Compositions and methods and uses relating thereto

Abstract

Method, use, and composition wherein at least one ester, amide, or imide compound and at least one small, functionalised aromatic compound are used to improve the lubricity of diesel fuel. The additiv

Description

The present invention relates to fuel compositions and to methods and uses relating thereto. In particular the present invention relates to additive combinations for improving the lubricity of diesel fuels. Natural untreated mineral diesel fuel comprises sulfur containing compounds and polyaromatic compounds which impart lubricity to the fuel. However hydrodesulfurisation of these fuels for environmental reasons reduces the amount of sulfur, polyaromatic compounds and other polar species present in the fuel. Fuels treated in this way therefore have low lubricity and thus are unable to adequately lubricate and protect parts of the fuel injection system leading to increased wear. It is therefore common practice to add lubricity improving additives to such fuels. In recent years there has been an increased demand to find alternatives to mineral diesel fuels. Biodiesel fuels comprising fatty acid esters have been known for some time. These fuels have a level of natural lubricity. However this is not always sufficient to prevent wear in an engine, especially when biodiesel is blended with other fuels. More recent developments have led to the introduction of renewable fuels obtained by the hydrotreatment of triglyceride oils, for example vegetable oils. These materials consist primarily of saturated hydrocarbons and do not contain significant levels of aromatic or polar species. Consequently such fuels (commonly referred to as hydrotreated vegetable oils or HVO) have very low natural lubricity. Other fuels comprising high levels of paraffinic species also have poor lubricity. Such fuels include, for example, synthetic fuels made via Fischer Tropsch synthesis, also known as gas to-liquid fuels. Although it is known to add lubricity improvers to fuels to protect engine parts which come into contact with the fuel from wear, achieving acceptable lubricity performance can be challenging, especially for highly paraffinic fuels which contain few or no components that provide natural lubricity. The industry standard test method for evaluating lubricity improving additives measures the wear between two metals components in a high frequency reciprocating rig (HFRR). The present inventors have surprisingly found that the addition of certain aromatic compounds in combination with ester, amide or imide compounds can significantly improve the lubricity of diesel fuel compositions, including highly paraffinic diesel fuel compositions. According to a first aspect of the present invention there is provided a method of improving the lubricity of a diesel fuel composition, the method comprising admixing into the composition: (a) at least one ester, amide or imide compound; and (b) at least one small, functionalised aromatic compound. According to a second aspect of the present invention there is provided the use of the combination of: (a) at least one ester, amide or imide compound and (b) at least one small, functionalised aromatic compound to improve the lubricity of a diesel fuel composition. According to a third aspect of the present invention there is provided a diesel fuel composition comprising a major proportion of a diesel fuel and: (a) at least one ester, amide or imide compound; and (b) at least one small, functionalised aromatic compound. According to a fourth aspect of the present invention there is provided a method of preparing a diesel fuel composition, the method comprising dosing into a diesel fuel: (a) at least one ester, amide or imide compound; and (b) at least one small, functionalised aromatic compound. Preferred features of the first, second, third and fourth aspects of the invention will now be described. The methods and use of the first, second and fourth aspects preferably provide a fuel composition of the third aspect. The diesel fuel composition provided by the present invention comprises a major proportion of a diesel fuel. By diesel fuel we mean any fuel suitable for use in a diesel engine, either for road use or nonroad use. This includes but is not limited to fuels described as diesel, marine diesel, railway diesel, heavy fuel oil, industrial fuel oil, etc. The diesel fuel composition used in the present invention may comprise a petroleum-based fuel oil, especially a middle distillate fuel oil. Such distillate fuel oils generally boil within the range of from 110°C to 500°C, e.g. 150°C to 400°C. The diesel fuel may comprise atmospheric distillate or vacuum distillate, cracked gas oil, or a blend in any proportion of straight run and refinery streams such as thermally and / or catalytically cracked and hydro-cracked distillates. The diesel fuel composition may comprise non-renewable Fischer-Tropsch fuels such as those described as GTL (gas-to-liquid) fuels, CTL (coal-to-liquid) fuels and OTL (oil sands-to-liquid). In some embodiments the diesel fuel comprises mineral diesel. In some embodiments the diesel fuel comprises biodiesel. In some embodiments the diesel fuel comprises renewable diesel. In some preferred embodiments the diesel fuel comprises mineral diesel and one or more further components selected from biodiesel, renewable diesel and mixtures thereof. In some embodiments the diesel fuel comprises mineral diesel and biodiesel. In some embodiments the diesel fuel comprises mineral diesel and renewable diesel. In some embodiments the diesel fuel comprises mineral diesel, biodiesel and renewable diesel. In some embodiments the diesel fuel comprises biodiesel and renewable diesel. By mineral fuels herein we mean fuels derived wholly from mineral (i.e. petroleum) sources. In this specification by biodiesel we mean to refer to esters of fatty acids. Such fuels are commonly referred to as first generation biodiesel. Biodiesel as defined herein contains esters of, for example, vegetable oils, animal fats and used cooking fats. This form of biodiesel may be obtained by transesterification of oils, with an alcohol, usually a monoalcohol, usually in the presence of a catalyst. The fatty acids used to produce the fuel may originate from a wide variety of natural sources including, but not limited to, vegetable oil, canola oil, safflower oil, sunflower oil, nasturtium seed oil, mustard seed oil, olive oil, sesame oil, soybean oil, com oil, peanut oil, cottonseed oil, rice bran oil, babassu nut oil, castor oil, palm oil, rapeseed oil, low erucic acid rapeseed oil, palm kernel oil, lupin oil, jatropha oil, coconut oil, flaxseed oil, evening primrose oil, jojoba oil, camelina oil, tallow, beef tallow, butter, chicken fat, lard, dairy butterfat, shea butter, used frying oil, oil miscella, used cooking oil, yellow trap grease, hydrogenated oils, derivatives of the oils, fractions of the oils, conjugated derivatives of the oils, and mixtures of any thereof. The diesel fuel composition may comprise renewable diesel, obtained by the hydrodeoxygenation of fats and oils. In some embodiments the diesel fuel composition may comprise a pyrolysis fuel oil, i.e. a middle distillate fraction obtained from distillation of a pyrolysis oil. The pyrolysis oil may be obtained from the pyrolysis of any type of waste, and the components and properties of the pyrolysis oil and the distillate fraction obtained therefrom will depend on the types of waste that was pyrolysed and the pyrolysis conditions. The pyrolysis oil may, for example, be obtained from the pyrolysis of plastic waste, agricultural waste, forestry waste, waste cooking oils, algae waste, used tyres and rubber waste. Preferred pyrolysis oils are plastic pyrolysis oils. These may be obtained from the pyrolysis of any type of plastic. However preferred plastic pyrolysis oils are obtained from the pyrolysis of one or polymers selected from low density polyethylene, high density polyethylene, ultra high density polyethylene, polypropylene, PET, polyacrylate, polynitrile and mixtures thereof. Middle distillate fuel oils obtained from pyrolysis oils may optionally be hydrotreated and / or treated using a cracking process. However, due to the typically low aromatic and sulfur content of middle distillate fuel oils obtained from pyrolysis oils it is possible to use straight run distillates. The diesel fuel composition used in the present invention may contain blends of any or all of the above diesel fuel compositions. In some embodiments the diesel fuel composition may be a blended diesel fuel comprising biodiesel. In such blends the biodiesel may be present in an amount of (by volume), for example up to 0.5%, up to 1%, up to 2%, up to 3%, up to 4%, up to 5%, up to 10%, up to 20%, or up to 30%. A fuel which comprises 100% biodiesel is denoted as B100, a fuel which comprises 90% mineral diesel and 10% biodiesel (by volume) is known as B10; fuel comprising 50% mineral diesel and 50% biodiesel (by volume) is known as B50; and so on. Preferred diesel fuel compositions for use herein comprise less than 10% biodiesel, preferably less than 5% (by volume). In some embodiments the diesel fuel composition may be a blended diesel fuel comprising renewable diesel. In such blends the renewable diesel may be present in an amount of (by volume), for example up to 0.5%, up to 1%, up to 2%, up to 3%, up to 4%, up to 5%, up to 10%, up to 20%, up to 30%, up to 40%, up to 50%, up to 60%, up to 70%, up to 80%, up to 90%, up to 95% or up to 99%. In some embodiments the fuel composition may comprise neat renewable diesel. A fuel which comprises 100% renewable diesel is denoted as R100, a fuel which comprises 90% mineral diesel and 10% renewable diesel (by volume) is known as R10; fuel comprising 50% mineral diesel and 50% renewable diesel (by volume) is known as R50; and so on. In some embodiments the diesel fuel comprises mineral diesel and one or more further components selected from biodiesel, renewable diesel and mixtures thereof. In some embodiments the diesel fuel comprises mineral diesel and at least 5 vol% of a fuel selected from biodiesel, renewable diesel and mixtures thereof. In some embodiments the diesel fuel comprises mineral diesel and at least 5 vol% biodiesel. In some embodiments the diesel fuel comprises mineral diesel and at least 5 vol% renewable diesel. In some embodiments the diesel fuel comprises mineral diesel and from 1 to 30 vol%, preferably from 1 to 20 vol%, more preferably from 1 to 10 vol% of a fuel selected from biodiesel, renewable diesel and mixtures thereof. In some embodiments the diesel fuel comprises mineral diesel and from 1 to 30 vol%, preferably from 1 to 20 vol%, more preferably from 1 to 10 vol% biodiesel. In some embodiments the diesel fuel comprises mineral diesel and from 1 to 30 vol%, preferably from 1 to 20 vol%, more preferably from 1 to 10 vol% renewable diesel. In some embodiments the diesel fuel comprises mineral diesel; from 1 to 30 vol%, preferably from 1 to 20 vol%, more preferably from 1 to 10 vol% biodiesel; and from 1 to 30 vol%, preferably from 1 to 20 vol%, more preferably from 1 to 10 vol% renewable diesel. The present invention is particularly useful for improving the lubricity of fuels having very low natural lubricity. In some embodiments the diesel fuel composition comprises a paraffinic fuel. In some embodiments the diesel fuel composition may be a blended fuel comprising a paraffinic fuel and a further fuel component, for example a mineral diesel or biodiesel component. In some preferred embodiments the paraffinic fuel provides at least 90 vol%, preferably at least 99 vol% of all fuel present in the fuel composition. By a paraffinic fuel we mean to refer to a fuel that is highly paraffinic in nature. Paraffinic fuels typically comprise predominantly paraffin compounds. By paraffin compounds we mean to refer to saturated hydrocarbon compounds, commonly known as alkanes. Preferably the paraffinic fuel comprises less than 5 wt% non-paraffinic compounds, preferably less than 3 wt%, more preferably less than 1 wt%, suitably less than 0.5 wt%, for example less than 0.1 wt% or less than 0.01 wt% Preferably the paraffinic fuel comprises less than 10 wt% aromatic compounds, preferably less than 5 wt% more preferably less than 1 wt%, suitably less than 0.5 wt%. Preferably the paraffinic fuel comprises less than 10000 ppm aromatic compounds, preferably less than 5000 ppm, suitably less than 2500 ppm. In some embodiments the paraffinic fuel comprises less than 1000 ppm aromatic compounds, for example less than 500 ppm or less than 350 ppm. In this specification, unless otherwise specified ppm refers to parts per million by weight. Aromatic content may be measured by any suitable method. Such methods will be known to the person skilled in the art. Preferably aromatic content is measured according to the standard method described in IP 391. Preferably the paraffinic fuel has a sulfur content of less than 100 ppm, preferably less than 50 ppm, more preferably less than 10 ppm, for example less than 5 ppm. Preferably the paraffinic fuel comprises less than 5 wt% oxygenated compounds, preferably less than 3 wt%, more preferably less than 1 wt%, suitably less than 0.5 wt%, for example less than 0.1 wt% or less than 0.01 wt%. By oxygenated compounds we mean to refer to compounds including an oxygen-containing functional group, for example esters, ethers and alcohols. Preferably the paraffinic fuel comprises less than 5 wt% unsaturated compounds, preferably less than 3 wt%, more preferably less than 1 wt%, suitably less than 0.5 wt%, for example less than 0.1 wt% or less than 0.01 wt%. One class of suitable paraffinic fuels are synthetic fuels. These fuels include Fischer-Tropsch fuels such as those described as GTL (gas-to-liquid) fuels, CTL (coal-to-liquid) fuels and OTL (oil sands-to-liquid). In a preferred embodiment the paraffinic fuel comprises hydrotreated triglyceride oil. This fuel is sometimes referred to as renewable diesel fuel. By hydrotreated triglyceride oil or renewable diesel we mean to refer to diesel fuel obtained by the hydrodeoxygenation of fats and oils. Such fuels are also often referred to as second generation biodiesel and are derived from renewable resources such as vegetable oils, fish oils and animal oils. These oils are processed, often in the refinery, using, for example, hydroprocessing. Hydroprocessing processes include the H-Bio process developed by Petrobras. Especially preferred hydrotreated triglyceride oils are hydrotreated vegetable oils or HVO fuel. HVO fuel is marketed by ConocoPhillips as Renewable Diesel and by Neste as NExBTL. The paraffinic fuel used in the present invention may comprise fuel commonly known as third generation biodiesel. Third generation biodiesel utilises gasification and Fischer-Tropsch technology including those described as BTL (biomass-to-liquid) fuels. Third generation biodiesel does not differ widely from some second generation biodiesel or hydrotreated triglyceride oil fuels, but aims to exploit the whole plant (biomass) and thereby widens the feedstock base. The paraffinic fuel is preferably produced from raw materials of biological origin. These may suitably be selected from vegetable oils, animal fats, fish oils and mixtures thereof. Examples include rapeseed oil, canola oil, tall oil, sunflower oil, soybean oil, hemp oil, olive oil, linseed oil, mustard oil, carinata oil, palm oil, palm kernel oil, peanut oil, castor oil, coconut oil, animal fats such as tallow or recycled food fats, raw materials resulting from genetic engineering, and biological raw materials produced from microorganisms such as algae and bacteria. Preferably, the paraffinic fuel is provided by a process involving hydrodeoxygenation (HDO) and optionally isomerization steps. The hydrodeoxygenation (HDO) step results in the decomposition of the structures of the biological esters or of the triglyceride constituents, in the elimination of the oxygen-bearing, phosphorus-bearing and sulfur-bearing compounds and in the hydrogenation of olefinic bonds. The product resulting from the hydrodeoxygenation reaction may then be isomerized. A fractionation step may optionally follow the hydrodeoxygenation and isomerization steps. Preferably the paraffinic fuel has a cetane number of between 50 and 90, preferably between 55 and 90, more preferably between 60 and 85. Cetane number is suitably measured by the standard test method set out in IP 498. Preferably the paraffinic fuel has a cloud point of less than 25°C, more preferably less than 10°C. Suitably the paraffinic fuel has a cloud point of less than -5°C, for example less than -10°C. Cloud point may suitably be measured using the standard test method described in IP 219. Preferably the paraffinic fuel has a kinematic viscosity at 40°C of 1 to 20 mm2s'1, preferably from 2 to 15 mm2s'1, more preferably from 2 to 10 mm2s1, most preferably from 2 to 4.5 mm2s'1. Kinematic viscosity may be measured according to ASTM D445. Preferably the paraffinic fuel has an initial boiling point (IBP) and a final boiling point (FBP) within the range 135 to 380°C, such as 265 to 380°C, more preferably within the range 275 to 380°C and most preferably within the range 290 to 375°C. Preferably the paraffinic fuel has a boiling range (final boiling point - initial boiling point) of less than 180°C, suitably less than 120°C, such as less than 80°C, preferably less than 70°C, suitably less than 60°C, for example from 30 to 60°C. Boiling range is used to refer to the difference between the final boiling point and the initial boiling point. The initial boiling point, final boiling point and boiling range can be determined according to the method set out in IP 123. The paraffinic fuel used suitably consists essentially of paraffinic compounds. Preferably the fuel may comprise n-paraffins (or straight chain alkanes), isoparaffins (i-paraffins or branched alkanes) or mixtures thereof. In some embodiments the paraffinic fuel may further comprise cycloalkanes (also known as naphthenes). Examples of paraffinic fuels comprising cycloalkanes are described, for example in WO2021 / 250115. In preferred embodiments the paraffinic fuel comprises predominately straight chain alkanes and branched alkanes. Preferably the paraffinic fuel comprises less than 20 wt% cycloalkanes, preferably less than 10 wt%, suitably less than 5 wt%, preferably less than 1 wt%, for example less than 0.1 wt%. For the avoidance of doubt by the term cycloalkane or napthene is used to refer to any saturated hydrocarbon compound which includes a non-aromatic cyclic moiety. Preferably the weight of ratio n-paraffins to i-paraffins present in the paraffinic fuel is from 99:1 to 1:99, more preferably from 90:10 to 10:90, preferably from 75:25 to 25:75. Techniques for determining the ratio of n-paraffins to i-paraffins are known to the person skilled in the art and include gas chromatography. In some preferred embodiments the weight of ratio n-paraffins to i-paraffins present in the paraffinic fuel is from 1:99 to 20:80. Ratios of n-paraffins and i-paraffins present in a fuel typically depend on the hydrotreatment method used to prepare the fuel, which may also include an isomerisation step. The paraffinic fuel used may comprise greater than 4 wt%, preferably greater than 5 wt%, of C14 to C16 n-alkanes. The paraffinic fuel may comprise greater than 5 wt%, preferably greater than 7 wt%, more preferably greater than 10 wt%, of C14 to C18 n-alkanes. The paraffinic fuel may comprise less than 8 wt%, preferably less than 6 wt%, of C14 to C16 n-alkanes. The paraffinic fuel may comprise less than 20 wt%, preferably less than 18 wt%, more preferably less than 16 wt%, of C14 to C18 n-alkanes. The paraffinic fuel may comprise greater than 4 wt%, preferably greater than 5 wt%, of C14 to C16 n-alkanes and less than 8 wt%, preferably less than 6 wt%, of C14 to C16 n-alkanes. The paraffinic fuel may comprise greater than 5 wt%, preferably greater than 7 wt%, more preferably greater than 10 wt%, of C14 to C18 n-alkanes and less than 20 wt%, preferably less than 18 wt%, more preferably less than 16 wt%, of C14 to C18 n-alkanes. The paraffinic fuel may comprise from 4 to 8 wt%, preferably from 5 to 6 wt%, of C14 to C16 n-alkanes. The paraffinic fuel may comprise from 5 to 20 wt%, preferably from 7 to 18 wt%, more preferably from 10 to 16 wt%, of C14 to C18 n-alkanes. The paraffinic fuel may comprise from 3 to 30 wt% of C6 to C24 n-alkanes (i.e. n-paraffin). Suitably the paraffinic fuel complies with the standard specification set out in EN15940. In preferred embodiments the paraffinic fuel is a hydrotreated triglyceride oil, for example a hydrotreated vegetable oil. Preferably the paraffinic fuel is a hydrotreated triglyceride oil having an aromatic content of less than 2500 ppm, preferably less than 500 ppm; and a sulfur content of less than 50 ppm. Preferably the paraffinic fuel is a hydrotreated triglyceride oil fuel having a cetane number of between 50 and 90, preferably between 55 and 90 (according to IP 498); a cloud point of less than 10°C, preferably less than -5°C (according to IP 219); and a kinematic viscosity at 40°C of from 1 to 20 mm2s'1, preferably from 2 to 10 mm2s'1 (according to ASTM 445). Preferably the paraffinic fuel is a hydrotreated triglyceride oil having an initial boiling point and a final boiling point within the range 135 to 380°C, such as 265 to 380°C, preferably 290 to 375°C and a boiling range (final boiling point - initial boiling point) of less than 180°C, suitably less than 120°C, such as less than 80°C, preferably 30 to 60°C. In the present invention (a) at least one ester, amide or imide compound and (b) at least one small, functionalised aromatic compound are added to a diesel fuel. Any suitable ester, amide or imide compound may be used as component (a). Suitably component (a) comprises an ester lubricity additive, a hydrocarbyl substituted amide compound, a hydrocarbyl substituted imide compound or mixtures thereof. The ester, amide or imide compounds of component (a) are suitably obtained by reaction of the carboxylic acid or reactive equivalent thereof with an alcohol or an amino compound. Ester compounds are suitably the reaction product of a carboxylic acid or reactive equivalent thereof and an alcohol. Amide and imide compounds are suitably the reaction product of a carboxylic acid or a reactive equivalent thereof and an amino compound. Mixed compounds comprising an ester functional group and an amide functional group are also within the scope of the invention. By a reactive equivalent of a carboxylic acid we mean to include any functional group which may react in the same way as a carboxylic acid to form an ester, amide or imide moiety. Such compounds are sometimes referred to as acylating agents. Suitable acylating agents include acid anhydrides and acid chlorides, as well as carboxylic acids. Preferred acylating agents are hydrocarbyl substituted. The carboxylic acid or reactive equivalent thereof used to prepare the ester lubricity additive may comprise or be derived from a monocarboxylic acid, a dicarboxylic acid, or a polycarboxylic acid. Preferably the acylating agent includes a hydrocarbyl moiety. The hydrocarbyl moiety is suitably a straight chain or branched alkyl or alkenyl chain. As used herein, the term "hydrocarbyl substituent", "hydrocarbyl moiety" or "hydrocarbyl group" is used in its ordinary sense, which is well-known to those skilled in the art. Specifically, it refers to a group having a carbon atom directly attached to the remainder of the molecule and having predominantly hydrocarbon character. Examples of hydrocarbyl groups include: (i) hydrocarbon groups, that is, aliphatic (which may be saturated or unsaturated, linear or branched, e.g., alkyl or alkenyl), alicyclic (e.g., cycloalkyl, cycloalkenyl) substituents, and aromatic-, aliphatic-, and alicyclic-substituted aromatic substituents, as well as cyclic substituents wherein the ring is completed through another portion of the molecule (e.g., two substituents together form a ring); (ii) substituted hydrocarbon groups, that is, substituents containing non-hydrocarbon groups which, in the context of this invention, do not alter the predominantly hydrocarbon nature of the substituent (e.g., halo (e.g. chloro,fluoro or bromo), hydroxy, alkoxy (e.g. Ci to C4 alkoxy), keto, acyl, cyano, mercapto, amino, amido, nitro, nitroso, sulfoxy, nitryl and carboxy); (iii) hetero substituents, that is, substituents which, while having a predominantly hydrocarbon character, in the context of this invention, contain other than carbon in a ring or chain otherwise composed of carbon atoms. Heteroatoms include sulphur, oxygen, nitrogen, and encompass substituents as pyridyl, furyl, thienyl and imidazolyl. In general, no more than two, preferably no more than one, non-hydrocarbon substituent will be present for every ten carbon atoms in the hydrocarbyl group; typically, there will be no non-hydrocarbon substituents in the hydrocarbyl group. Preferably the hydrocarbyl moiety has from 2 to 200 carbon atoms, suitably from 4 to 150 carbon atoms, for example from 6 to 100 carbon atoms. Preferably the hydrocarbyl moiety is aliphatic. Preferably the hydrocarbyl moiety does not include any aromatic functional groups. In some embodiments the carboxylic acid or reactive equivalent thereof (or acylating agent) includes a hydrocarbyl moiety having from 2 to 50 carbon atoms, suitably from 4 to 40 carbon atoms, for example from 6 to 36 carbon atoms. The hydrocarbyl moiety is suitably a straight chain or branched alkyl or alkenyl chain. Preferably the acylating agent does not include an aromatic functional group. In some preferred embodiments the carboxylic acid or reactive equivalent thereof is selected from a monocarboxylic acid, a dicarboxylic acid or an anhydride. In some embodiments the carboxylic acid or reactive equivalent thereof is a polycarboxylic acid, for example a dicarboxylic acid, or a reactive equivalent thereof, for example an anhydride. In some embodiments the carboxylic acid or reactive equivalent thereof may be a hydrocarbyl substituted succinic acid or a hydrocarbyl substituted succinic anhydride. Hydrocarbyl substituted succinic acids or anhydrides are typically prepared by the reaction of an alkene with maleic anhydride. The product may be optionally hydrolysed to form the diacid which is then reacted with an alcohol or amino compound to form an ester, amide or imide or the anhydride can be directly reacted with an alcohol or amino compound. A variety of alkenes having a range of different molecular weights may be reacted with maleic anhydride to form the hydrocarbyl substituted succinic acid or anhydride. In some embodiments the alkene may suitably have from 2 to 50 carbon atoms, preferably from 4 to 40, for example from 6 to 36 carbon atoms. In some embodiments the alkene may be a polyolefin, for example a polyisobutene. In some embodiments the alkene may be an a-olefin. The term a-olefin is used to refer to an alkene compound having a terminal double bond. Such compounds are also commonly described as terminal alkenes. In some preferred embodiments the alkene is an internal olefin. The term internal olefin is used to refer to any alkene compound in which the alkene group is not terminal. In one embodiment an internal olefin may be a p-olefin. Internal olefins may be prepared by isomerisation of an a-olefin. In some embodiments the carboxylic acid or reactive equivalent thereof is a hydrocarbyl substituted succinic acid or anhydride thereof in which the hydrocarbyl substituent has from 4 to 40 carbon atoms, suitably from 6 to 36 carbon atoms, preferably from 10 to 32 carbon atoms, for example from 12 to 30 carbon atoms, preferably from 12 to 24 carbon atoms, for example from 12 to 20 carbon atoms. In some embodiments the hydrocarbyl substituent may have from 14 to 18 carbon atoms. In some embodiments the carboxylic acid or reactive equivalent thereof is a hydrocarbyl substituted succinic acid or anhydride thereof having a substituent derived from an a-olefin having 6 to 36 carbon atoms; preferably from 10 to 30 carbon atoms, suitably from 12 to 24 carbon atoms, for example from 14 to 18 carbon atoms. In some embodiments the carboxylic acid or reactive equivalent thereof is a hydrocarbyl substituted succinic acid or anhydride thereof having a substituent derived from an internal olefin having 6 to 36 carbon atoms preferably from 10 to 32 carbon atoms, suitably from 12 to 24 carbon atoms, for example 14 to 18 carbon atoms. In some preferred embodiments a mixture of olefins having from 15 to 18 carbon atoms are used. Such mixtures of homologues are preferred to the use of a single homologue. In some embodiments the acylating agent is a polyisobutenyl substituted succinic acid or anhydride thereof. Preferably the polyisobutenyl substituent has a number average molecular weight of from 80 to 5000, preferably from 100 to 2000, more preferably from 200 to 1300. In some preferred embodiments the polyisobutenyl substituent has a number average molecular weight of from 500 to 1500, preferably from 800 to 1300, for example about 1000. In some preferred embodiments the polyisobutenyl substituent has a number average molecular weight of from 400 to 1300, preferably from 500 to 1300, for example about 750. In some preferred embodiments the polyisobutenyl substituent has a number average molecular weight of from 100 to 600, preferably from 300 to 450, for example about 370. Other preferred polyisobutenyl substituted succinic acids or anhydrides include those having a polyisobutenyl substituent with a number average molecular weight of from 200 to 600, for example of about 260 or about 550. A polyisobutenyl substituent number average molecular weight of about 260 is especially preferred. In some embodiments the carboxylic acid or reactive equivalent thereof may comprise a monocarboxylic acid, for example a fatty acid. Suitable fatty acids include compounds of formula RXCOOH in which Rx is an alkyl or alkenyl group having 6 to 36 carbon atoms, preferably 8 to 30 carbon atoms or 12 to 24 carbon atoms. Suitable fatty acids include caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, lignoceric acid, cerotic acid, myristoleic acid, palmitoleic acid, sapienic acid, oleic acid, elaidic acid, vaccenic acid, linoleic acid, linoelaidic acid, arachidonic acid, eicosapentaenoic acid, erucic acid, undecylenic acid and docosahexenoic acid. One preferred fatty acid is oleic acid. A mixture of fatty acids may also be used, for example, tall oil fatty acid. Tall oil fatty acid comprises a mixture of predominantly unsaturated fatty acids having mostly 18 carbon atoms. Typically tall oil fatty acids comprise mainly oleic acid and linoleic acids. The skilled person will appreciate that fatty acids derived from natural sources typically comprise mixtures of compounds. Preferably the acylating agent used to form the additives of component (a) is selected from hydrocarbyl substituted succinic acids, hydrocarbyl substituted succinic anhydrides and fatty acids of formula RXCOOH in which Rx is an alkyl or alkenyl group having 6 to 36 carbon atoms. In some embodiments component (a) comprises an ester lubricity additive. Any compound which includes an ester functional group and improves the lubricity of a fuel to which it is added may be regarded as an ester lubricity additive. In some embodiments of the present invention at least one ester lubricity additive is added to the fuel. The fuel composition may therefore comprise one ester lubricity additive or two or more ester lubricity additives. References herein to “an” ester lubricity additive or “the” ester lubricity additive, unless otherwise specified, also apply to embodiments in which two or more such additives are present. As the skilled person will appreciate each ester lubricity additive may comprise a mixture of compounds. Suitably the or each ester lubricity additive comprises the reaction product of a carboxylic acid ora reactive equivalent thereof and an alcohol. In embodiments in which the carboxylic acid or reactive equivalent thereof comprises more than one carboxylic acid group or reactive equivalent thereof, one or more than one of the carboxylic acid groups may react with the alcohol to form an ester. Thus the ester lubricity additive may comprise a monoester of a dicarboxylic acid, a diester of dicarboxylic acid, a monoester of a polycarboxylic acid, a diester of a polycarboxylic acid or a polyester of a polycarboxylic acid. Any suitable alcohol may be used. In some embodiments the alcohol may be a short chain monohydric alcohol, such as a C1 to C6 alcohol, for example methanol, ethanol, propanol or butanol. Preferably the alcohol is an aliphatic alcohol. In some preferred embodiments the alcohol is not methanol. In some preferred embodiments the alcohol is not methanol or ethanol. In some preferred embodiments the alcohol does not comprise any aromatic functional groups. Preferably the alcohol does not include any amino groups. In some preferred embodiments the alcohol is not methanol. In some preferred embodiments the alcohol is not methanol or ethanol. Preferably the or each ester lubricity additive comprises the reaction product of a carboxylic acid or reactive equivalent thereof and a polyhydric alcohol. The term polyhydric alcohol is used to refer to any compound including two or more OH functional groups. In embodiments in which the ester lubricity additive is prepared from a polyhydric alcohol one or more than one of the hydroxy groups may be esterified. Preferred polyhydric alcohols suitable for use herein are compounds having from 2 to 10 hydroxy groups, preferably from 2 to 6 hydroxy groups, more preferably 2 or 3 hydroxy groups. In some preferred embodiments the carboxylic acid or reactive equivalent thereof is reacted with a polyhydric alcohol of formula H-(OR)n-OH, wherein R is an optionally substituted alkylene group and n is at least 1. Preferably n is from 1 to 10, more preferably from 1 to 4. R is an optionally substituted alkylene group. In some embodiments the alcohol of formula H-(OR)n-OH has more than 2 hydroxy groups and the group R is a hydroxy substituted alkylene group. Such a group may have 1, 2 or more hydroxy groups. In some embodiments the alcohol of formula H-(OR)n-OH may be a sugar derived unit in which R includes one or more hydroxy residues. R may be substituted to form a cyclic alkylene unit. One or more heteroatoms may be present in the cyclic alkylene unit. For example the unit may contain an ether linkage. In some embodiments R may be one or more saccharide units or may be substituted with one or more saccharide units. In some embodiments H-(OR)n-OH may be selected from glycerol, pentaerythritol and trimethylolpropane. In some embodiments H-(OR)n-OH may be a sugar component for example, trehalose or sorbitol. In some embodiments R is an unsubstituted alkylene group. Preferably R is an optionally substituted alkylene group having 1 to 50 carbon atoms, preferably 1 to 40 carbon atoms, preferably 1 to 30 carbon atoms, more preferably 1 to 20 carbon atoms, suitably 1 to 10 carbon atoms, for example 2 to 6 or 2 to 4 carbon atoms. Preferably R is an unsubstituted alkylene group having 1 to 50 carbon atoms, preferably 1 to 20, more preferably 1 to 10, suitably 2 to 6, for example 2 to 4 carbon atoms. R may be straight chained or branched. Suitably R may be an ethylene, propylene, butylene, pentylene, or hexylene group. When R has more than 2 carbon atoms any isomer may be present. In some preferred embodiments R is an ethylene or a propylene group, most preferably an ethylene group. In some embodiments in which n is 1, R may be a group of formula (CH2)x wherein x is from 2 to 12, preferably from 2 to 6. In some embodiments in which n is 1, R is a straight chain or branched alkylene group and the polyhydric alcohol is selected from ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,6-hexanediol and neopentyl glycol. In some preferred embodiments in which n is 1, R is a straight chain or branched alkylene group having 2 to 6, preferably 2 to 5 carbon atoms. Suitable compounds of this type include ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol and neopentyl glycol. R may comprise a mixture of isomers. For example when R is propylene, the polyhydric alcohol may include moieties -CH2CH(CH3)- and -CH(CH3)CH2- in any order within the chain. R may comprise a mixture of different groups for example ethylene, propylene or butylene units. R may be an ethylene, propylene or butylene group. R may be an n-propylene or n-butylene group or an isopropylene or isobutylene group. For example R may be -CH2CH2-, -CH2CH(CH3)-, -CH2CH2CH2-, -CH2C(CH3)2, -CH2CH2CH2CH2-, -CH(CH3)CH(CH3)- or -CH2CH(CH2CH3)-. Preferably R is selected from is -CH2CH2-, -CH2CH2CH2-, -CH(CH3)CH2- or-CH2CH(OH)CH2. In some embodiments the polyhydric alcohol is selected from ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, glycerol and diglycerol. The or each ester lubricity additive is prepared by the reaction of a carboxylic acid or reactive equivalent thereof and an alcohol. In embodiments in which the carboxylic acid or reactive equivalent thereof comprises two or more carboxylic acid functional groups and / or wherein the alcohol comprises a polyhydric alcohol, a mixture of products may be obtained. In some embodiments the carboxylic acid or reactive equivalent thereof and the alcohol are reacted in a molar ratio of from 5:1 to 1:30, suitably from 2:1 to 1:20, preferably from 1:1 to 1:10, based on the ratio of COOH groups (or reactive equivalent thereof) in the carboxylic acid compound to OH groups present in the alcohol compound. In preferred embodiments an excess of alcohol groups are present relative to carboxylic acid groups or reactive equivalents thereof in the reaction used to prepare the ester lubricity additive. For the avoidance of doubt an anhydride functional group is a reactive equivalent of two COOH groups. In some embodiments in which the carboxylic acid or reactive equivalent thereof is a dicarboxylic acid this may be initially reacted with a first alcohol and then subsequently further reacted with a second different alcohol. In such embodiments the reaction product may comprise a diester compound including two different ester functional groups. For example the first alcohol may be a monohydric alcohol and the second alcohol may be a polyhydric alcohol. The or each ester lubricity additive preferably comprises the reaction product of a polyhydric alcohol and a carboxylic acid or reactive equivalent thereof selected from fatty acids, hydrocarbyl substituted succinic acids and hydrocarbyl substituted succinic anhydrides. A first class of ester lubricity additives suitable for use herein comprises an ester of a monocarboxylic acid or a reactive equivalent thereof and a polyhydric alcohol. Preferably the monocarboxylic acid is a fatty acid of formula RXCOOH wherein Rx is an alkyl or alkenyl group having 6 to 36 carbon atoms, preferably 8 to 30 carbon atoms, more preferably 12 to 24 carbon atoms. The fatty acid may be saturated or unsaturated. Preferably the fatty acid is unsaturated and Rx is an alkenyl group. The alkenyl group may have one or more double bonds, for example 1,2 or 3 double bonds. Examples of suitable saturated carboxylic acids include capric acid, lauric acid, myristic acid, palmitic acid and behenic acid. Examples of suitable unsaturated carboxylic acids include oleic acid, elaidic acid, palmitoleic acid, petroselic acid, ricinoleic acid, eleostearic acid, linoleic acid, linolenic acid, eicosanoic acid, galoleic acid, erucic acid and hypogeic acid. Unsaturated fatty acids are preferred. Preferably the monocarboxylic acid is an unsaturated carboxylic acid heaving from 12 to 24 carbon atom, preferably 16 to 18 atoms. Mixtures of fatty acids can be used, including natural mixtures, for example tall oil fatty acid. The alcohol which is reacted with the monocarboxylic acid is a polyhydric alcohol. Examples of suitable polyhydric alcohols include aliphatic, saturated or unsaturated, straight chain or branched alcohols having 2 to 10, preferably 2 to 6, more preferably 2 to 4, hydroxy groups, and having 2 to 90, preferably 2 to 30, more preferably 2 to 12, most preferably 2 to 5, carbon atoms in the molecule. Suitably the polyhydric alcohol may be a glycol or diol, or a trihydric alcohol, for example glycerol. In some embodiments the polyhydric alcohol has at least 3 hydroxy groups. Preferably the polyhydric alcohol is glycerol or diglycerol. Most preferably the polyhydric alcohol is glycerol. Examples of esters of polyhydric alcohols in the first class of ester lubricity additives include those where all of the hydroxy groups are esterified, those where not all of the hydroxy groups are esterified, and mixtures thereof. Preferred compounds include for example glycerol monooleate, glycerol dioleate and partial esters of glycerol and tall oil fatty acid. The ester may have one or more free hydroxy groups. Preferably not all of the alcohol groups are esterified. Other preferred features of the first class of suitable ester lubricity additives are described in EP680506. A second class of ester lubricity additives suitable for use herein comprises the reaction product of one or more alcohols and a hydrocarbyl substituted succinic acid or anhydride thereof. In preferred embodiments the ester lubricity additives comprise the reaction product of a polyhydric alcohol and a hydrocarbyl substituted succinic acid or anhydride thereof. The ester lubricity additives of the second class are preferably the reaction product of a succinic acid of formula (I) ora succinic anhydride of formula (II): wherein one of R1 and R3 is an alkyl or alkenyl group, and the other of R1 and R3 is hydrogen. Preferably one of R1 and R3 is an alkenyl group, and the other of R1 and R3 is hydrogen. Preferably the second class of ester lubricity additives comprises the reaction product of a compound of formula (I) or (II) and a polyhydric alcohol. The reaction product may comprise compounds of formula (III), (IV) or (V): wherein R2 is an optionally substituted alkylene moiety. The reaction product may also comprise oligomers of formula (VI): (VI) wherein n is 0 or an integer from 1 to 20, in each succinic acid moiety one of R1 and R3 is an alkyl or alkenyl group, and the other of R1 and R3 is hydrogen; and R2 is an optionally substituted alkylene moiety. R2 is an optionally substituted alkylene moiety and is suitably derived from a polyhydric alcohol. R2 may include one of more hydroxy substituents and / or one or more ether linkages within the alkylene chain. In any individual succinic acid moiety, if R1 is alkyl or alkenyl then R3 is hydrogen and vice versa. However the pattern of substitution along the oligomer chain need not be identical. Preferably n is at least 1, more preferably at least 2. Suitably n may be up to 11, more preferably up to 10, more preferably up to 8, more preferably up to 6 and most preferably up to 5. In some embodiments the ester lubricity additive may comprise the reaction product of a compound of formula (I) or (II) with a polyhydric alcohol and a monohydric alcohol. In such embodiments the reaction product may comprise compounds of formula (VII) or (VIII): (VII) (VIII) wherein R4is the residue of the monohydric alcohol and R1, R2 and R3 are as previously defined. In preferred embodiments the ester lubricity additives comprise the reaction product of a compound of formula (I) or (II) and a polyhydric alcohol and the reaction product comprises compounds of formula (III), (IV), (V) and / or (VI). Preferably one of R1 and R3 is a C12to C32 group, for example a C14 to C18 group, especially a C16 group. R1 or R3 may comprise a mixture of chain lengths and there can be some branching such as methyl, ethyl and higher alkyl branching. R1 and R3 can be derived from polymerised olefins, for example polymerised ethylene, polymerised propylene, polymerised butylene, polymerised isobutylene or polymerised mixtures of such olefins. In some embodiments one of R1 and R3 is derived from an internal olefin. Internal olefins contain predominantly a non-terminal double bond, such as a p or higher olefin. Preferably such materials are substantially completely p or higher olefins, for example containing less than 10% by weight a olefin, more preferably less than 5% by weight or less than 2% by weight. Typical internal olefins include Neodene 151810 available from Shell. Internal olefins are sometimes known as isomerised olefins and can be prepared from a-olefins by isomerisation. In some embodiments he number average molecular weight of the alkenyl group R1 or R3 is preferably at least 168, most preferably at least 180. The number average molecular weight of the alkenyl group R1 or R3 is preferably up to 1300, more preferably up to 1120, most preferably up to 448. In some embodiments one of R1 and R3 is a Ci to C150 alkenyl group, such as an olefin or polyolefin. In some embodiments one of R1 and R3 is the residue of an a-olefin having 6 to 36 carbon atoms; preferably from 10 to 30 carbon atoms, suitably from 12 to 24 carbon atoms, for example from 12 to 18 carbon atoms or from 14 to 18 carbon atoms. In some embodiments one of R1 and R3 is the residue of an internal olefin having 6 to 36 carbon atoms; preferably from 10 to 30 carbon atoms, suitably from 12 to 24 carbon atoms, for example from 12 to 18 carbon atoms or from 14 to 18 carbon atoms. In some embodiments one of R1 and R3 is preferably a C16 to C80 group and more preferably a polyisobutene (PIB) group. The number average molecular weight of the PIB is preferably from 200 to 2000, more preferably 260 to 1000, for example about 260, 320, 350, 550, 750 or 1000. Conventional PIBs and so-called "high-reactivity" PIBs (for example as described in EP565285) are suitable. High reactivity in this context is defined as a PIB wherein at least 50%, preferably 70% or more, of the terminal olefinic double bonds are of the vinylidene type, for example the GLISSOPAL compounds available from BASF. R2 is an optionally substituted alkylene group, preferably derived from a polyhydric alcohol. The polyhydric alcohol from which R2 is derived may, for example, be selected from ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, glycerol and diglycerol. In preferred embodiments the polyhydric alcohol is a dihydroxy alcohol, preferably having primary hydroxyl groups, at the respective ends of the carbon backbone. Suitably the polyhydric alcohol is selected from ethylene glycol, glycerol or diethylene glycol. In particularly preferred embodiments the polyhydric alcohol is ethylene glycol. In embodiments in which the succinic acid oranhydride thereof is also reacted with a monohydric alcohol R4OH, R4 in formulae (VII) and (VIII) is the residue of the monohydric alcohol. R4 may be an optionally substituted alkyl, alkenyl or aryl group. Preferably R4 is an alkyl group, preferably an unsubstituted alkyl group. R4 is preferably a C1 to C6 alkyl group, preferably methyl, ethyl, propyl or isopropyl. An especially preferred monohydric alcohol is isopropanol. The succinic acid / anhydride ester lubricity additives comprise compounds that are always at least semi-esterified. Mixtures of the fully and semi-esterified compounds in a variety of ratios are also within the invention. The degree of esterification may be determined according to the acid number, i.e. the amount of NaOH required to neutralise 1 g of the compound. The acid number is less than 90 mg NaOH / g, preferably less than 50 mg NaOH / g, for example less than 20 mg NaOH / g, less than 10 mg NaOH / g or less than 5 mg NaOH / g. Suitably in preparing the second class of ester lubricity additives the compound of formula (I) or (II) and a polyhydric alcohol are reacted in a molar ratio of from 5:1 to 1:30, suitably from 2:1 to 1:20, preferably from 1:1 to 1:10, based on the ratio of COOH groups (or reactive equivalent thereof) in the compound of formula (I) or (II) to OH groups present in the polyhydric alcohol. Further details of succinic acid derived ester lubricity additives and methods of preparing the same are described in EP1910504, GB2381789 and EP902804. Preferred ester lubricity additives may be selected from one or more of: (i) esters of a polyhydric alcohol having at least three hydroxy groups and one or more unsaturated fatty acids having 12 to 24 carbon atoms wherein not all of the alcohol groups are este rified; (ii) the reaction product of a polyhydric alcohol and a compound of formula (I) or (II) in which one of R1 and R3 is derived from an internal olefin having 12 to 32 carbon atoms and the other one of R1 and R3 is hydrogen; (iii) the reaction product of a compound of formula (I) or (II) in which one of R1 and R3 is a hydrocarbyl group having 10 to 32 carbon atoms, a polyhydric alcohol and a monohydric alcohol wherein the reaction product comprises compounds of formula (VII) and (VIII); (iv) the reaction product of a polyhydric alcohol and a compound of formula (I) or (II) in which one of R1 and R3 is derived from an a-olefin having 10 to 30 carbon atoms and the other one of R1 and R3 is hydrogen; and (v) the reaction product of a polyhydric alcohol and a compound of formula (I) or (II) in which one of R1 and R3 is a polyisobutylene group and the other one of R1 and R3 is hydrogen. Preferred ester lubricity additives may be selected from one or more of: (i) monoesters and / or diesters of glycerol and one or more unsaturated fatty acids having 12 to 24 carbon atoms; (ii) the reaction product of a polyhydric alcohol selected from ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, glycerol and diglycerol and compound of formula (I) or (II) in which one of R1 and R3 is is derived from an internal olefin having 12 to 24 carbon atoms and the other one of R1 and R3 is hydrogen; (iii) the reaction product of a compound of formula (I) or (II) in which one of R1 and R3 is a polyisobutene (PIB) group and the number average molecular weight of the PIB is from 100 to 1300; a monohydric alcohol selected from methanol, ethanol, propanol or isopropanol; and a polyhydric alcohol selected from ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, glycerol and diglycerol wherein the reaction product comprises compounds of formula (VII) and (VIII); (iv) the reaction product of a polyhydric alcohol selected from ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, glycerol and diglycerol and a compound of formula (I) or (II) in which one of R1 and R3 is derived from an a-olefin having 12 to 24 carbon atoms and the other one of R1 and R3 is hydrogen; and (iv) the reaction product of a polyhydric alcohol selected from ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, glycerol and diglycerol and a compound of formula (I) or (II) in which one of R1 and R3 is a polyisobutene (PIB) group and the number average molecular weight of the PIB is from 100 to 1300 and the other one of R1 and R3 is hydrogen. Preferred ester lubricity additives may be selected from one or more of: (i) monoesters and / or diesters of glycerol and one or more unsaturated fatty acids selected from oleic acid and tall oil fatty acid; (ii) the reaction product of a polyhydric alcohol selected from ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, glycerol and diglycerol and compound of formula (I) or (II) in which one of R1 and R3 is derived from an internal olefin having 14 to 18 carbon atoms and the other one of R1 and R3 is hydrogen, wherein the compound of formula (I) or (II) and the polyhydric alcohol are reacted in a molar ratio of from 2:1 to 1:20 based on the ratio of COOH groups (or reactive equivalent thereof) in the compound of formula (I) or (II) to OH groups present in the polyhydric alcohol; (iii) the reaction product of a compound of formula (I) or (II) in which one of R1 and R3 is a polyisobutene (PIB) group and the number average molecular weight of the PIB is from 200 to 600; a monohydric alcohol selected from methanol, ethanol, propanol or isopropanol; and ethylene glycol wherein the reaction product comprises compounds of formula (VII) and (VIII); (iv) the reaction product of a polyhydric alcohol selected from ethylene glycol and propylene glycol and a compound of formula (I) or (II) in which one of R1 and R3 is derived from an a-olefin having 12 to 18 carbon atoms and the other one of R1 and R3 is hydrogen; and (v) the reaction product of a polyhydric alcohol selected from ethylene glycol and propylene glycol and a compound of formula (I) or (II) in which one of R1 and R3 is a polyisobutene (PIB) group and the number average molecular weight of the PIB is from 200 to 600 and the other one of R1 and R3 is hydrogen. Preferred ester lubricity additives may be selected from one or more of: (i) monoesters and / or diesters of glycerol and one or more unsaturated fatty acids selected from oleic acid and tall oil fatty acid; (ii) the reaction product of a polyhydric alcohol selected from ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, glycerol and diglycerol and compound of formula (I) or (II) in which one of R1 and R3 is derived from an internal olefin having 14 to 18 carbon atoms and the other one of R1 and R3 is hydrogen, wherein the compound of formula (I) or (II) and the polyhydric alcohol are reacted in a molar ratio of from 1:1 to 1:10 based on the ratio of COOH groups (or reactive equivalent thereof) in the compound of formula (I) or (II) to OH groups present in the polyhydric alcohol; (iii) the reaction product of a compound of formula (I) or (II) in which one of R1 and R3 is a polyisobutene (PIB) group and the number average molecular weight of the PIB is approximately 260; isopropanol; and ethylene glycol wherein the reaction product comprises compounds of formula (VII) and (VIII); (iv) the reaction product of ethylene glycol and a compound of formula (I) or (II) in which one of R1 and R3 is derived from an a-olefin having 12 to 18 carbon atoms and the other one of R1 and R3 is hydrogen; and (v) the reaction product of ethylene glycol and a compound of formula (I) or (II) in which one of R1 and R3 is is a polyisobutene (PIB) group and the number average molecular weight of the PIB is approximately 260 and the other one of R1 and R3 is hydrogen. In some embodiments component (a) comprises an amide or imide compound, suitably a hydrocarbyl substituted amide compound ora hydrocarbyl substituted imide compound. The fuel composition may therefore comprise one hydrocarbyl substituted amide or imide compound or two or more hydrocarbyl substituted amide or imide compounds. References herein to “a” hydrocarbyl substituted amide or imide compound or “the” hydrocarbyl substituted amide or imide compound, unless otherwise specified, also apply to embodiments in which two or more such additives are present. As the skilled person will appreciate each hydrocarbyl substituted amide or imide compound may comprise a mixture of compounds. The hydrocarbyl substituted amide or imide compounds are suitably the reaction product of a hydrocarbyl substituted carboxylic acid or reactive equivalent thereof and an amino compound. When additive (a) comprises a hydrocarbyl substituted imide the amino compound reacts with two acylating functional groups to form the imide. These acylating functional groups may be provided by the same acylating agent or by two different acylating agents. To form the hydrocarbyl substituted amide or imide additives used in the present invention, the acylating agent is reacted with an amino compound. Any suitable amino compound may be reacted with the acylating agent. Suitable amino compounds include ammonia, hydrazine, primary amines and secondary amines. By primary amines we mean to include any compound which contains a primary amino group, NH2. By secondary amines we mean to include any compound which contains a secondary amino group having a single NH bond. Preferably the amino compound is an aliphatic amino compound. In some preferred embodiments the amino compound does not comprise any aromatic functional groups. In some embodiments the amino compound may further comprise one or more tertiary amine groups. However these will not react with the acylating agent. The amino compound may include one or more than one amino functional groups. In embodiments in which the amino compound includes more than one reactive amino functional groups, one or more than one of the reactive amino functional groups may react with the one or more acylating agents to form an amide or imide. In embodiments in which the acylating agent comprises more than one carboxylic acid group or reactive equivalent thereof, one or more than one of the carboxylic acid groups or reactive equivalent thereof may react with the amino compound to form an amide or imide. Thus the hydrocarbyl substituted amide or imide may comprise a mixture of compounds. Component (a) may comprise an amide of formula (IX): O R3 (IX) or an imide of formula (X): wherein R1 is an optionally substituted hydrocarbyl group; each of R2 and R5 is hydrogen, NH2 or an optionally substituted hydrocarbyl group; R3 is hydrogen or an optionally substituted hydrocarbyl group; and each of R4 and R6 is an optionally substituted hydrocarbyl group or R4 and R6are linked together to form a cyclic moiety including a hydrocarbyl substituent. In embodiments in which R2 and R3 are both hydrocarbyl groups these may be linked to from a cyclic moiety. The amide of formula (IX) is suitably prepared by the reaction of an acylating agent and an amino compound of formula R2R3NH. The amino compound may be ammonia (R2=R3=H), hydrazine (R2=NH2, R3=H) a primary amine (R3=H, R2^H) ora secondary amine (R2^H, R3^H). The imide of formula (X) is suitably prepared by reaction of one or two acylating agents and amino compound of formula R5NH2. This amino compound of formula R5NH2 may be ammonia (R5=H), hydrazine (R5=NH2) ora primary amine (R5^H). The skilled person will appreciate that the moieties R1COO, R4COO and R6COO in formula (IX) and formula (X) represent the residues of the acylating agents and are suitably as previously described herein. R1COO is suitably the residue of a fatty acid or reactive equivalent thereof. R1 is suitably an alkyl or alkenyl group having 6 to 36 carbon atoms, preferably 8 to 30 carbon atoms or 12 to 24 carbon atoms. R4 and R6 may be the same or different. Preferably R4 and R6 are connected to from a cyclic group including a hydrocarbyl substituent. Preferred imides are of formula (XI): wherein R7 is an optionally substituted hydrocarbyl group and R5 is hydrogen, NH2 or an optionally substituted hydrocarbyl group. R7 is preferably an alkyl or alkenyl group. In some embodiments R7 is alkyl or alkenyl group having 4 to 40 carbon atoms, preferably from 10 to 32 carbon atoms, for example 12 to 24 carbon atoms. In some embodiments R7 is derived from an internal olefin. In some embodiments R7 is derived from a-olefin. In some embodiments R7 is a polyisobutylene (PIB) group, suitably PIB groups have a number average molecular weight of from 100 to 5000, preferably 200 to 1000, for example about 260, about 550 or about 750. The hydrocarbyl substituted amide or imide additives of component (a) of the present invention are prepared by the reaction of an acylating agent and an amino compound. In order to react with the acylating agent the amino compound must comprise at least one reactive N-H group. The amino compound may be selected from ammonia, hydrazine, compounds including a primary amine group and compounds including a secondary amine group. The amino compound can be a monoamine or a polyamine. Thus the amino compound may include one, two or more than two amine functional groups. In some embodiments the amino compound is ammonia. In some embodiments the amino compound is hydrazine. However in preferred embodiments the amino compound is not hydrazine. In some embodiments the amino compound is a primary amine, preferably a primary alkyl or alkenyl amine ora primary alkanolamine. In some embodiments the amino compound may be a fatty amine, suitably a primary alkyl or alkenyl amine having from 6 to 30 carbon atoms. Suitable amines of this type include cocoamine, stearylamine, tallow amine, hydrogenated tallow amine, lauryl amine, oleylamine, octylamine, octadecylamine, hexadecylamine, decylamine and dodecylamine. In some embodiments the amino compound may be a primary alkyl amine having from 1 to 10 carbon atoms, for example 1 to 8 carbon atoms. Amines of this type include methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine, heptylamine, 2-ethyl hexylamine and n-octylamine. In some embodiments the amino compound may be a monoalkanolamine, suitably having 1 to 20 carbon atoms, preferably from 1 to 10 carbon atoms, for example 1 to 6 carbon atoms. For example, suitable monoalkanolamines include ethanolamine, propanolamine, butanolamine, pentanolamine and hexanolamine. In some embodiments, the amino compound may be a dialkylamine, dialkenylamine or alkylalkenylamine of formula R2R3NH wherein each of R2 and R3 is independently an alkyl or alkenyl group having from 6 to 36 carbon atoms, for example 10 to 20 carbon atoms. Compounds of this type include dicocoamine, diahexadecyl amine, distearyl amine, ditallow amine, dihydrogenated tallow amine and dilaurylamine. In some embodiments the amino compound may be a compound of formula R2R3NH in which each of R2 and R3 is independently an alkyl group or hydroxyalkyl group, preferably having 1 to 20 or more preferably 1 to 10 carbon atoms. In some embodiments the amino compound may be a dialkylamine or dialkanolamine, suitably having from 1 to 20 carbon atoms, suitably 1 to 12 carbon atoms, for example 1 to 8 or 1 to 6 carbon atoms. In some embodiments the amino compound may be a dialkylamine having from 1 to 12, for example 1 to 8 or 1 to 6 carbon atoms. Each of the alkyl groups may be the same or different. In some of the preferred embodiments each of the alkyl groups is the same. For example the amino compound may be selected from diethylamine, dipropylamine, dibutylamine, dipentylamine and dihexylamine. In some embodiments the amino compound may be a dialkanolamine in which each hydroxyalkyl may be the same or different. Preferably each hydroxyalkyl group is the same. Suitable dialkanolamines include diethanolamine, dipropanolamine, and dibutanolamine. In some embodiments the amino compound is a hydroxy-substituted and / or an ether containing poly amino compound. For example the amino compound may be of the formula H2N-[-R8-X-]n-H wherein n is at least 1, each R8 is an alkylene group and each X is independently NR9 or O, wherein R9 is hydrogen or Ci to C4 alkyl. Preferably n is from 1 to 10, more preferably from 1 to 6, for example from 1 to 4. R8 is an alkylene group, preferably having from 1 to 6 carbon atoms, preferably 2 to 4 carbon atoms, most preferably R8 is an ethylene group. R9 is preferably hydrogen. One preferred compound of this type is hydroxyethylethylene diamine. In some embodiments the amino compound is a polyamine. Polyamines may be selected from any compound including two or more amine groups. Preferably the polyamine is a (poly)alkylene polyamine (by which is meant an alkylene polyamine ora polyalkylene polyamine; including in each case a diamine, within the meaning of “polyamine”). Preferably the polyamine is a (poly)alkylene polyamine in which the alkylene component has 1 to 6, preferably 1 to 4, most preferably 2 to 3 carbon atoms. Most preferably the polyamine is a (poly) ethylene polyamine (that is, an ethylene polyamine or a polyethylene polyamine). Preferably the polyamine has 2 to 15 nitrogen atoms, preferably 2 to 10 nitrogen atoms, more preferably 2 to 8 nitrogen atoms. The polyamine may, for example, be selected from ethylenediamine, dimethyl amino propylamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, hexaethyleneheptamine, heptaethyleneoctamine, propane-1,2-diamine, 2(2-amino-ethylamino)ethanol, and N ,N-bis (2-aminoethyl) ethylenediamine (N(CH2CH2NH2)3). Preferred polyamines include ethylenediamine and especially tetraethylenepentamine. Commercially available sources of polyamines typically contain mixtures of isomers and / or oligomers, and products prepared from these commercially available mixtures fall within the scope of the present invention. Further amino compounds suitable for use herein include 1-aminopiperidine, 1-(2-aminoethyl)piperidine, 1- (3-aminopropyl)-2-pipecoline, 1-methyl-(4-methylamino)piperidine, 4-(1 -pyrrolidinyl)piperidine, 1 -(2-aminoethyl)pyrrolidine, 2-(2-aminoethyl)-1 - methylpyrrolidine, N,N-diethylethylenediamine, N,N-dimethylethylenediamine, N,N-dibutylethylenediamine, N,N-diethyl-l,3-diaminopropane, N,N-dimethyl-1,3-diaminopropane, N,N,N'- trimethylethylenediamine, N,N-dimethyl-N'-ethylethylenediamine, N,N-diethyl-N'-methylethylenediamine, N,N,N'- triethylethylenediamine, 3-dimethylaminopropylamine, 3-diethylaminopropylamine, 3-dibutylaminopropylamine, N,N,N'-trimethyl- 1,3- propanediamine, N,N,2,2-tetramethyl-l,3-propanediamine, 2-amino-5-diethylaminopentane, N,N,N',N'-tetraethyldiethylenetriamine, 3,3'-diamino-N-methyldipropylamine, 3,3'-iminobis(N,N-dimethylpropylamine), 1-(3-aminopropyl)imidazole and 4-(3-aminopropyl)morpholine, 1-(2-aminoethyl)piperidine, 3,3-diamino-N-methyldipropylamine, 3,3-aminobis(N,N- dimethyl propyl (amine) and N'-(3-(dimethylamino)propyl)-N,N-dimethyl 1,3-propanediamine. In some embodiments component (a) comprises the reaction product of a hydrocarbyl substituted acylating agent and an amino compound. Preferred acylating agents are fatty acids, hydrocarbyl substituted succinic acids and hydrocarbyl substituted succinic anhydrides. In some preferred embodiments component (a) comprises the reaction product of a fatty acid or a hydrocarbyl substituted succinic acid or anhydride thereof and an amino compound. Preferred amino compounds include ammonia, polyalkylene polyamines, dialkanolamines and hydroxyalkyl diamines. One preferred class of hydrocarbyl substituted imide compounds suitable for use herein comprise the reaction product of a hydrocarbyl substituted succinic acid or anhydride thereof and ammonia. In such embodiments the hydrocarbyl substituent of the succinic acid or anhydride is preferably an alkenyl group, more preferably an alkenyl group derived from an internal olefin. Preferably the hydrocarbyl substituent has from 6 to 36 carbon atoms, more preferably from 10 to 20 carbon atoms. A further preferred class of hydrocarbyl substituted imide compounds suitable for use herein comprises the reaction product of a hydrocarbyl substituted succinic acid or anhydride thereof and hydroxyalkyl substituted polyamine. In such embodiments the hydrocarbyl substituent of the succinic acid or anhydride thereof is preferably an alkenyl group, preferably derived from an internal olefin. Preferably the alkene substituent has from 4 to 40 carbon atoms, preferably 6 to 36 carbon atoms. The hydroxyalkyl substituted polyamine is preferably a compound of formula H2N4-R10-NH4mR11-OH in which m is from 1 to 4 and each of R10 and R11 is independently a Ci to Ce alkylene group. A preferred compound is hydroxyethylethylene diamine. A further preferred class of hydrocarbyl substituted imide compounds suitable for use herein comprise the reaction product of a hydrocarbyl substituted succinic acid or anhydride thereof and a polyalkylene polyamine. Preferably the hydrocarbyl substituent of the succinic acid or anhydride thereof is a polyisobutenyl substituent, preferably having a number average molecular weight from 100 to 2000, more preferably from 200 to 1500, preferably from 300 to 1300, more preferably from 500 to 1000, for example from 600 to 900. A number average molecular weight of around 750 is especially preferred. The polyalkylene polyamine is preferably a polyethylene polyamine, preferably having from 2 to 10 carbon atoms, for example, from 2 to 8 carbon atoms, more preferably from 3 to 7 carbon atoms, for example 5 carbon atoms. An especially preferred compound is tetraethylene pentamine. Preferred hydrocarbyl substituted amide compounds suitable for use herein comprise the reaction product of a fatty acid or a reactive equivalent thereof and a secondary amine. The fatty acid preferably has from 6 to 36 carbon atoms, preferably from 8 to 30 carbon atoms, more preferably from 10 to 30 carbon atoms, for example from 12 to 24 carbon atoms. The fatty acid may be alkyl or alkenyl derived. A particular preferred fatty acid is tall oil fatty acid. The secondary amine is preferably a preferably a dialkyl amine or a dialkanolamine. Preferably the secondary amine is a dialkanolamine, most preferably diethanolamine. In preferred embodiments component (a) comprises substantially aliphatic compounds. Preferably component (a) comprises less than 10 wt% aromatic compounds, preferably less that 5 wt%, more preferably less than 1 wt%. By aromatic compounds we mean to include any compound which contains an aromatic functional group. In preferred embodiments component (a) does not include any compounds having aromatic functional groups. In embodiments wherein component (a) is an ester, preferred esters do not include any amino groups. In embodiments wherein component (a) is an ester, preferred esters do not include any N-containing functional groups. In some embodiments component (a) comprises the reaction product of: an acylating agent selected from fatty acids, hydrocarbyl substituted succinic acids and hydrocarbyl substituted succinic anhydrides; and an amino compound selected from ammonia, polyalkylene polyamines, dialkyl amines, dialkanolamines and compounds of formula H2N-[-R8-X-]n-H in which each X is independently NR9 or O, n is from 1 to 10, R8 is a Ci to Ce alkylene group and R9 is hydrogen or Ci to C4 alkyl. In some embodiments component (a) comprises the reaction product of: an acylating agent selected from fatty acids, hydrocarbyl substituted succinic acids and hydrocarbyl substituted succinic anhydrides; and an amino compound selected from ammonia, polyethylene polyamines, dialkyl amines having 1 to 8 carbon atoms, dialkanolamines having 1 to 8 carbon atoms and compounds of formula H2N-[-R8-X-]n-H in which each X is independently NR9 or O, n is from 1 to 4 and R8 is a C2 to C4 alkylene group, and R9 is hydrogen orO to C4 alkyl. In some embodiments component (a) comprises the reaction product of: an acylating agent selected from fatty acids, hydrocarbyl substituted succinic acids and hydrocarbyl substituted succinic anhydrides; and an amino compound selected from ammonia, diethanolamine, tetraethylenepentamine and hydroxyethyl ethylene diamine. In some embodiments component (a) comprises the reaction product of: an acylating agent selected from fatty acids having 10 to 30 carbon atoms, polyisobutene substituted succinic acids or anhydrides thereof, and hydrocarbyl substituted succinic acids or anhydrides thereof having an alkenyl substituent of 8 to 30 carbon atoms; and an amino compound selected from ammonia, polyalkylene polyamines, dialkyl amines, dialkanolamines and compounds of formula H2N-[-R8-X-]n-H in which each X is independently NR9 or O, n is from 1 to 10, R8 is a Ci to Ce alkylene group and R9 is hydrogen or Ci to C4 alkyl. In some embodiments component (a) comprises the reaction product of: an acylating agent selected from fatty acids having 10 to 30 carbon atoms, polyisobutene substituted succinic acids or anhydrides thereof, and hydrocarbyl substituted succinic acids or anhydrides thereof having an alkenyl substituent of 8 to 30 carbon atoms; and an amino compound selected from ammonia, polyethylene polyamines, dialkyl amines having 1 to 8 carbon atoms, dialkanolamines having 1 to 8 carbon atoms and compounds of formula H2N-[-R8-X-]n-H in which each X is independently NH or O, n is from 1 to 4 and R8 is a C2 to C4 alkylene group. In some embodiments component (a) comprises the reaction product of: an acylating agent selected from fatty acids having 10 to 30 carbon atoms, polyisobutene substituted succinic acids or anhydrides thereof, and hydrocarbyl substituted succinic acids or anhydrides thereof having an alkenyl substituent of 8 to 30 carbon atoms; and an amino compound selected from ammonia, diethanolamine, tetraethylenepentamine and hydroxyethyl ethylene diamine. In some embodiments component (a) comprises the reaction product of: an acylating agent selected from fatty acids having 16 to 20 carbon atoms, polyisobutene substituted succinic acids or anhydrides thereof having a PIB number average molecular weight of 600 to 900, and hydrocarbyl substituted succinic acids or anhydrides thereof having an alkenyl substituent of 12 to 20 carbon atoms; and an amino compound selected from ammonia, polyalkylene polyamines, dialkyl amines, dialkanolamines and compounds of formula H2N-[-R8-X-]n-H in which each X is independently NR9 or O, n is from 1 to 10, R8 is a Ci to Ce alkylene group and R9 is hydrogen or Ci to C4 alkyl. In some embodiments component (a) comprises the reaction product of: an acylating agent selected from fatty acids having 16 to 20 carbon atoms, polyisobutene substituted succinic acids or anhydrides thereof having a PIB number average molecular weight of 600 to 900, and hydrocarbyl substituted succinic acids or anhydrides thereof having an alkenyl substituent of 12 to 20 carbon atoms; and an amino compound selected from ammonia, polyethylene polyamines, dialkyl amines having 1 to 8 carbon atoms, dialkanolamines having 1 to 8 carbon atoms and compounds of formula H2N-[-R8-X-]n-H in which each X is independently NH or O, n is from 1 to 4 and R8 is a C2 to C4 alkylene group. In some embodiments component (a) comprises the reaction product of: an acylating agent selected from fatty acids having 16 to 20 carbon atoms, polyisobutene substituted succinic acids or anhydrides thereof having a PIB number average molecular weight of 600 to 900, and hydrocarbyl substituted succinic acids or anhydrides thereof having an alkenyl substituent of 12 to 20 carbon atoms; and an amino compound selected from ammonia, diethanolamine, tetraethylenepentamine and hydroxyethyl ethylene diamine. Preferred hydrocarbyl substituted amide or imide compounds may be selected from: the reaction product of an alkenyl substituted succinic acid or anhydride thereof and ammonia, preferably wherein the alkenyl substituent has from 6 to 36 carbon atoms; the reaction product of an alkenyl substituted succinic acid or anhydride thereof, preferably wherein the alkenyl substituent has from 6 to 36 carbon atoms and a compound of formula H2N-[-R8-X-]n-H in which each X is independently NH or O, n is from 1 to 4 and R8 is a C2 to C4 alkylene group; the reaction product of a polyisobutenyl substituted succinic acid or anhydride thereof, preferably having a PIB number average molecular weight of 500 to 1000 and a polyethylene polyamine; and the reaction product of a fatty acid, preferably having 8 to 30 carbon atoms and a dialkanolamine. Preferred hydrocarbyl substituted amide or imide compounds may be selected from: the reaction product of an alkenyl substituted succinic acid or anhydride thereof and ammonia, wherein the alkenyl substituent has from 12 to 20 carbon atoms; the reaction product of an alkenyl substituted succinic acid or anhydride thereof, wherein the alkenyl substituent has from 12 to 20 carbon atoms and hydroxyethyl ethylene diamine; the reaction product of a polyisobutenyl substituted succinic acid or anhydride thereof having a PIB number average molecular weight of 750 and tetraethylenepentamine; and the reaction product of oleic acid and diethanolamine. In alternative embodiments the acylating agent used the prepare the ester, amide or imide compound is a polymeric compound comprising a carboxylic acid or reactive equivalent thereof. Preferred such compounds are polymers of an a, p unsaturated carboxylic acid or reactive equivalent thereof and optionally one or more further unsaturated monomers. Suitable such polymers will be known to the person skilled in the art. Suitable carboxylic acid based monomers include those based on acrylic acid, methacrylic acid, fumaric acid and maleic acid. An especially preferred carboxylic acid derived monomer is maleic anhydride. The polymeric acylating agents may be homopolymers or copolymers. Preferred are copolymers of a carboxylic acid based monomer and one or more further unsaturated monomers. Preferred further monomers are olefins, especially a-olefins. Preferred a-olefins have from 2 to 50 carbon atoms, preferably from 4 to 40 carbon atoms, more preferably from 6 to 35 carbon atoms, suitably from 8 to 30 carbon atoms, for example from 10 to 24 carbon atoms. In some preferred embodiments the a-olefins have from 16 to 18 carbon atoms. As the skilled person will appreciate, mixtures of a-olefins may be used and this can be advantageous. Preferred copolymeric acylating agents are 1:1 copolymers of maleic anhydride and an a-olefin, preferably having 6 to 36, especially 10 to 24 carbon atoms. The copolymers suitably have a number average molecular weight of from 200 to 20000, for example from 500 to 10000. Number average molecular weight may be measured, for example, by Gel Permeation Chromotography (GPC) relative to polystyrene standards. To form the ester, amide or imide compounds suitable for use as component (a) the polymeric acylating agent is reacted with an alcohol or amino compound. Suitable such compounds are as previously defined herein. Preferably the polymeric acylating agent is reacted with an amino compound. Preferred amino compounds include a primary amino group. When the polymeric acylating agent includes an anhydride functional group the primary amine is suitably reacted with the polymeric acylating agent in a molar ratio of 2:1 to 1:2, preferably from 1.5:1 to 1:1.5 based on the number of anhydride groups. A ratio of approximately 1:1 is especially preferred. Preferred amino compounds for reacting with the polymeric acylating agent are primary alkyl or alkenyl amines having from 6 to 30 carbon atoms. Suitable amines of this type include cocoamine, stearylamine, tallow amine, hydrogenated tallow amine, lauryl amine, oleylamine, octylamine, octadecylamine, hexadecylamine, decylamine and dodecylamine. In one embodiment component (a) comprises the reaction product of hydrogenated tallow amine and a copolymer of maleic anhydride and a C16 to C18 a-olefin. The fuel composition provided by the present invention further comprises (b) at least one small, functionalised aromatic compound. The fuel composition may comprise one small, functionalised aromatic compound or two or more small, functionalised aromatic compounds. References herein to “a” small, functionalised aromatic compound or “the” small functionalised aromatic compound include embodiments in which two or more such compounds are present, unless otherwise stated. Component (b) is different to component (a). Suitably component (a) and component (b) do not form part of the same molecule. By small, functionalised aromatic compound we mean to refer to a compound which includes at least one functional group and which is suitably not polymeric. The small functionalised aromatic compounds include at least one functional group. The small functionalised aromatic compounds may include one (i.e. a single) functional group or more than one, for example two or more functional groups. Suitable functional groups include alkenes, alkynes, hydroxy, alkoxy, hydroxyalkyl, esters, amides, imides, halo, amino, aldehyde, keto, nitro, nitrile, cyano substituted amino, carboxy, mercapto and sulfonic acid. In preferred embodiments the small, aromatic compounds do not comprise any halo or sulfur containing groups. Preferred functional groups contain one or more hetero atoms, especially nitrogen and / or oxygen atoms. Preferred functional groups include electron withdrawing groups. Other functional groups may also be present. Preferably the small, functionalised aromatic compound includes at least one electron withdrawing group and optionally one or more further functional groups. Preferably the small, functionalised aromatic compound includes one or more electron withdrawing groups selected from aldehydes, esters, amides, ketones and nitro groups and optionally one or more further functional groups. The one or more further functional groups are preferably hydroxy or hydroxyalkyl. Preferably the or each small, functionalised aromatic compound includes less than 5 aromatic rings, preferably less than 4 aromatic rings. Preferably the or each small, functionalised aromatic compound includes one or two aromatic rings. In some preferred embodiments the or each small, functionalised aromatic compound includes a single aromatic ring. One or more functional groups may be present within the molecule. The single aromatic ring may be entirely carbon based or it may be a heterocyclic aromatic ring. The aromatic ring may suitably comprise 5 to 7 carbon atoms. Preferably the aromatic ring comprises 6 carbon atoms. The aromatic ring may comprise one or more heteroatoms within the aromatic ring. Such heteroatoms are suitably selected from oxygen and / or nitrogen. Preferred aromatic rings are based on benzene. Preferably the small, functionalised aromatic compound is based on a benzene ring including an electron withdrawing group as a substituent and optionally one or more further substituents. Preferably the one or more further substituents are not ortho to the electron withdrawing group. In some preferred embodiments component (b) comprises a compound of formula (XII): (XII) wherein EWG is an electron withdrawing group and each of Ra, Rb, Rc, Rd and Re is independently selected from hydrogen, OH, alkyl, alkoxy, hydroxyalkyl, amino, and alkenyl. When any of Ra, Rb, Rc, Rd or Re is alkyl, alkenyl or alkoxy, such a group may contain from 1 to 30, preferably 2 to 10 carbon atoms. In some embodiments an alkenyl substituent may be a polyisobutenyl group, for example having number average molecular weight of from 200 to 1000, for example around 260. Preferred alkyl substituents are Ci to C20 alkyl groups, preferably C4 to C12 alkyl groups. These groups may be straight chain or branched. Preferred are straight chain alkyl groups. Preferred alkenyl substituents are Ci to C20 alkenyl groups, preferably C4 to C12 alkenyl groups. These groups may be straight chain or branched. Preferred are straight chain alkenyl groups. Preferred alkoxy substituents are Ci to C20 alkoxy groups, preferably C4 to C12 alkoxy groups. These groups may be straight chain or branched. Preferred are straight chain alkoxy groups. For example R may be CH3(CH2)nO wherein n is from 0 to 20, preferably from 2 to 16, suitably from 6 to 12, for example 8. Preferred hydroxyalkyl substituents are of the formula (CHRf)yOH wherein y is at least 1, each Rf is independently hydrogen or Ci to C4 alkyl, preferably methyl. Most preferably each Rf is hydrogen and y is from 1 to 10, preferably from 1 to 6, suitably from 1 to 4, for example 1 or 2. Preferably Ra is hydrogen. Preferably Re is hydrogen. Preferably Ra is hydrogen and Re is hydrogen. Preferably Rb is hydrogen or OH. Preferably Rd is hydrogen or OH Preferably Rc is hydrogen, NH2, hydroxyalkyl (especially HOCH2) or OH. In some preferred embodiments Rc is OH. Preferably EWG is COR12 or NO2 wherein R12 is hydrogen, alkyl, NR13R14, or OR15 wherein each of R13 and R14 is a hydrogen or an alkyl group and R15 is an alkyl group. Preferred alkyl groups for R12, R13, R14 and R15 are Ci to C20 alkyl groups, preferably Ci to C12 or Ci to C10 alkyl groups. Preferably R13 and R14 are hydrogen. Preferably R12 is hydrogen, Ci to C4 alkyl, NH2 or OR15. In some preferred embodiments Rb is hydrogen, Rd is hydrogen and Rc is OH. In some embodiments Ra and Re are hydrogen; Rc is hydrogen or OH; Rb is hydrogen, NH2, HO(CH2)y or OH; Rd is hydrogen or OH; and EWG is CO2R15, CONHR14, CHO, COR16 or NO2 wherein y is 1 to 6, preferably 1 or 2; R15 is Ci to C20 alkyl, preferably Ci to C12 alkyl, preferably Ci to C10 alkyl; R14 is hydrogen or Ci to C12 alkyl, preferably Ci to Ca alkyl, preferably Ci to C4 alkyl; and R16 is Ci to Ca alkyl, preferably Ci to C4 alkyl. In some embodiments Ra and Re are hydrogen, Rb is hydrogen or OH, Rd is hydrogen or OH, Rc is OH and EWG is CO2R15, CHO or NO2 wherein R15 is Ci to C20 alkyl, preferably Ci to C12 alkyl, preferably Ci to Ca alkyl. In some embodiments Ra, Rb, Rd and Re are all hydrogen, Rc is OH, NH2 or HOCH2 and EWG is selected from CHO, NO2, CONHR14, CO2R15and COR16 wherein R15 is Ci to C20 alkyl, suitably Ci to C10 alkyl; R14 is Ci to C12 alkyl, preferably Ci to Ca alkyl and R15 is Ci to C12 alkyl, preferably Ci to C4 alkyl. In some embodiments Ra, Rb, Rd and Re are all hydrogen, Rc is OH and EWG is selected from CHO, NO2, CONH2 and CO2R15wherein R15 is Ci to C20 alkyl, suitably Ci to C12 alkyl, preferably Ci to C10 alkyl. In some embodiments Ra and Re are hydrogen, Rb, Rc and Rd are all OH and EWG is CO2R15 wherein R15 is Ci to C12 alkyl, suitably Ci to Ce alkyl. In some embodiments each of Ra, Rb, Rc, Rd and Re is hydrogen and EWG is CONH2. Suitable compounds for use as component (b) include benzamide, 4-hydroxy benzaldehyde, Ci to C10 alkyl esters of 4-hydroxybenzoic acid, Ci to C4 alkyl esters of 3,4,5-(trihydroxy)benzoic acid, 4-nitro phenol, Ci to C4 alkyl esters of 4-aminobenzoic acid, 4-hydroxyacetophenone, Ci to C4 alkyl esters of 4-(hydroxyalkyl)benzoic acid and Ci to Ce alkyl or dialkyl amides of 4-hydroxybenzoic acid. Preferably component (b) does not comprise a reaction product of an aromatic polyamine and an acylating agent. Preferably component (b) does not comprise a compound including a quaternary ammonium functional group. Preferably component (b) does not comprise the product of a Mannich reaction. Preferred small, functionalised aromatic compounds for use herein have a molecular weight of less than 500 gmol'1, for example less than 300 gmol'1. Component (a) is preferably present in the fuel composition in an amount of at least 1 ppm, preferably at least 10 ppm, more preferably at least 20 ppm, preferably at least 50 ppm, for example at least 70 ppm. Component (a) may be present in the fuel composition in an amount of up to 5000 ppm, preferably up to 1000 ppm, more preferably up to 500 ppm, suitably up to 250 ppm, for example up to 150 ppm. Component (a) is preferably present in the fuel composition in an amount from 5 to 300 ppm, preferably 10 to 200 ppm, more preferably 50 to 150 ppm. Component (b) is preferably present in the fuel composition in an amount of at least 1 ppm, preferably at least 3 ppm, more preferably at least 5 ppm, preferably at least 10 ppm, for example at least 15 ppm. Component (b) is preferably present in the fuel composition in an amount of up to 1000 ppm, preferably up to 200 ppm, more preferably up to 100 ppm, suitably up to 60 ppm, preferably up to 49 ppm, for example up to 40 ppm. Component (b) is preferably present in the fuel composition in an amount of less than 50 ppm. Component (b) is preferably present in the fuel composition in an amount from 1 to 100 ppm, preferably from 5 to 50 ppm, more preferably from 10 to 40 ppm. The weight ratio of component (a) to component (b) in the fuel composition is preferably from 100:1 to 1:10, preferably from 10:1 to 1:1, more preferably from 6:1 to 2:1. Component (a) may comprise a mixture of compounds. Component (b) may comprise a mixture of compounds. For the avoidance of doubt in embodiments in which components (a) and / or (b) comprise a mixture of compounds the above amounts refer to the total amount of all such compounds present in the composition. In some embodiments the present invention provides a diesel fuel composition comprising at least 50 ppm of component (a) and less than 50 ppm of component (b), for example from 60 to 200 ppm of component (a) and from 1 to 45 ppm of component (b). The fourth aspect of the present invention relates to a method of preparing a diesel fuel composition by the addition of component (a) and component (b) to a diesel fuel. Preferably component (a) and component (b) are provided as separate entities. They may be admixed prior to addition to the fuel, for example they may be provided as part of an additive composition. In some embodiments the fourth aspect of the present invention may involve the addition to a diesel fuel composition of an additive combination comprising a mixture of (a) at least one ester, amide or imide compound; and (b) at least one small, functionalised aromatic compound. The additive combination may suitably be provided as part of an additive composition further comprising a diluent or carrier and optionally one or more further additives. The fuel composition provided by the present invention may further comprise one or more additional fuel additives. These may include dispersants, detergents, metal deactivating compounds, wax anti-settling agents, cold flow improvers, cetane improvers, dehazers, stabilisers, demulsifiers, antifoams, corrosion inhibitors, dyes, markers, combustion improvers, metal deactivators, odour masks, drag reducers and conductivity improvers. The fuel composition provided by the present invention has advantageous lubricity properties. Lubricity of a fuel may be measured according to the HFRR test. This is a standard test method that is known to the person skilled in the art and is described, for example, in ISO 12156-1:2023. Preferably the fuel composition provided by the present invention has a wear scar diameter as measured in the HFRR test of less than 420 pm, preferably less than 400 pm. The addition of additives (a) and (b) enhances the lubricity properties of the fuel. The method and use of the first and second aspects of the present invention involve the use of a combination of additives to improve the lubricity of a diesel fuel. Suitably the additive combination of additives (a) and (b) reduces the wear scar of a diesel fuel by at least 50 pm, preferably at least 70 pm, for example at least 100 pm compared with the wear scar of the base fuel without additive (a) or additive (b). The wear scar is suitably measured according to the HFRR test. Preferably the method and use of the first and second aspect of the present invention reduce the wear scar by an amount greater than the sum of the reduction in wear scar when using each of additives (a) and (b) alone. In another embodiment the present invention may involve further improving the lubricity of a fuel composition which already contains a hydrocarbyl substituted ester, amide or imide compound. According to a fifth aspect of the present invention there is provided a method of improving the lubricity of a diesel fuel composition comprising (a) at least one ester, amide or imide compound; the method comprising adding to the diesel fuel composition (b) at least one small, functionalised aromatic compound. According to a sixth aspect of the present invention there is provided the use of (b) at least one small, functionalised aromatic compound to improve the lubricity of a diesel fuel composition comprising (a) at least one ester, amide or imide compound. Preferred features of the fifth and sixth aspects of the present invention are suitably as defined in relation to the first, second and third aspects. In the fifth and sixth aspects of the present invention, the diesel fuel, the ester, amide or imide compound and the small functionalised aromatic compounds are preferably as defined in relation to the first, second, third and fourth aspects. Preferably the method and use of the fifth and sixth aspects reduce the wear scar of a diesel fuel composition comprising (a) at least one hydrocarbyl substituted ester, amide or imide compound by at least 30 pm, preferably at least 60 pm, more preferably at least 80 pm. The use of the sixth aspect may also reduce the treat rate of ester, amide or imide compound needed to achieve an equivalent lubricating performance compared with when the small, functionalised aromatic compound is not present. The invention will now be further described with reference to the following non-limiting examples. Example 1 Fuel compositions were prepared by adding small functionalised aromatic compounds listed below and additive A to a hydrotreated vegetable oil (HVO) fuel, as detailed in table 1. Additive A comprises an ester reaction product of ethylene glycol and a succinic anhydride obtained by the reaction of maleic anhydride and a C15 to C18 internal olefin. The HVO fuel complied with the specification set out in EN15940 (2023). The cloud point of the fuel was - 23°C (measured according to IP 219) and the pour point was -38°C (measured according to IP 15). Table 1 Example Fuel Composition Additive A (ppm) Aromatic compound (@25 ppm) Inventive (I) or comparative (C) 1 0 - C 2 125 - C 3 100 Benzamide I 4 100 4-hydroxy benzaldehyde I 5 100 Methyl 4-hydroxybenzoate I 6 100 Octyl 4-hydroxybenzoate I 7 100 2-ethyl hexyl 4-hydroxybenzoate I 8 100 Propyl 4-hydroxybenzoate I 9 100 4-nitro phenol I 10 100 Ethyl 3,4,5-trihydroxybenzoate I 11 100 methyl 4-aminobenzoate I 12 100 methyl 4-(hydroxymethyl) benzoate I 13 100 N,N dibutyl 4-hydroxybenzamide I 14 100 Isosnonyl 4-hydroxy benzoate I 15 100 4-hydroxyacetophenone I Example 2 The example fuel compositions of example 1 were tested according to the standard HFRR procedure set out in ISO 12156-1:2023 (modified to 70 minutes run time rather than 75 minutes) as follows: A steel ball is in contact with a steel disc with a normal load of 200 g. The contact is submerged in 2 ml of the test fuel composition and then heated to 60 °C. The ball slides in a reciprocating sliding motion over a length of 1 mm at a rate of 50 Hz. The ball is taken off after 70 mins and the wear scar is measured. The results are given in Table 2: Table 2 Example Fuel Composition Wear scar (pm) 1 580 2 445 3 345 4 230 5 350 6 345 7 365 8 360 9 320 10 380 11 370 12 360 13 355 14 335 15 355 Example 3 Diesel fuel compositions were prepared by adding the components listed in table 3 into a hydrotreated vegetable oil (HVO) fuel having the properties described in example 1. The wear scar of the resultant compositions was determined using the HFRR test method described in example 2. Additive B comprises the reaction product of an alkenyl substituted succinic anhydride (prepared from maleic anhydride and a C18 olefin) and ammonia. Ester lubricity additive C comprises an ester reaction product of a polyisobutenyl substituted succinic acid or anhydride thereof, ethylene glycol and isopropanol. Additive D comprises glycerol esters of tall oil fatty acid, with glycerol monooleate as the major component. Additive E comprises the reaction product of tall oil fatty acid and diethanolamine. Table 3 Example Fuel Composition Ester / amide / imide additive Treat rate (PPm) Aromatic compound (25 ppm) Inventive (I) or comparative (C) Wear scar (pm) 16 B 125 - C 405 17 C 125 - C 445 18 D 125 - C 450 19 E 125 - C 440 20 - 4-hydroxybenzaldehyde C 480 21 - - Methyl 4-hydroxybenzoate C 560 22 B 100 4-hydroxybenzaldehyde I 332 23 C 100 4-hydroxybenzaldehyde I 300 24 D 100 4-hydroxybenzaldehyde I 365 25 A 100 benzamide I 345 26 C 100 benzamide I 360 27 A 100 4-nitrophenol I 320 28 C 100 4-nitrophenol I 390 29 A 100 Methyl 4-hydroxybenzoate I 320 30 B 100 Methyl 4-hydroxybenzoate I 320 31 C 100 Methyl 4-hydroxybenzoate I 365 32 D 100 Methyl 4-hydroxybenzoate I 400 33 E 100 Methyl 4-hydroxybenzoate I 350 Example 4 Diesel fuel compositions were prepared by adding the components listed in table 4 into an EN590 compliant BO base fuel. The wear scar of the resultant compositions was determined using the HFRR test method described in example 2. Table 4 Example Fuel Composition Ester / amide / imide additive Treat rate (PPm) Aromatic compound (25 ppm) Inventive (I) or comparative (C) Wear scar (pm) 35 B 125 - C 405 36 C 125 - C 405 37 D 125 - c 430 38 E 125 - c 580 39 - - 4-hydroxybenzaldehyde c 540 40 - - Methyl 4-hydroxybenzoate c 590 41 C 100 4-hydroxybenzaldehyde I 345 42 D 100 4-hydroxybenzaldehyde I 335 43 E 100 4-hydroxybenzaldehyde I 462 44 B 100 Methyl 4-hydroxybenzoate I 300 45 C 100 Methyl 4-hydroxybenzoate I 242 46 D 100 Methyl 4-hydroxybenzoate I 345 47 E 100 Methyl 4-hydroxybenzoate I 450