to be introduced, or of a vehicle powered by such an engine.

2. The use of Claim 1, wherein the viscosity increasing component is a viscosity index (VI) improving additive.

3. The use of Claim 2, wherein the VI improving

additive comprises a block copolymer, which contains one or more monomer blocks selected from ethylene, propylene, butylene, butadiene, isoprene and styrene monomers.

4. The use of Claim 2 or Claim 3, wherein the VI improving additive comprises a polystyrene-polyisoprene stellate copolymer.

5. The use of any of Claims 2 to 4, wherein the VI improving additive is used at a concentration of between:

(i) 0.01% w/w and 1.0% w/w;

(ii) 0.05% w/w and 0.5% w/w; or

(iii) 0.1% w/w and 0.3% w/w;

based on the total weight of the fuel composition.

6. The use of Claim 1, wherein the viscosity increasing component is a minor amount of a high viscosity diesel fuel or oil component having a higher kinematic viscosity at 40°C than the major component of the diesel fuel composition.

7. The use of Claim 6, wherein the high viscosity diesel fuel or oil component is used at a concentration of between:

(i) 1% w/w and 30% w/w;

(ii) 2% w/w and 20% w/w; (iii) 3% w/w and 10% w/w; or

(iv) 5% w/w and 8% w/w;

based on the total weight of the fuel composition.

8. The use of any preceding claim, wherein the

viscosity increasing component is used in an amount sufficient to increase the kinematic viscosity of the diesel fuel composition at 40°C by:

(i) at least 0.2 mm²/s;

(ii) 0.25 mm²/s to 1.0 mm²/s; or

(iii) 0.32 mm²/s to 0.67 mm²/s;

compared to the viscosity of the fuel composition prior to the addition of the viscosity increasing component.

9. The use of any preceding claim, wherein the

kinematic viscosity at 40°C of the diesel fuel

composition comprising the viscosity increasing component is in the range of:

(i) up to 4.5 mm²/s;

(ii) between 2.0 mm²/s and 4.0 mm²/s; or

(iii) between 3.0 mm²/s and 3.8 mm/s.

10. The use of any preceding claim, wherein the diesel fuel composition comprises a biofuel.

11. The use of any preceding claim, wherein the diesel fuel composition comprises:

(i) at least 2% w/w fatty acid methyl esters

(FAME) ;

(ii) between 5% w/w and 50% w/w FAME; or

(iii) approximately 5% w/w FAME;

based on the total weight of the fuel composition.

12. The use of any preceding claim, which comprises two or more viscosity increasing components.

13. A method for improving the fuel economy of an engine or of a vehicle powered by such an engine, the method comprising introducing into a combustion chamber of the engine a {diesel) fuel composition comprising a viscosity increasing component.

14. The method of Claim 13, wherein the viscosity increasing component is defined according to any of Claims 1 to 12.

15. A method of operating a compression ignition engine and/or a vehicle which is powered by such an engine, which method involves introducing into a combustion chamber of the engine a fuel composition obtained with the use according to any of Claims 1 to 12, or with the methods of Claims 13 or 14.