Improvements relating to fuel compositions

Abstract

A lubricity additive for a diesel fuel composition comprising an ester, where the ester is an ester of a polyhydric alcohol (polyol) and an acylating compound, where the acylating compound is either a

Description

The present invention relates to methods and uses relating to fuel compositions, especially diesel fuel compositions. In particular the invention relates to methods of improving the lubricity of diesel fuel compositions. 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 materials have a level of natural lubricity. However the lubricating properties are not always sufficient to prevent wear, particularly when blended fuels are used. 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, the effects of these additives often diminishes over time and increasing wear is seen, particularly when engine parts have been in use for a long time. The industry standard test method for evaluating the performance of lubricity improving additives measures the wear between two metal components in a high frequency reciprocating rig (HFRR). This is reported in terms of a wear scar. Even when lubricity improving additives are included in a fuel composition, it is still common for the wear scar diameter measured in the HFRR test to increase over the duration of the test. This indicates that wear will increase over prolonged periods. Because the effects of engine wear can be catastrophic it is desirable to provide additives which protect against wear which have long lasting performance. The present inventors have surprisingly found that certain ester containing lubricity additives can be used to provide a longer lasting improvement in lubricity. Such a longer lasting performance may be observable in an HFRR test carried out over an extended period. In such a test the advantage of the present invention is evident when the wear scar does not increase significantly over the extended test period. According to a first aspect of the present invention there is provided the use of at least 100 ppm of a lubricity improving additive to provide an extended lubricating effect in a diesel fuel composition wherein the lubricity improving additive comprises an ester of a polyhydric alcohol and an acylating compound selected from fatty acids and hydrocarbyl substituted succinic acid compounds. According to a second aspect of the present invention there is provided a method of improving the lubricity of a diesel fuel composition for an extended period, the method comprising dosing into the fuel at least 100 ppm of a lubricity improving additive comprising an ester of a polyhydric alcohol and an acylating compound selected from fatty acids and hydrocarbyl substituted succinic acid compounds. The acylating compound used to prepare the lubricity improving additives suitably includes a hydrocarbyl moiety, preferably 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. The acylating compound is selected from fatty acids and hydrocarbyl substituted succinic acid compounds. In some embodiments the acylating compound is 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. 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. 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. 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. One preferred fatty acid is oleic acid. Mixture of fatty acids may also be used. Such mixtures may result from combining different fatty acids or may be derived from natural occurring mixtures, for example, tall oil fatty acid. The skilled person will appreciate that fatty acids derived from natural sources typically comprise mixtures of compounds. An example of such natural sources is tall oil fatty acid. Tall oil fatty acid is especially suitable for use in the present invention. 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. In some embodiments the acylating compound is a hydrocarbyl substituted succinic acid compound. By hydrocarbyl substituted succinic acid compound we mean to include hydrocarbyl substituted succinic acids in which both acid groups are present as the free acid; hydrocarbyl substituted succinic anhydrides; and monoesters of hydrocarbyl substituted succinic acids, i.e. compounds in which one of the acid groups of the succinic acid moiety has been esterified and the other acid group is present as the free acid. In the case of monoesters it is the free acid group which functions as the acylating agent. In preferred embodiments the hydrocarbyl substituted succinic acid compound is a hydrocarbyl substituted succinic acid or a hydrocarbyl substituted succinic anhydride. Hydrocarbyl substituted succinic acid compounds are typically prepared by the reaction of an alkene with maleic anhydride. The product may be optionally hydrolysed to form the diacid which may be reacted with an alcohol to form an ester or the anhydride can be directly reacted with an alcohol. 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 acylating agent is a hydrocarbyl substituted succinic acid compound 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 acylating agent is a hydrocarbyl substituted succinic acid compound 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 acylating agent is a hydrocarbyl substituted succinic acid compound 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 embodiments acylating agent is a polyisobutenyl substituted succinic acid compound. Preferably the polyisobutenyl substituent has a number average molecular weight of from 80 to 5000, preferably from 100 to 1000, more preferably from 200 to 600. Preferred polyisobutenyl substituted compounds include those having a polyisobutenyl substituent with a number average molecular weight of about 260 or about 550. A polyisobutenyl substituent number average molecular weight of about 260 is especially preferred. To form the lubricity improving additives used in the present invention, the acylating compound is reacted with a polyhydric alcohol. In embodiments in which the acylating compound comprises a hydrocarbyl substituted succinic acid or anhydride, one or both of the carboxylic acid groups may react with the alcohol to form an ester. Thus the lubricity improving additive may comprise a monoester of a succinic acid or a diester of succinic acid. The lubricity improving additive comprises the reaction product of an acylating compound and a polyhydric alcohol. The term polyhydric alcohol is used to refer to any compound including two or more OH functional groups. In the lubricity improving additive compounds one or more than one of the hydroxy groups of the polyhydric alcohol 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 polyhydric alcohol is a compound 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 an acylating agent and a polyhydric alcohol. When preparing the lubricity improving additives of the present invention which are esters, since the polyhydric alcohol has multiple functional groups, and the carboxylic acid compound may have multiple functional groups, a mixture of products may be obtained. In some embodiments the acylating compound and the 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 acylating 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 acylating agent comprises a succinic acid compound 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. In embodiments in which the first alcohol is a monohydric alcohol and the second alcohol is a polyhydric alcohol the acylating agent is a monoester of a hydrocarbyl substituted succinic acid. The lubricity improving additives used in the present invention comprise the reaction product of a polyhydric alcohol and an acylating compound selected from fatty acids and hydrocarbyl substituted succinic acid compounds. For the avoidance of doubt the lubricity improving additives may comprise a mixture of compounds. For avoidance of doubt mixtures of lubricity improving additive compounds that may be present include mixtures formed by reacting a mixture of different polyhydric alcohols with an acylating compound and / or mixtures formed by reacting a polyhydric alcohol with a mixture of acylating compounds and / or compounds formed by reacting a mixture of polyhydric alcohols with a mixture of acylating compounds. Already formed ester compounds may also be combined to form mixtures. The use of mixtures may arise due to the availability of starting materials or a particular mixture may be deliberately selected. In this specification any reference to “an additive” or “the additive” of the invention includes embodiments in which a single additive compound is present and embodiments in which two or more additive compounds are present. In embodiments in which two or more compounds are present the mixtures may be present due to a mixture of starting materials being used to prepare the additive compounds (e.g. a mixture of polyhydric alcohols and / or a mixture of acylating compounds). Alternatively and / or additionally two or more pre-formed ester compounds may be mixed into a fuel composition. A first class of ester lubricity additives suitable for use herein comprises an ester of a fatty acid and a polyhydric alcohol. Preferably the fatty acid is 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 polyhydroxy 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 compound. 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) or a 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 polyhydroxy 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 R4 is 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 C12 to 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 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 1200, 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 or anhydride 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. In preferred embodiments the lubricity improving additive is 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. Preferably the lubricity improving additive is 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 1000; 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 (v) 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 1000 and the other one of R1 and R3 is hydrogen. Preferably the lubricity improving additive is 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. Preferably the lubricity improving additive is 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. The lubricity improving additives used in the present invention provide an extended lubricating effect in a diesel fuel composition. 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 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 of less than 80°C, preferably 30 to 60°C. The lubricity improving additives are dosed in the diesel fuel according to the present invention in an amount of at least 100 ppm. Preferably the lubricity improving additives are included in the diesel fuel composition in an amount of at least 120 ppm, preferably at least 140 ppm, more preferably at least 160 ppm. The lubricity improving additive may be present in the diesel fuel composition in an amount of up to 5000 ppm, preferably 2000 ppm, suitably up to 1000 ppm, for example up to 500 ppm or up to 350 ppm. Preferably the ester lubricity additives are dosed in the diesel fuel according to the present invention in an amount of from 100 to 500 ppm, preferably from 120 to 500 ppm, more preferably from 140 to 300 ppm. For the avoidance of doubt, when mixtures of lubricity improving additives are present, the above amounts refer to the total amount of all lubricity improving additives present in the composition. The lubricity improving additives used in the present invention provide an extended lubricating effect. By an extended lubricating effect we mean that wear resistance is maintained and does not diminish overtime. The extended lubricating effect may be demonstrated by no further increase in wear being observed after an initial period. Such an effect can be illustrated in the HFRR test. The standard procedure for the HFRR (high frequency reciprocating rig) test is set out in ISO 12156-1:2023 and described in example 2. The standard HFRR test involves measuring the wear scar at the end of a 75 minute period. The present inventors have surprisingly found that when using the lubricity improving additives of the present invention, although the wear scar initially increases at the start of the test as routinely observed, a plateau is reached whether no further significant increase is observed overtime. Thus the extended lubricating effect provided by the present invention may involve reaching a maximum wear scar diameter in an HFRR test above which no significant further increase in wear scar is observed. Preferably the maximum wear scar diameter is achieved during the standard period of running the test, i.e. within 75 minutes. Preferably the maximum wear scar diameter is achieved within 70 minutes of starting the HFRR test, preferably within 65 minutes, suitably within 60 minutes. In some instances the maximum wear scar diameter is achieved within 55 or 50 minutes of starting the HFRR test. In some instances the maximum wear scar diameter is achieved within 35 or 40 minutes of starting the HFRR test. In some instances the maximum wear scar diameter is achieved within 15 or 20 minutes of starting the HFRR test. At high treat rates of the lubricity improving additives, for example more than 300 or more than 400 ppm, the maximum wear scar diameter may be achieved within 15 minutes or within 10 minutes. The present invention provides a prolonged or extended improvement in lubricity. Such an effect is not achieved by all lubricity improvers known in the art. Thus according to the present invention the lubricity improving additives provide an extended increase in lubricity such that when the time for which the HFRR test is run is extended, no significant further increase in wear scar is observed. Suitably according to the present invention no further significant increase in wear scar is observed on extending the duration of the HFRR test from 75 to 90 minutes. Preferably no further significant increase in wear scar is observed on extending the duration of the HFRR test from 75 to 100 minutes. Preferably no further significant increase in wear scar is observed on extending the duration of the HFRR test from 75 to 110 minutes. Preferably no further significant increase in wear scar is observed on extending the duration of the HFRR test from 75 to 120 minutes. Preferably on running an extended HFRR test no significant increase in wear scar is seen between 70 and 100 minutes. Preferably on running an extended HFRR test no significant increase in wear scar is seen between 70 and 120 minutes. Preferably on running an extended HFRR test no significant increase in wear scar is seen between 60 and 100 minutes. Preferably on running an extended HFRR test no significant increase in wear scar is seen between 60 and 120 minutes. By an extended HFRR test we mean to refer to an HFRR test which is run following the standard procedure but for a longer period of time, i.e. more than 75 minutes. In the statements above by no further significant increase in wear scar we mean that the wear scar does not increase more than 10% over the specified time period. In some embodiments the method of the second aspect of the present invention may involve dosing into a diesel fuel composition at least 100 ppm of a lubricity improving additive comprising an ester of a polyhydric alcohol and an acylating compound selected from fatty acids and hydrocarbyl substituted succinic acid compounds; performing a test on the diesel fuel composition according to the standard procedure for the HFRR (high frequency reciprocating rig) test; taking a first wear scar measurement after 75 minutes; and taking a second wear scar measurement after 90 minutes; wherein the difference between the first wear scar measurement and the second wear scar measurement is less than 10%. The invention will now be further described with reference to the following non-limiting examples. Example 1 Fuel compositions were prepared by adding ester lubricity additives and further additive compounds to a hydrotreated vegetable oil (HVO) fuel, as detailed in table 1. The HVO fuel complied with the specification set out in EN15940 (2023). The cloud point of the fuel was - 23°C and the pour point was -38°C. Table 1 Fuel Composition Additive Treat rate (ppm) 1 None 0 2 A 200 3 B 200 4 C 200 5 (comparative) D 200 Additive A comprises glycerol esters of tall oil fatty acid, with glycerol monooleate as the major component. Additive B 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. Additive C comprises a mixed ester reaction product of ethylene glycol and isopropanol and a polyisobutenyl substituted succinic anhydride having a PIB number average molecular weight of 260. Additive D is a comparative example comprising an acid based lubricity improving additive comprising tall oil fatty acid. 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 as outlined below. 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 every 10 minutes and the wear scar is measured. The results are given in Table 2 and illustrated in figure 1: Composition Wear scar (p.m) Time: 10 min 20 min 30 min 40 min 50 min 60 min 70 min 1 271 337 400 459 506 537 580 2 211 241 274 284 285 283 292 3 222 264 295 317 326 329 332 4 230 287 320 352 372 387 383 5 229 262 305 333 371 392 419 Example 3 Further fuel compositions were prepared by adding ester lubricity additives and further additive compounds to a B0 fuel, as detailed in table 3. The fuel was an EN590 compliant diesel fuel supplied by Coryton Advanced Fuels, UK. Table 3 Composition Additive Treat rate (ppm) 6 None 0 7 D 200 8 B 200 Example 4 The example fuel compositions of example 1 were tested according to the standard HFRR procedure set out in ISO 12156-1:2023, modified to extend the test period. The results are given in Table 4 and illustrated in figure 2: Composition 6 7 8 Time Wear scar (p.m) 10 min 230 215 225 20 min 310 265 270 30 min 360 295 305 40 min 400 330 335 50 min 450 370 340 60 min 495 400 335 70 min 520 430 340 80 min 550 450 350 90 min 570 470 355 100 min 595 490 350 110 min 610 520 355 120 min 640 555 360